Mohan Jacob

Non-Linear Surface Impedance of YBCO

Su perconductors Microwave Superconductivity

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NON-LINEAR SURFACE IMPEDANCE OF YBCO SUPERCONDUCTORS

MOHAN V. JACOB·

School of Engineering and Physical Sciences James Cook University Townsvi lle, OLD 4811

AUSTRALIA

&111;5 PhD "'ork WQ.I' dOIlf! 01 Deparlmem of Electrollic Sciellces, University of Delhi. I"dio /ll/der Ihe slIpen'isioll of Prof. G. P. SrimSlura

TABLE OF CONTENTS

CHAPTER - [ MICROWAVE PROPERT IES OF HIGH TEMPERATURE SUPERCONDUCTORS

1.1 lntroduction 1.2 Surface Impedance 1.3 Effect of Phys ical Parameters on Surface Impedance

I. 3.1 Temperature Dependence 1.3.2 Frequency Dependence

Frequency Dependence Due to the Vortices 1.3.3 Power Dependence 1.3.4 Dependence on Microstructure 1.3.5 Dependence on Thickness 1.3.6 Effect of Si lver Doping 1.4 Conclusions

CHAPTER -II ONSET OF NON-LrNEAR LOSSES IN HIGH TEMPERATURE SUPERCONDUCTING MICROWAVE DEVICES

2.1 Introduction 2.2 Granular Superconductors 2.3 Residual Surface Resistance 2.4 Theoretical Studies 2.5 Kinetic lnductance 2.6 Variation of Surface Resistance 2.7 Variation of Resonant Frequency 2.8 Experiments 2.9 Results and Discussion 2. 10 Conclusions

CHAPTER - III NON-LINEAR SURFACE IMPEDANCE OF HIGH T, SUPERCONDUCTORS AT HIGH MICROWAVE POWE R LEVELS

3. 1 Introduction 3.2 Non-linear Phenomena and Models 3.3 Causes of Non-linear Surface Impedance 3.4 Beans Critical State Model 3.5 Modified Critical State Models 3.6 Calculation of~ and fo at High Power Leve ls 3.7 Result s and Discussion

3.7.1 Stripline Resonator 3.7.2 Dielectric Resonator

3.8 Conclusions Appendix 3. 1

ii

2 3 8 8 12 13 16 24 24 25 25

27

28 29 30 30 31 32 33 36 36 42

44

45 45 47 48 48 49 52 53 59 64 65

CHAPTER - IV 66 MEASUREMENT TECHNIQUES

4.1 Introduction 67 4.2 Cavil)' Method 67

4.2.1 End·Wall Replacement Method 70 4.2.2 Endplate Substitution Method 72 4.2.3 Conical Cavity 74 4.2.4 Coaxial Cavity 74 4.2.5 Helical Cavity 75

4.3 Modified Confocal Resonator 77 4.4 Parallel Plate Resonator 78 4.5 Microstrip Method 79

4.5.1 Microstrip Line Resonator 80 4.5.2 Ring resonator 81 4.5.3 Stripline Resonator 82

4.6 Dielectric Resonator 83 4.7 Comparison of Errors 90 4.8 Conclusions 91

Appendix 4.1 Calculation of Resonant Frequency 92 Appendix 4.2 Calculation of Magnetic Field 93 Appendix 4.3 Calculation of Electric Field 94

CHAPTER - V 95 SURFACE [MPEDANCE MEASUREMENT OF YBCO THIN FILMS USING DIELECTRIC RESONATOR

5.1 Introduction 96 5.2 Preparation of YBCO thin film by pulsed laser deposition technique 97 5.3 Analysis of the YBCO thin film by SEM and XRD 98 5.4 Microwave measurements 103

5.4.1 Cal ibration ofHP 8722A Network Analyser 104 5.4.2 Calculation of Unloaded Q factor 104

5.5 Resu lts and Disclission lOS 5.5.1 YBCO thin films prepared by themlul co-evaporat ion method lOS

5.5.2 YBCO thin films prepared by PLD technique 109 5.6 Conclusions 114

CHAPTER- VI CONCLUSIONS 116

6.1 Conclusions of the Work 117 6.2 Suggestion for Future Work 118

REFERENCES 119

'"

Preface The American Physical Society conference of March 1987 announced the remarkable discovery of a superconductor with a transition temperature (Te) of 90 K. Now a decade has passed since the inventi on of High TemperalUre Superconductors (HTS), and many conceivable combinat ions of ceramics and metals have been tested to procure rOOm

temperature superconductivity. So far the goal has not been achieved, but, higher Te have been obtained. Now researchers are concentrating all the application side of superconductors. Many prototype devices have been successful. We can expect a major change in the next few years especially in the field of communications. The study of superconductivity is exciting and results range frol11 new superconductillg compounds through the detection of novel physical phenomena and the development of new theories to the demonstration of new devices.

