V. Nasretdinova, Ya. A. Gerasimenko, J. Mravlje, G. Gatti, P. Sutar, D. Svetin, A. Meden, V. Kabanov, A. Yu. Kuntsevich, M. Grioni & D. Mihailovic in Scientific Reports Volume 9, November 2019, Article number: 15959 (2019), DOI: doi:10.1038/s41598-019-52231-4
Mo8O23 is a low-dimensional chemically robust transition metal oxide coming from a prospective family of functional materials, MoO3−x, ranging from a wide gap insulator (x = 0) to a metal (x = 1). The large number of stoichometric compounds with intermediate x have widely different properties. In Mo8O23, an unusual charge density wave transition has been suggested to occur above room temperature, but its low temperature behavior is particularly enigmatic. We present a comprehensive experimental study of the electronic structure associated with various ordering phenomena in this compound, complemented by theory. Density-functional theory (DFT) calculations reveal a cross-over from a semi-metal with vanishing band overlap to narrow-gap semiconductor behavior with decreasing temperature. A buried Dirac crossing at the zone boundary is confirmed by angle-resolved photoemission spectroscopy (ARPES). Tunneling spectroscopy (STS) reveals a gradual gap opening corresponding to a metal-to-insulator transition at 343 K in resistivity, consistent with CDW formation and DFT results, but with large non-thermal smearing of the spectra implying strong carrier scattering. At low temperatures, the CDW picture is negated by the observation of a metallic Hall contribution, a non-trivial gap structure in STS below ∼170 K and ARPES spectra that together represent evidence for the onset of the correlated state at 70 K and the rapid increase of gap size below ∼30 K. The intricate interplay between electronic correlations and the presence of multiple narrow bands near the Fermi level set the stage for metastability and suggest suitability for memristor applications.
Mo8O23 je dvodimenzionalen kemijsko odporen oksid prehodnih kovin. Spada v skupino funkcionalnih materialov MoO3-x, kateri segajo od izolatorjev z veliko režo (x = 0) do kovin (x = 1). Veliko število stehiometričnih spojin z vmesnim x ima širok spekter različnih lastnosti. V spojini Mo8O23 je bil predpostavljen neobičajen prehod v stanje z valom gostote naboja, ki se pojavi nad sobno temperature. Hkrati pa je njeno nizkotemperaturno obnašanje velika uganka.
V članku predstavljamo obsežno eksperimentalno študijo elektronske strukture, povezane z različnimi fenomeni urejanja v spojini, dopolnjeno s teorijo. Izračuni na podlagi teorije gostotnih funkcionalov (DFT) pri zniževanju temperature razkrivajo preskok iz polkovine z manjkajočim prekrivanjem pasov, do polprevodnikov z ozkimi režami. Dirac-ovo sedlo na meji Brillouinove cone je potrjeno s pomočjo kotno-ločljivostne fotoemisijske spektroskopije (ARPES). Tunelska spektroskopija (STS) s spremembo upornosti razkriva postopno odpiranje vrzeli, ki ustreza prehodu kovina-izolator pri 343 K, kar je konsistentno s tvorbo vala gostote naboja in DFT rezultati, vendar pa z velikim netermičnim zabrisanjem spektrov kaže na močno sipanje nosilca. Pri nizkih temperaturah se CDW slika izniči z opazovanjem prispevka kovinskega Hallovega efekta, netrivialno strukturo reže v STS pod ∼170 K in ARPES spektri, ki skupaj predstavljajo dokaz za začetek spremenjenega stanja pri 70 K in hitro povečanje velikosti vrzeli pod ∼30 K.
Zapletena medsebojna povezava med elektronskimi korelacijami in prisotnostjo več ozkih pasov blizu Fermijeve ravni je postavila metastabilnost materiala na mesto, kjer se je pokazal kot primeren za uporabo v memristorskih aplikacijah.
