40 research outputs found
Inter-species Tunneling in One-dimensional Bose Mixtures
We study the ground-state properties and quantum dynamics of few-boson
mixtures with strong inter-species repulsion in one-dimensional traps. If one
species localizes at the center, e.g., due to a very large mass compared to the
other component, it represents an effective barrier for the latter and the
system can be mapped onto identical bosons in a double well. For weaker
localization, the barrier atoms begin to respond to the light component,
leading to an induced attraction between the mobile atoms that may even
outweigh their bare intra-species repulsion. To explain the resulting effects,
we derive an effective Hubbard model for the lighter species accounting for the
backaction of the barrier in correction terms to the lattice parameters. Also
the tunneling is drastically affected: Varying the degree of localization of
the "barrier" atoms, the dynamics of intrinsically noninteracting bosons can
change from Rabi oscillations to effective pair tunneling. For identical
fermions (or fermionized bosons) this leads to the tunneling of attractively
bound pairs.Comment: 13 pages, 11 figures; v2 reflects major revisio
Beyond two-stage models for lung carcinogenesis in the Mayak workers: Implications for Plutonium risk
Mechanistic multi-stage models are used to analyze lung-cancer mortality
after Plutonium exposure in the Mayak-workers cohort, with follow-up until
2008. Besides the established two-stage model with clonal expansion, models
with three mutation stages as well as a model with two distinct pathways to
cancer are studied. The results suggest that three-stage models offer an
improved description of the data. The best-fitting models point to a mechanism
where radiation increases the rate of clonal expansion. This is interpreted in
terms of changes in cell-cycle control mediated by bystander signaling or
repopulation following cell killing. No statistical evidence for a two-pathway
model is found. To elucidate the implications of the different models for
radiation risk, several exposure scenarios are studied. Models with a radiation
effect at an early stage show a delayed response and a pronounced drop-off with
older ages at exposure. Moreover, the dose-response relationship is strongly
nonlinear for all three-stage models, revealing a marked increase above a
critical dose
Material-barrier Tunneling in One-dimensional Few-boson Mixtures
We study the quantum dynamics of strongly interacting few-boson mixtures in
one-dimensional traps. If one species is strongly localized compared to the
other (e.g., much heavier), it can serve as an effective potential barrier for
that mobile component. Near the limit of infinite localization, we map this to
a system of identical bosons in a double well. For realistic localization, the
backaction of the light species on the "barrier" atoms is explained--to lowest
order--in terms of an induced attraction between these. Even in equilibrium,
this may outweigh the bare intra-species interaction, leading to unexpected
correlated states. Remarkably, the backaction drastically affects the
inter-species dynamics, such as the tunneling of an attractively bound pair of
fermionized atoms.Comment: 10 pages, 3 figure
Few-boson tunneling in a double well with spatially modulated interaction
We study few-boson tunneling in a one-dimensional double well with a
spatially modulated interaction. The dynamics changes from Rabi oscillations in
the non-interacting case to a highly suppressed tunneling for intermediate
coupling strengths followed by a revival near the fermionization limit. With
extreme interaction inhomogeneity in the regime of strong correlations we
observe tunneling between the higher bands. The dynamics is explained on the
basis of the few-body spectrum and stationary eigenstates. For higher number of
particles, N > 2, it is shown that the inhomogeneity of the interaction can be
tuned to generate tunneling resonances. Finally, a tilted double-well and its
interplay with the interaction asymmetry is discussed.Comment: 10 Pages. Published with minor change
Ground states of dipolar gases in quasi-1D ring traps
We compute the ground state of dipoles in a quasi-one-dimensional ring trap
using few-body techniques combined with analytic arguments. The effective
interaction between two dipoles depends on their center-of-mass coordinate and
can be tuned by varying the angle between dipoles and the plane of the ring.
