arXiv subjects

M. Isobe

Publications and source records attributed to M. Isobe.

At least 37 records · Page 2Linked to original sources

arXiv2014

Bulk Nature of Layered Perovskite Iridates beyond the Mott Scenario : An Approach from Bulk Sensitive Photoemission Study

We present genuine bulk Ir 5d jeff states of layered perovskite iridates obtained by hard-x-ray photoemission spectroscopy (HAXPES) with s- and p-polarized lights. HAXPES spectra of Sr2IrO4 and Ba2IrO4 are well reproduced by the quasi-particle densities of states calculated by the local density approximation with dynamical mean-field theory (LDA+DMFT). It is demonstrated that the insulating nature of the iridates is triggered by antiferromagnetic correlation (Slater type) combined with electron correlation (Mott type). The extremely-low-energy bulk-sensitive photoemission spectroscopy reveals "bad metallic" states in the paramagnetic phase of the iridates, suggesting strongly renormalized metallic states above the Neel temperature as predicted by the LDA+DMFT.

cond-mat.str-el↗

arXiv2014

Magnetic pyroxenes LiCrGe2O6 and LiCrSi2O6: dimensionality crossover in a non-frustrated S=3/2 Heisenberg model

The magnetism of magnetoelectric $S$ = 3/2 pyroxenes LiCrSi$_2$O$_6$ and LiCrGe$_2$O$_6$ is studied by density functional theory (DFT) calculations, quantum Monte Carlo (QMC) simulations, neutron diffraction, as well as low-field and high-field magnetization measurements. In contrast with earlier reports, we find that the two compounds feature remarkably different, albeit non-frustrated magnetic models. In LiCrSi$_2$O$_6$, two relevant exchange integrals, $J_1 \simeq$ 9 K along the structural chains and $J_{\text{ic1}}$ $\simeq$ 2 K between the chains, form a 2D anisotropic honeycomb lattice. In contrast, the spin model of LiCrGe$_2$O$_6$ is constituted of three different exchange couplings. Surprisingly, the leading exchange $J_{\text{ic1}}$ $\simeq$ 2.3 K operates between the chains, while $J_1$ $\simeq$ 1.2 K is about two times smaller. The additional interlayer coupling $J_{\text{ic2}}$ $\simeq$ $J_1$ renders this model 3D. QMC simulations reveal excellent agreement between our magnetic models and the available experimental data. Underlying mechanisms of the exchange couplings, magnetostructural correlations, as well as implications for other pyroxene systems are discussed.

cond-mat.str-el↗

arXiv2013

Orbital occupancies and the putative jeff = 1/2 groundstate in Ba2IrO4: a combined oxygen K edge XAS and RIXS study

The nature of the electronic groundstate of Ba2IrO4 has been addressed using soft X-ray absorption and inelastic scattering techniques in the vicinity of the oxygen K edge. From the polarization and angular dependence of XAS we deduce an approximately equal superposition of xy, yz and zx Ir4+ 5d orbitals. By combining the measured orbital occupancies, with the value of the spin-orbit coupling provided by RIXS, we estimate the crystal field splitting associated with the tetragonal distortion of the IrO6 octahedra to be small, Δ=50(50) meV. We thus conclude definitively that Ba2IrO4 is a close realization of a spin-orbit Mott insulator with a jeff = 1/2 groundstate, thereby overcoming ambiguities in this assignment associated with the interpretation of X-ray resonant scattering experiments.

cond-mat.str-el↗

arXiv2013

The electronic structure of the high-symmetry perovskite iridate Ba2IrO4

We report angle-resolved photoemission (ARPES) measurements, density functional and model tight-binding calculations on Ba$_2$IrO$_4$ (Ba-214), an antiferromagnetic ($T_N=230$ K) insulator. Ba-214 does not exhibit the rotational distortion of the IrO$_6$ octahedra that is present in its sister compound Sr$_2$IrO$_4$ (Sr-214), and is therefore an attractive reference material to study the electronic structure of layered iridates. We find that the band structures of Ba-214 and Sr-214 are qualitatively similar, hinting at the predominant role of the spin-orbit interaction in these materials. Temperature-dependent ARPES data show that the energy gap persists well above $T_N$, and favour a Mott over a Slater scenario for this compound.

cond-mat.str-el↗

arXiv2012

Robustness of basal-plane antiferromagnetic order and the $J_{eff}=1/2$ state in single-layer iridate spin-orbit Mott insulators

