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Victor Leca

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Papers by Victor Leca

Termination Control of (001) and (110) NdGaO3 Single-Crystal Substrates by Selective Chemical Etching

Crystals

Controlling the surface morphology and composition of the perovskite substrates is a critical asp... more Controlling the surface morphology and composition of the perovskite substrates is a critical aspect in tuning the final properties of the deposited films and of their interfaces. The paper reports on a chemical etching method developed for (110) and (001) NdGaO3 single crystal substrates in order to obtain a well-defined GaO2−x-terminated surface. The etching process is based on a HF + NH4OH solution and includes an annealing step performed in air or under O2 flow at temperatures of 800–1000 °C. In order to obtain the desired composition and surface morphology, the etching procedure was optimized for the vicinal step density at the surface and substrate crystal orientation. Growth nucleation studies of one-unit-cell MeO films (Me = Ti, Sr, Ba) on chemically etched and on only annealed substrates were performed in order to determine the composition of the substrate topmost layer. The results indicate that the chemically etched NdGaO3 substrate surface has a predominantly GaO2−x term...

Status of the project for a positron laboratory at ELI-NP

We report on the plans of a positron laboratory to be built at the Extreme Light Infrastructure -... more We report on the plans of a positron laboratory to be built at the Extreme Light Infrastructure - Nuclear Physics (ELI-NP) facility. The slow e+ source is based on e−–e+ pair production in a converter made of tungsten foils by a brilliant gamma beam (Eγ<3.5 MeV, Iγ = 2.4×1010 γ s-1) which will be generated by Compton back–scattering of photons from a high power laser on electrons from a LINAC. Numerical simulations of γ–ray interactions with the designed converter showed that, if the tungsten foils are used for moderation of the created fast e+, a slow e+ beam of an intensity of ~1×106 s-1 can be obtained. Circular polarization of the γ-beam is proposed to be one of the ELI-NP's upgrades. The slow e+ will be extracted perpendicularly to the γ–beam and will have ~ 30% transversal polarization degree. By using a transmission re-moderation stage and electrostatically turning the re–moderated beam by 90°, the transversal polarization will be changed to longitudinal. Longitudinal ...

An overview of the design of ELIPS—A new slow positron beam line

Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2019

In this work, the progress made by the positron group from ELI-NP in the design of a new slow pos... more In this work, the progress made by the positron group from ELI-NP in the design of a new slow positron beam line is summarized. The rays of a brilliant gamma beam will be used to produce a slow positron beam of high intensity. The main results of the simulations, done for the geometry optimization of the Converter/Moderator Assembly (CMA) for γ-to-slow e + conversion, are reported. By using the γ-beam with energy of up to 3.5 MeV and intensity of 2.4 × 10 10 γ s −1 , the slow e + beam can be obtained with a maximum intensity of ∼ 2 × 10 6 s −1 with tungsten or platinum CMA. Solid neon moderation is applicable, and a successful implementation will result in a slow e + beam of intensity > 10 7 s −1. The CMA can be operated with a central hole (= 2.5 mm) in the foils in parasitic mode, i.e., simultaneously with other γ-beam experiments, at the cost of reduced intensity by 18%. Furthermore, details on the slow positron transport and the spectroscopic systems, which are under construction, are provided.

Nanoscale Control of Structure and Composition for Nanocrystalline Fe Thin Films Grown by Oblique Angle RF Sputtering

Materials

The use of Fe films as multi-element targets in space radiation experiments with high-intensity u... more The use of Fe films as multi-element targets in space radiation experiments with high-intensity ultrashort laser pulses requires a surface structure that can enhance the laser energy absorption on target, as well as a low concentration and uniform distribution of light element contaminants within the films. In this paper, (110) preferred orientation nanocrystalline Fe thin films with controlled morphology and composition were grown on (100)-oriented Si substrates by oblique angle RF magnetron sputtering, at room temperature. The evolution of films key-parameters, crucial for space-like radiation experiments with organic material, such as nanostructure, morphology, topography, and elemental composition with varying RF source power, deposition pressure, and target to substrate distance is thoroughly discussed. A selection of complementary techniques was used in order to better understand this interdependence, namely X-ray Diffraction, Atomic Force Microscopy, Scanning and Transmission...

