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Recently, a superconducting (SC) transition from low-field (LF) to high-field (HF) SC states was reported in CeRh$_2$As$_2$, indicating the existence of multiple SC states. It has been theoretically noted that the existence of two Ce sites in the unit cell, the so-called sub-lattice degrees of freedom owing to the local inversion symmetry breaking at the Ce sites, can lead to the appearance of multiple SC phases even under an interaction inducing spin-singlet superconductivity. CeRh$_2$As$_2$ is considered as the first example of multiple SC phases owing to this sub-lattice degree of freedom. However, microscopic information about the SC states has not yet been reported. In this study, we measured the SC spin susceptibility at two crystallographically inequivalent As sites using nuclear magnetic resonance for various magnetic fields. Our experimental results strongly indicate a spin-singlet state in both SC phases. In addition, the antiferromagnetic phase, which appears within the SC phase, only coexists with the LF SC phase; there is no sign of magnetic ordering in the HF SC phase. The present work reveals unique SC properties originating from the locally noncentrosymmetric characteristics., Comment: 9 pages, 7 figures, including Supplementary Materials, selected as Editors' Suggestion

Spatial inversion symmetry in crystal structures is closely related to the superconducting (SC) and magnetic properties of materials. Recently, several theoretical proposals that predict various interesting phenomena caused by the breaking of the local inversion symmetry have been presented. However, experimental validation has not yet progressed owing to the lack of model materials. Here we present evidence for antiferromagnetic (AFM) order in CeRh₂As₂ (SC transition temperature T[SC]∼0.37 K), wherein the Ce site breaks the local inversion symmetry. The evidence is based on the observation of different extents of broadening of the nuclear quadrupole resonance spectrum at two crystallographically inequivalent As sites. This AFM ordering breaks the inversion symmetry of this system, resulting in the activation of an odd-parity magnetic multipole. Moreover, the onset of antiferromagnetism T[N] within an SC phase, with T[N], 縺（もつ）れ結晶に起因した「超伝導反強磁性」状態を発見. 京都大学プレスリリース. 2022-02-07.

Investigating the pressure dependence of the superconducting (SC) transition temperature $T_{\rm c}$ is crucial for understanding the SC mechanism. Herein, we report on the pressure dependence of $T_{\rm c}$ in the nonmagnetic topological line-nodal material CaSb$_2$, based on measurements of electric resistance and alternating current magnetic susceptibility. $T_{\rm c}$ initially increases with increasing pressure and peaks at $\sim$ 3.1~GPa. With a further increase in pressure, $T_{\rm c}$ decreases and finally becomes undetectable at 5.9~GPa. Because no signs of phase transition or Lifshitz transition are observed in the normal state, the peculiar peak structure of $T_{\rm c}$ suggests that CaSb$_2$ has an unconventional SC character., Comment: 5 pages, 5 figures

To investigate spin susceptibility in a superconducting (SC) state, we measured the $^{125}\mathrm{Te}$-NMR Knight shifts at magnetic fields ($H$) up to 6.5 T along the $b$ and $c$ axes of single-crystal ${\mathrm{UTe}}_{2}$, a promising candidate for a spin-triplet superconductor. In the SC state, the Knight shifts along the $b$ and $c$ axes (${K}_{b}$ and ${K}_{c}$, respectively) decreased slightly, and the decrease in ${K}_{b}$ was almost constant up to 6.5 T. The reduction in ${K}_{c}$ decreased with increasing $H$, and ${K}_{c}$ was unchanged through the SC transition temperature at 5.5 T, excluding the possibility of spin-singlet pairing. Our results indicate that spin susceptibilities along the $b$ and $c$ axes slightly decrease in the SC state in low $H$, and the $H$ response of SC spin susceptibility is anisotropic on the $bc$ plane. We discuss the possible $\mathbit{d}$-vector state within the spin-triplet scenario and suggest that the dominant $\mathbit{d}$-vector component for the case of $H\ensuremath{\parallel}b$ changes above 13 T, where ${T}_{\mathrm{c}}$ increases with increasing $H$.

