arXiv subjects

S. Kitagawa

Publications and source records attributed to S. Kitagawa.

At least 19 recordsLinked to original sources

arXiv2026

Role of d-electron density of states in the quantum size effect of Pt-Ni and Pt-Pd nanoparticles

We investigated the quantum size effect (QSE) in bimetallic Pt$_{1-x}$Pd$_x$ and Pt$_{1-x}$Ni$_x$ nanoparticles, using $^{195}$Pt nuclear magnetic resonance measurements. The temperature and size dependencies of the anomaly in the nuclear spin-lattice relaxation rate divided by temperature $1/T_1T$ in the Pt$_{1-x}$Pd$_x$ nanoparticles suggest similar electron states between Pt and Pd atoms and are well understood by the QSE. The temperature and composition variations of $1/T_1T$ and Knight shift reveal a systematic increase in the density of states and reduction of the characteristic energy scale $T^*$ with increasing Ni content, consistent with the Kubo gap $\delta_{\mathrm{Kubo}}$. In contrast to Pt$_{1-x}$Cu$_x$ nanoparticles where the QSE is suppressed, the Pt$_{1-x}$Ni$_x$ nanoparticles exhibit clear signatures of quantum energy discretization. This discrepancy highlights the essential role of $d$-electrons in the manifestation of the QSE. Furthermore, analysis of the modified Korringa parameter $K(\alpha)$ suggests enhanced ferromagnetic correlations with increasing Ni concentration, approaching a ferromagnetic quantum critical regime. These results provide experimental evidence that $d$-electron density of states plays a crucial role in the manifestation of the QSE in the nanoparticles formed by the metallic $d$-electron atoms.

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arXiv2026

Reply to Comment on: Microscopic signatures of an imaginary charge density wave in a kagome metal

We address a recent Comment [I. Nikolov {\it et al.}, arXiv:2608.13579 (2026)] proposing crystalline mosaicity as an alternative explanation for the asymmetric nuclear magnetic resonance (NMR) spectra reported in our study [S. Suetsugu {\it et al.}, Nat. Phys. {\bf 22}, 1251--1256 (2026)]. We show that this scenario requires substantial temperature- and site-dependent distributions of crystallographic orientations and additional site-dependent distributions of the electric field gradient (EFG) asymmetry parameter, none of which follow from ordinary crystalline mosaicity. We therefore conclude that crystalline mosaicity cannot account for the observed site-selective spectral asymmetry and does not provide an alternative explanation for the central spectroscopic observation underlying our interpretation.

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arXiv2026

Microscopic evidence for imaginary charge density wave in a kagome metal

Dissipationless charge transport without any energy loss is one of the most fascinating phenomena in condensed matter physics. This extraordinary state manifests in two well-established systems: superconductors and quantum Hall systems. A proposed third category is associated with chiral loop current order, characterized by the spontaneous formation of microscopic electric current loops. The microscopic origin of these currents stems from imaginary hopping terms, conceptualized as an imaginary charge density wave (iCDW). Despite extensive investigations, its existence remains highly controversial. Here we report site-selective spectroscopic evidence for a pure iCDW in the kagome nonmagnetic metal CsV$_3$Sb$_5$. Nuclear quadrupole resonance spectra at out-of-plane $^{121}$Sb site sensitive to in-plane currents reveal anomalous broadening below $T^*\approx$120 K, coinciding with the nematic transition well above the real charge density wave (CDW). Under magnetic fields, the spectra exhibit asymmetric lineshapes, demonstrating that this broadening purely originates from magnetic effects rather than from electric quadrupolar effects associated with CDW fluctuations. The observed lineshapes are quantitatively consistent with ~1 mT local fields induced by chiral loop currents, indicating spontaneous time-reversal symmetry breaking. This microscopic identification of the long-sought pure iCDW establishes a novel form of quantum order, potentially revolutionizing our understanding of exotic electronic states in quantum materials.

