arXiv subjects

T. Yanagisawa

Publications and source records attributed to T. Yanagisawa.

At least 19 recordsLinked to original sources

arXiv2025

Hybridized and Localized 4f Electronic States of Nd-based Intermetallic Compounds in Cubic Symmetry Probed by High-Energy Photoemission

We have performed soft and hard X-ray photoemission spectroscopies on NdTi2Al20 and NdBe13 which show antiferromagnetic ordering at low temperatures. The Nd 3d core-level photoemission and Nd 3d-4f valence-band resonant photoemission spectra show finite 4f4 initial-state components in addition to the 4f3 configurations attributed to the c-f hybridization effects in NdTi2Al20, while the 4f3 initial-state components with localized character are dominant in NdBe13. These results imply the emergence of overscreening channel due to the two-channel Kondo effect in NdTi2Al20 through the the strong c-f hybridization effect.

cond-mat.str-el↗

arXiv2025

Spin-strain interactions under hydrostatic pressure in $α$-RuCl$_3$

We investigate the effects of hydrostatic pressure on $α$-RuCl$_3$, a prototypical material for the Kitaev spin model on a honeycomb lattice with a possible spin-liquid ground state. Using ultrasound measurements at pressures up to 1.16 GPa, we reveal significant modifications of the acoustic properties and the $H$-$T$ phase diagram of this material. Hydrostatic pressure suppresses the three-dimensional magnetic order and induces a dimerization transition at higher pressures. At low pressures, the sound attenuation exhibits a linear temperature dependence, while above 0.28 GPa, it becomes nearly temperature independent, suggesting a shift in the phonon scattering regime dominated by Majorana fermions. These findings provide new insights into spin-strain interactions in Kitaev magnets and deliver a detailed characterization of the $H$-$T$ phase diagram of $α$-RuCl$_3$ under hydrostatic pressure.

cond-mat.str-el↗

arXiv2021

Electric Quadrupolar Contributions in the Magnetic Phases of UNi$_4$B

We present acoustic signatures of the electric quadrupolar degrees of freedom in the honeycomb-layer compound UNi$_4$B. The transverse ultrasonic mode $C_{66}$ shows softening below 30 K both in the paramagnetic phase and antiferromagnetic phases down to $\sim0.33$ K. Furthermore, we traced magnetic field-temperature phase diagrams up to 30 T and observed a highly anisotropic elastic response within the honeycomb layer. These observations strongly suggest that $Γ_6$(E$_{\rm 2g}$) electric quadrupolar degrees of freedom in localized $5f^2$ ($J = 4$) states are playing an important role in the magnetic toroidal dipole order and magnetic-field-induced phases of UNi$_4$B, and evidence some of the U ions remain in the paramagnetic state even if the system undergoes magnetic toroidal ordering.

cond-mat.str-el↗

arXiv2019

Unusual phase boundary of the magnetic-field-tuned valence transition in CeOs$_4$Sb$_{12}$

The phase diagram of the filled skutterudite CeOs$_4$Sb$_{12}$ has been mapped in fields $H$ of up to 60 T and temperatures $T$ down to 0.5 K using resistivity, magnetostriction, and MHz conductivity. The valence transition separating the semimetallic, low-$H$, low-$T$, $\cal{L}$ phase from the metallic high-$H$, high-$T$ $\cal{H}$ phase exhibits a very unusual, wedge-shaped phase boundary, with a non-monotonic gradient alternating between positive and negative. This is quite different from the text-book "elliptical" phase boundary usually followed by valence transitions. Analysis of Shubnikov-de Haas oscillations within the $\cal{H}$ phase reveals an effective mass that increases as $H$ drops toward the $\cal{H-L}$ phase boundary, suggesting proximity to a quantum-critical point. The associated magnetic fluctuations may be responsible for the anomalous $H,T$ dependence of the valence transition at high $H$, whereas the low$-H$, high$-T$ portion of the phase boundary may rather be associated with the proximity of CeOs$_4$Sb$_{12}$ to a topological semimetal phase induced by uniaxial stress.

cond-mat.str-el↗

arXiv2019

Muon spin rotation and relaxation in Pr$_{1-x}$Nd$_x$Os$_4$Sb$_{12}$: superconductivity and magnetism in Pr-rich alloys

The Pr-rich end of the alloy series Pr$_{1-x}$Nd$_x$Os$_4$Sb$_{12}$ has been studied using muon spin rotation and relaxation. The end compound PrOs$_4$Sb$_{12}$ is an unconventional heavy-fermion superconductor, which exhibits a spontaneous magnetic field in the superconducting phase associated with broken time-reversal symmetry. No spontaneous field is observed in the Nd-doped alloys for x $>$ 0.05. The superfluid density is insensitive to Nd concentration, and no Nd$^{3+}$ static magnetism is found down to the lowest temperatures of measurement. Together with the slow suppression of the superconducting transition temperature with Nd doping, these results suggest anomalously weak coupling between Nd spins and conduction-band states.

