Research Highlights

Mott insulators & bad-metal physics
We study the effects of strong electron-electron repulsion in materials that could be expected to be normal metals, but are instead very bad metals or even insulators. These problems are considered from different perspectives: a) conceptually, studying the very definition of Mott phases and the associated order, b) within effective simplified models, such as the Hubbard model, using non-perturbative approaches such as direct diagonalization or dynamical mean-field theory (DMFT), and c) in the context of real materials using band-structures techniques coupled with DMFT. Researchers: R. Žitko, Ž. Osolin, J. Mravlje, J. Kokalj, A. Horvat more ...
Kondo effect and quantum impurity physics
Quantum impurity is a name given to a defect in a host material with non-trivial internal dynamics on the time-scale of the experiment, for instance the magnetic moment of a dopant atom or of an adsorbate on a surface of a metal. The coupling between the impurity and the host leads to anomalies in thermodynamic and transport properties at low temperatures, collectively known as the Kondo effect. Recently, the focus has been on the localized states induced by magnetic impurities coupled to superconducting host materials which are found inside the superconducting gap. These are relevant in the context of search for Majorana states in solid-state systems. Researchers: R. Žitko more ...
Lattice QCD
We compute the hadron spectrum and study the interactions between hadrons from the first principles of quantum chromodynamics on the lattice. By simulating the scattering between hadrons, we determine the corresponding scattering phase shift. This lead to the first lattice determination of the mass and the width for a number of hadronic resonances in the light, strange and charm sector. Experimental candidates for the exotic hadrons often lie near thresholds, and we are performing pioneering simulations that take into account the effect of threshold on these states. The exotic charmonium-like XYZ states, which are candidates for mesonic molecules or tetraquarks, are of particular interest in light of recent experimental discoveries. Researchers: S. Prelovsek. more ...
Quark & lepton flavor phenomenology
Quark flavor and CP violation phenomena are described in the Standard Model by a total of 4 parameters of the Cabibbo-Kobayashi-Maskawa matrix and 6 quark masses. On the other hand, realistic Standard Model extensions contain additional parameters that are subject to strong constraints from precision flavor measurements. Our studies of such new physics effects focus on processes that are either rare or can be precisely predicted in the Standard Model. They are explored both model independently and in explicit new physics scenarios, predicting correlations among quark and lepton flavor observables, including CP violation as well as baryon and lepton number violation. Researchers: S. Fajfer, J. F. Kamenik, N. Kosnik. more ...
Top and Higgs physics
The top quark and the Higgs boson are the heaviest known elementary particles. Due to their large coupling they exhibit an extreme sensitivity to dynamics at energies far above the electroweak scale of the Standard Model. As such they also serve as excellent probes of new physics. We study top quark and Higgs boson properties and processes, which are well predicted or very suppressed within the Standard Model, with the aim of disentangling possible non-standard effects. These include collider signatures of hypothetical cosmological dark matter particles or light remnants of high scale gauge and matter unification. Researchers: S. Fajfer, J. F. Kamenik, N. Kosnik more ...
Physics beyond the standard model
We are looking for signals of physics beyond the standard model, like proton decay, neutrino masses, heavy neutrinos at LHC, neutrino-less double beta decay, and flavor physics. The models we are studying are mainly grand unified theories (SU(5), SO(10), E_6) and left-right symmetric theories, with or without their supersymmetric extensions. We are interested in phenomenological and cosmological consequences of these ideas, as well as theoretical issues like spontaneous supersymmetry breaking and study of strongly interacting systems through the AdS/CFT correspondence. Researchers: B. Bajc, M. Nemevsek more ...
Clifford Algebra as a Possible Clue to the Unification of Fundamental Interactions
Geometric calculus based on Clifford algebra is a very useful mathematical tool for description of physics. In addition, it enables formulation of a new theory in which the 4-dimensional spacetime is replaced by a more general, 16-dimensional space, called Clifford space (C-space). This is a space whose elements are not only points, but also lines, areas, 3-volumes and 4-volumes. Such generalization brings many new theoretical possibilities that are being explored. If we release the constraint of flat C-space, then we encounter a fascinating possibility, namely that curved C-space provides a realization of Kaluza-Klein theory [gr-qc...], and since all of its 16 dimensions are by assumption physically observable, there is no need for compactification of the "extra" dimensions. The concept of C-space is related to the concept of the configuration space associated with the extended objects, the so called branes. Researchers: M. Pavsic, more....
