arXiv subjects

Shun Asano

Publications and source records attributed to Shun Asano.

12 recordsLinked to original sources

arXiv2026

Winding charge density wave: intertwining of structural chirality and phase topology of electronic order

We propose a class of chiral charge density waves (CDWs), dubbed winding CDWs, that exhibit macroscopic chirality despite a single ordering wavevector. In screw-symmetric chiral crystals, chiral phonons drive a Peierls instability that selects a definite crystal angular momentum channel, thereby endowing the CDW with an integer azimuthal phase winding dictated by the selection rule governing electron-phonon coupling. We further extend this framework to achiral crystals with discrete rotational symmetry and demonstrate that spontaneous symmetry breaking stabilizes a winding CDW with either handedness, realizing an achiral-to-chiral phase transition. Our results reveal a fundamental link between the geometry of chiral structures and the phase topology of electronic orders.

cond-mat.str-el↗

arXiv2025

Spontaneous spin-selective structural phase transition in chiral crystals

In this Letter, we predict a structural phase transition unique to chiral crystals with screw symmetry. In chiral crystals, the phonon frequency renormalized by the electron-phonon coupling depends on the handedness of circular polarization. Consequently, the soft mode encoding phonon angular momentum induces spin-selective Peierls gaps in the electronic band, entailing a helical spin density wave and chiral lattice distortion. We also elucidate the chiral signatures and functional implications of collective modes. Our findings offer crucial insights into the emergence of chirality and highlight novel functional aspects of chiral materials and their design strategy.

cond-mat.str-el↗

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., moir\'e 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↗

arXiv2021

$^{139}$La-NMR study of spin-dynamics coupled with hole mobility in $T$*-type La$_{0.86}$Eu$_{0.86}$Sr$_{0.28}$CuO$_{4-δ}$

In $T$*-type cuprate oxides with five oxygen coordination, little is known about the relationship between the spin correlations and dope carriers. We performed $^{139}$La-nuclear magnetic resonance (NMR) and electrical resistivity measurements on an as-sintered (AS) and oxidation annealed (OA) polycrystalline $T$*-type La$_{0.86}$Eu$_{0.86}$S$_{0.28}$CuO$_4$ (LESCO) to investigate its magnetic and superconducting (SC) properties. Upon cooling, the NMR spectrum of AS LESCO broadened below 3 K, at which the nuclear spin-lattice relaxation rate $1/T_1$ against the temperature exhibited a maximum, thereby indicating the appearance of static magnetism. The temperature dependence of $1/T_1$ between 3 K and 20 K was similar to that of the resistivity displaying the semiconducting behavior. Furthermore, the energy scale of the transport gap and spin-dynamics estimated was found to be comparable. These results suggest a close connection between the mobility of the doped carriers and low-energy spin-dynamics, as reported for lightly doped $T$-type La$_{2-x}$Sr$_x$CuO$_4$. In the OA SC sample, we confirmed the absence of a magnetic order and the Korringa relation above 10 K. Therefore, in the $T$*-type LESCO with $x$ = 0.28, the magnetic state coupled with holes drastically turns to the weakly correlated metallic state by oxidation annealing.

cond-mat.str-el↗

arXiv2020

Fermi-liquid state in $T$*-type La$_{1-x/2}$Eu$_{1-x/2}$Sr$_x$CuO$_4$ revealed via element substitution effects on magnetism

Despite its unique structural features, the magnetism of single-layered cuprate with five oxygen coordination ($T$*-type structure) has not been investigated thus far. Here, we report the results of muon spin relaxation and magnetic susceptibility measurements to elucidate the magnetism of $T$*-type La$_{1-x/2}$Eu$_{1-x/2}$Sr$_x$CuO$_4$ (LESCO) via magnetic Fe- and non-magnetic Zn-substitution. We clarified the inducement of the spin-glass (SG)-like magnetically ordered state in La$_{1-x/2}$Eu$_{1-x/2}$Sr$_x$Cu$_y$Fe$_{1-y}$O$_4$ with $x = 0.24 + y$, and the non-magnetic state in La$_{1-x/2}$Eu$_{1-x/2}$Sr$_x$Cu$_y$Zn$_{1-y}$O$_4$ with $x$ = 0.24 after the suppression of superconductivity for $y$ $\geq$ 0.025. The SG state lies below $\sim$7 K in a wide Sr concentration range between 0.19 and 0.34 in 5$\%$ Fe-substituted LESCO. The short-range SG state is consistent with that originating from the Ruderman-Kittel-Kasuya-Yosida interaction in a metallic state. Thus, the results provide the first evidence for Fermi liquid (FL) state in the pristine $T$*-type LESCO. Taking into account the results of an oxygen $K$-edge X-ray absorption spectroscopy measurement $[$J. Phys. Soc. Jpn. 89, 075002 (2020)$]$ reporting the actual hole concentrations in LESCO, our results demonstrate the existence of the FL state in a lower hole-concentration region, compared to that in $T$-type La$_{2-x}$Sr$_x$CuO$_4$. The emergence of the FL state in a lower hole-concentration region is possibly associated with a smaller charge transfer gap energy in the parent material with five oxygen coordination.

