Junyun Li

Current sensing noise thermometry using a low Tc DC SQUID preamplifier

We describe here the design and performance of a current sensing noise thermometer using a low Tc DC SQUID as the front end amplifier. The DC SQUID is used to measure the thermal noise current in a resistor and the temperature is then obtained from the Nyquist formula. The thermometer is fast, absolute and precise and is usable over a wide temperature range below 4.2 K, in principle down to well below 1 mK. The excellent energy sensitivity of the DC SQUID, operated at fixed temperature, enables the use of a relatively large noise resistor, in the mΩ range. This requires relatively short averaging times when measuring the spectrum of noise fluctuations. We have shown that it is possible to determine absolute temperature with a precision of 1% in a measuring time of 10 seconds with an amplifier noise temperature, TN, of the order of 30 µK, and to an accuracy better than 0.3%. The percentage precision is independent of temperature for temperatures much greater than TN. Our method of heat sinking the noise resistor ensures proper cooling of the electrons. We incorporate a fixed point device for checking the gain calibration. We have cooled the thermometer successfully to below 1 mK, achieving a minimum electron temperature of 300 µK. We present the results of a preliminary comparison with a 3He melting curve thermometer (MCT) above 4.5 mK, and with a platinum NMR thermometer down to the lowest temperatures.

Current-sensing noise thermometry from 4.2 K to below 1 mK using a DC SQUID preamplifier

Abstract We are using a low-Tc DC SQUID to perform current-sensing noise thermometry, by measuring the thermal noise currents in a copper resistor. The temperature is obtained from the Nyquist formula. This is a practical thermometer for use from 4.2 K to below 1 mK , with a percentage precision independent of temperature. Using a 0.34 mΩ resistor, the thermometer had an amplifier noise temperature TN of 8 μK . A precision of 1.5% was obtained in 200 s . The thermometer was in good agreement with the PLTS-2000 3 He melting curve scale down to 4.5 mK . We have cooled the thermometer successfully below 1 mK , achieving a minimum electron temperature of 300 μK .

DC SQUID Spectrometers for NMR

We report experiments in which we have used DC SQUIDs in pulsed NMR spectrometers to observe directly the free precession of nuclear spins. A broadband spectrometer, which operates in flux-locked loop mode with a bandwidth of 3.4 MHz using a SQUID with additional positive feedback and an untuned superconducting input circuit, has been used to observe NMR signals from platinum powder at frequencies of 38, 65, 85, 240 and 513 kHz. The performance is compared with a second system in which a DC SQUID is operated open loop as a small signal rf amplifier with a series tank input circuit tuned at 1 MHz.

Low field DC SQUID nuclear magnetic resonance on single crystal UPt3

Abstract A SQUID spectrometer is being used to study high-quality single crystals of UPt 3 in low magnetic fields by performing pulsed NMR on 195 Pt. The system uses a multiloop DC SQUID with additional positive feedback and operates in flux-locked loop mode from DC to 3 MHz. It has an overall coupled energy sensitivity of 800 h and a dead time of ∼5 μs. NMR signals from UPt 3 have been observed in both the superconducting mixed state and in the normal state. A bulk platinum marker is used to determine the magnetic field. Measurements of 195 Pt Knight shifts in UPt 3 are reported.

Low-frequency broadband NMR on UPt3 using DC SQUIDs

Abstract An NMR spectrometer has been constructed based on a DC SQUID with additional positive feedback, operating in flux locked loop mode. The system has an overall bandwidth of 3MHz and a coupled energy sensitivity of 800 h . The spectrometer is being used to study high-quality single-crystal samples of UPt 3 , in order to investigate the superconducting order parameter. NMR signals have been observed in the superconducting mixed state at significantly lower frequencies than hitherto, down to 100 kHz.

