*Corresponding author.E-mail address: samvel@sc.kyushu-u.ac.jp, samvel@ipr.sci.am

(S. Gevorgyan).

Nuclear Instruments and Methods in Physics Research A 444 (2000) 471}475

Advantages of j measurement in #at geometry high-¹#cuprates

by an open-#at coil magnetometer demonstrating its widepossibilities for detection

S. Gevorgyan!,",#,*, T. Kiss!, A. Movsisyan", H. Shirinyan", T. Ohyama!,M. Takeo!, T. Matsushita!, K. Funaki#

!Graduate School of Information Science and Electrical Engineering, Kyushu University, Fukuoka 812-8581, Japan"Institute for Physical Research, National Academy of Sciences, Ashtarak-2 378410, Armenia

#Research Institute of Superconductivity, Kyushu University, Fukuoka 812-8581, Japan

Abstract

We developed an open-#at coil (OFC) preparation technology allowing creation of circular shape #at coils with highQ-factors. It enabled improving of an OFC magnetometer resolution up to *j&0.5 As (*j/j&3]10~7) at j measure-ments in thin HTS at 30 MHz. This permitted to reach the sensitivity &2]10~12 W/cm2 at detection of 0.63 lm laserradiation, which is better than the possibilities of the similar `screeninga type detector based however, on square shape#at coils. Besides, the resolution &30 nA/cm2 was registered at detection of the currents passing through the HTS thin"lms. We used this technique also to study the "ne peculiarities of the vortex motion in high-¹

#cuprates. ( 2000

Elsevier Science B.V. All rights reserved.

1. Introduction

Various aspects of low-temperature physics ofmatter, including many properties of `heliuma orlow-¹

#superconductors (LTS) were suggested as

methods for detection of the particles and radiationand also for determination of the physical para-meters of various external "elds, etc. The summaryof the progress, which has been made and possibleoutlook plans in this area can be found in Refs.[1,2]. However, an inspection of the scienti"c andtechnological advances in this "eld leads to con-clusion that there are yet only a few suggestions to

use the high-¹#

superconductors (HTS) for low-temperature detection. While, if we could use themfor these purposes in fact, at least great savings incryogenic systems' cost and also complexity are outof any doubts. Moreover, our tests made ground tobelieve that HTS-based detectors' possibilities willnot yield the `heliuma temperature ones so much infuture, if appropriately improved. Probably, theycan successfully substitute the LTS ones at least forsome practical applications. Thus, aimed to "ll thisde"ciency in detection area we developed and re-ported recently our considerations on some possi-bilities of use of HTS for detection of particles andradiation, magnetic "elds, currents, etc. [3,4]. Thebasic principles and some designs of new detectorswere o!ered and discussed suitable for both theradio frequency (RF) and microwave applications.

0168-9002/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved.PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 1 4 2 5 - 4 SECTION X.

Fig. 1. Schematics of the OFC magnetometer serving as a de-tecting part for so-called radio frequency `screeninga typedetectors.

The test results of one of them (the so-called RF`screeninga type detector) were reported showingthe feasibility of its creation [5]. In this paper wereport about the improvement of its characteristicsand design. We present also some results of kmeasurement we get by an OFC technique, showingits wide possibilities for application in various "eldsof low-temperature detection.

2. Distinctive peculiarity of HTS materials usedfor high-resolution detection

The main aspect of high-¹#

superconductivity,used as a new method for detection, is a signi"-cantly higher value of the skin depth in HTS atnormal state compared to the ones in LTS (due tomuch higher resistivity of HTS in normal state). Asa consequence, the testing RF or even microwave"eld, applied at the surface of a few micrometerthick HTS material, penetrates easily across thewhole thickness of the HTS in normal state and isscreened (shielded) almost completely in supercon-ducting state. In contrast, the same thick LTSshields strongly the testing "eld in both states dueto small values of the skin depth, d, and the penetra-tion depth, j in LTS. This strong di!erence intesting "eld's screening in normal and supercon-ducting states by a thin HTS material is used asa sensitive method for study of the S/N transitionsin HTS [6]. We suggested its application for par-ticle and radiation detection too [3,4,7].

3. OFC-based detecting technique forhigh-resolution study of the 6at structures

To use this remarkable peculiarity of thin HTSmaterials for the low-temperature detection a `suit-ablea testing technique for detection of small cha-nges of penetration of the external "elds into thin(mainly #at con"guration) high-¹

#cuprates is ne-

cessary. In this connection j is the most appropri-ate detecting parameter. It is one of the basicallyimportant physical parameters, which characterizesthe superconducting state from many sides. More-over, it is sensitive to changes induced by varioustypes of external in#uences. Thus, j can serve as

a convenient detecting parameter. For this, the #attesting con"gurations may have evident advant-ages especially, for study of #at HTS materials.

