Presentation at Stripe Club, january 2011

Today Kai Wu and I gave a presentation at Jan Zaanen’s group meeting called the “Stripe Club”. Our presentation was named “The motion of a single exciton in a bilayer quantum antiferromagnet” (PDF, 1.9 MB). In summary: we developed a spin wave theory for the bilayer Heisenberg model in order to describe the motion of an exciton through such a bilayer, which we solved using the self-consistent Born approximation. Recently, we obtained numerical results for the exciton spectral function. It appears to be that the Ising-type ladder spectrum reappears even though quantum fluctuations are taken into account, as you can see in the following ‘teaser’ result for alpha=0.2.

Exciton spectral function for alpha=0.2.

arXiv preprint: Prediction of the quantization of magnetic flux in double layer exciton superfluids

I just posted a preprint paper on the arXiv, with the following abstract.

Currently a way is lacking to detect unambiguously the possible phase coherence of an exciton condensate in an electron-hole double layer. Here we show that despite the fact that excitons are charge-neutral, the double layer exciton superfluid exhibits a diamagnetic response. In devices with specific circular geometry the magnetic flux threading between the layers must be quantized in units of $latex \frac{h}{e} \chi_m$ where $latex \chi_m$ is the diamagnetic susceptibility of the device. We discuss possible experimental realizations of the predicted unconventional flux quantization.

See the whole article at arXiv:1009.1793.

Presentation at Casimir Spring School 2010

Last week at the Casimir Spring School 2010 I was invited to give a talk on my work on ‘Flux Quantization in Double Layer Exciton Superfluids’. With this talk I was awarded the prize for best oral presentation! šŸ™‚ You can download the presentation here in Powerpoint-format. Note that this talk is intended for general physics PhD-audiences. A more theoretical talk can be found here.

Title:Ā Flux Quantization in Double Layer Exciton Superfluids (pptx, 7.5 MB)
Abstract: We predict an unconventional magnetic flux quantization effect to occur in double layer exciton superfluids and we discuss designs for a device to measure this universal electromagnetic signature of the exciton superfluid. This would provide an unambiguous test for the macroscopic phase coherence associated with an exciton Bose-Einstein Condensate.

Vici project “Opposites attract” starts

Today I start officially as PhD-student in Theoretical Physics under the supervision of Hans Hilgenkamp, Jan Zaanen and Jeroen van den Brink. The latter two are professors in theoretical physics at the Lorentz Institute, Leiden University. Professor Hans Hilgenkamp, from Twente University, is the main supervisor since he got a Vici-grant for our research.

The Vici is granted to Hilgenkamps’ proposal which is titled “Opposites attract; Electron-hole dances in coupled p- and n-type Mott-conductors“. The goal of the research is to realize and investigate new states of matter by coupling p-type and n-type Mott materials.

Continue reading “Vici project “Opposites attract” starts”

Masters thesis: Phase Transitions in Matrix Models

In september 2008 I received my Masters degree in Theoretical Physics cum laude, with the research I did under supervision of Koenraad Schalm at the Leiden University. My Masters thesis was titled “Phase Transitions in Matrix Models” and can be downloaded here (pdf, 716 kB). The summary of the thesis is:

Matrix models are toy models applicable in various ļ¬elds of physics. The overall propertiesĀ of such a matrix model are deļ¬ned by its partition function, which is an integral over $latex N \times N$ Hermitian matrices M with energy/action S[M] invariant under similarity transformations.

Upon integrating over the rotational degrees of freedom, the action can be described in termsĀ of the eigenvalues of M.Ā If the action has one unique absolute minimum, then the free energy $latex F = – \log Z_N$ canĀ be approximated via a perturbation series around that minimum. Generically, however, theĀ matrix model action will have multiple extrema, e.g. in the $latex gM^4$ model. Using the eigenvalueĀ representation, we show that the $latex gM^4$ model exhibits a phase transition for a speciļ¬c range ofĀ coupling constants. For high $latex \mu_C = m^2/4g$ (the depth of the potential well) the ground stateĀ consists of a superposition of multiple solitons. For low $latex \mu_C$ there exists one single minimumĀ of the action, which allows a perturbation expansion of the free energy.

We ļ¬nd that the phase transition of the $latex gM^4$ model is analytic in the macroscopic parameters, but is non-analytic when the action is coupled to external sources for eigenvalues.Ā This can be veriļ¬ed by computing the correlation function in both phases. Physically theĀ source term preselects one speciļ¬c set of microscopic variables. The non-analyticity in thisĀ microscopic parameter while analytic in all macroscopic parameters suggests that we areĀ dealing with a Kosterlitz-Thouless phase transition.

Finally we construct a renormalization group ļ¬‚ow of the theory with respect to changesĀ in the matrix dimension N and show that the lines of constant Z (aka the renormalizationĀ group ļ¬‚ows) do cross the line of critical Āµin the phase diagram.