Spin polarization in the electron gas by a magnetic impurity - Theory of the excess Knight shift
Excess Knight shift theory of spin polarization in electron gas by magnetic impurities
Ritchie, B.↗
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Spin polarization in the electron gas by a magnetic impurity - Theory of the excess Knight shift
Excess Knight shift due to spin polarization in electron gas by magnetic impurity Fermi contact coupling as function of distance from nucleus
Ritchie, B.↗
$μ^+$ Knight shift in UTe 2 : Evidence for relocalization in a Kondo lattice
The local magnetic susceptibility of the spin-triplet superconductor UTe 2 has been investigated by positive muon (μ + ) Knight shift measurements in the normal state. Three distinct μ + Knight shift components are observed for a magnetic field applied parallel to the c axis. Two of these exhibit a breakdown in the linear relationship with the bulk magnetic susceptibility (χ) below a temperature T*~30K, which points to a gradual emergence of a correlated Kondo liquid. Below T r ~12K linearity is gradually restored, indicating relocalization of the Kondo liquid quasiparticles. The third Knight shift component is two orders of magnitude larger, and despite the c-axis alignment of the external field, scales with the a axis χ above T r ~12K. We conjecture that this component is associated with magnetic clusters and the change in the temperature dependence of all three Knight shift components below T r is associated with a change in magnetic correlations. Finally, our findings indicate that prior to the onset of superconductivity the development of the itinerant heavy-electron fluid is halted by a gradual development of local U 5ƒ-moment fluctuations.
36 MATERIALS SCIENCE↗
Sub-millisecond 125 Te NMR spin-lattice relaxation times and large Knight shifts in complex tellurides: Validation of a quadratic relation across the spectrum
125 Te NMR spectra and spin-lattice relaxation times, T 1 , have been measured for several GeTe-based materials with Te excess. In this paper, the spectra show inhomogeneous broadening by several thousand ppm and a systematic variation in T 1 relaxation time with resonance frequency. The quadratic dependence of the spin-lattice relaxation rate, 1/T 1 , on the Knight shift in the Korringa relation is found to be valid over a wide range of Knight shifts. This result confirms that T 1 relaxation in GeTe-based materials is mostly dominated by hyperfine interaction between nuclei and free charge carriers. In GeTe with 2.5% excess of Te, about 15% of the material exhibits a Knight shift of ≥4500 ppm and a T 1 of only 0.3 ms, indicating a high hole concentration that could correspond to close to 50% vacancies on the Ge sublattice in this component. Lastly, our findings provide a basis for determining the charge carrier concentration and its distribution in complex thermoelectric and phase-change tellurides, which should lead to a better understanding of electronic and thermal transport properties as well as chemical bonding in these materials.
125Te NMR↗
Measurements of the NMR Knight shift tensor and nonlinear magnetization in URu 2 Si 2
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 nd 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.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Universal Knight shift anomaly in the periodic Anderson model
Here, we report a Determinant Quantum Monte Carlo investigation which quantifies the behavior of the susceptibility and the entropy in the framework of the periodic Anderson model (PAM), focussing on the evolution with different degree of conduction electron (c) -local moment (f) hybridization. These results capture the behavior observed in several experiments, including the universal behavior of the NMR Knight shift anomaly below the crossover temperature, T*. We find that T* is a measure of the onset of c-f correlations and grows with increasing hybridization. Our results suggest that the NMR Knight shift and spin-lattice relaxation rate measurements in non-Fermi liquid materials are strongly influenced by temperature-dependent hybridization processes. Furthermore, our results provide a microscopic basis for the phenomenological two-fluid model of Kondo lattice behavior, and its evolution with pressure and temperature.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Knight Shift and Leading Superconducting Instability from Spin Fluctuations in Sr 2 RuO 4
