524 research outputs found
Probing electron-electron interaction in quantum Hall systems with scanning tunneling spectroscopy
Using low-temperature scanning tunneling spectroscopy applied to the
Cs-induced two-dimensional electron system (2DES) on p-type InSb(110), we probe
electron-electron interaction effects in the quantum Hall regime. The 2DES is
decoupled from p-doped bulk states and exhibits spreading resistance within the
insulating quantum Hall phases. In quantitative agreement with calculations we
find an exchange enhancement of the spin splitting. Moreover, we observe that
both the spatially averaged as well as the local density of states feature a
characteristic Coulomb gap at the Fermi level. These results show that
electron-electron interaction effects can be probed down to a resolution below
all relevant length scales.Comment: supplementary movie in ancillary file
Density matrix renormalization group study of the interacting Kitaev chain with quasi-periodic disorder
We document the ground state phase diagram of the one-dimensional Kitaev
chain with quasi-periodic disorder in the presence of two-body interactions.
Our data was obtained for systems of sites using large-scale density
matrix renormalization group numerics and is benchmarked against known results
for the clean system. We demonstrate that moderate quasi-periodic disorder
stabilizes the topological phase both for repulsive and attractive
interactions. For larger disorder strengths, the system features re-entrance
behavior and multiple phase transitions
Time-evolving a matrix product state with long-ranged interactions
We introduce a numerical algorithm to simulate the time evolution of a matrix
product state under a long-ranged Hamiltonian. In the effectively
one-dimensional representation of a system by matrix product states,
long-ranged interactions are necessary to simulate not just many physical
interactions but also higher-dimensional problems with short-ranged
interactions. Since our method overcomes the restriction to short-ranged
Hamiltonians of most existing methods, it proves particularly useful for
studying the dynamics of both power-law interacting one-dimensional systems,
such as Coulombic and dipolar systems, and quasi two-dimensional systems, such
as strips or cylinders. First, we benchmark the method by verifying a
long-standing theoretical prediction for the dynamical correlation functions of
the Haldane-Shastry model. Second, we simulate the time evolution of an
expanding cloud of particles in the two-dimensional Bose-Hubbard model, a
subject of several recent experiments.Comment: 5 pages + 3 pages appendices, 4 figure
Investigation of the coupling asymmetries at double-slit interference experiments
Double-slit experiments inferring the phase and the amplitude of the
transmission coefficient performed at quantum dots (QD), in the Coulomb
blockade regime, present anomalies at the phase changes depending on the number
of electrons confined. This phase change cannot be explained if one neglects
the electron-electron interactions. Here, we present our numerical results,
which simulate the real sample geometry by solving the Poisson equation in 3D.
The screened potential profile is used to obtain energy eigenstates and
eigenvalues of the QD. We find that, certain energy levels are coupled to the
leads stronger compared to others. Our results give strong support to the
phenomenological models in the literature describing the charging of a QD and
the abrupt phase changes.Comment: conference paper, 50th anniversary of Aharonov-Bohm effec
The role of mesoscale hydrography on microbial dynamics in the northeast Atlantic: Results of a spring bloom experiment
During RV Meteor cruise No. 10 from May to June 1989 (JGOFS pilot study) bacterial and picocyanobacterial abundance, biomass, and bacterial production were estimated at two drift stations close to 47N, 20W and 58N, 20W in the northeast Atlantic. At 47N two different mesoscale hydrographic structures were sampled which divided the drift experiment into a cyclonic and an anticyclonic circulation phase. Transition from one phase to the next was clearly reflected by changes of the biological structure in the upper water column. Phytoplankton stocks maintained during the cyclonic phase were about 1.8 times higher than those of the anticyclonic phase (1552 mg C m−2 and 880 mg C m−2, resp., integrated over the mixed layer, Deckers, 1991). Integrated stocks of bacteria showed an opposite pattern of distribution. Picocyanobacterial biomass (PCB) was 3.4 times higher during the anticyclonic phase than during the cyclonic phase (96 mg C m−2 and 28 mg C m−2, resp.), and the respective factor for total bacterial biomass (TBB) was 3.7 (830 mg C m−2 and 225 mg C m−2, resp.). Our analysis indicates that the combined bacterial biomass dominated within the mixed layer during the anticyclonic phase, while the cyclonic phase was clearly dominated by eucaryotic phytoplankton. Additional evidence for a shift of biology toward the microbial food web was indicated by a strong increase of bacteria during the anticyclonic phase. Thus, simultaneously and side by side, an autotrophic and a heterotrophic system were supported by the prevailing hydrographic conditions. At 58N within an anticyclonic mesoscale hydrographic structure the phytoplankton bloom was at a developing stage, characterized by low biomass (730 mg C m−2 in the mixed layer, Deckers, 1991) but relatively high primary production. In contrast, bacterial stocks were quite high, but bacterial production was low in comparison to the anticyclonic phase at 47N (90 mg C m−2 d−1 and 153 mg C m−2 d−1, resp., integrated from 0–300 m). It was calculated that bacterial gross production averaged 42% (47N, anticyclonic phase) and 25% (58N) of primary production. These results suggest that within a specific type of hydrographic structure either a heterotrophic or an autotrophic system can be established, depending on the stage of bloom development. In conclusion: Depending on their origin and age, mesoscale hydrographic structures can be correlated with different stages of biological development. This leads to the mesoscale patchiness of biological measurements, which is a characteristic feature of the northeast Atlantic
