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The identification of distinct charge transport features, deriving from nontrivial bulk band and surface states, has been a challenging subject in the field of topological systems. In topological Dirac and Weyl semimetals, nontrivial conical bands with Fermi-arc surface states give rise to negative longitudinal magnetoresistance due to chiral anomaly effect and unusual thickness dependent quantum oscillation from Weyl-orbit effect, which were demonstrated recently in experiments. In this work, we report the experimental observations of large nonlinear and nonreciprocal transport effects for both longitudinal and transverse channels in an untwinned Weyl metal of SrRuO_{3} thin film grown on a SrTiO_{3} substrate. From rigorous measurements with bias current applied along various directions with respect to the crystalline principal axes, the magnitude of nonlinear Hall signals from the transverse channel exhibits a simple sinα dependence at low temperatures, where α is the angle between bias current direction and orthorhombic [001]_{o}, reaching a maximum when current is along orthorhombic [11[over ¯]0]_{o}. On the contrary, the magnitude of nonlinear and nonreciprocal signals in the longitudinal channel attains a maximum for bias current along [001]_{o}, and it vanishes for bias current along [11[over ¯]0]_{o}. The observed α-dependent nonlinear and nonreciprocal signals in longitudinal and transverse channels reveal a magnetic Weyl phase with an effective Berry curvature dipole along [11[over ¯]0]_{o} from surface states, accompanied by 1D chiral edge modes along [001]_{o}.

In a noncentrosymmetric system, an intrinsic electric polarization is allowed and may lead to unusual nonreciprocal charge transport phenomena. As a result, a current-dependent resistance, arising from the magnetoelectric anisotropy term of k·E[over ⃗]×B[over ⃗], appears and acts as a current rectifier with the amount of rectification being linearly proportional to the magnitude of both current and applied magnetic field. In this work, a different type of nonreciprocal transport effect was demonstrated in a graphene-based device, which requires no external magnetic field. Due to the unique pseudospin (valley) degree of freedom in chiral fermions with trigonal warping, a large nonreciprocal transport effect was uncovered in a gapped bilayer graphene, where electric-field tunabilities of the band gap and valley polarization play an important role. The exact cancellation of nonreciprocal effect between two different valleys is effectively removed by breaking the inversion symmetry via electric gatings. The magnitude of the current rectification appears to be at a maximum when the Fermi surface undergoes a Lifshitz transition near the band edges, which is proportional to the current and the displacement field strength. The full electric-field tuning of the nonreciprocal transport effect without a magnetic field opens up a new direction for valleytronics in two-dimensional-based devices.

The intrinsic mechanism of the spin-orbit coupling at the LaAlO_{3}/SrTiO_{3} interface remains a debatable issue. Rashba-type spin-orbit coupling is an appealing candidate that has been demonstrated by several magnetotransport results. On the other hand, the atomic spin-orbit coupling was also shown to play an important role, particularly when the Fermi level is close to the Lifshitz point. In this paper, we focus on the measurements of the anisotropic superconducting upper critical fields in gated LaAlO_{3}/SrTiO_{3} devices. By rigorous fittings of the H_{c2}-T curves using both the Werthamer-Helfand-Hohenberg theory and Klemm-Luther-Beasley model, an upper limit of the spin-orbit scattering lifetime can be determined in the two-dimensional limit of the superconducting state. We found that the extracted spin-orbit scattering lifetime monotonically increases with the effective transport lifetime that spans over two orders of magnitude in the regime with sheet density close to and higher than that at the Lifshitz point. Those results suggest the dominant role of Elliott-Yafet type spin relaxation. The comparison to the weak localization fittings on magnetoconductance reveals a striking difference. Such a model dependence infers an electronic phase separation at the LaAlO_{3}/SrTiO_{3} interface, where the superconductivity originates from a selective d-subband occupancy.

