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We investigate the monoclinic distortion that occurs at 4.7 GPa at room temperature in the frustrated Shastry-Sutherland model quantum magnet SrCu 2(BO3)2 as a function of pressure and temperature by means of powder and single crystal angle dispersive synchrotron X-ray diffraction. Our results indicate that the onset of the structural distortion varies in a narrow pressure range between ~4.0 and 5.0 GPa. This result will be useful in order to distinguish between magnetic transitions related to structural changes and potential intrinsic quantum phase transitions that various reports have suggested to take place in SrCu2(BO 3)2 at high pressure and low temperature. Swiss National Science Foundation grant (No. 200021-1080). Scopus

The oxygen isotope ($^{16}$O/$^{18}$O) effect (OIE) on the pseudogap (charge-stripe ordering) temperature $T^{\ast}$ is investigated for the cuprate superconductor La$_{2-x}$Sr$_{x}$CuO$_{4}$ as a function of doping $x$ by means of x-ray absorption near edge structure (XANES) studies. A strong $x$ dependent and sign reversed OIE on $T^{\ast}$ is observed. The OIE exponent $\alpha_{T^{\ast}}$ systematically decreases from $\alpha_{T^{\ast}} = - 0.6(1.3)$ for $x = 0.15$ to $\alpha_{T^{\ast}} = - 4.4(1.1)$ for $x = 0.06$, corresponding to increasing $T^{\ast}$ and decreasing superconducting transition temperature $T_{c}$. Both $T^{\ast}(^{16}{\rm O})$ and $T^{\ast}(^{18}{\rm O})$ exhibit a linear doping dependence with different slopes and critical end points (where $T^{\ast}(^{16}{\rm O})$ and $T^{\ast}(^{18}{\rm O})$ fall to zero) at $x_{c}(^{16}{\rm O}) = 0.201(4)$ and $x_{c}(^{18}{\rm O}) = 0.182(3)$, indicating a large positive OIE of $x_{c}$ with an exponent of $\alpha_{x_{c}} = 0.84(22)$. The remarkably large and strongly doping dependent OIE on $T^{\ast}$ signals a substantial involvement of the lattice in the formation of the pseudogap, consistent with a polaronic approach to cuprate superconductivity and the vibronic character of its ground state.

A novel negative oxygen-isotope (16O/18O) effect (OIE) on the low-temperature tetragonal phase transition temperature T_{LTT} was observed in La_{2-x}Ba_xCuO_4 (x = 1/8) using high-resolution neutron powder diffraction. The corresponding OIE exponent alpha(T_{LTT}) = - 0.36(5) has the same sign as alpha(T_{so}) = -0.57(6) found for the spin-stripe order temperature T_so. The fact that the LTT transition is accompanied by charge ordering (CO) implies the presence of an OIE also for the CO temperature T_co. Furthermore, a temperature dependent shortening of the c-axis with the heavier isotope is observed. These results combined with model calculations demonstrate that anharmonic electron-lattice interactions are essential for all transitions observed in the stripe phase of cuprates., 7 pages and 7 figures, including supplemental material

The temperature dependence of the gapped triplet excitations (triplons) in the 2D Shastry-Sutherland quantum magnet SrCu$_2$(BO$_3$)$_2$ is studied by means of inelastic neutron scattering. The excitation amplitude rapidly decreases as a function of temperature while the integrated spectral weight can be explained by an isolated dimer model up to 10~K. Analyzing this anomalous spectral line-shape in terms of damped harmonic oscillators shows that the observed damping is due to a two-component process: one component remains sharp and resolution limited while the second broadens. We explain the underlying mechanism through a simple yet quantitatively accurate model of correlated decay of triplons: an excited triplon is long-lived if no thermally populated triplons are near-by but decays quickly if there are. The phenomenon is a direct consequence of frustration induced triplon localization in the Shastry--Sutherland lattice., 5 pages, 4 figures