We performed 121/123Sb-nuclear quadrupole resonance (NQR) measurements on the superconducting (SC) line-nodal material CaSb2 in order to investigate electronic properties in the normal and SC states from a microscopic point of view. In the normal state, the nuclear spin-lattice relaxation rate 1/T1 for the Sb(1) site, which is responsible for the line-nodal parts, is approximately proportional to temperature, indicating the conventional Fermi liquid state. From comparison with band structure calculations, it is considered that the NQR properties related to the line-nodal character are hidden because the conventional behavior originating from Fermi-surface parts away from the nodes is dominant. In the SC state, a clear coherence peak just below the transition temperature and an exponential decrease at lower temperatures were observed in 1/T1. These results strongly suggest that conventional s-wave superconductivity with a full gap is realized in CaSb2., Comment: 6 pages, 5 figures

UTe$_2$ is a recently discovered promising candidate for a spin-triplet superconductor. In contrast to conventional spin-singlet superconductivity, spin-triplet superconductivity possesses spin and angular momentum degrees of freedom. To detect these degrees of freedom and obtain the solid evidence of spin-triplet superconductivity in UTe$_2$, we performed $^{125}$Te-NMR measurement. We previously reported that the shoulder signal appears in NMR spectra below the superconducting (SC) transition temperature $T_{\rm c}$ in $H \parallel b$, and that a slight decrease in the Knight shift along the $b$ and $c$ axes ($K_b$ and $K_c$, respectively) below $T_{\rm c}$ at a low magnetic field $H$. To clarify the origin of the shoulder signal and the trace of the decrease in $K_b$, we compared the $^{125}$Te-NMR spectra obtained when $H~\parallel~b$ and $H~\parallel~c$ and measured the $^{125}$Te-NMR spectra for $H~\parallel~b$ up to 14.5~T. The intensity of the shoulder signal observed for $H~\parallel~b$ has a maximum at $\sim 6$~T and vanishes above 10~T, although the superconductivity is confirmed by the $\chi_{\rm AC}$ measurements, which can survive up to 14.5~T (maximum $H$ in the present measurement). Moreover, the decrease in $K_b$ in the SC state starts to be small around 7~T and almost zero at 12.5~T. This indicates that the SC spin state gradually changes with the application of $H$. Meanwhile, in $H~\parallel~c$, unexpected broadening without the shoulder signals was observed below $T_{\rm c}$ at 1~T, and this broadening was quickly suppressed with increasing $H$. We construct the $H$--$T$ phase diagram for $H~\parallel~b$ and $H~\parallel~c$ based on the NMR measurements and discuss possible SC states with the theoretical consideration. We suggest that the inhomogeneous SC state characterized by the broadening of the NMR spectrum originates from the spin degrees of freedom., Comment: 8 pages, 7 figures

We present nuclear spin-lattice relaxation rate (1/T1) at the Co site and ac-susceptibility results in the normal and superconducting (SC) states of CeCoIn5 for H || c near the SC upper critical field Hc2 above 0.1 K. At 4.2 T, 1/T1 rapidly decreases below the SC transition temperature, consistent with the previous reports. Although the field dependence of 1/T1T at 0.1 K shows a peak at 5.2 T above Hc2, the temperature dependence of 1/T1T at 5.2 T is independent of temperature below 0.2 K, showing a Fermi-liquid behavior. In addition, we found no NMR-spectrum broadening by the appearance of internal fields around Hc2 at 0.1 K. We could not detect any field-induced magnetic instability around Hc2 down to 0.1 K although the remarkable non-Fermi-liquid behavior towards Hc2 was observed in various physical quantities., 6 pages, 3 figures

The phase separation of the ferromagnetic (FM) and paramagnetic (PM) phases in the superconducting (SC) state of UCoGe at the FM critical region was investigated using $^{59}$Co nuclear quadrupole resonance (NQR) technique by taking advantage of its site-selective feature. The NQR measurements revealed that the first-order quantum phase transition occurs between the FM and the PM states. The nuclear spin-lattice relaxation rate $1/T_1$ exhibited a clear drop at the SC state in the PM phase, whereas it was not detected in the FM phase, which indicates that the superconductivity in the FM phase becomes weaker at the FM critical region due to the presence of the PM SC state. This result suggests that the SC condensation energy of the PM SC state is equal or larger than that of the FM SC state in this region. The pressure-temperature phase diagram of UCoGe was modified by taking the results from this study into account., 4 pages, 4 figures

