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The competition of magnetic order and superconductivity is a key element in the physics of all unconventional superconductors, for example in high-transition-temperature cuprates, heavy fermions and organic superconductors. Here superconductivity is often found close to a quantum critical point where long-range antiferromagnetic order is gradually suppressed as a function of a control parameter, for example charge-carrier doping or pressure. It is believed that dynamic spin fluctuations associated with this quantum critical behaviour are crucial for the mechanism of superconductivity. Recently, high-temperature superconductivity has been discovered in iron pnictides, providing a new class of unconventional superconductors. Similar to other unconventional superconductors, the parent compounds of the pnictides show a magnetic ground state and superconductivity is induced on charge-carrier doping. In this Letter the structural and electronic phase diagram is investigated by means of X-ray scattering, muon spin relaxation and Mössbauer spectroscopy on the series LaO1−xFxFeAs. We find a discontinuous first-order-like change of the Néel temperature, the superconducting transition temperature and the respective order parameters. Our results strongly question the relevance of quantum critical behaviour in iron pnictides and prove a strong coupling of the structural orthorhombic distortion and the magnetic order both disappearing at the phase boundary to the superconducting state. [ABSTRACT FROM AUTHOR]

The magnetic phase diagram of the system Ce(Cu1−xNix)2Ge2 with 0⩽x⩽0.9 is studied by muon spin relaxation (μSR). Static magnetic order has been found up to x=0.5 whereas for x=0.8 and 0.9, no magnetic relaxation has been detected for temperatures down to 20 mK. [Copyright &y& Elsevier]

The magnetic order in La1.8−xEu0.2SrxCuO4 (x⩽0.2) with low-temperature tetragonal (LTT) structure has been investigated with muon spin relaxation. The magnetic phase diagram of the LTT phase resembles the generic phase diagram of the cuprates where superconductivity is replaced by a second antiferromagnetic phase. A crossover from magnetic order to superconductivity is observed at a charge doping of x=0.2. Changing the degree of the LTT lattice distortion via a variation of the Eu content the ground state at x=0.2 can be tuned from magnetism to superconductivity. [Copyright &y& Elsevier]

Abstract: We present a study of the magnetically ordered state of single-crystalline . As has been shown previously [A. Otop, et al., J. Appl. Phys. 95 (2004) 6702], this compound orders antiferromagnetically below and shows magnetic irreversibility below . We carried out muon spin relaxation experiments in the temperature range 2–40K, in order to study the homogeneity of the magnetic state below and in the intermediate range . From our data a homogeneous antiferromagnetic state below is established. In the intermediate temperature we find evidence of the presence of diluted magnetic clusters as result of frozen in crystallographic disorder. [Copyright &y& Elsevier]

Abstract: We report high field specific heat measurements on the structural two-leg ladder compound at zero magnetic field and at . The magnetic specific heat is well described using an isolated dimer model with on the rungs on the spin ladder consistent with previous studies of the magnetic properties [C. Mennerich et al., Phys. Rev. B 73 (2006) 174415]. [Copyright &y& Elsevier]

Room-temperature precipitation from aqueous solutions yields the hitherto unknown metastable stoichiometric iron selenide (ms-FeSe) with tetragonal anti-PbO type structure. Samples with improved crystallinity are obtained by diffusion-controlled precipitation or hydrothermal recrystallization. The relations of ms-FeSe to superconducting &bgr;-FeSe1-x and other neighbor phases of the iron--selenium system are established by high-temperature X-ray diffraction, DSC/TG/MS (differential scanning calorimetry/thermogravimetry/mass spectroscopy), Fe Mössbauer spectroscopy, magnetization measurements, and transmission electron microscopy. Above 300 °C, ms-FeSe decomposes irreversibly to &bgr;-FeSe1-x and Fe7Se8. The structural parameters of ms-FeSe (P4/nmm, a = 377.90(i) pm, c = S51.1l{3) pm, Z = 2), obtained by Rjetveld refinement, differ significantly from hterature data for &bgr;-FeSe1-x The Mössbauer spectrum rules out interstitial iron atoms or additional phases. Magnetization data suggest canted antiferromagnetism below TN, = 50 K. Stoichiometric non-superconducting ms-FeSe can be regarded as the tme "parent" compound for the "11" iron-chalcogenide superconductors and may serve as starting point for new chemical modifications. [ABSTRACT FROM AUTHOR]

Topological materials ranging from topological insulators to Weyl and Dirac semimetals form one of the most exciting current fields in condensed-matter research. Many half-Heusler compounds, RPtBi (R = rare earth), have been theoretically predicted to be topological semimetals. Among various topological attributes envisaged in RPtBi, topological surface states, chiral anomaly, and planar Hall effect have been observed experimentally. Here, we report an unusual intrinsic anomalous Hall effect (AHE) in the antiferromagnetic Heusler Weyl semimetal compounds GdPtBi and NdPtBi that is observed over a wide temperature range. In particular, GdPtBi exhibits an anomalous Hall conductivity of up to 60 Ω-1⋅cm-1 and an anomalous Hall angle as large as 23%. Muon spin-resonance (μSR) studies of GdPtBi indicate a sharp antiferromagnetic transition (TN) at 9 K without any noticeable magnetic correlations above TN. Our studies indicate that Weyl points in these half-Heuslers are induced by a magnetic field via exchange splitting of the electronic bands at or near the Fermi energy, which is the source of the chiral anomaly and the AHE. [ABSTRACT FROM AUTHOR]

