arXiv subjects

Naoto Nagaosa

Publications and source records attributed to Naoto Nagaosa.

At least 109 records · Page 6Linked to original sources

arXiv2020

Photocurrent of exciton polaritons

We study photocurrent of exciton polaritons in inversion broken systems. We use an effective Hamiltonian and Green's function approaches to deduce the formula for polariton photocurrent. The obtained formula shows the polariton photocurrent is proportional to the polariton density and shows opposite signs for the upper and lower branches. Nonvanishing polariton photocurrent requires time reversal symmetry breaking in addition to inversion symmetry breaking. We show that exciton states in transition metal dichalcogenides such as MoS$_2$ support polariton photocurrent once we break time reversal symmetry by applying magnetic fields or using photons with circular polarization. We also perform a simulation based on a rate equation and study a time profile of polariton photocurrent after pulse excitation, which indicates that the photocurrent provides a useful nonlinear probe to study nonequilibrium dynamics of exciton polaritons.

cond-mat.mes-hall↗

arXiv2020

Current response of nonequilibrium steady states in Landau-Zener problem: Nonequilibrium Green's function approach

The carrier generation in insulators subjected to strong electric fields is characterized by the Landau-Zener formula for the tunneling probability with a nonperturbative exponent. Despite its long history with diverse applications and extensions, study of nonequilibrium steady states and associated current response in the presence of the generated carriers has been mainly limited to numerical simulations so far. Here, we develop a framework to calculate the nonequilibrium Green's function of generic insulating systems under a DC electric field, in the presence of a fermionic reservoir. Using asymptotic expansion techniques, we derive a semi-quantitative formula for the Green's function with nonperturbative contribution. This formalism enables us to calculate dissipative current response of the nonequilibrium steady state, which turns out to be not simply characterized by the intraband current proportional to the tunneling probability. We also apply the present formalism to noncentrosymmetric insulators, and propose nonreciprocal charge and spin transport peculiar to tunneling electrons.

cond-mat.str-el↗

arXiv2020

Feedback-type thermoelectric effect in correlated solids

A new thermoelectric effect mechanism inspired by an autonomous Maxwell's demon [P. Strasberg, G. Schaller, T. Brandes, and M. Esposito, Phys. Rev. Lett. 110, 040601 (2013)] is proposed. In contrast to the former work where a model for microscopic systems is proposed, a specific model for the thermoelectric effect in solid is formulated and its response to the electric field and temperature gradient is calculated in the framework of stochastic thermodynamics. The results show that relatively high $ZT$, which represents efficiency of the thermoelectric material, can be achieved within a range of realistic parameters.

cond-mat.mes-hall↗

arXiv2020

Giant anomalous Hall effect from spin-chirality scattering in a chiral magnet

The electrical Hall effect can be significantly enhanced through the interplay of the conduction electrons with magnetism, which is known as the anomalous Hall effect (AHE). Whereas the mechanism related to band topology has been intensively studied towards energy efficient electronics, those related to electron scattering have received limited attention. Here we report the observation of giant AHE of electron-scattering origin in a chiral magnet MnGe thin film. The Hall conductivity and Hall angle respectively reach 40,000 Ω-1cm-1 and 18 % in the ferromagnetic region, exceeding the conventional limits of AHE of intrinsic and extrinsic origins, respectively. A possible origin of the large AHE is attributed to a new type of skew-scattering via thermally-excited spin-clusters with scalar spin chirality, which is corroborated by the temperature-magnetic-field profile of the AHE being sensitive to the film-thickness or magneto-crystalline anisotropy. Our results may open up a new platform to explore giant AHE responses in various systems, including frustrated magnets and thin-film heterostructures.

cond-mat.mtrl-sci↗

arXiv2020

Combing the helical phase of chiral magnets with electric currents

The competition between the ferromagnetic exchange interaction and anti-symmetric Dzyaloshinskii-Moriya interaction can stabilize a helical phase or support the formation of skyrmions. In thin films of chiral magnets, the current density can be large enough to unpin the helical phase and reveal its nontrivial dynamics. We theoretically study the dynamics of the helical phase under spin-transfer torques that reveal distinct orientation processes, driven by topological defects in the bulk or induced by edges, limited by instabilities at higher currents. Our experiments confirm the possibility of on-demand switching the helical orientation by current pulses. This helical orientation might serve as a novel order parameter in future spintronics applications.

