arXiv subjects

Ryoi Ohashi

Publications and source records attributed to Ryoi Ohashi.

9 recordsLinked to original sources

arXiv2026

Phase-shift instanton approach to tunneling duality in Read--Rezayi state

We study the duality between quasi-particle and electron tunneling in point-contact geometries of fractional quantum Hall states. To treat non-Abelian edge operators, we introduce a "phase-shift instanton" that incorporates phase factors from primary fields into the instanton gas framework. Using this method, we reformulate the Moore--Read duality and obtain an explicit dual description for the $k=3$ Read-Rezayi state. Our results clarify how quasi-particle tunneling produces characteristic phase shifts in instantons and how these shifts map strong quasi-particle tunneling to weak electron tunneling. Based on this dual description, we analytically evaluate the non-linear differential conductance in the strong-coupling regime. We reveal that, due to the physical requirement that the tunneling particle across the vacuum gap must be a true fermion, the transport behavior universally converges to a $G \propto V^4$ scaling for both the Moore--Read and Read--Rezayi states. This universal transport signature highlights a fundamental topological constraint underlying non-Abelian fractional quantum Hall edges.

cond-mat.mes-hall↗

arXiv2026

Universal Transport Theory for Paired Fractional Quantum Hall States in the Quantum Point Contact Geometry

Even-denominator fractional quantum Hall (FQH) states can be viewed as topological superconductors of composite fermions, supporting a charged chiral mode and $|\mathcal{C}_{cf}|$ neutral Majorana modes set by the Chern number $\mathcal{C}_{cf}$. Despite ongoing efforts, distinguishing the many competing paired phases remains an open problem. In this work, we propose a unified theory of charge transport across a quantum point contact (QPC) for general paired FQH states described by an $so(N)_1 \times u(1)$ conformal field theory. We derive the boundary effective action for an arbitrary number of Majorana fermions $N=|\mathcal{C}_{cf}|$ and develop a non-perturbative instanton approximation to describe tunneling processes. We establish a weak-strong duality relating strong quasiparticle tunneling to weak electron tunneling. We calculate the scaling dimensions of the tunneling operators and demonstrate that while the weak-coupling fixed point is generally unstable, the strong-coupling fixed point is stable for physically relevant filling fractions and number of Majorana fermions. These transport exponents provide a distinct experimental fingerprint to identify the topological phases of even-denominator FQH states.

cond-mat.mes-hall↗

arXiv2025

Magnetic penetration depth in topological superconductors: Effect of Majorana surface states and application for UTe$_2$

In this study, we examine how orbital degrees of freedom and Majorana surface states influence the magnetic penetration depth in the superconductor UTe$_2$. Using a two-orbital model, we analyze pairing states belonging to the irreducible representations of the $D_{2h}$ crystal symmetry: $A_u$, $B_{1u}$, $B_{2u}$, and $B_{3u}$. For bulk nodal states such as $B_{2u}$, we find that the penetration depth for screening currents along the antinodal direction and the cylindrical axis scales as $T^2$, in strong contrast to the conventional $T^4$ law. This behavior originates from quasiparticles near the point nodes contributing to the interorbital paramagnetic current. We further show that Majorana surface states can dominate the low-temperature response. The fully gapped $A_u$ state hosts Majorana cones, which produce a $T^3$ dependence of the penetration depth when the ratio of penetration depth to coherence length ($\kappa$) is small. In contrast, the other pairing states exhibit Majorana Fermi arcs: the exponent is $n=2$ along the dispersive direction, while along the dispersionless direction it depends on whether the arcs terminate at endpoints. The exponent $n=2$ in the dispersive direction is robust, while it in the dispersionless direction relies on the presence or absence of the endpoints of the arcs and deviates from $n=2$ when endpoints are absent. Our results demonstrate that penetration-depth measurements provide a direct probe of Majorana surface states in low-$\kappa$ superconductors. For larger $\kappa$, the surface contribution becomes negligible and the temperature dependence is governed by bulk quasiparticles.

cond-mat.supr-con↗

arXiv2024

Collective modes in Fulde-Ferrell-Larkin-Ovchinnikov superconductors: The role of long-range Coulomb interaction and signatures in density response

We theoretically investigate collective excitations in the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states of Pauli-limited superconducting films. When the long-range Coulomb interaction is absent, excitation spectra consist of two gapless and three gapped modes. The gapless modes are the Nambu-Goldstone modes associated with the spontaneous breaking of the ${\rm U}(1)$ symmetry and the translational symmetry. The gapped modes include the Higgs mode and the twofold degenerate modes that cause the oscillation of the domain width and grayness of FFLO nodal planes. We find that the long-range Coulomb interaction only gaps out the gapless phase mode through the Anderson-Higgs mechanism, while the other modes remain unaffected. Furthermore, the field evolution of the dispersion of the gapless elastic mode, the Nambu-Goldstone mode associated with the translational symmetry breaking, is associated with that of the bandwidth of the mid-gap Andreev bound states. We demonstrate that the signature of the elastic mode can be detected by measuring the density-density response function.

cond-mat.supr-con↗

arXiv2024

Anisotropic paramagnetic response of topological Majorana surface states in the superconductor $\text{UTe}_2$

