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  • 1.
    Andersson, Andreas
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Statistical Physics.
    Lidmar, Jack
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Statistical Physics.
    Anomalous Nernst effect and heat transport by vortex vacancies in granular superconductors2010In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 81, no 6, p. 060508-1-060508-4Article in journal (Refereed)
    Abstract [en]

    We study the Nernst effect due to vortex motion in two-dimensional granular superconductors using simulations with Langevin or resistively shunted Josephson-junction dynamics. In particular, we show that the geometric frustration of both regular and irregular granular materials can lead to thermally driven transport of vortices from colder to hotter regions, resulting in a sign reversal of the Nernst signal. We discuss the underlying mechanisms of this anomalous behavior in terms of heat transport by mobile vacancies in an otherwise pinned vortex lattice.

  • 2.
    Andersson, Andreas
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Statistical Physics.
    Lidmar, Jack
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Statistical Physics.
    Influence of vortices and phase fluctuations on thermoelectric transport properties of superconductors in a magnetic field2011In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 83, no 17, p. 174502-Article in journal (Refereed)
    Abstract [en]

    We study heat transport and thermoelectric effects in two-dimensional superconductors in a magnetic field. These are modeled as granular Josephson-junction arrays, forming either regular or random lattices. We employ two different models for the dynamics: relaxational model-A dynamics or resistively and capacitively shunted Josephson junction dynamics. We derive expressions for the heat current in these models, which are then used in numerical simulations to calculate the heat conductivity and Nernst coefficient for different temperatures and magnetic fields. At low temperatures and zero magnetic field the heat conductivity in the RCSJ model is calculated analytically from a spin wave approximation, and is seen to have an anomalous logarithmic dependence on the system size, and also to diverge in the completely overdamped limit C -> 0. From our simulations we find at low magnetic fields that the Nernst signal displays a characteristic "tilted hill" profile similar to experiments and a nonmonotonic temperature dependence of the heat conductivity. We also investigate the effects of granularity and randomness, which become important for higher magnetic fields. In this regime geometric frustration strongly influences the results in both regular and random systems and leads to highly nontrivial magnetic field dependencies of the studied transport coefficients.

  • 3.
    Andersson, Andreas
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Statistical Physics.
    Lidmar, Jack
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Statistical Physics.
    Modeling and simulations of quantum phase slips in ultrathin superconducting wiresManuscript (preprint) (Other academic)
  • 4.
    Andersson, Andreas
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics.
    Lidmar, Jack
    KTH, School of Engineering Sciences (SCI), Theoretical Physics.
    Modeling and simulations of quantum phase slips in ultrathin superconducting wires2015In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 91, no 13, article id 134504Article in journal (Refereed)
    Abstract [en]

    We study quantum phase slips (QPS) in ultrathin superconducting wires. Starting from an effective one-dimensional microscopic model, which includes electromagnetic fluctuations, we map the problem to a (1+1)-dimensional gas of interacting instantons. We introduce a method to calculate the tunneling amplitude of quantum phase slips directly from Monte Carlo simulations. This allows us to go beyond the dilute instanton gas approximation and study the problem without any limitations of the density of QPS. We find that the tunneling amplitude shows a characteristic scaling behavior near the superconductor-insulator transition. We also calculate the voltage-charge relation of the insulating state, which is the dual of the Josephson current-phase relation in ordinary superconducting weak links. This evolves from a sinusoidal form in the regime of dilute QPS to more exotic shapes for higher QPS densities, where interactions are important.

  • 5.
    Andersson, Andreas
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Statistical Physics.
    Lidmar, Jack
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Statistical Physics.
    Scaling, Finite Size Effects, and Crossovers of the Resistivity and Current-Voltage Characteristics in Two-Dimensional SuperconductorsManuscript (preprint) (Other academic)
    Abstract [en]