The concept of a topological Kondo insulator (TKI) has been brought forward as a new class of topological insulators in which non-trivial surface states reside in the bulk Kondo band gap at low temperature due to the strong spin-orbit coupling [1-3]. In contrast to other three-dimensional (3D) topological insulators (e.g. Bi2Se3), a TKI is truly insulating in the bulk [4]. Furthermore, strong electron correlations are present in the system, which may interact with the novel topological phase. Applying spin- and angle-resolved photoemission spectroscopy (SARPES) to the Kondo insulator SmB6, a promising TKI candidate, we reveal that the surface states of SmB6 are spin polarized, and the spin is locked to the crystal momentum. Counter-propagating states (i.e. at k and -k) have opposite spin polarizations protected by time-reversal symmetry. Together with the odd number of Fermi surfaces of surface states between the 4 time-reversal invariant momenta in the surface Brillouin zone [5], these findings prove, for the first time, that SmB6 can host non-trivial topological surface states in a full insulating gap in the bulk stemming from the Kondo effect. Hence our experimental results establish that SmB6 is the first realization of a 3D TKI. It can also serve as an ideal platform for the systematic study of the interplay between novel topological quantum states with emergent effects and competing order induced by strongly correlated electrons., 4 figures

Large negative oxygen-isotope (16O/18O) effects (OIE's) on the static spin-stripe ordering temperature T_so and the magnetic volume fraction V_m were observed in La_(2-x)Ba_xCuO_4 (x = 1/8) by means of muon spin rotation experiments. The corresponding OIE exponents were found to be alpha_(T_so) = -0.57(6) and alpha_(V_m) = -0.71(9), which are sign reversed to alpha_(T_c) = 0.46(6) measured for the superconducting transition temperature T_c. This indicates that the electron-lattice interaction is involved in the stripe formation and plays an important role in the competition between bulk superconductivity and static stripe order in the cuprates., Comment: 5 pages, 4 figures

Large negative oxygen isotope (16Oand 18O) effects (OIEs) on the static spin stripe ordering temperature Tso and the magnetic volume fraction Vm were observed in La2-xBaxCuO4 (x=1/8) by means of muon spin rotation experiments. The corresponding OIE exponents were found to be aTso=-0.57(6) and aVm = -0.71(9) which are sign reversed to aTc = 0.46(6) measured for the superconducting transition temperature Tc. This indicates that the electron lattice interaction is involved in the stripe formation and plays an important role in the competition between bulk superconductivity and static stripe order in the cuprates.

Systematic angle-resolved photoemission spectroscopy (ARPES) experiments have been carried out to investigate the bulk and (100) surface electronic structures of a topological mixed-valence insulator candidate, YbB6. The bulk states of YbB6 were probed with bulk-sensitive soft X-ray ARPES, which show strong three-dimensionality as required by cubic symmetry. Surprisingly the measured Yb 4f states are located around 1 eV and 2.3 eV below the Fermi level (EF), instead of being near EF as indicated by first principle calculations. The dispersive bands near EF are B 2p states, which hybridize with the 4f states. Using surface-sensitive vacuum-ultraviolet ARPES, we revealed two-dimensional surface states which form three electron-like Fermi surfaces (FSs) with Dirac-cone-like dispersions. The odd number of surface FSs gives the first indication that YbB6 is a moderately correlated topological insulator. The spin-resolved ARPES measurements provide further evidence that these surface states are spin polarized with spin locked to the crystal momentum. We have observed a second set of surface states with different topology (hole-like pockets). Clear folding of the bands suggest their origin from a 1X2 reconstructed surface. The topological property of the reconstructed surface states needs further studies., Comment: 5 figures

The relaxation of Yb 3+ in YBa 2Cu 3O x (x = 6.1 and 6.4) was studied using Electron Paramagnetic Resonance (EPR). In these samples the Yb 3+ relaxation is dominated by a phonon mechanism. It was shown that the conventional Raman two-phonon process involving acoustic phonons can not describe the temperature dependence of the Yb 3+ relaxation. Instead, the Raman process involving optical phonons or an Orbach-like process via the excited vibronic levels of Cu 2+ ions with energy Ω = 500(50) K is responsible for the phononic part of the Yb 3+ relaxation in YBa 2Cu 3O x. The present results provide clear experimental evidence that optical phonons or local vibrations are the dominant source of spin-lattice relaxation at sufficiently high temperatures, which cannot be described by the traditional approach using the Debye approximation.

