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We investigate the properties of the fermionic Fulling-Rindler vacuum for a massive Dirac field in a general number of spatial dimensions. As important local characteristics, the fermionic condensate and the expectation value of the energy-momentum tensor are evaluated. The renormalization is reduced to the subtraction of the corresponding expectation values for the Minkowski vacuum. It is shown that the fermion condensate vanishes for a massless field and is negative for nonzero mass. Unlike the case of scalar fields, the fermionic vacuum stresses are isotropic for general case of massive fields. The energy density and the pressures are negative. For a massless field the corresponding spectral distributions exhibit thermal properties with the standard Unruh temperature. However, the density-of-states factor is not Planckian for general number of spatial dimensions. Another interesting feature is that the thermal distribution is of the Bose-Einstein type in even number of spatial dimensions. This feature has been observed previously in the response of a particle detector uniformly accelerating through the Minkowski vacuum. In an even number of space dimensions the fermion condensate and the mean energy-momentum tensor coincide for the fields realizing two inequivalent irreducible representations of the Clifford algebra. In the massless case, we consider also the vacuum energy-momentum tensor for Dirac fields in the conformal vacuum of the Milne universe, in static open universe and in the hyperbolic vacuum of de Sitter spacetime., Comment: 28 pages, 5 figures, Discussion added

For a massive scalar field with general curvature coupling we evaluate the Wightman function in the geometry of two parallel branes perpendicular to the AdS boundary. On the separate branes, the field operator is constrained by Robin boundary conditions, in general, with different coefficients. In the region between the branes their contribution to the Wightman function is explicitly separated. By using this decomposition, the brane-induced effects on the vacuum expectation values (VEVs) for the field squared and energy-momentum tensor are investigated. The behavior of those expectation values is studied in various asymptotic regions of the parameters. The vacuum energy-momentum tensor in addition to the diagonal components has a nonzero off-diagonal stress. Depending on the boundary conditions and also on the distance from the branes, the vacuum energy density can be either positive or negative. The Casimir forces acting on the branes have two components. The first one corresponds to the standard normal force and the second one is parallel to the branes and presents the vacuum shear force. Unlike to the problem of parallel plates in the Minkowski bulk, the normal Casimir forces acting on separate branes differ if the boundary conditions on the branes are different. They can be either repulsive or attractive. In a similar way, depending on the coefficients in the boundary conditions, the shear force is directed toward or from the AdS boundary. The separate components may also change their signs as functions of the interbrane separation. At large proper separations between the branes, compared to the AdS curvature radius, both the components of the Casimir forces exhibit a power-law decay. For a massive scalar field this behavior is in contrast to that for the Minkowski bulk, where the decrease is exponential., Comment: 31 pages, 7 figures, Keywords: Casimir effect, AdS spacetime, branes

We investigate the combined effects of a magnetic flux-carrying cosmic string and a brane on the fermionic condensate (FC) and on the vacuum expectation value (VEV) of the energy-momentum tensor for a massive charged fermionic field in background of 5-dimensional anti-de Sitter (AdS) spacetime. The brane is parallel to the AdS boundary and it divides the space into two regions with distinct properties of the fermionic vacuum. For two types of boundary conditions on the field operator and for the fields realizing two inequivalent representations of the Clifford algebra, the brane-induced contributions in VEVs are explicitly separated. The VEVs are even periodic functions of the magnetic flux, confined in the core, with the period of flux quantum. Near the horizon the FC and the vacuum energy-momentum tensor are dominated by the brane-free contribution, whereas the brane-induced part dominates in the region near the brane. Both the contributions vanish on the AdS boundary. At large distances from the cosmic string, the topological contributions in the VEVs, as functions of the proper distance, exhibit an inverse power-law decrease in the region between the brane and AdS horizon and an exponential decrease in the region between the brane and AdS boundary. We show that the FC and the vacuum energy density can be either positive or negative, depending on the distance from the brane. Applications are discussed in fermionic models invariant under the charge conjugation and parity transformation and also in $Z_{2}$% -symmetric braneworld models. By the limiting transition we derive the expressions of the FC and the vacuum energy-momentum tensor for a cosmic string on 5-dimensional Minkowski bulk in the presence of a boundary perpendicular to the string., Comment: 34 pages, 7 figures. Version accepted for publication in JHEP

We analytically solve the Landau-Lifshitz equations for the collective magnetization dynamics in a synthetic antiferromagnet (SAF) nanoparticle and uncover a regime of barrier-free switching under a short small-amplitude magnetic field pulse applied perpendicular to the SAF plane. We give examples of specific implementations for forming such low-power and ultra-fast switching pulses. For fully optical, resonant, barrier-free SAF switching we estimate the power per write operation to be $ \sim 100 $ pJ, 10-100 times smaller than for conventional quasi-static rotation, which should be attractive for memory applications., Comment: 12 pages, 4 figures

