Studies

Author: VIDMANTAS TOMKUS

Dissertation title: Microstructural and optical properties of metal oxide optical coatings deposited by ion beam sputtering and their application in UV spectral range
Fields of science: Material Engineering (T008)
Scientific supervisor: dr. Kęstutis Regelskis
Defence of the dissertation: February 4, 2021

SUMMARY: Permeability of yeast cells to TPP+ is limited by cell walls and not the plasma membranes even though exposure to PEF improves overall permeability of both structures. Exposure to electric field pulse with a strength of ≈ 6 kV/cm induces irreversible damage to the plasma membrane and not the cell wall. Fall time of voltage after electric field pulse formation depends on load conductivity. Active regulation of fall time can lead to a decrease in effective pulse duration by up to 35 %, which in turn can diminish the lethal effects of pulsed electric field. Such unaccounted pulse duration defects can cause up to 10 % of death in yeast cells.  Mediated amperometry with hydrophilic mediator ferricyanide can be employed to evaluate irreversible electroporation in yeast cells. Current strengths generated by electroporated cells (E=16 kV/cm, t=300 μs) increased by more than one order of magnitude when compared to currents from untreated yeast cells.  A decrease in menadione-mediated current strength by up to 6 times in electroporated yeast cells (E=16 kV/cm, t=300 μs) represents rise in membrane permeability and subsequent leakage of NAD(P)H which are needed for menadione reduction. The decrease in current strength can be compensated entirely by adjusting the extracellular concentration of NADH to 1 mM.  Exposure to 5 consecutive electric field pulses with nanosecond duration (E≥220 kV/cm, t=90 ns) results in the expression of markers related to caspase-dependent death in ≈ 73 % of yeast cells.  Electric field pulses with nanosecond duration can selectively inactivate yeast cells in acid whey solution. Yeast cells were more susceptible to PEF treatment (E=95 kV/cm, t=90 ns, pn=100) by up to 14 times when compared to bacteria cells.

