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A DC-SQUID magnetometer operated at 77K
Physica C 153-155 (1988) 1413-1414 North-Holland, Amsterdam
A D C - S Q U I D MAGNETOMETER OPERATED AT 77K C.X. FAN, Y.J. QIAN, L.S. DAI and Z.Y. HUA Fudan University, Shanghai, China A prototype l ak'ge bridge D C - S Q U I D magnetometer is prepared, the equivalent flux noise o f which can be down to 4 x 10"* ~bo Hz -72. 1. THE N E T W O R K M O D E L The structure and characteristics of sintered oxide
under microwave irradiation. All these results proved that the proposed "network'model seems to be acceptable.
superconductors, such as YBa2Cu3OT-x, have been extensively studied ( 1, 2). The results o f measured critical current Ic and resistance R at current-induced resistive state as functions of
2. THE L A R G E BRIDGE
applied magnetic field B are shown in Fig. 1 (3). Those results give two distinct features: Ic dramatically decreases at very low field ( < 10 G) and R sharply varies with B. For interpreting these phenomena, we have proposed a "network" model (3), i. e. a sintered bulk sample which can be described as a network of random Josephson weak links. Magnetic field effectively depresses Ic o f these weak links and when the current reaches the effective critical current of "network', the weak links lose their superconductivity gradually, the resistance R appears and afterwards give the sharp change o f R - - B curve. The I c ~ B curve differs from the diffraction pattern of a single Josephson junction. No periodical structure can be found and Ic does not go to zero even at high B; however, the experimental data can be well fitted in with the theoretical calculation if a proper distribution function o f diffraction period is assumed. More experiments have been then performed to check this model. The most important results are (4) :high hysterises in Ic B curves at B = 0-3 Tesla; two resistive jumps on the V ~ I curve; two steps on the AC susceptibility signal vs. transport current, and the depression of zero-bias voltage supercurrent
Keeping the random Josephson weak link assembly in mind, we have carved a bulk material of YBa2Cu3OT-xto a bridge" with a cross-sectional area o f 0.1 x 0.5ram 2 and 0.3mm in length, and the V - - I characteristics o f which under microwave irradiation (to / 2n = 5.8 GHz) is investigated. It is surprising that the observed V ~ I curves are similar to the characteristics o f a single Josephson junction. As shown in Fig. 2 (5), clear Shapiro steps appear with the correct relation of V = (2eta/'fi)n Where n is an integer. Moreover, with this kind of "bridge" being narrowed to smaller cross-section, the Shapiro steps still exist even though the Ic is reduced to 1 / 1000 o f the original value. Since the coherence length o f YBa2Cu307-x has been found to be only a few nm (6), which is much smaller than the size o f our "bridge', it seems unlikely that one can assign it as a conventional mierobridge. Therefore we termed it a "large bridge" representing the new intrinsic weak link properties. It is not clear enough now why this "large bridge" behaves as a single Josephson junction. Only some possibilities are assumed as follows. Instead o f independent weak links in the "network", there might be a rather complicated coupling between the links. The observed AC Josephson effect was a cooperative result o f this coupled system. Let the V - - I characteristics o f a weak link be represented by the following nonlinear vector function o f R: f(R) = ( x ( R ) , y ( R ) , z ( R ) ) . In the presence o f small number o f weak links, for example, two weak links only, the combined effect may not equal that o f
I //,,5~
a single junction, i.e.
E
~ 4 >
t//// -8
I
-4
I 0 B (GI
FIGURE
,oo
I
I
4
8
1
0921-4534/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
f(Rl)+f(R2):C=f(R1, R2). But when the number o f weak links is very large (--~'~), then the equivalent effect may turn back into that o f a single junction, i.e., F(~) = f ( R ) where F is an operator representing the cooperative mode, and = (f(Ri), f(R2)""f(Ri)"")
1414
C.X. Fan et a L / SQUID magnetometer operated at 77 K .4 40
2O
°°=
"~.2
~ ~
.-~ O, > 7
,
?,
,
0
-20
1
2
T.77K ! • 5.8 GHz
-6
-2
-4
0
I
I
I
2
4
6
..
l(mA)
3. THE DC-SQUID Based on this "large bridge" conception, a DC-SQUID has been made in this laboratory (7). Initially we supposed that ff a hole is pierced through a large bridge, the later might be divided into two bridges to form a DC-SQUID. The typical result of several devices at LN2 temperature is shown in Fig. 3 (a), which is the transmission function (sinusoidal wave) of a DC-SQUID. An inductive coil attached to this device and some electronics added serve as a closed-loop magnetometer.
==
~
1
~"--1/
FIGURE 2 when the bridge region is narrowed down, the array of junctions is divided, yet the Josephson behaviour persists provided i is large enough even though Jc dramatically decreases. Another possible explanation is that only one weak link which has the smallest Jc responses the measured sensitivity to B. The rest of the weak links is still under superconducting state at given current and does not show up on V ~ I curves. Of course, other mechanism should be possible, too.