We investigate topological effects of a cosmic string and compactification of a spatial dimension on the vacuum expectation value (VEV) of the energy-momentum tensor for a fermionic field in (4+1)-dimensional locally AdS spacetime. The contribution induced by the compactification is explicitly extracted by using the Abel-Plana summation formula. The mean energy-momentum tensor is diagonal and the vacuum stresses along the direction perpendicular to the AdS boundary and along the cosmic string are equal to the energy density. All the components are even periodic functions of the magnetic fluxes inside the string core and enclosed by compact dimension, with the period equal to the flux quantum. The vacuum energy density can be either positive or negative, depending on the values of the parameters and the distance from the string. The topological contributions in the VEV of the energy-momentum tensor vanish on the AdS boundary. Near the string the effects of compactification and gravitational field are weak and the leading term in the asymptotic expansion coincides with the corresponding VEV in (4+1)-dimensional Minkowski spacetime. At large distances, the decay of the cosmic string induced contribution in the vacuum energy-momentum tensor, as a function of the proper distance from the string, follows a power law. For a cosmic string in the Minkowski bulk and for massive fields the corresponding fall off is exponential. Within the framework of the AdS/CFT correspondence, the geometry for conformal field theory on the AdS boundary corresponds to the standard cosmic string in (3+1)-dimensional Minkowski spacetime compactified along its axis., Comment: 24 pages, 6 figures

We investigate the fermion condensate (FC) for a massive spinor field on background of the 5-dimensional locally anti-de Sitter (AdS) spacetime with a compact dimension and in the presence of a cosmic string carrying a magnetic flux. The FC is decomposed into two contributions. The first one corresponds to the geometry without compactification and the second one is induced by the compactification. Depending on the values of the parameters, the total FC can be either positive or negative. As a limiting case, the expression for the FC on locally Minkowski spacetime is derived. It vanishes for a massless fermion field and the nonzero FC on the AdS bulk in the massless case is an effect induced by gravitation. This shows that the gravitational field may essentially influence the parameters space for phase transitions. For a massive field the FC diverges on the string as the inverse cube of the proper distance from the string. In the case of a massless field, depending on the magnetic flux along the string and planar angle deficit, the limiting value of the FC on the string can be either finite or infinite. At large distances, the decay of the FC as a function of the distance from the string is power law for both cases of massive and massless fields. For a cosmic string on the Minkowski bulk and for a massive field the decay is exponential. The topological part in the FC vanishes on the AdS boundary. We show that the FCs coincide for the fields realizing two inequivalent irreducible representations of the Clifford algebra. In the special case of the zero planar angle deficit, the results presented in this paper describe Aharonov-Bohm-type effects induced by magnetic fluxes in curved spacetime., Comment: 28 pages, 6 figures. Version accepted for publication in Symmetry

We investigate the effects of a brane and magnetic-flux-carrying cosmic string on the vacuum expectation value (VEV) of the current density for a charged fermionic field in the background geometry of 4+1 dimensional anti-de Sitter (AdS) spacetime. The brane is parallel to the AdS boundary and the cosmic string is orthogonal to the brane. Two types of boundary conditions are considered on the brane that include the MIT bag boundary condition and the boundary conditions in Z2-symmetric braneworld models. The brane divides the space into two regions with different properties of the vacuum state. The only nonzero component of the current density is along the azimuthal direction and in both the regions the corresponding VEV is decomposed into the brane-free and brane-induced contributions. The latter vanishes on the string and near the string the total current is dominated by the brane-free part. At large distances from the string and in the region between the brane and AdS horizon the decay of the brane-induced current density, as a function of the proper distance, is power-law for both massless and massive fields. For a massive field this behavior is essentially different from that in the Minkowski bulk. In the region between the brane and AdS boundary the large-distance decay of the current density is exponential. Depending on the boundary condition on the brane, the brane-induced contribution is dominant or subdominant in the total current density at large distances from the string. By using the results for fields realizing two inequivalent irreducible representations of the Clifford algebra, the vacuum current density is investigated in C- and P-symmetric fermionic models. Applications are given for a cosmic string in the Randall-Sundrum-type braneworld model with a single brane., Comment: JHEP, in press, 29 pages, 11 figures, 1 table, Keywords: Cosmic string, AdS space, Brane, Vacuum polarization

In this paper, we consider a massive charged fermionic quantum field and investigate the current densities induced by a magnetic flux running along the core of an idealized cosmic string in the background geometry of a 5-dimensional anti-de Sitter spacetime, assuming that an extra dimension is compactified. Along the compact dimension quasi-periodicity condition is imposed on the field with a general phase. Moreover, we admit the presence of a magnetic flux enclosed by the compactified axis. The latter gives rise to Ahanorov-Bohm-like effect on the vacuum expectation value of the currents. In this setup, only azimuthal and axial current densities take place. The former presents two contributions, with the first one due to the cosmic string in a 5-dimensional AdS spacetime without compact dimension, and the second one being induced by the compactification itself. The latter is an odd function of the magnetic flux along the cosmic string and an even function of the magnetic flux enclosed by the compactified axis with period equal to the quantum flux. As to the induced axial current, it is an even function of the magnetic flux along the string's core and an odd function of the magnetic flux enclosed by the compactification perimeter. For untwisted and twisted field along compact dimension, the axial current vanishes. The massless field case is presented as well as some asymptotic limits for the parameters of the model., Comment: 21 pages, 5 figures, PACS numbers: 03.70.+k 04.62.+v 04.20.Gz 11.27.+d, replaced typo in paragraph after eq. (3.18)

