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World record in low temperatures

Researchers of the YKI-group of the Low Temperature Laboratory have recently achieved the lowest temperature ever produced.

The record-low temperature was reached in a piece of rhodium metal, which was cooled to 100 pK, or 0.000 000 000 1 degrees above the absolute zero. The absolute zero is the limit of all temperatures, —273.15... C, a temperature one can never reach. However, at the Low Temperature Laboratory, the researchers have for more than 20 years been reaching closer and closer. The previous Low Temperature World Record of 280 pK was in fact set here in 1993.

The present results have been almost 9 years in preparation. Many scientists have contributed to the project, most recently the team was: T.A. Knuuttila, J.T. Tuoriniemi, K.I. Juntunen, and K.K. Nummila from the Low Temperature Laboratory, and two Danish researchers: K. Lefmann (Risø National Laboratory), and F.B. Rasmussen (University of Copenhagen).

This long lasting effort was not made with the sole purpose of achieving even lower temperatures. The main objective was to study the magnetism of the nuclei of the rhodium atoms. Each of the billions of nuclei in rhodium has very small magnetic moment like a small compass needle. By lowering the temperature the researchers expected the moments to "freeze" as known from e.g. household magnets, and also seen in similar ultra-cold experiments with copper, silver, and other nuclei. Unexpectedly, the rhodium nuclear moments did not freeze even at this record-low temperature, although the moments to some degree follow the movement of their neighbors. The study of this magnetic behaviour is related to a much wider scope of physical phenomena, e.g. the study of high-temperature superconductivity.

As an additional feature, Rhodium metal itself becomes superconducting, with the lowest known transition temperature of all elements. The researchers were able to study how nuclear magnetism affects the superconducting electrons. This study has been highly successful and has created several interesting results.

The apparatus reaching the record low temperatures consists of several consecutive cooling stages. The central parts of the cryostat are a dilution refrigerator reaching a temperature of 3 mK, and two nuclear cooling stages utilizing the method of adiabatic nuclear demagnetization. The first nuclear stage cools to a temperature of 50 mK, whereas the second nuclear stage,the rhodium sample, reaches the record low temperatures in the picokelvin-range.

8.12.2000 Tauno Knuuttila