Gutzwiller study of extended Hubbard models with fixed boson densities

We studied all possible ground states, including supersolid (SS) phases and phase separations of hard-core- and soft-core-extended Bose--Hubbard models with fixed boson densities by using the Gutzwiller variational wave function and the linear programming method. We found that the phase diagram of the soft-core model depends strongly on its transfer integral. Furthermore, for a large transfer integral, we showed that an SS phase can be the ground state even below or at half filling against the phase separation. We also found that the density difference between nearest-neighbor sites, which indicates the density order of the SS phase, depends strongly on the boson density and transfer integral.Comment: 14 pages, 14 figures, to be published in Phys. Rev.

Internal magnetic field effect on magnetoelectricity in orthorhombic RMnO3RMnO_3 crystals

We have investigated the role of the 4$f$ moment on the magnetoelectric (ME) effect of orthorhombic $R$MnO$_{3}$ ($R$=rare earth ions). In order to clarify the role of the 4$f$ moment, we prepared three samples: (Eu,Y)MnO$_{3}$ without the 4$f$ moment, TbMnO$_{3}$ with the anisotropic 4$f$ moment, and (Gd,Y)MnO$_{3}$ with the isotropic 4$f$ moment. The ferroelectric behaviors of these samples are different from each other in a zero magnetic field. (Eu,Y)MnO$_{3}$ and (Gd,Y)MnO$_{3}$ show the ferroelectric polarization along the a axis in the ground state, while TbMnO$_{3}$ shows it along the c axis. Such difference may arise from the influence of the anisotropic Tb$^{3+}$ 4$f$ moment. The direction of the ferroelectric polarization of $R$MnO$_{3}$ is determined by the internal magnetic field arising from the 4$f$ moment.Comment: 2 pages, 1 figure, the proceeding of International Conference of Magnetism, to be published in the Journal of Magnetism and Magnetic Material

Observation of an optical non-Fermi-liquid behavior in the heavy fermion state of YbRh2_{2}Si2_{2}

We report far-infrared optical properties of YbRh$_{2}$Si$_{2}$ for photon energies down to 2 meV and temperatures 0.4 -- 300 K. In the coherent heavy quasiparticle state, a linear dependence of the low-energy scattering rate on both temperature and photon energy was found. We relate this distinct dynamical behavior different from that of Fermi liquid materials to the non-Fermi liquid nature of YbRh$_{2}$Si$_{2}$ which is due to its close vicinity to an antiferromagnetic quantum critical point.Comment: 5 pages, 4 figures. submitte

Limitations on the extent of off-center displacements in TbMnO3 from EXAFS measurements

We present EXAFS data at the Mn K and Tb L3 edges that provide upper limits on the possible displacements of any atoms in TbMnO3. The displacements must be less than 0.005-0.01A for all atoms which eliminates the possibility of moderate distortions (0.02A) with a small c-axis component, but for which the displacements in the ab plane average to zero. Assuming the polarization arises from a displacement of the O2 atoms along the c-axis, the measured polarization then leads to an O2 displacement that is at least 6X10^{-4}A, well below our experimental limit. Thus a combination of the EXAFS and the measured electrical polarization indicate that the atomic displacements likely lie in the range 6X10^{-4} - 5X10^{-3}A.Comment: submitted to PRB; 11 pages (preprint form) 7 figure

Iron-Based Heavy Quasiparticles in SrFe4_{4}Sb12_{12}: An Infrared Spectroscopic Study

Temperature-dependent infrared reflectivity spectra of SrFe$_{4}$Sb$_{12}$ has been measured. A renormalized Drude peak with a heavy effective mass and a pronounced pseudogap of 10 meV develops in the optical conductivity spectra at low temperatures. As the temperature decreases below 100 K, the effective mass ($m^{*}$) rapidly increases, and the scattering rate ($1/\tau$) is quenched. The temperature dependence of $m^{*}$ and $1/\tau$ indicates that the hybridization between the Fe 3d spins and the charge carriers plays an important role in determining the physical properties of SrFe$_{4}$Sb$_{12}$ at low temperatures. This result is the clear evidence of the iron-based heavy quasiparticles.Comment: 5 pages, 5 figure

Lazy states: sufficient and necessary condition for zero quantum entropy rates under any coupling to the environment

We find the necessary and sufficient conditions for the entropy rate of the system to be zero under any system-environment Hamiltonian interaction. We call the class of system-environment states that satisfy this condition lazy states. They are a generalization of classically correlated states defined by quantum discord, but based on projective measurements of any rank. The concept of lazy states permits the construction of a protocol for detecting global quantum correlations using only local dynamical information. We show how quantum correlations to the environment provide bounds to the entropy rate, and how to estimate dissipation rates for general non-Markovian open quantum systems.Comment: 4 page

Change of Electronic Structure Induced by Magnetic Transitions in CeBi

The temperature dependence of the electronic structure of CeBi arising from two types of antiferromagnetic transitions based on optical conductivity ($\sigma(\omega)$) was observed. The $\sigma(\omega)$ spectrum continuously and discontinuously changes at 25 and 11 K, respectively. Between these temperatures, two peaks in the spectrum rapidly shift to the opposite energy sides as the temperature changes. Through a comparison with the band calculation as well as with the theoretical $\sigma(\omega)$ spectrum, this peak shift was explained by the energy shift of the Bi $6p$ band due to the mixing effect between the Ce $4f \Gamma_8$ and Bi $6p$ states. The single-layer antiferromagnetic ($+-$) transition from the paramagnetic state was concluded to be of the second order. The marked changes in the $\sigma(\omega)$ spectrum at 11 K, however, indicated the change in the electronic structure was due to a first-order-like magnetic transition from a single-layer to a double-layer ($++--$) antiferromagnetic phase.Comment: 4 pages, to be published in J. Phys. Soc. Jpn. 73 Aug. (2004

Optical conductivity of the Kondo insulator YbB_12: Gap formation and low-energy excitations

Optical reflectivity experiments have been conducted on single crystals of the Kondo insulator YbB_12 in order to obtain its optical conductivity, \sigma(\omega). Upon cooling below 70 K, a strong supression of \sigma(\omega) is seen in the far-infrared region, indicating the opening of an energy gap of ~ 25 meV. This gap development is coincident with a rapid decrease in the magnetic susceptibility, which shows that the gap opening has significant influence on magnetic properties. A narrow, asymmetric peak is observed at ~40 meV in \sigma(\omega), which is attributed to optical transitions between the Yb 4f-derived states across the gap. In addition, a broad peak is observed at ~0.25 eV. This peak is attributed to transitions between Yb 4f-derived states and p-d band, and is reminiscent of similar peaks previously observed for rare-earth hexaborides.Comment: 4 pages, 4 figure

P-Wave Charmed-Strange Mesons

We examine charmed-strange mesons within the framework of the constituent quark model, focusing on the states with L=1. We are particularly interested in the mixing of two spin-states that are involved in $D_{s1}(2536)$ and the recently discovered $D_{sJ}(2460)$. We assume that these two mesons form a pair of states with J=1. These spin-states are mixed by a type of the spin-orbit interaction that violates the total-spin conservation. Without assuming explicit forms for the interactions as functions of the interquark distance, we relate the matrix elements of all relevant spin-dependent interactions to the mixing angle and the observed masses of the L=1 quartet. We find that the spin-spin interaction, among various types of the spin-dependent interactions, plays a particularly interesting role in determining the spin structure of $D_{s1}(2536)$ and $D_{sJ}(2460)$