3.1. Solenoid coil-based traditional testing technique

Comparison among the known methods forj measurement permits to conclude that the `LCresonatora method, used widely for tests at a MHzrange, has advantages compared to others [8]. Dueto low frequency it enables to avoid the possiblequasiparticle excitation by the testing "eld. How-ever, for high resolution massive crystals are used,as a rule, in this technique for LTS. This increasesthe "lling factor and allows to achieve *j&0.2 Asresolution at a MHz range (with the relative resolu-tion *j/j&2]10~4). But this method, applied forthin HTS, makes it possible to achieve the resolu-tion at most *j&10}20 As because of very low"lling factor of the solenoid coils (typically &10~4)arising from the small sample volume for thinplate-like HTS materials.

3.2. Open-yat coil-based improved testing technique

We improved the `LC resonatora method forhigh-resolution detection of changes of j attransition of #at geometry thin HTS substitutingthe solenoid coil by a #at one driven by tunneldiode (TD) oscillator [3,4,9]. Fig. 1 shows schematics

472 S. Gevorgyan et al. / Nuclear Instruments and Methods in Physics Research A 444 (2000) 471}475

Fig. 2. Technological details of design of some of tested #at coils. (a) Typical circular shape coil; (b) typical square shape coil.

of such a testing technique } OFC magnetometer,which serves as a detecting part for the `screeningatype detector [4]. As a sensitive element serves anythin HTS material. The measuring e!ect is deter-mined by a distortion of the coil's "eld con"gura-tion near its face by a HTS material, due to theexternal in#uences, resulting in the shift of the oscil-lator's frequency. As an external in#uence for thesensitive element can serve any elementary particle,various types of an ionizing radiation, applied"elds, the currents penetrating the HTS material,etc. In contrast to the traditional method, in im-proved one the testing "eld's con"guration is verydense near coil's surface. Besides, due to the #atgeometry (providing high "lling factor) the "lm atits transition distorts it much strongly. We meanthe penetration of the #ux of testing "eld throughthe sensitive element, which was previously shiel-ded (screened) by it, in superconducting state. Ac-cording to this work principle we named suchtechnique in Refs. [3,4] as the `screeninga typedetector. The changes, *j, in penetration depth(due to external in#uences) are detected in thismethod as the resonant frequency shifts, dF, of theoscillator by *j"!G]dF where G is a geomet-ric factor. It depends strongly on the geometry oftesting "eld's con"guration near coil and the ge-ometry of sensitive element used. It depends expo-nentially also on the position of the latter from thecoil's face [8]. For high resolution a G-factor, as

small as possible, is required. Thus, we tested vari-ous designs for #at coil (Fig. 2) to maximize thedensity of "eld con"guration near it. We optimizedalso the distance between the sensitive element andthe coil [8]. In these senses the circular #at coilswith dense winding provide better G-factors for theplate-like objects, pressed to theirs surface, than therectangular ones. The stability of oscillators withthe circular #at coils is better also. All this enablesus to reach higher resolution in j tests and hence, inlow-temperature detection too.

4. OFC magnetometer real applications

The creation of the low-power stable-frequencyTD oscillators with a #at (open-faced) coils, operat-ing up to 300 K, is not so easy because of lowQ-factors of the open coils, especially winded fromthe "ne wires (even made of a good conductors). Wesucceeded in high Q-factor #at coil preparationtechnology (that is about 50 at F&10 MHz forrectangular and better than 80 at F&30 MHz forthe circular coils) by optimizing the coil's designand the testing "eld's energy losses inside the coil.Higher stability of circular shape #at coil basedmagnetometers (better than 1 Hz in 30 MHz at300 K) in comparison with the square coil ones,and the used low-loss #at coil preparation techno-logy both, enabled the resolution to reach about

S. Gevorgyan et al. / Nuclear Instruments and Methods in Physics Research A 444 (2000) 471}475 473

SECTION X.

Fig. 3. Magnetic transition curves at di!erent temperatures: (a)experiment: Top inset } "eld con"guration. Bottom inset } be-havior of the curves for small "eld values. (b) Schematic diagram.

*j&0.5 As . At this, the relative resolution is betterthan *j/j&3]10~7).

4.1. Circular coil `screeninga type detector testresults

Such an optimization enabled to improve theparameters of the test detector by 3}7 times incomparison with the ones preliminary reported inRefs. [3}5,7] for the square coil detector. The pres-ently achieved sensitivity is 2]10~12 W/cm2 forthe 0.63 lm wavelength laser radiation, detected bya 1 cm2 circular coil detector. Besides, the resolu-tion &25 lOe/cm2 was achieved at a detection ofthe changes of DC magnetic "eld less than 50 Oemagnitudes and about 1 mOe/cm2 for the "eldsH&104 Oe. The resolution of about 30 nA/cm2

was also registered at a detection of the changes ofDC currents passing through the HTS thin "lms.The achieved parameters are not a limit for theOFC-type testing technique. It is estimated to bebetter more than 2}5 orders of magnitude [3,7].One of the most real ways to increase its resolutionis a reduction of the coil sizes as close as possible tothe specimen sizes for at least 50 lm size specimens.This enables to achieve a high signal-to-noise ratioas large as the method permits (better than 106)even for 50 lm size samples. In this case the sensi-tivities more than 10~17 W for the 0.63 lm radi-ation for 50 lm size HTS "lms is predicted toachieve. This is close to the typical values of theNoise Equivalent Power (NEP) achieved for LTSdetectors [1,2]. An additional way to increase theresolution is substitution of tunnel diodes by theHTS made SIS structures [9] with a few orders ofmagnitude less steepnesses of their I}< curve'snegative di!erential resistance branch. This can in-crease the oscillator frequency stability and hence,will enable to reach the sensitivities at least twoorders of magnitude more. Such improvement maypermit to reach the level of detection of the sup-pression of superconductivity in a few micron-sizeplate-like HTS by a #ux quantum.

