Ferroelectric Exchange Bias Affects Interfacial Electronic States

In polar oxide interfaces phenomena such as superconductivity, magnetism, 1D conductivity, and quantum Hall states can emerge at the polar discontinuity. Combining controllable ferroelectricity at such interfaces can affect the superconducting properties and sheds light on the mutual effects between the polar oxide and the ferroelectric oxide. Here, the interface between the polar oxide LaAlO₃ and the ferroelectric Ca-doped SrTiO₃ is studied by means of electrical transport combined with local imaging of the current flow with the use of scanning a superconducting quantum interference device (SQUID). Anomalous behavior of the interface resistivity is observed at low temperatures. The scanning SQUID maps of the current flow suggest that this behavior originates from an intrinsic bias induced by the polar LaAlO₃ layer. Such intrinsic bias combined with ferroelectricity can constrain the possible structural domain tiling near the interface. The use of this intrinsic bias is recommended as a method of controlling and tuning the initial state of ferroelectric materials by the design of the polar structure. The hysteretic dependence of the normal and the superconducting state properties on gate voltage can be utilized in multifaceted controllable memory devices.     

Stabilization Time of Josephson Tunnel Junctions

The stabilization time is from the initial state to the steady state. The resistive-capacitive-inductive shunted junction (RCLSJ) model was used to study the nonlinear dynamical behavior of Josephson tunnel junction (JJ). The stabilization time of JJs is calculated by the information of voltage waveform. With the certain irradiation, four-dimensional image of stabilization time of JJs is calculated in the three-dimensional parameter space composed of the junction capacitance C, shunt inductance L, and the bias current I. The average of stabilization time under different conditions was applied to thr RCLSJ model and the superconducting quantum interference device (SQUID) model. In this paper, the average of stabilization time at different initial conditions, irradiation frequencies, and intensity was simulated. The average of stabilization time was nearly unvaried with initial value of normalized voltage and increased with the intensity of irradiation. The average of stabilization time fluctuates with the frequency of irradiation. Similar results are obtained in SQUID model. The average of stabilization time in SQUID model is less than that in JJ.     

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.     

Effects on DC SQUID characteristics of damping of input coil resonances

The possibility of improving dc SQUID performance by damping the input circuit resonances caused by parasitic capacitances is studied experimentally. A high-quality dc SQUID was coupled to a first-order axial gradiometer built for neuromagnetic research, and a resistor-capacitor shunt was connected in parallel with the input coil of the SQUID. Ten differentRC shunts were studied with the SQUID operating in a flux-locked loop, carefully shielded against external disturbances. It was found that increasing the shunt resistance resulted in smoother flux-voltage characteristics and smaller noise. At best, the minimum obtainable equivalent flux noise level was one-fourth that for the unshunted SQUID. The noise level is a function of the shunt resistanceR _s only, except for shunt capacitance values bringing the low-frequency resonance of the input coil close to the flux modulation frequency. At a constant bias current level, where the amplitude of the flux-voltage characteristics is at maximum, the equivalent flux noise varies asR _s ^/–0.7 . The results agree reasonably well with recently published predictions based on numerical simulations where the whole input circuit with parasitic capacitances was taken into account.     

The effect of the resonant mode structure on flux tunneling in SQUIDs

The problem of macroscopic quantum tunneling in SQUIDs is discussed, taking into account the resonant mode structure of typical devices. These are evaluated for the particular case of a SQUID formed from a conical point intersecting a hemispherical cavity, and it is shown that the conventional representation of a SQUID as a Josephson junction in parallel with an inductor and a capacitor is a good first approximation in most cases, provided the inductance and capacitance used are those of the whole device rather than of the weak link alone. The discussion is extended to another case of practical importance, where such a cavity is connected to an outer hole by a flange, and it is found that if the capacitance of the flange is large, the tunneling behavior is largely independent of the presence of the outer hole, apart from the effects of any dc bias.     

An integrated superconductive magnetic nanosensor for high-sensitivity nanoscale applications

