Orthorhombic C2v Research Articles

Optically modulated magnetic resonance of erbium implanted silicon

Er implanted Si is a candidate for quantum and photonic applications; however, several different Er centres are generated, and their symmetry, energy level structure, magnetic and optical properties, and mutual interactions have been poorly understood, which has been a major barrier to the development of these applications. Optically modulated magnetic resonance (OMMR) gives a spectrum of the modulation of an electron paramagnetic resonance (EPR) signal by a tuneable optical field. Our OMMR spectrum of Er implanted Si agrees with three independent measurements, showing that we have made the first measurement of the crystal field splitting of the 4I13/2 manifold of Er implanted Si, and allows us to revise the crystal field splitting of the 4I15/2 manifold. This splitting originates from a photoluminescence (PL) active O coordinated Er centre with orthorhombic C2v symmetry, which neighbours an EPR active O coordinated Er centre with monoclinic C1h symmetry. This pair of centres could form the basis of a controlled NOT (CNOT) gate.

Tetrahedral Order in Liquid Crystals

We review the impact of tetrahedral order on the macroscopic dynamics of bent-core liquid crystals. We discuss tetrahedral order comparing with other types of orientational order, like nematic, polar nematic, polar smectic, and active polar order. In particular, we present hydrodynamic equations for phases, where only tetrahedral order exists or tetrahedral order is combined with nematic order. Among the latter we discriminate between three cases, where the nematic director (a) orients along a 4-fold, or (b) along a 3-fold symmetry axis of the tetrahedral structure. For the optically isotropic T_d phase, which only has tetrahedral order, we focus on the coupling of flow with e.g. temperature gradients and on the specific orientation behavior in external electric fields. For the transition to the nematic phase, electric fields lead to a temperature shift that is linear in the field strength. Electric fields induce nematic order, again linear in the field strength. If strong enough, electric fields can change the tetrahedral structure and symmetry leading to a polar phase. We briefly deal with the T phase that arises when tetrahedral order occurs in a system of chiral molecules. To case (a) belong (i) the non-polar, achiral, optically uniaxial D2d phase with ambidextrous helicity and with orientational frustration in external fields, (ii) the non-polar tetragonal S4 phase, (iii) the non-polar, orthorhombic D2 phase that is structurally chiral featuring ambidextrous chirality, (iv) the polar orthorhombic C2v phase, and (v) the polar monoclinic C2 phase. Case (b) results in a trigonal C3v phase that behaves like a biaxial polar nematic phase. Finally we discuss some experiments that show typical effects related to the existence of tetrahedral order.

Room-temperature single phase multiferroic magnetoelectrics: Pb(Fe, M)x(Zr,Ti)(1−x)O3 [M = Ta, Nb

We describe extensive studies on a family of perovskite oxides that are ferroelectric and ferromagnetic at ambient temperatures. The data include x-ray diffraction, Raman spectroscopy, measurements of ferroelectric and magnetic hysteresis, dielectric constants, Curie temperatures, electron microscopy (both scanning electron microscope and transmission electron microscopy (TEM)) studies, and both longitudinal and transverse magnetoelectric constants α33 and α31. The study extends earlier work to lower Fe, Ta, and Nb concentrations at the B-site (from 15%–20% down to 5%). The magnetoelectric constants increase supralinearly with Fe concentrations, supporting the earlier conclusions of a key role for Fe spin clustering. The room-temperature orthorhombic C2v point group symmetry inferred from earlier x-ray diffraction studies is confirmed via TEM, and the primitive unit cell size is found to be the basic perovskite Z = 1 structure of BaTiO3, also the sequence of phase transitions with increasing temperature from rhombohedral to orthorhombic to tetragonal to cubic mimics barium titanate.

Crystal symmetry breaking of wurtzite to orthorhombic in nonpolar a-ZnO epifilms

Crystal symmetry breaking of wurtzite C6V to orthorhombic C2V due to in-plane anisotropic strain was investigated for nonpolar (112¯0) ZnO epifilms grown on the R-sapphire. X-ray diffraction results reveal the epilayer is subjected to a compressive strain along the polar c-axis and tensile strains along both a-[112¯0] surface normal and in-plane p-[11¯00] axis. The polarized Raman spectra of E2 modes reveal violation of the C6V selection rules. Oppositely, the C2V configuration satisfies the selection rules for the Raman modes. The observed E1 and E2 bands in polarized optical reflection and photoluminescence spectra confirm the anisotropic strain causes the structure change to the orthorhombic one.

