292 research outputs found
Nonlinear Viscous Vortex Motion in Two-Dimensional Josephson-Junction Arrays
When a vortex in a two-dimensional Josephson junction array is driven by a
constant external current it may move as a particle in a viscous medium. Here
we study the nature of this viscous motion. We model the junctions in a square
array as resistively and capacitively shunted Josephson junctions and carry out
numerical calculations of the current-voltage characteristics. We find that the
current-voltage characteristics in the damped regime are well described by a
model with a {\bf nonlinear} viscous force of the form , where is the vortex velocity,
is the velocity dependent viscosity and and are
constants for a fixed value of the Stewart-McCumber parameter. This result is
found to apply also for triangular lattices in the overdamped regime. Further
qualitative understanding of the nature of the nonlinear friction on the vortex
motion is obtained from a graphic analysis of the microscopic vortex dynamics
in the array. The consequences of having this type of nonlinear friction law
are discussed and compared to previous theoretical and experimental studies.Comment: 14 pages RevTex, 9 Postscript figure
Vortex reflection at boundaries of Josephson-junction arrays
We study the propagation properties of a single vortex in square
Josephson-junction arrays (JJA) with free boundaries and subject to an applied
dc current. We model the dynamics of the JJA by the resistively and
capacitively shunted junction (RCSJ) equations. For zero Stewart-McCumber
parameter we find that the vortex always escapes from the array when
it gets to the boundary. For and for low currents we find
that the vortex escapes, while for larger currents the vortex is reflected as
an antivortex at one edge and the antivortex as a vortex at the other, leading
to a stationary oscillatory state and to a non-zero time-averaged voltage. The
escape and the reflection of a vortex at the array edges are qualitatively
explained in terms of a coarse-grained model of a vortex interacting
logarithmically with its image. We also discuss the case when the free
boundaries are at degrees with respect to the direction of the vortex
motion. Finally, we discuss the effect of self-induced magnetic fields by
taking into account the full-range inductance matrix of the array, and find
qualitatively equivalent results.Comment: 14 pages RevTex, 9 Postscript figure
Fabrication of a Few-Layer Graphene Electrodes for Molecular Electronics Devices
We report on thefabrication of a molecular transistor based on a single molecule trapped in a few-layergraphene nanogap. The device is pre-patterned with He-ion beam milling oroxygen plasma etching prior to nanogap formation. Pre-patterning helps tolocalize the gap, and to make it narrower, so that only a few or a singlemolecule can be trapped in it. The nanogap is formed by an electroburning techniqueat room temperature. In order to test the functionality of the device wedeposited diamino-terphenyl molecules in the nanogap. Three-terminal electricalmeasurements showed an increase of the current after deposition, and a gatevoltage dependence at low temperatures. Hence, pre-patterned few-layer graphenejunctions can be used for electron transport measurements through a terphenylmolecule with a future prospective towards more complex molecularconfigurations
Quantum Landauer erasure with a molecular nanomagnet
The erasure of a bit of information is an irreversible operation whose minimal entropy production of kB ln 2 is set by the Landauer limit1. This limit has been verified in a variety of classical systems, including particles in traps2, 3 and nanomagnets4. Here, we extend it to the quantum realm by using a crystal of molecular nanomagnets as a quantum spin memory and showing that its erasure is still governed by the Landauer principle. In contrast to classical systems, maximal energy efficiency is achieved while preserving fast operation owing to its high-speed spin dynamics. The performance of our spin register in terms of energy–time cost is orders of magnitude better than existing memory devices to date. The result shows that thermodynamics sets a limit on the energy cost of certain quantum operations and illustrates a way to enhance classical computations by using a quantum system
Fabrication of a Few-Layer Graphene Electrodes for Molecular Electronics Devices
We report on thefabrication of a molecular transistor based on a single molecule trapped in a few-layergraphene nanogap. The device is pre-patterned with He-ion beam milling oroxygen plasma etching prior to nanogap formation. Pre-patterning helps tolocalize the gap, and to make it narrower, so that only a few or a singlemolecule can be trapped in it. The nanogap is formed by an electroburning techniqueat room temperature. In order to test the functionality of the device wedeposited diamino-terphenyl molecules in the nanogap. Three-terminal electricalmeasurements showed an increase of the current after deposition, and a gatevoltage dependence at low temperatures. Hence, pre-patterned few-layer graphenejunctions can be used for electron transport measurements through a terphenylmolecule with a future prospective towards more complex molecularconfigurations
Finite Size and Current Effects on IV Characteristics of Josephson Junction Arrays
The effects of finite size and of finite current on the current-voltage
characteristics of Josephson junction arrays is studied both theoretically and
by numerical simulations. The cross-over from non-linear to linear behavior at
low temperature is shown to be a finite size effect and the non-linear behavior
at higher temperature, , is shown to be a finite current effect.
