Low-frequency Anomalies Research Articles

Thermal relaxation and low-frequency vibrational anomalies in simple models of glasses: A study using nonlinear Hamiltonians

Glasses exist because they are not able to relax in a laboratory time scale toward the most stable structure: a crystal. At the same time, glasses present low-frequency vibrational-mode (LFVM) anomalies. We explore in a systematic way how the number of such modes influences thermal relaxation in one-dimensional models of glasses. The model is a Fermi-Pasta-Ulam chain with nonlinear springs that join second neighbors at random, which mimics the adding of bond constraints in the rigidity theory of glasses. The corresponding number of LFVMs decreases linearly with the concentration of these springs, and thus their effect upon thermal relaxation can be studied in a systematic way. To do so, we performed numerical simulations using lattices that were thermalized and afterwards placed in contact with a zero-temperature bath. The results indicate that the time required for thermal relaxation has two contributions: one depends on the number of LFVMs and the other on the localization of modes due to disorder. By removing LFVMs, relaxation becomes less efficient since the cascade mechanism that transfers energy between modes is stopped. On the other hand, normal-mode localization also increases the time required for relaxation. We prove this last point by comparing periodic and nonperiodic chains that have the same number of LFVMs.

Droughts and Persistent Wet Spells over the United States and Mexico

Abstract Droughts and persistent wet spells over the United States and northwest Mexico have preferred regions of occurrence and persistence. Wet or dry conditions that persist more than 1 yr tend to occur over the interior United States west of 90°–95°W and northwest Mexico. In contrast, events over the eastern United States are less likely to occur and often last less than 6 months. The long persistent drought and wet spells are often modulated by low-frequency sea surface temperature anomalies (SSTAs). The persistent dry or wet conditions over northwest Mexico and the Southwest are associated with decadal variability of SSTAs over the North Pacific. Persistent events over the northwestern mountains are associated with two decadal SSTA modes. One mode has loadings over three southern oceans and another one is an El Niño–Southern Oscillation (ENSO) like decadal mode. Wet and dry conditions over the Pacific Northwest and the Great Plains are often associated with ENSO. The seasonal cycle of precipitation over the central-eastern United States, the East Coast, and the Ohio Valley is weak. Drought and wet spells over these regions are less persistent because the ENSO events have opposite impacts on precipitation for summer and winter.

Relations between Annular Modes and the Mean State: Southern Hemisphere Winter

Abstract In a zonally symmetric climatology with a single eddy-driven jet, such as prevails in the Southern Hemisphere summer, the midlatitude variability is dominated by fluctuations of the jet around its mean position, as described by the Southern Hemisphere annular mode (SAM). To study whether this result holds for a zonally asymmetric climatology, the observed variability of the Southern Hemisphere winter is analyzed. The mean state in this case is characterized by relatively weak stationary waves; yet there exist significant zonal variations in the mean strength and meridional structure of the subtropical jet stream. As in summer, the winter SAM signature is annular in shape and the corresponding wind anomalies are dipolar; but it is associated with two different behaviors of the eddy-driven jet in different longitudinal ranges. Over the Indian Ocean, the SAM is associated primarily with a latitudinal shift of the jet around its mean position. Over the Pacific sector, it is instead characterized by a seesaw in the wind speed between two distinct latitudes, corresponding to the positions of the midlatitude and subtropical jets. Composites of eddy forcing and baroclinicity over both sectors appear consistent with the two different behaviors. As in the zonal-mean case, high-frequency eddies both force and maintain the low-frequency wind anomalies associated with the SAM. The positive feedback by eddies is, however, not local: changes in the eddy forcing are influenced most strongly by zonal wind anomalies located upstream.

Seasonal Dependence of the MJO–ENSO Relationship

AbstractObservations of the development of recent El Niño events suggest a pivotal role for the Madden–Julian oscillation (MJO). Previous attempts to uncover a systematic relationship between MJO activity and the El Niño–Southern Oscillation (ENSO), however, have yielded conflicting results. In this study the MJO–ENSO relationship is stratified by season, and the focus is on MJO activity in the equatorial western Pacific. The results demonstrate that MJO activity in late boreal spring leads El Niño in the subsequent autumn–winter for the period 1979–2005. Spring is the season when MJO activity is least asymmetric with respect to the equator and displays the most sensitivity to SST variations at the eastern edge of the warm pool. Enhanced MJO activity in the western Pacific in spring is associated with an eastward-expanded warm pool and low-frequency westerly surface zonal wind anomalies. These sustained westerly anomalies in the western Pacific are hypothesized to project favorably onto a developing El Niño in spring.

