Evan Telford | Searcharxiv

arXiv2024

Quantum Noise Spectroscopy of Criticality in an Atomically Thin Magnet

Dynamic critical fluctuations in magnetic materials encode important information about magnetic ordering in the associated critical exponents. Using nitrogen-vacancy centers in diamond, we implement $T_2$ (spin-decoherence) noise magnetometry to study critical dynamics in a 2D Van der Waals magnet CrSBr. By analyzing NV decoherence on time scales approaching the characteristic correlation time $\tau_c$ of critical fluctuations, we extract the critical exponent $\nu$ for the correlation length. Our result deviates from the Ising prediction and highlights the role of long-range dipolar interactions in 2D CrSBr. Furthermore, analyzing the divergence of the correlation length suggests the possibility of 2D-XY criticality in CrSBr in a temperature window near $T_C$ where static magnetic domains are absent. Our work provides a first demonstration of $T_2$ noise magnetometry to quantitatively analyze critical scaling behavior in 2D materials.

cond-mat.mes-hall↗

arXiv2019

Blockade of vortex flow by thermal fluctuations in atomically thin clean-limit superconductors

Resistance in superconductors arises from the motion of vortices driven by flowing supercurrents or external electromagnetic fields and may be strongly affected by thermal or quantum fluctuations. The common expectation borne out in previous experiments is that as the temperature is lowered, vortex motion is suppressed, leading to a decreased resistance. A new generation of materials provides access to the previously inaccessible regime of clean-limit superconductivity in atomically thin superconducting layers. We show experimentally that for few-layer 2H-NbSe$_2$ the resistance below the superconducting transition temperature may be non-monotonic, passing through a minimum and then increasing again as temperature is decreased further. The effects exists over a wide range of current and magnetic fields, but is most pronounced in monolayer devices at intermediate currents. Analytical and numerical calculations confirm that the findings can be understood in a two-fluid vortex model, in which a fraction of vortices flow in channels while the rest are pinned but thermally fluctuating in position. We show theoretically that the pinned, fluctuating vortices effectively control the mobility of the free vortices. The findings provide a new perspective on fundamental questions of vortex mobility and dissipation in superconductors.

cond-mat.mes-hall↗