1,226 research outputs found
Microscopic scale of quantum phase transitions: from doped semiconductors to spin chains, cold gases and moir\'e superlattices
In the vicinity of continuous quantum phase transitions (QPTs), quantum
systems become scale-invariant and can be grouped into universality classes
characterized by sets of critical exponents. We have found that despite
scale-invariance and universality, the experimental data still contain
information related to the microscopic processes and scales governing QPTs. We
conjecture that near QPTs, various physical quantities follow the generic
exponential dependence predicted by the scaling theory of localization; this
dependence includes as a parameter a microscopic seeding scale of the
renormalization group, . We also conjecture that for interacting systems,
the temperature cuts the renormalization group flow at the length travelled by
a system-specific elementary excitation over the life-time set by the Planckian
time, =. We have adapted this approach for QPTs in several
systems and showed that extracted from experiment is comparable to
physically-expected minimal length scales, namely (i) the mean free path for
metal-insulator transition in doped semiconductors, (ii) the distance between
spins in Heisenberg and Ising chains, (iii) the period of an optical lattice
for cold atom boson gases, and (iv) the period of a moir\'e superlattice for
the Mott QPT in dichalcogenide bilayers. In the first companion paper, we show
that in superconducting films and nanowires, as well as in the high temperature
superconductor LaSrCuO, is comparable to
superconducting coherence length. In the second companion paper, we show that
in quantum Hall systems, is comparable to the magnetic length. The
developed theoretical approach quantitatively explains and unifies a large body
of experimental data and can be expanded to other complex systemsComment: 12 pages, 6 figure
Quantum phase transitions in quantum Hall and other topological systems: role of the Planckian time
Transformations between the plateau states of the quantum Hall effect (QHE)
are an archetypical example of quantum phase transitions (QPTs) between phases
with non-trivial topological order. These transitions appear to be
well-described by the single-particle network theories. The long-standing
problem with this approach is that it does not account for Coulomb
interactions. In this paper, we show that experimental data in the quantum
critical regime for both integer and fractional QHEs can be quantitatively
explained by the recently developed phenomenological model of QPTs in
interacting systems. This model assumes that all effects of interactions are
contained in the life-time of fluctuations as set by the Planckian time
. The dephasing length is taken as the distance traveled by
a non-interacting particle along the bulk edge state over this time. We show
that the model also provides quantitative description of QPTs between the
ground states of anomalous QHE and axion and Chern insulators. These analyzed
systems are connected in that the QPTs occur via quantum percolation. Combining
the presented results with the results of two companion papers, we conclude
that the Planckian time is the encompassing characteristic of QPTs in
interacting systems, independent of dimensionality and microscopic physics.Comment: 6 pages, 3 figure
Adaptive observers for nonlinearly parameterized systems subjected to parametric constraints
We consider the problem of adaptive observer design in the settings when the
system is allowed to be nonlinear in the parameters, and furthermore they are
to satisfy additional feasibility constraints. A solution to the problem is
proposed that is based on the idea of universal observers and non-uniform
small-gain theorem. The procedure is illustrated with an example.Comment: 19th IFAC World Congress on Automatic Control, 10869-10874, South
Africa, Cape Town, 24th-29th August, 201
Peculiarities of Electron-Beam Formation of Hydrophobic and Superhydrophobic Coatings Based on Hydrocarbons of Various Molecular Weights and PTFE
The paper studies the possibility of superhydrophobic coatings formations at exposure of powder mixture of polytetrafluorethylene and hydrocarbons having various molecular weights to low-energy electron beam in vacuum. It is shown that paraffin and PTFE based thin composite coatings may be characterized by superhydrophobic properties. The superhydrophobic properties are attained due to low surface energy of the fluorine-containing component and structured surface due to peculiarities of composite layer formation. The chemical processes observed in electron beam exposed area determine the molecular structure, morphology and the contact angle of thin organic coatings deposited. It is shown that high-molecular-weight hydrocarbon compounds should not be recommended for vacuum electron-beam deposition of superhydrophobic thin coatings because of deep changes in the molecular structure exposed to electron beam. These processes are responsible for high degree of unsaturation of the thin layer formed and for occurrence of oxygen-containing polar groups. The influence of substrate temperature on molecular structure, morphology and hydrophobic properties of thin coatings deposited is investigated. Potentially such coatings may be applied for deposition on the surface of metal capillaries used in biotechnological analyzers
Superconducting properties of polycrystalline Nb nanowires templated by carbon nanotubes
Journal ArticleContinuous Nb wires, 7-15 nm in diameter, have been fabricated by sputter-coating single fluorinated carbon nanotubes. Transmission electron microscopy revealed that the wires are polycrystalline, having grain sizes of about 5 nm. The critical current of wires thicker than ~12 nm is very high (107 A/cm2) and comparable to the expected depairing current. The resistance versus temperature curves measured down to 0.3 K are well described by the Langer-Ambegaokar-McCumber-Halperin theory of thermally activated phase slips. Quantum phase slips are suppressed
Microscopic scale of pair-breaking quantum phase transitions in superconducting films, nanowires and LaSrCuO
The superconducting ground state in a large number of two-dimensional (2d)
systems can be created and destroyed through quantum phase transitions (QPTs)
driven by non-thermal parameters such as the carrier density or magnetic field.
The microscopic mechanism of QPTs has not been established in any 2d
superconductor, in part due to an emergent scale-invariance near the critical
point, which conceals the specific processes driving the transitions. In this
work, we find that the pair-breaking mechanism causing the suppression of the
Cooper pair density gives a unifyingly consistent description of
magnetic-field-driven QPTs in amorphous MoGe, Pb and TaN films, as well as in
quasi-2d high-temperature superconductor LaSrCuO. This
discovery was facilitated by the development of a novel theoretical approach,
one which goes beyond the standard determination of critical exponents and
allows for the extraction of a microscopic seeding length scale of the
transitions. Remarkably, for the materials studied, and also for MoGe
nanowires, this scale matches the superconducting coherence length. Further,
this approach has been successfully applied to many other complex,
non-superconducting systems.Comment: 9 pages, 5 figure
Magnetic-field enhancement of superconductivity in ultranarrow wires
Journal ArticleWe study the effect of an applied magnetic field on sub-10-nm wide MoGe and Nb superconducting wires. We find that magnetic fields can enhance the critical supercurrent at low temperatures, and do so more strongly for narrower wires. We conjecture that magnetic moments are present, but their pair-breaking effect, active at lower magnetic fields, is suppressed by higher fields. The corresponding microscopic theory, which we have developed, quantitatively explains all experimental observations, and suggests that magnetic moments have formed on the wire surfaces
- β¦