arXiv subjects

F. M. Marchetti

Publications and source records attributed to F. M. Marchetti.

At least 19 recordsLinked to original sources

arXiv2024

Rydberg excitons and polaritons in monolayer transition metal dichalcogenides in a magnetic field

We develop a microscopic theory for excitons and cavity exciton polaritons in transition metal dichalcogenide (TMD) monolayers under a perpendicular static magnetic field. We obtain numerically exact solutions for the ground and excited states, accounting for the interplay between arbitrarily large magnetic fields and light-matter coupling strengths. This includes the very strong coupling regime, where light-induced modifications of the exciton wavefunction become essential and the approximate coupled oscillator description breaks down. Our results show excellent agreement with recent experimental measurements of the diamagnetic shift of the ground and excited exciton states in WS$_2$, MoS$_2$, MoSe$_2$, and MoTe$_2$ monolayers. For polaritons, we consider experimentally relevant system parameters and demonstrate that the diamagnetic shifts of both the ground and excited states at high magnetic fields exhibit clear signatures of the very strong coupling regime, highlighting the necessity of our microscopic and numerically exact treatment over perturbative approaches. Furthermore, our microscopic approach allows us to evaluate the exciton-exciton and polariton-polariton interaction strengths. Comparing results specific to TMD monolayers with those applicable to quantum wells, we find that variational approaches overestimate the TMD excitons' interaction strength. We also observe that magnetic fields weaken the interaction strength for both excitons and polaritons, with a less pronounced effect in TMDs than in quantum wells, and that light-induced modifications to the matter component in TMD polaritons can enhance interaction strengths beyond those of purely excitonic interactions.

cond-mat.mes-hall↗

arXiv2024

Multiple polaron quasiparticles with dipolar fermions in a bilayer geometry

We study the Fermi polaron problem with dipolar fermions in a bilayer geometry, where a single dipolar particle in one layer interacts with a Fermi sea of dipolar fermions in the other layer. By evaluating the polaron spectrum, we obtain the appearance of a series of attractive branches when the distance between the layers diminishes. We relate these to the appearance of a series of bound two-dipole states when the interlayer dipolar interaction strength increases. By inspecting the orbital angular momentum component of the polaron branches, we observe an interchange of orbital character when system parameters such as the gas density or the interlayer distance are varied. Further, we study the possibility that the lowest energy two-body bound state spontaneously acquires a finite center of mass momentum when the density of fermions exceeds a critical value, and we determine the dominating orbital angular momenta that characterize the pairing. Finally, we propose to use the tunneling rate from and into an auxiliary layer as an experimental probe of the impurity spectral function.

cond-mat.quant-gas↗

arXiv2022

Crossover from exciton polarons to trions in doped two-dimensional semiconductors at finite temperature

We study systematically the role of temperature in the optical response of doped two-dimensional semiconductors. By making use of a finite-temperature Fermi-polaron theory, we reveal a crossover from a quantum-degenerate regime with well-defined polaron quasiparticles to an incoherent regime at high temperature or low doping where the lowest energy "attractive" polaron quasiparticle is destroyed, becoming subsumed into a broad trion-hole continuum. We demonstrate that the crossover is accompanied by significant qualitative changes in both absorption and photoluminescence. In particular, with increasing temperature (or decreasing doping), the emission profile of the attractive branch evolves from a symmetric Lorentzian to an asymmetric peak with an exponential tail involving trions and recoil electrons at finite momentum. We discuss the effect of temperature on the coupling to light for structures embedded into a microcavity, and we show that there can exist well-defined polariton quasiparticles even when the exciton-polaron quasiparticle has been destroyed, where the transition from weak to strong light-matter coupling can be explained in terms of the polaron linewidths and spectral weights.

cond-mat.mes-hall↗

arXiv2022

Exact Quantum Virial Expansion for the Optical Response of Doped Two-Dimensional Semiconductors

We present a quantum virial expansion for the optical response of a doped two-dimensional semiconductor. As we show, this constitutes a perturbatively exact theory in the high-temperature or low-doping regime, where the electrons' thermal wavelength is smaller than their interparticle spacing. The virial expansion predicts new features of the photoluminescence, such as a non-trivial shape of the attractive branch related to universal low-energy exciton-electron scattering and an associated shift of the attractive peak from the trion energy. Our results are in excellent agreement with recent experiments on doped monolayer MoSe$_2$ [Zipfel et al., Phys. Rev. B 105, 075311 (2022)] and they imply that the trion binding energy is likely to have been overestimated in previous measurements. Our theory furthermore allows us to formally unify two distinct theoretical pictures that have been applied to this system, with the conventional trion picture results emerging as a high-temperature and weak-interaction limit of Fermi polaron theory.

