2,349 research outputs found
Individual differences in discount rate are associated with demand for self‐administered cocaine, but not sucrose
Substance abusers, including cocaine abusers, discount delayed rewards to a greater extent than do matched controls. In the current experiment, individual differences in discounting of delayed rewards in rats (choice of one immediate over three delayed sucrose pellets) were assessed for associations with demand for either sucrose pellets or an intravenous dose of 0.1 mg/kg/infusion cocaine. Twenty‐four male Sprague Dawley rats were split into three groups based on sensitivity to delay to reinforcement. Then, demand for sucrose pellets and cocaine was determined across a range of fixed‐ratio values. Delay discounting was then reassessed to determine the stability of this measure over the course of the experiment. Individual differences in impulsive choice were positively associated with elasticity of demand for cocaine, a measure of reinforcer value, indicating that rats having higher discount rates also valued cocaine more. Impulsive choice was not associated with the level of cocaine consumption as price approached 0 or with any parameter associated with demand for sucrose. Individual sensitivity to delay was correlated with the initial assessment when reassessed at the end of the experiment, although impulsive choice increased for this cohort of rats as a whole. These findings suggest that impulsive choice in rats is positively associated with valuation of cocaine, but not sucrose.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95135/1/adb361.pd
Diagrammatic calculation of energy spectrum of quantum impurity in degenerate Bose-Einstein condensate
In this paper we considered a quantum particle moving through delute
Bose-Einstein condensate at zero temperature. In our formulation the impurity
particle interacts with the gas of uncoupled Bogoliubov's excitations. We
constructed the perturbation theory for the Green's function of the impurity
particle with respect to the impurity-condensate interaction employing the
coherent-state path integral approach. The perturbative expansion for the
Green's function is resumed into the expansion for its poles with the help of
the diagrammatic technique developed in this work. The dispersion relation for
the impurity clothed by condensate excitations is obtained and effective mass
is evaluated beyond the Golden rule approximation
Statistics of layered zigzags: a two-dimensional generalization of TASEP
A novel discrete growth model in 2+1 dimensions is presented in three
equivalent formulations: i) directed motion of zigzags on a cylinder, ii)
interacting interlaced TASEP layers, and iii) growing heap over 2D substrate
with a restricted minimal local height gradient. We demonstrate that the
coarse-grained behavior of this model is described by the two-dimensional
Kardar-Parisi-Zhang equation. The coefficients of different terms in this
hydrodynamic equation can be derived from the steady state flow-density curve,
the so called `fundamental' diagram. A conjecture concerning the analytical
form of this flow-density curve is presented and is verified numerically.Comment: 5 pages, 4 figure
Finite formation time effects in quasi-elastic scattering on nuclear targets
The problem of the final state interaction in quasi-elastic
scattering at large , is investigated by exploiting the idea that the
ejected nucleon needs a finite amount of time to assume its asymptotic form. It
is shown that when the dependence of the scattering amplitude of the ejected
nucleon on its virtuality is taken into account, the final state interaction is
decreased. The developed approach is simpler to implement than the one based on
the color transparency description of the damping of the final state
interaction, and is essentially equivalent to the latter in the case of the
single rescattering term. The process on the deuteron is numerically
investigated and it is shown that, at , appreciable finite formation time
effects at of the order of 10 (GeV/c) are expected.Comment: 23 pages, 3 figure
Light emission and spin-polarised hole injection in InAs/GaAs quantum dot heterostructures with Schottky contact
EPL draftWe demonstrate the feasibility to obtain electroluminescence (EL), up to room temperature, from InGaAs self-assembled quantum dots (QDs) included in a forward-biased Schottky
diode. Moreover, using a ferromagnet (FM) as the contact layer, sizable circular polarization of
the EL emission in the presence of an external magnetic eld is obtained. A resonant behav-
ior of the degree of circular polarization (P) as a function of applied voltage (V ), for a given
value of magnetic eld, is observed. We explain our ndings using a model including tunneling
of (spin-polarised) holes through the metal-semiconductor interface, transport in the near surface
region of the heterostructure and out-of-equilibrium statistics of the injected carriers occupying
the available states in the QD heterostructure. In particular, the resonant P(V ) dependence is
related to the splitting of the qusi-Fermi level for two spin orientations in the FM.FCT, Portugal (project
POCI/FIS/58524/2004), the RFBR, Russia (grant 10-02-
00501), MEC (grants MAT2008-01555, QOIT-CSD2006-
00019) and CAM (S-2009/ESP-1503) (Spain)
Multiple scattering of matter waves: an analytic model of the refractive index for atomic and molecular gases
We present an analytic model of the refractive index for matter waves
propagating through atomic or molecular gases. The model, which combines a WKB
treatment of the long range attraction with the Fraunhofer model treatment of
the short range repulsion, furnishes a refractive index in compelling agreement
with recent experiments of Jacquey et al. [Phys. Rev. Lett. 98, 240405 (2007)]
on Li atom matter waves passing through dilute noble gases. We show that the
diffractive contribution, which arises from scattering by a two dimensional
"hard core" of the potential, is essential for obtaining a correct imaginary
part of the refractive index.Comment: 5 pages, 1 figure, 2 table
Pulsed current-voltage electrodeposition of stoichiometric Bi2Te3 nanowires and their crystallographic characterization by transmission electron backscatter diffraction
Bi2Te3 nanowires with diameters ranging from 25 to 270 nm, ultra-high aspect ratio, and uniform growth front were fabricated by electrodeposition, pulsing between zero current density during the off time and constant potential during the on time (pulsed-current-voltage method, p-IV). The use of zero current density during the off time is to ensure no electrodeposition is carried out and the system is totally relaxed. By this procedure, stoichiometric nanowires oriented perpendicular to the c-axis is obtained for the different diameters of porous alumina templates. In addition, the samples show a uniform growth front with ultra-high aspect ratio single crystal nanowires. The high degree of crystallinity was verified by transmission electron backscatter diffraction. This characterization revealed that the nanowires present both large single crystalline areas and areas with alternating twin configurations
Polar magneto-optical Kerr effect for low-symmetric ferromagnets
The polar magneto-optical Kerr effect (MOKE) for low-symmetric ferromagnetic
crystals is investigated theoretically based on first-principle calculations of
optical conductivities and a transfer matrix approach for the electrodynamics
part of the problem. Exact average magneto-optical properties of polycrystals
are described, taking into account realistic models for the distribution of
domain orientations. It is shown that for low-symmetric ferromagnetic single
crystals the MOKE is determined by an interplay of crystallographic
birefringence and magnetic effects. Calculations for single and bi-crystal of
hcp 11-20 Co and for a polycrystal of CrO_2 are performed, with results being
in good agreement with experimental data.Comment: 14 pages, 7 figures, accepted for publication in Phys. Rev.
A surface containing a line and a circle through each point is a quadric
We prove that a surface in real 3-space containing a line and a circle
through each point is a quadric. We also give some particular results on the
classification of surfaces containing several circles through each point.Comment: Improved exposition, 4 figures adde
- …