• Friday, September 5, 1:30PM
  Peter Abbamonte
  (BNL)
  Imaging density disturbances in water with 41.3 attosecond time resolution
  Recent years have seen great progress in the generation and detection of attosecond laser pulses, which have heralded a new age of attophysics in which electronic motion will be witnessed in real time [1-3]. The stated need for time-resolved methods is based on the fact that energy-domain techniques provide little detail about electron dynamics. In this talk I will present a simple phase-inversion algorithm for momentum resolved energy domain techniques, in this case inelastic x-ray scattering, which allows charge dynamics to be viewed explicitly. The method will be demonstrated by showing the screening of a point disturbance in liquid water with resolutions dt=41.3 attosecond and dx=1.27 angstrom. These results will be used to construct the screening cloud around a photoexcited chromophore in solution, as well as the wake field induced by a 9 MeV gold ion.
  [1] J. Giles, Nature, 420, 737 (2002)
  [2] L. DiMauro, Nature, 419, 789 (2002)
  [3] Lewenstein, Science, 297, 1131 (2002)
  Host: Allen
  
  
• Friday, September 12, 1:30PM
  Jukka Pekola
  Helsinki University of Technology
  Fundamental limitations and non-thermal energy distribution of mesoscopic electron coolers
  We demonstrate two fundamental limitations in cooling electrons using biased tunnel junctions to extract heat from a normal metal into a superconductor. Firstly, when the injection rate of electrons exceeds the internal relaxation rate in the metal to be cooled, the electrons do no more obey the Fermi-Dirac distribution, and the concept of temperature cannot be applied as such. Secondly, at low bath temperatures, states within the gap induce anomalous heating and determine the minimum achievable temperature. Experimental results indicate that our devices are close to these limits.
  Host: Averin
  
  
• Friday, September 19, 1:30PM Cancelled due to storm
  Mark Pederson
  (Naval Research Laboratory)
  DFT for Molecular Magnet Tunnel Splittings: Role of Solvents, Vibrons and Exchange
  
  
• Friday, September 26, 1:30PM
  Tero Heikkilä
  (Helsinki University of Technology)
  Ultralow-dissipation supercurrent transistor
  The critical current of a superconductor - normal-metal - superconductor Josephson junction can be controlled by driving a current through the weak link from extra control probes. Depending on the relative rates of control current injection and internal relaxation, the electron system in the weak link is either in a true nonequilibrium state or in quasiequilibrium with a well-defined electron temperature. In this talk, I will describe how the supercurrent behaves in these different regimes, and how the behavior differs between systems with normal-metal or superconducting control probes. In the latter case, one can construct a supercurrent transistor with a high current gain and very low dissipation.
  Host: Averin
  
  
• Friday, October 3, 1:30PM
  Nikolai Zhitenev
  (Lucent)
  Conductance in molecular nanojunctions: low-energy hopping, tunneling and the Coulomb blockade.
  Many different approaches to contact a small number of molecules have been developed recently. Some of the methods exploit the flexibility of tunable contacts to molecules afforded by scanning probes or break junctions. Other methods use a fixed contact arrangement. We employ new techniques that combine some advantages of both tunable and fixed contacts and experimentally compare these with other known approaches. The new methods are similar to the fabrication of single-electron transistors using the shadow angle evaporation. The device performance can be monitored during fabrication as with tunable contacts, but the contact geometry is fixed, providing good mechanical and thermal stability. Different conductivity mechanisms are identified depending on the granularity of the metal used as a substrate for assembling the monolayer. Unexpectedly, the energy scale controlling the dominant conductance channels is quite low in comparison with the molecular level spacing. In single-grain junctions, the dominant conductance mechanism is hopping with energy scale of the order of 10-100 meV determined by the nature of metal contacts. In the case of multi-grain junctions, additional tunnel conductance is observed with low-energy Coulomb-blockade features.
  Host: Likharev
  
  
• Friday, October 17, 1:30PM
  Richard Gaál
  (EPFL, Lausanne )
  BaVS3 : a strongly correlated electron system
  The d-electron system BaVS₃ has a Fermi liquid, a non-Fermi liquid, or an antiferromagnetic insulating ground state, depending on pressure. At ambient pressure the material has a second-order metal-insulator transition from a bad metallic state to an insulator, but the order parameter has long been a mystery. Results of magnetotransport and infrared spectroscopy experiments will be presented showing the nature of this transition. Secondly, the different ground states and the transitions between them will be outlined. Two important aspects will be addressed: i., the relevance of spin and orbital degrees of freedom in the insulator side, and ii., the role of quantum fluctuations above the critical pressure.
  Host: Mihaly
  
  
• Friday, October 24, 1:30PM
  Eugene M. Chudnovsky
  (The City University of New York )
  Conservation of Angular Momentum in the Dynamics of the Magnetic Flux in Superconductors
  Conservation of angular momentum makes profound effects on classical and quantum dynamics of the magnetic flux in superconductors. One [1] is a large contribution of the transversal displacements of the crystal lattice to the inertial mass of the Abrikosov vortex. Another [2] is a parameter-free mechanism of decoherence of quantum oscillations of the superconducting current between opposite directions in a SQUID.
  [1] E.M.Chudnovsky and A.B.Kuklov, Phys. Rev. Lett. 91, 067004 (2003).
  [2] E.M.Chudnovsky and A.B.Kuklov, Phys. Rev. B67,064515 (2003).
  Host: Lukens
  