To identify the key parameter for optimal superconductivity in iron pnictides, we measured the $^{31}\mathrm{P}$-NMR relaxation rate on ${\mathrm{BaFe}}_{2}{({\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{P}}_{x})}_{2}\phantom{\rule{4pt}{0ex}}(x=0.22$ and 0.28) under pressure and compared the effects of chemical substitution and physical pressure. For $x=0.22$, structural and antiferromagnetic (AFM) transition temperatures both show minimal changes with pressure up to 2.4 GPa, whereas the superconducting transition temperature ${T}_{\mathrm{c}}$ increases to twice its former value. In contrast, for $x=0.28$ near the AFM quantum critical point (QCP), the structural phase transition is quickly suppressed by pressure and ${T}_{\mathrm{c}}$ reaches a maximum. The analysis of the temperature-dependent nuclear relaxation rate indicates that these contrasting behaviors can be quantitatively explained by a single curve of the ${T}_{\mathrm{c}}$ dome as a function of Weiss temperature $\ensuremath{\theta}$, which measures the distance to the QCP. Moreover, the ${T}_{\mathrm{c}}\text{\ensuremath{-}}\ensuremath{\theta}$ curve under pressure precisely coincides with that with a chemical substitution, which is indicative of the existence of a universal relationship between low-energy AFM fluctuations and superconductivity on ${\mathrm{BaFe}}_{2}{({\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{P}}_{x})}_{2}$.

Quite recently, a pronounced drop of $^{17}$O NMR Knight shift in the superconducting (SC) state of an unstrained Sr$_2$RuO$_4$ was reported by Pustogow and Luo ${\it et al.}$ They revealed such behavior from the free-induction decay (FID) signal after a weak RF pulse. We examined this behavior with our single-crystalline Sr$_2$RuO$_4$, and reproduced their result: the peaks of the $^{17}$O-NMR spectra shift in the SC state as long as the RF-pulse power is smaller than a threshold. Furthermore, we measured the temperature variation of the Knight shift by a standard spin-echo method with small-power RF pulses, and found that the spin susceptibility decreases in the SC state. We conclude that our previous results of the invariance of the Knight shift in the SC state were due to instantaneous destruction of superconductivity by the RF pulses. The heat-up effect was characterized by the temperature variation of the Knight shift under various measurement conditions., 18 pages, 7 figures

A $^{59}$Co nuclear magnetic resonance (NMR) measurement was performed on the single-crystalline ferromagnetic (FM) superconductor UCoGe under a pressure of 1.09 GPa, where the FM state is suppressed and superconductivity occurs in the paramagnetic (PM) state, to study the superconducting (SC) state in the PMstate. $^{59}$Co-NMR spectra became broader but hardly shifted across the SC transition temperature. The Knight-shift change determined from fitting the spectral peak with a Gaussian was much smaller than the spin part of the Knight shift; this is in good agreement with the spin-triplet pairing suggested from the large upper critical field. The spectrum broadening in the SC state cannot be attributed to the SC diamagnetic effect but is related to the properties of spin-triplet superconductivity. The origins of the broadening are discussed herein., 8 pages, 11 figures

We have performed 31P-NMR measurements on the s-wave superconductor LaRu4P12 to investigate the magnetic field effect of the nuclear spin-lattice relaxation rate 1/T1 on a conventional full-gap superconductor. With increasing magnetic field, the Hebel-Slichter peak immediately below Tc in 1=T1 was suppressed, and the magnetic field dependence of 1/T1 at 0.8 K, well below Tc, was proportional to H2. These behaviors can be fully understood by the orbital pair-breaking effect in a single-band s-wave superconductor, 7pages, 4figures

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., Comment: 5 pages, 4 figures

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., Comment: 30 pages, 59 figures, accepted for publication in J. Phys. Soc. Jpn as an invited review paper

We have performed the $^{125}$Te-nuclear magnetic resonance (NMR) measurement in the field along the $b$ axis on the newly discovered superconductor UTe$_2$, which is a candidate of a spin-triplet superconductor. The nuclear spin-lattice relaxation rate divided by temperature $1/T_1T$ abruptly decreases below a superconducting (SC) transition temperature $T_c$ without showing a coherence peak, indicative of UTe$_2$ being an unconventional superconductor. It was found that the temperature dependence of $1/T_1T$ in the SC state cannot be understood by a single SC gap behavior but can be explained by a two SC gap model. The Knight shift, proportional to the spin susceptibility, decreases below $T_c$, but the magnitude of the decrease is much smaller than the decrease expected in the spin-singlet pairing. Rather, the small Knight-shift decrease as well as the absence of the Pauli-depairing effect can be interpreted by the spin triplet scenario., Comment: 10 pages, 8 figures, including supplemental materials, chosen as Editors' choice, open access

