Influence of light on a ceramic YBa2Cu3O7−δ SQUID at 77 K

ELSEVIER

Physiea C 223 (1994) 123-130

Influence of light on a ceramic

YBa2Cu3OT_6S Q U I D

at 77 K

R. L a i h o * Wihuri PhysicalLaboratory, Universityof Turku, 20500 Turku, Finland

E.V. II'ichev, V.M. Z a k o s a r e n k o Institute of Microelectronics Technology,Academyof Sciences ofRussia, 142432 Chernogolovka, Russian Federation

Received 13 December 1993 Revised manuscript received25 January 1994

Abstract

Flow of magnetic flux, initiated by visible light, has been observed in a ceramic YBa2Cu3Oxrf SQUID operated at 77 K in the nonhysteretic mode. The response of the SQUID to the applied optical power density is linear and has a coefficient of conversion ~ 90 co/roW Oe, in units of the flux quantum ¢0. Experiments made with illuminating a piece of the SQUID material inserted into its coupling hole show that the induced flux motion has exponential time dependence, especially at small time values. No influence of light on the 1/fnoise figure of the SQUID was observed.

1. Introduction

It has been clear from almost the discovery of highTc metal-oxide superconductors that their applications would be restricted by the relatively low critical current density. In spite of this limitation the possibility to develop superconducting electronics working at liquid nitrogen temperatures seems feasible. It is usually assumed [ 1 ] that operation of a quantum interferometer (SQUID) requires conditions where the Josephson coupling energy is higher than the thermal energy, i.e. I¢~o/2n > kBT, where I¢ is the critical current of the junction and ~o=nh/ e ~ 2.07 × 10- t5 Wb is the flux quantum. However, it has been shown that an rf SQUID made out of ceramic YBa2Cu3Ox is capable of nonhysteretic (dispersive) operation in liquid nitrogen even when the condition 7=2~kBT/Id~o < 1 is not full'died [2]. In * Corresponding author.

the same work it was also shown that the thermal noise appearing in the weak links can be explained rather well on the basis of a resistive model of the Josephson contact. The problem of the thermal noise in ffSQUIDS working in the hysteretic (dissipative) mode [3,4] at 77 K is more complex. Besides the noise mentioned above at least thermally activated irregular jumps between stable states of the SQUID ring will contribute to the noise figure. This is not the case when the SQUID is working in the dispersive mode [ 5 ]. There is evidence that electronic devices made out from high-To superconductors exhibit significant 1 / f noise. According to a theoretical analysis [6 ] this noise is originated from magnetic flux motion and is closely related with the low flux pinning energy and the giant flux creep usually observed in these materials. Another reason for the 1/fnoise was proposed to be in thermally activated transitions typical to twoor multi-level systems [ 7 ].

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R. Laiho et al. / Physica C 223 (1994) 123-130

Our earlier experiments with the ceramic superconductor Lal.sBao.2CuO4 have shown that visible light can influence significantly the magnetic moment of this material [ 8 ]. Recently optically induced increase of the magnetic relaxation rate has been observed in Bi-2212 single crystals at helium temperature [9]. These examples suggest that optically induced flux motion is a common phenomenon in highTc superconductors. It has been attributed to the breaking of Cooper pairs and creation of quasiparticles under illumination as well as to nonequilibrium phonons resulting from the relaxation of the quasiparticles [8,9]. In the present paper we discuss the effect of illumination on a SQUID made out of ceramic YBa2Cu307_ 6.