cond-mat.supr-con↗

arXiv2020

Localized character of charge excitations for La$_{2-x}$Sr$_{x}$NiO$_{4+δ}$ revealed by oxygen $K$-edge resonant inelastic X-ray scattering

We performed a resonant inelastic X-ray scattering (RIXS) study of La$_{2-x}$Sr$_{x}$NiO$_{4+δ}$ (LSNO) at the oxygen $K$ edge to investigate the nature of the doped holes with regard to charge excitations. Charge excitations of the hole-doped nickelates are found to be almost independent of momentum transfer, indicating that the doped holes are strongly localized in character. Additionally, conspicuous changes in energy position are in temperature dependence. These characters are observed in stark contrast to those of the high-$T_{c}$ cuprate La$_{2-x}$Sr$_{x}$CuO$_{4}$ (LSCO), where delocalized doped holes form charge excitations with sizable momentum dependence in the CuO$_2$ plane. This distinct nature of charge excitations of doped holes is consistent with the metallicity of the materials and could be caused by strong electron-phonon coupling and weak quantum spin fluctuation in the nickelates.

cond-mat.str-el↗

arXiv2020

Revisiting the phase diagram of T*-type La$_{1-x/2}$Eu$_{1-x/2}$Sr$_x$CuO$_4$ using Oxygen $K$-edge X-ray absorption spectroscopy

Oxygen $K$-edge X-ray absorption spectroscopy measurements were conducted on T*-type La$_{1-x/2}$Eu$_{1-x/2}$Sr$_x$CuO$_4$ (LESCO) to estimate the hole density ($n_{\rm h}$) and investigate the oxidation annealing effect on $n_{\rm h}$. A drastic increase in $n_{\rm h}$ due to annealing was found. The increase in $n_{\rm h}$ cannot be explained solely by the oxygen gain due to annealing, suggesting that delocalized holes are introduced into the CuO$_2$ plane. A phase diagram of LESCO was redrawn against $n_{\rm h}$.

cond-mat.supr-con↗

arXiv2020

Post-growth annealing effects on charge and spin excitations in Nd$_{2-x}$Ce$_x$CuO$_4$

We report a Cu K- and L$_3$-edge resonant inelastic x-ray scattering study of charge and spin excitations of bulk Nd$_{2-x}$Ce$_x$CuO$_4$, with focus on post-growth annealing effects. For the parent compound Nd$_2$CuO$_4$ ($x = 0$), a clear charge-transfer gap is observed in the as-grown state, whereas the charge excitation spectra indicate that electrons are doped in the annealed state. This is consistent with the observation that annealed thin-film and polycrystalline samples of RE$_2$CuO$_4$ (RE = rare earth) can become metallic and superconducting at sufficiently high electron concentrations without Ce doping. For $x = 0.16$, a Ce concentration for which it is known that oxygen reduction destroys long-range antiferromagnetic order and induces superconductivity, we find that the high-energy spin excitations of non-superconducting as-grown and superconducting annealed crystals are nearly identical. This finding is in stark contrast to the significant changes in the low-energy spin excitations previously observed via neutron scattering.

cond-mat.supr-con↗

arXiv2020

Distinct variation of electronic states due to annealing in La$_{1.8}$Eu$_{0.2}$CuO$_4$ and Nd$_{2}$CuO$_4$

