1,788 research outputs found
Exact ground states of generalized Hubbard models
We present a simple method for the construction of exact ground states of
generalized Hubbard models in arbitrary dimensions. This method is used to
derive rigorous criteria for the stability of various ground state types, like
the -pairing state, or N\'eel and ferromagnetic states. Although the
approach presented here is much simpler than the ones commonly used, it yields
better bounds for the region of stability.Comment: Revtex, 8 page
The Influence of Transnational Actors on the Enforcement of Sanctions Against Rhodesia
When the Rhodesian Government in November 1965 declared independence from Great Britain, first the British Government and then the United Nations imposed economic sanctions against Rhodesia to compel that Government to return to legality, The experience of the past 10 years shows that not only have economic sanctions not brought about the intended political changes, but also that Rhodesia has, in fact, prospered
A Field Experience Involving the Development of a Handbook for Beginning School Board Members
A field study involving the development of a handbook for beginning school board members, by the writer, under the direction of Dr. Matzner of the Eastern Illinois University Department of School Service Personnel, is summarized in this paper. A handbook for beginning school board members was developed to assist them in the difficult areas of school operations as designated by board members now serving school districts in central Illinois.
The field study involved gathering information from boards of education on the problems they encountered as beginning school board members. The problems were categorized into major topics and a handbook was developed discussing pertinent information for a new board member on each topic.
The author gathered information for the individual topics based on his sixteen years of personal experiences in public education as a teacher and administrator. School board members, superintendents, and instructional staff were used as resource persons to give a wide spectrum of opinion as to the knowledge needed to function efficiently as a school board member. The library served as a valuable resource since much has been written in recent years on the duties, powers, and responsibilities of school board members.
The largest part of the paper is the actual handbook. The topics addressed are duties, powers, and responsibilities of school board members, revenue, employment practices for the district\u27s staff, evaluation, collective bargaining, federal funded programs, expenditures, curriculum, and budget. The topics are presented in such a manner that the average layperson in the community would be able to understand the material.
The writer includes recommended materials from the National School Boards Association and the Illinois School Boards Association as a supplement on the various topics. Tables, charts, and data were selected for use, by the writer, from the Illinois State Board of Education.
The handbook prepared is an attempt to present items which school board members designated as being important when they were first elected to the board of education. The information should serve only as a starting point for new board members and should not be intended to serve as a comprehensive guide for all board members
Ferromagnetism in Correlated Electron Systems: Generalization of Nagaoka's Theorem
Nagaoka's theorem on ferromagnetism in the Hubbard model with one electron
less than half filling is generalized to the case where all possible
nearest-neighbor Coulomb interactions (the density-density interaction ,
bond-charge interaction , exchange interaction , and hopping of double
occupancies ) are included. It is shown that for ferromagnetic exchange
coupling () ground states with maximum spin are stable already at finite
Hubbard interaction . For non-bipartite lattices this requires a hopping
amplitude . For vanishing one obtains as in
Nagaoka's theorem. This shows that the exchange interaction is important
for stabilizing ferromagnetism at finite . Only in the special case
the ferromagnetic state is stable even for , provided the lattice allows
the hole to move around loops.Comment: 13 pages, uuencoded postscript, includes 1 table and 2 figure
Cloning and characterisation of a maize carotenoid cleavage dioxygenase (ZmCCD1) and its involvement in the biosynthesis of apocarotenoids with various roles in mutualistic and parasitic interactions
Colonisation of maize roots by arbuscular mycorrhizal (AM) fungi leads to the accumulation of apocarotenoids (cyclohexenone and mycorradicin derivatives). Other root apocarotenoids (strigolactones) are involved in signalling during early steps of the AM symbiosis but also in stimulation of germination of parasitic plant seeds. Both apocarotenoid classes are predicted to originate from cleavage of a carotenoid substrate by a carotenoid cleavage dioxygenase (CCD), but the precursors and cleavage enzymes are unknown. A Zea mays CCD (ZmCCD1) was cloned by RT-PCR and characterised by expression in carotenoid accumulating E. coli strains and analysis of cleavage products using GC¿MS. ZmCCD1 efficiently cleaves carotenoids at the 9, 10 position and displays 78% amino acid identity to Arabidopsis thaliana CCD1 having similar properties. ZmCCD1 transcript levels were shown to be elevated upon root colonisation by AM fungi. Mycorrhization led to a decrease in seed germination of the parasitic plant Striga hermonthica as examined in a bioassay. ZmCCD1 is proposed to be involved in cyclohexenone and mycorradicin formation in mycorrhizal maize roots but not in strigolactone formatio
Determining ethylene group disorder levels in -(BEDT-TTF)Cu[N(CN)]Br
We present a detailed structural investigation of the organic superconductor
-(BEDT-TTF)Cu[N(CN)]Br at temperatures from 9 to 300 K.
Anomalies in the dependence of the lattice parameters are associated with a
glass-like transition previously reported at = 77 K. From structure
refinements at 9, 100 and 300 K, the orthorhombic crystalline symmetry, space
group {\it Pnma}, is established at all temperatures. Further, we extract the
dependence of the occupation factor of the eclipsed conformation of the
terminal ethylene groups of the BEDT-TTF molecule. At 300 K, we find 67(2) %,
with an increase to 97(3) % at 9 K. We conclude that the glass-like transition
is not primarily caused by configurational freezing-out of the ethylene groups
Spintronic properties of one-dimensional electron gas in graphene armchair ribbons
We have investigated, using effective mass approach (EMA), magnetic
properties of a one-dimensional electron gas in graphene armchair ribbons when
the electrons of occupy only the lowest conduction subband. We find that
magnetic properties of the one-dimensional electron gas may depend sensitively
on the width of the ribbon. For ribbon widths , a critical point
separates ferromagnetic and paramagnetic states while for
paramagnetic state is stable ( is an integer and is the length of
the unit cell). These width-dependent properties are a consequence of
eigenstates that have a subtle width-dependent mixture of and
states, and can be understood by examining the wavefunction
overlap that appears in the expression for the many-body exchange self-energy.
Ferromagnetic and paramagnetic states may be used for spintronic purposes.Comment: 5 pages, 6 figure
Improved stability regions for ground states of the extended Hubbard model
The ground state phase diagram of the extended Hubbard model containing
nearest and next-to-nearest neighbor interactions is investigated in the
thermodynamic limit using an exact method. It is found that taking into account
local correlations and adding next-to-nearest neighbor interactions both have
significant effects on the position of the phase boundaries. Improved stability
domains for the -pairing state and for the fully saturated ferromagnetic
state at half filling have been constructed. The results show that these states
are the ground states for model Hamiltonians with realistic values of the
interaction parameters.Comment: 21 pages (10 figures are included) Revtex, revised version. To be
published in Phys. Rev. B. E-mail: [email protected]
Temperature and filling dependence of the superconducting -phase in the Penson-Kolb-Hubbard model
We investigate in the Hartree Fock approximation the temperature and filling
dependence of the superconducting -phase for the Penson-Kolb-Hubbard
model. Due to the presence of the pair-hopping term, the phase survives for
repulsive values of the on-site Coulomb interaction, exhibiting an interesting
filling and temperature dependence. The structure of the self-consistent
equations peculiar to the -phase of the model allows to explicitly solve
them for the chemical potential. The phase diagrams are shown and discussed in
dimension 2 and 3. We also show that, when a next-nearest neighbours hopping
term is included, the critical temperature of the superconducting region
increases, and the corresponding range of filling values is shifted away from
half-filling. Comparison with known exact results is also discussed.Comment: 20 pages, REVTEX, 8 eps figure
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