arXiv subjects

S. Iguchi

Publications and source records attributed to S. Iguchi.

At least 19 recordsLinked to original sources

arXiv2024

Interband spectroscopy of Landau levels and magnetoexcitons in bulk black phosphorus

Low-dimensional systems based on black phosphorus (BP) have recently attracted much interest. Since the crystal structure of BP consists of 2D layers with a strong in-plane anisotropy, its electronic properties at a high magnetic field ($B$) are quite interesting. Here, we report a magneto-optical study of bulk BP at high $B$ to 12 T perpendicular to the 2D layers. In the obtained optical conductivity spectra, periodic peaks are clearly observed corresponding to Landau levels of up to $n$=6 quantum number. They exhibit almost linear shifts with $B$, and from their analysis an electron-hole reduced mass of 0.13 $m_0$ is obtained, where $m_0$ is the electron mass. Many of the peaks appear in pairs, which is interpreted in terms of Zeeman splitting and an electron-hole combined $g$-factor of 6.6 is obtained. In addition, magnetoexciton peaks are observed to shift quadratically with $B$, and the exciton binding energy at $B$=0 is estimated to be 9.7 meV.

cond-mat.mes-hall↗

arXiv2022

A Monomer Mott Insulator (BEDT-TTF)Cu[N(CN)$_2$]$_2$ as a Potential Nodal Line System

We report the band structure calculations and the experimental results of resistivity and magnetic susceptibility in a spin-1/2 (BEDT-TTF)$^{\bullet +}$ monomer Mott insulator (BEDT-TTF)Cu[N(CN)$_2$]$_2$. The band calculations indicate a Dirac semimetal state with nodal lines at the Fermi level. The resistivity and the magnetic susceptibility as functions of temperature are well interpreted in terms of the monomer Mott insulating state instead of the expected semimetal state probably owing to strong electron correlation. In addition, we find that an Arrhenius-type steep reduction of the paramagnetic susceptibility appears below approximately 25 K, which indicates a spin-singlet ground state.

cond-mat.str-el↗

arXiv2019

Structural Alternation Correlated to the Conductivity Enhancement of PEDOT:PSS Films by Secondary Doping

"Secondary doping" in poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (PEDOT:PSS) is quite effective and a frequently used method for the conductivity enhancement. This simple approach, the addition of co-solvents to the PEDOT:PSS solution before film fabrication, is an essential way to derive the high electrical performance of PEDOT:PSS, but the mechanism still remains unclear. In this study, nanoscale structural changes synchronized with the conductivity enhancement via secondary doping in PEDOT:PSS films were investigated. During secondary doping with ethylene glycol near the critical dopant concentration, non-crystalized PEDOT molecules uncoupled from PSS chains and then underwent nano-crystallization. These structural changes might be the key driving force for conductivity enhancement via secondary doping.

cond-mat.mtrl-sci↗

arXiv2018

Low temperature magnetism of gold nano particles contained in electrochemical sugar recognition system

Low temperature magnetic properties were investigated on the gold nano particles (GNP) with an average size of 11.5 nm, assembled with molecules of ruthenium complex (Ru0) and phenylboronic acid (B0) by the proton nuclear resonance (1H-NMR) and susceptibility measurements. The temperature dependence of the NMR shift and the uniform susceptibility was described as the sum of the Curie-Weiss term and a positive constant term. From the former, the average number of Ru0 on each GNP was estimated to be 118, which is 23 % of the calculation. The finite positive constant term shows a clear contrast with the well-known fact that the bulk gold is diamagnetic. Finally, a disappearance of the motional narrowing effect in the proton NMR spectra below 60 K indicates that the wavering motion of Ru0 complexes on GNP at room temperature is frozen at low temperatures.

cond-mat.mes-hall↗

arXiv2017

Crystallization and vitrification of electrons in a glass-forming charge liquid

Charge ordering (CO) is a phenomenon in which electrons in solids crystallize into a periodic pattern of charge-rich and charge-poor sites owing to strong electron correlations. This usually results in long-range order. In geometrically frustrated systems, however, a glassy electronic state without long-range CO has been observed. We found that a charge-ordered organic material with an isosceles triangular lattice shows charge dynamics associated with crystallization and vitrification of electrons, which can be understood in the context of an energy landscape arising from the degeneracy of various CO patterns. The dynamics suggest that the same nucleation and growth processes that characterize conventional glass-forming liquids guide the crystallization of electrons. These similarities may provide insight into our understanding of the liquid-glass transition.

cond-mat.str-el↗

arXiv2017

Dimer-Mott and charge-ordered insulating states in the quasi-one-dimensional organic conductors $δ'_{P}$- and $δ'_{C}$-(BPDT-TTF)$_2$ICl$_2$

