Z. R. Ye

Fudan University, Shanghai, Shanghai Shi, China

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Publications (26)119.28 Total impact

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    ABSTRACT: We have studied the low-lying electronic structure of a new ThCr$_2$Si$_2$-type superconductor KNi$_2$Se$_2$ with angle-resolved photoemission spectroscopy. Three bands intersect the Fermi level, forming complicated Fermi surface topology, which is sharply different from its isostructural superconductor K$_x$Fe$_{2-y}$Se$_2$. The Fermi surface shows weak variation along the $k_z$ direction, indicating its quasi-two-dimensional nature. Further comparison with the density functional theory calculations demonstrates that there exist relatively weak correlations and substantial hybridization of the Ni 3$d$ and the Se 4$p$ orbitals in the low-lying electronic structure. Our results indicate that the large density of states at the Fermi energy leads to the reported mass enhancement based on the specific heat measurements. Moreover, no anomaly is observed in the spectra when entering the fluctuating charge density wave state reported earlier.
    11/2014;
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    ABSTRACT: We report the electronic structure reconstruction of Ca$_{1-x}$Pr$_x$Fe$_2$As$_2$ ($x$ = 0.1 and 0.15) in the low temperature collapsed tetragonal (CT) phase observed by angle-resolved photoemission spectroscopy. Different from Ca(Fe$_{1-x}$Rh$_x$)$_2$As$_2$ and the annealed CaFe$_2$As$_2$ where all hole Fermi surfaces are absent in their CT phases, the cylindrical hole Fermi surface can still be observed in the CT phase of Ca$_{1-x}$Pr$_x$Fe$_2$As$_2$. Furthermore, we found at least three well separated electron-like bands around the zone corner in the CT phase of Ca$_{1-x}$Pr$_x$Fe$_2$As$_2$, which are more dispersive than the electron-like bands in the high temperature tetragonal phase. Based on these observations, we propose that the weakening of correlations (as indicated by the reduced effective mass), rather than the lack of Fermi surface nesting, might be responsible for the absence of magnetic ordering and superconductivity in the CT phase.
    10/2014;
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    ABSTRACT: Eu(Fe0.79Ru0.21)2As2 is suggested to be a nodeless superconductor based on the empirical correlation between pnictogen height (hPn) and superconducting gap behavior, in contrast to BaFe2(As0.7P0.3)2 and Ba(Fe0.65Ru0.35)2As2. We studied the low-lying electronic structure of Eu(Fe0.79Ru0.21)2As2 with angle-resolved photoemission spectroscopy (ARPES). By photon energy dependence and polarization dependence measurements, we resolved the band structure in the three-dimensional momentum space and determined the orbital character of each band. In particular, we found that the -originated ? band does not contribute spectral weight to the Fermi surface around Z, unlike BaFe2(As0.7P0.3)2 and Ba(Fe0.65Ru0.35)2As2. Since BaFe2(As0.7P0.3)2 and Ba(Fe0.65Ru0.35)2As2 are nodal superconductors and their hPn's are less than 1.33??, while the hPn of Eu(Fe0.79Ru0.21)2As2 is larger than 1.33 ?, our results provide more evidence for a direct relationship between nodes, orbital character and hPn. Our results help to provide an understanding of the nodal superconductivity in iron-based superconductors.
    Journal of Physics Condensed Matter 06/2014; 26(26):265701. · 2.22 Impact Factor
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    ABSTRACT: The diversities in crystal structures and ways of doping result in extremely diversified phase diagrams for iron-based superconductors. With angle-resolved photoemission spectroscopy (ARPES), we have systematically studied the effects of chemical substitution on the electronic structure of various series of iron-based superconductors. In addition to the control of Fermi surface topology by heterovalent doping, we found two more extraordinary effects of doping: 1. the site and band dependencies of quasiparticle scattering; and more importantly 2. the ubiquitous and significant bandwidth-control by both isovalent and heterovalent dopants in the iron-anion layer. Moreover, we found that the bandwidth-control could be achieved by either applying the chemical pressure or doping electrons, but not by doping holes. Together with other findings provided here, these results complete the microscopic picture of the electronic effects of dopants, which facilitates a unified understanding of the diversified phase diagrams and resolutions to many open issues of various iron-based superconductors.
    04/2014;
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    ABSTRACT: We report the electronic structure of YbB6, a recently predicted moderately correlated topological insulator, measured by angle-resolved photoemission spectroscopy. We directly observed linearly dispersive bands around the time-reversal invariant momenta {\Gamma} and X with negligible kz dependence, consistent with odd number of surface states crossing the Fermi level in a Z2 topological insulator. Circular dichroism photoemission spectra suggest that these in-gap states possess chirality of orbital angular momentum, which is related to the chiral spin texture, further indicative of their topological nature. The observed insulating gap of YbB6 is about 100 meV, larger than that reported by theoretical calculations. Our results present strong evidence that YbB6 is a correlated topological insulator and provide a foundation for further studies of this promising material.
