J. Flokstra - Academia.edu (original) (raw)
Papers by J. Flokstra
Sensor Technology in the Netherlands: State of the Art, 1998
Cryogenic radiation and particle detectors are widely investigated nowadays. Their high intrinsic... more Cryogenic radiation and particle detectors are widely investigated nowadays. Their high intrinsic energy sensitivity makes them suitable for a wide range of applications. In order to take full advantage of the sensitivity of these newly developed cryogenic detectors, in general current sensors are needed with a white noise level of the order of 10-12 A/√HZ and a bandwidth larger than 1 MHz. We applied a Double Relaxation Oscillation SQUID (DROS) as a low-noise current preamplifier for measuring the current-voltage characteristics of a superconducting Nb/Al-AlOx/Al/Nb Josephson tunnel junction. A white current noise level smaller than 2 pA/√Hz was obtained. Thanks to the large flux-to-voltage transfer of a DROS (typically 1 mV/O o), no modulation techniques are required, allowing simple room-temperature flux locked loop electronics. Since the Josephson junction was voltage biased, no magnetic field had to be applied to suppress the Josephson supercurrent, which eliminates distortion of the subgap I-V characteristics by Fiske resonances. The measurements show that a DROS is a serious alternative for the high resolution readout of an X-ray spectrometer based on a superconducting tunnel junction.
Physica B+C, 1978
A comparison is made between the results of low-frequency relaxation measurements on potassium ch... more A comparison is made between the results of low-frequency relaxation measurements on potassium chromium alum placed in liquid helium and in vacuum, respectively. It is found that only a part of the lattice oscillations is involved in the spin-lattice relaxation process. The spin-lattice relaxation time was found after correcting the time constants of the vacuum measurements by experimental shortening factors. The differences between the spin-lattice relaxation time and the time constants from the liquid-helium measurements can be explained for the greater part by the finite thermal conductivity of the liquid helium.
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 1996
Injection-detection experiments have been performed to study quasiparticle diffusion and quasipar... more Injection-detection experiments have been performed to study quasiparticle diffusion and quasiparticle losses in Nb strips using a series array of Nb/AI junctions. The I-Vcurve of the detector junction was measured as a function of the spacing between the (high-quality) detector junction and the simultaneously biased injector junction. For our sample geometry, the quasiparticle gas is described in terms of a one-dimensional diffusion model. It was found that trapping losses at the interface between the Nb base electrode and the anodised NbZO, dominate. In addition, a very low quasiparticle diffusion constant at 1 K, D = I .2 X IO-' m'/s has been found. We estimate that this corresponds to an electron mean free path of I = 7 nm. Due to the difference between the bandgaps of the base and top electrode and to the peak in the density of states at the gap edge, the excess detector current shows a characteristic voltage dependence for bias voltages below 0.5 mV.
1998 Conference on Precision Electromagnetic Measurements Digest (Cat. No.98CH36254), 1998
We have designed and constructed a dedicated cryogenic current comparator (CCC) for the measureme... more We have designed and constructed a dedicated cryogenic current comparator (CCC) for the measurement of very small currents. The essential element of the system is a home-made SQUID with input inductance equal to that of the CCC overlapping shield (which we use as pick-up coil)
Journal of Low Temperature Physics, 1993
On the basis of a microscopic model of the proximity effect in an SS" sandwich we have calculated... more On the basis of a microscopic model of the proximity effect in an SS" sandwich we have calculated effective trapping, excitation, and tunneling rates of the reduced gap region in an SS'IS'S junclion as a function of temperature, voltage over the junction and strength of the proximity effect. We developed a simple model to describe the effect of magnetic fields on the gap of the superconductor, and calculated the trapping rate as function of the field. The experimentally determined gap reduction and quasiparticle loss times as function of the field were described in terms of this model.
Physica B: Condensed Matter, 1994
45 degrees grain boundary junctions have been made using (100) MgO substrates, a CeO 2 template l... more 45 degrees grain boundary junctions have been made using (100) MgO substrates, a CeO 2 template layer and an YBa2Cu307 top layer. To minimize the damage to the MgO surface, which will occur if the CeO 2 is structured using ion milling, the CeO 2 layer has been structured using the CaO lift-off technique. Electrical measurements of these junctions as a function of temperature, microwave irradiation and magnetic field will be discussed in this paper.
The anomalous behaviour of the spin-lattice relaxation observed for single crystals of ytterbium ... more The anomalous behaviour of the spin-lattice relaxation observed for single crystals of ytterbium chloride hexahydrate at fields stronger than 5 kOe is due to the poor heat transfer in the liquid-helium bath. The thermal conduction effects can be explained by means of a thermal conduction model for the relaxation.
