Low-noise Dc SQUID Research Articles

A new limit of the 129Xenon Electric Dipole Moment

We report on the first preliminary result of our 129Xe EDM measurement performed by the MIXed collaboration. The aim of this report is to demonstrate the feasibility of a new method to set limits on nuclear EDMs by investigating the EDM of the diamagnetic 129Xe atoms. In our setup, hyperpolarized 3He serves as a comagnetometer needed to suppress magnetic field fluctuations. The free induction decay of the two polarized spin species is directly measured by low noise DC SQUIDs, and the weighted phase difference extracted from these measurements is used to determine a preliminary upper limit on the 129Xe EDM.

Development of cryogenic alpha spectrometers using metallic magnetic calorimeters

Abstract Cryogenic particle detectors have recently been adopted in radiation detection and measurement because of their high energy resolution. Many of these detectors have demonstrated energy resolutions better than the theoretical limit of semiconductor detectors. We report the development of a micro-fabricated magnetic calorimeter coupled to a large-area particle absorber. It is based on a planar, 1 mm2 large paramagnetic temperature sensor made of sputtered Au:Er, which covers a superconducting meander-shaped pickup coil coupled to a low-noise dc-SQUID to monitor the magnetization of the sensor. A piece of gold foil of 2.5×2.5×0.07 mm3 was glued to the Au:Er film to serve as an absorber for incident alpha particles. The detector performance was investigated with an 241Am source. The signal size comparison for alpha and gamma peaks with a large difference in energy demonstrated that the detector had good linear behavior. An energy resolution of 2.83±0.05 keV in FWHM was obtained for 5.5 MeV alpha particles.

Performance of Micro-fabricated Magnetic Calorimeters Arrays for X-Ray Spectroscopy

We have been developing array technology for fabricating magnetic calorimeters for X-ray astronomy. The magnetization change in each pixel of the paramagnetic sensor material due to the heat input of an absorbed X-ray is sensed by a meander shaped coil. With this geometry it is possible to obtain excellent energy sensitivity, low magnetic cross-talk and large format arrays fabricated on wafers that are separate from the SQUID read-out. A magnetic bias field for each pixel is generated by the use of a persistent current that is stored. We report on the results from our prototype arrays, which are coupled to low noise DC-SQUIDs. The first test results are presented and the sensitivity is compared with calculations.

Low-noise S-band DC SQUID based amplifier

A low-noise rf amplifier based on a dc SQUID (SQA) is tested in the frequency range 3.3-4.1 GHz. A new signal launching system for the SQA rf coupling has been developed and successfully implemented. The following parameters have been measured at 3.65 GHz using a band-pass filter at the input of a single-stage SQA: gain (11.0/spl plusmn/1.0) dB, 3 dB bandwidth of 300 MHz and noise temperature (4.0/spl plusmn/1.0)K. This figure corresponds to a flux noise S/sub /spl Phi///sup 1/2 /=0.6 /spl mu//spl Phi//sub 0//Hz/sup 1/2 / and an energy sensitivity /spl epsi//sub i//spl ap/75 h. The input saturation power, P/sub s/, (1 dB gain compression) is measured for different bandwidths of the input band-pass filter, A corresponding input signal saturation temperature (normalized for a 1 GHz bandwidth) T/sub SAT//sup 1GHZ/=P/sub SAT//k/sub B/ is estimated to be 11.5 K GHz at an SQA bias voltage 27 /spl mu/V (condition for minimum noise temperature). The dependencies of the SQA gain, noise temperature and saturation level on the operation point are studied. The reason for the SQA saturation is discussed.

