arXiv subjects

Kenya Ohgushi

Publications and source records attributed to Kenya Ohgushi.

33 records · Page 2Linked to original sources

arXiv2016

Magnetic resonances of multiferroic TbFe$_3$(BO$_3$)$_4$

Low-energy magnetic excitations of the easy-axis antiferromagnet TbFe$_3$(BO$_3$)$_4$ are investigated by far-infrared absorption and reflection spectroscopy in high magnetic fields up to 30 T. The observed field dependence of the resonance frequencies and the magnetization are reproduced by a mean-field spin model for magnetic fields applied both along and perpendicular to the easy axis. Based on this model we determined the full set of magnetic interactions, including Fe-Fe and Fe-Tb exchange interactions, single-ion anisotropy for Tb ions and $g$-factors, which describe the ground-state spin texture and the low-energy spin excitations of TbFe$_3$(BO$_3$)$_4$. Compared to earlier studies we allow a small canting of the nearly Ising-like Tb moments to achieve a quantitative agreement with the magnetic susceptibility measurements. The additional high energy magnetic resonance lines observed, besides the two resonances expected for a two-sublattice antiferromagnet, suggest a more complex six-sublattice magnetic ground state for TbFe$_3$(BO$_3$)$_4$.

cond-mat.mtrl-sci↗

arXiv2016

The magnetic precursor of the pressure-induced superconductivity in Fe-ladder compound

The pressure effects on the antiferromagentic orders in iron-based ladder compounds CsFe$_2$Se$_3$ and BaFe$_2$S$_3$ have been studied using neutron diffraction. With identical crystal structure and similar magnetic structures, the two compounds exhibit highly contrasting magnetic behaviors under moderate external pressures. In CsFe$_2$Se$_3$ the ladders are brought much closer to each other by pressure, but the stripe-type magnetic order shows no observable change. In contrast, the stripe order in BaFe$_2$S$_3$, undergoes a quantum phase transition where an abrupt increase of N$\acute{e}$el temperature by more than 50$\%$ occurs at about 1 GPa, accompanied by a jump in the ordered moment. With its spin structure unchanged, BaFe$_2$S$_3$ enters an enhanced magnetic phase that bears the characteristics of an orbital selective Mott phase, which is the true neighbor of superconductivity emerging at higher pressures.

cond-mat.supr-con↗

arXiv2015

The Effects of Stoichiometry and Substitution in quasi-one-dimensional iron-chalcogenide BaFe$_{2}$S$_{3}$

The effects of off-stoichiometry and elemental substitution on electronic properties of iron-based ladder compound BaFe$_{2}$S$_{3}$ are investigated. Resistivity and magnetization are revealed to be quite sensitive to the stoichiometry of Fe atoms, and 10 \% deficiency at Fe sites reduces the antiferromagnetic transition temperature by 40 K. The antiferromagnetic transition temperature decreases even faster and collapse to zero with hole doping through 10 \% K substitution at Ba site, while the antiferromagnetic ordering phase remains with electron doping through 20 \% Co substitution at Fe site. Such electron-hole asymmetry is opposite to two-dimensional iron-based superconductors, and can be explained on the basis of both itinerant and localized electronic pictures.

cond-mat.str-el↗

arXiv2015

Pressure-induced Superconductivity in the Iron-based Ladder Material BaFe2S3

All the iron-based superconductors identified to date share a square lattice composed of Fe atoms as a common feature, despite having different crystal structures. In copper-based materials, the superconducting phase emerges not only in square lattice structures but also in ladder structures. Yet iron-based superconductors without a square lattice motif have not been found despite being actively sought out. Here, we report the discovery of pressure-induced superconductivity in the iron-based spin-ladder material BaFe2S3, a Mott insulator with striped-type magnetic ordering below ~120 K. On the application of pressure this compound exhibits a metal-insulator transition at about 11 GPa, followed by the appearance of superconductivity below Tc = 14 K, right after the onset of the metallic phase. Our findings indicate that iron-based ladder compounds represent promising material platforms, in particular for studying the fundamentals of iron-based superconductivity.

cond-mat.supr-con↗

arXiv2015

Temperature and composition phase diagram in the iron-based ladder compounds Ba1-xCsxFe2Se3

