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At least 73 records · Page 4Linked to original sources
arXiv2007

Dissipation in finite systems: Semiconductor NEMS, graphene NEMS, and metallic nanoparticles

This thesis studies three kinds of mesoscopic systems and how one of their collective degrees of freedom is affected by a dissipative environment: a)Nanoelectromechanical systems based on semiconductor materials, and how surface friction processes limit the quality factor of their vibrational eigenmodes at low temperatures, causing as well a frequency shift; b)Graphene - based nanoresonators, and several damping mechanisms limiting the quality factor of its fundamental bending (flexural) eigenmode, and c)The dissipative dynamics of the surface plasmon created in a metallic nanocluster by a laser pulse, discussing the validity and limitations of a very common theoretical model used for its description.

cond-mat.mes-hall↗
arXiv2007

The Origin and Control of the Sources of AMR in (Ga,Mn)As Devices

We present details of our experimental and theoretical study of the components of the anisotropic magnetoresistance (AMR) in (Ga,Mn)As. We develop experimental methods to yield directly the non-crystalline and crystalline AMR components which are then independently analyzed. These methods are used to explore the unusual phenomenology of the AMR in ultra thin (5nm) (Ga,Mn)As layers and to demonstrate how the components of the AMR can be engineered through lithography induced local lattice relaxations. We expand on our previous [Phys. Rev. Lett. \textbf{99}, 147207 (2007)] theoretical analysis and numerical calculations to present a simplified analytical model for the origin of the non-crystalline AMR. We find that the sign of the non-crystalline AMR is determined by the form of spin-orbit coupling in the host band and by the relative strengths of the non-magnetic and magnetic contributions to the impurity potential.

cond-mat.mes-hall↗
arXiv2008

The Bose-Fermi Kondo model with a singular dissipative spectrum: Exact solutions and their implications

Quantum dissipation induces a critical destruction of a Kondo screened state, which is of interest in the contexts of quantum critical heavy fermion metals and magnetic nanostructures. The sub-ohmic Bose-Fermi Kondo model provides a setting to study this effect. We find that this many-body problem is exactly solvable when the spectrum of the dissipative bosonic bath, J(ω), is singular, corresponding to J(τ)=const.. We determine the local spin correlation functions, showing that the singular LONGITUDINAL fluctuations of the bosonic bath dominate over the transverse ones. Our results provide evidence that the local quantum critical solution, derived within the extended dynamical mean field approach to the Kondo lattice model, has a zero residual entropy.

cond-mat.str-el↗
arXiv2009

Twisting versus bending in quantum waveguides

We make an overview of spectral-geometric effects of twisting and bending in quantum waveguides modelled by the Dirichlet Laplacian in an unbounded three-dimensional tube of uniform cross-section. We focus on the existence of Hardy-type inequalities in twisted tubes of non-circular cross-section.

math-ph↗
arXiv2008

Hybrid-impurity resonances in anisotropic quantum dots

The absorption of electromagnetic radiation of an anisotropic quantum dot is theoretically investigated taking into account the processes associated with simultaneous scattering from ionized impurities. It is shown that the scattering of electrons by impurities leads to the resonance absorption even if we have only one impurity in the quantum dot. Explicit formula is derived for the absorption coefficient. The positions of the resonances peaks are found. The effects of external magnetic field on the resonance absorption are studied.

cond-mat.mes-hall↗
arXiv2010

Spontaneously Localized Photonic Modes Due to Disorder in the Dielectric Constant

We present the first experimental evidence for the existence of strongly localized photonic modes due to random two dimensional fluctuations in the dielectric constant. In one direction, the modes are trapped by ordered Bragg reflecting mirrors of a planar, one wavelength long, microcavity. In the cavity plane, they are localized by disorder, which is due to randomness in the position, composition and sizes of quantum dots located in the anti-node of the cavity. We extend the theory of disorder induced strong localization of electron states to optical modes and obtain quantitative agreement with the main experimental observations.

cond-mat.mes-hall↗
arXiv2008

Simulations of Nanowire Transistors: Atomistic vs. Effective Mass Models

The ballistic performance of electron transport in nanowire transistors is examined using a 10 orbital sp3d5s* atomistic tight-binding model for the description of the electronic structure, and the top-of-the-barrier semiclassical ballistic model for calculation of the transport properties of the transistors. The dispersion is self consistently computed with a 2D Poisson solution for the electrostatic potential in the cross section of the wire. The effective mass of the nanowire changes significantly from the bulk value under strong quantization, and effects such as valley splitting strongly lift the degeneracies of the valleys. These effects are pronounced even further under filling of the lattice with charge. The effective mass approximation is in good agreement with the tight binding model in terms of current-voltage characteristics only in certain cases. In general, for small diameter wires, the effective mass approximation fails.

cond-mat.mtrl-sci↗
arXiv2008

Spin-orbit interaction effect on transport of Dirac fermions in graphene

We study theoretically the quantum transport properties of the Dirac fermions with spin-orbit interactions (SOIs) in graphene by using the method of Schwinger proper time together with decomposition over Landau level poles and Kubo formula. The analytical expressions for both longitudinal and Hall conductivities are derived explicitly. It is found that, from some numerical examples, when the Rashba SOI is taken into account the Shubnikov-de Haas (SdH) oscillation peaks of the longitudinal conductivity versus the chemical potential are split, while the SdH oscillation of the longitudinal conductivity versus a external magnetic field exhibits a beating pattern. Furthermore, the Rashba SOI tends to suppress the quantum Hall effect in graphene.

