We investigate the organized formation of strain, ripples, and suspended features in macroscopic graphene sheets transferred onto corrugated substrates made of an ordered array of silica pillars with variable geometries. Depending on the pitch and sharpness of the corrugated array, graphene can conformally coat the surface, partially collapse, or lie fully suspended between pillars in a fakir-like fashion over tens of micrometers. With increasing pillar density, ripples in collapsed films display a transition from random oriented pleats emerging from pillars to organized domains of parallel ripples linking pillars, eventually leading to suspended tent-like features. Spatially resolved Raman spectroscopy, atomic force microscopy, and electronic microscopy reveal uniaxial strain domains in the transferred graphene, which are induced and controlled by the geometry. We propose a simple theoretical model to explain the structural transition between fully suspended and collapsed graphene. For the arrays of high density pillars, graphene membranes stay suspended over macroscopic distances with minimal interaction with the pillars' apexes. It offers a platform to tailor stress in graphene layers and opens perspectives for electron transport and nanomechanical applications.

Although signatures of superconductivity in Dirac semimetals have been reported, for instance by applying pressure or using point contacts, our understanding of the topological aspects of Dirac semimetal superconductivity is still developing. Here, we utilize nanoscale phase-sensitive junction technology to induce superconductivity in the Dirac semimetal BiSb. Our radiofrequency irradiation experiments then reveal a significant contribution of 4π-periodic Andreev bound states to the supercurrent in Nb-BiSb-Nb Josephson junctions. The conditions for a substantial 4π contribution to the supercurrent are favourable because of the Dirac cone's very broad transmission resonances and a measurement frequency faster than the quasiparticle poisoning rate. In addition, we show that a magnetic field applied in the plane of the junction allows tuning of the Josephson junctions from 0 to π regimes. Our results open the technologically appealing avenue of employing the topological bulk properties of Dirac semimetals for topological superconductivity research and topological quantum computer development.

We investigate the superconducting proximity effect through graphene in the long diffusive junction limit, at low and high magnetic field. The interface quality and sample phase coherence lead to a zero resistance state at low temperature, zero magnetic field, and high doping. We find a striking suppression of the critical current near graphene's charge neutrality point, which we attribute to specular reflexion of Andreev pairs at the interface of charge puddles. This type of reflexion, specific to the Dirac band structure, had up to now remained elusive. At high magnetic field the use of superconducting electrodes with high critical field enables the investigation of the proximity effect in the Quantum Hall regime. Although the supercurrent is not directly detectable in our two wire configuration, interference effects are visible which may be attributed to the injection of Cooper pairs into edge states.

Superconducting nanowires can be fabricated by decomposition of an organometallic gas using a focused beam of Ga ions. However, physical damage and unintentional doping often results from the exposure to the ion beam, motivating the search for a means to achieve similar structures with a beam of electrons instead of ions. This has so far remained an experimental challenge. We report the fabrication of superconducting tungsten nanowires by electron-beam-induced-deposition, with critical temperature of 2.0 K and critical magnetic field of 3.7 T, and compare them with superconducting wires made with ions. This work opens up new possibilities for the realization of nanoscale superconducting devices, without the requirement of an ion beam column.Comment: 5 pages, 4 figure

New polynitro-substituted bispyrazoles were synthesised and fully characterised in this study. Ammonium 4-(4 0 -amino-3 0 ,5 0 -dinitro-1 0 -pyrazol)-3,5-dinitropyrazole (3), 4-(4 0 -amino-3 0 ,5 0 -dinitro-1 0 -pyrazol)-1-amino-3,-5-initropyrazole (6), diaminoguanidinium 4-(4 0 -amino-3 0 ,5 0 -dinitro-1 0 -pyrazol)-3,5-dinitropyrazole (9), and 3,4,5-triamino-1,2,4-triazolium 4-(4 0 -amino-3 0 ,5 0 -dinitro-1 0 -pyrazol)-3,5-dinitropyrazole (12) were further confirmed using single-crystal X-ray diffraction. The compounds exhibited excellent thermal stabilities, insensitivities and high detonation performance. The measured LC 50 of the compounds suggested that their toxicities were lower than that of TNT and that for compound 3 even lower than that of TATB.Compound 5 demonstrates unprecedented overall performance: higher detonation velocity (V D ¼ 8760-8981 m s À1 ), detonation heat (Q v ¼ 7551 kJ kg À1 ) and explosive power (A ¼ 1712 kJ g À1 ) than RDX, a decomposition temperature (T d ¼ 297 C) higher than that of HMX, and much lower toxicity (LC 50 ¼ 7 mg mL À1 ) than that of TNT, ranking it in a new generation of heat resistant, less sensitive and low environmental impact high energetic materials.

