Splay Nematic Phase

Researchers from the Department for Complex Matter Jožef Stefan Institute Alenka Mertelj, Nerea Sebastián, Luka Cmok in Martin Čopič have in collaboration with researchers from University of York, UK, studied a recently designed nematic phase, which appears in materials made of polar wedge-shaped molecules. They discovered that average molecular orientation in the new phase organizes in a manner that resembles a modulated array of Japanese fans. The modulated structure is biaxial and antiferroelectric. The described phase is a major step forward towards the realization of an often-speculated polar nematic phase, which could lead to materials with optical and electrical behaviors desired for a wide range of applications. The study was published in the paper Splay Nematic Phase in Physical Review X.

Raziskovalci Odseka za kompleksne snovi Instituta “Jožef Stefan” Alenka Mertelj, Nerea Sebastián, Luka Cmok in  Martin Čopič so skupaj s kolegi iz Univerze v Yorku, Velika Britanija, preučili novo nematično fazo, ki se pojavi v snoveh, zgrajenih iz močno polarnih molekul klinaste oblike. Ugotovili so, da ima povprečna ureditev molekul  v novi fazi modulirano pahljačasto strukturo, kar povzroči, da je faza dvoosna in antiferoelektrična. Fazo so poimenovali pahljačasta nematična faza. Opisana faza predstavlja pomemben korak k uresničitvi polarne nematične faze, ki lahko vodi do materialov z optičnimi in električnimi lastnostmi, želenimi za raznovrstne aplikacije. Rezultati raziskave z naslovom Splay Nematic Phase so bili objavljeni v reviji Physical Review X.

Hidden quantum resistance of a superconductor nanowire, Science Advances

Superconductors are known for zero electrical resistance, which is one of the consequences of superconductivity as a macroscopic quantum phenomenon.
However, in an unusual experiment on extremely narrow superconducting nanowires performed by Ivan Madan and Jože Buh under the leadership of Dragan Mihailovic, the superconductor is switched by ultrashort laser pulses to a hidden metastable resistive quantum state with the introduction of quantum phase slip centers. While the work demonstrates a new fundamental quantum phenomenon of nonequilibrium superconducting states related to quantum chaos, it is also of interest for designing new single photon detectors for use in quantum encrypted communications.
The nanowires were synthesised by Aleš Mrzel, while the theory calculations were performed by Viktor Kabanov and his student Vladimir Baranov, so everything from nanomaterial synthesis, nanocircuit technology to electrical and optical measurements, including theory is proprietary knowledge developed at the Jozef Stefan Institute and the CENN Nanocenter. The work was published on March 30 in Science Advances.

____________________________

Skrita kvantna upornost superprevodne nanožice

Superprevodniki so znani po tem, da nimajo električnega upora, kar je ena izmed posledic makroskopske kvantne narave tega pojava. Vendar pa se izkaže, da v to ne velja vedno. V eksperimentu, ki sta ga izvedla Ivan Madan in Jože Buh pod vodstvom Dragana Mihailovića s kratkimi laserskimi sunki so raziskovalci pokazali, da je možno superprevodne nanožice z majhnimi premeri spraviti v metastabilno kvantno stanje v katerih je upor končen. Poleg tega, da delo odkriva nov temeljni kvantni pojav neravnovesnih superprevodnih stanj povezan s kvantnim kaosom, je odkritje zanimivo tudi iz vidika razvoja novih detektorjev posameznih fotonov v kvantnih šifriranih komunikacijah. Nanožice je sintentiziral Aleš Mrzel, teoretične izračune pa opravil Viktor Kabanov s svojim študentom Vladimirom Baranovim. Celotno delo, ki vključuje sintezo nanožic po novi poti, priprave vezij nanometrskih dimenzij, električnih in optičnih meritev ter teoretičnimi izračuni, je rezultat lastnega znanja z Inštituta »Jožef Stefan« in CO Nanocentra. Delo je objavljeno 30. marca v ugledni reviji Science Advances.

Anisotropic magnetic nanoparticles: A review of their properties, synthesis and potential applications

A review article Anisotropic magnetic nanoparticles: A review of their properties, synthesis and potential applications has been published in the journal Materials Science (impact factor 31.140), after an invitation from the Editor, by Prof. Dr. Darja Lisjak (Department for Materials Synthesis) and Dr. Alenka Mertelj (Department of Complex Matter). The research on magnetic nanoparticles has been increasing in the last two decades or so due to their interesting properties and a wide range of applications in techniques, ecology and bio-medicine. The authors focused on the anisotropic magnetic nanoparticles that, in addition to their nano dimensions, show scientifically relevant and applicable properties due to their anisotropic shapes. Progress of Materials Science, Vol. 95, June 2018, 286-328

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]

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]

Best poster Award LEES at 2018 for Mimoza Naseska

Young researcher Mimoza Naseska was awarded Best Poster Award at the 2018 Low Energy Electrodynamics of Solids (LEES) conference, June 24-29, 2018, Portonovo, Ancona, Italy for poster “Optical quench and recovery of SDW state SeFe2As2 and EuFe2As2”.

