Temperature Dependent Polarized XANES Spectra for Zn-doped LSCO system

The cuprates seem to exhibit statistics, dimensionality and phase transitions in novel ways. The nature of excitations [i.e. quasiparticle or collective], spin-charge separation, stripes [static and dynamics], inhomogeneities, psuedogap, effect of impurity dopings [e.g. Zn, Ni] and any other phenomenon in these materials must be consistently understood. Zn-doped LSCO single crystal were grown by TSFZ technique. Temperature dependent Polarized XANES [near edge local structure] spectra were measured at the BL13-B1 [Photon Factory] in the Flourescence mode from 10 K to 300 K. Since both stripes and nonmagnetic Zn impurities substituted for Cu give rise to inhomogeneous charge and spin distribution it is interesting to understand the interplay of Zn impurities and stripes. To understand these points we have used Zn-doping and some of the results obtained are as follows: The spectra show a strong dependence with respect to the polarization angle, $\theta$, as is evident at any temperature by comparing the spectra where the electric field vector is parallel with ab-plane to the one where it is parallel to the c-axis. By using the XANES [temperature] difference spectra we have determined T* [experimentally we find, T* $\approx$ 160-170 K] for this sample. The XANES difference spectra shows that the changes in XANES features are larger in the ab-plane than the c-axis, this trend is expected since zinc is doped in the ab-plane at the copper site. Our study also complements the results in literature namely that zinc doping does not affect the c-axis transport.Comment: To appear in Physica C [ISS2001 Special Issue], related talk presented at ISS2001 as PC-16, 10 pages revtex and 7 pages of figures (pdf

Magnetic Hamiltonian and phase diagram of the quantum spin liquid Ca10Cr7O28

A spin liquid is a new state of matter with topological order where the spin moments continue to fluctuate coherently down to the lowest temperatures rather than develop static long range magnetic order as found in conventional magnets. For spin liquid behavior to arise in a material the magnetic Hamiltonian must be frustrated , where the combination of lattice geometry, interactions, and anisotropies gives rise to competing spin arrangements in the ground state. Theoretical Hamiltonians which produce spin liquids are spin ice, the Kitaev honeycomb model, and the kagome antiferromagnet. Spin liquid behavior, however, in real materials is rare because they can only approximate these Hamiltonians and often have weak higher order terms that destroy the spin liquid state. Ca10Cr7O28 is a new quantum spin liquid candidate with magnetic Cr5 ions that possess quantum spin number S . The spins are entirely dynamic in the ground state and the excitation spectrum is broad and diffuse, as is typical of spinons which are the excitations of a spin liquid. In this paper we determine the Hamiltonian of Ca10Cr7O28 using inelastic neutron scattering under high magnetic field to induce a field polarized paramagnetic ground state and spin wave excitations that can be fitted to extract the interactions. We further explore the phase diagram by using inelastic neutron scattering and heat capacity measurements and establish the boundaries of the spin liquid phase as a function of magnetic field and temperature. Our results show that Ca10Cr7O28 consists of distorted kagome bilayers with several isotropic ferromagnetic and antiferromagnetic interactions where, unexpectedly, the ferromagnetic interactions are stronger than the antiferromagnetic ones. This complex Hamiltonian does not correspond to any known spin liquid model and points to new directions in the search for quantum spin liquid behavio

Signatures for Berezinskii Kosterlitz Thouless critical behavior in the planar antiferromagnet BaNi2V2O8

We investigate the critical properties of the spin 1 honeycomb antiferromagnet BaNi2V2O8, both below and above the ordering temperature TN using neutron diffraction and muon spin rotation measurements. Our results characterize BaNi2V2O8 as a two dimensional 2D antiferromagnet across the entire temperature range, displaying a series of crossovers from 2D Ising like to 2D XY and then to 2D Heisenberg behavior with increasing temperature. In particular, the extracted critical exponent of the order parameter reveals a narrow temperature regime close to TN , in which the system behaves as a 2D XY antiferromagnet. Above TN , evidence for Berezinsky Kosterlitz Thouless behavior driven by vortex excitations is obtained from the scaling of the correlation length. Our experimental results are in accord with classical and quantum Monte Carlo simulations performed for microscopic magnetic model Hamiltonians for BaNi2V2O

