Closure of the Mott gap and formation of a superthermal metal in the Fröhlich-type nonequilibrium polaron Bose-Einstein condensate in UO2+x

Paper [link]

Field-controlled structures in ferromagnetic cholesteric liquid crystals

One of the advantages of anisotropic soft materials is that their structures and, consequently, their properties can be controlled by moderate external fields. Whereas the control of materials with uniform orientational order is straightforward, manipulation of systems with complex orientational order is challenging. We show that a variety of structures of an interesting liquid material, which combine chiral orientational order with ferromagnetic one, can be controlled by a combination of small magnetic and electric fields. In the suspensions of magnetic nanoplatelets in chiral nematic liquid crystals, the platelet’s magnetic moments orient along the orientation of the liquid crystal and, consequently, the material exhibits linear response to small magnetic fields. In the absence of external fields, orientations of the liquid crystal and magnetization have wound structure, which can be either homogeneously helical, disordered, or ordered in complex patterns, depending on the boundary condition at the surfaces and the history of the sample. We demonstrate that by using different combinations of small magnetic and electric fields, it is possible to control reversibly the formation of the structures in a layer of the material. In such a way, different periodic structures can be explored and some of them may be suitable for photonic applications. The material is also a convenient model system to study chiral magnetic structures, because it is a unique liquid analog of a solid helimagnet.

Full text at advances.sciencemag.org [link]

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.

Dynamic Magneto-optic Coupling in a Ferromagnetic Nematic Liquid Crystal, Physical Review Letters, 01.09.2017

Hydrodynamics of complex fluids with multiple order parameters is governed by a set of dynamic equations with many material constants, of which only some are easily measurable. We present a unique example of a dynamic magneto-optic coupling in a ferromagnetic nematic liquid, in which long-range orientational order of liquid crystalline molecules is accompanied by long-range magnetic order of magnetic nanoplatelets. We investigate the dynamics of the magneto-optic response experimentally and theoretically and find out that it is significantly affected by the dissipative dynamic cross-coupling between the nematic and magnetic order parameters. The cross-coupling coefficient determined by fitting the experimental results with a macroscopic theory is of the same order of magnitude as the dissipative coefficient (rotational viscosity) that governs the reorientation of pure liquid crystals.

Full text at Physical Review Letters [link]

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.

Jan Ravnik; Best poster award at the International School and Workshop on Electronic Crystals ECRYS-2017, Cargese, France (21.08.-02.09.2017)

Jan Ravnik received the Best Poster Award at the International School and Workshop on Electronic Crystals ECRYS-2017, for his paper: Reaching a Hidden State in Half a Picosecond.

 

A high- temperature quantum spin liquid with polaron spins, Nature Physics, 31.07.2017

The existence of a quantum spin liquid (QSL) in which quantum fluctuations of spins are sufficiently strong to preclude spin ordering down to zero temperature was originally proposed theoretically more than 40 years ago, but its experimental realization turned out to be very elusive. Here we report on an almost ideal spin liquid state that appears to be realized by atomic-cluster spins on the triangular lattice of a charge-density wave state of 1T-TaS2. In this system, the charge excitations have a well-defined gap of ∼0.3 eV, while nuclear quadrupole resonance and muon-spin-relaxation experiments reveal that the spins show gapless QSL dynamics and no long-range magnetic order at least down to 70 mK. Canonical T2 power-law temperature dependence of the spin relaxation dynamics characteristic of a QSL is observed from 200 K to Tf = 55 K. Below this temperature, we observe a new gapless state with reduced density of spin excitations and high degree of local disorder signifying new quantum spin order emerging from the QSL.

Full text at Nature Physics [link]

10th Liquid Matter Conference, 17-21 July 2017, Ljubljana, Slovenia

The conference will be held from July 17 through 21 2017 at the Cankarjev dom Cultural and Congress Centre in Ljubljana, Slovenia. Previous conferences in the series were hosted by Lyon (1990), Florence (1993), Norwich (1996), Granada (1999), Konstanz (2002), Utrecht (2005), Lund (2008), Vienna (2011) and Lisbon (2014). The purpose of the conference is to review recent advances in experimental, theoretical, and applied studies in the liquid state of matter, bringing together researchers working in the physics and chemistry of liquids as well as in several closely related fields such as soft condensed matter and biophysics. The 10th Liquid Matter Conference is an Europhysics Conference.

The conference will open Monday morning July 17 2017 and close around lunchtime on Friday July 21 2017. The conference will consist of plenary lectures and of topical symposia including keynote lectures, contributed oral presentations, and poster sessions.

