Institut für Festkörper- und Materialphysik

A recent article suggested that the saturation of low energy spectral weight observed by X-ray absorption spectroscopy in the cuprates at high hole doping could be explained within the single-band Hubbard model. We show that this result is an artifact of inappropriate integration limits.

Extremely high magnetoresistance (XMR) in the lanthanum monopnictides LaX (X = Sb, Bi) has recently attracted interest in these compounds as candidate topological materials. However, their perfect electron-hole compensation provides an alternative explanation, so the possible role of topological surface states requires verification through direct observation. Our angle-resolved photoemission spectroscopy (ARPES) data reveal multiple Dirac-like surface states near the Fermi level in both materials. Intriguingly, we have observed circular dichroism in both surface and near-surface bulk bands. Thus the spin-orbit coupling-induced orbital and spin angular momentum textures may provide a mechanism to forbid backscattering in zero field. Our data suggest that surface and near-surface bulk bands likely contribute strongly to XMR in LaX. The extremely simple rock salt structure of these materials and the ease with which high-quality crystals can be prepared suggests that they may be an ideal platform for further investigation of topological matter.

We have grown cubic centimeter-size crystals of YBa2Cu3Ox suitable for neutron studies, by a top-seeded melt-growth technique. Growth conditions were optimized with an eye toward maximizing phase purity. It was found that the addition of 2% Y2BaCuO5 and 0.5% Pt (by mass) were required to prevent melt loss and to obtain the highest crystallinity. A neutron diffraction study on a mosaic of six such crystals found that the final Y2BaCuO5 concentration was 5%, while other impurity phases comprised less than 1% by volume. The oxygen content was set to x = 6.5, the crystals were detwinned, and then carefully annealed to give the well-ordered ortho-II phase. The neutron study determined that 70% of the mosaic's volume was in the majority orthorhomic domain. The neutron (006) and (110) rocking curve widths were ∼1° per crystal and ∼2.2° for the mosaic, and the oxygen chain correlation lengths were >100 Å in the a- and b-directions and ∼50 Å in the c-direction.

X-ray absorption spectroscopy on oxygen-annealed, self-flux-grown single crystals of Tl2Ba2CuO6−δ suggests a microscopic doping mechanism whereby interstitial oxygens are attracted to copper substituted on the thallium site, contributing holes to both the planes and to these coppers, and typically promoting only one hole to the plane rather than two. These copper substituents would provide an intrinsic hole doping. The evidence for this is discussed, along with an alternative interpretation.

We report the growth of millimetre-sized single crystals of Tl2Ba2CuO6+x by a copper-rich self-flux technique. The crystals were annealed under controlled oxygen partial pressures and characterized. The interplay of oxygen non-stoichiometry, orthorhombicity, Cu substitution on the Tl site, and the superconducting properties is discussed.

We present measurements of the microwave surface impedance of the single-layer cuprate Tl2Ba2CuO6+δ, deep in the overdoped regime, with Tc≈25 K. Measurements have been made using cavity perturbation of a dielectric resonator at 17 discrete frequencies ranging from 2.50 to 19.16 GHz, and at temperatures from 0.12 to 27.5 K. From the surface impedance we obtain the microwave conductivity, penetration depth, and superfluid density. The superfluid density displays a strong linear temperature dependence from 2 to 14 K, indicative of line nodes in the energy gap. The microwave data are compared with theoretical predictions for a d-wave superconductor with pointlike impurities, with the conclusion that disorder in Tl2Ba2CuO6+δ acts predominantly in the weak-to-intermediate-strength scattering regime, and that small-angle scattering is important.

Pb(Fe0.5Nb0.5)O3  (PFN), one of the few relaxor multiferroic systems, has a G-type antiferromagnetic transition at TN=143K and a ferroelectric transition at TC=385K. By using high-resolution neutron-diffraction experiments and a total scattering technique, we paint a comprehensive picture of the long- and short-range structures of PFN: (i) a clear sign of short-range structural correlation above TC, (ii) no sign of the negative thermal expansion behavior reported in a previous study, and (iii) clearest evidence thus far of magnetoelectric coupling below TN. We conclude that at the heart of the unusual relaxor multiferroic behavior lies the disorder between Fe3+ and Nb5+ atoms. We argue that this disorder gives rise to short-range structural correlations arising from O disorder in addition to Pb displacement.

A recent article suggested that the saturation of low energy spectral weight observed by X-ray absorption spectroscopy in the cuprates at high hole doping could be explained within the single-band Hubbard model. We show that this result is an artifact of inappropriate integration limits.

Magnetometry, electrical transport, and neutron scattering measurements were performed on single crystals of the Fe 4+-containing perovskite-related phase Sr 3 Fe 2 O 7−δ as a function of oxygen content. Although both the crystal structure and electron configuration of this compound are closely similar to those of well-studied ruthenates and manganates, it exhibits very different physical properties. The fully oxygenated compound (δ = 0) exhibits a charge-disproportionation transition at T D = 340 K, and an antiferromagnetic transition at T N = 115 K. For temperatures T T D , the material is a small-gap insulator; the antiferromagnetic order is incommensurate, which implies competing exchange interactions between the Fe 4+ moments. The fully deoxygenated compound (δ = 1) is highly insulating, and its Fe 3+ moments exhibit commensurate antiferromagnetic order below T N ∼ 600 K. Compounds with intermediate δ exhibit different order with lower T N , likely as a consequence of frustrated exchange interactions between Fe 3+ and Fe 4+ sublattices. A previous proposal that the magnetic transition temperature reaches zero is not supported.

