1,242 research outputs found
Back and forth from cool core to non-cool core: clues from radio-halos
X-ray astronomers often divide galaxy clusters into two classes: "cool core"
(CC) and "non-cool core" (NCC) objects. The origin of this dichotomy has been
the subject of debate in recent years, between "evolutionary" models (where
clusters can evolve from CC to NCC, mainly through mergers) and "primordial"
models (where the state of the cluster is fixed "ab initio" by early mergers or
pre-heating). We found that in a well-defined sample (clusters in the GMRT
Radio halo survey with available Chandra or XMM-Newton data), none of the
objects hosting a giant radio halo can be classified as a cool core. This
result suggests that the main mechanisms which can start a large scale
synchrotron emission (most likely mergers) are the same that can destroy CC and
therefore strongly supports "evolutionary" models of the CC-NCC dichotomy.
Moreover combining the number of objects in the CC and NCC state with the
number of objects with and without a radio-halo, we estimated that the time
scale over which a NCC cluster relaxes to the CC state, should be larger than
the typical life-time of radio-halos and likely shorter than about 3 Gyr. This
suggests that NCC transform into CC more rapidly than predicted from the
cooling time, which is about 10 Gyr in NCC systems, allowing the possibility of
a cyclical evolution between the CC and NCC states.Comment: Accepted for publication in A&
Pump frequency resonances for light-induced incipient superconductivity in YBaCuO
Optical excitation in the cuprates has been shown to induce transient
superconducting correlations above the thermodynamic transition temperature,
, as evidenced by the terahertz frequency optical properties in the
non-equilibrium state. In YBaCuO this phenomenon has so far
been associated with the nonlinear excitation of certain lattice modes and the
creation of new crystal structures. In other compounds, like
LaBaCuO, similar effects were reported also for excitation at
near infrared frequencies, and were interpreted as a signature of the melting
of competing orders. However, to date it has not been possible to
systematically tune the pump frequency widely in any one compound, to
comprehensively compare the frequency dependent photo-susceptibility for this
phenomenon. Here, we make use of a newly developed optical parametric
amplifier, which generates widely tunable high intensity femtosecond pulses, to
excite YBaCuO throughout the entire optical spectrum (3 - 750
THz). In the far-infrared region (3 - 25 THz), signatures of non-equilibrium
superconductivity are induced only for excitation of the 16.4 THz and 19.2 THz
vibrational modes that drive -axis apical oxygen atomic positions. For
higher driving frequencies (25 - 750 THz), a second resonance is observed
around the charge transfer band edge at ~350 THz. These observations highlight
the importance of coupling to the electronic structure of the CuO planes,
either mediated by a phonon or by charge transfer.Comment: 47 pages, 21 figures, 2 table
Coherent Modulation of the YBa2Cu3O6+x Atomic Structure by Displacive Stimulated Ionic Raman Scattering
We discuss the mechanism of coherent phonon generation by Stimulated Ionic
Raman Scattering, a process different from conventional excitation with near
visible optical pulses. Ionic Raman scattering is driven by anharmonic coupling
between a directly excited infrared-active phonon mode and other Raman modes.
We experimentally study the response of YBa2Cu3O6+x to the resonant excitation
of apical oxygen motions at 20 THz by mid-infrared pulses, which has been shown
in the past to enhance the interlayer superconducting coupling. We find
coherent oscillations of four totally symmetric (Ag) Raman modes and make a
critical assessment of the role of these oscillatory motions in the enhancement
of superconductivity.Comment: 12 pages, 4 figure
A river model of space
Within the theory of general relativity gravitational phenomena are usually
attributed to the curvature of four-dimensional spacetime. In this context we
are often confronted with the question of how the concept of ordinary physical
three-dimensional space fits into this picture. In this work we present a
simple and intuitive model of space for both the Schwarzschild spacetime and
the de Sitter spacetime in which physical space is defined as a specified set
of freely moving reference particles. Using a combination of orthonormal basis
fields and the usual formalism in a coordinate basis we calculate the physical
velocity field of these reference particles. Thus we obtain a vivid description
of space in which space behaves like a river flowing radially toward the
singularity in the Schwarzschild spacetime and radially toward infinity in the
de Sitter spacetime. We also consider the effect of the river of space upon
light rays and material particles and show that the river model of space
provides an intuitive explanation for the behavior of light and particles at
and beyond the event horizons associated with these spacetimes.Comment: 22 pages, 5 figure
Achieving geodetic motion for LISA test masses: ground testing result
The low-frequency resolution of space-based gravitational wave observatories
such as LISA (Laser Interferometry Space Antenna) hinges on the orbital purity
of a free-falling reference test mass inside a satellite shield. We present
