Upcoming seminar: Dr. Victor Vega-Mayoral, 28.3.2019, 13:00, Seminar room F7

Fabrication of thin film transistors and energy storage devices from liquid phase exfoliated nanosheets

Dr. Victor Vega-Mayoral, CRANN & AMBER research centers, Trinity College Dublin, School of physics, Trinity College Dublin

Abstract:

Transition metal dichalcogenides has been one of the most promising family of materials for a lustrum. Strong light matter interaction and high mobilities when exfoliated down to the monolayer. Liquid phase exfoliation produces dispersions of few-layer TMDCs. Due to the liquid nature of the final sample it is possible to print or spray these dispersions to produce thin films of 2d flakes. We will talk about the fabrication of transistors and energy storage devices.

TiS2 is a promising material for energy storage devices. Its application has been reduced due to a fast degradation in open atmosphere. Our latest results proofs how degradation can be avoided by carefully choosing the solvent. We have fabricated stable batteries with high cyclability and capacities close to the theoretical value.

Mixed networks of conducting and non-conducting nanoparticles show promise in a range of applications where fast charge transport is important. While the dependence of network conductivity on the conductive mass fraction (Mf) is well understood, little is known about the Mf-dependence of mobility and carrier density. I  will show here a detailed characterization of thin film transistors as a function of the WS2-graphene Mf. Here, we use electrolytic gating to investigate the transport properties of spray-coated composite networks of graphene and WS2 nanosheets.

Poster [PDF]

Jan Ravnik, 14.2.2019 at 11:00, F7 Seminar room

Direct detection of ultrafast switching in quantum cryo-memory devices

Jan Ravnik, Department of Complex Matter, Jožef Stefan Institute, Ljubljana, Slovenia

Abstract:

Giant leaps in information technology place demands for devices that can no longer be met by current semiconductor technologies, while the power consumption of the IT sector is an urgent global environmental concern. Cryo-cooled supercomputers and quantum computation are prospective directions, which both face the same barrier: No high-performance storage memory is available for low-temperature operation.
In this short seminar, I will present my proposed post-doc project using the SwissFEL facility at PSI. The ultimate goal of the project is to develop fundamentally new storage-class memory devices, which overcome major challenges in speed and switching energy. We will follow a concept based on electrical and optical switching between the metallic/insulating charge-ordered topologically protected quantum states of 1T-TaS2. While the optical switching is well understood and achieved in record-breaking 0.5 ps, the less explored electrical switching has yet greater potential for an ultrafast memory device, due to its ease of implementation in standard electronics. I want to clarify the processes which drive the electronic transition, and determine the rate of the domain growth and its limits on switching speed. To achieve this, I will conduct a novel kind of position-dependent switching experiment using ultrafast electronic pump pulses and the SwissFEL X-ray free-electron laser as the structural probe.

Poster [PDF]

Aleksander Yu. Kuntsevich, 22.11.2018 at 11:00, F7 Seminar room

SrₓBi₂Se₃-nematicity, superconductivity, crystals and thin films

Aleksander Yu. Kuntsevich, P. N. Lebedev Physical Institute, Moscow, Russia

Abstract:

3D topological insulator Bi₂Se₃ attracts much attention as a platform for future low consumption spintronics and quantum computations. Recently, nematic (and possibly topological) superconductivity was discovered in AₓBi₂Se₃, where A = Cu, Sr, Nb. In my talk I will discuss phenomenology of the nematicity, observed in SrₓBi₂Se₃ single crystals in both superconducting and normal states. I will also review our efforts on molecular beam epitaxy growth of both parent compound Bi₂Se₃ and Sr-doped Bi₂Se₃ thin films. The latter appear to be non-superconducting because Sr atoms in the films get different positions than in the crystals. Our results call for novel growth approaches for design of superconducting SrₓBi₂Se₃ thin films.

Poster [PDF]

Andrej Kranjec, 13.11.2018 at 14:30, F7 Seminar room

Vertex directed analysis of the hidden state relaxation in 1T-TaS2

Andrej KranjecDepartment for Complex Matter, JSI

Abstract:

I will be presenting a brief summary of our latest results obtained by applying a global interdomain shifts recognition algorithm on a high quality STM sequence of the relaxation of the H-state in 1T-TaS2. We show how the results of the algorithm provide a basis for the study of dislocations by determining trivial and non-trivial vertices where domain walls meet by constructing Burgers circuits and vectors around selected vertices. The analysis shows that some of the relaxation dynamics could be consistent within the scope of dislocation theory.