cond-mat.mes-hall↗

arXiv2021

Effect of fermion indistinguishability on optical absorption of doped two-dimensional semiconductors

We study the optical absorption spectrum of a doped two-dimensional semiconductor in the spin-valley polarized limit. In this configuration, the carriers in the Fermi sea are indistinguishable from one of the two carriers forming the exciton. Most notably, this indistinguishability requires the three-body trion state to have p-wave symmetry. To explore the consequences of this, we evaluate the system's optical properties within a polaron description, which can interpolate from the low density limit -- where the relevant excitations are few-body bound states -- to higher density many-body states. In the parameter regime where the trion is bound, we demonstrate that the spectrum is characterized by an attractive quasiparticle branch, a repulsive branch, and a many-body continuum, and we evaluate the doping dependence of the corresponding energies and spectral weights. In particular, at low doping we find that the oscillator strength of the attractive branch scales with the square of the Fermi energy as a result of the trion's p-wave symmetry. Upon increasing density, we find that both the repulsive and attractive branches blueshift, and that the orbital character of the states associated with these branches interchanges. We compare our results with previous investigations of the scenario where the Fermi sea involves carriers distinguishable from those in the exciton, for which the trion ground state is s-wave.

cond-mat.mes-hall↗

arXiv2019

Extremely imbalanced two-dimensional electron-hole-photon systems

We investigate the phases of two-dimensional electron-hole systems strongly coupled to a microcavity photon field in the limit of extreme charge imbalance. Using variational wave functions, we examine the competition between different electron-hole paired states for the specific cases of semiconducting III-V single quantum wells, electron-hole bilayers, and transition metal dichalcogenide monolayers embedded in a planar microcavity. We show how the Fermi sea of excess charges modifies both the electron-hole bound state (exciton) properties and the dielectric constant of the cavity active medium, which in turn affects the photon component of the many-body polariton ground state. On the one hand, long-range Coulomb interactions and Pauli blocking of the Fermi sea promote electron-hole pairing with finite center-of-mass momentum, corresponding to an excitonic roton minimum. On the other hand, the strong coupling to the ultra-low-mass cavity photon mode favors zero-momentum pairs. We discuss the prospect of observing different types of electron-hole pairing in the photon spectrum.

cond-mat.mes-hall↗

arXiv2018

Early-time dynamics of Bose gases quenched into the strongly interacting regime

We study the early-time dynamics of a degenerate Bose gas after a sudden quench of the interaction strength, starting from a weakly interacting gas. By making use of a time-dependent generalization of the Nozières-Saint-James variational formalism, we describe the crossover of the early-time dynamics from shallow to deep interaction quenches. We analyze the coherent oscillations that characterize both the density of excited states and the Tan's contact as a function of the final scattering length. For shallow quenches, the oscillatory behaviour is negligible and the dynamics is universally governed by the healing length and the mean-field interaction energy. By increasing the final scattering length to intermediate values, we reveal a universal regime where the period of the coherent atom-molecule oscillations is set by the molecule binding energy. For the largest scattering lengths we can numerically simulate in the unitary regime, we find a universal scaling behaviour of the typical growth time of the momentum distribution in agreement with recent experimental observations [C. Eigen et al., Nature 563, 221 (2018)].

cond-mat.quant-gas↗

arXiv2018

Spontaneous patterns in coherently driven polariton microcavities

We consider a polariton microcavity resonantly driven by two external lasers which simultaneously pump both lower and upper polariton branches at normal incidence. In this setup, we study the occurrence of instabilities of the pump-only solutions towards the spontaneous formation of patterns. Their appearance is a consequence of the spontaneous symmetry breaking of translational and rotational invariance due to interaction induced parametric scattering. We observe the evolution between diverse patterns which can be classified as single-pump, where parametric scattering occurs at the same energy as one of the pumps, and as two-pump, where scattering occurs at a different energy. For two-pump instabilities, stripe and chequerboard patterns become the dominant steady-state solutions because cubic parametric scattering processes are forbidden. This contrasts with the single-pump case, where hexagonal patterns are the most common arrangements. We study the possibility of controlling the evolution between different patterns. Our results are obtained within a linear stability analysis and are confirmed by finite size full numerical calculations.

