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The a.c. and d.c. Josephson effects in a Bose–Einstein condensate S. Levy, E. Lahoud, I. Shomroni & J. Steinhauer The alternating- and direct-current (a.c. and d.c.) Josephson effects were first discovered in a system of two superconductors, the macroscopic wavefunctions of which are weakly coupled via a tunnelling barrier. In the a.c. Josephson effect, a constant chemical potential difference (voltage) is applied, which causes an oscillating current to flow through the barrier. Because the frequency is proportional to the chemical potential difference only, the a.c. Josephson effect serves as a voltage standard. In the d.c. Josephson effect, a small constant current is applied, resulting in a constant supercurrent flowing through the barrier. In a sense, the particles do not 'feel' the presence of the tall tunnelling barrier, and flow freely through it with no driving potential. Bose–Einstein condensates should also support Josephson effects; however, while plasma oscillations have been seen in a single Bose–Einstein condensate Josephson junction, the a.c. Josephson effect remains elusive. Here we observe the a.c. and d.c. Josephson effects in a single Bose–Einstein condensate Josephson junction. The d.c. Josephson effect has been observed previously only in superconducting systems; in our study, it is evident when we measure the chemical potential–current relation of the Bose–Einstein condensate Josephson junction. Our system constitutes a trapped-atom interferometer with continuous readout, which operates on the basis of the a.c. Josephson effect. In addition, the measured chemical potential–current relation shows that the device is suitable for use as an analogue of the superconducting quantum interference device, which would sense rotation. Nature 449, 579 (4.10.2007)
Charles A. Sackett: Cold
meeting at a junction. Nature
449, 546 (4.10.2007)
Jon Cartwright: Josephson
effect seen in atomic gas (3.10.2007)
 Fluctuating superconductivity
in organic molecular metals close to the Mott transition
On cooling through the transition temperature Tc of a conventional superconductor, an energy gap develops as the normal-state charge carriers form Cooper pairs; these pairs form a phase-coherent condensate that exhibits the well-known signatures of superconductivity: zero resistivity and the expulsion of magnetic flux (the Meissner effect). However, in many unconventional superconductors, the formation of the energy gap is not coincident with the formation of the phase-coherent superfluid. Instead, at temperatures above the critical temperature a range of unusual properties, collectively known as 'pseudogap phenomena', are observed. Here we argue that a key pseudogap phenomenon—fluctuating superconductivity occurring substantially above the transition temperature—could be induced by the proximity of a Mott-insulating state. The Mott-insulating state in the -(BEDT-TTF)2X organic molecular metals can be tuned, without doping, through superconductivity into a normal metallic state as a function of the parameter t/U, where t is the tight-binding transfer integral characterizing the metallic bandwidth and U is the on-site Coulomb repulsion. By exploiting a particularly sensitive probe of superconducting fluctuations, the vortex-Nernst effect, we find that a fluctuating regime develops as t/U decreases and the role of Coulomb correlations increases. Nature 449, 584 (4.10.2007)
Single artificial-atom
lasing
Solid-state superconducting circuits are versatile systems in which quantum states can be engineered and controlled. Recent progress in this area has opened up exciting possibilities for exploring fundamental physics as well as applications in quantum information technology; in a series of experiments it was shown that such circuits can be exploited to generate quantum optical phenomena, by designing superconducting elements as artificial atoms that are coupled coherently to the photon field of a resonator. Here we demonstrate a lasing effect with a single artificial atom—a Josephson-junction charge qubit—embedded in a superconducting resonator. We make use of one of the properties of solid-state artificial atoms, namely that they are strongly and controllably coupled to the resonator modes. The device is essentially different from existing lasers and masers; one and the same artificial atom excited by current injection produces many photons. Nature 449, 588 (4.10.2007)
Hamish Johnston: Artifical-atom
laser debuts (5.10.2007)
Ultrastrong and Stiff
Layered Polymer Nanocomposites
Nanoscale building blocks are individually exceptionally strong because they are close to ideal, defect-free materials. It is, however, difficult to retain the ideal properties in macroscale composites. Bottom-up assembly of a clay/polymer nanocomposite allowed for the preparation of a homogeneous, optically transparent material with planar orientation of the alumosilicate nanosheets. The stiffness and tensile strength of these multilayer composites are one order of magnitude greater than those of analogous nanocomposites at a processing temperature that is much lower than those of ceramic or polymer materials with similar characteristics. A high level of ordering of the nanoscale building blocks, combined with dense covalent and hydrogen bonding and stiffening of the polymer chains, leads to highly effective load transfer between nanosheets and the polymer. Science 318, 80
(5.10.2007)
Preparation and Detection
of Magnetic Quantum Phases in Optical Superlattices
We describe a novel approach to prepare, detect, and characterize magnetic quantum phases in ultracold spinor atoms loaded in optical superlattices. Our technique makes use of singlet-triplet spin manipulations in an array of isolated double-well potentials in analogy to recently demonstrated control in quantum dots. We also discuss the many-body singlet-triplet spin dynamics arising from coherent coupling between nearest neighbor double wells and derive an effective description for such systems. We use it to study the generation of complex magnetic states by adiabatic and nonequilibrium dynamics. Phys. Rev. Lett. 99,
