Seminar 2: High Temperature Superconductivity

Transcription

1 Seminar : High Temperature Superconductivity Author:Marko Viršek ( ) Adviser:prof.Dragan Mihailovi Department of Physics, University of Ljubljana May 4, 005 Abstract Bednorz and Muller discovered high-temperature superconductivity in In few years many cooper oxide (cuprates) high-temperature superconductors (HTSCs) were synthesized, with a record at thallium doped, mercury based cuprate with Tc at 138K. Cuprates are layered perovskites, constructed of supercoducting, insulating and hole-donating layers. Most of them contain mixed-valence atoms, such as copper. Cuprates are all antiferromagnetic at zero level of doping and exhibit a complex phase diagram. BCS theory fails at explaining high values of Tc. Superconductive and normal phase of HTSCs exhibit completely different properties as the same phases in LTSCs. None of the proposed theories was successful although there exist a consensus about some basic properties the final theory should predict. Many application are proposed, some of them based on HTSCs properties alone, others just being advanced solutions of LTSCs applications. Magnets, SQUIDs and microwave filter devices made of HTSCs are already in use, while many power supply and electronics devices are being tested.

2 WHAT IS SUPERCONDUCTIVITY? Character of a superconducting state Superconductivity is more then perfect conductivity: Meissner effect Critical fields: type I and type II superconductors More about energy gap... 4.HISTORY STRUCTURE THEORY Low temperature superconductivity: from dicovery to theory Phenomenological London theory BCS theory HTSC vs LTSC Phase diagram Building blocks for HTSC theory HTSC theories APPLICATIONS Generaly on applications Magnets Cables and power applications Electronics and sensors CONCLUSION References:... 14

3 WHAT IS SUPERCONDUCTIVITY? 1.1 Character of a superconducting state Superconductivity is an electronic state of matter characterized by zero resistance (1911, Kamerlingh-Onnes), perfect diamagnetism (1934, Meissner), energy gap and long-range quantum mechanical order [1]. A superconductor (SC) can behave as if it had no measurable DC electrical resistivity. Currents have been established in SCs that, in the absence of any driving field, have shown no discernible decay for very long period of time. A SC can behave as a perfect diamagnet. A sample in thermal equilibrium in an applied magnetic field provided the field is not too strong, carries electrical surface currents. These currents give rise to an additional magnetic field that precisely cancels the applied magnetic field in the interior of the SC. A SC usually behaves as if there were a gap in energy of width centred about the Fermi energy, in the set of allowed one-electron levels. The energy gap increases in size as the temperature drops, having a maximum value (0) at very low temperatures. There are also limitations for supercurrent under Tc. Superconductivity is destroyed by application of a sufficiently large magnetic field or if a current exceeds a critical current (Silsbee effect). The state is also destroyed for AC electric field, where dissipation response occurs at frequency of order /, where is the energy gap. 1. Superconductivity is more then perfect conductivity: Meissner effect It is easy to understand that when having a piece of metal in the superconducting state and then turning on the magnetic field that is not too strong, the magnetic field cannot penetrate the metal. If any of the magnetic field were to build up inside the SC, there would be a rate of change of flux, which would produce an electric field, and an electric field would generate a current, which, by Lentz s law, would oppose the flux. An infinitesimal electric field will generate enough current to oppose completely any applied magnetic field. This effect is implied by perfect conductivity alone. Meissner effect is more. If you have a piece of SC above Tc and establish magnetic field through it, and then lower the temperature below Tc, the field is expelled. In other words, it starts up its own current. The perfect conductivity implies a time-independent magnetic field in the interior; in a SC the value of field is zero. 1.3 Critical fields: type I and type II superconductors An important characteristic of all SCs is that the superconductivity is quenched when the material is exposed to a sufficiently high magnetic field. This magnetic field, H c, is called the critical field. In the early SCs, including all of the elemental SCs except niobium, the superconductivity is quenched in relatively low magnetic fields. We call these type I SCs. In contrast, type II SCs have two critical fields. The first is a low-intensity field H c1, which partially suppresses the superconductivity. The second is a much higher critical field, H c, which totally quenches the superconductivity. The upper critical field of type II SCs tends to be two orders of magnitude or more above the critical fields of a type I SC. Between the lower and the upper fields the superconducting state coexists with a magnetic field. This is called mixed state. The normal state is entering in the form of vortices, which form a lattice. When 3

