Free Electron Value Research Articles

Boltzmann Description of Non-Interacting Electrons in Weakly Localized Regime

Transport process of non-interacting electrons is reformulated, in view of the exact treatment of interacting fermions reported by the present authors in this conference. It makes correspondence to the Boltzmann theory, covering the weakly localized regime, and also eliminates the hidden ultraviolet divergence by incorporating the particle-particle and hole-hole pairs. A mass renormalization of purely two-particle nature appears in the transport equation, giving rise to the conductivity of the Drude form. Taking the maximally crossed diagrams, the 2D conductivity is shown to vanish in the elastic scattering limit, while the distribition function preserves its sharp peak and the transport mass reduces to the free-electron value. The description differs from the self-consistent theory by Vollhardt and Wölfle, in which the Bethe-Salpeter structure of the scattering vertex is abandoned in favour of the time-reversal symmetry so the correspondence to the Boltzmann theory is lost.

Spin-orbit coupling and its effects in organic solids

We present a detailed analysis of spin-orbit coupling (SOC) in $\ensuremath{\pi}$-conjugated organic materials and its effects on spin characteristics including the spin-relaxation time, spin-diffusion length, and $g$ factor. While $\ensuremath{\pi}$ electrons are responsible for low-energy electrical and optical processes in $\ensuremath{\pi}$-conjugated organic solids, $\ensuremath{\sigma}$ electrons must be explicitly included to properly describe the SOC. The SOC mixes up- and down-spin states and, in the context of spintronics, can be quantified by an admixture parameter in the electron and hole polaron states in $\ensuremath{\pi}$-conjugated organics. Molecular geometry fluctuations such as ring torsion, which are common in soft organic materials and may depend on sample preparation, are found to have a strong effect on the spin mixing. The SOC-induced spin mixing leads to spin flips as polarons hop from one molecule to another, giving rise to spin relaxation and diffusion, which are examined by the time-dependent perturbation theory and density-matrix theory. The spin-relaxation rate is found to be proportional to the carrier hopping rate, or equivalently, carrier mobility. The spin-diffusion length depends on the spin mixing and hopping distance but is insensitive to the carrier mobility. An applied electric field causes spin drift and gives rise to upstream and downstream spin-diffusion lengths in the hopping-conduction regime. The SOC influences the $g$ factor of the polaron state and makes it deviate from the free-electron value. The deviation is due to the mixing of different orbitals in the polaron state, which does not include the spin mixing within the same orbital, and therefore underestimates the SOC strength. In particular, the $g$ factor is not sensitive to the molecular geometry fluctuations, where the spin mixing within the same orbital is dominant. The SOCs in tris-(8-hydroxyquinoline) aluminum (Alq${}_{3}$) and in copper phthalocyanine (CuPc) are particularly strong, due to the orthogonal arrangement of the three ligands in the former and Cu $3d$ orbitals in the latter. The theory quantitatively explains the recent measured spin-diffusion lengths in Alq${}_{3}$ from muon spin rotation and in CuPc from spin-polarized two-photon photoemission.

Structural, EPR, and Mössbauer characterization of (μ-alkoxo)(μ-carboxylato)diiron(II,III) model complexes for the active sites of mixed-valent diiron enzymes.