We report that recent nuclear magnetic resonance studies [A. Pustogow et al. , Nature 574, 72 (2019)] have challenged the prevalent chiral triplet pairing scenario proposed for Sr 2 RuO 4 . To provide guidance from microscopic theory as to which other pair states might be compatible with the new data, we perform a detailed theoretical study of spin fluctuation mediated pairing for this compound. We map out the phase diagram as a function of spin-orbit coupling, interaction parameters, and band structure properties over physically reasonable ranges, comparing when possible with photoemission and inelastic neutron scattering data information. We find that even-parity pseudospin singlet solutions dominate large regions of the phase diagram, but in certain regimes spin-orbit coupling favors a near-nodal odd-parity triplet superconducting state, which is either helical or chiral depending on the proximity of the γ band to the van Hove points. A surprising near degeneracy of the nodal s ' and d x 2 - y 2 wave solutions leads to the possibility of a near-nodal time-reversal symmetry broken s ' + i d x 2 - y 2 pair state. Lastly, predictions for the temperature dependence of the Knight shift for fields in and out of plane are presented for all states.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NMR characterization of sulphur substitution effects in the K x Fe 2 - y Se 2 - z S z high- T c superconductor
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phonons, thermal conductivity, energy storage (inc↗
YPdSn and YPd{sub 2}Sn: Structure, {sup 89}Y solid state NMR and {sup 119}Sn Moessbauer spectroscopy
The stannides YPdSn and YPd{sub 2}Sn were synthesized by high-frequency melting of the elements in sealed tantalum tubes. Both structures were refined on the basis of single crystal X-ray diffractometer data: TiNiSi type, Pnma, a=715.4(1), b=458.8(1), c=789.1(1) pm, wR2=0.0461, 510 F{sup 2} values, 20 variables for YPdSn and MnCu{sub 2}Al type, Fm3 Macron m, a=671.44(8), wR2=0.0740, 55 F{sup 2} values, 5 parameters for YPd{sub 2}Sn. The yttrium atoms in the new stannide YPdSn are coordinated by two tilted Pd{sub 3}Sn{sub 3} hexagons (ordered AlB{sub 2} superstructure). In the Heusler phase YPd{sub 2}Sn each yttrium atom has octahedral tin coordination and additionally eight palladium neighbors. The cubic site symmetry of yttrium is reflected in the {sup 119}Sn Moessbauer spectrum which shows no quadrupole splitting. In contrast, YPdSn shows a single signal at {delta}=1.82(1) mm/s subjected to quadrupole splitting of {Delta}E{sub Q}=0.93(1) mm/s. Both compounds have been characterized by high-resolution {sup 89}Y solid state NMR spectroscopy, which indicates the presence of strong Knight shifts. The spectrum of YPd{sub 2}Sn is characterized by an unusually large linewidth, suggesting the presence of a Knight shift distribution reflecting local disordering effects. The range of {sup 89}Y Knight shifts of several binary and ternary intermetallic yttrium compounds is briefly discussed. - Graphical abstract: YPdSn and YPd{sub 2}Sn: Structure, {sup 89}Y solid state NMR and {sup 119}Sn Moessbauer spectroscopy. Highlights: Black-Right-Pointing-Pointer Synthesis and structure of ternary stannides YPdSn and YPd{sub 2}Sn. Black-Right-Pointing-Pointer {sup 119}Sn Moessbauer spectroscopic investigation of YPdSn and YPd{sub 2}Sn. Black-Right-Pointing-Pointer {sup 89}Y solid state NMR of intermetallics.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NMR Characterization of Sulphur Substitution Effects in the KxFe2−ySe2−zSz High-Tc Superconductor
We present a {sup 77}Se NMR study of the effect of S substitution in the high-T{sub c} superconductor K{sub x}Fe{sub 2-y}Se{sub 2-z}S{sub z} in a temperature range up to 250 K. We examine two S concentrations, with z = 0.8 (T{sub c} {approx} 26 K) and z = 1.6 (nonsuperconducting). The samples containing sulphur exhibit broader NMR line shapes than the K{sub x}Fe{sub 2}Se{sub 2} sample due to local disorder in the Se environment. Our Knight shift {sup 77}K data indicate that in all samples, uniform spin susceptibility decreases with temperature, and that the magnitude of the Knight shift itself decreases with increased S concentration. In addition, S substitution progressively suppresses low-frequency spin fluctuations. None of the samples exhibit an enhancement of low-frequency antiferromagnetic spin fluctuations near T{sub c} in 1/T{sub 1}T, as seen in FeSe.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Nuclear magnetic resonance investigation of the heavy fermion system Ce 2 CoAl 7 Ge 4
