Thermal activation of vortex-antivortex pairs in quasi-two-dimensional Bose-Einstein condensates

Here we show, by performing ab initio classical field simulations that two distinct superfluid phases, separated by thermal vortex-antivortex pair creation, exist in experimentally producible quasi-2D Bose gas. These results resolve the debate on the nature of the low temperature phase(s) of a trapped interacting 2D Bose gas.     

Onset and dynamics of vortex-antivortex pairs in polariton optical parametric oscillator superfluids

We study, both theoretically and experimentally, the occurrence of topological defects in polariton superfluids in the optical parametric oscillator (OPO) regime. We explain in terms of local supercurrents the deterministic behavior of both the onset and dynamics of vortex-antivortex pairs generated by perturbing the system with a pulsed probe. Using a generalized Gross-Pitaevskii equation, including photonic disorder, pumping and decay, we elucidate the reason why topological defects form in couples and can be detected by direct visualizations in multishot OPO experiments.     

Direct observation of condon domains in silver by Hall probes

Using a set of micro Hall probes for the detection of the local induction, the inhomogeneous Condon domain structure has been directly observed at the surface of a pure silver single crystal under strong Landau quantization in magnetic fields up to 10 T. The inhomogeneous induction occurs in the theoretically predicted part of the Condon domain phase diagram. Information about size, shape, and orientation of the domains is obtained by analyzing Hall probes placed along and across the long sample axis and by tilting the sample. On a beryllium surface the induction inhomogeneity is almost absent although the expected induction splitting here is at least 10 times higher than in silver.     

Group analysis of domains and domain pairs

Basic symmetry properties of transformation twins and of ferroelectric or ferromagnetic domains are examined in terms of the abstract group theory. It is shown that the crystallographical relations between domains (twin components) and between domain pairs can be deduced from the decomposition of the symmetry group of the high symmetry phase into the left and double cosets of the group of the low symmetry phase. Expressions are derived for the numbers of proper and improper domains, for the number of crystallographically equivalent low symmetry phases, and for the number of crystallographically non-equivalent domain pairs. A classification of domain pairs according to their symmetry is proposed. The domain structure of the monoclinic phase in WO₃ and the Dauphiné twinning in quartz are analysed as illustrative examples.     

Direct observation of large electronic domains with memory effect in doped manganites

We use a spatially resolved, direct spectroscopic probe for electronic structure with an additional sensitivity to chemical compositions to investigate high-quality single crystal samples of La(1/4)Pr(3/8)Ca(3/8)MnO3, establishing the formation of distinct insulating domains embedded in the metallic host at low temperatures. These domains are found to be at least an order of magnitude larger in size compared to previous estimates and exhibit memory effects on temperature cycling in the absence of any perceptible chemical inhomogeneity, suggesting long-range strains as the probable origin.     

Switching current noise and relaxation of ferroelectric domains

We simulate field-induced nucleation and switching of domains in a three-dimensional model of ferroelectrics with quenched disorder and varying domain sizes. We study (1) bursts of the switching current at slow driving along the hysteresis loop (electrical Barkhausen noise) and (2) the polarization reversal when a strong electric field was applied and back-switching after the field was removed. We show how these processes are related to the underlying structure of domain walls, which in turn is controlled by the pinning at quenched local electric fields. When the depolarization fields of bound charges are properly screened we find that the fractal switching current noise may appear with two distinct universal behaviors. The critical depinning of plane domain walls determines the universality class in the case of weak random fields, whereas for large randomness the massive nucleation of domains in the bulk leads to different scaling properties. In both cases the scaling exponents decay logarithmically when the driving frequency is increased. The polarization reverses in the applied field as a power-law, while its relaxation in zero field is a stretch exponential function of time. The stretching exponent depends on the strength of pinning. The results may be applicable for uniaxial relaxor ferroelectrics, such as doped SBN:Ce. Received 7 February 2002 / Received in final form 10 April 2002 Published online 9 July 2002     

Visualization of ferroelectric domains with coherent light

Illumination of a lithium niobate or lithium tantalate crystal along the crystallographic c axis with coherent light while simultaneously applying an external electrical field through transparent electrodes allows real-time, in situ, nondestructive monitoring of ferroelectric domain patterns. Imaging of the optical near-field through a lens directly visualizes the domain walls, whereas the far-field yields averaged information about the spatial orientation of the domain boundaries.     

