Nuclear Magnetic Resonance Knight Shift Research Articles

Fully gapped pairing state in spin-triplet superconductor UTe2.

The recently discovered superconductor UTe2 is a promising candidate for spin-triplet superconductors, but the symmetry of the superconducting order parameter remains highly controversial. Here, we determine the superconducting gap structure by the thermal conductivity of ultraclean UTe2 single crystals. We find that the a-axis thermal conductivity divided by temperature κ/T in zero-temperature limit is vanishingly small for both magnetic field H‖a and H‖c axes up to H/Hc2 ∼ 0.2, demonstrating the absence of nodes around the a axis contrary to the previous belief. The present results, combined with the reduction of nuclear magnetic resonance Knight shift, indicate that the superconducting order parameter belongs to the isotropic Au representation with a fully gapped pairing state, analogous to the B phase of superfluid 3He. These findings reveal that UTe2 is likely to be a long-sought three-dimensional strong topological superconductor, hosting helical Majorana surface states on any crystal plane.

Al27 NMR local study of the Al0.5TiZrPdCuNi alloy in high-entropy alloy and metallic glass forms

We report a $^{27}\mathrm{Al}$ nuclear magnetic resonance (NMR) local spectroscopic study of the NMR lineshape and Knight shift of a six-component ${\mathrm{Al}}_{0.5}\mathrm{TiZrPdCuNi}$ metallic alloy that can be prepared either as a crystalline high-entropy alloy (HEA) or as an amorphous metallic glass (MG) at the same chemical composition. For both structural modifications of the material (HEA and MG), we have determined the distribution of electric-field-gradient (EFG) tensors and the local electronic density of states (DOS) $g({\ensuremath{\varepsilon}}_{\mathrm{F}})$ at the Fermi level at the position of $^{27}\mathrm{Al}$ nuclei. A theoretical $I=\frac{5}{2}$ quadrupole-perturbed NMR spectrum, pertinent to both cubic HEAs and amorphous MGs, has been derived using the Gaussian isotropic model of the EFG tensor distribution, and excellent fits of the experimental spectra were obtained. The EFG distribution function of the MG state is about twice broader than that of the HEA state, reflecting the existence of a (distorted) crystal lattice in the latter and its absence in the former. The ${T}^{2}$ dependence of the Knight shift indicates that the DOS is changing rapidly with energy within the Fermi level region for both structural modifications. The local DOS at the $^{27}\mathrm{Al}$ sites of the HEA sample is $\ensuremath{\sim}10%$ larger than that of the MG state, indicating comparable degrees of disorder.

Drastic change in magnetic anisotropy of UTe2 under pressure revealed by Te125 -NMR

To investigate the normal-state magnetic properties of UTe2 under pressure, we perform 125Te nuclear magnetic resonance (NMR) measurements up to 2 GPa. Below 1.2 GPa, the b-axis NMR Knight shift shows a broad maximum at the so-called T_chimax on cooling, which is consistent with the magnetization measurement under pressure. T_chimax decreases with increasing pressure and disappears at the critical pressure Pc = 1.7 GPa, above which superconductivity is destroyed. This tendency is also observed in the temperature dependence of the nuclear spin-lattice relaxation rate 1/T1. At low pressures, 1/T1 shows a conventional Fermi-liquid behavior (1/T1T = constant) at low temperatures, indicating the formation of the heavy-fermion state. Above Pc, 1/T1T follows a 1/T behavior without any crossover to the heavy-fermion state down to the lowest temperature (~3 K). In addition, the NMR signals disappear below 3~K, due to the influence of the magnetically ordered moments. From the pressure dependence of the T_chimax and Knight shift, it was found that the Fermi surface character is abruptly changed at Pc, and that superconductivity is observed only in the heavy-fermion state.