    We revisit the scaling properties of the resistivity and the current-voltage characteristics at and below the Berezinskii-Kosterlitz-Thouless transition, both in zero and nonzero magnetic field. The scaling properties are derived by integrating the renormalization group flow equations up to a scale where they can be reliably matched to simple analytic expressions. The vortex fugacity turns out to be dangerously irrelevant for these quantities below $T_c$, thereby altering the scaling behavior. We derive the possible crossover effects as the current, magnetic field or system size is varied, and find a strong multiplicative logarithmic correction near $T_c$, all which is necessary to account for when interpreting experiments and simulation data. Our analysis clarifies a longstanding discrepancy between the finite size dependence found in many simulations and the current-voltage characteristics of experiments. We further show that the logarithmic correction can be avoided by approaching the transition in a magnetic field, thereby simplifying the scaling analysis. We confirm our results by large scale numerical simulations, and calculate the dynamic critical exponent $z$, for relaxational Langevin dynamics and for resistively and capacitively shunted Josephson junction dynamics.

  • 6.
    Andersson, Andreas
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Statistical Physics.
    Lidmar, Jack
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Statistical Physics.
    Scaling, finite size effects, and crossovers of the resistivity and current-voltage characteristics in two-dimensional superconductors2013In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 87, no 22, p. 224506-Article in journal (Refereed)
    Abstract [en]

    We revisit the scaling properties of the resistivity and the current-voltage characteristics at and below the Berezinskii-Kosterlitz-Thouless transition, both in zero and nonzero magnetic fields. The scaling properties are derived by integrating the renormalization group flow equations up to a scale where they can be reliably matched to simple analytic expressions. The vortex fugacity turns out to be dangerously irrelevant for these quantities below T-c, thereby altering the scaling behavior. We derive the possible crossover effects as the current, magnetic field, or system size is varied, and find a strong multiplicative logarithmic correction near T-c, all of which is necessary to account for when interpreting experiments and simulation data. Our analysis clarifies a longstanding discrepancy between the finite size dependence found in many simulations and the current-voltage characteristics of experiments. We further show that the logarithmic correction can be avoided by approaching the transition in a magnetic field, thereby simplifying the scaling analysis. We confirm our results by large-scale numerical simulations, and calculate the dynamic critical exponent z, for relaxational Langevin dynamics and for resistively and capacitively shunted Josephson junction dynamics.

  • 7.
    Andersson, Magnus
    et al.
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Espinosa-Arronte, Beatriz
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    van der Beek, C. J.
    Nikolaou, M.
    Lidmar, J.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics.
    Wallin, M.
    A vortex solid-to-liquid transition with fully anisotropic scaling2009In: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 150, no 5Article in journal (Refereed)
    Abstract [en]

    The vortex solid-to-liquid transition has been studied in heavy ion irradiated untwinned single crystals of YBa2Cu3O7-delta with an inclined applied magnetic field. For magnetic fields tilted at angles about 45 degrees away from the columnar defects, we find that the electric resistivity in the vortex liquid regime approaches zero with power laws in the reduced temperature T - T-c that have different exponents in all three spatial directions. Since the symmetry in the problem has been broken in two non-collinear directions by i) the direction of the columnar defects and ii) the direction of the applied magnetic field, our findings give evidence for a new type of critical behavior with fully anisotropic critical exponents. A possible view of the vortex topology for the transition is also suggested.

  • 8.
    Ergül, Adem
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Lidmar, Jack
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Statistical Physics.
    Johansson, Jan
    Azizoglu, Yagiz
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Schaeffer, David
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Haviland, David B.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Localizing quantum phase slips in one-dimensional Josephson junction chains2013In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 15, p. 095014-Article in journal (Refereed)
    Abstract [en]

    We studied quantum phase-slip (QPS) phenomena in long one-dimensional Josephson junction series arrays with tunable Josephson coupling. These chains were fabricated with as many as 2888 junctions, where one sample had a separately tunable link in the middle of the chain. Measurements were made of the zero-bias resistance, R-0, as well as current-voltage characteristics (IVC). The finite R-0 is explained by QPS and shows an exponential dependence on root E-J/E-C with a distinct change in the exponent at R-0 = R-Q = h/4e(2). When R-0 > R-Q, the IVC clearly shows a remnant of the Coulomb blockade, which evolves to a zero-current state with a sharp critical voltage as E-J is tuned to a smaller value. The zero-current state below the critical voltage is due to coherent QPSs and we show that these are enhanced when the central link is weaker than all other links. Above the critical voltage, a negative, differential resistance is observed, which nearly restores the zero-current state.

