Knight Shift Measurements Research Articles

Impurity effects and spin polarizations in a narrow quantum Hall system

The temperature dependence of electron spin polarization for a narrow quantum Hall system shows behavior analogous to that of a two-dimensional system at major filling factors. At the lowest half-filled quantum Hall state for which no two-dimensional analog exists, we find a stable spin partially-polarized system. Periodic Gaussian repulsive impurities (antidots) in such a system leads to novel spin transitions at $\nu=\frac13$ and $\nu=\frac12$ and the pair-correlation functions provide clues about nature of different ground states in the system. These results can be explored in optical spectroscopy and optically pumped NMR Knight shift measurements.

Nonunitary Spin-Triplet Superconductivity inUPt3: Evidence fromP195tKnight Shift Study

${}^{195}\mathrm{Pt}$ Knight shift (KS) measurements covering the superconducting multiple phases for major field ( $H$) orientations have been carried out on the high-quality single crystal ${\mathrm{UPt}}_{3}$. For $Hg5$ kOe, the KS does not change below the superconducting transition temperature ${T}_{c}$ down to 28 mK, regardless of major crystal orientations, which provides evidence that the odd-parity superconductivity with the parallel spin pairing is realized. By contrast, the KS decreases below ${T}_{c}$ for ${H}_{b}\ensuremath{\parallel}b$ axis and ${H}_{b}l5$ kOe and for ${H}_{c}\ensuremath{\parallel}c$ axis and ${H}_{c}l2.3$ kOe, whereas the KS for ${H}_{a}\ensuremath{\parallel}a$ axis is $T$ independent across ${T}_{c}$ down to ${H}_{a}\ensuremath{\sim}1.764$ kOe. These novel findings entitle ${\mathrm{UPt}}_{3}$ as the first spin-triplet odd-parity superconductor including a nonunitary pairing characterized by the two-component $\mathbf{d}$ vector like ${\mathbf{d}}_{b}+i{\mathbf{d}}_{c}$ at low $T$ and low $H$.

Study of Sodium Colloids in Heavily Irradiated NaCl Crystals by Nuclear Magnetic Resonance

AbstractStrong irradiation of NaCl crystals results in a formation of sodium colloids which exhibit metallic properties. In consequence, besides the 23Na line of NaCl a second well‐separated 23Na line occurs in the NMR spectrum caused by the Knight shift of the conduction electrons. We present Knight shift and nuclear spin relaxation measurements performed on the “metallic” 23Na line in e–‐irradiated NaCl single‐crystals between about 4 and 390 K. Evaluation of the data leads to detailed information on the electronic properties, atomic self‐diffusion, and melting behavior of the colloids. The results are discussed in view of corresponding data obtained in bulk sodium.

Temperature dependence of the muon Knight shift in graphite measured using a μSR\times 2 method

We describe a simple method by which time differential μSR spectra are collected on two different samples simultaneously. One application is to make accurate measurements of the muon precession frequency in a sample relative to a reference. As an example we report precise measurements of the muon Knight shift on HOPG graphite with H parallel to \hat c as a function of temperature.

Do the measurements of 27Al and 7Li Knight shifts have any analytical role in studies of freshly prepared lithium aluminium alloys?

27Al and 7Li Knight shifts have been measured for a series of freshly prepared Li/Al alloys. The phase diagram implies that three phases should be accessible for the compositions (up to 6% Li) considered and it is shown that, particularly in the 27Al spectra, three Knight-shifted resonances are observable for several compositions. It is suggested that these might correspond to the three phases (α, δ, δ′). The resonance believed to be associated with the δ′ phase is sensitive to the lithium content, but changes discontinuously when precipitation of the cubic δ phase (LiAl) occurs.

