Spectral function of the one-dimensional Hubbard model away from half filling

We calculate the photoemission spectral function of the one-dimensional Hubbard model away from half filling using the dynamical density matrix renormalization group method. An approach for calculating momentum-dependent quantities in finite open chains is presented. Comparison with exact Bethe Ansatz results demonstrates the unprecedented accuracy of our method. Our results show that the photoemission spectrum of the quasi-one-dimensional conductor TTF-TCNQ provides evidence for spin-charge separation on the scale of the conduction band width.Comment: REVTEX, 4 pages including 4 EPS figures (changed); correct chemical potential used to define excitation energies in figures and tex

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

A model for the phase separation controlled by doping and the internal chemical pressure in different cuprate superconductors

In the framework of a two-band model, we study the phase separation regime of different kinds of strongly correlated charge carriers as a function of the energy splitting between the two sets of bands. The narrow (wide) band simulates the more localized (more delocalized) type of charge carriers. By assuming that the internal chemical pressure on the CuO$_2$ layer due to interlayer mismatch controls the energy splitting between the two sets of states, the theoretical predictions are able to reproduce the regime of phase separation at doping higher than 1/8 in the experimental pressure-doping-$T_c$ phase diagram of cuprates at large microstrain as it appears in overoxygenated La$_2$CuO$_4$.Comment: 8 pages, 5 figures, submitted to Phys. Rev.

Climate or rural development policy?

Being heavily energy dependent, it is not much of a surprise that Europe pays special attention to reducing the use of fossil fuels. Each one of the ten new member states is characterized by relatively low per capita energy consumption and relatively low energy efficiency, and the share of renewables in their energy mix tends to be low, too. The paper examines the problem when policy measures create a decrease in environmental capital instead of an increase. In this case it hardly seems justified to talk about environmental protection. The authors describe a case of a Hungarian rapeseed oil mill which would not be of too much interest on its own but given that almost all similar plants went bankrupt, there are some important lessons to learn from its survival. The enterprise the authors examined aimed at establishing a micro-regional network. They completed a brown-field development to establish a small plant on the premises of a former large agricultural cooperative. By partnering with the former employees and suppliers of the sometime cooperative, they enjoyed some benefits which all the other green-field businesses focusing on fuel production could not. The project improved food security, energy security and population retention as well

Spin-Charge Decoupling and Orthofermi Quantum Statistics

Currently Gutzwiller projection technique and nested Bethe ansatz are two main methods used to handle electronic systems in the $U$ infinity limit. We demonstrate that these two approaches describe two distinct physical systems. In the nested Bethe ansatz solutions, there is a decoupling between the spin and charge degrees of freedom. Such a decoupling is absent in the Gutzwiller projection technique. Whereas in the Gutzwiller approach, the usual antisymmetry of space and spin coordinates is maintained, we show that the Bethe ansatz wave function is compatible with a new form of quantum statistics, viz., orthofermi statistics. In this statistics, the wave function is antisymmetric in spatial coordinates alone. This feature ultimately leads to spin-charge decoupling.Comment: 12 pages, LaTex Journal_ref: A slightly abridged version of this paper has appeared as a brief report in Phys. Rev. B, Vol. 63, 132405 (2001

Strong-coupling expansions for the anharmonic Holstein model and for the Holstein-Hubbard model

A strong-coupling expansion is applied to the anharmonic Holstein model and to the Holstein-Hubbard model through fourth order in the hopping matrix element. Mean-field theory is then employed to determine transition temperatures of the effective (pseudospin) Hamiltonian. We find that anharmonic effects are not easily mimicked by an on-site Coulomb repulsion, and that anharmonicity strongly favors superconductivity relative to charge-density-wave order. Surprisingly, the phase diagram is strongly modified by relatively small values of the anharmonicity.Comment: 34 pages, typeset in ReVTeX, 11 encapsulated postscript files include

Towards analytic description of a transition from weak to strong coupling regime in correlated electron systems. I. Systematic diagrammatic theory with two-particle Green functions