Pu239 nuclear magnetic resonance in the candidate topological insulator PuB4

We present a detailed nuclear magnetic resonance (NMR) study of ${}^{239}$Pu in bulk and powdered single-crystal plutonium tetraboride (PuB$_4$), which has recently been investigated as a potential correlated topological insulator. This study constitutes the second-ever observation of the ${}^{239}$Pu NMR signal, and provides unique on-site sensitivity to the rich $f$-electron physics and insight into the bulk gap-like behavior in PuB$_4$. The ${}^{239}$Pu NMR spectra are consistent with axial symmetry of the shift tensor showing for the first time that ${}^{239}$Pu NMR can be observed in an anisotropic environment and up to room temperature. The temperature dependence of the ${}^{239}$Pu shift, combined with a relatively long spin-lattice relaxation time ($T_1$), indicate that PuB$_4$ adopts a non-magnetic state with gap-like behavior consistent with our density functional theory (DFT) calculations. The temperature dependencies of the NMR Knight shift and $T_1^{-1}$--microscopic quantities sensitive only to bulk states--imply bulk gap-like behavior confirming that PuB$_4$ is a good candidate topological insulator. The large contrast between the ${}^{239}$Pu orbital shifts in the ionic insulator PuO$_2$ ($\sim$~+24.7~\%) and PuB$_4$ ($\sim$~-0.5~\%) provides a new tool to investigate the nature of chemical bonding in plutonium materials.

NMR probe of bulk electronic structures in [formula omitted] beam irradiated Bi2Te3 topological insulator

We report a nuclear magnetic resonance (NMR) study on H+ beam irradiated Bi2Te3 powdered single crystals. In this work, we demonstrate that the beam creates defects within its penetration range giving rise to delocalized charge carriers, thereby making further 125Te NMR Knight shift and line broadening. Upon increasing temperature, the NMR line narrowing manifests the activated motions of thermally excited charge carriers in the irradiated sample. In contrast, it reveals that in the unirradiated sample the free-charge carriers at the Fermi level dominantly contribute to the Knight shift. Our results show that the orbital contribution to the Knight shift in the bulk state of Bi2Te3 becomes predominant in the system with the higher density of defects, as evidenced by modified electronic structures induced by the beam irradiation.

Measurements of the NMR Knight shift tensor and nonlinear magnetization in URu2Si2

URu$_2$Si$_2$ exhibits an anomalous peak in the nonlinear magnetic susceptibility at the hidden order transition. In order to investigate this anomaly, we conducted direct magnetization measurements and investigated the detailed angular dependence of the $^{29}$Si nuclear magnetic resonance Knight shift tensor. We find that the nonlinear magnetization is smaller than previously reported, and the analogous nonlinear Knight shift tensor is below the detection limit. Our results suggest that the magnitude of the anomalous peak is sample dependent.

Semimetallic behavior in Heusler-type Ru2TaAl and thermoelectric performance improved by off-stoichiometry