We investigate the combined effects of boundaries and topology on the vacuum expectation values (VEVs) of the charge and current densities for a massive 2D fermionic field confined on a conical ring threaded by a magnetic flux. Different types of boundary conditions on the ring edges are considered for fields realizing two inequivalent irreducible representations of the Clifford algebra. The related bound states and zero energy fermionic modes are discussed. The edge contributions to the VEVs of the charge and azimuthal current densities are explicitly extracted and their behavior in various asymptotic limits is considered. On the ring edges the azimuthal current density is equal to the charge density or has an opposite sign. We show that the absolute values of the charge and current densities increase with increasing planar angle deficit. Depending on the boundary conditions, the VEVs are continuous or discontinuous at half-integer values of the ratio of the effective magnetic flux to the flux quantum. The discontinuity is related to the presence of the zero energy mode. By combining the results for the fields realizing the irreducible representations of the Clifford algebra, the charge and current densities are studied in parity and time-reversal symmetric fermionic models. If the boundary conditions and the phases in quasiperiodicity conditions for separate fields are the same the total charge density vanishes. Applications are given to graphitic cones with edges (conical ribbons)., Comment: 29 pages, 10 figures

The vacuum expectation value (VEV) of the fermionic current density is investigated in the geometry of two parallel branes in locally AdS spacetime with a part of spatial dimensions compactified to a torus. Along the toral dimensions quasiperiodicity conditions are imposed with general phases and the presence of a constant gauge field is assumed. Different types of boundary conditions are discussed on the branes, including the bag boundary condition and the conditions arising in $Z_{2}$-symmetric braneworld models. Nonzero vacuum currents appear along the compact dimensions only. In the region between the branes they are decomposed into the brane-free and brane-induced contributions. Both these contributions are periodic functions of the magnetic flux enclosed by compact dimensions with the period equal to the flux quantum. Depending on the boundary conditions, the presence of the branes can either increase or decrease the vacuum current density. For a part of boundary conditions, a memory effect is present in the limit when one of the branes tends to the AdS boundary. Unlike to the fermion condensate and the VEV of the energy-momentum tensor, the VEV of the current density is finite on the branes. Applications are given to higher-dimensional generalizations of the Randall-Sundrum models with two branes and with toroidally compact subspace. The features of the fermionic current are discussed in odd-dimensional parity and time-reversal symmetric models. The corresponding results for three-dimensional spacetime are applied to finite length curved graphene tubes threaded by a magnetic flux. It is shown that a nonzero current density can also appear in the absence of the magnetic flux if the fields corresponding to two different points of the Brillouin zone obey different boundary conditions on the tube edges., Comment: 33 pages, 7 figures, PACS numbers: 04.62.+v, 03.70.+k, 98.80.-k, 61.46.Fg

We investigate the influence of a brane on the vacuum expectation value (VEV) of the current density for a charged fermionic field in background of locally AdS spacetime with an arbitrary number of toroidally compact dimensions and in the presence of a constant gauge field. Along compact dimensions the field operator obeys quasiperiodicity conditions with arbitrary phases and on the brane it is constrained by the bag boundary condition. The VEVs for the charge density and the components of the current density along uncompact dimensions vanish. The components along compact dimensions are decomposed into the brane-free and brane-induced contributions. The behavior of the latter in various asymptotic regions of the parameters is investigated. It particular, it is shown that the brane-induced contribution is mainly located near the brane and vanishes on the AdS boundary and on the horizon. An important feature is the finiteness of the current density on the brane. Applications are given to $Z_2$-symmetric braneworlds of the Randall-Sundrum type with compact dimensions for two classes of boundary conditions on the fermionic field. In the special case of three-dimensional spacetime, the corresponding results are applied for the investigation of the edge effects on the ground state current density induced in curved graphene tubes by an enclosed magnetic flux., Comment: 32 pages, 9 figures, PACS numbers: 04.62.+v, 03.70.+k, 98.80.-k, 61.46.Fg

In the present work, the observational consequences of a subclass of of the Horndeski theory have been investigated. In this theory a scalar field (tachyon field) non-minimally coupled to the Gauss-Bonnet invariant through an arbitrary function of the scalar field. By considering a spatially flat FRW universe, the free parameters of the model have been constrained using a joint analysis from observational data of the Type Ia supernovae and Baryon Acoustic Oscillations measurements. The best fit values obtained from these datasets are then used to reconstruct the equation of state parameter of the scalar field. The results show the phantom, quintessence and phantom divide line crossing behavior of the equation of state and also cosmological viability of the model., Comment: 15 pages, 6 figures, 2 tables, PACS numbers: 95.36.+x, 98.80.-k, 04.50.kd, Keywords: Dark energy; Gauss-Bonnet coupling; Observational cosmology. Typos corrected, some clarifying remarks added, a few references added

It has been recently shown that tachyonic chameleon model of dark energy in which tachyon scalar field non-minimally coupled to the matter admits stable scaling attractor solution that could give rise to the late-time accelerated expansion of the universe and hence alleviate the coincidence problem. In the present work, we use data from Type Ia supernova (SN Ia) and Baryon Acoustic Oscillations to place constraints on the model parameters. In our analysis we consider in general exponential and non-exponential forms for the non-minimal coupling function and tachyonic potential and show that the scenario is compatible with observations., Comment: 11 Pages, 12 figures

R\&D activity on Cu photocathodes is under development at the SPARC\_LAB test facility to fully characterize each stage of the photocathode "life" and to have a complete overview of the photoemission properties in high brightness photo-injectors. The nano(n)-machining process presented here consists in diamond milling, and blowing with dry nitrogen. This procedure reduces the roughness of the cathode surface and prevents surface contamination introduced by other techniques, such as polishing with diamond paste or the machining with oil. Both high roughness and surface contamination cause an increase of intrinsic emittance and consequently a reduction of the overall electron beam brightness. To quantify these effects, we have characterized the photocathode surface in terms of roughness measurement, and morphology and chemical composition analysis by means of Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), and Atomic Force Microscopy (AFM) techniques. The effects of n-machining on the electron beam quality have been also investigated through emittance measurements before and after the surface processing technique. Finally, we present preliminary emittance studies of yttrium thin film on Cu photocathodes.

