647 research outputs found
Anomalous dispersion and negative-mass dynamics of exciton polaritons in an atomically thin semiconductor
Dispersion engineering is a powerful and versatile tool that can vary the
speed of light signals and induce negative-mass effects in the dynamics of
electrons, quasiparticles, and quantum fluids. Here, we demonstrate that
dissipative coupling between bound electron-hole pairs (excitons) and photons
in an optical microcavity can lead to the formation of exciton polaritons with
an inverted dispersion of the lower polariton branch and hence, a negative
mass. We perform a direct measurement of the anomalous dispersion in an
atomically thin WS crystal embedded in a planar microcavity, and
demonstrate that the propagation direction of the negative-mass polaritons is
opposite to their momentum. Our study introduces a new concept of non-Hermitian
dispersion engineering for exciton polaritons and shows a pathway for realising
new phases of quantum matter in a solid state.Comment: 7 pages, 4 figure
Paired atom laser beams created via four-wave mixing
A method to create paired atom laser beams from a metastable helium atom
laser via four-wave mixing is demonstrated. Radio frequency outcoupling is used
to extract atoms from a Bose Einstein condensate near the center of the
condensate and initiate scattering between trapped and untrapped atoms. The
unequal strengths of the interactions for different internal states allows an
energy-momentum resonance which leads to the creation of pairs of atoms
scattered from the zero-velocity condensate. The resulting scattered beams are
well separated from the main atom laser in the 2-dimensional transverse atom
laser profile. Numerical simulations of the system are in good agreement with
the observed atom laser spatial profiles, and indicate that the scattered beams
are generated by a four-wave mixing process, suggesting that the beams are
correlated.Comment: 5 pages, 3 figure
Phonon spectrum and dynamical stability of a quantum degenerate Bose-Fermi mixture
We calculate the phonon excitation spectrum in a zero-temperature
boson-fermion mixture. We show how the sound velocity changes due to the
boson-fermion interaction and we determine the dynamical stability regime of a
homogeneous mixture. We identify a resonant phonon-exchange interaction between
the fermions as the physical mechanism leading to the instability.Comment: 4 pages, 3 figure
Comparing the use of conventional and three-dimensional printing (3DP) in mandibular reconstruction
This is the final version. Available on open access from BMC via the DOI in this recordAvailability of data and materials:
The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.Background
There are a number of clinical disorders that require mandibular reconstruction (MR). Novel three-dimensional (3D) printing technology enables reconstructions to be more accurate and beneficial to the patient. However, there is currently no evidence identifying which techniques are better suited for MR, based on the type of clinical disorder the patient has. In this study, we aim to compare 3D techniques with conventional techniques to identify how best to reconstruct the mandible based on the clinical cause that necessitates the reconstructive procedure: cancerous or benign tumours, clinical disorders, infection or disease and trauma or injury.
Methods
PubMed, Scopus, Embase and Medline were searched to identify relevant papers that outline the clinical differences between 3D and conventional techniques in MR. Data were evaluated to provide a clear outline of suitable techniques for surgery.
Results
20 of 2749 papers met inclusion criteria. These papers were grouped based on the clinical causes that required MR into four categories: malignant or benign tumour resection; mandibular trauma/injury and other clinical disorders.
Conclusions
The majority of researchers favoured 3D techniques in MR. However, due to a lack of standardised reporting in these studies it was not possible to determine which specific techniques were better for which clinical presentations
Resonator-Enhanced Optical Dipole Trap for Fermionic Lithium Atoms
We demonstrate a novel optical dipole trap which is based on the enhancement
of the optical power density of a Nd:YAG laser beam in a resonator. The trap is
particularly suited for experiments with ultracold gases, as it combines a
potential depth of order 1 mK with storage times of several tens of seconds. We
study the interactions in a gas of fermionic lithium atoms in our trap and
observe the influence of spin-changing collisions and off-resonant photon
scattering. A key element in reaching long storage times is an ultra-low noise
laser. The dependence of the storage time on laser noise is investigated.Comment: 4 pages 3 figures Revised 17.07.2001; Corrected calibration of noise
measm
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