An integrated magnetic nanosensor based on a niobium dc SQUID (superconducting quantum interference device) for nanoscale applications is presented. The sensor, having a washer shape with a hole of 200?nm and two Josephson-Dayem nanobridges of 80?nm × 100?nm, consists of a Nb(30?nm)/Al(30?nm) bilayer patterned by electron beam lithography (EBL) and shaped by lift-off and reactive ion etch (RIE) processes. The presence of the niobium coils, integrated on-chip and tightly coupled to the SQUID, allows us to easily excite the sensor in order to get the voltage-flux characteristics and to flux bias the SQUID at its optimal point. The measurements were performed at liquid helium temperature. A voltage swing of 75?μV and a maximum voltage-flux transfer coefficient (responsivity) as high as 1?mV/Φ(0) were directly measured from the voltage-flux characteristic. The noise measurements were performed in open loop mode, biasing the SQUID with a dc magnetic flux at its maximum responsivity point and using direct-coupled low-noise readout electronics. A white magnetic flux noise spectral density as low as 2.5?μΦ(0)?Hz(-1/2) was achieved, corresponding to a magnetization or spin sensitivity in units of the Bohr magneton of 100?spin?Hz(-1/2). Possible applications of this nanosensor can be envisaged in magnetic detection of nanoparticles and small clusters of atoms and molecules, in the measurement of nanoobject magnetization, and in quantum computing.     

dc SQUID: Current noise

The computer model used by Tesche and Clarke to calculate the voltage noise across the dc SQUID is extended to calculate the circulating current noise around the SQUID loop, and the correlation between the circulating current noise and the voltage noise across the SQUID. The parameters chosen are % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4baFfea0dXde9vqpa0lb9% cq0dXdb9IqFHe9FjuP0-iq0dXdbba9pe0lb9hs0dXda91qaq-xfr-x% fj-hmeGabaqaciGacaGaaeqabaWaaeaaeaaakeaacqaHYoGycqGH9a% qpcaaIYaacbaGaa8htaiaa-LeadaWgaaWcbaacbiGaa4hmaaqabaGc% caGGVaacciGae0NPdy0aaSbaaSqaaiab9bcaGiab9bdaWaqabaGccq% GH9aqpcaaIXaGaaiilaiaabccacqqFtoWrcqqF9aqpcqqFYaGmcqaH% apaCcaWGRbWaaSbaaSqaaiaadkeaaeqaaOGaa8hvaiaa-9cacaWFjb% WaaSbaaSqaaiaa+bcacaGFWaaabeaakiab9z6agnaaBaaaleaacqqF% GaaicqqFWaamaeqaaOGaeyypa0JaaeiiaiaabcdacaqGUaGaaeimai% aabwdaaaa!51F9!\[\beta = 2LI_0 /\Phi _{ 0} = 1,{\rm{ }}\Gamma = 2\pi k_B T/I_{ 0} \Phi _{ 0} = {\rm{ 0}}{\rm{.05}}\], and an applied flux of ₀/4 (L is the SQUID inductance, I ₀ is the critical current per junction, T is the temperature, and ₀ is the flux quantum). At frequencies well below the Josephson frequency and at the optimum current bias, the voltage power spectral density is approximately 16 kBTR, the current power spectral density is approximately 11k_B T/R and the voltage-current correlation spectral density is approximately 12k _B T, where R is the resistance per junction.This work was supported by the Division of Materials Sciences, Office of Basic Energy Sciences, U.S. Department of Energy.Guggenheim Fellow.     

Research on Intelligent Control System of DC SQUID Magnetometer Parameters for Multi-channel System

In a multi-channel SQUID measurement system, adjusting device parameters to optimal condition for all channels is time-consuming. In this paper, an intelligent control system is presented to determine the optimal working point of devices which is automatic and more efficient comparing to the manual one. An optimal working point searching algorithm is introduced as the core component of the control system. In this algorithm, the bias voltage \(V_\mathrm{bias}\) is step scanned to obtain the maximal value of the peak-to-peak current value \(I_\mathrm{pp}\) of the SQUID magnetometer modulation curve. We choose this point as the optimal one. Using the above control system, more than 30 weakly damped SQUID magnetometers with area of \(5 \times 5 \, \hbox {mm}^2\) or \(10 \times 10 \, \hbox {mm}^2\) are adjusted and a 36-channel magnetocardiography system perfectly worked in a magnetically shielded room. The average white flux noise is \(15 \, {\upmu \Phi }_0/\hbox {Hz}^{1/2}\).     

Recent Results of a New Microwave SQUID Multiplexer

We present recent results of a prototype microwave SQUID multiplexer containing four SQUIDs coupled to GHz frequency resonant circuits and fed with a single microwave readout line. The system is operating at a readout frequency range of 8–10 GHz. All four SQUIDs share a common DC bias and modulation lines. A new modulation scheme is tested to eliminate the need for individual flux biasing of the SQUIDs, which extends the dynamic range of the readout. In this scheme a common modulation signal is imposed on each SQUID and the received signal is demodulated at one and two times the modulation frequency to maintain sensitivity at any flux state. We also demonstrated a microwave RF bias scheme eliminating the necessity of the DC current bias to the SQUID. Our preliminary performance tests at 4.2 K show that the input noise of the device is ∼5 pA/ .      