Dielectric Relaxation in K0.5Na0.5NbO3-PVDF Composites

K1−x Na x NbO3 is considered as an alternative system for lead-based functional ceramics widely used in transducers, sensors and actuators. In some applications however, the composites of electroactive ceramic and polymer, with low acoustic impedance and dielectric permittivity that can be tailored to various requirement, are more useful. We studied dielectric relaxation processes in polyvinylidene fluoride (PVDF) loaded with K0.5Na0.5NbO3 powder in wide temperature and frequency range. The powder was prepared by solid state synthesis and characterized by HRTEM and NIR Raman spectroscopy. The dielectric response of (K0.5Na0.5NbO3)0.32(PVDF)0.68 composite, of (0–3) connectivity, was found to be determined by the relaxation processes of the polymer matrix with an additional broad low-frequency anomaly at ∼ 305 K that we relate to a response of water absorbed by the shell of KNN grains.

Energy distributions of the large-scale horizontal currents caused by wind in the baroclinic ocean

Ocean current data for nearly 3 months in the South China Sea (SCS), combined with the NCEP/NCAR reanalysis wind data, are analyzed. The results indicate that the wind energy enters the upper mixed layer in a wide continuous frequency band. In addition, the interaction between the low-frequency wind anomaly and the low-frequency current anomaly is the most 'effective' way for the energy input from the wind to the upper ocean. However, only the inertial and the near inertial energy propagate downwards through the upper mixed layer. The downward-propagating energy is distributed into the barotropic currents, the baroclinic currents and each mode of the baroclinic currents following the normal distributions. The energy change ratios between the barotropic motion to the baroclinic motion induced by the wind present a normal distribution of N(0.0242, 0.3947). The energy change ratios of the first 4 baroclinic modes to the whole baroclinic currents also follow the normal distributions. The first baroclinic mode follow; N (0.2628, 0.1872), the second N (0.1979, 0.1504), the third N (0.1331, 0.1633), and the fourth N (0.0650, 0.1540), respectively.

A preliminary investigation of basin water response to climate forcing in a Scottish fjord: evaluating the influence of the NAO

The sea lochs (fjords) of NW Scotland bridge the land–ocean interface in a region of Europe which is particularly well situated to monitor changes in westerly air streams. Inter-annual atmospheric circulation changes at this latitude are largely governed by the North Atlantic Oscillation (NAO). Comparing two recent extreme NAO years, a two-dimensional model of Loch Sunart, NW Scotland, is used to examine the potential effects of climate oscillations on the magnitude and frequency of deep-water renewal events and the resulting water properties in the fjord basins. In the upper basin of the fjord, meteorological forcing during the high NAO index year (1988–89) resulted in less-frequent deep-water renewal, greater variability in basin salinity and a lower annual-mean salinity (by 0.52) than that predicted for the low NAO index year (1995–96). In the main basin, variations in meteorological forcing had much less effect on basin water properties. In both basins, predicted deep water inflow was significantly greater during the negative phase NAO, with annual inflow to the upper and main basins being respectively 50% and 300% greater during 1995–96 relative to 1988–99. Through a sensitivity analysis, the NAO is shown to affect upper basin water properties through the influence of low-frequency anomalies in the meteorological forcing, particularly the enhanced westerly wind stress associated with positive phases, which inhibits deep water renewal over the winter months. The salinity of the main and upper basins respond differently to the boundary forcing due to differential tidal mixing above the respectively sub- and super-critical entrance sills. Predictions of basin water isotope ratios are made by applying the salinity: δ 18O water mixing line for the region to the salinity results; the combination of the weak dependence of δ 18O water on salinity and the stable salinity predicted for the main basin suggests that some fjord basins may provide ideal sites for palaeotemperature studies.

Nuclear Ordering in Lithium and an Upper Limit on its Ambient Pressure Superconducting Transition Temperature

We have discovered spontaneous ordering of nuclear spins in lithium metal by NMR measurements at very low temperatures. In low magnetic fields, B<0.2 mT, the NMR spectra show a pronounced low-frequency anomaly. Also, nonadiabatic response to a slowly varying magnetic field was observed. A rich phase diagram with three different nonparamagnetic regions is proposed. We estimate a critical spin temperature T(c) approximately 350 nK at B=0. We also report the absence of superconductivity in lithium at normal pressure down to T(e) approximately 100 microK (B<10 nT).

