Strong Spin Polarization Research Articles

Spin-dependent pseudopotentials

Near degeneracies and strong spin polarization are characteristics of transition metals that offer challenges to the pseudopotential approximation. Here we investigate the spin dependence of pseudopotentials generated from atomic, all-electron density-functional calculations. Different spins are found to require different pseudopotentials for any spin-polarized atom. Ignoring this leads to significant errors in the representation of all but the nonmagnetic configurations. Including a correction that is linear in the local spin polarization results in a spin-dependent expression, which dramatically improves the transferability of the atomic pseudopotential beyond previous nonlinear core corrections.

FT-EPR study of alkyl radicals formed in the photochemical reaction of Re(alkyl)(α-diimine) and Ru(alkyl)(α-diimine) complexes

A Fourier transform EPR (FT-EPR) study was made of the photochemistry of [Re(R)(CO)3 (α-diimine)] and [Ru(E)(R)(CO)2(α-diimine)] complexes, where R = alkyl or benzyl, E = I or SnPh3, and α-diimine = 4,4′-dimethyl-2,2′-bipyridine (DMB) orN,N′-diisopropyl-1,4-diazabutadiene (iPr-DAB). Photoexcitation of these complexes leads to homolysis of the metal-alkyl (benzyl) bonds as evident from the detection of the spectra of the alkyl (benzyl) radicals. FT-EPR spectra display strong spin polarization effects attributed to Triplet Mechanism (TM) and Radical Pair Mechanism (RPM) Chemically Induced Dynamic Electron Polarization (CIDEP). CIDEP patterns point to bond dissociation via a triplet state precursor. For a number of complexes, spin polarization was found to exhibit unusually large solvent effects, whereas for one complex the CIDEP pattern proved to be sensitive to the wavelength of laser light used to initiate bond dissociation. These effects reflect the strong dependence of CIDEP on the character of the excited states involved in the photochemical reactions and contribute to the understanding of the reaction mechanism.

CHARACTERIZATION OF THE MAGNETIC STRUCTURE OF Cr on Pd(001) BY SPIN-POLARIZED AND ANGLE-RESOLVED PHOTOELECTRON DIFFRACTION

Recent investigations of the magnetic structure of Cr layers on Pd (001) have shown that two structures appear dependent on the number of Cr layers which have an in-plane antiferromagnetic structure and a layer antiferromagnetic structure, respectively. It is shown that these magnetic structures should be distinguishable experimentally by spin-polarized photoelectron diffraction experiments. The photoelectron intensity from the Cr 2 p 1/2 core level is calculated for normal incidence of left and right circularly polarized light. It is shown that there is a strong dichroism and spin polarization in the intensities. Dichroism and spin polarization are analyzed with respect to their sources, and thus the contribution of the magnetic scattering is separated. It is an essential contribution for a monolayer Cr on Pd (001).

Polarized neutron reflection study of Gd/W multilayers

Magnetic properties of a series of Gd/W multilayers with constant W-layer thickness (18 Å) and variable Gd-layer thickness have been studied by polarized neutron reflectometry and SQUID magnetometry. Our results are not consistent with enhanced Gd moments and/or strong ferrogmagnetic spin polarizations in the W layers as claimed by Heys et al. [1]. As an unexpected and remarkable feature we have observed for the multilayers with thin Gd films of 14 and 19 Å strong unusual peaks in the reflectivity spectra which depend on the external magnetic field and on the cooling process. These peaks depend on the magnetic structure of the layer system and do not occur in multilayers with thicker Gd films.

