arXiv subjects

Naoto Nagaosa

Publications and source records attributed to Naoto Nagaosa.

At least 181 records · Page 10Linked to original sources

arXiv2015

Purely electrical detection of a skyrmion in constricted geometry

How to detect the skyrmion position is a crucial problem in future skyrmionics since it corresponds to the reading process of information. We propose a method to detect the skyrmion position purely electrically by measuring the Hall conductance in a constricted geometry. The Hall conductance becomes maximum when a skyrmion is at the lead position. It is possible to detect the skyrmion position even at room temperature. We find an optimized width of the sample determined by the skyrmion radius. We also investigate the effects of elastic and inelastic scatterings, and finite temperature. We find that the local density of states become minimum at the skyrmion position. Our results will be a basis of future skyrmion electronics.

cond-mat.mes-hall↗

arXiv2015

Spontaneous modulation of superconducting phase in Kitaev ladder

We study theoretically the two Kitaev chains put in parallel, i.e., Kitaev ladder, coupled by Josephson junction. The $π$-junction between the Majorana bound states at the ends of the chains competes with the usual Josephson coupling along the chain, and this frustration leads to the modulation of the phase difference of the superconducting order parameter between the two chains. We show that this modulation gives the double degeneracy of the ground states, which can be manipulated by external electric and magnetic fields.

cond-mat.supr-con↗

arXiv2015

Domain wall of a ferromagnet on a three-dimensional topological insulator

Topological insulators (TIs) show rich phenomena and functions which can never be realized in ordinary insulators. Most of them come from the peculiar surface or edge states. Especially, the quantized anomalous Hall effect (QAHE) without an external magnetic field is realized in the two-dimensional ferromagnet on a three-dimensional TI which supports the dissipationless edge current. Here we demonstrate theoretically that the domain wall of this ferromagnet, which carries edge current, is charged and can be controlled by the external electric field. The chirality and relative stability of the Neel wall and Bloch wall depend on the position of the Fermi energy as well as the form of the coupling between the magnetic moments and orbital of the host TI. These findings will pave a path to utilize the magnets on TI for the spintronics applications.

cond-mat.mes-hall↗

arXiv2015

Skyrmionic magnetization configurations at chiral magnet/ferromagnet heterostructures

We consider magnetization configurations at chiral magnet (CM)/ferromagnet (FM) heterostructures. In the CM, magnetic skyrmions and spin helices emerge due to the Dzyaloshinskii-Moriya interaction, which then penetrate into the adjacent FM. However, because the non-uniform magnetization structures are energetically unfavorable in the FM, the penetrated magnetization structures are deformed, resulting in exotic three-dimensional configurations, such as skyrmion cones, sideways skyrmions, and twisted helices and skyrmions. We discuss the stability of possible magnetization configurations at the CM/FM and CM/FM/CM hybrid structures within the framework of the variational method, and find that various magnetization configurations appear in the ground state, some of which cause nontrivial emergent magnetic field.

cond-mat.mtrl-sci↗

arXiv2015

Emergent non-Fermi liquid at the quantum critical point of a topological phase transition in two dimensions

We study the effects of Coulomb interaction between 2D Weyl fermions with anisotropic dispersion which displays relativistic dynamics along one direction and Newtonian dynamics along the other. Such a dispersion can be realized in phosphorene under electric field or strain, in TiO$_2$/VO$_2$ superlattices, and, more generally, at the quantum critical point between a nodal semimetal and an insulator in systems with a chiral symmetry. Using the one-loop renormalization group approach in combination with the large-$N$ expansion, we find that the system displays interaction-driven non-Fermi liquid behavior in a wide range of intermediate frequencies and marginal Fermi liquid behavior at the smallest frequencies. In the non-Fermi liquid regime, the quasiparticle residue $Z$ at energy $E$ scales as $Z \propto E^a$ with $a >0$, and the parameters of the fermionic dispersion acquire anomalous dimensions. In the marginal Fermi-liquid regime, $Z \propto (|\log E|)^{-b}$ with universal $b = 3/2$.

