67 research outputs found
Observation of the ground-state-geometric phase in a Heisenberg XY model
Geometric phases play a central role in a variety of quantum phenomena,
especially in condensed matter physics. Recently, it was shown that this
fundamental concept exhibits a connection to quantum phase transitions where
the system undergoes a qualitative change in the ground state when a control
parameter in its Hamiltonian is varied. Here we report the first experimental
study using the geometric phase as a topological test of quantum transitions of
the ground state in a Heisenberg XY spin model. Using NMR interferometry, we
measure the geometric phase for different adiabatic circuits that do not pass
through points of degeneracy.Comment: manuscript (4 pages, 3 figures) + supporting online material (6 pages
+ 7 figures), to be published in Phys. Rev. Lett. (2010
Valley Contrasting Magnetoluminescence in Monolayer MoS Quantum Hall Systems
The valley dependent optical selection rules in recently discovered monolayer
group-VI transition metal dichalcogenides (TMDs) make possible optical control
of valley polarization, a crucial step towards valleytronic applications.
However, in presence of Landaul level(LL) quantization such selection rules are
taken over by selection rules between the LLs, which are not necessarily valley
contrasting. Using MoS as an example we show that the spatial
inversion-symmetry breaking results in unusual valley dependent inter-LL
selection rules, which directly locks polarization to valley. We find a
systematic valley splitting for all Landau levels (LLs) in the quantum Hall
regime, whose magnitude is linearly proportional to the magnetic field and in
comparable with the LL spacing. Consequently, unique plateau structures are
found in the optical Hall conductivity, which can be measured by the
magneto-optical Faraday rotations
Many-body effects in nonlinear optical responses of 2D layered semiconductors
We performed ultrafast degenerate pump-probe spectroscopy on monolayer WSe2
near its exciton resonance. The observed differential reflectance signals
exhibit signatures of strong many-body interactions including the
exciton-exciton interaction and free carrier induced band gap renormalization.
The exciton-exciton interaction results in a resonance blue shift which lasts
for the exciton lifetime (several ps), while the band gap renormalization
manifests as a resonance red shift with several tens ps lifetime. Our model
based on the many-body interactions for the nonlinear optical susceptibility
fits well the experimental observations. The power dependence of the spectra
shows that with the increase of pump power, the exciton population increases
linearly and then saturates, while the free carrier density increases
superlinearly, implying that exciton Auger recombination could be the origin of
these free carriers. Our model demonstrates a simple but efficient method for
quantitatively analyzing the spectra, and indicates the important role of
Coulomb interactions in nonlinear optical responses of such 2D materials
Electrical Control of Two-Dimensional Neutral and Charged Excitons in a Monolayer Semiconductor
Monolayer group VI transition metal dichalcogenides have recently emerged as
semiconducting alternatives to graphene in which the true two-dimensionality
(2D) is expected to illuminate new semiconducting physics. Here we investigate
excitons and trions (their singly charged counterparts) which have thus far
been challenging to generate and control in the ultimate 2D limit. Utilizing
high quality monolayer molybdenum diselenide (MoSe2), we report the unambiguous
observation and electrostatic tunability of charging effects in positively
charged (X+), neutral (Xo), and negatively charged (X-) excitons in field
effect transistors via photoluminescence. The trion charging energy is large
(30 meV), enhanced by strong confinement and heavy effective masses, while the
linewidth is narrow (5 meV) at temperatures below 55 K. This is greater
spectral contrast than in any known quasi-2D system. We also find the charging
energies for X+ and X- to be nearly identical implying the same effective mass
for electrons and holes.Comment: 11 pages main text with 4 figures + 7 pages supplemental material
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