Head: Kashin V.V.
2025
The purpose of this scientific work is to study the physical effects of electronic transport in micro- and nanostructures caused by the influence of external acoustic and electric fields. Acousto-electronic delay lines on lithium niobate plates LiNbO3 and plate modes of acoustic waves SHn, An and Sn serve as a convenient experimental and theoretical model for generating acoustic and electric fields. The electromechanical coupling coefficient for the SH0 wave in lithium niobate YX can exceed 30%, which is significantly higher than for the surface acoustic wave (surfactant) in the crystallographic orientation 128YX (5.6%). For this reason, the acoustoelectric current for the lamellar mode in the YX plate should be higher than for surfactants.
The project investigates:
1) Structures created on the surface of the substrate.
The acoustic effect on the conductor will be measured using a stand for studying electrophysical characteristics, consisting of a probe station, a generator, a vector circuit analyzer, a synchronous amplifier, an oscilloscope, and a picoammeter. In the case of tunnel contact between the STM probe and the conductor on the surface of the delay line, the acoustic part of the measuring stand is transferred to the STM table or located nearby, the signal is detected by the internal circuits of the STM.
The sensitivity of the method can be increased if the measured current flowing in the external circuit is increased by reducing the shunting internal current. There are several possible ways to create high-resistance conductors.
a) The use of conductive materials with high resistance. The use of a nichrome target for magnetron sputtering is available. Another way is to use conductive inks, for example, based on conductive polymers. Using water-soluble ink based on PEDOT:PSS with the addition of 1% graphene, a conductive layer with a width of 300 microns and a thickness of 1 microns is created. Such a conductor with a length of 1 mm has a resistance of 4 kOhm.
b) Creating low-dimensional conductors by reducing the conductor cross-section as much as possible. There are two ways here:
1. Reducing the thickness of the conductor during, for example, spraying to ensure that there are sufficient inhomogeneities in the thin-film electrode.
2. Reducing the width of the conductor. If the first path is easily achievable in the standard process of magnetron sputtering of a material, then one of the ways to create a conductor of a controlled width of the order of several tens of nanometers is the method of electron lithography with a two-layer resistive mask. As a result, nanostructures with a geometric width of up to 20 nm are obtained.
2) Structures fixed on the surface of the substrate.
The project investigates crystals of quasi-one-dimensional conductors TaS3, NbS3 and (TaSe4)2I (whiskers), which must be fixed on a piezoactive substrate in the range of an acoustic wave. The ends of the whiskers are glued to the substrate with a drop of epoxy resin. Thermal indium spraying is used to make contacts to them. The masks for the samples are pieces of indium.
Another method of manufacturing contacts for whisker samples is the method of laser ablation of gold. The application of this method is due to the high energy of gold ions in plasma, which allows to overcome the oxide layer on the surface of TaS3 and provides ohmic contacts.
For quasi-one-dimensional conductors TaS3, NbS3 and (TaSe4)2, the project investigates the effect of acoustic waves on the dynamics of a charge density wave(CDW). CDW is a collective state of electrons (an electronic crystal), which can be considered as an elastic medium capable of deforming and sliding when an electric field is applied, localized on defects, impurities and inhomogeneities. It is characterized by the pinning energy of the CDW, an energy barrier that holds the CDW in a fixed position in the crystal lattice, preventing its movement and, as a result, the flow of electric current until an electric field exceeding a threshold value is applied. It is assumed that the interaction of acoustic waves with the CDW will manifest itself in cyclic stretching and compression of the crystal lattice, which will directly affect the structure of the CDW, since the field and pinning energy of the CDW are sensitive to deformation. An acoustic wave creates not only a mechanical effect, but also a traveling electric field that propagates synchronously with the deformation. This field interacts with charged excitations in a two-dimensional material and can change the phase and velocity of the CDW, enhancing or weakening the effect of mechanical modulation. The combination of these two contributions — deformation and electric — makes the effect especially strong: the deformation changes the local pinning energy, and the electric field creates an additional force tending to displace the CDW.
The expected manifestation of the interaction is the suppression of the threshold electric field at the voltage characteristic, which is necessary for the start of the slip of the CDW. Under the influence of an acoustic wave, pinning weakens, and the VZP can switch to a dynamic mode at lower voltages. The second key effect is the appearance of Shapiro steps on the waveforms that occur when synchronizing the intrinsic dynamics of a charge density wave with an external periodic excitation set by the frequency of the acoustic wave.
The project executors have at their disposal crystals of quasi-one-dimensional conductors TaS3, NbS3 and (TaSe4)2I. The TaS3 compound differs from other quasi-one-dimensional conductors in the most pronounced relationship between the dynamics of the CDW and the deformation of the lattice. Synchronization of the CDW in TaS3 is optimal at frequencies from 1 to 10 MHz. To study the effect of acoustic waves on the dynamics of the CDW, a sample was selected, the length of which was on the order of the length of the antisymmetric wave A0 at a frequency of 1.131 MHz and amounted to 742 microns. The whisker was placed on the surface of the substrate between the HSPs so that its main axis was parallel to the direction of wave propagation. When working out the methodology for measuring the transport properties of a sample at a temperature of 120 ° K, special features were found on the VAC when voltage was applied to the IDT at a frequency of 1.131 MHz. The effect of acoustic waves leads to suppression of the threshold field and the appearance of Shapiro steps. When the RF field is applied directly to the sample, Shapiro steps also occur, moreover, at the same CDW currents.
From a practical point of view, the result opens up wide possibilities for studying the effects of mechanical vibrations in piezoelectric materials on the dynamics of the CDW.
