Low frequency elastic measurements on solid 4^{4}He in Vycor using a torsional oscillator

Torsional oscillator experiments involving solid $^{4}$He confined in the nanoscale pores of Vycor glass showed anomalous frequency changes at temperatures below 200 mK. These were initially attributed to decoupling of some of the helium's mass from the oscillator, the expected signature of a supersolid. However, these and similar anomalous effects seen with bulk $^{4}$He now appear to be artifacts arising from large shear modulus changes when mobile dislocations are pinned by $^{3}$He impurities. We have used a torsional oscillator (TO) technique to directly measure the shear modulus of the solid $^{4}$He/Vycor system at a frequency (1.2 kHz) comparable to that used in previous TO experiments. The shear modulus increases gradually as the TO is cooled from 1 K to 20 mK. We attribute the gradual modulus change to the freezing out of thermally activated relaxation processes in the solid helium. The absence of rapid changes below 200 mK is expected since mobile dislocations could not exist in pores as small as those of Vycor. Our results support the interpretation of a recent torsional oscillator experiment that showed no anomaly when elastic effects in bulk helium were eliminated by ensuring that there were no gaps around the Vycor sample.Comment: Accepted by Journal of Low Temperature Physic

Non-linear Elastic Response in Solid Helium: critical velocity or strain

Torsional oscillator experiments show evidence of mass decoupling in solid 4He. This decoupling is amplitude dependent, suggesting a critical velocity for supersolidity. We observe similar behavior in the elastic shear modulus. By measuring the shear modulus over a wide frequency range, we can distinguish between an amplitude dependence which depends on velocity and one which depends on some other parameter like displacement. In contrast to the torsional oscillator behavior, the modulus depends on the magnitude of stress, not velocity. We interpret our results in terms of the motion of dislocations which are weakly pinned by 3He impurities but which break away when large stresses are applied

Pancreatic Beta Cell Plasticity and Involvement of Insulin-Expressing Progenitor Cells

Islet transplants have been successfully used as treatment for diabetes, but are limited by shortages of cadaveric insulin-producing β-cells. An alternate source may be the dedifferentiation, expansion, and subsequent redifferentiation of pancreatic islets or β-cells using in vitro techniques prior to transplant. Based on protocols which described the dedifferentiation of human islets to ductal-like cells, we hypothesized that neonatal mouse islets could be similarly dedifferentiated in vitro. Dedifferentiation techniques produced significant duct-like cells, but redifferentiation to insulin-expressing cells was limited. RIPCre;Z/AP+/+ mice were consequently utilized to lineage trace β-cell fate during culture by a human placental alkaline phosphatase (HPAP) reporter. The proportion of HPAP+ β-cells decreased significantly in culture, but the rare remaining cells expressed the ductal marker CK19. Flow cytometric sorting of β-cell subpopulations from whole pancreas showed that the HPAP+CK19+ cells had derived from insulin-positive, glucose-transporter-2-negative (Ins+Glut2-) cells, previously purported as insulin-expressing progenitor cells. In P7 mouse pancreas, these Ins+Glut2- cells represent 3.5% of all insulin+ cells, the majority of which were found outside of islets within β-cell aggregates (BCA, \u3c5 β-cells). Ins+Glut2- cells demonstrated greater proliferation rates in vivo and in vitro as compared to Ins+Glut2+ cells, and a subset could differentiate into endocrine, ductal, and neural lineages. We sought to quantify the presence of these progenitor cells throughout life in both mouse and human pancreata, and how their abundance and location changes with age. The presence of Ins+Glut2- cells in human and mouse pancreas demonstrated similar distributions and iii ontogenies, being more abundant in BCA than islets at all ages sampled, and decreasing with age. Finally, neonatal rodents can regenerate β-cell mass after streptozotocin (STZ) exposure, but this response is mitigated in adulthood. As STZ accesses the β-cell via Glut2, we hypothesized that the β-cell regenerative capacity in early life involves mobilization of Ins+Glut2- cells, and used RIPCreER;Z/AP+/+ mice to trace these during damage and regeneration. We found that Ins+Glut2- cells indeed survived STZ exposure, and subsequently matured into Ins+Glut2+ cells. These Ins+Glut2- cells represent a transitional cell type, the majority of which contribute to pancreas maturation, and a subset of which retains multipotential lineage capability.

For such a time as this : ecclesiastical response to human sexual trafficking in North America

https://place.asburyseminary.edu/ecommonsatsdissertations/2033/thumbnail.jp

Dislocation networks in helium-4 crystals

The mechanical behavior of crystals is dominated by dislocation networks, their structure and their interactions with impurities or thermal phonons. However, in classical crystals, networks are usually random with impurities often forming non-equilibrium clusters when their motion freezes at low temperature. Helium provides unique advantages for the study of dislocations: crystals are free of all but isotopic impurities, the concentration of these can be reduced to the ppb level, and the impurities are mobile at all temperatures and therefore remain in equilibrium with the dislocations. We have achieved a comprehensive study of the mechanical response of 4He crystals to a driving strain as a function of temperature, frequency and strain amplitude. The quality of our fits to the complete set of data strongly supports our assumption of string-like vibrating dislocations. It leads to a precise determination of the distribution of dislocation network lengths and to detailed information about the interaction between dislocations and both thermal phonons and 3He impurities. The width of the dissipation peak associated with impurity binding is larger than predicted by a simple Debye model, and much of this broadening is due to the distribution of network lengths.Comment: accepted by Phys. Rev.

