Search

We report a study of the Mott insulator ${\mathrm{La}}_{2}{\mathrm{O}}_{3}{\mathrm{Fe}}_{2}{\mathrm{Se}}_{2}$ by means of $^{139}\mathrm{La}$ nuclear quadrupole resonance (NQR). The NQR spectra evidence a single La site in the paramagnetic phase and two inequivalent La sites, La1 and La2, in the antiferromagnetic phase. These two sites are characterized by different quadrupole couplings, indicative of distinct lattice configurations segregated in domains. The dependence of the quadrupole coupling for La2 on temperature suggests that the structural distortion is driven by the magnetic order parameter. The nuclear transverse relaxation rate $1/{T}_{2}$ evidences fluctuations in the paramagnetic phase with characteristic frequencies well below the Heisenberg exchange frequency and likely associated with nematic fluctuations.

A family of layered manganese-pnictide antiferromagnets ${\mathrm{BaMn}}_{2}{Pn}_{2}$ ($\mathit{Pn}$ stands for P, As, Sb, and Bi) has been recently shown to host anomalously large magnetoresistance (MR) with both positive and negative MR components. In search for the microscopic picture of MR in this family, we here report a local-probe $^{55}\mathrm{Mn}$ nuclear magnetic resonance (NMR) study. The zero-field NMR spectra and the temperature dependence of the spin-lattice relaxation rates are fully consistent with the proposed G-type antiferromagnetic order. However, a close inspection of the $^{55}\mathrm{Mn}$ NMR spectra reveals a fine structure, which is due to the weakly localized charge carriers. As these carriers localize and segregate in the presence of electron correlations, they also undergo collective spin fluctuations which freeze-out at low temperatures and thus contribute to the $^{55}\mathrm{Mn}$ spin-lattice relaxation. The characteristic temperatures of the appearance of these additional features in the magnetic response probed by $^{55}\mathrm{Mn}$ NMR correlate well with the anomalies in the resistivity and anomalously large MR, which hints that the two phenomena are connected.

Selected members of the $\mathrm{La}(\mathrm{Fe},\mathrm{Mn})\mathrm{As}({\mathrm{O}}_{0.89}{\mathrm{F}}_{0.11})$ system were analyzed using high-resolution synchrotron x-ray powder diffraction. The tetragonal to orthorhombic structural transition is progressively recovered in the optimally electron-doped $\mathrm{LaFeAs}({\mathrm{O}}_{0.89}{\mathrm{F}}_{0.11})$ phase by very light Mn substitution; at the same time, superconductivity is suppressed whereas magnetic ordering is restored. Distinct incommensurate satellite peaks develop within different thermal ranges and mark the occurrence of charge density waves characterized by distinct propagation wave vectors, as well as multiple incommensurate structural transitions; in particular, some of them arise in conjunction with the structural transformation process, disappearing after the completion of the dissymmetrization. The thermal evolution of satellite reflections observed at $\mathbf{Q}\ensuremath{\sim}1.93\phantom{\rule{0.16em}{0ex}}{\AA{}}^{--1}$ indicates a strong competition between the charge density waves and the superconductive state. A phase diagram of the $\mathrm{La}(\mathrm{Fe},\mathrm{Mn})\mathrm{As}({\mathrm{O}}_{0.89}{\mathrm{F}}_{0.11})$ system is drawn on the basis of the structural, magnetic, and electronic properties of the analyzed samples.

