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  • 1.
    Bergkvist, Sara
    et al.
    KTH, Superseded Departments, Physics.
    Henelius, Patrik
    KTH, Superseded Departments, Physics.
    Rosengren, Anders
    KTH, Superseded Departments, Physics.
    Ground state of the random-bond spin-1 Heisenberg chain2002In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 66, p. 134407-1-134407-9Article in journal (Refereed)
    Abstract [en]

    Stochastic series expansion quantum Monte Carlo is used to study the ground state of the antiferromagnetic spin-1 Heisenberg chain with bond disorder. Typical spin- and string-correlation functions behave in accordance with real-space renormalization group predictions for the random-singlet phase. The average string-correlation function decays algebraically with an exponent of -0.378(6), in very good agreement with the prediction of -(3-root5)/2similar or equal to-0.382, while the average spin-correlation function is found to decay with an exponent of about -1, quite different from the expected value of -2. By implementing the concept of directed loops for the spin-1 chain we show that autocorrelation times can be reduced by up to two orders of magnitude.

  • 2.
    Bergkvist, Sara
    et al.
    KTH, Superseded Departments, Physics.
    Henelius, Patrik
    KTH, Superseded Departments, Physics.
    Rosengren, Anders
    KTH, Superseded Departments, Physics.
    Local-density approximation for confined bosons in an optical lattice2004In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 70, p. 053601-1-053601-7Article in journal (Refereed)
    Abstract [en]

    We investigate local and global properties of the one-dimensional Bose-Hubbard model with an external confining potential, describing an atomic condensate in an optical lattice. Using quantum Monte Carlo techniques we demonstrate that a local-density approximation, which relates the unconfined and the confined model, yields quantitatively correct results in most of the interesting parameter range. We also examine claims of universal behavior in the confined system, and demonstrate the origin of a previously calculated fine structure in the experimentally accessible momentum distribution.

  • 3.
    Bergkvist, Sara
    et al.
    KTH, Superseded Departments, Physics.
    Henelius, Patrik
    KTH, Superseded Departments, Physics.
    Rosengren, Anders
    KTH, Superseded Departments, Physics.
    Reduction of the sign problem using the meron-cluster approach2003In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 68, p. 016122-1-016122-8Article in journal (Refereed)
    Abstract [en]

    The sign problem in quantum Monte Carlo calculations is analyzed using the meron-cluster solution. A meron is a loop that alters the sign of the configuration, and the concept of merons can be used to solve the sign problem for a limited class of models. Here we show that the method can be used to reduce the sign problem in a wider class of models. We investigate how the meron solution evolves between a point in parameter space where it eliminates the sign problem and a point where it does not affect the sign problem at all. In this intermediate regime, the merons can be used to reduce the sign problem. The average sign still decreases exponentially with system size and inverse temperature, but with a different prefactor. The sign exhibits the slowest decrease in the vicinity of points where the meron-cluster solution eliminates the sign problem. We have used stochastic series expansion quantum Monte Carlo combined with the concept of directed loops.

  • 4.
    Biltmo, Anders
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics.
    Henelius, Patrik
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Glauber Dynamics of Dilute Ising DipolesManuscript (preprint) (Other academic)
  • 5.
    Biltmo, Anders
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics.
    Henelius, Patrik
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Low-temperature properties of the dilute dipolar magnet LiHoxY1-xF42008In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 78, no 5Article in journal (Refereed)
    Abstract [en]

    We analyze recent experiments on the dilute rare-earth compound LiHoxY1-xF4 in the context of an effective Ising dipolar model. Using a Monte Carlo method we calculate the low-temperature behavior of the specific heat and linear susceptibility and compare our results to measurements. In our model the susceptibility follows a Curie-Weiss law at high temperature, X similar to 1 / (T- T-cw), with a Curie-Weiss temperature that scales with dilution, T-cw similar to x, consistent with early experiments. We also find that the peak in the specific heat scales linearly with dilution, C-max(T)similar to x, in disagreement with recent experiments. This difference could be caused by the hyperfine interaction which is not included in our calculation. Experimental studies do not reach a consensus on the functional form of the susceptibility and specific heat, and in particular, we do not see reported scalings of the form X similar to T-0.75 and X similar to exp(-T/T-0). Furthermore, we calculate the ground-state magnetization as a function of dilution and re-examine the phase diagram around the critical dilution x, = 0.24 +/- 0.03. We find that the spin-glass susceptibility for the Ising model does not diverge below x, while some recent experiments give strong evidence for a stable spin-glass phase in LiHo0.167Y0.833F4.

