At the EXCON 2012 conference in Groningen and at the `Innovations in Strongly Correlated Electron Systems’ Summer School and Workshop at the ICTP (Trieste, Italy) I presented this poster about the most recent developments in the exciton business. You can download the poster here (PDF, 1.6 MB).

## Talk@Bristol: Mott insulator bilayers

As a quasi-member of the Interfaces and Correlated Electron systems (ICE) group from Hans Hilgenkamp I joined the ICE2UK-tour. Together with Marcel Hoek, who is also on the VICI project, I gave a talk in Bristol – the hometown of Nevill F. Mott! We couldn’t resist, the talk is called ‘Mott insulator bilayers’, and can be downloaded here (PDF, 10 MB).

## arXiv preprint: Dynamics of a single exciton in strongly correlated bilayers

Title: Dynamics of a single exciton in strongly correlated bilayers

Abstract:We formulated an effective theory for a single interlayer exciton in a bilayer quantum antiferromagnet, in the limit that the holon and doublon are strongly bound onto one interlayer rung by the Coulomb force. Upon using a rung linear spin wave approximation of the bilayer Heisenberg model, we calculated the spectral function of the exciton for a wide range of the interlayer Heisenberg coupling \alpha=J_{\perp}/Jz. In the disordered phase at large \alpha, a coherent quasiparticle peak appears representing free motion of the exciton in a spin singlet background. In the N\'{e}el phase, which applies to more realistic model parameters, a ladder spectrum arises due to Ising confinement of the exciton. The exciton spectrum is visible in measurements of the dielectric function, such as c-axis optical conductivity measurements.

Reference:Louk Rademaker, Kai Wu and Jan Zaanen, arXiv:1202.3616

## Poster Physics@FOM Veldhoven 2012

At this years edition of the annual Dutch Physics@FOM conference in Veldhoven I will present a poster entitled “*Exciton condensation in strongly correlated electron bilayers*“. Actually, for the first time the poster will actually be about exciton condensation, the initial goal of my PhD research.

In the picture here you see an excerpt from the poster, explaining the exciton t-J model Hamiltonian. The poster itself can be downloaded here (pdf, 1.3 MB).

## Publication: The dynamical frustration of interlayer excitons delocalizing in bilayer quantum antiferromagnets

The paper on the properties of a single exciton is accepted and published in the January issue of Europhysics Letters! (See the completely different arXiv version here.)

Title: The dynamical frustration of interlayer excitons delocalizing in bilayer quantum antiferromagnet

Abstract:Using the self-consistent Born approximation we study the delocalization of interlayer excitons in the bilayer Heisenberg quantum antiferromagnet. Under realistic conditions we find that the coupling between the exciton motion and the spin system is strongly enhanced as compared to the case of a single carrier, to a degree that it mimics the confinement physics of carriers in Ising spin systems. We predict that the “ladder spectrum” associated with this confinement physics should be visible in the c-axis exciton spectra of insulating bilayer cuprates such as YBa2Cu3O6. Our discovery indicates that finite density systems of such excitons should show very rich physical behavior.

Reference:Louk Rademaker, Kai Wu, Hans Hilgenkamp and Jan Zaanen, EPL97, 27004 (2012).

## Presentation: Frustrated interlayer excitons in antiferromagnets

On 9 September 2011 I was presenting my work on interlayer excitons (see this preprint) at the University of Utrecht in the so-called ‘BEC group meeting’ (that is: the groups of Henk Stoof, Rembert Duine and Cristiane de Morais Smith combined).

Title:Frustrated interlayer excitons in antiferromagnets (click to download the presentation in PDF, 3.6 MB)

## Poster at NSPM 2011, Erice (and OECS12)

In Erice (a beautiful place in Sicily, Italy) both a summer school and a workshop was held about “Quantum Phenomena in Graphene, other Low-Dimensional Materials, and Optical Lattices“. Needless to say I was there for the “other low-dimensional materials”, with lectures on bilayer exciton condensation (by Allan MacDonald) and oxide heterostructures. At the workshop part I presented a poster on my latest work on excitons in bilayer quantum antiferromagnets, which you of course can download here.

**Poster title:** Dynamical frustration of interlayer excitons in bilayer quantum antiferromagnets (PDF, 397 kB)

In addition, I discovered that Shou-Cheng Zhang used the picture Jeroen Huijben made for me in his presentation of topological insulators. Zhang’s presentation (amongst others) can be downloaded here.

**Update:** I presented this very same poster at the international conference on Optics of Excitons in Confined Systems (OECS12) in Paris, 12-16 September 2011.

## arXiv preprint: The dynamical frustration of interlayer excitons delocalizing in bilayer quantum antiferromagnets

Last week I put my work on exciton dynamics in strongly correlated bilayers onto the arXiv!

Title: The dynamical frustration of interlayer excitons delocalizing in bilayer quantum antiferromagnets

Abstract:Using the self-consistent Born approximation we study the delocalization of interlayer excitons in the bilayer Heisenberg quantum antiferromagnet (see figure above). Under realistic conditions we find that the coupling between the exciton motion and the spin system is strongly enhanced as compared to the case of a single carrier, to a degree that it mimics the confinement physics of carriers in Ising spin systems. We predict that the ‘ladder spectrum’ associated with this confinement physics should be visible in the c-axis exciton spectra of insulating bilayer cuprates such as YBa2Cu3O6. Our discovery indicates that finite density systems of such excitons should show very rich physical behavior.

Reference:Louk Rademaker, Kai Wu, Hans Hilgenkamp and Jan Zaanen, arXiv:1106.5347

## Physics@FOM Veldhoven 2010 / Lorentz Workshop April 2011 Poster

At the Physics@FOM Veldhoven 2010 (January 19, 2010) conference I presented a poster with the title “How a Neutral and Massless Superfluid can still exhibit Flux Quantization” (pdf, 2.8 MB).

*Image: In concentric bilayers a supercurrent of excitons will exhibit a quantization of the flux in between the two cylinders. The magnetic field lines are shown in red. (Figure made by Jeroen Huijben.)*

**Update, March 12, 2010**: Recently we found that the results, as presented in Veldhoven, were not completely correct. Flux quantization does appear in exciton double layers, however, the trapped flux must be a multiple of $latex \frac{h}{e} \chi_m$ where $latex \chi_m$ is the diamagnetic susceptibility of the exciton double layer.

**Update, April 4, 2011**: At the Lorentz workshop ‘100th Anniversary of Superconductivity’ I presented a corrected version of the poster. The results have by now been peer-reviewed and published in PRB. On this website I replaced the incorrect old version with the new correct version!

## Presentation at the DRSTP Spring School 2011

This year at the Dutch Research School for Theoretical Physics AIO/OIO School for Statistical Physics and Theory of Condensed Matter (aka the DRSTP SPTCM School 2011) I gave some introduction to the idea of linear spin-wave theory and how it can be applied to interlayer excitons. More information on the school itself can be found here, the presentation itself can be downloaded below.

Title: Excitons and spins in strongly correlated systems (pdf, 741 kB)