arXiv subjects

Ke Yang

Publications and source records attributed to Ke Yang.

At least 199 records · Page 11Linked to original sources

arXiv2015

Black hole solution and strong gravitational lensing in Eddington-inspired Born-Infeld gravity

A new theory of gravity called Eddington-inspired Born-Infeld (EiBI) gravity was recently proposed by Bañados and Ferreira. This theory leads to some exciting new features, such as free of cosmological singularities. In this paper, we first obtain a charged EiBI black hole solution with a nonvanishing cosmological constant when the electromagnetic field is included in. Then based on it, we study the strong gravitational lensing by the asymptotic flat charged EiBI black hole. The strong deflection limit coefficients and observables are shown to closely depend on the additional coupling parameter $κ$ in the EiBI gravity. It is found that, compared with the corresponding charged black hole in general relativity, the positive coupling parameter $κ$ will shrink the black hole horizon and photon sphere. Moreover, the coupling parameter will decrease the angular position and relative magnitudes of the relativistic images, while increase the angular separation, which may shine new light on testing such gravity theory in near future by the astronomical instruments.

gr-qc↗

arXiv2015

Breakdown of Three-dimensional Dirac Semimetal State in pressurized Cd3As2

We report the first observation of a pressure-induced breakdown of the 3D-DSM state in Cd3As2, evidenced by a series of in-situ high-pressure synchrotron X-ray diffraction (XRD) and single crystal transport measurements. We find that Cd3As2 undergoes a structural phase transition from a metallic tetragonal (T) phase in space group I41/acd to a semiconducting monoclinic (M) phase in space group P21/c at critical pressure 2.57 GPa, above this pressure, an activation energy gap appears, accompanied by distinct switches in Hall resistivity slope and electron mobility. These changes of crystal symmetry and corresponding transport properties manifest the breakdown of the 3D-DSM state in pressurized Cd3As2.

cond-mat.supr-con↗

arXiv2015

Brane worlds in gravity with auxiliary fields

Recently, Pani, Sotiriou, and Vernieri explored a new theory of gravity by adding nondynamical fields, i.e., gravity with auxiliary fields [Phys. Rev. D 88, 121502(R) (2013)]. In this gravity theory, higher-order derivatives of matter fields generically appear in the field equations. In this paper we extend this theory to any dimensions and discuss the thick braneworld model in five dimensions. Domain wall solutions are obtained numerically. The stability of the brane system under the tensor perturbation is analyzed. We find that the system is stable under the tensor perturbation and the gravity zero mode is localized on the brane. Therefore, the four-dimensional Newtonian potential can be realized on the brane.

hep-th↗

arXiv2015

Superconductivity emerging from suppressed large magnetoresistant state in WTe2

The recent discovery of large and non-saturating magnetoresistance (LMR) in WTe2 provides a unique playground to find new phenomena and significant perspective for potential applications. Here we report the first observation of superconductivity near the proximity of suppressed LMR state in pressurized WTe2 through high-pressure synchrotron X-ray diffraction, electrical resistance, magnetoresistance, and ac magnetic susceptibility measurements. It is found that the positive magnetoresistance effect can be turned off at a critical pressure of 10.5 GPa without crystal structure change and superconductivity emerges simultaneously. The maximum superconducting transition temperature can be reached to 6.5 K at ~15 GPa and it decreases down to 2.6 K at ~25 GPa. In-situ high pressure Hall coefficient measurements at 10 K demonstrate that elevating pressure decreases hole carrier's population but increases electron carrier's population. Significantly, at the critical pressure, we observed a sign change in the Hall coefficient, indicating a possible Lifshitz-type quantum phase transition in WTe2.

cond-mat.supr-con↗

arXiv2014

Stability and (quasi-)localization of gravitational fluctuations in Eddington-Inspired Born-Infeld brane system

Stability and localization of the gravitational perturbations for a special brane system in Eddington-inspired Born-Infeld (EiBI) gravity were studied in [Phys. Rev. D 85, 124053 (2012)]. In this paper, we show that the gravitational perturbations for a general brane system are stable, the four-dimensional graviton (massless KK graviton) can be localized on the brane, and the mass spectrum of massive KK gravitons are gapless and continuous. Two models are constructed as examples. In the first model, which is a generalization of [Phys. Rev. D 85, 124053 (2012)], the brane has no inner structure and there is no gravitational resonance (quasi-localized KK gravitons). In the second one, the background scalar field is a double-kink when the parameter in the model approaches its critical value. Correspondingly, the brane has inner structure and some gravitational resonances appear.

hep-th↗

arXiv2014

Large Scale Structure Formation in Eddington-inspired Born-Infeld Gravity

We study the large scale structure formation in Eddington-inspired Born-Infeld (EiBI) gravity. It is found that the linear growth of scalar perturbations in EiBI gravity deviates from that in general relativity for modes with large wave numbers ($k$), but the deviation is largely suppressed with the expansion of the Universe. We investigate the integrated Sachs-Wolfe effect in EiBI gravity, and find that its effect on the angular power spectrum of the anisotropy of the cosmic microwave background (CMB) is almost the same as that in the Lambda-cold dark matter ($Λ$CDM) model. We further calculate the linear matter power spectrum in EiBI gravity and compare it with that in the $Λ$CDM model. Deviation is found on small scales ($k\gtrsim 0.1 h$ Mpc$^{-1}$), which can be tested in the future by observations from galaxy surveys.