  • 6.
    Biltmo, Anders
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics.
    Henelius, Patrik
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Phase diagram of the dilute magnet LiHoxY1-xF42007In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 76, no 5Article in journal (Refereed)
    Abstract [en]

    We study the effective long-range Ising dipole model with a local exchange interaction appropriate for the dilute magnetic compound LiHoxY1-xF4. Our calculations yield a value of 0.12 K for the nearest-neighbor exchange interaction. Using a Monte Carlo method, we calculate the phase boundary T-c(x) between the ferromagnetic and paramagnetic phases. We demonstrate that the experimentally observed linear decrease in T-c with dilution is not the simple mean-field result, but a combination of the effects of fluctuations and the exchange interaction. Furthermore, we find a critical dilution x(c)=0.21(2), below which there is no ordering. In agreement with recent Monte Carlo simulations on a similar model, we find no evidence of the experimentally observed freezing of the glassy state in our calculation. We apply the theory of Stephen and Aharony to LiHoxY1-xF4 and find that the theory does predict a finite-temperature freezing of the spin glass. Reasons for the discrepancies are discussed.

  • 7.
    Biltmo, Anders
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics.
    Henelius, Patrik
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    The ferromagnetic transition and domain structure in LiHoF42009In: Europhysics letters, ISSN 0295-5075, E-ISSN 1286-4854, Vol. 87, no 2Article in journal (Refereed)
    Abstract [en]

    Using Monte Carlo simulations we verify that the rare-earth compound LiHoF4 is a very good realization of a dipolar Ising model. With only one free parameter our calculations for the magnetization, specific heat and inverse susceptibility match experimental data at a quantitative level in the 0.5-3 kelvin range, including the ferromagnetic transition at 1.53 K. Using parallel tempering methods and reaching system sizes up to 32000 dipoles with periodic boundary conditions, we are able to give evidence of the logarithmic corrections predicted in renormalization group theory. Due to the long range and angular dependence of the dipolar model, sample shape and domains play a crucial role in the ordered state. We consider surface corrections to Griffiths's theorem, which arise infinite macroscopic samples and lead to a theory of magnetic domains. We find a domain wall energy of 0.059 erg/cm(2) and predict that the ground-state domain structure for cylinders with a demagnetization factor N > 0 consists of thin parallel sheets of opposite magnetization, with a width depending on the demagnetization factor.

  • 8.
    Biltmo, Anders
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics.
    Henelius, Patrik
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Unreachable glass transition in dilute dipolar magnet2012In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 3, p. 857-Article in journal (Refereed)
    Abstract [en]

    In magnetic systems the combined effects of disorder and frustration may cause the moments to freeze into a disordered state at a spin-glass transition. Recent experiments have shown that the rare earth compound LiHo0.045Y0.955F4 freezes, but that the transition is unreachable because of dynamics that are 10(7) times slower than in ordinary spin-glass materials. This conclusion refutes earlier investigations reporting a speed-up of the dynamics into an exotic anti-glass phase caused by entanglement of quantum dipoles. Here we present a theory, backed by numerical simulations, which describes the material in terms of classical dipoles governed by Glauber dynamics. The dipoles freeze and we find that the ultra-slow dynamics are caused by rare, strongly ordered clusters, which give rise to a previously predicted, but hitherto unobserved, Griffths phase between the paramagnetic and spin-glass phases. In addition, the hyperfine interaction creates a high energy barrier to flipping the electronic spin, resulting in a clear signature in the dynamic correlation function.

