arXiv subjects

Yoshifumi Tokiwa

Publications and source records attributed to Yoshifumi Tokiwa.

16 recordsLinked to original sources

arXiv2026

Paramagnetic electron-nuclear spin entanglement in HoCo2Zn20

We investigated electron-nuclear spin entanglement in the paramagnetic ground state of the Ho-based cubic compound HoCo2Zn20. From analyses of magnetization and specific heat data, we determined the cubic crystalline electric field (CEF) parameters, the magnetic exchange constant, and the hyperfine coupling constant between the 4f magnetic moment and the 165Ho nuclear spin. Our results show that the Gamma5 CEF ground state is split by the hyperfine coupling, with an energy width of 1.3 K at 0 T, and that the true paramagnetic ground state is a quasi-sextet arising primarily from entanglement between the f-electron effective spin S = 1 and the 165Ho nuclear spin I = 7/2. We further demonstrate that, depending on the CEF parameters, the paramagnetic ground state can switch to an electron-nuclear coupled dectet. These findings underscore the importance of accurately identifying the electron-nuclear level scheme for understanding the low-temperature properties of rare-earth compounds containing spin-active nuclei.

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arXiv2026

Electron-Hole Scattering Dichotomy and Anisotropic Warping in Quasi-Two-Dimensional Fermi Surfaces of UTe2

We present a combined experimental and theoretical study of the detailed Fermi-surface (FS) geometry of UTe2, a heavy-fermion superconductor that has recently attracted considerable attention as a promising candidate for spin-triplet pairing. Using angle-dependent magnetoresistance oscillations, a bulk- and low-energy-sensitive transport probe for quasi-two-dimensional (Q2D) electronic structures, we directly determine the in-plane FS geometry. We found that the Q2D FS exhibits a rectangular cross-sectional shape with strongly anisotropic warping, originating from the hybridization of two orthogonal quasi-one-dimensional bands. Through a quantitative comparison between experiment and theoretical calculations, we further reveal a large electron-hole scattering dichotomy: the quasiparticle lifetime on the electron FS is substantially shorter than that on the hole FS. This dichotomy is naturally explained by anisotropic, low-dimensional antiferromagnetic fluctuations, which selectively enhance scattering on the electron FS. This suggests a dominant role of the electron pockets for the emergence of superconductivity. Our results clarify a direct relation between FS geometry, magnetic fluctuations, and momentum-dependent quasiparticle lifetimes, and thus providing a crucial basis for the microscopic understanding of pairing mechanism, and impose stringent constraints on the gap symmetry of spin-triplet superconductivity in UTe2.

cond-mat.supr-con↗

arXiv2025

Spin dynamics in natural multiferroic pyroxene NaFeSi$_2$O$_6$

Spin dynamics in the natural mineral aegirine, NaFeSi$_2$O$_6$, a member of the pyroxene family, was studied by elastic and inelastic neutron scattering. Magnetization and specific heat measurements as well as single-crystal neutron diffraction maps, taken in the temperature range 2 - 20 K, confirm two successive magnetic transitions at 8.8 and 5.8 K, consistent with previous studies. The observed spin-wave excitations emerge from the incommensurate magnetic Bragg peaks corresponding to the propagation vector $k_{\rm ICM} = (0, 0.77, 0)$, and extend up to energies of about 1.5 meV. In the low-temperature helical phase, the spin dynamics of the Fe$^{3+}$ ions is well described by a simple linear spin-wave model. The observed excitations can be modeled using a spin Hamiltonian that includes three primary exchange interactions - intrachain coupling $J=0.142(2)$ meV, interchain couplings $J_1=0.083(1)$ meV and $J_2=0.186(1)$ meV - and an easy-plane anisotropy $D=0.020(6)$ meV. Our results show that no single exchange interaction dominates the spin dynamics. The similar strengths of the intrachain and interchain couplings point to the fact that the magnetic interactions in aegirine are three-dimensional rather than confined along one direction. As a result, the system cannot be considered quasi-one-dimensional, as previously suggested, and calls for further investigations.