In this Rapid Communication, a set of $^{209}\mathrm{Bi}$-nuclear magnetic resonance (NMR)/nuclear quadrupole resonance (NQR) measurements has been performed to investigate the physical properties of superconducting (SC) ${\mathrm{BaTi}}_{2}{\mathrm{Bi}}_{2}\mathrm{O}$ from a microscopic point of view. The NMR and NQR spectra at 5 K can be reproduced with a nonzero in-plane anisotropic parameter $\ensuremath{\eta}$, indicating the breaking of the in-plane fourfold 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, $1/{T}_{1}T$, does not change even below ${T}_{\mathrm{c}}$, while a clear SC transition was observed with a diamagnetic signal. This observation can be attributed to the strong two dimensionality in ${\mathrm{BaTi}}_{2}{\mathrm{Bi}}_{2}\mathrm{O}$. Comparing the NMR/NQR results among ${\mathrm{BaTi}}_{2}P{n}_{2}\mathrm{O}$ ($Pn=\text{As}$, Sb, and Bi), it was found that the normal and SC properties of ${\mathrm{BaTi}}_{2}{\mathrm{Bi}}_{2}\mathrm{O}$ were considerably different from those of ${\mathrm{BaTi}}_{2}{\mathrm{Sb}}_{2}\mathrm{O}$ and ${\mathrm{BaTi}}_{2}{\mathrm{As}}_{2}\mathrm{O}$, which might explain the two-dome structure of ${T}_{\mathrm{c}}$ in this system.

We performed $^{185/187}$Re nuclear quadrupole resonance (NQR) measurements under pressure to investigate the superconducting properties of noncentrosymmetric superconductor Cd$_{2}$Re$_{2}$O$_{7}$ under various crystal structures. The pressure dependence of superconducting transition temperature $T_{\rm c}$ determined through ac susceptibility measurements is consistent with the results of previous resistivity measurements [T. C. Kobayashi $et al$., J. Phys. Soc. Jpn. 80, 023715 (2011).]. Below 2.2 GPa, in the nuclear spin-lattice relaxation rate $1/T_{1}$, a clear coherence peak was observed just below $T_{\rm c}$, indicating conventional $s$-wave superconductivity. In contrast, the coherence peak disappears at 3.1 GPa, suggesting a change in superconducting symmetry to the $p$-wave dominant state against pressure., Comment: 5 pages, 5 figures

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_{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_{1}T abruptly decreased below the SC transition temperature T_{c}(H), a phenomenon understood within the framework of conventional spin-singlet superconductivity. In contrast, 1/T_{1}T was enhanced just below T_{c}(H) and exhibited a broad maximum when magnetic fields close to μ_{0}H_{c2}(0) were applied parallel or perpendicular to the c axis; although the Knight shift decreased just below T_{c}(H). This enhancement of 1/T_{1}T, 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.

We have performed $^{119}\mathrm{Sn}$-NMR measurements on the antiperovskite oxide superconductor ${\mathrm{Sr}}_{3\ensuremath{-}x}\mathrm{SnO}$ 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 ${\mathrm{Sr}}_{3\ensuremath{-}x}\mathrm{SnO}$ 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.

Recent progress in the fabrication techniques of superlattices (SLs) has made it possible to sandwich several-layer-thick block layers (BLs) of heavy-fermion superconductor CeCoIn5 between conventional-metal YbCoIn5 BLs or spin-density-wave-metal CeRhIn5 BLs of a similar thickness. However, the magnetic state in each BL, particularly at the interface, is not yet understood, as experimental techniques applicable to the SL system are limited. Here, we report measurements of 59Co nuclear magnetic resonance, which is a microscopic probe of the magnetic properties inside the target BLs. In the CeCoIn5/YbCoIn5 SL, the low-temperature magnetic fluctuations of the CeCoIn5 BL are weakened as expected from the Rashba spin-orbit effect. However, in the CeCoIn5/CeRhIn5 SL, the fluctuations show an anomalous enhancement below 6 K, highlighting the importance of the magnetic proximity effect occurring near a magnetic-ordering temperature TN ~ 3 K of the CeRhIn5 BL. We suggest that the magnetic properties of the BLs can be altered by the interfacial interaction, which is an alternative route to modify the magnetic properties., 5 pages, 4 figures

$^{73}$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.