MECANISM DE CREŞTERE ŞI PROPRIETĂŢI ALE FILMELOR SUBŢIRI DE YBa 2 Cu 3 O 7-δ DEPUSE PRIN ABLAŢIE LASER PE (001) SrTiO 3 GROWTH MECHANISM AND PROPERTIES OF YBa 2 Cu 3 O 7-δ THIN FILMS DEPOSITED BY LASER ABLATION ON (001) SrTiO 3

Filme subţiri monocristaline de YBa 2 Cu 3 O 7-δ cu proprietăţi supraconductoare au fost crescute... more Filme subţiri monocristaline de YBa 2 Cu 3 O 7-δ cu proprietăţi supraconductoare au fost crescute epitaxial prin ablaţie laser pe substraturi de SrTiO 3 (001). Morfologia suprafeţei filmelor a fost studiată prin difracţie prin reflexie de electroni cu energie înaltă (DREEI) la presiuni mari, microscopie de forţă atomică (MFA) şi microscopie electronică de baleiaj (ME), în timp ce proprietăţile structurale au fost analizate prin difracţie de raze X (DRX). Pentru parametrii optimizaţi de creştere, filmele rezultate au o suprafaţă cu o rugozitate de ~ 10-15 nm, conform rezultatelor obţinute de la MFA şi ME. Creşterea filmelor, de tip Stransky-Krastanov, este influenţată de proprietăţile interfeţei substrat-film, datorită prezenţei stresului epitaxial. Din punct de vedere al proprietăţilor electrice, filmele sunt supraconductoare, cu valori ale temperaturii critice de T c =85-91 K. Sunt prezentate rezultate obţinute asupra fabricării şi proprietăţilor electrice ale joncţiunilor Josephso...

Fabrication of micron-sized periodic gratings in free-standing metallic foils for laser-plasma experiments

High Power Laser Science and Engineering

Engineered targets are expected to play a key role in the future high-power laser experiments cal... more Engineered targets are expected to play a key role in the future high-power laser experiments calling for a joined, extensive knowledge in materials properties, engineering techniques, and plasma physics. In this work, we propose a novel patterning procedure of self-supported 10 µm thick Au and Cu foils for obtaining periodic micron-size gratings as targets for high-power laser applications. Accessible techniques were considered, by using cold rolling, electron beam lithography, and Arion milling process. The developed patterning procedure allows efficient control of the gratings and foils surface on large area. Targets consisting of patterned regions of 450x450 µm 2 , with 2 µm periodic gratings, were prepared on 25×25 mm 2 Au and Cu

New Low-Stress PECVD Poly-SiGe Layers

OPTIMIZAREA MORFOLOGIEI SUPRAFEŢEI SUBSTRATURILOR DE NdGaO3 (110) PRIN TRATAMENTE TERMICE ŞI CHIMICE IMPROVED SURFACE MORPHOLOGY OF (110) NdGaO3 SUBSTRATES BY THERMAL AND CHEMICAL TREATMENTS

The results of the influence of thermal and chemical treatments on the surface morphology of (110... more The results of the influence of thermal and chemical treatments on the surface morphology of (110) NdGaO3 substrates are described in this paper. The treated surfaces were analysed by Atomic Force Microscopy (AFM), in air, and by in-situ high pressure Reflection High Energy Electron Diffraction (RHEED). The thermal treatment of substrates resulted in a NdO1+x single terminated surface, while a surface with GaO2-x terminating layer and atomically flat terraces without etch pits could be obtained by chemical etching in a HF + NH4F + H2O solution, followed by an annealing step at high temperatures (900-1000C) in air or in oxygen flow, for surface recrystalization. In acest articol sunt prezentate rezultatele influenţei pe care tratamentele termice si chimice le au asupra morfologiei suprafeţei substraturilor de NdGaO3 cu orientare (110). Suprafeţele astfel tratate au fost analizate prin intermediul microscopului de forţă atomică (MFA), in aer, si difracţiei prin reflexie de electroni c...

Termination control of NdGaO3 crystal surfaces by selective chemical etching

arXiv: Materials Science, 2012

A chemical etching method was developed for (110) and (001) NdGaO3 single crystal substrates in o... more A chemical etching method was developed for (110) and (001) NdGaO3 single crystal substrates in order to obtain an atomically flat GaO2-x - terminated surface. Depending on the surface step density the substrates were etched in pH-controlled NH4F- or NH4Cl-based solutions, followed by an annealing step at temperatures of 800-1000oC, in air or in oxygen flow, in order to recrystallize the surface. Atomic Force Microscopy (AFM) and high-pressure Reflection High Energy Electron Diffraction (RHEED) were used to analyse the surface morphology of the samples after every treatment. Studies on the chemistry and characteristics of the terminating layer showed that the chemically etched NdGaO3 substrate surface has a GaO2-x termination and that the (110) and (001) NdGaO3 surfaces are characterized by a different free surface energy, which is lower for latter.

