International Journal of Modern Physics B

The scattering matrix (S-matrix), relating the initial and final states of a physical system undergoing a scattering process, is a fundamental object in quantum mechanics and quantum field theory. The study of factorized S-matrices, in which many-body scattering factorizes into a product of two-body terms to yield an integrable model, has long been considered the domain of mathematical physics. Many beautiful results have been obtained over several decades for integrable models of this kind, with far reaching implications in both mathematics and theoretical physics. The viewpoint that these were only toy models changed dramatically with brilliant experimental advances in realizing low-dimensional quantum many-body systems in the lab. These recent experiments involve both the traditional setting of condensed matter physics and the trapping and cooling of atoms in optical lattices to engineer and study quasi-one-dimensional systems. In some cases the quantum physics of one-dimensional systems is arguably more interesting than their three-dimensional counterparts, because the effect of interactions is more pronounced when atoms are confined to one dimension. This article provides a brief overview of these ongoing developments, which highlight the fundamental importance of integrability.

We performed first-principles calculations for two Mn-doped structures in which Mn atoms substitute Ti atoms to determine whether (i) it is more conducive to Mn ion doping and (ii) ferromagnetism can occur in F adsorption anatase TiO₂ surfaces. Ferromagnetic (FM) coupling is more stable than antiferromagnetic (AFM) coupling for all doping configurations as the adsorption of F atoms on the surface significantly lowers the formation energy of the TiO₂:Mn system. The magnetic moments of the Mn ions are reduced, whereas those of O atoms on the surface are increased. The magnetic moment of the O atoms is mainly derived from the spin polarization p_x and p_y orbitals. F adsorption promotes doping of Mn atoms and to a certain extent improves the stability of the structure, magnetism and metallicity.

Li-doped NKLN ceramic (Na_0.5 K_0.5)_0.935Li_0.065NbO₃ (NKLN935) in pure and MnO₂ doped compositions have been revisited to carry out a detailed analysis of the structural and optical properties. Rietveld analysis of the X-ray diffraction (XRD) pattern reveals the system to be tetragonal (space group P4mm). UV–Visible (UV–Vis) spectroscopy and an equivalent Kubelka–Munk function is used to obtain the optical band gap values. It is reported that with increasing Mn doping, the band gap values decreases, which has been analyzed and understood in terms of the tetragonal structure and is found to be consistent with dielectric properties.

Efficient absorption of light in visible range and enhance separation of photoexcited electron-hole pairs (EHPs) are crucial for improving the photoactivity of metal nonmetal codoped TiO₂. By using density functional theory (DFT) calculations, an effective metal (Ag) and nonmetal (N) codoping approach is described to modify the photoelectrochemical properties of titanium dioxide ( TiO₂). Nitrogen (N) doping introduces isolated N-2p states above the valence band maximum (VBM) which acts as an electron trap to promote EHP recombination. For Ag-doped TiO₂, Ag-4d states are introduced above the VBM which leads to the band gap narrowing. Silver (Ag) and nitrogen codoped TiO₂ possess stable configuration, narrowed band gap and best visible light absorption. Defect pair binding energy calculation shows that individual dopants, located at a distance of 8.951 Å bind each other, which indicates that the defect pair is stable compared to the isolated impurities in the host lattice. Ag and N codoped TiO₂ shows better visible light absorption as compared to other doped models due to the reduced band gap. N doping reduces the band gap of TiO₂ while Ag doping enhances the EHPs separation, so their combined presence in a sample would improve the photocatalytic activity due to their synergistic codoping effect. Our calculations provide reasonable explanation for the experimental findings.

In this paper, the effects of external DC electric fields on the neuro-computational properties are investigated in the context of Morris–Lecar (ML) model with bifurcation analysis. We obtain the detailed bifurcation diagram in two-dimensional parameter space of externally applied DC current and trans-membrane potential induced by external DC electric field. The bifurcation sets partition the two-dimensional parameter space in terms of the qualitatively different behaviors of the ML model. Thus the neuron's information encodes the stimulus information, and vice versa, which is significant in neural control. Furthermore, we identify the electric field as a key parameter to control the transitions among four different excitability and spiking properties, which facilitates the design of electric fields based neuronal modulation method.

In this work, the energy transport of phonon and electron in graphene nanoribbons (GNRs) are investigated by the nonequilibrium Green's function method without considering the interaction of phonon and electron. The heat current of phonon contribution comes from the gradient of temperature. While for the electron contribution, it stems from the gradient of both temperature and electrochemical potential. The corresponding intermediate functions satisfy the Onsager relationship. Thermal conductances are calculated in GNR and compared to those in square lattice ribbon model respectively. It is found that both the phonon and electron thermal conductances in square lattice ribbon are smaller than those in GNRs at low temperature and surpass those in armchair and zigzag GNRs respectively, as the temperature increases. Meanwhile, the heat transport is related to the edges of GNRs. These phenomena depend on their dispersion relations and energy band structures.

In this work we investigate the dependence of spin observables, like the total spin alignment and the spin squeezing factor, for electrons confined inside an elliptic quantum corral and interacting with a pair of impurities located on the semi-major axis of the ellipse. The results suggest that such a system exhibits some of the characteristic features of a qubit, concerning the persistence of the orientation and squeezing of a component of the total spin.

In this paper, we have studied optical properties of an inhomogeneous quantum wire system. In this regard, we have calculated the absorption coefficients and refractive index changes using density matrix method. We have investigated the effect of size variation on these optical properties. The wires are considered to be triangle with infinite potential at the boundaries. We have described the size nonuniformity distribution by a Gaussian function. It is shown that the optical properties of the quantum wires depend strongly on the wire size distribution described by the parameters a₀ and D (the average side length and standard deviation of quantum wire, respectively). It is deduced that the effect of size distribution on the optical properties of triangle quantum wire is noticeable.

The possible transition states, minimum energy paths (MEPs) and migration mechanisms of defect clusters and xenon-vacancy defect clusters in uranium dioxide (UO₂) have been investigated using the dimer and the nudged elastic-band (NEB) methods. The nearby O atom can easily hop into the oxygen vacancy position by overcoming a small energy barrier, which is much lower than that for the migration of a uranium vacancy. A simulation for a vacancy cluster consisting of two oxygen vacancies reveals that the energy barrier of the divacancy migration tends to decrease with increasing the separation distance of divacancy. For an oxygen interstitial, the migration barrier for the hopping mechanism is almost three times larger than that for the exchange mechanism. Xe moving between two interstitial sites is unlikely a dominant migration mechanism considering the higher energy barrier. A net migration process of a Xe-vacancy pair containing an oxygen vacancy and a xenon interstitial is identified by the NEB method. We expect the oxygen vacancy-assisted migration mechanism to possibly lead to a long distance migration of the Xe interstitials in UO₂. The migration of defect clusters involving Xe substitution indicates that Xe atom migrating away from the uranium vacancy site is difficult.

Freeman Dyson has had a distinguished career as a scientist, but perhaps this notable body of work might be eclipsed in importance by his many writings about society, especially those dealing with the dilemma of how improved living standards can be brought about without despoiling the land. Dyson is one of the few prominent commentators who directly addresses what might shape up as a culture war between two viewpoints — environmentalism and humanism — that otherwise have many aims in common. The first part of this essay looks at the broad outline of Dyson's career while the second part looks more particularly at his contributions to the humanist debate.