Quasi-two-dimensional superconductivity in 1$T$-Ti$_{1-x}$Ta$_x$Se$_2$

TL;DR

This work demonstrates quasi‑2D superconductivity in bulk 1T‑Ti1−xTa xSe2 with x = 0.2, achieving Tc ≈ 2.3 K. Through comprehensive magnetization, transport, and specific‑heat measurements, the authors show weak interlayer coupling, anisotropic type‑II behavior, and a quasi‑two‑dimensional superconducting state evidenced by a 2D Tinkham fit to angle‑dependent Hc2 and a Berezinskii‑Kosterlitz‑Thouless transition at TBKT ≈ 2.22 K. The superconductivity is well described by conventional weakly coupled isotropic s‑wave BCS theory, with a Debye temperature around 151 K and λe‑ph ≈ 0.61, indicating a phonon‑mediated pairing mechanism. The findings establish a bulk Ti‑based TMD as a platform to explore intrinsic 2D superconductivity and pave the way for studying few‑layer TiSe2‑like systems and related low‑dimensional quantum phenomena.

Abstract

The emergence of two-dimensional (2D) superconductivity in bulk transition metal dichalcogenides (TMDs) is a fascinating area of research, as their weak interlayer coupling leads to novel superconducting behavior and offers a rich platform to host nontrivial gap structures and interactions with other electronic orders. In this work, we present a comprehensive study of the superconducting properties of bulk single-crystalline $1T$-Ti$_{1-x}$Ta$_x$Se$_2$ for x = 0.2. Our results confirm the weakly coupled anisotropic superconductivity. Angle-dependent upper critical field measurements and observation of a Berezinskii-Kosterlitz-Thouless transition confirm the quasi-2D nature of the superconducting state. These results position $1T$-Ti$_{1-x}$Ta$_x$Se$_2$ as a promising platform for exploring low-dimensional superconducting physics and highlight bulk TMD crystals as a promising platform for realizing intrinsic 2D superconductivity, opening avenues for future quantum applications.

Figures (4)

• Figure 1: (a) A single crystal-image with Laue spots in the inset. (b) Rietveld refined powder XRD patterns. The Bragg position, theoretical refinement, and experimental results are symbolized in that sequence by marks, lines, and vertical bars, respectively. The difference between the computed and experimental data is indicated by the line at the bottom. Inset: Crystal structure of 1$T$-Ti$_{0.8}$Ta$_{0.2}$Se$_2$ is viewed along the out-of-plane direction. Purple, brown, and blue spheres correspond to Ti, Se, and Ta atoms, respectively. (c) Single-crystal XRD patterns of undoped and Ta-doped TiSe$_2$, represented by pink and teal colors, respectively. Inset: The observed shift corresponds to a change in the 2$\theta$. (d) Temperature-dependent resistivity at zero field. Inset shows the Hall resistivity with positive slope at T = 5 K. (e) Abrupt drop in resistivity occurs at $T$$_{c,}$$_{onset}$ = 2.32(1) K. (f) Magnetization versus temperature measurements show superconducting transition at a temperature of 2.22(3) K.
• Figure 2: (a) and (b) Temperature-dependent normalized electrical resistivity for $H \parallel c$ and $H\perp c$ with different magnetic fields, respectively. (c) Upper critical fields in both directions are also fitted by the GL model as a function of temperature. The Pauli limit is 4.31(5) T, as shown by the purple dotted lines. Inset: Temperature-dependent lower critical fields in both directions. GL-fitting is shown by the red lines.
• Figure 3: (a) Field-dependent electrical resistivity with various angles shows anisotropy in this sample. (b) Upper critical fields versus angle for $\rho = 0.9\rho_n$ are best described by the two-dimensional Tinkham model rather than the three-dimensional anisotropic GL model. Inset: Expanded view of the fitting near 90$^{\circ}$ angle. (c) The temperature-dependent $V$-$I$ curves are shown on a logarithmic scale. (d) The evolution of the slopes of the $V$-$I$ curves, identifying the BKT jump in $\alpha (T)$ at 2.22(2) K. The slope of $\alpha (T)$ = 3 is marked by the purple dotted line. Inset: The variation of critical current $I_c$ with temperature.
• Figure 4: The electronic specific heat data are fitted by an isotropic s-wave model, rendering conventional superconductivity. Inset: Normal-state specific heat data are fitted using the Debye-Sommerfeld model.