Evaluation of Half-metallic Antiferromagnetism in A2{\cal A}_2CrFeO6_6 ({A\cal A}=La, Sr

The nearly well-ordered double perovskite La$_2$CrFeO$_6$ has been synthesized recently. Contrary to previous theoretical predictions, but in agreement with experimental observations, our first principle calculations indicate an insulating ferrimagnet La$_2$CrFeO$_6$ with antialigned S=3/2 Cr$^{3+}$ and S=5/2 Fe$^{3+}$ ions,using the local spin density approximation (LSDA), a correlated band theory LDA+U, and a semilocal functional modified Becke-Johnson method. Additionally, we investigated the double perovskite Sr$_2$CrFeO$_6$, which is as yet unsynthesized. In LSDA calculations, this system shows formally tetravalent Cr and Fe ions both having antialigned $S$=1 moments, but is a simple metal. Once applying on-site Coulomb repulsion U on both Cr and Fe ions, this system becomes half-metallic and the moment of Fe is substantially reduced, resulting in zero net moment. These results are consistent with our fixed spin moment studies. Our results suggest a precisely compensated half-metallic Sr$_2$CrFeO$_6$.Comment: 7 page

Remarks on the Upper Bounds on the Higgs Boson Mass from Triviality

We study the effects of the one-loop matching conditions on Higgs boson and top quark masses on the triviality bounds on the Higgs boson mass using $\beta_{\lambda}$ with corrected two-loop coefficients. We obtain quite higher results than previous ones and observe that the triviality bounds are not nearly influenced by varying top quark mass over the range measured at CDF and D0. The effects of typo errors in $\beta_{\lambda}^{(2)}$ and the one-loop matching condition on the top quark mass are negligible. We estimate the size of effects on the triviality bounds from the one-loop matching condition on the Higgs boson mass.Comment: 9 pages, tar'ed gzip'ed uuencoded files, LaTex, 5 PostScript figures. To appear in Physical Review

Effects of metallic spacer in layered superconducting Sr2(Mgy_yTi1−y_{1-y})O3FeAs

The highly two-dimensional superconducting system Sr2(Mg$_y$Ti$_{1-y}$)O3FeAs, recently synthesized in the range of 0.2 < y < 0.5, shows an Mg concentration-dependent $T_c$. Reducing the Mg concentration from y=0.5 leads to a sudden increase in $T_c$, with a maximum $T_c$ ~40 K at y=0.2. Using first principles calculations, the unsynthesized stoichiometric y=0 and the substoichiometric y=0.5 compounds have been investigated. For the 50% Mg-doped phase (y=0.5), Sr2(Mg$_y$Ti$_{1-y}$)O3 layers are completely insulating spacers between FeAs layers, leading to the fermiology such as that found for other Fe pnictides. At y=0, representing a phase with metallic Sr2TiO3 layers, the $\Gamma$-centered Fe-derived Fermi surfaces (FSs) considerably shrink or disappear. Instead, three $\Gamma$-centered Ti FSs appear, and in particular two of them have similar size, like in MgB2. Interestingly, FSs have very low Fermi velocity in large fractions: the lowest being 0.6$\times10^6$ cm/s. Furthermore, our fixed spin moment calculations suggest the possibility of magnetic ordering, with magnetic Ti and nearly nonmagnetic Fe ions. These results indicate a crucial role of Sr2(Mg$_y$Ti$_{1-y}$)O3 layers in this superconductivity.Comment: 7 pages; Proceedings of ICSM-201

Delayed and localized pemphigus vulgaris after breast cancer radiotherapy

Breast cancer treatment involving ionizing radiation causes characteristic radiation dermatitis in the majority of patients. The DNA damaging effects of radiation can rarely predispose to primary inflammatory dermatoses, such as pemphigus vulgaris. In such cases, the disease presents with all the hallmarks of the primary dermatosis, but the eruption is limited to the field of irradiation and is often amenable to treatment. In contrast, occurrence of generalized pemphigus vulgaris in this setting may mean cancer recurrence. The mechanism by which radiotherapy induces localized disease remains unknown, but there is likely a loss of self-tolerance which maybe coupled to antigen exposure

Heisenberg-picture approach to the exact quantum motion of a time-dependent forced harmonic oscillator

In the Heisenberg picture, the generalized invariant and exact quantum motions are found for a time-dependent forced harmonic oscillator. We find the eigenstate and the coherent state of the invariant and show that the dispersions of these quantum states do not depend on the external force. Our formalism is applied to several interesting cases.Comment: 15 pages, two eps files, to appear in Phys. Rev. A 53 (6) (1996

Constraints on the R-parity- and Lepton-Flavor-Violating Couplings from B0 Decats to Two Charged Leptons

We derive the upper bounds on certain products of R-parity- and lepton-flavor-violating couplings from the decays of the neutral $B$ meson into two charged leptons. These modes of $B^0$ decays can constrain the product combinations of the couplings with one or more heavy generation indices. We find that most of these bounds are stronger than the previous ones.Comment: Table is changed; version to appear in Phys. Rev.

Born Effective Charges and Infrared Response of LiBC

Calculations of the zone center optical mode frequencies (including LO-TO splitting), Born effective charges Z$^*_{\alpha\alpha}$ for each atom, dielectric constants $\epsilon_{0}$ and $\epsilon_{\infty}$, and the dielectric response in the infrared, using density functional linear response theory, are reported. Calculated Raman modes are in excellent agreement with experimental values (170 cm$^{-1}$ and 1170 cm$^{-1}$), while it will require better experimental data to clarify the infrared active mode frequencies. The Born effective charges Z$^*_{\alpha \alpha}$ (i) have surprisingly different values for B and C, and (ii) show considerable anisotropy. Relationships between the effective charges and LO-TO splitting are discussed, and the predicted reflectivity in the range 0 -- 1400 cm$^{-1}$ is presented. These results hold possible implications for Li removal in LiBC, and C substition for B in MgB$_2$.Comment: 6 pages, 3 figure

The effective action of (2+1)-dimensional QED: the effect of finite fermion density

The effective action of (2+1)-dimensional QED with finite fermion density is calculated in a uniform electromagnetic field. It is shown that the integer quantum Hall effect and de Haas-van Alphen like phenomena in condensed matter physics are derived directly from the effective action.Comment: 10 pages, Revtex, No figure

Universal expansion of vortex clusters in a dissipative two-dimensional superfluid

A large ensemble of quantum vortices in a superfluid may itself be treated as a novel kind of fluid that exhibits anomalous hydrodynamics. Here we consider the dynamics of vortex clusters with thermal friction, and present an analytic solution that uncovers a new universality class in the out-of-equilibrium dynamics of dissipative superfluids. We find that the long-time dynamics of the vorticity distribution is an expanding Rankine vortex (i.e.~top-hat distribution) independent of initial conditions. This highlights a fundamentally different decay process to classical fluids, where the Rankine vortex is forbidden by viscous diffusion. Numerical simulations of large ensembles of point vortices confirm the universal expansion dynamics, and further reveal the emergence of a frustrated lattice structure marked by strong correlations. We present experimental results in a quasi-two-dimensional Bose-Einstein condensate that are in excellent agreement with the vortex fluid theory predictions, demonstrating that the signatures of vortex fluid theory can be observed with as few as $N\sim 11$ vortices. Our theoretical, numerical, and experimental results establish the validity of the vortex fluid theory for superfluid systems.Comment: V1: 6 pages, 3 figures in main text. 5 pages, 5 figures in supplemental material. V2: Updated in response to reviewer comments: Improved introduction and discussion, additional simulation data provided in supplemental material