Physics Department Seminar

Wednesday, 15^th November, 1.30pm, Rm 1311HN

ULTRAFAST OPTICAL STUDIES OF CONDENSED MATTER SYSTEMS

Marshall Onellion

Physics Department

University of Wisconsin-Madison

'Ultrafast' optical studies means an optical pulse width as narrow as 10- 20 fs and a time 'window' of 20 fs- 10 ns. This time window complements the time windows of muon spin rotation (down to ~ 100 ps) and neutron scattering (down to sub-femtosecond with Neutron Compton Scattering methods), with the limitation that due to the negligible photon linear momentum the measurements involve only wavevector |k| ~ 0 excitations. In this talk, I illustrate some of the science found by using ultrafast optical probes, including semiconductors [1], cuprates [2-5], heavy fermions [6], colossal magnetoresistance materials [7-9] other unconventional superconductors, in particular Sr₂RuO₄ ,[10,11] collective excitations [12- 14] and ultrafast demagnetization [15,16].

Based on the data from my collaborations and independent scientists, there are five scientific topics I will briefly discuss:

* Electron- electron thermalization [2-4, 6, 10, 11]: The main point to recognize is the difference between the electron- electron interactions in the Fermi liquid regime of correlated electron systems [6, 10, 11] and the not- Fermi liquid regime of correlated electron systems [2-4, 10, 11];

* Relaxation (dissipation) [1-11]: Here the main point is to compare behaviors of semiconductors, which are independent electron systems having real electronic bandgaps, heavy fermion and cuprates, which exhibit bottlenecks in the relaxation behavior, and the Fermi liquid normal state of an unconventional superconductor, Sr₂RuO₄, which exhibit relaxation times that scale with the electrical conductivity;

* Coherent longitudinal acoustic phonons in cuprates,[5] in which a simple model [12] allows us to account quantitatively for the oscillation period, dispersion, phase and amplitude decay with time;

* Collective excitations observed in correlated electron systems, including phonon- polariton oscillations in colossal magnetoresistance materials,[7] charge density waves in chalcogenides,[13] and surface plasmons in silver nanoparticles;[14]

* Ultrafast demagnetization in both ferromagnetic [15] and antiferromagnetic [16] materials.

1. F. Rossi and T. Kuhn, Rev. Mod. Phys. 74 (2002) 895.

2. M.L. Schneider et al, Euro. Phys. J. B 36 (2003) 327.

3. N. Gedik et al, Science 300 (2003) 1410.

4. V.V. Kabanov et al, Phys. Rev. Lett. 95 (2005) 147002.

5. I. Bozovic et al, Phys. Rev. B 69 (2004) 132503.

6. J. Demsar et al, Phys. Rev. Lett. 91 (2003) 027401.

7. R.D. Averitt et al, Phys. Rev. Lett. 87 (2001) 017401.

8. Y. Ren et al, Phys. Rev. B 64 (2001) 144401.

9. E. Dagotto et al, Phys. Rep. 344 (2001) 1.

10. P. Guptasarma et al, J. Phys. Chem. Sol. 67 (2006) 525.

11. M. Onellion et al, in preparation.

12. C. Thomsen et al, Phys. Rev. B 34 (1986) 4129.

13. J. Demsar et al, Phys. Rev. Lett. 83 (1999) 800; J. Demsar et al, Phys. Rev. B 66 (2002) 041101.

14. J. Lehmann et al, Phys. Rev. Lett. 85 (2000) 2921.

15. E. Beaurepaire et al, Phys. Rev. Lett. 76 (1996) 4250; M. Vomir et al, J. Appl. Phys. 99 (2006) 08A501.

16. T. Ogasawara et al, Phys. Rev. Lett. 94 (2005) 087202 and references therein.