SUMMARY: It was found that the electrical resistance decreased most significantly during the first annealing stage in graphene vdW structures with Au and Ni layers. This stage lasted 4 minutes. The decrease of RC was equal to 57 % for Au CTLM samples after 4 minutes of annealing. Moreover, the reduction of the contact resistance RC of Ni CTLM samples reached 75 % after 4 minutes of annealing. The further annealing (tan. >4 min) did not decrease the RC significantly. The charge transfer length LC in CTLM samples with both Au and Ni films decreased almost twice after the 4 minutes annealing step. However, when annealing time was increased the change in LC was not observed. The steep decrease in LC could be associated with the decrease in current spreading and the decrease of the distance between the graphene layer and metal surface due to contact annealing. Sheet resistance changed smoothly during the thermal formation of the metal-graphene contacts. However, different stages of RSh were not observed over annealing time. RSh was found to decrease slightly during the entire contact annealing process. It has been shown that the contact annealing in the graphene layer on the surface of the metal contacts resulted in change of compressive strain. ε increased over the annealing time. It has been found that the change in doping was dominant due to the annealing in graphene sheet on insulator surface. The density of charge carriers varied from p ~ 0 to p=(0.18–0.5)×10¹³ cm⁻² in the graphene layer. Meanwhile, the strain changed slightly during the annealing from ε=(−0.28 – −0.36) % to ε=(−0.37 – −0.43) % in the graphene layer on SiO₂. Investigations of deformations in graphene structures with metal films revealed different statistical distributions of Raman G and 2D peaks on the flat areas of the sample (one component of the normal distribution) and on the transition area of the contact step (several components of the normal distribution). It is shown that long-distance deformations are created at the contact step area, which are widespread in the graphene layer far away from the boundary of the contact edge. Compressive strain was found in the transition zone from the contact edge areas to the SiO₂ zone for both Au-based and Ni-based samples. However, the stresses in the graphene did not reach ε>−0.25 % in graphene samples with metal films. The appearance of data clusters was observed in the constructed pos(2D) from pos(G) diagrams. Each cluster is assigned to a specific area of the structure surface. It is shown that using different (1σ or 2σ) intervals, the overlap of the ze cluster with adjacent flat-area clusters can be estimated. The study showed that the charge density p in the graphene samples with Au was the same on the SiO₂ surface and on metal. Doping was equal to p=(2–4)×10¹³ cm⁻². Meanwhile, a strong p change between metal and SiO₂ domains was observed in Ni samples. The equivalent electric circuit model was proposed that incorporates p and ε changes in the graphene layer. It has been shown that long-range deformations in a graphene sheet can affect the contact and sheet resistances. Long-term stability study showed that changes of electrical characteristics in graphene-based structures are caused by environmental conditions. It was shown that after the production of resistive structures, the electrical characteristics have changed over the time. However, the annealing helped to restore the resistance of the samples. The drift of the samples was attributed to the change of density of the charge carriers in the graphene sheet on the contact step zone. The change of density of the charge carriers is this region was the largest compared to other regions. The temperature process increased the p-type doping in the graphene layer. The analysis of train in graphene lattice showed that the largest change occurred in the graphene layer on the flat metal area. These changes were dependent on the metal. During the drift of graphene samples an increase in compressive strain was obtained the samples with Au films. Meanwhile, for samples with Ni films a decrease in tensile strain was observed. The work function in graphene samples with Au and Ni samples changed differently. The largest change in WGr was found to occur in the graphene layer on the flat Au and Ni zones and on the contact step region. The contact step zone has been found to be the most sensitive regions to drift and annealing. Meanwhile, the charge density, mechanical deformation, and the work function in the graphene layer on the insulator changed the least. An equivalent circuit model was presented which should relate the changes in the density of the charge carriers and the work function in the graphene sheet at the contact step zone with the change of the resistance Rcrowd and RQ components. The decrease in the electrical conductivity of graphene resistive structure with Au and Ni layers was observed a varying the temperature from 293 K to 523 K. TCR in graphene structures with the Au film was found to be α=0.003 K−1, while in structures with the Ni film α=0.002 K⁻¹. The growing of temperature increased the contact resistance RC. The contact resistance response to the changing temperature was found to be more sensitive than the layer resistance. It was found that the relative value of the charge transfer length LC increased differently depending on the metal layer used for construction of graphene structures.

Author: TADAS BARTULEVIČIUS
Dissertation title: Compact high pulse energy fiber laser systems for industrial and scientific applications
Fields of science: Physics (N 002)
Scientific supervisor: dr. Andrejus  Michailovas 
Defence of the dissertation: June 07, 2021

SUMMARY: Fiber lasers have a strong interest and growth in a field of photonics. In this doctoral dissertation, the use of versatile and flexible fiber technology by means of theoretical and experimental research allowed the realization of ultrashort pulse fiber laser sources which can be successfully applied in various fields of science and industry. The main goals in the dissertation were consistently approached - the realization of a compact high energy (µJ level) ultrashort pulse fiber laser system, the demonstration of high average power (>20 W) of laser radiation and the expansion of the regimes of the laser operation. A compact fiber chirped pulse amplification system exploiting a tandem of a chirped fiber Bragg grating stretcher and a chirped volume Bragg grating compressor with matched dispersion profiles, realizing high fidelity transform-limited ultrashort laser pulses (<300 fs) at the output of the system was proposed and experimentally demonstrated. The successful demonstration of these technologies and the implementation of a chirped pulse amplification system allowed the development of high-energy and high average power fiber laser systems. Finally, a unique method to synthesize ultra-high (>2 GHz) repetition rate bursts of ultrashort laser pulses has been demonstrated which allowed to expand the regimes of the laser operation and thus to increase the pulse/burst energies at the output of a fiber laser system.

