Independent Spins Research Articles

Electron-Rich Iron/Ruthenium Arylalkynyl Complexes for Third-Order Nonlinear Optics: Redox-Switching between Three States

The new [(η(2)-dppe)(η(5)-C(5)Me(5))Fe(C≡C-1,4-C(6)H(4)C≡C)Ru(η(2) -dppe)(2) C≡C(C(6)H(5))] complex (3-H) and its hexanuclear relative [{(η(2)-dppe)(η(5)-C(5) Me(5))Fe(C≡C-1,4-C(6)H(4)-C≡C)Ru(η(2)-dppe)(2)(C≡C-1,4-C(6)H(4)C≡C)(3)(1,3,5-C(6)H(3))] (4) have been synthesized and characterized. The linear and cubic nonlinear optical properties of these compounds in their various redox states have been studied along with those of the analogous complexes [(η(2)-dppe)(η(5)-C(5)Me(5))Fe(C≡C-1,4-C(6)H(4)C≡C)Ru(η(2)-dppe)(2)R][PF(6)](n) (n=0-2; R=Cl, 2-Cl; R=C≡C(4-C(6)H(4)NO(2)),3-NO(2)). We show that molecules exhibiting large third-order nonlinearities can be obtained by assembling such dinuclear Fe/Ru units around a central 1,3,5-substituted C(6)H(3) core. These data are discussed with a particular emphasis on the large changes in their nonlinear (third-order) optical properties brought about by oxidation. Experimental and computational (DFT) evidence for the electronic structures of these compounds in their various redox states is presented using 3-H(n+) as a prototypical model. Single crystals of this complex in its mono-oxidized state (3-H[PF(6)]) provide the first structural data for such carbon-rich Fe(III) /Ru(II) heteronuclear mixed-valent (MV) systems. Although experimental evidence for the structure of the dioxidized states was more difficult to obtain, the theoretical study reveals that 3-H(2+) can be considered to have a biradical structure with two independent spins. The low-lying absorptions that appear in the near-infrared (NIR) range for all these compounds following oxidation correspond to intervalence charge-transfer (IVCT) bands for the mono-oxidized states and to ligand-to-metal charge-transfer (LMCT) transitions for the dioxidized states. These play a crucial role in the strong optical modulation achieved. The possibility of accessing additional states with distinct linear or nonlinear optical properties is also briefly discussed.

ChemInform Abstract: Magnetic Properties of Radical, Crystalline Mixed Cyclopentadienyl/Dithiolene Complexes

AbstractReview: 76 refs.

Magnetic strong coupling in a spin-photon system and transition to classical regime

We study the energy level structure of the Tavis-Cumming model applied to an ensemble of independent magnetic spins $s=1/2$ coupled to a variable number of photons. Rabi splittings are calculated and their distribution is analyzed as a functin of photon number $n_{\rm max}$ and spin system size $N$. A sharp transition in the distribution of the Rabi frequency is found at $n_{\rm max}\approx N$. The width of the Rabi frequency spectrum diverges as $\sqrt{N}$ at this point. For increased number of photons $n_{\rm max}>N$, the Rabi frequencies converge to a value proportional to $\sqrt{n_{\rm max}}$. This behavior is interpreted as analogous to the classical spin resonance mechanism where the photon is treated as a classical field and one resonance peak is expected. We also present experimental data demonstrating cooperative, magnetic strong coupling between a spin system and photons, measured at room temperature. This points towards quantum computing implementation with magnetic spins, using cavity quantum-electrodynamics techniques.

Two destructive effects of decoherence on Bell inequality violation

We consider a system of two spin-(1/2) particles, initially in an entangled Bell state. If one of the particles is interacting with an environment (e.g., a collection of N independent spins), the two-particle system undergoes decoherence. Using a simple model of decoherence, we show that this process has two consequences. First, the maximal amount by which the Clauser-Horne-Shimony-Holt inequality is violated decays to zero. Second, the set of directions of measurement for which the inequality is violated is reduced in the course of decoherence. The volume of that set is bounded above by constx|r|{sup 2}, where r is the decoherence factor. We obtain similar results for the case when each of the two particles is in interaction with a separate environment.

