arXiv subjects

A. Ueda

Publications and source records attributed to A. Ueda.

At least 19 recordsLinked to original sources

arXiv2024

Gliese 12 b: A temperate Earth-sized planet at 12 pc ideal for atmospheric transmission spectroscopy

Recent discoveries of Earth-sized planets transiting nearby M dwarfs have made it possible to characterize the atmospheres of terrestrial planets via follow-up spectroscopic observations. However, the number of such planets receiving low insolation is still small, limiting our ability to understand the diversity of the atmospheric composition and climates of temperate terrestrial planets. We report the discovery of an Earth-sized planet transiting the nearby (12 pc) inactive M3.0 dwarf Gliese 12 (TOI-6251) with an orbital period ($P_{\rm{orb}}$) of 12.76 days. The planet, Gliese 12b, was initially identified as a candidate with an ambiguous $P_{\rm{orb}}$ from TESS data. We confirmed the transit signal and $P_{\rm{orb}}$ using ground-based photometry with MuSCAT2 and MuSCAT3, and validated the planetary nature of the signal using high-resolution images from Gemini/NIRI and Keck/NIRC2 as well as radial velocity (RV) measurements from the InfraRed Doppler instrument on the Subaru 8.2 m telescope and from CARMENES on the CAHA 3.5 m telescope. X-ray observations with XMM-Newton showed the host star is inactive, with an X-ray-to-bolometric luminosity ratio of $\log L_{\rm X}/L_{\rm bol} \approx -5.7$. Joint analysis of the light curves and RV measurements revealed that Gliese 12b has a radius of 0.96 $\pm$ 0.05 $R_\oplus$, a 3$\sigma$ mass upper limit of 3.9 $M_\oplus$, and an equilibrium temperature of 315 $\pm$ 6 K assuming zero albedo. The transmission spectroscopy metric (TSM) value of Gliese 12b is close to the TSM values of the TRAPPIST-1 planets, adding Gliese 12b to the small list of potentially terrestrial, temperate planets amenable to atmospheric characterization with JWST.

astro-ph.EP↗

arXiv2024

The Discovery and Follow-up of Four Transiting Short-period Sub-Neptunes Orbiting M dwarfs

Sub-Neptunes with $2-3R_\oplus$ are intermediate in size between rocky planets and Neptune-sized planets. The orbital properties and bulk compositions of transiting sub-Neptunes provide clues to the formation and evolution of close-in small planets. In this paper, we present the discovery and follow-up of four sub-Neptunes orbiting M dwarfs (TOI-782, TOI-1448, TOI-2120, and TOI-2406), three of which were newly validated by ground-based follow-up observations and statistical analyses. TOI-782 b, TOI-1448 b, TOI-2120 b, and TOI-2406 b have radii of $R_\mathrm{p} = 2.740^{+0.082}_{-0.079}\,R_\oplus$, $2.769^{+0.073}_{-0.068}\,R_\oplus$, $2.120\pm0.067\,R_\oplus$, and $2.830^{+0.068}_{-0.066}\,R_\oplus$ and orbital periods of $P = 8.02$, $8.11$, $5.80$, and $3.08$\,days, respectively. Doppler monitoring with Subaru/InfraRed Doppler instrument led to 2$\sigma$ upper limits on the masses of $<19.1\ M_\oplus$, $<19.5\ M_\oplus$, $<6.8\ M_\oplus$, and $<15.6\ M_\oplus$ for TOI-782 b, TOI-1448 b, TOI-2120 b, and TOI-2406 b, respectively. The mass-radius relationship of these four sub-Neptunes testifies to the existence of volatile material in their interiors. These four sub-Neptunes, which are located above the so-called ``radius valley'', are likely to retain a significant atmosphere and/or an icy mantle on the core, such as a water world. We find that at least three of the four sub-Neptunes (TOI-782 b, TOI-2120 b, and TOI-2406 b) orbiting M dwarfs older than 1 Gyr, are likely to have eccentricities of $e \sim 0.2-0.3$. The fact that tidal circularization of their orbits is not achieved over 1 Gyr suggests inefficient tidal dissipation in their interiors.

