arXiv subjects

K. Miyagawa

Publications and source records attributed to K. Miyagawa.

At least 19 recordsLinked to original sources

arXiv2026

$^{13}$C NMR observation of a nonmagnetic charge-ordered state in the organic conductor $κ$-(ET)$_{2}$Hg(SCN)$_{2}$Cl

We investigated the local magnetism of the organic conductor, $κ$-(ET)$_{2}$Hg(SCN)$_{2}$Cl, with a quasi-triangular lattice of weakly dimerized molecules through $^{13}$C NMR spectroscopy. The NMR spectra and nuclear relaxation show that charge disproportionation occurs, associated with the metal-insulator transition at 31 K. The relaxation rate indicates that the paramagnetic spins in the insulating phase undergo a transition into a spin-singlet ground state with the emergence of orphan spins, a possible valence-bond-glass state. The present results are in high contrast to the spin-cluster paramagnetism of the electric dipole-liquid candidate, $κ$--(ET)$_{2}$Hg(SCN)$_{2}$Br, having nearly identical material parameters. This fact indicates that these two systems are on the verge between distinct phases in both charge and spin degrees of freedom; a spin-singlet charge-ordered state versus a spin-active Mott insulating state, competing with each other on a triangular lattice of dimerized sites.

cond-mat.str-el↗

arXiv2025

Photoinduced Frustration Modulation in $κ$-type Quantum Spin Liquid Candidates

Geometric frustration is a key parameter controlling electronic and magnetic properties of quantum spin liquid systems, yet remains challenging to tune. Here, we coherently drive molecular vibrations with midinfrared pulses in two organic quantum spin liquid candidates, the insulating $κ$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$ and the metallic $κ$-(BEDT-TTF)$_4$Hg$_{2.89}$Br$_8$, and probe their electronic response through ultrafast reflectivity measurements. We observe a nonlinear coupling between local molecular vibrations and nonlocal phonons, which is expected to directly modulate the geometric frustration of their triangular lattice. Our findings establish a promising route to dynamically control frustration in nonbipartite quantum materials.

cond-mat.str-el↗

arXiv2023

Contributions of $2π$-exchange, $1π$-exchange, and contact three-body forces in NNLO ChEFT to $_Λ^3$H

Faddeev calculations of hypertriton ($_Λ^3$H) separation energy are performed, incorporating all next-to-next-to-leading-order $Λ$NN three-body forces (3BFs) in chiral effective field theory: $2π$-exchange, $1π$-exchange, and contact interactions. The $1π$-exchange and contact interactions are rewritten in a form suitable for evaluating partial-wave matrix elements. The $Λ$-deuteron folding potentials constructed from these 3BFs are evaluated to demonstrate their contributions to \h3t. The $1π$-exchange interaction provides an attractive effect in which the d-state component of the deuteron wave function plays an important role. The attractive contribution tends to cancel the repulsive ones from the $2π$-exchange and contact 3BFs. Faddeev calculations show that the net effect of the 3BFs to the \h3t separation energy is small in a range between $-5$ to $+20$ keV, depending on the NN interaction used. Although these results are based on speculative low-energy constants, they can serve as a reference for further investigations.

nucl-th↗

arXiv2023

Faddeev Calculation of $_Λ^3$H incorporating 2π-exchange $Λ$NN Interaction

Faddeev calculations of hypertriton ($_Λ^3$H) separation energy are performed, incorporating $2π$-exchange $Λ$NN three-baryon force. Repulsive contributions of the three-baryon force in the order of 20 keV are found, depending on the NN interactions employed. The effect is not negligible compared with the small separation-energy of $_Λ^3$H.

nucl-th↗

arXiv2022

Partial-wave expansion of $ΛNN$ three-baryon interactions in chiral effective field theory

An expression of partial wave expansion of three-baryon interactions in chiral effective field theory is presented. The derivation follows the method by Hebeler et al. [Phys. Rev. C{\bf 91}, 044001 (2015)], but the final expression is more general. That is, a systematic treatment of the higher-rank spin-momentum structure of the interaction becomes possible. Using the derived formula, a $Λ$-deuteron folding potential is evaluated. This information is valuable for inferring the possible contribution of the $ΛNN$ three-baryon forces to the hypertriton as the basis of further studies by sophisticated Faddeev calculations. A microscopic understanding of $ΛNN$ three-baryon forces together with two-body $ΛN$ interactions is essential for the description of hypernuclei and neutron-star matter.

