When the energy eigenvalues of two coupled quantum states approach each other in a certain parameter space, their energy levels repel each other and level crossing is avoided. Such level repulsion, or avoided level crossing, is commonly used to descr
ibe the dispersion relation of quasiparticles in solids. However, little is known about the level repulsion when more than two quasiparticles are present; for example, in an open quantum system where a quasiparticle can spontaneously decay into many particle continuum. Here we show that even in this case level repulsion exists between a long-lived quasiparticle state and a continuum. In our fine resolution neutron spectroscopy study of magnetic quasiparticles in a frustrated quantum magnet BiCu2PO6, we observe a renormalization of quasiparticle dispersion relation due to the presence of the continuum of multi-quasiparticle states. Our results have a broadimplication for understanding open quantum systems described by non-hermitian Hamiltonian.
The recent determination of a robust spin Hamiltonian for the anti-ferromagnetic XY pyrochlore Er2Ti2O7 reveals a most convincing case of the order by quantum disorder (ObQD) mechanism for ground state selection. This mechanism relies on quantum fluc
tuations to remove an accidental symmetry of the magnetic ground state, and selects a particular ordered spin structure below T_N=1.2K. The removal of the continuous degeneracy results in an energy gap in the spectrum of spin wave excitations, long wavelength pseudo-Goldstone modes. We have measured the ObQD spin wave gap at a zone center in Er2Ti2O7, using low incident energy neutrons and the time-of-flight inelastic scattering method. We report a gap of Delta =0.053 +/- 0.006 meV, which is consistent with upper bounds placed on it from heat capacity measurements and roughly consistent with theoretical estimate of ~ 0.02 meV, further validating the spin Hamiltonian that led to that prediction. The gap is observed to vary with square of the order parameter, and goes to zero for T ~ T_N.
We report a spin S = 3/2 triangular antiferromagnet with nearest-neighbor coupling J = 0.29 meV in La2Ca2MnO7. A genuinely two-dimensional, three-sublattice order develops below 2.80 K << the Weiss constant (25 K). The spin excitations deviate substa
ntially from linear spin-wave theory, suggesting that magnon breakdown occurs in the material. Such a breakdown has been anticipated in recent theoretical studies, although the excitation spectrum remains to be accounted for.
Magnetic spin fluctuations is one candidate to produce the bosonic modes that mediate the superconductivity in the ferrous superconductors. Up until now, all of the LaOFeAs and BaFe2As2 structure types have simple commensurate magnetic ground states,
as result of nesting Fermi surfaces. This type of spin-density-wave (SDW) magnetic order is known to be vulnerable to shifts in the Fermi surface when electronic densities are altered at the superconducting compositions. Superconductivity has more recently been discovered in alpha-Fe(Te,Se), whose electronically active antifluorite planes are isostructural to the FeAs layers found in the previous ferrous superconductors and share with them the same quasi-two-dimensional electronic structure. Here we report neutron scattering studies that reveal a unique complex incommensurate antiferromagnetic order in the parent compound alpha-FeTe. When the long-range magnetic order is suppressed by the isovalent substitution of Te with Se, short-range correlations survive in the superconducting phase.
The transition temperature Tc~26 K of the recently discovered superconductor LaFeAs(O,F) has been demonstrated to be extremely sensitive to the lanthanide ion, reaching 55 K for the Sm containing oxypnictides. Therefore, it is important to determine
how the moment on the lanthanide affects the overall magnetism in these systems. Here we report a neutron diffraction study of the Nd oxypnictides. Long ranged antiferromagnetic order is apparent in NdFeAsO below 1.96 K. Rietveld refinement shows that both Fe and Nd magnetic ordering are required to describe the observed data with the staggered moment 1.55(4) Bohr magneton per Nd and 0.9(1) Bohr magneton per Fe at 0.3 K. The other structural properties such as the tetragonal-orthorhombic distortion are found to be very similar to those in LaFeAsO. Neither the magnetic ordering nor the structural distortion occur in the superconducting sample NdFeAsO0.80F0.20 at any temperatures down to 1.5 K.
In addition to higher Tc compared with the ubiquitous cuprates for a material composed of a single electronically active layer, the newly discovered LnFeAsO superconductors offer additional compositional variation. In a similar fashion to the CuO2 la
yers in cuprates, the FeAs layers now dominate the electronic states that produce superconductivity. Cuprate superconductors distinguish themselves structurally by adopting different stacking of the Cu-O and electronically inactive spacer layers. Using the same structural philosophy, materials with the formula (A,K)Fe2As2,A=Ba or Sr have been reported and possess a Tc~38 K. Here, we report the neutron diffraction studies of BaFe2As2 that shows, in contrast to previous studies on the LnFeAsO materials, an antiferromagnetic transition which concurs with first-order structural transition. Although the magnetic and structural transitions occur differently in the AFe2As2 and LnFeAsO-type materials, this work clearly demonstrates that the complete evolution to a low symmetry structure is a pre-requirement for the magnetic order.
The newly discovered superconductor La(O,F)FeAs (Tc = 26 K) was investigated using the neutron scattering technique. No spin-density-wave (SDW) order was observed in the normal state nor in the superconducting state, both with and without an applied
magnetic field of 9 T, consistent with the proposal that SDW and superconductivity are competing in the laminar materials. While our inelastic measurements offer no constraints on the spin dynamic response from d-wave pairing, an upper limit for the magnetic resonance peak predicted from an extended s-wave pairing mechanism is provided. Our measurements also support the energy scale of the calculated phonon spectrum which is used in electron-phonon coupling theory, and fails to produce the high observed Tc.