We report a study of the temperature-dependent electrical resistivity, Seebeck coefficient, thermal conductivity, specific heat, and $^{27}\mathrm{Al}$ nuclear magnetic resonance (NMR) in Heusler-type ${\mathrm{Ru}}_{2}\mathrm{TaAl}$, to shed light on its semimetallic behavior. While the temperature dependence of the electrical resistivity exhibits semiconductorlike behavior, the analysis of low-temperature specific heat reveals a residual Fermi-level density of states (DOS). Both observations can be realized by means of a semimetallic scenario with the Fermi energy located in the pseudogap of the electronic DOS. The NMR Knight shift and spin-lattice relaxation rate show activated behavior at higher temperatures, attributing to the thermally excited carriers across a pseudogap in ${\mathrm{Ru}}_{2}\mathrm{TaAl}$. From the first-principles band structure calculations, we further provide a clear picture that an indirect overlap between electron and hole pockets is responsible for the formation of a pseudogap in the vicinity of the Fermi level of ${\mathrm{Ru}}_{2}\mathrm{TaAl}$. In addition, an effort for improving the thermoelectric performance of ${\mathrm{Ru}}_{2}\mathrm{TaAl}$ has been made by investigating the thermoelectric properties of ${\mathrm{Ru}}_{1.95}{\mathrm{Ta}}_{1.05}\mathrm{Al}$. We found significant enhancements in the electrical conductivity and Seebeck coefficient and marked reduction in the thermal conductivity via the off-stoichiometric approach. This leads to an increase in the figure-of-merit $ZT$ value from $6.1\ifmmode\times\else\texttimes\fi{}\phantom{\rule{0.16em}{0ex}}{10}^{\ensuremath{-}4}$ in ${\mathrm{Ru}}_{2}\mathrm{TaAl}$ to $3.4\ifmmode\times\else\texttimes\fi{}\phantom{\rule{0.16em}{0ex}}{10}^{\ensuremath{-}3}$ in ${\mathrm{Ru}}_{1.95}{\mathrm{Ta}}_{1.05}\mathrm{Al}$ at room temperature. In this respect, a further improvement of thermoelectric performance based on ${\mathrm{Ru}}_{2}\mathrm{TaAl}$ through other off-stoichiometric attempts is highly probable.

Defect-induced magnetism in SiC probed by nuclear magnetic resonance

We give evidence for intrinsic defect-induced bulk paramagnetism in SiC by means of $^{13}\mathrm{C}$ and $^{29}\mathrm{Si}$ nuclear magnetic resonance (NMR) spectroscopy. The temperature dependence of the internal dipole-field distribution, probed by the spin part of the NMR Knight shift and the spectral linewidth, follows the Curie law and scales very well with the macroscopic dc susceptibility. In order to quantitatively analyze the NMR spectra, a microscopic model based on dipole-dipole interactions was developed. The very good agreement between these simulations and the NMR data establishes a direct relation between the frequency distribution of the spectral intensity and the corresponding real-space volumes of nuclear spins. The presented approach by NMR can be applied to a variety of similar materials and, thus, opens a new avenue for the microscopic exploration and exploitation of diluted bulk magnetism in semiconductors.

11B pulsed nuclear magnetic resonance study of DyNi2B2C single crystals

DyNi2B2C is the only compound in the RNi2B2C (R = rare-earth element) series where superconductivity at Tc ∼6.2 K coexists with the antiferromagnetic ordering below the Nèel temperature TN ∼10.3 K. This system is of great interest because superconductivity rarely coexists with magnetic ordering. 11B pulsed nuclear magnetic resonance (NMR) measurements were performed at 8.0056 T to investigate the local electronic structures and 4f spin dynamics of DyNi2B2C powders and single crystals. The spectrum for the single crystal showed three narrow resonance peaks at 295 K due to the nuclear Zeeman splitting of a nuclear spin I = 3/2 with quadrupolar perturbation. The 11B NMR Knight shift of the single crystal was very large and highly anisotropic at K = −0.60% and +0.27% for the fields parallel and perpendicular, respectively, to the c-axis at 295 K. Considering the anisotropy of the Knight shift, we were able to simulate the 11B NMR power pattern that agreed well with the measured spectrum. The linewidth for DyNi2B2C was also large and anisotropic, and the linewidth value increased rapidly at low temperatures. The 11B NMR shift and linewidth were found to be proportional to the magnetic susceptibility, indicating that the hyperfine field at the B site originates from the 4f spins of Dy. Above TN, the values for 1/T1 (the spin-lattice relaxation rate) and 1/T2 (the spin-spin relaxation rate) were very large, showing slight increases at low temperatures. Below TN, the values of 1/T1 and 1/T2 were suppressed significantly because of the slowing of the 4f spin fluctuation. This confirmed the huge change in Dy 4f spin dynamics across the antiferromagnetic transition.

Detection of a spin-triplet superconducting phase in oriented polycrystalline U(2)PtC(2) samples using ^(195)Pt nuclear magnetic resonance.