The Stokes parameters have been found in the framework of quantum electrodynamics for the description of polarization of radiation emitted by relativistic positrons channeled between (110) planes in Si crystal. The degree of polarization, which is simply given by the contribution of channeling radiation, has been analyzed. Numerical calculation are presented for the frequencies that are most interesting for the sources of polarized high-energy photons., Comment: 8 pages, Keywords: Positrons Channeling in Crystals, Radiation from Channeled Positrons, Polarization of Radiation

We derive a closed expression for the vacuum expectation value (VEV) of the fermionic current density in a (D+1)-dimensional locally AdS spacetime with an arbitrary number of toroidally compactified Poincare spatial dimensions and in the presence of a constant gauge field. The latter can be formally interpreted in terms of a magnetic flux treading the compact dimensions. In the compact subspace, the field operator obeys quasiperiodicity conditions with arbitrary phases. The VEV of the charge density is zero and the current density has nonzero components along the compact dimensions only. They are periodic functions of the magnetic flux with the period equal to the flux quantum and tend to zero on the AdS boundary. Near the horizon, the effect of the background gravitational field is small and the leading term in the corresponding asymptotic expansion coincides with the VEV for a massless field in the locally Minkowski bulk. Unlike the Minkowskian case, in the system consisting an equal number of fermionic and scalar degrees of freedom, with same masses, charges and phases in the periodicity conditions, the total current density does not vanish. In these systems, the leading divergences in the scalar and fermionic contributions on the horizon are canceled and, as a consequence of that, the charge flux, integrated over the coordinate perpendicular to the AdS boundary, becomes finite. We show that in odd spacetime dimensions the fermionic fields realizing two inequivalent representations of the Clifford algebra and having equal phases in the periodicity conditions give the same contribution to the VEV of the current density. Combining the contributions from these fields, the current density in odd-dimensional C-,P- and T -symmetric models are obtained. As an application, we consider the ground state current density in curved carbon nanotubes., Comment: 22 pages, 6 figures, PACS numbers: 04.62.+v, 03.70.+k, 98.80.-k, 61.46.Fg

The paper devoted to investigation of volume reflection and channeling processes of ultrarela- tivistic positive charged particles moving in germanium single crystals. We demonstrate that the choice of atomic potential on the basis of Hartree-Fock method and correct choice of Debye tem- perature allow us to describe the above mentioned processes in a good agreement with the recent experiments. Moreover, the presented in the paper universal form of equations for volume reflection gives true description of the process at a wide range of particle energies. Standing on this study we make predictions for mean angle reflection (as a function of bending radius) of positive and negative particles for germanium (110) and (111) crystallographic planes., Comment: 5 pages, 2 figures, PACS number: 41.60.-m

For a massive quantum fermionic field, we investigate the vacuum expectation values (VEVs) of the charge and current densities induced by an external magnetic flux in a two-dimensional circular ring. Both the irreducible representations of the Clifford algebra are considered. On the ring edges the bag (infinite mass) boundary conditions are imposed for the field operator. This leads to the Casimir type effect on the vacuum characteristics. The radial current vanishes. The charge and the azimuthal current are decomposed into the boundary-free and boundary-induced contributions. Both these contributions are odd periodic functions of the magnetic flux with the period equal to the flux quantum. An important feature that distinguishes the VEVs of the charge and current densities from the VEV of the energy density, is their finiteness on the ring edges. The current density is equal to the charge density for the outer edge and has the opposite sign on the inner edge. The VEVs are peaked near the inner edge and, as functions of the field mass, exhibit quite different features for two inequivalent representations of the Clifford algebra. We show that, unlike the VEVs in the boundary-free geometry, the vacuum charge and the current in the ring are continuous functions of the magnetic flux and vanish for half-odd integer values of the flux in units of the flux quantum. Combining the results for two irreducible representations, we also investigate the induced charge and current in parity and time-reversal symmetric models. The corresponding results are applied to graphene rings with the electronic subsystem described in terms of the effective Dirac theory with the energy gap. If the energy gaps for two valleys of the graphene hexagonal lattice are the same, the charge densities corresponding to the separate valleys cancel each other, whereas the azimuthal current is doubled., Comment: 29 pages, 9 figures, PACS numbers: 03.70.+k, 11.27.+d, 04.60.Kz

