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Banda, J., D. Hafner, J. F. Landaeta, E. Hassinger, K. Mitsumoto, M. Giovannini, J. G. Sereni, C. Geibel, and M. Brando. "Electronuclear Quantum Criticality." arXiv (2023): 2308.15294.
Abstract: We present here a rare example of electronuclear quantum criticality in a metal. The compound YbCu4.6Au0.4 is located at an unconventional quantum critical point (QCP). In this material the relevant Kondo and RKKY exchange interactions are very weak, of the order of 1 K. Furthermore, there is strong competition between antiferromagnetic and ferromagnetic correlations, possibly due to geometrical frustration within the fcc Yb sublattice. This causes strong spin fluctuations which prevent the system to order magnetically. Because of the very low Kondo temperature the Yb3+ 4f-electrons couple weakly with the conduction electrons allowing the coupling to the nuclear moments of the 171Yb and 173Yb isotopes to become important. Thus, the quantum critical fluctuations observed at the QCP do not originate from purely electronic states but from entangled electronuclear states. This is evidenced by the anomalous temperature and field dependence of the specific heat at low temperatures.
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Zabala, J., P. Pedrazzini, F. J. Castro, and V. F. Correa. "Enhanced weak superconductivity in trigonal γ-PtBi2." arXiv 2023 (2023): 2309.00105.
Abstract: Electrical transport experiments show superconductivity in a high-quality single crystal of trigonal γ-PtBi2. The critical temperature shows a large dependence on the electrical current and in the limit of very low currents, a Tc = 1.1 K is observed, while a zero temperature critical field Bc(0) ≈ 1.5 Tesla is estimated. These are the highest superconducting parameters reported (at ambient pressure) in a stoichiometric γ-PtBi2 bulk sample so far. Under a magnetic field a strict zero resistance state is no longer observed even though an incipient superconducting transition is seen. Such a behavior is most probably associated with very low critical currents and is reminiscent of filamentary superconductivity. The superconducting state is elusive to magnetization measurements discarding a bulk phase down to T= 0.3 K.
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Sereni, J. G. First observation of Ce volume collapse in CeN., 2017.
Abstract: On the occasion of the 80th anniversary of the first observation of Ce volume collapse in CeN a remembrance of the implications of that transcendent event is presented, along with a review of the knowledge of Ce physical properties available at that time. Coincident anniversary corresponds to the first proposal for Ce as a mix valence element, motivating to briefly review how the valence instability of Ce was investigated since that time.
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Schimmel, S., Y. Fasano, S. Hoffmann, J. Puig, G. Shipunov, D. Baumann, S. Aswartham, B. Büchner, and C. Hess. "High-TC surface superconductivity in topological Weyl semimetal t-PtBi2." arXiv (2023): 2302.08968.
Abstract: Topological superconductivity is a very promising concept for generating fault tolerant qubits in quantum computation[1-3]. Early experimental achievements study hybrid systems[4-5] as well as doped intrinsic topological or superconducting materials[6-8]. These pioneering results concern either inconclusive phenomenology or very-low temperatures. In a very recent work intrinsic two-dimensional low-temperature superconductivity has been reported in the type-I Weyl semimetal trigonal PtBi2[9]. However, larger critical temperatures are indispensable for technological exploitation. Here we show that trigonal PtBi2 possesses high-temperature surface superconductivity. By means of scanning tunnelling spectroscopy, we reveal that the superconducting gap can be as large as 20 meV, suggesting a critical temperature in the 100 K range, like in cuprate high-temperature superconductors. To our knowledge, this is the largest superconducting gap that has been observed so far in a topological material. The superconducting state at 5 K persists up to Tesla-range applied fields and the upper critical field at this temperature is of roughly 12 Tesla. Our results therefore render trigonal PtBi2 a prime candidate for topological superconductivity at technologically relevant temperatures.
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Blatter, G., M. Sirena, Y. Lee, J. Kim, and N. Haberkorn. "Tuning vortex critical velocity in Mo2N thin films via striped magnetic domain configuration." arXiv (2023): arXiv:2310.18524.
Abstract: We report on the impact of the magnetic domain stripe configuration on the critical velocity of vortices in superconducting/ferromagnetic bilayers. Using a 23 nm thick Mo$_2$N film, covered by a 48 nm FePt layer with tunable nanosized striped domains, we demonstrate that flux instability at low magnetic fields depends on the orientation of the stripes. When the stripes are perpendicular to the applied current and act as vortex guides, the velocity values reach 5 km/s, duplicating those found when configured parallel to the current, creating winding vortex paths. Our results indicate that vortex critical velocities can be tuned by configuring different domain structures, providing a platform to understand vortex dynamics in superconducting microstrips.
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