For weak enough interactions, the state resembles a weakly interacting Fermi
gas or an (inhomogeneous) Lieb-Liniger gas. A mapping between the Lieb-Liniger
and the dipolar-gas parameters in and beyond the Born approximation is
established, and we discuss the effect of inhomogeneities based on a
local-density approximation. For strongly repulsive interactions, the system
exhibits crystal-like localization of the particles. Their inhomogeneous
distribution may be understood in terms of a simple few-body model as well as a
local-density approximation. In the case of partially attractive interactions,
clustered states form for strong enough coupling, and the dependence of the
state on particle number and orientation angle of the dipoles is discussed
analytically.Comment: 15 pages, 10 figure
One-dimensional Few-boson Systems in Single- and Double-well Traps
This thesis studies the one-dimensional Bose gas in harmonic and double-well traps from a few-body perspective. The main emphasis is on the crossover from weak interactions to the fermionization limit of infinite repulsion, where the system maps to an ideal Fermi gas. To explore the structure as well as the quantum dynamics throughout that crossover, we both develop an exact-diagonalization approach and resort to a multi-configurational time-dependent method (MCTDH). The basic mechanism of the fermionization crossover for the ground state is shown to consist in the formation of a correlation hole in the two-body density, which culminates in a localization of the individual particles for strong repulsion. This is accompanied by a reduction of coherence. We demonstrate how the concrete pathway depends on the trap geometry, on the shape of the interaction, as well as on the atom number. By extension, we also investigate the lowest excitations, whose understanding is a base for studying the impact of the fermionization crossover on the tunneling dynamics in a double well. In symmetric wells, a pathway from single-particle to fragmented-pair tunneling shows up. By energetically offsetting the two wells, tunnel resonances become accessible, which may be used to extract single atoms
Polarons and Molecules in a Two-Dimensional Fermi Gas
We study an impurity atom in a two-dimensional Fermi gas using variational
wave functions for (i) an impurity dressed by particle-hole excitations
(polaron) and (ii) a dimer consisting of the impurity and a majority atom. In
contrast to three dimensions, where similar calculations predict a sharp
transition to a dimer state with increasing interspecies attraction, we show
that the polaron ansatz always gives a lower energy. However, the exact
solution for a heavy impurity reveals that both a two-body bound state and
distortions of the Fermi sea are crucial. This reflects the importance of
particle-hole pairs in lower dimensions and makes simple variational
calculations unreliable. We show that the energy of an impurity gives important
information about its dressing cloud, for which both ans\"atze give inaccurate
results.Comment: 5 pages, 2 figures, minor change
Tunneling dynamics of few bosons in a double well
We study few-boson tunneling in a one-dimensional double well. As we pass
from weak interactions to the fermionization limit, the Rabi oscillations first
give way to highly delayed pair tunneling (for medium coupling), whereas for
very strong correlations multi-band Rabi oscillations emerge. All this is
explained on the basis of the exact few-body spectrum and without recourse to
the conventional two-mode approximation. Two-body correlations are found
essential to the understanding of the different tunnel mechanisms. The
investigation is complemented by discussing the effect of skewing the double
well, which offers the possibility to access specific tunnel resonancesComment: 10 pages, 8 figure
Ultracold Few-Boson Systems in a Double-Well Trap
We investigate the transition of a quasi-one-dimensional few-boson system
from a weakly correlated to a fragmented and finally a fermionized ground
state. Our numerically exact analysis, based on a multi-configurational method,
explores the interplay between different shapes of external and inter-particle
forces. Specifically, we demonstrate that the addition of a central barrier to
an otherwise harmonic trap may supports the system's fragmentation, with a
symmetry-induced distinction between even and odd atom numbers. Moreover, the
impact of inhomogeneous interactions is studied, where the effective coupling
strength is spatially modulated. It is laid out how the ground state can be
displaced in a controlled way depending on the trap and the degree of
modulation. We present the one- and two-body densities and, beyond that,
highlight the role of correlations on the basis of the natural occupations
Correlations in Ultracold Trapped Few-Boson Systems: Transition from Condensation to Fermionization
We study the correlation properties of the ground states of few ultracold
bosons, trapped in double wells of varying barrier height in one dimension.
Extending previous results on the signature of the transition from a
Bose-condensed state via fragmentation to the hard-core limit, we provide a
deeper understanding of that transition by relating it to the loss of coherence
in the one-body density matrix and to the emerging long-range tail in the
momentum spectrum. These are accounted for in detail by discussing the natural
orbitals and their occupations. Our discussion is complemented by an analysis
of the two-body correlation function.Comment: 22 pages, 7 figure