Local antiferromagnetic exchange and collaborative Fermi surface as key ingredients of high temperature superconductors

Cuprates, ferropnictides and ferrochalcogenides are three classes of unconventional high-temperature superconductors, who share similar phase diagrams in which superconductivity develops after a magnetic order is suppressed, suggesting a strong interplay between superconductivity and magnetism, although the exact picture of this interplay remains elusive. Here we show that there is a direct bridge connecting antiferromagnetic exchange interactions determined in the parent compounds of these materials to the superconducting gap functions observed in the corresponding superconducting materials. High superconducting transition temperature is achieved when the Fermi surface topology matches the form factor of the pairing symmetry favored by local magnetic exchange interactions. Our result offers a principle guide to search for new high temperature superconductors.Comment: 12 pages, 5 figures, 1 table, 1 supplementary materia

Magnetic field-temperature phase diagram of multiferroic (NH4)2FeCl5??H2O

Owing to their overall low energy scales, flexible molecular architectures, and ease of chemical substitution, molecule-based multiferroics are extraordinarily responsive to external stimuli and exhibit remarkably rich phase diagrams. Even so, the stability and microscopic properties of various magnetic states in close proximity to quantum critical points are highly under-explored in these materials. Inspired by these opportunities, we combined pulsed-field magnetization, first-principles calculations, and numerical simulations to reveal the magnetic field???temperature (B???T) phase diagram of multiferroic (NH4)2FeCl5???H2O. In this system, a network of intermolecular hydrogen and halogen bonds creates a competing set of exchange interactions that generates additional structure in the phase diagram???both in the vicinity of the spin flop and near the 30 T transition to the fully saturated state. Consequently, the phase diagrams of (NH4)2FeCl5???H2O and its deuterated analog are much more complex than those of other molecule-based multiferroics. The entire series of coupled electric and magnetic transitions can be accessed with a powered magnet, opening the door to exploration and control of properties in this and related materials

From (pi, 0) magnetic order to superconductivity with (pi, pi) magnetic resonance in Fe1.02(Te1-xSex)

The iron chalcogenide Fe1+y(Te1-xSex) is structurally the simplest of the Fe-based superconductors. Although the Fermi surface is similar to iron pnictides, the parent compound Fe1+yTe exhibits antiferromagnetic order with in-plane magnetic wave-vector (pi, 0). This contrasts the pnictide parent compounds where the magnetic order has an in-plane magnetic wave-vector (pi, pi) that connects hole and electron parts of the Fermi surface. Despite these differences, both the pnictide and chalcogenide Fe-superconductors exhibit superconducting spin resonances around (pi, pi), suggesting a common symmetry for their superconducting order parameter. A central question in this burgeoning field is therefore how (pi, pi) superconductivity can emerge from a (pi, 0) magnetic instability. Here, we report that the magnetic soft mode evolving from the (pi, 0)-type magnetic long-range order is associated with weak charge carrier localization. Bulk superconductivity occurs only as the magnetic mode at (pi, pi) becomes dominant upon doping. Our results suggest a common magnetic origin for superconductivity in iron chalcogenide and pnictide superconductors.Comment: 17 pages, 4 figure

Spin-chain correlations in the frustrated triangular lattice material CuMnO2

The Ising triangular lattice remains the classic test-case for frustrated magnetism. Here we report neutron scattering measurements of short range magnetic order in CuMnO$_2$, which consists of a distorted lattice of Mn$^{3+}$ spins with single-ion anisotropy. Physical property measurements on CuMnO$_2$ are consistent with 1D correlations caused by anisotropic orbital occupation. However the diffuse magnetic neutron scattering seen in powder measurements has previously been fitted by 2D Warren-type correlations. Using neutron spectroscopy, we show that paramagnetic fluctuations persist up to $\sim$25 meV above TN= 65 K. This is comparable to the incident energy of typical diffractometers, and results in a smearing of the energy integrated signal, which hence cannot be analysed in the quasi-static approximation. We use low energy XYZ polarised neutron scattering to extract the purely magnetic (quasi)-static signal. This is fitted by reverse Monte Carlo analysis, which reveals that two directions in the triangular layers are perfectly frustrated in the classical spin-liquid phase at 75 K. Strong antiferromagnetic correlations are only found along the b-axis, and our results hence unify the pictures seen by neutron scattering and macroscopic physical property measurements

