Two-dimensional short-range interacting attractive and repulsive Fermi gases at zero temperature

We study a two-dimensional two-component Fermi gas with attractive or repulsive short-range interactions at zero temperature. We use Diffusion Monte Carlo with Fixed Node approximation in order to calculate the energy per particle and the opposite spin pair distribution functions. We show the relevance of beyond mean field effects and verify the consistency of our approach by using Tan’s Contact relations.     

Exchange interaction potential in a fluctuating field of a disordered system of electron spins

We studied spin interactions within the strong coupling approximation using a contour representation of operators. Based on this representation, the exchange interaction potential was found. It was shown that the effective exchange interaction is caused not only by pair correlations, but has a more complex origin that depends on four-particle interactions.     

Quantitative time-resolved EPR CIDEP study of the photodecomposition oftrans-azocumene in solution

The cumyl radical system, which is created after laser flash irradiation oftrans-azocumene in benzene solution at room temperature, is investigated using time-resolved EPR spectroscopy. From the quantitative analysis of EPR time-profiles at different microwave powers the spin relaxation timesT ₁=3.5±0.3 μs andT ₂=2.5±0.1 μs are evaluated as well as the magnitude of the chemically induced electron polarization (CIDEP), which is generated by the radical pair mechanism (RPM). The geminate RPM polarization is found to be considerably smaller than the F-pair one, 32±2 and 48±5 in units of the Boltzmann polarization, respectively. This is attributed to an initial radical separation in the geminate pair, caused by the cleavage reaction. Besides cleavage, the photoexcitedtrans-azocumene also decays via isomerization to the thermally unstablecis-isomer, the lifetime of which is found to be 14±3 μs at 293 K in benzene, three times longer than in cyclohexane. The quantum yield of free radicals, escaping from the primary cage, is determined as 0.28±0.06 for the decay of the excitedtrans-azocumene and 0.18±0.04 for the thermal cleavage of thecis-isomer. The self-termination of cumyl radicals proceeds with a rate constant 2k _t=7±1)·10⁸ M^?1s^?1 in benzene at RT.     

Spin-vibrational interactions in geminate radical pairs: Effective spin Hamiltonian including anharmonicity

An equation for the effective spin Hamiltonian including the interaction of radical pair spins with vibrations in both linear and higher approximations was obtained. Even in the linear approximation, the Hamiltonian was found to be non-Heisenberg. The influence of a sound field on the probability of geminate radical pair recombination is considered in the Appendix. The results obtained can be of use for solving certain nanochemistry problems.     

The electronic structure and the ferromagnetic intermolecular interactions in the crystal of a diphenyl nitroxide derivative

Based on the generalized gradient approximation, full potential linearized augmented plane-wave calculations have been performed to study the electronic band structure and the ferromagnetic (FM) interactions of the 9,9-dipropyl-9,10-dihydroacridin-10-yloxyl (DPAO) organic radical. The total and partial density of states and the atomic spin magnetic moments are calculated and discussed. It is found that the unpaired electrons in this radical are localized in a molecular orbital constituted primarily of the π* (NO) orbital, and the main contribution of the spin magnetic moment comes from the NO free radical, with little contributions from other C atoms. The origin of FM interactions is also studied in detail.     

Multifrequency time-resolved EPR (9.5GHz and 95GHz) on covalently linked porphyrin-quinone model systems for photosynthetic electron transfer: effect of molecular dynamics on electron spin polarization

Covalently linked porphyrin–quinone model systems for photosynthetic electron transfer were examined by using time-resolved electron paramagnetic resonance (TREPR) at intermediate magnetic field and microwave frequency (0.34T/9.5GHz, X-band) and high field and frequency (3.4T/95GHz, W-band). The paramagnetic transients studied were the light-induced spin-correlated radical pair states of the donor–acceptor complex in polar solvents below the melting point and in the soft glass phase of a liquid crystal. It is shown that the systems form strongly exchange-coupled radical pairs, whose TREPR lineshapes are determined mainly by fast electron recombination together with both spin–lattice relaxation and modulation of the exchange interaction. Below the melting point the spin–lattice relaxation rate naturally slows down, but that of the spin on the quinone site is still of the order of 10⁶ s^-1. Most probably this is due to contributions from spin–rotation interaction, and dependent on the molecular orientation with respect to the magnetic field. This relaxation anisotropy is related to anisotropic motion of the quinone site in the solvent cage. The results allow conclusions to be drawn concerning the molecular dynamics and flexibility of the systems. To yield long-lived radical pair states that would mimic photosynthetic electron transfer, the two mechanisms described, modulation of exchange and spin–rotation interactions, have to be suppressed by reducing the molecular flexibility of the complex.     