Author: LINA GRINEVIČIŪTĖ
Dissertation title: Nanostructured optical coatings for the manipulation of laser radiation
Fields of science: Material Engineering (T008)
Scientific supervisor: dr. Ramutis Drazdys 
Defence of the dissertation: July 09, 2021

SUMMARY: Permanent research in material science is of crucial importance to develop novel optical components with complex functionalities and novel optical characteristics. This thesis is focused on improving the properties of optical elements by changing the thin film’s internal structure. The aim of this work is to investigate the possibility of forming thin films with controllable internal structures by using physical vapor deposition technologies, and to extend these methods to the manufacture of optical elements used in the manipulation of laser light. The main novelty of this dissertation from both sides, scientific and practical, is the demonstration of the new possibilities to control the internal structure of the deposited layer by: i) selecting the appropriate technology, ii) optimizing deposition parameters, and iii) choosing substrates of suitable profiles. Firstly, nanostructured all-silica anisotropic layers were investigated in detail, and their advantages in phase retardation plates and polarizer formation were demonstrated. Secondly, the possibility of forming 2D photonic crystals based on periodically pre-patterned substrates coated in a multilayer structure was established. Such methods allow for the creation of nanostructured optical elements as coatings, which can be applied to microlasers and to high-power complex systems.

Author: VILIUS VERTELIS
Dissertation title: Magnetic field diffusion in metallic and superconducting cylinders
Fields of science: Physics (N 002)
Scientific supervisor: prof. habil.dr. Saulius Balevičius 
Defence of the dissertation: September 17, 2021

SUMMARY: A Cylindrical aluminum payload causes a noticeable disturbance of the magnetic field dynamics inside the bore of a serial augmented hexagonal railgun that is the highest during the abrupt change of magnetic field pulse and decays sharply with the distance from the surface of the payload. This should be considered when designing launch packages. The lumped parameter approach is capable of describing the magnetic field dynamics and the trapped flux inside a thin-walled superconducting tube. The critical current density - magnetic field relationship and the power law exponent can be evaluated using a single magnetic pulse measurement inside a tube. Thin YBCOfilms prepared by the PI-MOCVD on Al2O3 substrates with CeO2 layers below their Tcexposed to a DC magnetic field slightly higher than Bc display a linear magneto-resistive effect in accordance to Bardeen-Stephen model. Thin YBCOfilms prepared by the PI-MOCVD on Al2O3 substrates with CeO2 layers exposed to DC voltage exhibit S-shaped current-voltage characteristics. Under constant current these films show dynamic resistance triggered by an external DC magnetic field. These phenomena can be well explained by uniform Joule heating caused by the flux flow-induced resistive state of the superconducting film. Motion of a superconducting disk-shaped YBCO armature cooled to 77 K electromagnetically accelerated by a pancake coil and magnetic field dynamics between them can be well described numerically with simulations based on Maxwell’s equations in their H-formulation, assuming a power-law electric field-current density relationship for the superconductor. The mechanical energy transferred to a superconducting armature from a pulsed magnetic field source increases with the increasing critical current density of the superconducting armature. This increase has a limit for a given accelerating coil current pulse and armature starting position. Energy transferred to a type II superconducting armature in a coilgun depends on the shape, amplitude and duration of the coil current pulse. Superconducting armatures outperform normal metal armatures in the lower current amplitude range and when pulses with lower time derivatives are used. Type II superconducting armatures exhibit strong magnetic braking when step-like current pulses are used for their acceleration, however, they outperform normal metal armatures in the lower step amplitude range. 

