arXiv subjects

Jun-ichi Yamaura

Publications and source records attributed to Jun-ichi Yamaura.

At least 37 records · Page 2Linked to original sources

arXiv2016

Semimetallic "Electride Bands" Derived from Interlayer Electrons in Quasi-Two-Dimensional Electride Y$_2$C

Two-dimensional (2D) electrides are a new concept material in which anionic electrons are confined in the interlayer space between positively charged layers. We have performed angle-resolved photoemission spectroscopy measurements on Y$_2$C, which is a possible 2D electride, in order to verify the formation of 2D electride states in Y$_2$C. We clearly observe the existence of semimetallic "electride bands" near the Fermi level, as predicted by ${ab}$ ${initio}$ calculations, conclusively demonstrating that Y$_2$C is a quasi-2D electride with electride bands derived from interlayer anionic electrons.

cond-mat.mtrl-sci↗

arXiv2015

Strong charge density wave fluctuation and sliding state in PdTeI with quasi-1D PdTe chains

In quasi-one-dimensional (quasi-1D) system, the charge density wave (CDW) transition temperature TCDW is usually lower than the mean-field-theory predicted TMF and a CDW fluctuation region exists between them. Here, we investigate the physical properties of PdTeI single crystal containing quasi-1D PdTe chains. Surprisingly, we find that the carrier concentration decreases gradually before the long-range CDW ordering state occurring at T1 ~ 110 K, reflecting the existence of strong CDW fluctuation with possible pseudogap state at T >> T1 because of dynamic charge separation of Pd ions (Pd3+ -> Pd2+ + Pd4+). Moreover, the sliding CDW state appears below T2 ~ 6 K. Combined such low T2 with the feature of multiple quasi-1D bands, PdTeI exhibits exotic crossover behavior from negative to huge positive magnetoresistance under magnetic field and field-induced localization. Thus, PdTeI provides a novel platform for studying the CDW fluctuation and the interplay between magnetic field and CDW state.

cond-mat.str-el↗

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↗

arXiv2014

All-in/all-out magnetic domains: X-ray diffraction imaging and magnetic field control

Long-range non-collinear all-in/all-out magnetic order has been directly observed for the first time in real space in the pyrochlore Cd$_2$Os$_2$O$_7$ using resonant magnetic microdiffraction at the Os L$_3$ edge. Two different antiferromagnetic domains related by time-reversal symmetry could be distinguished and have been mapped within the same single crystal. The two types of domains are akin to magnetic twins and were expected - yet unobserved so far - in the all-in/all-out model. Even though the magnetic domains are antiferromagnetic, we show that their distribution can be controlled using a magnetic field-cooling procedure.

cond-mat.mtrl-sci↗

arXiv2014

Hydrogen Ordering and New Polymorph of Layered Perovskite Oxyhydrides: Sr2VO4-xHx

Compositionally tunable vanadium oxyhydrides Sr2VO4-xHx (x = 0 - 1) without considerable anion vacancy were synthesized by high-pressure solid state reaction. The crystal structures and their properties were characterized by powder neutron diffraction, synchrotron X-ray diffraction, thermal desorption spectroscopy, and first-principles density functional theory (DFT) calculations. The hydrogen anions selectively replaced equatorial oxygen sites in the VO6 layers via statistical substitution of hydrogen in the low x region (x < 0.2). A new orthorhombic phase (Immm) with an almost entirely hydrogen-ordered structure formed from the K2NiF4-type tetragonal phase with x > 0.7. Based on the DFT calculations, the degree of oxygen/hydrogen anion ordering is strongly correlated with the bonding interaction between vanadium and the ligands.

cond-mat.mtrl-sci↗

arXiv2014

Layered Compounds BaM2Ge4Ch6 (M = Rh, Ir and Ch = S, Se) with Pyrite-Type Building Blocks and Ge-Ch Heteromolecule-Like Anions

