SMN Theory Seminar

The goal of the seminars is to bring together participants of the Centre for Materials Science and Nanotechnology involved in theory, simulations and experiments, to create a proper infrastructure and initiate joint projects.

Information about the seminars is distributed via funmat-theory mailinglist. To post a message to all the list members, send email to funmat-theory[at]fys.uio.no, to join the list or unsubscribe, visit the mailinglist page or send email to dansh[at]fys.uio.no.

Time
Place
Author, Title
Abstract
22.06.2007

FV102
13:15
Xiaoxing Xi
Department of Physics and Department of Materials Science and Engineering
Penn State University, USA


Properties of MgB2 Thin Films in Magnetic Field
The technique of hybrid physical-chemical vapor deposition (HPCVD) has been very successful in depositing very clean epitaxial MgB2 thin films with low residual resistivity (much smaller than 1 microOhm*cm), as well as dirty films showing very high upper critical field. One demonstration of the cleanness of the pure MgB2 films is the absence of the dendritic flux penetration due to the reduced electron scattering. In the normal state, a large anisotropic magnetoresistance is observed, which changes its angular dependence dramatically as the temperature increased. This is attributed to the multiple bands with different scattering rates, which vary differently with temperature. For high magnetic field applications, carbon is doped into the MgB2 films, which modifies the interband and intraband scattering and leads to extraordinary high Hc2 of over 60 T. Unique properties of flux pinning and vortex states in this two-band superconductor will be discussed.

Links: Web-page of Xiaoxing Xi
10.05.2007

Store Fy
14:30
Andrei Varlamov
Italian National Institute for Condensed Matter Physics


Physics in the Kitchen
The lecture is devoted to the description of the physical aspects of the processes of the preparation of tea, coffee, wine, spaghetti etc. The physical aspects of filtration processes, heat propagation in a piece of meat, interaction of electromagnetic fields with water molecules, NMR characterization of wine quality, the origin of the kettle noise before boiling and dependence of its frequency on temperature, physical reasons for the advantage of espresso coffee with respect to other methods of preparation,...

Links:
Homepage of Andrei Varlamov
Book "The Wonders of Physics"
05.02.2007

FV102
13:15
Yuli V. Nazarov
Kavli Institute of Nanoscience Delft
Delft University of Technology


Exciting half-integer charges in a quantum point contact
We study a voltage-driven quantum point contact (QPC) strongly coupled to a qubit. We predict pronounced observable features in the QPC current that can be interpreted in terms of half-integer charge transfers. Our analysis is based on the Keldysh generating functional approach and contains general results, valid for all coherent conductors.

Links: cond-mat/0701017
19.01.2007

FV102
14:15
Christian Kjølseth and Harald Fjeld
SMN & Department of Chemistry, UiO


Quantitative MO imaging and MO indicator films prepared by MOD method
Our best oxygen ion and proton conducting oxides tend to exhibit high resistivities across grain boundaries, commonly assigned to charged boundary cores and resulting space charge layers in which the charge carrying ionic defect is depleted. The phenomenon is a problem for practical application of solid electrolytes, and many features are not well understood theoretically. In the talk we present theoretical models, experimental methods and results, interpretations, and current challenges.

Links: Group for Solid-State Electrochemistry
12.10.2006

FV102
12:15
Takayuki Ishibashi
Institute of Symbiotic Science and Technology Tokyo University of Agriculture and Technology, Japan


Quantitative MO imaging and MO indicator films prepared by MOD method
Polarization modulation method widely used to measure magneto-optical (MO) spectra of magnetic materials will be reviewed. Utilizing this method allows us to obtain quantitative high-resolution MO images visualizing magnetic field spatial distribution. I will give a detailed description of the quantitative MO microscope and will discuss the properties of MO indicator films. The indicator films are fabricated by a very simple thin-film preparation technique, metal-organic decomposition (MOD) method.
12.09.2006

FV102
16:00
Rinke J. Wijngaarden
Department of Physics and Astronomy, Free University, Amsterdam, The Netherlands


The dynamics of patterns in type-I superconductors
We report on patterns and their dynamics as observed in magneto-optical experiments on type-I superconductors. We observe: (1) A stripe-spot transition that is hysteretic, leading to two modes of stripe formation: slow continuous growth and avalanche growth. (2) A wiggling instability, similar to that in ferrimagnetic garnet films. (3) A zigzag instability when a pattern of parallel lines is rotated through a sample with low pinning. (4) Breaking and reconnection of stripes as such a pattern is rotated in a sample with strong pinning. (5) Random telegraph behavior close to the depinning of such pattern in the presence of a constant driving force. The observed patterns consist of superconducting and normal domains of macroscopic size in thin lamina of type-I superconductors and are observed by an advanced magneto-optical technique. The patterns are manipulated by changing the applied magnetic field vector or by applying an electrical transport current.

