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.
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.
Noncollinear Spin-Orbit Magnetic Fields in a Carbon Nanotube Double Quantum Dot
M. C. Hels, B. Braunecker, K. Grove-Rasmussen, J. Nygård
Phys. Rev. Lett. 117, 276802 (2016)
We demonstrate experimentally that noncollinear intrinsic spin-orbit magnetic fields can be realized in a
curved carbon nanotube two-segment device. Each segment, analyzed in the quantum dot regime, shows
near fourfold degenerate shell structure allowing for identification of the spin-orbit coupling and the angle
between the two segments. Furthermore, we determine the four unique spin directions of the quantum states
for specific shells and magnetic fields. This class of quantum dot systems is particularly interesting when
combined with induced superconducting correlations as it may facilitate unconventional superconductivity
and detection of Cooper pair entanglement. Our device comprises the necessary elements.
Non-Kondo many-body physics in a Majorana-based Kondo type system
I. J. van Beek, B. Braunecker
Phys. Rev. B 94, 115416 (2016)
We carry out a theoretical analysis of a prototypical Majorana system, which demonstrates the existence of a
Majorana-mediated many-body state and an associated intermediate low-energy fixed point. Starting from two
Majorana bound states, hosted by a Coulomb-blockaded topological superconductor and each coupled to a separate
lead, we derive an effective low-energy Hamiltonian, which displays a Kondo-like character. However, in contrast
to the Kondo model which tends to a strong- or weak-coupling limit under renormalization, we show that this
effective Hamiltonian scales to an intermediate fixed point, whose existence is contingent upon teleportation
via the Majorana modes. We conclude by determining experimental signatures of this fixed point, as well as
the exotic many-body state associated with it.