245 research outputs found
DC-assisted microwave quenching of YBa2Cu3O7-{\delta} coplanar waveguide to a highly dissipative state
The paper reports on finding the effect of a strong change in the microwave
losses in an HTS-based coplanar waveguide (CPW) at certain values of the input
power Pin and direct current Idc. CPW on the basis of 150 nm thick
YBa2Cu3O7-{\delta} epitaxial film on a single crystal MgO substrate was studied
experimentally. A sharp and reversible transition of the CPW into a strongly
dissipative state at the certain meanings of Pin and Idc depending on
temperature was observed. Apparently the effect can be explained by
self-heating of HTS structure caused by magnetic flux flow under the joint
influence of MW and DC.Comment: 9 pages, 5 figures, 17 reference
Millimeter-wave surface impedance of optimally-doped Ba(Fe1-xCox)2As2 single crystals
Precision measurements of active and reactive components of in-plane
microwave surface impedance were performed in single crystals of
optimally-doped Fe-based superconductor Ba(Fe1-xCox)2As2 (x = 0.074, Tc = 22.8
K). Measurements in a millimeter wavelength range (Ka band, 35-40 GHz) were
performed using whispering gallery mode excitations in the ultrahigh quality
factor quasioptical sapphire disk resonator with YBa2Cu2O7 superconducting (Tc
= 90 K) end plates. The temperature variation of the London penetration depth
is best described by a power-law function, delta {\lambda}(T) is proportional
to T with the exponent n, n = 2.8, in reasonable agreement with radio-frequency
measurements on crystals of the same batch. This power-law dependence is
characteristic of a nodeless superconducting gap in the extended s-wave pairing
scenario with a strong pair-breaking scattering. The quasiparticle conductivity
of the samples, {\sigma}1(T), gradually increases with the decrease of
temperature, showing no peak below or at Tc, in notable contrast with the
behavior found in the cuprates. The temperature-dependent quasiparticle
scattering rate was analyzed in a two-fluid model, assuming the validity of the
Drude description of conductivity and generalized expression for the scattering
rate. This analysis allows us to estimate the range of the values of a residual
surface resistance from 3 to 6 mOhm.Comment: 7 pages, 7 figure
Electric Current and Noise in Long GaN Nanowires in the Space-Charge Limited Transport Regime
We studied electric current and noise in planar GaN nanowires (NWs). The
results obtained at low voltages provide us with estimates of the depletion
effects in the NWs. For larger voltages, we observed the space-charge limited
current (SCLC) effect. The onset of the effect clearly correlates with the NW
width. For narrow NWs the mature SCLC regime was achieved. This effect has
great impact on fluctuation characteristics of studied NWs. At low voltages, we
found that the normalized noise level increases with decreasing NW width. In
the SCLC regime, a further increase in the normalized noise intensity (up to
1E4 times) was observed, as well as a change in the shape of the spectra with a
tendency towards slope -3/2. We suggest that the features of the electric
current and noise found in the NWs are of a general character and will have an
impact on the development of NW-based devices.Comment: 12 pages, 4 figures in Fluctuation and Noise Letters (2017
Design and characterization of all-cryogenic low phase-noise sapphire K-band oscillator for sattelite communication
An all-cryogenic oscillator consisting of a frequency-tunable sapphire resonator, a high-temperature superconducting filter and a pseudomorphic high electron-mobility transistor amplifier was designed for the K-band frequency range and investigated. Due to the high quality factor of the resonator above 1000 000 and the low amplifier phase noise of approximately -133 dBc/Hz at a frequency offset of 1kHz from the carrier, we have achieved oscillator phase-noise values superior to quartz-stabilized oscillators at the same carrier frequency for offset frequencies higher than 100 Hz. In addition to, low phase noise, our prototype oscillator possesses mechanical and electrical frequency tunability. We have implemented a two-step electrical tuning arrangement consisting of a varactor phase shifter integrated within the amplifier circuit (fine tuning by 5'kHz) and a dielectric plunger moved by a piezomechanical transducer inside the resonator housing (course tuning by 50 kHz). This tuning range is sufficient for phase locking and for electronic compensation of temperature drifts occurring during operation of the device employing a miniaturized closed-cycle Stirling-type cryocooler
Noise and Transport Characterization of Single Molecular Break Junctions with Individual Molecule
We studied the noise spectra of molecule-free and molecule-containing
mechanically controllable break junctions. Both types of junctions revealed
typical 1/ f noise characteristics at different distances between the contacts
with square dependence of current noise power spectral density on current.