Lattice effects and magnetic structure in the layered colossal magnetoresistance manganite La2-2xSr1+2xMn2O7, x = 0.3

We report on the temperature dependence of the crystal and magnetic structure of the layered colossal magnetoresistive manganite, La2-2xSr1+2xMn2O7, x=0.3. Neutron-diffraction measurements show that the insulator-metal (IM) transition (7IM) at ∼100 K is accompanied by a ferromagnetic (FM) ordering of spins within MnO6 bilayers (intrabilayer coupling), but with an antiferromagnetic coupling between neighboring bilayers (interbilayer coupling). Below TIM, the Mn spins rotate from ∼45° inclination to the c axis until they are almost parallel to the c axis at 5 K. Coincident with this spin reorientation, a FM c axis component develops below 75 K. Evidence from both neutron and synchrotron x-ray-diffraction experiments suggest that the FM c-axis magnetic moment results from a second layered manganite phase with composition 0.3≲x ≲0.32. This observation emphasizes the need for thorough examination of the homogeneity when measuring bulk properties (e.g., magnetization, transport) of nominally x=0.3 samples. Associated with the electronic and magnetic transitions, a pronounced lattice response along the c axis (observed in both phases) signals a transfer of charge into dx2-y2 orbitals in the low-temperature phase. That the lattice effects here are opposite in sign to those observed in the x=0.4 layered manganite points to the sensitivity of the spin-lattice-charge coupling to dopant concentration in these reduced-dimensionality manganites. ©1999 The American Physical Society

Interlayer tuning of electronic and magnetic properties in honeycomb ordered Ag3LiRu2O6.

We report a switching of electronic, magnetic and lattice properties in honeycomb ruthenates by interlayer cation exchange. The new material Ag3LiRu2O6 was made by ion-exchange of the ordered Li/Ru honeycomb material Li2RuO3 in an AgNO3 melt at 200°C. Neutron powder diffraction and electron microscopy show that the Li/Ru order is preserved in the honeycomb layers, however, significant stacking disorder is found between layers. In contrast to Li2RuO3, which is insulating, dimerised and diamagnetic, Ag3LiRu2O6 has low electrical resistivity (0.01 ohm cm−1) and a large magnetic susceptibility at room temperature. This is attributed to the electronic influence of the highly polarisable interlayer Ag+ cation. The combination of two dimensionality, good conductivity and stacking disorder means this family of materials have potential for thermoelectric applications. © 2010, Royal Society of Chemistr

Structural effects on the magnetic and transport properties of perovskite A(1-x)A(x)'MnO3 (x=0.25, 0.30)

The evolution of the structural properties of A1-xA′xMnO3 was determined as a function of temperature, average A-site radius , and applied pressure for the "optimal" doping range x = 0.25, 0.30, by using high-resolution neutron powder diffraction. The metal-insulator transition, which can be induced both as a function of temperature and of , was found to be accompanied by significant structural changes. Both the paramagnetic charge-localized phase, which exists at high temperatures for all values of , and the spincanted ferromagnetic charge-ordered phase, which is found at low temperatures for low values of , are characterized by large metric distortions of the MnO6 octahedra. These structural distortions are mainly incoherent with respect to the space-group symmetry, with a significant coherent component only at low . These distortions decrease abruptly at the transition into the ferromagnetic metal phase. These observations are consistent with the hypothesis that, in the insulating phases, lattice distortions of the Jahn-Teller type, in addition to spin scattering, provide a charge-localization mechanism. The evolution of the average structural parameters indicates that the variation of the electronic bandwidth is the driving force for the evolution of the insulator-to-metal transition at Tc as a function of "chemical" and applied pressure.</ra