arXiv subjects

Y. Segawa

Publications and source records attributed to Y. Segawa.

At least 19 recordsLinked to original sources

arXiv2024

Calibration of detector time constant with a thermal source for the POLARBEAR-2A CMB polarization experiment

The Simons Array (SA) project is a ground-based Cosmic Microwave Background (CMB) polarization experiment. The SA observes the sky using three telescopes, and POLARBEAR-2A (PB-2A) is the receiver system on the first telescope. For the ground-based experiment, atmospheric fluctuation is the primary noise source that could cause polarization leakage. In the PB-2A receiver system, a continuously rotating half-wave plate (HWP) is used to mitigate the polarization leakage. However, due to the rapid modulation of the polarization signal, the uncertainty in the time constant of the detector results in an uncertainty in the polarization angle. For PB-2A, the time constant of each bolometer needs to be calibrated at the sub-millisecond level to avoid introducing bias to the polarization signal. We have developed a new calibrator system that can be used to calibrate the time constants of the detectors. In this study, we present the design of the calibration system and the preliminary results of the time constant calibration for PB-2A.

astro-ph.IM↗

arXiv2022

The POLARBEAR-2 and Simons Array Focal Plane Fabrication Status

We present on the status of POLARBEAR-2 A (PB2-A) focal plane fabrication. The PB2-A is the first of three telescopes in the Simon Array (SA), which is an array of three cosmic microwave background (CMB) polarization sensitive telescopes located at the POLARBEAR (PB) site in Northern Chile. As the successor to the PB experiment, each telescope and receiver combination is named as PB2-A, PB2-B, and PB2-C. PB2-A and -B will have nearly identical receivers operating at 90 and 150 GHz while PB2-C will house a receiver operating at 220 and 270 GHz. Each receiver contains a focal plane consisting of seven close-hex packed lenslet coupled sinuous antenna transition edge sensor bolometer arrays. Each array contains 271 di-chroic optical pixels each of which have four TES bolometers for a total of 7588 detectors per receiver. We have produced a set of two types of candidate arrays for PB2-A. The first we call Version 11 (V11) and uses a silicon oxide (SiOx) for the transmission lines and cross-over process for orthogonal polarizations. The second we call Version 13 (V13) and uses silicon nitride (SiNx) for the transmission lines and cross-under process for orthogonal polarizations. We have produced enough of each type of array to fully populate the focal plane of the PB2-A receiver. The average wirebond yield for V11 and V13 arrays is 93.2% and 95.6% respectively. The V11 arrays had a superconducting transition temperature (Tc) of 452 +/- 15 mK, a normal resistance (Rn) of 1.25 +/- 0.20 Ohms, and saturations powers of 5.2 +/- 1.0 pW and 13 +/- 1.2 pW for the 90 and 150 GHz bands respectively. The V13 arrays had a superconducting transition temperature (Tc) of 456 +/-6 mK, a normal resistance (Rn) of 1.1 +/- 0.2 Ohms, and saturations powers of 10.8 +/- 1.8 pW and 22.9 +/- 2.6 pW for the 90 and 150 GHz bands respectively.

astro-ph.IM↗

arXiv2022

Improved upper limit on degree-scale CMB B-mode polarization power from the 670 square-degree POLARBEAR survey

We report an improved measurement of the degree-scale cosmic microwave background $B$-mode angular-power spectrum over 670 square-degree sky area at 150 GHz with POLARBEAR. In the original analysis of the data, errors in the angle measurement of the continuously rotating half-wave plate, a polarization modulator, caused significant data loss. By introducing an angle-correction algorithm, the data volume is increased by a factor of 1.8. We report a new analysis using the larger data set. We find the measured $B$-mode spectrum is consistent with the $\Lambda$CDM model with Galactic dust foregrounds. We estimate the contamination of the foreground by cross-correlating our data and Planck 143, 217, and 353 GHz measurements, where its spectrum is modeled as a power law in angular scale and a modified blackbody in frequency. We place an upper limit on the tensor-to-scalar ratio $r$ < 0.33 at 95% confidence level after marginalizing over the foreground parameters.

