Publications
46 results found
Barnett R, Podolsky D, Refael G, 2009, Geometrical approach to hydrodynamics and low-energy excitations of spinor condensates, Physical Review B, Vol: 80, Pages: 024420-024420
Barnett R, Refael G, Porter MA, et al., 2008, Vortex lattice locking in rotating two-component Bose-Einstein condensates, NEW JOURNAL OF PHYSICS, Vol: 10, ISSN: 1367-2630
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Barnett R, Mukerjee S, Moore JE, 2008, Vortex lattice transitions in cyclic spinor condensates, Physical Review Letters, Vol: 100, Pages: 240405-240405
Barnett RL, Maragakis P, Turner A, et al., 2007, Multiscale model of electronic behavior and localization in stretched dry DNA, Journal of Materials Science, Vol: 42, Pages: 8894-8903, ISSN: 0022-2461
When the DNA double helix is subjected to external forces it can stretch elastically to elongations reaching 100% of its natural length. These distortions, imposed at the mesoscopic or macroscopic scales, have a dramatic effect on electronic properties at the atomic scale and on electrical transport along DNA. Accordingly, a multiscale approach is necessary to capture the electronic behavior of the stretched DNA helix. To construct such a model, we begin with accurate density-functional-theory calculations for electronic states in DNA bases and base pairs in various relative configurations encountered in the equilibrium and stretched forms. These results are complemented by semi-empirical quantum mechanical calculations for the states of a small size [18 base pair poly(CG)–poly(CG)] dry, neutral DNA sequence, using previously published models for stretched DNA. The calculated electronic states are then used to parametrize an effective tight-binding model that can describe electron hopping in the presence of environmental effects, such as the presence of stray water molecules on the backbone or structural features of the substrate. These effects introduce disorder in the model hamiltonian which leads to electron localization. The localization length is smaller by several orders of magnitude in stretched DNA relative to that in the unstretched structure.
Turner AM, Barnett R, Demler E, et al., 2007, Nematic order by disorder in spin-2 bose-einstein condensates, Physical Review Letters, Vol: 98, ISSN: 0031-9007
We show that quantum and thermal fluctuations in spin-2 Bose-Einstein condensates lift the accidental degeneracy of the mean-field phase diagram. Fluctuations select the uniaxial (square biaxial) nematic state for scattering lengths a4>a2 (a4<a2). Paradoxically, the order is stronger at higher temperatures. For spin-2 87Rb and 23Na, a continuous Ising-type transition is predicted on raising the magnetic field, from a fluctuation stabilized uniaxial state to a field stabilized square biaxial order state. This is a promising experimental system to realize the “order-by-disorder” phenomenon.
Barnett R, Turner A, Demler E, 2007, Classifying vortices in S= 3 Bose-Einstein condensates, Physical Review A, Vol: 76, Pages: 013605-013605
Barnett R, Turner A, Demler E, 2006, Classifying novel phases of spinor atoms, Physical Review Letters, Vol: 97, ISSN: 0031-9007
We consider many-body states of bosonic spinor atoms which, at the mean-field level, can be characterized by a single-particle wave function for the Bose-Einstein condensation and Mott insulating states. We describe and apply a classification scheme that makes explicit the spin symmetries of such states and enables one to naturally analyze their collective modes and topological excitations. Quite generally, the method allows classification of a spin F system as a polyhedron with 2F vertices. We apply the method to the many-body states of bosons with spins two and three. For spin-two atoms we find the ferromagnetic state, a continuum of nematic states, and a state having the symmetry of the point group of the regular tetrahedron. For spin-three atoms we obtain similar ferromagnetic and nematic phases as well as states having symmetries of various types of polyhedra with six vertices.
