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We present a detailed study of the Bose-Hubbard model in a $p$-band triangular lattice by focusing on the evolution of orbital order across the superfluid-Mott insulator transition. Two distinct phases are found in the superfluid regime. One of these phases adiabatically connects the weak interacting limit. This phase is characterized by the intertwining of axial $p_\pm=p_x \pm ip_y$ and in-plane $p_\theta=\cos\theta p_x+\sin\theta p_y$ orbital orders, which break the time-reversal symmetry and lattice symmetries simultaneously. In addition, the calculated Bogoliubov excitation spectrum gaps the original Dirac points in the single-particle spectrum but exhibits emergent Dirac points. The other superfluid phase in close proximity to the Mott insulator with unit boson filling shows a detwined in-plane ferro-orbital order. Finally, an orbital exchange model is constructed for the Mott insulator phase. Its classical ground state has an emergent SO$(2)$ rotational symmetry in the in-plane orbital space and therefore enjoys an infinite degeneracy, which is ultimately lifted by the orbital fluctuation via the order by disorder mechanism. Our systematic analysis suggests that the in-plane ferro-orbital order in the Mott insulator phase agrees with and likely evolves from the latter superfluid phase., Comment: 9 pages, 4 figures

Recently, the intrinsic magnetic topological insulator ${\mathrm{MnBi}}_{2}{\mathrm{Te}}_{4}$ has attracted great attention. It has an out-of-plane antiferromagnetic order, which is believed to open a sizable energy gap in the surface states. This gap, however, was not always observable in the latest angle-resolved photoemission spectroscopy (ARPES) experiments. To address this issue, we analytically derive an effective model for the two-dimensional (2D) surface states by starting from a three-dimensional (3D) Hamiltonian for bulk ${\mathrm{MnBi}}_{2}{\mathrm{Te}}_{4}$ and taking into account the spatial profile of the bulk magnetization. Our calculations suggest that the diminished surface gap may be caused by a much smaller and more localized intralayer ferromagnetic order. In addition, we calculate the spatial distribution and penetration depth of the surface states, which indicates that the surface states are mainly embedded in the first two septuple layers from the terminating surface. From our analytical results, the influence of the bulk parameters on the surface states can be found explicitly. Furthermore, we derive a $\mathbf{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{p}$ model for ${\mathrm{MnBi}}_{2}{\mathrm{Te}}_{4}$ thin films and show the oscillation of the Chern number between odd and even septuple layers. Our results will be helpful for the ongoing explorations of the ${\mathrm{MnBi}}_{x}{\mathrm{Te}}_{y}$ family.

We study the in-plane critical magnetic field of two-dimensional Ising superconducting systems and propose the microscopic calculation for these disordered two-dimensional superconducting systems with or without inversion symmetry. Protected by certain specific spin-orbit interaction which polarizes the electron spin to the out-of-plane direction, the in-plane critical fields largely surpass the Pauli limit and show remarkable upturn in the zero-temperature limit. The impurity scattering and Rashba spin-orbit interaction, treated on equal footing in the microscopic framework, both weaken the critical field but in qualitatively different manners. The microscopic theory is consistent with recent experimental results in stanene and Pb superconducting ultrathin films.

We perform magnetotransport and microwave (MW) photovoltage (PV) measurements in bulk tantalum arsenide (TaAs) crystal with $c$-(001) crystalline surface at low temperature ($T$). Two sets of quantum oscillations from different topological Weyl electron pockets are found in magnetoresistance and PV. Similar to the magnetoresistance, PV oscillates with the density of states around the Fermi level. Our observation qualitatively countenances a model which holds that the PV oscillations originate from nonequilibrium chiral Weyl fermions. Moreover, as a probe for detecting local electron states, PV is proved to be more sensitive than magnetoresistances. Microwave photovoltaic effect provides an effective method to explore topological electron structure and novel quantum phases in three-dimensional Weyl systems.