Poster [PDF]

Prof. Dr. Michele Fabrizio, 4.10.2018 at 13:00, Physics seminar room

Transient cooling of quasiparticles in K3C60  by mid-infrared laser pulses 

 Prof. Dr. Michele Fabrizio, International School for Advanced Studies SISSA, Trieste, Italy

Abstract:

We propose an explanation of the transient superconducting-like optical behavior observed at temperatures as high as ten times Tc in K3C60 irradiated by mid-infrared laser pulses.  In our theory the phenomenon is due to the laser pulse effectively cooling down low energy quasiparticles much below the external temperature. The mechanism is quite general and relies on the existence of  localised excitations, in K3C60 these are spin-triplet excitons,  that act as entropy sink when the laser is on, while, when the laser is off, they release back the stolen entropy very gradually.

Poster [PDF]

Mimoza Naseska, 19.9.2018 at 13:00, F7 Seminar room

Spin density wave dynamics in iron based pnictides 

Mimoza Naseska, Department of Complex Matter, JSI

Abstract:

We present ultrafast optical time-resolved spectroscopy measurements of the ultrafast system trajectory through the spin density wave (SDW) phase transition in SrFe2As2 and EuFe2As2. Using the standard pump-probe technique we determined the threshold fluence for the nonthermal destruction of the SDW order (Fth ≈ 0.2 mJ/cm2) at two different pump-photon energies (1.55 eV and 3.1 eV). The SDW destruction timescale obtained from the multipulse measurements is ~ 150 fs. We found that the destruction pulse penetration depth in the mJ/cm2  excitation-fluences range significantly exceeds the equilibrium optical penetration depth suggesting the absorption saturation. The recovery dynamics of the SDW order was simulated using an extended three temperature model (3TM). The analysis suggests that the optical-phonons energy-relaxation surprisingly plays an important role in the recovery of almost exclusively electronically driven SDW order.

Poster [PDF]

Maria D’Antuono, 18.7.2018 at 13:00 in F7 Seminar room

Magnetotransport properties of 2DEG formed in LAO/ETO/STO heterostructures studied using the electric field effect

Maria D’Antuono, University of Naples Federico II, Naples, Italy

Abstract:

In 2004 Ohtomo and Hwang reported the formation of a high mobility 2-dimensional electron gas (2DEG) at the interface between two wide bandgap insulators oxides, namely LaAlO3 (LAO) and SrTiO3 (STO). In this work, we show that the 2DEG created at the LAO/STO interface becomes both electric-field-tunable spin polarized and superconducting by introducing a few atomic layers of EuTiO3 (ETO) which is an antiferromagnetic (AF) insulator iso-structural to STO . Among the most interesting characteristics of this 2DEG are the remarkably large Rashba-spin-orbit interactions and unconventional superconductivity and magnetism, possibly related to the presence of strong correlations in quantum-confined 3d-bands. The occurrence of magnetic interactions, superconductivity and spin-orbit interactions in the same 2DEG system makes the LAO/ETO/STO an intriguing platform for the emergence of novel quantum phases in low-dimensional devices. The main goal of this work was to investigate the electrical transport of the LAO/ETO/STO interface and to shed more light on the complex nature of this system and its phase diagram. In particular, we focused on the interplay between Rashba-spin-orbit interactions and ferromagnetism. In fact, the LAO/ETO/STO 2DEG is one of the few systems where such interplay can be studied and it is therefore of great interest for future spintronic applications.

Poster [PDF]

Vitomir Sever, 6.7.2018 at 13:00 in F7 Seminar room

Study of the efficiency of the solar updraft tower as a function of its height
Vitomir Sever, Material Science and Engineering Department at Institute National des Sciences  Appliquées (INSA), Lyon

Abstract:

With the constant increase in pollution, the need to find new systems that provide clean energy becomes more and more important. One of these solutions could be the solar updraft tower. This tower allows us to create electrical energy from the solar energy. Air is heated by solar radiation under a circular transparent roof open at the periphery. In the middle of this collector is a vertical tower with air inlets at its base. As hot air is lighter than cold air it rises up the tower. Suction from the tower then draws in more hot air from the collector, and cold air comes in from the outer perimeter. Therefore, turbines at the base of the tower generates electricity.

