Electronic Asymmetry Research Articles

Elastic ν e− scattering of solar neutrinos with electromagnetic moments

We consider the azimuthal asymmetry of the recoil electrons in elastic ν e − scattering of solar neutrinos, which can arise if neutrinos have electromagnetic moments and there is a large solar magnetic field. We show that using this effect it is not possible to distinguish between magnetic and electric dipole moments in the 1-Dirac and 2-Majorana neutrino cases and that averaging over neutrino energy is important and suppresses the azimuthal asymmetry in the 2-Majorana neutrino case.

Asymmetry of electron and hole doping inYMnO3

We synthesized hexagonal ${\mathrm{YMnO}}_{3}$ doped with tetravalent Zr ions and studied the electronic and magnetic properties. Zr substitution creates a mixed ${\mathrm{Mn}}^{3+}\ensuremath{-}{\mathrm{Mn}}^{2+}$ system, instead of the conventional ${\mathrm{Mn}}^{3+}\ensuremath{-}{\mathrm{Mn}}^{4+}$ of colossal magnetoresistance manganites. The ${\mathrm{YMnO}}_{3}$ system displays a pronounced asymmetry for electron and hole doping. Hole doping results in a conducting state, whereas electron doping retains the insulating state. This asymmetry is a consequence of the crystal field splitting of the Mn ions in trigonal bipyramidal coordination.

An Application of Electronic Asymmetry to Highly Enantioselective Catalytic Diels−Alder Reactions

The compounds (RRu)-[CyRuCl(S)-BINPO]SbF6 and [CyRuCl(S)-TolBINPO]SbF6 (Cy = eta6-cymene), were synthesized from (CyRuCl2)(2) and the appropriate non-C2-symmetric bisphosphine monoxide ligands (S)-BINPO and (S)-TolBINPO (BINPO = 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl) in the presence of NaSbF6. When these complexes were mixed with AgSbF6 the resulting Lewis acids catalyzed the Diels-Alder cycloaddition of cyclopentadiene and methacrolein. The product (2S)-methylbicyclo[2.2.1]hept-5-ene-2-carboxaldehyde was obtained with excellent diastereoselectivity (up to 99%) and enantioselectivity (up to 99%) in several cases. When the complexes containing the analogous C2-symmetric bisphosphine ligands (S)-BINAP and (S)-TolBINAP were employed as catalysts, the Diels-Alder cycloadducts were obtained with much lower enantioselectivity (19 to 50%) for the opposite antipode. Although some of the effect may arise from chelate ring size change, much of the enhanced stereoselectivity of (RRu)-[CyRuCl(S)-BINPO]SbF6 and [CyRuCl(S)-TolBINPO]SbF6 can be attributed to the electronic asymmetry at the stereogenic Ru center.

A Theoretical Analysis of Absorption Spectra of Photosynthetic Reaction Centers: Mechanism of Temperature Dependent Peak Shift

Detailed analysis of the absorption spectra of the R26.Phe-a mutant RCs at various temperatures has been presented constructing a model Hamiltonian based on the previous spectral analysis results. The temperature dependence of the electronic asymmetry and the coupling between the Bchls in the special pair can be obtained by using the thermal expansion model. The major contributions to the energy gaps between the exciton states of the special pair result from the protein-induced electronic asymmetry Δ and the intra-dimer coupling β. The values of Δ and β are obtained to be 147 and 405.19 cm-1 at room temperature, respectively, and they increase with a decrease in temperature. Peak intensity ratios of each electronic state to the lowest electronic state at various temperatures are calculated using the delocalized basis set. The absorption spectra at 77 and 278 K have been theoretically predicted by using the model presented in this paper. Anisotropy is also studied using both delocalized and dimer basis set...

