Report to the 1999 General Assembly for 1996-99
Associate Member 94-97:
Commission C5 on Low Temperature Physics is the most important international body which on the world-wide scale attends to the interests of low temperature physicists. Low temperature physics deals with the properties of matter in condensed forms, which often are unusual and only exist at low temperatures. The most well known example cases are the superfluid and superconducting phases, which exhibit quantization on macroscopic scales. Central to low temperature physics is the development of new refrigeration techniques and low-noise high-sensitivity measuring methods for the purpose to study matter at low temperatures or to improve measurement sensitivity and resolution.
There are a number of established engineering conferences in cryogenics and superconductivity which provide large international platforms for meetings of the low temperature community. However, the most important forum for physicists in basic research is the triennial IUPAP-sponsored International Conference on Low Temperature Physics. Commission C5 finds the host for this meeting and supervises the conference arrangements.
The last conference, known as LT21, took place in 1996 in Prague and was organized jointly by the Faculty of Mathematics and Physics of the Charles University (Prague), the Institute of Physics of the Academy of Sciences of the Czech Republic, and the Physics Department of the University of Bayreuth. There were 1421 registered participants at this 6-day meeting, 1823 abstracts were submitted to the conference programme, and 1460 papers were published in the conference proceedings of 3386 pages as a supplement to the Czechoslovak Journal of Physics (Vol. 46, 1996).
At the LT conferences the Fritz London Memorial Award, one of the 9 awards for scientific excellence sponsored by IUPAP, is given to recognize outstanding experimental and theoretical contributions in low temperature physics. In 1996 at LT21, the prize was received by M.H.W. Chan, for his work on helium liquids in restricted geometries, and by E.A. Cornell and C. Wieman for their discovery of Bose-Einstein condensation.
Commission C5 held a business meeting during LT21 in Prague and decided to accept the proposal from the Low Temperature Laboratory of the Helsinki University of Technology to host the next conference, LT22 in August 1999 in Helsinki. The preparations for this meeting are in progress and interest is building up at several places to prepare for a bid to host LT23 in 2002. The decision about this conference will be formed by C5 at its coming meeting in Helsinki in August 1999.
The LT conference has demonstrated its vitality and functionality during the last fifty years. New fields of research have been born at regular intervals, sometimes from the midst of traditional low temperature research, sometimes from its fringe areas. Today, as before, the prime concern of C5 is to maintain the LT conference in the forefront of new development in basic research. The LT conferences cover a wide area of topics, including quantum fluids and solids, superconductivity, electronic and nuclear magnetism, systems of mesoscopic scale, materials, devices, and cryogenic applications. Most of these fields also organize regular specialized meetings. Most recently after the last Quantum Fluids and Solids conferences, QFS97 in Paris and QFS98 in Amherst, which both attracted more than 250 participants, the decision was taken to maintain contact with the C5 commission and to organize a QFS meeting annually in those years when the LT conference does not take place. We hope that in a similar way C5 would be closely connected with all other subfields, to fulfill its function as a coordinating body on the world-wide scale.
2. Onnes temperature: the superconducting transition temperature
Professors B. Goodman, N. Kurti, H.B.G. Casimir, J. de Nobel, R. de Bruyn Outboter, and J. Huiskamp have recommended that the (zero-magnetic-field) superconducting transition temperature be called the "Onnes Temperature", by analogy with the "Curie " and "Néel" temperatures. C5 is discussing the practical implementation of this recommendation, which is becoming more timely in view of the approaching centennial anniversary of Kamerlingh Onnes's two most outstanding accomplishments: the liquefaction of helium in 1908 and the discovery of superconductivity in 1911.
During the last three years the Nobel Prizes in physics have been awarded to discoveries which deal with low temperatures:
1996: D.M. Lee, D.D. Osheroff,and R.C. Richardson: for the discovery of superfluidity in liquid helium-3.
1997: S. Chu, C. Cohen-Tannoudji, and W.D. Phillips: for the development of methods to cool and trap atoms with laser light.
1998: R.B. Laughlin, H.L. Störmer, D.C. Tsui: for the discovery of a new form of quantum fluid with fractionally charged excitations ("fractional quantum Hall effect").
While these discoveries were made ten to twenty years ago, striking advances have been made
at ever faster pace in more recent years. Below we describe some of the general trends and list
randomly some areas where progress has been fast.
1) Bose-Einstein condensates
The general impression of research in low temperature physics has been that it deals with the quantum properties of the solid and liquid phases of matter, but recently quantum gases have moved in the forefront. After a challenging effort of more than twenty years, two research teams have now succeeded in cooling a gas of spin-aligned hydrogen atoms to a degenerate coherent quantum state. In one case at MIT in Boston this was achieved in the bulk gas phase, which produced the largest and densest example of a Bose-Einstein condensed state. The key to success were two different sequential processes of evaporative cooling, by which the high temperature tails can be cut off from the Maxwellian velocity distribution of the gas atoms and the average temperature can be reduced to 50 microkelvin. The second experiment, conducted in the University of Turku (Finland), registered a transition in the 2-D atomic hydrogen gas adsorbed on a superfluid 4He film, when presumably a quasicondensate with local coherence was formed. The required density at temperatures from 0.1 to 0.2 K was achieved with magnetic compression. The road to these bosonic hydrogen condensates has been a complicated one, but it has taught us the most beautiful example of low temperature chemistry, the processes responsible for the recombination of atoms to molecules, which is of importance, for instance, in outer space.