The quantum Hall effect (QHE) is broadly studied in the films of semiconductors, conductors, topological insulators, and topological semimetals. Wedge-shaped samples, guaranteeing the uniform properties of materials, bring obscure characteristics connected to the sample geometry. Here we study the QHE in wedge-shaped samples by using a simple electron gas model. We find that the Hall resistance shows anomalous behaviors: the Hall plateau value shifts when measuring from different lateral terminals, the Hall resistances in positive and negative magnetic fields are not equal, and the Hall resistance depends on the direction of the tilted magnetic field. In addition, the longitudinal resistance also has anomalous characteristics in wedge-shaped samples. These results are very similar to the QHE based on the Weyl orbits observed in a recent experiment, although they are different in origin. These anomalous behaviors in the trivial wedge-shaped sample originate from the spatial redistribution of Landau levels; that is, the edge state can follow an equal-thickness path in the bulk.

In this Letter, we study an Anderson-localization-induced quantized transport in disordered Chern insulators (CIs). By investigating the disordered CIs with a step potential, we find that the chiral interface states emerge along the interfaces of the step potential, and the energy range for such quantized transport can be manipulated through the potential strength. Furthermore, numerical simulations on cases with a multi-step potential demonstrate that such chiral state can be spatially shifted by varying the Fermi energy, and the energy window for quantized transport is greatly enlarged. Experimentally, such chiral interface states can be realized by imposing transverse electric field, in which the energy window for quantized transport is much broader than the intrinsic band gap of the corresponding CI. These phenomena are quite universal for disordered CIs due to the direct phase transition between the CI and the normal insulator., 20 pages, 20 figures

Majorana zero modes (MZMs)---bearing potential applications for topological quantum computing---are verified in quasi-one-dimensional (1D) fermion systems, including semiconductor nanowires, magnetic atomic chains, and planar Josephson junctions. However, the existence of multibands in these systems makes the MZMs fragile to the influence of disorder. Moreover, in practical perspective, the proximity induced superconductivity may be difficult and restricted for 1D systems. Here, we propose a flexible route to realize MZMs through 1D topological kink states by engineering a 2D electron gas with antidot lattices in which both the aforementioned issues can be avoided owing to the robustness of kink states and the intrinsically attainable superconductivity in high-dimensional systems. The MZMs are verified to be quite robust against disorders and the bending of kink states and can be conveniently tuned by varying the Rashba spin-orbit coupling strength. Our proposal provides an experimental feasible platform for MZMs with systematic manipulability and assembleability based on the present techniques in a 2D electron gas system.

We show that Floquet engineering with circularly polarized light (CPL) can selectively split the valley degeneracy of a twisted multilayer graphene (TMG), and thus generate a controlled valley-polarized Floquet Chern flat band with a tunable large Chern number. It offers a feasible optical way to manipulate the valley degree of freedom in moir\'e flat bands, and hence opens new opportunities to study the valleytronics of moir\'e flat band systems. We thus expect that many of the valley-related properties of TMG, e.g., orbital ferromagnetism, can be switched by CPL with proper doping. We reveal a Chern number hierarchy rule for the Floquet flat bands in a generic $(M+N)$-layer TMG. We also illustrate that the CPL effects on TMG strongly rely on the stacking chirality, which is a unique feature of TMG. All these phenomena could be tested in the twisted double bilayer graphene systems, which is the simplest example of TMG and has already been realized in experiments.

In this work, we predict a hinged quantum spin-Hall (HQSH) effect featured by a pair of helical hinge modes in antiferromagnetic (AFM) topological insulator (TI) multilayers. This pair of helical hinge modes are localized on the hinges of the top and bottom surfaces of the AFM TI multilayers. Unlike the conventional QSH effect, the HQSH effect survives the breaking of time-reversal symmetry (TRS) and thus represents a different kind of topological phenomenon. The helical hinge modes are sustainable to inelastic scattering and TRS-breaking disorder, which can be observed in macroscopic samples. We show that this HQSH effect can be understood as a three-dimensional generalization of the Su-Schrieffer-Heeger model and its topology is characterized by the spin Chern number. At last, we propose that the HQSH effect can be realized in newly found intrinsic AFM TI materials (MnBi$_2$Te$_4$)$_m$(Bi$_2$Te$_3$)$_n$ or magnetic-doped TI multilayers by current experimental setups., 6 pages, 4 figures

