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We propose detailed numerical simulations of a class of material systems that strongly discriminate light based primarily on the angle of incidence, over a broad range of frequencies, and independent of the polarization. Unique properties of these systems emerge from investigating photonic crystals whose constituents have an anisotropic dielectric response.
An external magnetic field can cause a transition in α – RuCl 3 from an ordered zigzag state to a disordered state that is possibly related to the Kitaev quantum spin liquid. Here, we present field-dependent inelastic neutron scattering and magnetocaloric effect measurements implying the existence of an additional transition out of the quantum spin-liquid phase at an upper field limit B u . The neutron scattering reveals three distinct regimes of magnetic response. In the low-field ordered state the response shows magnon peaks; the intermediate-field regime shows only continuum scattering, and above B u the response shows sharp magnon peaks at the lower bound of a strong continuum. Measurable dispersion of magnon modes along the (0,0,L) direction implies non-negligible interplane interactions. Combining the magnetocaloric effect measurements with other data, a T–B phase diagram is constructed. The results constrain the range where one might expect to observe quantum spin-liquid behavior in α – RuCl 3 .
In this work, we study the effect of engineered disorder on the local magnetic response of proximity-coupled superconducting island arrays by comparing scanning superconducting quantum interference device (SQUID) susceptibility measurements to a model in which we treat the system as a network of one-dimensional (1D) superconductor-normal-metal-superconductor Josephson junctions, each with a Josephson coupling energy E-J determined by the junction length or distance between islands. We find that the disordered arrays exhibit a spatially inhomogeneous diamagnetic response which, for low local applied magnetic fields, is well described by this junction network model, and we discuss these results as they relate to inhomogeneous 2D superconductors. Our model of the static magnetic response of the arrays does not fully capture the onset of nonlinearity and dissipation with increasing applied field, as these effects are associated with vortex motion due to the dynamic nature of the scanning SQUID susceptometry measurement. This work demonstrates a model 2D superconducting system with engineered disorder, and it highlights the impact of dissipation on the local magnetic properties of 2D superconductors and Josephson junction arrays.
The band structure of β–LiGaO 2 is calculated using the quasiparticle self-consistent QSGWˆ method where the screened Coulomb interaction Wˆ is evaluated including electron-hole interaction ladder diagrams and G is the one-electron Green's function. Improved convergence compared to previous calculations leads to a significantly larger band gap of about 7.0 eV. However, exciton binding energies are found to be large and lead to an exciton gap of about 6.0 eV if also a zero-point-motion correction of about –0.4 eV is included. Furthermore, these results are in excellent agreement with recent experimental results on the onset of absorption. Besides the excitons observed thus far, the calculations indicate the existence of a Rydberg-like series of exciton excited states, which is however modified from the classical Wannier exciton model by the anisotropies of the material and the more complex mixing of Bloch states in the excitons resulting from the Bethe-Salpeter equation. The exciton fine structure and the exciton wave functions are visualized and analyzed in various ways.
In this work, we measured the local magnetic response of a niobium thin film by applying a millitesla-scale AC magnetic field using a micron-scale field coil and detecting the response with a micron-scale pickup loop in a scanning superconducting quantum interference device (SQUID) susceptometry measurement. Near the film's critical temperature, we observed a steplike nonlinear and dissipative magnetic response due to the dynamics of a small number of vortex-antivortex pairs induced in the film by the local applied AC field. We modeled the dynamics of the measurement using a combined two-dimensional London-Maxwell and time-dependent Ginzburg-Landau approach, allowing us to construct a detailed real-space picture of the vortex motion causing the observed dissipative response. This work pushes scanning SQUID susceptometry of two-dimensional superconductors beyond the regime of linear response and lays the foundation for microscopic studies of vortex dynamics and pinning in superconducting devices and more exotic materials systems.
The magnetotropic susceptibility is the thermodynamic coefficient associated with the rotational anisotropy of the free energy in an external magnetic field and is closely related to the magnetic susceptibility. It emerges naturally in frequency-shift measurements of oscillating mechanical cantilevers, which are becoming an increasingly important tool in the quantitative study of the thermodynamics of modern condensed-matter systems. Here, in this work, we discuss the basic properties of the magnetotropic susceptibility as they relate to the experimental aspects of frequency-shift measurements, as well as to the interpretation of those experiments in terms of the intrinsic properties of the system under study.
