155 research outputs found
Cryogenic Calibration Setup for Broadband Complex Impedance Measurements
Reflection measurements give access to the complex impedance of a material on
a wide frequency range. This is of interest to study the dynamical properties
of various materials, for instance disordered superconductors. However
reflection measurements made at cryogenic temperature suffer from the
difficulty to reliably subtract the circuit contribution. Here we report on the
design and first tests of a setup able to precisely calibrate in situ the
sample reflection, at 4.2 K and up to 2 GHz, by switching and measuring, during
the same cool down, the sample and three calibration standards.Comment: (6 pages, 6 figures
Atomic manipulation of the gap in BiSrCaCuO
Single atom manipulation within doped correlated electron systems would be
highly beneficial to disentangle the influence of dopants, structural defects
and crystallographic characteristics on their local electronic states.
Unfortunately, their high diffusion barrier prevents conventional manipulation
techniques. Here, we demonstrate the possibility to reversibly manipulate
select sites in the optimally doped high temperature superconductor
BiSrCaCuO using the local electric field of the tip.
We show that upon shifting individual Bi atoms at the surface, the spectral gap
associated with superconductivity is seen to reversibly change by as much as 15
meV (~5% of the total gap size). Our toy model that captures all observed
characteristics suggests the field induces lateral movement of point-like
objects that create a local pairing potential in the CuO2 plane.Comment: Published in Science, this is the originally submitted manuscript
prior to changes during the review proces
Evidence for Two Time Scales in Long SNS Junctions
We use microwave excitation to elucidate the dynamics of long superconductor
/ normal metal / superconductor Josephson junctions. By varying the excitation
frequency in the range 10 MHz - 40 GHz, we observe that the critical and
retrapping currents, deduced from the dc voltage vs. dc current characteristics
of the junction, are set by two different time scales. The critical current
increases when the ac frequency is larger than the inverse diffusion time in
the normal metal, whereas the retrapping current is strongly modified when the
excitation frequency is above the electron-phonon rate in the normal metal.
Therefore the critical and retrapping currents are associated with elastic and
inelastic scattering, respectively
High-Quality Planar high-Tc Josephson Junctions
Reproducible high-Tc Josephson junctions have been made in a rather simple
two-step process using ion irradiation. A microbridge (1 to 5 ?m wide) is
firstly designed by ion irradiating a c-axis-oriented YBa2Cu3O7-? film through
a gold mask such as the non-protected part becomes insulating. A lower Tc part
is then defined within the bridge by irradiating with a much lower fluence
through a narrow slit (20 nm) opened in a standard electronic photoresist.
These planar junctions, whose settings can be finely tuned, exhibit
reproducible and nearly ideal Josephson characteristics. This process can be
used to produce complex Josephson circuits.Comment: 4 pages, 5 figures, to be published in Applied Physics Letter
Two-dimensional Ising model with competing interactions and its application to clusters and arrays of -rings and adiabatic quantum computing
We study planar clusters consisting of loops including a Josephson
-junction (-rings). Each -ring carries a persistent current and
behaves as a classical orbital moment. The type of particular state associated
with the orientation of orbital moments at the cluster depends on the
interaction between these orbital moments and can be easily controlled, i.e. by
a bias current or by other means. We show that these systems can be described
by the two-dimensional Ising model with competing nearest-neighbor and diagonal
interactions and investigate the phase diagram of this model. The
characteristic features of the model are analyzed based on the exact solutions
for small clusters such as a 5-site square plaquette as well as on a mean-field
type approach for the infinite square lattice of Ising spins. The results are
compared with spin patterns obtained by Monte Carlo simulations for the 100
100 square lattice and with experiment. We show that the -ring
clusters may be used as a new type of superconducting memory elements. The
obtained results may be verified in experiments and are applicable to adiabatic
quantum computing where the states are switched adiabatically with the slow
change of coupling constants.Comment: 32 pages, 22 figures, RevTe
Interaction-driven quantum phase transition of a single magnetic impurity in Fe(Se,Te)
Understanding the interplay between individual magnetic impurities and
superconductivity is crucial for bottom-up construction of novel phases of
matter. For decades, the description by Yu, Shiba and Rusinov (YSR) of single
spins in a superconductor and its extension to include quantum effects has
proven highly successful: the pair-breaking potential of the spin generates
sub-gap electron- and hole excitations that are energetically equidistant from
zero. By tuning the energy of the sub-gap states through zero, the impurity
screening by the superconductor makes the ground state gain or lose an
electron, signalling a parity breaking quantum phase transition. Here we show
that in multi-orbital impurities, correlations between the in-gap states can
conversely lead to a quantum phase transition where more than one electron
simultaneously leave the impurity without significant effect of the screening
by the superconductor, while the parity may remain unchanged. This finding
implies that the YSR treatment is not always valid, and that intra-atomic
interactions, particularly Hund's coupling that favours high spin
configurations, are an essential ingredient for understanding the sub-gap
states. The interaction-driven quantum phase transition should be taken into
account for impurity-based band-structure engineering, and may provide a
fruitful basis in the search for novel physics.Comment: Main text and supplementar
Coupled Superconducting Phase and Ferromagnetic Order Parameter Dynamics
Via a direct coupling between the magnetic order parameter and the singlet
Josephson supercurrent, we detect spin-wave resonances, and their dispersion,
in ferromagnetic Josephson junctions in which the usual insulating or metallic
barrier is replaced with a weak ferromagnet. The coupling arises within the
Fraunhofer interferential description of the Josephson effect, because the
magnetic layer acts as a time dependent phase plate. A spin-wave resonance at a
frequency ws implies a dissipation that is reflected as a depression in the
current-voltage curve of the Josephson junction when hbar ws = 2eV. We have
thereby performed a resonance experiment on only 10^7 Ni atoms.Comment: 4 pages, 4 figure
Phase Separation and the Phase Diagram in Cuprates Superconductors
We show that the main features of the cuprates superconductors phase diagram
can be derived considering the disorder as a key property of these materials.
Our basic point is that the high pseudogap line is an onset of phase separation
which generates compounds made up of regions with distinct doping levels. We
calculate how this continuous temperature dependent phase separation process
occurs in high critical temperature superconductors (HTSC) using the
Cahn-Hilliard approach, originally applied to study alloys. Since the level of
phase separation varies for different cuprates, it is possible that different
systems with average doping level pm exhibit different degrees of charge and
spin segregation. Calculations on inhomogeneous charge distributions in form of
stripes in finite clusters performed by the Bogoliubov-deGennes superconducting
approach yield good agreement to the pseudogap temperature T*(pm), the onset of
local pairing amplitudes with phase locked and concomitantly, how they develop
at low temperatures into the superconducting phase at Tc(pm) by percolation.Comment: 9 pages, 9 figures. Submitted to Phys. Rev.
Spontaneous supercurrent induced by ferromagnetic pi-junctions
We present magnetization measurements of mesoscopic superconducting niobium
loops containing a ferromagnetic (PdNi) pi-junction. The loops are prepared on
top of the active area of a micro Hall-sensor based on high mobility
GaAs/AlGaAs heterostructures. We observe asymmetric switching of the loop
between different magnetization states when reversing the sweep direction of
the magnetic field. This provides evidence for a spontaneous current induced by
the intrinsic phase shift of the pi-junction. In addition, the presence of the
spontaneous current near zero applied field is directly revealed by an increase
of the magnetic moment with decreasing temperature, which results in half
integer flux quantization in the loop at low temperatures.Comment: 4 pages, 4 figure
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