Collaboration: Mariela Menghini

Links: Phys. Rev. B 72, 172503 (2005)
08.09.2006

FV102
13:15
Alexey V. Pan
Institute for Superconducting and Electronic Materials, University of Wollongong, Australia


The role of multilayering in the significant enhancement of structural and superconducting properties in high-Tc films
By introducing a multilayered structure to YBa2Cu3O7 (YBCO) films, the significant enhancement of superconducting and structural properties in the obtained structure has been achieved. The critical current density (Jc) is significantly larger (by a factor of up to 3) than in monolayer YBCO films with an arbitrary thickness. This remarkable result can be of a very high technological importance for overcoming the well-known degradation of Jc as the function of the film thickness. The role of the interfaces for the Jc enhancement is underlined and attributed to a larger filling factor of the multilayer (denser structure), better grain alignment and additional formation of dislocations at the interfaces between the layers. The role of “multilayered” Ag-doping for the Jc-enhancement is also discussed.

Collaboration: Serhiy V. Pysarenko, Marie Roussel, and Shi X. Dou

Links: ISEM, University of Wollongong
07.09.2006

FV102
13:15
Fabiano Colauto
Departamento de Fisica, Universidade Federal de Sao Carlos, Brazil


Fluctuation on the magnetic response of superconducting thin films of Nb and MgB2
The main results presented:
M(H) curves show jumps for MgB2 films made in Pohang and Nb films made in Cambridge, but do not for MgB2 made in PennState.
AC-Susceptibility: (i) Fluctuates for measurements made in MPMS, and depends on threshold values of h, f and T. (ii) Does not fluctuate for measurements made in PPMS. (iii) When fluctuates, it depends on thickness of the sample. (iv)AC-field is able to trigger avalanches of vortices when its excitation amplitude is increasing.

Collaboration: Wilson A. Ortiz
15.05.2006

FV102
13:15
Alexander Ulyashin
Center for Material Science and Nanotechnology, UiO


Renewable hydrogen and solar cells: expectations and problems
Hydrogen as energy carrier, primary derived from water, can address issues of sustainability, environmental emissions, and energy security. An analysis of renewable hydrogen production pathways using solar cells and related devices will be done. Some examples of solar cell related activity at the Center for Material Science and Nanotechnology will be presented. A possible extension of such activity will be discussed.
11.05.2006

FV102
13:15
Alexander Zyuzin
Ioffe Physico-Technical Institute, St.Petersburg, Russia


Transport in Granular Metals
A granular metal is an array of metallic nano-particles embedded into an insulating matrix. Granular conductors combine the unique properties of individual nanocrystals and the collective properties of coupled nanocrystals thus opening a new route for novel electronic applications. A review of electronic transport properties of granular metals will be given covering both sides of the metal-insulator transition. The Hall effect in the regime of strong Coloumb blockade will also be discussed.
20.04.2006

FV102
11:15
Simen Braeck
PGP, UiO


Spontaneous dissipation of elastic energy by self-localizing thermal runaway
Melts are often observed in the geological record as pseudotachylytes in fault zones. Yet, apart from the fact that dissipation of energy during deformation leads to heating above the melting temperature of rocks, little is understood of the deformation mechanism behind the melting process. A central question is whether the rock-melting deformation is brittle or ductile, that is, whether the dissipation of energy is due to frictional or viscous forces. Is it at all possible to melt rocks by viscous dissipation during ductile deformations? In this study we address the problem of a ductile deformation process with account of viscous dissipation from a theoretical viewpoint. Our model consists of a viscoelastic slab which is subjected to simple shear and is in ideal thermal contact with the surroundings. The heat conduction and momentum equations, coupled through temperature dependent viscosity, are solved both analytically and numerically. It is demonstrated analytically that if a generalized Brinkman number exceeds a critical value, the thermal-mechanical interaction is inherently unstable. The critical number is a function of a characteristic dimensionless relaxation time. Hence, the stability of the solution is controlled by two dimensionless variables only. Numerical solution of the full non-linear problem verifies the analytical findings. The results show that if the Brinkman number is larger than the critical value, the stored elastic energy spontaneously dissipates as heat in a self-localizing thermal runaway. The deformation process thus terminates in highly localized shear zones characterized by extreme peak temperatures. Based on our findings we propose thermal runaway as a possible mechanism for rapid ductile deformation and generation of melts.

Collaboration: Yuri Podladchikov, Sergei Medvedev
23.03.2006

FV102
13:15
Oyvind Haugen
AMCS, Dept. of Physics, UiO


High resolution thermal imaging of hotspots in superconducting films
Thermal imaging of hotspots in bridge structures of YBaCuO thin films is presented with micrometer resolution. Hotspots formed by passing transport currents are observed using a method based on the temperature dependence in the photoluminescence of a polymer thin film deposited on the superconductor. It is shown that, e.g., passing a current of 10 mA in a 9 micrometer wide bridge the temperature distribution over the bridge and in its vicinity can be observed with a resolution better than 0.1 K. The local temperature rise has a maximum of 4.5 K. In a larger superconducting network several hotspots are observed in different branches of the network depending on the magnitude of the applied current. In an external magnetic field the hotspots are formed at lower transport current and at several seconds delay. It is also found that states with hotspots show hysteretic or bistable behavior, in agreement with earlier theoretical predictions.
01.03.2006

