Ruibao Tao

Quantum computing of molecular magnet Mn 12

Quantum computation in molecular magnets is studied by solving the time-dependent Schrodinger equation numerically. Following Leuenberger and Loss [Nature (London) 410, 789 (2001)], an external alternating magnetic field is applied to populate and manipulate a superposition of single-spin states in molecular magnet Mn-12. The conditions to realize parallel recording and reading databases of Grover algorithms in molecular magnets are discussed in detail. It is found that an accurate time duration of the magnetic pulse as well as the discrete frequency spectrum and the amplitudes are required to design a quantum computing device.

Effective thermal conductivity of a periodic composite with contact resistance

Periodic composite materials with contact resistance on the inclusion surface are studied. A generalized Rayleigh identity, which is applicable to such systems, is derived. Calculations are made for composites containing a simple cubic array of spherical inclusions. These solutions are derived by applying the generalized Rayleigh identity. A definition of the effective conductivity suitable for systems with a discontinuity of the temperature field on surface of inclusions is suggested, and it produced reasonable effective conductivities over all the possible range of Biot’s number. It turned out that the contact resistance can change the effective conductivity dramatically and is rich in its effects on the thermal properties of composites.

DYNAMIC RESPONSE TIMES OF ELECTRORHEOLOGICAL FLUIDS IN STEADY SHEAR

Transient responses of electrorheological fluids to square-wave electric fields in steady shear are investigated by computational simulation method. The structure responses of the fluids to the field with high frequency are found to be very similar to that to the field with very low frequency or the sudden applied direct current field. The stress rise processes are also similar in both cases and can be described by an exponential expression. The characteristic time τ of the stress response is found to decrease with the increase of the shear rate γ and the area fraction of the particles φ2. The relation between them can be roughly expressed as τ∝γ−3/4φ2−3/2. The simulation results are compared with experimental measurements. The aggregation kinetics of the particles in steady shear is also discussed according to these results.

Many-body effect in electrorheological responses

The many-body effect in the kinetic responses of ER fluids is studied by a moleculardynamic simulation method. The mutual polarization effects of the particles are considered by self-consistently calculating the dipole strength on each particle according to the external field and the dipole field due to all the other particles in the fluids. The many-body effect is found to increase with the enhancement of the particle concentration and the permittivity ratio between the solvent and the particles. The calculated response times are shorter than that predicted with the ‘point-dipole’ model and agree very well with experimental results. The many-body effect enhances the shear stresses of the fluids by several times. But they are not proportional to the many-body correction factor λ as expected. This is due to the fact that larger interaction forces between the particles lead to coarsening of the fibers formed in the suspensions. The results show that the many-body and multipolar interaction between the particles must be treated comprehensively in the simulations in order to get more reliable results.

Exchange bias in ferromagnetic/compensated antiferromagnetic bilayers

By means of micromagnetic spin dynamics calculations, a quantitative calculation is carried out to explore the mechanism of exchange bias (EB) in ferromagnetic (FM)/compensated antiferromagnetic IAFM) bilayers. The antiferromagnets with low and high Neel temperatures have been both considered, and the crossover from negative to positive EB is found in the case with low Neel temperature. We propose that the mechanism of EB in FM/compensated AFM bilayers is due to the symmetry breaking of AFM that yields some net ferromagnetic components during cooling process.

Structure and viscoelasticity of an electrorheological fluid in oscillatory shear : computer simulation investigation

The three-dimensional molecular dynamics simulation method has been used to study the dynamic responses of an electrorheological (ER) fluid in oscillatory shear. The structure and related viscoelastic behaviour of the fluid are found to be sensitive to the amplitude of the strain. With the increase of the strain amplitude, the structure formed by the particles changes from isolated columns to sheet-like structures which may be perpendicular or parallel to the oscillating direction. Along with the structure evolution, the field-induced moduli decrease significantly with an increase in strain amplitude. The viscoelastic behaviour of the structures obtained in the cases of different strain amplitudes was examined in the linear response regime and an evident structure dependence of the moduli was found. The reason for this lies in the anisotropy of the arrangement of the particles in these structures. Short-range interactions between the particles cannot be neglected in determining the viscoelastic behaviour of ER fluids at small strain amplitude, especially for parallel sheets. The simulation results were compared with available experimental data and good agreement was reached for most of them.

On soliton excitations in a one-dimensional Heisenberg ferromagnet

Soliton excitations in a one-dimensional Heisenberg ferromagnet are studied by means of the Holstein-Primakoff representation. Writing the Hamiltonian and the equation of motion into dimensionless forms, the authors show that the relative ratio of epsilon to eta is important for the determination of the modified terms of the nonlinear Schrodinger equation; epsilon =1/ square root S (S is the spin length) is the small dimensionless parameter used in the semiclassical approximation and eta =a/ lambda 0 (a is the lattice space and lambda 0 is the characteristic wavelength of the excitations) is another small dimensionless parameter used in the long-wave approximation. The soliton solutions are given in three cases ( eta =O( epsilon ), eta =O( epsilon 32/) and eta =O( epsilon 2)) which correspond to the different physical conditions of the system. The results obtained by Pushkarov and Pushkarov (1977), de Azevedo et al. (1982), and Skrinjar et al. (1989) are included in this approach.

Coherent transport of armchair graphene constrictions

(\mathrm{L}{\mathrm{i}}_{0.8}\mathrm{F}{\mathrm{e}}_{0.2})\mathrm{OHFeSe}

Influences of spin accumulation on the intrinsic spin Hall effect in two-dimensional electron gases with Rashba spin-orbit coupling

is a newly discovered intercalated iron-selenide superconductor with a