List of regular department seminars 2008

 

dr. Christoph Gadermaier, Josef Stefan Institute
Stark spectroscopy of excited state transitions in a conjugated polymer
10.01.2008

 

prof. Wilfried Schranz, Department of Physics, University of Vienna
Glass transition dynamics in nanostructured media studied by Dynamic Mechanical Analysis
29.01.2008 abstract

 

Miran Kambič, Faculty of Mathematics and Physics, University of Ljubljana
Merjenje nelinearnega lomnega kolicnika tekocih kristalov
07.02.2008

 

dr. Francesco Federiconi, Universita Politecnica delle Marche, Ancona, Italy
Effect of multivalent ions on solutions of guanosine 5-monophosphate
14.02.2008

 

Iztok Umek, absolvent Fakultete za matematiko in fiziko
Nanašanje tankih plasti s tehniko Langmuir - Blodgett
28.02.2008

 

prof. dr. Adrijan Levstik, Condensed Matter Physics Department
Magnetoelektricni relaksor in polaroni v ABO3 perovskitih
12.03.2008 abstract

 

Tadej Gabrič, absolvent Fakultete za matematiko in fiziko
Merjenje viskoznosti tekočin na osnovi dinamičnega sipanja svetlobe
20.03.2008

 

dr. Viktor Kabanov, Jožef Stefan institute
Magnetic quantum oscillations in doped antiferromagnetic insulators
10.04.2008

 

prof. Gregory Goltsman, Moscow State Pedagogical, Univ. Moscow, Russia
Terahertz range hot-electron bolometers, mixers, and infra-red photon counters based on non-equilibrium phenomena in ultrathin superconducting films
10.06.2008 abstract

 

Matija Milanič, PhD student, F7
Development and evaluation of pulsed Photothermal Radiometry for temperature profiling in biological tissues
12.06.2008 abstract

 

prof. dr. Dragan Mihailović, Jožef Stefan institute
Coherent control and recovery of the broken symmetry state in tri-tellurides: Studying the kinetics and internal structure of the "Order Parameter"
26.06.2008 abstract

 

Andrej Ošlak, Fakulteta za matematiko in fiziko
Časovna odvisnost magnetooptične Kerrove rotacije v fotovzbujenih monokristalih (La, Sr)MnO3 in tankem filmu la(Mn, Ni)O3 - diplomska naloga
11.09.2008

 

prof. dr. Guoquan Zhang, TEDA Applied Physics School, Nankai University, Kitajska Slow and fast lights with moving and stationary gratings
24.09.2008 abstract

 

Jerneja Milavec, Fakulteta za matematiko in fiziko
Dvodimenzionalne holografske mrežice v tekočekristalnih disperzijah - diplomska naloga
09.10.2008

 

Andrej Petelin , PhD Student, F7
Stress dependence of the nematic fluctuation relaxation in liquid crystal elastomers
23.10.2008

 

dr. Christoph Gadermaier, Jozef Stefan Institute, F7
Distinct pseudogap and superconducting state quasiparticle relaxation dynamics in near-optimally doped SmFeAsO0:8F0:2 single crystals
06.11.2008 abstract

 

dr. Goran Pichler, Institute of Physics, Zagreb, Croatia
Velocity selection by the accumulation effects of the optical frequency comb
13.11.2008

Blaž Kavčič, univ.dipl. fiz., Fakulteta za matematiko in fiziko
Izdelava in karakterizacija mikrofluidične celice z mikročrpalko
04.12.2008

 

Abstracts

prof. Wilfried Schranz, Department of Physics, University of Vienna
Glass transition dynamics in nanostructured media studied by Dynamic Mechanical Analysis
29.01.2008
Dynamics in confined surroundings appears in many fields, i.e. in chemistry, physics,
biology, material science, etc. Using a dynamic mechanical analyser (DMA) we
measured1 the low frequency elastic response of the glass formers salol and o-TP
confined in silica based nanoporous media of various pore sizes (2.5nm – 11 nm). In addition to the glass transition of the bulk material of salol we find a “second freezing process”, showing up in the real and imaginary parts of the complex elastic response. This is explained by a radial distribution of Vogel-Fulcher temperatures inside the pores, an assumption which is consistent with recent computer simulations2 showing an increase of the molecular relaxation time with decreasing distance from rough pore surfaces. The observed glass transition temperatures decrease with decreasing pore size.
The mechanism of the glass transition reduction will be discussed.
Acknowledgements: Support by the Austrian FWF (P19284-N20) and the University
of Vienna (IK 1022-N) is gratefully acknowledged.
[1] W. Schranz, M.R. Puica, H. Kabelka and A.V. Kityk. Europhys. Lett. 79, 36003
(2007)
[2] P. Scheidler, W. Kob and K. Binder, Europhys. Lett. 59, 701 (2002).