The spin gap evolution upon Ca doping in Sr14-xCaxCu24O41 was systematically investigated using inelastic neutron scattering. We discover that the singlet-triplet spin gap excitation survives in this series with x up to 13, indicating the singlet dimer ground state in these compounds. This observation corrects the previous speculation that the spin gap collapses at x~13 by the NMR technique. The strong intensity modulation along QH in x=0 gradually evolves into a Q-independent feature in x>11. This could be attributed to the localized Cu moment magnetism developing into an itinerant magnetism with increasing x. It is a surprise that the spin gap persists in the normal state of this spin ladder system with metallic behaviour, which evidences the possibility of magnetically-mediated carrier pairing mechanism in a two-leg spin ladder lattice., 17 pages, 6 figures

We present a detailed investigation of the magnetic and structural properties of magnetically doped 3D topological insulator Bi2Se3. From muon spin relaxation measurements in zero magnetic field, we find that even 5% Fe doping on the Bi site turns the full volume of the sample magnetic at temperatures as high as ~250 K. This is also confirmed by magnetization measurements. Two magnetic "phases" are identified; the first is observed between ~10-250 K while the second appears below ~10 K. These cannot be attributed to impurity phases in the samples. We discuss the nature and details of the observed magnetism and its dependence on doping level., 7 pages and 6 figures

Recently discovered A-Fe-Se (A - alkali metal) materials have questioned the most popular theories of iron-based superconductors because of their unusual electronic structure [1]. Controversial photoemission data taken in the superconducting state [2-7] are in conflict with highly magnetic state seen by neutron-, muSR-spectroscopies and transport/thermodynamic probes [8-10]. These results lead to suggestions to consider all iron-based materials as originating from Mott-insulators or semiconductors, thus once again raising the question of close relation between the cuprates and Fe-based superconductors [e.g. 2]. Here we study electronic and magnetic properties of Rb0.77Fe1.61Se2 (Tc = 32.6 K) in normal and superconducting states by means of photoemission and muSR spectroscopies as well as band structure calculations. We demonstrate that the puzzling behavior of these novel 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 byproduct of Rb-intercalation and its magnetic properties have hardly any relation to the superconductivity. Instead, we find that also in this, already third class of iron-based compounds, the key ingredient for superconductivity is a certain proximity of a van Hove singularity to the Fermi level. These findings set the direction for effective search of new superconducting materials., Comment: 11 pages, PDF format, Related work: http://www.ifw-dresden.de/institutes/iff/research/SC/arpes

The topological insulator Bi2Se3 shows a Raman scattering response related to topologically protected surface states amplified by a resonant interband transition. Most significantly this signal has a characteristic Lorentzian lineshape and spin-helical symmetry due to collision dominated scattering of Dirac states at the Fermi level E_F on bulk valence states. Its resonance energy, temperature and doping dependence points to a high selectivity of this process. Its scattering rate (Gamma=40 cm-1=5 meV) is comparable to earlier observations, e.g. in spin-polaron systems. Although the observation of topological surface states in Raman scattering is limited to resonance conditions, this study represents a quite clean case which is not polluted by symmetry forbidden contributions from the bulk, 12 pages, 5 figures, 1 table

Neutron diffraction measurements on multiferroic LuFe2O4 show changes in the antiferromagnetic (AFM) structure characterized by wavevector q = (1/3 1/3 1/2) as a function of electric field cooling procedures. The increase of intensity from all magnetic domains and the decrease in the 2D magnetic order observed below the Neel temperature are indicative of increased ferroelectric charge order. The AFM order changes the dynamics of the CO state, and stabilizes it. It is determined that the increase in electric polarization observed at the magnetic ordering temperature is due to a transition from paramagnetic 2D charge order to AFM 3D charge order., Comment: 5 pages, 3 figures

