1,717 research outputs found
Fermionic currents in AdS spacetime with compact dimensions
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.F
Current-biased Transition-edge Sensors Based on Re-entrant Superconductors
AbstractTransition-edge sensors are widely recognized as one of the most sensitive tools for the photon and particles detection in many areas, from astrophysics to quantum computing. Their application became practical after understanding that rather than being biased in a constant current mode, they should be biased in a constant voltage mode. Despite the methods of voltage biasing of these sensors are well developed since then, generally the current biasing is more convenient for superconducting circuits. Thus transition-edge sensors designed inherently to operate in the current-biased mode are desirable. We developed a design for such detectors based on re-entrant superconductivity. In this case constant current biasing takes place in the normal state, below the superconducting transition, so that following the absorption of a photon it does not yield a latching. Rather, the sensor gains energy and shifts towards the lower resistant (e.g., superconducting) state, and then cools down faster (since Joule heating is now reduced), and resets in a natural way to be able to detect the next photon. We prototyped this kind of transition edge sensors and tested them operational in accordance with the outlined physics. The samples used in experiments were modified compositions of YBCO-superconductors in a ceramic form (which, as we discovered, reproducibly demonstrates re-entrant superconductivity). In this presentation we report their composition, methods of preparation, and the detection results. This approach, in some areas, may have practical advantage over the traditional voltage-biased devices
Giant vortices, vortex rings and reentrant behavior in type-1.5 superconductors
We predict that in a bulk type-1.5 superconductor the competing magnetic
responses of the two components of the order parameter can result in a vortex
interaction that generates group-stabilized giant vortices and unusual vortex
rings in the absence of any extrinsic pinning or confinement mechanism. We also
find within the Ginzburg-Landau theory a rich phase diagram with successions of
behaviors like type-1 -> type-1.5 -> type-2 -> type-1.5 as temperature
decreases.Comment: 5 pages, 4 figure
Fermionic vacuum currents in topologically nontrivial braneworlds: Two-brane geometry
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 -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.F
Fermionic currents in topologically nontrivial braneworlds
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
-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.F
Induced vacuum currents in anti-de Sitter space with toral dimensions
We investigate the Hadamard function and the vacuum expectation value of the
current density for a charged massive scalar field on a slice of anti-de Sitter
(AdS) space described in Poincar\'{e} coordinates with toroidally compact
dimensions. Along compact dimensions periodicity conditions are imposed on the
field with general phases. Moreover, the presence of a constant gauge field is
assumed. The latter gives rise to Aharonov-Bohm-like effects on the vacuum
currents. The current density along compact dimensions is a periodic function
of the gauge field flux with the period equal to the flux quantum. It vanishes
on the AdS boundary and, near the horizon, to the leading order, it is
conformally related to the corresponding quantity in Minkowski bulk for a
massless field. For large values of the length of the compact dimension
compared with the AdS curvature radius, the vacuum current decays as power-law
for both massless and massive fields. This behavior is essentially different
from from the corresponding one in Minkowski background, where the currents for
a massive field are suppressed exponentially.Comment: 15 pages, 4 figure
The landscape, the swampland and the era of precision cosmology
We review the advanced version of the KKLT construction and pure d=4" role="presentation" style="display: inline; line-height: normal; font-size: 13.6px; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border-width: 0px; border-style: initial; color: rgb(0, 0, 0); font-family: arial, verdana, sans-serif; position: relative;">=4d=4 de Sitter supergravity, involving a nilpotent multiplet, with regard to various conjectures that de Sitter state cannot exist in string theory. We explain why we consider these conjectures problematic and not well motivated, and why the recently proposed alternative string theory models of dark energy, ignoring vacuum stabilization, are ruled out by cosmological observations at least at the 3σ" role="presentation" style="display: inline; line-height: normal; font-size: 13.6px; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border-width: 0px; border-style: initial; color: rgb(0, 0, 0); font-family: arial, verdana, sans-serif; position: relative;">33σ level, i.e. with more than 99.7%" role="presentation" style="display: inline; line-height: normal; font-size: 13.6px; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border-width: 0px; border-style: initial; color: rgb(0, 0, 0); font-family: arial, verdana, sans-serif; position: relative;">99.7 .7%confidence
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