In the temperature (T)-nuQ3 phase diagram of NaxCoO2 \dot yH2O, there exist two superconducting regions of nuQ3 separated by the nonsuperconducting region, where nuQ3 is usually estimated from the peak position of the 59Co-NQR spectra of the 5/2-7/2 transition and well-approximated here as nuQ3~3nuQ,nuQ being the interaction energy between the nuclear quadrupole moment and the electric field gradient. We have carried out measurements of the 59Co-NMR Knight shift (K) for a single crystal in the higher-nuQ3 superconducting phase and found that K begins to decrease with decreasing T at Tc for both magnetic field directions parallel and perpendicular to CoO2-planes. The result indicates together with the previous ones that the superconducting pairs are in the spin-singlet state in both phases, excluding the possibility of the spin-triplet superconductivity in this phase diagram. The superconductivity of this system spreads over the wide nuQ3 regions, but is suppressed in the narrow region located at the middle point of the region possibly due to charge instability.
We report systematic 17O-NMR measurements on the high-Tc cuprate (Ca_xLa_{1-x})(Ba_{1.75 - x}La_{0.25 + x})Cu_3O_y, for four different families (different x). Using Knight shift data, we show that the pseudogap lines for all families are inconsistent with a quantum critical point inside the superconducting dome. In addition, at constant doping the pseudogap temperature does not vary with x, in contrast to Tc. We therefore argue that pseudogap and superconductivity are separate phenomena in these cuprates. Using Knight shift data, we show that the pseudogap opening temperature T* is much higher…
The superconducting state in the quasi-two-dimensional and strongly correlated Sr$_2$RuO$_4$ is uniquely held up as a solid state analog to superfluid $^3$He-$A$, with an odd-parity order parameter that also breaks time reversal symmetry, and for which the vector order parameter has the same direction in spin space for all electron momenta. The recent discovery that uniaxial pressure causes a steep rise and maximum in transition temperature ($T_c$) in strained samples motivated the study of $^{17}$O nuclear magnetic resonance (NMR) that we describe in this article. A reduction of Knight shifts…
We calculate ab initio the NMR relaxation rates and the Knight shifts in MgB_2. We show that the dominant relaxation mechanism at the B nucleus is the interaction with the electronic orbital moment, and we give a simple explanation of that using a simple $sp$ tight binding model. When Stoner enhancement (also calculated ab-initio) is accounted for, we obtain good agreement with reported experimental values. For the 25Mg nucleus, we predict that the dominant relaxation mechanism is the Fermi-contact interaction, which also dominates the Mg Knight shift.
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-…
The superconducting state of NaxCoO2.yH2O with three CoO2 layers in a unit cell has been studied by 59Co-NMR. The Knight shift measured for a peak of the NMR spectra corresponding to the external magnetic field H along one of the principal directions within the CoO2 plane, exhibits a rapid decrease with decreasing temperature T below the superconducting transition temperature Tc, indicating that the spin susceptibility is suppressed in the superconducting phase, at least, for this field direction. Because differences of the superconducting properties are rather small between this three-layer N…
$^{75}$As NMR investigation of a single crystal of superconducting LiFeAs is presented. The Knight shift and the \textit{in situ} ac susceptibility measurements as a function of temperature and external field are indicative of two superconducting (SC) transition temperatures, each of which is associated with its own upper critical field. Strikingly, the Knight shift maintains its normal state value over a temperature range in the SC state before it drops abruptly being consistent with spin-singlet pairing. Together with our previous NMR study, the anomalous SC state featured by the constant Kn…