The goal of this project was to find out how the height of the tower influences its efficiency. Using a very simple theoretical model and making measurements on a real model, it can be seen that the height varies with the air velocity in power of 1/2, and with the recoverable power in power of 5/2. This model remains correct for heights lower than 1m which is very limiting. Finally, many improvements remain to be made, such as the theoretical model, geometry,  materials and precision.

Poster [PDF]

Dr. Satoshi Tsuchiya, 22.6.2018 at 13:00 in F7 Seminar room

Spectrally resolved and three pulse pump-probe spectroscopy in strongly correlated organic conductors
Dr. Satoshi Tsuchiya, Complex Matter Department, Jozef Stefan Institute, Slovenia; Department of Applied Physics, Hokkaido University, Japan

Abstract:

The series of k-(BEDT-TTF)2X (X: anion molecules) has attracted much interest in fundamental physics related with strong electron correlation and application of devices because of a rich variety of electronic phases under equilibrium such as the Mott insulator and superconductor, as well as nonequilibrium related with photoinduced phase transition. In my long-term stay in JSI, spectrally-resolved pump probe spectroscopy was carried out to investigate superconducting phase in this system. On the other hand, three-pulse pump probe spectroscopy was applied for photoinduced phase separation. In the seminar, I will show the obtained results briefly and discuss in detail.

Poster [PDF]

Koichi Nakagawa, 1.6.2018 at 13:30 in the Physics Seminar room

Polarized pump-probe measurements in organic superconductors near Mott boundary
Koichi Nakagawa, Department of Applied Physics, Hokkaido University, Sapporo, Japan

Abstract:

A series of κ-type organic superconductors offers an ideal electronic system to investigate crucial electronic properties such as pseudogap and fluctuating superconductivity, for understanding mechanism of unconventional superconductivity since chemical substitution in insulating/conducting molecules can lead to change of effective electron correlation, contributed by its flexible nature. Recently, behavior of the superconducting electronic state near the Mott boundary has attracted much attention since the Nernst effect measurements, which can detect finite superconducting quantum vortices even above Tc, reported the huge onset temperature divergently up to ~5 Tc with reaching the boundary.   However, the consensus about the fluctuating state has yet to be reached due to the inconsistencies from the other experimental methods. Thus, spectroscopic methods, which can characterize the superconducting gap formation, have been required. In the presentation, I would like to talk about the results of the polarized pump-probe spectroscopic measurements, which are sensitive to the gap formation near EF, for κ-type organics, in which the electron correlation is finely tunable around the Mott boundary by a ratio of deuterated conducting molecules.

Poster [PDF]

Asst. Prof. Dr. Hiroyoshi Nobukane, 1.6.2018 at 13:00 in the Physics Seminar room

High-Tc superconductivity in ruthenates
Asst. Prof. Dr. Hiroyoshi Nobukane, Department of Physics, Hokkaido University, Sapporo, Japan

Abstract:

Layered perovskite materials can control quantum states from superconductivity to a Mott insulator by tuning physical and chemical pressure. By reducing the thickness of a layered crystal to nanometer range, a nanofilm due to a negative pressure effect leads to novel quantum states that have not been found yet in bulk crystals. We will discuss the emergence of high-Tc superconductivity near 100 K in Ca2RuO4 nanofilms. A thin film of Ca2RuO4 exhibits supercurrent and typical Berezinskii-Kosterlitz-Thouless transition behavior. We also found that the tuned  film thickness and the induced bias current cause a superconductor-insulator transition. Our results demonstrate the presence of two-dimensional superconductivity from a high temperature. The fabrication of nanofilms made of layered material enables us to discuss rich superconducting phenomena in ruthenates.

Poster [PDF]

Jaka Vodeb, 25.5.2018 at 13:00 in F7 Seminar room

Charged lattice gas of polarons in 1T-TaS2
Jaka Vodeb, Department for Complex Matter, Jozef Stefan Institute, Ljubljana, Slovenia

Abstract:

Motivation for this work comes from the discovery of a new metastable amorphous (A) state in 1T-TaS2 (TAS)1. The A state represents a disordered polaron pattern, which exhibits metallic behavior. We employed a classical charged lattice gas of polarons with screened Coulomb interaction and a fixed polaron concentration as a model for the A state. The idea has been used successfully in the work of Brazovskii2 who first proposed the model and used it to model the commensurate state. Karpov et al.3 also used it to model the hidden state, observed in TAS4. Our work has shown that the A state can be successfully modeled using classical charged polarons. Further investigation of the model has shown that there exists an infinite number of crystalline phases within the model as a function of polaron concentration. There also exist other amorphous phases which lie in between the crystalline phases. They exhibit glassy behavior and in contrast to the current paradigm of glass formation5 appear to be the ground state of the system.

