arXiv subjects

H. Noh

Publications and source records attributed to H. Noh.

At least 37 records · Page 2Linked to original sources

arXiv2001

On the 1/3 factor in the CMB Sachs-Wolfe effect

We point out that a pseudo-Newtonian interpretations of the $(1/3)$ factor in the Sachs-Wolfe effect, which relates the fluctuations in temperature and potential, $(δT/ T) = (1/ 3) δΦ$, is not supported by the General Relativistic analysis. Dividing the full gravitational effect into separate parts depends on the choice of time slicing (gauge) and there exist infinitely many different choices. More importantly, interpreting the parts as being due to the gravitational redshift and the time dilation is not justified in the rigorous relativistic perturbation theory. We suggest to regard the $(1/3)$ factor as the General Relativistic result which applies in a restricted situation of adiabatic perturbation in the $K = 0 = Λ$ model with the last scattering occuring in the matter dominated era. For an isocurvature initial condition the corresponding result, $(δT/ T) = 2 δΦ$, has a different numerical coefficient.

astro-ph↗

arXiv2001

Gauge-ready formulation of the cosmological kinetic theory in generalized gravity theories

We present cosmological perturbations of kinetic components based on relativistic Boltzmann equations in the context of generalized gravity theories. Our general theory considers an arbitrary number of scalar fields generally coupled with the gravity, an arbitrary number of mutually interacting hydrodynamic fluids, and components described by the relativistic Boltzmann equations like massive/massless collisionless particles and the photon with the accompanying polarizations. We also include direct interactions among fluids and fields. The background FLRW model includes the general spatial curvature and the cosmological constant. We consider three different types of perturbations, and all the scalar-type perturbation equations are arranged in a gauge-ready form so that one can implement easily the convenient gauge conditions depending on the situation. In the numerical calculation of the Boltzmann equations we have implemented four different gauge conditions in a gauge-ready manner where two of them are new. By comparing solutions solved separately in different gauge conditions we can naturally check the numerical accuracy.

astro-ph↗

arXiv2001

Inflationary spectra in generalized gravity: Unified forms

The classical evolution and the quantum generation processes of the scalar- and tensor-type cosmological perturbations in the context of a broad class of generalized gravity theories are presented in unified forms. The exact forms of final spectra of the two types of structures generated during a generalized slow-roll inflation are derived. Results in generalized gravity are characterized by two additional parameters which are the coupling between gravity and field, and the nonminimal coupling in the kinetic part of the field. Our general results include widely studied gravity theories and inflation models as special cases, and show how the well known consistency relation and spectra in ordinary Einstein gravity inflation models are affected by the generalized nature of the gravity theories.

astro-ph↗

arXiv2001

Quintessential perturbations during scaling regime

The scalar field with an exponential potential allows a scaling solution where the the density of the field follows the density of the dominating fluid. Such a scaling regime is often used as an important ingredient in many models of quintessence. We analyse evolution of perturbations while the background follows the scaling. As the results, the perturbed scalar field also scales with the perturbed fluid, and the perturbations accompany the adiabatic as well as the isocurvature mode between the fluid and the field.

astro-ph↗

arXiv2001

The Sachs-Wolfe Effect: Gauge Independence and a General Expression

In this paper we address two points concerning the Sachs-Wolfe effect: (i) the gauge independence of the observable temperature anisotropy, and (ii) a gauge-invariant expression of the effect considering the most general situation of hydrodynamic perturbations. The first result follows because the gauge transformation of the temperature fluctuation at the observation event only contributes to the isotropic temperature change which, in practice, is absorbed into the definition of the background temperature. Thus, we proceed without fixing the gauge condition, and express the Sachs-Wolfe effect using the gauge-invariant variables.

astro-ph↗

arXiv2001

Anomalous Rashba spin splitting in two-dimensional hole systems

It has long been assumed that the inversion asymmetry-induced Rashba spin splitting in two-dimensional (2D) systems at zero magnetic field is proportional to the electric field that characterizes the inversion asymmetry of the confining potential. Here we demonstrate, both theoretically and experimentally, that 2D heavy hole systems in accumulation layer-like single heterostructures show the opposite behavior, i.e., a decreasing, but nonzero electric field results in an increasing Rashba coefficient.