Nuclear magnetic resonance (NMR) measurements on the ^{195}Pt nucleus in an aligned powder of the moderately heavy-fermion material U_{2}PtC_{2} are consistent with spin-triplet pairing in its superconducting state. Across the superconducting transition temperature and to much lower temperatures, the NMR Knight shift is temperature independent for field both parallel and perpendicular to the tetragonal c axis, expected for triplet equal-spin pairing superconductivity. The NMR spin-lattice relaxation rate 1/T_{1}, in the normal state, exhibits characteristics of ferromagnetic fluctuations, compatible with an enhanced Wilson ratio. In the superconducting state, 1/T_{1} follows a power law with temperature without a coherence peak giving additional support that U_{2}PtC_{2} is an unconventional superconductor. Bulk measurements of the ac susceptibility and resistivity indicate that the upper critical field exceeds the Pauli limiting field for spin-singlet pairing and is near the orbital limiting field, an additional indication for spin-triplet pairing.

27Al pulsed nuclear magnetic resonance study of CeAl2

The magnetic properties and the electronic structures of a rare-earth aluminum intermetallic compound CeAl2 are investigated by magnetic susceptibility measurements and 27Al pulsed nuclear magnetic resonance (NMR) techniques. The magnetic susceptibility is strongly temperature-dependent, following a Curie–Weiss law down to ∼12 K, and shows an antiferromagnetic transition at 4 K. The 27Al NMR spectra show a typical powder pattern for a nuclear spin I of 5/2 with the second-order nuclear quadrupole interaction at high temperature and an additional large dipolar broadening between the 4f electron spins of cerium and the 27Al nuclear spins at low temperature. The 27Al NMR Knight shift follows the same temperature dependence as the magnetic susceptibility, suggesting that the 27Al NMR Knight shift originates from the transferred hyperfine field of the Ce 4f electron spins with the hyperfine coupling constant of A = +5.7 kOe/μB. The spin-lattice relaxation rate 1/T1 is roughly proportional to temperature, as with most non-magnetic metals at high temperature, and then strongly temperature-dependent, increasing rapidly with a peak near the antiferromagnetic transition temperature and decreasing at lower temperature. The temperature dependence of the Korringa ratio K, however, suggests that the antiferromagnetic spin fluctuation signature, which is an enhancement in the Korringa ratio, is washed out owing to the geometrical cancellation of Ce 4f fluctuations at the Al sites.

Electron transport and magnetic properties of semimetallic LuAs

Abstract Polycrystalline samples of LuAs with a residual resistivity ratio (RRR) between 2.52 and 3.38 were examined by: X-ray diffraction, magnetisation, resistivity, magnetoresistivity, Hall coefficient and thermoelectric power S(T), in addition to the nuclear magnetic resonance (NMR) of 75As and 175Lu nuclei. The density and mobility of carriers for the valence and conduction bands were determined for the RRR = 2.52 sample and used for the calculation of thermoelectric power, Sc(T). The value of Sc(T) fits well with that determined by experiment. The sample shows a charge imbalance with the number of electrons n, (holes, p) equal to 1.52 × 1020 (1.12 × 1020) e/cm3 at 4 K. This imbalance decreases with temperature, reversing after passing a minimum at ∼80 K and then increasing to n (p) = 9.4 × 1019 (1.42 × 1020) e/cm3 at 300 K. The 75As NMR Knight shift is found to be temperature independent and equal to 0.102% and the 175Lu NMR resonance frequency is shifted to higher values by ca. 0.011% with respect to that of LuSb. All data indicate that LuAs belongs to semimetals and the clearly observed 175Lu NMR multiquadrupolar echoes suggest the existence of defects in the cubic structure of the investigated samples.

