by Professor Atac Imamoglu, Quantum Photonics Group, Department of Physics, ETH Zürich, Switzerland
Hosted by Prof. Darrick Chang
If the Coulomb repulsion between the electrons becomes significantly stronger than their kinetic energy, the itinerant electrons in the two-dimensional systems are expected to form a spatially-ordered state, termed a Wigner crystal . According to former Quantum Monte Carlo calculations , the ratio rs of the two energy scales must exceed 30 for such a crystallization to occur in the absence of the magnetic field (B = 0). Owing to severe difficulties in satisfying this condition for conventional semiconductors (e.g., GaAs), prior experimental studies of the crystalline electronic states have mainly focused on the electrons confined to single Landau level under strong external magnetic field, which almost completely quenches the kinetic energy. In this talk, I will describe recent experiments in atomically-thin transition metal dichalcogenides (TMDs) where it is possible to reach rs > 40. Our measurements provide a direct evidence that the electrons at densities < 3 · 1011 cm-2 in a pristine MoSe2 monolayer spontaneously break the continuous translation symmetry and form a Wigner crystal even at B = 0 . This is revealed by our low-temperature (T = 80 mK) magneto-optical spectroscopy experiments that utilize a newly developed technique allowing to unequivocally detect charge order in an electronic Mott-insulator state . This method relies on the modification of excitonic band structure arising due to the periodic potential experienced by the excitons interacting with a crystalline electronic lattice. Under such conditions, optically-inactive exciton states with finite momentum matching the reciprocal Wigner lattice vector k = k¬W get Bragg scattered back to the light cone, where they hybridize with the zero-momentum bright exciton states. This leads to emergence of a new, umklapp peak in the optical spectrum heralding the presence of periodically-ordered electronic lattice.
Atac Imamoglu earned his BS from the Middle East Technical University in 1985. He then earned an MS and Ph.D. from Stanford University in 1987 and 1991 respectively. After graduating, Imamoglu became an assistant professor at the University of California, Santa Barbara, becoming a full professor in 1999. In 2002, he moved to ETH Zurich, Switzerland, where he is today. He is a Fellow of OSA and APS. He has received a number of awards for his work, including the 2001 Alexander von Humboldt Foundation Wolfgang Paul Award, 2009 IEEE Quantum Electronics Award and OSA’s Charles Hard Townes Award in 2010 “for his seminal contribution to electromagnetically induced transparency and pioneering work on quantum information processing with quantum dots.