Study of semileptonic and nonleptonic decays of the B<Subscript>c</Subscript><Superscript>-</Superscript> meson

We evaluate semileptonic and two-meson nonleptonic decays of the B _c ^- meson in the framework of a nonrelativistic quark model. The former are done in spectator approximation using one-body current operators at the quark level. Our model reproduces the constraints of heavy-quark spin symmetry obtained in the limit of infinite heavy-quark mass. For the two-meson nonleptonic decays we work in factorization approximation. We compare our results to the ones obtained in different relativistic approaches.     

Top quark physics at hadron colliders

The top quark, discovered at the FERMILAB TEVATRON collider in 1995, is the heaviest known elementary particle. Today, ten years later, still relatively little is known about its properties. The strong and weak interactions of the top quark are not nearly as well studied as those of the other quarks and leptons. The strong interaction is most directly measured in top quark pair production. The weak interaction is measured in top quark decay and single top quark production, which remains thus far unobserved. The large top-quark mass of about 175 GeV/c² suggests that it may play a special role in nature. It behaves differently from all other quarks due to its large mass and its correspondingly short lifetime. The top quark decays before it hadronises, passing its spin information on to its decay products. Therefore, it is possible to measure observables that depend on the top quark spin, providing a unique environment for tests of the Standard Model and for searches for physics beyond the Standard Model. This report summarises the latest measurements and studies of top quark properties and rare decays from the TEVATRON in Run II. With more than 1 fb^-1 of luminosity delivered to each experiment, CDF and DO, top quark physics at the TEVATRON is at a turning point from first studies to precision measurements with sensitivity to new physics. An outlook onto top quark physics at the Large Hadron Collider (LHC) at CERN, planned to begin operation in the year 2007, is also given.     

Free radicals in irradiated pepper: An electron spin resonance study

With electron spin resonance (ESR) spectroscopy, we revealed various free radicals in commercially available pepper in Japan before and after γ-irradiation. The representative ESR spectrum of the pepper is composed of a sextet centered atg=2.0, a singlet at the sameg-value and a singlet atg=4.0. The first signal is attributable to a signal with hyperfine interactions of the Mn²⁺ ion (hyperfine constant, 7.4 mT). The second signal is due to an organic free radical apparently induced by a sterilization process. The third signal may originate from the Fe³⁺ ion in the nonheme proteins. The progressive saturation behavior at various microwave power levels indicated quite different relaxation behaviors of those radicals. Namely, the peak intensity of the organic free radical component decreases in a monotonic fashion, whereas the Mn²⁺ and Fe³⁺ ESR signals substantially remain constant. This evidences the presence of three independent radicals in the pepper before irradiation. Upon γ-irradiation, a new pair of signals appeared in the pepper. The progressive saturation behavior of the pair peaks after the irradiation showed a quite different behavior as compared with the free radical centered atg=2.0. For the measure of irradiation effects, we propose a universal index for the ESR analysis of irradiated dry foods.     

Steady state and transient Overhauser effect in systems of three spins

The rate equations describing spin polarization in a system of three spins are derived and solved for the case of a free radical dissolved in a solvent containing two nuclear spins. Triple irradiation experiments indicate that a nuclear spin A can be effectively coupled to an electron spin C via a second nuclear spin B and measurements of both the steady state and transient Overhauser effects are in accord with the theoretical predictions for a three-spin system. The ‘three spin effect’ is found to operate only in dilute solutions of free radicals in which case the probabilities for transitions between different nuclear or electronic energy levels can be determined. It was found to be effective for fluorine nuclei—in the presence of both protons and a free radical and for carbon [13] nuclei in the presence of either protons or fluorine nuclei and a free radical. Detailed measurements have been performed for CHFCl₂, para-difluorobenzene, and meta-fluorotoluene containing the tritertiary butyl phenoxyl radical.     