Poster [PDF]

Michele Diego, 17.5.2018 at 11:00 in F7 Seminar room

Out of equilibrium electronic properties of ZrTe5
Michele Diego, Department of Complex Matter, JSI

Abstract:

Transition metal pentatelluride, ZrTe5, displays a set of unique and exotic transport properties, which make this material an ideal candidate for magnetic and thermoelectric devices. In particular, the best known anomaly is its resistivity trend, which shows a metallic behavior or a semiconductor behavior,  respectively, for temperatures below or above a critical temperature. Several theoretical models have been proposed to interpret this anomaly, based on charge density wave or polaron formation, but with no direct experimental verification. In this work we report on the temperature dependence of the ZrTe5 valence band, studied at equilibrium and out of equilibrium, by means of time and angle resolved photoemission spectroscopy. Our results unveil the dependence of the band structure across the Fermi level on the sample temperature. By performing temperature dependent experiments, we are able to observe an energy shift of the ZrTe5 valence band. The same effect is observed both by varying the equilibrium sample temperature and by optically exciting the system out-of equilibrium.
Finally, by following the out-of-equilibrium time evolution of the valance band, we report the relaxation times of its position, width and intensity.

Poster [PDF]

Matjaž Ličen, 20.4.2018 at 13:00 in F7 Seminar room

Photoregulating the self-assembly of lipophilic guanosine derivaters at the air-water interface
Odsek za kompleksne snovi, IJS

Abstract:

Basic blocks of DNA are very interesting constituents for designing supramolecular single-layer and multilayer surface  architectures. Our recent studies of self-assembly of nucleoside derivatives in monolayer films at the air-water interface (Langmuir films) reveal that guanosine derivatives exhibit very different behaviour from analogous derivatives containing other nucleobases [1]. We also demonstrated that the number of lipophilic chains attached to the sugar hydroxyl groups and the type/concentration of ions present in the water subphase strongly affected molecular organization of guanosine derivatives in Langmuir monolayers as well as in Langmuir-Blodgett (LB) films deposited on various solid substrates [2,3,4]. Modifications of these parameters hence provide a possibility of tuning intermolecular organization in thin film configurations.  An appealing strategy for control and manipulation of intermolecular organization is to use optical irradiation. To test the applicability of this method in case of guanosine derivatives we investigated Langmuir films of azo-functionalised guanosine molecules in which the isomerization can be switched from trans to cis and vice versa by irradiation with UV and visible light. Photoinduced modifications within Langmuir films were studied by film balance experiments and by Brewster angle microscopy (BAM). We were able to reversibly induce changes in the surface pressure of the film by alternatingly irradiating with UV and blue light, indicating a light-induced change in the film structure. The measurements for several different films mixed from a photoactive guanosine derivative and other non-photoactive nucleosides were compared to a simplified model of the film. The results from the model are in good agreement with the measurements for all films, except for the film formed by mixing guanosine and cytidine molecules, which hints at specific bonding between these two molecules.

[1] L. Coga, T. Ilc, M. Devetak, S. Masiero, L. Gramigna, G. P. Spada, and I. Drevensek Olenik, Colloid. Surface B103, 45 – 51 (2013).
[2] M. Devetak, S. Masiero, S. Pieraccini, G. P. Spada, M. Copic, and I. Drevensek Olenik, Appl. Surf. Sci. 256,2038  2043 (2010)
[3] L. Coga, S. Masiero, and I. Drevensek Olenik, Colloid. Surface B 121, 114 – 121 (2014)
[4] L. Coga, S. Masiero, and I. Drevensek Olenik, Langmuir 31, 4837 – 4843 (2015)

Dr. Andrey Leontyev, 12.4.2018 at 13:00 in F7 Seminar room

Nonresonant multi-pulse selective spectroscopy of interaction-induced responses in liquids using optical Kerr effect

Kazan E.K. Zavoisky Physical-Technical Institute, Kazan, Russia

Abstract:

An ultrafast selective spectroscopic measurements of vibrational-rotational dynamics of molecules in liquids were implemented using optical Kerr effect (OKE) detection under multi-pulse nonresonant xcitation. Introducing additional excitation pulses and creating either constructive or destructive interference of corresponding wave packets allows one to isolate the responses of certain molecular motions (orientational,inter-orintramolecular)in OKE signal. As a result,the  parameters of molecular rotations and coherent vibrations could be evaluated with significantly greater precision compared to conventional single pulse excitation scenario.