2. Experimental Ceramic pellets of YBa2Cu307_6 were prepared by the conventional solid state synthesis. The material had the superconducting transition at T¢~ 90 K and the critical current density, measured with the fourterminal probe, about 100 A / c m 2. To fabricate a SQUID a block with dimensions of 3 X 3 X 4 mm 3 was cut from a pellet and a hole with a diameter of I mm was drilled through it. Using a diamond saw a 0.2 mm wide slit was opened between the hole and the outer space leaving a 50 ~tm thick bridge to close the current path around the hole. The structure of the SQUID is schematically shown in Fig. 4. A single-layer copper coil was inserted in the SQUID hole and connected to a tank circuit with Q ~ 50. To the coil were supplied a driving current Irf ( f ~ 2 5 MHz) and a DC bias current IDC to generate a static magnetic field in the hole. The rf voltage applied across the tank circuit was fed to a resonant amplifier followed by an amplitude detector. The detected rf voltage, U, was plotted vs. I~f to investigate the characteristics of the SQUID. In some experiments the SQUID was used as a magnetometer according to a method [ 10 ] in which a 1 kHz modulation current was supplied to the tank coil and U was fed to a lock-in amplifier. The output of the amplifier was connected back to the tank coil to compensate the change of the static magnetic flux through the SQUID. Occasionally this feedback loop was not used and the number of oscillations appear-

ing in the output signal was counted directly to determine the magnetic flux in units of the flux quantum ~o=2.07 × 10 -15 Wb. The noise figure of the SQUID was measured as follows. Arrays of U(t), consisting of averages of U over time intervals r, were measured at the frequency r - 1and stored in a computer memory. The measurements were made at four frequencies corresponding to r=0.1, 1.0, 10 and 100 ms. Each array of U(t) contained N = 1024 or N = 2048 values. Fast Fourier transformation was used to obtain the spectral density of noise within the frequency interval from fmin = 1/2N~ to fmax= 1/2r. For measurements the SQUID was fixed in a specimen holder near to one end of an optical fiber. An Ar ion or a He-Ne laser followed by a variable speed chopper was used for illumination. External magnetic field was shielded out with a combination of cylindrical /t metal and ceramic superconducting shields. For measurements in variable magnetic fields a copper solenoid was wound around the superconducting cylinder. To quarantee efficient cooling of the illuminated surface the specimen was immersed during measurements in liquid nitrogen.

3. Results and discussion 3.1. Characteristics o f the ceramic rf SQUID The properties of ceramic high-To superconducting materials, including the magnetic behaviour, are largely dependent on intergranular currents ascribed to Josephson coupling. When a ring is made from such a material a long-range superconducting order parameter can be established around it [ 1 1 ]. Consequently there will be a direct relationship between the phase difference, O, across a junction and the magnetic flux contained in the ring, O = 2n~/~)o. Supposing that the ring has one weak link the supercurrent flowing through it is I=I~ sin(2n~/0o).

(1)

In practice the ring will contain a great number of intergranular links. Then the parameters in Eq. ( 1 ) are related with the weakest point of the ring [ 11 ]. When an external magnetic flux ~e is applied a

R. Laiho et al. / Physica C 223 (1994) 123-130 2.0

shielding current is induced so that the net flux through the ring takes the form [ 1 ]

O=G -LI¢ sin(2x0/0o),

(2)

~-

(Do o) ~ , ~ _ -

where L is the inductance o f the ring. It is clear that 0 (0e) will be a single- or a multi-valued function depending on the value o f the product LIe. For values of the dimensionless parameter fl~= 2~LI¢/~o > 1 the function 0 ( ~ ) is multi-valued corresponding to jumps between stable states along a hysteresis loop when the external field is cycled up and down. This is called the hysteretic or dissipative operation mode of the SQUID. For fie< 1 the nonlinearity of ~ ( ~ ) will cause a change in the reactive part of the tank circuit impedance which is usually ascribed to the parametric inductance of the Josephson contact. This mode o f operation is known as the nonhysteretic or dissipative mode. In Fig. 1 is plotted the rf voltage across the tank circuit vs. the driving current I~f o f our ceramic YBa2Cu3Ox SQUID. These measurements indicate slightly nonlinear behaviour and a clear frequency dependence at higher values o f I,f. The response o f t4-

SQUID

12

TemperaLure 77 K /

24.76 MHz

.