4.2. OFC magnetometer use for study of the mixed state

We used the OFC technique to measure themagnetic transition curves of 3 mm in diameter and

200 lm in line width thin-"lm YBCO ring too.Typical curves for dF (or *j) vs. H (Ec) at di!erenttemperatures are shown in Fig. 3. The "eld con"g-uration is shown in top inset to the "gure. The ringsample used was patterned by the typical chemicaletching method from the c-axis oriented and 0.2 lmthick "lm [10]. At small magnetic "elds (at thebeginnings of the mixed state) the measured dataobey the root dependence of dF vs. "eld. At higher

474 S. Gevorgyan et al. / Nuclear Instruments and Methods in Physics Research A 444 (2000) 471}475

"elds the dependence of dF(H) changes from theroot to the linear, then quadratic and then thecurves become sharper compared with quadraticlow, near the end of the S/N transition. Finally, thefrequency is saturated at the same constant value ifthe magnetic "eld H becomes high enough. There-fore, one can determine the value of the uppercritical "eld, H

C2, by the magnetic "eld value at the

saturation point [8]. The achieved large signal-to-noise ratio (&105) for tested small ring shows thatone can use such technique for many-sided study ofthe peculiarities of the vortex dynamics in thin HTSstructures with high resolution. In particular, theregistered root behavior of the transition curves (atsmall "eld values) with subsequent deviation fromit at H

$%1enables to study the peculiarities of a vor-

tex depinning in the mixed state. Besides, the sharpshapes of the curves dF(H) at the end of the S/Ntransition permits, in principle, to separate possiblemechanisms responsible for the "nal destruction ofthe superconductivity in HTS materials at a "eldsclose to H

C2.

5. Conclusion

We developed a circular shape open-#at coil-based magnetometer reaching the resolution*j&0.5 As (relative resolution is better than*j/j&3]10~7) at j measurements. It enabled toimprove the parameters of the RF `screeninga typedetector with square shape pick-up coils by almostan order of magnitude. By reduction of coil sizesit is feasible to reach the sensitivities better than10~17 W at detection of the visible spectrum radi-

ation by a 50 lm size HTS "lms. We suggest touse it widely to study the weakly expressed pecu-liarities of the vortex motion in thin HTS. Suchhigh resolution allows to use this technique also forimprovement of the characteristics of some LTSdetectors (for example, SSG based and `stripa typedetectors [1,2]).

References

[1] Proceedings of LTD6, Nucl. Instr. and Meth. A 370 (1996).[2] S. Cooper (Ed.), Proceedings of LTD7, Munich, Germany,

1997, published by the Max Planck Institute of Physics,1997.

[3] S.G. Gevorgyan, A.L. Movsisyan, G.D. Movsesyan, V.A.Shindyan, H.G. Shirinyan, Mod. Phys. Lett. B 11 (1997)1123.

[4] S.G. Gevorgyan, G.D. Movsesyan, A.A. Movsisyan, A.L.Gyulamiryan, V.A. Shindyan, H.G. Shirinyan, J. Contemp.Phys. (NAS of Armenia) 32 (1997) 33.

[5] S. Gevorgyan, A. Movsisyan, V. Shindyan, H. Shirinyan,G. Movsesyan, in: S. Cooper (Ed.), Proceedings of theLTD7, Munich, Germany, 1997, published by the MaxPlanck Institute of Physics, 1997, pp. 215}216.

[6] S. Gevorgyan, A. Movsisyan, T. Kiss, Y. Hanayama, T.Matsushita, M. Takeo, V. Vysotsky, Advances in Super-conductivity XI, Springer, Tokyo, 1999, pp. 601}604.

[7] S.G. Gevorgyan, A.A. Movsisyan, IEEE Trans. Appl.Supercond. 9 (1999) 3190.

[8] S.G. Gevorgyan, T. Kiss, A.A. Movsisyan, H. Shirinyan, Y.Hanayama, H. Katsube, T. Ohyama, M. Takeo, T. Mat-sushita, K. Funaki, Rev. Sci. Instr. 71 (3) (2000), in press.

[9] S.G. Gevorgyan, G.D. Movsesyan, A.A. Movsisyan,V.T. Tatoyan, H.G. Shirinyan, Rev. Sci. Instr. 69 (9) (1998)2550.

[10] T. Nakamura, Y. Hanayama, T. Kiss, V. Vysotsky, H.Okamoto, T. Matsushita, M. Takeo, F. Irie, K. Yamafuji,1997, Proceedings of the EUCAS'97, pp. 1017}1020.

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