548 research outputs found
Determination of reaction rate constants and T-2 relaxation times using integrated NMR power spectra
Magnetic Field Dependent Tunneling in Glasses
We report on experiments giving evidence for quantum effects of
electromagnetic flux in barium alumosilicate glass. In contrast to expectation,
below 100 mK the dielectric response becomes sensitive to magnetic fields. The
experimental findings include both, the complete lifting of the dielectric
saturation by weak magnetic fields and oscillations of the dielectric response
in the low temperature resonant regime. As origin of these effects we suggest
that the magnetic induction field violates the time reversal invariance leading
to a flux periodicity in the energy levels of tunneling systems. At low
temperatures, this effect is strongly enhanced by the interaction between
tunneling systems and thus becomes measurable.Comment: 4 pages, 4 figure
Evidence for a Second Order Phase Transition in Glasses at Very Low Temperatures -- A Macroscopic Quantum State of Tunneling Systems
Dielectric measurements at very low temperature indicate that in a glass with
the eutectic composition BaO-AlO-SiO a phase transition occurs at
5.84 mK. Below that temperature small magnetic fields of the order of 10 T
cause noticeable changes of the dielectric constant although the glass is
insensitive to fields up to 20 T above 10 mK. The experimental findings may be
interpreted as the signature of the formation of a new phase in which many
tunneling systems perform a coherent motion resulting in a macroscopic wave
function.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Let
Poroelastic responses of confined aquifers to subsurface strain and their use for volcano monitoring
Well water level changes associated with magmatic unrest can be interpreted as a result of pore pressure changes in the aquifer due to crustal deformation, and so could provide constraints on the subsurface processes causing this strain. We use finite element analysis to demonstrate the response of aquifers to volumetric strain induced by pressurized magma reservoirs. Two different aquifers are invoked – an unconsolidated pyroclastic deposit and a vesicular lava flow – and embedded in an impermeable crust, overlying a magma chamber. The time-dependent, fully coupled models simulate crustal deformation accompanying chamber pressurization and the resulting hydraulic head changes as well as flow through the porous aquifer, i.e. porous flow. The simulated strain leads to centimetres (pyroclastic aquifer) to metres (lava flow aquifer) of hydraulic head changes; both strain and hydraulic head change with time due to substantial porous flow in the hydrological system.
Well level changes are particularly sensitive to chamber volume, shape and pressurization strength, followed by aquifer permeability and the phase of the pore fluid. The depths of chamber and aquifer, as well as the aquifer's Young's modulus also have significant influence on the hydraulic head signal. While source characteristics, the distance between chamber and aquifer and the elastic stratigraphy determine the strain field and its partitioning, flow and coupling parameters define how the aquifer responds to this strain and how signals change with time.
We find that generic analytical models can fail to capture the complex pre-eruptive subsurface mechanics leading to strain-induced well level changes, due to aquifer pressure changes being sensitive to chamber shape and lithological heterogeneities. In addition, the presence of a pore fluid and its flow have a significant influence on the strain signal in the aquifer and are commonly neglected in analytical models. These findings highlight the need for numerical models for the interpretation of observed well level signals. However, simulated water table changes do indeed mirror volumetric strain, and wells are therefore a valuable addition to monitoring systems that could provide important insights into pre-eruptive dynamics
Correlated Persistent Tunneling Currents in Glasses
Low temperature properties of glasses are derived within a generalized
tunneling model, considering the motion of charged particles on a closed path
in a double-well potential. The presence of a magnetic induction field B
violates the time reversal invariance due to the Aharonov-Bohm phase, and leads
to flux periodic energy levels. At low temperature, this effect is shown to be
strongly enhanced by dipole-dipole and elastic interactions between tunneling
systems and becomes measurable. Thus, the recently observed strong sensitivity
of the electric permittivity to weak magnetic fields can be explained. In
addition, superimposed oscillations as a function of the magnetic field are
predicted.Comment: 4 page
Magnetic field effect on the dielectric constant of glasses: Evidence of disorder within tunneling barriers
The magnetic field dependence of the low frequency dielectric constant
(H) of a structural glass a - SiO2 + xCyHz was studied from 400 mK to 50
mK and for H up to 3T. Measurement of both the real and the imaginary parts of
is used to eliminate the difficult question of keeping constant the
temperature of the sample while increasing H: a non-zero (H) dependence is
reported in the same range as that one very recently reported on multicomponent
glasses. In addition to the recently proposed explanation based on
interactions, the reported (H) is interpreted quantitatively as a
consequence of the disorder lying within the nanometric barriers of the
elementary tunneling systems of the glass.Comment: latex Bcorrige1.tex, 5 files, 4 figures, 7 pages [SPEC-S02/009
On the occurrence of phreatic eruptions at Ruapehu: statistics and probabilistic hazard forecast for ballistics
Low temperature acoustic properties of amorphous silica and the Tunneling Model
Internal friction and speed of sound of a-SiO(2) was measured above 6 mK
using a torsional oscillator at 90 kHz, controlling for thermal decoupling,
non-linear effects, and clamping losses. Strain amplitudes e(A) = 10^{-8} mark
the transition between the linear and non-linear regime. In the linear regime,
excellent agreement with the Tunneling Model was observed for both the internal
friction and speed of sound, with a cut-off energy of E(min) = 6.6 mK. In the
non-linear regime, two different behaviors were observed. Above 10 mK the
behavior was typical for non-linear harmonic oscillators, while below 10 mK a
different behavior was found. Its origin is not understood.Comment: 1 tex file, 6 figure
Electrothermal Icing Protection of Aerosurfaces Using Conductive Polymer Nanocomposites
Ice protection systems (IPS) are critical components for many aerospace flight vehicles, including commercial transports and unmanned aerial systems (UAS), and can include anti-icing, de-icing, ice sensing, etc. Here, an IPS is created using nanomaterials to create a surface-modified external layer on an aerosurface based on observations that polymer nanocomposites have tailorable and attractive heating properties. The IPS uses Joule heating of aligned carbon nanotube (CNT) arrays to create highly efficient de-icing and anti-icing of aerosurfaces. An ice wind tunnel test of a CNT enhanced aerosurface is performed to demonstrate the system under a range of operating regimes (temperature, wind speed, water content in air) including operation down to -20.6°C (-5°F) at 55.9 m/s (125 mph) under heavy icing. Manufacturing, design considerations, and further improvements to the materials and systems are discussed.United States. Dept. of the Navy. Small Business Innovation Research (Contract N68335-11-C-0424)National Science Foundation (U.S.). Materials Research Science and Engineering Centers (Program) (Grant DMR-0819762
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