cond-mat.mes-hall↗

arXiv2020

Low-frequency divergence and quantum geometry of the bulk photovoltaic effect in topological semimetals

We study the low-frequency properties of the bulk photovoltaic effect in topological semimetals. The bulk photovoltaic effect is a nonlinear optical effect that generates DC photocurrents under uniform irradiation, allowed by noncentrosymmetry. It is a promising mechanism for a terahertz photodetection based on topological semimetals. Here, we systematically investigate the low-frequency behavior of the second-order optical conductivity in point-node semimetals. Through symmetry and power-counting analysis, we show that Dirac and Weyl points with tilted cones show the leading low-frequency divergence. In particular, we find new divergent behaviors of the conductivity of Dirac and Weyl points under circularly polarized light, where the conductivity scales as $ω^{-2}$ and $ω^{-1}$ near the gap-closing point in two and three dimensions, respectively. We provide a further perspective on the low-frequency bulk photovoltaic effect by revealing the complete quantum geometric meaning of the second-order optical conductivity tensor. The bulk photovoltaic effect has two origins, which are the transition of electron position and the transition of electron velocity during the optical excitation, and the resulting photocurrents are respectively called the shift current and the injection current. Based on an analysis of two-band models, we show that the injection current is controlled by the quantum metric and Berry curvature, whereas the shift current is governed by the Christoffel symbols near the gap-closing points in semimetals. Finally, for further demonstrations of our theory beyond simple two-band models, we perform first-principles calculations on magnetic Dirac semimetal MnGeO$_3$and Weyl semimetal PrGeAl. Our work brings out new insights into the structure of nonlinear optical responses as well as for the design of semimetal-based terahertz photodetectors.

cond-mat.mes-hall↗

arXiv2020

Fermi blockade of the electron-phonon interaction: why strong coupling effects may not be seen in optimally doped high temperature superconductors

We study how manifestations of strong electron-phonon interaction (EPI) depend on the carrier concentration by solving the two-dimensional Holstein model for the spin-polarized fermions using an approximation free bold-line diagrammatic Monte Carlo (BDMC) method. We show that the strong EPI, obviously present at very small Fermion concentration, is masked by the Fermi blockade effects and Migdal's theorem to the extent that it manifests itself as moderate one at large carriers densities. Suppression of strong EPI fingerprints is in agreement with experimental observations in doped high temperature superconductors

cond-mat.supr-con↗

arXiv2020

Bulk and surface topological indices for a skyrmion string

The magnetic skyrmion is a topological magnetic vortex, and its topological nature is characterized by an index called skyrmion number which is a mapping of the magnetic moments defined on a two-dimensional space to a unit sphere. In three-dimensions, a skyrmion, i.e., a vortex penetrating though the magnet naturally forms a string, which terminates at the surfaces of the magnet or in the bulk. For such a string, the topological indices, which control its topological stability are less trivial. Here, we show theoretically, in terms of numerical simulation for the current-driven motion of a skyrmion string in a film sample with the step edges on the surface, that the topological indices relevant to the stability are the followings; (i) skyrmion number along the developed surface, and (ii) the monopole charge in the bulk defined as the integral over the surface enclosing a singular magnetic configuration. As long as the magnetic configuration is slowly varying, the former is conserved while its changes is associated with nonzero monopole charge. The skyrmion number and the monoplole charge offer a coherent understanding of the stability of the topological magnetic texture and the nontrivial dynamics of skyrmion strings.

cond-mat.mes-hall↗

arXiv2020

Theory of shift current in Anderson insulator

Shift current is a photovoltaic current in bulk noncentrosymmetric insulator. Studies on shift current have so far focused on the extended Bloch waves, such as in semiconductors and perovskites. In contrast, it is unknown whether the localized wavefunctions support the dc photocurrent. Here, we show theoretically that the dc shift current appears in a noncentrosymmetric disordered one-dimensional insulator with random potential. When the light illuminates the entire sample, the photocurrent forms in presence of the electron-phonon coupling. We find this photocurrent remains robustly even when the energy scale of random potential is larger than the bandwidth. On the other hand, the photocurrent decays exponentially when the excitation is local, or the relaxation is only due to the contact with the electrodes. These results open a route to design high-efficiency solar cells and photodetectors.