4 field is increasing vortices are coming close to each other, and at some field vortex cores overlap, suppressing the superconductivity. If vortices are moving, they will quench the superconductivity in a very unpredictable way. Magnetic flux gets stable in a so-called pinning process when there is sufficient amount of impurities. The new ceramic oxide SCs (high temperature SCs, HTSCs) are type II SCs with very high critical fields. The H c of yttrium-barium-copper-oxide is 14 T at liquid nitrogen temperature (77 K) and at least 60 T at liquid helium temperature (4K). The similar rare earth ceramic oxide, thallium-barium-copper-oxide, was reported to have a H c of 36 T at 77 K and 100 T or greater at 4K. 1.4 More about energy gap There are some properties indicating the existance of the gap. One is specific heat, which behavior is substantially altered at the Tc. For the normal state, the specific heat is a potentional function of temperature (for normal metal it has the form AT + BT 3 ). At the Tc specific heat jumps to a higher value and at very low temperatures has a dominant behavior of the form Exp (- /k b T). This is the characteristic thermal behavior of the sistem whose excited levels are separated from the ground state by an energy. Other manifestation of the energy gap is in frequency dependent response of a SC to EM radiation. Energy / is typically in the range between microwave and infrared frequencies. So the SC behaves as in normal state at optical frequencies, the deviations first appear in the infrared, and the lack of absorption because of the energy gap is fully displayed at microwave frequencies. Again energy gap can be seen when measuring normal tunnelling. When we put the potential difference between two metals separated with insulator, the Ohm s law is obeyed. But when instead of metal there is SC well below Tc, no current is observed until the potential V reaches the value where ev=. As the temperature is raised, the threshold voltage declines, indicating the gap is declining with increasing temperature..history Heike Kamerlingh Onnes discovered superconductivity in He noticed that the resistance of mercury dropped abruptly at 4. K to a value below the resolution of his instruments. Later, in 1913, he won a Nobel Prize in physics for his research in this area. In 1933, W. Meissner and R.Oschenfeld discovered that a metal cooled into the superconducting state in a weak magnetic field expels the magnetic field from its interior. The SC is a perfect diamagnet. In 1945, the Russian physicist V.Arkadiev first performed the experiment of using this expulsion of a magnetic field to levitate a small bar magnet above the surface of a SC. In the years between 1910 and 1986 superconductivity was only known for some metals and alloys and also ceramic materials, but in all cases critical temperatures were extremely low [6]. In 1973, a niobium alloy was produced with a critical temperature of 3. K. The 1980's were a decade of discoveries in field of superconductivity. The first theoreticaly predicted organic (carbon-based) SCs were successfully synthesized in (TMTSF) PF 6 has transition at 1.K and subjected to high pressure. The search for a ceramic SC began with research conducted by several laboratories around the world, with the most productive research coming from K. Alex Mueller and J. Georg Bednorz of IBM Zurich. In 1986 the IBM group published results of research showing indications of superconductivity at about 30 K. Building on their work, several other 4