We study the energy spectrum of Weyl fermions in a topological semimetal in the presence of a central charged impurity. When the strength of the charge exceeds a critical value, an infinite family of quasibound states appear that show characteristic signatures of discrete scale invariance. The invariance is broken by the presence of screening or a finite mass. If these terms are small enough the quasibound states still exist, but there can be perturbations of the shallowest quasibound states. These special quasibound states manifest themselves in the local density of states as well as the relative phase, which can be observed by scanning tunneling microscopy. Such effects that are usually associated with the relativistic quantum electrodynamics of strong fields become observable in topological materials.

Besides the giant peak of the nonlocal resistance $R_{NL}$, an anomalous negative value of $R_{NL}$ has been observed in graphene systems, while its formation mechanism is not quite understood yet. In this work, utilizing the non-equilibrium Green's function method, we calculate the local-current flow in an H-shaped non-interacting graphene system locating in the ballistic regime. Similar to the previous conclusions made from the viscous regime, the numerical results show that a local-current vortex appears between the nonlocal measuring terminals, which induces a backflow current and a remarkable negative voltage drop at the probe. Specifically, the stronger the vortex exhibits, the more negative $R_{NL}$ manifests. Besides, a spin-orbital coupling is added as an additional tool to study this exotic vortex, which is not a driving force for the arising vortex at all. Moreover, a breakdown of the nonlocal Wiedemann-Franz law is obtained in this ballistic system, and two experimental criteria are further provided to confirm the existence of this exotic vortex. Notably, a discussion is made that the vortex actually originates from the collision between the flowing current and the boundaries, due to the long electron mean free path and the consequent ballistic transport caused by the specific linear spectrum of graphene., Comment: 9 pages, 7 figures

The quantized thermal Hall effect is an important probe for detecting chiral topological order and revealing the nature of chiral gapless edge states. The standard Kubo formula approach for the thermal Hall conductance $\kappa_{xy}$ based on the linear-response theory faces difficulties in practical application due to the lack of a reliable numerical method for calculating dynamical quantities in microscopic models at finite temperature. In this work, we propose an approach for calculating $\kappa_{xy}$ in two-dimensional lattice models displaying chiral topological order. Our approach targets at the edge current localized at the boundary which involves only thermal averages of local operators in equilibrium, thus drastically lowering the barrier for the calculation of $\kappa_{xy}$. We use the chiral $p$-wave superconductor (with and without disorder) and the Hofstadter model as benchmark examples to illustrate several sources of finite-size effects, and we suggest the infinite (or sufficiently long) strip as the best geometry for carrying out numerical simulations., Comment: 12 pages, 9 figures, published version

Superconductor-insulator/metal transition (SIT/SMT) represents a prototype of quantum phase transition, where quantum fluctuation plays a dominant role and dramatically changes the physical properties of low-dimensional superconducting systems. Recent observation of quantum Griffiths singularity (QGS) offers an essential perspective to understand the subtleties of quantum phase transition in two-dimensional superconductors. Here we study the magnetic field induced SMT in ultrathin crystalline Pb films down to ultralow temperatures. The divergent critical exponent is observed when approaching zero temperature quantum critical point, indicating QGS. Distinctively, the anomalous phase boundary of SMT that the onset critical field decreases with decreasing temperatures in low temperature regime distinguishes our observation from previous reports of QGS in various two-dimensional superconductors. We demonstrate that the anomalous phase boundary originates from the superconducting fluctuations in ultrathin Pb films with pronounced spin-orbit interaction. Our findings reveal a novel aspect of QGS of SMT in two-dimensional superconductors with anomalous phase boundary., Evidence of quantum phase transitions is normally difficult to be detected. Here, Liu and Wang et al. report divergent critical exponent in ultrathin Pb films with superconducting fluctuations and spin-orbit interaction, indicating an anomalous quantum Griffiths singularity of superconductor-metal transition.

