Femtosecond X-Ray Diffraction and Absorption: A New Probe for Phase Transition Dynamics

Andrea Cavalleri, Lawrence Berkeley National Laboratory

Many spectacular effects observed in strongly-correlated systems derive from coupling between structural, electronic and magnetic degrees of freedom of a solid. However, because different effects appear “simultaneous” in time-integrated measurements, elementary correlation mechanisms are difficult to establish. Yet, intrinsic response times are dramatically different if observed on the ultrafast timescale and selective excitation/probing of different degrees of freedom of the system can provide important clues.

In this talk, I will discuss experimental work on the case of VO₂, a non-magnetic oxide undergoing an insulator-to-metal transition upon small structural distortion at 340 K [1]. The nature of the low-T phase of VO₂ has been heavily debated in the past [2,3], the issue being the role of structural distortion (Peierls) [4]and electronic correlations (Mott-Hubbard) in determining its insulating properties. A combination of femtosecond optical and x-ray diffraction experiments allows for new insight into this problem [5]. I will then discuss recent progress in the development of soft x-ray absorption techniques including absorption experiments performed with femtosecond, tunable x-ray pulses at the Advanced Light Source.

1. F. Morin Physical Review Letters 3, 34 (1959).
2. J.B. Goodenough 117, 1442 (1960).
3. A. Zylberstein, N. Mott Physical Review B 11, 4383 (1975).
4. R.M. Wentzcovic, W.W. Schultz, P.B. Allen Physical Review Letters 72, 3389 (1994).
5. A. Cavalleri et al. Physical Review Letters 87, 237401 (2001).

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