 

prof. dr. Adrijan Levstik, Condensed Matter Physics Department
Magnetoelektricni relaksor in polaroni v ABO3 perovskitih
12.03.2008
Substitucijsko neurejena trdna raztopina

0.8Pb(Fe1/2Nb1/2)O3-0.2Pb(Mg1/2W1/2)O3 (0.8PFN-0.2PMW) je magnetoelektrični relaksor, ki kaže široke in frekvenčno odvisne maksimume v električni in magnetni susceptibilnosti.
Vogel-Fulcherjeva zamrznitev električnega relaksorja je povezana z znatno magnetno anomalijo, ki demonstrira magnetoelektrično sklopitev.
0.3Pb(Fe1/2Nb1/2)O3-0.7Pb(Mg1/2W1/2)O3 (0.3PFN-0.7PMW) trdna raztopina je magnetni in električni relaksor, saj je magnetoelektrična sklopitev zanemarljiva.
Študirali smo električno prevodnost KTaO3;Li  in PbTiO3;La,Cu kot funkcijo frekvence in temperature. Realni del kompleksne prevodnosti kaže univerzalni dielektični odziv s¢ µ ns. Teoretska analiza temperaturne odvisnosti parametra s kaže, da je pri nizkih temperaturah tuneliranje majhnih polaronov dominantni mehanizem transporta naboja v ABO3 perovskitih.

 

prof. Gregory Goltsman, Moscow State Pedagogical, Univ. Moscow, Russia
Terahertz range hot-electron bolometers, mixers, and infra-red photon counters based on non-equilibrium phenomena in ultrathin superconducting films
10.06.2008
Presently non-equilibrium phenomena in ultrathin superconducting films is widely used in fast and sensitive detectors as diverse as bolometers for THz-frequency radiation and single-photon detectors operated in near infrared. Superconducting Hot-Electron Bolometers (HEB) can be operated in either a direct detection mode (as a bolometer) or in a heterodyne mode (as a mixer). At frequencies above 1 THz NbN HEB and HEB mixers outperform their counterparts such as superconductor-insulator-superconductor (SIS) tunnel junctions, and Schottky diodes.
HEB mixers exhibit a gain bandwidth of about 5-6 GHz and a noise temperature just several times higher than the quantum limit which makes them suitable for most sensitive heterodyne instruments being developed in the far IR region.
Being operated as direct detectors, HEBs demonstrate about 50 ps response time and a noise equivalent power (NEP) of 3x10-13 WHz-1/2 which allows one to reach the background limited performance (BLP). Despite of all the progress of THz detectors, single-photon counting at THz frequency is still an ambitious task. Meanwhile, ultrafast response and high sensitivity of HEB enable the detection of picosecond-long pulsed THz frequency signals with an energy in the pulse as low as several attojoules (10-18 J). Although a pulse with such energy contains many THz photons the total energy of such a pulse is still lower than the energy of a single photon of near infrared range which are traditionally used for data transmission. This is a fruitful idea to create low power data transmission lines for applications in which transmitter power is of vital concern.
Another approach to single-photon counting is a nanowire superconducting single-photon detector (SSPD). It is successfully realized in near infrared (1.3 mm): quantum efficiency of about 30% with the dark count level as low as 2x10-4 s-1, meanwhile a single-photon response was observed up to 60 THz frequency. At 1.6 K operation temperature SSPD exhibited quantum efficiency of ~1% at 5.6 mm.

 

Matija Milanič, PhD student
Development and evaluation of pulsed Photothermal Radiometry for temperature profiling in biological tissues
12.06.2008
In this dissertation we present the development and evaluation of pulsed photothermal radiometric (PPTR) temperature depth profiling in biological tissues. Motivation for this work is incomplete success in laser therapy of port-wine stain birthmarks (PWS). PPTR technique, which utilizes infrared (IR) emission from materials following pulsed laser exposure, can provide information about PWS depth and epidermal thickness, both required to optimize the therapy.
We develop three original reconstruction codes, which are based on truncated singular value decomposition (TSVD), conjugated gradient (CG) and the υ-method, respectively. All codes involve a non-negativity constraint to the sought temperature vector and automatic regularization. When applied to different test objects, all three codes produce reconstruction results, which are much more accurate than the results published in earlier PPTR studies.
Calibration of PPTR signals and the error due to linearization of PPTR signal expression is analyzed. We find that the linearization error depends on temperature amplitude, absorber depth, acquisition time, and the spectral acquisition band.
PPTR measurements commonly employ broad-band signal acquisition to increase signal-to-noise ratio (SNR), but all reported studies use a fixed effective IR absorption coefficient (μeff). We show that in samples with large spectral variation of μ(λ) in mid-IR, which includes most biological tissues, selection of μeff strongly affects the accuracy of the results. A novel analytical approach to determination of optimal μeff from spectral properties of the sample and radiation detector is presented. In extensive numerical simulation of PPTR temperature profiling in human skin using different IR detectors and spectral bands, we demonstrate that our approach predicts viable values of μeff.
The influence of spectral filtering on the accuracy of temperature profiles is studied by a systematic experimental comparison of PPTR temperature profiling on agar tissue phantoms utilizing the customary spectral band of the InSb detector (λ = 3.0-5.6 μm) and a narrowed acquisition spectral band (λ = 4.5-5.6 μm). To support our experimental observations, we present also a detailed numerical simulation of the experimental procedure utilizing spectral acquisition bands with the lower wavelength limit varied between λl = 3.0 μm and 5.0 μm and the upper wavelength limit fixed at 5.6 μm. The experimental and numerical simulation results indicate that spectral filtering reduces reconstruction error and broadening of temperature profiles, especially for shallower and more complex absorbing structures. Analogously, we performed the experiment and numerical simulation involving gelatin tissue phantoms, which more closely resemble human skin. Again, we find that a suitable spectral filtering (λl = 4.0–4.5 μm) is beneficial, despite the associated reduction of SNR.
We determine experimentally the accuracy of PPTR temperature depth profiling in custom tissue phantoms composed of agar gel layers separated by single very thin absorbing layers. The laser-induced temperature depth profiles, reconstructed from measured PPTR signals, correlate very well with absorber depths determined by magnetic resonance imaging and optical microscopy. We observe significant broadening and attenuation of reconstructed profiles with increasing depth of absorbing layer. Corresponding numerical analysis indicates that the broadening equals to ~13% of the absorber depth. Using a numerical simulation we also analyzed, how the accuracy of reconstructed temperature profiles depends on sampling frequency.