Identifying the superconducting gap symmetry and topological signatures in the putative spin-triplet superconductor $\text{UTe}_2$ is an important issue. Especially, a smoking-gun detection scheme for Majorana surface states hallmarking topological superconductivity in $\text{UTe}_2$ is still lacking. In this study, we examine the surface spin susceptibility of $\text{UTe}_2$ with a particular focus on the contribution of the surface states. We find that Majorana surface states contribute significantly to the surface spin susceptibility, and give rise to an Ising-like anisotropy and anomalous enhancement in the surface spin susceptibility. We calculate the surface spin susceptibility as well as the local density of states using the recursive Green's function method and examine the anisotropy of the surface spin susceptibility in terms of the topological surface states and symmetry for all irreducible representations of odd-parity pairing states. Our results indicate that the Ising anisotropy and the anomalous enhancement are attributed to the Majorana surface state protected by the crystalline symmetry. These findings suggest the possibility of detecting the Majorana surface state via magnetic measurements.

cond-mat.supr-con↗

arXiv2024

Andreev-like Reflection in the Pfaffian Fractional Quantum Hall Effect

We studied the tunnel transport between the edge of a Pfaffian fractional quantum Hall state and that of an integer quantum Hall state. Based on the duality argument between the strong and weak tunnelings, we found that an Andreev-like reflection appeared in the strong tunneling regime. We calculated the charge conductance in the weak and strong tunneling regimes for the low-voltage limit. In the weak tunneling limit, $dI}/dV$ was proportional to $V^{1/ν}$ with bias voltage $V$ and $ν=1/2$. By contrast, in the strong tunneling limit, $dI/dV$ was expressed by $(e^{2}/h)2ν/(1+ν)$ with a correction term. We expect that this condition can be realized experimentally at the point contact between a fractional quantum Hall state with $ν=5/2$ and an integer quantum Hall state with $ν=3$.

cond-mat.mes-hall↗

arXiv2023

Surface density of states and tunneling spectroscopy of a spin-3/2 superconductor with Bogoliubov Fermi Surfaces

Bogoliubov Fermi surfaces of superconducting states arise from point or line nodes by breaking time-reversal symmetry. Because line and point nodes often accompany topologically protected zero-energy surface Andreev bound states (ASBSs) and thereby lead to a characteristic zero-bias conductance peak (ZBCP) in tunneling spectroscopy, we investigate how these properties change when the line and point nodes deform into BFSs. In this paper, we consider spin-quintet $J_{\rm pair}=2$ pairing states of spin-3/2 electrons with BFSs and calculate the surface density of states and the charge conductance. Comparing the obtained results with the cases of spin-singlet $d$-wave pairing states having the same symmetry, we find that the ZBCP associated with point and/or line nodes is blunted or split in accordance with the appearance of the BFSs. On the other hand, when the spin-singlet $d$-wave state has point nodes but does not have SABS on the surface, we obtain a nonzero small electron conductivity at zero bias through the zero-energy states on the BFSs.

cond-mat.supr-con↗

arXiv2021

Possible topological phases in quantum anomalous Hall insulator/unconventional superconductor hybrid systems

Quantum anomalous Hall insulator (QAH)/$s$-wave superconductor (SC) hybrid systems are known to be an ideal platform for realizing two-dimensional topological superconductors with chiral Majorana edge modes. In this paper we study QAH/unconventional SC hybrid systems whose pairing symmetry is $p$-wave, $d$-wave, chiral $p$-wave, or chiral $d$-wave. The hybrid systems are a generalization of the QAH/$s$-wave SC hybrid system. In view of symmetries of the QAH and pairings, we introduce three topological numbers to classify topological phases of the hybrid systems. One is the Chern number that characterizes chiral Majorana edge modes and the others are topological numbers associated with crystalline symmetries. We numerically calculate the topological numbers and associated surface states for three characteristic regimes that feature an influence of unconventional SCs on QAHs. Our calculation shows a rich variety of topological phases and unveils the following topological phases that are no counterpart of the $s$-wave case: crystalline symmetry-protected helical Majorana edge modes, a line node phase (crystalline-symmetry-protected Bogoliubov Fermi surface), and multiple chiral Majorana edge modes. The phenomena result from a nontrivial topological interplay between the QAH and unconventional SCs. Finally, we discuss tunnel conductance in a junction between a normal metal and the hybrid systems, and show that the chiral and helical Majorana edge modes are distinguishable in terms of the presence/absence of zero-bias conductance peak.

cond-mat.mes-hall↗

arXiv2020

Theory of Tunneling Effect in 1D AIII-class Topological Insulator (Nanowire) Proximity Coupled with a Superconductor

We study the tunneling effect in an AIII-class insulator proximity coupled with a spin-singlet $s$-wave superconductor, in which three phases are characterized by the integer topological invariant $\mathcal{N}$. By solving the Bogoliubov-de Gennes equation explicitly, we analytically obtain a normal reflection coefficient $R_{σσ'}$ and an Andreev reflection coefficient $A_{σσ'}$, and derive a charge conductance formula,where $σ(σ')$ is the spin index of a reflected (injected) wave. The resulting conductance indicates a wide variety of line shapes: (i)gap structure without coherence peaks for $\mathcal{N}=0$, (ii)quantized zero-bias conductance peak (ZBCP) with height $2e^{2}/h$ for $\mathcal{N}=1$, and (iii)ZBCP spitting for $\mathcal{N}=2$. At zero bias voltage $eV=0$, $\sum_{σσ'} R_{σσ'} = \sum_{σσ'} A_{σσ'}$ is satisfied and the spin direction of an injected electron is rotated at approximately $90^\circ$ for the $\mathcal{N}=1$ state. Meanwhile, $A_{σσ'}=0$ is satisfied for the $\mathcal{N}=2$ state, and the spin rotation angle can become $180^\circ$.

cond-mat.supr-con↗