Quantum Monte Carlo study of static potential in graphene

In this paper the interaction potential between static charges in suspended graphene is studied within the quantum Monte Carlo approach. We calculated the dielectric permittivity of suspended graphene for the set of temperatures and extrapolated our results to zero temperature. The dielectric permittivity at zero temperature has the following properties. At zero distance $\epsilon=2.24\pm0.02$. Then it rises and at a large distance the dielectric permittivity reaches the plateau $\epsilon\simeq4.20\pm0.66$. The results obtained in this paper allow to draw a conclusion that full account of many-body effects in the dielectric permittivity of suspended graphene gives $\epsilon$ very close to the one-loop results. Contrary to the one-loop result, the two-loop prediction for the dielectric permittivity deviates from our result. So, one can expect large higher order corrections to the two-loop prediction for the dielectric permittivity of suspended graphene.Comment: 6 pages, 2 figure

Realization of anisotropic compass model on the diamond lattice of Cu2+^{2+} in CuAl2_2O4_4

Spin-orbit (SO) Mott insulators are regarded as a new paradigm of magnetic materials, whose properties are largely influenced by SO coupling and featured by highly anisotropic bond-dependent exchange interactions between the spin-orbital entangled Kramers doublets, as typically manifested in $5d$ iridates. Here, we propose that a very similar situation can be realized in cuprates when the Cu$^{2+}$ ions reside in a tetrahedral environment, like in spinel compounds. Using first-principles electronic structure calculations, we construct a realistic model for the diamond lattice of the Cu$^{2+}$ ions in CuAl$_2$O$_4$ and show that the magnetic properties of this compound are largely controlled by anisotropic compass-type exchange interactions that dramatically modify the magnetic ground state by lifting the spiral spin-liquid degeneracy and stabilizing a commensurate single-$\boldsymbol{q}$ spiral

Non-linear BFKL dynamics: color screening vs. gluon fusion

A feasible mechanism of unitarization of amplitudes of deep inelastic scattering at small values of Bjorken $x$ is the gluon fusion. However, its efficiency depends crucially on the vacuum color screening effect which accompanies the multiplication and the diffusion of BFKL gluons from small to large distances. From the fits to lattice data on field strength correlators the propagation length of perturbative gluons is $R_c\simeq 0.2-0.3$ fermi. The probability to find a perturbative gluon with short propagation length at large distances is suppressed exponentially. It changes the pattern of (dif)fusion dramatically. The magnitude of the fusion effect appears to be controlled by the new dimensionless parameter $\sim R_c^2/8B$, with the diffraction cone slope $B$ standing for the characteristic size of the interaction region. It should slowly $\propto 1/\ln Q^2$ decrease at large $Q^2$. Smallness of the ratio $R_c^2/8B$ makes the non-linear effects rather weak even at lowest Bjorken $x$ available at HERA. We report the results of our studies of the non-linear BFKL equation which has been generalized to incorporate the running coupling and the screening radius $R_c$ as the infrared regulator.Comment: 16 pages, 2 figures, version accepted for publication, references adde

Exchange Field Induced Magnetoresistance in Colossal Magnetoresistance Manganites

The effect of an exchange field on electrical transport in thin films of metallic ferromagnetic manganites has been investigated. The exchange field was induced both by direct exchange coupling in a ferromagnet/antiferromagnet multilayer and by indirect exchange interaction in a ferromagnet/paramagnet superlattice. The electrical resistance of the manganite layers was found to be determined by the absolute value of the vector sum of the effective exchange field and the external magnetic field.Comment: 5 pages, 4 figure

Nonlinear kT factorization for Forward Dijets in DIS off Nuclei in the Saturation Regime

We develop the QCD description of the breakup of photons into forward dijets in small-x deep inelastic scattering off nuclei in the saturation regime. Based on the color dipole approach, we derive a multiple scattering expansion for intranuclear distortions of the jet-jet transverse momentum spectrum. A special attention is paid to the non-Abelian aspects of the propagation of color dipoles in a nuclear medium. We report a nonlinear $k_{\perp}$-factorization formula for the breakup of photons into dijets in terms of the collective Weizs\"acker-Williams (WW) glue of nuclei as defined in ref. \cite{Saturation,NSSdijet}. For hard dijets with the transverse momenta above the saturation scale the azimuthal decorrelation (acoplanarity) momentum is of the order of the nuclear saturation momentum QA. For minijets with the transverse momentum below the saturation scale the nonlinear kT-factorization predicts a complete disappearance of the jet-jet correlation. We comment on a possible relevance of the nuclear decorrelation of jets to the experimental data from the STAR-RHIC Collaboration.Comment: 40 pages, 7 figure

