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[show abstract]
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ABSTRACT: Using resonant inelastic x-ray scattering, we observe in the bilayer iridate Sr_{3}Ir_{2}O_{7}, a spin-orbit coupling driven magnetic insulator with a small charge gap, a magnon gap of ≈92 meV for both acoustic and optical branches. This exceptionally large magnon gap exceeds the total magnon bandwidth of ≈70 meV and implies a marked departure from the Heisenberg model, in stark contrast to the case of the single-layer iridate Sr_{2}IrO_{4}. Analyzing the origin of these observations, we find that the giant magnon gap results from bond-directional pseudodipolar interactions that are strongly enhanced near the metal-insulator transition boundary. This suggests that novel magnetism, such as that inspired by the Kitaev model built on the pseudodipolar interactions, may emerge in small charge-gap iridates.
Physical Review Letters 10/2012; 109(15):157402. · 7.37 Impact Factor
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ABSTRACT: Using resonant x-ray diffraction, we observe an easy c-axis collinear antiferromagnetic structure for the bilayer Sr3Ir2O7, a significant contrast to the single layer Sr2IrO4 with in-plane canted moments. Based on a microscopic model Hamiltonian, we show that the observed spin-flop transition as a function of number of IrO2 layers is due to strong competition among intra- and interlayer bond-directional pseudodipolar interactions of the spin-orbit entangled J(eff)=1/2 moments. With this we unravel the origin of anisotropic exchange interactions in a Mott insulator in the strong spin-orbit coupling regime, which holds the key to the various types of unconventional magnetism proposed in 5d transition metal oxides.
Physical Review Letters 07/2012; 109(3):037204. · 7.37 Impact Factor
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N Driza,
S Blanco-Canosa,
M Bakr,
S Soltan,
M Khalid,
L Mustafa,
K Kawashima,
G Christiani,
H-U Habermeier, G Khaliullin,
C Ulrich,
M Le Tacon,
B Keimer
[show abstract]
[hide abstract]
ABSTRACT: The electron-phonon interaction is of central importance for the electrical and thermal properties of solids, and its influence on superconductivity, colossal magnetoresistance and other many-body phenomena in correlated-electron materials is the subject of intense research at present. However, the non-local nature of the interactions between valence electrons and lattice ions, often compounded by a plethora of vibrational modes, presents formidable challenges for attempts to experimentally control and theoretically describe the physical properties of complex materials. Here we report a Raman scattering study of the lattice dynamics in superlattices of the high-temperature superconductor YBa(2)Cu(3)O(7) (YBCO) and the colossal-magnetoresistance compound La(2/3)Ca(1/3)MnO(3) that suggests a new approach to this problem. We find that a rotational mode of the MnO(6) octahedra in La(2/3)Ca(1/3)MnO(3) experiences pronounced superconductivity-induced line-shape anomalies, which scale linearly with the thickness of the YBCO layers over a remarkably long range of several tens of nanometres. The transfer of the electron-phonon coupling between superlattice layers can be understood as a consequence of long-range Coulomb forces in conjunction with an orbital reconstruction at the interface. The superlattice geometry thus provides new opportunities for controlled modification of the electron-phonon interaction in complex materials.
Nature Material 07/2012; 11(8):675-81. · 32.84 Impact Factor
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N. Driza,
S. Blanco-Canosa,
M. Bakr,
S. Soltan,
M. Khalid,
L. Mustafa,
K. Kawashima,
G. Christiani,
H. -U. Habermeier, G. Khaliullin,
C. Ulrich,
M. Le Tacon,
B. Keimer
[show abstract]
[hide abstract]
ABSTRACT: The electron-phonon interaction is of central importance for the electrical
and thermal properties of solids, and its influence on superconductivity,
colossal magnetoresistance, and other many-body phenomena in
correlated-electron materials is currently the subject of intense research.