The magnetic properties of polycrystaline La0.5Ba0.5CoO2.5+d (d = 0.44 and 0.485) where investigated using muon spin relaxation and bulk magnetization experiments. Below a Curie temperature of T_C = 160 K and 180 K, respectively, a magnetic phase with a macroscopic ferromagnetic moment forms. muSR proves that the full volume inhomogeneously orders at T_C but that there is microscopically a phase separation into phase volumes possessing a fully static and a slow magnetic dynamic behavior. A peak in the dynamic relaxation rate 35 K respectively 55 K below the Curie temperature indicates the freezing of spin components. The spin fluctuations are thermally activated with a typical Jahn-Teller like phonon energy of E_a = 340 K. Together with co-operative Jahn-Teller distortions appearing below the spin freezing temperature, this points to a strong magneto elastic coupling as the cause for the magnetic fluctuations and the spin freezing.

We report on synthesis of single crystals of BaFe2Se3 and study of their crystal and magnetic structures by means of synchrotron single crystal X-ray and neutron powder diffraction. The crystal structure has orthorhombic symmetry and consists of double chains of FeSe4 edge connected tetrahedra intercalated by barium. Below 240 K long range block-spin checkerboard antiferromagnetic (AFM) order is developed. The magnetic structure is similar to one observed in A0.8Fe1.6Se2 (A=K, Rb or Cs) superconductors. The crystals exhibit a transition to the diamagnetic state with an onset transition temperature of Tc ~11 K. Though we observe FeSe as an impurity phase (, 12 pages, 6 figures, added erratum (page 12) for Figure 4b showing tau2 structure

We investigate the doping dependence of the nanoscale electronic and magnetic inhomogeneities in the hole-doping range 0.0020.05, dominated by Co^{3+}-Co^{3+} intracluster interactions. Our conclusions are in good agreement with a recently proposed model of the phase separation in cobalt perovskites [He et al., Europhys. Lett. 87, 27006 (2009)].

In high temperature copper oxides superconductors, a novel magnetic order associated with the pseudogap phase has been identified in two different cuprate families over a wide region of temperature and doping. We here report the observation below 120 K of a similar magnetic ordering in the archetypal cuprate ${\rm La_{2-x}Sr_xCuO_4}$ (LSCO) system for x=0.085. In contrast to the previous reports, the magnetic ordering in LSCO is {\it\bf only} short range with an in-plane correlation length of $\sim$ 10 \AA\ and is bidimensional (2D). Such a less pronounced order suggests an interaction with other electronic instabilities. In particular, LSCO also exhibits a strong tendency towards stripes ordering at the expense of the superconducting state., Comment: 4 figures, submitted to Phys. Rev. Lett

We report muon spin relaxation measurements on the magnetic structures of RBaCo_2O_5.5 with R=Y, Tb, Dy, and Ho. Three different phases, one ferrimagnetic and two antiferromagnetic, are identified below 300 K. They consist of different ordered spin state arrangements of high-, intermediate-, and low-spin Co^3+ of CoO_6 octahedra. Phase separation into well separated regions with different spin state order is observed in the antiferromagnetic phases. The unusual strongly anisotropic magnetoresistance and its onset at the FM-AFM phase boundary is explained., 4 pages, accepted for publication in Phys. Rev. Lett

The various phases observed in all cuprate superconductors [superconducting (SC), spin-glass (SG), and antiferromagnetic (AFM)] were investigated with respect to oxygen-isotope (O-16/O-18) effects, using here as a prototype system of cuprates Y1-xPrxBa2Cu3O7-delta. All phases exhibit an isotope effect which is strongest where the respective phase terminates. In addition, the isotope effects on the magnetic phases (SG and AFM) are sign reversed as compared to the one on the superconducting phase. In the coexistence regime of the SG and SC phase a two-component behavior is observed where the isotope induced decrease of the superfluid density leads to a corresponding enhancement in the SG related density.