  • 9.
    Ergül, Adem
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics.
    Schaeffer, David
    KTH.
    Lindblom, Magnus
    KTH, School of Engineering Sciences (SCI), Applied Physics.
    Haviland, David B.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Lidmar, Jack
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Statistical Physics.
    Johansson, Jan
    Phase sticking in one-dimensional Josephson junction chains2013In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 88, no 10, p. 104501-Article in journal (Refereed)
    Abstract [en]

    We studied current-voltage characteristics of long one-dimensional Josephson junction chains with Josephson energy much larger than charging energy, E-J >> E-C. In this regime, typical I-V curves of the samples consist of a supercurrent-like branch at low-bias voltages followed by a voltage-independent chain current branch, I-chain at high bias. Our experiments showed that I-chain is not only voltage-independent but it is also practically temperature-independent up to T = 0.7T(C). We have successfully model the transport properties in these chains using a capacitively shunted junction model with nonlinear damping.

  • 10.
    Espinosa-Arronte, Beatriz
    et al.
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Andersson, Magnus
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    van der Beek, C. J.
    Laboratoire des Solides Irradiés, CNRS-UMR 7642 and CEA/DSM/DRECAM, Ecole Polytechnique, Palaiseau.
    Nikolaou, Marios
    KTH, School of Engineering Sciences (SCI), Applied Physics.
    Lidmar, Jack
    KTH, School of Engineering Sciences (SCI), Applied Physics.
    Wallin, Mats
    KTH, School of Engineering Sciences (SCI), Applied Physics.
    Fully anisotropic superconducting transition in ion-irradiated YBa2Cu3O7-delta with a tilted magnetic field2007In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 75, no 10, p. 100504-Article in journal (Refereed)
    Abstract [en]

    We consider the superconducting vortex solid-to-liquid transition in heavy ion-irradiated untwinned YBa2Cu3O7-delta single crystals in the case where the magnetic field direction does not coincide with that of the irradiation-induced linear columnar defects. For a certain range of angles, the resistivities measured in three orthogonal spatial directions vanish at the transition as three clearly different powers of reduced temperature. At previously known second-order phase transitions, scaling of physical quantities has either been isotropic or anisotropic in one direction. Thus, our findings yield evidence for a new type of critical behavior with fully anisotropic critical exponents.

  • 11.
    Hutter, Carsten
    et al.
    Stockholm University, Physics.
    Stannigel, Kai
    Stockholm University, Physics.
    Tholén, Erik
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Lidmar, Jack
    KTH, School of Engineering Sciences (SCI), Theoretical Physics.
    Haviland, David
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Josephson junction transmission lines as tunable artificial crystals2011In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 83, no 1, p. 014511-Article in journal (Refereed)
    Abstract [en]

    We investigate one-dimensional Josephson junction arrays with generalized unit cells, beyond a single junction or SQUID, as a circuit approach to engineer band gaps. Within a specific frequency range, of the order of the single junction plasma frequency, the dispersion relation becomes gapped and the impedance becomes purely imaginary. We derive the parameter dependence of this gap and suggest designs to lower it to microwave frequencies. The gap can be tuned in a wide frequency range by applying external flux, and persists in the presence of small imperfections. These arrays, which can be thought of as tunable artificial crystals, may find use in applications ranging filters to the protection of quantum bits.

  • 12.
    Langmann, Edwin
    et al.
    KTH, Superseded Departments, Physics.
    Lidmar, Jack
    KTH, Superseded Departments, Physics.
    Salmhofer, Manfred
    Wallin, Mats
    KTH, Superseded Departments, Physics.
    Mean field analysis of a model for superconductivity in an anti-ferromagnetic background1998In: Physica. C, Superconductivity, ISSN 0921-4534, E-ISSN 1873-2143, Vol. 296, p. 119-136Article in journal (Refereed)
    Abstract [en]

    We study a lattice fermion model for superconductivity in the presence of an anti-ferromagnetic background, described as a fixed external staggered magnetic field. We discuss the possibility that our model provides an effective description of coexistence of anti-ferromagnetic correlations and superconductivity. and its possible application to high-temperature superconductivity. We also argue that, under certain conditions, this model describes a variant of the periodic Anderson model for heavy fermions. Using a path integral formulation we construct mean field equations, which we study in some detail. We evaluate the superconducting critical temperature and show thar it is strongly enhanced by anti-ferromagnetic order. We also evaluate the superconducting gap, the superconducting density of states, and the tunneling conductivity, and show that the most stable channel usually has a d(x2-y2)-wave gap.

