Domain Walls in Superfluid 3He-B

No Heading We consider domain walls between regions of superfluid ³He-B in which one component of the order parameter has the opposite sign in the two regions far from one another. We report calculations of the order parameter profile and the free energy for two types of domain wall, and discuss how these structures are relevant to superfluid ³He confined between two surfaces.PACS numbers: 67.57.Np     

Intrinsic Conductance of Domain Walls in BiFeO3

Ferroelectric domain walls exhibit a number of new functionalities that are not present in their host material. One of these functional characteristics is electrical conductivity that may lead to future device applications. Although progress has been made, the intrinsic conductivity of BiFeO₃ domain walls is still elusive. Here, the intrinsic conductivity of 71° and 109° domain walls is reported by probing the local conductance over a cross section of the BiFeO₃/TbScO₃ (001) heterostructure. Through a combination of conductive atomic force microscopy, high‐resolution electron energy loss spectroscopy, and phase‐field simulations, it is found that the 71° domain wall has an inherently charged nature, while the 109° domain wall is close to neutral. Hence, the intrinsic conductivity of the 71° domain walls is an order of magnitude larger than that of the 109° domain walls associated with bound‐charge‐induced bandgap lowering. Furthermore, the interaction of adjacent 71° domain walls and domain wall curvature leads to a variation of the charge distribution inside the walls, and causes a discontinuity of potential in the [110]_p direction, which results in an alternative conductivity of the neighboring 71° domain walls, and a low conductivity of the 71° domain walls when measurement is taken from the film top surface.     

Interaction Between Domain Walls in Chiral p-Wave Superfluids

We calculate microscopically the interaction energy of domain walls separating degenerate ground states in a chiral p-wave fermionic superfluid. The interaction is mediated by the quasiparticles experiencing Andreev scattering at the domain walls. As a by-product, we derive a useful general expression for the free energy of an arbitrary nonuniform texture of the order parameter in terms of the quasiparticle scattering matrix.     

Atomic Level 1D Structural Modulations at the Negatively Charged Domain Walls in BiFeO3 Films

Charged domain walls (CDWs) show great potentials to mediate the properties of ferroelectrics. Direct mapping of these domain walls at an atomic scale is of critical importance for understanding the domain wall dominated properties. Here, based on aberration‐corrected scanning transmission electron microscopy, tail‐to‐tail CDWs at 71°, 109°, and 180° domains in BiFeO₃ thin films have been identified. 2D mappings demonstrate 1D structural modulations with alternate lattice expansions and clockwise/counterclockwise lattice rotations at these CDWs. Such behaviors of CDWs reveal a remarkable contrast to the uncharged domain walls and imply delicate interactions between bound charges and structural compensations of domain wall. These results are expected to provide new information on domain wall structures and shed some light on the understanding of domain wall properties in ferroelectrics.     

Quantum phase transitions in 2D antiferromagnets

Quantum Monte Carlo results for the T = 0 order-disorder transition of two-dimensional dimerized quantum Heisenberg antiferromagnets are presented. For a two-layer model, with the dimers having one member on each plane, the finite-size scaling of the staggered structure factor and the staggered susceptibility is consistent with the 3D classical Heisenberg universality class, as required if the transition is described by the 2+1-dimensional non-linear sigma-model. However, for models with the dimers in a single plane, the behavior does not appear to be consistent with this universality class.     

Incommensurate fluctuations in Bi2Sr2CaCu2O8

A special neutron scattering technique has been used to discover an incommensurate fluctuation in Bi₂Sr₂CaCu₂O₈ that appears below T_c. The fluctuation is identified as a dynamic charge density wave since its scattering intensity appears to increase with increasing momentum transfer. The fluctuation is found at a wavevector near 2k_F and could be associated with a dynamic stripe phase.     

Quantum Phases of 7Li Atoms in an Optical Lattice

We study the quantum phases that may arise when ⁷Li atoms that possess an attractive two-body interaction are loaded into an optical lattice. The phase diagram that we have obtained for ⁷Li atoms within a mean-field approximation is composed of the superfluid and Mott-insulator phases as for bosonic atoms with a repulsive two-body interaction. We have found that the repulsive three-body interaction plays a vital role in stabilizing the quantum phases of ⁷Li atoms in an optical lattice. The ground-state energy and the average atom number per lattice site in the quantum phases have been calculated and found to be consistent with the nature of these quantum phases. The excitation spectra of quasi-particles and quasi-holes in the Mott-insulator phase have also been calculated. In our general investigation of the effects of the three-body interaction that is allowed to vary, we have found that the energies of quasi-particles are substantially increased as the strength of the repulsive three-body interaction is raised, whereas the energies of quasi-holes are slightly decreased at low-filling factors of atoms and substantially decreased at large filling factors.     

