Nematic Correlation Length in Iron-Based Superconductors Probed by Inelastic X-Ray Scattering.

Nematicity is ubiquitous in electronic phases of high-T_{c} superconductors, particularly in the Fe-based systems. We used inelastic x-ray scattering to extract the temperature-dependent nematic correlation length ξ from the anomalous softening of acoustic phonon modes in FeSe, underdoped Ba(Fe_{0.97}Co_{0.03})_{2}As_{2}, and optimally doped Ba(Fe_{0.94}Co_{0.06})_{2}As_{2}. In all cases, we find that ξ is well described by a power law (T-T_{0})^{-1/2} extending over a wide temperature range. Combined with the previously reported Curie-Weiss behavior of the nematic susceptibility, these results point to the mean-field character of the nematic transition, which we attribute to a sizable nematoelastic coupling that is likely detrimental to superconductivity.

Scaling of the Fano Effect of the In-Plane Fe-As Phonon and the Superconducting Critical Temperature in Ba_{1-x}K_{x}Fe_{2}As_{2}.

By means of infrared spectroscopy, we determine the temperature-doping phase diagram of the Fano effect for the in-plane Fe-As stretching mode in Ba_{1-x}K_{x}Fe_{2}As_{2}. The Fano parameter 1/q^{2}, which is a measure of the phonon coupling to the electronic particle-hole continuum, shows a remarkable sensitivity to the magnetic and structural orderings at low temperatures. Most strikingly, at elevated temperatures in the paramagnetic tetragonal state we observe a linear correlation between 1/q^{2} and the superconducting critical temperature T_{c}. Based on theoretical calculations and symmetry considerations, we identify the relevant interband transitions that are coupled to the Fe-As mode. In particular, we show that a sizable xy orbital component at the Fermi level is fundamental for the Fano effect and, thus, possibly also for the superconducting pairing.

SANS study of vortex lattice structural transition in optimally doped (Ba1-x K x )Fe2As2.

Small-angle neutron scattering on high quality single crystalline Ba1-x K x Fe2As2 reveals the transition from a low-field vortex solid phase with orientational order to a vortex polycrystal at high magnetic field. The vortex order-disorder transition is correlated with the second-peak feature in isothermal hysteresis loops, and is interpreted in terms of the generation of supplementary vortex solid dislocations. The sharp drop of the structure factor above the second peak field is explained by the dynamics of freezing of the vortex ensemble in the high field phase.

Infrared Study of the Spin Reorientation Transition and Its Reversal in the Superconducting State in Underdoped Ba(1-x)K(x)Fe(2)As(2).

With infrared spectroscopy we investigated the spin-reorientation transition from an orthorhombic antiferromagnetic (o-AF) to a tetragonal AF (t-AF) phase and the reentrance of the o-AF phase in the superconducting state of underdoped Ba(1-x)K(x)Fe(2)As(2). In agreement with the predicted transition from a single-Q to a double-Q AF structure, we found that a distinct spin density wave develops in the t-AF phase. The pair breaking peak of this spin density wave acquires much more low-energy spectral weight than the one in the o-AF state which indicates that it competes more strongly with superconductivity. We also observed additional phonon modes in the t-AF phase which likely arise from a Brillouin-zone folding that is induced by the double-Q magnetic structure with two Fe sublattices exhibiting different magnitudes of the magnetic moment.

Magnetic blue phase in the chiral itinerant magnet MnSi.

Chiral nematic liquid crystals sometimes form blue phases characterized by spirals twisting in different directions. By combining model calculations with neutron-scattering experiments, we show that the magnetic analogue of blue phases does form in the chiral itinerant magnet MnSi in a large part of the phase diagram. The properties of this blue phase explain a number of previously reported puzzling features of MnSi such as partial magnetic order and a two-component specific-heat and thermal-expansion anomaly at the magnetic transition.

Evidence of a precursor superconducting phase at temperatures as high as 180 K in RBa2Cu3O(7-δ) (R=Y, Gd, Eu) superconducting crystals from infrared spectroscopy.

We show that a multilayer analysis of the infrared c-axis response of RBa2Cu3O(7-δ) (R=Y, Gd, Eu) provides important new information about the anomalous normal-state properties of underdoped cuprate high temperature superconductors. In addition to competing correlations which give rise to a pseudogap that depletes the low-energy electronic states below T*≫T(c), it enables us to identify the onset of a precursor superconducting state below T(ons)>T(c). We map out the doping phase diagram of T(ons) which reaches a maximum of 180 K at strong underdoping and present magnetic field dependent data which confirm our conclusions.

Evidence for two separate energy gaps in underdoped high-temperature cuprate superconductors from broadband infrared ellipsometry.

We present broadband infrared ellipsometry measurements of the c-axis conductivity of underdoped RBa_{2}Cu_{3}O_{7-delta} (R=Y, Nd, and La) single crystals. Our data show that separate energy scales are underlying the redistributions of spectral weight due to the normal state pseudogap and the superconducting gap. Furthermore, they provide evidence that these gaps do not share the same electronic states and do not merge on the overdoped side. Accordingly, our data are suggestive of a two gap scenario with a pseudogap that is likely extrinsic with respect to superconductivity.

