Knight Shift Measurements Research Articles

NMR and NQR studies of the pairing symmetry in the superconducting state of [formula omitted

We report intensive studies on the cobalt oxide superconductors Na x CoO 2 · 1.3 H 2 O via 59Co NMR and NQR measurements. In the sample with x = 0.26 and T c = 4.6 , we find that the spin–lattice relaxation rate 1 / T 1 shows a T 3 variation below T c and down to very low temperatures, which indicates the presence of line nodes in the superconducting (SC) gap function. We have also studied the spin susceptibility below T c via the Knight shift measurement. The spin part of the Knight shift decreases below T c in both the a - and c -directions indicating singlet spin state of the Cooper pairs. Our results strongly suggest that the superconducting state is a d-wave state.

NMR study of the spin gap in the vanadium bronze [formula omitted

V 51 NMR measurements have been made on a single crystal to study the spin gap in the vanadium bronze η - Na 1.286 V 2 O 5 . We observe four sets of V 51 NMR spectra coming from the nonmagnetic V1 and V2 sites above T 0 ∼ 110 K and eight sets below T 0 due to the doubling of the V1 and V2 sites accompanied by the structural transition. The V 51 Knight shift measurements for the V1 and V2 sites indicate that the magnetic vanadium sites show the spin-gap behavior with the gap energy dependent on the vanadium sites below T 0 .

NMR study of Al substitution effects in FeSi

We report the results of 27Al NMR Knight shift measurement on FeSi 1− x Al x ( x=0.02 and 0.05) as related to the influence of Al substitution on the magnetic properties and energy gap features of FeSi. For both compositions, the temperature-dependent 27Al NMR Knight shifts exhibit activation-type behavior with the broad maximum at around 400 K. The observed data can be well reproduced by using a model band structure with two narrow peaks in the density of states separated by a small energy gap. The deduced gap was found to be insensitive to the Al content, indicative of little modification on the band structure of FeSi with addition of itinerant carriers. However, the absolute value of Knight shift increases with Al concentration, revealing an enhancement of the Pauli-type spin susceptibility via Al substitution.

Spin singlet pairing in the superconducting state ofNaxCoO2∙1.3H2O: Evidence from aCo59Knight shift in a single crystal

We report a ^{59}Co Knight shift measurement in a single crystal of the cobalt oxide superconductor Na_{x}CoO_2\cdot1.3H_2O (T_c=4.25 K). We find that the shift due to the spin susceptibility, K^s, is substantially large and anisotropic, with the spin shift along the a-axis K^s_a being two times that along the c-axis K^s_c. The shift decreases with decreasing temperature (T) down to T\sim100 K, then becomes a constant until superconductivity sets in. Both K^s_a and K^s_c decrease below T_c. Our results indicate unambiguously that the electron pairing in the superconducting state is in the spin singlet form.

Anomalous magnetic correlations in heavy fermion Pauli paramagnet [formula omitted]: 11B NMR Study

We report 11B NMR results on UIr 3 B 2 , which is an enhanced Pauli paramagnet with electronic specific heat coefficient γ = 63.8 mJ / mol K 2 . The Knight shift and nuclear spin-lattice relaxation measurement revealed two distinct temperature ( T) regimes, in which the magnetic state dynamics are different one another. From the standpoint of general susceptibility ( χ q , ω ) , we suggest that ferromagnetic (FM) correlations is dominant in high T-region, on the other hand, antiferromagnetic (AFM) correlations emerge below T * ≃ 50 K . Since χ q , ω does not show the tendency to disappear its magnitude at q = 0 , FM correlations still survive with AFM correlations even in the low- T state.

O17NMR Studies of a Triangular-Lattice SuperconductorNaxCoO2·yH2O

We report the first 17O NMR studies of a triangular-lattice superconductor Na(1/3)CoO2 x 4/3H(2)O and the host material Na(x)CoO2 (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 CoO2 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.

