Quantum Thermalization and the Expansion of Atomic Clouds

I also presented this work in the form of a poster at the SPICE workshop ’Non-equilibrium Quantum Matter’, Mainz, Germany, from 30 May to 2 June 2017. The poster can be downloaded here (pdf, 1.3 MB).

Title: Quantum Thermalization and the Expansion of Atomic Clouds

Authors: Louk Rademaker, Jan Zaanen

quantumthermalization

Abstract: The ultimate consequence of quantum many-body physics is that even the air we breathe is governed by strictly unitary time evolution. The reason that we perceive it nonetheless as a completely classical high temperature gas is due to the incapacity of our measurement machines to keep track of the dense many-body entanglement of the gas molecules. The question thus arises whether there are instances where the quantum time evolution of a macroscopic system is qualitatively different from the equivalent classical system? Here we study this question through the expansion of noninteracting atomic clouds. While in many cases the full quantum dynamics is indeed indistinguishable from classical ballistic motion, we do find a notable exception. The subtle quantum correlations in a Bose gas approaching the condensation temperature appear to affect the expansion of the cloud, as if the system has turned into a diffusive collision-full classical system.

arXiv:1703.02489

 

Phonon linewidth due to electron-phonon interactions with strong forward scattering in FeSe thin films on oxide substrates

Title: Phonon linewidth due to electron-phonon interactions with strong forward scattering in FeSe thin films on oxide substrates

Authors: Yan Wang, Louk Rademaker, Elbio Dagotto, Steven Johnston

Abstract: The discovery of an enhanced superconducting transition temperature Tc in monolayers of FeSe grown on several oxide substrates has opened a new route to high-Tc superconductivity through interface engineering. One proposal for the origin of the observed enhancement is an electron-phonon (e-ph) interaction across the interface that peaked at small momentum transfers. In this paper, we examine the implications of such a coupling on the phononic properties of the system. We show that a strong forward scattering leads to a sizable broadening of phonon lineshape, which may result in charge instabilities at long-wavelengths. However, we further find that the inclusion of Coulombic screening significantly reduces the phonon broadening. Our results show that one might not expect anomalously broad phonon linewidths in the FeSe interface systems, despite the fact that the e-ph interaction has a strong peak in the forward scattering (small q) direction.

ArXiv:1703.02013

Publication: Bilayer Excitons in Two-Dimensional Nanostructures for Greatly Enhanced Thermoelectric Efficiency

Title: Bilayer Excitons in Two-Dimensional Nanostructures for Greatly Enhanced Thermoelectric Efficiency

thermoAbstract: Currently, a major nanotechnological challenge is to design thermoelectric devices that have a high figure of merit. To that end, we propose to use bilayer excitons in two-dimensional nanostructures. Bilayer-exciton systems are shown to have an improved thermopower and an enhanced electric counterflow and thermal conductivity, with respect to regular semiconductor-based thermoelectrics. We suggest an experimental realization of a bilayer-exciton thermocouple. Based on current experimental parameters, a bilayer-exciton heterostructure of p- and n-doped Bi2Te3 can enhance the figure of merit an order of magnitude compared to bulk Bi2Te3. Another material suggestion is to make a bilayer out of electron-doped SrTiO3 and hole-doped Ca3Co4O9.

Reference: Kai Wu, Louk Rademaker, and Jan Zaanen, Phys. Rev. Applied 2, 054013 (2014).

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)

Publication: Influence of long-range interactions on charge ordering phenomena on a square lattice

TitleInfluence of long-range interactions on charge ordering phenomena on a square lattice

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Abstract: Usually complex charge ordering phenomena arise due to competing interactions. We have studied how such ordered patterns emerge from the frustration of a long-ranged interaction on a lattice. Using the lattice gas model on a square lattice with fixed particle density, we have identified several interesting phases, such as a generalization of Wigner crystals at low particle densities and stripe phases at densities between ρ=1/3 and 1/2. These stripes act as domain walls in the checkerboard phase present at half-filling. The phases are characterized at zero temperatures using numerical simulations, and mean field theory is used to construct a finite temperature phase diagram.
ReferenceLouk Rademaker, Yohanes Pramudya, Jan Zaanen, and Vladimir Dobrosavljević, Phys. Rev. E 88, 032121 (2013)

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).

Publication: Dynamics of a single exciton in strongly correlated bilayers

The extended paper on the properties of a single exciton, following up on my earlier Europhys Lett, is accepted and published in the New Journal of Physics.

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 when 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 α = J⊥/Jz. In the disordered phase at large α, a coherent quasi-particle peak appears, representing free motion of the exciton in a spin singlet background. In the Néel 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, New J. Phys. 14, 083040 (2012).

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, EPL 97, 27004 (2012).

Publication: Prediction of quantization of magnetic flux in double-layer exciton superfluids

My work on flux quantization in exciton superfluids finally culminated into a paper published in PRB. A preprint version (which is slightly different) can be found on the arXiv.

Title: Prediction of quantization of magnetic flux in double-layer exciton superfluids

Abstract: Currently, there is no way 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 h/e χm, where χm is the diamagnetic susceptibility of the device. We discuss possible experimental realizations of the predicted unconventional flux quantization.

Reference: Louk Rademaker, Jan Zaanen and Hans Hilgenkamp, Phys. Rev. B 83, pp. 012504 (2011)