FV102
14:15
Sanyalak Niratisairak (Pop)
AMCS, Dept. of Physics, UiO


Introduction to Magnetic Garnets thin films
The talk is devoted to thin films of the magnetic Garnet-structure. This kind of films is used for magneto-optical imaging and for optical isolation. The general overview about structures and optical properties will be given. The talk will cover YIG (Ytterium Iron Garnet) films, FGF (Ferrite Garnet Films) as well as description of their growth techniques (liquid phase epitaxy and pulsed laser deposition). The most important optical properties of the garnets to be discussed include the Faraday rotation, reflectance, transmittance and absorption spectra.
16.02.2006

FV102
13:15
Daniel Shantsev
AMCS, Dept. of Physics, UiO


Small thermal avalanches of vortices in superconductors
The critical state of type-II superconductors is a metastable state characterized by a certain gradient of flux density. In many respects it is similar to a sandpile, where the role of sand grains is played by vortices that get pinned by microscopic defects, which prevents them from reaching the equilibrium uniform distribution. If the flux density gradient exceeds the critical value, the vortices are set in motion and continue moving until the critical gradient is restored. We study small avalanches of vortices occurring when the external magnetic field is increased. The avalanches in superconducting films are visualized using magneto-optical imaging and their size distribution is found to have a peak. We propose a model of vortex avalanches that takes into account local heating due to energy released by moving vortices.

Links:
Phys. Rev. B 72, 024541 (2005)
Vortex avalanches in superconductors
09.02.2006

FV102
13:15
Joakim Bergli
AMCS, Dept. of Physics, UiO


Spin echo in an arbitrary field: How to undo an unknown rotation
Spin echo is a well known technique in Nuclear Magnetic Resonance (NMR). By applying a control pulse, one can make a nuclear spin return to its starting point, even if the rate of precession is unknown. However, the method is limited by the fact that you have to know the direction of the external magnetic field, but not its magnitude. Motivated by recent experiments with electron spins in quantum dots, were the nuclear field provides magnetic field that is unknown both in magnitude and direction we have studied the possibility of making echo experiments in this situation.

Collaboration: Leonid Glazman, University of Minnesota

Links: Joakim Bergli's homepage
05.10.2005

FV102
13:15
Lasse Vines
Physical Electronics, Dept. of Physics, UiO


Ion induced electronic nanochannels in Si
During penetration of an ion into a solid it induces electrically active point defects. It has been proposed that for implantation of heavy ions in semiconductors, e.g. Si, electrically active point defects are generated predominantly along the ion trajectory. This can result in a local modification of the Fermi-level, creating “electronic nanochannels” along the ion track. The technological advantage of such nanochannels is that, unlike the “traditional” nanowires, the ion induced nanochannels are readily embedded into the Si crystal by conventional ion implantation. However, no observation of such nanochannels has been reported in elemental semiconductors by conventional techniques measuring structurally (transmission electron microscopy) or topographically (atomic force microscopy) related techniques.

In an ongoing project at the physical electronics group in MiNa lab, the ion induced nanochannels are studied using scanning capacitance microscopy (SCM) and deep level transient spectroscopy (DLTS). In particular, SCM studies show a pattern of increased charge concentration in regions of 150-600nm after 3 and 5 MeV Au2+ implantations in Si. The pattern shows a distinct dose and temperature dependence, in accordance with measured energy levels in the forbidden band gap, and has been ascribed the ion induced nanochannels.

Links: Semiconductror nanostructure research at Microtechnology Lab

09.05.2005

FV102
12:15
Kevin Bassler
Dept. of Physics, University of Houston, TX, USA


The Self-Organized Evolution of Canalization
Canalization, which is a form of robustness in complex networks, has long been recognized to be an important property of evolving biological systems. The mechanism for the evolution of canalization, however, remains controversial. In this colloquium, a novel mechanism that produces canalization by selecting for diversity of expression patterns based on a frustrated competition between nodes (genes) will be presented. It will be shown that the mechanism produces a canalized, critical steady state that retains a capacity for further evolution in a simple model of a genetic regulatory network. Recent results that suggest small networks may evolve in a fundamentally different way than large networks will also be presented.

Links: Phys. Rev. Lett. 93, 038101 (2004)
11.11.2004

FV102
13:15
Vitaliy V. Yurchenko
Department of Physics, University of Oslo
&
Institute of Physics AS CR, Prague


Interaction of Vortices with Artificial Defects in Type-II Superconductors
In the talk I will highlight the most important features of large cylinder cavities, i.e. antidots, in superconductors in the context of their interaction with vortex medium. Technology of antidots production, energy of interaction between the holes and vortices (theory and effective experimental values), principles of flux localization in perforated samples, possible mechanisms of vortex guidance along the arrays of antidots are the subjects to be discussed.

Collaboration:
M. Jirsa, A. Stupakov, Institute of Physics AS CR, Prague
R. Wordenweber, Research Centre of Julich, Germany
03.11.2004

FV102
13:15
Joern Inge Vestgaarden
Department of Physics, University of Oslo


Phonon-vortex interaction in superfluid helium II
- Description of the two-fluid model, quantized vortices, and phonon excitations in helium II.
- Outline of how phonons are scattered on a vortex, and how the transverse force on the quantized vortex from the phonons can be found.
- Finally, I present a controversy about the existence of the force based on other results.