51V NMR Study in the Paramagnetic State of LaVO3

The magnetic susceptibility (χ) and the 51 V Knight shift ( K ) measurements are performed on the perovskite LaVO 3 in the paramagnetic state. By analyzing the K -χ diagram, independent contributions of spin and orbital susceptibilities to the observed one are determined experimentally. The spin susceptibility was found to be suppressed from the Curie-Weiss law for S =1 spins well above the Neel temperature. It was also found that the hyperfine coupling constant for d spin is largely reduced from the usual core-polarization contribution. The analysis based on a local-moment model shows that this reduction may be caused by a large transferred hyperfine coupling between the neighboring V 3+ ions rather than by covalency with O 2- ions. This accounts for all the NMR data rather quantitatively, though, the sign and the magnitude of the transferred hyperfine field are not compatible with any existing microscopic theories.

Thermodynamics and spin polarizations of the fractional quantum Hall states.

The filling factor and temperature dependence of the electron spin polarization $〈{S}_{z}(T)〉$ of a two-dimensional electron system have been studied for various values of Zeeman coupling. At $T\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0$, there are sharp spin transitions for all the filling fractions considered here except $\ensuremath{\nu}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\frac{1}{3}$. At low $T$, the appearance of a peak in $〈{S}_{z}(T)〉$ at $\ensuremath{\nu}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\frac{2}{3}$ and $\ensuremath{\nu}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\frac{2}{5}$ is interpreted as a manifestation of the reentrant behavior observed earlier. At $\ensuremath{\nu}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\frac{3}{5}$, one sees a gradual trasition from one spin state to another at low $T$. At large $T$, $〈{S}_{z}(T)〉$ generally decays as ${T}^{\ensuremath{-}1}$. These can be explored in the optically pumped nuclear magnetic resonance Knight shift measurements.

Rotational dynamics of C604- and electronic excitation in Rb4C60.

10% $^{13}\mathrm{C}$ enriched ${\mathrm{Rb}}_{4}$${\mathrm{C}}_{60}$ was investigated by $^{13}\mathrm{C}$ NMR. The relaxation rates ${\mathit{T}}_{1}^{\mathrm{\ensuremath{-}}1}$ and ${\mathit{T}}_{2}^{\mathrm{\ensuremath{-}}1}$ are basically determined by reorientation of the ${\mathrm{C}}_{60}^{4\mathrm{\ensuremath{-}}}$ molecules. In addition, ${\mathit{T}}_{1}^{\mathrm{\ensuremath{-}}1}$ exhibits an activated electronic contribution with activation energy 104 meV. The same activation energy is also found in Knight-shift measurements. Both compounds, ${\mathrm{Rb}}_{4}$${\mathrm{C}}_{60}$ and ${\mathrm{K}}_{4}$${\mathrm{C}}_{60}$, exhibit the same electronic excitation with the same activation energy within experimental error. We attribute this to a local excitation into a high-spin state of the Jahn-Teller distorted ${\mathrm{C}}_{60}^{4\mathrm{\ensuremath{-}}}$.

Redetermination of the 13C Knight shift in K 3C 60

Due to the absence of suitable reference substance, previous determinations of the 13C Knight shift in K 3C 60 are incorrect. We propose that K 3C 60(THF) 3 should be a good reference for the Knight shift measurement and with this new reference we find that the 13C Knight shift in K 3C 60 is only 6ppm, indicating that the electronic density of states for K 3C 60 at the Fermi level in normal state is relatively lower, which agrees well with previous photoemission results.

Observation of precipitation in Mg-Al alloys by 25Mg and 27Al NMR

A combination of 25Mg and 27Al NMR has been shown to be an effective structural probe for the intermetallic phase Mg17Al12, able to detect precipitation of this phase in dilute alloys of aluminium in magnesium during age hardening, with well-resolved signals from the precipitate and the alloy. Knight shift and spin-lattice relaxation measurements for the intermetallic phase are examined for evidence of a quasigap at the Fermi surface, recently proposed as the origin for the stability of this phase.

