RESEARCH INTERESTS

The Low Temperature Laboratory is one of the world centres in ultralow temperature physics. Applications of superconductivity, ranging from electric power technology and large high-field magnets to the measurement of weak biomagnetic signals, have been pursued as well.

The adiabatic nuclear demagnetization method has been developed in the LTL to lower temperatures than anywhere else. Over the years, many world records have been established. By means of a two-stage nuclear refrigerator, spontaneous antiferromagnetic order has been found in the spin systems of copper and silver at T>0 and ferromagnetic order in silver at T<0. In early 1993, the nuclear spins of rhodium were cooled to T = 280 pK and to T = -750 pK; these are, respectively, the current records on the positive and negative sides of absolute zero.

Fig. 3. Shrinking interference rings in superfluid ³He-B, caused by the fountain effect. The three pictures were taken at 20-ms intervals.

Another ultralow temperature topic intensively pursued in the LTL is ³He in rotation. Quantized vortex lines in the ³He superfluids, discovered by our ROTA1 group in 1982, have revealed several new and unexpected phenomena. Our measurements show, for example, that there are seven vortex structures, all quite different from the previously known simple vortex of ⁴He. Superfluid ³He has been observed, for the first time ever, in our ROTA2 cryostat by optical means. It is conceivable that vortices, analogous to those in ³He, are present in quickly rotating neutron stars. A rapidly spinning spherical sample of ³He thus might be a useful laboratory model of pulsars composed of neutron superfluid.

Neuromagnetic research, carried out by an interdisciplinary group of physiologists, physicists, mathematicians, and psychologists, using the magnetoencephalographic (MEG) technique, is another priority area of the LTL. This work involves measurements of the extremely weak magnetic fields caused by currents that flow in the human brain. The signal source in the cortex can be localized, under favorable conditions, with an accuracy of a few millimeters; the temporal resolution of the MEG method is better than 1 ms. The tiny magnetic fields are recorded by using superconducting SQUID-sensors. In the middle of 1992, a 122-channel neuromagnetometer, surrounding the whole head, became operational. Extensive software development and improvements of mathematical analysis methods have been carried out as well. Signal processing in the human brain, including work on basic neurophysiological phenomena and cognitive processes, have been studied intensively using measurements of evoked responses to a variety of stimuli. Investigations of spontaneous cortical activity are also carried out and localization of epileptic foci has shown considerable promise.