cond-mat.str-el↗

arXiv2015

Topological magneto-electric effects in thin films of topological insulators

We propose that the topological magneto-electric (ME) effect, a hallmark of topological insulators (TIs), can be realized in thin films of TIs in the $ν=0$ quantum Hall state under magnetic field or by doping two magnetic ions with opposite signs of exchange coupling. These setups have the advantage compared to previously proposed setups that a uniform configuration of magnetic field or magnetization is sufficient for the realization of the topological ME effect. To verify our proposal, we numerically calculate ME response of TI thin films in the cylinder geometry and that of effective 2D models of surface Dirac fermions. The ME response is shown to converge to the quantized value corresponding to the axion angle $θ=\pm π$ in the limit of the large top and bottom surface area of TI films, where non-topological contributions from the bulk and the side surface are negligible.

cond-mat.mes-hall↗

arXiv2015

Current-Driven Motion of Magnetic Domain Wall with Many Bloch Lines

The current-driven motion of a domain wall (DW) in a ferromagnet with many Bloch lines (BLs) via the spin transfer torque is studied theoretically. It is found that the motion of BLs changes the current-velocity ($j$-$v$) characteristic dramatically. Especially, the critical current density to overcome the pinning force is reduced by the factor of the Gilbert damping coefficient $α$ even compared with that of a skyrmion. This is in sharp contrast to the case of magnetic field driven motion, where the existence of BLs reduces the mobility of the DW.

cond-mat.mes-hall↗

arXiv2015

Quantized Anomalous Hall Effects in Skyrmion Crystal

We theoretically study the quantized anomalous Hall effect (QAHE) in skyrmion crystal (SkX) without external magnetic field. The emergent magnetic field in SkX could be gigantic as much as $\sim4000$T when its lattice constant is $\sim1$nm. The band structure is not flat but has a finite gap in the low electron-density regime. We also study the conditions to realize the QAHE for the skyrmion size, carrier density, disorder strength and temperature. Comparing the SkX and the system under the corresponding uniform magnetic field, the former is more fragile against the temperature compared with the latter since the gap is reduced by a factor of $\sim$ 1/5, while they are almost equally robust against the disorder. Therefore, it is expected that the QAHE of SkX system is realized even with strong disorder at room temperature when the electron density of the order of one per a skyrmion.

cond-mat.mes-hall↗

arXiv2015

Charge and spin transport in edge channels of a $ν=0$ quantum Hall system on the surface of topological insulators

Three-dimensional topological insulators of finite thickness can show the quantum Hall effect (QHE) at the filling factor $ν=0$ under an external magnetic field if there is a finite potential difference between the top and bottom surfaces. We calculate energy spectra of surface Weyl fermions in the $ν=0$ QHE and find that gapped edge states with helical spin structure are formed from Weyl fermions on the side surfaces under certain conditions. These edge channels account for the nonlocal charge transport in the $ν=0$ QHE which is observed in a recent experiment on (Bi$_{1-x}$Sb$_x$)$_2$Te$_3$ films. The edge channels also support spin transport due to the spin-momentum locking. We propose an experimental setup to observe various spintronics functions such as spin transport and spin conversion.

cond-mat.mes-hall↗

arXiv2015

Control of Dzyaloshinskii-Moriya interaction in Mn$_{1-x}$Fe$_x$Ge: a first-principles study

Motivated by the recent experiment on the size and helicity control of skyrmions in Mn$_{1-x}$Fe$_x$Ge [K. Shibata et al., Nature Nanotechnology 8, 732 (2013)], we study how the Dzyaloshinskii-Moriya (DM) interaction changes its size and sign in metallic helimagnets. By means of first-principles calculations, we successfully reproduce the non-trivial sign change of the DM interaction observed in the experiment. While the DM interaction sensitively depends on the carrier density or the detail of the electronic structure such as the size of the exchange splitting, its behavior can be systematically understood in terms of the distribution of anticrossing points in the band structure. By following this guiding principle, we can even induce gigantic anisotropy in the DM interaction by applying a strain to the system. These results pave the new way for skyrmion crystal engineering in metallic helimagnets.