  • 9. Bovo, L.
    et al.
    Twengström, Mikael
    KTH, School of Engineering Sciences (SCI), Physics.
    Petrenko, O. A.
    Fennell, T.
    Gingras, M. J. P.
    Bramwell, S. T.
    Henelius, Patrik
    KTH, School of Engineering Sciences (SCI), Physics.
    Special temperatures in frustrated ferromagnets2018In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 9, no 1, article id 1999Article in journal (Refereed)
    Abstract [en]

    The description and detection of unconventional magnetic states, such as spin liquids, is a recurring topic in condensed matter physics. While much of the efforts have traditionally been directed at geometrically frustrated antiferromagnets, recent studies reveal that systems featuring competing antiferromagnetic and ferromagnetic interactions are also promising candidate materials. We find that this competition leads to the notion of special temperatures, analogous to those of gases, at which the competing interactions balance, and the system is quasi-ideal. Although induced by weak perturbing interactions, these special temperatures are surprisingly high and constitute an accessible experimental diagnostic of eventual order or spin-liquid properties. The well characterised Hamiltonian and extended low-temperature susceptibility measurement of the canonical frustrated ferromagnet Dy2Ti2O7 enables us to formulate both a phenomenological and microscopic theory of special temperatures for magnets. Other members of this class of magnets include kapellasite Cu3Zn(OH)6Cl2 and the spinel GeCo2O4.

  • 10. Chakraborty, P. B.
    et al.
    Henelius, Patrik
    KTH, Superseded Departments, Physics.
    Kjonsberg, H.
    Sandvik, A. W.
    Girvin, S. M.
    Theory of the magnetic phase diagram of LiHoF42004In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 70, no 14Article in journal (Refereed)
    Abstract [en]

    The properties of LiHoF4 are believed to be well described by a long-range dipolar Ising model. We go beyond mean-field theory and calculate the phase diagram of the Ising model in a transverse field using a quantum Monte Carlo method. The relevant Ising degrees of freedom are obtained using a nonperturbative projection onto the low-lying crystal-field eigenstates. We explicitly take the domain structure into account, and the strength of the near-neighbor exchange interaction is obtained as a fitting parameter. The on-site hyperfine interaction is approximately taken into account through a renormalization of the transverse applied magnetic field. Finally, we propose a spectroscopy experiment to precisely measure the most important parameter controlling the location of the phase boundary.

  • 11.
    Edberg, Richard
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Sandberg, L. Orduk
    Univ Copenhagen, Nanosci Ctr, Niels Bohr Inst, Univ Pk 5, DK-2100 Copenhagen O, Denmark..
    Bakke, I. M. Bergh
    Univ Oslo, Ctr Mat Sci & Nanotechnol, N-0315 Oslo, Norway..
    Haubro, M. L.
    Univ Copenhagen, Nanosci Ctr, Niels Bohr Inst, Univ Pk 5, DK-2100 Copenhagen O, Denmark..
    Folkers, L. C.
    Lund Univ, Dept Chem, Lund, Sweden..
    Mangin-Thro, L.
    Inst Laue Langevin, F-38042 Grenoble, France..
    Wildes, A.
    Inst Laue Langevin, F-38042 Grenoble, France..
    Zaharko, O.
    Paul Scherrer Inst, Lab Neutron Scattering, CH-5232 Villigen, Switzerland..
    Guthrie, M.
    European Spallat Source ERIC, S-22363 Lund, Sweden.;Univ Edinburgh, Sch Phys & Astron, Edinburgh, Midlothian, Scotland.;Univ Edinburgh, Ctr Sci Extreme Condit, Edinburgh, Midlothian, Scotland..
    Holmes, A. T.
    European Spallat Source ERIC, S-22363 Lund, Sweden..
    Sorby, M. H.
    Inst Energy Technol, Dept Neutron Mat Characterizat, POB 40, N-2027 Kjeller, Norway..
    Lefmann, K.
    Univ Copenhagen, Nanosci Ctr, Niels Bohr Inst, Univ Pk 5, DK-2100 Copenhagen O, Denmark..
    Deen, P. P.
    Univ Copenhagen, Nanosci Ctr, Niels Bohr Inst, Univ Pk 5, DK-2100 Copenhagen O, Denmark.;European Spallat Source ERIC, S-22363 Lund, Sweden..
    Henelius, Patrik
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory. Abo Akad Univ, Fac Sci & Engn, Turku, Finland..
    Dipolar spin ice under uniaxial pressure2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 100, no 14, article id 144436Article in journal (Refereed)
    Abstract [en]

