The anisotropy of the nuclear spin-lattice relaxation rate $1/T_{1}$ of $^{75}$As was investigated in the iron-based superconductor LaFeAs(O$_{1-x}$F$_{x}$) ($x = 0.07, 0.11$ and 0.14) as well as LaFeAsO. While the temperature dependence of the normal-state $1/T_1T$ in the superconducting (SC) $x = 0.07$ is different from that in the SC $x = 0.11$, their anisotropy of $1/T_1$, $R \equiv (1/T_{1})_{H \parallel ab}/(1/T_{1})_{H \parallel c}$ in the normal state is almost the same ($\simeq$ 1.5). The observed anisotropy is ascribable to the presence of the local stripe correlations with $Q = (π, 0)$ or $(0, π)$. In contrast, $1/T_1$ is isotropic and $R$ is approximately 1 in the overdoped $x = 0.14$ sample, where superconductivity is almost suppressed. These results suggest that the presence of the local stripe correlations originating from the nesting between hole and electron Fermi surfaces is linked to high-$T_c$ superconductivity in iron pnictides.
75As-nuclear magnetic resonance (NMR) on an iron-based superconductor LaFeAsO1-xFx (x=0.14) was performed under a pressure of 3GPa. Enhancement of superconducting transition temperature Tc was confirmed from the relaxation rate 1/T1; Tc goes up to 40K by applying pressure up to 3GPa. 1/T1T, which is temperature independent just above Tc and gives a measure of the density of states (DOS) at the Fermi energy, enhances by applying pressure. These facts suggest that the increase of the DOS leads to the enhancement of Tc. On the other hand, anomalous behavior of 1/T1T observed at high temperatures …
Results of 75As NMR and NQR measurements are reported for superconducting samples of LaFeAsO1-xFx with various x values. The x dependent widths of the superconducting transition and those of the NQR spectra indicate that the optimally doped LaFeAsO1-xFx sample with the superconducting transition temperature Tc ~28 K has the smallest width or smallest inhomogeneity of the superconducting order parameter Δ. In the analyses of the temperature (T) dependence of the NMR longitudinal relaxation rate 1/T1, we have tried to use a simple relation 1/T1 ~ T^n to see how fast 1/T1 decreases when the super…
$^{19}$F NMR measurements in SmFeAsO$_{1-x}$F$_x$, for $0.15\leq x\leq 0.2$, are presented. The nuclear spin-lattice relaxation rate $1/T_1$ increases upon cooling with a trend analogous to the one already observed in CeCu$_{5.2}$Au$_{0.8}$, a quasi two-dimensional heavy-fermion intermetallic compound with an antiferromagnetic ground-state. In particular, the behaviour of the relaxation rate either in SmFeAsO$_{1-x}$F$_x$ or in CeCu$_{5.2}$Au$_{0.8}$ can be described in the framework of the self-consistent renormalization theory for weakly itinerant electron systems. Remarkably, no effect of t…
The purpose of this brief invited paper is to summarize what we have (not) learned from NMR on stripes and inhomogeneity in La{2-x}Sr{x}CuO{4}. We explain that the reality is far more complicated than generally accepted.
Site-dependent NMR relaxation time $T_1({\bf r})$ is calculated in the vortex state using the Bogoliubov-de Gennes theory, taking account of possible "field-induced stripe'' states in which the magnetism arises locally around a vortex core in d-wave superconductivity. The recently observed huge enhancement $T_1^{-1}({\bf r})$ below $T_c$ at a core site in Tl$_2$Ba$_2$CuO$_6$ is explained. The field-induced stripe picture explains consistently other relevant STM and neutron experiments.
Charge and spin ordering in La2NiO4.17 has been studied by 139La NMR using the field and temperature dependence of the linewidth and relaxation rates. The stripes are site centered and the domain spins freeze at ~ 180 K, followed by a second transition at ~ 50 K, where the spins on the domain walls freeze. Although consistent with the neutron measurements, the results are quantitatively different from the NMR results in a nickelate with quenched Sr disorder.
The experimental observations of stripes in superconducting cuprates and insulating nickelates clearly show the modulation in charge and spin density. However, these have proven to be rather insensitive to the harmonic structure and (site or bond) ordering. Using 139La NMR in La2NiO4.17, we show that in the 1/3 hole doped nickelate below the freezing temperature the stripes are strongly solitonic and site ordered with Ni3+ ions carrying S=1/2 in the domain walls and Ni2+ ions with S=1 in the domains.
