Academic Interests
My main interest is the physics of strongly correlated electron systems and quantum magnetism. I'm also interested in developing better numerical methods for studying such systems.
Teaching
Educational background
- 09- Professor, UiO
- 07-09: Associate Professor UiO
- 01-07: Assistant Professor/Lecturer, NORDITA
- 99-01: Nordic Fellow, NORDITA
- 97-99: Postdoc, MIT
- 92-97: Phd., UCLA
- 86-91: Siv.Ing., NTH
Tags:
Physics,
Theoretical physics,
Computational Physics,
Condensed matter physics,
Strongly Correlated Electrons,
Quantum Magnetism
Publications
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Syljuåsen, Olav Fredrik & Paaske, Jens
(2026).
Nematostriction in frustrated two-dimensional classical Heisenberg models.
Physical review B (PRB).
ISSN 2469-9950.
113(22).
doi:
10.1103/n8z4-ygz1.
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We investigate the nematic phase transition in the Heisenberg 𝐽1−𝐽2 model on square and triangular lattices, accounting for finite lattice compressibility and bond-length-dependent magnetic exchange. Using Nematic Bond Theory, a diagrammatic self-consistent method, we study the nematostriction that happens when the onset of nematic order in the spin-system drives a concomitant structural phase transition. We analyze the mechanisms by which the magnetoelastic couplings renormalize the critical temperature and modify the phonon spectrum. The magnetoelastic feedback can also alter fundamentally the nature of the phase transition. Specifically, on the square lattice, the transition shifts from continuous to weakly first-order (discontinuous) beyond a critical magnetoelastic coupling threshold. Conversely, on the triangular lattice, the transition remains discontinuous regardless of coupling strength.
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Glittum, Cecilie & Syljuåsen, Olav Fredrik
(2026).
The finite-temperature behavior of a triangular Heisenberg antiferromagnet.
Journal of Physics: Condensed Matter.
ISSN 0953-8984.
38(13),
p. 135801–135801.
doi:
10.1088/1361-648x/ae4f4b.
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We investigate the classical antiferromagnetic Heisenberg model on the triangular lattice with up to third-nearest neighbor exchange couplings using the Nematic Bond Theory. This approach allows us to compute the free energy and the neutron scattering static structure factor at finite temperatures. We map out the phase diagram with a particular emphasis on finite-temperature phase transitions that break lattice-rotational symmetries, spiral spin liquids and the broad specific heat hump that is ubiquitous in the antiferromagnetic 120 degree phase. We identify this specific heat hump as signaling the onset of an exponentially increasing correlation length. Further, we map out the temperature of the specific heat hump and the transition temperatures of the symmetry-breaking transitions throughout the exchange-coupling space. Along the line J3 = J2 / 2 , the Fourier-transformed exchange coupling exhibits a degenerate ring-like minimum, giving rise to spiral spin liquid behavior at intermediate temperatures. We investigate the structure factor of the spiral spin liquid as function of J2 and identify the corresponding low-temperature order, which coincides with the single-q spiral states of maximum spin-wave entropy along the degenerate ring.
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Glittum, Cecilie & Syljuåsen, Olav Fredrik
(2023).
Sublattice pairing in pyrochlore Heisenberg antiferromagnets.
Physical review B (PRB).
ISSN 2469-9950.
108(1),
p. 1–6.
doi:
10.1103/PhysRevB.108.014413.
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We argue that classical pyrochlore Heisenberg antiferromagnets with small further-neighbor couplings can order in a state where pairs of sublattices form antiparallel spirals. The spiral ordering wave vectors of the two pairs are in general different from each other, and are constrained by which sublattices are being paired. This sublattice pairing state generally breaks inversion and most rotation symmetries. Its existence depends on the antiferromagnetic nearest-neighbor coupling which favors the spins on each tetrahedron to sum to zero. To substantiate our argument, we extend the nematic bond theory; a diagrammatic large-Ns method, to non-Bravais lattices, and demonstrate that the predicted state is indeed realized at low temperatures in a large region of exchange coupling space. We also carry out a spin wave calculation which suggests that the sublattice pairing state is coplanar.