63Cu NMR study in high-T c superconductor TlSr 2(Lu 1− xCa x)Cu 2o y

63Cu NMR measuements have been carried out in heavily-doped TlSr 2CaCu 2O 7−δ with T c = 70K, 52K and OK, respectively, and in lightly-doped TlSr 2(Lu 0.7Ca 0.3)Cu 2O y with T c = 40K under a magnetic field of 11T. From a systematic measurement of the nuclear-spin lattice relaxation rate, 63(1/T 1) , the antiferromagnetic (AF) spin correlation is suppressed with increasing hole content, i.e. with decreasing T c as in the case of Tl 2Ba 2CuO 6+y. From the Knight shift measurement, it has been found that the T-independent behavior of the spin susceptibility above T c in heavily-doped system changes into a T-decreasing one in the lightly-doped TlSr 2(Lu 0.7Ca 0.3)Cu 2O y, which is the same as in YBa 2Cu 3O 6.63.

63Cu NMR study in non-doped and Y-doped Bi 2Sr 2CaCu 2O 8 single crystals

The normal and superconducting properties in single crystals Bi 2Sr 2CaCu 2O 8 (Bi2212) and Bi 2Sr 2(Ca 0.95Y 0.05)Cu 2O 8 (5%Y-doped Bi2212) have been investigated by systematic measurements of the 63Cu Knight shift, 63K(T) and the nuclear spin-lattice relaxation time, 63T 1.

NMR studies of alkali C60 superconductors

The authors report13C,87Rb,39K, and133Cs nuclear magnetic resonance (NMR) measurements of lineshapes, Knight shifts, and spin-lattice relaxation rates in the normal and superconducting states of M3C60, where M3 = Rb3, K3, Rb2K, RbK2, Rb2Cs, and RbCs2. Measurements are used as a guide to a new ammonia solvent synthesis technique. Temperature dependence of the superconducting state electron spin susceptibility is found to follow BCS weak coupling predictions. The issue of the Hebel-Slichter coherence peak is addressed.

Martensitic transformation in Ag-Cd and Cu-Zn alloys studied by nuclear magnetic resonance.

$^{113}\mathrm{Cd}$, $^{109}\mathrm{Ag}$, and $^{63}\mathrm{Cu}$ nuclear-magnetic-resonance line-shape and Knight-shift measurements are reported for ${\mathrm{Ag}}_{53.5}$${\mathrm{Cd}}_{46.5}$ and ${\mathrm{Cu}}_{61}$${\mathrm{Zn}}_{39}$ over a temperature interval covering the martensitic phase transformation (MPT) displayed by these alloys. The changes observed in the line shape and width are explained in terms of the changes in atomic coordination and crystal symmetry occurring at the MPT. A small but non-negligible temperature dependence of the Knight shift is observed in the austenitic phase with small discontinuous changes at the transformation for all the nuclei investigated. The difference of resonance frequency of $^{113}\mathrm{Cd}$ in the two phases has been utilized to obtain the curve of the percentage of martensite in the Ag-Cd alloy as a function of temperature during the thermoelastic transformation. It is shown that our growth curve reveals details which cannot be seen by dynamical methods such as differential scanning calorimetry.

Nmr Studies of Electronic and Local Structure in Cu-Au Alloys

ABSTRACTWe have studied ordered and disordered Cu-Au alloys via 63Cu NMR, probing local electronic structure in the bulk and near anti-phase boundaries in CuAuII. A line-shape model has provided good agreement for disordered and partially ordered alloys, and we thus have obtained a measure of local site symmetries, and Knight shifts and local densities of states within the alloys. We have combined Knight shift and relaxation measurements to obtain detailed local information. We report effects of the ordering process in Cu3Au, CuAu, and CuAu3 intermetallics. We find enhanced susceptibility at the CuAuII anti-phase boundary which is heavily anti-site populated. We compare these results to other measurements of these properties.

NMR measurement of superconducting-state spin susceptibility in alkali fullerides.

The authors report $^{13}\mathrm{C}$, $^{87}\mathrm{Rb}$, and $^{133}\mathrm{Cs}$ nuclear magnetic resonance measurements of line shapes and Knight shifts in the normal and superconducting states of ${\mathrm{Rb}}_{3}$${\mathrm{C}}_{60}$ and ${\mathrm{Rb}}_{2}$${\mathrm{CsC}}_{60}$, and deduce the temperature dependence of the electron spin susceptibility. The results are in good agreement with the BCS theory in the weak-coupling limit: 2${\mathrm{\ensuremath{\Delta}}}_{0}$=3.52${\mathit{kT}}_{\mathit{c}}$.