We evaluate the fermion condensate and the expectation values of the charge and current densities for a massive fermionic field in (2+1)-dimensional conical spacetime with a magnetic flux located at the cone apex. The consideration is done for both irreducible representations of the Clifford algebra. The expectation values are decomposed into the vacuum expectation values and contributions coming from particles and antiparticles. All these contributions are periodic functions of the magnetic flux with the period equal to the flux quantum. Related to the non-invariance of the model under the parity and time-reversal transformations, the fermion condensate and the charge density have indefinite parity with respect to the change of the signs of the magnetic flux and chemical potential. The expectation value of the radial current density vanishes. The azimuthal current density is the same for both the irreducible representations of the Clifford algebra. It is an odd function of the magnetic flux and an even function of the chemical potential. The behavior of the expectation values in various asymptotic regions of the parameters are discussed in detail. In particular, we show that for points near the cone apex the vacuum parts dominate. For a massless field with zero chemical potential the fermion condensate and charge density vanish. Simple expressions are derived for the part in the total charge induced by the planar angle deficit and magnetic flux. Combining the results for separate irreducible representations, we also consider the fermion condensate, charge and current densities in parity and time-reversal symmetric models. Possible applications to graphitic nanocones are discussed., Comment: 30 pages and 9 figures

We evaluate the Hadamard function and the vacuum expectation value (VEV) of the current density for a charged scalar field in the region between two co-dimension one branes on the background of locally AdS spacetime with an arbitrary number of toroidally compactified spatial dimensions. Along compact dimensions periodicity conditions are considered with general values of the phases and on the branes Robin boundary conditions are imposed for the field operator. In addition, we assume the presence of a constant gauge field. The latter gives rise to Aharonov-Bohm type effect on the vacuum currents. There exists a range in the space of the Robin coefficients for separate branes where the vacuum state becomes unstable. Compared to the case of the standard AdS bulk, in models with compact dimensions the stability condition imposed on the parameters is less restrictive. The current density has nonzero components along compact dimensions only. These components are decomposed into the brane-free and brane-induced contributions. The component along a given compact dimension is a periodic function of the gauge field flux, enclosed by that dimension, with the period of the flux quantum. An important feature, that distinguishes the current density from the expectation values of the field squared and energy-momentum tensor, is its finiteness on the branes. In particular, for Dirichlet boundary condition the current density vanishes on the branes. We show that, depending on the constants in the boundary conditions, the presence of the branes may either increase or decrease the current density compared with that for the brane-free geometry. Applications are given to the Randall--Sundrum 2-brane model with extra compact dimensions., Comment: 29 pages, 7 figures, PACS numbers: 04.62.+v, 04.50.-h, 11.10.Kk, 11.25.-w. arXiv admin note: text overlap with arXiv:1508.07255

We apply Ferrara-Porrati-Sagnotti approach to the case of one-dimensional supersymmetric model with N=2 supersymmetry spontaneously broken to the N=1 one. We explicitly demonstrate that only one superfield can be treated as the Goldstone one, while the second one has the meaning of the matter superfield. The general action for such a system is constructed and also two relevant particular cases are considered., Comment: 12 pages, PACS:11.30.Pb,12.60.Jv; to appear in the Czechoslovak Journal of Physics, Reports on Mathematical Physics, Symmetry, Integrability and Geometry: Methods and Applications and Journal of Physics: Conference Series, proceedings of The XXIIIth International Conference on Integrable Systems and Quantum symmetries (ISQS-23), Prague, Czech Republic, June 23 - 27 2015, talk by S. Bellucci

The two-point function and VEV of the current density are investigated for a massive charged scalar field with arbitrary curvature coupling in the geometry of a brane on background of AdS spacetime with partial toroidal compactification.The presence of a gauge field flux enclosed by compact dimensions is assumed.On the brane the field obeys Robin boundary condition and along compact dimensions periodicity conditions with general phases are imposed.There is a range in the space of values for the coefficient in the boundary condition where Poincare vacuum is unstable.This range depends on the brane location.In models with compact dimensions the stability condition is less restrictive than for AdS bulk with trivial topology.Vacuum charge density and components of current along non-compact dimensions vanish. VEV of the current density along compact dimensions is a periodic function of the gauge field flux with a period equal to the flux quantum.It is decomposed into the boundary-free and brane-induced contributions.The asymptotic behavior of the latter is investigated near the brane, AdS boundary and horizon.In contrast to VEVs of the field squared and energy-momentum tensor, current density is finite on brane and vanishes for the special case of Dirichlet boundary condition.Both boundary-free and brane-induced contributions vanish on AdS boundary.Brane-induced contribution vanishes on the horizon and for points near the horizon the current is dominated by the boundary-free part.In the near-horizon limit, the latter is connected to the corresponding quantity for a massless field in the Minkowski bulk by a simple conformal relation.Depending on the value of the Robin coefficient, the presence of the brane can either increase or decrease the vacuum currents. Applications are given for a higher-dimensional version of the Randall-Sundrum 1-brane model, Comment: 30 pages, 8 figures, PACS numbers: 04.62.+v, 04.50.-h, 11.10.Kk, 11.25.-w