astro-ph.EP↗

arXiv2022

Validation and atmospheric exploration of the sub-Neptune TOI-2136b around a nearby M3 dwarf

The NASA space telescope $TESS$ is currently in the extended mission of its all-sky search for new transiting planets. Of the thousands of candidates that TESS is expected to deliver, transiting planets orbiting nearby M dwarfs are particularly interesting targets since they provide a great opportunity to characterize their atmospheres by transmission spectroscopy. We aim to validate and characterize the new sub-Neptune-sized planet candidate TOI-2136.01 orbiting a nearby M dwarf ($d = 33.36 \pm 0.02$ pc, $T_{eff} = 3373 \pm 108$ K) with an orbital period of 7.852 days. We use TESS data, ground-based multicolor photometry, and radial velocity measurements with the InfraRed Doppler (IRD) instrument on the Subaru Telescope to validate the planetary nature of TOI-2136.01 and estimate the stellar and planetary parameters. We also conduct high-resolution transmission spectroscopy to search for helium in its atmosphere. We confirmed that TOI-2136.01 (now named as TOI-2136b) is a bona fide planet with a planetary radius of $R_p = 2.2 \pm 0.07$ $R_{Earth}$ and a mass of $M_p = 4.7^{+3.1}_{-2.6}$ $M_{Earth}$. We also search for helium 10830 \r{A} absorption lines and place an upper limit on the equivalent width of $<$ 7.8 m\r{A} and on the absorption signal of $<$ 1.44 % with 95 % confidence. TOI-2136b is a sub-Neptune transiting a nearby and bright star (J=10.8) and is a potentially hycean planet, which is a new class of habitable planets with large oceans under a H$_2$-rich atmosphere, making it an excellent target for atmospheric studies to understand the formation, evolution, and habitability of the small planets.

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arXiv2021

Zodiacal Exoplanets in Time (ZEIT) XIII: Planet Orbits and Atmospheres in the V1298 Tau System, a Keystone in Studies of Early Planetary Evolution

Studies of planetary systems of stars in star-forming regions and young clusters open a window on the formative stages of planetary evolution. We obtained high-cadence high-resolution infrared spectroscopy of the solar-mass Taurus association-member V1298 Tau during a transit of its 10R$_{\oplus}$-size "b" planet. We measured the systemic radial velocity and find that the kinematics of V1298 Tau suggest an affiliation with a $\gtrsim$6 Myr-old subgroup. A comparison of V1298 Tau and the nearby, co-moving star 2M0405 with stellar evolution models suggests an age of $\sim$10-25 Myr. We measured the projected spin-orbit angle of "b" as $\lambda=15_{-16}^{+15}$ and $\lambda = 2_{-4}^{+12}$ degrees using the apparent RV shift and change in line profile, respectively, induced by the transient occultation of the rotating star by the planet. These values indicate a prograde orbit like that of the interior "c" planet of V1298 Tau and point to a co-planar multi-planet system that formed within a disk. We also measured variation in the strength of the 1083 nm triplet of neutral orthohelium as a probe of any extended/escaping atmosphere around "b". We detect a steady decrease in absorption over the transit which appears to arise from the star or its planetary system. While this variation could be ascribed to "b" or possibly to the immediately preceding transit of "d", we cannot rule out that this is due to rapid variation in the stellar disk-integrated flux in the triplet. The amplitude of variation ($\sim$0.04 nm) is consistent with moderate estimates of atmospheric escape driven by XUV radiation from the central star.

astro-ph.EP↗

arXiv2021

Zodiacal Exoplanets in Time (ZEIT) XII: A Directly-Imaged Planetary-Mass Companion to a Young Taurus M Dwarf Star

