Collisional Conductivity Research Articles

Magneto-optical properties in thin film topological insulators with quadratic momentum term

In this work we investigate the influence of quadratic in momentum term (Schrodinger term) on magneto-transport properties of thin film topological insulators. The Schrodinger term modifies the Dirac cones into an hourglass shape which results in inter and intraband Landau levels crossings. Breaking of the particle–hole symmetry in Landau level spectrum in the presence of k 2 term leads to asymmetrical density of states profile. We calculate collisional and Hall conductivity for mixed Dirac–Schrodinger system in linear response regime and show oscillatory behavior in collisional conductivity, while Zeeman and hybridization terms provide a doubly split peak structure in collisional conductivity for the case m/m e → ∞. We calculate Hall conductivity analytically and show that for mixed system filling factor is not symmetric about Fermi energy unlike symmetric plateaus for pure Dirac case.

Magnetotransport properties of the α-T3 model

Using the well-known Kubo formula, we evaluate magnetotransport quantities, such as the collisional and Hall conductivities of the α-T3 model. The collisional conductivity exhibits a series of peaks at a strong magnetic field. Each of the conductivity peaks for (graphene) splits into two in the presence of a finite α. This splitting occurs due to a finite phase difference between the contributions coming from the two valleys. The density of states is also calculated to explore the origin of the splitting of conductivity peaks. As α approaches 1, the right split part of a conductivity peak comes closer to the left split part of the next conductivity peak. At , they merge with each other to produce a new series of the conductivity peaks. On the other hand, the Hall conductivity undergoes a smooth transition from to with n = 0,1,2,... as we tune α from 0–1. For intermediate α, we obtain the Hall plateaus at values 0,2,4,6,8,... in units of e2/h.

Quantum magnetotransport for the surface states of three-dimensional topological insulators in the presence of a Zeeman field

We show that the surface states of magnetic topological insulators realize an activated behavior and Shubnikov de Haas oscillations. Applying an external magnetic field perpendicular to the surface of the topological insulator in the presence of Zeeman interaction, we investigate the opening of a gap at the Dirac point, making the surface Dirac fermions massive, and the effects on the transport properties. Analytical expressions are derived for the collisional conductivity for elastic impurity scattering in the first Born approximation. We also calculate the Hall conductivity using the Kubo formalism. Evidence for a transition from gapless to gapped surface states at n = 0 and activated transport is found from the temperature and magnetic-field dependence of the collisional and Hall conductivities.

Quantum magnetotransport properties of topological insulators under strain

We present a detailed theoretical investigation of the quantum magnetotransport properties of topological insulators under strain. We consider an external magnetic field perpendicular to the surface of the topological insulator in the presence of strain induced by the substrate. The strain effects mix the lower and upper surface states of neighboring Landau levels into two unequally spaced energy branches. Analytical expressions are derived for the collisional conductivity for elastic impurity scattering in the first Born approximation. We also calculate the Hall conductivity using the Kubo formalism. Evidence for the beating of Shubnikov--de Haas oscillations is found from the temperature and magnetic field dependence of the collisional and Hall conductivities. In the regime of a strong magnetic field, the beating pattern is replaced by a splitting of the magnetoresistance peaks due to finite strain energy. These results are in excellent agreement with recent HgTe transport experiments.

Gap opening in the zeroth Landau level in gapped graphene: pseudo-Zeeman splitting in an angular magnetic field

We present a theoretical study of gap opening in the zeroth Landau level in gapped graphene as a result of pseudo-Zeeman interaction. The applied magnetic field couples with the valley pseudospin degree of freedom of the charge carriers leading to the pseudo-Zeeman interaction. To investigate its role in transport at the charge neutrality point (CNP), we study the integer quantum Hall effect in gapped graphene in an angular magnetic field in the presence of pseudo-Zeeman interaction. Analytical expressions are derived for the Hall conductivity using the Kubo–Greenwood formula. We also determine the longitudinal conductivity for elastic impurity scattering in the first Born approximation. We show that pseudo-Zeeman splitting leads to a minimum in the collisional conductivity at high magnetic fields and a zero plateau in the Hall conductivity. Evidence for activated transport at CNP is found from the temperature dependence of the collisional conductivity.