Author: EDITA SODAITIENĖ
Dissertation title: Removal of anionic and cationic pollutants by groundwater water treatment waste
Fields of science: Chemistry (N003)
Scientific supervisor: dr. Audronė Gefenienė
Defence of the dissertation: September 23, 2021

SUMMARY: Groundwater and surface water are very important components of the environment, so the supervision and control of water resources are the responsibility of each state. The developed legislation helps regulate water management issues, protects these natural resources from pollution, and creates a system of restrictions, prohibitions, and legal liability [1]. Industrial wastewater can be connected directly to natural water resources or discharged into urban sewers. Wastewater discharged in this way affects water bodies or the operation of sewerage and wastewater treatment plants [2]. In Lithuania, wastewater discharge is regulated by the Wastewater Management Regulation (Order No. D1-236) which was signed in 2006 and specifies the requirements for the discharge of both domestic and industrial wastewater into the environment. Controlled substances include total chromium, with MPLs (maximum permissible limits) of 2 mg L-1 in the sewage system and 0.5 mg L-1 in the natural system [3]. Anodizing is an important and widely used electrochemical process for the surface treatment of metal and electronic parts [4]. The anodizing process consists of surface cleaning and preparation followed by coating and finishing [5]. To give color to the anodized coatings, the surfaces are dyed. The process is constantly improved to enhance the penetration of the dye into the pores [4]. The effluent streams generated during the whole process are mixed before treatment, resulting in colored effluents containing various metal ions [5]. It is very important to treat such wastewater in order to meet strict environmental requirements. Azo dyes are the largest group of azo compounds and the most widely used dyes in the industry. They are suitable for fiber dyeing, photoelectronic, printing systems, food technology, and biological reactions [6]. There is currently a fairly large selection of dyes that are used for anodized aluminum dyeing. The use of complex metal dyes is a major concern for their environmental impact, as heavy metals in dyes are considered to be nonbiodegradable pollutants. In order to reduce environmental pollution by colored wastewater, the search for cheap and efficient adsorbents is very important. The need for environmental sustainability supports the idea of using a material generated during the treatment of clean groundwater as an adsorbent for the treatment of contaminated water. The purpose and objective of the groundwater treatment are to ensure the quality of drinking water so that it tastes good and is safe to use. During the process, a natural nanomaterial 124 rich in iron(III) compounds (groundwater treatment (GWT) waste), as a byproduct is produced. The physicochemical properties of GWT waste are similar to those of specially synthesized Fe(III)-containing adsorbents and catalysts. This dissertation presents research on sustainable processes for the removal of anionic chromium complex dye (Green) from aqueous solutions and real anodized aluminum dyeing effluents using GWT waste as an adsorbent or catalyst. As the dye contains chromium, the effluent from the anodized aluminum dyeing must be treated to remove not only the color but also the heavy metal ions. For this reason, the dye removal efficiency was evaluated not only by the change in the solution color but also by the change of the chromium concentration in the solution to be cleaned. In addition, the chemical behavior of the entire adsorption system was observed during the dye removal: changes in solution pH, iron, calcium, and aluminum concentrations. The adsorption properties of groundwater treatment (GWT) waste were also investigated by treating water contaminated with cationic pollutants (Pb(II) and Cu(II) ions). The catalytic properties of GWT waste were investigated in the Fenton reaction of oxidative degradation of chromium complex dye (Green). The efficiency of the removal of cationic and anionic pollutants was evaluated by conducting kinetic, equilibrium, and thermodynamic studies.

 

Author: MINDAUGAS KAMARAUSKAS
Dissertation title: Intentional modification of silicon photovoltaic devices by deep surface structuring and two dimensional material coatings
Fields of science: Material Engineering (T008)
Scientific supervisor: dr. Arūnas Šetkus 
Defence of the dissertation: November 5, 2021

SUMMARY: Tasks for this work: 1. To investigate the properties of deeply textured silicon surface and to describe relationships between characteristics and technological parameters when metal assisted chemical etching method is modified by substituting noble metals with other metals acceptable to form catalytic nanoparticles, that result in the directional etching and the key mechanisms of the related process. 2. To investigate applicability of metal assisted chemical etching for texturing large areas of silicon surfaces dimensions of that are comparable with the substrates used in commercial device fabrication and to investigate influence of technological parameters on properties of these surfaces. 3. To create surface structures that can be accepted being analogous to the pn heterojunctions in the bulk semiconductors but fabricated by deposition of the two-dimensional material films on monocrystalline silicon and to investigate their characteristics and dependencies on technological parameters and surface properties. 4. To develop a laboratory model of photovoltaic light detector based on heterojunction between silicon and 2D material and to investigate its characteristics.

