Anisotropic Knight Shift Research Articles

Al27 NMR insight into the phase transition in BaFe2Al9

We have applied $^{27}\mathrm{Al}$ nuclear magnetic resonance (NMR) spectroscopy to investigate the iron-based aluminide of $\mathrm{Ba}{\mathrm{Fe}}_{2}{\mathrm{Al}}_{9}$. This material has been a subject of current interest due to indications of charge density wave behavior below the transition temperature ${T}_{C}\ensuremath{\simeq}100$ K. Two sets of the $^{27}\mathrm{Al}$ NMR resonance lines that are associated with two nonequivalent crystallographic aluminum sites have been well resolved. We have discussed the obtained electric field gradient and anisotropic Knight shift for each individual aluminum site, revealing the strong $ab$ plane bonding configuration for the structural properties of $\mathrm{Ba}{\mathrm{Fe}}_{2}{\mathrm{Al}}_{9}$. Pronounced features in the isotropic Knight shift and nuclear spin-lattice relaxation rate have been observed in the vicinity of ${T}_{C}$. Furthermore, the detailed $^{27}\mathrm{Al}$ NMR analyses have provided evidence for the decrease of the electronic density of states upon lowering temperature below ${T}_{C}$.

Orbital ordering and fluctuations in a kagome superconductor CsV3Sb5

Recently, competing electronic instabilities, including superconductivity and density-wave-like order, have been discovered in vanadium-based kagome metals AV3Sb5 (A = K, Rb, Cs) with a nontrivial band topology. This finding stimulates considerable interest to study the interplay of these competing electronic orders and possible exotic excitations in the superconducting state. Here, we performed 51V and 133Cs nuclear magnetic resonance (NMR) measurements on a CsV3Sb5 single crystal to clarify the nature of density-wave-like transition in these kagome superconductors. A first-order structural transition is unambiguously revealed below Ts ∼ 94 K by observing the sudden splitting of Knight shift in 51V NMR spectrum. Moreover, combined with 133Cs NMR spectrum, the present result confirms a three-dimensional structural modulation. By further analyzing the anisotropy of Knight shift and 1/T1T at 51V nuclei, we proposed that the orbital order is the primary electronic order induced by the first-order structural transition, which is supported by further analysis on electric field gradient at 51V nuclei. In addition, the evidence for possible orbital fluctuations is also revealed above Ts. The present work sheds light on a rich orbital physics in kagome superconductors AV3Sb5.

105Pd NMR and NQR study of the cubic heavy fermion system Ce3Pd20Si6

We report 105Pd nuclear magnetic resonance (NMR) and nuclear quadrupolar resonance (NQR) measurements on a single crystal of Ce3Pd20Si6, where antiferroquadrupolar and antiferromagnetic orders develop at low temperature. From the analysis of NQR and NMR spectra, we have determined the electric field gradient (EFG) tensors and the anisotropic Knight shift (K) components for both inequivalent Pd sites—Pd(32f) and Pd(48h). The observed EFG values are in excellent agreement with our state-of-the-art density functional theory calculations. The principal values of the quadrupolar coupling are MHz and MHz, for the Pd(32f) and Pd(48h) sites, respectively, which is large compared to the Larmor frequency defined by the gyromagnetic constant MHz/T for 105Pd. Therefore, the complete knowledge of K and the EFG tensors is crucial to establish the correspondence between NMR spectra and crystallographic sites, which is needed for a complete analysis of the magnetic structure, static spin susceptibility, and the spin-lattice relaxation rate data and a better understanding of the groundstate of Ce3Pd20Si6.

NMR studies on antiferromagnetic fluctuation in [formula omitted

We have carried out 195Pt-NMR measurements on a heavy Fermion superconductor UPt3 at H∼1.5−2.0T to investigate the in-plane anisotropy in the normal state. For an applied field parallel to [112¯0] direction, split 195NMR line are observed, which has not been reported previously. Such a line splitting is explained by anisotropic Knight shift tensors associated with the local symmetry of Pt-site in UPt3.

