Antiferromagnetic Correlations in Strongly Valence Fluctuating CeIrSn.

CeIrSn with a quasikagome Ce lattice in the hexagonal basal plane is a strongly valence fluctuating compound, as we confirm by hard x-ray photoelectron spectroscopy and inelastic neutron scattering, with a high Kondo temperature of T_{K}∼480 K. We report a negative in-plane thermal expansion α/T below 2 K, which passes through a broad minimum near 0.75 K. Volume and a-axis magnetostriction for B∥a are markedly negative at low fields and change sign before a sharp metamagnetic anomaly at 6 T. These behaviors are unexpected for Ce-based intermediate valence systems, which should feature positive expansivity. Rather they point towards antiferromagnetic correlations at very low temperatures. This is supported by muon spin relaxation measurements down to 0.1 K, which provide microscopic evidence for a broad distribution of internal magnetic fields. Comparison with isostructural CeRhSn suggests that these antiferromagnetic correlations emerging at T≪T_{K} result from geometrical frustration.

Evidence of a Nodal Line in the Superconducting Gap Symmetry of Noncentrosymmetric ThCoC_{2}.

The newly discovered noncentrosymmetric superconductor ThCoC_{2} exhibits numerous types of unconventional behavior in the field dependent heat capacity data. Here we present the first measurement of the gap symmetry of ThCoC_{2} by muon spin rotation and relaxation (µSR) measurements. The temperature dependence of the magnetic penetration depth measured using the transverse field µSR experiment reveals the evidence of a nodal pairing symmetry. To understand this finding, we carry out calculations of the superconducting pairing eigenvalue and eigenfunction (pairing symmetry) due to the spin-fluctuation mechanism by directly implementing the ab initio band structures. We find that the system possesses a single Fermi surface with considerable three dimensionality and a strong nesting along the k_{z} direction. Such nesting promotes a superconducting state with a cosk_{z}-like pairing symmetry with a prominent nodal line on the k_{z}=±π/2 plane. The result agrees well with the experimental data.

Spin Waves in Detwinned BaFe_{2}As_{2}.

Understanding magnetic interactions in the parent compounds of high-temperature superconductors forms the basis for determining their role for the mechanism of superconductivity. For parent compounds of iron pnictide superconductors such as AFe_{2}As_{2} (A=Ba, Ca, Sr), although spin excitations have been mapped out throughout the entire Brillouin zone, the respective measurements were carried out on twinned samples and did not allow for a conclusive determination of the spin dynamics. Here we use inelastic neutron scattering to completely map out spin excitations of ∼100% detwinned BaFe_{2}As_{2}. By comparing observed spectra with theoretical calculations, we conclude that the spin excitations can be well described by an itinerant model when taking into account moderate electronic correlation effects.

Multiband One-Dimensional Electronic Structure and Spectroscopic Signature of Tomonaga-Luttinger Liquid Behavior in K_{2}Cr_{3}As_{3}.

We present angle-resolved photoemission spectroscopy measurements of the quasi-one-dimensional superconductor K_{2}Cr_{3}As_{3}. We find that the Fermi surface contains two Fermi surface sheets, with linearly dispersing bands not displaying any significant band renormalizations. The one-dimensional band dispersions display a suppression of spectral intensity approaching the Fermi level according to a linear power law, over an energy range of ∼200 meV. This is interpreted as a signature of Tomonoga-Luttinger liquid physics, which provides a new perspective on the possibly unconventional superconductivity in this family of compounds.

Exotic magnetism on the quasi-fcc lattices of the d3 double Perovskites La2NaB'O6 (B'=Ru, Os).

We find evidence for long-range and short-range (ζ=70 Å at 4 K) incommensurate magnetic order on the quasi-face-centered-cubic (fcc) lattices of the monoclinic double perovskites La2NaRuO6 and La2NaOsO6, respectively. Incommensurate magnetic order on the fcc lattice has not been predicted by mean field theory, but may arise via a delicate balance of inequivalent nearest neighbor and next nearest neighbor exchange interactions. In the Ru system with long-range order, inelastic neutron scattering also reveals a spin gap Δ âˆ¼ 2.75 meV. Magnetic anisotropy is generally minimized in the more familiar octahedrally coordinated 3d3 systems, so the large gap observed for La2NaRuO6 may result from the significantly enhanced value of spin-orbit coupling in this 4d(3) material.

