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arXiv2008

Non-linear graphene optics for terahertz applications

The linear electrodynamic properties of graphene -- the frequency-dependent conductivity, the transmission spectra and collective excitations -- are briefly outlined. The non-linear frequency multiplication effects in graphene are studied, taking into account the influence of the self-consistent-field effects and of the magnetic field. The predicted phenomena can be used for creation of new devices for microwave and terahertz optics and electronics.

cond-mat.mes-hall↗

arXiv2009

Incoherence induced sign change in noise cross-correlations: A case study in the full counting statistics of a pure spin pump

The full counting statistics of a non adiabatic pure spin pump are calculated with particular emphasis on the second and third moments. We show that incoherence can change the sign of spin shot noise cross-correlations from negative to positive, implying entanglement for spin-singlet electronic sources, a truly counterintuitive result. The third moment on the other hand is shown to be much more resilient to incoherence.

quant-ph↗

arXiv2012

Magnetocapacitance of a graphene monolayer

We present a theoretical study of magnetocapacitance in a graphene monolayer at finite temperature taking into account the effects of disorder. The density of states (DOS) and magnetocapacitance found for graphene are compared to those found in standard two dimensional electron gas (2DEG) systems. The magnetic oscillations in DOS and magnetocapacitance are found to be enhanced and much more robust with respect to temperature damping in monolayer graphene in comparison with a 2DEG. Furthermore, we find that there is a $π$ phase shift between magnetic oscillations in the two systems which can be attributed to Dirac electrons in graphene acquiring a Berry's phase as they traverse a closed path in a magnetic field.

cond-mat.mes-hall↗

arXiv2009

Aharonov-Casher Effect in Wigner Crystal Exchange Interactions

We theoretically study the effects of spin-orbit coupling on spin exchange in a low-density Wigner crystal. In addition to the familiar antiferromagnetic Heisenberg exchange, we find general anisotropic interactions in spin space if the exchange paths allowed by the crystal structure form loops in real space. In particular, it is shown that the two-electron exchange interaction can acquire ferromagnetic character.

cond-mat.mes-hall↗

arXiv2008

Gate control of the tunneling magnetoresistance in double-barrier junctions

We calculate the conductances and the tunneling magnetoresistance (TMR) of double magnetic tunnel junctions, taking as a model example junctions composed of Fe/ZnSe/Fe/ZnSe/Fe (001). The calculations are done as a function of the gate voltage applied to the in-between Fe layer slab. We find that the application of a gate voltage to the in-between Fe slab strongly affects the junctions' TMR due to the tuning or untuning of conductance resonances mediated by quantum well states. The gate voltage allows a significant enhancement of the TMR, in a more controllable way than by changing the thickness of the in-between Fe slab. This effect may be useful in the design of future spintronic devices based on the TMR effect, requiring large and controllable TMR values.

cond-mat.mes-hall↗

arXiv2009

The Marginal Fermi Liquid - An Exact Derivation Based on Dirac's First Class Constraints Method

Dirac's method for constraints is used for solving the problem of exclusion of double occupancy for Correlated Electrons. The constraints are enforced by the pair operator $Q(\vec{x})=ψ_{\downarrow}(\vec{x})ψ_{\uparrow}(\vec{x})$ which annihilates the ground state $|Ψ^0>$. Away from half fillings the operator $Q(\vec{x})$ is replaced by a set of $first$ $class$ Non-Abelian constraints $Q^{(-)}_α(\vec{x})$ restricted to negative energies. The propagator for a single hole away from half fillings is determined by modified measure which is a function of the time duration of the hole propagator. As a result: a) The imaginary part of the self energy - is linear in the frequency. At large hole concentrations a Fermi Liquid self energy is obtained. b) For the Superconducting state the constraints generate an asymmetric spectrum excitations between electrons and holes giving rise to an asymmetry tunneling density of states.

cond-mat.mes-hall↗

arXiv2008

Decoherence of one-dimensional electron system

Recently, there have been many attempts to implement quantum computation experimentally. For this purpose, quantum coherence should be maintained during gate operations. Therefore, the control of decoherence is a very important problem. In present study, we examine the decoherence of one-dimensional electron system, which coupled with acoustic phonon. The time-dependent Hatree-Fock approximation is applied for electron system, the Schroinger equation is applied for phonon.The metal-insulator transition found for coherence time. The results are the election interaction reduces the coherence time. We also exmined the Bang-Bang control, the results shows recovery of coherence.

cond-mat.mes-hall↗

arXiv2008

Quantum spin pump on a finite antiferromagnetic chain through bulk states

We studied the possibility of the spin pump in a S=1/2 antiferromagnetic chain. The spin chain is mapped into a fermion system and bosonization is utilized to transform the equation of motion to a sine-Gordon equation. The sine-Gordon equation on a finite chain with different boundary conditions is solved. Among numerous solutions, the static soliton is compatible with the original physical system. By varying adiabatically a angle in the phase space composed of applied electric and magnetic fields, the spin states change between the Neel state and dimer state and a quantized spin is transported by the bulk state from one end of the system to the other.

cond-mat.str-el↗

arXiv2009

Quantum Impurity Problems in Condensed Matter Physics

Impurities are ubiquitous in condensed matter. Boundary Conformal Field Theory (BCFT) provides a powerful method to study a localized quantum impurity interacting with a gapless continuum of excitations. The results can also be implied to nanoscopic devices like quantum dots. In these lecture notes, I review this field, including the following topics: I. General Renormalization Group (RG) framework for quantum impurity problems: example of simplest Kondo model II. Multi-channel Kondo model III. Quantum Dots: experimental realizations of one and two channel Kondo models IV. Impurities in Luttinger liquids: point contact in a quantum wire V. Quantum impurity entanglement entropy VI. Y-junctions of Luttinger liquids VII. Boundary condition changing operators and the X-ray edge problem.

