243 research outputs found
Mott insulators in strong electric fields
Recent experiments on ultracold atomic gases in an optical lattice potential
have produced a Mott insulating state of Rb atoms. This state is stable to a
small applied potential gradient (an `electric' field), but a resonant response
was observed when the potential energy drop per lattice spacing (E), was close
to the repulsive interaction energy (U) between two atoms in the same lattice
potential well. We identify all states which are resonantly coupled to the Mott
insulator for E close to U via an infinitesimal tunneling amplitude between
neighboring potential wells. The strong correlation between these states is
described by an effective Hamiltonian for the resonant subspace. This
Hamiltonian exhibits quantum phase transitions associated with an Ising density
wave order, and with the appearance of superfluidity in the directions
transverse to the electric field. We suggest that the observed resonant
response is related to these transitions, and propose experiments to directly
detect the order parameters. The generalizations to electric fields applied in
different directions, and to a variety of lattices, should allow study of
numerous other correlated quantum phases.Comment: 17 pages, 14 figures; (v2) minor additions and new reference
Correlations between Ground and Excited State Spectra of a Quantum Dot
We have studied the ground and excited state spectra of a semiconductor
quantum dot for successive numbers of electron occupancy using linear and
nonlinear magnetoconductance measurements. We present the first observation of
direct correlation between the mth excited state of the N electron system and
the ground state of the N+m electron system for m up to 4. Results are
consistent with a non-spin-degenerate single particle picture of the filling of
levels. Electron-electron interaction effects are also observed as a
perturbation to this model. Magnetoconductance fluctuations of ground states
are shown as anticrossings where wavefunction characteristics are exchanged
between adjacent levels.Comment: 8 pages pdf; gzipped ps available at
http://www-leland.stanford.edu/group/MarcusLab/grouppubs.htm
Temperature and ac Effects on Charge Transport in Metallic Arrays of Dots
We investigate the effects of finite temperature, dc pulse, and ac drives on
the charge transport in metallic arrays using numerical simulations. For finite
temperatures there is a finite conduction threshold which decreases linearly
with temperature. Additionally we find a quadratic scaling of the
current-voltage curves which is independent of temperature for finite
thresholds. These results are in excellent agreement with recent experiments on
2D metallic dot arrays. We have also investigated the effects of an ac drive as
well as a suddenly applied dc drive. With an ac drive the conduction threshold
decreases for fixed frequency and increasing amplitude and saturates for fixed
amplitude and increasing frequency. For sudden applied dc drives below
threshold we observe a long time power law conduction decay.Comment: 6 pages, 7 postscript figure
Mesoscopic Coulomb Blockade in One-channel Quantum Dots
Signatures of "mesoscopic Coulomb blockade" are reported for quantum dots
with one fully transmitting point-contact lead, T1 = 1, T2 << 1. Unlike Coulomb
blockade (CB) in weak-tunneling devices (T1, T2 << 1), one-channel CB is a
mesoscopic effect requiring quantum coherence. Several distinctive features of
mesoscopic CB are observed, including a reduction in CB upon breaking
time-reversal symmetry with a magnetic field, relatively large fluctuations of
peak position as a function of magnetic field, and strong temperature
dependence on the scale of the quantum level spacing.Comment: 12 pages, including 4 figure
Distributions of the Conductance and its Parametric Derivatives in Quantum Dots
Full distributions of conductance through quantum dots with single-mode leads
are reported for both broken and unbroken time-reversal symmetry. Distributions
are nongaussian and agree well with random matrix theory calculations that
account for a finite dephasing time, , once broadening due to finite
temperature is also included. Full distributions of the derivatives of
conductance with respect to gate voltage are also investigated.Comment: 4 pages (REVTeX), 4 eps figure
Demographic and clinical characteristics of patients with sustained and switching treatments using biological and targeted synthetic disease-modifying antirheumatic drugs: A multicenter, observational cross-sectional study for rheumatoid arthritis