Journal of Physics D: Applied Physics, 1988
A recent preparation method by citrate synthesis in combination with pyrolysis has been applied t... more A recent preparation method by citrate synthesis in combination with pyrolysis has been applied to the new high-T, superconductors. A step-by-step procedure is given to obtain the ultra-fine and homogeneous powder. X-ray diffraction analyses were performed to characterise the powder and to study the influence of the calcination temperature. Resistance measurements exhibit complete superconductivity at 91.2 K.
Cryogenics, 1990
A 19 channel d.c. SQUID magnetometer for biomagnetic research is under construction. The system n... more A 19 channel d.c. SQUID magnetometer for biomagnetic research is under construction. The system needs compactly built control and detection electronics and to facilitate the test an electronic circuit simulating the typical periodic characteristics of a SQUID was developed. In total nine steps of triangular shape were implemented with a step voltage reference generator and precision full-wave rectifier. The transfer function of the SQUID electronics and the maximum slew rate of the system could be easily measured.
Journal of Micromechanics and Microengineering, 2006
Wideband 100 kHz-4 GHz power sensors are presented, which are based on sensing the electrostatic ... more Wideband 100 kHz-4 GHz power sensors are presented, which are based on sensing the electrostatic force between an RF signal line and a suspended membrane. The electrostatic force, which is proportional to the square of the rms signal voltage and thus to the signal power, results in a displacement of the suspended membrane. This displacement is detected capacitively, allowing the sensing of the signal power with extremely low dissipative losses; therefore the sensor can be placed in a transmission line with negligible disturbance of the signal. Devices have been designed and fabricated successfully by aluminum surface micromachining using photoresist as the sacrificial layer. Optimization of the design with SONNET has resulted in measured return and insertion losses (S 11 and S 21 ) better than −30 dB and −0.15 dB, respectively, up to 4 GHz, and a sensitivity of 90 aF mW -1 .
Journal of the Less Common Metals
The physical properties of laser-deposited YBaCuO on Si using a single buffer layer of ZrOa and a... more The physical properties of laser-deposited YBaCuO on Si using a single buffer layer of ZrOa and a double layer of Nisi2 and ZrOz have been studied.
High Tc Superconductor Thin Films, 1992
A new method for measuring power of high-frequency signals based on MEMS technology is described.... more A new method for measuring power of high-frequency signals based on MEMS technology is described. A capacitive low-loss MEMS component for frequencies up to 7 GHz has been designed. The design aspects including high frequency simulations to achieve low reflections and calculations of forces, capacitances and spring constant are presented.
Physica B+C, 1983
ABSTRACT
Physica, 1974
In low-frequency relaxation experiments on paramagnetic crystals, placed in liquid helium, often ... more In low-frequency relaxation experiments on paramagnetic crystals, placed in liquid helium, often dispersion-and absorption curves are found, strongly deviating from those following from the thermodynamic theory of Casimir and Du Pr6. For the relaxation time tabs, related to the maximum in the absorption curve, a temperature dependence Tab s ~ T -°t with 1 < t~ < 6 has been found frequently, accompanied sometimes with a jump at the helium-transition temperature T x. We computed, starting from a model including the heat conduction of lattice and helium bath and a heat resistance from crystal to bath, dispersion-and absorption curves and found that, assuming the direct process for the spin-lattice relaxation, ral~(T) (related to the maximum of the calculated absorption curve) can be described roughly as mentioned above with a jump at T h.
Physica B+C, 1978
ABSTRACT
Physica B+C, 1980
The crystallographic and magnetic structure of RbCoCI 3 • 2 D20 were determined mainly by means o... more The crystallographic and magnetic structure of RbCoCI 3 • 2 D20 were determined mainly by means of neutron diffraction measurements. Below T N = 2.79 K the magnetic moments are ordered in a canted antiferromagnetic pattern. A metamagnetic phase transition is observed at unusually small field values (H = 18 Oe at T = 2 K).
Physica B+C, 1985
The low-frequency relaxation behaviour of the linear-chain antiferromagnet CsMnC13.2H20 at the sp... more The low-frequency relaxation behaviour of the linear-chain antiferromagnet CsMnC13.2H20 at the spin-flop transition has been determined from dynamic susceptibility measurements on a single crystal placed in direct contact with liquid helium. The experiments were performed between 1.4 and 4.2 K in the frequency range 0,1 Hz-3.0 kHz with a frequencysweeping SQUID susceptometer. Below Ta =2.17K, the relaxation rate ~.-1 manifests an exponential temperature dependence, ~.-t = too e -EIkT, where E/k = 3.19 +-0.04 K is approximately equal to the magnitude of the intrachain exchange interaction constant Jo/k. Above Ta the apparent deviation from the exponential behaviour has been explained satisfactorily by using the thermal conduction model of relaxation. The field-dependent factor too is directly proportional to the ratio of the adiabatic Xs to the isothermal XT susceptibilities.
