arXiv subjects

Kenji Ishida

Publications and source records attributed to Kenji Ishida.

At least 73 records · Page 4Linked to original sources

arXiv2019

Enhancement of superconductivity by pressure-induced critical ferromagnetic fluctuations in UCoGe

A $^{59}$Co nuclear quadrupole resonance (NQR) was performed on a single-crystalline ferromagnetic (FM) superconductor UCoGe under pressure. The FM phase vanished at a critical pressure $P_c$, and the NQR spectrum just below $P_c$ showed phase separation of the FM and paramagnetic (PM) phases below Curie temperature $T_{\textrm{Curie}}$, suggesting first-order FM quantum phase transition (QPT). We found that the internal field was absent above $P_c$, but the superconductivity is almost unchanged. This result suggests the existence of the nonunitary to unitary transition of the superconductivity around $P_c$. Nuclear spin-lattice relaxation rate $1/T_1$ showed the FM critical fluctuations around $P_c$, which persist above $P_c$ and are clearly related to superconductivity in the PM phase. This FM QPT is understood to be a weak first order with critical fluctuations. $1/T_1$ sharply decreased in the superconducting (SC) state above $P_c$ with a single component, in contrast to the two-component $1/T_1$ in the FM SC state, indicating that the inhomogeneous SC state is a characteristic feature of the FM SC state in UCoGe.

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arXiv2019

Review of U-based Ferromagnetic Superconductors: Comparison between UGe2, URhGe, and UCoGe

The discovery in 2000 that the ferromagnetic (FM) compound UGe2 (T_Curie = 52 K at ambient pressure) becomes superconducting under a pressure of P = 1.1 GPa until it enters the paramagnetic (PM) phase above Pc = 1.6 GPa was a surprise. Successive searches for new materials (URhGe and UCoGe) led to the discovery of the coexistence of superconductivity (SC) and ferromagnetism at ambient pressure. Furthermore in UCoGe, it was found that SC survives in the PM regime from P_c = 1.1 to 4 GPa. Focus has been on low-temperature experiments under extreme conditions of magnetic field (H), pressure, and uniaxial stress. In UGe2, strong interplay exists between Fermi surface (FS) reconstructions in the cascade of different FM and PM ground states and their magnetic fluctuations. Similar phenomena occur in URhGe and UCoGe but, at first glance, the SC seems to be driven by the FM fluctuations. In UCoGe, a longitudinal field scan leads to a drastic decrease in the FM fluctuations, while a transverse field scan leads to suppression of the Curie temperature, T_Curie; the consequence is a boost in FM fluctuations, leading to a reinforcement of SC. The singularity in URhGe is the weakness of the anisotropy between c- and b-axes; the most noteworthy feature is the detection of reentrant SC near H_R. All the experimental results give evidence that the SC in these three materials originates from the FM fluctuations, which are amplitude modes of magnetic excitations in the FM state. Spin-triplet pairing has been anticipated in the FM superconductors and was actually observed by Knight-shift measurements in the SC state of UCoGe. Their fascinating (p, T, H) phase diagrams are now well established. Discussion is presented on how different theoretical approaches can describe the various phenomena discovered by experimentalists.

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arXiv2018

Nematic transition and highly two-dimensional superconductivity in BaTi$_2$Bi$_2$O revealed by $^{209}$Bi-nuclear magnetic resonance/nuclear quadrupole resonance measurements

