arXiv subjects

Naoto Nagaosa

Publications and source records attributed to Naoto Nagaosa.

At least 145 records · Page 8Linked to original sources

arXiv2018

Robustness of anomaly-related magnetoresistance in doped Weyl semimetals

Weyl semimetal with Weyl fermions at Fermi energy is one of the topological materials, and is a condensed-matter realization of the relativistic fermions. However, there are several crucial differences such as the shift of Fermi energy, which can hinder the expected interesting physics. Chiral anomaly is a representative nontrivial phenomenon associated with Weyl fermions, which dictates the transfer of fermions between the Weyl fermions with opposite chirality; it is manifested as the negative magnetoresistance. Here we demonstrate that the magnetoresistance is robust against the deviation from the ideal Weyl Hamiltonian such as the shifted Fermi energy and nonlinear dispersions. We study a model with the energy dispersion containing two Weyl nodes, and find that the magnetoresistance persists even when the Fermi level is far away from the node, even above the saddle point that separates the two nodes. Surprisingly, the magnetoresistance remains even after the pair annihilation of the nodes.

cond-mat.mes-hall↗

arXiv2018

Anomalous Hall Effect and Spin Fluctuations in Ionic Liquid Gated SrCoO$_3$ Thin Films

The recent realization of epitaxial SrCoO$_3$ thin films has triggered a renewed interest in their electronic, magnetic, and ionic properties. Here we uncover several unusual magneto-transport properties of this compound, suggesting that it hosts persistent spin fluctuation down to low temperatures. We achieve the metallic SrCoO$_3$ with record-low resistivity from insulating SrCoO$_{2.5}$ by the ionic liquid gating. We find a linear relationship between the anomalous Hall resistivity and the longitudinal resistivity, which cannot be accounted for by the conventional mechanisms. We theoretically propose that the impurity induced chiral spin fluctuation gives rise to such a dependence. The existence of spin fluctuation manifests itself as negatively enhanced magneto-resistance of SrCoO$_3$ when the temperature approaches zero. Our study brings further insight into the unique spin state of SrCoO$_3$ and unveils a novel skew scattering mechanism for the anomalous Hall effect.

cond-mat.mes-hall↗

arXiv2018

Singular nonlinear response in metals due to divergent Berry curvature: application to magneto-resistance in metals with type-II Weyl nodes

The physical properties of metals are often given by the sum of the contributions from the electrons consisting the Fermi surface (FS), and therefore, fine structures of the electronic bands and Bloch functions are often masked by the integral over FS. As a consequence, usually, the singular structures in the electronic bands are often not reflected to the macroscopic quantities. In this work, we investigate the anomaly-related magnetoresistance in metals with type-II Weyl nodes close to the FS, and find that the anomaly-related current increases divergently, showing a singular structure. Detailed analysis on a simple model with multiple Weyl nodes shows that the contribution to the magnetoresistance is dominated by the electrons in the vicinity of the Weyl nodes; this is related to the fact that the current is given by the square of the Berry curvature, which enhances the contribution from the electrons around the Weyl nodes. The above results potentially allows an estimate of the anomaly-induced current without precise information of the entire Band structure.

cond-mat.mes-hall↗

arXiv2018

Capture and chaotic scattering of a charged particle by magnetic monopole under uniform electric field

Motivated by the realization of magnetic monopole of Berry curvature by the energy crossing point, we theoretically study the effect of magnetic monopole under a uniform electric field in the semiclassical dynamics, which is relevant to many physical situations such as relaxation through the diabolic point. We found that the competition between the backward scattering by the monopole magnetic field and the acceleration by the electric field leads to the bound state, i.e., capture of a particle near the monopole. Furthermore, the nonlinearity induced by the magnetic monopole leads to the chaotic behavior in the transient dynamics, i.e., the transient chaos. We computed characteristic quantities of the strange saddle which gives rise to the transient chaos, and verified that the abrupt bifurcation occurs as we tune the system parameter toward the parameter region in which the system is solvable.

cond-mat.mes-hall↗

arXiv2018

Electron-phonon coupling in undoped cuprate $YBa_2Cu_3O_6$ estimated from Raman and optical conductivity spectra

We study experimentally the Raman response of the undoped high-Tc parent compound $YBa_2Cu_3O_6$, and give a unified theory of the two-magnon Raman peak and optical conductivity based on the Hubbard-Holstein model with electron-phonon coupling (EPC). The Hubbard model without EPC can qualitatively account for the experimentally observed resonance of the Raman response, but only the Hubbard-Holstein model (i) reproduces asymmetry of the Raman spectrum, (ii) validates experimental visibility of the two-magnon peak, and (iii) predicts the correct shape and energy of the lower edge of the charge transfer gap in optical conductivity. Comparison of experiments with the theory gives the EPC strength $λ$ = 0.6. This result convincingly indicates the vital role of EPC in high-Tc cuprates providing a clue to the mechanism of high-Tc.