We investigated the electronic states of the quasi-one-dimensional organic conductors $δ'_{P}$-(BPDT-TTF)$_2$ICl$_2$ and $δ'_{C}$-(BPDT-TTF)$_2$ICl$_2$, both of which are insulating at room temperature owing to strong electron correlations. Through measurements of electrical resistivity, optical conductivity, and magnetic susceptibility, as well as band-structure calculations, we have revealed that the two materials possess completely different ground states, even though they have the same chemical composition and stacking configuration of the donor molecules. We have found that the $δ_P'$-type salt with an effective half-filled band behaves as a dimer-Mott (DM) insulator and exhibits a nonmagnetic transition at 25 K, whereas the $δ'_C$-type salt with a 3/4-filled band shows a charge ordering (CO) transition just above room temperature and becomes nonmagnetic below 20 K. The optical spectra of the $δ_P'$-type salt are composed of two characteristic bands due to intra- and interdimer charge transfers, supporting the DM insulating behavior arising from the strong on-site Coulomb interaction. By contrast, in the $δ'_C$-type salt, a single band characterizing the formation of CO arising from the off-site Coulomb interactions is observed. Upon lowering the temperature, the shape of the optical spectra in the $δ_C'$-type salt becomes asymmetric and shifts to much lower frequencies, suggesting the emergence of domain-wall excitations with fractional charges expected in a one-dimensional CO chain. The temperature dependence of the magnetic susceptibility of the $δ_P'$-type salt is well described by a 2D spin-1/2 Heisenberg AFM model on an anisotropic square lattice in the dimerized picture, while in the $δ_C'$-type salt, it can be explained by a 2D spin-1/2 Heisenberg AFM model on an anisotropic honeycomb lattice formed in the CO state.

cond-mat.str-el↗

arXiv2017

WISDOM Project - I: Black Hole Mass Measurement Using Molecular Gas Kinematics in NGC 3665

As a part of the mm-Wave Interferometric Survey of Dark Object Masses (WISDOM) project, we present an estimate of the mass of the supermassive black hole (SMBH) in the nearby fast-rotator early-type galaxy NGC 3665. We obtained Combined Array for Research in Millimeter Astronomy (CARMA) B and C array observations of the $^{12}$CO$(J=2-1)$ emission line with a combined angular resolution of $0".59$. We analysed and modelled the three-dimensional molecular gas kinematics, obtaining a best-fit SMBH mass $M_{\rm BH}=5.75^{+1.49}_{-1.18} \times 10^{8}$ $M_{\odot}$, a mass-to-light ratio at $H$-band $(M/L)_{H}=1.45\pm0.04$ $(M/L)_{\odot, H}$, and other parameters describing the geometry of the molecular gas disc (statistical errors, all at $3σ$ confidence). We estimate the systematic uncertainties on the stellar $M/L$ to be $\approx0.2$ $(M/L)_{\odot, H}$, and on the SMBH mass to be $\approx0.4\times10^{8}$ $M_{\odot}$. The measured SMBH mass is consistent with that estimated from the latest correlations with galaxy properties. Following our older works, we also analysed and modelled the kinematics using only the major-axis position-velocity diagram, and conclude that the two methods are consistent.

astro-ph.GA↗

arXiv2017

Quantum-disordered state of magnetic and electric dipoles in a hydrogen-bonded Mott system

Strongly enhanced quantum fluctuations often lead to a rich variety of quantum-disordered states. A representative case is liquid helium, in which zero-point vibrations of the helium atoms prevent its solidification at low temperatures. A similar behaviour is found for the internal degrees of freedom in electrons. Among the most prominent is a quantum spin liquid (QSL), in which localized spins are highly correlated but fluctuate even at absolute zero. Recently, a coupling of spins with other degrees of freedom has been proposed as an innovative approach to generate even more fascinating QSLs such as orbital--spin liquids. However, such ideas are limited to the internal degrees of freedom in electrons. Here, we demonstrate that a coupling of localized spins with the zero-point motion of hydrogen atoms (proton fluctuations) in a hydrogen-bonded organic Mott insulator provides a new class of QSLs. We find that a divergent dielectric behaviour towards a hydrogen-bond order is suppressed by the quantum proton fluctuations, resulting in a quantum paraelectric (QPE) state. Furthermore, our thermal-transport measurements reveal that a QSL state with gapless spin excitations rapidly emerges upon entering the QPE state. These findings indicate that the quantum proton fluctuations give rise to a novel QSL --- a quantum-disordered state of magnetic and electric dipoles --- through the coupling between the electron and proton degrees of freedom.

cond-mat.str-el↗

arXiv2015

Emergence of charge degrees of freedom under high pressure in an organic dimer-Mott insulator $β^{\prime}$-(BEDT-TTF)$_2$ICl$_2$

