In-plane Principal Axes Research Articles

Anisotropic Infrared Response and Orientation-Dependent Strain-Tuning of the Electronic Structure in Nb2SiTe4

Two-dimensional materials with tunable in-plane anisotropic infrared response promise versatile applications in polarized photodetectors and field-effect transistors. Black phosphorus is a prominent example. However, it suffers from poor ambient stability. Here, we report the strain-tunable anisotropic infrared response of a layered material Nb2SiTe4, whose lattice structure is similar to the 2H-phase transition metal dichalcogenides (TMDCs) with three different kinds of building units. Strikingly, some of the strain-tunable optical transitions are crystallographic axis-dependent, even showing an opposite shift when uniaxial strain is applied along two in-plane principal axes. Moreover, G0W0-BSE calculations show good agreement with the anisotropic extinction spectra. The optical selection rules are obtained via group theory analysis, and the strain induced unusual shift trends are well explained by the orbital coupling analysis. Our comprehensive study suggests that Nb2SiTe4 is a good candidate for tunable polarization-sensitive optoelectronic devices.

An open-endcap blade trap for radial-2D ion crystals

We present the design and experimental demonstration of an open-endcap radio frequency trap to confine ion crystals in the radial-two dimensional (2D) structural phase. The central axis of the trap is kept free of obstructions to allow for site-resolved imaging of ions in the 2D crystal plane, and the confining potentials are provided by four segmented blade electrodes. We discuss the design challenges, fabrication techniques, and voltage requirements for implementing this open-endcap trap. Finally, we validate its operation by confining up to 29 ions in a 2D triangular lattice, oriented such that both in-plane principal axes of the 2D crystal lie in the radial direction.

Disorder-induced suppression of charge density wave order: STM study of Pd-intercalated ErTe3

Pd-intercalated ${\mathrm{ErTe}}_{3}$ is studied as a model system to explore the effect of increasing disorder on an incommensurate two-component charge density wave (CDW). The ordering vectors of the CDW components lie along the two in-plane principal axes of the nearly tetragonal crystal structure. Using scanning tunneling microscopy (STM), we show that introducing Pd intercalants (i.e., disorder) induces CDW dislocations, which appear to be associated with each CDW component separately. Increasing Pd concentration has a stronger effect on the secondary CDW order, manifested in a higher density of dislocations, and thus increases the anisotropy (nematic character) of the CDW. Suggestive evidence of Bragg glass phases at weak disorder is also discussed.

Scaling Behavior of Individual Nanoparticle Plasmon Resonances

By using experiments and an intuitive model, this study reports on the dependence of different dipole resonances in gold nanoparticles (NPs) with rectangular, elliptical, and diamond-like footprints on the length of the three principal axes and on particle geometry. The length of the two in-plane principal axes of the NPs studied is between 50 and 300 nm, while the particle height is between 10 and 50 nm. The scattering experiments reveal characteristic dependencies of the in-plane dipolar resonances on axis lengths and particle geometry. These experimental findings are discussed according to an intuitive resonance condition based on the electrostatic eigenmode method extended to include terms for retardation (dynamic depolarization) and particle curvature. Focus is laid on the dependence of the retardation term on the relevant depolarization factor, the contributions of static and dynamic depolarization to the resonance wavelength, and to a generalization of the depolarization factors that describe the s...

Anomalous anticrossing of neutral exciton states in GaAs/AlGaAs quantum dots

We study the effects of heavy hole--light hole (HH-LH) mixing on fine-structure and polarization properties of neutral excitons (${\text{X}}^{0}$) confined in single GaAs/AlGaAs quantum dots (QDs) under the application of anisotropic biaxial stress. In the large HH-LH mixing regime, these properties are substantially different from the usually observed properties in the case of small or no mixing. By varying the applied stress, the mixing in the initially strain-free QDs changes from $\ensuremath{\sim}$0 to $\ensuremath{\sim}$70$%$ and an anomalous anticrossing of the ${\text{X}}^{0}$ bright states is observed. The latter is attributed to stress-induced rotation of the in-plane principal axis of the QD confinement potential. We show that the analysis of free-excitonic emission of bulk GaAs surrounding the QDs not only allows estimation of the stress and mixing in the QDs, but also provides the quantum-confinement-induced HH-LH splitting of the as-grown QDs.

Multi-step etching of three-dimensional sub-millimeter curved silicon microstructures with in-plane principal axis

We report a micromachining method for batch fabrication of three-dimensional curved microstructures exhibiting in-plane principal axis with respect to the wafer substrate. The target profile in the out-of-plane direction is controlled by a multi-step technique involving etching and protection alternatively with pre-designed timing of each step. The versatility of the method is studied theoretically and found to be suitable for curved microstructure realization with height-to-radius-of-curvature ratios in the order of 0.25 to 1. The method is applied experimentally on silicon substrates and an inductively coupled plasma reactor is used to carry out the multiple steps of etching and protection without human interruption. Three-dimensional curved microstructures with sub-millimeter radii of curvature are successfully demonstrated.

