220 research outputs found
Measurements of thermoelectric power in annealed and quenched gold-platinum alloys
Report gives measurements of absolute thermoelectric powers of dilute gold-platinum alloys and influence of quenched-in lattice vacancies on their thermoelectric powers. It investigates phonon-drag component of thermoelectric power as a function of platinum concentration, and change in phonon-drag thermoelectric power by lattice vacancies
Instability driven formation of domains in the intermediate state of type-I superconductors
The formation of normal-state domains in type-I superconducting indium films
is investigated using the high resolution magneto-optical imaging technique.
The observed patterns consist of coexisting circular and lamellar normal-phase
domains surrounded by the superconducting phase. The distribution of domain
surface areas is found to exhibit a threshold, above which only the lamellar
shape is observed. We show that this threshold coincides with the predicted
critical surface area for the elongation instability of the circular shape. The
partition of the normal phase into circular and lamellar domains is determined
by the combined effects of the elongation instability and the penetration of
magnetic flux by bursts at the early stage of pattern formation. It is not
governed by mutual interactions between domains, as usually assumed for
self-organized systems
Room Temperature Electrical Detection of Spin Coherence in C60
An experimental demonstration of electrical detection of coherent spin motion
of weakly coupled, localized electron spins in thin Fullerene C60 films at room
temperature is presented. Pulsed electrically detected magnetic resonance
experiments on vertical photocurrents through Al/C60/ZnO samples showed that an
electron spin Rabi oscillation is reflected by transient current changes. The
nature of possible microscopic mechanisms responsible for this spin to charge
conversion as well as its implications for the readout of endohedral Fullerene
(N@C60) spin qubits are discussed.Comment: 4 pages, 3 figure
Impeded Growth of Magnetic Flux Bubbles in the Intermediate State Pattern of Type I Superconductors
Normal state bubble patterns in Type I superconducting Indium and Lead slabs
are studied by the high resolution magneto-optical imaging technique. The size
of bubbles is found to be almost independent of the long-range interaction
between the normal state domains. Under bubble diameter and slab thickness
proper scaling, the results gather onto a single master curve. On this basis,
in the framework of the "current-loop" model [R.E. Goldstein, D.P. Jackson and
A.T. Dorsey, Phys. Rev. Lett. 76, 3818 (1996)], we calculate the equilibrium
diameter of an isolated bubble resulting from the competition between the
Biot-and-Savart interaction of the Meissner current encircling the bubble and
the superconductor-normal interface energy. A good quantitative agreement with
the master curve is found over two decades of the magnetic Bond number. The
isolation of each bubble in the superconducting matrix and the existence of a
positive interface energy are shown to preclude any continuous size variation
of the bubbles after their formation, contrary to the prediction of mean-field
models.Comment: \'{e}quipe Nanostructures Quantique
Numerical simulations of two dimensional magnetic domain patterns
I show that a model for the interaction of magnetic domains that includes a
short range ferromagnetic and a long range dipolar anti-ferromagnetic
interaction reproduces very well many characteristic features of
two-dimensional magnetic domain patterns. In particular bubble and stripe
phases are obtained, along with polygonal and labyrinthine morphologies. In
addition, two puzzling phenomena, namely the so called `memory effect' and the
`topological melting' observed experimentally are also qualitatively described.
Very similar phenomenology is found in the case in which the model is changed
to be represented by the Swift-Hohenberg equation driven by an external
orienting field.Comment: 8 pages, 8 figures. Version to appear in Phys. Rev.
Quasiparticle scattering time in superconducting films: from dirty to clean limit
We study the quasiparticle energy relaxation processes in superconducting Nb
films of different thicknesses corresponding to different electron mean free
paths in a state far from equilibrium, that is the highly dissipative flux-flow
state driven up to the instability point. From the measured current-voltage
curves we derive the vortex critical velocity for several temperatures.
From the values, the quasiparticle energy relaxation time
is evaluated within the Larkin-Ovchinnikov model and
numerical calculations of the quasiparticle energy relaxation rates are carried
out to support the experimental findings. Besides the expected constant
behavior of for the dirty samples, we observe a strong
temperature dependence of the quasiparticle energy relaxation time in the clean
samples. This feature is associated with the increasing contribution from the
electron-phonon scattering process as the dirty limit is approached from the
clean regime
Nucleation and Collapse of the Superconducting Phase in Type-I Superconducting Films
The phase transition between the intermediate and normal states in type-I
superconducting films is investigated using magneto-optical imaging. Magnetic
hysteresis with different transition fields for collapse and nucleation of
superconducting domains is found. This is accompanied by topological hysteresis
characterized by the collapse of circular domains and the appearance of
lamellar domains. Magnetic hysteresis is shown to arise from supercooled and
superheated states. Domain-shape instability resulting from long-range magnetic
interaction accounts well for topological hysteresis. Connection with similar
effects in systems with long-range magnetic interactions is emphasized
Parental well-being in times of Covid-19 in Germany
We examine the effects of Covid-19 and related restrictions on individuals with dependent children in Germany. We specifically focus on the role of day care center and school closures, which may be regarded as a “disruptive exogenous shock” to family life. We make use of a novel representative survey of parental well-being collected in May and June 2020 in Germany, when schools and day care centers were closed but while other measures had been relaxed and new infections were low. In our descriptive analysis, we compare well-being during this period with a pre-crisis period for different groups. In a difference-in-differences design, we compare the change for individuals with children to the change for individuals without children, accounting for unrelated trends as well as potential survey mode and context effects. We find that the crisis lowered the relative well-being of individuals with children, especially for individuals with young children, for women, and for persons with lower secondary schooling qualifications. Our results suggest that public policy measures taken to contain Covid-19 can have large effects on family well-being, with implications for child development and parental labor market outcomes
The role of temperature in the magnetic irreversibility of type-I Pb superconductors
Evidence of how temperature takes part in the magnetic irreversibility in the
intermediate state of a cylinder and various disks of pure type-I
superconducting lead is presented. Isothermal measurements of first
magnetization curves and magnetic hysteresis cycles are analyzed in a reduced
representation that defines an equilibrium state for flux penetration in all
the samples and reveals that flux expulsion depends on temperature in the disks
but not in the cylinder. The magnetic field at which irreversibility sets in
along the descending branch of the hysteresis cycle and the remnant
magnetization at zero field are found to decrease with temperature in the
disks. The contributions to irreversibility of the geometrical barrier and the
energy minima associated to stress defects that act as pinning centers on
normal-superconductor interfaces are discussed. The differences observed among
the disks are ascribed to the diverse nature of the stress defects in each
sample. The pinning barriers are suggested to decrease with the magnetic field
to account for these results
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