Physica B+C, 1982
ABSTRACT
Sensor Technology in the Netherlands: State of the Art, 1998
Cryogenic radiation and particle detectors are widely investigated nowadays. Their high intrinsic... more Cryogenic radiation and particle detectors are widely investigated nowadays. Their high intrinsic energy sensitivity makes them suitable for a wide range of applications. In order to take full advantage of the sensitivity of these newly developed cryogenic detectors, in general current sensors are needed with a white noise level of the order of 10-12 A/√HZ and a bandwidth larger than 1 MHz. We applied a Double Relaxation Oscillation SQUID (DROS) as a low-noise current preamplifier for measuring the current-voltage characteristics of a superconducting Nb/Al-AlOx/Al/Nb Josephson tunnel junction. A white current noise level smaller than 2 pA/√Hz was obtained. Thanks to the large flux-to-voltage transfer of a DROS (typically 1 mV/O o), no modulation techniques are required, allowing simple room-temperature flux locked loop electronics. Since the Josephson junction was voltage biased, no magnetic field had to be applied to suppress the Josephson supercurrent, which eliminates distortion of the subgap I-V characteristics by Fiske resonances. The measurements show that a DROS is a serious alternative for the high resolution readout of an X-ray spectrometer based on a superconducting tunnel junction.
Physica B+C, 1978
A comparison is made between the results of low-frequency relaxation measurements on potassium ch... more A comparison is made between the results of low-frequency relaxation measurements on potassium chromium alum placed in liquid helium and in vacuum, respectively. It is found that only a part of the lattice oscillations is involved in the spin-lattice relaxation process. The spin-lattice relaxation time was found after correcting the time constants of the vacuum measurements by experimental shortening factors. The differences between the spin-lattice relaxation time and the time constants from the liquid-helium measurements can be explained for the greater part by the finite thermal conductivity of the liquid helium.
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 1996
Injection-detection experiments have been performed to study quasiparticle diffusion and quasipar... more Injection-detection experiments have been performed to study quasiparticle diffusion and quasiparticle losses in Nb strips using a series array of Nb/AI junctions. The I-Vcurve of the detector junction was measured as a function of the spacing between the (high-quality) detector junction and the simultaneously biased injector junction. For our sample geometry, the quasiparticle gas is described in terms of a one-dimensional diffusion model. It was found that trapping losses at the interface between the Nb base electrode and the anodised NbZO, dominate. In addition, a very low quasiparticle diffusion constant at 1 K, D = I .2 X IO-' m'/s has been found. We estimate that this corresponds to an electron mean free path of I = 7 nm. Due to the difference between the bandgaps of the base and top electrode and to the peak in the density of states at the gap edge, the excess detector current shows a characteristic voltage dependence for bias voltages below 0.5 mV.
1998 Conference on Precision Electromagnetic Measurements Digest (Cat. No.98CH36254), 1998
We have designed and constructed a dedicated cryogenic current comparator (CCC) for the measureme... more We have designed and constructed a dedicated cryogenic current comparator (CCC) for the measurement of very small currents. The essential element of the system is a home-made SQUID with input inductance equal to that of the CCC overlapping shield (which we use as pick-up coil)
Journal of Low Temperature Physics, 1993
On the basis of a microscopic model of the proximity effect in an SS" sandwich we have calculated... more On the basis of a microscopic model of the proximity effect in an SS" sandwich we have calculated effective trapping, excitation, and tunneling rates of the reduced gap region in an SS'IS'S junclion as a function of temperature, voltage over the junction and strength of the proximity effect. We developed a simple model to describe the effect of magnetic fields on the gap of the superconductor, and calculated the trapping rate as function of the field. The experimentally determined gap reduction and quasiparticle loss times as function of the field were described in terms of this model.
Physica B: Condensed Matter, 1994
45 degrees grain boundary junctions have been made using (100) MgO substrates, a CeO 2 template l... more 45 degrees grain boundary junctions have been made using (100) MgO substrates, a CeO 2 template layer and an YBa2Cu307 top layer. To minimize the damage to the MgO surface, which will occur if the CeO 2 is structured using ion milling, the CeO 2 layer has been structured using the CaO lift-off technique. Electrical measurements of these junctions as a function of temperature, microwave irradiation and magnetic field will be discussed in this paper.
The anomalous behaviour of the spin-lattice relaxation observed for single crystals of ytterbium ... more The anomalous behaviour of the spin-lattice relaxation observed for single crystals of ytterbium chloride hexahydrate at fields stronger than 5 kOe is due to the poor heat transfer in the liquid-helium bath. The thermal conduction effects can be explained by means of a thermal conduction model for the relaxation.
