117 research outputs found
Law without law or "just" limit theorems?
About 35 years ago Wheeler introduced the motto `law without law' to
highlight the possibility that (at least a part of) Physics may be understood
only following {\em regularity principles} and few relevant facts, rather than
relying on a treatment in terms of fundamental theories. Such a proposal can be
seen as part of a more general attempt (including the maximum entropy approach)
summarized by the slogan `it from bit', which privileges the information as the
basic ingredient. Apparently it seems that it is possible to obtain, without
the use of physical laws, some important results in an easy way, for instance,
the probability distribution of the canonical ensemble. In this paper we will
present a general discussion on those ideas of Wheeler's that originated the
motto `law without law'. In particular we will show how the claimed simplicity
is only apparent and it is rather easy to produce wrong results. We will show
that it is possible to obtain some of the results treated by Wheeler in the
realm of the statistical mechanics, using precise assumptions and nontrivial
results of probability theory, mainly concerning ergodicity and limit theorems.Comment: 9 pages, 3 figure
Theory of charge-spin conversion at oxide interfaces: The inverse spin-galvanic effect
We evaluate the non-equilibrium spin polarization induced by an applied
electric field for a tight-binding model of electron states at oxides
interfaces in LAO/STO heterostructures. By a combination of analytic and
numerical approaches we investigate how the spin texture of the electron
eigenstates due to the interplay of spin-orbit coupling and inversion asymmetry
determines the sign of the induced spin polarization as a function of the
chemical potential or band filling, both in the absence and presence of local
disorder. With the latter, we find that the induced spin polarization evolves
from a non monotonous behavior at zero temperature to a monotonous one at
higher temperature. Our results may provide a sound framework for the
interpretation of recent experiments.Comment: Submitted to Proceedings of SPIE Nanoscience + Engineering 2018,
Spintronics XI, 23 pages, 9 figure
The Ancient Romans’ Route to Charge Density Waves in Cuprates
An account is given of the main steps that led the research group in Rome, to which the author belongs, to the formulation of the charge-density-wave scenario for high-T-c superconducting cuprates. The early finding of the generic tendency of strongly correlated electron systems with short range interactions to undergo electron phase separation was subsequently contrasted with the homogenizing effect of the long-range Coulomb interaction. The two effects can find a compromise in the formation of incommensurate charge density waves. These charge density waves are inherently dynamical and are overdamped as a consequence of the possibility to decay in electron-hole pairs, yet tend to maintain a (quantum) critical character, which is mirrored in their marked momentum and frequency dependence and in their strong variation with temperature and doping. These dynamical incommensurate charge density waves act as mediators of pairing lading to high-T-c superconductivity, and provide the scattering mechanism that produces the observed violation of the Fermi-liquid paradigm in the metallic phase
The Mott Metal-Insulator transition in the half-filled Hubbard model on the Triangular Lattice
We investigate the metal-insulator transition in the half-filled Hubbard
model on a two-dimensional triangular lattice using both the
Kotliar-Ruckenstein slave-boson technique, and exact numerical diagonalization
of finite clusters. Contrary to the case of the square lattice, where the
perfect nesting of the Fermi surface leads to a metal-insulator transition at
arbitrarily small values of U, always accompanied by antiferromagnetic
ordering, on the triangular lattice, due to the lack of perfect nesting, the
transition takes place at a finite value of U, and frustration induces a
non-trivial competition among different magnetic phases. Indeed, within the
mean-field approximation in the slave-boson approach, as the interaction grows
the paramagnetic metal turns into a metallic phase with incommensurate spiral
ordering. Increasing further the interaction, a linear spin-density-wave is
stabilized, and finally for strong coupling the latter phase undergoes a
first-order transition towards an antiferromagnetic insulator. No trace of the
intermediate phases is instead seen in the exact diagonalization results,
indicating a transition between a paramagnetic metal and an antiferromagnetic
insulator.Comment: 5 pages, 4 figure
Intrinsic spin Hall effect in systems with striped spin-orbit coupling
The Rashba spin-orbit coupling arising from structure inversion asymmetry
couples spin and momentum degrees of freedom providing a suitable (and very
intensively investigated) environment for spintronic effects and devices. Here
we show that in the presence of strong disorder, non-homogeneity in the