We report the discovery of a resolved (0".9) substellar companion to a member of the 1-5 Myr Taurus star-forming region. The host star (2M0437) is a single mid-M type ($T_{eff}\approx$3100K) dwarf with a position, space motion, and color-magnitude that support Taurus membership, and possible affiliation with a $\sim$2.5 Myr-old sub-group. A comparison with stellar models suggests a 2-5 Myr age and a mass of 0.15-0.18M$_{\odot}$. Although K2 detected quasi-periodic dimming from close-in circumstellar dust, the star lacks detectable excess infrared emission from a circumstellar disk and its H$\alpha$ emission is not commensurate with accretion. Astrometry based on three years of AO imaging shows that the companion (2M0437b) is co-moving, while photometry of two other sources at larger separation indicates they are likely heavily-reddened background stars. A comparison of the luminosity of 2M0437b with models suggests a mass of 3-5$M_{\rm jup}$, well below the deuterium burning limit, and an effective temperature of 1400-1500K, characteristic of a late L spectral type. The $H$-$K$ color is redder than the typical L dwarf, but comparable to other directly detected young planets, e.g. those around HR 8799. The discovery of a super-Jupiter around a very young, very low mass star challenges models of planet formation by either core accretion (which requires time) or disk instability (which requires mass). We also detected a second, co-moving, widely-separated (75") object which appears to be a heavily-extincted star. This is certainly a fellow member of this Taurus sub-group and statistically likely to be a bound companion.

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arXiv2020

Discovery of a hot, transiting, Earth-sized planet and a second temperate, non-transiting planet around the M4 dwarf GJ 3473 (TOI-488)

We present the confirmation and characterisation of GJ 3473 b (G 50--16, TOI-488.01), a hot Earth-sized planet orbiting an M4 dwarf star, whose transiting signal ($P=1.1980035\pm0.0000018\mathrm{\,d}$) was first detected by the Transiting Exoplanet Survey Satellite (TESS). Through a joint modelling of follow-up radial velocity observations with CARMENES, IRD, and HARPS together with extensive ground-based photometric follow-up observations with LCOGT, MuSCAT, and MuSCAT2, we determined a precise planetary mass, $M_b = 1.86\pm0.30\,\mathrm{M_\oplus},$ and radius, $R_b = {1.264\pm0.050}\,\mathrm{R_\oplus}$. Additionally, we report the discovery of a second, temperate, non-transiting planet in the system, GJ 3473 c, which has a minimum mass, $M_c \sin{i} = {7.41\pm0.91}\,\mathrm{M_\oplus,}$ and orbital period, $P_c={15.509\pm0.033}\,\mathrm{d}$. The inner planet of the system, GJ 3473 b, is one of the hottest transiting Earth-sized planets known thus far, accompanied by a dynamical mass measurement, which makes it a particularly attractive target for thermal emission spectroscopy.

astro-ph.EP↗

arXiv2020

Zodiacal Exoplanets in Time. XI. The Orbit and Radiation Environment of the Young M Dwarf-Hosted Planet K2-25b

M dwarf stars are high-priority targets for searches for Earth-size and potentially Earth-like planets, but their planetary systems may form and evolve in very different circumstellar environments than those of solar-type stars. To explore the evolution of these systems, we obtained transit spectroscopy and photometry of the Neptune-size planet orbiting the ~650 Myr-old Hyades M dwarf K2-25. An analysis of the variation in spectral line shape induced by the Doppler "shadow" of the planet indicate that the planet's orbit is closely aligned with the stellar equator (lambda = -1.7+5.8/-3.7 deg), and that an eccentric orbit found by previous work could arise from perturbations by another planet on a co-planar orbit. We detect no significant variation in the depth of the He I line at 1083 nm during transit. A model of atmospheric escape as a isothermal Parker wind with a solar composition show that this non-detection is not constraining compared to escape rate predictions of ~0.1 Mearth/Gyr; at such rates, at least several Gyr are required for a Neptune-like planet to evolve into a rocky super-Earth.

astro-ph.EP↗

arXiv2020

Enhanced exciton-exciton collisions in an ultra-flat monolayer MoSe2 prepared through deterministic flattening

