11,217 research outputs found
Swing surfaces and holographic entanglement beyond AdS/CFT
We propose a holographic entanglement entropy prescription for general states
and regions in two models of holography beyond AdS/CFT known as flat/BMSFT
and (W)AdS/WCFT. Flat/BMSFT is a candidate of holography for
asymptotically flat three-dimensional spacetimes, while (W)AdS/WCFT is
relevant in the study of black holes in the real world. In particular, the
boundary theories are examples of quantum field theories that feature an
infinite dimensional symmetry group but break Lorentz invariance. Our
holographic entanglement entropy proposal is given by the area of a swing
surface that consists of ropes, which are null geodesics emanating from the
entangling surface at the boundary, and a bench, which is a spacelike geodesic
connecting the ropes. The proposal is supported by an extension of the
Lewkowycz-Maldacena argument, reproduces previous results based on the Rindler
method, and satisfies the first law of entanglement entropy.Comment: 45 pages, 4 figures; v2: corrected typos and added comments on strong
subadditivity, matches published versio
Modular Hamiltonians in flat holography and (W)AdS/WCFT
We study several aspects of holographic entanglement in two models known as
flat/BMSFT and (W)AdS/WCFT. These are two examples of holography beyond
AdS/CFT where the the boundary field theories are not Lorentz invariant but
still feature an infinite set of local symmetries. In the first example,
BMS-invariant field theories (BMSFTs) are conjectured to provide a holographic
description of quantum gravity in asymptotically flat three-dimensional
spacetimes; while in the second example, warped conformal field theories
(WCFTs) are proposed to describe quantum gravity in warped AdS or AdS
backgrounds with Dirichlet-Neumann boundary conditions. In particular, we
derive the modular Hamiltonian for single intervals in both BMSFTs and WCFTs
and find the holographic duals in the bulk using the covariant formulation of
gravitational charges. We also extend the first law of entanglement entropy to
these models of non-AdS holography and discuss the bound on "modular chaos"
introduced recently in the context of the AdS/CFT correspondence.Comment: 46 pages, 3 figures; v2: corrected typos, matches published versio
Crystal Structure Manipulation of the Exchange Bias in an Antiferromagnetic Film
Exchange bias is one of the most extensively studied phenomena in magnetism,
since it exerts a unidirectional anisotropy to a ferromagnet (FM) when coupled
to an antiferromagnet (AFM) and the control of the exchange bias is therefore
very important for technological applications, such as magnetic random access
memory and giant magnetoresistance sensors. In this letter, we report the
crystal structure manipulation of the exchange bias in epitaxial hcp Cr2O3
films. By epitaxially growing twined (10-10) oriented Cr2O3 thin films, of
which the c axis and spins of the Cr atoms lie in the film plane, we
demonstrate that the exchange bias between Cr2O3 and an adjacent permalloy
layer is tuned to in-plane from out-of-plane that has been observed in (0001)
oriented Cr2O3 films. This is owing to the collinear exchange coupling between
the spins of the Cr atoms and the adjacent FM layer. Such a highly anisotropic
exchange bias phenomenon is not possible in polycrystalline films.Comment: To be published in Scientific Reports, 12 pages, 6 figure
Probe of Spin Dynamics in Superconducting NbN Thin Films via Spin Pumping
The emerging field of superconductor (SC) spintronics has attracted intensive
attentions recently. Many fantastic spin dependent properties in SC have been
discovered, including the observation of large magnetoresistance, long spin
lifetimes and the giant spin Hall effect in SC, as well as spin supercurrent in
Josephson junctions, etc. Regarding the spin dynamic in SC films, few studies
has been reported yet. Here, we report the investigation of the spin dynamics
in an s-wave superconducting NbN film via spin pumping from an adjacent
insulating ferromagnet GdN layer. A profound coherence peak of the Gilbert
damping is observed slightly below the superconducting critical temperature of
the NbN layer, which is consistent with recent theoretical studies. Our results
further indicate that spin pumping could be a powerful tool for investigating
the spin dynamics in 2D crystalline superconductors.Comment: 11 pages, 4 figures, and S
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