Dirac Fermions In Presence Research Articles

Re-assessing special aspects of Dirac fermions in presence of Lorentz-symmetry violation

This paper focuses on additional inspections concerning the fermionic sector of the Standard Model Extension (SME). In this context, our main effort in this contribution is to investigate effects of Lorentz-symmetry violation (LSV) on the Klein Paradox, the Zitterbewegung and its phenomenology in connection to Condensed Matter Physics, Atomic Physics, and Astrophysics. Finally, we discuss a particular realization of LSV in the Dirac equation, considering an asymmetry between space and time due to a scale factor present in the linear momentum of the fermion, but which does not touch its time derivative. We go further and extend the implications of this asymmetry in the situation the scale factor becomes space–time dependent to compute its influence on the kinematics of the Compton effect with the extended dispersion relation for the fermion that scatters the photon.

Stability of the Néel quantum critical point in the presence of Dirac fermions

We investigate the stability of the N\'eel quantum critical point of two-dimensional quantum antiferromagnets, described by a nonlinear $\ensuremath{\sigma}$ model, in the presence of a Kondo coupling to ${N}_{f}$ flavors of two-component Dirac fermion fields. The long-wavelength order parameter fluctuations are subject to Landau damping by electronic particle-hole fluctuations. Using the momentum-shell renormalization group (RG), we demonstrate that the Landau damping is weakly irrelevant at the N\'eel quantum critical point, despite the fact that the corresponding self-energy correction dominates over the quadratic gradient terms in the IR limit. In the ordered phase, the Landau damping increases under the RG, indicative of damped spin-wave excitations. Although the Kondo coupling is weakly relevant, sufficiently strong Landau damping renders the N\'eel quantum critical point quasistable for ${N}_{f}\ensuremath{\ge}4$ and thermodynamically stable for ${N}_{f}<4$. In the latter case, we identify a multicritical point which describes the transition between the N\'eel critical and Kondo runaway regimes. The symmetry breaking at this fixed point results in the opening of a gap in the Dirac fermion spectrum. Approaching the multicritical point from the disordered phase, the fermionic quasiparticle residue vanishes, giving rise to non-Fermi-liquid behavior.

Super-Klein tunneling of Dirac fermions through electrostatic gratings in graphene

We use the Wick-rotated time-dependent supersymmetry to construct models of two-dimensional Dirac fermions in presence of an electrostatic grating. We show that there appears omnidirectional perfect transmission through the grating at specific energy. Additionally to being transparent for incoming fermions, the grating hosts strongly localized states.

Two-dimensional Dirac fermion in presence of an asymmetric vector potential

We introduce the new, exactly solvable model of the two-dimensional Dirac fermion in presence of an asymmetric, Pöschl–Teller-like vector potential. Utilizing the translation invariance of the system, the effective one-dimensional stationary equation is brought into the form of the Heun equation and its fundamental solutions are found as an irreducible combination of two Gauss hypergeometric functions. The energy spectrum and the scattering is studied dependant on the conserved longitudinal momentum as well as on the strength of the coupling.

Shubnikov–de Haas measurements on a high mobility monolayer graphene flake sandwiched between boron nitride sheets

A flake of monolayer graphene was sandwiched between boron nitride sheets. Temperature dependent Shubnikov–de Haas measurements were performed to access how this technique influences the electronic properties of the graphene sample. The maximum mobility found in this configuration was approximately 105 cm2 Vs −1. From the phase of the oscillations a Berry phase β of 1/2 was obtained indicating the presence of Dirac fermions. We obtained Fermi velocities around m s−1 which imply hopping energies close to 2.5 eV. Furthermore, the carrier lifetime is typically higher than that found in graphene supported by SiO2, reaching values higher than 700 fs.

Magnetotransport study of Dirac fermions inYbMnBi2antiferromagnet

We report quantum transport and Dirac fermions in YbMnBi2 single crystals. YbMnBi2 is a layered material with anisotropic conductivity and magnetic order below 290 K. Magnetotransport properties, nonzero Berry phase, and small cyclotron mass indicate the presence of Dirac fermions. Lastly, angular-dependent magnetoresistance indicates a possible quasi-two-dimensional Fermi surface, whereas the deviation from the nontrivial Berry phase expected for Dirac states suggests the contribution of parabolic bands at the Fermi level or spin-orbit coupling.