  
• Friday, October 31, 1:30PM
  Wei Ku
  (Department of Physics, Brookhaven National Laboratory )
  Magnetic Coupling in Insulating Quasi-1D Cu-O Spin Chains: Toward Fully First-Principles Approaches for Strong Correlation
  Insulating quasi-1D edge-sharing Cu-O chains possess rich behavior from ferromagnetic (FM) order to antiferromagnetic (AFM) order featuring spin-Peierl transition. In such low dimension, it is important to properly account for magnetic couplings beyond first-neighbors along the chain, as well as the weak inter-chain couplings. In this talk, microscopic origin of the magnetic couplings in three prototype insulating quasi-1D materials will be compared: FM Li₂CuO₂, AFM CuGeO₃, and AFM CuSiO₃, based on recent theoretical development of the first-principles construction of symmetry-respecting, energy-resolved Wannier states (e.g. "spin orbitals" in these materials). In addition to the standard argument based on the bond-angle, the crystal potential that breaks the x-y symmetry is shown to have crucial impact on the magnetic behavior via modification of hybridization tails of the Wannier states that control both the AFM superexchange and the FM direct exchange. The novel approach will be illustrated with previous study on exceptional insulating ferromagnetism in La₄Ba₂Cu₂O₁₀ (*), a byproduct of High-T_c cuprates. Finally, future development toward fully first-principles treatment of strong correlation will be outlined.
  (*) Wei Ku, H. Rosner, W. E. Pickett, and R. T. Scalettar, Phys. Rev. Lett. 89, 167204 (2002)
  Host: Allen
  
  
• Friday, November 14, 1:30PM Rescheduled from Sept. 19
  Mark Pederson
  (Naval Research Laboratory)
  DFT for Molecular Magnet Tunnel Splittings: Role of Solvents, Vibrons and Exchange
  Observation of resonant tunneling of magnetization in Mn12-Acetate has focused significant attention on a novel class of spin-ordered organometallic molecules. These molecular magnets consist of approximately 70-200 atoms and are typically composed of 4-15 transition metal atoms locked in place by organic ligands and anions. In addition to very interesting chemistry and physics, such molecular systems could form the basis of future nanoscale devices. Potential applications include high-density magnetic storage devices, q-bits for quantum computing, biomedical imaging, and tunable high-frequency radiation sources. The fundamental figure of merit that determines the resonant tunneling fields is the second-order magnetic anisotropy Hamiltonian that is governed primarily by the spin-orbit interaction. The large size of these systems and the small energy scales of interest also provide an interesting challenge to quantum-mechanical-based computational methods. In this talk I will give some background on the relevant experiments and then describe our efforts at predicting properties of molecular magnets with an all-electron density-functional formalism. A brief review of the massively parallel computational method, NRLMOL, will be given followed by applications to molecules of current interest. With respect to Mn12-Acetate, I will first discuss the calculation of the second-order anisotropy Hamiltonian followed by calculations aimed at understanding the observed symmetry breaking in this molecule. To further benchmark the reliability of density-functional theory for such systems, similar results on several other molecular magnets will be presented.
  Various parts of this work were performed in collaboration with S.N. Khanna, J. Kortus, S. Hellberg, T. Baruah, N. Bernstein and K. Park.
  Host: Allen
  
  
• Friday, December 5, 1:30PM
  Ashvin Vishwanath
  (MIT)
  Landau Forbidden Phase Transitions and Emergent Photons in Quantum Antiferromagnets
  Our modern understanding of phase transitions is built on Landau's theory, which, among other things, dictates when it may be possible to have a continuous transition between two phases. For example, a continuous transition between a superfluid state and a density wave is forbidden without special fine tuning in this theory since different orders exist on either side of the transition . In this talk I describe how in certain two dimensional systems (e.g. the spin half antiferromagnet on the square lattice) interference effects from Berry phases can lead to precisely such `Landau forbidden' quantum phase transitions. Moreover, the critical point, at which certain topological defects - hedgehog configurations of the `spins' - are absent, is most naturally described in terms of fractionalized excitations (e.g. spinons with spin one half) coupled to emergent photons. In contrast, the phases on either side of the transition are conventional. I also describe how these ideas may be important to understanding phenomena in strongly correlated materials.
  Host: Abanov
  
  
• Friday, December 12, 1:30PM
  Myron Strongin
  (Physics Deptartment, BNL)
  Experiments on the Insulator/Superconductor Transition in Thin Films
  Early and recent experiments on the transition temperature and properties of ultra-thin films will be discussed, along with fond reminiscences of the 1970's and the interactions with V.L. Ginzburg, John Bardeen and Bernd Matthias. Emphasis is given to how the early experiments eventually led to the discovery of the I/S transition. Recent experiments indicating the possibility of a "Josephson Phase" near the I/S transition will also be discussed.
  Host: Allen
  
  

Send comments to Laszlo Mihaly; updated 30/8/2003.