Broadband Superconducting Quantum MagnetometerR. A. Kamper and M. B. Simmonds Citation: Applied Physics Letters 20, 270 (1972); doi: 10.1063/1.1654144 View online: http://dx.doi.org/10.1063/1.1654144 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/20/8?ver=pdfcov Published by the AIP Publishing Articles you may be interested in A low field superconducting quantum interference device magnetometer for dynamic measurements Rev. Sci. Instrum. 68, 3761 (1997); 10.1063/1.1148023 A miniature magnetometer based on the superconducting quantum interference device with direct readoutelectronics Rev. Sci. Instrum. 68, 1461 (1997); 10.1063/1.1147633 Magnetometers based on double relaxation oscillation superconducting quantum interference devices Appl. Phys. Lett. 66, 2274 (1995); 10.1063/1.113190 An 8T superconducting quantum interference device magnetometer Rev. Sci. Instrum. 62, 1824 (1991); 10.1063/1.1142428 Superconducting magnetometer Appl. Phys. Lett. 56, 2037 (1990); 10.1063/1.103010

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270 T. YAMAMOTO AND M. MORIMOTO

Insulator

t/-/ / /

region N-reglon / / Metal

/ p-

e ,~

~~//1

I Jc eVQ

E

---- -, r--L----\ , tt,. --- -~------'----------- J, 6l6lee~

-~ Jy •

Inversion ! layer

FIG. 3. Band model for p-n-I-M structure where band bending is neglected at V= 0 and where V= Va'

action is perhaps much faster than any other S-type negative-resistance diode. The rise time including the delay of a pulse response is below 100 nsec, and the decay time is too short to be measured in our labora­tory. Also optical switching is possible, but its photo­sensitivity is not high.

The regenerative nature of this device is not clearly understood. The hole injection from the p region into the n region, the tunnel or Schottky emission from met­al through the thin Si02 layer, and the interface states between the n layer and the insulator are perhaps re­sponsible for this regenerative mechanism. When a positive voltage is applied to the electrode of the p sub­strate, hole current would be injected into the n layer such that the inversion layer is formed at the interface between the n layer and the insulator. This changes the S-1 interface potential l/!s which gives a much higher electric field across the insulator. This in turn induces

three possible tunneling currents from the metal, name­ly Jv' the tunneling of a metal electron into an unoc­cupied valence band of the semiconductor; Je, the tun­neling of a metal electron into an unoccupied conduction band; and J p the tunneling of a metal electron into emp­ty interface states and the follOwing recombination with holes. 3_5 Figure 3 shows these processes. It is thought that the J e current component passes through the n re­gion in the form of a hot-electron current and then re­combines with the majority carriers in the p region, and the J1 current component recombines with the holes at the interface via interface states, and thus these ex­hausted holes must be furnished from the p region. These two current components in effect "short out" the n layer, and the sustaining voltage becomes 2-3 V which is sufficient to sustain the tunneling currents.

We also succeeded in the experiments to obtain negative­resistance characteristics using the complementary­type n-p-l-M diode.

This device may be useful as a high-speed Switching element in microwave circuits, in future integrated circuits of the high-speed shift resistor, or a shift pulse generator for solid-state imaging and sensing de­vices and the flying-spot scanner. Other materials such as GaAs, GaP, etc., instead of Si may also be used. In addition, the microwave switching transistor might be fabricated using a p-n-l-M or an n-p-l-M structure.

The authors wish to thank Professor T. Horii and Pro­fessor I. Takao for their support of this work, our colleagues for their cooperation, and also Professor U. Shibata of Tohoku University for his helpful discussions.

1T. Yamamoto and Y. Takas e, Trans. I EC E 53-C, 10 (1970). 2T. Yamamoto, Trans. IECE 53-C, 11 (1970). 3W. E. Dahlke, Appl. Phys. Letters 10, 10 (1967). 4L. B. Freeman and W. E. Dahlke, Solid-State Electron. 13, 1483 (1970).

5Samares Kar and W. E. Dahlke, Appl. Phys. Letters 18, 9 (1971).

Broadband Superconducting Quantum Magnetometer*

R. A. Kamper and M. B. Simmondst

National Bureau of Standards, Institute for Basic Standards, Boulder, Colorada 80302 (Received 2 December 1971)

We describe the design, operation, and performance of a superconducting quantum inter­ference device (SQUID) which operates at a frequency of 9 GHz. It is sensitive to variations of magnetic field in a frequency band from 0 to 1 GHz.