However, the non-local nature of the interactions between valence electrons and
lattice ions, often compounded by a plethora of vibrational modes, present
formidable challenges for attempts to experimentally control and theoretically
describe the physical properties of complex materials. Here we report a Raman
scattering study of the lattice dynamics in superlattices of the
high-temperature superconductor $\bf YBa_2 Cu_3 O_7$ and the
colossal-magnetoresistance compound $\bf La_{2/3}Ca_{1/3}MnO_{3}$ that suggests
a new approach to this problem. We find that a rotational mode of the MnO$_6$
octahedra in $\bf La_{2/3}Ca_{1/3}MnO_{3}$ experiences pronounced
superconductivity-induced lineshape anomalies, which scale linearly with the
thickness of the $\bf YBa_2 Cu_3 O_7$ layers over a remarkably long range of
several tens of nanometers. The transfer of the electron-phonon coupling
between superlattice layers can be understood as a consequence of long-range
Coulomb forces in conjunction with an orbital reconstruction at the interface.
The superlattice geometry thus provides new opportunities for controlled
modification of the electron-phonon interaction in complex materials.
06/2012;
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[show abstract]
[hide abstract]
ABSTRACT: Using resonant inelastic x-ray scattering, we observe in the bilayer iridate
Sr3Ir2O7, a spin-orbit coupling driven magnetic insulator with a small charge
gap, a magnon gap of ~92 meV for both acoustic and optical branches. This
exceptionally large magnon gap exceeds the total magnon bandwidth of ~70 meV
and implies a marked departure from the Heisenberg model, in stark contrast to
the case of the single-layer iridate Sr2IrO4. Analyzing the origin of these
observations, we find that the giant magnon gap results from bond-directional
pseudo-dipolar interactions that are strongly enhanced near the metal-insulator
transition boundary. This suggests that novel magnetism, such as that inspired
by the Kitaev model built on the pseudo-dipolar interactions, may emerge in
small charge-gap iridates.
05/2012;
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Jungho Kim,
D Casa,
M H Upton,
T Gog,
Young-June Kim,
J F Mitchell,
M van Veenendaal,
M Daghofer,
J van den Brink, G Khaliullin,
B J Kim
[show abstract]
[hide abstract]
ABSTRACT: We used resonant inelastic x-ray scattering to reveal the nature of magnetic interactions in Sr2IrO4, a 5d transition-metal oxide with a spin-orbit entangled ground state and J(eff)=1/2 magnetic moments. The magnon dispersion in Sr2IrO4 is well-described by an antiferromagnetic Heisenberg model with an effective spin one-half on a square lattice, which renders the low-energy effective physics of Sr2IrO4 much akin to that in superconducting cuprates. This point is further supported by the observation of exciton modes in Sr2IrO4, whose dispersion is strongly renormalized by magnons, which can be understood by analogy to hole propagation in the background of antiferromagnetically ordered spins in the cuprates.
Physical Review Letters 04/2012; 108(17):177003. · 7.37 Impact Factor
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Jungho Kim,
D. Casa,
M. H. Upton,
T. Gog,
Young-June Kim,
J. F. Mitchell,
M. van Veenendaal,
M. Daghofer,
J. van den Brink, G. Khaliullin,
B. J. Kim
[show abstract]
[hide abstract]
ABSTRACT: We used resonant inelastic x-ray scattering to reveal the nature of magnetic
interactions in Sr2IrO4, a 5d transition-metal oxide with a spin-orbit
entangled ground state and Jeff=1/2 magnetic momemts, referred to as
'isospins'. The magnon dispersion in Sr2IrO4 is well described by an
antiferromagnetic Heisenberg model with isospin one-half moments on a square
lattice, which renders the low-energy effective physics of Sr2IrO4 much akin to
that in superconducting cuprates. This is further supported by the observation
of exciton modes in Sr2IrO4 whose dispersion is strongly renormalized by
magnons, which can be understood by analogy to the hole propagation in the
background of antiferromagnetically ordered spins in the cuprates.