cond-mat.supr-con↗

arXiv2019

Evidence for the Single-Site Quadrupolar Kondo Effect in the Dilute non-Kramers System Y$_{1-x}$Pr$_x$Ir$_2$Zn$_{20}$

Acoustic signatures of the single-site quadrupolar Kondo effect in Y$_{0.966}$Pr$_{0.034}$Ir$_2$Zn$_{20}$ are presented. The elastic constant ($C_{11}-C_{12}$)/2, corresponding to the $Γ_3$(E)-symmetry electric-quadrupolar response, reveals a logarithmic temperature dependence of the quadrupolar susceptibility in the low-magnetic-field region below $\sim$0.3 K. Furthermore, the Curie-type divergence of the elastic constant down to $\sim$1 K indicates that the Pr ions in this diluted system have a non-Kramers ground-state doublet. These observations evidence the single-site quadrupolar Kondo effect, as previously suggested based on specific-heat and electrical resistivity data.

cond-mat.str-el↗

arXiv2018

Magnetoelastic phenomena in antiferromagnetic uranium intermetallics: the $\mathrm{UAu_{2}Si_{2}}$ case

Thermal expansion, magnetostriction and magnetization measurements under magnetic field and hydrostatic pressure were performed on a $\mathrm{UAu_{2}Si_{2}}$ single crystal. They revealed a large anisotropy of magnetoelastic properties manifested by prominent length changes leading to a collapse of the unit-cell volume accompanied by breaking the fourfold symmetry (similar to that in $\mathrm{URu_{2}Si_{2}}$ in the hidden-order state) in the antiferromagnetic state as consequences of strong magnetoelastic coupling. The magnetostriction curves measured at higher temperatures confirm a bulk character of the 50 K weak ferromagnetic phase. The large positive pressure change of the ordering temperature predicted from Ehrenfest relation contradicts the more than an order of magnitude smaller pressure dependence observed by the magnetization and specific heat measured under hydrostatic pressure. A comprehensive magnetic phase diagram of $\mathrm{UAu_{2}Si_{2}}$ in magnetic field applied along the $c$ axis is presented. The ground-state antiferromagnetic phase is suppressed by a field-induced metamagnetic transition that changes its character from the second to the first order at the tricritical point.

cond-mat.str-el↗

arXiv2018

Superconductors that do not expel magnetic flux

An ultrathin superconducting bilayer creates a coreless fractional vortex when only the second layer has a hole. The quantization is broken by the hole, and the normal core disappears. The magnetic flux is no longer confined near the normal core, and its density profile around the hole becomes similar to that of a crème caramel; the divergence of the magnetic flux density is truncated around the center. We propose basic design of a practical device to realize a coreless fractional vortex.

cond-mat.supr-con↗

arXiv2018

Search for Multipolar Instability in URu$_2$Si$_2$ Studied by Ultrasonic Measurements under Pulsed Magnetic Field

The elastic properties of URu$_2$Si$_2$ in the high-magnetic field region above 40 T, over a wide temperature range from 1.5 to 120 K, were systematically investigated by means of high-frequency ultrasonic measurements. The investigation was performed at high magnetic fields to better investigate the innate bare 5$f$-electron properties, since the unidentified electronic thermodynamic phase of unknown origin, so called `hidden order'(HO) and associated hybridization of conduction and $f$-electron ($c$-$f$ hybridization) are suppressed at high magnetic fields. From the three different transverse modes we find contrasting results; both the $Γ_4$(B$_{\rm 2g}$) and $Γ_5$(E$_{\rm g}$) symmetry modes $C_{66}$ and $C_{44}$ show elastic softening that is enhanced above 30 T, while the characteristic softening of the $Γ_3$(B$_{\rm 1g}$) symmetry mode $(C_{11}-C_{12})/2$ is suppressed in high magnetic fields. These results underscore the presence of a hybridization-driven $Γ_3$(B$_{\rm 1g}$) lattice instability in URu$_2$Si$_2$. However, the results from this work cannot be explained by using existing crystalline-electric field (CEF) schemes applied to the quadrupolar susceptibility in a local $5f^2$ configuration. Instead, we present an analysis based on a band Jahn-Teller effect.

cond-mat.str-el↗

arXiv2017

Uncompensated Antiferromagnetic Ordering of UAu$_2$Si$_2$ Studied by $^{29}$Si-NMR

