We describe a mechanism for insulator-to-metal transition triggered by spin-canting following fs laserexcitation of insulating anti-ferromagnetic (AFM) states of colossal magneto-resistive (CMR) manganites. We show that photoexcitation of composite fermion quasi-particles dressed by spin fluctuations results in the population of a broad metallic conduction band due to canting of the AFM background spins via strong electron-spin local correlation. By inducing spin-canting, photoexcitation can increase the quasi-particle energy dispersion and quench the charge excitation energy gap. This increases the critical Jahn-Teller (JT) lattice displacement required to maintain an insulating state. We present fs-resolved pump-probe measurements showing bi-exponential relaxation of the differential reflectivity below the AFM transition temperature. We observe a nonlinear dependence of the ratio of the fs and ps relaxation component amplitudes at the same pump fluence threshold where we observe femtosecond magnetization photoexcitation. We attribute this correlation between nonlinear fs spin and charge dynamics to spin/charge/lattice coupling and population inversion between the polaronic majority carriers and metallic quasi-electron minority carriers as the lattice displacement becomes smaller than the critical value required to maintain an insulating state following laser-induced spin canting.
The origin of the anisotropic, paramagnetic phase associated with electronic nematicity in the iron pnictides is yet to be resolved. Furthermore, the detwinning technique used to study the nematic order in single crystals is known to introduce extra anisotropy into the sample, which can smear out the transition and even modify intrinsic characteristics associated with "spontaneous" Ising, Z2, symmetry breaking. Here we use a strain and stress free, twinned sample to show that there is a significant reduction in the energy relaxation times of the hot electrons following non-equilibrium femtosecond laser excitation on both the high and low temperature sides of the nematic phase transition. This femtosecond, critical speeding-up behavior provides an alternative way to study complex, electronically-driven nematicity, invoking neither external strain nor measuring a small anisotropy in twinned crystals. Particularly, a detailed analysis of the observed ultrafast decay time and the amplitude associated with an initial electronic relaxation provides compelling implications on the physical origin of nematicity in iron pnictides: (1) nematic fluctuations strongly influence the dynamics of electron cooling; and (2) spin fluctuations determine the part of amplitude arising from the nematicity. Finally, we discuss ultrafast coherent phonon generation which may contribute to the measured transition temperature in our ultrafast measurements.
High intensity laser pulses were recently shown to induce a population inverted transient state in graphene [T. Li et al. Phys. Rev. Lett. 108, 167401 (2012)]. Using a combination of hydrodynamic arguments and a kinetic theory we determine the post-transient state relaxation of hot, dense, population inverted electrons towards equilibrium. The cooling rate and charge-imbalance relaxation rate are determined from the Boltzmann-equation including electron-phonon scattering.We show that the relaxation of the population inversion, driven by inter-band scattering processes, is much slower than the relaxation of the electron temperature, which is determined by intra-band scattering processes. This insight may be of relevance for the application of graphene as an optical gain medium. 1 arXiv:1303.7014v1 [cond-mat.mes-hall]
Abstract. We report a pump threshold behavior in fs photoinduced magnetization in a strongly correlated manganite, which indicates the establishment of thermally-inaccessible ferromagnetic ground state and buildup of new magnetic order parameters at fs time scales
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