    The magnetically frustrated spin ice family of materials is host to numerous exotic phenomena such as magnetic monopole excitations and macroscopic residual entropy extending to low temperature. A finite-temperature ordering transition in the absence of applied fields has not been experimentally observed in the classical spin ice materials Dy2Ti2O7 and Ho2Ti2O7. Such a transition could be induced by the application of pressure, and in this work we consider the effects of uniaxial pressure on classical spin ice systems. Theoretically, we find that the pressure-induced ordering transition in Dy2Ti2O7 is strongly affected by the dipolar interaction. We also report measurements of the neutron structure factor of Ho2Ti2O7 under pressure and compare the experimental results to the predictions of our theoretical model.

  • 12.
    Giblin, S. R.
    et al.
    Cardiff Univ, Sch Phys & Astron, Cardiff CF24 3AA, S Glam, Wales..
    Twengström, Mikael
    KTH, School of Engineering Sciences (SCI), Physics.
    Bovo, L.
    UCL, London Ctr Nanotechnol, 17-19 Gordon St, London WC1H 0AH, England.;UCL, Dept Phys & Astron, 17-19 Gordon St, London WC1H 0AH, England.;UCL, Dept Innovat & Enterprise, 90 Tottenham Court Rd, London W1T 4TJ, England..
    Ruminy, M.
    Paul Scherrer Inst, Lab Neutron Scattering & Imaging, CH-5232 Villigen, Switzerland..
    Bartkowiak, M.
    Paul Scherrer Inst, Lab Neutron Scattering & Imaging, CH-5232 Villigen, Switzerland..
    Manuel, P.
    Rutherford Appleton Lab, ISIS Facil, Didcot OX11 0QX, Oxon, England..
    Andresen, J. C.
    Ben Gurion Univ Negev, Dept Phys, IR-84105 Beer Sheva, Israel..
    Prabhakaran, D.
    Univ Oxford, Phys Dept, Clarendon Lab, Oxford OX1 3PU, England..
    Balakrishnan, G.
    Univ Warwick, Dept Phys, Coventry CV4 7AL, W Midlands, England..
    Pomjakushina, E.
    Paul Scherrer Inst, Lab Sci Dev, CH-5232 Villigen, Switzerland..
    Paulsen, C.
    Univ Joseph Fourier, CNRS, Inst Neel, BP 166, F-38042 Grenoble, France..
    Lhotel, E.
    Univ Joseph Fourier, CNRS, Inst Neel, BP 166, F-38042 Grenoble, France..
    Keller, L.
    Paul Scherrer Inst, Lab Neutron Scattering & Imaging, CH-5232 Villigen, Switzerland..
    Frontzek, M.
    Oak Ridge Natl Lab, Neutron Scattering Div, Oak Ridge, TN USA..
    Capelli, S. C.
    Rutherford Appleton Lab, ISIS Facil, Didcot OX11 0QX, Oxon, England..
    Zaharko, O.
    Paul Scherrer Inst, Lab Neutron Scattering & Imaging, CH-5232 Villigen, Switzerland..
    McClarty, P. A.
    Max Planck Inst Phys Komplexer Syst, Nothnitzer Str 38, D-01187 Dresden, Germany..
    Bramwell, S. T.
    UCL, London Ctr Nanotechnol, 17-19 Gordon St, London WC1H 0AH, England.;UCL, Dept Phys & Astron, 17-19 Gordon St, London WC1H 0AH, England..
    Henelius, Patrik
    KTH, School of Engineering Sciences (SCI), Physics.
    Fennell, T.
    Paul Scherrer Inst, Lab Neutron Scattering & Imaging, CH-5232 Villigen, Switzerland..
    Pauling Entropy, Metastability, and Equilibrium in Dy2Ti2O7 Spin Ice2018In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 121, no 6, article id 067202Article in journal (Refereed)
    Abstract [en]