Lattice Dynamics and Central-Mode Phenomena in the Dielectric Response of Ferroelectrics and Related Materials

In this review we describe and discuss recent results on the linear dielectric response of high-permittivity dielectric crystals and ceramics in the wide frequency range 102–1014Hz. Our attention is paid to materials which exhibit some interesting low-frequency polar-phonon anomalies in combination with some additional dielectric dispersion below the polar phonon region. The following compounds are discussed: microwave ceramics Bi1.5Zn1.0Nb1.5O7, antiferroelectric PbZrO3ceramics and AgNbO3crystals, ferroelectric LiNaGe4O9, LaBGeO5, Cd2Nb2O7and SrBi2Ta2O9crystals, incommensurate ferroelectric Sr2Nb2O7, Ba2NaNb5O15and BCCD crystals, dipolar-glass crystal of Rb1/2(ND4)1/2D2PO4, relaxor ferroelectric Pb(Sc1/2Ta1/2)O3and PLZT ceramics and PMN crystals, antiferroelectric PLZT 2/95/5 ceramics and relaxor-based PMN-PT, PZN-PT and BiScO3-PT crystals and ceramics. For these materials the polar phonon spectra are discussed together with their flow-frequency dielectric response and dispersion regions in between, in the 10–300 K or higher temperature range, depending on the existing phase transitions of interest. Some universal features of the disorder and anharmonicities are pointed out: central-mode phenomena near displacive phase transitions and polar nano-clusters which appear at rather high temperatures and remain present down to low temperatures if the material remains structurally disordered. The manifestation of the latter in the dielectric spectra consists in extreme broadening of the relaxation region on cooling which at low temperatures results in constant loss spectra (1/fnoise).

An improved gaas device model for the simulation of analog integrated circuit

This paper describes an improved device model of GaAs MESFETs and heterojunction FETs for the design and analysis of analog integrated circuits. The proposed device model provides a new expression for the current and the capacitance of the device,which gives excellent agreements with experimental data for all regions of device operation. For the expression of the low frequency anomalies of GaAs devices, an improved technique with an equivalent circuit are presented to model the frequency dispersion of the transconductance and the drain conductance of the device, which give a good agreement with the experimental data of both the frequency dispersion and the lag effect of the device. The new device model proposed here clearly provides a superior prediction of the performance of GaAs analog integrated circuit.

Infrared and Raman spectroscopy of [Pb(Zn1/3Nb2/3)O3]0.92–[PbTiO3]0.08 and [Pb(Mg1/3Nb2/3)O3]0.71–[PbTiO3]0.29 single crystals

Far-infrared reflectivity spectra of [Pb(Zn1/3Nb2/3)O3]0.92–[PbTiO3]0.08 and [Pb(Mg1/3Nb2/3)O3]0.71–[PbTiO3]0.29 single crystals were investigated between 10 and 530 K, micro-Raman spectra were recorded between 300 and 800 K. No phonon softening was observed near either of the ferroelectric phase transitions. The low-frequency dielectric anomaly in the paraelectric phase is caused by contribution of dynamic polar nanoclusters with the main dispersion in the microwave range. Infrared and Raman spectra confirm the locally doubled unit cell (Zprim=2) in the paraelectric and ferroelectric phases due to the ordering in the perovskite B sites and occurrence of polar nanoclusters in the paraelectric phase. The lowest-frequency transverse optical (TO1) phonon mode active in the infrared spectra is underdamped in contrast to the recent result of inelastic neutron scattering, where no TO1 mode could be observed for the wave vectors q⩽0.2 Å−1. This discrepancy was explained by different q vectors probed in infrared and neutron experiments. The infrared probe couples with very long-wavelength phonons (q≈10−5 Å−1) which see the homogeneous medium averaged over the nanoclusters, whereas the neutron probe couples with phonons whose wavelength is comparable to the nanocluster size (q⩾10−2 Å−1).

Low-energy librational excitations in vitreous poly(methyl methacrylate)

The infrared (IR) and Raman spectra of vitreous poly(methyl methacrylate) (PMMA) are measured and investigated in the frequency range 10–150 cm−1. A comparison of the results obtained from IR and Raman spectroscopic measurements permits the assignment of the low-frequency anomaly (boson peak) observed in the spectra to librational vibrations occurring in a segment of the main chain that is comparable in length to the statistical chain segment. It is demonstrated that coherent librational excitations are associated with the relaxation processes proceeding in the polymers.