To elucidate the pressure evolution of the electronic structure in an antiferromagnetic dimer-Mott (DM) insulator $β^{\prime}$-(BEDT-TTF)$_2$ICl$_2$, which exhibits superconductivity at 14.2 K under 8 GPa, we measured the polarized infrared (IR) optical spectra under high pressure. At ambient pressure, two characteristic bands due to intra- and interdimer charge transfers have been observed in the IR spectra, supporting that this salt is a typical half-filled DM insulator at ambient pressure. With increasing pressure, however, the intradimer charge transfer excitation shifts to much lower energies, indicating that the effective electronic state changes from half-filled to 3/4-filled as a result of weakening of dimerization. This implies that the system approaches a charge-ordered state under high pressure, in which charge degrees of freedom emerge as an important factor. The present results suggest that charge fluctuation inside of dimers plays an important role in the high-temperature superconductivity.

cond-mat.str-el↗

arXiv2014

Interface-dependent magnetotransport properties for thin Pt films on ferrimagnetic Y3Fe5O12

We have studied magnetoresistance and Hall effects for 1.8-nm-thick Pt films grown on a ferrimagnetic insulator Y3Fe5O12 in a wide temperature (0.46-300 K) and magnetic-field (-15-15 T) region. In the low-temperature regime where quantum corrections to conductivity are observed, weak antilocalization behavior observed in Pt films is critically suppressed when the film is attached to Y3Fe5O12. Hall resistance in the Pt film is also affected by Y3Fe5O12, and it exhibits logarithmic temperature dependence in a broad temperature range. The magnetotransport properties in the high-field range are significantly influenced by the interface between Pt and Y3Fe5O12.

cond-mat.mes-hall↗

arXiv2014

Collective excitation of a short-range charge ordering in $θ$-$\mbox{(BEDT-TTF)}_2\mbox{CsZn(SCN)}_4$

We find a characteristic low-energy peak structure located in the range of 100-300 cm$^{-1}$ in the optical conductivity spectra of a quasi-two-dimensional organic compound with a triangular lattice, $θ$-$\mbox{(BEDT-TTF)}_2\mbox{CsZn(SCN)}_4$, in which two different types of short-range charge orderings (COs) coexist. Upon lowering the temperature, the low-energy peak becomes significant and shifts to much lower frequencies only for the polarization of ${E \parallel a}$, in contrast to the other broad electronic bands in the mid-infrared region. On introducing disorder, the low-energy peak is strongly suppressed in comparison with the broad electronic bands. This result indicates that the low-energy peak is attributed to a collective excitation that originates from the short-range CO with a relatively long-period $3\times3$ pattern. The present results shed light on the understanding of the low-energy excitation in the glassy electronic state, where the charge degrees of freedom remain at low temperatures.

cond-mat.str-el↗

arXiv2013

Relaxor ferroelectricity induced by electron correlations in a molecular dimer Mott insulator

We have investigated the dielectric response in an antiferromagnetic dimer-Mott insulator beta'-(BEDT-TTF)2ICl2 with square lattice, compared to a spin liquid candidate kappa-(BEDT-TTF)2Cu2(CN)3. Temperature dependence of the dielectric constant shows a peak structure obeying Curie-Weiss law with strong frequency dependence. We found an anisotropic ferroelectricity by pyrocurrent measurements, which suggests the charge disproportionation in a dimer. The ferroelectric actual charge freezing temperature is related to the antiferromagnetic interaction, which is expected to the charge-spin coupled degrees of freedom in the system.

cond-mat.str-el↗

arXiv2012

Giant Molecular Clouds and Star Formation in the Tidal Molecular Arm of NGC 4039

The properties of tidally induced arms provide a means to study molecular cloud formation and the subsequent star formation under environmental conditions which in principle are different from quasi stationary spiral arms. We report the properties of a newly discovered molecular gas arm of likely tidal origin at the south of NGC 4039 and the overlap region in the Antennae galaxies, with a resolution of 1"68 x 0"85, using the Atacama Large Millimeter/submillimeter Array science verification CO(2-1) data. The arm extends 3.4 kpc (34") and is characterized by widths of ~ 200 pc (2") and velocity widths of typically \DeltaV ~ 10-20 km/s . About 10 clumps are strung out along this structure, most of them unresolved, with average surface densities of Σ_gas ~ 10-100 Msun pc^{-2}, and masses of (1-8) x 10^6 Msun. These structures resemble the morphology of beads on a string, with an almost equidistant separation between the beads of about 350 pc, which may represent a characteristic separation scale for giant molecular associations. We find that the star formation efficiency at a resolution of 6" (600 pc) is in general a factor of 10 higher than in disk galaxies and other tidal arms and bridges. This arm is linked, based on the distribution and kinematics, to the base of the western spiral arm of NGC 4039, but its morphology is different to that predicted by high-resolution simulations of the Antennae galaxies.