We performed Cu {\it K}-edge X-ray absorption fine structure measurements on T'-type La$_{1.8}$Eu$_{0.2}$CuO$_4$ (LECO) and Nd$_2$CuO$_4$ (NCO) to investigate the variation in the electronic state associated with the emergence of superconductivity due to annealing. The X-ray absorption near-edge structure spectra of as-sintered (AS) LECO are quite similar to those of AS NCO, indicating that the ground state of AS T'-type LECO is a Mott insulator. We found a significant variation of the electronic state at the Cu sites in LECO due to annealing. The electron density after annealing ($n_{\rm AN}$) was evaluated for both superconducting LECO and non-superconducting NCO and found to be 0.40 and 0.05 electrons per Cu, respectively. In LECO but not in NCO, extended X-ray absorption fine structure analysis revealed a softening in the strength of the Cu-O bond in the CuO$_2$ plane due to annealing, which is consistent with the screening effect on phonons in the metallic state. Since the amounts of oxygen loss due to annealing ($δ$) for LECO and NCO are comparable with each other, these results suggest distinct electron-doping processes in the two compounds. That electron-doping in NCO approximately follows the relation $n_{\rm AN}=2δ$ can be understood if electrons are doped through oxygen deficiency, but the anneal-induced metallic nature and large $n_{\rm AN}$ of LECO suggest a variation of the electronic band structure causes self-doping of carriers. The origin of the difference in doping processes due to annealing is discussed in connection with the size of the charge transfer gap.

cond-mat.supr-con↗

arXiv2019

Oxidation annealing effects on the spin-glass-like magnetism and appearance of superconductivity in T*-type La$_{1-x/2}$Eu$_{1-x/2}$Sr$_x$CuO$_4$ (0.14 $\leq x \leq$ 0.28)

We investigated the magnetism and superconductivity in as-sintered (AS) and oxidation annealed (OA) T*-type La$_{1-x/2}$Eu$_{1-x/2}$Sr$_x$CuO$_4$ (LESCO) with 0.14 $\leq x \leq$ 0.28 by the first comprehensive muon spin rotation/relaxation ($μ$SR), magnetic susceptibility, and electrical resistivity measurements. In OA superconducting samples, no evidence of magnetic order was observed, whereas AS semiconducting samples exhibited evidence of a disordered magnetic state in the measured temperature range between $\sim$4 K and $\sim$8 K. Therefore, the ground state in LESCO drastically varies with oxidation annealing and the magnetic phase competitively exists with the superconducting (SC) phase. The magnetic phase in the AS LESCO is quite robust against Sr doping, while the SC phase degrades with increasing $x$. A monotonous decrease of the SC transition temperature from 24.5 K in $x$ = 0.14 to 9.0 K in $x$ = 0.28 suggests the disappearance of the SC phase at $x$ $\sim$ 0.34. Furthermore, we clarified the simultaneous development of (quasi) static magnetism and the electrical resistivity at a low temperature in AS samples, suggesting the inducement of magnetism by the suppression of carrier mobility. The variation in magnetism due to annealing is discussed from a viewpoint of structural defects, which was previously reported from neutron diffraction measurements.

cond-mat.supr-con↗

arXiv2018

Ce-doping and reduction annealing effects on electronic states in Pr$_{2-x}$Ce$_{x}$CuO$_4$ studied by Cu ${K}$-edge X-ray absorption spectroscopy

We investigated Ce-substitution and reduction annealing effects on the electronic states at copper sites by Cu ${K}$-edge x-ray absorption near-edge structure measurements in Pr$_{2-x}$Ce$_x$CuO$_{4+α-δ}$ (PCCO) with varying $x$ and $δ$ (the amount of oxygen loss during annealing) values. Absorption near-edge spectra were modified by Ce-substitution and reduction annealing in a similar manner with increasing $x$ and $δ$. Considering electron doping by Ce-substitution, this similarity indicates an increase of electron number at the copper sites due to annealing $n_{\rm AN}$. Thus, the total number of electrons is determined by the amount of Ce and oxygen ions. Furthermore, quantitative analyses of the spectra clarified that the number of Cu$^+$ sites, corresponding to the induced electron number by Ce-substitution $n_{\rm Ce}$ increases linearly with $x$ in the as-sintered PCCO ($δ=0$), whereas $n_{\rm AN}$ is not exactly equal to twice of $δ$, which is expected from charge neutrality. For each $x$-fixed sample, $n_{\rm AN}$ tends to exceed 2$δ$ with increasing $δ$, suggesting the emergence of two types of carrier due to annealing.

cond-mat.supr-con↗