Monte Carlo without chains

A sampling method for spin systems is presented. The spin lattice is written as the union of a nested sequence of sublattices, all but the last with conditionally independent spins, which are sampled in succession using their marginals. The marginals are computed concurrently by a fast algorithm; errors in the evaluation of the marginals are offset by weights. There are no Markov chains and each sample is independent of the previous ones; the cost of a sample is proportional to the number of spins (but the number of samples needed for good statistics may grow with array size). The examples include the Edwards-Anderson spin glass in three dimensions.

Counting 1/8-BPS dual-giants

We count 1/8-BPS states in type IIB string theory on AdS_5 x S^5 background which carry three independent angular momenta on S^5. These states can be counted by considering configurations of multiple dual-giant gravitons up to N in number which share at least four supersymmetries. We map this counting problem to that of counting the energy eigen states of a system of N bosons in a 3-dimensional harmonic oscillator. We also count 1/8-BPS states with two independent non-zero spins in AdS_5 and one non-zero angular momentum on S^5 by considering configurations of arbitrary number of giant gravitons that share at least four supersymmetries.

Meso–meso‐Linked Corrole Dimers with Modified Cores: Synthesis, Characterization, and Properties

The synthesis and characterization of the first examples of core-modified corrole dimers linked through the meso positions are described. The dimers are obtained by a simple Ag(I)- or Fe(III)-catalyzed coupling reaction in near-quantitative yields. The corroles obtained are metalated with metal ions, such as Cu(II) and Ni(II). The electronic-absorption spectral studies reveal weak electronic interaction between the two subunits and the exciton coupling observed for the free-base corrole dimer (1717 cm(-1)) is lower than that for the corresponding protonated derivative (4081 cm(-1)). The solution-state structure derived from (1)H and two-dimensional NMR spectral studies reveals a noncoplanar arrangement of two corrole units. Geometry optimization at the B3LYP-631G level also confirms the noncoplanar arrangement of corroles with a dihedral angle of 64.8 degrees between two corrole planes. The electron paramagnetic resonance (EPR) and magnetic characterization studies on the paramagnetic copper-metalated dimer indicate that both copper ions behave as independent spins without any noticeable interaction. Results of fluorescence studies reveal a bathochromic shift of about 60 nm upon dimerization. The first hyperpolarizability (beta) measured by using the hyper Rayleigh scattering (HRS) method reveals doubling of the beta values on progressing from monomer to dimer, attributed to enhanced pi conjugation. The use of copper dimers in the photocleavage of DNA is also explored. It is shown that the bimetallic copper dimer selectively cleaves the nucleic acids without affecting the proteins, suggesting a possible application of the copper complex in the removal of nucleic acid contaminants from protein extracts through a simple photolytic pathway.

Photoinduced Electron Transfer in Solid C60 Donor/Acceptor Complexes Studied by Light-Induced Electron-Spin Resonance

ABSTRACT Light-induced ESR (LESR) spectroscopy was used to study photoinduced electron transfer (PIET) in the crystals of the C60 donor/acceptor complexes. High-field LESR signals attributed to (g = 1.9984−1.9992 and ΔH = 3−6 G at 100 K) were observed in fullerene complexes with Leuco Crystal Violet (LCV·C60·C6H5Cl, 1), N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPDA·C60, 2), and bianthrone ((BA)2C60, 3), whereas a low-field LESR signal in 2 was attributed to TMPDA•+. Microwave power studies reveal two independent spins in 2 and show high relaxation rates for photoinduced spins on . The LESR signals consist of a major long-lived “persistent” component and a minor “prompt” one, which disappears immediately after light is off. The maximal intensity of the LESR signals was observed for the excitation of 1–3 below 2 eV. This range of photon energy corresponds to direct intermolecular charge transfer from donor to C60 molecules. PIET is temperature activated with Ea = 23, 43 and 49 meV for 1, 2, and 3, respectively. Photoinduced spin susceptibility attains the maximum at 30 K in 1 and decreases with temperature down to 4 K indicating a possible antiferromagnetic interaction of spins, while in 2 it increases with the temperature decrease down to 4 K.

Critical properties of two-level atom systems interacting with a radiation field

The thermodynamic properties of $N$ two-level atoms coupled to radiation field are analyzed taking into account the diamagnetic term by a slight modification to the usual approach to quantizing the electromagnetic field. Following this procedure, we are able to solve the discrepancy originating in the use of different states as a basis for expressing the state of radiation field. We develop a perturbative expansion of partition function and a simple analytic expression is found for a high coupling constant. We show that, in this limit, the interaction of independent atomic spins with a single photon mode can be interpreted as an effective spin-spin interaction of long range nature. Even in this approach, if the diamagnetic term is taken into account, a phase transition at nonzero temperature cannot occur due to sum-rule arguments.

