Thermal nanophysics and superconducting devices

Complete tunneling of acoustic waves between piezoelectric crystals
Zhuoran Geng and Ilari J. Maasilta
Communications Physics 6, 178 (2023)

In this paper, we show that in certain situations a sound wave can jump or “tunnel” fully (100 % efficiency) without any reflections across a vacuum gap between two solids, if the materials in question are piezoelectric. In such materials, vibrations (sound waves) produce an electrical response, as well, and since an electric field can exist in vacuum, it can transmit the sound waves across. The requirement is that the size of the gap is smaller than the wavelength of the sound wave. This effect works not only in audio range of frequencies (Hz-kHz), but also in ultrasound (MHz) and hypersound (GHz) frequencies, as long as the vacuum gap is made smaller as the frequencies increase.  

Effective medium theory for the low-temperature heat capacity of a metasolid plate
Tuomas A. Puurtinen, Ilari J. Maasilta
Communications Materials 4, 1 (2023)

This work developed a theory for a thermal metamaterial, which can be used to strongly modify the phononic heat capacity. The derived analytic theory describes a nanoscale periodic holey plate with the help of effective elastic parameters. It describes the thermal response in the temperature range where the holey material looks continuous, but with modified elastic properties and density because of the structuring. The modified elastic properties in turn change the vibrational spectrum of the material (phonon energies), which directly affects the capacity to store heat.

Controlling thermal conductance using three-dimensional phononic crystals
Samuli Heiskanen, Tuomas A. Puurtinen, and Ilari J. Maasilta
APL Materials 9, 081108 (2021)

We fabricated for the first time 3D phononic crystals with integrated heater and thermometer structures using the 3D direct laser write lithography, and measured the thermal conduction of such structure at low temperatures. A puzzling enhancement of conduction was observed, which is not theoretically understood yet. 

Imaging Bacterial Colonies and Phage–Bacterium Interaction at Sub-Nanometer Resolution Using Helium-Ion Microscopy
Miika Leppänen, Lotta-Riina Sundberg, Elina Laanto, Gabriel Magno de Freitas Almeida, Petri Papponen and Ilari J. Maasilta
Advanced Biosystems 1, 1700070 (2017)

This paper, in collaboration with the biology department, demonstrates the first helium ion microscope (HIM) imaging of bacterial colonies interacting with viruses of bacteria, or bacteriophages. Furthermore, by controlling the He beam dose or by using heavier Ne ions, we milled out material in the samples, allowing for subsurface imaging and in situ sectioning. HIM seems to offer great opportunities to advance the studies of microbial imaging.

Broadband Ultrahigh-Resolution Spectroscopy of Particle-Induced X Rays: Extending the Limits of Nondestructive Analysis
M. R. J. Palosaari, M. Käyhkö, K. M. Kinnunen, M. Laitinen, J. Julin, J. Malm, T. Sajavaara, W. B. Doriese, J. Fowler, C. Reintsema, D. Swetz, D. Schmidt, J. N. Ullom, and I. J. Maasilta 
Phys. Rev. Applied 6, 024002 (2016)

PIXE spectroscopy is a widely used accelerator-based method for elemental analysis, down to trace level impurities. This work, performed in collaboration with the JyU Accelerator laboratory and NIST Boulder, demonstrates for the first time that practical PIXE spectroscopy can be performed with a cryogenic X-ray transition-edge sensors array, with over an order of magnitude improved energy resolution to previous state-of-the-art.  We show that the technique offers great promise in the elemental analysis of thin-film and bulk samples, especially in the difficult cases where tens of different elements with nearly overlapping emission lines have to be identified down to trace concentrations. In the demonstrated cases, standard silicon drift detectors could not resolve all elements.

Engineering thermal conductance using a two-dimensional phononic crystal 
Nobuyuki Zen, Tuomas A. Puurtinen, Tero J. Isotalo, Saumyadip Chaudhuri, and Ilari J. Maasilta
Nature Communications 5, 3435 (2014)

Here we show for the first time that phonon thermal conductance can be strongly modified coherently, by orders of magnitude, using two-dimensional hole array phononic crystals. Calculations performed using finite element method agreed with the experiment and demonstrated that the effect arises from strong reduction of the group velocities of the modified phonon modes.