SQUID photoscanning: an imaging technique for NDE of semiconductor wafers and devices based on photomagnetic detection

The SQUID Photoscanning technique enables the noninvasive evaluation of semiconductor wafers and photovoltaic devices. The basic idea of the method is to detect photogenerated currents via their magnetic field by means of sensitive SQUID magnetometers. A magnetic imaging with high spatial resolution is performed by scanning the sample with a focused laser beam and synchronously measuring the magnetic field of the net photocurrents. Objects of analysis are semiconductor wafers with doping level fluctuations or electrically active defects, such as grain boundaries. Furthermore, the SQUID Photoscanning allows for the localization of artefacts in photovoltaic devices. The system uses sensitive, low-noise dc-SQUID magnetometers operated in a flux-locked loop (FLL) at 4.2 K or 77 K, respectively. The FLL electronics is adapted to the operation of the SQUID Photoscanning system in the presence of large, low-frequency interferences by implementing a frequency dependent feedback range. A digital signal processor (DSP) based control and data acquisition unit controls the amplitude modulation of the laser illumination, the xy-motion of the sample and the phase sensitive detection of the SQUID signal. The SQUID Photoscanning signal strengths obtained from the samples under investigation cover a range of about 100 fT for slight doping inhomogeneities in high purity silicon wafers up to several nT for photocurrent distributions in solar cells. The results of numerical simulations of SQUID Photoscanning signals are qualitatively and quantitatively in fair agreement with the experimental findings.

Characterization of the Input Noise Sources of a dc SQUID

From the theory of the intrinsic noise in a dc SQUID by Tesche and Clarke, we derive the expressions of the current and voltage input noise spectral densities in a dc SQUID current amplifier operated in a flux locked mode. The expected current and voltage noises are compared, at audio frequencies, with the experimental results obtained with a low noise dc SQUID in which the input load (resonant and not) and the operating temperature (1–4 K) are changed. In order to evaluate the input voltage noise, which is directly related to the current noise around the SQUID loop and is usually neglected, we have used as the input circuit a LC resonator with a very high quality factor (≈106). Both the voltage and current input noises exceed the expected values by the same factor of about 8. This means that the modulus the optimum source impedance of the SQUID amplifier is still in agreement with the value expected from the theory, which is approximately given by the product of the input coil inductance and the angular frequency. To explain the excess noise results, we propose a model in which the voltage and current input noises are due to a thermal magnetic noise source which is present near the SQUID.

Sub-kelvin current amplifier using a DC-SQUID

We have set up a system where a low-noise DC-SQUID is used as a current amplifier. The SQUID output is read using a wide band electronics unit based on the noise cancellation scheme. The SQUID has been installed in a compact Nanoway PDR50 dilution refrigerator, and superconducting transitions of Ti/Au thermometer strips for X-ray calorimeter applications have been measured. We can operate at 100 mK using a SQUID with Pd shunt resistors. Noise and bandwidth results of the setup are presented.

Back action of a DC SQUID current amplifier

We present temperature measurements of the back action of a low noise DC SQUID on a strongly coupled electrical resonator operating at 1480 Hz with quality factors up to Q=1.2×10 6. The characteristics of the voltage noise of the SQUID current amplifier are discussed and compared with the predictions.

Low-Tc magnetometer used for active shielding in the frequency range dc-500 Hz in biomagnetic measurements

Ultra low-noise dc SQUIDS have been used to detect disturbances in biomagnetic measurements and to feed an active shielding system that compensates these disturbances. This work describes some properties of the SQUIDs used and the principle of active shielding that has been improved for frequencies from dc to 500 Hz. So far an additional attenuation of up to 50 dB for near-dc and almost 20 dB for 50 Hz magnetic disturbances has been achieved.

High-resolution magnetoneurography

Abstract Low noise dc-SQUID systems allow noninvasive measurements of magnetic fields generated by electric currents in human peripheral nerves. High-resolution magnetoneurography techniques were used to detect the magnetic fields of stimulated leg nerves with amplitudes of only 10–20 fT over the lower spine. A signal-to-noise ratio of about 10 was achieved after a special designed signal processing routine which was sufficient for an analysis of the source current, i.e. in particular the estimation of its location, strength, and spatial extent.

