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["Erik Ylvisaker" <eylvisaker@physics.ucdavis.edu>]

My own understanding of LO-TO splitting is fairly rudimentary, so anyone feel free to correct me where I'm wrong.  But I'll explain what I understand of it.

 

Optical phonons at the gamma point involve shifting of one or more atomic sublattices with respect to another.  In insulating materials, this creates a long range electric field which effectively increases the spring constant (if you want to think of it that way) and frequency of the vibrational mode.  This causes the LO-TO splitting.  It's not an issue in metals because the electric field is screened out.  In the DFPT formalism, this effect is caused by a term which has 1/q in it, which presents problems for a Gamma point calculation, but other q points are treated correctly.  The resolution of the problem is to take the limit at q goes to zero along a particular direction of that term.  This results in LO-TO splitting which depends on direction, which is relevant for, say, a hexagonal system, but for a cubic system like CsCl I'd imagine there's only one relevant splitting.  LO-TO splitting is important to have the phonon dispersion along a particular direction be continuous from q=0 to q != 0

 

You can't switch on LO-TO splitting in abinit while doing the phonon calculation.  You need to use the post-processing code anaddb.  There's some good discussion of how to do this in the second half of the response functions 1 tutorial.

----- Original Message -----

From: zhangtingPKU

To: forum@abinit.org

Sent: Tuesday, May 09, 2006 2:32 AM

Subject: [abinit-forum] What is a LO-TO spliting in phonon calculations?

Dear everyone

 

      When I calculating the phonon at Gamma point of CsCl crystal, I found that there are great mismatch between the results from LO-TO swith on & off calculations.

When the LO-TO is off, the three optical mode at Gamma are degenerate, and the frequency is about 102.7 cm-1; While, when the LO-TO is on(using CASTEP code,for I do not know how to switch on LO-TO calculation in ABINIT), the highest phonon at Gamma rises to 170 cm-1.

What does the LO-TO spliting mean? Is it important in real CsCl system? And, how to switch on LO-TO calculation in ABINIT code?

      Thanks!

 

　　　　　　　　　　　　　　zhangtingPKU

　　　　　　　　　　　　　　zhangting@pku.edu.cn

　　　　　　　　　　　　　　　　　2006-05-09