M. Mootz

Quantum droplets of electrons and holes

Interacting many-body systems are characterized by stable configurations of objects—ranging from elementary particles to cosmological formations—that also act as building blocks for more complicated structures. It is often possible to incorporate interactions in theoretical treatments of crystalline solids by introducing suitable quasiparticles that have an effective mass, spin or charge which in turn affects the material’s conductivity, optical response or phase transitions. Additional quasiparticle interactions may also create strongly correlated configurations yielding new macroscopic phenomena, such as the emergence of a Mott insulator, superconductivity or the pseudogap phase of high-temperature superconductors. In semiconductors, a conduction-band electron attracts a valence-band hole (electronic vacancy) to create a bound pair, known as an exciton, which is yet another quasiparticle. Two excitons may also bind together to give molecules, often referred to as biexcitons, and even polyexcitons may exist. In indirect-gap semiconductors such as germanium or silicon, a thermodynamic phase transition may produce electron–hole droplets whose diameter can approach the micrometre range. In direct-gap semiconductors such as gallium arsenide, the exciton lifetime is too short for such a thermodynamic process. Instead, different quasiparticle configurations are stabilized dominantly by many-body interactions, not by thermalization. The resulting non-equilibrium quantum kinetics is so complicated that stable aggregates containing three or more Coulomb-correlated electron–hole pairs remain mostly unexplored. Here we study such complex aggregates and identify a new stable configuration of charged particles that we call a quantum droplet. This configuration exists in a plasma and exhibits quantization owing to its small size. It is charge neutral and contains a small number of particles with a pair-correlation function that is characteristic of a liquid. We present experimental and theoretical evidence for the existence of quantum droplets in an electron–hole plasma created in a gallium arsenide quantum well by ultrashort optical pulses.

Coherent cyclotron motion beyond Kohn’s theorem

Kohn’s theorem states that the electron cyclotron resonance is unaffected by many-body interactions in a static magnetic field. Yet, intense terahertz pulses do introduce Coulomb effects between electrons—holding promise for quantum control of electrons.

Sequential build-up of quantum-optical correlations

The build-up dynamics of quantum-optical correlations in a Jaynes–Cummings model for semiconductor quantum-dot systems is characterized using the cluster-expansion scheme. Assuming an excitation with a coherent state source under strong- and weak-coupling conditions, it is found that higher-order correlations are sequentially generated. Even though the influence of dephasing hinders their development, significant correlations build up even in the presence of strong dissipation showing that quantum-spectroscopy studies are possible even in interacting many-body systems like semiconductors.

Theory of ultrafast spin-charge quantum dynamics in strongly correlated systems controlled by femtosecond photoexcitation: an application to insulating antiferromagnetic manganites

We use a non-equilibrium many-body theory that engages the elements of transient coherence, correlation, and nonlinearity to describe changes in the magnetic and electronic phases of strongly correlated systems induced by femtosecond nonlinear photoexcitation. Using a generalized tight–binding mean field approach based on Hubbard operators and including the coupling of the laser field, we describe a mechanism for simultaneous insulator–to–metal and anti- to ferro–magnetic transition to a transient state triggered by non–thermal ultrafast spin and charge coupled excitations. We demontrate, in particular, that photoexcitation of composite fermion quasiparticles induces quasi-instantaneous spin canting that quenches the energy gap of the antiferromagnetic insulator and acts as a nonadiabatic “initial condition” that triggers non-thermal lattice dynamics leading to an insulator to metal and antiferromagnetic (AFM) to ferromagnetic (FM) transitions. Our theoretical predictions are consistent with recent ultrafast pump-probe spectroscopy experiments that revealed a magnetic phase transition during 100fs laser pulse photoexcitation of the CE–type AFM insulating phase of colossal magnetoresistive manganites. In particular, experiment observes two distinct charge relaxation components, fs and ps, with non- linear threshold dependence at a pump fluence threshold that coincides with that for femtosecond magnetization photoexcitation. Our theory attributes the correlation between femtosecond spin and charge nonlinearity leading to transition in the magnetic and electronic state to spin/charge/lattice coupling and laser-induced quantum spin canting that accompanies the driven population inversion between two quasi–particle bands with different properties: a mostly occupied polaronic band and a mostly empty metallic band, whose dispersion is determined by quantum spin canting.