Squeezing bubbles and impurities out of interlayer spaces by applying force through a few-layer graphene capping layer leads to van der Waals heterostructures with ultra-flat structure free from random electrostatic potential arising from charged impurities. Without the graphene capping layer, a squeezing process with an AFM tip induces applied-force-dependent charges of n ~ 2 x 10^12 cm^-2 uN^-1, resulting in strong intensity of trions in photoluminescence spectra of MoSe2 at low temperature. We found that a hBN/MoSe2/hBN prepared with the "modified nano-squeezing method" shows a strong excitonic emission with negligible trion peak, and the residual linewidth of the exciton peak is only 2.2 meV, which is comparable to the homogeneous limit. Furthermore, in this high-quality sample, we found that formation of biexciton occurs even at extremely low excitation power (Phi ~ 2.3 x 10^19 cm^-2 s^-1) due to the enhanced collisions between excitons.

cond-mat.mes-hall↗

arXiv2020

Exciton diffusion in a hBN-encapsulated monolayer MoSe2

Excitons, quasi particles composed of an electron and a hole, play an important role in optical responses in low-dimensional nanostructures. In this work, we have investigated exciton diffusion in a monolayer MoSe2 encapsulated between flakes of hexagonal boron nitrides (hBN/MoSe2/hBN). Through PL imaging and numerical solving the 2D diffusion equation, we revealed that temperature dependence of exciton mobility in the hBN/MoSe2/hBN shows non-saturating increase at low temperature, which is qualitatively different from those of quantum wells composed of compound semiconductors. Ultraflat structure of monolayer MoSe2 in the hBN/MoSe2/hBN probably leads to the suppression of charged-impurity scattering and surface-roughness scattering.

cond-mat.mes-hall↗

arXiv2018

Size effects on supercooling phenomena in strongly correlated electron systems: IrTe$_2$ and $\theta$-(BEDT-TTF)$_2$RbZn(SCN)$_4$

We report that the sample miniaturization of first-order-phase-transition bulk systems causes a greater degree of supercooling. From a theoretical perspective, the size effects can be rationalized by considering two mechanisms: (i) the nucleation is a rare and stochastic event, and thus, its rate is correlated with the volume and/or surface area of a given sample; (ii) when the sample size decreases, the dominant heterogeneous nucleation sites that play a primary role for relatively large samples are annealed out. We experimentally verified the size effects on the supercooling phenomena for two different types of strongly correlated electron systems: the transition-metal dichalcogenide IrTe$_2$ and the organic conductor $\theta$-(BEDT-TTF)$_2$RbZn(SCN)$_4$. The origin of the size effects considered in this study does not depend on microscopic details of the material; therefore, they may often be involved in the first-order-transition behavior of small-volume specimens.

cond-mat.str-el↗

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.

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arXiv2016

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↗

arXiv2016

AC transport and full-counting statistics of molecular junctions in the weak electron-vibration coupling regime

The coupling of the charge carriers passing through a molecule bridging two bulky conductors with local vibrational modes of the molecule, gives rise to distinct features in the electronic transport properties on one hand, and to nonequilibrium features in the vibrations' properties, e.g., their population, on the other. Here we explore theoretically a generic model for a molecular junction biased by an arbitrary dc voltage in the weak-coupling regime. We analyze the signature of the electron-vibration interaction on the full-counting statistics of the current fluctuations (i.e., the cumulant generating-function of the current correlations), we give a detailed account of the response to an ac field exerted on the junction (on top of the dc bias voltage), we study the nonequilibrium distribution of the vibrational modes and the fluctuations they cause in the displacement of the molecule center of mass. The calculations use the technique of nonequilibrium Green's functions, and treat the electron-vibration coupling in perturbation theory, within the random-phase approximation when required.

cond-mat.mes-hall↗

arXiv2015

Phase-change memory function of correlated electrons in organic conductors

Phase-change memory (PCM), a promising candidate for next-generation non-volatile memories, exploits quenched glassy and thermodynamically stable crystalline states as reversibly switchable state variables. We demonstrate PCM functions emerging from a charge-configuration degree of freedom in strongly correlated electron systems. Non-volatile reversible switching between a high-resistivity charge-crystalline (or charge-ordered) state and a low-resistivity quenched state, charge glass, is achieved experimentally via heat pulses supplied by optical or electrical means in organic conductors $\theta$-(BEDT-TTF)$_2$$X$. Switching that is one order of magnitude faster is observed in another isostructural material that requires faster cooling to kinetically avoid charge crystallization, indicating that the material's critical cooling rate can be useful guidelines for pursuing a faster correlated-electron PCM function.