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Fogh, Ellen; Klemke, Bastian; Reehuis, Manfred; Bourges, Philippe; Niedermayer, Christof & Holm-Dahlin, Sonja
[Show all 23 contributors for this article]
(2023).
Tuning magnetoelectricity in a mixed-anisotropy antiferromagnet.
Nature Communications.
14(1),
p. 3408–3408.
doi:
10.1038/s41467-023-39128-7.
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Abstract Control of magnetization and electric polarization is attractive in relation to tailoring materials for data storage and devices such as sensors or antennae. In magnetoelectric materials, these degrees of freedom are closely coupled, allowing polarization to be controlled by a magnetic field, and magnetization by an electric field, but the magnitude of the effect remains a challenge in the case of single-phase magnetoelectrics for applications. We demonstrate that the magnetoelectric properties of the mixed-anisotropy antiferromagnet LiNi 1− x Fe x PO 4 are profoundly affected by partial substitution of Ni 2+ ions with Fe 2+ on the transition metal site. This introduces random site-dependent single-ion anisotropy energies and causes a lowering of the magnetic symmetry of the system. In turn, magnetoelectric couplings that are symmetry-forbidden in the parent compounds, LiNiPO 4 and LiFePO 4 , are unlocked and the dominant coupling is enhanced by almost two orders of magnitude. Our results demonstrate the potential of mixed-anisotropy magnets for tuning magnetoelectric properties.
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Hansen, Ursula Bengaard; Syljuåsen, Olav Fredrik; Jensen, Jens; Schaffer, Turi K.; Boehm, Martin & Andersen, Christopher R.
[Show all 9 contributors for this article]
(2022).
Magnetic Bloch oscillations and domain wall dynamics in a near-Ising ferromagnetic chain.
Nature Communications.
13(1).
doi:
10.1038/s41467-022-29854-9.
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Abstract When charged particles in periodic lattices are subjected to a constant electric field, they respond by oscillating. Here we demonstrate that the magnetic analogue of these Bloch oscillations are realised in a ferromagnetic easy axis chain. In this case, the “particles” undergoing oscillatory motion in the presence of a magnetic field are domain walls. Inelastic neutron scattering reveals three distinct components of the low energy spin-dynamics including a signature Bloch oscillation mode. Using parameter-free theoretical calculations, we are able to account for all features in the excitation spectrum, thus providing detailed insights into the complex dynamics in spin-anisotropic chains.
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Syljuåsen, Olav Fredrik; Paaske, Jens & Schecter, Michael
(2019).
Interplay between magnetic and vestigial nematic orders in the layered J1-J2 classical Heisenberg model.
Physical review B (PRB).
ISSN 2469-9950.
99(17).
doi:
10.1103/PhysRevB.99.174404.
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We study the layered J1− J 2 classical Heisenberg model on the square lattice using a self-consistent bond theory. We derive the phase diagram for fixed J1 as a function of temperature T, J2, and interplane coupling Jz. Broad regions of (anti)ferromagnetic and stripe order are found, and are separated by a first-order transition near J2≈ 0.5 (in units of |J1|). Within the stripe phase the magnetic and vestigial nematic transitions occur simultaneously in first-order fashion for strong Jz. For weaker Jz, there is in addition, for J∗ 2 < J 2 < J * * 2 , an intermediate regime of split transitions implying a finite temperature region with nematic order but no long-range stripe magnetic order. In this split regime, the order of the transitions depends sensitively on the deviation from J∗ 2 and J**2, with split second-order transitions predominating for J∗ 2 ≪ J 2 ≪ J * * 2 . We find that the value of J∗ 2 depends weakly on the interplane coupling and is just slightly larger than 0.5 for |Jz|≲ 0.01 . In contrast, the value of J**2 increases quickly from J∗ 2 at |Jz|≲ 0.01 as the interplane coupling is further reduced. In addition, the magnetic correlation length is shown to directly depend on the nematic order parameter and thus exhibits a sharp increase (or jump) upon entering the nematic phase. Our results are broadly consistent with the predictions based on itinerant electron models of the iron-based superconductors in the normal state and, thus, help substantiate a classical spin framework for providing a phenomenological description of their magnetic properties.