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., 30 pages, 7 Figures, Accepted for publication in Philosophical Magazine

We have performed $^{63}\mathrm{Cu}$ nuclear-magnetic-resonance/nuclear-quadrupole-resonance measurements to investigate the magnetic and superconducting (SC) properties on a ``superconductivity dominant'' ($S$-type) single crystal of ${\mathrm{CeCu}}_{2}{\mathrm{Si}}_{2}$. 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 ${T}_{\mathrm{c}}\ensuremath{\sim}0.6$ K. These behaviors were expected in $S$-type ${\mathrm{CeCu}}_{2}{\mathrm{Si}}_{2}$. 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 ${s}_{\ifmmode\pm\else\textpm\fi{}}$-wave model rather than the two-gap ${s}_{++}$-wave model down to 90 mK. Under magnetic fields, the spin susceptibility in both directions clearly decreased below ${T}_{\mathrm{c}}$, which is 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 $1/{T}_{1}T$, 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.

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., 12 pages, 6 figures

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., 5 pages, 5 figures

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 $��_0H_{\rm c2}$ = 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 $< ��_0 H < ��_0 H_{\rm c2}$. 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., 5 pages, 5 figures

We investigated the magnetism and superconductivity in as-sintered (AS) and oxidation annealed (OA) T*-type La1−x/2Eu1−x/2SrxCuO4 (LESCO) with 0.14 ≤ x ≤ 0.28 by the first comprehensive muon spin r...

We performed $^{17}\mathrm{O}$ nuclear magnetic resonance measurements on superconducting (SC) ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ under in-plane magnetic fields. We found that no new signal appears in the SC state and that the $^{17}\mathrm{O}$ 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 ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$, 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.

We carried out $^{101}\mathrm{Ru}$ nuclear quadrupole resonance (NQR) measurement on superconducting (SC) ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ under zero magnetic field $(H=0)$ and found that the nuclear spin-spin relaxation rate $1/{T}_{2}$ is enhanced in the SC state. The $1/{T}_{2}$ measurement in the SC state under $H=0$ is effective for detecting slow magnetic fluctuations parallel to the quantized axis of the nuclear spin. Our results indicate that low-energy magnetic fluctuations perpendicular to the ${\mathrm{RuO}}_{2}$ plane emerge when the superconductivity sets in, which is consistent with the previous $^{17}\mathrm{O}\text{-NQR}$ result that the nuclear spin-lattice relaxation rate $1/{T}_{1}$ of the in-plane O site exhibits anomalous behavior in the SC state. The enhancement of the magnetic fluctuations in the SC state is unusual and suggests that the fluctuations are related to the unconventional SC pairing. We suggest that this phenomenon is a consequence of the spin degrees of freedom of the spin-triplet pairing.

This review presents a summary of our NMR experiments on LaFeAs ( O 1 − x F x ) and BaFe 2 ( As 1 − x P x ) 2 . The authors introduce the key NMR experimental results which reveal their physical properties in the normal and superconducting states. From the comparison of the NMR results between the two systems, we point out that the superconducting gap structure and the relationship between superconductivity and magnetic fluctuations probed with NMR are quite different between the two systems.

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 $T_\mathrm{c}(H)$., 10 pages, 4 figures

We report experimental results of 75As and 139La nuclear magnetic resonance (NMR) in the layered oxypnictide system LaFeAs ( O 1 - x F x ) ( x = 0.0 , 0.04, 0.07 and 0.11) where superconductivity occurs in x greater than 0.04. In the undoped LaFeAsO, 1 / T 1 of 139La exhibits a distinct peak at T N ~ 142 K below which the La NMR spectra become broadened due to the internal magnetic field attributed to an antiferromagnetic (AFM) ordering. In the x = 0.04 sample, 1 / T 1 T of 75As exhibits a Curie–Weiss temperature dependence down to 30 K, suggesting the development of AFM spin fluctuations with decreasing temperature. In the x = 0.11 sample, in contrast, pseudogap behavior is observed in 1 / T 1 T at the 75As site with a gap value of Δ PG ~ 175 K . The spin dynamics vary markedly with F-doping, which is ascribed to the change of the Fermi-surface structure by the electron-doping. As for the superconducting properties for x = 0.04 , 0.07 and 0.11, 1 / T 1 of 75As in all compounds does not exhibit a coherence peak just below T c and follows a T 3 dependence at low temperatures. These results seemingly suggests that unconventional superconductivity with zero gap along lines, whereas the lack of the residual density of states at the low temperatures is incompatible with the presence of the line-nodes. We discuss similarity and difference between LaFeAs ( O 1 - x F x ) and cuprates, and also discuss the relationship between spin dynamics and superconductivity on the basis of F-doping dependence of T c and 1 / T 1 .

