Electron transport in Coulomb- and tunnel-coupled one-dimensional systems

We develop a linear theory of electron transport for a system of two identical quantum wires in a wide range of the wire length L, unifying both the ballistic and diffusive transport regimes. The microscopic model, involving the interaction of electrons with each other and with bulk acoustical phonons allows a reduction of the quantum kinetic equation to a set of coupled equations for the local chemical potentials for forward- and backward-moving electrons in the wires. As an application of the general solution of these equations, we consider different kinds of electrical contacts to the double-wire system and calculate the direct resistance, the transresistance, in the presence of tunneling and Coulomb drag, and the tunneling resistance. If L is smaller than the backscattering length l_P, both the tunneling and the drag lead to a negative transresistance, while in the diffusive regime (L >>l_P) the tunneling opposes the drag and leads to a positive transresistance. If L is smaller than the phase-breaking length, the tunneling leads to interference oscillations of the resistances that are damped exponentially with L.Comment: Text 14 pages in Latex/Revtex format, 4 Postscript figure

The unusual protoplanetary disk around the T Tauri star ET Cha

We present new continuum and line observations, along with modelling, of the faint (6-8) Myr old T Tauri star ET Cha belonging to the eta Chamaeleontis cluster. We have acquired HERSCHEL/PACS photometric fluxes at 70 mic and 160 mic, as well as a detection of the [OI] 63 mic fine-structure line in emission, and derived upper limits for some other far-IR OI, CII, CO and o-H2O lines. The HERSCHEL data is complemented by new ANDICAM B-K photometry, new HST/COS and HST/STIS UV-observations, a non-detection of CO J=3-2 with APEX, re-analysis of a UCLES high-resolution optical spectrum showing forbidden emission lines like [OI] 6300A, [SII] 6731A and 6716A, and [NII] 6583A, and a compilation of existing broad-band photometric data. We used the thermo-chemical disk code ProDiMo and the Monte-Carlo radiative transfer code MCFOST to model the protoplanetary disk around ET Cha. Based on these models we can determine the disk dust mass Mdust = (2.E-8 - 5.E-8) Msun, whereas the total disk gas mass is found to be only little constrained, Mgas = (5.E-5 - 3.E-3) Msun. In the models, the disk extends from 0.022 AU (just outside of the co-rotation radius) to only about 10 AU. Larger disks are found to be inconsistent with the CO J=3-2 non-detection. The low velocity component of the [OI] 6300A emission line is consistent with being emitted from the inner disk. The model can also reproduce the line flux of H2 v=1-0 S(1) at 2.122 mic. An additional high-velocity component of the [OI] 6300A emission line, however, points to the existence of an additional jet/outflow of low velocity (40 - 65) km/s with mass loss rate ~1.E-9 Msun/yr. In relation to our low estimations of the disk mass, such a mass loss rate suggests a disk lifetime of only ~(0.05 - 3) Myr, substantially shorter than the cluster age. The evolutionary state of this unusual protoplanetary disk is discussed.Comment: accepted by Astronomy & Astrophysics (18 pages, 11 figures and 7 tables). Additional 9-page appendix with 6 figures, 3 tables and 37 equation

Spin-triplet superconductivity in repulsive Hubbard models with disconnected Fermi surfaces: a case study on triangular and honeycomb lattices

We propose that spin-fluctuation-mediated spin-triplet superconductivity may be realized in repulsive Hubbard models with disconnected Fermi surfaces. The idea is confirmed for Hubbard models on triangular (dilute band filling) and honeycomb (near half-filling) lattices using fluctuation exchange approximation, where triplet pairing order parameter with f-wave symmetry is obtained. Possible relevance to real superconductors is suggested.Comment: 5 pages, 6 figures, RevTeX, uses epsf.sty and multicol.st

The Eccentricity-Mass Distribution of Exoplanets: Signatures of Different Formation Mechanisms?

