Projects in Holger Frahm's Research Group

FOR 2316: Correlations in Integrable Quantum Many-Body Systems

The goal of the research unit is the development of many-body standard reference systems with known static and dynamical correlation functions at arbitrary temperature, on the lattice and in the continuum, very much like Bosonization provides standard reference systems near \(T = 0\), namely conformal field theories with central charges \(c = 1\) and known static low-temperature properties.

Participating Instutions

Project: Spin chains and vertex models based on superalgebras

Spin chains and two-dimensional vertex models with an underlying superalgebra structure appear naturally in certain statistical physics models, e.g. intersecting loops, and disordered electron systems. In several examples for such models the low energy effective theory describing their critical behaviour has displayed rather unusual properties: the lack of unitarity in these systems allows for continua of critical exponents leading to a fine structure with strong subleading corrections to scaling in the finite size spectrum. This is a signature of non-compact degrees of freedom emerging in the continuum limit of these models. In this project we plan to study the properties of these systems in the context of integrable superspin chains. In particular we want to identify the corresponding conformal field theories and characterize the continuous part of their spectrum. In addition, the effect of boundary conditions on the critical properties will be addressed. To deal with the strong finite-size effects present in these systems we shall develop new analytical methods for the analysis of the spectral problem.


Frahm, H., & Gehrmann, S. (2023). Integrable boundary conditions for staggered vertex models. Journal of Physics A: Mathematical and Theoretical, 56(2), [025001].


Frahm, H., & Martins, M. J. (2022). \(OSp(n|2m)\) quantum chains with free boundaries. Nuclear Physics B, 980, [115799].

Frahm, H., & Gehrmann, S. (2022). Finite size spectrum of the staggered six-vertex model with \(U_q(sl(2))\)-invariant boundary conditions. Journal of High Energy Physics, 2022(1), [70].

Hobuß, K. (2019). Spin chains and vertex models based on superalgebras. [Doctoral thesis, Gottfried Wilhelm Leibniz Universität Hannover]. Gottfried Wilhelm Leibniz Universität Hannover.

Frahm, H., Hobuß, K., & Martins, M. J. (2019). On the critical behaviour of the integrable q-deformed OSp(3|2) superspin chain. Nuclear Physics B, 946, [114697].


Frahm, H., & Martins, M. J. (2018). The fine structure of the finite-size effects for the spectrum of the OSp(n|2m) spin chain. Nuclear Physics B, 930, 545-562.


Frahm, H., & Hobuß, K. (2017). Spectral flow for an integrable staggered superspin chain. Journal of Physics A: Mathematical and Theoretical, 50(29), [294002].


Frahm, H.,Hobuß, K., & Martins, M. J. (2019). Dataset: Finite size data for the q-deformed OSp(3|2) superspin chain.

Project: Non-Abelian anyons

Quasi particles in topological quantum liquids such as the fractional Quantum Hall states and certain two-dimensional frustrated magnets display unconventional quantum statistics. The conserved topological charge of these non-Abelian anyons is protected and has spawned interest for such systems in the context of quantum computation. In this project we plan to study the properties of interacting many-anyon systems whose construction is based on the mathematical structures describing the fundamental operations of fusion and braiding. Upon fine-tuning of the interactions these models can be embedded into a family of commuting operators. We shall develop functional methods to exploit local identities present in these integrable models for the solution of their spectral problem. Our investigation of integrable anyon chains will be complemented by studies of non-integrable deformations thereof to gain understanding into the emergence of unconventional boundary degrees of freedom and their realization as topological quantum impurities in electronic systems.


Westerfeld, D. (2022). Functional methods for correlation functions of integrable face and anyon models. [Doctoral thesis, Gottfried Wilhelm Leibniz Universität Hannover]. Leibniz Universität Hannover.

Ardonne, E., Finch, P. E., & Titsworth, M. (2021). Classification of Metaplectic Fusion Categories. Symmetry, 13(11), [2102].

Frahm, H., & Westerfeld, D. (2021). Density matrices in integrable face models. SciPost Physics, 11(3), [057].

Borcherding, D., & Frahm, H. (2019). Condensates of interacting non-Abelian SO(5)Nf anyons. Journal of High Energy Physics, 2019(10), [54].

Borcherding, D., & Frahm, H. (2018). Condensation of non-Abelian SU(3) Nf anyons in a one-dimensional fermion model. Journal of Physics A: Mathematical and Theoretical, 51(49), [495002].

Borcherding, D., & Frahm, H. (2018). Signatures of non-Abelian anyons in the thermodynamics of an interacting fermion model. Journal of Physics A: Mathematical and Theoretical, 51(19), [195001].

Finch, P. E., Flohr, M., & Frahm, H. (2018). Zn clock models and chains of so(n)2 non-Abelian anyons: Symmetries, integrable points and low energy properties. Journal of Statistical Mechanics: Theory and Experiment, 2018(2), [023103].