Additional Lorentzian-shape (1/ f 2) noise components were recorded only when
nanoelectrodes were bridged by individual 1,4 benzenediamine molecule. The
characteristic frequency of the revealed 1/ f 2 noise related to a single
bridging molecule correlates with the lock-in current amplitudes. The recorded
behavior of Lorentzian-shape noise component as a function of current is
interpreted as the manifestation of a dynamic reconfiguration of molecular
coupling to the metal electrodes. We propose a phenomenological model that
correlates the charge transport via a single molecule with the reconfiguration
of its coupling to the metal electrodes. Experimentally obtained results are in
good agreement with theoretical ones and indicate that coupling between the
molecule metal electrodes is important aspect that should be taken into
account.Comment: 15 pages, 7 figure
Hacia un nuevo socialismo
Nos encontramos ante el testamento
polÃtico de uno de los lÃderes norteamericanos
del socialismo democrático.
M. Harrington murió en agosto de
1989 poco después de haber aparecido
este libro. Intuyendo su próximo final
hace Un balance histórico del movimiento socialista y apunta hacia su potencial
futuro. Es el testamento de un
militante y un intelectual que reflexiona
sobre lo que es el socialismo desde
Estados Unidos, la sociedad occidental
del capitalismo más desarrollada y más
refractaria a la tradición socialista.Peer reviewe
Dynamic redistribution of the electric field of the channel in AlGaN/GaN high electron mobility transistor with nanometer-scale gate length
Transport peculiarities and the physical origin of noise properties in AlGaN/GaN-based high electron mobility transistors (HEMTs) with a large ratio of channel length to gate length were investigated. Dependence of deviations of low-frequency noise spectra from the 1/f law on applied gate voltages was studied in an extended range of frequencies. The behavior is explained in terms of a model based on the dynamic redistribution of the electric field along the two-dimensional channel of the HEMT. The results show that the main contribution to the noise originates from the region under the gate and adjacent to the gate channel regions. (C) 2005 American Institute of Physics
Millimeter-wave study of London penetration depth temperature dependence in Ba(Fe0.926Co0.074)2As2 single crystal
In-plane surface Ka-band microwave impedance of optimally doped single
crystals of the Fe-based superconductor Ba(Fe0.926Co0.074)2As2 (Tc= 22.8K) was
measured. Sensitive sapphire disk quasi-optical resonator with high-Tc cuprate
conducting endplates was developed specially for Fe-pnictide superconductors.
It allowed finding temperature variation of London penetration depth in a form
of power law, namely \Delta \lambda (T)~ Tn with n = 2.8 from low temperatures
up to at least 0.6Tc consisted with radio-frequency measurements. This exponent
points towards nodeless state with pairbreaking scattering, which can support
one of the extended s-pairing symmetries. The dependence \lambda(T) at low
temperatures is well described by one superconducting small-gap (\Delta \cong
0.75 in kTc units, where k is Boltzman coefficient) exponential dependence.Comment: 6 pages, 2 figures, to be published in Low Temperature
Physics,vol.37, August 201
Spin splitting of X-related donor impurity states in an AlAs barrier
We use magnetotunneling spectroscopy to observe the spin splitting of the
ground state of an X-valley-related Si-donor impurity in an AlAs barrier. We
determine the absolute magnitude of the effective Zeeman spin splitting factors
of the impurity ground state to be g= 2.2 0.1. We also investigate
the spatial form of the electron wave function of the donor ground state, which
is anisotropic in the growth plane
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