Field-Tunable Anisotropic Fulde-Ferrell Phase in NbSe$_2$/CrSiTe$_3$ Heterostructures

TL;DR

This work addresses how to realize and control finite-momentum pairing in two-dimensional superconductors through symmetry engineering. By constructing NbSe2/CrSiTe3 heterostructures with proximity-induced Rashba SOC and reduced rotational symmetry, the authors identify an anisotropic FF phase under in-plane magnetic fields via magnetoresistance and nonreciprocal transport, supported by mean-field BdG calculations. Key findings include a half-dome $B$-$T$ region where $B_{c2}$ surpasses the Pauli limit and a finite second-harmonic response $R^{2\omega}$, with strong in-plane anisotropy and twist-angle dependence showing that interfacial coupling governs FF stability. The results demonstrate that heterostructure stacking is a powerful approach to engineer superconducting states and pave the way for FF-based devices and potential topological superconductivity in atomically thin materials.

Abstract

The emergence of superconductivity in two-dimensional transition metal dichalcogenides with strong spin orbit coupling (SOC) has opened new avenues for exploring exotic superconducting states. Here, we report experimental observation of an anisotropic Fulde-Ferrell (FF) phase in few-layer NbSe$_2$/CrSiTe$_3$ heterostructures under in-plane magnetic fields. Through combined magnetoresistance and nonreciprocal transport measurements, we find that due to the couplings from the ferromagnetic CrSiTe$_3$, a half-dome-shaped region emerges in the magnetic field-temperature ($B$-$T$) diagram. Importantly, the half-dome-shaped region exhibits finite second harmonic resistance with in-plane anisotropy, indicating that the superconducting state is an anisotropic FF phase. Through a symmetry analysis combined with mean field calculations, we attribute the emergent anisotropic FF phase to the CrSiTe$_3$ layer induced Rashba SOC and three-fold rotational symmetry breaking. These results demonstrate that heterostructure stacking is a powerful tool for symmetry engineering in superconductors, which can advance the design of quantum devices in atomically thin superconducting materials.

Figures (5)

• Figure 1: Superconductivity and nonreciprocal transport properties in pristine NbSe$_2$ and NbSe$_2$/CrSiTe$_3$ heterostructures.a, Optical image of the device structure for electrical transport measurement. The voltages signal of V$_1$ and V$_2$ correspond to the pristine NbSe$_2$ and NbSe$_2$/CrSiTe$_3$ heterostructures. The red, orange, and navy blue dashed boxes in the figure correspond to the NbSe$_2$, CrSiTe$_3$, and h-BN thin flakes, respectively, with NbSe$_2$ having a thickness of 9 nm. b, The top-view crystal structures of monolayer NbSe$_2$ and CrSiTe$_3$, as well as their stacked crystal structure. c, d, The evolution of R-H curves at various temperatures for pristine NbSe$_2$ and NbSe$_2$/CrSiTe$_3$ heterostructures. e, f, The corresponding $B$-$T$ phase diagrams of the normalized magnetoresistance $R/R_\textrm{N}$ for pristine NbSe$_2$ and NbSe$_2$/CrSiTe$_3$ heterostructures, respectively. The yellow dashed lines represent the Pauli limit. During the measurement, the magnetic field is applied along the y-direction. Here the superconducting transition temperature at zero magnetic field is $T_{\textrm{c}}=6.1$ K. g, h, The second-harmonic magnetoresistance $R^{2\omega}$ of pristine NbSe$_2$ and NbSe$_2$/CrSiTe$_3$ heterostructures under in-plane and out-of-plane magnetic fields at $T$=3 K.
• Figure 2: FF phase of NbSe$_2$/CrSiTe$_3$ with different thickness of NbSe$_2$.a - c, $B$-$T$ phase diagrams of the normalized magnetoresistance $R/R_\textrm{N}$ for the NbSe$_2$/CrSiTe$_3$ heterostructures with different thickness of NbSe$_2$ as 20, 14 and 4 nm, respectively. The blue triangles and brown diamonds labeled the $B_{\textrm{c}2}$ in different regions. The green solid lines represent the fitting curves by BCS theory of zero momentum pairing.
• Figure 3: Anisotropic FF phase in NbSe$_2$/CrSiTe$_3$.a, b$B$-$T$ phase diagrams of $R/R_\textrm{N}$ when the current is applied along the x-direction and y-direction, respectively. c, d, The corresponding $B$-$T$ phase diagrams of $R^{2\omega}$. During the measurement, the magnetic field is applied in-plane and perpendicular to the current.
• Figure 4: Twist angle engineering of the FF phase. a The optical image of the NbSe$_2$/CrSiTe$_3$ heterostructure composed of one NbSe$_2$ layer and two CrSiTe$_3$ layers. The regions outlined by the red, sapphire and yellow dashed boxes correspond to NbSe$_2$, CrSiTe$_3$ and the capping h-BN, respectively. b, c The schematic diagrams of the twist angles between NbSe$_2$ and CrSiTe$_3$, which are approximate b 0$^\circ$ and c 30$^\circ$. d, e The corresponding $B$-$T$ phase diagrams of $R/R_\textrm{N}$ for both twist angle configurations.
• Figure 5: Anisotropic FF phase diagram.a and b The $B$-$T$ phase diagram for the superconducting state with anisotropic FF phase under an in-plane magnetic field along the $x$- and $y$-directions, respectively. At $T=0$K, $x$- and $y$-directional magnetic fields generate finite momentum $\bm{q}=\left(0,q_{x0}\right)$ and $\bm{q}=\left(q_{y0},0\right)$, respectively. c The two-fold asymmetric pairing gap in the $\Gamma$ Fermi pocket. d The polar pllot of the in-plane upper critical field $B_{\textrm{c}2}$. The angle $\theta$ denotes the angle between the in-plane magnetic field and the $x$-axis. e The finite momentum $\bm{q}$ as a function of the in-plane magnetic field $|\bm{B}|=2B_{\textrm{P}}$ with varying in-plane directions. The colorbar $\braket{\bm{q}, \bm{B}}$ denotes the angle between the $\bm{q}$ and the applied in-plane $\bm{B}$. Due to the SOC of C$_{1v}$ symmetry group, $\bm{q}$ is mainly perpendicular to $\bm{B}$.