Photoinduced reordering in thin azo-dye films and light-induced reorientation dynamics of the nematic liquid-crystal easy axis

We theoretically study the kinetics of photoinduced reordering triggered by linearly polarized (LP) reorienting light in thin azo-dye films that were initially illuminated with LP ultraviolet pumping beam. The process of reordering is treated as a rotational diffusion of molecules in the light intensity-dependent mean-field potential. The two-dimensional diffusion model which is based on the free energy rotational Fokker-Planck equation and describes the regime of in-plane reorientation is generalized to analyze the dynamics of the azo-dye order parameter tensor at varying polarization azimuth of the reorienting light. It is found that, in the photosteady state, the intensity of LP reorienting light determines the scalar order parameter (the largest eigenvalue of the order parameter tensor), whereas the steady state orientation of the corresponding eigenvector (the in-plane principal axis) depends solely on the polarization azimuth. We show that, under certain conditions, reorientation takes place only if the reorienting light intensity exceeds its critical value. Such threshold behavior is predicted to occur in the bistability region provided that the initial principal axis lies in the polarization plane of reorienting light. The model is used to interpret the experimental data on the light-induced azimuthal gliding of the liquid-crystal easy axis on photoaligned azo-dye substrates.

Wave propagation along a non-principal direction in a compressible pre-stressed elastic layer

The dispersive behavior of small amplitude waves propagating along a non-principal direction in a pre-stressed, compressible elastic layer is considered. One of the principal axes of stretch is normal to the elastic layer and the direction of propagation makes an angle θ with one of the in-plane principal axes. The dispersion relations which relate wave speed and wavenumber are obtained for both symmetric and anti-symmetric motions by formulating the incremental boundary value problem for a general strain energy function. The behavior of the dispersion curves for symmetric waves is for the most part similar to that of the anti-symmetric waves at the low and high wavenumber limits. At the low wavenumber limit, depending on the pre-stress and propagation angle, it may be possible for both the fundamental mode and the next lowest mode to have finite phase speeds, while other higher modes have an infinite phase speed. At the high wavenumber limit, the phase speeds of the fundamental mode and the higher modes tend to the Rayleigh surface wave speed and the limiting wave speeds of the layer, respectively. Numerical results are presented for a Blatz–Ko material and the effect of the propagation angle is clearly illustrated.

Incommensurate transverse anisotropy induced by disorder and spin-orbit-vibron coupling in Mn12 acetate

It has been shown within density-functional theory that in Mn12 acetate there are effects due to disorder by solvent molecules and a coupling between vibrational and electronic degrees of freedom. We calculate the in-plane principal axes of the second-order anisotropy caused by the second effect and compare them with those of the fourth-order anisotropy due to the first effect. We find that the two types of the principal axes are not commensurate with each other, which results in a complete quenching of the tunnel-splitting oscillation as a function of an applied transverse field.

Numerical simulation of ZnSe/GaAs interface reflectance difference spectroscopy

A numerical method based on the matrices established by Yeh [Optical Waves in Layered Media (Wiley, New York, 1988)] is developed to simulate the non-normal incidence reflectance difference spectroscopy (RDS) spectra of biaxial anisotropic (εx≠εy≠εz) multilayer systems. The main features of the RDS spectra obtained from the biaxial anisotropic ZnSe/GaAs interface are reproduced by the numerical method. It has demonstrated that in the cases of near-normal incidence and when the anisotropy within the layer plane (in-plane anisotropy) is small (εx−εy≪εx) a RDS spectrum can be separated into two spectra, namely, the in-plane anisotropic spectrum and the off-plane anisotropic spectrum. The reflectance of the s wave and the p wave can be calculated separately when the in-plane principal axes are at certain orientations, making it possible to obtain the anisotropic dielectric tensor directly from the measured spectra.

