Collective electronic excitation in a trapped ensemble of photogenerated dipolar excitons and free holes revealed by inelastic light scattering

Abstract

Photogenerated excitonic ensembles confined in coupled GaAs quantum wells are probed by a complementary approach of emission spectroscopy and resonant inelastic light scattering.Lateral electrostatic trap geometries are used to create dense systems of spatially indirect excitons and excess holes with similar densities in the order of 10 11 cm -2. Inelastic light scattering spectra reveal a very sharp low-lying collective mode that is identified at an energy of 0.44 meV and a FWHM of only ~50 µeV. This mode is interpreted as a plasmon excitation of the excess hole system coupled to the photogenerated indirect excitons. The emission energy of the indirect excitons shifts under the application of a perpendicular applied electric field with the quantum-confined Stark effect unperturbed from the presence of free charge carriers. Our results illustrate the potential of studying low-lying collective excitations in photogenerated exciton systems to explore the many-body phase diagram, related phase transitions, and interaction physics. [1] . BEC has been observed for the first time in atomic systems [2,3] and later also in more exotic solid-state systems like exciton-polaritons [4][5][6] . Already more than 50 years ago, BEC has been predicted for excitons in semiconductors [7] . Most of such experiments are based on two tunnelcoupled semiconductor quantum wells (CQWs) hosting the excitons. These are electron-hole pairs coupled by the attractive Coulomb interaction. The bosonic quasiparticles exhibit a rich quantum phase diagram including a free exciton gas at elevated temperatures and low densities, which is expected to condense into a BEC with a macroscopic coherence at low temperatures.At high densities, an electron-hole plasma is predicted at higher temperatures that undergoes a Bardeen-Cooper-Schrieffer (BCS) transition to an excitonic insulator at low temperatures [1] .Also, signatures for a (classical) exciton liquid formed by repulsive exciton interactions have been reported [8] . The phase transition between the different phases might be gradual, and the coexistence of different (classical and quantum) phases is possible. Each phase inherently holds a characteristic spectrum of low energy collective excitations of strongly or weakly coupled electron-hole pairs including phenomena such as roton instabilities in their wave vector dispersion [9,10] . In this context, it has been shown for electron bilayers at a total filling factor of one, where exciton condensation occurs [11][12][13][14] , that deep and soft magnetorotons as well as collective spin excitations exist [15,16] . These excitations are linked to quantum phase transitions [15,16] .In general, there are two types of CQW systems hosting excitons. In one case, both CQWs are doped with electrons, and a strong perpendicular magnetic field is applied such that in each quantum well, the lowest Landau level is exactly half filled with electrons and consequently also half filled with holes. If the Coulomb interaction is strong enough in such CQWs, the...

Long-Lived Direct and Indirect Interlayer Excitons in van der Waals Heterostructures

“…IXs are composite bosons that feature enlarged lifetimes due to the reduced overlap of the electronhole wave functions resulting in dense IX ensembles that are thermalized to the lattice temperature. Besides IX ensembles in III-V HS [5][6][7][8][9][10][11][12][13][14][15], hetero-bilayers prepared from semiconducting transition metal dichalcogenides (TMDs) exhibit superior potential for studying interacting IX ensembles [3,[16][17][18][19][20][21][22][23][24][25] with intriguing spin-and valley-properties [26][27][28]. At the same time, TMDs are truly two-dimensional (2D) crystals coupled to each other or to substrates by van der Waals forces.…”

On the parabolicity of dipolar exciton traps and their population of excess charge carriers

“…Since we observe the SEP only in the so-called trap configuration (i.e. V T <V G ), we can deduce that the excess charge carriers in the CQWs are holes and in turn, that within the trion-picture, the SEP describes the photon emission from positively charged excitons [25,35,36]. Given the high electric field strength along the growth direction for the investigated range of V T and the relatively small tunneling barrier in-between the two quantum wells, we infer that both holes forming the trion complex are located in the same QW.…”

“…Generally, the polarity and escape dynamics of the unbound charge carrier are determined by the 'trap'and 'anti-trap'-configuration of both electrodes [36]. In the discussed 'trap'configuration (figure 2), the unbound holes remain confined below the central gate, and the trapped IX ensemble is in coexistence with a two dimensional hole gas [24,35,36]. However, we find that the FEP intensity is enhanced at the electrode edges by a factor of maximum 2.3, in apparent contradiction to an assumed IX dissociation at these positions.…”

“…For this reason, in-plane and out-of-plane flow of excess charge carriers is possible in such electrostatic trap geometries. As a direct consequence, the introduced decoherence [33] and the loss of exciton oscillator strength [34] makes it experimentally challenging, but not impossible to detect the phase-coherent properties of such charged IX ensembles [35]. As soon as excess charge carriers are coexistent to the excitons, a second emission line emerges.…”

“…As soon as excess charge carriers are coexistent to the excitons, a second emission line emerges. The corresponding exciton complexes have been attributed to so-called trion states with a neutral exciton being bound to a free charge carrier [24,[35][36][37][38][39][40][41][42][43][44][45][46][47][48][49], albeit recent theory and experiments point out an alternative explanation in terms of attractive and repulsive exciton-polaron branches of excitons being dressed with the interaction with excess charge carriers [50,51]. Generally, it is unclear whether such exciton complexes impairs the formation of a BEC of trapped composite boson ensembles.…”