1
Superconductivity
Brief History
Characteristic of Superconductors
Applications
Important Superconductors
What Is a Superconductor?
A superconductor is an element, inter-
metallic alloy, or compound that will conduct
electricity without resistance below a certain
temperature.
Once set in motion, electrical current will
flow forever in a closed loop of
superconducting material –making it the
closest thing to perpetual motion in nature.
Superconductivity is a “macroscopic quantum
phenomenon”.
Superconductivity
Materials become
superconductors below some
critical temperature, TC.
The temperature dependent
change between
superconducting and normal
conduction is abrupt!
The temperature at which
this drastic decrease in
resistance occurs is the critical
temperature of a
superconductor. Abrupt change!
Resistance goes to
zero. This is the
critical temperature.
Superconductors Compared to
Other Conductors
Semiconductors show a increase in
resistance as the temperature is
decreased.
Fewer electrons are excited from the
donor band (in n-type extrinsic semi-
conductors), into the acceptor band (in
p-type extrinsic semiconductors), and
from the valence band to the conduction
band (in intrinsic semiconductors).
Metal conductors show a decrease in
resistance as the temperature is
decreased.
Fewer vibrations result in a more
‘perfect’lattice.
Superconducting Critical Temperature Tc
= the temperature at which the system (sample)
undergoes a phase transition from a normal
conducting state into a superconducting state,
characterized by zero dc electrical resistivity.
normalsuperconductor
Three Temperatures
Tc(onset) — onset transition
temperature, when the R?T curve
begin to departure from the linear
relation of normal resistance Rn.
Tc(min) — the middle transition
temperature, which correspond the
point that resistance drops to Rn/2.
T∞ — the temperature when
resistance drops to zero.
For YBa 2Cu3O7-δ :
?Tc(onset)=95K，
?Tc(min)=91K
?T∞=90.5K and
△ Tc=1K
2
Mercury Superconducting Transition
Mercury was
historically the first to
show superconductivity.
Its practical usefulness
is limited by the fact
that its critical
magnetic field is only
0.019 T, so the amount
of electric current it can
carry is also limited.
Discovery of Superconductivity
by H. Kamerlingh Onnes (1911)
How the Superconductivity Was First Discovered?
(This story was told by Prof. P. Kes of Leiden in 1993 at a NATO summer
school in Erice, Italy.)
There were two assistants working for Onnes, Horst and Dorshman (these
names need to be confirmed). The son of Dorshman told Prof. Kes in 1992
the story of the discovery of superconductivity his father used to tell to his
son.
They were studying the resistance of mercury with a resistance bridge.
One day, by pumping on liquid He in the cryostat, they realized that for
some reason the resistance bridge did not seem to be working properly
because it was not giving any signal.
After they stopped the pump, by mistake, they forgot to re-open the valve
to release the evaporated He gas from the cryostat. The pressure increased
beyond atmosphere and the temperature increased. It was THEN that they
noticed that the resistance of mercury recovered!
This is how the superconductivity was first discovered. (Not on cooling,
but on warming mercury!)
In the Leiden Communication article, there is a description that “the tap
(valve) Eak2”was used to increase the temperature.
Superconductive Elements
Table from Burns
A15
compounds
alloy
Materials Transition temp.(K)
Al 1.2 (-272°C)
Sn 3.4 (-270°C)
Pb 7.2 (-266°C)
Nb3Sn 23.8 (-249°C)
LaSrCuO 40 (-233°C)
YBaCuO 90 (-178°C)
BiSrCaCuO 107 (-166°C)
TlBaCaCuO 125 (-148°C)
HgBaCaCuO 135 ~ 165K
Li: Element With the Highest TC
K. Shimizu et al.,
Nature
2002, 419, 587.
Superconductivity at high temperatures is expected
in elements with low atomic numbers. For example,
it has been predicted that when hydrogen is
compressed to its dense metallic phase (at pressures
exceeding 400 GPa), it will become superconducting
with a transition temperature above room
temperature. Such pressures are difficult to produce
in a laboratory setting, so the predictions are not
easily confirmed. Under normal conditions lithium
is the lightest metal of all the elements, and may
become superconducting at lower pressures. In this
work, Li shows superconducting at pressures
greater than 30 GPa, with a pressure dependent
transition temperature (Tc) of 20 K at 48GPa. This
is the highest observed Tc of any element; it
confirms the expectation that elements with low
atomic numbers will have high transition
temperatures, and suggests that metallic hydrogen
will have a very high Tc.
