Fiber Bragg Grating Sensor for Strain Sensing in Low Temperature Superconducting Magnet

Fiber Bragg grating (FBG) sensor for monitoring the electromagnetic strain in a low temperature superconducting (LTS) magnet was studied. Before used to LTS magnet strain sensing, the strain response of the sensor with 1.54-mu m wavelength at liquid helium was experimentally studied. It was found that the wavelength shift showed good linearity with longitudinal applied loads and the strain sensitivity is constant at 4.2 K. And then, the hoop strain measurement of a LTS magnet was carried out on the basis of measured results. Furthermore, the finite element method (FEM) was used to simulate the magnet strain. The difference between the experimental and numerical analysis results is very small.

Fiber Bragg Grating Sensor for Strain Sensing in Low Temperature Superconducting Magnet

Fiber Bragg grating (FBG) sensor for monitoring the electromagnetic strain in a low temperature superconducting (LTS) magnet was studied. Before used to LTS magnet strain sensing, the strain response of the sensor with 1.54-mu m wavelength at liquid helium was experimentally studied. It was found that the wavelength shift showed good linearity with longitudinal applied loads and the strain sensitivity is constant at 4.2 K. And then, the hoop strain measurement of a LTS magnet was carried out on the basis of measured results. Furthermore, the finite element method (FEM) was used to simulate the magnet strain. The difference between the experimental and numerical analysis results is very small.

Calculations of the spectral shifts and line profiles of alkaline earth atoms in liquid helium environment

Liquid configurations generated by Metropolis Monte Carlo simulations are used in time-dependent density functional theory calculations of the spectral line shifts and line profiles of the lowest lying excitation of the alkaline earth atoms, Be, Mg, Ca, Sr and Ba embedded in liquid helium. The results are in very good agreement with the available experimental data. Special attention is given to the calculated spectroscopic shift and the associated line broadening. The analysis specifies the inhomogeneous broadening of the three separate contributions due to the splitting of the s -> p transition of the alkaline earth atom in the liquid environment. (C) 2012 Elsevier B. V. All rights reserved.

Liquid crystal temperature sensor based on a micrometric structure and a metallic nanometric layer

This letter presents a novel temperature sensor, which consists of an interdigitated comb electrode structure with a micrometric-scale size, nanometric metallic layer, and nematic liquid crystal (NLC) film. This sensor exploits the permittivity dependence of the NLC with temperature and principle of electrical conductivity above the percolation threshold in thin film metallic layers. The latter has been demonstrated to increase the temperature sensitivity considerably. The high impedance input reduces the power dissipation, and the high enough voltage output makes it easy to measure the output signal with high precision. The operation principle and fabrication process as well as the characterization of the temperature sensor are presented. Experimental results show that the device offers a sensitivity of 9 mV/°C and is dependent on the applied voltage. This is six times greater than the same structure without the use of a nanometric layer.

A novel high-sensitivity, low-power, liquid crystal temperature sensor

A novel temperature sensor based on nematic liquid crystal permittivity as a sensing magnitude, is presented. This sensor consists of a specific micrometric structure that gives considerable advantages from other previous related liquid crystal (LC) sensors. The analytical study reveals that permittivity change with temperature is introduced in a hyperbolic cosine function, increasing the sensitivity term considerably. The experimental data has been obtained for ranges from −6 °C to 100 °C. Despite this, following the LC datasheet, theoretical ranges from −40 °C to 109 °C could be achieved. These results have revealed maximum sensitivities of 33 mVrms/°C for certain temperature ranges; three times more than of most silicon temperature sensors. As it was predicted by the analytical study, the micrometric size of the proposed structure produces a high output voltage. Moreover the voltage’s sensitivity to temperature response can be controlled by the applied voltage. This response allows temperature measurements to be carried out without any amplification or conditioning circuitry, with very low power consumption.

Low temperature magneto-photoluminescence characterization of high purity gallium arsenide and indium phosphide