cond-mat.mes-hall↗

arXiv2020

Quantum Kinetic Theory of Thermoelectric and Thermal Transport in a Magnetic Field

We present a general quantum kinetic theory that accounts for the interplay between a temperature gradient, momentum-space Berry curvatures of Bloch electrons, and Bloch-state scattering. Using a theory that incorporates the presence of a temperature gradient by introducing a "thermal vector potential", we derive a quantum kinetic equation for Bloch electrons in the presence of disorder and a temperature gradient. Taking also into account the presence of electric and magnetic fields, the quantum kinetic equation we derive makes it possible to compute transport coefficients at arbitrary orders of electric-field $\vec{E}$, magnetic-field $\vec{B}$, and temperature-gradient $\nabla T$ strengths $|\vec{E}|^a |\vec{B}|^b |\nabla T|^c$. Our theory enables a systematic calculation of magnetothermoelectric and magnetothermal conductivities of systems with momentum-space Berry curvatures. As an illustration, we derive from a general microscopic electron model a general expression for the rate of pumping of electrons between valleys in parallel temperature gradient and magnetic field. From this expression we find a relation, which is analogous to the Mott relation, between the rate of pumping due to a temperature gradient and that due to an electric field. We also apply our theory to a two-band model for Weyl semimetals to study thermoelectric and thermal transport in a magnetic field. We show that the Mott relation is satisfied in the chiral-anomaly induced thermoelectric conductivity, and that the Wiedemann-Franz law is violated in the chiral-anomaly induced thermal conductivity, which are both consistent with the results obtained by invoking semiclassical wave-packet dynamics.

cond-mat.mes-hall↗

arXiv2020

Slow steady flow of a skyrmion lattice in a confined geometry probed by resistance narrow-band noise

Using resistance fluctuation spectroscopy, we observe current-induced narrow-band noise (NBN) in the magnetic skyrmion-lattice phase of micrometer-sized MnSi. The NBN appears only when electric-current density exceeds a threshold value, indicating that the current-driven motion of the skyrmion lattice triggers the NBN. The observed NBN frequency is 10-10$^4$ Hz at $\sim$10$^{9}$ A/m$^{2}$, implying a skyrmion steady flow velocity of 1-100 $μ$m/s, 3-5 orders of magnitude slower than previously reported. The temperature evolution of the NBN frequency suggests that the steady flow entails thermally activated processes, which are most likely due to skyrmion creation and annihilation at the sample edges. This scenario is qualitatively supported by our numerical simulations considering boundary effects, which reveals that the edges limit the steady flow of skyrmions, especially at low temperatures. We discuss a mechanism that dramatically slows the skyrmion steady flow in a microfabricated specimen.

cond-mat.mes-hall↗

arXiv2020

Possible phason-polaron effect on purely one dimensional charge order of Mo6Se6 nanowires

In one-dimensional (1D) metallic systems, the diverging electron susceptibility and electron-phonon coupling collaboratively drive the electrons into a charge density wave (CDW) state. However, strictly 1D system is unstable against perturbations, whose effect on CDW order requires clarification ideally with altered coupling to surroundings. Here, we fabricate such a system with nanowires of Mo6Se6 bundles, which are either attached to edges of monolayer MoSe2 or isolated freely, by post-annealing the preformed MoSe2. Using scanning tunneling microscopy (STM), we visualized charge modulations and CDW gaps with prominent coherent peaks in the edge-attached nanowires. Astonishingly, the CDW order becomes suppressed in the isolated nanowires, showing CDW correlation gaps without coherent peaks. The contrasting behavior, as revealed with theoretical modeling, is interpreted as the effect of phason-polarons on the 1D CDW state. Our work elucidates a possibly unprecedented many body effect that may be generic to strictly 1D system but undermined in quasi-1D system.

cond-mat.mes-hall↗

arXiv2020

Theory of nonreciprocal Josephson effect

The hetero junction between the different materials often gives the rectification effect; e.g., pn junction is used for diode. On the other hand, the Josephson junction between the two superconductors is assumed to show symmetric response between the two directions of the current, i.e., the voltage drop $V$ is anti-symmetric with respect to the sign change of the current $I$. However, there should be an asymmetry between the states of charge accumulation on right and left sides of the Josephson junction, which can lead to the nonreciprocal responses. Here we demonstrate theoretically that nonreciprocal $I$-$V$ characteristic appears due to this charging energy difference both in the classical and quantum regimes. This result will pave a route to design and develop the Josephson diode.

