2,150 research outputs found
Nonequilibrium structures and dynamic transitions in driven vortex lattices with disorder
We review our studies of elastic lattices driven by an external force in
the presence of random disorder, which correspond to the case of vortices in
superconducting thin films driven by external currents. Above a critical force
we find two dynamical phase transitions at and , with
. At there is a transition from plastic flow to smectic flow
where the noise is isotropic and there is a peak in the differential
resistance. At there is a sharp transition to a frozen transverse solid
where both the transverse noise and the diffussion fall down abruptly and
therefore the vortex motion is localized in the transverse direction. From a
generalized fluctuation-dissipation relation we calculate an effective
transverse temperature in the fluid moving phases. We find that the effective
temperature decreases with increasing driving force and becomes equal to the
equilibrium melting temperature when the dynamic transverse freezing occurs.Comment: 8 pages, 3 fig
Intrinsic leakage of the Josephson flux qubit and breakdown of the two-level approximation for strong driving
Solid state devices for quantum bit computation (qubits) are not perfect
isolated two-level systems, since additional higher energy levels always exist.
One example is the Josephson flux qubit, which consists on a mesoscopic SQUID
loop with three Josephson junctions operated at or near a magnetic flux of half
quantum. We study intrinsic leakage effects, i.e., direct transitions from the
allowed qubit states to higher excited states of the system during the
application of pulses for quantum computation operations. The system is started
in the ground state and rf- magnetic field pulses are applied at the qubit
resonant frequency with pulse intensity . A perturbative calculation of
the average leakage for small is performed for this case, obtaining that
the leakage is quadratic in , and that it depends mainly on the matrix
elements of the supercurrent. Numerical simulations of the time dependent
Schr\"odinger equation corresponding to the full Hamiltonian of this device
were also performed. From the simulations we obtain the value of above
which the two-level approximation breaks down, and we estimate the maximum Rabi
frequency that can be achieved. We study the leakage as a function of the ratio
among the Josephson energies of the junctions of the device, obtaining
the best value for minimum leakage (). The effects of flux
noise on the leakage are also discussed.Comment: Final improved version. Some figures have changed with new results
added. To be published in Phys. Rev.
Influence of Print Orientation on Surface Roughness in Fused Deposition Modeling (FDM) Processes
In the present paper, we address the influence of print orientation angle on surface roughnessobtained in lateral walls in fused deposition modelling (FDM) processes. A geometrical model isdefined that considers the shape of the filaments after deposition, in order to define a theoreticalroughness profile, for a certain print orientation angle. Different angles were considered between 5¿and 85¿. Simulated arithmetical mean height of the roughness profile, Ra values, were calculated fromthe simulated profiles. The Ra simulated results were compared to the experimental results, whichwere carried out with cylindrical PLA (polylactic acid) samples. The simulated Ra values were similarto the experimental values, except for high angles above 80¿, where experimental roughness decreasedwhile simulated roughness was still high. Low print orientation angles show regular profiles withrounded peaks and sharp values. At a print orientation angle of 85¿, the shape of the profile changeswith respect to lower angles, showing a gap between adjacent peaks. At 90¿, both simulated andexperimental roughness values would be close to zero, because the measurement direction is parallelto the layer orientation. Other roughness parameters were also measured: maximum height ofprofile, Rz, kurtosis, Rku, skewness, Rsk, and mean width of the profile elements, Rsm. At high printorientation angles, Rz decreases, Rku shifts to positive, Rsk slightly increases, and Rsk decreases,showing the change in the shape of the roughness profiles.Postprint (published version
Large Amplitude Harmonic Driving of Highly Coherent Flux Qubits
The device for the Josephson flux qubit (DJFQ) can be considered as a solid
state artificial atom with multiple energy levels. When a large amplitude
harmonic excitation is applied to the system, transitions at the energy levels
avoided crossings produce visible changes in the qubit population over many
driven periods that are accompanied by a rich pattern of interference
phenomena. We present a Floquet treatment of the periodically time-dependent
Schr\"odinger equation of the strongly driven qubit beyond the standard two
levels approach. For low amplitudes, the average probability of a given sign of
the persistent current qubit exhibits, as a function of the static flux
detuning and the driving amplitude, Landau-Zener-St\"uckelberg interference
patterns that evolve into complex diamond-like patterns for large amplitudes.
In the case of highly coherent flux qubits we find that the higher order
diamonds can not be simply described relying on a two-level approximations. In
addition we propose a new spectroscopic method based on starting the system in
the first excited state instead of in the ground state, which can give further
information on the energy level spectrum and dynamics in the case of highly
coherent flux qubits. We compare our numerical results with recent experiments
that perform amplitude spectroscopy to probe the energy spectrum of the
artificial atom.Comment: 12 Pages and 12 Figures Phys. Rev. B (in press
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