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.


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)

 Module 5 Lesson 24 fig15(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)

Module 5 Lesson 24 fig15(2)

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

    Module 5 Lesson 24 fig15(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



Tc (K)

No. of Cu-O planes
in unit cell

Crystal structure

























































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

Time adjust response function

Full power protection cycles


  • 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


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

RF and Microwave Filters-

Medical Diagnosis-

Last modified: Friday, 7 February 2014, 6:00 AM