NMR spectrum analysis for CrAs at ambient pressure

We report NMR spectrum analysis for CrAs, which was recently reported to be superconducting under pressure. The NMR spectrum obtained by the powdered single crystals shows a typical powder pattern reproduced by the electric field gradient (EFG) parameters and isotropic Knight shift, indicating anisotropy of Knight shift is not remarkable in CrAs. For the oriented sample, the spectrum can be understood by considering that the crystals are aligned for H∥b. The temperature dependence of Knight shift was successfully obtained from NMR spectrum with large nuclear quadrupole interaction.

Single crystal 27Al-NMR study of the cubic Γ3 ground doublet system PrTi2Al20

We report results of 27Al-NMR measurements on a single crystal of PrTi2Al20, in which the ground state of Pr3+ ions in the crystalline electric field is the nonmagneitc Γ3 doublet. From the analysis of NMR spectra with the magnetic field applied precisely along the ⟨111⟩ and ⟨100⟩ directions, we determined the electric field gradient tensors for all three inequivalent Al sites (Al(1) ∼ Al(3) sites) and the anisotropic Knight shifts for the Al(3) sites. The hyperfine coupling tensor at the Al(3) sites is strongly anisotropic and much larger than the classical dipolar coupling, indicating importance of the anisotropic hybridization between the conduction and ƒ electron states.

NMR Study of Layered Transition Metal Ditelluride (Ir,Pt)Te2

We report the results of 125Te and 195Pt NMR measurements on (Ir,Pt)Te2 in order to elucidate the characteristic electronic states. For PtTe2, the NMR spectrum exhibits a sharp line, which shows the uniaxially symmetric powder pattern due to the anisotropic Knight shift. The Knight shift is almost independent of temperature and is larger than that for IrTe2. Also, the nuclear spin-lattice relaxation rate 1/T1 of PtTe2 is proportional to the temperature in a wide temperature range, that is, obeys the Korringa relation as expected for simple metallic systems. From the analyses of the Knight shift and 1/T1, it is suggested that the antiferromagnetic correlations slightly exist.

Solid state 27Al NMR investigation of Knight shift tensors of LnAl2 (Ln=Y, La and Lu) and correlation with DFT calculations

The atomic and electronic structures of LnAl2 (Ln=Y, La and Lu) on Al sites were characterized by 27Al solid state NMR spectroscopy. 27Al isotropic metallic shifts have been determined in these cubic laves phases under both static and magic angle spinning (MAS) conditions. Based on band structure calculations, we found a strong correlation of 27Al isotropic metallic shifts and s character of the density of states at the Fermi level on Al sites. We also found tetrahedral distortion of Al centered clusters, which can be well characterized by quadruple coupling constants and Knight shift anisotropy.

Metal-insulator transition in the hollanditeK2V8O16with a frustrated zigzag ladder probed byV51NMR

We report the experimental results of $^{51}\mathrm{V}$ NMR measurements on the hollandite ${\mathrm{K}}_{2}$V${}_{8}$O${}_{16}$ consisting of a frustrated zigzag ladder with the orbital degrees of freedom. The metal-insulator transition is found to involve the spin-singlet formation by the $^{51}\mathrm{V}$ Knight shift $K$, the nuclear spin-lattice relaxation rate $1/{T}_{1}$, and the spin-echo decay rate $1/{T}_{2}$ measurements. In the insulating state, the anisotropic electric-field gradient supports the ${d}_{\mathit{xy}}$ orbital order with the spin singlet along the chain. The ${d}_{\mathit{xy}}$ orbital is magnetically most active in the metallic state, as observed by the anisotropic Knight shift, which suggests the strong electron correlation in the ${d}_{\mathit{xy}}$ band. Despite the large enhancement of the spin susceptibility, no apparent spin correlation is developed in the frustrated metallic state. Pressure suppresses the electron correlation continuously, as highlighted in the decrease of the metal-insulator transition and the spin susceptibility keeping the largest ${d}_{\mathit{xy}}$ contribution in the metallic state. A robust spin-singlet insulating phase with the large spin gap and paramagnetic spins appears above 1 GPa, which suggests a competition of the charge-orbital ordering pattern.

