Spin-echo Decay Rate Research Articles

Local moments in the heterogeneous electronic state of Cd-substituted CeCoIn5: NQR relaxation rates

We have used nuclear quadrupole resonance (NQR) to probe microscopically the response of a prototypical quantum critical metal CeCoIn5 to substitutions of small amounts of Cd for In. Approximately half of the Cd substituents induce local Ce moments in their close proximity, as observed by site-dependent longitudinal nuclear spin relaxation rates 1/T1. To reaffirm that localized f moments are induced around the Cd substituents, we find a Gaussian spin-echo decay rate 1/T2G of transverse nuclear spin relaxation. Further, for the NQR subpeak is found to be proportional to temperatures, again indicating local moments fluctuations around the Cd substituents, while that for the NQR main peak shows a T0.7-dependence. The latter temperature dependence is close to 0.75 in pure CeCoIn5 and indicates that the bulk electronic state is located close to a two dimensional quantum critical instability.

Persistence of slow fluctuations in the overdoped regime ofBa(Fe1−xRhx)2As2superconductors

We present nuclear magnetic resonance evidence that very slow ($\leq 1$ MHz) spin fluctuations persist into the overdoped regime of Ba(Fe$_{1-x}$Rh$_{x}$)$_2$As$_2$ superconductors. Measurements of the $^{75}$As spin echo decay rate, obtained both with Hahn Echo and Carr Purcell Meiboom Gill pulse sequences, show that the slowing down of spin fluctuations can be described by short-range diffusive dynamics, likely involving domain walls motions separating $(\pi/a,0)$ from $(0,\pi/a)$ correlated regions. This slowing down of the fluctuations is weakly sensitive to the external magnetic field and, although fading away with doping, it extends deeply into the overdoped regime.

Evidence of unconventional low-frequency dynamics in the normal phase of Ba(Fe1−xRhx)2As2iron-based superconductors

This work presents 75As NMR spin echo decay rate (1/T2) measurements in Ba(Fe1-xRhx)2As2 superconductors, for 0.041 < x < 0.094. It is shown that 1/T2 increases upon cooling, in the normal phase, suggesting the onset of an unconventional very low-frequency activated dynamic. The correlation times of the fluctuations and their energy barriers are derived. The motion is favored at large Rh content, while it is hindered by the application of a magnetic field perpendicular to the FeAs layers. The same dynamic is observed in the spin-lattice relaxation rate, in a quantitatively consistent manner. These results are discussed in the light of nematic fluctuations involving domain wall motion. The analogies with the behaviour observed in the cuprates are also outlined.

NMR investigation of vortex dynamics in the Ba(Fe0.93Rh0.07)2As2superconductor

75As NMR spin-lattice relaxation (1/T1) and spin-echo decay (1/T2) rate measurements were performed in a single crystal of Ba(Fe0.93Rh0.07)2As2 superconductor. Below the superconducting transition temperature Tc, when the magnetic field H is applied along the c axes, a peak in both relaxation rates is observed. Remarkably that peak is suppressed for H || ab. Those maxima in 1/T1 and 1/T2 have been ascribed to the flux lines lattice motions and the corresponding correlation times and pinning energy barriers have been derived on the basis of an heuristic model. Further information on the flux lines motion was derived from the narrowing of 75As NMR linewidth below Tc and found to be consistent with that obtained from 1/T2 measurements. All the experimental results are described in the framework of thermally activated vortices motions.

Observation of the Partial Fermi Surface Quenching in the Noncentrosymmetric Superconductor Mo3Al2C

We study the normal state properties of a noncentrosymmetric superconductor Mo 3 Al 2 C by means of the nuclear magnetic resonance (NMR) experiment. The change of NMR line shape and the peak of the nuclear spin-echo decay rate at ≃130 K indicate that the Mo 3 Al 2 C exhibits the phase transition prior to the emergence of superconductivity. The reduction in the conduction electron density is inferred from the decrease in the nuclear spin–lattice relaxation rate. By considering the result of the energy band calculation, we discussed the nature of the newly found phase transition.

Heterogeneous spin state in the field-induced phase of volborthite as seen via51V nuclear magnetic resonance

We report results of ${}^{51}$V NMR in the field-induced phase of volborthite Cu${}_{3}$V${}_{2}$O${}_{7}$(OH)${}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}$2H${}_{2}$O, a spin-1/2 antiferromagnet on a distorted kagom\'e lattice. In magnetic fields above 4.5 T, two types of V sites with different spin-echo decay rates are observed. The hyperfine field at the fast decaying sites has a distribution, while it is more homogeneous at the slowly decaying sites. Our results indicate a heterogeneous state consisting of two spatially alternating Cu spin systems, one of which exhibits anomalous spin fluctuations contrasting with the other showing a conventional static order.

