Olav Fredrik Syljuåsen

Publikasjonsliste

• Schecter, Michael; Syljuåsen, Olav Fredrik & Paaske, Jens (2018). Cooper pair induced frustration and nematicity of two-dimensional magnetic adatom lattices. Physical Review B.  ISSN 2469-9950.  97(17) . doi: 10.1103/PhysRevB.97.174412
• 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 Fulltekst i vitenarkiv. Vis sammendrag

  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.

• 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 Fulltekst i vitenarkiv. Vis sammendrag

  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.

• 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 1098-0121.  89(8) . doi: 10.1103/PhysRevB.89.085113 Fulltekst i vitenarkiv. Vis sammendrag

  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.

• 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 Fulltekst i vitenarkiv.
• 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.  ISSN 1050-2947.  85(5:053642) . doi: 10.1103/PhysRevA.85.053642
• 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 Fulltekst i vitenarkiv.
• 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 1050-2947.  84(4) . doi: 10.1103/PhysRevA.84.041603
• 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
• 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 0295-5075.  89(5) . doi: 10.1209/0295-5075/89/56003
• 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 Vis sammendrag

  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.

• 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 1050-2947.  80(3) . doi: 10.1103/PhysRevA.80.033622 Vis sammendrag

  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.

• Cheianov, Vadim; Falko, Vladimir; Syljuåsen, Olav Fredrik & Altshuler, BL (2009). Hidden Kekule ordering of adatoms on graphene. Solid State Communications.  ISSN 0038-1098.  149(37-38), 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 0038-1098.  149, s 1499- 1501 . doi: 10.1016/j.ssc.2009.07.008 Vis sammendrag

  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.

• 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 1098-0121.  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 1098-0121.  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 1098-0121.  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 1063-651X.  66 . doi: 10.1103/PhysRevE.66.046701 Vis sammendrag

  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.

Se alle arbeider i Cristin

• Syljuåsen, Olav Fredrik (2017). Bloch oscillations. Vis sammendrag

  Numerical simulations of Bloch oscillations

• Syljuåsen, Olav Fredrik (2017). Quantum Monte Carlo simulations.
• Schecter, Mike; Syljuåsen, Olav Fredrik & Paaske, Jens (2016). Frustrated square-lattice magnetism from indirect exchange through a 3D superconductor. Vis sammendrag

  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.

• Shinkevich, Sergey & Syljuåsen, Olav Fredrik (2016). Magnetic Bloch Oscillations.
• Syljuåsen, Olav Fredrik (2016). Introduction to Quantum Monte Carlo: The directed-loop algorithm. Vis sammendrag

  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.

• Syljuåsen, Olav Fredrik (2016). Magnetic Bloch Oscillations. Vis sammendrag

  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. Vis sammendrag

  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.

Se alle arbeider i Cristin