Effects of electron correlation and spin projection on rotational barriers of trithiocarbenium ion [C(SH)3]+ and Radical Dication [C(SH)3]?,2+

Theoretical level dependencies are discussed of relative isomer stabilities and rotational barriers of trithiomethyl cation [C(SH)3]+ (a) and of radical dication [C(SH3)]⋅,2+ (b). Spin polarization and dynamic electron correlation are very important for the radical dictation. Removal of an electron from one of the degenerate π-HOMOs of C3h symmetric [C(SH)3]+ stabilizes the remaining π electron to such an extent that the unpaired electron is not in the HOMO of the dictation. The radial π MO's “diving below the Fermi level” facilitates strong spin polarization because of its energetic proximity to σ MOs. Projection of the first three higher spin states eliminates spin contaminations, and the terms E(PUHF(s+3))-E(UHF) and E(PMP4(s+3))-E(MP4) are discussed. The combination of annihilation of spin contamination and electron correlation is essential for the determination of relative energies and rotational barriers of the radical dication. The results obtained at this level match the results of high level QCISD(T) calculations in a near-quantitative fashion. Perturbation theory alone does not correct for spin contamination even if it is carried to full fourth order and includes triple excitations; the E(PMP4(s+3))-E(MP4) values are all negative and can exceed 5 kcal/mol in magnitude. Previous studies showed that annihilation of spin contaminations is important in regions of potential energy surfaces where σ bonds are broken (homolytic dissociation), formed (radical addition), or both (H abstraction by radical). Our findings stress that the annihilation of spin contaminations can be just as important for any process that greatly alters spin polarization and even if that process proceeds without breaking or forming of σ bonds. For comparison, density functional theory also was employed in the potential energy surface analyses. The results obtained with the B3LYP formalism were found to be less susceptible to spin contamination and resulted in rather good agreement with the best pertubation and configuration interaction results. © 1997 John Wiley & Sons, Inc. J Comput Chem 18:1023–1035, 1997

Effects of electron correlation and spin projection on rotational barriers of trithiocarbenium ion [C(SH)3] and Radical Dication [C(SH)3],2

Theoretical level dependencies are discussed of relative isomer stabilities and rotational barriers of trithiomethyl cation [C(SH)3]+ (a) and of radical dication [C(SH3)]⋅,2+ (b). Spin polarization and dynamic electron correlation are very important for the radical dictation. Removal of an electron from one of the degenerate π-HOMOs of C3h symmetric [C(SH)3]+ stabilizes the remaining π electron to such an extent that the unpaired electron is not in the HOMO of the dictation. The radial π MO's “diving below the Fermi level” facilitates strong spin polarization because of its energetic proximity to σ MOs. Projection of the first three higher spin states eliminates spin contaminations, and the terms E(PUHF(s+3))-E(UHF) and E(PMP4(s+3))-E(MP4) are discussed. The combination of annihilation of spin contamination and electron correlation is essential for the determination of relative energies and rotational barriers of the radical dication. The results obtained at this level match the results of high level QCISD(T) calculations in a near-quantitative fashion. Perturbation theory alone does not correct for spin contamination even if it is carried to full fourth order and includes triple excitations; the E(PMP4(s+3))-E(MP4) values are all negative and can exceed 5 kcal/mol in magnitude. Previous studies showed that annihilation of spin contaminations is important in regions of potential energy surfaces where σ bonds are broken (homolytic dissociation), formed (radical addition), or both (H abstraction by radical). Our findings stress that the annihilation of spin contaminations can be just as important for any process that greatly alters spin polarization and even if that process proceeds without breaking or forming of σ bonds. For comparison, density functional theory also was employed in the potential energy surface analyses. The results obtained with the B3LYP formalism were found to be less susceptible to spin contamination and resulted in rather good agreement with the best pertubation and configuration interaction results. © 1997 John Wiley & Sons, Inc. J Comput Chem 18:1023–1035, 1997

The ST_-relaxation mechanism of magnetic field effects on radical pair recombination. Intraradical relaxation versus ST_-relaxation transitions

The nonadiabatic ST_ transition induced by the fluctuating (anistropic) part of the hyperfine and Zeeman interactions are shown to make a significant contribution to the magnetic field effects in the radical pair recombination in liquids and micelles. The anisotropic Zeeman interaction induced ST_ transitions are found to cause a strong decreased in the radical yield Y and strong net electron and nuclear spin polarization at high magnetic field B. The experimentally observed dependence Y( B) is accurately described within the theory developed.