astro-ph.CO↗

arXiv2012

The parsec-scale jet of PKS 1749+096

PKS 1749+096 is a BL Lac object showing weak extended jet emission to the northeast of the compact VLBI core on parsec scales. We aim at better understanding the jet kinematics and variability of this source and finding clues that may applicable to other BL Lac objects. The jet was studied with multi-epoch multi-frequency high-resolution VLBI observations. The jet is characterized by a one-sided curved morphology at all epochs and all frequencies. The VLBI core, located at the southern end of the jet, was identified based on its spectral properties. The equipartition magnetic field of the core was investigated, through which we derived a Doppler factor of 5, largely consistent with that derived from kinematics (component C5). The study of the detailed jet kinematics at 22 and 15 GHz, spanning a period of more than 10 years, indicates the possible existence of a bimodal distribution of the jet apparent speed. Ballistic and non-ballistic components are found to coexist in the jet. Superluminal motions in the range of 5-21 c were measured in 11 distinct components. We estimated the physical jet parameters with the minimum Lorentz factor of 10.2 and Doppler factors in the range of 10.2-20.4 (component C5). The coincidence in time of the component's ejection and flares supports the idea that, at least in PKS 1749+096, ejection of new jet components is connected with major outbursts in flux density. For the best-traced component (C5) we found that the flux density decays rapidly as it travels downstream the jet, accompanied by a steepening of its spectra, which argues in favor of a contribution of inverse Compton cooling. These properties make PKS 1749+096 a suitable target for an intensive monitoring to decipher the variability phenomenon of BL Lac objects.

astro-ph.CO↗

arXiv2011

Digital Spectro-Correlator System for the Atacama Compact Array of the Atacama Large Millimeter/submillimeter Array

We have developed an FX-architecture digital spectro-correlator for the Atacama Compact Array (ACA) of the Atacama Large Millimeter/submillimeter Array. The correlator is able to simultaneously process four pairs of dual polarization signals with the bandwidth of 2 GHz, which are received by up to sixteen antennas. It can calculate auto- and cross-correlation spectra including cross-polarization in all combinations of all the antennas, and output correlation spectra with flexible spectral configuration such as multiple frequency ranges and multiple frequency resolutions. Its spectral dynamic range is estimated to be higher than 10^4 relative to Tsys from processing results of thermal noise for eight hours with a typical correlator configuration. The sensitivity loss is also confirmed to be 0.9 % with the same configuration. In this paper, we report the detailed design of the correlator and the verification results of the developed hardware.

astro-ph.IM↗

arXiv2011

Impact of geometrical frustration on charge transport near the Mott transition in pyrochlore (Y$_{1-x}$Cd$_{x}$)$_{2}$Mo$_{2}$O$_{7}$

To search for novel spin-charge coupled phenomena in the strongly correlated electron system on a frustration lattice, charge transport and dynamics have been investigated for the hole-doped spin-glass (Mott) insulator (Y$_{1-x}$Cd$_{x}$)$_{2}$Mo$_{2}$O$_{7}$ with pyrochlore structure. Coincident with the disappearance of spin-glass state by hole doping, a diffusive metallic transport with strongly renormalized electron mass shows up with no long range spin order, which is caused by strong antiferromagnetic spin fluctuation due to the geometrical frustration.

cond-mat.str-el↗

arXiv2008

Performance Measurements of 8-Gsps 1-bit ADCs Developed for Wideband Radio Astronomical Observations

8-Gsps 1-bit Analog-to-Digital Converters (ADCs) were newly developed toward the realization of the wideband observation. The development of the wideband ADCs is one of the most essential developments for the radio interferometer. To evaluate its performance in interferometric observations, the time (phase) stability and frequency response were measured with a noise source and a signal generator. The results of these measurements show that the developed ADCs can achieve the jitter time less than 0.05 psec at the time interval of 1 sec, the passband frequency response with the slope less than 0.73 dB/GHz and the ripple less than 1.8 dB, and the aperture time less than 20 psec. The details of the developed ADC design, the measurement methods, and the results of these measurements are presented in this paper.

astro-ph↗

arXiv2007

Multi-epoch VLBA observations of 3C 66A

We present the results of six-epoch Very Long Baseline Array (VLBA) observations of 3C~66A. The high-resolution Very Long Baseline Interferometer (VLBI) maps obtained at multi-frequency (2.3, 8.4, and 22.2 GHz) simultaneously enabled us to identify the brightest compact component with the core. We find that the spectrum of the core can be reasonably fitted by the synchrotron self-absorption model. Our VLBA maps show that the jet of 3C~66A has two bendings at about 1.2 and 4 mas from the core. We also give possible identifications of our jet components with the components in previous VLBA observations by analysing their proper motions. We find consistent differences of the position from the core in one component between different frequencies at six epochs.

astro-ph↗