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SUMMARY:Excitonic properties and valley dynamics in monolayer transitioin 
 metal dichalcogenides - Bernhard Urbaszek (Toulouse\, France)
DTSTART:20151116T153000Z
DTEND:20151116T170000Z
UID:TALK60779@talks.cam.ac.uk
CONTACT:6270
DESCRIPTION:Transition metal dichalcogenides (TMDCs) such as WSe2 and MoS2
  are layered semiconductors\, with strong in-plane bonds and Van-der-Waals
  bonding between the individual layers. Although indirect semiconductors i
 n bulk form\, when thinned down to one monolayer (ML) they become direct s
 emiconductors in the visible/IR region of the optical spectrum [1]. These 
 atomically flat 2D materials have unique physical properties for manipulat
 ing electrons in non-equivalent valleys in momentum space and can potentia
 lly serve as building blocks in Van-der-Waals heterostructures for optoele
 ctronics applications. In this talk we will review the optical properties 
 of ML TMDCs governed by robust excitons\, Coulomb bound electron-hole pair
 s. We discuss valley state manipulation with polarized lasers and the nume
 rous open questions and challenges in this fast growing field.\n\nML TMDCs
  share common properties: The ML crystals have no inversion centre\, contr
 ary to graphene or bilayer TMDCs. In addition\, the spin-orbit (SO) coupli
 ng is strong due to the heavy transition metal atoms. This combination lea
 ds to a unique coupling of electron spin and k-space valley physics\, init
 ially described in a single particle picture [2]. When electrons and holes
  are simultaneously present\, they will form excitons as the Coulomb inter
 action is enhanced by the strong quantum confinement\, the large effective
  masses\, and the reduced dielectric screening in these ideal 2D systems. 
 Excitons with typical binding energies of 500 meV dominate the optical pro
 perties of ML TMDCs as the 1s\, 2s\, 2p…. exciton states are resonantly 
 excited\, also Raman scattering amplitudes are strongly affected by excito
 n resonances [3]. Due to the high oscillator strength of excitonic transit
 ions\, the intrinsic radiative lifetimes are expected to be short\, and in
  time resolved photoluminescence experiments on ML MoS2\, WSe2 and MoSe2 w
 e find indeed emission times in the ps range [4]. \n\nIn addition to these
  common properties for ML TMDCs we also discuss important differences. We 
 show how valley polarization and coherence can be optically initialized in
  ML WSe2 but not ML MoSe2. We also discuss the competition between bright 
 and dark excitons that determines the light emission yield in these binary
  materials and their ternary alloys MoWSe2 [5]. \n\nReferences\n[1] Mak et
  al\, PRL 105\, 136805 (2010)\; Lezema et al NanoLett 15\, 2336 (2015)\n[2
 ] Di Xiao et al\, PRL 108\, 196802 (2012)\n[3] Wang et al\, PRL 114\, 0974
 03 (2015) & Wang et al PRL 115\, 117401 (2015)
\n[4] Lagarde et al\, PRL
  112\, 047401 (2014) & Wang et al PRB 90\, 075413 (2014)
\n[5] Wang et a
 l\, arXiv 1506.08114\n
LOCATION:Rutherford building\, Seminar Room B
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