Large Anisotropy Values Research Articles

Virtual excitations and quantum correlations in ultra-strongly coupled harmonic oscillators under intrinsic decoherence

We study the effect of Milburn intrinsic decoherence in a system of ultra-strongly coupled quantum harmonic oscillators, putting emphasis on the virtual excitations of the ground state and their interconnection with quantum correlations like entanglement and steering. For isotropic (equal frequency) oscillators we derive an analytic expression for the common steady state value of virtual excitations and then show analytically and numerically that the influence of intrinsic decoherence can be delayed by increasing the ultra-strong coupling. When increasing the anisotropy in the oscillator frequencies, we numerically observe an asymmetric redistribution of virtual excitations among the oscillators, which leads to an asymmetry in quantum steering. Furthermore, virtual excitations are increased, causing also the strengthening of entanglement and steering immediately after their generation. For large anisotropy values we observe quantum synchronization between the oscillators, with both excitations and quantum correlations being significantly enhanced. When increasing the ultra-strong coupling in the case of large anisotropy, a considerable reinforcement of quantum correlations is observed, over long simulation times and in spite of the presence of decoherence. This is the most important result of this work, since it suggests how to properly design the experimental parameters of the system in order to delay the effect of intrinsic decoherence.

Anisotropy-mediated reentrant localization

We consider a 2d dipolar system, d=2d=2, with the generalized dipole-dipole interaction \sim r^{-a}∼r−a, and the power aa controlled experimentally in trapped-ion or Rydberg-atom systems via their interaction with cavity modes. We focus on the dilute dipolar excitation case when the problem can be effectively considered as single-particle with the interaction providing long-range dipolar-like hopping. We show that the spatially homogeneous tilt \betaβ of the dipoles giving rise to the anisotropic dipole exchange leads to the non-trivial reentrant localization beyond the locator expansion, a<da<d, unlike the models with random dipole orientation. The Anderson transitions are found to occur at the finite values of the tilt parameter \beta = aβ=a, 0<a<d0<a<d, and \beta = a/(a-d/2)β=a/(a−d/2), d/2<a<dd/2<a<d, showing the robustness of the localization at small and large anisotropy values. Both exact analytical methods and extensive numerical calculations show power-law localized eigenstates in the bulk of the spectrum, obeying recently discovered duality a↔2d-aa↔2d−a of their spatial decay rate, on the localized side of the transition, a>a_{AT}a>aAT. This localization emerges due to the presence of the ergodic extended states at either spectral edge, which constitute a zero fraction of states in the thermodynamic limit, decaying though extremely slowly with the system size.

Co nanodot arrays grown on a GdAu2 template: substrate/nanodot antiferromagnetic coupling.

Controlling anisotropy and exchange coupling in patterned magnetic nanostructures is the key for developing advanced magnetic storage and spintronic devices. We report on the antiferromagnetic interaction between a Co nanodot array and its supporting GdAu2 nanotemplate that induces large anisotropy values in individual Co nanodots. In clear contrast with nonmagnetic Au substrates, GdAu2 triggers an earlier switch from out-of-plane anisotropy in monatomic high dots to in-plane when the dot height becomes biatomic.

The Na2X Superalkali Species (X=SH, SCH3, OCH3,CN, N3) as Building Blocks in the Na2XY Salts (Y=MgCl3, Cl, NO2). An Ab Initio Study of the Electric Propertiesof the Na2XY Salts

Vertical and adiabatic ionization potentials of Na2X (X = SH, SCH3, OCH3, CN, and N3) superalkali molecules have been studied using the ab initio methods. The smallest vertical ionization potential was calculated for the Na2OCH3 system (4.365 eV). The Na2X molecules were found to be capable of forming stable [Na2X]+[Y]– salts with the species exhibiting various electron affinities (Y = MgCl3, Cl, NO2). The dipole moments, polarizabilities, and first-order hyperpolarizabilities of the Na2XY ionic salts were calculated and discussed. It was found that the Na2XNO2 molecules possess extraordinary large values of anisotropy of polarizabilities (in the 15.5–28.2 × 10–24 esu range) and the first-order hyperpolarizbilities (spanning the 597.8–1295.7 × 10–30 esu range).