nucl-th↗

arXiv2022

Reduced superfluid density in a doped spin liquid candidate

A quantum spin liquid (QSL) would be an exotic stage for superconductivity. A promising candidate for a doped QSL is the organic triangular-lattice system, kappa-(BEDT-TTF)4Hg2:89Br8, which hosts a non-Fermi liquid and magnetism of a QSL nature and shows superconductivity upon cooling. Here, we report that its superfluid density is anomalously reduced, indicating the existence of substantial incoherent spectral weight and weak superconducting phase stiffness. Moreover, the ratio of the superconducting transition temperature to the nominal Fermi temperature is as large as 0.1, orders of magnitude beyond typical BCS values. These observations in a system free from competing orders that complicate the similar issue in underdoped cuprates give a clue to the enigmatic missing superfluid density in doped Mott insulators.

cond-mat.supr-con↗

arXiv2022

Thermoelectric signature of quantum critical phase in a doped spin liquid candidate

Quantum spin liquid is a nontrivial magnetic state of longstanding interest, in which spins are strongly correlated and entangled but do not order1, 2; further intriguing is its doped version, which possibly hosts strange metal and unconventional superconductivity3. Promising and currently the only candidate of the doped spin liquid is a triangular-lattice organic conductor, kappa-(BEDT-TTF)4Hg2.89Br8, recently found to hold metallicity, spin-liquid-like magnetism and BEC-like superconductivity4-6. The nature of the metallic state with the spin-liquid behaviour is awaiting to be further clarified. Here, we report the thermoelectric signature that mobile holes in the spin liquid background is in a quantum critical state and it pertains to the BEC-like superconductivity. The Seebeck coefficient divided by temperature, S/T, is enhanced on cooling with logarithmic divergence indicative of quantum criticality. Furthermore, the logarithmic enhancement is correlated with the superconducting transition temperature under pressure variation, and the temperature and magnetic field profile of S/T upon the superconducting transition change with pressure in a consistent way with the previously suggested BEC-BCS crossover. The present results reveal that the quantum criticality in a doped spin liquid emerges in a phase, not at a point, and is involved in the unconventional BEC-like nature.

cond-mat.str-el↗

arXiv2022

Mott-driven BEC-BCS crossover in a doped spin liquid candidate, kappa-(BEDT-TTF)4Hg2.89Br8

The pairing of interacting fermions leading to superfluidity has two limiting regimes: the Bardeen-Cooper-Schrieffer (BCS) scheme for weakly interacting degenerate fermions and the Bose-Einstein condensation (BEC) of bosonic pairs of strongly interacting fermions. While the superconductivity that emerges in most metallic systems is the BCS-like electron pairing, strongly correlated electrons with poor Fermi liquidity can condense into the unconventional BEC-like pairs. Quantum spin liquids harbor extraordinary spin correlation free from order and the superconductivity that possibly emerges by carrier doping of the spin liquids is expected to have a peculiar pairing nature. The present study experimentally explores the nature of the pairing condensate in a doped spin-liquid candidate material and under varying pressure, which changes the electron-electron Coulombic interactions across the Mott critical value in the system. The transport measurements reveal that the superconductivity at low pressures is a BEC-like condensate from a non-Fermi liquid and crosses over to a BCS-like condensate from a Fermi liquid at high pressures. The Nernst-effect measurements distinctively illustrate the two regimes of the pairing in terms of its robustness to the magnetic field. The present Mott tuning of the BEC-BCS crossover can be compared to the Feshbach tuning of the BEC-BCS crossover of fermionic cold atoms.

cond-mat.supr-con↗

arXiv2022

Experimental Verification of Charge Soliton Excitations in the Ionic Mott-Peierls Ferroelectric, TTF-CA

Strong coupling of charge, spin, and lattice in solids brings about emergent elementary excitations with their intertwining and, in one dimension, solitons are known as such. The charge-transferred organic ferroelectric, TTF-CA, has been argued to host charge solitons; however, the existence of the charge solitons remains unverified. Here, we demonstrate that the charge-transport gap in the ionic Mott-Peierls insulating phase of TTF-CA is an order of magnitude smaller than expected from quasiparticle excitations, however, being entirely consistent with the charge soliton excitations. We further suggest that charge and spin solitons move with similar diffusion coefficients in accordance with their coexistence. These results provide a basis for the thermal excitations of the emergent solitons.