The Mott insulator ..alpha..-RuCl3 has generated great interest in the community due to its possible field-induced Kitaev quantum spin liquid state. Despite enormous effort spent trying to obtain the form of the low-energy Hamiltonian, there is currently no agreed upon set of parameters which is able to explain all of the data. A key piece of missing information lies in the determination of the magnetic form factor of Ru3+, particularly for a true quantitative treatment of inelastic neutron scattering data. Here we present the experimentally derived magnetic form factor of Ru3+ in the low spin 4d5 state using polarized neutron diffraction within the paramagnetic regime on high-quality single crystals of ..alpha..-RuCl3. We observe strong evidence of an anisotropic form factor, expected of the spin-orbit coupled jeff=12 ground state. We model the static magnetization density in increasing complexity from simple isotropic cases, to a multipolar expansion, and, finally, ab initio calculations of the generalized jeff=12 ground state. Comparison of both single ion models and inclusion of Cl- anions support the presence of hybridization of Ru3+ with the surrounding Cl- ligands.
The superconducting transition temperature T{sub c} of multilayers of electron-doped cuprates, composed of underdoped (or undoped) and overdoped La{sub 2-x}Ce{sub x}CuO{sub 4} (LCCO) and Pr{sub 2-x}Ce{sub x}CuO{sub 4} (PCCO) thin films, is found to increase significantly with respect to the T{sub c} of the corresponding single-phase films. By investigating the critical current density of superlattices with different doping levels and layer thicknesses, we find that the T{sub c} enhancement is caused by a redistribution of charge over an anomalously large distance.
The nature of a striking pattern of extended defects formed by very high dose implantation of helium in a complex oxide, for example, ferroelectricz-cut LiNbO{sub 3}, is studied. After irradiation, a high concentration of defects is found to collect and create a network of thick prismatic planar defects, which have typical dimensions of {approx}1.5 {micro}m and 200 nm parallel and perpendicular to the z axis, respectively. This defect network and its effect on the lattice were studied using a set of x-ray and electron microscopy probes of the lattice structure and spatial variation. Optical microscopy shows that there is strong temperature dependence for forming the network; the density of these extended defects reaches a maximum value for an annealing temperature of 250 C but is fully eliminated by a temperature of 380 C. High-resolution transmission electron microscopy studies indicate that these extended defects are probably localized twinning and dislocation pileups due to plastic deformation of the lattice to relieve He-implantation-induced stress. During this deformation, He accumulates at the twin boundaries. The study also shows that the He interstitials evolve into bubbles causing high stress and resulting in a formation of thick prismatic planar defects. Finally, a mechanism is proposed for defect creation and elimination.
LaAlO{sub 3} epitaxial films with La:Al cation ratios ranging from 0.9 to 1.2 were grown on TiO{sub 2}-terminated SrTiO{sub 3} (001) substrates by off-axis pulsed laser deposition. Although all films are epitaxial, rocking curve measurements show that the crystallographic quality degrades with increasing La:Al ratio. Films with La:Al ratios of 0.9, 1.0, and 1.1 were coherently strained to the substrate. However, the out-of-plane lattice parameter increases over this range, revealing a decrease in film tetragonality. Although all film surfaces exhibit hydroxylation, the extent of hydroxylation is greater for the La-rich films. Rutherford backscattering spectrometry reveals that La from the film diffuses deeply into the SrTiO{sub 3} substrate and secondary-ion-mass spectroscopy shows unambiguous Sr outdiffusion into the films.
Low-temperature magnetic and structural behavior of the extended Kagome system YBaCo{sub 4}O{sub 7} has been studied by single-crystal neutron diffraction and high-resolution powder x-ray diffraction. Long-range magnetic ordering associated with a structural transition from orthorhombic Pbn2{sub 1} to monoclinic P2{sub 1} symmetry has been found at T{sub 1}-100 K. The interplay between the structural and magnetic degrees of freedom indicates that the degeneracy of the magnetic ground state, present in the orthorhombic phase, is lifted through a strong magnetoelastic coupling, as observed in other frustrated systems. At T{sub 2}-60 K, an additional magnetic transition is observed, though isosymmetric. Models for the magnetic structures below T{sub 1} and T{sub 2} are presented, based on refinements using a large number of independent reflections. The results obtained are compared with previous single-crystal and powder-diffraction studies on this and related compositions.
