Science Advances has published the article Sculpting stable structures in pure liquids, which has been done under supervision of Uroš Tkalec from Condensed matter physics JSI by Tadej Emeršič, Žiga Kos, Simon Čopar and Natan Osterman from University of Ljubljana and Rui Zhang and Juan J. de Pablo from University of Chicago. Authors are reporting about the creation of reconfigurable micro-domains with polar order in liquid crystals, achieved by flow and light pulses. This work represents an important step in structuring complex fluids with outer fields, because it is the first example of stabilizing dynamic segments in one-component anisotropic fluids. The used approach allows manipulation of border between orientational phases, encapsulation of other molecules and active biological system integration.
Revija Science Advances je objavila članek Sculpting stable structures in pure liquids, ki so ga pod vodstvom Uroša Tkalca z Odseka za fiziko trdne snovi IJS oblikovali Tadej Emeršič, Žiga Kos, Simon Čopar in Natan Osterman z Univerze v Ljubljani ter Rui Zhang in Juan J. de Pablo z Univerze v Chicagu. Avtorji poročajo o kontroliranem tvorjenju mikrodomen s polarnim redom v neravnovesnem nematskem tekočem kristalu, ki ga dosežejo z laserskimi in tokovnimi pulzi. Delo predstavlja pomemben korak v strukturiranju kompleksnih tekočin z zunanjimi polji, saj gre za prvi primer stabilizacije dinamičnih segmentov v enokomponentni anizotropni tekočini. Uporabljen pristop omogoča manipulacijo meje med orientacijskima fazama, enkapsulacijo drugih molekul in aplikacije v aktivnih bioloških sistemih.
Molybdenum suboxides have been gaining attention lately due to their potential as charge storage materials and memory elements. In spite of increased interest in their functional properties, the phase diagrams, transient, and even equilibrium optical properties have not been systematically studied for many molybdenum suboxides, such as, for example, monoclinic semimetallic Mo8O23. This compound undergoes an incommensurate ordering transition at TIC ∼ 350 K, followed by a structural transition to commensurate order at TIC−C = 285 K, in addition, an enigmatic resistance maximum is observed at Tel ∼ 150 K, whose origin has so far proved elusive. In our combined polarized transient optical spectroscopy and Raman spectroscopy studies of the electronic relaxation dynamics and lattice vibrational modes in Mo8O23 single crystals we find evidences of the gapped state appearance below Tel that has so far eluded detection by structural analyses. The study was published in the paper Time-resolved reflectivity and Raman studies of the interplay of electronic orders in Mo8 O23 in Physical Review B.
Molibdenovim suboksidom je v zadnjem času namenjene vse več pozornosti zaradi njihovega potenciala kot snovi za shranjevanje naboja in spominske elemente. Kljub zanimanju za njihove funkcionalne lastnosti fazni diagrami, dinamične in celo ravnovesne optične lastnosti niso bili sistematično raziskani za večino molibdenovih suboksidov. Mednje spada tudi monoklinski polkovinski Mo8O23. Ta snov kaže prehod v inkomenzurabilno fazo pri TIC ∼ 350 K, ki mu sledi strukturni prehod v komenzurabilno fazo pri TIC−C = 285 K. Poleg tega je pri Tel ∼ 150 K opazen enigmatični maksimum električne upornosti, katerega izvor je doslej neznan. V naših optičnih raziskavah elektronske relaksacijske dinamike in vibracijskih načinov rešetke v kombinaciji z ramansko spektroskopijo monokristalov Mo8O23 smo našli dokaze o dodatnem faznem prehodu, katerega posledica je nastanek energijske reže pod Tel. Dodatnega faznega prehoda prejšnje, pretežno strukturne raziskave niso zaznale. Rezultati raziskave z naslovom Time-resolved reflectivity and Raman studies of the interplay of electronic orders in Mo8 O23 so bili objavljeni v reviji Physical Review B.
Domain walls in insulating materials may be orders of magnitude more conducting than the bulk, allowing one to design the material functionality by their patterning. Studies of inhomogeneities such as domain walls in the presence of strong electronic interactions is a very challenging problem. A particularly interesting case is that of Mott insulators, where electrons are […]
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.
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.
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
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]
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]
On Friday, 18th January, 2019, the Dean of the University of Ljubljana Faculty of Mathematics and Physics, Prof. Dr. Anton Ramšak, presented the Faculty Prešeren Awards to the students of the Faculty of Mathematics and Physics. Mimoza Naseska, a young researcher from our department, was one of the recipients of the Faculty Prešeren Award for her work titled “Ultrafast Spin Density Wave dynamics at intense optical pulse excitation”. The work was supervised by Assist. Prof. Dr. Tomaž Mertelj.
About the work
The experimental work for her master’s thesis was carried out at the Jožef Stefan Institute Department of Complex Matter. With the multipulse time-resolved optical spectroscopy, she investigated the femtosecond dynamics in iron pnictides with a spin density wave (SDW) state.
Using this technique she determined the fluence dependence of the timescale for optical destruction and recovery of the SDW state. She also found that the measured heating is significantly smaller than expected on the basis of the optical penetration depth data published in literature.
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 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).
Interview in Slovenian language, Tromba agency [link]
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.
Project Brief [link]
Development of gradual optical shutter – OPTIGRAD, dr. Luka Cmok; The Ministry of Education, Science and Sport, Republic of Slovenia
- Femtosecond time-resolved scanning tunneling electron microscopy of complex materials, 01.05.2017 – 01.05.2020, prof. dr. Dragan Mihailović
- Electrically tunable ferromagnetic liquids, 01.05.2017 – 01.05.2020, asst. dr. Alenka Mertelj
- Magnetically reconfigurable elastomeric optical surfaces, 01.04.2017 – 31.03.2019, prof. dr. Dragan Mihailović, prof. dr. Irena Drevenšek Olenik
ARRS Results [link]
- NFFA-Europe facilities, Scalling properties of CDW memory, prof. dr. Dragan Mihailović
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
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.
At the Jožef Stefan Institute scientists have discovered a new type of memory element and achieve world speed record for an optical element.
New memory element and a world speed record – article [link]
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.
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]
Rosen Plevneliev’s visit at the “Jožef Stefan” Institute
Article in slovenian language [link]
Quantum Technologies – new development
Article at Tromba Agency [link]
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]
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]
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]
Interview with Dr. Igor Vaskivskyi
Full text in Slovenian language at Tromba Agency [link]
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]
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]
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]