Neutral Spin Research Articles

Temperature Dependence of Interactions between Stable Piperidine‐1‐yloxyl Derivatives and an Ionic Liquid

2,2,6,6-Tetramethylpiperidine-1-yloxyl derivatives substituted with either hydrogen bonding [-OH, -OSO(3)H] or ionic [-OSO(3) (-)Na(+), -OSO(3) (-)K(+), N(+)(CH(3))(3)I(-), N(+)(CH(3))(3) N(-)(SO(2)-CF(3))(2)] substituents are investigated in 1-butyl-3-methylimidazolium tetrafluoroborate over a wide temperature range covering both glassy and viscous states. The Vogel-Fulcher-Tammann equation describes the temperature dependence of the ionic liquid viscosity. Quantum chemical calculations of the spin probes at the UB3LYP/6-311(2d,p++) level are done to describe the dependence of the spin density on nitrogen on the substitution pattern of the 4-position of the probe. The results of these calculations are also used to understand the experimental results obtained by applying the Spernol-Gierer-Wirtz theory to analyze the viscosity dependence of the rotational correlation time of the spin probes. Significant differences are found between 2,2,6,6-tetramethylpiperidine-1-yloxyl and its derivatives containing substituents that are able to form hydrogen bonds with the ionic liquid. Moreover, derivatives substituted with ionic groups at the 4-position have a large effect on temperature-induced solvent viscosity, as this is particularly dependent on the nature of the substituent at the 4-position. These dependencies include the temperature region that can be used to describe interactions between the spin probes and the ionic liquid, diffusion into the free volume during non-activated (neutral spin probes) and activated (charged spin probes) processes. Additional parameters are the radii of the ionic liquid and the spin probes, which are calculated and measured approximately. In addition, the temperature dependence of the isotropic hyperfine coupling constants of the spin probes results in information about the micropolarity of the ionic liquid. At room temperature, this is comparable to that of the solvent dimethylsulfoxide.

Single-spin optical read-out in CdTe/ZnTe quantum dot studied by photon correlation spectroscopy

Received 9 February 2008; revised manuscript received 5 May 2008; published 5 June 2008Spin dynamics of a single electron and an exciton confined in CdTe/ZnTe quantum dot is investigated bypolarization-resolved correlation spectroscopy. Spin memory effects extending over at least a few tens ofnanoseconds have been directly observed in magnetic field and described quantitatively in terms of a simplerate equation model. We demonstrate an effective 68% all-optical read-out of the single carrier spin statethrough probing the degree of circular polarization of exciton emission after the capture of an oppositelycharged carrier. The perturbation introduced by the pulsed optical excitation serving to study the spin dynamicshas been found to be the main source of the polarization loss in the read-out process. In the limit of low laserpower, the read-out efficiency extrapolates to a value close to 100%. The measurements allowed us as well todetermine neutral exciton spin relaxation time ranging from 3.4 0.1 ns at

Spin Texture and Magnetoroton Excitations atν=1/3

Neutral spin texture (ST) excitations at nu=1/3 are directly observed for the first time by resonant inelastic light scattering. They are determined to involve two simultaneous spin flips. At low magnetic fields, the ST energy is below that of the magnetoroton minimum. With increasing in-plane magnetic field these mode energies cross at a critical ratio of the Zeeman and Coulomb energies of eta(c)=0.020+/-0.001. Surprisingly, the intensity of the ST mode grows with temperature in the range in which the magnetoroton modes collapse. The temperature dependence is interpreted in terms of a competition between coexisting phases supporting different excitations. We consider the role of the ST excitations in activated transport at nu=1/3.

Spinons and triplons in spatially anisotropic frustrated antiferromagnets

The search for elementary excitations with fractional quantum numbers is a central challenge in modern condensed-matter physics. It has long been speculated that two-dimensional frustrated magnets might support quantum disordered states with neutral spin-1/2 excitations known as spinons. Despite decades of search, however, no clear experimental examples have been found. We explore the possibility for several materials using a realistic model, the spin-1/2 spatially anisotropic frustrated Heisenberg antiferromagnet in two dimensions. Here, we derive an effective Schrodinger equation valid in the weak interchain coupling regime. The dynamical spin correlations from this approach agree quantitatively without fitting parameters with inelastic neutron measurements of the triangular antiferromagnet Cs2CuCl4. In such antiferromagnets, the spectrum is composed of an incoherent continuum arising from the effects of one-dimensional spinons of individual chains, and a sharp dispersing peak, due to coherently propagating ‘triplon’ bound states of two spinons. We argue that triplons are generic features of spatially anisotropic frustrated antiferromagnets, which arise because the bound spinon pair lowers its kinetic energy by propagating between chains.