Barnett RL, Polkovnikov A, Demler E, et al., 2006, Coexistence of gapless excitations and commensurate charge-density wave in the 2H transition metal dichalcogenides, Physical Review Letters, Vol: 96, ISSN: 0031-9007
An unexpected feature common to 2H transition metal dichalcogenides (2H TMDs) is revealed with a first-principles Wannier function analysis of the electronic structure of the prototype 2H TaSe2: The low-energy Ta “5dz2” bands governing the physics of a charge-density wave (CDW) is dominated by hopping between next-nearest neighbors. With this motivation we develop a minimal effective model for the CDW formation, in which the unusual form of the hopping leads to an approximate decoupling of the three sublattices. In the CDW phase one sublattice remains undistorted, leaving the bands associated with it ungapped everywhere in the Fermi surface, resolving the long-standing puzzle of the coexistence of gapless excitations and commensurate CDW in the 2H TMDs.
Barnett R, Petrov D, Lukin M, et al., 2006, Quantum magnetism with multicomponent dipolar molecules in an optical lattice, Physical Review Letters, Vol: 96, Pages: 190401-190401
Barnett R, Demler E, Kaxiras E, 2005, Superconducting and charge–density wave instabilities in ultrasmall-radius carbon nanotubes, Solid State Communications, Vol: 135, Pages: 335-339, ISSN: 0038-1098
We perform a detailed analysis of the band structure, phonon dispersion, and electron–phonon coupling of three types of small-radius carbon nanotubes (CNTs): (5,0), (6,0), and (5,5) with diameters 3.9, 4.7, and 6.8 Å respectively. The large curvature of the (5,0) CNTs makes them metallic with a large density of states at the Fermi energy. The density of states is also strongly enhanced for the (6,0) CNTs compared to the results obtained from the zone-folding method. For the (5,5) CNTs the electron–phonon interaction is dominated by the in-plane optical phonons, while for the ultrasmall (5,0) and (6,0) CNTs the main coupling is to the out-of-plane optical phonon modes. We calculate electron–phonon interaction strengths for all three types of CNTs and analyze possible instabilities toward superconducting and charge–density wave phases. For the smallest (5,0) nanotube, in the mean-field approximation and neglecting Coulomb interactions, we find that the charge–density wave transition temperature greatly exceeds the superconducting one. When we include a realistic model of the Coulomb interaction we find that the charge–density wave is suppressed to very low temperatures, making superconductivity dominant with the mean-field transition temperature around one K. For the (6,0) nanotube the charge–density wave dominates even with the inclusion of Coulomb interactions and we find the mean-field transition temperature to be around five Kelvin. We find that the larger radius (5,5) nanotube is stable against superconducting and charge–density wave orders at all realistic temperatures.
Barnett R, Demler E, Kaxiras E, 2005, Electron-phonon interaction in ultrasmall-radius carbon nanotubes, Physical Review B, Vol: 71, Pages: 035429-035429
Hartman AZ, Jouzi M, Barnett RL, et al., 2004, Theoretical and experimental studies of carbon nanotube electromechanical coupling, Physical Review Letters, Vol: 92, Pages: 236804-236804
Cronin SB, Barnett R, Tinkham M, et al., 2004, Electrochemical gating of individual single-wall carbon nanotubes observed by electron transport measurements and resonant Raman spectroscopy, Applied Physics Letters, Vol: 84, Pages: 2052-2052
Maragakis P, Barnett RL, Kaxiras E, et al., 2002, Electronic structure of overstretched DNA, Physical Review B - Condensed Matter and Materials Physics, Vol: 66, Pages: 2411041-2411044, ISSN: 0163-1829
Minuscule molecular forces can transform DNA into a structure that is elongated by more than half its original length. We demonstrate that this pronounced conformational transition is of relevance to ongoing experimental and theoretical efforts to characterize the conducting properties of DNA wires. We present quantum-mechanical calculations for acidic, dry, poly(CG)-poly(CG) DNA that has undergone elongation of up to 90% relative to its natural length, along with a method for visualizing the effects of stretching on the electronic eigenstates. We find that overstretching leads to a drastic drop of the hopping matrix elements between localized occupied electronic states, suggesting a dramatic decrease in the conductivity through holes.
Maragakis P, Barnett RL, Kaxiras E, et al., 2002, Electronic structure of overstretched DNA, Physical Review B, Vol: 66, Pages: 241104-241104
Barnett R, Gibson GN, 1999, Static field tunneling ionization of H2+, Physical Review A, Vol: 59, Pages: 4843-4843
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