In this Rapid Communication, a set of $^{209}$Bi-nuclear magnetic resonance (NMR)/nuclear quadrupole resonance (NQR) measurements has been performed to investigate the physical properties of superconducting (SC) BaTi$_2$Bi$_2$O from a microscopic point of view. The NMR and NQR spectra at 5~K can be reproduced with a non-zero in-plane anisotropic parameter $η$, indicating the breaking of the in-plane four-fold symmetry at the Bi site without any magnetic order, i.e., `the electronic nematic state'. In the SC state, the nuclear spin-lattice relaxation rate divided by temperature, $1/T_1T$, does not change even below $T_{\rm c}$, while a clear SC transition was observed with a diamagnetic signal. This observation can be attributed to the strong two-dimensionality in BaTi$_2$Bi$_2$O. Comparing the NMR/NQR results among BaTi$_2$$Pn$$_2$O ($Pn$ = As, Sb, and Bi), it was found that the normal and SC properties of BaTi$_2$Bi$_2$O were considerably different from those of BaTi$_2$Sb$_2$O and BaTi$_2$As$_2$O, which might explain the two-dome structure of $T_{\rm c}$ in this system.

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arXiv2018

Evidence for the presence of the Fulde-Ferrell-Larkin-Ovchinnikov state in CeCu$_2$Si$_2$ revealed using $^{63}$Cu NMR

Nuclear magnetic resonance measurements were performed on CeCu$_{2}$Si$_{2}$ in the presence of a magnetic field close to the upper critical field $μ_{0} H_{\rm c2}$ in order to investigate its superconducting (SC) properties near pair-breaking fields. In lower fields, the Knight shift and nuclear spin-lattice relaxation rate divided by temperature $1/T_1T$ abruptly decreased below the SC transition temperature $T_{\rm c}(H)$, a phenomenon understood within the framework of conventional spin-singlet superconductivity. In contrast, $1/T_1T$ was enhanced just below $T_{\rm c}(H)$ and exhibited a broad maximum when magnetic fields close to $μ_0H_{\rm c2}(0)$ were applied parallel or perpendicular to the $c$ axis, although the Knight shift decreased just below $T_{\rm c}(H)$. This enhancement of $1/T_1T$, which was recently observed in the organic superconductor $κ$-(BEDT-TTF)$_{2}$Cu(NCS)$_{2}$, suggests the presence of high-density Andreev bound states in the inhomogeneous SC region, a hallmark of the Fulde-Ferrell-Larkin-Ovchinnikov phase.

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arXiv2018

Normal-state properties of the antiperovskite oxide Sr$_{3-x}$SnO revealed by $^{119}$Sn-NMR

We have performed $^{119}$Sn-NMR measurements on the antiperovskite oxide superconductor Sr$_{3-x}$SnO to investigate how its normal state changes with the Sr deficiency. A two-peak structure was observed in the NMR spectra of all the measured samples. This suggests that the phase separation tends to occur between the nearly stoichiometric and heavily Sr-deficient Sr$_{3-x}$SnO phases. The measurement of the nuclear spin-lattice relaxation rate $1/T_1$ indicates that the Sr-deficient phase shows a conventional metallic behavior due to the heavy hole doping. In contrast, the nearly stoichiometric phase exhibits unusual temperature dependence of $1/T_1$, attributable to the presence of a Dirac-electron band.

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arXiv2018

Single ferromagnetic fluctuations in UCoGe revealed by 73Ge- and 59Co-NMR studies

$^{73}$Ge and $^{59}$Co nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements have been performed on a $^{73}$Ge-enriched single-crystalline sample of the ferromagnetic superconductor UCoGe in the paramagnetic state. The $^{73}$Ge NQR parameters deduced from NQR and NMR are close to those of another isostructural ferromagnetic superconductor URhGe. The Knight shifts of the Ge and Co sites are well scaled to each other when the magnetic field is parallel to the $b$ or $c$ axis. The hyperfine coupling constants of Ge are estimated to be close to those of Co. The large difference of spin susceptibilities between the $a$ and $b$ axes could lead to the different response of the superconductivity and ferromagnetism with the field parallel to these directions. The temperature dependence of the nuclear spin-lattice relaxation rates $1/T_1$ at the two sites is similar to each other above 5 K. These results indicate that the itinerant U-$5f$ electrons are responsible for the ferromagnetism in this compound, consistent with previous studies. The similarities and differences in the three ferromagnetic superconductors are discussed.

