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Yuki Yamazaki

Publications and source records attributed to Yuki Yamazaki.

12 recordsLinked to original sources
arXiv2026

Majorana-assisted nonlocal spin correlation in quasi-one-dimensional Kitaev spin liquids

We propose Majorana-assisted nonlocal spin correlation as a manifestation of Majorana nonlocality in quasi-one-dimensional (1D) Kitaev spin liquids. Focusing on the flux-free sector of the Kitaev honeycomb model in a quasi-1D geometry, we uncover its topological nature and show that it hosts Majorana zero modes localized at both ends, which are stabilized by finite-size-induced topology. We further show that the nonlocal Majorana fermion parity operator, $P_{\text{MF}}=i\gamma_{\text{L}}\gamma_{\text{R}}$, is mapped to a nonlocal spin-string operator, producing an end-to-end spin correlation proportional to the product of $P_{\text{MF}}$ and total fermion parity operators when local perturbations remove redundant ground-state degeneracies while preserving the Majorana and total fermion parities in the flux-free sector. Numerical calculations confirm a finite nonlocal spin correlation generated by these Majorana zero modes without any local magnetization. Our results establish a concrete signature of intrinsic Majorana nonlocality in quantum spin liquids.

cond-mat.str-el↗
arXiv2026

Raman response of collective modes in multicomponent superconductors

We formulate a microscopic theory of the Raman response of superconducting collective modes in multicomponent superconductors. Starting from a general Bogoliubov--de Gennes (BdG) Hamiltonian with a separable pairing interaction, we derive a gauge-invariant expression for the Raman susceptibility, including a long-range Coulomb interaction. The resulting Raman susceptibility is directly computable for an arbitrary BdG Hamiltonian, which contains single- and multiband systems, spin-singlet and triplet order parameters, and time-reversal-symmetric and time-reversal-symmetry-breaking superconducting states. Based on the microscopic coupling between a Raman source field and collective modes, we derive a symmetry selection rule for Raman-active collective modes and show a group-theoretical classification for all crystalline point groups. This classification provides a unified framework based on the ``higher-order Lifshitz-invariant'' to identify Raman-active collective modes such as Leggett mode, Bardasis-Schrieffer (BS) mode, and clapping mode. As an application, we focus on an effective model of the heavy-fermion superconductor UTe$_2$ with a fully gapped multicomponent odd-parity pairing state. We find sharp in-gap Raman resonances below the quasiparticle continuum, which do not correspond to a conventional Leggett mode but arise from the {\it intraband} relative modes between different pairing components.

cond-mat.supr-con↗
arXiv2025

Higher-order topological phases for time-reversal-symmetry breaking superconductivity in UTe$_2$

The recent discovery of heavy-fermion superconductor UTe$_2$ has broadened the possibility of realizing exotic time-reversal-symmetry-breaking superconductivity. However, a comprehensive understanding of the topological phases in the superconducting states of UTe$_2$ is still lacking. Here, we present an exhaustive classification of topological phases for all time-reversal symmetry breaking pairing symmetries of UTe$_2$. Using the K theoretical classification approach, we uncover that 25 out of 36 possible pairing states are classified as higher-order topological phases, with some demonstrating hybrid-order topology through an intricate interplay of hinge and corner states. Furthermore, under the weak-coupling condition of the pair potentials, the possible pairing symmetries are constrained to $B_{ju} + i B_{ku}$, $A_{u} + i B_{j u}$, and $B_{j g} + iA_u$ ($j,k = 1,2,3$; $j \neq k$), where these symbols denote the irreducible representations of the point group $D_{2h}$. For these pairing states, the topological invariants are related to the Fermi surface topology via the Fermi-surface formula, enabling us to systematically diagnose higher-order topological phases. Using a tight-binding model, we demonstrate the higher-order topological phases of the mixed-parity $A_u + iB_{1g}$ superconductors, where the second-order and hybrid-order topological phases emerge as the number of Fermi surfaces enclosing the time-reversal invariant momentum evolves from two to four. The findings suggest that UTe$_2$ serves as a compelling platform for exploring higher-order topological superconductors with diverse topological surface states.

cond-mat.supr-con↗
arXiv2024

Majorana multipole response with magnetic point group symmetry

Majorana fermions (MFs) in a topological superconductor exhibit anisotropic electromagnetic responses, called Majorana multipole responses, when MFs are degenerate under time-reversal and crystalline symmetries. In time-reversal symmetric systems, the Majorana multipole response relates to Cooper pair symmetry in the underlying superconducting material, which provides a way to identify pairing symmetries through surface-spin-sensitive measurements. Here, we extend the concept of Majorana multipole response to systems with magnetic point group symmetry that break time-reversal symmetry and clarify how the response of MFs includes information about underlying superconductors. From a topological classification of symmetry-protected MFs and an effective surface theory, we classify possible magnetic and electric responses for MFs, which manifests a direct connection to Cooper pair symmetry for a symmetry-enforced pair of MFs. Additionally, we find several time-reversal-even higher-order multipole responses, such as the quadrupole response, which are forbidden in time-reversal symmetric systems, whereby indicating breaking of time-reversal symmetry. The theory is applied to the odd-parity chiral superconductor UTe$_2$ and the ferromagnetic superconductor UCoGe, demonstrating the appearance of a magnetic quadrupole response on a surface.

cond-mat.supr-con↗
arXiv2023

Majorana Spin Current Generation by Dynamic Strain

Majorana fermions that emerge on the surface of topological superconductors are charge neutral but can have higher-rank electric multipoles by allowing for account time-reversal and crystalline symmetries. Applying the general classification of these multipoles, we show that the spin current of Majorana fermions is driven by spatially nonuniform dynamic strains on the (001) surface of superconducting antiperovskite Sr3SnO. We also find that the frequency dependence of the Majorana spin current reflects the energy dispersion of Majorana fermions. Our results suggest that the spin current can be a probe for Majorana fermions.

