Academic Interests
My main interest is the physics of strongly correlated electron systems. I'm also interested in developing better numerical methods for studying such systems.
Teaching
Educational background
 09 Professor, UiO
 0709: Associate Professor UiO
 0107: Assistant Professor/Lecturer, NORDITA
 9901: Nordic Fellow, NORDITA
 9799: Postdoc, MIT
 9297: Phd., UCLA
 8691: Siv.Ing., NTH
Tags:
USA,
Quantum Physics,
Theoretical physics,
Condensed matter physics,
Nordic,
Computational Physics,
Physics,
Strongly correlated electrons
Publications

Schecter, Michael; Syljuåsen, Olav Fredrik & Paaske, Jens (2017). Nematic Bond Theory of Heisenberg Helimagnets. Physical Review Letters.
ISSN 00319007.
119(15) . doi:
10.1103/PhysRevLett.119.157202
Full text in Research Archive.
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We study classical twodimensional frustrated Heisenberg models with generically incommensurate ground states. A new theory for the latticenematic “order by disorder” transition is developed based on the selfconsistent determination of the effective exchange coupling bonds. In our approach, fluctuations of the constraint field imposing conservation of the local magnetic moment drive nematicity at low temperatures. The critical temperature is found to be highly sensitive to the peak helimagnetic wave vector, and vanishes continuously when approaching rotation symmetric Lifshitz points. Transitions between symmetry distinct nematic orders may occur by tuning the exchange parameters, leading to lines of bicritical points.

Syljuåsen, Olav Fredrik (2015). Dynamical structure factor of magnetic Bloch oscillations at finite temperatures. European Physical Journal B : Condensed Matter Physics.
ISSN 14346028.
88(10) . doi:
10.1140/epjb/e2015604865
Full text in Research Archive.
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Domainwalls in onedimensional Ising ferromagnets can undergo Bloch oscillations when sub jected to a skew magnetic field. Such oscillations imply finite temperature nondispersive lowfrequency peaks in the dynamical structure factor which can be probed in neutron scattering. We study in detail the spectral weight of these peaks. Using an analytical approach based on an approximate treatment of a gas of spincluster excitations we give an explicit expression for the momentum and temperaturedependence of the spectral weights. Generally the spectral weights increase with temperature T and approaches the same order of magnitude as the spinwave spectral weights at high temperatures. We compare the analytical expression to numerical exact diagonalizations and find that it can, without any adjustable parameters, account for the T and momentumtransfer dependence of the numerically obtained spectral weights in the parameter regime where the ratio of magnetic fields hx/hz ≪ 1 and the temperature is hx < T <∼ Jz/2. We also carry out numerical calculations pertinent to the material CoNb2O6, and find qualitatively similar results.

Häfliger, Petra S.; Gerber, Simon; Ravindra, Pramod; Schnells, Vera I.; Piazza, Bastien Dalla; Chati, Ralph; Pomjakushin, Vladimir Yu; Conder, Kazimierz; Pomjakushina, Ekaterina; Le Dreau, Loic; Christensen, Niels Bech; Syljuåsen, Olav Fredrik; Normand, Bruce & Rønnow, Henrik Moodysson (2014). Quantum and thermal ionic motion, oxygen isotope effect, and superexchange distribution in la 2 CuO 4. Physical Review B. Condensed Matter and Materials Physics.
ISSN 10980121.
89(8) . doi:
10.1103/PhysRevB.89.085113
Full text in Research Archive.
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We study the zeropoint and thermal ionic motion in La2CuO4 by means of highresolution neutrondiffraction experiments. Our results demonstrate anisotropic motion of O and, to a lesser extent, Cu ions, both consistent with the structure of coupled CuO6 octahedra, and quantify the relative effects of zeropoint and thermal contributions to ionic motion. By substitution of O18, we find that the oxygen isotope effect on the lattice dimensions is small and negative (−0.01%), while the isotope effect on the ionic displacement parameters is significant (−6 to 50%). We use our results as input for theoretical estimates of the distribution of magnetic interaction parameters, J, in an effective oneband model for the cuprate plane. We find that ionic motion causes only small (1%) effects on the average value ⟨J⟩, which vary with temperature and O isotope, but results in dramatic (10–20%) fluctuations in J values that are subject to significant (8–12%) isotope effects. We demonstrate that this motional broadening of J can have substantial effects on certain electronic and magnetic properties in cuprates.

Shinkevich, Sergey & Syljuåsen, Olav Fredrik (2013). Numerical simulations of laserexcited magnetic Bloch oscillations. Physical Review B. Condensed Matter and Materials Physics.
ISSN 10980121.
87(6) . doi:
10.1103/PhysRevB.87.060401
Full text in Research Archive.