Temperature and Field Dependence of the Order Parameter in the Antiferroquadrupolar Phase of CeB6 from μ+ Knight Shift Measurements

The Fermi contact hyperfine contribution to the Knight shift of positive muons, implanted at the interstitial 3d sites in CeB6, is found to exhibit the same temperature dependence below T(Q) in phase II as the quadrupolar order parameter determined from resonant and nonresonant x-ray scattering. Furthermore, the contact coupling parameter is shown to be anisotropic and field dependent. These unanticipated features are interpreted to arise from the RKKY induced conduction electron spin polarization, which depends on the orientation and expectation value of the ordered 4f quadrupole moments.

Study of the Electronic Properties of CeRhSn by µ+ Knight Shift and Relaxation Measurements in Single Crystals

We report on measurements of the Knight shift and relaxation behavior of spin polarized positive muons (µ + ) implanted in monocrystalline CeRhSn. A two-component signal is seen below 100 K which appears to indicate the presence of two different types of domains or phases in which Ce is either in an intermediate, valence fluctuating state, occupying a volume fraction of ∼ 80% above 1.3 K, or in the 3+ ionic state, respectively. The latter domains become dominant below 0.5 K and can be partially suppressed by an external field H ext . The two associated Knight shifts and µSR line width's as well as the bulk susceptibility χ bulk reveal subtle changes at 30 K in the magnetic response. Strong anomalies, not present in χ bulk , are seen in the two Knight shifts for H ext ⊥ c axis at ∼ 7 K. Some of the observed subtleties may originate from the conduction electron system. The small observed µSR line width's are inconsistent with atomic disorder involved to explain certain non-Fermi liquid features. The absenc...

Influence of confined geometry on nuclear spin relaxation and self-diffusion in liquid indium

The spin-lattice relaxation times for the $^{115}\mathrm{In}$ isotope in liquid indium embedded into artificial opals and porous glass were found to be shortened remarkably compared to the bulk indium melt. The spin relaxation acceleration was ascribed mainly to the enhancement of the quadrupole contribution caused by the translational diffusion in liquid indium; the magnetic contribution was implied unchanged on the base of the Knight shift measurements and the Korringa relation. Calculations made for quadrupole relaxation showed that it dominated the spin relaxation process for indium in nanopores and that the correlation times of atomic motion increased by more than a factor of 6 depending on pore sizes. The increase in the correlation time evidenced drastic slowing down the atomic diffusion for confined melted indium.

Characterization of the electronic properties of YB4 and YB6 using 11B NMR and first-principles calculations

Two compounds, tetragonal YB4 and cubic YB6, have been investigated by electric-field gradient (EFG) and Knight shift measurements at the boron sites using the 11B nuclear magnetic resonance (NMR) technique and by performing first-principles calculations. In YB611B (I=3/2) NMR spectra reveal patterns typical for an axially symmetric field gradient with a quadrupole coupling frequency of νQ=600±15kHz. In the second boride (YB4) three different EFGs were observed corresponding to the three inequivalent crystallographic sites for the boron atoms (4h, 4e, and 8j). They correspond to: νQ(4h)=700±30kHz with an asymmetry parameter η=0.02±0.02, νQ(4e)=515±30kHz, η=0.00+0.02/−0.00, and νQ(8j)=515±40kHz, η=0.46±0.08. The Knight shifts measured by magic-angle spinning (MAS) NMR at room temperature are very small being 0.6±8 and −1±8ppm for YB4 and YB6, respectively. For the theoretical calculations structure optimizations were performed as a first step. For the obtained structural parameters the EFGs were computed within the local-density approximation. Very satisfactory agreement between experimental and theoretical results is obtained both for the structural parameters and the B EFGs, thus confirming the underlying structural models. In addition to the EFGs, band structures, densities of states, and valence-electron densities are presented and the bonding situation in the two yttrium borides is discussed. The band-structure results are compatible with the very low values for the Knight shifts mentioned above.