cond-mat.str-el↗

arXiv2018

Impurity-induced vector spin chirality and anomalous Hall effect in ferromagnetic metals

Scattering by multiple scatterers sometimes gives rise to nontrivial consequences such as anomalous Hall effect. We here study a mechanism for anomalous Hall effect which originates from the correlation of nonmagnetic impurities and localized moments; a Hall effect induced by vector spin chirality. Using a scattering theory approach, we study the skew scattering induced by the scattering processes that involve two magnetic moments and a non-magnetic impurity, which is proportional to the vector spin chirality of the spins in the vicinity of the non-magnetic impurity. Furthermore, we show that a finite vector spin chirality naturally exists around an impurity in the usual ferromagnetic metals at finite temperature due to the local inversion-symmetry breaking by the impurity. The result is potentially relevant to magnetic oxides which the anomalous Hall effect is enhanced at finite temperatures.

cond-mat.mes-hall↗

arXiv2018

Photogalvanic Effect in Weyl Semimetals from First Principles

Using first-principles calculations, we investigate the photogalvanic effect in the Weyl semimetal material TaAs. We find colossal photocurrents caused by the Weyl points in the band structuure in a wide range of laser frequency. Our calculations reveal that the photocurrent is predominantly contributed by the three-band transition from the occupied Weyl band to the empty Weyl band via an intermediate band away from the Weyl cone, for excitations both by linearly and circularly polarized lights. Therefore, it is significant to sum over all three-band transitions by considering a full set of Bloch bands (both Weyl bands and trivial bands) in the first-principles band structure while it does not suffice to only consider the two-band direct transition within a Weyl cone. Calculated photoconductivities are well consistent with recent experiment measurements. Our work provides the first first-principles calculation on nonlinear optical phenomena of Weyl semimetals and serves as a deep understanding of the photogalvanic effects in complexed materials.

cond-mat.str-el↗

arXiv2018

Nonreciprocal charge transport in two-dimensional noncentrosymmetric superconductors

Nonreciprocal charge transport phenomena are studied theoretically for two-dimensional noncentrosymmetric superconductors under an external magnetic field $B$. Rashba superconductors, surface superconductivity on the surface of three-dimensional topological insulators, and transition metal dichalcogenides (TMD) are representative systems, and the current-voltage $I$-$V$ characteristics, i.e., $V=V(I)$, for each of them is analyzed. $V(I)$ can be expanded with respect to the current $I$ as $V(I)= \sum_{j=1,\infty} a_j(B,T) I^j$, and the $(B,T)$-dependence of $a_j$ depends on the mechanism of the charge transport. Above the mean field transition temperature $T_0$, the fluctuation of the superconducting order parameter gives the additional conductivity, i.e., paraconductivity. Extending the analysis to the nonlinear response, we obtain the nonreciprocal charge transport expressed by $a_2(B,T) = a_1(T) γ(T) B$, where $γ$ converges to a finite value at $T=T_0$. Below $T_0$, the vortex motion is relevant to the voltage drop, and the dependence of $a_j$ on $B,T$ is different depending on the system and mechanisms. For the superconductors under the in-plane magnetic field, the Kosterlitz-Thouless (KT) transition occurs at $T_{\rm KT}$. In this case $γ$ has the characteristic temperature dependences such as $γ\sim (T-T_{\rm KT})^{-3/2}$ near $T_{\rm KT}$. On the other hand, for TMD with out-plane magnetic field, the KT transition is gone, and there are two possible mechanisms for the nonreciprocal response. One is the anisotropy of the damping constant for the motion of the vortex. In this case, $a_1(B) \sim B$ and $a_2(B) \sim B^2$. The other one is the ratchet potential acting on the vortex motion, which gives $a_1(B) \sim B$ and $a_2(B) \sim B$. Based on these results, we propose the experiments to identify the mechanism of the nonreciprocal charge transport.

cond-mat.supr-con↗

arXiv2018

Electrical detection of a skyrmion in a magnetic tunneling junction

We theoretically investigated a method to detect a single skyrmion in a magnetic tunneling junction (MTJ) geometry. Using the tunneling Hamiltonian approach, we calculated the tunneling magnetoresistance (TMR) ratio of the skyrmion-ferromagnet bilayer system. We show the TMR ratio is determined sorely by the spin profile of the skyrmion and geometrical factor of the device, if only the system is reasonably clean such that the spectral broadening is smaller than the exchange coupling between the local and the itinerant magnetic moment. The TMR ratio in that case can amount to $30\%$ or higher when the diameter of the skyrmion is as large as the size of the device. Since this criterion is easily achievable in real systems, MTJ geometry can be a good candidate of the electrical detection of a single skyrmion i.e., the reading process of the information in the future skyrmionics memory devices.

