No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Three-Dimensional Bulk Fermiology of CeRu 2 Ge 2 in the Paramagnetic Phase by Soft X-Ray h ν -Dependent (700–860 eV) ARPES
Abstract
By virtue of the soft x-ray angle-resolved photoelectron spectroscopy, the three-dimensional bulk fermiology has been successfully performed for a strongly correlated Ce compound, ferromagnet ${\mathrm{CeRu}}_{2}{\mathrm{Ge}}_{2}$ in the paramagnetic phase. A clear difference of the Fermi surface topology from either band calculation or de Haas\char21{}van Alphen results in the ferromagnetic phase is observed and interpreted by considering the difference of the $4f$ contribution to the Fermi surfaces in the paramagnetic phase.
... Although the electronic structure and Fermi surface topology are very important for discussing the itinerant or localized 4f nature in the system, quantum oscillation measurements have not been successful to date for CeCu 2 Ge 2 . Soft x-ray angle-resolved photoemission spectroscopy (ARPES) at hν > 500 eV has an advantage in probing the three-dimensional bulk electronic structure of strongly correlated electron systems as well as their Fermi surface topology, suppressing their surface contributions highly deviated from the bulk ones [6,7,8,9] . In this paper we report the electronic structure of CeCu 2 Ge 2 probed by the soft x-ray ARPES and the resonance photoemission study near the 3d 5/2 edges for LaCu 2 Ge 2 (hν ∼830 eV) and CeCu 2 Ge 2 (hν ∼880 eV). ...
... The clean surface was obtained by cleaving in situ providing a (001) plane at the measuring temperature of 20 K in the base pressure of ∼ 1 × 10 −8 Pa. It is known that compounds with the ThCr 2 Si 2 crystal structure can be reproducibly cleaved by a commonly used a "lever-a-post" method [7,8,9]. Neither O nor C 1s photoelectron signal was detected, which suggests the cleanliness of the cleaved surface. ...
... The spectral weight of the tail of the Kondo resonance is much weaker than the spin-orbit partner in contrast to the case of a typical heavy fermion system CeRu 2 Si 2 [16,18], which suggests that the possible 4f -derived quasi-particle spectral weight is negligible in the total 4f spectral weight [9] for CeCu 2 Ge 2 . In the case of CeRu 2 Ge 2 where the 4f 1 5/2 final-state peak height has been rather comparable to the 4f 1 7/2 one [16,18], its non-4f electronic structure probed by the soft x-ray ARPES has been basically consistent with the band-structure calculation for LaRu 2 Ge 2 corresponding to the 4f -localized model [7]. Therefore, the result of the Ce 3dedge resonance photoemission and that of the soft soft x-ray ARPES probing the non-4f states are mutually consistent. ...
We have performed bulk-sensitive soft x-ray angle-resolved photoemission spectroscopy for CeCu2Ge2 isostructural to a heavy fermion superconductor CeCu2Si2, indicating the localized 4f character in ambient pressure. Resonance enhancement is seen at the La 3d5/2-edge for LaCu2Ge2 in both angle-resolved and integrated photoemission although the magnitude of enhancement is less than that in the Ce 3d5/2-edge resonance photoemission for CeCu2Ge2, which indicates that the rare-earth 5d contributions can also be enhanced in addition to the 4f contributions at the resonance conditions.
... For example, PES experiments revealed limitations of the single impurity models for describing dense Kondo systems [36][37][38]. ARPES experiments carried on varieties of Kondo lattice systems like CeRu 2 Si 2 [54][55][56], CeRu 2 Ge 2 [57], CeBi [58], CeNiSn [59] and YbRh 2 Si 2 [60][61][62] showed large FSs due to the coherent participation of 4f electrons. ARPES experiments also permitted a direct observation of dispersive Kondo resonance peaks in CeCoGe 1.2 Si 0.8 [63] and surface and bulk hybridizations in AF Kondo lattice CeRh 2 Si 2 [64]. ...
... For all values considered for the Kondo interaction and the electronic filling, we find that the FS of the Kondo lattice (x = 1) is large and it includes the contributions from both the conduction electrons and the Kondo spins. This universal feature is in good agreement with previous theoretical and experimental results [52][53][54][55][56][57][58][59][60][61][62]. It can be well understood in terms of Luttinger theorem, which stipulates that all fermionic degrees of freedom participate in the formation of the Fermi liquid ground state. ...
We study the Kondo alloy model on a square lattice using dynamical mean-field theory for Kondo substitution and disorder effects, together with static mean-field approximations. We computed and analyzed photoemission properties as a function of electronic filling n c , Kondo impurity concentration x, and strength of Kondo temperature T K. We provide a complete description of the angle resolved photoemission spectroscopy (ARPES) signals expected in the paramagnetic (PM) Kondo phases. By analyzing the Fermi surface (FS), we observe the Lifshitz-like transition predicted previously for strong T K at x = n c and we discuss the evolution of the dispersion from the dense coherent to the dilute Kondo regimes. At smaller T K, we find that this transition marking the breakdown of coherence at x = n c becomes a crossover. However, we identify another transition at a smaller concentration x ⋆ where the effective mass continuously vanishes. x ⋆ separates the one-branch and the two-branches ARPES dispersions characterizing respectively dilute and dense Kondo PM regimes. The x − T K phase diagrams are also described, suggesting that the transition at x ⋆ might be experimentally observable since magnetically ordered phases are stabilized at much lower T K. FS reconstructions in antiferromagnetic and ferromagnetic phases are also discussed.
... For example, PES experiments revealed limitations of the single impurity models for describing dense Kondo systems [36,37,38]. ARPES experiments carried on varieties of Kondo lattice systems like CeRu 2 Si 2 [54,55,56], CeRu 2 Ge 2 [57], CeBi [58],CeNiSn [59] and YbRh 2 Si 2 [60,61,62] showed large Fermi surfaces due to the coherent participation of 4f electrons. ARPES experiments also permitted a direct observation of dispersive Kondo resonance peaks in CeCoGe 1.2 Si 0.8 [63] and surface and bulk hybridizations in antiferromagnetic Kondo lattice CeRh 2 Si 2 [64]. ...
... For all values considered Photo-emission signatures of coherence breakdown in Kondo alloys: dynamical mean-field theory approach13 for the Kondo interaction and the electronic filling, we find that the Fermi surface of the Kondo lattice (x = 1) is large and it includes the contributions from both the conduction electrons and the Kondo spins. This universal feature is in good agreement with previous theoretical and experimental results [52,53,54,55,56,57,58,59,60,61,62]. It can be well understood in terms of Luttinger theorem, which stipulates that all fermionic degrees of freedom participate in the formation of the Fermi liquid ground state. ...
We study the Kondo alloy model on a square lattice using dynamical mean-field theory (DMFT) for Kondo substitution and disorder effects, together with static mean-field approximations. We computed and analyzed photoemission properties as a function of electronic filling $n_c$, Kondo impurity concentration $x$, and strength of Kondo temperature $T_K$. We provide a complete description of the Angle Resolved Photoemission Spectroscopy (ARPES) signals expected in the paramagnetic Kondo phases. By analyzing the Fermi surface, we observe the Liftshitz-like transition predicted previously for strong $T_K$ at $x=n_c$ and we discuss the evolution of the dispersion from the dense coherent to the dilute Kondo regimes. At smaller $T_K$, we find that this transition marking the breakdown of coherence at $x=n_c$ becomes a crossover. However, we identify another transition at a smaller concentration $x^\star$ where the effective mass continuously vanishes. $x^\star$ separates the one-branch and the two-branches ARPES dispersions characterizing respectively dilute and dense Kondo paramagnetic regimes. The $x-T_K$ phase diagrams are also described, suggesting that the transition at $x^\star$ might be experimentally observable since magnetically ordered phases are stabilized at much lower $T_K$. Fermi surface reconstructions in antiferromagnetic and ferromagnetic phases are also discussed.
... where v F , E,hk, andhk F denote the Fermi velocity, binding energy, momentum, and Fermi momentum, respectively. Since ARPES is advantageous to investigate momentumdependent electronic structures compared to other probes, such as quantum oscillation experiments, intensive studies have been performed to reveal the quasiparticle band structures in HF materials [11][12][13][14][15][16][17][18][19][20][21][22][23][24]. Nevertheless, only a few ARPES works have been reported for the renormalized (heavy) bands of HF systems [12,13,21]. ...
... The sample qualities were examined on the basis of the absence of O and C 1s core-level peaks caused by possible impurities or surface oxidization [44]. Figure 1 shows an evolution of the Fermi surface (FS) from LaNi 2 Ge 2 to CeNi 2 Ge 2 obtained by hν-dependent ARPES, which has an advantage of clarifying the bulk FS topology in the 3D Brillouin zone (BZ) [17]. The FS topology in the k x -k y plane is circular for LaNi 2 Ge 2 , as shown in Fig. 1(b), but that for CeNi 2 Ge 2 in Fig. 1(a) is elliptical with the major axis along the -X line. ...
