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arXiv:1906.10174 · Physical Review A

Tuning quantum reflection in graphene with an external magnetic field

Marcius Silvestre, Tarik P. Cysne, Daniela Szilard, F. A. Pinheiro, Carlos Farina

Published Sep 4, 2019• 7 citations
arXivHTMLPDF

Abstract

We theoretically demonstrate that an external magnetic field can be used to control quantum reflection of matter waves in graphene due to its extraordinary magneto-optical properties. We calculate the quantum reflection probabilities in graphene for three experimentally relevant atomic species (He, Na, and Rb) using the full Casimir-Polder potential computed by the Lifshitz formula valid at all distance regimes, going beyond the traditional approach to quantum reflection, based on power-law potentials, which are known to be valid only in the short-distance (nonretarded van der Waals) or in the large-distance (retarded) regimes. We predict the energy range for which quantum reflection is more likely to occur as a function of the magnetic field and show that the quantum reflection probabilities exhibit discontinuities that reflect the structure of Landau levels in graphene. Altogether our findings suggest an alternative way to control quantum reflection at the nanoscale and pave the way for the design of alternative, magnetically tuned reflective diffraction elements for matter waves.

DOI
10.1103/physreva.100.033605

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We theoretically demonstrate that an external magnetic field can be used to control quantum reflection of matter waves in graphene due to its extraordinary magneto-optical properties. We calculate the quantum reflection probabilities in graphene for three experimentally relevant atomic species (He, Na, and Rb) using the full Casimir-Polder potential computed by the Lifshitz formula valid at all distance regimes, going beyond the traditional approach to quantum reflection, based on power-law potentials, which are known to be valid only in the short-distance (nonretarded van der Waals) or in the large-distance (retarded) regimes. We predict the energy range for which quantum reflection is more likely to occur as a function of the magnetic field and show that the quantum reflection probabilities exhibit discontinuities that reflect the structure of Landau levels in graphene. Altogether our findings suggest an alternative way to control quantum reflection at the nanoscale and pave the way for the design of alternative, magnetically tuned reflective diffraction elements for matter waves.
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 Among the fundamental quantum effects, quantum reflection (QR) is one of the most notable phenomena. Approximating arbitrary potentials in the Schrödinger equation as multistep potentials allows us to determine the reflection coefficient by means of a Riccati equation. We introduce a measure for the wave function's characteristic reflection distance and derive an analytical expression for this reflection point in an arbitrary power-law potential. We study the associated QR rate in the context of Casimir-Polder atom surface interactions, revealing its strong dependence on critical parameters such as velocity and interaction duration. As an application, we demonstrate how the matter-wave diffraction of a low-velocity metastable argon beam impinging on a rotated grating facilitates high-precision potential analysis.
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.
We demonstrate experimentally that an electric field can be used to control quantum reflection of matter waves off periodically microstructured surfaces. Applying a voltage of alternating polarity between neighboring grating bars induces an electric field which modifies the interaction between the surface and the impinging matter wave so that quantum reflection is gradually reduced. We find that the measured reflectivities are in good agreement with our numerical simulations and that their suppression is well captured by a simple analytic model. This experiment constitutes a step towards electrically tuned reflective diffraction elements for atomic and molecular matter waves.
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.
The properties of an electron in an atom or molecule are not fixed; rather they are a function of the optical environment of the emitter. Not only is the spontaneous emission a function of the optical environment, but also the underlying wave functions and energy levels, which are modified by the potential induced by quantum fluctuations of the electromagnetic field. In free space, this modification of atomic levels and wave functions is very weak and generally hard to observe due to the prevalence of other perturbations like fine structure. Here, we explore the possibility of highly tailorable electronic structure by exploiting large Lamb shifts in tunable electromagnetic environments such as graphene, whose optical properties are dynamically controlled via doping. The Fermi energy can be chosen so that the Lamb shift is very weak, but it can also be chosen so that the shifts become more prominent than the fine structure of the atom and even potentially the Coulomb interaction with the nucleus. Potential implications of this idea include being able to electronically shift an unfavorable emitter structure into a favorable one, a new approach to probe near-field physics in fluorescence, and a way to access regimes of physics where vacuum fluctuations are not a weak perturbation but rather the dominant physics.