Structure and chemical features of layered compounds BaM2Ge4Ch6 (M = Rh, Ir; Ch = S, Se) synthesized under high-pressure and high-temperature method are systematically studied. These compounds crystallize in an orthorhombic phase with space group Pbca (No. 61). The remarkable structural feature is to have the M-Ge-Ch pyrite-type building units, stacking with Ba-Ch layers alternatively along c axis. It is very rare and novel that pyrite-type subunits as the building blocks in the layered compounds. Theoretical calculations and experimental results indicate that there are strongly polarized covalent bonds between Ge and Ch atoms, forming heteromolecule-like anions in these compounds. Moreover, Ge atoms in this structure exhibit unusual valence state (~ +1) due to the tetrahedral coordination environment of Ge atoms along with M and Ch atoms simultaneously.

cond-mat.mtrl-sci↗

arXiv2014

Bipartite magnetic parent phases in the iron-oxypnictide superconductor

High-temperature (high-$T_{\rm c}$) superconductivity appears as a consequence of the carrier-doping of an undoped parent compound exhibiting antiferromagnetic order; thereby, ground-state properties of the parent compound are closely relevant to the superconducting state. On the basis of the concept, a spin-fluctuation has been addressed as an origin of pairing of the superconducting electrons in cuprates. Whereas, there is growing interest in the pairing mechanism such as an unconventional spin-fluctuation or an advanced orbital-fluctuation due to the characteristic multi-orbital system in iron-pnictides. Here, we report the discovery of an antiferromagnetic order as well as a unique structural transition in electron-overdoped LaFeAsO$_{1-x}$H$_x$ ($x$ ~ 0.5), whereby another parent phase was uncovered, albeit heavily doped. The unprecedented two-dome superconducting phases observed in this material can be interpreted as a consequence of the carrier-doping starting from the original at $x\sim0$ and advanced at $x\sim0.5$ parent phases toward the intermediate region. The bipartite parent phases with distinct physical properties in the second magnetic phase provide us with an interesting example to illustrate the intimate interplay among the magnetic interaction, structural change and orbital degree of freedom in iron-pnictides.

cond-mat.str-el↗

arXiv2013

Novel Rattling of K Atoms in Aluminium-Doped Defect Pyrochlore Tungstate

Rattling dynamics have been identified as fundamental to superconductivity in defect pyrochlore osmates and aluminium vanadium intermetallics, as well as low thermal conductivity in clathrates and filled skutterudites. Combining inelastic neutron scattering (INS) measurements and ab initio molecular dynamics (MD) simulations, we use a new approach to investigate rattling in the Al-doped defect pyrochlore tungstates: AAl0.33W1.67O6 (A = K, Rb, Cs). We find that although all the alkali metals rattle, the rattling of the K atoms is unique, involving local diffusion within a cage as well as vibrations at a range of frequencies. We show that the novel K rattling is driven by the strong anharmonicity and anisotropy of the local potentials around the K atoms, both of which are a consequence of the small size of the K rattler. Our findings may open new possibilities for phonon engineering in thermoelectric materials.

cond-mat.mtrl-sci↗

arXiv2013

Spinodal Decomposition in the TiO2-VO2 System

Spinodal decomposition is a ubiquitous phenomenon leading to phase separation from a uniform solution. We show that a spinodal decomposition occurs in a unique combination of two rutile compounds of TiO2 and VO2, which are chemically and physically distinguished from each other: TiO2 is a wide-gap insulator with photo catalytic activities and VO2 is assumed to be a strongly correlated electron system which exhibits a dramatic metal-insulator transition at 342 K. The spinodal decomposition takes place below 830 K at a critical composition of 34 mol% Ti, generates a unidirectional composition modulation along the c axis with a wavelength of approximately 6 nm, and finally results in the formation of self-assembled lamella structures made up of Ti-rich and V-rich layers stacked alternately with 30-50 nm wavelengths. A metal-insulator transition is not observed in quenched solid solutions with intermediate compositions but emerges in the thin V-rich layers as the result of phase separation. Interestingly, the metal-insulator transition remains as sharp as in pure VO2 even in such thin layers and takes place at significantly reduced temperatures of 310-340 K, which is probably due to a large misfit strain induced by lattice matching at the coherent interface.

