Search

We have measured the relaxation of charge-density waves in chromium from out-of-equilibrium states achieved through rapid thermal quenching. Although pinned density waves have been predicted to relax through stick-slip dynamics, in which long-lived metastable states are linked by abrupt rearrangements of the microscopic structure, these microscopic behaviors have not been observed. Using synchrotron x rays, we have measured metastability of the order parameter and microscopic pinning phenomena including an avalanche of the charge-density wave phase. Molecular dynamical simulations based on the Fukuyama-Lee-Rice model show that these behaviors may be attributed to nucleation, pinning, and propagation of phase solitons. [ABSTRACT FROM AUTHOR]

We report on the dynamics of the structural order parameter in a chromium film using synchrotron radiation in response to photoinduced ultrafast excitations. Following transient optical excitations the effective lattice temperature of the film rises close to the Néel temperature and the charge-density wave (CDW) amplitude is reduced but does not appear to ever be fully destroyed. The persistence of the CDW diffraction signal demonstrates that the CDW, if destroyed by the laser pulse, must be reestablished within the 100-ps time resolution of the synchrotron x-ray pulses. Furthermore, at all times after photoexcitation, the CDW retains its low-temperature periodicity, rather than regenerating with its high-temperature period shortly after photoexcitation. The long-term evolution shows that the CDW reverts to its ground state on a time scale of 370 ± 40 ps. We attribute the apparent persistence of the CDW to the long-lived periodic lattice displacement in chromium. This study highlights the fundamental role of the lattice distortion and its impact on the recondensation dynamics of the charge ordered state in strongly correlated materials. [ABSTRACT FROM AUTHOR]

The resistivity of vanadium dioxide (VO2) decreased by over one order of magnitude upon localized illumination with x rays at room temperature. Despite this reduction, the structure remained in the monoclinic phase and had no signature of the high-temperature tetragonal phase that is usually associated with the lower resistance. Once illumination ceased, relaxation to the insulating state took tens of hours near room temperature. However, a full recovery of the insulating state was achieved within minutes by thermal cycling. We show that this behavior is consistent with random local-potential fluctuations and random distribution of discrete recombination sites used to model residual photoconductivity. [ABSTRACT FROM AUTHOR]

We investigate the electronic and structural changes at the nanoscale in vanadium dioxide (VO2) in the vicinity of its thermally driven phase transition. Both electronic and structural changes exhibit phase coexistence leading to percolation. In addition, we observe a dichotomy between the local electronic and structural transitions. Nanoscale x-ray diffraction reveals local, nonmonotonic switching of the lattice structure, a phenomenon that is not seen in the electronic insulator-to-metal transition mapped by near-field infrared microscopy. [ABSTRACT FROM AUTHOR]