Enhanced charge density wave with mobile superconducting vortices in La1.885Sr0.115CuO4.

Superconductivity in the cuprates is found to be intertwined with charge and spin density waves. Determining the interactions between the different types of order is crucial for understanding these important materials. Here, we elucidate the role of the charge density wave (CDW) in the prototypical cuprate La1.885Sr0.115CuO4, by studying the effects of large magnetic fields (H) up to 24 Tesla. At low temperatures (T), the observed CDW peaks reveal two distinct regions in the material: a majority phase with short-range CDW coexisting with superconductivity, and a minority phase with longer-range CDW coexisting with static spin density wave (SDW). With increasing magnetic field, the CDW first grows smoothly in a manner similar to the SDW. However, at high fields we discover a sudden increase in the CDW amplitude upon entering the vortex-liquid state. Our results signify strong coupling of the CDW to mobile superconducting vortices and link enhanced CDW amplitude with local superconducting pairing across the H - T phase diagram.

Momentum Dependence of the Nematic Order Parameter in Iron-Based Superconductors.

The momentum dependence of the nematic order parameter is an important ingredient in the microscopic description of iron-based high-temperature superconductors. While recent reports on FeSe indicate that the nematic order parameter changes sign between electron and hole bands, detailed knowledge is still missing for other compounds. Combining angle-resolved photoemission spectroscopy with uniaxial strain tuning, we measure the nematic band splitting in both FeSe and BaFe_{2}As_{2} without interference from either twinning or magnetic order. We find that the nematic order parameter exhibits the same momentum dependence in both compounds with a sign change between the Brillouin center and the corner. This suggests that the same microscopic mechanism drives the nematic order in spite of the very different phase diagrams.

Band-Resolved Imaging of Photocurrent in a Topological Insulator.

We study the microscopic origins of photocurrent generation in the topological insulator Bi_{2}Se_{3} via time- and angle-resolved photoemission spectroscopy. We image the unoccupied band structure as it evolves following a circularly polarized optical excitation and observe an asymmetric electron population in momentum space, which is the spectroscopic signature of a photocurrent. By analyzing the rise times of the population we identify which occupied and unoccupied electronic states are coupled by the optical excitation. We conclude that photocurrents can only be excited via resonant optical transitions coupling to spin-orbital textured states. Our work provides a microscopic understanding of how to control photocurrents in systems with spin-orbit coupling and broken inversion symmetry.

Three-dimensional collective charge excitations in electron-doped copper oxide superconductors.

High-temperature copper oxide superconductors consist of stacked CuO2 planes, with electronic band structures and magnetic excitations that are primarily two-dimensional1,2, but with superconducting coherence that is three-dimensional. This dichotomy highlights the importance of out-of-plane charge dynamics, which has been found to be incoherent in the normal state3,4 within the limited range of momenta accessible by optics. Here we use resonant inelastic X-ray scattering to explore the charge dynamics across all three dimensions of the Brillouin zone. Polarization analysis of recently discovered collective excitations (modes) in electron-doped copper oxides5-7 reveals their charge origin, that is, without mixing with magnetic components5-7. The excitations disperse along both the in-plane and out-of-plane directions, revealing its three-dimensional nature. The periodicity of the out-of-plane dispersion corresponds to the distance between neighbouring CuO2 planes rather than to the crystallographic c-axis lattice constant, suggesting that the interplane Coulomb interaction is responsible for the coherent out-of-plane charge dynamics. The observed properties are hallmarks of the long-sought 'acoustic plasmon', which is a branch of distinct charge collective modes predicted for layered systems8-12 and argued to play a substantial part in mediating high-temperature superconductivity10-12.

The effects of post-growth annealing on the structural and magnetic properties of BaFe2As2.

We investigate the effects of post-growth annealing on the structural and magnetic properties of BaFe2As2. Magnetic susceptibility measurements, which exhibit a signal corresponding to the magnetic phase transition, and high-resolution x-ray diffraction measurements, which directly probe the structural order parameter, show that annealing causes the ordering temperatures of both the phase transitions to increase, sharpen and converge. In the as grown sample, our measurements show two distinct transitions corresponding to structural and magnetic ordering, which are separated in temperature by approximately 1 K. After 46 days (d) of annealing at 700 °C, the two become concurrent in temperature. These measurements demonstrate that the structural phase transition is second-order like when the magnetic and structural phase transitions are separated in temperature, and first-order like when the two phase transition temperatures coincide. This observation indicates that annealing causes the system to cross a hitherto undiscovered tricritical point. In addition, x-ray diffraction measurements show that the c-axis lattice parameter increases with annealing up to 30 d, but remains constant for longer annealing times. Comparisons of BaFe2As2 to SrFe2As2 are made when possible.

High-pressure resistivity technique for quasi-hydrostatic compression experiments.

Diamond anvil cell techniques are now well established and powerful methods for measuring materials properties to very high pressure. However, high pressure resistivity measurements are challenging because the electrical contacts attached to the sample have to survive to extreme stress conditions. Until recently, experiments in a diamond anvil cell were mostly limited to non-hydrostatic or quasi-hydrostatic pressure media other than inert gases. We present here a solution to the problem by using focused ion beam ultrathin lithography for a diamond anvil cell loaded with inert gas (Ne) and show typical resistivity data. These ultrathin leads are deposited on the culet of the diamond and are attaching the sample to the anvil mechanically, therefore allowing for measurements in hydrostatic or nearly hydrostatic conditions of pressure using noble gases like Ne or He as pressure transmitting media.

