2,705 research outputs found
Effective mass in quasi two-dimensional systems
The effective mass of the quasiparticle excitations in quasi two-dimensional
systems is calculated analytically. It is shown that the effective mass
increases sharply when the density approaches the critical one of
metal-insulator transition. This suggests a Mott type of transition rather than
an Anderson like transition.Comment: 3 pages 3 figure
Phase diagram for interacting Bose gases
We propose a new form of the inversion method in terms of a selfenergy
expansion to access the phase diagram of the Bose-Einstein transition. The
dependence of the critical temperature on the interaction parameter is
calculated. This is discussed with the help of a new condition for
Bose-Einstein condensation in interacting systems which follows from the pole
of the T-matrix in the same way as from the divergence of the medium-dependent
scattering length. A many-body approximation consisting of screened ladder
diagrams is proposed which describes the Monte Carlo data more appropriately.
The specific results are that a non-selfconsistent T-matrix leads to a linear
coefficient in leading order of 4.7, the screened ladder approximation to 2.3,
and the selfconsistent T-matrix due to the effective mass to a coefficient of
1.3 close to the Monte Carlo data
Adenylate effects on protein phosphorylation in the interenvelope lumen of pea chloroplasts
A 64-kilodalton (kDa) protein, situated in the lumen between the inner and outer envelopes of pea (Pisum sativum L.) chloroplasts (Soll and Bennett 1988, Eur. J. Biochem., 175, 301–307) is shown to undergo reversible phosphorylation in isolated mixed envelope vesicles. It is the most conspicuously labelled protein after incubation of envelopes with 33 nmol·1-1 [-32P]ATP whereas incubation with 50 mol·1-1 [-32P]ATP labels most prominently two outer envelope proteins (86 and 23 kDa). Half-maximum velocity for phosphorylation of the 64-kDa protein occurs with 200 nmol·1-1 ATP, and around 40 mol·1-1 ATP for phosphorylation of the 86- and 23-kDa proteins, indicating the operation of two distinct kinases. GGuanosine-, uridine-, cytidine 5-triphosphate and AMP are poor inhibitors of the labelling of the 64-kDa protein with [-32P]ATP. On the other hand, ADP has a potent influence on the extent of labelling (half-maximal inhibition at 1–5 mol·1-1). The ADP-dependent appearance of 32P in ATP indicates that ADP acts by reversal of kinase activity and not as a competitive inhibitor. However, the most rapid loss of 32P from pre-labelled 64-kDa protein occurs when envelope vesicles are incubated with ATP t1/2=15 s at 20 molsd1-1 ATP). This induced turnover of phosphate appears to be responsible for the rapid phosphoryl turnover seen in situ
Semiclassical corrections to the interaction energy of a hard-sphere Boltzmann gas
Quantum effects in statistical mechanics are important when the thermal
wavelength is of the order of, or greater than, the mean interatomic spacing.
This is examined at depth taking the example of a hard-sphere Boltzmann gas.