We evaluate the Hadamard function and the vacuum expectation value (VEV) of the current density for a charged scalar field, induced by flat boundaries in spacetimes with an arbitrary number of toroidally compactified spatial dimensions. The field operator obeys the Robin conditions on the boundaries and quasiperiodicity conditions with general phases along compact dimensions. In addition, the presence of a constant gauge field is assumed. The latter induces Aharonov-Bohm-type effect on the VEVs. There is a region in the space of the parameters in Robin boundary conditions where the vacuum state becomes unstable. The stability condition depends on the lengths of compact dimensions and is less restrictive than that for background with trivial topology. The vacuum current density is a periodic function of the magnetic flux, enclosed by compact dimensions, with the period equal to the flux quantum. It is explicitly decomposed into the boundary-free and boundary-induced contributions. In sharp contrast to the VEVs of the field squared and the energy-momentum tensor, the current density does not contain surface divergences. Moreover, for Dirichlet condition it vanishes on the boundaries. The normal derivative of the current density on the boundaries vanish for both Dirichlet and Neumann conditions and is nonzero for general Robin conditions. When the separation between the plates is smaller than other length scales, the behavior of the current density is essentially different for non-Neumann and Neumann boundary conditions. In the former case, the total current density in the region between the plates tends to zero. For Neumann boundary condition on both plates, the current density is dominated by the interference part and is inversely proportional to the separation., Comment: 25 pages, 5 figures, PACS numbers: 03.70.+k, 11.10.Kk, 04.20.Gz

We derive the component on-shell action of the space-filling D3-brane, {\it i.e.} $N=1$ supersymmetric Born-Infeld action, within the nonlinear realization approach. The covariant Bianchi identity defining the $N=1$, $d=4$ vector supermultiplet has been constructed by introducing a new bosonic Goldstone superfield associated with the generator of the $U(1)$ group, which transforms to each other the spinor generators of unbroken and spontaneously broken $N=1$, $d=4$ supersymmetries. The first component of this Goldstone superfield is the auxiliary field of the vector supermultiplet and, therefore, the Bianchi identity can be properly defined. The component action of the D3-brane has a very simple form, being written in terms of derivatives covariant with respect to spontaneously broken supersymmetry - it just mimics its bosonic counterpart., Comment: 12 pages, PACS: 11.30.Pb, 12.60.Jv

We apply the approach of S. Ferrara, M. Porrati and A. Sagnotti \cite{FPS} to the one dimensional system described by the $N=2, d=1$ supersymmetric action for two particles in which one of $N=1$ supersymmetries is spontaneously broken. Using the nonlinear realization approach we reconsider the system in the basis where only one superfield has the Goldstone nature while the second superfield can be treated as the matter one, being invariant under transformations of the spontaneously broken $N=1$ supersymmetry. We establish the transformations relating the two selected FPS-like cases with our more general one, and find the field redefinitions which relate these two cases. Thus we demonstrate, at least in one dimension, that the only difference between two FPS cases lies in the different choice of the actions, while the supermultiplets specified by the FPS-like constraints are really the same. Going further with the nonlinear realization approach, we construct the most general action for the system of two $N=1$ superfields possessing one additional hidden spontaneously broken $N=1$ supersymmetry. The constructed action contains two arbitrary functions and reduces to the FPS actions upon specification of these functions. Unfortunately, the exact form of these functions corresponding to FPS actions is not very informative and gives no explanation on why the FPS cases are selected., Comment: 13 pages, PACS: 11.30.Pb, 12.60.Jv

In the present paper we explicitly construct the on-shell supersymmetric component action for a 3-brane moving in D=8 within the nonlinear realizations framework. Similarly to the previously considered case of the super 3-brane in D=6, all ingredients entering the component action follow from the nonlinear realizations approach. The component action of the 3-brane possesses N=4, d=4 supersymmetry partially broken to N=2, d=4 one. The basic Goldstone superfield is the generalized version of N=2,d=4 hypermultiplet. The action has a structure, such that all terms of higher orders in the fermions are hidden inside the covariant derivatives and vielbeins. The main part of the component action mimics its bosonic cousin in which the ordinary space-time derivatives and the bosonic worldvolume are replaced by their covariant (with respect to broken supersymmetry) supersymmetric analogs. The spontaneously broken supersymmetry fixes the Ansatz for the component action, up to two constant parameters. The role of the unbroken supersymmetry is just to fix these parameters., Comment: 16 pages, PACS: 11.30.Pb, 12.60.Jv

We demonstrated that the new N=2 Born-Infeld action with two N=1 vector supermultiplets, i.e. n=2 case considered as the example in the recent paper by S. Ferrara, M. Porrati and A. Sagnotti, is some sort of complexification of J. Bagger and A. Galperin construction of N=2 Born-Infeld action. Thus, novel features could be expected only for n>2 cases, if the standard action is considered., Comment: 2 pages, PACS: 11.30.Pb, 12.60.Jv, minor rephrasing of a few sentences. A note added

We evaluate the Wightman function, the mean field squared and the vacuum expectation value (VEV) of the energy-momentum tensor for a scalar field with Robin boundary condition on a spherical shell in the background of a constant negative curvature space. For the coefficient in the boundary condition there is a critical value above which the scalar vacuum becomes unstable. In both interior and exterior regions, the VEVs are decomposed into the boundary-free and sphere-induced contributions. For the latter, rapidly convergent integral representations are provided. In the region inside the sphere, the eigenvalues are expressed in terms of the zeros of the combination of the associated Legendre function and its derivative and the decomposition is achieved by making use of the Abel-Plana type summation formula for the series over these zeros. The sphere-induced contribution to the VEV of the field squared is negative for Dirichlet boundary condition and positive for Neumann one. At distances from the sphere larger than the curvature scale of the background space the suppression of the vacuum fluctuations in the gravitational field corresponding to the negative curvature space is stronger compared with the case of the Minkowskian bulk. In particular, the decay of the VEVs with the distance is exponential for both massive and massless fields. The corresponding results are generalized for spaces with spherical bubbles and for cosmological models with negative curvature spaces., Comment: 28 pages, 3 figures, LaTeX file