Screening of the middle CuO2 layer in Bi1.6Pb0.4Sr2Ca2Cu3O10 determined from Cu NMR.

Knight-shift measurements of the $^{63}\mathrm{Cu}$ nuclei in the sandwiching Cu(2)${\mathrm{O}}_{2}$ layers and the middle Cu(1)${\mathrm{O}}_{2}$ layer in the three-layer bismuth oxide superconductor have been performed. Spin-lattice-relaxation measurements reveal a larger spin-fluctuation enhancement at the Cu(1) sites relative to the Cu(2) sites. This and a reduced spin susceptibility in the Cu(1)${\mathrm{O}}_{2}$ layer indicates that the Cu(2)${\mathrm{O}}_{2}$ layers screen the charge transfer from the bismuth bilayer to the Cu(1)${\mathrm{O}}_{2}$ layer. Our results show that the Cu(1)${\mathrm{O}}_{2}$ layer has distinctly different physical properties than the Cu(2)${\mathrm{O}}_{2}$ layer.

Knight shifts in Si:P and Si:(P,B) in the vicinity of the metal-insulator transition.

Recent experimental and theoretical work has suggested that a two-fluid model can be used to describe the magnetic properties of the just-metallic phase of metal-insulator systems such as Si:P. Alternative descriptions based on a Fermi liquid approach have also been put forward to explain the magnetic and other behaviors of these systems. In the present work $^{29}\mathrm{Si}$ Knight-shift measurements have been made on a number of samples of Si:P and Si:(P,B) in the just-metallic region as a function of temperature and electron concentration. The magnetization of the $^{29}\mathrm{Si}$ spin system follows Curie law behavior down to 50 mK. The mean Knight shift is found to be temperature independent in the range 50 mK--4.2 K for both compensated and uncompensated systems. This result is consistent with a proposal that the $^{29}\mathrm{Si}$ spins that are observed in the Knight-shift measurements interact only with the itinerant electron fluid, even though the susceptibility is controlled by the other fluid of localized moments.

17O Knight shifts of the various types of CuO2 planes in Bi-cuprates high-Tc superconductors

A 17 O NMR study has been performed on a 17 O enriched powder sample of (Bi, Pb)2 Sr2 Ca2 Cu3 Oy , also called n = 3 phase (with Tc = 110K) which belongs to the Bi-based superconductors family (Bi, Pb)2 Sr2 Can-1 Cun O2n+4 The n = 3 compound which contains in its unit cell two types of CuO2 planes (labelled type I and II), Is compared to the n = 2 compound where only one type of CuCO2 planes (type I) is present. 17 O Knight shift measurements versus temperature, in the normal phase, have allowed us to evidence the distinct behaviours of the two types of planes present in the n = 3 compound. The results are consistent with the existence of stronger electron correlations, or smaller charge carrier density in the type II planes.

Magnetic resonance determination of the electronic density of states and the metal-non-metal transition in the LaH 2LaH 3 system

Proton spin-lattice relaxation rate R 1 and 139La Knight shift measurements have been utilized to study the electronic density of states N( E F ) at the Fermi level and the metal-non-metal (MNM) transition in the lanthanum hydrides, LaH x (2 ⩽ x ⩽ 3), in samples prepared from highest purity Ames Laboratory lanthanum as well as from lanthanum containing controlled low levels (less than 300 ppm) of gadolinium. In the pure samples, free-electron-like metallic behavior is found for all compositions up to x ≈ 2.8 at temperatures T ⩽ 200 K. In the gadolinium-doped samples the Gd 3+ ion spin fluctuations, characterized by the ion spin-lattice relaxation time τ i ,contribute an additional proton spin relaxation rate. Measurements of this rate for x < 2.8 show that τ i ∝ T −1 at all temperatures, indicating metallic behavior. However, for x ⩾ 2.8 and T > 250 K, τ i ∝ T −5, showing that Gd 3+ spin relaxation is then governed by the two-phonon process typical of insulating solids.