  • 13.
    Lidmar, Jack
    KTH, Superseded Departments, Physics.
    Amorphous vortex glass phase in strongly disordered superconductors2003In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 91, no 9Article in journal (Refereed)
    Abstract [en]

    We introduce a model describing vortices in strongly disordered three-dimensional superconductors. The model focuses on the topological defects, i.e., dislocation lines, in an elastic description of the vortex lattice. The model is studied using Monte Carlo simulations, revealing a glass phase at low temperatures, separated by a continuous phase transition to the high temperature resistive vortex liquid phase. The critical exponents nuapproximate to1.3 and etaapproximate to-0.4 characterizing the transition are obtained from finite size scaling.

  • 14.
    Lidmar, Jack
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Statistical Physics.
    Comment on "€œCritical Dynamics of a Vortex-Loop Model for the Superconducting Transition"2002In: Physical Review Letters, Vol. 89, no 10Article in journal (Refereed)
  • 15.
    Lidmar, Jack
    KTH, School of Engineering Sciences (SCI), Theoretical Physics.
    Improving the efficiency of extended ensemble simulations: The accelerated weight histogram method2012In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics, ISSN 1539-3755, E-ISSN 1550-2376, Vol. 85, no 5, p. 056708-Article in journal (Refereed)
    Abstract [en]

    We propose a method for efficient simulations in extended ensembles, useful, e. g., for the study of problems with complex energy landscapes and for free energy calculations. The main difficulty in such simulations is the estimation of the a priori unknown weight parameters needed to produce flat histograms. The method combines several complementary techniques, namely, a Gibbs sampler for the parameter moves, a reweighting procedure to optimize data use, and a Bayesian update allowing for systematic refinement of the free energy estimate. In a certain limit the scheme reduces to the 1/t algorithm of B. E. Belardinelli and V. D. Pereyra [Phys. Rev. E 75, 046701 (2007)]. The performance of the method is studied on the two-dimensional Ising model, where comparison with the exact free energy is possible, and on an Ising spin glass.

  • 16.
    Lidmar, Jack
    KTH, Superseded Departments, Physics.
    Ordering and phase transitions in models of high temperature superconductors1997Licentiate thesis, comprehensive summary (Other scientific)
  • 17.
    Lidmar, Jack
    KTH, Superseded Departments, Physics.
    Phase transitions in high-temperature superconductors1998Doctoral thesis, comprehensive summary (Other scientific)
    Abstract [en]

    Thermal fluctuations and disorder strongly influence the behaviour of hightemperature superconductors. In particular the vortices play a key role in determining their properties. In this thesis the main focus lies on phase transitions, both in ultra-thin films and in three-dimensional systems, which are driven by vortex fluctuations. The last paper concerns the influence of antiferromagnetism on superconductivity in a simple model. A brief review of these topics is given in the introductory part.

    The main results are:

    The phase transition in ultra-thin superconducting/superfluid films is studied within the two-dimensional Coulomb gas model, which is known to have a Berezinskii-Kosterlitz-Thouless transition at low vortex densities. We construct the phase diagram from grand canonical Monte Carlo simulations on a continuum, without any restrictions on the vortex density.

    The dynamical universality classes for vortices in superconductors in zero magnetic field are studied by means of Monte Carlo simulations, with particular attention to the role of screening of the vortex interaction.

    We construct a formula for the k = 0 helicity modulus directly in terms of the vortex line fluctuations, which can serve as a useful way to detect superconducting coherence in model calculations. A method for simulating vortex lines on a continuum is developed, and used to study the melting of the Abrikosov vortex lattice.

