arXiv subjects

Yasuhiro Tada

Publications and source records attributed to Yasuhiro Tada.

36 records · Page 2Linked to original sources

arXiv2020

Quantum criticality of magnetic catalysis in two-dimensional correlated Dirac fermions

We study quantum criticality of the magnetic field induced charge density wave (CDW) order in correlated spinless Dirac fermions on the $π$-flux square lattice at zero temperature as a prototypical example of the magnetic catalysis, by using the infinite density matrix renormalization group. It is found that the CDW order parameter $M(B)$ exhibits an anomalous magnetic field $(B)$ scaling behavior characteristic of the $(2+1)$-dimensional chiral Ising universality class near the quantum critical point, which leads to a strong enhancement of $M(B)$ compared with a mean field result. We also establish a global phase diagram in the interaction-magnetic field plane for the fermionic quantum criticality.

cond-mat.str-el↗

arXiv2020

Cyclotron resonance in Kondo insulator

Motivated by the recent experimental observations of quantum oscillations in the Kondo insulators SmB$_6$ and YbB$_{12}$, we study magneto-optical conductivity of Kondo insulators. We show that there exist non-trivial cyclotron resonance peaks in a frequency domain below the hybridization gap as a result of the characteristic Landau level structure of the system. Furthermore, these low energy peaks are enhanced near the insulator-metal phase transition driven by an applied magnetic field and stable even at low temperatures. They are smoothly connected via Kondo effect to resonance peaks for unhybridized conduction and $f$-electrons at high temperature. The non-trivial cyclotron resonance is a dynamical counterpart of quantum oscillations in static quantities, and through this intimate connection it will provide a useful insight for the latter.

cond-mat.str-el↗

arXiv2019

Quantum phase transition and criticality in quasi one-dimensional spinless Dirac fermions

We study quantum criticality of spinless fermions on the quasi one dimensional $π$-flux square lattice in cylinder geometry, by using the infinite density matrix renormalization group and abelian bosonization. For a series of the cylinder circumferences $L_y=4n+2=2, 6, \cdots$ with the periodic boundary condition, there are quantum phase transitions from gapped Dirac fermion states to charge density wave (CDW) states. We find that the quantum phase transitions for such circumferences are continuous and belong to the (1+1)-dimensional Ising universality class. On the other hand, when $L_y=4n=4, 8, \cdots$, there are gapless Dirac fermions at the non-interacting point and the phase transition to the CDW state is Gaussian. Both of these two criticalities are described in a unified way by the bosonization. We clarify their intimate relationship and demonstrate that a central charge $c=1/2$ Ising transition line arises as a critical state of an emergent Majorana fermion from the $c=2$ Gaussian transition point.

cond-mat.str-el↗

arXiv2019

Accessing electromagnetic properties of matter with cylindrical vector beams

Cylindrical vector beam (CVB) is a structured lightwave characterized by its topologically nontrivial nature of the optical polarization. The unique electromagnetic field configuration of CVBs has been exploited to optical tweezers, laser accelerations, and so on. However, use of CVBs in research fields outside optics such as condensed matter physics has not progressed. In this paper, we propose potential applications of CVBs to those fields based on a general argument on their absorption by matter. We show that pulse azimuthal CVBs around terahertz (THz) or far-infrared frequencies can be a unique and powerful mean for time-resolved spectroscopy of magnetic properties of matter and claim that an azimuthal electric field of a pulse CVB would be a novel way of studying and controlling edge currents in topological materials. We also demonstrate how powerful CVBs will be as a tool for Floquet engineering of nonequilibrium states of matter.

physics.optics↗

arXiv2018

Non-thermodynamic nature of the orbital angular momentum in neutral fermionic superfluids

We discuss the orbital angular momentum (OAM) and the edge mass current in neutral fermionic superfluids with broken time reversal symmetry. Recent mean field studies imply that total OAM of a uniform superfluid depends on boundary conditions and is not a thermodynamic quantity. We point out that this does not conflict with thermodynamics, because there is no intensive external field conjugate to OAM with which a uniform superfluid is stable in the thermodynamic limit, in sharp contrast to the orbital magnetization in a non-superfluid system. We establish a simple physical picture for the sensitivity of OAM to boundaries by introducing the notion of "unpaired fermions" and "fermionic Landau criterion" within a mean field description. In order to go beyond the mean field approximation, we perform a density matrix renormalization group calculation and conclude that the mean field understanding is essentially correct.

cond-mat.supr-con↗

arXiv2018

Decay of Superconducting Correlations for Gauged Electrons in Dimensions $D\le 4$

We study lattice superconductors coupled to gauge fields, such as an attractive Hubbard model in electromagnetic fields, with a standard gauge fixing. We prove upper bounds for a two-point Cooper pair correlation at finite temperatures in spatial dimensions $D\le 4$. The upper bounds decay exponentially in three dimensions, and by power law in four dimensions. These imply absence of the superconducting long-range order for the Cooper pair amplitude as a consequence of fluctuations of the gauge fields. Since our results hold for the gauge fixing Hamiltonian, they cannot be obtained as a corollary of Elitzur's theorem.