5 laboratories soon demonstrated bulk superconductivity at 39 K in an oxide of lanthanum, strontium, and copper. Bednorz and Mueller received the 1988 Nobel Prize in Physics for their groundbreaking work The group of physicists at the University of Houston, led by C.W.Chu and his associate M.K.Wu of the University of Alabama at Huntsville, discovered that they could raise the critical temperature to 5.5 K by applying a considerable amount of mechanical pressure (more than 10,000 atmospheres) to the sample. Finally the University of Houston and University of Alabama groups came up with the idea of pressurizing the crystal from the inside by substituting similar atoms of a different size rather than using an external mechanical press. In January of 1987 they produced ceramic SC, the oxide YBa Cu 3 O 7 (also known as 13 YBaCuO or YBCO), with a critical temperature 9K. The material has sparked the current technological revolution. At least ten elements from the lanthanide series have been successfully substituted for yttrium in making the new oxide SCs. The current class of ceramic SCs with the highest transition temperatures are the mercuric-cuprates (cuprate = copper oxide). The world record from 1993 with Tc of 138 K is now held by a thallium-doped, mercuric-cuprate comprised of the elements mercury, thallium, barium, calcium, copper and oxygen. Under extreme pressure its Tc is 5 to 30 degrees more at 300,000 atmospheres. There is more: in 1997 researchers discovered that at a temperature very near absolute zero an alloy of gold and indium was both a SC and a natural magnet. Conventionally was held that a material with such properties could not exist! Since then, over a half-dozen such compounds have been found. Recent years have also seen the discovery of the first hightemperature SC that does NOT contain any copper (000), and the first all-metal perovskite SC (001) 3.STRUCTURE Figure 1: Isometric perovskite structure: Metallic + valence ions (grey) and a valence oxides (red) form a FCC unit cell. The metallic +4 valence ion (green) is situated at the centre of the cell. In the first report on HTSC Bednorz and Muller referred to their samples as metallic, oxygen-deficient, perovskite-like mixed-valence copper oxides. Subsequent work has confirmed that the new SCs do indeed possess these characteristics. Perovskites are large family of crystalline ceramics with a metal-to-oxygen ratio of approximately -to-3. The general formulation of perovskite structure is AXO 3, where A is a metal ion with a + valence and X is a metal ion with a +4 valence [7]. In the perovskite structure the A + and O - ions form a FCC unit cell with the A + ions at the corners of the unit cell. The O - ions are in the centre of the faces of the unit cell. The X 4+ ion is located at the octahedral interstitial site at the centre of the unit cell and is coordinated to six O -. This simplified ideal structure is isometric in symmetry. Members of this group would all be isometric if it were not for the fact that the octahedrons of most of the natural members of the group are twisted or rotated so as to kink or bend the structure. The twisting or bending is to accommodate the large ions between the octahedrons. The result is a variety of symmetries from isometric to tetragonal to orthorhombic to monoclinic depending on the degree of distortion to the basic ideal structure. Most perovskite minerals show some pseudocubic tendencies due to the close to, but not quite, isometric structure. 5

6 HTSCs can be called layered perovskites. Layers of Co and O atoms alternate with layers of other (heavy) metals. We are dealing with ceramic, flaky oxides, which are poor metals or insulators at room temperature. They contain few charge carriers compared to normal metals, and display highly anisotropic electrical and magnetic properties which are very senstive to oxygen content. Oxides like silica are normally insulators because the electrons are strongly bond. But there are oxides that can become metallic conductors, or even SCs. The most interesting contain mixed valence atoms such as copper that can give up a variable number of electrons when bonding. By looking at the coordination number of these atoms, we can usually tell something about their valence[8]. okok Figure 3: Structure of a superconducting YBa Cu 33O 7.. Cu are green atoms, the O are red, Ba is blue and the Y is cyan. The green squares are chain layers. In YBa Cu 3 O 7 (YBCO) the Cu are green atoms, the O are red, Ba is blue and the Y is cyan. Clearly there are two kinds of Cu atom, those that are coordinated by 4 O atoms (green squares), typical of divalent Cu +, and those that have a fifth oxygen atom (green pyramids). If we heat this SC in the absence of oxygen it loses one of its O atoms and becomes the insulator YBa Cu 3 O 6 with a very similar structure. The O is lost from one particular site, the chains of CuO 4 squares. Cu in these squares is left with only two O atoms, typical of monovalent Cu + ; Cu has been reduced from Cu + to Cu +. Oxidising the Cu again from Cu + to Cu + can restore oxygen and superconductivity. This solid state chemistry is clearly responsible for the unusual electrical properties. Yet it is not the CuO 4 chains that are responsible for the superconductivity in YBCO. Similar materials, which superconduct at higher temperatures, can be made by replacing chains by layers of other materials, such as heavy metal oxides. Neutron diffraction from oxide SCs indicated that oxidation of these charge reservoir layers results in the formal oxidation of the planes of copper oxide pyramids (Cu + to Cu 3+ ), due to charge transfer. This empirical understanding leads directly to the discovery of many other similar superconducting materials. The highest Tc (138K) so far obtained is for a material, where the charge reservoir consists of mercury oxide. Hg-O layers are drawn as a yellow rock salt type structure, but actually the charge reservoir structure is much more complex, being typically not commensurate with the copper oxide layers and so-called lone-pair electrons on mercury further complicate the real structure. Figure : Struture of an insulating YBa Cu 3O 6. Colors as in figure 1. Compared to figure 1, oxygen is lost from chain layers. Figure 4: Simpified structure of a Hg 0.8 Tl 0. Ba Ca Cu 3 O Colors as in figure 1. Hg-O layers are drawn as a yellow rock salt type structure. Compared to figure 1, chain layers are replaced by Hg-O layers. These SC structures are related to the simple perovskite structure. The formula YBa Cu 3 O 7 may be considered as (YBa )Cu 3 O 9 or 3 units of perovskite 3x AXO 3 with of the oxygen atoms removed. More precisely: YBa Cu 3 O 7 can be looked upon as stacking of three perovskite units BaCuO 3, YCuO and BaCuO. Oxygen atoms must be removed to preserve charge/valence balance for the formula Y 3+ 1Ba + Cu 3+ 1Cu + O - 7. Although more than 50 superconducting cuprates are now known, they are mostly all lightly doped copper-oxide planes. 6