NMR investigation of the Knight shift anomaly in CeIrIn5at high magnetic fields

We report nuclear magnetic resonance Knight shift data in the heavy fermion material CeIrIn5 at fields up to 30 T. The Knight shift of the In displays a strong anomaly, and we analyze the results using two different interpretations. We find that the Kondo lattice coherence temperature and the effective mass of the heavy electrons remains largely unaffected by the magnetic field, despite the fact that the Zeeman energy is on the order of the coherence temperature.

Evolution of the pseudogap in Sr1−xYxAl2Si2(x=0, 0.05, 0.1, and 0.15) probed viaAl27nuclear magnetic resonance

With the aim of providing microscopic information about the electronic characteristics of SrAl${}_{2}$Si${}_{2}$ and effects of chemical substitution on its pseudogap features, we carried out a study on Sr${}_{1\ensuremath{-}x}$Y${}_{x}$Al${}_{2}$Si${}_{2}$ ($x=$ 0, 0.05, 0.1, and 0.15) by means of $^{27}\mathrm{Al}$ nuclear magnetic resonance (NMR) spectroscopy. For stoichiometric SrAl${}_{2}$Si${}_{2}$, the temperature-dependent NMR Knight shift and spin-lattice relaxation rate are associated with a sharp feature in the electronic density of state within a pseudogap at around the Fermi level. On the other hand, the NMR observations for Y-substituted compounds exhibit ordinary metallic behavior, suggesting that the Fermi level has moved out of the pseudogap for these materials, resulting in the dominant Korringa process responsible for the observed relaxation rates.

NMR evidence for the partially gapped state inCeOs2Al10

We report the results of a $^{27}\text{A}\text{l}$ nuclear magnetic resonance (NMR) study of ${\text{CeOs}}_{2}{\text{Al}}_{10}$ at temperatures between 4 and 300 K. This material has been of current interest due to indications of hybridization gap behavior below the transition temperature ${T}_{o}\ensuremath{\simeq}29\text{ }\text{K}$. Five $^{27}\text{A}\text{l}$ NMR resonance lines that are associated with five nonequivalent crystallographic aluminum sites have been resolved. For each individual aluminum site, the low-temperature NMR Knight shift goes over a thermally activated response. The temperature-dependent spin-lattice-relaxation rate exhibits a rapid drop below ${T}_{o}$, indicative of the formation of an energy gap in this material. We interpret the Knight shift and the relaxation-rate data in light of the presence of a pseudogap with residual electronic density of states at the Fermi level. Moreover, the magnitude of the pseudogap of 120 K is extracted from NMR results, in agreement with the value obtained from the inelastic neutron-scattering experiment.

27Al and 63Cu NMR studies on intermetallic Kondo compound CeCu 3Al 2

Detailed 27Al and 63Cu nuclear magnetic resonance (NMR) experiments have been performed in between 4.5 and 295 K on a polycrystalline sample of CeCu 3Al 2 which has a Kondo temperature ∼ 11 K . The NMR spectral features and Knight shift measurements show that in this compound, Cu occupy both c and g lattice sites, while Al occupy only the g sites. From the temperature dependence of Knight shift ( K), the hyperfine fields for 63Cu and 27Al at the g sites have been estimated as 2.02(5) and 1.67 ( 5 ) kOe / μ B , respectively, and that for 63Cu at c sites as − 3.83 ( 5 ) kOe / μ B . Nuclear spin-lattice relaxation time ( T 1) measurements indicate that though the relaxation process is governed by fluctuations of magnetic interaction, nuclear quadrupolar interaction also has a significant contribution, especially towards relaxation of 63Cu in both c and g sites. For 27Al, K and T 1 have yielded the effective 4f-spin correlation rate 1 / τ 4 f . The behavior of its temperature dependence indicates that at low temperatures in CeCu 3Al 2, the valence state of cerium might not be stable.

