how two ibm scientists gets this compound

1. PEROVSKITE-TYPE OXIDES based
HIGH-TC SUPERCONDUCTIVITY
sanjeev ranjan(160122032)

2. What is perovskite-type oxide ?
:-same type of crystal structure as calcium titanium oxide (CaTiO3) or as ABO3
:-oxygen occupies face center
:-'A' atoms are larger than the 'B' atoms

3. What are superconductors?
At very very low temp. this type of material shows zero electrical resistance(T<TC)
and ,expel magnetic flux fields(T<Tc&H<Hc)=>Meissner effect
superconductor can carry a current indefinitely without losing any energy as there
is no resistance.
In 1911 superconductivity was first observed in mercury by Dutch physicist Heike
Kamerlingh Onnes.
he cooled mercury to the temperature of liquid helium, 4 degrees Kelvin (-269C),
its resistance suddenly disappeared.

4. superconductor

5. magnetic levitating

6. BCS theory
The properties of some superconductors were modeled successfully by the efforts of John Bardeen, Leon
Cooper, and Robert Schrieffer in what is commonly called the BCS theory.
A key conceptual element in this theory is the pairing of electrons close to the Fermi level into Cooper
pairs through interaction with the crystal lattice. This pairing results from a slight attraction between the
electrons related to lattice vibrations; the coupling to the lattice is called a phonon interaction.
Pairs of electrons can behave very differently from single electrons which and must obey the Pauli
exclusion principle. The pairs of electrons can condense into the same energy level. The electron pairs
have a slightly lower energy and leave an energy gap above them on the order of .001 eV which inhibits
the kind of collision interactions which lead to ordinary resistivity. For temperatures such that the thermal
energy is less than the band gap, the material exhibits zero resistivity.

7. High Tc superconductor
High-temperature superconductors are materials that behave as superconductors at unusually high
temperatures.The first high-Tc superconductor Yttrium barium copper oxide( YBa2Cu3O7) was discovered in
1986 by IBM researchers Georg Bednorz and K. Alex Müller,who were awarded the 1987 Nobel Prize in
Physics.
Tc<30 k =>superconductors
Tc>30 k -> high-Tc superconductors

8. The Background
He did investigation of perovskites like SrTiO3 and , as a model crystal to study structural and ferroelectric
phase transition .
The key material, pure SrTiO3, could even be turned into a superconductor if it were reduced, i.e., if
oxygen were partially removed from its lattice [1.3]. The transition temperature of 0.3 K, however, was too
low Tc. Nevertheless,it was interesting that superconductivity occurred at all, because the carrier densities
were so low compared to superconducting NbO, which has carrier densities like a normal metal.
So, For increasing Tc increase carrier density,
Doping Nb in samples n = 2X1020 cm-3, enhanced electron phonon coupling led to a Tc of 0.7 K . By
further increasing the dopant concentration, the Tc even rose to 1.2 K, but not more than 1.2K.
Metallic grains of aluminum are surrounded by oxide layers acting as Josephson junctions. In granular
systems, the Tc were higher, up to 2.8 K, as compared to pure Al with Tc = 1.1 K
.

9. Involvement with the Problem
casual observation of the development of the
increase of superconducting transition
temperatures, lead to the conviction that
intermetallic compounds should not be pursued
any further. This because since 1973 the highest
Tc of 23.3 K could not be raised.
But nevertheless, the fact that superconductivity
had been observed in several complex oxides
evoked his special interest. Figure 1.1. Development of the superconducting
transition temperatures after the discovery of the
phenomenon in 1911. The materials listed are
metals or intermetallic compounds and reflect the
respective highest Tc's.

10. Involvement with the Problem
The second oxide after SrTiO3 is Li1+xTi2-xO4 spinel to exhibit surprisingly high Tc of 13 K in the Li-Ti-O
[1.8] and BaPb1-xBixO3 perovskite also exhibited the Tc of 13 K.
According to the BCS theory [1.10]
k x Tc= I.13h⍵De-1(N(E))V*
λ(Electron-phonon Coupling Constant)= N(E)xV*
N(E) =density of state at fermi level
He expected other metallic oxides to show even higher T
C
’s by increasing N(E) and/or the
electron-phonon coupling. Possibly we could enhance the coupling by polaron formation or
by the introduction of mixed valencies.

