## LESSON 24. High Temperature SuperConductor (HTS)

Introduction to High ${T_c}$ superconductors

Basic concept of High Temperature Superconductors (HTS)

• High-temperature superconductors are materials that behave as superconductors at unusually high temperatures.

• Symbolically, it is denoted by high- ${T_c}$ or HTS.

• The first high- ${T_c}$ superconductor was discovered in 1986 by the researchers Karl Müller and Johannes Bednorz.

• The “ordinary” or “metallic” superconductors usually have transition temperatures (temperatures below which they exhibit the superconductivity phenomena) below 30 K (-243.2°C ).

• The HTS have been observed with transition temperatures as high as 138 K (-135°C ).

• The compounds of Copper and Oxygen (so-called "cuprates") were believed to have HTS properties, and the term high-temperature superconductor was used interchangeably with cuprites superconductor for compounds such as Bismuth Strontium Calcium Copper Oxide (BSCCO) and Yttrium Barium Copper Oxide (YBCO).

• However, several iron-based compounds are now known to be superconducting at high temperatures.

History

The phenomenon of superconductivity was discovered by Kamerlingh Onnes in 1911, in metallic Mercury below 4 K (-269.15°C ). For seventy-five years after that, researchers attempted to observe superconductivity at higher and higher temperatures.

In 970, superconductivity was observed in certain metal oxides at temperatures as high as 13 K (-260.2°C ), which were much higher than those for elemental metals.

In 1987, K Alex Müller and J. Georg Bednorz were exploring a new class of ceramics for superconductivity. Bednorz encountered a Barium-doped compound of Lanthanum and Copper oxide whose resistance dropped down to zero at a temperature around 35 K (-238.2°C).

The superconductor with the highest transition temperature that has been confirmed by more than one independent research groups is mercury barium calcium copper oxide ( $HgB{a_2}C{a_2}C{u_3}{O_8}$ ) at around 133 K.

Crystal structures of high-temperature ceramic superconductors

• The structure of high- ${T_c}$ Copper oxide or cuprate superconductors are often closely related to perovskite  structure [A perovskite structure is any material with the same type of crystal structure as Calcium Titanium Oxide ( $CaTi{O_3}$ )].
• One of the properties of the crystal structure of oxide superconductors is an alternating multi-layer of $Cu{O_2}$ planes with superconductivity taking place between these layers.
• This structure causes a large anisotropy in normal conducting and superconducting properties, since electrical currents are carried by holes induced in the oxygen sites of the $Cu{O_2}$  sheets.

$YBaCuO$ Superconductors Fig 15(1)

The first superconductor found with ${T_c}$ > 77 K (liquid nitrogen boiling point) is Yttrium Barium Copper Oxide ( $YB{a_2}C{u_3}{O_{7 - x}}$ )

The proportions of the 3 different metals in the $YB{a_2}C{u_3}{O_{7}}$ superconductor are in the mole ratio of 1:2:3 for yttrium to barium to copper, respectively. So it is referred to as the 123 superconductor.

The unit cell of $YB{a_2}C{u_3}{O_{7}}$ consists of three pseudocubic elementary perovskite unit cells.

Each perovskite unit cell contains a Y or Ba atom at the center: Ba  in the bottom unit cell, Y in the middle one, and Ba in the top unit cell.

Thus, Y and  Ba a are stacked in the sequence [Ba - Y  - Ba ] along the c-axis.

All corner sites of the unit cell are occupied by Cu, which has two different coordination, Cu(1) and Cu(2) , with respect to oxygen.

There are four possible crystallographic sites for oxygen: O(1) , O(2) , O(3) and O(4) . The coordination polyhedral of Y  and Ba with respect to oxygen is different.

The tripling of the perovskite unit cell leads to nine oxygen atoms, whereas $YB{a_2}C{u_3}{O_7}$ has seven oxygen atoms and, therefore, is referred to as an oxygen-deficient perovskite structure. The structure has a stacking of different layers: $\left( {CuO} \right)\left( {BaO} \right)\left( {Cu{O_2}} \right)\left( Y \right)\left( {Cu{O_2}} \right)\left( {BaO} \right)\left( {CuO} \right)$ .