Collective processes in intense charged particle beams described self-consistently by the Vlasov-Maxwell equations are studied using a 3D multispecies nonlinear perturbative particle simulation method. The newly developed beam equilibrium, stability, and transport (BEST) code is used to simulate the nonlinear stability properties of intense beam propagation, surface eigenmodes in a high-intensity beam, and the electron-proton (e-p) two-stream instability observed in the Proton Storage Ring (PSR) experiment. Detailed simulations in a parameter regime characteristic of the PSR experiment show that the dipole-mode two-stream instability is stabilized by a modest spread (about 0.1%) in axial momentum of the beam particles.

We study the RKKY interaction mediated by the helical edge states of a quantum spin Hall insulator in the presence of the Rashba spin-orbital coupling induced by an external electric field and the electron-electron interaction. We show that in the presence of the Rashba coupling, the RKKY interaction induced by the helical edge states contains not only the Heisenberg-like and the Dzyaloshinskii-Moria terms, but also the nematic-type term that is not present in the absence of the Rashba coupling. Moreover, the range functions depend on the strength of the Rashba coupling. We also show that the electron-electron interaction changes the strength of the RKKY interaction by modifying the power of the 1//x/ dependence of the range functions. In particular, by varying the strength of the interaction or the Rashba coupling, there is an (impurity) quantum phase transition involving the sign change of the RKKY interaction at the value of the Luttinger liquid parameter K=1/2. Since both the strength of the Rashba coupling and the chemical potential of the helical edge states are electrically controllable by external gate voltages, our results not only shed light on the nature of magnetic impurity correlations in the edge of a two-dimensional topological insulator, but also pave a way to manipulate the qubits in quantum computing. [ABSTRACT FROM AUTHOR]

We study the quantum point contact between the topological superconductor and the helical Luttinger liquid. The effects of the electron-electron interactions in the helical Luttinger liquid on the low-energy physics of this system are analyzed by the renormalization group. Among the various couplings at the point contact which arises from the tunneling via the Majorana edge channel, the induced backscattering in the helical Luttinger liquid is the most relevant for repulsive interactions. Hence, at low temperatures, the helical Luttinger liquid is effectively cut into two separated half wires. As a result, the low-temperature physics is described by a fixed point consisting of two leads coupled to the topological superconductor, and the electrical transport properties through the point contact at low temperature and low bias are dominated by the tunneling via the Majorana edge channel. We compute the temperature dependence of the zero-bias tunneling conductance and study the full counting statistics for the tunneling current at zero temperature. [ABSTRACT FROM AUTHOR]

We study a quantum dot coupled to two edge states of a quantum spin Hall insulator through electron tunnelings in the presence of a Rashba spin-orbital interaction induced by an external electric field. We show that if the electron interactions on the edge states are repulsive, there are two possible phases, depending on the Luttinger liquid parameter K. For 1/2

We investigate theoretically the nonequilibrium finite-frequency current noise in a four-terminal quantum point contact of interacting helical edge states at a finite bias voltage. Special focus is put on the effects of the single-particle and two-particle scattering between the two helical edge states on the fractional charge quasiparticle excitations shown in the nonequilibrium current noise spectra. Via the Keldysh perturbative approach, we find that the effects of the single-particle and the two-particle scattering processes on the current noise depend sensitively on the Luttinger liquid parameter. Moreover, the Fano factors for the auto- and cross correlations of the currents in the terminals are distinct from the ones for tunneling between the chiral edge states in the quantum Hall liquid. The current noise spectra in the single-particle-scattering-dominated and the two-particle-scattering-dominated regime are shown. Experimental implications of our results on the transport through the helical edges in two-dimensional topological insulators are discussed. [ABSTRACT FROM AUTHOR]