Matej Prijatelj, 23.3.2018 at 13:00 in F7 Seminar room

Expansion of transition metal dichalcogenides

Hella Saturnus Slovenija d.o.o., HSS – Ljubljana

Abstract:

Layered two-dimensional materials have unique physical and chemical properties due to their highly anisotropic nature.
A few of the layered crystals can be further expanded to low density / high specific surface area materials by introducing molecules to the inter planar spaces. These can be rapidly transformed to gas which inflates the layers and transforms the compact crystals to low density foam like structures. The most notable examples of expanded layered crystals are expanded vermiculite and expanded graphite. Both have numerous uses both in science and industry.
Transition metal dichalcogenides (TMDs) form layered crystals of stoichiometry MX2 (M=transition metal, X=S, Se, or Te). The best known compound, MoS2, is an earth abundant mineral and has been used for decades as a solid lubricant and as a catalyst for petroleum desulphurization.
In this seminar I will present our results in preparation and expansion of air stable expandable MoS2 and expanded TiS2 and TaS2. The quality of the expanded crystals has been shown by various analytical techniques not to differ from the parent compounds and our findings show that expandable MoS2 can be considered an analogue to expandable graphite.

Prof. dr. Roman Sobolewski, 16.3.2018 at 13:00 in the Physics Seminar room

Nanostructured superconducting single-photon detectors as photon energy, number, and polarization resolving devices

Departments of Electrical and Computer Engineering and Physics and Astronomy, and the Materials Science Program and Laboratory for Laser Energetics, University of Rochester, Rochester, NY

Abstract:

We present an overview of the physics of operation of superconducting single-photon detectors (SSPDs) and their implementation as the photon-energy, photon-number, and polarization resolving devices. The detection mechanism of SSPDs is based on photon-induced hotspot formation and, subsequent, generation of a voltage transient across a nanostructured superconducting NbN meander (~4-nm-thick and ~100-nm-wide stripe). The NbN SSPD operates in the 4.2-2 K temperature range. The best devices exhibit quantum efficiency of up to near 100% (when encapsulated in a cavity) in the near-infrared (1550 nm) wavelength range, dark count rates <1 Hz, and the noise-equivalent power (NEP) of ~510-21 W/Hz.1/2 For our photon-energy resolution studies, we have adopted a statistical method based on a well-documented fact that quantum efficiency (QE) of SSPDs very strongly (quasi-exponentially) depends on the photon wavelength and the normalized current bias. Thus, by measuring the SSPD QE at different bias levels, we were able to resolve the wavelength of the incident photons with a 50-nm resolution. In another approach, we have implemented a low-noise, cryogenic high-electron-mobility transistor (HEMT) as a very high impedance element to separate the 50-Ω output transmission line from the SSPD. This arrangement allowed us to achieve some amplitude resolution of the recorded output transients and, subsequently, photons with different energies could be distinguished by comparing the output transient amplitude distributions. Next, by designing SSPDs with different physical geometries, we could unambiguously demonstrate their sensitivity to photon polarization. At the end of our presentation, we will present new directions of the SSPD research, focusing on superconductor/ferromagnetic nanostripes.

Yelyzaveta Chernolevska, 14.3.2018 at 13:00, Seminar room F7

Cluster structure of monohydric alcohol

Yelyzaveta Chernolevska, Faculty of Physics of Taras Scevchenko, National University of Kyiv, Ukraine

dr. Satoshi Tsuchiya, 22.09. 2017 at 13:00, Seminar room F7

Photoinduced carrier dynamics in organic superconducting system: Previous results and recent progress

Dr. Satoshi Tsuchiya
Department of Applied Physics, Hokkaido University, Hokkaido, Japan

Tinkara Troha, 14.09. 2017 at 13:00, Seminar room F7

UV second harmonic generation in structured AIN optical wavequides

Tinkara Troha
Fakulteta za matematiko in fiziko in Institut »Jožef Stefan«, Ljubljana

Abstract:

We investigated the potential of aluminium nitride (AlN) for generation of UV light through a nonlinear optical process of second harmonic generation (SHG). AlN is an excellent candidate since it is transparent to 200 nm and has a large nonlinear susceptibility. High quality thin AlN films were grown on sapphire substrate forming optical waveguies. In order to make SHG efficient, phase matching between the pump and SH waves must be established. The most efficient method is quasi phase matching, where material’s crystalographic c-axis is periodically inverted. Another method is modal dispersion phase matching where the appropriate combination of interacting waveguide modes is selected. We observed several phase matched peaks in UV spectral region in single polar waveguides that could be well explained by the theoretical model for modal dispersion phase matching. The first demonstration of SHG into the visible and near UV spectral region was achieved using structured AlN waveguides with 10 µm periodicity. The theoretical model including higher order quasi phase matching interactions was used for the interpretation of the resulting SH peaks.