math-ph↗

arXiv2018

Fulde-Ferrell state in ferromagnetic chiral superconductor with magnetic domain wall

Motivated by the recent theoretical and experimental progress in the heavy fermion system UCoGe, we study ferromagnetic chiral superconductors in the presence of magnetic domains. Within mean field approximations, it is shown that chiral superconducting domains are naturally induced by the ferromagnetic domains. The domain wall current flows in the opposite direction to the naively expected one as in $^3$He-A phase due to contributions from "unpaired electrons". Consequently, the domain wall current flows in the same direction with that of surface currents when the magnetic domain wall lies parallel to the sample surface, and therefore they contribute to the net current along the whole sample. We find that, due to the non-cancellation between the domain wall current and surface current, a Fulde-Ferrell-like superconducting state can be stabilized in an anisotropic sample for all the temperatures below the superconducting transition temperature.

cond-mat.supr-con↗

arXiv2016

Two No-Go Theorems on Superconductivity

We study lattice superconductors such as attractive Hubbard models. As is well known, Bloch's theorem asserts absence of persistent current in ground states and equilibrium states for general fermion systems. While the statement of the theorem is true, we can show that the theorem cannot exclude possibility of a surface persistent current. Such a current can be stabilized by boundary magnetic fields which do not penetrate into the bulk region of a superconductor, provided emergence of massive photons, i.e., Meissner effect. Therefore, we can expect that a surface persistent current is realized for a ground/equilibrium state in the sense of stability against local perturbations. We also apply Elitzur's theorem to superconductors at finite temperatures. As a result, we prove absence of symmetry breaking of the global $U(1)$ phase of electrons for almost all gauge fields. These observations suggest that the nature of superconductivity is the emergence of massive photons rather than the symmetry breaking of the $U(1)$ phase of electrons.

math-ph↗

arXiv2016

Magnetism in f electron superlattices

We analyze antiferromagnetism in $f$ electron superlattices. We show that the competition between the Kondo effect and the RKKY interaction in $f$ electron materials is modified by the superlattice structure. Thus, the quantum critical point which separates the magnetic phase and the Fermi liquid phase depends on the structure of the $f$ electron superlattice. The competition between the Kondo effect and the RKKY interaction is also reflected in the magnetic interlayer coupling between different $f$ electron layers. We demonstrate that in the case of weak Kondo effect the magnetic interlayer coupling behaves similar to other magnetic heterostructures without Kondo effect. However, close to the quantum phase transition, the dependence of the interlayer coupling on the distance between the $f$ electron layers is modified by the Kondo effect. Another remarkable effect, which is characteristic for $f$ electron superlattice, is that the magnetic interlayer coupling does vanish stepwise depending on the distance between different $f$ electron layers. As a consequence, the quantum critical point depends also stepwise on this distance.

cond-mat.str-el↗

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↗

arXiv2015

Spin fluctuations and superconductivity in layered $f$-electron superlattices

We investigate magnetic and superconducting properties of layered $f$-electron superlattices within the fluctuation exchange approximation (FLEX). We show that spin fluctuations, which are characterized by the maximum value of the spin susceptibility in the 3-dimensional (3D) Brillouin zone, are strongly suppressed in $f$-electron superlattices. However, effective 2D spin fluctuations can be increased due to the spatial confinement of the $f$-electrons. Therefore, the tendency towards $d_{x^2-y^2}$-wave superconductivity, mediated by these spin fluctuations, can be strongly increased in $f$-electron-superlattices. This is in sharp contrast to superlattices composed of conventional $s$-wave superconductors, where superconductivity is generally suppressed.

cond-mat.supr-con↗

arXiv2015

Equilibrium Surface Current and Role of U(1) Symmetry: sum rule and surface perturbations

We discuss effects of surface perturbations on equilibrium surface currents which contribute to orbital magnetization and orbital angular momentum in systems without time reversal symmetry. We show that, in a U(1) particle number conserving system, disorder and other perturbations at a surface do not affect the equilibrium surface current and corresponding orbital magnetization due to a sum rule which is analogous to Luttinger's theorem. On the other hand, for a superfluid, the sum rule is no longer applicable and hence the surface mass current and corresponding orbital angular momentum can depend on details of a surface.

cond-mat.supr-con↗

arXiv2014

Orbital Angular Momentum and Spectral Flow in Two Dimensional Chiral Superfluids

We study the orbital angular momentum (OAM) $L_z$ in two dimensional chiral $(p_x+ip_y)^ν$-wave superfluids (SF) of $N$ fermions on a disc at zero temperature, in terms of spectral asymmetry and spectral flow. It is shown that $L_z=νN/2$ for any integer $ν$, in the BEC regime. In contrast, in the BCS limit, while the OAM is $L_z=N/2$ for the $p+ip$-wave SF, for chiral SF with $ν\geq2$, the OAM is remarkably suppressed as $L_z=N\times O(Δ_0/\varepsilon_F)\ll N$, where $Δ_0$ is the gap amplitude and $\varepsilon_F$ is the Fermi energy. We demonstrate that the difference between the $p+ip$-wave SF and the other chiral SFs in the BCS regimes originates from the nature of edge modes and related depairing effects.