We review our nuclear-magnetic resonance (NMR) and nuclear-quadrupole-resonance (NQR) studies in superconducting Sr2RuO4, which have been performed in order to investigate the gap structure and the pairing symmetry in the superconducting state and magnetic fluctuations in the normal state. The spin-lattice relaxation rate (1/T1) of a high-quality sample with Tc∼1.5K shows a sharp decrease without a coherence peak just below Tc, followed by a T3 behavior down to 0.15 K. This result indicates that the superconducting gap in pure Sr2RuO4 is a highly anisotropic character with a line-node gap. The Knight shift, which is related to the spin susceptibility, is unchanged in the superconducting state irrespective of the direction of the applied fields and various magnitude of the field. This result strongly suggests that the superconducting pairs are in the spin-triplet state, and the spin direction of the triplet pairs is considered to be changed by small fields of several hundred Oe.

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., 5 pages, 4 figures, accepted for publication as a Rapid Communication in Physical Review B

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., Comment: 3 pages, 2 figures

Nuclear-magnetic-resonance (NMR) studies were performed in Na0.7CoO2 to investigate its magnetic properties from the microscopic point of view. Na0.7CoO2 is a starting compound of the recently discovered superconductor Na0.35CoO2·1.3H2O, in which Co atoms form a tri- angular structure. Although 23 Na-NMR results show no magnetic anomaly down to 1.5 K, Knight shift ( 23 K) and 23 (1/T1T) of 23 Na continue to increase, suggestive of the vicinity of the magnetic phase. From the relation between 23 K and 23 (1/T1T), it is shown that ferromagnetic fluctuations are dominant below 100 K. In addition to the ferromagnetic correlations, antifer- romagnetic fluctuations are also developed in a low temperature region below 4 K. Magnetic fluctuations suggested by our experiment are discussed. Since the superconductivity in NaxCoO2·yH2O (x � 0.35, y � 1.3) with the transition temperature Tc � 5 K was discovered by Takada et al, 1 understanding of mag- netic properties in the CoO2 plane is one of the most important topics in strongly correlated electron systems. This is because the magnetic properties in the plane might be related to the mechanism of the unconventional superconductivity in the compound. In addition, the Co atoms form a triangular lattice in the plane, which is an- ticipated to possess the geometrical frustrations. Novel types of magnetic transitions and ground states are ex- pected due to a suppression of a long-range magnetic ordering by the geometrical frustrations associated with the triangular structure. In particular, only a few com- pounds have been known to date which possess a metallic property down to the lowest temperature with the geo- metrical frustrations, e.g. (YSc)Mn2 2 and LiV2O4. 3 The NaxCoO2 (0.5 < x < 0.78) compound is considered to be one of the promising metallic compounds with the geometrical frustrations at low temperature. 4 In this paper, we report the magnetic properties in Na0.7CoO2 revealed by our 23 Na-NMR experiments. Powder samples of Na0.7CoO2 were prepared from Na2CO3 (99.99%) and Co3O4(99.9%) by solid-state re- action. Its quality was checked by X-ray powder diffrac- tion. The Na content, which is very sensitive to physical properties, is estimated to be 0.702 by the inductive- coupled plasma atomic emission spectroscopy (ICP- AES) method. Using this well-characterized powder sample, 23 Na- and 59 Co-NMR measurements were per- formed.

111 Cd NMR measurements have been performed in the superconducting (SC) state of the pyrochlore Cd 2 Re 2 O 7 at T c ∼1 K, and in a field of less than 3 kOe below T c . The upper critical field at 0.1 K has been determined to be 4 kOe from in situ measurements of the ac susceptibility. A reduction of the Knight shift in the SC state is confirmed. The present results provide strong evidence that this compound has a singlet SC pairing symmetry.