29Si-NMR study of magnetic anisotropy and hyperfine interactions in the uranium-based ferromagnet UNiSi2

UNiSi2 orders ferromagnetically below TCurie = 95 K. This material crystallizes in the orthorhombic CeNiSi2-type structure. The uranium atoms form double-layers, which are stacked along the crystallographic b axis (the longest axis). From magnetization measurements the easy (hard) magnetization axis is the c axis (b axis). 29Si-NMR measurements have been performed in the paramagnetic state. In UNiSi2, two crystallographic Si sites exist with orthorhombic local symmetry. The Knight shifts on each Si sites have been estimated from these spectra of random and oriented powders. The transferred hyperfine couplings have been also derived. It is found that the transferred hyperfine coupling constants on each Si sites are nearly isotropic, and that their Knight shift anisotropy comes from that of the bulk susceptibility. The nuclear-spin lattice relaxation rate 1/T1 shows temperature-independent behaviour, which indicates the existence of localized 5f electrons.

In-plane anisotropy of the magnetic fluctuations in [formula omitted

We found two components of the in-plane Co 59 NMR Knight shift K and different nuclear spin–lattice relaxation rates 1 / T 1 in bilayer hydrated superconductor Na x CoO 2 · y H 2 O , an oriented powder sample by a magnetic field in Fluorinert FC70. Distinguishable two peaks in the central transition line of the Co 59 NMR spectrum at 10 K can be explained by an anisotropic Knight shift in a triangular plane. The hyperfine coupling constants seem to change around 150 K, in which the bulk susceptibility χ shows a broad minimum, suggesting a change of the electronic state in the CoO 2 plane.

Studying nanocrystalline superconducting Nb3Sn layers in Nb/Cu-Sn composites of various design using NMR and magnetic susceptibility methods

The paper presents the results of measurements of the temperature dependence of the shape of the NMR lines of 93Nb and of the real part of the dynamic magnetic ac susceptibility χ′(T) for multifilamentary superconducting Nb3Sn-based composites with various geometry of niobium filaments (ordinary cylindrical, paired cylindrical, and ringlike (tubular)) produced by the bronze technology. In all the composites studied, a superconducting (SC) transition was observed in diffusional Nb3Sn layers at T c ∼ 17 K. Its width is ΔT c ≈ 3 K and is independent of the composite design. The difference in the absolute value of χ′(T) in the composites of ordinary assembly and with paired filaments is less by a factor of 1.5 than that of a composite with ring filaments. In the experimental NMR spectra of 93Nb, subspectra of different intensities have been distinguished, whose number is three in ordinary assemblies and in assemblies with paired filaments, and two in composites with ring filaments. Estimates of the intensities of each subspectrum in each experimental spectrum of the composites studied has been correlated with a definite diffusional layer of Nb3Sn; thus, the number of grains of a corresponding layer in each composite has been estimated. In addition, it has been established that in the first two composites the shape of the NMR line of 93Nb is asymmetric, which indicates the existence of an anisotropy of the Knight shift in Nb3Sn layers in both the normal and superconducting states, whereas in the composite with ring filaments the line is symmetric and the Knight shift is isotropic. The magnitude of the anisotropic Knight shift in composites with isolated and paired cylindrical filaments is 93 K an ≈ 0.02%. The anisotropy in them appears to be due to the different character and magnitude of interaction of grains of the Nb3Sn layers with the bronze matrix and the residual Nb, which leads to different shifts of the subspectrum frequencies and, correspondingly, to the shape asymmetry of the NMR line of 93Nb. At the same time, in a composite with ring filaments the interaction of the Nb3Sn layer with the bronze matrix occurs on both sides, is, apparently, of the same character and is compensated in magnitude. In this case, in the first two composites the tin distribution across the diffusional layers of Nb3Sn is nonuniform and a significant deviation from the stoichiometric composition is observed.