We present here nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements performed on single crystalline ${\mathrm{Ce}}_{2}{\mathrm{CoAl}}_{7}{\mathrm{Ge}}_{4}$, a member of a recently discovered family of heavy fermion materials ${\mathrm{Ce}}_{2}M{\mathrm{Al}}_{7}{\mathrm{Ge}}_{4}$ ($M=\mathrm{Co}$, Ir, Ni, or Pd). Previous measurements indicated a strong Kondo interaction as well as magnetic order below ${T}_{M}=1.8\mathrm{K}$. Our NMR spectral measurements show that the Knight shift $K$ is proportional to the bulk magnetic susceptibility ${\chi}$ at high temperatures. A clear Knight shift anomaly ($K\neg{}{\propto}{\chi}$) is observed at coherence temperatures ${T}^{*}{\sim}17.5\mathrm{K}$ for ${H}_{0}{\parallel}\stackrel{\hat{}{}{}}{c}$ and 10 K for ${H}_{0}{\parallel}\stackrel{\hat{}{}{}}{a}$ at the $^{59}\mathrm{Co}$ site, and ${T}^{*}{\sim}12.5\mathrm{K}$ at the $^{27}\mathrm{Al}$(3) site for ${H}_{0}{\parallel}\stackrel{\hat{}{}{}}{a}$ characteristic of the heavy fermion nature of this compound. At high temperatures, the $^{59}\mathrm{Co}$ NMR spin-lattice relaxation rate ${T}_{1}^{{-}1}$ is dominated by spin fluctuations of the $4f$ local moments with a weak metallic background. The spin fluctuations probed by $^{59}\mathrm{Co}$ NMR are anisotropic and larger in the basal plane than in the $c$ direction. Furthermore, we find ${({T}_{1}TK)}^{{-}1}{\propto}{T}^{{-}1/2}$ at the $^{59}\mathrm{Co}$ site as expected for a Kondo system for $T>{T}^{*}$ and $T>{T}_{K}$. $^{59}\mathrm{Co}$ NQR ${T}_{1}^{{-}1}$ measurements at low temperatures indicate slowing down of spin fluctuations above the magnetic ordering temperature ${T}_{M}{\sim}1.8\mathrm{K}$. A weak ferromagnetic character of fluctuations around $\mathbf{q}=0$ is evidenced by an increase of ${\chi}T$ versus $T$ above the magnetic ordering temperature. We also find good agreement between the observed and calculated electric field gradients at all observed sites.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Nuclear magnetic resonance in Kondo lattice systems
Nuclear magnetic resonance has emerged as a vital tool to explore the fundamental physics of Kondo lattice systems. Because nuclear spins experience two different hyperfine couplings to the itinerant conduction electrons and to the local f moments, the Knight shift can probe multiple types of spin correlations that are not accessible via other techniques. Furthermore, the Knight shift provides direct information about the onset of heavy electron coherence and the emergence of the heavy electron fluid.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Residual spin susceptibility in the spin-triplet orbital-singlet model
Nuclear magnetic resonance 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. In conclusion, we observe a non-negligible residual spin-susceptibility at low temperature.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Long range order and two-fluid behavior in heavy electron materials
The heavy electron Kondo liquid is an emergent state of condensed matter that displays universal behavior independent of material details. Properties of the heavy electron liquid are best probed by NMR Knight shift measurements, which provide a direct measure of the behavior of the heavy electron liquid that emerges below the Kondo lattice coherence temperature as the lattice of local moments hybridizes with the background conduction electrons. Because the transfer of spectral weight between the localized and itinerant electronic degrees of freedom is gradual, the Kondo liquid typically coexists with the local moment component until the material orders at low temperatures. The two-fluid formula captures this behavior in a broad range of materials in the paramagnetic state. In order to investigate two-fluid behavior and the onset and physical origin of different long range ordered ground states in heavy electron materials, we have extended Knight shift measurements to URu 2 Si 2 , CeIrIn 5 , and CeRhIn 5 . In CeRhIn 5 we find that the antiferromagnetic order is preceded by a relocalization of the Kondo liquid, providing independent evidence for a local moment origin of antiferromagnetism. In URu 2 Si 2 the hidden order is shown to emerge directly from the Kondo liquid and so is not associated with local moment physics. Lastly, our results imply that the nature of the ground state is strongly coupled with the hybridization in the Kondo lattice in agreement with phase diagram proposed by Yang and Pines.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NMR study of topological insulator Bi{sub 2}Te{sub 3} in a wide temperature range