    Scientific Reports 03/2014; 4. · 5.08 Impact Factor
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    ABSTRACT: NdO$_{0.5}$F$_{0.5}$BiS$_{2}$ is a new layered superconductor. We have studied the low-lying electronic structure of a single crystalline NdO$_{0.5}$F$_{0.5}$BiS$_{2}$ superconductor, whose superconducting transition temperature is 4.87K, with angle-resolved photoemission spectroscopy. The Fermi surface consists of two small electron pockets around the X point and shows little warping along the $k_z$ direction. Our results demonstrate the multi-band and two-dimensional nature of the electronic structure. The good agreement between the photoemission data and the band calculations gives the renormalization factor of 1, indicating the rather weak electron correlations in this material. Moreover, we found that the actual electron doping level and Fermi surface size are much smaller than what are expected from the nominal composition, which could be largely explained by the bismuth dificiency. The small Fermi pocket size and the weak electron correlations found here put strong constraints on theory, and suggest that the BiS$_2$-based superconductors could be conventional BCS superconductors mediated by the electron-phonon coupling.
    02/2014;
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    ABSTRACT: SmB6, a well-known Kondo insulator, exhibits a transport anomaly at low temperature. This anomaly is usually attributed to states within the hybridization gap. Recent theoretical work and transport measurements suggest that these in-gap states could be ascribed to topological surface states, which would make SmB6 the first realization of topological Kondo insulator. Here by performing angle-resolved photoemission spectroscopy experiments, we directly observe several dispersive states within the hybridization gap of SmB6. These states show negligible kz dependence, which indicates their surface origin. Furthermore, we perform photoemission circular dichroism experiments, which suggest that the in-gap states possess chirality of the orbital angular momentum. These states vanish simultaneously with the hybridization gap at around 150 K. Together, these observations suggest the possible topological origin of the in-gap states.
    Nature Communications 12/2013; 4:3010. · 10.74 Impact Factor
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    ABSTRACT: With angle-resolved photoemission spectroscopy, we studied the electronic structure of TaFe$_{1.23}$Te$_3$, which is a two-leg spin ladder compound with a novel antiferromagnetic ground state. Quasi-two-dimensional Fermi surface is observed, indicating sizable inter-ladder hopping, which would facilitate the in-plane ferromagnetic ordering through double exchange interactions. Moreover, an energy gap is not observed at the Fermi surface in the antiferromagnetic state. Instead, the shifts of various bands have been observed. Combining these observations with density-functional-theory calculations, we propose that the large scale reconstruction of the electronic structure, caused by the interactions between the coexisting itinerant electrons and local moments, is most likely the driving force behind the magnetic transition. TaFe$_{1.23}$Te$_3$ thus provides a simpler system that contains similar ingredients as the parent compounds of iron-based superconductors, which yet could be readily modeled and understood.
    10/2013;
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    ABSTRACT: We studied the low-lying electronic structure of the newly discovered iron-platinum-arsenide superconductor, Ca10(Pt4As8)(Fe2−xPtxAs2)5 (Tc=22 K) with angle-resolved photoemission spectroscopy. We found that the Pt4As8 layer contributes to a small electronlike Fermi surface, indicative of metallic charge reservoir layers that are rare for iron based superconductors. Moreover, the electronic structure of the FeAs layers resembles those of other prototype iron pnictides to a large extent. However, there is only dxy-orbital originated holelike Fermi surface near the zone center, which is rather strange for an iron pnictide superconductor with relatively high superconducting transition temperature; and the dxz and dyz originated bands are not degenerate at the zone center. Furthermore, all bands near the Fermi energy show negligible kz dependence, indicating the strong two-dimensional nature of this material. Our results indicate this material possesses a rather interesting electronic structure, which enriches our current knowledge of iron based superconductors.
    Physical Review B 09/2013; 88(11). · 3.66 Impact Factor
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    ABSTRACT: The in-plane resistivity anisotropy has been studied with the Montgomery method on the detwinned parent compound of the iron-based superconductor FeTe. The observed resistivity in the antiferromagnetic (AFM) direction is larger than that in the ferromagnetic (FM) direction, which is different from that observed in BaFe2As2 before. We show that the opposite resistivity anisotropy behavior in FeTe could be attributed to the strong Hund's rule coupling effects, which should be understood in a localized picture: Hund's rule coupling makes hopping along the FM direction easier than along the AFM direction in FeTe, similar to the colossal magnetoresistance observed in some manganites.