Collaboration:
Jon Magne Leinaas, Department of Physics, UiO
21.10.2004

FV102
13:15
Oleg Fefelov
Department of Physics, University of Oslo
&
State Research Center of Virology and Biotechnology VECTOR, Kolcovo, Russia


Interaction of biological cells with nonuniform AC electric field
The method of cell destruction by AC electric field and experimental data on investigation of virus-cell interaction by dielectrophoresis (motion of cells in inhomogeneous AC electric field) will be introduced. The following virus-cell pairs were studied: tick-borne encephalitis-goose erythrocytes, influenza virus-chicken erythrocytes, virus-Vero cells. The performed studies have revealed that the amplitude-frequency characteristics of the erythrocyte polarization and the equilibrium frequency change considerably at the stage of the virus adsorption to the erythrocyte membrane. The higher concentration of viruses in cell suspension, the more considerable changes occur. At the same time in case of low virus concentration the cells with characteristics similar to that of intact ones were observed along with infected cells.

Links: State Research Center of Virology and Biotechnology VECTOR
20.10.2004

FV102
13:15
Sanyalak Niratisairak
Department of Physics, University of Oslo
&
Linkoping University, Sweden


Ni/V multilayer X-ray mirrors for the Water Window
Carbon and nitrogen have a high absorption, compared to oxygen and hydrogen, in the region of soft X-ray wavelengths. One can take advantage of this to study biological specimens using a microscope operating in this wavelength range. The region is called the water window, 2.4 - 4.4 nm. Nickel (Ni) and Vanadium (V) multilayers have been designed to obtain the highest reflectivity for the normal incident of the X-ray wavelength at 2.48 nm. The optimal parameters, which are predicted to give highest reflectivity for Ni/V at 2.48 nm, was determined to be L=1.242 nm and G=0.411. Ni/V multilayer films were deposited on Silicon (100) substrates by magnetron D.C. sputtering with Argon as the sputtering gas. Furthermore, hard X-rays with wavelength 0.154 nm were used in the structural characterisation and compared with computer simulations. It was found that a substrate bias of -33 volts was required during deposition in order to achieve the highest peak reflectivity and the lowest diffuse scattering.

Collaboration:
Fredrik Eriksson, Linkoping University, Sweden
14.10.2004

FV102
13:15
Dmitry Denisov
Department of Physics, University of Oslo
&
Ioffe Physico-Technical Institute, St.Petersburg, Russia


Negative-U centers model. Similarity in the superconducting properties of chalcogenides and cuprate oxides
The idea of Anderson pairs has been put forward for explanation of many extraordinary properties of chalcogenides glassy semiconductors. Recent decades made obvious that these pairs localized on the centers with negative effective correlation energy (negative-U centers) really exist in chalcogenides. If the concentration of negative-U centers is enough to create the pair band states, this can lead to superconductivity because Anderson pairs are Bose particles. Several properties of chalcogenides and HTSC cuprates in normal and superconducting states are similar for these groups of materials and can be naturally explained in the frame of negative U centers model of superconductivity.

Collaboration:
Konstantin D. Tsendin, Boris P. Popov, Ioffe Physico-Technical Institute, St.Petersburg, Russia

Links: Supercond. Sci. Techn. 16 80, 2003
01.09.2004

FV102
13:15
Joakim Bergli
Department of Physics, University of Oslo
&
Princeton University, USA


Dephasing of superconducting qubits
I will discuss recent experiments on superconducting qubits. In particular I will explain how to understand the change in dephasing and decoherence rates as the working point of the qubit is changed. Also I will discuss the effect of low frequency noise that gives a non-Gaussian distribution of the phase and contrast it to the conventional Gaussian description.


Links: Phys. Rev. Lett. 87, 246601 (2001).
26.05.2004

FV102
10:15
Valerii A. Yampol'skii
A. Ya. Usikov Institute for Radiophysics and Electronics, Kharkov, Ukraine


Symbolic stochastic dynamical systems viewed as binary N-step Markov chains and the Zipf law
A theory of systems with long-range correlations based on the consideration of binary N-step Markov chains is developed. In the model, the conditional probability that the ith symbol in the chain equals zero or unity is a linear function of the number of unities among the preceding N symbols. The correlation and distribution functions as well as the variance of the number of symbols in the words of arbitrary length L are obtained analytically and numerically. A self-similarity of the studied stochastic process is revealed and the similarity group transformation of the chain parameters is presented. The diffusion Fokker-Planck equation governing the distribution function of the L words is explored. If the persistent correlations are not extremely strong, the distribution function is shown to be the Gaussian with the variance being nonlinearly dependent on L.
The binary many-step Markov chain with the step-like memory function is considered as a model for the analysis of rank distributions of words in stochastic symbolic dynamical systems. We prove that the envelope curve for this distribution obeys the power law with the exponent of the order of unity in the case of rather strong persistent correlations. The Zipf law is shown to be valid for the rank distribution of words with lengths about and shorter than the correlation length in the Markov sequence. A self-similarity in the rank distribution with respect to the decimation procedure is observed. The applicability of the developed theory to the coarse-grained written and DNA texts is discussed.

