Search for Dislocation Free Helium 4 Crystals
F Souris1, A D Fefferman2, A Haziot3
1Laboratoire de Physique Statistique de l'École Normale Supérieure associé au CNRS et aux Universités P.M. Curie et D. Diderot, 24 rue Lhomond, 75231 Paris Cedex 05, France.
Journal of Low Temperature Physics
|June 13, 2015
Summary
Researchers found significantly higher dislocation densities in helium 4 crystals than previously reported. This challenges existing explanations for crystal plasticity and highlights the need for further study into dislocation nucleation mechanisms.
Area of Science:
- Condensed Matter Physics
- Materials Science
- Low-Temperature Physics
Background:
- The plasticity of helium 4 ([He]) crystals is attributed to high dislocation mobility.
- Previous studies reported very low screw dislocation densities (<100 cm⁻²) in [He] crystals.
- Dislocation-free crystals were expected to exhibit distinct mechanical properties.
Purpose of the Study:
- To investigate the dislocation density in helium 4 crystals.
- To compare measured dislocation densities with previous findings.
- To re-evaluate the role of dislocations in [He] crystal plasticity.
Main Methods:
- Growth of helium 4 crystals under conditions similar to previous studies.
- Extraction of dislocation density from measured mechanical properties.
- Comparison of experimental results with literature data.
Main Results:
- Measured dislocation densities ranging from 10⁵ to 10⁷ cm⁻², orders of magnitude higher than previously reported.
- Contradiction with prior studies by Ruutu et al. regarding screw dislocation density.
- Identification of potential discrepancies due to indirect measurement techniques and different dislocation types.
Conclusions:
- The high dislocation density found challenges the prevailing explanation for giant plasticity in [He] crystals.
- Discrepancies suggest that previous measurements may have been indirect or focused on specific dislocation types.
- The mechanism of dislocation nucleation in [He] crystals remains an open question requiring further investigation.
Related Concept Videos
01:26Imperfections in Crystal Structure: Stoichiometric Point Defects
91
Schottky defects arise when some lattice points in a crystal, such as those in NaCl, remain unoccupied, creating lattice vacancies without disturbing the overall electrical neutrality of the crystal. This defect is common in ionic crystals where the positive and negative ions are similar in size, as seen in sodium chloride and cesium chloride. The presence of Schottky defects enables the crystal to conduct electricity to a small extent through an ionic mechanism. Electric fields cause nearby...
91
02:54Noble Gases
23.5K
The elements in group 18 are noble gases (helium, neon, argon, krypton, xenon, and radon). They earned the name “noble” because they were assumed to be nonreactive since they have filled valence shells. In 1962, Dr. Neil Bartlett at the University of British Columbia proved this assumption to be false.
23.5K
01:25Imperfections in Crystal Structure: Point, Line and Plane Defects
103
A perfect crystal, in theory, has a uniform structure with the same unit cell and lattice points throughout. However, any deviation from this periodic arrangement is known as an imperfection or defect. These defects can be categorized into three types: point, line, and plane defects.Point defects occur when there is a deviation from the ideal due to missing atoms, displaced atoms, or additional atoms. These imperfections might occur due to imperfect packing during crystallization or because of...
103
01:29Imperfections in Crystal Structure: Non-Stoichiometric Defects
92
Non-stoichiometric defects refer to a type of defect in the crystal structure of a compound where the ratio of its constituent elements deviates from the ideal stoichiometric ratio. There are two main types of non-stoichiometric defects: metal excess defects and metal deficiency defects.Metal excess defects occur when there is a slight surplus of metal ions than what is required by the stoichiometric ratio of the compound. For example, heating a sodium chloride crystal in sodium vapor results...
92
01:29Determination of Crystal Structures
114
In the late 1800s, the revelation that light extended beyond visible wavelengths led to the discovery of X-rays by Wilhelm Roentgen. Recognized as high-energy electromagnetic radiation with short wavelengths, X-rays prompted exploration into their interaction with crystals. Max von Laue proposed in 1912 that the periodic arrangement of atoms, ions, or molecules in crystals would cause them to diffract X-rays, a hypothesis confirmed through experiments with copper sulfate and zinc sulfide...
114