Mimoza Naseska, 11.09. 2017 at 13:00, Seminar room F7

Ultrafast spin density wave dynamics at intense optical pulse excitation

Mimoza Naseska
Fakulteta za matematiko in fiziko in Institut »Jožef Stefan«, Ljubljana

Abstract:

Ultrafast time-resolved optical spectroscopy has become an important tool for characterizing the various energy transfer processes occurring after absorption of intense laser pulses. The relaxation timescales observed after photoexcitation can in some case reveal the microscopic origins of a specific macroscopic material property. In this work I studied the properties of photoinduced excitations of the spin density wave state in EuFe_2As_2 and SrFe_2As_2 by systematic measurements of the temperature and fluence dependence of the transient reflectivity. The results obtained from the standard pump-probe measurements reveal the relaxation dynamics of the photoexcited quasiparticles while the three pulse technique uncovered the temporal evolution of the SDW order of the system undergoing photoinduced phase transition.

Jaka Vodeb, 08.09.2017 at 13:00, Seminar room F7

Urejanje polaronov v dveh dimenzijah

Jaka Vodeb
Fakulteta za matematiko in fiziko in Institut »Jožef Stefan«, Ljubljana

Abstract:

V magistrski nalogi je obravnavano modeliranje nizkotemperaturnih faz kristala 1T-TaS$_2$ (TAS). V splošnem je za kristal TAS značilen bogat fazni diagram in mikroskopska struktura pojava vala gostote naboja (VGN). Do sedaj je bilo možno mikroskopsko teorijo povprečnega polja, ki seobičajno uporablja za opis VGN, uporabiti le za opis faznega prehoda, ki se zgodi pri višjih temperaturah. Glavna ideja magistrske naloge je uporabiti pojav kvazidelca, ki se imenuje polaron, za opis nizkotemperaturnih faz TAS. Predpostavljena interakcija med polaroni temelji na skrajno anizotropni kvazidvodimenzionalni naravi kristala, ki je tudi uporabljena za utemeljitev obravnave samo ene plasti Tantalovih atomov v TAS. Izkaže se, da je možno kvantno mehansko obravnavo kristala poenostaviti na obravnavo klasičnih interagirajočih polaronov na trikotni kristalni rešetki, kar je obravnavano z metodo Monte Carlo in Metropolisovim algoritmom. Primerjave nizkotemperaturnih konfiguracij polaronov z eksperimentalno opaženimi fazami TAS nakazujejo, da je možno fazo s skoraj komenzurabilnim VGN opisati s predlaganim modelom le z dodatkom zunanjega polja, ki vsiljuje eksperimentalno opaženo daljnosežno ureditev. Predlagana interakcija med polaroni v ozkem območju prostih parametrov sicer podpira eksperimentalno opaženo ureditev, vendar le ob prisotnosti zunanjega polja. Novo odkrite metastabilne faze TAS oziroma tako imenovanega skritega stanja, ki ni prisotno na termodinamičnem faznem diagramu TAS, ni možno opisati z vpeljavo tovrstnega polja. Razlog leži v odsotnosti daljnosežne urejenosti domen, ki je prisotna v primeru faze s skoraj komenzurabilnim VGN. Izkaže se, da tudi z modelom brez zunanjega polja, ki sicer podpira naključno urejenost domenske strukture skritega stanja, ni možno popolnoma opisati vseh njegovih značilnosti.

prof. dr. Xinzheng Zhang, 22.06.2017 at 13:00, Seminar room F7

Control of Light Field Based on Liquid Crystal/Polymer Composite Structures

prof. dr. Xinzheng Zhang
TEDA Institute of Applied Physics & School of Physics, Nankai University, P. R. China

Abstract:

