A LOOK AT THE HISTORY OF SUPERCONDUCTIVITY IN BULGARIA

 

Elena Nazarova,

Georgi Nadjakov Institute of Solid State Physics, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee Blvd., 1784 Sofia, Bulgaria, nazarova@issp.bas.bg

 

 

Abstract. The development of Low Temperature Physics and Superconductivity in Bulgaria begins with establishment of Low Temperature Laboratory at the former Physical Institute with Atomic Scientific Experimental Base in 1963. Three main stages exist in almost half century history of Bulgarian Low Temperature Physics. The first ten years period is connected with elaboration of technology for production and conservation of liquid nitrogen and helium. The second (fourteen years) period (1973-1986) is characterized with investigations of conventional superconductors: the “spin-glass” state, coexistence of antiferromagnetic and superconducting phases, AC losses in type II superconductors ect. The third period is initiated by the discovery of non-conventional high temperature superconductors. The good perspectives for practical application of “nitrogen” superconductors stimulated many scientific organizations to start experimental and theoretical research on this topic. Based on international collaboration Bulgaria developed its own specialists on superconductivity. Many of them worked and others still work at the authoritative centers of superconductivity all over the world.

 

1 Introduction

 

In 2011 the scientific community notes the 100 -th anniversary of the discovery of superconductivity. The Netherlandish physicist and Nobel laureate Heike Kamerlingh Onnes observed this remarkable phenomenon in 1911. At this time Bulgarian Literary Society (established in Braila, Romania – 1869) is transformed to Bulgarian Academy of Sciences –BAS (6.03.1911). Academician G. Nadjakov founded and headed the first Physical Institute with Atomic Scientific Experimental Base (FI with ASEB) in the frame of BAS in 1946. Research investigations in the field of low temperature physics and superconductivity in Bulgaria started with the officially establishment (5.07.1963) of the Low Temperature Laboratory (LTL) at the former FI with ASEB [1]. The main initiator for that was Prof. Eugeny Leyarovski. The enthusiasm and efforts of young colleagues was supported by the deputy director of the institute Prof. Sazdo Ivanov, who was the first head of the Laboratory.

 

2 First period

 

Conventional superconductivity, discovered by H. K. Onnes, is a low temperature phenomenon and investigations in this field needed special equipment. In the first ten years period (until 1973) the technology of production and conservation of liquid nitrogen and helium was developed. The institute obtained first helium liquefier and about ten nitrogen stations for production of liquid nitrogen. This initiated the research work on the heat conductivity of pure superconducting metals and alloys and the influence of crystal defects on the kinetic properties of solids at low temperatures. Special attention was paid to the application of adiabatic demagnetization for production of low temperatures and cryogenic engineering problems. The research work focused on studying the adsorption/desorption of inert gases at cryogenic temperatures, transport properties of metals and deformation defects behavior at low temperatures was also conducted. Heat conductivity of polycrystalline indium in superconducting state is investigated and phonon mechanism is established below T=0.6 K, while electron mechanism is found above this temperature [3].

In this period Physical Department at Sofia University also obtained helium liquefier from Czech Republic. Unfortunately the liquefier’s compressor was problematic and the installation was unsuccessful. However new elective course on Superconductivity attracted students attention. Lectures were presented by the young assistant P. Vassilev.

The initial steps of a new Low Temperatures Lab are extraordinary not only in the frame of our country, but also had international significance. Two important projects between LTL and National Committee for Science and Technical Progress were accomplished: (a) the contract for research and development of small helium turboexpander and (b) the application of multiple adsorption/desorption method for attaining pure gases of He and Ne from wasted gases in nitrogen/oxygen production in industrial plants. These projects gave the financial support for new equipment supply, enlargement of staff and validated the LTL as the only group in Bulgaria dealing with R&D in the area of low temperature physics and engineering [1]. The scientific results from one of the project brought the first international recognition for the LTL. The presented “New method for attaining pure gases of He and Ne from wasted gases” by Prof. E. Leyarovski won two gold medals awards from the World patents exhibition in Geneva (1973) and Brussels (1975) [2]. The other important event was the agreement signed in 1968 between the Academies of Sciences of Poland, Bulgaria, the former GDR and USSR on the foundation of International Laboratory of High Magnetic Fields and Low Temperatures in Wroclaw, Poland. The purpose of establishing the Laboratory was best described in the introduction of the Agreement: “With a view to conducting theoretical and experimental research in the area of high magnetic fields and low temperatures, the BAS, the PAS and Russian Academy of Sciences have agreed…”.