We present clear experimental evidence for the momentum-dependent heavy fermionic electronic structures of the 4f-based strongly correlated system CeNi2Ge2 by soft x-ray angle-resolved photoemission spectroscopy. A comparison between the experimental three-dimensional quasiparticle dispersion of LaNi2Ge2 and CeNi2Ge2 has revealed that heavy fermionic electronic structures are seen in the region surrounding a specific momentum. Furthermore, the wave vectors between the observed “heavy spots” are consistent with a result of neutron scattering reflecting magnetic correlations, which could be a trigger for the superconductivity in CeNi2Ge2.
... On Kondo alloys, it was first Park et al. [221] conducted ARPES experiments on multiple Ce-based Kondo lattice systems, since then this probe was employed extensively to study the electronic structure of correlated electronic systems [15]. These experimental technics have already been proven to be very useful to investigate the physics of −electron systems, including Kondo [223][224][225], CeRu 2 Ge 2 [226], CeBi [227],CeNiSn [228] and YbRh 2 Si 2 [135,229,230] showed large Fermi surfaces due to the coherent participation of 4 electrons. ARPES can also reveal anisotropic properties present in some Kondo alloys. ...
n this thesis, we will study f -electron systems under two different aspects: the formation and the breakdown of lattice coherence, and the nature of f -electrons, which can be either localized, itinerant, or dual. In the first part, we study lattice coherence in 4f systems with the atomic substitution of magnetic atoms by non-magnetic atoms. We will deal with the ubstitutional disorder by using the dynamic mean-field, theory. We start by generalizing the Doniach type phase diagram with substitution by considering the phases: ferromagnetic, antiferromagnetic, and paramagnetic Kondo. We also study the relevance of our phase diagrams by comparing the experimental data of various cerium-based Kondo alloys. Next, we will focus on the Kondo paramagnetic phase on a square lattice in order to study the signatures of lattice coherence breakdown with the dilution of magnetic impurities. To do this, we analyze the photoemission signals, the effective masses, the local potential scattering, and the charge order. We confirmed previous predictions of a Lifshitz-type transition between dilute and dense impurity systems. In addition, we detect a new critical concentration for a Fermi liquid instability. The latter is highlighted by a vanishing effective mass. The second part of this thesis deals with the apparent dual character of 5f electrons in actinide-based heavy-fermion compounds, where itinerant and localized 5f -degrees of freedom seem to coexist. We adopt the rotationally invariant slave-boson method to study the influence of intra-atomic, i. e., Hund’s rule type correlations. Our results confirm the conjecture that intra-atomic correlations may enhance anisotropies in the effective 5f -5f hopping and thus eventually lead to orbital-selective partial localization. Finally, we analyze the different partially localized phases in terms of orbital-dependent quasiparticle mass and occupation, magnetization, and valency configurations.
... An especially strong Cu 3d-Ce 4f hybridization had also been conjectured from the extreme pair-breaking capability of tiny substitutions of nonmagnetic (Rh, Pd) as well as magnetic (Mn) impurities on the Cu sites [52]. We mention that the experimental FS of CeCu 2 Si 2 is different from other Ce-based 122 HF compounds, such as CeRu 2 Ge 2 [53], CeRu 2 Si 2 [43], and CeRu 2 ðSi 0.82 Ge 0.18 Þ 2 [54]. ...
The superconducting order parameter of the first heavy-fermion superconductor CeCu2Si2 is currently under debate. A key ingredient to understand its superconductivity and physical properties is the quasiparticle dispersion and Fermi surface, which remains elusive experimentally. Here, we present measurements from angle-resolved photoemission spectroscopy. Our results emphasize the key role played by the Ce 4f electrons for the low-temperature Fermi surface, highlighting a band-dependent conduction-f electron hybridization. In particular, we find a very heavy quasi-two-dimensional electron band near the bulk X point and moderately heavy three-dimensional hole pockets near the Z point. Comparison with theoretical calculations reveals the strong local correlation in this compound, calling for further theoretical studies. Our results provide the electronic basis to understand the heavy-fermion behavior and superconductivity; implications for the enigmatic superconductivity of this compound are also discussed.
... The electronic structure is three-dimensional despite the chainlike arrangement of Ce, but the resulting k-space anisotropy in V can be naturally linked to the proposed quasi-1D magnetic anisotropy, which could be key for the observed FM QCP [1]. Since the electronic structure of FM Kondo-lattice systems, particularly their momentumresolved 4f bands, has been studied much less by ARPES [37][38][39] compared to their AFM counterparts, our results can be useful for a basic understanding of these materials. It is interesting to note that FM rare-earth Kondolattice systems are rare compared to AFM systems [40,41]. ...
Heavy fermion compounds exhibiting a ferromagnetic quantum critical point have attracted considerable interest. Common to two known cases, i.e., CeRh6Ge4 and YbNi4P2, is that the 4f moments reside along chains with a large interchain distance, exhibiting strong magnetic anisotropy that was proposed to be vital for the ferromagnetic quantum criticality. Here, we report an angle-resolved photoemission study on CeRh6Ge4 in which we observe sharp momentum-dependent 4f bands and clear bending of the conduction bands near the Fermi level, indicating considerable hybridization between conduction and 4f electrons. The extracted hybridization strength is anisotropic in momentum space and is obviously stronger along the Ce chain direction.The hybridized 4f bands persist up to high temperatures, and the evolution of their intensity shows clear band dependence. Our results provide spectroscopic evidence for anisotropic hybridization between conduction and 4f electrons in CeRh6Ge4, which could be important for understanding the electronic origin of the ferromagnetic quantum criticality.
... An especially strong Cu 3d -Ce 4f hybridization had also been conjectured from the extreme pair-breaking capability of tiny substitutions of nonmagnetic (Rh, Pd) as well as magnetic (Mn) impurities on the Cu sites [52]. We mention that the experimental FS of CeCu 2 Si 2 is different from other Ce-based 122 HF compounds, such as CeRu 2 Ge 2 [53], CeRu 2 Si 2 [43] and CeRu 2 (Si 0.82 Ge 0.18 ) 2 [54]. ...
The superconducting order parameter of the first heavy-fermion superconductor CeCu2Si2 is currently under debate. A key ingredient to understand its superconductivity and physical properties is the quasiparticle dispersion and Fermi surface, which remains elusive experimentally. Here we present measurements from angle-resolved photoemission spectroscopy. Our results emphasize the key role played by the Ce 4f electrons for the low-temperature Fermi surface, highlighting a band-dependent conduction-f electron hybridization. In particular, we find a very heavy quasi-two-dimensional electron band near the bulk X point and moderately heavy three-dimensional hole pockets near the Z point. Comparison with theoretical calculations reveals the strong local correlation in this compound, calling for further theoretical studies. Our results provide the electronic basis to understand the heavy fermion behavior and superconductivity; implications for the enigmatic superconductivity of this compound are also discussed.
... The electronic structure is three-dimensional despite the chain-like arrangement of Ce, but the resulting k-space anisotropy in V can be naturally linked to the proposed quasi-1D magnetic anisotropy, which could be key for the observed FM QCP [1]. Since the electronic structure of FM Kondolattice systems, particularly their momentum-resolved 4f bands, are much less studied by ARPES [33][34][35], compared to their AFM counterparts, our results can be useful for a basic understanding of these materials. It is interesting to note that FM rare-earth Kondo-lattice systems are rare compared to AFM systems [36,37]. ...
Heavy fermion compounds exhibiting a ferromagnetic quantum critical point have attracted considerable interest. Common to two known cases, i.e., CeRh6Ge4 and YbNi4P2, is that the 4f moments reside along chains with a large inter-chain distance, exhibiting strong magnetic anisotropy that was proposed to be vital for the ferromagnetic quantum criticality. Here we report an angle-resolved photoemission study on CeRh6Ge4, where we observe sharp momentum-dependent 4f bands and clear bending of the conduction bands near the Fermi level, indicating considerable hybridization between conduction and 4f electrons. The extracted hybridization strength is anisotropic in momentum space and is obviously stronger along the Ce chain direction. The hybridized 4f bands persist up to high temperatures, and the evolution of their intensity shows clear band dependence. Our results provide spectroscopic evidence for anisotropic hybridization between conduction and 4f electrons in CeRh6Ge4, which could be important for understanding the electronic origin of the ferromagnetic quantum criticality.