gr-qc↗

arXiv2014

Pure Geometric Branes and Mass Hierarchy

We consider a toy model with flat thin branes embedded in a 5-dimensional Weyl integrable manifold, where the geometric Weyl scalar provides the material that constitute the brane configurations. The brane configuration is similar to the Randall-Sundrum model. However, it is found that the massless graviton is localized on the brane with negative tension. So in order to solve the gauge hierarchy problem our world should be confined on the positive tension brane, and this is crucial to reproduce a correct Friedmann-like equation on the brane. The spacings of graviton mass spectrum are very tiny, but these massive gravitons are hidden in low energy experiments because they are weakly coupled with matter on the brane.

hep-th↗

arXiv2014

Dirac dynamical resonance states around Schwarzschild black holes

Recently, a novel kind of scalar wigs around Schwarzschild black holes---scalar dynamical resonance states were introduced in [Phys. Rev. D 84, 083008 (2011)] and [Phys. Rev. Lett. 109, 081102 (2012)]. In this paper, we investigate the existence and evolution of Dirac dynamical resonance states. First we look for stationary resonance states of a Dirac field around a Schwarzchild black hole by using the Schrödinger-like equations reduced from the Dirac equation in Schwarzschild spacetime. Then Dirac pseudo-stationary configurations are constructed from the stationary resonance states. We use these configurations as initial data and investigate their numerical evolutions and energy decay. These dynamical solutions are the so-called "Dirac dynamical resonance states". It is found that the energy of the Dirac dynamical resonance states shows an exponential decay. The decay rate of energy is affected by the resonant frequency, the mass of Dirac field, the total angular momentum, and the spin-orbit interaction. In particular, for ultra-light Dirac field, the corresponding particles can stay around a Schwarzschild black hole for a very long time, even for cosmological time-scales.

gr-qc↗

arXiv2014

Linear perturbations in Eddington-inspired Born-Infeld gravity

We study the full linear perturbations of a homogeneous and isotropic spacetime in the Eddington-inspired Born-Infeld gravity. The stability of the perturbations are analyzed in the Eddington regime. We find that, for positive $κ$, the scalar modes are stable in the infinite wavelength limit ($k=0$) but unstable for $k\neq0$. The vector modes are stable and the tensor mode is unstable in the Eddington regime, independent of the wave vector $k$. However, these modes are unstable and hence cause the instabilities for negative $κ$.

gr-qc↗

arXiv2012

Gravity localization and mass hierarchy in scalar-tensor branes

We consider a braneworld model in the scalar-tensor gravity. In order to solve the gauge hierarchy problem in this model, our world should be confined on the positive tension brane rather than on the negative one. This is crucial to reproduce a correct Friedmann-like equation on the brane. Interestingly, it is found that the spacing of mass spectrum in this scenario is very tiny, but the light gravitons cannot be observed individually in colliders because of their sufficiently weak interaction with matter fields on the visible brane.

hep-th↗

arXiv2012

Strong field limit analysis of gravitational lensing in Kerr-Taub-NUT spacetime

In this paper, we investigate the strong gravitational lensing by the stationary, axially-symmetric black hole in Kerr-Taub-NUT spacetime in the strong field limit. The deflection angle of light ray and other strong deflection limit coefficients are obtained numerically and they are found to be closely dependent on the NUT charge $n$ and spin $a$. The magnification and the positions of the relativistic images are computed. The caustics are studied and the results show that these caustics drift away from the optical axis, which is quite different from the Schwarzschild black hole case. Moreover, the intersections of the critical curves on the equatorial plane are obtained and it is shown that they increase with the NUT charge. These results show that there is a significant effect of the NUT charge on the strong gravitational lensing.

hep-th↗

arXiv2012

Emergence of Cosmic Space and the Generalized Holographic Equipartition

Recently, a novel idea about our expanding Universe was proposed by T. Padmanabhan [arXiv:1206.4916]. He suggested that the expansion of our Universe can be thought of as the emergence of space as cosmic time progresses. The emergence is governed by the basic relation that the increase rate of Hubble volume is linearly determined by the difference between the number of degrees of freedom on the horizon surface and the one in the bulk. In this paper, following this idea, we generalize the basic relation to derive the Friedmann equations of an $(n+1)$-dimensional Friedmann-Robertson-Walker universe corresponding to general relativity, Gauss-Bonnet gravity, and Lovelock gravity.