Statistical mechanics in the extended Gaussian ensemble.

The extended Gaussian ensemble (EGE) is introduced as a generalization of the canonical ensemble. This ensemble is a further extension of the Gaussian ensemble introduced by Hetherington [J. Low Temp. Phys. 66, 145 (1987)]. The statistical mechanical formalism is derived both from the analysis of the system attached to a finite reservoir and from the maximum statistical entropy principle. The probability of each microstate depends on two parameters beta and gamma which allow one to fix, independently, the mean energy of the system and the energy fluctuations, respectively. We establish the Legendre transform structure for the generalized thermodynamic potential and propose a stability criterion. We also compare the EGE probability distribution with the q-exponential distribution. As an example, an application to a system with few independent spins is presented.

Design and synthesis of a new binucleating ligand via cobalt-promoted C-N bond fusion reaction. Ligand isolation and its coordination to nickel, palladium, and platinum.

A new polydentate bridging ligand, NH(4)C(5)N=NC(6)H(4)N(H)C(5)H(4)N (HL(2)), is synthesized by the cobalt-mediated phenyl ring amination of coordinated NH(4)C(5)N=NC(6)H(5). The green cobalt complex intermediate [Co(L(2))(2)](ClO(4)), [1](ClO(4)), and the free ligand HL(2) were isolated and characterized. The X-ray structure of [H(2)L(2)](ClO(4)) is reported. The ligand, upon deprotonation, behaves as a bridging ligand. It reacts with NiCl(2).6H(2)O and Na(2)[PdCl(4)] to produce dimetallic complexes, [Ni(2)Cl(2)(L(2))(2)], 2, and [Pd(2)(L(2))(2)](ClO(4))(2), [3](ClO(4))(2), respectively. X-ray structures of these two dimetallic complexes are reported. The structure of the dinickel complex, in particular, is unique. In this complex, the two deprotonated secondary amine nitrogens of the two [L(2)](-) ligands bind to two nickel centers simultaneously forming a planar Ni(2)N(2) arrangement. The complex [3](ClO(4))(2) is diamagnetic while the complex 2 is paramagnetic. The results of magnetic measurements on the dinickel complex in the temperature range 1.8-300 K are reported. The system can be described as a single spin S = 2 in the low-temperature range T << J/k whereas at high temperatures, T >> J/k, it behaves as two independent spins S = 1.The reaction of [L(2)](-) with K(2)[PtCl(4)], however, yielded a monometallic platinum complex, [PtCl(3)(L(2))], 5, where the pyridyl nitrogen of the aminopyridyl function remained unused. The X-ray structure of the complex 4a is reported. The bond lengths along the ligand backbones in all the complexes indicate extensive pi-delocalization. Spectral data of the complexes are reported and compared.

Radikal-markierte Metallocene (M = Fe, Co). Metallocenylen verbrückte Bis(3-imidazolin-l-oxyle) / Radical Labeled M etallocenes (M = Fe, Co). M etallocenylene B ridged B is(3-im idazolin-l-oxyls)

Abstract Radical Labeled M etallocenes (M = Fe, Co). M etallocenylene B ridged B is(3-im idazolin-l-oxyls) The addition of the lithium salt of the silyl protected hydroxylamine 1-hydroxy-2,2,4,5,5-pentamethyl-3-imidazoline (1) to 6 ,6 -dimethylfulvalene gives the lithium cyclopentadienide 2 from which the 1,1 '-substituted ferrocene and cobaltocinium derivatives 3 and 4 were obtained. Deprotection and oxidation of 3 and 4 afford the metallocenylene bridged bis(3-imidazolin-l-oxyls) 5 and 7. The ferrocene bridged diradical 5 forms with DDQ a charge transfer complex 8 . The complexes 5 and 7 were characterized by X-ray diffraction and by their EPR spectra. Magnetic measurements show at room temperature two independent spins for 5 and 7 and four unpaired electrons for 8 , the latter arising from three different sources [Fe(III) low spin d5, two nitroxides and DDQ- radical anion].

