Drexel

   Goran Karapetrov

Ultrafast dynamics in normal and CDW phase of NbSe2

2H-NbSe2 is a layered van der Waals material with trigonal Nb layer stacked between two trigonal Se layers having periodicity of a double Se-Nb-Se trilayer. It remains metallic from room temperature down to its superconducting transition at Tc=7.2 K. At temperatures below TCDW=33.5 K an incommensurate charge density wave (CDW) sets in with periodicty of ~3a0 along the three in-plane axes. In this paper we investigate carrier and collective mode dynamics in 2H-NbSe2 using time-resolved optical pump-probe spectroscopy and compare the results with first-principle calculations. Broadband ultrafast reflectivity studies of 2H-NbSe2 in a wide temperature interval covering the normal, CDW and superconducting phase were performed.

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Transient optical reflectivity in CuTiSe2

1T-TiSe2 is a layered dichalcogenide with unique excitonically-driven charge density wave (CDW) state below TCDW=200 K. The commensurate CDW has a chiral character with nanoscale domains of opposite chirality. Understanding the mechanism of formation and evolution of this CDW state could enable the control of the individual nanoscale CDW domains. In our recent paper we report on a transient optical reflectivity study in 1T-TiSe2 and CuxTiSe2 single crystals. Results from the experiments show that 1T-TiSe2 undergoes a quantum phase transition upon copper intercalation from a state of commensurate charge order (x<0.04) to a state  with a different symmetry in which a new charge order coexists with the superconducting phase at low temperatures (x>0.04). The results point to intricate co-evolution of the CDW and superconducting order parameters in this material.

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Electronic properties of MoS2 films

MoS2 is a layered 2D transition metal dichalcogenide that shows great promise for next generation logic circuits, optoelectronics, and sensing. Many of these applications require high quality films and crystals with small defect concentration. The stability with respect to temperature excursions and electromagnetic radiation is essential as well. In our recent article we report on structural and electronic properties of defects in chemical vapor-deposited monolayer and few-layer MoS2 films.  We track the evolution of defects that are formed under heating and electron beam irradiation. We observe formation of metastable domains with different work function values after annealing the material in ultra-high vacuum to moderate temperatures. We attribute these metastable values of the work function to evolution of crystal defects forming during the annealing. The results provide initial estimates of the thermal budgets available for reliable fabrication of MoS2 -based integrated electronics and indicate the importance of defect control and layer passivation.

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Photon-induced selenium migration in 1T-TiSe2

1T-TiSe2 is a member of transition metal dichalcogenide family of layered van-der-Waals materials that exhibits some distinctive electronic and optical properties. We performed Raman spectroscopy studies on single crystal TiSe2 to investigate photon-induced defects associated with formation of selenium vacancies. Temperature dependent studies of the threshold temperature and laser intensity necessary to initiate selenium migration to the surface show an activation barrier of 1.55 eV for this process. These findings reported in Applied Physics Letters article could have significant ramifications for integration of transition metal dichalcogenides in electronic applications.

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