Measurements of proton nuclear magnetic resonance ($^{1}\mathrm{H}\phantom{\rule{0.16em}{0ex}}\mathrm{NMR}$) spectra and relaxation and of muon spin relaxation (${\ensuremath{\mu}}^{+}\mathrm{SR}$) have been performed as a function of temperature and external magnetic field on two isostructural lanthanide complexes, Er(trensal) and Dy(trensal) [where ${\mathrm{H}}_{3}\mathrm{trensal}=2,{2}^{\ensuremath{'}},{2}^{\ensuremath{'}\ensuremath{'}}\ensuremath{-}\mathrm{tris}\ensuremath{-}(\mathrm{salicylideneimino})\mathrm{triethylamine}$], featuring crystallographically imposed trigonal symmetry. Both the nuclear $1/{T}_{1}$ and muon \ensuremath{\lambda} longitudinal relaxation rates (LRRs) exhibit a peak for temperatures $T$ 30 K, associated to the slowing down of the spin dynamics, and the width of the NMR absorption spectra starts to increase significantly at $T$ \ensuremath{\sim} 50 K, a temperature sizably higher than the one of the LRR peaks. The LRR peaks have a field and temperature dependence different from those previously reported for all molecular nanomagnets. They do not follow the Bloembergen-Purcell-Pound scaling of the amplitude and position in temperature and field and thus cannot be explained in terms of a single dominating correlation time ${\ensuremath{\tau}}_{\mathrm{c}}$ determined by the spin slowing down at low temperature. Further, for $T50\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ the spectral width does not follow the temperature behavior of the magnetic susceptibility \ensuremath{\chi}. We suggest, using simple qualitative considerations, that the observed behavior is due to a combination of two different relaxation processes characterized by the correlation times ${\ensuremath{\tau}}_{\mathrm{LT}}$ and ${\ensuremath{\tau}}_{\mathrm{HT}}$, dominating for $T$ 30 K and $T50\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, respectively. Finally, the observed flattening of LRR for $T$ 5 K is suggested to have a quantum origin.

We discuss the results of $^{75}\mathrm{As}$ nuclear quadrupole resonance (NQR) and muon spin relaxation measurements in $A{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ ($A$ = Cs, Rb) iron-based superconductors. We point out that the crossover detected in the nuclear spin-lattice relaxation rate $1/{T}_{1}$ (around 150 K in ${\mathrm{RbFe}}_{2}{\mathrm{As}}_{2}$ and around 75 K in ${\mathrm{CsFe}}_{2}{\mathrm{As}}_{2}$ ), from a high temperature nearly localized to a low temperature delocalized behavior, is associated with the onset of an inhomogeneous local charge distribution causing the broadening or even the splitting of the NQR spectra as well as an increase in the muon spin relaxation rate. We argue that this crossover, occurring at temperatures well above the phase transition to the nematic long-range order, is due to a charge disproportionation at the Fe sites induced by competing Hund's and Coulomb couplings. In ${\mathrm{RbFe}}_{2}{\mathrm{As}}_{2}$ around 35 K, far below that crossover temperature, we observe a peak in the NQR $1/{T}_{1}$ which is possibly associated with the critical slowing down of electronic nematic fluctuations on approaching the transition to the nematic long-range order.

Digital quantum simulators are among the most appealing applications of a quantum computer. Here we propose a universal, scalable, and integrated quantum computing platform based on tunable nonlinear electromechanical nano-oscillators. It is shown that very high operational fidelities for single and two qubits gates can be achieved in a minimal architecture, where qubits are encoded in the anharmonic vibrational modes of mechanical nanoresonators, whose effective coupling is mediated by virtual fluctuations of an intermediate superconducting artificial atom. An effective scheme to induce large single-phonon nonlinearities in nano-electromechanical devices is explicitly discussed, thus opening the route to experimental investigation in this direction. Finally, we explicitly show the very high fidelities that can be reached for the digital quantum simulation of model Hamiltonians, by using realistic experimental parameters in state-of-the art devices, and considering the transverse field Ising model as a paradigmatic example., 14 pages, 8 figures

We discuss the impact of concomitant substitution of Fe by Mn and La by Y in optimally F-doped ${\mathrm{LaFeAsO}}_{0.89}{\mathrm{F}}_{0.11}$. Mn has a known poisoning effect on superconductivity which is particularly strong in the La1111 system, where 0.2% of Mn were reported to completely suppress superconductivity. Through isovalent substitution of La by the much smaller Y we are able to inflict chemical pressure on the structure, which we show is stabilizing the superconducting state, resulting in a drastically larger amount of Mn needed to completely quench superconductivity. Interestingly, we find that the lattice parameter $\mathit{c}$ changes significantly even for small amounts of Mn substitution within a series, which is unexpected taking only the differences between ionic radii into account. We discuss our findings in the light of electron localization caused by small amounts of paramagnetic Mn impurities in ${\mathrm{La}}_{1\ensuremath{-}y}{\mathrm{Y}}_{y}{\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}{\mathrm{AsO}}_{0.89}{\mathrm{F}}_{0.11}$ also indicated by resistivity and M\"o\ss{}bauer measurements.

