43 research outputs found
Interference effects in the Coulomb blockade regime: current blocking and spin preparation in symmetric nanojunctions
We consider nanojunctions in the single-electron tunnelling regime which, due
to a high degree of spatial symmetry, have a degenerate many body spectrum. As
a consequence, interference phenomena which cause a current blocking can occur
at specific values of the bias and gate voltage. We present here a general
formalism to give necessary and sufficient conditions for interference blockade
also in the presence of spin polarized leads. As an example we analyze a triple
quantum dot single electron transistor (SET). For a set-up with parallel
polarized leads, we show how to selectively prepare the system in each of the
three states of an excited spin triplet without application of any external
magnetic field.Comment: 10 pages, 9 figures. Corrected typos and updated reference
Effects of spin-orbit coupling and many-body correlations in STM transport through copper phthalocyanine
The interplay of exchange correlations and spin-orbit interaction (SOI) on
the many-body spectrum of a copper phtalocyanine (CuPc) molecule and their
signatures in transport are investigated. We first derive a minimal model
Hamiltonian in a basis of frontier orbitals which is able to reproduce
experimentally observed singlet-triplet splittings; in a second step SOI
effects are included perturbatively. Major consequences of the SOI are the
splitting of former degenerate levels and a magnetic anisotropy, which can be
captured by an effective low-energy spin Hamiltonian. We show that STM-based
magnetoconductance measurements can yield clear signatures of both these SOI
induced effects.Comment: 12 pages, 6 figure
Non-equilibrium spin-crossover in copper phthalocyanine
We demonstrate the tip induced control of the spin state of copper
phthalocyanine (CuPc) on an insulator coated substrate. Accounting for
electronic correlations, we find that, under the condition of energetic
proximity of neutral excited states to the anionic groundstate, the system can
undergo a population inversion towards these excited states. The resulting
state of the system is accompanied by a change in the total spin quantum
number. Experimental signatures of the crossover are the appearance of
additional nodal planes in the topographical STM images as well as a strong
suppression of the current near the center of the molecule. The robustness of
the effect against moderate charge conserving relaxation processes has also
been tested.Comment: 5 pages, 4 figures; added supplemental material (+ 5 pages
Dynamical symmetry breaking in transport through molecules
We analyze the interplay between vibrational and electronic degrees of
freedom in charge transport across a molecular single-electron transistor. We
focus on the wide class of molecules which possess quasi-degenerate vibrational
eigenstates, while no degeneracy occurs for their anionic configuration. We
show that the combined effect of a thermal environment and coupling to leads,
involving tunneling events charging and discharging the molecule, leads to a
dynamical symmetry breaking where quasi-degenerate eigenstates acquire
different occupations. This imbalance gives rise to a characteristic asymmetry
of the current versus an applied gate voltage.Comment: 4 pages, 2 figures, revised final published versio
Quantum Shuttle in Phase Space
We present a quantum theory of the shuttle instability in electronic
transport through a nanostructure with a mechanical degree of freedom. A phase
space formulation in terms of the Wigner function allows us to identify a
cross-over from the tunnelling to the shuttling regime, thus extending the
previously found classical results to the quantum domain. Further, a new
dynamical regime is discovered, where the shuttling is driven exclusively by
the quantum noise.Comment: 4 pages, 2 figures; minor changes; final version published in Phys.
Rev. Let
Current and current fluctuations in quantum shuttles
We review the properties of electron shuttles, i.e. nanoelectromechanical
devices that transport electrons one-by-one by utilizing a combination of
electronic and mechanical degrees of freedom. We focus on the extreme quantum
limit, where the mechanical motion is quantized. We introduce the main
theoretical tools needed for the analysis, e.g. generalized master equations
and Wigner functions, and we outline the methods how the resulting large
numerical problems can be handled. Illustrative results are given for current,
noise, and full counting statistics for a number of model systems. Throughout
the review we focus on the physics behind the various approximations, and some
simple examples are given to illustrate the theoretical concepts. We also
comment on the experimental situation.Comment: Minireview; technical level aimed at general audience, based on an
invited talk at "Transport Phenomena in Micro and Nanodevices", October 17-21
Kona, Hawai
Quantum theory of shuttling instability in a movable quantum dot array
We study the shuttling instability in an array of three quantum dots the
central one of which is movable. We extend the results by Armour and MacKinnon
on this problem to a broader parameter regime. The results obtained by an
efficient numerical method are interpreted directly using the Wigner
distributions. We emphasize that the instability should be viewed as a
crossover phenomenon rather than a clear-cut transition.Comment: 4 pages, 2 figures, presented at HCIS-13, Modena, July 200
Theory of STM junctions for \pi-conjugated molecules on thin insulating films
A microscopic theory of the transport in a scanning tunnelling microscope
(STM) set-up is introduced for \pi-conjugated molecules on insulating films,
based on the density matrix formalism. A key role is played in the theory by
the energy dependent tunnelling rates which account for the coupling of the
molecule to the tip and to the substrate. In particular, we analyze how the
geometrical differences between the localized tip and extended substrate are
encoded in the tunnelling rate and influence the transport characteristics.
Finally, using benzene as an example of a planar, rotationally symmetric
molecule, we calculate the STM current voltage characteristics and current maps
and analyze them in terms of few relevant angular momentum channels.Comment: 19 pages, 12 figures, minor changes to conform to published versio