arXiv subjects

Youichi Yanase

Publications and source records attributed to Youichi Yanase.

At least 55 records · Page 3Linked to original sources

arXiv2025

Vertex correction for the linear and nonlinear optical responses in superconductors: multiband effect and topological superconductivity

Intensive research has revealed intriguing optical responses in topological materials. This paper focuses on the optical responses in $s$-wave superconductors with a Rashba spin-orbit coupling and a magnetic field, one of the platforms of topological superconductivity. On the one hand, to satisfy some conservation laws in superconducting responses, it is essential to take into account collective excitation modes. On the other hand, the optical response is a promising phenomenon for detecting hidden collective modes in superconductors. In this paper, we investigate the effect of collective excitation modes on the linear and second-order optical responses based on the self-consistent response approximation, which is formulated using the Kadanoff-Baym method. Our main results reveal that the Higgs mode enhances the optical responses when the Fermi level is close to the Dirac point. The enhancement is due to the multiband effects characterized by interband pairing. We also demonstrate the sign reversal of the photocurrent conductivity around the topological transition with increasing the Zeeman field. This finding supports the prediction in our previous work without considering collective excitation modes [H. Tanaka, et al., Phys. Rev. B 110, 014520 (2024)]. The sign reversal phenomenon is attributed to the magnetic injection current modified by the Higgs mode, and is proposed for a bulk probe of topological superconductors. We also discuss the interplay of quantum geometry and collective modes.

cond-mat.supr-con↗

arXiv2025

Unidirectional superconductivity and superconducting diode effect induced by dissipation

A general principle of condensed matter physics prohibits the electric current in equilibrium. This prevents a zero-resistance state realized solely under a finite electric current, namely unidirectional superconductivity. In this paper, we propose a setup to realize the unidirectional superconductivity as a nonequilibrium steady state. We focus on the in-plane transport of atomically thin bilayer superconductors lacking the in-plane inversion symmetry and introduce dissipation by applying the out-of-plane electric field and current. By analyzing the time-dependent Ginzburg-Landau equations, we show that locally stable steady-state solutions appear only under the in-plane supercurrent when the out-of-plane electric field exceeds a threshold value. Our system also realizes the dissipation-induced superconducting diode effect up to 100% efficiency by purely electric means.

cond-mat.supr-con↗

arXiv2024

Quantum geometry encoded to pair potentials

Bloch wave functions of electrons have properties called quantum geometry, which has recently attracted much attention as the origin of intriguing physical phenomena. In this paper, we introduce the notion of the quantum-geometric pair potentials (QGPP) based on the generalized band representation and thereby clarify how the quantum geometry of electrons is transferred to the Cooper pairs they form. QGPP quantifies the deviation of multiband superconductors from an assembly of single-band superconductors and has a direct connection to the quantum-geometric corrections to thermodynamic coefficients. We also discuss their potential ability to emulate exotic pair potentials and engineer intriguing superconducting phenomena including topological superconductivity.

cond-mat.supr-con↗

arXiv2024

Correlation-induced Fermi surface evolution and topological crystalline superconductivity in CeRh2As2

Locally noncentrosymmetric structures in crystals are attracting much attention owing to emergent phenomena associated with the sublattice degree of freedom. The newly discovered heavy fermion superconductor CeRh$_2$As$_2$ is considered to be an excellent realization of this class. Angle-resolved photoemission spectroscopy experiments recently observed low-energy spectra of electron and hole bands and characteristic Van Hove singularities, stimulating us to explore the electronic correlation effect on the band structure. In this Letter, we theoretically study the electronic state and topological superconductivity from first principles. Owing to the Coulomb repulsion $U$ of Ce 4$f$ electrons, the low-energy band structure is modified in accordance with the experimental result. We show that Fermi surfaces change significantly from a complicated three-dimensional structure to a simple two-dimensional one. Fermi surface formulas for one-dimensional $\mathbb{Z}_2$ invariants in class D indicate topological crystalline superconductivity protected by the glide symmetry in a broad region for $U$. The classification of superconducting gap structure reveals topologically protected excitation gap and node. Our findings of the correlation-induced evolution of electronic structure provide a basis to clarify the unusual phase diagram of CeRh$_2$As$_2$ including superconductivity, magnetic order, and quadrupole density wave, and accelerate the search for topological superconductivity in strongly correlated electron systems.

