arXiv subjects

Maria Hermanns

Publications and source records attributed to Maria Hermanns.

24 records · Page 2Linked to original sources

arXiv2015

Spin-Peierls Instability of Three-Dimensional Spin Liquids with Majorana Fermi Surfaces

Three-dimensional (3D) variants of the Kitaev model can harbor gapless spin liquids with a Majorana Fermi surface on certain tricoordinated lattice structures such as the recently introduced hyperoctagon lattice. Here we investigate Fermi surface instabilities arising from additional spin exchange terms (such as a Heisenberg coupling) which introduce interactions between the emergent Majorana fermion degrees of freedom. We show that independent of the sign and structure of the interactions, the Majorana surface is always unstable. Generically the system spontaneously doubles its unit cell at exponentially small temperatures and forms a spin liquid with line nodes. Depending on the microscopics further symmetries of the system can be broken at this transition. These spin-Peierls instabilities of a 3D spin liquid are closely related to BCS instabilities of fermions.

cond-mat.str-el↗

arXiv2015

Spin-orbit physics of j=1/2 Mott insulators on the triangular lattice

The Heisenberg-Kitaev (HK) model on the triangular lattice is conceptually interesting for its interplay of geometric and exchange frustration. HK models are also thought to capture the essential physics of the spin-orbital entanglement in effective $j=1/2$ Mott insulators studied in the context of various 5d transition metal oxides. Here we argue that the recently synthesized Ba$_3$IrTi$_2$O$_9$ is a prime candidate for a microscopic realization of the triangular HK model. We establish that an infinitesimal Kitaev exchange destabilizes the 120$^\circ$ order of the quantum Heisenberg model and results in the formation of an extended $\mathbb{Z}_2$-vortex crystal phase in the parameter regime most likely relevant to the real material. Using a combination of analytical and numerical techniques we map out the entire phase diagram of the model, which further includes various ordered phases as well as an extended nematic phase around the antiferromagnetic Kitaev point.

cond-mat.str-el↗

arXiv2015

Weyl spin liquids

The fractionalization of quantum numbers in interacting quantum-many body systems is a central motif in condensed matter physics with prominent examples including the fractionalization of the electron in quantum Hall liquids or the emergence of magnetic monopoles in spin-ice materials. Here we discuss the fractionalization of magnetic moments in three-dimensional Kitaev models into Majorana fermions (and a $\mathbb Z_2$ gauge field) and their emergent collective behavior. We analytically demonstrate that the Majorana fermions form a Weyl superconductor for the Kitaev model on the recently synthesized hyperhoneycomb structure of $β$-Li$_2$IrO$_3$ when applying a magnetic field. We characterize the topologically protected bulk and surface features of this state, which we dub a Weyl spin liquid, including thermodynamic and transport signatures.

cond-mat.str-el↗

arXiv2014

Entanglement Spectrum and Entanglement Hamiltonian of a Chern insulator with open boundaries

We study the entanglement spectrum of a Chern insulator on a cylinder geometry, with the cut separating the two partitions taken parallel to the cylinder edge, at varying distances from the edge. In contrast to similar studies on a torus, there is only one cut, and hence only one virtual edge mode in the entanglement spectrum. The entanglement spectrum has a gap when the cut is close enough to the physical edge of the cylinder such that the edge mode spatially extends over the cut. This effect is suppressed for parameter choices where the edge mode is sharply localized at the edge. In the extreme case of a perfectly localized edge mode, the entanglement spectrum is gapless even if the smaller partition consists of a single edge row. For the single-row cut, we construct the corresponding entanglement Hamiltonian, which is a one-dimensional tight-binding Hamiltonian with complex long-range hopping and interesting properties. We also study and explain the effect of two different schemes of flux insertion through a ring described by such an entanglement Hamiltonian.

cond-mat.str-el↗

arXiv2011

From Irrational to Non-Unitary: on the Haffnian and Haldane-Rezayi wave functions

We study the Haffnian and Haldane-Rezayi quantum Hall wave functions and their quasihole excitations by means of their `root configurations', and point out a close connection between these seemingly different states. For both states, we formulate a `generalized Pauli-principle', which allows to count the degeneracies of these states. The connection between these states might elucidate the underlying theory describing the `irrational' Haffnian state.

cond-mat.str-el↗

arXiv2008

Order in 2D nodal superconductors

A topological theory of d-wave superconductors is derived in this thesis. Ginzburg-Landau theory describes superconductivity by defining a complex order parameter and applying Landau's theory for phase transitions. However, there is no local, gauge invariant order parameter for a superconductor and classical order is no appropriate description. Topological order has proven to be a powerful tool for describing the Quantum Hall effect and it gives also an appropriate description of s-wave superconductors. For d-wave superconductors there are gapless excitations at four points on the Fermi surface. The topological theory for superconductors exhibits a similar structure for the ground state as found in the s-wave case. However, the gapless excitations destroy the topological degeneracy of the ground state and introduce an additional degeneracy in one of the flux sectors. In search for regularities which reflect the topological order, I include a magnetic point impurity and focus on the effects it has on the Casimir energy. Indeed, it is shown that the energy shift reflects the topology to some extend. However, this is work in progress and further computations will be done.

cond-mat.supr-con↗