The crystal electric field (CEF) scheme of YbNi4P2 is determined, based on experimental data from inelastic neutron scattering, heat capacity, susceptibility andNMRmeasurements. Despite the tetragonal crystal structure, 9 parameters are needed to describe the crystal field in YbNi4P2 due to the orthorhombic site symmetry of the Yb ion. Alarge basal plane anisotropy is detected by the local probe NMR. Our analysis yields CEF excitation energies of 8.5, 12.5 and roughly 30 meV and a ground state wave function that is dominated by the 5 2 state. Furthermore, we present an analysis of the CEF scheme based on density functional theory calculations, which confirms the large basal plane anisotropy. [ABSTRACT FROM AUTHOR]

We report experimental results of the static magnetization, ESR and NMR spectroscopic measurements of the Ni-hybrid compound NiCl3C6H5CH2CH2NH3. In this material NiCl3 octahedra are structurally arranged in chains along the crystallographic a axis. According to the static susceptibility and ESR data Ni2+ spins S=1 are isotropic and are coupled antiferromagnetically (AFM) along the chain with the exchange constant J=25.5 K. These are important prerequisites for the realization of the so-called Haldane spin-1 chain with the spin-singlet ground state and a quantum spin gap. However, experimental results evidence AFM order at TN≈10K presumably due to small interchain couplings. Interestingly, frequency-, magnetic field-, and temperature-dependent ESR measurements, as well as the NMR data, reveal signatures which could presumably indicate an inhomogeneous ground state of co-existent mesoscopically spatially separated AFM ordered and spin-singlet state regions similar to the situation observed before in some spin-diluted Haldane magnets. [ABSTRACT FROM AUTHOR]

A combination of phenomenological analysis and Mössbauer spectroscopy experiments on the tetragonal Fe1+yTe system indicates that the magnetic ordering transition in compounds with higher Fe excess, y=0.11, is unconventional. Experimentally, a liquidlike magnetic precursor with quasistatic spin order is found from significantly broadened Mössbauer spectra at temperatures above the antiferromagnetic transition. The incommensurate spin-density wave order in Fe1+yTe is described by a magnetic free energy that violates the weak Lifshitz condition in the Landau theory of second-order transitions. The presence of multiple Lifshitz invariants provides the mechanism to create multidimensional, twisted, and modulated solitonic phases. [ABSTRACT FROM AUTHOR]

We report a combined experimental study of magnetic properties of a single crystal of the frustrated diamond lattice antiferromagnet CoAl2O4 with Co2+ electron spin resonance, 27Al nuclear magnetic resonance, and muon spin rotation/relaxation techniques. With our local probes, we show that the frustration of spin interactions and the Co/Al site disorder strongly affect the spin dynamics. The experimental results evidence inhomogeneous and slow magnetic fluctuations and the occurrence of short-range electron spin correlations far above a characteristic temperature T* = 8 K at which the spin system turns into in a quasistatic state. Our data indicate that this spin order is likely short range and unconventional with spin fluctuations persistent even at T ⪡ T*. The results of three spectroscopy techniques highlight a nontrivial role of structural disorder for the magnetism of a frustrated diamond spin lattice at the proximity to the critical point. [ABSTRACT FROM AUTHOR]

Using a combination of muon-spin relaxation (μSR), inelastic neutron scattering (INS), and nuclear magnetic resonance (NMR), we investigated the novel iron-based superconductor with a triclinic crystal structure (CaFe1-xPtxAs)10Pt3As8 (Tc=13 K), containing platinum-arsenide intermediary layers. The temperature dependence of the superfluid density obtained from the μSR relaxation-rate measurements indicates the presence of two superconducting gaps, Δ1⪢Δ2. According to our INS measurements, commensurate spin fluctuations are centered at the (π,0) wave vector, like in most other iron arsenides. Their intensity remains unchanged across Tc, indicating the absence of a spin resonance typical for many Fe-based superconductors. Instead, we observed a peak in the spin-excitation spectrum around ℏω0=7 meV at the same wave vector, which persists above Tc and is characterized by the ratio ℏω0/kBTc≈6.2, which is significantly higher than typical values for the magnetic resonant modes in iron pnictides (~4.3). The temperature dependence of magnetic intensity at 7 meV revealed an anomaly around T*=45 K related to the disappearance of this new mode. A suppression of the spin-lattice relaxation rate, 1/T1T, observed by NMR immediately below T* without any notable subsequent anomaly at Tc, indicates that T* could mark the onset of a pseudogap in (CaFe1-xPtxAs)10Pt3As8, which is likely associated with the emergence of preformed Cooper pairs. [ABSTRACT FROM AUTHOR]