Layered Co oxide NaxCoO2.yH2O with the superconducting transition temperature Tc =4.5 K has been studied by 59Co-NMR. The Knight shift K estimated from the observed spectra for powder sample exhibits almost temperature(T)-independent behavior above Tc and decreases with decreasing T below Tc. This result and the existence of the coherence peak in the spin-lattice-relaxation-rate versus T curve reported by the present authors indicate, naively speaking, that the singlet order parameter with s-wave symmetry is realized in NaxCoO2.yH2O. Differences of the observed behaviors between the spectra of…
Temperature (T) dependence of 59Co-NMR Knight shifts K of Na0.3CoO2.1.3H2O has been studied, where samples of randomly oriented powder and aligned crystals have been used for the applied magnetic fields H // ab plane and H //c-axis, respectively. For both directions of H, the shift K decreases below the superconducting transition temperature Tc(H) with decreasing T, indicating that the superconducting electron pairs are in the singlet state. The upper critical fields Hc2(T) determined from the K(H)-T curves are found to be consistent with the values reported by the resistivity measurements for…
We report a detailed study of $^{11}$B and $^{27}$Al NMR spin-lattice relaxation rates ($1/T_1$), as well as of $^{27}$Al Knight shift (K) of Mg$_{1-x}$Al$_x$B$_2$, $0\leq x\leq 1$. The obtained ($1/T_1T$) and K vs. x plots are in excellent agreement with ab initio calculations. This asserts experimentally the prediction that the Fermi surface is highly anisotropic, consisting mainly of hole-type 2-D cylindrical sheets from bonding $2p_{x,y}$ boron orbitals. It is also shown that the density of states at the Fermi level decreases sharply on Al doping and the 2-D sheets collapse at $x\approx 0.…
The unconventional temperature variation of the Knight shift (static spin susceptibility) that has been observed in Fe-based superconductors AFe$_2$As$_2$ (A = K, Rb, Cs) is explained in terms of proximity to a van Hove singularity. Using the Hubbard model we show that when the Fermi energy is in the vicinity of a van Hove singularity, a downturn in spin susceptibility occurs as the temperature is lowered. This behavior is characterized by a temperature, $T^*$, which is determined by the difference in energy between the Fermi level and the van Hove singularity. When vertex corrections are take…
We report the measurements of the $^{29}$Si Knight shift $^{29}K$ on the noncentrosymmetric heavy-fermion compound CePt$_{3}$Si in which antiferromagnetism (AFM) with $T_{\rm N}=2.2$ K coexists with superconductivity (SC) with $T_{c}=0.75$ K. Its spin part $^{29}K_{\rm s}$, which is deduced to be $K_{\rm s}^{c}\ge 0.11$ and 0.16% at respective magnetic fields $H=2.0061$ and 0.8671 T, does not decrease across the superconducting transition temperature $T_{c}$ for the field along the c-axis. The temperature dependence of nuclear spin-lattice relaxation of $^{195}$Pt below $T_{c}$ has been accoun…
We report 59Co NMR studies on the magnetically oriented powder samples of Co-oxide superconductors NaxCoO2yH2O with Tc ~ 4.7 K. From two-dimensional powder pattern in the NMR spectrum, the ab-plane Knight shift in the normal state was estimated by the magnetic field dependence of second-order quadrupole shifts at various temperatures. Below 50 K, the Knight shift shows a Curie-Weiss-like temperature dependence, similarly to the bulk magnetic susceptibility chi. From the analysis of so-called K-chi plot, the spin and the orbital components of K and the positive hyperfine coupling constant were …
The 17O and 207Pb NMR spectra were measured in ceramic samples in the metallic phase of BaPb_{1-x}Bi_{x}O_3 oxides (0<x< 0.33). The inhomogeneous magnetic broadening which appears due to a distribution of the Knight shifts was analyzed in detail. It is shown that Bi atoms, which are randomly incorporated in BaPbO_3 parent compound give rise to an increased conduction electron spin density within an area which is delimited by its two first cation shells. According to NMR data the percolative overlap of these areas occurs in superconducting compositions and it is accompanied by a sharp growth of…
