The emission from open cavities with non-integrable features remains a challenging problem of practical as well as fundamental relevance. Square-shaped dielectric microcavities provide a favorable case study with generic implications for other polygonal resonators. We report on a joint experimental and theoretical study of square-shaped organic microlasers exhibiting a far-field emission that is strongly concentrated in the directions parallel to the side walls of the cavity. A semiclassical model for the far-field distributions is developed that is in agreement with even fine features of the experimental findings. Comparison of the model calculations with the experimental data allows the precise identification of the lasing modes and their emission mechanisms, providing strong support for a physically intuitive ray-dynamical interpretation. Special attention is paid to the role of diffraction and the finite side length.
The study of insulator-to-metal transitions is of interest from the viewpoint of fundamental understanding of the underlying physics, and materials at the brink of such transitions possess useful functionality. Driving this transition through compositional tuning can help engineer useful material properties. Here we study the role of disorder in the form of cation off-centering on the compositionallycontrolled insulator-to-metal transition in the solid solution oxide pyrochlore (Pr1−xBix)2Ru2O7. Prior work has established site disorder by the Bi 3+ cations shifting incoherently away from their ideal crystallographic site in the Bi end-member pyrochlore as a consequence of stereochemical activity of the lone pair of electrons. However, less is known about the consequences of such off-centering in solid solutions and its role in determining the electronic ground state. Here we demonstrate through total scattering studies that even a small substitution of Bi on the pyrochlore A site leads to site disorder that enhances the average effective size of the A-site cation. This indirectly increases Ru-O-Ru covalency, which appears to play a crucial role in the cross-over from insulating to metallic behavior in the solid solution. Further, density functional electronic structure calculations suggest the combination of primary and secondary (due to size) electronic effects of the lone pair-driven incoherent cation displacements drive the solid solution into a metallic state.
The arrangement of cations on the triangular pyrochlore lattice leads to a wealth of interesting physical phenomena influenced by geometric frustration. Although uncommon, several pyrochlore materials overcome this frustration and exhibit polar structures. Unraveling the origin of such behavior is key to understanding how broken inversion symmetry arises in complex crystal structures. Here we investigate the effect of varying degrees of covalency in the pyrochlore lattice through a detailed structural and lattice dynamical analysis of the pyrochlore oxysulfide series Cd 2 Nb 2 O 7−x S x above and below the ferroelectric transition temperatures (T C) using synchrotron x-ray diffraction (SXRD) and first principles calculations. All compositions exhibit the cubic F d3m pyrochlore aristotype above T C , whereas the amplitude and character of various structural distortions are found to be composition dependent below T C. For x = 0, large Cd and Nb cation displacements occur to produce the polar Ima2 structure accompanied by a change in translational symmetry. Our symmetry and lattice dynamical calculations indicate Cd 2 Nb 2 O 7 undergoes a proper ferroelectric transition through T C. Analysis of the sulfur-substituted niobates indicates that although the polar space group F dd2 is adopted by the nominal x = 0.25 sample, the transition into the polar phase is improper. For the nominally x = 0.7 composition, the lattice remains nearly cubic, but exhibits a high degree of structural disorder in the pyrochlore channel, with a deviation from the linear Cd-X'-Cd bond by nearly 15 • to accommodate the large size of S while preventing extreme stretching of the Nb-O bond. This highly distorted Cd-X' network is accompanied by a highly distorted NbO 6 network, which is accommodated by the polarizable NbO 6 coordination environment. This sheds light on the limited existence of oxysulfide pyrochlores, for example, the lack of reported S substitution in the case of the similar yet less-polarizable Cd 2 Ta 2 O 7. Our work both provides new understanding of how inversion-symmetry lifting displacements arise and how anion substitution, which tunes covalent cation-anion interactions, is a useful strategy for manipulating polar behavior in the pyrochlore lattice.
The metal−amine complex Co(en) 3 , where en = ethylenediamine, intercalates between layers of cobalt sulfide (CoS) to form a polar, ferromagnetic metal. We solve the structure of the hybrid compound [Co(en) 3 ](CoS) 12 •en in the polar group Pca2 1 with lattice parameters a = 14.778(3) Å, b = 11.066(3) Å, and c = 20.095(5) Å using single-crystal X-ray diffraction. The [Co(en) 3 ] 2+ complexes order between CoS layers and break their inherent fourfold symmetry. Moreover, the chiral Co(en) 3 complexes hydrogen bond to the terminal sulfides of the layers and break inversion symmetry, thereby inducing a polar state. The shortest hydrogen bond of the amino group is H•••S = 2.41(1) Å. From 1.8 to 300 K, the title compound displays metallic electrical resistivity and an anomaly at 43 K. Through magnetization measurements, we find that Co(en) 3 exhibits spontaneous ferromagnetic order below 43 K. First-principles calculations reproduce the ferromagnetic structure and illustrate decoupling between the conducting electrons and the inversion-lifting distortion. Our work shows that hybrid materials created from intercalation chemistry of functional 2D hosts provides a pathway for uniting contraindicated properties.
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