> =L ca v
~
0
'
I
I 1
2
'b
l 3
T (rain).
FIGURE 4 The output voltage and the noise level are shown in Fig. 4(a) and (b), respectively. The flux sensitivity can be further calculated as 4 x 10-400 / Hz I / 2 for this prototype magnetometer. As shown in Fig. 3(b) and 3 ( c ) , either B ramps up or down, only the first few sinusoidal patterns are perfect, and the latter waveforms are usually distorted and aperiodic. This hysterises phenomenon may be caused by the flux trapping in the material. Amplitude modulation of these sinusoidal patterns are also obtained which has been observed at a DC-SQUID with a third near-by junction. The main advantage of this LN: DC-SQUID is that it can suffer strong electric transients. The high flux sensitivity of our device is comparable with conventional liquid helium devices. However, the field sensitivity is low since the aperture is small (several micrometers in diameter) which was originally designed to avoid noise because no magnetic shielding was used in these experiments. 4. CONCLUSIONS A random assembly of Josephson weak links for the sintered YBazCu3OT-x superconductor has been proposed through several different measurements. A prototype DC-SQUID magnetometer has" been made of such material with excellent flux sensitivity under 77 K, yet for practical applications there still exist many problems which should be improved for this kind of device.
b $C
REFERENCES
SC
I I I B (tGIdiv)
FIGURE
I
3
(1) R.J. Cava, R. B. VanDover, B. Batlogg and E. A. Rietman, Phy. Rev. Left. 58 (1987) 408. (2) K. A. M'*iller, M. Takashise and J. G. Bednorz, Phys. Rev. Left. 58 (1987) 1143. (3) B.Zhou, J. W. Qiu, Z. M. Tang, B. C. Miao, and Y. J. Qian, Int. J. Mod. Phys. IB (1987) 521. (4) Y.J. Qian, J. W. Qin, K. Y. Chen, B. Zhou, B. C. Miao, Y. M. Cai and Z. M. Tang, in print. (5) C.X. Fan, L. Sun, B. C. Miao and J. Liu, Solid State Commun. 64 (| 987) 589. (6) B. Oh, K. Char, A. D. Kent, M. Naito, M. R. Beasley, T. H. Geballe, R . H . Hammond and A. Kapitulnik, in print. (7) C. X. Fan, L. S. Dai, J. Z. Ruan, L, Sun, L. K. Yu and B. C. Miao, in print.
A D C - S Q U I D MAGNETOMETER OPERATED AT 77K C.X. FAN, Y.J. QIAN, L.S. DAI and Z.Y. HUA Fudan University, Shanghai, China A prototype l ak'ge bridge D C - S Q U I D magnetometer is prepared, the equivalent flux noise o f which can be down to 4 x 10"* ~bo Hz -72. 1. THE N E T W O R K M O D E L The structure and characteristics of sintered oxide
under microwave irradiation. All these results proved that the proposed "network'model seems to be acceptable.
superconductors, such as YBa2Cu3OT-x, have been extensively studied ( 1, 2). The results o f measured critical current Ic and resistance R at current-induced resistive state as functions of
2. THE L A R G E BRIDGE
applied magnetic field B are shown in Fig. 1 (3). Those results give two distinct features: Ic dramatically decreases at very low field ( < 10 G) and R sharply varies with B. For interpreting these phenomena, we have proposed a "network" model (3), i. e. a sintered bulk sample which can be described as a network of random Josephson weak links. Magnetic field effectively depresses Ic o f these weak links and when the current reaches the effective critical current of "network', the weak links lose their superconductivity gradually, the resistance R appears and afterwards give the sharp change o f R - - B curve. The I c ~ B curve differs from the diffraction pattern of a single Josephson junction. No periodical structure can be found and Ic does not go to zero even at high B; however, the experimental data can be well fitted in with the theoretical calculation if a proper distribution function o f diffraction period is assumed. More experiments have been then performed to check this model. The most important results are (4) :high hysterises in Ic B curves at B = 0-3 Tesla; two resistive jumps on the V ~ I curve; two steps on the AC susceptibility signal vs. transport current, and the depression of zero-bias voltage supercurrent
Keeping the random Josephson weak link assembly in mind, we have carved a bulk material of YBa2Cu3OT-xto a bridge" with a cross-sectional area o f 0.1 x 0.5ram 2 and 0.3mm in length, and the V - - I characteristics o f which under microwave irradiation (to / 2n = 5.8 GHz) is investigated. It is surprising that the observed V ~ I curves are similar to the characteristics o f a single Josephson junction. As shown in Fig. 2 (5), clear Shapiro steps appear with the correct relation of V = (2eta/'fi)n Where n is an integer. Moreover, with this kind of "bridge" being narrowed to smaller cross-section, the Shapiro steps still exist even though the Ic is reduced to 1 / 1000 o f the original value. Since the coherence length o f YBa2Cu307-x has been found to be only a few nm (6), which is much smaller than the size o f our "bridge', it seems unlikely that one can assign it as a conventional mierobridge. Therefore we termed it a "large bridge" representing the new intrinsic weak link properties. It is not clear enough now why this "large bridge" behaves as a single Josephson junction. Only some possibilities are assumed as follows. Instead o f independent weak links in the "network", there might be a rather complicated coupling between the links. The observed AC Josephson effect was a cooperative result o f this coupled system. Let the V - - I characteristics o f a weak link be represented by the following nonlinear vector function o f R: f(R) = ( x ( R ) , y ( R ) , z ( R ) ) . In the presence o f small number o f weak links, for example, two weak links only, the combined effect may not equal that o f
I //,,5~
a single junction, i.e.