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arXiv2025

Superconducting Properties on Two-dimensional Quasicrystal (Ta$_{0.95}$Cu$_{0.05}$)$_{1.6}$Te Studied with $^{125}$Te-NMR

Physical properties in the normal and superconducting (SC) state are investigated with $^{125}$Te-nuclear magnetic resonance (NMR) measurements in a quasicrystal $\mathrm{(Ta_{0.95}Cu_{0.05})_{1.6}Te}$, which was a recently discovered superconductor with the SC transition temperature $T_{\mathrm{c}}$ = 0.94 K. The nuclear spin-lattice relaxation rate $1/T_1$ shows a coherence peak just below $T_{\mathrm{c}}$, followed by an exponential decrease down to 0.1 K. The overall temperature dependence of $1/T_1$ is in good agreement with an $s$-wave SC model with a SC gap slightly smaller than the BCS value. However, the coherence peak is unusually small, which may be attributable to a reduced Bogoliubov peak theoretically predicted for quasicrystals. Furthermore, $^{125}$Te-NMR spectra show almost no broadening nor shift in the SC state, suggesting that an unusual SC state such as parity mixing might be realized in the Ta$_{1.6}$Te superconductor.

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arXiv2024

Pressure evolution of the normal- and superconducting-state properties of the line-nodal material CaSb$_2$ revealed by $^{123}$Sb nuclear quadrupole resonance

CaSb$_2$ is the Dirac line-nodal material that exhibits a superconducting (SC) transition at 1.7 K. In spite of its conventional SC state at ambient pressure, the transition temperature $T_{\mathrm{c}}$ shows a peak structure against hydrostatic pressure. We performed ac magnetic susceptibility and $^{123}$Sb nuclear quadrupole resonance (NQR) measurements on single-crystalline CaSb$_2$ under pressures up to 2.08 GPa. $T_{\mathrm{c}}$ monotonically increased in this pressure region, which is consistent with a previous study. We observed continuous broadening of the NQR spectrum against pressure, which is a sign of unique compression behavior of the lattice. In the normal state, the nuclear spin-lattice relaxation rate 1/$T_1$ is proportional to temperature in all pressure values; typical of a metal. However, 1/$T_1T$ in the normal state is independent of pressure, indicating that the density of states at the Fermi energy $N(E_{\mathrm{F}})$, which is one of the parameters governing $T_{\mathrm{c}}$, is insensitive to pressure. From these results, we conclude that $N(E_{\mathrm{F}})$ does not govern the origin of the enhancement in $T_{\mathrm{c}}$. This is unusual for a weak electron-phonon coupling superconductor. In the SC state, we revealed that the SC gap becomes larger and more isotropic under pressure.

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arXiv2023

Change of superconducting character in UTe2induced by magnetic field

UTe2 is a recently discovered spin-triplet superconductor. One of the characteristic features of UTe2 is a magnetic field (H)-boosted superconductivity above 16 T when H is applied exactly parallel to the b axis. To date, this superconducting (SC) state has not been thoroughly investigated, and the SC properties as well as the spin state of this high-H SC (HHSC) phase are not well understood. In this study, we performed AC magnetic susceptibility and nuclear magnetic resonance (NMR) measurements and found that, up to 24.8 T, the HHSC state is intrinsic to UTe2 and quite sensitive to the H angle, and that its SC character is different from that in the low-H SC (LHSC) state. The dominant spin component of the spin-triplet pair is along the a axis in the LHSC state but is changed in the HHSC state along the b axis. Our results indicate that H-induced multiple SC states originate from the remaining spin degrees of freedom.

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arXiv2022

First-Order Antiferromagnetic Transition and Novel Gapless Excitation in a 4f Zigzag Chain Compound YbCuS2

We report on the 63/65Cu-nuclear magnetic resonance and nuclear quadrupole resonance (NQR) studies of trivalent Yb zigzag chain compound YbCuS2. Sharp NQR signals were observed in the paramagnetic (PM) state. Below TO = 0.95 K, the multi peaks induced by the internal magnetic fields arising from the antferromagnetic (AFM) ordered moments appear and coexist with the PM signal down to 0.85 K, evidencing the first-order AFM phase transition at TO. In addition, the nuclear spin-lattice relaxation rate 1/T1 abruptly decreases below TO and shows the T-linear behavior below 0.5 K. The significant large 1/T1T value strongly suggests the presence of the novel gapless spin excitation in low temperature region.

