9 research outputs found
Giant paramagnetism induced valley polarization of electrons in charge-tunable monolayer MoSe2
For applications exploiting the valley pseudospin degree of freedom in
transition metal dichalcogenide monolayers, efficient preparation of electrons
or holes in a single valley is essential. Here, we show that a magnetic field
of 7 Tesla leads to a near-complete valley polarization of electrons in MoSe2
monolayer with a density 1.6x10^{12} cm^{-2}; in the absence of exchange
interactions favoring single-valley occupancy, a similar degree of valley
polarization would have required a pseudospin g-factor exceeding 40. To
investigate the magnetic response, we use polarization resolved
photoluminescence as well as resonant reflection measurements. In the latter,
we observe gate voltage dependent transfer of oscillator strength from the
exciton to the attractive-Fermi-polaron: stark differences in the spectrum of
the two light helicities provide a confirmation of valley polarization. Our
findings suggest an interaction induced giant paramagnetic response of MoSe2,
which paves the way for valleytronics applications
Valley Zeeman effect in elementary optical excitations of monolayer WSe2
A monolayer of a transition metal dichalcogenide such as WSe2 is a two-dimensional direct-bandgap valley-semiconductor(1,2) having an effective honeycomb lattice structure with broken inversion symmetry. The inequivalent valleys in the Brillouin zone could be selectively addressed using circularly polarized light fields', suggesting the possibility for magneto-optical measurement and manipulation of the valley pseudospin degree of freedom(6-8). Here we report such experiments that demonstrate the valley Zeeman effect-strongly anisotropic lifting of the degeneracy of the valley pseudospin degree of freedom using an external magnetic field. The valley-splitting measured using the exciton transition deviates appreciably from values calculated using a three-band tight-binding model(9) for an independent electron-hole pair at +/- K valleys. We show, on the other hand, that a theoretical model taking into account the strongly bound nature of the exciton yields an excellent agreement with the experimentally observed splitting. In contrast to the exciton, the trion transition exhibits an unexpectedly large valley Zeeman effect that cannot be understood within the same framework, hinting at a different contribution to the trion magnetic moment. Our results raise the possibility of controlling the valley degree of freedom using magnetic fields in monolayer transition metal dichalcogenides or observing topological states of photons strongly coupled to elementary optical excitations in a microcavity(10)
Optically active quantum dots in monolayer WSe2
Semiconductor quantum dots have emerged as promising candidates for the implementation of quantum information processing, because they allow for a quantum interface between stationary spin qubits and propagating single photons(1-3). In the meantime, transition-metal dichalcogenide monolayers have moved to the forefront of solid-state research due to their unique band structure featuring a large bandgap with degenerate valleys and non-zero Berry curvature(4). Here, we report the observation of zero-dimensional anharmonic quantum emitters, which we refer to as quantum dots, in monolayer tungsten diselenide, with an energy that is 20-100 meV lower than that of two-dimensional excitons. Photon antibunching in second-order photon correlations unequivocally demonstrates the zero-dimensional anharmonic nature of these quantum emitters. The strong anisotropic magnetic response of the spatially localized emission peaks strongly indicates that radiative recombination stems from localized excitons that inherit their electronic properties from the host transition-metal dichalcogenide. The large similar to 1 meV zero-field splitting shows that the quantum dots have singlet ground states and an anisotropic confinement that is most probably induced by impurities or defects. The possibility of achieving electrical control in van der Waals heterostructures(5) and to exploit the spin-valley degree of freedom(6) renders transition-metal-dichalcogenide quantum dots interesting for quantum information processing
Fermi polaron-polaritons in charge-tunable atomically thin semiconductors
The dynamics of a mobile quantum impurity in a degenerate Fermi system is a fundamental problem in many-body physics. The interest in this field has been renewed due to recent ground-breaking experiments with ultracold Fermi gases. Optical creation of an exciton or a polariton in a two-dimensional electron system embedded in a microcavity constitutes a new frontier for this field due to an interplay between cavity coupling favouring ultralow-mass polariton formation6 and exciton–electron interactions leading to polaron or trion formation. Here, we present cavity spectroscopy of gate-tunable monolayer MoSe2 exhibiting strongly bound trion and polaron resonances, as well as non-perturbative coupling to a single microcavity mode. As the electron density is increased, the oscillator strength determined from the polariton splitting is gradually transferred from the higher-energy repulsive exciton-polaron resonance to the lower-energy attractive exciton-polaron state. Simultaneous observation of polariton formation in both attractive and repulsive branches indicates a new regime of polaron physics where the polariton impurity mass can be much smaller than that of the electrons. Our findings shed new light on optical response of semiconductors in the presence of free carriers by identifying the Fermi polaron nature of excitonic resonances and constitute a first step in investigation of a new class of degenerate Bose–Fermi mixtures.Physic
Many-Body Effects in Optical Excitations of Transition Metal Dichalcogenides
This dissertation treats a quantum impurity problem in a semiconductor system.
Quantum impurity problems describe the interaction between a single quantum
object and a complex environment. They are ubiquitous in physical systems and
represent a fundamental eld of research in many-body physics. Prominent examples
are the Anderson orthogonality catastrophe and the Kondo effect. In both cases, the
impurity is much heavier than the constituents of the interacting environment. If the
mass of the impurity is comparable to the surrounding particles, we have a mobile
impurity. These systems are usually harder to solve, as evident in the case of lattice
polarons, which were rst proposed in 1933 by Lev Landau. A complex but accurate
description was found years later in 1955 by Richard Feynman. In recent years,
strong coupling between single, mobile quantum impurities and a fermionic bath
was realized in cold atoms. The interaction results in the formation of new quasiparticles
called Fermi polarons. In contrast to other mobile quantum impurities such
as lattice polarons, Fermi polarons can be described with a simple and quantitatively
accurate model, which renders them an especially attractive eld of research of
many-body physics.
In this work, we report the observation of Fermi polarons in a solid state environment,
namely a new class of semiconductors called transition metal dichalcogenides
(TMDs). TMDs consisting of the transition metal Tungsten or Molybdenum and
the chalcogenide Sulphur or Selenium are semiconductors. In the monolayer limit,
they feature a direct bandgap, a large Coulomb interaction and a large effective electron
and hole mass as compared to GaAs. In combination with the two-dimensional
con nement, these result in a large binding energy of the exciton. As a consequence,
the exciton remains a rigid particle even when it is surrounded by a two-dimensional
electron system (2DES) with a large electron density.
When the exciton is surrounded by a 2DES, a second resonance emerges in the
optical spectrum. Previously, this resonance was attributed to the trion, a bound
state of two electrons and a hole. In this dissertation, we demonstrate that this
emerging red-shifted resonance has to be described as a Fermi polaron. Thanks to
the large binding energy of excitons in TMDs, we can test the predictions of our
model qualitatively and quantitatively for a large range of electron densities.
For our experimental investigations, we employ cavity quantum electrodynamics
in a zero-dimensional, tunable micro-cavity to investigate the optical spectrum of
the TMD monolayer for different electron densities. The possibility to reduce the
cavity length to a few wavelengths allows the formation of polaron-polariton modes.
The strong light-matter coupling cannot be explained with the trion model, and
provides solid evidence for the validity of the Fermi polaron model to describe optical
resonances in a 2DES