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A. Sudbo

Publications and source records attributed to A. Sudbo.

At least 37 records · Page 2Linked to original sources
arXiv2004

Comment on "Variation of the superconducting transition temperature of hole-doped copper oxides"

We point out the incorrect derivation of the gap equation in X.-J. Chen and H. Q. Lin [Phys. Rev. B 69 (2004) 104518; cond-mat/0306680] within the interlayer tunneling (ILT) model for multilayered cuprates. There, the local structure in k-space of the ILT effective interaction has not been taken into due account when the ILT model is generalized to the case of n layers per unit cell. This is a specific characteristic of the ILT model that, apart from giving rise to a highly nontrivial k-dependence of the gap function, is known to enhance the critical temperature Tc in a natural way. As a consequence, we argue that Chen and Lin's results cannot be employed, in their present form, for a quantitative interpretation of the high-pressure dependence of Tc in Bi-2212, as is done by X.-J. Chen et al. [cond-mat/0408587]. Moreover, when the generalization of X.-J. Chen et al. [cond-mat/0408587] is applied to the case n=2, it fails to reproduce the original ILT gap equation. However, a more careful analysis of the ILT model for multilayered cuprates, taking into account the nonuniform hole distribution among inequivalent layers, has been earlier suggested to describe the observed pressure dependence of Tc in homologous series of high-Tc cuprates.

cond-mat.supr-con↗
arXiv2004

Phase structure of Abelian Chern-Simons gauge theories

We study the effect of a Chern-Simons (CS) term in the phase structure of two different Abelian gauge theories. For the compact Maxwell-Chern-Simons theory, we obtain that for values $g=n/2π$ of the CS coupling with $n=\pm 1,\pm 2$, the theory is equivalent to a gas of closed loops with contact interaction, exhibiting a phase transition in the $3dXY$ universality class. We also employ Monte Carlo simulations to study the noncompact U(1) Abelian Higgs model with a CS term. Finite size scaling of the third moment of the action yields critical exponents $α$ and $ν$ that vary continuously with the strength of the CS term, and a comparison with available analytical results is made.

cond-mat.str-el↗
arXiv2004

Critical properties of the N-color London model

The critical properties of $N$-color London model are studied in $d=2+1$ dimensions. The model is dualized to a theory of $N$ vortex fields interacting through a Coulomb and a screened potential. The model with N=2 shows two anomalies in the specific heat. From the critical exponents $α$ and $ν$, the mass of the gauge field, and the vortex correlation functions, we conclude that one anomaly corresponds to an {\it inverted} \xy fixed point, while the other corresponds to a \xy fixed point. There are $N$ fixed points, namely one corresponding to an inverted \xy fixed point, and $N-1$ corresponding to neutral \xy fixed points. This represents a novel type of quantum fluid, where superfluid modes arise out of charged condensates.

cond-mat.supr-con↗
arXiv2004

Metal-Insulator Transition in Two- and Three-Dimensional Logarithmic Plasmas

We consider scaling of the mean square dipole moments in a plasma with logarithmic interactions in a two- and three-dimensional system. In both cases, we establish the existence of a low-temperature regime where the mean square dipole moment does not scale with system size and a high-temperature regime where it does scale with system size. Thus, there is a nonanalytic change in the polarizability of the system as a function of temperature, and hence a metal-insulator transition in both cases. The relevance of this transition in three dimensions to quantum phase transitions in 2+1-dimensional systems is briefly discussed.

cond-mat.str-el↗
arXiv2003

Criticality versus q in the 2+1-dimensional $Z_q$ clock model

Using Monte Carlo simulations we have studied the $d=3$ $Z_q$ clock model in two different representations, the phase-representation and the loop/dumbbell-gas (LDG) representation. We find that for $q \ge 5$ the critical exponents $α$ and $ν$ for the specific heat and the correlation length, respectively, take on values corresponding to the case $q\to \infty$, where $\lim_{q \to \infty} Z_q = 3DXY$ model, i.e. in terms of critical properties the limiting behaviour is reached already at $q=5$.

cond-mat.stat-mech↗
arXiv2003

Phase Structure of d=2+1 Compact Lattice Gauge Theories and the Transition from Mott Insulator to Fractionalized Insulator