Nonequilibrium dynamics of many body quantum systems has become one of the most advanced contemporary research areas in solid state physics, ultra cold atoms, nanophysics and quantum devices. Understanding of such dynamics, which in some cases leads to unusual nonequilibrium steady states, presents a challenge to experimentalists and theory alike. Equally concurrent is an open question related the role of ergodicity breaking mechanisms in many body quantum systems, e.g., in systems subject to strong disorder. At the same time, new and sometimes unexpected experimental discoveries provide stimulus for theory. This workshop continues the tradition with the 9th focused workshop on nonequilibrium phenomena. The aim is to bring together experiment and theory in an informal setting that has been extremely stimulating on previous occasions. The workshop intends to bring together scientists working in the field of:
- Nonequilibrium dynamics in correlated solid state systems such as charge density wave, superconducting and magnetic systems
- Nonequilibrium dynamics in “artificial” quantum systems such as ultra cold atoms in optical lattices and quantum dots
- Nonequilibrium dynamics and measures of ergodicity in quantum many body systems
- Finite-temperature properties of systems with strong electron or electron phonon interactions
Due to the COVID-19 situation the conference will be online only.
Jaka Vodeb, Viktor V Kabanov, Yaroslav A Gerasimenko, Rok Venturini, Jan Ravnik, Marion A van Midden, Erik Zupanic, Petra Sutar and Dragan Mihailovic in New Journal of Physics, Volume 21, August 2019, PDF, DOI: https://dx.doi.org/10.1088/1367-2630/ab3057
Mesoscopic irregularly ordered and even amorphous self-assembled electronic structures were recently reported in two-dimensional metallic dichalcogenides (TMDs), created and manipulated with short light pulses or by charge injection. Apart from promising new all-electronic memory devices, such states are of great fundamental importance, since such aperiodic states cannot be described in terms of conventional charge-density-wave (CDW) physics. In this paper, we address the problem of metastable mesoscopic configurational charge ordering in TMDs with a sparsely filled charged lattice gas model in which electrons are subject only to screened Coulomb repulsion. The model correctly predicts commensurate CDW states corresponding to different TMDs at magic filling fractions Doping away from results either in multiple near-degenerate configurational states, or an amorphous state at the correct density observed by scanning tunnelling microscopy. Quantum fluctuations between degenerate states predict a quantum charge liquid at low temperatures, revealing a new generalized viewpoint on both regular, irregular and amorphous charge ordering in transition metal dichalcogenides.
Graphene has great application prospects in the field of optoelectronics. We investigate a field effect transistor with a graphene channel. Carrier density and chemical potential of the channel can be spatially modified by topping the channel with dielectric structures consisting of pure and lithium enriched SU-8 layers. As an example, we demonstrate that application of a single-gate voltage can induce a P–N junction to a channel with an appropriate dielectric architecture. Electronic and photoelectric properties of the junction are studied. The photocurrent mapping is investigated, which clearly shows the origin of the photocurrent from the P–N junction. The proposed technology makes fabrication of graphene-based photodetectors simple and flexible, and may also be interesting for the development of future optoelectronic components using other two-dimensional materials.
The article was chosen as an Editor’s Suggestion!
The impulsive generation of two-magnon modes in antiferromagnets by femtosecond optical pulses, so-called femto-nanomagnons, leads to coherent longitudinal oscillations of the antiferromagnetic order parameter that cannot be described by a thermodynamic Landau-Lifshitz approach. We argue that this dynamics is triggered as a result of a laser-induced modification of the exchange interaction. In order to describe the oscillations, we have formulated a quantum mechanical description in terms of magnon pair operators and coherent states. Such an approach allowed us to derive an effective macroscopic equation of motion for the temporal evolution of the antiferromagnetic order parameter. An implication of the latter is that the photoinduced spin dynamics represents a macroscopic entanglement of pairs of magnons with femtosecond period and nanometer wavelength. By performing magneto-optical pump-probe experiments with 10 femtosecond resolution in the cubic KNiF3 and the uniaxial K2NiF4 collinear Heisenberg antiferromagnets, we observed coherent oscillations at the frequency of 22 and 16 THz, respectively. The detected frequencies as a function of the temperature fit the two-magnon excitation up to the Néel point. The experimental signals are described as dynamics of magnetic linear dichroism due to longitudinal oscillations of the antiferromagnetic vector.
