Joakim Hove

Order of the metal-to-superconductor transition

We have studied various aspects of the critical properties of the Abelian Higgs model. The initial motivation to study this model is its relation to superconductivity, but the results extend beyond the realms of superconductivity. This thesis contains an introductory part and three research papers, all related to different aspects of the Abelian Higgs model.Paper 1: We have investigated the properties of the model using a dual vortex representation. By focusing on the propagators of the gauge field A and the dual gauge field h we find a nice demonstration of the fact that the dual of a neutral condensate is isomorphic to a charged condensate. Finally this also provides firm support for the existence of a stable charged fixed point in the theory, distinct from the 3DXY fixed point.Paper 2: The critical fluctuations in the Abelian Higgs model are vortex loops. We have studied the geometrical properties of these loops, and by using duality we have obtained scaling relations between the fractal dimension DH of the loops and the anomalous dimension ηφ of the dual field theory.Paper 3: We have calculated the GL parameter κtri separating a first order metal to superconductor transition from a second order one, κtri =(0.76±0.04))/√2. We also argue qualitatively that this κtri is the value separating type-I and type-II behavior, in contrast to the conventional value 1=√2. The calculations have been done including fluctuations in the amplitude and the phase of the matter-field, as well as fluctuations in the gauge field.Paper 4: We have determined the effective interaction between vortices in the Ginzburg-Landau model from large-scale Monte-Carlo simulations. We find a change, in the form of a crossover, from attractive to repulsive effective vortex interactions in an intermediate range of Ginzburg-Landau parameters κe[0.76; 1]=√2, depending on temperature. We present a simple physical picture of the crossover, and relate it to observations in Ta and Nb elemental superconductors which have low-temperature values of κ in the relevant range.

Vortex interactions and thermally induced crossover from type-I to type-II superconductivity

We have studied various aspects of the critical properties of the Abelian Higgs model. The initial motivation to study this model is its relation to superconductivity, but the results extend beyond the realms of superconductivity. This thesis contains an introductory part and three research papers, all related to different aspects of the Abelian Higgs model.Paper 1: We have investigated the properties of the model using a dual vortex representation. By focusing on the propagators of the gauge field A and the dual gauge field h we find a nice demonstration of the fact that the dual of a neutral condensate is isomorphic to a charged condensate. Finally this also provides firm support for the existence of a stable charged fixed point in the theory, distinct from the 3DXY fixed point.Paper 2: The critical fluctuations in the Abelian Higgs model are vortex loops. We have studied the geometrical properties of these loops, and by using duality we have obtained scaling relations between the fractal dimension DH of the loops and the anomalous dimension ηφ of the dual field theory.Paper 3: We have calculated the GL parameter κtri separating a first order metal to superconductor transition from a second order one, κtri =(0.76±0.04))/√2. We also argue qualitatively that this κtri is the value separating type-I and type-II behavior, in contrast to the conventional value 1=√2. The calculations have been done including fluctuations in the amplitude and the phase of the matter-field, as well as fluctuations in the gauge field.Paper 4: We have determined the effective interaction between vortices in the Ginzburg-Landau model from large-scale Monte-Carlo simulations. We find a change, in the form of a crossover, from attractive to repulsive effective vortex interactions in an intermediate range of Ginzburg-Landau parameters κe[0.76; 1]=√2, depending on temperature. We present a simple physical picture of the crossover, and relate it to observations in Ta and Nb elemental superconductors which have low-temperature values of κ in the relevant range.

Hausdorff dimension of critical fluctuations in abelian gauge theories

The geometric properties of the critical fluctuations in Abelian gauge theories such as the Ginzburg-Landau model are analyzed in zero background field. Using a dual description, we obtain scaling relations between exponents of geometric and thermodynamic nature. In particular, we connect the anomalous scaling dimension eta of the dual matter field to the Hausdorff dimension D(H) of the critical fluctuations, which are fractal objects. The connection between the values of eta and D(H), and the possibility of having a thermodynamic transition in finite background field, is discussed.

Phase structure of (2+1)-dimensional compact lattice gauge theories and the transition from Mott insulator to fractionalized insulator

Phase structure of (2+1)-dimensional compact lattice gauge theories and the transition from Mott insulator to fractionalized insulator

Criticality in the (2+1)-dimensional compact Higgs model and fractionalized insulators.

We use a novel method of computing the third moment M3 of the action of the (2+1)-dimensional compact Higgs model in the adjoint representation with q=2 to extract correlation length and specific heat exponents nu and alpha without invoking hyperscaling. Finite-size scaling analysis of M3 yields the ratios (1+alpha)/nu and 1/nu separately. We find that alpha and nu vary along the critical line of the theory, which however exhibits a remarkable resilience of Z2 criticality. We propose this novel universality class to be that of the quantum phase transition from a Mott-Hubbard insulator to a charge-fractionalized insulator in two spatial dimensions.

First-Order Phase Transition in Easy-Plane Quantum Antiferromagnets

Quantum phase transitions in Mott insulators do not fit easily into the Landau-Ginzburg-Wilson paradigm. A recently proposed alternative to it is the so-called deconfined quantum criticality scenario, providing a new paradigm for quantum phase transitions. In this context it has recently been proposed that a second-order phase transition would occur in a two-dimensional spin 1/2 quantum antiferromagnet in the deep easy-plane limit. A check of this conjecture is important for understanding the phase structure of Mott insulators. To this end we have performed large-scale Monte Carlo simulations on an effective gauge theory for this system, including a Berry-phase term that projects out the S=1/2 sector. The result is a first-order phase transition, thus contradicting the conjecture.

Methods to determine the Hausdorff dimension of vortex loops in the three-dimensional X Y model

The geometric properties of critical fluctuations in the 3D XY model are analyzed. The 3D XY model is a lattice model describing superfluids. We present a direct evaluation of the Hausdorff dimension D_H of the vortex loops which are the critical fluctuations of the 3D XY model. We also present analytical arguments for why \vartheta in the scaling relation \eta_{\phi} + D_H = 2 + \vartheta between D_H and the anomalous scaling dimension of the corresponding field theory, must be zero.

Anomalous scaling dimensions and critical points in type-II superconductors

The existence of a stable critical point, separate from the Gaussian and XY critical points, of the Ginzburg–Landau theory for superconductors, is demonstrated by direct extraction via Monte-Carlo simulations, of a negative anomalous dimension ηφ of a complex scalar field φ forming a dual description of a neutral superfluid. The dual of the neutral superfluid is isomorphic to a charged superfluid coupled to a massless gauge-field. The anomalous scaling dimension of the superfluid order-field is positive, while we find that the anomalous dimension of the dual field is negative. The dual gauge-field does not decouple from the dual complex matter-field at the critical point. These two critical theories represent separate fixed points. The physical meaning of a negative ηφ is that the vortex-loop tangle of the superfluid at the critical point fills space more efficiently than random walkers, without collapsing. This is due to the presence of the massless dual gauge-field, and the resulting long-ranged vectorial Biot–Savart interaction between vortex-loop segments, which is a relevant perturbation to the steric |ψ|4 repulsion term. Hence, the critical dual theory is not in the universality class of the |ψ|4-theory.