575 research outputs found
Localized - delocalized electron quantum phase transitions
Metal--insulator transitions and transitions between different quantum Hall
liquids are used to describe the physical ideas forming the basis of quantum
phase transitions and the methods of application of theoretical results in
processing experimental data. The following two theoretical schemes are
discussed and compared: the general theory of quantum phase transitions, which
has been developed according to the theory of thermodynamic phase transitions
and relies on the concept of a partition function, and a theory which is based
on a scaling hypothesis and the renormalization-group concept borrowed from
quantum electrodynamics, with the results formulated in terms of flow diagrams.Comment: 27 pages, 20 figure
Quantum Metallicity on the High-Field Side of the Superconductor-Insulator Transition
We investigate ultrathin superconducting TiN films, which are very close to
the localization threshold. Perpendicular magnetic field drives the films from
the superconducting to an insulating state, with very high resistance. Further
increase of the magnetic field leads to an exponential decay of the resistance
towards a finite value. In the limit of low temperatures, the saturation value
can be very accurately extrapolated to the universal quantum resistance h/e^2.
Our analysis suggests that at high magnetic fields a new ground state, distinct
from the normal metallic state occurring above the superconducting transition
temperature, is formed. A comparison with other studies on different materials
indicates that the quantum metallic phase following the magnetic-field-induced
insulating phase is a generic property of systems close to the disorder-driven
superconductor-insulator transition.Comment: 4 pages, 4 figures, published versio
Superconductor insulator transition in thin films driven by an orbital parallel magnetic field effect
We study theoretically orbital effects of a parallel magnetic field applied
to a disordered superconducting film. We find that the field reduces the phase
stiffness and leads to strong quantum phase fluctuations driving the system
into an insulating behavior. This microscopic model shows that the critical
field decreases with the sheet resistance, in agreement with recent
experimental results. The predictions of this model can be used to discriminate
spin and orbital effects. We find that experiments conducted by A. Johansson
\textit{et al.} are more consistent with the orbital mechanism.Comment: 4 pages, 2 figure
Width of the Zero-Field Superconducting Resistive Transition in the Vicinity of the Localization Threshold
Resistive superconducting zero-field transition in amorphous In-O films in
states from the vicinity of the insulator-superconductor transition is analyzed
in terms of two characteristic temperatures: the upper one, , where the
finite amplitude of the order parameter is established and the lower one,
, where the phase ordering takes place. It follows from the
magnetoresistance measurements that the resistance in between, ,
cannot be ascribed to dissipation by thermally dissociated vortex pairs. So, it
is not Kosterlitz-Thouless-Berezinskii transition that happens at .Comment: 4 pages, 3 figure
Influence of chemical pressure effects on nonlinear thermal conductivity of intrinsically granular superconductors
Using a 2D model of capacitively coupled Josephson junction arrays (created
by a network of twin boundary dislocations with strain fields acting as an
insulating barrier between hole-rich domains in underdoped crystals), we study
the influence of chemical pressure on nonlinear thermal conductivity (NLTC) of
an intrinsically granular superconductor. Quite a substantial enhancement of
NLTC is predicted when intrinsic chemoelectric field closely matches the
externally produced thermoelectric field. The estimates of the model parameters
suggest a realistic possibility to experimentally monitor this effect in
non-stoichiometric superconductors.Comment: 10 pages, 2 figure
Superconductivity on the localization threshold and magnetic-field-tuned superconductor-insulator transition in TiN films
Temperature- and magnetic-field dependent measurements of the resistance of
ultrathin superconducting TiN films are presented. The analysis of the
temperature dependence of the zero field resistance indicates an underlying
insulating behavior, when the contribution of Aslamasov-Larkin fluctuations is
taken into account. This demonstrates the possibility of coexistence of the
superconducting and insulating phases and of a direct transition from the one
to the other. The scaling behavior of magnetic field data is in accordance with
a superconductor-insulator transition (SIT) driven by quantum phase
fluctuations in two-dimensional superconductor. The temperature dependence of
the isomagnetic resistance data on the high-field side of the SIT has been
analyzed and the presence of an insulating phase was confirmed. A transition
from the insulating to a metallic phase is found at high magnetic fields, where
the zero-temperature asymptotic value of the resistance being equal to h/e^2.Comment: 5 pages, 4 eps figures, RevTeX4, Published versio
Theory of Magneto-resistance of Disordered Superconducting Films
Recent experimental studies of magneto-resistance in disordered
superconducting thin films reveal a huge peak (about 5 orders of magnitude).
While it may be expected that magnetic field destroys superconductivity,
leading to an enhanced resistance, attenuation of the resistance at higher
magnetic fields is surprising.
We propose a model which accounts for the experimental results in the entire
range of magnetic fields, based on the formation of superconducting islands due
to fluctuations in the superconducting order parameter amplitude. At strong
magnetic fields Coulomb blockade in these islands gives rise to negative
magneto-resistance. As the magnetic field is reduced the effect of Coulomb
blockade diminishes and eventually the magneto-resistance changes sign.
Numerical calculations show good qualitative agreement with experimental data.Comment: 4 pages, 4 figure
- …