We present a $^{75}\mathrm{As}$ nuclear magnetic resonance (NMR) and resistivity study of the effect of 5.5 MeV proton irradiation on the optimal electron doped $(x=0.068)$ and overdoped $(x=0.107)$ $\mathrm{Ba}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Rh}}_{x})}_{2}{\mathrm{As}}_{2}$ iron based superconductors. While the proton induced defects only mildly suppress the critical temperature and increase residual resistivity in both compositions, sizable broadening of the NMR spectra was observed in all the irradiated samples at low temperature. The effect is significantly stronger in the optimally doped sample where the Curie Weiss temperature dependence of the line width suggests the onset of ferromagnetic correlations coexisting with superconductivity at the nanoscale. $1/{T}_{2}$ measurements revealed that the energy barrier characterizing the low energy spin fluctuations of these compounds is enhanced upon proton irradiation, suggesting that the defects are likely slowing down the fluctuations between $(0,\ensuremath{\pi})$ and $(\ensuremath{\pi}$,0) nematic ground states.

Antiferromagnetic molecular rings are widely studied both for fundamental quantum-mechanical issues and for technological applications, particularly in the field of quantum information processing. Here we present a detailed first-principles study of two families---purple and green---of ${{\mathrm{Cr}}_{7}M}$ antiferromagnetic rings, where $M$ is a divalent transition metal ion ($M={\mathrm{Ni}}^{2+}, {\mathrm{Mn}}^{2+}$, and ${\mathrm{Zn}}^{2+}$). We employ a recently developed flexible and efficient scheme to build ab initio system-specific Hubbard models. From such many-body models we systematically derive the low-energy effective spin Hamiltonian for the rings. Our approach allows us to calculate isotropic as well as anisotropic terms of the spin Hamiltonian, without any a priori assumption on its form. For each compound we calculate magnetic exchange couplings, zero-field splitting tensors, and gyromagnetic tensors, finding good agreement with experimental results.

Angle-resolved photoemission spectroscopy (ARPES) is one of most powerful techniques to unravel the electronic properties of layered materials and, in recent decades, it has lead to significant progress in the understanding of the band structures of cuprates, pnictides, and other materials of current interest. On the other hand, its application to Mott-Hubbard insulating materials where a Fermi surface is absent has been more limited. Here we show that in these latter materials, where electron spins are localized, ARPES may provide significant information on the spin correlations which can be complementary to the one derived from neutron-scattering experiments. Sr2Cu1-xZnxO2Cl2, a prototype of a diluted spin S=1/2 antiferromagnet (AF) on a square lattice, was chosen as a test case and a direct correspondence between the amplitude of the spectral weight beyond the AF zone boundary derived from ARPES and the spin-correlation length ξ estimated from Cl35 NMR was established. It was found that even for correlation lengths of a few lattice constants, a significant spectral weight in the backbended band is present which depends markedly on ξ. Moreover, the temperature dependence of that spectral weight is found to scale with the x-dependent spin stiffness. These findings prove that the ARPES technique is very sensitive to short-range correlations and its relevance in the understanding of the electronic correlations in cuprates is discussed.

Muon spin rotation ($\ensuremath{\mu}\mathrm{SR}$) and $^{19}\mathrm{F}$ nuclear magnetic resonance (NMR) measurements were performed to investigate the effect of Mn for Fe substitutions in ${\mathrm{La}}_{1\ensuremath{-}y}{\mathrm{Y}}_{y}{\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}{\mathrm{AsO}}_{0.89}{\mathrm{F}}_{0.11}$ superconductors. While for $y=0$ a very low critical concentration of Mn ($x=0.2%$) is needed to quench superconductivity, as $y$ increases the negative chemical pressure introduced by Y for La substitution stabilizes superconductivity and for $y=20%$ it is suppressed at Mn contents an order of magnitude larger. A magnetic phase arises once superconductivity is suppressed both for $y=0$ and for $y=20%$. Low-energy spin fluctuations give rise to a peak in $^{19}\mathrm{F}$ NMR $1/{T}_{1}$ with an onset well above the superconducting transition temperature and whose magnitude increases with $x$. Also the static magnetic correlations probed by $^{19}\mathrm{F}$ NMR linewidth measurements show a marked increase with Mn content. The disruption of superconductivity and the onset of the magnetic ground state are discussed in the light of the proximity of ${\mathrm{LaFeAsO}}_{0.89}{\mathrm{F}}_{0.11}$ to a quantum critical point.

