Static and Dynamic Electronic Properties of Weyl Semimetal NbP -- A Single Crystal $^{93}$Nb-NMR Study

TL;DR

This study uses $^{93}$Nb-NMR to investigate the static and dynamic electronic properties of the Weyl semimetal NbP, revealing details about its density of states, band structure, and nuclear relaxation behaviors.

Contribution

It provides the first detailed NMR analysis of NbP, determining electric quadrupole and hyperfine parameters, and compares its relaxation dynamics with related Weyl semimetals.

Findings

Knight shift reveals nearly massless fermion density of states

Quantum oscillations match previous bulk measurements

Distinct temperature dependence of spin-lattice relaxation rate

Abstract

Nuclear magnetic resonance (NMR) techniques have been used to study the static and dynamic microscopic properties of the Weyl semimetal NbP. From a complete analysis of the angular dependence of the $^{93}$Nb-NMR spectra in a single crystal, the parameters for the electric quadrupole interactions and the magnetic hyperfine interactions were determined to be $\nu_{\rm Q} = 0.61$\,MHz, $\eta = 0.20$, $(K_{XX}, K_{YY}, K_{ZZ}) = (- 0.06, 0.11, - 0.11)$\% at 4.5\,K. The temperature and field dependence of the $^{93}$Nb Knight shift revealed a characteristic feature of the shape of the density of states with nearly massless fermions. We clearly observed a quantum oscillation of the Knight shift associated with the band structure, whose frequency was in good agreement with the previous bulk measurements. The temperature dependence of the spin-lattice relaxation rate, $1 / T_{1} T$, showed an almost constant behavior for $30 < T < 180$\,K, while a weak temperature dependence was observed below $\sim 30$\,K. This contrasts with the behavior observed in TaP and TaAs, where the $1 / T_{1} T$ measured by the $^{181}$Ta nuclear quadrupole resonance (NQR) shows $1 / T_{1} T \propto T^{2}$ and $T^{4}$ above approximately 30\,K. In TaP, the temperature dependent orbital hyperfine interaction plays a signficant role in nuclear relaxation, whereas this contribution is not observed in TaAs. Two-component spin echo oscillations were observed. The shorter-period oscillation is attributed to the origin of quadrupole coupling, while the longer-period oscillation indicates the presence of indirect nuclear spin-spin coupling, as discussed in other Weyl semimetal like TaP.