Superconductor Overview
The focus of our current research is high Tc superconductors (HTSCs).
Superconductivity is an interesting phonomenon that some materials lose their resistivity below a critical temperature, Tc . They become superconductors, through which the electric current can flow without any applied voltage. Another unique aspect of a superconductor is Meissner effect: magnetic field will be expelled out from the bulk of a superconducting region.
For the historical reason, they are two major catagories of superconductors: conventional superconductors and high-Tc superconductors.
Conventional superconductors (CSCs)
The first conventional superconductor, mercury, was discovered at the beginning of twenth century. Most CSCs are single elements or binary alloys. Their critical temperatures are low, normally in liquid helium region. Some binary alloys have Tc up to 23K. Therefore they are also called low Tc superconductors.
BCS theory was developed in 1950's to explain the mechanism of the conventional superconductivity. The central concept of the theory is the Cooper pair-two electrons form a pair by attractive force. Unlike individual electrons, which have to stack up in energy levels, Cooper pairs can all condensate to the lowest energy level, therefore create an energy gap at the top energy level. The energy gap protects the superconducting state from small perturbations, and therefore electric current can be carried by Cooper pairs without resistance.
Cooper pairs of the CSCs are mediated by phonons, which represents the vibration of crystal lattice. Cooper pair formes a spin singlet state (S=0) and carries charge 2e. The energy gap (order parameter) is isotropic (s-wave) in conventional superconductors.
High-Tc superconductors (HTSCs)
In 1986, Georg Bednorz and Alex Müller discovered a material La2-xBaxCuO4 with the Tc of 30K. This discovery quickly led to discoveries of more cuprates oxides with higher transition temperatures. Here are some examples of HTSCs
| compound | Tc(K) |
| La2-xSrxCuO4 | 38 |
| Nd2-xCexCuO4-y | 30 |
| R1Ba2Cu2+mO6+m R: Y, La, Nd, sm, Eu, Ho Er, Tm, Lu |
92 (m=1) 95 (m=1.5) 82 (m=2) |
| Bi2Sr2Can-1CunO2n+4
|
~10 (n=1) 85 (n=2) 110 (n=3) |
| Tl2Ba2Can-1CunO2n+4
|
80 (n=1) 100 (n=2) 125 (n=3) |
| HgBa2Ca3Cu4O10 | 133 |
The crystal structure of HTSCs are quite
complicated . Most of the structure of HTSC materials are layered cuprates.
The picture below shows the unit cell of Bi2Sr2CaCu2O8.
The (super)conductivity essentially takes place within quasi two-dimensional CuO2 planes. Two CuO2 are separated by Ca plane. BiO2 layers are interlayer regions which serve as charge resevoir. The chemical bonding between BiO2 layers are the weak Van Der Vaals bonding. After cleave, it can generate a flat bulk representing surface for ARPES study.
The HTSCs have some properties similar to BCS superconductors. Pairing of electrons still exists, as experiments show that supercurrent carrier charge is 2e. Cooper pairs are spin singlet state, or S=0.
However, there are many differences between HTSCs and BCS superconductors. For example, unlike BCS superconductors which have an isotropic (s-wave) order parameter, HTSCs have a highly anisotropic (d-wave) order parameter. In addition, HTSCs have weak isotope effect, suggesting a non-phonon mediating pairing mechanism. With these and other differences, it is now accepted that HTSCs can not be described by BCS theory.
The normal state properties of HTSCs are also quite anomalous. Electronic transport properties are anisotropic, resistivity along the direction perpendicular to the CuO2 plane is hundred times larger than that of parallel to the CuO2 plane. Even the in-plane transport is not Fermi liquid like, which can describe the normal state of BCS superconductor . The in-plane resistivity has a linear temperature(T) dependence while BCS superconductors have T2 dependence, a typical behavior.
The picture on top shows a universal phase diagram of HSTCs. The parent compounds of HTSCs are Mott insulators and in antiferromagnetic ground state. With the increase of hole doping, the materials turn into superconductors. This implies that in HTSCs, correlations and magnetism are important. Different competing phase may interplay with superconducting. All of these make the HTSCs problem difficult to solve.