Local thermometry of NbSe$_2$ flake with delta-$T$ noise measurements

TL;DR

This work develops a local thermometry method based on delta-T noise to probe nonequilibrium superconductivity in a NbSe2 flake. It uses a NbSe2/Au tunnel junction as an energy-preserving sensor, combining conductance spectroscopy with shot-noise measurements to map local temperature and energy flow. The authors identify Andreev reflection through shot-noise doubling and extract electron-phonon relaxation parameters, finding le-ph ≈ 0.8 μm/(T[K])^{1.1} and VΣe-ph ≈ 5×10^{-9} W/K^{4.2} in the normal state, indicating strong energy relaxation on micron scales; in the superconducting state the delta-T noise data qualitatively agree with expectations. The approach provides a general, spectrally featureless tool to study nonequilibrium configurations in 2D superconductors and TMDCs.

Abstract

We perform transport and noise measurements for device consisting of a thin NbSe$_2$ flake laid onto the predefined gold electrodes and covered with a thin hBN flake. In the shot noise of a NbSe$_2$/Au tunnel junction (TJ), we identify Andreev reflection regime by demonstrating the effective charge doubling. Further, by creating temperature gradient across the TJ and measuring its delta-$T$ noise in the normal state, we extract electron-phonon scattering length in NbSe$_2$ and its $T$-dependence. The results of delta-$T$ noise measurements in the absence of a magnetic field when the flake is superconducting are in qualitative agreement with expectations. The introduced approach is promising for the study of nonequilibrium configurations in superconductors.

Figures (9)

• Figure 1: (a) Optical micrograph of the device. Exfoliated NbSe$_2$ flake (marked with white line) is put on pre-patterned gold contacts and is then covered with hBN flake. Working contacts are marked with C$1$, C$2$ and C$3$. (b) Temperature dependence of the normalized resistances $R_{12}$ (blue circles) and $R_{3}$ (red crosses). The inset demonstrates magnetic field dependence of $R_{12}$ at $T=4.2\,\text{K}$ (red) and $0.7\,\text{K}$ (blue). (c) Measurement scheme for characterization of the tunnel junction realized at C$3$/NbSe$_2$ interface (up) and for conductance/noise spectroscopy (down).
• Figure 1: (a) Optical micrograph of another thin device. Similarly to the main text, exfoliated NbSe$_2$ flake is put on pre-patterned gold contacts and is then covered with hBN flake. (b) Temperature dependence of the four-terminal linear-response resistance $R_{23,16}=dV_{23}/dI_{16}$. (c) Color-scale plot of the four-terminal differential resistance $dV_{45}/dI_{16}$ as a function of the magnetic field and the bias current. (d) Current-voltage characteristics of data from panel (c) at specific values of $B=0,\,0.46,\, 0.97,\, 1.43,\, 1.94$ and $2.91\,\text{T}$.
• Figure 2: (a) Normalized differential conductance of the tunnel probe. The dashed lines are fits according to the BTK model using $\Delta=0.69\,\text{meV}$, $Z=1.03$ at $4.2\,\text{K}$ and $\Delta=0.88\,\text{meV}$, $Z=1.12$ at $0.7\,\text{K}$. (b) Current noise spectral density of the tunnel probe at $T=0.7\,\text{K}$ in a magnetic field of $B=0$ and $5.1\,\text{T}$. Dashed lines are the fits with $F=0.7$ and $q=e$ in $B=5.1\,\text{T}$ and $q=2e$ in $B=0\,\text{T}$ (see text).
• Figure 2: (a) Optical micrograph of a relatively thick device with lithographically realized contacts. (b) Magnetic field dependence of a four-terminal resistance at $T=4.2\,\text{K}$ (red curve) and $T=0.5\,\text{K}$ (blue curve). (c) Four-terminal current-voltage characteristics measured at $T=4.2\,\text{K}$ for two opposite current sweep directions.
• Figure 3: (a) Differential resistance of the flake as a function of bias voltage in magnetic fields of $B=0,\,2.1,\,3.1,\,4.1$ and $5.7\,\text{T}$ (from blue to green curves). (b) $I$-$V$ curve of the flake demonstrating the voltage steps and their evolution in a magnetic field at $T=4.2\,\text{K}$. (c) Differential conductance of the tunnel probe as a function of flake current $I_{12}$ and the tunnel probe bias voltage at $T=4.2\,\text{K}$. (d) Linecuts of differential conductance at specific values of $I_{\text{flake}}=0,\,0.21,\, 0.389$ and $0.397\,\text{mA}$ indicated by dashed lines on panel (c).