Author: GIEDRIUS SINKEVIČIUS
Dissertation title: Research of piezoelectric ringing suppression in pockels cells
Fields of science: Electrical and Electronic Engineering (T001)
Scientific supervisor: prof. dr. Algirdas Baškys 
Defence of the dissertation: November 9, 2021

SUMMARY: The dissertation conducts research and proposes solutions to increase the optical contrast of Pockels cells by suppressing the piezoelectric ringing caused by the high voltage pulses, which are used for the control of the cell. The dependence of the optical contrast of Pockels cells on the duration and frequency of high voltage pulses was investigated experimentally. Resonant frequencies were detected and the propagation of acoustic waves in the Pockels cell crystal was investigated. An active piezoelectric oscillation suppression method is proposed and a high voltage high frequency pulse generator is developed to implement this method. The dissertation consists of an introduction, three chapters, general conclusions, references and the list of author’s publications on the topic of the dissertation. The introductory chapter discusses the research problem, the relevance of the work, describes the object of research, formulates the aim and objectives of the work, describes the research methodology, scientific novelty of the work, the practical significance of the results, defended statements. At the end of the introduction, the author's publications and conference papers on the topic of the dissertation and the structure of the dissertation are presented. The first chapter analyzes the types of light modulators used in high power laser systems and their properties. The materials of the crystals used in Pockels cells, the phenomenon reducing the contrast ratio and main optical schemes are reviewed. Topologies of high voltage pulse generators, used for the control of Pockels cells, are analyzed, their advantages and disadvantages are discussed. The second chapter presents the investigation of the dependence of the optical contrast of Pockels cells with beta barium borate (BBO) and potassium dideuterium phosphate (DKDP) crystals on the duration and frequency of high voltage pulses. The propagation of acoustic waves caused by a piezoelectric phenomenon in Pockels cell crystals was investigated. Passive suppression of piezoelectric ringing in a Pockels cell with a BBO crystal was investigated. The third chapter proposes an active piezoelectric ringing suppression method for Pockels cells with a DKDP crystal and describes a high voltage pulse generator developed for the implementation of this method. Experimental investigation of active piezoelectric ringing suppression method was performed.

Author: KAROLIS MADEIKIS
Dissertation title: Investigation of the ultrafast fiber and hybrid laser systems and their nonlinear wavelength conversion methods in the infrared spectral region
Fields of science: Physics (N 002)
Scientific supervisor: dr. Andrejus Michailovas 
Defence of the dissertation: November16, 2021

SUMMARY:  Combination of two-channel fiber seed laser with subsequent amplification in narrowband picosecond Nd:YVO4 regenerative amplifier and in multiple mid-IR OPA stage where efficient low-energy SC radiation pre-amplification in BBO crystal is employed, allows to realize hybrid broadband (up to 550 cm-1 and up to 850 cm-1 bandwidth by using the crystal scanning technique) high resolution (<3 cm-1) SFG spectrometer system, that is tunable in the spectral range from 2 μm to 10 μm. Feedback loop incorporation in an Er-doped fiber phase shifter setup allows increasing frequency response speed of 100 times. Furthermore, this could be increased even more with a higher pump offset, but it requires suppressing delay instabilities, arising due to the increased thermal load. A combination of a low total cavity dispersion (-0.27 ps2) fiber oscillator and an Er-doped fiber phase shifter allows realizing a frequency-locked mode-locked fiber oscillator operating at 1030 nm wavelength, which exhibits stable locking of pulse repetition rate to an external RF source with a timing jitter below 200 fs. Supercontinuum seeded stimulated Raman active crystal amplifier setup allows realizing compact and efficient μJ-level femtosecond 1 μm laser wavelength conversion to biological tissues transparent NIR region (1.1-1.35 μm). Furthermore, supercontinuum self-seeded stimulated Raman amplification scheme combined with GHz burst providing fiber-based pump source, synchronous pumping, and all-fiber repetition rate locking techniques paves a way for improved efficiency, compact, and robust wavelength conversion concept.