3
Superconductivity of Iron (Fe)
Shimizu et al., Nature 2001, 412, 316
?Temperature dependence of the
electrical resistivity of iron at 25
GPa. A 10% drop in resistivity
indicates the onset of
superconductivity at around 1.5 K.
The temperature dependence of the magnetization
of iron under pressure obtained by cooling the
sample at a magnetic field of 130 G. The signal at
21 GPa (the area enclosed by the dotted line is
expanded in the upper inset) shows the appearance
of diamagnetism at temperatures below 1.7 K,
which is confirmed by the signal given by tin at 2.7
K. The lower inset shows the disappearance of the
Meissner signal in iron when the pressure is
decreased to 3.5 GPain the b.c.c. phase.
1908, Kammerlingh-Onnes experiments on liquid He ( a
few ml)
Hg resistance: 0.08 ohm @ 5K to 0.000003 ohm @ 4.2 K
1986, J. G. Bednorz , K. H. Muller (IBM)
La-Ba -Cu-O
Oxide: Tc = 35 K
Superconductivity
Discovery of Superconductivity
in La-Ba-Cu-O (1986)
“At the extreme
forefront of research
in superconductivity
is the empirical
search for new
materials.”(1983)
Brief History of Superconductivity
1911 Kamerlingh Onnes discovered superconductivity in Hg
at Tc=4K
1913 Kamerlingh Onnes won the Nobel Prize in Physics
1933 Meissner and Ochsenfeld discovered the Meissner Effect
1941 Superconductivity was reported in Nb nitride at Tc=16K
1953 Superconductivity was reported in V3Si at Tc=17.5K
1962 Development of first superconducting wire
1972 Bardee, Cooper & Schrieffer won the Nobel Prize in
Physics
1986 Müller and Bednorz (IBM-Zurich) discovered High
Temperature Superconductivity in La-Ba -Cu-O at Tc=35K !
1987 Müller and Bednorz won the Nobel Prize in Physics
1987 Superconductivity was found in YBCO copper oxide at
Tc=92K !!!
1988 Tcwas pushed to 120K in a ceramic containing Ca and
Tl
1993 HgBa2Ca2Cu3O8 was found to superconduct at Tc=133K
39K Superconductivity in MgB2
In MgB2, hexagonal honeycomb
layers of boron atoms alternate
with layers of magnesium atoms,
centered on the hexagons.
MgB2, like graphite, has strong σ bonds in the planes and weak pi
bonds between them, but since boron atoms have fewer electrons
than carbon atoms, not all the σ bonds in the boron planes are
occupied. And because not all the σ bonds are filled, lattice vibration
in the boron planes has a much stronger effect, resulting in the
formation of strong electron pairs confined to the planes.
Nagamatsu et al.
Nature 2001, 410, 63
Characteristics of Superconductors
Loss of Resistance! ?? Zero electrical resistivity. This means that
an electrical current in a superconducting ring continues indefinitely
(at least for a very long time ~ years … ), without dissipation through
the ring or until a force is applied to oppose the current.
MeissnerEffect! ?? Superconductors expel all magnetic flux in a
process called the Meissner effect. The magnetic field inside a bulk
sample is zero. When a magnetic field is applied, current flows in the
outer skin of the material, leading to an induced magnetic field that
exactly opposes the applied field. The material is strongly
diamagnetic as a result.
A superconductor excludes magnetic
flux. In this experiment, this is used
to levitate a magnet above the
surface of the superconductor.
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Meissner Effect
When a superconducting sample is cooled below
Tc in the presence of an external magnetic field,
the magnetic field (i.e., lines of the induction B) are
pushed out.
A superconductor is a perfect diamagnet !
Two Easy Experiments Showing
Meissner Effect
Liquid nitrogen is added to a reservoir
beneath the superconductor. (The
superconductor is actually just out of
sight beneath the rim of the cup.) A
smaller magnet levitates about a
centimeter above it.
Three Barriers of Superconducting Materials
High Tc (critical temperature)
High Hc (critical magnetic field)
High Jc (critical current density)
Critical Magnetic Field
A sufficiently strong external magnetic field can
destroy the superconducting state
critical magnetic field
phase diagram of I-
type superconductor experimentaldata
Application
wire
Existing wire
- Energy loss by resistance
- High voltage needed
Wire with superconductor
- No energy loss
- No high voltage needed.