Natural abundance solid-state carbon NMR studies of photosynthetic reaction centers with photoinduced polarization.

Solid-state NMR spectra of natural abundance 13C in reaction centers from photosynthetic bacteria Rhodobacter sphaeroides R-26 was measured. When the quinone acceptors were removed and continuous visible illumination of the sample was provided, exceptionally strong nuclear spin polarization was observed in NMR lines with chemical shifts resembling those of the aromatic carbons in bacteriochlorophyll and bacteriopheophytin. The observation of spin polarized 15N nuclei in bacteriochlorophyll and bacteriopheophytin was previously demonstrated with nonspecifically 15N-labeled reaction centers. Both the carbon and the nitrogen NMR studies indicate that the polarization is developed on species that carry unpaired electrons in the early electron transfer steps, including the bacteriochlorophyll dimer donor P860 and probably the bacteriopheophytin acceptor. I. Both enhanced-absorptive and emissive polarization were seen in the carbon spectrum; most lines were absorptive but the methine carbons of the porphyrin ring (alpha, beta, gamma, ) exhibited emissive polarization. The change in the sign of the hyperfine coupling at these sites indicates the existence of nodes in the spin density distribution on the tetrapyrrole cofactors flanking each methine carbon bridge.

Porphyrins linked to high acceptor strength cyano quinones as models for the photosynthetic reaction center

The synthesis of biomimetic photosynthetic model compounds, composed of 10, 15, 20-tritolylporphyrins linked to different quinones by cyclohexylene bridges, is described. Cyclic voltammetry measurements show a significant change of the reduction potential of the quinone moiety through introduction of cyano groups as electron withdrawing substituents. EPR and ENDOR spectra of the semiquinone anion radical derivatives reveal a considerable spin density redistribution within the semiquinone moiety caused by the substituents. Lamp irradiation in situ of the porphyrin hydroquinones, dissolved in reversed Triton X-100 micelles, through the resonator slits of the EPR spectrometer does not only yield hyperfine resolved absorptive, but also emissive EPR spectra. This is indicative of strong electron spin polarization effects. From the polarization pattern it can be deduced that the radical/triplet pair mechanism between two photoactive species, located in one micelle, gives rise to this effect.

Magnetic field dependence of the carrier concentration in a modulation doped dilute magnetic semiconductor quantum well structure

The subband structure of a dilute magnetic semiconductor (DMS) quantum well embedded in a modulation doped non-magnetic host is evaluated as a function of applied magnetic field. Because of the large effective g-value of carriers in the DMS layer, the application of a dc magnetic field can lead to a strong spin polarization of the electron gas in the quantum well. For a quantum well containing few carriers at zero magnetic field, a substantial increase in carrier concentration can result from the application of a dc magnetic field.

FT-EPR Study of the Photochemical Homolysis of Re− and Ru−Alkyl Bonds in Re(R)(CO)3(α-Diimine) and Ru(I)(R)(CO)2(α-Diimine) Complexes. Evidence for the Reactive σ(M−R)π* State

A Fourier transform EPR (FT-EPR) study was made of the photochemistry of Re(R)(CO){sub 3}({alpha}-diimine) and Ru(I)(R)(CO){sub 2}({alpha}-diimine) complexes, where R = benzyl, 2-propyl, or ethyl and {alpha}-diimine = 4,4`-dimethyl-2, 2`-bipyridine or N,N`-diisopropyl-1,4-diazabutadiene. The FT-EPR technique was used to monitor the formation and decay of alkyl radicals generated by metal-R bound homolysis. The spectra showed pronounced chemically induced dynamic electron polarization (CIDEP) effects with polarization patterns that depend strongly on metal ion, alkyl substituent, and solvent. To varying degrees, all spectra shows a contribution form ST{sub 0} radical pair mechanism (RPM) CIDEP. From the low-field-emission/high-field-adsorption polarization pattern given by this mechanism, it can be deduced that the free radicals are formed via a triplet-state precursor, confirming conclusions reached in earlier spectroscopic studies. In addition to the RPM contribution, some spectra also show a net-absorptive CIDEP contribution while others show a net-emissive contribution. These net spin polarization contributions are attributed tentatively to triplet mechanism and spin-orbit coupling CIDEP. From the fact that the alkyl radical spectra exhibit strong spin polarization, it is concluded that metal ion atomic orbital contributions to the electronic states of precursor triplet and radical pair must be small. 17 refs., 4 figs.