cond-mat.str-el↗

arXiv2014

Systematic Variations in the Charge-Glass-Forming Ability of Geometrically Frustrated $\theta$-(BEDT-TTF)$_2X$ Organic Conductors

The critical cooling rate $R_{\rm c}$ above which charge ordering is kinetically avoided upon cooling, which results in charge-glass formation, was investigated for the geometrically frustrated system $\theta$-(BEDT-TTF)$_2X$. X-ray diffraction experiments revealed that $\theta$-(BEDT-TTF)$_2$TlCo(SCN)$_4$ exhibits a horizontally charge-ordered state, and kinetic avoidance of this state requires rapid cooling of faster than 150 K/min. This value is markedly higher than that reported for two other isostructural $\theta$-type compounds, thus demonstrating the lower charge-glass-forming ability of $X$ $=$ TlCo(SCN)$_4$. In accounting for the systematic variations of $R_{\rm c}$ among the three $\theta$-(BEDT-TTF)$_2X$, we found that stronger charge frustration leads to superior charge-glass former. Our results suggest that charge frustration tends to slow the kinetics of charge ordering.

cond-mat.str-el↗

arXiv2012

Full-counting statistics for molecular junctions: Fluctuation theorem and singularities

We study the full-counting statistics of charges transmitted through a single-level quantum dot weakly coupled to a local Einstein phonon which causes fluctuations in the dot energy. An analytic expression for the cumulant generating function, accurate up to second order in the electron-phonon coupling and valid for finite voltages and temperatures, is obtained in the extended wide-band limit. The result accounts for nonequilibrium phonon distributions induced by the source-drain bias voltage, and concomitantly satisfies the fluctuation theorem. Extending the counting field to the complex plane, we investigate the locations of possible singularities of the cumulant generating function, and exploit them to identify regimes in which the electron transfer is affected differently by the coupling to the phonons. Within a large-deviation analysis, we find a kink in the probability distribution, analogous to a first-order phase transition in thermodynamics, which would be a unique hallmark of the electron-phonon correlations. This kink reflects the fact that although inelastic scattering by the phonons once the voltage exceeds their frequency can scatter electrons opposite to the bias, this will never generate current flowing against the bias at zero temperature, in accordance with the fluctuation theorem.

cond-mat.mes-hall↗

arXiv2011

Effects of coupling to vibrational modes on the ac conductance of molecular junctions

We theoretically examine the effect of the coupling of the transport electrons to a vibrational mode of the molecule on the ac linear-response conductance of molecular junctions. Representing the molecule by a single electronic state, we find that at very low temperatures the frequency-dependent conductance is mainly enhanced (suppressed) by the electron-vibration interaction when the chemical potential is below (above) the energy of that state. The vertex corrections of the electron-vibration interaction induce an additional peak structure in the conductance, which can be observed by tuning the tunnel couplings with the leads.

cond-mat.mes-hall↗

arXiv2010

Phonon Spectroscopy by Electric Measurements of Coupled Quantum Dots

We propose phonon spectroscopy by electric measurements of the low-temperature conductance of coupled-quantum dots, specifically employing dephasing of the quantum electronic transport by the phonons. The setup we consider consists of a T-shaped double-quantum-dot (DQD) system in which only one of the dots (dot 1) is connected to external leads and the other (dot 2) is coupled solely to the first one. For noninteracting electrons, the differential conductance of such a system vanishes at a voltage located in-between the energies of the bonding and the anti-bonding states, due to destructive interference. When electron-phonon (e-ph) on the DQD is invoked, we find that, at low temperatures, phonon emission taking place on dot 1 does not affect the interference, while phonon emission from dot 2 suppresses it. The amount of this suppression, as a function of the bias voltage, follows the effective e-ph coupling reflecting the phonon density of states and can be used for phonon spectroscopy.

cond-mat.mes-hall↗