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.

NMR Studies of Half-Heusler Type Compounds YbPtSb and LuPtSb

The 121/123 Sb and 195 Pt nuclear magnetic resonance (NMR) results are presented for the half-Heusler compound YbPtSb. The characteristics of localized 4 f spins were observed in the Cure–Weiss type behavior of Knight shifts. The spin–lattice relaxation rate (1/ T 1 ) gradually increases with decreasing temperature and shows a maximum at around 20 K. An approximate analysis of the 4 f spins correlation rate 1/τ f was performed using the measured values of 1/ T 1 and bulk susceptibility. We observed a √T dependence of 1/τ f between 10 and 220 K, which is indicative of an existence of isolated local moment fluctuations. The spin–echo decay rate exhibits a distinct peak at 10 K. We suppose this peak is associated with the magnetic fluctuation originating from 4 f electrons. The crystalline defects in the half-Heusler compounds are also discussed together with the NMR results in LuPtSb.

NMR studies in the half-Heusler type compound YbPtSb

121Sb and 19BPt nuclear magnetic resonance (NMR) has been studied in the half-Heusler type compound YbPtSb to obtain information on local magnetic behavior. The characteristics of the localized 4f spins are observed in the Cuire-Weiss type behavior of the Knight shifts K for both 121Sb and 19BPt. From the slope of K-χ plots we estimated hyperfine coupling constants of -3.8 and -4.6 kOe/μB at Sb and Pt sites, respectively. It was found that the spin-echo decay rate 1/T2 of 121Sb shows a clear peaks at 10 K. Similar tendency was also observed in case of 19BPt. However, static properties do not show any anomalies near 10 K.

Quantum criticality in a uranium heavy fermion system revealed with NMR spin-spin relaxation

In order to clarify quantum criticality in heavy fermion systems, we propose to use the Gaussian spin-echo decay rate 1/T2G of the NMR spin-spin relaxation. In contrast to the spin-lattice relaxation rate 1/T1, which is connected with the dynamical susceptibility Imχ(q,ω), I/T2G is driven by the static susceptibility Reχ(q,0) at finite q for an antiferromagnet. The combined quantity T1T/T22G is found to provide key information in determining the nature of quantum criticality. The T-dependence of T1T/T22G for the Sn site in the heavy fermion compound USn3 is presented as an example.

Phase Diagram and Spin Dynamics in Volborthite with a Distorted Kagome Lattice

We report a (51)V-NMR study on a high-quality powder sample of volborthite Cu3V2O7(OH)2 . 2H2O, a spin-1/2 Heisenberg antiferromagnet on a distorted kagome lattice formed by isosceles triangles. In the magnetic fields below 4.5 T, a sharp peak in the nuclear spin-lattice relaxation rate 1/T(1) accompanied with line broadening revealed a magnetic transition near 1 K. The low temperature phase shows anomalies such as a Lorentzian line shape, a 1/T(1) proportional, variantT behavior indicating dense low-energy excitations, and a large spin-echo decay rate 1/T(2) pointing to unusually slow fluctuations. Another magnetic phase appears above 4.5 T with less anomalous spectral shape and dynamics.

Crossover from the Quantum Critical to Overdamped Regime in the Heavy-Fermion SystemUSn3

We report measurements of the 119Sn nuclear spin-echo decay rate 1/T2G in the heavy-fermion compound USn3. From 1/T2G, the magnetic spin-spin correlation length xi is found to vary as xi approximately T(-3/4) above approximately 100 K, which is expected for a quantum critical regime at high temperatures. Combined with the spin-lattice relaxation rate 1/T1, T1T/T2G2 is found to be temperature independent in the heavy-fermion state below T* approximately 30 K. This indicates that the heavy-fermion state of USn3 is categorized in the overdamped regime with a dynamical critical exponent z=2. These observations are consistent with a spin density wave magnetic instability at the quantum critical point.

