Microwave Response and Spin Waves in Superconducting Ferromagnets

Excitation of spin waves is considered in a superconducting ferromagnetic slab with the equilibrium magnetization both perpendicular and parallel to the surface. The surface impedance is calculated and its behavior near propagation thresholds is analyzed. Influence of non-zero magnetic induction at the surface is considered in various cases. The results provide a basis for investigation of materials with coexisting superconductivity and magnetism by microwave response measurements.Comment: 10 pages, 7 figure

Comment on "Domain Structure in a Superconducting Ferromagnet"

According to Faure and Buzdin [Phys. Rev. Lett. 94, 187202 (2005)], in a superconducting ferromagnet a domain structure with a period small compared with the London penetration depth can arise. They claim that this contradicts to the conclusion of Sonin [Phys. Rev. B, 66, 100504 (2002)] that ferromagnetic domain structure in the Meissner state of a superconducting ferromagnet is absent in equilibrium. This contradiction is imaginary, based on misinterpretation of the results of these two papers.Comment: 1 page, no figures, final version published in Phys.Rev.Let

Associations Between Neighborhoods and Summer Meals Sites: Measuring Access to Federal Summer Meals Programs

While the challenge of access to healthy food features prominently in the literature, access to federal nutrition programs, specifically USDA’s Summer Meals Program, is relatively undocumented. Participation in the program depends on a number of complex variables, including site availability, neighborhood dynamics, and community investment. The purpose of this study is to determine neighborhood indicators that correlate with the coverage and density of summer meals sites in Texas. Attributes of the community and the number of meal sites were collected at the census tract level, and paired t-tests revealed the statistical significance of differences between tracts with and without meal sites. We also implemented a regression model to predict the number of sites within a tract as a function of neighborhood variables. Urban areas have the greatest access to summer meals sites, while access is limited in suburban and rural areas. In general, method of transportation proved significant. We find that the site coverage and density in areas of different urbanicity depends on the availability of transportation

Paramagnetic-diamagnetic interplay in quantum dots for non-zero temperatures

In the usual Fock-and Darwin-formalism with parabolic potential characterized by the confining energy \eps_o := \hbar\omega_o= 3.37 meV, but including explicitly also the Zeeman coupling between spin and magnetic field, we study the combined orbital and spin magnetic properties of quantum dots in a two-dimensional electron gas with parameters for GaAs, for N =1 and N >> 1 electrons on the dot. For N=1 the magnetization M(T,B) consists of a paramagnetic spin contribution and a diamagnetic orbital contribution, which dominate in a non-trivial way at low temperature and fields rsp. high temperature and fields. For N >> 1, where orbital and spin effects are intrinsically coupled in a subtle way and cannot be separated, we find in a simplified Hartree approximation that at N=m^2, i.e. at a half-filled last shell, M(T,B,N) is parallel (antiparallel) to the magnetic field, if temperatures and fields are low enough (high enough), whereas for N\ne m^2 the magnetization oscillates with B and N as a T-dependent periodic function of the variable x:=\sqrt{N}eB/(2m^*c\omega_o), with T-independent period \Delta x =1 (where m^* := 0.067 m_o is the small effective mass of GaAs, while m_o is the electron mass). Correspondingly, by an adiabatic demagnetization process, which should only be fast enough with respect to the slow transient time of the magnetic properties of the dot, the temperature of the dot diminishes rsp. increases with decreasing magnetic field, and in some cases we obtain quite pronounced effects.Comment: LaTeX, 28 pages; including three .eps-figures; final version accepted by J. Phys. CM, with minimal changes w.r.to v

Domain Walls in a Tetragonal Chiral p-Wave Superconductor

Domain walls in a tetragonal chiral p-wave superconductors with broken time reversal symmetry are analyzed in the framework of the Ginsburg-Landau theory. The energy and the jump of the magnetic induction on the wall were determined for different types of walls as functions of the parameters of the Ginzburg-Landau theory and orientation of the domain wall with respect to the crystallographic axes. We discuss implications of the analysis for $Sr_{2}RuO_{4}$, where no stray magnetic fields from domain walls were detected experimentally.Comment: 8 pages, 2 figure

Conventional Magnetic Superconductors: Coexistence of Singlet Superconductivity and Magnetic Order

The basic physics of bulk magnetic superconductors (MS) related to the problem of the coexistence of singlet superconductivity (SC) and magnetic order is reviewed. The interplay between exchange (EX) and electromagnetic (EM) interaction is discussed and argued that the singlet SC and uniform ferromagnetic (F) order practically never coexist. In case of their mutual coexistence the F order is modified into a domain-like or spiral structure depending on magnetic anisotropy. It turns out that this situation is realized in several superconductors such as $ErRh_{4}B_{4}$, $HoMo_{6}S_{8}$, $HoMo_{6}Se_{8}$ with electronic and in $AuIn_{2}$ with nuclear magnetic order. The later problem is also discussed here. The coexistence of SC and antiferromagnetism is more favorable than with the modified F order. Very interesting physics is realized in systems with SC and weak-ferromagnetism which results in an very reach phase diagram. The properties of magnetic superconductors in magnetic field are very peculiar, especially near the (ferro)magnetic transition temperature where the upper critical field becomes smaller than the thermodynamical critical field. The extremely interesting physics of Josephson junctions based on MS with spiral magnetic order is also discussed. The existence of the triplet pairing amplitude $F_{\uparrow \uparrow}$ ($F_{\downarrow \downarrow}$) in MS with rotating magnetization (the effect recently rediscovered in SFS junctions) gives rise to the so called $\pi$-contact. Furthermore, the interplay of the superconducting and magnetic phase in such a contact renders possibilities for a new type of coupled Josephson-qubits.Comment: 28 pages, 3 figures; submitted for the Special Issue Comptes de l'Academie des Sciences: Problems of the Coexistence of Magnetism and Superconductivity, edited by A. Buzdi

