1,182 research outputs found
Is room-temperature superconductivity with phonons possible?
By recognizing the vital importance of two-hole Cooper pairs (CPs) in
addition to the usual two-electron ones in a strongly-interacting many-electron
system, the concept of CPs was re-examined with striking conclusions. Based on
this, Bose-Einstein condensation (BEC) theory has been generalized to include
not boson-boson interactions (also neglected in BCS theory) but rather
boson-fermion (BF)interaction vertices reminiscent of the Frohlich
electron-phonon interaction in metals. Unlike BCS theory, the GBEC model is not
a mean-field theory restricted to weak-coupling as it can be diagonalized
exactly. In weak coupling it reproduces the BCS condensation energy. Each kind
of CP is responsible for only half the condensation energy. The GBEC theory
reduces to all the old known statistical theories as special cases including
the so-called "BCS-Bose crossover" picture which in turn generalizes BCS theory
by not assuming that the electron chemical potential equals the Fermi energy.
Indeed, a BCS condensate is precisely the weak-coupling limit of a GBE
condensate with equal numbers of both types of CPs. With feasible Cooper/BCS
model interelectonic interaction parameter values, and even without BF
interactions, the GBEC theory yields transition temperatures [including
room-temperature superconductivity (RTSC)] substantially higher than the BCS
ceiling of around 45K, without relying on non-phonon dynamics involving
excitons, plasmons, magnons or otherwise purely-electronic mechanisms.Comment: 14 pages, 2 figures, Mini-course delivered at "X Training Course in
the Physics of Correlated-Electron Systems and High Tc Superconductors"
Salerno, Italy, 3-14 October, 200
Anomalous behavior of ideal Fermi gas below two dimensions
Normal behavior of the thermodynamic properties of a Fermi gas in
dimensions, integer or not, means monotonically increasing or decreasing of its
specific heat, chemical potential or isothermal sound velocity, all as
functions of temperature. However, for dimensions these properties
develop a ``hump'' (or ``trough'') which increases (or deepens) as .
Though not the phase transition signaled by the sharp features (``cusp'' or
``jump'') in those properties for the ideal Bose gas in (known as the
Bose-Einstein condensation), it is nevertheless an intriguing structural
anomaly which we exhibit in detail.Comment: 14 pages including 3 figure
Testing Bayes Rule and the Representativeness Heuristic: Some Experimental Evidence
The psychological literature has identified a number of heuristics which individuals may use in making judgments or choices under uncertainty. Mathematically equivalent problems may be treated differently depending upon details of the decision setting (Gigerenzer et al. (1988), Hinz et al. (1988), Birnbaum and Mellers (1983), Ginossar and Trope (1987)) or upon how the decisions are framed (Tversky and Kahneman (1986)). The results presented in this paper are consistent with those findings and are unsettling. In equivalent problems subjects appear to adopt different strategies in response to observing different data. All problems were inference problems about populations represented by bingo cages and all randomization was operational and observed by the subjects. Thus one cannot explain the change of decision strategy by appeal to changing reference points nor should difference between surface and deep structure of problems apply (Wagenaar et al. (1988)). A striking observation from the experiments is the result of employing financial incentives. Some experiments included financial incentives for accuracy and some did not. In the latter experiments the number of nonsense or incoherent responses increased by a factor of three. The majority of subjects in both treatments behaved reasonably, but of those lacking financial incentives a larger proportion gave obviously absurd responses. This suggests that data from decision experiments in which no financial incentives were should be treated as possibly contaminated and statistical methods robust against outliers employed
Financial Incentive Effects and Individual Decision-making
Though much of the literature of experimental psychology covers topics that seem relevant to economics, the literature is generally ignored by economists. Possibly the reason for this is that psychologists seldom use financial incentives to motivate subjects' choices. This paper provides an example of an individual decision-making experiment in which the presence or absence of financial incentives affects the subjects' behavior. The observed effects are not marginal but often involve qualitatively different types of responses
Origin of nonlinear contribution to the shift of the critical temperature in atomic Bose-Einstein condensates
We discuss a possible origin of the experimentally observed nonlinear
contribution to the shift of the critical
temperature in an atomic Bose-Einstein condensate (BEC) with respect to
the critical temperature of an ideal gas. We found that accounting
for a nonlinear (quadratic) Zeeman effect (with applied magnetic field closely
matching a Feshbach resonance field ) in the mean-field approximation
results in a rather significant renormalization of the field-free nonlinear
contribution , namely (where is the s-wave scattering length, is the thermal wavelength at ) with and . In particular, we predict for the resonance observed in the
BEC.Comment: Accepted for publication in JETP Letter
Research and Development Expenditures as a Competitive Strategy
Making a better mousetrap has been one of the standard methods of achieving competitive advantages in American industries. Of course, making an equally good mousetrap with lower costs can be just as effective. To the extent that businesses compete with each other with product and process improvements, then one would expect investment in these activities (research and development [R&D]) to be a primary competitive tool. It is natural to ask which types of firms tend to engage heavily in R&D. A clearly related question one might ask is which market structures are conducive to R&D activity and which are not. In addition, it is important to remember that market structure itself may be affected by firms' R&D activities--raising the question of R&D's impact on market structure (see the related paper by Preston in this volume). The most obvious situation in which market structure is affected by R&D activity is, of course, that of a monopoly position achieved and maintained by patents. This latter question; that is, essentially asking if market structure is really exogenous, is often not directly addressed in the literature.
The purpose of this paper is to survey one portion of the so-called market structure literature, viz., the empirical literature dealing with the relation between market structure and the level of research and development activity. Weiss (1969) has surveyed the empirical literature in the entire field of industrial organization, and Kamien and Schwartz (1975) more recently surveyed the literature concerning innovative activity in general. In order to allow for an intensive examination of one body of literature, the scope of this paper has been kept narrow. Readers interested in other issues, for example, the rate of adoption or imitation and the diffusion of technological information, are referred to these other studies
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