Physicists in Finland and Japan have invented a new type of electronic thermometer that relates temperature directly to the Boltzmann constant. Although not the first device to do so, the team say that their thermometer could easily be mass produced and therefore could be used as a highly accurate laboratory instrument as well as a calibration standard.

The current definition of the unit of absolute temperature is very messy indeed — the International Committee for Weights and Measures (CIPM) in Paris defines the Kelvin as 1/273.16 of the temperature difference between absolute zero and the triple point of pure water (roughly 0 °C) at a certain pressure. However, the CIPM would prefer to define the Kelvin, along with other SI units, in terms of fundamental constants — the Boltzmann constant kB, in the case of temperature.

As a result, teams of physicists around the world are dreaming up new techniques that relate temperature directly to kB. The latest is “single-junction thermometry” (SJT), which has been unveiled by Jukka Pekola and colleagues at the Helsinki University of Technology and NEC’s Nano Electronics Research Laboratories in Tsukuba (Phys. Rev. Lett. 101 206801).

A variation on Coulomb blockade

Their technique is a variation on Coulomb blockade thermometry (CBT), which was invented by Pekola a decade ago and is currently used in some commercial devices. CBT is based on the fact that the electrical conductance of an array of tunnel junctions — tiny bits of insulator sandwiched between two metals — changes with temperature.

While CBT works very well at temperatures above about 1K, small variations in the electronic properties of individual junctions results in an unacceptably large measurement uncertainty at very low temperatures.

Now, Pekola and colleagues have got around this problem by arranging a collection of tunnel junctions in a circuit such that the conductance depends on the properties of just one junction.

The tunnel junctions are created by first allowing a very thin layer of aluminium oxide to grow on the surface of micrometre-wide aluminium electrodes. Another electrode is then deposited on the oxide, creating metal-insulator-metal junctions through which electrons can tunnel.

Drop in conductance

Applying a voltage across the electrodes causes a current to flow through the junction. In principle, the size of the current depends on the number of electrons that can pile into the negative electrode — the more electrons available for tunnelling, the greater the conductance. At voltages above about 0.4 mV, however, this number is limited by a compromise between the Coulomb repulsion between electrons — which tends to reduce the number — and thermal energy of the electrons, which tends to boost the number. The upshot of this is that at these voltages the conductance does not vary with voltage.

At smaller voltages, however, the conductance drops off rapidly, until it falls to a minimum value at 0 V, before rising again as a negative voltage is applied. The dramatic drop in conductance occurs because at lower voltages, the junction behaves more like a capacitor, with the number of electrons that can pile into an electrode being proportional to the applied voltage as well as the thermal energy.

Works down to 150 mK

According to Pekola, the width of the dip (which can be measured by scanning the applied voltage and measuring the current through the junction) is directly proportional to the Boltzmann constant multiplied by the temperature. The team measured this width at several temperatures ranges (the lowest being 150–450 mK) and confirmed that the width is directly proportional to temperature.

As well as providing a way to express temperature in terms of the Boltzmann constant, Pekola says that the device is suitable for mass production and could therefore form the basis of a new thermometry system for use in low-temperature labs.

Sam Benz, a thermometry expert at NIST in the US, told physicsworld.com that the HUT-NEC team have done an “interesting experimental demonstration of a primary electronic-based thermometer that may prove useful at low temperatures”.

Pekola says that the team may try to commercialize the technology for use as an electronic thermometer, however there are several challenges that must be overcome. For example, he pointed out that their SJT devices are not optimized for any particular temperature range — something that would have to done to make them useful in the lab. For measuring temperatures lower than about 150 mK, for example, the junction electrodes would have to be made with a relatively large volume to ensure that the electrons are in thermal equilibrium with their surroundings.