140601 (5.10.2007)
Constraining Unparticle
Physics with Cosmology and Astrophysics
It has recently been suggested that a scale-invariant “unparticle” sector with a nontrivial infrared fixed point may couple to the standard model (SM) via higher-dimensional operators. The weakness of such interactions hides the unparticle phenomena at low energies. We demonstrate how cosmology and astrophysics can place significant bounds on the strength of unparticle-SM interactions. We also discuss the possibility of a having a non-negligible unparticle relic density today. Phys. Rev. Lett. 99,
141301 (1.10.2007)
Vibration-Induced Climbing
of Drops
We report an experimental study of liquid drops moving against gravity, when placed on a vertically vibrating inclined plate, which is partially wetted by the drop. The frequency of vibrations ranges from 30 to 200 Hz, and, above a threshold in vibration acceleration, drops experience an upward motion. We attribute this surprising motion to the deformations of the drop, as a consequence of an up or down symmetry breaking induced by the presence of the substrate. We relate the direction of motion to contact angle measurements. This phenomenon can be used to move a drop along an arbitrary path in a plane, without special surface treatments or localized forcing. Phys. Rev. Lett. 99,
144501 (3.10.2007)
Observation of Immobilized
Water Molecules around Hydrophobic Groups
We have used femtosecond midinfrared spectroscopy to study the orientational mobility of water molecules in the hydration shells of hydrophobic groups. Our results show that hydrophobic groups are surrounded by a number of water molecules that display much slower orientational dynamics than the bulk liquid and that are therefore effectively immobilized. It turns out that each methyl group is surrounded by four immobilized water OH groups. Phys. Rev. Lett. 99,
148301 (1.10.2007)
Michael Schirber: Oil Holds
Water Transfixed (4.10.2007)
Neutrino-driven instabilities
in very dense plasmas
Nonlinear interactions between intense neutrino bursts and electrostatic plasma oscillations in a very dense Fermi plasma are considered. By using the fluid description for intense neutrino bursts and the quantum hydrodynamic model for a dense Fermi plasma, we derive a system of equations that exhibit nonlinear couplings between neutrinos and electrostatic electron plasma waves/ion-acoustic oscillations. The latter incorporate the appropriate electron pressure law and the quantum force involving the strong electron density correlation in a dense Fermi plasma. The governing equations are Fourier transformed and combined to deduce the dispersion relations, which admit instabilities. It is found that for dense Fermi plasmas under extreme conditions, such as those in the interior of massive white dwarfs, the neutrino driven electrostatic instabilities develop rapidly, and they can be responsible for the neutrino energy absorption in dense astrophysical Fermi plasmas. EPL 80, 35001 (28.9.2007)
The Jarzynski relation,
fluctuation theorems, and stochastic thermodynamics for non-Markovian processes
We prove the Jarzynski relation for general stochastic processes including non-Markovian systems with memory. The only requirement for our proof is the existence of a stationary state, therefore excluding non-ergodic systems. We then show how the concepts of stochastic thermodynamics can be used to prove further exact non-equilibrium relations like the Crooks relation and the fluctuation theorem on entropy production for non-Markovian dynamics. J. Stat. Mech. (2007) L09002
(26.9.2007)
A Ge/Si heterostructure
nanowire-based double quantum dot with integrated charge sensor
One proposal for a solid-state-based quantum bit (qubit) is to control coupled electron spins on adjacent semiconductor quantum dots1, 2. Most experiments have focused on quantum dots made from III–V semiconductors; however, the coherence of electron spins in these materials is limited by hyperfine interactions with nuclear spins3, 4, 5, 6. Ge/Si core/shell nanowires seem ideally suited to overcome this limitation, because the most abundant nuclei in Ge and Si have spin zero and the nanowires can be chemically synthesized defect-free with tunable properties7. Here, we present a double quantum dot based on Ge/Si nanowires in which we can completely control the coupling between the dots and to the leads. We also demonstrate that charge on the double dot can be detected by coupling it capacitively to an adjacent nanowire quantum dot. The double quantum dot and integrated charge sensor serve as an essential building block to form a solid-state qubit free of nuclear spin. Nature Nanotechnology 2,
622 (30.9.2007)
Mark A. Eriksson and Mark
Friesen: Nanowires charge towards integration. Nature Nanotechnology 2,
595 (30.9.2007)
Relativistic analysis
of magnetoelectric crystals: Extracting a new 4-dimensional P odd and T
odd pseudoscalar from Cr2O3 data
Earlier, the linear magnetoelectric effect of chromium sesquioxide Cr2O3 has been determined experimentally as a function of temperature. One measures the electric field-induced magnetization on Cr2O3 crystals or the magnetic field-induced polarization. From the magnetoelectric moduli of Cr2O3 we extract a 4-dimensional relativistic invariant pseudoscalar a. It is temperature dependent and of the order of 10-4Y0, with Y0 as vacuum admittance. We show that the new pseudoscalar a is odd under parity transformation and odd under time inversion. Moreover, a is for Cr2O3 what Tellegen's gyrator is for two port theory, the axion field for axion electrodynamics, and the PEMC (perfect electromagnetic conductor) for electrical engineering. Phys. Lett. A: In pess (17.9.2007)
R. S.: Neues von magnetoelektrischen
Kristallen. (5.10.2007)
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