arXiv subjects

Fumiyuki Ishii

Publications and source records attributed to Fumiyuki Ishii.

At least 37 records · Page 2Linked to original sources

arXiv2018

Doping-induced persistent spin helix with large spin splitting in monolayer SnSe

Finding a new class of materials supporting a long spin lifetime is essential in development of energy-saving spintronics, which is achievable by using a persistent spin helix (PSH) materials. However, for spintronic devices, the PSH states with large spin splitting is required for operation at room temperature. By employing first-principles calculations, we show that the PSH states with large spin splitting are achieved in the SnSe monolayer (ML) functionalized by a substitutional halogen impurity. We find the PSH states in the Fermi level where k-space Fermi surface is characterized by the shifted two loops, dominated by out-of-plane spin orientations. We clarify the PSH states in term of an effective k.p Hamiltonian obtained from symmetry consideration. Finally, large spin-orbit strength in the PSH states with a substantially small wavelength are found, rendering that this system is promising for the development of an efficient and high-density scalable spintronic devices operating at room temperatures.

cond-mat.str-el↗

arXiv2018

Implementation of Generalized Bloch Theorem Using Linear Combination of Pseudo-Atomic Orbitals

We have implemented the generalized Bloch theorem based on first-principles calculations using a linear combination of pseudo-atomic orbitals (LCPAO) as the basis sets. In order to test our implementation in a code, we examined three systems that have been reported in experiments or other calculations, namely, the carrier-induced spin-spiral ground state in the one-dimensional model system, the spin stiffness of bcc-Fe, and the spin stiffness in a zigzag graphene nanoribbon. We confirmed that our implementation gives good agreement with experiments. Based on these results, we believe that our implementation of the generalized Bloch theorem using an LCPAO is useful for predicting the properties of complex magnetic materials. We also predicted a large reduction (enhancement) of spin stiffness for the electron (hole) doping of zigzag-edge graphene nanoribbon ferromagnetic states.

cond-mat.mtrl-sci↗

arXiv2018

First-principles Study of Rashba Spin Splitting at Strained SrTiO3(001) Surfaces

We investigated the Rashba spin splittings in a compressive-strained SrTiO$_3$(001) ultra thin-film using first-principles calculations. The effect of the polarization due to the compressive strain leads to the 2DEG with large Rashba spin splittings, where the sheet carrier density is of the same order of magnitude as that of the heterostructure LaAlO$_3$/SrTiO$_3$ ($\sim10^{14}$ cm$^{-2}$). Some localized surface states (SSs) show the giant Rashba coefficient $α_R$ larger than 100 meV$\cdot$Å.

cond-mat.mtrl-sci↗

arXiv2018

Strong Rashba effect in the localized impurity states of halogen-doped monolayer PtSe2

The recent epitaxial growth of 1T-phase of PtSe2 monolayer (ML) has opened a possibility for its novel applications, in particular for spintronics device. However, in contrast to 2H-phase of transition-metal dichalcogenides (TMDs), the absence of spin splitting in the PtSe2 ML may limit the functionality for spintronics application. Through fully-relativistic density-functional theory calculations, we show that large spin splitting can be induced in the PtSe2 ML by introducing a substitutional halogen impurity. Depending on the atomic number (Z) of the halogen dopants, we observe an enhancement of the spin splitting in the localized impurity states (LIS), which is due to the increased contribution of the p-d orbitals coupling. More importantly, we identify very large Rashba splitting in the LIS near Fermi level around the Gamma point characterized by hexagonal warping of the Fermi surface. We show that the Rashba splitting can be controlled by adjusting the doping concentration. Therefore, this work paves a possible way to induce the significant Rashba splitting in the two-dimensional TMDs, which is useful for spintronic devices operating at room temperature.

cond-mat.str-el↗

arXiv2018

Large anomalous Nernst coefficient in an oxide Skyrmion crystal Chern insulator

A sizable transverse thermoelectric coefficient N , large to the extent that it potentially serves applications, is predicted to arise, by means of first-principles calculations, in a Skyrmion crystal assumed on EuO monolayer where carrier electrons are introduced upon a quantum anomalous Hall insulating phase of Chern number C = 2. This encourages future experiments to pursue such an effect.

