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SUMMARY:Ultrafast electronic band gap control and inhibition of the photoi
 nduced structural phase transition in an excitonic insulator - LACUS Semin
 ar
DTSTART:20180615T110000
DTEND:20180615T130000
DTSTAMP:20260428T182321Z
UID:012158b8142f590927ee007b4591476236cb1ab1b48f71854129f24d
CATEGORIES:Conferences - Seminars
DESCRIPTION:Selen Mor    \nUltrafast control of matter phases is of bo
 th fundamental and technological interest. Here\, we study the ultrafast d
 ynamics of Ta 2 NiSe 5 by means of time- and angle-resolved photoelectron 
 spectroscopy (trARPES) [1] and transient reflectivity measurements [2]. Ta
  2 NiSe 5 is proposed to support an excitonic insulator (EI) phase below T
  C ≈ 328 K\, combined with a structural change from orthorombic to monoc
 linic symmetry. Such an EI phase is expected to occur in small gap semicon
 ductors with strong electron-hole interaction as excitons can form spontan
 eously and condense into a ground state. Below T C\, trARPES around  sh
 ows a strong fluence-dependent valence band depopulation\, until absorptio
 n saturates at a critical fluence F sat = 0.2 mJ cm −2 . This is reflect
 ed in a saturation of the mid-IR optical response at F sat . A coherent ph
 onon at 4 THz\, marker of the EI/monoclinic phase\, persists above T C \, 
 indicating that the photoinduced structural transition is hindered by pump
  absorption saturation. trARPES shows that below F sat the band gap shrink
 s transiently due to photoenhanced screening of the Coulomb interaction\, 
 while above F sat it widens and recovers to its equilibrium value after ~1
 .5 ps. Hartree-Fock calculations reveal that the band gap widening is due 
 to photoenhancement of the exciton condensate\, persisting until the syste
 m undergoes interband relaxation. Our results prove it is possible to mani
 pulate exciton condensates optically and gain ultrafast control of semicon
 ductor band gaps.\n\n[1] Mor et al.\, Phys. Rev. Lett. 119\, 086401 (2017)
 .
LOCATION:CH G1 495 https://plan.epfl.ch/?room==CH%20G1%20495
STATUS:CONFIRMED
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