Best Poster Award at QCM 2018 for Mimoza Naseska

Young researcher Mimoza Naseska was awarded Best Poster Award at the International Workshop Quantum Complex Matter (QCM 2018) and Frontiers Condensed Matter Physics (FCMP),  June 11-15, 2018, Rome, Italy for poster “Optical quench and recovery of SDW state SeFe2As2 and EuFe2As2”.

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]

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).

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. 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.

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.

Research cooperation between the Republic of Slovenia and the People’s Republic of China

prof. dr. Irena Drevenšek Olenik- selected with the projects on Public tender for co-financing scientific research cooperation between the Republic of Slovenia and the People’s Republic of China in the years 2017 – 2018.

ARRS [link]

Jan Fišer, the recipient of the Faculty Prešeren Prize (FMF) in Physics

Award announcement [link]

Fakultetne nagrade za študijsko leto 2015/16
Sedem fakultetnih Prešernovih nagrad, nagrada Franca Močnika, 45 Dekanovih priznanj
6. decembra 2016 smo podelili nagrade za najboljše dosežke študentom naše fakultete.

Fakultetno Prešernovo nagrado so s področja matematike prejeli Matej Petković, Rok Brence, Blaž Koroša in Petra Poklukar, s področja fizike pa Jan Fišer, Tanja Kaiba in Lara Ulčakar. Nagrado Franca Močnika je dobila Neda Tompa.

Dekanova priznanja študentom Fakultete za matematiko in fiziko UL je prejelo 20 študentov z Oddelka za matematiko in 25 študentov z Oddelka za fiziko.

Vsem nagrajencem iskreno čestitamo. [www.fmf.uni-lj.si]

Ultrafast optical spectroscopy of strongly correlated materials and high-temperature superconductors: a non-equilibrium approach, Advances in Physics

In the last two decades, non-equilibrium spectroscopies have evolved from avant-garde studies to crucial tools for expanding our understanding of the physics of strongly correlated materials. The possibility of obtaining simultaneously spectroscopic and temporal information has led to insights that are complementary to (and in several cases beyond) those attainable by studying the matter at equilibrium. From this perspective, multiple phase transitions and new orders arising from competing interactions are benchmark examples where the interplay among electrons, lattice and spin dynamics can be disentangled because of the different timescales that characterize the recovery of the initial ground state. For example, the nature of the broken-symmetry phases and of the bosonic excitations that mediate the electronic interactions, eventually leading to superconductivity or other exotic states, can be revealed by observing the sub-picosecond dynamics of impulsively excited states. Furthermore, recent experimental and theoretical developments have made it possible to monitor the time-evolution of both the single-particle and collective excitations under extreme conditions, such as those arising from strong and selective photo-stimulation. These developments are opening the way for new, non-equilibrium phenomena that can eventually be induced and manipulated by short laser pulses. Here, we review the most recent achievements in the experimental and theoretical studies of the non-equilibrium electronic, optical, structural and magnetic properties of correlated materials. The focus will be mainly on the prototypical case of correlated oxides that exhibit unconventional superconductivity or other exotic phases. The discussion will also extend to other topical systems, such as iron-based and organic superconductors, and charge-transfer insulators. With this review, the dramatically growing demand for novel experimental tools and theoretical methods, models and concepts, will clearly emerge. In particular, the necessity of extending the actual experimental capabilities and the numerical and analytic tools to microscopically treat the non-equilibrium phenomena beyond the simple phenomenological approaches represents one of the most challenging new frontiers in physics.

Full text at Taylor and Francis online [link]

Real-time measurement of the emergence of superconducting order in a high-temperature superconductor, Physical Review B

Systems which rapidly evolve through symmetry-breaking transitions on timescales comparable to the fluctuation timescale of the single-particle excitations may behave very differently than under controlled near-ergodic conditions. A real-time investigation with high temporal resolution may reveal insights into the ordering through the transition that are not available in static experiments. We present an investigation of the system trajectory through a normal-to-superconductor transition in a prototype high-temperature superconducting cuprate in which such a situation occurs. Using a multiple pulse femtosecond spectroscopy technique we measure the system trajectory and time evolution of the single-particle excitations through the transition in La1.9 Sr0.1 CuO4 and compare the data to a simulation based on the time-dependent Ginzburg-Landau theory, using the laser excitation fluence as an adjustable parameter controlling the quench conditions in both experiment and theory. The comparison reveals the presence of significant superconducting fluctuations which precede the transition on short timescales. By including superconducting fluctuations as a seed for the growth of the superconducting order we can obtain a satisfactory agreement of the theory with the experiment. Remarkably, the pseudogap excitations apparently play no role in this process.