Spin-wave spectrum of the quantum ferromagnet on the pyrochlore lattice Lu2V2O7.

Neutron inelastic scattering has been used to probe the spin dynamics of the quantum (S=1/2) ferromagnet on the pyrochlore lattice Lu(2)V(2)O(7). Well-defined spin waves are observed at all energies and wave vectors, allowing us to determine the parameters of the Hamiltonian of the system. The data are found to be in excellent overall agreement with a minimal model that includes a nearest-neighbor Heisenberg exchange J = 8.22(2) meV and a Dzyaloshinskii-Moriya interaction (DMI) D = 1.5(1) meV. The large DMI term revealed by our study is broadly consistent with the model originally used to explain the magnon Hall effect in this compound [Onose et al., Science 329, 297 (2010) and Ideue et al., Phys. Rev. B 85, 134411 (2012)]. However, our ratio of D/J = 0.18(1) is roughly half of their value, and is much larger than those found in other theoretical studies [Xiang et al., Phys. Rev. B 83, 174402 (2011) and Mook et al., Phys. Rev. B 89,134409 (2014)].

Competing exchange interactions on the verge of a metal-insulator transition in the two-dimensional spiral magnet Sr3Fe2O7.

We report a neutron scattering study of the magnetic order and dynamics of the bilayer perovskite Sr(3)Fe(2)O(7), which exhibits a temperature-driven metal-insulator transition at 340 K. We show that the Fe(4+) moments adopt incommensurate spiral order below T(N) = 115 K and provide a comprehensive description of the corresponding spin-wave excitations. The observed magnetic order and excitation spectra can be well understood in terms of an effective spin Hamiltonian with interactions ranging up to third-nearest-neighbor pairs. The results indicate that the helical magnetism in Sr(3)Fe(2)O(7) results from competition between ferromagnetic double-exchange and antiferromagnetic superexchange interactions whose strengths become comparable near the metal-insulator transition. They thus confirm a decades-old theoretical prediction and provide a firm experimental basis for models of magnetic correlations in strongly correlated metals.

Muon spin relaxation and emergence of disorder-induced unconventional dynamic magnetic fluctuations in Dy2Zr2O7.

The disordered pyrochlore oxide Dy2Zr2O7shows the signatures of field-induced spin freezing with remnant zero-point spin-ice entropy at 5 kOe magnetic field. We have performed zero-field and longitudinal field Muon spin relaxation (µSR) studies on Dy2Zr2O7. Our zero field studies reveal the absence of both long-range ordering and spin freezing down to 62 mK. TheµSR relaxation rate exhibits a temperature-independent plateau below 4 K, indicating a dynamic ground state of fluctuating spins similar to the well-known spin ice system Dy2Ti2O7. The low-temperature spin fluctuations persist in the longitudinal field of 20 kOe as well and show unusual field dependence of the relaxation rate, which is uncommon for a spin-liquid system. Our results, combined with the previous studies do not show any evidence of spin ice or spin glass ground state, rather point to a disorder-induced dynamic magnetic ground state in the Dy2Zr2O7material.

Disordered magnetic ground state in a quasi-1-Dd4columnar iridate Sr3LiIrO6.

Spin-orbit coupling offers a large variety of novel and extraordinary magnetic and electronic properties in otherwise 'ordinary pool' of heavy ion oxides. Here we present a detailed study on an apparently isolated hexagonal 2Hspin-chaind4iridate Sr3LiIrO6with geometric frustration. Our structural studies reveal Li-Ir chemical order with desired stoichiometry in this compound, while x-ray absorption together with x-ray photoemission spectroscopic characterizations establish pure 5+ valence of Ir. We have established a magnetic ground state with finite Ir5+magnetic moments in this compound, contrary to the anticipated nonmagneticJeff= 0 state, through combined dc susceptibility,7Li nuclear magnetic resonance (NMR), muon spin relaxation (µSR) andab-initioelectronic structure studies. These investigations together with ac magnetic susceptibility and specific heat measurements reveal that despite having noticeable antiferromagnetic correlation among the Ir5+local moments, this system does not magnetically order down to at least 0.05 K, possibly due to geometrical exchange frustration, arising from the comparable nearest- and next-nearest-neighbor interchain Ir-O-O-Ir superexchange interaction strengths with opposite signs. However, the zero-fieldµSR analysis shows emergence of a considerable proportion of spin-freezing on top of a spin-fluctuating dynamic magnetic background down to the lowest measured temperature of 1.7 K, possibly due to some inhomogeneity and/or the much stronger intra-column Ir-Ir magnetic exchange interaction strength relative to the inter-column Ir-Ir ones. The linear temperature dependence of the magnetic specific heat (Cm) in both zero and applied magnetic fields, plus the power-law behavior of the NMR spin-lattice relaxation rate suggest a gapless spinon density of states in this charge gapped disordered magnetic ground state of Sr3LiIrO6.

