431 research outputs found
New Magnetic-Field-Induced Macroscopic Quantum Phenomenon in High- Cuprates: Confined Field-Induced Density Waves in the Superconducting State
We reveal a novel macroscopic quantum phenomenon induced by a magnetic field.
It corresponds to the {\it non-integer quantization of the superfluid density}
in a superconductor with gap nodes due to the generation of confined
field-induced density waves (CFIDW) over a portion of the Fermi surface (FS).
The Landau numbers are not sufficient to index these macroscopic quantum
states and new quanrum numbers must be added. Distinct qualitative
implications of this quantization are evident in a number of
puzzling experiments in high- cuprates including the plateaus behavior in
the field profile of thermal conductivity, field induced magnetic moments,
charge textures around the vortices, and field induced vortex-solid to cascade
vortex glass transitions.Comment: M2S Rio 200
Magnetic field induced singlet - triplet phase transition in quasi one-dimensional organic superconductors
We propose a theoretical model of quasi-one-dimensional superconductors, with
attractive electron-electron interactions dominant in the singlet d-wave
channel and sub-dominant in the p-wave channel. We discuss, in the mean field
approximation, the effect of a magnetic field applied perpendicularly to the
direction of the lowest conductivity. The lowest free energy phase corresponds
to a singlet d-wave symmetry in low fields, but to a triplet symmetry in high
fields. A first order singlet-triplet phase transition is expected at moderate
applied fields of a few teslas. We propose to ascribe the recent critical field
and NMR experimental data, observed in superconducting (TMTSF)2ClO4 to such an
effect.Comment: 6 pages, 2 figures, accepted in EP
Coexistence of Superconductivity and Spin Density Wave orderings in the organic superconductor (TMTSF)_2PF_6
The phase diagram of the organic superconductor (TMTSF)_2PF_6 has been
revisited using transport measurements with an improved control of the applied
pressure. We have found a 0.8 kbar wide pressure domain below the critical
point (9.43 kbar, 1.2 K) for the stabilisation of the superconducting ground
state featuring a coexistence regime between spin density wave (SDW) and
superconductivity (SC). The inhomogeneous character of the said pressure domain
is supported by the analysis of the resistivity between T_SDW and T_SC and the
superconducting critical current. The onset temperature T_SC is practically
constant (1.20+-0.01 K) in this region where only the SC/SDW domain proportion
below T_SC is increasing under pressure. An homogeneous superconducting state
is recovered above the critical pressure with T_SC falling at increasing
pressure. We propose a model comparing the free energy of a phase exhibiting a
segregation between SDW and SC domains and the free energy of homogeneous
phases which explains fairly well our experimental findings.Comment: 13 pages, 10 figures, revised v: fig.9 added, section 4.2 rewritten,
accepted v: sections 4&5 improve
The Bi-O-edge wavefront sensor: How Foucault-knife-edge variants can boost eXtreme Adaptive Optics
Direct detection of exoplanets around nearby stars requires advanced Adaptive
Optics (AO) systems. High order systems are needed to reach high Strehl Ratio
(SR) in near infrared and optical wavelengths on future Giant Segmented Mirror
Telescopes (GSMTs). Direct detection of faint exoplanets with the ESO ELT will
require some tens of thousand of correction modes. Resolution and sensitivity
of the wavefront sensor (WFS) are key requirements for this science case. We
present a new class of WFSs, the Bi-Orthogonal Foucault-knife-edge Sensors (or
Bi-O-edge), that is directly inspired by the Foucault knife edge test (Foucault
1859). The idea consists of using a beam-splitter producing two foci, each of
which is sensed by an edge with orthogonal direction to the other. We describe
two implementation concepts: The Bi-O-edge sensor can be realised with a sharp
edge and a tip-tilt modulation device (sharp Bi-O-edge) or with a smooth
gradual transmission over a grey edge (grey Bi-O-edge). A comparison between
the Bi-O-edge concepts and the 4-sided classical Pyramid Wavefront Sensor (PWS)
gives some important insights into the nature of the measurements.Our analysis
shows that the sensitivity gain of the Bi-O edge with respect to the PWS
depends on the system configuration. The gain is a function of the number of
control modes and the modulation angle. We found that for the sharp Bi-O-edge,
the gain in reduction of propagated photon noise variance approaches a
theoretical factor of 2 for a large number of control modes and small
modulation angle, meaning that the sharp Bi-O-edge only needs half of the
photons of the PWS to reach similar measurement accuracy.Comment: Accepted for publication in Astronomy and Astrophysic
Solving the Richardson equations for Fermions
Forty years ago Richardson showed that the eigenstates of the pairing
Hamiltonian with constant interaction strength can be calculated by solving a
set of non-linear coupled equations. However, in the case of Fermions these
equations lead to singularities which made them very hard to solve. This letter
explains how these singularities can be avoided through a change of variables
making the Fermionic pairing problem numerically solvable for arbitrary single
particle energies and degeneracies.Comment: 5 pages, 4 figures, submitted to Phys.Rev.
