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Centrosymmetric compounds which host three-dimensional topological spin structures comprise a distinct subclass of materials in which multiple-$\mathbf{q}$ magnetic order is stabilized by anisotropy and bond frustration in contrast to the more common path of antisymmetric exchange interactions. Here we investigate static and dynamic magnetic properties of the cubic perovskite SrFeO$_3$ $\unicode{x2013}$ a rare example of a centrosymmetric material hosting two types of topological spin textures: skyrmion- and hedgehog-lattice phases. Our detailed magnetization and dilatometry measurements describe the domain selection processes and phase transitions in SrFeO$_3$. Spin excitations are investigated using inelastic neutron scattering for all three zero-field phases. In the higher-temperature ordered phases, high-energy magnons increasingly lose coherence, so that spin fluctuations are dominated by a distinct quasielastic component at low energies. We anticipate that this could be generic to symmetric helimagnets in which the chiral symmetry is spontaneously broken by the magnetic order.

Magnetic dilution of a well-established Kitaev candidate system is realized in the substitutional Ru$_{1-x}$Rh$_x$Cl$_3$ series ($x = 0.02-0.6$). Optimized syntheses protocols yield uniformly-doped single crystals and polycrystalline powders that are isostructural to the parental $\alpha$-RuCl$_3$ as per X-ray diffraction. The Rh content $x$ is accurately determined by the quantitative energy-dispersive X-ray spectroscopy technique with standards. We determine the magnetic phase diagram of Ru$_{1-x}$Rh$_x$Cl$_3$ for in-plane magnetic fields from magnetization and specific-heat measurements as a function of $x$ and stacking periodicity, and identify the suppression of the magnetic order at $x \approx 0.2$ towards a disordered phase, which does not show any clear signature of freezing into a spin glass. Comparing with previous studies on the substitution series Ru$_{1-x}$Ir$_x$Cl$_3$, we propose that chemical pressure would contribute to the suppression of magnetic order especially in Ru$_{1-x}$Ir$_x$Cl$_3$ and that the zigzag magnetic ground state appears to be relatively robust with respect to the dilution of the Kitaev--$\Gamma$--Heisenberg magnetic lattice. We also discovered a slight dependence of the magnetic properties on thermal cycling, which would be due to an incomplete structural transition.

The conformation of molecules on surfaces is decisive for their functionality. For the case of the endofullerene paramagnet Sc2TbN@C80 the conformation is linked to an electric and a magnetic dipole moment. Therefore a workfunction change of a substrate with adsorbed molecules, qualifies the system to be magnetoelectric. One monolayer of Sc2TbN@C80 has been studied on h-BN/Ni(111). The molecules assume a hexagonally close packed lattice aligned with the substrate high symmetry directions. The structure is incommensurate and arranges at a periodicity of about 4.3x4.3 substrate unit cells. At low temperatures a (2 x 2) superstructure is observed. Angular resolved valence band photoemission spectroscopy shows a temperature induced 0.3 eV shift on the C80 molecular orbitals to lower binding energies that is parallel to a workfunction increase. From comparison of the molecular orbital angular photoemission intensity distributions it is conjectured that the molecules undergo a change in conformation between 30 and 300 K. This phase transition is centred at 125 K as observed with high resolution x-ray photoelectron spectroscopy that shows the core levels of the atomic species on the molecules to shift parallel to the workfunction. The temperature dependence of the workfunction can be described with a two level model that accounts for the disordering with an excitation energy of 60 meV into a highly degenerate ensemble. The experimental findings are backed by density functional theory calculations for the diamagnetic sibling of Sc2TbN@C80 : Sc2YN@C80 that rationalize the incommensurate structure, show a permanent dipole moment of Sc2YN@C80 and a relation between the workfunction and the orientation of the endohedral cluster., Comment: 7 pages 6 figures

Electronic correlations were long suggested not only to be responsible for the complexity of many novel materials, but also to form essential prerequisites for their intriguing properties. Electronic behavior of iron-based superconductors is far from conventional, while the reason for that is not yet understood. Here we present a combined study of the electronic spectrum in the iron-based superconductor FeSe by means of angle-resolved photoemission spectroscopy (ARPES) and dynamical mean field theory (DMFT). Both methods in unison reveal strong deviations of the spectrum from single-electron approximation for the whole 3$d$ band of iron: not only the well separated coherent and incoherent parts of the spectral weight are observed, but also a noticeable dispersion of the lower Hubbard band (LHB) is clearly present. This way we demonstrate correlations of the most puzzling intermediate coupling strength in iron superconductors.