Wightman function, the vacuum expectation values (VEVs) of the field squared and the energy-momentum tensor are investigated for a massive scalar field with general curvature coupling in a spherically symmetric static background geometry described by two distinct metric tensors inside and outside a spherical boundary. The exterior and interior geometries can correspond to different vacuum states of the same theory. In the region outside the sphere, the contributions in the VEVs, induced by the interior geometry, are explicitly separated. For the special case of the Minkowskian exterior geometry, the asymptotics of the VEVs near the boundary and at large distances are discussed in detail. In particular, it has been shown that the divergences on the boundary are weaker than in the problem of a spherical boundary in Minkowski spacetime with Dirichlet or Neumann boundary conditions. As an application of general results, dS and AdS spaces are considered as examples of the interior geometry. For AdS interior there are no bound states. In the case of dS geometry and for nonminimally coupled fields, bound states appear for a radius of the separating boundary sufficiently close to the dS horizon. Starting from a critical value of the radius the Minkowskian vacuum in the exterior region becomes unstable. For small values of the AdS curvature radius, to the leading order, the VEVs in the exterior region coincide with those for a spherical boundary in Minkowski spacetime with Dirichlet boundary condition. The exceptions are the cases of minimal and conformal couplings: for a minimal coupling the VEVs are reduced to the case with Neumann boundary condition, whereas for a conformally coupled field there is no reduction to Dirichlet or Neumann results., Comment: 31 pages, 5 figures

We propose a method to construct on-shell component actions for theories with 1/2 partial breaking of global supersymmetry within the nonlinear realization (coset) approach. In contrast with the standard superfield approach in which unbroken supersymmetry plays the leading role, we have shifted the attention to the spontaneously broken supersymmetry. It turns out that in theories in which half of supersymmetries is spontaneously broken, all physical fermions are just the fermions of the nonlinear realization. Moreover, transformation properties of these fermions with respect to broken supersymmetry are the same as in the Volkov-Akulov model. Just this completely fixed all possible appearances of fermions in the component action: they can enter the action through the determinant of the vielbein and covariant derivatives, only. In our parametrization of the coset the rest of physical components, i.e. all bosonic components, transform as "matter fields" with respect to broken supersymmetry. Clearly, the component action acquires the form of the Volkov-Akulov action for these "matter fields". The complete form of the action can be further fixed by two additional requirements: a) to reproduce the bosonic limit, which is explicitly known in many interesting cases, and b) to have a proper linearized form, which has to be invariant with respect to linearized unbroken supersymmetry. In some cases additional Wess-Zumino terms (which disappear in the bosonic limit) have to be added to the action. We supply the general consideration by detailed examples of actions for the superparticle in D=3,5, the on-shell component action for N=1, D=5 supermembrane and its dual cousins and the component action of N=1 supermembrane in D=4, providing the exact proof of the invariance of the constructed component actions with respect to both broken and unbroken supersymmetries., Comment: 31 pages, to appear in Breaking of supersymmetry and Ultraviolet Divergences in extended Supergravity Workshop, Proceedings of the INFN-Laboratori Nazionali di Frascati School 2013, Springer Heidelberg New York Dordrecht London, Springer Proceedings in Physics (2014), PACS: 11.30.Pb, 12.60.Jv

We investigate the finite temperature fermionic condensate and the expectation values of the charge and current densities for a massive fermion field in a spacetime background with an arbitrary number of toroidally compactified spatial dimensions in the presence of a non-vanishing chemical potential. Periodicity conditions along compact dimensions are taken with arbitrary phases and the presence of a constant gauge field is assumed. The latter gives rise to Aharonov-Bohm-like effects on the expectation values. They are periodic functions of magnetic fluxes enclosed by compact dimensions with the period equal to the flux quantum. The current density has nonzero components along compact dimensions only. Both low- and high-temperature asymptotics of the expectation values are studied. In particular, it has been shown that at high temperatures the current density is exponentially suppressed. This behavior is in sharp contrast with the corresponding asymptotic in the case of a scalar field, where the current density linearly grows with the temperature. The features for the models in odd dimensional spacetimes are discussed. Applications are given to cylindrical and toroidal nanotubes described within the framework of effective Dirac theory for the electronic subsystem., Comment: 31 pages, 7 figures

Based on the connection between partial breaking of global supersymmetry, coset approach, which realized the given pattern of supersymmetry breaking, and the Nambu-Goto actions for the extended objects, we have constructed on-shell component action for N=1, D=5 supermembrane and its dual cousins. We demonstrate that the proper choice of the components and the use of the covariant (with respect to broken supersymmetry) derivatives drastically simplify the action: it can be represented as a sum of four terms each having an explicit geometric meaning., Comment: 14 pages, PACS: 11.30.Pb, 12.60.Jv

