Steady-state distributions for models of bubbles: their existence and econometric implications

The purpose of this paper is to examine the properties of bubbles in the light of steady state results for threshold auto-regressive (TAR) models recently derived by Knight and Satchell (2011). We assert that this will have implications for econometrics. We study the conditions under which we can obtain a steady state distribution of asset prices using our simple model of bubbles based on our particular definition of a bubble. We derive general results and further extend the analysis by considering the steady state distribution in three cases of a (I) a normally distributed error process, (II) a non normally (exponentially) distributed steady-state process and (III) a switching random walk with a fairly general i.i.d error process We then examine the issues related to unit root testing for the presence of bubbles using standard econometric procedures. We illustrate as an example, the market for art, which shows distinctly bubble-like characteristics. Our results shed light on the ubiquitous finding of no bubbles in the econometric literature

Are there bubbles in the art market? The detection of bubbles when fair value is unobservable

The purpose of this paper is to look for bubbles in the Art Market using a structure based on steady state results for TAR models and appropriate definitions of bubbles recently put forward by Knight, Satchell and Srivastava (2011). The usual method for investigating bubbles is to measure prices as deviations from fair value. We assess whether it is meaningful to define a fair value of art and conclude that it is very challenging empirically to implement any definition. We then treat fair value as zero in one instance and unobservable in the other case and in both cases provide evidence of bubbles in the art market

Supercurrent in ferromagnetic Josephson junctions with heavy metal interlayers

The length scale over which supercurrent from conventional BCS, s-wave superconductors (S) can penetrate an adjacent ferromagnetic (F) layer depends on the ability to convert singlet Cooper pairs into triplet Cooper pairs. Spin-aligned triplet Cooper pairs are not dephased by the ferromagnetic exchange interaction and can thus penetrate an F layer over much longer distances than singlet Cooper pairs. These triplet Cooper pairs carry a dissipationless spin current and are the fundamental building block for the fledgling field of superspintronics. Singlet-triplet conversion by inhomogeneous magnetism is well established. Here, we describe an attempt to use spin-orbit coupling as an alternative mechanism to mediate singlet-triplet conversion in S-F-S Josephson junctions. We report that the addition of thin Pt spin-orbit-coupling layers in our Josephson junctions significantly increases supercurrent transmission, however the decay length of the supercurrent is not found to increase. We attribute the increased supercurrent transmission to Pt acting as a buffer layer to improve the growth of the Co F layer

Probing the Spiral Magnetic Phase in 6 nm Textured Erbium using Polarised Neutron Reflectometry

We characterise the magnetic state of highly-textured, sputter deposited erbium for a film of thickness 6 nm. Using polarised neutron reflectometry it is found the film has a high degree of magnetic disorder, and we present some evidence that the films’ local magnetic state is consistent with bulk-like spiral magnetism. This, combined with complementary characterisation techniques, show that thin film erbium is a strong candidate material for incorporation into device structures

Observation of anomalous Meissner screening in Cu/Nb and Cu/Nb/Co thin films

We have observed the spatial distribution of magnetic flux in Nb, Cu/Nb and Cu/Nb/Co thin films using muon-spin rotation. In an isolated 50 nm thick Nb film we find a weak flux expulsion (Meissner effect) which becomes significantly enhanced when adding an adjacent 40 nm layer of Cu. The added Cu layer exhibits a Meissner effect (due to induced superconducting pairs) and is at least as effective as the Nb to expel flux. These results are confirmed by theoretical calculations using the quasiclassical Green’s function formalism. An unexpected further significant enhancement of the flux expulsion is observed when adding a thin (2.4 nm) ferromagnetic Co layer to the bottom side of the Nb. This observed cooperation between superconductivity and ferromagnetism, by an unknown mechanism, forms a key ingredient for developing superconducting spintronics

Control of superconductivity with a single ferromagnetic layer in niobium/erbium bilayers

Superconducting spintronics in hybrid superconductor{ferromagnet (S{F) heterostructures provides an exciting potential new class of device. The prototypical super-spintronic device is the superconducting spin-valve, where the critical temperature, Tc, of the S-layer can be controlledby the relative orientation of two (or more) F-layers. Here, we show that such control is also possible in a simple S/F bilayer. Using eld history to set the remanent magnetic state of a thin Er layer, we demonstrate for a Nb/Er bilayer a high level of control of both Tc and the shape of the resistive transition, R(T), to zero resistance. We are able to model the origin of the remanent magnetization, treating it as an increase in the e ective exchange eld of the ferromagnet and link this, using conventional S{F theory, to the suppression of Tc. We observe stepped features in the R(T) which we argue is due to a fundamental interaction of superconductivity with inhomogeneous ferromagnetism, a phenomena currently lacking theoretical description

Supercurrent diode effect in thin film Nb tracks

We demonstrate nonreciprocal critical current in 65 nm thick polycrystalline and epitaxial Nb thin films patterned into tracks. The nonreciprocal behavior gives a supercurrent diode effect, where the current passed in one direction is a supercurrent and the other direction is a normal state (resistive) current. We study the variation of the diode effect with temperature and magnetic field, and find an unexpected dependence with the width of the Nb tracks from 2-10 $\mu$m. For both polycrystalline and epitaxial samples, we find that tracks of width 4 $\mu$m provides the largest supercurrent diode efficiency of up to $\approx30\%$, with the effect reducing or disappearing in the widest tracks of 10 $\mu$m. It is anticipated that the supercurrent diode will become a ubiquitous component of the superconducting computer.Comment: 15 pages, 4 figure

Neurophysiological modeling of bladder afferent activity in the rat overactive bladder model

The overactive bladder (OAB) is a syndrome-based urinary dysfunction characterized by “urgency, with or without urge incontinence, usually with frequency and nocturia”. Earlier we developed a mathematical model of bladder nerve activity during voiding in anesthetized rats and found that the nerve activity in the relaxation phase of voiding contractions was all afferent. In the present study, we applied this mathematical model to an acetic acid (AA) rat model of bladder overactivity to study the sensitivity of afferent fibers in intact nerves to bladder pressure and volume changes. The afferent activity in the filling phase and the slope, i.e., the sensitivity of the afferent fibers to pressure changes in the post-void relaxation phase, were found to be significantly higher in AA than in saline measurements, while the offset (nerve activity at pressure ~0) and maximum pressure were comparable. We have thus shown, for the first time, that the sensitivity of afferent fibers in the OAB can be studied without cutting nerves or preparation of single fibers. We conclude that bladder overactivity induced by AA in rats is neurogenic in origin and is caused by increased sensitivity of afferent sensors in the bladder wall