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Probing nanoscale phase inhomogeneities in strongly correlated transition metal oxides with ultrafast infrared spectroscopy


The contribution of phase inhomogeneities to various phenomena in transition metal oxides is an intense area of contemporary research. These phase inhomogeneities are believed to play an important role in phenomena including high-temperature superconductivity and colossal magnetoresistance (CMR).  In this work, we use ultrafast infrared (IR) spectroscopy to time-resolve the dynamics of nanoscale phase inhomogeneities in the strongly correlated oxides Nd 0.5 Sr 0.5 MnO 3 , Tl 2 Mn 2 O 7 , and VO 2 . Through comparison of our time-resolved measurements on Nd 0.5 Sr 0.5 MnO 3 with x-ray scattering measurements, we find that sub-picosecond dynamics in manganites are governed by the excitation and redressing of uncorrelated Jahn-Teller lattice polarons [1,2]. In Tl 2 Mn 2 O 7 , our measurements indicate that quasiparticle dynamics are governed by spin disorder, with the static spin disorder that governs carrier relaxation at low temperatures ( T ≤0.75 T C ) gradually transitioning to dynamic spin disorder at high temperatures ( T≥ 1.4 T C ) through an intermediate temperature range (0.75 T C T ≤1.4 T C ) in which the photoexcited carrier density is extraordinarily long lived.  This suggests that CMR in TMO may originate from spin disorder and furthermore, that TMO may be a particularly simple example where the transport properties are determined by intrinsic nanoscale inhomogeneity occurring at a second order phase transition [2-4].  In addition, optical-pump terahertz-probe spectroscopy was used to directly measure conductivity dynamics in VO 2 , revealing the existence of metallic inhomogeneities in the insulating phase that facilitate the growth of a homogeneous metallic phase after photoexcitation [5]. Finally, we will briefly overview some of our more recent measurements of quasiparticle dynamics in multiferroic oxides and manganite-superconductor superlattices and also describe a newly developed system for performing ultrafast mid-IR pump, terahertz-probe spectroscopy. Our results demonstrate that ultrafast spectroscopy is sensitive to the dynamics of nanoscale phase inhomogeneities in complex oxides and strongly support the universality of nanoscale phase separation in these systems.

[1] R. P. Prasankumar, et al, Phys. Rev. B 76 , 020402(R) (2007).

[2] R. P. Prasankumar, et al, Physica B 403 , 1401 (2008).

[3] R. P. Prasankumar, et al, Phys. Rev. Lett. 95 , 267404 (2005).

[4] D. J. Hilton, et al, J. Phys. Soc. Jpn . 75 , 011006 (2006).

[5] D. J. Hilton, et al, Phys. Rev. Lett. 99 , 226401 (2007).