On brane-induced gravity in warped backgrounds

We study whether modification of gravity at large distances is possible in warped backgrounds with two branes and a brane-induced term localized on one of the branes. We find that there are three large regions in the parameter space where the theory is weakly coupled up to high energies. In one of these regions gravity on the brane is four-dimensional at arbitrarily large distances, and the induced Einstein term results merely in the renormalization of the 4d Planck mass. In the other two regions the behavior of gravity changes at ultra-large distances; however, radion becomes a ghost. In parts of these regions, both branes have positive tensions, so the only reason for the appearance of the ghost field is the brane-induced term. In between these three regions, there are domains in the parameter space where gravity is strongly coupled at phenomenologically unacceptable low energy scale.Comment: 12 pages, 2 fig, JHEP3 style required, typos correcte

Exploring improved holographic theories for QCD: Part I

Various holographic approaches to QCD in five dimensions are explored using input both from the putative non-critical string theory as well as QCD. It is argued that a gravity theory in five dimensions coupled to a dilaton and an axion may capture the important qualitative features of pure QCD. A part of the higher alpha' corrections are resummed into a dilaton potential. The potential is shown to be in one-to-one correspondence with the exact beta-function of QCD, and its knowledge determines the full structure of the vacuum solution. The geometry near the UV boundary is that of AdS_5 with logarithmic corrections reflecting the asymptotic freedom of QCD. We find that all relevant confining backgrounds have an IR singularity of the "good" kind that allows unambiguous spectrum computations. Near the singularity the 't Hooft coupling is driven to infinity. Asymptotically linear glueball masses can also be achieved. The classification of all confining asymptotics, the associated glueball spectra and meson dynamics are addressed in a companion paper, ArXiv:0707.1349Comment: 37+23 pages, 11 figures. (v3) Some clarifications and typo corrections. Journal versio

Tkachenko waves, glitches and precession in neutron star

Here I discuss possible relations between free precession of neutron stars, Tkachenko waves inside them and glitches. I note that the proposed precession period of the isolated neutron star RX J0720.4-3125 (Haberl et al. 2006) is consistent with the period of Tkachenko waves for the spin period 8.4s. Based on a possible observation of a glitch in RX J0720.4-3125 (van Kerkwijk et al. 2007), I propose a simple model, in which long period precession is powered by Tkachenko waves generated by a glitch. The period of free precession, determined by a NS oblateness, should be equal to the standing Tkachenko wave period for effective energy transfer from the standing wave to the precession motion. A similar scenario can be applicable also in the case of the PSR B1828-11.Comment: 6 pages, no figures, accepted to Ap&S

Cosmological particle production, causal thermodynamics, and inflationary expansion

Combining the equivalence between cosmological particle creation and an effective viscous fluid pressure with the fact that the latter represents a dynamical degree of freedom within the second-order Israel-Stewart theory for imperfect fluids, we reconsider the possibility of accelerated expansion in fluid cosmology. We find an inherent self-limitation for the magnitude of an effective bulk pressure which is due to adiabatic (isentropic) particle production. For a production rate which depends quadratically on the Hubble rate we confirm the existence of solutions which describe a smooth transition from inflationary to noninflationary behavior and discuss their interpretation within the model of a decaying vacuum energy density. An alternative formulation of the effective imperfect fluid dynamics in terms of a minimally coupled scalar field is given. The corresponding potential is discussed and an entropy equivalent for the scalar field is found.Comment: 16 pages, revtex file, submitted to Phys. Rev.

Role Of Oxygen Vacancies In The Magnetic And Dielectric Properties Of The High-dielectric-constant System Cacu3 Ti4 O12: An Electron-spin Resonance Study