Yaroslav A. Gerasimenko, Igor Vaskivskyi, Maksim Litskevich, Jan Ravnik, Jaka Vodeb, Michele Diego, Viktor Kabanov, Dragan Mihailovic in Nature Material, Vol. 18 Issue 8, July 2019, https://rdcu.be/bKxRq, DOI: https://www.nature.com/articles/s41563-019-0423-3
Distinct many-body states may be created under non-equilibrium conditions through different ordering paths, even when their constituents are subjected to the same fundamental interactions. The phase-transition mechanism to such states remains poorly understood. Here we show that controlled optical or electromagnetic perturbations can lead to an amorphous metastable state of strongly correlated electrons in a quasi-two-dimensional dichalcogenide. Scanning tunnelling microscopy reveals a hyperuniform pattern of localized charges, whereas multitip surface nanoscale conductivity measurements and tunnelling spectroscopy show an electronically gapless conducting state that is different from conventional Coulomb glasses and manybody localized systems. The state is stable up to room temperature and shows no signs of either local charge order or phase separation. The mechanism for its formation is attributed to a dynamical localization of electrons through mutual interactions. Theoretical calculations confirm the correlations between localized charges to be crucial for the state’s unusual stability.
Med poskusi, namenjenimi ustvarjanju novih oblik kvantnih materialov pod močno neravnovesnimi pogoji v kristalu tantalovega disulfida, je skupina raziskovalcev na Institutu »Jožef Stefan« s kratkimi laserskimi sunki ustvarila nenavadno gosto amorfno elektronsko snov, v kateri se zaradi medsebojnih interakcij elektroni zagozdijo. Odkritje spada na področje kvantne fizike in je fundamentalno pomembno, saj odpira novo področje. Razumevanje pojava predstavlja nov velik izziv za današnjo kvantno fiziko. Pojav zagozdenja elektronov lahko nastane vsepovsod tam, kjer imamo opravka s hitro kompresijo osnovnih delcev pri velikih gostotah, npr. v jedrih ali v nevtronskih zvezdah. Ima tudi potencialno uporabo, saj je pojav možno kontrolirati, ob njem pa se močno spremeni električna upornost snovi.
Scienta Omicron, 1.8.2019:
Scienta Omicron Newsflyer Fall 2019, 16.10.2019: https://www.scientaomicron.com/en/newsletter
In a paper in the Nature physics journal of Quantum Materials, published on the 26 th of June, the structure of newly created quantum matter is revealed. The work follows the fundamental idea that the constituents of interacting many body systems in complex quantum materials may self-organize into new and unexpected artificial quantum states. However, in spite of the efforts of a large number of groups worldwide to determine the structure of such matter, and huge recent advances in experimental techniques, demonstrating new emergent order has proven surprisingly difficult. In the work by the group at the Jozef Stefan Institute, the authors use femtosecond-light pulse-induced using scanning tunneling microscopy, reporting for the first time a surprisingly intricate chiral long-range topologically non-trivial charge order emerging in a prototypical two-dimensional material.
The discovery of the principles that lead to metastability in charge-ordered systems opens the way
to designing novel emergent functionalities, particularly ultrafast all-electronic non-volatile cryo-
V članku, ki so ga 26. junija objavili v reviji Nature Physics Journal of Quantum Materials, avtorji razkrivajo detajlno strukturo povsem nove zvrsti kvantne snovi, ustvarjene daleč iz ravnovesja. Delo sledi ideji, da se lahko elektroni v kompleksnih kvantnih materialih samoorganizirajo v nove in nepričakovane umetne kvantne tvorbe pod neravnovesnimi pogoji. Žal pa kljub velikemu napredku eksperimentalnih tehnik in delu velikega števila vrhunskih skupin po svetu določitev strukture tovrstnih kvantnih snovi doslej ni bila uspešna. Skupini z Instituta “Jožef Stefan” je prvi na svetu uspelo s povsem novo metodo določiti presenetljivo zapleteno kiralno ureditev elektronov v tovrstni snovi. V članku skupina opisuje uporabo femtosekundnih svetlobnih sunkov za tvorbo nove vrste metastabilne kvantne ureditve v prototipnem dvodimenzionalnem dihalkogenidnem kristalu, v katerem so potem z uporabo tunelske mikroskopije z atomsko ločljivostjo raziskali podrobno razporeditev eletronov. Odkritje mehanizma, ki vodi k metastabilnosti v narejenih sistemih, odpira pot k novi uporabi, zlasti računalniškega spomina.
