arXiv subjects

Haricharan Padmanabhan

Publications and source records attributed to Haricharan Padmanabhan.

4 recordsLinked to original sources

arXiv2018

Linear and nonlinear optical probe of the ferroelectric-like phase transition in a polar metal, LiOsO3

LiOsO3 is one of the first materials identified in a recent literature as a 'polar metal', a class of materials that are simultaneously noncentrosymmetric and metallic. In this work, the linear and nonlinear optical susceptibility of LiOsO3 is studied by means of ellipsometry and optical second harmonic generation (SHG). Strong optical birefringence is observed using spectroscopic ellipsometry. The nonlinear optical susceptibility extracted from SHG polarimetry reveals that the tensor components are of the same magnitude as in isostructural insulator LiNbO3, except the component along the polar axis d33, which is suppressed by an order of magnitude. Temperature-dependent SHG measurements in combination with Raman spectroscopy indicate a continuous order-disorder type polar phase transition at 140 K. Linear and nonlinear optical microscopy techniques reveal 109 deg/71 deg ferroelastic domain walls, like in other trigonal ferroelectrics. No 180 deg polar domain walls are observed to emerge across the phase transition.

cond-mat.mtrl-sci↗

arXiv2018

A new symmetry-based framework for discovering minimum energy pathways

Physical systems evolve from one state to another along paths of least energy barrier. Without a priori knowledge of the energy landscape, multidimensional search methods aim to find such minimum energy pathways between the initial and final states of a kinetic process. However in many cases, the user has to repeatedly provide initial guess paths, thus ensuring that the reliability of the final result is heavily user-dependent. Recently, the idea of "distortion symmetry groups" as a complete description of the symmetry of a path has been introduced. Through this, a new framework is enabled that provides a powerful means of classifying the infinite collection of possible pathways into a finite number of symmetry equivalent subsets, and then exploring each of these subsets systematically using rigorous group theoretical methods. The method is shown to lead to the discovery of new, previously hidden pathways for the case studies of bulk ferroelectric switching and domain wall motion in proper and improper ferroelectrics, as well as in multiferroic switching. This approach is applicable to a wide variety of physical phenomena ranging from structural, electronic and magnetic distortions, diffusion, and phase transitions in materials.

cond-mat.mtrl-sci↗

arXiv2017

Spatio-temporal Symmetry - Point Groups with Time Translations

Spatial symmetries occur in combination with temporal symmetries in a wide range of physical systems in nature, including time-periodic quantum systems typically described by the Floquet formalism. In this context, groups formed by three-dimensional point group symmetry operations in combination with time translation operations are discussed in this work. The derivation of these 'spatio-temporal' groups from conventional point groups and their irreducible representations is outlined, followed by a complete listing. The groups are presented in a template similar to space group operations, and are visualized using a modified version of conventional stereographic projections. Simple examples of physical processes that simultaneously exhibit symmetry in space and time are identified and used to illustrate the application of spatio-temporal groups.

cond-mat.mtrl-sci↗

arXiv2016

Intertwined Lattice Deformation and Magnetism in Monovacancy Graphene

Using density functional calculations we have investigated the local spin moment formation and lattice deformation in graphene when an isolated vacancy is created. We predict two competing equilibrium structures: a ground state planar configuration with a saturated local moment of 1.5 $μ_B$, and a metastable non-planar configuration with a vanishing magnetic moment, at a modest energy expense of ~50 meV. Though non-planarity relieves the lattice of vacancy-induced strain, the planar state is energetically favored due to maximally localized defect states (v$σ$, v$π$). In the planar configuration, charge transfer from itinerant (Dirac) states weakens the spin-polarization of v$π$ yielding a fractional moment, which is aligned parallel to the unpaired v$σ$ electron through Hund's coupling. In the non-planar configuration, the absence of orthogonal symmetry allows interaction between v$σ$ and local d$π$ states, to form a hybridized v$σ^\prime$ state. The non-orthogonality also destabilizes the Hund's coupling, and an antiparallel alignment between v$σ$ and v$π$ lowers the energy. The gradual spin reversal of v$π$ with increasing non-planarity opens up the possibility of an intermediate structure with balanced v$π$ spin population. If such a structure is realized under external perturbations, diluted vacancy concentration may lead to v$σ$ based spin-1/2 paramagnetism.

cond-mat.mtrl-sci↗