Measurements of the electrical resistivities, Hall coefficients, thermoelectric powers, electronic specific heat coefficients have been carried out for samples of LnFe1-yMyAsO1-xFx (Ln=La, Nd; M=Co, Mn; x=0.11) obtained by M atom dopings to the superconducting LnFeAsO1-xFx (Ln1111) system. The NMR longitudinal relaxation rates 1/T1 have also been measured for samples of LaFe1-yCoyAsO1-xFx with various x values. Co atoms doped to the superconducting LnFeAsO1-xFx are nonmagnetic, and the Tc-suppression by the Co atoms has been found to be too weak to understand by the pair breaking effect expect…
Stripe fluctuations in La2NiO4.17 have been studied by 139La NMR using the field and temperature dependence of the linewidth and relaxation rates. In the formation process of the stripes the NMR line intensity is maximal below 230K, starts to diminish around 140K, disappears around 50K and recovers at 4K. These results are shown to be consistent with, but completely complementary to neutron measurements, and to be generic for oxygen doped nickelates and underdoped cuprates.
We report on 139La and 63Ccu NMR/NQR measurements in the high-Tc superconductor La1.90Sr0.10CuO4 with Tc=26.5 K. Spin fluctuations probed by 139La spin-lattice relaxation (T1), continuously slow down on cooling through Tc. We argue that spin-freezing and superconductivity are bulk effects in this sample. Thus, both phenomena have to coexist microscopically. The distribution of 139La T1 values at low temperature reveals a wide spread of spin fluctuation frequencies in CuO2 planes. A simple estimate shows that Cu nuclei at sites where electronic fluctuations are the slowest are not observable be…
We report results of 75As nuclear magnetic resonance (NMR) experiments on a self-flux grown single crystal of BaFe2As2. A first-order antiferromagnetic (AF) transition near 135 K was detected by the splitting of NMR lines, which is accompanied by simultaneous structural transition as evidenced by a sudden large change of the electric field gradient tensor at the As site. The NMR results lead almost uniquely to the stripe spin structure in the AF phase. The data of spin-lattice relaxation rate indicate development of anisotropic spin fluctuations of the stripe-type with decreasing temperature i…
We perform an analysis of the experimentally known temperature dependence of the staggered magnetization in the antiferromagnetic phase. This analysis allows us to put an upper limit on the unknown value of the spin wave velocity along the stripes of equal spin direction (spin stripes). The velocity is about ten times smaller than the velocity perpendicular to the spin stripes. The strongly anisotropic spin-wave dispersion implies a high density of low energy magnetic excitations. We demonstrate that this high density strongly enhances the $^{75}$As NMR spin-lattice relaxation via the Raman sc…
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 report results of 23Na and 75As nuclear magnetic resonance (NMR) experiments on a self-flux grown high-quality single crystal of stoichiometric NaFeAs. The NMR spectra revealed a tetragonal to twinned-orthorhombic structural phase transition at T_O = 57 K and an antiferromagnetic (AF) transition at T_AF = 45 K. The divergent behavior of nuclear relaxation rate near T_AF shows significant anisotropy, indicating that the critical slowing down of stripe-type AF fluctuations are strongly anisotropic in spin space. The NMR spectra at low enough temperatures consist of sharp peaks showing a comme…
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 report 139La, 57Fe and 75As nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements on powders of the new LaO1-xFxFeAs superconductor for x = 0 and x = 0.1 at temperatures up to 480 K, and compare our measured NQR spectra with local density approximation (LDA) calculations. For all three nuclei in the x = 0.1 material, it is found that the local Knight shift increases monotonically with an increase in temperature, and scales with the macroscopic susceptibility, suggesting a single magnetic degree of freedom. Surprisingly, the spin lattice relaxation rates for al…
We present $^{77}$Se-NMR measurements on single-crystalline FeSe under pressures up to 2 GPa. Based on the observation of the splitting and broadening of the NMR spectrum due to structural twin domains, we discovered that static, local nematic ordering exists well above the bulk nematic ordering temperature, $T_{\rm s}$. The static, local nematic order and the low-energy stripe-type antiferromagnetic spin fluctuations, as revealed by NMR spin-lattice relaxation rate measurements, are both insensitive to pressure application. These NMR results provide clear evidence for the microscopic cooperat…
Recent nuclear magnetic resonance (NMR) measurements revealed the coexistence of stripe-type antiferromagnetic (AFM) and ferromagnetic (FM) spin correlations in both the hole- and electron-doped BaFe$_2$As$_2$ families of iron-pnictide superconductors by a Korringa ratio analysis. Motivated by the NMR work, we investigate the possible existence of FM fluctuations in another iron pnictide superconducting family, Ca(Fe$_{1-x}$Co$_x$)$_2$As$_2$. We re-analyzed our previously reported data in terms of the Korringa ratio and found clear evidence for the coexistence of stripe-type AFM and FM spin co…
