Browsing by Author "Kao, H. C."

Now showing 1 - 9 of 9

Ballistic transport in graphene beyond linear response
(2010-01-15)
Rosenstein, B.
;
Lewkowicz, M.
;
Kao, H. C.
;
Korniyenko, Y.
;
電子物理學系
The process of coherent creation of particle-hole excitations by an electric field in graphene is quantitatively described beyond linear response. We calculate the evolution of current density, number of pairs and energy in ballistic regime for electric field E using the tight-binding model. While for ballistic flight times smaller than t(nl)proportional to E(-1/2) current is linear in E and independent of time, for larger ballistic times the current increases after t(nl) as J proportional to E(3/2)t and finally at yet larger times (t > t(B) proportional to E(-1)) Bloch oscillations set in. It is shown that the number of pairs follows the 2D generalization of the Schwinger's creation rate n proportional to E(3/2) only on certain time segments with a prefactor different from that obtained using the asymptotic formula.
Ballistic transport, chiral anomaly, and emergence of the neutral electron-hole plasma in graphene
(2010-07-07)
Kao, H. C.
;
Lewkowicz, M.
;
Rosenstein, B.
;
電子物理學系
;
Department of Electrophysics
The process of coherent creation of particle-hole excitations by an electric field in graphene is quantitatively described using a dynamic "first-quantized" approach. We calculate the evolution of current density, number of pairs, and energy in ballistic regime using the tight-binding model. The series in electric field strength E up to third order in both dc and ac are calculated. We show how the physics far from the two Dirac points enters various physical quantities in linear response and how it is related to the chiral anomaly. The third harmonic generation and the imaginary part of conductivity are obtained. It is shown that at certain time scale t(nl) alpha E-1/2 the physical behavior dramatically changes and the perturbation theory breaks down. Beyond the linear-response physics is explored using an exact solution of the first-quantized equations. While for small electric fields the I-V curve is linear characterized by the universal minimal resistivity sigma=pi/2(e(2)/h), at t>t(nl) the conductivity grows fast. The copious pair creation (with rate E-3/2), analogous to Schwinger's electron-positron pair creation from vacuum in QED, leads to creation of the electron-hole plasma at ballistic times of order t(nl). This process is terminated by a relaxational recombination.
Covariant gaussian approximation in Ginzburg-Landau model
(2017-05-01)
Wang, J. F.
;
Li, D. P.
;
Kao, H. C.
;
Rosenstein, B.
;
電子物理學系
Condensed matter systems undergoing second order transition away from the critical fluctuation region are usually described sufficiently well by the mean field approximation. The critical fluctuation region, determined by the Ginzburg criterion, vertical bar T/T-c, 1 vertical bar << Gi, is narrow even in high T, superconductors and has universal features well captured by the renormalization group method. However recent experiments on magnetization, conductivity and Nernst effect suggest that fluctuations effects are large in a wider region both above and below T-c. In particular some "pseudogap" phenomena and strong renormalization of the mean field critical temperature T-mf can be interpreted as strong fluctuations effects that are nonperturbative (cannot be accounted for by "gaussian fluctuations"). The physics in a broader region therefore requires more accurate approach. Self consistent methods are generally "non-conserving" in the sense that the Ward identities are not obeyed. This is especially detrimental in the symmetry broken phase where, for example, Goldstone bosons become massive. Covariant gaussian approximation remedies these problems. The Green's functions obey all the Ward identities and describe the fluctuations much better. The results for the order parameter correlator and magnetic penetration depth of the Ginzburg-Landau model of superconductivity are compared with both Monte Carlo simulations and experiments in high T-c, cuprates. (C) 2017 Elsevier Inc. All rights reserved.
Dynamical approach to ballistic transport in graphene
(2011-01-01)
Kao, H. C.
;
Lewkowicz, M.
;
Korniyenko, Y.
;
Rosenstein, B.
;
電子物理學系
The process of the coherent creation of particle-hole excitations by an electric field in graphene is quantitatively described beyond linear response We calculate the evolution of the current density and the number of pairs in the ballistic regime using the tight binding model While for small electric fields the I-V curve is linear characterized by the universal minimal resistivity sigma = pi/2(e(2)/h) for larger fields after a certain time interval the linear regime crosses over to a quadratic one and finally at larger times Bloch oscillations set in (C) 2010 Elsevier BV All rights reserved
Long-range electron-electron interactions in graphene make its electrodynamics nonlocal
(2014-07-28)
Rosenstein, B.
;
Kao, H. C.
;
Lewkowicz, M.
