1,711 research outputs found
Coherent switching of semiconductor resonator solitons
We demonstrate switching on and off of spatial solitons in a semiconductor
microresonator by injection of light coherent with the background illumination.
Evidence results that the formation of the solitons and their switching does
not involve thermal processes.Comment: 3 pages, 5 figure
Enhanced quantized current driven by surface acoustic waves
We present the experimental realization of different approaches to increase
the amount of quantized current which is driven by surface acoustic waves
through split gate structures in a two dimensional electron gas. Samples with
driving frequencies of up to 4.7 GHz have been fabricated without a
deterioration of the precision of the current steps, and a parallelization of
two channels with correspondingly doubled current values have been achieved. We
discuss theoretical and technological limitations of these approaches for
metrological applications as well as for quantum logics.Comment: 3pages, 4eps-figure
Heat transport by turbulent Rayleigh-B\'enard convection for $\Pra\ \simeq 0.83\times 10^{12} \alt \Ra\ \alt 10^{15}\Gamma = 0.50$
We report experimental results for heat-transport measurements, in the form
of the Nusselt number \Nu, by turbulent Rayleigh-B\'enard convection in a
cylindrical sample of aspect ratio ( m is
the diameter and m the height). The measurements were made using
sulfur hexafluoride at pressures up to 19 bars as the fluid. They are for the
Rayleigh-number range 3\times 10^{12} \alt \Ra \alt 10^{15} and for Prandtl
numbers \Pra\ between 0.79 and 0.86. For \Ra < \Ra^*_1 \simeq 1.4\times
10^{13} we find \Nu = N_0 \Ra^{\gamma_{eff}} with , consistent with classical turbulent Rayleigh-B\'enard convection in a
system with laminar boundary layers below the top and above the bottom plate.
For \Ra^*_1 < \Ra < \Ra^*_2 (with \Ra^*_2 \simeq 5\times 10^{14})
gradually increases up to . We argue that above
\Ra^*_2 the system is in the ultimate state of convection where the boundary
layers, both thermal and kinetic, are also turbulent. Several previous
measurements for are re-examined and compared with the present
results.Comment: 44 pages, 18 figures, submitted to NJ
Heat transport by turbulent Rayleigh-B\'enard convection for $\Pra\ \simeq 0.84\times 10^{11} \alt \Ra\ \alt 2\times10^{14}\Gamma = 1.00$
We report experimental results for heat-transport measurements by turbulent
Rayleigh-B\'enard convection in a cylindrical sample of aspect ratio ( m is the diameter and m the height).
They are for the Rayleigh-number range 4\times10^{11} \alt \Ra \alt
2\times10^{14} and for Prandtl numbers \Pra\ between 0.79 and 0.86.
For \Ra < \Ra^*_1 \simeq 2\times 10^{13} we find \Nu = N_0
\Ra^{\gamma_{eff}} with and ,
consistent with classical turbulent Rayleigh-B\'enard convection in a system
with laminar boundary layers below the top and above the bottom plate and with
the prediction of Grossmann and Lohse.
For \Ra > \Ra_1^* the data rise above the classical-state power-law and
show greater scatter. In analogy to similar behavior observed for , we interpret this observation as the onset of the transition to the
ultimate state. Within our resolution this onset occurs at nearly the same
value of \Ra_1^* as it does for . This differs from an earlier
estimate by Roche {\it et al.} which yielded a transition at \Ra_U \simeq
1.3\times 10^{11} \Gamma^{-2.5\pm 0.5}. A -independent \Ra^*_1 would
suggest that the boundary-layer shear transition is induced by fluctuations on
a scale less than the sample dimensions rather than by a global
-dependent flow mode. Within the resolution of the measurements the
heat transport above \Ra_1^* is equal for the two values, suggesting
a universal aspect of the ultimate-state transition and properties. The
enhanced scatter of \Nu\ in the transition region, which exceeds the
experimental resolution, indicates an intrinsic irreproducibility of the state
of the system.Comment: 17 pages, including 2 pages of data tables and 56 references.