“Symmetry-enforced Dirac points in antiferromagnetic semiconductors” has been published online today, 27 June 2019, in the 15 June 2019 issue of Physical Review B (Vol. 99, No. 23):
Dirac points usually exist in systems where both the time-reversal and the inversion symmetries are present. In magnetic systems the time-reversal symmetry is broken. In antiferromagnets often the time-reversal symmetry is present in combination with some rotation, inversion or translation. It is shown that this combined symmetry is responsible for the formation of a Dirac point in antiferromagnetic semiconductors.
Danes, 27. junija 2019, je bil na spletu objavljen članek “Symmetry-enforced Dirac points in antiferromagnetic semiconductors” v številki Physical Review B (Vol. 99, No. 23), ki je izšla 15. junija 2019:
Diracove točke tipično obstajajo v sistemih z zlomljeno simetrijo na obrat časa kot tudi zlomljeno simetrijo inverzije prostora. V magnetnih sistemih je zlomljena simetrija na obrat časa. Pri antiferomagnetih je simetrija na obrat časa pogosto prisotna v kombinaciji z nekakšno rotacijo, inverzijo, ali translacijo. Pokazano je, da je prisotnost takšne kombinirane simetrije odgovorna za tvorbo Diracovih točk v antiferomagnetnih polprevodnikih.
We are pleased to inform you that the Letter Fabrication of High-Temperature Quasi-Two-Dimensional Superconductors at the Interface of a Ferroelectric Ba0.8Sr0.2TiO3 Film and an Insulating Parent Compound of La2CuO4 by Dmitrii P. Pavlov, Rustem R. Zagidullin (Zavoisky Physical-Technical Institute, FRC KazanSC of RAS, Kazan, Russia), Vladimir M. Mukhortov (Southern Scientific Center of RAS, Rostov-on-Don, Russia), Viktor V. Kabanov (Department for Complex Matter, Jožef Stefan Institute, Ljubljana, Slovenia), Tadashi Adachi (Department of Engineering and Applied Sciences, Sophia University, Tokyo, Japan), Takayuki Kawamata, Yoji Koike (Department of Applied Physics, Tohoku University, Sendai, Japan), and Rinat F. Mamin (Zavoisky Physical-Technical Institute, FRC KazanSC of RAS, Kazan, Russia), published in Phys. Rev. Lett. 122, 237001 on 14 June 2019, has been highlighted by the editors as an Editors’ Suggestion. A highlighted Letter has additional significance, because only about one Letter in six is highlighted as a Suggestion due to its particular importance, innovation, and broad appeal.
The fabrication of the two-dimensional superconductors is the challenging problem of solid state physics. Recently it was reported the first observation of superconductivity in a heterostructure consisting of an insulating ferroelectric film (Ba0.8Sr0.2TiO3) grown on an insulating parent compound of La2CuO4. It was demonstrated that superconductivity is confined near the interface region. Application of a weak magnetic field perpendicular to the interface leads to the suppression of the superconductivity and the appearance of the finite resistance. The proposed concept promises ferroelectrically controlled interface superconductivity which offers the possibility of novel design of electronic devices.
S ponosom sporočamo, da si je članek z naslovom Fabrication of High-Temperature Quasi-Two-Dimensional Superconductors at the Interface of a Ferroelectric Ba0.8Sr0.2TiO3 Film and an Insulating Parent Compound of La2CuO4, katerega avtorji so Dmitrii P. Pavlov, Rustem R. Zagidullin (Zavoisky Physical-Technical Institute, FRC KazanSC of RAS, Kazan, Russia), Vladimir M. Mukhortov (Southern Scientific Center of RAS, Rostov-on-Don, Russia), Viktor V. Kabanov (Department for Complex Matter, Jožef Stefan Institute, Ljubljana, Slovenia), Tadashi Adachi (Department of Engineering and Applied Sciences, Sophia University, Tokyo, Japan), Takayuki Kawamata, Yoji Koike (Department of Applied Physics, Tohoku University, Sendai, Japan), and Rinat F. Mamin (Zavoisky Physical-Technical Institute, FRC KazanSC of RAS, Kazan, Russia), objavljen pa je bil 14. junija 2019 v Phys. Rev. Lett. 122, 237001, prislužil priporočilo po izboru urednikov.