We investigate the fermionic condensate (FC) and the vacuum expectation value (VEV) of the energy-momentum tensor for a charged massive fermionic field in the geometry of a cosmic string compactified along its axis. In addition, we assume the presence of two types of magnetic fluxes: a flux running along the cosmic string and another enclosed by the compact dimension. These fluxes give rise to Aharanov-Bohm-like effects on the VEVs. The VEVs are decomposed into two parts corresponding to the geometry of a straight cosmic string without compactification plus a topological part induced by the compactification of the string axis. Both contributions are even periodic functions of the magnetic fluxes with period equal to the flux quantum. The vacuum energy density is equal to the radial stress for the parts corresponding to the straight cosmic string and the topological one. Moreover, the axial stress is equal to the energy density for the parts corresponding to the straight cosmic string; however, for massive fermionic field this does not occur for the topological contributions. With respect to the dependence on the magnetic fluxes, both, the fermionic condensate and the vacuum energy density, can be either positive or negative. Moreover, for points near the string, the main contribution to the VEVs comes from the straight cosmic string part, whereas at large distances the topological ones dominate. In addition to the local characteristics of the vacuum state, we also evaluate the part in the topological Casimir energy induced by the string., Comment: 28 pages, 6 figures

We determined the frequency dependent effective permittivity of a large ternary network of randomly positioned resistors, capacitors, and diodes. A linear circuit analysis of such systems is shown to match the experimental dielectric response of single-walled carbon nanotube (SWCNT) filled polymers. This modeling method is able to reproduce the two most important features of SWCNT filled composites, i.e. the low frequency dispersion and dipolar relaxation. As a result of the modeling important physical conclusion proved by the experimental data was done: the low frequency behavior of SWCNT-filled polymer composites is mostly caused by the fraction of semiconducting SWCNTs.

We argue the conjecture that the on-shell component super particle actions have a universal form, in which the physical fermions enter the action through the ein-bein and the space-time derivatives of the matter fields, only. We explicitly constructed the actions for the super particles in $D=3$ realizing the $N=4\cdot 2^{k} \rightarrow N=2\cdot 2^k$ pattern of supersymmetry breaking, and in $D=5$ with the $N=16$ supersymmetry broken down to the $N=8$ one. All constructed actions have indeed a universal form, confirming our conjecture. Our construction is strictly based on the assumption that in the system we have one half breaking of the global supersymmetry, and on the very special choice of the superspace coordinates and component fields., Comment: 13 pages, PACS: 11.30.Pb, 12.60.Jv, conclusion modified, removed first sentence in the last paragraph, revised references

We evaluate the two-point functions of the electromagnetic field in (D+1) -dimensional spatially flat Friedmann-Robertson-Walker universes with a power-law scale factor, assuming that the field is prepared in the Bunch-Davies vacuum state. The range of powers are specified in which the two-point functions are infrared convergent and the Bunch-Davies vacuum for the electromagnetic field is a physically realizable state. The two-point functions are applied for the investigation of the vacuum expectation values of the field squared and the energy-momentum tensor, induced by a single and two parallel conducting plates. Unlike to the case of conducting plates in the Minkowski bulk, in the problem under consideration the stresses along the directions parallel to the plates are not equal to the energy density. We show that, in addition to the diagonal components, the vacuum energy-momentum tensor has a nonzero off-diagonal component which describes energy flux along the direction normal to the plates. For a single plate this flux is directed from the plate. The Casimir forces are investigated in the geometry of two plates. At separations between the plates smaller than the curvature radius of the background spacetime, to the leading order, we recover the corresponding result in the Minkowski spacetime and in this case the forces are attractive. At larger separations, the influence of the curvature on the Casimir forces is essential with different asymptotic behavior for decelerated and accelerated expansions. In particular, for the latter case there is a range of powers of the expansion law in which the forces become repulsive at large separations between the plates., Comment: 25 pages, 4 figures

We propose a method to construct the on-shell component actions for the theories with $1/2$ partial breaking of global supersymmetry within the nonlinear realization (coset) approach. In contrast with the standard superfield approach in which unbroken supersymmetry plays the leading role, we have shifted the attention to the spontaneously broken supersymmetry. It turns out that in the theories in which half of supersymmetries is spontaneously broken, all physical fermions are just the fermions of the nonlinear realization. Moreover, the transformation properties of these fermions with respect to the broken supersymmetry are the same as in the famous Volkov-Akulov model. Just this fact completely fixed all possible appearances of the fermions in the component action: they can enter the action through the determinant of the vielbein (to compensate the transformation of the volume form) and the covariant derivatives, only. It is very important that in our parametrization of the coset the rest of physical components, i.e. all bosonic components, transform as ``matter fields'' with respect to the broken supersymmetry. Clearly, in such a situation the component action acquires the form of the Volkov-Akulov action for these ``matter fields''. The complete form of the action can be further fixed by two additional requirements: a) to reproduce the bosonic limit, which is explicitly known in many interesting cases, and b) to have a proper linearized form, which has to be invariant with respect to the linearized unbroken supersymmetry. We supply the general consideration by a detailed example of the component action of $N=1$ supermembrane in $D=4$ constructed within our procedure. In this case we provide the exact proof of the invariance of the constructed component action with respect to both, broken and unbroken supersymmetries., Comment: 12 pages, PACS: 11.30.Pb, 12.60.Jv