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arXiv2017

Pseudogap Behavior of the Nuclear Spin-lattice Relaxation Rate in FeSe Probed by $^{77}$Se-NMR

We conducted $^{77}$Se-nuclear magnetic resonance studies of the iron-based superconductor FeSe in magnetic fields of 0.6 to 19 T to investigate the superconducting and normal-state properties. The nuclear spin-lattice relaxation rate divided by the temperature $(T_1T)^{-1}$ increases below the structural transition temperature $T_\mathrm{s}$ but starts to be suppressed below $T^*$, well above the superconducting transition temperature $T_\mathrm{c}(H)$, resulting in a broad maximum of $(T_1T)^{-1}$ at $T_\mathrm{p}(H)$. This is similar to the pseudogap behavior in optimally doped cuprate superconductors. Because $T^*$ and $T_\mathrm{p}(H)$ decrease in the same manner as $T_\mathrm{c}(H)$ with increasing $H$, the pseudogap behavior in FeSe is ascribed to superconducting fluctuations, which presumably originate from the theoretically predicted preformed pair above $T_\mathrm{c}(H)$.

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arXiv2017

Spatially Inhomogeneous Superconducting State near $H_{\rm c2}$ in UPd$_2$Al$_3$

We have performed $^{27}$Al-NMR measurements on single-crystalline UPd$_2$Al$_3$ with the field parallel to the $c$ axis to investigate the superconducting (SC) properties near the upper critical field of superconductivity $H_{\rm c2}$. The broadening of the NMR linewidth below 14~K indicates the appearance of the internal field at the Al site, which originates from the antiferromagnetically ordered moments of U 5$f$ electrons. In the SC state well below $μ_0H_{\rm c2}$ = 3.4~T, the broadening of the NMR linewidth due to the SC diamagnetism and a decrease in the Knight shift are observed, which are well-understood by the framework of spin-singlet superconductivity. In contrast, the Knight shift does not change below $T_{\rm c}(H)$, and the NMR spectrum is broadened symmetrically in the SC state in the field range of 3~T $< μ_0 H < μ_0 H_{\rm c2}$. The unusual NMR spectrum near $H_{\rm c2}$ suggests that a spatially inhomogeneous SC state such as the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state would be realized.

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arXiv2017

Magnetic and superconducting properties of the heavy-fermion CeCoIn5 epitaxial film probed by nuclear quadrupole resonance

Since the progress in the fabrication techniques of thin-films of exotic materials such as strongly correlated heavy-fermion compounds, microscopic studies of the magnetic and electronic properties inside the films have been needed. Herein, we report the first observation of 115In nuclear quadrupole resonance (NQR) in an epitaxial film of the heavy-fermion superconductor CeCoIn5, for which the microscopic field gradient within the unit cell as well as magnetic and superconducting properties at zero field are evaluated. We find that the nuclear spin-lattice relaxation rate in the film is in excellent agreement with that of bulk crystals, whereas the NQR spectra show noticeable shifts and significant broadening indicating a change in the electric-field distribution inside the film. The analysis implies a displacement of In layers in the film, which however does not affect the magnetic fluctuations and superconducting pairing. This implies that inhomogeneity of the electronic field gradient in the film sample causes no pair breaking effect.

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arXiv2017

Absence of the Pauli-Paramagnetic Limit in a Superconducting U$_6$Co

We performed $^{59}$Co nuclear magnetic resonance (NMR) measurements of single-crystalline U$_6$Co. There is a small decrease in the Knight shift in the superconducting (SC) state, but this change mainly arises from the SC diamagnetic effect. The negligible change of the spin part of the Knight shift, together with the absence of the Pauli-paramagnetic effect in the SC U$_6$Co, is understood as a consequence of the small spin susceptibility. The nuclear spin-lattice relaxation rate $1/T_1$ is also measured in the SC state under the magnetic field, and exhibits a tiny Hebel-Slichter peak just below the SC transition temperature and exponential behavior at lower temperatures. These behaviors are in agreement with the full-gap s-wave pairing in U$_6$Co.

