Browsing by Author "Lewkowicz, M."

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Ballistic transport in graphene beyond linear response
(2010-01-15)
Rosenstein, B.
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Lewkowicz, M.
;
Kao, H. C.
;
Korniyenko, Y.
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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.
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Lewkowicz, M.
;
Rosenstein, B.
;
電子物理學系
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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.
Chiral Anomaly and Strength of the Electron-Electron Interaction in Graphene
(2013-02-08)
Rosenstein, B.
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Lewkowicz, M.
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Maniv, T.
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電子物理學系
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Department of Electrophysics
The long-standing controversy concerning the effect of electron-electron interaction on the electrical conductivity of an ideal graphene sheet is settled. Performing the calculation directly in the tight binding approach without the usual prior reduction to the massless Dirac (Weyl) theory, it is found that, to leading order in the interaction strength alpha = e(2)/(h) over barv(0), the dc conductivity sigma/sigma(0) = 1 + C alpha + O(alpha(2)) is significantly enhanced with respect to the independent-electron result sigma(0), i.e., with the value C = 0.26. The ambiguity characterizing the various existing approaches is nontrivial and related to the chiral anomaly in the system. In order to separate the energy scales in a model with massless fermions, contributions from regions of the Brillouin zone away from the Dirac points have to be accounted for. Experimental consequences of the relatively strong interaction effect are briefly discussed. DOI: 10.1103/PhysRevLett.110.066602
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
Dynamics of electric transport in interacting Weyl semimetals
(2013-07-03)
Rosenstein, B.
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Lewkowicz, M.
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電子物理學系
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Department of Electrophysics
The response to an electric field (dc and ac) of electronic systems in which the Fermi "surface" consists of a number of three-dimensional (3D) Weyl points (such as some pyrochlore iridates) exhibits a peculiar combination of characteristics usually associated with insulating and conducting behavior. Generically a neutral plasma in clean materials can be described by a tight-binding model with a strong spin-orbit interaction. A system of that type has a vanishing dc conductivity; however the current response to the dc field is very slow: The current decays with time in a powerwise manner, different from an insulator. The ac conductivity, in addition to a finite real part sigma'(Omega) which is linear in frequency, exhibits an imaginary part sigma '' (Omega) that increases logarithmically as a function of the UV cutoff (atomic scale). This leads to a substantial dielectric response like a large dielectric constant at low frequencies. This is in contrast to a two-dimensional (2D) Weyl semimetal-like graphene at a neutrality point where the ac conductivity is purely pseudodissipative. The Coulomb interaction between electrons is long range and sufficiently strong to make a significant impact on transport. The interaction contribution to the ac conductivity is calculated within the tight-binding model. The result for the real part expressed via the renormalized (at frequency (Omega) over bar) Fermi velocity nu is Delta sigma'(Omega) = e(4)Omega/(9 pi(2)(h) over bar nu)[2 log(Omega/ (Omega) over bar)-5].
Dynamics of Particle-Hole Pair Creation in Graphene
(2009-03-13)
Lewkowicz, M.
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Rosenstein, B.
;
電子物理學系
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Department of Electrophysics
The process of coherent creation of particle-hole excitations by an electric field in graphene is quantitatively described. We calculate the evolution of the current density, number of pairs, and energy after switching on the electric field. In particular, it leads to a dynamical visualization of universal finite resistivity without dissipation in pure graphene. We show that the dc conductivity of pure graphene is pi/2e(2)/h rather than the often cited value of 4/pi e(2)/h. This value coincides with the ac conductivity calculated and measured recently at optical frequencies. The effect of temperature and random chemical potential (charge puddles) are considered and explain the recent experiment on suspended graphene. A possibility of Bloch oscillations is discussed within the tight binding model.
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.
Nonlocal electrodynamics in Weyl semimetals
(2017-02-28)
Rosenstein, B.
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Kao, H. C.
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Lewkowicz, M.
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電子物理學系
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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.
Two distinct ballistic processes in graphene
(2012-01-01)
Lewkowicz, M.
;
Rosenstein, B.
;
Nghiem, D.
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電子物理學系
;
Department of Electrophysics
A dynamical approach to ballistic transport in mesoscopic graphene samples of finite length Land contact potential difference with leads U is developed. It is shown that at ballistic times shorter than both relevant time scales, t(L) = L/v(g) (v(g) - Fermi velocity) and t(U) = (h) over bar/(eU), the major effect of electric field is to creates the electron - hole pairs, namely causes interband transitions. At ballistic times lager than the two scales the mechanism is very different. The conductivity has its "nonrelativistic" or intraband value equal to the one obtained within the Landauer-Butticker approach for the barrier U-resulting from evanescent waves tunneling through the barrier.
Two distinct ballistic processes in graphene at the Dirac point
(2011-09-15)
Lewkowicz, M.
;
Rosenstein, B.
;
Nghiem, D.
;
電子物理學系
;
Department of Electrophysics
The dynamical approach is applied to ballistic transport in mesoscopic graphene samples of length L and contact potential U. At times shorter than both relevant time scales, the flight time t(L) = L/v(g) (v(g) is Fermi velocity) and t(U) = (h) over bar /U, the major effect of the electric field is to create electron-hole pairs, i.e., causing interband transitions. In linear response, this leads (for width W >> L) to conductivity sigma(2) = pi/2 e(2)/h. On the other hand, at times lager than the two scales, the mechanism and value are different. It is shown that the conductivity approaches its intraband value, equal to the one obtained within the Landauer-Butticker approach resulting from evanescent waves. It is equal to sigma(1) = 4/pi e(2)/h for W >> L and t(U) << t(L). The interband transitions, within linear response, are unimportant in this limit. Between these extremes there is a crossover behavior dependent on the ratio between the two time scales t(L)/t(U). At strong electric fields (beyond linear response) the interband process dominates. The electron-hole mechanism is universal, namely, does not depend on geometry (aspect ratio, topology of boundary conditions, properties of leads), while the evanescent modes mechanism depends on all of them. On basis of the results we determine that while in absorption measurements and in dc transport in suspended graphene sigma(2) was measured, sigma(1) would appear in experiments on small ballistic graphene flakes on substrate.