7 Some physicists proposed a classification of superconducting oxide structures in terms of the sequence of (1) superconducting layers (CuO in YBCO), () insulating layers (Y) and (3) hole-donating (charge reservoir) layers (CuO)[9]. Doping is most important in changing electrical properties in SCs. The real YBCO formula must be written as YBa Cu 3 O 7-. For different, the Tc is different, and at some value it is no more a SC. Depending on the temperature and the level of doping, the cuprates can be insulators, metals or SCs. 4.THEORY 4.1 Low temperature superconductivity: from dicovery to theory For almost twenty years after Onnes discovery of superconductivity physicists did not possess the basic building blocks needed to formulate a solution, the quantum theory of normal metals. At first theorists have built phenomenological description of superconducting flow. Fritz London pointed out in 1935 that ''superconductivity is a quantum phenomenon on a macroscopic scale,...with the lowest energy state separated by a finite interval from the excited states'' and that '' diamagnetism is the fundamental property.'' The theoretical understanding of superconductivity came in 1957 by John Bardeen, Leon Cooper, and John Schrieffer who won a Nobel prize in 197 for their BCS thory. In 196 Brian D. Josephson predicted that electrical current would flow between superconducting materials, even when a non-superconductor or insulator separates them. His prediction won him a share of the 1973 Nobel Prize in Physics. This tunnelling phenomenon is today known as the Josephson effect and has been applied to electronic devices such as the SQUID. These theories are explaining conventional SCs. Even eighteen years after the discovery of HTSC in ceramic compounds containing copper-oxide planes, there is no commonly accepted theory for this. 4. Phenomenological London theory It is important to look at the London s equations [1] in order to understand the penetration depth. The asumption of the theory is a two-fluid model, where only a part n s of all electrons is superconducting. Normal electron are ignored. The superconducting electrons will be freely accelarated without dissipation, so they satisfy: m dv s = - ee dt Current density is j = - ev s n s,so we have: dj n se = E dt m Substituing this into Faradey s lay of induction E = - db/dt gives this: n se ( j + B) = 0 t m This is true for all perfect conductors. Any time-intependent B and j are solutions to the upper equation, but Londons now considered Meissner effect: magnetic field inside the SC must be zero. So the solution must be: n e s j = B m 7