cond-mat.mes-hall↗
arXiv2008

The Noise Susceptibility of a Photo-excited Coherent Conductor

We report the theory of the \emph{dynamical response of current fluctuations} of a photo-excited conductor. We have performed the calculation for a coherent conductor described by arbitrary energy-dependent transmissions and for arbitrary frequencies. We consider two experimental setups that correspond to different ways of symmetrizing the current operators, leading to different predictions. Our results are in very good, quantitative agreement with a recent measurement. We demonstrate that the dynamical response of noise that we have calculated is the key concept that relates Dynamical Coulomb Blockade, i.e. the electron-electron correction to the conductance due to the presence of an external impedance, to quantum noise.

cond-mat.mes-hall↗
arXiv2008

Magnetic structure at zigzag edges of graphene bilayer ribbons

We study the edge magnetization of bilayer graphene ribbons with zigzag edges. The presence of flat edge-state bands at the Fermi energy of undoped bilayer, which gives rise to a strong peak in the density of states, makes bilayer ribbons magnetic at the edges even for very small on-site electronic repulsion. Working with the Hubbard model in the Hartree Fock approximation we show that the magnetic structure in bilayer ribbons with zigzag edges is ferromagnetic along the edge, involving sites of the two layers, and antiferromagnetic between opposite edges. It is also shown that this magnetic structure is a consequence of the nature of the edge states present in bilayer ribbons with zigzag edges. Analogously to the monolayer case, edge site magnetization as large as $m \approx0.2 μ_{B}$ (per lattice site) even at small on-site Hubbard repulsion $U \approx 0.3 {eV}$ is realized in nanometer wide bilayer ribbons.

cond-mat.mes-hall↗
arXiv2008

Quantum Field Theory on Star Graphs

We discuss some basic aspects of quantum fields on star graphs, focusing on boundary conditions, symmetries and scale invariance in particular. We investigate the four-fermion bulk interaction in detail. Using bosonization and vertex operators, we solve the model exactly for scale invariant boundary conditions, formulated in terms of the fermion current and without dissipation. The critical points are classified and determined explicitly. These results are applied for deriving the charge and spin transport, which have interesting physical features.

hep-th↗
arXiv2008

Transmission Eigenvalue Densities and Moments in Chaotic Cavities from Random Matrix Theory

We point out that the transmission eigenvalue density and higher order correlation functions in chaotic cavities for an arbitrary number of incoming and outgoing leads $(N_1,N_2)$ are analytically known from the Jacobi ensemble of Random Matrix Theory. Using this result and a simple linear statistic, we give an exact and non-perturbative expression for moments of the form $<λ_1^m>$ for $m>-|N_1-N_2|-1$ and $β=2$, thus improving the existing results in the literature. Secondly, we offer an independent derivation of the average density and higher order correlation functions for $β=2,4$ which does not make use of the orthogonal polynomials technique. This result may be relevant for an efficient numerical implementation avoiding determinants.

cond-mat.mes-hall↗
arXiv2008

Photoconductivity of an intrinsic graphen

We examine the photoconductivity of an intrinsic graphene associated with far- and mid-infrared irradiation at low temperatures. The model under consideration accounts for the excitation of the electron-hole pairs by incident radiation, the interband generation-recombination transitions due to thermal radiation, and the intraband energy relaxation due to acoustic phonon scattering. The momentum relaxation is assumed to be caused by elastic scattering. The pertinent collision integrals are adapted for the case of the massless energy spectrum of carriers that interact with the longitudinal acoustic mode and the thermal radiation. It is found that the photoconductivity is determined by an interplay between weak energy relaxation and generation-recombination processes. Due to this the threshold of nonlinear response is fairly low.

cond-mat.mtrl-sci↗
arXiv2008

Influence of spin and interactions on quantum dots and nano-wires

In this dissertation we use sophisticated numerical methods in order to examine ground-state (GS) properties of two types of quantum systems with electron electron interactions: A quantum dot (QD) and a nano-wire. In the first half of the work we study a system of a single level coupled to a one-dimensional wire with interacting spinless electrons, when the wire is either clean or disordered. We utilize the density-matrix renormalization-group (DMRG) method to investigate the influence of the level on several thermodynamic properties of the clean interacting wire, which can be in one of two phases: Tomonaga-Luttinger liquid and charge density wave phases. When the wire is disordered, we investigate the Friedel oscillations, exploring the difference between the two phases and comparing them to the clean non-interacting case, for which we develop an exact formula for the oscillations. In the second half of the dissertation we study two cases of an isolated two-dimensional QD. We begin by an investigation of a new numerical method, the particle-hole DMRG (PH-DMRG), which is used to calculate the GS energy of a disordered QD consisting of interacting spinless electrons. We show that this method is much more accurate than the Hartree-Fock method, and we suggest an improvement of the algorithm, which reduces the error rate by almost 30 percents. Finally we study the magnetization of a QD with spin 1/2 electrons, in the presence of spin-orbit coupling and interactions. We calculate the g-factor and the expectation values of the spin operators in the GS, and find that when the QD is occupied by an even number of electrons, the GS can have a finite magnetization.

cond-mat.str-el↗
arXiv2008

What is Quantum in Quantum Pumping: The Role of Phase and Asymmetries

We show that quantum pumping does not always require a quantum description or a quantum phase. Quantum pumping is shown to encompass different types of processes, some of which intrinsically rely on phase while others do not. We also show that many pumping processes have a hidden antisymmetric component that contributes significantly to the instantaneous current at the terminals without causing net charge transfer in a period. We have also computed the exact pumped current for some cases over a full range of time variation from adiabatic to non-adiabatic.

cond-mat.mes-hall↗