$^{75}$As nuclear magnetic resonance (NMR) measurements carried out on underdoped, non-superconducting Ca$_{10}$(Pt$_3$As$_8$)(Fe$_2$As$_2$)$_5$ reveal physical properties that are similar but not identical to the 122 superconductor parent compounds such as BaFe$_2$As$_2$. Results from the single crystal study indicate a phase transition to an antiferromagnetic (AF) state on cooling through T~100K, albeit nonuniformly. Specifically, the NMR lineshape reflects the presence of staggered hyperfine fields on the As sites associated with a striped AF order. The variation of the internal hyperfine f…
In the iron pnictide superconductors, theoretical calculations have consistently shown enhancements of the static magnetic susceptibility at both the stripe-type antiferromagnetic (AFM) and in-plane ferromagnetic (FM) wavevectors. However, the possible existence of FM fluctuations has not yet been examined from a microscopic point of view. Here, using $^{75}$As NMR data, we provide clear evidence for the existence of FM spin correlations in both the hole- and electron-doped BaFe$_2$As$_2$ families of iron-pnictide superconductors. These FM fluctuations appear to compete with superconductivity …
To elucidate the magnetic structure and the origin of the nematicity in FeSe, we perform a high-pressure $^{77}$Se NMR study on FeSe single crystals. We find a suppression of the structural transition temperature with pressure up to about 2 GPa from the anisotropy of the Knight shift. Above 2 GPa, a stripe-order antiferromagnetism that breaks the spatial four-fold rotational symmetry is determined by the NMR spectra under different field orientations and with temperatures down to 50 mK. The magnetic phase transition is revealed to be first-order type, implying the existence of a concomitant st…
We report detailed 17O, 139La, and 63Cu Nuclear Magnetic Resonance (NMR) and Nuclear Quadrupole Resonance (NQR) measurements in a stripe ordered La1.875Ba0.125CuO4 single crystal and in oriented powder samples of La1.8-xEu0.2SrxCuO4. We observe a partial wipeout of the 17O NMR intensity and a simultaneous drop of the 17O electric field gradient (EFG) at low temperatures where the spin stripe order sets in. In contrast, the 63Cu intensity is completely wiped out at the same temperature. The drop of the 17O quadrupole frequency is compatible with a charge stripe order. The 17O spin lattice relax…
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…
Interrelationships between nematicity, antiferromagnetic spin fluctuations and superconductivity: Role of hotspots in FeSe$_{1-x}$S$_{x}$ revealed by high pressure $^{77}$Se NMR study
We report $^{75}$As nuclear magnetic resonance (NMR) measurements of single-crystalline Ca(Fe$_{1-x}$Co$_x$)$_2$As$_2$ ($x$ = 0.023, 0.028, 0.033, and 0.059) annealed at 350~$^{\circ}$C for 7 days. From the observation of a characteristic shape of $^{75}$As NMR spectra in the stripe-type antiferromagnetic (AFM) state, as in the case of $x$ = 0 ($T_{\rm N}$ = 170 K), clear evidence for the commensurate AFM phase transition with the concomitant structural phase transition is observed in $x$ = 0.023 ($T_{\rm N}$ = 106 K) and $x$ = 0.028 ($T_{\rm N}$ = 53 K). Through the temperature dependence of …
We report $^{31}$P- and $^{75}$As-NMR studies on (Ca$_4$Al$_2$O$_{6}$)Fe$_2$(As$_{1-x}$P$_x$)$_2$ with an isovalent substitution of P for As. We present the novel evolution of emergent phases that the nodeless superconductivity (SC) in 0$\le x \le$0.4 and the nodal one around $x$=1 are intimately separated by the onset of a commensurate stripe-type antiferromagnetic (AFM) order in 0.5$\le x \le$ 0.95, as an isovalent substitution of P for As decreases a pnictogen height $h_{Pn}$ measured from the Fe plane. It is demonstrated that the AFM order takes place under a condition of 1.32Å$\le h_{Pn} …
We report 75As-NMR/NQR results on new iron-arsenide compounds (La0.5-xNa0.5+x)Fe2As2. The parent compound x=0 exhibits a stripe-type antiferromagnetic (AFM) order below T_N=130 K. The measurement of nuclear spin relaxation rate at hole-doped x=+0.3 and heavily electron-doped x=-0.5 revealed that the normal-state properties are dominated by AFM spin fluctuations (AFMSFs), which are more significant at x=+0.3 than at x=-0.5. Their superconducting (SC) phases are characterized by unconventional multi-gap SC state, where the smaller SC gaps are particularly weaken in common. The experimental resul…
We have performed ^{75}As nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) on single crystalline Ba_{1-x}K_{x}Fe_{2}As_{2} for x = 0.27-1. ^{75}As nuclear quadruple resonance frequency (ν_{Q}) increases linearly with increasing x. The Knight shift K in normal state shows Pauli paramagnetic behavior with slight temperature T dependence. The value of K increases gradually with increasing x. By contrast, nuclear spin- lattice relaxation rate 1/T_{1} in normal state has a large T-dependence, which indicates existence of large antiferomagnetic (AF) spin fluctuations for all x…