10/2011;
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M. Le Tacon,
G. Ghiringhelli,
J. Chaloupka,
M. Moretti Sala,
V. Hinkov,
M. W. Haverkort,
M. Minola,
M. Bakr,
K. J. Zhou,
S. Blanco-Canosa,
C. Monney,
Y. T. Song,
G. L. Sun,
C. T. Lin,
G. M. De Luca,
M. Salluzzo, G. Khaliullin,
T. Schmitt,
L. Braicovich,
B. Keimer
Nature Physics 01/2011; 7(9):725-730. · 18.97 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: We present two different theories for Raman scattering and Resonant Inelastic X-ray Scattering (RIXS) in the low temperature ferromagnetic phase of YTiO$_3$ and compare this to the available experimental data. For description of the orbital ground-state and orbital excitations, we consider two models corresponding to two theoretical limits: one where the $t_{2g}$ orbitals are degenerate, and the other where strong lattice distortions split them. In the former model the orbitals interact through superexchange. The resulting superexchange Hamiltonian yields an orbitally ordered ground state with collective orbital excitations on top of it - the orbitons. In the orbital-lattice model, on the other hand, distortions lead to local dd-transitions between crystal field levels. Correspondingly, the orbital response functions that determine Raman and RIXS lineshapes and intensities are of cooperative or single-ion character. We find that the superexchange model yields theoretical Raman and RIXS spectra that fit very well to the experimental data. Comment: Summary of changes: 1) Calculation of Raman spectrum in local model added. 2) RIXS treatment is generalized, the Ultrashort Core hole Lifetime is no longer needed. The RIXS sections are rearranged
09/2009;
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C Ulrich,
L J P Ament,
G Ghiringhelli,
L Braicovich,
M Moretti Sala,
N Pezzotta,
T Schmitt, G Khaliullin,
J van den Brink,
H Roth,
T Lorenz,
B Keimer
[show abstract]
[hide abstract]
ABSTRACT: High-resolution resonant inelastic x-ray scattering has been used to determine the momentum dependence of orbital excitations in Mott-insulating LaTiO(3) and YTiO(3) over a wide range of the Brillouin zone. The data are compared to calculations in the framework of lattice-driven and superexchange-driven orbital ordering models. A superexchange model in which the experimentally observed modes are attributed to two-orbiton excitations yields the best description of the data.
Physical Review Letters 09/2009; 103(10):107205. · 7.37 Impact Factor
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[show abstract]
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ABSTRACT: We present a comprehensive ellipsometric study of an untwinned, nearly stoichiometric LaMnO_3 crystal in the spectral range 1.2-6.0 eV at temperatures 20 K < T < 300 K. The complex dielectric response along the b and c axes of the Pbnm orthorhombic unit cell, \epsilon^b(\nu) and \epsilon^c(\nu), is highly anisotropic over the spectral range covered in the experiment. The difference between \epsilon^b(\nu) and \epsilon^c(\nu) increases with decreasing temperature, and the gradual evolution observed in the paramagnetic state is strongly enhanced by the onset of A-type antiferromagnetic long-range order at T_N = 139.6 K. In addition to the temperature changes in the lowest-energy gap excitation at 2 eV, there are opposite changes observed at higher energy at 4 - 5 eV, appearing on a broad-band background due to the strongly dipole-allowed O 2p -- Mn 3d transition around the charge-transfer energy 4.7 eV. Based on the observation of a pronounced spectral-weight transfer between low- and high-energy features upon magnetic ordering, they are assigned to high-spin and low-spin intersite d^4d^4 - d^3d^5 transitions by Mn electrons. The anisotropy of the lowest-energy optical band and the spectral weight shifts induced by antiferromagnetic spin correlations are quantitatively described by an effective spin-orbital superexchange model. An analysis of the multiplet structure of the intersite transitions by Mn e_g electrons allowed us to estimate the effective intra-atomic Coulomb interaction, the Hund exchange coupling, and the Jahn-Teller splitting energy between e_g orbitals in LaMnO_3. This study identifies the lowest-energy optical transition at 2 eV as an intersite d-d transition, whose energy is substantially reduced compared to that obtained from the bare intra-atomic Coulomb interaction. Comment: 10 pages, 14 figures