Journal of Physics D: Applied Physics, 1988
A recent preparation method by citrate synthesis in combination with pyrolysis has been applied t... more A recent preparation method by citrate synthesis in combination with pyrolysis has been applied to the new high-T, superconductors. A step-by-step procedure is given to obtain the ultra-fine and homogeneous powder. X-ray diffraction analyses were performed to characterise the powder and to study the influence of the calcination temperature. Resistance measurements exhibit complete superconductivity at 91.2 K.
Cryogenics, 1990
A 19 channel d.c. SQUID magnetometer for biomagnetic research is under construction. The system n... more A 19 channel d.c. SQUID magnetometer for biomagnetic research is under construction. The system needs compactly built control and detection electronics and to facilitate the test an electronic circuit simulating the typical periodic characteristics of a SQUID was developed. In total nine steps of triangular shape were implemented with a step voltage reference generator and precision full-wave rectifier. The transfer function of the SQUID electronics and the maximum slew rate of the system could be easily measured.
Journal of Micromechanics and Microengineering, 2006
Wideband 100 kHz-4 GHz power sensors are presented, which are based on sensing the electrostatic ... more Wideband 100 kHz-4 GHz power sensors are presented, which are based on sensing the electrostatic force between an RF signal line and a suspended membrane. The electrostatic force, which is proportional to the square of the rms signal voltage and thus to the signal power, results in a displacement of the suspended membrane. This displacement is detected capacitively, allowing the sensing of the signal power with extremely low dissipative losses; therefore the sensor can be placed in a transmission line with negligible disturbance of the signal. Devices have been designed and fabricated successfully by aluminum surface micromachining using photoresist as the sacrificial layer. Optimization of the design with SONNET has resulted in measured return and insertion losses (S 11 and S 21 ) better than −30 dB and −0.15 dB, respectively, up to 4 GHz, and a sensitivity of 90 aF mW -1 .
Journal of the Less Common Metals
The physical properties of laser-deposited YBaCuO on Si using a single buffer layer of ZrOa and a... more The physical properties of laser-deposited YBaCuO on Si using a single buffer layer of ZrOa and a double layer of Nisi2 and ZrOz have been studied.
High Tc Superconductor Thin Films, 1992
A new method for measuring power of high-frequency signals based on MEMS technology is described.... more A new method for measuring power of high-frequency signals based on MEMS technology is described. A capacitive low-loss MEMS component for frequencies up to 7 GHz has been designed. The design aspects including high frequency simulations to achieve low reflections and calculations of forces, capacitances and spring constant are presented.
Physica B+C, 1983
ABSTRACT
Physica, 1974
In low-frequency relaxation experiments on paramagnetic crystals, placed in liquid helium, often ... more In low-frequency relaxation experiments on paramagnetic crystals, placed in liquid helium, often dispersion-and absorption curves are found, strongly deviating from those following from the thermodynamic theory of Casimir and Du Pr6. For the relaxation time tabs, related to the maximum in the absorption curve, a temperature dependence Tab s ~ T -°t with 1 < t~ < 6 has been found frequently, accompanied sometimes with a jump at the helium-transition temperature T x. We computed, starting from a model including the heat conduction of lattice and helium bath and a heat resistance from crystal to bath, dispersion-and absorption curves and found that, assuming the direct process for the spin-lattice relaxation, ral~(T) (related to the maximum of the calculated absorption curve) can be described roughly as mentioned above with a jump at T h.
Physica B+C, 1978
ABSTRACT
Physica B+C, 1980
The crystallographic and magnetic structure of RbCoCI 3 • 2 D20 were determined mainly by means o... more The crystallographic and magnetic structure of RbCoCI 3 • 2 D20 were determined mainly by means of neutron diffraction measurements. Below T N = 2.79 K the magnetic moments are ordered in a canted antiferromagnetic pattern. A metamagnetic phase transition is observed at unusually small field values (H = 18 Oe at T = 2 K).
Physica B+C, 1985
The low-frequency relaxation behaviour of the linear-chain antiferromagnet CsMnC13.2H20 at the sp... more The low-frequency relaxation behaviour of the linear-chain antiferromagnet CsMnC13.2H20 at the spin-flop transition has been determined from dynamic susceptibility measurements on a single crystal placed in direct contact with liquid helium. The experiments were performed between 1.4 and 4.2 K in the frequency range 0,1 Hz-3.0 kHz with a frequencysweeping SQUID susceptometer. Below Ta =2.17K, the relaxation rate ~.-1 manifests an exponential temperature dependence, ~.-t = too e -EIkT, where E/k = 3.19 +-0.04 K is approximately equal to the magnitude of the intrachain exchange interaction constant Jo/k. Above Ta the apparent deviation from the exponential behaviour has been explained satisfactorily by using the thermal conduction model of relaxation. The field-dependent factor too is directly proportional to the ratio of the adiabatic Xs to the isothermal XT susceptibilities.
Physica B+C, 1982
ABSTRACT