8-9th August, 2008 School of Materials Science & Nanotechnology,Jadavpur University
Metastable orbital ordered state in nanowires (~20-50 nm) of LaMnO3
Parthasarathi Mondal and Dipten Bhattacharya,*
Sensor & Actuator Division, Central Glass and Ceramic Research Institute, CSIR,
Kolkata 700032
Dipshikha Bhattacharya† and Omprakash Chakrabarti
Non-Oxide and Ceramic Composite Division, Central Glass and Ceramic Research Institute, CSIR, Kolkata 700032
The study of phase stability and transition in confined geometry (e.g., nanoscale system) assumes significance both from the thermodynamic and kinetic points of view because of the fundamentally new physics associated with it. It is all the more important to address this issue as its detailed understanding is intimately related to the design and synthesis of nanoscale systems with different architectures capable of offering novel application opportunities. Although, this has been addressed theoretically more than 30 years back [1], direct measurement of phase stability and transition enthalpy as well as kinetics is being reported [2-4] only very recently. Several novel approaches – such as scanning tunneling microscopy together with perturbed angular correlation [2], laser irradiation coupled with local calorimetry [3], ultrasensitive nanocalorimetry [4] etc. – have been adopted for measuring the melting point and latent heat of melting in nanoscale atomic clusters. It has been observed that nanoscale system offers fundamental instabilities (e.g., Rayleigh instability [5]) which give rise not only to a precipitous decrease in latent heat and melting point but also to a sharp rise in conductivity noise. This, in turn, limits the application potential of such systems in nano-electronics based devices. We need to find out the origin as well as the size limit within which such instabilities can severely restrict the application potential of the nanoscale systems. Deeper understanding of the size related phase fluctuation will help in designing more stable and novel nano-architectures suitable for different applications.
The strongly correlated electron systems – such as perovskite manganites – with active 3d orbitals and orbital degeneracy are currently occupying the centre stage of condensed matter and materials physics research. The electronic phases, e.g., charge, spin, and orbital are intimately coupled with each other and with or without significant influence from the lattice degrees of freedom they organize in several commensurate or incommensurate patterns across the entire phase diagram of the manganites [6]. Such patterns govern the metal-insulator transition, colossal magnetoresistance, colossal electroresistance, ferroelectric instability across magnetic transition point, electronic ferroelectricity etc. in a series of these systems. In pure LaMnO3, the orbital order superstructure develops as a result of degeneracy in eg levels in Mn3+O6 octahedra which is lifted via Jahn-Teller effect and ultimately gives rise to cooperative ordering of d3x2-r2 and d3y2-r2 orbitals within the ab-plane and their staggered ordering along the c-axis (so-called ‘d’-type orbital order). Such orbitals result from mixing of Jahn-Teller split eg orbitals – dx2-y2 and dz2. The d-type orbital ordered structure is strongly influenced by tilt of Mn3+O6 octahedra as La-site is progressively replaced by smaller rare-earth ions – Pr, Nd, Sm, Gd, Tb, Dy etc. The orbital ordered structure undergoes an order-disorder transition at a characteristic transition point TOO. This is often accompanied by a structural transition as well – from O’ orthorhombic to O*/O orthorhombic. It is important to find out whether such orbital order-disorder transition can be triggered by application of other stimulations – light, magnetic or electric field etc.
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*corresponding author; dipten@cgcri.res.in
†currently at the Department of Chemistry, Indian Institute of Technology, Kharagpur 721302
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