Domain boundary engineering – recent progress and many open questions

Some domain boundaries contain functionalities which do not exist in the bulk. Typical examples are (super-) conducting twin walls in WO₃, highly conducting walls in BiFeO₃ and in structural interfaces between SrTiO₃ and LaAlO₃, and ferroelectric walls in CaTiO₃. The emerging field of ‘Domain Boundary Engineering’ endeavors to generate such functional interfaces in a multitude of materials for applications in device materials. Some of the recent successes are reviewed together with suggestions for further research.     

Experimental Investigation into the Low-Molecular-Weight Fluoropolymer for the Storage of Ultracold Neutrons

The experimental setup for examining the low-molecular-weight CF₃(CF₂)₃–O–CF₂–O–(CF₂)₃CF₃ fluoropolymer, which is a promising coating material for the walls of storage chambers for ultracold neutrons, is described. The results are detailed. The measurement data are interpreted in the model of a multilayer complex quantum-mechanical potential of the chamber walls.     

Enhanced reactivity of domain walls in with sodium

A possible new high temperature superconducting phase was recently reported in WO ₃ :Na. We have examined the reaction between sodium vapour and WO ₃ , and compared the phases formed by the reaction to previously known WO ₃ phases. By using light microscopy and electron microprobe analysis, domain walls from the interior of the crystal are shown to have a much higher Na content than bulk material after reaction with Na vapour. This indicates preferential transport along the domain walls. The result is very similar to a reduction reaction of WO ₃ crystals in which twin walls lose oxygen preferentially. Oxygen deficient twin walls are superconducting with . Received 3 September 1999 and Received in final form 15 December 1999     

Magnetic contrast from domain walls in BaFe12O19 by tunneling stabilized magnetic force microscopy

The magnetic structure of BaFe₁₂O₁₉ is imaged with a scanning tunneling microscope having a flexible, magnetic tip. We find that Fe thin films evaporated on a silicon tip, integrated with a cantilever, behave as magnetically soft tips. Therefore, we are able to image domain walls with high lateral resolution. A different contrast along the domain wall due to surface magnetic charges is observed. We explain the data using previously established models for wavy domain walls. The obtained images are the first experimental evidence of magnetic charges induced on the wavy domain walls in BaFe₁₂O₁₉.     

Domain-wall mechanism of nucleation in a magnetic structural phase transition in La<Subscript>2</Subscript>CuO<Subscript>4</Subscript>

The structure of domain walls and new-phase nucleation are investigated in a four-sublattice antiferromagnet (AFM) of the La₂CuO₄ type placed in a magnetic field which initiates an AFM-weak-ferromagnet (WFM) magnetic structural phase transition. The critical fields for nucleus growth are found in the case of two types of domain walls present. The magnetization curve is calculated and a two-step mechanism is proposed for the AFM-WFM phase transition observed in La₂CuO₄.     

Domain wall theory of elementary excitations in anisotropic antiferromagneticS=1 chains

We present a theoretical investigation of elementary excitations in anisotropic antiferromagneticS=1 chains using the concept of domain walls in string (hidden) order. Domain walls are classified by the internal spin projectionS _dw ^z . We calculate energies and string correlation 0 functions of low lying excited states of the domain wall type in the Haldane phase and compare the results to those of numerical computations. The boundaries of the Haldane phase are determined from the instability of these excitations with respect to the ground state. The interaction between two domain walls is found to be proportional to the productS _dw ^z , S _dw ^z ₂, it is effectively repulsive 0140 for equal spin projections.     

Analysis of the ferroelastic domains and phase transition in the K2SO4 crystal

Abstract

The domain structures of the β-K₂SO₄ crystal were analyzed by group theory. We obtained the permissible kinds of domain association and domain walls from the results of the group theory. It is suggested from the analysis that the (110) and (130) planes are W_mb walls. The value of the spontaneous strain of K₂SO₄ wast = 6.32 × 10^?3 at room temperature and also its temperature dependence was observed.     

Collisions of domain walls in a supersymmetric model

A collision of two parallel domain walls in a supersymmetric model is analyzed by using both the effective-Lagrangian approximation and a numerical solution to the equations of motion for the scalar components of the superfields involved. Two cases—that where a configuration belonging to the type of two parallel walls is saturated in the sense of Bogomol’nyi, Prasad, and Sommerfeld (BPS) and that where such a configuration is not BPS-saturated—are considered individually. For the first case, it is shown that, at low initial velocities, a collision of the walls is virtually an elastic reflection somewhat delayed in time. It is also demonstrated that, in this case, it is possible to introduce a collective variable that has the meaning of an internal parameter of the configuration and which can be treated as a dynamical (time-dependent) variable and to describe the dynamics of the system in terms of an effective Lagrangian. For the second case, it is found that, for collisions, there is a critical value of v _cr≈0.9120 for the initial velocity v _i. For v _i<v _cr, the reflection of the walls occurs, the vacuum between the walls remaining unchanged. For v _i>v _cr, the collision process is accompanied by a change in the vacuum state between the walls.     