11. Involvement with the Problem
Increase N(E) ,by changing the Pb:Bi ratio, but the
compound underwent a metal-insulator transition with a
different structure, thus these attempts failed.
in Figure 1.2. There are three phases, a metallic one for
small λ and an insulating bipolaronic one for large λ, with a
superconductive phase between them, i.e., a metal-
insulator transition occurs for large λ. For intermediate λ, a
high-T, superconductor might be expected.
The question was, in which systems to look for
superconductive transitions?
Fig. 1.2. Phase diagram as a
function of electron-phonon coupling
strength(adapted from (1.1 I]).

12. The concept:-Jahn-Teller (JT) polaron model,
The guiding idea in developing the concept was influenced by the Jahn-Teller (JT) polaron model,
Jahn-Teller (JT) theorem :A nonlinear molecule or a molecular complex exhibiting an electronic
degeneracy will spontaneously distort to remove or reduce this degeneracy. Complexes containing
specific transition metal (TM) as central ions with special valency show this effect.
In the linear chain model,for small JT distortions with a stabilization energy EJT, smaller than the
bandwidth of the metal, only a slight change of the traveling electrons is present. With increasing EJT the
tendency to localization is enhanced, and for the case where EJT is comparable with the bandwidth, the
formation of JT polarons was postulated.
These composites of an electron and a surrounding lattice distortion with a high effective mass can travel
through the lattice as a whole, and a strong electron-phonon coupling exists. He knew there were many of
them. Oxides containing TM ions with partially filled eg orbitals, like Ni3+, Fe4+ or Cu2+ exhibit a strong JT
effect (Fig. 1.3), andconsidered these as possible candidates for new superconductors.

13. Copper Ions in the Oxide Octahedron
Fig. 1.3. Schematic representation of
electron orbitals for octahedrally
coordinated copper ions in oxides. For
Cu3+ with 3dn configuration, the orbitals
transforming as base functions of the cubic
eg group are half-filled, thus a
singlet ground state is formed. In the
presence of Cu2+ with 3d9 configuration,
the ground state is degenerate, and a
spontaneous distortion of the octahedron
occurs to remove this degeneracy. This is
known as the Jahn-Teller effect.

14. The Search and Breakthrough
ln La-Ni-O system. LaNiO3 is a metallic conductor with the
transfer energy of the JT-eg, electrons larger than the JT
stabilization energy, and thus the JT distortion of the oxygen
octahedra surrounding the Ni3+ is suppressed.
He replaced Ni3+ by Al3+ partially to reduce the metallic
bandwidth of the Ni3+ ions and make it comparable to the Ni3+ J-T
stabilization energy. With increasing Al3+ concentration, the
metallic characteristics (see Fig. 1.4) of the pure LaNiO3,
The idea did not work out the way he thought, so he considered
the introduction of some internal strain within the LaNiO3 lattice to
reduce the bandwidth. By replacing the La3+ ion by the smaller
Y3+ ion, keeping the Ni3+ site unaffected. Fig. 1.4. Temperature dependence of
the resistivity for metallic LaNiO3 and
LaAI1-xNixO3, where substitution of
Ni3+ by Al3+ leads to insulating
behavior for x=0.4.

15. The Search and Breakthrough
Some other scientists studied Ba-La-Cu oxide. In the
Ba-La-Cu oxide with a perovskite-type structure
containing Cu in two different valencies, all our
concept requirements seemed to be fulfilled.
By varying compositions well as the thermal
treatment , he was able to get Tc of 35 K (Fig. 1.5).
This was an incredibly high value compared to the
highest T, in the Nb3Ge superconductor(27k).
Figure 1.5. Low-temperature resistivity of a
sample with x(Ba) = 0.75, recorded for
different current densities. From [1.19]

