Logan Bishop-Van Horn

Logan Bishop-Van Horn

Physics PhD student @ Stanford

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portfolio

publications

The signature of inhomogeneous superconductivity

Published in Journal of Low Temperature Physics, 2016

This work describes the RF magnetic response of organic superconductors at low temperature and high magnetic field, where the field is applied parallel to the quasi-2D superconducting layers of the organic crystal. Under certain conditions, these materials are thought to host an unconventional phase known as the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase, in which Cooper pairs acquire a finite center of mass momentum leading to a spatially modulated superconducting order parameter.

Citation: Charles C. Agosta, Logan Bishop-Van Horn, Max Newman, "The Signature of Inhomogeneous Superconductivity", Journal of Low Temperature Physics 185, 220–229 (2016). https://doi.org/10.1007/s10909-016-1657-y

Cryogen-free variable temperature scanning SQUID microscope

Published in Review of Scientific Instruments, 2019

This paper describes the construction of a variable temperature scanning SQUID microscope in a dry cryostat, enabling precision magnetometry and susceptometry at sample temperatures from 2.8 K to over 100 K.

Citation: Logan Bishop-Van Horn, Zheng Cui, John R. Kirtley, and Kathryn A. Moler , "Cryogen-free variable temperature scanning SQUID microscope", Review of Scientific Instruments 90, 063705 (2019). https://doi.org/10.1063/1.5085008

Imaging anisotropic vortex dynamics in FeSe

Published in Physical Review B, 2019

This paper describes measurements and modeling of anisotropic vortex pinning in the iron-based superconductor FeSe.

Citation: Irene P. Zhang, Johanna C. Palmstrom, Hilary Noad, Logan Bishop-Van Horn, Yusuke Iguchi, Zheng Cui, Eli Mueller, John R. Kirtley, Ian R. Fisher, and Kathryn A. Moler, "Imaging anisotropic vortex dynamics in FeSe", Phys. Rev. B 100, 024514 (2019). https://doi.org/10.1103/PhysRevB.100.024514

SuperScreen: An open-source package for simulating the magnetic response of two-dimensional superconducting devices

Published in Computer Physics Communications, 2022

This paper introduces SuperScreen, an open-source Python package for simulating the response of 2D superconductors to trapped flux and applied time-independent or quasi-DC magnetic fields for any value of the effective magnetic penetration depth, Λ.

Citation: Logan Bishop-Van Horn and Kathryn A. Moler , "SuperScreen: An open-source package for simulating the magnetic response of two-dimensional superconducting devices", Computer Physics Communications Volume 280, November 2022, 108464. https://doi.org/10.1016/j.cpc.2022.108464

Local imaging of diamagnetism in proximity-coupled niobium nanoisland arrays on gold thin films

Published in Physical Review B, 2022

This paper describes measurements and modeling of the local magnetic response of disordered arrays of proximity-coupled superconducting nano-islands.

Citation: Logan Bishop-Van Horn*, Irene P. Zhang*, Emily N. Waite, Ian Mondragon-Shem, Scott Jensen, Junseok Oh, Tom Lippman, Malcolm Durkin, Taylor L. Hughes, Nadya Mason, Kathryn A. Moler, and Ilya Sochnikov, "Local imaging of diamagnetism in proximity-coupled niobium nanoisland arrays on gold thin films", Phys. Rev. B 106, 054521 (2022) (Editors' Suggestion). https://doi.org/10.1103/PhysRevB.106.054521

talks

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

Published:

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.

Cryogen-free variable temperature scanning SQUID microscope

Published:

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.

Simulating the static magnetic response of thin film superconducting devices

Published:

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.

Vortex dynamics induced by scanning SQUID susceptometry

Published:

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.

teaching

Teaching Assistant, Physics 43 (Spring 2018)

Undergraduate course, Stanford University, Department of Physics, 2018

Teaching assistant for Physics 43 (introductory electricity & magnetism for all non-physics STEM undergraduates at Stanford) in the 2018 Spring quarter (April - June). I led two weekly discussion sections (~20 students each), and held weekly office and tutoring hours.

Teaching Assistant, Physics 67 (Spring 2022)

Undergraduate course, Stanford University, Department of Physics, 2022

Teaching assistant for Physics 67, Introduction to Laboratory Physics with a focus on statistical data analysis, in the 2022 Spring quarter (April - June). This course is intended to teach practical data analysis skills, and all work is performed in Jupyter notebooks (see course website here). I led two weekly discussion sections (~20 students each) and held weekly office hours.

Teaching Assistant, Physics 22 (Autumn 2022)

Undergraduate course, Stanford University, Department of Physics, 2022

Teaching assistant for Physics 22, Mechanics, Fluids, and Heat Laboratory for non-physics majors, in the 2022 Autumn quarter (August - December). I led two weekly discussion sections (~20 students each) and held weekly tutoring hours.