We report detailed 75As nuclear quadrupole resonance investigations of the locally noncentrosymmetric superconductor SrPtAs. The spin-lattice relaxation studies prove weakly coupled multigap superconductivity. A retardation of the decay in 1/T1)T evidences a nodeless (fully gapped) superconducting state on the complex multipocket Fermi surface, which is consistent with an anisotropic s-wave order parameter and with proposed unconventional f-wave and chiral d-wave symmetries. A quantitative analysis of these models favors the unconventional f-wave state. [ABSTRACT FROM AUTHOR]

We present a detailed local probe study of the magnetic order in the oxychalcogenide La2O2Fe2OSe2 utilizing 57Fe Mossbauer, l39La NMR, and muon-spin relaxation spectroscopy. This system can be regarded as an insulating reference system of the Fe arsenide and chalcogenide superconductors. From the combination of the local probe techniques we identify a noncollinear magnetic structure similar to Sr2F2Fe2OS2. The analysis of the magnetic order parameter yields an ordering temperature TN = 90.1 K and a critical exponent of β = 0.133, which is close to the two-dimensional Ising universality class as reported in the related oxychalcogenide family. [ABSTRACT FROM AUTHOR]

In this study, we investigate the electronic and magnetic properties of Rb0.77Fe1.61Se2 (Tc = 32.6 K) in normal and superconducting states by means of photoemission and μSR spectroscopies as well as band-structure calculations. We demonstrate that the unusual behavior of these materials is the result of separation into metallic (~12%) and insulating (~88%) phases. Only the former becomes superconducting and has a usual electronic structure of electron-doped FeSe slabs. Our results thus imply that the antiferromagnetic insulating phase is just a by-product of Rb intercalation and its magnetic properties have no direct relation to the superconductivity. Instead, we find that also in this class of iron-based compounds, the key ingredient for superconductivity is a certain proximity of a Van Hove singularity to the Fermi level. [ABSTRACT FROM AUTHOR]

We report the magnetic and superconducting properties of locally noncentrosymmetric SrPtAs obtained by muon-spin-rotation/relaxation (μSR) measurements. Zero-field μSR reveals the occurrence of small spontaneous static magnetic fields with the onset of superconductivity. This finding suggests that the superconducting state of SrPtAs breaks time-reversal symmetry. The superfluid density as determined by transverse field μSR is nearly flat approaching T = 0 K proving the absence of extended nodes in the gap function. By symmetry, several superconducting states supporting time-reversal-symmetry breaking in SrPtAs are allowed. Out of these, a dominantly d + id (chiral d-wave) order parameter is most consistent with our experimental data. [ABSTRACT FROM AUTHOR]

We provide direct experimental evidence for the identical effect of the in-plane Fe1-xCox and of the out-of-plane O1-xFx chemical dilution on the itinerant spin-density-wave (SDW) magnetic phase in CeFeAsO. Remarkably, the suppression of SDW is not sensitive at all to the different kinds of disorder introduced in the two cases. Still, it is clearly shown that the sizable in-plane disorder induced by the Fe1-xCox, substitution is highly effective in suppressing Tc. Differently from what is observed in CeFeAsO1-xFx, the ordered magnetic phase of the Ce sublattice is preserved throughout the whole phase diagram in CeFe1-xCoxAsO (x ⩽ 0.2). An intriguing effect is encountered, whereby the magnetic coupling among Ce3+ ions is enhanced by the superconducting phase. [ABSTRACT FROM AUTHOR]

Metastable FeSe is synthesized by diffusion-controlled precipitation from Li2Se and FeI2 powders at room temperature using water as diffusion medium (Ar, H-shaped tube, 40 d) and by hydrothermal recrystallization from aqueous solutions of Li2Se and FeI2 (autoclave, 200 °C, 3 d). [ABSTRACT FROM AUTHOR]

Cerium 4f electronic spin dynamics in single crystals of the heavy-fermion system CeFePO is studied by means of ac susceptibility, specific heat, and muon-spin relaxation (μSR). Short-range static magnetism occurs below the freezing temperature Tg ≈ 0.7 K, which prevents the system from accessing a putative ferromagnetic quantum critical point. In the μSR, the sample-averaged muon asymmetry function is dominated by strongly inhomogeneous spin fluctuations below 10 K and exhibits a characteristic time- field scaling relation expected from glassy spin dynamics, strongly evidencing cooperative and critical spin fluctuations. The overall behavior can be ascribed neither to canonical spin glasses nor other disorder-driven mechanisms. [ABSTRACT FROM AUTHOR]