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arXiv2023

Longitudinal spin fluctuations driving field-reinforced superconductivity in UTe$_2$

Our measurements of $^{125}$Te NMR relaxations reveal an enhancement of electronic spin fluctuations above $μ_0H^*\sim15$ T, leading to their divergence in the vicinity of the metamagnetic transition at $μ_0H_m\approx35$ T, below which field-reinforced superconductivity appears when a magnetic field ($H$) is applied along the crystallographic $b$ axis. The NMR data evidence that these fluctuations are dominantly longitudinal, providing a key to understanding the peculiar superconducting phase diagram in $H\|b$, where such fluctuations enhance the pairing interactions.

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arXiv2022

Ferromagnetic Crossover within the Ferromagnetic Order of U$_{7}$Te$_{12}$

We investigate the physical properties of a single crystal of uranium telluride U$_{7}$Te$_{12}$. We have confirmed that U$_{7}$Te$_{12}$ crystallizes in the hexagonal structure with three nonequivalent crystallographic uranium sites. The paramagnetic moments are estimated to be approximately 1 $μ_{\rm B}$ per the uranium site, assuming a uniform moment on all the sites. A ferromagnetic phase transition occurs at $T_{\rm C}=48$ K, where the in-plane magnetization increases sharply, whereas the out-of-plane component does not increase significantly. With decreasing temperature further below $T_{\rm C}$ under field-cooling conditions, the out-of-plane component increases rapidly around $T^{\star}=26$ K. In contrast, the in-plane component hardly changes at $T^{\star}$. Specific heat measurement indicates no $λ$-type anomaly around $T^{\star}$, so this is a cross-over suggesting a reorientation of the ordering moments or successive magnetic ordering on the part of the multiple uranium sites.

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arXiv2022

First Observation of de Haas-van Alphen Effect and Fermi Surfaces in Unconventional Superconductor UTe2

We report the first observation of the de Haas-van Alphen (dHvA) effect in the novel spin-triplet superconductor UTe2 using high quality single crystals with the high residual resistivity ratio (RRR) over 200. The dHvA frequencies, named alpha and beta, are detected for the field directions between c and a-axes. The frequency of branch beta increases rapidly with the field angle tilted from c to a-axis, while branch alpha splits, owing to the maximal and minimal cross-sectional areas from the same Fermi surface. Both dHvA branches, alpha and beta reveal two kinds of cylindrical Fermi surfaces with a strong corrugation at least for branch alpha. The angular dependence of the dHvA frequencies is in very good agreement with that calculated by the generalized gradient approximation (GGA) method taking into account the on-site Coulomb repulsion of U = 2 eV, indicating the main Fermi surfaces are experimentally detected. The detected cyclotron effective masses are large in the range from 32 to 57 m0 . They are approximately 10-20 times lager than the corresponding band masses, consistent with the mass enhancement obtained from the Sommerfeld coefficient and the calculated density of states at the Fermi level. The local density approximation (LDA) calculations of ThTe2 assuming U4+ with the 5f^2 localized model are in less agreement with our experimental results, in spite of the prediction for two cylindrical Fermi surfaces, suggesting a mixed valence states of U4+ and U3+ in UTe2.

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arXiv2022

Slow Electronic Dynamics in the Paramagnetic State of UTe$_2$

$^{125}$Te NMR experiments in field ($H$) applied along the easy magnetization axis (the $a$-axis) revealed slow electronic dynamics developing in the paramagnetic state of UTe$_2$. The observed slow fluctuations are concerned with a successive growth of long-range electronic correlations below 30$-$40 K, where the spin susceptibility along the hard magnetization axis (the $b$-axis) shows a broad maximum. The experiments also imply that tiny amounts of disorder or defects locally disturb the long-range electronic correlations and develop an inhomogeneous electronic state at low temperatures, leading to a low temperature upturn observed in the bulk-susceptibility in $H\|a$. We suggest that UTe$_2$ would be located on the paramagnetic side near an electronic phase boundary, where either magnetic or Fermi-surface instability would be the origin of the characteristic fluctuations.

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arXiv2020

Field evolution of the spin-liquid candidate YbMgGaO$_4$

We report magnetization, heat capacity, thermal expansion, and magnetostriction measurements down to millikelvin temperatures on the triangular antiferromagnet YbMgGaO$_4$. Our data exclude the formation of the distinct $\frac13$ plateau phase observed in other triangular antiferromagnets, but reveal plateaulike features in second derivatives of the free energy, magnetic susceptibility and specific heat, at $μ_0H$ = 1.0 - 2.5 T for $H\parallel{}c$ and 2 - 5 T for $H\perp{}c$. Using Monte-Carlo simulations of a realistic spin Hamiltonian, we ascribe these features to nonmonotonic changes in the magnetization and the $\frac12$ plateau that is smeared out by the random distribution of exchange couplings in YbMgGaO$_4$.