astro-ph.CO↗

arXiv2020

A measurement of the CMB E-mode angular power spectrum at subdegree scales from 670 square degrees of POLARBEAR data

We report a measurement of the E-mode polarization power spectrum of the cosmic microwave background (CMB) using 150 GHz data taken from July 2014 to December 2016 with the POLARBEAR experiment. We reach an effective polarization map noise level of $32\,\mu\mathrm{K}$-$\mathrm{arcmin}$ across an observation area of 670 square degrees. We measure the EE power spectrum over the angular multipole range $500 \leq \ell <3000$, tracing the third to seventh acoustic peaks with high sensitivity. The statistical uncertainty on E-mode bandpowers is $\sim 2.3 \mu {\rm K}^2$ at $\ell \sim 1000$ with a systematic uncertainty of 0.5$\mu {\rm K}^2$. The data are consistent with the standard $\Lambda$CDM cosmological model with a probability-to-exceed of 0.38. We combine recent CMB E-mode measurements and make inferences about cosmological parameters in $\Lambda$CDM as well as in extensions to $\Lambda$CDM. Adding the ground-based CMB polarization measurements to the Planck dataset reduces the uncertainty on the Hubble constant by a factor of 1.2 to $H_0 = 67.20 \pm 0.57 {\rm km\,s^{-1} \,Mpc^{-1}}$. When allowing the number of relativistic species ($N_{eff}$) to vary, we find $N_{eff} = 2.94 \pm 0.16$, which is in good agreement with the standard value of 3.046. Instead allowing the primordial helium abundance ($Y_{He}$) to vary, the data favor $Y_{He} = 0.248 \pm 0.012$. This is very close to the expectation of 0.2467 from Big Bang Nucleosynthesis. When varying both $Y_{He}$ and $N_{eff}$, we find $N_{eff} = 2.70 \pm 0.26$ and $Y_{He} = 0.262 \pm 0.015$.

astro-ph.CO↗

arXiv2019

A Measurement of the Degree Scale CMB B-mode Angular Power Spectrum with POLARBEAR

We present a measurement of the $B$-mode polarization power spectrum of the cosmic microwave background (CMB) using taken from July 2014 to December 2016 with the POLARBEAR experiment. The CMB power spectra are measured using observations at 150 GHz with an instantaneous array sensitivity of $\mathrm{NET}_\mathrm{array}=23\, \mu \mathrm{K} \sqrt{\mathrm{s}}$ on a 670 square degree patch of sky centered at (RA, Dec)=($+0^\mathrm{h}12^\mathrm{m}0^\mathrm{s},-59^\circ18^\prime$). A continuously rotating half-wave plate is used to modulate polarization and to suppress low-frequency noise. We achieve $32\,\mu\mathrm{K}$-$\mathrm{arcmin}$ effective polarization map noise with a knee in sensitivity of $\ell = 90$, where the inflationary gravitational wave signal is expected to peak. The measured $B$-mode power spectrum is consistent with a $\Lambda$CDM lensing and single dust component foreground model over a range of multipoles $50 \leq \ell \leq 600$. The data disfavor zero $C_\ell^{BB}$ at $2.2\sigma$ using this $\ell$ range of POLARBEAR data alone. We cross-correlate our data with Planck high frequency maps and find the low-$\ell$ $B$-mode power in the combined dataset to be consistent with thermal dust emission. We place an upper limit on the tensor-to-scalar ratio $r < 0.90$ at 95% confidence level after marginalizing over foregrounds.

astro-ph.CO↗

arXiv2019

Internal delensing of Cosmic Microwave Background polarization B-modes with the POLARBEAR experiment