Recent breakthrough in search for the analogs of fundamental particles in condensed matter systems lead to experimental realizations of 3D Dirac and Weyl semimetals. Weyl state can be hosted either by non-centrosymmetric or magnetic materials and can be of the first or the second type. Several non-centrosymmetric materials have been proposed to be type-II Weyl semimetals, but in all of them the Fermi arcs between projections of multiple Weyl points either have not been observed directly or they were hardly distinguishable from the trivial surface states which significantly hinders the practical application of these materials. Here we present experimental evidence for type-II non-centrosymmetric Weyl state in TaIrTe$_4$ where it has been predicted theoretically. We find direct correspondence between ARPES spectra and calculated electronic structure both in the bulk and the surface and clearly observe the exotic surface states which support the quasi-1D Fermi arcs connecting only four Weyl points. Remarkably, these electronic states are spin-polarized in the direction along the arcs, thus highlighting TaIrTe$_4$ as a novel material with promising application potential., Comment: 9 pages, 4 figures

Muon spin rotation technique is used to study magnetic ordering in ultra-pure samples of SrCu$_{1-x}$Ni$_x$O$_2$, an archetypical $S=1/2$ antiferromagnetic Heisenberg chain system with a small amount of $S=1$ defects. The ordered state in the parent compound is shown to be highly homogeneous, contrary to previous report [M. Matsuda et al., Phys. Rev. B \textbf{55}, R11953 (1997)]. Even minute amount of Ni impurities result in inhomogeneous order and a decrease of the transition temperature. At as little as $0.5$~\% Ni concentration, magnetic ordering is entirely suppressed. The results are compared to previous theoretical studies of weakly coupled spin chains with site-defects.

We have studied the magnetic ordering in Na doped BaFe$_2$As$_2$ by unpolarized and polarized neutron diffraction using single crystals. Unlike previously studied FeAs-based compounds that magnetically order, Ba$_{1-x}$Na$_x$Fe$_2$As$_2$ exhibits two successive magnetic transitions: For x=0.35 upon cooling magnetic order occurs at $\sim$70\ K with in-plane magnetic moments being arranged as in pure or Ni, Co and K-doped BaFe$_2$As$_2$ samples. At a temperature of $\sim$46\ K a second phase transition occurs, which the single-crystal neutron diffraction experiments can unambiguously identify as a spin reorientation. At low temperatures, the ordered magnetic moments in Ba$_{0.65}$Na$_{0.35}$Fe$_2$As$_2$ point along the $c$ direction. Magnetic correlations in these materials cannot be considered as Ising like, and spin-orbit coupling must be included in a quantitative theory., Comment: 5 pages, 4 figures

The S=1/2 spin chain material SrCuO2 doped with 1% S=1 Ni-impurities is studied by inelastic neutron scattering. At low temperatures, the spectrum shows a pseudogap \Delta ~ 8 meV, absent in the parent compound, and not related to any structural phase transition. The pseudogap is shown to be a generic feature of quantum spin chains with dilute defects. A simple model based on this idea quantitatively accounts for the exprimental data measured in the temperature range 2-300 K, and allows to represent the momentum-integrated dynamic structure factor in a universal scaling form., Comment: 5 pages, 3 figures