Interlayer electronic transport inCaMnBi2antiferromagnet

We report interlayer electronic transport in CaMnBi$_{2}$ single crystals. Quantum oscillations and angular magnetoresistance suggest coherent electronic conduction and valley polarized conduction of Dirac states. Small cyclotron mass, large mobility of carriers and nontrivial Berry's phase are consistent with the presence of Dirac fermions on the side wall of the warped cylindrical Fermi surface. Similar to SrMnBi$_{2}$ that features an anisotropic Dirac cone, our results suggest that magnetic field-induced changes in the interlayer conduction are also present in layered bismuth-based materials with zero-energy line in momentum space created by the staggered alkaline earth atoms.

Electron-hole asymmetry, Dirac fermions, and quantum magnetoresistance inBaMnBi2

We report two-dimensional quantum transport and Dirac fermions in BaMnBi2 single crystals. BaMnBi2 is a layered bad metal with highly anisotropic conductivity and magnetic order below 290 K. Magnetotransport properties, nonzero Berry phase, small cyclotronmass, and the first-principles band structure calculations indicate the presence of Dirac fermions in Bi square nets. Quantum oscillations in the Hall channel suggest the presence of both electron and hole pockets, whereas Dirac and parabolic states coexist at the Fermi level.

Topology, cosmic strings and quantum dynamics – a case study with graphene

We explore the possibility to study the quantum dynamics of Dirac fermions in presence of a cosmic string by introducing a conical topological defect in gapped graphene in the presence of a Coulomb charge. When the Coulomb charge exceeds a certain critical strength, quantum instability sets in. Below the critical regime and for certain values of the system parameters, the allowed boundary conditions in gapped graphene cone can be classified in terms of a single real quantity. Observables such as local density of states, scattering phase shifts and the bound state spectra are dependent on the value of this real parameter, which has to be determined empirically. For a supercritical Coulomb charge, we analyze the system with a regularized potential as well as with a zigzag boundary condition and find the effect of the sample topology on the observable features of the system.

Giant magnetoresistance in layered manganese pnictide CaMnBi2

We report the physical properties of a layered transition metal pnictide, CaMnBi2, which has a crystal structure similar to that of the superconducting iron pnictides. This compound is a bad metal with a long-range antiferromagnetic order at TN = 270 K. The linear temperature dependence of magnetic susceptibility above TN suggests that strong antiferromagnetic correlations exist in the paramagnetic state. A linear magnetic field dependence of the magnetoresistance implies the existence of the linear energy dispersion, which may result in the giant in-plane magnetoresistance (about 105% in 10 T at 2.5 K for H∥c). The results of de Haas-van Alphen effect are consistent with the presence of Dirac fermions.

Millikelvin de Haas–van Alphen and magnetotransport studies of graphite

Recent studies of the electronic properties of graphite have produced conflicting results regarding the positions of the different carrier types within the Brillouin zone, and the possible presence of Dirac fermions. In this paper we report a comprehensive study of the de Haas--van Alphen, Shubnikov--de Haas, and Hall effects in a sample of highly orientated pyrolytic graphite, at temperatures in the range 30$ $mK to 4$ $K and magnetic fields up to 12$ $T. The transport measurements confirm the Brillouin-zone locations of the different carrier types assigned by Schroeder, Dresselhaus, and Javan, Phys. Rev. Lett. 20, 1292 (1968): electrons are at the $K$ point, and holes are near the $H$ points. We extract the cyclotron masses and scattering times for both carrier types from the temperature- and magnetic-field-dependences of the magneto-oscillations. Our results indicate that the holes experience stronger scattering and hence have lower mobility than the electrons. We utilize phase-frequency analysis and intercept analysis of the $1/B$ positions of magneto-oscillation extrema to identify the nature of the carriers in graphite, whether they are Dirac or normal (Schr\"odinger) fermions. These analyses indicate normal holes and electrons of indeterminate nature.

Derivative expansion for the effective action of chiral gauge fermions. The abnormal parity component

Explicit exact formulas are presented, for the leading order term in a strict chiral covariant derivative expansion, for the abnormal parity component of the effective action of two- and four-dimensional Dirac fermions in presence of scalar, pseudo-scalar, vector and axial vector background fields. The formulas hold for completely general internal symmetry groups and general configurations. In particular the scalar and pseudo-scalar fields need not be on the chiral circle.