Unveiling the structural phase transition and magnetic structure of ternary boride PrIr3B2.

We present a comprehensive study of PrIr3B2, which includes a detailed investigation of its crystal and magnetic structure using neutron diffraction. AC and DC magnetization and heat capacity data reveal antiferromagnetic ordering atTN= 10 K. The heat capacity measurements further exhibit a broad peak near 270 K which is related to a structural transition fromP6/mmmtoC2/mseen in low temperature x-ray diffraction and neutron diffraction. High intensity neutron diffraction data confirm the long-range ordering of Pr3+spins, with no apparent magnetic moment on either of the Iridium sites. Two possible magnetic structures with eitherk1= [1,0,0] ork2= [½,½,0] fit nearly equally well the neutron diffraction data. However, based on previous magnetization studies on a single crystalline sample it is argued that the second solution withk2corresponds to the appropriate magnetic structure of PrIr3B2below 10 K. In this magnetic structure, the Pr3+moments are oriented at ∼45° to both theaandbaxes, with thec-axis being the hard axis of magnetization. Overall, our results provide new insights into the magnetic and structural properties of PrIr3B2.

Probing the strongly correlated magnetic state of Co2C nanoparticles at low temperatures usingµSR.

Co2C nanoparticles (NPs) are amongst transition metal carbides whose magnetic properties have not been well explored. An earlier study (Royet al2021J. Phys.: Condens. Matter33375804) showed that a pellet made from Co2C NPs exhibits exchange bias (EB) effect below a temperature,TEB= 50 K and a spin glass (SG) feature emerges belowTSG= 5 K. In the current study we use magnetic, electrical transport, specific heat, and muon spin rotation (µSR) measurements to explore further the magnetic properties of a pellet made with 40 nm diameter pure Co2C NPs. We uncover the onset of Kondo localization at Kondo temperatureTK(= 40.1 K), which is close to the onset temperature (TEB) of the EB effect. A crossover from the Kondo-screened scenario to the Ruderman-Kittel-Kasuya-Yosida interaction-dominated regime is also observed forT

Vibron quasibound state in the noncentrosymmetric tetragonal heavy-fermion compound CeCuAl3.

We have investigated the noncentrosymmetric tetragonal heavy-fermion antiferromagnetic compound CeCuAl3 (T(N)=2.5 K) using inelastic neutron scattering (INS). Our INS results unequivocally reveal the presence of three magnetic excitations centered at 1.3, 9.8, and 20.5 meV. These spectral features cannot be explained within the framework of crystal-electric-field models and recourse to Kramers' theorem for a 4f(1) Ce(3+) ion. To overcome these interpretational difficulties, we have generalized the vibron model of Thalmeier and Fulde for cubic CeAl(2) to tetragonal point-group symmetry with the theoretically calculated vibron form-factor. This extension provides a satisfactory explanation for the position and intensity of the three observed magnetic excitations in CeCuAl3, as well as their dependence on momentum transfer and temperature. On the basis of our analysis, we attribute the observed series of magnetic excitations to the existence of a vibron quasibound state.

High coercive field and magnetization reversal in core-shell cum nanotwin driven Ni/NiO nanospheres.

We report here nanotwin-core-shell Ni(core)NiO(shell) spheres of average size 25 nm prepared through polyol method. They exhibit high coercive field at 2 K, sharp peak at approximately 20 K in magnetization curve and magnetization reversal. Interestingly, exchange bias due to antiferromagnetic NiO shell is absent. Among other possibilities, anisotropy variations due to particle size distribution and twinning associated with disorder appear to play an important role. Further, magnetic interactions of twinned bigger spheres, which may also act as superferrimagnetic-like Ni multilayer cores, with superparamagnetic Ni of smaller spheres, might be the additional causes. These nanostructures therefore seem to have potential interest in memory effect.