cond-mat.str-el↗

arXiv2018

First-principles study on thermoelectric properties of half-Heusler compounds CoMSb(M=Sc, Ti, V, Cr, and Mn)

We have performed systematic density functional calculations and evaluated thermoelectric properties, See- beck coefficient and anomalous Nernst coefficient of half-Heusler comounds CoMSb(M=Sc, Ti, V, Cr, and Mn). The carrier concentration dependence of Seebeck coefficients in nonmagnetic compounds are in good agreement with experimental values. We found that the half-metallic ferromagnetic CoMnSb show large anomalous Nernst effect originating from Berry curvature at the Brillouin zone boundary. These results help to understanding for the mechanism of large anomalous Nernst coefficient and give us a clue to design high performance magnetic thermoelectric materials.

cond-mat.mtrl-sci↗

arXiv2018

Clear variation of spin splitting by changing electron distribution at non-magnetic metal/Bi2O3 interfaces

Large spin splitting at Rashba interface, giving rise to strong spin-momentum locking, is essential for efficient spin-to-charge conversion. Recently, a Cu/Bismuth oxide (Bi2O3) interface has been found to exhibit an efficient spin-to-charge conversion similar to a Ag/Bi interface with large Rashba spin splitting. However, the guiding principle of designing the metal/oxide interface for the efficient conversion has not been clarified yet. Here we report strong non-magnetic (NM) material dependence of spin splitting at NM/Bi2O3 interfaces. We employed spin pumping technique to inject spin current into the interface and evaluated the magnitude of interfacial spin-to-charge conversion. We observed large modulation and sign change in conversion coefficient which corresponds to the variation of spin splitting. Our experimental results together with first-principles calculations indicate that such large variation is caused by material dependent electron distribution near the interface. The results suggest that control of interfacial electron distribution by tuning the difference in work function across the interface may be an effective way to tune the magnitude and sign of spin-to-charge conversion and Rashba parameter at interface.

cond-mat.mtrl-sci↗

arXiv2017

First-principles study of electric-field-induced topological phase transition in one-bilayer Bi(111)

Using first-principles calculations, we found the topological phase transition induced by electric fields in one-bilayer Bi(111). The bandgap decreased with increasing electric field strength, and it is closed at 2.1 V/Å. For fields exceeding 2.1 V/Å, the bandgap increased with increasing electric field strength, reaching 0.34 eV at 4.0 V/Å. We computed the $Z_{2}$ invariant that characterizes topological insulator phases. As results, one-bilayer Bi(111) showed a topological phase transition induced by the electric field, from the topological insulator phase to the trivial insulator phase through a Dirac semimetal. This topological phase transition could be applied to novel devices.

cond-mat.mtrl-sci↗

arXiv2017

Strain-induced large spin splitting and persistent spin helix at LaAlO$_3$/SrTiO$_3$ interface

We investigated the effect of the tensile strain on the spin splitting at the n-type interface in LaAlO$_3$/SrTiO$_3$ in terms of the spin-orbit coupling coefficient $α$ and spin texture in the momentum space using first-principles calculations. We found that the $α$ could be controlled by the tensile strain and be enhanced up to 5 times for the tensile strain of 7%, and the effect of the tensile strain leads to a persistent spin helix, which has a long spin lifetime. These results support that the strain effect on LaAlO$_3$/SrTiO$_3$ is important for various applications such as spinFET and spin-to-charge conversion.

