The ferroelectric domain wall is an ideal interface for studies of the boundary scattering of high frequency acoustic phonons. These internal interfaces have no contamination and can be reversibly created and destroyed by application of an electric field at the phase transition-122 K in KDP. Images of phonons in single-and multiple-domain cases show remarkable differences in the ballistic heat flux. Theoretical modeling based on acoustic wave reflection reveals that the domain walls act as nearly perfect grain boundaries.PACS numbers: 66.70.+f, 63.20.Mt, 68.35.Gy, 77.80.Dj The transport of thermal energy across crystalline boundaries has great practical importance, yet there are few cases where measurements agree with microscopic models of the interface. The classic example is the anomalously high thermal boundary conductance observed between a crystal and liquid helium, as first reported by Kapitza [1]. Also, at temperatures of a few kelvin the thermal conductance across solid-solid interfaces is generally not well described by acoustic-mismatch theory [2]. These large discrepancies are generally attributed to diffusive scattering of phonons by defects at the interface [3]. Acoustic-mismatch theory, in contrast, computes the transmission and reflection coefficients of an elastic wave under the assumption that k\\, the wave vector parallel to the interface, is conserved (i.e., Snell's law), which is the condition for specular scattering [4]. While there are some notable exceptions [5], this condition is difficult to satisfy at ordinary (imperfect) interfaces for phonons with wavelengths of a few hundred angstroms or less. In this paper, we report the observation of scattering of high frequency phonons from "internal" interfaces-ferroelectric domain walls in KDP-and find that these interfaces act as nearly perfect "specular" interfaces. Our phonon imaging experiments provide detailed information about the propagation of all phonon modes, leading to an understanding of unusual anisotropics observed in the thermal conductivity of this crystal at low temperatures.At room temperature KDP is a paraelectric material that has applications in nonlinear optics, principally as an efficient frequency doubler of laser photons, and in electro-optic devices [6]. The crystal symmetry about 122 K is tetragonal [7]. At T = 122 K, KDP undergoes a ferroelectric phase transition, in which domains of opposing electric polarization spontaneously appear, as shown in Fig. 1(a) [8]. The positive energy needed to create domain walls between regions of opposing polarization is more than offset by the reduction in overall electrostatic energy. If, however, the crystal is placed in an electric field greater than about 1 kV/cm along the fourfold axis as it passes through the transition, a single domain is formed, which persists at lower temperature even when the field is removed.There is a structural change from tetragonal to orthorhombic which accompanies the ferroelectric phase transition [the actual distortion is much smaller than that s...

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We launch surface acoustic waves (SAW) along both the 〈 〉 110 and the 〈 〉 110 directions of a Hall bar and measure the anisotropic conductivity in a high purity GaAs two-dimensional electron system in the quantum Hall regime of the stripe and the bubble phases. In the anisotropic stripe phase, SAW propagating along the 'easy' 〈 〉 110 direction sense a compressible behavior (finite resistance) which is seen in standard transport measurement only if current flows along the 'hard' 〈 〉 110 direction. In the isotropic bubble phase, the SAW data show compressible behavior in both directions, in marked contrast to the incompressible quantum Hall behavior seen in transport measurements. These results challenge models that assume that both the stripe and the bubble phase consist of incompressible domains and raise important questions about the role of domain boundaries in SAW propagation.