cond-mat.supr-con↗

arXiv2014

Dimensional crossover in layered $f$-electron superlattices

Motivated by the remarkable experimental realizations of $f$-electron superlattices, e.g. CeIn$_3$/LaIn$_3$- and CeCoIn$_5$/YbCoIn$_5$- superlattices, we analyze the formation of heavy electrons in layered $f$-electron superlattices by means of the dynamical mean field theory. We show that the spectral function exhibits formation of heavy electrons in the entire system below a temperature scale $T_0$. However, in terms of transport, two different coherence temperatures $T_x$ and $T_z$ are identified in the in-plane- and the out-of-plane-resistivity, respectively. Remarkably, we find $T_z < T_x \sim T_0$ due to scatterings between different reduced Brillouin zones. The existence of these two distinct energy scales implies a crossover in the dimensionality of the heavy electrons between two and three dimensions as temperature or layer geometry is tuned. This dimensional crossover would be responsible for the characteristic behaviors in the magnetic and superconducting properties observed in the experiments.

cond-mat.str-el↗

arXiv2013

Kondo effect in f-electron superlattices

We demonstrate the importance of the Kondo effect in artificially created {\it f}-electron superlattices. We show that the Kondo effect does not only change the density of states of the {\it f}-electron layers, but is also the cause of pronounced resonances at the Fermi energy in the density of states of the non-interacting layers in the superlattice, which are between the {\it f}-electron layers. Remarkably, these resonances strongly depend on the structure of the superlattice; due to interference, the density of states at the Fermi energy can be strongly enhanced or even shows no changes at all. Furthermore, we show that by inserting the Kondo lattice layer into a three-dimensional (3D) metal, the gap of the Kondo insulating state changes from a full gap to a pseudo gap with quadratically vanishing spectral weight around the Fermi energy. Due to the formation of the Kondo insulating state in the {\it f}-electron layer, the superlattice becomes strongly anisotropic below the Kondo temperature. We prove this by calculating the in-plane and out-of-plane conductivity of the superlattice.

cond-mat.str-el↗

arXiv2010

Upper critical field in superconductors near ferromagnetic quantum critical points; UCoGe

We study the strong-coupling superconductivity near ferromagnetic quantum critical points, mainly focusing on the upper critical fields $H_{c2}$. Based on our simple model calculations, we discuss experimentally observed unusual behaviors of $H_{c2}$ in a recently discovered ferromagnetic superconductor UCoGe. Especially, the large anisotropy between $H_{c2}\parallel a$-axis and $H_{c2}\parallel c$-axis, and the strong-coupling behaviors in $H_{c2}^{\parallel a}$ are investigated. We also examine effects of non-analytic corrections in the spin susceptibility on the superconductivity, which can arise from effective long range interactions due to particle-hole excitations.

cond-mat.supr-con↗

arXiv2009

Pairing state at an interface of Sr$_2$RuO$_4$: parity-mixing, restored time-reversal symmetry, and topological superconductivity

We investigate pairing states realized at the (001) interface of a spin-triplet superconductor Sr$_2$RuO$_4$ on the basis of microscopic calculations. Because of a Rashba-type spin-orbit interaction induced at the interface, strong parity-mixing of Cooper pairs between a spin-singlet state and a spin-triplet state occurs in this system. There are also strong inter-band pair correlations between the spin-orbit split bands, in spite of the considerably large spin-orbit splitting. This is due to frustration between the spin-orbit interaction and pairing interactions. In this pairing state, time-reversal symmetry is restored, in contrast to the bulk Sr$_2$RuO$_4$ which is believed to be a chiral $p+ip$ superconductor with broken time-reversal symmetry. It is demonstrated that, because of these features, the pairing state at the interface is a promising candidate for the recently proposed time-reversal invariant topological superconductor.

cond-mat.supr-con↗

arXiv2007

Microscopic Mechanism and Pairing Symmetry of Superconductivity in the Noncentrosymmetric Heavy Fermion Systems CeRhSI$_3$ and CeIrSi$_3$

We study the pairing symmetry of the noncentrosymmetric heavy fermion superconductors CeRhSi$_3$ and CeIrSi$_3$ under pressures, which are both antiferromagnets at ambient pressure. We solve the Eliashberg equation by means of the random phase approximation and find that the mixed state of extended s-wave and p-wave rather than the $d+f$ wave state could be realized by enhanced antiferromagnetic spin fluctuations. It is elucidated that the gap function has line nodes on the Fermi surface and the resulting density of state in the superconducting state shows a similar character to that of usual d-wave superconductors, resulting in the NMR relaxation rate $1/(T_1T)$ that exhibits no coherence peak and behaves like $1/(T_1T)\propto T^2$ at low temperatures.

cond-mat.str-el↗