Anomalous superconducting diode effect in a polar superconductor

A superconductor with broken time reversal and inversion symmetry may exhibit nonreciprocal charge transport, including a nonreciprocal critical current, also known as superconducting diode effect. We report an intrinsic superconducting diode effect in a polar strontium titanate film. Differential resistance measurements reveal a superconducting state whose depairing current is polarity dependent. There is, however, no measurable deviation from Ohmic behavior, implying that this state does not arise from a bulk magnetochiral anisotropy. In the entire measurement range, the only deviation from linearity in the differential resistance is on the edge of the superconducting transition at high magnetic fields, likely due to the motion of flux vortices. Furthermore, the magnitude of the effect is preserved even when the in-plane magnetic field is oriented parallel to the current, indicating that this effect truly does not originate from a bulk magnetochiral anisotropy

Induced superconductivity in the two-dimensional topological insulator phase of cadmium arsenide

Hybrid structures between conventional, s-wave superconductors and two-dimensional topological insulators (2D TIs) are a promising route to topological superconductivity. Here, we investigate planar Josephson junctions fabricated from hybrid structures that use thin films of cadmium arsenide (Cd3As2) as the 2D TI material. Measurements of superconducting interference patterns in a perpendicular magnetic field are used to extract information about the spatial distribution of the supercurrent. We show that the interference patterns are distinctly different in junctions with and without mesa-isolation, respectively. In mesa-defined junctions, the bulk of the 2D TI appears to be almost completely shunted by supercurrent flowing along the edges, while the supercurrent is much more uniform across the junction when the Cd3As2 film extends beyond the device. We discuss the possible origins of the observed behaviors.Comment: Accepted for publication in APL Material

Exchange biased anomalous Hall effect driven by frustration in a magnetic kagome lattice.

Co[Formula: see text]Sn[Formula: see text]S[Formula: see text] is a ferromagnetic Weyl semimetal that has been the subject of intense scientific interest due to its large anomalous Hall effect. We show that the coupling of this material's topological properties to its magnetic texture leads to a strongly exchange biased anomalous Hall effect. We argue that this is likely caused by the coexistence of ferromagnetism and geometric frustration intrinsic to the kagome network of magnetic ions, giving rise to spin-glass behavior and an exchange bias

Observation of two-dimensional Fermi surface and Dirac dispersion in YbMnSb2_2

We present the crystal structure, electronic structure, and transport properties of the material YbMnSb$_2$, a candidate system for the investigation of Dirac physics in the presence of magnetic order. Our measurements reveal that this system is a low-carrier-density semimetal with a 2D Fermi surface arising from a Dirac dispersion, consistent with the predictions of density functional theory calculations of the antiferromagnetic system. The low temperature resistivity is very large, suggesting scattering in this system is highly efficient at dissipating momentum despite its Dirac-like nature.Comment: 8 pages, 6 figure

Magnetic torque anomaly in the quantum limit of the Weyl semi-metal NbAs

Electrons in materials with linear dispersion behave as massless Weyl- or Dirac-quasiparticles, and continue to intrigue physicists due to their close resemblance to elusive ultra-relativistic particles as well as their potential for future electronics. Yet the experimental signatures of Weyl-fermions are often subtle and indirect, in particular if they coexist with conventional, massive quasiparticles. Here we report a large anomaly in the magnetic torque of the Weyl semi-metal NbAs upon entering the "quantum limit" state in high magnetic fields, where topological corrections to the energy spectrum become dominant. The quantum limit torque displays a striking change in sign, signaling a reversal of the magnetic anisotropy that can be directly attributed to the topological properties of the Weyl semi-metal. Our results establish that anomalous quantum limit torque measurements provide a simple experimental method to identify Weyl- and Dirac- semi-metals.Comment: 5 pages, 4 figure

Competition between magnetic order and charge localization in Na2_2IrO3_3 thin crystal devices

Spin orbit assisted Mott insulators such as sodium iridate (Na$_2$IrO$_3$) have been an important subject of study in the recent years. In these materials, the interplay of electronic correlations, spin-orbit coupling, crystal field effects and a honeycomb arrangement of ions bring exciting ground states, predicted in the frame of the Kitaev model. The insulating character of Na$_2$IrO$_3$ has hampered its integration to an electronic device, desirable for applications, such as the manipulation of quasiparticles interesting for topological quantum computing. Here we show through electronic transport measurements supported by Angle Resolved Photoemission Spectroscopy (ARPES) experiments, that electronic transport in Na$_2$IrO$_3$ is ruled by variable range hopping and it is strongly dependent on the magnetic ordering transition known for bulk Na$_2$IrO$_3$, as well as on external electric fields. Electronic transport measurements allow us to deduce a value for the localization length and the density of states in our Na$_2$IrO$_3$ thin crystals devices, offering an alternative approach to study insulating layered materials

