Ultrafast photonic control of colossal magnetoresistive materials
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One challenge in condensed matter physics is the active control of quantum phases of functional transition metal oxides (TMOs) using photons. The realization of active control falls into the research field of photo-induced phase transitions using ultrafast spectroscopy, which has attracted research effort for the past two decades. Early research demonstrated photo-control of spin crossover compounds and the neutral-to-ionic transition in organic crystals. The realization of photo-control of the macroscopic properties of metal oxides is not a simple extension of the old subject, but a rebirth and extension of this research area as reflected in the following ways: 1. Ultrafast optical spectroscopy (UOS) provides a dynamical route to analyze the coupled degrees of freedom in the equilibrium state of TMOs, providing a new approach to understand fundamental issues in condensed matter physics. 2. UOS studies of complex materials drive the development of new high precision experimental techniques to facilitate mode-selective excitation. 3. The active control of magnetism, ferro-electricity, superconductivity and metal-insulator transitions in TMOs opens new avenues for the design and application of novel optoelectronic devices. An even greater challenge for photo-induced phase transition is to realize nonthermal switching between the ground state and a meta-stable phase, having a well-defined order parameter. For this purpose we perform ultrafast optical-pump THz-probe spectroscopy on the strain-engineered colossal magnetoresistance (CMR) material La2/3Ca1/3MnO3. We utilize the anisotropic strain applied by NdGaO3 (001) substrates to tune the La2/3Ca1/3MnO3 into a charge ordered insulating (COI) phase, originating from the enhanced orthorhombicity of the lattice, such that Mn-O-Mn bonding angle deviates from 180^o. Both octahedral tilting and Jahn-Teller-like distortion suppress the ferromagnetism and itinerant nature of the d-electrons. Thus, a charge ordered insulating phase dominates at low temperatures. Using ultrafast spectroscopy, we demonstrate a persistent and single-laser-pulse driven insulator-to-metal phase transition in these La2/3Ca1/3MnO3 thin films. The experimental results demonstrate that the light-induced phase transition is of a cooperative nature involving multiple degrees of freedom, with the key ingredient of the switching being magnetoelastic coupling. Such active photo-control provides a dynamical perspective to understand how the delicate balance of competing orders decide the quantum phases in colossal-magnetoresistance materials.