Orbital properties ofSr3Ru2O7and related ruthenates probed byO17NMR

We report a site-separated $^{17}$O-NMR study of the layered perovskite ruthenate Sr$_3$Ru$_2$O$_7$, which exhibits nearly two-dimensional transport properties and itinerant metamagnetism at low temperatures. The local hole occupancies and the spin densities in the oxygen $2p$ orbitals are obtained by means of tight-binding analyses of electric field gradients and anisotropic Knight shifts. These quantities are compared with two other layered perovskite ruthenates: the two-dimensional paramagnet Sr$_2$RuO$_4$ and the three-dimensional ferromagnet SrRuO$_3$. The hole occupancies at the oxygen sites are very large, about one hole per ruthenium atom. This is due to the strong covalent character of the Ru-O bonding in this compound. The magnitude of the hole occupancy might be related to the rotation or tilt of the RuO$_6$ octahedra. The spin densities at the oxygen sites are also large, 20-40% of the bulk susceptibilities, but in contrast to the hole occupancies, the spin densities strongly depend on the dimensionality. This result suggests that the density-of-states at the oxygen sites plays an essential role for the understanding of the complex magnetism found in the layered perovskite ruthenates.

Te125NMR Studies of single-crystalCeTe3

We report $^{125}\mathrm{Te}$ NMR studies for single crystals of $\mathrm{Ce}{\mathrm{Te}}_{3}$ between 10 and $307\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, under an applied field of $H=7.4847\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ along the $a$ or $b$ axis $H\ensuremath{\Vert}ab$ and the $c$ axis $H\ensuremath{\Vert}c$, respectively. We observed that the $^{125}\mathrm{Te}$ NMR spectra consist of a superposition of broad and sharp peaks, which we assign to the signals of $^{125}\mathrm{Te}(1)$ in double Te(1) sheets and $^{125}\mathrm{Te}(2)$ in CeTe(2) bilayers, respectively. For $H\ensuremath{\Vert}ab$, a broad $^{125}\mathrm{Te}(1)$ NMR spectrum shows a powder pattern with three split lines, which is evidence for an incommensurate charge-density wave modulation. The $^{125}\mathrm{Te}(1)$ NMR spectrum with three lines broadens with cooling. Knight shifts widely distributed from $\ensuremath{-}0.16%$ to $+0.58%$ at $110\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ indicate that a spin density of conduction electron is polarized from Ce local moments. For $H\ensuremath{\Vert}c$, a spectrum has a fine structure with three lines, and the linewidth of the $^{125}\mathrm{Te}(1)$ NMR spectrum is narrow compared with that for $H\ensuremath{\Vert}ab$. On the other hand, the anisotropic Knight shift of the $^{125}\mathrm{Te}(2)$, which is located at the CeTe(2) bilayer, is positive with proportion to the anisotropic bulk susceptibility for $H\ensuremath{\Vert}ab$ and $H\ensuremath{\Vert}c$. The hyperfine coupling constants of $^{125}\mathrm{Te}(2)$ are nearly isotropic but the nuclear spin-lattice relaxation rate is strongly anisotropic, indicating the anisotropic spin fluctuations in this system. The nuclear spin-lattice relaxation rate of the $^{125}\mathrm{Te}(2)$ diverges due to the slowing down of spin fluctuations.

Comparative NMR study of hybridization effect and structural stability inD022-typeNbAl3andNbGa3