We present a first principles study of NMR and spin orbit effects in the unconventional superconductor Sr2RuO4. We have calculated the uniform magnetic susceptibility, which agrees rather well with the experiment in amplitude, but, as in an earlier model result we found the calculated hard axis to be z, opposite to the experiment. We have also calculated the Knight shifts and the NMR relaxation rates for all atoms, and again found an overall good agreement, but important deviations from the experiment in same particular characteristic, such as the Knight shift anisotropy. Our results suggest t…
NMR longitudinal relaxation rates 1/T1 and Knight shifts K have been measured for superconducting samples of LaFe1-yCoyAsO1-xFx with y=0.0 and 0.0075 and for a nonsuperconducting metallic sample with y=0.1, where the x values are always fixed at 0.11. The temperature (T) dependence the relaxation rates 1/T1 of the superconducting samples has been found to be markedly different from the behavior 1/T1 T^(2.5-3.0) reported by many groups in the entire T range measured (from the temperature immediately below the superconducting transition temperature Tc down to (0.1-0.2)Tc). The nonexistence of th…
We present $^{57}$Fe-NMR measurements of the novel normal and superconducting-state characteristics of the iron-arsenide superconductor Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ ($T_c$ = 38 K). In the normal state, the measured Knight shift and nuclear spin-lattice relaxation rate $(1/T_1)$ demonstrate the development of wave-number ($q$)-dependent spin fluctuations, except at $q$ = 0, which may originate from the nesting across the disconnected Fermi surfaces. In the superconducting state, the spin component in the $^{57}$Fe-Knight shift decreases to almost zero at low temperatures, evidencing a spin-s…
We report the 77Se NMR Knight shift and spin-lattice relaxation studies on the superconducting state of the ternary iron selenide K0.8Fe2-xSe2 with Tc~30 K. Just below Tc, the Knight shift 77Kn shows an immediate drop, indicating a singlet pairing. The spin-lattice relaxation rate 1/77T1 decreases rapidly in the temperature range from TC to Tc/2, which can be fit with an isotropic gap of Δ~3.8\pm 0.5 k_BTc. The Hebel-Slichter coherence peak is not observed. These data give bulk evidence for a strongly coupled superconductivity with isotropic gaps in K0.8Fe2-xSe2, which is similar to other iron…
Experiment and theory indicate that UPt3 is a topological superconductor in an odd-parity state, based in part from temperature independence of the NMR Knight shift. However, quasiparticle spin-flip scattering near a surface, where the Knight shift is measured, might be responsible. We use polarized neutron scattering to measure the bulk susceptibility with H||c, finding consistency with the Knight shift but inconsistent with theory for this field orientation. We infer that neither spin susceptibility nor Knight shift are a reliable indication of odd-parity.
From the spatial structure of vortex lattice state calculated by Eilenberger theory, we study the resonance line shape of Knight shift of the paramagnetic moments in the s-wave and the d-wave superconductors, comparing with the Redfield pattern of the internal field distribution. We discuss the deviation from the temperature dependence of the Yosida function, and the magnetic field dependence of the paramagnetic susceptibility. In addition to the calculation in the clean limit, influences of the impurity scattering are estimated in the Born limit and in the unitary limit. These results are hel…
