Department of Physics

Thermal properties of nanostructures

Controlling phonon thermal transport using 2D phononic crystals

We have shown experimentally and theoretically that it is possible to change the phonon thermal conductance strongly based on the wave properties of the phonons [Zen2014]. This was achieved by fabricating a 2D nanoscale mesh structure (or a so called phononic crystal), whose period is of the same order as the wavelength of the phonons that carry heat, about a micrometer in this case. Then the phonon waves interact strongly with the phononic crystal structure and change their speed by almost an order of magnitude. Because the waves move much more slowly, the thermal conductance is strongly reduced. In the future, the demonstrated concept can possibly be used in many ways. At low temperatures, there are direct applications in the development of ultrasensitive radiation detectors, where the control of heat transport is essential. In addition, if the demonstrated concept can be made to work at room temperature range, it might have impact on the development of future more efficient thermoelectric devices, which can be harnessed to generate electricity from waste heat. Future research will concentrate in optimizing the effect using modified geometries.

2D phononic crystal sample

Fabrication of 3D phononic crystals using colloidal crystallization

We are also working on fabricating three-dimensional periodic nanoscale structures using colloidal crystallization of polymeric nanospheres, for use as phononic structures to control heat transport. Thick and large crystals have been successfully deposited in a single-step vertical dipping process [Isotalo2014]. We have recently also succeeded in depositing micron-scale metal wires on top of the polymer structures using e-beam lithography, paving the way for real device applications.

3D phononic crystal sample

Phonon transport in low-D insulators

ISuspended 1d devicensulators carry heat by quanta of the lattice vibrations (phonons). In one-dimensional suspended-beam samples at low temperatures, this phonon transport can reach a limit where the thermal conductance is ultralow, below the single channel quantum limit. We have shown that such structures can be fabricated, and that the phonon transport seems to be limited by the interface between the beam and substrate, as phonon transport is ballistic [Koppinen2009i]. We plan to engineer further low-D structures, which offer control over the transmission probability of phonons [Koppinen2009ii]. Theoretical work on 2D phonon transport in the surface scattering (Casimir) limit has also been put forward [Maasilta2011].





Electron-phonon interaction in metals


We have studied the effects of phonon dimensionality, disorder and metal film thickness on electron-phonon (e-p) scattering rates in thin film normal metal wires (Cu,Au, AlMn etc.), using NIS tunnel junctions as ultrasensitive thermometers [Karvonen2007i, Karvonen2007ii, Taskinen2006]. We have shown that if the phonons become 2D, the e-p coupling is increased, and its temperature dependence changed. For thin films on 3D substrates, on the other hand, e-p interaction weakens when films are made thinner, and even more strongly, if impurity atoms are added. Most measurements are performed at sub-Kelvin temperatures.

We have also developed a novel ac-technique to measure the electron-phonon scattering rate directly without any fitting parameters [Taskinen2004], and are developing low-temperature preamplifiers to increase the readout bandwidth of the NIS-thermometers and to enable fluctuation measurements [Geng2012].


Development of tunnel junction thermometers and coolers

We have investigated possibilities to use novel superconducting materials for the NIS junctions. This is interesting, since by increasing or decreasing the energy gap from the standard material, Al, thermometers and coolers can be developed which work better at higher of lower temperatures. We have recently succeeded in developing Nb [Nevala2012], NbN [Chaudhuri2013i], and even TaN based, NIS junctions, which all have energy gaps of the order of 1 meV as compared to 0.2 meV fro Al. Especially interesting are the future possibilities to develop NIS microrefrigerators for higher operational temperature than the current 0.3 K. The more complex nitride superconducting films are deposited using infrared pulsed laser deposition (PLD) [Chaudhuri2011, Chaudhuri2013ii].

We have also discovered that by annealing Al/AlOx/Al tunnel junctions, they become more stable [Koppinen2007] and their 1/f-noise is lowered [Julin2010]. This might help to lower the decoherence in superconducting quantum bits in the future, where these kind of junctions are heavily used.

In terms of theoretical studies of NIS junctions, we have clarified the effect of non-idealities on their performance [Chaudhuri2012], as well as the effect of single-electron tunneling (charging) on NIS thermometry [Koppinen2009iii].

Pulsed laser deposition (PLD) setup

Novel nanofabrication techniques for phononic and photonic metamaterials

"A microbridge" fabricated with 3D lithography

With the help of EU regional funds, we have recently purchased and installed a novel 3D nanolithography tool (Nanoscribe GmbH), which utilizes direct laser writing in 3D. In combination with atomic layer deposition (ALD) also now available at NSC, we are in a position to develop novel 3D metamaterial structures and utilize them in thermal transport studies for phonons and photons.