Search for: All records

The recently-discovered ferroelectric nematic (NF) liquid crystal material presents a host of defect phenomena due to its unique polar nature and long-ranged electrostatic interactions. Much like the solid state ferroelectrics, the depolarization field in the material favors a spontaneous spatial variation of the polarization P, manifesting in myriad ways including a twist in the bulk and different arrangements of alternating polarization domains. Unlike the solid state materials with a specific bulk crystalline structure, the configuration of the NF materials is determined not only by the reduction of depolarization fields but also by the alignment of molecules at interfaces. In this work, we will consider an NF confined to a thin cell pre-patterned with various types of apolar surface anchoring of the molecules produced by photoalignment. For uniform planar alignment, we find that the sample forms a series of striped domains, reminiscent of striped configurations found in solid uniaxial ferroic thin films. For a cell pre-patterned with a radial +1 defect pattern, the NF breaks up into ”pie-slice” polarization domains. We calculate the elastic and electrostatic energy balance, which determines the observed configurations. We also consider how ionic screening modifies the dipolar interactions and the ensuing patterns of polarization. 

The domain structure of a fluid ferroelectric nematic is dramatically different from the domain structure of solid ferroelectrics since it is not restricted by rectilinear crystallographic axes and planar surface facets. We demonstrate that thin films of a ferroelectric nematic seeded by colloidal inclusions produce domain walls (DWs) in the shape of conics such as a parabola. These conics reduce the bound charge within the domains and at the DWs. An adequate description of the domain structures requires one to analyze the electrostatic energy, which is a challenging task. Instead, we demonstrate that a good approximation to the experimentally observed polydomain textures is obtained when the divergence of spontaneous polarization—which causes the bound charge—is heavily penalized by assuming that the elastic constant of splay in the Oseen-Frank energy is much larger than those for twist and bend. The model takes advantage of the fact that the polarization vector is essentially parallel to the nematic director throughout the sample. Published by the American Physical Society2024 

Ferroelectric nematic liquid crystals are formed by achiral molecules with large dipole moments. Their three-dimensional orientational order is described as unidirectionally polar. We demonstrate that the ground state of a flat slab of a ferroelectric nematic unconstrained by externally imposed alignment directions is chiral, with left- and right-handed twists of polarization. Although the helicoidal deformations and defect walls that separate domains of opposite handedness increase the elastic energy, the twists reduce the electrostatic energy and become weaker when the material is doped with ions. This work shows that the polar orientational order of molecules could trigger chirality in soft matter with no chemically induced chiral centers. 

Abstract Spontaneous electric polarization of solid ferroelectrics follows aligning directions of crystallographic axes. Domains of differently oriented polarization are separated by domain walls (DWs), which are predominantly flat and run along directions dictated by the bulk translational order and the sample surfaces. Here we explore DWs in a ferroelectric nematic (N F ) liquid crystal, which is a fluid with polar long-range orientational order but no crystallographic axes nor facets. We demonstrate that DWs in the absence of bulk and surface aligning axes are shaped as conic sections. The conics bisect the angle between two neighboring polarization fields to avoid electric charges. The remarkable bisecting properties of conic sections, known for millennia, play a central role as intrinsic features of liquid ferroelectrics. The findings could be helpful in designing patterns of electric polarization and space charge. 

Abstract Surface interactions are responsible for many properties of condensed matter, ranging from crystal faceting to the kinetics of phase transitions. Usually, these interactions are polar along the normal to the interface and apolar within the interface. Here we demonstrate that polar in-plane surface interactions of a ferroelectric nematic N F produce polar monodomains in micron-thin planar cells and stripes of an alternating electric polarization, separated by $${180}^{{{{{{\rm{o}}}}}}}$$ 180 o domain walls, in thicker slabs. The surface polarity binds together pairs of these walls, yielding a total polarization rotation by $${360}^{{{{{{\rm{o}}}}}}}$$ 360 o . The polar contribution to the total surface anchoring strength is on the order of 10%. The domain walls involve splay, bend, and twist of the polarization. The structure suggests that the splay elastic constant is larger than the bend modulus. The $${360}^{{{{{{\rm{o}}}}}}}$$ 360 o pairs resemble domain walls in cosmology models with biased vacuums and ferromagnets in an external magnetic field. 

Surface alignment and edge dislocations are explored in the recently discovered twist‐bend ferroelectric nematic, N_TBF, in which the vector of spontaneous polarization follows an oblique helicoidal trajectory around a polar twist‐bend axis. In a planar cell, the polar axis aligns at some angle to the rubbing direction to mitigate the surface electric charge. It is demonstrated that the pseudolayers in planar cells form chevron defects, a hallmark defect of one‐dimensionally positionally ordered phases, such as smectic A and smectic C. The polar character of the twist‐bend axis prevents the cores of N_TBFedge dislocations from splitting into semi‐integer disclinations, in stark contrast to dislocations in paraelectric and ferroelectric chiral nematics. The tilt of pseudolayers around the defect core allows estimation of the elastic penetration length as being close to the pitch of N_TBF. Compression/dilation stresses around the core modify the heliconical tilt angle of molecules as evidenced by a substantial variation in local birefringence. The climb of dislocations exhibits high mobility, allowing the system to equilibrate the temperature‐dependent pitch. The uncovered properties facilitate the development of N_TBFmaterials for electro‐optical applications, such as electrically controlled diffraction lattices and structural colors. 

Abstract Many measurements at the LHC require efficient identification of heavy-flavour jets, i.e. jets originating from bottom (b) or charm (c) quarks. An overview of the algorithms used to identify c jets is described and a novel method to calibrate them is presented. This new method adjusts the entire distributions of the outputs obtained when the algorithms are applied to jets of different flavours. It is based on an iterative approach exploiting three distinct control regions that are enriched with either b jets, c jets, or light-flavour and gluon jets. Results are presented in the form of correction factors evaluated using proton-proton collision data with an integrated luminosity of 41.5 fb -1 at  √s = 13 TeV, collected by the CMS experiment in 2017. The closure of the method is tested by applying the measured correction factors on simulated data sets and checking the agreement between the adjusted simulation and collision data. Furthermore, a validation is performed by testing the method on pseudodata, which emulate various mismodelling conditions. The calibrated results enable the use of the full distributions of heavy-flavour identification algorithm outputs, e.g. as inputs to machine-learning models. Thus, they are expected to increase the sensitivity of future physics analyses.