Session E03: Condensed Matter Physics II

Novel transport phenomena in noncollinear antiferromagnets

Despite the significant academic interest in them and their richness in nature, antiferromagnets have always been overshadowed by ferromagnets in real-life applications based on magnetism or spintronics. This is primarily due to the fact that antiferromagnet order parameters, in contrast to the ferromagnetic magnetization, are only weakly coupled to magnetic fields, and are hence difficult, in conventional view, to be manipulated. In this talk I will discuss a number of recent theoretical and experimental developments that counter this conventional wisdom, in a class of antiferromagnets that have stable noncollinear magnetic orders. As an introduction I will talk about the discovery of the anomalous Hall effect (AHE) in noncollinear antiferromagnets. Then I will explain a theory for the recent experimental discovery of time-reversal-symmetry-breaking counterparts of the conventional SHE and ISHE in the noncollinear antiferromagnet Mn$_3$Sn, which we name as the magnetic spin Hall effect (MSHE) and the magnetic inverse spin Hall effect (MISHE), respectively. Finally I will discuss the concept of spin density polarization, and how to use it to describe the spin-Hall effect in a magnetic insulator as a bulk effect, without using the spin current language.   

Creating a Probabilistic Quantum Superposition using Nuclear Magnetic Resonance.

The no-superposition theorem states that given two arbitrary quantum states and two complex coefficients, an appropriate superposition of these two states cannot be created. By including a third quantum state as an ancilla however, a probabilistic procedure has been proposed that creates an un-normalized superposition [M. Oszmaniec \textit{et al.} Phys. Rev. Lett. \textbf{116} 110403 (2016)]. We will review the challenges of creating a quantum superposition. We develop an automated program to output superpositions, we explore the possibility of applying this process to physical molecules. We introduce the concept of pseudo-pure states, and elaborate on the difficulty of procuring and detecting molecules in these states.   

Structural Studies of Synthetic Ferromagnetic Sample CeTiGe3 Under Pressure

Under extreme conditions the interactions, and thereby the arrangement of atoms, in materials is affected. We can change the temperature and pressure which a material is experiencing to observe what changes occur; such as structural changes, magnetic property changes, and in some cases superconductivity. This research was a case study of CeTiGe3, a ferromagnetic material, under extreme pressures using helium as pressure transmitting medium. The measurements were conducted under pressure using single crystal x-ray diffraction in a synchrotron. No phase transitions were observed, but structural shifts were seen at ambient temperature with increased pressure from ~0-10 GPa. Further investigations of the magnetic properties of CeTiGe3 can be carried out to relate structural properties to magnetic properties.   

Modeling Magnetic Correlations in Magnetite Nanoparticle Assemblies Using X-ray Magnetic Scattering Data

Magnetic nanoparticles are increasingly used in nanotechnologies and biomedical applications, such as drug targeting, MRI, and bio-separation. Magnetite (Fe3O4) nanoparticles stand to be effective in these roles due to the non-toxic nature of magnetite and its ease of manufacture. To be more effective in these applications, a greater understanding of the magnetic behavior of the individual magnetite nanoparticles is needed when a collection of them is used. This research seeks to discover the local magnetic ordering of ensembles of magnetite nanoparticles occurring at various stages of the magnetization process. To complete this study, we use resonant x-ray magnetic scattering, which provides information about the magnetic orders in the material. Here we discuss the modeling of the magnetic scattering data using a one-dimensional chain of nanoparticles with a mix of ferromagnetic, anti-ferromagnetic, and random order. The model utilizes twelve variable parameters and we used a Levenberg-Marquardt algorithm to find the best fit parameters. By fitting the model to the experimental data, we extracted information about the magnetic correlations in the nanoparticle assembly.   

Group Theory and Domain-boundary defects in crystals.

The symmetries of a crystal form a space group. A phase transition gives rise to a physical order parameter that breaks some of the space-group symmetries, while preserving others. Those that are preserved form a subgroup of the original space group. The cosets of the subgroup can be used to construct a graph that illuminates the defects that arise where domains of the order parameter intersect.   

Using the Pair Distribution Function to Explore Superconductivity in BaFe2As2

Superconductors have a great potential to revolutionize energy, but conventional superconductors can only operate at temperatures near absolute zero and/or under extreme pressure. New materials showing superconductivity at higher temperatures have been discovered in recent years, yet the mechanism of superconductivity remains unknown. Research suggests that changing the symmetry of the local structure plays a large role in superconductivity in these new materials. The method of pair distribution function (PDF) analysis allows us to determine the local, short range structure at various temperatures, providing a glimpse of the behavior of superconducting materials as their local symmetry changes. We focus on data acquired from samples of BaFe2As2 analyzed using the PDF method over temperatures ranging from 2K to 300K.