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Yan, Zheng; Wu, Yongzheng; Liu, Chenrong; Syljuåsen, Olav Fredrik; Chen, Yan & Lou, Jie
(2019).
Sweeping cluster algorithm for quantum spin systems with strong geometric restrictions.
Physical review B (PRB).
ISSN 2469-9950.
99(16).
doi:
10.1103/PhysRevB.99.165135.
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Quantum spin systems with strong geometric restrictions give rise to rich quantum phases such as valence bond
solids and spin liquid states. However, the geometric restrictions often hamper the application of sophisticated
numerical approaches. Based on the stochastic series expansion method, we develop an efficient and exact
quantum Monte Carlo “sweeping cluster” algorithm which automatically satisfies the geometrical restrictions.
Here we use the quantum dimer model as a benchmark to demonstrate the reliability and power of this algorithm.
Comparing to existing numerical methods, we can obtain higher accuracy results for a wider parameter region
and much more substantial system sizes.
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Schecter, Michael; Syljuåsen, Olav Fredrik & Paaske, Jens
(2018).
Cooper pair induced frustration and nematicity of two-dimensional magnetic adatom lattices.
Physical review B (PRB).
ISSN 2469-9950.
97(17).
doi:
10.1103/PhysRevB.97.174412.
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We propose utilizing the Cooper pair to induce magnetic frustration in systems of two-dimensional
(2D) magnetic adatom lattices on s-wave superconducting surfaces. The competition between singlet
electron correlations and the RKKY coupling is shown to lead to a variety of hidden order states
that break the point-group symmetry of the 2D adatom lattice at finite temperature. The phase
diagram is constructed using a newly developed effective bond theory [M. Schecter et al., Phys.
Rev. Lett. 119, 157202 (2017)], and exhibits broad regions of long-range vestigial nematic order.
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Schecter, Michael; Syljuåsen, Olav Fredrik & Paaske, Jens
(2017).
Nematic Bond Theory of Heisenberg Helimagnets.
Physical Review Letters.
ISSN 0031-9007.
119(15).
doi:
10.1103/PhysRevLett.119.157202.
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We study classical two-dimensional frustrated Heisenberg models with generically incommensurate ground states. A new theory for the lattice-nematic “order by disorder” transition is developed based on the self-consistent 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.
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Syljuåsen, Olav Fredrik
(2015).
Dynamical structure factor of magnetic Bloch oscillations at finite temperatures.
European Physical Journal B : Condensed Matter Physics.
ISSN 1434-6028.
88(10).
doi:
10.1140/epjb/e2015-60486-5.
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Domain-walls in one-dimensional Ising ferromagnets can undergo Bloch oscillations when sub- jected to a skew magnetic field. Such oscillations imply finite temperature non-dispersive low-frequency 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 spin-cluster excitations we give an explicit expression for the momentum- and temperature-dependence of the spectral weights. Generally the spectral weights increase with temperature T and approaches the same order of magnitude as the spin-wave 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 momentum-transfer 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.
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Häfliger, Petra S.; Gerber, Simon; Ravindra, Pramod; Schnells, Vera I.; Piazza, Bastien Dalla & Chati, Ralph
[Show all 14 contributors for this article]
(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 1098-0121.
89(8).
doi:
10.1103/PhysRevB.89.085113.
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We study the zero-point and thermal ionic motion in La2CuO4 by means of high-resolution neutron-diffraction 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 zero-point 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 one-band 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.
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Shinkevich, Sergey & Syljuåsen, Olav Fredrik
(2013).
Numerical simulations of laser-excited magnetic Bloch oscillations.
Physical Review B. Condensed Matter and Materials Physics.
ISSN 1098-0121.
87(6).
doi:
10.1103/PhysRevB.87.060401.
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We propose to use lasers to excite magnetic Bloch oscillations in one-dimensional easy-axis ferromagnets at low temperatures. This proposal is investigated numerically in detail for material parameters relevant for CoCl2⋅2H2O. © 2013 American Physical Society
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Shinkevich, Sergey & Syljuåsen, Olav Fredrik
(2012).