Effect of electron correlations on magnetic excitations in the isovalently doped iron-based superconductor Ba(Fe(1-x)Ru(x))(2)As(2).

Magnetic correlations in isovalently doped Ba(Fe(1-x)Ru(x))(2)As(2) (x = 0.25, T(c) = 14.5 K; x = 0.35, T(c) = 20 K) are studied by elastic and inelastic neutron scattering techniques. A relatively large superconducting spin gap accompanied by a weak resonance mode is observed in the superconducting state in both samples. In the normal state, the magnetic excitation intensity is dramatically reduced with increasing Ru doping toward the optimally doped regime. Our results favor that the weakening of the electron-electron correlations by Ru doping is responsible for the dampening of the resonance mode, as well as the suppression of the normal state antiferromagnetic correlations near the optimally doped regime in this system.

Comment on "Retention of the tetragonal to orthorhombic structural transition in F-substituted SmFeAsO: a new phase diagram for SmFeAs(O(1-x)F((x))".

A Comment on the Letter by A. Martinelli, A. Palenzona, M. Tropeano, M. Putti, C. Ferdeghini, G. Profeta, and E. Emerich, Phys. Rev. Lett. 106, 227001 (2011). The authors of the Letter offer a Reply.

Signatures of a hybridization gap in magnetic susceptibility of Ce(1-x)La(x)Os(4)Sb(12).

The magnetic susceptibility of Ce(1-x)La(x)Os(4)Sb(12), with x ≈ 0.2, exhibits a pronounced maximum at T(m) = 80 K. This T(m) coincides roughly with the temperature below which a small gap, believed to be a hybridization gap, is observed in spectroscopic measurements in undoped CeOs(4)Sb(12). However, a similar anomaly, at a lower temperature of 50 K, is observed in LaOs(4)Sb(12). Furthermore, there is a monotonic variation of T(m) with x, for x > 0.2, suggesting the same origin of the two anomalies and undermining a simple hybridization gap interpretation of the susceptibility of Ce(1-x)La(x)Os(4)Sb(12) alloys, with x < 1. A possibility of the hybridization gap opening, induced by freezing out of local phonons, strongly coupled with electronic degrees of freedom, is discussed.

Investigation of the heavy fermion state and superconductivity in Pr(1-x)La(x)Os4Sb12 by the upper critical field slope at Tc.

Measurements of the upper critical field H(c2) near T(c) of Pr(1 - x)La(x)Os(4)Sb(12) were performed by specific heat. The results support the hypothesis of a non-single-ion origin of m* enhancement in PrOs(4)Sb(12), derived from the analysis of the C/T discontinuity at T(c). Both sets of measurements indicate the existence of a crossover concentration, x(cr)≈0.2-0.3, separating heavy fermion-like alloys (x < x(cr)) from normal metals (x > x(cr)). Heavy fermion-like alloys exhibit field-induced long-range antiferro-quadrupolar (AFQ) anomalies in the specific heat, while those with x > x(cr) do not. The curvature in H(c2) versus T near T(c), observed in different measurements on pure PrOs(4)Sb(12), is sample- and method-dependent.

The dilute limit of heavy fermions: evidence of the coherent nature of CeAl(3).

Strongly diluted Ce(x)La(1-x)Al(3) alloys have been studied by the low temperature specific heat in order to elucidate the mechanism that determines their ground state. The results for alloys with Ce concentrations of 0.0005≤x≤0.1 are fairly consistent with a S = 1/2 Kondo behavior. However, the single-ion scaling is observed only below x = 0.01. The true single-ion Kondo temperature is small, 0.2 K, approximately equal to that for dilute CeAl(2). It is about 20 times smaller than that for CeAl(3), indicating the coherent nature of CeAl(3). The single-ion contribution to thermodynamic properties of CeAl(3) is negligible even at temperatures as high as 5 K.

Tomonaga-Luttinger liquid in a quasi-one-dimensional S = 1 antiferromagnet observed by specific heat measurements.

Specific-heat experiments on single crystals of the S = 1 quasi-one-dimensional bond-alternating antiferromagnet Ni(C9H24N4)(NO2)ClO2 (NTENP) have been performed in magnetic fields applied both parallel and perpendicular to the spin chains. We have found for the parallel field configuration that the magnetic specific heat (C(mag)) is proportional to temperature (T) above a critical field H(c), at which the energy gap vanishes, in a temperature region above that of the long-range ordered state. The ratio C(mag)/T increases as the magnetic field approaches H(c) from above. The data are in good quantitative agreement with the prediction of the c= 1 conformal field theory in conjunction with the velocity of the excitations calculated by a numerical diagonalization, providing conclusive evidence for a Tomonaga-Luttinger liquid.

High magnetic field phase diagram of PrOs4Sb12.

The magnetic phase diagram of PrOs4Sb12 has been investigated by specific heat measurements between 8 and 32 T. A new Schottky anomaly, due to excitations between two lowest crystalline-electric-field (CEF) singlets, has been found for both H parallel (100) and H parallel (110) above the field where the field-induced ordered phase (FIOP) is suppressed. The constructed H-T phase diagram shows weak magnetic anisotropy and implies a crossing of the two CEF levels at about 8-9 T for both field directions. These results provide an unambiguous evidence for the Gamma(1) singlet being the CEF ground state and suggest the level crossing (involving lowest CEF levels) as the driving mechanism of FIOP.