Terahertz-light quantum tuning of a metastable emergent phase hidden by superconductivity

Abstract‘Sudden’ quantum quench and prethermalization have become a cross-cutting theme for discovering emergent states of matter1–4. Yet this remains challenging in electron matter5–9, especially superconductors10–14. The grand question of what is hidden underneath superconductivity (SC)15 appears universal, but poorly understood. Here we reveal a long-lived gapless quantum phase of prethermalized quasiparticles (QPs) after a single-cycle terahertz (THz) quench of a Nb3Sn SC gap. Its conductivity spectra is characterized by a sharp coherent peak and a vanishing scattering rate that decreases almost linearly towards zero frequency, which is most pronounced around the full depletion of the condensate and absent for a high-frequency pump. Above a critical pump threshold, such a QP phase with coherent transport and memory persists as an unusual prethermalization plateau, without relaxation to normal and SC thermal states for an order of magnitude longer than the QP recombination and thermalization times. Switching to this metastable ‘quantum QP fluid’ signals non-thermal quench of coupled SC and charge-density-wave (CDW)-like orders and hints quantum control beneath the SC.THz pump–probe experiments of superconducting Nb3Sn reveal that a metastable phase is induced for excitation fields around 250 kV cm−1.

Extending nonlinear coherent quantum control with intense terahertz pulses

Dynamics in solid-state systems are governed by many-body interactions that are inherently tied to the high density of electrons and ions. For most elementary excitations, however, Coulomb (i.e., elastic) scattering leads to dephasing within a few to a fewhundreds of femtoseconds. Coherent quantum control (the precise manipulation of the phases of quantum states) is therefore usually considered to be a daunting challenge in many-body systems. In 1961, however, Walter Kohn found that the cyclotron resonance (CR) of Landau electrons (i.e., the harmonic motion of electrons in a magnetic field) is immune to electron–electron Coulomb interactions. Kohn’s theorem thus shows that the CR is one of the most robust manifestations of the quantum harmonic oscillator, and excludes the possibility of any nonlinear light– matter interactions.1 Although the CR has given rise to a number of sophisticated quantum phenomena, such as ultrastrong light–matter coupling,2 superradiance,3 coherent control,4 and superfluorescence,5 the complete absence of nonlinearities suggests that many intriguing possibilities (e.g., quantum logic operations) are excluded. In this work,6 we show how strong terahertz (THz) pulses can be used to create non-perturbative THz excitations of a magnetically biased, 2D electron gas (2DEG). The pulses induce strong, coherent nonlinearities and facilitate coherent quantum control of multiple Landau levels, leading to population inversion. In our approach, the 2DEG is contained within two 30nm-wide gallium arsenide quantum wells (each n-doped at 1.6 1011cm 2) Figure 1. (a) The transmitted terahertz (THz) field is used to monitor the coherent inter-Landau level polarization. Results are shown for different amplitudes of the driving field (between 0.7 and 8.7kVcm 1), as a function of the electro-optic sampling (EOS) delay time (t). (b) The decay constant ( c) that is extracted from the data in (a), as a function of the initial field (E0). The Landau-level population, for fields of 4.3 and 8.7kVcm 1 (blue and red bars, respectively), is shown in the inset. f: Population density. h̄!LO: Longitudinal optical phonon energy.

Nonperturbative THz nonlinearities for many-body quantum control in semiconductors

Quantum computing and ultrafast quantum electronics constitute pivotal technologies of the 21st century and revolutionize the way we process information. Successful implementations require controlling superpositions of states and coherence in matter, and exploit nonlinear effects for elementary logic operations. In the THz frequency range between optics and electronics, solid state systems offer a rich spectrum of collective excitations such as excitons, phonons, magnons, or Landau electrons. Here, single-cycle THz transients of 8.7 kV/cm amplitude centered at 1 THz strongly excite inter-Landau-level transitions of magnetically biased GaAs quantum wells, facilitating coherent Landau ladder climbing by more than six rungs, population inversion, and coherent polarization control. Strong, highly nonlinear pump-probe and four- and six-wave mixing signals, entirely unexpected for this paragon of the harmonic oscillator, are revealed through two-time THz spectroscopy. In this scenario of nonperturbative polarization dynamics, our microscopic theory shows how the protective limits of Kohn’s theorem are ultimately surpassed by dynamically enhanced Coulomb interactions, opening the door to exploiting many-body dynamics for nonlinear quantum control.

Quantum optics with dropletons

Dropletons are highly correlated quasiparticles, recently found in GaAs quantum wells. We demonstrate that they can be controlled by adjusting light sources quantum fluctuations and that their size grows with increasing temperature.

Quantum Electron-Hole Droplets in GaAs Quantum Wells

We present evidence from transient-absorption spectra for quantum electron-hole droplets in GaAs quantum wells. Quantum droplets have a two-particle correlation function characteristic of a liquid, but, unlikemacroscopic droplets, have quantized binding energy.

Quantum theory of dropletons

A quantum theory is presented for the identification of dropletons as new quasiparticles in GaAs quantum wells. Dropletons consist of more than four electron-hole pairs in a liquid-like state and have quantized binding energy due to their small size.