- Storage of electricity.
Cut end of superconductor wire
Wire with superconductor
Uses of Superconductors [Levitation]
“MagLev”trains have been under development in Japan
for the past two decades
The train floats above the track using superconducting
magnets.
There’s no friction between the train and the “rail”so less
energy is lost and the train can reach much higher speeds.
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Application
Magnetic levitating Vehicle
?principle
?car with superconductor?
?train
Application
Josephson device
SQUID (Superconducting QUantum Interference Device )
A superconducting loop interrupted in 2 places by
Josephson junctions. When sufficient electrical current is
conducted across the squid body, a voltage is generated
proportional to the strength of any nearby magnetic field.
Uses of Superconductors
[Magnetic Resonance Imaging]
MRI is a technique developed in the 1940s that allows
doctors to see what is happening inside the body without
directly performing surgery.
The development of superconductors has improved the
field of MRI as the superconducting magnet can be smaller
and more efficient than an equivalent conventional magnet.
Application
Super computer
- Without superconductor
: large heat, large electric power use
- with superconductor
: no heat, small electric power use
§ Particle colliders are very large running tracks that are
used to accelerate particles (i.e. electrons, positrons,
hadrons and more) to speeds approaching the speed of
light before they are collided with one another.
–The collision usually possess enough energy to split
the particles into smaller particles.
–Particle colliders were used to discover many sub -
nuclear particles such as taus and neutrinos.
§ They do this by cycling the particle using magnetic
fields, continually increasing the speed of the particle.
Application ? Particle Colliders High Temperature Superconductors
Cuprate superconductors have been the focus of researchers
because they conduct at relatively high temperature (Tc >
77K).
In the Y, Ba , Cu, O compounds: Y, Ba , and O have
oxidation states of +3, +2, and ?2, respectively.
This results in copper having mixed oxidation states +2
and +3.
A similar result is obtained for the other materials.
Their structures are related to that of perovskite (CaTiO3).
Compound Tc/K Compound Tc/K
YBa2Cu 3O 7 93 Tl2CaBa2Cu 2O 8 119
YBa2Cu 4O 8 80 Tl2Ca2 Ba2Cu 2O 7 128
Y 2 Ba4Cu 7O 15 93 TlCaBa2 Cu2 O7 103
Bi2 CaSr2 Cu2O 6 92 TlCa2Ba2Cu 3O 8 110
Bi2 Ca2Sr2Cu3 O 10 110 Tl0.5Pb0.5Ca2Sr2Cu 3O 9 120
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Perovskite Structure
Octahedronal
coordination of Ti
Perovskite Structure
Rocksalt Structure and Fluorite Structure
Rocksalt Structure Fluorite Structure
The Perovskite (CaTiO3) Unit Cell
If a polyhedron is used to represent the Ti centered
coordination environments, then an O atom lies at each
vertex, the Ca 2+ is shown in black.
An O atom lies at
each vertex, the Ca 2+
is shown in black.
Perovskite
structure: ABO3
B
A
a
a
)O(R)B(R
)O(R)A(R
2
1t =
+
+?=
tolerance factor:
R: ionic radius: R(O 2-) ≡ 1.40 ?
aA and aB: natural size of each layer
AO layer:
2/a)O(R)A(R A=+
2/a)O(R)B(R B=+
BO2 layer:
B O
O
A
The actual cell size a is determined from aA and aB.
t ≈ 1 is needed. Layered perovskites are more
tolerant.
a = 3.8 ~ 4.0 ?
R(A) = 1.3 ~ 1.4 ?, R(B) = 0.5 ~ 0.6 ?
Three factors to consider:
1. valence
2. oxygen coordination number
3. ionic radius
Tolerance Factor:
B
A
a
a
)O(R)B(R
)O(R)A(R
2
1t =
+
+?= Structure of Cuprate Superconductors
Oxygen's from a CuO2 layer are “shared”by the
perovskite unit cell.
In the perovskite cell, the Ba2+ (Black) and Y3+
(Gray) ions substitute for Ca2+. The Cu (blue)
centers substitute for Ti(IV).
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§The Cuprate superconductors
all contain a layer with
stoichiometry CuO2.
–This layer can be planer
§The other high-temperature
cuprate superconductors have
layer structures as well.
