Novel two-dimensional, atomically thin materials, notably graphene and transition-metal dichalcogenides, exhibit exotic Dirac quasiparticle spectra and can serve as experimental and theoretical laboratories for understanding fundamental physical phenomena, such as the nature of the van der Waals force between atoms and material surfaces. van der Waals forces are the most common manifestations of electromagnetism in Nature and govern the interactions between neutral objects on mesoscopic scales. Yet they are very hard to “engineer” and manipulate effectively, and here the newly-discovered graphene-type materials can lead to fundamental progress. I will discuss recent progress and my results in the following main directions: (1) I will describe how van der Waals interactions between cold atoms and graphene arise and the way physicists calculate them. The atoms have to be fairly “cold” so that thermal motion and other dissipation effects do not interfere with the main phenomenon. (2) I will present recent results on how we can effectively change the strength of these interactions. In particular I will discuss how changing the number of electrons in the graphene sheet (via application of voltage) can lead to changes in the van der Waals force. Other ways of manipulating, or “quantum engineering” such forces will also be mentioned, namely effects of strain and using other atomically thin materials instead of graphene.
Novel two-dimensional, atomically thin materials, notably graphene and transition-metal dichalcogenides, exhibit exotic Dirac quasiparticle spectra and can serve as experimental and theoretical laboratories for understanding fundamental physical phenomena, such as the nature of the van der Waals force between atoms and material surfaces. van der Waals forces are the most common manifestations of electromagnetism in Nature and govern the interactions between neutral objects on mesoscopic scales. Yet they are very hard to “engineer” and manipulate effectively, and here the newly-discovered graphene-type materials can lead to fundamental progress. I will discuss recent progress and my results in the following main directions: (1) I will describe how van der Waals interactions between cold atoms and graphene arise and the way physicists calculate them. The atoms have to be fairly “cold” so that thermal motion and other dissipation effects do not interfere with the main phenomenon. (2) I will present recent results on how we can effectively change the strength of these interactions. In particular I will discuss how changing the number of electrons in the graphene sheet (via application of voltage) can lead to changes in the van der Waals force. Other ways of manipulating, or “quantum engineering” such forces will also be mentioned, namely effects of strain and using other atomically thin materials instead of graphene.
Abstract Novel two-dimensional atomically flat materials, such as graphene and transition-metal dichalcogenides, exhibit unconventional Dirac electronic spectra. We propose to effectively engineer their interactions with cold atoms in microgravity, leading to a synergy between complex electronic and atomic collective quantum phases and phenomena. Dirac materials are susceptible to manipulation and quantum engineering via changes in their electronic properties by application of strain, doping with carriers, adjustment of their dielectric environment, etc. Consequently the interaction of atoms with such materials, namely the van der Waals/Casimir–Polder interaction, can be effectively manipulated, leading to the potential observation of physical effects such as quantum reflection off atomically thin materials and confined Bose–Einstein condensate frequency shifts.
We discuss how the presence of two-dimensional (2D) materials, such as graphene, can significantly affect two intrinsically quantum phenomena that have traditionally served as probes of the fundamental van der Waals (VDW) interaction. First, we show theoretically that Quantum Reflection (QR) of slow atoms off attractive VDW potential tails (due to interactions with 2D materials) is very strongly dependent on material characteristics (such as band structure, doping and screening level, etc). Secondly, we analyze manifestations of such 2D effects for many atoms forming a confined Bose-Einstein condensate (BEC) placed near 2D materials, which in turn makes the BEC frequency sensitive to the interface. In both cases we find that relatively small 2D material changes (either by external factors such as strain or doping, or by using gapped 2D materials instead of graphene) can have a profound effect on the above phenomena. In particular, Quantum Reflection at a given energy can experience a significant enhancement or supression (relative to conventionally used bulk materials) making 2D quantum materials an attractive playground for the study of many-body phenomena at the interface of atomic and solid state physics. *This work received support under NASA grant number 80NSSC19M0143.
Abstract Atoms deposited on 2D electronic materials, such as graphene, can exhibit unconventional many‐body correlations, not accessible in other settings. All of these are driven by van der Waals forces: between the atoms themselves and atom‐material interactions. For example 4He atoms on 2D materials can potentially form a variety of exotic quantum states of matter, such as 2D supersolids and superfluids, in addition to solid phases. For the “most quantum” case of a single helium layer we discuss, from a theoretical perspective, how the effective low‐energy (Bose–Hubbard) description can take advantage of the extreme sensitivity of this unique system to the interplay between the atomic (helium) and solid‐state (graphene) components. Due to the extraordinary variety and tunability of 2D electronic materials, it is envisaged that a wide range of correlated atomic phases can be realized under favorable conditions. Exciting possibilities in the opposite extreme of many atomic layers forming a liquid on top of graphene are also outlined —in this case a so‐called “spinodal dewetting” pattern can form at the liquid–vapor interface which reflects the presence and electronic properties of graphene underneath. Such patterns could be manipulated by choosing different atoms and materials, with potential technological applications.
We aim to understand how the van der Waals force between neutral adatoms and a graphene layer is modified by uniaxial strain and electron correlation effects. A detailed analysis is presented for three atoms (He, H, and Na) and graphene strain ranging from weak to moderately strong. We show that the van der Waals potential can be significantly enhanced by strain, and present applications of our results to the problem of elastic scattering of atoms from graphene. In particular, we find that quantum reflection can be significantly suppressed by strain, meaning that dissipative inelastic effects near the surface become of increased importance. Furthermore, we introduce a method to independently estimate the Lennard-Jones parameters used in an effective model of He interacting with graphene, and determine how they depend on strain. At short distances, we find that strain tends to reduce the interaction strength by pushing the location of the adsorption potential minima to higher distances above the deformed graphene sheet. This opens up the exciting possibility of mechanically engineering an adsorption potential, with implications for the formation and observation of anisotropic low-dimensional superfluid phases.
A tutorial overview is given on interactions between atoms and surfaces that are mediated by the electromagnetic field. The emphasis is on dispersion (or van der Waals) forces and transitions induced by thermal fluctuations in the near field of a surface. Applications with cold atoms held in microscopic traps near surfaces ("atom chips") provide the experimental background for this theory lecture. We do not cover the chemical physics aspects that are relevant at distances comparable to the atomic scale, like adsorption and diffusion on surfaces, growth of nanostructures, or atomic beam diffraction from crystalline surfaces. These are covered by other lecturers.
Atoms deposited on atomically thin substrates are a playground for exotic quantum many-body physics due to the highly tunable, atomic-scale nature of the interaction potentials. The ability to engineer strong interparticle interactions can lead to the emergence of collective states of matter, not possible in the context of dilute atomic gases confined in optical lattices. While it is known that the first layer of adsorbed helium on graphene is permanently locked into a solid phase, we motivate, with a physically intuitive mean-field calculation, and confirm, with quantum Monte Carlo simulations, that simple isotropic graphene lattice expansion unlocks a large variety of two-dimensional ordered commensurate, incommensurate, cluster atomic solid, and superfluid states for adsorbed atoms. It is especially significant that an atomically thin superfluid phase of matter emerges under experimentally feasible strain values, with potentially supersolid phases in close proximity on the phase diagram.