Low-frequency Spin Fluctuations Research Articles

La139 and Cu63 NMR investigation of charge order in La2CuO4+y ( Tc=42 K)

We report $^{139}$La and $^{63}$Cu NMR investigation of the successive charge order, spin order, and superconducting transitions in super-oxygenated La$_2$CuO$_{4+y}$ single crystal with stage-4 excess oxygen order at $T_{stage}\simeq 290$ K. We show that the stage-4 order induces tilting of CuO$_6$ octahedra below $T_{stage}$, which in turn causes $^{139}$La NMR line broadening. The structural distortion continues to develop far below $T_{stage}$, and completes at $T_{charge}\simeq 60$ K, where charge order sets in. This sequence is reminiscent of the the charge order transition in Nd co-doped La$_{1.88}$Sr$_{0.12}$CuO$_4$ that sets in once the low temperature tetragonal (LTT) phase is established. We also show that the paramagnetic $^{63}$Cu NMR signals are progressively wiped out below $T_{charge}$ due to enhanced low frequency spin fluctuations, but the residual $^{63}$Cu NMR signals continue to exhibit the characteristics expected for optimally doped superconducting CuO$_2$ planes. This indicates that charge order in La$_2$CuO$_{4+y}$ does not take place uniformly in space. Low frequency Cu spin fluctuations as probed by $^{139}$La nuclear spin-lattice relaxation rate are mildly glassy, and do not exhibit critical divergence at $T_{spin}$($\simeq T_{c}$)=42 K. These findings, including the spatially inhomogeneous nature of the charge ordered state, are qualitatively similar to the case of La$_{1.885}$Sr$_{0.115}$CuO$_4$ [T. Imai et al., Phys. Rev. B 96 (2017) 224508, and A. Arsenault et al., Phys. Rev. B 97 (2018) 064511], but both charge and spin order take place more sharply in the present case.

La139 NMR investigation of the charge and spin order in a La1.885Sr0.115CuO4 single crystal

$^{139}\mathrm{La}$ NMR is suited for investigations into magnetic properties of ${\mathrm{La}}_{2}{\mathrm{CuO}}_{4}$-based cuprates in the vicinity of their magnetic instabilities, owing to the modest hyperfine interactions between $^{139}\mathrm{La}$ nuclear spins and Cu electron spins. We report comprehensive $^{139}\mathrm{La}$ NMR measurements on a single-crystal sample of high-${T}_{c}$ superconductor ${\mathrm{La}}_{1.885}{\mathrm{Sr}}_{0.115}{\mathrm{CuO}}_{4}$ in a broad temperature range across the charge and spin order transitions $({T}_{\mathrm{charge}}\ensuremath{\simeq}80$ K, ${T}_{\mathrm{spin}}^{\mathrm{neutron}}\ensuremath{\simeq}{T}_{c}=30$ K). From the high-precision measurements of the linewidth for the nuclear spin ${I}_{z}=+1/2$ to $\ensuremath{-}1/2$ central transition, we show that paramagnetic line broadening sets in precisely at ${T}_{\mathrm{charge}}$ due to enhanced spin correlations within the ${\mathrm{CuO}}_{2}$ planes. Additional paramagnetic line broadening ensues below $\ensuremath{\sim}35$ K, signaling that Cu spins in some segments of ${\mathrm{CuO}}_{2}$ planes are on the verge of three-dimensional magnetic order. A static hyperfine magnetic field arising from ordered Cu moments along the $ab$ plane, however, begins to develop only below ${T}_{\mathrm{spin}}^{\ensuremath{\mu}SR}=15--20$ K, where earlier muon spin rotation measurements detected Larmor precession for a small volume fraction $(\ensuremath{\sim}20%)$ of the sample. Based on the measurement of $^{139}\mathrm{La}$ nuclear-spin-lattice relaxation rate $1/{T}_{1}$, we also show that charge order triggers enhancement of low-frequency Cu spin fluctuations inhomogeneously; a growing fraction of $^{139}\mathrm{La}$ sites is affected by enhanced low-frequency spin fluctuations toward the eventual magnetic order, whereas a diminishing fraction continues to exhibit a behavior analogous to the optimally superconducting phase even below ${T}_{\mathrm{charge}}$. These $^{139}\mathrm{La}$ NMR results corroborate our recent $^{63}\mathrm{Cu}$ NMR observation that a very broad, anomalous winglike signal gradually emerges below ${T}_{\mathrm{charge}}$, whereas the normally behaving, narrower main peak is gradually wiped out [T. Imai et al., Phys. Rev. B 96, 224508 (2017)]. Furthermore, we show that the enhancement of low-energy spin excitations in the low-temperature regime below ${T}_{\mathrm{spin}}^{\mathrm{neutron}}\phantom{\rule{4pt}{0ex}}(\ensuremath{\simeq}{T}_{c})$ depends strongly on the magnitude and orientation of the applied magnetic field.