~

MHz

a

Q I

~c 6 Z

4 2

0

20

40

60

125

~ _

_

b)

d ~ - - ~ - ~ - ~ , _ ~

~-

L =(-~-_~_

O > t.0 2:3

~

-

0.0

~2

_

2~

2~, z~ z'6 d7 Frequency (,MHz) Fig. 2. Amplitude-frequency characteristics of the tank circuit with the ceramic SQUID as reconstructed from a set of the signal characteristics for an integer, integer and quarter and integer and a half number of flux quanta in the SQUID. The examples of the signal characteristics are shown for the frequencies of 24.5 (a), 24.75 (b) and 25.5 Mtlz (c). the S Q U I D to a change of the D C magnetic flux (the pair of the curves observed at 25.00 M H z ) at low levels o f Irf and the fact that detuning leads to a rise o f this response are characteristic to the dispersive operation mode. In Fig. 2 are shown amplitude-frequency characteristics o f the tank circuit connected to the SQUID. The measurements were made at a fixed driving current indicated by the arrow in Fig. 1. It is clear from Fig. 2 that response of the S Q U I D is mainly due to the reactive part o f the circuit impedance caused by the change o f the parametric inductance o f the Josephson contact. The foregoing results were obtained with the S Q U I D immersed in liquid nitrogen at 77 K. They lead us to conclude that it is operated in the dispersive mode with the parameter fie near unity. The fluxto-voltage conversation at the optimal working conditions was found to be ~ / = d U / d ¢ ~ 6 ~V/0o.

80

Drive curreM (nA)

Fig. 1. Current-voltage characteristics of the tank circuit with the ceramic SQUID. The driving current frequencies are 24.76 and 25.00 MHz. The lower curves show the difference between the cases when either an integer or an integer and a half number of flux quanta are traversing the SQUID hole. The arrow indicates the driving current used to determine the amplitude-frequency characteristics.

3.2. Influence of visible light on the flux in the

SQUID An effect o f illumination on the magnetic m o m e n t of ceramic high-Tc superconductor YBa2Cu307_6 has already been investigated in samples placed inside the

R. Laiho et al. / Physica C 223 (1994) 123-130

126

specimen chamber of a SQUID magnetometer [ 12 ]. The phenomenon was observable in the temperature range of T < T¢ although it increased steeply with lowering of the temperature below about 20 K. To investigate the influence of light on the behaviour of our SQUID its outer surface was exposed to laser light through an optical fiber. In the first series of experiments the SQUID fixed on the sample holder with the magnetic shielding cylinders around it was magnetized at a temperature above Tc by using the copper solenoid and then immersed in liquid nitrogen. As a result a distribution of current loops corresponding roughly to the resultant of the applied field and the field of the Earth was frozen in the SQUID sample. In Fig. 3 is shown the dependence of the flux flow through the hole of the SQUID on the intensity of light applied to the input of the optical fiber ( 100 mW of laser power corresponds to ~ 1 m W / m m 2 power density on the surface of the SQUID). It has been suggested that the optically induced magnetic relaxation of high-T~ superconductors is caused by quasiparticles generated when Cooper pairs are broken by light [ 8,9 ]. At low intensities this results in a change of the energy gap by ~A=--2nA o where n is the number of the quasiparticles and Ao is the unperturbed gap parameter. The supercurrent flowing through a junction connecting grains i a n d j is [ 13 ]

I o = I ¢ ( T) s i n ( ~ i - ~ , j - A u ) ,

(3)

!30 !::82brrl/X. Ceramic

40

~1}

60

~->

;70

T

=

77

K

,//

/

/~ ~

I±

•

/*

SQUIB

~

,o / Q 0

r , 0

i/

( ;?!.,r / 0 ~

j,

~

y-{

~ ~ 7~-~ 50

• i

Laser

T i 100

~TbmOeH

: , T I T , , i 1 i 150 200

power

!

-* i

• I I 250

(roW)

Fig. 3. Dependence of the optically induced magnetic flux change

in the SQUID hole on pumping intensity (output power of the laser). The magneticfield values correspond to fields applied by the copper solenoid on the superconducting cylinder used as the shielding.

where ~/i and ~ujare the phases of the supercurrent, A is the vector potential and the critical current can be expressed as [ 14 ]

io(v)_ 2eR.~ tanh \-~ff-~-/~_ot 2 k T "

(4)