Spectral signatures of magnetic Bloch oscillations in one-dimensional easy-axis ferromagnets.
Physical Review B. Condensed Matter and Materials Physics.
ISSN 1098-0121.
85(10).
doi:
10.1103/PhysRevB.85.104408.
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Domain walls in a one-dimensional gapped easy-axis ferromagnet can exhibit Bloch oscillations in an applied magnetic field. We investigate how exchange couplings modify this behavior within an approximation based on noninteracting domain-wall bound states. In particular, we obtain analytical results for the spectrum and the dynamic structure factor, and show where in momentum space to expect equidistant energy levels, the Wannier-Zeeman ladder, which is the spectral signature of magnetic Bloch oscillations. We compare our results to previous calculations employing a single domain-wall approximation, and make predictions relevant for the material CoCl2⋅2H2O. © 2012 American Physical Society
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Pedersen, Kim Georg Lind; Andersen, Brian Møller; Syljuåsen, Olav Fredrik; Bruun, Georg M. & Sørensen, Anders Søndberg
(2012).
Inducing spin-dependent tunneling to probe magnetic correlations in optical lattices.
Physical Review A. Atomic, Molecular, and Optical Physics (PRA).
ISSN 1050-2947.
85(5:053642).
doi:
10.1103/PhysRevA.85.053642.
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Shinkevich, Sergey; Syljuåsen, Olav Fredrik & Eggert, Sebastian
(2011).
Spin-wave calculation of the field-dependent magnetization pattern around an impurity in Heisenberg antiferromagnets.
Physical Review B. Condensed Matter and Materials Physics.
ISSN 1098-0121.
83(5).
doi:
10.1103/PhysRevB.83.054423.
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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 (PRA).
ISSN 1050-2947.
84(4).
doi:
10.1103/PhysRevA.84.041603.
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Andersen, BM; Syljuåsen, Olav Fredrik & Hedegård, P
(2009).
Magnetic-field-induced soft mode in spin-gapped high-T-c superconductors.
Physical Review B. Condensed Matter and Materials Physics.
ISSN 1098-0121.
80(5).
doi:
10.1103/PhysRevB.80.052509.
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We present an explanation of the dynamical in-gap 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 spin-only 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.
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Cheianov, Vadim; Falko, Vladimir; Syljuåsen, Olav Fredrik & Altshuler, Boris
(2009).
Partial Kekule Ordering of Adatoms on Graphene.
Solid State Communications.
ISSN 0038-1098.
149,
p. 1499–1501.
doi:
10.1016/j.ssc.2009.07.008.
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Electronic and transport properties of Graphene, a one-atom 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.
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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 (PRA).
ISSN 1050-2947.
80(3).
doi:
10.1103/PhysRevA.80.033622.
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We analyze how noise correlations probed by time-of-flight experiments reveal antiferromagnetic (AF) correlations of fermionic atoms in two-dimensional and three-dimensional 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 spin-resolved 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.
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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 1098-0121.
78(18).
doi:
10.1103/PhysRevB.78.180413.
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Syljuåsen, Olav Fredrik & Sandvik, Anders W.
(2002).
Quantum Monte Carlo with Directed Loops.
Physical Review E. Statistical, Nonlinear, and Soft Matter Physics.
ISSN 1539-3755.
66.
doi:
10.1103/PhysRevE.66.046701.
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We introduce the concept of directed loops in stochastic series expansion and path-integral 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 directed-loop 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 back-tracking probability vanishes continuously as the isotropic Heisenberg point is approached. For the XY-model, 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 directed-loop simulations to study the magnetization process in the two-dimensional 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. Finite-size 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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Schecter, Mike; Syljuåsen, Olav Fredrik & Paaske, Jens
(2016).
Frustrated square-lattice 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 spin-orbit coupling. The adatoms experience an effective long-ranged 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 spin-rotation 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.
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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.
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Syljuåsen, Olav Fredrik
(2016).
Introduction to Quantum Monte Carlo: The directed-loop 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 directed-loop updates.
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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.
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View all works in NVA
Published
Nov. 4, 2010 12:38 PM
- Last modified
Feb. 5, 2025 4:15 PM
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