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Dirac material LaCuSb2 shows anisotropic superconducting response to applied magnetic fields. 

Superconducting nickelates are a new family of strongly correlated electron materials with a phase diagram closely resembling that of superconducting cuprates. While analogy with the cuprates is natural, very little is known about the metallic state of the nickelates, making these comparisons difficult. We probe the electronic dispersion of thin-film superconducting five-layer ( $n=5$) and metallic three-layer ( $n=3$) nickelates by measuring the Seebeck coefficient $S$. We find a temperature-independent and negative $S/T$for both $n=5$and $n=3$nickelates. These results are in stark contrast to the strongly temperature-dependent $S/T$measured at similar electron filling in the cuprate ${\mathrm{La}}_{1.36}{\mathrm{Nd}}_{0.4}{\mathrm{Sr}}_{0.24}{\mathrm{CuO}}_4$. The electronic structure calculated from density-functional theory can reproduce the temperature dependence, sign, and amplitude of $S/T$in the nickelates using Boltzmann transport theory. This demonstrates that the electronic structure obtained from first-principles calculations provides a reliable description of the fermiology of superconducting nickelates and suggests that, despite indications of strong electronic correlations, there are well-defined quasiparticles in the metallic state. Finally, we explain the differences in the Seebeck coefficient between nickelates and cuprates as originating in strong dissimilarities in impurity concentrations. Our study demonstrates that the high elastic scattering limit of the Seebeck coefficient reflects only the underlying band structure of a metal, analogous to the high magnetic field limit of the Hall coefficient. This opens a new avenue for Seebeck measurements to probe the electronic band structures of relatively disordered quantum materials. Published by the American Physical Society2024 

Adiabatic decompression of paraquadrupolar materials has significant potential as a cryogenic cooling technology. We focus on TmVO ${}_4$, an archetypal material that undergoes a continuous phase transition to a ferroquadrupole-ordered state at 2.15 K. Above the phase transition, each Tm ion contributes an entropy of $k_{B}ln2$due to the degeneracy of the crystal electric field groundstate. Owing to the large magnetoelastic coupling, which is a prerequisite for a material to undergo a phase transition via the cooperative Jahn–Teller effect, this level splitting, and hence the entropy, can be readily tuned by externally induced strain. Using a dynamic technique in which the strain is rapidly oscillated, we measure the adiabatic elastocaloric response of single-crystal TmVO ${}_4$, and thus experimentally obtain the entropy landscape as a function of strain and temperature. The measurement confirms the suitability of this class of materials for cryogenic cooling applications and provides insight into the dynamic quadrupole strain susceptibility. 

We present in situ calorimetry, thermal conductivity, and thermal diffusivity measurements of materials using temperature-sensing optical wireless integrated circuits (OWiCs). These microscopic and untethered optical sensors eliminate input wires and reduce parasitic effects. Each OWiC has a mass of ∼100 ng, a 100-μm-scale footprint, and a thermal response time of microseconds. We demonstrate that they can measure the thermal properties of nearly any material, from aerogels to metals, on samples as small as 100 ng and over thermal diffusivities covering four orders of magnitude. They also function over a broad temperature range, and we present proof-of-concept measurements of the thermodynamic phase transitions in both liquid crystal 5CB and gadolinium. 

Topological semimetals are predicted to exhibit unconventional electrodynamics, but a central experimental challenge is singling out the contributions from the topological bands. TaAs is the prototypical example, where 24 Weyl points and 8 trivial Fermi surfaces make the interpretation of any experiment in terms of band topology ambiguous. We report magneto-infrared reflection spectroscopy measurements on TaAs. We observed sharp inter-Landau level transitions from a single pocket of Weyl Fermions in magnetic fields as low as 0.4 tesla. We determine the W2 Weyl point to be 8.3 meV below the Fermi energy, corresponding to a quantum limit—the field required to reach the lowest LL—of 0.8 tesla—unprecedentedly low for Weyl Fermions. LL spectroscopy allows us to isolate these Weyl Fermions from all other carriers in TaAs, and our result provides a way for directly exploring the more exotic quantum phenomena in Weyl semimetals, such as the chiral anomaly.