Possible Inverse Isotope effect in High Tc Superconductors Using the Non Variational Quasi-particles Formulation

IOSR Journal of Applied Physics (IOSR-JAP)
e-ISSN: 2278-4861.Volume 7, Issue 1 Ver. I (Jan.-Feb. 2015), PP 27-30
www.iosrjournals.org
DOI: 10.9790/4861-07112730 www.iosrjournals.org 27 | Page
Possible Inverse Isotope effect in High Tc Superconductors Using
the Non Variational Quasi-particles Formulation
1
Chijioke P. Igwe, 2
Nwakanma Mike, 3
Oguazu E. Chinenye, 4
E. G. Nneji
Department of Industrial Physics, Renaissance University Ugbawka,Enugu
Department of Physics and Astronomy, University of Nigeria, Nsukka, Nigeria,
Abstract: This theory proposes a possible isotope shift in high Tc superconductors. The attractive electron-
electron pairing in the non-variational quasi-particles Hamiltonian formulation by Bogoliubov and Valatin
leads due to deeply overlap of electron wave functions around
Z
Fe ion, providing an initiation for the
covalent mixing of electron wave functions to form a singlet pair in the superconductivity regime. The
mathematical solution yields a reversal of signs (stable and unstable energy) in the solution of the quadratic
function and leads to negative isotope effect exponent.
I. Introduction
The Bardeen, Cooper and Scheriffer (BCS)[1] bilinear model Hamiltonian (BMH) is of the form;
  1  







ks k
kkkkkkkkksksksm bCCCCCCH 
    2
1
11
1
1   
bVCCVH kkkk
Since this form of mean field Hamiltonian is bilinear in the creation and annihilation operators. We diagonalized
by using a linear canonical transformation of these operators introduced by Bogoliubov and Valatin (BV)[2-4].




 11 , kkkokkkkkokk
UVCVUC  3
1, kko  are fermions annihilation operators,the coefficient kk VU , are chosen to make the Hamiltonian
diagonal and also for the coefficients of

10 kk  and 01 kk  in the MH to vanish and are required to satisfy
412222
 kkkkkk UVVUVU
Putting Eqn(3) into Eqn(1), upon simplification, we obtain
        
     
 




k
kk
k
kkkokkkkokk
ks k
kkkokkkkokkkkkokkkkokksm
bUVVU
VUUVUVVUH
511
1111


The Hamiltonian is diagonalized if we select kU and kV so that the co-efficient of

10 kk  and 01 kk 
vanish. This means that the Hamiltonian is carried into a pair containing only constant plus terms proportional to
the occupation number

1kko . The coefficient of undesired terms is zero, than get
    7)1(20
2
  
k
Kkkkkk
ks
kks VUVUV

Possible Inverse Isotope effect in High Tc Superconductors Using the Non Variational …
form;theoffunctionquadraticayieldstionsimplificaandgrearrangin,
U
tion withmultiplicaUpon
1,applyingand,bysidesbothgMultiplyin
2
k
k
2



   dkVdkVVdkUUdkVU kkk
k
kkkk
802
2












k
k
k
kk
k
k
U
V
U
V

Resolving Eqn (8) by completing the square method,
11,
10
2
2
9
2
2
22
2
22
2
222
2
k
kk
k
k
k
k
k
kk
k
k
k
k
k
kk
k
k
k
k
k
k
k
k
k
k
k
k
U
V
U
V
U
V
U
V
U
V






































Observing the math’s solutions[4-6] reveals the existence reversal of signs in k and kE arise naturally from
the method of solution of the quadratic function resulting to the stable and the unstable solutions of the energy
    12,
222222
kkkkkkk
k
k
E
U
V







The negative sign is chosen in other to constructively criticized, challenged the statement that the positive sign
corresponds to the stable state solution of energy and not the negative. kE , gives the energy of excitation, k
plays the role of an energy gap or minimum(maximum) energy excitation since at the Fermi level where
,0k 0 kkE . Applying
k
k
kk
E
UV
2

