62 research outputs found
Gate-modulated thermopower in disordered nanowires: I. Low temperature coherent regime
Using a one-dimensional tight-binding Anderson model, we study a disordered
nanowire in the presence of an external gate which can be used for depleting
its carrier density (field effect transistor device configuration). In this
first paper, we consider the low temperature coherent regime where the electron
transmission through the nanowire remains elastic. In the limit where the
nanowire length exceeds the electron localization length, we derive three
analytical expressions for the typical value of the thermopower as a function
of the gate potential, in the cases where the electron transport takes place
(i) inside the impurity band of the nanowire, (ii) around its band edges and
eventually (iii) outside its band. We obtain a very large enhancement of the
typical thermopower at the band edges, while the sample to sample fluctuations
around the typical value exhibit a sharp crossover from a Lorentzian
distribution inside the impurity band towards a Gaussian distribution as the
band edges are approached.Comment: 13 pages, 8 figures, final version as publishe
Scanning Gate Microscopy of Quantum Contacts Under Parallel Magnetic Field: Beating Patterns Between Spin-Split Transmission Peaks or Channel Openings
We study the conductance of an electron interferometer created in a two
dimensional electron gas between a nanostructured contact and the depletion
region induced by the charged tip of a scanning gate microscope. Using
non-interacting models, we study the beating pattern of interference fringes
exhibited by the images giving as a function of the tip position when a
parallel magnetic field is applied. The analytical solution of a simplified
model allows us to distinguish between two cases: (i) If the field is applied
everywhere, the beating of Fabry-P\'erot oscillations of opposite spins gives
rise to interference rings which can be observed at low temperatures when the
contact is open between spin-split transmission resonances. (ii) If the field
acts only upon the contact, the interference rings cannot be observed at low
temperatures, but only at temperatures of the order of the Zeeman energy. For a
contact made of two sites in series, a model often used for describing an
inversion-symmetric double-dot setup, a pseudo-spin degeneracy is broken by the
inter-dot coupling and a similar beating effect can be observed without
magnetic field at temperatures of the order of the interdot coupling.
Eventually, numerical studies of a quantum point contact with quantized
conductance plateaus confirm that a parallel magnetic field applied everywhere
or only upon the contact gives rises to similar beating effects between
spin-split channel openings.Comment: 11 pages, 17 figure
Absorbing/Emitting Phonons with one dimensional MOSFETs
We consider nanowires in the field effect transistor device configuration.
Modeling each nanowire as a one dimensional lattice with random site
potentials, we study the heat exchanges between the nanowire electrons and the
substrate phonons, when electron transport is due to phonon-assisted hops
between localized states. Shifting the nanowire conduction band with a metallic
gate induces different behaviors. When the Fermi potential is located near the
band center, a bias voltage gives rise to small local heat exchanges which
fluctuate randomly along the nanowire. When it is located near one of the band
edges, the bias voltage yields heat currents which flow mainly from the
substrate towards the nanowire near one boundary of the nanowire, and in the
opposite direction near the other boundary. This opens interesting perspectives
for heat management at submicron scales: Arrays of parallel gated nanowires
could be used for a field control of phonon emission/absorption.Comment: 9 pages, 11 figure
A numerical finite size scaling approach to many-body localization
We develop a numerical technique to study Anderson localization in
interacting electronic systems. The ground state of the disordered system is
calculated with quantum Monte-Carlo simulations while the localization
properties are extracted from the ``Thouless conductance'' , i.e. the
curvature of the energy with respect to an Aharonov-Bohm flux. We apply our
method to polarized electrons in a two dimensional system of size . We
recover the well known universal one
parameter scaling function without interaction. Upon switching on the
interaction, we find that is unchanged while the system flows toward
the insulating limit. We conclude that polarized electrons in two dimensions
stay in an insulating state in the presence of weak to moderate
electron-electron correlations.Comment: 5 pages, 4 figure
Thermoelectric study of the time-dependent Resonant Level Model
We study the non-interacting time-dependent resonant level model mimicking a
driven quantum dot connected through leads to two electronic reservoirs held at
different temperatures and electrochemical potentials. Using a scattering
approach, we provide analytical formulas of the time-dependent particle
currents, heat currents, and input driving power under the wide-band limit
