Control of tetrahedral coordination and superconductivity in FeSe0.5Te0.5 thin films

We demonstrate a close relationship between superconductivity and the dimensions of the Fe-Se(Te) tetrahedron in FeSe0.5Te0.5. This is done by exploiting thin film epitaxy, which provides controlled biaxial stress, both compressive and tensile, to distort the tetrahedron. The Se/Te height within the tetrahedron is found to be of crucial importance to superconductivity, in agreement with the theoretical proposal that (pi,pi) spin fluctuations promote superconductivity in Fe superconductors

Field-driven phase transitions in a quasi-two-dimensional quantum antiferromagnet

We report magnetic susceptibility, specific heat, and neutron scattering measurements as a function of applied magnetic field and temperature to characterize the $S=1/2$ quasi-two-dimensional frustrated magnet piperazinium hexachlorodicuprate (PHCC). The experiments reveal four distinct phases. At low temperatures and fields the material forms a quantum paramagnet with a 1 meV singlet triplet gap and a magnon bandwidth of 1.7 meV. The singlet state involves multiple spin pairs some of which have negative ground state bond energies. Increasing the field at low temperatures induces three dimensional long range antiferromagnetic order at 7.5 Tesla through a continuous phase transition that can be described as magnon Bose-Einstein condensation. The phase transition to a fully polarized ferromagnetic state occurs at 37 Tesla. The ordered antiferromagnetic phase is surrounded by a renormalized classical regime. The crossover to this phase from the quantum paramagnet is marked by a distinct anomaly in the magnetic susceptibility which coincides with closure of the finite temperature singlet-triplet pseudo gap. The phase boundary between the quantum paramagnet and the Bose-Einstein condensate features a finite temperature minimum at $T=0.2$ K, which may be associated with coupling to nuclear spin or lattice degrees of freedom close to quantum criticality.Comment: Submitted to New Journal of Physic

Spin-Dynamics of the antiferromagnetic S=1/2-Chain at finite magnetic Fields and intermediate Temperatures

We present a study of the dynamic structure factor of the antiferromagnetic spin-1/2 Heisenberg chain at finite temperatures and finite magnetic fields. Using Quantum-Monte-Carlo based on the stochastic series expansion and Maximum-Entropy methods we evaluate the longitudinal and the transverse dynamic structure factor from vanishing magnetic fields up to and above the threshold $B_c$ for ferromagnetic saturation, as well as for high and for intermediate temperatures. We study the field-induced redistribution of spectral weight contrasting longitudinal versus transverse excitations. At finite fields below saturation incommensurate low-energy modes are found consistent with zero temperature Bethe-Ansatz. The crossover between the field induced ferromagnet above $B_c$ and the Luttinger liquid below $B_c$ is analyzed in terms of the transverse spin-dynamics. Evaluating sum-rules we assess the quality of the analytic continuation and demonstrate excellent consistency of the Maximum-Entropy results.Comment: 10 pages, 6 figure