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SUMMARY:Electrostatic\, polarisation and redox effects in oxide nano-struc
 tures - Nick Bristowe (Dept. Earth Sciences/Cavendish Lab)
DTSTART:20110314T160000Z
DTEND:20110314T170000Z
UID:TALK29442@talks.cam.ac.uk
CONTACT:Ian Farnan
DESCRIPTION:The interface between LaAlO3 (LAO) and SrTiO3 (STO) has receiv
 ed worldwide attention since the discovery of the interface quasi two-dime
 nsional electron gas (2DEG) in 2004 [1]. It has since been shown that the 
 2DEG\, which can be spin-polarised\, superconducting or charge ordered\, i
 s reversibly switched on or off with an external electric field\, promisin
 g potential applications in field-effect transistors with reduced feature 
 sizes\, and added functionality over conventional semiconductor systems. D
 espite this progress the origin of the 2DEG is still debated\, which is no
 t only important for basic scientific understanding\, but also for the des
 ign and engineering of improved and novel devices. One of the debated mech
 anisms\, called the ‘polar-catastrophe’ [2]\, is based on the result t
 hat LAO has a fixed spontaneous\, but non-switchable\, polarisation of pre
 cisely 0.5 electrons per unit cell [3\,4]\, which requires screening charg
 es at its surfaces via an electronic transfer from the LAO surface to the 
 interface STO Ti 3d conduction band [5]. However several experimental find
 ings have raised doubts to the validity of this explanation: missing inter
 face electrons\, lack of surface holes\, no internal field in thin insulat
 ing films and an inconsistent LAO critical thickness for metallicity.\n	We
  present a paradigm that plausibly explains several pieces to this puzzle.
  It is based on a surface redox reaction (e.g. surface oxygen vacancies) a
 nd a subsequent electronic transfer across the film that screens the LAO p
 olarization charge [6]. The process is studied with a simple model describ
 ing the formation energy of such redox screening processes\, supported by 
 first principles calculations. The model predicts the growth of the densit
 y of related surface defects and hence STO Ti 3d population with LAO thick
 ness\, showing a minimum critical thickness\, in agreement with spectrosco
 pic data. We also show that the surface defects generate trapping potentia
 ls that will affect the mobility of the interface 2DEG. \n	We build upon t
 hese models to include thin films of a ferroelectric material whose polari
 zation can be switched\, unlike LAO. The redox process in these systems ma
 y now not only be responsible for the interface carriers\, but also the fa
 cilitation of ferroelectricity itself\, which is again thickness dependent
 . Results from first principles calculations will be presented.\n\n1.	A. O
 hotmo and H. Hwang\, Nature 427\, 423 (2004)\n2.	N. Nakagawa\, H. Hwang an
 d D. Muller\, Nat. Mater. 5\, 204 (2006)\n3.	M. Stengel and D. Vanderbilt\
 , Phys. Rev. B 80\, 241103 (2009)\n4.	N.C. Bristowe\, P.B. Littlewood and 
 E. Artacho\, J. Phys.: Condens. Matter 23\, 081001 (2011) \n5.	N.C. Bristo
 we\, E. Artacho and P.B. Littlewood\, Phys. Rev. B 80\, 045425 (2009)\n6.	
 N.C. Bristowe\, P.B. Littlewood and E. Artacho\, arXiv:1008.1951 (2010)\n
LOCATION:Harker 2 lecture room\, Department of Earth Sciences\, Downing Si
 te
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