The superfluid density and superconducting gaps of superconducting RbFe2As2 have been determined as a function of temperature, magnetic field, and hydrostatic pressure by susceptibility and muon-spin spectroscopy measurements. From the data, fundamental microscopic parameters of the superconducting state like the London penetration depth λ, the gap values △, the upper critical field B2, and the Ginzburg-Landau parameter K have been obtained. In accordance with earlier measurements the ratio of the superfluid density ns α λ-2 to the superconducting transition temperature Tc = 2.52(2) K at ambient pressure is found to be much higher in the strongly hole-overdoped RbFe2As2 than in high-7"c Fe-based and other unconventional superconductors. As a function of pressure, Tc strongly decreases, at the rate of Tc/dp = --1.32 K GPa-1, i.e., it is reduced by 52% at p = 1 GPa. The temperature dependence of ns is best described by a two-gap S-wave model, with both superconducting gaps being decreased by hydrostatic pressure until the smaller gap completely disappears at p = 1 GPa. [ABSTRACT FROM AUTHOR]

We report on superconductivity in CeFeAs1-xPxO and the possible coexistence with Ce ferromagnetism (FM) in a small homogeneity range around x = 30% with ordering temperatures of TSC ≅ TC ≅ 4 K. The antiferromagnetic (AFM) ordering temperature of Fe at this critical concentration is suppressed to TNFe ≈ 40 K and does not shift to lower temperatures with a further increase of the P concentration. Therefore, a quantum-critical-point scenario with TNFe → 0 K which is widely discussed for the iron based superconductors can be excluded for this alloy series. Surprisingly, thermal expansion and x-ray powder diffraction indicate the absence of an orthorhombic distortion despite clear evidence for short-range AFM Fe ordering from muon-spin-rotation measurements. Furthermore, we discovered the formation of a sharp electron spin resonance signal unambiguously connected with the emergence of FM ordering. [ABSTRACT FROM AUTHOR]

The local 4f-electronic spin dynamics and magnetic order in YbNi4P2 were studied by means of muon-spin relaxation measurements. Zero-field muon-spin relaxation proves static magnetic order with a strongly reduced ordered Yb3+ moment of (2.5-4.6) x 10-2μB, below Tc = 140 mK. Above Tc, the muon-spin polarization P(t,B) is dominated by quasihomogeneous spin fluctuations and exhibits atime-field scaling relation P(t,B) = P(t/BY), indicating cooperative critical spin dynamics in the system. At T = 190 mK, slightly above Tc, y = 0.81(5), suggesting time-scale invariant power-law behavior for the dynamic electronic spin-spin autocorrelation function. [ABSTRACT FROM AUTHOR]

We report on the superconducting properties of AxFe2-ySe2 (A=Rb, K) single crystals studied with the muon spin relaxation or rotation (&mgr;SR) technique. At low temperatures, close to 90% of the sample volumes exhibit large-moment magnetic order which impedes the investigation of their superconducting properties by &mgr;SR. On the other hand, about 10% of the sample volumes remain paramagnetic and clearly show a superconducting response. The temperature dependence of the superconducting carrier density was analyzed within the framework of a single i-wave gap scenario. The zero-temperature values of the in-plane magnetic penetration depths &lgr;ab(0) = 258(2) and 225(2) nm and the superconducting gaps Δ(0) = 7.7(2) and 6.3(2) meV have been determined for A = Rb and K, respectively. The microscopic coexistence and/or phase separation of superconductivity and magnetism is discussed. [ABSTRACT FROM AUTHOR]

A comprehensive comparison between the magnetic field- and temperature-dependent low-frequency spin dynamics in the antiferromagnetic spin-1/2 Heisenberg chain system copper pyrazine dinitrate, probed via the 13C nuclear magnetic resonance relaxation rate T1-1, and the field theoretical approach in the Luttinger liquid regime was performed. We found very good agreement between experiment and theory in the investigated temperature and field range. Our results demonstrate how spin-spin interactions strongly affect the spin dynamics of Heisenberg spin chain compounds in the vicinity of the quantum critical point. [ABSTRACT FROM AUTHOR]

We report upper critical field Bc2(T) data for LaO0.9F0.1FeAs1-δ in a wide temperature and field range up to 60 T. The large slope of Bc2 ≈ -5.4 to -6.6 TK-1 near an improved Tc ≈ 28.5 K of the in-plane Bc2(T) contrasts with a flattening starting near 23 K above 30 T we regard as the onset of Paulilimited behaviour (PLB) with Bc2(0) ≈ 63-68 T. We interpret a similar hitherto unexplained flattening of the Bc2(T) curves reported for at least three other disordered closely related systems, Co-doped BaFe2As2, (Ba,K) Fe2As2 and NdO0.7F0.3FeAs (all single crystals), for applied fields H ∥ (a, b), also as a manifestation of PLB. Their Maki parameters have been estimated by analysing their Bc2 (T) data within the Werthamer-Helfand-Hohenberg approach. The pronounced PLB of (Ba, K)Fe2As2 single crystals obtained from an Sn flux is attributed also to a significant As deficiency detected by wavelength dispersive x-ray spectroscopy as reported by Ni et al (2008 Phys. Rev. B 78 014507). Consequences of our results are discussed in terms of disorder effects within conventional superconductivity (CSC) and unconventional superconductivity (USC). USC scenarios with nodes on individual Fermi surface sheets (FSS), e.g. p- and d-wave SC, can be discarded for our samples. The increase of dBc2/dT∣Tc by sizeable disorder provides evidence for an important intraband (intra-FSS) contribution to the orbital upper critical field. We suggest that it can be ascribed either to an impurity-driven transition from s± USC to CSC of an extended s++- wave state or to a stabilized s±-state provided As-vacancies cause predominantly strong intraband scattering in the unitary limit. We compare our results with Bc2 data from the literature, which often show no PLB for fields below 60-70 T probed so far. A novel disorder-related scenario of a complex interplay of SC with two different competing magnetic instabilities is suggested. [ABSTRACT FROM AUTHOR]