    We study the critical dynamics for vortices in the presence of columnar defects. The linear resistivity and current-voltage characteristics are calculated in Monte Carlo simulations, and the critical behaviour extracted using finite size scaling. We reconsider the scaling properties as the magnetic field is tilted away from the direction of the columns.

    The influence of antiferromagnetic correlations on the superconducting properties is studied in a simplified lattice fermion model for superconductivity in the presence of an antiferromagnetic background. We find that the superconducting critical temperature is enhanced by antiferromagnetic order, and that a gap with dx2-y2-wave symmetry is the most stable.

  • 18.
    Lidmar, Jack
    et al.
    KTH, Superseded Departments, Physics.
    Mirny, L.
    Nelson, D. R.
    Virus shapes and buckling transitions in spherical shells2003In: 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, no 5Article in journal (Refereed)
    Abstract [en]

    We show that the icosahedral packings of protein capsomeres proposed by Caspar and Klug for spherical viruses become unstable to faceting for sufficiently large virus size, in analogy with the buckling instability of disclinations in two-dimensional crystals. Our model, based on the nonlinear physics of thin elastic shells, produces excellent one-parameter fits in real space to the full three-dimensional shape of large spherical viruses. The faceted shape depends only on the dimensionless Foppl-von Karman number gamma=YR2/kappa, where Y is the two-dimensional Young's modulus of the protein shell, kappa is its bending rigidity, and R is the mean virus radius. The shape can be parametrized more quantitatively in terms of a spherical harmonic expansion. We also investigate elastic shell theory for extremely large gamma, 10(3)

  • 19. Lidmar, Jack
    et al.
    Nelson, D. R.
    Gorokhov, D. A.
    Vortex wandering among splayed columnar defects2001In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 6414, no 14Article in journal (Refereed)
    Abstract [en]

    We investigate the scaling properties of single flux lines in a random pinning landscape consisting of splayed columnar defects. Such correlated defects can be injected into type II superconductors by inducing nuclear fission or via direct heavy ion irradiation. The result is often very efficient pinning of the vortices which gives, e.g., a strongly enhanced critical current. The wandering exponent zeta and the free energy exponent omega of a single flux line in such a disordered environment are obtained analytically from scaling arguments combined with extreme-value statistics. In contrast to the case of point disorder, where these exponents Lire universal, we find a dependence of the exponents on details in the probability distribution of the low lying energies of the columnar defects. The analytical results show excellent agreement with numerical transfer matrix calculations in two and three dimensions.

  • 20.
    Lidmar, Jack
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Statistical Physics.
    Wallin, Mats
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Statistical Physics.
    Critical properties of Bose-glass superconductors1999In: Europhysics letters, ISSN 0295-5075, E-ISSN 1286-4854, Vol. 47, no 4Article in journal (Refereed)
    Abstract [en]

    We study vortex lines in high-temperature superconductors with columnar defects produced by heavy-ion irradiation. We reconsider scaling theory for the Bose-glass transition with tilted magnetic fields, and propose, e.g. , a new scaling form for the shape of the Bose glass phase boundary, which is relevant for experiments. We also consider Monte Carlo simulations for a vortex model with a screened interaction. Critical exponents are determined from scaling analysis of Monte Carlo data for current-voltage characteristics and other quantities. The dynamic critical exponent is found to be z = 4.6 ± 0.3.

  • 21.
    Lidmar, Jack
    et al.
    KTH, Superseded Departments, Physics.
    Wallin, Mats
    KTH, Superseded Departments, Physics.
    Monte Carlo simulation of a two-dimensional continuum Coulomb gas1997In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 55, p. 522-530Article in journal (Refereed)
    Abstract [en]

    We study the classical two-dimensional (2D) Coulomb-gas model for thermal vortex fluctuations in thin superconducting/superfluid films by Monte Carlo simulation of a grand-canonical vortex ensemble defined on a continuum. The Kosterlitz-Thouless transition is well understood at low vortex density, but at high vortex density the nature of the phase diagram and of the vortex phase transition is less clear. From our Monte Carlo data we construct phase diagrams for the 2D Coulomb gas without any restrictions on the vortex density. For negative vortex chemical potential (positive vortex core energy) we always find a Kosterlitz-Thouless transition. Only if the Coulomb interaction is supplemented with a short-distance repulsion is a first-order transition Line found, above some positive value of the vortex chemical potential.

