Study of orbital excitation, phase separation and quantum spin liquid state in correlated electronic systems: A Raman spectroscopic study

Raman spectroscopy has proven to be a valuable tool to investigate the dynamics of correlated electrons offering unique insight into coupled dynamics of magnetic, electronic and lattice degrees of freedom, able to prove local development of charge, orbital, spin ordering. Moreover, Raman scattering is also able to detect changes in the electronic band topology and quantum spin liquid state [1, 2]. 
Colossal dielectric material CaCu3Ti4O12 has been investigated by detailed low temperature polarized Raman spectroscopy. Based on our study, a new phenomenological description is proposed to explain colossal dielectric constant. An evidence of orbital order-disorder transition is found around 100K and above this temperature, orbital ordered (insulating) phase coexist with orbital disordered (metallic) phase at nanoscales [3]. Ag(1) Raman mode of the system shows asymmetric Fano- line shape owing to composite electron-phonon scattering due to onset of metallic fractions in the system. Temperature evolution of Fano line shape affirms existence of nanoscale phase-separation and prominence of orbitally disrupted metallic regions above 100K [4]. The possibility of twin domain structure as an origin for electrical heterogeneity is also ruled out by polarized Raman spectroscopy and Raman mapping [5].  
Structural aspect of coexisting phases in prototypical phase separated La5/8−yPryCa3/8MnO3 system has been explored by low temperature Raman measurements and Raman mapping that revealed that charge-ordered insulating and ferromagnetic metallic phases are structurally dissimilar and possess P21/m and R-3C like symmetries respectively [6]. 
Spin orbit induced coupling of lattice vibration and magnetic interaction is studied in Na2IrO3 system by Raman scattering measurements. By exploiting group theory, total -point phonon modes of the system are calculated; all Raman active modes are assigned by their polarization characteristics. Low temperature Raman scattering traversing magnetic transition temperature is employed to explore the temperature evolution of phonon modes and its correlation to magnetic order. One of the phonon mode Ag(3) exhibits magnetic order induced renormalisation of phonon frequencies owing to spin-phonon coupling. At low temperatures, signature of quantum spin liquid is evidenced [7].
1.	Achintya Bera et al.  Phys. Rev. Lett. 110, 107401 (2013).
2.	J. Knolle et al., arXiv:1406.3944 (2014).
3.	D K Mishra et al., J. Phys.: Condens. Matter 23 072203 (2011).
4.	D K Mishra et al., J. Phys.: Condens. Matter 24 252202 (2012).
5.	A Ahalawat, D K Mishra et al., J. Phys.: Condens. Matter 25,  025902 (2013).
6.	D K Mishra et al., (Under Review App. Phys. Letter)
7.	S N Gupta, D K Mishra, et al.,  arXiv:1408.2239v1.