    Physical Review B 09/2013; · 3.66 Impact Factor
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    ABSTRACT: We studied the low-lying electronic structure of the newly discovered iron-platinum-arsenide superconductor, Ca10(Pt4As8)(Fe2-xPtxAs2)5 (Tc=22 K) with angle-resolved photoemission spectroscopy. We found that the Pt4As8 layer contributes to a small electron-like Fermi surface, indicative of metallic charge reservoir layers that are rare for iron based superconductors. Moreover, the electronic structure of the FeAs-layers resembles those of other prototype iron pnictides to a large extent. However, there is only dxy-orbital originated hole-like Fermi surface near the zone center, which is rather unique for an iron pnictide superconductor with relatively high superconducting transition temperature; and the dxz and dyz originated bands are not degenerate at the zone center. Furthermore, all bands near the Fermi energy show negligible kz dependence, indicating the strong two-dimensional nature of this material. Our results indicate this material possesses rather unique electronic structure, which enriches our current knowledge of iron based superconductors.
    08/2013;
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    ABSTRACT: Angle-resolved photoemission spectroscopy (ARPES) has played an important role in determining the band structure and the superconducting gap structure of iron-based superconductors. Here from the ARPES perspective, we briefly review the main results from our group in the recent years on the iron-based superconductors and their parent compounds, and depict our current understanding on the antiferromagnetism and superconductivity in these materials.
    Chinese Physics B 07/2013; 22(8). · 1.15 Impact Factor
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    ABSTRACT: The correlations between the superconductivity in iron pnictides and their electronic structures are elusive and controversial so far. Here through angle-resolved photoemission spectroscopy measurements, we show that such correlations are rather distinct in AFe$_{1-x}$Co$_x$As (A=Li, Na), but only after one realizes that they are orbital selective. We found that the superconductivity is enhanced by the Fermi surface nesting, but only when it is between $d_{xz}/d_{yz}$ Fermi surfaces, while for the $d_{xy}$ orbital, even nearly perfect Fermi surface nesting could not induce superconductivity. Moreover, the superconductivity is completely suppressed just when the $d_{xz}/d_{yz}$ hole pockets sink below Fermi energy and evolve into an electron pocket. Our results thus establish the orbital selective relation between the Fermiology and the superconductivity in iron-based superconductors, and substantiate the critical role of the $d_{xz}/d_{yz}$ orbitals. Furthermore, around the zone center, we found that the $d_{xz}/d_{yz}$-based bands are much less sensitive to impurity scattering than the $d_{xy}$-based band, which explains the robust superconductivity against heavy doping in iron-based superconductors.
    03/2013;
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    ABSTRACT: The in-plane resistivity anisotropy has been studied with the Montgomery method on two detwinned parent compounds of the iron-based superconductors, NaFeAs and FeTe. For NaFeAs, the resistivity in the antiferromagnetic (AFM) direction is smaller than that in the ferromagnetic (FM) direction, similar to that observed in BaFe2As2 before. While for FeTe, the resistivity in the AFM direction is larger than that in the FM direction. We show that these two opposite resistivity anisotropy behaviors could be attributed to the strong Hund's rule coupling effects: while the iron pnictides are in the itinerant regime, where the Hund's rule coupling causes strong reconstruction and nematicity of the electronic structure; the FeTe is in the localized regime, where Hund's rule coupling makes hopping along the FM direction easier than along the AFMdirection, similar to the colossal magnetoresistance observed in some manganites.
    10/2012;
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    ABSTRACT: The coexisting regime of spin density wave (SDW) and superconductivity in the iron pnictides represents a novel ground state. We have performed high resolution angle-resolved photoemission measurements on NaFe1-xCoxAs (x = 0.0175) in this regime and revealed its distinctive electronic structure, which provides some microscopic understandings of its behavior. The SDW signature and the superconducting gap are observed on the same bands, illustrating the intrinsic nature of the coexistence. However, because the SDW and superconductivity are manifested in different parts of the band structure, their competition is non-exclusive. Particularly, we found that the gap distribution is anisotropic and nodeless, in contrast to the isotropic superconducting gap observed in an SDW-free NaFe1-xCoxAs (x=0.045), which puts strong constraints on theory.
