arXiv subjects

H. Noh

Publications and source records attributed to H. Noh.

At least 19 recordsLinked to original sources

arXiv2013

Anomalous Nernst Effects of [CoSiB/Pt] Multilayer Films

We report a measurement for the anomalous Nernst effects induced by a temperature gradient in [CoSiB/Pt] multilayer films with perpendicular magnetic anisotropy. The Nernst voltage shows a characteristic hysteresis which reflects the magnetization of the film as in the case of the anomalous Hall effects. With a local heating geometry, we also measure the dependence of the anomalous Nernst voltage on the distance d from the heating element. It is roughly proportional to 1/d^1.3, which can be conjectured from the expected temperature gradient along the sample from the heat equation.

cond-mat.mtrl-sci↗

arXiv2011

Observational constraints on decaying vacuum dark energy model

The decaying vacuum model (DV), treating dark energy as a varying vacuum, has been studied well recently. The vacuum energy decays linearly with the Hubble parameter in the late-times, $ρ_Λ(t) \propto H(t)$, and produces the additional matter component. We constrain the parameters of the DV model using the recent data-sets from supernovae, gamma-ray bursts, baryon acoustic oscillations, CMB, the Hubble rate and x-rays in galaxy clusters. It is found that the best fit of matter density contrast $Ω_m$ in the DV model is much lager than that in $Λ$CDM model. We give the confidence contours in the $Ω_m-h$ plane up to $3σ$ confidence level. Besides, the normalized likelihoods of $Ω_m$ and $h$ are presented, respectively. %

astro-ph.CO↗

arXiv2009

Radiative energy transfer in disordered photonic crystals

The difficulty of description of the radiative transfer in disordered photonic crystals arises from the necessity to consider on the equal footing the wave scattering by periodic modulations of the dielectric function and by its random inhomogeneities. We resolve this difficulty by approaching this problem from the standpoint of the general multiple scattering theory in media with arbitrary regular profile of the dielectric function. We use the general asymptotic solution of the Bethe-Salpeter equation in order to show that for a sufficiently weak disorder the diffusion limit in disordered photonic crystals is presented by incoherent superpositions of the modes of the ideal structure with weights inversely proportional to the respective group velocities. The radiative transfer and the diffusion equations are derived as a relaxation of long-scale deviations from this limiting distribution. In particular, it is shown that in general the diffusion is anisotropic unless the crystal has sufficiently rich symmetry, say, the square lattice in 2D or the cubic lattice in 3D. In this case, the diffusion is isotropic and only in this case the effect of the disorder can be characterized by the single mean-free-path depending on frequency.

cond-mat.dis-nn↗

arXiv2007

Third-order cosmological perturbations of zero-pressure multi-component fluids: Pure general relativistic nonlinear effects

Present expansion stage of the universe is believed to be mainly governed by the cosmological constant, collisionless dark matter and baryonic matter. The latter two components are often modeled as zero-pressure fluids. In our previous work we have shown that to the second-order cosmological perturbations, the relativistic equations of the zero-pressure, irrotational, multi-component fluids in a spatially near flat background effectively coincide with the Newtonian equations. As the Newtonian equations only have quadratic order nonlinearity, it is practically interesting to derive the potential third-order perturbation terms in general relativistic treatment which correspond to pure general relativistic corrections. Here, we present pure general relativistic correction terms appearing in the third-order perturbations of the multi-component zero-pressure fluids. We show that, as in a single component situation, the third-order correction terms are quite small (~ 5 x10^{-5} smaller compared with the relativistic/Newtonian second-order terms) due to the weak level anisotropy of the cosmic microwave background radiation. Still, there do exist pure general relativistic correction terms in third-order perturbations which could potentially become important in future development of precision cosmology. We include the cosmological constant in all our analyses.

astro-ph↗

arXiv2007

Second-order perturbations of cosmological fluids: Relativistic effects of pressure, multi-component, curvature, and rotation