cond-mat.supr-con↗

arXiv2024

Magnetic parity violation and parity-time-reversal-symmetric magnets

Parity-time-reversal symmetry ($\mathcal{PT}$ symmetry), a symmetry for the combined operations of space inversion ($\mathcal{P}$) and time reversal ($\mathcal{T}$), is a fundamental concept of physics and characterizes the functionality of materials as well as $\mathcal{P}$ and $\mathcal{T}$ symmetries. In particular, the $\mathcal{PT}$-symmetric systems can be found in the centrosymmetric crystals undergoing the parity-violating magnetic order which we call the odd-parity magnetic multipole order. While this spontaneous order leaves $\mathcal{PT}$ symmetry intact, the simultaneous violation of $\mathcal{P}$ and $\mathcal{T}$ symmetries gives rise to various emergent responses that are qualitatively different from those allowed by the nonmagnetic $\mathcal{P}$-symmetry breaking or by the ferromagnetic order. In this review, we introduce candidates hosting the intriguing spontaneous order and overview the characteristic physical responses. Various off-diagonal and/or nonreciprocal responses are identified, which are closely related to the unusual electronic structures such as hidden spin-momentum locking and asymmetric band dispersion.

cond-mat.mtrl-sci↗

arXiv2024

Twisted Bogoliubov quasiparticles in the superconducting NbSe$_2$ monolayer on graphene

The superconducting properties of layered materials can be controlled by thinning, stacking, and twisting, demanding investigation of electronic states by spectroscopic means at the nanometer scale. Here, we reveal the spatial variations of the electronic states in heterostructures of the superconducting monolayer NbSe$_2$/graphene using spectroscopic-imaging scanning tunneling microscopy. The NbSe$_2$ monolayer grown by molecular beam epitaxy is naturally twisted with respect to the graphene substrate and exhibits interference patterns of Bogoliubov quasiparticles twisted with respect to the NbSe$_2$ and graphene lattices. We find that the twisted interference patterns originate from a sextet of regions in momentum space where the Fermi surfaces of NbSe$_2$ and graphene overlap. The Fermi surface overlap is sensitive to the twist angle, providing a knob to tune superconductivity.

cond-mat.supr-con↗

arXiv2024

Tuning monolayer superconductivity in twisted NbSe$_2$ graphene heterostructures

The recent advent of artificial structures has triggered the emergence of fascinating phenomena that could not exist in natural compounds. A prime example is twisted multilayers, i.e., moiré superlattices represented by magic-angle twisted bilayer graphene (MATBG). As in the case of MATBG, unconventional band hybridization can induce a new type of superconductivity: artificial band engineering by twist induces properties different from the original systems. Here, we apply this perspective to a monolayer superconductor NbSe$_2$ stacked with a twist on doped graphene. We show that the superconducting states of the NbSe$_2$ layer change dramatically by varying the twist angle. Our result shows that twist tuning, in addition to substrate effects, will provide a strategy for designing monolayer superconductors with high controllability.

cond-mat.supr-con↗

arXiv2024

Superconducting meron phase in locally noncentrosymmetric superconductors

Theory of the superconducting parity transition is extended by incorporating the vortex degree of freedom. We employ the bilayer Rashba model representing locally noncentrosymmetric layered superconductors and derive the Ginzburg-Landau free energy functional. This formulation reveals the parity transition, where the even-parity superconducting state changes to the odd-parity one upon increasing the magnetic field under the vortex states. The H-T phase diagram of CeRh${}_{2}$As${}_{2}$ is quantitatively reproduced and a novel superconducting state with a meron (half-skyrmion) lattice pseudospin texture is predicted.

cond-mat.supr-con↗

arXiv2024

Rectification and nonlinear Hall effect by fluctuating finite-momentum Cooper pairs

Nonreciprocal charge transport is attracting much attention as a novel probe and functionality of noncentrosymmetric superconductors. In this work, we show that both the longitudinal and transverse nonlinear paraconductivity are hugely enhanced in helical superconductors in moderate and high magnetic fields, which can be observed by second-harmonic resistance measurements. The discussion is based on the generalized formulation of nonlinear paraconductivity in combination with the microscopically determined Ginzburg-Landau coefficients. The enhanced nonreciprocal transport would be observable even with the cyclotron motion of fluctuating Cooper pairs, which is elucidated with a Kubo-type formula of nonlinear paraconductivity. Nonreciprocal charge transport in the fluctuation regime is thereby established as a promising probe of helical superconductivity regardless of the sample dimensionality. Implications on the other finite-momentum superconducting states are briefly discussed.

cond-mat.supr-con↗

arXiv2024

Nonlinear optical responses in superconductors under magnetic fields: quantum geometry and topological superconductivity

