1,149 research outputs found
Non-equilibrium transport through a vertical quantum dot in the absence of spin-flip energy relaxation
We investigate non-equilibrium transport in the absence of spin-flip energy
relaxation in a few-electron quantum dot artificial atom. Novel non-equilibrium
tunneling processes involving high-spin states which cannot be excited from the
ground state because of spin-blockade, and other processes involving more than
two charge states are observed. These processes cannot be explained by orthodox
Coulomb blockade theory. The absence of effective spin relaxation induces
considerable fluctuation of the spin, charge, and total energy of the quantum
dot. Although these features are revealed clearly by pulse excitation
measurements, they are also observed in conventional dc current characteristics
of quantum dots.Comment: accepted for publication in Phys. Rev.Let
Transient current spectroscopy of a quantum dot in the Coulomb blockade regime
Transient current spectroscopy is proposed and demonstrated in order to
investigate the energy relaxation inside a quantum dot in the Coulomb blockade
regime. We employ a fast pulse signal to excite an AlGaAs/GaAs quantum dot to
an excited state, and analyze the non-equilibrium transient current as a
function of the pulse length. The amplitude and time-constant of the transient
current are sensitive to the ground and excited spin states. We find that the
spin relaxation time is longer than, at least, a few microsecond.Comment: 5 pages, 3 figure
Co-tunneling current through the two-level quantum dot coupled to magnetic leads: A role of exchange interaction
The co-tunneling current through a two-level doubly occupied quantum dot
weakly coupled to ferromagnetic leads is calculated in the Coulomb blockade
regime. It is shown that the dependence of the differrential conductance on
applied voltage has a stair-case structure with different sets of "stairs" for
parallel and anti-parallel configurations of magnetization of the leads.
Contributions to the current from elastic and inelastic processes are
considered distinctly. It is observed that the interference part of the
co-tunneling current involves terms corresponding to inelastic processes.
Dependence of the co-tunneling current on the phases of the tunneling
amplitudes is studied.Comment: LaTex, 14 page
Charge occupancy of two interacting electrons on artificial molecules - exact results
We present exact solutions for two interacting electrons on an artificial
atom and on an artificial molecule made by one and two (single level) quantum
dots connected by ideal leads. Specifically, we calculate the accumulated
charge on the dots as function of the gate voltage, for various strengths of
the electron-electron interaction and of the hybridization between the dots and
the (one-dimensional) leads. With increasing of the (negative) gate voltage,
the accumulated charge in the two-electron ground state increases in gradual
steps from 0 to 1 and then to 2. The value 0 represents an "insulating" state,
where both electrons are bound to shallow states on the impurities. The value
of 1 corresponds to a "metal", with one electron localized on the dots and the
other extended on the leads. The value of 2 corresponds to another "insulator",
with both electrons strongly localized. The width of the "metallic" regime
diverges with strength of the electron-electron interaction for the single dot,
but remains very narrow for the double dot. These results are contrasted with
the simple Coulomb blockade picture.Comment: 12 pages, 7 figure
Understanding pressure-induced phase-transformation behavior in silicon through in situ electrical probing under cyclic loading conditions
Cyclic indentation of crystalline silicon exhibits interesting pressure-induced phase-transformation behavior whereby sequential changes in the phase composition ultimately lead to a catastrophic (“pop-out”) event during subsequent cycles and complete transformation to high pressure Si-III and Si-XII phases. This study combines in situ electrical measurements with cyclic loading to monitor such phase-transformation behavior. We find that, if a pop-out is not observed on the unloading curve, the end phase is predominantly amorphous but a small and increasing volume of Si-III/Si-XII results with each cycle. At a critical Si-III/Si-XII volume, pop-out can occur on a subsequent cycle, whereafter Si-III/Si-XII dominates the indent volume.The authors would like to acknowledge WRiota Pty Ltd
and the Australian Research Council for funding this project
Geometrical effects and signal delay in time-dependent transport at the nanoscale
The nonstationary and steady-state transport through a mesoscopic sample
connected to particle reservoirs via time-dependent barriers is investigated
within the reduced density operator method. The generalized Master equation is
solved via the Crank-Nicolson algorithm by taking into account the memory
kernel which embodies the non-Markovian effects that are commonly disregarded.
