arXiv subjects

J. Hwang

Publications and source records attributed to J. Hwang.

At least 55 records · Page 3Linked to original sources

arXiv2008

Evidence for a pseudogap in underdoped $Bi_2Sr_2CaCu_2O_{8+δ}$ and $YBa_2Cu_3O_{6.50}$ from in-plane optical conductivity measurements

The real part of the in-plane optical self-energy data in underdoped Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+δ}$ (Bi-2212) and ortho II YBa$_{2}$Cu$_{3}$O$_{6.5}$ contains new and important information on the pseudogap. Using a theoretical model approach we find that the density of state lost below the pseudogap $Δ_{pg}$ is accompanied with a pileup just above this energy. The pileup along with a sharp mode in the bosonic spectral function leads to an unusually rapid increase in the optical scattering rate and a characteristically sloped peak in the real part of the optical self-energy. These features are not found in optimally doped and overdoped samples and represent a clearest signature so far of the opening of a pseudogap in the in-plane optical conductivity.

cond-mat.supr-con↗

arXiv2008

Bosonic spectral density of epitaxial thin-film La1.83Sr0.17CuO4 superconductors from infrared conductivity measurements

We use optical spectroscopy to investigate the excitations responsible for the structure in the optical self-energy of thin epitaxial films of La1.83Sr0.17CuO4. Using Eliashberg formalism to invert the optical spectra we extract the electron-boson spectral function and find that at low temperature it has a two component structure closely matching the spin excitation spectrum recently measured by magnetic neutron scattering. We contrast the temperature evolution of the spectral density and the two-peak behavior in La2-xSrxCuO4 with another high temperature superconductor Bi2Sr2CaCu2O8+d. The bosonic spectral functions of the two materials account for the low Tc of LSCO as compared to Bi-2212.

cond-mat.supr-con↗

arXiv2007

Role of magnetism and electron-lattice interactions in STM spectra of cuprates

The study of bosonic modes that couple to the charge carriers is a key element in understanding superconductivity. Lee et al. have used atomic-resolution scanning tunneling spectroscopy (STM) in an attempt to extract the spectrum of of these modes in the high temperature superconductor Bi2Sr2CaCu2O8+d and they found a mode whose frequency did not depend on doping but changed on 16O to 18O isotopic substitution leading them to suggest a role for lattice modes (phonons). A careful examination of their published data reveals a weaker but distinct feature that has all the characteristics of the magnetic excitation identified as the bosonic mode in other competing experiments. We suggest the lattice mode seen by Lee et al. is not relevant to superconductivity and is due to inelastic tunneling through the insulating oxide layer.

cond-mat.supr-con↗

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

High energy scales in electronic self-energy imaged by optics

We use a new technique to directly extract an estimate of the quasiparticle self-energy from the optical conductivity which can be easily related to both theory and angle-resolved photoemission spectroscopy (ARPES) experiments. In the high Tc cuprate Bi-2212 we find evidence for a new high energy scale at 900 meV in addition to the two previously well known ones at roughly 50 and 400 meV. The intermediate scale at 400 meV has been recently seen in ARPES as a large kink which optics finds to be weaker and shifted. In YBCO, the three energy scales are shifted to lower energy relative to Bi-2212 and we observe the emergence of a possible fourth high energy feature at 600 meV.

cond-mat.supr-con↗

arXiv2006

Evolution of the Spin Susceptibility of High-$T_c$ Superconductors

We demonstrate that a new tool, a model independent numerical Eliashberg inversion of the optical self-energy, based on maximum entropy considerations can be used to extract the magnetic excitation spectra of high-transition-temperature superconductors. In Bi-2212 we explicitly show that the magnetic mode that dominates the self-energy at low temperatures directly evolves out of a smooth transfer of spectral weight to the mode from the continuum just above it. This redistribution starts already at 200 K in optimally doped materials but is much weaker in overdoped samples. This provides evidence for the magnetic origin of the superconductivity and presents a challenge to theories of the spin susceptibility and to neutron scattering experiments in high-temperature superconductors.

cond-mat.supr-con↗

arXiv2006

Doping dependent optical properties of Bi2Sr2CaCu2O8+d

We report on the ab-plane reflectance of the high temperature superconductor Bi2Sr2CaCu2O8+d (Bi-2212). Samples spanning the doping range from under doped with Tc = 67 K(UD), to optimally doped with Tc = 96 K (OPT), to over doped with Tc = 60 K (OD) were measured from room temperature down to the superconducting state. The measured reflectance data were analyzed to extract the optical conductivity and the real and imaginary parts of the free carrier optical self-energy. We get an estimate of the dc resistivity from the low frequency extrapolation of the optical conductivity and the superfluid density from the imaginary part of the optical conductivity. The conductivity sum rule can be related to the changes of the kinetic energy of the system. When this system becomes a superconductor, the kinetic energy decreases in the underdoped samples and increases in overdoped ones. The optical self-energy, obtained from the extended Drude model, is dominated by two channels of interaction, a sharp mode and a broad background. The amplitude of the mode is strongly doping and temperature dependent whereas the background decreases weakly with doping and is nearly temperature independent.

