30 research outputs found
Restrictions on the lifetime of sterile neutrinos from primordial nucleosynthesis
We analyze the influence of decaying sterile neutrinos with the masses in the
range 1-140 MeV on the primordial Helium-4 abundance, explicitly solving the
Boltzmann equations for all particle species, taking into account neutrino
flavour oscillations, and paying special attention to systematic uncertainties.
We show that the Helium abundance depends only on the sterile neutrino lifetime
and not on the way the active-sterile mixing is distributed between flavours,
and derive an upper bound on the lifetime. We also demonstrate that the recent
results of Izotov & Thuan [arXiv:1001.4440], who find 2sigma higher than
predicted by the standard primordial nucleosynthesis value of Helium-4
abundance, are consistent with the presence in the plasma of sterile neutrinos
with the lifetime 0.01-2 seconds. The decay of these particles perturbs the
spectra of (decoupled) neutrinos and heats photons, changing the ratio of
neutrino to photon energy density, that can be interpreted as extra neutrino
species at the recombination epoch.Comment: 17 pp. + Appendices. Analysis of deuterium bounds and more accurate
account of CMB bounds on Helium-4 is added. Final version to appear in JCA
Two-Dimensional Phase-Fluctuating Superconductivity in Bulk-Crystalline NdOFBiS
We present a combined growth and transport study of superconducting
single-crystalline NdOFBiS. Evidence of two-dimensional
superconductivity with significant phase fluctuations of preformed Cooper pairs
preceding the superconducting transition is reported. This result is based on
three key observations. (1) The resistive superconducting transition
temperature (defined by resistivity ) increases with
increasing disorder. (2) As , the conductivity diverges
significantly faster than what is expected from Gaussian fluctuations in two
and three dimensions. (3) Non-Ohmic resistance behavior is observed in the
superconducting state. Altogether, our observations are consistent with a
temperature regime of phase-fluctuating superconductivity. The crystal
structure with magnetic ordering tendencies in the NdOF layers
and (super)conductivity in the BiS layers is likely responsible for the
two-dimensional phase fluctuations. As such, NdOFBiS falls
into the class of unconventional ``laminar" bulk superconductors that include
cuprate materials and 4Hb-TaS
Damped spin excitations in a doped cuprate superconductor with orbital hybridization
A resonant inelastic x-ray scattering study of overdamped spin excitations in slightly underdoped La2−x Srx CuO4 (LSCO) with x = 0.12 and 0.145 is presented. Three high-symmetry directions have been investigated: (1) the antinodal (0,0) → ( 1 ,0), (2) the nodal (0,0) → ( 1 , 1 ), and (3) the zone-boundary direction
2 4 4 ( 1 1 1 2 ,0) → ( 4 ,4 ) connecting these two. The overdamped excitations exhibit strong dispersions along (1) and (3), whereas a much more modest dispersion is found along (2). This is in strong contrast to the undoped compound
La2CuO4 (LCO) for which the strongest dispersions are found along (1) and (2). The t − t i − t ii − U Hubbard model used to explain the excitation spectrum of LCO predicts—for constant U/t —that the dispersion along (3) scales with (t i/t )2. However, the diagonal hopping t i extracted on LSCO using single-band models is low (t i/t ∼ −0.16) and decreasing with doping. We therefore invoked a two-orbital (dx2 −y2 and dz2 ) model which implies that t i is enhanced. This effect acts to enhance the zone-boundary dispersion within the Hubbard model. We thus conclude that hybridization of dx2 −y2 and dz2 states has a significant impact on the zone-boundary dispersion in LSCO
In situ uniaxial pressure cell for x-ray and neutron scattering experiments
We present an in situ uniaxial pressure device optimized for small angle x-ray and neutron scattering experiments at low-temperatures and high magnetic fields. A stepper motor generates force, which is transmitted to the sample via a rod with an integrated transducer that continuously monitors the force. The device has been designed to generate forces up to 200 N in both compressive and tensile configurations, and a feedback control allows operating the system in a continuous-pressure mode as the temperature is changed. The uniaxial pressure device can be used for various instruments and multiple cryostats through simple and exchangeable adapters. It is compatible with multiple sample holders, which can be easily changed depending on the sample properties and the desired experiment and allow rapid sample changes
Designing the stripe-ordered cuprate phase diagram through uniaxial-stress
The ability to efficiently control charge and spin in the cuprate high-temperature superconductors is crucial for fundamental research and underpins technological development. Here, we explore the tunability of magnetism, superconductivity, and crystal structure in the stripe phase of the cuprate La[Formula: see text]Ba[Formula: see text]CuO[Formula: see text], with [Formula: see text] = 0.115 and 0.135, by employing temperature-dependent (down to 400 mK) muon-spin rotation and AC susceptibility, as well as X-ray scattering experiments under compressive uniaxial stress in the CuO[Formula: see text] plane. A sixfold increase of the three-dimensional (3D) superconducting critical temperature [Formula: see text] and a full recovery of the 3D phase coherence is observed in both samples with the application of extremely low uniaxial stress of [Formula: see text]0.1 GPa. This finding demonstrates the removal of the well-known 1/8-anomaly of cuprates by uniaxial stress. On the other hand, the spin-stripe order temperature as well as the magnetic fraction at 400 mK show only a modest decrease under stress. Moreover, the onset temperatures of 3D superconductivity and spin-stripe order are very similar in the large stress regime. However, strain produces an inhomogeneous suppression of the spin-stripe order at elevated temperatures. Namely, a substantial decrease of the magnetic volume fraction and a full suppression of the low-temperature tetragonal structure is found under stress, which is a necessary condition for the development of the 3D superconducting phase with optimal [Formula: see text]. Our results evidence a remarkable cooperation between the long-range static spin-stripe order and the underlying crystalline order with the three-dimensional fully coherent superconductivity. Overall, these results suggest that the stripe- and the SC order may have a common physical mechanism