Before hydrogen atoms, Bose-Einstein condensation was first achieved in 1995 with optically cooled rubidium (JILA & NIST, Boulder, Colorado) and sodium (MIT, Boston) atoms in magnetic traps. Since then an entirely new genre of experiments has become possible. One central question has been what happens when two condensates are joined. This has been studied with condensates consisting of rubidium atoms in two different states of total angular momentum. Another new step by the MIT group was to demonstrate that a condensate remains in a coherent state when it is released from the trap and that thus an atom laser with pulses of coherent atoms has become possible.
2) Helium superfluids
The closest relatives to the gaseous Bose-Einstein condensates are the stable helium superfluids. The measurement of Josephson oscillations in the particle current across a weak link between two superfluid reservoirs has been one of the long-sought goals in this area. The initial experiments of 1987 at Saclay-Orsay in France with superfluid 3He have been repeated by a group from the University of California in Berkeley with a regular array of 100 nanometre size apertures. Suprisingly the Josephson tunnel current, which is too weak to be sensed individually in one hole, is coherent while passing through the array and becomes observable with a SQUID-based ultra-sensitive pressure measurement. These observations open new opportunities for macroscopic studies of quantum phase slip which are not possible with superconducting Josephson junctions.
Currently an important area in the study of superfluids is the effort to understand disorder when He liquids are confined in Aerogel. Randomly oriented strands of Aerogel at low density introduce weak disorder, in analogy with impurities in superconductors, which suppresses correlations and phase transitions. In 3He, the p-wave paired fermionic superfluid, the suppression of the critical temperature and the superfluid density have been measured and are in reasonable agreement with quantitative theoretical interpretation. Another active area is the study of topological defects and phase transitions in the pure bulk superfluid, to test analogies with other models in quantum field theory. The most attractive application has been the creation of topological defects in a quench-cooled symmetry breaking superfluid transition, to be compared with models of cosmic string production in phase transitions of the Early Universe.
An interesting bridge between fermionic superfluids and superconductors is established by recent measurements in the University of Manchester, where the mutual friction in the motion of quantized vortex lines in 3He superflow was determined. The results fit quantitatively a theory which describes the interaction between fermionic quasiparticle excitations localized in the vortex core and those in the bulk superfluid. The localized vortex-core states, which were originally proposed by de Gennes and his colleagues in the sixties, have finally also been directly observed in the vortices of a high-temperature superconductor. In scanning tunneling microscope measurements at the University of Geneva the quantized core states were imaged as peaks in the density of states at energies considerably below the superconducting energy gap.
Josephson tunneling or the search for nodes in the energy gap from the specific heat of high-temperature superconductors, to determine the symmetry of the superconducting order parameter, has been a hotly debated area in the recent past. The overwhelming evidence now favors d-wave Cooper pairing for the standard copper oxide superconductor like yttrium barium copper oxide YBCO. The study of the normal state, especially at low temperatures, when superconductivity is suppressed in a high magnetic field, has revealed that a metal-insulator transition as a function of the composition of the material, by which the carrier concentration is controlled, seems to be a key element in understanding these layered structures. Other striking observations have been that high-temperature superconductivity is not restricted to square CuO2 planes, as shown by the "ladder" cuprate compounds, and that there may appear "stripe-like" antiferromagnetic ordering within the copper oxide planes, which is incommensurable with the crystalline lattice.
During the last decade a revolution in the understanding of matter on the atomic scale has been brought about by the development of different microscopic scanning probes for surface analysis. The scanning tunnel microscope, adapted to the low temperature environment, most recently even to the 3He-4He dilution refrigerator, has become a unique high-resolution tool. It is used for imaging the vortex distribution in superconductors in the mixed state, or the interactions between magnetic single-atom impurities as a function of their separation on a clean superconducting surface. High expectations are placed on the injection of a spin-polarized current into a superconductor, which may serve as the basis for a new class of electronic devices. The STM is equally important in other areas, as far apart as semiconductor structures, such as quantum wells, quantum wires, and quantum dots, or as studies of adsorption, sticking, and diffusion of gases on solid substrates.
One often finds in condensed matter physics that resolution on the quantum level is available only at the lowest temperatures, more than often requiring refrigeration to well below 1 K. This is especially true of systems of lower dimensionality and of mesoscopic size which have become the new growth areas of low temperature physics.
Matti Krusius and Henri Godfrin