The establishment of the International Laboratory in Wroclaw had big influence on the development of low temperature and superconductivity investigations in Bulgaria in the second (fourteen years) period (1973-1986). Many researchers from Bulgarian Academy of Sciences and Sofia University visited Wroclaw to use the experimental equipment and share their knowledge. It is a great honour for the Bulgarian low temperature science that Prof. E. Leyarovski was elected as deputy director of the International Laboratory in the period of 1974-1977.

 

3. Second period

 

In the beginning of second period some administrative changes take place in BAS. The former Institute of Physics with ASEB was split into Institute of Solid State Physics (ISSP) and Institute of Nuclear Research and Nuclear Energy by a Decree of the Bulgarian Ministry Council from October 16, 1972. However the real existence of these scientific institutions dates since January 1973. ISSP was directed to specialize in fundamental and applied research in the field of condensed matter physics, optics, spectroscopy and laser physics. A new department of Magnetism and Low Temperatures at ISSP incorporated three groups: Magnetism from the Faculty of Physics in Sofia University (headed by Prof. A. Apostolov), Low temperature Physics (headed by Prof. E. Leyarovski) and Applied superconductivity and cryogenics (headed by Prof. V. Kovachev).

At this period special attention is paid to the development of experimental techniques for obtaining of low temperatures (below 1 K) and nanokelvin range (300 nK) has been reached. From today's point of view especially important was the search for superconductivity below 1K in transition metal borides (in 1979). A lot of compounds with the formula MeB2 (Me=Ti, Zr, Hf, V, Nb, Ta, Cr, Mo) were examined. Superconductivity was observed only in NbB2 with Tc=0.62 K [4]. Unfortunately MgB2 was not among the investigated compounds and superconducting transition with 39 K was observed in it as late as 2001. The other important results are the discovery of new superconductors in the system Nb-Al [5], and the coexistence of antiferromagnetic and superconducting phases [6].

The other specific direction of investigations was the alternating current (AC) losses in type II superconductors: A-15 (Nb3Sn, Nb3Ge) [7], C-15 (V2Hf and other ternary Lave’s phases compounds) [8] and B-1 (NbN) [9]. Loss values were obtained by an electronic wattmeter multiplying two signals: one proportional to the voltage induced in the sample and second proportional to the AC component of the magnetic field. For the first time in the literature a minimum of AC loss in Nb3Sn under AC and DC (direct current) magnetic fields was reported, analyzed and modeled in details [10-12]. All the results of the group working on this topic have been presented and discussed in the monograph of Prof. V. Kovachev “Energy dissipation in Superconducting Materials”, Oxford Science Publication, Clarendon Press, 1991, Oxford, UK.

The group of “Applied superconductivity and cryogenics” headed by Prof. V. Kovachev participated in the Cryogenics program of countries from former Council for Mutual Economic Assistance.  As a leading group of AC loss measurements in superconducting materials it organized the Workshop where scientists from Moscow, Kiev, Wroclaw and Bratislava have been introduced to the measurement technique. In 1982 in the frame of this program a Conference “Electro-conductance in electro-energetic and electro-technique” take place in Varna, Bulgaria with 60 specialists from Russia, Poland, Chehoslovakia and Bulgaria. Among the Russian participants was Alexei Abrikosov. He was awarded the 2003 Nobel Prize in Physics for his research in superconductivity. In 1971 and 1983 Bulgaria hosted the 10-th and 21-th International Conferences of countries from the Council for Mutual Economic Assistance on Low Temperature Physics and Techniques.

