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SUMMARY:Topological and Charge-Ordered Phases in Transition-Metal Dichalco
 genides - Professor Phil King\, School of Physics and Astronomy\, Universi
 ty of St. Andrews\, UK
DTSTART:20190417T130000Z
DTEND:20190417T140000Z
UID:TALK123052@talks.cam.ac.uk
CONTACT:Dr Kaveh Delfanazari
DESCRIPTION:Transition-metal dichalcogenides (TMDs) are renowned for their
  rich and varied properties.  They range from metals and superconductors t
 o strongly spin-orbit-coupled semiconductors and charge-density-wave syste
 ms\, with their single-layer variants one of the most prominent current ex
 amples of two-dimensional materials beyond graphene.1\,2  In this talk\, I
  will focus on two aspects of TMD physics. First\, I will show our spin- a
 nd angle-resolved photoemission measurements that demonstrate how both the
  1T and 2H-structured TMDs are natural hosts of ladders of type-I and type
 -II bulk Dirac cones and topological surface states and resonances.3\,4  T
 hese arise from the chalcogen p-orbital manifold as a very general consequ
 ence of their trigonal crystal field\, and as such can be expected across 
 a large number of compounds\, opening routes to tune and ultimately exploi
 t the topological physics of TMDs. Second\, I will discuss the evolution o
 f the electronic structure and charge density wave (CDW) phases of TiSe2 a
 nd VSe2 when their thickness is reduced to a single monolayer. Three-dimen
 sionality is a core feature of the electronic structure and ordering wavev
 ectors of both of these parent compounds. Nonetheless\, by fabricating epi
 taxial monolayers using molecular-beam epitaxy\, we show how their CDW pha
 ses not only persist\, but are in fact strengthened\, in the two-dimension
 al limit. In TiSe2\, this can be attributed to a loss of kz-selectivity in
  band hybridisation at the CDW ordering instability.5 In VSe2\, we show ho
 w a strong-coupling CDW-like instability in turn drives a metal-insulator 
 transition in the monolayer\, in contrast to small partial gap opening in 
 bulk\, which we argue removes a competing instability to ferromagnetism th
 at is predicted for monolayer VSe2.6 Together\, this work points to the de
 licate balance that can be realized between competing electronic\, topolog
 ical\, and quantum many-body states and phases in bulk and monolayer trans
 ition-metal dichalcogenides.\n\nThis work was performed in close collabora
 tion with O.J. Clark\, M.D. Watson\, A. Rajan\, J. Feng\, D. Biswas\, M.S.
  Bahramy\, and colleagues from the Universities of St Andrews\, Tokyo\, Ox
 ford\, Keil\, Diamond\, SNU\, and NTNU.\n\nReferences\n1.	Q. H. Wang et al
 .\, Nature Nano. 7\, 669 (2012).\n2.	X. Xu et al.\, Nature Phys. 10\, 343 
 (2014).\n3.	M.S. Bahramy\, O.J. Clark et al.\, Nature Materials 17\, 21 (2
 018).\n4.	O.J. Clark et al.\, Phys. Rev. Lett. 120\, 156401 (2018).\n5.	M.
 D. Watson et al.\, Phys. Rev. Lett.  122\, 076404 (2019).\n6.	J. Feng et a
 l. Nano Lett. 18\, 4493 (2018).\n
LOCATION:Mott Seminar Room (531)\, Cavendish Laboratory\, Department of Ph
 ysics
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