Laser-phase directional control of photofragments in dissociative ionization ofH2+using two-color intense laser pulses

Exact non-Born-Oppenheimer numerical solutions of the time-dependent Schr\odinger equation for the one-dimensional ${\mathrm{H}}_{2}^{+}$ molecule in an intense, two-color $(\ensuremath{\omega}+2\ensuremath{\omega})$ laser field, with relative phase $\ensuremath{\varphi},$ have been obtained. Both electron and proton kinetic energy spectra show spatial, correlated, asymmetric distributions. The calculated spectra exhibit the same unusual correlations as seen in experiments, in which both positively charged nuclear fragments and negatively charged photoelectrons are preferentially emitted in the same direction. It is found that, for the most asymmetric combined electric field $(\ensuremath{\varphi}=0),$ the electron is ionized in a ``counterintuitive'' direction: i.e., more electrons follow the direction of the maximum electric field. This unexpected behavior of electrons is not specific to molecules: we show that the same effect occurs in atoms. The above asymmetries of photoemission of electrons are interpreted in the framework of a quasistatic tunneling model and it is shown that for $\ensuremath{\varphi}=0$ the electron asymmetry is induced by the Coulomb attraction from the parent ion. Proton asymmetries found in the dissociation $\mathrm{H}+p$ channel depend strongly on the initialization: proton asymmetry for low vibrational state initialization is opposite that for high vibrational states. In the latter case, for $\ensuremath{\varphi}=0,$ protons follow the direction of the maximum field. Coulomb explosion spectra are shown to arise from an enhanced ionization mechanism, which is phase dependent also.

Diastereoselective Synthesis and Electronic Asymmetry of Chiral Nonracemic Rhenium(V) Oxo Complexes Containing the Hydrotris(1-pyrazolyl)borate Ligand

The compounds [TpReO(η2-N−X)] (X = N, O; Tp = hydrotrispyrazolylborate) were prepared from TpReOCl2 and chiral, bidentate ligands in which η2-N−X = the alcoholates or amidates derived from (1S,2R)-ephedrine, (1S,2S)-diphenylethylenediamine, and l-proline. The three complexes have been fully characterized by 1H NMR, IR, circular-dichroism, elemental analysis, polarimetry, and X-ray crystallography. All have a chiral center at rhenium. The complexes derived from (1S,2R)-ephedrine and l-proline were formed diastereoselectively as a single diastereomer. All of the complexes exhibited similar Re−O bond distances, ranging from 1.687(7) to 1.707(7) Å, consistent with distances found in other Re(V) oxo complexes. The electronic asymmetry at the metal caused by the different donor properties of the ligands leads to tilting of the Re−oxo bond relative to the plane of the equatorial ligands. The compounds were found to be robust and resisted oxo transfer even when subjected to harsh conditions.

Impact of $e^+$ and $e^-$ beam polarization on chargino and neutralino production at a linear collider

We study the production processes e^+ e^- -> chi^+_i chi^-_j, i,j=1,2, and e^+ e^- -> chi^0_m chi^0_n, m,n=1,...,4, working out the advantages of polarizing both beams. For e^+ e^- -> chi^+_1 chi^-_1 with chi^-_1 -> chi^0_1 e^- nu and e^+ e^- -> chi^0_1 chi^0_2 with chi^0_2 -> chi^0_1 e^+ e^- we perform a detailed analysis, including the complete spin correlations between production and decay. We analyze the forward--backward asymmetry of the decay electron for various beam polarizations. We also study polarization asymmetries in e^+ e^- -> chi^0_1 chi^0_2. These asymmetries strongly constrain the gaugino parameter M_1 and the masses m_{\tilde{e}_L}, m_{\tilde{e}_R}, m_{\tilde{nu}} also if m_{\tilde{e}_{L,R}}, m_{\tilde{nu}} > sqrt{s}/2. We give numerical predictions for three scenarios for a linear collider with sqrt{s}=500-1000 GeV.

Parity violation in elastic electron-proton scattering and the Proton's strange magnetic form factor

We report a new measurement of the parity-violating asymmetry in elastic electron scattering from the proton at backward scattering angles. This asymmetry is sensitive to the strange magnetic form factor of the proton as well as electroweak axial radiative corrections. The new measurement of A = -4.92+/-0.61+/-0.73 ppm provides a significant constraint on these quantities. The implications for the strange magnetic form factor are discussed in the context of theoretical estimates for the axial corrections.