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arXiv2021

Pairing interaction in superconducting UCoGe tunable by magnetic field

The mechanism of unconventional superconductivity, such as high-temperature-cuprate, Fe-based, and heavy-fermion superconductors, has been studied as a central issue in condensed-matter physics. Spin fluctuations, instead of phonons, are considered to be responsible for the formation of Cooper pairs, and many efforts have been made to confirm this mechanism experimentally. Although a qualitative consensus seems to have been obtained, experimental confirmation has not yet been achieved. This is owing to a lack of the quantitative comparison between theory and experiments. Here, we show a semi-quantitative comparison between the superconducting-transition temperature ($T_{\rm SC}$) and spin fluctuations derived from the nuclear magnetic resonance (NMR) experiment on the ferromagnetic (FM) superconductor UCoGe, in which the FM fluctuations and superconductivity are tunable by external fields. The enhancement and abrupt suppression of $T_{\rm SC}$ by applied fields, as well as the pressure variation of $T_{\rm SC}$ around the FM criticality are well understood by the change in the FM fluctuations on the basis of the single-band spin-triplet theoretical formalism. The present comparisons strongly support the theoretical formalism of spin-fluctuation-mediated superconductivity, particularly in UCoGe.

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arXiv2021

Charge neutral fermions and magnetic field driven instability in insulating YbIr$_3$Si$_7$

Materials where localized magnetic moments are coupled to itinerant electrons, the so-called Kondo lattice materials, provide a very rich backdrop for strong electron correlations. They are known to realize many exotic phenomena, including unconventional superconductivity, strange metals, and correlated topological phases of matter. Here, we report what appears to be electron fractionalization in insulating Kondo lattice material YbIr$_3$Si$_7$, with emergent neutral excitations that carry heat but not electric current and contribute to metal-like specific heat. We show that these neutral particles change their properties as the material undergoes a transformation between two antiferromagnetic phases in an applied magnetic field. In the low-field AF-I phase, we find that the low temperature linear specific heat coefficient $γ$ and the residual linear term in the thermal conductivity $κ/T(T\rightarrow 0)$ are finite, demonstrating itinerant gapless excitations. These results, along with a spectacular violation of the Wiedemann-Franz law, directly indicate that YbIr$_3$Si$_7$ is a charge insulator but a thermal metal. Nuclear magnetic resonance spectrum reveals a spin-flop transition to a high field AF-II phase. Near the transition field, $γ$ is significantly enhanced. Most surprisingly, inside the AF-II phase, $κ/T$ exhibits a sharp drop below $\sim300$ mK, indicating either opening of a tiny gap or a linearly vanishing density of states. This finding demonstrates a transition from a thermal metal into an insulator/semimetal driven by the spin-flop magnetic transition. These results suggest that spin degrees of freedom directly couple to the neutral fermions, whose emergent Fermi surface undergoes a field-driven instability at low temperatures.

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arXiv2018

Interplay between unconventional superconductivity and heavy-fermion quantum criticality: CeCu$_2$Si$_2$ versus YbRh$_2$Si$_2$

In this paper the low-temperature properties of two isostructural canonical heavy-fermion compounds are contrasted with regards to the interplay between antiferromagnetic (AF) quantum criticality and superconductivity. For CeCu$_2$Si$_2$, fully-gapped d-wave superconductivity forms in the vicinity of an itinerant three-dimensional heavy-fermion spin-density-wave (SDW) quantum critical point (QCP). Inelastic neutron scattering results highlight that both quantum critical SDW fluctuations as well as Mott-type fluctuations of local magnetic moments contribute to the formation of Cooper pairs in CeCu$_2$Si$_2$. In YbRh$_2$Si$_2$, superconductivity appears to be suppressed at $T\gtrsim~10$ mK by AF order ($T_N$ = 70 mK). Ultra-low temperature measurements reveal a hybrid order between nuclear and 4f-electronic spins, which is dominated by the Yb-derived nuclear spins, to develop at $T_A$ slightly above 2 mK. The hybrid order turns out to strongly compete with the primary 4f-electronic order and to push the material towards its QCP. Apparently, this paves the way for heavy-fermion superconductivity to form at $T_c$ = 2 mK. Like the pressure - induced QCP in CeRhIn$_5$, the magnetic field - induced one in YbRh$_2$Si$_2$ is of the local Kondo-destroying variety which corresponds to a Mott-type transition at zero temperature. Therefore, these materials form the link between the large family of about fifty low-$T$ unconventional heavy - fermion superconductors and other families of unconventional superconductors with higher $T_c$s, notably the doped Mott insulators of the cuprates, organic charge-transfer salts and some of the Fe-based superconductors. Our study suggests that heavy-fermion superconductivity near an AF QCP is a robust phenomenon.