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arXiv2019

Quantum-critical phase out of frustrated magnetism in a strongly correlated metal

Strange-metal phenomena often develop at the border of antiferromagnetic order in strongly correlated metals. It has been well established that they can originate from the fluctuations anchored by the point of continuous quantum phase transition out of the antiferromagnetic order, i.e., a quantum critical point. What has been unclear is how these phenomena can be associated with a potential new phase of matter at zero temperature. Here we show that magnetic frustration of the 4f-local moments in the distorted Kagome intermetallic compound CePdAl gives rise to such a paramagnetic quantum-critical phase. Moreover, we demonstrate that this phase turns into a Fermi liquid through a Mott-like crossover; in a two-dimensional parameter space of pressure and magnetic field, this crossover is linked to a line of zero-temperature 4f-electron localization-delocalization phase transitions at low and moderate pressures. Our discovery motivates a new design principle for strongly correlated metallic states with unconventional excitations that may underlie the development of such effects as high temperature superconductivity.

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arXiv2019

Rearrangement of uncorrelated valence bonds evidenced by low-energy spin excitations in YbMgGaO4

DC-magnetization data measured down to 40 mK speak against conventional freezing and reinstate YbMgGaO$_4$ as a triangular spin-liquid candidate. Magnetic susceptibility measured parallel and perpendicular to the $c$-axis reaches constant values below 0.1 and 0.2 K, respectively, thus indicating the presence of gapless low-energy spin excitations. We elucidate their nature in the triple-axis inelastic neutron scattering experiment that pinpoints the low-energy ($E$ $\leq$ $J_0$ $\sim$ 0.2 meV) part of the excitation continuum present at low temperatures ($T$ $<$ $J_0$/$k_B$), but \emph{completely} disappearing upon warming the system above $T$ $\gg$ $J_0$/$k_B$. In contrast to the high-energy part at $E$ $>$ $J_0$ that is rooted in the breaking of nearest-neighbor valence bonds and persists to temperatures well above $J_0$/$k_B$, the low-energy one originates from the rearrangement of the valence bonds and thus from the propagation of unpaired spins. We further extend this picture to herbertsmithite, the spin-liquid candidate on the kagome lattice, and argue that such a hierarchy of magnetic excitations may be a universal feature of quantum spin liquids.

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arXiv2018

Gapped ground state in the zigzag pseudospin-1/2 quantum antiferromagnetic chain compound PrTiNbO6

We report a single-crystal study on the magnetism of the rare-earth compound PrTiNbO$_6$ that experimentally realizes the zigzag pseudospin-$\frac{1}{2}$ quantum antiferromagnetic chain model. Random crystal electric field caused by the site mixing between non-magnetic Ti$^{4+}$ and Nb$^{5+}$, results in the non-Kramers ground state quasi-doublet of Pr$^{3+}$ with the effective pseudospin-$\frac{1}{2}$ Ising moment. Despite the antiferromagnetic intersite coupling of about 4 K, no magnetic freezing is detected down to 0.1 K, whilst the system approaches its ground state with almost zero residual spin entropy. At low temperatures, a sizable gap of about 1 K is observed in zero field. We ascribe this gap to off-diagonal anisotropy terms in the pseudospin Hamiltonian, and argue that rare-earth oxides open an interesting venue for studying magnetism of quantum spin chains.

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arXiv2018

Absence of zero-point entropy in a triangular Ising antiferromagnet

Frustrated Ising magnets host exotic excitations, such as magnetic monopoles in spin ice. The ground state (GS) in this case is characterized by an extensive degeneracy and associated residual entropy going back to the pioneering work by G. Wannier who established large residual entropy of nearly 50%Rln2 per mole spins in a triangular Ising antiferromagnet (TIAF) already in 1950. Here, we endeavor to verify this result experimentally using TmMgGaO4, a novel rare-earth-based frustrated antiferromagnet with Ising spins arranged on a perfect triangular lattice. Contrary to theoretical expectations, we find almost no residual entropy and ascribe this result to the presence of a weak second-neighbor coupling J2zz ~ 0.09J1zz that lifts the GS degeneracy and gives rise to several ordered states, the stripe order, 1/3-plateau, and 1/2-plateau. TmMgGaO4 gives experimental access to these novel phases of Ising spins on the triangular lattice.