 ,
kk
k
k
k
k
kk
k
k
kk
EV
UE
U
V
VU







 ,,1 22
to get the unstable square solution in kk UV , and
the equivalent solution in BCS we obtain
163
2
1
,3
2
1
,
22
3
22
1
1
15
2
1
2
1
2
1
222
2











































k
k
k
k
k
k
k
k
k
k
k
k
k
k
k
k
k
k
kk
k
k
k
k
k
k
k
E
V
E
U
EE
U
E
E
E
EU
V
E
U


While Eqn (15) and Eqn (16) differ slightly with the result of BSC in signs and in number, It is also greater than
the stable solution of Eqn (17) ( which agrees very well with BCS ) by the factor 3.
171
2
1
,1
2
1 22













k
k
k
k
k
k
E
V
E
U

The diagonalized model Hamiltonian becomes
    181100 

k
kkkkk
k
kkkkm EbEH 

The first sum in the equation of the band is constant which differ from the corresponding sum for the normal
state. T = 0, kk   , 0k by exactly the condensation energy )0()0(
2
1
)0()0(  NUU ns
,
U(T) = internal energy, 0k is the energy gap at T=0. The second sum gives the increase in energy above
the ground state in terms of the number operators for Fermions.   kkk CCb applying the values of
 kk
CC into the equation and dropping off-diagonal terms in quasi-particle operator. Equation emphasizes
the rudiment of the superconductivity model for the superconducting iron oxy-pnictide materials, attractive
electron-electron pairing due to deep overlapping of electron wave functions around
Z
Fe ion of effective
valence (Z), provides an initiation for the covalent mixing of electron wave functions to form a singlet pair,
 kk
CC HM[7-9].
191 1100  
 kkkkkkkkk VUCCb 
While the energy gap expressions are
201 1100
1
111
1
1
1
111  
  kkkkkk
k
kkk
k
k
k
kkk VUVCCVbV 
At T= 0, equations reduces to Eqn (1) but at T > 0, the probability of a fermions quasi-particle with excitation
energy, kE is the Fermi- Dirac distribution,   1
1exp)(

 kkEF 
  21)(21
2 1
1
1
1 k
k
kk EF
E
V 

 
k , being temperature dependent, the integral equation has a trivial solution for 0k which corresponds
to the normal state (NS). Non trivial solution exists if the NS is unstable and the system becomes
superconducting. The equation above gives the superconducting state as long as the gap parameter for  is
non-zero.
 kkk ,1
, approaching this equation Coopers way[10]
22
,0
,, !
1



 

otherwise
V
V kk
kk
 
Where  , sph VVV is constant less than unity. For weak superconductors, N(0)V << 1 and
N(0)V >> 1 for strong coupling superconductors. Changing the sum over k to the density of states )( kN 
   
23
2
2/tanh
)(
2
2/tanh
1
, k
ck
k
k k
ck TK
NV
TK
V
D
Dk




 



 



D , is Debye cut-off frequency such that[11] DD K   . Since the density of state in the NS varies little
within an energy D of the Fermi level and also )( kN  can be approximated by the constant value N(0) at
the Fermi level
 
24
2
2/tanh2
)0(1
0

D
ck TKd
VN





Upon simplification of Eqn(24 ), we obtain the gap equation, critical temperature and apply it to weak
coupling limits.
  25
6.13
exp2
)(
,exp13.1 




 



ZTK
T
T
c
effDc


Since isotope effect exponent is as a results of the discovery of the interaction of electron-electron in the self
consistence phonon mediated superconductivity. Hence, it value for iron pnictides can be got from the transition

temperature or the gap equations. Using this relation [12],
2
1







m
K
w

 and solving with Eqn(25), we obtain
that
  26exp)(13.1 2
1
2
1
2
1









 cmmT effc 
Therefore,  









effC  exp)(13.1 2
1

Energy gap, transition temperature and isotope effect exponent can be predicted in the iron based
superconducting materials from Eqn(25) and Eqn(26).Thus, isotope effect exponent is BCS and negative (-0.2).
II. Summary and Conclusion
This theory shows that mathematical calculations of energy gap, transition temperature, isotope effect
exponent and assumes from mathematical deductions that coherent length, penetration depth, specific heat and
other properties have negative values. These contradict experimental and theoretical findings. However, It
remains valid for property, such as isotope effect exponent and in agreement with theories and experimental
predictions of isotope shift in iron pnictides and cuprates.
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Possible Inverse Isotope effect in High Tc Superconductors Using the Non Variational Quasi-particles Formulation

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