We present general relativistic correction terms appearing in Newton's gravity to the second-order perturbations of cosmological fluids. In our previous work we have shown that to the second-order perturbations, the density and velocity perturbation equations of general relativistic zero-pressure, irrotational, single-component fluid in a flat background coincide exactly with the ones known in Newton's theory. Here, we present the general relativistic second-order correction terms arising due to (i) pressure, (ii) multi-component, (iii) background curvature, and (iv) rotation. In case of multi-component zero-pressure, irrotational fluids under the flat background, we effectively do not have relativistic correction terms, thus the relativistic result again coincides with the Newtonian ones. In the other three cases we generally have pure general relativistic correction terms. In case of pressure, the relativistic corrections appear even in the level of background and linear perturbation equations. In the presence of background curvature, or rotation, pure relativistic correction terms directly appear in the Newtonian equations of motion of density and velocity perturbations to the second order. In the small-scale limit (far inside the horizon), relativistic equations including the rotation coincide with the ones in Newton's gravity.

astro-ph↗

arXiv2006

Second-order perturbations of a zero-pressure cosmological medium: comoving vs. synchronous gauge

Except for the presence of gravitational wave source term, the relativistic perturbation equations of a zero-pressure irrotational fluid in a flat Friedmann world model coincide exactly with the Newtonian ones to the second order in perturbations. Such a relativistic-Newtonian correspondence is available in a special gauge condition (the comoving gauge) in which all the variables are equivalently gauge invariant. In this work we compare our results with the ones in the synchronous gauge which has been used often in the literature. Although the final equations look simpler in the synchronous gauge, the variables have remnant gauge modes. Except for the presence of the gauge mode for the perturbed order variables, however, the equations in the synchronous gauge are gauge invariant and can be exactly identified as the Newtonian hydrodynamic equations in the Lagrangian frame. In this regard, the relativistic equations to the second order in the comoving gauge are the same as the Newtonian hydrodynamic equations in the Eulerian frame. We resolve several issues related to the two gauge conditions often to fully nonlinear orders in perturbations.

astro-ph↗

arXiv2005

Third order perturbations of a zero-pressure cosmological medium: Pure general relativistic nonlinear effects

We consider a general relativistic zero-pressure irrotational cosmological medium perturbed to the third order. We assume a flat Friedmann background but include the cosmological constant. We ignore the rotational perturbation which decays in expanding phase. In our previous studies we discovered that, to the second-order perturbation, except for the gravitational wave contributions, the relativistic equations coincide exactly with the previously known Newtonian ones. Since the Newtonian second-order equations are fully nonlinear, any nonvanishing third and higher order terms in the relativistic analyses are supposed to be pure relativistic corrections. In this work we derive such correction terms appearing in the third order. Continuing our success in the second-order perturbations we take the comoving gauge. We discover that the third-order correction terms are of $ϕ_v$-order higher than the second-order terms where $ϕ_v$ is a gauge-invariant combination related to the three-space curvature perturbation in the comoving gauge; compared with the Newtonian potential we have $δΦ\sim {3 \over 5} ϕ_v$ to the linear order. Therefore, the pure general relativistic effects are of $varphi_v$-order higher than the Newtonian ones. The corrections terms are independent of the horizon scale and depend only on the linear order gravitational potential perturbation strength. From the temperature anisotropy of cosmic microwave background we have ${δT \over T} \sim {1 \over 3} δΦ\sim {1 \over 5} ϕ_v \sim 10^{-5}$. Therefore, our present result reinforces our previous important practical implication that near current era one can use the large-scale Newtonian numerical simulation more reliably even as the simulation scale approaches near the horizon.

gr-qc↗

arXiv2005

Relativistic-Newtonian correspondence of the zero-pressure but weakly nonlinear cosmology

It is well known that couplings occur among the scalar-, vector-, and tensor-type perturbations of Friedmann world model in the second perturbational order. Here, we prove that, except for the gravitational wave contribution, the relativistic zero-pressure irrotational fluid perturbed to second order in a flat Friedmann background coincides exactly with the Newtonian result. Since we include the cosmological constant, our results are relevant to currently favoured cosmology. As we prove that the Newtonian hydrodynamic equations are valid in all cosmological scales to the second order, our result has an important practical implication that one can now use the large-scale Newtonian numerical simulation more reliably even as the simulation scale approaches and even goes beyond the horizon. That is, our discovery shows that, in the zero-pressure case, except for the gravitational wave contribution, there are no relativistic correction terms even near and beyond the horizon to the second-order perturbation.

gr-qc↗

arXiv2005

Second order perturbations of a zero-pressure cosmological medium: Proofs of the relativistic-Newtonian correspondence