8 Combining this with = j we arrive at: B 0 s 1 B = λ where λ = m n se is penetration depth. The field B in SC is exponentialy decreasing with penetration depth. Calculated values for are 10 nm, which is true for low temperature SC, while in HTSC values of about 100 nm are observed. 4.3 BCS theory In a momentum space description of a simple metal, the ground state is a sphere with states filled to Fermi level. The energy of the outermost electrons, E f = p f / m is very large compared to their average thermal energy, kt. As a result, only a fraction of the electrons, kt/e f are excited above the ground state. How can these interacting electrons undergo a transition to the superconducting state? In 1950 researchers at the Rutgers University discovered that the Tc of lead depended on its isotopic mass, being inversely proportional to M 1/. Since the lattice vibrational energy displays the same dependence on M 1/, phonons must be involved[]. We can imagine an electron emitting a phonon which is subsequently absorbed by a second electron. In BCS theory [15] the Cooper pairs are structureless objects, i.e. the two partners form a spin-singlet in a relative s-wave orbital state, i.e. pairs of electrons having opposite spin and momentum and can to a good approximation be thought of as composite bosons that undergo Bose-Einstein condensation into a condensate characterized by macroscopic quantum coherence. The SC state is characterized by two distinct components: a superfluid, the condensate, and a normal fluid made up of the single particle excitations which result from the break up of the condensate pairs at finite temperatures. The potencial, net interaction, between electron with wave vectors k and k has the form: 4πe ω V k,k ' = q + k ο ω - ωq where is the difference in electronic energies k o is Fermi wave vector, q is difference in electron wave vectors, and q is a frequency of a phonon of wave vector q. The potencial is atractive for electrons with energies sufficiently close together (separated less then D, a measure of the typical phonon energy). Cooper demonstrated that in the presence of Fermi sphere the exlusion priciple alters two-electron problem so that the bound state exists no matter how weak the interaction. Cooper s argument applies to a single electron pair. BCS theory takes step forward, constructing a ground state in which all electrons form bound pairs. If (rs, r s ) is a bound state wave function for one pair, then BCS ground state is just a product of identical two-electron wave functions: BCS (r 1 s 1,, r N s N ) = A(r 1 s 1, r s ) (r N-1 s N-1, r N s N ) A is antysimmetrizer which takes all permutations of arguments, weighted with +1 or 1. may be thought as an order parameter. The characteristic length over which coherent behavior can occur is coherence length 0, which also represents the spatial range of the pair wave function. ( 0 equals in pure SCs well below T c ). We can estimate 0 as follows: 8

9 p p f = δε = δ( ) = ( ) δp = vf δp, m m ξ 0 v δp f Calculated 0 value is about 1000 nm, which is true for low temperature SC, while in HTSC lower values are observed. YBCO has ξ c (T=0) =0.4 nm and ξ ab (T=0) = 1.5 nm. For HTSCs with even higher anisotropy this values are even smaller, up to 0. nm. These values are order of nearly thousand interparticle spacings in LTSCs and just about order of interparticle spacing in HTSCs. From this result we see that the appearance of Cooper pairs is very different in HTSCs, and the question is if they really can exist in such a form. 4.4 HTSC vs LTSC All HTSC cuprates are lightly doped copper-oxide planes. All cuprates poses great anisotropy, compared to conventional SCs. As may be seen in table below, both the charge response (measured in transport and optical experiments), and the spin response (measured in static susceptibility, nuclear magnetic resonance (NMR) experiments and inelastic neutron scattering (INS) experiments) of the HTSCTc are dramatically different from their low Tc counterparts both in normal state[10]. low temperature SCs HTSCs Penetration depth* ~10nm ~100nm Coherence length* ~100 nm ~1nm Resistivity at at Quasiparticles lifetime at + b at + b Spin excitation spectrum flat Peaked at q(/a,/a) Max strength of spin 1 state/ev states/ev excitations Characteristic spin excitation ~E f << E f energy Antiferromagnetic none Strong, with AF a correlations Magnetic susceptibility flat Varies with T, maximum at T>Tc for underdoped systems Table 1:Comparison between low temperature SCs and HTSCs *in a superconducting state 9