Study of Li Diffusion on Metal and Semiconductor Surfaces via NQI Presented at the XVth International Symposium on Nuclear Quadrupole Interactions

Nuclear magnetic resonance has been used to study the diffusion of lithium on a Ru(001) single crystal surface. The quadrupolar interaction of the radioactive probe nucleus 8Li was utilized for this. In an online experiment the 8Li nuclei are produced, thermalized and highly polarized before they land on the surface studied, the parity violating β-decay revealing the sought after NMR/NQI nuclear information through the spatial asymmetry of the decay electrons. As a function of substrate temperature, alkali metal coverage and magnetic field the nuclear spin-lattice relaxation measurements show the existence of two distinctly different diffusion barriers on the surface, valued at 0.45 eV and 0.15 eV. The former is attributed to jumps from step to terrace sites, whereas the latter is the barrier between adjacent terrace sites. On the substrate Si(l 11)7x7 the relaxation measurements suggest a much higher diffusion barrier around 0.8 eV.

A measurement of the beta asymmetry in neutron decay with PERKEO II

The recent measurement of the electron asymmetry in neutron decay using the spectrometer PERKEO II is presented. In a preliminary analysis we arrive at the following result: A=−0.1189(8), implying λ=−1.2740(21).

Spin-dependent electron transport in ferromagnet/semiconductor Schottky barrier structures

ABSTRACTClear evidence for high efficiency spin-polarized electron transport across ferromagnet/semiconductor Schottky barrier interfaces was observed in Ni80Fe20/GaAs structures. Circularly polarized light was used to excite electrons with a spin polarization perpendicular to the film plane. At negative bias, an almost constant difference between the helicity-dependent photocurrent obtained for the magnetization parallel and perpendicular to the photon helicity was detected. An effective asymmetry, A, was also estimated from the helicity-dependent photocurrent difference, attributed to spin-polarized electron tunneling from GaAs to NiFe (spin filtering). A decreases with increasing photon energy, which is consistent with the energy-dependence of the asymmetry of photoexcited electrons in GaAs. Weak spin injection from NiFe to GaAs was seen at a bias corresponding to the Schottky barrier height, which is likely to occur via a ballistic process.

Small S0–S1 energy gaps for certain twisted conformations of unsymmetric polymethine dyes: quantum chemical treatment and spectroscopic manifestations

The AM1–CI (Austin model 1 configuration interaction) method has been used to calculate energy gaps between the ground state (S0) and the lowest biradicaloid (BR) excited state as a measure for roughly estimating the probability of reaching an S0–BR conical intersection by bond twisting of one of the bonds along the polymethine chain of unsymmetric cyanine dyes. Invoking qualitative concepts for interpreting the computational data, a pronounced alternation of successive bonds is shown and explained. The magnitude of the S0–BR energy gap was found to be governed by the position of the corresponding bond with respect to the polymethine chain ends and by the degree of electronic asymmetry of the chain, i.e. by the difference in electron donor abilities of its end groups. These results are compared to experimental photophysical data for a series of differently bridged asymmetric cyanine dyes and suggest that the magnitude of the energy gap for a given twisted bond can be modulated by bridge-induced perturbations. Nonreactive low-lying minima caused by the accessibility of several twisted excited-state conformers have to be invoked in order to explain the experimentally observed ‘‘ inverse-loose-bolt’’ effect of the nonradiative decay rates and should be taken into account in the treatment of possible pathways of fluorescence quenching and in the design of high-quantum-yield fluorescent dyes.

Excited-state electronic asymmetry of the special pair in photosynthetic reaction center mutants: absorption and Stark spectroscopy.

