Charge Configuration Memory Devices
Department for Complex Matter, JSI
Cross-differential dynamic microscopy
Dr. Andrej Petelin
Department for Complex Matter, JSI and University of Ljubljana, Faculty of Mathematics and Physics
Influence of dopants on critical behavior of room-temperature ferroelectric Sn2P2S6
Dr. Vasyl Shvalya
Scientific Researcher of Department of gaseous electronic F6, JSI
Prof. dr. Aleš Mrzel
Department for Complex Matter, JSI
Report on current research work
Polar order in liquids
Prof. dr. Alenka Mertelj
Department for Complex Matter, JSI
Report on current research work
High-Tc cuprates | story of two electronic subsystems
Prof. Neven Barišić, Viena University of Technology, Austria and University of Zagreb, Croatia
19th November 2019 at 11:15, Seminar room for physics
The high-Tc cuprates are amongst the most intensively studied correlated materials. Nevertheless, pivotal questions regarding their principal phases and regimes, as well as the transitions between them, remain unanswered. This is, largely, due to the complexity of these materials that renders the extraction of intrinsic properties difficult. We have performed a thorough experimental study of HgBa2CuO4+δ, which, in many respects, is a model cuprate compound. From the comparison of our measurements with data for other cuprates, we are able to separate universal behavior from compound-specific features. This exercise leads to a series of remarkable findings, the most important of which are that the effective mass and the scattering rate remain essentially unchanged across the phase diagram [1,2], and that the scattering rate is dominated by an umklapp process . These novel insights enabled an accurate count of charges across the phase diagram. The electronic system is thus found to consist of 1+p charges, where p corresponds to doping. At low dopings, within the pseudogap regime, exactly one hole is localized per planar copper-oxygen unit. Upon increasing doping and temperature, the hole is gradually delocalized and becomes itinerant . The overall behaviors are consistent with a gradual extension of Fermi arcs to a full Fermi surface, without an essential change of the underlying Fermi surface that encloses 1+p states. Finally, we have established that the itinerant Fermi-liquid holes become superconducting while the localized hole provides the glue.
 N. Barišić et al, New J. Phys. 21, 113007 (2019)
 Li et al, Phys. Rev. Lett. 117, 197001 (2016)
 N. Barišić et al, Proc. Natl. Acad. Sci. 110, 12235 (2013); S. I. Mirzaei et al, Proc.
Natl. Acad. Sci. 110, 5774 (2013); M. Chan et al, Phys. Rev. Lett. 113, 177005 (2014); P.
Popčević et al, Quantum Mater. 3, 42 (2018)
 D. Pelc et al, Sci. Adv. 5, eaau4538 (2019)
Symmetry forbidden Raman lines activated by photorefractivity of Fe-doped Lithium Niobate crystal
Prof. Ninel Kokanyan
Laboratory of Optical Materials Photonics & Systems, CentraleSupélec, Metz, France
Lithium niobate (LN) has various advantageous properties such as large nonlinear optical coefficients, high transparency in the visible and near infrared range, technology for the manufacturing of waveguides and domain structures. It is widely used in various applications such as light modulation, frequency conversion, SAW sensors, photonic devices. LN, especially when doped with iron (Fe) shows another very remarkable property which is photorefractivity.
Transmission Raman spectra were recorded in photorefractive iron-doped LN crystal within a priori equivalent configurations, Y(XZ)Y and Y(ZX)Y showed completely different behaviors as function of time. In Y(ZX)Y only E[TO] modes are present in accordance with selection rules, while in Y(XZ)Y configuration spectra showed a strong dependence on time with a rise of A1[TO] Raman modes. The intensity of the forbidden activated lines reveals a time evolution of the conversion from the o- polarization to the e- polarization after crossing the sample. The intensity ratio of activated A1[TO]* and E[TO8] reflects the conversion efficiency as a function of time
The evolution order parameters in an auxetic Liquid Crystal Elastomers under strain
School of Physics and Astronomy, University of Leeds, UK
Liquid crystal elastomers combine the fields of liquid crystal physics with polymer physics with surprising consequences. Our group has reported a negative order parameter of the polymer backbone of an LCE based on stress strain curves. This coincides with a negative Poisson ratio and occurs during a reorientation of the nematic director.
The evolution of order parameters, P2 and P4, for an LCE under strain has been determined using Raman spectroscopy. This revealed that, for our LCE, P4>P2 for certain values of strain. A behaviour not seen in conventional nematic thermotropic liquid crystals. The region where P4>P2 coincides with the beginning of the mechanical Freedrickz transition. Subsequent experiments on the LCE prepared in different states to understand the deformation modes. These are reported herein.
