Condensation of collective charge ordering in chromium

Abstract

We report on the dynamics of the structural order parameter in a chromium film using synchrotron radiation in response to photo-induced 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 CDWs 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 longterm evolution shows that the CDW reverts to its ground state on a timescale 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 in charge ordered systems and its impact on the recondensation dynamics of the charge ordered state in strongly correlated materials.

Laser-Induced Charge-Density-Wave Transient Depinning in Chromium

“…Contrary to many other CDW systems [19][20][21][22][23][24][25][26], the ultrafast dynamics of chromium has never been studied in bulks. Femtosecond reflectivity experiments have been reported in films, validating that the ultrafast electronic response is well accounted for by the two-temperature model [27,28], and confirmed by a recent study of a commensurate CDW in a Cr film as well [29].In this work, the CDW modulation (and the superimposed periodic lattice distortion) and the average lattice have been studied as a function of time after femtosecond laser excitation by picosecond time-resolved x-ray diffraction in a bulk. The strength of this technique is its wavevector selectivity, which allows to track the timedependent behaviour of the CDW and average lattice independently.…”

Ultrafast Formation of a Charge Density Wave State in 1T−TaS2 : Observation at Nanometer Scales Using Time-Resolved X-Ray Diffraction

“…Diffraction techniques are especially well adapted to study CDW compounds [5,10,12,14,15,22,[24][25][26][27][28][29]. Indeed, a structural modulation with wavevector q gives rise to satellite peaks located at positions ± q with respect to each regular lattice peak (Fig.…”

Photoinduced Enhancement of the Charge Density Wave Amplitude

“…1(c) and reveals that the CDW has a wave vector normal to the film surface, is pinned by the film surfaces, and is quantized with 8.5 periods in the film, as expected from earlier studies [3,20,21]. Here, the satellite peaks form constructive and destructive x-ray interference with the Laue oscillations, which leads to an increase of the scattered intensity at [0,0, 2-2δ] or decrease at [0,0, 2+2δ] [22,23] (2δ being the momentum transfer of the CDW). We used short optical laser pulses to excite ultrafast dynamics in the Cr thin film and the time dependent CDW amplitude was monitored via ultrafast x-ray diffraction (insets of Fig.…”

“…At a time delay of 0.11 ps after photoexcitation the dynamic x-ray data is in agreement with static x-ray data recorded above the Néel temperature T N and shows an undistorted lattice. At 0.22 ps we observed a reversal of the CDW amplitude, as revealed by the change of sign of the interference term in the diffraction signal [22,23]; the CDW still has nodes at the interfaces, however, the amplitude is inverted.…”