Abstract: In this short review, we attempt to give a comprehensive discussion of studies performed to date by muon-spin spectroscopy (more precisely the relaxation and rotation technique, also know as μSR) on the recently discovered layered iron-based superconductors. On one side, μSR has been used to characterize the magnetic state of different families of layered iron-based systems. Similarly the subtle interplay of the magnetic state and the structural transition present in some families has been investigated. We will also discuss the information provided by this technique on the interaction between the magnetic state and the superconducting phase. Finally the μSR technique has been used to investigate the magnetic penetration depth of the superconducting ground state. The study of its absolute value, temperature and magnetic field dependence provides crucial tests for investigating possible unconventional superconducting states in such systems. [Copyright &y& Elsevier]

We present a clamp-type pressure cell for high energy x-ray diffraction. The pressure cell was specifically designed for studies of weak superstructure reflections at low temperatures in transition metal oxides, resulting from, e.g., charge density modulations. Using a photon energy of E=100 keV, the bulk properties of single crystals with a volume of typically 2–5 mm3 can be studied in transmission geometry. To demonstrate the performance of the pressure cell, we present data on the charge stripe order in the high-temperature superconductor La1.875Ba0.125CuO4. [ABSTRACT FROM AUTHOR]

Abstract: In recent X-ray diffraction experiments on orthorhombic nickelates, we have observed a significant coupling between the stripe correlations and the lattice distortion. In particular, the diagonal charge stripes of this insulating material exhibit a preferred orientation parallel to the orthorhombic a-axis, similar to the diagonal spin stripes in La2−x Sr x CuO4 below the metal-insulator transition. In the light of this finding, we draw a comparison between the lightly doped parts of the phase diagrams of La2−x Sr x CuO4 with low-temperature orthorhombic (LTO) structure, and La1.8−x Eu0.2Sr x CuO4 with low-temperature tetragonal (LTT) structure. In La1.8−x Eu0.2Sr x CuO4, a local minimum in the Sr dependence of the LTO/LTT transition temperature, as well as reduced spin-glass transition temperatures, may be indicators for the presence of diagonal charge correlations. Above the metal-insulator transition the LTT phase is suited to pin parallel stripes, while below that transition the LTT phase seems instead to compete with the formation of diagonal stripes. [Copyright &y& Elsevier]

Abstract: We present C-NMR investigations of the S = antiferromagnetically coupled Heisenberg chain system [CuPM(NO)(HO)], (PM==pyrimidine). For this material, as result of the Dzyaloshinskii–Moriya interaction and an alternating g-tensor, the ground state is characterized by a giant static spin canting and an anisotropic magnetic field-induced spin excitation gap. We measured the spin–lattice relaxation rate at three inequivalent carbon sites in the pyrimidine molecule as function of temperature. From these studies we deduce the magnetic excitation spectrum of copper pyrimidine dinitrate at a constant applied magnetic field of 9.3T. The experimental data are discussed in context with recent ESR investigations, which have been interpreted in terms of breather, soliton and multi-particle excitations based on the quantum sine-Gordon field theory. [Copyright &y& Elsevier]

We present 13C NMR experiments on the S=1/2 antiferromagnetically coupled spin chain compound [PM·Cu(NO3)2·(H2O)2]n (PM=pyrimidine) at temperatures 5–120 K. In the NMR spectra, we observe three pairs of double lines. From the angular dependence of the NMR shift, we assign the three line pairs to the three inequivalent C sites on the pyrimidine molecule C4N2H4, with the splitting of the signal from each site into two lines caused by the hydrogen atom bound to C. We obtain the temperature dependence of the NMR shift, which exhibits a qualitatively different behavior for the three inequivalent C sites. [Copyright &y& Elsevier]