Local-spin-selective x-ray absorption and x-ray magnetic circular dichroism of MnP.

The local-spin-selective x-ray-absorption spectrum and the magnetic-circular-dichroism spectrum are measured at the manganese K edge of MnP. Both spectra show the presence of a strong spin polarization at the edge. A comparison of the two techniques makes it possible to determine the energy dependence of the Fano factor. It is found that the Fano factor is -4% at the edge but it decreases quickly over an energy range of 10 eV to values lower than 1%. The local-spin-selective x-ray-absorption spectrum is compared to the empty density of states as determined from a local-spin-density band-structure calculation. The use of the K\ensuremath{\beta} detection yields a spectrum with considerably better resolution, which offers new possibilities for more detailed comparisons with electronic-structure models.

Magnetic 4d systems studied by implanted ions

By application of the perturbed γ-ray distribution method following heavy-ion reactions and recoil implantation techniques, we have found an experimental way of producing and investigating magnetic 4d states in metals. Strong 4d magnetism has been found for 4d ions in alkali metal hosts and in Pd hosts. In alkali metals, 4d ions reflect the phenomena of well-defined ionic ground states, orbital magnetism, mixed valence, and crystal field splittings smaller than theLS coupling. Magnetic 4d states in alkali metals cannot be described by one-electron approaches based on Anderson-type models, but requires an analysis in terms of many electron ionic configurations exhibiting basic features common to the physics of stable and unstable f stales in metals. In contrast, the local moment formation of 4d and 3d ions in Pd is governed by inter-atomic interactions of the magnetic d states with host d-band electrons, giving rise to spin magnetic behavior of the 4d impurity and to strong spin polarizations of the 4d electrons of the Pd host. Thus, the magnetism and electronic structure of 4d ions in metals exhibit qualitative differences in alkali metal hosts compared to Pd. The existence of magnetic 4d systems strongly depends on the 4d ion species and the host matrix, and on spin fluctuation rates or the corresponding Kondo temperatures. The results can be directly compared to theoretical work and also to the magnetic behavior of 3d ions in sp metal hosts and in hosts with d-band electrons.

Photoelectron Spectroscopic Studies of an α-nitronyl Nitroxide Radical: Mechanism of Ferromagnetic Intermolecular Interaction

An ultraviolet photoelectron spectrum of phenyl α-nitronyl nitroxide (PNN) has been studied in the gas phase, in order to understand the potentiality of the α-nitronyl nitroxide radical family yielding ferromagnetic intermolecularcoupling. The observed spectrum has been well interpreted with the help of CNDO-CI calculation. Very strong spin polarization effect has been suggested, which is advantageous for the intermolecular ferromagnetic coupling

Anisotropic suppression of the energy gap in CeNiSn by high magnetic fields.

The formation of the pseudogap in CeNiSn has been investigated by means of resistivity and specific-heat measurements in high magnetic fields. When the field is raised from 0 to 12 T along the orthorhombic a axis, the semiconductorlike behavior in the resistivity below 6 K disappears and the linear electronic term in the specific heat increases from 57 to 100 mJ/${\mathrm{K}}^{2}$ mol. However, no significant effect was observed for fields along the b and c axes up to 15 T. These results suggest an anisotropic suppression of the pseudogap, which can be interpreted as a result of strong spin polarization of the renormalized narrow band.