An on-chip superconducting clock with two modes

A superconducting clock based on a 2-junction SQUID (superconducting quantum interference device) flip-flop with feedback is presented. The feedback consists of transmission lines that emanate from and return to the SQUID; the entire clock is built as a compact integrated circuit. The period of the clock is mainly determined by the length of transmission line, and there are two modes of operation that can be separately excited whose periods are in a ratio of 2:1. I-V data for a pair of clocks with different designed periods confirm the presence of the two modes, show how period depends on length, and give information on the switching time of the SQUID flip-flops. Specifically, the I-V data show that there exist both a fundamental model for flux bias at odd multiples of Φ ₀/2 (half-flux quantum) and a doubled mode with precisely half the period flux bias at even multiples. Clocks with longer transmission lines have longer periods, but simple scaling does not occur due to other sources of time delay     

Current noise measured in the dc SQUID

Measurements are presented of the current noise in a dc superconducting quantum interference device (SQUID) and of its correlation with the voltage noise across the SQUID. The measured spectral densities are in good agreement with analog simulations. When a resonant circuit is connected across the output of the SQUID, there is a reduction in the current noise for frequencies at which the impedance of the circuit is low. This effect is also in good agreement with predictions.     

Performance of TES X-ray Microcalorimeters with AC Bias Read-Out at MHz Frequencies

At SRON we are developing Frequency Domain Multiplexing for the read-out of superconducting transition edge sensor microcalorimeters for future X-ray astrophysical missions. We will report on the performance of Goddard Space Flight Center pixels under AC bias in the MHz frequency range. Superconducting flux transformers are used to improve the impedance matching between the low ohmic TESs and the SQUID. We connected 5 pixels to the LC filters with resonant frequencies ranging between 1 and 5 MHz. For X-ray photons of 6 keV we measured a best X-ray energy resolution of 3.6 eV at 1.4 MHz, consistent with the integrated Noise Equivalent Power. In addition, we improved the electrical circuit by optimizing the coupling ratio of the impedance matching transformer. In addition, we improved electrical circuit for impedance matching; modified transformer coupling ratio. As a result, we got the integrated noise equivalent power resolution of 2.7 eV at 2.5 MHz. A characterization of the detector response as a function of the AC bias voltage, bias frequency and the applied magnetic field is presented.     

A thermal activation model for noise in the dc SQUID

A thermal activation model is described for the dc SQUID. The equations of motion for the junction phase differences are shown to develop in time like the coordinates of a particle performing Brownian motion in a viscous medium in a two-dimensional potential field. Expressions are derived relating the average voltage, transfer function, and current and voltage noise spectral densities to the features of the potential determined by the device parameters. Comparison with a numerical simulation is presented. Calculations of the current noise as a function of loop inductance and critical current asymmetry are performed. An anomalously large current noise is predicted at certain values of the device characteristics. The correlation spectral density is also calculated as a function of loop inductance, and related to the optimal source resistance for a tuned SQUID amplifier. A theory of low-frequency noise sources in the SQUID is developed in a fashion compatible with the thermal activation model. Equilibrium temperature fluctuations as a possible source of 1/f noise in the SQUID are discussed. A scheme for optimizing the resolution at low frequencies is presented. Proper exploitation of low-capacitance Pb-PbOx-Pb junction technology is shown to increase the resolution at 1 Hz by at least a factor of 8, provided that the temperature fluctuations are the dominant source of noise.Work supported in part by ONR under contract #NR319-154.     

Optimization of the R-SQUID noise thermometer

The Josephson junction can be used to convert voltage into frequency and thus it can be used to convert voltage fluctuations generated by Johnson noise in a resistor into frequency fluctuations. As a consequence, the temperature of the resistor can be defined by measuring the variance of the frequency fluctuations. Unfortunately, the absolute determination of temperature by this approach is disturbed by several undesirable effects: a rolloff introduced by the bandwidth of the postdetection filter, additional noise caused by rf amplifiers, and a mixed noise effect caused by the nonlinearity of the Josephson junction together with rf noise in the tank circuit. Furthermore, the variance is a statistical quantity and therefore the limited number of frequency counts produces inaccuracy in a temperature measurement. In this work the total inaccuracy of the noise thermometer is analyzed and the optimal choice of the parameters is derived. A practical way to find the optimal conditions for the Josephson junction noise thermometer is discussed. The inspection shows that under the optimal conditions the total error is dependent only on the temperature under determination, the equivalent noise temperature of the preamplifier, the bias frequency of the SQUID, and the total time used for the measurement.     