A Transport and Microwave Study of Superconducting and Magnetic RuSr2EuCu2O8

We have performed susceptibility, thermopower, dc resistance and microwave measurements on RuSr2EuCu2O8. This compound has recently been shown to display the coexistence of both superconducting and magnetic order. We find clear evidence of changes in the dc and microwave resistance near the magnetic ordering temperature (132 K). The intergranular effects were separated from the intragranular effects by performing microwave measurements on a sintered ceramic sample as well as on a powder sample dispersed in an epoxy resin. We show that the data can be interpreted in terms of the normal-state resistivity being dominated by the CuO2 layers with exchange coupling to the Ru moments in the RuO2 layers. Furthermore, most of the normal-state semiconductor-like upturn in the microwave resistance is found to arise from intergranular transport. The data in the superconducting state can be consistently interpreted in terms of intergranular weak-links and an intragranular spontaneous vortex phase due to the ferromagnetic component of the magnetization arising from the RuO2 planes.Comment: 20 pages including 6 figures in pdf format. To be published in Phys. Rev.

Absence of Meissner State and Robust Ferromagnetism in the Superconducting State of UCoGe: Possible Evidence of Spontaneous Vortex State

We report ac magnetic susceptibility and dc magnetization measurements on the superconducting ferromagnet UCoGe (with superconducting and Curie temperatures of $T_{{\rm SC}} \sim 0.5$~K and $T_{{\rm Curie}} \sim 2.5$~K, respectively). In the normal, ferromagnetic state ($T_{{\rm SC}} < T < T_{{\rm Curie}}$), the magnetization curve exhibits a hysteresis loop similar to that of a regular itinerant ferromagnet. Upon lowering the temperature below $T_{{\rm SC}}$, the spontaneous magnetization is unchanged, but the hysteresis is markedly enhanced. Even deeply inside the superconducting state, ferromagnetism is not completely shielded, and there is no Meissner region, a magnetic field region of $H < H_{\rm c1}$ (a lower critical field). From these results, we suggest that UCoGe is the first material in which ferromagnetism robustly survives in the superconducting state and a spontaneous vortex state without the Meissner state is realized.Comment: 5 pages, 4 figures, to be published in J. Phys. Soc. Jp

A diverse panel of hepatitis C virus glycoproteins for use in vaccine research reveals extremes of monoclonal antibody neutralization resistance

Despite significant advances in the treatment of hepatitis C virus (HCV) infection, the need to develop preventative vaccines remains. Identification of the best vaccine candidates and evaluation of their performance in preclinical and clinical development will require appropriate neutralization assays utilizing diverse HCV isolates. We aimed to generate and characterize a panel of HCVE1E2 glycoproteins suitable for subsequent use in vaccine and therapeutic antibody testing. Full-length E1E2 clones were PCR amplified from patient- derived serum samples, cloned into an expression vector, and used to generate viral pseudoparticles (HCVpp). In addition, some of these clones were used to generate cell culture infectious (HCVcc) clones. The infectivity and neutralization sensitivity of these viruses were then determined. Bioinformatic and HCVpp infectivity screening of approximately 900 E1E2 clones resulted in the assembly of a panel of 78 functional E1E2 proteins representing distinct HCV genotypes and different stages of infection. These HCV glycoproteins differed markedly in their sensitivity to neutralizing antibodies. We used this panel to predict antibody efficacy against circulating HCV strains, highlighting the likely reason why some monoclonal antibodies failed in previous clinical trials. This study provides the first objective categorization of cross-genotype patient-derived HCVE1E2 clones according to their sensitivity to antibody neutralization. It has shown that HCV isolates have clearly distinguishable neutralization-sensitive, -resistant, or -intermediate phenotypes, which are independent of genotype. The panel provides a systematic means for characterization of the neutralizing response elicited by candidate vaccines and for defining the therapeutic potential of monoclonal antibodies

Generalizing with perceptrons in case of structured phase- and pattern-spaces

We investigate the influence of different kinds of structure on the learning behaviour of a perceptron performing a classification task defined by a teacher rule. The underlying pattern distribution is permitted to have spatial correlations. The prior distribution for the teacher coupling vectors itself is assumed to be nonuniform. Thus classification tasks of quite different difficulty are included. As learning algorithms we discuss Hebbian learning, Gibbs learning, and Bayesian learning with different priors, using methods from statistics and the replica formalism. We find that the Hebb rule is quite sensitive to the structure of the actual learning problem, failing asymptotically in most cases. Contrarily, the behaviour of the more sophisticated methods of Gibbs and Bayes learning is influenced by the spatial correlations only in an intermediate regime of $\alpha$, where $\alpha$ specifies the size of the training set. Concerning the Bayesian case we show, how enhanced prior knowledge improves the performance.Comment: LaTeX, 32 pages with eps-figs, accepted by J Phys