We report results of 75As nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) experiments as well as 7Li NMR on different samples of self flux grown LiFeAs and 5 % Co doped LiFeAs single crystals, and a polycrystalline LiFeAs sample. We were able to distinguish the samples by their slightly different quadrupole frequencies, $ν_Q$, which is a direct measure of the electric field gradient (EFG) at the As site. Interestingly, samples with a large quadrupole frequency appear to show a different Knight shift and spin lattice relaxation in the superconducting state from those wit…
$^{11}$B NMR measurements have been performed in $^{11}$B enriched MgB$_2$ powder samples in external fields of 0.813, 1.55, 4.7 and 7.2 T both in the normal phase and in the superconducting phase. A previously unreported dipolar Pake doublet has been observed in the quadrupole perturbed NMR spectrum. The Knight shift can thus be accurately determined by narrowing the line with the Magic Angle Spinning (MAS) technique. Results of Knight shift ($K$) and relaxation rates ($1/T_1$) for both $^{11}$B and $^{27}$Al nuclei are reported also for AlB$_2$. The comparison of the data in the two compound…
We report both 77Se and 87Rb NMR studies on Tl0.47Rb0.34Fe1.63Se2 single-crystalline superconductors (T_c about 32 K). Singlet superconductivity is decisively determined by a sharp drop of the Knight shift K(T) below T_c, after subtracting the superconducting diamagnetic effect. However, the Hebel-Slichter coherence peak below T_c is not observed in the spin-lattice relaxation rate 1/T_1, even with a low in-plane NMR field of 2.6 Tesla. Just above T_c, no evidence of low-energy spin fluctuation is found in the spin-lattice relaxation rate on both the 77Se and the 87Rb sites. Upon warming, howe…
We report the $^{207}$Pb nuclear magnetic resonance (NMR) measurements on polycrystalline sample of PbTaSe$_2$ with noncentrosymmetric crystal structure and topological electronic band. The nuclear spin-lattice relaxation rate $1/T_1$ shows a suppressed coherence peak below the superconducting transition temperature $T_{\rm c}$ = 4.05 K and decreases as an exponential function of temperature. The penetration depth derived from the NMR spectrum is almost temperature independent below $T$ = 0.7 $T_{\rm c}$. The Knight shift $K$ decreases below $T_{\rm c}$. These results suggest spin-singlet supe…
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 pa…
NMR spectra and nuclear spin-lattice relaxation rate $T_1^{-1}$ of $^{11}$B have been measured in superconducting polycrystalline MgB$_{2}$ with $T_C^{ons}\cong 39.5K$. It is shown that $(T_{1}T)^{-1}$ and the Knight shift $K_{s}$ are independent of temperature and nearly isotropic above $T_C$. Both of these quantities are decreased gradually in going to superconducting state. According NMR data the density of states (DOS) near the Fermi level is flat at the scale $\approx$ 500K. Some conclusions on the orbital content of the DOS at the Fermi level was done and compared with the results of the…
We report bulk superconductivity (SC) in Ca$_3$Ir$_4$Sn$_{13}$ by means of $^{119}$Sn nuclear magnetic resonance (NMR) experiments. Two classical signatures of BCS superconductivity in spin-lattice relaxation rate ($1/T_1$), namely the Hebel-Slichter coherence peak just below the $T_c$ and the exponential decay in the superconducting phase, are evident. The noticeable decrease of $^{119}$Sn Knight shift below $T_c$ indicates spin-singlet superconductivity. The temperature dependence of the spin-lattice relaxation rate ($1/T_1$) is convincingly described by the multigap isotropic superconductin…
Nuclear magnetic resonance (NMR) and Knight shift measurements are critical tools in the identification of spin-triplet superconductors. We discuss the effects of spin orbit coupling on the Knight shift and susceptibilities for a variety of spin triplet multi-orbital gap functions with orbital-singlet character and compare their responses to "traditional" single band spin-triplet ($p_x+ip_y$) superconductors. We observe a non-negligible residual spin-susceptibility at low temperature.
