Magnetic order in the Shastry-Sutherland model

We investigate the influence of energetic disorder, viscous damping and an external field on the electron transfer (ET) in DNA. The double helix structure of the λ-form of DNA is modeled by a steric oscillator network. In the context of the base-pair picture two different kinds of modes representing twist motions of the base pairs and H-bond distortions are coupled to the electron amplitude. Through the nonlinear interaction between the electronic and the vibrational degrees of freedom localized stationary states in the form of standing electron-vibron breathers are produced which we derive with a stationary map method. We show that in the presence of additional energetic disorder the degree of localization of such breathers is enhanced. It is demonstrated how an applied electric field initiates the long-range coherent motion of breathers along the bases of a DNA strand. These moving electron-vibron breathers, absorbing energy from the applied field, sustain energetic losses due to the viscous friction caused by the aqueous solvent as well as the impact of a moderate amount of energetic disorder. Moreover, it is illustrated that with the choice of the amplitude and frequency of the external field, the breather can be steered to a desired lattice position achieving control of the ET. Received 5 July 2002 Published online 29 November 2002     

Second order self-energy of the two-dimensional Hubbard model

The self-energy Σ(k; ω) of the 2D Hubbard model on the square lattice is calculated numerically in second order perturbation theory. In the limit of small frequencies the imaginary part of Σ(k; ω) is also obtained analytically. We find that at half filling ImΣ(k; ω) ～ ω for k on the Fermi surface, with a logarithmically divergent prefactor close to the corners k = (0,±π) and (±π,0) in agreement with the numerical results.     

Off-diagonal long-range order in the ground state of the two-dimensional antiferromagnetic Heisenberg model

A two-dimensional Jordan-Wigner transformation, together with a fermion-boson mapping, has been used to transform the two-dimensional antiferromagnetic Heisenberg model (2D AFMHM) into an interacting boson system with a fixed number of particles. We found that the excitation energy depends linearly on the momentum for small energy, and it is 1.80J for (π,π). Our results suggest that the ground state of the 2D AFMHM is a quantum liquid, and the order is off-diagonal long-range order associated with the Bose-Einstein condensation.     

Magnetic susceptibility of the two-dimensional two-sublattice Hubbard model calculated in the static-fluctuation approximation

The transverse dynamic susceptibility of the 2D two-sublattice Hubbard model is calculated in the static-fluctuation approximation. The static magnetic susceptibility is studied as a function of various parameters of the system. The results for the special case of the one-dimensional Hubbard model are compared to the exact solution.     

Magnetic reconnection in the two-dimensional Kelvin-Helmholtz instability

Magnetic reconnection in the two-dimensional Kelvin-Helmholtz instability is studied. The flow is modeled by the reduced MHD equations with constant resistivity and viscosity. For super-Alfvénic flow, localized transient reconnection is observed on the Kelvin-Helmholtz time scale (this is not new). We study this transient reconnection and consider the peak reconnection rate which occurs with the initial vortex formation. Over the range of resistivities considered, it is shown that this peak reconnection rate is not a function of resistivity, and is a function of the initial flow shear. Additionally, it is demonstrated that there is a fundamental difference between the evolution of a problem at S = 200 and S = 10,000.     

Magnetic susceptibility and order parameter of the spin-glass-like phase of the double-exchange model

The magnetic susceptibility and Edwards-Anderson order parameter q of the spin-glass-like (SGL) phase of the double-exchange model are evaluated in the weak-coupling or RKKY limit. Dynamical mean-field theory is used to show that q = M(T/T(SGL))2, where M is the classical Brillouin function and T(SGL) is the SGL transition temperature. The correlation length of the SGL phase is determined by a correlation parameter Q that maximizes T(SGL) and minimizes the free energy. The magnetic susceptibility has a cusp at T(SGL) and reaches a nonzero value as T --> 0.     

Magnetic relaxation in two-dimensional molecular magnets

Magnetic properties of planar molecular magnets have been investigated by Mössbauer spectroscopy. A significant increase in the linewidth of the magnetic hyperfine structure with an increase in temperature is observed. A fluctuation model of magnetism for magnets lacking long-range magnetic order is proposed. This model describes the magnetic field distributions in these magnets in a wide temperature range with the use of a fixed set of coupling constants.     

Magnetic barrier in a two-dimensional hole gas

Transport properties of a magnetic barrier in a Ga_xAl_1−xAs based two-dimensional hole gas are reported. A ferromagnetic cobalt film, separated by an AlO_x layer from the semiconductor in order to prevent leakage currents, is magnetized in-plane, such that the fringe field generates a localized perpendicular magnetic field acting as a magnetic barrier. The resistance as a function of the in-plane magnetic field shows a characteristic minimum at the coercive field of the ferromagnetic film. Semiclassical simulations based on the Landauer–Büttiker formalism show good agreement with the experiment.