Hydride superconductivity is here to stay
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Recent advances in high-entropy superconductors
High-entropy materials (HEMs) exhibit significant potential for diverse applications owing to their tunable properties, which can be precisely engineered through the selection of specific elements and the modification of stoichiometric ratios. The discovery of superconductivity in HEMs has garnered considerable interest, leading to accelerated advancements in this field in recent years. This review provides an overview of various high-entropy superconductors, highlighting their distinct features, such as disordered crystal structure, factors affecting the critical temperature (Tc), unconventional superconductivity, and topological bands. A perspective on this field is subsequently proposed, drawing upon insights from recently published academic literature. The objective is to provide researchers with a comprehensive and clear understanding of the newly developed high-entropy superconductivity, thereby catalyzing further advancements in this domain.
Absence of diode effect in chiral type-I superconductor NbGe2
Symmetry elegantly governs the fundamental properties and derived functionalities of condensed matter. For instance, realizing the superconducting diode effect (SDE) demands breaking space-inversion and time-reversal symmetries simultaneously. Although the SDE is widely observed in various platforms, its underlying mechanism remains debated, particularly regarding the role of vortices. Here, we systematically investigate the nonreciprocal transport in the chiral type-I superconductor NbGe2. Moreover, we induce type-II superconductivity with elevated superconducting critical temperature on the artificial surface by focused ion beam irradiation, enabling control over vortex dynamics in NbGe2 devices. Strikingly, we observe negligible diode efficiency (Q < 2%) at low magnetic fields, which rises significantly to Q ~ 50% at high magnetic fields, coinciding with an abrupt increase in vortex creep rate when the superconductivity of NbGe2 bulk is suppressed. These results unambiguously highlight the critical role of vortex dynamics in the SDE, in addition to the established symmetry rules.
Enhanced superconductivity near a pressure-induced quantum critical point of strongly coupled charge density wave order in 2H-Pd0.05TaSe2
Interplay between charge density wave (CDW) order and superconductivity (SC) in quasi-two-dimensional materials remains poorly understood due to their diverse experimental varieties. Here, we investigate the pressure-dependent electrical transport and Raman scattering spectra of 2H-Pd0.05TaSe2, which exhibits a CDW transition at TCDW = 115 K and a superconducting transition at Tc = 2.6 K at ambient pressure conditions. As pressure increases, TCDW, identified by the resistivity anomaly, shifts towards lower temperatures and approaches zero at a critical pressure of Pc ~ 21.5 GPa. At this critical pressure, both Tc and upper critical field Hc2 reach their maximum values of ~ 8.5 K and ~ 6.4 T, respectively. Analysis of the Raman scattering spectra demonstrates that increasing pressure systematically suppresses both the two-phonon spectral weight above TCDW and the CDW amplitudon energies below TCDW, leading to their simultaneous disappearance at Pc. These observations provide direct evidence for the formation of a CDW quantum critical point (QCP) at Pc, indicating that charge and lattice fluctuations associated with the QCP of strongly coupled CDW order may enhance SC in pressurized 2H-Pd0.05TaSe2.
Tuning a magnetic energy scale with pressure and field in UTe2
When a fragile ordered state is suppressed to zero temperature, a quantum phase transition occurs, which is often marked by the appearance of unconventional superconductivity. While the quantum critical point can be hidden, the influence of the quantum criticality extends to fairly high temperatures, manifesting non-Fermi liquid behavior in a wide range of the field-temperature-pressure phase space. Here, we report the tuning of a magnetic energy scale in the heavy-fermion superconductor UTe2, previously identified with a peak in the c-axis electrical transport temperature dependence, using applied hydrostatic pressures and a-axis-oriented magnetic fields as complementary (and opposing) tuning parameters: the characteristic peak in c-axis resistivity decreases in temperature with applied pressure before vanishing near the critical pressure of 15 kbar (1.5 GPa), while the application of field shifts the peak to a higher temperature and broadens it under all studied pressures. At the critical pressure, the transport behavior deviates from Fermi liquid behavior, exhibiting a nearly linear temperature dependence of resistivity with an enhanced pre-factor. Our results shed light on the microscopic origin of the c-axis resistivity peak and provide a clear picture of magnetic energy scale evolution relevant to quantum criticality in UTe2.
Long-range crossed Andreev reflection in a topological insulator nanowire proximitized by a superconductor
Crossed Andreev reflection is a non-local transport phenomenon that creates and detects Cooper pair correlations between distant locations. It is also the basis of Cooper pair splitting to generate remote entanglement. Although crossed Andreev reflection has been extensively studied in semiconductors proximity-coupled to a superconductor, observing it in a topological insulator has been very difficult. Here we report the observation of this effect in a proximitized topological insulator nanowire. We perform local and non-local conductance spectroscopy on mesoscopic devices in which superconducting niobium and metallic contacts are connected to a bulk-insulating nanowire. In our local conductance measurements we detect a hard gap and the appearance of Andreev bound states that can reach zero bias. We also occasionally observe a negative non-local conductance when sweeping the chemical potential, providing evidence of crossed Andreev reflection. This signal is detected even over length scales much longer than the expected superconducting coherence length of either niobium or the proximitized nanowire. We suggest that this long-range effect is due to the intricate role of disorder in proximitized nanowires.
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