We report that Bi₂Se₃ thin films can be epitaxially grown on SrTiO₃ substrates, which allow for very large tunablity in carrier density with a back gate. The observed low field magnetoconductivity due to weak antilocalization (WAL) has a very weak gate-voltage dependence unless the electron density is reduced to very low values. Such a transition in WAL is correlated with unusual changes in longitudinal and Hall resistivities. Our results suggest a much suppressed bulk conductivity at large negative gate voltages and a possible role of surface states in the WAL phenomena.

The Griffiths singularity in a phase transition, caused by disorder effects, was predicted more than 40 years ago. Its signature, the divergence of the dynamical critical exponent, is challenging to observe experimentally. We report the experimental observation of the quantum Griffiths singularity in a two-dimensional superconducting system. We measured the transport properties of atomically thin gallium films and found that the films undergo superconductor-metal transitions with increasing magnetic field. Approaching the zero-temperature quantum critical point, we observed divergence of the dynamical critical exponent, which is consistent with the Griffiths singularity behavior. We interpret the observed superconductor-metal quantum phase transition as the infinite-randomness critical point, where the properties of the system are controlled by rare large superconducting regions.

Kondo resonances are a very precise measure of spin-polarized transport through magnetic impurities. However, the Kondo temperature, indicating the thermal range of stability of the magnetic properties, is very low. By contrast, we find for iron phthalocyanine a Kondo temperature in spectroscopic measurements which is well above room temperature. It is also shown that the signal of the resonance depends strongly on the adsorption site of the molecule on a gold surface. Experimental data are verified by extensive numerical simulations, which establish that the coupling between iron states and states of the substrate depends strongly on the adsorption configuration.

The Weyl semimetal (WSM) is a newly proposed quantum state of matter. It has Weyl nodes in bulk excitations and Fermi arcs surface states. We study the effects of disorder and localization in WSMs and find three exotic phase transitions. (I) Two Weyl nodes near the Brillouin zone boundary can be annihilated pairwise by disorder scattering, resulting in the opening of a topologically nontrivial gap and a transition from a WSM to a three-dimensional (3D) quantum anomalous Hall state. (II) When the two Weyl nodes are well separated in momentum space, the emergent bulk extended states can give rise to a direct transition from a WSM to a 3D diffusive anomalous Hall metal. (III) Two Weyl nodes can emerge near the zone center when an insulating gap closes with increasing disorder, enabling a direct transition from a normal band insulator to a WSM. We determine the phase diagram by numerically computing the localization length and the Hall conductivity, and propose that the exotic phase transitions can be realized on a photonic lattice.PACS numbers: 72.15. Rn, 73.20.Fz, Topological quantum states of matter have emerged as an important and growing field in condensed matter and materials physics recently [1,2]. The Weyl semimetal (WSM) is a newly proposed quantum state of the kind that breaks time-reversal symmetry or inversion symmetry [3][4][5][6][7][8][9][10]. A WSM exhibits a set of paired zero-energy Weyl nodes (linearly touching points of conduction and valence bands) in its bulk spectrum and Fermi arcs excitations localized on the surface. A number of candidate materials have been predicted to be WSMs, including pyrochlore iridates and magnetic topological insulator multilayers [3][4][5][6]. Recently, following the theoretical prediction [7], angle resolved photoemission experiments confirmed that TaAs is a WSM by the observation of the Fermi arcs surface states [8]. Both the Weyl nodes and the Fermi arcs have been observed in NbAs using a combination of soft X-ray and ultraviolet photoemission experiments [9]. Furthermore, the Weyl points have been predicted and subsequently observed remarkably in gyroid photonic crystals [10].In this Letter, we study both numerically and analytically the stability of the gapless Weyl nodes and Fermi arcs against random potential scattering and the novel disorder-induced metal-insulator transitions in WSM systems. Previous studies have concentrated on the properties of a single Weyl node, assuming that the disorder potential is smooth enough to avoid scattering between different nodes [11][12][13][14]. Indeed, a system with a single Weyl node is not subject to Anderson localization even for strong disorder [15]. However, the theorem of Nielsen and Ninomiya states that gapless Weyl nodes with opposite chirality must appear in pairs [16]. Thus, it is essential to study the localization properties of a pair of Weyl nodes since they can be annihilated pairwise when approaching each other in momentum space or by strong intervalley scattering [17,18]. To this end, we study a model syste...