NMR studies of {sup 125}Te in the topological insulator bismuth telluride Bi{sub 2}Te{sub 3} in a wide temperature range from room temperature to 12.5 K are performed. The pulsed NMR spectrometer Bruker Avance 400 is applied. The NMR spectra are obtained for the powder from Bi2Te3 single crystal and monocrystalline plates with the orientations c || B and c ⊥ B. At room temperature, the spectra consist of two lines related to two nonequivalent positions of tellurium nuclei Te1 and Te2. The parameters of the NMR frequency shift tensor are found from the powder spectrum. The temperature dependences of the spectra for the powder and plates with the orientation c ⊥ B agree with each other. The line shift with decreasing temperature is explained by the reduction of the Knight shift. The thermal activation energy of charge carriers is estimated. The spectra for the plates with the orientation c || B demonstrate peculiar behavior below 91 K. The spin-lattice relaxation time for the powder and monocrystalline plates with both orientations at room temperature is measured.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Bias voltage dependence of the electron spin depolarization in quantum wires in the quantum Hall regime detected by the resistively detected NMR
We performed the resistively-detected nuclear magnetic resonance (RDNMR) to study the electron spin polarization in the non-equilibrium quantum Hall regime. By measuring the Knight shift, we derive source-drain bias voltage dependence of the electron spin polarization in quantum wires. The electron spin polarization shows minimum value around the threshold voltage of the dynamic nuclear polarization.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
RFe{sub 2}Mg{sub x}Al{sub 8−x} (R=La–Nd and Sm; x≈0.8): Flux synthesis, structure, magnetic and electrical properties
Single crystals of Mg-substituted CeFe{sub 2}Al{sub 8} type intermetallics RFe{sub 2}Mg{sub x}Al{sub 8–x} (R=La–Nd and Sm; x≤1) were grown by reacting iron and rare earth metals in 1:1 Mg/Al mixed flux. The structure features mono-capped and bi-capped trigonal prismatic FeAl{sub 6} units. Electronic structure calculations indicate that magnesium substitution reduces the valence electron count, shifting the Fermi level away from a pseudo-gap. This changes the electronic nature of the cerium analog; the previously reported ternary CeFe{sub 2}Al{sub 8} shows strong hybridization between the cerium states and the conduction electrons, resulting in no magnetic moment on Ce atoms. On the other hand, magnetic susceptibility measurements on CeFe{sub 2}Mg{sub x}Al{sub 8–x} indicates a localized moment on cerium. The newly synthesized Pr, Nd and Sm analogs exhibit antiferromagnetic ordering at 2.8 K, 7.8 K and 12 K respectively. Solid state {sup 27}Al NMR of LaFe{sub 2}Mg{sub x}Al{sub 8–x} exhibits a broad Knight shift at ~1200 ppm, consistent with the metallic behavior shown by electrical resistivity data. - Graphical abstract: Mg substitution into CeFe{sub 2}Al{sub 8} modifies cerium valence due to changing valence electron count. - Highlights: • RFe{sub 2}Mg{sub x}Al{sub 8−x} (R=La–Nd, Sm) grow as large crystals from reactions in Mg/Al flux. • Products are magnesium-substituted variants of CeFe{sub 2}Al{sub 8}, with CaCo{sub 2}Al{sub 8} structure. • Ce magnetic moment in CeFe{sub 2}Mg{sub x}Al{sub 8−x} varies from that in CeFe{sub 2}Al{sub 8} due to VEC change. • Antiferromagnetic ordering observed for Pr, Nd, Sm analogs of RFe{sub 2}Mg{sub x}Al{sub 8−x}.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Electron density distribution in BaPb{sub 1-x}Sb{sub x}O{sub 3} superconducting oxides studied by double nuclear magnetic resonance methods
The effect of charge disorder on the formation of an inhomogeneous state of the electron system in the conduction band in BaPb{sub 1-x}Sb{sub x}O{sub 3} superconducting oxides is investigated experimentally by NMR methods. The NMR spectra of {sup 17}O are measured systematically, and the contributions from {sup 17}O atoms with different cation nearest surroundings are identified. It is found that microscopic regions with an elevated spin density of charge carriers are formed within two coordination spheres near antimony ions. Nuclei of the superconducting phase of the oxide (regions with an elevated antimony concentration) microscopically distributed over the sample are detected in compounds with x = 0.25 and 0.33. Experiments in which a double resonance signal of the spin echo of {sup 17}O-{sup 207}Pb and {sup 17}O-{sup 121}Sb are measured in the metal phase of BaPb{sub 1-x}Sb{sub x}O{sub 3} oxides are carried out for the first time. The constants of indirect heteronuclear spin-spin {sup 17}O-{sup 207}Pb interaction are determined as functions of the local Knight shift {sub 207}Ks. The estimates of the constants of the indirect interaction between the nuclei of the nearest neighbors (O-Pb and Pb-Pb atoms) and analysis of evolution of the NMR spectra of {sup 17}O upon a change in the antimony concentration are convincing evidence in favor of the development of a microscopically inhomogeneous state of the electron system in the metal phase of BaPb{sub 1-x}Sb{sub x}O{sub 3} oxides.
36 MATERIALS SCIENCE↗