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Jun Goryo

Publications and source records attributed to Jun Goryo.

At least 19 recordsLinked to original sources
arXiv2026

Possible Pairing Symmetry of BaPtAs$_{1-x}$Sb$_{x}$ with an Ordered Honeycomb Network

We investigate the possible pairing symmetry of superconducting $\rm{BaPtAs}_{1-\it{x}}\rm{Sb}_{\it{x}}$ solid solution with an ordered-honeycomb network of Pt and pnictogens. A spontaneous internal magnetic field below the superconducting transition temperature is observed in BaPtSb ($x = 1$) via the muon-spin relaxation measurement. We then pursue a scenario where the pairing symmetry is changed from a time-reversal symmetry-breaking (TRSB) state to another one by changing the Sb-concentration utilizing the effective tight-binding model obtained from the first principles calculations for $x = 0$ and $x = 1$, at which we see a significant difference in the shape of the dominant Fermi surfaces. We find that the chiral $d$-wave state with TRSB is most stable at $x = 1$, whereas the nodal $f$-wave or the conventional $s$-wave states without TRSB are competitive at $x = 0$.

cond-mat.supr-con↗
arXiv2026

Interband pairing in two-band superconductors with spin-orbit and Zeeman couplings

Interband pairing in multiband superconductors is often neglected because of its higher energetic cost compared with intraband pairing. We show that, in multiband systems, a Zeeman magnetic field can stabilize interband pairing through the near degeneracy of spin-split branches from different bands, even within a minimal on-site attractive interaction. Using hexagonal tight-binding models with locally broken inversion symmetry, we find a Zeeman-driven transition between a conventional intraband s-wave state and an interband-dominated superconducting Mixing state. The resulting quasiparticle spectrum is intrinsically gapless, leading to anomalous thermodynamic behavior, including a T-linear specific heat at low temperatures, reflecting a finite zero-energy density of states.

cond-mat.supr-con↗
arXiv2025

Reentrant Superconductivity from Competing Spin-Triplet Instabilities

Reentrant superconductivity in strong magnetic fields challenges the conventional expectation that magnetic fields necessarily suppress superconductivity. We show that reentrant superconducting instability can arise from the competition between spin-unpolarized and spin-polarized superconducting channels. Using a minimal Ginzburg--Landau theory with two coupled spin-triplet order parameters, we demonstrate that a magnetic field can reorganize the hierarchy of superconducting instabilities, yielding a characteristic reentrant instability curve over a broad parameter range.

cond-mat.supr-con↗
arXiv2023

Study on Susceptibilities of Superconductors BaPtSb and BaPtAs with Honeycomb Structure

The low-energy electronic properties of the new superconductors BaPtSb and BaPtAs with an ordered honeycomb network are investigated in the normal phase, where the former compound is a candidate for time-reversal symmetry-breaking superconductors. By means of the first-principles calculation, we show that there exist two-dimensional cylinder-like Fermi surfaces around the kz axis and one outer spherelike Fermi surface around the K and K' points in both compounds, which are mainly composed of Pt 5d and Sb 5p/As 4p electrons. We construct low-energy effective models, which are well described by using three bands consisting of two Pt 5d orbitals and one Sb 5p/As 4p orbital. By evaluating susceptibilities using effective models, we find that dominant contributions to those susceptibilities result from the outer spherelike Fermi surface. Whereas out-of-plane fluctuations are enhanced in both compounds in the higher-temperature region, in-plane fluctuations become dominant in the very low-temperature region in BaPtSb owing to a better nesting condition in the k_z=0 plane from the outer spherelike Fermi surface. These fluctuations yield instabilities to ordered states, such as superconductivity, and might be associated with the occurrence of the superconducting state with time-reversal symmetry breaking in BaPtSb.

cond-mat.supr-con↗
arXiv2020

Phenomenology of the chiral $d$-wave state in the hexagonal pnictide superconductor SrPtAs

The pairing symmetry of the hexagonal pnictide superconductor SrPtAs is discussed with taking into account its multiband structure. The topological chiral $d$-wave state with time-reversal-symmetry breaking has been anticipated from the spontaneous magnetization observed by the muon-spin-relaxation experiment. We point out in this paper that the recent experimental reports on the nuclear-spin-lattice relaxation rate $T_1^{-1}$ and superfluid density $n_s(T)$, which seemingly support the conventional $s$-wave pairing, are also consistent with the chiral $d$-wave state. The compatibility of the gap and multiband structures is crucial in this argument. We propose that the measurement of the bulk quasiparticle density of states would be useful for the distinction between two pairing states.