Noncentrosymmetric superconductors offer fascinating phenomena of quantum transport and optics such as nonreciprocal and nonlinear responses. Time-reversal symmetry breaking often plays an essential role in the emergence and enhancement of nonreciprocal transport. In this paper, we show the nonreciprocal optical responses in noncentrosymmetric superconductors arising from time-reversal symmetry breaking by demonstrating them in $s$-wave superconductors with a Rashba spin-orbit coupling and a magnetic field. Numerical results reveal the superconductivity-induced bulk photocurrent and second harmonic generation, which are forbidden at the zero magnetic field. We discuss the properties and mechanisms of the superconducting nonlinear responses emerging under the magnetic field. In particular, we investigate the magnetic-field dependence of the photocurrent conductivity and clarify the essential ingredients which give a contribution unique to superconductors under the magnetic field. This contribution is dominant in the low carrier density regime although the corresponding joint density of states is tiny. We attribute the enhancement to the quantum geometry. Moreover, the nonlinear conductivity shows peculiar sign reversal at the transition to the topological superconducting state. We propose a bulk probe of topological transition and quantum geometry in superconductors.

cond-mat.supr-con↗

arXiv2024

Chiral superconducting diode effect by Dzyaloshinsky-Moriya interaction

A two-component quasi-two-dimensional superconductor with Dzyaloshinsky-Moriya interaction is studied based on the Ginzburg-Landau and Bogoliubov-de Gennes theories. Under external in-plane magnetic fields, the order parameter of the superconducting state is a type of the Fulde-Ferrell state with a finite momentum of Cooper pairs due to the Dzyaloshinsky-Moriya interaction. It is shown that the superconducting diode effect can emerge when a supercurrent flows parallel to the external magnetic field, characteristic of chiral crystals. In the Bogoliubov-de Gennes theory, phase diagrams associated with the transition of the Cooper-pair momentum and the Josephson phase between spin-singlet and spin-triplet Cooper pairs are derived, and a close relationship with the diode quality factor is demonstrated. Implications of critical currents in the aspect of thermodynamics are also discussed. Based on such an argument, it is argued that the first-order phase transition in terms of Cooper-pair momentum and the coexistence of phases with different Cooper-pair momentum and Josephson phase can occur. The argument also implies the issue with the definition of critical currents calculated from the extremes of the supercurrent when metastable states exist. Comments on purely two-dimensional superconductors are also given.

cond-mat.supr-con↗

arXiv2023

Field-induced superconductivity mediated by odd-parity multipole fluctuation

Field-induced superconductivity has long presented a counterintuitive phenomenon and a pivotal challenge in condensed matter physics. In this Letter, we introduce a mechanism for achieving field-induced superconductivity wherein the sublattice degree of freedom and the Coulomb interaction are tightly entwined. Our multipole-resolved analysis elucidates that lifting the fluctuation degeneracy results in an unconventional Cooper pairing channel, thereby realizing field-induced superconductivity. This research substantively augments the exploration of the latent potential of strongly correlated electron systems with sublattice degrees of freedom.

cond-mat.supr-con↗

arXiv2023

Spin-triplet superconductivity from quantum-geometry-induced ferromagnetic fluctuation

We show that quantum geometry induces ferromagnetic fluctuation resulting in spin-triplet superconductivity. The criterion for ferromagnetic fluctuation is clarified by analyzing contributions from the effective mass and quantum geometry. When the non-Kramers band degeneracy is present near the Fermi surface, the Fubini-Study quantum metric strongly favors ferromagnetic fluctuation. Solving the linearized gap equation with the effective interaction obtained by the random phase approximation, we show that the spin-triplet superconductivity is mediated by quantum-geometry-induced ferromagnetic fluctuation.

cond-mat.supr-con↗

arXiv2023

Coexistence of near-EF flat band and van Hove singularity in a two-phase superconductor

In quantum many-body systems, particularly, the ones with large near-EF density states, like flat bands or van Hove singularity (VHS), electron correlations often give rise to rich phase diagrams with multiple coexisting/competing orders occurring at similar energy scales. The recently discovered locally noncentrosymmetric heavy fermion superconductor CeRh2As2 has stimulated extensive attention due to its unusual H-T phase diagram, consisting of two-phase superconductivity, antiferromagnetic order, and possible quadrupole-density wave orders. However, despite its great importance, the near-EF electronic structure remains experimentally elusive. Here, we provide this key information by combining soft X-ray and vacuum ultraviolet (VUV) angle-resolved photoemission spectroscopy measurements and atom-resolved DFT+U calculations. With bulk-sensitive soft X-rays, we reveal quasi-2D hole- and 3D electron- pockets with a pronounced nesting feature. Most importantly, we observe a symmetry-protected fourfold VHS coexisting with the Ce 4f flat bands near the EF, which, to the best of our knowledge, has never been reported before. Such a rare coexistence is expected to lead to a large density of states at the zone edge, enhancement in electron correlations, and a large upper critical field of the odd-parity superconducting phase. Uniquely, it will also result in a new type of f-VHS hybridization that alters the order and fine electronic structure of the symmetry-protected VHS and flat bands. These peculiarities offer important dimensions for understanding the reported rich phase diagram and are discussed as an origin of superconductivity with two phases. Our findings not only provide key insights into the nature of multiple phases in CeRh$_2$As$_2$, but also open up new prospects for exploring the novelties of many-body systems with f-VHS hybridization.