In the semiclassical regime, the chiral anomaly of the Weyl semimetals is demonstrated by the quadratic longitudinal magnetoconductivity. Recently the planar Hall effect is proposed in the same systems, where the Hall conductivity depends quadratically on the magnetic field. If the Weyl cones are tilted, both the chiral chemical potential and the anomalous velocity contribute a linear term to the conductivities. We investigate the electronic transport of a tilted Weyl semimetal in the presence of an in-plane magnetic field. The results show that both the longitudinal and the Hall conductivities contain a linear term in magnetic field besides the quadratic terms and the angular dependence of them is different from the case without tilting. The correction of the phase space volume factor is also studied, which shows effects on both the angular dependence as well as the amplitudes of the conductivities. The angular dependence of the conductivities, showing the signature of the planar Hall effect, is experimentally verifiable.

We study the confined bound states in circular bilayer graphene quantum dots. Using the semiclassical Einstein-Brillouin-Keller quantization rule, we predict that valley levels inside the quantum dot undergo three stages: splitting, crossing, and recombining when varying the applied magnetic field. This exotic phenomenon originates from the Berry phase, which increases from zero to $2\ensuremath{\pi}$ with the magnetic field and has opposite signs for the two inequivalent valleys. To further confirm this phenomenon, we perform a fully quantum mechanical calculation based on a low-energy continuum model and a real space tight-binding model, and show that the valley effect can be explicitly observed by scanning-tunneling spectroscopy measurements. Moreover, we propose an efficient valley filter device as an application of this valley effect, which can be used to manipulate the valley degree of freedom in valleytronics.

We extend the scope of the Klein bottle entropy, originally introduced by [Tu, Phys. Rev. Lett. 119, 261603 (2017)] in the rational conformal field theory (CFT), to the compactified boson CFT, which are relevant to the studies of Luttinger liquids. We first review the Klein bottle entropy in rational CFT and discuss details of how to extract the Klein bottle entropy from lattice models using the example of the transverse field Ising model. We then go beyond the scope of rational CFT and study the Klein bottle entropy $\ln g$ in the compactified boson CFT, which turns out to have a straightforward relation to the compactification radius $R$, $\ln g = \ln R$. This relation indicates a convenient and efficient method to extract the Luttinger parameter from lattice model calculations. Our numerical results on the Klein bottle entropy in the spin-$1/2$ XXZ chain show excellent agreement with the CFT predictions, up to some small deviations near the isotropic point, which we attribute to the marginally irrelevant terms. For the $S = 1$ XXZ chain that cannot be exactly solved, our numerical results provide an accurate numerical determination of the Luttinger parameter in this model., Comment: 18 pages, 6 figures, published version

A planar Josephson junction with a normal metal attached on its top surface will form a hollow nanowire structure due to its three dimensional nature. In such hollow nanowire structure, the magnetic flux induced by a small magnetic field (about 0.01T) will tune the system into topologically non-trivial phase and therefore two Majorana zero-modes will form at the ends of the nanowire. Through tuning the chemical potential of the normal metal, the topologically non-trivial phase can be obtained for almost all energy within the band. Furthermore, the system can be conveniently tuned between the topologically trivial and non-trivial phases via the phase difference between the superconductors. Such device, manipulable through flux, can be conveniently fabricated into desired 2D networks. Finally, we also propose a cross-shaped junction realizing the braiding of Majorana zero-modes through manipulating the phase differences., Comment: 5 pages, 4 figures

Unconventional responses upon breaking discrete or crystal symmetries open avenues for exploring emergent physical systems and materials. By breaking inversion symmetry, a nonlinear Hall signal can be observed, even in the presence of time-reversal symmetry, quite different from the conventional Hall effects. Low-symmetry two-dimensional materials are promising candidates for the nonlinear Hall effect, but it is less known when a strong nonlinear Hall signal can be measured, in particular, its connections with the band-structure properties. By using model analysis, we find prominent nonlinear Hall signals near tilted band anticrossings and band inversions. These band signatures can be used to explain the strong nonlinear Hall effect in the recent experiments on two-dimensional WTe$_{2}$. This Letter will be instructive not only for analyzing the transport signatures of the nonlinear Hall effect but also for exploring unconventional responses in emergent materials.