spin-orbit coupling gives rise to a finite spin Hall conductivity in contrast
with the corresponding case of a homogeneous linear spin-orbit coupling. In
particular, we examine the inhomogeneity arising from a striped structure for a
two-dimensional electron gas, affecting both density and Rashba spin-orbit
coupling. We suggest that this situation can be realized at oxide interfaces
with periodic top gating.Comment: 9 pages, 8 figure
Metal-superconductor transition in low-dimensional superconducting clusters embedded in two-dimensional electron systems
Motivated by recent experimental data on thin film superconductors and oxide interfaces, we propose a random-resistor network apt to describe the occurrence of a metal-superconductor transition in a two-dimensional electron system with disorder on the mesoscopic scale. We consider low-dimensional (e.g. filamentary) structures of a superconducting cluster embedded in the two-dimensional network and we explore the separate effects and the interplay of the superconducting structure and of the statistical distribution of local critical temperatures. The thermal evolution of the resistivity is determined by a numerical calculation of the random-resistor network and, for comparison, a mean-field approach called effective medium theory (EMT). Our calculations reveal the relevance of the distribution of critical temperatures for clusters with low connectivity. In addition, we show that the presence of spatial correlations requires a modification of standard EMT to give qualitative agreement with the numerical results. Applying the present approach to an LaTiO3/SrTiO3 oxide interface, we find that the measured resistivity curves are compatible with a network of spatially dense but loosely connected superconducting islands
Superfluid properties of superconductors with disorder at the nanoscale: a random impedance model
Some two-dimensional superconductors like, e.g., LaAlO 3 /SrTiO 3 heterostructures or thin films of transition metal dichalcogenides, display peculiar properties that can be understood in terms of electron inhomogeneity at the nanoscale. In this framework, unusual features of the metal-superconductor transition have been interpreted as due to percolative effects within a network of superconducting regions embedded in a metallic matrix. In this work we use a mean-field-like effective medium approach to investigate the superconducting phase below the critical temperature Tc at which the resistivity vanishes. Specifically, we consider the finite frequency impedance of the system to extract the dissipative part of the conductance and the superfluid stiffness in the superconducting state. Intriguing effects arise from the metallic character of the embedding matrix: upon decreasing the temperature below Tc proximity effects may rapidly increase the superfluid stiffness. Then, a rather fragile superconducting state, living on a filamentary network just below Tc , can be substantially consolidated by additional superconducting regions induced by proximity effect in the interstitial metallic regions. This mean-field prediction should call for further theoretical analyses and trigger experimental investigations of the superconducting properties of the above systems
Thermodynamic phase diagram of the competition between superconductivity and charge order in cuprates
We argue that there is a special doping point in the phase diagram of
cuprates, such that the condensation of holes into a charge-ordered and into a
superconducting phase are degenerate in energy but with an energy barrier in
between. We present a Monte Carlo simulation of this problem without and with
quenched disorder. While in the clean case charge order and superconductivity
are separated by a first-order line which is nearly independent of temperature,
in the presence of quenched disorder, charge order is fragmented into domains
separated by superconducting filaments reminiscent of the supersolid behavior
in He. The resulting phase diagram is in good agreement with the
experiments.Comment: 30 pages, 17 figure
Dissipative quantum criticality as a source of strange metal behavior
The strange metal behavior, usually characterized by a linear-in-temperature (T) resistivity, is a still unsolved mystery in solid-state physics. It is often associated with the proximity to a quantum critical point (a second order transition at temperature T=0, leading to a broken symmetry phase) focusing on the related divergent order parameter correlation length. Here, we propose a paradigmatic shift, focusing on a divergent characteristic time scale due to a divergent dissipation acting on the fluctuating critical modes while their correlation length stays finite. To achieve a divergent dissipation, we propose a mechanism based on the coupling between a local order parameter fluctuation and electron density diffusive modes that accounts both for the linear-in-T resistivity and for the logarithmic specific heat versus temperature ratio C_V/T∼log(1/T), down to low temperatures
- …