1,081 research outputs found
Redistribution of phase fluctuations in a periodically driven cuprate superconductor
We study the thermally fluctuating state of a bi-layer cuprate superconductor
under the periodic action of a staggered field oscillating at optical
frequencies. This analysis distills essential elements of the recently
discovered phenomenon of light enhanced coherence in YBaCuO,
which was achieved by periodically driving infrared active apical oxygen
distortions. The effect of a staggered periodic perturbation is studied using a
Langevin and Fokker-Planck description of driven, coupled Josephson junctions,
which represent two neighboring pairs of layers and their two plasmons. In a
toy model including only two junctions, we demonstrate that the external
driving leads to a suppression of phase fluctuations of the low-energy plasmon,
an effect which is amplified via the resonance of the high energy plasmon. When
extending the modeling to the full layers, we find that this reduction becomes
far more pronounced, with a striking suppression of the low-energy
fluctuations, as visible in the power spectrum. We also find that this effect
acts onto the in-plane fluctuations, which are reduced on long length scales.
All these findings provide a physical framework to describe light control in
cuprates
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
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&
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
Dynamical decoherence of the light induced interlayer coupling in YBaCuO
Optical excitation of apical oxygen vibrations in
YBaCuO has been shown to enhance its c-axis
superconducting-phase rigidity, as evidenced by a transient blue shift of the
equilibrium inter-bilayer Josephson plasma resonance. Surprisingly, a transient
c-axis plasma mode could also be induced above T by the same apical
oxygen excitation, suggesting light activated superfluid tunneling throughout
the pseudogap phase of YBaCuO. However, despite the
similarities between the above T transient plasma mode and the
equilibrium Josephson plasmon, alternative explanations involving high mobility
quasiparticle transport should be considered. Here, we report an extensive
study of the relaxation of the light-induced plasmon into the equilibrium
incoherent phase. These new experiments allow for a critical assessment of the
nature of this mode. We determine that the transient plasma relaxes through a
collapse of its coherence length rather than its carrier (or superfluid)
density. These observations are not easily reconciled with quasiparticle
interlayer transport, and rather support transient superfluid tunneling as the
origin of the light-induced interlayer coupling in
YBaCuO.Comment: 27 pages (17 pages main text, 10 pages supplementary), 5 figures
(main text
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
Optical melting of the transverse Josephson plasmon: a comparison between bilayer and trilayer cuprates
We report on an investigation of the redistribution of interlayer coherence in the trilayer cuprate Bi2Sr2Ca2Cu3O10. The experiment is performed under the same apical-oxygen phonon excitation discussed in the past for the bilayer cuprate YBa2Cu3O6.5. In Bi2Sr2Ca2Cu3O10, we observe a similar spectral weight loss at the transverse plasma mode resonance as that seen in YBa2Cu3O6.5. However, this feature is not accompanied by the light-enhanced interlayer coherence that was found in YBa2Cu3O6+x, for which the transverse plasma mode is observed at equilibrium even in the normal state. These new observations offer an experimental perspective in the context of the physics of light-enhanced interlayer coupling in various cuprates
Dynamics of photo-induced ferromagnetism in oxides with orbital degeneracy
By using intense coherent electromagnetic radiation, it may be possible to
manipulate the properties of quantum materials very quickly, or even induce new
and potentially useful phases that are absent in equilibrium. For instance,
ultrafast control of magnetic dynamics is crucial for a number of proposed
spintronic devices and can also shed light on the possible dynamics of
correlated phases out of equilibrium. Inspired by recent experiments on
spin-orbital ferromagnet YTiO we consider the nonequilibrium dynamics of
Heisenberg ferromagnetic insulator with low-lying orbital excitations. We model
the dynamics of the magnon excitations in this system following an optical
pulse which resonantly excites infrared-active phonon modes. As the phonons
ring down they can dynamically couple the orbitals with the low-lying magnons,
leading to a dramatically modified effective bath for the magnons. We show this
transient coupling can lead to a dynamical acceleration of the magnetization
dynamics, which is otherwise bottlenecked by small anisotropy. Exploring the
parameter space more we find that the magnon dynamics can also even completely
reverse, leading to a negative relaxation rate when the pump is blue-detuned
with respect to the orbital bath resonance. We therefore show that by using
specially targeted optical pulses, one can exert a much greater degree of
control over the magnetization dynamics, allowing one to optically steer
magnetic order in this system. We conclude by discussing interesting parallels
between the magnetization dynamics we find here and recent experiments on
photo-induced superconductivity, where it is similarly observed that depending
on the initial pump frequency, an apparent metastable superconducting phase
emerges.Comment: 16 pages, 11 figures + 5 pages, no figure
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