The emerging field of superconductor (SC) spintronics has attracted intensive attentions recently. Many fantastic spin dependent properties in SC have been discovered, including the observation of large magnetoresistance, long spin lifetimes and the giant spin Hall effect in SC, as well as spin supercurrent in Josephson junctions, etc. Regarding the spin dynamic in SC films, few studies has been reported yet. Here, we report the investigation of the spin dynamics in an s-wave superconducting NbN film via spin pumping from an adjacent insulating ferromagnet GdN layer. A profound coherence peak of the Gilbert damping is observed slightly below the superconducting critical temperature of the NbN layer, which is consistent with recent theoretical studies. Our results further indicate that spin pumping could be a powerful tool for investigating the spin dynamics in 2D crystalline superconductors., 11 pages, 4 figures, and SI

The conductance measurement of a half quantized plateau in a quantum anomalous Hall insulator-superconductor structure is reported by a recent experiment [Q. L. He \textit{et al.}, Science 357, 294-299 (2017)], which suggests the existence of the chiral Majorana fermion modes. However, such half quantized conductance plateau may also originates from a disorder-induced metallic phase. To identify the exact mechanism, we study the transport properties of such a system in the presence of strong disorders. Our results show that the local current density distributions of these two mechanisms are different. In particular, the current noises measurement can be used to distinguish them without any further fabrication of current experimental setup.

Spin superfluid is a novel emerging quantum matter arising from the Bose-Einstein condensate (BEC) of spin-1 bosons. We demonstrate the spin superfluid ground state in canted antiferromagnetic Cr2O3 thin film at low temperatures via nonlocal spin transport. A large enhancement of the nonlocal spin signal is observed below ~ 20 K, and it saturates from ~ 5 K down to 2 K. We show that the spins can propagate over very long distances (~ 20 micro meters) in such spin superfluid ground state and the nonlocal spin signal decreases very slowly as the spacing increases with an inverse relationship, which is consistent with theoretical prediction. Furthermore, spin superfluidity has been investigated in the canted antiferromagnetic phase of the (11-20)-oriented Cr2O3 film, where the magnetic field dependence of the associated critical temperature follows a two-thirds power law near the critical point. The experimental demonstration of the spin superfluid ground state in canted antiferromagnet will be extremely important for the fundamental physics on the BEC of spin-1 bosons and paves the way for future spin supercurrent devices, such as spin-Josephson junctions., Comment: Main text 8 pages, 4 figures, Supplementary Materials 5 pages, 15 SI figures

We study the nonlocal transport property of Majorana quasiparticle states (MQPs) in a normal lead--topological superconducting wire--normal lead system. We find that the tunneling coefficient of the electron transmission process displays an interesting even-odd interference behavior. However, this even-odd interference behavior is difficult to observe in the tunneling coefficient of the crossed Andreev reflection process (CAR). We show that this even-odd interference behavior is directly related to the self-Hermitian property of MQPs. Due to the self-Hermitian property, the correction to the transport in the electron transmission process is in the first order while the correction to the CAR process is in the higher order. Thus, the interference behavior is more significant in the electron transmission process than in the CAR process. Such a unique transport property demonstrates the nonlocal and self-Hermitian characteristics of MQPs.

We study spin transport through a normal metal-spin superconductor junction. A spin-flip reflection is demonstrated at the interface, where a spin-up electron incident from the normal metal can be reflected as a spin-down electron and the spin $2\times \hbar/2$ will be injected into the spin superconductor. When the (spin) voltage is smaller than the gap of the spin superconductor, the spin-flip reflection determines the transport properties of the junction. We consider both graphene-based (linear-dispersion-relation) and quadratic-dispersion-relation normal metal-spin superconductor junctions in detail. For the two-dimensional graphene-based junction, the spin-flip reflected electron can be along the specular direction (retro-direction) when the incident and reflected electron locates in the same band (different bands). A perfect spin-flip reflection can occur when the incident electron is normal to the interface, and the reflection coefficient is slightly suppressed for the oblique incident case. As a comparison, for the one-dimensional quadratic-dispersion-relation junction, the spin-flip reflection coefficient can reach 1 at certain incident energies. In addition, both the charge current and the spin current under a charge (spin) voltage are studied. The spin conductance is proportional to the spin-flip reflection coefficient when the spin voltage is less than the gap of the spin superconductor. These results will help us get a better understanding of spin transport through the normal metal-spin superconductor junction., 11 pages, 9 figures