Many-body localization and the area law in two dimensions
We study the high-energy phase diagram of a two-dimensional
spin- Heisenberg model on a square lattice in the presence of
disorder. The use of large-scale tensor network numerics allows us to compute
the bi-partite entanglement entropy for systems of up to lattice
sites. We demonstrate the existence of a finite many-body localized phase for
large disorder strength for which the eigenstate thermalization hypothesis
is violated. Moreover, we show explicitly that the area law holds for excited
states in this phase and determine an estimate for the critical
where the transition to the ergodic phase occurs.Comment: 5 pages, 5 figure
A renormalization group approach to time dependent transport through correlated quantum dots
We introduce a real time version of the functional renormalization group
which allows to study correlation effects on nonequilibrium transport through
quantum dots. Our method is equally capable to address (i) the relaxation out
of a nonequilibrium initial state into a (potentially) steady state driven by a
bias voltage and (ii) the dynamics governed by an explicitly time-dependent
Hamiltonian. All time regimes from transient to asymptotic can be tackled; the
only approximation is the consistent truncation of the flow equations at a
given order. As an application we investigate the relaxation dynamics of the
interacting resonant level model which describes a fermionic quantum dot
dominated by charge fluctuations. Moreover, we study decoherence and relaxation
phenomena within the ohmic spin-boson model by mapping the latter to the
interacting resonant level model
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Bilingualism Is Associated with a Delayed Onset of Dementia but Not with a Lower Risk of Developing it: a Systematic Review with Meta-Analyses.
Some studies have linked bilingualism with a later onset of dementia, Alzheimer's disease (AD), and mild cognitive impairment (MCI). Not all studies have observed such relationships, however. Differences in study outcomes may be due to methodological limitations and the presence of confounding factors within studies such as immigration status and level of education. We conducted the first systematic review with meta-analysis combining cross-sectional studies to explore if bilingualism might delay symptom onset and diagnosis of dementia, AD, and MCI. Primary outcomes included the age of symptom onset, the age at diagnosis of MCI or dementia, and the risk of developing MCI or dementia. A secondary outcome included the degree of disease severity at dementia diagnosis. There was no difference in the age of MCI diagnosis between monolinguals and bilinguals [mean difference: 3.2; 95% confidence intervals (CI): -3.4, 9.7]. Bilinguals vs. monolinguals reported experiencing AD symptoms 4.7 years (95% CI: 3.3, 6.1) later. Bilinguals vs. monolinguals were diagnosed with dementia 3.3 years (95% CI: 1.7, 4.9) later. Here, 95% prediction intervals showed a large dispersion of effect sizes (-1.9 to 8.5). We investigated this dispersion with a subgroup meta-analysis comparing studies that had recruited participants with dementia to studies that had recruited participants with AD on the age of dementia and AD diagnosis between mono- and bilinguals. Results showed that bilinguals vs. monolinguals were 1.9 years (95% CI: -0.9, 4.7) and 4.2 (95% CI: 2.0, 6.4) older than monolinguals at the time of dementia and AD diagnosis, respectively. The mean difference between the two subgroups was not significant. There was no significant risk reduction (odds ratio: 0.89; 95% CI: 0.68-1.16) in developing dementia among bilinguals vs. monolinguals. Also, there was no significant difference (Hedges' g = 0.05; 95% CI: -0.13, 0.24) in disease severity at dementia diagnosis between bilinguals and monolinguals, despite bilinguals being significantly older. The majority of studies had adjusted for level of education suggesting that education might not have played a role in the observed delay in dementia among bilinguals vs. monolinguals. Although findings indicated that bilingualism was on average related to a delayed onset of dementia, the magnitude of this relationship varied across different settings. This variation may be due to unexplained heterogeneity and different sources of bias in the included studies. Registration: PROSPERO CRD42015019100
Transmission phase lapses in quantum dots: the role of dot-lead coupling asymmetry
Lapses of transmission phase in transport through quantum dots are ubiquitous
already in the absence of interaction, in which case their precise location is
determined by the signs and magnitudes of the tunnelling matrix elements.
However, actual measurements for a quantum dot embedded in an Aharonov-Bohm
interferometer show systematic sequences of phase lapses separated by Coulomb
peaks -- an issue that attracted much attention and generated controversy.
Using a two-level quantum dot as an example we show that this phenomenon can be
accounted for by the combined effect of asymmetric dot-lead couplings (left
lead/right lead asymmetry as well as different level broadening for different
levels) and interaction-induced "population switching" of the levels, rendering
this behaviour generic. We construct and analyse a mean field scheme for an
interacting quantum dot, and investigate the properties of the mean field
solution, paying special attention to the character of its dependence
(continuous vs. discontinuous) on the chemical potential or gate voltage.Comment: 34 LaTeX pages in IOP format, 9 figures; misprints correcte
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