cond-mat.mes-hall↗

arXiv2001

Non-monotonic magnetic field and density dependence of in-plane magnetoresistance in dilute two-dimensional holes in GaAs/AlGaAs

We studied low temperature (T=50mK) in-plane magnetoresistance of a dilute two-dimensional hole system in GaAs/AlGaAs heterostructure that exhibits an apparent metal-insulator transition. We found an anisotropic magnetoresistance, which changes dramatically at high in-plane fields ($B_{\parallel}\agt$5T) as the hole density is varied. At high densities where the system behaves metallic at $B_{\parallel}=0$, the transverse magnetoresistance is larger than the longitudinal magnetoresistance. With decreasing the hole density the difference becomes progressively smaller, and at densities near the "critical" density and lower, the longitudinal magnetoresistance becomes larger than the transverse magnetoresistance.

cond-mat.mes-hall↗

arXiv2001

f(R) gravity theory and CMBR constraints

We consider the large-scale cosmic structure generation by an inflation based on a pure f(R)-type gravity theory. Comparison with recent CMBR observations gives the following results: (1) The near Zel'dovich spectral conditions uniquely choose R^2-type gravity. (2) The R^2 gravity predicts specific nearly scale-invariant Zel'dovich spectra for both the scalar- and tensor-type perturbations. Thus, (3) the considered model survives current observational data. The COBE-DMR quadrupole data (4) give constraints on the coupling constant and the energy scale during inflation, and (5) require the gravitational wave contribution to be suppressed. (6) Therefore, future observations of (2) and (5) can provide strong tests of the inflation scenario based on R^2 gravity. Parallel analyses made in the conformally transformed Einstein frame give the observationally identical results.

astro-ph↗

arXiv2001

Observational constraints on inflation models with nonminimal scalar field

We present the power spectra of the scalar- and tensor-type structures generated in an inflation model based on the nonminimally coupled scalar field with a self coupling. By comparing the contributions of these structures to the anisotropy of the cosmic microwave background radiation with the four year COBE DMR data we derive strong constraints on model parameters and the inflation model.

astro-ph↗

arXiv2000

Cosmological Perturbations with Multiple Scalar Fields

We consider the evolution of perturbed cosmological spacetime with multiple scalar fields in Einstein gravity. A complete set of scalar-type perturbation equations is presented in a gauge-ready form, and we derived the closed set of second-order differential equations in several useful forms. Conserved behaviors of the perturbed three-space curvature in the comoving gauge, $ϕ_v$, under several conditions are clarified. Under the slow-roll conditions, the adiabatic and isocurvature modes decouple from each other, and in the large-scale limit we have (i) the adiabatic mode is generally conserved, (ii) for a couple of special potential the isocurvature modes decouple from each other and are described by conserved quantities, (iii) in the two field system, the isocurvature mode is described by a conserved quantity for the general potential.

astro-ph↗

arXiv1999

Conserved cosmological structures in the one-loop superstring effective action

A generic form of low-energy effective action of superstring theories with one-loop quantum correction is well known. Based on this action we derive the complete perturbation equations and general analytic solutions in the cosmological spacetime. Using the solutions we identify conserved quantities characterizing the perturbations: the amplitude of gravitational wave and the perturbed three-space curvature in the uniform-field gauge both in the large-scale limit, and the angular-momentum of rotational perturbation are conserved independently of changing gravity sector. Implications for calculating perturbation spectra generated in the inflation era based on the string action are presented.

astro-ph↗

arXiv1999

COBE constraints on inflation models with a massive non-minimal scalar field

We derive power spectra of the scalar- and tensor-type structures generated in an inflation model based on a massive non-minimally coupled scalar field with the strong coupling assumption. We make analyses in both the original-frame and the conformally transformed Einstein-frame. We derive contributions of both structures to the anisotropy of the cosmic microwave background radiation, and compare the contributions with the four-year COBE-DMR data. Previous study showed that sufficient amount of inflation requires a small coupling parameter. In such a case the spectra become near Zeldovich spectra, and the gravitational wave contribution becomes negligible compared with the scalar-type contribution which is testable in future CMBR experiments.

