Magnetic Field Induced Vortex Lattice Transition in HgBa$_{2}$CuO$_{4+\delta}$

TL;DR

This study uses $^{17}$O NMR to investigate vortex lattice structures in HgBa$_{2}$CuO$_{4+eta}$, revealing a magnetic field-induced transition from an oblique to a triangular vortex lattice near 15 T and analyzing vortex dynamics.

Contribution

First to identify a vortex lattice structural transition in HgBa$_{2}$CuO$_{4+eta}$ using NMR and Ginzburg-Landau theory, linking it to magnetic field and anisotropy effects.

Findings

Vortex lattice transitions from oblique to triangular near 15 T.

Vortex melting temperature is independent of magnetic field at high fields.

Structural parameters like penetration depth vary with magnetic field.

Abstract

Measurements of the $^{17}$O nuclear magnetic resonance (NMR) quadrupolar spectrum of apical oxygen in HgBa$_{2}$CuO$_{4+\delta}$ were performed over a range of magnetic fields from 6.4 to 30\,T in the superconducting state. Oxygen isotope exchanged single crystals were investigated with doping corresponding to superconducting transition temperatures from 74\,K underdoped, to 78\,K overdoped. The apical oxygen site was chosen since its NMR spectrum has narrow quadrupolar satellites that are well separated from any other resonance. Non-vortex contributions to the spectra can be deconvolved in the time domain to determine the local magnetic field distribution from the vortices. Numerical analysis using Brandt's Ginzburg-Landau theory was used to find structural parameters of the vortex lattice, penetration depth, and coherence length as a function of magnetic field in the vortex solid phase. From this analysis we report a vortex structural transition near 15\,T from an oblique lattice with an opening angle of $73^{\circ}$ at low magnetic fields to a triangular lattice with $60^{\circ}$ stabilized at high field. The temperature for onset of vortex dynamics has been identified with vortex lattice melting. This is independent of the magnetic field at sufficiently high magnetic field similar to that reported for YBa$_2$Cu$_3$O$_7$ and Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ and is correlated with mass anisotropy of the material. This behavior is accounted for theoretically only in the limit of very high anisotropy.