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Probing Vortex Dynamics in 2D Superconductors with Scanning Quantum Microscope

Sreehari Jayaram, Malik Lenger, Dong Zhao, Lucas Pupim, Takashi Taniguchi, Kenji Watanabe, Ruoming Peng, Marc Scheffler, Rainer Stöhr, Mathias S. Scheurer, Jurgen Smet, Jörg Wrachtrup

TL;DR

This work leverages scanning quantum microscopy with NV-center magnetometry to image nanoscale vortex dynamics in a 2D superconductor (NbSe2). It reveals a distorted, non-hexagonal vortex glass at low thickness, with vortex configurations and melting driven by cooling rate, and detects dynamic magnetic noise persisting below the superconducting transition. The observed phenomena are interpreted through Pearl-vortex physics, boundary/disorder effects, and a Langevin-based model of vortex motion, linking microscopic magnetic fluctuations to macroscopic responses. The approach provides a powerful, high-resolution window into low-energy fluctuations and non-equilibrium vortex states in 2D superconductors, with potential implications for transport and device performance.

Abstract

The visualization of the magnetic responses of a two-dimensional (2D) superconducting material on the nanoscale is a powerful approach to unravel the underlying supercurrent behavior and to investigate critical phenomena in reduced dimensions. In this study, scanning quantum microscopy is utilized to explore the local magnetic response of the 2D superconductor 2H-NbSe2. Our technique enables both static and dynamic sensing of superconducting vortices with high sensitivity and a spatial resolution down to 30 nm, unveiling unexpected phenomena linked to the intrinsic 2D nature of the superconductor, which are challenging to detect with more conventional local probes. Vortices do not arrange in a hexagonal lattice, but form a distorted vortex glass with expanding vortex size. A vortex can exhibit strong local dynamics due to thermal excitation. As the critical temperature is approached, a clear melting of the vortex glass is identified, leading to distinct configurations under different cooling conditions. Vortex fluctuations can also be probed through spin Hahn-echo measurements, which reveal the spin decoherence even well below the critical temperature -- and, intriguingly, enhanced decoherence at lower temperatures. Spatiotemporal microscopy of the magnetic dynamics associated with vortex excitations and fluctuations provides direct evidence of 2D superconducting phenomena at the nanoscale.

Probing Vortex Dynamics in 2D Superconductors with Scanning Quantum Microscope

TL;DR

This work leverages scanning quantum microscopy with NV-center magnetometry to image nanoscale vortex dynamics in a 2D superconductor (NbSe2). It reveals a distorted, non-hexagonal vortex glass at low thickness, with vortex configurations and melting driven by cooling rate, and detects dynamic magnetic noise persisting below the superconducting transition. The observed phenomena are interpreted through Pearl-vortex physics, boundary/disorder effects, and a Langevin-based model of vortex motion, linking microscopic magnetic fluctuations to macroscopic responses. The approach provides a powerful, high-resolution window into low-energy fluctuations and non-equilibrium vortex states in 2D superconductors, with potential implications for transport and device performance.

Abstract

The visualization of the magnetic responses of a two-dimensional (2D) superconducting material on the nanoscale is a powerful approach to unravel the underlying supercurrent behavior and to investigate critical phenomena in reduced dimensions. In this study, scanning quantum microscopy is utilized to explore the local magnetic response of the 2D superconductor 2H-NbSe2. Our technique enables both static and dynamic sensing of superconducting vortices with high sensitivity and a spatial resolution down to 30 nm, unveiling unexpected phenomena linked to the intrinsic 2D nature of the superconductor, which are challenging to detect with more conventional local probes. Vortices do not arrange in a hexagonal lattice, but form a distorted vortex glass with expanding vortex size. A vortex can exhibit strong local dynamics due to thermal excitation. As the critical temperature is approached, a clear melting of the vortex glass is identified, leading to distinct configurations under different cooling conditions. Vortex fluctuations can also be probed through spin Hahn-echo measurements, which reveal the spin decoherence even well below the critical temperature -- and, intriguingly, enhanced decoherence at lower temperatures. Spatiotemporal microscopy of the magnetic dynamics associated with vortex excitations and fluctuations provides direct evidence of 2D superconducting phenomena at the nanoscale.
Paper Structure (6 sections, 4 figures)

This paper contains 6 sections, 4 figures.

Table of Contents

  1. Introduction
  2. Local vortex excitation in the 2D superconductor
  3. Arrangement of the vortices in 2D superconductor
  4. Melting transition of the vortex lattice
  5. Dynamics in the vortex state
  6. Conclusion

Figures (4)

  • Figure 1: Scanning quantum microscopy of the vortex state in 2D superconductors. (a) Schematic of the scanning quantum microscope used to probe local magnetic responses at the nanoscale. (b) Phase diagram of a 5.1nm thick NbSe$_2$ flake as a function of temperature and magnetic field, identifying three distinct phases: vortex glass, vortex liquid, and a metallic state. (c) Magnetic field map of the vortex glass phase, showing pronounced field contrast and highlighting the sample boundary. (d) High-resolution scan resolving a single vortex in the NbSe$_2$ flake.
  • Figure 2: Vortex arrangements in 2D NbSe$_2$ with varying thickness. (a, b) Magnetic field map and corresponding autocorrelation of a thin (5.1nm) NbSe$_2$ sample S1 on an oxide substrate and encapsulated with hBN. The vortex configuration appears disordered, showing weak spatial correlations and broad, smeared autocorrelation peaks. (c, d) Magnetic field map and autocorrelation of a thicker (11.59nm) NbSe$_2$ sample S2, fully encapsulated with hBN, revealing enhanced vortex hexagonal ordering. All scale bars are 1µm.
  • Figure 3: Cooling rate–dependent vortex arrangements in 5.1nm NbSe$_2$ sample S1. (a) SQM stray field map of a thin NbSe$_2$ flake after rapid cooling (from 6.3K to 2K within 1 minute), showing weak magnetic contrast. (b) Corresponding autocorrelation reveals a disordered vortex configuration with poor spatial correlation. (c) Magnetic field map after slow cooling (from 6.3K to 2K over several hours), showing enhanced vortex contrast. (d) Autocorrelation of (c) indicates more distinct local vortex ordering and stronger magnetic response. All scale bars are 1µm.
  • Figure 4: Unexpected magnetic noise in 2D superconductors. (a) Schematic of a local NV probe detecting supercurrent-induced magnetic fluctuations in a 2D superconductor. Magnetic fields are only partially screened in the thin superconducting layer. (b) Spin Hahn echo $T_2$ measurements with the NV probe in contact with and retracted from the sample, revealing enhanced dephasing near the surface. Inset: Spin Hahn echo pulse sequence (see SI section 2.4). (c) Extracted $T_2$ times as a function of lift-off distance. These data suggest that there is a critical length scale for magnetic fluctuations $>$100nm. (d) Enhanced decoherence rate between in-contact and out-of-contact (tip-sample distance greater than 1µm) conditions (1/$T_{2}^{IC}$-1/$T_{2}^{OOC}$), measured below the superconducting transition temperature (black dots). This rate reveals magnetic noise originating from the superconducting sample and exhibits an inverse temperature dependence, in good agreement with numerical simulations (blue dots).