We report bulk superconductivity (SC) in Ca 3 Ir 4 Sn 13 by means of 119 Sn nuclear magnetic resonance (NMR) experiments. Two classical signatures of BCS superconductivity in spin-lattice relaxation rate (1/ T 1 ), namely the Hebel–Slichter coherence peak just below the T c , and the exponential decay in the superconducting phase, are evident. The noticeable decrease of 119 Sn Knight shift below T c indicates spin-singlet superconductivity. The temperature dependence of the spin-lattice relaxation rate 119 (1/ T 1 ) is convincingly described by the multigap isotropic superconducting gap. NMR experiments do not witness any sign of enhanced spin fluctuations. [ABSTRACT FROM AUTHOR]

Abstract: Zero-field muon spin rotation () and DC magnetization investigations have been performed on the antiferromagnetic-ferromagnetic (AFM–FM) and the ferromagnetic-paramagnetic (FM–PM) transitions of two oxygen isotope (O or O) substituted powders. While the FM–PM phase transition shows nearly no isotope effect, a small but clear shift of 1.5K on the transition temperature is observed for the AFM–FM transition. Together with the recent observation of an isotope effect on the metal–insulator transition on the same sample [E. Pomjakushina, et al., Phys. Rev. B 73 (2006) 113105.], our observations clearly show the coupling of electronic, magnetic and structural degrees of freedom in this system. [Copyright &y& Elsevier]

We report a combined experimental and theoretical study on magnetism and magneto-electric coupling in howardevansite-type mixed-spin chain compounds ACuFe 2 (VO 4) 3 (A = Li, Na). While LiCuFe 2 (VO 4) 3 shows long-range magnetic order and spin-order-induced ferroelectricity, i.e., multiferroicity of type II, neither magnetic order nor magneto-electric effects are observed in the Na-based sister compound. Notably, our local magnetic probes studies by means of NMR, ESR, and Mössbauer spectroscopy evidence similar magnetic behavior of both compounds at the time scales of these experiments. This is confirmed by first principles calculations which imply a similar hierarchy of exchange interaction parameters. Our results suggest that the absence of magneto-electric effects in NaCuFe 2 (VO 4) 3 as seen in bulk measurements is a disorder effect associated with lower mobility of Na ions within the channels of the crystal structure as compared to the mobility of Li ions, thereby presenting a yet disregarded degree of freedom to tune the multiferroicity. • Thermodynamic and resonant properties evidence new features of multiferroic ACuFe2(VO4)3 (A = Li, Na). • Local probes by means of NMR, ESR and Mössbauer spectroscopy provide information on the magnetic properties. • Multiferroicity is associated with interplay of a magnetic subsystem with the mobile electric subsystem of the alkali ions. [ABSTRACT FROM AUTHOR]

We report muon spin rotation and magnetic susceptibility experiments on in-plane stress effects on the static spin-stripe order and superconductivity in the cuprate system La2-xBaxCuO4 with x=0.115. An extremely low uniaxial stress of ~0.1 GPa induces a substantial decrease in the magnetic volume fraction and a dramatic rise in the onset of 3D superconductivity, from ~10 to 32 K; however, the onset of at-least-2D superconductivity is much less sensitive to stress. These results show not only that large-volume-fraction spin-stripe order is anticorrelated with 3D superconducting coherence but also that these states are energetically very finely balanced. Moreover, the onset temperatures of 3D superconductivity and spin-stripe order are very similar in the large stress regime. These results strongly suggest a similar pairing mechanism for spin-stripe order and the spatially modulated 2D and uniform 3D superconducting orders, imposing an important constraint on theoretical models. [ABSTRACT FROM AUTHOR]

Abstract: The crystal structure of Na2T2(ox)2(H2O)2 with , and Mn(II) consists of isostructural compounds with an anionic transition metal oxalate network with a 2-leg ladder topology. We performed zero external field , longitudinal-field quenching (LFQ) and avoided level-crossing (ALC) measurements on the Co-compound of this series. The measurements show that a significant fraction of the implanted muons forms a muoniated radical state by muonium addition to the resonant electron system of the group. The LFQ measurements reveal electronic spin fluctuation rates of several GHz. For reference, ALC measurements on the nonmagnetic compounds Li2(ox)2 and Na2(ox)2 were carried out and the muon hyperfine coupling constant were deduced to 115 and 150MHz, respectively. [Copyright &y& Elsevier]

Abstract: We present 63Cu NMR studies of the CuO2 chains in Sr13LaCu24O41 and Sr14Cu24O41. In both compounds the temperature dependence of the NMR frequency shift is described by an antiferromagnetic dimer model for the local susceptibility. For Sr14Cu24O41 the resulting spin excitation gap (133K) is consistent with macroscopic susceptibility and inelastic neutron scattering measurements and supports the suggested static charge order model for a hole-doping level of 0.6/Cu chain site. Surprisingly, Sr13LaCu24O41, a system with a reduced doping of ∼0.5 holes per Cu atom, for which macroscopic magnetic susceptibility measurements suggest a gapless antiferromagnetic chain, shows a similar behavior with a slightly reduced spin excitation gap of 108K. [Copyright &y& Elsevier]