;
電子物理學系
;
Department of Electrophysics
Graphene, a one-layer-thick hexagonal array of carbon atoms, when undoped, exhibits a curious mixture of properties pertinent to either metals or insulators. On the one hand, despite near absence of both charge carriers and impurities, it has a finite conductivity like a metal. On the other hand, the Coulomb interaction between electrons is unscreened like in a dielectric and hence is long range. The chemical potential is pinned right between the conical valence and conduction bands causing quasiparticles to move like massless relativistic particles. We demonstrate at small coupling that the electrodynamics of graphene exhibits nonlocality on a macroscopic level due to the combination of the long-range interactions and the linear dispersion relation. The frequency and wave vector k-dependent conductivity tensor, in addition to a local pseudo-Ohmic part sigma(T)delta(ij), possesses a nonlocal contribution sigma(n1)k(i)k(j)/k(2). While the coefficient of the local part is sigma(T) approximate to e(2)/4 (h) over bar, the coefficient of the nonlocal part is proportional to the Coulomb interaction strength alpha, sigma(n1) = sigma(T)alpha. This leads to several remarkable effects in transport and optical response. In particular, the resistance of the graphene flake depends on the location and the geometry of the source, drain, and probes. A voltage perpendicular to the current appears in a time-reversal symmetric situation and the polarization of reflected and transmitted light is modified, without either the magnetic field (like in the Faraday effect) or anisotropy.
Mean field theory of short-range order in strongly correlated low dimensional electronic systems
(2019-09-18)
Rosenstein, Baruch
;
Li, Dingping
;
Ma, Tianxing
;
Kao, H. C.
;
電子物理學系
Mean field approach, although a generally reliable tool that captures major short-range correlations, often fails in symmetric low dimensional strongly correlated electronic systems like those described by the Hubbard model. In these situations a symmetry is almost broken. The problem is linked to the restoration of the symmetry due to strong fluctuations (both quantum and thermal) on all scales. The restoration of symmetry in statistical models of scalar order parameter fields was treated recently successfully on the Gaussian approximation level by symmetrization of the correlators. Here the idea is extended to fermionic systems in which the order parameter is composite. Furthermore, the precision of the correlators can be improved perturbatively. Such a scheme (based on covariant Gaussian approximation) is demonstrated on the one dimensional (1D) and 2D one band Hubbard models by comparison of the correlator with exact diagonalization and MC simulations, respectively.
Nonlocal electrodynamics in Weyl semimetals
(2017-02-28)
Rosenstein, B.
;
Kao, H. C.
;
Lewkowicz, M.
;
電子物理學系
;
Department of Electrophysics
Recently synthesized three-dimensional materials with Dirac spectrum exhibit peculiar electric transport qualitatively different from its two-dimensional analog, graphene. By neglecting impurity scattering, the real part of the conductivity is strongly frequency dependent, while the imaginary part is nonzero unlike in undoped, clean graphene. The Coulomb interaction between electrons is unscreened as in a dielectric and hence is long range. We demonstrate that the interaction correction renders the electrodynamics nonlocal on a mesoscopic scale. The longitudinal conductivity sigma(L) and the transverse conductivity sigma(T) are different in the long-wavelength limit and consequently the standard local Ohm's law description does not apply. This leads to several remarkable effects in optical response. The p-polarized light generates in these materials bulk plasmons as well as the transversal waves. At a specific frequency the two modes coincide, a phenomenon impossible in a local medium. For any frequency there is a Brewster angle where total absorption occurs, turning the Weyl semimetals opaque. The effect of the surface, including the Fermi arcs, is discussed.
Signature of Schwinger's pair creation rate via radiation generated in graphene by strong electric current
(2012)
Rosenstein, B.
;
Lewkowicz, M.
;
Kao, H. C.
;
電子物理學系
;
Department of Electrophysics
Electron - hole pairs are copuously created by an applied electric field near the Dirac point in graphene or similar 2D electronic systems. It was shown recently that for sufficiently large electric fields E and ballistic times the I-V characteristics become strongly nonlinear due to Schwinger's pair creation rate, proportional to E-3/2. Since there is no energy gap the radiation from the pairs' annihilation is enhanced. The spectrum of radiation is calculated and exhibits a maximum at omega = root eEvg/(h) over bar. The angular and polarization dependence of the emitted photons with respect to the graphene sheet is quite distinctive. For very large currents the recombination rate becomes so large that it leads to the second Ohmic regime due to radiation friction.
Signature of the Schwinger pair creation rate via radiation generated in graphene by a strong electric current
(2011-07-20)
Lewkowicz, M.
;
Kao, H. C.
;
Rosenstein, B.
;
電子物理學系
;
Department of Electrophysics
Electron-hole pairs are copiously created by an applied electric field near the Dirac point in graphene or similar two-dimensional electronic systems. It was shown recently that for sufficiently large electric fields E and ballistic times the I-V characteristics become strongly nonlinear due to Schwinger's pair creation rate, proportional to E(3/2). Since there is no energy gap the radiation from the pairs' annihilation is enhanced. The spectrum of radiation is calculated and exhibits a maximum at omega = root eEv(g)/h. The angular and polarization dependence of the emitted photons with respect to the graphene sheet is quite distinctive. For very large currents the recombination rate becomes so large that it leads to the second Ohmic regime due to radiation friction.