cond-mat.mtrl-sci↗

arXiv2017

Defect-induced large spin-orbit splitting in the monolayer of PtSe$_2$

The effect of spin-orbit coupling (SOC) on the electronic properties of monolayer (ML) PtSe$_2$ is dictated by the presence of the crystal inversion symmetry to exhibit spin polarized band without characteristic of spin splitting. Through fully-relativistic density-functional theory calculations, we show that large spin-orbit splitting can be induced by introducing point defects. We calculate stability of native point defects such as a Se vacancy (V$_{\texttt{Se}}$), a Se interstitial (Se$_{i}$), a Pt vacancy (V$_{\texttt{Pt}}$), and a Pt interstitial (Pt$_{i}$), and find that both the V$_{\texttt{Se}}$ and Se$_{i}$ have the lowest formation energy. We also find that in contrast to the Se$_{i}$ case exhibiting spin degeneracy in the defect states, the large spin-orbit splitting up to 152 meV is observed in the defect states of the V$_{\texttt{Se}}$. Our analyses of orbital contributions to the defect states show that the large spin splitting is originated from the strong hybridization between Pt-$d_{x{^2}+y{^2}}+d_{xy}$ and Se-$p_{x}+p_{y}$ orbitals. Our study clarifies that the defects play an important role in the spin splitting properties of the PtSe$_2$ ML, which is important for designing future spintronic devices.

cond-mat.str-el↗

arXiv2017

First-principles study of spin texture and Fermi lines in Bi(111) multi-layer nanofilm

We have performed a fully relativistic first-principles density functional calculation examining the surface state of bismuth (Bi) (111) multi-layer nanofilm, with up to 20 Bi bilayers, and investigated the Rashba effect and spin texture on the Bi surfaces. We have revealed a giant out-of-plane spin states on the Fermi lines, and the maximum value of the out-of-plane spin component being approximately 40% of the magnitude of the total spin. We have also evaluated the Rashba parameter $α_R \simeq 1.9 {\rm eV}\cdot$Åusing the surface state bands which is buried in the bulk state, at -0.32 eV below the Fermi energy.

cond-mat.mtrl-sci↗

arXiv2017

Polarity tuning of spin-orbit-induced spin splitting in two-dimensional transition metal dichalcogenides semiconductors

The established spin splitting in monolayer (ML) of transition metal dichalcogenides (TMDs) that is caused by inversion symmetry breaking is dictated by mirror symmetry operations to exhibit fully out-of-plane direction of spin polarization. Through first-principles density functional theory calculations, we show that polarity-induced mirror symmetry breaking leads to new sizable spin splitting having in-plane spin polarization. These splittings are effectively controlled by tuning the polarity using biaxial strain. Furthermore, the admixtures of the out-of-plane and in-plane spin-polarized states in the strained polar systems are identified, which is expected to influence the spin relaxation through the Dyakonov-Perel mechanism. Our study clarified that the polarity-induced mirror symmetry breaking plays an important role in controlling the spin splitting and spin relaxation in the TMDs ML, which is useful for designing future spintronic devices.

cond-mat.str-el↗

arXiv2016

First-principles design of the spinel iridate Ir2O4 for high-temperature quantum spin ice

Insulating magnetic rare-earth pyrochlores related to spin ice host emergent bosonic monopolar spinons, which obey a magnetic analogue of quantum electrodynamics and may open a route to a magnetic analogue of electronics. However, the energy scales of the interactions among rare-earth moments are so low as 1 K that the possible quantum coherence can only be achieved at a sub-Kelvin. Here, we desgin high-temperature quantum spin ice materials from first principles. It is shown that the A-site deintercalated spinel iridate Ir2O4, which has been experimentally grown as epitaxial thin films, is a promising candidate for quantum spin ice with a spin-ice-rule interaction of a few tens of meV. Controlling electronic structures of Ir2O4 through substrates, it is possible to tune magnetic interactions so that a magnetic Coulomb liquid persists at high temperatures.