The magnetic structure and electronic groundstate of the layered perovskite Ba2IrO4 have been investigated using x-ray resonant magnetic scattering (XRMS). Our results are compared with those for Sr2IrO4, for which we provide supplementary data on its magnetic structure. We find that the dominant, long-range antiferromagnetic order is remarkably similar in the two compounds, and that the electronic groundstate in Ba2IrO4, deduced from an investigation of the XRMS $L_3/L_2$ intensity ratio, is consistent with a $J_{eff}=1/2$ description. The robustness of these two key electronic properties to the considerable structural differences between the Ba and Sr analogues is discussed in terms of the enhanced role of the spin-orbit interaction in 5d transition metal oxides.

cond-mat.str-el↗

arXiv2012

Partially disordered state and spin-lattice coupling in an S=3/2 triangular lattice antiferromagnet Ag2CrO2

Ag2CrO2 is an S=3/2 frustrated triangular lattice antiferromagnet without orbital degree of freedom. With decreasing temperature, a 4-sublatice spin state develops. However, a long-range partially disordered state with 5 sublattices abruptly appears at TN=24 K, accompanied by a structural distortion, and persists at least down to 2 K. The spin-lattice coupling stabilizes the anomalous state, which is expected to appear only in limited ranges of further-neighbor interactions and temperature. It was found that the spin-lattice coupling is a common feature in triangular lattice antiferromagnets with multiple-sublattice spin states, since the triangular lattice is elastic.

cond-mat.str-el↗

arXiv2011

Peierls Mechanism of the Metal-Insulator Transition in Ferromagnetic Hollandite K2Cr8O16

Synchrotron X-ray diffraction experiment shows that the metal-insulator transition occurring in a ferromagnetic state of a hollandite K$_2$Cr$_8$O$_{16}$ is accompanied by a structural distortion from the tetragonal $I4/m$ to monoclinic $P112_{1}/a$ phase with a $\sqrt{2}\times\sqrt{2}\times 1$ supercell. Detailed electronic structure calculations demonstrate that the metal-insulator transition is caused by a Peierls instability in the quasi-one-dimensional column structure made of four coupled Cr-O chains running in the $c$-direction, leading to the formation of tetramers of Cr ions below the transition temperature. This furnishes a rare example of the Peierls transition of fully spin-polarized electron systems.

cond-mat.str-el↗

arXiv2011

Pressure-induced changes in the magnetic and valence state of EuFe2As2

We present the results of electrical resistivity, ac specific heat, magnetic susceptibility, X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) of the ternary iron arsenide EuFe2As2 single crystal under pressure. Applying pressure leads to a continuous suppression of the antiferromagnetism associated with Fe moments and the antiferromagnetic transition temperature becomes zero in the vicinity of a critical pressure Pc ~2.5-2.7 GPa. Pressure-induced re-entrant superconductivity, which is highly sensitive to the homogeneity of the pressure, only appears in the narrow pressure region in the vicinity of Pc due to the competition between superconductivity and the antiferromagnetic ordering of Eu2+ moments. The antiferromagnetic state of Eu2+ moments changes to the ferromagnetic state above 6 GPa. We also found that the ferromagnetic order is suppressed with further increasing pressure, which is connected with a valence change of Eu ions.

cond-mat.supr-con↗

arXiv2010

Electronic structure of an antiferromagnetic metal: CaCrO3

We report on the electronic structure of the perovskite oxide CaCrO3 using valence-band, core-level, and Cr 2p - 3d resonant photoemission spectroscopy (PES). Despite its antiferromagnetic order, a clear Fermi edge characteristic of a metal with dominant Cr 3d character is observed in the valence band spectrum. The Cr 3d single particle density of states are spread over 2 eV, with the photoemission spectral weight distributed in two peaks centered at ~ 1.2 eV and 0.2 eV below EF, suggestive of the coherent and incoherent states resulting from strong electron-electron correlations. Resonant PES across the Cr 2p - 3d threshold identifies a 'two-hole' correlation satellite and yields an on-site Coulomb energy U ~4.8 eV. The metallic DOS at EF is also reflected through the presence of a well-screened feature at low binding energy side of the Cr 2p core-level spectrum. X-ray absorption spectroscopy (XAS) at Cr L3,2 and O K edges exhibit small temperature dependent changes that point towards a small change in Cr-O hybridization. The multiplet splitting in Cr 2p core level spectrum as well as the spectral shape of the Cr XAS can be reproduced using cluster model calculations which favour a negative value for charge transfer energy between the Cr 3d and O 2p states. The overall results indicate that CaCrO3 is a strongly hybridized antiferromagnetic metal, lying in the regime intermediate to Mott-Hubbard and charge-transfer systems.

cond-mat.str-el↗

arXiv2009

Condensation of a tetrahedra rigid-body libration mode in HoBaCo4O7 : the origin of phase transition at 355 K