Direct observation of ferroelectric domain switching in varying electric field regimes using in situ TEM

In situ Transmission Electron Microscopy (TEM) techniques can potentially fill in gaps in the current understanding interfacial phenomena in complex oxides. Select multiferroic oxide materials, such as BiFeO(3) (BFO), exhibit ferroelectric and magnetic order, and the two order parameters are coupled through a quantum-mechanical exchange interaction. The magneto-electric coupling in BFO allows control of the ferroelectric and magnetic domain structures via applied electric fields. Because of these unique properties, BFO and other magneto-electric multiferroics constitute a promising class of materials for incorporation into devices such as high-density ferroelectric and magnetoresistive memories, spin valves, and magnetic field sensors. The magneto-electric coupling in BFO is mediated by volatile ferroelastically switched domains that make it difficult to incorporate this material into devices. To facilitate device integration, an understanding of the microstructural factors that affect ferroelastic relaxation and ferroelectric domain switching must be developed. In this article, a method of viewing ferroelectric (and ferroelastic) domain dynamics using in situ biasing in TEM is presented. The evolution of ferroelastically switched ferroelectric domains in BFO thin films during many switching cycles is investigated. Evidence of partial domain nucleation, propagation, and switching even at applied electric fields below the estimated coercive field is revealed. Our observations indicate that the occurrence of ferroelastic relaxation in switched domains and the stability of these domains is influenced the applied field as well as the BFO microstructure. These biasing experiments provide a real time view of the complex dynamics of domain switching and complement scanning probe techniques. Quantitative information about domain switching under bias in ferroelectric and multiferroic materials can be extracted from in situ TEM to provide a predictive tool for future device development.     

Direct observation of discommensurations in the incommensurate superlattice of ferroelectric Rb2ZnCl4 by transmission electron microscopy

The insulating orthorhombic crystal Rb₂ZnCl₄ forms an incommensurate superlattice below 30°C which locks into a commensurate one at −81°C. The discommensurations in the incommensurate phase were made visible for the first time by dark field electron microscopy. The topological features of the discommensurations as well as the characteristics of the corresponding electron diffraction pattern agree with previous results obtained by indirect methods.     

Atomic-force microscopy of polarization domains in ferroelectric films

Single-crystal (001)-oriented PbZr_0.47Ti_0.53O₃ and polycrystalline (111)-oriented PbZr_0.47Ti_0.53O₃ thin ferroelectric films were studied using contact electrostatic force microscopy. Local electromechanical response measurements permitted study of the polarization vector distribution in natural and intentionally created polarization nanodomains in films. The principal components of an electromechanical response signal encountered in studies of ferroelectric films (the piezoelectric response and the additional capacitive contribution) were isolated and analyzed. The effect of tip-surface contact stiffness on the capacitive contribution to the electromechanical response signal was demonstrated experimentally and in terms of a model. It was shown that more accurate information on the distribution of the polarization vector in ferroelectric films can be gained by monitoring local variations in the tip-surface contact stiffness.     