The topological semimetal $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ag}}_{2}\mathrm{Se}$ features a Kramers-Weyl node at the origin in momentum space and a quadruplet of spinless Weyl nodes, which are annihilated by spin-orbit coupling. We show that single-crystalline $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ag}}_{2}\mathrm{Se}$ manifests giant Shubnikov--de Haas oscillations in the longitudinal magnetoresistance, which stem from a small electron pocket that can be driven beyond the quantum limit by a field less than 9 T. This small electron pocket is a remainder of the spin-orbit annihilated Weyl nodes and thus encloses a Berry-phase structure. Moreover, we observed a negative longitudinal magnetoresistance when the magnetic field is beyond the quantum limit. Our experimental findings are complemented by thorough theoretical band-structure analyses of this Kramers-Weyl semimetal candidate, including first-principles calculations and an effective $\mathbit{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbit{p}$ model.

We study the conductance fluctuation in topological semimetals. Through statistic distribution of energy levels of topological semimetals, we determine the dominant parameters of universal conductance fluctuation (UCF), i.e., the number of uncorrelated bands $k$, the level degeneracy $s$, and the symmetry parameter $\beta$. These parameters allow us to predict the zero-temperature intrinsic UCF of topological semimetals by the Altshuler-Lee-Stone theory. Then, we obtain numerically the conductance fluctuations for topological semimetals of quasi-1D geometry. We find that for Dirac/Weyl semimetals, the theoretical prediction coincides with the numerical results. However, a non-universal conductance fluctuation behavior is found for topological nodal line semimetals, i.e., the conductance fluctuation amplitude increases with the enlargement of SOC strength. We find that such unexpected parameter-dependent phenomena of conductance fluctuation are related to Fermi surface shape of 3D topological semimetals. These results will help us to understand the existing and future experimental results of UCF in 3D topological semimetals., Comment: 9 pages, 8 figures

Using a nonequilibrium-Green-function method, we numerically studied the transport properties of a superconductor-graphene-superconductor Josephson junction hybrid system in the quantum Hall regime. Our numerical calculations show that there are two interference patterns of the critical current due to the unique band structure of graphene. One is caused by the usual intraband Andreev retroreflection process, and the other one is caused by the interband specular Andreev reflection process. In the Andreev retroreflection regime, chiral Andreev edge states are formed and a distinct supercurrent can be observed. The critical current displays an AB oscillation behavior and the period is approximately $2{\mathrm{\ensuremath{\Phi}}}_{0}=h/e$. As for the specular Andreev refection process, the reflected holes are bent back to the reverse direction of the incident electrons and the supercurrent flows along both edges. It is similar to a superconductor ring Josephson junction and the period is ${\mathrm{\ensuremath{\Phi}}}_{0}=h/2e$. However, the critical current for the specular Andreev reflection process is very small and is unlikely to be observable in an experiment. Thus, we conclude that our numerical calculations are inconsistent to the experimental findings by Amet et al. [Science 352, 966 (2016)].

We investigate the quantum tunneling and Andreev reflection in a top-gated quantum anomalous Hall insulator proximity coupled with a superconductor junction. A quantized perfect crossed Andreev reflection with its coefficient being integer 1 is obtained and all other scattering processes (the normal reflection, normal tunneling, and local Andreev reflection) are completely suppressed when the topological superconductor phase with Chern number $\mathcal{N}=1$ is realized. This perfect crossed Andreev reflection originates from the tunneling of the chiral Majorana edge states, and the phase of tunneling amplitude only being 0 and $\ensuremath{\pi}$ plays a decisive role. Furthermore, because of the chiral characteristic of the Majorana edge states, the perfect crossed Andreev reflection is robust against the disorder and can work in a wide range of system parameters.