The linear thermal expansion coefficient of diamond crystals of type IIa and type Ia was measured in the temperature range from 10 to 295 K. Neither negative thermal expansion nor any substantial difference in the thermal expansion coefficient in crystals of the different types were observed. An empirical expression was obtained that approximates the temperature dependence of the thermal expansion coefficient of diamond. The T{sup 3} temperature dependence of a Debye solid holds below {approx}100 K with an accuracy of {approx}10{sup -8} K{sup -1}. A slight increase in the value of the lattice parameter was found for the Ia-type crystal, which suggests lattice dilatation by nitrogen impurity. The measurements were performed using Bragg diffraction in backscattering from diamond crystals of highly monochromatic 23.7 keV x rays with the recently demonstrated high relative accuracy of 1.2 x 10{sup -8} in the determination of the lattice parameter [S. Stoupin and Yu. Shvyd'ko Phys. Rev. Lett. 104 085901 (2010)].
We performed an x-ray spectroscopic study combining resonant x-ray emission spectroscopy (RXES) and photoelectron spectroscopy on the superconducting ternary silicide YbGa 1.15 Si 0.85 and nonsuperconducting ternary germanide YbGa x Ge 2-x (x = 1.0 and 1.1). The Yb valence for all three compounds is found to be about 2.3. In YbGa 1.15 Si 0.85 no temperature dependence of the Yb valence is observed in the RXES spectra in the temperature range of 7–300 K, while the valence shows a drastic increase under pressure from the Yb 2+ state partially including itinerant electrons to the localized Yb 3+ state. Differences are observed in the valence-band spectra of the photoelectron spectroscopy between YbGa 1.15 Si 0.85 and YbGa x Ge 2-x , which may be attributed to the difference of crystal structure. We conclude that both the crystal structure of the planar GaSi layer in YbGa 1.15 Si 0.85 and the resultant electronic structure may have a crucial role in the occurrence of superconductivity.
Oxygen octahedral rotations have been measured in short-period (LaNiO{sub 3}){sub n}/(SrMnO{sub 3}){sub m} superlattices using synchrotron diffraction. The in-plane and out-of-plane bond angles and lengths are found to systematically vary with superlattice composition. Rotations are suppressed in structures with m > n, producing a nearly unrotated form of LaNiO{sub 3}. Large rotations are present in structures with m < n, leading to reduced bond angles in SrMnO{sub 3}. The metal-oxygen-metal bond lengths decrease as rotations are reduced, in contrast to behavior previously observed in strained, single-layer films. This result demonstrates that superlattice structures can be used to stabilize nonequilibrium octahedral behavior in a manner distinct from epitaxial strain, providing a novel means to engineer the electronic and ferroic properties of oxide heterostructures.
The structural transformations induced in Gd{sub 2}Ti{sub 2}O{sub 7} single crystals irradiated at high energies (870-MeV Xe), where ionization processes (electronic stopping) dominate, and at low energies (4-MeV Au), where ballistic processes (nuclear stopping) dominate, have been studied via the combination of Rutherford backscattering spectrometry and channeling (RBS/C), Raman spectroscopy, and transmission electron microscopy (TEM) experiments. At high energy, amorphization occurs directly in individual ion tracks from the extreme electronic-energy deposition, and full amorphization results from the overlapping of these tracks as described by a direct impact model. The track diameters lie in the range 6-9 nm. At low energy, amorphization occurs via indirect processes, driven by ballistic nuclear energy deposition from the ions, that is accounted for in the framework of both direct-impact/defect-stimulated and multi-step damage accumulation models. The ion fluence for total amorphization of the irradiated layer is much higher at low energy (0.5 ion nm{sup -2}) than at high energy (0.05 ion nm{sup -2}), consistent with the nuclear stopping at low energy (5.2 keV/nm) compared to the electronic stopping at high energy (29 keV/nm).
Epitaxial films along the Fe3-xTixO4 (titanomagnetite) compositional series from pure end-members magnetite (Fe3O4) to ulvöspinel (Fe2TiO4) were successfully grown by pulsed laser deposition on MgO(100) substrates. Spectroscopic characterization including high resolution x-ray diffraction, x-ray photoelectron spectroscopy, and synchrotron-based x-ray absorption and magnetic circular dichroism consistently shows that Ti(IV) substitutes for Fe(III) in the inverse spinel lattice with a proportional increase in lattice Fe(II) concentration. No evidence of Ti interstitials, spinodal decomposition, or secondary phases was found in the bulk of the grown films. At the uppermost few nanometers of the Ti-bearing film surfaces, evidence suggests that Fe(II) is susceptible to facile oxidation, and that an associated lower Fe/Ti ratio in this region is consistent with surface compositional incompleteness or alteration to a titanomaghemite-like composition and structure. The surface of these films nonetheless appear to remain highly ordered and commensurate with the underlying structure despite facile oxidation, a surface condition that is found to be reversible to some extent by heating in low oxygen environments.