Rietveld profiles, Moessbauer spectra and x-ray absorption fine structure (XAFS) were analyzed through the structural phase transition at Ts = 355 K in HoBaCo4O7. Excess of the oxygen content over O7 was avoided via annealing the samples in argon flow at 600 degree C. Space groups (S.G.) Pbn21c and P63mc were used to refine the structure parameters in the low- and high-temperature phases, respectively. Additionally, the Cmc21 symmetry was considered as a concurrent model of structure of the low-temperature phase. In the high-temperature phase, severe anisotropy of thermal motion of the major part of the oxygen atoms was observed. This anisotropic motion turns to be quenched as the sample is cooled below Ts. The variation of quadrupole splitting near Ts is not similar to a steplike anomaly frequently seen at the charge-ordering transition. We observe instead a dip-like anomaly of the average quadrupole splitting near Ts. Narrow distribution of the electric field gradient (EFG) over different cobalt sites is observed and explained on the basis of point-charge model. XAFS spectra show no evidence of significant difference between YBaCo4O7 (T > Ts) and HoBaCo4O7 (T < Ts). The origin of the transition at Ts is ascribed to the condensation of the libration phonon mode associated with the rigid-body rotational movements of the starlike tetrahedral units, the building blocks of kagome network. It is shown that the condensation of the libration mode is not compatible with translation symmetry for the hexagonal S.G., but compatible for the orthorhombic S.G. The orthorhombic lattice parameters and EFG components (Vxx, Vyy, Vzz) vary smoothly with temperature at approaching Ts and closely follow each other.

cond-mat.mtrl-sci↗

arXiv2008

CaCrO3: an anomalous antiferromagnetic metallic oxide

Combining infrared reflectivity, transport, susceptibility and several diffraction techniques, we find compelling evidence that CaCrO3 is a rare case of a metallic and antiferromagnetic transition-metal oxide with a three-dimensional electronic structure. LSDA calculations correctly describe the metallic behavior as well as the anisotropic magnetic ordering pattern of C type: The high Cr valence state induces via sizeable pd hybridization remarkably strong next-nearest neighbor interactions stabilizing this ordering. The subtle balance of magnetic interactions gives rise to magneto-elastic coupling, explaining pronounced structural anomalies observed at the magnetic ordering transition.

cond-mat.str-el↗

arXiv2006

Muon-spin relaxation measurements on the dimerized spin-1/2 chains NaTiSi2O6 and TiOCl

We report muon spin relaxation (muSR) and magnetic susceptibility investigations of two Ti3+ chain compounds which each exhibit a spin gap at low temperature, NaTiSi2O6 and TiOCl. From these we conclude that the spin gap in NaTiSi2O6 is temperature independent, with a value of 2*Delta=660(50)K, arising from orbital ordering at Too = 210K; the associated structural fluctuations activate the muon spin relaxation rate up to temperatures above 270K. In TiOCl we find thermally activated spin fluctuations corresponding to a spin gap 2*Delta=420(40)K below Tc1=67K. We also compare the methods used to extract the spin gap and the concentration of free spins within the samples from muSR and magnetic susceptibility data.

cond-mat.str-el↗

arXiv2006

Physical property characterization of Fe-tube encapsulated and vacuum annealed bulk MgB2

We report phase formation, and detailed study of magnetization and resistivity under magnetic field of MgB2 polycrystalline bulk samples prepared by Fe-tube encapsulated and vacuum (10-5 torr) annealed (750 0C) route. Zero-field-cooled magnetic susceptibility (cZFC) measurements exhibited sharp transition to superconducting state with a sizeable diamagnetic signal at 39 K (Tc). The measured magnetization loops of the samples, despite the presence of flux jumps, exhibited a stable current density (Jc) of around 2.4 x 105 A/cm2 in up to 2 T (Tesla) field and at temperatures (T) up to 10 K. The upper critical field is estimated from resistivity measurements in various fields and shows a typical value of 8 T at 21 K. Further, cFC measurements at an applied field of 0.1 T reveal paramagnetic Meissner effect (PME) that is briefly discussed.

cond-mat.supr-con↗

arXiv2006

Optical phonons in new ordered perovskite Sr2Cu(Re0.69Ca0.31) Oy system observed by infrared reflectance spectroscopy

We report infrared reflectivity spectra for a new correlated cupric oxide system Sr2Cu(Re0.69Ca0.31)Oy with y ~ 0.6 at several temperatures ranging between 8 and 380 K. The reflectivity spectrum at 300 K comprises of several optical phonons. A couple of residual bands located around 315 and 653 cm-1 exhibit exceptionally large intensity as compared to the other ones. The overall reflectivity spectrum lifts up slightly with increasing temperature. The energy and damping factor of transverse-optical phonons are determined by fitting the imaginary dielectric constant by Lorentz oscillator model and discussed as a function of temperature in terms of lattice anharmonicity.

