arXiv subjects

Satoshi Fujimoto

Publications and source records attributed to Satoshi Fujimoto.

At least 55 records · Page 3Linked to original sources

arXiv2019

Disorder-induced exceptional and hybrid rings in Weyl/Dirac semimetals

Non-Hermiticity in Weyl Hamiltonian leads to the realization of Weyl exceptional rings and flat bands inside the Weyl exceptional rings. Recently, the platform of non-Hermitian physics is extended to many-body or disordered systems where quasiparticles possess finite lifetime. Here, we clarify that the deviation from unitarity limit in a disordered Weyl semimetal leads to the generation of Weyl exceptional rings regardless of the detail of the scattering potential. In the case of topological Dirac semimetals, hybrid rings and flat bands without vorticity of complex-energy eigenvalues are realized. This scenario is applicable to any Weyl or Dirac semimetals. These effects are detectable by using photoemission or quasiparticle interference experiments.

cond-mat.mes-hall↗

arXiv2019

Impact of off-diagonal exchange interactions on the Kitaev spin liquid state of $α$-RuCl$_3$

Motivated by the recent experimental observation of the half-quantized thermal Hall conductivity for the candidate material of the Kitaev spin liquid, $α$-RuCl$_3$[ Y. Kasahara et al., Nature 559, 227 (2018)], we investigate effects of non-Kitaev exchange interactions, which exist in the real material, on the gapped chiral spin liquid state realized in the case with an applied magnetic field. It is found that off-diagonal exchange interactions enhance significantly the mass gap of Majorana fermions. This result provides a possible explanation for robust quantization of the thermal Hall conductivity observed in the above-mentioned experiment. Furthermore, we demonstrate that if the Kitaev spin liquid state and the zigzag antiferromagnetic order coexist around the border of these two phases, off-diagonal exchange interactions can induce Fermi surfaces of Majorana fermions, leading to a state similar to the U(1) spin liquid.

cond-mat.str-el↗

arXiv2019

Torsional chiral magnetic effect due to skyrmion textures in a Weyl superfluid $^3$He-A

We investigate torsional chiral magnetic effect (TCME) induced by skyrmion-vortex textures in the A phase of the superfluid $^3$He. In $^3$He-A, Bogoliubov quasiparticles around point nodes behave as Weyl fermions, and the nodal direction represented by the $\ell$-vector may form a spatially modulated texture. $\ell$-textures generate a chiral gauge field and a torsion field directly acting on the chirality of Weyl-Bogoliubov quasiparticles. It has been clarified by G. E. Volovik [Pi'sma Zh. Eksp. Teor. Fiz. {\bf 43}, 428 (1986)] that, if the $\ell$-vector is twisted, the chiral gauge field is responsible for the chiral anomaly, leading to an anomalous current along ${\ell}$. Here we show that, even for non-twisted $\ell$-vector fields, a torsion arising from $\ell$-textures brings about contributions to the equilibrium currents of Weyl-Bogoliubov quasiparticles along ${\rm curl}{\ell}$. This implies that while the anomalous current appears only for the twisted (Bloch-type) skyrmion of the $\ell$-vector, the extra mass current due to TCME always exists regardless of the skyrmion type. Solving the Bogoliubov-de Gennes equation, we demonstrate that both Bloch-type and Néel-type skyrmions induce chiral fermion states with spectral asymmetry, and possess spatially inhomogeneous structures of Weyl bands in the real coordinate space. Furthermore, we discuss the contributions of Weyl-Bogoliubov quasiparticles and continuum states to the mass current density in the vicinity of the topological phase transition. In the weak coupling limit, continuum states give rise to backflow to the mass current generated by Weyl-Bogoliubov quasiparticles, which makes a non-negligible contribution to the orbital angular momentum. As the topological transition is approached, the mass current density is governed by the contribution of continuum states.

cond-mat.supr-con↗

arXiv2018

Negative thermal magnetoresistivity as a signature of chiral anomaly in Weyl superconductors

