Recent studies have shown a number of surprising vortex dynamics phenomena both in low and high temperature superconductors, which include: low frequency noise, slow voltage oscillations, history dependent dynamic response, memory of the direction, amplitude, duration, and frequency of the previously applied current, suppression of a large ac vortex response by a very small dc bias, and a strong frequency dependence. Taken together, these phenomena are incompatible with the current understanding of bulk vortex dynamics. We propose a generic mechanism to account for these observations in terms of the competition between the injection of a disordered vortex phase through the surface barriers at the sample edges, and the annealing of this metastable disorder by the transport current. The model is confirmed by investigating the current distribution across NbSe2 single crystals using arrays of Hall sensors. For an ac current only narrow regions near the edges are in the pinned disordered phase resulting in a large response. In the presence of a dc bias a wide region of the sample is filled by the disordered phase preventing vortex motion. The resulting spatial variation of the disorder across the sample acts as an active memory of the previously applied current sequence.Comment: 15 pages 4 figures. Accepted for publication in Natur

Transport studies in a Corbino disk geometry suggest that the Bragg glass phase undergoes a first-order transition into a disordered solid. This transition shows a sharp reentrant behavior at low fields. In contrast, in the conventional strip configuration, the phase transition is obscured by the injection of the disordered vortices through the sample edges, which results in the commonly observed vortex instabilities and smearing of the peak effect in NbSe 2 crystals.These features are found to be absent in the Corbino geometry, in which the circulating vortices do not cross the sample edges.

Hydrogen sensors based on single Pd nanowires show promising results in speed, sensitivity, and ultralow power consumption. The utilization of single Pd nanowires, however, face challenges in nanofabrication, manipulation, and achieving ultrasmall transverse dimensions. We report on hydrogen sensors that take advantage of single palladium nanowires in high speed and sensitivity and that can be fabricated conveniently. The sensors are based on networks of ultrasmall (<10 nm) palladium nanowires deposited onto commercially available filtration membranes. We investigated the sensitivities and response times of these sensors as a function of the thickness of the nanowires and also compared them with a continuous reference film. The superior performance of the ultrasmall Pd nanowire network based sensors demonstrates the novelty of our fabrication approach, which can be directly applied to palladium alloy and other hydrogen sensing materials.

Extremely large magnetoresistance (XMR) was recently discovered in WTe2, triggering extensive research on this material regarding the XMR origin. Since WTe2 is a layered compound with metal layers sandwiched between adjacent insulating chalcogenide layers, this material has been considered to be electronically two-dimensional (2D). Here we report two new findings on WTe2: (1) WTe2 is electronically 3D with a mass anisotropy as low as 2, as revealed by the 3D scaling behavior of the resistance R(H, θ) = R(ε θ H) with ε θ = (cos 2 θ + γ −2 sin 2 θ) 1/2 , θ being the magnetic field angle with respect to c-axis of the crystal and γ being the mass anisotropy; (2) the mass anisotropy γ varies with temperature and follows the magnetoresistance behavior of the Fermi liquid state. Our results not only provide a general scaling approach for the anisotropic magnetoresistance but also are crucial for correctly understanding the electronic properties of WTe2, including the origin of the remarkable 'turn-on' behavior in the resistance versus temperature curve, which has been widely observed in many materials and assumed to be a metal-insulator transition.

Three-dimensional topological Dirac semimetals have a linear dispersion in 3D momentum space and are viewed as the 3D analogues of graphene. Here, we report angle-dependent magnetotransport on the newly revealed Cd 3 As 2 single crystals and clearly show how the Fermi surface evolves with crystallographic orientations. Remarkably, when the magnetic field lies in the [112] or ½441 axis, magnetoresistance oscillations with only single period are present. However, the oscillation shows double periods when the field is applied along the ½110 direction. Moreover, aligning the magnetic field at certain directions also gives rise to double period oscillations. We attribute the observed anomalous oscillation behavior to the sophisticated geometry of Fermi surface and illustrate a complete 3D Fermi surface with two nested anisotropic ellipsoids around the Dirac points. Additionally, a submillimeter mean-free path at 6 K is found in Cd 3 As 2 crystals, indicating ballistic transport in this material. By measuring the magnetoresistance up to 60 T, we reach the quantum limit (n ¼ 1 Landau level) at about 43 T. These results improve the knowledge of the Dirac semimetal material Cd 3 As 2 and also pave the way for proposing new electronic applications based on 3D Dirac materials.