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Register a new userCooperative photoinduced metastable phase control in strained manganite films
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A major challenge in condensed matter physics is active control of quantum phases. Dynamic control with pulsed electromagnetic fields can overcome energetic barriers enabling access to transient or metastable states that are not thermally accessible. Here we demonstrate strain-engineered tuning of La2/3Ca1/3MnO3 into an emergent charge-ordered insulating phase with extreme photo-susceptibility where even a single optical pulse can initiate a transition to a long-lived metastable hidden metallic phase. Comprehensive single-shot pulsed excitation measurements demonstrate that the transition is cooperative and ultrafast, requiring a critical absorbed photon density to activate local charge excitations that mediate magnetic-lattice coupling that, in turn, stabilize the metallic phase. These results reveal that strain engineering can tune emergent functionality towards proximal macroscopic states to enable dynamic ultrafast optical phase switching and control.
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Magnetic properties of polycrystalline Sm0.1Ca0.84Sr0.06MnO3 in pristine and metastable states have been investigated in wide range of temperatures and magnetic fields. It was found that below Curie temperature TC = 105 K the pristine state exhibits phase separation comprising ferromagnetic and antiferromagnetic phases. The metastable states with reduced magnetization were obtained by successive number of quick coolings of the sample placed in container with kerosene-oil mixture. By an increasing number of quick coolings (> 6) the long time relaxation appeared at 10 K and the magnetization reversed its sign and became strongly negative in wide temperature range, even under an applied magnetic field of 15 kOe. The observed field and temperature dependences of the magnetization in this state are reversed in comparison with the ordinary ferromagnetic ones. Above TC, the observed diamagnetic susceptibility of the reversed magnetization state at T = 120 K is ~ - 0.9 x 10-4 emu g-1 Oe-1. Only after some storage time at room temperature, the abnormal magnetic state is erasable. It is suggested that the negative magnetization observed results from a specific coupling of the nano/micro-size ferromagnetic regions with a surrounding diamagnetic matrix formed, in a puzzled way, by the repeating training (quick cooling) cycles.
Utrafast control of material physical properties represents a rapid developing field in condensed matter physics. Yet, accessing to the long-lived photoinduced electronic states is still in its early stage, especially with respect to an insulator to metal phase transition. Here, by combing transport measurement with ultrashort photoexcitation and coherent phonon spectroscopy, we report on photoinduced multistage phase transitions in Ta2NiSe5. Upon excitation by weak pulse intensity, the system is triggered to a short-lived state accompanied by a structural change. Further increasing the excitation intensity beyond a threshold, a photoinduced steady new state is achieved where the resistivity drops by more than four orders at temperature 50 K. This new state is thermally stable up to at least 350 K and exhibits the lattice structure different from any of the thermally accessible equilibrium states. Transmission electron microscopy reveals an in-chain Ta atom displacement in the photoinduced new structure phase. We also found that nano-sheet samples with the thickness less than the optical penetration depth are required for attaining a complete transition.
Transition metal oxides possess complex free energy surfaces with competing degrees of freedom. Photoexcitation allows shaping of such rich energy landscapes. In epitaxially strained $mathrm{La_{0.67}Ca_{0.33}MnO_3}$, optical excitation with a sub-100 fs pulse above $2 mathrm{mJ/cm^2}$ leads to a persistent metallic phase below 100 K. Using single-shot optical and terahertz spectroscopy, we show that this phase transition is a multi-step process. We conclude that the phase transition is driven by partial charge order melting, followed by growth of the persistent metallic phase on longer timescales. A time-dependent Ginzburg-Landau model can describe the fast dynamics of the reflectivity, followed by longer timescale in-growth of the metallic phase.
Deterministic oscillations of current-induced metastable resistivity in changing voltage have been detected in La$_{0.82}$Ca$_{0.18}$MnO$_3$ single crystals. At low temperatures, below the Curie point, application of specific bias procedures switches the crystal into metastable resistivity state characterized by appearance of pronounced reproducible and random structures in the voltage dependence of the differential conductivity. In certain bias range equally spaced broad conductivity peaks have been observed. The oscillating conductivity has been tentatively ascribed to resonances in a quantum well within the double tunnel barrier of intrinsic weak-links associated with twin-like defect boundaries.
Epitaxial strain provides important pathways to control the magnetic and electronic states in transition metal oxides. However, the large strain is usually accompanied by a strong reduction of the oxygen vacancy formation energy, which hinders the direct manipulation of their intrinsic properties. Here using a post-deposition ozone annealing method, we obtained a series of oxygen stoichiometric SrCoO3 thin films with the tensile strain up to 3.0%. We observed a robust ferromagnetic ground state in all strained thin films, while interestingly the tensile strain triggers a distinct metal to insulator transition along with the increase of the tensile strain. The persistent ferromagnetic state across the electrical transition therefore suggests that the magnetic state is directly correlated with the localized electrons, rather than the itinerant ones, which then calls for further investigation of the intrinsic mechanism of this magnetic compound beyond the double-exchange mechanism.
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