Two postgraduates, Artyom Grebenko (MS program) and Artyom Baranov (PhD program), both working for MIPT Interdisciplinary Center of Fundamental Studies (ICFS) and the Department of General and Applied Physics (DGAP) have begun tuning of a unique atomic force microscope (AFM). The work is supervised by V. Dryomov, a Candidate of Physical and Mathematical Sciences, and a group of German specialists.
This is the first installation of its kind in Russia. It will allow scientists to study the properties of nanostructures in strong magnetic fields (up to 9 T), and with temperatures as low as 3 °K without using liquid helium.
The experimental installation devised by the German company attocube systems AG, makes it possible to do without costly liquid helium to achieve low temperatures. “In several weeks’ time an ordinary working cryogenic plant may use hundreds of liters of liquid helium, which equals hundreds of thousands ofrubles spent”, said VyacheslavDryomov, a Senior Researcher at Quantum Nanostructures Laboratory of ICFS. “With this device, one can achieve temperature as low as 3 °K in 10 hours, and it takes just one hour to replace the sample”. Using the atomicforce microscope, the scientists intend to study the transport properties of various substances and materials (topological insulators in particular) and carry out experiments on controlling spin currents.
The cooling unit functions on the acoustothermal principle. The unit’s most important part is the pulse tube, inside of which the gas first gets compressed, releasing heat, and is then expanded and cooled. The compression and expansion are achieved bythe reciprocating motion of a pistoninside the tube. Since the 1980s such systems have been actively developed, and are currently being produced for research laboratories.
Acoustothermal refrigerators, efficient and economical as they are, have but one drawback: the moving parts produce vibrations, which interferes with such a precise research method as atomic force microscopy. The need to avoid such vibrations hassled to the creation of installations like the one on the first floor of the MIPT Laboratory building.
The MIPT campus in Dolgoprudny (i.e., the Laboratory building) is intended to be used for the creation of a high-capacity research base. This is the plan of the scientists of several “nanophysical” laboratories - winners of the TOP-100 Program first scientific competition, including the Quantum Nanostructures Laboratory (led by Mikhail Troonin, the Dean of DGAP), the Laboratory of Topological Quantum Phenomena in Superconducting Systems (headed by Alexander Golubov, the winner of the latest mega-grant contest) and others.
According to Mikhail Troonin, “the physics of condensed matter is all about ultra-low temperatures, which is why, this year, so-called ‘dry fridges’ will be set up set up along with the AFM. ‘Dry fridges’ are liquid helium-free refrigerators, in which various nanostructures (including those made at the unique MIPT nanolithograph) will be cooled to a temperature of 10 millikelvin”.
Vyacheslav Dryomov says, “previously one could work with such low temperatures only at 2-3 external MIPT base departments, but now we are able to combine the training of future specialists and research work right here.
Abroad, it is a rule to have well-grounded scientific research centers both in colleges and universities, and rightly so.”
The AFM helps to see (or, rather, feel) the surface of various specimens placed in low temperatures and a strong magnetic field by scanning the materials with a sharp needle on a flexible cantilever suspension. Besides, this new device will also help to study the electrical properties of the materials in those conditions, e.g., to measure the tunneling density of electron states or examine the spread of electric defects on the surface of a sample.
“Topological insulators are materials which have non-conducting properties in their core, but closer to the surface they form a two-dimensional layer over which the electrons are spread without dispersion. Topological insulators reveal their qualities only in low temperatures, and our installation will help us to move closer to the surface which conducts current without resistance. I need to stress that this is not the same as superconductivity, the mechanisms at work here are quite different, which makes the topological insulators even more intriguing,” said Vyacheslav Dryomov in his interview with the MIPT Press Service.
There are many reasons why topological insulators are “intriguing”. For example, the electron spin in them is orthogonal to the impulse, which means that the directed electron flow creates not only the charge current but also the spin current. Such materials, thus, can serve as a basis for spintronics, i.e., the electronic devices which operate not only according to the charge, but also the spin. Spintronic circuits are supposed to be faster-functioning, more compact and economical than electronic circuits, but producing them requires further fundamental research.
Topological insulators can be used not only when solving applied tasks, but also in modeling such exotic particles as magnetic monopoles or Majorana fermions. A magnetic monopole, or “an isolated magnet with only one magnetic pole”, is only a hypothetical elementary particle in particle physics; yet, if a dipole is placed at the topological insulator’s surface, the resulting magnetic field will be like that of a monopole. In such a way one can treat the perturbations which “behave” like real particles, i.e., quasiparticles. Studying those, in turn, can be of interest not only for physicists but also for the specialists who are trying to find the existing prefigurations of quasiparticles.
Finally, topological insulators can help in finding ways to apply quantum entangled electrons. Entangled particles form an integral quantum object which sustains its integrity regardless of the distance separating the particles. Quantum entangled photons are being closely studied at present, and a number of practical solutions have already been found – e.g., devising a mobile phone line which cannot be tapped unnoticed by the recipient.
Thus, the new AFM device will help, first of all, to obtain fundamental knowledge of nanostructure properties. Yet, these days many fundamental tasks of quantum nanophysics are realized in the sphere of applied technology.
The MIPT Press Service expresses gratitude to Artyom Baranov, VyacheslavDryomov and Mikhail Troonin for the tour of the laboratory and help in preparing the material.