Electron spin resonance of Er–oxygen complexes in GaAs grown by metal organic chemical vapor deposition

We have performed electron spin resonance (ESR) measurements on Er-doped GaAs grown with oxygen codoping by metal organic chemical vapor deposition. An isotropic line (an effective g value, g=5.95) which had been already reported was observed in samples without oxygen codoping. On the other hand, for samples with oxygen codoping other strong anisotropic ESR lines originated from four kinds of Er3+(4f11) centers (A, B, C, and D) were newly observed in addition to the weaker isotropic line. The anisotropic g tensors obtained by analyzing the angular dependence of the ESR lines indicate that B and C centers are of orthorhombic C2v symmetry, A center has lower symmetry than orthorhombic symmetry, and D center is of trigonal C3i symmetry. The ESR intensities of A, B, and C centers were approximately two orders of magnitude higher than that of the isotropic line with g=5.95. The ESR intensity of D center was one order of magnitude lower than those of A, B, and C. The Er concentration dependence of the relative ESR intensities of these centers was investigated, which indicates (i) the ESR intensities of A and D increase with increasing Er concentration, and (ii) those of B and C are saturated above the Er concentration [Er]⩾1018 cm−3. The ESR measurement under light illumination, as well as the Er concentration dependence, suggests that the B center with C2v symmetry corresponds to the dominant Er luminescent center under host photoexcitation.

Phonon spectra and unit cell analysis of 2‐cyanothiophene

AbstractVibrational spectra of 2‐cyanothiophene in the solid state reveal that the crystals are ordered below the freezing point and have an orthorhombic C2v unit cell. Intermolecular forces between the two molecules occupying the Bravais unit are not strong enough to promote the separation of all the Davidov doublets of the phonon states.

Analysis of the spin–Hamiltonian parameters for Mn2+ in orthorhombic and axial symmetry sites of the alkali chlorides. Binding energies of the Mn2+ cation vacancy complexes

The superposition model is used to analyze for the first time the second-order spin–Hamiltonian parameters b02 and b22 for the Mn2+ ion in orthorhombic C2v symmetry sites of the alkali chlorides LiCl, NaCl, KCl, and RbCl. Values for the intrinsic parameter ?2 and for the power-law exponent t2 are reported for the divalent manganese ion surrounded by chlorine ligands. In order to test the applicability of the superposition model a calculation was made of the quadrupole b02 term describing the 6S5/2 ground state splitting of the Mn2+ ion in axial symmetry sites of the alkali chlorides, using the intrinsic parameters ?2 and the power-law exponent t2 previously obtained from the analysis of the spin–Hamiltonian parameters corresponding to the orthorhombic symmetry site. In order to carry out the analysis mentioned above, a theoretical calculation was made of the lattice distortion around the impurity ion caused by its substitution for the host alkali ion in all the alkali halides. Values for the binding energy of the ground and first excited states of the impurity Mn2+ -cation vacancy pair are also reported.

Analysis of the spin-Hamiltonian parameters for Eu2+ in the alkali halides

An analysis is made, in terms of the superposition model, of the second- and fourth-order spin-Hamiltonian parameters for the Eu2+ ion in orthorhombic C2v symmetry sites of the alkali halides. Values for the intrinsic parameters b̄2 and b̄4 and for the power law exponents t2 and t4 are reported for the divalent europium ion surrounded by fluorine, chlorine, bromine, and iodine ions as ligands. It is found that the b̄4 value of Eu2+ decreases as the polarizability of the ligand increases in agreement with the results previously reported for the isomorphic Gd3+ ion and for the transition metal ions Mn2+ and Fe3+. It is established, for the first time, that the value for the intrinsic parameter b̄2 of Eu2+ also decreases as the polarizability of the ligand increases. In order to carry out the analysis mentioned above, a theoretical calculation was made of the lattice distortion around the impurity ion caused by its substitution for the host alkali ion in all the alkali halides.