The superconducting quantum interference device (SQUID) has been developed, over the past few years, into a very sensitive and versatile magnetic sensor. 1-3

It consists of a small loop of superconducting metal closed by a weak contact. Because of quantum mechani­cal interference the contact behaves electrically like a lossy nonlinear inductance which varies periodically with the magnetic flux enclosed by the loop, with a period equal to the magnetic flux quantum h/2e (2 x 10-15 Wb).

Appl. Phys. Lett., Vol. 20, No.8, 15 April 1972

This variation can be sensed by monitoring the imped­ance of the loop at some convenient radio frequency. A tradition has grown up to choose about 30 MHz for this operating frequency, originally because of the conveni­ent availability of components. More recently, Zimmer­man and Frederick4 have demonstrated the benefit of operation at 300 MHz, with a resulting increase in sig­nal level (and hence sensitivity). Theoretically, 5 the signal power available from the device should be pro-

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BROADBAND SUPERCONDUCTING QUANTUM MAGNETOMETER 271

(a)

COAXIAL LINE

(b)

~~;tsSOFT SOLDER

BABBITT

- A/4--Al4-

FIG. 1. SQUID: (a) longitudinal section; (b) top view. Some dimensions have been exaggerated for clarity.

portional to operating frequency up to some (as yet un­known) limit. In this letter we describe a SQUID which operates at 9 GHz. It generates the high signal level expected for this operating frequency, and also has a usable response to variations in magnetic flux from dc to 1 GHz (limited only by the tunnel-diode amplifiers used in the microwave system).

A prime consideration in designing any type of SQUID is to achieve optimum coupling of the device to the sensing system. In normal operation, the microwave field sweeps the magnetic flux in the loop through one flux quantum % (peak to peak) during each cycle. The match­ing criterion we used was that at this drive level, the energy absorbed by the SQUID (- cp~/ L per cycle, where L is the inductance of the loop) should be equal to the energy absorbed by the receiver. This is equivalent to the criterion commonly used for resonant coupling to SQUID's at lower frequency, 6 and subsequent experience has shown that it does indeed give the desired result.

Figure 1 shows a schematic diagram of one of our broadband SQUID's. It consists of a short section of a rectangular X-band waveguide, reduced in the E direc­tion (0.25 mm high) in order to give a characteristic impedance of 10 Q (at the midpoint). It is lined with superconducting metal. The point contact clOSing the loop is a small, pointed niobium SCrew placed on the center line a quarter-wavelength from a short-circuit termination. It can be adjusted in the cryostat with a simple arrangement of a lever and micrometer-driven push rod. In the version shown in Fig. 1, magnetic flux is coupled to the SQUID via a coaxial line passing through the waveguide parallel to the E field. This is a natural arrangement for detecting current. For detect­ing magnetic field the coaxial line would be connected to a pickup coil, the entire circuit being superconduct­ing. The relatively high characteristic impedance (50 Q) of the coaxial line is helpful in avoiding undesirable perturbation of the microwave field in the SQUID. Other methods of coupling the SQUID to external fields which

we have tried successfully include a small coil with its axis in the H plane close to the point contact, and nonsu­perconducting sections in the narrow sidewalls of the device to permit direct linking of external magnetic flux. The coaxial line arrangement is superior at the highest frequencies.

The SQUID was fabricated by facing two brass blocks with babbitt' (85% lead, 10% antimony, 5% tin) and mill­ing the faces flat. The blocks were then clamped togeth­er with O. 25-mm plastic spacers and soldered together around the edges with 50-50 lead-tin solder. No flux was required to make a good super conducting joint. The babbitt is sufficiently hard to take a good surface finish and satisfactory SCrew threads.

The microwave system is shown schematically in Fig. 2. It uses WR90 waveguide and is connected to the SQUID via a taper section 10 cm long. The drive power is gen­erated by a klystron oscillator and passes to the SQUID via several attenuators and a directional coupler (about 90-dB total attenuation). The signal reflected from the SQUID is amplified by a chain of tunnel-diode amplifiers (-40-dB total gain, 1-GHz bandwidth) and detected with a crystal rectifier. This detected signal may be dis­played directly with an oscilloscope Or other recorder, Or individual sidebands may be picked out by using a communications receiver for a second detector.

