Research

Superconductivity

■ Magnetic penetration depth measurements

Characterizing the performance of superconductors in the presence of magnetic fields is of critical importance not only for finding and optimizing materials for applications, but also for understanding and comparing basic properties in different classes of superconductors, ranging from conventional metallic, via high temperature cuprate and pnictide, to heavy fermion systems.

Using our unique He-3 MFM with vector magnet capabilities and a straightforward comparative method, we aim to directly probe the absolute value of the London penetration depth in a spatially resolved manner, as well as obtaining the absolute value of the pinning force for single Abrikosov vortices.

■ One dimensional linear potential system

Superconductors that are geometrically confined often struggle to maintain the typical quantization of magnetic flux because they can’t fully block external magnetic fields. We found that magnetic field confinement happens regardless of the system’s dimensionality, even in 1D linear systems. Using a vector-field magnetic force microscope, we created a vortex‒antivortex pair linked by a 1D unquantized magnetic flux in ultrathin superconducting films. By studying the pair’s behavior, we discovered a long-range interaction mediated by this flux, suggesting that unquantized magnetic flux can form universally, independent of geometry. This opens new possibilities for demonstrating braiding operations in topological superconducting vortices.

Kim, et al., NPG Asia Mater 16, 44 (2024)

■ Superconductor/ferromagnet hybrid system

The interaction between superconductivity and magnetism has been studied for years, but the influence of superconducting states on magnetic domains has received less attention. We show that thermally activated superconducting vortices can manipulate magnetic domains in a ferromagnet/superconductor hybrid. Specifically, we observe a reversible transition between magnetic stripes and bubbles in a Nb/CeRu2Ga2B system, driven by vortex motion in the superconductor. The close proximity of Curie and superconducting critical temperatures, along with magnetic metastable states, makes this manipulation possible.

Yun, et al., Appl. Phys. Lett. 124, 052601 (2024)

■ Superconducting device

We investigate the Larkin–Ovchinnikov instability in epitaxial NbN microbridges, revealing how structural homogeneity affects vortex dissipation. Our analysis of current–voltage curves under varying magnetic fields and temperatures shows vortex velocities reaching 6 km/s, decreasing with higher magnetic fields and stabilizing near the critical temperature. Recombination times (τ), ranging around 10 ps and decreasing with temperature, highlight the role of thermally activated phonons, with thinner films exhibiting shorter τ values, emphasizing the impact of film thickness on vortex dynamics. Comparing our results with existing literature demonstrates that disorder and surface roughness play crucial roles in determining vortex velocity. These insights into vortex behavior in high-quality thin films offer valuable implications for advancing superconducting technologies.

Magnetism

The interplay between various degrees of freedom in correlated electron systems is the origin of rich phase diagrams, where oftentimes small details determine the occurrence of unexpected and exotic phases. While bulk magnetic characterization studies of such systems can give a hint about the kind of magnetic order, the actual domain structure remains elusive. Moreover, domain structures with similar fingerprints in thermodynamic measurements can differ dramatically.

Uncovering the microscopic magnetic structure and its field- and temperature behavior has important implications ranging from conceiving next-generation magnetic data storage devices to understanding the formation of exotic, topologically non-trivial phenomena, such as skyrmions.

■ Magnetic domains in a van der Waals ferromagnets

We reveal the intricate magnetic domain configuration in the van der Waals ferromagnets using advanced vector-field cryogenic magnetic force microscopy. Our findings show how varying magnetic field strength and angle lead to the coexistence of striped and spike-like magnetic domains. By uncovering the role of uniaxial magnetic anisotropy in shaping these domains, this research provides critical insights into controlling and stabilizing diverse domain patterns in van der Waals ferromagnets, paving the way for future innovations in magnetic materials and device engineering.

■ Magnetic domains in uniaxial ferromagnets

In the search for advanced materials for magnetic data storage and spintronic devices, compounds with intrinsic anisotropies or competing interactions are crucial for tailoring magnetic domain shapes and sizes. Using vector magnetic fields, we demonstrate precise control over domain shapes and sizes in the uniaxial ferromagnets, enabling smooth transitions from stripe to bubble domains, paving the way for future applications in magnetic domain engineering.

Weyl metal

Weyl metals, or Weyl semimetals, are topological quantum materials that host Weyl fermions—quasi-particles behaving like massless chiral fermions. A key feature of these materials is the chiral anomaly, where chiral charge is not conserved in the presence of parallel electric and magnetic fields, leading to negative magnetoresistance. This results in a violation of Ohm’s law, as the current becomes dependent on the orientation of the fields, rather than following a linear response. This non-ohmic behavior highlights the unique quantum properties of Weyl metals, offering insights into topological phenomena and potential applications in advanced electronics.

■ Violation of Ohm’s law

Ohm’s law, a cornerstone of electrical transport in metals, is fundamentally challenged by our discovery of its breakdown in the Weyl metal phase due to the chiral anomaly’s topological structure. In Bi₀.₉₆Sb₀.₀₄ single crystals, we observe nonlinear I–V characteristics in the diffusive limit, emerging only when the electric and magnetic fields are aligned (E∥B). Using Boltzmann transport theory with charge pumping, we reveal a topologically driven nonlinear conductivity, leading to a universal scaling function that fully explains our experimental results. This nonlinear conductivity, a hallmark of Weyl metals, opens new possibilities for nonlinear electronics, optical applications, and advances in topological Fermi-liquid theory beyond traditional Landau Fermi-liquid models.

■ Supernonlocality in a Weyl metal

We have discovered a macroscopic quantum phenomenon, termed supernonlocality, in the Weyl state, a topological metal known for both surface and bulk topological transport. Supernonlocality is defined by the striking similarity between nonlocal and local resistances, pointing to a global transport property. We observe an extraordinary nonlocal decay length of 0.6 mm, far exceeding that of other semimetals. This behavior is strongly linked to the system’s nonlinear conductivity, suggesting that supernonlocality originates from chiral charge pumping through a one-dimensional topological channel. Our findings offer new insights into macroscopic quantum phenomena in topological metals, with implications for future advancements in topological nonlocal electronics.