[Pw_forum] [Fwd: paramagnetic structure]

[degironc]

Dear  Subhradip Ghosh,

This is the problem of describing a disordered system (magnet,  alloy, 
... ) using a periodic system.
A classical way is to assume a model, Ising or more refined one like in 
the cluster expansion idea
[De Fontaine in Solid State Phys vol 51,  Phys. Rev. B 36, 4163-4185 
(1987) or any of the yearly
encyclicals by Alex Zunger on the subject]  calculate a few 
configurations that allow you to extract
interaction parameters defining the model and solve the model. This 
provides you with the phase
diagram of your system, thus much more than just a single total energy.

If you wish to simulate the disordered state the Special Quasi-random 
Stractures (SQS) idea is a
rather clever one [ Phys. Rev. Lett. 65, 353 (1990) and Phys. Rev. B 42, 
9622  (1990)].

One important point in favour of supercell calculations (whether used to 
extract model parameters or
directly in SQS) vs KKR or LMTO CPA-like calculations is that they take 
into accounts the relaxation
around atoms in a proper way, which is usually not the case for KKR or 
LMTO CPA-like calculatons.

Notice that if coupling with lattice relaxation is important then the 
interatomic interaction constants can
be very long ranged [Phys. Rev. Lett.  66, 2116 (1991),  Phys. Rev. 
Lett. 72, 4001 (1994)].

Hope this helps,

  stefano

Subhradip Ghosh wrote:

>Thanks Davide. Therefore, looks like there is no simple way to simulate PM
>phase with espresso. My question originated from the fact that in KKR
>based calculations people do simulate the PM phase by the so called
>'disordered local moment' model. I wanted to know if something similar has
>been done within espresso.
>
>Regards,
>
>Subhradip
>
>
>
>  
>
>>Subhradip Ghosh wrote:
>>    
>>
>>>Dear Davide,
>>>
>>>All I wanted to know is the following thing: How does espresso capture
>>>the
>>>randomness in local moments in paramagnetic phase?
>>>      
>>>
>>Ah, that's a different story! that's the right question!
>>
>>All calculations are zero temperature and they are like a snapshot of the
>>system, with all spin frozen.
>>
>>At any finite T there will be spin excitation (magnons). In the past
>>Baroni and Gebauer computed the magnon dispersion in a metal. The role
>>of temperature is to destroy the magnetic order (FM and AFM) and
>>above a transition temperature (T_Curie for FM and T_Neel for AFM) the
>>system will behave as a paramagnet.
>>This is not straightforward to do with a single ground state calculation.
>>
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>>    
>>
>
>
>************************************************************************
>              Dr. Subhradip Ghosh
>              Assistant Professor
>              Department of Physics
>            Indian Institute of Technology
>               Guwahati,Assam-781039
>                    India
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