... Actually, changes of the electronic structure between Ceand the reference La-based compounds can also be found in other heavy fermion compounds, such as CeRu 2 Ge 2 and its reference compound LaRu 2 Ge 2 [27]. E F shift of CeRu 2 Ge 2 in the paramagnetic phase compared with LaRu 2 Ge 2 is thought to be due to the increased number of the electrons contributing to the near E F bands in the paramagnetic CeRu 2 Ge 2 , where the weak but nonnegligible hybridization of the Ce-4f electron should be additionally taken into account. ...
LaIrIn5 is a reference compound of the heavy-fermion superconductor CeIrIn5. The lack of f electrons in LaIrIn5 indicates that there should not be any f electron participating in the construction of its Fermi surface. Thus the electronic structure comparison between LaIrIn5 and CeIrIn5 provides a good platform to study the properties of f electrons. Here angle-resolved photoemission spectroscopy (ARPES) measurements and density functional theory (DFT) calculations are performed to study the electronic structures of LaIrIn5 and CeIrIn5. We find the valence band structures of the two materials are similar to each other, except for the absence of f bands in LaIrIn5. By analyzing the Fermi crossings of the three conduction bands of the two materials quantitatively, we find the volumes of the electron pockets α and β around the M′ point become larger from LaIrIn5 to CeIrIn5, while the hole pocket γ around the Γ′ point becomes smaller. Together with the calculation results, we confirm that this is mainly originated from the f-electron contribution, while the lattice-constant difference between LaIrIn5 and CeIrIn5 only has a finite influence. We also give a summary of the f-electron character in its related Ce-115 heavy fermion compounds. Our results may be essential for the complete microscopic understanding of the 115 compounds and the related heavy-fermion systems.
... Since the f -electron cross section is strongly hνdependent, with diminishing intensity below ∼ 50 eV (Yeh and Lindau, 1985), hν > 100 eV is routinely employed to exploit the enhanced f -electron signal. Moreover, soft x-ray (> 500 eV) ARPES has been useful for suppressing the contribution of surface states and achieving true bulk sensitivity, albeit at the cost of compromised energy resolution (Yano et al., 2007). Another technical yet important limitation is imposed by safety protocols concerning transuranic compounds, spurring the development of separate dedicated ARPES facilities for these materials (Graham et al., 2013). ...
The physics of quantum materials is dictated by many-body interactions and mathematical concepts such as symmetry and topology that have transformed our understanding of matter. Angle-resolved photoemission spectroscopy (ARPES), which directly probes the electronic structure in momentum space, has played a central role in the discovery, characterization, and understanding of quantum materials ranging from strongly-correlated states of matter to those exhibiting non-trivial topology. Over the past two decades, ARPES as a technique has matured dramatically with ever-improving resolution and continued expansion into the space-, time-, and spin-domains. Simultaneously, the capability to synthesize new materials and apply non-thermal tuning parameters \emph{in-situ} has unlocked new dimensions in the study of all quantum materials. We review these developments, and survey the scientific contributions they have enabled in contemporary quantum materials research.
... There are two more benefits in using SX-ARPES for studying Ir j eff states: (i) a high Ir 5d-O 2p sensitivity ratio, which is about 60 times higher than that in the vacuum ultraviolet (VUV) region [22], and (ii) a large survey area that covers more than one whole Brillouin zone (BZ) in the 3D k space. This is made possible by scanning a photoelectron acceptance angle of about 10 • and a photon energy of over 350 eV [23,24]. ...
In this study, we systematically investigate three-dimensional (3D) momentum (k)-resolved electronic structures of Ruddlesden-Popper-type iridium oxides Sr n+1 Ir n O 3n+1 using soft-x-ray (SX) angle-resolved photoemission spectroscopy (ARPES). Our results provide direct evidence of an insulator-to-metal transition that occurs upon increasing the dimensionality of the IrO 2-plane structure. This transition occurs when the spin-orbit-coupled j eff = 1/2 band changes its behavior in the dispersion relation and moves across the Fermi energy. In addition, an emerging band along the (0,0,0)-R(π,π,π) direction is found to play a crucial role in the metallic characteristics of SrIrO 3. By scanning the photon energy over 350 eV, we reveal the 3D Fermi surface in SrIrO 3 and k z-dependent oscillations of photoelectron intensity in Sr 3 Ir 2 O 7. In contrast to previously reported results obtained using low-energy photons, folded bands derived from lattice distortions and/or magnetic ordering make significantly weak (but finite) contributions to the k-resolved photoemission spectrum. At the first glance, this leads to the ambiguous result that the observed k-space topology is consistent with the unfolded Brillouin zone (BZ) picture derived from a nonrealistic simple square or cubic Ir lattice. Through careful analysis, we determine that a superposition of the folded and unfolded band structures has been observed in the ARPES spectra obtained using photons in both ultraviolet and SX regions. To corroborate the physics deduced using low-energy ARPES studies, we propose to utilize SX-ARPES as a powerful complementary technique, as this method surveys more than one whole BZ and provides a panoramic view of electronic structures.
... [15][16][17] In contrast, soft X-ray ARPES is more sensitive to the bulk electronic structure while still having reasonable momentum resolution. [18][19][20] In this work, we carry out a combined spectroscopic study with soft X-ray absorption (XAS), magnetic circular dichroism (XMCD), and ARPES to reveal the relationship between the magnetism and the hybridization between the Ce 4f and the conduction electrons (c-f hybridization) for CeAgSb 2 . ...
... Polycrystal photoemission measurements 22 , angular correlation of the electron-positron annihilation radiation 23 , and de Haas-van Alphen (dHvA) 24 measurements all reveal the localized nature of f electrons at ambient pressure. Photoemission spectroscopy is a powerful tool to detect the reconstruction of the electronic structure and is often used to judge the nature of f electrons [25][26][27][28] . Photoemission spectroscopy can even be used to detect the SC energy gap in HF systems 29 . ...
We show the three-dimensional electronic structure of the Kondo lattice CeIn3 using soft x-ray angle resolved photoemission spectroscopy in the paramagnetic state. For the first time, we have directly observed the three-dimensional topology of the Fermi surface of CeIn3 by photoemission. The Fermi surface has a complicated hole pocket centred at the Γ-Z line and an elliptical electron pocket centred at the R point of the Brillouin zone. Polarization and photon-energy dependent photoemission results both indicate the nearly localized nature of the 4f electrons in CeIn3, consistent with the theoretical prediction by means of the combination of density functional theory and single-site dynamical mean-field theory. Those results illustrate that the f electrons of CeIn3, which is the parent material of CeMIn5 compounds, are closer to the localized description than the layered CeMIn5 compounds.
... Moreover, it is well known that the conventional low energy ARPES probes only the surface electronic structures which is often quite different from the bulk ones especially for the strongly correlated 4f electron systems [15][16][17] . In contrast, the soft X-ray ARPES is more sensitive to the bulk electronic structure with still reasonable momentum resolution [18][19][20] . ...
We report a combined study for the electronic structures of ferromagnetic CeAgSb$_2$ using soft X-ray absorption (XAS), magnetic circular dichroism (XMCD), and angle-resolved photoemission (ARPES) spectroscopies. The Ce $M_{4, 5}$ XAS spectra show very small satellite structures, reflecting a strongly localized character of the Ce $4f$ electrons. The linear dichroism effects in the Ce $M_{4, 5}$ XAS spectra demonstrate the ground state Ce $4f$ symmetry of $\Gamma{_6}$, the spatial distribution of which is directed along the $c$-axis. The XMCD results give support to the picture of local-moment magnetism in CeAgSb$_2$. Moreover it is also found that the theoretical band dispersions for LaAgSb$_2$ provides better description of the ARPES band structures than those for CeAgSb$_2$. Nevertheless, ARPES spectra at the Ce $3d$-$4f$ resonance show the momentum dependence for the intensity ratio between Ce $4f^{1}_{5/2}$ and $4f^{1}_{7/2}$ peaks in a part of the Brillouin zone, suggesting the non-negligible momentum dependent hybridization effect between the Ce $4f$ and the conduction electrons. This is associated with the moderate mass enhancement in CeAgSb$_2$.
... Polycrystal photoemission measurements 22 , angular correlation of the electron-positron annihilation radiation 23 , and de Haas-van Alphen (dHvA) 24 measurements all reveal the localized nature of f electrons at ambient pressure. Photoemission spectroscopy is a powerful tool to detect the reconstruction of the electronic structure and is often used to judge the nature of f electrons [25][26][27][28] . Photoemission spectroscopy can even be used to detect the SC energy gap in HF systems 29 . ...