Phase-sensitive evidence for dx 2 − y 2-pairing symmetry in the parent-structure high-Tc cuprate superconductor

Jochen Tomaschko,1 Sebastian Scharinger,1 Victor Leca,1,2 Joachim Nagel,1 Matthias Kemmler,1 Tere... more Jochen Tomaschko,1 Sebastian Scharinger,1 Victor Leca,1,2 Joachim Nagel,1 Matthias Kemmler,1 Teresa Selistrovski,1 Dieter Koelle,1 and Reinhold Kleiner1,* 1Physikalisches Institut–Experimentalphysik II and Center for Collective Quantum Phenomena in LISA+, Universität Tübingen, Auf der Morgenstelle 14, 72076 Tübingen, Germany 2National Institute for Research and Development in Microtechnologies, Molecular Nanotechnology Laboratory, Erou Iancu Nicolae Street 126A, RO-077190, Bucharest, Romania (Received 23 March 2012; published 7 September 2012)

MECANISM DE CREŞTERE ŞI PROPRIETĂŢI ALE FILMELOR SUBTIRI DE YBa2Cu3O7-δ DEPUSE PRIN ABLAŢIE LASER PE (001 ) srTiO3

High quality, single phase c-axis oriented YBa 2 Cu 3 O 7-δ thin films with superconducting prope... more High quality, single phase c-axis oriented YBa 2 Cu 3 O 7-δ thin films with superconducting properties were grown by laser ablation on (001) SrTi0 3 substrates. The surface morphology of the films has been investigated by means of high-pressure reflection high energy electron diffraction (RHEED), atomic force microscopy (AFM), and scanning electron microscopy (SEM), while the structural properties were studied by X-ray diffraction (XRD). Deposition under optimum conditions produces films with relatively smooth surface, with a roughness of about 10-15 nm, as confirmed by AFM and SEM data. The growth follows a Stransky-Krastanov mechanism governed by the substrate-film interface properties due to presence of epitaxial strain. The films show good superconducting properties with T c values of 85-91 K. Results on the fabrication and electrical transport properties of ramp-type Josephson junctions with YBa 2 Cu 3 O 7-δ electrodes and PrBa 2 Cu 3 O 7-δ barrier are presented.

Publisher’s Note: Phase-Sensitive Order Parameter Symmetry Test Experiments Utilizing Nd2-xCexCuO4-y/Nb Zigzag Junctions [Phys. Rev. Lett. 94, 167001 (2005)]

Physical Review Letters, 2005

Studies of Defect Structure in Epitaxial AlN/GaN Films Grown on (111) 3C-SiC

Nanomaterials

Several aspects such as the growth relation between the layers of the GaN/AlN/SiC heterostructure... more Several aspects such as the growth relation between the layers of the GaN/AlN/SiC heterostructure, the consistency of the interfaces, and elemental diffusion are achieved by High Resolution Transmission Electron Microscopy (HR-TEM). In addition, the dislocation densities together with the defect correlation lengths are investigated via High-Resolution X-ray Diffraction (HR-XRD) and the characteristic positron diffusion length is achieved by Doppler Broadening Spectroscopy (DBS). Moreover, a comparative analysis with our previous work (i.e., GaN/AlN/Si and GaN/AlN/Al2O3) has been carried out. Within the epitaxial GaN layer defined by the relationship F4¯3m (111) 3C-SiC || P63mc (0002) AlN || P63mc (0002) GaN, the total dislocation density has been assessed as being 1.47 × 1010 cm−2. Compared with previously investigated heterostructures (on Si and Al2O3 substrates), the obtained dislocation correlation lengths (Le = 171 nm and Ls =288 nm) and the mean distance between two dislocation...

