Superconducting Properties on Two-dimensional Quasicrystal (Ta$_{0.95}$Cu$_{0.05}$)$_{1.6}$Te Studied with $^{125}$Te-NMR

TL;DR

This study uses $^{125}$Te-NMR to probe superconductivity in the two-dimensional quasicrystal $(\mathrm{Ta_{0.95}Cu_{0.05})_{1.6}Te}$ with $T_c = 0.94\ \text{K}$. Measurements of the Knight shift and spin-lattice relaxation rate $1/T_1$ reveal a coherence peak just below $T_c$ and an exponential decay at lower $T$, with the peak being unusually suppressed, suggesting DOS-edge smearing due to quasiperiodicity. The Knight shift exhibits almost no diamagnetic shift and only a small decrease in the SC state, hinting at potential parity mixing or a triplet component, while the diamagnetic contribution $K_{\mathrm{dia}}$ is estimated to be negligible. Numerical modeling of $1/T_1$ supports an $s$-wave full-gap with $2\Delta(0)/k_B T_c \approx 3.04$ and DOS broadening, though the quasiperiodic lattice may induce unconventional features that merit further single-crystal NMR studies under high fields. Overall, the work highlights how quasiperiodicity can modify superconducting signatures in QC Ta$_{1.6}$Te and motivates exploration of possible parity-mixed or triplet components.

Abstract

Physical properties in the normal and superconducting (SC) state are investigated with $^{125}$Te-nuclear magnetic resonance (NMR) measurements in a quasicrystal $\mathrm{(Ta_{0.95}Cu_{0.05})_{1.6}Te}$, which was a recently discovered superconductor with the SC transition temperature $T_{\mathrm{c}}$ = 0.94 K. The nuclear spin-lattice relaxation rate $1/T_1$ shows a coherence peak just below $T_{\mathrm{c}}$, followed by an exponential decrease down to 0.1 K. The overall temperature dependence of $1/T_1$ is in good agreement with an $s$-wave SC model with a SC gap slightly smaller than the BCS value. However, the coherence peak is unusually small, which may be attributable to a reduced Bogoliubov peak theoretically predicted for quasicrystals. Furthermore, $^{125}$Te-NMR spectra show almost no broadening nor shift in the SC state, suggesting that an unusual SC state such as parity mixing might be realized in the Ta$_{1.6}$Te superconductor.

Figures (10)

• Figure 1: (a) Temperature dependence of the ac susceptibility under various fields. Arrows indicate the SC transition temperature $T_{\mathrm{c}}$ determined from the onset of the diamagnetic signal. (b) Magnetic-field–temperature $H$–$T$ phase diagram of (Ta_0.95Cu_0.05)_1.6Te together with the $H_{\mathrm{c2}}$ result of Ta_1.6Te by Terashima et al.Terashima_npj_2024. The dashed line shows the orbital critical field based on WHH theory Werthamer_PR_1966.
• Figure 2: (a) Temperature dependence of the Knight shift of (Ta_0.95Cu_0.05)_1.6Te at 6.0T and 1.1T in the normal state. The inset shows typical NMR spectrum in normal state at 1.6K and 100K at 6.0T. (b) ^125Te nuclear spin-lattice relaxation rate $1/T_{1}T$ of (Ta_0.95Cu_0.05)_1.6Te at 6.0T and 1.1T in the normal state. The dashed line is a guide to the eye.
• Figure 3: ^125Te nuclear spin-lattice relaxation rate divided by $T$$1/T_{1}T$ of (Ta_0.95Cu_0.05)_1.6Te at 0.38T, 0.26T, and 0.19T below 1.5K. The solid curve is the calculation for an conventional $s$-wave full-gap model with $2\Delta(0)/k_{\mathrm{B}}T_{\mathrm{c}} = 3.04$ and $\delta/\Delta(0) = 0.4$. The Inset shows the plot of $(T_{1}T)_{\mathrm{s}}/(T_{1}T)_{\mathrm{n}}$ versus $T_{\mathrm{c}}(H)/T$ at 0.26T and 0.38T. The linear dependence (dashed line) indicates an exponential decay of $1/T_{1}T$.
• Figure 4: Height of the HS peak as a function of $H/H_{\mathrm{c2}}$. Star symbols show the present result on (Ta_0.95Cu_0.05)_1.6Te. The solid line (clean limit) and square symbols (Born limit) are theoretical results from the Eilenberger calculations Tanaka_PRB_2015.
• Figure 5: Typical ^125Te NMR spectra in the normal (a) and SC (b) states under 0.26T and 0.43T, respectively. Arrows indicate the spectral peak. (c) Temperature dependence of the Knight shift measured at various fields. The dot-dashed and dashed lines are guides to the eye.