Here R,n is the resistance of the junction in the normal state and ot is a constant. The SQUID was cooled down to 77 K in an external field less than 1 0 e parallel to its hole. This is less than the first critical field, W H~t, of the sample material and therefore the supercurrent can be approximated by a set of current filaments circulating the body of the SQUID [ 14 ]. The net flux through its hole can be influenced by reducing I~(T) of the current loops (see Eq. ( 2 ) ) . When this is made by illumination a linear response is expected at modest pumping levels because 8A ~ n ~ light intensity. This agrees with the results presented in Fig. 3. In oxygen deficient YBa2Cu3Ox superconductivity can be generated and enhanced by illumination with visible light [ 15,16 ]. As to the applied photon dose this effect is cumulative in the time scale of hours when the optical pumping is made with a medium power Ar laser. When the illumination is interrupted the relaxation back to the original state is very slow at a low temperature such as 77 K. These features contradict clearly with the response time of our SQUID to illumination, which is in the time scale of seconds. Therefore it cannot be explained with "photoinduced enhancement of superconductivity". The flux change induced by illumination does not depend in a simple way on the strength of the magnetizating field (see Fig. 3 ). This may be explained by differences between the current distributions frozen in the sample when cooling it down in different magnetic fields. In Fig. 4 is shown the optically induced flow of flux through the SQUID hole when the external magnetic field applied with the copper solenoid is swept from zero to 10e. The solid line fitted to the experimental points intercepts the x-axis at a field of ~ 0.45 Oe which corresponds to the compensation of the vertical component of the Earth's field (this direction parallels with the axis of the copper solenoid and that of the SQUID hole).

R. Laiho et aL / Physica C 223 (1994) 123-130

127

60

8o 60".

40-

TP = 7 7 1 " S K m W / m

P = 6 mW/mm 2 T = 77 K

,e~ 402 o

20-

20 © o',

0-

£5,

x -201

-20

ee

~ 1.e5 ,',g

t~ - 4 0

E -4o-

-6Q2 --80

t

0.0

l;It

0

~

i

I

0.2

i

i

i

I

0.4

I

i

i

I

0.6

i

~

~

I

0.8

~

i

0.0

1.0

Magnetic field (Oe) Fig. 4. Light-induced change of the flux in the SQUID as a function of the magneticfield. The conversionfactor is ~ 90 ~o/mW Oe.

It was observed that immediately after each change of the magnetic field the response of the SQUID to illumination was much stronger than the values presented in Fig. 4. To obtain a "steady state" light with 5 times larger power density than that quoted in Fig. 4 was applied to the sample. After this the intensity was reduced momentarily to zero and the subsequent measurements were performed at a power density 1.5 m W / m m 2. The large increase of A~ at the moment when the sample was exposed to light for the first time shows that the trapped flux strongly contributes to the optically induced flow of flux in the SQUID during the first few seconds. This question will be further discussed in section 3.3.

3.3. Influence of light on the magnetic moment of YBazCu307_~ To investigate the response of the basic YBa2Cu307_6 material to illumination two samples weighing 1.85 and 0.4 mg were cut from the original pellet and inserted on top of an optical fiber into the hole of the ceramic SQUID. The fiber and the sample were surrounded with a thin aluminium foil in order to prevent the illumination of the SQUID itself. As can be seen by comparing Figs. 4 and 5 the opticaUy induced change of the magnetic moment of the basic material and the light induced flux flow in the SQUID are very similar. The fact that the change of the magnetic moment, M, is equal in both samples

0.2 0.4 Magnetic

L,ght

0.6 0.8 f i e l d (Oe)

1.0

Fig. 5. Magnetic field dependence of the optically induced change of the magnetic moment in ceramic YBa2Cu30~_~ samples weighing 0.4 and 1.85 nag. 40D

"G"

?