Pedersen, Kim Georg Lind; Andersen, Brian Møller; Syljuåsen, Olav Fredrik; Bruun, Georg M. & Sørensen, Anders Søndberg (2012). Inducing spindependent tunneling to probe magnetic correlations in optical lattices. Physical Review A. Atomic, Molecular, and Optical Physics.
ISSN 10502947.
85(5:053642) . doi:
10.1103/PhysRevA.85.053642

Shinkevich, Sergey & Syljuåsen, Olav Fredrik (2012). Spectral signatures of magnetic Bloch oscillations in onedimensional easyaxis ferromagnets. Physical Review B. Condensed Matter and Materials Physics.
ISSN 10980121.
85(10) . doi:
10.1103/PhysRevB.85.104408
Full text in Research Archive.

Pedersen, Kim Georg; Andersen, Brian Møller; Bruun, Georg; Syljuåsen, Olav Fredrik & Sørensen, Anders (2011). Measuring spin correlations in optical lattices using superlattice potentials. Physical Review A. Atomic, Molecular, and Optical Physics.
ISSN 10502947.
84(4) . doi:
10.1103/PhysRevA.84.041603

Shinkevich, Sergey; Syljuåsen, Olav Fredrik & Eggert, Sebastian (2011). Spinwave calculation of the fielddependent magnetization pattern around an impurity in Heisenberg antiferromagnets. Physical Review B. Condensed Matter and Materials Physics.
ISSN 10980121.
83(5) . doi:
10.1103/PhysRevB.83.054423

Cheianov, VV; Syljuåsen, Olav Fredrik; Altshuler, B. L. & Falko, Vladimir (2010). Sublattice ordering in a dilute ensemble of monovalent adatoms on graphene. Europhysics letters.
ISSN 02955075.
89(5) . doi:
10.1209/02955075/89/56003

Andersen, BM; Syljuåsen, Olav Fredrik & Hedegård, P (2009). Magneticfieldinduced soft mode in spingapped highTc superconductors. Physical Review B. Condensed Matter and Materials Physics.
ISSN 10980121.
80(5) . doi:
10.1103/PhysRevB.80.052509
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We present an explanation of the dynamical ingap spin mode in La2−xSrxCuO4 induced by an applied magnetic field H as recently observed experimentally [J. Chang et al., Phys. Rev. Lett. 102, 177006 (2009)]. Our model consists of a phenomenological spinonly Hamiltonian, and the softening of the spin mode is caused by vortex pinning of dynamical stripe fluctuations which we model by a local ordering of the exchange interactions. The spin gap vanishes experimentally around H=7 T, which in our scenario corresponds to the field required for overlapping vortex regions.

Bruun, Georg; Syljuåsen, Olav Fredrik; Pedersen, Kim Georg; Andersen, Brian Møller; Demler, Eugene & Sørensen, Anders (2009). Antiferromagnetic noise correlations in optical lattices. Physical Review A. Atomic, Molecular, and Optical Physics.
ISSN 10502947.
80(3) . doi:
10.1103/PhysRevA.80.033622
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We analyze how noise correlations probed by timeofflight experiments reveal antiferromagnetic (AF) correlations of fermionic atoms in twodimensional and threedimensional optical lattices. Combining analytical and quantum Monte Carlo calculations using experimentally realistic parameters, we show that AF correlations can be detected for temperatures above and below the critical temperature for AF ordering. It is demonstrated that spinresolved noise correlations yield important information about the spin ordering. Finally, we show how to extract the spin correlation length and the related critical exponent of the AF transition from the noise.

Cheianov, Vadim; Falko, Vladimir; Syljuåsen, Olav Fredrik & Altshuler, BL (2009). Hidden Kekule ordering of adatoms on graphene. Solid State Communications.
ISSN 00381098.
149(3738), s 1499 1501 . doi:
10.1016/j.ssc.2009.07.008

Cheianov, Vadim; Falko, Vladimir; Syljuåsen, Olav Fredrik & Altshuler, Boris (2009). Partial Kekule Ordering of Adatoms on Graphene. Solid State Communications.
ISSN 00381098.
149, s 1499 1501 . doi:
10.1016/j.ssc.2009.07.008
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Electronic and transport properties of Graphene, a oneatom thick crystalline material, are sensitive to the presence of atoms adsorbed on its surface. An ensemble of randomly positioned adatoms, each serving as a scattering center, leads to the Boltzmann–Drude diffusion of charge determining the resistivity of the material. An important question, however, is whether the distribution of adatoms is always genuinely random. In this Communication we demonstrate that dilute adatoms on graphene may have a tendency towards a spatially correlated state with a hidden Kekulé mosaic order. This effect emerges from the interaction between the adatoms mediated by the Friedel oscillations of the electron density in graphene. The onset of the ordered state, as the system is cooled below the critical temperature, is accompanied by the opening of a gap in the electronic spectrum of the material, dramatically changing its transport properties.