Three-dimensional (3D) Dirac semimetals, which possess 3D linear dispersion in the electronic structure as a bulk analogue of graphene, have lately generated widespread interest in both materials science and condensed matter physics. Recently, crystalline Cd3As2 has been proposed and proved to be a 3D Dirac semimetal that can survive in the atmosphere. Here, by using point contact spectroscopy measurements, we observe exotic superconductivity around the point contact region on the surface of Cd3As2 crystals. The zero-bias conductance peak (ZBCP) and double conductance peaks (DCPs) symmetric around zero bias suggest p-wave-like unconventional superconductivity. Considering the topological properties of 3D Dirac semimetals, our findings may indicate that Cd3As2 crystals under certain conditions could be topological superconductors, which are predicted to support Majorana zero modes or gapless Majorana edge/surface modes in the boundary depending on the dimensionality of the material.

A novel uniform multi-level matrix of vertically erected graphene walls is directly grown on a dielectric substrate by plasma enhanced chemical vapor deposition (PECVD) at 900 C without the presence of any catalyst and post-transfer treatment. Such a two-level structure is composed of continuous vertically erected graphene sheets (the second level) on a nanocrystalline graphene film (the first level). A nanocrystalline film is formed in the first stage (<15 min), and the graphene walls initialize on the boundary C sp 3 atoms as nucleation centers to grow the erected graphene walls as the second-level component. The microstructure of the graphene walls can be modified by plasma power, growth time and seed layer coating. The unique three-dimensional graphene structure possessed high hydrophobicity (contact angle: 141 ), outstanding electron conductivity (sheet resistance: 198 U sq À1 ), and tunable transparency (91.9-38.0% at 550 nm). The three-dimensional structure enables the graphene to act as an excellent electron transport network with high surface area in many aspects. The highly conductive graphene walls were used as the counter electrode of dye-sensitized solar cells (DSSC) with a photovoltaic efficiency of 6.01%, comparable to FTO-based DSSCs (6.10%). This in situ one-step growth indicates the great potential to fabricate excellent electrodes for photovoltaic and electronic applications.

The nonlinear Hall effect has opened the door towards deeper understanding of topological states of matter. Disorder plays indispensable roles in various linear Hall effects, such as the localization in the quantized Hall effects and the extrinsic mechanisms of the anomalous, spin, and valley Hall effects. Unlike in the linear Hall effects, disorder enters the nonlinear Hall effect even in the leading order. Here, we derive the formulas of the nonlinear Hall conductivity in the presence of disorder scattering. We apply the formulas to calculate the nonlinear Hall response of the tilted 2D Dirac model, which is the symmetry-allowed minimal model for the nonlinear Hall effect and can serve as a building block in realistic band structures. More importantly, we construct the general scaling law of the nonlinear Hall effect, which may help in experiments to distinguish disorder-induced contributions to the nonlinear Hall effect in the future.

Atomically thin hexagonal boron nitride ( h -BN) is often regarded as an elastic film that is impermeable to gases. The high stabilities in thermal and chemical properties allow h -BN to serve as a gas barrier under extreme conditions. Here, we demonstrate the isolation of hydrogen in bubbles of h -BN via plasma treatment. Detailed characterizations reveal that the substrates do not show chemical change after treatment. The bubbles are found to withstand thermal treatment in air, even at 800 °C. Scanning transmission electron microscopy investigation shows that the h -BN multilayer has a unique aligned porous stacking nature, which is essential for the character of being transparent to atomic hydrogen but impermeable to hydrogen molecules. In addition, we successfully demonstrated the extraction of hydrogen gases from gaseous compounds or mixtures containing hydrogen element. The successful production of hydrogen bubbles on h -BN flakes has potential for further application in nano/micro-electromechanical systems and hydrogen storage.