Fabrication of step-edge Josephson junctions for low-noise dc-SQUIDs

We report the development of a fabrication process of high-resistance step-edge Josephson junctions on sharp argon ion beam etched substrate steps. For a reproducible fabrication, the redeposition of material at the step during the etch process has to be avoided. This is achieved by an alternating angle of ion incidence parallel to the edge of the photoresist mask. The reproducible straight step profile exhibits an angle around and is reflected in reproducible electrical properties of the junctions. Critical current and asymptotic normal state resistance both scale with the junction width w. The current-voltage (I-V) characteristics depend on the ratio of film thickness t to step height h. For > 0.85 they are flux-flow-like; for < 0.8 they correspond to the resistively shunted junction (RSJ) model. Lower ratios of correspond to higher specific resistances resulting at 77 K typically in asymptotic normal state resistances between 5 and per junction, depending on w. At 77 K we achieve products of more than . The junctions are applied to dc superconducting quantum interference devices (dc-SQUIDs) with inductances between 25 and 35 pH. At 77 K transfer functions of more than are achieved. noise of the step-edge dc-SQUIDs is observed, but using bias current modulation it is entirely suppressed resulting in a spectral density of flux noise of measured at 1 Hz and 77 K for a SQUID with an inductance of 30 pH.

Use of low-noise dc-SQUIDs in fundamental physics experiments

The noise performance of dc-SQUIDs (Superconducting Quantum Interference Devices), which nowadays can reach values close to the quantum limit, makes them very attractive as amplifiers for experiments using low temperatures and demanding for extremely high sensitivity. After reviewing the working principle of the dc-SQUID, we describe a thin-film device with a new structure, which allowed us to reach an energy sensitivity of 5.5 ∞ at 0.9 K and a good coupling with a large input coil. At last we discuss some fundamental physics experiments which take advantage from the SQUID use.

SQUID-susceptometry up to 10Tesla: An improved method for magnetization studies on a two-dimensional electron system

We report on a high-sensitive SQUID-susceptometer for magnetization studies on two-dimensional electron systems (2DES). It consists of a low-noise thin-film dc SQUID and a wire-wound first-order gradiometer and is operated inside a superconducting 10 T-magnet. At zero magnetic field the system's flux noise level is frequency-independent and only 40×10−6Φ0/√Hz, which is almost the noise level of the bare SQUID. In magnetic fields up to 10 T the sensitivity can be better than 10−13 J/T at frequencies above 1 kHz. With this setup, we have studied the de Haas–van Alphen effect on a gated single layer 2DES revealing spin splitting and the enhanced Lande factor.

DC SQUID series arrays with intracoil damping to reduce resonance distortions

Abstract We report on low-noise DC SQUID series arrays incorporating intracoil damping, which show smooth DC characteristics. The voltage-flux characteristics of these devices are reproducible upon repeated cooling and do not require multiple heating/cooling cycles to maximize peak-to-peak output voltage modulation depth. The devices consist of 100 DC SQUIDs with individual signal and feedback coils connected in series. The total input inductance is 600 nH, and the peak-to-peak output modulation is 4 mV with a transfer function of 350 V/A. The bandwidth is 5 MHz and the equivalent input current noise is approximately 2.5 pA/√ Hz . With these characteristics, these devices are useful as preamplifiers for microcalorimeter X-ray detectors.

AC resistance bridge based on the cryogenic current comparator

This paper describes a low-frequency AC resistance bridge based on a Cryogenic Current Comparator (CCC). A low-noise dc SQUID is used as a null detector. The bridge is the main tool to trace resistance to 10 k/spl Omega/, 100 /spl Omega/ and 1 /spl Omega/ levels from the quantum Hall resistance (QHR). The ratio of the resistors ranging from 1 /spl Omega/ to 10 k/spl Omega/ can be measured with an uncertainty of the order of 1 ppb. The bridge is typically operated with a PC generating a sinusoidal ac signal ranging from 0.1 Hz-2 Hz. Using a vector analyzer as a signal source the bridge is used up to 1 kHz. Both sinusoidal and pseudorandom signals have been used to verify the frequency dependence of the standard resistors against the QHR. At present, the bridge can be used to perform accurate calibrations only up to 10 Hz.