To obtain structural and spectroscopic models for the diiron(II,III) centers in the active sites of diiron enzymes, the (μ-alkoxo)(μ-carboxylato)diiron(II,III) complexes [Fe(II)Fe(III)(N-Et-HPTB)(O(2)CPh)(NCCH(3))(2)](ClO(4))(3) (1) and [Fe(II)Fe(III)(N-Et-HPTB)(O(2)CPh)(Cl)(HOCH(3))](ClO(4))(2) (2) (N-Et-HPTB = N,N,N',N'-tetrakis(2-(1-ethyl-benzimidazolylmethyl))-2-hydroxy-1,3-diaminopropane) have been prepared and characterized by X-ray crystallography, UV-visible absorption, EPR, and Mössbauer spectroscopies. Fe1-Fe2 separations are 3.60 and 3.63 Å, and Fe1-O1-Fe2 bond angles are 128.0° and 129.4° for 1 and 2, respectively. Mössbauer and EPR studies of 1 show that the Fe(III) (S(A) = 5/2) and Fe(II) (S(B) = 2) sites are antiferromagnetically coupled to yield a ground state with S = 1/2 (g= 1.75, 1.88, 1.96); Mössbauer analysis of solid 1 yields J = 22.5 ± 2 cm(-1) for the exchange coupling constant (H = JS(A)·S(B) convention). In addition to the S = 1/2 ground-state spectrum of 1, the EPR signal for the S = 3/2 excited state of the spin ladder can also be observed, the first time such a signal has been detected for an antiferromagnetically coupled diiron(II,III) complex. The anisotropy of the (57)Fe magnetic hyperfine interactions at the Fe(III) site is larger than normally observed in mononuclear complexes and arises from admixing S > 1/2 excited states into the S = 1/2 ground state by zero-field splittings at the two Fe sites. Analysis of the "D/J" mixing has allowed us to extract the zero-field splitting parameters, local g values, and magnetic hyperfine structural parameters for the individual Fe sites. The methodology developed and followed in this analysis is presented in detail. The spin Hamiltonian parameters of 1 are related to the molecular structure with the help of DFT calculations. Contrary to what was assumed in previous studies, our analysis demonstrates that the deviations of the g values from the free electron value (g = 2) for the antiferromagnetically coupled diiron(II,III) core in complex 1 are predominantly determined by the anisotropy of the effective g values of the ferrous ion and only to a lesser extent by the admixture of excited states into ground-state ZFS terms (D/J mixing). The results for 1 are discussed in the context of the data available for diiron(II,III) clusters in proteins and synthetic diiron(II,III) complexes.

ChemInform Abstract: Production, Spectroscopy, and Electronic Structure of Soluble Fullerene Ions.

Taking advantage of the well-separated redox potentials of the mono-, di-, and trianions of C{sub 60} and C{sub 70}, their room-temperature-stable, soluble monoanion radicals have been selectively prepared in bulk quantities by controlled-potential coulometry. Upon reduction, strong bands in the near-IR are observed for C{sub 60}{sup {minus}} but not for C{sub 70}{sup {minus}}. Absorptions at 1,064, 995, and 917 nm are tentatively assigned to the allowed HOMO-LUMO t{sub 1u}-t{sub 1g} transition of C{sub 60}{sup {minus}}, in agreement with theoretical calculations. Vibrational (Raman) bands of excited C{sub 60}{sup {minus}} radical anions at 1,507 and 652 cm{sup {minus}1} and/or t{sub 1g} orbital splittings of similar magnitudes can be calculated from the electronic spectrum. For C{sub 70}{sup {minus}} the allowed HOMO-LUMO 3{sub 1}-a{sub 1} transition is calculated to occur in the IR region. ESR signals with g values below the free electron value are observed for solutions of both C{sub 60}{sup {minus}} and C{sub 70}{sup {minus}}. For C{sub 60}{sup {minus}} the occurrence of facile electron self-exchange in solution is suggested.

Electron Dispersion in Liquid Alkali and Their Alloys

Ashcroft's local empty core (EMC) model pseudopotential in the second-order perturbation theory is used to study the electron dispersion relation, the Fermi energy, and deviation in the Fermi energy from free electron value for the liquid alkali metals and their equiatomic binary alloys for the first time. In the present computation, the use of pseudo-alloy-atom model (PAA) is proposed and found successful. The influence of the six different forms of the local field correction functions proposed by Hartree (H), Vashishta–Singwi (VS), Taylor (T), Ichimaru–Utsumi (IU), Farid et al. (F), and Sarkar et al. (S) on the aforesaid electronic properties is examined explicitly, which reflects the varying effects of screening. The depth of the negative hump in the electron dispersion of liquid alkalis decreases in the order Li → K, except for Rb and Cs, it increases. The results of alloys are in predictive nature.