We report muon spin rotation and magnetization measurements under pressure on Fe1+δSe1-xSx with x ≈ 0.11. Above p ≈ 0.6 GPa we find a microscopic coexistence of superconductivity with an extended dome of long range magnetic order that spans a pressure range between previously reported separated magnetic phases. The magnetism initially competes on an atomic scale with the coexisting superconductivity leading to a local maximum and minimum of the superconducting Tc(p). The maximum of Tc corresponds to the onset of magnetism while the minimum coincides with the pressure of strongest competition. A shift of the maximum of Tc(p) for a series of single crystals with x up to 0.14 roughly extrapolates to a putative magnetic and superconducting state at ambient pressure for x ≥ 0.2. [ABSTRACT FROM AUTHOR]

We present results of 23 Na and 19F nuclear magnetic resonance (NMR) measurements on NaCaCo2F7, a frustrated pyrochlore magnet with a Curie-Weiss temperature Θ CW ≈-140 K and intrinsic bond disorder. Below 3.6 K both the 23Na and 19F spectra broaden substantially in comparison to higher temperatures accompanied by a considerable reduction (80%) of the NMR signal intensity: This proves a broad quasistatic field distribution. The 19F spin-lattice relaxation rate 19(1/T 1 ) exhibits a peak at 2.9 K already starting to develop below 10 K. We attribute the spin freezing to the presence of bond disorder. This is corroborated by large-scale Monte Carlo simulations of a classical bond-disordered XY model on the pyrochlore lattice. The low freezing temperature, together with the very short magnetic correlation length not captured by the simulations, suggests that quantum effects play a decisive role in NaCaCo2F7. [ABSTRACT FROM AUTHOR]

The cubic double perovskite Ba2 YIrO6 has been investigated by the local probe techniques nuclear magnetic resonance (89Y NMR) and muon spin rotation (μ SR). Both methods confirm the absence of magnetic long-range order in this compound but find evidence for diluted localized paramagnetic moments. NMR spin-lattice relaxation rate T1-1 measurements suggest a slowing down of localized spin moments at low temperatures. An increase of the μ SR spin-lattice relaxation rate λ confirms the presence of weak magnetism in Ba2 YIrO6. However, these findings cannot be explained by the recently suggested excitonic type of magnetism. Instead, they point towards tiny amounts of localized paramagnetic spin centers leading to this magnetic response on the background of a simple nonmagnetic ground state of the 5d4 (J = 0) electronic configuration of Ir5+. [ABSTRACT FROM AUTHOR]

Over the past years a lot of theoretical and experimental effort has been made to find states with broken time-reversal symmetry (BTRS) in multiband superconductors. In particular, it was proposed theoretically that in the Ba1-x Kx Fe2 As2 system either an s + is or an s + id BTRS state may exist at high doping levels in a narrow region of the phase diagram. Here we report the observation of an enhanced zero-field muon spin-relaxation rate below the superconducting transition temperature for a high quality crystalline sample with x ≈ 0.73. This indicates that indeed the time-reversal symmetry is broken in superconducting Ba1-x Kx Fe2 As2 at this doping level. [ABSTRACT FROM AUTHOR]

In the quasi-one-dimensional heavy-fermion system YbNi4(P1-xAsx)2 the presence of a ferromagnetic (FM) quantum critical point (QCP) at xc ≈0.1 with unconventional quantum critical exponents in the thermodynamic properties has been recently reported. Here, we present muon-spin relaxation (μSR) experiments on polycrystals of this series to study the magnetic order and the low-energy 4f-electronic spin dynamics across the FM QCP. The zero-field μSR measurements on pure YbNi4P2 proved static long-range magnetic order and suggested a strongly reduced ordered Yb moment of about 0.04μB. With increasing As substitution, the ordered moment is reduced by half at x=0.04 and to less than 0.005μB at x=0.08. The dynamic behavior in the μSR response shows that magnetism remains homogeneous upon As substitution, without evidence of a disorder effect. In the paramagnetic state across the FM QCP the dynamic muon-spin relaxation rate follows 1/T1T∝T-n with 1.01±0.04≤n≤1.13±0.06. The critical fluctuations are very slow and become even slower when approaching the QCP. [ABSTRACT FROM AUTHOR]

We report the structural, magnetic, and thermodynamic properties of the double perovskite compound La2CuIrO6 from x-ray, neutron diffraction, neutron depolarization, dc magnetization, ac susceptibility, specific heat, muon-spin-relaxation (μSR), electron-spin-resonance (ESR) and nuclear magnetic resonance (NMR) measurements. Below ∼113 K, short-range spin-spin correlations occur within the Cu2+ sublattice. With decreasing temperature, the Ir4+ sublattice is progressively involved in the correlation process. Below T=74 K, the magnetic sublattices of Cu (spin s=1/2) and Ir (pseudospin j=1/2) in La2CuIrO6 are strongly coupled and exhibit an antiferromagnetic phase transition into a noncollinear magnetic structure accompanied by a small uncompensated transverse moment. A weak anomaly in ac susceptibility as well as in the NMR and μSR spin lattice relaxation rates at 54 K is interpreted as a cooperative ordering of the transverse moments which is influenced by the strong spin-orbit coupled 5d ion Ir4+. We argue that the rich magnetic behavior observed in La2CuIrO6 is related to complex magnetic interactions between the strongly correlated spin-only 3d ions with the strongly spin-orbit coupled 5d transition ions where a combination of the spin-orbit coupling and the low symmetry of the crystal lattice plays a special role for the spin structure in the magnetically ordered state. [ABSTRACT FROM AUTHOR]