Friedel-oscillations-induced surface magnetic anisotropy

We present detailed numerical studies of the magnetic anisotropy energy of a magnetic impurity near the surface of metallic hosts (Au and Cu) that we describe in terms of the tight-binding surface Green's-function technique. We study the case when spin-orbit coupling originates from the $d$ band of the host material and we also investigate the case of a strong local spin-orbit coupling on the impurity itself. The splitting of the impurity's spin states is calculated to leading order in the exchange interaction between the impurity and the host atoms using the diagrammatic Green's-function technique. The magnetic anisotropy constant is an oscillating function of the separation $d$ from the surface. It asymptotically decays as $\ensuremath{\sim}1/{d}^{2}$ and its oscillation period is determined by the extremal vectors of the host's Fermi surface. Our results clearly show that the host-induced magnetic anisotropy energy is by several orders of magnitude smaller than the anisotropy induced by the local mechanism, which provides sufficiently large anisotropy values to explain the size dependence of the Kondo resistance observed experimentally.

Tricritical transition in the classicalXYmodel on the kagome lattice under local anisotropy

Using mean-field theory and high-resolution Monte Carlo simulation technique based on multihistogram method, we have investigated the critical properties of an antiferromagnetic $XY$ model on the two-dimensional (2D) kagome lattice, with single-ion easy-axes anisotropy. The mean-field theory predicts second-order phase transition from disordered to all-in all-out state for any value of anisotropy for this model. However, Monte Carlo simulations result in first-order transition for small values of anisotropy, which turns to second order with increasing strength of anisotropy, indicating the existence of a tricritical point for this model. The critical exponents, obtained by finite-size scaling methods, show that the transition is in Ising universality class for large values of anisotropy, while the critical behavior of the system deviates from 2D ${\ensuremath{\phi}}^{6}$ model near the tricritical point. This suggests the possibility for existence of another tricritical universality in two dimensions.

On the formation of Widmanstätten ferrite in binary Fe–C – phase-field approach

A phase-field method, based on a Gibbs energy functional, is formulated for γ→ α transformation in Fe–C. The derived phase-field model reproduces the following important types of phase transitions: from C diffusion controlled growth through Widmanstätten microstructures to massive growth without partitioning of C. Applying thermodynamic functions assessed by the Calphad technique and diffusional mobilities available in the literature, we study two-dimensional growth of ferrite side plates emanating from an austenite grain boundary. The morphology of the ferrite precipitates is defined by a highly anisotropic interfacial energy. As large values of anisotropy lead to an ill-posed phase-field equation we present a regularization method capable of circumvent non-differentiable domains of interfacial energy.

Emission Spectral Properties of Cadmium Sulfide Nanoparticles with Multiphoton Excitation.

We describe the luminescence spectral properties of CdS nanoparticles with multiphoton excitation. Three types of CdS nanoparticles were examined which were a CdS/dendrimer composite which displays high anisotropy, Cd2+-enriched nanoparticles which display two emission maxima, and polyphosphate-stabilized nanoparticles which display long wavelength emission. Illumination with long wavelengths near 700-790 nm resulted in two-photon excitation. Essentially the same emission spectra and intensity decays were observed with one-photon and two-photon excitation. Comparison with fluorescein indicates the NPs display large two-photon cross sections near 100 GM. The CdS/dendrimer and Cd2+-enriched CdS nanoparticles displayed large anisotropy values with two-photon excitation, substantially larger than with one-photon excitation. It appears that semiconductor nanoparticles are comparable to organic fluorophores which display the same spectral properties with one-photon and two-photon excitation.

Near-field scanning optical microscopy (NSOM) study of alkyl-substituted polyfluorene films: the affect of alkyl substituent length on nanoscale polymer ordering and cluster formation.

