Pawel Jakubczyk

Singular nonordering susceptibility at a Pomeranchuk instability

We study magnetic susceptibilities of two-dimensional itinerant electron systems exhibiting symmetry-breaking Fermi surface distortions, the so-called d-wave Pomeranchuk instability, in a magnetic field. In a pure forward scattering model, the longitudinal susceptibility chi^{zz} is found to exhibit a jump at a critical point. The magnitude of this jump diverges at a tricritical point. When scattering processes involving finite momentum transfers are allowed for, chi^{zz} is expected to diverge also at a critical point. The system displays multiple critical fluctuations. We argue that the features of chi^{zz} are general properties associated with singularities of a non-ordering susceptibility, leading to implications for a variety of materials including Sr_3Ru_2O_7.

Nematic quantum criticality without order

We consider a two-dimensional interacting Fermi system which displays a nematic phase within mean-field theory. The system is analyzed using a nonperturbative renormalization-group scheme. We find that order-parameter fluctuations can suppress the nematic order obtained in mean-field theory at any temperature and any filling, even at van Hove filling. For a suitable choice of parameters, a quantum critical point surrounded by a disordered phase can be realized, giving rise to quantum critical behavior in the absence of an ordered regime in the phase diagram.

Longitudinal fluctuations in the Berezinskii-Kosterlitz-Thouless phase

We analyze the interplay of longitudinal and transverse fluctuations in a

Critical Casimir forces for O(N) models from functional renormalization

U(1)

Repulsive Casimir forces at quantum criticality

symmetric two-dimensional

Critical temperature and Ginzburg region near a quantum critical point in two-dimensional metals

\phi^4

Interfacial correlation function for adsorption on a disc

-theory. To this end, we derive coupled renormalization group equations for both types of fluctuations obtained from a linear (cartesian) decomposition of the order parameter field. Discarding the longitudinal fluctuations, the expected Berezinskii-Kosterlitz-Thouless (BKT) phase characterized by a finite stiffness and an algebraic decay of order parameter correlations is recovered. Renormalized by transverse fluctuations, the longitudinal mass scales to zero, so that longitudinal fluctuations become increasingly important for small momenta. Within our expansion of the effective action, they generate a logarithmic decrease of the stiffness, in agreement with previous functional renormalization group calculations. The logarithmic terms imply a deviation from the vanishing beta-function for the stiffness in the non-linear sigma model describing the phase fluctuations at three-loop order. To gain further insight, we also compute the flow of the parameters characterizing longitudinal and transverse fluctuations from a density-phase representation of the order parameter field, with a cutoff on phase fluctuations. The power-law flow of the longitudinal mass and other quantities is thereby confirmed, but the stiffness remains finite in this approach. We conclude that the marginal flow of the stiffness obtained in the cartesian representation is an artifact of the truncated expansion of momentum dependences.

Phase diagram and correlation functions of the anisotropic imperfect Bose gas in d dimensions

We consider the classical O(N)-symmetric models confined in a d-dimensional slab-like geometry and subject to periodic boundary conditions. Applying the one-particle-irreducible variant of functional renormalization group (RG) we compute the critical Casimir forces acting between the slab boundaries. The applied truncation of the exact functional RG flow equation retains interaction vertices of arbitrary order. We evaluate the critical Casimir amplitudes \Delta_f(d,N) for continuously varying dimensionality between two and three and N = 1,2. Our findings are in very good agreement with exact results for d=2 and N=1. For d=3 our results are closer to Monte Carlo predictions than earlier field-theoretic RG calculations. Inclusion of the wave function renormalization and the corresponding anomalous dimension in the calculation has negligible impact on the computed Casimir forces.

Quantum wetting transitions in two dimensions: An alternative path to non-universal interfacial singularities

We study the Casimir effect in the vicinity of a quantum critical point. As a prototypical system we analyze the

Quantum interface unbinding transitions

d