We report both 23Na and 75As NMR studies on hole-doped Ca1-xNaxFe2As2 superconducting single crystals (x\approx 0.67) with Tc =32 K. Singlet superconductivity is suggested by a sharp drop of the Knight shift 75K below Tc. The spin-lattice relaxation rate 1/T1 does not show the Slichter-Hebel coherence peak, which suggests an unconventional pairing. The penetration depth is estimated to be 0.24 μm at T=2 K. 1/75T1T shows an anisotropic behavior and a prominent low-temperature upturn, which indicates strong low-energy antiferromagnetic spin fluctuations and supports a magnetic origin of supercon…
Magnetic Field Dependence of the Spin Susceptibility on Conventional s-wave Superconductor LaRu$_4$P$_{12}$ Revealed by $^{31}$P-NMR and $^{139}$La-NMR
To investigate the superconducting (SC) state near a charge instability, we performed ^{13}C NMR experiments on the molecular superconductor beta"-(BEDT-TTF)_{4}[(H_{3}O)Ga(C_{2}O_{4})_{3}]C_{6}H_{5}NO_{2}, which exhibits a charge anomaly at 100 K. The Knight shift which we measured in the SC state down to 1.5 K demonstrates that Cooper pairs are in spin-singlet state. Measurements of the nuclear spin-lattice relaxation time reveal strong electron-electron correlations in the normal state. The resistivity increase observed below 10 K indicates that the enhanced fluctuation has an electric orig…
We have performed $^{139}$La,$^{31}$P-NMR studies on a filled skutterudite superconductor LaFe$_{4}$P$_{12}$ with a critical temperature $T_c=4.1$ K. In the normal state, the presence of antiferromagnetic (AFM) fluctuations at finite wave vectors is suggested from the relation between the nuclear spin-lattice relaxation rate $1/T_1$ and the Knight shift. In the superconducting (SC) state, the distinct coherence peak was observed just below $T_c$, and is easily suppressed by the applied field. An exponential decrease of $1/T_1$ was observed, suggestive of the isotropic superconducting gap with …
We report on NMR measurements in underdoped Pb2Sr2(Y,Ca)Cu3O8+d crystals. A pseudogap is observed in the Knight shift and spin-lattice relaxation rate. In contrast to other underdoped compounds, the pseudogap observed in the Knight shift is weak and occurs at a significantly lower temperature. On the other hand, the effect the pseudogap has on spin-lattice relaxation is quite similar to that in other compounds. The contrast between weak and strong pseudogaps is discussed.
We revealed novel phase deagram of Fe-pnictide high-Tc superconductor LaFe(As_{1-x}P_{x})O in wide doping level (0.3<x<1) by P-NMR. Systematic 31P-NMR studies revealed the emergence of the antiferromagnetic ordered phase (AFM-2) in 0.4 < x < 0.7 that intervenes between two superconductivity (SC-1/SC-2) phases. The 31P-NMR Knight shift points to the appearance of the sharp density of states at the Fermi level that is derived from d_{3Z^2?r^2} orbit, which is less relevant with the onset of the SC-2. On the other hand, we remark that the AFM spin fluctuations arising from the interband nesting o…
$^{11}B$ NMR and nuclear spin-lattice relaxation rate (NSLR) are reported at 7.2 Tesla and 1.4 Tesla in powder samples of the intermetallic compound $MgB_2$ with superconducting transition temperature in zero field $T_c$ = 39.2 K. From the first order quadrupole perturbed NMR specrum a quadrupole coupling frequency of 835 $\pm$ 5 kHz is obtained. The Knight shift is very small and it decreases to zero in the superconducting phase. The NSLR follows a linear law with $T_1T$ = 165 $\pm$ 10 (sec K) . The results in the normal phase indicate a negligible $s$-character of the wave function of the co…
We report $^{75}$As nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) studies on the superconductor Rb$_{2}$Cr$_{3}$As$_{3}$ with a quasi one-dimensional crystal structure. Below $T\sim$ 100 K, the spin-lattice relaxation rate (1/$T_{1}$) divided by temperature, 1/$T_{1}T$, increases upon cooling down to $T_{\rm c}$ = 4.8 K, showing a Curie-Weiss-like temperature dependence. The Knight shift also increases with decreasing temperature. These results suggest ferromagnetic spin fluctuation. In the superconducting state, 1/$T_{1}$ decreases rapidly below $T_{\text{c}}$ withou…