Exploring a low temperature glassy state, exchange bias effect, and high magnetic anisotropy in Co2C nanoparticles.

It is interesting to explore the connections between the exchange bias effect (EBE) and magnetic anisotropy (MA). It is often found that materials exhibiting a strong EBE also have enhanced MA. Here we explore 40 nm diameter Co2C nanoparticles (NPs) that exhibit ferromagnetism with a blocking temperature exceeding 300 K. We report the first observation of EBE in these Co2C NPs below 50 K. The effect arises from the exchange coupling of frozen ferromagnetic spins with a freely rotatable spin component. The dynamics of the freely rotatable component freezes in a temperature range between 5 K to 20 K resulting in low-temperature coexistence of a glassy behavior along with ferromagnetism. In fact, Co2C displays a unique separation of onset temperatures of spin freezing (∼20 K), vanishing of EBE (∼50 K), and magnetic blocking (⩾450 K). Our calculations show that Co2C NPs have a core-shell structure. Our study suggests that modifying chemical co-ordination in the shell is one of the effective routes to manipulating MA compared to manipulating EBE.

Magnetic structure and crystal field excitations of NdOs2Al10: a neutron scattering study.

We have investigated the magnetic properties of a polycrystalline sample of NdOs2Al10using neutron diffraction and inelastic neutron scattering, magnetic susceptibility and heat capacity measurements. The magnetic susceptibility data reveal an antiferromagnetic transition atTN1= 2.2 K while the heat capacity data which extend to lower temperatures show a further transition atTN2= 1.1 K. The INS measurements detect four well-resolved crystal field excitations at 7.4, 12.4, 17.6 meV and 19.2 meV at 5 K. The positions and intensities of the observed CEF excitations and the temperature dependence of previously published single crystal susceptibility data are analysed based on the orthorhombic CEF model of theJ= 9/2 ground state multiplet of Nd3+ion. The neutron diffraction study reveals the magnetic structure atT= 1.6 K to be of sine wave type (κ= [0, 0.723, 0]) with an ordered state moment ofµNd3+= 1.59(1)µBaligned along thea-axis in agreement with the single ion crystal field anisotropy. This indicates that the magnetism of NdOs2Al10is governed by the RKKY interactions in agreement with otherRT2Al10(R= Nd, Sm, Tb,T= Fe, Ru and Os), but different to that ofR= Ce based compounds where anisotropic two ions interaction as well as Kondo interaction govern the anomalous direction of the ordered state moment.

Electron-phonon superconductivity in C-doped topological nodal-line semimetal Zr5Pt3: a muon spin rotation and relaxation (µSR) study.

In the present work, we demonstrate that C-doped Zr5Pt3is an electron-phonon superconductor (with critical temperatureTC= 3.8 K) with a nonsymmorphic topological Dirac nodal-line semimetal state, which we report here for the first time. The superconducting properties of Zr5Pt3C0.5have been investigated by means of magnetization, resistivity, specific heat, and muon spin rotation and relaxation (µSR) measurements. We find that at low temperatures, the depolarization rate is almost constant and it can be well described by a single-bands-wave model with a superconducting gap of 2Δ(0)/kBTC= 3.84, somewhat higher than the value of BCS theory. From the transverse field µSR analysis, we estimate the London penetration depthλL= 469 nm, superconducting carrier densityns= 1.83 × 1026 m-3, and effective massm* = 1.428me. The zero field µSR confirms the absence of any spontaneous magnetic field in the superconducting ground state. In order to gain additional insights into the electronic ground state of C-doped Zr5Pt3, we also performed first-principles calculations within the framework of density functional theory (DFT). The observed homogenous electronic character of the Fermi surface as well as the mutual decrease ofTCand density of states at the Fermi level are consistent with the experimental findings of this study. However, the band structure reveals the presence of robust, gapless fourfold-degenerate nodal lines protected by 63screw rotations and glide mirror planes. Therefore, Zr5Pt3represents a novel, unprecedented condensed matter system to investigate the intricate interplay between superconductivity and topology.

Transition from heavy-fermion to mixed-valence behavior in Ce(1-x)Y(x)Al3: a quantitative comparison with the anderson impurity model.

