arXiv subjects

Tamio Oguchi

Publications and source records attributed to Tamio Oguchi.

34 records · Page 2Linked to original sources

arXiv2019

Topology analysis for anomalous Hall effect in the non-collinear antiferromagnetic states of Mn$_3\mathit{A}$N ($A$ = Ni, Cu, Zn, Ga, Ge, Pd, In, Sn, Ir, Pt)

We investigate topological features of electronic structures which produce large anomalous Hall effect in the non-collinear antiferromagnetic metallic states of anti-perovskite manganese nitrides by first-principles calculations. We first predict the stable magnetic structures of these compounds to be non-collinear antiferromagnetic structures characterized by either $T_{1g}$ or $T_{2g}$ irreducible representation by evaluating the total energy for all of the magnetic structures classified according to the symmetry and multipole moments. The topology analysis is next performed for the Wannier tight-binding models obtained from the first-principles band structures. Our results reveal the small Berry curvature induced through the coupling between occupied and unoccupied states with the spin-orbit coupling, which is widely spread around the Fermi surface in the Brillouin zone, dominantly contributes after the $k$-space integration to the anomalous Hall conductivity, while the local divergent Berry curvature around Weyl points has a rather small contribution to the anomalous Hall conductivity.

cond-mat.mtrl-sci↗

arXiv2019

Evidence for bulk nodal loops and universality of Dirac-node arc surface states in ZrGeXc (Xc = S, Se, Te)

We have performed angle-resolved photoemission spectroscopy (ARPES) on layered ternary compounds ZrGeXc (Xc = S, Se, and Te) with square Ge lattices. ARPES measurements with bulk-sensitive soft-x-ray photons revealed a quasi-two-dimensional bulk-band structure with the bulk nodal loops protected by glide mirror symmetry of the crystal lattice. Moreover, high-resolution ARPES measurements near the Fermi level with vacuum-ultraviolet photons combined with first-principles band-structure calculations elucidated a Dirac-node-arc surface state traversing a tiny spin-orbit gap associated with the nodal loops. We found that this surface state commonly exists in ZrGeXc despite the difference in the shape of nodal loops. The present results suggest that the spin-orbit coupling and the multiple nodal loops cooperatively play a key role in creating the exotic Dirac-node-arc surface states in this class of topological line-node semimetals.

cond-mat.mes-hall↗

arXiv2017

Metal-Insulator Transition and Jeff = 1/2 Spin-Orbit Insulating State in Rutile-based IrO2/TiO2 Superlattices

By combining 5d transition-metal oxides showing pronounced spin-orbit interactions and oxide-based heterostructures, we propose rutile-based IrO2/TiO2 superlattices as promising candidates for unconventional electronic properties. By means of density-functional-theory simulations complemented with Hubbard-like corrections, we focus on the evolution of the electronic structure as a function of the IrO2 layer thickness and predict the heterostructures to exhibit a thickness-controlled metal-to-insulator transition, crucially related to the connectivity among IrO6 octahedra. The subtle interplay between electron correlation and spin-orbit coupling leads to an almost pure Jeff = 1/2 spin-orbit insulating state at the level of atomically-thin IrO2 monolayer with almost isolated IrO6 octahedra, leading to a predicted emerging state awaiting for experimental confirmation.

cond-mat.str-el↗

arXiv2016

Electronic structure and phase transition in polar ScFeO3 from First Principles Calculations

The properties of newly discovered polar ScFeO3 with magnetic ordering are examined using Ab initio calculations and symmetry mode analysis. The GGA+U calculation confirms the stability of polar R3c Phase in ScFeO3 and the pressure induced phase transition to non-polar Pnma phase. Octahedron tilting and structural properties as a function of applied pressure have been analyzed. The origin of polar phase is associated with instability of non-polar R-3c phase and group theory using the symmetry mode analysis has been applied to understand this instability as well as the spontaneous polarization of polar R3c phase. The magnetic phase transition shows G-type AFM ordering of Fe3+ ion within Goodenough-Kanamori theory and the possibility of magnetic spin structure has been analyzed by using energy analysis including spin canting possibility.

cond-mat.mtrl-sci↗

arXiv2015

Coupling ferroelectricity with spin-valley physics in oxide-based heterostructure

The coupling of spin and valley physics is nowadays regarded as a promising route toward next-generation spintronic and valleytronic devices. In the aim of engineering functional properties for valleytronic applications, we focus on the ferroelectric heterostructure BiAlO3/BiIrO3, where the complex interplay among trigonal crystal field, layer degrees of freedom and spin-orbit coupling mediates a strong spin-valley coupling. Furthermore, we show that ferroelectricity provides a non-volatile handle to manipulate and switch the emerging valley-contrasting spin polarization.