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arXiv2016

Absence of $^{17}$O Knight-Shift Changes across the First-Order Phase Transition Line in Sr$_2$RuO$_4$

We performed $^{17}$O nuclear magnetic resonance measurements on superconducting (SC) Sr$_2$RuO$_4$ under in-plane magnetic fields. We found that no new signal appears in the SC state and that the $^{17}$O Knight shifts obtained from the double-site measurements remain constant across the first-order phase-transition line, as well as across the second-order phase-transition line as already reported. The present results indicate that the SC spin susceptibility does not decrease in the high-field region, although a magnetization jump in the SC state was reported at low temperatures. Because the spin susceptibility is unchanged in the SC state in Sr$_2$RuO$_4$, we suggest that the first-order phase transition across the upper critical field should be interpreted as a depairing mechanism other than the conventional Pauli-paramagnetic effect.

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arXiv2015

The interface between heavy fermions and normal electrons investigated by spatially-resolved nuclear magnetic resonance

We have studied the superlattices with alternating block layers (BLs) of heavy-fermion superconductor CeCoIn5 and conventional-metal YbCoIn5 by site-selective nuclear magnetic resonance(NMR) spectroscopy, which uniquely offers spatially-resolved dynamical magnetic information. We find that the presence of antiferromagnetic fluctuations is confined to the Ce-BLs, indicating that magnetic degrees of freedom of f-electrons are quenched inside the Yb-BLs. Contrary to simple expectations that the two-dimensionalization enhances fluctuations, we observe that antiferromagnetic fluctuations are rapidly suppressed with decreasing Ce-BL thickness. Moreover, the suppression is more prominent near the interfaces between the BLs. These results imply significant effects of local inversion-symmetry breaking at the interfaces.

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arXiv2015

Effect of Geomagnetism on 101Ru Nuclear Quadrupole Resonance Measurements of CeRu2

We performed 101Ru nuclear quadrupole resonance (NQR) measurements on the s-wave superconductor CeRu2 and found oscillatory behavior in the spin-echo amplitude at the +-1/2 <-> +-3/2 transitions but not at the +-3/2 <-> +-5/2 transitions. The modulation disappears in the superconducting state or in a magnetic shield, which implies a geomagnetic field effect. Our results indicate that the NQR spin-echo decay curve at the +-1/2 <-> +-3/2 transitions is sensitive to a weak magnetic field.

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arXiv2015

Suppression of Nonmagnetic Insulating State by Application of Pressure in Mineral Tetrahedrite Cu$_{12}$Sb$_{4}$S$_{13}$

The mineral tetrahedrite Cu$_{12}$Sb$_{4}$S$_{13}$ exhibits a first-order metal--insulator transition (MIT) at $T_{\rm MI}$ = 85 K and ambient pressure. We measured the $^{63}$Cu-NMR at ambient pressure and the resistivity and magnetic susceptibility at high pressures. $^{63}$Cu-NMR results indicate a nonmagnetic insulating ground state in this compound. The MIT is monotonically suppressed by pressure and disappears at $\sim1.0$ GPa. Two other anomalies are observed in the resistivity measurements, and the pressure -- temperature phase diagram of Cu$_{12}$Sb$_{4}$S$_{13}$ is constructed.