Low-temperature magneto-photoluminescence is a very powerful technique to characterize high purity GaAs and InP grown by various epitaxial techniques. These III-V compound semiconductor materials are used in a wide variety of electronic, optoelectronic and microwave devices. The large binding energy differences of acceptors in GaAs and InP make possible the identification of those impurities by low-temperature photoluminescence without the use of any magnetic field. However, the sensitivity and resolution provided by this technique rema1ns inadequate to resolve the minute binding energy differences of donors in GaAs and InP. To achieve higher sensitivity and resolution needed for the identification of donors, a magneto-photoluminescence system 1s installed along with a tunable dye laser, which provides resonant excitation. Donors 1n high purity GaAs are identified from the magnetic splittings of "two-electron" satellites of donor bound exciton transitions 1n a high magnetic field and at liquid helium temperature. This technique 1s successfully used to identify donors 1n n-type GaAs as well as 1n p-type GaAs in which donors cannot be identified by any other technique. The technique is also employed to identify donors in high purity InP. The amphoteric incorporation of Si and Ge impurities as donors and acceptors in (100), (311)A and (3ll)B GaAs grown by molecular beam epitaxy is studied spectroscopically. The hydrogen passivation of C acceptors in high purity GaAs grown by molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) 1s investigated using photoluminescence. Si acceptors ~n MBE GaAs are also found to be passivated by hydrogenation. The instabilities in the passivation of acceptor impurities are observed for the exposure of those samples to light. Very high purity MOCVD InP samples with extremely high mobility are characterized by both electrical and optical techniques. It is determined that C is not typically incorporated as a residual acceptor ~n high purity MOCVD InP. Finally, GaAs on Si, single quantum well, and multiple quantum well heterostructures, which are fabricated from III-V semiconductors, are also measured by low-temperature photoluminescence.

Cr3+ electron paramagnetic resonance study of Sn(1-x)CrxO(2) (0.00 <= x <= 0.10)

This paper reports on the liquid-helium-temperature (5 K) electron paramagnetic resonance (EPR) spectra of Cr3+ ions in the nanoparticles of SnO2 synthesized at 600 degrees C with concentrations of 0%, 0.1%, 0.5%, 1%, 1.5%, 2.0%, 2.5%, 3.0%, 5.0%, and 10%. Each spectrum may be simulated as overlap of spectra due to four magnetically inequivalent Cr3+ centers characterized by different values of the spin-Hamiltonian parameters. Three of these centers belong to Cr3+ ions in orthorhombic sites, situated near oxygen vacancies, characterized by very large zero-field splitting parameters D and E, presumably due to the presence of nanoparticles in the samples. The fourth EPR spectrum belongs to the Cr3+ ions situated at sites with tetragonal symmetry, substituting for the Sn4+ ion, characterized by a very small value of D. In addition, there appears a ferromagnetic resonance line due to oxygen defects for samples with Cr3+ concentrations of <= 2.5%. Further, in samples with Cr3+ concentrations of >2.5%, there appears an intense and wide EPR line due to the interactions among the Cr3+ ions in the clusters formed due to rather excessive doping; the intensity and width of this line increase with increasing concentration. The Cr3+ EPR spectra observed in these nanopowders very different from those in bulk SnO2 crystals.

光纤Bragg光栅液氦环境下温度传感特性的研究

进行了液氦温度（4．2K）到室温（298K）温区内光纤Bragg光栅（FHG）温度传感性能的实验研究。重点分析了液氦温度（4．2K）到液氮温度（77K）FBG的温度传感特性。实验表明：FHG传感特性与温度相关。在50K以下，温度响应基本没有变化；50K-77K，波长偏移量随温度上升变化不规律；150K-298K传感特性近似成线性。对比裸光栅与涂敷光栅，涂敷光栅的温度灵敏度远大于裸光栅的温度灵敏度。选用外加热膨胀系数大的聚合物封装，可以显著提高FHG的温敏系数和线性度。

Investigations of noise and of quantum interference in proximity effect bridges

This work reports investigations upon weakly superconducting proximity effect bridges. These bridges, which exhibit the Josephson effects, are produced by bisecting a superconductor with a short (<1µ) region of material whose superconducting transition temperature is below that of the adjacent superconductors. These bridges are fabricated from layered refractory metal thin films whose transition temperature will depend upon the thickness ratio of the materials involved. The thickness ratio is changed in the area of the bridge to lower its transition temperature. This is done through novel photolithographic techniques described in the text, Chapter 2.

If two such proximity effect bridges are connected in parallel, they form a quantum interferometer. The maximum zero voltage current through this circuit is periodically modulated by the magnetic flux through the circuit. At a constant bias current, the modulation of the critical current produces a modulation in the dc voltage across the bridge. This change in dc voltage has been found to be the result of a change in the internal dissipation in the device. A simple model using lumped circuit theory and treating the bridges as quantum oscillators of frequency ω = 2eV/h, where V is the time average voltage across the device, has been found to adequately describe the observed voltage modulation.

The quantum interferometers have been converted to a galvanometer through the inclusion of an integral thin film current path which couples magnetic flux through the interferometer. Thus a change in signal current produces a change in the voltage across the interferometer at a constant bias current. This work is described in Chapter 3 of the text.

The sensitivity of any device incorporating proximity effect bridges will ultimately be determined by the fluctuations in their electrical parameters. He have measured the spectral power density of the voltage fluctuations in proximity effect bridges using a room temperature electronics and a liquid helium temperature transformer to match the very low (~ 0.1 Ω) impedances characteristic of these devices.