Visualization of ferroelectric domains in bulk single crystals

In recent years ferroelectric domain patterning has become a popular topic of physical research because it enables photonic applications as well as data storage. For generation of tailored domain structures and for further understanding of ferroelectricity, a visualization of the domain patterns is required. A large number of imaging techniques have therefore been developed. This review summarizes these techniques and highlights systematically their strengths and weaknesses. PACS 77.84.-s; 78.20.-e; 68.37.-d     

基于静态电滞回线的铁电电容模型

根据铁电体的特征电滞回线和微观结构特点，将构成铁电体的晶胞等效为偶极子. 通过分析偶极子在电场作用下的极化反转机理，运用统计物理学基本原理建立了新的铁电电容模型. 该模型不仅适用于饱和极化的情况，对非饱和、电滞回线不对称以及输入电压中途转向等各种情况也都适用. 模型数学表达简洁，易于结合到实际的电路仿真软件中去，仿真结果与试验结果符合非常好. 关键词： 铁电电容 建模 电滞回线 偶极子     

Role of nanosecond processes in the formation of domains in ferroelectric crystals

The contribution from pyroelectric and photovoltaic effects to the formation of light-induced domains in crystals of oxide ferroelectrics of lithium niobate type upon irradiation by nanosecond focused laser beams is considered. It is established that inverted domains are formed in irradiated regions under the combined effect of generated pyroelectric and photovoltaic fields with a sharp increase in temperature. It is shown that the most stable domains are formed in lithium niobate samples containing Fe²⁺ and Fe³⁺ impurity ions.     

Properties of the peaks of second harmonic light through Fibonacci-class ferroelectric domains

After establishing the method of constructing a class of one-dimensional (1D) Fibonacci-class quasiperiodic (FC(n)) ferroelectric domains system, we have studied the properties of the electric field of the second harmonic generation (SHG) by means of the small-signal approximation in the case of the vertically transmission. It was found that only the second harmonic light (SHL) peaks which were indexed by two special integers q and p would be the brightest and the spectra whose positions were decided by successive FC(n) integers and were perfect self-similar without considering the dispersive effect of the refractive index on SHL. The effect of the vacancies for some special spectral lines was also studied generally. The analytic results were confirmed by the numerical simulations. Received 26 October 1999 and Received in final form 6 January 2000     

Photoluminescence observation of donor pairs in silicon

The asymmetric broadening with a tail on the lower energy side and the peak shift to lower energies are observed in the luminescence band due to bound excitons in silicon crystals which contain phosphorus impurities below the critical concentration for semiconductor-metal transition (8 × 10¹⁶ -3 × 10¹⁸ cm^-3). These behaviors are understood in terms of an exciton bound to a donor pair in silicon.     

A comparison of the visibility of ferroelectric domains in BaTiO3 and ferromagnetic domains in Fe-Si on X-ray diffraction topographs

An X-ray diffraction image of ferroelectric domain walls in BaTiO₃ was studied by Lang's technique. The visibility of the walls follows, in principle, the same rule that was found to be valid for the visibility of ferromagnetic domain walls in Fe-Si. The type of contrast is, however, strongly affected by the relatively large c/a ratio.The author expresses her thanks to Dr. J. Kaczér and Dr. J. Fousek for their valuable discussions.     

Anti-parallel ferroelectric domains in surface space-charge layers of BaTiO3

It is shown on simplified models of a surface space-charge layer of a single crystal of BaTiO ₃ that in the surface region, in which the direction of the electric field is opposite to the direction of spontaneous polarization inside the crystal, anti-parallel domains can exist. The dimensions of these domains are determined. The use of the results obtained is proposed for interpreting the microrelief of the etch figures and for explaining the different phases of the polarization reversal process of a single-domain single crystal of BaTiO ₃.

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BatiO₃

BaTiO₃ , , , . . BaTiO₃.

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The author would like to thank J. Kaczer, Z. Málek and V. Dvoák of the Institute of Physics, Czechoslovak Academy of Sciences, for valuable remarks on this paper and H. Havlíková for carefully carrying out the numerical calculations.     

Direct observation of the Aharonov-Casher phase

Ring structures fabricated from HgTe/HgCdTe quantum wells have been used to study Aharonov-Bohm type conductance oscillations as a function of Rashba spin-orbit splitting strength. We observe nonmonotonic phase changes indicating that an additional phase factor modifies the electron wave function. We associate these observations with the Aharonov-Casher effect. This is confirmed by comparison with numerical calculations of the magnetoconductance for a multichannel ring structure within the Landauer-Büttiker formalism.