Izdelava dvodimenzionalnih superprevodnikov je eden najpomembnejših in najzahtevnejših problemov fizike trdne snovi. Nedavno so poročali o prvem opažanju superprevodnosti v heterostrukturi tanke plasti feroelektričnega izolatorja (Ba0.8Sr0.2TiO3), pripravljenega na matični spojini La2CuO4. Pokazana je bila ujetost superprevodnosti v regijo stika. Šibko magnetno polje, usmerjeno pravokotno na stik, povzroči zatretje superprevodnosti in pojav končne prevodnosti. Predlagani koncept omogoča feroelektrično upravljanje superprevodnosti na stiku, kar odpira možnosti za razvoj novih oblik elektronskih naprav.
Jan Ravnik, Igor Vaskivskyi, Yaroslav Gerasimenko, Michele Diego, Jaka Vodeb, Viktor Kabanov and Dragan D. Mihailovic in ACS Applied Nano Materials, Vol. 2, Iss. 6, https://pubs.acs.org/doi/pdf/10.1021/acsanm.9b00644?rand=vc8zknx2, DOI: https://doi.org/10.1021/acsanm.9b00644
ACS Applied Nano Materials has published the paper Strain-Induced Metastable Topological Networks in Laser-Fabricated TaS2 Polytype Heterostructures for Nanoscale Devices which has been done at the complex matter department at JSI by Jan Ravnik, Igor Vaskivskyi, Yaroslav Gerasimenko, Michele Diego, Jaka Vodeb, Viktor Kabanov and Dragan D. Mihailović. The authors report on a laser induced single layer polytype transformation from 1T to 1Hpolytype in TaS2, where local heating and fast quench play a crucial role in formation of the new state. The polytype transformed layer shows a thermal phase transition between the stripe phase at low temperatures and a network of hexagonal vertices at high temperatures, as seen with STM. The vertices show interesting topological properties that reflect the symmetry of the underlying lattice.
Revija ACS Applied Nano Materials je objavila članek Strain-Induced Metastable Topological Networks in Laser-Fabricated TaS2 Polytype Heterostructures for Nanoscale Devices, ki so ga napisali Jan Ravnik, Igor Vaskivskyi, Yaroslav Gerasimenko, Michele Diego, Jaka Vodeb, Viktor Kabanov in Dragan D. Mihailović. Avtorji poročajo o politipni transformaciji ene plasti TaS2 iz 1T v 1H politip z uporabo ultrahitrih laserjev. Ključno vlogo pri transformaciji igra lokalno segrevanje in hitro ohlajanje. V transformirani plasti opazimo fazni prehod med črtasto fazo pri nizkih temperaturah in mrežo šest valentnih vozlišč pri visokih temperaturah. Vozlišča kažejo zanimive topološke lastnosti, ki odražajo simetrijo mreže.
Junior researcher Yevhenii Vaskivskyi was awarded SPO-2019 prize for the best poster presentation Investigation of metastable states in 1T-TaS2 by combining ultrafast spectroscopy with scanning tunneling microscopy in the 20th International Young Scientists Conference Optics and High Technology Material Science – SPO 2019, September 26 – 29, 2019, Kyiv, Ukraine.
Young researcher Jaka Vodeb was awarded SMEC Student Prize 2019 for an outstanding oral presentation Correlated Configurational States and a Quantum Charge Liquid in Layered Metallic Dichalcogenides in the international meeting Study of matter at extreme conditions (SMEC 2019), March 30 – April 06, 2019, Miami – East Caribbean – Miami.
Inštitut danes zaposluje skoraj tisoč ljudi, od tega jih je skoraj polovica doktorjev znanosti. Kapaciteta znanja na enem mestu je neprimerljiva s čimerkoli v državi in je spoštovanja vredna intelektualna sila tudi v širšem prostoru. Na leto objavijo v povprečju 15 patentov, so partner podjetij, kot so Domel, Droga, Balder, Kolektor, Danfoss, Cinkarna, Eti, Gen, Knauf, Cosylab, Xlab. Sodelujejo tudi z vsemi pomembnejšimi inštituti in univerzami v Evropi ter mnogimi v ZDA, Južni Koreji in na Japonskem. Direktor v pogovoru napol v šali pove, da niso nacionalni, temveč evropski inštitut.
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]