Higgs mode in superconducting Titanium nanostructures

Abstract

We report observations of Higgs modes in superconducting Titanium nanostructures at very low temperature. They appear as anomalies in the microwave complex impedance of the samples revealed by the presence of a dc supercurrent. By varying the sample geometry and contact material, we probe how the Higgs modes are sensitive to the dimensionality of superconductivity, the penetration of the dc and ac current densities in the sample and the dissipation in the contacts.

Figures (4)

• Figure 1: Experimental setup. Bottom left: photograph of the picoprobe contacting the Ti wire: the center tip of the probe is connected to the center of the CPW, leading to the Ti wire; the two external tips contact the ground plane.
• Figure 2: (a): Real and imaginary parts of $\delta Z$ vs. frequency for sample WTn at $T=7$ mK, for various dc currents $I$ between $0$ (blue) and 75 $\mu$A (green), with $I_c=76.6\mu$A. Dotted black lines are Gaussian fits. Inset: peak height of Re($\delta Z$), $\Delta R_{H}$, vs. $I$ in log-log scale. For comparison, the dashed line corresponds to $I^2$.
• Figure 3: Normalized real and imaginary parts of $\delta Z$ vs. normalized frequency $hf/(2\Delta(T))$ for all the samples. $\Delta(T)$ is calculated numerically using the BCS integral equation of the superconducting gap at temperature $T$. Green: sample NTk at $T/T_c=0.85$, $I/I_c=0.89$. Violet: sample NTk(Al) at $T/T_c=0.85$, $I/I_c=0.82$. Red: sample NTk(Ti) at $T/T_c=0.85$, $I/I_c=0.83$. Blue: sample WTn at $T/T_c=0.88$ ; $I/I_c=0.85$. Orange: sample WTk at $T/T_c=0.87$, $I/I_c=0.87$.
• Figure 4: (a) Temperature dependence of the position $f_H$ (blue) and full width at half maximum FWHM (orange) of the HM for sample NTk. For comparison, $2\Delta(T)/h$ (black dashed line) is the calculated superconducting BCS gap. (b) Temperature dependence of the HM peak height $\Delta R_H(T)$ for the same sample for $I\sim I_c$.