Broadband DC SQUID NMR Spectrometry on Bulk Metals

We report on experiments in which we have used a DC SQUID with additional positive feedback (APF) in a broadband pulsed NMR spectrometer, of bandwidth 3.4 MHz, to observe directly the free precession of nuclear spins in bulk metallic samples. The sample is located inside a superconducting receiver coil, which forms a flux transformer with the input coil of the SQUID. The broadband response of the flux transformer, combined with the relatively large bandwidth of the APF SQUID when operated in flux-locked-loop mode, results in a short recovery time. This enables the study of solid-state samples with a short spin-spin relaxation time at frequencies below 1MHz, appropriate at millikelvin temperatures. A further advantage of this scheme is that the NMR frequency can be varied without changes in the detection electronics being necessary. The sensitivity of the DC SQUID detection system, compared with conventional NMR, allows the study of small single crystal samples. The decay of eddy currents following the transmitter pulse is governed by sample size, geometry and resistivity. Typical decay times of less than 10 us are expected on 0.5 mm diameter cylindrical samples. We discuss progress on experiments with aluminium wires at a temperature of 20 mK.

NMR on Systems of Low Spin Density Using DC Squids

We discuss the use of SQUID NMR for the study of systems of low spin density. The sample is located inside a coil which forms part of a series tuned resonant circuit attached to the input coil of a SQUID. Such a scheme was first discussed by Freeman et al.1We have studied the pulsed NMR response at 1 MHz of a3He film adsorbed on the surface of closely packed Mylar sheets. In this case a monolayer corresponds to a spin density a factor of order 104 smaller than that of bulk liquid and a factor of order 102smaller than with a Grafoil substrate. For our particular SQUID and input coil the calculated noise temperature is 60 mK, significantly better than that so far achieved with a cooled semiconductor preamplifier. We evaluate the present and potential performance of the spectrometer, some of the practicalities involved in its implementation, and discuss the minimum number of detectable spins.

European Dissemination of the Ultra-low Temperature Scale, PLTS-2000

The first phase of the EU collaborative project on sub‐kelvin thermometry, ‘ULT Dissemination’, is nearing completion, leading to the development of several thermometers and devices, and the instrumentation needed to disseminate the new Provisional Low Temperature Scale, PLTS‐2000, to users. Principal among these are a current‐sensing noise thermometer (CSNT), a CMN thermometer adapted for industrial use, a Coulomb blockade thermometer, a second‐sound acoustic thermometer and a superconductive reference device SRD‐1000. Several partners have set up 3He melting‐pressure thermometers to realise the PLTS‐2000, and will check it using Pt‐NMR, CMN and other thermometers. The scale, which was formally adopted by the Comite International des Poids et Mesures in October 2000, covers the range of temperature from 1 K down to 0.9 mK, and is defined by an equation for the melting pressure of 3He. The SRD employs novel fabrication and detection techniques with up to 10 samples, and is expected to meet the requirement...

EU dissemination of the provisional ultra-low-temperature scale, PLTS-2000

Following the introduction of the provisional low-temperature scale from 0.9 mK to 1K, PLTS-2000, there is a need for primary and secondary thermometers and fixed points, which can disseminate the scale to users. This paper reports on the progress, within the EU collaborative project ‘ULT Dissemination’, in the development and evaluation of several devices with associated instrumentation. Principal among them are a current-sensing noise thermometer, a CMN thermometer adapted for industrial use, a Coulomb blockade thermometer, a second-sound thermometer, a 3He melting pressure thermometer for a direct realisation of the PLTS-2000. A superconductive reference device has also been developed, as a replacement for the NBS SRM-768 which is no longer available.

A tuned NMR spectrometer using a DC SQUID for systems of low spin density

Abstract We are using a low Tc DC SQUID as the first-stage amplifier in an NMR spectrometer designed for the study of systems of low spin density. The NMR pickup coil forms part of a series tuned tank circuit attached to the input coil of the SQUID. The tank circuit resonant frequency is close to 1 MHz. The SQUID amplifier uses additional positive feedback (APF) and operates in flux-locked loop mode with a bandwidth of 1.6 MHz. Preliminary measurements on 3He samples are reported.