A time-dependent order parameter for ultrafast photoinduced phase transitions

Abstract

Strongly correlated electron systems often exhibit very strong interactions between structural and electronic degrees of freedom that lead to complex and interesting phase diagrams. For technological applications of these materials it is important to learn how to drive transitions from one phase to another. A key question here is the ultimate speed of such phase transitions, and to understand how a phase transition evolves in the time domain. Here we apply time-resolved X-ray diffraction to directly measure the changes in long-range order during ultrafast melting of the charge and orbitally ordered phase in a perovskite manganite. We find that although the actual change in crystal symmetry associated with this transition occurs over different timescales characteristic of the many electronic and vibrational coordinates of the system, the dynamics of the phase transformation can be well described using a single time-dependent 'order parameter' that depends exclusively on the electronic excitation.

Magnetic order dynamics in optically excited multiferroicTbMnO3

“…An additional possibility for the slow decay of the (0q0) reflection might be related to the fact that the current measurements probe a secondary order parameter caused by spin canting from the very weak Dzyaloshinskii-Moriya interaction, which could imply that the primary (0q1) Fourier component of the magnetic order decays more quickly. Further time-resolved studies directly sensitive to lattice distortions, as have been done on other systems [28,29], or designed to probe the (0q1) reflection would be required to confirm these possibilities in TbMnO3. An alternative explanation for the slow decay could be that the spin system itself requires time to thermalize by transferring energy from high wavevector magnons, which couple more directly to the excitation, to the lower frequency excitations that drive the changes in <S> [30].…”

“…Using this decay time, we additionally show that the spin system temperature increases linearly across our entire range of pump fluences; the pump laser excites the electronic system, which eventually transfers energy, heating the spin system. Whether this energy is mediated by the lattice due to the creation of a small polaron or simple lattice heating could be addressed in the future by probing other diffraction peaks sensitive to lattice and charge order [28]. Finally, the lack of large changes in the scattering wavevector indicates that despite the ultrafast pump, the extremely low magnon group velocity may prohibit fast changes in the spin-cycloid wavevector on these time scales.…”

Evolution of Hot Polaron States with a Nanosecond Lifetime in a Manganite Perovskite

“…This frequency band is also observed in ultrafast X-ray crystallography of the JT distortion. [31] This mode is only visible, while probing the ΔOD at low energies close to the JT peak maximum. Figure 5d shows the normalized temperature evolution of the amplitude of this phonon mode.…”

Photoinduced Demagnetization and Insulator-to-Metal Transition in Ferromagnetic InsulatingBaFeO3Thin Films

“…Beaurepaire et al proposed a phenomenological "three-temperature model" in order to understand the ultrafast demagnetization of Ni, which considers three interacting reservoirs of electrons, spins, and lattice, and suggested the importance of direct electron-spin interactions. Since the electron, spin, and lattice systems are quite tightly coupled to each other in strongly correlated 3d transition metal oxides, it is interesting to investigate the photoinduced dynamics with respect to the electronic states and magnetism [4][5][6][7][8][9][10][11][12][13][14][15].For this study, we chose fully oxidized single crystalline BaFeO 3 thin films, which show unusual behaviors of ferromagnetic and insulating properties with saturation magnetization and Curie temperature of 3.2 µ B /formula unit and 115 K, respectively [16]. The large magnetic moment of BaFeO 3 thin films results in quite large peak intensity of Fe 2p x-ray magnetic circular dichroism (XMCD), ∼ 18 % of the x-ray absorption peak * Electronic address: wadati@issp.u-tokyo.ac.jp; URL: http://www.geocities.jp/qxbqd097/index2.htm intensity [17].…”

“…Beaurepaire et al proposed a phenomenological "three-temperature model" in order to understand the ultrafast demagnetization of Ni, which considers three interacting reservoirs of electrons, spins, and lattice, and suggested the importance of direct electron-spin interactions. Since the electron, spin, and lattice systems are quite tightly coupled to each other in strongly correlated 3d transition metal oxides, it is interesting to investigate the photoinduced dynamics with respect to the electronic states and magnetism [4][5][6][7][8][9][10][11][12][13][14][15].…”