With the aim of providing experimental information for the correlation between $p\text{\ensuremath{-}}d$ hybridization and phase stability in the $D{0}_{22}$ structure, we performed a comparative investigation on $\mathrm{Nb}{\mathrm{Al}}_{3}$ and $\mathrm{Nb}{\mathrm{Ga}}_{3}$ using $^{93}\mathrm{Nb}$ NMR spectroscopy. The quadrupole splittings, Knight shifts, and spin-lattice relaxation times (${T}_{1}$'s) for each individual compound have been identified. The larger quadrupole interaction and higher anisotropic Knight shift have been observed in $\mathrm{Nb}{\mathrm{Al}}_{3}$, indicative of the stronger hybridization effect for this material, as compared with its isostructural compound $\mathrm{Nb}{\mathrm{Ga}}_{3}$. Results of experimental ${T}_{1}$ together with theoretical band structure calculations provide a measure of $d$-character Fermi-level density of states ${N}_{d}({E}_{F})$ and an indication of orbital weights. In addition, we found evidence that ${N}_{d}({E}_{F})$ correlates with the structural stability of the studied materials. Our NMR measurements confirm that $\mathrm{Nb}{\mathrm{Al}}_{3}$ is more stable than $\mathrm{Nb}{\mathrm{Ga}}_{3}$ with respect to the $D{0}_{22}$ structure, attributed to the stronger $p\text{\ensuremath{-}}d$ hybridization in the former material.

First-principles study of spin-orbit effects and NMR inSr2RuO4

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 that correlations in Sr2RuO4 lead to underestimations of the orbital effects in density-functional based calculations. We also argue that the accepted ``experimental'' value for the relative contribution of orbital polarization in susceptibility, 10-15%, is also an underestimation. We discuss the puzzling invariance of the the O and Ru Knight shift across the superconducting transition for all directions of the applied field. We show that this fact cannot be explained by accidental cancellations or spin-flip scattering, as it happens in some elemental superconductors. We also point out that large contribution of the dipole and orbital hyperfine field into the Knight shifts in Sr2RuO4, combined with the possibility of an orbital-dependent superconductivity, calls for a revision of the standard theory of the Knight shift in the superconducting state.

Knight shift of triangular lattice superconductor [formula omitted

The field-swept nuclear magnetic resonance (NMR) spectra of 59Co in the superconductor Na 0.35 CoO 2 · 1.3 H 2 O have been measured at several magnetic fields below T c . The anisotropic Knight shift K y seems to decrease with decreasing temperature without broadening of the spectrum, indicating either the site-selective diamagnetic effect, which may result in the spin-triplet superconductivity, or the decrease of the intrinsic spin shift. Analysis of the magnetic field dependence of the Knight shift at 2 K revealed that the superconductivity occurs in an unconventional way.

NMR features of a decagonalAl72.6Ni10.5Co16.9quasicrystal

The nuclear magnetic resonance (NMR) spectra of a decagonal ${\mathrm{Al}}_{72.6}{\mathrm{Ni}}_{10.5}{\mathrm{Co}}_{16.9}$ single-grain quasicrystal (QC) were investigated by field- and frequency-sweep techniques in two magnetic fields and the $^{27}\mathrm{Al}$ relaxation rate was determined. The spectra are strongly inhomogeneously broadened by the electric quadrupole interaction, as evident from the magnetic-field independence of the width of the satellite part of the spectrum and the inverse-field dependence of the width of the central line. The temperature-independent $^{27}\mathrm{Al}$ isotropic Knight shift is anomalously low similarly to icosahedral Al-based QCs, whereas the line broadening due to anisotropic Knight shift is minute as compared to the quadrupolar broadening. The $^{59}\mathrm{Co}$ resonance was found centered at its Larmor frequency and its temperature-independent frequency shift demonstrates that cobalt is in a nonmagnetic state. The NMR spectra exhibit orientation dependence in a magnetic field. In the twofold rotation pattern, the satellite intensity shows a pronounced orientation-dependent shape and structure, whereas this was not detected in the tenfold pattern. The tenfold rotation pattern of the $^{27}\mathrm{Al}$ central line, obtained with the Fourier-transform technique, exhibits a tiny, but significant orientation dependence of the line shape and its center of gravity ${M}_{1}$ with the quasiperiodic symmetry $\ensuremath{\pi}∕5$. The orientation dependence is stronger for rotation about the twofold axis, where the rotation pattern exhibits a periodic symmetry $\ensuremath{\pi}$. We introduce a model that we name as ``stacked planar,'' which reproduces the orientation-dependent shape of the NMR spectra of the decagonal AlNiCo QC for rotation about the periodic axis, whereas---due to assumed amorphous order within the atomic planes---it does not yield orientation-dependent spectra for rotation about the quasiperiodic axis.