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arXiv2017

Magnetic and superconducting properties on S-type single-crystal CeCu$_2$Si$_2$ probed by $^{63}$Cu nuclear magnetic resonance and nuclear quadrupole resonance

We have performed $^{63}$Cu nuclear magnetic resonance/nuclear quadrupole resonance measurements to investigate the magnetic and superconducting (SC) properties on a "superconductivity dominant" ($S$-type) single crystal of CeCu$_2$Si$_2$. Although the development of antiferromagnetic (AFM) fluctuations down to 1~K indicated that the AFM criticality was close, Korringa behavior was observed below 0.8~K, and no magnetic anomaly was observed above $T_{\rm c} \sim$ 0.6 K. These behaviors were expected in $S$-type CeCu$_2$Si$_2$. The temperature dependence of the nuclear spin-lattice relaxation rate $1/T_1$ at zero field was almost identical to that in the previous polycrystalline samples down to 130~mK, but the temperature dependence deviated downward below 120~mK. In fact, $1/T_1$ in the SC state could be fitted with the two-gap $s_{\pm}$-wave rather than the two-gap $s_{++}$-wave model down to 90~mK. Under magnetic fields, the spin susceptibility in both directions clearly decreased below $T_{\rm c}$, indicative of the formation of spin singlet pairing. The residual part of the spin susceptibility was understood by the field-induced residual density of states evaluated from $1/T_1T$, which was ascribed to the effect of the vortex cores. No magnetic anomaly was observed above the upper critical field $H_{c2}$, but the development of AFM fluctuations was observed, indicating that superconductivity was realized in strong AFM fluctuations.

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arXiv2013

Pressure-Temperature-Magnetic Field Phase Diagram of Ferromagnetic Kondo Lattice CeRuPO

We report the temperature-pressure-magnetic field phase diagram made from electrical resistivity measurements for the ferromagnetic (FM) Kondo lattice CeRuPO. The ground state at zero field changes from the FM state to another state, which is suggested to be an antiferromagnetic (AFM) state, above ~0.7 GPa, and the magnetically ordered state is completely suppressed at ~2.8 GPa. In addition to the collapse of the AFM state under pressure and a magnetic field, a metamagnetic (MM) transition from a paramagnetic state to a polarized paramagnetic state appears. CeRuPO will give us a rich playground for understanding the mechanism of the MM transition under comparable FM and AFM correlations in the Kondo lattice.

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arXiv2013

Normal state spin dynamics in the iron-pnictide superconductors BaFe2(As1-xPx)2 and Ba(Fe1-xCox)2As2 probed with NMR measurements

The NMR results in iron pnictides BaFe2(As1-xPx)2 and Ba(Fe1-xCox)2As2 are analyzed based on the self-consistent renormalization (SCR) spin fluctuation theory. The temperature dependence of the NMR relaxation rate (T_1)^{-1} as well as the electrical resistivity is well reproduced by a SCR model where two-dimensional antiferromagnetic (AF) spin fluctuations are dominant. The successful description of the crossover feature from non-Fermi liquid to Fermi liquid behavior strongly suggests that low-lying spin fluctuations in BaFe2(As1-xPx)2 and Ba(Fe1-xCox)2As2 possess an itinerant AF nature, and that chemical substitution in the two compounds tunes the distance of these systems to an AF quantum critical point. The close relationship between spin fluctuations and superconductivity is discussed compared with the other unconventional superconductors, cuprate and heavy fermion superconductors. In addition, it is suggested that magnetism and lattice instability in these pnictides are strongly linked via orbital degrees of freedom.