approximation. We also derive Landauer formulas for the corresponding
time-integrated quantities when the perturbation applied on the dot is of
finite duration. Then, we focus on the case of a single square pulse, benchmark
our analytical results against numerical ones that are valid beyond the
wide-band limit, and perform numerical simulations for a smooth square pulse
and a periodic square pulse train. Finally, we discuss whether the efficiency
of the device in a stationary Seebeck configuration can be enhanced by driving
the dot potential. We find numerically that the transient increase of the
efficiency observed in some cases is eventually cancelled out at long times.Comment: 11 pages, 6 figures, 1 appendix; final version as publishe
Scanning Gate Microscopy of Kondo Dots: Fabry-Pérot Interferences and Thermally Induced Rings
5 pages, 4 figuresWe study the conductance of an electron interferometer formed in a two dimensional electron gas between a nanostructured quantum contact and the charged tip of a scanning gate microscope. Measuring the conductance as a function of the tip position, thermally induced rings may be observed in addition to Fabry-Pérot interference fringes spaced by half the Fermi wavelength. If the contact is made of a quantum dot opened in the middle of a Kondo valley, we show how the location of the rings allows to measure by electron interferometry the magnetic moment of the dot above the Kondo temperature
The energy scale behind the metallic behaviors in low-density Si-MOSFETs
We show that the unexpected metallic behavior (the so-called two-dimensional
metal-insulator transition) observed in low-density Silicon
metal-oxide-semiconductor field-effect transistors (Si-MOSFETs) is controlled
by a unique characteristic energy scale, the polarization energy. On one hand,
we perform Quantum Monte Carlo calculations of the energy needed to polarize
the two dimensional electron gas at zero temperature, taking into account
Coulomb interactions, valley degeneracy and electronic mobility (disorder). On
the other hand, we identify the characteristic energy scale controlling the
physics in eight different sets of experiments. We find that our {\it
ab-initio} polarization energies (obtained without any adjustable parameters)
are in perfect agreement with the observed characteristic energies for all
available data, both for the magnetic field and temperature dependence of the
resistivities. Our results put strong constraints on possible mechanisms
responsible for the metallic behavior. In particular, there are strong
indications that the system would eventually become insulating at low enough
temperature.Comment: two references added, corrected typos, minor changes, final version
as publishe
Using Activated Transport in Parallel Nanowires for Energy Harvesting and Hot Spot Cooling
12 pages, 8 figures, 4 appendicesInternational audienceWe study arrays of parallel doped semiconductor nanowires in a temperature range where the electrons propagate through the nanowires by phonon assisted hops between localized states. By solving the Random Resistor Network problem, we compute the thermopower , the electrical conductance , and the electronic thermal conductance of the device. We investigate how those quantities depend on the position -- which can be tuned with a back gate -- of the nanowire impurity band with respect to the equilibrium electrochemical potential. We show that large power factors can be reached near the band edges, when self-averages to large values while is small but scales with the number of wires. Calculating the amount of heat exchanged locally between the electrons inside the nanowires and the phonons of the environment, we show that phonons are mainly absorbed near one electrode and emitted near the other when a charge current is driven through the nanowires near their band edges. This phenomenon could be exploited for a field control of the heat exchange between the phonons and the electrons at submicron scales in electronic circuits. It could be also used for cooling hot spots
Gate-modulated thermopower of disordered nanowires: II. Variable-range hopping regime
International audienceWe study the thermopower of a disordered nanowire in the field effect transistorconfiguration. After a first paper devoted to the elastic coherent regime (Bosisio R., Fleury G.and Pichard J.-L. 2014 New J. Phys. 16 035004), we consider here the inelastic activated regimetaking place at higher temperatures. In the case where charge transport is thermally assisted byphonons (Mott Variable Range Hopping regime), we use the Miller-Abrahams random resistornetwork model as recently adapted by Jiang et al. for thermoelectric transport. This approachpreviously used to study the bulk of the nanowire impurity band is extended for studying itsedges. In this limit, we show that the typical thermopower is largely enhanced, attaining valueslarger that 10 kB/e ∼ 1 mV K−1 and exhibiting a non-trivial behaviour as a function of thetemperature. A percolation theory by Zvyagin extended to disordered nanowires allows us toaccount for the main observed edge behaviours of the thermopower
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