Spin- and angle-resolved photoemission from single crystals and epitaxial films using circularly polarized synchrotron radiation

We describe some novel results on the experimental spin-resolved electronic structure of a layered system, consisting of ultrathin films of fcc-Cobalt epitaxially grown onto a Cu(100) single crystal surface. Films more than 2 mono-atomic layers (ML) thick are found to be ferromagnetic at room temperature with an in-plane remanent magnetization. The electronic structure of 5 ML fcc-Co films turns out to be already bulk-like. On the other hand, the excitation with circularly polarized light leads to strong spin-polarization effects from the clean Cu(100) surface. This proves a significant influence of the spin-orbit interaction even in copper, resulting in strongly hybridized electronic energy bands.

Spin polarization of secondary electrons from amorphous 3d metallic ferromagnets.

It is shown that the spin polarization of secondary electrons from Fe-Ni-based metallic glasses (${\mathrm{Fe}}_{82}$${\mathrm{B}}_{12}$${\mathrm{Si}}_{6}$, ${\mathrm{Fe}}_{60}$${\mathrm{Ni}}_{20}$${\mathrm{B}}_{20}$, ${\mathrm{Fe}}_{44}$${\mathrm{Ni}}_{37}$${\mathrm{B}}_{19}$) in the energy range of 10-20 eV directly reflects the bulk magnetization of the material. All samples show a strong spin-polarization enhancement at low secondary energy ( 10 eV). No additional structure is observed for these amorphous materials, contrary to single crystals where the spin polarization is affected by band-structure (low-energy electron diffraction) effects.

Chemically induced dynamic nuclear polarization studies of yeast tRNAPhe

Chemically induced dynamic nuclear polarization (CIDNP) has been observed from yeast tRNAPhe following reaction with photoexcited riboflavin. At 20 degrees C, several resonances of tRNA in the native form show polarization; previous work predicts that only guanosine and its derivatives in single-stranded regions are likely to become polarized [ McCord , E.F., Morden , K. M., Pardi , A., Tinoco , I., Jr., & Boxer, S. G. (1984) Biochemistry (preceding paper in this issue)]. The methyl protons of m22G -26 show strong negative spin polarization, indicating that this residue is accessible. The solvent accessibility of this residue has not been previously demonstrated. In addition, two positively polarized aromatic resonances are observed, which are likely due to two or more G(H8) protons, including those of G-20, m22G -26, and/or Gm-34. For temperatures below 50 degrees C, a negatively polarized signal in the aromatic region is shown to arise from cross relaxation with the methyl group protons of m22G -26. This indicates the proximity of an aromatic proton, probably H2 of A-44, to the methyl groups of m22G -26. At higher temperatures, the CIDNP spectra show polarization of several additional G resonances, including those of m2G -10. These changes in the CIDNP spectra reflect melting of the tertiary and secondary structure of the tRNA. This work is the first use of CIDNP to study a large nucleic acid molecule and exemplifies the value of this technique in probing single-stranded and solvent-accessible regions of tRNA.

ESR and DF ODMR studies of photoexcited triplets in single crystals of t-stilbene-1,2,4,5-tetracyanobenzene (1:2) complex

The photoexcited triplets in single crystals of the trans-stilbene-1,2,4,5-tetracyanobenzene (1:2) complex have been investigated by means of the ESR and DF ODMR techniques. The ESR lines attributed to localized species exhibit strong spin polarization whose dependence on the direction of the magnetic field has been examined. The zero-field splitting parameters have been measured: X = −308±15 MHz, Y = 2010±15 MHz, Z = −1702±15 MHz with the z axis directed along the normal to the t-stilbene phenyl rings. The negative sign of the DF ODMR signal has been attributed to annihilation of unlike triplets (exciton and X-trap), a process whose occurrence is supported also by decay measurements of the fluorescence.

Electron Spin Polarization in Inner-Shell Excitations in a Solid

Using a ferromagnetic solid as an array of spin-polarized atoms allows one to measure directly the spin dependence of electron scattering in core excitations. Strong spin polarization of the $3p\ensuremath{\rightarrow}3d$ energy-loss electrons scattered from ferromagnetic ${\mathrm{Fe}}_{83}$${\mathrm{B}}_{17}$ has been observed. The large spin polarization in the autoionization emission satellite above the $\mathrm{MMM}$ Auger line is discussed.