Topological nodal-line semimetals are characterized by one-dimensional Dirac nodal rings that are protected by the combined symmetry of inversion P and time-reversal T . The stability of these Dirac rings is guaranteed by a quantized ±π Berry phase and their low-energy physics is described by a one-parameter family of (2+1)-dimensional quantum field theories exhibiting the parity anomaly. Here we study the Berry-phase supported topological transport of PT invariant nodal-line semimetals. We find that small inversion breaking allows for an electric-field induced anomalous transverse current, whose universal component originates from the parity anomaly. Due to this Hall-like current, carriers at opposite sides of the Dirac nodal ring flow to opposite surfaces when an electric field is applied. To detect the topological currents, we propose a dumbbell device, which uses surface states to filter charges based on their momenta. Suggestions for experiments and device applications are discussed.The last decade witnessed a growing interest in anomalous transport properties of topological semimetals 1-6 , such as the axial current in Weyl semimetals 7 and the valley Hall effect in graphene 8,9 . These topological currents have their origin in quantum anomalies of the relativistic field theories describing the low-energy physics of semimetals. Quantum anomalies arise whenever a symmetry of the classical theory is broken by the regularization of the quantum theory. For example, in Weyl semimetals the (3+1)-dimensional chiral anomaly 10-16 manifests itself by the non-conservation of the chiral charge, i.e., as an axial current flowing between Weyl points with opposite chiralities. Experiments on TaAs 17,18 and on Na 3 Bi 19 have revealed signatures of the chiral anomaly in magneto-transport measurements. The chiral anomaly of Weyl semimetals is intimately connected to the nontrivial topology of the Berry bundle 20-23 , which endows the Weyl points with a nonzero topological charge.Another example of an anomaly leading to topological currents is the (2+1)-dimensional parity anomaly 24-27 , which is realized in graphene 28-33 . The parity anomaly also appears on the surface of topological (crystalline) insulators 34,35 and in quantum spin Hall systems 36 . The fermionic excitations near the Dirac cones of graphene are described by a (2+1)-dimensional quantum field theory exhibiting the parity anomaly. Any gauge symmetric regularization of this quantum field theory must break spacetime inversion symmetry, which manifests itself by a parity-breaking Chern-Simons term in the electromagnetic response theory of a single graphene Dirac cone. This Chern-Simons term gives rise to the valley Hall effect, where fermions from different Dirac cones flow to opposite transverse edges, upon applying an electric field. The valley Hall effect has been observed experimentally 28-31 and has attracted attention due to possible applications in valleytronics devices 28,37 .Parallel to these developments, recent research has shown that there exist t...

A new spintronic theory has been developed for the magnetic tunnel junction (MTJ) with single-crystal barrier. The barrier will be treated as a diffraction grating with intralayer periodicity, the diffracted waves of tunneling electrons thus contain strong coherence, both in charge and especially in spin. The theory can answer the two basic problems present in MgO-based MTJs: (1) Why does the tunneling magnetoresistance (TMR) oscillate with the barrier thickness? (2) Why is the TMR still far away from infinity when the two electrodes are both half-metallic? Other principal features of TMR can also be explained and reproduced by the present work. It also provides possible ways to modulate the oscillation of TMR, and to enhance TMR so that it can tend to infinity. Within the theory, the barrier, as a periodic diffraction grating, can get rid of the confinement in width, it can vary from nanoscale to microscale. Based on those results, a future-generation MTJ is proposed where the three pieces can be fabricated separately and then assembled together, it is especially appropriate for the layered materials, e.g., MoS2 and graphite, and most feasible for industries.

Majorana fermions feature non-Abelian exchange statistics and promise fascinating applications in topological quantum computation. Recently, second-order topological superconductors (SOTSs) have been proposed to host Majorana fermions as localized quasiparticles with zero excitation energy, pointing out a new avenue to facilitate topological quantum computation. We provide a minimal model for SOTSs and systematically analyze the features of Majorana zero modes with analytical and numerical methods. We further construct the fundamental fusion principles of zero modes stemming from single or multiple SOTS islands. Finally, we propose concrete schemes in different setups formed by SOTSs, enabling us to exchange and fuse the zero modes for non-Abelian braiding and holonomic quantum gate operations.

We report on the experimental and theoretical studies of cooling field (HFC) and temperature (T) dependent exchange bias (EB) in FexAu1 − x/Fe19Ni81 spin glass (SG)/ferromagnet (FM) bilayers. When x varies from 8% to 14% in the FexAu1 − x SG alloys, with increasing T, a sign-changeable exchange bias field (HE) together with a unimodal distribution of coercivity (HC) are observed. Significantly, increasing in the magnitude of HFC reduces (increases) the value of HE in the negative (positive) region, resulting in the entire HE ∼ T curve to move leftwards and upwards. In the meanwhile, HFC variation has weak effects on HC. By Monte Carlo simulation using a SG/FM vector model, we are able to reproduce such HE dependences on T and HFC for the SG/FM system. Thus this work reveals that the SG/FM bilayer system containing intimately coupled interface, instead of a single SG layer, is responsible for the novel EB properties.

A canonical spin glass (SG) FeAu layer is fabricated to couple to a soft ferromagnet (FM) FeNi layer. Below the SG freezing temperature, exchange bias (EB) and training are observed. Training in SG/FM bilayers is insensitive to cooling field and may suppress the EB or change the sign of the EB field from negative to positive at specific temperatures, violating from the simple power-law or the single exponential function derived from the antiferromagnet based systems. In view of the SG nature, we employ a double decay model to distinguish the contributions from the SG bulk and the SG/FM interface to training. Dynamical properties during training under different cooling fields and at different temperatures are discussed, and the nonzero shifting coefficient in the time index as a signature of slowing-down decay for SG based systems is interpreted by means of a modified Monte Carlo Metropolis algorithm.