Discovery of log-periodic quantum oscillations may open a new chapter in the nearly 90-year history of quantum oscillations., Quantum oscillations are usually the manifestation of the underlying physical nature in condensed matter systems. Here, we report a new type of log-periodic quantum oscillations in ultraquantum three-dimensional topological materials. Beyond the quantum limit (QL), we observe the log-periodic oscillations involving up to five oscillating cycles (five peaks and five dips) on the magnetoresistance of high-quality single-crystal ZrTe5, virtually showing the clearest feature of discrete scale invariance (DSI). Further, theoretical analyses show that the two-body quasi-bound states can be responsible for the DSI feature. Our work provides a new perspective on the ground state of topological materials beyond the QL.

We report comprehensive studies of the single crystal growth and electrical transport properties for various samples of TaAs, the first experimentally confirmed inversion symmetry-breaking Weyl semimetal. The transport parameters for different samples are obtained through the fitting of the two band model and the analysis of Shubnikov de Haas oscillations. We find that the ratio factor of transport lifetime to quantum lifetime is intensively enhanced when the Fermi level approaches the Weyl node. This result is consistent with the side-jump interpretation derived from a chirality-protected shift in the scattering process for a Weyl semimetal., This is a modified version of arXiv:1502.00251

The search for long electron phase coherence length, which is the length that an electron can keep its quantum wave-like properties, has attracted considerable interest in the last several decades. Here, we report the long phase coherence length of ~ 3.7 micro meters in La-doped CdO thin films at 2 K. Systematical investigations of the La doping and the temperature dependences of the electron mobility and the electron phase coherence length reveal contrasting scattering mechanisms for these two physical properties. Furthermore, these results show that the oxygen vacancies could be the dominant scatters in CdO thin films that break the electron phase coherence, which would shed light on further investigation of phase coherence properties in oxide materials., Comment: 13 pages, 6 figure. SI: 8 pages. To appear in Phys. Rev. B

Recently, chiral-induced spin selectivity has been attracting intense interest. Here, we report a theoretical study of spin-dependent electron transport in achiral nanotubes contacted by nonmagnetic leads. Our results reveal that by properly connecting to the leads, the achiral nanotubes can present a pronounced spin filtering phenomenon even if the spin-orbit coupling is very weak. In addition, the spin selectivity effect holds for various achiral nanotubes with different radii and is still significant in the presence of strong disorder and dephasing. These findings open new opportunities of using achiral molecules in spintronic applications and could motivate further studies on spin transport along achiral systems.

The Higgs amplitude mode in superconductors is the condensed matter analogy of Higgs bosons in particle physics. We investigate the time evolution of Higgs amplitude mode in massless Dirac systems, induced by a weak quench of an attractive interaction. We find that the Higgs amplitude mode in the half-filling honeycomb lattice has a logarithmic decaying behaviour, qualitatively different from the $1/\sqrt{t}$ decay in the normal superconductors. Our study is also extended to the doped cases in honeycomb lattice. As for the 3D Dirac semimetal at half filling, we obtain an undamped oscillation of the amplitude mode. Our finding is not only an important supplement to the previous theoretical studies on normal fermion systems, but also provide an experimental signature to characterize the superconductivity in 2D or 3D Dirac systems., 6 pages, 8 figures

The behavior of phase coherence is studied in two-dimensional hole gas through the integer quantum Hall plateau-to-plateau transition. From the plateau transition as a function of temperature, scaling properties of multiple transitions are analyzed. Our results are in good agreement with the assumption of the zero-point fluctuations of the coherent holes, and support the intrinsic saturation of the coherence time at low temperature limit. The critical exponent $p$ can also be determined under the scheme of the zero-point fluctuations. The similarity and difference in experimental observations between quantum Griffiths singularity and plateau transition is discussed. The spin-orbit coupling effect's influence on the plateau transition is explored by comparing the results from different transitions.

