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.

We report an anomalous matching effect in MoGe thin films containing pairs of circular holes arranged in such a way that four of those pairs meet at each vertex point of a square lattice. A remarkably pronounced fractional matching was observed in the magnetic field dependences of both the resistance and the critical current. At the half matching field the critical current can be even higher than that at zero field. This has never been observed before for vortices in superconductors with pinning arrays. Numerical simulations within the nonlinear Ginzburg-Landau theory reveal a square vortex ice configuration in the ground state at the half matching field and demonstrate similar characteristic features in the field dependence of the critical current, confirming the experimental realization of an artificial ice system for vortices for the first time.PACS numbers: 74.78. Na, 74.40.Gh Artificial ice systems [1][2][3][4][5][6][7][8][9][10][11][12][13][14] that can have properties similar to atomic spin ices [15][16][17][18][19][20] have been gaining tremendous interest in recent years in areas ranging from solid state systems, magnetism, and soft matter. Among them the two-dimensional (2D) artificial spin ices created using e.g., nanomagnetic arrays [1-8] and charged colloidal particle assemblies [9][10][11][12][13][14] have opened a new avenue in the study of novel phenomena such as geometrical frustration [7,8,[15][16][17][18][19][20][21][22][23][24] which can elucidate, e.g., exotic spin states, [16] charge quantization in magnetic monopoles, [21,22] and mechanisms of high-T c superconductivity. [24] In artificial nanomagnetic square spin ices, [1,8] however, the ice rule states with spin arrangements following "two spins in, two spins out" orders [17] have been only partially observed, which could be due to the weak interactions between the magnetic islands.Vortex matter in a superconductor has much stronger interactions relative to the pinning strength due to the much smaller size scale of the pinning array and could therefore permit a true ice rule obeying ground state. In a recent theoretical work Libal et al. proposed to create artificial square and Kagome ices with vortices in superconductors containing nanostructured arrays of pinning centers.[25] Using elongated double-well pinning sites arranged in a square lattice, for example, they were able to obtain the ground state of a square vortex ice which follows the "two vortices in, two vortices out" rule at each vertex, where the state of each double-well site is defined as "in" if the vortex sits close to the vertex and "out" otherwise. Such vortex systems can offer several advantages over the other artificial ices: [25] i) the ground state can be reached more rapidly as compared to nanomagnet systems due to the larger vortex-vortex interaction strength; ii) defect formation processes can be studied by changing the magnetic field to create vacancies or interstitials that locally break the ice rules; iii) different dynamical annealing protocols can be real...

The maximum current (critical current) a type-II superconductor can transmit without energy loss is limited by the motion of the quantized magnetic flux penetrating into a superconductor. Introducing nanoscale holes into a superconducting film has been long pursued as a promising way to increase the critical current. So far the critical current enhancement was found to be mostly limited to low magnetic fields. Here we experimentally investigate the critical currents of superconducting films with a conformal array of nanoscale holes that have non-uniform density while preserving the local ordering. We find that the conformal array of nanoscle holes provides a more significant critical current enhancement at high magnetic fields. The better performance can be attributed to its arching effect that not only gives rise to the gradient in hole-density for pinning vortices with a wide range of densities but also prevent vortex channeling occurring in samples with a regular lattice of holes. PACS numbers: 74.25.Sv, 74.25.Wx, 74.78.Na Critical current (I c ), below which a superconductor can transmit electrical power without energy loss, is a parameter of primary importance for potential applications of the material. 1 In a type-II superconductor, it is limited essentially by the motion of vortices, each consisting of exactly one quantum of flux (Φ 0 = hc/2e = 20.7 G · µm 2 ) surrounded by circulating supercurrents in the plane perpendicular to the magnetic field. 2 In order to overcome this limitation, various types of artificial pinning centers are introduced into a superconductor to immobilize the vortices. 1-19 Among them, regular arrays of holes in superconducting films 9-21 have been explored to enhance vortex pinning and hence the current-carrying capacity of a superconductor. It has been shown that an enhancement in the critical current can be achieved when the vortex lattice is commensurate with the underlying periodic hole array. [12][13][14][15][16] That is, local maxima appear in the magnetic field dependence of the critical current I c (H) at matching fields at which the density of vortices equals an integer multiple of that of the holes. Away from the matching fields, however, the enhancement of the critical current is reduced. In order to overcome this shortcoming, more complicated pinning topologies such as quasiperiodic arrays were proposed. [22][23][24] Due to the existence of multiple periodicities in these arrays, more maxima or extended peaks are expected to occur and have been experimentally observed in Nb films 14,24 and Pb film 25,26 containing quasiperiodic Penrose lattices of artificial pinning centers.Theoretical and experimental studies indicate that graded pinning landscapes can be an excellent candidate for controlling vortex motion. [27][28][29][30] By changing the hole density in the equally spaced rows parallel to the current flow, Wu et al. observed a ratchet effect in patterned Nb films. 27 Motta et al. conducted magnetization measure-ments on a MoGe film with rows of equal density of...