International Journal of Modern Physics B

We construct a class of exact ground states for correlated electrons on pentagon chains in the high density region and discuss their physical properties. In this procedure the Hamiltonian is first cast in a positive semidefinite form using composite operators as a linear combination of creation operators acting on the sites of finite blocks. In the same step, the interaction is also transformed to obtain terms which require for their minimum eigenvalue zero at least one electron on each site. The transformed Hamiltonian matches the original Hamiltonian through a nonlinear system of equations whose solutions place the deduced ground states in restricted regions of the parameter space. In the second step, nonlocal product wave functions in position space are constructed. They are proven to be unique ground states which describe non-saturated ferromagnetic and correlated half metallic states. These solutions emerge when the strength of the Hubbard interaction U_i is site-dependent inside the unit cell. In the deduced phases, the interactions tune the bare dispersive band structure such to develop an effective upper flat band. We show that this band flattening effect emerges for a broader class of chains and is not restricted to pentagon chains. For the characterization of the deduced solutions, uniqueness proofs, exact ground state expectation values for long-range hopping amplitudes and correlation functions are also calculated.

The study of physical reasons which lead to the appearance of ferromagnetism has revealed a new mechanism for the emergence of an ordered phase, described here in detail. This works as follows: starting from a completely dispersive bare band structure, the interactions quench the kinetic energy, hence the ordered phase is obtained solely by a drastic decrease of the interaction energy. Since U_i are site dependent, this determinative decrease is obtained by a redistribution of the double occupancy d_i such to attain small d_i where the on-site Coulomb repulsion U_i is high, and vice versa. The kinetic energy quench leads to the upper effective flat band, whose role is to enhance by its degeneracy the switching to the ordered phase dictated and stabilized by the interactions present. It is shown that for this phenomenon to occur, a given degree of complexity is needed for the chain, and the mechanism becomes inactive when the U_i interactions are homogeneous, or are missing from the ground state wave function.

In the frame of density functional theory, first-principles calculations have been carried out to investigate the structures, elastic constants, structural phase transition between B1 and B2 phases and thermodynamic properties of the zirconium nitride (ZrN) by means of the generalized gradient approximation. The equilibrium lattice parameter we obtained for ZrN in B1 phase is closer to the experiment results than previous theoretical results. In addition, the calculations of the elastic constants show that ZrN is a brittle material. What is more, based on third-order natural strain equation of state, the phase transition pressure 338 GPa for ZrN is predicted for B1–B2 transition. According to the quasi-harmonic Debye model, the thermodynamic parameters of ZrN have been investigated systematically.

The four site and eight site 1D anti-ferromagnetic Heisenberg chains in the Jordan–Wigner representation are investigated within the standard Hartree–Fock and random phase approximation (RPA) approaches, both in the symmetry unbroken and in the symmetry broken phases. A translation invariant groundstate, obtained by the projection method as a linear combination of a symmetry-broken HF state and its image under reflection, is also considered, for each chain type. It is found that the projection method considerably improves the HF treatment for instance as far as the groundstate energy is concerned, but also with respect to the RPA energies. The results are furthermore confronted with the ones obtained within so-called SCRPA scheme.

In this communication we report the interplay of the normal electron-phonon (EP) interaction, dynamic Jahn–Teller (DJT) distortion and superconductivity in high temperature superconductors in presence of a static lattice strain. This model consists of a degenerate two orbital band separated by Jahn–Teller (JT) energy modified by the DJT interaction in the conduction band. The superconductivity is assumed to be s-wave type present in the same band. The interaction Hamiltonian is solved by Green's function method and a modified BCS gap equation is obtained with a modified conduction band energy and modified BCS order parameter with α = 1, 2 designating the two orbitals. This interplay displays some new interesting results which are different from the effect of the static lattice strain on superconducting (SC) order parameter. The interplay is studied by varying the normal EP coupling, the DJT coupling, the SC coupling, the phonon vibration frequency, the phonon wave vector and other model parameters of the system.

The density functional theory (DFT) is used to investigate the adsorption of H₂ molecules on the Al-adsorbed single-walled carbon nanotube (Al-SWCNT) and the hydrogen storage capacity. The most stable position for the Al atoms adsorbed on the outer surface of SWCNT is confirmed with the binding energy of -2.510 eV. The research results indicate that the hydrogen storage capacity can be greatly improved when the adsorbed H₂ molecules are arranged into a layered structure, and the analyzing of electronic density also supports this point. It is found that the H₂ molecules can be absorbed in six layers at most with a suitable average binding energy of -0.131 eV/H₂, and the hydrogen storage capacity of Al-(7, 7) SWCNT (Al₇C₇₀) is approximately 28.0 wt% which is much higher than the DOE's target.

We investigate numerically the dynamic properties of a system of globally coupled oscillators driven by periodic symmetry-breaking fields in the presence of noise. The phase distribution of the oscillators is computed and a dynamic transition is disclosed. It is further found that the stochastic resonance is closely related to the behavior of the dynamic order parameter, which is in turn explained by the formation of a bi-cluster in the system. Here noise tends to symmetrize the motion of the oscillators, facilitating the bi-cluster formation. The observed resonance appears to be of the same class as the resonance present in the two-dimensional Ising model under oscillating fields.

We calculate here the Raman frequencies of some lattice modes as a function of pressure at constant temperatures for the solid and liquid phases of benzene. The observed data for the molar volume from literature is used to calculate the Raman frequencies through the mode Grüneisen parameter in benzene.

Our calculated frequencies are in good agreement with the observed data when the mode Grüneisen parameter is taken as a constant at one particular pressure in solid benzene.

It is shown here that the Raman frequencies can be calculated from the volume data, as demonstrated for benzene here.

We investigate spontaneously generated coherence (SGC) and Autler–Townes Splitting (ATS) in various three-level systems. Through detailed analytical calculations on absorption spectrum of probe laser field, we show that in V-type system the SGC can completely eliminate the absorption of the probe field and at the same time significantly reduce the group velocity. By using residue theorem and spectrum decomposition method, we prove that there exists a crossover from SGC to ATS for both cold and warm atoms when the magnitude of SGC is changed. Different contributions of SGC and ATS to probe-field absorption spectrum are clearly clarified in different parameter regions. In addition, our results show that there is no SGC and hence no SGC-ATS crossover in Λ- and Ξ-type systems.

We investigate the torque on two rotating particles and two rotating plates due to the thermal fluctuation and vacuum fluctuation. A general formula has been developed including the temperature difference on each plate and vacuum, and the different angular velocity on each plate. We show that the Casimir torque in absorbing material occurs whenever there is a difference in temperature or a relative angular velocity between the plates and the vacuum.