here a torsion pendulum study of the forces that will disturb an orbiting test
mass inside a LISA capacitive position sensor. The pendulum, with a measured
torque noise floor below 10 fNm/sqrt{Hz} from 0.6 to 10 mHz, has allowed
placement of an upper limit on sensor force noise contributions, measurement of
the sensor electrostatic stiffness at the 5% level, and detection and
compensation of stray DC electrostatic biases at the mV level.Comment: 4 pages (revtex4) with 4 figure
The interaction between stray electrostatic fields and a charged free-falling test mass
We present an experimental analysis of force noise caused by stray
electrostatic fields acting on a charged test mass inside a conducting
enclosure, a key problem for precise gravitational experiments. Measurement of
the average field that couples to test mass charge, and its fluctuations, is
performed with two independent torsion pendulum techniques, including direct
measurement of the forces caused by a change in electrostatic charge. We
analyze the problem with an improved electrostatic model that, coupled with the
experimental data, also indicates how to correctly measure and null the stray
field that interacts with test mass charge. Our measurements allow a
conservative upper limit on acceleration noise, of 2 fm/s\rthz\ for
frequencies above 0.1 mHz, for the interaction between stray fields and charge
in the LISA gravitational wave mission.Comment: Minor edits in PRL publication proces
Microscopic theory for the light-induced anomalous Hall effect in graphene
We employ a quantum Liouville equation with relaxation to model the recently
observed anomalous Hall effect in graphene irradiated by an ultrafast pulse of
circularly polarized light. In the weak-field regime, we demonstrate that the
Hall effect originates from an asymmetric population of photocarriers in the
Dirac bands. By contrast, in the strong-field regime, the system is driven into
a non-equilibrium steady state that is well-described by topologically
non-trivial Floquet-Bloch bands. Here, the anomalous Hall current originates
from the combination of a population imbalance in these dressed bands together
with a smaller anomalous velocity contribution arising from their Berry
curvature. This robust and general finding enables the simulation of electrical
transport from light-induced Floquet-Bloch bands in an experimentally relevant
parameter regime and creates a pathway to designing ultrafast quantum devices
with Floquet-engineered transport properties
Nonlinear lattice dynamics as a basis for enhanced superconductivity in YBa2Cu3O6.5
THz-frequency optical pulses can resonantly drive selected vibrational modes
in solids and deform their crystal structure. In complex oxides, this method
has been used to melt electronic orders, drive insulator to metal transitions
or induce superconductivity. Strikingly, coherent interlayer transport strongly
reminiscent of superconductivity can be transiently induced up to room
temperature in YBa2Cu3O6+x. By combining femtosecond X-ray diffraction and ab
initio density functional theory calculations, we determine here the crystal
structure of this exotic non-equilibrium state. We find that nonlinear lattice
excitation in normal-state YBa2Cu3O6+x at 100 K causes a staggered
dilation/contraction of the Cu-O2 intra/inter- bilayer distances, accompanied
by anisotropic changes in the in-plane O-Cu-O bond buckling. Density functional
theory calculations indicate that these motions cause dramatic changes in the
electronic structure. Amongst these, the enhancement in the dx2-y2 character of
the in-plane electronic structure is likely to favor superconductivity.Comment: 28 pages, including Supplemen
Kinematics and hydrodynamics of spinning particles
In the first part (Sections 1 and 2) of this paper --starting from the Pauli
current, in the ordinary tensorial language-- we obtain the decomposition of
the non-relativistic field velocity into two orthogonal parts: (i) the
"classical part, that is, the 3-velocity w = p/m OF the center-of-mass (CM),
and (ii) the so-called "quantum" part, that is, the 3-velocity V of the motion
IN the CM frame (namely, the internal "spin motion" or zitterbewegung). By
inserting such a complete, composite expression of the velocity into the
kinetic energy term of the non-relativistic classical (i.e., newtonian)
lagrangian, we straightforwardly get the appearance of the so-called "quantum
potential" associated, as it is known, with the Madelung fluid. This result
carries further evidence that the quantum behaviour of micro-systems can be
adirect consequence of the fundamental existence of spin. In the second part
(Sections 3 and 4), we fix our attention on the total 3-velocity v = w + V, it
being now necessary to pass to relativistic (classical) physics; and we show
that the proper time entering the definition of the four-velocity v^mu for
spinning particles has to be the proper time tau of the CM frame. Inserting the
correct Lorentz factor into the definition of v^mu leads to completely new
kinematical properties for v_mu v^mu. The important constraint p_mu v^mu = m,
identically true for scalar particles, but just assumed a priori in all
previous spinning particle theories, is herein derived in a self-consistent
way.Comment: LaTeX file; needs kapproc.st
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