–For example:
Tl2Ca2Ba2Cu3O10
Structure of Cuprate Superconductors Cuprate Superconductors
§The CuO2 layers are responsible for the
superconducting properties.
§The other layers serve as sources of electrons.
§The copper 3d and oxygen 2p atomic orbitals
are close enough to allow for significant orbital
mixing à band structure.
–This band is half filled because Cu(II) has a d9
configuration
§The half-filled band is tuned electronically by
the effects of the neighboring layers in the lattice.
Tc vs Number of CuO2 Layers Why Do They Superconduct?
§In compounds such as YBa 2Cu3O7-x the metal ion (i.e.
copper) is partially oxidized.
§But an individual metal ion cannot be ‘partially’oxidized.
–Instead, the lattice will be comprised of a ratio of Cu2+ to
Cu3+ ions, depending on x.
–There will be ‘holes' of positive charge (Cu3+ ions) within
the lattice.
–This type of superconductor is referred to as a p-type
superconductor
§Compounds can also be doped to insert extra electrons into
the lattice (i.e. a reduction), e.g. La2CuO4+x
–This is called an n-type superconductor.
Why Do They Superconduct?
One explanation involves the use of holes within the
superconductor.
§When a current is applied to the superconductor, the
electrons travel along the ion planes in the lattice.
§As an electron passes a positive hole (due to oxidized cation,
Cu3+) in a neighboring plane, it will push negative charge
from orbitals on a reduced cation (such as Cu2+) towards the
hole.
–This is due to electrostatic repulsion.
§The oxidized cation (Cu3+) then reduces, and the reduced
ion (Cu2+) oxidizes
–Effectively, the hole moves backwards (as an electron
moves forwards).
–This “extra”current that is caused by the normal current
is the supercurrent.
Lanthanum-Barium-Copper Oxide
Superconductor
This ceramic material
was the first of a new
class of high temperature
superconductors. It is
made by randomly
substituting some
barium atoms into the
lattice of lanthanum-
copper-oxide in what is
termed a solid solution.
8
Yttrium-Barium-Copper Oxide
This ceramic
material was the first
of the high
temperature
superconductors to
make the phase
change at a
temperature above the
liquid nitrogen
temperature (77 K).
Superconductors
YBa2Cu3O7-δ
Perovskite?
(YBa2)Cu3O9-x
Oxygen Deficient
Triple Perovskite
Crosses mark
absent oxygen
Properties
YBa2Cu3O7 superconductor
- resistance lost completely
at temperature Tc
YBa2Cu3O6 semiconductor
- oxygen lost from base of
unit cell
YBa2Cu3O7-δ
As δ increases:
1) Tc decreases
2) symmetry changes
from orthorhombic to
tetragonal
(oxygen atoms
rearrange in base) O = orthorhombicT = tetragonal
δ value
Changing Properties?
Can substitute many elements into YBa2Cu3O7
structure:
Y ? lanthanides - no change in Tc
Y ? other elements - decrease in Tc
Ba ? Sr, Ca - decrease in Tc
Cu ? transition metals - decrease in Tc
Cu ? Au - very slight increase?
Ba ? La - very slight increase?
Generally detrimental!
Oxides Superconductors
vs
Perovskite Structure
Left: 3perovskite unit cells, CaTiO3 × 3 = Ca3Ti3O9
Center: Replace Ca with Ba, Y; Replace Ti with Cu→ YBa2Cu3O9
orthorhombic unit cell count
Right: Removal of 2/9 of oxygens gives defect perovskite structure,
YBa2Cu3O7 -δ(x≈0.2) “123”Superconductor
C.N. Ba = 10, C.N. Y = 8
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YBa2Cu3O7 as a Defective Perovskite Two types of Cu site Layers of CuO
5 square pyramids
Chains of vertex-linked CuO4 squares
These are indicated in a Polyhedral Representation
CuO2
BaO
CuO
BaO
CuO2
Y
CuO2
K2NiF4 Structure
K2NiF4 structure and its
(110) projection
K2NiF4 is a derivative
structure of perovskite
structure. This structure
can be regarded as the
alternate stacking of 2D
perovskite layer and
Rocksalt Layer.
BaxLa2-xCuO4-y is K2NiF4
structure， Tc(0)=38K
(La,Sr)2CuO4(T Phase)
A superconductor
with Derivative
structure of K2NiF4
Can be regarded as
the alternate stacking
of perovskite structure
unit containing Cu-O
layer and rocksalt
structure unit along c
axis.
