摘要:

An adaptation of the mechanically controllable break junction technique using thin metallic films to create point contacts and tunnel junctions is presented. The junctions that are created using this very simple method are extremely stable, and can easily be adjusted from rather large point contacts (R≊0.1 &OHgr;) to high‐resistance (1010&OHgr;) tunnel junctions. The electrodes can be kept very clean by breaking the sample in ultrahigh vacuum or in a very pure helium atmosphere at 1–4 K. Point‐contact spectra of good quality were measured for contacts of 0.1–500 &OHgr;. For very small contacts, the expected steplike increase of junction resistance with decreasing contact size was observed, with a jump to tunneling behavior occurring at resistances of 12–14 k&OHgr;. ©1996 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1147089

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A fully automated precise electrical resistance measurement system for more than one sample has been constructed. Conventional four‐probe measurements with van der Pauw and Montgomery configurations are possible with this system. Resistance measurements in the range of a few &mgr;&OHgr; to a few G&OHgr; are possible for six samples at a time from room temperature down to liquid‐helium or liquid‐nitrogen temperatures with a temperature control accuracy of better than 10 mK. The design features of the system with special reference to the low‐noise switching methods of currents and voltages are described in detail. Precision of the results thus obtained using this system are highlighted for a few superconducting and semiconducting samples. ©1996 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1147118

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A combination high‐sensitivity alternating current (ac) susceptometer and high‐frequencyB–Hlooper is described. Based on the well‐known inductance technique, the design uses modern low‐noise components and stable temperature control to achieve the high sensitivity. The mechanical assembly is designed for use inside a glass Dewar, enabling measurements to be made at any temperature between 4.2 and 300 K. Initial ac susceptibility can be measured over a frequency range of 10 Hz to 120 kHz with sensitivity of 1×10−8emu, comparable to a superconducting quantum interference device (SQUID) magnetometer.B–Hloops are measured at 2 kHz with a low duty cycle, damped oscillatory field pulse, and have sensitivity of 4×10−8emu. The instrument can thus measure ac susceptibility at higher frequencies than ac‐equipped SQUID magnetometers and commercially available inductive ac susceptometers, and can also measureB–Hloops at much higher frequencies and with greater applied fields than steady‐state loop tracers. ©1996 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1147119

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Two methods have been developed to measure variations with magnetic field of electrical resistances in the range 0.1 n&OHgr;–1 &mgr;&OHgr;, for applied magnetic fields up to 3 T, and for temperatures less than 9 K. The first method provides direct access toR(H). Its absolute sensitivity is 3×10−14V/&sqrt;Hz with a short circuit at input. It becomes on a 1 &mgr;&OHgr; sample: 6×10−14V/&sqrt;Hz in zero field, and 2×10−12V/&sqrt;Hz under 3 T. Averaging the signal over 100 s, the sensitivity is 6×10−15V in zero field and 2×10−13V under 3 T. In terms of resistance, using a sensing current of 100 mA, it corresponds to a sensitivity of 6×10−14&OHgr; in zero field and 2×10−12&OHgr; in the maximum field. The reproducibility of this method is 0.3%. The second method is a sinusoidal field modulation technique giving the field derivative of the resistance:dR/dH. It has the same absolute sensitivity as theR(H) method and it achieves ultrahigh resolution. It is sensitive to variations of the resistance as small as 6×10−14&OHgr; in zero field and 2×10−12&OHgr; in 3 T independent of the absolute resistance of the sample, thus the resulting resolution &Dgr;R/Ris, for example, 6×10−8in zero field and 2×10−6in the maximum field for a 1 &mgr;&OHgr; sample. To demonstrate the high sensitivity of the setups, we measured the magnetoresistance of a magnetic multilayer with the current applied perpendicular to the multilayer plane. ©1996 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1147120

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A novel diamond‐anvil cell (DAC) adaptation for far infrared (FIR) cryogenic magnetospectroscopy in a small‐bore magnet is reported. A design featuring a multibellows ram is employed to generate amplified force forinsitupressure tuning of the DAC at cryogenic temperatures without increasing the overall diameter of the apparatus. A constructed version, using a 26‐mm‐diam double‐bellows ram, is capable of producing 5 kN force (scalable to 10 kN with four bellows) when driven by4He at 4.2 K. Signals are enhanced using paraboloidal cones to focus the FIR radiation on the sample and collect the transmitted signal. It is feasible to record transmission spectra to energies as low as 80 cm−1with this apparatus using appropriate gasket‐hole sizes and detectors. The pressure, magnetic field, and temperature can be tuned independently in the ranges 0–20 GPa (in steps as small as 0.05 GPa), 0–15 T, and 2–300 K. We present high pressure (4.2 K) Fourier‐transform FIR spectra at fixed magnetic fields, and laser (118.8 &mgr;m) magnetospectroscopy data on the 1s–2p+transition of Si donors in GaAs measured with this apparatus. ©1996 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1147121