We examine the distributions of eccentricity and host star metallicity of exoplanets as a function of their mass. Planets with M sin i >~ 4 M_J have an eccentricity distribution consistent with that of binary stars, while planets with M sin i <~ 4 M_J are less eccentric than binary stars and more massive planets. In addition, host star metallicities decrease with planet mass. The statistical significance of both of these trends is only marginal with the present sample of exoplanets. To account for these trends, we hypothesize that there are two populations of gaseous planets: the low-mass population forms by gas accretion onto a rock-ice core in a circumstellar disk and is more abundant at high metalliticities, and the high-mass population forms directly by fragmentation of a pre-stellar cloud. Planets of the first population form in initially circular orbits and grow their eccentricities later, and may have a mass upper limit from the total mass of the disk that can be accreted by the core. The second population may have a mass lower limit resulting from opacity-limited fragmentation. This would roughly divide the two populations in mass, although they would likely overlap over some mass range. If most objects in the second population form before the pre-stellar cloud becomes highly opaque, they would have to be initially located in orbits larger than ~30 AU, and would need to migrate to the much smaller orbits in which they are observed. The higher mean orbital eccentricity of the second population might be caused by the larger required intervals of radial migration, and the brown dwarf desert might be due to the inability of high-mass brown dwarfs to migrate inwards sufficiently in radius.Comment: 7 pages, 4 figures. Version with expanded discussion section. Accepted for publication in A&

Theory of the anomalous Hall effect from the Kubo formula and the Dirac equation

A model to treat the anomalous Hall effect is developed. Based on the Kubo formalism and on the Dirac equation, this model allows the simultaneous calculation of the skew-scattering and side-jump contributions to the anomalous Hall conductivity. The continuity and the consistency with the weak-relativistic limit described by the Pauli Hamiltonian is shown. For both approaches, Dirac and Pauli, the Feynman diagrams, which lead to the skew-scattering and the side-jump contributions, are underlined. In order to illustrate this method, we apply it to a particular case: a ferromagnetic bulk compound in the limit of weak-scattering and free-electrons approximation. Explicit expressions for the anomalous Hall conductivity for both skew-scattering and side-jump mechanisms are obtained. Within this model, the recently predicted ''spin Hall effect'' appears naturally

Silicon-based molecular electronics

Molecular electronics on silicon has distinct advantages over its metallic counterpart. We describe a theoretical formalism for transport through semiconductor-molecule heterostructures, combining a semi-empirical treatment of the bulk silicon bandstructure with a first-principles description of the molecular chemistry and its bonding with silicon. Using this method, we demonstrate that the presence of a semiconducting band-edge can lead to a novel molecular resonant tunneling diode (RTD) that shows negative differential resistance (NDR) when the molecular levels are driven by an STM potential into the semiconducting band-gap. The peaks appear for positive bias on a p-doped and negative for an n-doped substrate. Charging in these devices is compromised by the RTD action, allowing possible identification of several molecular highest occupied (HOMO) and lowest unoccupied (LUMO) levels. Recent experiments by Hersam et al. [1] support our theoretical predictions.Comment: Author list is reverse alphabetical. All authors contributed equally. Email: rakshit/liangg/ ghosha/[email protected]

Magnetoresistance of one-dimensional subbands in tunnel-coupled double quantum wires

We study the low-temperature in-plane magnetoresistance of tunnel-coupled quasi-one-dimensional quantum wires. The wires are defined by two pairs of mutually aligned split gates on opposite sides of a < 1 micron thick AlGaAs/GaAs double quantum well heterostructure, allowing independent control of their widths. In the ballistic regime, when both wires are defined and the field is perpendicular to the current, a large resistance peak at ~6 Tesla is observed with a strong gate voltage dependence. The data is consistent with a counting model whereby the number of subbands crossing the Fermi level changes with field due to the formation of an anticrossing in each pair of 1D subbands

Atomic Tunneling from a STM/AFM tip: Dissipative Quantum Effects from Phonons

We study the effects of phonons on the tunneling of an atom between two surfaces. In contrast to an atom tunneling in the bulk, the phonons couple very strongly, and qualitatively change the tunneling behavior. This is the first example of {\it ohmic} coupling from phonons for a two-state system. We propose an experiment in which an atom tunnels from the tip of an STM, and show how its behavior would be similar to the Macroscopic Quantum Coherence behavior predicted for SQUIDS. The ability to tune and calculate many parameters would lead to detailed tests of the standard theories. (For a general intro to this work on the on the World-Wide-Web: http://www.lassp.cornell.edu. Click on ``Entertaining Science Done Here'' and ``Quantum Tunneling of Atoms'')Comment: 12 pages, ReVTex3.0, two figures (postscript). This is a (substantially) revised version of cond-mat/9406043. More info (+ postscript text) at : http://www.lassp.cornell.edu/ardlouis/publications.htm