Vortex Avalanches Induced by Single High-Frequency Pulses in?MgB2 Films

We report on avalanche-like flux features in the response of superconducting films of MgB₂, prepared by magnetron sputtering and an ex-situ annealing process, when a single high-frequency pulse is used as a perturbative source. The instability is observed as an abrupt jump in the output voltage signal of the SQUID magnetometer where the sample is measured as a function of temperature, magnetic field, and pulse width, at fixed nominal power and frequency values. The interaction between the high-frequency surface currents and the relaxing vortex system is suggested to explain the nucleation of avalanches. Local heating due to the pulse application is also considered as a contributing mechanism.     

Measurement of the input resistance of a rf biased SQUID

Measurements of the real part of the input impedance of a RF biased SQUID are described. The experimental data are in good agreement with the predictions of the recently published theory. In particular the dependence of the input resistance on the detuning of the rf bias frequency from tank circuit resonance frequency is confirmed.     

The Design of a Frequency Multiplexed Ir-Au TES Array

The IrAu TESs which have been developed in Genoa in past years, will be assembled in large numbers in a detector to reach the required accuracy in neutrino mass measurement (by β decay of ¹⁸⁷Re). When the number of bolometers increases a multiplexed readout is needed to reduce the thermal load on the low temperature refrigerator and to keep low the cost of SQUID electronics. We present the scheme of the circuit that distributes the AC supply to the TESs in the array and collects the signal from a row of devices, decoding the address by the signal frequency. The TESs are polarized by voltage sources of different frequencies. The TES working at different frequencies can be read by a single SQUID, which reads the current induced in a Superconducting transformer, and feeds in the transformer itself a feedback flux. The signal from the TES in the idle state will be compensated. The circuit will be built with planar film technology and will act as heat sink for each TES. The requirements for the SQUID will be discussed. The construction of a small prototype of the circuit is under way.      

Anomalous kink behavior in the current-voltage characteristics of suspended carbon nanotubes

Electrically-heated suspended, nearly defect-free, carbon nanotubes (CNTs) exhibiting negative differential conductance in the high bias regime experience a sudden drop in current (or “kink”). The bias voltage at the kink (V _kink) is found to depend strongly on gate voltage, substrate temperature, and gas environment. After subtracting the voltage drop across the contacts, however, the kink bias voltages converge around 0.2 V, independent of gate voltage and gas environment. This bias voltage of 0.2 V corresponds to the threshold energy of optical phonon emission. This phenomenon is corroborated by simultaneously monitoring the Raman spectra of these nanotubes as a function of bias voltage. At the kink bias voltage, the G band Raman modes experience a sudden downshift, further indicating threshold optical phonon emission. A Landauer model is used to fit these kinks in various gas environments where the kink is modeled as a change in the optical phonon lifetime, which corresponds to a change in the non-equilibrium factor that describes the existence of hot phonons in the system.      

Capacitively-Coupled SQUID Bias for Time Division Multiplexing

The multiplexing scheme presented in this paper is part of the readout chain of the QUBIC instrument devoted to cosmic microwave background polarization observations. It is based on time domain multiplexing using superconducting quantum interference devices (SQUIDs) to read out a large array of superconducting bolometers. The originality of the multiplexer presented here lies in the use of capacitors for the SQUID addressing. Capacitive coupling allows us to bias many SQUIDs in parallel (in a 2D topology), with low crosstalk and low power dissipation of the cryogenic front-end readout. However, capacitors in series with the SQUID require a modification of the addressing strategy. This paper presents a bias reversal technique adopted to sequentially address the SQUIDs through capacitors using a cryogenic SiGe integrated circuit. We further present the different limitations of this technique and how to choose the proper capacitance for a given multiplexing frequency and current source compliance.     

Cross-Correlated Dynamic Resistance of a Direct Current Superconducting Quantum Interference Device

This paper presents experimental data on a comparative study of a dc SQUID with voltage and current bias. We introduce a cross-correlated dynamic resistance of the device defined as a ratio R _dCV = _VC/_IV, where _VC = (V/) _A is the slope of the voltage-to-flux characteristics measured in the current bias mode and _IV = (I/) _A is the slope of the current-to-flux characteristics measured with voltage bias. It has been found that R _dCV may deviate strongly from the dynamic resistance observed in the current bias mode of operation. The intrinsic energy resolution of the SQUID remains unchanged for both modes of operation, but the current noise of the voltage biased device scales with the cross-correlated dynamic resistance. In our SQUID with the loop inductance L = 105 pH, is equal to 37 h in the white noise region at a temperature of 4.2 K.