Using only cosmic microwave background polarization data from the POLARBEAR experiment, we measure $B$-mode polarization delensing on subdegree scales at more than $5\sigma$ significance. We achieve a 14% $B$-mode power variance reduction, the highest to date for internal delensing, and improve this result to 2% by applying for the first time an iterative maximum a posteriori delensing method. Our analysis demonstrates the capability of internal delensing as a means of improving constraints on inflationary models, paving the way for the optimal analysis of next-generation primordial $B$-mode experiments.

astro-ph.CO↗

arXiv2017

A Measurement of the Cosmic Microwave Background $B$-Mode Polarization Power Spectrum at Sub-Degree Scales from 2 years of POLARBEAR Data

We report an improved measurement of the cosmic microwave background (CMB) $B$-mode polarization power spectrum with the POLARBEAR experiment at 150 GHz. By adding new data collected during the second season of observations (2013-2014) to re-analyzed data from the first season (2012-2013), we have reduced twofold the band-power uncertainties. The band powers are reported over angular multipoles $500 \leq \ell \leq 2100$, where the dominant $B$-mode signal is expected to be due to the gravitational lensing of $E$-modes. We reject the null hypothesis of no $B$-mode polarization at a confidence of 3.1$σ$ including both statistical and systematic uncertainties. We test the consistency of the measured $B$-modes with the $Λ$ Cold Dark Matter ($Λ$CDM) framework by fitting for a single lensing amplitude parameter $A_L$ relative to the Planck best-fit model prediction. We obtain $A_L = 0.60 ^{+0.26} _{-0.24} ({\rm stat}) ^{+0.00} _{-0.04}({\rm inst}) \pm 0.14 ({\rm foreground}) \pm 0.04 ({\rm multi})$, where $A_{L}=1$ is the fiducial $Λ$CDM value, and the details of the reported uncertainties are explained later in the manuscript.

astro-ph.CO↗

arXiv2015

The POLARBEAR-2 and the Simons Array Experiment

We present an overview of the design and status of the \Pb-2 and the Simons Array experiments. \Pb-2 is a Cosmic Microwave Background polarimetry experiment which aims to characterize the arc-minute angular scale B-mode signal from weak gravitational lensing and search for the degree angular scale B-mode signal from inflationary gravitational waves. The receiver has a 365~mm diameter focal plane cooled to 270~milli-Kelvin. The focal plane is filled with 7,588 dichroic lenslet-antenna coupled polarization sensitive Transition Edge Sensor (TES) bolometric pixels that are sensitive to 95~GHz and 150~GHz bands simultaneously. The TES bolometers are read-out by SQUIDs with 40 channel frequency domain multiplexing. Refractive optical elements are made with high purity alumina to achieve high optical throughput. The receiver is designed to achieve noise equivalent temperature of 5.8~$μ$K$_{CMB}\sqrt{s}$ in each frequency band. \Pb-2 will deploy in 2016 in the Atacama desert in Chile. The Simons Array is a project to further increase sensitivity by deploying three \Pb-2 type receivers. The Simons Array will cover 95~GHz, 150~GHz and 220~GHz frequency bands for foreground control. The Simons Array will be able to constrain tensor-to-scalar ratio and sum of neutrino masses to $σ(r) = 6\times 10^{-3}$ at $r = 0.1$ and $\sum m_ν(σ=1)$ to 40 meV.

astro-ph.IM↗

arXiv2013

Magneto-photoluminescence of charged excitons from MgZnO/ZnO heterojunctions

We report on the photoluminescence (PL) properties of MgZnO/ZnO heterojunctions grown by plasma-assisted molecular-beam epitaxy. Influence of the applied magnetic field (B) on the radiative recombination of the two-dimensional electron gas (2DEG) is investigated up to 54 T. An increase in magnetic field in the range of B <= 20 T results in a redshift in the PL. Abrupt lineshape changes in the PL spectra are observed at higher magnetic fields, in correlation with the integer quantum Hall states. We attempt to interpret these features using the conventional model for the 2DEG-related PL based on the transition between the 2DEG and a hole as well as a model taking a bound state effect into account, i.e., a charged exciton. The comparison about the adequateness of these models was made, being in favor of the charged exciton model.