  • 22.
    Lidmar, Jack
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Statistical Physics.
    Wallin, Mats
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Statistical Physics.
    Superconducting coherence and the helicity modulus in vortex line models1999In: Physical Review B, Vol. 59, no 13, p. 8451-8454Article in journal (Refereed)
  • 23.
    Lidmar, Jack
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Statistical Physics.
    Wallin, Mats
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Statistical Physics.
    Wengel, Carsten
    Girvin, S. M.
    Young, A. P.
    Dynamical universality classes of the superconducting phase transition1998In: Physical Review B, Vol. 58, no 5, p. 2827-2833Article in journal (Refereed)
  • 24.
    Lindahl, Viveca
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Theoretical & Computational Biophysics.
    Lidmar, Jack
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Statistical Physics.
    Hess, Berk
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Theoretical & Computational Biophysics.
    Accelerated weight histogram method for exploring free energy landscapes2014In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 141, no 4, p. 044110-Article in journal (Refereed)
    Abstract [en]

    Calculating free energies is an important and notoriously difficult task for molecular simulations. The rapid increase in computational power has made it possible to probe increasingly complex systems, yet extracting accurate free energies from these simulations remains a major challenge. Fully exploring the free energy landscape of, say, a biological macromolecule typically requires sampling large conformational changes and slow transitions. Often, the only feasible way to study such a system is to simulate it using an enhanced sampling method. The accelerated weight histogram (AWH) method is a new, efficient extended ensemble sampling technique which adaptively biases the simulation to promote exploration of the free energy landscape. The AWH method uses a probability weight histogram which allows for efficient free energy updates and results in an easy discretization procedure. A major advantage of the method is its general formulation, making it a powerful platform for developing further extensions and analyzing its relation to already existing methods. Here, we demonstrate its efficiency and general applicability by calculating the potential of mean force along a reaction coordinate for both a single dimension and multiple dimensions. We make use of a non-uniform, free energy dependent target distribution in reaction coordinate space so that computational efforts are not wasted on physically irrelevant regions. We present numerical results for molecular dynamics simulations of lithium acetate in solution and chignolin, a 10-residue long peptide that folds into a beta-hairpin. We further present practical guidelines for setting up and running an AWH simulation.

  • 25.
    Nikolaou, Marios
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics.
    Lidmar, Jack
    KTH, School of Engineering Sciences (SCI), Theoretical Physics.
    Wallin, Mats
    KTH, School of Engineering Sciences (SCI), Theoretical Physics.
    Monte Carlo Simulations of a Lattice Gas with Logarithmic InteractionsManuscript (Other academic)
  • 26. Vestergren, A.
    et al.
    Lidmar, Jack
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Statistical Physics.
    Topological order and the deconfinement transition in the (2+1)-dimensional compact Abelian Higgs model2005In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 72, no 17Article in journal (Refereed)
    Abstract [en]

    We study an Abelian compact gauge theory minimally coupled to bosonic matter with charge q, which may undergo a confinement-deconfinement transition in 2+1 dimensions. The transition is analyzed using a nonlocal order parameter W, which is related to large Wilson loops for fractional charges. We map the model to a dual representation with no gauge field but only a global q-state clock symmetry and show that W corresponds to the domain wall energy of that model. W is also directly connected to the concept of topological order. We exploit these facts in Monte Carlo simulations to study the detailed nature of the deconfinement transition.

  • 27. Vestergren, A.
    et al.
    Lidmar, Jack
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Statistical Physics.
    Hansson, T. H.
    Topological order in the (2+1)D compact lattice superconductor2005In: Europhysics letters, ISSN 0295-5075, E-ISSN 1286-4854, Vol. 69, no 2, p. 256-262Article in journal (Refereed)
    Abstract [en]

    We study topological aspects of a compact lattice superconductor, and show that the characteristic energy splitting, Delta, between almost degenerate ground states, is simply related to a novel order parameter (W) over tilde, which is closely related to large Wilson loops. Using Monte Carlo methods, we study the scaling properties of (W) over tilde close to the deconfining phase transition, and conclude that Delta similar to e(-L/xi), where L is the size of the system, thus giving quantitative support to the vortex tunneling scenario proposed by Wen.