Radical ions from and spin adducts to 1,2-dimethyl-3,4-bis[(2,4,6-tri-tert-butylphenyl) phosphinidene]-1-cyclobutene

The radical cation and anion of the title compound were obtained either by treatment with mercury trifluorocatetate in trifluoroacetic acid or by reduction with potassium in tetrahydrofurane. For both species the ESR spectra indicated magnetic non-equivalence of the two phosphorus atoms, as well as of the two methyl groups. Spectra attributed to the corresponding spin adducts were also observed when reacting the title compound with in situ-generated alkoxyl and thiyl radicals. MO calculations carried out for both the ionic radicals and the neutral spin adducts led to questioning the actual nature of some of the latter species.

Lyman-alpha radiative transfer during the epoch of reionization: contribution to 21-cm signal fluctuations

During the epoch of reionization, Ly-α photons emitted by the first stars can couple the neutral hydrogen spin temperature to the kinetic gas temperature, providing an opportunity to observe the gas in emission or absorption in the 21-cm line. Given the bright foregrounds, it is particularly important to determine the fluctuation signature of the signal precisely, so as to be able to extract it by its correlation power. LICORICE is a Monte-Carlo radiative transfer code, coupled to the dynamics via an adaptative Tree-SPH code. We present here the Ly-α part of the implementation and validate it through three classical tests. Unlike previous works, we do not assume that , the number of scatterings of Ly-α photons per atom per second, is proportional to the Ly-α background flux, but take the scatterings in the Ly-α line wings into account. The latter have the effect of steepening the radial profile of around each source, and re-inforce the contrast of the fluctuations. In the particular geometry of cosmic filaments of baryonic matter, photons are scattered out of the filament, and the large-scale structure of is significantly anisotropic. This could have strong implications for the possible detection of the 21-cm signal.

EPR Study of the Interactions between Dendrimers and Peptides Involved in Alzheimer's and Prion Diseases

Spin-probe and spin-label techniques were used to study the interactions of the Abeta 1-28 peptide involved in Alzheimer disease and the PrP 106-126 peptide suspected to be preferentially involved in spongiform encephalopathies with three different types of dendrimers. A computer-aided EPR analysis of a positively charged and a neutral spin probe was performed by comparing the pure dendrimer and peptide systems with the dendrimer-peptide ones. Also spin-labeled PAMAM dendrimers were used to test the interactions. The results show the interactions between dendrimer and peptide monomer to be stronger for Abeta 1-28 than for PrP 106-126. PAMAM dendrimers perturb the aggregation of the peptides more than PPI dendrimers do.

DARK MATTER CANDIDATE FROM CONFORMALITY

Abelian quiver gauge theories provide candidates for the conformality approach to physics beyond the standard model which possess novel cancellation mechanisms for quadratic divergences. A Z2 symmetry ( R parity) can be imposed and leads naturally to a dark matter candidate which is the Lightest Conformality Particle (LCP), a neutral spin-1 / 2 state with weak interaction annihilation cross-section, mass in the 100 GeV region and relic density of non-baryonic dark matter Ωdm which can be consistent with the observed value Ωdm≃0.24.

Radiation and self-polarization of neutral fermions in quasi-classical description

A Lorentz invariant formalism for quasi-classical description of electromagnetic radiation from a neutral spin 1/2 particle with an anomalous magnetic moment moving in an external electromagnetic field is developed. In the high symmetry fields, for which analytical solutions to the Bargmann--Michel--Telegdi equation are known, the so called self-polarization axes, i. e. directions of preferred polarization of particles in the radiation process, are found. Expressions for the radiative transition probability and spectral-angular distribution of the radiation emitted by a polarized particle are obtained in the fields under consideration.

SUSY approach to Pauli Hamiltonians with an axial symmetry

A two-dimensional Pauli Hamiltonian describing the interaction of a neutral spin-1/2 particle with a magnetic field having axial and second-order symmetries is considered. After separation of variables, the one-dimensional matrix Hamiltonian is analysed from the point of view of supersymmetric quantum mechanics. Attention is paid to the discrete symmetries of the Hamiltonian and also to the Hamiltonian hierarchies generated by intertwining operators. The spectrum is studied by means of the associated matrix shape invariance. The relation between the intertwining operators and the second-order symmetries is established, and the full set of ladder operators that complete the dynamical algebra is constructed.