cond-mat.str-el↗

arXiv2006

Fluxiod jumps coupled high critical current density of nano-Co3O4 doped MgB2

Polycrystalline MgB2 samples with 0, 2, 4 and 6 percent added nano-Co3O4 being synthesized by vacuum (10-5 Torr) annealing at 750 0C for three hours each are found to be nearly single phase with presence of only a small quantity of Mg/MgO in pristine sample and in addition the Co2O3 in doped compounds. All the samples exhibited clear and sharp diamagnetic transitions at around 38 K, in Zero-field-cooled (ZFC) magnetic susceptibility measurements with sizeable signal. The Field cooled (FC) measurements though having sharp transitions, but showed a very small signal, indicating high level of pinning centers in these samples. Further some of the doped samples exhibited Paramagnetic-Meissner-Effect (PME) in applied field of 5 Oe. The critical current density (Jc), being estimated by invoking Bean model for the pristine compound increase by nearly an order of magnitude for 2 and 4 percent nano-Co3O4 doping and later the same decreases sharply for 6 percent sample at nearly all studied temperatures and applied fields. Further the increased Jc (~ 10^8 A/cm2) is coupled with fluxiod jumps (T = 20 K and H = 1 T). Fluxiod Jumps are not seen in relatively low Jc pristine or 6 percent sample. This means the fluxiod-jumps are intrinsic only to the high Jc samples.

cond-mat.supr-con↗

arXiv2005

New Misfit-Layered Cobalt Oxide (CaOH)1.14CoO2

We found a new cobalt oxide (CaOH)1.14CoO2 by utilizing the high-pressure technique. X-ray and electron diffraction studies revealed that the compound has layer structure which consists of CdI2-type CoO2 layers and rock-salt-type double CaOH atomic layers. The two subcells have incommensurate periodicity along the a-axis, resulting in modulated crystal structure due to the inter-subcell interaction. The structural modulation affects carrier conduction through the potential randomness. We found that the two-dimensional (2-D) variable-range hopping (VRH) regime with hole conduction is dominant at low temperature for this compound, and that the conduction mechanism undergoes crossover from the 2-D VRH regime to thermal activation-energy type one with increasing temperature. Based on the experimental results of resistivity, thermoelectric power, magnetic susceptibility and specific heat measurements, we suggested a possible electronic-band structure model to explain these results. The cobalt t2g-derivative band crosses Fermi energy level near the band edge, yielding small finite density of localized states at the Fermi level in the band. The observed resistivity, Seebeck coefficient, large Pauli paramagnetic component in the magnetic susceptibility and comparatively small Sommerfeld constant in the specific heat are principally attributed to the holes in the t2g-derivative band. We estimated the Wilson ratio to be about 2.8, suggesting the strong electron correlation realized in this compound.

cond-mat.mtrl-sci↗

arXiv2004

Orbital and spin chains in ZnV2O4

Our powder inelastic neutron scattering data indicate that \zvo is a system of spin chains that are three dimensionally tangled in the cubic phase above 50 K due to randomly occupied $t_{2g}$ orbitals of V$^{3+}$ ($3d^2$) ions. Below 50 K in the tetragonal phase, the chains become straight due to antiferro-orbital ordering. This is evidenced by the characteristic wave vector dependence of the magnetic structure factor that changes from symmetric to asymmetric at the cubic-to-tetragonal transition.

cond-mat.str-el↗

arXiv2003

Raman scattering study of charge ordering in beta-Ca0.33V2O5

Polarized Raman spectra of the calcium vanadium oxide bronze beta-Ca0.33V2O5 are measured in the temperature range between 300 K and 20 K. The charge ordering phase transition at about 150 K is characterized by the appearance of the new Raman-active modes in the spectra, and by anomalies in the electronic background scattering. The high temperature Raman scattering spectra of beta-Ca0.33V2O5 are in apparent resemblance with those of alpha'-NaV2O5, which suggests that there is a similar charge-phonon dynamics in both compounds. The study of dynamical properties and the symmetry analysis of the Raman modes show that in the mixed valence state of beta-Ca0.33V2O5 the electrons are delocalized into V1-O5-V3 orbitals. We propose that in the charge ordered state below 150 K the d-electrons localize within V1-V3 ladders, either in "zig-zag" fashion like in alpha'-NaV2O5 or in the forms of the double chains like in gamma-LiV2O5.

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