    Physical Review X 09/2012; · 8.39 Impact Factor
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    ABSTRACT: BaFe2(As1-xPx)2 is a unique iron-based superconductor, where the superconductivity is induced by the isovalent substitution of phosphorus (P) for arsenic (As). Unlike other iron pnictides, the superconducting gap in BaFe2(As1-xPx)2 has been suggested to contain nodal lines by various experiments. The exact nature of the isovalent doping and nodal gap are key open issues in building a comprehensive picture of the iron-based superconductors. With angle-resolved photoemission spectroscopy, we found that the P substitution in BaFe2(As1-xPx)2 alters the electronic structure significantly. With P doping, the hole and electron Fermi surface sheets expand simultaneously and the band velocities are enhanced indicating a suppression of electron correlations. Moreover, the P doping induces strong kz dispersion on the dxz-originated band with significant mixing of the dz2 orbital around Z, while the dxy-originated band and the electron pockets are relatively intact. These rule out theories suggesting that the nodal gap is due to the vanishing dxy hole pocket, while support those considering a dz2-dominated hole Fermi surface around Z being responsible. Our results are thus helpful to explain the nodal superconductivity in BaFe2(As1-xPx)2 and understand the role of lattice parameter or pressure effect in iron-based superconductors.
    Physical review. B, Condensed matter 07/2012; · 3.66 Impact Factor
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    ABSTRACT: Although a nodeless superconducting gap has been observed on the large Fermi pockets around the zone corner in KxFe2−ySe2, whether its pairing symmetry is s wave or nodeless d wave is still under intense debate. Here we report an isotropic superconducting gap distribution on the small electron Fermi pocket around the Z point in KxFe2−ySe2, which favors the s-wave pairing symmetry.
    Physical review. B, Condensed matter 05/2012; 85(22). · 3.66 Impact Factor
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    ABSTRACT: We present the ultra-low-temperature heat-transport study of iron-based superconductors Ba(Fe1-xRux)2As2 and BaFe2(As1-xPx)2. For optimally doped Ba(Fe0.64Ru0.36)2As2, a large residual κ0/T at zero field and a √H dependence of κ0(H)/T are observed, which provide strong evidences for nodes in the superconducting gap. This result demonstrates one more nodal superconductor in iron pnictides. The similarities between isovalent Fe and P dopings strongly suggest that the nodal superconductivity in Ba(Fe0.64Ru0.36)2As2 may have the same origin as in BaFe2(As0.67P0.33)2. Furthermore, in underdoped Ba(Fe0.77Ru0.23)2As2 and strongly underdoped BaFe2(As0.82P0.18)2, κ0/T manifests similar nodal behavior, which result shows the robustness of nodal superconductivity in the underdoped regime and puts constraint on theoretical models.
    Physical Review X 02/2012; 2(1). · 8.39 Impact Factor
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    ABSTRACT: The superconductivity discovered in iron pnictides is intimately related to a nematic ground state, where the C4 rotational symmetry is broken via the structural and magnetic transitions. We here study the nematicity in NaFeAs with polarization-dependent angle-resolved photoemission spectroscopy. A uniaxial pressure was applied on the sample to overcome the twinning effect in the low temperature C2-symmetric state and obtain a much simpler electronic structure than that of a twinned sample. We found the electronic structure undergoes an orbital-dependent reconstruction in the nematic state, primarily involving the dxy- and dyz-dominated bands. These bands strongly hybridize with each other, inducing a band splitting, while the dxz-dominated bands only exhibit an energy shift without any reconstruction. These findings suggest that the spin fluctuations at high temperatures and their coupling with the orbital degree of freedom could be the dominant force to drive the nematicity, while the ferro-orbital ordering between dxz and dyz orbitals can only play a minor role here.
    Physical review. B, Condensed matter 02/2012; 85(8). · 3.66 Impact Factor
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    ABSTRACT: The nature of the parent compound of a high-temperature superconductor (HTS) often plays a pivotal role in determining its superconductivity. The parent compounds of the cuprate HTSs are antiferromagnetically ordered Mott insulators, while those of the iron-pnictide HTSs are metals with spin-density-wave order. Here we report the electronic identification of two insulating parental phases and one semiconducting parental phase of the newly discovered family of KxFe2-ySe2 superconductors. The two insulating phases exhibit Mott-insulator-like signatures, and one of the insulating phases is even present in the superconducting and semiconducting KxFe2-ySe2 compounds. However, it is mesoscopically phase-separated from the superconducting or semiconducting phase. Moreover, we find that both the superconducting and semiconducting phases are free of the magnetic and vacancy orders present in the insulating phases, and that the electronic structure of the superconducting phase could be developed by doping the semiconducting phase with electrons. The rich electronic properties discovered in these parental phases of the KxFe2-ySe2 superconductors provide the foundation for studying the anomalous behavior in this new class of iron-based superconductors.
    Physical Review X 12/2011; 1(2). · 8.39 Impact Factor