The dynamic world model and its linear perturbations were first studied in Einstein's gravity. In the system without pressure the relativistic equations coincide exactly with the later known ones in Newton's gravity. Here we prove that, except for the gravitational wave contribution, even to the second-order perturbations, equations for the relativistic irrotational zero-pressure fluid in a flat Friedmann background coincide exactly with the previously known Newtonian equations. Thus, to the second order, we correctly identify the relativistic density and velocity perturbation variables, and we expand the range of applicability of the Newtonian medium without pressure to all cosmological scales including the super-horizon scale. In the relativistic analyses, however, we do not have a relativistic variable which corresponds to the Newtonian potential to the second order. Mixed usage of different gauge conditions is useful to make such proofs and to examine the result with perspective. We also present the gravitational wave equation to the second order. Since our correspondence includes the cosmological constant, our results are relevant to currently favoured cosmology. Our result has an important practical implication that one can use the large-scale Newtonian numerical simulation more reliably even as the simulation scale approaches near horizon.

gr-qc↗

arXiv2005

Why Newton's gravity is practically reliable in the large-scale cosmological simulations

Until now, it has been common to use Newton's gravity to study the non-linear clustering properties of the large-scale structures. Without confirmation from Einstein's theory, however, it has been unclear whether we can rely on the analysis, for example, near the horizon scale. In this work we will provide a confirmation of using Newton's gravity in cosmology based on relativistic analysis of weakly non-linear situations to the third order in perturbations. We will show that, except for the gravitational wave contribution, the relativistic zero-pressure fluid equations perturbed to the second order in a flat Friedmann background coincide exactly with the Newtonian results. We will also present the pure relativistic correction terms appearing in the third order. The third-order correction terms show that these are the linear-order curvature perturbation strength higher than the second-order relativistic/Newtonian terms. Thus, the pure general relativistic corrections in the third order are independent of the horizon scale and are small in the large-scale due to the low-level temperature anisotropy of the cosmic microwave background radiation. Since we include the cosmological constant, our results are relevant to currently favoured cosmology. As we prove that the Newtonian hydrodynamic equations are valid in all cosmological scales to the second order, and that the third-order correction terms are small, our result has a practically important implication that one can now use the large-scale Newtonian numerical simulation more reliably as the simulation scale approaches and even goes beyond the horizon.

astro-ph↗

arXiv2004

Classical evolution and quantum generation in generalized gravity theories including string corrections and tachyon: Unified analyses

We present cosmological perturbation theory based on generalized gravity theories including string theory correction terms and a tachyonic complication. The classical evolution as well as the quantum generation processes in these variety of gravity theories are presented in unified forms. These apply both to the scalar- and tensor-type perturbations. Analyses are made based on the curvature variable in two different gauge conditions often used in the literature in Einstein's gravity; these are the curvature variables in the comoving (or uniform-field) gauge and the zero-shear gauge. Applications to generalized slow-roll inflations and its consequent power spectra are derived in unified forms which include wide range of inflationary scenarios based on Einstein's gravity and others.

gr-qc↗

arXiv2004

Coulomb Drag near the metal-insulator transition in two-dimensions

We studied the drag resistivity between dilute two-dimensional hole systems, near the apparent metal-insulator transition. We find the deviations from the $T^{2}$ dependence of the drag to be independent of layer spacing and correlated with the metalliclike behavior in the single layer resistivity, suggesting they both arise from the same origin. In addition, layer spacing dependence measurements suggest that while the screening properties of the system remain relatively independent of temperature, they weaken significantly as the carrier density is reduced. Finally, we demonstrate that the drag itself significantly enhances the metallic $T$ dependence in the single layer resistivity.

cond-mat.mes-hall↗

arXiv2004

Spin Polarization Dependence of the Coulomb Drag at Large $r_{s}$

We find that the temperature dependence of the drag resistivity ($ρ_{D}$) between two dilute two-dimensional hole systems exhibits an unusual dependence upon spin polarization. Near the apparent metal-insulator transition, the temperature dependence of the drag, given by $T^α$, weakens with the application of a parallel magnetic field ($B_{||}$), with $α$ saturating at half its zero field value for $B_{||} > B^{*}$, where $B^{*}$ is the polarization field. Furthermore, we find that $α$ is roughly 2 at the parallel field induced metal-insulator transition, and that the temperature dependence of $ρ_{D}/T^{2}$ at different $B_{||}$ looks strikingly similar to that found in the single layer resistivity. In contrast, at higher densities, far from the zero field transition, the temperature dependence of the drag is roughly independent of spin polarization, with $α$ remaining close to 2, as expected from a simple Fermi liquid picture.