The dynamics of a single magnetic skyrmion in an atomic spin system under the influence of a scanning tunneling microscope is investigated by computer simulations solving the Landau-Lifshitz-Gilbert equation. Two possible scenarios are described: manipulation with aid of a spin-polarized tunneling current and by an electric field created by the scanning tunneling microscope. The dynamics during the creation and annihilation process is studied and the possibility to move single skyrmions is showed.

In quantum mechanics, a particle is best described by the wave packet instead of the plane wave. Here, we study the wave-packet scattering problem in Weyl semimetals with the low-energy Weyl fermions of different chiralities. Our results show that the wave packet acquires a chirality-protected shift in the single impurity scattering process. More importantly, the chirality-protected shift can lead to an anomalous scattering probability, thus, affects the transport properties in Weyl semimetals. We find that the ratio between the transport lifetime and the quantum lifetime increases sharply when the Fermi energy approaches to the Weyl nodes, providing an explanation on the experimentally observed the ultrahigh mobility in topological (Weyl or Dirac) semimetals.

Using non-equilibrium Green's functions, we studied numerically the transport properties of a Josephson junction, superconductor-topological insulator-superconductor hybrid system. Our numerical calculation shows first that proximity-induced superconductivity is indeed observed in the edge states of a topological insulator adjoining two superconducting leads and second that the special characteristics of topological insulators endow the edge states with an enhanced proximity effect with a superconductor but do not forbid the bulk states to do the same. In a size-dependent analysis of the local current, it was found that a few residual bulk states can lead to measurable resistance, whereas because these bulk states spread over the whole sample, their contribution to the interference pattern is insignificant when the sample size is in the micrometer range. Based on these numerical results, it is concluded that the apparent disappearance of residual bulk states in the superconducting interference process as described in Ref. [\onlinecite{HartNautrePhys2014f}] is just due to the effects of size: the contribution of the topological edge states outweighs that of the residual bulk states., Comment: 8 pages and 4 figures

Light-field imaging is a crucial and straightforward way of measuring and analyzing surrounding light worlds. In this paper, a dual-polarized light-field imaging micro-system based on a twisted nematic liquid-crystal microlens array (TN-LCMLA) for direct three-dimensional (3D) observation is fabricated and demonstrated. The prototyped camera has been constructed by integrating a TN-LCMLA with a common CMOS sensor array. By switching the working state of the TN-LCMLA, two orthogonally polarized light-field images can be remapped through the functioned imaging sensors. The imaging micro-system in conjunction with the electric-optical microstructure can be used to perform polarization and light-field imaging, simultaneously. Compared with conventional plenoptic cameras using liquid-crystal microlens array, the polarization-independent light-field images with a high image quality can be obtained in the arbitrary polarization state selected. We experimentally demonstrate characters including a relatively wide operation range in the manipulation of incident beams and the multiple imaging modes, such as conventional two-dimensional imaging, light-field imaging, and polarization imaging. Considering the obvious features of the TN-LCMLA, such as very low power consumption, providing multiple imaging modes mentioned, simple and low-cost manufacturing, the imaging micro-system integrated with this kind of liquid-crystal microstructure driven electrically presents the potential capability of directly observing a 3D object in typical scattering media.

$\mathrm{HfT}{\mathrm{e}}_{5}$ is predicted to be a promising platform for studying topological phases. Here through an electrical transport study, we present an observation of chiral anomaly and ultrahigh mobility in $\mathrm{HfT}{\mathrm{e}}_{5}$ crystals. Negative magnetoresistivity in $\mathrm{HfT}{\mathrm{e}}_{5}$ is observed when the external magnetic and electrical fields are parallel (B//E) and quickly disappears once B deviates from the direction of E. Quantitative fitting further confirms the chiral anomaly as the underlying physics. Moreover, by analyzing the conductivity tensors of longitudinal and Hall traces, ultrahigh mobility and ultralow carrier density are revealed in $\mathrm{HfT}{\mathrm{e}}_{5}$, which paves the way for potential electronic applications.