07/2009;
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07/2009;
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[show abstract]
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ABSTRACT: Recent studies exposed many remarkable properties of layered cobaltates NaxCoO2. Surprisingly, many-body effects have been found to increase at sodium-rich compositions of NaxCoO2 where one expects a simple, nearly free motion of the dilute S=1/2 holes doped into a band insulator NaCoO2. Here we discuss the origin of enigmatic correlations that turn a doped NaCoO2 into a strongly correlated electronic system. A minimal model including orbital degeneracy is proposed and its predictions are discussed. The model is based on a key property of cobalt oxides - the spin-state quasidegeneracy of CoO6 octahedral complex - which has been known, e.g., in the context of an unusual physics of LaCoO3 compound. Another important ingredient of the model is the 90-degree Co-O-Co bonding in NaxCoO2 which allows nearest-neighbor $t_{2g}-e_g$ hopping. This hopping introduces a dynamical mixture of electronic configurations $t_{2g}^6, S=0$ and $t_{2g}^5e_g^1, S=1$ of neighboring cobalt ions. We show that scattering of charge carriers on spin-state fluctuations suppresses their coherent motion and leads to the spin-polaron physics at $x\sim 1$. At larger doping when coherent fermionic bands are formed, the model predicts singlet superconductivity of extended s-wave symmetry. The presence of low-lying spin states of Co$^{3+}$ is essential for the pairing mechanism. Implications of the model for magnetic orderings are also discussed.
07/2008;
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[show abstract]
[hide abstract]
ABSTRACT: We present numerical results on a spin-orbital polaron for lightly doped cobaltates with x≲ 0.7 < 1. We compare magnetic susceptibility of the polaron with realistic parameters to experiment and find perfect agreement.
Further, we analyze magnetic excitations and argue that they are responsible for observed spin-wave scattering.
The cobaltates Na
x
CoO2have recently enjoyed much interest for a variety of reasons. One of their intriguing features is an unconventional superconductivity,
which can be observed in the hydrated compound Na
x
CoO2:yH2O for x≈ 0.3, y ≈ 1.4 [1, 2]. The half-doped Na0.5CoO2 is a charge ordered insulator and separates two different metallic phases: Low sodium content leads to paramagnetism (this
doping range contains the superconducting compounds) while high sodium induces a Curie-Weiss-like susceptibility [3]. The
latter part of the phase diagram is of interest because of its high thermo-electric power [4, 5], i.e., the capability to
transform temperature differences into electricity, combined with relatively low resistivity. The thermoelectric properties
are strongly affected by a magnetic field, which seems to suggest a strong role of the spin degree of freedom and to indicate
strong electron-electron correlation effects [6]. The magnetic properties for 0.7 ≲x < 1 — where thermopower is particularly enhanced [2] — are rather peculiar: While neither of the endpoints x = 1 and x≲ 0.7 shows magnetic structure, A-type antiferromagnetism below T
N
≈ 20K [7, 8] is found in between, where ferromagnetic planes are stacked antiferromagnetically. This regime is rather unconventional,
because its high-temperature magnetic behavior shows a negative Curie-Weiss temperature [5, 9, 6, 10, 11], while a positive
one can be inferred from its low-temperature spin-wave data, as pointed out in Ref. [7]. This contradiction can be resolved
by assuming that the charge carriers are spin-orbital polarons [12, 13]. Spin-orbital polarons can likewise explain why the
ferromagnetic in-plane coupling is rather small — only as large as the antiferromagnetic inter-layer coupling, despite the
much short in-plane bond lengths.