Oxygen isotope (^{16}O/^{18}O) effects (OIE's) on the superconducting transition (T_c), the spin-glass ordering (T_g), and the antiferromagnetic ordering (T_N) temperatures were studied for Y_1-xPr_xBa_2Cu_3O_7-\delta as a function of Pr content (0.0\leq x \leq 1.0). The OIE on T_c increases with increasing x up to x\approx0.55, where superconductivity disappears. For decreasing x the OIE's on T_N and T_g increase down to x\approx 0.7 where antiferromagnetic order and down to x\approx0.3 where spin-glass behavior vanish, respectively. The OIE's on T_g and T_N are found to have {\it opposite signs} as compared to the OIE on T_c. All OIE's are suggested to arise from the isotope dependent mobility (kinetic energy) of the charge carriers., Comment: 5 pages, 3 figures, 1 table

Magnetic properties of high purity stoichiometric La$_2$CuO$_4$ nanoparticles are systematically investigated as a function of particle size. Ferromagnetic single-domain spin clusters are shown to spontaneously form at the surface of fine grains as well as paramagnetic defects. Hysteresis loops and thermomagnetic irreversibility are observed in a wide temperature range $5 - 350$ K with the remnant moment and coercivity gradually decreasing with increasing temperature. Possible origins of the spontaneous surface ferromagnetic clusters and the relation of our data to the appearance of unusual magnetic phenomena and phase separation of doped cuprates are discussed., 10 pages, 15 figures

Magnetic properties of high purity stoichiometric La2 Cu O4 nanoparticles are systematically investigated as a function of particle size. Ferromagnetic single-domain spin clusters are shown to spontaneously form at the surface of fine grains as well as paramagnetic defects. Hysteresis loops and thermomagnetic irreversibility are observed in a wide temperature range 5-350 K with the remnant moment and coercivity gradually decreasing with increasing temperature. Possible origins of the spontaneous surface ferromagnetic clusters and the relation of our data to the appearance of unusual magnetic phenomena and phase separation of doped cuprates are discussed. © 2007 The American Physical Society.

The crystal and magnetic structures of the specified CMR manganite system have been studied as a function of $y=(0.2-1)$ across the metal-insulator (MI) transition, and of the oxygen mass ($^{16}$O, $^{18}$O). We quantitatively show how the polaronic narrowing of the carrier bandwidth and the crystal lattice micro-strains control the volume fractions of the mesoscopic ferro- and antiferromagnetic clusters. A well-defined dip in the transition temperatures and the suppression of all the types of long range ordering seen near the MI-transition at $y\simeq0.9$ indicate a key role of the quenched disorder for the formation of the long-scale phase separated state., Comment: 21 pages (REVTeX preprint), 1 Table, 13 Figures, accepted in PRB

We review the results of our recent studies of La2-x Sr x CuO4 cuprate superconductor using electron paramagnetic resonance (EPR). It is shown that the EPR of Mn2+ doped into La 2-x Sr x CuO4 (LSCO) provides an unique microscopic information concerning the magnetic, electronic and lattice properties of cuprates. The main conclusions followed from these experiments are discussed. Most attention is given to important and highly debated questions, such as the role of the lattice, electron-phonon interaction, polaron formation and microscopic electronic phase separation in cuprates. © 2007 Springer-Verlag Berlin Heidelberg.

We present an inelastic neutron scattering study of the low energy crystal-field excitations in the lightly doped cobalt perovskite La_0.998Sr_0.002CoO_3. In contrast to the parent compound LaCoO_3 an inelastic peak at energy transfer ~0.75 meV was found at temperatures below 30 K. This excitation apparently corresponds to a transition between a ground state orbital singlet and a higher excited orbital doublet, originating from a high-spin triplet split by a small trigonal crystal field. Another inelastic peak at an energy transfer ~0.6 meV was found at intermediate temperatures starting from T > 30 K. This confirms the presence of a thermally induced spin-state transition from the low-spin Co^3+ to a magnetic high-spin state in the non-disturbed LaCoO_3 matrix. We suggest that hole doping of LaCoO_3 leads to the creation of a magnetic polaron and hence to the low-to-high spin state transition on the relevant Co sites., 4 pages, 2 figures; based on a talk given at ICM'06, Kyoto; to appear in JMMM