One of the key features of the unit cell of $YB{a_2}C{u_3}{O_{7 - x}}$ is the presence of two layers of $Cu{O_2}$ . The role of the Y plane is to serve as a space between two $Cu{O_2}$ planes.

In $YBCO$ ,  the $Cu$ - $O$ chains are known to play an important role for superconductivity.

${T_c}$ is maximal near 92 K when $x$ $\approx$ 0.15 and the structure is orthorhombic.

Superconductivity disappears at $x$ $\approx$  0.6, where the structural transformation of   $YBCO$  occurs from orthorhombic to tetragonal.

${B_i}$ - ,  $Tl$  - and ${H_g}$ - based high- ${T_c}$ superconductors

• The crystal structure of  ${B_i}$ - ,  $Tl$  - and ${H_g}$ - based high- ${T_c}$ superconductors are very similar. Like $YBCO$ ,

• It is perovskite-type feature and the presence of  $Cu{O_2}$  layers also exists in these superconductors.

• However, unlike $YBCO$ , - $Cu$ - $O$ chains are not present in these superconductors. The  superconductor has an orthorhombic structure, whereas the other high-   superconductors have a tetragonal structure.

The $B{i_2}$-$Sr$ -$Ca$ -$Cu$ -$O$  -system has three superconducting phases forming a homologous series as $B{i_2}S{r_2}C{a_{n - 1}}C{u_n}{O_{4 + 2n + x}}$  (n = 1, 2 and 3)

These three phases are $Bi$-2201, $Bi$-2212 and $Bi$-2223, having transition temperatures of 20, 85 and 110 K, respectively, where the numbering system represent number of atoms for   $Bi$$Si$ , $Cu$  Ca and Cu respectively.

The two phases have a tetragonal structure which consists of two sheared crystallographic unit cells. The unit cell of these phases has double $Bi - O$  planes which are stacked in a way that the Bi atom of one plane sits below the oxygen atom of the next consecutive plane. The Ca atom forms a layer within the interior of the $Cu{O_2}$ layers in both   $Bi$-2212 and   $Bi$-2223; there is no $Ca$  layer in the   $Bi$-2201 phase. The three phases differ with each other in the number of    $Cu{O_2}$ planes; $Bi$-2201, $Bi$-2212 and $Bi$-2223 phases have one, two and three $Cu{O_2}$ planes, respectively. The c axis of these phases increases with the number of $Cu{O_2}$ planes (see table below). The coordination of the $Cu$ atom is different in the three phases. The $Cu$ atom forms an octahedral coordination with respect to oxygen atoms in the 2201 phase, whereas in 2212, the $Cu$ atom is surrounded by five oxygen atoms in a pyramidal arrangement. In the 2223 structure, $Cu$  have two co-ordinations with respect to oxygen: one $Cu$ tom is bonded with four oxygen atoms in square planar configuration and another   $Cu$ atom is coordinated with five oxygen atoms in a pyramidal arrangement.

$Tl - Ba - Ca - Cu - O$   Superconductor:

• The first series of the $TL$ -based superconductor containing one  $TL$–$O$ layer has the general formula $TlB{a_2}C{a_{n - 1}}C{u_n}{O_{2n + 3}}$ whereas the second series containing two $TL$ –$O$ layers has a formula of $T{l_2}B{a_2}C{a_{n - 1}}C{u_n}{O_{2n + 4}}$ with n = 1, 2 and 3

• In the structure of  $T{l_2}B{a_2}Cu{O_6}$ ( $TL$-2201), there is one $Cu{O_2}$ layer with the stacking sequence ( $TL$–$O$ ) ( $TL$–$O$ ) ( $Ba$–$O$ ) ( $Cu$–$O$ ) ( $Ba$–$O$ )$TL$–$O$ )( $TL$–$O$ ).

• In $T{l_2}B{a_2}CaC{u_2}{O_8}$ ( $TL$-2212), there are two  $Cu$–$O$ layers with a $Ca$ layer in between. Similar to the $T{l_2}B{a_2}Cu{O_6}$ structure,  $TL$–$O$  layers are present outside the   $Ba$–$O$  layers.

• In $T{l_2}B{a_2}C{a_2}C{u_3}{O_{10}}$ ( $TL$-2223), there are three $Cu{O_2}$ layers enclosing  $Ca$ layers between each of these.