hep-th↗

arXiv2012

q-Form fields on p-branes

In this paper, we give one general method for localizing any form (q-form) field on p-branes with one extra dimension, and apply it to some typical p-brane models. It is found that, for the thin and thick Minkowski branes with an infinite extra dimension, the zero mode of the q-form fields with q<(p-1)/2 can be localized on the branes. For the thick Minkowski p-branes with one finite extra dimension, the localizable q-form fields are those with q<p/2, and there are also some massive bound Kaluza-Klein modes for these q-form fields on the branes. For the same q-form field, the number of the bound Kaluza-Klein modes (but except the scalar field (q=0)) increases with the dimension of the p-branes. Moreover, on the same p-brane, the q-form fields with higher q have less number of massive bound KK modes. While for a family of pure geometrical thick p-branes with a compact extra dimension, the q-form fields with q<p/2 always have a localized zero mode. For a special pure geometrical thick p-brane, there also exist some massive bound KK modes of the q-form fields with q<p/2, whose number increases with the dimension of the p-brane.

hep-th↗

arXiv2012

Localization of 5D Elko Spinors on Minkowski Branes

Recently, a new spin-1/2 fermionic quantum field with mass dimension one in four dimensions--Elko field $λ$ was introduced as a candidate of dark matter. In this paper, we investigate the localization of 5D Elko {spinors} on Minkowski branes by presenting the equation of {the} Elko KK modes. For the 5D free massless Elko field, the zero mode can be localized on Randall-Sundrum thin brane but can not be localized on the majority of thick branes. There {do} not exist bound massive KK modes on all these branes. If the 5D mass term is introduced, there will exist bound Elko zero mode in Randall-Sundrum brane model. And when we introduce the Yukawa type coupling $ηϕ^2 \mathop λ\limits^\negλ$ with $ϕ$ the background scalar field, the Elko zero mode can be localized on some special thick branes with a particular coupling constant $η$. Nevertheless, the massive KK modes still can not be localized on these branes. These results are very different from that of the conventional Dirac spinor {field} and the scalar field.

hep-th↗

arXiv2012

Domain Wall Brane in Eddington Inspired Born-Infeld Gravity

Recently, inspired by Eddington's theory, an alternative gravity called Eddington-inspired Born-Infeld gravity was proposed by Ba$\tilde{\text{n}}$ados and Ferreira. It is equivalent to Einstein's general relativity in vacuum, but deviates from it when matter is included. Interestingly, it seems that the cosmological singularities are prevented in this theory. Based on the new theory, we investigate a thick brane model with a scalar field presenting in the five-dimensional background. A domain wall solution is obtained, and further, we find that at low energy the four-dimensional Einstein gravity is recovered on the brane. Moreover, the stability of gravitational perturbations is ensured in this model.

hep-th↗

arXiv2012

Reemerging superconductivity at 48 K across quantum criticality in iron chalcogenides

Pressure plays an essential role in the induction1 and control2,3 of superconductivity in iron-based superconductors. Substitution of a smaller rare-earth ion for the bigger one to simulate the pressure effects has surprisingly raised the superconducting transition temperature Tc to the record high 55 K in these materials4,5. However, Tc always goes down after passing through a maximum at some pressure and the superconductivity eventually tends to disappear at sufficiently high pressures1-3. Here we show that the superconductivity can reemerge with a much higher Tc after its destruction upon compression from the ambient-condition value of around 31 K in newly discovered iron chalcogenide superconductors. We find that in the second superconducting phase the maximum Tc is as high as 48.7 K for K0.8Fe1.70Se2 and 48 K for (Tl0.6Rb0.4)Fe1.67Se2, setting the new Tc record in chalcogenide superconductors. The presence of the second superconducting phase is proposed to be related to pressure-induced quantum criticality. Our findings point to the potential route to the further achievement of high-Tc superconductivity in iron-based and other superconductors.

cond-mat.supr-con↗

arXiv2011

Tensor perturbations of $f(R)$-branes

We analyze the tensor perturbations of flat thick domain wall branes in $f(R)$ gravity. Our results indicate that under the transverse and traceless gauge, the metric perturbations decouple from the perturbation of the scalar field. Besides, the perturbed equation reduces to the familiar Klein-Gordon equation for massless spin-2 particles only when the bulk curvature is a constant or when $f(R)=R$. As an application of our results, we consider the possibility of localizing gravity on some flat thick branes. The stability of these brane solutions is also shortly discussed.

hep-th↗

arXiv2010

de Sitter Thick Brane Solution in Weyl Geometry

In this paper, we consider a de Sitter thick brane model in a pure geometric Weyl integrable five-dimensional space-time, which is a generalization of Riemann geometry and is invariant under a so-called Weyl rescaling. We find a solution of this model via performing a conformal transformation to map the Weylian structure into a familiar Riemannian one with a conformal metric. The metric perturbations of the model are discussed. For gravitational perturbation, we get the effective modified P$\ddot{\text{o}}$schl-Teller potential in corresponding Schr$\ddot{\text{o}}$dinger equation for Kaluza-Klein (KK) modes of the graviton. There is only one bound state, which is a normalizable massless zero mode and represents a stable 4-dimensional graviton. Furthermore, there exists a mass gap between the massless mode and continuous KK modes. We also find that the model is stable under the scalar perturbation in the metric. The correction to the Newtonian potential on the brane is proportional to $e^{-3 r β/2}/r^2$, where $β$ is the de Sitter parameter of the brane. This is very different from the correction caused by a volcano-like effective potential.

hep-th↗