111 research outputs found
Modeling Green's function measurements with two-tip scanning tunneling microscopy
A double-tip scanning tunneling microscope with nanometer-scale tip separation has the ability to access the single-electron Green's function in real and momentum spaces based on second-order tunneling processes. Experimental realization of such measurements has been limited to quasi-one-dimensional systems due to the extremely small signal size. Here we propose an alternative approach to obtain such information by exploiting the current-current correlations from the individual tips and present a theoretical formalism to describe it. To assess the feasibility of our approach we make a numerical estimate for an ∼25-nm Pb nanoisland and show that the wave function in fact extends from tip to tip and the signal depends less strongly on increased tip separation in the diffusive regime than the one in alternative approaches relying on tip-to-tip conductance.Quantum Matter and Optic
Shot noise in resonant tunneling through a zero-dimensional state with a complex energy spectrum
We investigate the noise properties of a GaAs/AlGaAs resonant tunneling
structure at bias voltages where the current characteristic is determined by
single electron tunneling. We discuss the suppression of the shot noise in the
framework of a coupled two-state system. For large bias voltages we observed
super-Poissonian shot noise up to values of the Fano factor .Comment: 4 pages, 4 figures, accepted for Phys. Rev.
Statistics of Coulomb Blockade Peak Spacings within the Hartree-Fock Approximation
We study the effect of electronic interactions on the addition spectra and on
the energy level distributions of two-dimensional quantum dots with weak
disorder using the self-consistent Hartree-Fock approximation for spinless
electrons. We show that the distribution of the conductance peak spacings is
Gaussian with large fluctuations that exceed, in agreement with experiments,
the mean level spacing of the non-interacting system. We analyze this
distribution on the basis of Koopmans' theorem. We show furthermore that the
occupied and unoccupied Hartree-Fock levels exhibit Wigner-Dyson statistics.Comment: 5 pages, 2 figures, submitted for publicatio
Persistent currents in diffusive metallic cavities: Large values and anomalous scaling with disorder
The effect of disorder on confined metallic cavities with an Aharonov-Bohm
flux line is addressed. We find that, even deep in the diffusive regime, large
values of persistent currents may arise for a wide variety of geometries. We
present numerical results supporting an anomalous scaling law of the average
typical current with the strength of disorder , with . This is contrasted with previously
reported results obtained for cylindrical samples where a scaling has been found. Possible links to, up to date, unexplained
experimental data are finally discussed.Comment: 5 pages, 4 figure
Fractal Noise in Quantum Ballistic and Diffusive Lattice Systems
We demonstrate fractal noise in the quantum evolution of wave packets moving
either ballistically or diffusively in periodic and quasiperiodic tight-binding
lattices, respectively. For the ballistic case with various initial
superpositions we obtain a space-time self-affine fractal which
verify the predictions by Berry for "a particle in a box", in addition to
quantum revivals. For the diffusive case self-similar fractal evolution is also
obtained. These universal fractal features of quantum theory might be useful in
the field of quantum information, for creating efficient quantum algorithms,
and can possibly be detectable in scattering from nanostructures.Comment: 9 pages, 8 postscript figure
Ultrafast control of magnetic interactions via light-driven phonons
Resonant ultrafast excitation of infrared-active phonons is a powerful technique with which to control the electronic properties of materials that leads to remarkable phenomena such as the light-induced enhancement of superconductivity1,2, switching of ferroelectric polarization3,4 and ultrafast insulator-to-metal transitions5. Here, we show that light-driven phonons can be utilized to coherently manipulate macroscopic magnetic states. Intense mid-infrared electric field pulses tuned to resonance with a phonon mode of the archetypical antiferromagnet DyFeO3 induce ultrafast and long-living changes of the fundamental exchange interaction between rare-earth orbitals and transition metal spins. Non-thermal lattice control of the magnetic exchange, which defines the stability of the macroscopic magnetic state, allows us to perform picosecond coherent switching between competing antiferromagnetic and weakly ferromagnetic spin orders. Our discovery emphasizes the potential of resonant phonon excitation for the manipulation of ferroic order on ultrafast timescales6
Shot noise of coupled semiconductor quantum dots
The low-frequency shot noise properties of two electrostatically coupled
semiconductor quantum dot states which are connected to emitter/collector
contacts are studied. A master equation approach is used to analyze the bias
voltage dependence of the Fano factor as a measure of temporal correlations in
tunneling current caused by Pauli's exclusion principle and the Coulomb
interaction. In particular, the influence of the Coulomb interaction on the
shot noise behavior is discussed in detail and predictions for future
experiments will be given. Furthermore, we propose a mechanism for negative
differential conductance and investigate the related super-Poissonian shot
noise.Comment: submitted to PR
Entanglement in Mesoscopic Structures: Role of Projection
We present a theoretical analysis of the appearance of entanglement in
non-interacting mesoscopic structures. Our setup involves two oppositely
polarized sources injecting electrons of opposite spin into the two incoming
leads. The mixing of these polarized streams in an ideal four-channel beam
splitter produces two outgoing streams with particular tunable correlations. A
Bell inequality test involving cross-correlated spin-currents in opposite leads
signals the presence of spin-entanglement between particles propagating in
different leads. We identify the role of fermionic statistics and projective
measurement in the generation of these spin-entangled electrons.Comment: 5 pages, 1 figur
Density functional theory of spin-polarized disordered quantum dots
Using density functional theory, we investigate fluctuations of the ground
state energy of spin-polarized, disordered quantum dots in the metallic regime.
To compare to experiment, we evaluate the distribution of addition energies and
find a convolution of the Wigner-Dyson distribution, expected for noniteracting
electrons, with a narrower Gaussian distribution due to interactions. The tird
moment of the total distribution is independent of interactions, and so is
predicted to decrease by a factor of 0.405 upon application of a magnetic field
which transforms from the Gaussian orthogonal to the Gaussian unitary ensemble.Comment: 13 pages, 2 figure
Conductance Peak Height Correlations for a Coulomb-Blockaded Quantum Dot in a Weak Magnetic Field
We consider statistical correlations between the heights of conductance peaks
corresponding to two different levels in a Coulomb-blockaded quantum dot.
Correlations exist for two peaks at the same magnetic field if the field does
not fully break time-reversal symmetry as well as for peaks at different values
of a magnetic field that fully breaks time-reversal symmetry. Our results are
also relevant to Coulomb-blockade conductance peak height statistics in the
presence of weak spin-orbit coupling in a chaotic quantum dot.Comment: 5 pages, 3 figures, REVTeX 4, accepted for publication in Phys. Rev.
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