arXiv subjects

S. Akutagawa

Publications and source records attributed to S. Akutagawa.

7 recordsLinked to original sources

arXiv2024

Imaging quantum interference in a monolayer Kitaev quantum spin liquid candidate

Single atomic defects are prominent windows to look into host quantum states because collective responses from the host states emerge as localized states around the defects. Friedel oscillations and Kondo clouds in Fermi liquids are quintessential examples. However, the situation is quite different for quantum spin liquid (QSL), an exotic state of matter with fractionalized quasiparticles and topological order arising from a profound impact of quantum entanglement. Elucidating the underlying local electronic property has been challenging due to the charge neutrality of fractionalized quasiparticles and the insulating nature of QSLs. Here, using spectroscopic-imaging scanning tunneling microscopy, we report atomically resolved images of monolayer $α-RuCl_3$, the most promising Kitaev QSL candidate, on metallic substrates. We find quantum interference in the insulator manifesting as incommensurate and decaying spatial oscillations of the local density of states around defects with a characteristic bias dependence. The oscillation differs from any known spatial structures in its nature and does not exist in other Mott insulators, implying it is an exotic oscillation involved with excitations unique to $α-RuCl_3$. Numerical simulations suggest that the observed oscillation can be reproduced by assuming that itinerant Majorana fermions of Kitaev QSL are scattered across the Majorana Fermi surface. The oscillation provides a new approach to exploring Kitaev QSLs through the local response against defects like Friedel oscillations in metals.

cond-mat.str-el↗

arXiv2023

Exotic heavy fermion superconductivity in atomically thin CeCoIn$_5$ films

We report an {\it in-situ} scanning tunneling microscopy study of atomically thin films of CeCoIn$_5$, a $d$-wave heavy-fermion superconductor. Both hybridization and superconducting gaps are observed even in monolayer CeCoIn$_5$, providing direct evidence of superconductivity of heavy quasiparticles mediated by purely two-dimensional bosonic excitations. In these atomically thin films, $T_c$ is suppressed to nearly half of the bulk, but is similar to CeCoIn$_5$/YbCoIn$_5$ superlattices containing CeCoIn$_5$ layers with the same thickness as the thin films. Remarkably, the out-of-plane upper critical field $μ_0H_{c2\perp}$ at zero temperature is largely enhanced from those of bulk and superlattices. The enhanced $H_{c2\perp}$ well exceeds the Pauli and bulk orbital limits, suggesting the possible emergence of unusual superconductivity with parity mixing caused by the inversion symmetry breaking.

cond-mat.str-el↗

arXiv2011

Evidence of nodal gap structure in the non-centrosymmetric superconductor Y2C3

The magnetic penetration depth $λ(T)$ and the upper critical field $% μ_{0}H_{c2}(T_{c})$ of the non-centrosymmetric (NCS) superconductor Y$_{2} $C$_{3}$ have been measured using a tunnel-diode (TDO) based resonant oscillation technique. We found that the penetration depth $λ(T)$ and its corresponding superfluid density $ρ_{s}(T)$ show linear temperature dependence at very low temperatures ($T\ll T_{c}$), indicating the existence of line nodes in the superconducting energy gap. Moreover, the upper critical field $μ_{0}H_{c2}(T_{c})$ presents an upturn at low temperatures with a rather high value of $μ_{0}H_{c2}(0)$ $\simeq 29$T, which slightly exceeds the weak-coupling Pauli limit. We discuss the possible origins for these nontrivial superconducting properties, and argue that the nodal gap structure in Y$_{2}$C$_{3}$ is likely attributed to the absence of inversion symmetry, which allows the admixture of spin-singlet and spin-triplet pairing states.

cond-mat.supr-con↗

arXiv2011

Large upper critical field in non-centrosymmetric superconductor Y2C3

We determine the upper critical field $μ_0 H_{c2}(T_c)$ of non-centrosymmetric superconductor $Y_2 C_3$ using two distinct methods: the bulk magnetization M(T) and the tunnel-diode oscillator (TDO) based impedance measurements. It is found that the upper critical field reaches a value of 30T at zero temperature which is above the weak-coupling Pauli paramagnetic limit. We argue that the observation of such a large $μ_0 H_{c2}(0)$ in $Y_2 C_3$ could be attributed to the admixture of spin-singlet and spin-triplet pairing states as a result of broken inversion symmetry.

cond-mat.supr-con↗

arXiv2008

Conventional $s$-Wave Superconductivity in Noncentrosymmetric Ir$_2$Ga$_9$: $^{71}$Ga-NQR Evidence

We report a $^{71}$Ga nuclear-quadrupole-resonance (NQR) study on the characteristics of superconductivity in noncentrosymmetric Ir$_2$Ga$_9$ at zero field (H=0). The $^{71}$Ga-NQR measurements have revealed that $1/T_1$ has the clear coherence peak just below $T_{\rm c}$, and decreases exponentially upon further cooling in Ir$_2$Ga$_9$. From these results, Ir$_2$Ga$_9$ is concluded to be the conventional s-wave superconductor. Despite the lack of spatial centrosymmetry, there are no evidence for unconventional superconducting state ascribed to ASOC in Ir$_2$Ga$_9$.

cond-mat.supr-con↗

arXiv2006

Multigap Superconductivity in Y$_2$C$_3$: A $^{13}$C-NMR Study

We report on the superconducting (SC) properties of Y$_2$C$_3$ with a relatively high transition temperature $T_{\rm c}=15.7$ K investigated by $^{13}$C nuclear-magnetic-resonance (NMR) measurements under a magnetic field. The $^{13}$C Knight shift has revealed a significant decrease below $T_{\rm c}$, suggesting a spin-singlet superconductivity. From an analysis of the temperature dependence of the nuclear spin-lattice relaxation rate $1/T_1$ in the SC state, Y$_2$C$_3$ is demonstrated to be a multigap superconductor that exhibits a large gap $2Δ/k_{\rm B}T_{\rm c}=5$ at the main band and a small gap $2Δ/k_{\rm B}T_{\rm c}=2$ at other bands. These results have revealed that Y$_2$C$_3$ is a unique multigap s-wave superconductor similar to MgB$_2$.

cond-mat.supr-con↗

arXiv2003

The enhancement of superconducting transition temperature in yttrium sesquicarbide system, Y2C3, with the maximum Tc of Tc=18K

We discovered the enhancement of superconducting transition temperature, Tc, in yttrium sesquicarbide system with the maximum Tc of Tc=18K and their superconducting properties were discussed. The crystal structure of Y2C3 is the body-centered cubic (Pu2C3-type) structure, and the lattice parameters are varied with the heat treatment conditions. The magnetization (M-H) curves of this compound showed a typical type-II superconducting behavior, and the lower critical field, Hc1(0), is 3.5mT. The Tc-value in this system was changed in the range from 15K to 18K, depending on the sintering conditions. In a previous report by Krupka et al., maximum Tc in Y2C3 was observed at 11.5K by the magnetic measurements. So we successfully synthesized the new high-Tc phase in Y2C3.

cond-mat.supr-con↗