Pollen and Isotope Investigations of an Ice Core from Vavilov Ice Cap, October Revolution Island, Severnaya Zemlya Archipelago, Russia

The Vavilov Ice Cap (79°27'N, 95° 21'E) was cored during February and March of 1988. The corer passed through 457.18 m of glacier ice, 2.15 m of moraine-containing ice, and 2.28 m of underlying rocks. Structural-stratigraphical and isotope analysis show the glacier ice is of Holocene in age; the ice layer covered by frozen deposits is Pleistocene glacier ice; and the ground (ice wedge?) ice from underlying sediments was formed during the Last Interglacial. Palynological studies of this core, carried out for the first time in the Russian Arctic demonstrate that the pollen spectra have a unique pattern. It reduces the possibility of correlation between the Vavilov Ice Cape spectra and pollen spectra from other surficial deposits, because the ice retains pollen and spores brought from enormous distances. Only the upper 65 m of the core is easily dated, to the last millennium, by the presence of cereals, Plantago lanceolata, Centaurea cyanus, Cannabis pollen. That is in good agreement with the model of age distribution based upon depth. The presence of considerable amounts of Tilia cordifolia pollen, a West-European species in the upper layers suggests that summer air masses have been dominantly from the southwest during the last 500 years. The pollen data do not contradict the conclusion the Vavilov ice core is composed of a section of Holocene ice, moraine-containing ice representing the Pleistocene episode, and a ground ice formed during an earlier warm period (Last Interglacial?).Une carotte de glace prélevée en 1988 au sein de la calotte glaciaire de Vavilov (79°27'N, 95°21'E) a fait l'objet d'analyses isotopiques et palynologiques. Le sondage comprend 457,18 m de glace « pure », suivis de 2,15 m de glace chargée de sédiments et 2,28 m de roches gelés du socle. Les résultats des analyses structurales, stratigraphiques et isotopiques sont les suivants : la glace pure datée de l'Holocène, les 2,15 de la couche chargée de sédiments datent du Pléistocène, tandis que la glace présente dans les fentes du socle s'est formée au cours du dernier interglaciaire. Les spectres polliniques de cette carotte sont caractérisés par des grains de pollen d'origine lointaine préservés dans la glace. Cette constatation réduit grandement les possibilités de corrélation avec les données sédimentaires de la région, qui reflètent la composition de la végétation locale. Un âge inférieur à 1000 ans peut être attribué aux 65 m supérieurs de la carotte en raison de la présence de pollen de céréales, de Plantago lanceolata, Centaurea cyanus et de Cannabis. Cette interprétation concorde avec le modèle du taux d'accumulation de la glace. Dans la partie supérieure de la carotte, la présence en quantité considérable de Tilia cordifolia, une espèce de tilleul d'Europe de l'Ouest, laisse supposer que les masses d'air en provenance du sud-ouest ont prédominé au cours des étés des 500 dernières années. En conclusion, les données palynologiques ne contredisent pas les résultats antérieurs selon lesquels la carotte glaciaire étudiée couvre une partie de l'Holocène, que la glace contenant la moraine représente l'épisode du Pléistocène et que la glace du socle se soit formée pendant une période chaude (le dernier interglaciaire?)

Exclusive photoproduction of hard dijets and magnetic susceptibility of QCD vacuum

We argue that coherent production of hard dijets by linearly polarized real photons can provide direct evidence for chirality violation in hard processes, the first measurement of the magnetic susceptibility of the quark condensate and the photon distribution amplitude. It can also serve as a sensitive probe of the generalized gluon parton distribution. Numerical calculations are presented for HERA kinematics.Comment: 4 pages, 4 figure

Ultrahigh energy neutrinos and non-linear QCD dynamics

The ultrahigh energy neutrino-nucleon cross sections are computed taking into account different phenomenological implementations of the non-linear QCD dynamic s. Based on the color dipole framework, the results for the saturation model supplemented by DGLAP evolution as well as for the BFKL formalism in the geometric scaling regime are presented. They are contrasted with recent calculations using NLO DGLAP and unified BFKL-DGLAP formalisms.Comment: 5 pages, 2 figures. Version to be published in Physical Review