Precision hyperfine spectroscopy of an individual nuclear-spin-9/2

TL;DR

This study demonstrates nanoscale NMR spectroscopy of a single nuclear spin using an Er3+ center in CaWO4 at millikelvin temperatures, revealing detailed hyperfine interactions and new Hamiltonian terms.

Contribution

It introduces a method for high-resolution single-spin NMR spectroscopy and uncovers previously unobserved spin Hamiltonian components.

Findings

Achieved Hertz spectral resolution in single-spin NMR.

Determined the 93Nb impurity site and quadrupolar tensor.

Identified two new terms in the spin Hamiltonian.

Abstract

Single-spin magnetic resonance spectroscopy promises to yield structural and chemical information at the level of individual atoms or molecules, in a non-invasive way. Here, we use an Er3+ paramagnetic center in a CaWO4 crystal, detected by microwave photon counting at 10 mK, as a nanoscale magnetic sensor to measure the NMR spectrum of a proximal individual nuclear-spin-9/2 93Nb impurity with Hertz spectral resolution. From these measurements, we determine the 93Nb insertion site, its position relative to the Er3+ , and its complete quadrupolar tensor. We moreover harness the high spectral resolution of our measurements to establish the presence of two previously unobserved terms in the spin Hamiltonian. The first describes a coupling between the Er3+ spin and the 93Nb nuclear quadrupole; it possibly originates from a spin-dependent electrostatic interaction between the two systems. The second is a nuclear hexadecapolar term, and may be caused by the coupling of the electric field third derivative to the 93Nb nuclear hexadecapolar moment.