Liquid Crystal/Polymer Composites (LCPCs) consisting of liquid crystal (LC) and polymer are a new class of functional materials. They possess excellent anisotropy and reconfigurability by external fields. In composites, the polymer plays a role of fixing and confining the liquid crystal molecules. The control of liquid crystal directors is the key to improving the performance of liquid crystal displays or functional liquid crystal devices. Recently, the compartmentalized or multiple states LC alignment has become the research highlight. Control of light field is a perpetual and pivotal challenge in optics and photonics. By adjusting the amplitude, polarization or phase of one incident light, a Gaussian beam can be converted into different specific optical beam. In order to realize the control of the LC molecules and light field, we develop a new alignment technology which can realize the accurate and arbitrary azimuthal angle control of liquid crystals. The specific optical beams can be generated via the predesigned liquid crystal/polymer composite structures. The main contents include realization of patterned LC alignment in arbitrary microstructures and generating the specific optical beams by q-plates based on Liquid Crystal/Polymer Composite structures.

Prof. dr. Rupert Schreiner, 15.06.2017 at 13:00, Seminar room F7

Semiconductor Field Emission Electron Source for Application in Sensors and X-ray Sources

prof. dr. Rupert Schreiner
Ostbayerische Technische Hochschule Regensburg, Fakultät Allgemeinwissenschaften und Mikrosystemtechnik, Germany

Abstract:

Semiconductor field emitter arrays are suitable candidates for applications which require a very stable field emission (FE) current and a high emission uniformity over the entire emission area. By using various semiconductor materials, doping profiles, geometries and coatings of the apex it is possible to vary the emission characteristics of these cathodes within a wide range. Each specific application requires its own optimized design for the cathode as well as for the other parts of the FE electron source. To meet as many of these requirements as possible while using only a limited number of different prefabricated components, we established a modular system concept for our FE electron source. It consists of a semiconductor cathode (with an optional integrated gate electrode) and a grid electrode (metallized silicon nitride membrane). For pre-characterization of the FE electron source, an UHV measurement system is used. An anode is mounted on a movable isolated cantilever and is placed at a defined distance over the extraction grid. Different cathode, gate, grid and anode potentials can be chosen while the corresponding currents are monitored. In addition to the integral FE measurement, the emission characteristics of individual tips out of various cathode materials and using different geometries (e.g. black Si) were investigated by means of FE scanning microscopy. For the actual device application (e.g. vacuum sensor or x-ray source), the electron source can be directly integrated and characterized in the complete system without any further modification.

Peter Topolovšek, 24.05.2017 at 14:00, Seminar room F7

Nanometric thin organic and inorganic layers and their use in perovskite solar cells and elastomers

Peter Topolovšek
Jožef Stefan International Postgraduate School, Ljubljana

Invitation and abstract [PDF]

Abstract:

Realization of highly performing vertically built opto-electronic devices typically requires a close control over the thickness of the constituing layers in order to find the balance between the conformity and optical and electronic properties of the layers. Specifically in solar cells, charge selective layers should be thick enough to allow pinhole free coverage and thin enough to minimize the electrical series resistance and the parasitic absorption. In the talk I will first present recently developed applications of thin organic and inorganic layers in perovskite solar cells which provide conformal coverage and minimization of material consumption while retaining selectivity of the charge transport over nanometric distances. Furthermore I will briefly describe how the 2D nature of PbI2 can be exploited for environmentally friendly synthesis of MAPbI3 submicron sized particles and how the surface modification of chemically exfoliated MoS2 can promote its homogeneous incorporation into elastomers.

Tetiana Borzda, 18.05.2017 at 13:00, Seminar room F7

Non-equilibrium electron dynamics in semiconducting transition metal

Tetiana Borzda
Jožef Stefan International Postgraduate School, Ljubljana
Invitation and abstract [PDF]

Abstract:

In the talk I will discuss equilibrium and non-equilibrium optical properties of two-dimensional materials, in particular transition metal dichalcogenides (TMDs) such as MoS2 and WSe2. After an introduction to TMD materials and their photophysics as well as the experimental methods, the first first part of the talk addresses the apparent conflict between strongly bound excitons on the one hand and efficient photovoltaics and sensitive photodetectors on the other. I use continuous wave photomodulation and femtosecond pump-probe spectroscopy to identify the spectral features of photogenerated charges and trace their dynamics, starting with their generation either by direct impulsive excitation into the charge continuum or via exciton dissociation.
But the origin of the femtosecond transient absorption changes can have an alternative interpretation. It can be formed not due to population dynamics but due to band gap renormalization and peak shift. I will discuss influence of many-body interaction on early pump-probe signal in the second part of the talk.
In the last part of the talk I present the attempt to develop metrics for characterization of the doping level of two-dimensional materials. The investigation of the three main Raman peaks of WSe2 in a field effect transistor with ionic liquid gating, which allows to control the doping level over a wide range was done. I track the positions and intensities of these peaks as a function of doping level and find several combinations – peak distances or intensity ratios that can be used as metrics for fast characterization.