One basic concept in heavy fermions systems is the entanglement of localized
spin state and itinerant electron state. It can be tuned by two competitive
intrinsic mechanisms, Kondo effect and Ruderman-Kittel-Kasuya-Yosida
interaction, with external disturbances. The key issue regarding heavy fermions
properties is how the two mechanisms work in the same phase region. To
investigate the relation of the two mechanisms, the cubic antiferromagnetic
heavy fermions compound CeIn3 was investigated by soft x-ray angle resolved
photoemission spectroscopy. The hybridization between f electrons and
conduction bands in the paramagnetic state is observed directly, providing
compelling evidence for Kondo screening scenario and coexistence of two
mechanisms. The hybridization strength shows slight and regular anisotropy in K
space, implying that the two mechanisms are competitive and anisotropic. This
work illuminates the concomitant and competitive relation of the two mechanisms
and supplies some evidences for the anisotropic superconductivity of CeIn3.
... Another important concept in heavy-fermion systems is a hybridization effect between conduction (c) and f electrons. The c-f hybridization-derived electronic structure has been verified by ARPES studies [2][3][4][5][6][7][8][9][10][11][12][13] . However, most of those studies have been performed on the basis of the surface Brillouin zone (BZ) [2][3][4][5][6] or by using the 4d-4f resonant process [7][8][9] , in which k z (the momentum normal to surface) dependence on the electronic structure is neglected owing to its quasi-two-dimensionality. ...
We report the electronic structure of a prototypical valence fluctuation
system, YbAl2, using angle-resolved photoemission spectroscopy. The observed
band dispersions and Fermi surfaces are well described in terms of band
structure calculations based on local density approximation. Strong
hybridization between the conduction and 4f bands is identified on the basis of
the periodic Anderson model. The evaluated small mass enhancement factor and
the high Kondo temperature qualitatively agree with those obtained from
thermodynamic measurements. Such findings suggest that the strong hybridization
suppresses band renormalization and is responsible for the valence fluctuations
in YbAl2.
... To clarify this issue, polarization-dependent soft x-ray (SX)-ARPES with hν ∼ 400-800 eV has been carried out. [2][3][4][5][7][8][9] Within a theoretical SX-ARPES approach, which was recently developed and applied to Ag(001) (Ref. 8) and W(110), 9 we present a detailed comparison with experimental band maps, EDCs, and FSs of bulk ferromagnetic Ni(001), the prototype of moderate electron-electron correlations. ...
Within this soft x-ray photoemission study we present a detailed experimental and theoretical view on the bulk-related electronic structure of ferromagnetic nickel. Our results resolve the long-standing issue of the Fermi surfaces of bulk Ni and thereby establish the validity of a local correlation picture for its electronic structure. We performed complementary theoretical and experimental soft x-ray angle-resolved photomission spectroscopy studies to determine the Fermi surfaces and correlation effects in the bulk states of Ni. The electronic structure, obtained from the local-spin density approximation with dynamical mean-field theory and one-step photoemission calculations including matrix elements, is based on a fully relativistic treatment using a complex and energy-dependent self-energy Σ(E). The experimental band dispersions, the circular dichroism in the spectral functions, and the experimentally extracted self-energy Σ(E) are in agreement with theoretical findings.
As discussed in Chaps. 4 and 5, the bulk sensitivity becomes essential in photoelectron spectroscopy when one studies the electronic structure of strongly correlated electron systems where the effect of the short range Coulomb repulsion U becomes decisive.
The progress of photoelectron spectroscopy has strongly been backed up by the development of synchrotron radiationsynchrotron radiation (SR) from the late 1970s and, in particular, undulator radiation since the 1980s.
The physics of quantum materials is dictated by many-body interactions and mathematical concepts such as symmetry and topology that have transformed our understanding of matter. Angle-resolved photoemission spectroscopy (ARPES), which directly probes the electronic structure in momentum space, has played a central role in the discovery, characterization, and understanding of quantum materials ranging from strongly correlated states of matter to those exhibiting nontrivial topology. Over the past two decades, ARPES as a technique has matured dramatically with ever-improving resolution and continued expansion into the space, time, and spin domains. Simultaneously, the capability to synthesize new materials and apply nonthermal tuning parameters in situ has unlocked new dimensions in the study of all quantum materials. These developments are reviewed, and the scientific contributions they have enabled in contemporary quantum materials research are surveyed.
Graphite is a carbon allotrope with a unique anisotropy. The in-plane bonds of carbon have a strong covalent bonding characteristic, while out-of-plane bonding of carbon is due to the weak van der Waals interaction. Graphite can be easily exfoliated, i.e., a sheet of graphite called graphene can be easily separated. By intercalating metallic atoms, the property of graphite can be drastically altered. For example, CaC6 is known as a graphite intercalation compound (GIC) superconductor having transition temperature of 11.5 K at ambient pressure. In the first half of this chapter, the brief history of GIC discovery as a superconducting material is outlined, and then the current understanding of GIC superconducting mechanism is described. In the second half of this chapter, the experimental verification of the atomic and electronic structures of the pristine graphite and GIC using photoelectron spectroscopy and diffraction is introduced. Finally, this chapter is summarized and expanded into future prospects.
We report the soft X-ray angle-resolved photoemission study for LaNi2Ge2 to reveal the electronic structures derived from non-4f bands of the heavy fermion compound CeNi2Ge2. The photoemission spectra recorded at the La M4,5 absorption edges clearly show the enhancement of the La 5d components in the valence band spectra. The circular dichroism of photoemission spectra reveals the band-dependent dichroic response due to the orbital symmetry.
Recent remarkable progress in angle-resolved photoelectron spectroscopy (ARPES) has enabled the direct observation of the band structures of 4f and 5f materials. In particular, ARPES with various light sources such as lasers () or high-energy synchrotron radiations () has shed light on the bulk band structures of strongly correlated materials with energy scales of a few millielectronvolts to several electronvolts. The purpose of this paper is to summarize the behaviors of 4f and 5f band structures of various rare-earth and actinide materials observed by modern ARPES techniques, and understand how they can be described using various theoretical frameworks. For 4f-electron materials, ARPES studies of (, , and ) and with various incident photon energies are summarized. We demonstrate that their 4f electronic structures are essentially described within the framework of the periodic Anderson model, and that the band-structure calculation based on the local density approximation cannot explain their low-energy electronic structures. Meanwhile, electronic structures of 5f materials exhibit wide varieties ranging from itinerant to localized states. For itinerant compounds such as , their electronic structures can be well-described by the band-structure calculation assuming that all electrons are itinerant. In contrast, the band structures of localized compounds such as and are essentially explained by the localized model that treats electrons as localized core states. In regards to heavy fermion -based compounds such as the hidden-order compound , their electronic structures exhibit complex behaviors. Their overall band structures are generally well-explained by the band-structure calculation, whereas the states in the vicinity of E
F show some deviations due to electron correlation effects. Furthermore, the electronic structures of in the paramagnetic and hidden-order phases are summarized based on various ARPES studies. The present status of the field as well as possible future directions are also discussed.
As discussed in Chap. 4 and 5, the bulk sensitivity becomes essential in photoelectron spectroscopy when one studies the electronic structure of strongly correlated electron systems where the effect of the short range Coulomb repulsion U becomes decisive.
The progress of photoelectron spectroscopy has strongly been backed up by the development of synchrotron radiation (SR) from the late 1970s and, in particular, undulator radiation since the 1980s.
Angle integrated as well as angle resolved photoemission in the soft and hard X-ray regime became a very important tool to investigate the bulk properties of various materials. In practise enhanced bulk sensitivity can be achieved by so called threshold photoemission. Increased bulk sensitivity may suggest that LSDA band structure or density of states calculations can be directly compared to the measured spectra. However, various important effects, like matrix elements, the photon momentum or phonon excitations, are this way neglected. Here, we present a generalization of the one-step model that describes the excitation of the photoelectron, its transport to the surface and the escape into the vacuum in a coherent way. First, a short introduction to the main features of the one-step model implementation within the Munich SPR-KKR program package is given. The capability to account for correlation effects and chemical disorder using the LSDA+DMFT (dynamical mean field theory) scheme in combination with the coherent potential approximation (CPA) method will be demonstrated by various examples. Special emphasis is put on the description of phonon-assisted transitions which lead to the so-called XPS-limit in the hard X-ray and/or high temperature regime.
Soft X-ray angle-resolved photoemission has been performed for metallic V
2
O
3
. By combining a microfocus beam (40 µm × 65 µm) and micro-positioning techniques with a long-working-distance microscope, it has been possible to observe band dispersions from tiny cleavage surfaces with a typical size of several tens of µm. The photoemission spectra show a clear position dependence, reflecting the morphology of the cleaved sample surface. By selecting high-quality flat regions on the sample surface, it has been possible to perform band mapping using both photon-energy and polar-angle dependences, opening the door to three-dimensional angle-resolved photoemission spectroscopy for typical three-dimensional correlated materials where large cleavage planes are rarely obtained.