    Determining the fate of the Pauling entropy in the classical spin ice material Dy2Ti2O7 with respect to the third law of thermodynamics has become an important test case for understanding the existence and stability of ice-rule states in general. The standard model of spin ice-the dipolar spin ice model-predicts an ordering transition at T approximate to 0.15 K, but recent experiments by Pomaranski et al. suggest an entropy recovery over long timescales at temperatures as high as 0.5 K, much too high to be compatible with the theory. Using neutron scattering and specific heat measurements at low temperatures and with long timescales ( 0.35 K/10(6) s and 0.5 K/10(5) s, respectively) on several isotopically enriched samples, we find no evidence of a reduction of ice-rule correlations or spin entropy. High-resolution simulations of the neutron structure factor show that the spin correlations remain well described by the dipolar spin ice model at all temperatures. Furthermore, by careful consideration of hyperfine contributions, we conclude that the original entropy measurements of Ramirez et al. are, after all, essentially correct: The short-time relaxation method used in that study gives a reasonably accurate estimate of the equilibrium spin ice entropy due to a cancellation of contributions.

  • 13. Gingras, M. J. P.
    et al.
    Henelius, Patrik
    KTH, School of Engineering Sciences (SCI), Theoretical Physics.
    Collective phenomena in the LiHo xY 1-xF 4 quantum ising magnet: Recent progress and open questions2011In: Journal of Physics: Conference Series, ISSN 1742-6588, Vol. 320, p. 012001-Article in journal (Refereed)
    Abstract [en]

    In LiHo xY 1-xF 4, the magnetic Holmium Ho 3+ ions behave as effective Ising spins that can point parallel or antiparallel to the crystalline c-axis. The predominant inter-Ho 3+ interaction is dipolar, while the Y 3+ ions are non-magnetic. The application of a magnetic field B x transverse to the c-axis Ising direction leads to quantum spin-flip fluctuations, making this material a rare physical realization of the celebrated transverse field Ising model. The problems of classical and transverse-field-induced quantum phase transitions in LiHo xY 1-xF 4 in the dipolar ferromagnetic (x = 1), diluted ferromagnetic (0.25 ≤∼ x < 1) and highly diluted x ≤∼ 0.25 dipolar spin glass regimes have attracted much experimental and theoretical interest over the past twenty-five years. Two questions have received particular attention: (i) is there an antiglass (quantum disordered) phase at low Ho 3+ concentration and (ii) what is the mechanism responsible for the fast B x-induced destruction of the ferromagnetic (0.25 ≤∼ x < 1) and spin glass (x ≤∼ 0.25) phases? This paper reviews some of the recent theoretical and experimental progress in our understanding of the collective phenomena at play in LiHo xY 1-xF 4, in both zero and nonzero B x.

  • 14.
    Henelius, Patrik
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Fishman, R. S.
    Hybrid quantum-classical Monte Carlo study of a molecule-based magnet2008In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 78, no 21Article in journal (Refereed)
    Abstract [en]

    Using a Monte Carlo (MC) method, we study an effective model for the Fe(II)Fe(III) bimetallic oxalates. Within a hybrid quantum-classical MC algorithm, the Heisenberg S=2 and S-'=5/2 spins on the Fe(II) and Fe(III) sites are updated using a quantum MC loop while the Ising-type orbital angular momenta on the Fe(II) sites are updated using a single-spin classical MC flip. The effective field acting on the orbital angular momenta depends on the quantum state of the system. We find that the mean-field phase diagram for the model is surprisingly robust with respect to fluctuations. In particular, the region displaying two compensation points shifts and shrinks but remains finite.

  • 15.
    Henelius, Patrik
    et al.
    KTH, Superseded Departments, Physics.
    Frobrich, P.
    Kuntz, P. J.
    Timm, C.
    Jensen, P. J.
    Quantum Monte Carlo simulation of thin magnetic films2002In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 66, no 9Article in journal (Refereed)
    Abstract [en]

    The stochastic series expansion quantum Monte Carlo method is used to study thin ferromagnetic films, described by a Heisenberg model including local anisotropies. The magnetization curve is calculated, and the results compared to Schwinger boson and many-body Green's function calculations. A transverse field is introduced in order to study the reorientation effect, in which the magnetization changes from out of plane to in plane. Since the approximate theoretical approaches above differ significantly from each other, and the Monte Carlo method is free of systematic errors, the calculation provides an unbiased check of the approximate treatments. By studying quantum spin models with local anisotropies, varying spin size, and a transverse field, we also demonstrate the general applicability of the recent cluster-loop formulation of the stochastic series expansion quantum Monte Carlo method.