We present a neutron scattering investigation of Ce1-xYxAl3 as a function of chemical pressure, which induces a transition from heavy-fermion behavior in CeAl3 (T{K}=5 K) to a mixed-valence state at x=0.5 (T{K}=150 K). The crossover can be modeled accurately on an absolute intensity scale by an increase in the k-f hybridization, V{kf}, within the Anderson impurity model. Surprisingly, the principal effect of the increasing V{kf} is not to broaden the low-energy components of the dynamic magnetic susceptibility but to transfer spectral weight to high energy.

Antiferromagnetic fluctuations and charge carrier localization in ferromagnetic bilayer manganites: electrical resistivity scales exponentially with short-range order controlled by temperature and magnetic field.

The compound La2-2x Sr1+2x Mn2O7, x = 0.30-0.40, consists of bilayers of ferromagnetic metallic MnO2 sheets that are separated by insulating layers. The materials show colossal magnetoresistance-a reduction in resistivity of up to two orders of magnitude in a field of 7 T-at their three-dimensional ordering temperatures, T C = 90-126 K, and are the layered analogues of the widely studied pseudo-cubic perovskite manganites, R1-x A x MnO3 (R = rare earth, A = Ca, Sr, Ba, Pb). Two distinct short-range orderings-antiferromagnetic fluctuations and correlated polarons, which are related to the magnetic and the lattice degrees of freedom respectively-have previously been discovered in La2-2x Sr1+2x Mn2O7, x = 0.40, and have each been qualitatively connected to the resistivity. Here, in a comprehensive study as a function of both temperature and magnetic field for the different hole-concentrations per Mn site of x = 0.30 and 0.35, we show that antiferromagnetic fluctuations also appear at temperatures just above T C, and that the intensities of both the antiferromagnetic fluctuations and polaron correlations closely track the resistivity. In particular, for x = 0.35 we show that there is a simple scaling relation between the intensities of the antiferromagnetic fluctuations and the in-plane resistivity that applies for the temperatures and magnetic fields used in the experiments. The results show that antiferromagnetic fluctuations are a common feature of La2-2x Sr1+2x Mn2O7 with ferromagnetic bilayers, and that there is a close connection between the antiferromagnetic fluctuations and polarons in these materials.

Ir 5d-band derived superconductivity in LaIr3.

The superconducting properties of rhombohedral LaIr3 were examined using susceptibility, resistivity, heat capacity, and zero-field (ZF) and transverse-field (TF) muon spin relaxation and rotation ([Formula: see text]SR) measurements. The susceptibility and resistivity measurements confirm a superconducting transition below [Formula: see text] K. Two successive transitions are observed in the heat capacity data, one at [Formula: see text] K and a second at 1.2 K below [Formula: see text]. The heat capacity jump is [Formula: see text], which is lower than 1.43 expected for Bardeen-Cooper-Schrieffer (BCS) weak-coupling limit. TF-[Formula: see text]SR measurements reveal a fully gapped s-wave superconductivity with [Formula: see text], which is small compared to the BCS value of 3.56, suggesting weak-coupling superconductivity. The magnetic penetration depth, [Formula: see text], estimated from TF-[Formula: see text]SR gives [Formula: see text] nm, a superconducting carrier density [Formula: see text] carriers m-3 and a carrier effective-mass enhancement factor [Formula: see text]. ZF-[Formula: see text]SR data show no evidence for any spontaneous magnetic fields below [Formula: see text], which demonstrates that time-reversal symmetry is preserved in the superconducting state of LaIr3.

Observation of a neutron spin resonance in the bilayered superconductor CsCa2Fe4As4F2.

We report inelastic neutron scattering (INS) investigations on the bilayer Fe-based superconductor CsCa2Fe4As4F2 above and below its superconducting transition temperature T c ≈ 28.9 K to investigate the presence of a neutron spin resonance. This compound crystallises in a body-centred tetragonal lattice containing asymmetric double layers of Fe2As2 separated by insulating CaF2 layers and is known to be highly anisotropic. Our INS study clearly reveals the presence of a neutron spin resonance that exhibits higher intensity at lower momentum transfer (Q) at 5 K compared to 54 K, at an energy of 15 meV. The energy E R of the observed spin resonance is broadly consistent with the relationship E R = 4.9k B T c, but is slightly enhanced compared to the values observed in other Fe-based superconductors. We discuss the nature of the electron pairing symmetry by comparing the value of E R with that deduced from the total superconducting gap value integrated over the Fermi surface.