Angle-resolved photoelectron spectroscopy was measured in the Ce 3d→4f resonance energy region for the paramagnetic state of CeRu2Si 2, Ce0.84La0.16Ru2Si2 and LaRu2Si2 to investigate a variation of band structures around the quantum critical point (QCP). While the results clearly demonstrate the difference of the band structures between CeRu2Si2 and LaRu2Si2, the observed band structures of CeRu 2Si2 and Ce0.84La0.16Ru 2Si2 resemble each other. The results indicate that the Ce 4 f electrons in the paramagnetic state have an itinerant character in either side of the critical composition, and Ce1-xLaxRu 2Si2 near QCP is similar to CeRu2(Si 1-xGex)2.
Angle-resolved photoelectron spectroscopy (ARPES) measurements were made in the Ce resonance energy region for the paramagnetic state of CeRu2Si2, CeRu2(Si0.82Ge0.18)2, and LaRu2Si2 to investigate a variation of band structures around the quantum critical point (QCP). The observed band structures are very similar between CeRu2Si2 and CeRu2(Si0.82Ge0.18)2. The results indicate that the Ce 4f electrons in the paramagnetic state have an itinerant character and participate in the formation of energy bands both in CeRu2Si2 and CeRu2(Si0.82Ge0.18)2, and the change of the band structures in the paramagnetic states should be continuous around the QCP of the CeRu2(Si1−xGex)2 system.
Traditional angle-resolved photoemission (ARPES) with excitation in the ca. 20 to 150 eV range has clearly evolved to be the technique of choice for studying the electronic structure of surfaces and complex new strongly correlated and magnetic materials. However, it is clear that ARPES with excitation only up to 150 eV or so remains a very surface-sensitive probe, thus necessitating careful in-situ sample treatment, cleaving, or even synthesis to avoid the measurement of surface-associated artifacts. A key measure of this surface sensitivity is the electron inelastic mean free path (IMFP orΛe), which measures the mean depth of electron emission without inelastic scattering, and both experimental [1, 2] and theoretical [3] IMFP studies showing that the only reliable way to increase bulk or buried layer/interface sensitivity for all material types is to go to higher photon energies in the soft X-ray (ca. 0.5–2 keV) or hard X-ray (ca. 2–10 keV) regime.
The information obtained by all-direction-resolved photoelectron spectroscopy for valence band and core levels are described. By measuring two-dimensional photoelectron intensity angular distribution (PIAD) from valence band, the iso-energy cross section of valence band, e.g., the Fermi surface can be observed. In the case of linearly-polarized-light excitation, the symmetry relation in the photoelectron excitation process is also displayed as “angular distribution from atomic orbital”, which is used to distinguish the atomic orbitals constituting the energy band. Another important effect in angular distribution is the “photoelectron structure factor (PSF)”, which originates from the interference among photoelectron waves from individual atoms. The bonding character of the energy band can be clarified from the intensity inequivalency between Brillouin zones determined by PSF. On the other hand, the photoelectron from a localized core level is an excellent probe for element-specific atomic structure analysis. Photoelectron diffraction provides information on the surrounding atomic configuration, which is recorded as forward focusing peaks at local interatomic directions and diffraction patterns in PIAD. By combining this diffraction technique with core level spectroscopy–we call it diffraction spectroscopy, one can get access to each atomic site structure and have their electronic property information individually. Direct three-dimensional atomic structure visualization and site specific electronic property analysis methods are reviewed.
We present a theory of temperature-dependent photoemission which accurately describes phonon effects in soft and hard x-ray angle-resolved photoemission. Our approach is based on a fully relativistic one-step theory of photoemission that quantitatively reproduces the effects of phonon-assisted transitions beyond the usual k-conserving dipole selection rules which lead to the so-called XPS limit in the hard x-ray and/or high temperature regime. Vibrational atomic displacements have been included using the coherent potential approximation in analogy to the treatment of disordered alloys. The applicability of this alloy analogy model is demonstrated by direct comparison to experimental soft x-ray data from W(110) showing very satisfying agreement.
We have applied angle-resolved Ce 3\textit{d}→ 4\textit{f}
resonance photoemission spectroscopy (hν˜ 882 eV) in the soft
X-ray region to the analysis of the non-centrosymmetric pressure-induced
superconductor CeIrSi3 and obtained the 4f band structure and
Fermi surfaces. We have found that the Ce 4f states are located mainly
near the Fermi level and that the photoemission feature corresponding to
the dispersive conduction bands across the Fermi level shows
considerable resonant enhancement. In addition, the band structure and
Fermi surfaces of CeIrSi3 differ from those of the non- f
reference compound LaIrSi3, and the differences are well
explained by the band structure calculated within local density
approximation, in which the Ce 4f states are assumed to be itinerant.
These results strongly suggest that the Ce 4f electrons in
CeIrSi3 hybridized well with conduction bands and form
itinerant electronic states.
X-ray absorption (XAS) and its magnetic circular dichroism (XMCD) were measured at the Ce M4,5 absorption edges of ferromagnetic CeRu2Ge2 and paramagnetic CeRu2Si2: both compounds are considered to be located near the boundary of delocalization of Ce 4f electrons. While the XAS line shape varies clearly reflecting the variation in the 4f delocalization, the line-shape variation in XMCD is hardly discernible under various conditions of temperature and magnetic field. The XAS line-shape variation can be explained as effects of the variations in the 4f occupation number and in the ratio of J=7/2 states in the ground states, both of which are closely related to the 4f delocalization. The 4f delocalization also causes a decrease in the ratio of the orbital magnetic moment to the spin magnetic moment. The magnetic-field dependence of XAS suggests that the Ce 4f electrons retain a delocalized character across the metamagnetic transition in CeRu2Si2.
In this overview, I will briefly explore some of the basic concepts and observable effects in X-ray photoelectron spectroscopy, including references to some key first publications, as well as other papers in this issue that explore many of them in more detail. I will then turn to some examples of several present and promising future applications of this diverse technique. Some of the future areas explored will be the use chemical shifts, multiplet splittings, and hard X-ray excitation in the study of strongly correlated materials; photoelectron diffraction and holography for atomic structure determinations; standing wave and hard X-ray excited photoemission for probing buried interfaces and more bulk-like properties of complex materials; valence-band mapping with soft and hard X-ray excitation; and time-resolved measurements with the sample at high ambient pressures in the multi-torr regime.
We report the magnetic, transport and de Haas--van Alphen (dHvA) effect measurements to investigate the evolutions of metamagnetic behavior and electronic structure with Ge concentration x in CeRu2(Si1-xGex)2. With decreasing x, the ground state changes from ferromagnetic state to antiferromagnetic state at xa = 0.58, then to paramagnetic state at xc = 0.065. When a magnetic field is applied parallel to the [001] direction, first order metamagnetic transition takes place in the anitferromagnetic state and it changes to metamagnetic crossover in the paramagnetic state at xc. It is also found that the metamagnetic behavior as well as the magnetic phase diagram qualitatively change at xb = 0.29. The transport and dHvA effect measurements have been performed with fields applied in the (001) plane for which no metamagnetic transition takes place. With decreasing x, the magnitudes of the residual resistivity at 0.5 K, the coefficient A of T2 term in the temperature dependence of resistivity, the magnetoresistivity and the Hall resistivity increase discontinuously across xa. In the antiferromagnetic states they do not change much between xa and xb and then increases rapidly with decreasing x down to xc. Then, in the paramagnetic state they decrease with decreasing x, indicating that they are enhanced around xc in the antiferromagnetic and paramagnetic states. However, they seem to be maximum between x= 0.2 and xc, but not at xc. There may be also discontinuous changes in the transport properties across xb and xc. In the ferromagnetic state, we can observe the dHvA oscillations which are attributed to the electron surfaces as well as from the ellipsoidal hole surfaces in CeRu2Ge2 where the f electron is localized. In the antiferromagnetic state for x < xa we can not observe the dHvA oscillations from the electron surface, while we can observe those from the ellipsoidal hole surfaces which are also present in CeRu2Si2 where the f electron is itinerant. The effective mass is found to be enhanced around xc. We discuss the characteristic behavior of the metamagnetic transition in this system compared with other system. From the discontinuous changes at xa found in the transport and dHvA effect measurements, we argue that the f electron is likely to change its nature form localized to itinerant at xa.
Electronic structure of heavily over-doped Bi1.72Pb0.38Sr1.88CuO6+delta (Bi2201) superconductor was investigated by using the angle-resolved photoemission spectroscopy with soft x-ray as the incident photon source (SX-ARPES), which is known as one of the best bulk sensitive experimental techniques to investigate the electronic structure. By using the Cu 2p-3d resonance condition, we succeeded in determining the energy-momentum dispersion near the Fermi level and the Fermi surface. It is clearly shown that both SX- and VUV-ARPES are capable of determining the energy-momentum dispersion near \varepsilonF at least for the homologous series of Bi2Sr2CanCun+1O6+2n+delta cuprate superconductors, and that the combinational use of SX- and VUV-ARPES is one of the best experimental methods to determine the electronic structure of bulk materials.