Collaboration:
O. V. Usatenko, Institute for Radiophysics and Electronics, Kharkov, Ukraine
K. E. Kechedzhy and S. S. Mel'nyk, Kharkov National University, Kharkov, Ukraine

Links: Phys. Rev. Lett. 90, 110601 (2003), Phys. Rev. E 68, 061107 (2003).
10.03.2004

FV102
13:15
Joachim Grillenberger
Physical electronics, Physics Dept, University of Oslo


Impurities in SiC
Silicon Carbide (SiC) is a wide band gap semiconductor attracting great interest because it is expected to replace Si or GaAs in high-temperature, high-power, and high-frequency devices. Intrinsic defects and impurities in the crystal may create energy levels in the band gap and are therefore crucial for the functionality of any semiconductor device. Consequently, it is of high technological interest to establish a correlation between band gap states and a certain defect structure. Within extensive studies in the last years, the electronic properties of transition metals in SiC were revealed by means of radiotracer Deep Level Spectroscopy (DLTS-RI). In each case a definite chemical assignment of band gap states was established and in some cases the defect structure could be clarified. The radiotracer DLTS experiments have lead to the most extensive and reliable dataset about impurity atoms in SiC available.

Links: Physical Electronics at UiO
25.02.2004

FV102
13:15
Yuri Podladchikov
Physics of Geological Processes, University of Oslo


Reverse Modelling of Geological Phenomena
The talk is about mathematical and numerical aspects of geological "restorations". Geological past is reconstructed using present day observations. It is an inverse problem known to be ill posed. Several cases (gravity, seismic) are treated in classical geophysics. Numerous new modeling directions became feasible due to growth of computer power. Most of these new modeling attempts are "forward" in time because they deal with irreversible processes. However, geological structures often formed by instabilities. Amazingly, instabilities are often easier to simulate inverse (reverse) in time. Case studies on inversion of several instabilities will be reported (folding, Rayleigh-Taylor).

Links: Yuri Podladchikov homepage
04.02.2004

FV102
13:15
Nicholas R. Moloney
Imperial College, London, United Kingdom


Universal Fluctuations in Critical Phenomena
Systems displaying critical phenomena are often distinguished by the "universality classes" they belong to. Traditionally, the assignment of a universality class is made on the basis of the critical exponents for a system at the critical point. Alternatively, the probability distribution for the order parameter can be used instead. I will give examples of this latter approach in both equilibrium and non-equilibrium systems.

Collaboration:
Z.Racz, G.Gyorgyi, ELTE, Budapest

Links: Nicholas R. Moloney homepage
17.12.2003

Store fysiske
13:00
Alexei Abrikosov
Argonne National Laboratory, USA


Superconductivity: History and present state
The Royal Swedish Academy of Sciences has decided to award the Nobel Prize in Physics for 2003 "for pioneering contributions to the theory of superconductors and superfluids" jointly to Alexei A. Abrikosov, Vitaly L. Ginzburg, and Anthony J. Leggett.

Landau's pupil, Alexei Abrikosov, realised almost immediately that Ginzburg and Landau's theory can also describe those superconductors (type II) that can coexist with strong magnetic fields. According to Abrikosov's theory this occurs because the superconductor allows the magnetic field to enter through vortices in the electron superfluid. These vortices can form regular structures, Abrikosov lattices, but disordered structures can also occur.


Links: Press Release and Illustrated presentation from The Royal Swedish Academy of Sciences; Gallery of Abrikosov Lattices in Superconductors
03.12.2003

FV102
13:15
Yuri Galperin
AMCS group, Department of Physics, University of Oslo


Quantum bit in dynamic disorder
With the growing efforts in isolating solid-state qubits from external decoherence sources, the material-inherent sources of noise start to play crucial role. One representative example is electron traps in the device material or substrate. Electrons can tunnel or hop between a charged and an empty trap, or between a trap and a gate electrode. A single trap typically produces telegraph noise and can hence be modeled as a bistable fluctuator. Since the distribution of hopping rates is exponentially broad, many traps produce flicker-noise with spectrum close to 1/f. Here we develop a theory of decoherence of a qubit in the environment consisting of two-state fluctuators, which experience transitions between their states induced by interaction with thermal bath. Due to interaction with the qubit the fluctuators produce 1/f-noise in the qubit's eigenfrequency. We calculate the results of qubit manipulations - free induction and echo signals - in such environment. The main problem is that in many important cases the relevant random process is both non-Markovian and non-Gaussian. Consequently the results in general cannot be represented by pair correlation function of the qubit eigenfrequency fluctuations. Our calculations are based on analysis of the density matrix of the qubit using methods developed for stochastic differential equations. The proper generating functional is then averaged over different fluctuators using the so-called Holtsmark procedure. The analytical results are compared with simulations allowing checking accuracy of the averaging procedure and evaluating mesoscopic fluctuations. The results allow understanding some observed features of the echo decay in Josephson qubits.

Collaboration:
Boris Altshuler, Princeton University
Daniel Shantsev, University of Oslo

Links: Boris Altshuler homepage
26.11.2003

FV102
13:15
Melissa Bailey
AMCS group, Department of Physics, University of Oslo
&
University of Arizona


Near-Field Modeling
Near-Field systems can produce sub-wavelength diameter optical spots used for data recording and reading. The purpose of this talk is to introduce XFDTD, a finite difference time domain program used to calculate electro-magnetic phenomena in Near-Field Optical Data Storage. The talk will discuss several geometries studied using XFDTD and their applications to Data Storage.