The Italian affair: The employment of parallel‐plate ionization chambers for dose measurements in high dose‐per‐pulse IORT electron beams

At the high dose-per-pulse rates used by some intraoperative radiotherapy (IORT) units, the employment of ionization chambers for dose measurements needs an appropriate correction (k(sat)) for ion recombination. Through a revision of the existing literature, the authors compared different methods for the determination of the recombination correction factor and their impact on clinical dosimetry. A dosimetric characterization of IORT electron beams from a Linac Hitesys Novac7 (Aprilia-Latina, Italy) was performed. Dose-to-water (D(w)) values were measured with dose-per-pulse independent chemical dosimeters (operated by the Italian Primary Standard Dosimetry Laboratory, ENEA) and compared to doses obtained by two different parallel-plate ionization chamber models (Markus and Advanced Markus, PTW, Freiburg, Germany). For dose measurements using ionization chambers, the authors applied two different methods for the determination of the ion recombination correction factor (k(sat)), as suggested in previous articles. The first method is based on the experimental estimation of the free-electron fraction values p; the second one is based on the "nonstandard" two-voltage analysis including the free-electron component. For a Markus type chamber, there is a good agreement between the results on k(sat) and those reported in the literature for both methods, while for the Advanced Markus chamber, no data are available for comparison. Comparing values of D(w) obtained by dose-per-pulse independent dosimeters and by ionization chambers measurements corrected using the two different approaches, the authors can conclude that the k(sat) factor determination is very critical and that only an experimental protocol using ionization chamber intercalibration can be considered reliable.

Study of electron dispersion curves in liquid alkalis

Ashcroft's empty core local model of pseudopotentials in the second-order perturbation theory is used to study the electron dispersion relation, the Fermi energy and deviation in the Fermi energy from free-electron values for the liquid alkali metals. The influence of the six different forms of the local-field correction functions proposed by Hartree, Vashishta–Singwi, Taylor, Ichimaru–Utsumi, Farid et al. and Sarkar et al. on the aforesaid electronic properties is examined explicitly, which reflects the varying effects of screening. The depth of the negative hump in the electron dispersion of liquid alkalis decreases in the order Li→K; except for Rb and Cs, where it increases.

Shifts of g Values in the Excited Triplet States of Metal Complexes Studied by Time-Resolved W-band EPR

A highly time-resolved high-frequency/high-field W-band electron paramagnetic resonance (EPR) (ν ~ 94 GHz) is a powerful technique to determine small g anisotropies of transient paramagnetic species. We applied this method to studies of the lowest excited triplet (T1)3ππ* states in metal complexes such as a platinum (Pt) diimine complex and metal (Zn and Mg) porphines in rigid glasses. From the analyses of time-resolved EPR spectra, g anisotropies were obtained as gz = 2.0048, gx = gy = 2.0035 for Pt(b-iq)(CN)2 (b-iq = 3,3′bi-isoquinoline) and gz = 1.9968, gx = gy = 2.0022 for zinc tetraphenylporphine (ZnTPP). No measurable anisotropies were found for magnesium (Mg) TPP. The g values of the Pt complex are larger than ge (=2.0023, g value of free electron) and that gz of ZnTPP is smaller than ge. These results were interpreted in terms of the nature of the perturbed states: the higher triplet ππ′* state mixes with T1(ππ*) via spin–orbit coupling in ZnTPP. In contrast, the higher triplet dπ* state is involved in this coupling for the Pt complex. Thus, the nature of the perturbed state can be distinguished from the anisotropic g values of the T1(ππ*) state.

