Cryogenic spin 3/2 nuclear quadrupole resonance: Spin relaxation and electric field gradient via Rabi frequency goniometry

TL;DR

This paper demonstrates a method using nuclear quadrupole resonance to determine the electric field gradient principal axes on single crystal samples with spin 3/2 nuclei, measuring relaxation times across a range of temperatures in a cryogen-free setup.

Contribution

It introduces a geometrical Rabi frequency approach to determine EFG principal axes and extends relaxation time measurements to colder temperatures using a cryogen-free cryostat.

Findings

EFG principal axes can be determined via Rabi frequency dependence.

Relaxation times T1 and T2* measured from 17K to 200K.

Cryogen-free cryostat operation is feasible for NQR experiments.

Abstract

A straightforward way to determine the electric field gradient $-$ principal axes frame (EFG-PAF) on single crystal samples with spin 3/2 nuclei is demonstrated. Nuclear quadrupole resonance (NQR) spectroscopy is used to determine the Rabi frequency for $^{35}$Cl in a single crystal of potassium chlorate (KClO$_3$) by comparing the NQR signal for powder and single crystal samples. By exploiting the geometrical dependence of the Rabi frequency with respect to the excitation direction, EFG-PAF is readily determined. Furthermore, relaxation times $T_1$ and $T_2^*$ were measured at multiple temperatures ranging from $17~\textrm{K}$ to $200 ~$K, extending the results of previous works to colder temperatures where new relaxation mechanisms become dominant. The experiments were performed in a cryogen-free cryostat, which posed distinct challenges compared to a conventional cryogenic cooling setup. The successful operation of the NQR probe within a cryogen-free cryostat has the potential to make the technique more accessible and widen applications.