Transport anomalies in a simplified model for a heavy-electron quantum critical point

We discuss the transport anomalies associated with the development of heavy electrons out of a neutral spin fluid using the large-N treatment of the Kondo-Heisenberg lattice model. At the phase transition in this model the spin excitations suddenly acquire charge. The Higgs process by which this takes place causes the constraint gauge field to loosely ``lock'' together with the external, electromagnetic gauge field. From this perspective, the heavy fermion phase is a Meissner phase in which the field representing the difference between the electromagnetic and constraint gauge field, is excluded from the material. We show that at the transition into the heavy fermion phase, both the linear and the Hall conductivity jump together. However, the Drude weight of the heavy electron fluid does not jump at the quantum critical point, but instead grows linearly with the distance from the quantum critical point, forming a kind of ``gossamer'' Fermi-liquid.

Spin distribution in neutral polyene radicals: Pariser–Parr–Pople model studied with the density matrix renormalization group method

The geometries and pi electron spin distributions induced by neutral soliton defects in trans-polyacetylene radicals (from C(7)H(9) to C(49)H(51)) are studied using Pariser-Parr-Pople (PPP) model, solved by the density matrix renormalization group (DMRG) method. Comparisons with other quantum chemical methods as well as the experimental observations on heptatrienyl (C(7)H(9)) and nonatetraenyl (C(9)H(11)) radical species show that the semiempirical PPP method is in the list of the very few theories that can give correct description of the spin distributions for such extended pi-conjugated systems. By virtue of DMRG's power in dealing with large one-dimensional systems, we predicted that the half-width of a neutral spin soliton in polyacetylene is about 14 atoms, and the spin distributions in the center of the soliton is calculated as rho(0)=0.25, rho(1)=-0.12 with rho(1)rho(0)=-0.48, rho(-)rho(+)=-0.52, which agree well with the results from electron-nuclear double resonance experiments.

A highly stabilized hydrogenated amorphous silicon film having very low hydrogen concentration and an improved Si bond network

A highly stabilized hydrogenated amorphous silicon film was developed using a triode deposition system in which a mesh was installed between cathode and substrate in a plasma-enhanced chemical vapor deposition system. The hydrogen concentrations of the resulting films are very low compared with those of conventional films prepared at the same substrate temperature. It was observed that light-soaking stability of the obtained film is promising, which is confirmed by measuring a neutral spin density as well as an I-V characteristic of a Schottky diode. Reduction of densities of higher silane radicals during film growth is an important factor for controlling degradation.

Theory of neutron scattering for gapless neutral spin-1 collective mode in graphite

Using tight binding band picture for 2D graphite, and the Hubbard interaction, recently we obtained a gapless, neutral spin-1 collective mode in graphite \cite{SZS}. In this paper we present a detailed RPA analysis of the Neutron Scattering cross section for this collective mode. Near $K-$point and very close to $\Gamma-$point, the intensity of neutron scattering peaks vanishes as $q^3$. This is shown using a simple Dirac cone model for the graphite band structure, which captures the small$-q$ behavior of the system. As we move away from the $\Gamma-$ and $K-$points in the Brillouin zone of the collective mode momenta, we can identify our collective mode quanta with spin triplet excitons with the spatial extent of the order of a few to a couple of lattice parameter $a$, with more or less anisotropic character, which differs from point to point. We also demonstrate that the inclusion of the long range tail of the Coulomb interaction in real graphite, does not affect our spin-1 collective mode qualitatively. This collective mode could be probed at different energy scales by thermal, hot and epithermal neutron scattering experiments. However, the smallness of the calculated scattering intensity, arising from a reduced form factor of carbon $2p_z$ orbital makes the detection challenging.

Spectral properties and lifetimes of neutral spin-12fermions in a magnetic guide

We investigate the resonant motion of neutral spin-$\frac{1}{2}$-fermions in a magnetic guide. A wealth of unitary and antiunitary symmetries is revealed in particular giving rise to a twofold degeneracy of the energy levels. To compute the energies and decay widths of a large number of resonances the complex scaling method is employed. We discuss the dependence of the lifetimes on the angular momentum of the resonance states. In this context the existence of so-called quasibound states is shown. In order to approximately calculate the resonance energies of such states a radial Schr\odinger equation is derived which improves the well-known adiabatic approximation. The effects of an additionally applied homogeneous Ioffe field on the resonance energies and decay widths are also considered. The results are applied to the case of the $^{6}\mathrm{Li}$ atom in the $F=\frac{1}{2}$ hyperfine ground state.