cond-mat.mtrl-sci↗

arXiv2015

Renormalization group analysis of multi-Dirac-node materials

We theoretically study the electromagnetic interaction in Dirac systems with $N$ nodes by using the renormalization group, which is relevant to the quantum critical phenomena of topological phase transition ($N=1$) and Weyl semimetals ($N=4$ or $N=12$). Compared with the previous work for $N=1$ [H. Isobe and N. Nagaosa, Phys. Rev. B 86, 165127 (2012); arXiv:1205.2427], we obtained the analytic solution for the large $N$ limit, which differs qualitatively for the scaling of the speed of light $c$ and that of electron $v$, i.e., $v$ does notchange while $c$ is reduced to $v$. We also found a reasonably accurate approximate analytic solution for generic $N$, which well interpolates between $N=1$ and large $N$ limit, and it concludes that $c^2 v^N$ is almost unrenormalized. The temperature dependence of the physical properties, the dielectric constant, magnetic susceptibility, spectral function, DC conductivity, and mass gap are discussed based on these results.

cond-mat.str-el↗

arXiv2015

Inertia, diffusion and dynamics of a driven skyrmion

Skyrmions recently discovered in chiral magnets are a promising candidate for magnetic storage devices because of their topological stability, small size ($\sim 3-100$nm), and ultra-low threshold current density ($\sim 10^{6}$A/m$^2$) to drive their motion. However, the time-dependent dynamics has hitherto been largely unexplored. Here we show, by combining the numerical solution of the Landau-Lifshitz-Gilbert equation and the analysis of a generalized Thiele's equation, that inertial effects are almost completely absent in skyrmion dynamics driven by a time-dependent current. In contrast, the response to time-dependent magnetic forces and thermal fluctuations depends strongly on frequency and is described by a large effective mass and a (anti-) damping depending on the acceleration of the skyrmion. Thermal diffusion is strongly suppressed by the cyclotron motion and is proportional to the Gilbert damping coefficient $α$. This indicates that the skyrmion position is stable, and its motion responds to the time-dependent current without delay or retardation even if it is fast. These findings demonstrate the advantages of skyrmions as information carriers.

cond-mat.str-el↗

arXiv2014

Diagrammatic Monte Carlo method for many-polaron problems

We introduce the first bold diagrammatic Monte Carlo approach to deal with polaron problems at finite density non-perturbatively, i.e., by including vertex corrections to high orders. Using Holstein model on a square lattice as a prototypical example, we demonstrate that our method is capable of providing accurate results in the thermodynamic limit in all regimes from renormalized Fermi-liquid to single polarons, across the non-adiabatic region where Fermi and Debye energies are of the same order of magnitude. By accounting for vertex corrections the accuracy of theoretical description is increased by orders of magnitude relative to the lowest-order self-consistent Born approximation employed in most studies. We also find that for electron-phonon coupling typical for real materials, the quasiparticle effective mass increases and the quasiparticle residue decreases with increasing the system density.

cond-mat.str-el↗

arXiv2014

Generalized Hund's rule for two-atom systems

Hund's rule is one of the fundamentals of the correlation physics at the atomic level, determining the ground state multiplet of the electrons. It consists of three laws: (i) maximum $S$ (total spin), (ii) maximum $L$ (total orbital angular momentum) under the constraint of (i), and (iii) the total angular momentum $J$ is $|L-S|$ for electron number less than half, while $J=L+S$ for more than half due to the relativistic spin-orbit interaction (SOI). In real systems, the electrons hop between the atoms and gain the itinerancy, which is usually described by the band theory. The whole content of theories on correlation is to provide a reliable way to describe the intermediate situation between the two limits. Here we propose an approach toward this goal, i.e., we study the two-atom systems of three $t_{2g}$ orbitals and see how the Hund's rule is modified by the transfer integral $t$ between them. It is found that the competition between $t$ and the Hund's coupling $J$ at each atom determines the crossover from the molecular orbital limit to the strong correlation limit. Especially, the focus is on the generalization of the third rule, i.e., the inter-and intra-atomic SOI's in the presence of the correlation. We have found that there are cases where the effective SOI's are appreciably enhanced by the Hund's coupling. The conditions for the enhancement are the intermediate Hund's coupling and the filling of four or five electrons.

cond-mat.str-el↗

arXiv2014

Spin-to-charge conversion in lateral and vertical topological-insulator/ferromagnet heterostructures with microwave-driven precessing magnetization