The electronic absorption line shape and Stark spectrum of the lowest energy Q(y)() transition of the special pair in bacterial reaction centers contain a wealth of information on mixing with charge transfer states and electronic asymmetry. Both vary greatly in mutants that perturb the chemical composition of the special pair, such as the heterodimer mutants, and in mutants that alter interactions between the special pair and the surrounding reaction center protein, such as those that add or remove hydrogen bonds. The conventional and higher-order Stark spectra of a series of mutants are presented with the aim of developing a systematic description of the electronic structure of the excited state of the special pair that initiates photosynthetic charge separation. The mutants L168HF, M197FH, L131LH and L131LH/M160LH/M197FH are known to have different hydrogen-bonding patterns to the special pair; however, they exhibit Stark effects that are very similar to wild type. By contrast, the addition of a hydrogen bond to the M-side keto carbonyl group of the special pair in M160LH greatly affects both the absorption and Stark spectra. The heterodimer special pairs, L173HL and M202HL, exhibit much larger Stark effects than wild type, with the greatest effect in the M-side mutant. Double mutants that combine the M-side heterodimer and a hydrogen-bond addition to the L-side of the special pair decrease the magnitude of the Stark effect. These results suggest that the electronic asymmetry of the dimer can be perturbed either by the formation of a heterodimer or by adding or deleting a hydrogen bond to a keto carbonyl group. From the pattern observed, it is concluded that the charge transfer state P(L)(+)P(M)(-) has a larger influence on the excited state of the dimer in wild type than the P(L)(-)P(M)(+)charge transfer state. Furthermore, asymmetry can be varied continuously, from extreme cases in which the heterodimer and hydrogen-bond effects work together, to cases in which hydrogen bonding offsets the effects of the heterodimer, to cases in which the homodimer is perturbed by hydrogen bonds. This leads to a unified model for understanding the effects of perturbations on the electronic symmetry of the special pair, and this can be connected with perturbations on the properties of many other systems such as donor-acceptor-substituted polyenes.

Theory of tunneling magnetoresistance in a junction with a nonmagnetic metallic interlayer

It is demonstrated by numerical evaluation of the real-space Kubo formula that the tunneling magnetoresistance (TMR) due to tunneling between two cobalt electrodes separated by a vacuum gap remains nonzero when one of the electrodes is covered with a copper layer. This contradicts the classical theory of tunneling that predicts zero TMR. It is shown that a nonzero TMR is due to quantum well states in the Cu layer that do not participate in transport. Since these only occur in the down-spin channel, their loss from transport creates a spin asymmetry of electrons tunneling from a Cu overlayer, i.e., nonzero TMR. This mechanism could provide an explanation of the observed nonzero TMR for junctions with Cu or Ag interlayers. A simple method for modifying the classical theory of tunneling so that it can describe correctly tunneling in the presence of quantum well states is proposed and implemented for the Co junction with a Cu interlayer.

Bonding Characteristics Mediated by Saddle-Shaped Porphyrin Deformation: A Theoretical Approach to the Control of Spin State of Iron(III) Porphyrins

The novel concept of a “symmetry switch” for spin transfer is proposed. EHT and INDO calculations on a series of iron(III) porphyrin complexes indicate that factors that can contribute to the stability of the intermediate-spin state can be explained clearly with crystal-field theory. A decrease in symmetry causes the extensive hybridization of d and d (shown here) orbitals and induces large electronic structure asymmetry in the saddle-shaped iron(III) porphyrin complexes.

Influence of hydrostatic pressure and temperature on two-photon absorption of aC60-2-thioxo-1,3-dithiole cycloadduct

Two-photon absorption (TPA) at $\ensuremath{\lambda}=532\mathrm{nm}$ versus applied hydrostatic pressure (p) (up to 20 GPa) and temperature (T) (in the 77--300 K range) was used to study intermolecular dynamics in powderlike fullerene-1,3-dithiolic adduct and ${\mathrm{C}}_{60}$ systems. For ${\mathrm{C}}_{60}$-1,3-dithiole adduct, we observed a sharplike maxim of the TPA versus $p\ensuremath{-}T$ below 230 K. Comparing the obtained results with those of ${\mathrm{C}}_{60},$ we have unambiguously shown that the observed behaviors are connected with the adding of the 1,3-dithiole core. To explain the experimental results, we performed ab initio molecular-dynamics geometry optimization, taking into account the superposition of all possible molecular conformations with appropriate weighting factors and higher-order intermolecular multipole interactions. We have shown an increase of the absolute value of the TPA coefficient in the ${\mathrm{C}}_{60}$ cycloadduct by more than 30%. This fact is mainly caused by the asymmetry of intramolecular electronic charge-density asymmetry. The modulatedlike dependence of the TPA versus p and T have been observed. The latter one reflects the vibration and rotational contributions to the TPA values.