89Ynuclear magnetic resonance study of Ca-dopedY1−xCaxBa2Cu3Oyfrom the underdoped to the overdoped superconducting regime

${}^{89}\mathrm{Y}$ NMR linewidth, Knight shift, spin-echo dephasing, and spin-lattice relaxation measurements have been carried out in ${\mathrm{Y}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{y}.$ Underdoped and overdoped samples have been obtained by means of ${\mathrm{Y}}^{3+}$ for ${\mathrm{Ca}}^{2+}$ substitutions in the parent chain-empty antiferromagnetic (AF) ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6.1}$ and in the chain-full ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7},$ respectively. Unexpected effects, as the divergence of relaxation rate with the concurrent broadening of the NMR line in the underdoped superconducting phase and the inadequacy of the Korringa relation between the relaxation rate ${1/T}_{1}$ and Knight shift, even in the overdoped regime, suggest that a revision of the commonly accepted view of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6+y}$ compounds is required. In particular the linear temperature dependence of ${T}_{1}^{\ensuremath{-}1}$ and the temperature behavior of Knight shift cannot be accounted for over all the temperature range. In the underdoped superconducting phase the divergence of ${1/T}_{1}$ on cooling is associated with the slowing down of excitations possibly related to sliding motions of orbital currents, or with the concurrent freezing of AF correlated spins. Echo-dephasing measurements evidence an extreme slowing down of longitudinal spin fluctuations which appear to be driven by a different dynamic, related either to flux line motions or to ${}^{63,65}\mathrm{Cu}$ spin-lattice relaxation.

Extended Quasiparticle States Outside the Vortex Cores in a d-Wave Superconductor

We a describe a measurement of the spin Knight shift (Ks) around the middle point between two vortices as a function of magnetic field (H) up to 28 T in a high-Tc superconductor TlSr 2 CaCu 2 O 6.8. At low temperatures, Ks increases substantially with increasing field, which indicates that the quasiparticle states with an ungapped spectrum are extended outside the vortex cores in a d-wave superconductor. The density of such states is found to be [Formula: see text], with N0 being the normal-state density of states and Hc2 being the upper critical field.

Magnetism and superconductivity in UT2Al3 (T=Pd, Ni) and UGe2

We review magnetic and superconducting properties of UT2Al3 (T=Pd, Ni) and UGe2 together with URu2Si2. For UPd2Al3, we have suggested that the superconducting attractive paring interaction is mediated by magnetic excitons. Knight shift measurements of UNi2Al3 show an odd parity pairing in contrast to an even parity formation in UPd2Al3. The ac magnetic susceptibility measurements of UGe2 indicate that the volume fraction of superconductivity grows with increasing pressure, suggesting inhomogeneous coexistence of superconductivity and ferromagnetism. Recent dc magnetization measurements on UGe2 exhibit a step like hysteresis loop at low temperatures. An analysis of the data suggests that a magnetic domain size is smaller than a superconducting coherence length. Finally, we report the observation of an anomaly appearing in the thermal expansion of URu2Si2 under high pressures, which can be ascribed to the phase transition between a hidden and antiferromagnetic order.

Al Knight-shift measurement in the superconducting state of UNi2Al3

We report 27Al Knight-shift (27K) measurement on a single-crystal UNi2Al3 that reveals a coexistence of superconductivity and a spin-density-wave (SDW) type of magnetic ordering (TSDW=4.5 K). The spin part of 27K, 27Ks does not change down to 50 mK across the superconducting (SC) transition temperature Tc∼0.9 K. In contrast with the isostructural compound UPd2Al3 (Tc∼2 K), which was identified to be a spin-singlet d-wave superconductor, the behavior of 27K strongly supports that UNi2Al3 belongs to a class of spin-triplet SC pairing state superconductor.