16. It was now confirmed that Ba-doped
La2CuO4, as the superconducting
compound as it shown the Meissner
effect, and Combining the result of
the X-ray analysis, resistivity and
susceptibility measurements.
Replacing La also by other alkaline
earth In particular Sr2+ had the same
ionic radius as La3+,TC was
approaching 40 K and the
diamagnetism was even higher (see
Fig. l.12).
Fig. 1 12. Top: Resistivity as a function of temperature for Ca (O), Sr ( A ) ,and Ba
(0) substitution with substituent to La ratios of 0.2/1.8, 0.2/1.8,0.15/1.85,
respectively. Bottom: Magnetic susceptibility of these samples. The substituents
are Ca (0). Sr ( A ) , and Ba (0), with total sample masses of 0.14, 0.21. and
0.13 g, respectively.adapted from [ 1.251).

17. YBa2Cu3O7With TC of 92 K
Modification of the original
oxides by introducing the
smaller yttrium for the larger
lanthanum ion resulted in a
giant jump of TC, to 92 K in
multiphase samples.
even more impressive was
the fact that the Meissner
effect could now be
demonstrated without any
experimental difficulties with
liquid nitrogen as the coolant.

18. YBCO unit cell
The unit cell of YBa2Cu3O7-x 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 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 coordinations, 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).[23] The coordination polyhedron of Y and Ba with respect to
oxygen are different. The tripling of the perovskite unit cell leads to nine
oxygen atoms, whereas YBa2Cu3O7 has seven oxygen atoms and,
therefore, is referred to as an oxygen-deficient perovskite structure. The
structure has a stacking of different layers:
(CuO)(BaO)(CuO2)(Y)(CuO2)(BaO)(CuO). One of the key feature of the
unit cell of YBa2Cu3O7−x (YBCO) is the presence of two layers of CuO2.
The role of the Y plane is to serve as a spacer between two CuO2 planes.
In YBCO, the Cu–O chains are play an important role for
superconductivity. Tc is maximal near 92 K when x ≈ 0.15 and the
structure is orthorhombic. Superconductivity disappears at x ≈ 0.6, where
the structural transformation of YBCO occurs from orthorhombic to
tetragonal.

19. Characteristic part of the X-ray diffraction pattern, showing the
orthorhombic- to-tetragonal structural phase transition with
increasing Ba : La ratio. Concentration axis not to scale (adapted
from [1.20]).

20. Properties of the 123 Superconductors
What type of superconductivity
is it? Does one again have
Cooper pairing or not?
From I-V characteristics of
compound and From the well-
known Josephson formula;
VS=hv/q
they obtained q = 2e, i.e.,
Cooper pairs were present.
Steps induced by microwave
irradiation at frequency v=9.4
GHz

21. Nature of charge carrier
In La2CuO4 very little doping of divalent Ba2+ or Sr2+ and Ca2+;it was most likely that these ions substituted
for the trivalent La3+ ions. Thus,from charge-neutrality requirements, the compounds had to contain holes.
The holes were thought to be localized on the Cu ions., doping would create Cu3+ ions. Thus a mixed
Cu2+/Cu3+ state had to be present. Early photoelectron core level spectra (XPS and UPS), did not reveal
a 2p3d8 final state arising from a Cu3+ 3d8 state .
However, the excitation was consistent with the formation of holes L in the oxygen derived band, i.e., a
predominant 3d9 configuration . Photo-X-ray absorption near the edge structure was also interpreted in the
same manner by comparison with other known Cu compounds. Also the emission spectra pointed in the
same direction,
since the excitation thresholds were compatible with the presence of holes in Cu-O hybrid bands.

22. Nature of charge carrier
The electron deficiency is hereafter written in the form [Cu-O]+ as a peroxide complex in which the
probability of the hole is about 70% 3d92p, in oxygen hybrid band, and 30% 3d8 as in Cu3+ .

23. TC ∝ 1/M1/2, where M is the reduced mass.
In lower TC-> isotopes effect is more
But in ,YBa2Cu3O7 this effect is nearly absent or very small ,because of high TC
Effect of isotopes,replacing O16 by O18

24. Reference
Perovskite-Type Oxides- the New Approach to High-T, Superconductivity **
By J. Georg Bednorz* and K. Alex Miiller*
Thank you