The thermodynamic one-particle Green function in the renormalized spin wave approximation for isotropic cubic ferromagnets

The thermodynamic one-particle Green function in the renormalized spin wave approximation for isotropic cubic ferromagnetic insulators with Dyson's spin wave theory as a base is derived. In quantitative respect, dynamic and kinematic effects of spin waves are approximated by the graphs deficient in the energy denominators, wherefore at low temperature kinematic interaction turns out to be too strong. As against the one-particle Green function for independent spin waves, dynamic interaction of ferromagnons is shown to effect the renormalization of the spin wave energy, whereas kinematic interaction directly modifies the average ferromagnon population numbers. In the matter of magnetization, its formula based on the Green function assumes a similar form as in the spin wave theory without interactions on the understanding that it remains valid within the entire range of temperatures from absolute zero up to the critical point.     

Gaussian approximation in the theory of MR signal formation in the presence of structure-specific magnetic field inhomogeneities

A detailed theoretical analysis of the free induction decay (FID) and spin echo (SE) MR signal formation in the presence of mesoscopic structure-specific magnetic field inhomogeneities is developed in the framework of the Gaussian phase distribution approximation. The theory takes into account diffusion of nuclear spins in inhomogeneous magnetic fields created by arbitrarily shaped magnetized objects with permeable boundaries. In the short-time limit the FID signal decays quadratically with time and depends on the objects' geometry only through the volume fraction, whereas the SE signal decays as 5/2 power of time with the coefficient depending on both the volume fraction of the magnetized objects and their surface-to-volume ratio. In the motional narrowing regime, the FID and SE signals for objects of finite size decay mono-exponentially; a simple general expression is obtained for the relaxation rate constant deltaR2. In the case of infinitely long cylinders in the motional narrowing regime the theory predicts non-exponential signal decay lnS approximately -tlnt in accordance with previous results. For specific geometries of the objects (spheres and infinitely long cylinders) exact analytical expressions for the FID and SE signals are given. The theory can be applied, for instance, to biological systems where mesoscopic magnetic field inhomogeneities are induced by deoxygenated red blood cells, capillary network, contrast agents, etc.     

Spin compensation temperatures induced by longitudinal fields in a mixed spin-3/2 and spin-5/2 Ising ferrimagnet

I studied the ferrimagnetic Ising model with nearest neighbour interactions for a square lattice and simple cubic one, using mean field theory. The free energy of a mixed spin Ising ferrimagnetic model was calculated from a mean field approximation of the Hamiltonian. By minimizing the free energy, I obtained the equilibrium magnetizations and the compensation temperatures. Clear indications of the single-ion anisotropies on the compensation points of the mixed spin-3/2 and spin-5/2 ferrimagnetic lattices are found. Some interesting behaviors of these systems are obtained depending not only on the values of magnetic anisotropies for both sublattice sites but also on the lattice structure. The longitudinal magnetic fields dependence of the spin compensation temperature is the main focus of research. The possibility of many compensation temperatures is indicated.     

Effect of halogen atom on electron spin polarization studied by CIDEP,using halogen substituted aromatic acyl compounds

The reaction and spin dynamics of the photocleavage reaction of 2-chloro-2′-acetylnaphthalene were studied by time-resolved FT-EPR and transient absorption (TA) spectroscopy. The photocleavage reaction from both singlet and triplet states was observed by TA and EPR experiments, although the radical cleavage reaction in the excited triplet state is energetically unfavourable. This feature has been explained by the ionic cleavage reaction due to the electro-negativity of the chlorine atoms. The time-resolved FT-EPR spectra were similar to those observed in the bromine substituted compound, 2-BAN, reported in a previous paper. The origin of the electron spin polarization was assigned to the radical triplet pair mechanism (RTPM) and free radical pair mechanism (F-pair RPM) from analysis of the time profiles of the spin polarization.     