Large-scale Monte Carlo simulations are employed to study phase transitions in the three-dimensional compact abelian Higgs model in adjoint representations of the matter field, labelled by an integer q, for q=2,3,4,5. We also study various limiting cases of the model, such as the $Z_q$ lattice gauge theory, dual to the $3DZ_q$ spin model, and the 3DXY spin model which is dual to the $Z_q$ lattice gauge theory in the limit $q \to \infty$. We have computed the first, second, and third moments of the action to locate the phase transition of the model in the parameter space $(β,κ)$, where $β$ is the coupling constant of the matter term, and $κ$ is the coupling constant of the gauge term. We have found that for q=3, the three-dimensional compact abelian Higgs model has a phase-transition line $β_{\rm{c}}(κ)$ which is first order for $κ$ below a finite {\it tricritical} value $κ_{\rm{tri}}$, and second order above. We have found that the $β=\infty$ first order phase transition persists for finite $β$ and joins the second order phase transition at a tricritical point $(β_{\rm{tri}}, κ_{\rm{tri}}) = (1.23 \pm 0.03, 1.73 \pm 0.03)$. For all other integer $q \geq 2$ we have considered, the entire phase transition line $β_c(κ)$ is critical.

cond-mat.str-el↗
arXiv2002

Vortex Interactions and Thermally Induced Crossover from Type-I to Type-II Superconductivity

We have computed the effective interaction between vortices in the Ginzburg-Landau model from large-scale Monte-Carlo simulations, taking thermal fluctuations of matter fields and gauge fields fully into account close to the critical temperature. 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 $κ\in [0.76-1]/\sqrt{2}$ upon increasing the temperature in the superconducting state. This corresponds to a thermally induced crossover from \typeI to \typeII superconductivity around a temperature $T_{\rm{Cr}}(κ)$, which we map out in the vicinity of the metal-to-superconductor transition. In order to see this crossover, it is essential to include amplitude fluctuations of the matter field, in addition to phase-fluctuations and gauge-field fluctuations. We present a simple physical picture of the crossover, and relate it to observations in \metal{Ta} and \metal{Nb} elemental superconductors which have low-temperature values of $κ$ in the relevant range.

cond-mat.supr-con↗
arXiv2001

Staggered Orbital Currents in a Model of Strongly Correlated Electrons

This preprint has been withdrawn, as the evidence for persistent currents found with the "infinite-size" DMRG algorithm has not been reproduced with the (presumably more accurate) "finite-size" version of the algorithm (M. Troyer and U. Schollwoeck, private communiation). A much clearer example of long-range order in orbital currents is presented in: J. O. Fjaerestad and J. B. Marston, ``Staggered Orbital Currents in the Half-Filled Two-Leg Ladder,'' cond-mat/0107094.

cond-mat.str-el↗
arXiv2001

The order of the metal to superconductor transition

We present results from large-scale Monte Carlo simulations on the full Ginzburg-Landau (GL) model, including fluctuations in the amplitude and the phase of the matter-field, as well as fluctuations of the non-compact gauge-field of the theory. {}From this we obtain a precise critical value of the GL parameter $\kct$ separating a first order metal to superconductor transition from a second order one, $\kct = (0.76\pm 0.04)/\sqrt{2}$. This agrees surprisingly well with earlier analytical results based on a disorder theory of the superconductor to metal transition, where the value $\kct=0.798/\sqrt{2}$ was obtained. To achieve this, we have done careful infinite volume and continuum limit extrapolations. In addition we offer a novel interpretation of $\kct$, namely that it is also the value separating \typeI and \typeII behaviour.<

cond-mat.supr-con↗
arXiv2001

Fermion-pairing on a square lattice in extreme magnetic fields

We consider the Cooper-problem on a two-dimensional, square lattice with a uniform, perpendicular magnetic field. Only rational flux fractions are considered. An extended (real-space) Hubbard model including nearest and next nearest neighbor interactions is transformed to "k-space", or more precisely, to the space of eigenfunctions of Harper's equation, which constitute basis functions of the magnetic translation group for the lattice. A BCS-like truncation of the interaction term is performed. Expanding the interactions in the basis functions of the irreducible representations of the point group $C_{4ν}$ of the square lattice simplify calculations. The numerical results indicate enhanced binding compared to zero magnetic field, and thus re-entrant superconducting pairing at extreme magnetic fields, well beyond the point where the usual semi-classical treatment of the magnetic field breaks down.