Structuring Free-Standing Foils for Laser-Driven Particle Acceleration Experiments

Frontiers in Physics

The recent development of petawatt-class laser systems sets a focus on the development of ultra-t... more The recent development of petawatt-class laser systems sets a focus on the development of ultra-thin free-standing targets to access enhanced particle acceleration schemes vital for future applications, such as, medical and laser-driven nuclear physics. Specific strategies are required to improve the laser-to-particle energy conversion efficiency and increase the maximum particle energy. One of the promising approaches is based on the target design optimization; either by tuning key parameters which will strongly affect the laser-matter interaction process (e.g., material, composition, density, thickness, lateral dimensions, and shape) or by using micro/nanostructures on the target surface. At ELI-NP, considerable efforts are dedicated to extend the target capabilities beyond simple planar target design and develop complex targets with tailored properties suitable for high-power laser-plasma interaction experiments, as well as for studies with gamma and positrons beams. The paper pr...

Defect Structure Determination of GaN Films in GaN/AlN/Si Heterostructures by HR-TEM, XRD, and Slow Positrons Experiments

Nanomaterials

The present article evaluates, in qualitative and quantitative manners, the characteristics (i.e.... more The present article evaluates, in qualitative and quantitative manners, the characteristics (i.e., thickness of layers, crystal structures, growth orientation, elemental diffusion depths, edge, and screw dislocation densities), within two GaN/AlN/Si heterostructures, that alter their efficiencies as positron moderators. The structure of the GaN film, AlN buffer layer, substrate, and their growth relationships were determined through high-resolution transmission electron microscopy (HR-TEM). Data resulting from high-resolution X-ray diffraction (HR-XRD) was mathematically modeled to extract dislocation densities and correlation lengths in the GaN film. Positron depth profiling was evaluated through an experimental Doppler broadening spectroscopy (DBS) study, in order to quantify the effective positron diffusion length. The differences in values for both edge ( ρ d e ) and screw ( ρ d s ) dislocation densities, and correlation lengths (Le, Ls) found in the 690 nm GaN film, were associ...

Study of Edge and Screw Dislocation Density in GaN/Al2O3 Heterostructure

Materials

This study assesses the characteristics (edge and screw dislocation density) of a commercially av... more This study assesses the characteristics (edge and screw dislocation density) of a commercially available GaN/AlN/Al2O3 wafer. The heterostructure was evaluated by means of high-resolution X-ray diffraction (HR-XRD), high-resolution transmission electron microscopy (HR-TEM), and Doppler-Broadening Spectroscopy (DBS). The results were mathematically modeled to extract defect densities and defect correlation lengths in the GaN film. The structure of the GaN film, AlN buffer, Al2O3 substrate and their growth relationships were determined through HR-TEM. DBS studies were used to determine the effective positron diffusion length of the GaN film. Within the epitaxial layers, defined by a [GaN P 63 m c (0 0 0 2) || P 63 m c AlN (0 0 0 2) || (0 0 0 2) R 3 ¯ c Al2O3] relationship, regarding the GaN film, a strong correlation between defect densities, defect correlation lengths, and positron diffusion length was assessed. The defect densities ρ d e = 6.13 × 1010 cm−2, ρ d s = 1.36 × 1010 cm−2,...

Project for a Source of Polarized Slow Positrons at ELI-NP

Defect and Diffusion Forum

We describe the status of a project for obtaining an intense beam of polarized slow positrons at ... more We describe the status of a project for obtaining an intense beam of polarized slow positrons at the Extreme Light Infrastructure - Nuclear Physics (ELI-NP) at Magurele (near Bucharest, Romania) [1]. Positrons will be created via pair production and moderated at a tungsten target using the pulsed brilliant gamma beam which will be produced by Compton back-scattering of circularly polarized laser photons on electrons from a warm linac beam [2]. Simulations of the interaction of circularly polarized γ‑rays of energies up to 3.5 MeV and an intensity of 2.4×1010γ/s with the target, moderation of created positrons and beam formation are discussed. The optimization of the target design showed that the primary slow positron beam can be obtained with intensity of 1‑2×106e+/s. The primary beam will be transversally polarized with a degree of polarization of ~30%. We discuss the necessity of changing the e+ beam polarization from transversal to longitudinal by an electrostatic 90˚ bender whic...