Charge compensation in an irradiation-induced phase of δ-Sc4Zr3O12

Robust materials that are durable under irradiation are needed by a resurgent nuclear industry, and oxide ceramics are leading candidates for a number of nuclear applications, such as the storage of nuclear waste [1]. The delta (d) phase of Sc4Zr3O12 is an example of an oxide that has recently been shown to have excellent radiation tolerance/ amorphization resistance [2, 3]. It has been proposed that the origin of the pronounced radiation tolerance in such materials is linked to crystal structure characteristics [3]. The structure of d-Sc4Zr3O12 is similar to that of fluorite (CaF2), but with rhombohedral (not cubic) symmetry (space group R 3 [4]). Ion irradiation damage experiments on d-Sc4Zr3O12 using 300 keV Kr 2? ions, have revealed an order-to-disorder (O-D) phase transformation from an ordered d-phase to a disordered fluorite structure at low ion dose [2]. This O-D transformation is similar to that observed in other fluorite derivative oxides, especially pyrochlore compounds [5]. However, in d-Sc4Zr3O12 at high ion dose, a surprising second transformation, a disorder-to-order (D-O) transformation, was observed [6]. This D-O transformation was shown to involve a structural rearrangement from a disordered fluorite to an ordered bixbyite structure [6]. Sickafus et al. [7] found that the irradiation-induced bixbyite phase contained a high concentration of Zr. The Sc:Zr ratio in the bixbyite phase was found to be 4:3, just as it is in the d-phase (to within the error of the experimental measurements). This is unexpected because bixbyites are usually sesquioxide compositions with a trivalent cation. This said, we would propose to write the composition of the irradiation-induced bixbyite as ðSc4=7Zr3=7Þ2O3; where the compound is made solely from 3 cations. It is, however, well known that Zr strongly desires a 4? valence. Thus, the bixbyite formula, ðSc4=7Zr3=7Þ2O3; runs counter to intuition. Here we attempt to identify the charge compensation mechanism for incorporation of excess Zr in a Sc–Zr–O bixbyite. As mentioned previously, one of the possible charge compensation mechanisms for Zr incorporation in a Sc– Zr–O bixbyite involves Zr. This ion is a paramagnetic species, so we used EPR spectroscopy to determine the relative abundance of Zr in both a pristine d-Sc4Zr3O12 sample and in an irradiated d-Sc4Zr3O12 sample that contains the bixbyite phase. Figure 1 shows mass-corrected, first-derivative EPR spectra, measured at 5 K, for both pristine and irradiated d-Sc4Zr3O12. Both samples display a weak signal (approximately 10 spins/g) with a g-value of approximately 2.0027, calibrated against DPPH. This is close to the free electron value of 2.00238. Figure 1 also shows an EPR spectrum obtained from a zirconia sample that was annealed in a reducing environment to produce a nominal composition of *ZrO1.98 containing Zr 3? ions [2]. The EPR spectrum from this reduced compound is remarkably similar to that from the ion-irradiated d-Sc4 Zr3O12 sample containing the bixbyite phase. Similar studies [8, 9] on reduced ZrO2 samples have associated M. W. Blair (&) Earth and Environmental Science Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA e-mail: mblair@lanl.gov

Dense Low-Coordination Phases of Lithium

Ab initio density-functional-theory calculations and a structure-searching technique are used to identify candidate high-pressure phases of lithium (Li). We predict threefold coordinated structures to be stable in the pressure range 40-450 GPa and fourfold structures at higher pressures. We describe these low-coordination phases as elemental electrides. All of the stable phases are metallic but the Cmca-24 structure, and two distortions of it which are marginally the most stable in the pressure range 86-106 GPa, are nearly semiconducting with densities of electronic states at the Fermi energy of only a few percent of the free-electron value.

EPR and Luminescence of ${\hbox {F}}^{+}$ Centers in Bulk and Nanophosphor Oxyorthosilicates

The main thermally stimulated luminescence glow peak in irradiated oxyorthosilicates occurs near 360-400 K and has been postulated to comprise an electron trapped at an oxygen vacancy (F+ center). We have used electron paramagnetic resonance spectroscopy to identify this defect in Ln2SiO5:Ce (Ln = Lu and Y) and show that it consists of a single electron trapped at a non-silicon-bonded oxygen vacancy in both bulk and nanophosphor oxyorthosilicates. Both Lu- and Y-based nanophosphors form seven- and nine-oxygen coordinated structures (P21/c) whereas the bulk phosphors form six- and seven-oxygen coordinated structures (C2/c). In each case the F+ center predominately forms at the larger oxygen site. A typical resonance comprises a single Gaussian line broadened by hyperflne interaction with g-values near the free electron value and hyperflne coupling ~0.4 mT. The F+ center can be annealed and radiation-induced, consistent with the thermally stimulated luminescence glow peak behavior.