Links: Optical Data Storage Center, University of Arizona
12.11.2003

FV102
13:15
Øyvind Haugen
AMCS group, Department of Physics, University of Oslo
&
SINTEF


Downhole instrumentation in oil wells
Part 1: Signal analysis and sensors for measuring flow rates of oil, water and gas in an oil producing well.

Part 2: Acoustic sensor for flow rates, gauge level sensor for water separation in the well, wireless communication in oil wells and ultra sound imaging in oil wells.

Part 3: Other projects like estimating airplane brake distance on icy Norwegian air fields

Part 4: New project: thermo-optical imaging of superconductors.

Links: SINTEF

21.05.2003

FV146C
12:15
Thomas Kuehn
University of Jyvaskyla, Finland


Heat transport along mesoscopic dielectric bridges
Thermodynamic properties, including heat transport, of thin, isotropic free standing membranes, have been studied. The dispersion relation in this geometry is different from the disperion relation of an infinite solid and, due to that, thermodynamics differ as well.

Further we cut the membrane into a narrow bridge. We are especially interested in the heat transport along the bridge. Due to the small width of the bridge, the scattering on its edges may become the dominant scattering process and the mean free path of the phonons should become a constant that depends on the width of the bridge

Collaboration:
Dragos-Victor Anghel, University of Oslo

Links: Department of Physics, University of Jyvaskyla, Dragos Anghel

13.05.2003

FV146C
12:15
Lev N. Bulaevskii
Theoretical Division, Los Alamos National Laboratory, USA


Tunneling measurement of quantum spin oscillations
The problem of tunneling between two leads via a localized spin 1/2 or any other microscopic system (e.g., a quantum dot) which can be modeled by a two-level Hamiltonian is considered.

We assume that a constant magnetic field acts on the spin, that electrons in the leads are in a voltage driven thermal equilibrium and that the tunneling electrons are coupled to the spin through exchange interaction. Using the non-equilibrium Keldysh formalism we find the dependence of the spin-spin and current-current correlation functions on the applied voltage between leads, temperature, and on the degree and orientation of spin polarization of the electrons in the right and left leads. We found conditions under which both the spin-spin and current-current correlation functions exhibit a peak at the Larmor frequency corresponding to the effective magnetic field acting upon the spin.

We compare our results of a full quantum-mechanical treatment of the tunneling-via-spin model with those previously obtained in the quasi-classical approach, and discuss the experimental results observed using STM dynamic probes of the localized spin.

Collaboration:
M. Hruska, G. Ortiz, Los Alamos National Laboratory

Links: cond-mat/0212049

02.04.2003

FV146C
12:15
Ponniah Ravindran
Department of Chemistry, University of Oslo


Exited state properties from density functional calculation
The density functional theory (DFT) is originally deviced to describe the ground state properties of materials. But, recent years it has been shown that it can be used to study the excited state properties of the materials also. We have recently calcualted the linear optical properties such as optical dielectric functions, reflectivity, absorption coefficient, refractive index, electron energy loss function etc. for a series of LaXO3 (X=Sc-Cu) type oxides. We found that gradient corrected DFT describe correctly the semiconducting behavior of ionic insulators LaScO3 and CT insulators such as LaCrO3, LaFeO3 and LaMnO3 though it underestimate the bandgap. But it failed to describe the insulating behavior for Mott insulators such as LaTiO3 and LaVO3.

In order to probe the occupied and unoccupied electronic structure of the semiconductors we have calculated the XPS, BIS and UPS spectra for the semiconducting oxides. Another experimental technique that provides information about CB is XANES. I will present some examples where comparision is made between experimental and theoretical spectra. The investigation of compounds with high magneto-optical activity is of great importance both for applications (the search for potential materials for magneto-optical disks) and for achieving a deeper understanding of the electronic structure and magnetic properties of solids. We have calculated the MO properties such as Kerr and Faraday spectra for series of ferromagnetic materials.

From these study I will show that from the density functional calculations we can predict the excited state properties of the materials and also explain the features of experimentally observed spectra.

Links: Theory group in solid state chemistry

19.03.2003

FV146C
12:15
Grunde Løvoll
Condensed matter physics, Department of Physics, University of Oslo


Front scaling: from capillary fingering to viscous fingering
When one fluid phase displace another in a porous media the displacement is stable or unstable depending on wetting properties (capillary effects), gravity, viscous contrast and invasion speed. We have experimentally studied the transition between two unstable regimes. One controlled by "pore scale forces" (capillary fingering) the other controlled by the viscous pressure field (viscous fingering). The experiments are performed by injecting a low viscous non-wetting liquid (air) into a quasi two dimensional porous medium saturated with a high viscous liquid (glycerol/water) at various injection speeds

Collaboration:
Knut Jørgen Måløy, Dep. of Physics, UiO
Yves Méheust, Dep. of Physics, NTNU
Jean Schmittbuhl, Dep. of Geology ENS, Paris