Gap formation in the semimetal U 2Ru 2Sn: evidence from [formula omitted] NMR investigations

We report measurements of the 119 Sn nuclear spin-lattice relaxation rate 1/ T 1 and the Knight shift K for polycrystalline U 2Ru 2Sn powder samples as a function of temperature. The T dependence of 1/ T 1 is very similar to that of CeNiSn with the only difference that the temperature scale is increased roughly by a factor of 10. Therefore a first estimate gives a (pseudo-) gap value of Δ/k B ≈140 K ( ≈14 K for CeNiSn). At low temperatures similar to CeNiSn a T 1 T=constant behaviour is observed which indicates the presence of a residual density of states at the Fermi level N( E F) in the pseudo-gap. Knight shift measurements on oriented polycrystalline powder samples show nearly no magnetic anisotropy, whereas the temperature dependence roughly follows the behaviour of the magnetic susceptibility. First investigations on the non-magnetic reference compound Th 2Ru 2Sn are also discussed.

The interplay of charge order and magnetism in the one-dimensional quantum spin system Sr 14Cu 24O 41

We report muon Knight shift measurements on single crystals of Sr 14Cu 24O 41 and Ca 9La 5Cu 24O 41 to determine the muon stopping site in the isostructural compounds of the (Sr,Ca,La) 14Cu 24O 41 series. A site close to the ladder oxygen (0, 0.29, 0) is found. In addition, the magnetic correlations in the system Sr 14Cu 24O 41 have been studied by zero field μ +SR between room temperature and 2 K . Quasi-static muon spin relaxation is found below 10 K , whereas thermodynamic and neutron scattering reveal a singlet ground state.

Strong anomalous temperature dependence of the contact hyperfine contribution to the muon Knight shift in HoB 2C 2

Tetragonal HoB 2C 2 is one of the few compounds which show antiferroquadrupolar ordering below the onset of antiferromagnetic order (T Q =4.5 K, T N =5.9 K) . We have started the study of this system with the ultimate aim to learn more about the interaction of the magnetic and quadrupolar degrees of freedom in this context. In the contribution we present results of Knight-shift measurements on a single crystal above T N. The results, firstly, allowed to determine the muon site, which is identified to be the 8i interstitial site, and secondly, revealed an unusual temperature dependence of the contact hyperfine constant A c .

Spin-freezing in the two-dimensional spin-gap systems SrCu 2− xMg x(BO 3) 2 ( x=0, 0.04, 0.12)

The magnetic properties of the two-dimensional dimer spin-gap system SrCu 2(BO 3) 2 were investigated by the μ +SR technique. The relatively slow fluctuations of spin-dimers slow down with decreasing temperature and an unusual spin-freezing process is unraveled at T f<3.75 K, well within the spin-gap temperature range ( T SG≈20 K). This quasi-static phase displays a Gaussian field distribution with a remarkable stability with applied longitudinal fields. In support of the criticality of the SrCu 2(BO 3) 2 spin-gap ground state towards an antiferromagnetic transition, Knight-shift measurements suggest that implanted muons may liberate spin density at T< T SG that undergoes spin-freezing at very low temperatures. On the other hand, non-magnetic impurity-doping of the copper sublattice does not suppress the spin-gap ground state and does not lead to magnetic ordering effects of static nature.

μ +-Knight shift in the superconducting state of U 1− xTh xBe 13, x=0 and 0.035 single crystals

We report μ +-Knight shift measurements as a function of temperature in U 1− x Th x Be 13, x=0 and 0.035, single crystals. An anomalous change in the hyperfine coupling at low temperatures is observed which is similar to that reported in nuclear magnetic resonance studies of U- and Ce-based heavy fermion compounds. In the superconducting phase the static spin susceptibility, χ spin, determined from the μ +-Knight shift decreases continuously with decreasing temperature in pure UBe 13, but saturates below the second transition temperature in U 0.965Th 0.035Be 13. We briefly discuss these behaviors in terms of a two-component spin-triplet order parameter and propose μSR experiments that can test our hypothesis.

25Mg NMR determination of Knight shift, spin–lattice relaxation and electric field gradient in MgB 2

A measurement of the isotropic Knight shift ( K iso), nuclear spin–lattice relaxation time ( T 1) and electric field gradient V zz at the Mg site is reported from a 25Mg NMR investigation of a polycrystalline specimen of MgB 2. A comparison is made between the present experimental data and theoretical predictions.