,D

~

Deooo

3O

D~

E

20 E]~

Sample 1.85 mg T = 77 K P = 10 mW/mm 2

*

Cb

D

10

2'0

4'o

6'o

Time (s) Fig. 6. Time dependence of the optically induced magnetic moment change when the light is switched on (squares) or off (stars). The experiment is made in the field of 50 mOe.

although their masses differ by a factor of 5 confirms that the phenomenon is confined in a thin surface layer of the sample. In Fig. 6 is shown the time dependence of the change of the magnetic moment when the illumination falling on the ceramic sample is switched on (squares) and off (stars). As can be observed from Fig. 7 the plot of the time derivative dM/dt vs. t in a semilogarithmic scale gives a straight line showing that the change of M follows an exponential law

M(t) =Mo exp(-t/z) ,

(5)

R. Laiho et al. /Physica C 223 (1994) 123-130

128

,,rrl i:

~

l

','~ !!

I

/ ,~ !< 1,} fl v'v.."l/ ;~

I: d~ 'i ¸

() t

.

t

,[I C, 1

\,

':;' U() I

i

C'

i ~'()

f I

i

, 4r)

p

1 I

I

Fig. 7. Time derivatives o f the magnetic m o m e n t change M from Fig. 6 (logarithmic scale ) vs. time. The straight line is a fit to Eq. ( 5 ) with r = 4.9 s. Light is switched on (squares), light is switched off (stars).

with z ~ 5 s. When the light is switched off the magnetic moment changes for the first 10-20 s according to this time constant and then starts to stabilize slowly. It is clear from our results that the rate of the magnetization relaxation is much larger under illumination than in ceramic YBa2Cu307_a without illumination [ 17 ]. This agrees with observations made in Bi-2212 single crystals [ 9 ]. Formally the flux motion caused by illumination is similar to that initiated by normal heating of the lattice. Light can excite in the sample A s and BI~ (outof-phase oxygen plane vibration) optic phonon modes which exhibit strong anharmonic linewidth broadening due to an effective electron-phonon coupling [ 18 ]. These phonons thermalize by interacting with electrons and escape partially from the surface of the sample to the surrounding N2 bath. Another source of nonequilibrium phonons is the breaking of Cooper pairs under illumination and the quasiparticles produced during recombination of the electrons [ 8,9 ]. At the moment of their generation the quasiparticles are not in a thermal equilibrium with the surrounding lattice. Due to the bottleneck processes of their relaxation paths the lifetime of the quasiparticles may be quite long, r * = 1 0 - 9 - 1 0 - 6 S [ 19,20]. Estimating the diffusion length of the quasiparticles from the equation L ~ (vqz*l) 1/2 where vq is their velocity and l is their mean free path (comparable to the distance

between the C H O 2 layers in the lattice, about 1 nm) and assuming that Vq< vF (Fermi velocity of electrons in metals, 105 m / s ) one finds that L can be of the order of microns, approaching the dimensions of grains in the ceramic sample. In this way the illumination will lead to heating of the flux pinning centers in a thin surface layer of the specimen. For the next we estimate the overheating of the SQUID surface, AT, required to reduce Ic in accordance with the observed flux change (see Figs. 3 and 4). Inserting in Eqs. (2) and (4) the values of L = 0.5 nH (as inferred from the dimensions of the SQUID), Rnn = 5 ~'], and A ( T ) ~: ( 1 - T / T c ) 1 / 2 (gap parameter of a BCS superconductor) we get AT=0.2 K for a flux change of 30 ¢o. This result is realistic although the value of Rnn is open to question. The resistance 5 f~ corresponds to a high-To YBaCuO thin film grain boundary junction [ 21 ] and may well be larger in our ceramic material. This could easily reduce the value of A T t o less than 0.1 K. In a relaxation experiment on superconductors one investigates the time evolution of the total magnetic moment of the sample due to decay of metastable current configurations or vortices. In Ref. [ 9 ] a nearly logarithmic time dependence of the remanent magnetization, obtained in fields of 200- 1600 Oe, was observed under illumination. Magnetization in this kind of fields brings the sample into the Abrikosov vortex state. Our results correspond to the opposite case or magnetization in a weak field (less than 1 0 e ) resulting in circulation of current around the edges of the specimen and trapping of flux in intergranular weak links. Another important difference is that the data of ref. [ 9 ] were obtained at 4.2 K and our measurements were made at 77 K. However, these differences cannot provide a straightforward explanation to the qualitative difference of the flux creep data under illumination observed by us (exponential decay) and in ref. [9] (logarithmic decay). Eq. (5) represents a limiting case of stretched exponential decay defined by the Kohlrausch expression [22] M ( T ) =Mo e x p ( - t / r ) B ,

