What Is A Superconductor?
A superconductor is a material that can conduct electricity or transport electrons from one atom to another with no resistance. This means no heat, sound or any other form of energy would be released from the material when it has reached "critical temperature" (Tc), or the temperature at which the material becomes superconductive. Unfortunately, most materials must be in an extremely low energy state (very cold) in order to become superconductive. Research is underway to develop compounds that become superconductive at higher temperatures. Currently, an excessive amount of energy must be used in the cooling process making superconductors inefficient and uneconomical.
Superconductors come in two different flavors: type I and type II. (1)
Type I Superconductors
A type I superconductor consists of basic conductive elements that are used in everything from electrical wiring to computer microchips. At present, type I superconductors have Tcs between 0.000325 °K and 7.8 °K at standard pressure. Some type I superconductors require incredible amounts of pressure in order to reach the superconductive state. One such material is sulfur which, requires a pressure of 9.3 million atmospheres (9.4 x 1011 N/m2) and a temperature of 17 °K to reach superconductivity. Some other examples of type I superconductors include Mercury - 4.15 °K, Lead - 7.2 °K, Aluminum - 1.175 °K and Zinc - 0.85 °K. Roughly half of the elements in the periodic table are known to be superconductive. (1)
Type II Superconductors
A type II superconductor is composed of metallic compounds such as copper or lead. They reach a superconductive state at much higher temperatures when compared to type I superconductors. The cause of this dramatic increase in temperature is not fully understood. The highest Tc reached at stardard pressure, to date, is 135 °K or -138 °C by a compound (HgBa2Ca2Cu3O8) that falls into a group of superconductors known as cuprate perovskites. This group of superconductors generally has a ratio of 2 copper atoms to 3 oxygen atoms, and is considered to be a ceramic. Type II superconductors can also be penetrated by a magnetic field whereas a type I can not. (2)
Conventional superconductors are materials that display superconductivity as described by BCS theory or its extensions.
Critical temperatures of some simple metals:
ElementTc (Kelvin) Aluminum (Al)1.20 Mercury (Hg)4.15 Molybdenum (Mo)0.92 Niobium (Nb)9.26 Lead (Pb)7.19 Tantalum (Ta)4.48 Titanium (Ti)0.39 Vanadium (V)5.30 Zinc (Zn)0.88
Niobium and vanadium are type-II superconductors, while most other superconducting elements are type-I materials. Almost all compound and alloy superconductors are type-II materials.
The most commonly used conventional superconductor in applications is a niobium-titanium alloy - this is a type-II superconductor with a Tc of 11 K. The highest critical temperature so far achieved in a conventional superconductor was 39 K (-234 °C in magnesium diboride.
HIGH TEMPERATURE SUPERCONDUCTORS
Our department has an experimental research effort in the area of low-temperature physics, with emphasis on the study of the transport and magnetic behaviors of the high temperature superconductors. The picture shows the typical structure of such a material.
The high temperature superconductors represent a new class of materials which bear extraordinary superconducting and magnetic properties and great potential for wide-ranging technological applications. The importance of understanding the transport and magnetic behaviors of these novel materials is two-fold. First, it could lead to a better understanding of the basic phenomena of superconductivity in these materials. Second, it could provide ways to improve the magnetic quality of the presently known materials by enhancing flux pinning in a controllable manner.
When a current is applied to a type II superconductor (blue rectangular box) in the mixed state, the magnetic vortices (blue cylinders) feel a force (Lorentz force) that pushes the vortices at right angles to the current flow. This movement dissipates energy and produces resistance [from D. J. Bishop et al., Scientific American, 48 (Feb. 1993)].
When a type II superconductor is placed in a magnetic field Hc1 < H < Hc2, where Hc1 and Hc2 are the lower and upper critical fields, respectively, the magnetic vortices that penetrate the material should form a uniform triangular lattice (Abrikosov vortex lattice) with a lattice spacing determined by the strength of H. If H is increased, the vortices become more closely spaced and their cores start to overlap. At Hc2 the vortex lattice and the pairing of the electrons disappear and the material becomes normal. However, within the family of high-temperature superconductors, the vortices have been found to be capable of forming a number of exotic new phases of matter besides the triangular lattice. The weak pinning of the flux lines of high-temperature superconductors gives rise to energy dissipation in these materials at finite currents, which limits the maximum value of the critical current (the current required to destroy superconductivity) and, hence, a variety of applications of the high-temperature superconductors. Knowing how the vortices move and arrange themselves under various temperature and magnetic-field conditions, as well as how these phenomena are influenced by the physical properties of the material, will be critical in controlling the flux motion and maintaining the supercurrent flow in these materials.
Our research is conducted on several systems of copper-oxide superconductors, some of which exhibit superconductivity at temperatures as high as about 130 K, well above the boiling point of liquid nitrogen (77 K). Measurements such as magnetic susceptibility, magnetoresistance, Hall effect, and current-voltage characteristics over a wide range of temperatures, applied magnetic fields, and/or applied pressures are used to probe the physics of these materials. These experiments are carried out on two major pieces of equipment: (1) a low temperature platform suitable for transport measurements which consists of a 4He cryostat with pumping station and temperature controller for measurements from room temperature to 1.8 K, a 9 tesla magnet, and the required computer-interfaced electronics; (2) a SQUID (superconducting quantum interference device) magnetometer which incorporates all the hardware and software needed for precise magnetic measurements from room temperature to 1.8 K and magnetic fields up to 5 tesla.