Nonequilibrium quasiparticle relaxation dynamics is reported in superconducting CaFe1.89Co0.11As2 single crystal using femtosecond time-resolved pump-probe spectroscopy. The carrier dynamics reflects a three-channel decay of laser deposited energy with characteristic time scales varying from few hundreds of femtoseconds to order of few nanoseconds where the amplitudes and time-constants of the individual electronic relaxation components show significant changes in the vicinity of the spin density wave (T_SDW ~ 85 K) and superconducting (T_SC ~ 20 K) phase transition temperatures. The quasiparticles dynamics in the superconducting state reveals a charge gap with reduced gap value of 2$\Delta$_0/k_BT_SC ~ 1.8. We have determined the electron-phonon coupling constant $\lemda$ to be ~ 0.14 from the temperature dependent relaxation time in the normal state, a value close to those reported for other types of pnictides. From the peculiar temperature-dependence of the carrier dynamics in the intermediate temperature region between the superconducting and spin density wave phase transitions, we infer a temperature scale where the charge gap associated with the spin ordered phase is maximum and closes on either side while approaching the two phase transition temperatures., Comment: 6 pages, 4 figures (revised manuscript); http://dx.doi.org/10.1016/j.ssc.2013.02.008

We report temperature evolution of coherently excited acoustic and optical phonon dynamics in superconducting iron pnictide single crystal Ca(Fe_0.944Co_0.056)_2As_2 across the spin density wave transition at T_SDW ~ 85 K and superconducting transition at T_SC ~20 K. Strain pulse propagation model applied to the generation of the acoustic phonons yields the temperature dependence of the optical constants, and longitudinal and transverse sound velocities in the temperature range of 3.1 K to 300 K. The frequency and dephasing times of the phonons show anomalous temperature dependence below T_SC indicating a coupling of these low energy excitations with the Cooper-pair quasiparticles. A maximum in the amplitude of the acoustic modes at T ~ 170 is seen, attributed to spin fluctuations and strong spin-lattice coupling before T_SDW., Comment: 6 pages, 4 figures (revised manuscript)

We report ultrafast quasiparticle (QP) dynamics and coherent acoustic phonons in undoped CaFe_2As_2 iron pnictide single crystals exhibiting spin-density wave (SDW) and concurrent structural phase transition at temperature TSDW ~ 165 K using femtosecond time-resolved pump-probe spectroscopy. The contributions in transient differential reflectivity arising from exponentially decaying QP relaxation and oscillatory coherent acoustic phonon mode show large variations in the vicinity of T_SDW. From the temperature-dependence of the QP recombination dynamics in the SDW phase, we evaluate a BCS-like temperature dependent charge gap with its zero-temperature value of ~(1.6+/-0.2)k_BT_SDW, whereas, much above T_SDW, an electron-phonon coupling constant of ~0.13 has been estimated from the linear temperature-dependence of the QP relaxation time. The long-wavelength coherent acoustic phonons with typical time-period of ~100 ps have been analyzed in the light of propagating strain pulse model providing important results for the optical constants, sounds velocity and the elastic modulus of the crystal in the whole temperature range of 3 K to 300 K., Comment: Revised version (to appear as Full Paper in Journal of Physical Society of Japan (2013)); http://jpsj.ipap.jp/link?JPSJ/82/044715/

In our tunneling investigation using Andreev superconductor - normal metal - superconductor contacts on LiFeAs single crystals we observed two reproducible independent subharmonic gap structures at dynamic conductance characteristics. From these results, we can derive the energy of the large superconducting gap $\Delta_L=(2.5 \div 3.4)$ meV and the small gap $\Delta_L=(0.9 \div 1)$ meV at $T = 4.2$ K for the $T_C^{local} \approx (10.5 \div 14)$ K (the contact area critical temperature which deviation causes the variation of $\Delta_L$). The BCS-ratio is found to be $2\Delta_L/k_BT_C = (4.6 \div 5.6)$, whereas $2\Delta_S/k_BT_C \ll 3.52$ results from induced superconductivity in the bands with the small gap., Comment: 7 pages, 5 figures. Published in Pis'ma v ZhETF 95, 604-610 (2012)

