scholarly journals CORRELATED MEAN FIELD ANSATZ FOR THE KONDO NECKLACE

1994 ◽  
Vol 08 (10) ◽  
pp. 1355-1373
Author(s):  
H.Y. KEE ◽  
P. FAZEKAS
Keyword(s):  
Phase Diagram ◽  
Strong Coupling ◽  
Ground State ◽  
Mean Field ◽  
Spin Correlation ◽  
Ground State Phase Diagram ◽  
Field Solution ◽  
Trial State

We study the ground state phase diagram of the pseudospin model introduced by Doniach to describe the essential physics of Kondo lattices. We use variational trial states which augment the usual mean-field solution by incorporating various intersite correlations. A composite spin correlation describing the antiparallel alignment of fluctuating triplets is found to be particularly favorable for large Kondo couplings. With this trial state, the magnetic-to-Kondo transition is suppressed and the strong coupling ground state is ordered with strongly reduced moments. The relevance of the findings is discussed.

scholarly journals Mean-field Solution of the mixed spin-1 and spin-5/2Ising system with different single-ion anisotropies

2009 ◽  
Vol 6 (4) ◽  
pp. 784-789
Author(s):  
Baghdad Science Journal
Keyword(s):  
Phase Diagram ◽  
Ground State ◽  
Mean Field ◽  
Field Approximation ◽  
Mean Field Approximation ◽  
Two Dimensional ◽  
Ground State Phase Diagram ◽  
Mixed Spin ◽  
Bogoliubov Inequality ◽  
The Mean

The mixed-spin ferrimagnetic Ising system consists of two-dimensional sublattices A and B with spin values and respectively .By used the mean-field approximation MFA of Ising model to find magnetism( ).In order to determined the best stabile magnetism , Gibbs free energy employ a variational method based on the Bogoliubov inequality .The ground-state (Phase diagram) structure of our system can easily be determined at , we find six phases with different spins values depend on the effect of a single-ion anisotropies .these lead to determined the second , first orders transition ,and the tricritical points as well as the compensation phenomenon .

η-PAIRING SUPERCONDUCTIVITY IN THE NEGATIVE-U HUBBARD MODEL: EFFECT OF FLUCTUATIONS AND DISORDER

1994 ◽  
Vol 08 (15) ◽  
pp. 2041-2058
Author(s):  
JÜRGEN STEIN
Keyword(s):  
Phase Diagram ◽  
Asymptotic Expansion ◽  
Hubbard Model ◽  
Strong Coupling ◽  
Mean Field ◽  
Higher Order ◽  
Field Phase ◽  
Effective Pair ◽  
The Mean ◽  
Pair Hopping

We have studied the influence of singular fluctuations around the mean-field solution as well as higher order contributions to the geometry controlled asymptotic expansion of the propagators on η-pairing superconductivity in the strong coupling negative-U Hubbard model in the presence of unpaired electrons. The modifications of the mean-field results due to the introduction of disorder and allowance for finite U are also calculated. Besides the singularities already present in the O(2) nonlinear σ-model we find a filling-depending singular depression of the repulsive effective pair-hopping interaction which strongly alters the mean-field phase diagram and appears to suppress the η-pairing phase near the band edges.

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