We investigate the effects of long-ranged Coulomb interactions in a tilted Dirac semimetal in two dimensions by using the perturbative renormalization-group (RG) method. Depending on the magnitude of the tilting parameter, the undoped system can have either Fermi points (type I) or Fermi lines (type II). Previous studies usually performed the renormalization-group transformations by integrating out the modes with large momenta. This is problematic when the Fermi surface is open, like type-II Dirac fermions. In this work we study the effects of Coulomb interactions, following the spirit of Shankar [Rev. Mod. Phys. 66, 129 (1994)], by introducing a cutoff in the energy scale around the Fermi surface and integrating out the high-energy modes. For type-I Dirac fermions, our result is consistent with that of the previous work. On the other hand, we find that for type-II Dirac fermions, the magnitude of the tilting parameter increases monotonically with lowering energies. This implies the stability of type-II Dirac fermions in the presence of Coulomb interactions, in contrast with previous results. Furthermore, for type-II Dirac fermions, the velocities in different directions acquire different renormalization even if they have the same bare values. By taking into account the renormalization of the tilting parameter and the velocities due to the Coulomb interactions, we show that while the presence of a charged impurity leads only to charge redistribution around the impurity for type-I Dirac fermions, for type-II Dirac fermions, the impurity charge is completely screened, albeit with a very long screening length. The latter indicates that the temperature dependence of physical observables are essentially determined by the RG equations we derived. We illustrate this by calculating the temperature dependence of the compressibility and specific heat of the interacting tilted Dirac fermions. [ABSTRACT FROM AUTHOR]

We investigate the nature of the magnetic phase transition induced by the short-ranged electron-electron interactions in a Weyl semimetal by using the perturbative renormalization-group method. We find that the critical point associated with the quantum phase transition is characterized by a Gaussian fixed point perturbed by a dangerously irrelevant operator. Although the low-energy and long-distance physics is governed by a free theory, the velocities of the fermionic quasiparticles and the magnetic fluctuations suffer from nontrivial renormalization effects. In particular, their ratio approaches one at low energies, which indicates an emergent Lorentz symmetry at the quantum critical point. We further investigate the stability of the fixed point in the presence of weak disorder preserving the chiral symmetry. We show that while the fixed point is generally stable against weak disorder, a moderately strong random chemical potential and/or random vector potential may induce a quantum phase transition towards a disorder-dominated phase. We propose a global phase diagram of the Weyl semimetal in the presence of both electron-electron interactions and disorder based on our results. [ABSTRACT FROM AUTHOR]

We study the behavior of a topological Josephson junction in which two topological superconductors are coupled through a quantum dot. We focus on the case with the bulk superconducting gap being the largest energy scale. Two parameter regimes are investigated: a weak tunneling between the dot and the superconductors, with the dot near its charge degeneracy point, and a strong-tunneling regime in which the transmission between the dot and the superconductors is nearly perfect. We show that in the former situation, the Andreev spectrum for each sector with fixed fermion parity consists of only two levels, which gives rise to the nontrivial current-phase relation. Moreover, we study the Rabi oscillation between the two levels and indicate that the corresponding frequency is a 4π-periodic function of the phase difference between the two superconductors, which is immune to the quasiparticle poisoning. In the latter case, we find that the Coulomb charging energy enhances the effect of backscattering at the interfaces between the dot and the superconductors. Both the temperature and the gate-voltage dependence of the critical Josephson current are examined. [ABSTRACT FROM AUTHOR]

We investigate the quantum anomalous Hall effect (QAHE) and related chiral transport in the millimetersize (Cr0.12Bi0.26Sb0.62 )2Te3 films. With high sample quality and robust magnetism at low temperatures, the quantized Hall conductance of e2/h is found to persist even when the film thickness is beyond the two-dimensional (2D) hybridization limit. Meanwhile, the Chem insulator-featured chiral edge conduction is manifested by the nonlocal transport measurements. In contrast to the 2D hybridized thin film, an additional weakly field-dependent longitudinal resistance is observed in the ten-quintuple-layer film, suggesting the influence of the film thickness on the dissipative edge channel in the QAHE regime. The extension of the QAHE into the three-dimensional thickness region addresses the universality of this quantum transport phenomenon and motivates the exploration of new QAHE phases with tunable Chem numbers. In addition, the observation of scale-invariant dissipationless chiral propagation on a macroscopic scale makes a major stride towards ideal low-power interconnect applications. [ABSTRACT FROM AUTHOR]