We propose that chiral anomaly of Weyl superconductors gives rise to negative thermal magnetoresistivity induced by emergent magnetic fields, which are generated by vortex textures of order parameters or lattice strain. We establish this scenario by combining the argument based on Berry curvatures, and the quasi-classical theory of the Eilenberger equation with quantum corrections arising from inhomogeneous structures. It is found that the chiral anomaly contribution of the thermal conductivity exhibits characteristic temperature dependence, which can be a smoking-gun signature of this effect.

cond-mat.supr-con↗

arXiv2018

Charge/Spin Supercurrent and the Fulde-Ferrell State Induced by Crystal Deformation in Weyl/Dirac Superconductors

It has been predicted that emergent chiral magnetic fields can be generated by crystal deformation in Weyl/Dirac metals and superconductors. The emergent fields give rise to chiral anomaly phenomena as in the case of Weyl semimetals with usual electromagnetic fields. Here, we clarify effects of the chiral magnetic field on Cooper pairs in Weyl/Dirac superconductors on the basis of the Ginzburg-Landau equation microscopically derived from the quasiclassical Eilenberger formalism. It is found that Cooper pairs are affected by the emergent chiral magnetic field in a dramatic way, and the pseudo-Lorentz force due to the chiral magnetic field stabilizes the Fulde-Ferrell state and causes a charge/spin supercurrent which flows parallel to the chiral magnetic field in the case of Weyl/Dirac superconductors. This effect is in analogy with the chiral magnetic effect (CME) of Weyl semimetals. In addition, we elucidate that neither Meissner effect nor vortex state due to chiral magnetic fields occurs.

cond-mat.supr-con↗

arXiv2018

Quantum oscillations and Dirac-Landau levels in Weyl superconductors

When magnetic field is applied to metals and semimetals quantum oscillations appear as individual Landau levels cross the Fermi level. Quantum oscillations generally do not occur in superconductors (SC) because magnetic field is either expelled from the sample interior or, if strong enough, drives the material into the normal state. In addition, elementary excitations of a superconductor -- Bogoliubov quasiparticles -- do not carry a well defined electric charge and therefore do not couple in a simple way to the applied magnetic field. We predict here that in Weyl superconductors certain types of elastic strain have the ability to induce chiral pseudo-magnetic field which can reorganize the electronic states into Dirac-Landau levels with linear band crossings at low energy. The resulting quantum oscillations in the quasiparticle density of states and thermal conductivity can be experimentally observed under a bending deformation of a thin film Weyl SC and provide new insights into this fascinating family of materials.

cond-mat.supr-con↗

arXiv2017

Adiabatic and nonadiabatic spin torques induced by spin-triplet supercurrent

We study spin transfer torques induced by a spin-triplet supercurrent in a magnet with the superconducting proximity effect. By a perturbative approach, we show that spin-triplet correlations realize new types of torques, which are analogous to the adiabatic and non-adiabatic ($β$) torques, without extrinsic spin-flip scattering. Remarkable advantages compared to conventional spin-transfer torques are highlighted in domain wall manipulation. Oscillatory motions of a domain wall do not occur for a small Gilbert damping, and the threshold current density to drive its motion becomes zero in the absence of extrinsic pinning potentials due to the nonadiabatic torque controlled by the triplet correlations.

cond-mat.mes-hall↗

arXiv2016

Supercurrent-induced Skyrmion dynamics and Tunable Weyl points in Chiral Magnet with Superconductivity

Recent studies show superconductivity provides new perspectives on spintronics. We study a heterostructure composed of an s-wave superconductor and a cubic chiral-magnet that can stabilize a topological spin texture, a skyrmion. We propose a supercurrent-induced spin torque that originates from the spin-orbit coupling, and we show that the spin torque can drive a skyrmion in an efficient way that reduces Joule heating. We also study the band structure of Bogoliubov quasiparticles and show the existence of Weyl points, whose positions can be controlled by the direction of the magnetization. This results in an effective magnetic field acting on the quasiparticles in the presence spin textures. Furthermore, the tilt of the Weyl cones can also be tuned by the strength of the spin-orbit coupling, and we propose a possible realization of type-II Weyl points.