NMR investigation of spin correlations inBaCo2As2

We use NMR techniques to investigate the magnetic properties of BaCo2As2 single crystals, the non-superconducting end member of the Co-substituted iron-pnictide high Tc superconductor Ba(Fe1-xCox)2As2 with x = 1. We present 75As NMR evidence for enhancement of low frequency spin fluctuations below ~ 100 K. This enhancement is accompanied by that of static uniform spin susceptibility at the wave vector q = 0, suggesting that the primary channel of the spin correlations is ferromagnetic rather than antiferromagnetic. Comparison between the NMR Knight shift 75K and bulk susceptibility chi(bulk) data uncovers the presence of two separate components of spin susceptibility with distinct temperature dependences, presumably because multiple electronic bands crossing the Fermi energy play different roles in the electronic properties of BaCo2As2.

Strongly anisotropic spin response as a signature of the helical regime in Rashba nanowires

Rashba nanowires in a magnetic field exhibit a helical regime when the spin-orbit momentum is close to the Fermi momentum, k_F \approx k_{SO}. We show that this regime is characterized by a strongly anisotropic electron spin susceptibility, with an exponentially suppressed signal along one direction in spin space, and that there are no low frequency spin fluctuations along this direction. Since the spin response in the gapless regime k_F \not \approx k_{SO} has a power law behavior in all three directions, spin measurements provide a signature of the helical regime that complements spin-insensitive conductance measurements.

Low-Frequency Spin Fluctuations in Skyrmions Confined by Wires: Measurements of Local Nuclear Spin Relaxation

We investigate low-frequency electron spin dynamics in a quantum Hall system with wire confinement by nuclear spin relaxation measurements. We developed a technique to measure the local nuclear spin relaxation rate T(1)(-1). T(1)(-1) is enhanced on both sides of the local filling factor ν(wire)=1, reflecting low-frequency fluctuations of electron spins associated with Skyrmions inside the wire. As the wire width is decreased, the fast nuclear spin relaxation is suppressed in a certain range of Skyrmion density. This suggests that the multi-Skyrmion state is modified and the low-frequency spin fluctuations are suppressed by the wire confinement.

Structural and dynamical study of moment localization inβ-Mn1−xInx

We have used neutron scattering and muon spin relaxation (muSR) to investigate the structural and magnetic properties of the beta-phase of elemental manganese doped with dilute amounts of indium. beta-Mn is an example of a topologically frustrated antiferromagnetically correlated metal - but which remains paramagnetic at all temperatures. The addition of In to beta-Mn results in a vast volume expansion of the lattice, and would therefore be expected to have a major effect on the stability and localization of the Mn moment - as observed in, for example, Ru and Al doped beta-Mn alloys. We find that In doping in beta-Mn results in a short-range ordered spin-glass like ground state, similar to that of Al-doped beta-Mn but with residual low frequency spin fluctuations. This is in contrast to Ru doping which results in the stabilization of a long-range ordered Mn moment

Spin Susceptibility, Phase Diagram, and Quantum Criticality in the Eelctron-Doped High Tc Superconductor Ba(Fe1-xCox)2As2

We report a systematic investigation of Ba(Fe 1- x Co x ) 2 As 2 based on transport and 75 As NMR measurements, and establish the electronic phase diagram. We demonstrate that doping progressively suppresses the uniform spin susceptibility and low frequency spin fluctuations. The optimum superconducting phase emerges at x c ≃0.08 when the tendency toward spin ordering completely diminishes. Our findings point toward the presence of a quantum critical point near x c between the SDW (spin density wave) and superconducting phases.

F19NMR investigation of the iron pnictide superconductorLaFeAsO0.89F0.11

We report on $^{19}\text{F}$ nuclear magnetic resonance (NMR) investigation of the high-temperature superconductor ${\text{LaFeAsO}}_{0.89}{\text{F}}_{0.11}$ $({T}_{c}\ensuremath{\sim}28\text{ }\text{K})$. We demonstrate that low-frequency spin fluctuations exhibit pseudogap behavior above ${T}_{c}$. We also deduce the London penetration depth $\ensuremath{\lambda}$ from NMR line broadening below ${T}_{c}$.

