


Research Highlights
Mott insulators & badmetal physics

We study the effects of strong electronelectron 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 nonperturbative approaches such as direct diagonalization or dynamical meanfield theory (DMFT), and c) in the context of real materials using bandstructures techniques coupled with DMFT.
Researchers: R. Žitko, Ž. Osolin, J. Mravlje, J. Kokalj, A. Horvat
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Kondo effect and quantum impurity physics

Quantum impurity is a name given to a defect in a host material with nontrivial internal dynamics on the timescale 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 solidstate systems.
Researchers: R. Žitko
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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 charmoniumlike XYZ states, which are candidates for mesonic molecules or tetraquarks, are of particular interest in light of recent experimental discoveries.
Researchers: S. Prelovsek.
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Quark & lepton flavor phenomenology

Quark flavor and CP violation phenomena are described in the Standard Model by a total of 4 parameters of the CabibboKobayashiMaskawa 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.
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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 nonstandard 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
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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, neutrinoless double beta decay, and flavor physics. The models we are studying are mainly grand unified theories (SU(5), SO(10), E_6) and leftright 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
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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 4dimensional spacetime is replaced by a more general,
16dimensional space,
called Clifford space (Cspace). This is a space whose elements are not
only points, but also lines,
areas, 3volumes and 4volumes. Such generalization brings many new
theoretical possibilities that
are being explored.
If we release the constraint of flat Cspace, then we encounter a
fascinating possibility, namely
that curved Cspace provides a realization of KaluzaKlein theory [grqc...],
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 Cspace is related to the concept of the configuration
space associated
with the extended objects, the so called branes. Researchers: M. Pavsic,
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Fewbody problem

The CFHHM (Correlation Function Hyperspherical Harmonic Method) is a direct
(nonvariational) solution of the threebody Schroedinger equation. It
removes the Coulomb singularities analytically and expands the smooth
remainder in HH. CFHHM is being developed in a SlovenianIsraeli
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 muoncatalyzed 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 twobody Dirac equation as formulated by
Malenfant for the BreitCoulomb problem in just 6 iterations.
Researchers: R. Krivec,
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Network Modeling of Complex Systems

In complex dynamical systems various types of interactions are appropriately represented by graphs (networks) having 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 stochastic processes on networks; Applications of network theory to complex systems in physics and beyond. Specifically, we study information traffic, modeling biomolecular interactions, nanonetworks, brain imaging data, as well as data from online social systems.
Researchers: B. Tadic. Collaborations: Complexity Science Hub Vienna, Brunel Univ. London, IITM Chennai, Nanjing University, University of Nijmegen, more ...


Nanostructured Materials: Geometry and Function

To manufacture functional devices of nanoscale objects, often assembly of nanoparticles, biomolecules etc into macroscopic structures is required. Selfassembly of nanoscale objects is a process without guidance from an outside source. Recently, study of selfassembled 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 (selfassembly of nanoparticles and their clusters into largescale structures) and their topology, as well as modeling and simulations of dynamical processes on these structures (magnetization reversal, singleelectron tunnelings, etc) and quantifying their dependence on the topology of the assembly.
Researchers: M. Suvakov & B. Tadic. Collaborations: Univ. of Nottingham; Marie Curie network MRTNCT2004005728.
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Agentbased 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 machinelearning methods of text analysis to capture the content (emotional, cognitive) in messages exchanged between users, we investigate highresolution data on various Web portals and identify the structure of social groups and the emergence of collective behaviors. Furthermore, by designing agentbased 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
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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 nonclosepacked 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 hardcore/squareshoulder 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,
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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 manybody interactions. At low
concentrations of added salt in the system the interactions are not pair
wise additive and manybody effects are important. The noticeable
manybody effects have recently been observed on the solidliquid phase
diagram and three as well as fourbody interactions have been directly
calculated and measured.
We study questions like: What are the macroscopic properties of
complicated manybody 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 manybody interactions and cannot be observed with
pairwise additive interaction? Researchers: J. Dobnikar,
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Nanophysics

When two quantum dots each occupied by one electron are attached to
the leads is the low temperature physics similar to the twoimpurity
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 spinsinglet 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 solidstate theory. Besides not yet understood
mechanism of superconductivity, also very anomalous normalstate
electronic properties require proper theoretical description. We follow
the widespread belief that strong correlations are responsible
for nonFermiliquid 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 electrondoped 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 pocketlike to a large
Fermi surface. Researchers: P. Prelovsek, I. Sega, M. Zemljic.


Relaxor Ferroelectrics

In compositionally disordered perovskites, such as
PbMg_{1/3}Nb_{2/3}O_{3} (PMN), the lowfrequency quasistatic
dielectric constant can reach very high values in a
broad temperature range, but spontaneous ferroelectric
longrange 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 longrange interactions of spinglass type
in the presence of local random electric fields.
In collaboration with the NMR and dielectric spectroscopy
groups, the Spherical Random BondRandom 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
EdwardsAnderson order parameter in PMN compared with
NMR experiments (circles). (b) Local polarization
distribution W(p) in dipolar and quadrupolar glasses.
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NonFermiliquid Behavior in Coupled Quantum Dots

The search for Non_Fermiliquid 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
twochannel Kondo effect with nonFermiliquid properties. The figure
represents NRG eigenvalue flow of the coupled threedot system. Red
hashed strips represent values, corresponding to the twochannel Kondo model
as obtained using conformal field theory. NonFermiliquid properties
are reflected in a nonequidistant spacing between the energies.
Collaborators: R. Zitko and J. Bonca.
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Spinpolaron Excitations in Doped Spinliquid Systems

In lowdimensional quantum spin systems quantum fluctuations often
lead to disordered ground states that exhibit no magnetic ordering and
a gapped, nondegenerate singlet ground state. Such states, also
called spin liquids, are realized in one dimension as well as in the
two dimensional ShastrySutherland (SHS) model. Recent suggestion that
superconductivity may arise by doping a spinliquid state lead
to investigations of a SrCu2(BO3)2, doped with nonmagnetic
impurities. Figure represents the spinstructure factor of a doped
ShastrySutherland lattice. Most interesting features are ingap
states, appearing below the 3meV  the gap of the undoped compound.
Collaborators: S. El Shawish, I. Sega, and J. Bonca
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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 bottomup 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.
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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 abinitio computer simulations, the densityfunctional 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
Mobased nanowires, like the Mo_{6}S_{3}I_{6},
which has been synthesized recently and which has very interesting physical properties. The picture shows a longitudinal crosscut through the nanowire
and the valence electron density. Researchers: I. Vilfan.