Coupled Electron-Hole Dynamics at theSi/SiO2Interface

We report a new and surprising enhancement of the electric field at the Si/SiO2 interface following the cessation of intense pulsed near-infrared radiation. The phenomenon, measured by optical second-harmonic generation, occurs only for photon energies and oxide film thickness that exceed respective thresholds. We attribute the new effect to multiphoton hole injection into the oxide and to an asymmetry in electron and hole dynamics, in particular to distinctly different trapping and detrapping processes.

Constraints on four-Fermi contact interactions from low-energy electroweak experiments

We investigate the constraints on four-Fermi contact interactions from low-energy lepton-quark and lepton-lepton scattering experiments — polarization asymmetries in electron (muon)-nucleon scattering experiments, cesium and thallium atom parity violation measurements, neutrino-nuclei and neutrino-electron scattering experiments. These constraints are then combined by assuming the lepton and quark universalities and SU(2)L x U(l)y gauge invariance of the contact interaction, which leave independent six lepton-quark and three pure-leptonic interactions. Impacts of these constraints on models with an additional Z-boson are briefly discussed. We also present updates of the low-energy constraints on the S and T parameters.

Picosecond to Microsecond Photodynamics of a Nonplanar Nickel Porphyrin: Solvent Dielectric and Temperature Effects

The lifetime of the (dz2,dx2-y2) metal excited state of nickel(II) 5,10,15,20-tetra-tert-butylporphyrin (NiT(t-Bu)P) exhibits an extraordinary dependence on solvent dielectric properties and temperature. At room temperature, the excited-state deactivation time varies from 2 ps in highly polar solvents to about 50 ns in nonpolar media. The lifetimes increase to several microseconds in both polar and nonpolar solvents near 80 K. In contrast, the (d,d) lifetimes of nominally planar nickel porphyrins such as nickel tetraphenylporphyrin (NiTPP) vary only weakly with solvent dielectric properties and temperature, and typically fall in the range of 100 to 300 ps. All available evidence indicates that NiT(t-Bu)P in solution is highly ruffled (nonplanar) in the ground electronic state. It is proposed that the photoinduced conformational changes that occur in NiT(t-Bu)P in order to accommodate the excited-state electronic distribution are limited by the severe steric constraints imposed by the bulky meso tert-butyl substituents, and result in molecular and electronic asymmetry and thus a polar excited state. Solvent dielectric properties and temperature modulate these conformational excursions and thus the electronic deactivation rates by affecting the excited-state energies, porphyrin/solvent reorganizations, and the populations of low-frequency out-of-plane vibrations of the macrocycle. The novel findings for this nonplanar nickel porphyrin demonstrate the intimate connectivity that exists between the static and dynamic molecular structures of porphyrins and their ground- and excited-state electronic properties. Furthermore, the results obtained provide insights into the interactions between tetrapyrrole chromophores and their host proteins, and suggest the potential use of nonplanar porphyrins as building blocks for molecular photonics applications.

Up-down asymmetry: A diagnostic for neutrino oscillations

We propose a simple test of underground neutrino data to discriminate amongst neutrino oscillation models. It uses the asymmetry between downward-going events and upward-going events, for electron and muon events separately. Because of the symmetry of typical underground detectors, an asymmetry can be compared with calculations with little need for the intermediary of a simulation program. Furthermore, we show that the various oscillation scenarios give rise to dramatically differing trajectories of asymmetry versus energy for muons and electrons. This permits a clean distinction to be drawn between models.