We find the voltage noise to agree quite well with that predicted by phonon noise in the normal conduction through the bridge plus a contribution from the superconducting pair current through the bridge which is proportional to the ratios of this current to the time average voltage across the bridge. The total voltage fluctuations are given by <V^2(f ) > = 4kTR^2_d I/V where R_d is the dynamic resistance, I the total current, and V the voltage across the bridge . An additional noise source appears with a strong 1/f^(n) dependence , 1.5 < n < 2, if the bridges are fabricated upon a glass substrate. This excess noise, attributed to thermodynamic temperature fluctuations in the volume of the bridge, increases dramatically on a glass substrate due to the greatly diminished thermal diffusivity of the glass as compared to sapphire.

Structure and properties of an amorphous ferromagnetic alloy

The structure and the electrical and magnetic properties of an amorphous alloy containing approximately 80 at .% iron, 13 at.% phos phorus and 7 at.% carbon (Fe_(80)Fe_(13)C_7) obtained by rapid quenching from the liquid state have been studied. Transmission electron diffraction data confirm the amorphous nature of this alloy. An analysis of the radial distribution function obtained from X-ray diffraction data indicates that the number of nearest neighbors is approximately seven, at a distance of 2.6A. The structure of the alloy can be related to that of silicate glasses and is based on a random arrangement of trigonal prisms of Fe_2P and Fe_3C types in which the iron atoms have an average ligancy of seven. Electrical resistance measurements show that the alloys are metallic. A minimum in the electrical resistivity vs. temperature curve is observed between 10° K to 50° K depending on the specimen, and the temperature at which the minimum occurs is related to the degree of local ordering. The Fe-P-C amorphous alloys are ferromagnetic. The Curie temperature measured by the induction method and by Mossbauer spectroscopy is 315° C. The field dependence of the magneto-resistance at temperatures from liquid helium to room temperature is similar to that found in crystalline iron. The ordinary Hall coefficient is approximately 10^(-11) volt-cm/amp-G. The spontaneous Hall coefficient is about 0.6 x 10^(-9) volt-cm/amp-G and is practically independent of temperature from liquid helium temperature up to 300° c.

High-frequency hydrogen-related infrared modes in silicon grown in a hydrogen atmosphere

High-frequency vibrational modes have been observed at liquid-helium temperature in silicon samples grown in a H-2 or D-2 atmosphere. The highest-frequency ones are due to the overtones and combination modes of SiH fundamentals. Others are CH modes due to (C,H) complexes, but the simultaneous presence of NH modes due to (N,H) complexes cannot be excluded. The present results seem to show also the existence of centers including both SiH and CH or NH bonds. One sharp mode at 4349 cm-l is related to a weak SiH fundamental at 2210 cm(-1). The related center is ascribed to a vacancy fully decorated with hydrogen with a nearest-neighbor C atom. [S0163-1829(99)00911-X].

A device to investigate the axial strain dependence of the critical current density in superconductors

We have developed an instrument to study the behavior of the critical current density (J(c)) in superconducting wires and tapes as a function of field (mu(0)H), temperature (T), and axial applied strain (epsilon(a)). The apparatus is an improvement of similar devices that have been successfully used in our institute for over a decade. It encompasses specific advantages such as a simple sample layout, a well defined and homogeneous strain application, the possibility of investigating large compressive strains and the option of simple temperature variation, while improving the main drawback in our previous systems by increasing the investigated sample length by approximately a factor of 10. The increase in length is achieved via a design change from a straight beam section to an initially curved beam, placed perpendicular to the applied field axis in the limited diameter of a high field magnet bore. This article describes in detail the mechanical design of the device and its calibrations. Additionally initial J(c)(epsilon(a)) data, measured at liquid helium temperature, are presented for a bronze processed and for a powder-in-tube Nb3Sn superconducting wire. Comparisons are made with earlier characterizations, indicating consistent behavior of the instrument. The improved voltage resolution, resulting from the increased sample length, enables J(c) determinations at an electric field criterion E-c=10 muV/m, which is substantially lower than a criterion of E-c=100 muV/m which was possible in our previous systems. (C) 2004 American Institute of Physics.

Giant magnetoresistance in ultrasmall graphene based devices

By computing spin-polarized electronic transport across a finite zigzag graphene ribbon bridging two metallic graphene electrodes, we demonstrate, as a proof of principle, that devices featuring 100% magnetoresistance can be built entirely out of carbon. In the ground state a short zigzag ribbon is an antiferromagnetic insulator which, when connecting two metallic electrodes, acts as a tunnel barrier that suppresses the conductance. The application of a magnetic field makes the ribbon ferromagnetic and conductive, increasing dramatically the current between electrodes. We predict large magnetoresistance in this system at liquid nitrogen temperature and 10 T or at liquid helium temperature and 300 G.