• In $TL$ -based superconductors, ${T_c}$  is found to increase with the increase in $Cu{O_2}$ layers. However, the value of ${T_c}$ decreases after four layers in $TlB{a_2}C{a_{n - 1}}C{u_n}{O_{2n + 3}}$ and in the $TlB{a_2}C{a_{n - 1}}C{u_n}{O_{2n + 4}}$ compound, it decreases after three $Cu{O_2}$ layers.

$Hg - Ba - Ca - Cu - O$ Superconductor:

• The crystal structure of $HgB{a_2}Cu{O_4}$ ( $Hg$ -1201), $HgB{a_2}CaC{u_2}{O_6}$ ( $Hg$ -1212) and $HgB{a_2}C{a_2}C{u_3}{O_8}$ ( $Hg$ -1223) is similar to that of  $TL$-1201, $TL$-1212 and $TL$-1223, with  in place of  $TL$ .

• It is noteworthy that the ${T_c}$ of the Hg compound ($Hg$-1201) containing one $Cu{O_2}$ layer is much larger as compared to the one- $Cu{O_2}$ -layer compound of thallium ($TL$-1201).

• In the $Hg$ -based superconductor,  ${T_c}$ is also found to increase as the $Cu{O_2}$ layer increases.

• For $Hg$-1201, $Hg$-1212 and $Hg$-1223, the values of  ${T_c}$ are 94, 128 and the record value at ambient pressure 134 K, respectively.

Table-1 High temperature superconductors

 Formula Notation Tc (K) No. of Cu-O planes in unit cell Crystal structure $YB{a_2}C{u_3}{O_7}$ 123 92 2 Orthorhombic $B{i_2}S{r_2}Cu{O_6}$ Bi-2201 20 1 Tetragonal $B{i_2}S{r_2}CaC{u_2}{O_8}$ Bi-2212 85 2 Tetragonal $B{i_2}S{r_2}C{a_2}C{u_3}{O_6}$ Bi-2223 110 3 Tetragonal $T{l_2}B{a_2}Ca{O_6}$ Tl-2201 80 1 Tetragonal $T{l_2}B{a_2}CaC{u_2}{O_8}$ Tl-2212 108 2 Tetragonal $T{l_2}B{a_2}C{a_2}C{u_3}{O_{10}}$ Tl-2223 125 3 Tetragonal $TlB{a_2}C{a_3}C{u_4}{O_{11}}$ Tl-1234 122 4 Tetragonal $HgB{a_2}Cu{O_4}$ Hg-1201 94 1 Tetragonal $HgB{a_2}CaC{u_2}{O_6}$ Hg-1212 128 2 Tetragonal $HgB{a_2}C{a_2}C{u_3}{O_8}$ Hg-1223 134 3 Tetragonal

Properties of High Temperature Superconductors.

• HTS are brittle in nature

• The properties of the normal state of these materials are highly anisotropic.

• The Hall Coefficient is positive indicating the charge carriers are hole.

• Their behavior can’t be explained by BCS theory.

• The isotopic effect is almost absent in these materials.

• The magnetic properties of these materials are highly anisotropic.

• The effect of pressure is different on different materials e.g the application of pressure increases the critical temperature of  $LBCO$ compounds but decreases the critical temperature of   $YBCO$

Application of HTSC

Commercial quantities of HTSC wire based on $BSCCO$ are now available and applications include transformers, fault current limiters, power storage, motors and fusion reactors etc.

HTSC cables-

• Lower voltage at the same power rating

• Low impedance power guiding

• Lower life cycle costs

• No soil heating

• No electromagnetic spray fields.

Electrical Machines-

• High quality of generated power

• Improved voltage stability

• More reactive power is available at a given rating

• Low total harmonic distortion

• Low maintenance

• Size and Weight reduction

• High efficiency and low operating cost

Fault current limiters-

Over 100times faster response time

Full power protection cycles

Magnets-

• Higher operational temperature range up to 77 K

• Compact and high weight

• Higher magnetic field of the order of 100 Tesla

• Greater stability and thermal efficiency

• Less complex cryogenic system

• Higher signal to noise ratio

Transformers-

• HTS turns power transformer into compact, highly efficient and environment friendly performers.

RF and Microwave Filters-

Medical Diagnosis-