Superconducting single-phase Sr1-xLaxCuO2 thin films with improved crystallinity grown by pulsed laser deposition

Applied Physics Letters, Aug 28, 2006

Heteroepitaxial growth of copper oxide superconductors by Pulsed Laser Deposition

Phys Rev E, 2003

In this thesis, a systematic study of thin film preparation of p- and n-type infinite layer (IL) ... more

Source of slow polarized positrons using the brilliant gamma beam at ELI-NP. Converter design and simulations

Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2015

Termination Control of (001) and (110) NdGaO3 Single-Crystal Substrates by Selective Chemical Etching

Crystals

Controlling the surface morphology and composition of the perovskite substrates is a critical asp... more Controlling the surface morphology and composition of the perovskite substrates is a critical aspect in tuning the final properties of the deposited films and of their interfaces. The paper reports on a chemical etching method developed for (110) and (001) NdGaO3 single crystal substrates in order to obtain a well-defined GaO2−x-terminated surface. The etching process is based on a HF + NH4OH solution and includes an annealing step performed in air or under O2 flow at temperatures of 800–1000 °C. In order to obtain the desired composition and surface morphology, the etching procedure was optimized for the vicinal step density at the surface and substrate crystal orientation. Growth nucleation studies of one-unit-cell MeO films (Me = Ti, Sr, Ba) on chemically etched and on only annealed substrates were performed in order to determine the composition of the substrate topmost layer. The results indicate that the chemically etched NdGaO3 substrate surface has a predominantly GaO2−x term...

Status of the project for a positron laboratory at ELI-NP

We report on the plans of a positron laboratory to be built at the Extreme Light Infrastructure -... more We report on the plans of a positron laboratory to be built at the Extreme Light Infrastructure - Nuclear Physics (ELI-NP) facility. The slow e+ source is based on e−–e+ pair production in a converter made of tungsten foils by a brilliant gamma beam (Eγ<3.5 MeV, Iγ = 2.4×1010 γ s-1) which will be generated by Compton back–scattering of photons from a high power laser on electrons from a LINAC. Numerical simulations of γ–ray interactions with the designed converter showed that, if the tungsten foils are used for moderation of the created fast e+, a slow e+ beam of an intensity of ~1×106 s-1 can be obtained. Circular polarization of the γ-beam is proposed to be one of the ELI-NP's upgrades. The slow e+ will be extracted perpendicularly to the γ–beam and will have ~ 30% transversal polarization degree. By using a transmission re-moderation stage and electrostatically turning the re–moderated beam by 90°, the transversal polarization will be changed to longitudinal. Longitudinal ...

An overview of the design of ELIPS—A new slow positron beam line

Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2019

In this work, the progress made by the positron group from ELI-NP in the design of a new slow pos... more In this work, the progress made by the positron group from ELI-NP in the design of a new slow positron beam line is summarized. The rays of a brilliant gamma beam will be used to produce a slow positron beam of high intensity. The main results of the simulations, done for the geometry optimization of the Converter/Moderator Assembly (CMA) for γ-to-slow e + conversion, are reported. By using the γ-beam with energy of up to 3.5 MeV and intensity of 2.4 × 10 10 γ s −1 , the slow e + beam can be obtained with a maximum intensity of ∼ 2 × 10 6 s −1 with tungsten or platinum CMA. Solid neon moderation is applicable, and a successful implementation will result in a slow e + beam of intensity > 10 7 s −1. The CMA can be operated with a central hole (= 2.5 mm) in the foils in parasitic mode, i.e., simultaneously with other γ-beam experiments, at the cost of reduced intensity by 18%. Furthermore, details on the slow positron transport and the spectroscopic systems, which are under construction, are provided.

Nanoscale Control of Structure and Composition for Nanocrystalline Fe Thin Films Grown by Oblique Angle RF Sputtering

Materials

The use of Fe films as multi-element targets in space radiation experiments with high-intensity u... more The use of Fe films as multi-element targets in space radiation experiments with high-intensity ultrashort laser pulses requires a surface structure that can enhance the laser energy absorption on target, as well as a low concentration and uniform distribution of light element contaminants within the films. In this paper, (110) preferred orientation nanocrystalline Fe thin films with controlled morphology and composition were grown on (100)-oriented Si substrates by oblique angle RF magnetron sputtering, at room temperature. The evolution of films key-parameters, crucial for space-like radiation experiments with organic material, such as nanostructure, morphology, topography, and elemental composition with varying RF source power, deposition pressure, and target to substrate distance is thoroughly discussed. A selection of complementary techniques was used in order to better understand this interdependence, namely X-ray Diffraction, Atomic Force Microscopy, Scanning and Transmission...

