arXiv subjects

A. V. Avdeenkov

Publications and source records attributed to A. V. Avdeenkov.

11 recordsLinked to original sources

arXiv2008

Electronic and transport properties of rectangular graphene macromolecules and zigzag carbon nanotubes of finite length

We study one dimensional (1D) carbon ribbons with the armchair edges and the zigzag carbon nanotubes and their counterparts with finite length (0D) in the framework of the Hückel model. We prove that a 1D carbon ribbon is metallic if its width (the number of carbon rings) is equal to $2+3n$. We show that the dispersion law (electron band energy) of a 1D metallic ribbon or a 1D metallic carbon nanotube has a universal {\it sin-}like dependence at the Fermi energy which is independent of its width. We find that in case of metallic graphene ribbons of finite length (rectangular graphene macromolecules) or nanotubes of finite length the discrete energy spectrum in the vicinity of $\varepsilon=0$ (Fermi energy) can be obtained exactly by selecting levels from the same dispersion law. In case of a semiconducting graphene macromolecule or a semiconducting nanotube of finite length the positions of energy levels around the energy gap can be approximated with a good accuracy. The electron spectrum of 0D carbon structures often include additional states at energy $\varepsilon=0$, which are localized on zigzag edges and do not contribute to the volume conductivity.

cond-mat.mes-hall↗

arXiv2008

Generalized Mean Field Approach to a Resonant Bose-Fermi Mixture

We formulate a generalized mean-field theory of a mixture of fermionic and bosonic atoms, in which the fermion-boson interaction can be controlled by a Feshbach resonance. The theory correctly accounts for molecular binding energies of the molecules in the two-body limit, in contrast to the most straightforward mean-field theory. Using this theory, we discuss the equilibrium properties of fermionic molecules created from atom pairs in the gas. We also address the formation of molecules when the magnetic field is ramped across the resonance, and present a simple Landau-Zener result for this process.

cond-mat.other↗

arXiv2004

Ultracold Collisions of Fermionic OD Radicals

We discuss consequences of Fermi exchange symmetry on collisions of polar molecules at low temperatures (below 1 K), considering the OD radical as a prototype. At low fields and low temperatures, Fermi statistics can stabilize a gas of OD molecules against state-changing collisions. We find, however, that this stability does not extend to temperatures high enough to assist with evaporative cooling. In addition, we establish that a novel ``field-linked'' resonance state of OD dimers exists, in analogy with the similar states predicted for bosonic OH.

physics.atom-ph↗

arXiv2004

Pair Wave Functions in Atomic Fermi Condensates

Recent experiments have observed condensation behavior in a strongly interacting system of fermionic atoms. We interpret these observations in terms of a mean-field version of resonance superfluidity theory. We find that the objects condensed are not bosonic molecules composed of bound fermion pairs, but are rather spatially correlated Cooper pairs whose coherence length is comparable to the mean spacing between atoms. We propose experiments that will help to further probe these novel pairs.

cond-mat.soft↗

arXiv2003

Field-linked States of Ultracold Polar Molecules

We explore the character of a novel set of ``field-linked'' states that were predicted in [A. V. Avdeenkov and J. L. Bohn, Phys. Rev. Lett. 90, 043006 (2003)]. These states exist at ultralow temperatures in the presence of an electrostatic field, and their properties are strongly dependent on the field's strength. We clarify the nature of these quasi-bound states by constructing their wave functions and determining their approximate quantum numbers. As the properties of field-linked states are strongly defined by anisotropic dipolar and Stark interactions, we construct adiabatic surfaces as functions of both the intermolecular distance and the angle that the intermolecular axis makes with the electric field. Within an adiabatic approximation we solve the 2-D Schrodinger equation to find bound states, whose energies correlate well with resonance features found in fully-converged multichannel scattering calculations.

physics.atom-ph↗

arXiv2002

Linking Ultracold Polar Molecules

We predict that pairs of polar molecules can be weakly bound together in an ultracold environment, provided that a dc electric field is present. The field that links the molecules together also strongly influences the basic properties of the resulting dimer, such as its binding energy and predissociation lifetime. Because of their long-range character these dimers will be useful in disentangling cold collision dynamics of polar molecules. As an example, we estimate the microwave photoassociation yield for OH-OH cold collisions.