cond-mat.supr-con↗

arXiv2006

The infrared conductivity of Na$_x$CoO$_2$: evidence of gapped states

We present infrared ab-plane conductivity data for the layered cobaltate Na$_x$CoO$_2$ at three different doping levels ($x=0.25, 0.50$, and 0.75). The Drude weight increases monotonically with hole doping, $1-x$. At the lowest hole doping level $x$=0.75 the system resembles the normal state of underdoped cuprate superconductors with a scattering rate that varies linearly with frequency and temperature and there is an onset of scattering by a bosonic mode at 600 \cm. Two higher hole doped samples ($x=0.50$ and 0.25) show two different-size gaps (110 \cm and 200 \cm, respectively) in the optical conductivities at low temperatures and become insulators. The spectral weights lost in the gap region of 0.50 and 0.25 samples are shifted to prominent peaks at 200 \cm and 800 \cm, respectively. We propose that the two gapped states of the two higher hole doped samples ($x$=0.50 and 0.25) are pinned charge ordered states.

cond-mat.supr-con↗

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↗

arXiv2005

The a-axis optical conductivity of detwinned ortho-II YBa_2Cu_3O_6.50

The a-axis optical properties of a detwinned single crystal of YBa_2Cu_3O_6.50 in the ortho II phase (Ortho II Y123, T_c= 59 K) were determined from reflectance data over a wide frequency range (70 - 42 000 cm^-1) for nine temperature values between 28 and 295 K. Above 200 K the spectra are dominated by a broad background of scattering that extends to 1 eV. Below 200 K a shoulder in the reflectance appears and signals the onset of scattering at 400 cm^-1. In this temperature range we also observe a peak in the optical conductivity at 177 cm^-1. Below 59 K, the superconducting transition temperature, the spectra change dramatically with the appearance of the superconducting condensate. Its spectral weight is consistent, to within experimental error, with the Ferrell-Glover-Tinkham (FGT) sum rule. We also compare our data with magnetic neutron scattering on samples from the same source that show a strong resonance at 31 meV. We find that the scattering rates can be modeled as the combined effect of the neutron resonance and a bosonic background in the presence of a density of states with a pseudogap. The model shows that the decreasing amplitude of the neutron resonance with temperature is compensated for by an increasing of the bosonic background yielding a net temperature independent scattering rate at high frequencies. This is in agreement with the experiments.

cond-mat.supr-con↗

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

On the Relation between Optical Conductivity and Quasiparticle Dynamics: Boson Structures

An extended Drude form is often used to analyze optical data in terms of an optical scattering rate and renormalized mass corresponding, respectively, to the real and imaginary part of the memory function. We study the relationship between memory function and quasiparticle self energy for an isotropic system. We emphasize particularly boson signatures. We find it useful to introduce a new auxiliary model scattering rate and its Kramers-Kronig transform determined solely from optics which are much closer to the self energy than is the memory function itself in the normal state. In the superconducting state the simplification fails because the quasiparticle density of states acquires an essential energy dependence.

cond-mat.supr-con↗

arXiv2004

Marginal Fermi liquid analysis of 300 K reflectance of Bi2Sr2CaCu2O8+x

We use 300 K reflectance data to investigate the normal-state electrodynamics of the high temperature superconductor Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+δ}$ over a wide range of doping levels. The data show that at this temperature the free carriers are coupled to a continuous spectrum of fluctuations. Assuming the Marginal Fermi Liquid (MFL) form as a first approximation for the fluctuation spectrum, the doping-dependent coupling constant $λ(p)$ can be estimated directly from the slope of the reflectance spectrum. We find that $λ(p)$ decreases smoothly with the hole doping level, from underdoped samples with $ p=0.103$ ($T_c = 67$ K) where $λ(p)= 0.93$ to overdoped samples with $p=0.226$, ($T_c= 60$ K) where $λ(p)= 0.53$. An analysis of the intercept and curvature of the reflectance spectrum shows deviations from the MFL spectrum symmetrically placed at the optimal doping point $p=0.16$. The Kubo formula for the conductivity gives a better fit to the experiments with the MFL spectrum up to 2000 cm$^{-1}$ and with an additional Drude component or an additional Lorentz component up to 7000 cm$^{-1}$. By comparing three different model fits we conclude that the MFL channel is necessary for a good fit to the reflectance data. Finally, we note that the monotonic variation of the reflectance slope with doping provides us with an independent measure of the doping level for the Bi-2212 system.

cond-mat.supr-con↗