E
~ 4 >
t//// -8
I
-4
I 0 B (GI
FIGURE
,oo
I
I
4
8
1
0921-4534/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
f(Rl)+f(R2):C=f(R1, R2). But when the number o f weak links is very large (--~'~), then the equivalent effect may turn back into that o f a single junction, i.e., F(~) = f ( R ) where F is an operator representing the cooperative mode, and = (f(Ri), f(R2)""f(Ri)"")
1414
C.X. Fan et a L / SQUID magnetometer operated at 77 K .4 40
2O
°°=
"~.2
~ ~
.-~ O, > 7
,
?,
,
0
-20
1
2
T.77K ! • 5.8 GHz
-6
-2
-4
0
I
I
I
2
4
6
..
l(mA)
3. THE DC-SQUID Based on this "large bridge" conception, a DC-SQUID has been made in this laboratory (7). Initially we supposed that ff a hole is pierced through a large bridge, the later might be divided into two bridges to form a DC-SQUID. The typical result of several devices at LN2 temperature is shown in Fig. 3 (a), which is the transmission function (sinusoidal wave) of a DC-SQUID. An inductive coil attached to this device and some electronics added serve as a closed-loop magnetometer.
==
~
1
~"--1/
FIGURE 2 when the bridge region is narrowed down, the array of junctions is divided, yet the Josephson behaviour persists provided i is large enough even though Jc dramatically decreases. Another possible explanation is that only one weak link which has the smallest Jc responses the measured sensitivity to B. The rest of the weak links is still under superconducting state at given current and does not show up on V ~ I curves. Of course, other mechanism should be possible, too.
> =L ca v
~
0
'
I
I 1
2
'b
l 3
T (rain).
FIGURE 4 The output voltage and the noise level are shown in Fig. 4(a) and (b), respectively. The flux sensitivity can be further calculated as 4 x 10-400 / Hz I / 2 for this prototype magnetometer. As shown in Fig. 3(b) and 3 ( c ) , either B ramps up or down, only the first few sinusoidal patterns are perfect, and the latter waveforms are usually distorted and aperiodic. This hysterises phenomenon may be caused by the flux trapping in the material. Amplitude modulation of these sinusoidal patterns are also obtained which has been observed at a DC-SQUID with a third near-by junction. The main advantage of this LN: DC-SQUID is that it can suffer strong electric transients. The high flux sensitivity of our device is comparable with conventional liquid helium devices. However, the field sensitivity is low since the aperture is small (several micrometers in diameter) which was originally designed to avoid noise because no magnetic shielding was used in these experiments. 4. CONCLUSIONS A random assembly of Josephson weak links for the sintered YBazCu3OT-x superconductor has been proposed through several different measurements. A prototype DC-SQUID magnetometer has" been made of such material with excellent flux sensitivity under 77 K, yet for practical applications there still exist many problems which should be improved for this kind of device.
b $C
REFERENCES
SC
I I I B (tGIdiv)
FIGURE
I
3
(1) R.J. Cava, R. B. VanDover, B. Batlogg and E. A. Rietman, Phy. Rev. Left. 58 (1987) 408. (2) K. A. M'*iller, M. Takashise and J. G. Bednorz, Phys. Rev. Left. 58 (1987) 1143. (3) B.Zhou, J. W. Qiu, Z. M. Tang, B. C. Miao, and Y. J. Qian, Int. J. Mod. Phys. IB (1987) 521. (4) Y.J. Qian, J. W. Qin, K. Y. Chen, B. Zhou, B. C. Miao, Y. M. Cai and Z. M. Tang, in print. (5) C.X. Fan, L. Sun, B. C. Miao and J. Liu, Solid State Commun. 64 (| 987) 589. (6) B. Oh, K. Char, A. D. Kent, M. Naito, M. R. Beasley, T. H. Geballe, R . H . Hammond and A. Kapitulnik, in print. (7) C. X. Fan, L. S. Dai, J. Z. Ruan, L, Sun, L. K. Yu and B. C. Miao, in print.