Evidence for an antiferroquadrupolar ordering in YbSb probed bySb121andSb123nuclear magnetic resonances

Formation of the antiferroquadrupolar (AFQ) ordering in YbSb (NaCl-type structure) was evidenced microscopically with the $^{121}\mathrm{Sb}$ and $^{123}\mathrm{Sb}$ nuclear magnetic resonance (NMR) measurements. We have observed in the NMR linewidth below ${T}_{Q}=5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ the appearance of an additional field-independent contribution that is proportional to the nuclear electric-quadrupole moment. Thus, the field-independent contribution can be addressed not to any long-range magnetic orderings but to a broken symmetry of the charge distribution associated with the AFQ ordering. The AFQ ordering also gives rise to a large anisotropic Knight shift broadening caused by the anisotropy in the transferred hyperfine field and/or magnetic susceptibility. Fluctuations of the $\mathrm{Yb}\phantom{\rule{0.3em}{0ex}}4f$ moments are also investigated by using the $^{121,123}\mathrm{Sb}$ spin-lattice relaxation rate measurements.

High-temperature magnetic order and coexisting spin dynamics inPrCu2:Manifestations of multipolar effects

We present a comprehensive muon-spin-rotation/relaxation $(\ensuremath{\mu}\mathrm{SR})$ study of the high-temperature magnetic order ${(T}_{N}\ensuremath{\simeq}65\mathrm{K})$ in ${\mathrm{PrCu}}_{2},$ first observed quite unexpectedly in 1998 [A. Schenck et al., Phys. Rev. B $58,$ 5205 (1998)]. Measurements in zero and longitudinal fields as a function of temperature, orientation of the single-crystal sample, and strength of applied magnetic field ${H}_{\mathrm{ext}}$ confirmed the previous findings and allowed us to determine the properties of the internal spontaneous fields $\mathit{B}$ in detail. It is found that $\mathit{B}$ is strictly confined to the crystallographic $(a,c)$ plane and encloses on average an angle $\ensuremath{\theta}$ of $30\ifmmode^\circ\else\textdegree\fi{}\ensuremath{-}45\ifmmode^\circ\else\textdegree\fi{}$ with the a axis, depending on temperature. Both $|\mathit{B}|$ and $\ensuremath{\theta}$ display a large variation. This is consistent with, but does not unambiguously prove, the incommensurately modulated antiferromagnetic structure observed below $\ensuremath{\sim}50\mathrm{mK}$ by neutron scattering [S. Kawarazaki and J. Arthur, J. Phys. Soc. Jpn. $57,$ 1077 (1988)]. The estimated ordered electronic moment of $\ensuremath{\sim}0.29{\ensuremath{\mu}}_{B}$ is smaller than the value quoted by Kawarazaki et al. $(0.54{\ensuremath{\mu}}_{B}).$ The temperature dependence of $|\mathit{B}|$ is highly unusual and scales perfectly with the temperature dependence of the elastic constant ${C}_{66}$ [R. Settai et al., J. Phys. Soc. Jpn $67,$ 636 (1998)], implying that the ordered moment is proportional to the strain susceptibility involving the ${O}_{\mathrm{xy}}$ quadrupole moment. ${\ensuremath{\mu}}^{+}$ is also found to be exposed to fluctuating field components inducing spin-lattice relaxation. The relaxation rate $\ensuremath{\lambda}$ consists of a field-independent but rather anisotropic term and an essentially isotropic but field-dependent term. The first term reflects the Jahn-Teller transition at 7.5 K by a cusplike anomaly. Transverse-field measurements at 6 kOe and above 100 K allowed us to determine the anisotropic Knight shift, or the dipolar coupling tensor, from which it follows that ${\ensuremath{\mu}}^{+}$ are located at $4e$ sites. At lower temperatures, we find evidence that the contact-hyperfine coupling constant becomes temperature and orientation dependent.