The magnetically ordered phase of body-centered tetragonal UAu$_2$Si$_2$ was explored through $^{29}$Si-NMR experiments. The field-swept NMR spectra show asymmetric peak splitting below magnetic phase transition temperature, $T_{\rm m}$ = 20 K. This splitting is well explained by the occurrence of a magnetic order with propagation vector of $q$ = (2/3, 0, 0) and magnetic moments pointing parallel to the tetragonal [001] axis, offering evidence for uncompensated "up-up-down"-type antiferromagnetic ordering with a spontaneous [001] magnetization component. A symmetry analysis of the hyperfine-coupling tensor with phenomenological corrections including orbital and dipolar-field contributions yields the magnitude of the ordered moment to be approximately 1.4 $\pm$ 0.2 Bohr magneton per uranium ion, and evaluates the site-dependent hyperfine-coupling constants.

cond-mat.str-el↗

arXiv2016

Study of Localized Character of 4f Electrons and Ultrasonic Dispersions in SmOs4Sb12 by High-Pressure High-Frequency Ultrasonic Measurements

We present high-frequency ultrasonic measurements on the filled skutterudite SmOs$_4$Sb$_{12}$ under hydrostatic pressure. The results clarify that the 4$f$ electrons in this compound transform from delocalized at ambient pressure to localized at high pressures with a crossover pressure of approximately 0.7 GPa. This drastic change in the 4$f$ electrons under pressure is apparently related to the non-Fermi liquid state, which appears in an intermediate-pressure range of 0.5-1.5 GPa. The results or our analysis strongly suggest that the ferro-octupolar interaction becomes dominant at high pressure. Moreover, we report the pressure dependence of the ultrasonic dispersion, which is due to rattling, over a wide range of ultrasonic frequencies up to 323 MHz. The drastic change in the ultrasonic dispersions and the frequency dependent elastic anomaly in the $C_{11}$ mode at lower temperatures imply a possible coupling between rattling phonons and 4$f$ electrons.

cond-mat.str-el↗

arXiv2014

Experimental observation of a first-order phase transition below the superconducting transition temperature in the multilayer cuprate superconductor HgBa$_2$Ca$_4$Cu$_5$O$_y$

A first-order phase transition is found in the multilayer cuprate superconductor, HgBa$_2$Ca$_4$Cu$_5$O$_y$ (Hg-1245), with a superconducting transition temperature of 108 K, under zero magnetic field. We observed a hysteretic specific heat jump around 41 K. We conclude that the Bardeen-Cooper-Schrieffer pairs have a residual entropy due to fluctuations in the phase difference between the five CuO$_2$ planes in a unit cell of Hg-1245, and that this fluctuation freezes below the first-order phase transition temperature.

cond-mat.supr-con↗

arXiv2014

Muon spin rotation and relaxation in Pr$_{1-x}$Nd$_x$Os$_4$Sb$_{12}$: Paramagnetic states

Positive-muon ($μ^+$) Knight shifts have been measured in the paramagnetic states of Pr$_{1-x}$Nd$_x$Os$_4$Sb$_{12}$ alloys, where $x =$ 0, 0.25, 0.45, 0.50, 0.55, 0.75, and 1.00. In Pr-substituted NdOs$_4$Sb$_{12}$ ($x \le$ 0.75), but not in NdOs$_4$Sb$_{12}$, Clogston-Jaccarino plots of $μ^+$ Knight shift~$K$ versus magnetic susceptibility~$χ$ exhibit an anomalous saturation of $K(χ)$ at $\sim-$0.5% for large susceptibilities (low temperatures), indicating a reduction of the coupling strength between $μ^+$ spins and $4f$ paramagnetism for temperatures $\lesssim$ 15~K. We speculate that itinerant Pr$^{3+}$ quadrupolar excitations, invoked to mediate the superconducting Cooper-pair interaction, might modify the $μ^+$-$4f$ ion indirect spin-spin interaction.

cond-mat.str-el↗

arXiv2014

Muon spin rotation and relaxation in Pr$_{1-x}$Nd$_x$Os$_4$Sb$_{12}$: Magnetic and superconducting ground states