cond-mat.mes-hall↗

arXiv2018

Spontaneous symmetry breaking of domain walls in phase-competing regions

We study the nature of domain walls in an ordered phase in the phase-competing region of two Ising-type order parameters. Considering a two-component $ϕ^4$ theory, we show that the domain wall of the ground-state (primary) order parameter shows a second-order phase transition associated with the secondary order parameter of the competing phase; the effective theory of the phase transition is given by the Landau theory of Ising-type phase transition. We find that the phase boundary of this phase transition is different from the spinodal line of the competing order. Experimentally, the phase transition is detected by the divergence of the susceptibility corresponding to the secondary order when the temperature is quenched to introduce the domain walls.

cond-mat.stat-mech↗

arXiv2018

Nonreciprocal current from electron interactions in noncentrosymmetric crystals: roles of time reversal symmetry and dissipation

In noncentrosymmetric crystals with broken inversion symmetry $\mathcal{I}$, the $I-V$ ($I$: current, $V$: voltage) characteristic is generally expected to depend on the direction of $I$, which is known as nonreciprocal response and, for example, found in p-n junction. However, it is a highly nontrivial issue in translationally invariant systems since the time-reversal symmetry ($\mathcal{T}$) plays an essential role, where the two states at crystal momenta $k$ and $-k$ are connected in the band structure. Therefore, it has been considered that the external magnetic field ($B$) or the magnetic order which breaks the $\mathcal{T}$-symmetry is necessary to realize the nonreciprocal $I-V$ characteristics, i.e., magnetochiral anisotropy. Here we theoretically show that the electron correlation in $\mathcal{I}$-broken multi-band systems can induce nonreciprocal $I-V$ characteristics {\it without} $\mathcal{T}$-breaking. An analog of Onsager's relation shows that nonreciprocal current response without $\mathcal{T}$-breaking generally requires two effects: dissipation and interactions. By using nonequilibrium Green's functions, we derive general formula of the nonreciprocal response for two-band systems with onsite interaction. The formula is applied to Rice-Mele model, a representative 1D model with inversion breaking, and some candidate materials are discussed. This finding offers a coherent understanding of the origin of nonreciprocal $I-V$ characteristics, and will pave a way to design it.

cond-mat.str-el↗

arXiv2018

Diffusive real-time dynamics of a particle with Berry curvatures

We study theoretically the influence of Berry phase on the real-time dynamics of the single particle focusing on the diffusive dynamics, i.e., the time-dependence of the distribution function. Our model can be applied to the real-time dynamics of intraband relaxation and diffusion of optically excited excitons, trions or particle-hole pair. We found that the dynamics at the early stage is deeply influenced by the Berry curvatures in real-space ($B$), momentum-space ($Ω$), and also the crossed space between these two ($C$). For example, it is found that $Ω$ induces the rotation of the wave packet and causes the time-dependence of the mean square displacement of the particle to be linear in time $t$ at the initial stage; it is qualitatively different from the $t^3$ dependence in the absence of the Berry curvatures. It is also found that $Ω$ and $C$ modifies the characteristic time scale of the thermal equilibration of momentum distribution. Moreover, the dynamics under various combinations of $B$, $Ω$ and $C$ shows singular behaviors such as the critical slowing down or speeding up of the momentum equilibration and the reversals of the direction of rotations. The relevance of our model for time-resolved experiments in transition metal dichalcogenides is also discussed.

cond-mat.mes-hall↗

arXiv2017

Theory of magnetic skyrmion glass

Skyrmions and skyrmion crystal (SkX) discovered in chiral magnets show unique physical properties due to their nontrivial topology such as the stability against the annihilation and the motion driven by the ultralow current density, which can be advantageous for the device applications such as magnetic memories. Especially, the chiral dynamics, i.e., the velocity perpendicular to the force acting on a skyrmion, is a key to avoid the impurity potential and enhances its mobility. However, the collective pinning of SkX occurs by the disorder, which is crucial for its low energy properties. Here, we study theoretically the low energy dynamics of SkX in the presence of disorder effects in terms of replica field theory, and reveal nonreciprocal collective modes and their electromagnetic responses along the direction of external magnetic field. The physical quantities such as the relaxation rate of $μ$SR/NMR and the pinning frequency show a dramatic change associated with the topological phase transition from the helical state to SkX. These results provide a firm basis to explore the glassy state of SkX.

