Nematic-fluctuation-mediated superconductivity in CuxTiSe2

TL;DR

The study addresses the interplay among electronic nematicity, charge density wave (CDW), and superconductivity in Cu_xTiSe2 by probing nematic fluctuations. It uses elastoresistivity measurements on Cu_xTiSe2 single crystals, employing a modified Montgomery setup to extract nematic susceptibility in the $E_g$ channel, revealing Curie–Weiss behavior with a characteristic temperature $T^{*}$. In pristine TiSe2, the anisotropic elastoresistivity coefficient $-m_{E_g}$ diverges on cooling toward $T^{*}$ near $T_{CDW}$; with Cu intercalation, $T^{*}$ shifts to lower temperature and approaches zero near optimal superconductivity at $x \\sim 0.08$, then becomes negative as doping increases further, while $T_{CDW}$ is suppressed and $T_c$ peaks around $3.7$ K. These results indicate that nematic phase transitions or fluctuations intimately enhance superconductivity, making Cu_xTiSe2 a platform to study nematic-fluctuation-mediated superconductivity, and revealing a phase diagram where $T^{*}$ tracks $T_{CDW}$ at low $x$ and vanishes near optimal doping.

Abstract

The interplay among electronic nematicity, charge density wave, and superconductivity in correlated electronic systems has induced extensive research interest. Here, we discover the existence of nematic fluctuations in TiSe2 single crystal and investigate its evolution with Cu intercalation. It is observed that the elastoresistivity coefficient mEg exhibits a divergent temperature dependence following a Curie-Weiss law at high temperature. Upon Cu intercalation, the characteristic temperature T* of nematic fluctuation is progressively suppressed and becomes near zero when the superconductivity is optimized. Further intercalation of Cu leads to the sign change of T* and the suppression of superconductivity. These results strongly indicate that nematic phase transition may play a vital role in enhancing superconductivity in CuxTiSe2. Therefore, CuxTiSe2 provides a unique material platform to explore the nematic-fluctuation-mediated superconductivity.

Figures (4)

• Figure 1: (a) XRD pattern of a Cu$_{x}$TiSe$_{2}$ single crystal. Inset: crystal structure of Cu$_{x}$TiSe$_{2}$. The small blue, large red and medium orange balls represent Cu, Ti and Se atoms, respectively. (b) Temperature dependence of $\rho_{xx}(T)$ for different Cu content $x$. Inset: enlarged view of $\rho_{xx}(T)$ curves below 4 K. (c) Temperature dependence of magnetization $4\pi\chi(T)$ measured at 1 mT with zero-field-cooling (ZFC) mode. Inset: enlarged view of $4\pi\chi(T)$ curves below 4 K.
• Figure 2: Relative changes in resistivity (a) ($\Delta\rho/\rho)_{xx}$ and (b) ($\Delta\rho/\rho)_{yy}$ of TiSe$_{2}$ single crystal at various temperatures above $T_{\rm CDW}$ as a function of strain $\epsilon_{xx}$ applied via an attached piezoelectric actuator. Inset of (b) shows a schematic experimental setup using a modified Montgomery technique with the crystallographic $a$-axis parallel to $\epsilon_{xx}$.
• Figure 3: (a) -- (d) Temperature dependence of $-m_{E_{g}}(T)$ of Cu$_{x}$TiSe$_{2}$ for $x$ = 0, 0.03, 0.07 and 0.09. (e) -- (h) Corresponding $(-m_{Eg}-m_{Eg}^{0})^{-1}$ as a function of temperature. The red solid lines represent the fits using the CW formula and the gray-shaded areas in (a) -- (d) display the fitted regions.
• Figure 4: Phase diagram of Cu$_{x}$TiSe$_{2}$. The $T^{*}$ is denoted by pink open circles (error bars indicate fitting uncertainty). $T_{\rm {CDW}}$ and $T_c$ are represented by black triangles and red squares, respectively. The color scale shows the magnitude of $-m_{E_{g}}$.