The response of the system to slow variations in mag­netic flux is shown in Fig. 3(a), which is an oscilloscope display of rectified output vs magnetic flux. The postde­tection bandwidth was approximately 100 kHz. Allowing for differences in inductance, the signal power produc­ing this display can be compared with that from a SQUID operating at 30 MHz. 8 It appears to be superior by 20 dB, a factor somewhat short of that expected from the ratio of frequencies (-27 dB).

The response to variations in magnetic flux at high fre­quencies is best displayed in a slightly different fashion. Figure 3(b) shows an oscilloscope display of the recti­fied output of the system vs amplitude of an amplitude­modulated signal at 65 MHz. Each point on this display corresponds to an average over a segment of the re­sponse shown in Fig. 3(a), the length of the segment be­ing proportional to the rf amplitude. The postdetection bandwidth was 100 kHz. The display in Fig. 3(b) showed

or SECOND

DETECTOR

FIG. 2. Microwave system.

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272 R. A. KAMPER AND M. B. SIMMONDS

(a)

(b)

FIG. 3. Oscilloscope displays of the response of the system to: (a) slow variations in magnetic flux; (b) variations in amplitude of a signal at 65 MHz.

a striking insensitivity to variations in signal frequency. A rough measurement showed the response to be un­changed within a few percent from dc to 400 MHz, and we have observed a usable response up to a frequency

limited by the bandwidth of the tunnel-diode amplifiers, about 1 GHz.

The sensitivity of the SQUID shown in Fig. 1 to current through the coaxial line is approximately one flux quan­tum per 70 /lA. Another SQUID with a small coil with its axis in the H plane close to the point contact required about 5 /lA per flux quantum. Other designs could extend this range considerably.

In conclusion, we have shown that a broadband SQUID can operate at a microwave frequency with approximate­ly the theoretically expected performance. It has excit­ing possibilities as a sensor of current and magnetic field. For transient measurements it has a rise time of approximately 1 nsec. It could be used for digital mea­surement of current by counting flux quanta at rates up to l07/sec, enabling it to follow fast variations. It could be used for measuring rf current and attenuation over a very wide frequency range, using the flux quantum as a natural yardstick.

We would like to thank our colleagues N. V. Frederick, J. E. Zimmerman, and J. D. Siegwarth for the ideas and techniques they contributed to this work.

"Work partly supported by the U. S. Department of Defense, CCG.

tNAS-NRC Postdoctoral Associate. 1J.E. Zimmerman, P. Thiene, andJ.T. Harding, J. Appl. Phys. 41, 1572 (1970).

2J.E. Mercereau, Rev. Phys. Appl. 5, 13 (1970). 3J.E. Lukens, R.J. Warburton, and W.W. Webb, J. Appl. Phys. 42, 27 (1971).

4J. E. Zimmerman and N. V. Frederick, Appl. Phys. Letters 19, 16 (1971).

5A.H. Silver and J.E. Zimmerman, Phys. Rev. 157, 317 (1967).

SM. B. Simmonds and W. H. Parker, J. Appl. Phys. 42, 38 (1971).

1J. D. Siegwarth and D. B. Sullivan, Rev. Sci. Instr. (to be published) •

8J. E. Zimmerman, J. Appl. Phys. 42, 4483 (1971).

Nonlinear Susceptibility of GaP; Relative Measurement and Use of Measured Values to Determine a Better Absolute Value

B. F. Levine and C. G. Bethea Bell Telephone Laboratories, Murray Hill, New Jersey 07974

(Received 27 December 1971)

We have measured the ratio d3S(GaP)/d11(Si02)=185± 10% at 1.31811. By using this value together with other values in the literature, we determine a better set of absolute values for the nonlinear susceptibility.

Due to the increasing interest in nonlinear optical de­vices and the increasing accuracy of theoretical calcu­lations,l it is important to have an accurate set of ab­solute values for the nonlinear susceptibility d jJk • By appropriately averaging the best available absolute mea­surements of dlJk for various crystals and constraining these values to be consistent with the experimental ra-

Appl. Phys. Lett., Vol. 20, No.8, 15 April 1972

tios for these djik' an accurate set of absolute values can be obtained.

Unfortunately, there now exist two essentially separate sets of absolute measurements. One for materials with small band gaps (e. g., GaAs, GaP, Ag3SbS3 , etc.) for which GaAs or Ag3 SbS3 is the standard, and another set

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