We present nuclear magnetic resonance evidence that very slow $(\ensuremath{\le}1$ MHz) spin fluctuations persist into the overdoped regime of $\mathrm{Ba}({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Rh}}_{x}){}_{2}{\mathrm{As}}_{2}$ superconductors. Measurements of the $^{75}\mathrm{As}$ 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 $(\ensuremath{\pi}/a,0)$ from $(0,\ensuremath{\pi}/a)$ 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.

Implementing complex sequences of gates is crucial for any quantum computing architecture to become practical. This requires long-lived qubits which can be manipulated many times without errors. Here we propose a scheme to process hybrid qubits consisting of spin ensembles coupled to superconducting resonators in a cavity-protection regime, which enhances their coherence time by orders of magnitude. We perform numerical experiments for the quantum simulation of the $XY$ model and the quantum Fourier transform, by including all the main decoherence mechanisms and assuming system parameters that are guaranteed by present technology.

We investigate the magnetic properties and the phonon-induced relaxation dynamics of the first regular ${\mathrm{Cr}}_{9}$ antiferromagnetic (AF) ring, which represents a prototype frustrated AF ring. Geometrical frustration in ${\mathrm{Cr}}_{9}$ yields an energy spectrum with twofold degenerate low-lying levels and a low-spin ground state. The electronic relaxation dynamics is probed by ${}^{1}\mathrm{H}$-NMR through the temperature dependence of the spin-lattice relaxation rate $1/{T}_{1}$. We develop a microscopic model that reproduces $1/{T}_{1}(T)$ curves, taking also into account the wipeout effect. By interpreting these measurements we determine the spin-phonon coupling strength and we investigate the decay of the cluster magnetization due to the spin-phonon interaction. We find that at very low temperatures, the relaxation is characterized by a single dominating Arrhenius-type relaxation process, whereas several relevant processes emerge at higher temperatures. In addition, we calculate the temperature and magnetic field dependence of level lifetimes.

$^{19}\mathrm{F}$ NMR measurements in optimally electron-doped ${\mathrm{La}}_{1\text{\ensuremath{-}}y}{\mathrm{Y}}_{y}{\mathrm{Fe}}_{1\text{\ensuremath{-}}x}{\mathrm{Mn}}_{x}{\mathrm{AsO}}_{0.89}{\mathrm{F}}_{0.11}$ superconductors are presented. The effect of Mn doping on the superconducting phase is studied for two series of compounds $(y=0$ and $y=0.2)$ where the chemical pressure is varied by substituting La with Y. In the $y=0.2$ series superconductivity is suppressed for Mn contents an order of magnitude larger than for $y=0$. For both series a peak in the $^{19}\mathrm{F}$ NMR nuclear spin-lattice relaxation rate $1/{T}_{1}$ emerges upon Mn doping and becomes significantly enhanced on approaching the quantum phase transition between the superconducting and magnetic phases. $^{19}\mathrm{F}$ NMR linewidth measurements show that for similar Mn contents magnetic correlations are more pronounced in the $y=0$ series, at variance with what one would expect for $\stackrel{P\vec}{Q}=(\ensuremath{\pi}/a,0)$ spin correlations. These observations suggest that Mn doping tends to reduce fluctuations at $\stackrel{P\vec}{Q}=(\ensuremath{\pi}/a,0)$ and to enhance other low-frequency modes. The effect of this transfer of spectral weight on the superconducting pairing is discussed along with the charge localization induced by Mn.

