Publication: Exciton condensation in strongly correlated electron bilayers

Title: Exciton condensation in strongly correlated electron bilayers

bilayertj

Abstract: We studied the possibility of exciton condensation in Mott insulating bilayers. In these strongly correlated systems, an exciton is the bound state of a double occupied and empty site. In the strong coupling limit, the exciton acts as a hard-core boson. Its physics is captured by the exciton t-J model, containing an effective XXZ model describing the exciton dynamics only. Using numerical simulations and analytical mean-field theory, we constructed the ground-state phase diagram. Three homogeneous phases can be distinguished: the antiferromag- net, the exciton checkerboard crystal, and the exciton superfluid. For most model parameters, however, we predict macroscopic phase separation between these phases. The exciton superfluid exists only for large exciton hopping energy. Additionally, we studied the collective modes and susceptibilities of the three phases. In the superfluid phase, we find the striking feature that the bandwidth of the spin-triplet excitations, potentially detectable by resonant inelastic x-ray scattering (RIXS), is proportional to the superfluid density. The superfluid phase mode is visible in the charge susceptibility, measurable by RIXS or electron energy loss spectroscopy (EELS).

Reference: Louk Rademaker, Jeroen van den Brink, Jan Zaanen, and Hans Hilgenkamp, Phys. Rev. B 88, 235127 (2013).

Publication: Determinant quantum Monte Carlo study of exciton condensation in the bilayer Hubbard model

Title: Determinant quantum Monte Carlo study of exciton condensation in the bilayer Hubbard model

bilayerhubbard

Abstract: We studied the possibility of exciton condensation in a strongly correlated bilayer extended Hubbard model using determinant quantum Monte Carlo. To model both the on-site repulsion U and the interlayer interaction V we introduced an update scheme extending the standard Sherman-Morrison update. We observe that the sign problem increases dramatically with the inclusion of the interlayer interaction V, which prohibits at this stage an unequivocal conclusion regarding the presence of exciton condensation. However, enhancement of the interlayer tunneling results suggest that the strongest exciton condensation tendency lies around 10–20% p/n doping. Magnetic properties and conductivity turn out to be relatively independent of the interlayer interaction.

Reference: Louk Rademaker, Steve Johnston, Jan Zaanen, and Jeroen van den Brink, Phys. Rev. B 88, 235115 (2013)

PhD defense – “Fermions and Bosons: excitons in strongly correlated materials”

Here I get my PhD diploma handed out by Prof. Pierre van Baal. Photo by Bart Versteeg.
Here I get my PhD diploma handed out by Prof. Pierre van Baal. Photo by Bart Versteeg.

This Wednesday 11 December 2013 I obtained by PhD degree, after completing all the traditional Dutch ceremonial requirements (as you can see on the photo).

Cover of the thesis "Fermions and Bosons: excitons in strongly correlated materials"
Cover of the thesis “Fermions and Bosons: excitons in strongly correlated materials”

For those of you who want to read more:

Furthermore, there has been some press coverage of the defense.

arXiv preprint: Exciton condensation in strongly correlated electron bilayers

Title: Exciton condensation in strongly correlated electron bilayers

Phase diagram of the exiton t-J model

Abstract:We studied the possibility of exciton condensation in Mott insulating bilayers. In these strongly correlated systems an exciton is the bound state of a double occupied and empty site. In the strong coupling limit the exciton acts as a hard-core boson. Its physics are captured by the exciton t-J model, containing an effective XXZ model describing the exciton dynamics only. Using numerical simulations and analytical mean field theory we constructed the ground state phase diagram. Three homogeneous phases can be distinguished: the antiferromagnet, the exciton checkerboard crystal and the exciton superfluid. For most model parameters, however, we predict macroscopic phase separation between these phases. The exciton superfluid exists only for large exciton hopping energy. Additionally we studied the collective modes and susceptibilities of the three phases. In the superfluid phase we find the striking feature that the bandwidth of the spin-triplet excitations, potentially detectable by resonant inelastic x-ray scattering (RIXS), is proportional to the superfluid density. The superfluid phase mode is visible in the charge susceptibility, measurable by RIXS or electron energy loss spectroscopy (EELS).

Reference: Louk Rademaker, Jeroen van den Brink, Jan Zaanen, Hans Hilgenkamp, arXiv:1310.0685

arXiv preprint: Determinant quantum Monte Carlo study of exciton condensation in the bilayer Hubbard model

Title: Determinant quantum Monte Carlo study of exciton condensation in the bilayer Hubbard model

The Bilayer Hubbard model.

Abstract: We studied the possibility of exciton condensation in a strongly correlated bilayer extended Hubbard model using Determinant Quantum Monte Carlo. To model both the onsite repulsion U and the interlayer interaction V we introduced a novel update scheme extending the standard Sherman-Morrison update. We observe that the sign problem increases dramatically with the inclusion of the interlayer interaction V, which prohibits at this stage a unequivocal conclusion regarding the presence of exciton condensation. However, enhancement of the interlayer tunneling results suggest that the strongest exciton condensation tendency lies around 10-20 % p/n-doping. Magnetic properties and conductivity turn out to be relatively independent of the interlayer interaction.

Reference: Louk Rademaker, Steve Johnston, Jan Zaanen, Jeroen van den Brink, arXiv:1310.0623

Publication: Enhancement of spin propagation due to interlayer exciton condensation

Our latest paper, just published on-line in Phys. Rev. B Rapid Communications!


Title: Enhancement of spin propagation due to interlayer exciton condensation

prbrfigure

Abstract: We show that an interlayer exciton condensate doped into a strongly correlated Mott insulator exhibits a remarkable enhancement of the bandwidth of the magnetic excitations (triplons). This triplon is visible in the dynamical magnetic susceptibility and can be measured using resonant inelastic x-ray scattering. The bandwidth of the triplon scales with the exciton superfluid density, but only in the limit of strong correlations. As such the triplon bandwidth acts as a probe of exciton-spin interactions in the condensate.

Reference: Louk Rademaker, Jeroen van den Brink, Hans Hilgenkamp and Jan Zaanen, Phys. Rev. B 88, 121101(R) (2013).

arXiv preprint: Enhancement of spin propagation due to interlayer exciton condensation

Title: Enhancement of spin propagation due to interlayer exciton condensation

Abstract: We show that an interlayer exciton condensate doped into a strongly correlated Mott insulator exhibits a remarkable enhancement of the bandwidth of the magnetic excitations (triplons). This triplon is visible in the dynamical magnetic susceptibility and can be measured using resonant inelastic x-ray scattering. The bandwidth of the triplon scales with the exciton superfluid density, but only in the limit of strong correlations. As such the triplon bandwidth acts as a probe of exciton-spin interactions in the condensate.

Reference: Louk Rademaker, Jeroen van den Brink, Hans Hilgenkamp, Jan Zaanen, arXiv:1304.3643

Poster@Veldhoven2013 and MagLab Winter School

In January 2013 I managed to be at two places at the same time: in FOM Veldhoven and at the Winter School in the National High Magnetic Field Laboratory in Tallahassee, FL, USA. Okay, to be honest: I was physically in Florida, whilst Robert-Jan and Kai made sure my poster was presented in Veldhoven. Thanks guys!

The poster, entitled “Exciton condensation in strongly correlated electron bilayers“, can de downloaded here (PDF, 4.4 MB).