Hanna A. Dabkowska

Comment on the origin(s) of the giant permittivity effect in CaCu3Ti4O12 single crystals and ceramics

Impedance spectroscopy has been performed on CaCu3Ti4O12 (CCTO) single crystals and ceramics using Au and InGa alloy electrodes. The extrinsic effect responsible for the giant permittivity observed at radio-frequencies near room temperature is shown to be different for single crystals and ceramics. For single crystals it is attributed to a non-ohmic electrode contact to semiconducting CCTO. For ceramics it is attributed to an internal barrier layer capacitor mechanism associated with semiconducting CCTO grains and insulating grain boundaries.

Thermal contraction behavior in Al2(WO4)3 single crystal

Abstract The anisotropic thermal expansion behavior of a single crystal of Al 2 (WO 4 ) 3 was obtained for the first time and the anisotropic contraction behavior was identified. The direct clarification of the anisotropic behavior contributes greatly towards designing the thermal expansion characteristics along every axis, realizing unique materials such as a zero expansion material.

Magnetostriction and magnetic texture to 100.75 Tesla in frustrated SrCu2(BO3)2

Strong geometrical frustration in magnets leads to exotic states such as spin liquids, spin supersolids, and complex magnetic textures. SrCu2(BO3)2, a spin-1/2 Heisenberg antiferromagnet in the archetypical Shastry–Sutherland lattice, exhibits a rich spectrum of magnetization plateaus and stripe-like magnetic textures in applied fields. The structure of these plateaus is still highly controversial due to the intrinsic complexity associated with frustration and competing length scales. We discover magnetic textures in SrCu2(BO3)2 via magnetostriction and magnetocaloric measurements in fields up to 100.75 T. In addition to observing low-field fine structure with unprecedented resolution, the data also reveal lattice responses at 73.6 T and at 82 T that we attribute, using a controlled density matrix renormalization group approach, to a unanticipated 2/5 plateau and to the long-predicted 1/2 plateau.

Spin ice: magnetic excitations without monopole signatures using muon spin rotation.

Theory predicts the low temperature magnetic excitations in spin ices consist of deconfined magnetic charges, or monopoles. A recent transverse-field (TF) muon spin rotation (μSR) experiment [S. T. Bramwell et al., Nature (London) 461, 956 (2009)] reports results claiming to be consistent with the temperature and magnetic field dependence anticipated for monopole nucleation-the so-called second Wien effect. We demonstrate via a new series of μSR experiments in Dy(2)Ti(2)O(7) that such an effect is not observable in a TF μSR experiment. Rather, as found in many highly frustrated magnetic materials, we observe spin fluctuations which become temperature independent at low temperatures, behavior which dominates over any possible signature of thermally nucleated monopole excitations.

Spin Waves and Quantum Criticality in the Frustrated XY Pyrochlore Antiferromagnet Er2Ti2O7

We report detailed measurements of the low temperature magnetic phase diagram of Er2Ti2O7. Heat capacity and time-of-flight neutron scattering studies of single crystals reveal unconventional low-energy states. Er3+ magnetic ions reside on a pyrochlore lattice in Er2Ti2O7, where local XY anisotropy and antiferromagnetic interactions give rise to a unique frustrated system. In zero field, the ground state exhibits coexisting short and long-range order, accompanied by soft collective spin excitations previously believed to be absent. The application of finite magnetic fields tunes the ground state continuously through a landscape of noncollinear phases, divided by a zero temperature phase transition at micro{0}H{c} approximately 1.5 T. The characteristic energy scale for spin fluctuations is seen to vanish at the critical point, as expected for a second order quantum phase transition driven by quantum fluctuations.

Czochralski-Grown SrLaGaO4

The structure of Czochralski-grown lanthanum strontium tetraoxogallate, LaSrGaO 4 , a K 2 Ni 2 F 4 -type compound, has been redetermined by X-ray diffraction. The improved quality of the crystals resulted in a higher precision in the determination of the geometric parameters than reported previously [Ruter & Muller-Buschbaum (1990). Z. Anorg. Allg. Chem. 584, 119-124]. La and Sr atoms are equally distributed on a nine-coordinate site, and Ga is in a distorted octahedral site stretched along [001]. PIXE analysis gives excellent quantitative results.

Evaluation of LaSrGaO4 as a substrate for YBa2Cu3O7-δ

Abstract We present a comparison study between YBa2Cu3O7−δ thin films deposited on LaSrGaO4 substrates. The LaSrGaO4 crystals were grown, cut and polished in-house while the LaAlO3 substrates were prepared commercially. The deposited films were characterized by a variety of techniques including DC resistivity, critical current density, X-ray diffraction including rocking-curve analysis and scanning electron microscopy. Growth temperatures above 780°C on both substrates yielded highly oriented c-axis material with excellent transport properties. Because LaSrGaO4 has the advantage of being untwinned, a desirable property for microwave-device applications, we conclude that it has much promise as a potential substitute for LaAlO3.

Field-induced Bose-Einstein condensation of triplons up to 8 K in Sr3Cr2O8.

Single crystals of the spin dimer system Sr(3)Cr(2)O(8) have been grown for the first time. Magnetization, heat capacity, and magnetocaloric effect data up to 65 T reveal magnetic order between applied fields of H(c1) approximately 30.4 T and H(c2) approximately 62 T. This field-induced order persists up to T(c)(max) approximately 8 K at H approximately 44 T, the highest observed in any quantum magnet where H(c2) is experimentally accessible. We fit the temperature-field phase diagram boundary close to H(c1) using the expression T(c) = A(H-H(c1))(nu). The exponent nu = 0.65(2), obtained at temperatures much smaller than T(c)(max), is that of the 3D Bose-Einstein condensate (BEC) universality class. This finding strongly suggests that Sr(3)Cr(2)O(8) is a new realization of a triplon BEC where the universal regimes corresponding to both H(c1) and H(c2) are accessible at (4)He temperatures.

Dimensional evolution of spin correlations in the magnetic pyrochlore Yb2Ti2O7

The pyrochlore material Yb2Ti2O7 displays unexpected quasi-two-dimensional (2D) magnetic correlations within a cubic lattice environment at low temperatures, before entering an exotic disordered ground state below T=265mK. We report neutron scattering measurements of the thermal evolution of the 2D spin correlations in space and time. Short range three dimensional (3D) spin correlations develop below 400 mK, accompanied by a suppression in the quasi-elastic (QE) scattering below ~ 0.2 meV. These show a slowly fluctuating ground state with spins correlated over short distances within a kagome-triangular-kagome (KTK) stack along [111], which evolves to isolated kagome spin-stars at higher temperatures. Furthermore, low-temperature specific heat results indicate a sample dependence to the putative transition temperature that is bounded by 265mK, which we discuss in the context of recent mean field theoretical analysis.

Bose-Einstein condensation of triplons in Ba3Cr2O8

By performing heat-capacity, magnetocaloric effect, torque magnetometry, and force magnetometry measurements up to 33 T, we have mapped out the