Author: ALINA LEŠČINSKAITĖ
Dissertation title: Stellar populations in the dwarf irregular galaxy Leo A
Fields of science: Physics (N 002)
Scientific supervisor: prof. dr. Vladas Vansevičius 
Defence of the dissertation: November 26, 2021

SUMMARY: The main tasks of the thesis: Develop methods to decontaminate stellar photometry catalogues of foreground/background objects and stellar blends. Identify luminous asymptotic giant branch stars, emission stars and other peculiar objects in Leo A. Determine the extent of the Leo A galaxy based on the distribution of the old stellar populations at its outskirts.  Identify structural components of the Leo A galaxy, traced by red giant branch and asymptotic giant branch stars, and examine their radial variations.  Evaluate the age of young stellar populations by using main sequence stars, blue core-helium-burning stars and young star clusters; investigate the evolution of stellar structures in Leo A over the past ~300 Myr.

Author: LAURYNAS VESELIS
Dissertation title: High energy hybrid femtosecond lasers based on Yb doped fibers and YAG crystals
Fields of science: Physics (N 002)
Scientific supervisor: dr. Andrejus Michailovas 
Defence of the dissertation: November 26, 2021

SUMMARY: Axis-symmetric (1D) numerical model for three-level gain medium continuous-wave amplifier, which incorporates signal beam propagation description by Hankel transform, pump beam shape calculation by ABCD matrix method, gain medium parameters – thermal conductivity, thermo-optic coefficient, spectroscopic cross-sections – dependency on temperature, spectroscopic cross-section value dependency on wavelength, and operates in the iterative algorithm, where convergence parameters are end-pumped gain medium temperature (single-pass) and output power (double-pass), is capable to accurately predict amplifier output power, gain narrowing effect, amplified beam shape, and wave-front distortions. Application of CFBG as pulse stretcher and CVBG as matched dispersion pulse compressor in the hybrid laser system, consisting of FCPA laser as a seed source and a double-pass end-pumped Yb:YAG amplifier for further peak-power scaling, enables to construct a compact hybrid laser system, delivering nearly bandwidth-limited compressed femtosecond pulses. Incorporation of a pair of quarter-wave plates to the end-pumped double-pass Yb:YAG amplifier, placed before and after gain medium, and oriented in a way that enables amplification of circularly polarized signal beam inside the gain medium, in addition to spatial filtering technique, where the spatial aperture is placed near the focal plane of the refocused beam to filter out thermally-induced aberrations, provides practical means to significantly minimize bi-focusing and restore original beam quality, enabling to achieve diffraction-limited beam quality after amplification in the end-pumped Yb:YAG amplifier. Implementation of the SVWP enables significant reduction of the thermally-induced depolarization and astigmatism in the double-pass end-pumped Yb:YAG amplifier configuration.

Author: LUKAS RAZINKOVAS
Dissertation title: Vibrational properties and photoionization of color centers in diamond: theory and ab initio calculations
Fields of science: Physics (N 002)
Scientific supervisor: prof., dr. Audrius Alkauskas 
Defence of the dissertation: December 10, 2021