cond-mat.supr-con↗
arXiv2021

Majorana multipole response: General theory and application to wallpaper groups

Whereas identification of Cooper pair symmetry is the first and crucial step in the investigation of unconventional superconductors, only a few have been established so far because of its own difficulties. To solve this problem, we develop a theory for identification of pairing symmetry using knowledge of topological superconductivity. Establishing the multipole theory of emergent Majorana fermions in time-reversal-invariant topological superconductors, we discover a one-to-one correspondence between the electromagnetic response of Majorana fermions and Cooper pair symmetry. The emergent Majorana fermions host magnetic structures that share the same irreducible representation with Cooper pairs under crystalline symmetry. We furthermore reveal that Majorana fermions in high-spin or nonsymmorphic superconductors may exhibit magnetic octupole responses, which give a direct evidence of these exotic superconducting states. Electric responses of multiple Majorana Kramers pairs are also clarified. Our theory provides the fundamentals for identification of unconventional Cooper pairings through surface-spin-sensitive measurements as well as that for manipulation of Majorana fermions by external electromagnetic fields.

cond-mat.supr-con↗
arXiv2021

Electric Multipoles of Double Majorana Kramers Pairs

A single Majorana Kramers pair hosts only one component of the magnetic multipole. This can be used to determine the bulk Cooper-pair symmetry through surface-sensitive spectroscopic measurements, either by applying a magnetic field or by using a ferromagnet/superconductor junction. This paper proposes that the electric response, which is free from the Meissner effect, can be used an alternative method to measure the bulk Cooper-pair symmetry in time-reversal-invariant superconductors with double Majorana Kramers pairs. The relationships among electric multipoles, strain tensors and superconducting symmetries under a given wallpaper group on the surfaces of topological crystalline superconductors are shown. This study also reveals that only a specific irreducible representation of a uniform strain yields a gap in the double Majorana Kramers pairs for the topological-crystalline-superconductor candidate Sr$_3$SnO. This highlights the viability of electric detection regarding Cooper-pair symmetry.

cond-mat.supr-con↗
arXiv2021

Magnetic response of Majorana Kramers pairs with an order-two symmetry

We study an intrinsic relation between the topology of bulk electronic states and magnetic responses of Majorana Kramers pairs, Kramers pairs of Majorana fermions, on a surface of time-reversal-invariant topological superconductors. Majorana Kramers pairs respond to an applied magnetic field anisotropically due to the interplay between time-reversal and crystal symmetries. In this paper, we propose a systematic procedure to determine such surface magnetic responses in systems with an order-two symmetry. From the analysis of topological invariants associated with an order-two symmetry, it is found that magnetic responses are classified into four types, which are attributed to different topological invariants and exhibit distinguishable, characteristic magnetic responses. For a Kramers pair of Majorana fermions protected by $\mathbb{Z}_2$ topological invariants, we clarify that types of magnetic responses are determined only from Fermi--surface topology and symmetry of Cooper pairs. Finally, we apply our theory to the topological nonsymmorphic crystalline superconducting state in UCoGe, which exhibits a biaxially anisotropic magnetic response.

cond-mat.supr-con↗
arXiv2020

Magnetic Response of Majorana Kramers Pairs Protected by Z2 Invariants

On the surface of time-reversal-invariant topological superconductors, Kramers pairs of Majorana fermions with chiral and crystalline symmetries exhibit completely uniaxial or octupole anisotropic magnetic response. This paper reports possible types of magnetic responses of Majorana Kramers pairs with one-dimensional Z2 invariants defined by crystalline symmetry. In particular, the general theory predicts a new type of magnetic response where two Majorana Kramers pairs associated with the Z2 invariant show biaxially (quadrupolar) anisotropic magnetic response, which is a novel type of response that is rarely observed in conventional and Majorana fermions.

cond-mat.supr-con↗
arXiv2000

de Broglie-Bohm Interpretatin for Analytic Solutions of The Wheeler-DeWitt Equation in Spherically Symmetric Space-time

We discuss the implications of a wave function for quantum gravity, which involves nothing but 3-dimensional geometries as arguments and is invariant under general coordinate transformations. We derive an analytic wave function from the Wheeler-DeWitt equation for spherically symmetric space-time with the coordinate system arbitrary. The de Broglie-Bohm interpretation of quantum mechanics is applied to the wave function. In this interpretation, deterministic dynamics can be yielded from a wave function in fully quantum regions as well as in semiclassical ones. By introducing a coordinate system additionally, we obtain a cosmological black hole picture in compensation for the loss of general covariance. Our analysis shows that the de Broglie-Bohm interpretation gives quantum gravity an appropriate prescription to introduce coordinate systems naturally and extract information from a wave function as a result of breaking general covariance.

gr-qc↗
arXiv1998

de Broglie-Bohm interpretation for wave function of Reissner-Nordstrom-de Sitter black hole

We study the canonical quantum theory of the Reissner-Nordstrom-de Sitter black hole(RNdS). We obtain an exact general solution of the Wheeler-DeWitt equation for the spherically symmetric geometry with electro-magnetic field. We investigate the wave function form a viewpoint of the de Broglie-Bohm interpretation. The de Broglie-Bohm interpretation introduces a rigid trajectory on the minisuperspace without assuming an outside observer or causing collapse of the wave function. In our analysis, we obtain the boundary condition for the wave function which corresponds to the classical RNdS black hole and describe the quantum fluctuations near the horizons quantitatively.

gr-qc↗