cond-mat.mtrl-sci↗

arXiv2012

Precise calibration of Mg concentration in MgxZn1-xO thin films grown on ZnO substrates

The growth techniques for MgxZn1-xO thin films have advanced at a rapid pace in recent years, enabling the application of this material to a wide range of optical and electrical applications. In designing structures and optimizing device performances, it is crucial that the Mg content of the alloy be controllable and precisely determined. In this study, we have established laboratory-based methods to determine the Mg content of MgxZn1-xO thin films grown on ZnO substrates, ranging from the solubility limit of x ~ 0.4 to the dilute limit of x < 0.01. For the absolute determination of Mg content, Rutherford backscattering spectroscopy is used for the high Mg region above x = 0.14, while secondary ion mass spectroscopy is employed to quantify low Mg content. As a lab-based method to determine the Mg content, c-axis length is measured by X-ray diffraction and is well associated with Mg content. The interpolation enables the determination of Mg content to x = 0.023, where the peak from the ZnO substrate overlaps the MgxZn1-xO peak in standard laboratory equipment, and thus quantitative determination. At dilute Mg contents below x = 0.023, the localized exciton peak energy of the MgxZn1-xO films as measured by photoluminescence is found to show a linear Mg content dependence, which is well resolved from the free exciton peak of ZnO substrate down to x = 0.0043. Our results demonstrate that X-ray diffraction and photoluminescence in combination are appropriate methods to determine Mg content in a wide Mg range from x = 0.004 to 0.40 in a laboratory environment.

cond-mat.mtrl-sci↗

arXiv2008

Novel Quantum Criticality in CeRu$_2$Si$_2$ near Absolute Zero Observed by Thermal Expansion and Magnetostriction

We report linear thermal expansion and magnetostriction measurements for CeRu$_2$Si$_2$ in magnetic fields up to 52.6 mT and at temperatures down to 1 mK. At high temperatures, this compound showed Landau-Fermi-liquid behavior: The linear thermal expansion coefficient and the magnetostriction coefficient were proportional to the temperature and magnetic field, respectively. In contrast, a pronounced non-Fermi-liquid effect was found below 50 mK. The negative contribution of thermal expansion and magnetostriction suggests the existence of an additional quantum critical point.

cond-mat.str-el↗

arXiv2006

Exotic radiation from a photonic crystal excited by an ultra-relativistic electron beam

We report the observation of an exotic radiation (unconventional Smith-Purcell radiation) from a one-dimensional photonic crystal. The physical origin of the exotic radiation is direct excitation of the photonic bands by an ultra-relativistic electron beam. The spectrum of the exotic radiation follows photonic bands of a certain parity, in striking contrast to the conventional Smith-Purcell radiation, which shows solely a linear dispersion. Key ingredients for the observation are the facts that the electron beam is in an ultra-relativistic region and that the photonic crystal is finite. The origin of the radiation was identified by comparison of experimental and theoretical results.

cond-mat.mtrl-sci↗

arXiv2005

Majority-Carrier Mobilities in Undoped and \textit{n}-type Doped ZnO Epitaxial Layers

Transparent and conductive ZnO:Ga thin films are prepared by laser molecular-beam epitaxy. Their electron properties were investigated by the temperature-dependent Hall-effect technique. The 300-K carrier concentration and mobility were about $n_s \sim 10^{16}$ cm$^{-3}$ and 440 cm$^{2}$/Vs, respectively. In the experimental `mobility vs concentration' curve, unusual phenomenon was observed, i.e., mobilities at $n_s \sim 5\times$ 10$^{18}$ cm$^{-3}$ are significantly smaller than those at higher densities above $\sim 10^{20}$ cm$^{-3}$. Several types of scattering centers including ionized donors and oxygen traps are considered to account for the observed dependence of the Hall mobility on carrier concentration. The scattering mechanism is explained in terms of inter-grain potential barriers and charged impurities. A comparison between theoretical results and experimental data is made.