International audienceWe investigate proximity-induced superconductivity in micrometer-long bismuth nanowires connected to superconducting electrodes with a high critical field. At low temperature we measure a supercurrent that persists in magnetic fields as high as the critical field of the electrodes (above 11 T). The critical current is also strongly modulated by the magnetic field. In certain samples we find regular, rapid SQUID-like periodic oscillations occurring up to high fields. Other samples exhibit less periodic but full modulations of the critical current on Tesla field scales, with field-caused extinctions of the supercurrent. These findings indicate the existence of low dimensionality, phase coherent, interfering conducting regions through the samples, with a subtle interplay between orbital and spin contributions. We relate these surprising results to the electronic properties of the surface states of bismuth, strong Rashba spin-orbit coupling, large effective g factors, and their effect on the induced pair correlations. In particular, we emphasize the possible contribution of topological edge states of specific facets of the nanowires

The high tunability of the density of states of graphene [1] makes it an ideal probe of quantum transport in different regimes. In particular, the supercurrent that can flow through a nonsuperconducting (N) material connected to two superconducting electrodes, crucially depends on the lenghth of the N relative to the superconducting coherence length. Using graphene as the N material we have investigated the full range of the superconducting proximity effect, from short to long diffusive junctions. By combining several S/graphene/S samples with different contacts and lengths, and measuring their gate-dependent critical currents (Ic) and normal state resistance RN , we compare the product eRN Ic to the relevant energies, the Thouless energy in long junctions and the superconducting gap of the contacts in short junctions, over three orders of magnitude of Thouless energy. The experimental variations strikingly follow a universal law, close to the predictions of the proximity effect both in the long and short junction regime, as well as in the crossover region, thereby revealing the interplay of the different energy scales. Differences in the numerical coefficients reveal the crucial role played by the interfacial barrier between graphene and the superconducting electrodes, which reduces the supercurrent in both short and long junctions. Surprisingly the reduction of supercurrent is independent of the gate voltage and of the nature of the electrodes. A reduced induced gap and Thouless energy are extracted, revealing the role played by the dwell time in the barrier in the short junction, and an effective increased diffusion time in the long junction. We compare our results to the theoretical predictions of Usadel equations and numerical simulations which better reproduce experiments with imperfect NS interfaces. PACS numbers:The superconducting proximity effect describes the phenomena that occur when a superconductor (S) is placed in contact with a non-superconducting conductor ("normal" conductor, N), and superconducting properties are induced in the N due to the propagation of correlated Andreev pairs from the superconductor to the N [2]. Several experiments have revealed the striking effects of induced superconductivity: density of states measurements with tunnel probes have shown how the pair correlations develop as a function of distance to the NS interface [3]; how a minigap is induced in the N when it is connected to two S, and how this minigap is modulated by a magnetic flux that induces a phase difference between the two S electrodes [4]. In fact, not only the minigap but the entire energy spectrum of the Andreev eigenstates is phase-dependent, leading to a dissipationless supercurrent that can flow through the normal conductor. The phase dependence of the supercurrent has been probed both at high and low frequency [5]. It is remarkable that all the aforementioned experiments could be described by the theory of the proximity effect, irrespective of the superconducting and normal metals used, their length, as...

One of the consequences of Cooper pairs having a finite momentum in the interlayer of a Josephson junction, is π-junction behavior. The finite momentum can either be due to an exchange field in ferromagnetic Josephson junctions, or due to the Zeeman effect. Here, we report the observation of Zeeman effect induced 0-π transitions in Bi 1−x Sb x , 3D Dirac semimetal-based Josephson junctions. The large g-factor of the Zeeman effect from a magnetic field applied in the plane of the junction allows tuning of the Josephson junctions from 0-to π-regimes. This is revealed by sign changes in the modulation of the critical current by applied magnetic field of an asymmetric superconducting quantum interference device (SQUID). Additionally, we directly measure a nonsinusoidal current-phase relation in the asymmetric SQUID, consistent with models for ballistic Josephson transport. 1 arXiv:1807.07725v1 [cond-mat.mes-hall]