We report ^77Se-NMR measurements in the recent discovered K_0.8Fe_2Se_2 with the superconducting transition temperature T_c=32 K. The characteristic point of this system is that the high density of state is located in the vicinity of the Fermi level. The comparison between Knight shift and nuclear spin lattice relaxation rate 1/T_1 shows that the antiferromagnetic spin correlation is not strong in this system, but it is reasonable to consider that its development toward low temperatures occurs. 1/T_1 in the superconducting state can be reproduced well by an s+- wave model, but a d wave model a…
Quasi-particle spin susceptibility ($χ^{qp}$) for various heavy-fermion (HF) superconductors are discussed on the basis of the experimental results of electronic specific heat ($γ_{el}$), NMR Knight shift ($K$) and NMR relaxation rate ($1/T_1$) within the framework of the Fermi liquid model for a Kramers doublet crystal electric field (CEF) ground state. $χ^{qp}_γ$ is calculated from the enhanced Sommerfeld coefficient $γ_{el}$ and $χ^{qp}_{T_1}$ from the quasi-particle Korringa relation $T_1T(K^{qp}_{T_1})^2=const.$ via the relation of $χ^{qp}_{T_1}=(N_Aμ_B/A_{hf})K^{qp}_{T_1}$ where $A_{hf}$…
The static and the dynamic spin correlations in the low temperature collapsed tetragonal and the high temperature tetragonal phase in CaFe2As2 have been investigated by 75As nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements. Through the temperature (T) dependence of the nuclear spin lattice relaxation rates (1/T1) and the Knight shifts, although stripe-type antiferromagnetic (AFM) spin correlations are realized in the high temperature tetragonal phase, no trace of the AFM spin correlations can be found in the non-superconducting, low temperature, collapsed te…
The layered quasi-one-dimensional molecular superconductor (TMTSF)$_2$PF$_6$ is a very exotic material with a superconducting order parameter whose ground state symmetry has remained ill-defined. Here we present a pulsed NMR Knight shift (K) study of $^{77}$Se measured simultaneously with transport in pressurized (TMTSF)$_2$PF$_6$. The Knight shift is linearly dependent on the electron spin susceptibility $χ_s$, and is therefore a direct measure of the spin polarization in the superconducting state. For a singlet superconductor, the spin contribution to the Knight shift, K$_s$, falls rapidly o…
We report the $^{75}$As-NQR and NMR studies on the iron arsenide superconductor Li$_{x}$FeAs with $T_{\rm c} \sim 17$ K. The spin lattice relaxation rate, $1/T_{1}$, decreases below $T_{\rm c}$ without a coherence peak, and can be fitted by gaps with s$^{\pm}$-wave symmetry in the presence of impurity scattering. In the normal state, both $1/T_{1}T$ and the Knight shift decrease with decreasing temperature but become constant below $T \leq 50 K$. Estimate of the Korringa ratio shows that the spin correlations are weaker than that in other families of iron arsenides, which may account for the l…
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, whic…
We report on the superconducting (SC) properties of Y$_2$C$_3$ with a relatively high transition temperature $T_{\rm c}=15.7$ K investigated by $^{13}$C nuclear-magnetic-resonance (NMR) measurements under a magnetic field. The $^{13}$C Knight shift has revealed a significant decrease below $T_{\rm c}$, suggesting a spin-singlet superconductivity. From an analysis of the temperature dependence of the nuclear spin-lattice relaxation rate $1/T_1$ in the SC state, Y$_2$C$_3$ is demonstrated to be a multigap superconductor that exhibits a large gap $2Δ/k_{\rm B}T_{\rm c}=5$ at the main band and a s…
In nonsuperconducting, metallic paramagnetic SrCo2As2, inelastic neutron scattering measurements have revealed strong stripe-type $χ$(q) = ($π$,0) antiferromagnetic (AFM) spin correlations. Here, using nuclear magnetic resonance (NMR) measurements on 59Co and 75As nuclei, we demonstrate that stronger ferromagnetic (FM) spin correlations coexist in SrCo2As2. Our NMR data are consistent with density functional theory (DFT) calculations which show enhancements at both q = ($π$,0) and the in-plane FM q = 0 wavevectors in static magnetic susceptibility $χ$(q). We suggest that the strong FM fluctuat…
We have carried out direction-dependent ^{59}Co NMR experiments on a single crystal sample of the ferromagnetic superconductor UCoGe in order to study the magnetic properties in the normal state. The Knight shift and nuclear spin-lattice relaxation rate measurements provide microscopic evidence that both static and dynamic susceptibilities are ferromagnetic with strong Ising anisotropy. We discuss that superconductivity induced by these magnetic fluctuations prefers spin-triplet pairing state.