Muon spin rotation and relaxation ($μ$SR) experiments have been carried out to characterize magnetic and superconducting ground states in the Pr$_{1-x}$Nd$_x$Os$_4$Sb$_{12}$ alloy series. In the ferromagnetic end compound NdOs$_4$Sb$_{12}$ the spontaneous local field at positive-muon ($μ^+$) sites below the ordering temperature $T_C$ is greater than expected from dipolar coupling to ferromagnetically aligned Nd$^{3+}$ moments, indicating an additional indirect RKKY-like transferred hyperfine mechanism. For 0.45 $\le x \le$ 0.75, $μ^+$ spin relaxation rates in zero and weak longitudinal applied fields indicate that static fields at $μ^+$ sites below $T_C$ are reduced and strongly disordered. We argue this is unlikely to be due to reduction of Nd$^{3+}$ moments, and speculate that the Nd$^{3+}$-$μ^+$ interaction is suppressed and disordered by Pr doping. In an $x$ = 0.25 sample, which is superconducting below $T_c$ = 1.3 K, there is no sign of "spin freezing" (static Nd$^{3+}$ magnetism), ordered or disordered, down to 25 mK. Dynamic $μ^+$ spin relaxation is strong, indicating significant Nd-moment fluctuations. The $μ^+$ diamagnetic frequency shift and spin relaxation in the superconducting vortex-lattice phase decrease slowly below $T_c$, suggesting pair breaking and/or possible modification of Fermi-liquid renormalization by Nd spin fluctuations. For 0.25 $\le x \le$ 0.75, the $μ$SR data provide evidence against phase separation; superconductivity and Nd$^{3+}$ magnetism coexist on the atomic scale.

cond-mat.str-el↗

arXiv2013

Hybridization-Driven Orthorhombic Lattice Instability in URu2Si2

We have measured the elastic constant (C11-C12)/2 in URu2Si2 by means of high-frequency ultrasonic measurements in pulsed magnetic fields H || [001] up to 61.8 T in a wide temperature range from 1.5 to 116 K. We found a reduction of (C11-C12)/2 that appears only in the temperature and magnetic field region in which URu2Si2 exhibits a heavy-electron state and hidden-order. This change in (C11-C12)/2 appears to be a response of the 5f-electrons to an orthorhombic and volume conservative strain field ε_xx-ε_yy with Γ3-symmetry. This lattice instability is likely related to a symmetry-breaking band instability that arises due to the hybridization of the localized f electrons with the conduction electrons, and is probably linked to the hidden-order parameter of this compound.

cond-mat.str-el↗

arXiv2013

Soft confinement in a 3-d spin system

We consider a 1+3 dimensional spin system. The spin-wave (magnon) field is described by the O(3) non-linear sigma model with a symmetry-breaking potential. This interacts with a slow spin SU(2) doublet Schrodinger fermion. The interaction is described by a generalized nonperturbative Yukawa coupling, and the self-consistency condition is solved with the aid of a non-relativistic Gribov equation. When the Yukawa coupling is sufficiently strong, the solution exhibits supercriticality and soft confinement, in a way that is quite analogous to Gribov's light-quark confinement theory. The solution corresponds to a new type of spin polaron, whose condensation may lead to exotic superconductivity.

hep-ph↗

arXiv2012

On ground-state instability in two-dimensional antiferromagnetic systems

We discuss the stability of the antiferromagnetic ground state in two spatial dimensions. We start with a general analysis, based on Gribov's current-conservation techniques, of the bosonic modes in systems with magnetic order. We argue that the Goldstone $ϕ$ and Higgs $h$ modes mix in antiferromagnetic systems, and this leads to an effective $hhϕ$ three-point interaction. We then analyze the instability of the antiferromagnetic system in two spatial dimensions by studying the non-perturbative behaviour of the Higgs boson self-energy using the Dyson--Schwinger equations. The ground state turns out to be unstable for all values of the three-point coupling. We interpret this as being due to the formation of a (high-$T_C$) superconducting condensate. The carrier doping dependence of the energy gap has a general behaviour that is consistent with high-$T_C$ superconductivity. Superconductivity co-exists with antiferromagnetic order for large magnetization, or small doping.

cond-mat.supr-con↗

arXiv2011

Consequences of current conservation in systems with partial magnetic order

We discuss the consequences of spin current conservation in systems with SU(2) spin symmetry that is spontaneously broken by partial magnetic order, using a momentum-space approach. The long-distance interaction is mediated by Goldstone magnons, whose interaction is expressed in terms of the electron Green's functions. There is also a Higgs mode, whose excitation energy can be calculated. The case of fast magnons obeying linear dispersion relation in three spatial dimensions admits nonperturbative treatment using the Gribov equation, and the solution exhibits singular behaviour which has an interpretation as a tower of spin-1 electronic excitations. This occurs near the Mott insulator state. The electrons are more free in the case of slow magnons, where the perturbative corrections are less singular at the thresholds. We then turn our attention to the problem of high-Tc superconductivity, through the discussion of the stability of the antiferromagnetic ground state in two spatial dimensions. We argue that this is caused by an effective mixing of the Goldstone and Higgs modes, which in turn is caused by an effective Goldstone-boson condensation. The instability of the antiferromagnetic system is analyzed by studying the non-perturbative behaviour of the Higgs boson self-energy using the Dyson-Schwinger equations.

cond-mat.supr-con↗