A27lNMR study of the mixed valence compoundCeFe2Al10

We report the results of a $^{27}\text{A}\text{l}$ nuclear magnetic resonance (NMR) study on the mixed valence compound ${\text{CeFe}}_{2}{\text{Al}}_{10}$ at temperatures between 4 and 300 K. This material has been of current interest due to indications of Kondo insulator behavior. Five $^{27}\text{A}\text{l}$ NMR resonance lines that are associated with five nonequivalent crystallographic aluminum sites have been resolved. For each individual aluminum site, the temperature-dependent NMR Knight shift exhibits a character of valence fluctuation behavior with a broad maximum at around 70 K. At low temperatures, the Knight-shift results show a thermally activated feature, indicating the formation of an energy gap in this material. We interpret the Knight-shift data based on a proposed model density of states and the gap size of 110 K is properly estimated. A further analysis indicates that the observed gap in ${\text{CeFe}}_{2}{\text{Al}}_{10}$ should be characterized as a pseudogap with a finite number of carriers at the Fermi level.

Comparative NMR investigation of the Re-based borides

We report a systematic study of the rhenium-based borides, ${\text{ReB}}_{2}$, ${\text{Re}}_{7}{\text{B}}_{3}$, and ${\text{Re}}_{3}\text{B}$, by means of the $^{11}\text{B}$ nuclear magnetic resonance (NMR) spectroscopy. While ${\text{Re}}_{7}{\text{B}}_{3}$ and ${\text{Re}}_{3}\text{B}$ are superconductors, ${\text{ReB}}_{2}$ exhibits no superconducting signature but is of current interest due to its superhard mechanical property. Since the major focus of this investigation is their electronic characteristics in the normal states, we performed the measurements at temperatures between 77 and 295 K. For ${\text{Re}}_{7}{\text{B}}_{3}$ and ${\text{Re}}_{3}\text{B}$, $s$-character electrons were found to be responsible for the observed $^{11}\text{B}$ NMR Knight shift and spin-lattice relaxation rate $(1/{T}_{1})$. From ${T}_{1}$ analysis, we thus deduce the partial $\text{B}\text{ }s$ Fermi-level density of states (DOS) of both borides. On the other hand, the relaxation rate of ${\text{ReB}}_{2}$ is mainly associated with $p$ electrons, similar to the cases of ${\text{OsB}}_{2}$ and ${\text{RuB}}_{2}$. In addition, the extracted $\text{B}\text{ }2p$ Fermi-level DOS is in good agreement with the theoretical prediction from band-structure calculations.

NMR STUDIES OF LAYERED NITRIDE SUPERCONDUCTORS

Layered nitride superconductor, Li 0.48( THF )y HfNCl with Tc=25.5 K , was investigated by means of nuclear magnetic resonance (NMR). 7 Li -, 35 Cl -NMR studies clarified that the partial electron density of states at Fermi level at Li and Cl sites are negligibly small. From the 15 N NMR Knight shift studies, the spin susceptibility decreases toward zero below Tc, evidencing that the pairing symmetry is an even-parity spin-singlet. These results indicate that superconductivity is derived from the HfN layer. We also measured 15 N -, 7 Li -NMR spin-lattice relaxation rate in the temperature range of 4.2K - 100 K and observed the enhancement of the spin-lattice relaxation rate, 1/ T 1, by field fluctuations generated by the vortex dynamics.

Screening of transverse ac fields and influence of supercurrent on NMR Knight shift in high- Tc crystals

In the nuclear magnetic resonance (NMR) configuration with perpendicular constant (dc) field and radiofrequency (rf) magnetic field, a strong screening supercurrent is generated by the rf field. In order to estimate this current which may influence Knight shift data, we performed a model experiment in the NMR configuration on a YBa 2Cu 4O 8 single crystal. The differential magneto-optical imaging technique is used to determine the ac screening current distribution in the sample. We find that at T > 20 K, NMR experiments can be straightforwardly interpreted. At T < 20 K, the rf field is completely excluded. In this situation, NMR only probes the crystal edges; moreover, the Knight shift may be affected by the supercurrent flow.