The rare-earth ferroelectromagnet YMnO3 consists of weakly coupled triangular layers of S=2 spins. Below T_N ~ 70 K muon-spin relaxation data show two oscillatory relaxing signals due to magnetic order, with no relaxing signal resolvable (which would be expected for different coexisting spin distributions). The transition temperature T_N increases with applied hydrostatic pressure, even though the ordered moment decreases. These results suggest that pressure increases both the exchange coupling between the layers and the frustration within the layers., Comment: 4 pages, 3 figures, v2 (updated figures and minor changes to text)

The metallic phase in low doping range of x from 0.01 to 0.06 in La 2-xSrxCuO4 was analyzed using narrow electron paramagnetic resonance (EPR) line. It was observed that this line was distinct from the known broad line and both lines were due to probing Mn2+ ions. The narrow lines occurred due to metallic regions in material and its intensity increased exponentially upon cooling below ∼150 K. The results show that intensity of narrow EPR line follows same temperature dependence as in-plane resistivity anisotropy in lightly doped La2-xSr xCuO4 single crystals.

In the low doping range of $x$ from 0.01 to 0.06 in La$_{2-x}$Sr$_{x}$CuO$_{4}$, a narrow electron paramagnetic resonance (EPR) line has been investigated. This line is distinct from the known broad line, both due to probing Mn$^{2+}$ ions. The narrow line is ascribed to metallic regions in the material, and its intensity increases exponentially upon cooling below $\sim$ 150 K. The activation energy deduced $\Delta$ = 460(50) K is nearly the same as that found in the doped superconducting regime by Raman and neutron scattering. The intensity of the narrow EPR line follows the same temperature dependence as the resistivity anisotropy in lightly doped La$_{2-x}$Sr$_{x}$CuO$_{4}$ single crystals., Comment: 4 pages, 4 figures

We report site-selective oxygen isotope ($^{16}$O/$^{18}$O) effect (OIE) measurements on the in-plane penetration depth $\lambda_{ab} $ in underdoped Y$_{0.6}$Pr$_{0.4}$Ba$_2$Cu$_3$O$_{7-\delta}$, using the muon-spin rotation ($\mu$SR) technique. A pronounced OIE on the transition temperature $T_c$ as well as on $\lambda_{ab}^{-2}(0)$ was observed, which mainly arises from the oxygen sites within the superconducting CuO$_2$ planes (100 % within error bar). The values of the corresponding relative isotope shifts were found to be $T_c/T_c= -3.7(4)%$ and $\Delta\lambda^{-2}_{ab}(0)/\lambda^{-2}_{ab}(0)= -6.2(1.0)%$. Our results imply that in this compound the phonon modes involving the movement of planar oxygen are dominantly coupled to the electrons., Comment: 4pages, 4 figures

We report site-selective oxygen isotope ($^{16}$O/$^{18}$O) effect (OIE) measurements on the in-plane penetration depth $��_{ab} $ in underdoped Y$_{0.6}$Pr$_{0.4}$Ba$_2$Cu$_3$O$_{7-��}$, using the muon-spin rotation ($��$SR) technique. A pronounced OIE on the transition temperature $T_c$ as well as on $��_{ab}^{-2}(0)$ was observed, which mainly arises from the oxygen sites within the superconducting CuO$_2$ planes (100 % within error bar). The values of the corresponding relative isotope shifts were found to be $T_c/T_c= -3.7(4)%$ and $����^{-2}_{ab}(0)/��^{-2}_{ab}(0)= -6.2(1.0)%$. Our results imply that in this compound the phonon modes involving the movement of planar oxygen are dominantly coupled to the electrons., 4pages, 4 figures

Properties of quasiparticles in doped cuprates formed by an oxygen hole and two adjacent copper holes are investigated on the basis of the extended Hubbard model. The ground state energy, wave functions and the polaron-phonon coupling are calculated. We also analyzed the polaron-polaron interaction via the phonon field. It was found that this interaction is highly anisotropic and can explain the experimentally observed phase separation in the strongly underdoped LaSrCuO:Mn system.