cond-mat.supr-con↗
arXiv2019

Possibility of chiral $d$-wave state in the hexagonal pnictide superconductor SrPtAs

We discuss the type of pairing in the hexagonal pnictide superconductor SrPtAs, taking into account its multiband structure. The topological chiral $d$-wave state with time-reversal-symmetry breaking has been anticipated from the spontaneous magnetization observed by the muon-spin-relaxation experiment. We point out in this paper that the recent experimental reports on the nuclear-spin-lattice relaxation rate $T_1^{-1}$ and superfluid density $n_s(T)$, which seemingly support the conventional $s$-wave pairing, are also consistent with the chiral $d$-wave state. The compatibility of the gap and multiband structures is crucial in this argument.

cond-mat.supr-con↗
arXiv2017

Surface magnetism in a chiral d-wave superconductor with hexagonal symmetry

Surface properties are examined in a chiral d-wave superconductor with hexagonal symmetry, whose one-body Hamiltonian possesses the intrinsic spin-orbit coupling identical to the one characterizing the topological nature of the Kane-Mele honeycomb insulator. In the normal state spin-orbit coupling gives rise to spontaneous surface spin currents, whereas in the superconducting state there exist besides the spin currents also charge surface currents, due to the chiral pairing symmetry. Interestingly, the combination of these two currents results in a surface spin polarization, whose spatial dependence is markedly different on the zigzag and armchair surfaces. We discuss various potential candidate materials, such as SrPtAs, which may exhibit these surface properties.

cond-mat.supr-con↗
arXiv2017

Symmetry and Gap Classification of the non-symmorphic SrPtAs

The hexagonal superconductor SrPtAs exhibits time-reversal-symmetry breaking below $T_c$, hinting at an unconventional pairing state. Therefore, the symmetry of the underlying crystal is important for the classification of possible gap structures and their mixing. Here, we use the generating point group D$_{6h}$ of SrPtAs for a comprehensive classification of the gap functions and to construct a tight-binding model. Our work clarifies questions of symmetry and topology in this non-symmorphic material and allows for better comparison with other hexagonal systems.

cond-mat.supr-con↗
arXiv2016

Cooperon condensation and intra-valley pairing states in honeycomb Dirac systems

Motivated by recent developments in the experimental study of superconducting graphene and transition metal dichalcogenides, we investigate superconductivity of the Kane-Mele (KM) model with short-range attractive interactions on the two-dimensional honeycomb lattice. We show that intra-valley spin-triplet pairing arises from nearest-neighbor (NN) attractive interaction and the intrinsic spin-orbit coupling. We demonstrate this in two independent approaches: We study superconducting instability driven by condensation of Cooperons, which are in-gap bound states of two conduction electrons, within the $T$-matrix approximation and also study the superconducting ground state within the mean-field theory. We find that Cooperons with antiparallel spins condense at the $K$ and $K'$ points. This leads to the emergence of an intra-valley spin-triplet pairing state belonging to the irreducible representation A$_1$ of the point group $C_{6v}$. The fact that this pairing state has opposite chirality for $K$ and $K'$ identifies this state as a "helical" valley-triplet state, the valley-analog to the $^3$He-B phase in two dimension. Because of the finite center of mass momentum of Cooper pairs, the pair amplitude in NN bonds exhibits spatial modulation on the length scale of lattice constant, such that this pairing state may be viewed as a pair-density wave state. We find that the pair amplitude spontaneously breaks the translational symmetry and exhibits a $p$-Kekulé pattern. We also discuss the selection rule for pairing states focusing the characteristic band structure of the KM model and the Berry phase effects to the emergence of the intra-valley pairing state.

cond-mat.supr-con↗
arXiv2014

Chiral $d$-wave Superconductivity in SrPtAs

Recent $μ$SR measurements on SrPtAs revealed time-reversal-symmetry breaking with the onset of superconductivity [Biswas et al., Phys. Rev. B 87, 180503(R) (2013)], suggesting an unconventional superconducting state. We investigate this possibility via functional renormalization group and find a chiral $(d+\mathrm{i}d)$-wave order parameter favored by the multiband fermiology and hexagonal symmetry of SrPtAs. This $(d+\mathrm{i}d)$-wave state exhibits significant gap anisotropies as well as gap differences on the different bands, but only has point nodes on one of the bands at the Brillouin zone corners. We study the topological characteristics of this superconducting phase, which features Majorana-Weyl nodes in the bulk, protected surface states, and an associated thermal Hall response. The lack of extended nodes and the spontaneously broken time-reversal symmetry of the $(d+\mathrm{i}d)$-wave state are in agreement with the $μ$SR experiments. Our theoretical findings together with the experimental evidence thus suggests that SrPtAs is the first example of chiral $d$-wave superconductivity.