A myriad of applications has been successfully demonstrated. In the domain of microwave superconductivity, HTS resonators and filters have shown their potential. But the instability of device perfomlance with the input microwave power prevents the operation of these devices at high microwave power levels. The microwave losses are non·linear at high input power levels. The possible reasons for this non·linear loss are not clear.

Halbritter (1990, 92) modelled the non·Jinear phenomena by considering an HTS material as a network of grains and grai n boundaries (Herd et al., 1995). Nguyen et al. (1993) proposed a grain coupled model to explain the non-linear phenomena. They explained the results by a quadrat ic model. However, it was applicable only at low rf power levels. Sridhar (1994) proposed a hysteresis model to explain the nonlinear loss. Wasik et al. (1997) and Hein el al. ( 1997) attribute the non-linear losses to heating effects. So far, none of these models are able to explain the non linear phenomena under all physical cond itions such as temperature, frequency and power. A model which can elucidate the non·linear losses at all temperatures and frequencies is not ava ilable. An adequate model could shed light on this problem and this is the aim of this thesis.

In the rust chapter we discuss the behaviour of superconductors at different temperatures, frequencies, microwave fields, etc. Some of the important studies carried out to investigate the physical behaviour of surface resistance were reviewed in the first chapter. From the literature review it is clear that there is no unique model that explains all physical dependence. Most of the models are either empirical or phenomenological.

The second chapter deals mostly with the effect of microwave power at low power levels. In this chapter we have modelled the non· linear surface impedance. We have obtained a power model to study the non-linear effect's. A quadratic power model could explain the non-linear effects at low and intermediate «50 Oe) power leve ls. This model is similar to the empirical model proposed by Oates et al. (1993). Beyond the intennediate power level, the losses became faster than the quadratic power and hence the model failed beyond the intennediate power level. However, the model helped to explain the onset of non-linear losses in HTS. We used the measurement results of three microslrip resonators to substantiate the model.

We have discussed in detail the different causes of the non-linear loss in high temperature superconducting microwave devices in chapter Ill. It has been found from the critical state model that the current density is a function of the applied rf field. In thh chapter we have modelled the non-linear nlicrowave losses by assuming the exponential model for current density. We compared the model with the experimental results of a stripline resonator and a

dielectric resonator. The results are interesting because at all temperatures and frequencies this model matched the experi mental data for all input power leve ls.

We studied the different measurement techniques (resonant) used for microwave characterisation of high temperature superconducting samples and devices and the results are drawn in chapter rv. in this chapter we di scussed the cavity techniques, paraliel plate methods and microstrip methods. It is understood that a dielectri c resonator will give stable results. Also it is a non·destructi ve method for measuring HTS thin films. Hence we used the dielectric resonator for our measurement and carried out detai led analyses of the di electric resonator. We studied the electric and magnetic field di stributions inside and outside the sapphire rod.

In the fifth chapter we desc ribed the measurement system and the pulsed laser deposition (PLD) techniques used for the deposition of YBCO thin film. We carried out the measurement of two types of YBCO thin films here. The first was tbe pure YBCO thin film deposited by the co-evaporation technique on Lanthanum Aluminate Substrate. We measured the microwave power dependence of the samples at 10 GHz and 25 GHz frequencies at a temperature of 77 K. The second was silver doped YBCO thin films produced by the PLD technique on MgO substrates . We reported the results at a frequency of 25 GHz and a temperature of77 K at differing microwave power levels. We also studied the XRD pattern to investigate the presence of silver. Further, we carried out the SEM analysis of the Ag doped YBCO samples to study the effect of silver doping on the microstructure.

Finally we came to the conclusions of this work in chapter VI and suggest recommendations for future studies in the fie ld. We hope that the role by the semiconductor in thi s century will be played by the more powerful superconductors with less loss in the 21 st century. References are given at the end of the thesis.

This research was carried out at University of Delhi under the supervision of Prof. G. P. Srivastava, Department of Electronic Science. His supervision and suppon is acknowledged.

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