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Low Temperature Physics

Localized electrons in dense heavy noble gases

S. S. Nazin, V. B. Shikin

Published Apr 1, 2009• 0 citations
Publisher

Abstract

The paper addresses counterintuitive behavior of electrons injected into dense cryogenic media with negative scattering length a0. Instead of the expected polaronic effect (formation of density enhancement clusters) which should substantially reduce the electron mobility, an opposite picture is observed: with increasing ∣a0∣ (the trend taking place for inert gases with the growth of atomic number) and the medium density, the electrons remain practically free. An explanation of this behavior is provided based on consistent accounting for the nonlinearity of electron interaction with the gaseous medium in the gas atom number density.

DOI
10.1063/1.3115816

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The paper addresses counterintuitive behavior of electrons injected into dense cryogenic media with negative scattering length a0. Instead of the expected polaronic effect (formation of density enhancement clusters) which should substantially reduce the electron mobility, an opposite picture is observed: with increasing ∣a0∣ (the trend taking place for inert gases with the growth of atomic number) and the medium density, the electrons remain practically free. An explanation of this behavior is provided based on consistent accounting for the nonlinearity of electron interaction with the gaseous medium in the gas atom number density.
We report accurate measurements of excess electron mobility in high-density Ar gas at a temperature T\ensuremath{\simeq}163 K in the density range (4N70)\ifmmode\times\else\texttimes\fi{}1020{10}^{20}1020 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ as a function of the electric-field strength. The positive-density effect shown by the zero-field density-normalized mobility ${\mathrm{\ensuremath{\mu}}}_{0}$N can be explained within the same theoretical framework recently used for neon gas, where ${\mathrm{\ensuremath{\mu}}}_{0}$N shows a negative-density effect. Thus the analysis of our present measurements indicates that there is no need for two separate theories for both negative- and positive-scattering-length gases.
We report an experimental and theoretical study on the shift of the hyperfine splitting frequency of ground-state Li atoms in noble gases, He, Ne, Ar, and Xe. The frequency shift is due to the change in the electron-spin density at the Li nuclei induced by collisions to the noble-gas atoms. The electron density is calculated along the interatomic distance in a pseudopotential and a dispersion potential. Based on the measured and the calculated frequency shifts, we find the importance of attractive force in collisions to helium as well as heavy noble-gas atoms. Taking advantage of the simple energy structure of the Li atom, we obtain the s wave scattering length for free electrons on noble-gas atoms by using the hyperfine splitting frequency as a precise measure.
We consider the behavior of a small density of mobile impurities (polarons) immersed in a quantum gas, a generic scenario that can be realized in cold atomic gases, liquid helium mixtures, and doped semiconductors. We present a unified theoretical framework for understanding polaron quasiparticles beyond the single-impurity limit, and we identify two key factors that control the polaron-polaron interactions: (i) the statistics of the impurities, including whether or not they are degenerate, and (ii) the constraints on the medium response, i.e., whether the medium density or chemical potential is held fixed. By constructing wave functions for two bosonic, fermionic, or distinguishable impurities immersed in a Bose or Fermi gas, we derive rigorous results for the polaron interactions in the limit of weak impurity-medium coupling. We furthermore obtain an exact relationship between the polaron interactions at fixed medium density and at fixed chemical potential, a result which is valid for arbitrary interaction strength. Our work provides an important guide for understanding experiments, and it acts as a starting point for future strong-coupling theories of polaron interactions that capture all of the effects identified in this work.
A many-body system of fermion atoms with a model interaction characterized by the scattering length a is considered. We treat both a and the density as parameters assuming that the system can be created artificially in a trap. If a is negative the system becomes strongly correlated at densities ρ ∼ |a | −3, provided the scattering length is the dominant parameter of the problem. It means that we consider |a | to be much bigger than the radius of the interaction or any other relevant parameter of the system. The density ρc1 at which the compressibility vanishes is defined by ρc1 ∼ |a | −3. Thus, a system composed of fermion atoms with the scattering length a → − ∞ is completely unstable at low densities, inevitably collapsing until the repulsive core stops the density growth. As a result, any Fermi system possesses the equilibrium density and energy if the bare particle-particle interaction is sufficiently strong to make a negative and to be the dominant parameter. This behavior can be realized in a trap. Our results show that a low density neutron matter can have the equilibrium density. PACS numbers: 05.30.Fk, 24.10.Cn, 71.10.Ca Key Words: trapped Fermi gases; effective interaction
Local field effects on the rate of spontaneous emission and Lamb shift in a dense gas of atoms are discussed taking into account correlations of atomic center-of-mass coordinates. For this the exact retarded propagator in the medium is calculated in independent scattering approximation and employing a virtual-cavity model. The resulting changes of the atomic polarizability lead to modifications of the medium response which can be of the same order of magnitude but of opposite sign than those due to local field corrections of the dielectric function derived by Morice, Castin, and Dalibard [Phys.Rev.A 51, 3896 (1995)].
The remarkable single-particle control of individual ions combined with the versatility of ultracold atomic gases makes hybrid ion-atom systems an exciting new platform for quantum simulation of few- and many-body quantum physics. Here, we study theoretically the properties of a mobile ion immersed in a quantum degenerate gas of fermionic atoms. Using an effective low-energy atom-ion interaction together with a well-established approach that includes exactly two-body correlations, we calculate the full spectral response of the ion and demonstrate the existence of several quasiparticle branches, which are charged analogs of the Fermi polaron observed in neutral atomic gases. Due to the long-range nature of the atom-ion interaction, these ionic Fermi polarons have several properties distinct from their neutral counterparts such as the simultaneous presence of several stable states and smooth transitions from repulsive to attractive polarons with increasing interaction strength. Surprisingly, the residue of the ionic polaron is shown to increase with the Fermi density for fixed interaction strength, which is in marked contrast with the neutral polaron. The properties of the ionic polaron approach that of the neutral polaron only in the low-density limit where the average interparticle spacing is larger than the characteristic length of the atom-ion interaction. We finally analyze the effects of the Fermi gas on the molecular ions, which are bound atom-dimer states.