Domain wall theory of elementary excitations in anisotropic antiferromagneticS=1 chains

We present a theoretical investigation of elementary excitations in anisotropic antiferromagneticS=1 chains using the concept of domain walls in string (hidden) order. Domain walls are classified by the internal spin projectionS _dw ^z . We calculate energies and string correlation functions of low lying excited states of the domain wall type in the Haldane phase and compare the results to those of numerical computations. The boundaries of the Haldane phase are determined from the instability of these excitations with respect to the ground state. The interaction between two domain walls is found to be proportional to the productS _dw ^z , S _dw ^z ₂, it is effectively repulsive for equal spin projections.     

Thin <Emphasis Type="Italic">W</Emphasis>′ and <Emphasis Type="Italic">W</Emphasis> domain walls in ferroelastic Pb<Subscript>3</Subscript>(PO<Subscript>4</Subscript>)<Subscript>2</Subscript>

A model of ferroelastic domain walls consisting of matching interlayers of crystal lattices is proposed. The dependences of the parameters of the interlayers and of the parameters of the equations for W′ and W domain walls on the crystal lattice parameters of the ferroelastic phase in Pb₃(PO₄)₂ are determined. The problem concerning the number of possible orientational states and their interaction in a polydomain crystal is considered.     

Domain wall structure in BaTiO3 during switching

The increase in the complex permittivity during switching is calculated from a two-dimensional model of residual nuclei, assuming that domain walls possess a pyramidal step structure. The resulting formulae are compared with previous experimental data on BaTiO₃ and with corresponding formulae from the same model with smooth walls. It is shown that the real part of the permittivity during switching is very sensitive to the presence of steps on moving domain walls and that there is an indication that in BaTiO₃ such step structure actually exists.     

Simulation of first order confinement transition in relativistic heavy ion collision

We have carried out numerical simulation of first order phase transition in 2+1 dimensions to study the formation and evolution of Z(3) domain walls in relativistic Heavy Ion Collision using the effective potential proposed by Pisarski for QCD where Polyakov loop is the order parameter of the weak first order phase transition. Bubbles of the QGP phase are randomly nucleated on the lattice, which grow and coalesce. The spontaneous breaking of Z(3) symmetry in QGP phase gives rise to domain walls and topological strings. We discuss P _T enhancement due to reflection of quarks from the collapsing domain walls. We also discuss enhancement of doubly strange and triply strange hadrons due to larger concentration of s quarks inside collapsing wall. The decay of the domain walls when temperature drops below T _c results in the fluctuations of energy density.     

Pb(Zr0.52Ti0.48)O3陶瓷畴界粘滞运动的介电损耗模拟

对不同频率下(1—10kHz)Pb(Zr_0.52Ti_0.48)O₃多晶陶瓷的介电性能的测量表明：在接近铁电相变温度T_c以下存在一介电损耗峰,该峰具有弛豫特征但不满足Arrhenius关系．这一损耗峰被认为是由于畴界与晶格、缺陷钉扎的互作用引起的．用畴界粘滞运动的动力学方程,考虑陶瓷样品中T_c离散分布的情况,模拟了该介电损耗峰在不同频率下的行为,得到了与实验数据一致的结果．并由拟合参数计算了畴界运动     

Pb(Zr0.52Ti0.48)O3陶瓷畴界粘滞运动的介电损耗模拟

对不同频率下（１ —１０ ｋ Ｈｚ） Ｐｂ（ Ｚｒ０５２ Ｔｉ０４８） Ｏ３ 多晶陶瓷的介电性能的测量表明：在接近铁电相变温度 Ｔｃ 以下存在一介电损耗峰,该峰具有弛豫特征但不满足 Ａｒｒｈｅｎｉｕｓ 关系．这一损耗峰被认为是由于畴界与晶格、缺陷钉扎的互作用引起的．用畴界粘滞运动的动力学方程,考虑陶瓷样品中 Ｔｃ 离散分布的情况,模拟了该介电损耗峰在不同频率下的行为,得到了与实验数据一致的结果．并由拟合参数计算了畴界运动的粘滞系数和缺陷钉扎引起的回复力常量．     

Properties of domain walls and magnetoresistance in low-doped La<Subscript>2−<Emphasis Type="Italic">y</Emphasis></Subscript>Sr<Subscript>y</Subscript>CuO<Subscript>4</Subscript>