Designing the stripe-ordered cuprate phase diagram through uniaxial-stress
The ability to efficiently control charge and spin in the cuprate
high-temperature superconductors is crucial for fundamental research and
underpins technological development. Here, we explore the tunability of
magnetism, superconductivity and crystal structure in the stripe phase of the
cuprate La_2-xBa_xCuO_4, with x = 0.115 and 0.135, by employing
temperature-dependent (down to 400 mK) muon-spin rotation and AC
susceptibility, as well as X-ray scattering experiments under compressive
uniaxial stress in the CuO_2 plane. A sixfold increase of the 3-dimensional
(3D) superconducting critical temperature T_c and a full recovery of the 3D
phase coherence is observed in both samples with the application of extremely
low uniaxial stress of 0.1 GPa. This finding demonstrates the removal of the
well-known 1/8-anomaly of cuprates by uniaxial stress. On the other hand, the
spin-stripe order temperature as well as the magnetic fraction at 400 mK show
only a modest decrease under stress. Moreover, the onset temperatures of 3D
superconductivity and spin-stripe order are very similar in the large stress
regime. However, a substantial decrease of the magnetic volume fraction and a
full suppression of the low-temperature tetragonal structure is found at
elevated temperatures, which is a necessary condition for the development of
the 3D superconducting phase with optimal T_c. Our results evidence a
remarkable cooperation between the long-range static spin-stripe order and the
underlying crystalline order with the three-dimensional fully coherent
superconductivity. Overall, these results suggest that the stripe- and the SC
order may have a common physical mechanism.Comment: 11 pages, 5 figures. This work builds on our earlier findings on
LBCO, arXiv:2008.01159, and substantially expands i
Quantum Fluctuations in a Weakly Correlated Mott Insulator
Quantum fluctuations in low-dimensional systems and near quantum phase
transitions have significant influences on material properties. Yet, it is
difficult to experimentally gauge the strength and importance of quantum
fluctuations. Here we provide a resonant inelastic x-ray scattering study of
magnon excitations in Mott insulating cuprates. From the thin film of
SrCuO, single- and bi-magnon dispersions are derived. Using an effective
Heisenberg Hamiltonian generated from the Hubbard model, we show that the
single magnon dispersion is only described satisfactorily when including
significant renormalization stemming from quantum fluctuations. Comparative
results on LaCuO indicate that quantum fluctuations are much stronger
in SrCuO suggesting closer proximity to a magnetic quantum critical point.
Monte Carlo calculations suggest an exotic incommensurate magnetic order as the
ground state that competes with the antiferromagnetic N\'eel order. Our results
indicate that SrCuO -- due to strong quantum fluctuations -- is a unique
starting point for the exploration of novel magnetic ground states.Comment: Supplementary Information available upon reques
Charge order above room-temperature in a prototypical kagome superconductor La(RuFe)Si
The kagome lattice is an intriguing and rich platform for discovering, tuning
and understanding the diverse phases of quantum matter, which is a necessary
premise for utilizing quantum materials in all areas of modern and future
electronics in a controlled and optimal way. The system LaRuSi was
shown to exhibit typical kagome band structure features near the Fermi energy
formed by the Ru- orbitals and the highest superconducting transition
temperature 7K among the kagome-lattice materials.
However, the effect of electronic correlations on the normal state properties
remains elusive. Here, we report the discovery of charge order in
La(RuFe)Si ( = 0, 0.01, 0.05) beyond
room-temperature. Namely, single crystal X-ray diffraction reveals charge order
with a propagation vector of (,0,0) below
400K in all three compounds. At lower temperatures, we see the
appearance of a second set of charge order peaks with a propagation vector of
(,0,0). The introduction of Fe, which is known to quickly suppress
superconductivity, does not drastically alter the onset temperature for charge
order. Instead, it broadens the scattered intensity such that diffuse
scattering appears at the same onset temperature, however does not coalesce
into sharp Bragg diffraction peaks until much lower in temperature. Our results
present the first example of a charge ordered state at or above room
temperature in the correlated kagome lattice with bulk superconductivity.Comment: 15 pages, 8 figure
Analysis of reflex modulation with a biologically realistic neural network
In this study, a neuromusculoskeletal model was built to give insight into the mechanisms behind the modulation of reflexive feedback strength as experimentally identified in the human shoulder joint. The model is an integration of a biologically realistic neural network consisting of motoneurons and interneurons, modeling 12 populations of spinal neurons, and a one degree-of-freedom musculoskeletal model, including proprioceptors. The model could mimic the findings of human postural experiments, using presynaptic inhibition of the Ia afferents to modulate the feedback gains. In a pathological case, disabling one specific neural connection between the inhibitory interneurons and the motoneurons could mimic the experimental findings in complex regional pain syndrome patients. It is concluded that the model is a valuable tool to gain insight into the spinal contributions to human motor control. Applications lay in the fields of human motor control and neurological disorders, where hypotheses on motor dysfunction can be tested, like spasticity, clonus, and tremor