On interfacial waves in pre-stressed layered incompressible elastic solids

Interfacial waves along the plane boundary between two pre-stressed incompressible elastic solids are considered. One of the solids is a half-space while the other has arbitrary uniform thickness. The principal axes of the underlying pure homogeneous deformation in the two solids are aligned, with one axis normal to the interface. For propagation along an in-plane principal axis, the dispersion equation is derived in respect of a general strain-energy function. Conditions on the pre-strain, pre-stress and material parameters that ensure the existence of a unique interfacial wavespeed at low frequencies are obtained, and it is shown that, in special circumstances, non-dispersive waves can exist at the low-frequency limit. Asymptotic results at the high-frequency limit are also obtained. For the case of equibiaxial pre-strain, more specific conditions are derived for the existence of interfacial waves at the low- and high-frequency asymptotes, and these provide information on the existence of waves for the whole frequency range. A particular feature of the structure considered is that it may act as a mechanical filter in different frequency regimes depending on the pre-strain, pre-stress and material parameters. When the wavespeed vanishes, the dispersion equation reduces to a bifurcation equation, solutions of which define states of stress and deformation which form boundaries of the region of stability of the underlying state of stress and deformation in the two materials for given material properties. The bifurcation equation is examined separately and an explicit bifurcation criterion is given for equibiaxial deformations. The results are illustrated graphically by considering several numerical examples based on a certain class of strain-energy functions, which includes the neo-Hookean strain-energy function. The results highlight low- and high-frequency features and demonstrate the influence of pre-stress and deformation on the multiplicity of propagating modes.

Measurement of birefringence for optical recording of disk substrates

We experimentally investigate the birefringence of bare and coated substrates for magneto-optical recording, using ellipsometry at wavelengths of 632.8 and 780 nm. The polarization rotation and ellipticity of reflected or transmitted light are measured for different incident angles and for different orientations of the incident linear polarization. The measured data are then fitted by the MULTILAYER computer program, which solves the Maxwell equations for a plane wave propagating in a multilayer structure. This approach makes it possible to determine, with high accuracy, the orientations of the principal axes of the substrate and the corresponding refractive indices. The results show that one of the principal axes is always along the substrate's normal direction, but the orientations of the in-plane principal axes can be much different from the radial and track directions. A special feature of the ellipsometers that were used is that a glass hemisphere is placed in contact with the substrate to eliminate refraction of the incident beam. This enables a maximum propagation angle of 70° (with respect to the normal) in the substrate and hence increases the measurement sensitivity. Certain anomalies have been observed, which we believe are associated with the variation of birefringence properties along the thickness direction.

Optically detected proton and nitrogen ENDOR in the3Eustate of zinc porphine in ann-octane single crystal

Via optically detected ENDOR the hyperfine tensors of the protons, and partially of the nitrogen atoms, of zinc porphine in its 3 Eu state have been obtained. The results show that the in-plane principal axes of the fine-structure tensor make an angle of 7·8 ± 1·1° with the N-N symmetry axes. The spin densities are evaluated from the hyperfine data in a point-dipole model; a nuclear distortion of b1g symmetry has to be included in order to arrive at a consistent interpretation. The spin densities are compared with calculations and confirm a (eg ← a 2u ) orbital configuration for ZnP in its 3 Eu state.

Evolutionary change of the heme C electronic structure: Ferricytochrome c-551 from [formula omitted] and horse heart ferricytochrome c

Individual assignments of the 1H n.m.r. lines of heme c in reduced and oxidized cytochrome c-551 from Pseudomonas aeruginosa were obtained by nuclear Overhauser enhancement and saturation transfer experiments. Comparison with the corresponding data on horse heart cytochrome c showed that the locations of high spin density on the heme c periphery as well as the in-plane principal axes x and y of the electronic g-tensor are rotated by approximately 90° in ferricytochrome c-551 relative to horse ferricytochrome c. High spin density in ferricytochrome c-551 is thus localized on the pyrrole ring III. While this pyrrole ring is well shielded in the interior of mammalian-type cytochromes c, it is more easily accessible in cytochrome c-551. It is suggested that this evolutionary change of the heme c electronic structure would be compatible with the hypothesis that the electron transfer in both species is via solvent exposed peripheral ring carbon atoms.

The phosphorescent triplet state of p-xylene in an isotopically mixed crystal at 1·2K: an investigation by E.S.R. and MIDP

The phosphorescence spectrum of p-xylene-h 10 in a p-xylene-d 10 host is first presented. Secondly we report E.S.R. experiments performed on p-xylene-α,α′-d 6 in p-xylene-d 10; from an analysis of the orientations of the principal axes in combination with the crystal structure and of the hyperfine splittings it is concluded that the in-plane principal axes of the zfs tensor make an angle of 68° with the molecular axes. Thirdly we discuss Microwave-Induced Delayed Phosphorescence (MIDP) experiments on p-xylene-h 10 in p-xylene-d 10 in which the relative radiative rate constants for decay from the zero-field spin components of the triplet state to single vibronic bands of the ground state have been obtained. The results of the MIDP and E.S.R. measurements are shown to be mutually consistent in terms of a model in which the effect of the crystal field and the methyl substituents on the pseudo-Jahn-Teller unstable electronic structure of the triplet state of benzene is described.