cond-mat.mtrl-sci↗

arXiv2016

First-principles Study of Rashba Effect in Ultra-thin Bismuth Surface Alloys

We performed density functional calculations for ultra-thin bismuth surface alloys: surface alloys of bismuth and face-centered cubic metals Bi/$M$(111)-$(\sqrt{3}\times\sqrt{3})R30°$ ($M$=Cu, Ag, Au, Ni, Co, and Fe). Our calculated Rashba parameters for the Bi/Ag are consistent with the previous experimental and theoretical results. We predicted a trend in the Rashba coefficients $α_R$ of bands around the Fermi energy for noble metals as follows: Bi/Ag > Bi/Cu > Bi/Au. As for the transition metals, there is a trend in $α_R$: Bi/Ni > Bi/Co > Bi/Fe. Our finding may lead to design efficient spin-charge conversion materials.

cond-mat.mtrl-sci↗

arXiv2016

Strain-controlled spin splitting in the conduction band of monolayer WS2

Spin splitting bands that arises in conduction band minimum (CBM) of WS2 monolayer (ML) play an important role in the new spin-orbit phenomena such as spin-valley coupled electronics. However, application of strain strongly modifies electronic properties of the WS2 ML, which is expected to significantly affect to the properties of the spin splitting bands. Here, by using fully-relativistic first-principles calculations based on density-functional theory, we show that a substantial spin spliting bands observed in the CBM is effectively controlled and tuned by applying the biaxial strain. We also find that these spin splitting bands induce spin textures exhibiting fully out-of-plane spin polarization in the opposite direction between the K and Q points and their time reversals in the first Brillouin zone. Our study clarify that the strain plays an significant role in the spin-orbit coupling of the WS2 ML, which has very important implications in designing future spintronics devices.

cond-mat.str-el↗

arXiv2016

Large Anomalous Nernst Effect in a Skyrmion Crystal

Thermoelectric properties of a model Skyrmion crystal were theoretically investigated, and it was found that its large anomalous Hall conductivity, corresponding to large Chern numbers induced by its peculiar spin structure leads to a large transverse thermoelectric voltage through the anomalous Nernst effect. This implies the possibility of finding good thermoelectric materials among Skyrmion systems, and thus motivates our quests for them by means of the first-principles calculations as were employed here.

cond-mat.mtrl-sci↗

arXiv2015

Persistent spin helix on a wurtzite ZnO(10-10) surface: First-principles density-functional study

The persistent spin helix (PSH) that has been widely and exclusively studied in zinc-blende structures is revealed for the first time on the surface of a wurtzite structure. Through first-principles calculations of the ZnO(10-10) surface, a quasi-one dimensional orientation of the spin textures is identified. Further, the wavelength of this particular PSH is smaller than that observed with various zinc-blende quantum well structures, thus indicating that wurtzite-structured surfaces are suitable for spintronics applications.

cond-mat.mes-hall↗

arXiv2015

First-principles study on cubic pyrochlore iridates Y2Ir2O7 and Pr2Ir2O7

Fully relativistic first-principles electronic structure calculations based on a noncollinear local spin density approximation (LSDA) are performed for pyrochlore iridates Y$_2$Ir$_2$O$_7$ and Pr$_2$Ir$_2$O$_7$. The all-in, all-out antiferromagnetic (AF) order is stablized by the on-site Coulomb repulsion $U>U_c$ in the LSDA+$U$ scheme, with $U_c\sim1.1$~eV and 1.3~eV for Y$_2$Ir$_2$O$_7$ and Pr$_2$Ir$_2$O$_7$, respectively. AF semimetals with and without Weyl points and then a topologically trivial AF insulator successively appear with further increasing $U$. For $U=1.3$~eV, Y$_2$Ir$_2$O$_7$ is a topologically trivial narrow-gap AF insulator having an ordered local magnetic moment $\sim0.5μ_B$/Ir, while Pr$_2$Ir$_2$O$_7$ is barely a paramagnetic semimetal with electron and hole concentrations of $0.016$/Ir, in overall agreements with experiments. With decreasing oxygen position parameter $x$ describing the trigonal compression of IrO$_6$ octahedra, Pr$_2$Ir$_2$O$_7$ is driven through a non-Fermi-liquid semimetal having only an isolated Fermi point of $Γ_8^+$, showing a quadratic band touching, to a $Z_2$ topological insulator.

cond-mat.str-el↗