Electron magnetic resonance study of transition-metal magnetic nanoclusters embedded in metal oxides

Here, we report on the results of an electron magnetic resonance (EMR) study of a series of $\mathrm{Ni}∕\mathrm{Zn}\mathrm{O}$ and $\mathrm{Ni}∕\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ nanocomposites (NCs) to probe the resonance features of ferromagnetic (FM) Ni nanoclusters embedded in metal oxides. Interest in these NCs stems from the fact that they are promising for implementing the nonreciprocal functionality employed in many microwave devices, e.g., circulators. We observe that the EMR spectrum is strongly affected by the metallic FM content and its environment in the NC sample. We report the existence of broad and asymmetric features in the EMR spectra of these NCs. Our temperature dependent EMR data revealed larger linewidth and effective $g$ factor, in the range of 2.1--3.7 (larger than the free electron value of $\ensuremath{\approx}2$), for all samples as temperature is decreased from room temperature to $150\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The line broadening and asymmetry of the EMR features are not intrinsic properties of the metallic nanophase but reflect the local (nonmagnetic or magnetic) environment in which they are embedded. Furthermore, the results of a systematic dependence of the room temperature EMR linewidth and resonant field on the Ni content and the corresponding effective microwave losses measured in previous works show a remarkable correlation. This correlation has been attributed to the dipolar coupling between magnetic nanoparticles in the NCs.

Theoretical calculations of spin-Hamiltonian parameters for CsCdX 3:Ni 2+ (X = Cl, Br) crystals from the two-mechanism model

The high-order perturbation formulas of spin-Hamiltonian (SH) parameters ( g factors g ∥, g ⊥ and zero-field splitting D) for 3d 8 ions in trigonal octahedral sites of crystals are derived considering not only the crystal-field (CF) mechanism, but also the charge-transfer (CT) mechanism (which is neglected in the extensively used CF theory). From these formulas and by considering the suitable impurity-induced local lattice relaxation, the SH parameters of CsCdX 3:Ni 2+ (X = Cl, Br) crystals are calculated. The results are in reasonable agreement with the experimental values. The sign of Q CT ( Q = Δ g ∥, Δ g ⊥ or D, where the g-shift Δ g i = g i − g e, g e ≈ 2.0023 is the free-electron value) due to CT mechanism is the same as that of the corresponding Q CF due to CF mechanism. The relative importance of CT mechanism (characterized by Q CT/ Q CF) increases with the increasing atomic number of ligand X. So, for 3d n ion clusters in crystals with heavy element ligand ion (e.g., Br −), the reasonable explanations of SH parameters should contain the contributions from both CF and CT mechanisms.

Theoretical studies of spin-Hamiltonian parameters for Ni 2+ ions in CsMgX 3 (X=Cl, Br, I) crystals from the two-mechanism model

In this paper, the high-order perturbation formulas of spin-Hamiltonian (SH) parameters ( g factors g ∥, g ⊥ and zero-field splitting D), including both the crystal-field (CF) and for the first time charge-transfer (CT) mechanisms, are established for 3d 8 ions in trigonal octahedral clusters. By using these formulas, the SH parameters of Ni 2+ ions in CsMgX 3 (X=Cl, Br, I) crystals are calculated. The results are consistent with the experimental values. The calculations suggest that the sign of Q CT ( Q=Δ g ∥, Δ g ⊥ or D, where the g-shift Δ g i= g i− g e, g e≈2.0023 is the value of free-electron) due to CT mechanism is the same as that of the corresponding Q CF due to CF mechanism, and the relative importance of CT mechanism (characterized by Q CT /Q CF) increases with the increasing atomic number of ligand X. So, for the 3d n ML m clusters with ligand having large atomic number, the reasonable theoretical explanations of all SH parameters should take both CF and CT mechanisms into account. The defect structure of (NiX 6) 4− impurity centers in CsMgX 3:Ni 2+ crystals is also considered in our model.

Density Functional Theory calculations on the magnetic properties of the model tyrosine radical-histidine complex mimicking tyrosyl radical YD · in photosystem II

Results of Density Functional Theory (DFT) theoretical investigations, which use a model tyrosyl (Tyr) radical and tyrosyl-histidine (Tyr-His) complex to mimick the YD· radical in Photosystem II (PSII) are presented and compared to experimental results from 15N Electron-Nuclear Double Resonance spectroscopy (ENDOR) studies of the τ nitrogen coupling from His-189 in the PSII Tyr-His complex. The DFT calculations are performed using an optimized geometry of the tyrosine radical and Tyr-His complex. The conformational space of the Tyr-His tandem is explored by varying the relative geometry of the two components; relevant parameters, such as the spin distribution on the phenoxy-ring carbons of the Tyr radical and the EPR hyperfine tensors, are calculated at each geometry and compared with the available experimental data. The isotropic 15N-ENDOR signal arising from spin delocalization on the His hydrogen-bonded to the PSII tyrosine radical is analyzed in terms of the DFT obtained parameters. The calculations of the g tensor using the Gauge Independent Atomic Orbital (GIAO) approach are presented and the influence of the geometry of the Tyr-His complex on the deviation of the g-tensor elements from the free electron values is discussed.

