arXiv subjects

Naoto Nagaosa

Publications and source records attributed to Naoto Nagaosa.

At least 199 records · Page 11Linked to original sources

arXiv2014

Spin-Seebeck effect on the surface of topological insulator due to nonequilibrium spin-polarization parallel to the direction of thermally driven electronic transport

We study the transverse spin-Seebeck effect (SSE) on the surface of a three-dimensional topological insulator (TI) thin film, such as Bi$_2$Se$_3$, which is sandwiched between two normal metal leads. The temperature bias $ΔT$ applied between the leads generates surface charge current which becomes spin-polarized due to strong spin-orbit coupling on the TI surface, with polarization vector acquiring a component $P_x \simeq 60%$ {\em parallel to the direction of transport}. When the third nonmagnetic voltage probe is attached to the portion of the TI surface across its width $L_y$, pure spin current will be injected into the probe where the inverse spin Hall effect (ISHE) converts it into a voltage signal \mbox{$|V_\mathrm{ISHE}|^\mathrm{max}/ΔT \simeq 2.5$ $μ$V/K} (assuming the SH angle of Pt voltage probe and $L_y=1$ mm). The existence of predicted nonequilibrium spin-polarization parallel to the direction of electronic transport and the corresponding electron-driven SSE crucially relies on orienting quintuple layers (QLs) of Bi$_2$Se$_3$ {\em orthogonal} to the TI surface and {\em tilted} by $45^\circ$ with respect to the direction of transport. Our analysis is based on the Landauer-Büttiker-type formula for spin currents in the leads of a multi-terminal quantum-coherent junction, which is constructed using nonequilibrium Green function formalism within which we show how to take into account arbitrary orientation of QLs via the self-energy describing coupling between semi-infinite normal metal leads and TI.

cond-mat.mes-hall↗

arXiv2014

Quantum Criticality of Topological Phase Transitions in 3D Interacting Electronic Systems

Topological phase transitions in condensed matters accompany emerging singularities of the electronic wave function, often manifested by gap-closing points in the momentum space. In conventional topological insulators in three dimensions (3D), the low energy theory near the gap-closing point can be described by relativistic Dirac fermions coupled to the long range Coulomb interaction, hence the quantum critical point of topological phase transitions provides a promising platform to test the novel predictions of quantum electrodynamics. Here we show that a new class of quantum critical phenomena emanates in topological materials breaking either the inversion symmetry or the time-reversal symmetry. At the quantum critical point, the theory is described by the emerging low energy fermions, dubbed the anisotropic Weyl fermions, which show both the relativistic and Newtonian dynamics simultaneously. The interplay between the anisotropic dispersion and the Coulomb interaction brings about a new screening phenomena distinct from the conventional Thomas-Fermi screening in metals and logarithmic screening in Dirac fermions.

cond-mat.str-el↗

arXiv2014

Majorana Fermions and Multiple Topological Phase Transition in Kitaev Ladder Topological Superconductors

Motivated by the InSb nanowire superconductor system, we investigate a system where one-dimensional topological superconductors are placed in parallel. It would be simulated well by a ladder of the Kitaev chains. The system undergoes multiple topological phase transitions, where the number of Majorana fermions changes as a function of the interchain superconducting pairings. We analytically determine the topological phase diagram by explicitly calculating the topological number and the band structure. They show even-odd effects with respect to the number of legs of the ladder. When the relative phase between the inter- and intrachain superconducting pairings is 0 or $π$, the system belongs to the class BDI characterized by the $\mathbb{Z}$ index, and otherwise it belongs to the class D characterized by the $\mathbb{Z}_2$ index. This topological class change would be caused by applying the Josephson current or an external magnetic field, and could be observed by measuring the zero-bias differential conductance.

cond-mat.supr-con↗

arXiv2014

Correlation effects in (111) bilayers of perovskite transition-metal oxides

We investigate the correlation-induced Mott, magnetic, and topological phase transitions in artificial (111) bilayers of perovskite transition-metal oxides LaAuO$_3$ and SrIrO$_3$ for which the previous density-functional theory calculations predicted topological insulating states. Using the dynamical-mean-field theory with realistic band structures and Coulomb interactions, LaAuO$_3$ bilayer is shown to be far away from a Mott insulating regime, and a topological-insulating state is robust. On the other hand, SrIrO$_3$ bilayer is on the verge of an orbital-selective topological Mott transition and turns to a trivial insulator by an antiferromagnetic ordering. Oxide bilayers thus provide a novel class of topological materials for which the interplay between the spin-orbit coupling and electron-electron interactions is a fundamental ingredient.