We present an extended x-ray absorption fine structure study of the pyrochlore ${\mathrm{Y}}_{2}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$ (8--298 K). We find evidence, on a local scale, of a significant magnetoelastic coupling at the N\'eel temperature ${T}_{\mathrm{N}}\ensuremath{\sim}77\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ pointed out by a huge Debye-Waller ${\ensuremath{\sigma}}^{2}$ factor deviation from a correlated temperature dependent Debye-like local order behavior plus a temperature independent static contribution. Moreover, we notice the occurrence of a potential local order-disorder structural phase transition at ${T}^{*}=150$ K. This anomalous behavior is consistent with the pyrochlore's predisposition towards structural disorder and with a strong spin-phonon correlation. Remarkably the low-temperature order competes with the tendency of magnetic frustration to induce a less symmetric local structure.

We discuss the magnetic properties of a ${\mathrm{Sm}}_{2}{\mathrm{Mo}}_{2}{\mathrm{O}}_{7}$ single crystal as investigated by means of different experimental techniques. In the literature, a conventional itinerant ferromagnetic state is reported for the $\mathrm{Mo}{}^{4+}$ sublattice below $\ensuremath{\sim}78$ K. However, our results of dc magnetometry, muon spin spectroscopy (${\ensuremath{\mu}}^{+}$SR), and high-harmonics magnetic ac susceptibility unambiguously evidence highly disordered conditions in this phase, in spite of the crystalline and chemical order. This disordered magnetic state shares several common features with amorphous ferromagnetic alloys. This scenario for ${\mathrm{Sm}}_{2}{\mathrm{Mo}}_{2}{\mathrm{O}}_{7}$ is supported by the anomalously high values of the critical exponents, as mainly deduced by a scaling analysis of our dc magnetization data and confirmed by the other techniques. Moreover, ${\ensuremath{\mu}}^{+}$SR detects a significant static magnetic disorder at the microscopic scale. At the same time, the critical divergence of the third-harmonic component of the ac magnetic susceptibility around $\ensuremath{\sim}78$ K leads to additional evidence towards the glassy nature of this magnetic phase. Finally, the longitudinal relaxation of the ${\ensuremath{\mu}}^{+}$ spin polarization (also supported by results of ac susceptibility) evidences re-entrant glassy features similar to amorphous ferromagnets.

We have measured the nuclear spin-lattice relaxation of 59Co in the geometrically frustrated Ising-type spin-chain system Ca3Co2O6 as a function of temperature and applied magnetic field. In the nearly-saturated ferrimagnetic and ferromagnetic regimes considered here, the field-dependent energy scales governing the spin-lattice relaxation rates reflect the energy cost of a spin flip in the Co ions adjacent to the probed nuclei. This results in a thermally activated form for the nuclear relaxation rates, which can be exploited as a local probe of the exchange interactions. In particular, the measurements enable the values of intra- and interchain exchange constants J1, J2+J3 to be directly extracted from the data. By using a quasi-one-dimensional model, we then determine the value of J2 consistent with the spin-density-wave order observed in zero field.

Magnetization and ${}^{63}\mathrm{Cu}$ nuclear magnetic resonance-nuclear quadrupole resonance relaxation measurements are used to study the superconducting fluctuations in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6+x}$ (YBCO) oriented powders. In optimally doped YBCO the fluctuating negative magnetization ${M}_{\mathrm{fl}}(H,T)$ is rather well described by an anisotropic Ginzburg-Landau (GL) functional and the curves ${M}_{\mathrm{fl}}/\sqrt{H}$ cross at ${T}_{c}.$ In underdoped YBCO, instead, over a wide temperature range an anomalous diamagnetism is observed, stronger than in the optimally doped compound by about an order of magnitude. The field and temperature dependence of ${M}_{\mathrm{fl}}$ cannot be described either by an anisotropic GL functional or on the basis of scaling arguments. The anomalous diamagnetism is more pronounced in samples with a defined order in the Cu(1)O chains. The ${}^{63}\mathrm{Cu}(2)$ relaxation rate shows little, if any, field dependence in the vicinity of the transition temperature ${T}_{c}(H=0).$ It is argued how the results in the underdoped compounds can be accounted for by the presence of charge inhomogeneities, favored by chains ordering.