Molecular dynamics simulation of the soft mode for hydrogen-bonded ferroelectrics

Numerical simulations of dipole dynamics have been studied by employing a recently proposed modified strong dipole proton coupling (MSDPC) model of the phase transition in the hydrogen-bonded type of ferroelectrics. The obtained results in the form of the spectral density of the polarization fluctuation along the polar axis and along the lateral direction are simulated and correlated with known experimental data obtained primarily by Raman and infrared spectroscopy on KDP and DKDP lattices. A central peak (CP) appears in the calculated spectral density of the longitudinal polarization fluctuation in the ferroelectric phase of KDP and DKDP. The simulated CP indicates an order-disorder mechanism of excitation in the paraelectric phase near T c in both crystals. By increasing the temperature to 460 K, the crossover to the displacive behavior for KDP lattice is predicted. The model also predicts a low-frequency flat anomaly for KDP and a CP for DKDP in the spectral density of transversal polarization fluctuations, as was earlier detected by infrared and Raman spectroscopy. The superiority of the MSDPC model is additionally tested in the description of the experimentally obtained pressure-induced change of the CP line shape for KDP.

Atmospheric GCM Response to Extratropical SST Anomalies: Synthesis and Evaluation*

The advances in our understanding of extratropical atmosphere–ocean interaction over the past decade and a half are examined, focusing on the atmospheric response to sea surface temperature anomalies. The main goal of the paper is to assess what was learned from general circulation model (GCM) experiments over the recent two decades or so. Observational evidence regarding the nature of the interaction and dynamical theory of atmospheric anomalies forced by surface thermal anomalies is reviewed. Three types of GCM experiments used to address this problem are then examined: models with fixed climatological conditions and idealized, stationary SST anomalies; models with seasonally evolving climatology forced with realistic, time-varying SST anomalies; and models coupled to an interactive ocean. From representative recent studies, it is argued that the extratropical atmosphere does respond to changes in underlying SST although the response is small compared to internal (unforced) variability. Two types of interactions govern the response. One is an eddy-mediated process, in which a baroclinic response to thermal forcing induces and combines with changes in the position or strength of the storm tracks. This process can lead to an equivalent barotropic response that feeds back positively on the ocean mixed layer temperature. The other is a linear, thermodynamic interaction in which an equivalent-barotropic low-frequency atmospheric anomaly forces a change in SST and then experiences reduced surface thermal damping due to the SST adjustment. Both processes contribute to an increase in variance and persistence of low-frequency atmospheric anomalies and, in fact, may act together in the natural system.

Low-frequency Vibrational Anomalies in β-Lactoglobulin: Contribution of Different Hydrogen Classes Revealed by Inelastic Neutron Scattering

The low-frequency dynamics of the globular protein ‚-lactoglobulin has been investigated by inelastic neutron scattering, on both dry and D2O-hydrated samples. Both the dynamic structure factor and the density of states display an excess of vibrational modes with respect to the Debye level, with such an excess being reminiscent of the so-called boson peak. To establish which protein structural elements give origin to the anomalous peak, the samples were submitted to suitable H/D exchange procedures, which allowed us to highlight the following points: (i) in the dry samples, nonexchangeable, internal, hydrogens display a peak at 3 meV; (ii) exchangeable, mainly external, hydrogens provide a further peak, which is shifted to lower frequencies and displays a stronger intensity per atom; (iii) in the hydrated samples, no frequency difference between the peaks of external and internal hydrogens is found, meanwhile the stronger scattering intensity due to external hydrogens is still observed. These results are discussed in the framework of a model ascribing the anomalous bump to interactions of acoustic phonons with density fluctuations domains and the effect of hydration on the protein dynamics is revisited.

Design considerations for buried p-layers to suppress substrate-trapping effects in GaAs MESFETs

The effects of buried p-layers (BP-layers) on substrate-trap-induced frequency dispersion of drain conductance and drain lag in GaAs MESFETs have been analyzed. Our experiments show that the BP-layer concentration, profile, and configuration significantly affect its effectiveness in suppressing trap-induced low-frequency anomalies. BP-layer resistance should be low enough to enable fixing the electrostatic potential, which requires that the p-layers be partially neutralized. Our results can be explained by the self-backgating model and resistance measured for the BP-layers. However, while suppressing trap-induced phenomena, BP-layers may cause current transients at higher frequencies and degrade RF performance due to the charging and discharging of holes in the p-layer. This phenomenon is also investigated in detail.