The properties of the diagonal stripe structures of the Hubbard model are theoretically studied in relation to the incommensurate spin order and the magnetic effects detected in the dielectric phase of low-doped La_2?y Sr_yCuO₄ (y ≤ 0.05). The mean-field approximation is used to investigate the properties of the solutions with domain walls between antiphase antiferromagnetic domains that are centered on bonds. Such periodic structures with 2l sites in a unit cell are shown to have 2(l ? 1) levels in the lower and upper Hubbard subbands and two levels that are separated into the Hubbard gap and correspond to quasi-one-dimensional states localized on domain walls. The calculation results are employed to check the assumption that the low conduction of the dielectric LSCO phase occurs via the network of domain walls. The maximum relative change in the magnetoresistance during a spin-flop transition in a critical magnetic field is estimated, and the giant magnetoresistance is qualitatively explained.     

Influence of isomorphous substitution of metal ion on the low-frequency dielectric dispersion in NH2(CH3)2Al1−xCrx(SO4)2⋅6H2O ferroelectrics

This paper is devoted to the study of the influence of metal ion isomorphous substitution on the ferroelastic-ferroelectric phase transition and dispersion caused by the motion of domain walls in dimethylammonium metal sulfate hexahydrate DMAAl_1?xCr_xS ferroelectric crystals (x = 0, 0.065, 0.2). It is shown that such a substitution significantly changes the phase transition temperature and parameters of the dielectric dispersion. These changes are explained in terms of interaction between the metal-hydrate complexes and DMA groups that carry the dipole moment and due to this they are responsible for the phase transitions and motion of the domain walls.     

The effects of different nano particles of Al2O3 and Ag on the MHD nano fluid flow and heat transfer in a microchannel including slip velocity and temperature jump

The forced convection of nanofluid flow in a long microchannel is studied numerically according to the finite volume approach and by using a developed computer code. Microchannel domain is under the influence of a magnetic field with uniform strength. The hot inlet nanofluid is cooled by the heat exchange with the cold microchannel walls. Different types of nanoparticles such as Al₂O₃ and Ag are examined while the base fluid is considered as water. Reynolds number are chosen as Re=10 and Re=100. Slip velocity and temperature jump boundary conditions are simulated along the microchannel walls at different values of slip coefficient for different amounts of Hartmann number. The investigation of magnetic field effect on slip velocity and temperature jump of nanofluid is presented for the first time. The results are shown as streamlines and isotherms; moreover the profiles of slip velocity and temperature jump are drawn. It is observed that more slip coefficient corresponds to less Nusselt number and more slip velocity especially at larger Hartmann number. It is recommended to use Al₂O₃-water nanofluid instead of Ag-water to increase the heat transfer rate from the microchannel walls at low values of Re. However at larger amounts of Re, the nanofluid composed of nanoparticles with higher thermal conductivity works better.     

LOW-FREQUENCY INTERNAL FRICTION STUDY ON MODIFIED LEAD METANIOBATE CERAMICS

Six Q^-1 peaks in freshly-worked PbNb₂O₆ ceramics are observed by low-frequency internal friction measurement so far as we know for the first time. P₁, P₂, and P₄ appear at the first heating process with remarkable shear modulus anomalies. But they are not observed during the cooling process or in the well-annealed samples. Those peaks are considered to be related with the working processes. P₃ is a widened Debye relaxation peak in nature. The activation energy and pre-exponential factor are about 1.01 eV and 2.5×10^-12 s, respectively. This peak is attributed to the interaction of domain walls and oxygen vacancies. P₆ is corresponded with the paraelectric and ferroelectric phase transition. P₅, appearing near T_c, is thought to be caused by the viscous movement of the domain walls.     

Capacitance and H2SO4 adsorption in the pores of activated carbon fibers

H₂SO₄ adsorption was studied by solid-state ²H-NMRand temperature-programmed desorption (TPD) on a series of pitch-based activated carbon fibers (ACFs), and compared to their electric double-layer capacitance in H₂SO₄ and Et₄NBF₄. Three states of H₂SO₄ were found by magic angle spinning (MAS)²H-NMR, namely species adsorbed on the walls, trapped in the pores, and staying over the outer surface of the ACFs, respectively. Protons of H₂SO₄ are strongly fixed on the walls of small pores whereas in the larger pores they are rather mobile due to exchange. The adsorbed protons on the pore walls contribute to the capacitance while their exchange with the protons of H₂SO₄ in the pores appears to decrease their contribution to the capacitance. The capacitance of ACFs with smaller surface area is correlated to the amount of SO₂ desorbed by TPD up to 200 °C. By contrast, for other ACFs of larger surface area the amount up to the same temperature was found to be much larger than their capacitance. The contribution of H₂SO₄ adsorbed in the latter ACFs is much less effective for their capacitance.