Few-body problem
The CFHHM (Correlation Function Hyperspherical Harmonic Method) is a direct (nonvariational) solution of the three-body Schroedinger equation. It removes the Coulomb singularities analytically and expands the smooth remainder in HH. CFHHM is being developed in a Slovenian-Israeli collaboration. It yielded the best value of the annihilation rate of the positronium negative ion, resolved the Lamb shift discrepancy between theory and experiment for the first excited state of He, sorted out the alpha sticking probabilities and fusion rates in the muon-catalyzed hydrogen fusion as well as the hyperfine splitting in the muonic He atom, exposing unreliability of relativistic corrections in other works. Currently we are calculating high energy double photoionization cross sections. The QLM (Quasilinearization Method) for solving nonlinear differential equations has been generalized by lifting most restrictions on the potentials, and works even when perturbation method fails. QLM has quadratic convergence and is implemented numerically in an efficient iterative algorithm. For example, it gives 20 decimal places for the two-body Dirac equation as formulated by Malenfant for the Breit-Coulomb problem in just 6 iterations. Researchers: R. Krivec, more ....
Complex Networks
In complex dynamical systems various types of interactions are appropriately represented by graphs (networks). These networks often appear to have a complex multiscale structure and additional attributes of nodes and links, as opposed to random graphs well studied in the graph theory. Statistical physics of networks aims to reveal the underlying universal features of complex network topology and function. In our research, the emphases are on: Developing algorithms for network topology and evolution; Simulating dynamical processes on networks; Applications of network theory to real-world complex systems. In the application of networks, we study information traffic, modeling bio-molecular interactions, nanonetworks, as well as online social networks. Researchers: B. Tadic. Collaborations: Medical Univ. of Vienna, Brunel Univ. (London), Bogolubov Laboratory (Dubna), Nanjing University, University Nijmegen, more ...
Nanostructured
Materials:
Geometry and
Function
In order to manufacture functional devices of nanoscale objects, often assembly of nanoparticles, biomolecules etc into macroscopic structures is required. Self-assembly of nanoscale objects is a process without guidance from an outside source. Recently, study of self-assembled aggregates of nanosystems and physical processes on such structures appear as subjects of key importance due to emergent properties of the assemblies and their potential applications in nanotechnology. Our research focuses on mathematical modeling of the assembly processes into complex nanonetworks (self-assembly of nanoparticles and their clusters into large-scale structures and their topology), as well as modeling and simulations of dynamical processes on these structures (magnetization reversal, single-electron tunnelings, etc) and quantifying their dependence on the topology of the assembly. Researchers: M. Suvakov, B. Tadic. Collaborations: Nano- science group, Univ. of Nottingham; Marie Curie network: MRTN - CT - 2004 - 005728 more ...
Agent-based modeling of Web users
Recently, a huge amount of empirical data in the online communication systems triggered an interdisciplinary research of social dynamics, in which statistical physics theory plays a crucial role. Linking the physics theory of complex systems with machine-learning methods of text analysis, by which an emotion can be recognized in a written text of messages exchanged between users, we investigate high-resolution data on various Web portals and identify the structure of social groups and the emergence of collective emotional behaviors. Furthermore, by designing agent-based models, where Bots and agents with human features interact in the same way as on Blogs, Chat channels or in the online social networks, we simulate the processes on these communication systems and determine the underlying mechanisms which lead to the observed collective phenomena. Researchers:B. Tadic, M. Suvakov more ...; Collaborations: Institute of Physics, Belgrade, Aalto University, Helsinki, OFAI Vienna, University Wolverhampton, CYBEREMOTIONS community,
Lipid vesicles
It is amazing how closely do lipid vesicles mimic the behavior of some simple biological cells such as erythrocytes. Although the theoretical elastomechanics of bilayer membranes is not very complicated, the equilibrium shapes are often surprisingly complex. Presently we focus on vesicle aggregates, which could provide an insight into the physical aspects of certain tissues. - The cutaway views of vesicle doublets show that the contact zone may be either flat or sigmoidally curved. Researchers: P. Ziherl, S. Svetina more ...