 

4 Third period

 

The last period in superconductivity research in Bulgaria is connected with the high temperature superconductivity (HTS) discovered in 1986 by G. Bednorz and K. Muller. On the 20 April 1987 specialists from the Institute of Solid State Physics registered superconducting transition at Tc(0.5Rn)= 86.5 K in Y-Ba-Cu-Pt-O system [13, 14]. Thus the era of “nitrogen” superconductors has been started in Bulgaria too. The big enthusiasm in the beginning was connected with the more realistic perspectives for practical applications. On the other hand the difficulties supporting the experimental investigations of conventional superconductors were overcome. Many new scientific institutions were included in the preparation and investigations of new HTS materials in different form: single- and poly-crystals, thin and thick films, multi-structures, tapes and wires. Very often the investigations have been carried out on collaboration between the researchers from different scientific institutions. Some of them started his investigations in superconductivity with the discovery of HTS: Institute of Electronics – Bulgarian Academy of Sciences (BAS), Institute of General and Inorganic Chemistry-BAS, University of Chemical Technology and Metallurgy, Chemistry Department of Sofia University.  More experienced colleagues are from Institute of Solid State Physics (Superconductivity and superconducting materials laboratory; Low temperature and magnetic phenomena laboratory) and Physics Department of Sofia University. However every scientific institution has its own aspect in HTS investigations.

For example in the Institute of General and Inorganic Chemistry (BAS) synthesis of different superconducting materials have been performed [15-17], but particular emphasis was on X-ray diffraction methods including qualitative and quantitative phase analysis, diffraction line-broadening analysis (crystallite size distribution and micro-strains), as well as structure refinement by the Rietveld method on different superconducting materials.

In fact the superconductivity investigations started at the Institute of Electronics (BAS) a few years before discovery of HTS [18]. The earliest articles on superconductivity are connected with calculations of 1/f noise power spectrum for thin film devices (bolometers, Josephson junctions and SQUIDs) [19], analysis of the RF SQUID in regime, where the bias frequency (ω) is comparable to the natural frequency of the SQUID ring [20] and modeling the very high-frequency (VHF) phenomena in current driven Josephson junction [21]. In the beginning of high-Tc superconductivity metal additions and substitutions in polycristalline samples have been investigated [22, 23], as well tehnological aspects for thin films preparation by laser ablation [24] and radio frequency magnetron sputering [25], optical emission of laser induced plasma plume [26] and development of different methods for samples investigations [27]. In spite of all Institute of Electronics has his specific field of researh connected with development of cryoelectronics-especially in investigations of high – Tc Josephson Junctions and SQUIDs [28].

University of Chemical Technology and Metallurgy, Sofia, was included in high –Tc superconducting materials preparation and investigation almost in the beginning of this research [29]. The influence of different additives (Ag, Te, Sn) on the microstructure and phase formation in 1-2-3 and Bi-based superconducting systems is investigated systematically [30, 31]. Samples have been prepared by standard solid state reaction method and by melt-quenching method as well [32]. Spectrophotometric method for determination of oxygen content has been developed in cooperation of specialists from the chair of Analytical Chemistry and Institute of Solid State Physics (BAS) [33].