cond-mat.str-el↗

arXiv2014

Classification of stable three-dimensional Dirac semimetals with nontrivial topology

A three-dimensional (3D) Dirac semimetal is the 3D analog of graphene whose bulk band shows a linear dispersion relation in the 3D momentum space. Since each Dirac point with four-fold degeneracy carries a zero Chern number, a Dirac semimetal can be stable only in the presence of certain crystalline symmetries. In this work, we propose a general framework to classify stable 3D Dirac semimetals. Based on symmetry analysis, we show that various types of stable 3D Dirac semimetals exist in systems having the time-reversal, inversion, and uniaxial rotational symmetries. There are two distinct classes of stable 3D Dirac semimetals. In the first class, a pair of 3D Dirac points locate on the rotation axis, away from its center. Moreover, the 3D Dirac semimetals in this class have nontrivial topological properties characterized by 2D topological invariants, such as the $Z_{2}$ invariant or the mirror Chern number. These 2D topological indices give rise to stable 2D surface Dirac cones, which can be deformed to Fermi arcs when the surface states couple to the bulk states on the Fermi level. On the other hand, the second class of Dirac SM phases possess a 3D Dirac point at a time-reversal invariant momentum (TRIM) on the rotation axis and do not have surface states in general.

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arXiv2014

Emergent topological phenomena in thin films of pyrochlore iridates

Two quintessential ingredients governing the topological invariant of a system are the dimensionality and the symmetry of the system. Due to the recent development of thin film and artificial superstructure growth technique, it is possible to control the dimensionality of the system, smoothly between the two-dimensions (2D) and three-dimensions (3D). In this work we unveil the dimensional crossover of emergent topological phenomena in correlated topological materials. In particular, by focusing on the thin film of pyrochlore iridate antiferromagnets grown along the [111] direction, we demonstrate that it can show giant anomalous Hall conductance, which is proportional to the thickness of the film, even though there is no Hall effect in 3D bulk material. In addition, we uncover the emergence of a new topological phase, whose nontrivial topological properties are hidden in the bulk insulator but manifest only in thin films. This shows that the thin film of topological materials is a new platform to search for unexplored novel topological phenomena.

cond-mat.mes-hall↗

arXiv2014

Witnessing the formation and relaxation of massive quasi-particles in a strongly correlated electron system

The non-equilibrium semiconductors physics is based on the paradigm that different degrees of freedom interact on different timescales. In this context the photo-excitation is often treated as an impulsive injection of electronic energy that is transferred to other degrees of freedom only at later times. Here, by studying the ultrafast particles dynamics in a archetypal strongly correlated charge-transfer insulator (La2CuO4), we show that the interaction between electrons and bosons manifest itself directly in the photo-excitation processes of a correlated material. With the aid of a general theoretical framework (Hubbard Holstein Hamiltonian), we reveal that sub-gap excitation pilots the formation of itinerant quasi-particles which are suddently dressed (<100 fs) by an ultrafast reaction of the bosonic field.

cond-mat.str-el↗

arXiv2014

Giant thermoelectric effect in graphene-based topological insulators with nanopores

Designing thermoelectric materials with high figure of merit $ZT=S^2 G T/κ$ requires fulfilling three often irreconcilable conditions, i.e., the high electrical conductance $G$, small thermal conductance $κ$ and high Seebeck coefficient $S$. Nanostructuring is one of the promising ways to achieve this goal as it can substantially suppress lattice contribution to $κ$. However, it may also unfavorably influence the electronic transport in an uncontrollable way. Here we theoretically demonstrate that this issue can be ideally solved by fabricating graphene nanoribbons with heavy adatoms and nanopores. These systems, acting as a two-dimensional topological insulator with robust helical edge states carrying electrical current, yield a highly optimized power factor $S^2G$ per helical conducting channel. Concurrently, their array of nanopores impedes the lattice thermal conduction through the bulk. Using quantum transport simulations coupled with first-principles electronic and phononic band structure calculations, the thermoelectric figure of merit is found to reach its maximum $ZT \simeq 3$ at $T \simeq 40$ K. This paves a way to design high-$ZT$ materials by exploiting the nontrivial topology of electronic states through nanostructuring.

