786 research outputs found
Lasing in Bose-Fermi mixtures
No full textA.K. acknowledges the support from the EPSRC Established Career Fellowship. V.K., M.D., V.F.S. and A.K. acknowledge support from the Russian Ministry of Science and Education, contract (contract No. 11.G34.31.0067). P.G.S. acknowledges support from Greek GSRT program Aristeia (grant No. 1978). C.S., M. A. J.F., M.K and S.H. acknowledge support from the state of Bavaria.Light amplification by stimulated emission of radiation, well-known for revolutionising photonic science, has been realised primarily in fermionic systems including widely applied diode lasers. The prerequisite for fermionic lasing is the inversion of electronic population, which governs the lasing threshold. More recently, bosonic lasers have also been developed based on Bose-Einstein condensates of exciton-polaritons in semiconductor microcavities. These electrically neutral bosons coexist with charged electrons and holes. In the presence of magnetic fields, the charged particles are bound to their cyclotron orbits, while the neutral exciton-polaritons move freely. We demonstrate how magnetic fields affect dramatically the phase diagram of mixed Bose-Fermi systems, switching between fermionic lasing, incoherent emission and bosonic lasing regimes in planar and pillar microcavities with optical and electrical pumping. We collected and analyzed the data taken on pillar and planar microcavity structures at continuous wave and pulsed optical excitation as well as injecting electrons and holes electronically. Our results evidence the transition from a Bose gas to a Fermi liquid mediated by magnetic fields and light-matter coupling.Peer reviewe
Multi-scale hierarchical generation of PET parametric maps: Application and testing on [11C]DPN study
No full textIntroduction: We investigate a general approach to generate parametric maps that consists in a multi-stage hierarchical scheme whereas starting from the kinetic analysis of the whole brain we then cascade the kinetic information to anatomical systems that are akin in terms of receptor densities and then down to the pixel level. A-priori classes of voxels are generated either by anatomical segmentation or by functional segmentation using unsupervised clustering. Kinetic properties are then transmitted to the voxels in each class using Maximum a Posteriori (MAP) estimation approaches. We validate the algorithm on a test–retest data-sets of [11C]diprenorphine (DPN), which represents a challenge to estimation given its slow equilibration in tissue. We further offer internal validation by comparing resulting parametric maps generated from the anatomical and functional a-priori segmentation.Materialandmethods: We considered tracers that could be described by 1-tissue (1T) compartment model with 2 rate constants (K1 and k2). The parameters of the linearized 1T model have been obtained by using the MAP estimation approach in order to eliminate the bias introduced with the linearization of the model. Volume of distribution Vt was calculated as: Vt = K1/k2. The priors for the Bayesian estimation were derived from a weighted nonlinear least squares (WLQ) estimation done at region-of-interest (ROI) level. In order to investigate the impact of different ways to extract the priors, ROIs have been obtained A) using the anatomical atlas and B) using unsupervised clustering. Likelihood Estimation in Graphical Analysis (LEGA) [1] was also applied to quantify Vt at ROI level and its results have been compared to those obtained at pixel level. The 1T model was assumed to best describe [11C]DPN kinetics at pixel level, after comparison with more complex models (Akaike criterion). Five subjects (test and retest) underwent 95-min dynamic PET, following an injection with ∼185 MBq of [11C]DPN. Arterial plasma input functions corrected for radiolabelled metabolites were created. An individualized maximum probability atlas was created for each subject and used to derive 83 ROIs.Results: The regional distributions of Vt were consistent with opioid receptor distributions known from previous [11C]DPN studies [2]. When priors have been derived from the anatomical atlas (Fig. A), there was excellent agreement and strong correlation among pixel and LEGA ROI results (average R2 = 0.949) and excellent reliability test–retest for all subjects but the first (average R2 = 0.939).1T pixel level results didn't change when priors were defined from unsupervised clustering (Fig. B), i.e. the difference among the estimates varied between 0% and 2% among the subjects.Conclusion: The new method presented is fast (i.e. 15 min per subject) and robust. Applied to [11C]DPN data achieves accurate quantification of Vt as well as high quality Vt images. There is strong agreement between pixel level results and both LEGA ROI estimates and results from previous studies. Moreover, the way the priors are defined (i.e. using anatomical atlas or unsupervised clustering) doesn't affect the estimates
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Three-dimensional structure of quantized vortices in rotating Bose-Einstein condensates
No full textBose-Einstein condensates (BEC) are ideal superfluid systems to realize quantum turbulence (QT): vortex cores in BECs are larger than in superfluid Helium, making easier their observation. Recent experimental and numerical studies reported that vortex states in BEC can evolve towards a turbulent regime when an oscillatory excitation is applied. We discuss in this work how to accurately prepare initial states with vortices before running numerical simulations of QT based on the Gross-Pitaevskii equation. The case of a dense Abrikosov lattice in a fast rotating BEC is presented. High resolution numerical simulations using parallel computing are used to accurately capture physically important features of the vortices (vortex radius, inter-vortex spacing, vortex density profile)
Bose–Einstein condensation in the Rindler space