Theoretical investigations of the EPR parameters for Cr 3+ and Mn 4+ ions in PbTiO 3 crystals

The complete high-order perturbation formulas of EPR parameters ( g factors g ∥, g ⊥ and zero-field splitting D), containing the crystal-field (CF) mechanism and charge-transfer (CT) mechanism (the latter is omitted in crystal-field theory which is often used to study the EPR parameters), are established from a cluster approach for 3d 3 ions in tetragonal octahedral sites. According to the calculations based on these formulas, the EPR parameters g ∥, g ⊥ and zero-field splitting D for Cr 3+ and Mn 4+ ions in PbTiO 3 crystals are explained reasonably. The calculations show that (i) the sign of g-shift Δ g i CT (= g i − g s, where g s = 2.0023 is free-electron value and i = ∥ and ⊥) in CT mechanism is opposite to, but that of D CT is the same as, the corresponding signs in the CF mechanism and (ii) the relative importance of CT mechanism for the high valence state 3d 3 ion (e.g., Mn 4+) is large and so the contributions to EPR parameters from CT mechanism should be taken into account. The different sign of splitting D and the different defect structure for Cr 3+ and Mn 4+ impurity centers in PbTiO 3 crystals are also suggested from the calculations. The results are discussed.

Studies of the EPR g factor for Ni 2+ ion in CsMgX 3 (X = Cl, Br, I) crystals

The complete high-order perturbation formula of g factor, including not only the widely used crystal-field (CF) mechanism, but also the neglected change-transfer (CT) mechanism in the CF theory, is established for a 3d 8 ion in cubic octahedral site. From the formula, the g-shifts Δ g (= g − g s, where g s ≈ 2.0023, the value of free electron) of Ni 2+ ion in CsMgX 3 (X = Cl, Br, I) crystals are calculated. The results suggest that the g-shift Δ g CT due to the CT mechanism and the Δ g CF due to CF mechanism have the same sign and the importance of Δ g CT follows the order: CsMgI 3: Ni 2+ > CsMgBr 3: Ni 2+ > CsMgCl 3: Ni 2+. So, in the calculations of g or Δ g of 3d n MX m clusters in crystals, the contributions to g factor from both the CT and CF mechanisms should be taken into account in the case of heavy-element ligand ions, such as Br − and I − ions.

Synthesis and structure of the 2-(chlorophenylazo)pyridine chelated tricarbonylrhenium(I) complex and its anion radical derivatives via electrochemical reduction

Reacting Re(CO)5Cl with the azopyridine ligand (1) (L) in boiling benzene afford the complex Re(CO)3Cl(L), (2) in excellent yield [L=2-(p-Cl-C6H4NN)C5H4N]. The chelation of the azopyridine ligand accompanied by displacement of the two carbon monoxide ligands furnish a five-membered chelate ring. Structure determination of complex (2) has revealed a distorted octahedral ReC3N2Cl coordination sphere. The Re–N(pyridine) and, Re–N(azo) distances are 2.158(3) and 2.153(6) A respectively, and the N–N length [1.273(4) A], implicate relatively weak Re-azo(π*) back–bonding. The Re(CO)3Cl(L) lattice consists of C–H...Cl hydrogen bonding and Cl...O non-bonded interactions constituting a supramolecular network. Extended Huckel calculations reveal that the LUMO of Re(CO)3Cl(L) is Ca. 57% azo in character. One-electron quasireversible electrochemical reduction of the complex occurs near −0.3 V versus Saturated Calomel electrode(s.c.e.) The redox orbital is believed to belong to the above noted LUMO. Electrogenerated Re(CO)3Cl(L•–) underwent spontaneous solvolytic chloride displacement in MeCN, resulting in the isolation of Re(CO)3(MeCN)(L•–). The latter complex in turn reacted with imidazole and triphenylphosphine, furnishing Re(CO)3(C3H4N2)(L•–) and Re(CO)3(PPh3)(L•–), respectively. The pattern of carbonyl stretching frequencies of these radical anion complexes is similar to that of Re(CO)3Cl(L) but with shifts to lower frequencies by 10–20 cm−1. All three radical anion systems are one-electron paramagnetic (1.7–1.8 μ B ). The unpaired electron is primarily localized on the azoheterocycle ligand in a predominantly azo-π* orbital, but a small metal contribution (185, 187Re, I=5/2) is also present. Thus Re(CO)3(MeCN)(L•–) and Re(CO)3(C3H4N2)(L•–) display six-line e.p.r. spectra (A ˜ 28 G). The line shapes and intensities are characteristic of the presence of g-strain. In the case of Re(CO)3(PPh3)(L•–) seven nearly equispaced lines are observed due to virtually equal coupling between the metal and 31P (I=½) nuclei. The g-values of the radical species are slightly higher than the free-electron value of 2.0023.