cond-mat.supr-con↗
arXiv2012

Possible pairing symmetries in SrPtAs with a local lack of inversion center

We discuss possible pairing symmetries in the hexagonal pnictide superconductor SrPtAs. The local lack of inversion symmetry of the two distinct conducting layers in the unit cell results in a special spin-orbit coupling with a staggered structure. We classify the pairing symmetry by the global crystal point group D_3d, and suggest some candidates for the stable state using a tight-binding model with an in-plane, density-density type pairing interaction. We may have some unconventional states like s+f-wave and a mixture of chiral d-wave and chiral p-wave. The spin orbit coupling is larger than the interlayer hopping, and the mixing between spin-singlet and triplet states can be seen in spite of the fact that the system has a global inversion center.

cond-mat.supr-con↗
arXiv2011

Valley Spin Sum Rule for Dirac Fermions: Topological Argument

We consider a two-dimensional bipartite lattice system. In such a system, the Bloch band spectrum can have some valley points, around which Dirac fermions appear as the low-energy excitations. Each valley point has a valley spin +1 or -1. In such a system, there are two topological numbers counting vortices and merons in the Brillouin zone, respectively. These numbers are equivalent, and this fact leads to a sum rule which states that the total sum of the valley spins is absent even in a system without time-reversal and parity symmetries. We can see some similarity between the valley spin and chirality in the Nielsen-Ninomiya no-go theorem in odd-spatial dimensions.

cond-mat.mes-hall↗
arXiv2011

Magnetic Response in Quantized Spin Hall Phase of Correlated Electrons

We investigate the magnetic response in the quantized spin Hall (SH) phase of layered-honeycomb lattice system with intrinsic spin-orbit coupling lambda_SO and on-site Hubbard U. The response is characterized by a parameter g= 4 U a^2 d / 3, where a and d are the lattice constant and interlayer distance, respectively. When g< (sigma_{xy}^{s2} mu)^{-1}, where sigma_{xy}^{s} is the quantized spin Hall conductivity and mu is the magnetic permeability, the magnetic field inside the sample oscillates spatially. The oscillation vanishes in the non-interacting limit U -> 0. When g > (sigma_{xy}^{s2} mu)^{-1}, the system shows perfect diamagnetism, i.e., the Meissner effect occurs. We find that superlattice structure with large lattice constant is favorable to see these phenomena. We also point out that, as a result of Zeeman coupling, the topologically-protected helical edge states shows weak diamagnetism which is independent of the parameter g.

cond-mat.str-el↗
arXiv2010

The London Equation in Quantum Spin Hall System with Electronic Correlation

The Kane-Mele (KM) model is proposed to describe the quantum spin Hall effect of electrons on the two-dimensional honeycomb lattice. Here, we will show that, in a certain parameter region, the London equation is obtained from the effective field theory of the layered KM model with an electronic correlation.

cond-mat.str-el↗
arXiv2010

(De)confinement of supercurrent in Z_2 Topological Insulators

It is shown that the electric supercurrent flows in a Z_2 topological insulator with U_em(1) X U_z(1) (electromagnetic and spin) gauge symmetries. When U_z(1) is broken, a dissipationless electric current is still possible to flow locally but net charge transfer is absent, i.e., current is confined. In the Kane-Mele model for graphene, this confining-deconfining (superconducting) transition is driven by the Rashba spin-orbit interaction, which breaks U_z(1).

cond-mat.mes-hall↗
arXiv2010

Intrinsic and extrinsic origins of the polar Kerr effect in a chiral p-wave superconductor

Recently, the measurement of the polar Kerr effect (PKE) in the quasi two-dimensional superconductor Sr2RuO4, which is motivated to observe the chirality of px + i py-wave pairing, has been reported. We clarify that the PKE has intrinsic and extrinsic (disorder-induced) origins. The extrinsic contribution would be dominant in the PKE experiment.

cond-mat.supr-con↗
arXiv2008

Impurity Induced Polar Kerr Effect in A Chiral p-wave Superconductor

We discuss the polar Kerr effect (PKE) in a chiral p-wave (p_x+i p_y-wave) superconductor. It is found that the off-diagonal component of a current-current correlation function is induced by impurity scattering in the chiral p-wave condensate, and a nonzero Hall conductivity is obtained using the Kubo formula. We estimate the Kerr rotation angle by using this impurity-induced Hall conductivity and compare it with experimental results [Jing Xia et al., Phys. Rev. Lett. 97, 167002 (2006)].

cond-mat.supr-con↗