cond-mat.mes-hall↗

arXiv2016

Majorana Fermions and Topology in Superconductors

Topological superconductors are novel classes of quantum condensed phases, characterized by topologically nontrivial structures of Cooper pairing states. On the surfaces of samples and in vortex cores of topological superconductors, Majorana fermions, which are particles identified with their own anti-particles, appear as Bogoliubov quasiparticles. The existence and stability of Majorana fermions are ensured by bulk topological invariants constrained by the symmetries of the systems. Majorana fermions in topological superconductors obey a new type of quantum statistics referred to as non-Abelian statistics, which is distinct from bose and fermi statistics, and can be utilized for application to topological quantum computation. Also, Majorana fermions give rise to various exotic phenomena such as "fractionalization", non-local correlation, and "teleportation". A pedagogical review of these subjects is presented. We also discuss interaction effects on topological classification of superconductors, and the basic properties of Weyl superconductors.

cond-mat.supr-con↗

arXiv2016

Magnetism and Superconductivity in Ferromagnetic Heavy Fermion System UCoGe under In-plane Magnetic Fields

We study the ferromagnetic superconductor UCoGe at ambient pressure under $ab$-plane magnetic fields $\vec{H}$ which are perpendicular to the ferromagnetic easy axis. It is shown that, by taking into account the Dyaloshinskii-Moriya interaction arising from the zigzag chain crystal structure of UCoGe, we can qualitatively explain the experimentally observed in-plane anisotropy for critical magnetic fields of the paramagnetic transition. Because of this strong dependence on the magnetic field direction, upper critical fields of superconductivity, which is mediated by ferromagnetic spin fluctuations, also become strongly anisotropic. The experimental observation of "S-shaped" $H_{c2}\parallel b$-axis is qualitatively explained as a result of enhancement of the spin fluctuations due to decreased Curie temperature by the $b$-axis magnetic field. We also show that the S-shaped $H_{c2}$ is accompanied by a rotation of the $d$-vector, which would be a key to understand the experiments not only at ambient pressure but also under pressure.

cond-mat.supr-con↗

arXiv2016

Torsional chiral magnetic effect in Weyl semimetal with topological defect

We propose a torsional response raised by lattice dislocation in Weyl semimetals akin to chiral magnetic effect; i.e. a fictitious magnetic field arising from screw or edge dislocation induces charge current. We demonstrate that, in sharp contrast to the usual chiral magnetic effect which vanishes in real solid state materials, the torsional chiral magnetic effect exists even for realistic lattice models, which implies the experimental detection of the effect via SQUID or nonlocal resistivity measurements in Weyl semimetal materials.

cond-mat.mes-hall↗

arXiv2014

Giant Nernst and Hall Effects in Chiral Superconductors due to Berry Phase Fluctuations

We consider the Nernst and Hall effects in fluctuation regime of chiral superconductors above transition temperatures, that are raised not by conventional Lorentz force, but by asymmetric scattering due to fluctuations of the Berry phase of the Bogoliubov-de Gennes Hamiltonian. It is found that these effects can be more significant than conventional ones for cleaner samples, exhibiting qualitatively distinct behaviors. The results provide systematic and comprehensive understanding for recent experimental observations of the Nernst effect in a clean URu$_2$Si$_2$ sample, which is suggested to be a chiral superconductor.

cond-mat.supr-con↗

arXiv2014

Characterization of a topological Mott insulator in one dimension

We investigate properties of a topological Mott insulator in one dimension by examining the bulk topological invariant and the entanglement spectrum of a correlated electron model. We clarify how gapless edge states in a non-interacting topological band insulator evolve into spinon edge states in a topological Mott insulator. Furthermore, we propose a topological Mott transition, which is a new type of topological phase transition and never observed in free fermion systems. This unconventional transition occurs in spin liquid phases in the Mott insulator and is accompanied by zeros of the single-electron Green's function and a gap closing in the spin excitation spectrum.

