R.P. Reed

Development of miniature DC SQUID devices for the detection of single atomic spin-flips

We report progress toward a superconducting quantum interference device (SQUID)-based system capable of detecting a few atomic spin-flips. The scaling of the flux sensitivity with SQUID loop dimension of miniature niobium dc SQUID devices is examined and shown experimentally to vary as predicted. Our smallest device, with loop size 3 /spl mu/m/spl times/3 /spl mu/m, is capable of detecting 40 spins in a 1-Hz bandwidth. We address the task of depositing a sample, of nanoscale dimension, within the SQUID loop.

YBCO-noble metal resistors for HTS Josephson Noise Thermometry

Noise thermometry based on HTS Josephson devices has the potential to provide an absolute technique for the realisation of the temperature scale in the range 4 K-50 K. A resistor connected as a shunt across a YBCO grain boundary junction forms an HTS Josephson Noise Thermometer. We present experimental results and mathematical models for the resistivity of a noble metal film which connects two strips of superconducting YBCO. Accurate temperature measurement requires the resistor to be no more than 1/1000th of the intrinsic junction resistance. This sets an upper limit of 1 m /spl Omega/ for the value of the shunt resistor, including any contact resistance which exists between the metal and YBCO films. Essential aims are firstly to determine the lowest achievable contact resistivity, and secondly to demonstrate a very small variation with current of the contact resistance. A one dimensional model of the current transfer from superconducting to normal films has been used to define the necessary overlap length and also to investigate the possible influence of current dependent proximity effects at the interface. The model is compared with experimental behaviour of two designs of shunt resistance, one a simple rectangular patch configuration and the other a meander line.

The quantum roulette noise thermometer

A novel form of primary thermometer called the quantum roulette noise thermometer (QRNT), based on flux quantization in a high temperature superconducting (HTS) ring, interrupted by a Josephson junction, has been proposed. The basis of the QRNT is that the flux state of the ring is allowed to evolve freely, and is repeatedly interrogated to establish a probability distribution from which the temperature of the device is derived. Unlike other superconductivity-based noise thermometers, this device is essentially dissipationless, requires no input or output connections, and may be remotely interrogated. The QRNT has the potential to provide high accuracy with short measuring times. Prototype QRNT devices based on thin HTS films have been fabricated and results of initial experimental measurements are presented. Parameters of the experimental devices are compared with predictions of the theoretical model.

Small area, low-inductance niobium SQUID for the detection of single atomic spin-flips

Abstract We report progress towards a superconducting quantum interference device (SQUID) based system capable of detecting single atomic spin-flips. To date we have designed and had manufactured a niobium SQUID with a loop area of 3×10 −6 m by 3×10 −6 m. We present calculations which show that it is capable of detecting a few spins in a 1 Hz bandwidth, and experimental results are given which show that it operates close to the quantum-limit regime with the expected sensitivity. The SQUID is designed so that it will fit into a sample holder that will be loaded into a low-temperature ultra-high vacuum scanning tunnelling microscope (LT UHV STM). The STM will be used to image and manipulate trapped ad-atoms within the SQUID loop.

QRNT: a primary thermometer based on an HTS SQUID

Abstract We propose a novel form of cryogenic primary thermometer based on flux quantisation in a HTS superconducting ring, interrupted by a Josephson junction which we call the Quantum Roulette Noise Thermometer (QRNT). Unlike other superconductivity-based noise thermometers this device is essentially dissipationless, requires no input or output connections and may be remotely interrogated. The QRNT has the potential to provide high accuracy with short measuring times.

HTS SQUID application as a quantum roulette noise thermometer

We describe a cryogenic primary thermometer called the Quantum Roulette Noise Thermometer (QRNT), which is based on flux quantisation in a HTS superconducting ring. The basis of the QRNT is that the flux state of a double junction SQUID ring (in thermal contact with a heat bath whose temperature is to be measured) is allowed to evolve freely. The flux state is repeatedly interrogated to establish a probability distribution from which the absolute temperature may be derived. The QRNT is in principle dissipationless and has the potential to provide high accuracy with short measuring time. The QRNT prototype device which employs a YBCO thin film on an MgO bi-crystal substrate has been designed and fabricated. The superconducting ring of overall dimensions 6 mm/spl times/8 mm is broken by two grain boundary Josephson junctions of size 5 /spl mu/m/spl times/3 /spl mu/m. The state of the junctions is switched between superconducting and resistive by means of small currents applied to a superconducting strip line located near the junctions. A dc SQUID, which is used to read out the flux state of the QRNT ring, is operated at 77 K while the QRNT device can be set at any desired temperature above 10 K. Ongoing experimental measurements are described with reference to the expected theoretical predictions.

Shunted YBCO bi-crystal junctions for noise thermometry

Abstract We present experimental results on YBCO thin film bi-crystal junctions in which additional metallic shunt resistors are added by in-situ evaporation of gold or silver. These junctions are intended for use in the development of an HTS Josephson noise thermometer for primary temperature scale use. The temperature dependence of the critical current and effective shunt resistance are reported and the noise arising from the shunted junctions when biased into the finite voltage regime has been assessed as a function of frequency. This is compared with the predictions of a simple model and the implications for the potential accuracy of the proposed thermometer are considered.

Miniature DC SQUID devices for the detection of single atomic spin-flips: scaling of flux sensitivity with device dimension

We report progress towards a SQUID based system capable of detecting single atomic spin-flips. The scaling of the flux sensitivity with SQUID loop dimension of miniature niobium DC SQUID devices is examined and shown experimentally to vary as predicted. Our smallest device, with loop size 3 /spl mu/m/spl times/3 /spl mu/m, is capable of detecting a few spins in a 1 Hz bandwidth.

Properties of normal metal shunted YBCO bi-crystal junctions

Abstract We present experimental results on YBCO thin film bi-crystal junctions in which additional metallic shunt resistors are added by in-situ thin film evaporation of gold or silver. The current-voltage characteristics have been measured, as well as the broad band noise in the system between 1 Hz and 100 kHz, using a HTS dc SQUID as a low-noise preamplifier. The influence of the shunt resistors on the excess noise contributions is discussed and the measured shunt resistance results are compared with a resistive network model.