We have performed Co-nuclear quadrupole resonance (NQR) studies on Na$_{x}$CoO$_{2}\cdot y$H$_{2}$O compounds with different Na ($x$) and hydrate ($y$) contents. Two samples with different Na contents but nearly the same $T_c$ values ($x$ = 0.348, $T_c$ = 4.7 K ; $x$ = 0.339, $T_c$ = 4.6 K) were investigated. The spin-lattice relaxation rate $1/T_1$ in the superconducting (SC) and normal states is almost the same for the two samples except just above $T_c$. NQR measurements were also performed on different-hydrate-content samples with different $T_c$ values, which were prepared from the same Na-content ($x$ = 0.348) sample. From measurements of $1/T_1$ using the different-hydrate-content samples, it was found that a low-$T_c$ sample with $T_c = 3.9$ K has a larger residual density of states (DOS) in the SC state and a smaller increase of $1/T_1T$ just above $T_c$ than a high-$T_c$ sample with $T_c$ = 4.7 K. The former behavior is consistent with that observed in unconventional superconductors, and the latter suggests the relationship between $T_c$ and the increase in DOS just above $T_c$. This increase, which is seemingly associated with the two-dimensionality of the CoO$_2$ plane, is considered to be one of the most important factors for the occurrence of superconductivity.

We will report the present status of experimental results on heavy-fermion superconductors UPt 3 and UNi 2 Al 3 . The NMR studies of UPt 3 revealed that no reduction in the 195 Pt Knight shift below T c was observed for high-field C -phase. At low fields, the respective Knight shifts for the magnetic field parallel to b - and c -axis decrease at H H T c , whereas the shift for c -axis does not change across T c down to the lowest field of H ∼1.764 kOe. This unconventional behavior in Knight shift which depends on magnetic-field strength and crystal orientation cannot be explained in terms of any even-parity Cooper pairing state, but provides evidence for the odd-parity one being realized in UPt 3 . In UNi 2 Al 3 , 27 Al Knight shift dose not change across T c , giving evidence for the odd-parity one as well. This is in marked contrast to the even-parity d-wave superconductivity in UPd 2 Al 3 . In order to gain further insight into the odd-parity Cooper pairing state, we remark on the quasi-particle spin susceptibility in the heavy-fermion state.

Magnetic properties on Sr2−xCaxRuO4 have been investigated by a microscopic probe of 87Sr-NMR in order to understand the magnetic character on spin-triplet superconductor of Sr2RuO4, which has multibands on the Fermi surface. With substituting Ca for Sr which gives rise to crystal distortion, the Knight shift (K) and the nuclear spin-lattice relaxation rate divided by temperature (1/T1T) increases progressively up to x=1.5. The Korringa relation from K and 1/T1T becomes smaller, indicative of development of ferromagnetic fluctuations with increasing Ca content. This suggests that the q-independent spin fluctuations originating from the 2-dimensional γ band are changed to the ferromagnetic ones by the Ca doping.

Muon spin rotation measurements under high transverse fields (HTF-µSR) have been performed on polycrystalline samples of Ce 0.99 Cu 2.02 Si 2 . The muon hyperfine coupling constant was determined t...

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., Comment: 5 pages, 5 figures, to appear in J. Phys. Soc. Jpn

We report on the basis of Cu-NQR measurements that superconductivity (SC) and antiferromagnetism (AF) coexist on a microscopic level in CeCu_{2}(Si_{1-x}Ge_{x})_{2}, once a tiny amount of 1%Ge (x = 0.01) is substituted for Si. This coexistence arises because Ge substitution expands the unit-cell volume in nearly homogeneous CeCu2Si2 where the SC coexists with slowly fluctuating magnetic waves. We propose that the underlying exotic phases of SC and AF in either nearly homogeneous or slightly Ge substituted CeCu2Si2 are accountable based on the SO(5) theory that unifies the SC and AF. We suggest that the mechanism of the SC and AF is common in CeCu2Si2.

This paper introduces nuclear magnetic resonance works in the strongly correlated super-conductors: heavy-Fermion, high-T C superconductors and Sr2RuO4. The analyses strongly support the spin-fluctuation-mediated superconductivity model in a high-T C superconductor.

We have investigated a gap structure in the spin-triplet superconductor ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ through the measurement of the ${}^{101}\mathrm{Ru}$ nuclear spin-lattice relaxation rate ${}^{101}(1/{T}_{1})$ down to 0.09 K at zero magnetic field. In the superconducting state, $1/{T}_{1}$ in a high-quality sample with ${T}_{c}\ensuremath{\sim}1.5\mathrm{K}$ exhibits a sharp decrease without the coherence peak, followed by a ${T}^{3}$ behavior down to 0.15 K. This result is in marked contrast to the behavior observed below $\ensuremath{\sim}0.4\mathrm{K}$ in samples with lower ${T}_{c}$, where ${T}_{1}T$ is a constant. This behavior is demonstrated to be not intrinsic. We conclude that the gap structure in ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ is significantly anisotropic, consistent with line-node-like models.