cond-mat.str-el↗

arXiv2012

YCr6Ge6 as a Candidate Compound for a Kagome Metal

We show that YCr6Ge6, comprising a kagome lattice made up of Cr atoms, is a plausible candidate compound for a kagome metal that is expected to exhibit anomalous phenomena such as flat-band ferromagnetism. Resistivity, magnetization, and heat capacity are measured on single crystals of YCr6Ge6, and band structure calculations are performed to investigate the electronic structure. Curie-Weiss-like behavior in magnetic susceptibility, T2 dependence in resistivity, and a Sommerfeld coefficient doubly enhanced from a calculated value indicate a moderately strong electron correlation. Interestingly, the in-plane resistivity is twice as large as the interplane resistivity, which is contrary to the simple expectation from the layered structure. Band structure calculations demonstrate that there are partially flat bands slightly below the Fermi level near the Γ point, which is ascribed to Cr 3d3z2-r2 bands and may govern the properties of this compound.

cond-mat.mtrl-sci↗

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

Rattling and Superconducting Properties of the Cage Compound GaxV2Al20

Low-energy rattling modes and their effects on superconductivity are studied in the cage compound GaxV2Al20. A series of polycrystalline samples of 0 < x =< 0.6 are examined through resistivity, magnetic susceptibility, and heat capacity measurements. A weak-coupling BCS superconductivity is observed below Tc = 1.4-1.7 K for all the samples. For small Ga contents below 0.20, approximately 30% of the cages are occupied by rattling Al atoms having an Einstein temperature TE of 23 K, probably with most Ga atoms substituting for the cage-forming Al atoms. For higher Ga contents, approximately 0.05 Ga and 0.25-0.35 Al atoms coexist statistically inside the cages and behave as rattlers with TE ~ 8 and 23 K, respectively. A significant effect of Ga rattling on the superconductivity is clearly evidenced by the observation of a sharp rise in Tc by 8% at x = 0.20 when 0.05 Ga atoms are introduced into the case. Probably, the electron-phonon interaction is significantly enhanced by an additional contribution to the phonon density of states from the extremely low energy rattling modes of Ga atoms. In addition, a large softening of the acoustic modes is observed for x => 0.20, suggesting that the cage itself becomes anomalously soft in the presence of low-energy Ga rattling modes.

cond-mat.supr-con↗

arXiv2012

A novel crystal polymorph of volborthite, Cu3V2O7(OH)2 2H2O

A new polymorph of volborthite [tricopper(II) divanadium(V) heptaoxide dihydroxide dihydrate], Cu3V2O7(OH)2 2H2O, has been discovered in a single crystal prepared by hydrothermal synthesis. X-ray analysis reveals that the monoclinic structure has the space group C2/c at room temperature, which is different from that of the previously reported C2/m structure. Both structures have Cu3O6(OH)2 layers composed of edge-sharing CuO4(OH)2 octahedra, with V2O7 pillars and water molecules between the layers. The Cu atoms occupy two and three independent crystallographic sites in the C2/m and C2/c structures, respectively, likely giving rise to different magnetic interactions between CuII spins in the kagome lattices embedded in the Cu3O6(OH)2 layers.

cond-mat.mtrl-sci↗

arXiv2012

Large and homogeneous mass enhancement in the rattling-induced superconductor KOs$_2$O$_6$