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arXiv2015

Emergence of Orbital Nematicity in the Tetragonal Phase of BaFe$_2$(As$_{1-x}$P$_x$)$_2$

We report on $^{75}$As-NMR measurements in single crystalline BaFe$_{2}$(As$_{0.96}$P$_{0.04}$)$_{2}$ for magnetic fields parallel to the orthorhombic $[110]_{\rm o}$ and $[100]_{\rm o}$ directions above the structural transition temperature $T_{\rm S}\simeq121$ K. A large difference in the linewidth between the two field directions reveals in-plane anisotropy of the electric field gradient, even in the tetragonal phase. This provides microscopic evidence of population imbalance between As-$4p_{x}$ and $4p_{y}$ orbitals, which reaches $|n_x-n_y|/|n_x+n_y| \sim 15$\% at $T\rightarrow T_{\rm S}$ and is a natural consequence of the orbital ordering of Fe-3$d_{xz}$ and $d_{yz}$ electrons. Surprisingly, this orbital polarization is found to be already static near room temperature, suggesting that it arises from the pinning of anisotropic orbital fluctuations by disorder. The effect is found to be stronger below $\sim 160$ K, which coincides with the appearance of nematicity in previous torque and photoemission measurements. These results impose strong constraints on microscopic models of the nematic state.

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arXiv2015

Ferromagnetic Critical Behavior in U(Co$_{1-x}$Fe$_x$)Al ($0 \leq x \leq 0.02$) Studied by $^{59}$Co Nuclear Quadrupole Resonance Measurement

In order to investigate physical properties around a ferromagnetic (FM) quantum transition point and a tricritical point (TCP) in the itinerant-electron metamagnetic compound UCoAl, we have performed the $^{59}$Co nuclear quadrupole resonance (NQR) measurement for the Fe-substituted U(Co$_{1-x}$Fe$_x$)Al ($x$ = 0, 0.5, 1, and 2%) in zero external magnetic field. The Fe concentration dependence of $^{59}$Co-NQR spectra at low temperatures indicates that the first-order FM transition occurs at least above $x$ = 1%. The magnetic fluctuations along the $c$ axis detected by the nuclear spin-spin relaxation rate $1/T_2$ exhibit an anomaly at $T_\mathrm{max}$ $\sim$ 20 K and enhance with increasing $x$. These results are in good agreement with theoretical predictions and indicate the presence of prominent critical fluctuations at the TCP in this system.

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arXiv2014

Anisotropic Uniaxial Pressure Response in UCoAl Studied by Nuclear Magnetic Resonance Measurement

We have performed nuclear quadrupole resonance and nuclear magnetic resonance measurements on UCoAl with strong Ising-type anisotropy under $b$- and $c$-axes uniaxial pressure. In the $b$-axis uniaxial pressure ($P_{\parallel b}$) measurement, we observed an increase in the metamagnetic transition field with increasing $P_{\parallel b}$. In the $c$-axis uniaxial pressure ($P_{\parallel c}$) measurement, on the other hand, we observed a ferromagnetic transition in zero magnetic field along the $c$-axis above $P_{\parallel c}$ = 0.08 GPa. The anomaly of the nuclear spin-lattice relaxation rate divided by the temperature $\left[ (T_1 T)^{-1} \right]$ at $T$ = 20 K is suppressed by $P_{\parallel b}$ and slightly enhanced by $P_{\parallel c}$. The anisotropic uniaxial pressure response indicates that uniaxial pressure is a good parameter for tuning the Ising magnetism in UCoAl.

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arXiv2014

Relationship between Ferromagnetic Criticality and the Enhancement of Superconductivity Induced by Transverse Magnetic Fields in UCoGe

We have performed 59Co NMR experiments on the ferromagnetic superconductor UCoGe under magnetic fields (H) along the a- and b- axes to investigate the relationship between ferromagnetic properties and superconductivity. The ferromagnetic ordering temperature TCurie is suppressed and the nuclear spin-lattice relaxation rate 1/T1 at 2 K is enhanced in H || b, although TCurie and 1/T1 are unchanged in H || a, indicating that the ferromagnetic criticality is induced only when H is applied along the b axis. We show the close relationship between the magnetic anisotropies and the superconducting ones reported by Aoki et al.: the superconductivity is gradually suppressed in H || a, but enhanced in H || b above 5 T. We strongly suggest that the enhancement of the superconductivity observed in H || b originates from the field induced ferromagnetic criticality, as pointed out by Aoki et al and Mineev.

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