We investigated the iron-based ladder compounds (Ba,Cs)Fe$_2$Se$_3$. Their parent compounds, BaFe$_2$Se$_3$ and CsFe$_2$Se$_3$, have different space groups, formal valences of Fe and magnetic structures. Electrical resistivity, specific heat, magnetic susceptibility, X-ray diffraction and powder neutron diffraction measurements were conducted to obtain temperature and composition phase diagram of this system. Block magnetism observed in BaFe$_2$Se$_3$ is drastically suppressed with Cs doping. In contrast, stripe magnetism observed in CsFe$_2$Se$_3$ is not so fragile against Ba doping. New type of magnetic structure appears in intermediate compositions, which is similar to stripe magnetism of CsFe$_2$Se$_3$, but inter-ladder spin configuration is different. Intermediate compounds show insulating behavior, nevertheless finite $T$-linear contribution in specific heat was obtained at low temperatures.

cond-mat.str-el↗

arXiv2013

Mechanism of enhanced optical second-harmonic generation in the conducting pyrochlore-type Pb$_{2}$Ir$_{2}$O$_{7-x}$ oxide compound

The structural, electronic, and optical properties of pyrochlore-type Pb$_{2}$Ir$_{2}$O$_{6}$O'$_{0.55}$, which is a metal without spatial inversion symmetry at room temperature, were investigated. Structural analysis revealed that the structural distortion relevant to the breakdown of the inversion symmetry is dominated by the Pb-O' network but is very small in the Ir-O network. At the same time, gigantic second-harmonic generation signals were observed, which can only occur if the local environment of the Ir 5$d$ electrons features broken inversion symmetry. First-principles electronic structure calculations reveal that the underlying mechanism for this phenomenon is the induction of the noncentrosymmetricity in the Ir 5$d$ bands by the strong hybridization with O' 2$p$ orbitals. Our results stimulate theoretical study of inversion-broken iridates, where exotic quantum states such as a topological insulator and Dirac semimetal are anticipated.

cond-mat.str-el↗

arXiv2013

Observation of Phonon-Assisted Magnon Absorption in Spin-Orbit Coupling Induced Mott Insulator Sr$_{2}$IrO$_{4}$

The infrared transmission measurement was performed for a two-dimensional spin-orbit coupling induced Mott insulator Sr$_{2}$IrO$_{4}$ with a layered-perovskite structure. A phonon-assisted magnon mode was observed at around 1870 cm$^{-1}$ in the absorption spectrum below 180 K. The nearest-neighbor superexchange interaction $J$ was estimated to be 57 meV, which is about a half of the $J$ value of a copper oxide La$_{2}$CuO$_{4}$. Our results indicate the possibility that nontrivial properties such as superconductivity are realized at rather high temperature in iridium oxides.

cond-mat.str-el↗

arXiv2012

Complex Orbital State Stabilized by Strong Spin-Orbit Coupling in a Metallic Iridium Oxide IrO$_{2}$

Resonant x-ray diffraction experiments were performed for the metallic iridium oxide IrO$_{2}$. We observed anisotropic tensor of susceptibility (ATS) scattering, the spectrum of which shows a sharp contrast between the $L_{3}$ and $L_{2}$ edges. At the $L_{3}$ edge, resonance excitations were clearly observed from the core 2$p$ orbitals to both the 5$d$ $t_{2g}$ and $e_{g}$ orbitals. In contrast, the resonance mode associated with 5$d$ $t_{2g}$ orbitals was indiscernible at the $L_{2}$ edge. This contrasting behavior indicates that Ir 5$d$ $t_{2g}$ orbitals are fairly close to the $J_{\rm eff}$ = 1/2 state due to the strong spin--orbit coupling in 5$d$ transition metal ions, as in the Mott insulator Sr$_{2}$IrO$_{4}$. Our results clearly demonstrate that ATS scattering is a useful probe for investigating complex orbital states in a metallic state. Such states induce novel phenomena such as the spin Hall effect.

cond-mat.str-el↗

arXiv2012

Doping Dependence of Hall Coefficient and Evolution of Coherent Electronic State in the Normal State of Fe-based Superconductor Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$

We investigated the in-plane transport properties of the Fe-based superconductor Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ with a wide composition range $0 \leq x \leq 0.55$. We show that the doping dependence of the Hall coefficient is well-described by the Boltzmann equation for a two-band system with a rigid-band approximation. We successfully deduced transport parameters, which suggested that holes with heavier mass conduct more smoothly than electrons. Moreover, the temperature variation of the Hall coefficient indicated that an anomalous coherent state characterized by heavy quasiparticles in hole bands evolved below $\sim100$ K, predominantly in the optimal and overdoped regions. We argue that this phenomenon can be understood in relation to the pseudopeak structure observed in angle-resolved photoemission spectroscopy.

cond-mat.supr-con↗

arXiv2012

Block magnetism coupled with local distortion in the iron-based spin-ladder compound BaFe2Se3