We have determined the Fermi surface in KOs$_2$O$_6$ ($T_c$ = 9.6 K and $B_{c2} \sim$ 32 T) via de Haas-van Alphen (dHvA) oscillation measurements and a band structure calculation. We find effective masses up to 26(1) $m_e$ ($m_e$ is the free electron mass), which are unusually heavy for compounds where the mass enhancement is mostly due to electron-phonon interactions. Orbit-resolved mass enhancement parameters $λ_{dHvA}$ are large but fairly homogeneous, concentrated in the range 5 -- 8. We discuss origins of the large homogeneous mass enhancement in terms of rattling motion of the K ions.

cond-mat.supr-con↗

arXiv2012

Crystal Chemistry and Magnetic Properties of Manganese Zinc Alloy "YMn2Zn20" Comprising a Mn Pyrochlore Lattice

The chemical composition, crystal structure, and magnetic properties of a manganese zinc alloy with an ideal composition of YMn2Zn20, which comprises a pyrochlore lattice made of Mn atoms, are reported. The compound is stable only when In or Al is partially substituted for Zn. We have determined the actual chemical formula as YMn2+dZn20-x-dMx, with M = In or Al, and have identified the characteristic preferences with which the incorporated M and excess Mn atoms occupy the three crystallographic sites for Zn atoms. The Mn atoms in the pyrochlore lattice possess small magnetic moments that interact with each other antiferromagnetically but exhibit no long-range order above 0.4 K, probably owing to the geometrical frustration of the pyrochlore lattice. As a result, the effective mass of the conduction electrons is considerably enhanced, as observed in the related pyrochlore-lattice compounds (Y,Sc)Mn2 and LiV2O4. However, the presence of excess Mn atoms with large localized magnetic moments comparable to spin 5/2 tends to mask the inherent magnetism of the pyrochlore Mn atoms. It is suggested that "YMn2Zn18In2" with neither excess Mn atoms nor site disorder would be an ideal compound for further study.

cond-mat.mtrl-sci↗

arXiv2011

Superconductivity in an Einstein Solid AxV2Al20 (A = Al and Ga)

A cage compound AxV2Al20 (Al10V), that was called an Einstein solid by Caplin and coworkers 40 years ago, is revisited to investigate the low-energy, local vibrations of the A atoms and their influence on the electronic and superconducting properties of the compound. Polycrystalline samples with A = Al, Ga, Y, and La are studied through resistivity and heat capacity measurements. Weak-coupling BCS superconductivity is observed below Tc = 1.49, 1.66, and 0.69 K for Ax = Al0.3, Ga0.2, and Y, respectively, but not above 0.4 K for Ax = La. Low-energy modes are detected only for A = Al and Ga, which are approximately described by the Einstein model with Einstein temperatures of 24 and 8 K, respectively. A weak but significant coupling between the low-energy modes, which are almost identical to those called rattling in recent study, and conduction electrons manifests itself as anomalous enhancement in resistivity at around low temperatures corresponding to the Einstein temperatures.

cond-mat.supr-con↗

arXiv2011

Superconductivity and Rattling under High Pressure in the beta-Pyrochlore Oxide RbOs2O6

Rattling-induced superconductivity in the beta-pyrochlore oxide RbOs2O6 is investigated under high pressures up to 6 GPa. Resistivity measurements in a high-quality single crystal show that the superconducting transition temperature Tc increases gradually from 6.3 K at ambient pressure to 8.8 K at 3.5 GPa, surprisingly remains almost constant at 8.8 \pm 0.1 K in a wide pressure range between 3.5 (Po) and 4.8 GPa, and suddenly drops to 6.3 K at Ps = 4.9 GPa, followed by a gradual decrease with further pressure increase. Two anomalies in the temperature dependence of the normal-state resistivity are observed at Po < P < Ps and P > Ps, revealing the presence of two high-pressure phases corresponding to the changes in Tc. The rattling of the Rb ion inside a cage made of Os and O atoms may be slightly and seriously modified in these high-pressure phases that probably have cages of reduced symmetry, respectively, so that electron-rattler interactions that govern the superconducting and transport properties of beta-RbOs2O6 are significantly affected.

cond-mat.supr-con↗

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↗