arXiv subjects

Y. Tanabe

Publications and source records attributed to Y. Tanabe.

At least 19 recordsLinked to original sources

arXiv2025

A Unified Maxwell-Bloch Framework for Multi-periodic 6.7 GHz Methanol Flaring in G9.62+0.20E

We analyze a decade of 6.7 GHz methanol monitoring data in G9.62+0.20E, confirming the known periodicities of p1 = 241.3 +/- 2.3 d and p2 = 52.5 +/- 0.3 d, and identifying three new cycles at p3 = 127.0 +/- 1.6 d, p4 = 163.9 +/- 2.9 d, and p5 = 204.1 +/- 1.5 d. The 241.3-d and 204.1-d periods occur in multiple velocity channels, while the others are confined to single components. Despite their diverse morphologies and timescales, all flares can be reproduced within a unified Maxwell-Bloch framework operating in the fast-transient superradiance regime, driven by narrow periodic pump excitations. Model fits yield consistent environmental parameters across periodicities (temperatures, collisional timescales), pointing to broadly uniform physical conditions in the masing region. The discovery of new periodicities and their unified Maxwell-Bloch modeling provide a consistent picture of multi-periodic flaring in G9.62+0.20E and support superradiance as a general framework for maser flaring.

astro-ph.HE↗

arXiv2023

A Keplerian disk with a four-arm spiral birthing an episodically accreting high-mass protostar

High-mass protostars (M$_{\star} >$ 8 M$_{\odot}$) are thought to gain the majority of their mass via short, intense bursts of growth. This episodic accretion is thought to be facilitated by gravitationally unstable and subsequently inhomogeneous accretion disks. Limitations of observational capabilities, paired with a lack of observed accretion burst events has withheld affirmative confirmation of the association between disk accretion, instability and the accretion burst phenomenon in high-mass protostars. Following its 2019 accretion burst, a heat-wave driven by a burst of radiation propagated outward from the high-mass protostar G358.93-0.03-MM1. Six VLBI (very long baseline interferometry) observations of the raditively pumped 6.7 GHz methanol maser were conducted during this period, tracing ever increasing disk radii as the heat-wave propagated outward. Concatenating the VLBI maps provided a sparsely sampled, milliarcsecond view of the spatio-kinematics of the accretion disk covering a physical range of $\sim$ 50 - 900 AU. We term this observational approach `heat-wave mapping'. We report the discovery of a Keplerian accretion disk with a spatially resolved four-arm spiral pattern around G358.93-0.03-MM1. This result positively implicates disk accretion and spiral arm instabilities into the episodic accretion high-mass star formation paradigm.

astro-ph.SR↗

arXiv2023

Modelling of the multi-transition periodic flaring in G9.62+0.20E

We present detailed modeling of periodic flaring events in the 6.7 GHz and 12.2 GHz methanol lines as well as the OH 1665 MHz and 1667 MHz transitions observed in the G9.62+0.20E star-forming region. Our analysis is performed within the framework of the one-dimensional Maxwell-Bloch equations, which intrinsically cover the complementary quasi-steady state maser and transient superradiance regimes. We find that the variations in flaring time-scales measured for the different species/transitions, and sometimes even for a single spectral line, are manifestations of and are best modeled with Dicke's superradiance, which naturally accounts for a modulation in the duration of flares through corresponding changes in the inversion pump. In particular, it can explain the peculiar behaviour observed for some features, such as the previously published result for the OH 1667 MHz transition at $v_\mathrm{lsr}=+1.7$ km s$^{-1}$ as well as the methanol 6.7 GHz line at $v_\mathrm{lsr}=-1.8$ km s$^{-1}$, through a partial quenching of the population inversion during flaring events.