We present an angle resolved photoemission study of the surface and bulk electronic structure of the single layer ruthenate Sr$_2$RuO$_4$. As the early studies of its electronic structure by photoemission and scanning tunneling microscopy were confronted with a problem of surface reconstruction, surface aging was previously proposed as a possible remedy to access the bulk states. Here we suggest an alternative way by demonstrating that, in the case of Sr$_2$RuO$_4$, circularly polarised light can be used to disentangle the signals from the bulk and surface layers, thus opening the possibility of investigating many-body interactions both in bulk and surface bands. The proposed procedure results in improved momentum resolution, which enabled us to detect an unexpected splitting of the surface $\beta$ band. We propose that spin--orbit splitting might be responsible for this, and discuss possible relations of the newly observed surface feature to topological matter., Comment: Submitted to PRB. Related work: http://www.ifw-dresden.de/institutes/iff/research/SC/arpes

Superconductor/ferromagnet (S/F) spin valve effect theories based on the S/F proximity phenomenon assume that the superconducting transition temperature Tc of F1/F2/S or F1/S/F2 trilayers for parallel magnetizations of the F1- and F2-layers (TcP) are smaller than for the antiparallel orientations (TcAP). Here, we report for CoOx/Fe1/Cu/Fe2/In multilayered systems with varying Fe2-layer thickness the sign-changing oscillating behavior of the spin valve effect \Delta Tc=TcAP-TcP. Our measurements revealed the full direct spin valve effect with TcAP>TcP for Fe2-layer thickness dFe2<1 nm and the full inverse (TcAP=1 nm. Interference of Cooper pair wave functions reflected from both surfaces of the Fe2-layer appear as the most probable reason for the observed behavior of \Delta Tc., Comment: Accepted for publication in PRL

Superconductor/ferromagnet (S/F) proximity effect theory predicts that the superconducting critical temperature of the F1/F2/S or F1/S/F2 trilayers for the parallel orientation of the F1 and F2 magnetizations is smaller than for the antiparallel one. This suggests a possibility of a controlled switching between the superconducting and normal states in the S layer. Here, using the spin switch design F1/F2/S theoretically proposed by Oh et al. [Appl. Phys. Lett. 71, 2376 (1997)], that comprises a ferromagnetic bilayer separated by a non-magnetic metallic spacer layer as a ferromagnetic component, and an ordinary superconductor as the second interface component, we have successfully realized a full spin switch effect for the superconducting current., Comment: 5 pages, 4 figures

We investigate the critical current density and vortex dynamics in single crystals of Ca(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$ ($x$ = 0.051, 0.056, 0.065, and 0.073). The samples exhibit different critical temperatures and superconducting phase fractions. We show that in contrast to their Ba-based counterpart, the crystals do not exhibit a second peak in the field dependence of magnetization. The calculated composition-dependent critical current density ($j_{\rm c}$) increases initially with Co doping, maximizing at $x$ = 0.065, and then decreases. This variation in $j_{\rm c}$ follows the superconducting phase fractions in this series. The calculated $j_{\rm c}$ shows strong temperature dependence, decreasing rapidly upon heating. Magnetic relaxation measurements imply a nonlogarithmic dependence on time. We find that the relaxation rate is large, reflecting weak characteristic pinning energy. The analysis of temperature- and field-dependent magnetic relaxation data suggests that vortex dynamics in these compounds is consistent with plastic creeping rather than the collective creep model, unlike other 122 pnictide superconductors. This difference may cause the absence of the second peak in the field dependent magnetization of Ca(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$.

We present measurements of the thermal expansion coefficient $\alpha$ of polycrystalline RFeAsO (R = La,Ce,Pr,Sm,Gd). Anomalies at the magnetic ordering transitions indicate a significant magneto-elastic coupling and a negative pressure dependence of $T_{\rm N}$ . The structural transitions are associated by large anomalies in $\alpha$. Rare earth magnetic ordering in CeFeAsO, PrFeAsO, and SmFeAsO yields large positive anomalies at low temperatures., Comment: accepted