astro-ph↗

arXiv1999

Missing 2k_F Response for Composite Fermions in Phonon Drag

The response of composite Fermions to large wavevector scattering has been studied through phonon drag measurements. While the response retains qualitative features of the electron system at zero magnetic field, notable discrepancies develop as the system is varied from a half-filled Landau level by changing density or field. These deviations, which appear to be inconsistent with the current picture of composite Fermions, are absent if half-filling is maintained while changing density. There remains, however, a clear deviation from the temperature dependence anticipated for 2k_F scattering.

cond-mat.mes-hall↗

arXiv1999

Relativistic Hydrodynamic Cosmological Perturbations

Relativistic cosmological perturbation analyses can be made based on several different fundamental gauge conditions. In the pressureless limit the variables in certain gauge conditions show the correct Newtonian behaviors. Considering the general curvature ($K$) and the cosmological constant ($Λ$) in the background medium, the perturbed density in the comoving gauge, and the perturbed velocity and the perturbed potential in the zero-shear gauge show the same behavior as the Newtonian ones in general scales. In the first part, we elaborate these Newtonian correspondences. In the second part, using the identified gauge-invariant variables with correct Newtonian correspondences, we present the relativistic results with general pressures in the background and perturbation. We present the general super-sound-horizon scale solutions of the above mentioned variables valid for general $K$, $Λ$, and generally evolving equation of state. We show that, for vanishing $K$, the super-sound-horizon scale evolution is characterised by a conserved variable which is the perturbed three-space curvature in the comoving gauge. We also present equations for the multi-component hydrodynamic situation and for the rotation and gravitational wave.

astro-ph↗

arXiv1999

Phonon mediated drag in double layer two dimensional electron systems

Experiments studying phonon mediated drag in the double layer two dimensional electron gas system are reported. Detailed measurements of the dependence of drag on temperature, layer spacing, density ratio, and matched density are discussed. Comparisons are made to theoretical results [M. C. Bonsager et al., Phys. Rev. B 57, 7085 (1998)] which propose the existence of a new coupled electron-phonon collective mode. The layer spacing and density dependence at matched densities for samples with layer spacings below 2600 A do not support the existence of this mode, showing behavior expected for independent electron and phonon systems. The magnitude of the drag, however, suggests the alternate limit; one in which electrons and phonons are strongly coupled. The results for still larger layer spacing show significant discrepancies with the behavior expected for either limit.

cond-mat.mes-hall↗

arXiv1998

COBE-DMR constraints on a inflation model with non-minimal scalar field

We derive the power spectra of the scalar- and tensor-type structures generated in a chaotic-type inflation model based on non-minimally coupled scalar field with a self interaction. By comparing contributions of both-structures to the anisotropy of the cosmic microwave background radiation with the four-year COBE-DMR data on quadrupole anisotropy we derive constraints on the ratio of self-coupling and non-minimal coupling constants, and the expansion rate in the inflation era, Eq. (12). The requirement of successful amount of inflation further constrains the relative amount of tensor-type contribution, Eq. (16).

astro-ph↗

arXiv1998

Cosmological perturbations in a gravity with quadratic order curvature couplings

We present a set of equations describing the evolution of the scalar-type cosmological perturbation in a gravity with general quadratic order curvature coupling terms. Equations are presented in a gauge ready form, thus are ready to implement various temporal gauge conditions depending on the problems. The Ricci-curvature square term leads to a fourth-order differential equation for describing the spacetime fluctuations in a spatially homogeneous and isotropic cosmological background.

gr-qc↗

arXiv1998

Many-body correlations probed by plasmon-enhanced drag measurements in double quantum well structures

Electron drag measurements of electron-electron scattering rates performed close to the Fermi temperature are reported. While evidence of an enhancement due to plasmons, as was recently predicted [K. Flensberg and B. Y.-K. Hu, Phys. Rev. Lett. 73, 3572 (1994)], is found, important differences with the random-phase approximation based calculations are observed. Although static correlation effects likely account for part of this difference, it is argued that correlation-induced multiparticle excitations must be included to account for the magnitude of the rates and observed density dependences.

cond-mat.mes-hall↗