10 4.5 Phase diagram Figure 5: Example of phase diagram for cuprate HTSCs. It includes: antiferromagnetism, superconductivity, Fermi liquid phase, non-fermi liquid phase and pseudogap phase. HT superconductivity is only one aspect of the unique and complex phase diagram exhibited by cuprates[11]. Depending on the temperature and the level of doping, the cuprates can be insulators, metals or SCs. The non-superconducting or "normal" phase also exhibits unusual properties (figure5). Phase diagram includes antiferromagnetic ordering, a so-called pseudogap phase, superconductivity, Fermi liquid and non-fermi liquid phase. In antiferromagnetic phase (at small level of doping and low temperatures) the spins of Cu + are ordered in an array of antiparalel spins. In superconductive phase (at sufficient level of doping and low T) there exist a so-called optimal level of doping, where Tc is maximum. Fermi liquid [3] is metallic phase in which the Landau-Fermi liquid approach is correct. In this approach we consider electrons in metal that interact with each other by Coulomb's law. We can imagine an electron being surrounded by a cloud of other electrons, which screens an electrical field of this electron. When an electron moves, the cloud moves along, and the whole cloud behaves almost like an independent particle, which is called quasiparticle. Within this theory the states in interacting system have one-to-one correspondence with the states in a free-electron gas. A non-fermi liquid phase is similar, but some experiments show some ot the transport properties are altered. The interaction between quasipartcles is different. Some experiments show the existence of another gap in the pseudogap region. It arises at Tp>Tc. It is not clear whether this gap is directly connected to superconductivity gap, or it has another origin. Transitions among different regions in phase diagram are not sharp. 4.6 Building blocks for HTSC theory In low-temperature SCs the electrons pair together is s-wave state because of the phonon based pairing mechanism. In HTSCs the pairing mechanism is not known. It seems the electrons are in d-wave state, with the non-zero angular momentum. 10

11 There is a near consensus as well on the basic building blocks required to understand the HTSC. If we take well known sistems, YBa Cu 3 O 7-x (13 system) and La -x Sr x CuO 4 (14 system), these can be summarized as follows: 1.The action occurs primarily in the Cu-O planes, so that it suffices, in first approximation, to focus on the behavior of the planar excitations..at zero doping (YBa Cu 3 O 6, La CuO 4 ) and low temperatures, both systems are antiferromagnetic insulators, with an array of localized Cu + alternating spins. 3.One injects holes into the Cu-O planes of the 13 system by adding oxygen; for the 14 system this is accomplished by adding strontium. The resulting holes on the planar oxygen sites bond with the nearby Cu + spins, making it possible for the other Cu + spins to move, and, in the process, destroying the long range AF correlations found in the insulator. 4.If one adds sufficient holes, the system changes its ground state from an insulator to a SC. 5.In the normal state of the superconducting materials,the itinerant, but nearly localized Cu + spins form an unconventional Fermi liquid, with the quasiparticle spins displaying strong AF correlations even for systems at doping levels which exceed that at which Tc is maximum, the so-called overdoped materials. 4.7 HTSC theories What the final theory should predict? First, it should describe the full complex phase diagram. Second, it should reveal the special conditions in the cuprates that lead to this behaviour. From this should follow suggestion for other materials that would show similar behaviour. The final theory should give the correct order of magnitude for Tc and explain the trends that are observed in the cuprates like increasing in Tc as we move from single-layer cuprates to those containing two and three copper-oxide layers[1]. Here are some examples: Resonant valence bond. The RVB describes a lattice of antiferromagnetically coupled spins where the quantum fluctuations are so strong that long-range magnetic order is suppressed. The system resonates between states in which different pairs of spins form singlet s-wave states. But how does a doped RVB state behave? And why would doping stabilize a RVB state when the undoped state prefers an ordered magnetic state? Antiferromagnetic fluctuations. The parent compounds of HTSCs are antiferromagnetic insulators. Does the magnetic phase compete or cooperate with the superconductivity? Do the two phases coexist microscopically or form spatially separate phases? This is unknown. Density wave is a magnetic or spin order referring to pattern of the spins on the Cu +. In the antiferromagnetic state each spin is antiparallel to its nearest neighbor (figure 6a). The experiments on cuprate SCs suggest that the spins may have a more complex upand-down pattern, such as that shown in figure 6b. Figure 6: (a)antiferomagnetic ordering of Cu + ions in cuprates and (b) complex up-and-down pattern in doped material Spin order is expected to be accompanied by an ordered arrangement of charges. Spin electron coupling or d-wave theory considers d-wave symmetry of HTSC state. It includes two kinds of quasiparticles. First are spinons, which are uncharged carriers of spin with spin ½ and are fermions. Second are holons, which are spinless carriers of charge and are bosons. Typical connection between spin and charge is lost. We can imagine that when hole enters the antiferromagnetic structure it decays in a pair holon-spinon. Theorists are trying to build bound states from such quasiparticles, because it seems that electron description is not enough. 11

