2,077 research outputs found
Localized collapse and revival of coherence in an ultracold Bose gas
We study the collapse and revival of coherence induced by dipolar spin waves
in a trapped gas of Rb-87 atoms. In particular we observe spatially localized
collapse and revival of Ramsey fringe contrast and show how the pattern of
coherence depends on strength of the spin wave excitation. We find that the
spatial character of the coherence dynamics is incompatible with a simple model
based only on position-space overlap of wave functions. This phenomenon
requires a full phase-space description of the atomic spin using a quantum
Boltzmann transport equation, which highlights spin wave-induced coherent spin
currents and the ensuing dynamics they drive.Comment: 5 pages, 4 figure
Spin Diffusion in Trapped Gases: Anisotropy in Dipole and Quadrupole Modes
Recent experiments in a mixture of two hyperfine states of trapped Bose gases
show behavior analogous to a spin-1/2 system, including transverse spin waves
and other familiar Leggett-Rice-type effects. We have derived the kinetic
equations applicable to these systems, including the spin dependence of
interparticle interactions in the collision integral, and have solved for
spin-wave frequencies and longitudinal and transverse diffusion constants in
the Boltzmann limit. We find that, while the transverse and longitudinal
collision times for trapped Fermi gases are identical, the Bose gas shows
unusual diffusion anisotropy in both dipole and quadrupole modes. Moreover, the
lack of spin isotropy in the interactions leads to the non-conservation of
transverse spin, which in turn has novel effects on the hydrodynamic modes.Comment: 18 pages, 9 figure
Two-dimensional quantum antiferromagnets
This review presents some theoretical advances in the field of quantum
magnetism in two-dimensional systems, and quantum spin liquids in particular.
It is to be published as a chapter in the second edition of the book
"Frustrated spin systems", edited by H. T. Diep (World-Scientific). The section
(Sec. 7) devoted to the kagome antiferromagnet has been completely
rewritten/updated, as well as the concluding section (Sec. 8). The other
sections (Secs. 1-6) are unchanged from the first edition of the book
(published in 2005)Comment: 87 pages. 396 references. To be published as a chapter in the second
edition of the book "Frustrated spin systems", edited by H. T. Diep
(World-Scientific
MoS2-HgTe Quantum Dot Hybrid Photodetectors beyond 2 μm
Mercury telluride (HgTe) colloidal quantum dots (CQDs) have been developed as promising materials for the short and mid-wave infrared photodetection applications because of their low cost, solution processing and size tunable absorption in the short wave- and mid- infrared spectrum. However, the lowmobility and poor photo-gain have limited the responsivity of HgTe CQDs-based photodetectors to only tens of mA/W. Here, we integrated HgTe CQDs on a TiO2 encapsulated MoS2 transistor channel to form hybrid phototransistors with high responsivity of ~106 A/W, the highest reported to date for HgTe QDs. By operating the phototransistor in the depletion regime enabled by the gate modulated current of MoS2, the noise current is significantly suppressed leading to an experimentally measured specific detectivity D* of ~1012 Jones at a wavelength of 2 μm. This work demonstrates for the first time the potential of the hybrid 2D/QD detector technology in reaching out to wavelengths beyond 2 μm with compelling sensitivity.Peer ReviewedPostprint (author's final draft
The Antiferromagnetic Heisenberg Model on Fullerene-Type Symmetry Clusters
The nearest neighbor antiferromagnetic Heisenberg model is
considered for spins sitting on the vertices of clusters with the connectivity
of fullerene molecules and a number of sites ranging from 24 to 32. Using
the permutational and spin inversion symmetries of the Hamiltonian the low
energy spectrum is calculated for all the irreducible representations of the
symmetry group of each cluster. Frustration and connectivity result in
non-trivial low energy properties, with the lowest excited states being
singlets except for . Same hexagon and same pentagon correlations are the
most effective in the minimization of the energy, with the
symmetry cluster having an unusually strong singlet intra-pentagon correlation.
The magnetization in a field shows no discontinuities unlike the icosahedral
fullerene clusters, but only plateaux with the most pronounced for
. The spatial symmetry as well as the connectivity of the clusters appear
to be important for the determination of their magnetic properties.Comment: Extended to include low energy spectra, correlation functions and
magnetization data of clusters up to 32 site
Transport properties of mid-infrared colloidal quantum dot films
The transport and thermal properties of HgTe colloidal quantum dot films with
cut-off wavelengths in the mid-IR are investigated. The cut-off wavelength of
this material can be tuned over the 3-5 \mu m range, which makes it a promising
alternative to existing high cost detectors. Post deposition processes such as
ligand exchange and atomic layer deposition are investigated as a way to
increase the carrier mobility.Comment: 7 pages, 7 figure
An investigation of the quantum model on the honeycomb lattice
We have investigated the quantum model on the honeycomb lattice
with exact diagonalizations and linear spin-wave calculations for selected
values of , and antiferromagnetic () or
ferromagnetic () nearest neighbor interactions. We found a variety of
quantum effects: "order by disorder" selection of a N{\'e}el ordered
ground-state, good candidates for non-classical ground-states with dimer long
range order or spin-liquid like. The purely antiferromagnetic Heisenberg model
is confirmed to be N{\'e}el ordered. Comparing these results with those
observed on the square and triangular lattices, we enumerate some conjectures
on the nature of the quantum phases in the isotropic models.Comment: 14 pages, 22 Postscript figures, uses svjour.cls and svepj.clo,
submitted to European Physical Journal B: condensed matter physi
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