220 research outputs found
Transport in single-molecule transistors: Kondo physics and negative differential resistance
We report two examples of transport phenomena based on sharp features in the
effective density of states of molecular-scale transistors: Kondo physics in
C-based devices, and gate-modulated negative differential resistance
(NDR) in ``control'' devices that we ascribe to adsorbed contamination. We
discuss the need for a statistical approach to device characterization, and the
criteria that must be satisfied to infer that transport is based on single
molecules. We describe apparent Kondo physics in C-based single-molecule
transistors (SMTs), including signatures of molecular vibrations in the Kondo
regime. Finally, we report gate-modulated NDR in devices made without
intentional molecular components, and discuss possible origins of this
property.Comment: 15 pages, 8 figures. To appear in Oct. 2004 issue of Nanotechnology,
proceedings of International Conference on Nanoscale Devices and Systems
Integratio
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
Electron-Transport Properties of Na Nanowires under Applied Bias Voltages
We present first-principles calculations on electron transport through Na
nanowires at finite bias voltages. The nanowire exhibits a nonlinear
current-voltage characteristic and negative differential conductance. The
latter is explained by the drastic suppression of the transmission peaks which
is attributed to the electron transportability of the negatively biased plinth
attached to the end of the nanowire. In addition, the finding that a voltage
drop preferentially occurs on the negatively biased side of the nanowire is
discussed in relation to the electronic structure and conduction.Comment: 4 pages, 6 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
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&
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
Commensurate-incommensurate transitions of quantum Hall stripe states in double-quantum-well systems
In higher Landau levels (N>0) and around filling factors nu =4N+1, a
two-dimensional electron gas in a double-quantum-well system supports a stripe
groundstate in which the electron density in each well is spatially modulated.
When a parallel magnetic field is added in the plane of the wells, tunneling
between the wells acts as a spatially rotating effective Zeeman field coupled
to the ``pseudospins'' describing the well index of the electron states. For
small parallel fields, these pseudospins follow this rotation, but at larger
fields they do not, and a commensurate-incommensurate transition results.
Working in the Hartree-Fock approximation, we show that the combination of
stripes and commensuration in this system leads to a very rich phase diagram.
The parallel magnetic field is responsible for oscillations in the tunneling
matrix element that induce a complex sequence of transitions between
commensurate and incommensurate liquid or stripe states. The homogeneous and
stripe states we find can be distinguished by their collective excitations and
tunneling I-V, which we compute within the time-dependent Hartree-Fock
approximation.Comment: 23 pages including 8 eps figure
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]
Strongly Correlated Electrons on a Silicon Surface: Theory of a Mott Insulator
We demonstrate theoretically that the electronic ground state of the
potassium-covered Si(111)-B surface is a Mott insulator, explicitly
contradicting band theory but in good agreement with recent experiments. We
determine the physical structure by standard density-functional methods, and
obtain the electronic ground state by exact diagonalization of a many-body
Hamiltonian. The many-body conductivity reveals a Brinkman-Rice metal-insulator
transition at a critical interaction strength; the calculated interaction
strength is well above this critical value.Comment: 4 pages; 4 figures included in text; Revte
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