astro-ph.GA↗

arXiv2020

Lattice-Shifted Nematic Quantum Critical Point in FeSe$_{1-x}$S$_x$

We report the evolution of nematic fluctuations in FeSe$_{1-x}$S$_x$ single crystals as a function of Sulfur content $x$ across the nematic quantum critical point (QCP) $x_c\sim$ 0.17 via Raman scattering. The Raman spectra in the $B_{1g}$ nematic channel consist of two components, but only the low energy one displays clear fingerprints of critical behavior and is attributed to itinerant carriers. Curie-Weiss analysis of the associated nematic susceptibility indicates a substantial effect of nemato-elastic coupling which shifts the location of the nematic QCP. We argue that this lattice-induced shift likely explains the absence of any enhancement of the superconducting transition temperature at the QCP. The presence of two components in the nematic fluctuations spectrum is attributed to the dual aspect of electronic degrees of freedom in Hund's metals, with both itinerant carriers and local moments contributing to the nematic susceptibility.

cond-mat.supr-con↗

arXiv2017

Effects of strain on the electronic structure, superconductivity, and nematicity in FeSe studied by angle-resolved photoemission spectroscopy

One of central issues in iron-based superconductors is the role of structural change to the superconducting transition temperature (T_c). It was found in FeSe that the lattice strain leads to a drastic increase in T_c, accompanied by suppression of nematic order. By angle-resolved photoemission spectroscopy on tensile- or compressive-strained and strain-free FeSe, we experimentally show that the in-plane strain causes a marked change in the energy overlap (DeltaE_{h-e}) between the hole and electron pockets in the normal state. The change in DeltaE_{h-e} modifies the Fermi-surface volume, leading to a change in T_c. Furthermore, the strength of nematicity is also found to be characterized by DeltaE_{h-e}. These results suggest that the key to understanding the phase diagram is the fermiology and interactions linked to the semimetallic band overlap.

cond-mat.supr-con↗

arXiv2016

Metal-Insulator Transition and Pseudogap in Bi$_{1.76}$Pb$_{0.35}$Sr$_{1.89}$CuO$_{6+δ}$ High-$T_c$ Cuprates

It is inferred from bulk-sensitive muon Knight shift measurement for a Bi$_{1.76}$Pb$_{0.35}$Sr$_{1.89}$CuO$_{6+δ}$ single-layer cuprate that metal-insulator (MI) transition (in the low temperature limit, $T\rightarrow0$) occurs at the critical hole concentration $p=p_{\rm MI}=0.09(1)$, where the electronic density of states (DOS) at the Fermi level is reduced to zero by the pseudogap irrespective of the Néel order or spin glass magnetism. Superconductivity also appears for $p>p_{\rm MI}$, suggesting that this feature is controlled by the MI transition. More interestingly, the magnitude of the DOS reduction induced by the pseudogap remains unchanged over a wide doping range ($0.1\le p\le0.2$), indicating that the pseudogap remains as a hallmark of the MI transition for $p>p_{\rm MI}$.

cond-mat.str-el↗

arXiv2015

Argument on superconductivity pairing mechanism from cobalt impurity doping in FeSe: spin ($s_{\pm}$) or orbital ($s_{++}$) fluctuation

In high-superconducting transition temperature ($T_{\rm c}$) iron-based superconductors, interband sign reversal ($s_{\rm \pm}$) and sign preserving ($s_{\rm ++}$) $s$-wave superconducting states have been primarily discussed as the plausible superconducting mechanism. We study Co impurity scattering effects on the superconductivity in order to achieve an important clue on the pairing mechanism using single crystal Fe$_{1-x}$Co$_x$Se and depict a phase diagram of a FeSe system. Both superconductivity and structural transition / orbital order are suppressed by the Co replacement on the Fe sites and disappear above $x$ = 0.036. These correlated suppressions represent a common background physics behind these physical phenomena in the multiband Fermi surfaces of FeSe. By comparing experimental data and theories so far proposed, the suppression of $T_{\rm c}$ against the residual resistivity is shown to be much weaker than that predicted in the case of a general sign reversal and a full gap $s_{\pm}$ models. The origin of the superconducting paring in FeSe is discussed in terms of its multiband electronic structure.

cond-mat.supr-con↗

arXiv2015

Enhanced superconducting transition temperature in hyper-interlayer-expanded FeSe despite the suppressed electronic nematic order and spin fluctuations