cond-mat.str-el↗

arXiv2015

Impact of Ferroelectric Distortion on Thermopower in BaTiO3

We present a strategy for increasing thermoelectric performance by invoking ferroelectric distortion in a transition-metal oxide. By using an ab initio calculation approach with the Boltzmann transport equation, a large Seebeck coefficient is calculated in n-type BaTiO3. The polar structural distortion causes the peculiar dispersion of the lowest conduction band, which enhances the Seebeck coefficient along the polar direction. A microscopic mechanism of carrier concentration dependence of the Seebeck coefficient is discussed in terms of the band velocity distribution in the Brillouin zone.

cond-mat.mtrl-sci↗

arXiv2014

Signature of high Tc around 25K in higher quality heavily boron-doped diamond

Diamond has outstanding physical properties: the hardest known material, a wide band gap, the highest thermal conductivity, and a very high Debye temperature. In 2004, Ekimov et al. discovered that heavily boron-doped (B-doped) diamond becomes a superconductor around 4 K. Our group successfully controlled the boron concentration and synthesized homoepitaxially grown superconducting diamond films by a CVD method. By CVD method, we found that superconductivity appears when the boron concentration (nB) exceeds a metal-insulator transition concentration of 3.0x10^20 cm^-3 and its Tczero increases up to 7.4 K with increasing nB. We additionally elucidated that the holes formed at the valence band are responsible for the metallic states leading to superconductivity. The calculations predicted that the hole doping into the valence band induces strong attractive interaction and a rapid increase in Tc with increasing boron concentration. According to the calculations, if substitutional doped boron could be arranged periodically or the degree of disorder is reduced, a Tc of approximately 100 K could be achieved via minimal percent doping. In this work, we have successfully observed zero resistivity above 10 K and an onset of resistivity reduction at 25.2 K in heavily B-doped diamond film. However, the effective carrier concentration is similar to that of superconducting diamond with a lower Tc. We found that the carrier has a longer mean free path and lifetime than previously reported, indicating that this highest Tc diamond has better crystallinity compared to that of other superconducting diamond films. In addition, the susceptibility shows a small transition above 20 K in the high quality diamond, suggesting a signature of superconductivity above 20 K. These results strongly suggest that heavier carrier doped defect-free crystalline diamond could give rise to high Tc diamond.

cond-mat.mtrl-sci↗

arXiv2013

Electronic Structure of the Metallic Antiferromagnet PdCrO$_2$ Measured by Angle-Resolved Photoemission Spectroscopy

PdCrO$_2$ is material which has attracted interest due to the coexistence of metallic conductivity associated with itinerant Pd 4d electrons and antiferromagnetic order arising from localized Cr spins. A central issue is determining to what extent the magnetic order couples to the conduction electrons. Here we perform angle-resolved photoemission spectroscopy (ARPES) to experimentally characterize the electronic structure. We find that the Fermi surface has contributions from both bulk and surface states, which can be experimentally distinguished and theoretically verified by slab band structure calculations. The bulk Fermi surface shows no signature of electronic reconstruction in the antiferromagnetic state. This observation suggests that there is negligible interaction between the localized Cr spin structure and the itinerant Pd electrons measured by ARPES.

cond-mat.mtrl-sci↗

arXiv2013

Ab-initio prediction of magnetoelectricity in infinite-layer CaFeO2 and MgFeO2

Density functional based simulations are employed to explore magnetoelectric effects in iron-based oxides, showing a unique layered structure. We theoretically predict CaFeO2 to be a promising magnetoelectric, showing magnetically-controlled large electric polarization, possibly even above room temperature. The cross coupling between magnetic and dipolar degrees of freedom needs, as main ingredients, Fe-site spin-orbit coupling and a spin-dependent O p - Fe d hybridization, along with structural constraints related to the non-centrosymmetric point group and the peculiar geometry characterized by "flattened" FeO4 tetrahedrons. In order to enhance magnetoelectric effects, we performed a materials-design leading to a novel and optimized system, MgFeO2, where the larger O4 tetrahedral distortion leads to a stronger polarization.

cond-mat.str-el↗

arXiv2013

Extremely Large Magnetoresistance in the Nonmagnetic Metal PdCoO2

Extremely large magnetoresistance is realized in the nonmagnetic layered metal PdCoO2. In spite of a highly conducting metallic behavior with a simple quasi-two-dimensional hexagonal Fermi surface, the interlayer resistance reaches up to 35000% for the field along the [1-10] direction. Furthermore, the temperature dependence of the resistance becomes nonmetallic for this field direction, while it remains metallic for fields along the [110] direction. Such severe and anisotropic destruction of the interlayer coherence by a magnetic field on a simple Fermi surface is ascribable to orbital motion of carriers on the Fermi surface driven by the Lorentz force, but seems to have been largely overlooked until now.

cond-mat.str-el↗

arXiv2010

Direct evidence of spin-split one-dimensional metallic surface state on Si(557)-Au