The electron-doped Pr(2-x)Ce(x)CuO(4) (PCCO) compound in the pseudogap regime (x~0.15) was investigated using angle-resolved photoemission spectroscopy (ARPES) and the generalized dynamical mean-field theory (DMFT) with the k-dependent self-energy (LDA+DMFT+\Sigma_k). Model parameters (hopping integral values and local Coulomb interaction strength) for the effective one-band Hubbard model were calculated by the local density approximation (LDA) with numerical renormalization group method (NRG) employed as an "impurity solver" in DMFT computations. An "external" k-dependent self-energy \Sigma_k was used to describe interaction of correlated conducting electrons with short-range antiferromagnetic (AFM) pseudogap fluctuations. Both experimental and theoretical spectral functions and Fermi surfaces (FS) were obtained and compared demonstrating good semiquantitative agreement. For both experiment and theory normal state spectra of nearly optimally doped PCCO show clear evidence for a pseudogap state with AFM-like nature. Namely, folding of quasiparticle bands as well as presence of the "hot spots" and "Fermi arcs" were observed., Comment: 4 pages, 4 figures, as accepted to PRB Rapid Communications. Title is changed by Editors

We present measurements of the thermal expansion coefficient alpha of polycrystalline LaFeAsO1-xFx (x <= 0.1). The magnetic and structural transitions of the samples with x <= 0.04 give rise to large anomalies in alpha(T), while the onset of superconductivity in the crystals with x >= 0.05 is not resolved. Above the structural transition, the thermal expansion coefficient of LaFeAsO is significantly enhanced. This is attributed to fluctuations, which also affect the electrical transport properties of the compound. The complete absence of these fluctuations in the superconducting samples even for x = 0.05 is taken as evidence for an abrupt first-order type of suppression of the structural and magnetic transitions upon F doping., Comment: 6 pages, submitted to Phys. Rev. B

The precise momentum dependence of the superconducting gap in the iron-arsenide superconductor with Tc = 32K (BKFA) was determined from angle-resolved photoemission spectroscopy (ARPES) via fitting the distribution of the quasiparticle density to a model. The model incorporates finite lifetime and experimental resolution effects, as well as accounts for peculiarities of BKFA electronic structure. We have found that the value of the superconducting gap is practically the same for the inner Gamma-barrel, X-pocket, and "blade"-pocket, and equals 9 meV, while the gap on the outer Gamma-barrel is estimated to be less than 4 meV, resulting in 2Delta/kT_c=6.8 for the large gap, and 2Delta/kT_c<3 for the small gap. A large (77 \pm 3%) non-superconducting component in the photoemission signal is observed below T_c. Details of gap extraction from ARPES data are discussed in Appendix., Comment: Images revised; details of gap extraction from ARPES spectra are added as an Appendix

Resonant soft X-ray scattering with photon energies near the O K and the Cu L3 edges was used to study charge ordering in the system La_{1.8-x}Eu_{0.2}Sr_xCuO_4 as a function of temperature for x = 0.125 and 0.15. From the superstructure diffraction intensities a charge ordering with a doping dependent wave vector is derived which is in this system well below the transition temperature of the low-temperature tetragonal phase but well above the onset of spin ordering. This indicates that charge ordering is the primary driving force for the formation of stripe-like phases in two-dimensional doped cuprates. Analysis of the lineshape of the scattered intensity as a function of photon energy yields evidence for a high hole concentration in the stripes., Comment: 4 pages and 2 figures

We have measured a strictly linear pi-plasmon dispersion along the axis of individualized single wall carbon nanotubes, which is completely different from plasmon dispersions of graphite or bundled single wall carbon nanotubes. Comparative ab initio studies on graphene based systems allow us to reproduce the different dispersions. This suggests that individualized nanotubes provide viable experimental access to collective electronic excitations of graphene, and it validates the use of graphene to understand electronic excitations of carbon nanotubes. In particular, the calculations reveal that local field effects (LFE) cause a mixing of electronic transitions, including the 'Dirac cone', resulting in the observed linear dispersion.

Tweaking the properties of carbon nanotubes is a prerequisite for their practical applications. Here we demonstrate fine-tuning the electronic properties of single-wall carbon nanotubes via filling with ferrocene molecules. The evolution of the bonding and charge transfer within the tube is demonstrated via chemical reaction of the ferrocene filler ending up as secondary inner tube. The charge transfer nature is interpreted well within density functional theory. This work gives the first direct observation of a fine-tuned continuous amphoteric doping of single-wall carbon nanotubes.

