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Hwang, Jungseek The Korea Institute of Applied Superconductivity a 2019 한국초전도저온공학회논문지 Vol.21 No.3
Extended Drude model has been used to obtain information of correlations from measured optical spectra of strongly correlated electron systems. The optical self-energy can be defined by the extended Drude model formalism. One can extract the optical self-energy and the electron-boson spectral density function from measured reflectance spectra using a well-developed usual process, which is consistent with several steps including the extended Drude model and generalized Allen's formulas. Here we used a reverse process of the usual process to investigate the extended Drude analysis when an additional low-energy interband transition is included. We considered two typical electron-boson spectral density model functions for two different (normal and d-wave superconducting) material states. Our results show that the low-energy interband transition might give significant effects on the electron-boson spectral density function obtained using the usual process. However, we expect that the low-energy interband transition can be removed from measured spectra in a proper way if the transition is well-defined or well-known.
High-energy fluctuation spectra in cuprates from infrared optical spectroscopy
Hwang, Jungseek,Carbotte, J. P. American Physical Society 2014 Physical review. B, Condensed matter and materials Vol.89 No.2
Coupling of the charge carriers in the high-temperature superconducting oxides to bosonic modes has been widely reported using a variety of experimental probes. These include angular resolved photoemission (ARPES), scanning tunneling spectroscopy, Raman scattering, and infrared optical spectroscopy. The energy scale investigated has been mostly limited to a relatively small range up to 300 meV or so. Although some ARPES experiments report boson structure up to 800 meV in the dressed electron dispersion curves, the data are not analyzed to recover the spectral density of the fluctuation spectrum. We have extended to higher energies up to 2.2 eV the usual maximum entropy technique used to invert optical data so as to obtain an electron-boson spectral density. This has required that we include in our inversions, the calculated (local-density approximation) particle-hole symmetrized energy-dependent electronic density of states. Our analysis reveals that significant spectral weight remains in the fluctuation spectra up to 2.2 eV in the Bi-2212 family and 1.2 eV in Bi-2201 for all doping levels considered.
Conceptual understanding of ubiquitous superconductivity
Hwang, Jungseek The Korea Institute of Applied Superconductivity a 2020 한국초전도저온공학회논문지 Vol.22 No.4
Since the discovery of superconductivity, the unique and mysterious phenomenon has been observed in various metallic material systems. Now days, the superconductivity becomes ubiquitous because almost every metallic material system shows the superconductivity when it is cooled down enough. This ubiquity of the superconductivity is associated with the fermionic nature and itinerancy of electrons in metallic materials. Because fermions are governed by the Pauli's exclusion principle the total energy of fermions is much larger than that of bosons. Therefore, fermionic itinerant electrons are fundamentally instable. Itinerant electrons are able to find "a way" to lead them to their lowest possible energy state through an available bosonization (or pairing) process and Bose-Einstein condensation. Therefore, the lowest possible energy state of itinerant electrons will be a superconducting state, which is "their ultimate destination". This may explain the reason why the superconductivity is ubiquitous.
Jungseek Hwang* 한국초전도.저온공학회 2019 한국초전도저온공학회논문지 Vol.21 No.3
Extended Drude model has been used to obtain information of correlations from measured optical spectra of strongly correlated electron systems. The optical self-energy can be defined by the extended Drude model formalism. One can extract the optical selfenergy and the electron-boson spectral density function from measured reflectance spectra using a well-developed usual process, which is consistent with several steps including the extended Drude model and generalized Allen's formulas. Here we used a reverse process of the usual process to investigate the extended Drude analysis when an additional low-energy interband transition is included. We considered two typical electron-boson spectral density model functions for two different (normal and d-wave superconducting) material states. Our results show that the low-energy interband transition might give significant effects on the electron-boson spectral density function obtained using the usual process. However, we expect that the low-energy interband transition can be removed from measured spectra in a proper way if the transition is well-defined or well-known.
Jungseek Hwang 한국물리학회 2020 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.76 No.8
Correlation information in strongly correlated electron systems can be obtained using an extended Drude model. An interesting method related to the extended Drude model analysis of superconducting optical data was proposed recently. This method aims to extract the optical self-energy of quasiparticles (or residual unpaired electrons) from measured optical data in the superconducting state. However, this new optical self-energy is part of the (full) optical self-energy. The interpretation and the significance of this new optical self-energy is unclear. We investigate the proposed method by using a reverse process with simple electron-boson spectral density functions. With our obtained results, we conclude that the new optical self-energy is difficult to interpret because it provides an unphysical feature, which is a negative (optical) effective mass. The present study clarifies the extended Drude analysis method for superconducting optical data of correlated electron systems.
IOP 2016 Journal of physics, an Institute of Physics journa Vol.28 No.12
<P>We introduce an approximate method which can be used to simulate the optical conductivity data of correlated multiband systems for normal and superconducting cases by taking advantage of a reversed process in comparison to a usual optical data analysis, which has been used to extract the electron–boson spectral density function from measured optical spectra of single-band systems, like cuprates. We applied this method to optical conductivity data of two multiband pnictide systems (Ba<SUB>0.6</SUB>K<SUB>0.4</SUB>Fe<SUB>2</SUB>As<SUB>2</SUB> and LiFeAs) and obtained the electron–boson spectral density functions. The obtained electron–boson spectral density consists of a sharp mode and a broad background. The obtained spectral density functions of the multiband systems show similar properties as those of cuprates in several aspects. We expect that our method helps to reveal the nature of strong correlations in the multiband pnictide superconductors.</P>
Analysis of optical data using extended Drude model and generalized Allen’s formulas
IOP 2018 Journal of Physics, Condensed Matter Vol.30 No.40
<P>Extended Drude model formalism has been successfully utilized for analyzing optical spectra of strongly correlated electron systems including heavy-fermion systems and high-<I>T</I> <SUB>c</SUB> superconducting iron pnictides and cuprates. Furthermore, generalized Allen’s formulas have been developed and applied to extract the electron-boson spectral density function from measured optical data of high temperature superconductors including cuprates in various material phases. Here we used a reverse process to obtain various optical quantities starting from two typical electron-boson spectral density model functions for three intriguing (normal, pseudogap, and <I>d</I>-wave superconducting) material phases in cuprates. We also assigned the calculated optical results to designated regions in the phase diagram of hole-doped cuprates and compared them with the corresponding measured optical spectra of Bi<SUB>2</SUB>Sr<SUB>2</SUB>CaCu<SUB>2</SUB> <img ALIGN='MIDDLE' ALT='' SRC='http://ej.iop.org/images/0953-8984/30/40/405604/cmaaddcaieqn001.gif'/> (Bi-2212). This comparison suggested that this way of optical data analysis can be a convincing method to study correlated electrons in the copper oxide superconductors and other superconducting systems as well.</P>
Bhupathi, Pradeep,Hwang, Jungseek,Martin, Rodica M.,Blankstein, Jackson,Jaworski, Lukas,Mulders, Norbert,Tanner, David B.,Lee, Yoonseok The Optical Society 2009 Optics express Vol.17 No.13
<P>Optical transmission measurements were made on 98% porosity silica aerogel samples under various degrees of uniaxial strain. Uniaxially compressed aerogels exhibit large birefringence, proportional to the amount of compression, up to the 15% strain studied. The birefringence is mostly reversible and reproducible through multiple compression-decompression cycles. Our study demonstrates that uniaxially strained high porosity aerogels can be used as tunable waveplates in a broad spectral range.</P>