  • 28. Vestergren, A.
    et al.
    Lidmar, Jack
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Statistical Physics.
    Wallin, Mats
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Statistical Physics.
    Generalized anisotropic scaling theory and the transverse Meissner transition2005In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 94, no 8Article in journal (Refereed)
    Abstract [en]

    We consider a depinning transition in vortex systems with columnar disorder and tilted applied magnetic fields. From scaling arguments and Monte Carlo simulations, we find that this transverse Meissner transition is governed by a fixed point which is anisotropic in all three directions. This generalization of conventional anisotropic scaling means that the correlation length in different directions diverges with different rates, and we derive exact results for the anisotropy exponents. We make predictions which can be tested in experiments on superconductors with columnar disorder.

  • 29. Vestergren, A.
    et al.
    Lidmar, Jack
    KTH, Superseded Departments, Physics.
    Wallin, Mats
    KTH, Superseded Departments, Physics.
    Unscreened universality class for superconductors with columnar disorder2003In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 67, no 9Article in journal (Refereed)
    Abstract [en]

    The phase transition in a model for vortex lines in high temperature superconductors with columnar defects, i.e., linearly correlated quenched random disorder, is studied with finite size scaling and Monte Carlo simulations. Previous studies of critical properties have mainly focused on the limit of strongly screened vortex line interactions. Here the opposite limit of weak screening is considered. The simulation results provide evidence for a distinct universality class, with values of the critical exponents that differ from the case of strong screening of the vortex interaction. In particular, scaling is anisotropic and characterized by a nontrivial value of the anisotropy exponent zeta=nu(parallel to)/nu(perpendicular to). The exponents we find, zeta=1.25+/-0.1, nu(perpendicular to)=1.0+/-0.1, z=1.95+/-0.1, are similar to certain experimental results for YBa2Cu3O7.

  • 30. Vestergren, A.
    et al.
    Lidmar, Jack
    Wallin, Mats
    KTH, Superseded Departments, Physics.
    Vortex glass transition in a random pinning model2002In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 88, no 11Article in journal (Refereed)
    Abstract [en]

    We study the vortex glass transition in disordered high temperature superconductors using Monte Carlo simulations. We use a random pinning model with strong point-correlated quenched disorder, a net applied magnetic field, long-range vortex interactions, and periodic boundary conditions. From a finite size scaling study of the helicity modulus, the rms current, and the resistivity, we obtain critical exponents at the phase transition. The new exponents differ substantially from those of the gauge glass model, but are close to those of the pure three-dimensional XY model.

  • 31. Widom, M.
    et al.
    Lidmar, Jack
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Statistical Physics.
    Nelson, David R.
    Soft modes near the buckling transition of icosahedral shells2007In: Physical Review E, ISSN 1539-3755, Vol. 76, no 3Article in journal (Refereed)
    Abstract [en]

    Icosahedral shells undergo a buckling transition as the ratio of Young's modulus to bending stiffness increases. Strong bending stiffness favors smooth, nearly spherical shapes, while weak bending stiffness leads to a sharply faceted icosahedral shape. Based on the phonon spectrum of a simplified mass-and-spring model of the shell, we interpret the transition from smooth to faceted as a soft-mode transition. In contrast to the case of a disclinated planar network where the transition is sharply defined, the mean curvature of the sphere smooths the transition. We define elastic susceptibilities as the response to forces applied at vertices, edges, and faces of an icosahedron. At the soft-mode transition the vertex susceptibility is the largest, but as the shell becomes more faceted the edge and face susceptibilities greatly exceed the vertex susceptibility. Limiting behaviors of the susceptibilities are analyzed and related to the ridge-scaling behavior of elastic sheets. Our results apply to virus capsids, liposomes with crystalline order, and other shell-like structures with icosahedral symmetry.

1 - 31 of 31
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