Magnetic structure of the kagome mixed compound (Co(0.5)Ni(0.5))(3)V(2)O(8).

We report the magnetic structure of (Co(0.5)Ni(0.5))(3)V(2)O(8) (CNVO) deduced by single crystal neutron diffraction. This compound exhibits features which differ from that of its parent compounds, which are absolutely collinear along the a axis for Co(3)V(2)O(8) (CVO) or exhibit magnetic moments predominantly in the a-b plane with small components along c in the case of Ni(3)V(2)O(8) (NVO). The averaged magnetic moments of the statistically distributed Ni(2+) and Co(2+) ions in CNVO are oriented in the a-c plane and form loops of quasiferromagnetically coupled spins. These loops are connected along the a axis and separated along the c axis by cross-tie spins forming a quasiferromagnetic wave with the upper part of the respective neighbouring loops. The magnetic moments are sinusoidally modulated by the propagation vector k = (0.49,0,0) with an average amplitude of 1.59(1) µ(B) for a magnetic ion on a cross-tie site and 1.60(1) µ(B) for the spine site. In addition to neutron diffraction, specific heat and magnetization data, which confirm that the only magnetic phase transition above 1.8 K is the onset of antiferromagnetic order at T(N) = 7.4(1) K, are presented.

Importance of in-plane anisotropy in the quasi-two-dimensional antiferromagnet BaNi2V2O8.

The phase diagram of the quasi-two-dimensional antiferromagnet BaNi(2)V(2)O(8) is studied by specific heat, thermal expansion, magnetostriction, and magnetization for magnetic fields applied perpendicular to c. At micro(o)H* approximately 1.5 T, a crossover to a high-field state, where T(N)(H) increases linearly, arises from a competition of intrinsic and field-induced in-plane anisotropies. The pressure dependences of T(N) and H* are interpreted using the picture of a pressure-induced in-plane anisotropy. Even at zero field and ambient pressure, in-plane anisotropy cannot be neglected, which implies deviations from pure Berezinskii-Kosterlitz-Thouless behavior.

Nickel impurity-induced enhancement of the pseudogap of cuprate high-T(c) superconductors.

The influence of magnetic Ni and nonmagnetic Zn impurities on the normal-state pseudogap (PG) in the c-axis optical conductivity of (Sm,Nd)Ba(2){Cu(1-y)(Ni,Zn)(y)}(3)O(7-delta) crystals was studied by spectral ellipsometry. We find that these impurities, which strongly suppress superconductivity, have a profoundly different impact on the PG. Zn gives rise to a gradual and inhomogeneous PG suppression while Ni strongly enhances the PG. Our results challenge theories that relate the PG either to precursor superconductivity or to other phases with exotic order parameters, such as flux phase or d-density wave states, that should be suppressed by potential scattering. The apparent difference between magnetic and nonmagnetic impurities instead points towards an important role of magnetic correlations in the PG state.

Oxygen superstructures throughout the phase diagram of (Y,Ca)Ba2Cu3O6+x.

The doping dependence of short-range lattice superstructures in (Y,Ca)Ba2Cu3O6+x has been studied with high-energy x-ray scattering. We observe diffuse features with a well defined periodicity which depend on the oxygen concentration but not on the charge carrier concentration. In addition, we find that diffuse scattering is absent in underdoped YBa2Cu4O8, which does not sustain oxygen defects. Our combined data highlight that the diffuse scattering arises from short-range oxygen ordering and associated lattice distortions. Signatures of stripe ordering or fluctuations are not seen and therefore must be much weaker.

Investigating genotoxic and hematotoxic effects of N-nitrosodimethylamine, N-nitrosodiethylamine and N-nitrosodiethanolamine in the hen's egg-micronucleus test (HET-MN).

The hen's egg test for micronucleus induction (HET-MN) combines the use of the commonly accepted genetic endpoint "formation of micronuclei" with the well-characterized and complex model of the incubated hen's egg, which enables metabolic activation, elimination and excretion of xenobiotics including mutagens and promutagens and does not conflict with animal protection regulations and ethical aspects. N-nitrosodimethylamine (NDMA) and N-nitrosodiethylamine (NDEA) produced clearly positive, dose-dependent and reproducible results in this assay. NDMA revealed, in accordance with literature data, a much higher mutagenicity than NDEA. For both compounds the sensitivity of HETMN was to a large extent higher than published for the rodent micronucleus test, which is not capable of unequivocally identifying NDEA as positive. Additionally, NDEA induced severe anemia without obscuring the formation of micronucleated cells. N-nitrosodiethanolamine (NDELA), which in the literature is described as a non-mutagenic animal carcinogen, could clearly be confirmed as non-mutagenic in the HETMN without showing any disturbing effects on the formation of erythrocytes. The micronucleus frequencies of the concurrent negative controls of all experiments was in agreement with the historic negative control from 302 eggs and 412,532 cells. The same is true for the historic control of proliferation marker from 61 eggs and 13,020 cells. We interpret these results, which correspond well to published data from animal tests, as being further support for using the HET-MN as a reliable alternative genotoxicity assay system, which is physiologically closer to in vivo conditions than in vitro genotoxicity tests, and allows the observation of further local and systemic effects.