Numerical simulation results are presented to investigate whether it is possible to probe the Fermi surface (FS) of a heavy-fermion (HF) system with the experimental conditions of present state-of-the-art angle-resolved photoemission spectroscopy (ARPES) using synchrotron radiation. Since the dispersion of a HF band is one or two orders of magnitude smaller than the experimental energy resolution, conventional ARPES-intensity mapping at the Fermi level does not work, but its k-space gradient mapping can provide a FS much closer to the genuine one. If complications from non-4f conduction bands are to be removed, 4d(3d) → 4f resonant photoemission for a Ce-based HF system is quite essential.
We report the three-dimensional (3D) momentum-resolved soft x-ray photoemission spectroscopy of the Fermi liquid LaNiO3 . The out-of-plane and in-plane cuts of the 3D electron- and hole-Fermi surfaces (FSs) are observed by energy- and angle-dependent photoemission measurements. The energy bands forming the electron FS suggest an omega2 dependence of the imaginary part of the self-energy and a ``correlation kink'' at an energy scale of 0.25 eV. In contrast, the bands which form nesting character hole FSs do not show kinks and match local-density approximation calculations. The results indicate a momentum-dependent mass renormalization, leading to electron-hole asymmetry in strongly correlated LaNiO3 .
Direct three-dimensional atomic structure analysis by stereo atomscope, valence-band angular momentum analysis by circularly polarized photoelectron diffraction, and atomic-layer-resolved magnetic structure analysis by diffraction spectroscopy are reviewed. The circular dichroism of photoelectron forward focusing peak rotation around the incident-light axis reflects the orbital angular momentum of excited core level and is inversely proportional to the distance between the emitter and scatterer atoms. This is the basis for the stereo photograph of the atomic arrangements. Furthermore, these rotations are also found in the case of the valence band photoelectrons. The rotation corresponds to the orbital angular momentum quantum number of valence band. Simultaneously, the photoelectron structure factor, that corresponds to the interference of photoelectron waves from atomic orbitals within a unit cell, was observed. The origin of the dual behavior that appeared in the observation of local angular momentum from a delocalized valence band is discussed. Lastly, local electronic and magnetic structure information has been obtained by diffraction spectroscopy. X-ray absorption and magnetic circular dichroism spectra from different atomic layers of the Ni thin film were disentangled by use of Auger electron diffraction. Surface and interior core-level shifts and magnetic moments are determined for each atomic layer individually.
High-resolution and high-excitation energy photoemission spectroscopy has an advantage in probing the bulk electronic states in solids whereas the surface electronic states deviated from the bulk states are strongly reflected in low-excitation energy photoemission spectra which have so far extensively measured for many materials. We have succeeded in realization of the high-energy bulk-sensitive angle-resolved photoemission in order to probe the three-dimensional electronic structures and the Fermi surfaces. After introducing the angle-integrated photoemission spectra of 3d1-electron system Sr1-xCaxVO3 as an example for the bulk (surface) sensitivity of the high-(low-)energy photoemission, we show the recently obtained results of high-energy angle-resolved photoemission for strongly correlated electron systems such as Sr2RuO4 and ferromagnet CeRu2Ge2 in the paramagnetic phase, revealing their bulk Fermi surfaces. It is found that the Fermi surfaces of CeRu2Ge2 in the paramagnetic phase are substantially deviated from those in the ferromagnetic phase. Although our results for CeRu2Ge2 are roughly explained by the band-structure calculation for LaRu2Ge2, a few qualitative discrepancies are seen.
Soft and hard x-ray photoelectron spectroscopy has been performed for one of the heavy fermion system CeRu2Si2 and a 4f-localized ferromagnet CeRu2Ge2 in the paramagnetic phase. The three-dimensional band structures and Fermi surface shapes of CeRu2Si2 have been determined by soft x-ray hν-dependent angle-resolved photoelectron spectroscopy. The differences in the Fermi surface topology and the non-4f electronic structures between CeRu2Si2 and CeRu2Ge2 are qualitatively explained by the band-structure calculation for both 4f itinerant and localized models, respectively. The Ce valences in CeRu2X2 (X=Si,Ge) at 20 K are quantitatively estimated by the single impurity Anderson model calculation, where the Ce 3d hard x-ray core-level PES and Ce 3d x-ray absorption spectra have shown stronger hybridization and signature for the partial 4f contribution to the conduction electrons in CeRu2Si2.
We have measured three-dimensional band structure and Fermi surfaces (FSs) of YbCu2Ge2, which has recently become recognized as a nearly divalent Yb-based compound, by using soft x-ray angle-resolved photoemission spectroscopy. We clarified that the bands with finite Yb 4f contribution cross the Fermi level and are slightly involved in the formation of the FSs, in line with the valence-fluctuating state of YbCu2Ge2. The obtained valence-band dispersions and the FSs are well explained by a relativistic band-structure calculation based on a local-density-approximation (LDA). These results suggest that the LDA calculation is a good starting point for understanding of the electronic state of YbCu2Ge2 as well as nearly divalent Yb-based compounds.
Angle-resolved photoemission spectroscopy (ARPES) has generally been carried out at energies below ˜150 eV, but there is growing interest in going to higher energies so as to achieve greater bulk sensitivity. To this end, we have measured ARPES spectra from a tungsten (110) crystal in a plane containing the [100], [110], and [010] directions with a photon energy of 1253.6 eV. The experimental data are compared to free-electron final-state calculations in an extended zone scheme with no inclusion of matrix elements, as well as highly accurate one-step theory including matrix elements. Both models provide further insight into such future higher-energy ARPES measurements. Special effects occurring in a higher-energy ARPES experiment, such as photon momentum, phonon-induced zone averaging effects, and the degree of cryogenic cooling required are discussed, together with qualitative predictions via appropriate Debye-Waller factors for future experiments with a number of representative elements being presented.
We have performed soft x-ray angle-resolved photoemission spectroscopy (ARPES) for the electron-doped high-temperature superconducting cuprates (HTSCs) Nd2-xCexCuO4 (x=0.075 and 0.15) in order to disclose their genuine bulk electronic structures. Our soft x-ray ARPES has revealed that the bulk-derived nodal 'kink' (the abrupt change of the electron velocity in quasiparticle dispersions), being absent in the low-energy ARPES, is clearly observable for the electron-doped superconducting Nd1.85Ce0.15CuO4. This result confirms that the antiferromagnetic spin correlation is suppressed and the electron–phonon interactions are prevailing in the bulk of the electron-doped superconducting Nd2- xCexCuO4 at the optimal doping level. The nodal 'kink' behavior is equivalent to that observed in the hole-doped HTSCs. The anisotropic strong electron–phonon interactions seem to be inherent in the HTSCs irrespective of the type of doped carriers.
We have studied the temperature dependence of W(110) soft x-ray angle-resolved photoemission spectra excited at photon energies of 260 and 870 eV and between 300 and 780 K. The experimental results have been compared to both a free-electron final-state model and theoretical one-step model calculations of the photocurrent. At 300 K, clear band dispersions can be observed in the data. The temperature dependence of the data can be analyzed qualitatively in terms of a direct-transition band-dispersion regime (“UPS” limit) versus a nondirect-transition density-of-states regime (“XPS” limit). The ratio between direct and nondirect transitions is estimated from a Debye-Waller factor, which for example at h=870 eV predicts 70% direct transitions at 300 K, and 41% at 780 K, and these values qualitatively describe our data. Beyond this, the state-of-the-art one-step theoretical calculations reproduce well the band dispersions and matrix element effects in the measured spectra at room temperature. However, simulating the temperature dependence is more complicated, and including phonon effects via complex phase shifts accounts for the suppression of existing direct-transition features, but does not reproduce new, density-of-states-related background intensity which shows up in higher-temperature experimental spectra. Finally, we also discuss the implications of this work for future experiments on other materials and at even higher photon energies up to 10 keV.
Soft x-ray excited angle-resolved photoemission spectroscopy (ARPES) is performed for the valence bands of quasi-one-dimensional V6O13 and SrCuO2 in order to reveal behavior of the strongly correlated V 3d and Cu 3d states. The resonance enhancement of the V 3d state for the V 2p core excitation and the high photoionization cross section of the Cu 3d states compared with the O 2p states are fully utilized in addition to the high resolutions in energy and momentum facilitated by recent instrument developments. Clear differences from the results of low photon energy ARPES have been observed for both materials by virtue of the high 3d sensitivity as well as high bulk sensitivity. Coexistence of a quasiparticle peak with an incoherent peak is observed in the metallic phase of V6O13, whereas the quasiparticle peak collapses in the insulator phase, in which two incoherent peaks are observed. In SrCuO2, the dispersive behavior of the spectra is well understood on the basis of the one-dimensional half-filled Hubbard model with U∕t=7.5 and U=3.0 eV (U: Coulomb repulsive energy and t: transfer energy) and substantial coupling between the spin and charge excitations is suggested.
