High Temperature Superconductivity

HTS History

In 1911, Heike Kamerling Onnes (photo below left) discovered superconductivity (the ability of a material to carry electricity with no resistance) in mercury, cooled by expensive and rare liquid helium to below the critical temperature (T_c) of 4.2 K (Kelvin). During the next 75 years, applications were developed, such as powerful magnets built of these "low temperature" superconducting materials for medical magnetic resonance imaging (MRI), high energy accelators like the proposed Superconducting Supercollider (SSC), and very senstive magnetic field detectors called Superconducting Quantum Interference Devices (SQUIDs). Because of the expense and inconvenience of liquid helium refrigeration, however, other applications of the phenomenon were not considered economically feasible.

In April 1986, two researchers at IBM in Switzerland, K. Alex Muller and George Bednorz, detected superconductivity in (La-Ba)₂CuO₄ with a T_c up to 35 K, in contrast to the previous record of 23 K for which they were subsequently awarded the Nobel Prize. By the end of 1986, superconductivity research achieved revolutionary advances with the effort of Paul C. W. Chu (photo right) and colleagues at the University of Houston. Signs of superconductivity above 77 K were repeatedly observed in poorly-characterized samples during the period, strongly affirming the belief in the existence of superconductivity in the liquid-nitrogen temperature range. The scientific world knew that the textbooks had to be rewritten after January 1987, when the Houston group in collaboration with M. K. Wu, Chu's former student, achieved stable and reproducible superconductivity above 90 K in Y₁Ba₂Cu₃O_7-d (Y123), with T_c close to 100 K. Superconductivity at such high temperatures defies our common understanding of solids.

The observation of superconductivity above 77 K in such unusual classes of materials defied the predictions of earlier theories; but these materials are also intriguing because they behave unusually above their T_c's as well; e.g. the meissner effect. The causes for and consequences of these observations pose great challenges to physicists, chemists, and materials scientists. Even though "the liquid nitrogen barrier" has been broken, many of the great promises of superconductivity technology have yet to be realized. The difficulties with the materials can be attributed to many of the material and engineering problems of HTS's, e.g. making long HTS wire than can carry large current without energy loss and can retain excellent superconducting properties over long periods of time without chemical and physical degradation.

Earth based HTS epitaxial growth production technology was recently advanced based on University of Houston  Space Vacuum Epitaxy Center (SVEC), now the Texas Center for Superconductivity and Advanced Materials (TcSAM), basic science experiments flown on NASA space shuttle flights in 1994, 1995 and 1996.  The "Wake Shield Facility" shuttle experiments advanced earth based epitaxial growth science in the near perfect vacuum of earth orbit. Dr. Alex Ignatiev (photo left) and his team at SVEC designed the experiments and leveraged the orbital science into a modified metal organic chemical vapor deposition technology that makes the economical production of large quantities of high quality, low cost, epitaxially grown HTS coated conductor an achievable reality.

However, commercial applications of HTS technology in fields such as electric power, transportation, electronics and medicine are now appearing. Current applications of HTS include motors, transformers, power cables, electricity storage, power system stability devices, magnetic resonance imaging (MRI), wireless communication filters, and ultra-fast computer chips. By the year 2010, it is estimated that the global superconductivity market will be in excess of $50 billion.

After the discovery of the transistor in 1947, it took almost 40 years to introduce the one megabyte memory chip which is vital to today's powerful computers. Modern discoveries in superconductivity go far beyond piece-meal improvements in electric devices. They have opened the door on a totally new technology and stretch the imagination to the discovery of new applications. Future generations will witness significant changes in electricity generation, transmission and storage; impacts in microelectronics, communication, and computers; and advances in solid state science. If history serves as a guide, the wonderland of HTS applications is destined to be achievable in the foreseeable future with determination, persistence and patience all guided by vision and imaginative experimentation.

HTS Science

Following decades of work, there is now an experimental and theoretical consensus that the behavior of the elementary excitations in the Cu-O planes provides the key to understanding the normal state properties of these cuprate superconductors (see molecule left), and that practically no normal state property resembles that found in the normal state of a conventional, low , superconductor. Both the charge response (measured in transport and optical experiments), and the spin response (measured in static susceptibility, nuclear magnetic resonance (NMR) experiments and inelastic neutron scattering (INS) experiments) of the high materials are dramatically different from their low counterparts, as is the single particle spectral density measured in angle-resolved photoemission studies (ARPES).

The cuprates exhibit a wide range of unusual behavior depending on the temperature and the level of doping. This includes anti-ferromagnetic ordering (green region), a so-called "spin-gap" phase (blue), superconductivity (red) and anomalous metallic behavior above the superconducting transition temperature, T_c (black line) at "optimal" doping (see graph right).

Moreover, essentially no property of the superconducting state is that of a conventional superconductor, in which BCS pairing takes place in a singlet s-wave state, and the quasi particle energy gap at low temperatures is finite and isotropic as one moves around the Fermi surface. Despite the fact that something quite new and different is required to understand normal state behavior, there is also a consensus that BCS theory, suitably modified, will provide a satisfactory description of the transition to the superconducting state, and the properties of that state.

There is a near consensus as well on the basic building blocks required to understand the high temperature superconductors. These can be summarized as follows.

• The action occurs primarily in the Cu-O planes, so that it suffices, in first approximation, to focus both experimental and theoretical attention on the behavior of the planar excitations, and to focus as well on the two best-studied systems, the 1-2-3 system (YBaCuO) and the 2-1-4 system (LaSrCuO).

• At zero doping ( YBaCuO; LaCuO ) and low temperatures, both systems are anti-ferromagnetic insulators, with an array of localized Cu spins which alternate in sign throughout the lattice.

• One injects holes into the Cu-O planes of the 1-2-3 system by adding oxygen; for the 2-1-4 system this is accomplished by adding strontium. The resulting holes on the planar oxygen sites bond with the nearby Cu spins, making it possible for the other Cu spins to move, and, in the process, destroying the long range AF correlations found in the insulator.

• If one adds sufficient holes, the system changes its ground state from an insulator to a superconductor.

• In the normal state of the superconducting materials, the itinerant, but nearly localized Cu spins form an unconventional Fermi liquid, with the quasi particle spins displaying strong AF correlations even for systems at doping levels which exceed that at which is maximum, the so-called over doped materials.