Vanadium, molybdenum, and sodium triethanolamine complexes derived from an assembly system containing tetrathiometalate and triethanolamine.

The reaction system composed of triethanolamine (TEA) and tetrathiovanadate in the presence of MeONa (MeOLi) or NaNH2 was studied to afford vanadium and alkali metal TEA complexes. Complexes [M subsetV6O6[N(CH2CH2O)2(CH2CH2OH)]6]2S6 (M = Na (1), Li (2)) contain two cyclic V((IV)) [12]metallacrown-6 cations linked by a polysulfide S6(2-) anion. Also separated from the reaction system is a novel sodium TEA complex [[Na(TEA)]2S6]n (3), in which parallel coordination chains [Na(TEA)+]n are connected by O-H...S hydrogen bonds forming 3-D network structure. Variable-temperature conductance of 3 was determined to display semiconductor feature. The desulfurization of VS4(3-) was observed to form S6(2-) anion and/or S8 molecule in the synthetic reactions of 1-3. A striking contrast was noted that MoS4(2-) did not desulfurize in the similar reaction to that of VS4(3-) with TEA, and a Na/TEA complex containing MoS4(2-), (Et4N)2[Na2(mu-TEA)2(CH3OH)2](MoS4)2 (4), was obtained instead. The S...H-O and O...H-O hydrogen bonds play an important role in forming the one- or three-dimensional structures for all these complexes by the linkages between clusters and chains. The IR spectrum indicates the structural similarity of complexes 1 and 2. For complex 4, the Mo=S stretch vibration at 468 cm(-1) showing widening and slight red-shift and the slight movement of the 1H NMR signals of TEA ligand to downfield are considered to be the influence of the S...H-O hydrogen bonds. Variable temperature magnetic susceptibility data were collected for complex 1. The effect magnetic moment per V6 unit varies gradually from 4.04 muB at 300 K to 6.24 muB at 5 K, exhibiting ferromagnetic interaction. Heisenberg-type vector-coupling model and molecular field approximation were used to treat the interaction between the paramagnetic sites and gave a good fitting result: J = 3.97 cm(-1), J' = 1.99 cm(-1), g = 1.99, F = 3.64 x 10(-3). Density functional calculation was also performed to complex 1 and its Li, K analogues. The lowest total bonding energy of -874.001 eV was obtained for [Na subsetV6(mu3-O)6] complex containing six unpaired electrons, indicating the stability of a V6 system with six independent spins S = 1/2. When the alkali metal ion is inserted into the [V6(mu3-O)6] cavity, the positive values of the bonding interaction (E(b)) indicate that the insertion depresses the total energy and that complexes 1 and 2 together with K+ complex may exist. However, the lowest E(b) value of the K+ complex implies its instability.

Low temperature specific heat and magnetic properties of the pure organic ferromagnet Dupeyredioxyl

The bulk magnetic properties of the genuine pure organic ferromagnet Dupeyredioxyl ( N, N′-dioxy-1,3,5,7-tetramethyl-2,6-diazaadamantane, C 12N 2O 2H 20) are presented. Magnetization below T c=1.48 K presents no measurable hysteresis, and the anisotropy seems to be due only to crystal shape demagnetization effects. The specific heat presents a nice lambda peak at 1.44 K, and a T 3/2 power law below 1 K that is expected for spin wave excitations in the low T limit. The entropy change due to the phase transition does not give the expected value for two independent spins: 1/2 per molecule. The high-temperature susceptibility presents a Curie–Weiss temperature of 7.0 K. Magnetization curves up to 9 T above T c do not follow a universal behavior with H/ T below 50 K. The results are consistent with a model where intramolecular interactions between pairs of the ‘free spins’ form a triplet state well above T c and these S=1 molecules finally order into a true three-dimensional ferromagnetic transition due to intermolecular interactions below 1.5 K.