cond-mat.quant-gas↗

arXiv2016

Spontaneous rotating vortex rings in a parametrically driven polariton fluid

We present the theoretical prediction of spontaneous rotating vortex rings in a parametrically driven quantum fluid of polaritons -- coherent superpositions of coupled quantum well excitons and microcavity photons. These rings arise not only in the absence of any rotating drive, but also in the absence of a trapping potential, in a model known to map quantitatively to experiments. We begin by proposing a novel parametric pumping scheme for polaritons, with circular symmetry and radial currents, and characterize the resulting nonequilibrium condensate. We show that the system is unstable to spontaneous breaking of circular symmetry via a modulational instability, following which a vortex ring with large net angular momentum emerges, rotating in one of two topologically distinct states. Such rings are robust and carry distinctive experimental signatures, and so they could find applications in the new generation of polaritonic devices.

cond-mat.quant-gas↗

arXiv2016

Dipolar fermions in a multilayer geometry

We investigate the behavior of identical dipolar fermions with aligned dipole moments in two-dimensional multilayers at zero temperature. We consider density instabilities that are driven by the attractive part of the dipolar interaction and, for the case of bilayers, we elucidate the properties of the stripe phase recently predicted to exist in this interaction regime. When the number of layers is increased, we find that this "attractive" stripe phase exists for an increasingly larger range of dipole angles, and if the interlayer distance is sufficiently small, the stripe phase eventually spans the full range of angles, including the situation where the dipole moments are aligned perpendicular to the planes. In the limit of an infinite number of layers, we derive an analytic expression for the interlayer effects in the density-density response function and, using this result, we find that the stripe phase is replaced by a collapse of the dipolar system.

cond-mat.quant-gas↗

arXiv2015

Vortex and half-vortex dynamics in a spinor quantum fluid of interacting polaritons

Spinorial or multi-component Bose-Einstein condensates may sustain fractional quanta of circulation, vorticant topological excitations with half integer windings of phase and polarization. Matter-light quantum fluids, such as microcavity polaritons, represent a unique test bed for realising strongly interacting and out-of-equilibrium condensates. The direct access to the phase of their wavefunction enables us to pursue the quest of whether half vortices ---rather than full integer vortices--- are the fundamental topological excitations of a spinor polariton fluid. Here, we are able to directly generate by resonant pulsed excitations, a polariton fluid carrying either the half or full vortex states as initial condition, and to follow their coherent evolution using ultrafast holography. Surprisingly we observe a rich phenomenology that shows a stable evolution of a phase singularity in a single component as well as in the full vortex state, spiraling, splitting and branching of the initial cores under different regimes and the proliferation of many vortex anti-vortex pairs in self generated circular ripples. This allows us to devise the interplay of nonlinearity and sample disorder in shaping the fluid and driving the phase singularities dynamics

cond-mat.quant-gas↗

arXiv2015

On multicomponent polariton superfluidity in the optical parametric oscillator regime

Superfluidity, the ability of a liquid or gas to flow with zero viscosity, is one of the most remarkable implications of collective quantum coherence. In equilibrium systems like liquid 4He and ultracold atomic gases, superfluid behaviour conjugates diverse yet related phenomena, such as persistency of metastable flow in multiply connected geometries and the existence of a critical velocity for frictionless flow when hitting a static defect. The link between these different aspects of superfluid behaviour is far less clear in driven-dissipative systems displaying collective coherence, such as microcavity polaritons, which raises important questions about their concurrency. With a joint theoretical and experimental study, we show that the scenario is particularly rich for polaritons driven in a three-fluid collective coherent regime so-called optical parametric oscillator. On the one hand, the spontaneous macroscopic coherence following the phase locking of the signal and idler fluids has been shown to be responsible for their simultaneous quantized flow metastability. On the other hand, we show here that pump, signal and idler have distinct responses when hitting a static defect; while the signal displays hardly appreciable modulations, the ones appearing in pump and idler are determined by their mutual coupling due to nonlinear and parametric processes.

cond-mat.mes-hall↗

arXiv2015

Probing the collective excitations of a spinor polariton fluid

We propose a pump-probe set-up to analyse the properties of the collective excitation spectrum of a spinor polariton fluid. By using a linear response approximation scheme, we carry on a complete classification of all excitation spectra, as well as their intrinsic degree of polarisation, in terms of two experimentally tunable parameters only, the mean-field polarisation angle and a rescaled pump detuning. We evaluate the system response to the external probe, and show that the transmitted light can undergo a spin rotation along the dispersion for spectra that we classify as diffusive-like. We show that in this case, the spin flip predicted along the dispersion is enhanced when the system is close to a parametrically amplified instability.