Submitted to New J. Phy
Rayleigh-B\'{e}nard convection in a homeotropically aligned nematic liquid crystal
We report experimental results for convection near onset in a thin layer of a
homeotropically aligned nematic liquid crystal heated from below as a function
of the temperature difference and the applied vertical magnetic
field and compare them with theoretical calculations. The experiments cover
the field range 8 \alt h \equiv H/ H_{F} \alt 80 ( is the
Fr\'eedericksz field). For less than a codimension-two field the bifurcation is subcritical and oscillatory, with travelling- and
standing-wave transients. Beyond the bifurcation is stationary and
subcritical until a tricritical field is reached, beyond which it
is supercritical. The bifurcation sequence as a function of found in the
experiment confirms the qualitative aspects of the theoretical predictions.
However, the value of is about 10% higher than the predicted value and
the results for are systematically below the theory by about 2% at small
and by as much as 7% near . At , is continuous within
the experimental resolution whereas the theory indicates a 7% discontinuity.
The theoretical tricritical field is somewhat below the
experimental one. The fully developed flow above for is
chaotic. For the subcritical stationary bifurcation also
leads to a chaotic state. The chaotic states persist upon reducing the Rayleigh
number below , i.e. the bifurcation is hysteretic. Above the tricritical
field , we find a bifurcation to a time independent pattern which within
our resolution is non-hysteretic.Comment: 15 pages incl. 23 eps figure
Towards a Graphene-Based Quantum Impedance Standard
Precision measurements of the quantum Hall resistance with alternating
current (ac) in the kHz range were performed on epitaxial graphene in order to
assess its suitability as a quantum standard of impedance. The quantum Hall
plateaus measured with alternating current were found to be flat within one
part in 10^7. This is much better than for plain GaAs quantum Hall devices and
shows that the magnetic-flux-dependent capacitive ac losses of the graphene
device are less critical. The observed frequency dependence of about
-8x10^-8/kHz is comparable in absolute value to the positive frequency
dependence of plain GaAs devices, but the negative sign is attributed to stray
capacitances which we believe can be minimized by a careful design of the
graphene device. Further improvements thus may lead to a simpler and more
user-friendly quantum standard for both resistance and impedance
Compendium for precise ac measurements of the quantum Hall resistance
In view of the progress achieved in the field of the ac quantum Hall effect,
the Working Group of the Comite Consultatif d'Electricite et Magnetisme (CCEM)
on the AC Quantum Hall Effect asked the authors of this paper to write a
compendium which integrates their experiences with ac measurements of the
quantum Hall resistance. In addition to the important early work performed at
the Bureau International des Poids et Mesures and the National Physical
Laboratory, UK, further experience has been gained during a collaboration of
the authors' institutes NRC, METAS, and PTB, and excellent agreement between
the results of different national metrology institutes has been achieved. This
compendium summarizes the present state of the authors' knowledge and reviews
the experiences, tests and precautions that the authors have employed to
achieve accurate measurements of the ac quantum Hall effect. This work shows
how the ac quantum Hall effect can be reliably used as a quantum standard of ac
resistance having a relative uncertainty of a few parts in 10^8.Comment: 26 pages, 8 figure
Quantized charge pumping through a quantum dot by surface acoustic waves
We present a realization of quantized charge pumping. A lateral quantum dot
is defined by metallic split gates in a GaAs/AlGaAs heterostructure. A surface
acoustic wave whose wavelength is twice the dot length is used to pump single
electrons through the dot at a frequency f=3GHz. The pumped current shows a
regular pattern of quantization at values I=nef over a range of gate voltage
and wave amplitude settings. The observed values of n, the number of electrons
transported per wave cycle, are determined by the number of electronic states
in the quantum dot brought into resonance with the fermi level of the electron
reservoirs during the pumping cycle.Comment: 8 page
Continuous spin reorientation in antiferromagnetic films
We study anisotropic antiferromagnetic one-layer films with dipolar and
nearest-neighbor exchange interactions. We obtain a unified phase diagram as a
function of effective uniaxial D_e and quadrupolar C anisotropy constants. We
study in some detail how spins reorient continuously below a temperature T_s as
T and D_e vary.Comment: 3 LaTeX pages, 3 eps figures. Submitted to JMMM on 25 May 2006.
Accepted on 21 July 200
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