University of St Andrews
School of Physics and Astronomy
We are theoretical condensed matter physicists interested in many-body
and interaction effects
in systems that are attractive for future quantum technology.
Our current activities involve:
Topological states by self-organisation transitions in interacting low-dimensional conductors.
New physics achievable with Majorana bound states in condensed matter systems.
Generation and detection of entanglement in nanostructures.
For details on our recent activities click here.
Continuum description for sub-gap states at ferromagnetic and spiral ordered magnetic chains in two-dimensional superconductors
C. J. F. Carroll, B. Braunecker
We consider sub-gap bands induced in a two-dimensional superconductor by a densely packed chain of magnetic moments with ferromagnetic
or spiral alignments. We show that crucially all wavelengths must be taken into account in the calculation of the sub-gap properties,
and that in particular gap closings can occur at high momentum due to a mix of Shiba type and magnetic scattering states. The sub-gap
bands are always connected to the bulk bands such that it is impossible to single out a fully one-dimensional subband as required for
distinct topological properties. To obtain the latter a finite spacing between the impurities needs to be reintroduced such that by zone
folding the bands can disconnect from the continuum.
Spin liquid mediated RKKY interaction
H. F. Legg, B. Braunecker
We propose an RKKY-type interaction that is mediated by a spin liquid. If a spin liquid ground state exists such an
interaction could leave a fingerprint by ordering underlying localized moments such as nuclear spins. This interaction
has a unique phenomenology that is distinct from the RKKY interaction found in fermionic systems; most notably the
lack of a Fermi surface and absence of the requirement for itinerant electrons, since most spin liquids are insulators.
As a working example we investigate the two-dimensional spin-1/2 kagome antiferromagnet (KAFM), although the treatment
remains general and can be extended to other spin liquids and dimensions. We find that several different nuclear spin
orderings minimize the RKKY-type energy induced by the KAFM but are unstable due to a zero-energy flat magnon band.
Despite this we show that a small magnetic field is able to gap out this magnon spectrum for some of the orderings
resulting in an intricate nuclear magnetism.