Effects of in-plane magnetic field direction on the nuclear-spin relaxation in the [formula omitted] quantum Hall state

We investigated effects of the direction of in-plane magnetic field B ∥ on the phase transition in the quantum Hall ferromagnet at Landau-level filling factor ν = 2 3 by the nuclear-spin relaxation measurements. We controlled the angle φ between the direction of B ∥ and a current I; when B ∥ is parallel (perpendicular) to I, φ = 0 ∘ ( φ = 90 ∘ ) . We found that the nuclear spin relaxation rate T 1 - 1 at φ = 0 ∘ is about twice as much as that at φ = 90 ∘ . We discuss the anisotropic behavior of T 1 - 1 in term of a suppression of low-frequency electron spin fluctuations by B ∥ at φ = 90 ∘ .

μSR study of spin fluctuations in RMn 4Al 8 compounds ( [formula omitted], Dy, Ho, Er)

The compounds RMn 4Al 8 are characterized by very weak magnetic interactions between the rare earth (R) sites and the Mn sites. The Mn sites form one-dimensional chains and it likely that this inhibits moment formation and leads to low frequency spin fluctuations. ZF-μSR studies show similar behaviour in all four compounds with R = Tb , Dy, Ho and Er. The relaxation spectrum has two components: the slow relaxation rate peaks below the temperature where the ZFC susceptibility has a cusp, associated with ordering of the Mn moments. The second component has a high relaxation rate that increases rapidly at low temperatures. We tentatively attribute this to the slowing down of the rare earth spin dynamics.

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.

Spin excitations in the(Sr,Ca)14Cu24O41family of spin ladders:63Cuand17ONMR studies under pressure

We report the results of a NMR study of hole doped spin ladders belonging to the series ${\mathrm{Sr}}_{14\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{Cu}}_{24}{\mathrm{O}}_{41+\ensuremath{\delta}}$ performed on ${}^{63}\mathrm{Cu}$ and ${}^{17}\mathrm{O}$ nuclei. The new results obtained on Ca0 and Ca12 single crystals at ambient pressure and also under 32 kbar confirm the onset of low-lying spin fluctuations modes at zero energy coexisting with spin-gapped excitations when superconductivity can be stabilized under pressure in Ca12. We found that the theoretical two and three magnons mechanisms explain fairly well the spin-lattice relaxation data using the magnitude of the pressure dependent magnon spin gap ${\ensuremath{\Delta}}_{s}$ derived from the Knight shifts data as long as most of the spectral weight for low-frequency spin fluctuations is provided by the magnon dispersion of isolated ladders. The cross over between spin gapped and paramagnetic regimes of decoupled Heisenberg chains can be identified in heavily doped ladders via the temperature dependence of dynamical structure factors at $\mathbf{q}\ensuremath{\sim}(\ensuremath{\pi},\ensuremath{\pi})$ and $\mathbf{q}\ensuremath{\sim}(0,0)$ wave vectors. The cross over temperature scales under pressure with ${\ensuremath{\Delta}}_{s}/2.$

Low-frequency spins and the ground state in high- Tc cuprates

We study the spin fluctuations of pure and Zn-substituted high temperature superconductors (HTS) using the muon spin relaxation (μSR) technique. Superconductivity is found to coexist with low frequency spin fluctuations over a large region of the superconducting phase diagram. The characteristic temperature of spin fluctuations detected by μSR decreases with increasing doping and vanishes above a critical doping p c≈0.19 where the normal state pseudogap vanishes. Furthermore, it is at p c that our penetration depth studies show an abrupt change in the doping dependence of the superfluid density, ρ s. For p> p c the absolute value of ρ s is large and nearly independent of carrier concentration whereas for p< p c it drops rapidly, signalling a crossover to weak superconductivity. These results are discussed in terms of a quantum transition involving fluctuating short-range magnetic order that separates the superconducting phase diagram of HTS into two distinct ground states.

Systematic63CuNQR and89YNMR Study of Spin Dynamics inY1−zCazBa2Cu3Oyacross the Superconductor-Insulator Boundary

We demonstrate that the spin dynamics in underdoped Y(1-z)Ca(z)Ba(2)Cu(3)O(y) for y approximately equal to 6.0 exhibit qualitatively the same behavior to underdoped La(2-x)Sr(x)CuO(4) for an equal amount of hole concentration p = z/2 = x< or =0.11. However, a spin gap appears as more holes are doped into the CuO(2) plane by increasing the oxygen concentration to y approximately equal to 6.5 for a fixed value of Ca concentration z. Our results also suggest that Ca doping causes disorder effects that enhance the low frequency spin fluctuations.

