Residual Entropy of the Mott Insulator with No Symmetry Broken

The half-filled ground state of the Hubbard model on the hypercubic lattice in D dimensions is studied by the Kondo-lattice theory, which is none other than the 1/D expansion theory, but within the constrained Hilbert subspace where no symmetry is allowed to be broken. A gap can open in the single-particle excitation spectrum if and only if the residual entropy or entropy at T=+0 K is nonzero. The Mott insulator with no symmetry broken, if it is possible, is characterized by nonzero residual entropy or nonzero entropy at T=+0 K. This conclusion is consistent with Brinkman and Rice's theory and the dynamical mean-field theory. According to the well-known argument based on the Bethe-ansatz solution, on the other hand, the half-filled ground state in one dimension is the Mott insulator although its residual entropy per unit cell is vanishing in the thermodynamic limit. Two possible explanations are given for the contradiction between the present paper and the well-known argument.Comment: 27 page

Effect of Particle-Hole Asymmetry on the Mott-Hubbard Metal-Insulator Transition

The Mott-Hubbard metal-insulator transition is one of the most important problems in correlated electron systems. In the past decade, much progress has been made on examining a particle-hole symmetric form of the transition in the Hubbard model with dynamical mean field theory where it was found that the electronic self energy develops a pole at the transition. We examine the particle-hole asymmetric metal-insulator transition in the Falicov-Kimball model, and find that a number of features change when the noninteracting density of states has a finite bandwidth. Since, generically particle-hole symmetry is broken in real materials, our results have an impact on understanding the metal-insulator transition in real materials.Comment: 5 pages, 3 figure

Structural Change and Distribution of Support in Hungarian Agriculture following EU Accession: A Preliminary FADN Analysis

This paper, whilst preliminary and descriptive, highlights some significant changes in the structure of the Hungarian national farm around the time of EU accession, based on an examination of farm-level data. Between 2002 and 2005 gross value of total production changes little, but direct agricultural subsidies nearly double. There is a marked shift in favour of arable farming, which affects both private farms and economic organisations. Arable farms increase their share of total subsidies by 20 percentage points over the three years, at the expense of most animal and mixed farms. Overall, the distribution of subsidies, whilst highly unequal, is slightly less unequal in 2005.Agricultural and Food Policy,

Replications and Extensions in Marketing – Rarely Published But Quite Contrary

Replication is rare in marketing. Of 1,120 papers sampled from three major marketing journals, none were replications. Only 1.8% of the papers were extensions, and they consumed 1.1% of the journal space. On average, these extensions appeared seven years after the original study. The publication rate for such works has been decreasing since the 1970s. Published extensions typically produced results that conflicted with the original studies; of the 20 extensions published, 12 conflicted with the earlier results, and only 3 provided full confirmation. Published replications do not attract as many citations after publication as do the original studies, even when the results fail to support the original studies

Are Null Results Becoming an Endangered Species in Marketing?

Editorial procedures in the social and biomedical sciences are said to promote studies that falsely reject the null hypothesis. This problem may also exist in major marketing journals. Of 692 papers using statistical significance tests sampled from the Journal of Marketing, Journal of Marketing Research, and Journal of Consumer Research between 1974 and 1989, only 7.8% failed to reject the null hypothesis. The percentage of null results declined by one-half from the 1970s to the 1980s. The JM and the JMR registered marked decreases. The small percentage of insignificant results could not be explained as being due to inadequate statistical power. Various scholars have claimed that editorial policies in the social and medical sciences are biased against studies reporting null results, and thus encourage the proliferation of Type 1 errors (erroneous rejection of the null hypothesis). Greenwald (1975, p. 15) maintains that Type I publication errors are underestimated to the extent that they are: “. . . frightening, even calling into question the scientific basis for much published literature.” Our paper examines the publication frequency of null results in marketing. First, we discuss how editorial policies might foster an atmosphere receptive to Type I error proliferation. Second, we review the evidence on the publication of null results in the social and biomedical sciences. Third, we report on an empirical investigation of the publication frequency of null results in the marketing literature. Fourth, we examine power levels for statistically insignificant findings in marketing to see if they are underpowered and thus less deserving of publication. Finally, we provide suggestions to facilitate the publication of null results