cond-mat.str-el↗

arXiv2021

$Ξ$ hyper-nuclear states predicted by NLO chiral baryon-baryon interactions

The $Ξ$ single-particle potential obtained in nuclear matter with the next-to-leading order baryon-baryon interactions in chiral effective field theory is applied to finite nuclei by an improved local-density approximation method. As a premise, phase shifts of $ΞN$ elastic scattering and the results of Faddeev calculations for the $ΞNN$ bound state problem are presented to show the properties of the $ΞN$ interactions in the present parametrization. First, the $Ξ$ states in $^{14}$N are revisited because of the recent experimental progress, including the discussion on the $ΞN$ spin-orbit interaction that is relevant to the location of the $p$-state. Then the $Ξ$ levels in $^{56}$Fe are calculated. In particular, the level shift which is expected to be measured experimentally in the near future is predicted. The smallness of the imaginary part of the $Ξ$ single-particle potential is explicitly demonstrated.

nucl-th↗

arXiv2021

A phase diagram for light-induced superconductivity in $κ$-(ET)$_2$-X

Resonant optical excitation of certain molecular vibrations in $κ$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Br has been shown to induce transient superconducting-like optical properties at temperatures far above equilibrium $T_c$. Here, we report experiments across the bandwidth-tuned phase diagram of this class of materials, and study the Mott insulator $κ$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Cl and the metallic compound $κ$-(BEDT-TTF)$_2$Cu(NCS)$_2$. We find non-equilibrium photoinduced superconductivity only in $κ$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Br, indicating that the proximity to the Mott insulating phase and possibly the presence of preexisting superconducting fluctuations are pre-requisites for this effect.

cond-mat.supr-con↗

arXiv2021

A Realistic Approach to the $ΞNN$ Bound-State Problem based on Faddeev Equation

The Faddeev equations for the $ΞNN$ bound-state problem are solved where the three $S$=$-2$ baryon-baryon interactions of Jülich-Bonn-München chiral EFT, HAL QCD and Nijmegen ESC08c are used. The $T$-matrix $T_{ΞN, ΞN}$ obtained within the original $ΛΛ$-$ΞN$-$ΣΣ$ $/$ $ΞN$-$ΛΣ$-$ΣΣ$ coupled-channel framework is employed as an input to the equations. We found no bound state for Jülich-Bonn-München chiral EFT and HAL QCD but ESC08c generates a bound state with the total isospin and spin-parity $(T,J^π)=(1/2, 3/2^+)$ where the decays into $ΛΛN$ are suppressed.

nucl-th↗

arXiv2020

Magnetic excitations in an ionic spin-chain system with a non-magnetic ferroelectric instability

Cross-correlation between magnetism and dielectric is expected to offer novel emergent phenomena. Here, magnetic excitations in the organic donor-acceptor spin-chain system, TTF-BA, with a ferroelectric ground state is investigated by $^1$H-NMR spectroscopy. A nonmagnetic transition with a ferroelectric order is marked by sharp drops in NMR shift and nuclear spin relaxation rate $T_1^{-1}$ at 53 K. Remarkably, the analyses of the NMR shift and $T_1^{-1}$ dictate that the paramagnetic spin susceptibility in TTF-BA is substantially suppressed from that expected for the 1D Heisenberg spins. We propose that the spin-lattice coupling and the ferroelectric instability cooperate to promote precursory polar singlet formation in the ionic spin system with a nonmagnetic ferroelectric instability.

cond-mat.mtrl-sci↗

arXiv2020

Fate of a soliton matter upon symmetry-breaking ferroelectric order

In a one-dimensional (1D) system with degenerate ground states, their domain boundaries, dubbed solitons, emerge as topological excitations often carrying unconventional charges and spins; however, the soliton excitations are only vital in the non-ordered 1D regime. Then a question arises; how do the solitons conform to a 3D ordered state? Here, using a quasi-1D organic ferroelectric, TTF-CA, with degenerate polar dimers, we pursue the fate of a spin-soliton charge-soliton composite matter in a 1D polar-dimer liquid upon its transition to a 3D ferroelectric order by resistivity, NMR and NQR measurements. We demonstrate that the soliton matter undergoes neutral spin-spin soliton pairing and spin-charge soliton pairing to form polarons, coping with the 3D order. The former contributes to the magnetism through triplet excitations whereas the latter carries electrical current. Our results reveal the whole picture of a soliton matter that condenses into the 3D ordered state.