Critical behavior in the low-energy spin fluctuations in high-T(c) superconductors

We study the spin fluctuations of pure and Zn-substituted La2 − xSrxCuO4 using the muon spin relaxation technique. Superconductivity is found to coexist with low-frequency spin fluctuations over a large region of the superconducting phase diagram. The characteristic temperature of spin fluctuations detected by μSR decreases with increasing x and vanishes above a critical doping xc ≈ 0.19. This value of xc coincides with the doping at which the normal state pseudogap extrapolates to zero. These results are discussed in terms of a quantum transition that separates the superconducting phase diagram of high-temperature superconductors into two distinct ground states.

Spins in the vortices of a high-temperature superconductor.

Neutron scattering is used to characterize the magnetism of the vortices for the optimally doped high-temperature superconductor La(2-x)Sr(x)CuO4 (x = 0.163) in an applied magnetic field. As temperature is reduced, low-frequency spin fluctuations first disappear with the loss of vortex mobility, but then reappear. We find that the vortex state can be regarded as an inhomogeneous mixture of a superconducting spin fluid and a material containing a nearly ordered antiferromagnet. These experiments show that as for many other properties of cuprate superconductors, the important underlying microscopic forces are magnetic.

Nuclear Spin Relaxation in Hole-Doped Two-Leg Ladders

The nuclear spin-lattice relaxation rate ($1/T_{1}$) has been measured in the single crystals of hole doped two-leg ladder compounds Sr$_{14-x}$Ca$_{x}$Cu$_{24}$O$_{41}$ and in the undoped parent material La$_6$Ca$_8$Cu$_{24}$O$_{41}$. Comparison of $1/T_{1}$ at the Cu and the two distinct oxygen sites revealed that the major spectral weight of low frequency spin fluctuations is located near $q \sim (\pi, \pi)$ for most of the temperature and doping ranges investigated. Remarkable difference in the temperature dependence of $1/T_1$ for the two oxygen sites in the heavily doped $x$=12 sample revealed reduction of singlet correlations between two legs in place of growing antiferromagnetic correlations along the leg direction with increasing temperature. Such behavior is most likely caused by the dissociation of bound hole pairs.

Shot noise in ferromagnetic single-electron tunneling devices

Frequency dependent current noise in ferromagnetic double junctions with Coulomb blockade is studied theoretically in the limit of sequential tunneling. Two different relaxation processes are found in the correlations between spin polarized tunneling currents; low frequency spin fluctuations and high frequency charge fluctuations. Spin accumulation in strongly asymmetric junctions is shown to lead to a negative differential resistance. We also show that large spin noise activated in the range of negative differential resistance gives rise to a significant enhancement of the current noise.

SR evidence of low frequency spin fluctuations in the AF phase of hole-doped NiO

μSR measurements in the antiferromagnetic (AF) phase of Ni1-xLixO for \(\) are reported. While in pure NiO the muon longitudinal depolarization rate \(\) is found almost temperature independent, in the Li-doped compounds broad maxima around 130 K are observed. These maxima are associated with the progressive freezing of the spin fluctuations of S=1/2 defects induced by the localization of the extra-holes. From the temperature dependence of \(\) and the stretched exponential form of the depolarization, insights on the distribution of correlation times for the fluctuating field at the muon site are derived.

Low-frequency spin fluctuations in the superconducting La 2−xSr xCuO 4

Recent progress in low-energy neutron scattering study on the spatially modulated or so-called incommensurate spin fluctuations is reviewed. Well-defined incommensurate spin fluctuations are observed beyond x≈0.05 and up to the highest Sr-concentration x=0.25 in the present study. The incommensurability δ(x) is saturated at around 1/8 in the overdoped region and linearly scaled with the upper limit of T c(x) between the underdoped and optimally doped region. A characteristic energy E ∗ below which the dynamical magnetic susceptibility dramatically diminishes in the superconducting state can be determined to be around 7 meV in the energy spectrum of the spin fluctuations for x=0.15 and x=0.18 crystals. The disorder effect induces a low-frequency component of the incommensurate spin fluctuations below E ∗.