Cheianov, VV; Syljuåsen, Olav Fredrik; Altshuler, BL & Fal'ko, V (2009). Ordered states of adatoms on graphene. Physical Review B. Condensed Matter and Materials Physics.
ISSN 10980121.
80(23) . doi:
10.1103/PhysRevB.80.233409

Syljuåsen, Olav Fredrik (2008). Numerical evidence for unstable magnons at high fields in the Heisenberg antiferromagnet on the square lattice. Physical Review B. Condensed Matter and Materials Physics.
ISSN 10980121.
78(18) . doi:
10.1103/PhysRevB.78.180413

Syljuåsen, Olav Fredrik (2008). Using the average spectrum method to extract dynamics from quantum Monte Carlo simulations. Physical Review B. Condensed Matter and Materials Physics.
ISSN 10980121.
78(17) . doi:
10.1103/PhysRevB.78.174429

Syljuåsen, Olav Fredrik & Sandvik, Anders W. (2002). Quantum Monte Carlo with Directed Loops. Physical Review E. Statistical, Nonlinear, and Soft Matter Physics.
ISSN 1063651X.
66 . doi:
10.1103/PhysRevE.66.046701
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We introduce the concept of directed loops in stochastic series expansion and pathintegral quantum Monte Carlo methods. Using the detailed balance rules for directed loops, we show that it is possible to smoothly connect generally applicable simulation schemes (in which it is necessary to include backtracking processes in the loop construction) to more restricted loop algorithms that can be constructed only for a limited range of Hamiltonians (where backtracking can be avoided). The “algorithmic discontinuities” between general and special points (or regions) in parameter space can hence be eliminated. As a specific example, we consider the anisotropic S=1/2 Heisenberg antiferromagnet in an external magnetic field. We show that directedloop simulations are very efficient for the full range of magnetic fields (zero to the saturation point) and anisotropies. In particular, for weak fields and anisotropies, the autocorrelations are significantly reduced relative to those of previous approaches. The backtracking probability vanishes continuously as the isotropic Heisenberg point is approached. For the XYmodel, we show that back tracking can be avoided for all fields extending up to the saturation field. The method is hence particularly efficient in this case. We use directedloop simulations to study the magnetization process in the twodimensional Heisenberg model at very low temperatures. For L×L lattices with L up to 64, we utilize the step structure in the magnetization curve to extract gaps between different spin sectors. Finitesize scaling of the gaps gives an accurate estimate of the transverse susceptibility in the thermodynamic limit: χ⊥=0.0659±0.0002.
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Syljuåsen, Olav Fredrik (2017). Bloch oscillations.
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Numerical simulations of Bloch oscillations

Syljuåsen, Olav Fredrik (2017). Quantum Monte Carlo simulations.

Schecter, Mike; Syljuåsen, Olav Fredrik & Paaske, Jens (2016). Frustrated squarelattice magnetism from indirect exchange through a 3D superconductor.
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We study the magnetic ordering of a square lattice of magnetic adatoms deposited on the surface of a 3D superconductor without spinorbit coupling. The adatoms experience an effective longranged indirect exchange interaction that can be frustrated upon varying the lattice spacing. The frustration allows the adatom system to break the lattice rotation symmetry at low temperatures through an Ising phase transition, while maintaining the spinrotation symmetry. Near special values of the lattice spacing the susceptibility is peaked along a continuous manifold in momentum space, making the system an attractive candidate for the study of spin liquid physics and 2D topological superconductivity.

Shinkevich, Sergey & Syljuåsen, Olav Fredrik (2016). Magnetic Bloch Oscillations.

Syljuåsen, Olav Fredrik (2016). Introduction to Quantum Monte Carlo: The directedloop algorithm.
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There exists several Quantum Monte Carlo methods. In this talk I will give an introduction to one of the most powerful methods for lattice quantum spins and bosons:the stochastic series expansion method with directedloop updates.

Syljuåsen, Olav Fredrik (2016). Magnetic Bloch Oscillations.
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The Bloch oscillations phenomenon was predicted in the early days of quantum mechanics, and has been observed experimentally in a range of systems, however not yet in magnetism. In this talk I will present theoretical predictions for neutron scattering spectral signatures of Bloch oscillations in a one dimensional magnetic system. The talk will start with some hopefully pedagogical (undergraduate level) numerical simulations of Bloch oscillations.

Syljuåsen, Olav Fredrik (2016). Magnetic Bloch Oscillations.
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The Bloch oscillations phenomenon was for a long time controversial, but has now been observed experimentally in several different settings. However, not yet in magnetic systems. In this talk I will present theoretical predictions for neutron scattering spectral signatures of Bloch oscillations in a one dimensional magnetic system. The talk will start with some pedagogical (undergraduate level) numerical simulations of Bloch oscillations.

Syljuåsen, Olav Fredrik (2015). Bloch oscillations in one dimensional ferromagnets in magnetic fields.

Syljuåsen, Olav Fredrik (2015). Quantum Monte Carlo simulations of spin systems.

Syljuåsen, Olav Fredrik & Shinkevich, Sergey (2013). Magnetic Bloch Oscillations.
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Published Nov. 4, 2010 12:38 PM
 Last modified June 6, 2016 10:11 AM