2D linear friction weld modelling of a Ti-6Al-4V T-joint

Most examples of linear friction weld process models have focused on joining two identically shaped workpieces. This article reports on the development of a 2D model, using the DEFORM finite element package, to investigate the joining of a rectangular Ti-6Al-4V workpiece to a plate of the same material. The work focuses on how this geometry affects the material flow, thermal fields and interface contaminant removal. The results showed that the material flow and thermal fields were not even across the two workpieces. This resulted in more material expulsion being required to remove the interface contaminants from the weld line when compared to joining two identically shaped workpieces. The model also showed that the flash curves away from the weld due to the rectangular upstand "burrowing" into the base plate.Understanding these critical relationships between the geometry and process outputs is crucial for further industrial implementation of the LFW process.EPSRC, The Welding Institut

Pressure-driven flow of solid helium

The recent torsional oscillator results of Kim and Chan suggest a supersolid phase transition in solid 4He. We have used a piezoelectrically driven diaphragm to study the flow of solid helium through an array of capillaries. Our measurements showed no indication of low temperature flow, placing stringent restrictions on supersolid flow in response to a pressure difference. The average flow speed at low temperatures was less than 1.2x10-14 m/s, corresponding to a supersolid velocity at least 7 orders of magnitude smaller than the critical velocities inferred from the torsional oscillator measurements.Comment: 4 pages, 3 figure

High resolution spectral characteristics of the Earth-ionosphere cavity resonances

The natural resonances of the Earth-ionosphere cavity at frequencies between 5 and 100 Hz have been studied since the fundamental paper by Schumann. While the gross features of the phenomena are now well understood, considerable work remains to be done on their detailed behaviour. In the present study a high resolution, data adaptive spectral technique is applied to digital electromagnetic data obtained at a moderate latitude. A particular feature of the method employed is that spectral properties become available on the same time scale as many ELF events, thus both time local and time averaged resonance features can be readily established. The technique can thus be applied to both dynamic and steady-state descriptions of the cavity's properties. For the data set considered, the technique adequately resolves the first six resonance modes on a time scale of 0.75 s. The presence of higher order modes is also indicated. The time averaged frequencies obtained are in accord with those of previous experimental determinations. When the time local properties of individual transient waveforms are examined, however, we observe a number of detailed effects which are predicted by theory. The precise spectral structure of the resonance modes appears influenced by the differing locations of the sources of transient excitation. In the case of the first order resonance mode, the properties of the cavity consistently support both singlet and doublet resonance behaviour

Audiomagnetotelluric sounding using the Schumann resonances

The Schumann resonance waveforms in the lower ELF band (5-100 Hz) are produced within the Earth-ionosphere cavity by distant lightning discharges; they provide a useful source field for shallow audiomagnetotclluric (AMT) crustal sounding. In this study we investigate their waveform characteristics that are important to the assumptions of AMT sounding. A time-domain polarization analysis technique is applied to a variety of examples of Schumann resonance waveforms. The multiplicity of worldwide thunderstorm centres provides a background activity which is generally incoherent and, accordingly, displays incoherent polarization characteristics. Superimposed on the back­ground are larger-amplitude transient events (sferics) from individual thunderstorm centres; they represent the response of the Earth-ionosphere cavity to very large lightning discharges and are generally linearly polarized at a given azimuth. The analysis indicates that the Schuman resonance waveforms provide a plane-wave source field, as required for electromagnetic crustal sounding. The differences in the "received" characteristics between the two waveform types prompted an investigation of the extent to which the waveform type and its particular polarization characteristics influence the determination of a geoelectric sounding curve. A detailed study, carried out with a multivariate maximum entropy spectral analysis algorithm, indicates that the two types of Schumann resonance waveform provide repeatable and consistent results at the 95% confidence level and that the linear polarizations associated with the sferics do not influence the estimation of the Earth response

The role of airborne EM methods for environmental applications in different geological terrains

This paper reviews the increasing role of Airborne EM (AEM) methods for environmental purposes in a variety of geological contexts. The ability of AEM data to differentiate geological, cultural and environmental influences is considered using fixed-wing survey examples from Finland and the UK. The provision of AEM subsurface resistivity information constitutes a unique capability in relation to general remote-sensing information to which it is allied. To be fully exploited, the results of AEM surveys require both ground calibration and integration (e.g. through GIS techniques) with existing geological, hydrogeological and environmental databases