Quantum Seminars

“Out-of-equilibrium dynamics of supersolids in dipolar quantum gases”

Supersolidity is an exotic phase of matter originally proposed to explain solid helium, characterized by a crystal structure on top of a superfluid background. Supersolid states have been recently observed in dipolar quantum gases of magnetic atoms, and ongoing theoretical and experimental investigations aim to unravel their dynamical properties. The latter combine both the rigid structure of solids, along with the phase coherence of superfluids. 

We focus on out-of-equilibrium protocols that further illuminate these properties. In particular, by separating and subsequently merging quasi-1D supersolids, a directed motion of the supersolid stripes occurs, being out-of-phase with the one of the superfluid substrate. Moreover, quasi-2D supersolids are driven out-of-equilibrium by dynamically crossing the superfluid-supersolid phase transition. The crystal structure is destroyed by the superfluid excitations, and the dipolar gas reaches a non-equilibrium steady state, alluding to wave turbulence.

“Control and analysis of atomic multi-photon ionization processes”

The study of how matter responds to external electromagnetic radiation has profoundly shaped our understanding of atomic, molecular, and solid-state structure and dynamics over the past century. In recent decades, this research has been propelled by the development of advanced light sources that extend the limits of field parameters—reaching higher intensities, larger photon energies, shorter time scales, and improved spectral, temporal, and phase control. By exposing matter to these precisely controlled fields, we can now study and manipulate reactions with unprecedented precision, particularly when using simple atomic targets.

At Missouri S&T, our lab investigates the multiphoton ionization of one of the simplest atoms in the periodic table—lithium. We address fundamental questions of light-matter interaction, such as: How can electron emission be controlled by the properties of the laser field? How can we extract phase and amplitude information from the ejected electrons’ wave functions? How does ionization dynamics depend on the relative orientation (or helicity) of the target and the ionizing laser field? How do laser-excited wave packets evolve over time?

These experiments not only enhance our understanding of fundamental symmetries, which are crucial in many natural processes, but also provide versatile tools for coherent control of atomic and molecular dynamics.

“Dissociative capture in p – molecule collisions and detachment in anion – atom collisions”

Atomic fragmentation processes are particularly suitable to study the fundamentally important few-body problem (FBP).  The essence of the FBP is that the Schroedinger equation is not analytically solvable for more than 2 interacting particles even when the underlying forces are precisely known.  Recently, we focused on two fragmentation processes: in the last project at S&T we performed kinematically complete experiments on dissociative capture (DC) in p + H_2 and D_2 collisions.  In DC an electron is captured from the molecule to the projectile and subsequently the molecule breaks up into an atomic and an ionic fragment. The interference structure observed in the cross sections exhibit a surprising phase shift, for which a hypothetical explanation will be presented.

In the first project in Heidelberg, we measured multiple differential cross sections for detachment and for detachment with simultaneous target ionization (DI) in anion-atom collisions.  In detachment, the loss of an electron leads to projectile neutralization and in DI detachment is accompanied by target ionization.  Some features can be understood within a “quasi-free electron model”, in which the initial binding to the projectile is neglected.  Other observations are currently not understood.  More specifically, a correlated first-order process is dominant in causing DI although the projectile energy was well below the threshold for this mechanism.

“Generalized Sellmeier Model of the Dielectric Function with Application to Atom-Surface Interactions”

We present a generalized Sellmeier model (which we refer to as the Lorentz–Dirac model) that incorporates radiation reaction effects to provide a compact and physically motivated analytic description of the dielectric function. The model’s unique feature is the complex oscillator strengths, which we show can naturally arise from the response of coupled damped oscillators, thus offering a theoretical foundation for the Lorentz–Dirac form. This model is then applied to fit the experimental data of monocrystalline intrinsic silicon and calcium fluoride, materials of significant technological importance. We found that this model achieves high accuracy using only a few oscillators for different classes of materials across a wide frequency and temperature range.

This model enables us to study the transition from short-range (nonretarded) to long-range (retarded) behavior of the atom-surface interaction potential. Surprisingly, for simple non-alkali atoms (e.g., ground state hydrogen and helium), the nonretarded regime breaks down at a distance of about 10 nm (200 Bohr radii), which is much shorter than typical characteristic absorption wavelengths of solids. Larger transition regimes were observed for atoms with a large static polarizability, such as metastable helium. We present a simple analytic expression for the critical distance of onset of retardation. Our numerical analysis shows that it can reliably estimate the onset of retardation distance.

Our results provide theoretical insight and practical tools for modeling the optical properties of materials of technological significance. It also allows us to study fundamental phenomena such as atom-surface interactions in greater detail.