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arXiv2016

Quasiparticle Excitations in the Superconducting State of FeSe Probed by Thermal Hall Conductivity in the Vicinity of the BCS-BEC Crossover

There is growing evidence that the superconducting semimetal FeSe ($T_c\sim8$ K) is in the crossover regime between weak-coupling Bardeen-Cooper-Schrieffer (BCS) and strong-coupling Bose-Einstein-condensate (BEC) limits. We report on longitudinal and transverse thermal conductivities, $κ_{xx}$ and $κ_{xy}$, respectively, in magnetic fields up to 20 T. The field dependences of $κ_{xx}$ and $κ_{xy}$ imply that a highly anisotropic small superconducting gap forms at the electron Fermi-surface pocket whereas a more isotropic and larger gap forms at the hole pocket. Below $\sim1.0$ K, both $κ_{xx}$ and $κ_{xy}$ exhibit distinct anomalies (kinks) at the upper critical field $H_{c2}$ and at a field $H^*$ slightly below $H_{c2}$. The analysis of the thermal Hall angle ($κ_{xy}/κ_{xx}$) indicates a change of the quasiparticle scattering rate at $H^*$. These results provide strong support to the previous suggestion that above $H^*$ a distinct field-induced superconducting phase emerges with an unprecedented large spin imbalance.

cond-mat.supr-con↗

arXiv2011

Electronic Structure of Heavy Fermion Uranium Compounds Studied by Core-Level Photoelectron Spectroscopy

High-energy-resolution core-level and valence-band photoelectron spectroscopic studies were performed for the heavy Fermion uranium compounds UGe2, UCoGe, URhGe, URu2Si2, UNi2Al3, UPd2Al3, and UPt3 as well as typical localized and itinerant uranium compounds to understand the relationship between the uranium valence state and their core-level spectral line shapes. In addition to the main line and high-binding energy satellite structure recognized in the core-level spectra of uranium compounds, a shoulder structure on the lower binding energy side of the main lines of localized and nearly localized uranium compounds was also found. The spectral line shapes show a systematic variation depending on the U 5f electronic structure. The core-level spectra of UGe2, UCoGe, URhGe, URu2Si2, and UNi2Al3 are rather similar to those of itinerant compounds, suggesting that U 5f electrons in these compounds are well hybridized with ligand states. On the other hand, the core-level spectra of UPd2Al3 and UPt3 show considerably different spectral line shapes from those of the itinerant compounds, suggesting that U 5f electrons in UPd2Al3 and UPt3 are less hybridized with ligand states, leading to the correlated nature of U 5f electrons in these compounds. The dominant final state characters in their core-level spectra suggest that the numbers of 5f electrons in UGe2, UCoGe, URhGe, URu2Si2, UNi2Al3, and UPd2Al3 are close to but less than three, while that of UPt3 is close to two rather than to three.

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arXiv2006

Itinerant U 5f band states in the layered compound UFeGa5 observed by soft X-ray angle-resolved photoemission spectroscopy

We have performed angle-resolved photoemission spectroscopy (ARPES) experiments on paramagnetic UFeGa5 using soft X-ray synchrotron radiation (hn=500eV) and derived the bulk- and U 5f-sensitive electronic structure of UFeGa5. Although the agreement between the experimental band structure and the LDA calculation treating U 5f electrons as being itinerant is qualitative, the morphology of the Fermi surface is well explained by the calculation, suggesting that the U 5f states can be essentially understood within the itinerant-electron model.

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arXiv2006

Direct observation of quasi-particle band in CeIrIn$_5$: Angle-resolved photoemission spectroscopy study

We have performed a high-resolution angle resolved Ce 4$d-4f$ resonant photoemission experiment on the heavy fermion superconductor CeIrIn$_5$. We have observed a quasi-particle band which has an energy dispersion of $\sim 30$ meV in the Ce 4$f$ on-resonance spectra. The result suggests that although the 4$f$ spectra are dominated by the localized/correlated character, the small itinerant component is responsible for the superconductivity in this compound.

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