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arXiv2013

S wave superconductivity in newly discovered superconductor BaTi$_2$Sb$_2$O revealed by $^{121/123}$Sb-NMR/Nuclear Quadrupole Resonance measurements

We report the $^{121/123}$Sb-NMR/nuclear quadrupole resonance (NQR) measurements on the newly-discovered superconductor BaTi$_2$Sb$_2$O with a two-dimensional Ti$_2$O square-net layer formed with Ti$^{3+}$ (3$d^1$). NQR measurements revealed that the in-plane four-fold symmetry is broken at the Sb site below $T_{\rm A} \sim$ 40 K, without an internal field appearing at the Sb site. These exclude a spin-density wave (SDW)/ charge density wave (CDW) ordering with incommensurate correlations, but can be understood with the commensurate CDW ordering at $T_{\rm A}$. The spin-lattice relaxation rate $1/T_1$, measured at the four-fold symmetry breaking site, decreases below superconducting (SC) transition temperature $T_{\rm c}$, indicative of the microscopic coexistence of superconductivity and the CDW/SDW phase below $T_{\rm A}$. Furthermore, $1/T_1$ of $^{121}$Sb-NQR shows a coherence peak just below $T_{\rm c}$ and decreases exponentially at low temperatures. These results are in sharp contrast with those in cuprate and iron-based superconductors, and strongly suggest that its SC symmetry is classified to an ordinary s-wave state.

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arXiv2012

Universality and Critical Behavior at the Critical-End-Point on Itinerant-Metamagnet UCoAl

We performed nuclear-magnetic-resonance (NMR) measurements on itinerant-electron metamagnet UCoAl in order to investigate the critical behavior of the magnetism near a metamagnetic (MM) critical endpoint (CEP). We derived c-axis magnetization $M_c$ and its fluctuation $S_c$ from the measurements of Knight shift and nuclear spin-lattice relaxation rate $1/T_1$ as a function of the c-axis external field ($H_c$) and temperature ($T$). We developed contour plots of $M_c$ and $S_c$ on the $H_c$ - $T$ phase diagram, and observed the strong divergence of $S_c$ at the CEP. The critical exponents of $M_c$ and $S_c$ near the CEP are estimated, and found to be close to the universal properties of a three-dimensional (3-D) Ising model. We indicate that the critical phenomena at the itinerant-electron MM CEP in UCoAl have a common feature as a gas-liquid transition.

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arXiv2012

Microscopic Evidence of Direct Coupling between Magnetic and Superconducting Order Parameters in BaFe$_2$(As$_{1-x}$P$_x$)$_2$

The coexistence of magnetism and superconductivity in the isovalent-P-substituted BaFe$_2$(As$_{1-x}$P$_x$)$_2$ has been investigated microscopically by $^{31}$P-NMR measurements. We found that superconducting (SC) transition occurs in a magnetic region with static ordered moments and that the moments are reduced below the SC transition temperature $T_{\rm c}$ in the samples near the phase boundary of magnetism and superconductivity. Our results indicate that magnetism and superconductivity coexist spatially but compete with each other on the same Fermi surfaces. The coexistence state is qualitatively different from that observed in other unconventional superconductors and gives a strict constraint on the theoretical model for superconductivity in BaFe$_2$(As$_{1-x}$P$_x$)$_2$.

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arXiv2012

Metamagnetic Behavior and Kondo Breakdown in Heavy-Fermion CeFePO

We report that nonmagnetic heavy-fermion (HF) iron oxypnictide CeFePO with two-dimensional XY-type anisotropy shows a metamagnetic behavior at the metamagnetic field H_M \simeq 4 T perpendicular to the c-axis and that a critical behavior is observed around H_M. Although the magnetic character is entirely different from that in other Ce-based HF metamagnets, H_M in these metamagnets is linearly proportional to the inverse of the effective mass, or to the temperature where the susceptibility shows a peak. This finding suggests that H_M is a magnetic field breaking the local Kondo singlet, and the critical behavior around H_M is driven by the Kondo breakdown accompanied by the Fermi-surface instability.

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arXiv2011

Non-magnetic pair-breaking effect on La(Fe_{1-x}Zn_{x})AsO_{0.85} studied by NMR and NQR

$^{75}$As and $^{139}$La NMR and nuclear quadrupole resonance (NQR) studies on Zn-substituted LaFeAsO$_{0.85}$ have been performed to investigate the Zn-impurity effects microscopically. Although superconductivity in LaFeAsO$_{0.85}$ disappears by 3% Zn substitution, we found that NMR/NQR spectra and NMR physical quantities in the normal state are hardly changed, indicating that the crystal structure and electronic states are not modified by Zn substitution. Our results suggest that the suppression of superconductivity by Zn substitution is not due to the change of the normal-state properties, but due to strong non-magnetic pair-breaking effect to superconductivity.

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