Introduction Rheumatoid arthritis is a chronic inflammatory disease with different disease activity grades. Several registries have been designed to determine the appropriate regimens of disease-modifying antirheumatic drugs to obtain sustained clinical remission. We examined epidemiological and clinical characteristics of rheumatoid arthritis patients using a clinical registry database (BioSTaR) and analyzed the differences in patients with sustained and switched therapies. Methods A multicenter, observational cross-sectional study for rheumatoid arthritis was performed between February 2019 and September 2020 using the BioStaR-RA registry. Demographic and clinical characteristics were prospectively recorded into a specifically designed electronic database. The patients were divided into three groups due to the heterogeneity of the study cohort. Patients were grouped as Group I (Initial; within the first 6 months of treatment with biological/targeted synthetic drugs), Group ST (Sustained Treatment; any first drug lasting for at least 6 months without any change), and Group S (Switch; any switching to another drug). Comparative analysis was performed between sustained treatment (Group ST) and drug switching (Group S) groups. Results The study included a total of 565 patients. The mean age was 53.7 +/- 12.8 years, and the majority were female (80.4%). There were 104, 267, and 194 patients in Groups I, ST, and S, respectively. Erosive arthritis and hematological extra-articular involvement were more frequently detected in Group S than Group ST (p = 0.009 and p = 0.001). The patients in Group S had significantly higher disease activity scores (DAS28-CRP, CDAI, and SDAI) (p = 0.025, p = 0.010, and p = 0.003). There were significantly more patients with moderate disease activity in Group S (p < 0.05). Conclusions The groups with sustained treatment and switching included patients with different disease activity status, although higher disease activity was determined in switchers. Overall, moderate disease activity and remission were the most common disease activity levels. Lower disease activity scores, lower hematologic manifestations, better functional status, and lesser radiographic damage are associated with sustained treatment.Turkish Medicine and Medical Devices Agency ; Ankara Numune Egitim ve Arastirma Hastanes
Spin Degeneracy and Conductance Fluctuations in Open Quantum Dots
The dependence of mesoscopic conductance fluctuations on parallel magnetic
field is used as a probe of spin degeneracy in open GaAs quantum dots. The
variance of fluctuations at high parallel field is reduced from the low-field
variance (with broken time-reversal symmetry) by factors ranging from roughly
two in a 1 square-micron dot at low temperature, to four or greater in 8
square-micron dots. The factor of two is expected for simple Zeeman splitting
of spin degenerate channels. A possible explanation for the unexpected larger
factors in terms of field-dependent spin orbit scattering is proposed.Comment: Includes new reference to related theoretical work, cond-mat/0010064.
Other minor changes. Related papers at http://marcuslab.harvard.ed
Coulomb Blockade Fluctuations in Strongly Coupled Quantum Dots
Quantum fluctuations of Coulomb blockade are investigated as a function of
the coupling to reservoirs in semiconductor quantum dots. We use fluctuations
in the distance between peaks apart to characterize both the
amplitude and correlation of peak motion. For strong coupling, peak motion is
greatly enhanced at low temperature, but does not show an increase in
peak-to-peak correlation. These effects can lead to anomalous temperature
dependence in the Coulomb valleys, similar to behavior ascribed to Kondo
physics.Comment: figures made smaller so download works. Revised, including new data.
Related papers at http://www.stanford.edu/group/MarcusLab/grouppubs.htm
Statistics of Coulomb Blockade Peak Spacings
Distributions of Coulomb blockade peak spacing are reported for large
ensembles of both unbroken (magnetic field B = 0) and broken (B 0) time
reversal symmetry in GaAs quantum dots. Both distributions are symmetric and
roughly gaussian with a width ~ 2-6% of the average spacing, with broad,
non-gaussian tails. The distribution is systematically wider at B = 0 by a
factor of ~ 1.2 +- 0.1. No even-odd spacing correlations or bimodal structure
in the spacing distribution is found, suggesting an absence of spin-degeneracy.
There is no observed correlation between peak spacing and peak height.Comment: To appear in PRL; 13 pages, one table, 3 figures; pdf available at
http://www-leland.stanford.edu/group/MarcusLab/papers/Patel_peakspacing.pd
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