Thermodynamic picture of the glassy state gained from exactly solvable models

A picture for thermodynamics of the glassy state was introduced recently by us [Phys. Rev. Lett. 79, 1317 (1997); 80, 5580 (1998)]. It starts by assuming that one extra parameter, the effective temperature, is needed to describe the glassy state. This approach connects responses of macroscopic observables to a field change with their temporal fluctuations, and with the fluctuation-dissipation relation, in a generalized, nonequilibrium way. Similar universal relations do not hold between energy fluctuations and the specific heat. In the present paper, the underlying arguments are discussed in greater length. The main part of the paper involves details of the exact dynamical solution of two simple models introduced recently: uncoupled harmonic oscillators subject to parallel Monte Carlo dynamics, and independent spherical spins in a random field with such dynamics. At low temperature, the relaxation time of both models diverges as an Arrhenius law, which causes glassy behavior in typical situations. In the glassy regime, we are able to verify the above-mentioned relations for the thermodynamics of the glassy state. In the course of the analysis, it is argued that stretched exponential behavior is not a fundamental property of the glassy state, though it may be useful for fitting in a limited parameter regime.

Light-induced ESR studies in conjugated polymer-fullerene composites

In this contribution we discuss photo-induced electron transfer in conjugated polymer/fullerene composites and compare this with light induced effects in pure components. Two overlapping LESR lines are observed, from positive polarons on the polymer chains and negative charges on the fullerene moieties. Microwave power saturation studies show different relaxation times for these two spins, giving clear evidence for independent spins. The unusually high relaxation rate of the fullerene monoanionic spins is of intrinsic origin and discussed in terms of a splitting of the T 1u level by a Jahn-Teller type distortion as proposed in the literature. We observed two distinct contributions to LESR signals, a prompt and a persistent one. Excitation light intensity dependence of the prompt contributions into P + and C 60 − ESR signals is of bimolecular type (I 0.5) and implies the mutual annihilation within (P +…C 60 −) pair. The persistent contribution is found to be excitation intensity independent and proposed to originate from deep traps due to disorder.

Photoinduced charge carriers in conjugated polymer–fullerene composites studied with light-induced electron-spin resonance

Detailed studies on photoinduced spins in conjugated polymer/fullerene composites using (cw) light-induced electron-spin-resonance (LESR) technique are reported. Two overlapping LESR lines are observed, from positive polarons on the polymer chains and negative charges on the fullerene moieties. Microwave power saturation studies show different relaxation times for these two spins, ruling out spin-exchange correlations, giving clear evidence of independent spins. The unusually high relaxation rate of the fullerene monoanionic spins is of intrinsic origin, and discussed in terms of a splitting of the T1u level by a Jahn-Teller-type distortion as proposed in the literature. Further, we observed two distinct contributions to LESR signals: a prompt one and a persistent one. The excitation light intensity dependence of the prompt contributions into the P+ and C60- ESR signals is of bimolecular type (I0.5), and implies mutual annihilation within the created (P+...C60-) pair. The persistent contribution is found to be excitation intensity independent, and is proposed to originate from deep traps due to disorder.

Thermodynamic Properties of the One-Dimensional Kondo Insulators Studied by the Density Matrix Renormalization Group Method

Thermodynamic properties of the one-dimensional Kondo lattice model at half-filling are studied by the density matrix renormalization group method applied to the quantum transfer matrix. Spin susceptibility, charge susceptibility, and specific heat are calculated down to T=0.1t for various exchange constants. The obtained results clearly show crossover behavior from the high temperature regime of nearly independent localized spins and conduction electrons to the low temperature regime where the two degrees of freedom couple strongly. The low temperature energy scales of the charge and spin susceptibilities are determined and shown to be equal to the quasiparticle gap and the spin gap, respectively, for weak exchange couplings.

General 2+1 dimensional effective actions and soliton spin fractionalization

We propose actions for non-linear sigma models on cosets G H in 2+1 dimensions that include the most general non-linear realizations of Chern-Simons terms. When G is simply connected and H contains r commuting U(1) factors, there are r different topologically conserved charges and generically r different types of topological solitons. Soliton spin fractionalizes as a function of the Chern-Simons couplings, with independent spins associated to each species of soliton charge, as well as to pairs of different charges. This model of soliton spin fractionalization generalizes to arbitrary G H a model of Wilczek and Zee for one type of soliton.

Multiple Ising spins coupled to 2d quantum gravity

We study a model in which p independent Ising spins are coupled to 2d quantum gravity (in the form of dynamical planar φ3 graphs). Consideration is given to the p → ∞ limit in which the partition function becomes dominated by certain graphs; we identify most of these graphs. A truncated model is solved exactly providing information about the behaviour of the full model in the limit β → 0. Finally, we derive a bound for the critical value of the coupling constant, βc and examine the magnetization transition in the limit p → 0.