Magnetic susceptibility, ${}^{1}\mathrm{H}$ NMR and ${}^{63}\mathrm{Cu}$ NMR-NQR experiments on two slightly different species of the molecular $S=1/2$ antiferromagnetic (AF) ring Cu8, $[{\mathrm{Cu}}_{8}{(\mathrm{dmpz})}_{8}{(\mathrm{OH})}_{8}{]\mathrm{\ensuremath{\cdot}}2\mathrm{C}}_{5}{\mathrm{H}}_{5}\mathrm{N}$ (Cu8P) and $[{\mathrm{Cu}}_{8}{(\mathrm{dmpz})}_{8}{(\mathrm{OH})}_{8}{]\mathrm{\ensuremath{\cdot}}2\mathrm{C}}_{5}{\mathrm{H}}_{5}{\mathrm{NO}}_{2}$ (Cu8N), are presented. The magnetic energy levels are calculated exactly for an isotropic Heisenberg model Hamiltonian in zero magnetic field. From the magnetic susceptibility measurements we estimate the AF exchange coupling constant $J\ensuremath{\sim}1000\mathrm{K}$ and the resulting gap $\ensuremath{\Delta}\ensuremath{\sim}500\mathrm{K}$ between the ${S}_{T}=0$ ground state and the ${S}_{T}=1$ first excited state. The ${}^{63,65}\mathrm{Cu}$ NQR spectra indicate the presence of four crystallographically inequivalent copper nuclei in each ring. From the combination of the ${}^{63}\mathrm{Cu}$ NQR spectra and of the ${}^{63}\mathrm{Cu}$ NMR spectra at high magnetic field, we estimate the quadrupole coupling constant ${v}_{Q}$ of each site and the average asymmetry parameter \ensuremath{\eta} of the electric-field gradient tensor. The nuclear spin-lattice relaxation rate (NSLR) decreases exponentially on decreasing temperature for all nuclei investigated. The gap parameter extracted from ${}^{63}\mathrm{Cu}$ NQR-NSLR is the same as for the susceptibility while a smaller value is obtained from the ${}^{63}\mathrm{Cu}$ NMR-NSLR in an external magnetic field of 8.2 T.

The spin dynamics of Mn spins in the dodecanuclear manganese cluster of formula [Mn{sub 12}O{sub 12}(CH{sub 3}COO){sub 16}(H{sub 2}O){sub 4}]{center_dot}2CH{sub 3}-COOH{center_dot}4H{sub 2}O(Mn{sub 12}) has been investigated by {sup 1}HNMR and muon spin-lattice relaxation rate 1/T{sub 1} as a function of temperature (10{endash}400 K) and external magnetic field (0{endash}9.4 T). At room temperature, the proton 1/T{sub 1} depends on the measuring frequency. The results can be interpreted in terms of a slow decay of the Mn electronic-spin autocorrelation function, a feature characteristic of the almost zero dimensionality of the system. As the temperature is lowered, 1/T{sub 1} displays a critical enhancement that can be related to the slowing down of the local spin fluctuations as the cluster approaches the condensation into the total spin S=10 ground-state configuration. It is found that the application of an external magnetic field greatly depresses the enhancement of 1/T{sub 1} an effect that could be related to the superparamagnetic behavior of the Mn{sub 12} molecule. {copyright} {ital 1998} {ital The American Physical Society}

{sup 63}Cu NQR T{sub 1} and T{sub 2} relaxation measurements in La{sub 2}Cu{sub 1{minus}x}Zn{sub x}O{sub 4}, for 0{le}x{le}0.11 and in the temperature range T{sub N}{le}T{le}900 K, are presented. The results are used to derive insights into the Cu{sup 2+} correlated spin dynamics in the paramagnetic phase of the S=(1)/(2) two-dimensional (2D) Heisenberg (H) antiferromagnets (AF), and into the disorder effects associated with the spin vacancy due to Zn{sup 2+} (S=0) for Cu{sup 2+} substitution. In particular, by using scaling arguments for the static generalized susceptibility, {chi}({rvec q},0), and for the decay rate, {Gamma}{sub {rvec q}}, of the normal excitations, T{sub 2} and T{sub 1} are related to the in-plane correlation length {xi}{sub 2D}(x,T) and its dependence on temperature and Zn doping, x, is extracted. The experimental findings are analyzed in light of the quantum critical and renormalized classical behaviors for {xi}{sub 2D} predicted by recent theories for S=1/2 HAF on square lattices. It is shown that up to T{approx_equal}900 K, {xi}{sub 2D} is consistent with the assumption of a renormalized classical regime, in agreement with recent neutron scattering results and at variance with previous interpretations of the NQR data. It is discussed how Zn affects {xi}{sub 2D} through the modificationmore » in the spin stiffness and comparison with the disorder induced by itinerant extra holes is made.« less

