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1. Chiral ferroelectric nematic in Grandjean-Cano wedge cell

   https://doi.org/10.1117/12.2675751  

   Thapa, Kamal; Iadlovska, Olena S; Basnet, Bijaya; Wang, Hao; Paul, Ayusha; Gleeson, James T; Lavrentovich, Oleg D (September 2023, Proceedings SPIE)

   Khoo, Iam Choon (Ed.)

   We explore the structures and confinement-induced edge dislocations in Grandjean-Cano wedge cells filled with the recently discovered chiral ferroelectric nematic (N_F^*) and chiral antiferroelectric smectic-Z 〖(SmZ〗_A^*). The chiral mixture is formed by DIO mesogen doped with a chiral additive. Wedge cells with parallel and antiparallel rubbing at the opposite plates show quantitatively different structures which is attributed to the polar in-plane anchoring of the spontaneous polarization at the rubbed substrates. The helical pitch shows a non-monotonous temperature dependence upon cooling, increasing as the temperature is lowered to the N^*-SmZ_A^* phase transition. The SmZ_A^* formed from an untwisted N^* in the thin portion of the wedge shows a bookshelf (BK) geometry, whereas the twisted N^* transforms into a twisted planar (PA) SmZ_A^* structure. In the N_F^* phase, the untwisted N^* becomes twisted in a wedge with antiparallel assembly of plates and monodomain in wedges with parallel assembly. The twisted regions of N_F^* show only one type of Grandjean zones separated by thick edge dislocations with Burgers vector b=P; the neighboring regions differ by 2π- twist. 

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2. An Investigation into the Role of Dislocation Climb During Intermediate Temperature Flow of Mg Alloys

   https://doi.org/10.1007/978-3-030-36647-6_19  

   Ritzo, Michael A; Bhattacharyya, Jishnu J; Lebensohn, Ricardo A; Agnew, Sean R (January 2020, Magnesium Technology 2020)

   Textured Mg alloy sheet samples were tensile tested parallel to the transverse direction, at Zener–Hollomon parameter values ranging from Z ~ 50 at room temperature and 10−3 s−1 down to Z ~ 18 at 350 °C and 10−5 s−1. At high Z, the samples exhibit strong texture evolution indicative of significant prismatic slip of dislocations with  Burgers vectors. Correspondingly, the plastic anisotropy is high, r ~ 4. At low Z, the texture evolution is minimal and the response is nearly isotropic, r ~ 1. Previously, it has been asserted that the high ductility and low plastic anisotropy observed at low Z conditions is due to enhanced activity of non-basal slip modes, including prismatic slip of  dislocations and pyramidal slip of  and  dislocations. The present results call this understanding into question and suggest that the enhanced ductility is more closely associated with the climb of  dislocations. 

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3. Dislocation formation in the heteroepitaxial growth of PbSe/PbTe systems

   https://doi.org/10.1016/j.actamat.2023.119308  

   Li, Yang; Gu, Boyang; Diaz, Adrian; Phillpot, Simon R.; McDowell, David L.; Chen, Youping (November 2023, Acta Materialia)

   The paper presents a multiscale study of the kinetic processes of the heteroepitaxial growth of the PbSe/PbTe (111) and PbTe/PbSe(001) systems, using the Concurrent Atomistic-Continuum (CAC) method as the simulation tool. The CAC simulations have reproduced the Stranski–Krastanov growth mode and the layer-by-layer growth mode of the two systems, respectively; the pyramid-shaped island morphology of the PbSe epilayer on PbTe (111), the square-like misfit dislocation networks within the PbTe/PbSe(001) interface, and the critical thickness for the PbTe/PbSe(001) system at which coherent interfaces transit to semi-coherent interfaces with the formation of misfit dislocations, all in good agreement with experimental observations. Four types of misfit dislocations are found to form during the growth of the two PbTe/PbSe heterosystems, and hexagonal-like misfit dislocation networks are observed within the PbSe/PbTe(111) interfaces. The growth processes, including the formation of misfit dislocations, have been visualized. Dislocation half-loops have been observed to nucleate from the epilayer surfaces. These half-loops extend towards the interface by climb or glide motions, interact with other half-loops, and form misfit dislocation networks at the interfaces and threading dislocations extending from interfaces to epilayer surfaces. The dominant types of misfit dislocations in both systems are found to be those with Burgers vectors parallel to the interfaces, whereas the misfit dislocations with Burgers vectors inclined to the interface have a low likelihood of generation and tend to annihilate. The size of the substrate is demonstrated to have a significant effect on the formation, evolution, and distribution of dislocations on the growth of PbSe on PbTe(111). 

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4. Screw dislocations in the X-cube fracton model

   https://doi.org/10.21468/SciPostPhys.10.4.094  

   Manoj, Nandagopal; Slagle, Kevin; Shirley, Wilbur; Chen, Xie (January 2021, SciPost Physics)

   The X-cube model, a prototypical gapped fracton model, was shown in Ref. [1] to have a foliation structure. That is, inside the 3+1 D model, there are hidden layers of 2+1 D gapped topological states. A screw dislocation in a 3+1 D lattice can often reveal nontrivial features associated with a layered structure. In this paper, we study the X-cube model on lattices with screw dislocations. In particular, we find that a screw dislocation results in a finite change in the logarithm of the ground state degeneracy of the model. Part of the change can be traced back to the effect of screw dislocations in a simple stack of 2+1 D topological states, hence corroborating the foliation structure in the model. The other part of the change comes from the induced motion of fractons or sub-dimensional excitations along the dislocation, a feature absent in the stack of 2+1D layers. 

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5. Polarization near dislocation cores in SrTiO3 single crystals: The role of flexoelectricity

   https://doi.org/10.1063/5.0202453  

   Cheng, Xiaoxing; Wang, Bo; Chen, Long-Qing (March 2024, Applied Physics Letters)

   Spontaneous polarization as large as ∼28 μC/cm2 was recently observed around the dislocation cores in non-polar SrTiO3 bulk crystals, and its origin was attributed to the flexoelectric effect, i.e., polarization induced by strain gradients. However, the roles of flexoelectricity, relative to other electromechanical contributions, and the nature of dislocations, i.e., edge vs screw dislocations in the induced polarization, are not well understood. In this work, we study the role of flexoelectricity in inducing polarization around three types of dislocation cores in SrTiO3: b=a(100) edge dislocation, b=a(110) edge dislocation, and b=a(010) screw dislocation, where b is the Burgers vector. For the edge dislocations, polarization can be induced by electrostriction alone, while flexoelectricity is essential for stabilizing the symmetric polarization pattern. The shear component of the flexoelectric tensor has a dominant effect on the magnitude and spatial distribution of the flexoelectric polarization. In contrast, no polarization is induced around the b=a(010) screw dislocation through either electrostriction or flexoelectricity. Our findings provide an in-depth understanding of the role of flexoelectricity in inducing polarization around dislocation cores and offer insights into the defect engineering of dielectric/ferroelectric materials. 

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