The superconducting critical temperature, $T_{\rm c}$, of FeSe can be dramatically enhanced by intercalation of a molecular spacer layer. Here we report on a $^{77}$Se, $^7$Li and $^1$H nuclear magnetic resonance (NMR) study of the powdered hyper-interlayer-expanded Li$_{x}($C$_2$H$_8$N$_2$)$_y$Fe$_{2-z}$Se$_2$ with a nearly optimal $T_{\rm c}=45$~K. The absence of any shift in the $^7$Li and $^1$H NMR spectra indicates a complete decoupling of interlayer units from the conduction electrons in FeSe layers, whereas nearly temperature-independent $^7$Li and $^1$H spin-lattice relaxation rates are consistent with the non-negligible concentration of Fe impurities present in the insulating interlayer space. On the other hand, strong temperature dependence of $^{77}$Se NMR shift and spin-lattice relaxation rate, $1/^{77}T_1$, is attributed to the hole-like bands close to the Fermi energy. $1/^{77}T_1$ shows no additional anisotropy that would account for the onset of electronic nematic order down to $T_{\rm c}$. Similarly, no enhancement in $1/^{77}T_1$ due to the spin fluctuations could be found in the normal state. Yet, a characteristic power-law dependence $1/^{77}T_1\propto T^{4.5}$ still comply with the Cooper pairing mediated by spin fluctuations.

cond-mat.supr-con↗

arXiv2015

Emergence of the minority hole with high mobility on the electrical transport in the Fe-pnictides Ba(Fe$_{1-x}$Mn$_x$As)$_2$

In Fe pnictide (Pn) superconducting materials, neither Mn- nor Cr- doping to the Fe site induces superconductivity, even though hole carriers are generated. This is in strong contrast with the superconductivity appearing when holes are introduced by alkali metal substitution on the insulating blocking layers. We investigate in detail the effects of Mn doping on magneto-transport properties in Ba(Fe$_{1-x}$Mn$_x$As)$_2$ for elucidating the intrinsic reason. The negative Hall coefficient for $x$ = 0 estimated in the low magnetic field ($B$) regime gradually increases as $x$ increases, and its sign changes to a positive one at $x$ = 0.020. Hall resistivities as well as simultaneous interpretation using the magnetoconductivity tensor including both longitudinal and transverse transport components clarify that minority holes with high mobility are generated by the Mn doping via spin density wave (SDW) transition at low temperatures, while original majority electrons and holes residing in the parabolic-like Fermi surfaces (FSs) of the semimetallic Ba(FeAs)$_2$ are negligibly affected. Present results indicate that the mechanism of hole doping in Ba(Fe$_{1-x}$Mn$_x$As)$_2$ is greatly different from that of the other superconducting FePns family.

cond-mat.supr-con↗

arXiv2014

Reconstruction of Band Structure Induced by Electronic Nematicity in an FeSe Superconductor

We have performed high-resolution angle-resolved photoemission spectroscopy on FeSe superconductor (Tc ~ 8 K), which exhibits a tetragonal-to-orthorhombic structural transition at Ts ~ 90 K. At low temperature we found splitting of the energy bands as large as 50 meV at the M point in the Brillouin zone, likely caused by the formation of electronically driven nematic states. This band splitting persists up to T ~ 110 K, slightly above Ts, suggesting that the structural transition is triggered by the electronic nematicity. We have also revealed that at low temperature the band splitting gives rise to a van Hove singularity within 5 meV of the Fermi energy. The present result strongly suggests that this unusual electronic state is responsible for the unconventional superconductivity in FeSe.

cond-mat.supr-con↗

arXiv2014

Electric Transport of a Single Crystal Iron Chalcogenide FeSe Superconductor: Evidence of Symmetry Breakdown Nematicity and Additional Ultrafast Dirac cone-Like Carriers