Entanglement between individual spins can be detected by using thermodynamics quantities as entanglement witnesses. This applies to collective spins also, provided that their internal degrees of freedom are frozen, as in the limit of weakly coupled nanomagnets. Here, we extend such approach to the detection of entanglement between subsystems of a spin cluster, beyond such weak-coupling limit. The resulting inequalities are violated in spin clusters with different geometries, thus allowing the detection of zero- and finite-temperature entanglement. Under relevant and experimentally verifiable conditions, all the required expectation values can be traced back to correlation functions of individual spins, which are now made selectively available by four-dimensional inelastic neutron scattering. © 2013 American Physical Society.

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.

The appearance of static magnetism, nanoscopically coexisting with superconductivity, is shown to be a general feature of optimally electron-doped $L$Fe${}_{1\ensuremath{-}x}$Ru${}_{x}$AsO${}_{1\ensuremath{-}y}$F${}_{y}$ superconductors ($L$ represents a lanthanide ion) upon isovalent substitution of Fe by Ru. The magnetic ordering temperature ${T}_{N}$ and the magnitude of the internal field display a domelike dependence on $x$, peaked around $x=1/4$, with higher ${T}_{N}$ values for those materials characterized by a larger $z$ cell coordinate of As. Remarkably, the latter are also those with the highest superconducting transition temperatures (${T}_{c}$) for $x=0$. The reduction of ${T}_{c}(x)$ is found to be significant in the $x$ region of the phase diagram where the static magnetism develops. Upon increasing the Ru content superconductivity eventually disappears, but only at $x\ensuremath{\simeq}0.6$.

We investigate the direct correspondence between Co band ferromagnetism and structural parameters in the pnictide oxides $R\mathrm{CoPO}$ for different rare-earth ions ($R=\mathrm{La},\mathrm{Pr},\mathrm{Nd},\mathrm{Sm}$) by means of muon-spin spectroscopy and ab initio calculations, complementing our results published previously [G. Prando et al., Common effect of chemical and external pressures on the magnetic properties of $R\mathrm{CoPO}$ ($R=\mathrm{La},\mathrm{Pr}$), Phys. Rev. B 87, 064401 (2013)]. We find that both the transition temperature to the ferromagnetic phase ${T}_{\mathrm{C}}$ and the volume of the crystallographic unit cell $V$ are conveniently tuned by the $R$ ionic radius and/or external pressure. We report a linear correlation between ${T}_{\mathrm{C}}$ and $V$ and our ab initio calculations unambiguously demonstrate a full equivalence of chemical and external pressures. As such, we show that $R$ ions influence the ferromagnetic phase only via the induced structural shrinkage without involving any active role from the electronic $f$ degrees of freedom, which are only giving a sizable magnetic contribution at much lower temperatures.

Evidence is presented of superconducting fluctuations in the {sup 63}Cu NQR-NMR relaxation rate in YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}, as obtained from a careful comparison of measurements carried out in the absence and in the presence of a field parallel to the {ital c} axis. It is shown that the field causes a reduction of the relaxation rate {ital W} in a range of about 10 K above {ital T}{sub {ital c}}. This effect is related to the suppression by the magnetic field of the phase-sensitive positive Maki-Thompson contribution while the negative contribution from the DOS fluctuations is almost field independent. Furthermore, it is argued how the fluctuation effects on {ital W} can be used to discuss the pairing state symmetry, at variance with the insensitivity of the transport measurements. It is pointed out that the existence of the Maki-Thompson contribution to {ital W} evidences an {ital s}-wave symmetry component for the pairing in YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}. {copyright} {ital 1996 The American Physical Society.}