Using the charge-conserving Floquet-Green function approach to open quantum systems driven by external time periodic potential, we analyze how spin current pumped (in the absence of any dc bias voltage) by the precessing magnetization of a ferromagnetic (F) layer is injected {\em laterally} into the interface with strong spin-orbit coupling (SOC) and converted into charge current flowing in the same direction. In the case of metallic interface with the Rashba SOC used in experiments [Nature Comm. {\bf 4}, 2944 (2013)], both spin $I^{S_α}$ and charge $I$ current flow within it where $I/I^{S_α} \simeq$ 2--8\% (depending on the precession cone angle), while for F/topological-insulator (F/TI) interface employed in related experiments (arXiv:1312.7091) the conversion efficiency is greatly enhanced $I/I^{S_α} \simeq$ 40--60\% due to perfect spin-momentum locking on the surface of TI. The spin-to-charge conversion occurs also when spin current is pumped {\em vertically} through the F/TI interface with smaller efficiency $I/I^{S_α} \sim 0.001\%$, but with charge current signal being sensitive to whether the Dirac fermions at the interface are massive or massless.

cond-mat.mes-hall↗

arXiv2014

Fermion Fractionalization to Majorana Fermions in Dimerized Kitaev Superconductor

We study theoretically a one-dimensional dimerized Kitaev superconductor model which belongs to BDI class with time-reversal, particle-hole, and chiral symmetries. There are two sources of the particle-hole symmetry, i.e., the sublattice symmetry and superconductivity. Accordingly, we define two types of topological numbers with respect to the chiral indices of normal and Majorana fermions, which offers an ideal laboratory to examine the interference between the two different physics within the same symmetry class. Phase diagram, zero-energy bound states, and conductance at normal metal/superconductor junction of this model are unveiled from this viewpoint. Especially, the electron fractionalization to the Majorana fermions showing the splitting of the local density of states is realized at the soliton of the dimerization in this model.

cond-mat.supr-con↗

arXiv2014

Transport signatures of superconducting hybrids with mixed singlet and chiral triplet states

We propose a model for a superconductor where both spin-singlet and chiral triplet pairing amplitudes can coexist. By solving the Bogoliubov-de Gennes equations with a general pair potential that accounts for both spin states we study experimental signatures of normal metal and superconductor hybrids. The interplay between the spin-singlet and triplet correlations manifests in the appearance of two effective gaps. When the amplitude of the spin-triplet component is stronger than that of the spin-singlet, a topological phase transition into a non-trivial regime occurs. As a result, the normal metal-superconductor conductance evolves from a conventional gap profile onto an unconventional zero-bias peak. Additionally, in the topologically non-trivial phase, Andreev bound states formed at Josephson junctions present zero-energy modes; the number of those modes depends on the relative chirality of the junction. Finally, we present results for the current-phase relation and the temperature dependence of the Josephson critical current within both topological phases for several system parameters.

cond-mat.mes-hall↗

arXiv2014

Gauge Field and Confinement-Deconfinement Transition in Hydrogen-Bonded Ferroelectrics

Quantum melting of ferroelectric moment in the frustrated hydrogen-bonded system with "ice rule" is studied theoretically by using the quantum Monte Carlo simulation. The large number of nearly degenerate configurations are described as the gauge degrees of freedom, i.e., the model is mapped to a lattice gauge theory which shows the confinement-deconfinment transition (CDT). The dipole-dipole interaction $J_2$, on the other hand, explicitly breaks the gauge symmetry leading to the ferroelectric transition (FT) at finite temperature $T$. It is found that the crossover from FT to CDT manifests itself in the reduced correlation length of the polarization $ξ_{\text{FT}} \sim Δ(K-K_c)^{-ν}$ with $Δ\propto \sqrt{J_2}$ while $K_c$ and $ν$ remains finite in the limit $J_2 \to 0$. In contrast, the Currie-Weiss-like law for the susceptibility $χ$ and the spontaneous polarization behaves smoothly and the length scale $ξ_{\text{CDT}}$, related to the molecular symmetry and volume for CDT, does not reduce in this limit.

cond-mat.str-el↗