A section through the three-dimensional Fermi surface of Ni has been mapped perpendicular to the [110] direction using angle-scanned photoemission. Regions of minority and majority spin bands, well separated in k space, can clearly be identified through comparison with a band structure calculation. The behavior of the different bands is observed for temperatures below and above the Curie temperature T<sub>C</sub> . We find bands which move and bands which stay in place for going from T < T<sub>C</sub> to T > T<sub>C</sub> .
We report the Fermi surfaces of the superconductor Sr2RuO4 and the non-superconductor Sr1.8Ca0.2RuO4 probed by bulk-sensitive high-energy angle-resolved photoemission. It is found that there is one square-shaped hole-like, one square-shaped electron-like and one circle-shaped electron-like Fermi surface in both compounds. These results provide direct evidence for nesting instability giving rise to magnetic fluctuations. Our study clarifies that the electron correlation effects are changed with composition depending on the individual band. Comment: 5 pages, 3 figures including 2 color figures
Results of neutron diffraction, magnetization, specific heat and resistivity are reported for CeRu2Ge2 which orders ferromagnetically at 7.9 K with a spontaneous magnetization of 1.96 ±0.02μB along the tetragonal c axis. The anomalous behaviour of polycrystalline samples in the vicinity of the Curie temperature is not observed in sigle crystals.
We derive the Fermi surface for LaRu2Ge2 by a self-consistent relativistic APW method within the framework of a local-density approximation.The Fermi surfaces thus obtained are composed of four hole sheets and an electron sheet, all of which have no open orbits. The Fermi surface topology is consistent with the experimental results for the high-field transverse magnetoresistance. These Fermi surfaces explain reasonably well the absolute magnitude and the angular dependence of the de Haas-van Alphen (dHvA) frequency-branches. Moreover, we use the theoretical Fermi surface of LaRu2Ge2 to re-examine an origin of the dHvA frequency-branches for a ferromagnetic CeRu2Ge2 regarded as non-Kondo-lattice system.
On the basis of the itinerant-electron model for the 4f electrons, the energy band structure and the Fermi surface are calculated for the metamagnetic heavy-electron compound CeRu2Si2 having the low-temperature electronic specific heat coefficient gamma of 350 mJ/K2 mol. by a self-consistent symmetrized relativistic APW method with the exchange and correlation potential in a local-density approximation. The main Fermi surface consists of a large closed hole sheet and a complicated electron sheet like a jungle gym. The Fermi surface topology is consistent with the experimental result for the high-field magneto-resistance. By comparison with the electronic structure of LaRu2Si2, effects of the 4f bands on the Bloch states on the Fermi surface in CeRu2Si2 are investigated in detail. Strong evidences for existence of the electron sheet are found in available experimental de Haas-van Alphen frequencies. The enhancement factor for gamma is estimated as 38.
Based on an itinerant-electron model for the 4f electrons, the energy band structure is calculated for CeSn3, known to be the heavy-electron system having the Kondo temperature of about 200 K, by a self-consistent relativistic APW method with the exchange and correlation potential in the local-density approximation. An itinerant-electron model assures that this compound is a compensated metal. The large, nearly spherical hole sheet of the Fermi surface explains one of the major frequency branches of the de Haas-van Alphen effect of the order 107 Oe. The electron sheet is essentially spherical but highly distorted by the strong 4f-Sn 5p hybridization. It explains clearly origins of all other major frequency branches. These results support the view that the 4f electrons in the heavy-electron Ce compounds should be treated by an itinerant-electron picture.
The first measurements of Fermi-surface properties in a mixed-valent metal using the de Haas--van Alphen effect are presented. At least nine separate frequency branches in CeSnâ are seen and have measured effective masses as large as 9.2. The experiments show that hybridization between the conduction states and local f states occurs coherently in the mixed-valence compound CeSnâ, resulting in Bloch states with well-defined wave vector and energy.
This article attempts to review our present understanding of nearly-independent-particle excitations---quasiparticles---in heavy fermion systems. The emphasis is on a realistic, that is material-specific, description of the highly correlated Fermi liquid state which develops at low temperatures. After an initial discussion of Landau's Fermi liquid theory and its application to metals the concept of renormalized band structure calculation is introduced. The fundamental ideas are considered and explicit prescriptions are derived which allow a phenomenological account to be made of the strong local correlations. A comparison is made between the calculated quasiparticle bands and the detailed information now available from Fermi surface studies. Next, the interaction among the quasiparticles is discussed with particular reference to its influence on the electronic compressibility and the spin susceptibility. The correlation-related contribution to the quasiparticle-phonon coupling is derived. The question of quasiparticle collisions is also briefly discussed. Finally, a brief review is given of ideas and theoretical models which are used to describe superconductivity in heavy fermion compounds; in this context, a concept for constructing realistic order parameters is explored which accounts for the lattice structure and the anisotropy of the Fermi surface.
``Bulk-sensitive'' Ce 3d-4f resonance photoemission (RPES) with an unprecedentedly high resolution has revealed a clear difference of bulk Ce 4f spectral weights of heavy fermion system CeRu2Si2 and CeRu2Ge2 in the region of the tail of the Kondo peak and its spin-orbit partner. The significant spectral difference in both materials is in strong contrast to the mutually similar ``surface-sensitive'' 4d-4f RPES. The obtained bulk-sensitive spectra are well reproduced by a single impurity model. Consideration of the crystalline electric field splitting is important for estimating a realistic Kondo temperature from the bulk 4f spectra.
Based on the itinerant-electron model for the 4f electrons, the energy band structure and the Fermi surface is calculated for the mixed-valence cerium compound CeNi and its reference material LaNi by a self-consistent relativistic APW method with the exchange and correlation potential in the local-density approximation. Both CeNi and LaNi are compensated metals, and their hole and electron Fermi surfaces are multiply-connected sheets with open orbits, the direction of which is consistent with the high-field magnetoresistance. Though they are quite different from each other, the Fermi surfaces for CeNi and LaNi can explain reasonably well both the magnitude and the angle dependence of the de Haas-van Alphen effect frequency branches. These theoretical results suggest that the 4f electrons may be itinerant in the ground state in CeNi, as they are in CeSn3.
This paper reviews the most recent ARPES results on the cuprate superconductors and their insulating parent and sister compounds, with the purpose of providing an updated summary of the extensive literature in this field. The low energy excitations are discussed with emphasis on some of the most relevant issues, such as the Fermi surface and remnant Fermi surface, the superconducting gap, the pseudogap and d-wave-like dispersion, evidence of electronic inhomogeneity and nano-scale phase separation, the emergence of coherent quasiparticles through the superconducting transition, and many-body effects in the one-particle spectral function due to the interaction of the charge with magnetic and/or lattice degrees of freedom. The first part of the paper introduces photoemission spectroscopy in the context of strongly interacting systems, along with an update on the state-of-the-art instrumentation. The second part provides a brief overview of the scientific issues relevant to the investigation of the low energy electronic structure by ARPES. The rest of the paper is devoted to the review of experimental results from the cuprates and the discussion is organized along conceptual lines: normal-state electronic structure, interlayer interaction, superconducting gap, coherent superconducting peak, pseudogap, electron self energy and collective modes. Within each topic, ARPES data from the various copper oxides are presented.
Accurate valence-band dispersions E(k⃗) along the major symmetry directions Γ-K-X, Γ-Δ-X, and Γ-Λ-L have been determined for GaAs using simple angle-resolved photoemission techniques of general utility with synchrotron radiation for 25≤hν≤100 eV. At these photon energies, emission features can be understood within the direct-transition model, and spectral peaks can be classified roughly into two categories: one being those associated with primary cone emission with a lifetime-broadened free-electron-like final-state dispersion, and the other (usually weaker) being those associated with secondary cone-surface umklapp emission which emphasizes valence-band critical points with high state densities. Valence-band dispersions E(k⃗) along the Γ-K-X symmetry line perpendicular to the surface are determined using normal-emission spectra (primary cone peaks) from the (110) surface at various photon energies. Valence-band dispersions E(k⃗) along Γ-K-X, Γ-Δ-X, and Γ-Λ-L symmetry lines parallel to the surface are determined using off-normal emission spectra (primary cone peaks) from the same (110) surface with fixed perpendicular component of the electron momentum ℏk⃗⊥ at a zone center (extended-zone scheme) and varying parallel component of the electron momentum ℏk⃗∥, which are obtained by suitably varying hν and emission angles. Experimental valence-band dispersions and critical points are compared with other theoretical and experimental results. Simple formulas are derived to relate the widths of spectral peaks to electron and hole lifetimes. Initial hole lifetimes at valence critical points and typical final electron lifetimes are obtained. The latter yields final-state momentum broadenings (typically ≲ 10% of the Brillouin-zone size) which are consistent with the direct-transition model.