The electronic and magnetic properties of the collapsed-tetragonal CaPd$_2$As$_2$ superconductor (SC) with a transition temperature of 1.27 K have been investigated by $^{75}$As nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements. The temperature ($T$) dependence of the nuclear spin lattice relaxation rates (1/$T_1$) and the Knight shifts indicate the absence of magnetic correlations in the normal state. In the SC state, 1/$T_1$ measured by $^{75}$As NQR shows a clear Hebel-Slichter (HS) peak just below $T_{\rm c}$ and decreases exponentially at lower $T$, conf…
We present a 77Se NMR study of the newly discovered iron selenide superconductor KyFe2-xSe2, in which Tc = 32 K. Below Tc, the Knight shift 77K drops sharply with temperature, providing strong evidence for singlet pairing. Above Tc, Korringa-type relaxation indicates Fermi-liquid behavior. Our experimental results set strict constraints on the nature of possible theories for the mechanism of high-Tc superconductivity in this iron selenide system.
We report a study of the normal- and superconducting-state electronic properties of the centrosymmetric compound SrPt3P via 31P nuclear-magnetic-resonance (NMR) and magnetometry investigations. Essential features such as a sharp drop of the Knight shift at T < Tc and an exponential decrease of the NMR spin-lattice relaxation ratio 1/(T1T) below Tc are consistent with an s-wave electron pairing in SrPt3P, although a direct confirmation in the form of a Hebel-Slichter-type peak is lacking. Normal-state NMR data at T < 50 K indicate conventional features of the conduction electrons, typical of si…
The spin contributed Knight shift $^{59}K_{s}$ of $^{59}\mathrm{Co}$ in the triangular lattice superconductor, $\mathrm{Na_{0.35}CoO_{2} \cdot 1.3H_{2}O}$, has been estimated by NMR experiments in an oriented 2-dimensional powder sample. The Knight shift in the paramagnetic state was estimated by $δν/(γ\mathrm{_{N}}H_{i})$ vs. $(γ\mathrm{_{N}}H_{i})^{-2}$ plot at various temperatures. A Knight shift vs. magnetic susceptibility diagram revealed that at least 0.3 % for $K_{x}$ and 0.1 % for $K_{y}$ are attributed to the spin contribution. Below $T\mathrm{_{c}}$, $δν/(γ\mathrm{_{N}}H_{i})$ vs. $(…
We studied the Volovik effect on the NMR measurements of the two unconventional superconducting (SC) states, the d-wave and $\pm$s-wave states. We showed the generic field dependencies of the spin-lattice relaxation rate $1/T_1$ and Knight shift $K$ at low temperature limit in the pure cases as: $1/T_1 \propto H \log H$, $K \propto \sqrt{H}$ for the d-wave, and $1/T_1 \propto H$, $K \propto H$ for the $\pm$s-wave state, respectively. Performing numerical calculations we showed that these generic power laws survive for the good part of low field region with the realistic amount of impurities. W…
We report $^{77}$Se NMR data in the normal and superconducting states of a single crystal of FeSe for several different field orientations. The Knight shift is suppressed in the superconducting state for in-plane fields, but does not vanish at zero temperature. For fields oriented out of the plane, little or no reduction is observed below $T_c$. These results reflect spin-singlet pairing emerging from a nematic state with large orbital susceptibility and spin-orbit coupling. The spectra and spin-relaxation rate data reveal electronic inhomogeneity that is enhanced in the superconducting state,…
We report the first 17-O NMR studies of a triangular-lattice superconductor Na{1/3}CoO{2}-[4/3]H{2}O and the host material Na{x}CoO{2} (x=0.35 and 0.72). Knight shift measurements reveal that p-d hybridization induces sizable spin polarization in the O triangular-lattice layers. Water-intercalation makes CoO{2} planes homogeneous, and enhances low frequency spin fluctuations near T{c}=4.5 K at some finite wave-vectors different from both the ferromagnetic and ``120 degree'' modes.