We review recent NMR investigations on superconducting and magnetic properties in layered perovskite Sr 2 RuO 4 , isostructural to a high-T c cuprate. Measurements of the Knight shift and the nuclear spin-lattice relaxation rate 1/T 1 have identified its Cooper-pair spin state as a triplet and uncovered pronounced anisotropy in spin-fluctuation spectrum in Sr 2 RuO 4 The in-plane low-frequency components of the dynamical susceptibility are exchange-enhanced with a weak q dependence associated with the two-dimensional (2D) character in electronic structure. By contrast, its out-of-plane component is unexpectedly enhanced by AF spin fluctuations upon cooling below T * ∼130 K where the c-axis resistivity shows metallic behavior. We have found that the slightly-distorted cubic perovskite CaRuO 3 is a nearly ferromagnetic metal dominated by ferromagnetic spin fluctuations with a Stoner factor α=0.98. Contrasting behavior of spin fluctuations between Sr 2 RuO 4 and CaRuO 3 is due to their different dimensionality in electronic structure with the closeness to ferromagnetism. We remark that a simple ferromagnetic-spin-fluctuation-mediated mechanism with analogy to 3 He is not appropriate for an onset of spin-triplet p-wave superconductivity in Sr 2 RuO 4 . Altemately, we propose possible scenario that the in-plane local exchange interaction, which originates presumably from the Hund's coupling between the 4d and 4d xz,yz orbitals, may play a role for the occurrence of spin-triplet p-wave superconductivity.

Recent NMR measurements on high- T c and related material Sr 2 RuO 4 by the Osaka group are reviewed. The following results are presented: (1) Study of YBa 2 Cu 3 O 7 +Ni has revealed that the attractive force is spin in origin. (2) Analysis of T 1 , T 2 and Knight shift ( K ) in various high- T c materials supports the antiferromagnetic spin-fluctuation mediated superconducting model. (3) Temperatures at which a pseudogap starts to open obtained by 1/ T 1 T , d K /d T , photoemission and resistivity agree with each other in the Bi 2 Sr 2 CaCu 2 O 8+ δ system. (4) 17 O Knight shift measurements in the related material Sr 2 RuO 4 suggests that a triplet pairing is realized in this compound as in the heavy-fermion superconductor UPt 3 .

17O Knight shift measurements in Sr2RuO4 were performed over the wide range of magnetic field 3.2-11.4kOe parallel to the basal RuO2 planes. The spin susceptibility is totally unchanged through its Tc, evidencing that the spin-triplet superconducting state is realized in Sr2RuO4. The result indicates that the Cooper pairs consist of the parallel spin pairs | ↑↑> and | ↓↓> with their quantization axis perpendicular to the c-axis direction. The in-plane 2D nearly ferromagnetic spin fluctuations may play a role for the stabilization of this state among various representations of spin-triplet order parameter.

Recent NMR measurements on high-T c materials by our group are reviewed. Study of YBa2Cu3O7+Ni has revealed that the attractive force is spin in origin. Analysis of T1, the Knight shift K and T2G in Tl2Ba2Ca2CuO10 (Tl2223), HgBa2Ca2Cu3O8 (Hg1223), HgBa2Ca3Cu4O10 (Hg1234) and YBa2Cu4O8 (Y1248) supports the antiferromagnetic spin fluctuation (AFSF)-mediated-superconductivity-model. The temperatures at which a pseudogap starts to open obtained by 1/T1T, dK/dT, photoemission, resistivity and STM all agree with each other in Bi2Sr2CaCu2O8 (Bi2212). The origin of the pseudogap is discussed.

We have measured 101 Ru Knight shift ( 101 K) in the spin triplet superconductor Sr 2 RuO 4 in various magnetic fields applied parallel and perpendicular to the RuO 2 plane to determine the spin direction of the Cooper pair. We measured temperature dependence of 101 K in 0.15 and 0.055 T parallel to the RuO 2 plane using nuclear-quadrupole-resonance (NQR) technique. We found that 101 K is invariant with temperature in both fields on passing through the superconducting (SC) transition temperature. These results indicate that the spin susceptibility parallel to the RuO 2 plane does not change in the field greater than 0.055 T, suggesting that the SC d-vector is along the c-axis when the magnetic field greater than 0.055 T is applied along to the RuO 2 plane. The possible spin state in magnetic fields is discussed on the basis of the Knight-shift measurements so far.