Inhomogeneous state of the electron system in the Sr14 − x Ca x Cu24O41 superconducting cuprate: 63Cu-17O NMR study

Charge fluctuations in quasi-one-dimensional hole-doped Sr14Cu24O41 (Ca-0) and Sr2Ca12Cu24O41 (Ca-12) spin-ladder cuprates have been studied with the use of 63Cu-17O NMR. Spin-echo decay rates 17(1/T2) and 63(1/T2) have been measured in the temperature range of T = 10–300 K. The variation of 17(1/T2) and 63(1/T2) in the Ca-0 compound is monotonic in the entire temperature range of the NMR study. In the Ca-rich compound, pronounced peaks have been observed in the decay rates 17(1/T2) and 63(1/T2) at temperatures of 25 and 50 K, respectively. This result indicates the presence of collective low-frequency (τc ∼ T2) charge density fluctuations in superconducting Ca-12 with an activation energy of EA = 100(10) K. The fluctuations gradually slow down as the temperature decreases. The amplitude of the charge density fluctuations is only 0.01–0.02 hole per site. 17O-63Cu spin-echo double-resonance (SEDOR) experiments in Sr14 − xCaxCu24O41 oxides with x = 0 and 12 have been performed depending on the temperature and orientation of single crystals in a magnetic field. The constants of an indirect heteronuclear 17O-63Cu interaction of nuclear spins mediated by conduction electrons have been measured. The estimates of the indirect interaction constants for nearest neighbors O1-Cu and O2-Cu, as well as SEDOR experiments with selective excitation of separate sections of 17O and 63Cu NMR spectra, provide convincing evidence of the microscopically inhomogeneous spatial distribution of spin density developing in a crystal.

Copper valence, structural separation and lattice dynamics in tennantite (fahlore): NMR, NQR and SQUID studies

Electronic and magnetic properties of tennantite subfamily of tetrahedrite-group minerals have been studied by copper nuclear quadrupole resonance (NQR), nuclear magnetic resonance (NMR) and SQUID magnetometry methods. The temperature dependences of copper NQR frequencies and line-width, nuclear spin-lattice relaxation rate T 1 −1 and nuclear spin-echo decay rate T 2 −1 in tennantite samples in the temperature range 4.2–210 K is evidence of the presence of field fluctuations caused by electronic spins hopping between copper CuS3 positions via S2 bridging atom. The analysis of copper NQR data at low temperatures points to the magnetic phase transition near 65 K. The magnetic susceptibility in the range 2–300 K shows a Curie–Weiss behavior, which is mainly determined by Fe2+ paramagnetic substituting ions.

H1-NMR spin-echo measurements of the spin dynamic properties inλ−(BETS)2FeCl4

$^{1}$H-NMR spin-echo measurements of the spin-echo decay $M(2\tau)$ with a decay rate 1/$T_{2}$ and the frequency shift $\Delta\nu/\nu_{0}$ under applied magnetic field $\mathbf{B}$$_{0}$ = 9 T along the a-axis over a temperature range 2.0$-$180 K are reported for a single crystal of the organic conductor $\lambda$-(BETS)$_{2}$FeCl$_{4}$. It provides the spin dynamic and static properties in the paramagnetic metal (PM) and antiferromagnetic insulator (AFI) states as well as across the PM$-$AFI phase transition. A large slow beat structure in the spin-echo decay is observed with a typical beat frequency of $f$ $\sim$ 7 kHz and it varies across the spectrum. Its origin is attributed to the $^{1}$H$-$$^{1}$H dipole interactions rather than to the much larger dipolar field contribution from the Fe$^{3+}$ electrons (spin $S$ = 5/2). A simple phenomenological model provides an excellent fit to the data. The dominant $^{1}$H-NMR frequency shift comes from the dipolar field from the 3d Fe$^{3+}$ ions, and the Fe$^{3+}$ $-$ Fe$^{3+}$ exchange interactions ($J_{0}$) ($J_{0} $ includes the d$-$d exchange interactions through the $\pi-$electrons) have a substantial effect to the local field at the proton sites expecially at low temperatures. A good fit is obtained with $J_{0}$ = - 1.7 K. The data of the spin-echo decay rate 1/$T_{2}$ indicates that there is a significant change in the slow fluctuations of the local magnetic field at the $^{1}$H-sites on traversing the PM to AFI phase. This evidence supports earlier reports that the PM$-$AFI phase transition in $\lambda$-(BETS)$_{2}% $FeCl$_{4} $ is driven magnetically and first order.

