147 research outputs found
Double Quantum Dots in Carbon Nanotubes
We study the two-electron eigenspectrum of a carbon-nanotube double quantum
dot with spin-orbit coupling. Exact calculation are combined with a simple
model to provide an intuitive and accurate description of single-particle and
interaction effects. For symmetric dots and weak magnetic fields, the
two-electron ground state is antisymmetric in the spin-valley degree of freedom
and is not a pure spin-singlet state. When double occupation of one dot is
favored by increasing the detuning between the dots, the Coulomb interaction
causes strong correlation effects realized by higher orbital-level mixing.
Changes in the double-dot configuration affect the relative strength of the
electron-electron interactions and can lead to different ground state
transitions. In particular, they can favor a ferromagnetic ground state both in
spin and valley degrees of freedom. The strong suppression of the energy gap
can cause the disappearance of the Pauli blockade in transport experiments and
thereby can also limit the stability of spin-qubits in quantum information
proposals. Our analysis is generalized to an array of coupled dots which is
expected to exhibit rich many-body behavior.Comment: 14 pages, 11 pages and 1 table. Typos in text and Figs.4 and 6
correcte
Resonant Five-body Recombination in an Ultracold Gas of Bosonic Atoms
We combine theory and experiment to investigate five-body recombination in an
ultracold gas of atomic cesium at negative scattering length. A refined
theoretical model, in combination with extensive laboratory tunability of the
interatomic interactions, enables the five-body resonant recombination rate to
be calculated and measured. The position of the new observed recombination
feature agrees with a recent theoretical prediction and supports the prediction
of a family of universal cluster states at negative that are tied to an
Efimov trimer.Comment: 14 pages, 5 figure
Hyperspherical Description of the Degenerate Fermi Gas: S-wave Interactions
We present a unique theoretical description of the physics of the spherically
trapped -atom degenerate Fermi gas (DFG) at zero temperature based on an
ordinary Schr\"{o}dinger equation with a microscopic, two body interaction
potential. With a careful choice of coordinates and a variational wavefunction,
the many body Schr\"{o}dinger equation can be accurately described by a
\emph{linear}, one dimensional effective Schr\"{o}dinger equation in a single
collective coordinate, the rms radius of the gas. Comparisons of the energy,
rms radius and peak density of ground state energy are made to those predicted
by Hartree-Fock (HF). Also the lowest radial excitation frequency (the
breathing mode frequency) agrees with a sum rule calculation, but deviates from
a HF prediction
BEC-BCS Crossover of a Trapped Two-Component Fermi Gas with Unequal Masses
We determine the energetically lowest lying states in the BEC-BCS crossover
regime of s-wave interacting two-component Fermi gases under harmonic
confinement by solving the many-body Schrodinger equation using two distinct
approaches. Essentially exact basis set expansion techniques are applied to
determine the energy spectrum of systems with N=4 fermions. Fixed-node
diffusion Monte Carlo methods are applied to systems with up to N=20 fermions,
and a discussion of different guiding functions used in the Monte Carlo
approach to impose the proper symmetry of the fermionic system is presented.
The energies are calculated as a function of the s-wave scattering length a_s
for N=2-20 fermions and different mass ratios \kappa of the two species. On the
BEC and BCS sides, our energies agree with analytically-determined first-order
correction terms. We extract the scattering length and the effective range of
the dimer-dimer system up to \kappa = 20. Our energies for the
strongly-interacting trapped system in the unitarity regime show no shell
structure, and are well described by a simple expression, whose functional form
can be derived using the local density approximation, with one or two
parameters. The universal parameter \xi for the trapped system for various
\kappa is determined, and comparisons with results for the homogeneous system
are presented.Comment: 11 pages, 6 figures, extended versio
General Theoretical Description of \u3cem\u3eN\u3c/em\u3e-Body Recombination
Formulas for the cross section and event rate constant describing recombination of N particles are derived in terms of general S-matrix elements. Our result immediately yields the generalized Wigner threshold scaling for the recombination of N bosons. A semianalytical formula encapsulates the overall scaling with energy and scattering length, as well as resonant modifications by the presence of N-body states near the threshold collision energy in the entrance channel. We then apply our model to the case of four-boson recombination into an Efimov trimer and a free atom
First order phase transitions in optical lattices with tunable three-body onsite interaction
We study the two-dimensional Bose-Hubbard model in the presence of a
three-body interaction term, both at a mean field level and via quantum Monte
Carlo simulations. The three-body term is tuned by coupling the triply occupied
states to a trapped universal trimer. We find that, for sufficiently attractive
three-body interaction the n = 2 Mott lobe disappears and the system displays
first order phase transitions separating the n = 1 from the n = 3 lobes, and
the n = 1 and n = 3 Mott insulator from the superfluid. We have also analyzed
the effect of finite temperature and found that transitions are still of first
order at temperatures T\simJ where J is the hopping matrix element.Comment: introduction slightly changed, modified figure
Universality in Four-Boson Systems
We report recent advances on the study of universal weakly bound four-boson
states from the solutions of the Faddeev-Yakubovsky equations with zero-range
two-body interactions. In particular, we present the correlation between the
energies of successive tetramers between two neighbor Efimov trimers and
compare it to recent finite range potential model calculations. We provide
further results on the large momentum structure of the tetramer wave function,
where the four-body scale, introduced in the regularization procedure of the
bound state equations in momentum space, is clearly manifested. The results we
are presenting confirm a previous conjecture on a four-body scaling behavior,
which is independent of the three-body one. We show that the correlation
between the positions of two successive resonant four-boson recombination peaks
are consistent with recent data, as well as with recent calculations close to
the unitary limit. Systematic deviations suggest the relevance of range
corrections.Comment: Accepted for publication in special issue of Few-Body Systems devoted
to the Sixth Workshop on the Critical Stability of Quantum Few-Body Systems,
October 2011, Erice, Sicily, Ital
- …