cond-mat.supr-con↗
arXiv2000

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 $η$ of the dual matter field to the Hausdorff dimension $D_H$ of the critical fluctuations, {\it which are fractal objects}. The connection between the values of $η$ and $D_H$, and the possibility of having a thermodynamic transition in finite background field, is discussed.

cond-mat.supr-con↗
arXiv2000

Anomalous scaling dimensions and stable charged fixed-point of type-II superconductors

The critical properties of a type-II superconductor model are investigated using a dual vortex representation. Computing the propagators of gauge field $\vec{A}$ and dual gauge field $\vec{h}$ in terms of a vortex correlation function, we obtain the values $η_A=1$ and $η_h=1$ for their anomalous dimensions. This provides support for a dual description of the Ginzburg-Landau theory of type-II superconductors in the continuum limit, as well as for the existence of a stable charged fixed point of the theory, not in the 3DXY universality class.

cond-mat.supr-con↗
arXiv1999

Correlation functions for a two-dimensional electron system with bosonic interactions and a square Fermi surface

We calculate zero-temperature correlation functions for a model of 2D interacting electrons with short-range interactions and a square Fermi surface. The model was arrived at by mapping electronic states near a square Fermi surface with Hubbard-like interactions onto one-dimensional quantum chains, retaining terms which can be written in terms of bosonic density operators. Interactions between orthogonal chains, corresponding to orthogonal faces of the square Fermi surface, are neglected. The correlation functions become sums of Luttinger-type correlation functions due to the bosonic model. However, the correlation function exponents differ in form from those of the Luttinger model. As a consequence, the simple scaling relations found to exist between the Luttinger model exponents, do not carry over to the leading exponents of our model. We find that for repulsive effective interactions, charge-density wave/spin-density wave instabilities are dominant. We do not consider d-wave instabilities here.

cond-mat.str-el↗
arXiv1999

Non-k-diagonality in the interlayer pair-tunneling model of high-temperature superconductivity

We investigate the effect of k-space broadening of the interlayer pairing kernel on the critical temperature T_c and the k-dependence of the gap function in a one-dimensional version of the interlayer pair-tunneling model of high-T_c superconductivity. We consider constant as well as k-dependent intralayer pairing kernels. We find that the sensitivity to k-space broadening is larger the smaller the width of the peak of the Fermi-level gap calculated for zero broadening. This width increases with the overall magnitude of the interlayer tunneling matrix element, and decreases with the bandwidth of the single-electron intralayer excitation spectrum. The width also increases as the Fermi level is moved towards regions where the excitation spectrum flattens out. We argue that our qualitative conclusions are valid also for a two-dimensional model. This indicates that at or close to half-filling in two dimensions, when the Fermi-surface gap for zero broadening attains its peaks at $(\pm π/a,0)$ and $(0,\pmπ/a)$ where the excitation spectrum is flat, these peaks should be fairly robust to moderate momentum broadening.

cond-mat.supr-con↗
arXiv1998

Sharp k-space features in the order parameter within the interlayer pair-tunneling mechanism of high-T_c superconductivity

We study the k-dependence of the gap function of a bilayer superconductor, using standard mean-field techniques applied to a 2D extended Hubbard model, in the presence of coherent interlayer pair-tunneling and quenched coherent single-particle tunneling. The intralayer pairing potential thus defined is expandable in a finite number of basis functions for the irreducible representations of the point-group of the perfectly square lattice, C_{4v}. This gives rise to a competition between s- and d-wave symmetry, as the chemical potential is increased from the bottom to the top of a realistic band for most cuprates. It allows for mixed-symmetry paired state at temperatures below T_c, but never at T_c on a square lattice. Inclusion of the interlayer pair-tunneling into the effective pairing potential leads to highly non-trivial k-space structures, such as pronounced maxima along the Fermi line not seen in the absence of interlayer pair-tunneling. We show how such a gap structure evolves with temperature and with band filling, and how it affects various observables. In particular, a nonuniversal value of the normalized jump in the specific heat at T_c will be evidenced, at variance with the conventional universal BCS result.

cond-mat.supr-con↗