Highly sensitive YBa2Cu3O7 dc SQUID magnetometer with thin-film flux transformer

We have designed and fabricated a thin-film flux transformer by a YBa2Cu3O7/SrTiO3 (YBCO/STO) multilayer process. The flux transformer consists of a 20-turn input coil of 10 μm linewidth and a single-turn pickup loop surrounding an area of 7.5 by 7.5 mm2. All device levels are patterned by standard photolithography and Ar-ion-beam etching. The flux transformer has been combined in flip-chip configuration with the square washer of a low-noise YBCO dc SQUID. The dc SQUID magnetometer exhibits a magnetic field sensitivity of 1.4 nT/Φ0. The intrinsic white flux noise level of 70μΦ0/√Hz at 77 K is mainly dominated by the SQUID corresponding to a magnetic field resolution of the magnetometer of 100 fT/√Hz for frequencies above 40 Hz. At 1 Hz we measured 200 fT/√Hz. Biomagnetic measurements were performed in a magnetically shielded chamber.

11 channel magnetometer for biomagnetic measurements in unshielded environments

The authors describe a novel 11-channel magnetometer designed for operation in unshielded environments. The planar geometry of the system is particularly suitable for recording biomagnetic signals over a flat surface, as required for cardiac and gastrointestinal applications. The specific configuration of the sensor array also makes it possible to use the system for reliable and fast cardiac clinical measurements. Ambient noise rejection was achieved by the use of second-order gradiometers, optimized by means of additional fine balancing, whereas a sensitivity of less than 10 fT/Hz/sup 1/2/ was obtained by low-noise DC-SQUIDs (superconducting quantum interference devices), using Nb/Nb/sub 2/O/sub 5//PbAuIn technology for the Josephson junctions. The system is presently utilized for a systematic investigation of the localization properties of cardiac sources from data obtained during single-shot measurements, which are believed to be suitable for clinical studies. The results of localization are compared with anatomical information from magnetic resonance imaging data. >

An ultra low noise DC SQUID system for biomagnetic research

A ultra low noise DC SQUID magnetometer designed and constructed for high resolution magnetocardiographic measurements is presented. Details of the design considerations and the construction of the single-channel system are given, focusing in particular on the matching of the DC SQUID with the gradiometer and the room temperature electronics. Using a first-order gradiometer as antenna for biomagnetic fields, a white noise level of 2 fT Hz-1/2 has been attained with a 1/f corner frequency at 3 Hz. The intrinsic magnetic flux noise of the SQUID system of 2*10-6 Phi 0 Hz-1/2 corresponds to a noise limited sensitivity of 700 aT Hz-1/2.

Magnetic noise near the superconducting transition in bulk and thick film YBCO

We have used a conventional low noise dc SQUID to measure the magnetic noise in bulk YBCO ring and a thick film YBCO tube over the temperature range 4–100K and in very low magnetic fields. We observe noise levels which are higher than those found by other workers using epitaxial thin films. The noise in the bulk sample close to the transition exhibits two very closely spaced peaks and a spectral density which differs substantially from 1/f n. By cutting a thin slit in the sample, we have identified one of the peaks with the onset of connectivity around the sample.

Nb/AlOx/Nb trilayer process for the fabrication of submicron Josephson junctions and low-noise dc SQUIDs

A trilayer deposition process for making high quality all-refractory Nb/AlOx/Nb Josephson junctions is described. The sputtering parameters were optimized to deposit very uniform Al films, less than 5 nm in thickness. We found that it was essential to keep the substrate temperature below 40 °C during Al deposition but not during Nb depositions. Rapid anodization of the Nb counterelectrode allowed us to use positive photoresist as an anodization mask even for submicron geometries. The dc superconducting quantum interference devices made with this process have the lowest intrinsic energy sensitivity reported to date for an all-refractory technology.