4,243 research outputs found
The black hole dynamical horizon and generalized second law of thermodynamics
The generalized second law of thermodynamics for a system containing a black
hole dynamical horizon is proposed in a covariant way. Its validity is also
tested in case of adiabatically collapsing thick light shells.Comment: JHEP style, 8 pages, 2 figures, version to appear in JHEP with typos
correcte
Modelling and control of a high redundancy actuator
The high redundancy actuation concept is a completely new approach to fault tolerance, and it is important to appreciate that it provides a transformation of the characteristics of actuators so that the actuation performance (capability) degrades slowly rather than suddenly failing, even though individual elements themselves fail. This paper aims to demonstrate the viability of the concept by showing that a highly redundant actuator, comprising a relatively large number of actuation elements, can be controlled in such a way that faults in individual elements are inherently accommodated, although some degradation in overall performance will inevitably be found. The paper introduces the notion of fault-tolerant systems and the highly redundant actuator concept. Then a model for a two by two configuration with electro-mechanical actuation elements is derived. Two classical control approaches are then considered based on frequency domain techniques. Finally simulation results under a number of faults show the viability of the approach for fault accommodation without re-configuratio
Note on the energy-momentum tensor for general mixed tensor-spinor fields
This note provides an explicit proof of the equivalence of the Belinfante's
energy-momentum tensor and the metric energy-momentum tensor for general mixed
tensor-spinor fields.Comment: 7 pages, title changed, typos corrected, accepted for publication in
Communications in Theoretical Physic
A -Vertex Model: Creation Algebras and Quasi-Particles I
The infinite configuration space of an integrable vertex model based on
is studied at . Allowing four particular
boundary conditions, the infinite configurations are mapped onto the
semi-standard supertableaux of pairs of infinite border strips. By means of
this map, a weight-preserving one-to-one correspondence between the infinite
configurations and the normal forms of a pair of creation algebras is
established for one boundary condition. A pair of type-II vertex operators
associated with an infinite-dimensional -module
and its dual is introduced. Their existence is
conjectured relying on a free boson realization. The realization allows to
determine the commutation relation satisfied by two vertex operators related to
the same -module. Explicit expressions are provided for
the relevant R-matrix elements. The formal limit of these commutation
relations leads to the defining relations of the creation algebras. Based on
these findings it is conjectured that the type II vertex operators associated
with and give rise to part of the eigenstates of
the row-to-row transfer matrix of the model. A partial discussion of the
R-matrix elements introduced on is given.Comment: 45 pages, 5 figures, to appear in Nucl. Phys.
Altered Cortical Microarchitecture and Bone Metabolism in Patients with Monoclonal Gammopathy of Undetermined Significance
Patients with monoclonal gammopathy of undetermined significance (MGUS) are at increased fracture risk, and we have previously shown that MGUS patients have altered trabecular bone microarchitecture compared with controls. However, there are no data on whether the porosity of cortical bone, which may play a greater role in bone strength and the occurrence of fractures, is increased in MGUS. Thus, we studied cortical porosity and bone strength (apparent modulus) using high-resolution peripheral quantitative computed tomography imaging of the distal radius in 50 MGUS patients and 100 age-, gender-, and body mass index–matched controls. Compared with controls, MGUS patients had both significantly higher cortical porosity (+16.8%; P < .05) and lower apparent modulus (–8.9%; P < .05). Despite their larger radial bone size, MGUS patients have significantly increased cortical bone porosity and reduced bone strength relative to controls. This increased cortical porosity may explain the increased fracture risk seen in MGUS patients
Thermodynamics Inducing Massive Particles' Tunneling and Cosmic Censorship
By calculating the change of entropy, we prove that the first law of black
hole thermodynamics leads to the tunneling probability of massive particles
through the horizon, including the tunneling probability of massive charged
particles from the Reissner-Nordstr\"om black hole and the Kerr-Newman black
hole. Novelly, we find the trajectories of massive particles are close to that
of massless particles near the horizon, although the trajectories of massive
charged particles may be affected by electromagnetic forces. We show that
Hawking radiation as massive particles tunneling does not lead to violation of
the weak cosmic-censorship conjecture
Recommended from our members
The effect of inter-particle contact time in granular flows -- A network theory
