Peter Topolovšek, 22.12.2016 at 14:00, Seminar room F7

Ultrathin charge extraction layers in perovskite solar cells

Peter Topolovšek
Italian Institute of Technology, Milano, Italy

New Year’s dinner 2016

Daniele Vella, 15.12.2016 at 14:00, Seminar room F7

Photophysics of atomically thin MoS2 devices

Daniele Vella
Jožef Stefan International Postgraduate School, Ljubljana

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. []

dr. Tibor Tóth Katona, 10.11.2016 at 13:00, Seminar room F7

Biasing a ferronematic – a new way to detect weak magnetic field

dr. Tibor Tóth Katona
Hungarian Academy of Sciences, Budapest, Hungary

dr. Fabio Miletto Granozio, 27.09.2016 at 13:00, Seminar room F7

Nonvolatile resistive switches induced by field effect and light in 2DEGs at oxide interfaces

dr. Fabio Miletto Granozio
CNR-SPIN, Naples, Italy

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]