cond-mat.mtrl-sci↗

arXiv2020

Photo-molecular high temperature superconductivity

Superconductivity in organic conductors is often tuned by the application of chemical or external pressure. With this type of tuning, orbital overlaps and electronic bandwidths are manipulated, whilst the properties of the molecular building blocks remain virtually unperturbed.Here, we show that the excitation of local molecular vibrations in the charge-transfer salt $κ-(BEDT-TTF)_2Cu[N(CN)_2]Br$ induces a colossal increase in carrier mobility and the opening of a superconducting-like optical gap. Both features track the density of quasi-particles of the equilibrium metal, and can be achieved up to a characteristic coherence temperature $T^* \approxeq 50 K$, far higher than the equilibrium transition temperature $T_C = 12.5 K$. Notably, the large optical gap achieved by photo-excitation is not observed in the equilibrium superconductor, pointing to a light induced state that is different from that obtained by cooling. First-principle calculations and model Hamiltonian dynamics predict a transient state with long-range pairing correlations, providing a possible physical scenario for photo-molecular superconductivity.

cond-mat.supr-con↗

arXiv2018

Faddeev approach to the reaction $K^- d \to πΣn$ at $p_{K} = 1$ GeV/c

The reaction $K^-d \to πΣn$ is studied within a Faddeev-type approach, with emphasis on the specific kinematics of the E31 experiment at J-PARC, i.e. $K^-$ beam momentum of $p_K = 1$ GeV/c and neutron angle of $θ_n=0^\circ$. The employed Faddeev approach requires as main input amplitudes for the two-body subsystems $\bar KN \to \bar KN$ and $\bar KN \to πΣ$. For the latter results from recently published chiral unitary models of the $\bar KN$ interaction are utilized. The $\bar KN \to \bar KN$ amplitude itself, however, is taken from a recent partial-wave analysis. Due to the large incoming momentum of the $K^-$, the $\bar KN$ interaction is probed in a kinematical regime where those chiral potentials are no longer applicable. A comparison of the predicted spectrum for various $πΣ$ charge channels with preliminary data is made and reveals a remarkable agreement as far as the magnitude and the line shape in general is concerned. Noticeable differences observed in the $πΣ$ spectrum around the $\bar KN$ threshold, i.e. in the region of the $Λ$(1405) resonance, indicate a sensitivity to the details of the employed $\bar KN \to πΣ$ amplitudes and suggest that pertinent high-precision data could indeed provide substantial constraints on the structure of the $Λ$(1405).

nucl-th↗

arXiv2017

Resonant inelastic x-ray scattering probes the electron-phonon coupling in the spin-liquid kappa-(BEDT-TTF)2Cu2(CN)3

Resonant inelastic x-ray scattering at the N K edge reveals clearly resolved harmonics of the anion plane vibrations in the kappa-(BEDT-TTF)2Cu2(CN)3 spin-liquid insulator. Tuning the incoming light energy at the K edge of two distinct N sites permits to excite different sets of phonon modes. Cyanide CN stretching mode is selected at the edge of the ordered N sites which are more strongly connected to the BEDT-TTF molecules, while positionally disordered N sites show multi-mode excitation. Combining measurements with calculations on an anion plane cluster permits to estimate the sitedependent electron-phonon coupling of the modes related to nitrogen excitation.

cond-mat.mtrl-sci↗

arXiv2016

Spin-charge-entangled non-Fermi liquid in a candidate material for a doped spin liquid

Quantum spin liquids are exotic Mott insulators that carry extraordinary spin excitations and thus, when doped, expected to afford novel metallic states coupled to the unconventional magnetic excitations. The organic triangular-lattice system k-(ET)4Hg2.89Br8 is a promising candidate for the doped spin-liquid and hosts a non-Fermi liquid at low pressures. We show that, in the non-Fermi liquid regime, the charge transport confined in the layer gets deconfined sharply at low temperatures, coinciding with the entrance of spins into a quantum regime as signified by a steep decrease in spin susceptibility behaving like the triangular-lattice Heisenberg model indicative of spin-charge separation at high temperatures. This suggests a new type of non-Fermi liquid, where interlayer charge-deconfimement is associated with spin-charge entanglement.

cond-mat.str-el↗