We report experiments of electron spin resonance (ESR) of Cu2+ in polycrystalline samples of CaCu3 Ti4 O12 post-annealed in different atmospheres. After being synthesized by solid state reaction, pellets of CaCu3 Ti4 O12 were annealed for 24 h at 1000°C under air, Ar or O2. Our temperature dependent ESR data revealed for all samples nearly temperature independent g value (2.15(1)) and linewidth for T TN ≈25 K. However, the values of ESR linewidth are strongly affected by the oxygen content in the sample. For instance, argon post-annealed samples show a much larger linewidth than the O2 or air post-annealed samples. We attribute this broadening to an increase of the dipolar homogeneous broadening of the Cu2+ ESR lines due to the presence of oxygen vacancies which induce an S=1 2 spin inside the TiO6 octahedra. Correlation between a systematic dependence of the ESR linewidth on the oxygen content and the high dielectric constant of these materials is addressed. Also, ESR, magnetic susceptibility, and specific heat data for a single crystal of CaCu3 Ti4 O12 and for polycrystals of CdCu3 Ti4 O12 are reported. © 2006 The American Physical Society.7322Subramanian, M.A., Li, D., Duan, N., Reisner, B., Sleight, A.W., (2000) J. Solid State Chem., 151, p. 323. , JSSCBI 0022-4596 10.1006/jssc.2000.8703Ramirez, A.P., Subramanian, M.A., Gardel, M., Blumberg, G., Li, D., Vogt, T., Shapiro, S.M., (2000) Solid State Commun., 151, p. 217. , SSCOA4 0038-1098Homes, C.C., Vogt, T., Shapiro, S.M., Wakimoto, S., Ramirez, A.P., (2001) Science, 293, p. 673. , SCIEAS 0036-8075 10.1126/science.292.5517.673Lunkenheimer, P., Bobnar, V., Pronin, A.V., Ritus, A.I., Volkov, A.A., Loidl, A., (2002) Phys. Rev. B, 66, p. 052105. , PRBMDO 0163-1829 10.1103/PhysRevB.66.052105Homes, C.C., Vogt, T., Shapiro, S.M., Wakimoto, S., Subramanian, M.A., Ramirez, A.P., (2003) Phys. Rev. B, 67, p. 092106. , PRBMDO 0163-1829 10.1103/PhysRevB.67.092106Sinclair, D.C., Admas, T.B., Morrison, F.D., West, A.R., (2002) Appl. Phys. Lett., 80, p. 2153. , APPLAB 0003-6951 10.1063/1.1463211Giulloto, E., Mozzati, M.C., Azzoni, C.B., Massarotti, V., Bini, M., (2004) Ferroelectrics, 298, p. 61. , FEROA8 0015-0193Mozzati, M.C., Azzoni, C.B., Capsoni, D., Bini, M., Massarotti, V., (2003) J. Phys.: Condens. Matter, 15, p. 7365. , JCOMEL 0953-8984 10.1088/0953-8984/15/43/018Subramanian, M.A., Sleight, A.W., (2002) Solid State Sci., 4, p. 347. , SSSCFJ 1293-2558 10.1016/S1293-2558(01)01262-6Fang, L., Shen, M., Cao, W., (2004) J. Appl. Phys., 95, p. 6483. , JAPIAU 0021-8979 10.1063/1.1728308Koitzsch, A., Blumberg, G., Gozar, A., Dennis, B., Ramirez, A.P., Trebst, S., Wakimoto, S., (2002) Phys. Rev. B, 65, p. 052406. , PRBMDO 0163-1829 10.1103/PhysRevB.65.052406Bosman, A.J., Van Daal, H.J., (1970) Adv. Phys., 19, p. 1. , ADPHAH 0001-8732 10.1080/00018737000101071Lenjer, S., Schirmer, O.F., Hesse, H., Kool, T.W., (2002) Phys. Rev. B, 66, p. 165106. , PRBMDO 0163-1829 10.1103/PhysRevB.66.165106Bednorz, J.G., Mller, K.A., (1988) Rev. Mod. Phys., 60, p. 585. , RMPHAT 0034-6861 10.1103/RevModPhys.60.585Salamon, M.B., Jaime, M., (2001) Rev. Mod. Phys., 73, p. 583. , RMPHAT 0034-6861 10.1103/RevModPhys.73.583Scharfschwerdt, R., Mazur, A., Schirmer, O.F., Hesse, H., Mendricks, S., (1996) Phys. Rev. B, 54, p. 15284. , PRBMDO 0163-1829 10.1103/PhysRevB.54.15284Laguta, V.V., Slipenyuk, A.M., Bykov, I.P., Glinchuck, M.D., Maglione, M., Michau, D., Rosa, J., Jastrabik, L., (2005) Appl. Phys. Lett., 87, p. 022903. , APPLAB 0003-6951 10.1063/1.1954900Cohn, J.L., Peterca, M., Neumeier, J.J., (2005) J. Appl. Phys., 97, p. 034102. , JAPIAU 0021-8979 10.1063/1.1834976Abragam, A., Bleaney, B., (1670) Electron Paramagnetic Resonance of Transition Ions, , Clarendon, OxfordPoole, C.P., Farach, H.A., (1971) Relaxation in Magnetic Resonance, , Academic, New YorkVan Vleck, J.H., (1948) Phys. Rev., 74, p. 1168. , PHRVAO 0031-899X 10.1103/PhysRev.74.1168Anderson, P.W., Weiss, P.R., (1953) Rev. Mod. Phys., 25, p. 269. , RMPHAT 0034-6861 10.1103/RevModPhys.25.269Wu, L., Zhu, Y., Park, S., Shapiro, S., Shirane, G., Tafto, J., (1953) Rev. Mod. Phys., 25, p. 269. , RMPHAT 0034-6861 10.1103/RevModPhys.25.26

