Goal of research:
The objective of the project was to study basic physical phenomena at low temperatures of three types of low-dimensional quantum systems:
- mesoscopic-size superconducting nanostructures;
- low-dimensional magnetics;
- hybrid nanostructures containing superconductors, ferromagnets and topologic insulators.
All above systems have common features. First, all of them are essentially quantum objects. Second, the physics behind deals with collective behavior - so called, macroscopic quantum phenomena. Third the above unusual features are observable at low (<10 K) or ultra-low (<1K) temperatures.
The objects of study - the nanostructures - we fabricated using e-beam lift-off technique and directional vacuum deposition. Tunnel structures were obtained by oxidation of metal (e.g. aluminum) followed by deposition of the next layer without exposing the system to ambient atmosphere. Transport properties were measured using automatic measuring set-up based on 3He4He dilution refrigerator Oxford Instruments Kelvinox-400 at temperatures from 10 mK up to 1.5 K. Low-dimensional magnetics were fabricated using advanced solid state synthesis. The quality and composition of structures was controlled by X-ray analyses. Magnetic and thermodynamic properties were measured using Quantum Design PPMS at temperatures from 1.5 К to 77 К. 4-channel oscilloscope and sine generators were used to measure high-frequency characteristics of RLC filters from room down to liquid helium temperatures.
Theory analyses and modeling were made utilizing Bogolyubov- de Gennes formalism using Green function and Matsubara technique. Analytic methods were used, as well - in more general cases - numeric simulations using commercial software such as MATLAB, Wolfram Mathematica, PTC Mathcad.
Empirical base of research
Transport properties were measured using automatic measuring set-up based on 3He4He dilution refrigerator Oxford Instruments Kelvinox-400 at temperatures from 10 mK up to 1.5 K.
Amplitude - frequency dependencies of RF filters were measured using Agilent E8752D sine generator from 100 kHz up to 20 GHz, providing up to 20 dBm at 50 Ohm. Digital oscilloscope TDS-3024 with bandwidth 500 MHz and quantization frequency 5 GHz was used as the detector.
Amplitude vs. frequency characteristics of RLC filters were measured utilizing cryogenic insert positioned inside a standard 10 liter liquid helium Dewar. 4-probe measurement configuration is realized with standard 50 Ohm coaxial cable. The signal was analyzed with 4-channel digital 500 MHz oscilloscope Tektronix TM TDS3054 enabling 5GS/s sampling with 9-bit 10 000 records at all channels. As the generators Agilent Technologies TM PSG CW was used for measurements at high frequencies and Stanford Research Systems DS340 TM 15 MHz - for low frequencies.
Magnetic and thermodynamic properties of low-dimensional magnetics were measured using Quantum Design PPMS at temperatures from 1.5 К to 77 К.
Results of research:
First field of studies is mesoscopic superconductivity. Two directions were addressed: measurements of amplitude-frequency dependencies of RF filters; and experimental studies of quantum fluctuations of the order parameter in quasi-one-dimensional titanium channels.
It is known that utilization of RF filters enables a significant reduction of undesired EM environment contribution at ultra-low-T temperatures. Amplitude-frequency dependencies of RLC filters - self-made permalloy cores and commercial Wurth Elektronik beads - were studied. It was found that permalloy filters effectively suppress high-frequency up to tens of MHz, while Wurth Elektronik ones - up to 2 MHz. In both case the reduction of impedance at higher frequencies is determined by the capacitance of the coil. Parameters of the filters do not degrade at low temperatures, meaning that they can be used in cryogenic applications. It has been also demonstrated that cryogenic coaxial cables themselves act at filters at frequencies above 1 GHz. Hence, they can be considered as acceptable elements at low-frequency experiments, but not for high-frequency applications.
I-V characteristics of S1-I-S2 tunnel junctions, where one of the electrodes (S2) is the thin superconducting nanowire, were measured. The model gives a reasonable qualitative agreement with experiment. However, quantitative agreement is expected to be acceptable only when the direct impact of the order parameter amplitude and the density of states broadening contributions are considered. The corresponding theory is the subject of future studies.
Second field of studies was the low-dimensional magnetism. Thermodynamic (heat capacity С(Т)) and magnetic properties (magnetic susceptibility χ(T) and splitting ∆(T)) of the principle Nd3+ line of quasi-one-dimensional magnetic (Y1-xNdx)2BaNiO5 of various compositions x were studied. First-time magnetic properties and spin dynamics of Na3.70Co1.15TeO6 were studied. In this material one can realize the principally new structure with qausi-1-dimesional arrangement of cobalt in magnetically active layers, alternating with sodium layers.
Third field of research was hybrid nanostructures.
Hybrid SINIS turnstiles in the regime of Coulomb blockade was studied. New type of memory elements was developed: superconducting spintronic systems containing magnetic inclusions with non-collinear magnetic order.
Level of implementation, recommendations on implementation or outcomes of the implementation of the results
The topic of low-dimensional quantum systems is extremely important both from the point of view of basic research in solid state physics, and also from possibility of applications of such systems in nanoelectronic systems of new generation. The subject of the project fully corresponds to the Russian Federation roadmap for science, technology and technique development "Industry of nanosystems and materials". The technological know-how elaborated within the project will contribute to development of the critical technological fields of Russian Federation "Nanotechnology and nanomaterials" and "Technology of the electronics components development".
All results are new and fully correspond to high quality international-grade level of research in the field of: mesoscopic superconductivity; low-dimensional magnetics; hybrid nanostructures containing superconductors, ferromagnets and topological insulators. Main results have been published in international scientific journals indexed in WoS and Scopus databses.