Magnetic properties of nearly defect-free maghemite nanocrystals

Temperature variations of the magnetization $M$ and the electron magnetic resonance (EMR) parameters of maghemite $(\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3})$ nanocrystals are reported for the $4\phantom{\rule{0.3em}{0ex}}\mathrm{K}--300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ range. Transmission electron microscopy of the nanocrystals shows them to be nearly spherical samples (aspect ratio $a∕b=1.15$) with diameter $D=7(1)\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, and analysis of x-ray diffraction lines yields $D=6.4\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ with negligible strain. $M$ versus $T$ data show blocking temperatures ${T}_{B}\ensuremath{\simeq}101\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, $89\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, and $68\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in measuring fields $H=50$, 100, and $200\phantom{\rule{0.3em}{0ex}}\mathrm{Oe}$ respectively. $M$ versus $H$ data for $T>{T}_{B}$ fits the modified Langevin function $M={M}_{s}\mathcal{L}({\ensuremath{\mu}}_{p}H∕{k}_{B}T)+{\ensuremath{\chi}}_{a}H$ with ${\ensuremath{\mu}}_{p}=8000(500){\ensuremath{\mu}}_{B}∕\text{particle}$ and ${M}_{s}=80\phantom{\rule{0.3em}{0ex}}\mathrm{emu}∕\mathrm{g}$, identical to ${M}_{s}$ for bulk $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$. It is argued that this large value of ${M}_{s}$, the small value of coercivity ${H}_{c}\ensuremath{\simeq}20\phantom{\rule{0.3em}{0ex}}\mathrm{Oe}$ at $5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the lack of exchange bias in a field-cooled sample, and negligible strain point to nearly defect-free nanocrystals. In the EMR studies, two resonance lines are observed, one with the resonance field ${H}_{r}$ greater than that for the free-electron value and the other smaller. From the temperature variations of ${H}_{r}$ and the linewidths of the two lines, it is argued that the two lines are, respectively, due to nanocrystals with easy-axis aligned perpendicular and parallel to the applied field. The relatively narrow intrinsic linewidths $(\ensuremath{\simeq}400\phantom{\rule{0.3em}{0ex}}\mathrm{Oe})$ of the two lines in nearly defect-free nanocrystals facilitated their observation.

Thermal and electrical characterization of Cu/CoFe superlattices

The present work is directed at thermal and electrical characterization of the Cu/CoFe multilayer, which is made of extremely thin periodic layers, using steady-state Joule heating and thermometry in suspended bridges in the temperature range of 50–300 K. The total thickness of the layer is ds=144 nm, while the thickness of individual repeats are 12 and 21 Å for CoFe and Cu layers, respectively. The experimental data for thermal conductivity of a 144-nm-thick single Cu layer is also presented for comparison. The experimental data indicates that the spin-dependent electron scattering at the Cu/CoFe interface contributes to a strong reduction in thermal conductivity of these layers compared to the bulk values. The calculated Lorenz numbers (from the thermal and electrical conductivity data) varies by nearly a factor 2 from 4×10−8 W Ω K−2 at 50 K to 1.8×10−8 W Ω K−2 at 300 K and is different from the free electron value of L0=2.45×10−8 W Ω K−2. This implies that the Wiedemann-Franz law does not hold for Cu/CoFe thin films.