Colloids
What should be the radial profile of interaction between classical particles to stabilize a non-close-packed crystal lattice? This quesition remains the central topic of our research in the field. Recently, we have studied the condensed phases of repelling colloids with a hard-core/square-shoulder pair interaction, and we have shown that they include a number of structures formed by particle aggregates rather than by individual particles. - The hexagonal micellar aggregate phase (snapshot courtesy of Matt Glaser, University of Colorado): the blue filled circles and the green coronas correspond to hard cores of the particles and their shoulders, respectively. Researchers: M. A. Glaser, G. M. Grason, A. Kosmrlj, R. D. Kamien, C. D. Santangelo, P. Ziherl, more ...
Electrostatic interactions in complex fluids
Charge stabilized colloidal suspensions are complex fluids compsed of colloids (macroions) and microions. One of the keys to understand the properties of colloidal systems is to understand colloidal interactions and the connection between interactions on the microscopic and material properties on the macroscopic level. The electrostatic interactions dominate at large separations and are responsible for the phase behaviour of stable suspensions of charged colloids. They are mediated by micro ions and are therefore complicated many-body interactions. At low concentrations of added salt in the system the interactions are not pair wise additive and many-body effects are important. The noticeable many-body effects have recently been observed on the solid-liquid phase diagram and three- as well as four-body interactions have been directly calculated and measured. We study questions like: What are the macroscopic properties of complicated many-body systems? How well can we describe such systems with reduction to an effective pair description? Can we find structures that are only stable due to many-body interactions and cannot be observed with pairwise additive interaction? Researchers: J. Dobnikar, more...
Nanophysics
When two quantum dots each occupied by one electron are attached to the leads is the low temperature physics similar to the two-impurity Kondo problem studied twenty years ago for bulk materials. There two spin impurities form either two Kondo singlets with delocalized electrons or bind into a local spin-singlet state which is virtually decoupled from delocalized electrons. The crossover between the two regimes is determined by the relative values of the exchange magnetic energy and the Kondo temperature. The figure shows the density of states in such a double quantum dot as a function of energy and the chemical potential. Bright spots correspond to the Kondo regime and the two branches signal the local singlet formation -- the regime of entangled spin qubits. Currently active collaborations: Prof.dr. J.H. Jefferson, QinetiQ, Great Malvern; Prof.dr. C. Lambert, Lancaster University; Prof.dr. G.A.D. Briggs, University of Oxford; and members of RTN Marie Curie network: Fundamentals of nanoelectronics. Researchers: A. Ramsak, T. Rejec, J. Mravlje, partially J. Bonca and R. Zitko (F5); more...
High - temperature Super - conductors
Cuprates, which become superconducting at high temperatures, still represent one of the major challenges within the solid-state theory. Besides not yet understood mechanism of superconductivity, also very anomalous normal-state electronic properties require proper theoretical description. We follow the wide-spread belief that strong correlations are responsible for non-Fermi-liquid behaviour of electrons in cuprates and analyse the prototype microscopic models, using mostly numerical methods for finite systems. Recenly, a novel aspect on these problems has been been opened by ARPES spectroscopy experiments on electron-doped cuprates. We evaluated model spectral functions, in particular the development of Fermi surface (shown on the figure) with doping, revealing the change of topology from a pocket-like to a large Fermi surface. Researchers: P. Prelovsek, I. Sega, M. Zemljic.
Relaxor Ferroelectrics
In compositionally disordered perovskites, such as PbMg1/3Nb2/3O3 (PMN), the low-frequency quasistatic dielectric constant can reach very high values in a broad temperature range, but spontaneous ferroelectric long-range order does not occur. The system exhibits extremely slow relaxation and gradually freezes out below a dynamic freezing temperature. It has been suggested that these properties are due to polar nanoregions, which are coupled through randomly frustrated long-range interactions of spin-glass type in the presence of local random electric fields. In collaboration with the NMR and dielectric spectroscopy groups, the Spherical Random Bond--Random Field (SRBRF) model of relaxor ferroelectrics has been shown to be capable of describing a number of physical properties. Figure: (a) Calculated temperature dependence of the Edwards-Anderson order parameter in PMN compared with NMR experiments (circles). (b) Local polarization distribution W(p) in dipolar and quadrupolar glasses. more...