cond-mat.mes-hall↗

arXiv2014

Non-equilibrium Berezinskii-Kosterlitz-Thouless Transition in a Driven Open Quantum System

The Berezinskii-Kosterlitz-Thouless mechanism, in which a phase transition is mediated by the proliferation of topological defects, governs the critical behaviour of a wide range of equilibrium two-dimensional systems with a continuous symmetry, ranging from superconducting thin films to two-dimensional Bose fluids, such as liquid helium and ultracold atoms. We show here that this phenomenon is not restricted to thermal equilibrium, rather it survives more generally in a dissipative highly non-equilibrium system driven into a steady-state. By considering a light-matter superfluid of polaritons, in the so-called optical parametric oscillator regime, we demonstrate that it indeed undergoes a vortex binding-unbinding phase transition. Yet, the exponent of the power-law decay of the first order correlation function in the (algebraically) ordered phase can exceed the equilibrium upper limit -- a surprising occurrence, which has also been observed in a recent experiment. Thus we demonstrate that the ordered phase is somehow more robust against the quantum fluctuations of driven systems than thermal ones in equilibrium.

cond-mat.quant-gas↗

arXiv2014

Collective pairing of resonantly coupled microcavity polaritons

We consider the possible phases of microcavity polaritons tuned near a bipolariton Feshbach resonance. We show that, as well as the regular polariton superfluid phase, a "molecular" superfluid exists, with (quasi-)long-range order only for pairs of polaritons. We describe the experimental signatures of this state. Using variational approaches we find the phase diagram (critical temperature, density and exciton-photon detuning). Unlike ultracold atoms, the molecular superfluid is not inherently unstable, and our phase diagram suggests it is attainable in current experiments.

cond-mat.quant-gas↗

arXiv2013

Phase Equilibrium of Binary Mixtures in Mixed Dimensions

We study the stability of a Bose-Fermi system loaded into an array of coupled one-dimensional (1D) "tubes", where bosons and fermions experience different dimensions: Bosons are heavy and strongly localized in the 1D tubes, whereas fermions are light and can hop between the tubes. Using the 174Yb-6Li system as a reference, we obtain the equilibrium phase diagram. We find that, for both attractive and repulsive interspecies interaction, the exact treatment of 1D bosons via the Bethe ansatz implies that the transitions between pure fermion and any phase with a finite density of bosons can only be first order and never continuous, resulting in phase separation in density space. In contrast, the order of the transition between the pure boson and the mixed phase can either be second or first order depending on whether fermions are allowed to hop between the tubes or they also are strictly confined in 1D. We discuss the implications of our findings for current experiments on 174Yb-6Li mixtures as well as Fermi-Fermi mixtures of light and heavy atoms in a mixed dimensional optical lattice system.

cond-mat.quant-gas↗

arXiv2013

Control and ultrafast dynamics of a two-fluid polariton switch

We investigate the cross-interactions in a two-component polariton quantum fluid coherently driven by two independent pumping lasers tuned at different energies and momenta. We show that both the hysteresis cycles and the ON/OFF threshold of one polariton signal can be entirely controlled by a second polariton fluid. Furthermore, we study the ultrafast switching dynamics of a driven polariton state, demonstrating the ability to control the polariton population with an external laser pulse, in less than a few picoseconds.

cond-mat.mes-hall↗

arXiv2012

Density-wave phases of dipolar fermions in a bilayer

We investigate the phase diagram of dipolar fermions with aligned dipole moments in a two-dimensional (2D) bilayer. Using a version of the Singwi-Tosi-Land-Sjolander scheme recently adapted to dipolar fermions in a single layer [M. M. Parish and F. M. Marchetti, Phys. Rev. Lett. 108, 145304 (2012)], we determine the density-wave instabilities of the bilayer system within linear response theory. We find that the bilayer geometry can stabilize the collapse of the 2D dipolar Fermi gas with intralayer attraction to form a new density wave phase that has an orientation perpendicular to the density wave expected for strong intralayer repulsion. We thus obtain a quantum phase transition between stripe phases that is driven by the interplay between strong correlations and the architecture of the low dimensional system.

cond-mat.quant-gas↗