SUMMARY: The first goal of this research work is to develop a theoretical methodology for describing optical lineshapes and photoionization processes of deep-level defects. The second goal is the application of these methods to explain the vibrational structure of SiV⁻ and NV⁻ centers of diamond and photoionization mechanisms of NV⁻. To achieve these goals, we formulated the following tasks: Improve the computational methodology for the calculation of the defects’ vibrational structure in the dilute limit. Develop practical theoretical methods for the analysis of the dynamical multi-mode Jahn–Teller effect in color centers. Develop an ab initio methodology for the calculation of photoionization cross-sections in the supercell geometry. Explain the isotopic shift of the phonon side peak in the luminescence spectrum of the negatively charged silicon–vacancy center of diamond. Model theoretical luminescence and absorption lineshapes of the NV⁻ center.  Model absolute cross-sections of absorption, emission, and photoionization processes of the NV⁻ center.
Statements presented for the defense: (I) We suggest that the experimentally observed sharp vibrational feature in the photoluminescence spectrum of SiV⁻ is an eu-symmetry vibrational resonance. The appearance of this feature in the experimental spectrum cannot be explained in the Frank–Condon approximation. We hypoth-esize that this peak is due to the Herzberg–Teller effect, whereby the vibration modulates the optical transition dipole moment. (II) We developed a novel computationally tractable methodology for the ab initio description of the multi-mode Jahn–Teller effect in point defects. This methodology improves current theoretical techniques that are based on the single-mode approximation. (III) Our ab initio methodology to calculate luminescence and absorption line-shapes accurately reproduces the NV− center’s experimental lineshapes. (IV) After the photoionization from the ³E state, NV centers transition into the metastable ⁴A₂ state of NV0. This explains electron spin resonance experiments and has important consequences for the charge dynamics of the NV center. We determine the threshold for the photoionization to be 1.15 eV. (V) Our ab initio methodology for the photoionization cross-section calculations explains recent photoionization experiments and elucidates the charge-state dynamics of NV centers.

Author: IRENA BALČIŪNAITĖ
Dissertation title: Characterization of natural silicate garnets by means of non-destructive testing methods
Fields of science: Chemistry (N003)
Scientific supervisor: prof. habil. dr. Eugenijus Norkus 
Defence of the dissertation: December 14, 2021

SUMMARY: The aim of this work was to characterize the physico-chemical properties of natural garnets from known origin countries by means of the non-destructive methods of investigation, and demonstrate the suitability of such methods for sufficient identification of the investigated minerals without diminution of their jewellery value.  The main tasks of the work were as follows: 1. Selection of the optimal non-destructive research methods suitable for the characterization of jewellery garnets; 2. Determination of the chemical composition of the studied natural garnets; 3. Measurement of the specific weight and the values of the refractive index of the studied natural garnets; 4. Investigation of the spectral characteristics of the studied natural garnets – their light absorption in the visible part of the spectrum, Raman spectral features, and cathodoluminescent properties.

 

Author: VAKARIS RUDOKAS
Dissertation title: Magnetoresistive properties of nanostructured manganite-cobaltite films
Fields of science: Physics (N 002)
Scientific supervisor: prof. dr. Nerija Žurauskienė 
Defence of the dissertation: December 17, 2021

SUMMARY:  The main idea of this dissertation is to change the chemical composition of manganite films (part of Mn atoms with Co atoms) in order to increase the MR of the films, reduce the MRA at low magnetic fields and expand the operating temperature range of CMR-B-scalar sensors based on such films to cryogenic temperatures. It is also important to stabilize the basic parameters of the films over the time, thus the studies of their ageing phenomena are relevant. Investigation of the above phenomena in nanostructured manganite-cobaltite films would allow to control their magnetoresistive properties and use the films for the development of CMR-B-scalar magnetic field sensors.
The aim of this work: To investigate the magnetoresistance and its anisotropy phenomena of nanostructured manganite-cobaltite La_1-xSr_x(Mn_1-yCo_y)_zO₃ films, grown by Pulsed Injection (PI) MOCVD, in a wide range of magnetic fields and temperatures, to determine the underlying charge carrier transfer mechanisms and to develop prototypes of CMR-B-scalar pulsed magnetic field sensors operating at room (~290 K) and low (~80 K) temperatures exhibiting higher response and sensitivity to magnetic field in comparison to sensors based on manganite films.