Multiferroicity in the frustrated spinel cuprate GeCu2_2O4_4

Different from other magnetically frustrated spinel systems, GeCu$_{2}$O$_{4}$ is a strongly tetragonal distorted spinel cuprate in which edge-sharing CuO$_{2}$ ribbons are running along alternating directions perpendicular to the $c$-axis. Here, GeCu$_{2}$O$_{4}$ samples of high quality were prepared via high pressure synthesis (at 4 GPa) and the corresponding magnetic and dielectric properties were investigated. For the first time, we observed a ferroelectric polarization emerging at T$_{N}$ $\sim$ 33~K. Although the ferroelectric polarization is weak in GeCu$_{2}$O$_{4}$ ($P$ $\sim$ 0.2$\mu$C/m$^{2}$), the existence of spin-induced multiferroicity provides a strong constraint on the possible ground state magnetic structures and/or the corresponding theoretical models of multiferroicity for GeCu$_{2}$O$_{4}$.Comment: https://journals.aps.org/prmaterials/abstract/10.1103/PhysRevMaterials.2.04140

Electron g-2 in Light-Front Quantization

Basis Light-front Quantization has been proposed as a nonperturbative framework for solving quantum field theory. We apply this approach to Quantum Electrodynamics and explicitly solve for the light-front wave function of a physical electron. Based on the resulting light-front wave function, we evaluate the electron anomalous magnetic moment. Nonperturbative mass renormalization is performed. Upon extrapolation to the infinite basis limit our numerical results agree with the Schwinger result obtained in perturbation theory to an accuracy of 0.06%.Comment: 6 pages, 4 figure

Quantum Phase Transition, O(3) Universality Class and Phase Diagram of Spin-1/2 Heisenberg Antiferromagnet on Distorted Honeycomb Lattice: A Tensor Renormalization Group Study

The spin-1/2 Heisenberg antiferromagnet on the distorted honeycomb (DHC) lattice is studied by means of the tensor renormalization group method. It is unveiled that the system has a quantum phase transition of second-order between the gapped quantum dimer phase and a collinear Neel phase at the critical point of coupling ratio \alpha_{c} = 0.54, where the quantum critical exponents \nu = 0.69(2) and \gamma = 1.363(8) are obtained. The quantum criticality is found to fall into the O(3) universality class. A ground-state phase diagram in the field-coupling ratio plane is proposed, where the phases such as the dimer, semi-classical Neel, and polarized phases are identified. A link between the present spin system to the boson Hubbard model on the DHC lattice is also discussed.Comment: 6 pages, 5 figures, published in Phys. Rev.

The Density Profile of Cluster-scale Dark Matter Halos

We measure the average gravitational shear profile of 6 massive clusters (M_vir ~ 10^15 M_sun) at z=0.3 out to a radius ~2h^-1 Mpc. The measurements are fitted to a generalized NFW-like halo model \rho(r) with an arbitrary r -> 0 slope \alpha. The data are well fitted by such a model with a central cusp with \alpha ~ 0.9 - 1.6 (68% confidence interval). For the standard-NFW case \alpha = 1.0, we find a concentration parameter c_vir that is consistent with recent predictions from high-resolution CDM N-body simulations. Our data are also well fitted by an isothermal sphere model with a softened core. For this model, our 1\sigma upper limit for the core radius corresponds to a limit \sigma_star \leq 0.1 cm^2 g^-1 on the elastic collision cross-section in a self-interacting dark matter model.Comment: 4 pages, 3 figures; version accepted for publication by ApJ Letters. Three figures omitted to allow space for new fig. 3 and expanded results and discussion sections, including NSIS model fi

Evolution of magnetic component in Yang-Mills condensate dark energy models

The evolution of the electric and magnetic components in an effective Yang-Mills condensate dark energy model is investigated. If the electric field is dominant, the magnetic component disappears with the expansion of the Universe. The total YM condensate tracks the radiation in the earlier Universe, and later it becomes $w_y\sim-1$ thus is similar to the cosmological constant. So the cosmic coincidence problem can be avoided in this model. However, if the magnetic field is dominant, $w_y>1/3$ holds for all time, suggesting that it cannot be a candidate for the dark energy in this case.Comment: 12 pages, 4 figures, minor typos correcte

Detecting relic gravitational waves in the CMB: A statistical bias

Analyzing the imprint of relic gravitational waves (RGWs) on the cosmic microwave background (CMB) power spectra provides a way to determine the signal of RGWs. In this Letter, we discuss a statistical bias, which could exist in the data analysis and has the tendency to overlook the RGWs. We also explain why this bias exists, and how to avoid it.Comment: 4 pages, 1 figur

An exact equilibrium reduced density matrix formulation I: The influence of noise, disorder, and temperature on localization in excitonic systems

An exact method to compute the entire equilibrium reduced density matrix for systems characterized by a system-bath Hamiltonian is presented. The approach is based upon a stochastic unraveling of the influence functional that appears in the imaginary time path integral formalism of quantum statistical mechanics. This method is then applied to study the effects of thermal noise, static disorder, and temperature on the coherence length in excitonic systems. As representative examples of biased and unbiased systems, attention is focused on the well-characterized light harvesting complexes of FMO and LH2, respectively. Due to the bias, FMO is completely localized in the site basis at low temperatures, whereas LH2 is completely delocalized. In the latter, the presence of static disorder leads to a plateau in the coherence length at low temperature that becomes increasingly pronounced with increasing strength of the disorder. The introduction of noise, however, precludes this effect. In biased systems, it is shown that the environment may increase the coherence length, but only decrease that of unbiased systems. Finally it is emphasized that for typical values of the environmental parameters in light harvesting systems, the system and bath are entangled at equilibrium in the single excitation manifold. That is, the density matrix cannot be described as a product state as is often assumed, even at room temperature. The reduced density matrix of LH2 is shown to be in precise agreement with the steady state limit of previous exact quantum dynamics calculations.Comment: 37 pages, 12 figures. To appear in Phys. Rev.