06/2008: pages 49-55;
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ABSTRACT: Orbital reconstructions and covalent bonding must be considered as important factors in the rational design of oxide heterostructures with engineered physical properties. We have investigated the interface between high-temperature superconducting (Y,Ca)Ba(2)Cu3O7 and metallic La(0.67)Ca(0.33)MnO3 by resonant x-ray spectroscopy. A charge of about -0.2 electron is transferred from Mn to Cu ions across the interface and induces a major reconstruction of the orbital occupation and orbital symmetry in the interfacial CuO2 layers. In particular, the Cu d(3z(2)-r(2)) orbital, which is fully occupied and electronically inactive in the bulk, is partially occupied at the interface. Supported by exact-diagonalization calculations, these data indicate the formation of a strong chemical bond between Cu and Mn atoms across the interface. Orbital reconstructions and associated covalent bonding are thus important factors in determining the physical properties of oxide heterostructures.
Science 12/2007; 318(5853):1114-7. · 31.20 Impact Factor
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ABSTRACT: By muon spin rotation we investigated the magnetic properties of a series of highly Na-doped NaxCoO2 single crystals with 0.78(1)≤x≤0.97(1). Our data provide evidence for an intrinsically inhomogeneous magnetic state which can be described in terms of hole-doping (Na vacancy)-induced magnetic clusters that percolate at 1−x0.04 until they yield a bulk magnetic state near x=0.78. Evidence for a strong (likely geometrical) frustration of the magnetic order is obtained from the anomalous doping dependence of the spin fluctuation rate (above the ordering temperature) which is strongly enhanced at x=0.78 as compared to x=0.97.
EPL (Europhysics Letters) 09/2007; 80(2):27005. · 2.17 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: By muon spin rotation we investigated the magnetic properties of a series of highly Na doped NaxCoO2 single crystals with 0.78(1)<x<0.97(1). Our data provide evidence for an intrinsically inhomogeneous magnetic state which can be described in tems of hole doping (Na vacancy) induced magnetic clusters that percolate at 1-x>0.04 until they yield a bulk magnetic state near x=0.78. Evidence for a strong (likely geometrical) frustration of the magnetic order is obtained from the anomalous doping dependence of the spin fluctuation rate (above the ordering temperature) which is strongly enhanced at x=0.78 as compared to x=0.97.
02/2007;
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[show abstract]
[hide abstract]
ABSTRACT: Raman scattering is used to observe pronounced electronic excitations around 230 meV--well above the two-phonon range--in the Mott insulators LaTiO3 and YTiO3. Based on the temperature, polarization, and photon energy dependence, the modes are identified as orbital excitations. The observed profiles bear a striking resemblance to magnetic Raman modes in the insulating parent compounds of the superconducting cuprates, indicating an unanticipated universality of the electronic excitations in transition metal oxides.
Physical Review Letters 11/2006; 97(15):157401. · 7.37 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: We present a numerical treatment of a spin-orbital-polaron model for NaxCoO2 at small hole concentration (0.7 < x 1). We demonstrate how the polarons account for the peculiar magnetic properties of this layered compound: They explain the large susceptibility; their internal degrees of freedom lead both to a negative Curie-Weiss temperature and yet to a ferromagnetic intralayer interaction, thereby resolving a puzzling contradiction between these observations. We make specific predictions on the momentum and energy location of excitations resulting from the internal degrees of freedom of the polaron, and discuss their impact on spin-wave damping.
Physical Review Letters 07/2006; 96(21):216404. · 7.37 Impact Factor
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J. Chakhalian,
J. W. Freeland,
G. Srajer,
J. Strempfer, G. Khaliullin,
J. C. Cezar,
T. Charlton,
R. Dalgliesh,
C. Bernhard,
G. Cristiani,
H.-U. Habermeier,
B. Keimer
[show abstract]
[hide abstract]
ABSTRACT: Carefully controlled interfaces between two materials can give rise to novel physical phenomena and functionalities not exhibited by either of the constituent materials alone. Modern synthesis methods have yielded high-quality heterostructures of oxide materials with competing order parameters. Although magnetic correlations at the interface are expected to be important in determining the macroscopic properties of such systems, a quantitative determination of the interfacial magnetization profile has thus far not been reported. Here we examine superlattices composed of the half-metallic ferromagnet La
Nature Physics 03/2006; 2(4):244-248. · 18.97 Impact Factor