Model for entangled states with spin-spin interaction

A system consisting of two neutral spin-1/2 particles is analyzed for two magnetic field perturbations: (1) an inhomogeneous magnetic field over all space, and (2) external fields over a half space containing only one of the particles. The field is chosen to point from one particle to the other, which results in essentially a one-dimensional problem. A number of interesting features are revealed for the first case: the singlet, which has zero potential energy in the unperturbed case, remains unstable in the perturbing field. The spin-zero component of the triplet evolves into a bound state with a double well potential, with the possibility of tunneling. Superposition states can be constructed which oscillate between entangled and unentangled states. For the second case, we show that changes in the magnetic field around one particle affect measurements of the spin of the entangled particle not in the magnetic field nonlocally. By using protective measurements, we show it is possible in principle to establish a nonlocal interaction using the two particles, provided the dipole-dipole potential energy does not vanish and is comparable to the potential energy of the particle in the external field.

Light spin-12or spin-0 dark matter particles

We recall and precise how light spin-0 particles could be acceptable Dark Matter candidates, and extend this analysis to spin-1/2 particles. We evaluate the (rather large) annihilation cross sections required, and show how they may be induced by a new light neutral spin-1 boson U. If this one is vectorially coupled to matter particles, the (spin-1/2 or spin-0) Dark Matter annihilation cross section into e+e- automatically includes a v_dm^2 suppression factor at threshold, as desirable to avoid an excessive production of gamma rays from residual Dark Matter annihilations. We also relate Dark Matter annihilations with production cross sections in e+e- scatterings. Annihilation cross sections of spin-1/2 and spin-0 Dark Matter particles are given by exactly the same expressions. Just as for spin-0, light spin-1/2 Dark Matter particles annihilating into e+e- could be responsible for the bright 511 keV gamma ray line observed by INTEGRAL from the galactic bulge.

Efficient electron spin detection with positively charged quantum dots

We report the application of time- and polarization-resolved photoluminescence up-conversion spectroscopy to the study of spin capture and energy relaxation in positively and negatively charged, as well as neutral InAs self-assembled quantum dots. When compared to the neutral dots, we find that carrier capture and relaxation to the ground state is much faster in the highly charged dots, suggesting that electron–hole scattering dominates this process. The long spin lifetime, short capture time, and high radiative efficiency of the positively charged dots, indicates that these structures are superior to both quantum well and neutral quantum dot light-emitting diode spin detectors for spintronics applications.

High-Tcsuperconductivity, skyrmions, and the Berry phase

It is here pointed out that the antiferromagnetic spin fluctuation may be associated with a gauge field which gives rise to the antiferromagnetic ground state chirality. This is associated with the chiral anomaly and Berry phase when we consider the two dimensional spin system on the surface of a 3D sphere with a monopole at the centre. This realizes the RVB state where spinons and holons can be understood as chargeless spins and spinless holes attached with magnetic flux. The attachment of the magnetic flux of the charge carrier suggest, that this may be viewed as a skyrmion. The interaction of a massless fermion representing a neutral spin with a gauge field along with the interaction of a spinless hole with the gauge field enhances the antiferromagnetic correlation along with the pseudogap at the underdoped region. As the doping increases the antiferromagnetic long range order disappears for the critical doping parameter $\delta_{sc}$. In this framework, the superconducting pairing may be viewed as caused by skyrmion-skyrmion bound states.

Charged tensor matter fields and Lorentz symmetry violation via spontaneous symmetry breaking

We consider a model with a charged vector field along with a Cremmer-Scherk-Kalb-Ramond (CSKR) matter field coupled to a U(1) gauge potential. We obtain a natural Lorentz symmetry violation due to the local U(1) spontaneous symmetry breaking mechanism triggered by the imaginary part of the vector matter. The choice of the unitary gauge leads to the decoupling of the gauge-KR sector from the Higgs-KR sector. The excitation spectrum is carefully analyzed and the physical modes are identified. We propose an identification of the neutral massive spin-1 Higgs-like field with the massive Z' boson of the so-called mirror matter models.