  • 16.
    Henelius, Patrik
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Lin, T.
    Enjalran, M.
    Hao, Z.
    Rau, J. G.
    Altosaar, J.
    Flicker, F.
    Yavors'kii, T.
    Gingras, M. J. P.
    Refrustration and competing orders in the prototypical Dy2Ti2O7 spin ice material2016In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 93, no 2, article id 024402Article in journal (Refereed)
    Abstract [en]

    Spin ices, frustratedmagnetic materials analogous to common water ice, have emerged over the past 15 years as exemplars of high frustration in three dimensions. Recent experimental developments aimed at interrogating anew the low-temperature properties of these systems, in particular whether the predicted transition to long-range order occurs, behoove researchers to scrutinize our current dipolar spin ice model description of these materials. In this work, we do so by combining extensive Monte Carlo simulations and mean-field theory calculations to analyze data from previous magnetization, diffuse neutron scattering, and specific-heat measurements on the paradigmatic Dy2Ti2O7 spin ice material. In this work, we also reconsider the possible importance of the nuclear specific heat C-nuc in Dy2Ti2O7. We find that C-nuc is not entirely negligible below a temperature similar to 0.5 K and must therefore be taken into account in a quantitative analysis of the calorimetric data of this compound below that temperature. We find that in this material, small effective spin-spin exchange interactions compete with the magnetostatic dipolar interaction responsible for the main spin ice phenomenology. This causes an unexpected " refrustration" of the long-range order that would be expected from the incompletely self-screened dipolar interaction and which positions the material at the boundary between two competing classical long-range-ordered ground states. This allows for the manifestation of new physical low-temperature phenomena in Dy2Ti2O7, as exposed by recent specific-heat measurements. We show that among the four most likely causes for the observed upturn of the specific heat at low temperature [an exchange-induced transition to long-range order, quantum non-Ising (transverse) terms in the effective spin Hamiltonian, the nuclear hyperfine contribution, and random disorder], only the last appears to be reasonably able to explain the calorimetric data.

  • 17. Henelius, Patrik
    et al.
    Sandvik, A. W.
    Sign problem in Monte Carlo simulations of frustrated quantum spin systems2000In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 62, no 2, p. 1102-1113Article in journal (Refereed)
    Abstract [en]

    We discuss the: sign problem arising in Monte Carlo simulations of frustrated quantum spin systems. We show that for a class of semifrustrated systems [Heisenberg models with ferromagnetic couplings J(z)(r) < 0 along the; axis and antiferromagnetic couplings J(yx)(r) = -J(z)(r) in the xy plane, for arbitrary distances, 1. the sign problem present for algorithms operating in the z basis can be solved within a recent operator-loop formulation of the stochastic series expansion method [a cluster algorithm for sampling the diagonal matrix elements of the power series expansion of exp(-beta H) to all orders]. The solution relies on the identification of operator loops which change the configuration sign when updated (merons) and is similar to the meron-cluster algorithm recently proposed by Chandrasekharan and Wiese for solving the sign problem fur a class of fermion models [Phys. Rev. Lett. 83, 3116 (1999]. Some important expectation values, e.g., the internal energy, can be evaluated in the subspace with no merons, where the weight function is positive definite. Calculations of other expectation values require sampling of configurations with only a small number of merons (typically zero or two), with an accompanying sign problem which is not serious. We also discuss problems which arise in applying the meron concept to more general quantum spin models with frustrated interactions.