Modelling of DC Electric Fields Induced by RF Sheath in Front of ICRF Antenna

Reducing the ICRH (ion cyclotron range frequency) antenna-plasma interaction is one of the key points for reaching very long tokamak discharges. One problem which limits such discharges, is the appearance of hot spots on the surface of the antenna: Radio Frequency (RF) sheaths modify the properties of the edge plasma by rectifying the RF potential along open magnetic field lines and can induce hot spots. This paper investigates the corrections to sheath potentials introduced by the interactions between adjacent flux tubes. Our theoretical study started from an oscillating double Langmuir probe model, in which a transverse influx of current was included. This model was confronted with 1D PIC simulations along a magnetic field line, and demonstrated that current exchanges can decrease mean potentials. A 2D electrostatic fluid code was then developed, which couples adjacent flux tubes in a poloidal cross section with collisional conductivity or polarization currents. It showed that transverse currents are able to smooth structures smaller than a characteristic size in the sheath potential maps (results for Tore Supra). These computed rectified potentials can be used to obtain the DC electric fields in front of the antenna. And then, it gives an estimate of the particle drift and the energy flux on the antenna structure, which can explain hot spots.

Effects of Coulomb Collisions on the Dispersion Relation of Longitudinal Plasma Oscillations

The dispersion relation of longitudinal electron-plasma oscillation for a fully ionized plasma with equal electron and ion temperatures in the absence of an external magnetic field has been calculated by taking into account of both the electron-ion and electron-electron collisions. The calculation utilizes the absorptive parts of the collisional conductivity functions derived by DuBois and coworkers for obtaining the polarizability parts via Kramers-Kronig relation. The results are presented in graphical forms and show that the longitudinal plasma oscillations are shifted toward higher frequencies from the resonance of collisionless plasma predicted by the Bohm-Gross dispersion relation.

Electric Conductivity of Weakly Turbulent Plasmas

The static electric conductivity of weakly turbulent plasmas has been examined on the basis of a kinetic equation for the linear response to the one-particle distribution function. Within the regime of neglecting the mode-coupling terms, the high-frequency turbulent fluctuations associated with the electron plasma wave give rise to a modification of the ordinary collisional conductivity through the adiabatic interaction between the particles and the turbulent fluctuations. On the other hand, the low-frequency turbulent fluctuation due to the ion oscillation mode determines the relaxation process in the system through the resonant interaction, and gives rise to a finite conductivity even in the collisionless plasmas.

Conductivity without collisions or noise

It is usually assumed that there must be particle-particle collisions or wave-particle collisions (noise or radiation) in order to have a finite electrical conductivity. A finite conductivity is necessary for the reconnection of field lines at a magnetic neutral point. The above assumption would therefore imply that some sort of collisions are necessary for reconnection. Dungey has argued that collisions are not necessary to have a positive value of E · j (dissipation of energy), using an analogy with a diode. Indeed, for certain plasma geometries, an effective conductivity may be defined when the characteristic system length is small compared to a collisional mean free path. For the current sheet in the geomagnetic tail, the effective conductivity may be eleven orders of magnitude smaller than the collisional conductivity. The effective conductivity is essentially determined by the inertia of the particles, and therefore reconnection can proceed without collisions, with energy being carried off by accelerated particles. Inertial and gyro-conductivities and a new ‘current sheet adiabatic invariant’ are defined.

Effect of Ion Motion on the Collisional Conductivity of a Magnetoplasma

The collisional contribution to the electrical conductivity is obtained for a magnetoplasma consisting of an arbitrary mixture of charged particle species. The calculation is based on the Landau collision term and is carried out to second order in the wavenumber. Full account is taken of the ion motion.