^25Mg NMR spectra and nuclear spin-lattice relaxation time, T_1, have been measured in polycrystalline ^25MgB_2 with a superconducting transition temperature T_c = 39.0 K in zero magnetic field. From the first order and second order quadrupole perturbed NMR spectrum a quadrupole coupling frequency nu_Q = 222(1.5) kHz is obtained. T_1T = 1090(50) sK and Knight shift K_c = 242(4) ppm are temperature independent in the normal conducting phase. The ^25Mg Korringa ratio equals to 0.95 which is very close to the ideal value of unity for s-electrons. The comparison of the experimental nu_Q, T_1T, and K_c with the corresponding values obtained by LDA calculations shows an excellent agreement for all three quantities., Comment: 4 pages including 4 eps-figures, revtex4

We report on high-precision measurements of the temperature dependence of the plane-^63Cu NQR line frequency nu_Q(Cu2) and the linewidth in normal and superconducting ^16O and ^18O exchanged YBa_2Cu_4O_8. Whereas nu_Q(Cu2) passes T_c very smoothly without a discontinuity either in value nor in slope, the linewidth increases in the normal conducting phase down to T_c and starts to decrease sharply in the superconducting phase to finally resume its high-temperature value of the normal phase. There is a well discernible oxygen isotope effect on the nu_Q(Cu2) temperature dependencies. The temperature dependence of nu_Q(Cu2) is described by an empirical expression consisting of two parts: one related to the thermal expansion of the lattice and the other due to charge redistribution during the formation of new electronic structures in the CuO_2 planes. From the fit to the experimental data we determine for the conjectured formation of new electronic structures an energy scale Delta(^16O) = 188.0(1.6) K and Delta(^18O) = 180.0(1.6) K. This results in a partial oxygen isotope effect coefficient of 0.42(11) which is larger than both the spin-pseudogap coefficient [0.061(8)] and the T_c coefficient [0.056(12)]., Comment: 6 pages including 1 table, 3 eps-figures (revtex4)

Oxygen-isotope effect has been investigated in a recently discovered superconductor Sr_{0.4}K_{0.6}BiO_{3}. This compound has a distorted perovskite structure and becomes superconducting at about 12 K. Upon replacing ^{16}O with ^{18}O by 60-80%, the T_c of the sample is shifted down by 0.32-0.50 K, corresponding to an isotope exponent of alpha_{O} = 0.40(5). This isotope exponent is very close to that for a similar bismuthate superconductor Ba_{1-x}K_{x}BiO_{3} with T_c = 30 K. The very distinctive doping and T_c dependencies of alpha_{O} observed in bismuthates and cuprates suggest that bismuthates should belong to conventional phonon-mediated superconductors while cuprates might be unconventional supercondutors., 9 pages, 5 figures

We have resolved a controversial issue concerning the oxygen-isotope shift of the ferromagnetic transition temperature T_{C} in the manganite La_{0.8}Ca_{0.2}MnO_{3+y}. We show that the giant oxygen-isotope shift of T_C observed in the normal oxygen-isotope exchanged samples is indeed intrinsic, while a much smaller shift observed in the argon annealed samples is an artifact. The argon annealing causes the 18O sample to partially exchange back to the 16O isotope due to a small 16O contamination in the Ar gas. Such a contamination is commonly caused by the oxygen outgas that is trapped in the tubes, connectors and valves. The present results thus umambiguously demonstrate that the observed large oxygen isotope effect is an intrinsic property of manganites, and places an important constraint on the basic physics of these materials., 4 pages, 3 figures, submitted to PRB

The unoccupied electronic structure for the Sr_14Cu_24O_41 family of two-leg ladder compounds was investigated for different partial substitutions of Sr^2+ by Ca^2+, leaving the nominal hole count constant, and by Y^3+ or La^3+, reducing the nominal hole count from its full value of 6 per formula unit. Using polarization-dependent x-ray absorption spectroscopy on single crystals, hole states on both the chain and ladder sites could be studied. While for intermediate hole counts all holes reside on O sites of the chains, a partial hole occupation on the ladder sites in orbitals oriented along the legs is observed for the fully doped compound Sr_14Cu_24O_41. On substitution of Ca for Sr orbitals within the ladder planes but perpendicular to the legs receive some hole occupation as well., Comment: 10 pages RevTeX style with 7 embedded figures + 1 table; accepted by Phys. Rev. B

We performed high accuracy 63Cu NQR spin-lattice relaxation and SQUID magnetization measurements on 16O and 18O exchanged YBa2Cu4O8 to determine the isotope shift of the temperature of the opening of the spin gap, T*, and the superconducting transition temperature, Tc. The corresponding isotope exponents are αT = 0.061(8) and αT, = 0.056(12) which are the same within the error bars and suggest a common origin for the superconducting and the spin gap.