We report the spin Knight shift (K_s) and the nuclear spin-lattice relaxation rate (1/T_1) in the vortex state as a function of magnetic field (H) up to 28 T in the high-T_c superconductor TlSr_{2}CaCu_2O_{6.8} (T_c=68 K). At low temperatures well below T_{c}, both K_s and 1/T_1 measured around the middle point between two nearest vortices (saddle point) increase substantially with increasing field, which indicate that the quasiparticle states with an ungapped spectrum are extended outside the vortex cores in a d-wave superconductor. The density of states (DOS) around the saddle point is found…
The superconducting state of an optimally doped single crystal of Ba(Fe$_{0.93}$Co$_{0.07}$)$_2$As$_2$ was investigated by $^{75}$As NMR in high magnetic fields from 6.4 T to 28 T. It was found that the Knight shift is least affected by vortex supercurrents in high magnetic fields, $H>11$ T, revealing slow, possibly higher order than linear, increase with temperature at $T \lesssim 0.5 \, T_c$, with $T_c \approx 23 \, K$. This is consistent with the extended s-wave state with $A_{1g}$ symmetry but the precise details of the gap structure are harder to resolve. Measurements of the NMR spin-spin…
To investigate spin susceptibility in a superconducting (SC) state, we measured the $^{125}$Te-nuclear magnetic resonance (NMR) Knight shifts at magnetic fields ($H$) up to 6.5 T along the $b$ and $c$ axes of single-crystal 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 th…
We report a ^{11}B NMR measurement on the recently discovered superconductor Li_2Pd_3B. The nuclear spin lattice relaxation rate 1/T_1 shows a well-defined coherence peak just below T_c (H=1.46 T)=5.7 K, and the spin susceptibility measured by the Knight shift also decreases below T_c. These results indicate that the superconductivity is of conventional nature, with an isotropic gap. Our results also suggest that the $p$-electrons of boron and the d-electrons of palladium that hybridize with boron $p$-electrons are primarily responsible for the superconductivity.
Nuclear magnetic resonance (NMR) provides local, bulk information about the electronic properties of materials, and it has been influential for theory of high-temperature superconducting cuprates. Importantly, NMR found early that nuclear relaxation is much faster than what one expects from coupling to fermionic excitations above the critical temperature for superconductivity ($T_{\rm c}$), i.e. what one estimates from the Knight shift with the Korringa law. As a consequence, special electronic spin fluctuations have been invoked. Here, based on literature relaxation data it is shown that the …
We use NMR techniques to investigate the magnetic properties of BaCo2As2 single crystals, the non-superconducting end member of the Co-substituted iron-pnictide high Tc superconductor Ba(Fe1-xCox)2As2 with x = 1. We present 75As NMR evidence for enhancement of low frequency spin fluctuations below ~ 100 K. This enhancement is accompanied by that of static uniform spin susceptibility at the wave vector q = 0, suggesting that the primary channel of the spin correlations is ferromagnetic rather than antiferromagnetic. Comparison between the NMR Knight shift 75K and bulk susceptibility chi(bulk) d…
We report 29Si-NMR study on a single crystal of the heavy-fermion superconductor CeIrSi3 without an inversion symmetry along the c-axis. The 29Si-Knight shift measurements under pressure have revealed that the spin susceptibility for the ab-plane decreases slightly below Tc, whereas along the c-axis it does not change at all. The result can be accounted for by the spin susceptibility in the superconducting state being dominated by the strong antisymmetric (Rashba-type) spin-orbit interaction that originates from the absence of an inversion center along the c-axis and it being much larger than …
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…