Pristine and Annealed 170 nm films of the stiff-chain polyfluorene, bearing two hexyl (1) or dodecyl (2) groups at the 9 position were studied using Near-field Scanning Optical Microscopy (NSOM). NSOM images reveal two distinct types of nanoscale film morphology: clusters and long-range order (LRO). Dual polarization and dual wavelength fluorescence collection schemes are combined with the use of simple image math provide both qualitative and quantitative characterization. In pristine films, shorter alkyl substituents (1) reduce the solubility of the polyfluorene in the spinning solution used for spin-coating films which leads to the formation of 50-150 nm clusters which are not present in films made from 2. Clusters can be removed by annealing as a function of time. Polarization shows the clusters are amorphous with fluorescence from both intra and inter-polymer species. The second variety of nanoscale ordering, LRO, is characterized by regions of polarized emission in the polymer films. This polarization can be quantified with an anisotropy value and is barely present in pristine films of 1 but increases dramatically upon annealing to induce a liquid crystalline phase transition. Pristine films of 2 have larger anisotropy values than pristine films of 1, due to longer alkyl-substituent-induced ordering during the spin-coating procedure, but films of 2 have only a slight increase in their anisotropy value upon annealing. Both intra and inter-polymer emitting species are distributed homogeneously throughout both pristine and annealed films within the resolution of our microscope (40-70 nm).

Metastable states in the random anisotropy model for amorphous ferromagnets: the absence of ferromagnetism

Following a suggestion by Egami (1979), the author's earlier Monte-Carlo simulation calculations (Harris and Sung, 1978), for the random anisotropy model are shown to be incomplete. Using a modified algorithm, it is shown explicitly that the ferromagnetic (asperomagnetic) state is unstable against the reversal of a small number of spins, and that the energy of the resultant new ground state is significantly less than that of the asperomagnetic state, particularly for large anisotropy values. Thus, these new results appear to conform with the predictions of the renormalisation group calculations of Pelcovitz et al. (1978). Some implications for magnetisation processes and coercive field behaviour in real materials are also discussed.

Galvanomagnetic Effects, Magnetostriction, and Spin-Orbit Interaction in Cu-Ni-Fe and Other Ferromagnetic Nickel Alloys

In the triangular diagram of Cu-Ni-Fe alloys, the line of zero linear magnetostriction ${\ensuremath{\lambda}}_{s}$ approximately coincides with the line of zero extraordinary Hall constant ${R}_{s}$ and may be interpreted as the line where a $3d$ orbital degeneracy crosses the Fermi level. The "ferromagnetic anisotropy of resistivity" $\frac{({\ensuremath{\rho}}_{\mathrm{II}}\ensuremath{-}{\ensuremath{\rho}}_{\ensuremath{\perp}})}{{\ensuremath{\rho}}_{0}}$ of quenched Cu-Ni-Fe alloys containing more than 60 wt% Ni has been measured at 20\ifmmode^\circ\else\textdegree\fi{}K, where impurity scattering dominates. The large anisotropy values observed in Ni-Fe alloys decrease gradually by addition of copper. In particular, the anisotropy decrease along the line ${\ensuremath{\lambda}}_{s}=0$ suggests that the orbital degeneracy is partially lifted by the addition of copper. This line differs from a line of constant electron concentration, indicating a departure from the rigid-band model. A $3d$ band model with separate iron, nickel, and copper sub-bands correctly predicts the location of the line if the degeneracy is assumed to be located at the top of the nickel sub-band (bottom of the iron sub-band). The same assumption applies successfully to Me-Ni-Fe alloys, where Me is any metal (Cr, W, Mo, V, etc.) forming a nonmagnetic sub-band above the Fermi level. Existing data show that the extraordinary Hall conductivity ${\ensuremath{\gamma}}_{{H}_{s}}=\frac{{R}_{s}{M}_{s}}{{\ensuremath{\rho}}^{2}}$ is roughly proportional to the magnetostriction ${\ensuremath{\lambda}}_{s}$ for all fcc Ni-Fe, Cu-Ni, and Cu-Ni-Fe alloys at $T\ensuremath{\ll}{T}_{c}$, with a slope \ensuremath{\approx}2.0\ifmmode\times\else\texttimes\fi{}${10}^{+9}$ mks.