 

prof. dr. Dragan Mihailović, Jožef Stefan institute
Coherent control and recovery of the broken symmetry state in tri-tellurides: Studying the kinetics and internal structure of the "Order Parameter"
26.06.2008
Metallic rare-earth tri-tellurides represent an excellent model system for studying phase-interplay and electronic ordering in correlated electron materials. They have recently been discovered to display a variety of electronically-driven charge ordering co gurations: At high temperatures they undergo a Fermi-surface-nesting driven transition to a one-dimensional charge density wave state in a beautiful mean- eld like fashion. Chemical pressure however, exerted by RE element substitution appears to drive the system rst into an inhomogeneous electronic state, and eventually in a two- dimensionally ordered state at low temperatures, whose nature is presently the subject of intense debate.
In this work, we report on the discovery of a new metastable electronic state which forms in TbTe3 when illuminated with high-intensity femtosecond laser pulses. Using a newly developed multi-pulse sequence we are able to study the destruction and recovery of the order parameter by monitoring single particle and collective excitations of the coherently excited system in real time.
These appear to have very di erent recovery kinetics showing for the rst time that local order forms on a much shorter timescale than long-range order. The observation cannot be explained in terms of present concepts of equilibrium Ginzburg-Landau theory and has a fundamental signi cance in our understanding the kinetics of the formation of broken-symmetry states.

 

prof. dr. Guoquan Zhang, TEDA Applied Physics School, Nankai University, Kitajska
24.09.2008
Slow and fast lights with moving and stationary gratings
We discuss the slow and fast lights in solids at room temperature based on the dispersive phase coupling through moving gratings both theoretically and experimentally. Amplified ultraslow lights with group velocities less than 1 m/s and fast lights with negative group velocities can be achieved. In principle, such phasecoupling-induced slow and fast lights can be observed in any nonlinear wave mixing process with a dispersive phase coupling effect. The slow and fast lights in the stationary gratings are also discussed. The dispersion slope of the gratings can be modified and enhanced significantly by using structured gratings such as stratified and phase-shifted gratings. Therefore, it is possible to effectively control the group velocity of lights while keeping high light transparency with an appropriately designed structured grating. Issues such as the pulse broadening effect and the pulse distortion are also addressed. The slow and fast light has many important potential applications such as optical delay lines and optical buffers.

 

dr. Christoph Gadermaier, Jozef Stefan Institute, F7
The recent discovery of high-temperature superconductivity in iron-based pnictides has attracted a great deal of attention, partly because of their high critical temperatures, but more fundamentally because they appear to have some similarities and important differences compared to cuprate superconductors, which raises the question of the superconductivity mechanism.
Particularly pertinent are the questions regarding the existence of a pseudogap precursor state, which is believed by many to be essential for high-temperature superconductivity and is attributable to pre-formed pair formation above Tc.
Femtosecond spectroscopy has been instrumental in elucidating the nature of the electronic excitations in superconductors, since it allows to distinguish different components by their lifetimes. Moreover, the relaxation kinetics can yield valuable information on the mechanism for superconductivity. We investigate the quasiparticle relaxation and low-energy electronic structure in a near-optimally doped pnictide superconductor with Tc = 49.5 K.
Multiple relaxation processes are evident, with distinct superconducting state quasiparticle recombination dynamics exhibiting a T-dependent superconducting (SC) gap, and a clear "pseudogap"
(PG)-like feature with an onset around 200K indicating the existence of temperature-independent gap of magnitude DPG = 61 ± 9 meV above Tc. Both the SC and PG components show saturation as a function of fluence, where the threshold fluences scale approximately as (DPG/DSC)2, consistent with a BCS-like condensation energy for both gaps.