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A spectrometer based on a dc superconducting quantum interference device (SQUID) has been developed for the direct detection of nuclear magnetic resonance (NMR) or nuclear quadrupole resonance (NQR) at frequencies up to 5 MHz. The sample is coupled to the input coil of the niobium‐based SQUID via a nonresonant superconducting circuit. The flux locked loop involves the direct offset integration technique with additional positive feedback in which the output of the SQUID is coupled directly to a low‐noise preamplifier. Precession of the nuclear quadrupole spins is induced by a magnetic field pulse with the feedback circuit disabled; subsequently, flux locked operation is restored and the SQUID amplifies the signal produced by the nuclear free induction signal. The spectrometer has been used to detect27Al NQR signals in ruby (Al2O3[Cr3+]) at 359 and 714 kHz. ©1996 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1147122

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A cylindrical corundum‐filled resonator of the TE112‐type in the microwaveXband, coupled to a coaxial line, is used in two systems designed for the high‐pressure, low‐temperature electron paramagnetic resonance (EPR) studies. The first one is used up to 800 MPa and over a temperature range of 80–400 K. The second permits EPR measurements down to 4 K in the pressure range below 400 MPa. Pressure and temperature can be simultaneously varied and controlled with the high accuracy needed to study spectral anomalies in the vicinity of the phase transitions in solids. ©1996 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1147123

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

VIRGO is a detector of gravitational waves based on a laser interferometer with 3 km long arms, now under construction. In order to detect gravitational waves down to 10 Hz, the seismic noise has to be reduced by several orders of magnitude. New mechanical filters using cantilever springs as elastic elements have been developed for the VIRGO seismic isolation system. The spring total stiffness is reduced by more than one order of magnitude using magnetic antispring. A control system allows one to keep the spring length constant. The new filters provide an attenuation ≥40 dB each for frequencies above 10 Hz and, compared to the previously adopted solution (gas springs), are much less sensitive to temperature changes. ©1996 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1147069

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A new instrument is presented that measures the regression of a burning solid propellant surface. The system extracted the diffuse reflection of a laser beam from the burning surface using synchronous detection. The incoming beam had a 4000 Hz angular modulation that served as the basis of a synchronous detection scheme. A galvanometer kept the beam spot centered under a sensor as the burn surface regressed. Theoretical analysis of the system using the component response properties resulted in the design of a controller for optimal bandwidth and stability. Laboratory evaluation on diffuse surfaces revealed a system bandwidth of 200 Hz, a range of 50 mm, and a resolution of 0.03 mm. Experiments conducted on burning propellants showed that the instrument successfully tracked the position of the surface at combustion pressures up to 17 atm. Under these conditions, the combustion environment reduced the beam transmission by up to 24 dB. This lowered the system bandwidth to 20 Hz and the resolution to 0.08 mm. The instrument demonstrated the first tracking of a local area on a burning propellant with a laser device. ©1996 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1147070

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The design of a prototype instrument for measuring the end‐tidal concentration of carbon monoxide during human respiration is presented. The instrument automatically samples the final sixty cubic centimeters of exhaled breath, from successive breathing cycles, by coordinating a pump and the breathing cycle with a set of vacuum and pressure sensors. The concentration of carbon monoxide is measured using a nondispersive infrared spectrophotometer. The amount of carbon monoxide present is measured relative to the source air concentration eliminating the need for calibrating the instrument. The testing protocol and measurements can be controlled by a microcomputer connected to the instrument through a standard RS‐232 serial interface. When at equilibrium, the end‐tidal concentration of CO can be measured in a simple and reproducible fashion. This simplified technology allows for the construction of a small, portable, easy to use instrument that will allow the application of this new pulmonary function test at the point of contact with patients. ©1996 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1147071

出版商:AIP    

年代:1996

数据来源: AIP