We report local probe (μSR) measurements on the recently discovered tetragonal FeS superconductor which has been predicted to be electronically very similar to superconducting FeSe. Most remarkably, we find that low-moment (10-210-3μB) disordered magnetism with a transition temperature of TN≈20 K microscopically coexists with bulk superconductivity below Tc = 4.3(1) K. From transverse field μSR we obtain an in-plane penetration depth λab(0) = 223(2) nm for FeS. The temperature dependence of the corresponding superfluid density λab-2(T) indicates a fully gapped superconducting state and is consistent with a two gap s-wave model. Additionally, we find that the superconducting Tc of FeS continuously decreases for hydrostatic pressures up to 2.2 GPa. [ABSTRACT FROM AUTHOR]

The sodium iron phosphite NaFe3(HPO3)2(H2PO3)6 is studied by ac-magnetic susceptibility, pulsed-field magnetization, specific heat, and high-frequency electron spin resonance (HF-ESR) measurements combined with Mössbauer spectroscopy and density-functional calculations. We show that this compound develops ferrimagnetic order below TC=9.5K and reveals a magnetization plateau at 1/3 saturation. The plateau extends to Bc~8T, whereas above Bc the magnetization increases linearly until reaching saturation at Bs~27T. The Mössbauer spectroscopy reveals two magnetically nonequivalent iron sites with the 2:1 ratio. The HF-ESR spectra are consistent with a two-sublattice ferrimagnet and additionally pinpoint weak magnetic anisotropy as well as short-range spin order that persists well above TC. The ferrimagnetic order in the title compound is stabilized by a network of purely antiferromagnetic exchange interactions. The spin lattice comprises layers coinciding with the crystallographic (10-1) planes, with stronger couplings Ji~1.5K within the layers and weaker couplings Ji=0.3-0.4K between the layers. Both intralayer and interlayer couplings are frustrated. The ensuing ferrimagnetic order arises from a subtle interplay of the frustrated but nonequivalent exchange couplings. [ABSTRACT FROM AUTHOR]

Abstract: We present a temperature-dependent high-field magnetization study of the antiferromagnetic Heisenberg chain (AFHC) along the magnetically characteristic axis . The data are analyzed on basis of the Bethe ansatz equations and via exact diagonalization of a linear spin chain with 20 sites. Overall, we find a very good agreement between experimental data and theoretical calculations, and extract a magnetic coupling strength along the chain of 18.8(1)K. We discuss our results in context with the possibility for a magnetocaloric effect during the magnetization process of AFHC systems. [Copyright &y& Elsevier]

We present an investigation of the S=1/2 antiferromagnetically coupled Heisenberg spin chain [PM·Cu(NO3)2·(H2O)2]n (PM=pyrimidine=C4N2H4) with field-induced gap Δ by means of 13C–NMR experiments. From these studies, carried out as function of pressure, we determine the NMR shift and the spin-lattice relaxation rate 1/T1 and extract the magnetic exchange parameter J/kB and the size of the spin gap Δ. The observed pressure response is discussed in comparison to related molecular materials. [Copyright &y& Elsevier]

We examined the doping dependence of the charge order in the spin chains of Sr14Cu24O41 and Sr13LaCu24O41 by 63Cu-NMR. In Sr14Cu24O41we verified the existence of two distinct Zhang–Rice copper sites on the chains with a dimer local susceptibility in agreement with the macroscopic susceptibility. In Sr13LaCu24O41 one broad line associated with a Zhang–Rice site is observed. In contrast to expectations motivated by the charge order model this signal also exhibits a thermally activated local susceptibility. [Copyright &y& Elsevier]

Abstract: We report resistivity and upper critical field data for disordered (As-deficient) in a wide temperature and high field range up to 60T. These samples exhibit a slightly enhanced superconducting transition at and a significantly enlarged slope near which contrasts with a flattening of starting near 23K above 30T. The latter evidences Pauli-limiting behaviour (PLB) with . We compare our results with -data from the literature for clean and disordered samples. Whereas clean samples show almost no PLB for fields below 60–70T, the hitherto unexplained pronounced flattening of for applied fields observed for several disordered closely related systems is interpreted also as a manifestation of PLB. Consequences are discussed in terms of disorder effects within the frames of (un)conventional superconductivity, respectively. [ABSTRACT FROM AUTHOR]