cond-mat.mes-hall↗

arXiv2017

Quantum Hall states observed in thin films of Dirac semimetal Cd3As2

A well known semiconductor Cd3As2 has reentered the spotlight due to its unique electronic structure and quantum transport phenomena as a topological Dirac semimetal. For elucidating and controlling its topological quantum state, high-quality Cd3As2 thin films have been highly desired. Here we report the development of an elaborate growth technique of high-crystallinity and high-mobility Cd3As2 films with controlled thicknesses and the observation of quantum Hall effect dependent on the film thickness. With decreasing the film thickness to 10 nm, the quantum Hall states exhibit variations such as a change in the spin degeneracy reflecting the Dirac dispersion with a large Fermi velocity. Details of the electronic structure including subband splitting and gap opening are identified from the quantum transport depending on the confinement thickness, suggesting the presence of a two-dimensional topological insulating phase. The demonstration of quantum Hall states in our high-quality Cd3As2 films paves a road to study quantum transport and device application in topological Dirac semimetal and its derivative phases.

cond-mat.mtrl-sci↗

arXiv2017

Nonreciprocal current in noncentrosymmetric Rashba superconductors

Noncentrosymmetric superconductors with broken inversion symmetry $P$ offer rich physical phenomena such as the upper critical magnetic field $H_{c2}$ beyond the Pauli limit and magnetoelectric effect. The relativistic spin-orbit interaction (SOI) plays an essential role in these novel phenomena, which lifts the Kramers degeneracy at each $\mathbf{k}$-point, and leads to the mixing of the spin singlet-even parity and spin triplet odd-parity parings. On the other hand, the time-reversal symmetry $T$ relates the two states at $\mathbf{k}$ and $-\mathbf{k}$ points with the opposite spins, and the external magnetic field $B$ further breaking $T$ leads to the directional dependence of the nonlinear resistivity called magnetochiral anisotropy (MCA). Here we demonstrate theoretically that the two-component nature of the order parameter, i.e., even and odd pairings, leads to the gigantic enhancement of the MCA in the fluctuation region, in Rashba superconductor as a representative example of noncentrosymmetric system. This reveals the superconducting analogue of "ferroelectric" appearing in the transport phenomena.

cond-mat.supr-con↗

arXiv2017

Spin Transport and Accumulation in 2D Weyl Fermion System

In this work, we study the spin Hall effect and Rashba-Edelstein effect of a 2D Weyl fermion system in the clean limit using the Kubo formalism. Spin transport is solely due to the spin-torque current in this strongly spin-orbit coupled (SOC) system, and chiral spin-flip scattering off non-SOC scalar impurities, with potential strength $V$ and size $a$, gives rise to a skew-scattering mechanism for the spin Hall effect. The key result is that the resultant spin-Hall angle has a fixed sign, with $θ^{SH} \sim O \left(\tfrac{V^2}{v_F^2/a^2} (k_F a)^4 \right)$ being a strongly-dependent function of $k_F a$, with $k_F$ and $v_F$ being the Fermi wave-vector and Fermi velocity respectively. This, therefore, allows for the possibility of tuning the SHE by adjusting the Fermi energy or impurity size.

cond-mat.mes-hall↗

arXiv2017

Electromagnon on the Surface of Magnetic Topological Insulator

We investigate theoretically the electromagnon on the surfaces of the magnetic topological insulator thin films. It is found that when the magnetic asymmetry between the top and bottom surfaces is there, the ferromagnetic resonance is driven by the electric field which is two orders of magnitude more efficient compared with that by the magnetic field. The resonant frequency of the electromagnon is also estimated.

cond-mat.mes-hall↗

arXiv2017

Nonlinear spin current generation in noncentrosymmetric spin-orbit coupled systems

Spin current plays a central role in spintronics. In particular, finding more efficient ways to generate spin current has been an important issue and studied actively. For example, representative methods of spin current generation include spin polarized current injections from ferromagnetic metals, spin Hall effect, and spin battery. Here we theoretically propose a new mechanism of spin current generation based on nonlinear phenomena. By using Boltzmann transport theory, we show that a simple application of the electric field $\bf{E}$ induces spin current proportional to $\bf{E^2}$ in noncentrosymmetric spin-orbit coupled systems. We demonstrate that the nonlinear spin current of the proposed mechanism is supported in the surface state of three-dimensional topological insulators and two-dimensional semiconductors with the Rashba and/or Dresselhaus interaction. In the latter case, the angular dependence of the nonlinear spin current can be manipulated by the direction of the electric field and by the ratio of the Rashba and Dresselhaus interactions. We find that the magnitude of the spin current largely exceeds those in the previous methods for a reasonable magnitude of the electric field. Furthermore, we show that application of AC electric fields (e.g. terahertz light) leads to the rectifying effect of the spin current where DC spin current is generated. These findings will pave a new route to manipulate the spin current in noncentrosymmetric crystals.

cond-mat.mes-hall↗