169 research outputs found
Noise properties of nanoscale YBCO Josephson junctions
We present electric noise measurements of nanoscale biepitaxial YBa2Cu3O(7-x)
(YBCO) Josephson junctions fabricated by two different lithographic methods.
The first (conventional) technique defines the junctions directly by ion
milling etching through an amorphous carbon mask. The second (soft patterning)
method makes use of the phase competition between the superconducting YBCO
(Y123) and the insulating Y2BaCuO5 (Y211) phase at the grain boundary interface
on MgO (110) substrates. The voltage noise properties of the two methods are
compared in this study. For all junctions (having a thickness of 100 nm and
widths of 250-500 nm) we see a significant amount of individual charge traps.
We have extracted an approximate value for the effective area of the charge
traps from the noise data. From the noise measurements we infer that the soft
patterned junctions with a grain boundary (GB) interface manifesting a large
c-axis tunneling component have a uniform barrier and a SIS like behavior. The
noise properties of soft patterned junctions having a GB interface dominated by
transport parallel to the ab-planes are in accordance with a resonant tunneling
barrier model. The conventionally patterned junctions, instead, have suppressed
superconducting transport channels with an area much less than the nominal
junction area. These findings are important for the implementation of nanosized
Josephson junctions in quantum circuits.Comment: 11 Pages, 10 figure
Quantum Phase Dynamics in an LC shunted Josephson Junction
We have studied both theoretically and experimentally how an LC series
circuit connected in parallel to a Josephson junction influences the Josephson
dynamics. The presence of the shell circuit introduces two energy scales, which
in specific cases can strongly differ from the plasma frequency of the isolated
junction. Josephson junctions were manufactured using Nb/Al-AlOx/Nb fabrication
technology with various on-chip LC shunt circuits. Spectroscopic measurements
in the quantum limit show an excellent agreement with theory taking into
account the shunt inductance and capacitance in the Resistively and
Capacitively Shunted Junction model. The results clearly show that the dynamics
of the system are two-dimensional, resulting in two resonant modes of the
system. These findings have important implications for the design and operation
of Josephson junctions based quantum bits
Probing the phase diagram of cuprates with YBaCuO thin films and nanowires
We have grown and characterized 30 nm thick YBaCuO
(YBCO) films, deposited by pulsed laser deposition on both MgO (110) and
SrTiO (001) substrates, which induce opposite strain to the superconducting
layer. By carefully tuning the in-situ post-annealing oxygen pressure, we
achieved, in a reproducible way, films at different oxygen doping, spanning
from the slightly overdoped down to the strongly underdoped region of the phase
diagram. The transport properties of the films, investigated through resistance
versus temperature measurements, are in perfect qualitative agreement with
single crystals. Starting from these films, we have also successfully
fabricated nanowires with widths down to 65 nm, at different oxygen doping. The
nanostructures exhibit characteristic temperatures (as the critical temperature
and the pseudogap temperature ) similar to those of the
as-grown films and carry critical current densities close to
the critical depairing value, limited by vortex entry. This implies that the
superconducting and the normal state properties of underdoped YBCO are
preserved in our films, and they can be studied as a function of the
dimensionality of the system, down to the nanoscale.Comment: 11 pages, 9 figures, submitted to Phys. Rev. Material
Microwave losses in MgO, LaAlO3, and (La0.3Sr0.7)(Al0.65Ta0.35)O-3 dielectrics at low power and in the millikelvin temperature range
We have investigated both the temperature and the power dependence of microwave losses for various dielectrics commonly used as substrates for the growth of high critical temperature superconductor thin films. We present measurement of niobium superconducting lambda/2 coplanar waveguide resonators, fabricated on MgO, LaAlO3, and (La0.3Sr0.7)(Al0.65Ta0.35)O-3 (LSAT), at the millikelvin temperature range and at low input power. By comparing our results with the two-level system model, we have discriminated among different dominant loss mechanisms. LSAT has shown the best results as regards the dielectric losses in the investigated regimes
Ultra low noise YBCO nanoSQUIDs implementing nanowires
We present results on ultra low noise YBaCuO nano
Superconducting QUantum Interference Devices (nanoSQUIDs). To realize such
devices, we implemented high quality YBCO nanowires, working as weak links
between two electrodes. We observe critical current modulation as a function of
an externally applied magnetic field in the full temperature range below the
transition temperature . The white flux noise below 1 at T = 8 K makes our nanoSQUIDs very attractive for
the detection of small spin systems.Comment: 11 pages, 4 figures, submitted to Appl. Phys. Lett. 25/01/201
Soft nanostructuring of YBCO Josephson Junctions by phase separation
We have developed a new method to fabricate biepitaxial YBa2Cu3O(7-x) (YBCO)
Josephson junctions at the nanoscale, allowing junctions widths down to 100 nm
and simultaneously avoiding the typical damage in grain boundary interfaces due
to conventional patterning procedures. By using the competition between the
superconducting YBCO and the insulating Y2BaCuO5 phases during film growth, we
formed nanometer sized grain boundary junctions in the insulating Y2BaCuO5
matrix as confirmed by high resolution transmission electron microscopy.
Electrical transport measurements give clear indications that we are close to
probing the intrinsic properties of the grain boundaries.Comment: 16 pages, 6 figure
Direct transition from quantum escape to phase diffusion regime in YBaCuO biepitaxial Josephson Junctions
Dissipation encodes interaction of a quantum system with the environment and
regulates the activation regimes of a Brownian particle. We have engineered
grain boundary biepitaxial YBaCuO junctions to drive a direct transition from
quantum activated running state to phase diffusion regime. The cross-over to
the quantum regime is tuned by the magnetic field and dissipation is encoded in
a fully consistent set of junction parameters. To unravel phase dynamics in
moderately damped systems is of general interest for advances in the
comprehension of retrapping phenomena and in view of quantum hybrid technology
Electromigration tuning of the voltage modulation depth in YBa2Cu3O7-delta nanowire-based SQUIDs
Oxygen electromigration applied to a YBa2Cu3O7-delta nanowire can be used to tune its electrical properties. Here, we apply electromigration to YBCO nanowire-based superconducting quantum interference devices (SQUIDs) and study its effect on the voltage modulation depth of the devices. Using a dc electromigration current we replenish the oxygen of the weak links, improving the critical current symmetry of the SQUIDs. AC current electromigration is used to reduce the doping level of the weak links, thus reducing their critical current and increasing differential resistance. Both type of electromigration processes are found to improve the SQUIDs performance, although the best results are obtained with ac biased electromigration, which improved the voltage modulations of the SQUIDs by a factor as high as 8. This procedure can be instrumental to fine tune ex-situ the properties of superconducting electronics where a large number of weak links are required
Dispersion of the surface phonons in semiconductor/topological insulator Si/Bi2Te3 heterostructure studied by high resolution Brillouin spectroscopy
The dynamics and dispersion of surface phonons in heterostructure semiconductor/ topological insulator Si/Bi2Te3 was investigated using high resolution Brillouin light scattering method in the GHz frequency range. Both Rayleigh and Sezawa surface acoustic waves have been observed for wave vectors ranging from 0.006 to 0.023 nm−1. Anomaly in dispersion relations ω(q) for both surface waves were detected for the wave vector q = 0.016 nm−1. The finite element method (FEM) was used to simulate the observed shapes of ω(q) and to find the deformation profiles of surface acoustic waves. We attribute the observed changes to the coupling between low energy electrons and surface phonons. The coupling between helical Dirac states and surface phonons is discussed in the frame of accessible theoretical models
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