arXiv subjects

Shun Matsubara

Publications and source records attributed to Shun Matsubara.

5 recordsLinked to original sources

arXiv2023

A Rigorous Formalism of Unconventional Symmetry Breaking in Fermi Liquid Theory and Its Application to Nematicity in FeSe

Unconventional symmetry breaking due to nonlocal order parameters has attracted considerable attention in many strongly correlated metals. Famous examples are the nematic order in Fe-based superconductors and the star-of-David charge density order in kagome metals. Such exotic symmetry breaking in metals is a central issue of modern condensed matter physics, while its theoretical foundation is still unclear in comparison with the well-established theory of superconductivity. To overcome this difficulty, here we introduce the "form factor" that generalizes the nonlocal order parameter into the Luttinger-Ward (LW) Fermi liquid theory. We then construct a rigorous formalism of the "density-wave equation" that gives the thermodynamically stable form factor, similarly to the superconducting-gap equation. In addition, a rigorous expression of the Ginzburg-Landau free-energy for the unconventional order is presented to calculate various thermodynamic properties. In the next stage, we apply the derived formalism to a typical Fe-based superconductor FeSe, by using the one-loop LW function that represents the free-energy gain due to the interference among paramagnons. The following key experiments are naturally explained: (i) Lifshitz transition (=disappearance of an electron-pocket) due to the bond+orbital order below $T_c$. (ii) Curie-Weiss behavior of the nematic susceptibility at higher T, and the deviation from the Curie-Weiss behavior at lower T near the nematic quantum-critical-point. (iii) Scaling relation of the specific heat jump at $T_c$, $ΔC/T_c \propto T_c^b$ with $b \sim 3$. (Note that b=0 in the BCS theory.) These results lead to a conclusion that the nematicity in FeSe is the bond+orbital order due to the "paramagnon interference mechanism". The present theory paves the way for solving various unconventional phase transition systems.

cond-mat.str-el↗

arXiv2021

Generation of odd-frequency surface superconductivity with spontaneous spin current due to the zero-energy Andreev bound state

We propose that the odd-frequency $s$ wave ($s^{\rm{odd}}$ wave) superconducting gap function, which is usually unstable in the bulk, naturally emerges at the edge of $d$ wave superconductors. This prediction is based on the surface spin fluctuation pairing mechanism owing to the zero-energy surface Andreev bound state. The interference between bulk and edge gap functions triggers the $d+s^{\rm{odd}}$ state, and the generated spin current is a useful signal uncovering the ``hidden'' odd-frequency gap. In addition, the edge $s^{\rm{odd}}$ gap can be determined via the proximity effect on the diffusive normal metal. Furthermore, this study provides a decisive validation of the ``Hermite odd-frequency gap function,'' which has been an open fundamental challenge to this field.

cond-mat.supr-con↗

arXiv2020

Emergence of $d \pm ip$-wave superconducting state at the edge of $d$-wave superconductors mediated by Andreev-bound-state-driven ferromagnetic fluctuations

We propose a mechanism of spin-triplet superconductivity at the edge of $d$-wave superconductors. Recent theoretical research in $d$-wave superconductors predicted that strong ferromagnetic (FM) fluctuations are induced by large density of states due to edge Andreev bound states (ABS). Here, we construct the linearized gap equation for the edge-induced superconductivity, and perform a numerical study based on a large cluster Hubbard model with bulk $d$-wave superconducting (SC) gap. We find that ABS-induced strong FM fluctuations mediate the $d \pm ip$-wave SC state, in which the time-reversal symmetry is broken. The edge-induced $p$-wave transition temperature $T_{cp}$ is slightly lower than the bulk $d$-wave one $T_{cd}$, and the Majorana bound state may be created at the endpoint of the edge.

cond-mat.str-el↗

arXiv2020

Emergence of strongly correlated electronic states driven by the Andreev bound state in d-wave superconductors

As the surface Andreev bound state (ABS) forms at the open ($1,1$) edge of a $d_{x^2-y^2}$-wave superconductor, the local density of states (LDOS) increases. Therefore, a strong electron correlation and drastic phenomena may occur. However, a theoretical study on the effects of the ABS on the electron correlation has not been performed yet. To understand these effects, we study large cluster Hubbard model with an open ($1,1$) edge in the presence of a bulk $d$-wave gap. We calculate the site-dependent spin susceptibility by performing random-phase-approximation (RPA) and modified fluctuation-exchange (FLEX) approximation in the real space. We find that near the ($1,1$) edge, drastic ferromagnetic (FM) fluctuations occur owing to the ABS. In addition, as the temperature decreases, the system rapidly approaches a magnetic-order phase slightly below the transition temperature of the bulk $d$-wave superconductivity (SC). In this case, the FM fluctuations are expected to induce interesting phenomena such as edge-induced triplet SC and quantum critical phenomena.

cond-mat.str-el↗

arXiv2018

Edge-induced strongly correlated electronic states in two-dimensional Hubbard model: Enhancement of magnetic correlations and self-energy effects

To understand nontrivial edge electronic states in strongly-correlated metals such as cuprate superconductors, we study the two-dimensional Hubbard models with open edge boundary. The position-dependences of the spin susceptibility and the self-energy are carefully analyzed self-consistently, by using the fluctuation-exchange (FLEX) approximation. It is found that spin susceptibilities are strongly enlarged near the (1,1) open edge when the system is near the half-filling. The enhancement is large even if the negative feedback from the self-energy is considered in the FLEX approximation. The present study predicts the emergence of nontrivial spin-fluctuation-driven phenomena near the edge, like the quantum criticality, edge superconductivity, and the bond-density-wave order.

cond-mat.str-el↗