Recently a growing number of experiments have provided indications of the key role of polarons (composite particles formed by a charge strongly coupled with a local lattice deformation) in doped perovskites, hosting colossal magnetoresistance (CMR) and high Tc superconductivity. While the role of polarons is generally recognized in manganites due to the large amplitude of the local lattice deformation, the scientific debate remains open on the cuprates where the lattice deformation is smaller and the anomalous normal metallic phase becomes complex due to the coexistence of polarons with itinerant carriers. Moreover the segregation of polarons and itinerant charges in different spatial domains forming lattice-charge stripes as well as the slow dynamic 1D spin fluctuations have been observed. The debate on the driving force for the stripe formation remains object of discussion since it could be purely due to electronic interactions and/or due to strong electron-lattice (polaronic) interactions. In order to explore the important role of the latter in stripe charge segregation, we have studied isotope effects on the dynamical lattice fluctuations and polaron ordering temperature. Here we report a compelling evidence for a huge isotope effect on local lattice fluctuations of La(2-x)Sr(x)CuO(4) high Tc superconductor by x-ray absorption spectroscopy, a fast (~10^(-15) sec) and local probe (~5 A). Upon replacing $^{16}$O with $^{18}$O, the characteristic temperature T^{*} for polaron ordering in La(1.94)Sr(0.06)CuO(4) increases from about 110 K to 170 K., Comment: 4 pages, 3 eps figures, removed references in abstract

The spin-gap evolution upon Ca doping in Sr14-xCaxCu24O41 was systematically investigated using inelastic neutron scattering. We discover that the singlet-triplet spin-gap excitation survives in this series with x up to 13, indicating the singlet dimer ground state in these compounds. This observation corrects the previous speculation that the spin gap collapses at x similar to 13 by the NMR technique. The strong intensity modulation along Q(H) in x = 0 gradually evolves into a Q-independent feature in x > 11. This could be attributed to the localized Cu moment magnetism developing into an itinerant magnetism with increasing x. It is a surprise that the spin gap persists in the normal state of this spin-ladder system with metallic behavior, which evidences the possibility of magnetically mediated carrier pairing mechanism in a two-leg spin-ladder lattice.

The relaxation of Yb3+ in YBa2Cu3Ox (x = 6.1 and 6.4) was studied using Electron Paramagnetic Resonance (EPR). In these samples the Yb3+ relaxation is dominated by a phonon mechanism. It was shown that the conventional Raman two phonon process involving acoustic phonons can not describe the temperature dependence of the Yb3+ relaxation. Instead the Raman process involving optical phonons or an Orbach like process via the excited vibronic levels of Cu2+ ions with energy 500(50)K is responsible for the phononic part of the Yb3+ relaxation in YBa2Cu3Ox. The present results provide clear experimental evidence that optical phonons or local vibrations are the dominant source of spin lattice relaxation at sufficiently high temperatures which cannot be described by the traditional approach using the Debye approximation.

The doping of the honeycomb lattice compound Na2IrO3 with cations having smaller ionic radius such as lithium was supposed to drive the system into a Kitaev spin liquid phase. Even at the smallest doping level the long range magnetic order, present in the parent compound, got suppressed as evidenced by susceptibility and heat capacity measurements. At low doping levels, the long range ordered magnetic phase is accompanied by the formation of a glassy state with a transition temperature of T-F similar to 5 K as determined by dc and ac magnetic susceptibility. The magnetic excitation spectrum, measured by inelastic neutron scattering, shows a significant change with doping. Our inelastic neutron scattering data can be well described by linear spin wave theory, revealing the formation of a spiral magnetic phase.