Prof. Dr. Ivan K. Schuller
Department of Physics and Center for Advanced Nanoscience
University of California, San Diego, USA
Data acquisition (sensors) and manipulation (memory, computation, communications, data mining) in its many forms drives and fuels our civilization. Biology has evolved complex intelligent systems that can acquire and manipulate data in a very efficient and comprehensive fashion. On the other hand, scientific and technological developments have led to the invention of highly sophisticated data acquisition and manipulation machines, which have been continuously improving over the last 50 years. Since biological systems can, in many cases, outperform artificial systems a natural question arises. Can biology provide, some high level, guiding principles useful for the development of revolutionary, new concepts for the development of artificial, intelligent systems?
I will describe attempts to answer the US White House Nanotechnology-Inspired Grand Challenge for Future Technology: “Create a new type of computer that can proactively interpret and learn from data, solve unfamiliar problems using what it has learned, and operate with the energy efficiency of the human brain”.
The work was supported as part of the “Quantum Materials for Energy Efficient Neuromorphic Computing” an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science.
Tunable optical diffraction gratings based on a ferromagnetic liquid crystal
Mathias Fleisch, Faculty of Physics, University of Vienna, Austria
The director of a ferromagnetic liquid crystal can be realigned by small external magnetic fields. Transmission optical diffraction gratings composed of periodic slices of a ferromagnetic liquid crystal and a conventional photoresist polymer are demonstrated. Dependence of diffraction efficiencies of various diffraction orders on an in-plane external magnetic field is investigated. It is shown that diffraction properties can be effectively tuned by magnetic fields as low as a few mT. The experimental results are explained with analytical calculations based on the Jones calculus for polarising microscope measurements and simulations using the Rigorous Coupled Wave Analysis (RCWA) for the diffraction experiments.
S. Gao, M. Fleisch, R. A. Rupp, L. Cmok, P. Medle-Rupnik, A. Mertelj, D. Lisjak, X. Zhang, and I. Drevenšek-Olenik, “Magnetically tunable optical diffraction gratings based on a ferromagnetic liquid crystal,” Opt. Express 27 (2019)
Spectral properties and energy transfer in DNA fragment
Anton Hromov, Taras Shevchenko National University of Kyiv, Department of Experimental Physics
Spectral properties and energy transfer in DNA fragment.
Studied objects: single stranded HIV primers, telomeres fragment, polynucleotide poly(dAdT)2, the oligonucleotide d(CCCGGGTTTAAA), the trimer d(ATC) and the low molecular model compounds dGMP, dAMP, dCMP and dTMP.
Measurement of absorption spectra, fluorescence and phosphorescence were carried out at various temperatures (T = 4.2K,77K, 300K).
The system of energy sites and electronic processes in the oligonucleotides were examined.
The Jablonski diagram of studied samples was constructed, the paths of energy transfer were investigated.
The nature of the electronic excitations capturing centres in the DNA
V Yashchuk, V Kudrya, M Losytskyy, H Suga, T Ohul’chanskyy
Electrostatic interaction between magnetic nanoplatelets in alcohols
Patricija Hribar Boštjančič, Department of Complex Matter, Jožef Stefan Institute
In a room–temperature liquid magnet barium hexaferrite (BHF) nanoplatelets are spontaneously ordered and form a ferromagnetic nematic phase. In concentrated suspension in 1-butanol the nanoplatelets with magnetic moments perpendicular to their basal plane align in large macroscopic regions, forming magnetic domains. The key parameter for the suspension stability and the formation of the ferromagnetic nematic phase are electrostatic interactions, which can be influenced by the solvent and the concentration of surfactant, i.e., dodecylbenzenesulfonic acid (DBSA). We investigated the parameters that affect the electrostatic interaction between the BHF nanoplatelets in alcohol suspensions. The measurements of electrophoretic mobility and conductivity were performed in tert-butanol, 1-hexanol, 1-butanol and 2-propanol suspensions to consider also the effect of solvent’s dielectric constant. I will present our results of the alcohol’s dielectric constant effect on the equilibrium between dissolved and adsorbed DBSA and consequently on the electrostatic interaction and the Debye screening length.
Solid state generators and energy harvesters for waste heat recovery and thermal energy harvesting
Dr. Daniel Zabek, Institute of Heat Engineering, Warsaw University of Technology, Poland
Solid state thermal to electrical energy converters are capable of transforming low grade heat directly into electricity for waste heat recovery and thermal energy harvesting. Direct solid state heat engines, such as thermoelectric modules and thermionic converters for spatial temperature gradients, are compared with pyroelectric energy harvesters and thermomagnetic generators for transient changes in temperature. Temperature and size limitations along with the maturity of the technologies are discussed based on energy density and temperature range for the different generator technologies. Despite the low energy conversion efficiency with solid state generators, electric power density ranges from 4 nW/mm² to 324 mW/mm².
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
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.
