Logan Bishop-Van Horn

Logan Bishop-Van Horn

Physics PhD student @ Stanford

Talks and presentations

Vortex dynamics induced by scanning SQUID susceptometry

March 07, 2023

Talk, APS March Meeting 2023, Las Vegas, Nevada

Abstract: Using scanning superconducting quantum interference device (SQUID) susceptometry, one can phase-sensitively measure the local magnetic response of superconducting sample by applying a millitesla-scale AC magnetic field using a micron-scale field coil and detecting the response with a micron-scale pickup loop in a low-frequency lockin measurement. When Meissner screening is weak and the superconducting coherence length exceeds a few hundred nanometers, for example in a two-dimensional (2D) Josephson junction array or a thin film very close to its critical temperature, the local applied field from the SQUID can induce vortices in the superconductor, and subsequent motion of these vortices leads to dissipation and a change in the magnitude and phase of the measured magnetic response. Here, in an effort to quantitatively interpret these vortex-related nonlinearities and dissipative effects in measurements of 2D superconductors with long coherence lengths, we use a combination of London-Maxwell and time-dependent Ginzburg Landau (TDGL) techniques to model vortex dynamics in an AC SQUID susceptometry measurement. The model is in excellent agreement with measurements of the complex magnetic response of thin film niobium very close to its critical temperature. This work lays the foundation for scanning SQUID studies of vortex dynamics and pinning in more exotic materials systems.

Simulating the static magnetic response of thin film superconducting devices

March 14, 2022

Talk, APS March Meeting 2022, Chicago, Illinois (presented remotely)

Abstract: Quantitative understanding of the spatial distribution of magnetic fields and screening currents in two-dimensional (2D) superconductors and superconducting devices composed of thin films is critical to interpreting the results of magnetic measurements of such systems. A convenient numerical method for solving the static 2D London equation, which describes the linear magnetic response of 2D superconductors, was introduced by Brandt and Clem [Phys. Rev. B 69, 184509 (2004), Phys. Rev. B 72, 024529 (2005)]. Here, we outline the model and present an efficient, open-source Python implementation of Brandt and Clem’s matrix inversion method, which solves for the magnetic field and current distributions in devices composed of thin inhomogenous superconducting films of arbitrary geometry in the presence of trapped flux, vortices, and inhomogeneous applied fields. As a demonstration, we apply the model to scanning superconducting quantum interference device (SQUID) microscopy. Beyond magnetic microscopy, this tool can be used to model screening effects and calculate self- and mutual-inductance in superconducting devices, and simulate the magnetic response of inhomogeneous 2D superconductors.

Cryogen-free variable temperature scanning SQUID microscope

March 05, 2019

Talk, APS March Meeting 2019, Boston, Massachusetts

Abstract: Scanning Superconducting QUantum Interference Device (SQUID) microscopy is a powerful tool for imaging local magnetic properties, but it requires a low-vibration cryogenic environment, traditionally achieved by thermal contact with a bath of liquid helium or the mixing chamber of a “wet” dilution refrigerator. We mount a SQUID microscope on the 3 K plate of a Bluefors pulse tube cryocooler and characterize its vibrational spectrum by measuring SQUID noise in a region of sharp flux gradient. By implementing passive vibration isolation, we reduce relative sensor-sample vibrations to 20 nm in-plane and 15 nm out-of-plane. A variable-temperature sample stage that is thermally isolated from the SQUID sensor enables measurement at sample temperatures from 2.8 K to 110 K. We demonstrate these advances by imaging inhomogeneous susceptibility and vortex pinning in optimally doped YBCO above 90 K. Together with sub-micron spatial resolution and 350×350 μm2 scan range, these advances position us for further studies of magnetic and superconducting materials and devices over a temperature range not previously accessible to scanning SQUID microscopy.

New details in the superconducting phase diagram of λ-(BETS)2GaCl4

March 14, 2017

Talk, APS March Meeting 2017, New Orleans, Louisiana

Abstract: New low-noise rf penetration depth measurements of the high-field superconducting state in the quasi-2d organic superconductor $\lambda$-(BETS)2GaCl4 are presented and compared to previous measurements of the same material [Coniglio, et al., Phys. Rev. B 83, 224507 (2011); Mielke, et al., J. Phys.: Condens. Matter 13 (2001)]. The new data show very clear indication of a phase transition within the superconducting state, with the position of the $H_P$ phase line significantly lower than in less clean samples, while the $H_{c2}$ phase line is unchanged. Shubnikov-de Haas oscillations, previously never seen below 32 T in this material, are observed at fields as low as 13 T, indicating that there is less scattering in these new samples. $H_{c2}$ is usually sensitive to spin-orbit scattering, suggesting that the unchanged upper critical field is not traditional, but rather the destruction of a FFLO state. In contrast, $H_P$ should still be sensitive to changes of the spin-orbit scattering rate, consistent with the new data.