cond-mat.mes-hall↗

arXiv2004

Second-order Perturbations of the Friedmann World Model

We consider instability of the Friedmann world model to the second-order in perturbations. We present the perturbed set of equations up to the second-order in the Friedmann background world model with general spatial curvature and the cosmological constant. We consider systems with the completely general imperfect fluids, the minimally coupled scalar fields, the electro-magnetic field, and the generalized gravity theories. We also present the case of null geodesic equations, and the one based on the relativistic Boltzmann equation. In due stage a decomposition is made for the scalar-, vector- and tensor-type perturbations which couple each other to the second-order. Gauge issue is resolved to each order. The basic equations are presented without imposing any gauge condition, thus in a gauge-ready form so that we can use the full advantage of having the gauge freedom in analysing the problems. As an application we show that to the second-order in perturbation the relativistic pressureless ideal fluid of the scalar-type reproduces exactly the known Newtonian result. As another application we rederive the large-scale conserved quantities (of the pure scalar- and tensor-perturbations) to the second order, first shown by Salopek and Bond, now from the exact equations. Several other applications are made as well.

astro-ph↗

arXiv2003

Identification of perturbation modes and controversies in ekpyrotic perturbations

If the linear perturbation theory is valid through the bounce, the surviving fluctuations from the ekpyrotic scenario (cyclic one as well) should have very blue spectra with suppressed amplitude for the scalar-type structure. We derive the same (and consistent) result using the curvature perturbation in the uniform-field (comoving) gauge and in the zero-shear gauge. Previously, Khoury et al. interpreted results from the latter gauge condition incorrectly and claimed the scale-invariant spectrum, thus generating controversy in the literature. We also correct similar errors in the literature based on wrong mode identification and joining condition. No joining condition is needed for the derivation.

hep-th↗

arXiv2002

Cosmological perturbations in a generalized gravity including tachyonic condensation

We present unified ways of handling the cosmological perturbations in a class of gravity theory covered by a general action in eq. (1). This gravity includes our previous generalized $f(ϕ,R)$ gravity and the gravity theory motivated by the tachyonic condensation. We present general prescription to derive the power spectra generated from vacuum quantum fluctuations in the slow-roll inflation era. An application is made to a slow-roll inflation based on the tachyonic condensation with an exponential potential.

hep-th↗

arXiv2002

Non-singular big-bounces and evolution of linear fluctuations

We consider evolutions of linear fluctuations as the background Friedmann world model goes from contracting to expanding phases through smooth and non-singular bouncing phases. As long as the gravity dominates over the pressure gradient in the perturbation equation the growing-mode in the expanding phase is characterized by a conserved amplitude, we call it a C-mode. In the spherical geometry with a pressureless medium, we show that there exists a special gauge-invariant combination Φwhich stays constant throughout the evolution from the big-bang to the big-crunch with the same value even after the bounce: it characterizes the coefficient of the C-mode. We show this result by using a bounce model where the pressure gradient term is negligible during the bounce; this requires additional presence of an exotic matter. In such a bounce, even in more general situations of the equation of states before and after the bounce, the C-mode in the expanding phase is affected only by the C-mode in the contracting phase, thus the growing mode in the contracting phase decays away as the world model enters expanding phase. In the case the background curvature has significant role during the bounce, the pressure gradient term becomes important and we cannot trace C-mode in the expanding phase to the one before the bounce. In such situations, perturbations in a fluid bounce model show exponential instability, whereas the ones in a scalar field bounce model show oscillatory behaviors.

astro-ph↗

arXiv2002

Cosmological Perturbations with Multiple Fluids and Fields

We consider the evolution of perturbed cosmological spacetime with multiple fluids and fields in Einstein gravity. Equations are presented in gauge-ready forms, and are presented in various forms using the curvature (Φor ϕ_χ) and isocurvature (S_{(ij)} or δϕ_{(ij)}) perturbation variables in the general background with K and Λ. We clarify the conditions for conserved curvature and isocurvature perturbations in the large-scale limit. Evolutions of curvature perturbations in many different gauge conditions are analysed extensively. In the multi-field system we present a general solution to the linear order in slow-roll parameters.

astro-ph↗