Direct detection of ultrafast switching in quantum cryo-memory devices
Jan Ravnik, Department of Complex Matter, Jožef Stefan Institute, Ljubljana, Slovenia
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.
SrₓBi₂Se₃-nematicity, superconductivity, crystals and thin films
Aleksander Yu. Kuntsevich, P. N. Lebedev Physical Institute, Moscow, Russia
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.
Vertex directed analysis of the hidden state relaxation in 1T-TaS2
Andrej Kranjec, Department for Complex Matter, JSI
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.
Transient cooling of quasiparticles in K3C60 by mid-infrared laser pulses
Prof. Dr. Michele Fabrizio, International School for Advanced Studies SISSA, Trieste, Italy
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.
Spin density wave dynamics in iron based pnictides
Mimoza Naseska, Department of Complex Matter, JSI
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.
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
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.
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
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.
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
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.
Polarized pump-probe measurements in organic superconductors near Mott boundary
Koichi Nakagawa, Department of Applied Physics, Hokkaido University, Sapporo, Japan
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.
High-Tc superconductivity in ruthenates
Asst. Prof. Dr. Hiroyoshi Nobukane, Department of Physics, Hokkaido University, Sapporo, Japan
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.
Charged lattice gas of polarons in 1T-TaS2
Jaka Vodeb, Department for Complex Matter, Jozef Stefan Institute, Ljubljana, Slovenia
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.
Out of equilibrium electronic properties of ZrTe5
Michele Diego, Department of Complex Matter, JSI
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.
Photoregulating the self-assembly of lipophilic guanosine derivaters at the air-water interface
Odsek za kompleksne snovi, IJS
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 . 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.
 L. Coga, T. Ilc, M. Devetak, S. Masiero, L. Gramigna, G. P. Spada, and I. Drevensek Olenik, Colloid. Surface B103, 45 – 51 (2013).
 M. Devetak, S. Masiero, S. Pieraccini, G. P. Spada, M. Copic, and I. Drevensek Olenik, Appl. Surf. Sci. 256,2038 2043 (2010)
 L. Coga, S. Masiero, and I. Drevensek Olenik, Colloid. Surface B 121, 114 – 121 (2014)
 L. Coga, S. Masiero, and I. Drevensek Olenik, Langmuir 31, 4837 – 4843 (2015)
Nonresonant multi-pulse selective spectroscopy of interaction-induced responses in liquids using optical Kerr effect
Kazan E.K. Zavoisky Physical-Technical Institute, Kazan, Russia
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.
Expansion of transition metal dichalcogenides
Hella Saturnus Slovenija d.o.o., HSS – Ljubljana
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.
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
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 ~510-21 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.
Cluster structure of monohydric alcohol
Yelyzaveta Chernolevska, Faculty of Physics of Taras Scevchenko, National University of Kyiv, Ukraine
Photoinduced carrier dynamics in organic superconducting system: Previous results and recent progress
Dr. Satoshi Tsuchiya
Department of Applied Physics, Hokkaido University, Hokkaido, Japan
UV second harmonic generation in structured AIN optical wavequides
Fakulteta za matematiko in fiziko in Institut »Jožef Stefan«, Ljubljana
Ultrafast spin density wave dynamics at intense optical pulse excitation
Fakulteta za matematiko in fiziko in Institut »Jožef Stefan«, Ljubljana
Urejanje polaronov v dveh dimenzijah
Fakulteta za matematiko in fiziko in Institut »Jožef Stefan«, Ljubljana
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
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
Nanometric thin organic and inorganic layers and their use in perovskite solar cells and elastomers
Jožef Stefan International Postgraduate School, Ljubljana
Invitation and abstract [PDF]
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.
Non-equilibrium electron dynamics in semiconducting transition metal
Jožef Stefan International Postgraduate School, Ljubljana
Invitation and abstract [PDF]
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.
Graphene-related materials research @ Joanneum research – Materials
dr. Reinhard Kaindl
Joanneum Research, Graz, Austria
Phase changes in molybdenum oxides induced by AFM tip and laser excitation
Faculty of Mathematics and Physics, Ljubljana
Photophysics of transition metal dichalcogenides obtained from liquid phase exfoliation
Victor Vega Mayoral
Trinity College Dublin, Dublin, Ireland
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.
Ultrathin charge extraction layers in perovskite solar cells
Italian Institute of Technology, Milano, Italy
Photophysics of atomically thin MoS2 devices
Jožef Stefan International Postgraduate School, Ljubljana
Biasing a ferronematic – a new way to detect weak magnetic field
dr. Tibor Tóth Katona
Hungarian Academy of Sciences, Budapest, Hungary
Nonvolatile resistive switches induced by field effect and light in 2DEGs at oxide interfaces
dr. Fabio Miletto Granozio
CNR-SPIN, Naples, Italy