ELFT HÍRADÓ

[Mihaly Gyorgy <mihaly AT szfki.hu>]

                                 MEGHIVÓ
a BME Fizikai Intézet, Fizika Tanszék szemináriumára

                   Prof. C. A. Kuntscher

Filling of the Mott-Hubbard gap and structural phase transitions
in the oxyhalides TiOCl and TiOBr induced by external pressure

University of Augsburg
http://www.physik.uni-augsburg.de/exp2/main.en.shtml

Az elõadás helye: BME Fizikai Intézet szemináriumi szoba
Budafoki út 8. F-épület, III. lépcsõház, magasföldszint

Ideje: 2009. február 5. csütörtök, 10:00.

Abstract:
The titanium oxyhalides TiOCl and TiOBr are low-dimensional materials which 
undergo unconventional transitions to a spin-Peierls state via a structural 
incommensurate state.
With the electronic configuration 3d^1 these compounds are Mott-Hubbard 
insulators with a charge gap of approximately 2 eV. They have been 
discussed to exhibit a resonating valence bond state and high-temperature 
superconductivity upon doping. However, despite several attempts, up to now 
a metallization of TiOX (X= Cl, Br) upon doping was not successful.

Our recent pressure-dependent infrared spectroscopic investigations on TiOX 
[1] suggest that the application of external pressure is an alternative way 
to induce an insulator-to-metal transition in TiOX. By infrared 
microspectroscopy we could show the filling of the Mott-Hubbard gap at the 
critical pressure 16 GPa (14 GPa) for TiOCl (TiOBr) at room temperature. 
According to our pressure-dependent x-ray powder-diffraction data, the gap 
closure coincides with a structural phase transition. These findings are 
discussed in the context of recent electric transport experiments on TiOCl 
under pressure [2] and theoretical predictions of pressure-induced phase 
transitions in TiOCl by an ab initio molecular dynamics study [3].

[1] C. A. Kuntscher et al., Phys. Rev. B 74, 184402 (2006); C. A. Kuntscher 
et al., Phys. Rev. B 76, 24110 (R) (2007); C. A. Kuntscher et al., Phys. 
Rev. 78, 035106 (2008).
[2] Forthaus et al., Phys. Rev. B 77, 165121 (2008).
[3] Y-Z. Zhang et al., Phys. Rev. Lett. 101, 136406 (2008).

G. Mihaly
Department of Physics
Budapest University of Technology and Economics
http://dept.phy.bme.hu/