1,699 research outputs found
Spin-Charge Separation and Kinetic Energy in the t-J Model
I show that spin-charge separation in 2-D t-J model leads to an increase of
kinetic energy. Using a sum rule, I derive an exact expression for the lowest
possible KE (E_{bound}) for any state without doubly occupied sites. KE of
relevant slave-boson and Schwinger-boson mean-field states -- which exhibit
complete spin-charge separation -- are found to be much larger than E_{bound}.
Examination of n(k) shows that the large increse in KE is due to excessive
depletion of electrons from the bottom of the band (Schwinger boson) and of
holes from the top (slave boson). To see whether the excess KE is simply due to
poor treatment of the constraints, I solve the constraint problem analytically
for the Schwinger boson case in the J = 0 limit. This restores gauge
invariance, incorrectly violated in MF theories. The result is a generalized
Hartree-Fock state of the Hubbard model, but one that includes spin waves. Even
after constraints are imposed correctly, the KE remains much larger than
E_{bound}. These results support the notion, advanced earlier [PRB 61, 8663
(2000)] that spin-charge separation in the MF state costs excessive KE, and
makes the state unstable toward recombination processes which lead to
superconductivity in d = 2 and a Fermi liquid state in higher dimensions.Comment: 13 pages, LateX plus three figures. To appear in Phys Rev B Typos
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Rat floods and water floods: the ecological and sociological dynamics of rodent management in Bangladesh
Chakma, N., Belmain, S.R., Sarker, N.J., Sarker, S.U., Kamal, N.Q., Sarker, S.K
A strong-coupling expansion for the Hubbard model
We reconsider the strong-coupling expansion for the Hubbard model recently
introduced by Sarker and Pairault {\it et al.} By introducing slave particles
that act as projection operators onto the empty, singly occupied and doubly
occupied atomic states, the perturbation theory around the atomic limit
distinguishes between processes that do conserve or do not conserve the total
number of doubly occupied sites. This allows for a systematic expansion
that does not break down at low temperature ( being the intersite hopping
amplitude and the local Coulomb repulsion). The fermionic field becomes a
two-component field, which reflects the presence of the two Hubbard bands. The
single-particle propagator is naturally expressed as a function of a matrix self-energy. Furthermore, by introducing a time- and
space-fluctuating spin-quantization axis in the functional integral, we can
expand around a ``non-degenerate'' ground-state where each singly occupied site
has a well defined spin direction (which may fluctuate in time). This formalism
is used to derive the effective action of charge carriers in the lower Hubbard
band to first order in . We recover the action of the t-J model in the
spin-hole coherent-state path integral. We also compare our results with those
previously obtained by studying fluctuations around the large- Hartree-Fock
saddle point.Comment: 20 pages RevTex, 3 figure
Acute Pancreatitis Secondary to a Perivaterian Duodenal Diverticular Abscess
A 46-year-old previously fit lady was admitted with acute pancreatitis. She had no history of gallstones. She was not on any medications and consumed minimal amounts of alcohol. On subsequent investigations as to the causative factor, she was found at ultrasound to have an air-fluid filled cystic structure posterior to the head of pancreas which was compressing the common bile duct. Further magnetic resonance imaging and computer tomography scans showed that this cystic lesion was located around the ampulla of Vater. A diagnosis of a perivaterian abscess was made. At endoscopy, a large contained abscess was seen which was successfully drained. She made a full and uneventful recovery
spl(2,1) dynamical supersymmetry and suppression of ferromagnetism in flat band double-exchange models
The low energy spectrum of the ferromagnetic Kondo lattice model on a N-site
complete graph extended with on-site repulsion is obtained from the underlying
spl(2,1) algebra properties in the strong coupling limit. The ferromagnetic
ground state is realized for 1 and N+1 electrons only. We identify the large
density of states to be responsible for the suppression of the ferromagnetic
state and argue that a similar situation is encountered in the Kagome,
pyrochlore, and other lattices with flat bands in their one-particle density of
states.Comment: 7 pages, 1 figur
Canted Ferromagnetism in Double Exchange Model with on-site Coulomb Repulsion
The double exchange model with on-site Coulomb repulsion is considered.
Schwinger-bosons representation of the localized spins is used and two
spin-singlet Fermion operators are introduced. In terms of the new Fermi fields
the on-site Hund's interaction is in a diagonal form and the true magnons of
the system are identified. The singlet fermions can be understood as electrons
dressed by a cloud of repeatedly emitted and reabsorbed magnons. Rewritten in
terms of Schwinger-bosons and spin-singlet fermions the theory is U(1) gauge
invariant. We show that spontaneous breakdown of the gauge symmetry leads to
\emph{\textbf{canted ferromagnetism with on-site spins of localized and
delocalized electrons misaligned}}. On-site canted phase emerges in double
exchange model when Coulomb repulsion is large enough. The quantum phase
transition between ferromagnetism and canted phase is studied varying the
Coulomb repulsion for different values of parameters in the theory such as
Hund's coupling and chemical potential.Comment: 8 pages, 6 figure
Surface magnetic phase transition of the double-exchange ferromagnet: Schwinger-boson mean-field study
The surface magnetic phase transition of a double-exchange model for metallic
manganites is studied using a Schwinger-boson mean-field method. About three
unit-cells wide surface layers are identified. The magnetic moment in these
layers decreases more rapidly than that in the bulk when the temperature is
increased. This behavior is consistent with experimental observations. We also
discuss the implication of this behavior on the tunneling magnetoresistance
effect using manganites and possible improvement of the magnetoresistance
effect near the bulk Curie temperature.Comment: 7 pages, 9 figure
Integration of Remote Sensing Data and GIS Tools for Accurate Mapping of Flooded Area of Kurigram
Abstract Flood is the most devastating disaster in the present world which causes damage to environmental, social, economical and human lives at about 43% of all natural disasters. There are many flood hazard occurs in Bangladesh during the 19 th century and 20 th century in the different regions. These flood hazards have more catastrophic damages of huge area within human lives and other necessary properties of Bangladesh. The first step of flood management is to evaluate the area which is under threat of flood disaster. In this study here showed the importance of Remote Sensing (RS) data and Geographic Information System (GIS) tools to manage the flood related problems. Remote Sensing (RS) data and Geographic Information System (GIS) provide a lot of information to flood disaster management. ArcView GIS software tools are used for digitizing the base map and to create a flood risk zone of Kurigram, Bangladesh where images of remote sensing can be helped to determine the flood inundation areas. The integrated application of RS and GIS techniques for monitoring and flood mapping provides information for the decision makers. The study also grows attentions the need of cost-efficient methodology by creating a flood vulnerable map of Bangladesh
Temperature dependence of the resistivity in the double-exchange model
The resistivity around the ferromagnetic transition temperature in the double
exchange model is studied by the Schwinger boson approach. The spatial spin
correlation responsible for scattering of conduction electrons are taken into
account by adopting the memory function formalism. Although the correlation
shows a peak lower than the transition temperature, the resistivity in the
ferromagnetic state monotonically increases with increasing temperature due to
a variation of the electronic state of the conduction electron. In the
paramagnetic state, the resistivity is dominated by the short range correlation
of scattering and is almost independent of the temperature. It is attributed to
a cancellation between the nearest-neighbor spin correlation, the fermion
bandwidth, and the fermion kinetic energy. This result implies the importance
of the temperature dependence of the electronic states of the conduction
electron as well as the localized spin states in both ferromagnetic and
paramagnetic phases.Comment: RevTex, 4 pages, 4 PostScript figures, To appear in Phys. Rev.
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