Links: Grunde Løvoll homepage, Complex Systems group

05.03.2003

FV146C
12:15
Edward V. Monakhov
Physical Electronics, Department of Physics, University of Oslo


Observation of a "new" double acceptor center in silicon
This work has been initiated within the CERN collaboration on Radiation hard semiconductor devices for very high luminosity colliders. A considerable part of this collaboration is aimed to improvement of radiation hardness of silicon by means of defect engineering. During our studies of radiation-induced defects we have observed formation of a new double acceptor center (a defect with singly negative and doubly negative charge states, so it can capture up to 2 additional electrons) [Monakhov et al., Phys. Rev. B 65, 233207 (2002)]. After additional studies and independent theoretical works, it is believed now that we have observed a previously unknown second negative charge state of the divacancy-oxygen (V2O) complex. The V2O complex is considered one of the most important center with respect to radiation stability of silicon. Some believe that it is responsible for the radiation-induced degradation of silicon devices. Our studies will also help to resolve the mystery of divacancy annealing that persists despite of decades of silicon-based technology.

Collaboration:
G. Alfieri, B. Svensson, Dept. of Physics, UiO
B. S. Avset, SINTEF
A. Hallén, Royal Institute of Technology, Stockholm

Links: Physical Electronics at UiO

19.02.2003

FV102
12:15
Harald Hauglin
Condensed-matter physics, Department of Physics, University of Oslo
&
Institute of Nutrition Research, University of Oslo


Protein Folding
An introduction to the protein folding problem is given by a physicist making his first steps in biology. The talk is focused on review of the protein folding simulations, and related experiments.

For more information see:
Geometrical approach to protein folding: a tube picture, Banavar et al, Rev. Mod. Phys. 75 , 23 (2003).
Absolute comparison of simulated and experimental protein-folding dynamics, Snow et al., Nature 420, 102 (07 Nov 2002).
Mathematical Analysis of Coupled Parallel Simulations, Shirts et al., Phys. Rev. Lett. 86, 4983 (2001).
A surprising simplicity to protein folding, Baker, Nature 405, 39 (04 May 2000).
Heteropolymer freezing and design: Towards physical models of protein folding, Pande et al., Rev. Mod. Phys. 72, 259 (2000).


Links: Folding simulations @ Stanford, Searchable on-line textbooks on molecular cell biology
05.02.2003

FV102
12:15
Susanne F. Viefers
Theory group, Department of Physics, University of Oslo


A Carbon Nanotube Based Nanorelay
We investigate the operational characteristics of a nanorelay based on a conducting carbon nanotube placed on a terrace in a silicon substrate. The nanorelay is a three terminal device that acts as a switch in the GHz regime. Potential applications include logic devices, memory elements, pulse generators, and current or voltage amplifiers.

Collaboration:
J.M. Kinaret, T. Nord.  Chalmers University of Technology, Sweden

Links: cond-mat/0208427

22.01.2003

FV102
12:15
Yuri M Galperin
Condensed-matter physics, Department of Physics, University of Oslo


Coherent "shuttle" transfer of Cooper pairs by a movable superconducting grain
We consider coherent transfer of Cooper pairs between two bulk superconductors by a movable superconductor grain. It is assumed that the grain commutes between the leads by some external driving system (nano-pendulum). Devices of such type made of normal metals were recently studied experimentally. The main feature of the system under consideration is that by proper choice of the gates the states with N and N+1 Cooper pairs on the grain are made degenerate. As a result, during the time between contacts with the leads the grain persists in a hybrid quantum state as a movable quantum bit.

Two situations are discussed: (i) the phase different between the leads is kept constant, and DC current is measured. In this situation the movable grain stimulates DC Josephson effect, the critical current being dependent on the shuttle parameters Nature 411, 454 (2001); (ii) The leads are isolated, and the distribution of the phase difference between them is studied as a function of the number of the grain's excursions Phys. Rev. Lett. 89, 277002 (2002). In the talk I will concentrate on the role of interaction with environment which leads to decoherence.

Collaboration:
A. Isacsson, L. Y. Gorelik, R. I. Shekhter, and M. Jonson.  Chalmers University of Technology, Sweden

Links: homepage of Y.M. Galperin

11.12.2002

FV102
13:00
Morten Hjorth-Jensen
Nuclear and energy physics, Department of Physics, University of Oslo


Can we classify phase transitions in small systems of interacting fermions?
We employ a simple pairing interaction model in order to study and classify an eventual pairing phase transition in finite fermionic systems. We show that systems with as few as ~10-16 fermions can exhibit clear features reminiscent of a phase transition. To classify the nature of the transition we apply two different numerical methods, one based on standard thermodymanics, and another based on a recently proposed scheme by Borrmann et al. [Phys. Rev. Lett. 84, 3511 (2000)].

Links: homepage of M. Hjorth-Jensen
04.12.2002

FV102
12:15
David Dean
Oak Ridge National Lab, Oak Ridge
Department of Physics, University of Tennessee, Knoxville, Tennessee, USA


Computational chemistry for molecular electronics and related topics
Current through a benzine molecule between gold layers.

Phase transitions in a quantum dot in a strong magnetic field.

Review of computational methods.

