Thermodynamic evidence for full-gap superconductivity in the dodecagonal quasicrystal Cu-doped Ta$_{1.6}$Te

Abstract

We report the superconducting gap in the van der Waals layered quasicrystal Cu-doped Ta$_{1.6}$Te, using a fast relaxation technique that removes the large nuclear contribution of $^{181}$Ta. The initial-slope method enabled detection of the electronic specific heat down to 60~mK, revealing a fully gapped state with $Δ(0)/k_{\rm B} = 1.43$~K. Both the gap magnitude and specific-heat jump are smaller than BCS predictions, while quasiparticle excitations are strongly suppressed, consistent with a theoretical expectation for aperiodic systems. AC-susceptibility measurements show a large upper critical field and pronounced anisotropy, reflecting the quasi-two-dimensional structure. These results provide the first thermodynamic evidence for a full-gap superconducting state in a quasicrystal and highlight unconventional pairing mechanisms beyond periodic lattices.

Figures (5)

• Figure 1: Specific heat of Cu-doped Ta$_{1.6}$Te measured by the conventional heat-pulse method (HPM) and the initial-slope method (ISM). The solid curve represents an nuclear contribution fitted as $C_{\rm N} \propto T^{-2}$. Open circles denote the electronic and phonon contributions obtained by subtracting $C_{\rm N}$. The inset shows the same data on double logarithmic axes. The additional broken line indicates a nuclear contribution $C_{N}' \propto T^{-2}$ possibly remaining in the results of ISM.
• Figure 2: (a) $C/T$ vs $T^2$ for Cu-doped Ta$_{1.6}$Te below $T \lessapprox 1.7$ K. The solid line is a linear fit between $1.0$ and $1.5$ K used to estimate the phonon and electron contributions in the normal state excluding the contamination phases. (b) Electron contribution $C_{\rm el.} / T$ in the superconducting (0 T) and the normal (5 T) states. The horizontal line indicates an expected electronic contribution in the normal state.
• Figure 3: Semi-logarithmic plot of the specific heat $C$ vs inverse temperature $T^{-1}$ for Cu-doped Ta$_{1.6}$Te. The solid line represents a fit using $C \propto \exp(-\Delta(0)/k_{\rm B}T)$, resulting $\Delta(0)/k_{\rm B} = 1.43$ K.
• Figure 4: Magnetic field dependence of the ac magnetic susceptibility of Cu-doped Ta$_{1.6}$Te. Panels (a) and (b) denote the imaginary and real parts, respectively. The absolute value of the susceptibility was calibrated against a lead specimen of comparable size and shape as Cu-doped Ta$_{1.6}$Te.
• Figure 5: Temperature dependence of the upper critical field ($\mu_{0}H_{c2}$) of Cu-doped Ta$_{1.6}$Te, determined from our results. The superconducting transition temperature determined from the specific measurement is also shown. Error bars for $\mu_{0}H_{c2}$ and temperature stability are comparable to the symbol size.