These are argued to result from competition between the three length scales
characterizing the system. The importance of boundary effects is discussed and
it is shown that these may dominate the behavior in small arrays.Comment: 5 pages, figures included, to appear in PR
Hall effect and geometric phases in Josephson junction arrays
Since effectively the local contact vortex velocity dependent part of the Magnus force in a Josephson junction array is zero in the classical limit, we predict zero classical Hall effect. In the quantum limit because of the geometric phases due to the finite superfluid density at superconductor grains, rich and complex Hall effect is found in this quantum regime due to the Thouless-Kohmoto-Nightingale-den-Nijs effect
Decoupling in the 1D frustrated quantum XY model and Josephson junction ladders: Ising critical behavior
A generalization of the one-dimensional frustrated quantum XY model is
considered in which the inter and intra-chain coupling constants of the two
infinite XY (planar rotor) chains have different strengths. The model can
describe the superconductor to insulator transition due to charging effects in
a ladder of Josephson junctions in a magnetic field with half a flux quantum
per plaquette. From a fluctuation-effective action, this transition is expected
to be in the universality class of the two-dimensional classical XY-Ising
model. The critical behavior is studied using a Monte Carlo transfer matrix
applied to the path-integral representation of the model and a
finite-size-scaling analysis of data on small system sizes. It is found that,
unlike the previous studied case of equal inter and intra-chain coupling
constants, the XY and Ising-like excitations of the quantum model decouple for
large interchain coupling, giving rise to pure Ising model critical behavior
for the chirality order parameter and a superconductor-insulator transition in
the universality class of the 2D classical XY model.Comment: 15 pages with figures, RevTex 3.0, INPE-93/00
Transient thermal characterization of suspended monolayer MoS
We measure the thermal time constants of suspended single layer molybdenum
disulfide drums by their thermomechanical response to a high-frequency
modulated laser. From this measurement the thermal diffusivity of single layer
MoS is found to be 1.14 10 m/s on average. Using a
model for the thermal time constants and a model assuming continuum heat
transport, we extract thermal conductivities at room temperature between 10 to
40 W/(mK). Significant device-to-device variation in the thermal
diffusivity is observed. Based on statistical analysis we conclude that these
variations in thermal diffusivity are caused by microscopic defects that have a
large impact on phonon scattering, but do not affect the resonance frequency
and damping of the membrane's lowest eigenmode. By combining the experimental
thermal diffusivity with literature values of the thermal conductivity, a
method is presented to determine the specific heat of suspended 2D materials,
which is estimated to be 255 104 J/(kgK) for single layer MoS
Row-switched states in two-dimensional underdamped Josephson junction arrays
When magnetic flux moves across layered or granular superconductor
structures, the passage of vortices can take place along channels which develop
finite voltage, while the rest of the material remains in the zero-voltage
state. We present analytical studies of an example of such mixed dynamics: the
row-switched (RS) states in underdamped two-dimensional Josephson arrays,
driven by a uniform DC current under external magnetic field but neglecting
self-fields. The governing equations are cast into a compact
differential-algebraic system which describes the dynamics of an assembly of
Josephson oscillators coupled through the mesh current. We carry out a formal
perturbation expansion, and obtain the DC and AC spatial distributions of the
junction phases and induced circulating currents. We also estimate the interval
of the driving current in which a given RS state is stable. All these
analytical predictions compare well with our numerics. We then combine these
results to deduce the parameter region (in the damping coefficient versus
magnetic field plane) where RS states can exist.Comment: latex, 48 pages, 15 figs using psfi
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