256 research outputs found
Correlation Effects in Quantum Dot Wave Function Imaging
We demonstrate that in semiconductor quantum dots wave functions probed by
imaging techniques based on local tunneling spectroscopies like STM show
characteristic signatures of electron-electron Coulomb correlation. We predict
that such images correspond to ``quasi-particle'' wave functions which cannot
be computed by standard mean-field techniques (density functional theory,
Hartree-Fock) in the strongly correlated regime corresponding to low electron
density. From the configuration-interaction solution of the few-particle
problem for prototype dots, we find that quasi-particle wave function images
may display signatures of Wigner crystallization.Comment: Latex 2e + jjap2 style version 1.0. 4 pages, 3 postscript figures.
Submitted to the Japanese Journal of Applied Physics as Proceeding of STM05
Conference, Sapporo, Japan, July 3-8, 200
Financial Structure and Corporate Growth: Evidence from Italian Panel Data
We study the relationships between firm financial structure and growth for a large sample of Italian firms (1998-2003). We expand upon existing analyses testing whether liquidity constraints affect firm performance by considering among growth determinants also firm debt structure. Panel regression analyses show that more liquid firms tend to grow more. However, firms do not use their capital to expand, but rather to increase debt. We also find that firm growth is highly fragile as it is positively correlated with non-financial liabilities and it is not sustained by a long-term debt maturity. Finally, quantile regressions suggest that fast-growing firms are characterized by higher growth/cash-flow sensitivities and heavily rely on external debt, but seem to be less bank-backed than the rest of the sample. Overall, our findings suggest that the link between firms’ investment and expansion decisions is far more complicated than postulated by standard tests of investment/cash-flow sensitivities.Firm growth; Financial structure; Cash flow; Financial constraints; Gibrat law; Quantile regressions
Addition energies in semiconductor quantum dots: Role of electron-electron interaction
We show that the addition spectra of semiconductor quantum dots in the
presence of magnetic field can be studied through a theoretical scheme that
allows an accurate and practical treatment of the single particle states and
electron-electron interaction up to large numbers of electrons. The calculated
addition spectra exhibit the typical structures of Hund-like shell filling, and
account for recent experimental findings. A full three dimensional description
of Coulomb interaction is found to be essential for predicting the conductance
characteristics of few-electron semiconductor structures.Comment: LaTeX 2.09, RevTeX, 3 pages, 3 Postscript figure
Field-controlled suppression of phonon-induced transitions in coupled quantum dots
We calculate the longitudinal-acoustic phonon scattering rate for a vertical
double quantum dot system with weak lateral confinement and show that a strong
modulation of the single-electron excited states lifetime can be induced by an
external magnetic or electric field. The results are obtained for typical
realistic devices using a Fermi golden rule approach and a three-dimensional
description of the electronic quantum states.Comment: REVTex4 class, 6 pages, 3 figures, to be published in Applied Physics
Letter
Reduced electron relaxation rate in multi-electron quantum dots
We use a configuration-interaction approach and Fermi golden rule to
investigate electron-phonon interaction in realistic multi-electron quantum
dots. Lifetimes are computed in the low-density, highly correlated regime. We
report numerical evidence that electron-electron interaction generally leads to
reduced decay rates of excited electronic states in weakly confined quantum
dots, where carrier relaxation is dominated by the interaction with
longitudinal acoustic phonons.Comment: to appear in Phys. Rev. Let
Triplet-Singlet Spin Relaxation in Quantum Dots with Spin-Orbit Coupling
We estimate the triplet-singlet relaxation rate due to spin-orbit coupling
assisted by phonon emission in weakly-confined quantum dots. Our results for
two and four electrons show that the different triplet-singlet relaxation
trends observed in recent experiments under magnetic fields can be understood
within a unified theoretical description, as the result of the competition
between spin-orbit coupling and phonon emission efficiency. Moreover, we show
that both effects are greatly affected by the strength of the confinement and
the external magnetic field, which may give access to very long-lived triplet
states as well as to selective population of the triplet Zeeman sublevels.Comment: 5 pages, 3 figures. Closely related to recent experiments in
cond-mat/060972
: A Command-line Catalogue Cross-matching tool for modern astrophysical survey data
In the current data-driven science era, it is needed that data analysis
techniques has to quickly evolve to face with data whose dimensions has
increased up to the Petabyte scale. In particular, being modern astrophysics
based on multi-wavelength data organized into large catalogues, it is crucial
that the astronomical catalog cross-matching methods, strongly dependant from
the catalogues size, must ensure efficiency, reliability and scalability.
Furthermore, multi-band data are archived and reduced in different ways, so
that the resulting catalogues may differ each other in formats, resolution,
data structure, etc, thus requiring the highest generality of cross-matching
features. We present (Command-line Catalogue Cross-match), a
multi-platform application designed to efficiently cross-match massive
catalogues from modern surveys. Conceived as a stand-alone command-line process
or a module within generic data reduction/analysis pipeline, it provides the
maximum flexibility, in terms of portability, configuration, coordinates and
cross-matching types, ensuring high performance capabilities by using a
multi-core parallel processing paradigm and a sky partitioning algorithm.Comment: 6 pages, 4 figures, proceedings of the IAU-325 symposium on
Astroinformatics, Cambridge University pres
Effect of the Coulomb interaction on the electron relaxation of weakly-confined quantum dot systems
We study acoustic-phonon-induced relaxation of charge excitations in single
and tunnel-coupled quantum dots containing few confined interacting electrons.
The Full Configuration Interaction approach is used to account for the
electron-electron repulsion. Electron-phonon interaction is accounted for
through both deformation potential and piezoelectric field mechanisms. We show
that electronic correlations generally reduce intradot and interdot transition
rates with respect to corresponding single-electron transitions, but this
effect is lessened by external magnetic fields. On the other hand,
piezoelectric field scattering is found to become the dominant relaxation
mechanism as the number of confined electrons increases. Previous proposals to
strongly suppress electron-phonon coupling in properly designed single-electron
quantum dots are shown to hold also in multi-electron devices. Our results
indicate that few-electron orbital degrees of freedom are more stable than
single-electron ones.Comment: 20 pages (preprint format), 7 figures, submitted to Phys. Rev.
Quantum phases of correlated electrons in artificial molecules under magnetic fields
We investigate the stability of few-electron quantum phases in vertically coupled quantum dots under a magnetic field of arbitrary strength and direction. The orbital and spin stability diagrams of realistic devices containing up to five electrons, from strong to weak interdot coupling, is determined. Correlation effects and realistic sample geometries are fully taken into account within the full configuration interaction method. In general, the magnetic field drives the system into a strongly correlated regime by modulating the single-particle gaps. In coupled quantum dots different components of the field, either parallel or perpendicular to the tunneling direction, affect single-dot orbitals and tunneling energy, respectively. Therefore the stability of the quantum phases is related to different correlation mechanisms, depending on the field direction. Comparison of exact diagonalization results with simple models allows one to identify the specific role of correlations
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