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Stacking two-dimensional van der Waals (vdW) materials rotated with respect to each other show versatility for the study of exotic quantum phenomena. Especially, anisotropic layered materials have great potential for such twistronics applications, providing high tunability. Here, we report anisotropic superconducting order parameters in twisted Bi2Sr2CaCu2O8+x (Bi-2212) vdW junctions with an atomically clean vdW interface, achieved using the microcleave-and-stack technique. The vdW Josephson junctions with twist angles of 0{\deg} and 90{\deg} showed the maximum Josephson coupling, which was comparable to that of intrinsic Josephson (IJ) junctions in the bulk crystal. As the twist angle approaches 45{\deg}, Josephson coupling is suppressed, and eventually disappears at 45{\deg}. The observed twist angle dependence of the Josephson coupling can be explained quantitatively by theoretical calculation with the d-wave superconducting order parameter of Bi-2212 and finite tunneling incoherence of the junction. Our results revealed the anisotropic nature of Bi-2212 and provided a novel fabrication technique for vdW-based twistronics platforms compatible with air-sensitive vdW materials., Comment: 24 pages, 6 figures, to be published in Nano Letters

We introduce a novel two-dimensional electronic system with ultrastrong interlayer interactions, namely twisted bilayer graphene with a large twist angle, as an ideal ground for realizing interlayer-coherent excitonic condensates. In these systems, subnanometer atomic separation between the layers allows significant interlayer interactions, while interlayer electron tunneling is geometrically suppressed due to the large twist angle. By fully exploiting these two features we demonstrate that a sequence of odd-integer quantum Hall states with interlayer coherence appears at the second Landau level (N = 1). Notably the energy gaps for these states are of order 1 K, which is several orders of magnitude greater than those in GaAs. Furthermore, a variety of quantum Hall phase transitions are observed experimentally. All the experimental observations are largely consistent with our phenomenological model calculations. Hence, we establish that a large twist angle system is an excellent platform for high-temperature excitonic condensation.

Engineering quantum states through light-matter interaction has created a new paradigm in condensed matter physics. A representative example is the Floquet-Bloch state, which is generated by time-periodically driving the Bloch wavefunctions in crystals. Previous attempts to realise such states in condensed matter systems have been limited by the transient nature of the Floquet states produced by optical pulses, which masks the universal properties of non-equilibrium physics. Here, we report the generation of steady Floquet Andreev (F-A) states in graphene Josephson junctions by continuous microwave application and direct measurement of their spectra by superconducting tunnelling spectroscopy. We present quantitative analysis of the spectral characteristics of the F-A states while varying the phase difference of superconductors, temperature, microwave frequency and power. The oscillations of the F-A state spectrum with phase difference agreed with our theoretical calculations. Moreover, we confirmed the steady nature of the F-A states by establishing a sum rule of tunnelling conductance, and analysed the spectral density of Floquet states depending on Floquet interaction strength. This study provides a basis for understanding and engineering non-equilibrium quantum states in nano-devices., 23 pages, 3 Figures

Engineering quantum states through light-matter interaction has created a paradigm in condensed-matter physics. A representative example is the Floquet-Bloch state, which is generated by time-periodically driving the Bloch wavefunctions in crystals. Previous attempts to realize such states in condensed-matter systems have been limited by the transient nature of the Floquet states produced by optical pulses

The Majorana zero-energy modes (MZMs) residing at the boundary of topological superconductors have attracted a great deal of interest recently, as they provide a platform to explore fundamental physics such as non-Abelian statistics, as well as fault-tolerant quantum computation. Period doubling of Shapiro steps in a Josephson junction under microwave irradiation has been regarded as strong evidence for the emergence of the MZMs at the junction edges. However, questions remain as to how the Shapiro steps respond to the presence of a 4\ensuremath{\pi}-periodic Josephson current. In this study, we investigated the characteristic features of Shapiro steps with respect to the ratio ($\ensuremath{\alpha}$) of the 4\ensuremath{\pi}-periodic current to the topologically trivial 2\ensuremath{\pi}-periodic one, as well as the reduced microwave frequency $(\mathrm{\ensuremath{\Omega}})$ and McCumber parameter $(\ensuremath{\beta})$ of the junction. Our analysis reproduced Shapiro steps similar to those observed experimentally for specific parameter sets of $\ensuremath{\alpha},\mathrm{\ensuremath{\Omega}}(\ensuremath{\lesssim}0.1)$, and $\ensuremath{\beta}(\ensuremath{\gtrsim}1.0)$. Full suppression of the first lobe of the $n=1$ step guarantees the presence of a 4\ensuremath{\pi}-periodic Josephson current. In addition, we discuss the range of $\mathrm{\ensuremath{\Omega}}$ and $\ensuremath{\beta}$ needed for full suppression of the first lobe of the $n=1$ step, even for small $\ensuremath{\alpha}\phantom{\rule{4pt}{0ex}}(l0.1)$. To observe ``period-doubled Shapiro steps,'' even with a small $\ensuremath{\alpha}$, the junction should have a large ${I}_{c}{R}_{N}$ product and sufficiently large junction capacitance.

Correction to: Nature Materials https://doi.org/10.1038/s41563-020-0721-9, published online 6 July 2020.In the version of this Article originally published, the following sentence was missing from the end of the Acknowledgements: ‘M.N.A. acknowledges support from the Alexander von Humboldt Foundation’s Sofia Kovalevskaja Award and the BMBF MINERVA ARCHES Award.’ This has now been corrected in all versions of this Article.

Td-WTe

This paper reports micro-electro-discharge machining of vertically aligned carbon nanotube forests for the formation of high-aspect-ratio, three-dimensional microstructures in the material. The developed forest machining method is a dry process performed in air, generating high-frequency pulses of electrical discharge to locally machine the nanotubes in order to create target shapes in a forest. With this approach, forest microstructures can be fabricated to have varying shapes along their height, unachievable with conventional pre-patterned chemical vapor deposition growth techniques. The use of the pulses with a minimized discharge energy defined with 35 V and 10 pF in the discharge generation circuit leads to an aspect ratio of 20 with the smallest feature of 5 µm in forests without disordering the vertical orientation of the nanotubes. Micromachining of multilayer geometries as well as arrayed needle-like microstructures with angled surfaces is demonstrated.