Conductance and polarization in quantum junctions

We revisit the expression for the conductance of a general nanostructure -- such as a quantum point contact -- as obtained from the linear response theory. We show that the conductance represents the strength of the Drude singularity in the conductivity $\sigma(k,k';i\omega \to 0)$. Using the equation of continuity for electric charge we obtain a formula for conductance in terms of polarization of the system. This identification can be used for direct calculation of the conductance for systems of interest even at the {\it ab-initio} level. In particular, we show that one can evaluate the conductance from calculations for a finite system without the need for special ``transport'' boundary conditions

Transport in Molecular Junctions with Different Metallic Contacts

Ab initio calculations of phenyl dithiol connected to Au, Ag, Pd, and Pt electrodes are performed using non-equilibrium Green's functions and density functional theory. For each metal, the properties of the molecular junction are considered both in equilibrium and under bias. In particular, we consider in detail charge transfer, changes in the electrostatic potential, and their subsequent effects on the IV curves through the junctions. Gold is typically used in molecular junctions because it forms strong chemical bonds with sulfur. We find however that Pt and Pd make better electrical contacts than Au. The zero-bias conductance is found to be greatest for Pt, followed by Pd, Au, and then Ag

Spectrum of π\pi Electrons in Graphene as an Alternant Macromolecule and Its Specific Features in Quantum Conductance

An exact description of $\pi$ electrons based on the tight-binding model of graphene as an alternant, plane macromolecule is presented. The model molecule can contain an arbitrary number of benzene rings and has armchair- and zigzag-shaped edges. This suggests an instructive alternative to the most commonly used approach, where the reference is made to the honeycomb lattice periodic in its A and B sublattices. Several advantages of the macromolecule model are demonstrated. The newly derived analytical relations detail our understanding of $\pi$ electron nature in achiral graphene ribbons and carbon tubes and classify these structures as quantum wires.Comment: 13 pages 8 figures, revised in line with referee's comment

Memory erasure in small systems

We consider an overdamped nanoparticle in a driven double-well potential as a generic model of an erasable one-bit memory. We study in detail the statistics of the heat dissipated during an erasure process and show that full erasure may be achieved by dissipating less heat than the Landauer bound. We quantify the occurrence of such events and propose a single-particle experiment to verify our predictions. Our results show that Landauer's principle has to be generalized at the nanoscale to accommodate heat fluctuations.Comment: 4 pages, 4 figure

Dynamic generation of orbital quasiparticle entanglement in mesoscopic conductors

We propose a scheme for dynamically creating orbitally entangled electron-hole pairs through a time-dependent variation of the electrical potential in a mesoscopic conductor. The time-dependent potential generates a superposition of electron-hole pairs in two different orbital regions of the conductor, a Mach-Zehnder interferometer in the quantum Hall regime. The orbital entanglement is detected via violation of a Bell inequality, formulated in terms of zero-frequency current noise. Adiabatic cycling of the potential, both in the weak and strong amplitude limit, is considered.Comment: 4 pages, 2 figures; references update

Large magnetoresistance in bcc Co/MgO/Co and FeCo/MgO/FeCo tunneling junctions

By use of first-principles electronic structure calculations, we predict that the magnetoresistance of the bcc Co(100)/MgO(100)/bcc Co(100) and FeCo(100)/MgO(100)/FeCo(100) tunneling junctions can be several times larger than the very large magnetoresistance predicted for the Fe(100)/MgO(100)/Fe(100) system. The origin of this large magnetoresistance can be understood using simple physical arguments by considering the electrons at the Fermi energy travelling perpendicular to the interfaces. For the minority spins there is no state with $\Delta_1$ symmetry whereas for the majority spins there is only a $\Delta_1$ state. The $\Delta_1$ state decays much more slowly than the other states within the MgO barrier. In the absence of scattering which breaks the conservation of momentum parallel to the interfaces, the electrons travelling perpendicular to the interfaces undergo total reflection if the moments of the electrodes are anti-parallel. These arguments apply equally well to systems with other well ordered tunnel barriers and for which the most slowly decaying complex energy band in the barrier has $\Delta_1$ symmetry. Examples include systems with (100) layers constructed from Fe, bcc Co, or bcc FeCo electrodes and Ge, GaAs, or ZnSe barriers.Comment: 8 figure files in eps forma

Experimental Verification of the Quantized Conductance of Photonic Crystal Waveguides

We report experiments that demonstrate the quantization of the conductance of photonic crystal waveguides. To obtain a diffusive wave, we have added all the transmitted channels for all the incident angles. The conductance steps have equal height and a width of one half the wavelength used. Detailed numerical results agree very well with the novel experimental results.Comment: Phys. Rev. B (submitted

Exact Master Equation and Quantum Decoherence of Two Coupled Harmonic Oscillators in a General Environment

In this paper we derive an exact master equation for two coupled quantum harmonic oscillators interacting via bilinear coupling with a common environment at arbitrary temperature made up of many harmonic oscillators with a general spectral density function. We first show a simple derivation based on the observation that the two-harmonic oscillator model can be effectively mapped into that of a single harmonic oscillator in a general environment plus a free harmonic oscillator. Since the exact one harmonic oscillator master equation is available [Hu, Paz and Zhang, Phys. Rev. D \textbf{45}, 2843 (1992)], the exact master equation with all its coefficients for this two harmonic oscillator model can be easily deduced from the known results of the single harmonic oscillator case. In the second part we give an influence functional treatment of this model and provide explicit expressions for the evolutionary operator of the reduced density matrix which are useful for the study of decoherence and disentanglement issues. We show three applications of this master equation: on the decoherence and disentanglement of two harmonic oscillators due to their interaction with a common environment under Markovian approximation, and a derivation of the uncertainty principle at finite temperature for a composite object, modeled by two interacting harmonic oscillators. The exact master equation for two, and its generalization to $N$, harmonic oscillators interacting with a general environment are expected to be useful for the analysis of quantum coherence, entanglement, fluctuations and dissipation of mesoscopic objects towards the construction of a theoretical framework for macroscopic quantum phenomena.Comment: 35 pages, revtex, no figures, 2nd version, references added, to appear in PR

Intensity distribution of scalar waves propagating in random media

Transmission of the scalar field through the random medium, represented by the system of randomly distributed dielectric cylinders is calculated numerically. System is mapped to the problem of electronic transport in disordered two-dimensional systems. Universality of the statistical distribution of transmission parameters is analyzed in the metallic and in the localized regimes.In the metallic regime the universality of the transmission statistics in all transparent channels is observed. In the band gaps, we distinguish the disorder induced (Anderson) localization from the tunneling through the system due to the gap in the density of states. We show also that absorption causes rapid decrease of the mean conductance, but, contrary to the localized regime, the conductance is self-averaged with a Gaussian distribution

Landauer Conductance without Two Chemical Potentials

We present a theory of the four--terminal conductance for the multi-channel tunneling barrier, which is based on the self-consistent solution of Shrodinger, Poisson and continuity equations. We derive new results for the case of a barrier embedded in a long wire with and without disorder. We also recover known expressions for the conductance of the barrier placed into a ballistic constriction. Our approach avoids a problematic use of two chemical potentials in the same system.Comment: 12 page