19 research outputs found
Superposition of photon- and phonon- assisted tunneling in coupled quantum dots
We report on electron transport through an artificial molecule formed by two
tunnel coupled quantum dots, which are laterally confined in a two-dimensional
electron system of an AlGaAs/GaAs heterostructure. Coherent
molecular states in the coupled dots are probed by photon-assisted tunneling
(PAT). Above 10 GHz, we observe clear PAT as a result of the resonance between
the microwave photons and the molecular states. Below 8 GHz, a pronounced
superposition of phonon- and photon-assisted tunneling is observed. Coherent
superposition of molecular states persists under excitation of acoustic
phonons.Comment: 5 pages, 4 figure
Addition Spectra of Quantum Dots in Strong Magnetic Fields
We consider the magnetic field dependence of the chemical potential for
parabolically confined quantum dots in a strong magnetic field. Approximate
expressions based on the notion that the size of a dot is determined by a
competition between confinement and interaction energies are shown to be
consistent with exact diagonalization studies for small quantum dots. Fine
structure is present in the magnetic field dependence which cannot be explained
without a full many-body description and is associated with ground-state level
crossings as a function of confinement strength or Zeeman interaction strength.
Some of this fine structure is associated with precursors of the bulk
incompressible states responsible for the fractional quantum Hall effect.Comment: 11 pages, 3 figures (available from [email protected]). Revtex
3.0. (IUCM93-010
Scaling Of The Coulomb Energy Due To Quantum Fluctuations In The Charge Of A Quantum Dot
The charging energy of a quantum dot is measured through the effect of its
potential on the conductance of a second dot. This technique allows a
measurement of the scaling of the dot's charging energy with the conductance of
the tunnel barriers leading to the dot. We find that the charging energy scales
quadratically with the reflection probability of the barriers. In a second
experiment we study the transition from a single to a double-dot which exhibits
a scaling behavior linear in the reflection probability. The observed
power-laws agree with a recent theory.Comment: 5 pages, uuencoded and compressed postscript file, with figure
Quantum dots in magnetic fields: thermal response of broken symmetry phases
We investigate the thermal properties of circular semiconductor quantum dots
in high magnetic fields using finite temperature Hartree-Fock techniques. We
demonstrate that for a given magnetic field strength quantum dots undergo
various shape phase transitions as a function of temperature, and we outline
possible observable consequences.Comment: In Press, Phys. Rev. B (2001
Non-invasive detection of the evolution of the charge states of a double dot system
Coupled quantum dots are potential candidates for qubit systems in quantum
computing. We use a non-invasive voltage probe to study the evolution of a
coupled dot system from a situation where the dots are coupled to the leads to
a situation where they are isolated from the leads. Our measurements allow us
to identify the movement of electrons between the dots and we can also identify
the presence of a charge trap in our system by detecting the movement of
electrons between the dots and the charge trap. The data also reveals evidence
of electrons moving between the dots via excited states of either the single
dots or the double dot molecule.Comment: Accepted for publication in Phys. Rev. B. 4 pages, 4 figure
Coulomb Blockade of Tunneling Through a Double Quantum Dot
We study the Coulomb blockade of tunneling through a double quantum dot. The
temperature dependence of the linear conductance is strongly affected by the
inter-dot tunneling. As the tunneling grows, a crossover from
temperature-independent peak conductance to a power-law suppression of
conductance at low temperatures is predicted. This suppression is a
manifestation of the Anderson orthogonality catastrophe associated with the
charge re-distribution between the dots, which accompanies the tunneling of an
electron into a dot. We find analytically the shapes of the Coulomb blockade
peaks in conductance as a function of gate voltage.Comment: 11 pages, revtex3.0 and multicols.sty, 4 figures uuencode
Fano Resonances in Electronic Transport through a Single Electron Transistor
We have observed asymmetric Fano resonances in the conductance of a single
electron transistor resulting from interference between a resonant and a
nonresonant path through the system. The resonant component shows all the
features typical of quantum dots, but the origin of the non-resonant path is
unclear. A unique feature of this experimental system, compared to others that
show Fano line shapes, is that changing the voltages on various gates allows
one to alter the interference between the two paths.Comment: 8 pages, 6 figures. Submitted to PR
Universal Correlations of Coulomb Blockade Conductance Peaks and the Rotation Scaling in Quantum Dots
We show that the parametric correlations of the conductance peak amplitudes
of a chaotic or weakly disordered quantum dot in the Coulomb blockade regime
become universal upon an appropriate scaling of the parameter. We compute the
universal forms of this correlator for both cases of conserved and broken time
reversal symmetry. For a symmetric dot the correlator is independent of the
details in each lead such as the number of channels and their correlation. We
derive a new scaling, which we call the rotation scaling, that can be computed
directly from the dot's eigenfunction rotation rate or alternatively from the
conductance peak heights, and therefore does not require knowledge of the
spectrum of the dot. The relation of the rotation scaling to the level velocity
scaling is discussed. The exact analytic form of the conductance peak
correlator is derived at short distances. We also calculate the universal
distributions of the average level width velocity for various values of the
scaled parameter. The universality is illustrated in an Anderson model of a
disordered dot.Comment: 35 pages, RevTex, 6 Postscript figure
Resonant tunneling through ultrasmall quantum dots: zero-bias anomalies, magnetic field dependence, and boson-assisted transport
We study resonant tunneling through a single-level quantum dot in the
presence of strong Coulomb repulsion beyond the perturbative regime. The level
is either spin-degenerate or can be split by a magnetic field. We, furthermore,
discuss the influence of a bosonic environment. Using a real-time diagrammatic
formulation we calculate transition rates, the spectral density and the
nonlinear characteristic. The spectral density shows a multiplet of Kondo
peaks split by the transport voltage and the boson frequencies, and shifted by
the magnetic field. This leads to zero-bias anomalies in the differential
conductance, which agree well with recent experimental results for the electron
transport through single-charge traps. Furthermore, we predict that the sign of
the zero-bias anomaly depends on the level position relative to the Fermi level
of the leads.Comment: 27 pages, latex, 21 figures, submitted to Phys. Rev.
Non Linear Current Response of a Many-Level Tunneling System: Higher Harmonics Generation
The fully nonlinear response of a many-level tunneling system to a strong
alternating field of high frequency is studied in terms of the
Schwinger-Keldysh nonequilibrium Green functions. The nonlinear time dependent
tunneling current is calculated exactly and its resonance structure is
elucidated. In particular, it is shown that under certain reasonable conditions
on the physical parameters, the Fourier component is sharply peaked at
, where is the spacing between
two levels. This frequency multiplication results from the highly nonlinear
process of photon absorption (or emission) by the tunneling system. It is
also conjectured that this effect (which so far is studied mainly in the
context of nonlinear optics) might be experimentally feasible.Comment: 28 pages, LaTex, 7 figures are available upon request from
[email protected], submitted to Phys.Rev.