In a kinetic theory, it is usually assumed that the time duration of particle collision is vanishingly small and only binary collisions are considered. The validity of these assumptions depends on the ratio of collision time to mean free flight time. If this ratio is small, the kinetic theory description is appropriate. In a dense system, however, this ratio is usually large, and the dynamics of the multi-particle interactions have to be considered. For instance, during a collision, the contacting pair usually has a relative tangential velocity that causes a change in the direction of rebound. This implies a dependence of the granular stress on the vorticity of the mean flows field. Due to the inherent energy dissipation in a particle collision, and the consistent rearrangement of particles, there are relaxation times associated with them. In a binary collision, this energy dissipation is represented by coefficient of restitution. In a dense granular system, multi-particle interactions occur frequently. The energy dissipation and system relaxation have to be studied by the consideration of the dynamics in the duration of particle interaction and cannot be represented by a single coefficient of restitution. In this case, the relaxation times must be introduced explicitly. By modification of the network theory for rubber material, a constitutive model for dense granular material is developed based on the dynamics of multi-particle interaction. The finite particle interaction time and system relaxation times are considered
Defect reduction in overgrown semi-polar (11-22) GaN on a regularly arrayed micro-rod array template
We demonstrate a great improvement in the crystal quality of our semi-polar (11-22) GaN overgrown on regularly arrayed micro-rod templates fabricated using a combination of industry-matched photolithography and dry-etching techniques. As a result of our micro-rod configuration specially designed, an intrinsic issue on the anisotropic growth rate which is a great challenge in conventional overgrowth technique for semi-polar GaN has been resolved. Transmission electron microscopy measurements show a different mechanism of defect reduction from conventional overgrowth techniques and also demonstrate major advantages of our approach. The dislocations existing in the GaN micro-rods are effectively blocked by both a SiO2 mask on the top of each GaN micro-rod and lateral growth along the c-direction, where the growth rate along the c-direction is faster than that along any other direction. Basal stacking faults (BSFs) are also effectively impeded, leading to a distribution of BSF-free regions periodically spaced by BSF regions along the [-1-123] direction, in which high and low BSF density areas further show a periodic distribution along the [1-100] direction. Furthermore, a defect reduction model is proposed for further improvement in the crystalline quality of overgrown (11-22) GaN on sapphire
Microstructure investigation of semi-polar (11-22) GaN overgrown on differently designed micro-rod array templates
In order to realize semi-polar (11-22) GaN based laser diodes grown on sapphire, it is necessary to further improve the crystal quality of the (11-22) GaN obtained by using our overgrowth approach developed on regularly arrayed micro-rod templates [T. Wang, Semicond. Sci. Technol. 31, 093003 (2016)]. This can be achieved by carefully designing micro-rod templates. Based on transmission electron microscopy and photoluminescence measurements, it has been found that the micro-rod diameter plays a vital role in effectively reducing both the dislocation density and the basal staking fault (BSF) density of the overgrown (11-22) GaN, but in different manners. The BSF density reduces monotonically with increasing the micro-rod diameter from 2 to 5 μm, and then starts to be saturated when the micro-rod diameter further increases. In contrast, the dislocation density reduces significantly when the micro-rod diameter increases from 2 to 4 μm, and then starts to increase when the diameter further increases to 5 μm. Furthermore, employing shorter micro-rods is useful for removing additional BSFs, leading to further improvement in crystal quality. The results presented provide a very promising approach to eventually achieving (11-22) semi-polar III-nitride laser diodes
Urban climate change, livelihood vulnerability and narratives of generational responsibility in Jinja, Uganda
There is an urgent need to understand lived experiences of climate change in the context of African cities, where even small climate shocks can have significant implications for the livelihoods of the urban poor. This article examines narratives of climate and livelihood changes within Jinja Municipality, Uganda, emphasizing how Jinja's residents make sense of climate change through their own narrative frames rather than through the lens of global climate change discourses. We demonstrate how the onset of climate change in Jinja is widely attributed to perceived moral and environmental failings on the part of a present generation that is viewed as both more destructive than previous generations and unable to preserve land, trees and other resources for future generations. A focus on local ontologies of climate change highlights how the multiple, intersecting vulnerabilities of contemporary urban life in Jinja serve to obfuscate not only the conditions of possibility of an immediate future, but the longer-term horizons for future generations, as changing weather patterns exacerbate existing challenges people face in adapting to wider socio-economic changes and rising livelihood vulnerability. This form of analysis situates changing climate and environments within the context of everyday urban struggles and emphasizes the need for civic participation in developing climate change strategies that avoid the pitfalls of climate reductionism. The article draws on more than 150 qualitative interviews, generational dialogue groups, and creative methods based on research-led community theatre
- …