Using the virial expansion for the equation of state, it is shown that the
interaction energy of a classical hard-sphere gas is exactly zero. When the
(second) virial coefficient of such a gas is obtained quantum mechanically,
however, the quantum contribution to the interaction energy is shown to be
substantial. The importance of the semiclassical corrections to the interaction
energy shows up dramatically in such a system.Comment: 9 pages, 3 figures, submitted to Eur. J. Phy
Neural Signatures of Value Comparison in Human Cingulate Cortex during Decisions Requiring an Effort-Reward Trade-off
UNLABELLED: Integrating costs and benefits is crucial for optimal decision-making. Although much is known about decisions that involve outcome-related costs (e.g., delay, risk), many of our choices are attached to actions and require an evaluation of the associated motor costs. Yet how the brain incorporates motor costs into choices remains largely unclear. We used human fMRI during choices involving monetary reward and physical effort to identify brain regions that serve as a choice comparator for effort-reward trade-offs. By independently varying both options' effort and reward levels, we were able to identify the neural signature of a comparator mechanism. A network involving supplementary motor area and the caudal portion of dorsal anterior cingulate cortex encoded the difference in reward (positively) and effort levels (negatively) between chosen and unchosen choice options. We next modeled effort-discounted subjective values using a novel behavioral model. This revealed that the same network of regions involving dorsal anterior cingulate cortex and supplementary motor area encoded the difference between the chosen and unchosen options' subjective values, and that activity was best described using a concave model of effort-discounting. In addition, this signal reflected how precisely value determined participants' choices. By contrast, separate signals in supplementary motor area and ventromedial prefrontal cortex correlated with participants' tendency to avoid effort and seek reward, respectively. This suggests that the critical neural signature of decision-making for choices involving motor costs is found in human cingulate cortex and not ventromedial prefrontal cortex as typically reported for outcome-based choice. Furthermore, distinct frontal circuits seem to drive behavior toward reward maximization and effort minimization. SIGNIFICANCE STATEMENT: The neural processes that govern the trade-off between expected benefits and motor costs remain largely unknown. This is striking because energetic requirements play an integral role in our day-to-day choices and instrumental behavior, and a diminished willingness to exert effort is a characteristic feature of a range of neurological disorders. We use a new behavioral characterization of how humans trade off reward maximization with effort minimization to examine the neural signatures that underpin such choices, using BOLD MRI neuroimaging data. We find the critical neural signature of decision-making, a signal that reflects the comparison of value between choice options, in human cingulate cortex, whereas two distinct brain circuits drive behavior toward reward maximization or effort minimization
Correlated two-particle scattering on finite cavities
The correlated two-particle problem is solved analytically in the presence of
a finite cavity. The method is demonstrated here in terms of exactly solvable
models for both the cavity as well as the two-particle correlation where the
two-particle potential is chosen in separable form. The two-particle phase
shift is calculated and compared to the single-particle one. The two-particle
bound state behavior is discussed and the influence of the cavity on the
binding properties is calculated.Comment: Derivation shortened and corrected, 14 pages 10 figure
Enhancement of pairing due to the presence of resonant cavities
A correlated fermion system is considered surrounding a finite cavity with
virtual levels. The pairing properties are calculated and the influence of the
cavity is demonstrated. To this end the Gell-Mann and Goldberger formula is
generalized to many-body systems. We find a possible enhancement of pairing
temperature if the Fermi momentum times the cavity radius fulfills a certain
resonance condition which suggests an experimental realization.Comment: 4 pages 2 figure
Impossibility to describe repulsion with contact interaction
Contact interactions always lead to attractive behaviour. Arguments are
presented to show why a repulsive interacting system, e.g. Bose gases, cannot
be described by contact interactions and corresponding treatments are possibly
obscured by the appearance of bound states. The usually used cut-offs are
identified as finite range parameters.Comment: 2 pages commen
Remarks on the Zeros of the Associated Legendre Functions with Integral Degree
We present some formulas for the computation of the zeros of the
integral-degree associated Legendre functions with respect to the order.Comment: 7 pages, 2 figure
Brownian Thermal Noise in Multilayer Coated Mirrors
We analyze the Brownian thermal noise of a multi-layer dielectric coating,
used in high-precision optical measurements including interferometric
gravitational-wave detectors. We assume the coating material to be isotropic,
and therefore study thermal noises arising from shear and bulk losses of the
coating materials. We show that coating noise arises not only from layer
thickness fluctuations, but also from fluctuations of the interface between the
coating and substrate, driven by internal fluctuating stresses of the coating.
In addition, the non-zero photoeleastic coefficients of the thin films modifies
the influence of the thermal noise on the laser field. The thickness
fluctuations of different layers are statistically independent, however, there
exists a finite coherence between layers and the substrate-coating interface.
Taking into account uncertainties in material parameters, we show that
significant uncertainties still exist in estimating coating Brownian noise.Comment: 26 pages, 18 figure
- …