Compressibility of the Two-Dimensional infinite-U Hubbard Model

We study the interactions between the coherent quasiparticles and the incoherent Mott-Hubbard excitations and their effects on the low energy properties in the $U=\infty$ Hubbard model. Within the framework of a systematic large-N expansion, these effects first occur in the next to leading order in 1/N. We calculate the scattering phase shift and the free energy, and determine the quasiparticle weight Z, mass renormalization, and the compressibility. It is found that the compressibility is strongly renormalized and diverges at a critical doping $\delta_c=0.07\pm0.01$. We discuss the nature of this zero-temperature phase transition and its connection to phase separation and superconductivity.Comment: 4 pages, 3 eps figures, final version to appear in Phys. Rev. Let

Possible Solutions to the Radius Anomalies of Transiting Giant Planets

We calculate the theoretical evolution of the radii of all fourteen of the known transiting extrasolar giant planets (EGPs) for a variety of assumptions concerning atmospheric opacity, dense inner core masses, and possible internal power sources. We incorporate the effects of stellar irradiation and customize such effects for each EGP and star. Looking collectively at the family as a whole, we find that there are in fact two radius anomalies to be explained. Not only are the radii of a subset of the known transiting EGPs larger than expected from previous theory, but many of the other objects are smaller than the default theory would allow. We suggest that the larger EGPs can be explained by invoking enhanced atmospheric opacities that naturally retain internal heat. This explanation might obviate the necessity for an extra internal power source. We explain the smaller radii by the presence in perhaps all the known transiting EGPs of dense cores, such as have been inferred for Saturn and Jupiter. Importantly, we derive a rough correlation between the masses of our "best-fit" cores and the stellar metallicity that seems to buttress the core-accretion model of their formation. Though many caveats and uncertainties remain, the resulting comprehensive theory that incorporates enhanced-opacity atmospheres and dense cores is in reasonable accord with all the current structural data for the known transiting giant planets.Comment: 22 pages in emulateapj format, including 10 figures (mostly in color), accepted to the Astrophysical Journal (February 9, 2007); to appear in volume 661, June 200

Measuring compassion in young people

There has been increased scientific interest in compassion in recent years, and compassion is suggested to have many benefits for young people, reflecting the adult literature. However, research highlights issues related to the measurement of compassion in young people. The systematic review aimed to describe and critically appraise the compassion measures used with young people, and their psychometric properties. Three databases were searched (Medline, Psychinfo, Web of Science), and 15 papers were included which outlined the psychometric properties of eight compassion measures used with young people. The measures were critically reviewed and rated for quality, with quality ratings ranging from 2 to 9 out of 14. All measures had clear psychometric weaknesses. The majority of papers assessed self-compassion and no identified papers examined psychometric properties with young people in the UK. Overall, this review suggests that the majority of compassion measures are not suitable for use with young people. Recommendations for future research are discussed

Effects of Helium Phase Separation on the Evolution of Giant Planets

We present the first models of Saturn and Jupiter to couple their evolution to both a radiative-atmosphere grid and to high-pressure phase diagrams of hydrogen with helium. The purpose of these models is to quantify the evolutionary effects of helium phase separation in Saturn's deep interior. We find that prior calculated phase diagrams in which Saturn's interior reaches a region of predicted helium immiscibility do not allow enough energy release to prolong Saturn's cooling to its known age and effective temperature. We explore modifications to published phase diagrams that would lead to greater energy release, and find a modified H-He phase diagram that is physically reasonable, leads to the correct extension of Saturn's cooling, and predicts an atmospheric helium mass fraction Y_atmos in agreement with recent estimates. We then expand our inhomogeneous evolutionary models to show that hypothetical extrasolar giant planets in the 0.15 to 3.0 Jupiter mass range may have T_effs 10-15 K greater than one would predict with models that do not incorporate helium phase separation.Comment: 4 pages. Contribution to 'The Search for Other Worlds', Oct 2003, University of Marylan

Cold dilute neutron matter on the lattice II: Results in the unitary limit

This is the second of two papers which investigate cold dilute neutron matter on the lattice using pionless effective field theory. In the unitary limit, where the effective range is zero and scattering length is infinite, simple scaling relations relate thermodynamic functions at different temperatures. When the second virial coefficient is properly tuned, we find that the lattice results obey these scaling relations. We compute the energy per particle, pressure, spin susceptibility, dineutron correlation function, and an upper bound for the superfluid critical temperature.Comment: 16 pages, 9 figure