18 research outputs found
Emergent correlated phases in rhombohedral trilayer graphene induced by proximity spin-orbit and exchange coupling
The impact of proximity-induced spin-orbit and exchange coupling on the
correlated phase diagram of rhombohedral trilayer graphene (RTG) is
investigated theoretically. By employing \emph{ab initio}-fitted effective
models of RTG encapsulated by transition metal dichalcogenides (spin-orbit
proximity effect) and ferromagnetic CrGeTe (exchange proximity
effect), we incorporate the Coulomb interactions within the random-phase
approximation to explore potential correlated phases at different displacement
field and doping. We find a rich spectrum of spin-valley resolved Stoner and
intervalley coherence instabilities induced by the spin-orbit proximity
effects, such as the emergence of a \textit{spin-valley-coherent} phase due to
the presence of valley-Zeeman coupling. Similarly, proximity exchange removes
the phase degeneracies by biasing the spin direction, enabling a
magneto-correlation effect -- strong sensitivity of the correlated phases to
the relative magnetization orientations (parallel or antiparallel) of the
encapsulating ferromagnetic layers
Three-particle states and brightening of intervalley excitons in a doped MoS monolayer
Optical spectra of two-dimensional transition-metal dichalcogenides (TMDC)
are influenced by complex multi-particle excitonic states. Their theoretical
analysis requires solving the many-body problem, which in most cases, is
prohibitively complicated. In this work, we calculate the optical spectra by
exact diagonalization of the three-particle Hamiltonian within the Tamm-Dancoff
approximation where the doping effects are accounted for via the Pauli blocking
mechanism, modelled by a discretized mesh in the momentum space. The
single-particle basis is extracted from the {\it ab initio} calculations.
Obtained three-particle eigenstates and the corresponding transition dipole
matrix elements are used to calculate the linear absorption spectra as a
function of the doping level. Results for negatively doped MoS monolayer
(ML) are in an excellent quantitative agreement with the available experimental
data, validating our approach. The results predict additional spectral features
due to the intervalley exciton that is optically dark in an undoped ML but is
brightened by the doping. Our approach can be applied to a plethora of other
atomically thin semiconductors, where the doping induced brightening of the
many-particle states is also anticipated
Electrostatic and Environmental Control of the Trion Fine Structure in Transition Metal Dichalcogenide Monolayers
Charged excitons or trions are essential for optical spectra in low-dimensional doped monolayers (ML) of transitional metal dichalcogenides (TMDC). Using a direct diagonalization of
the three-body Hamiltonian, we calculate the low-lying trion states in four types of TMDC MLs as a function of doping and dielectric environment. We show that the fine structure of the trion is the result of the interplay between the spin-valley fine structure of the single-particle bands and the exchange interaction. We demonstrate that by variations of the doping and dielectric environment, the fine structure of the trion energy can be tuned, leading to anticrossing of the bright and dark states, with substantial implications for the optical spectra of the TMDC ML
Nonlinear spectroscopy of excitonic states in transition metal dichalcogenides
Second-harmonic generation (SHG) is a well-known nonlinear spectroscopy
method to probe electronic structure, specifically, in transition metal
dichalcogenide (TMDC) monolayers. This work investigates the nonlinear dynamics
of a strongly excited TMDC monolayer by solving the time evolution equations
for the density matrix. It is shown that the presence of excitons qualitatively
changes the nonlinear dynamics leading, in particular, to a huge enhancement of
the nonlinear signal as a function of the dielectric environment. It is also
shown that the SHG polarization angular diagram and its dependence on the
driving strength are very sensitive to the type of exciton state. This
sensitivity suggests that SHG spectroscopy is a convenient tool for analyzing
the fine structure of excitonic states.Comment: 13 pages, 5 figure
Emergent Trion-Phonon Coupling in Atomically-Reconstructed MoSe-WSe Heterobilayers
In low-temperature resonant Raman experiments on MoSe-WSe
heterobilayers, we identify a hybrid interlayer shear mode (HSM) with an
energy, close to the interlayer shear mode (SM) of the heterobilayers, but with
a much broader, asymmetric lineshape. The HSM shows a pronounced resonance with
the intralayer hybrid trions (HX) of the MoSe and WSe layers, only.
No resonance with the neutral intralayer excitons is found. First-principles
calculations reveal a strong coupling of Q-valley states, which are delocalized
over both layers and participate in the HX, with the SM. This emerging
trion-phonon coupling may be relevant for experiments on gate-controlled
heterobilayers.Comment: 6 pages, 3 figure
Strain control of exciton and trion spin-valley dynamics in monolayer transition metal dichalcogenides
The electron-hole exchange interaction is a fundamental mechanism that drives
valley depolarization via intervalley exciton hopping in semiconductor
multi-valley systems. Here, we report polarization-resolved photoluminescence
spectroscopy of neutral excitons and negatively charged trions in monolayer
MoSe and WSe under biaxial strain. We observe a marked
enhancement(reduction) on the WSe triplet trion valley polarization with
compressive(tensile) strain while the trion in MoSe is unaffected. The
origin of this effect is shown to be a strain dependent tuning of the
electron-hole exchange interaction. A combined analysis of the strain dependent
polarization degree using ab initio calculations and rate equations shows that
strain affects intervalley scattering beyond what is expected from strain
dependent bandgap modulations. The results evidence how strain can be used to
tune valley physics in energetically degenerate multi-valley systems
Trion induced photoluminescence of a doped MoS2 monolayer
We demonstrate that the temperature and doping dependencies of the photoluminescence (PL) spectra of a doped MoS2 monolayer have several peculiar characteristics defined by the trion radiative decay. While only zero-momentum exciton states are coupled to light, radiative recombination of non-zero momentum trions is also allowed. This leads to an asymmetric broadening of the trion spectral peak and redshift of the emitted light with increasing temperature. The lowest energy trion state is dark, which is manifested by the sharply non-monotonic temperature dependence of the PL intensity. Our calculations combine the Dirac model for the single-particle states, with parameters obtained from the first-principles calculations, and the direct solution of the three-particle problem within the Tamm-Dancoff approximation. The numerical results are well captured by a simple model that yields analytical expressions for the temperature dependencies of the PL spectra