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Snapshots of electronic structure by
Time and Angle Resolved Photo-emission

Charge density waves (CDW) underpin the electronic properties of many complex materials. Near-equilibrium CDW order is linearly coupled to a periodic, atomic-structural distortion, and the dynamics is understood in terms of amplitude and phase modes.

Figure: Snapshots of momentum-dependent photoemission intensity. In the first panel the equilibrium band positions are marked. As the insulator-metal transition proceeds, transfer of intensity across the Fermi level is evident throughout both Brillouin zones. The gap between bands at the zone boundary collapses on sub-vibrational timescales.


However, at the shortest timescales lattice and charge order may become de-coupled, highlighting the electronic nature of this many-body ground state. Using time and angle resolved photoemission spectroscopy with sub-30-fs XUV pulses near 20 eV photon energy, we have mapped the time- and momentum-dependent electronic structure in photo-stimulated 1T-TaS2, a two-dimensional charge density wave compound. Contrary to what was expected, we found that CDW order, melted well before relaxation of the underlying structural distortion [1].


       

Figure: Time-resolved maps of photoemission intensity at Γ– and M-point of the CDW compound after optical excitation. The collapse of the Mott gap is highlighted by the prompt transfer of intensity across the Fermi level (EB=0 meV) at different points of the Brillouin zone. The destruction of CDW order appears as the collapse of two distinct bands into a single feature. (see also the right diagram, showing individual energy distribution curves before and after photoexcitation).

These studies were combined with ultrabroadband optical spectroscopy to connect the physics of this unconventional photo-excited state with the polaronic transport in these transiently photometallic compounds [2].

 

In the future, we plan to extend studies of complex many-electron dynamics to few femtoseconds or even attosecond timescales, in a collaborative effort with the extreme timescales group of Adrian Cavalieri.

 


Related Publications:

[1] Clocking the Melting Transition of Charge and Lattice Order in 1T-TaS2 with Ultrafast Extreme-Ultraviolet Angle-Resolved Photoemission Spectroscopy
J. C. Petersen, S. Kaiser, N. Dean, A. Simoncig, H. Y. Liu, A. L. Cavalieri, C. Cacho, I. C. E. Turcu, E. Springate, F. Frassetto, L. Poletto, S. S. Dhesi, H. Berger, and A. Cavalleri
Physical Review Letters 107, 177402 (2011)  
→ press release

[2] Polaronic Conductivity in the Photoinduced Phase of 1T-TaS2
N. Dean, J. C. Petersen, D. Fausti, R. I. Tobey, S. Kaiser, L. V. Gasparov, H. Berger, and A. Cavalleri
Phys. Rev. Lett. 106, 016401 (2011)
→more

Experiment:

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