cond-mat.mes-hall↗

arXiv2020

Nonreciprocal Landau-Zener tunneling

Application of strong dc electric field to an insulator leads to quantum tunneling of electrons from the valence band to the conduction band, which is a famous nonlinear response known as Landau-Zener tunneling. One of the growing interests in recent studies of nonlinear responses is nonreciprocal phenomena where transport toward the left and the right differs. Here, we theoretically study Landau-Zener tunneling in noncentrosymmetric systems, i.e., the crystals without spatial inversion symmetry. A generalized Landau-Zener formula is derived taking into account the geometric nature of the wavefunctions. The obtained formula shows that nonreciprocal tunneling probability originates from the difference in the Berry connections of the Bloch wavefunctions across the band gap, i.e., shift vector. We also discuss application of our formula to tunneling in a one-dimensional model of a ferroelectrics.

cond-mat.mes-hall↗

arXiv2019

Shift current from electromagnon excitations in multiferroics

Electromagnon is the spin wave in multiferroic materials and is known to accompany electric polarization due to the cross correlation between the charge and spin. Here, we theoretically show that the electromagnons also induce dc current upon their photoexcitations. The proposed dc current response originates from the shift current mechanism which is characterized by the so called shift vector, a geometric quantity of the Bloch wavefunctions.

cond-mat.str-el↗

arXiv2019

Thermal Hall effect, spin Nernst effect, and spin density induced by thermal gradient in collinear ferrimagnets from magnon-phonon interaction

We theoretically study the intrinsic thermal Hall and spin Nernst effect in collinear ferrimagnets on a honeycomb lattice with broken inversion symmetry. The broken inversion symmetry allows in-plane Dzyaloshinskii-Moriya interaction between the nearest neighbors, which does not affect the magnon spectrum in the linear spin wave theory. However, the Dzyaloshinskii-Moriya interaction can induce large Berry curvature in the magnetoelastic excitation spectrum through the magnon-phonon interaction to produce thermal Hall current. Furthermore, we find that the magnetoelastic excitations transport spin, which is inherited from the magnon bands. Therefore, the thermal Hall current is accompanied by spin Nernst current. Because the magnon-phonon interaction does not conserve the spin, we also study the spin density induced by thermal gradient in the presence of magnon-phonon interaction. We find that the intrinsic part of the spin density shows no asymmetric spin accumulation near the boundary of the system having a stripe geometry. However, because of the magnon-phonon interaction, we find nonzero total spin density in the system having armchair edges. The extrinsic part of the spin density, on the other hand, shows asymmetric spin accumulation near the boundary for both armchair and zigzag edges because of the magnon-phonon interaction. In addition, we find nonzero total spin density in the system having zigzag edge.

cond-mat.mes-hall↗

arXiv2019

Critical spin fluctuation mechanism for the spin Hall effect

We propose mechanisms for the spin Hall effect in metallic systems arising from the coupling between conduction electrons and local magnetic moments that are dynamically fluctuating. Both a side-jump-type mechanism and a skew-scattering-type mechanism are considered. In either case, dynamical spin fluctuation gives rise to a nontrivial temperature dependence in the spin Hall conductivity. This leads to the enhancement in the spin Hall conductivity at nonzero temperatures near the ferromagnetic instability. The proposed mechanisms could be observed in $4d$ or $5d$ metallic compounds.

cond-mat.str-el↗

arXiv2019

Tunable Charged Domain Wall from Topological Confinement in Nodal-Line Semimetals

We study theoretically the electronic structure of topological nodal-line semimetals. We show that, in the presence of a gap-opening spatially dependent mass term that forms a domain wall, an in-gap charged localized mode emerges at the domain wall. It turns out that such a domain wall is realized by head-to-head (or tail-to-tail) bulk electric polarizations. The localized mode has a topological origin, i.e., a topological confinement is realized, which is understood by a semiclassical topological number defined in the semiclassical momentum-real space. In contrast to previous studies, our study demonstrates a topological confinement at the interface between two insulators without bulk topological numbers. Moreover, the dispersion of the localized mode evolves from gapless to gapped as the bulk bandgap increases, which means that its conductivity is externally tunable. We discuss a possible experimental realization of the stable, electrically-tunable charged domain wall.

cond-mat.mes-hall↗