12 Other competing theories include those based on fluctuating stripes (in certain cuprates the doped holes are observed to localize along parallel lines, called stripes, in the copperoxide planes), and those that propose to unite the superconducting and antiferromagnetic phases in a larger symmetry group (SO(5) theories). Other theories are based on the polaron mechanism (where electrons pairs with polarization lattice deformation) and seek to exploit the strong coupling between electrons and phonons in oxide materials. Some theorists state the oxygen located in the chain layer is the answer. The cooper planes serve to compresses the oxygen atoms into a superconducting state. The prove comes in observation that a magnetic ion positioned in the crystal structure next to the cuprate plane has no effect on superconductivity; but if the magnetic ion were placed close to the chain layer, it depressed the transition temperature and destroyed superconductivity. Well? 5.APPLICATIONS 5.1 Generaly on applications Applications currently being explored are mostly extensions of current technology used with the low temperature SCs [4]. Epitaxial HTSC thin films achieve excellent SC properties (Tc > 90 K, Jc(77 K, 0 T) > 106 A/cm ; microwave surface resistance Rs(77 K, = 10 GHz) < 500 ) that are well-suited for superconductive electronics. Melt-textured HTSC bulk material shows superb magnetic pinning properties and may be used as high-field permanent magnets. 5. Magnets The main factor limiting the field strength of the conventional (copper wire) electromagnet is the I R losses. In a SC, where R = 0, the I R losses are obviously not a problem. Cuprate SCs are type II SCs with very high critical fields. Upper critical field of YBCO is 14 T at 77K and at least 60 T at 4K. The similar rare earth ceramic oxide, thallium-barium-copper-oxide, was reported to have a critical field of 36 T at 77K and 100 T at 4K. Problem is in flux creep; a process where vortices penetrate the material can quench the superconductivity. Magnetic flux gets stable when there is sufficient amount of impurities or holes, so that vortices bind. On the other hand this solution can cause the formation of hot spots, destroying the material. The first commercial impact of HTSC on superconducting magnet systems are current leads that substantially reduce the heat load to the cold magnet system. Whereas for normal metals the ratio between thermal and electrical conduction is about 1 W/kA for charge transport to a 4 K heat load, HTSC ratio is by factor 5 smaller. Superconducting magnetic bearings (SMB) composed of melt-textured YBCO pellets and permanent magnets are being investigated for the levitation of rotors in flywheels and motors. SMB achieved extremely low coefficient of friction. Powerful magnets could be used in fusion reactors, particle accelerators, advanced transportation systems (such as maglev trains), and advanced medical imaging devices. Also it is likely that the capability of shaping a magnetic field and shielding magnetic fields will become one of the more important industrial applications of these SCs. The military use of SCs comes with the deployment of "E-bombs". These are devices that make use of strong, superconductor-derived magnetic fields to create a fast, high-intensity electro-magnetic pulse to disable an enemy's electronic equipment. 1