cond-mat.mes-hall↗

arXiv2014

Topological Phase Transition without Gap Closing

Topological phase transition is accompanied with a change of topological numbers. It has been believed that the gap closing and the breakdown of the adiabaticity at the transition point is necessary in general. However, the gap closing is not always needed to make the topological index ill-defined. In this paper, we show that the states with different topological numbers can be continuously connected \textit{without} gap closing in some cases, where the symmetry of the system changes during the process. Here we propose the generic principles how this is possible (impossible) by demonstrating various examples such as 1D polyacetylene with the charge-density-wave order, 2D silicene with the antiferromagnetic order, 2D silicene or quantum well made of HgTe with superconducting proximity effects and 3D superconductor Cu doped Bi$_{2}$Se$_{3}$. It is argued that such an unusual phenomenon can occur when we detour around the gap closing point provided the connection of the topological indices is lost along the detour path.

cond-mat.supr-con↗

arXiv2013

Two-dimensional p-wave superconducting states with magnetic moments on a conventional s-wave superconductor

Unconventional superconductivity induced by the magnetic moments in a conventional spin-singlet s-wave superconductor is theoretically studied. By choosing the spin directions of these moments, one can design spinless pairing states appearing within the s-wave superconducting energy gap. It is found that the helix spins produce px + py-wave state while the skyrmion crystal configuration px + ipy-wave like state. Nodes in the energy gap and the zero energy flat band of Majorana edge states exist in the former one, while the chiral Majorana channels along edges of the sample and the zero energy Majorana bound state at the core of the vortex appear in the latter case.

cond-mat.supr-con↗

arXiv2013

Current-induced skyrmion dynamics in constricted geometries

Magnetic skyrmions, vortex-like swirling spin structures with quantized topological number, observed in chiral magnets, are appealing for potential applications in spintronics because it is possible to control their motion with ultralow current density. In order to realise skyrmion-based spintronic devices, it is essential to understand the skyrmion motions in confined geometries. Here we show by micromagnetic simulations that the current-induced motion of skyrmions in the presence of geometrical boundaries is very different from that in an infinite plane. In a channel with finite width, the transverse confinement results in steady state characteristics of the skyrmion velocity as a function of current similar to those of domain walls in ferromagnets, whereas the transient behaviour depends on the initial distance of the skyrmion from the boundary. In addition, we show that single skyrmions can be created by electric current in a simple constricted geometry of a plate-shaped specimen of suitable size and geometry. These findings could guide the design of skyrmion-based devices, in which skyrmions are used as information carriers.

cond-mat.mes-hall↗

arXiv2013

Theory of Electrical Spin Manipulation in Spin-Orbit Coupling Systems

By associating a spin-orbit interaction with a non-Abelian gauge potential, we theoretically present a spin polarization in a quite general form using an effective Yang-Mills field and a usual electromagnetic field. In this gauge invariant result, we focus on a purely electrically-induced spin contribution. We find that both the inverse spin galvanic effect and the spin Hall effect arise from the same origin, i.e., the SU(2)$\times$U(1) Hall effect. We also discover that a large effective magnetic field of the order of 1T is induced in the Rashba system.

cond-mat.mes-hall↗

arXiv2013

Charge transport in pn and npn junctions of silicene

We investigate charge transport of pn and npn junctions made from silicene, Si analogue of graphene. The conductance shows the distinct gate-voltage dependences peculiar to the topological and non-topological phases, where the topological phase transition is caused by external electric field. Namely, the conductance is suppressed in the np regime when the both sides are topological, while in the nn regime when one side is topological and the other side is non-topological. Furthermore, we find that the conductance is almost quantized to be 0, 1 and 2. Our findings will open a new way to nanoelectronics based on silicene.