We propose a physically reasonable model for the lead-sample coupling which
takes into account the match between the energy of the incident electrons and
the levels of the isolated sample, as well as their overlap at the contacts.
Using a tight-binding description of the system we investigate the effects
induced in the transient current by the spectral structure of the sample and by
the localization properties of its eigenfunctions. In strong magnetic fields
the transient currents propagate along edge states. The behavior of populations
and coherences is discussed, as well as their connection to the tunneling
processes that are relevant for transport.Comment: 26 pages, 13 figures. To appear in New Journal of Physic
Resonant inelastic x-ray scattering in one-dimensional copper oxides
The Cu K-edge resonant inelastic x-ray scattering (RIXS) spectrum in
one-dimensional insulating cuprates is theoretically examined by using the
exact diagonalization technique for the extended one-dimensional
Hubbard model with nearest neighbor Coulomb interaction. We find the
following characteristic features that can be detectable by RIXS experiments:
(i) The spectrum with large momentum transfer indicates the formation of
excitons, i.e., bound states of holon and doublon. (ii) The spectrum with small
momentum transfer depends on the incident photon energy. We propose that the
RIXS provides a unique opportunity to study the upper
Hubbard band in one-dimensional cuprates.Comment: 3 pages with 4 figures, minor changes, to appear in Phys.Rev.
Electron transport through double quantum dots
Electron transport experiments on two lateral quantum dots coupled in series
are reviewed. An introduction to the charge stability diagram is given in terms
of the electrochemical potentials of both dots. Resonant tunneling experiments
show that the double dot geometry allows for an accurate determination of the
intrinsic lifetime of discrete energy states in quantum dots. The evolution of
discrete energy levels in magnetic field is studied. The resolution allows to
resolve avoided crossings in the spectrum of a quantum dot. With microwave
spectroscopy it is possible to probe the transition from ionic bonding (for
weak inter-dot tunnel coupling) to covalent bonding (for strong inter-dot
tunnel coupling) in a double dot artificial molecule. This review on the
present experimental status of double quantum dot studies is motivated by their
relevance for realizing solid state quantum bits.Comment: 32 pages, 31 figure
Tunable few-electron double quantum dots and Klein tunnelling in ultra-clean carbon nanotubes
Quantum dots defined in carbon nanotubes are a platform for both basic
scientific studies and research into new device applications. In particular,
they have unique properties that make them attractive for studying the coherent
properties of single electron spins. To perform such experiments it is
necessary to confine a single electron in a quantum dot with highly tunable
barriers, but disorder has until now prevented tunable nanotube-based
quantum-dot devices from reaching the single-electron regime. Here, we use
local gate voltages applied to an ultra-clean suspended nanotube to confine a
single electron in both a single quantum dot and, for the first time, in a
tunable double quantum dot. This tunability is limited by a novel type of
tunnelling that is analogous to that in the Klein paradox of relativistic
quantum mechanics.Comment: 21 pages including supplementary informatio
Conditional statistics of electron transport in interacting nanoscale conductors
Interactions between nanoscale semiconductor structures form the basis for
charge detectors in the solid state. Recent experimental advances have
demonstrated the on-chip detection of single electron transport through a
quantum dot (QD). The discreteness of charge in units of e leads to intrinsic
fluctuations in the electrical current, known as shot noise. To measure these
single-electron fluctuations a nearby coherent conductor, called a quantum
point contact (QPC), interacts with the QD and acts as a detector. An important
property of the QPC charge detector is noninvasiveness: the system physically
affects the detector, not visa-versa. Here we predict that even for ideal
noninvasive detectors such as the QPC, when a particular detector result is
observed, the system suffers an informational backaction, radically altering
the statistics of transport through the QD as compared to the unconditional
shot noise. We develop a theoretical model to make predictions about the joint
current probability distributions and conditional transport statistics. The
experimental findings reported here demonstrate the reality of informational
backaction in nanoscale systems as well as a variety of new effects, such as
conditional noise enhancement, which are in essentially perfect agreement with
our model calculations. This type of switching telegraph process occurs
abundantly in nature, indicating that these results are applicable to a wide
variety of systems.Comment: 16 pages, 3 figures, to appear in Nature Physic
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