MECANISM DE CREŞTERE ŞI PROPRIETĂŢI ALE FILMELOR SUBŢIRI DE YBa 2 Cu 3 O 7-δ DEPUSE PRIN ABLAŢIE LASER PE (001) SrTiO 3 GROWTH MECHANISM AND PROPERTIES OF YBa 2 Cu 3 O 7-δ THIN FILMS DEPOSITED BY LASER ABLATION ON (001) SrTiO 3

Filme subţiri monocristaline de YBa 2 Cu 3 O 7-δ cu proprietăţi supraconductoare au fost crescute... more Filme subţiri monocristaline de YBa 2 Cu 3 O 7-δ cu proprietăţi supraconductoare au fost crescute epitaxial prin ablaţie laser pe substraturi de SrTiO 3 (001). Morfologia suprafeţei filmelor a fost studiată prin difracţie prin reflexie de electroni cu energie înaltă (DREEI) la presiuni mari, microscopie de forţă atomică (MFA) şi microscopie electronică de baleiaj (ME), în timp ce proprietăţile structurale au fost analizate prin difracţie de raze X (DRX). Pentru parametrii optimizaţi de creştere, filmele rezultate au o suprafaţă cu o rugozitate de ~ 10-15 nm, conform rezultatelor obţinute de la MFA şi ME. Creşterea filmelor, de tip Stransky-Krastanov, este influenţată de proprietăţile interfeţei substrat-film, datorită prezenţei stresului epitaxial. Din punct de vedere al proprietăţilor electrice, filmele sunt supraconductoare, cu valori ale temperaturii critice de T c =85-91 K. Sunt prezentate rezultate obţinute asupra fabricării şi proprietăţilor electrice ale joncţiunilor Josephso...

Fabrication of micron-sized periodic gratings in free-standing metallic foils for laser-plasma experiments

High Power Laser Science and Engineering

Engineered targets are expected to play a key role in the future high-power laser experiments cal... more Engineered targets are expected to play a key role in the future high-power laser experiments calling for a joined, extensive knowledge in materials properties, engineering techniques, and plasma physics. In this work, we propose a novel patterning procedure of self-supported 10 µm thick Au and Cu foils for obtaining periodic micron-size gratings as targets for high-power laser applications. Accessible techniques were considered, by using cold rolling, electron beam lithography, and Arion milling process. The developed patterning procedure allows efficient control of the gratings and foils surface on large area. Targets consisting of patterned regions of 450x450 µm 2 , with 2 µm periodic gratings, were prepared on 25×25 mm 2 Au and Cu

New Low-Stress PECVD Poly-SiGe Layers

OPTIMIZAREA MORFOLOGIEI SUPRAFEŢEI SUBSTRATURILOR DE NdGaO3 (110) PRIN TRATAMENTE TERMICE ŞI CHIMICE IMPROVED SURFACE MORPHOLOGY OF (110) NdGaO3 SUBSTRATES BY THERMAL AND CHEMICAL TREATMENTS

The results of the influence of thermal and chemical treatments on the surface morphology of (110... more The results of the influence of thermal and chemical treatments on the surface morphology of (110) NdGaO3 substrates are described in this paper. The treated surfaces were analysed by Atomic Force Microscopy (AFM), in air, and by in-situ high pressure Reflection High Energy Electron Diffraction (RHEED). The thermal treatment of substrates resulted in a NdO1+x single terminated surface, while a surface with GaO2-x terminating layer and atomically flat terraces without etch pits could be obtained by chemical etching in a HF + NH4F + H2O solution, followed by an annealing step at high temperatures (900-1000C) in air or in oxygen flow, for surface recrystalization. In acest articol sunt prezentate rezultatele influenţei pe care tratamentele termice si chimice le au asupra morfologiei suprafeţei substraturilor de NdGaO3 cu orientare (110). Suprafeţele astfel tratate au fost analizate prin intermediul microscopului de forţă atomică (MFA), in aer, si difracţiei prin reflexie de electroni c...

Termination control of NdGaO3 crystal surfaces by selective chemical etching

arXiv: Materials Science, 2012

A chemical etching method was developed for (110) and (001) NdGaO3 single crystal substrates in o... more A chemical etching method was developed for (110) and (001) NdGaO3 single crystal substrates in order to obtain an atomically flat GaO2-x - terminated surface. Depending on the surface step density the substrates were etched in pH-controlled NH4F- or NH4Cl-based solutions, followed by an annealing step at temperatures of 800-1000oC, in air or in oxygen flow, in order to recrystallize the surface. Atomic Force Microscopy (AFM) and high-pressure Reflection High Energy Electron Diffraction (RHEED) were used to analyse the surface morphology of the samples after every treatment. Studies on the chemistry and characteristics of the terminating layer showed that the chemically etched NdGaO3 substrate surface has a GaO2-x termination and that the (110) and (001) NdGaO3 surfaces are characterized by a different free surface energy, which is lower for latter.