After the successful start in high-Tc superconductivity specialist from the Institute of Solid State Physics (BAS) continue their work. Single-phase polycrystalline samples have been obtained by “wet” nitrate and solid state reaction methods. The coexistence of Meissner domains and mixed state phase was established below the Earth’s field in YBCO system [34], the role of 4f electrons on the formation and coexistence of superconductivity and magnetism, thermodynamic fluctuations of the superconducting order parameter in 1-2-3 superconducting system were studied. The influence of different substitutions and additives on the magnetic properties of 1-2-3 and Bi-based superconducting systems is investigated systematically [32, 35]. Different new superconducting systems: (Pb, M)Sr2(Y, Ca)Cu2Oz with M=Sn; Ag, (Hg1-xSnxBa2Ca3Oy; cadmium analog of the mercury system; (Pb,М)Sr2YCu2Oz; Sn - doped  Ru-1222 have been synthesized and investigated [36-38]. Systematic investigations of the overdoped state in 1-2-3 superconducting system were carried out [39]. First generation superconducting tapes with Bi-based and Y(Ca)BCO superconducting core were produced and investigated [40, 41]. All this work has been carried out in the frame of many project with National Scientific Fund and international projects with NATO and Europen Euratom program.

The work on high-Tc superconductivity in Physics Department in Sofia University started immediately after the publication of J.G. Bednorz and K. A. Muller in Zeitachrift fur Physik B [42]. The first efforts are connected with obtaining the La-Ba-Cu-O superconducting system. It was late understood that as a result of possible replacement of La and Ba atoms this system is not so easy to be obtained. For that reason La based compound with 1-2-3 stoichiometry was the last obtained with critical temperature in the range of 90 K. In spite of this initial disappointment the colleagues from Physics Department have their great contribution to the development of high-Tc superconductivity in Bulgaria. The important theoretical articles appeared, where possible mechanisms of HTS were discussed [43-45]. A lot of experimental work has been done in the field of Raman spectroscopy of new materials [46-48]. Bulgarian group became one of the leading in the world. The head of the group, Prof. M. Iliev, was invited by P. Chu to establish the Raman Spectroscopy Lab at Superconductivity center in Huston, USA.

Bulgaria developed its own school of specialists on superconductivity. Many of them worked or still work at the authoritative centers of superconductivity all over the world: in USA- Texas Center for Superconductivity (M. Iliev), Superconducting Super Collider Laboratory, Dallas (V. Kovachev), University of California, Riverside (E. Leyarovski), Ohio State University (I. Kostadinov),  Ilinois Institute of Technology (N. Leyarovska); High Energy Accelerator Research Organization (KEK) in Japan (V. Kovachev), University of Wollongoug, Australia (K. Konstantinov), University of Gothenburg, Sweden (Z. Ivanov), in UK- University of Cambridge (R. Tomov, V. Tzaneva), University of Birmingham (R. Chakalov), in Germany- Technical University of Braunschweig and KfA Juelich (S. Tinchev),  Leibniz Institute for Solid State and Materials Research – Dresden (K. Nenkov), Karlsruhe Institute of Technology (S. Terzieva), in Spain - University of Barcelona (K. Zalamova), Catalan Institution for Research and Advanced Studies (V. Skumriev) and many others.

In fact Bulgarian low temperature and superconductivity research has approximately half century history. For this short historical period significant experimental and theoretical results have been obtained. More frequently this is a result of successful international collaboration between our leading scientific organizations in this field and prestigious Laboratories and Institutes in different countries. Especially important result is a creation of qualified specialists working at home and abroad. More difficult is the implementation of superconductivity in different fields of activity in the country. The first scientific apparatus consisting superconducting magnet were supplied at the Institute of Organic Chemistry with Centre of Phyto-chemistry (Bruker Nuclear Magnetic Resonance Spectrometer with 14 T magnet) and Institute of Solid State Physics of BAS (Physical Properties Measurement System with 9 T magnet). In the past 10-15 years many apparatus for Nuclear Magnetic Resonance for medical investigations have been bought (in Tokuda hospital, Losenetz hospital, Military hospital in Sofia and some hospitals in the country). Bulgarian Institute of Metrology plans to provide the voltage etalon based on the Josephson effect. Introducing the high temperature superconductors in different apparatus and machines will increase the applicability of superconductivity in various areas of human activity.

 

Acknowledgments The author is grateful to the colleagues for a critical reading of manuscript.

 

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