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Register a new userWaveguide Modes in Weyl Semimetals with Tilted Dirac Cones
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We theoretically study unattenuated electromagnetic guided wave modes in centrosymmetric Weyl semimetal layered systems. By solving Maxwells equations for the electromagnetic fields and using the appropriate boundary conditions, we derive dispersion relations for propagating modes in a finite-sized Weyl semimetal. Our findings reveal that for ultrathin structures, and proper Weyl cones tilts, extremely localized guided waves can propagate along the semimetal interface over a certain range of frequencies. This follows from the anisotropic nature of the semimetal where the diagonal components of the permittivity can exhibit a tunable epsilon-near-zero response. From the dispersion diagrams, we determine experimentally accessible regimes that lead to high energy-density confinement in the Weyl semimetal layer. Furthermore, we show that the net system power can vanish all together, depending on the Weyl cone tilt and frequency of the electromagnetic wave.These effects are seen in the energy transport velocity, which demonstrates a substantial slowdown in the propagation of electromagnetic energy near critical points of the dispersion diagrams. Our results can provide guidelines in designing Weyl semimetal waveguides that can offer efficient control in the velocity and direction of energy flow.
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We study the effects of pseudo-magnetic fields on Weyl semimetals with over-tilted Weyl cones, or type II cones. We compare the phenomenology of the resulting pseudo-Landau levels in the type II Weyl semimetal to the known case of type I cones. We predict that due to the nature of the chiral Landau level resulting from a magnetic field, a pseudo-magnetic field, or their combination, the optical conductivity can be utilized to detect a type II phase and deduce the direction of the tilt. Finally, we discuss ways to engineer homogeneous and inhomogeneous type II semimetals via generalizations of known layered constructions in order to create controlled pseudo-magnetic fields and over-tilted cones.
Broken symmetry and tilting effects are ubiquitous in Weyl semimetals (WSMs). Therefore, it is crucial to understand their impacts on the materials electronic and optical properties. Here, using a realistic four-band model for WSMs that incorporates both the symmetry breaking and tilting effects, we study its Landau quantization and the associated magneto-absorption spectrum. We show that the Landau levels in tilted Weyl bands can be obtained by considering a non-tilt Hamiltonian through Lorentz boost. However, broken symmetry effects can generate an additional term in the Hamiltonian, which equivalently leads to band reconstruction. Our work provides a more realistic view of the magnetic field response of WSMs that shall be taken into account in relevant future device applications.
In the presence of certain symmetries, three-dimensional Dirac semimetals can harbor not only surface Fermi arcs, but also surface Dirac cones. Motivated by the experimental observation of rotation-symmetry-protected Dirac semimetal states in iron-based superconductors, we investigate the potential intrinsic topological phases in a $C_{4z}$-rotational invariant superconducting Dirac semimetal with $s_{pm}$-wave pairing. When the normal state harbors only surface Fermi arcs on the side surfaces, we find that an interesting gapped superconducting state with a quartet of Majorana cones on each side surface can be realized, even though the first-order topology of its bulk is trivial. When the normal state simultaneously harbors surface Fermi arcs and surface Dirac cones, we find that a second-order time-reversal invariant topological superconductor with helical Majorana hinge states can be realized. The criteria for these two distinct topological phases have a simple geometric interpretation in terms of three characteristic surfaces in momentum space. By reducing the bulk material to a thin film normal to the axis of rotation symmetry, we further find that a two-dimensional first-order time-reversal invariant topological superconductor can be realized if the inversion symmetry is broken by applying a gate voltage. Our work reveals that diverse topological superconducting phases and types of Majorana modes can be realized in superconducting Dirac semimetals.
We investigate collective modes in three dimensional (3D) gapless multi-Weyl semimetals with anisotropic energy band dispersions (i.e., $Esim sqrt{ k_{parallel}^{2J} + k_z^2}$, where $k_{parallel}$ and $k_z$ are wave vectors and $J$ is a positive integer). For comparison, we also consider the gapless semimetals with the isotropic band dispersions (i.e., $Esim k^J$). We calculate analytically long-wavelength plasma frequencies incorporating interband transitions and chiral properties of carriers. For both the isotropic and anisotropic cases, we find that interband transitions and chirality lead to the depolarization shift of plasma frequencies. For the isotropic parabolic band dispersion (i.e., $N=2$, $Esim k^2$), the long-wavelength plasma frequencies lie outside the single particle excitation regions for all carrier densities, and thus the plasmons do not decay via Landau damping. For the higher-order band dispersions ($N ge 3$) the long-wavelength plasmons experience damping below a critical density. For systems with the anisotropic dispersion the density dependence of the long-wavelength plasma frequency along the direction of non-linear dispersion behaves like that of the isotropic linear band model ($N=1$), while along the direction of linear dispersion it behaves like that of the isotropic non-linear model ($N ge 2$). Plasmons along both directions remain undamped over a broad range of densities due to the chirality induced depolarization shift. Our results provide a comprehensive picture of how band dispersion and chirality affect plasmon behaviors in 3D gapless chiral systems with the arbitrary band dispersion.
We report the first observation of Shubnikov-de Haas (SdH) oscillations and quantized Hall resistance in the multilayered massless Dirac fermion system $alpha$-(BEDT-TTF)$_2$I$_3$ with tilted cones. Holes were injected into the thin crystal fixed on a polyethylene naphthalate (PEN) substrate by contact electrification. The detection of SdH oscillations whose phase was modified by Berrys phase $pi$ strongly suggested that the carrier doping was successful in this system. We succeeded in detecting the quantum Hall effect (QHE) with the steps which is the essence of two dimensional Dirac fermion systems. The number of effectively doped layers was examined to be two in this device. We reveal that the correlation between effective layers plays an important role in QHE.
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