Magnetic torque anomaly in the quantum limit of Weyl semimetals

Electrons in materials with linear dispersion behave as massless Weyl- or Dirac-quasiparticles, and continue to intrigue due to their close resemblance to elusive ultra-relativistic particles as well as their potential for future electronics. Yet the experimental signatures of Weyl-fermions are often subtle and indirect, in particular if they coexist with conventional, massive quasiparticles. Here we show a pronounced anomaly in the magnetic torque of the Weyl semimetal NbAs upon entering the quantum limit state in high magnetic fields. The torque changes sign in the quantum limit, signalling a reversal of the magnetic anisotropy that can be directly attributed to the topological nature of the Weyl electrons. Our results establish that anomalous quantum limit torque measurements provide a direct experimental method to identify and distinguish Weyl and Dirac systems

The Role of Industry, Geography and Firm Heterogeneity in Credit Risk Diversification

In theory the potential for credit risk diversification for banks could be substantial. Portfolio diversification is driven broadly by two characteristics: the degree to which systematic risk factors are correlated with each other and the degree of dependence individual firms have to the different types of risk factors. We propose a model for exploring these dimensions of credit risk diversification: across industry sectors and across different countries or regions. We find that full firm-level parameter heterogeneity matters a great deal for capturing differences in simulated credit loss distributions. Imposing homogeneity results in overly skewed and fat-tailed loss distributions. These differences become more pronounced in the presence of systematic risk factor shocks: increased parameter heterogeneity greatly reduces shock sensitivity. Allowing for regional parameter heterogeneity seems to better approximate the loss distributions generated by the fully heterogeneous model than allowing just for industry heterogeneity. The regional model also exhibits less shock sensitivity

Paths to interacting topological semimetals

Do topological electrons interact differently than normal electrons? We discuss three paths to answering this question.One route is to engineer a new material with nontrivial topology and long-range magnetic order. In the first part of this dissertation we present a new material, YbMnSb2, and we provide evidence from transport, thermodynamic, and spectroscopic measurements to show that it is an antiferromagnetic topological semimetal. The majority of this work was published as Kealhofer, et al., Phys. Rev. B 97, 045109.Another route to answer this question is to study the role of topology in strongly interacting systems. The cerium monopnictides CeSb and CeBi incorporate threads from f-electron magnetism, many-body (e.g. Kondo) physics, and topology. CeSb is a trivial semimetal, while CeBi is a topological semimetal. in the second part of this dissertation, we will untangle some of these threads and present our investigation of the relationship between topology, magnetism, and Kondo-like hybridization in CeSb and CeBi. The main tool we will use is angle-resolved photoemission spectroscopy (ARPES), taking advantage of the MERLIN beamline at the Advanced Light Source and its ability to resonantly tune to the Ce N-edge. This capability allows us to be exquisitely sensitive to the photoemission signal originating from the Ce f electrons. We will see that the signatures of magnetic interactions in CeSb and CeBi are markedly different, a difference whose origin may be the presence of topological band crossings in CeBi. Our work on CeSb was published as Jang, Kealhofer, et al., Sci. Adv. 5(3) eaat7158.A third route to studying interactions in topological semimetals is to somehow reduce the dimension of the material from three to one. Fermi liquids abound in three dimensions--metals (and semimetals) are everywhere. In one dimension, though, the the ground state for a large number of interacting fermions is not the Fermi liquid but the Tomonaga-Luttinger liquid. So it would seem that if, in the laboratory, there were a knob labeled ``Dimension'' which one could turn from "3" to "1," it would be easy to study non-Fermi liquids. In the final part of this dissertation, we discuss a future experiment which would attempt to turn this knob in TaAs, a Weyl semimetal. As one of the experimental consequences of the Fermi liquid is the proportionality of thermal conductivity and electrical conductivity at a fixed temperature (the Wiedemann-Franz law), the proposed experiment is to measure this ratio (the Lorenz number) in a strong applied magnetic field. Due to TaAs's low carrier density, at even relatively moderate fields of 5 T or lower the electrons will occupy only the lowest Landau level. As a result, conduction along the field direction is relatively unchanged, while the conduction perpendicular to the field is reduced, giving rise to a dimensional crossover. Measuring thermal conductivity through this crossover will allow us to observe signatures of the one dimensional conducting state, in which interactions between electrons cannot be ignored