Magnetic Phase Transitions in the double spin-chains compound LiCu2O2\rm LiCu_2O_2

We report high-resolution x-ray diffraction, muon-spin-rotation spectroscopic and specific heat measurements in the double spin-chains compound $\rm LiCu_2O_2$. The x-ray diffraction results show that the crystal structure of $\rm LiCu_2O_2$ ~is orthorhombic down to T=10K. Anisotropic line-broadening of the diffraction peaks is observed, indicating disorder along the spin chains. Muon spin relaxation and specific heat measurements show that $\rm LiCu_2O_2$ \~undergoes a phase transition to a magnetic ordered state at $\rm T_1\sim24K$. The specific heat data exhibits a second $\rm \lambda$-like peak at $\rm T_2\sim22.5 K$, which increases with increasing magnetic field similarly way to that found in spin-ladder compounds.Comment: 6 pages, 6 fifures, to appear in Physica

Ferro-type order of magneto-electric quadrupoles as an order-parameter for the pseudo-gap phase of a cuprate superconductor

Understanding the pseudo-gap phase from which a superconducting state emerges is a central problem in the physics of cuprates. The phase exists below a temperature T* for less than optimal doping of holes, and no clear jump in the specific heat or change in lattice symmetry is observed on crossing T*. Here we show that, two sets of intriguing experimental results in the literature concerning magnetic properties of underdoped YBCO are united in a single theory based on Cu quadrupole moments. The quadrupole is a product of spin and the electric dipole, so there is no magnetic dipole moment, and standard crystallography describes the magnetic motif. One experiment employed the Kerr effect and the second Bragg diffraction of polarized neutrons, and both are shown to imply an order-parameter for the pseudo-gap phase composed of magneto-electric quadrupoles at in-plane Cu sites in a ferro-type order. The successful interpretation of neutron diffraction data for YBCO yields the first experimental evidence that neutrons are deflected by magneto-electric multipoles. Predicted in 2014, our verification of the effect validates a new application for the experimental technique of choice in studies of magnetic motifs and excitations

Role of magnetic and orbital ordering at the metal-insulator transition in NdNiO3

Soft x-ray resonant scattering at the Ni L2,3 edges is used to test models of magnetic and orbital-ordering below the metal-insulator transition in NdNiO3. The large branching ratio of the L3 to L2 intensities of the (1/2,0,1/2) reflection and the observed azimuthal angle and polarization dependence originates from a non collinear magnetic structure. The absence of an orbital signal and the non collinear magnetic structure show that the nickelates are materials for which orbital ordering is absent at the metal-insulator transition.Comment: 10 pages, 4 figures, Physical Review B rapid communication, to be publishe