Magnetic and transport eddy-current anomalies in cylinders owing to magnetization rotations

For a magnetic conducting cylinder with a coaxial core, where the spontaneous magnetization M s is helical, the technical magnetization under an axial AC magnetic field or AC transport current is carried out by M s rotations, which induce normal and lateral eddy currents. The low-frequency eddy-current anomaly factors for the magnetic and the transport case, η, are calculated as functions of the radius ratio of the core to the cylinder, p= r b/ r 0, the scalar susceptibility of the shell normalized to the normal susceptibility of the core, q= χ s/ χ zz or χ s/ χ φφ , and the helical angle with respect to the z-axis, α. It is shown that for the typical case of α= π/4, the lateral-eddy-current loss is appreciably less and greater than the normal one in the magnetic and transport case, respectively, which is in contrast to the situation of a slab where both losses are equal.

A Monte Carlo analysis of the elastic incoherent neutron scattering data in hydrated azurin

We measured the incoherent neutron scattering elastic intensity of hydrated copper azurin at different temperatures in a wide momentum transfer q range. The results show a well-defined non-Gaussian q dependence above 180 K. To investigate the nature of the motions underlying this non-Gaussian dependence, we used a Monte Carlo method to interpret the elastic intensity as the superposition of individual non-dynamically equivalent Gaussian contributions. The calculated probability distribution functions corresponding to such a decomposition are analysed as a function of the temperature. The onset of large amplitude Gaussian motions is responsible for the mean square displacements behaviour, which show above 210 K a marked departure from the harmonic temperature dependence. In addition, such an approach allowed us to correlate the elastic intensity q dependence with the boson peak, a characteristic low-frequency anomaly of protein inelastic spectra.

Rectification of the Madden–Julian Oscillation into the ENSO Cycle

An ocean general circulation model, forced with idealized, purely oscillating wind stresses over the western equatorial Pacific similar to those observed during the Madden‐Julian oscillation (MJO), developed rectified low-frequency anomalies in SST and zonal currents, compared to a run in which the forcing was climatological. The rectification in SST resulted from increased evaporation under stronger than normal winds of either sign, from correlated intraseasonal oscillations in both vertical temperature gradient and upwelling speed forced by the winds, and from zonal advection due to nonlinearly generated equatorial currents. The net rectified signature produced by the MJO-like wind stresses was SST cooling (about 0.48C) in the west Pacific, and warming (about 0.18C) in the central Pacific, tending to flatten the background zonal SST gradient. It is hypothesized that, in a coupled system, such a pattern of SST anomalies would spawn additional westerly wind anomalies as a result of SST-induced changes in the low-level zonal pressure gradient. This was tested in an intermediate coupled

Trapping and migration of methane associated with the gas hydrate stability zone at the Blake Ridge Diapir: new insights from seismic data

Abstract The Blake Ridge Diapir is the southernmost of a line of salt diapirs along the Carolina trough. Diapirs cause faulting of the superjacent sediments, creating pathways for migration of fluids and gas to the seafloor. We analyzed reflection seismic data from the Blake Ridge Diapir, which is located in a region with known abundant gas hydrate occurrence. A striking feature in these data is a significant shallowing of the base of gas hydrate stability (BGHS) over the center of the diapir: The seafloor is warped up by about 100 m above the diapir, from about 2300 m to about 2200 m. The BGHS, as indicated by a bottom simulating reflection (BSR), is about 4.5 s off the flanks of the diapir, rising to about 4.15 s at the center. Above the diapir, a fault system appears to rise vertically from the BGHS to about 0.05 s below the seafloor (40–50 m); it then diverges into several steeply dipping faults that breach the seafloor and cover an area ∼700 m in diameter. Other secondary faults diverge from the main fault or emerge directly from the BGHS near the crest of the diapir. Gas and other fluids may migrate upward through the faults. We performed complex trace analysis to compare the reflection strength and instantaneous frequency along individual reflections. A low-frequency anomaly over the center of the diapir indicates high seismic attenuation. This is interpreted to be caused by migration of fluids (probably methane) along fault pathways. The migration of gas (i.e. probably mainly methane) through the gas hydrate stability zone is not yet understood. We speculate that pore fluids in the faults may be too warm and too salty to form gas hydrate, even at depths where gas hydrate is stable away from the diapir. Alternatively, gas hydrates may seal the fault walls such that water supply is too low to transform all the gas into gas hydrates. The shallowing of the BSR may reflect increased heatflow above the diapir either caused by the high thermal conductivity of the underlying salt or by advective heat transport along with fluids. High pore water salinity shifts the gas hydrate stability to lower temperatures and may also play a significant role in BSR shallowing. We, therefore, investigated the possible effect of pore water salinity on shallowing of the BSR. We found that BSR shallowing may theoretically be entirely caused by increased salinity over the diapir, although geologically this would not be reasonable. This observation demonstrates the potential importance of pore water salinity for lateral variations of BSR depths, in particular, above salt structures.