BiTeI exhibits large Rashba spin splitting due to its noncentrosymmetric crystal structure. The study of the chemical doping effect is important in order to either tune the Fermi level or refine the crystal quality. Here, we report the magnetotransport measurement in high quality BiTeI single crystals with different copper dopings. We found that a small amount of copper doping improves the crystal quality significantly, which is supported by the transport data showing higher Hall mobility and larger amplitude in Shubnikov-de Haas oscillation at low temperature. Two distinct frequencies in Shubnikov-de Haas oscillation were observed, giving extremal Fermi surface areas of AS=9.1×1012 cm-2 and AL=3.47×1014 cm-2 with corresponding cyclotron masses mS*=0.0353me and mL*=0.178me, respectively. Those results are further compared with relativistic band structure calculations using three reported Te and I positions. Our analysis infers the crucial role of Bi-Te bond length in the observed large bulk Rashba-type spin-splitting effect in BiTeI. [ABSTRACT FROM AUTHOR]

The thermoelectric power of a material, typically governed by its band structure and cartier density, can be varied by chemical doping that is often restricted by solubility of the dopant. Materials showing large thermoelectric power are useful for many industrial applications, such as the heat-to-electricity conversion and the thermoelectric cooling device. Here we show a full electric-field tuning of thermoelectric power in a dual-gated bilayer graphene device resulting from the opening of a band gap by applying a perpendicular electric field on bilayer graphene. We uncover a large enhancement in thermoelectric power at a low temperature, which may open up a new possibility in low temperature thermoelectric application using graphene-based device. [ABSTRACT FROM AUTHOR]

In a superconducting nanostructure, phase fluctuations are prominent and give rise to finite resistance below superconducting transition temperature Tc. By using a monolayer polymer/nanosphere hybrid we developed previously, we fabricated a large array of interconnected niobium (Nb) honeycomb lattices with the thinnest interconnected linewidth d ranging from 36 nm to 89 nm. The honeycomb cells form a highly ordered triangular lattice with more than 108 unit cells extending over few mm² area, which enables the detailed transport study at nanometer scales. We found Tc gradually drops with decreasing d due to the phase-slip effect, while the critical field at lower temperature tends to follow that of a continuous Nb thin film. One likely scenario is to consider a model system of numerous superconducting islands interconnected by short phase-slip junctions, where the phase coherence is dictated by the phase slippage in the nanoconstriction. This was strongly supported by the excellent fitting to the thermally activated phase-slip model and also the unusual phenomena of transition width narrowing in high fields. [ABSTRACT FROM AUTHOR]

Low-dimensional materials could display anomalous thermal conduction that the thermal conductivity (κ) diverges with increasing lengths, in ways inconceivable in any bulk materials. However, previous theoretical or experimental investigations were plagued with many finite-size effects, rendering the results either indirect or inconclusive. Indeed, investigations on the anomalous thermal conduction must demand the sample length to be sufficiently long so that the phenomena could emerge from unwanted finite-size effects. Here we report experimental observations that the κ's of single-wall carbon nanotubes continuously increase with their lengths over 1 mm, reaching at least 8640 W/mK at room temperature. Remarkably, the anomalous thermal conduction persists even with the presence of defects, isotopic disorders, impurities, and surface absorbates. Thus, we demonstrate that the anomalous thermal conduction in real materials can persist over much longer distances than previously thought. The finding would open new regimes for wave engineering of heat as well as manipulating phonons at macroscopic scales. [ABSTRACT FROM AUTHOR]