arXiv subjects

M. Graf

Publications and source records attributed to M. Graf.

5 recordsLinked to original sources

arXiv2024

Non-reciprocal neutral ferroelectric domain walls in BiFeO3

This paper analyzes a peculiar phenomenon of non-reciprocal domain wall pairs and illustrate the implications in ab-initio-based atomistic computational experiments with (112)-oriented planar R180 domain walls within the canonical multiferroic ferroelectric crystal of BiFeO3. Results show that parallel walls on the opposite sides of a given domain within a simply twinned lamellar domain structure can have considerably different polarization and oxygen octahedra tilt profiles, different thickness and energy densities. The spontaneous formation of zigzag walls and triangular domains suggests that these domain walls are actually the lowest energy planar R180 walls in pure insulating BiFeO3 crystals and films.

cond-mat.mtrl-sci↗

arXiv2015

Electrochemical reaction in single layer MoS2: nanopores opened atom by atom

Ultrathin nanopore membranes based on 2D materials have demonstrated ultimate resolution toward DNA sequencing. Among them, molybdenum disulphide (MoS2) shows long-term stability as well as superior sensitivity enabling high throughput performance. The traditional method of fabricating nanopores with nanometer precision is based on the use of focused electron beams in transmission electron microscope (TEM). This nanopore fabrication process is time-consuming, expensive, not scalable and hard to control below 1 nm. Here, we exploited the electrochemical activity of MoS2 and developed a convenient and scalable method to controllably make nanopores in single-layer MoS2 with sub-nanometer precision using electrochemical reaction (ECR). The electrochemical reaction on the surface of single-layer MoS2 is initiated at the location of defects or single atom vacancy, followed by the successive removals of individual atoms or unit cells from single-layer MoS2 lattice and finally formation of a nanopore. Step-like features in the ionic current through the growing nanopore provide direct feedback on the nanopore size inferred from a widely used conductance vs. pore size model. Furthermore, DNA translocations can be detected in-situ when as-fabricated MoS2 nanopores are used. The atomic resolution and accessibility of this approach paves the way for mass production of nanopores in 2D membranes for potential solid-state nanopore sequencing.

cond-mat.soft↗

arXiv2014

Phase Transitions and Antiferroelectrivity in BiFeO3 from Atomic Level Simulations

The structural and polar properties of BiFeO3 at finite temperature are investigated using an atomistic shell model fitted to first-principles calculations. Molecular Dynamics simulations show a direct transition from the low-temperature R3c ferroelectric phase to the Pbnm orthorhombic phase without evidence of any intermediate bridging phase between them. The high-temperature phase is characterized by the presence of two sublattices with opposite polarizations, and it displays the characteristic double-hysteresis loop under the action of an external electric field. The microscopic analysis reveals that the change in the polar direction and the large lattice strains observed during the antiferroelectric-ferroelectric phase transition originate from the interplay between polarization, oxygen octahedron rotations and strain. As a result, the induced ferroelectric phase recovers the symmetry of the low temperature R3c phase.

cond-mat.mtrl-sci↗

arXiv2011

The Influence Of Elastic Deformations On The Supersolid Transition

We study, within the Ginzburg-Landau (GL) theory of phase transitions, how elastic deformations in a supersolid lead to local changes in the supersolid transition temperature. The GL theory is mapped onto a Schrodinger-type equation with an effective potential that depends on local dilatory strain. The effective potential is attractive for local contractions and repulsive for local expansion. Different types of elastic deformations are studied. We find that a contraction (expansion) of the medium that may be brought about by, e.g., applied stress leads to a higher (lower) transition temperature as compared to the unstrained medium. In addition, we investigate edge dislocations and illustrate that the local transition temperature may be increased in the immediate vicinity of the dislocation core. Our analysis is not limited to supersolidity. Similar strain effects should also play a role in superconductors.

cond-mat.other↗

arXiv2005

Nonlinear Magneto-Optical Rotation with Amplitude-Modulated Light: AMOR

A new technique of nonlinear magneto-optical rotation with amplitude modulated light is developed. The technique is an alternative to its counterpart with frequency modulated light and can be applied to sensitive measurements of magnetic fields ranging from microgauss to the Earth-field level. The rotation signals exhibit nontrivial features like narrowed non-Lorentzian lineshapes and multi-component resonances.

physics.optics↗