Paris West University Nanterre La Défense

About: Paris West University Nanterre La Défense is a education organization based out in Paris, France. It is known for research contribution in the topics: Computer science & Politics. The organization has 895 authors who have published 1430 publications receiving 21712 citations.

First excitations of the spin 1/2 Heisenberg antiferromagnet on the kagomé lattice

Abstract: We study the exact low energy spectra of the spin 1/2 Heisenberg antiferromagnet on small samples of the kagome lattice of up to N=36 sites. In agreement with the conclusions of previous authors, we find that these low energy spectra contradict the hypothesis of Neel type long range order. Certainly, the ground state of this system is a spin liquid, but its properties are rather unusual. The magnetic ( $$(\Delta S = 1)$$ ) excitations are separated from the ground state by a gap. However, this gap is filled with nonmagnetic ( $$(\Delta S = 0)$$ ) excitations. In the thermodynamic limit the spectrum of these nonmagnetic excitations will presumably develop into a gapless continuum adjacent to the ground state. Surprisingly, the eigenstates of samples with an odd number of sites, i.e. samples with an unsaturated spin, exhibit symmetries which could support long range chiral order. We do not know if these states will be true thermodynamic states or only metastable ones. In any case, the low energy properties of the spin 1/2 Heisenberg antiferromagnet on the kagome lattice clearly distinguish this system from either a short range RVB spin liquid or a standard chiral spin liquid. Presumably they are facets of a generically new state of frustrated two-dimensional quantum antiferromagnets.

Order versus disorder in the quantum Heisenberg antiferromagnet on the kagomé lattice using exact spectra analysis

Abstract: A group-symmetry analysis of the low-lying levels of the spin-1/2 kagom\'e Heisenberg antiferromagnet is performed for small samples up to $N=27$. This approach allows one to follow the effect of quantum fluctuations when the sample size increases. The results contradict the scenario of ``order by disorder'' which has been advanced on the basis of large-$S$ calculations. A large enough second-neighbor ferromagnetic exchange coupling is needed to stabilize the $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ pattern: the finite-size analysis indicates a quantum critical transition at a nonzero coupling.

Signature of Néel order in exact spectra of quantum antiferromagnets on finite lattices.

Abstract: We show how the broken symmetries of the N\'eel state are embodied in the exact spectrum of the triangular Heisenberg antiferromagnet on finite lattices as small as N=21 (spectra up to N=36 have been computer). We present the first numerical evidence of an extensive set of low-lying levels that are below the softest magnons and collapse to the ground state in the thermodynamic limit. This set of quantum states represents the quantum counterpart of the classical N\'eel ground state. We develop an approach relying on the symmetry analysis and finite-size scaling and we provide new arguments in favor of an ordered ground state for the S=1/2 triangular Heisenberg model.

Independent phase modulation for quadruplex polarization channels enabled by chirality-assisted geometric-phase metasurfaces

Abstract: Geometric-phase metasurfaces, recently utilized for controlling wavefronts of circular polarized (CP) electromagnetic waves, are drastically limited to the cross-polarization modality. Combining geometric with propagation phase allows to further control the co-polarized output channel, nevertheless addressing only similar functionality on both co-polarized outputs for the two different CP incident beams. Here we introduce the concept of chirality-assisted phase as a degree of freedom, which could decouple the two co-polarized outputs, and thus be an alternative solution for designing arbitrary modulated-phase metasurfaces with distinct wavefront manipulation in all four CP output channels. Two metasurfaces are demonstrated with four arbitrary refraction wavefronts, and orbital angular momentum modes with four independent topological charge, showcasing complete and independent manipulation of all possible CP channels in transmission. This additional phase addressing mechanism will lead to new components, ranging from broadband achromatic devices to the multiplexing of wavefronts for application in reconfigurable-beam antenna and wireless communication systems.