cond-mat.str-el↗

arXiv2008

Nanoelectronic Devices: A Unified View

Nanoscale electronic devices are of great interest for all kinds of applications like switching, energy conversion and sensing. The objective of this chapter, however, is not to discuss specific devices or applications. Rather it is to convey the conceptual framework that has emerged over the last twenty years, which is important not only because of the practical insights it provides into the design of nanoscale devices, but also because of the conceptual insights it affords regarding the meaning of resistance and the essence of all non-equilibrium phenomena in general. We present a unified description applicable to a wide variety of devices from molecular conductors to carbon nanotubes to silicon transistors covering different transport regimes from the ballistic to the diffusive limit, based on what we call the NEGF-Landauer approach.

cond-mat.mes-hall↗

arXiv2008

Transport and noise in organic field effect devices

The transport and fluctuation properties of organic molecules ordered parallel between two Au contact leads are investigated by the method of surface Green function. From first-principles simulation the relevant hopping parameters are extracted and used to calculate nonlinear transport coefficients with respect to an external bias voltage. A staggering of conductance is found in dependence on the number of molecules squeezed in between the contacts. The thermal properties show an anomalous behavior whenever the voltage reaches the values of the molecular energy levels active for transport. The thermoelectric figure of merit shows a resonance allowing to reach values even larger than one.

cond-mat.mes-hall↗

arXiv2011

Fractional Quantum Hall Effect States on a Torus show a Liquid Insulator Direct Transition

From the Fractional Quantum Hall Effect States on a torus with filling factor nu = 1/p, p odd, we found that for small values of nu such states describe triangular Wigner crystals which are stable configurations. Therefore, the insulator phase of the two-dimensional electron system can be described with the same wave function that the liquid phase, and the incompressible liquid freezes (or the Wigner crystal melts) without any quantum transition between states, thus the system phase changes by means of a direct transition. We found at least one stable configuration of a Wigner crystal with F = 3 electrons for cell and we investigate the behavior of the crystal when the lengths ratio Ly/Lx changes.

cond-mat.mes-hall↗

arXiv2009

Catching the zitterbewegung

We demonstrate how the zitterbewegung charge oscillations can be detected through a charge conductance measurement in a three-terminal junction. By tuning the spin-orbit interaction strength or an external magnetic field the zitterbewegung period can be modulated, translating into complementary conductance oscillations in the two outgoing leads of the junction. The proposed experimental setup is within the reach of demonstrated technology and material parameters, and enables the observation of the so far elusive zitterbewegung phenomenon.

cond-mat.mes-hall↗

arXiv2008

Third order susceptibility: general formalism for photoinduced current density in semiconductors

This paper contains a detailed derivation of the photoinduced current density at third order in the coupling between a semiconductor and a multifrequency photon field, starting from its standard textbook expression as a third order time integral of a triple commutator. Due to a major intrinsic problem linked to this triple commutator, such a derivation has been made possible quite recently only, thanks to the tools developed in the composite-boson many-body theory we have just constructed. The photoinduced current density is shown to ultimately read in a compact form, in terms of the Pauli and Coulomb scatterings for exciton-exciton interactions introduced in this theory. Representation in Shiva diagrams is also given to better grasp the physics of the various contributions.

cond-mat.mes-hall↗

arXiv2008

Dynamical properties of quasiparticles in a gapped graphene sheet

We present numerical calculations of the impact of charge carriers-carriers interactions on the dynamical properties of quasiparticles such as renormalized velocity and quasiparticle inelastic scattering lifetime in a gapped graphene sheet. Our formalism is based on the many-body $G_0W$-approximation for the self-energy. We present results for the many-body renormalized velocity suppression and the renormalization constant over a broad range of energy gap values. We find that the renormalized velocity is almost independence of the carrier densities at large density regime. We also show that the quasiparticle inelastic scattering lifetime decreases by increasing the gap value. Finally, we present results for the mean free path of charge carriers suppression over the energy gap values.

cond-mat.mes-hall↗

arXiv2008

Nanogranular Thermoelectrics

We investigate thermopower and thermoelectric coefficient of nano-granular materials at large tunneling conductance between the grains, g_{T} >> 1. We show that at intermediate temperatures, T >= g_{T}δ, where δis the mean energy level spacing for a single grain, electron-electron interaction leads to an increase of the thermopower with decreasing grain size. We discuss our results in the light of new types of thermoelectric materials and present the behavior of the figure of merit depending on system parameters.

cond-mat.mes-hall↗

arXiv2009

Dipole emission and coherent transport in random media I

This is the first of a series of papers devoted to develop a microscopical approach to the dipole emission process and its relation to coherent transport in random media. In this Letter, we deduce general expressions for the decay rate of spontaneous emitters and the power emission of induced dipoles embedded in homogenous dielectric media. We derive formulae which apply generically to virtual cavities and, in the continuum approximation, to small real cavities.

physics.optics↗

arXiv2009

Dipole emission and coherent transport in random media II. Density of states vs. index of refraction

This is the second of a series of papers devoted to develop a microscopical approach to the dipole emission process and its relation to coherent transport in random media. In this Letter, we deduce a relation between the transverse decay rate of an emitter in a virtual cavity and the complex refraction index of the host medium. We argue on the possibility of a criterion for inhibition/enhancement of spontaneous emission in function of the transition frequency and the correlation length of the host scatterers. In addition, we study the radiative/non-radiative nature of the net power emission through a microscopical analysis of the scattering events involved. This study reveals essential discrepancies with previous interpretations.

physics.optics↗