Full text at Physical Review B [link]

Fast electronic resistance switching involving hidden charge density wave states, Nature Communications

The functionality of computer memory elements is currently based on multi-stability, driven either by locally manipulating the density of electrons in transistors or by switching magnetic or ferroelectric order. Another possibility is switching between metallic and insulating phases by the motion of ions, but their speed is limited by slow nucleation and inhomogeneous percolative growth. Here we demonstrate fast resistance switching in a charge density wave system caused by pulsed current injection. As a charge pulse travels through the material, it converts a commensurately ordered polaronic Mott insulating state in 1T–TaS2 to a metastable electronic state with textured domain walls, accompanied with a conversion of polarons to band states, and concurrent rapid switching from an insulator to a metal. The large resistance change, high switching speed (30 ps) and ultralow energy per bit opens the way to new concepts in non-volatile memory devices manipulating all-electronic states.

Full text at Nature Communications [link]

 

Critical femtosecond relaxation dynamics of collective and single-particle excitations through the phase transitions in single crystals of η−Mo4O11, Physical Review B

We present a systematic study of the single-particle and collective excitations by femtosecond transient reflectivity measurements in single crystals η−Mo4O11, investigating the dynamics as a function of temperature with two different pump photon energies (3.1 and 1.55 eV). A remarkable slowing down of the relaxation dynamics is observed at the first charge density wave (CDW) transition at TCDW1=105 K associated with hidden one-dimensional Fermi surface (FS) nesting. In contrast, the appearance of the second transition at TCDW2 associated with further CDW ordering is barely perceptible. The coherent response can be described well by the displacive coherent excitation model of Zeiger et al. [Phys. Rev. B 45, 768 (1992)] assuming a coupling of phonons to the photoexcited quasiparticles. The coupling of the collective modes to the electronic order parameter is found to be weak. The exponential relaxation is discussed in terms of single-particle relaxation and an overdamped collective mode.

Full text at Physical Review B [link]

Factors determining large observed increases in power conversion efficiency of P3HT:PCBM solar cells embedded with Mo6S9−xIx nanowires, Synthetic Metals

Power conversion efficiency (PCE) of bulk heterojunction solar cells is influenced by many factors, such as energy level alignment, light trapping and absorption, exciton diffusion, charge carrier mobility and non radiative recombination rate. Despite significant efforts towards improving all these aspects, the PCE remains relatively low and progress has been slow. Here we report a remarkable 52% relative increase in efficiency of solar cells embedded with small amounts of Mo6S9−xIx nanowires dispersed in P3HT:PCBM matrix. We present a detailed and systematic investigation of the numerous factors influencing this breakthrough increase in PCE. Raman spectroscopy and photocurrent imaging are used to investigate the spatial inhomogeneity of solar cell parameters and correlate them with the device performance. The largest effect appears to be improved hole mobility, which increases by a factor of 2.5. Surprisingly, only cells with highly regioregular P3HT show a dramatic effect with Mo6S9−xIx nanowires, while less regioregular P3HT:PCBM matrices show much smaller effect, pointing to level alignment as the crucial parameter in cell efficiency. A smaller PCE increase is attributed to absorbance of the active layer by surface-deposited Mo6S9−xIx nanowires.

Full text at ScienceDirect [link]

Spontaneous liquid crystal and ferromagnetic ordering of colloidal magnetic nanoplates, Nature Communications

Ferrofluids are familiar as colloidal suspensions of ferromagnetic nanoparticles in aqueous or organic solvents. The dispersed particles are randomly oriented but their moments become aligned if a magnetic field is applied, producing a variety of exotic and useful magnetomechanical effects. A longstanding interest and challenge has been to make such suspensions macroscopically ferromagnetic, that is having uniform magnetic alignment in the absence of a field. Here we report a fluid suspension of magnetic nanoplates that spontaneously aligns into an equilibrium nematic liquid crystal phase that is also macroscopically ferromagnetic. Its zero-field magnetization produces distinctive magnetic self-interaction effects, including liquid crystal textures of fluid block domains arranged in closed flux loops, and makes this phase highly sensitive, with it dramatically changing shape even in the Earth’s magnetic field.

Full text at Nature Communications [link]