Magnetism in the insulating BaFe$_2$Se$_3$ was examined through susceptibility, specific heat, resistivity and neutron diffraction measurements. After formation of a short-range magnetic correlation, a long-range ordering was observed below $T_{\rm N}\sim 255$ K. The transition is obscured by bulk properties. Magnetic moments ($\parallel a$) are arranged to form a Fe$_4$ ferromagnetic unit, and each Fe$_4$ stacks antiferromagnetically. This block magnetism is of the third type among magnetic structures of ferrous materials. The magnetic ordering drives unusually large distortion via magnetoelastic coupling.

cond-mat.str-el↗

arXiv2011

Resonant X-Ray Diffraction Study of Strongly Spin-Orbit-Coupled Mott Insulator CaIrO3

We performed resonant x-ray diffraction experiments at the $L$ absorption edges for the post-perovskite-type compound CaIrO$_{3}$ with $(t_{2g})^5$ electronic configuration. By observing the magnetic signals, we could clearly see that the magnetic structure was a striped order with an antiferromagnetic moment along the c-axis and that the wavefunction of a $t_{2g}$ hole is strongly spin-orbit entangled, the $J_{\rm eff} =1/2$ state. The observed spin arrangement is consistent with theoretical work predicting a unique superexchange interaction in the $J_{\rm eff} =1/2$ state and points to the universal importance of the spin-orbit coupling in Ir oxides, irrespective of the local coordination and lattice topology. We also propose that the non-magnetic resonant scattering is a powerful tool for unraveling an orbital state even in a metallic iridate.

cond-mat.str-el↗

arXiv2009

Intrinsic Properties of AFe2As2 (A = Ba, Sr) Single Crystal under Highly Hydrostatic Pressure Conditions

We measured the electrical resistivity and ac magnetic susceptibility of BaFe2As2 and SrFe2As2 single crystals under pressure using a cubic anvil apparatus. For BaFe2As2, the antiferromagnetic (AF) and structural transitions are suppressed with increasing pressure. Unexpectedly, these transitions persist up to 8 GPa, and no signature of a superconducting transition was observed in the pressure range investigated here. On the other hand, the AF and structural transitions of SrFe2As2 collapse at around the critical pressure Pc ~ 5 GPa, resulting in the appearance of bulk superconductivity. The superconducting volume fraction abruptly increases above Pc, and shows a dome centered at approximately 6 GPa. Our results suggest that the bulk superconducting phase competes with the AF/orthorhombic phase and only appears in the narrow pressure region of the tetragonal phase.

cond-mat.supr-con↗

arXiv2009

Antiferromagnetism of SrFe2As2 studied by Single-Crystal 75As-NMR

We report results of 75As nuclear magnetic resonance (NMR) experiments on a self-flux grown high-quality single crystal of SrFe2As2. The NMR spectra clearly show sharp first-order antiferromagnetic (AF) and structural transitions occurring simultaneously. The behavior in the vicinity of the transition is compared with our previous study on BaFe2As2. No significant difference was observed in the temperature dependence of the static quantities such as the AF splitting and electric quadrupole splitting. However, the results of the NMR relaxation rate revealed difference in the dynamical spin fluctuations. The stripe-type AF fluctuations in the paramagnetic state appear to be more anisotropic in BaFe2As2 than in SrFe2As2.

cond-mat.str-el↗

arXiv2008

Commensurate Itinerant Antiferromagnetism in BaFe2As2: 75As-NMR Studies on a Self-Flux Grown Single Crystal

We report results of 75As nuclear magnetic resonance (NMR) experiments on a self-flux grown single crystal of BaFe2As2. A first-order antiferromagnetic (AF) transition near 135 K was detected by the splitting of NMR lines, which is accompanied by simultaneous structural transition as evidenced by a sudden large change of the electric field gradient tensor at the As site. The NMR results lead almost uniquely to the stripe spin structure in the AF phase. The data of spin-lattice relaxation rate indicate development of anisotropic spin fluctuations of the stripe-type with decreasing temperature in the paramagnetic phase.

cond-mat.supr-con↗

arXiv1999

Spin Anisotropy and Quantum Hall Effect in the Kagome Lattice - Chiral Spin State based on a Ferromagnet-

A ferromagnet with spin anisotropies on the 2D Kagome lattice is theoretically studied. This is a typical example of the flat-band ferromagnet. The Berry phase induced by the tilting of the spins opens the band gap and quantized Hall conductance σ_{xy}=\pm e^2/h is realized without external magnetic field. This is the most realistic chiral spin state based on the ferromagnetism. We also discuss the implication of our results to anomalous Hall effect observed in the metallic pyrochlore ferromagnets R_2Mo_2O_7(R=Nd, Sm, Gd).

cond-mat.str-el↗