Non-Fermi-liquid Behavior in Coupled Quantum Dots
The search for Non_Fermi-liquid behavior in coupled nano devices has been stimulated by recent advances in nanotechnology that enables fabrication of tunable coupled quantum dots. By studying three coupled quantum dots connected to the leads of noninteractig electrons we discovered a wide temperature interval where the system exhibits the two-channel Kondo effect with non-Fermi-liquid properties. The figure represents NRG eigenvalue flow of the coupled three-dot system. Red hashed strips represent values, corresponding to the two-channel Kondo model as obtained using conformal field theory. Non-Fermi-liquid properties are reflected in a non-equidistant spacing between the energies. Collaborators: R. Zitko and J. Bonca. more...
Spin-polaron Excitations in Doped Spin-liquid Systems
In low-dimensional quantum spin systems quantum fluctuations often lead to disordered ground states that exhibit no magnetic ordering and a gapped, non-degenerate singlet ground state. Such states, also called spin liquids, are realized in one dimension as well as in the two dimensional Shastry-Sutherland (SHS) model. Recent suggestion that superconductivity may arise by doping a spin-liquid state lead to investigations of a SrCu2(BO3)2, doped with non-magnetic impurities. Figure represents the spin-structure factor of a doped Shastry-Sutherland lattice. Most interesting features are in-gap states, appearing below the 3meV - the gap of the undoped compound. Collaborators: S. El Shawish, I. Sega, and J. Bonca more...
Theoretical Biophysics
Macromolecules and macromolecular aggregates like DNA and phospholipid membranes, obtained through isolation and/or reconstitution of fundamental building blocks of the living matter, determine their basic modes of action as well as responses to changes in their molecular environment. The analysis of various macromolecular liquid crystalline phases, be they biological macromoleculs themselves or their complicated anorganic analogues, as well as their basic symmetries and fundamental biocolloidal interactions define the first step of the proposed research program. The intergration of these fundamental principles of the nature of the structures and interactions among the basic macromolecular building blocks in the understanding of the physical properties and even more importantly, the possibility of control of the type and resonse of the constituents of the cell or cells and larger cell aggregates on changes in their environment represents the second step of this research program. Its basic principle being the bottom-up physical reconstitution of various living systems, from simple, model macromolecules all the way to morte complicated macromolecular aggregates, cells, organs and tissues. Researchers: M. Brumen, M. Cepic, J. Dobnikar, R. Podgornik, S. Svetina, N. Vaupotic, P. Ziherl, B. Zeks. more...
Modeling Liquid Crystal Properties
Theoretical modelling of liquid crystals was always an important part of the scientific efforts at the Department for theoretical physics. Understanding of the physics in complex systems formed by complex molecules enables the insights and allows for hints for organic chemists heading towards novel materials with the desirable macroscopic properties and potentials for applications. Theoretical modeling is based on the analysis of the intermolecular interactions which originated from different, often competing interactions. Therefore liquid phases with very different properties and structures with modulations in nano regions appear in various liquid crystals. One may also find various phases stable within the single material. Researchers: M. cepic, N. Vaupotic, more...
Density- functional based simulations of nanomaterials
Among ab-initio computer simulations, the density-functional theory (DFT) represents a very powerful tool to investigate the structure, stability and physical properties of different systems, including surfaces and nanomaterials. Using the DFT code as implemented in the WIEN2k code we investigate different Mo-based nanowires, like the Mo6S3I6, which has been synthesized recently and which has very interesting physical properties. The picture shows a longitudinal cross-cut through the nanowire and the valence electron density. Researchers: I. Vilfan.
 
4 Nov, 2017 Abroad:
Invited Talk
Svjetlana Fajfer...
4 Nov, 2017 Abroad:
Invited Talk
Svjetlana Fajfer...
4 Nov, 2017 Abroad:
Invited Talk
Svjetlana Fajfer...
4 Nov, 2017 Abroad:
Invited Talk
Svjetlana Fajfer...
4 Nov, 2017 Abroad:
Invited Talk
Svjetlana Fajfer...
27 Oct, 2017 Abroad:
Invited Talk
Bosiljka Tadic...
27 Oct, 2017 Abroad:
Seminar
Bosiljka Tadic...
23 Oct, 2017 Seminar:
Oct 24: Conservation in...
Friedrich Krien...
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