cond-mat.mtrl-sci↗

arXiv2005

Analysis on reflection spectra in strained ZnO thin films

Thin films of laser molecular-beam epitaxy grown ZnO films were studied with respect to their optical properties. 4-K reflectivity was used to analyze various samples grown at different biaxial in-plane strain. The spectra show two structures at 3.37 eV corresponding to the A-free exciton transition and at 3.38 eV corresponding to the B-free exciton transition. Theoretical reflectivity spectra were calculated using the spatial dispersion model. Thus, the transverse energies, the longitudinal transversal splitting (ELT,), the oscillator strengths, and the damping parameters were determined for both the A- and B-free excitons of ZnO. As a rough trend, the strain dependence of the energy E_LT for the A-excitons is characterized by a negatively-peaking behavior with a minimum around the zero strain, while ELT for the B-excitons is an increasing function of the strain field values.

cond-mat.mtrl-sci↗

arXiv2005

Spectral shape analysis of ultraviolet luminescence in \textit{n}-type ZnO:Ga

Thin films of laser molecular-beam epitaxy grown \textit{n}-type Ga-doped ZnO were investigated with respect to their optical properties. Intense room-temperature photoluminescence (PL) in the near-band edge (NBE) region was observed. Moreover, its broadening of PL band was significantly larger than predicted by theoretical results modeled in terms of potential fluctuations caused by the random distribution of donor impurities. In addition, the lineshape was rather asymmetrical. To explain these features of the NBE bands, a vibronic model was developed accounting for contributions from a series of phonon replicas.

cond-mat.mtrl-sci↗

arXiv2005

Electron transport in ZnO thin films

Epitaxial, n-type ZnO films grown by a laser molecular-beam epitaxy method were investigated by the temperature-dependent Hall-effect technique. The 300-K carrier concentration and mobility were about $n_s \sim 10^{16}$ cm$^{-3}$ and 440 cm$^{2}$/Vs, respectively. Transport characteristics are calculated by solving the Boltzmann transport equation using a variational method. Mobility limit of 430 cm$^{2}$/Vs was calculated at 300 K. The temperature dependence of the mobility for an undoped film is calculated and agrees favorably well with experimental data if physical parameters are chosen so as to approach to those. In the experimental `mobility vs concentration' curve, unusual phenomenon was observed, i.e., mobilities at $n_s \sim 5\times$ 10$^{18}$ cm$^{-3}$ are significantly smaller than those at higher densities above $\sim 10^{20}$ cm$^{-3}$. It is qualitatively explained in terms of electron-plasmon interaction.

cond-mat.mtrl-sci↗

arXiv2005

Exciton-optical-phonon coupling: comparison with experiments for ZnO quantum wells

The temperature-dependent linewidths of excitons in ZnO quantum wells were studied by measuring absorption spectra from 5 K to room temperature. We deduced experimentally the exciton-longitudinal-optical (LO) phonon coupling strength, which showed reduction of coupling with decrease in well width. This reduction was explained in terms of confinement-induced enhancement of the excitonic binding energy by comparing the binding energy dependence of calculated coupling strength.

cond-mat.mtrl-sci↗

arXiv2004

Room-temperature stimulated emission from ZnO multiple quantum wells grown on lattice-matched substrates

The optical properties of ZnO quantum wells, which have potential application of short-wavelength semiconductor laser utilizing a high-density excitonic effect, were investigated. Stimulated emission of excitons was observed at temperatures well above room temperature due to the adoption of the lattice-matched substrates. The mechanism of stimulated emission from ZnO quantum wells is discussed in this chapter.

cond-mat.mtrl-sci↗