An SDW antiferromagnetic (SDW-AF) low temperature phase transition is generally observe and the AF spin fluctuations are considered to play an important role for the superconductivity paring mechanism in FeAs superconductors. However, a similar magnetic phase transition is not observed in FeSe superconductors, which has caused considerable discussion. We report on the intrinsic electronic states of FeSe as elucidated by transport measurements under magnetic fields using a high quality single crystal. A mobility spectrum analysis, an ab initio method that does not make assumptions on the transport parameters in a multicarrier system, provides very import and clear evidence that another hidden order, most likely the symmetry broken from the tetragonal C4 symmetry to the C2 symmetry nematicity associated with the selective d-orbital splitting, exists in the case of superconducting FeSe other than the AF magnetic order spin fluctuations. The intrinsic low temperature phase in FeSe is in the almost compensated semimetallic states but is additionally accompanied by Dirac cone like ultrafast electrons $\sim$ 10$^4$cm$^2$(VS)$^{-1}$ as minority carriers.

cond-mat.supr-con↗

arXiv2014

Kondo-like mass enhancement of Dirac fermion in iron pnictides Ba(Fe$_{1-x}$Mn$_x$As)$_2$

The effect of Mn substitution, acting as a magnetic impurity for Fe, on the Dirac cone was investigated in Ba(Fe$_{1-x}$Mn$_x$As)$_2$. Both magnetoresistance and Hall resistivity studies clearly indicate that the cyclotron effective mass ($m^{\ast}$) of the Dirac cone is anomalously enhanced at low temperatures by the impurity, although its evolution as a function of carrier number proceeds in a conventional manner at higher temperatures. Kondo-like band renormalization induced by the magnetic impurity scattering is suggested as an explanation for this, and the anomalous mass enhancement of the Dirac fermions is discussed.

cond-mat.supr-con↗

arXiv2012

Suppression of backward scattering of Dirac fermions in iron pnictides Ba(Fe$_{1-x}$Ru$_x$As)$_2$

We report electronic transport of Dirac cones when Fe is replaced by Ru, which has an isoelectronic electron configuration to Fe, using single crystals of Ba(Fe$_{1-x}$Ru$_x$As)$_2$. The electronic transport of parabolic bands is shown to be suppressed by scattering due to the crystal lattice distortion and the impurity effect of Ru, while that of the Dirac cone is not significantly reduced due to the intrinsic character of Dirac cones. It is clearly shown from magnetoresistance and Hall coefficient measurements that the inverse of average mobility, proportional to cyclotron effective mass, develops as the square root of the carrier number (n) of the Dirac cones. This is the unique character of the Dirac cone linear dispersion relationship. Scattering of Ru on the Dirac cones is discussed in terms of the estimated mean free path using experimental parameters.

cond-mat.supr-con↗

arXiv2012

Distinct Fe-induced magnetic states in the underdoped and overdoped regimes of La2-xSrxCu1-yFeyO4 revealed by muon spin relaxation

Zero-field and longitudinal-field muon-spin-relaxation measurements have been performed in partially Fe-substituted La2-xSrxCu1-yFeyO4 in a wide range of hole concentration, to investigate the magnetic state induced by the Fe substitution recently suggested from the neutron-scattering measurements [Phys. Rev. Lett. 107, 127002 (2011)]. It has been found that the magnetic transition temperature is notably enhanced through the 1% Fe substitution in a wide range of hole concentration where superconductivity appears in Fe-free La2-xSrxCuO4. In the underdoped regime, the Fe-induced magnetic order can be understood in terms of the concept of stripe pinning by Fe as in the case of the Zn-induced one in La2-xSrxCu1-yZnyO4. In the overdoped regime, on the other hand, the Fe-induced magnetic order is short-ranged, which is distinct from the stripes. It is plausible that a spin-glass state of Fe spins derived from the Ruderman-Kittel-Kasuya-Yosida interaction is realized in the overdoped regime, suggesting a change of the ground state from the strongly correlated state to the Fermi-liquid state with hole doping in La-214 high-Tc cuprates.