cond-mat.mes-hall↗

arXiv2013

Theory of magnon-skyrmion scattering in chiral magnets

We study theoretically the dynamics of magnons in the presence of a single skyrmion in chiral magnets featuring Dzyaloshinskii-Moriya interaction. We show by micromagnetic simulations that the scattering process of magnons by a skyrmion can be clearly defined although both originate in the common spins. We find that (i) the magnons are deflected by a skyrmion, with the angle strongly dependent on the magnon wavenumber due to the effective magnetic field of the topological texture, and (ii) the skyrmion motion is driven by magnon scattering through exchange of the momenta between the magnons and a skyrmion: the total momentum is conserved. This demonstrates that the skyrmion has a well-defined, though highly non-Newtonian, momentum.

cond-mat.str-el↗

arXiv2013

Edge States in Silicene Nanodisks

Silicene is a honeycomb-structure silicon atoms, which shares many intriguing properties with graphene. Silicene is expected to be a quantum spin-Hall insulator due to its spin-orbit interactions. We investigate the electronic properties of silicene nanodisks, which are silicene derivatives with closed edge. In case of the simplest model of graphene nanodisks, the number of the zero-energy modes is given by the lower bound of the Lieb theorem. They are standing wave states.When the spin-orbit interaction is introduced and the system becomes a topological insulator, they begin to propagate around a nanodisk. Helical edge currents flow around the edge of a nanodisk though the crystal momentum is ill-defined. We have newly found the counting rule of the zero-energy states, and constructed the low-energy theory of zigzag triangular silicene. We also show the validity of the bulk-edge correspondence in nanodisks with rough edge by calculating the local probability amplitude and current. Finally, the signatures of the topological phase transition in silicene nanodisks under an external electric field are revealed.Our results will be observable by means of STM experiments.

cond-mat.mes-hall↗

arXiv2013

Theory of topological quantum phase transitions in 3D noncentrosymmetric systems

We have constructed a general theory describing the topological quantum phase transitions in 3D systems with broken inversion symmetry. While the consideration of the system's codimension generally predicts the appearance of a stable metallic phase between the normal and topological insulators, it is shown that a direct topological phase transition between two insulators is also possible when an accidental band crossing (ABC) occurs along directions with high crystalline symmetry. At the quantum critical point (QCP), the energy dispersion becomes quadratic along one direction while the dispersions along the other two orthogonal directions are linear, which manifests the zero chirality of the band touching point (BTP). Due to the anisotropic dispersion at QCP, various thermodynamic and transport properties show unusual temperature dependence and anisotropic behaviors.

cond-mat.str-el↗

arXiv2013

Theory of giant magnetoresistance at misfit interfaces

We study theoretically the resistance at the interface between the two planar systems with different lattice constants a and b. The resistance and the effect of the magnetic field depends sensitively on the ratio a/b. The size of the enlarged unit cell $λ= n_Aa = n_Bb$ ($n_A$, $n_B$: integers) is the crucial quantity, and the magnetic flux penetrating this enlarged unit cell determines the oscillation of the resistance. Therefore, the magnetoresistance is very much enhanced at (nearly) incommensurate relation between a and b.

cond-mat.mes-hall↗

arXiv2013

Interference of Topologically Protected Edge States in Silicene Nanoribbons

Silicene is a two-dimensional quantum spin-Hall insulator. We study the edge channels of silicene nanoribbons from the viewpoint of the topological protection and the interference between the two edges. It is found that the behaviors of the helical edge channels (HECs) are completely different between the armchair and zigzag edges. The penetration depths $ξ$ of the HEC is antiproportional to the spin-orbit gap for the armchair edge $ξ_{\rm arm} \sim \hbar v_\text{F}/ Δ$ ($v_\text{F}$: the Fermi velocity, $Δ$: the gap due to the spin-orbit interaction), while it remains as short as the lattice constant for the zigzag edge. Zero-energy states disappear in armchair nanoribbons due to an interference of two edge states, while they remains in zigzag nanoribbons even if the width $W$ is quite narrow. The gap $δ$ of HECs behaves as $δ\sim \hbar v_\text{F}/W$ for $Δ=0$, and as $δ\sim Δ\exp[- W/ξ_{\rm arm}]$ for $Δ\not=0$ for armchair nanoribbons, while it remains essentially zero irrespectively of $Δ$ for zigzag nanoribbons.

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