The Weinberg-Salam model and weak radiative decays of KL mesons

Weak radiative decays K_L → 2γ and K_L → π⁺π^?γ are considered in the Weinberg-Salam model for weak interactions and in the framework of asymptotically free gauge theory (AFGT) for strong interactions (SI). The problem of calculation of the decays at short and large distances is discussed. It is shown that only the large distance contribution is essential for each decay. The estimates of these contributions being rough, it proves impossible to compare them with experiment. However, using anomalous PCAC theory we derive the relation connecting the contributions for these two decays and, hence, the decay amplitudes via the known constants. Experimental data confirm the obtained result. The important result of the paper is that the matrix elements of the effective Hamiltonian specifying the decays at short distances are not dynamically enhanced, unlike the case of weak non-leptonic amplitudes.     

Pulse-shift measurement: a direct observation of radical pair dynamics in a micelle

The hydrogen abstraction reactions of benzophenone in a micellar solution in the absence and presence of 1,4-cyclohexadiene are compared with time-resolved electron spin resonance (ESR) and optically detected (OD) ESR. “Pulse shift” measurement by OD-ESR, which observes the effect of a resonant microwave pulse at different delay times after laser excitation, reveals that the lifetime of the radical pair becomes much shorter in the presence of 1,4-cyclohexadiene. This explains the change of chemically induced dynamic electron polarization from spin-correlated radical pair polarization in the absence of 1,4-cyclohexadiene to conventional, E^*/A polarization in its presence. The rate constants determined by this technique indicate that the escaping rate of cyclohexadienyl radical is ten times larger than those of alkyl and benzophenone ketyl radicals.     

Composite weak bosons and their radiative decays

In composite models of leptons, quarks, and weak bosons, radiative decays of the Z-boson may occur with a relatively large rate. In these decays a photon and a new spin zero particle are produced. The latter subsequently decays into lepton or quark pairs, or into a pair of photons. Various phenomenological aspects of these decays are studied. Radiative decays of the W^± boson are suppressed with respect to those of the Z⁰.     

Modelling Moran Process with Network Dynamics for the Evolution of Cooperation

We introduce a simple model based on the Moran process with network dynamics. Using pair approximation, the cooperation frequencies at equilibrium states are deduced for general interactions. Three usual social dilemmas are discussed in the framework of our model. It is found that they all have a phase transition at the same value of cost-to-benefit ratio. For the prisoner＇s dilemma game, notably it is exactly the simple rule reported in the literature [Nature 441 （2006） 502]. In our model, the simple rule results from the parent-offspring link. Thus the basic mechanism for cooperation enhancement in network reciprocity is in line with the Hamilton rule of kin selection. Our simulations verify the analysis obtained from pair approximation.     

Radiative decays as a test for right-handed currents

It is shown that the purity violating amplitude for the decays Σ⁺ → pγ and Ξ^? → Σ^?γ vanish in the SU(3) symmetry limit if the charged weak currents possess a U spin symmetry for strangeness changing processes. This applies to Cabibbo currents and their SU (4) generalization. Assuming an asymptotically free model for the strong interactions this result applies to all models with only left-handed charged currents. Observation of a large parity violating amplitude would indicate right-handed charged weak currents. Tests involving other radiative decays are also considered.     

Phase separation in a lattice model of a superconductor with pair hopping

We have studied the extended Hubbard model with pair hopping in the atomic limit for arbitrary electron density and chemical potential. The Hamiltonian considered consists of (i) the effective on-site interaction U and (ii) the intersite charge exchange interactions I, determining the hopping of electron pairs between nearest-neighbour sites. The model can be treated as a simple effective model of a superconductor with very short coherence length in which electrons are localized and only electron pairs have a possibility of transferring. The phase diagrams and thermodynamic properties of this model have been determined within the variational approach, which treats the on-site interaction term exactly and the intersite interactions within the mean-field approximation. We have also obtained rigorous results for a linear chain (d = 1) in the ground state. Moreover, at T = 0 some results derived within the random phase approximation (and the spin-wave approximation) for d = 2 and 3 lattices and within the low-density expansions for d = 3 lattices are presented. Our investigation of the general case (as a function of the electron concentration n and as a function of the chemical potential μ) shows that, depending on the values of interaction parameters, the system can exhibit not only the homogeneous phases, superconducting (SS) and nonordered (NO), but also the phase separated states (PS: SS-NO). The system considered exhibits interesting multicritical behaviour including tricritical points.