The electrical resistivity ρ(T) and transverse magnetoresistivity of magnetically ordered CeRu2Ge2 (TN=8.55 K and TC=7.40 K) was measured as a function of pressure up to P=11 GPa and down to T=30 mK. Pressure first increases TN and suppresses TC. Then a second transition, corresponding probably to a modification of the AFM state, appears at TL<TN in the pressure range 3.5<P<7.2 GPa. The long-range magnetic order disappears at a critical pressure Pc=8.7 GPa like TN∝(Pc-P)m, with m=0.71(8). The ρ(T) curves at low temperature and low pressure (P<7.8 GPa) are well described by a power law ρ(T)∝Tn, with exponents n>2. Well above Pc, a Fermi liquid behavior (n=2) is observed and the A coefficient of the quadratic T dependence decreases with pressure. In the pressure interval Pc±0.8 GPa, exponents close to n=1.5 indicate a deviation from a Fermi liquid description. The magnetoresistivity curves show different anomalies at characteristic magnetic fields Ba, Bc, and BM. The latter occurs close to Pc and is reminiscent of the metamagneticlike field in CeRu2Si2. A quantitative comparison between the (T,P) data of CeRu2Ge2 and the (T,x) data of CeRu2(Si1-xGex)2 on the basis of the unit-cell volume as the crucial parameter shows that the same (T,V) diagram is obtained.
The temperature dependence of the thermoelectric power, S(T), and the electrical resistivity of the magnetically ordered CeRu2Ge2 (TN=8.55 K and TC=7.40 K) were measured for pressures p<16 GPa in the temperature range 1.2 K<T<300 K. Long-range magnetic order is suppressed at pc≈7.8 GPa. Pressure drives S(T) through a sequence of temperature dependences, ranging from a behavior characteristic for magnetically ordered heavy fermion compounds to a typical behavior of intermediate-valent systems. At intermediate pressures a large positive maximum develops above 10 K in S(T). Its origin is attributed to the Kondo effect and its position is assumed to reflect the Kondo temperature TK. The pressure dependence of TK is discussed in a revised and extended (T,p) phase diagram of CeRu2Ge2.
The results of elastic, quasielastic, and inelastic neutron-scattering studies on polycrystalline CeM2Ge2 (M=Ag, Au, and Ru) are presented. All compounds reveal long-range magnetic order at low temperatures. Ferromagnetic (M=Ru), antiferromagnetic (M=Au), and incommensurate (M=Ag) structures were detected. Using time-of-flight (TOF) techniques, the crystalline electric-field splittings were determined. With high-resolution TOF experiments the temperature and wave-vector dependence of the magnetic relaxation rate was studied. Korringa-like behavior was found for the Au and Ru compounds. In CeAg2Ge2 the temperature dependence of the magnetic relaxation rate exhibits a square-root dependence at high temperatures and shows a residual linewidth for low T. This behavior can be explained by strong hybridization effects of the f electrons with the delocalized band states which are active in heavy fermion systems. Previous results in CeCu2Ge2 and CeNi2Ge2 are included, whenever necessary. Detailed comparison is made with results obtained in the disilicides.
A detailed de Haas-van Alphen study of high-quality crystals of ferromagnetic CeRu2Ge2 (Tc ≈ 8 K) is presented. All expected Fermi surface sheets have been observed, including an extraordinarily large surface with a cross-sectional area bordering on the size of the Brillouin zone itself. A comparison to band structure calculations allows the formulation of a model for the Fermi surface consisting of five spin split sheets. For the first time in one of these systems the completeness of the de Haas-van Alphen data allows a rigorous analysis of the heat capacity and its accountability in terms of the observed quasiparticles. Analysis of the observed spin splitting yields values for the 4f to conduction electron interaction parameters for each sheet and enables a self-consistent understanding of the mass enhancement. A comparison to the related heavy-fermion system CeRu2Si2 provides new insight into the underlying nature of its quasiparticle properties.
We have performed a dHvA effect study on the ferromagnetic heavy fermion compound CeRu2Ge2. Six spin-split dHvA frequency branches are observed in the (1 0 0) and (1 1 0) planes. The angular dependence of the dHvA frequencies is mostly consistent with the result reported by King and Lonzarich (KL), except that we have successfully observed the dHvA signal from a large hole surface for fields parallel to [0 0 1] and a new low-frequency branch. The effective masses measured for fields in the (0 0 1) plane agree with those of KL and do not depend on field strength. On the other hand, those for fields parallel to [0 0 1] differ considerably from those of KL and are found to decrease with increasing field. Moreover, the measured effective mass of the large hole surface is less than half of the value expected from the electronic specific heat coefficient.
The magnetic relaxation rates Γ as determined via inelastic neutron scattering experiments are reported for the ternary compounds CeM2X2 (M Ni, Cu, Ag, Au, Ru; X Si, Ge). Strongly enhanced values of Γ and deviations from a Korringa-type of behaviour were found. A close correlation between the unit cell volume and the 4f-local spin / conduction electron hybridization is demonstrated.
Results of the susceptibility, specific heat and thermal expansion are reported for the compounds CeM2Ge2 (M: Ag, Au, Ru), which order antiferromagnetically below TN = 5 K (Ag) and 15 K (Au) and ferromagnetically below Tc = 7.5 K (Ru). Distinct maxima in the electronic specific heat coefficient γ(T) at 0.3 K for the Ag- and Au-based compounds hint at a coexistence of coherent electronic quasiparticles of “medium”-heavy mass with magnetic order.
The -resolved single particle excitations, as determined by angle-resolved photoemission spectroscopy (ARPES), are compared and contrasted for, LaRu2Si2, CeRu2Si2, ThRu2Si2, and URu2Si2, isostructural layered compounds with differing nominal f-occupations of f0, f1, f0, and f2, respectively. ARPES measurements include 4d and 5d-edge resonant photoemission to distinguish f-character and Fermi-energy intensity mapping of Fermi surface contours. Comparison to RLAPW band structure calculations shows very good agreement of the d-band structure away from EF. Discrepancies in the near EF region highlight -dependent effects of f-correlation and f–d hybridization. Approximately equal dimensions of Fermi contours for X=(La, Ce) suggest the exclusion of 4f electrons from the CeRu2Si2 Fermi surface at temperatures far above the Kondo temperature. High-resolution spectra for X=(Ce, U) allow comparison of f–d mixing to predictions of the Anderson lattice model.
Studies of the quasiparticle band structure in a series of heavy electron metals have been performed by means of angle resolved measurements of the de Haas-van Alphen effect in pure crystals. A review is presented of the principal results of these investigations in the magnetic transition metal MnSi, the incipient antiferromagnet Pr, the actinide heavy fermion superconductor UPt3 and rare earth compounds CeAl2 and CeRu2Si2.
Measurements of the de Haas-van Alphen effect in the heavy-electron metal CeCu6 in the coherent state are reported. Cyclotron masses of up to 40 m(0) are observed and a many-body enhancement 20 times larger than in LaCu6 is deduced. It is concluded from the absences of any light electrons that the many body renormalization leading to the coherent state influences all electrons, and not just those with primary f character.
Electron correlations are known to play an important role in determining the unusual physical properties of a variety of compounds. Such properties include high-temperature superconductivity, heavy fermion behaviour and metal-to-insulator transitions. High-resolution photoelectron spectroscopy (PES) provides a means of directly probing the electronic states (particularly those near the Fermi level) in these materials, but the short photoelectron mean free paths (< or = 5 A) associated with the low excitation energies conventionally used (< or = 120 eV) make this a surface-sensitive technique. Now that high-resolution PES is possible at much higher energies, with mean free paths as long as 15 A (ref. 6), it should become feasible to probe the bulk electronic states in these materials. Here we demonstrate the power of this technique by applying it to the cerium compounds CeRu2Si2 and CeRu2. Previous PES studies of these compounds revealed very similar spectra for the Ce 4f electronic states, yet it is expected that such states should be different owing to their differing degrees of hybridization with other valence bands. Our determination of the bulk Ce 4f electronic states of these compounds resolves these differences.
A very high resolution soft X-ray beamline, BL25SU, has been designed and is under construction at SPring-8. Completely right or left circularly polarized light is supplied on a common axis of a newly designed twin helical undulator. A helicity modulation up to 10 Hz can be performed using five kicker magnets. The fundamental radiation covers the region 0.5-3 keV. Higher-order radiation is rather weak on the axis. A monochromator with varied-line-spacing plane gratings is installed to cover the region below 1.5 keV. A very high resolution beyond 10(4) is expected for the whole energy region.