The same structure
with the first cupper
oxide superconductors
La2-xBaxCuO4(Tc=35K).
Doped La2CuO4
{La2-xSrxCuO4 and
La2-xBaxCuO4 } are the first
(1986) High-Tc
Superconducting Oxide (Tc ~
40 K)
for which Bednorz &
Müller were awarded a
Nobel Prize
La2CuO4 may be viewed as
if constructed from an
ABAB... arrangement of
perovskite cells
- known as an AB Perovskite!
Alternative Views of the La2CuO4 Structure Alternative Views of the La
2CuO4 Structure
We may view the
structure as based on:
1.Sheets of elongated
CuO6 octahedra,
sharing only vertices
2.Layered networks
of CuO46-, connected
only by La3+ ions
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(Nd,Sr)2(Nd,Ce)2Cu2O8 (T* Phase)
Derivated from K2NiF4
structure. CuO6 octahedron
lose one vertex and thus form
CuO5 square pyramids.
(Nd,Sr)2(Nd,Ce)2Cu2O8 can
be regarded as the alternate
stacking of CuO5 square
pyramids layers, fluorite
layer and rocksalt layer along
c axis.
Tc=28K
(Nd,Ce)2CuO4 (T’Phase)
Derivated from K2NiF4
structure. CuO6 octahedron
lose two vertexes and thus
form CuO4 plane.
(Nd,Ce)2CuO4 can be
regarded as the alternate
stacking of CuO4 plane and
fluorite structure unit along
c axis.
Tc=24K
TlBa2Can-1CunO2n+3 Superconductors
TlBa2Can-1CunO2n+3
(n=1,2,3… ) series
with single TlO layer,
Tc is 50K， 103K and
117K， respectively.
They are built up by
Rocksalt layers and
oxygen-deficient
Perovskite layers.
Ba
Pb2Sr2-xLaxCu2O6 (Pb2202)
Can be regarded as special
K2NiF4 structure, and derivated
from La2CuO4 ,in which one CuO6
octahedron loss all oxygens and
form liner coordination structure.
There are two kinds of Cu,one
in octahedron coordination
structure (+2 valence)and the
other in linear coordination
structure (+1 valence).
Tc=32K
Comparison of Pb2202 and La2CuO4
Pb2(Sr,La)2Cu2O6 (La,Sr)2CuO4
(T phase)
La2CaCu2O6
This structure contains two
face-to-face CuO5 square pyramid
layers. It is a result of two-fold
oxygen-deficient perovskite
structure from two layers of CuO6
octahedron losing the common
vertex oxygen.
This structure can also be
regarded as the derivative in
which the single perovskite layer
is replaced by two layers oxygen-
deficient perovskite.
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Comparison of La2CaCu2O6 and La2CuO4
La2CaCu2O6 La2CuO4 (T phase)
YBa2Cu3O7 ?the 123 Superconductor
the first material to
superconduct at liquid N2
temperature, Tc > 77 K
YBa2Cu3O7 can be viewed as
an Oxygen-Deficient Perovskite.
Two types of Cu site:
Layers of CuO5 square
pyramids
Chains of vertex-linked CuO4
squares
Oxide Superconductor and
Perovskite Structure
ABO3 3 ABO3 Y123
YBa2Cu4O8 (Y124 Phase)
Y124 phase connects two CuO5
square pyramids by structure
units of Cu-O double chain. Cu-O
double chain can be regarded as
the common edges of CuO6
octahedron of two layers of
perovskites and lossing oxygens
in the opposite planes.
It is a 7-fold oxygen-deficient
perovskite structure (c~7ap). Its
superconducting transition
temperature is 80K.
Comparison of Y124 and Y123
YBa2Cu4O8
(Y124)
YBa2Cu3O7
(Y123)
Y2Ba4Cu7O14 (Y-247 phase)
Besides oxygen-deficient
perovskite layers containing Cu-O
plane, Y247 unit also contains Cu-
O double chain and Cu-O linear
structure.
Can be regarded as the complex
structure of Y123 phase and Y124
phase.Its superconducting
temperature is 40K.
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Comparison of Y247 and Y124 and Y123
YBa2Cu4O8
(Y124)
YBa2Cu3O7
(Y123)
Y2Ba4Cu7O14
(Y247)
Bi2Sr2CuO6 (Bi2201) Phase
Bi2Sr2CuO6 is the first layered
cupper oxide superconductors
which do not contain rare earth
ions.