We analyze behavior of correlated electrons described by Hubbard-like models at intermediate and strong coupling. We show that with increasing interaction a pole in a generic two-particle Green function is approached. The pole signals metal-insulator transition at half filling and gives rise to a new vanishing ``Kondo'' scale causing breakdown of weak-coupling perturbation theory. To describe the critical behavior at the metal-insulator transition a novel, self-consistent diagrammatic technique with two-particle Green functions is developed. The theory is based on the linked-cluster expansion for the thermodynamic potential with electron-electron interaction as propagator. Parquet diagrams with a generating functional are derived. Numerical instabilities due to the metal-insulator transition are demonstrated on simplifications of the parquet algebra with ring and ladder series only. A stable numerical solution in the critical region is reached by factorization of singular terms via a low-frequency expansion in the vertex function. We stress the necessity for dynamical vertex renormalizations, missing in the simple approximations, in order to describe the critical, strong-coupling behavior correctly. We propose a simplification of the full parquet approximation by keeping only most divergent terms in the asymptotic strong-coupling region. A qualitatively new, feasible approximation suitable for the description of a transition from weak to strong coupling is obtained.Comment: 17 pages, 4 figures, REVTe

Validation of a laboratory method for evaluating dynamic properties of reconstructed equine racetrack surfaces.

BackgroundRacetrack surface is a risk factor for racehorse injuries and fatalities. Current research indicates that race surface mechanical properties may be influenced by material composition, moisture content, temperature, and maintenance. Race surface mechanical testing in a controlled laboratory setting would allow for objective evaluation of dynamic properties of surface and factors that affect surface behavior.ObjectiveTo develop a method for reconstruction of race surfaces in the laboratory and validate the method by comparison with racetrack measurements of dynamic surface properties.MethodsTrack-testing device (TTD) impact tests were conducted to simulate equine hoof impact on dirt and synthetic race surfaces; tests were performed both in situ (racetrack) and using laboratory reconstructions of harvested surface materials. Clegg Hammer in situ measurements were used to guide surface reconstruction in the laboratory. Dynamic surface properties were compared between in situ and laboratory settings. Relationships between racetrack TTD and Clegg Hammer measurements were analyzed using stepwise multiple linear regression.ResultsMost dynamic surface property setting differences (racetrack-laboratory) were small relative to surface material type differences (dirt-synthetic). Clegg Hammer measurements were more strongly correlated with TTD measurements on the synthetic surface than the dirt surface. On the dirt surface, Clegg Hammer decelerations were negatively correlated with TTD forces.ConclusionsLaboratory reconstruction of racetrack surfaces guided by Clegg Hammer measurements yielded TTD impact measurements similar to in situ values. The negative correlation between TTD and Clegg Hammer measurements confirms the importance of instrument mass when drawing conclusions from testing results. Lighter impact devices may be less appropriate for assessing dynamic surface properties compared to testing equipment designed to simulate hoof impact (TTD).Potential relevanceDynamic impact properties of race surfaces can be evaluated in a laboratory setting, allowing for further study of factors affecting surface behavior under controlled conditions

CROPSTATUS--A Computer Program to Assess the Effects of Seasonal Weather Changes on Nebraska\u27s Agriculture

CROPSTATUS is a series of programs residing in Nebraska\u27s AGNET system using daily weather data to assess seasonal changes in crops, livestock, and other agricultural conditions. Assessments are based on parameters developed from accumulations of current daily temperature and precipitation data collected from a network of synoptic, climate, and automated micrometeorological stations in Nebraska in comparison with daily normals. The daily normals were derived from monthly summaries using multiple regression models to compute daily values as a function of Julian day numbers. Crop phenology models based on growing degree days were used to monitor and forecast the progress of different crop strains and times of planting. Biological time scale statistical yield models are used for production estimates. Weather probability information is also available from CROPSTATUS. Long term climatic records have been used to determine spring and autumn freeze probabilities, preseason precipitation available for subsoil moisture recharge and the probabilities of weekly averages of daily maximum and minimum temperatures. These and other features are available in a menu of over 20 different agricultural weather information items developed from a network of 60 weather stations. CROPSTATUS is also used to prepare tabular data and computer maps showing changes in conditions throughout the state. These maps are used in meetings by an interdisciplinary committee of agricultural extension specialists to prepare weekly agweather situation/advisory reports

Variational Cluster Perturbation Theory for Bose-Hubbard models

We discuss the application of the variational cluster perturbation theory (VCPT) to the Mott-insulator--to--superfluid transition in the Bose-Hubbard model. We show how the VCPT can be formulated in such a way that it gives a translation invariant excitation spectrum -- free of spurious gaps -- despite the fact that if formally breaks translation invariance. The phase diagram and the single-particle Green function in the insulating phase are obtained for one-dimensional systems. When the chemical potential of the cluster is taken as a variational parameter, the VCPT reproduces the dimension dependence of the phase diagram even for one-site clusters. We find a good quantitative agreement with the results of the density-matrix renormalization group when the number of sites in the cluster becomes of order 10. The extension of the method to the superfluid phase is discussed.Comment: v1) 10 pages, 6 figures. v2) Final version as publishe