Mean Field theory for Structural Phase Transition in Y-Ba-Cu-O
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Date
2007-07
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Addis Ababa University
Abstract
The spring-defect-model has been developed earlier for describing the structural phase
transition in YBCO (from tetragonal to orthorhombic at 900K) in high Tc(. 92K)
superconductor is now casted into effective strain-strain interaction Hamiltonian. In
this model the connection between elastic distortion and oxygen ordering is described
using mean field theory. The lattice static of the Cu-O basal plane is treated in
the harmonic approximation in which oxygen and vacancy are modeled via pseudo
spin ˆ S = ±1. The strain model is suitable for describing paraelastic (tetragonal) to
ferroelastic (orthorhombic) transition in YBCO. The orthorhombic phase is known
to be ferroelastic and it develop spontaneous strain due to oxygen ordering in one
direction of lattice.
We present the results for ferroelastic order parameter, static compliance, frequency
dependent compliance and internal friction in terms of all the basic parameter like
spring constants K,K0 distortion parameter , lattice constants a0 of the model. The
signature of phase transition is occurred through ferroelastic and anelastic relaxation
from mean field approximation of the strain-strain model under the influence of an
external in homogeneous stress. These are very important results as far as formation
of tweed textures and mechanical behavior in YBCO is concerned. The model is 2D
as our focus is on the Cu-O basal plane of 3D structures
Our main aim is to describe the statics and kinetics of the ferroelastic transition in
YBCO
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Phase Transition in Y-Ba-Cu-O