Tc=7~22K
Bi-2201 phase is formed by the
ordered array of rocksalt
structures of Bi2O2 double layers
and pervoskite structures along c
axis.
Bi2Sr2CaCu2O8(Bi2212)
Tc=85K
Can be regarded as
the 2-fold oxygen-
deficient pervoskite
structure unit.
Ca
CuO2
SrO
BiO
BiO
SrO
CuO2
Ca
CuO2
SrO
BiO
BiO
SrO
CuO2
Ca
Correction！！！
Bi2Sr2Ca2Cu3O10 (Bi2223)
Bi2Sr2Ca2Cu3O10 can be
regarded as 3-fold
oxygen-deficient
pervoskite structure unit.
Tc=110K
CuO2Ca
CuO2
SrOBiO
BiO
SrOCuO
2Ca
CuO2Ca
CuO2
SrOBiO
BiOSrO
CuO2
CaCuO
2
Correction！！！
Bi2Sr2(Ln,Ce)2Cu2O10 (Bi2222)
Bi2222 is formed from the
insertion of one fluorite
layer into two CuO5 square
pyramids. Its structure is
the ordered array of three
rocksalt layer, oxygen-
deficient pervoskite
structure and fluorite layer
along c-axis.
Comparison of Bi2212 and Bi2222
Bi2Sr2(Ln,Ce)2Cu2O10
(Bi2222)Bi2Sr2CaCu2O8
（ Bi2212）
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(Bi,M)Sr2YCu2O7 (Bi1212)
It is difficult to form a cupper
oxide with intact single layer of
BiO. Suitable metal ions such as
Cd， Cu substitute Bi to stabilize
BiO layer, resulting in
(Bi,M)Sr2YCu2O7 (Bi1212) with
single layer of (Bi,M)O.
Its structure is the ordered array
of two rocksalt layers and oxygen-
deficient pervoskite structure
along c-axis.
When M=Cu, Tc=60K.
(Bi,M)Sr2(Ln,Ce)2Cu2O7 (Bi1222)
Insertion of one layer of
fluorite structure unit into
Bi1212 phase forms Bi1222.
When M=Cd,Tc=27K (Bi,M)Sr
2YCu2O7
(Bi1212)
(Bi,M)Sr2(Ln,Ce)3Cu2O7 (Bi1232)
Insertion of two-layers of
fluorite structure into Bi1212
phase forms Bi-1232.
When M=Cu,Tc=20K (Bi,M)Sr2YCu2O7 (Bi1212)
Pb2Sr2(Ln, Ca)Cu3O8+δ (Pb3212)
Pb2Sr2(Ln,Ca)Cu3O8+ δ is the first
Pb series of cupper oxide
superconductor discovered by
Cava.
Tc=68K
Can be regarded as the
derivation in which the two-fold
oxygen-deficient pervoskite
structure units containing two
CuO5 square pyramids replace the
CuO6 octahedron in Pb2(Sr,
La)2Cu2O6+ δ (Pb2202).
Comparison of Pb3212 and Pb2202
Pb2Sr2(Ln, Ca)Cu3O8+ δ
Pb3212
Pb2Sr2-xLaxCu2O6
(Pb2202)
Pb2Sr2(Nd,Ce)2Cu3Ox(Pb3222)
The insertion of fluorite layer
into the two CuO5 square
pyramids of Pb3212 phase can
form Pb3222 phase.
Pb2Sr2NdCu3O8+ δ
(Pb3212)
14
Pb(Sr,Ba)2(Y,Ca)Cu3O7 (Pb2212)
Tc=60K
Pb-2212 can be regarded as
taken one PbO rocksalt phase
from Pb-3212. In this unit cell,
Ba,Sr,Pb distribute orderly. Pb2Sr2(Ln,Ca)Cu3O8+ δ(Pb3212)
Pb(Sr,Ba)2(Nd,Ce)2Cu3Ox (Pb-2222)
Pb-2222 can be regarded as the
insertion of fluorite structure
into Pb-2212.
Pb(Sr,Ba )2(Y,Ca)Cu3O7
(Pb2212)
Tl2BaCan-1CuO2n+4
Up to now, this kind
of superconductors with
n from 1 to 5 has been
successfully synthesized.
The lattice parameter a
is similar while c
increases with n.
Such superconductors
have single TlO layer.