physics.atom-ph↗

arXiv2002

Rotational Feshbach Resonances in Ultracold Molecular Collisions

In collisions at ultralow temperatures, molecules will possess Feshbach resonances, foreign to ultracold atoms, whose virtual excited states consist of rotations of the molecules. We estimate the mean spacing and mean widths of these resonant states, exploiting the fact the molecular collisions at low energy display chaotic motion. As examples, we consider the experimentally relevant molecules O_2, OH, and PbO. The density of s-wave resonant states for these species is quite high, implying that a large number of narrow resonant states will exist.

physics.atom-ph↗

arXiv2001

On Cooper Pairing in Finite Fermi Systems

In order to analyse the role of the quasiparticle-phonon interaction in the origin of nuclear gap, we applied an approach which is similar to the Eliashberg theory for usual superconductors. We obtained that the averaged contribution of the quasiparticle-phonon mechanism to the observed value of the pairing gap for $^{120}$Sn is 26% and the BCS-type mechanism gives 74% . Thus, pairing is of a mixed nature at least in semi-magic nuclei -- it is due to the quasiparticle-phonon and BCS mechanisms, the first one being mainly a surface mechanism and the second one mainly a volume mechanism. The calculations of the strength distribution for the odd-mass nuclei $^{119}Sn$ and $^{121}Sn$ have shown that the quasiparticle-phonon mechanism mainly improves the description of the observed spectroscopic factors in these nuclei. For the case of nuclei with pairing in both proton and neutron systems it is necessary to go beyond the Eliashberg-Migdal approximations and include the vertex correction graphs in addition to the rainbow ones. The estimations for spectroscopic factors performed within a three-level model have shown that the contribution of the vertex correction graphs was rather noticeable.

nucl-th↗

arXiv2000

The Role of Ground State Correlations in the Single-Particle Strength of Odd Nuclei with Pairing

A method based on the consistent use of the Green function formalism has been developed to calculate the distribution of the single-particle strength in odd nuclei with pairing. The method takes into account the quasiparticle-phonon interaction, ground state correlations and a "refinement" of phenomenological single-particle energies and pairing gap values from the quasiparticle-phonon interaction under consideration. The calculations for 121Sn and 119Sn that were performed in the quasiparticle$\otimes$phonon approximation, have shown a reasonable agreement with experiment. The ground state correlations play a noticeable role and mostly improve the agreement with experiment or shift the results to the right direction.

nucl-th↗

arXiv2000

Description of Excitations in Odd Non-magic Nuclei by the Green's Function Method

General equations for one- particle Green function in non- magic nuclei have been derived where a pairing mechanism, which is analogous to the Bardeen- Cooper- Schrieffer one, has been singled out explicitely. A "refining" procedure for the phenomenological single- particle energies and pairing gaps is formulated to avoid double counting of the phonon mixing if the initial data of the problem are these phenomenological quantities. The general equations obtained have been written in the squared phonon creation amplitude approximation. This corresponds to taking quasiparticle$\otimes$phonon configurations into account and, as shown in the work, is a good approximation for semi- magic nuclei. Using this approximation the secular equation to calculate excitations of odd non- magic nuclei has been obtained. The equation takes ground state correlations into account completely and contains only the squared energy variable. Distributions of single- particle strength in 119Sn and 121}Sn have been calculated and reasonable agreement with the available experiment has been obtained.

nucl-th↗

arXiv2000

On the mechanisms of superfluidity in atomic nuclei

A system of equations is obtained for the Cooper gap in nuclei. The system takes two mechanisms of superfluidity into account in an approximation quadratic in the phonon- production amplitude : a Bardeen- Cooper- Schrieffer (BCS) type mechanism and a quasiparticle- phonon mechanism. These equations are solved for 120 Sn in a realistic approximation. If the simple procedures proposed are used to determine the new particle- particle interaction and to estimate the average effect, then the contribution of the quasiparticle- phonon mechanism to the observed width of the pairing gap is 26% and the BCS-type contribution is 74%. This means that at least in semimagic nuclei pairing is of a mixed nature - it is due to the two indicated mechanisms, the first being mainly a surface mechanism and the second mainly a volume mechanism.

nucl-th↗