cond-mat.supr-con↗

arXiv2012

Curie-like paramagnetism due to incomplete Zhang-Rice singlet formation in La2-xSrxCuO4

In an effort to elucidate the origin of the Curie-like paramagnetism that is generic for heavily-overdoped cuprates, we have performed high transverse-field muon spin rotation (TF-muSR) measurements of La2-xSrxCuO4 single crystals over the Sr content range 0.145 < x < 0.33. We show that the x-dependence of the previously observed field-induced broadening of the internal magnetic field distribution above the superconducting transition temperature Tc reflects the presence of two distinct contributions. One of these becomes less pronounced with increasing x and is attributed to diminishing antiferromagnetic correlations. The other grows with increasing x, but decreases above x ~ 0.30, and is associated with the Curie-like term in the bulk magnetic susceptibility. In contrast to the Curie-like term, however, this second contribution to the TF-muSR line width extends back into the underdoped regime. Our findings imply a coexistence of antiferromagnetically correlated and paramagnetic moments, with the latter becoming dominant beyond x ~ 0.185. This suggests that the doped holes do not neutralize all Cu spins via the formation of Zhang-Rice singlets. Moreover, the paramagnetic component of the TF-muSR line width is explained by holes progressively entering the Cu 3d_{x^2-y^2} orbital with doping.

cond-mat.supr-con↗

arXiv2011

Coexistence of Dirac-Cone States and Superconductivity in Iron Pnictide Ba(Fe$_{1-x}$Ru$_x$As)$_2$

The Ru doping effect on the Dirac cone states is investigated in iron pnictide superconductors Ba(Fe$_{1-x}$Ru$_x$As)$_2$ using the transverse magnetoresistance (MR) measurements as a function of temperature. The linear development of MR against magnetic field $B$ is observed for $x$ = 0 - 0.244 at low temperatures below the antiferromagnetic transition. The $B$-linear MR is interpreted in terms of the quantum limit of the Dirac cone states by using the model proposed by Abrikosov. An intriguing evidence is shown that the Dirac cone state persists on the electronic phase diagram where the antiferromagnetism and the superconductivity coexist.

cond-mat.supr-con↗

arXiv2011

Development of Cu-spin correlation in Bi_1.74_Pb_0.38_Sr_1.88_Cu_1-y_Zn_y_O_6+d_ high-temperature superconductors observed by muon spin relaxation

A systematic muon-spin-relaxation study in Bi-2201 high-Tc cuprates has revealed for the first time that the Cu-spin correlation (CSC) is developed at low temperatures below 2 K in a wide range of hole concentration where superconductivity appears. The CSC tends to become weak gradually with increasing hole-concentration. Moreover, CSC has been enhanced through the 3% substitution of Zn for Cu. These results are quite similar to those observed in La-214 high-Tc cuprates. Accordingly, it has been suggested that the intimate relation between the so-called spin-charge stripe correlations and superconductivity is a universal feature in hole-doped high-Tc cuprates. Furthermore, apparent development of CSC, which is suppressed through the Zn substitution oppositely, has been observed in non-superconducting heavily overdoped samples, being argued in the context of a recently proposed ferromagnetic state in heavily overdoped cuprates.

cond-mat.supr-con↗

arXiv2011

Similarity between Ni and Zn impurity effects on the superconductivity and Cu-spin correlation in La2-xSrxCu1-yNiyO4 high-Tc cuprates: A comparison based on the hole trapping by Ni

Ni-substitution effects on the superconductivity and Cu-spin correlation have been investigated in La2-xSrxCu1-yNiyO4 from the electrical resistivity and muon spin relaxation measurements, taking into account the hole trapping by Ni recently suggested. It has been found that the Ni-substitution suppresses the superconductivity, induces the localization of holes and develops the Cu-spin correlation to the same degree as the Zn substitution. These suggest that Ni with a trapped hole tends to give rise to potential scattering of holes in the CuO2 plane to the same degree as Zn, being discussed in relation to the so-called dynamical stripe correlations of spins and holes.

cond-mat.supr-con↗