Links: homepage of David J. Dean

13.11.2002

FV102
12:15
Ole Martin Løvvik
Nuclear and energy physics, Department of Physics, University of Oslo


Quantum mechanical modelling of hydrogen in metals
If hydrogen is to be used as an energy carrier in large scale, there are technical hurdles and fundamental problems remaining to be solved, both in the production, storage, and end use of hydrogen. One common feature of these problems is the interaction of hydrogen with metals, either when metals are used in heterogeneous catalysis (e.g. in fuel cells), when metal embrittlement is caused by hydrogen (e.g. in pipelines), or when metal serves as a hydrogen storage device (in metal hydrides). An ongoing project is working on such problems, with a main focus on new, light weight metal hydrides. This is a collaboration between UiO and Institute for Energy Technology (IFE) at Kjeller, where experiments are done on the same materials. This seminar introduces the main aspects of the theoretical methods being used, some results are shown, and possible applications are also discussed.

Links: folk.uio.no/olem/
30.10.2002

FV102
12:15
Ponniah Vajeeston
Department of Chemistry, University of Oslo


Theoretical investigations on metal hydrides from density functional calculations
I will present some of our recent work on metal hydrides which violate the "2 A rule" and also pressure induced strcutural transition in MgH2". More details about my presentation will be found in the following two references:
Violation of the "2 Å rule" for Metal Hydrides, Phys. Rev.Lett. 89, 106403 (2002).
Pressure-induced structural transitions in MgH2, Phys. Rev. Lett. 89, 175506 (2002).

Collaboration:
P.Ravindran, R.Vidya, A. Kjekshus, H. Fjellvåg.  Department of Chemistry, University of Oslo

Links: folk.uio.no/ravi
16.10.2002

FV102
12:15
Dragos Anghel
Condensed-Matter Physics Group, Department of Physics, University of Oslo


Universality and first order phase transitions in systems of generalized statistics
I make a brief introduction to the first order phase transitions, by showing the main aspects of Yang & Lee theory [Phys. Rev. 87, 404 (1952), Phys. Rev. 87, 410 (1952)] and Landau's phenomenological model (Landau and Lifshitz, Stat. Mech.). Nonanalyticity of the thermodynamic potential may be due to multiple, competing peaks of the probability distribution in the phase-space of a system. I show a concrete example of such a system: 2D interacting gas of generalized statistics (i.e. Bose, Fermi, or Haldane's fractional exclusion statistics). I turn off the interaction, and emphasize the universal thermodynamics of quantum gases in 2D. The paper is published in J. Phys. A (Abstract)

Links: folk.uio.no/dragos/
02.10.2002

FV102
12:15
Daniel V. Shantsev
Condensed-Matter Physics Group, Department of Physics, University of Oslo


Thermomagnetic instability in type-II superconductors
Magnetic flux penetrates in type-II superconductors in form of quantized flux lines or vortices. The vortices can be pinned by tiny material inhomogeneities, which prevents their motion. When votices are moving, a heat is dissipated, and the resulting temperature rise weakens the pinning. This can lead to a thermomagnetic instability accompanied by abundant heating and macroscopic redistribution of magnetic flux. Development of the instability is very sensitive to the material, sample geometry and external conditions. Theoretical efforts, both analytical and simulations, to understand the phenomenon are reviewed. MD simulations reproducing dendritic instability, found experimentally in thin superconducting films, are discussed in detail.

Collaboration:
T. H. Johansen, Y. M. Galperin, S. Braeck, M. Baziljevich, P. E. Goa.  Condensed-Matter Physics Group, Department of Physics, University of Oslo

Links: www.fys.uio.no/super/dend

18.09.2002

FV102
14:15
Maria G. Ganchenkova
Material Physics and Semiconductors, Royal Institute of Technology, Stockholm, Sweden


Formation and stability of radiation defect complexes in Si and Si:Ge: Pressure effects
Divacancies (V2) and E-centers (like P-V pairs) are known to be most common radiation defects in silicon based semiconductors after ion implantation. Based on experimental measurements it has been recently suggested that V2 and P-V exibit preferential configurations in SiGe system in respect with amount of Ge atoms surrounding the defect. In this work the stability of these fundamental defect complexes, specifically V2 and P-V pair, are studied as a function of amount of Ge atoms surrounding the defect in Si. We have investigated also the effects of hydrostatic pressure on the lattice relaxation associated with the formation of these types of defects. Pressure studies allows to reach a better understanding of the properties of defect states through the separation of the temperature effects at constant pressure into their volume- dependent and volume-independent contributions. The latter contributions arise because of anharmonic interactions. A new model to simulate the pressure effect on the formation energy and volume has been suggested. The methods used are molecular static with Tersoff™s many-body interatomic potential as well as ab-initio approach. Formation energies and volumes for V2 and P-V complexes are calculated and are compared with experimental data.

Collaboration:
A.Yu. Kuznetsov.  Material Physics and Semiconductors, Royal Institute of Technology, Stockholm, Sweden Physical Electronics, Department of Physics, University of Oslo, Oslo, Norway
A.V.Nazarov.  Department of Material Science, Moscow State Engineering-Physics Institute, Moscow, Russia
R.M.Nieminen.  Helsinki University of Technology, Department of Engineering Physics and Mathematics, Espoo, Finland