1. MgB2
Since 1973 the limiting transition temperature in conventional alloys and metals was 23K, first set by Nb3Ge, and then equaled by an Y-Pd-B-C compound in 1994
In February 2001 superconductivity was found in MgB2 at just below 40K Ironically this compound was routinely available in most laboratories.
Although the critical field is modest (10T) it may well find applications - it is relatively easy to make into wires.
The high transition temperature has led to suggestions that the superconductivity may be unconventional
However, it appears that it is the phonons themselves that are perhaps a little out of the ordinary
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2. The breakthrough -1986
George Bednorz
Alex Muller
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3. The breakthrough-1986 “BaxLa5-xCu5O5(3-y)” actually La2-xBaxCu2O4
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4. ….and higher
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5. …..and higher...
YBa2Cu3O7
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6. The High Temperature Superconductors
The high temperature superconductors are all mixed valent cuprates based upon the perovskite structure, usually associated with BaTiO3 Ba O Ti
Many minerals take the perovskite structure - it is extremely stable to substitution and many, much more complex, crystal forms (such as the high Tc cuprates can be derived from it
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7. The High Temperature Superconductors
Bednorz and Muller’s original HiTc was soon shown to be based upon La2CuO4 which is derived from the basic perovskite structure
However La2CuO4 itself is found to be an anti-ferromagnetic insulator with a Neel point of over 240K, only when doped with M=Ba or Sr, giving
La2-xMxCuO4 (ie hole-doped) is superconductivity found AF SC
tetragonal
orthorhombic
Metal
insulator x
0.125
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8. The High Temperature Superconductors
The 90K Y-Ba-Cu-O superconductor - in which there are two CuO2 planes rather than one - shows similar behaviour.
Here introduction of O in the chains provides the hole doping directly:
YBa2Cu3O6
YBa2Cu3O7 AF SC
tetragonal
orthorhombic
Metal
insulator x
YBa2Cu3O6+x 1
0.5
400K
90K Y Ba Cu Y Ba Cu O
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9. Doping
The onset of superconductivity, and the optimisation of the transition temperatures of all of the high temperature cuprates is associated with doping….
……..usually (but not always) with holes
This can be seen by examination of the formal valences:
Y3+ + 2Ba2+ + Cu1+ + 2Cu2+ + 6O insulating antiferromagnetic YBa2Cu3O6
Y3+ + 2Ba2+ + 3Cu O superconducting YBa2Cu3O7
2La3+ + Cu2+ + 4O insulating La2CuO4
1.875La Ba2+ + Cu O superconducting La2-xBaxCuO4
Overdoping, as well as underdoping, can lead to a reduction in Tc. Usually the optimal doping is 0.2 holes per Cu atom
We can therefore draw a generic phase diagram
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10. Generic phase diagram for the High Tcs
Non Fermi Liquid
Fermi Liquid
superconducting
pseudogap Tc
antiferromagnetic
temperature
0.2
Doping level (holes per CuO2)
underdoped
optimally
doped
overdoped
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11. Generic phase diagram for the High Tcs
In the non-Fermi-liquid region the thermodynamic properties are unexceptional and, within experimental uncertainties, are similar to the behaviour of a Fermi liquid. However, this region is characterized by exceptionally simple but unusual power laws in all of its transport properties as a function of temperature. These transport properties include the resistivity, the optical conductivity, the electronic Raman-scattering intensity, the thermal conductivity, various nuclear relaxation rates, the Hall conductivity and the magnetoresistance.
The so-called pseudogap regime is not well understood - evidence for its existence comes,eg, from the coefficient of electronic specific heat which starts to decrease at temperatures well above Tc. The region is characterised by spin and/or charge stripes and fluctuations
In a so-called Landau-Fermi liquid the properties of single electrons are "renormalized" by interactions with other electrons to form "quasiparticles". The properties of the material can then be understood in terms of the weak residual interactions between the quasiparticles and their excitations. A key feature of the quasiparticle concept is that low-energy single- particle excitations have very narrow linewidths: Dw~w2 where w is the energy of the excitation.
Non Fermi Liquid
Fermi Liquid
superconducting
pseudogap Tc
antiferromagnetic
temperature
0.2
Doping level (holes per CuO2)
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12. Other high Tc phases 214 123 2201 2212 2223 La2-xBaxCuO4 35K
La2-xSrxCuO4 38K
(La2-xSrx )CaCu2O6 60K
YBa2Cu3O7 92K
Bi2Sr2CuO6 20K
Bi2Sr2CaCu2O8 85K
Bi2Sr2Ca2Cu3O K
TlBa2CaCu2O7 80K
TlBa2Ca2Cu3O9 110K
TlBa2Ca2Cu4O K
HgBa2CuO4 94K
HgBa2Ca2Cu3O8 135K 1 2 3
Number of CuO2 layers
HgBa2Ca2Cu3O8
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13. Layered cuprates La2-xSrxCuO4 YBa2Cu3O7 HgBa2Ca2Cu3O8 N=1 N=2 N=3
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14. Other properties Highly anisotropic conductivity and superconductivity
In plane conductivity much higher than out of plane
Resistivity proportional to T at all temperatures above Tc
Close proximity to magnetic transition
Low temperature (few K) magnetic order when Y is replaced by Gd, Dy, Er, Ho etc
Very high critical upper critical fields (>60T), very low lower critical fields (mT)
Very long penetration depths (>120nm) but very short coherence lengths (1nm and less)
Very small isotope effect
Probably d-wave superconductors
Are they BCS superconductors?
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