[institut] najava seminara Centra za fiziku cvrstog stanja i nove materijale

[Zoran Popovic]

Postovane koleginice i kolege,

U cetvrtak 5.jula 2012, sa pocetkom u 12 casova u citaonici IF/a,

Dr Emil Bozin ce odrzati seminar pod naslovom

Детекција динамичких фероелектричних нанодомена у гвожђе телурид
термоелектрику из мерења неутронским расејањем

Наслов и абстракт на енглеском су

Detection of Dynamic Ferroelectric Nanodomains in Thermoelectric Lead
Telluride by Neutron Scattering Probes

Local symmetry breaking in complex materials is emerging as an 
important
contributor to materials properties but is inherently difficult to 
study. It
was a surprise to find such a symmetry breaking in the therмoelectric 
(TE)
material PbTe that has a simple rock-salt average crystal structure. 
Our
neutron total scattering and atomic pair distribution function (PDF)
analysis reveals the existence of a novel paraelectric state at and 
above
room temperature [1]. However, on cooling the structural dipoles do not
order, but disappear resulting in an undistorted rock-salt 
ground-state. I
will discuss these PDF results and results of our follow-up study of 
the
lattice dynamics using inelastic neutron scattering (INS) [2], which 
shows
that the lattice dynamics are responsive to the local symmetry broken 
phase
observed by PDF, giving not only key insights into the behavior of 
PbTe, but
also revealing INS as a powerful tool for studying local structure. The 
new
result of the INS study is the observation of the unexpected appearance 
on
warming of a new zone center phonon branch in PbTe. In a harmonic solid 
the
number of phonon branches is strictly determined by the contents and
symmetry of the unit cell. The appearance of the new mode appears to 
confirm
a crossover to a dynamic lower symmetry structure with increasing
temperature while no structural transition is seen 
crystallographically. The
observation resembles relaxor ferroelectricity, however, since there 
are no
inhomogeneous dopants in pure PbTe, this anomalous behavior is an 
intrinsic
response of the system. The results cannot be explained within the 
framework
of conventional phase transition theories such as soft-mode theory and
challenges our basic understanding of the physics of materials. We 
suggest
that new thermoelectrics should be sought among materials that, like 
PbTe
[1], are close to a ferroelectric instability.

[1] E.S. Bozin et aI., Science v330, pp1660 (2010).
[2] K.M.0. Jensen et aI., arXiv:1203.5943 (2012).
--


Sedacan pozdrav
Zoran V. Popovic

Institute of Physics Belgrade
Pregrevica 118, 11080 Belgrade, Serbia
http://www.ipb.ac.rs/