Phase-sensitive evidence for dx 2 − y 2-pairing symmetry in the parent-structure high-Tc cuprate superconductor

Jochen Tomaschko,1 Sebastian Scharinger,1 Victor Leca,1,2 Joachim Nagel,1 Matthias Kemmler,1 Tere... more Jochen Tomaschko,1 Sebastian Scharinger,1 Victor Leca,1,2 Joachim Nagel,1 Matthias Kemmler,1 Teresa Selistrovski,1 Dieter Koelle,1 and Reinhold Kleiner1,* 1Physikalisches Institut–Experimentalphysik II and Center for Collective Quantum Phenomena in LISA+, Universität Tübingen, Auf der Morgenstelle 14, 72076 Tübingen, Germany 2National Institute for Research and Development in Microtechnologies, Molecular Nanotechnology Laboratory, Erou Iancu Nicolae Street 126A, RO-077190, Bucharest, Romania (Received 23 March 2012; published 7 September 2012)

MECANISM DE CREŞTERE ŞI PROPRIETĂŢI ALE FILMELOR SUBTIRI DE YBa2Cu3O7-δ DEPUSE PRIN ABLAŢIE LASER PE (001 ) srTiO3

High quality, single phase c-axis oriented YBa 2 Cu 3 O 7-δ thin films with superconducting prope... more High quality, single phase c-axis oriented YBa 2 Cu 3 O 7-δ thin films with superconducting properties were grown by laser ablation on (001) SrTi0 3 substrates. The surface morphology of the films has been investigated by means of high-pressure reflection high energy electron diffraction (RHEED), atomic force microscopy (AFM), and scanning electron microscopy (SEM), while the structural properties were studied by X-ray diffraction (XRD). Deposition under optimum conditions produces films with relatively smooth surface, with a roughness of about 10-15 nm, as confirmed by AFM and SEM data. The growth follows a Stransky-Krastanov mechanism governed by the substrate-film interface properties due to presence of epitaxial strain. The films show good superconducting properties with T c values of 85-91 K. Results on the fabrication and electrical transport properties of ramp-type Josephson junctions with YBa 2 Cu 3 O 7-δ electrodes and PrBa 2 Cu 3 O 7-δ barrier are presented.

Publisher’s Note: Phase-Sensitive Order Parameter Symmetry Test Experiments Utilizing Nd2-xCexCuO4-y/Nb Zigzag Junctions [Phys. Rev. Lett. 94, 167001 (2005)]

Physical Review Letters, 2005

Studies of Defect Structure in Epitaxial AlN/GaN Films Grown on (111) 3C-SiC

Nanomaterials

Several aspects such as the growth relation between the layers of the GaN/AlN/SiC heterostructure... more Several aspects such as the growth relation between the layers of the GaN/AlN/SiC heterostructure, the consistency of the interfaces, and elemental diffusion are achieved by High Resolution Transmission Electron Microscopy (HR-TEM). In addition, the dislocation densities together with the defect correlation lengths are investigated via High-Resolution X-ray Diffraction (HR-XRD) and the characteristic positron diffusion length is achieved by Doppler Broadening Spectroscopy (DBS). Moreover, a comparative analysis with our previous work (i.e., GaN/AlN/Si and GaN/AlN/Al2O3) has been carried out. Within the epitaxial GaN layer defined by the relationship F4¯3m (111) 3C-SiC || P63mc (0002) AlN || P63mc (0002) GaN, the total dislocation density has been assessed as being 1.47 × 1010 cm−2. Compared with previously investigated heterostructures (on Si and Al2O3 substrates), the obtained dislocation correlation lengths (Le = 171 nm and Ls =288 nm) and the mean distance between two dislocation...