Structural and magnetic properties of the quantum magnet BaCuTe2O6

We investigate the structural and magnetic properties of the quantum magnet BaCuTe2O6. This compound is synthesized in powder and single crystal form for the first time. Synchrotron x ray and neutron diffraction reveal a cubic crystal structure P4132 where the magnetic Cu2 ions form a complex network. Heat capacity and static magnetic susceptibility measurements suggest the presence of antiferromagnetic interactions with a Curie Weiss temperature of amp; 8776; amp; 8722;33 K, while long range magnetic order occurs at the much lower temperature of amp; 8776;6.3 K. The magnetic structure, solved using neutron diffraction, reveals antiferromagnetic order along chains parallel to the a, b, and c crystal axes. This is consistent with the magnetic excitations which resemble the multispinon continuum typical of the spin 1 2 Heisenberg antiferromagnetic chain. A consistent intrachain interaction value of amp; 8776;34 K is achieved from the various techniques. Finally the magnetic structure provides evidence that the chains are coupled together in a noncollinear arrangement by a much weaker antiferromagnetic, frustrated hyperkagome interactio

Thermodynamics of Meissner effect and flux pinning behavior in the bulk of single crystal La2 xSrxCuO4 x 0.09

We have studied the evolution of magnetic flux pinning behavior in the Meissner phase and the mixed state for the high- T c single-crystal La 2 − x Sr x CuO 4 ( x = 0.09 ) superconductor using the polarized neutron imaging method with varying magnetic field and temperature. In the Meissner state expulsion of magnetic field (switched on during the measurements) is visualized, and the signatures of a mixed state with increasing temperature are observed. However, for flux pinning behavior in the range 5 K ≤ T ≤ 15 K and H ext = 63.5 mT (switched off during the measurements), the evolution of the fringe pattern indicates magnetic flux pinning inside the bulk of the sample. At 25 K ≤ T ≤ 32 K , a continuous decrease in inhomogeneously distributed pinned magnetic flux is observed, with the sample reaching a normal conducting state at T c ( ≈ 32 K). The flux pinning behavior is also explored as a function of H ext at T = 5 K . As expected, with increasing H ext an increase in fringe density is observed, indicating an increase in magnetic flux pinning in the bulk of the sample. A comparison between calculated and experimentally visualized pinned magnetic fluxes shows good agreement. This implies quantification of pinned magnetic flux inside the sample, which is not possible with any other technique for bulk samples

Critical properties of coupled anisotropic Haldane spin chains in a magnetic field

We use inelastic neutron scattering to measure magnetic fluctuations as a function of transverse magnetic field and temperature in a single crystal of the coupled Haldane chain compound SrNi2V2O8 with uniaxial anisotropy. At the base temperature 2 K , spin excitations confirm a field induced quantum phase transition from the disordered Haldane gapped singlet state to a gapped long range ordered state through a quantum critical point QCP amp; 956;0Hc amp; 8776; 11.5 T . At elevated temperatures in the vicinity of the QCP, both the energy and linewidth of the lowest triplet mode scale linearly with temperature, in accordance with quantum criticality. We also demonstrate that the experimental field dependence of the triplet excitation modes at the base temperature is in agreement with the bosonic and macroscopic models, but is in contrast to the fermionic and perturbative model

Order out of a Coulomb Phase and Higgs Transition Frustrated Transverse Interactions of Nd2Zr2O7

The pyrochlore material Nd2Zr2O7 with an all in all out AIAO magnetic order shows novel quantum moment fragmentation with gapped flat dynamical spin ice modes. The parametrized spin Hamiltonian with a dominant frustrated ferromagnetic transverse term reveals a proximity to a U 1 spin liquid. Here we study the magnetic excitations of Nd2Zr2O7 above the ordering temperature TN using high energy resolution inelastic neutron scattering. Wefind strong spin ice correlations at zero energy with the disappearance of gapped magnon excitations of the AIAO order. It seems that the gap to the dynamical spin ice closes above TN and the system enters a quantum spin ice state competing with and suppressing the AIAO order. Classical Monte Carlo simulations, molecular dynamics, and quantum boson calculations support the existence of a Coulombic phase above TN. Our findings relate the magnetic ordering of Nd2Zr2O7 with the Higgs mechanism and provide explanations for several previously reported experimental features

Multiple lattice instabilities resolved by magnetic field and disorder sensitivities in MgV2O4

Ultrasound velocity measurements of the orbitally degenerate frustrated spinel MgV2O4 are performed in a high purity single crystal which exhibits successive structural and antiferromagnetic phase transitions, and in the disorder introduced single crystal which exhibits spin glass like behavior. The measurements reveal that two types of unusual temperature dependence of the elastic moduli coexist in the cubic paramagnetic phase, which are resolved by magnetic field and disorder sensitivities huge Curie type softening with decreasing temperature convex temperature dependence , and concave temperature dependence with a characteristic minimum. These elastic anomalies suggest the coupling of the lattice to coexisting orbital fluctuations and spin cluster excitation