We report unprecedented evidence of a spin split one-dimensional metallic surface state for the system of Si(557)-Au obtained by means of high-resolution spin- and angle-resolved photoelectron spectroscopy combined with first principles calculations. The surface state shows double parabolic energy dispersions along the Au chain structure together with a reversal of the spin polarization with respect to the time-reversal symmetry point as is characteristic from the Rashba effect. Moreover, we have observed a considerably large out-of-plane spin polarization which we attribute to the highly anisotropic wave function of the gold chains.

cond-mat.mes-hall↗

arXiv2005

Electronic band structure and Fermi surface of Ag$_5$Pb$_2$O$_6$

We present electronic band structure of Ag$_5$Pb$_2$O$_6$ with layered hexagonal structure containing one-dimensional chains and two-dimensional Kagomé layers of silver. A half-filled conduction band shows extremely simple, single nearly-free-electron-like Fermi surface. The conduction band is composed of an antibonding state of Pb-$6s$ and O-$2p$ mixing with Ag-4d and $5s$. Mass enhancement in the state density at the Fermi energy is expected to be negligibly small by comparing with the specific-heat data. Calculated Fermi velocity is consistent with small anisotropy observed in transport properties. Doping effects on the electronic structure are also discussed.

cond-mat.str-el↗

arXiv2004

Holes in the valence band of superconducting boron-doped diamond film studied by soft X-ray absorption and emission spectroscopy

Carbon- and boron-2$p$ states of superconducting and non-superconducting boron-doped diamond samples are measured using soft X-ray emission and absorption spectroscopy. For the superconducting sample, a large density of hole states is observed in the valence band in addition to the states in the impurity band. The hole states in the valence band is located at about 1.3 eV below the valence band maximum regardless of the doping level, which cannot be interpreted within a simple rigid band model. Present experimental results, combined with the first principles calculations, suggest that superconductivity is to be attributed to the holes in the valence band.

cond-mat.supr-con↗

arXiv2001

Electronic Structure of B-2p State in AlB2 Single Crystal: Direct Observation ofpσand pπDensity of States

X-ray emission (XES) and absorption (XAS) spectra near the B-$K$ edge were measured on single-crystalline AlB$_2$ compound which is an isostructural diboride of superconducting MgB$_2$. The partial density of states (PDOS) of B-2$pσ$ and $pπ$ orbitals were derived from the polarization dependence of XES and XAS spectra. There are considerable amounts of PDOS near the Fermi energy in AlB$_2$ similarly to that in MgB$_2$, but there are almost no PDOS in $pσ$ orbitals of AlB$_2$ near the Fermi energy, i.e., a pseudo-gap in $pσ$ state and a broad metallic state in $pπ$ state are observed. The present result indirectly supports scenarios that the $pσ$ holes play an important role in the occurrence of superconductivity in MgB$_2$. The overall features of PDOS were found to be in good agreement with the result of band calculation of AlB$_2$, but a small discrepancy in the Fermi energy is observed, which is attributed to the Al vacancy in the compounds, i.e., the estimated concentration is Al$_{0.93}$B$_2$.

cond-mat.supr-con↗

arXiv2000

Band-theoretical prediction of magnetic anisotropy in uranium monochalcogenides

Magnetic anisotropy of uranium monochalcogenides, US, USe and UTe, is studied by means of fully-relativistic spin-polarized band structure calculations within the local spin-density approximation. It is found that the size of the magnetic anisotropy is fairly large (about 10 meV/unit formula), which is comparable with experiment. This strong anisotropy is discussed in view of a pseudo-gap formation, of which crucial ingredients are the exchange splitting of U 5f states and their hybridization with chalcogen p states (f-p hybridization). An anomalous trend in the anisotropy is found in the series (US>>USe<UTe) and interpreted in terms of competition between localization of the U 5f states and the f-p hybridization. It is the spin-orbit interaction on the chalcogen p states that plays an essential role in enlarging the strength of the f-p hybridization in UTe, leading to an anomalous systematic trend in the magnetic anisotropy.

cond-mat.mtrl-sci↗

arXiv1997

First-principles calculation of exchange interaction and exchange force between magnetic Fe films

First-principles calculations have been performed for the exchange interaction and exchange force between two magnetic Fe(100) thin films. It is shown that the magnitude of the calculated exchange force is of the order of $10^{-9} $N at $d/a < 1$ and of the order of $10^{-10} $N at $1 < d/a < 1.5$, where $d$ denotes the distance between surfaces of the two thin films and $a$ the lattice constant of bulk Fe. The obtained forces are sufficiently larger than the sensitivity of the current atomic force microscopy of $10^{-12} - 10^{-13}$N, which suggests the feasibility of the exchange force microscopy.

cond-mat.mes-hall↗