We report structural, magnetization, electrical resistivity and nuclear- and electron spin resonance data of the complex transition metal oxide In_2VO_5 in which structurally well-defined V-O chains are realized. An itinerant character of the vanadium d-electrons and ferromagnetic correlations, revealed at high temperatures, are contrasted with the insulating behavior and predominantly antiferromagnetic exchange between the localized V^{4+} S = 1/2-magnetic moments which develop below a certain characteristic temperature T* ~ 120 K. Eventually the compound exhibits short-range magnetic order at $T_SRO ~ 20 K. We attribute this crossover occurring around T* to the unusual anisotropic thermal contraction of the lattice which changes significantly the overlap integrals and the character of magnetic intra- and interchain interactions., Comment: submitted to Phys. Rev. B

Electron spin resonance and magnetization data in magnetic fields up to 55 T of a novel multicenter paramagnetic molecular complex [L_2Ni_4(N_3)(O_2C Ada)_4](Cl O_4) are reported. In this compound, four Ni centers each having a spin S = 1 are coupled in a single molecule via bridging ligands (including a \mu_4-azide) which provide paths for magnetic exchange. Analysis of the frequency and temperature dependence of the ESR signals yields the relevant parameters of the spin Hamiltonian, in particular the single ion anisotropy gap and the g factor, which enables the calculation of the complex energy spectrum of the spin states in a magnetic field. The experimental results give compelling evidence for tuning the ground state of the molecule by magnetic field from a nonmagnetic state at small fields to a magnetic one in strong fields owing to the spin level crossing at a field of ~25 T., Comment: revised version, accepted for publication in Physical Review B

We study the magnetism of the hole doped CuO2 spin chains in Sr14Cu24O41 by measuring the Electron Spin Resonance (ESR) and the static magnetization M in applied magnetic fields up to 14 T. In this compound, the dimerized ground state and the charge order in the chains are well established. Our experimental data suggest that at low temperatures the Curie-like increase of M as well as the occurrence of the related ESR signal are due to a small amount of paramagnetic centers which are not extrinsic defects but rather unpaired Cu spins in the chain. These observations qualitatively confirm recent ab initio calculations of the ground state properties of the CuO2 chains in Sr14Cu24O41. Our complementary quantum statistical simulations yield that the temperature and field dependence of the magnetization can be well described by an effective Heisenberg model in which the ground state configuration is composed of spin dimers, trimers, and monomers., Comment: revised version

We have studied the interlayer coupling in the antiferromagnetic (AF) phase of Sr and Zn doped La2CuO4 by analyzing the spin flip transition in the magnetization curves. We find that the interlayer coupling strongly depends on the mobility of the hole charge carriers. Samples with the same hole content as well as the same Neel temperature but a different hole mobility, which we adjusted by Zn co-doping, can have a very different interlayer coupling. Our results suggest that only mobile holes can cause a strong frustration of the interlayer coupling., Comment: 4 pages, 4 figures

We report measurements of the longitudinal ($^{139}T_1^{-1}$) and transverse ($^{139}T_2^{-1}$) decay rates of the magnetization of $^{139}$La nuclei performed in a high quality single crystal of La$_{1.65}$Eu$_{0.2}$Sr$_{0.15}$CuO$_{4}$. We observe a dramatic slowing of the Cu 3d spins manifested as a sharp increase of both $^{139}T_1^{-1}$ and $^{139}T_2^{-1}$ below 30 K. We find that in this temperature range the fluctuations involve a unique time scale $\tau $ which diverges as $(T-T_{\rm A})^{-1.9}$ with $T_{\rm A}\thickapprox 5$ K. This behavior is distinct from the continuous freezing observed in underdoped La$_{1-x}$Sr$_x$CuO$_4$ which involves a distribution of energy barriers. By contrast, in La$_{1.65}$Eu$_{0.2}$Sr$_{0.15}$CuO$_{4}$, the freezing below 30K is intrinsic to its magnetic ground state and the observed power law supports the existence of a glass forming "charge stripe liquid"., Comment: 5 pages, 3 Figures. Revised version, resubmitted