A General Method for Evaluation of 2D and 3D Domain Integrals Without Domain Discretization and its Application in BEM

A robust method is presented to evaluate 2D and 3D domain integrals without domain discretization. Each domain integral is transformed into a double integral, a boundary integral and a 1D integral. Both integrals are evaluated by adaptive Simpson quadrature method. The method can be used to evaluate domain integrals over simply or multiply connected regions with any arbitrary form of integrands. As an application of the method, domain integrals produced in boundary element formulation of potential and elastostatic problems are analyzed. Several examples are provided to show the validity and accuracy of the method.     

New Phases for the Orientational Ordering of Quantum Rotors: H2 on Hexagonal BN

The phase diagram for the orientational ordering of 2D films of ortho–para H ₂ mixtures has been explored for both commensurate and incommensurate monolayer coverages on hexagonal BN. NMR techniques were used to study ortho concentrations 0.30<x<0.74and temperatures 0.05 <T<4.2. Three distinctly different classes of line shapes were observed and we interpret these in terms of the different phases predicted for the orientational ordering in 2D. At high concentrations a Pake doublet line shape was observed which is characteristic of the pinwheel (PW) ordering predicted by Harris and Berlinsky. For intermediate concentrations a distinctive helmet shaped NMR line is observed which is the signature of a quadrupolar glass state. This line shape was seen only for commensurate coverages. At low concentrations (x<0.45) a new very sharply defined Pake doublet shape is seen which corresponds to a very narrow distribution of order parameters. We interpret this latter phase as a herringbone (HB) ordering.     

Melting and Evaporation of Monolayer H2-D2 Mixtures Adsorbed on D2-Plated Graphite

We report heat capacity measurements on second layer mixtures of H ₂ -D ₂ adsorbed on exfoliated graphite plated with one monolayer of D ₂ , for 3K ₂ concentration, between 0 and 1, across the three-phase melting region (T _m ) and the two-phase liquid-vapor coexistence region through its high temperature boundary (T _e ). Deviations from a linear dependence of T _m and T _e on x have been found. An estimate of the degree of fractionation is presented.     

Quantum Fluctuations and Vortex-Antivortex Unbinding in the 2D Bcs-Bec Crossover

Very recently, quasi two-dimensional (2D) systems made of attractive fermionic alkali-metal atoms with a widely tunable interaction due to Fano-Feshbach resonances have been realized. In this way, it has been achieved the 2D crossover from the Bardeen-Cooper-Schrieffer regime of weakly-interacting Cooper pairs to the Bose-Einstein condensate regime strongly bound dimers. These experiments pave the way to the investigation of 2D strongly-interacting attractive fermions during the Berezinskii-Kosterlitz-Thouless (BKT) transition from a low-temperature superfluid phase characterized by quasi-condensation to a high-temperature normal phase, where vortex proliferation driven by quantum and thermal fluctuations completely destroys superfluidity. In this paper, we discuss our preliminar theoretical results on the behavior of the BKT critical temperature across the crossover. Our microscopic calculations are based on functional integration taking into account renormalized Gaussian fluctuations and the crucial 2D effect of vortex-antivortex unbinding.     

Critical Dynamics of Sound Propagation and Response Function Near Phase Transitions to Incommensurate Magnetic Phases in Layered Media with a Tetragonal Structure

Journal of Superconductivity and Novel Magnetism - A symmetry analysis was described of possible magnetic phase transitions to incommensurate structures from the paramagnetic state in layered...     

Domain Walls Due to Anisotropic Diffusion in 3.8% 3He-4He Solutions

Nonlinear anisotropic spin diffusion is considered in ³He-⁴He solutions at the ³He concentration 3.8% where spin-rotation effects are absent. For large anisotropy D _∥ D _⊥ it is shown that a long-lived domain wall solution exists in which transverse magnetization is confined in a tube between two reservoirs of up and down spins, as in a longitudinal spin diffusion experiment. This is possible when D _∥ D _⊥ because diffusion of the transverse magnetization away from the domain wall is balanced by longitudinal diffusion toward the wall. Furthermore, the spin current between the up- and down-spin reservoirs is reduced because the spins must diffuse through the domain wall via transverse diffusion. The decay rate of the wall due to small external and demagnetizing field gradients is also calculated.     

Quantum Effects in Uniform and Staggered Moment of Frustrated Quasi-2D Antiferromagnets

We investigate the frustrated two-dimensional S=1/2 next nearest neighbor Heisenberg antiferromagnet on a square lattice. This model is relevant for a variety of V oxides and Cu compounds and its anisotropic version for the Fe pnictides. We use spin-wave theory and exact diagonalization in a magnetic field for finite tiles with a new method for the finite size scaling procedure. The induced uniform and the staggered moment in the antiferromagnetically ordered phases are calculated. They deviate strongly from classical behavior depending on frustration ratio J ₂/J ₁ and the exchange anisotropy. The former may be determined by comparison with experimental saturation fields. Applying a magnetic field up to one half of the saturation field stabilizes the staggered moment in the striped columnar (CAF) and Néel (NAF) antiferromagnetic phases, in particular close to the phase boundaries. The field dependence of the staggered moment uniquely determines the exchange parameters. This allows to derive the frustration ratio J ₂/J ₁ also from the field dependence of neutron diffraction data.