cond-mat.str-el↗

arXiv2014

Electrodynamics in Skyrmions Merging

In a recent study the coalescence of magnetic skyrmions was observed in a metallic chiral magnet Fe0.5Co0.5Si when the skyrmion phase is destroyed, and numerical simulations demonstrated the existence of a monopole at the merging point of two skyrmion lines. The exchange interaction between such magnetic textures and the conduction electrons can be described by emergent electromagnetism. In this paper, we investigate the effect of a skyrmions-merging process on conduction electrons by calculating induced electric currents. Here, in addition to the exchange interactions, we consider the antisymmetric spin-orbit couplings (SOC) due to broken inversion symmetry, which is an essential ingredient for the realization of skyrmion texture in the itinerant magnet Fe0.5Co0.5Si. We obtain an adiabatic current which is dissipationless, and dissipative currents driven by the effective electromagnetic fields including the effect of SOC. In terms of the effective fields, a moving monopole at the merging point turns out to be a dyon-like object; i.e. it has both electric charge and magnetic charge.

cond-mat.str-el↗

arXiv2014

Dynamical axion in topological superconductors and superfluids

We consider dynamical axion phenomena in topological superconductors and superfluids in three spatial dimensions in terms of the gravitoelectromagnetic topological action, in which the axion field couples with mechanical rotation under finite temperature gradient. The dynamical axion is induced by relative phase fluctuations between topological and s-wave superconducting orders. We show that an antisymmetric spin-orbit interaction which induces parity-mixing of Cooper pairs enlarges the parameter region in which the dynamical axion fluctuation appears as a low-energy excitation. We propose that the dynamical axion increases the moment of inertia, and in the case of ac mechanical rotation, i.e. a shaking motion with a finite frequency $ω$, as $ω$ approaches the dynamical axion fluctuation mass, the observation of this effect becomes feasible.

cond-mat.supr-con↗

arXiv2013

Topological phase in a two-dimensional metallic heavy-fermion system

We report on a topological insulating state in a heavy-fermion system away from half-filling, which is hidden within a ferromagnetic metallic phase. In this phase, the cooperation of the RKKY interaction and the Kondo effect, together with the spin-orbit coupling, induces a spin-selective gap, bringing about topologically non-trivial properties. This topological phase is robust against a change in the chemical potential in a much wider range than the gap size. We analyze these remarkable properties by using dynamical mean field theory and the numerical renormalization group. Its topological properties support a gapless chiral edge mode, which exhibits a non-Tomonaga-Luttinger liquid behavior due to the coupling with bulk ferromagnetic spin fluctuations. We also propose that the effects of the spin fluctuations on the edge mode can be detected via the NMR relaxation time measurement.

cond-mat.str-el↗

arXiv2013

Topological antiferromagnetic phase in a correlated Bernevig-Hughes-Zhang model

Topological properties of antiferromagnetic phases are studied for a correlated topological band insulator by applying the dynamical mean field theory to an extended Bernevig-Hughes-Zhang model including the Hubbard interaction. The calculation of the magnetic moment and the spin Chern number confirms the existence of a non-trivial antiferromagnetic (AF) phase beyond the Hartree-Fock theory. In particular, we uncover the intriguing fact that the topologically non-trivial AF phase is essentially stabilized by correlation effects but not by the Hartree shifts alone. This counterintuitive effect is demonstrated, through a comparison with the Hartree-Fock results, and should apply for generic topological insulators with strong correlations.

cond-mat.str-el↗

arXiv2013

Quantum Thermal Hall Effect in a Time-Reversal-Symmetry-Broken Topological Superconductor in Two Dimensions : Approach From Bulk Calculations

We discuss thermal transport of two-dimensional topological superconductors (TSCs) with broken time reversal symmetry, which are described by Bogoliubov-de Gennes (BdG) Hamiltonians. From the calculations of bulk quantities only, without refereeing to Majorana edge states, we show that the thermal Hall conductivity of two-dimensional TSCs in the low-temperature limit is quantized in multiples of $1/2\frac{πT}{6}$, which is exactly one half of the value of quantization in the case of the integer quantum Hall effect, and that this exact half-quantization is caused by the structure of the Nambu spinor and the particle-hole symmetry, which BdG Hamiltonians generally have. In the case of spinless chiral p-wave superconductors, this result is in perfect agreement with the argument based on the Ising conformal field theory with the central charge $c=1/2$, which is an effective low-energy theory of the Majorana edge states.

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