No full textAbstractBased on the Unruh effect, we calculate the critical acceleration of the Bose–Einstein condensation in a free complex scalar field at finite density in the Rindler space. Our model corresponds to an ideal gas performing constantly accelerating motion in a Minkowski space–time at zero-temperature, where the gas is composed of the complex scalar particles and it can be thought to be in a thermal-bath with the Unruh temperature. In the accelerating frame, the model will be in the Bose–Einstein condensation state at low acceleration; on the other hand, there will be no condensation at high acceleration by the thermal excitation brought into by the Unruh effect. Our critical acceleration is the one at which the Bose–Einstein condensation begins to appear in the accelerating frame when we decrease the acceleration gradually. To carry out the calculation, we assume that the critical acceleration is much larger than the mass of the particle
Fermi-Bose supersymmetry (supergauge symmetry in four dimensions)
No full textThe author explains the ideas of Fermi-Bose supersymmetry and presents examples to show how the construction of realistic models may be attempted. (24 refs)
Chemisorption on binary alloys with long-range order
No full textPT: J; CR: BERK NF, 1975, SURF SCI, V48, P289 BOSE SM, 1981, PHYS REV B, V24, P1934 FOO EN, 1970, PHYS REV LETT, V25, P1748 HELLWEGE KH, 1981, LANDOLTBORNSTEIN N A, V13, P406 KALKSTEIN D, 1971, SURF SCI, V26, P85 MORANLOPEZ JL, 1975, J PHYS F MET PHYS, V5, P1277 MORANLOPEZ JL, 1976, SURF SCI, V57, P540 NEWNS DM, 1969, PHYS REV, V178, P1123 SOVEN P, 1967, PHYS REV, V156, P809 SULSTON KW, 1986, PHYS REV B, V33, P2263 UEBA H, 1979, J CHEM PHYS, V70, P1745 UEBA H, 1979, PHYS STATUS SOLIDI B, V92, P307 VANSANTEN RA, 1975, SURF SCI, V53, P35 VANSANTEN RA, 1977, SURF SCI, V63, P358; NR: 14; TC: 2; J9: PHYS REV B; PG: 4; GA: P4638Source type: Electronic(1
Modification of Loop 1 Affects the Nucleotide Binding Properties of Myo1c, the Adaptation Motor in the Inner Ear
No full textMyo1c is one of eight members of the mammalian myosin I family of actin-associated molecular motors. In stereocilia of the hair cells in the inner ear, Myo1c presumably serves as the adaptation motor, which regulates the opening and closing of transduction channels. Although there is conservation of sequence and structure among all myosins in the N-terminal motor domain, which contains the nucleotide- and actin-binding sites, some differences include the length and composition of surface loops, including loop 1, which lies near the nucleotide-binding domain. To investigate the role of loop 1, we expressed in insect cells mutants of a truncated form of Myo1c, Myo1c1IQ, as well as chimeras of Myo1c1IQ with the analogous loop from other myosins. We found that replacement of the charged residues in loop 1 with alanines or the whole loop with a series of alanines did not alter the ATPase activity, transient kinetics properties, or Ca2+ sensitivity of Myo1c1IQ. Substitution of loop 1 with that of the corresponding region from tonic smooth muscle myosin II (Myo1c1IQ-tonic) or replacement with a single glycine (Myo1c1IQ-G) accelerated the release of ADP from A.M 2?3-fold in Ca2+, whereas substitution with loop 1 from phasic muscle myosin II (Myo1c1IQ-phasic) accelerated the release of ADP 35-fold. Motility assays with chimeras containing a single ?-helix, or SAH, domain showed that Myo1cSAH-tonic translocated actin in vitro twice as fast as Myo1cSAH-WT and 3-fold faster than Myo1cSAH-G. The studies show that changes induced in Myo1c via modification of loop 1 showed no resemblance to the behavior of the loop donor myosins or to the changes previously observed with similar Myo1b chimeras
Light-induced atomic desorption in a compact system for ultracold atoms
No full textThis work was supported by the UK EPSRC grant GR/T08272/01 and the Leverhulme Trust Research Project Grant RPG-2013-074. G.S. acknowledges support from a SUPA Advanced Fellowship.In recent years, light-induced atomic desorption (LIAD) of alkali atoms from the inner surface of a vacuum chamber has been employed in cold atom experiments for the purpose of modulating the alkali background vapour. This is beneficial because larger trapped atom samples can be loaded from vapour at higher pressure, after which the pressure is reduced to increase the lifetime of the sample. We present an analysis, based on the case of rubidium atoms adsorbed on pyrex, of various aspects of LIAD that are useful for this application. Firstly, we study the intensity dependence of LIAD by fitting the experimental data with a rate-equation model, from which we extract a correct prediction for the increase in trapped atom number. Following this, we quantify a figure of merit for the utility of LIAD in cold atom experiments and we show how it can be optimised for realistic experimental parameters.Peer reviewe
Bose-Einstein condensation of excitons in uniaxially stressed cuprous oxide
No full textExcitons in Cu\sb2O have been demonstrated to be a nearly ideal gas of bosons, providing a unique system for studying Bose-Einstein statistics and Bose-Einstein Condensation of an ideal gas in the laboratory. In this study, I have applied uniaxial stress to optimize the conditions for a Bose-Einstein Condensation phase transition of the excitons in this crystal. Using time-resolved photoluminescence, an enhanced quantum degeneracy of the orthoexcitons is observed in this stressed crystal due to the lowered multiplicity of the orthoexciton ground state, although no evidence of the condensation phase of the orthoexcitons is observed. In contrast, I have found compelling evidence for Bose-Einstein Condensation of the singlet paraexcitons in uniaxially stressed Cu\sp2O. When the paraexciton gas attains a particle density exceeding the critical density for Bose-Einstein condensation, the paraexciton spectrum develops an extra component at low energy which is about 0.1-0.3 meV above the zero-kinetic-energy state. I also observe an anomalous spatial transport of the paraexcitons when this low-energy component is present. This ballistic wave-like (or drift-like) motion occurs at an expansion rate 3 10\sp5 cm/sec to 4 10\sp5 cm/sec, roughly correlated with a blue-shift of 0.1-0.3 meV for the low-energy component of the paraexcitons. I therefore interpret the low-energy component of the paraexcitons as a Bose-Einstein condensation peak.U of I OnlyETDs are only available to UIUC Users without author permissio
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