Plasma Magnetosphere Formation Around Oscillating Magnetized Neutron Stars

The notion of death line of rotating pulsars is applied to model of oscillating neutron stars. It is shown that the magnetosphere of typical non-rotating oscillating stars may not contain secondary plasma to support the generation of radio emission in the region of open field lines of plasma magnetosphere.Comment: Accepted for publication in Astrophysics & Space Scienc

Chandra Smells a RRAT: X-ray Detection of a Rotating Radio Transient

"Rotating RAdio Transients" (RRATs) are a newly discovered astronomical phenomenon, characterised by occasional brief radio bursts, with average intervals between bursts ranging from minutes to hours. The burst spacings allow identification of periodicities, which fall in the range 0.4 to 7 seconds. The RRATs thus seem to be rotating neutron stars, albeit with properties very different from the rest of the population. We here present the serendipitous detection with the Chandra X-ray Observatory of a bright point-like X-ray source coincident with one of the RRATs. We discuss the temporal and spectral properties of this X-ray emission, consider counterparts in other wavebands, and interpret these results in the context of possible explanations for the RRAT population.Comment: 5 pages, 2 b/w figures, 1 color figure. To appear in the proceedings of "Isolated Neutron Stars", Astrophysics & Space Science, in pres

Recent Progress on Anomalous X-ray Pulsars

I review recent observational progress on Anomalous X-ray Pulsars, with an emphasis on timing, variability, and spectra. Highlighted results include the recent timing and flux stabilization of the notoriously unstable AXP 1E 1048.1-5937, the remarkable glitches seen in two AXPs, the newly recognized variety of AXP variability types, including outbursts, bursts, flares, and pulse profile changes, as well as recent discoveries regarding AXP spectra, including their surprising hard X-ray and far-infrared emission, as well as the pulsed radio emission seen in one source. Much has been learned about these enigmatic objects over the past few years, with the pace of discoveries remaining steady. However additional work on both observational and theoretical fronts is needed before we have a comprehensive understanding of AXPs and their place in the zoo of manifestations of young neutron stars.Comment: 10 pages, 6 figures; to appear in proceedings of the conference "Isolated Neutron Stars: From the Interior to the Surface" eds. S. Zane, R. Turolla, D. Page; Astrophysics & Space Science in pres

The Enhancon, Black Holes, and the Second Law

We revisit the physics of five-dimensional black holes constructed from D5- and D1-branes and momentum modes in type IIB string theory compactified on K3. Since these black holes incorporate D5-branes wrapped on K3, an enhancon locus appears in the spacetime geometry. With a `small' number of D1-branes, the entropy of a black hole is maximised by including precisely half as many D5-branes as there are D1-branes in the black hole. Any attempts to introduce more D5-branes, and so reduce the entropy, are thwarted by the appearance of the enhancon locus above the horizon, which then prevents their approach. The enhancon mechanism thereby acts to uphold the Second Law of Thermodynamics. This result generalises: For each type of bound state object which can be made of both types of brane, we show that a new type of enhancon exists at successively smaller radii in the geometry, again acting to prevent any reduction of the entropy just when needed. We briefly explore the appearance of the enhancon in the black hole interior.Comment: 22 pages, 2 figures, latex, epsfig (v2: Fixed trivial typos.

On Traversable Lorentzian Wormholes in the Vacuum Low Energy Effective String Theory in Einstein and Jordan Frames

Three new classes (II-IV) of solutions of the vacuum low energy effective string theory in four dimensions are derived. Wormhole solutions are investigated in those solutions including the class I case both in the Einstein and in the Jordan (string) frame. It turns out that, of the eight classes of solutions investigated (four in the Einstein frame and four in the corresponding string frame), massive Lorentzian traversable wormholes exist in five classes. Nontrivial massless limit exists only in class I Einstein frame solution while none at all exists in the string frame. An investigation of test scalar charge motion in the class I solution in the two frames is carried out by using the Plebanski-Sawicki theorem. A curious consequence is that the motion around the extremal zero (Keplerian) mass configuration leads, as a result of scalar-scalar interaction, to a new hypothetical "mass" that confines test scalar charges in bound orbits, but does not interact with neutral test particles.Comment: 18 page