NMR evidence for very slow carrier density fluctuations in the organic metal(TMTSF)2ClO4

We have investigated the origin of the large increase in spin-echo decay rates for the $^{77}\mathrm{Se}$ nuclear spins at temperatures near to $T=30\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in the organic superconductor ${(\mathrm{TMTSF})}_{2}{\mathrm{ClO}}_{4}$. The measured angular dependence of ${T}_{2}^{\ensuremath{-}1}$ demonstrates that the source of the spin-echo decays lies with carrier density fluctuations rather than fluctuations in TMTSF molecular orientation. The very long time scales are directly associated with the dynamics of the anion ordering occurring at $T=25\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, and the inhomogeneously broadened spectra at lower temperatures result from finite domain sizes. Our results are similar to observations of line-broadening effects associated with charge-ordering transitions in quasi-two-dimensional organic conductors.

Cu-NQR study for possible charge density wave state in CuO chain confined in insulating phase of Pr 1− xY xBa 2Cu 3O 7

We have performed NQR measurement on the Cu(1) site in the CuO chain in Pr 1− x Y x Ba 2Cu 3O 7 in order to investigate how the charge density wave (CDW) state observed previously in the end compound PrBa 2Cu 3O 7 develops in going metallic phase with increasing x. The Cu(1)-NQR evidence for the CDW transition, that is the unusual broadening of the NQR spectrum after the critical divergence of the nuclear spin echo decay rate and/or nuclear spin–lattice relaxation rate, was found to be confined in x ≲ 0.5 being essentially insulating phase in Pr 1− x Y x Ba 2Cu 3O 7. The disappearance of the CDW state is owing to a loss of perfect one-dimensionality in the CuO chain when the CuO 2 plane becomes metallic.

Inhomogeneous Electronic Structure Probed by Spin-Echo Experiments in the Electron Doped High-TcSuperconductorPr1.85Ce0.15CuO4−y

63Cu nuclear magnetic resonance spin-echo decay rate (T-12) measurements are reported for the normal and superconducting states of a single crystal of Pr(1.85)Ce(0.15)CuO(4-y) in a magnetic field B(0)=9 T over the temperature range 2<T<200 K. The spin-echo decay rate is temperature dependent for T<55 K and has a substantial dependence on the radio frequency (rf) pulse parameters below T approximately 25 K. This dependence indicates that T-12 is strongly effected by a local magnetic field distribution that can be modified by the rf pulses, including ones that are not at the nuclear Larmor frequency. The low-temperature results are consistent with the formation of a static inhomogeneous electronic structure that couples to the rf fields of the pulses.

Charge freezing in the zigzag chain PrBa2Cu4O8 cuprate.

We report nuclear quadrupole resonance (NQR) studies on the chain Cu sites of PrBa2Cu4O8, a quasi-one-dimensional conductor with a nearly quarter-filled band. The nuclear spin-lattice relaxation rate 1/T1 shows a pronounced peak near 100 K caused by fluctuations of electric field gradient. Similar peak was observed for the spin-echo decay rate 1/T2, however, at a different temperature near 50 K. These results and broadening of the NQR spectrum at low temperatures indicate that slow charge fluctuations of either electronic or ionic origin freeze gradually at low temperatures.

Two-dimensional t-J model at moderate doping

Using the method which retains the rotation symmetry of spin components in the paramagnetic state and has no preset magnetic ordering, spectral and magnetic properties of the two-dimensional t-J model in the normal state are investigated for the ranges of hole concentrations 0 <= x <= 0.16 and temperatures 0.01t <= T <= 0.2t. The used hopping t and exchange J parameters of the model correspond to hole-doped cuprates. The obtained solutions are homogeneous which indicates that stripes and other types of phase separation are not connected with the strong electron correlations described by the model. A series of nearly equidistant maxima in the hole spectral function calculated for low T and x is connected with hole vibrations in the region of the perturbed short-range antiferromagnetic order. The hole spectrum has a pseudogap in the vicinity of (0,\pi) and (\pi,0). For x \approx 0.05 the shape of the hole Fermi surface is transformed from four small ellipses around (\pm\pi/2,\pm\pi/2) to two large rhombuses centered at (0,0) and (\pi,\pi). The calculated temperature and concentration dependencies of the spin correlation length and the magnetic susceptibility are close to those observed in cuprate perovskites. These results offer explanations for the observed scaling of the static uniform susceptibility and for the changes in the spin-lattice relaxation and spin-echo decay rates in terms of the temperature and doping variations in the spin excitation spectrum of the model.