cond-mat.str-el↗

arXiv2023

Magnetism and superconductivity in mixed-dimensional periodic Anderson model for UTe$_{2}$

UTe$_{2}$ is a strong candidate for a topological spin-triplet superconductor, and it is considered that the interplay of magnetic fluctuation and superconductivity is essential for the origin of the superconductivity. Despite various experiments suggesting ferromagnetic criticality, neutron scattering measurements observed only antiferromagnetic fluctuation and called for theories of spin-triplet superconductivity near the antiferromagnetic quantum critical point. We construct a periodic Anderson model with one-dimensional conduction electrons and two- or three-dimensional $f$-electrons, reminiscent of the band structure of UTe$_2$, and show that ferromagnetic and antiferromagnetic fluctuations are reproduced depending on the Fermi surface of $f$ electrons. These magnetic fluctuations cooperatively stabilize spin-triplet $p$-wave superconductivity. We also study hybridization dependence as a possible origin of pressure-induced superconducting phases and find that moderately large hybridization drastically changes the antiferromagnetic wave vector and stabilizes $d$-wave superconductivity.

cond-mat.supr-con↗

arXiv2023

Orbital effect on intrinsic superconducting diode effect

Much ink has recently been spilled on nonreciprocal phenomena in superconductors, especially the superconducting diode effect (SDE) characterized by the nonreciprocity of the critical current $ΔJ_c$. Contrary to the fundamental and practical significance of the SDE, the precise underlying mechanism remains unclear. In this paper, we investigate the impact of an orbital effect on the intrinsic SDE in a bilayer superconductor with Rashba spin-orbit coupling and an in-plane magnetic field. We show that a small orbital effect leads to the sign reversal of $ΔJ_c$ and a crossover of the helical superconducting state at a lower magnetic field than the monolayer superconductor. On the other hand, a large orbital effect induces a decoupling transition, stabilizing a finite momentum Cooper pairing state called the orbital Fulde-Ferrell-Larkin-Ovchinnikov state, and results in the drastic change of the SDE. Owing to the orbital effect, the field dependence of the SDE may show oscillations several times. The results shed light on the mechanism of the SDE in atomically-thin multilayer superconductors.

cond-mat.supr-con↗

arXiv2023

de Haas-van Alphen Oscillations for the Field Along c-axis in UTe2

We performed de Haas-van Alphen (dHvA) experiments in the spin-triplet superconductor UTe2 for magnetic field along the c-axis above 15T. Three fundamental dHvA frequencies, named alpha1, alpha2 and beta corresponding to the cross sections of cylindrical Fermi surfaces (FSs) with large cyclotron effective masses (33-43 m0) were detected. No other fundamental dHvA frequencies were detected at high frequency range, suggesting a cylindrical-shaped electron FS without connecting at the Z point of the Brillouin zone. However, the existence of small pocket FSs associated with extremely heavy masses cannot be fully excluded.

cond-mat.str-el↗

arXiv2023

Quantum-geometry-induced anapole superconductivity

Anapole superconductivity recently proposed for multiband superconductors Commun. Phys. $\bm 5$, 39\ (2022) (at https://www.nature.com/articles/s42005-022-00804-7) is a key feature of time-reversal ($\mathcal{T}$)-symmetry-broken polar superconductors. The anapole moment was shown to arise from the asymmetric Bogoliubov spectrum, which induces a finite center of mass momenta of Cooper pairs at the zero magnetic field. In this paper, we show an alternative mechanism of anapole superconductivity: the quantum geometry induces the anapole moment when the interband pairing and Berry connection are finite. Thus, the anapole superconductivity is a ubiquitous feature of $\mathcal{T}$-broken multiband polar superconductors. Applying the theory to a minimal model of UTe$_2$, we demonstrate the quantum-geometry-induced anapole superconductivity. Furthermore, we show the Bogoliubov Fermi surfaces (BFS) in an anapole superconducting state and predict an unusual temperature dependence of BFS due to the quantum geometry. Experimental verification of these phenomena may clarify the superconducting state in UTe$_2$ and reveal the ubiquitous importance of quantum geometry in exotic superconductors.

cond-mat.supr-con↗