Quantum Limit in a Parallel Magnetic Field in Layered Conductors
We show that electron wave functions in a quasi-two-dimensional conductor in
a parallel magnetic field are always localized on conducting layers. Wave
functions and electron spectrum in a quantum limit, where the "sizes" of
quasi-classical electron orbits are of the order of nano-scale distances
between the layers, are determined. AC infrared measurements to investigate
Fermi surfaces and to test Fermi liquid theory in Q2D organic and high-Tc
materials in high magnetic fields, H = 10-45 T, are suggested.Comment: 9 pages, 2 figures; Submitted to Physical Review Letter
Unification Theory of Angular Magnetoresistance Oscillations in Quasi-One-Dimensional Conductors
We present a unification theory of angular magnetoresistance oscillations,
experimentally observed in quasi-one-dimensional organic conductors, by solving
the Boltzmann kinetic equation in the extended Brillouin zone. We find that, at
commensurate directions of a magnetic field, resistivity exhibits strong
minima. In two limiting cases, our general solution reduces to the results,
previously obtained for the Lebed Magic Angles and Lee-Naughton-Lebed
oscillations. We demonstrate that our theoretical results are in good
qualitative and quantitative agreement with the existing measurements of
resistivity in (TMTSF)ClO conductor.Comment: 6 pages, 2 figure
Soliton Wall Superlattice in Quasi-One-Dimensional Conductor (Per)2Pt(mnt)2
We suggest a model to explain the appearance of a high resistance high
magnetic field charge-density-wave (CDW) phase, discovered by D. Graf et al.
[Phys. Rev. Lett. v. 93, 076406 (2004)] in (Per)2Pt(mnt)2. In particular, we
show that the Pauli spin-splitting effects improve the nesting properties of a
realistic quasi-one-dimensional electron spectrum and, therefore, a high
resistance Peierls CDW phase is stabilized in high magnetic fields. In low and
very high magnetic fields, a periodic soliton wall superlattice (SWS) phase is
found to be a ground state. We suggest experimental studies of the predicted
phase transitions between the Peierls and SWS CDW phases in (Per)2Pt(mnt)2 to
discover a unique SWS phase.Comment: 10 pages, 3 figures. Submitted to Physical Review Letters (February
19, 2007
Laboratory Experiments of Model-based Reinforcement Learning for Adaptive Optics Control
Direct imaging of Earth-like exoplanets is one of the most prominent
scientific drivers of the next generation of ground-based telescopes.
Typically, Earth-like exoplanets are located at small angular separations from
their host stars, making their detection difficult. Consequently, the adaptive
optics (AO) system's control algorithm must be carefully designed to
distinguish the exoplanet from the residual light produced by the host star.
A new promising avenue of research to improve AO control builds on
data-driven control methods such as Reinforcement Learning (RL). RL is an
active branch of the machine learning research field, where control of a system
is learned through interaction with the environment. Thus, RL can be seen as an
automated approach to AO control, where its usage is entirely a turnkey
operation. In particular, model-based reinforcement learning (MBRL) has been
shown to cope with both temporal and misregistration errors. Similarly, it has
been demonstrated to adapt to non-linear wavefront sensing while being
efficient in training and execution.
In this work, we implement and adapt an RL method called Policy Optimization
for AO (PO4AO) to the GHOST test bench at ESO headquarters, where we
demonstrate a strong performance of the method in a laboratory environment. Our
implementation allows the training to be performed parallel to inference, which
is crucial for on-sky operation. In particular, we study the predictive and
self-calibrating aspects of the method. The new implementation on GHOST running
PyTorch introduces only around 700 microseconds in addition to hardware,
pipeline, and Python interface latency. We open-source well-documented code for
the implementation and specify the requirements for the RTC pipeline. We also
discuss the important hyperparameters of the method, the source of the latency,
and the possible paths for a lower latency implementation.Comment: Accepted for publication in JATI
- …