dr. Reinhard Kaindl, 19.05.2017 at 10:30, Seminar room F7

Graphene-related materials research @ Joanneum research – Materials

dr. Reinhard Kaindl
Joanneum Research, Graz, Austria
Invitation [PDF]

Miloš Borovšak, 17.05.2017 at 13:00, Seminar room F7

Phase changes in molybdenum oxides induced by AFM tip and laser excitation

Miloš Borovšak
Faculty of Mathematics and Physics, Ljubljana

Abstract:

Molybdenum oxides show many physical properties, especially electronic, that are similar to the physical properties of 1T-TaS2, where a phase transition from insulator to metal can occur with a single ultrashort laser pulse, and as such are very promising candidates for study, if it is possible to induce phase transitions in the material. With the use of biased tip on an atomic force microscope we were able to induce a topotactic changes at the surface of η –Mo4O11 crystals. We showed that we can influence the surface potential while the topography remains the same. We performed systematic study of femtosecond time-resolved optical spectroscopy on both molybdenum oxides. In the case of η –Mo4O11 we observed a remarkable slowing down of the relaxation dynamics at the first charge density wave transition at TCDW1 = 105 K associated with hidden one-dimensional Fermi surface nesting. We also observed that with higher fluences we induce a dynamical phase transition that cannot be explained by heating. In the case of Mo8O23 we performed some additional characterization measurements such as resistivity and Raman spectroscopy in search for CDW phase transitions but we did not observe any specific indication of it. In comparison to η –Mo4O11 we did not observe any remarkable slowing down in dynamics. We observed that the relaxation dynamics parameters have qualitatively similar behavior as the resistivity with a peak/dip structure at around 150 K.

Victor Vega Mayoral, 11.01.2017 at 14:00, Seminar room F7

Photophysics of transition metal dichalcogenides obtained from liquid phase exfoliation

Victor Vega Mayoral
Trinity College Dublin, Dublin, Ireland

Abstract:

Two research lines are presented in this seminar. The first one is focused on exfoliating WS2 via liquid phase exfoliation and the use of some basic, commonly available and fast characterization techniques. Exfoliation in water/polymer dispersions has been studied. Poly(vynil alcohol) (PVA) is proposed as a model polymeric stabilizer. Effects of polymer concentration and centrifugation size- selection protocol on concentration, thickness and lateral size are reported. Extinction, Raman and photoluminescence spectroscopy were used for a fast and semiquantitative characterization supported by the more conventional TEM and AFM microscopy. The second research line focuses on a deeper characterization of the photoexcited states dynamics of few-layer MoS2 and WS2 where we have used spectrally-resolved femtosecond pump–probe spectroscopy. Photoexcited states dynamics in few-layer WS2 have beeen studied using femtosecond pump-probe spectroscopy. Using a cascade model I obtained an exciton dissociation lifetime of 1.3 ps. After excitons dissociation the geminated charges diffuse till they are trapped in defects such as grain boundaries or sulphur vacancies. There is a longer relaxation process that lasts hundreds of picoseconds assigned to charge recombination.

Peter Topolovšek, 22.12.2016 at 14:00, Seminar room F7

Ultrathin charge extraction layers in perovskite solar cells

Peter Topolovšek
Italian Institute of Technology, Milano, Italy

Daniele Vella, 15.12.2016 at 14:00, Seminar room F7

Photophysics of atomically thin MoS2 devices

Daniele Vella
Jožef Stefan International Postgraduate School, Ljubljana

dr. Tibor Tóth Katona, 10.11.2016 at 13:00, Seminar room F7

Biasing a ferronematic – a new way to detect weak magnetic field

dr. Tibor Tóth Katona
Hungarian Academy of Sciences, Budapest, Hungary

dr. Fabio Miletto Granozio, 27.09.2016 at 13:00, Seminar room F7

Nonvolatile resistive switches induced by field effect and light in 2DEGs at oxide interfaces

dr. Fabio Miletto Granozio
CNR-SPIN, Naples, Italy