(6)

where 0 < fl< 1 is the dispersion parameter. Eq. 6 has been used to describe the decay of a number of physical properties of disordered matter [23 ]. Applying

R. Laiho et aL / Physica C 223 (1994) 123-130

the model of serial dynamics [24] we assume that when the temperature of the specimen slowly increases with time after switching on the illumination the distribution of the supercurrent changes in a constrained way - the system must pick a path through a sequence of sparse states (current configurations) sampling an appreciable fraction of all states in the process. In our case the constraints are the granular structure of the material and the distribution of the critical current values of intergranular weak links. This picture resembles the model of weakly linked two-dimensional clusters of superconducting grains which was shown to have magnetic properties similar to spin glasses [25 ]. Several authors have observed a stretched exponential decay in random magnets, including spin glasses [26-29]. The connection between this behaviour and hierarchically constrained dynamics has also been theoretically established [ 24 ]. In our data the value offl~ 1 indicates a weakly interacting system. Further conclusions warrant for some caution because the current understanding of the microscopic mechanism of photoinduced flux creep in high-T~ superconductors is still insufficient.

10

i

129

~ R s ;, ~'. ~'~I'"

..... *',,, ~aa~ Dauuu - -

H 0.85 0e H = 0 . 7 5 Oe H = 0.50e H = 0 electronics noise

~°'~f:-.

lo

~ ,gXxOD~

10 -4 ~' '"'"1o -{ ....... 1l ........lO' ' ' '"'"'1o £ ......1 ~) 3' ' '~ Frequency (Hz)

Fig. 8. Noise spectraldensity of the ceramic YBa2Cu307_6 SQUID in various magnetic fields either unilluminated (large symbols) or illuminated with ~ 1.5 r o W / r a m 2 H e - N e laser power (small symbols ).

tion of flux induced by illumination and by rising the temperature of the bulk of the sample. The fact that illumination did not increase the noise of the SQUID suggests that the thermal motion of flux is not the only source of the 1/fnoise behaviour.

3.4. Noise of the ceramic squid under illumination 4. Conclusions The noise spectrum of the SQUID was measured at 77 K by illuminating on one of its external surfaces with visible light. The measurements were made in applied magnetic fields between 0.85 mOe and the residual field inside the # metal shield (lower than 5 mOe). The residual field was estimated from the response of the SQUID to laser light assuming a linear dependence of A¢ on the field (see Fig. 4). As shown in Fig. 8 the noise spectrum of our SQUID obeys the 1/flaw over the whole range of frequencies and the magnetic fields used in the experiment. No difference of the noise figures could be observed between the cases when the sample was unilluminated (large symbols) or illuminated using the power density of ~1.5 m W / m m 2 (small symbols). The l / f n o i s e of SQUIDs made out of high-T¢ superconductors has been explained by the flux creep phenomenon [ 6 ] or by thermally assisted fluctuations of currents in Josephson junctions due to transitions in a two- or a multi-level system [ 7 ]. We have already noted the formal similarity between the mo-

Influence of visible fight on a ceramic Yla2Cu307_6 rfSQUID operated at 77 K in the dispersive mode with fie close to unity has been observed. The first response of the SQUID to illumination is a flow of flux due to the release of the field trapped on its surface. After that can be detected a signal which is proportional to the light intensity and the applied (small) magnetic field. It is likely that this part of the response results from weakening of the intergranular Josephson junctions under illumination. When a piece of the SQUID material was exposed to light inside its coupling hole a magnetic relaxation with an exponential time decay could be detected. Because the measurements were made in a field much less than H w this effect may be explained by motion of the flux trapped at intergranular weak links. The noise of the investigated SQUID was found to obey the 1 / f law at least over the frequency range 10-1 _ 103 Hz. No influence of an applied field up to 0.85 Oe or light ( P = 1.5 m W / m m 2) could be observed on the noise figure.

130

R. Laiho et al. /Physica C 223 (1994) 123-130

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