Magnetic brightening and control of dark excitons in monolayer WSe2.

Monolayer transition metal dichalcogenide crystals, as direct-gap materials with strong light-matter interactions, have attracted much recent attention. Because of their spin-polarized valence bands and a predicted spin splitting at the conduction band edges, the lowest-lying excitons in WX2 (X = S, Se) are expected to be spin-forbidden and optically dark. To date, however, there has been no direct experimental probe of these dark excitons. Here, we show how an in-plane magnetic field can brighten the dark excitons in monolayer WSe2 and permit their properties to be observed experimentally. Precise energy levels for both the neutral and charged dark excitons are obtained and compared with ab initio calculations using the GW-BSE approach. As a result of their spin configuration, the brightened dark excitons exhibit much-increased emission and valley lifetimes. These studies directly probe the excitonic spin manifold and reveal the fine spin-splitting at the conduction band edges.

[1]  S. Louie,et al.  Electron-hole excitations and optical spectra from first principles , 2000 .

[2]  Louie,et al.  Electron correlation in semiconductors and insulators: Band gaps and quasiparticle energies. , 1986, Physical review. B, Condensed matter.

[3]  Stefano de Gironcoli,et al.  QUANTUM ESPRESSO: a modular and open-source software project for quantum simulations of materials , 2009, Journal of physics. Condensed matter : an Institute of Physics journal.

[4]  H. Dery,et al.  Polarization analysis of excitons in monolayer and bilayer transition-metal dichalcogenides , 2015, 1506.06686.

[5]  A. Splendiani,et al.  Emerging photoluminescence in monolayer MoS2. , 2010, Nano letters.

[6]  D. Ralph,et al.  Breaking of valley degeneracy by magnetic field in monolayer MoSe2. , 2014, Physical review letters.

[7]  Wang Yao,et al.  Coupled spin and valley physics in monolayers of MoS2 and other group-VI dichalcogenides. , 2011, Physical review letters.

[8]  Ji Feng,et al.  Valley-selective circular dichroism of monolayer molybdenum disulphide , 2012, Nature Communications.

[9]  M. Mootz,et al.  Quantum droplets of electrons and holes , 2014, Nature.

[10]  X. Marie,et al.  Exciton fine structure and spin decoherence in monolayers of transition metal dichalcogenides , 2014, 1403.0108.

[11]  Xu Cui,et al.  Valley splitting and polarization by the Zeeman effect in monolayer MoSe2. , 2014, Physical Review Letters.

[12]  Timothy C. Berkelbach,et al.  Exciton binding energy and nonhydrogenic Rydberg series in monolayer WS(2). , 2014, Physical review letters.

[13]  Wang Yao,et al.  Spin and pseudospins in layered transition metal dichalcogenides , 2014, Nature Physics.

[14]  V. C. Moore,et al.  Optical Signatures of the Aharonov-Bohm Phase in Single-Walled Carbon Nanotubes , 2004, Science.

[15]  A. Balocchi,et al.  Valley dynamics probed through charged and neutral exciton emission in monolayer WSe2 , 2014, 1402.6009.

[16]  Yingchun Cheng,et al.  Giant spin-orbit-induced spin splitting in two-dimensional transition-metal dichalcogenide semiconductors , 2011 .

[17]  Yugui Yao,et al.  Three-band tight-binding model for monolayers of group-VIB transition metal dichalcogenides , 2013, 1305.6089.

[18]  S. Louie,et al.  Optical spectrum of MoS2: many-body effects and diversity of exciton states. , 2013, Physical review letters.

[19]  C. Robert,et al.  Splitting between bright and dark excitons in transition metal dichalcogenide monolayers , 2016, 1601.07351.

[20]  T. Heinz,et al.  Experimental Evidence for Dark Excitons in Monolayer WSe_{2}. , 2015, Physical review letters.

[21]  A. Forchel,et al.  Spectroscopic study of dark excitons in In x Ga 1-x As self-assembled quantum dots by a magnetic-field-induced symmetry breaking , 2000 .

[22]  Steven G. Louie,et al.  Probing excitonic dark states in single-layer tungsten disulphide , 2014, Nature.

[23]  P. Mallet,et al.  Single photon emitters in exfoliated WSe2 structures. , 2015, Nature nanotechnology.

[24]  Valley- and spin-polarized Landau levels in monolayer WSe2. , 2017, Nature nanotechnology.

[25]  P. L. McEuen,et al.  The valley Hall effect in MoS2 transistors , 2014, Science.

[26]  P. Tan,et al.  Carrier and polarization dynamics in monolayer MoS2. , 2013, Physical review letters.

[27]  A Gholinia,et al.  WSe₂ Light-Emitting Tunneling Transistors with Enhanced Brightness at Room Temperature. , 2015, Nano letters.

[28]  X. Marie,et al.  Exciton valley dynamics probed by Kerr rotation in WSe2 monolayers , 2014, 1407.5862.

[29]  R. Bratschitsch,et al.  Resonant internal quantum transitions and femtosecond radiative decay of excitons in monolayer WSe2. , 2015, Nature materials.

[30]  G. Burkard,et al.  k·p theory for two-dimensional transition metal dichalcogenide semiconductors , 2014, 1410.6666.

[31]  Timothy C. Berkelbach,et al.  Observation of biexcitons in monolayer WSe2 , 2015, Nature Physics.

[32]  C. Robert,et al.  Spin-orbit engineering in transition metal dichalcogenide alloy monolayers , 2015, Nature Communications.

[33]  K. Ko'smider,et al.  Large spin splitting in the conduction band of transition metal dichalcogenide monolayers , 2013, 1311.0049.

[34]  Aaron M. Jones,et al.  Optical generation of excitonic valley coherence in monolayer WSe2. , 2013, Nature nanotechnology.

[35]  Takashi Taniguchi,et al.  Probing dark excitons in atomically thin semiconductors via near-field coupling to surface plasmon polaritons. , 2017, Nature nanotechnology.

[36]  Andras Kis,et al.  Valley Zeeman effect in elementary optical excitations of monolayer WSe2 , 2014, Nature Physics.

[37]  Tao Yu,et al.  Valley depolarization due to intervalley and intravalley electron-hole exchange interactions in monolayer MoS 2 , 2013, 1401.0047.

[38]  David A. Strubbe,et al.  BerkeleyGW: A massively parallel computer package for the calculation of the quasiparticle and optical properties of materials and nanostructures , 2011, Comput. Phys. Commun..

[39]  Fengcheng Wu,et al.  Direct measurement of exciton valley coherence in monolayer WSe2 , 2016 .

[40]  H. Shtrikman,et al.  Exciton exchange splitting in wide GaAs quantum wells , 1999 .

[41]  J. Shan,et al.  Atomically thin MoS₂: a new direct-gap semiconductor. , 2010, Physical review letters.