13 5.3 Cables and power applications SCs can help to increase the efficiency of components for power applications by means of their lower losses as well as to reduce volume and weight by means of their potential for high power density [5]. The requirement for making cables is ability of shaping material in long, flexible wires. Flexible HTSC wire or tape conductor material is obtained either by embedding HTSC as thin filaments in a silver matrix or by coating of metal carrier tapes. YBCO wire is made in the shape of three-layered ribbons. First layer is metallic, second is oxide with similar structure to YBCO, which is the third layer. This material has Jc (77K, 0T) about 100kA/cm. Current flow through defects may lead to a local quenching of superconductivity and the creation of hot spots which may finally end up in the destruction of the SC. Ag-sheathed Bi-HTSC represent the only exception to the rule that strong biaxial texture is necessary to achieve technically meaningful currents. Good mechanical and electrical contact with the Ag matrix allows high current flow under the relaxed condition that a little detour via Ag is possible in case the direct current transfer between the HTSC grains is blocked. This Bi-HTSC bulk material is commercially available in sizes of several 10 cm with Jc (77K, 0T) of several ka/cm. Experiments with liquid He-cooled LTSC cables in the 70's were technically successful, but would economically justified only for power transfer > 5-10 GW. For liquid N -cooled HTSC cables, this limit may be reduced to MW. This would allow for an increase of power transfer by a factor 3 at a reduction of the voltage from 400 kv to 100 kv. The low operational self-fields of 0.1 T of power cables are within the limit of present commercial Bi-3/Ag tapes. In a Danish project, a 30 m 3-phase HTSC cable with a voltage rating of 30 kv and a power rating of 104 MW has been installed and tested under realistic conditions. The fault current limiter (FCU) is a component, inheritable in existing power grids. It prevents overloads from the grid components. FCU is based on the quench of a SC due to a current exceeding its critical current, resulting in a very rapid tremendous increase of its electrical resistance. A FCU based on Bi-1 of a 6.4MW was already tested. The time for recovery to the full operational state after a quench was few seconds. Superconductivity offers two ways for energy storage. In Superconductive Magnetic Energy Storage (SMES) systems, a SC coil stores magnetic field energy. The energy can be transferred very rapidly via power electronics to or from the grid. SMES systems based on classical SCs are already in commercial use for the improvement of the power quality. HTSC SMES systems are of interest with respect to size and volume reduction. In flywheels based on SC magnetic bearings, electric power is transformed to kinetic energy via a motor-generator combination controlled by power electronics. Due to the mechanics involved, the energy cannot be transferred as fast as by SMES. However, the density of storable energy can be much higher. 5.4 Electronics and sensors The low microwave losses of HTSC thin films [13] enable the coupling of a large number of resonators to microwave filter devices with much sharper frequency characteristics than conventional filters. HTSC filters are interesting in aircraft electronics (for better rejection of interference noise in aircraft radar systems), in mobile phone communication systems, (where coverage can be achieved by a smaller number of base stations) and in communication satellites. Superconducting QUantum Interference Devices (SQUIDs) are superconducting loops with integrated Josephson contacts, which can be used as the most sensitive magnetic field sensors. The magnetic field resolution of HTSC SQUIDs operating at liquid N temperature is only one order of magnitude above the LTS SQUIDs at 4 K. Josephson junctions are the basis for superconducting electronic devises; they can work at ps frequencies. 13

14 In superconducting bolometers [14], the sharp superconducting transition as a function of temperature is used as very sensitive thermometer, which allows to measure with high sensitivity the heating of a thermally connected absorber under electromagnetic irradiation. A Gd-13 based bolometer was used for a sensor for IR observation of OH molecules in the atmosphere in a satellite project.. The periodicity of the electric characteristics of superconducting loops as a function of the introduced magnetic flux can be used for the construction of superconducting AD converters. Flux quantization can help to implement feedback loops with quantum accuracy. 7.CONCLUSION HTSCs theory remains a mystery. For now, every attempt has failed. The pairing mechanism has not been found, and a question is which particles or quasiparticles pair. On the other hand, technical applications don't rely on theoretical aspects. Some applications are based on HTSCs properties alone, others are just a copy of what was made with LTSCs. This field is growing and the market for HTSCs devices is larger every year. References: 1.Neil W.Ashcroft, N.D.Mermin: Solid State Physics, Holt,Rinehart and Winston, 1975.V.P. Mineev, K.V.Samokhin:Introduction to Unconventional Superconductivity, Gordon and Breach Science Publisher, Charles P.Poole, H.A.Farach:Superconductivity, Academic Press, Archive/super-mechanism.html J.Hook,H.Hall: Solid state Physics, John Willey&Sons, 1994, second edition 14