Structuring Free-Standing Foils for Laser-Driven Particle Acceleration Experiments

Frontiers in Physics

The recent development of petawatt-class laser systems sets a focus on the development of ultra-t... more The recent development of petawatt-class laser systems sets a focus on the development of ultra-thin free-standing targets to access enhanced particle acceleration schemes vital for future applications, such as, medical and laser-driven nuclear physics. Specific strategies are required to improve the laser-to-particle energy conversion efficiency and increase the maximum particle energy. One of the promising approaches is based on the target design optimization; either by tuning key parameters which will strongly affect the laser-matter interaction process (e.g., material, composition, density, thickness, lateral dimensions, and shape) or by using micro/nanostructures on the target surface. At ELI-NP, considerable efforts are dedicated to extend the target capabilities beyond simple planar target design and develop complex targets with tailored properties suitable for high-power laser-plasma interaction experiments, as well as for studies with gamma and positrons beams. The paper pr...

Defect Structure Determination of GaN Films in GaN/AlN/Si Heterostructures by HR-TEM, XRD, and Slow Positrons Experiments

Nanomaterials

The present article evaluates, in qualitative and quantitative manners, the characteristics (i.e.... more The present article evaluates, in qualitative and quantitative manners, the characteristics (i.e., thickness of layers, crystal structures, growth orientation, elemental diffusion depths, edge, and screw dislocation densities), within two GaN/AlN/Si heterostructures, that alter their efficiencies as positron moderators. The structure of the GaN film, AlN buffer layer, substrate, and their growth relationships were determined through high-resolution transmission electron microscopy (HR-TEM). Data resulting from high-resolution X-ray diffraction (HR-XRD) was mathematically modeled to extract dislocation densities and correlation lengths in the GaN film. Positron depth profiling was evaluated through an experimental Doppler broadening spectroscopy (DBS) study, in order to quantify the effective positron diffusion length. The differences in values for both edge ( ρ d e ) and screw ( ρ d s ) dislocation densities, and correlation lengths (Le, Ls) found in the 690 nm GaN film, were associ...

Study of Edge and Screw Dislocation Density in GaN/Al2O3 Heterostructure

Materials

This study assesses the characteristics (edge and screw dislocation density) of a commercially av... more This study assesses the characteristics (edge and screw dislocation density) of a commercially available GaN/AlN/Al2O3 wafer. The heterostructure was evaluated by means of high-resolution X-ray diffraction (HR-XRD), high-resolution transmission electron microscopy (HR-TEM), and Doppler-Broadening Spectroscopy (DBS). The results were mathematically modeled to extract defect densities and defect correlation lengths in the GaN film. The structure of the GaN film, AlN buffer, Al2O3 substrate and their growth relationships were determined through HR-TEM. DBS studies were used to determine the effective positron diffusion length of the GaN film. Within the epitaxial layers, defined by a [GaN P 63 m c (0 0 0 2) || P 63 m c AlN (0 0 0 2) || (0 0 0 2) R 3 ¯ c Al2O3] relationship, regarding the GaN film, a strong correlation between defect densities, defect correlation lengths, and positron diffusion length was assessed. The defect densities ρ d e = 6.13 × 1010 cm−2, ρ d s = 1.36 × 1010 cm−2,...

Project for a Source of Polarized Slow Positrons at ELI-NP

Defect and Diffusion Forum

We describe the status of a project for obtaining an intense beam of polarized slow positrons at ... more We describe the status of a project for obtaining an intense beam of polarized slow positrons at the Extreme Light Infrastructure - Nuclear Physics (ELI-NP) at Magurele (near Bucharest, Romania) [1]. Positrons will be created via pair production and moderated at a tungsten target using the pulsed brilliant gamma beam which will be produced by Compton back-scattering of circularly polarized laser photons on electrons from a warm linac beam [2]. Simulations of the interaction of circularly polarized γ‑rays of energies up to 3.5 MeV and an intensity of 2.4×1010γ/s with the target, moderation of created positrons and beam formation are discussed. The optimization of the target design showed that the primary slow positron beam can be obtained with intensity of 1‑2×106e+/s. The primary beam will be transversally polarized with a degree of polarization of ~30%. We discuss the necessity of changing the e+ beam polarization from transversal to longitudinal by an electrostatic 90˚ bender whic...

Superconducting single-phase Sr1-xLaxCuO2 thin films with improved crystallinity grown by pulsed laser deposition

Applied Physics Letters, Aug 28, 2006

Heteroepitaxial growth of copper oxide superconductors by Pulsed Laser Deposition

Phys Rev E, 2003

In this thesis, a systematic study of thin film preparation of p- and n-type infinite layer (IL) ... more

Source of slow polarized positrons using the brilliant gamma beam at ELI-NP. Converter design and simulations

Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2015