Topics:

  1. Ionic Liquids, Liquid Metals
  2. Water, Solutions
  3. Liquid Crystals
  4. Polymers, Polyelectrolytes, Biopolymers
  5. Colloids
  6. Films, Foams, Surfactants, Emulsions
  7. Confined Fluids, Interfacial Phenomena
  8. Supercooled Liquids, Glasses, Gels
  9. Driven Systems, Rheology, Nanofluidics
  10. Active Matter
  11. Biological and Biomimetic Fluids

Dr. Ljupka Stojčevska Malbašić; Best poster award at the conference ICFSI 2017, Hannover, Germany (02.-07.07.2017)

Dr. Ljupka Stojčevska Malbašić received the Best Poster Award at the 16th International Conference on the Formation of Semiconductor Interfaces (ICFSI 2017), for her paper: Study of the photoinduced hidden state in 1T-TaS2 single crystals doped with selenium by means of time-resolved photoemission spectroscopy (Co-authors from JSI; Asst. Prof. Tomaž Mertelj, Prof. Dr. Dragan Mihailović and Petra Šutar).

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.

Jan Ravnik; Best poster award at the conference PIPT6, Sendai, Japan (04.-09.06.2017)

Jan Ravnik received the Best Poster Award at the 6th International Conference On Photoinduced Phase Transitions (PIPT6), for his paper:

Time evolution of a phase transition to a stable photoinduced state in 1T-TaS2

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.

Prof. Dr. Dragan Mihailović is the recipient of the ERC’s “Proof of concept” project

Ljubljana, 9 May 2017 – Prof. dr. Dragan Mihailović, Head of the Department of Complex Matter at the “Jožef Stefan” Institute has at the call of the European Research Council (ERC) obtained an ERC “Proof of concept” – project aimed at developing a commercial concept of research findings.

Proposal [PDF]

Project Brief [link]

March for Science

Scientists against research funding cuts:

 

Slovenian Research Agency – NEW PROJECTS APPROVED, 2017

ARRS Results [link]

NEW EU PROJECT APPROVED, 2017

  • NFFA-Europe facilities, Scalling properties of CDW memory, prof. dr. Dragan Mihailović

A possible boost for integrated photonics from the labs of the Jožef Stefan Institute

Fibre optical communication is increasingly replacing traditional electrical interconnects due to its higher speed and energy efficiency: A possible boost for integrated photonics from the labs of the Jožef Stefan Institute

Abstract:

A possible boost for integrated photonics from the labs of the Jožef StefanInstitute Fibre optical communication is increasingly replacingtraditional electrical interconnects due to its higher speedand energy efficiency. What has started with long distancecommunication such as transatlantic cables is now thedefault choice for connecting our offices and homes to theinternet and is making headway into ever shorter distances,such as connecting racks in a data centre, and may soonalso connect the chips on a circuit board and ultimatelyalso replace some of the copper leads inside a chip. Thedriving force behind this trend is the fast increasing energy consumption of informationtechnology. Data centres alone now consume 3% of the world’s electricity – up from almostnothing just ten years ago.The adoption of optical communication on ever smaller scales relies on the swift translation ofthe zeroes and ones – “voltage off” and “voltage on” – into “light off” and “light on” in anelectro-optical or electroabsorption modulator. Curiously, this is a device that has provenremarkably resilient against efforts to make it smaller, contrary to transistors and othercomponents that shrink in half every other year. Hence, to make optical communication feasiblefor short distances, an alternative concept that enables smaller devices is needed.The ideal material for compact electromodulators has a strong absorption that changes strongly ifan electric field is applied. Researchers from IJS’s Department of Complex Matter, together withtheir colleagues from the Swiss Federal Institute of Technology in Lausanne and the ItalianInstitute of Technology in Milan, may have just found the right material for the job. MoS2 is asemiconductor that forms layered crystals, with a single layer being just two thirds of ananometer thick, but absorbing 10% of the incident light. Upon applying a voltage of only 0.5 V,they found that the absorption decreased to 9.5%, which may seem insignificant at first glance.However, their experiment – published in the latest edition of the journal 2d Materials – is a firstproof of concept, and they have a clear strategy how to reach technologically required values –switching the absorbance between 80% and 40%.After IJS and the other organizations involved had filed a patent application for the proposedtechnology, Daniele Vella and Christoph Gadermaier from the Department of Complex Matterhave founded a spin-out company that is now seeking money to develop an actual deviceexploiting the presented concept for integrated photonic circuits that could greatly reduce thepower consumption of data centers and later also laptops and mobile phones.

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.

Bite into science, RTV 4

At the Jožef Stefan Institute scientists have discovered a new type of memory element and achieve world speed record for an optical element.