The Fe 4+-containing cubic perovskite phase SrFeO 3−δ is of interest both for its high-temperature, oxygen-conducting properties as a solid oxide fuel cell component, and at low temperatures, where it exhibits a plethora of helical magnetic phases and a candidate Skyrmion lattice. However, a sequence of structural phase transitions encountered on cooling to room temperature has limited the size of single crystals. We report the floating-zone growth and oxygen-annealing of multiple-cubic-centimetre-sized single crystals of SrFeO 3−δ , suitable for inelastic neutron scattering and other measurement techniques requiring large sample volumes.

We present results of inelastic light scattering experiments on singlecrystalline La2−xSrxCuO4 in the doping range 0.00 ≤ x = p ≤ 0.30 and Tl2Ba2CuO 6+δ at p = 0.20 and p = 0.24. The main emphasis is placed on the response of electronic excitations in the antiferromagnetic phase, in the pseudogap range, in the superconducting state, and in the essentially normal metallic state at x ≥ 0.26, where no superconductivity could be observed. In most of the cases we compare B1g and B2g spectra which project out electronic properties close to (π, 0) and (π/2, π/2), respectively. In the channel of electron-hole excitations we find universal behavior in B2g symmetry as long as the material exhibits superconductivity at low temperature. In contrast, there is a strong doping dependence in B1g symmetry: (i) In the doping range 0.20 ≤ p ≤ 0.25 we observe rapid changes of shape and temperature dependence of the spectra. (ii) In La2−xSrxCuO4 new structures appear for x < 0.13 which are superposed on the electron-hole continuum. The temperature dependence as well as model calculations support an interpretation in terms of charge-ordering fluctuations. For x ≤ 0.05 the response from fluctuations disappears at B1g and appears at B2g symmetry in full agreement with the orientation change of stripes found by neutron scattering. While, with a grain of salt, the particle-hole continuum is universal for all cuprates the response from fluctuating charge order in the range 0.05 ≤ p < 0.16 is so far found only in La2−xSrxCuO4. We conclude that La2−xSrxCuO4 is close to static charge order and, for this reason, may have a suppressed Tc.

A recent article suggested that the saturation of low energy spectral weight observed by X-ray absorption spectroscopy in the cuprates at high hole doping could be explained within the single-band Hubbard model. We show that this result is an artifact of inappropriate integration limits.

The spontaneous expulsion of applied magnetic field, the Meissner effect, is a defining feature of superconductors; in Type-II superconductors above the lower critical field, this screening takes the form of a lattice of magnetic flux vortices. Using implanted spin-1/2 positive muons, one can measure the vortex lattice field distribution through the spin precession and deduce key parameters of the superconducting ground state, and thereby fundamental properties of the superconducting pairing. Muon spin rotation/relaxation (µSR) experiments have indeed revealed much interesting physics in the underdoped cuprates, where superconductivity is closely related to, or coexistent with, disordered or fluctuating magnetic and charge excitations. Such complications should be absent in overdoped cuprates, which are believed to exhibit conventional Fermi liquid behaviour. These first transverse field (TF)-µ + SR experiments on heavily-overdoped single crystals reveal a superfluid density exhibiting a clear inflection point near 0.5Tc, with a striking doping-independent scaling. This reflects hitherto unrecognized physics intrinsic to d-wave vortices, evidently generic to the cuprates, and may offer fundamentally new insights into their still-mysterious superconductivity.

Extremely high magnetoresistance (XMR) in the lanthanum monopnictides LaX (X = Sb, Bi) has recently attracted interest in these compounds as candidate topological materials. However, their perfect electron-hole compensation provides an alternative explanation, so the possible role of topological surface states requires verification through direct observation. Our angle-resolved photoemission spectroscopy (ARPES) data reveal multiple Dirac-like surface states near the Fermi level in both materials. Intriguingly, we have observed circular dichroism in both surface and near-surface bulk bands. Thus the spin-orbit coupling-induced orbital and spin angular momentum textures may provide a mechanism to forbid backscattering in zero field, suggesting that surface and near-surface bulk bands may contribute strongly to XMR in LaX. The extremely simple rock salt structure of these materials and the ease with which high-quality crystals can be prepared suggests that they may be an ideal platform for further investigation of topological matter.

We found new two-dimensional (2D) quantum (S=1/2) antiferromagnetic systems: CuRE2Ge2O8 (RE=Y and La). According to our analysis of high-resolution X-ray and neutron diffraction experiments, the Cu-network of CuRE2Ge2O8 (RE=Y and La) exhibits a 2D triangular lattice linked via weak bonds along the perpendicular b-axis. Our bulk characterizations from 0.08 to 400 K show that they undergo a long-range order at 0.51(1) and 1.09(4) K for the Y and La systems, respectively. Interestingly, they also exhibit field induced phase transitions. For theoretical understanding, we carried out the density functional theory (DFT) band calculations to find that they are typical charge-transfer-type insulators with a gap of Eg ≅ 2 eV. Taken together, our observations make CuRE2Ge2O8 (RE=Y and La) additional examples of low-dimensional quantum spin triangular antiferromagnets with the lowtemperature magnetic ordering.

While the pure elements tend to exhibit Type-I rather than Type-II superconductivity, nearly all compound superconductors are Type-II, with only a few known exceptions. We report single crystal growth and physical characterization of the rhenium aluminide Al6Re, which we conclude is a Type-I superconductor based on magnetization, ac-susceptibility, and specific-heat measurements. This detection of superconductivity, despite the strong similarity of Al6Re to a family of W and Mo aluminides that do not superconduct, suggests that these aluminides are an ideal testbed for identifying the relative importance of valence electron count and inversion symmetry in determining whether a material will superconduct.