  • 18. Henelius, Patrik
    et al.
    Sandvik, A. W.
    Timm, C.
    Girvin, S. M.
    Monte Carlo study of a two-dimensional quantum ferromagnet2000In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 61, no 1, p. 364-374Article in journal (Refereed)
    Abstract [en]

    We present quantum Monte Carlo results for the field and temperature dependence of the magnetization and the spin lattice relaxation rate 1/T-1 of a two-dimensional S = 1/2 quantum Heisenberg ferromagnet. The Monte Carlo method, which yields results free of systematic errors, is described in detail. The high accuracy of the calculated magnetization allows for stringent tests of recent approximate analytical calculations. We also compare our results with recent experimental data for a nu = 1 quantum Hall ferromagnet, which is expected to be well described by the Heisenberg model. The dynamic response function needed to extract 1/T-1 is obtained using maximum-entropy analytic continuation of the corresponding imaginary-time dependent correlation function. We discuss the reliability of this approach.

  • 19. Laflorencie, N.
    et al.
    Rieger, H.
    Sandvik, A. W.
    Henelius, Patrik
    KTH, Superseded Departments, Physics.
    Crossover effects in the random-exchange spin-1/2 antiferromagnetic chain2004In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 70, no 5Article in journal (Refereed)
    Abstract [en]

    The random antiferromagnetic spin-1/2 XX and XXZ chain is studied numerically for varying strength of the disorder, using exact diagonalization and stochastic series expansion methods. The spin-spin correlation function as well as the stiffness display a clear crossover from the pure behavior (no disorder) to the infinite randomness fixed point or random singlet behavior predicted by the real space renormalization group. The crossover length scale is shown to diverge as xisimilar toD(-gamma), where D is the variance of the random bonds. Our estimates for the exponent gamma agrees well within the error bars with the one for the localization length exponent emerging within an analytical bosonization calculation. Exact diagonalization and stochastic series expansion results for the string correlation function are also presented.

  • 20. Revell, H. M.
    et al.
    Yaraskavitch, L. R.
    Mason, J. D.
    Ross, K. A.
    Noad, H. M. L.
    Dabkowska, H. A.
    Gaulin, B. D.
    Henelius, Patrik
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Kycia, J. B.
    Evidence of impurity and boundary effects on magnetic monopole dynamics in spin ice2013In: Nature Physics, ISSN 1745-2473, E-ISSN 1745-2481, Vol. 9, no 1, p. 34-37Article in journal (Refereed)
    Abstract [en]

    Electrical resistance is a crucial and well-understood property of systems ranging from computer microchips to nerve impulse propagation in the human body. Here we study the motion of magnetic charges in spin ice and find that extra spins inserted in Dy2Ti2O7 trap magnetic monopole excitations and provide the first example of how defects in a spin-ice material obstruct the flow of monopoles-a magnetic version of residual resistance. We measure the time-dependent magnetic relaxation in Dy2Ti2O7 and show that it decays with a stretched exponential followed by a very slow long-time tail. In a Monte Carlo simulation governed by Metropolis dynamics we show that surface effects and a very low level of stuffed spins (0.30%)-magnetic Dy ions substituted for non-magnetic Ti ions-cause these signatures in the relaxation. In addition, we find evidence that the rapidly diverging experimental timescale is due to a temperature-dependent attempt rate proportional to the monopole density.

  • 21.
    Twengström, Mikael
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Bovo, L.
    Gingras, M. J. P.
    Bramwell, S. T.
    Henelius, Patrik
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Microscopic aspects of magnetic lattice demagnetizing factors2017In: PHYSICAL REVIEW MATERIALS, ISSN 2475-9953, Vol. 1, no 4, article id 044406Article in journal (Refereed)
    Abstract [en]

    The demagnetizing factor N is of both conceptual interest and practical importance. Considering localized magnetic moments on a lattice, we show that for nonellipsoidal samples, N depends on the spin dimensionality (Ising, XY, or Heisenberg) and orientation, as well as the sample shape and susceptibility. The generality of this result is demonstrated by means of a recursive analytic calculation as well as detailed Monte Carlo simulations of realistic model spin Hamiltonians. As an important check and application, we also make an accurate experimental determination of N for a representative collective paramagnet (i.e., the Dy2Ti2O7 spin ice compound) and show that the temperature dependence of the experimentally determined N agrees closely with our theoretical calculations. Our conclusion is that the well-established practice of approximating the true sample shape with "corresponding ellipsoids" for systems with long-range interactions will in many cases overlook important effects stemming from the microscopic aspects of the system under consideration.

1 - 21 of 21
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