arXiv subjects

Divine Kumah

Publications and source records attributed to Divine Kumah.

5 recordsLinked to original sources

arXiv2026

Vortex Tunneling and Critical State in an Oxide Heterostructure

Two-dimensional superconductors offer an excellent platform for the study of vortex matter due to their low superfluid stiffness and inability to effectively screen applied magnetic fields. Here we explore vortices in a two-dimensional superconductor formed at the surface of the complex oxide KTaO$_3$. Multiple regimes of vortex-mediated transport are identified and studied, revealing switching behaviour attributed to nucleation of individual vortices. Analysis of this regime allows us to identify the quantum tunneling of vortices, which transitions to thermally activated behaviour at elevated temperatures. Magnetic field dependence reveals rich histograms of the switching currents which we attribute to different configurations of pinned vortices.

cond-mat.supr-con↗

arXiv2026

Electronic Structure of Epitaxial Films of the Bilayer Strontium Ruthenate: Sr$_{3}$Ru$_2$O$_{7}$

We report a combined study of the low-energy electronic band structure of epitaxial Sr$_3$Ru$_2$O$_7$ films using angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT). To investigate the effects of substrate dependence on the band structure, Sr$_3$Ru$_2$O$_7$ thin films are epitaxially grown on SrTiO$_3$ (STO) and (LaAlO$_{3}$)$_{0.3}$(Sr$_{2}$TaAlO$_{6}$)$_{0.7}$ (LSAT) substrates using molecular beam epitaxy. The measured and calculated Fermi-surfaces clearly show substantial changes in the Fermi surface topologies that originate from the underlying strain states. The compressively strained film grown on LSAT exhibits Fermi-surface features consistent with an orthorhombic-like symmetry; and the tensile-strained film grown on STO features a tetragonal-like surface symmetry. In addition, the ARPES data for both films indicate weakly dispersive spectral features within $\sim15~\text{meV}$ below the Fermi level. These observations underscore the strong sensitivity of the electronic structure of Sr$_3$Ru$_2$O$_7$ to epitaxial strain and establish a foundation for future efforts to tune correlated phases in this bilayer ruthenate.

cond-mat.str-el↗

arXiv2020

The Future of the Correlated Electron Problem

A central problem in modern condensed matter physics is the understanding of materials with strong electron correlations. Despite extensive work, the essential physics of many of these systems is not understood and there is very little ability to make predictions in this class of materials. In this manuscript we share our personal views on the major open problems in the field of correlated electron systems. We discuss some possible routes to make progress in this rich and fascinating field. This manuscript is the result of the vigorous discussions and deliberations that took place at Johns Hopkins University during a three-day workshop January 27, 28, and 29, 2020 that brought together six senior scientists and 46 more junior scientists. Our hope, is that the topics we have presented will provide inspiration for others working in this field and motivation for the idea that significant progress can be made on very hard problems if we focus our collective energies.

cond-mat.str-el↗

arXiv2016

The role of double TiO2 layers at the interface of FeSe/SrTiO3 superconductors

We determine the surface reconstruction of SrTiO3 used to achieve superconducting FeSe films in experiments, which is different from the 1x1 TiO2 terminated SrTiO3 assumed by most previous theoretical studies. In particular, we identify the existence of a double TiO2 layer at the SrTiO3-FeSe interface that plays two important roles. First, it facilitates the epitaxial growth of FeSe. Second, ab initio calculations reveal a strong tendency for electrons to transfer from an oxygen deficient SrTiO3 surface to FeSe when the double TiO2 layer is present. As a better electron donor than previously proposed interfacial structures, the double layer helps to remove the hole pocket in the FeSe at the Γ point of the Brillouin zone and leads to a band structure characteristic of superconducting samples. The characterization of the interface structure presented here is a key step towards the resolution of many open questions about this novel superconductor.

cond-mat.str-el↗

arXiv2014

Active silicon integrated nanophotonics: ferroelectric BaTiO3 devices

The integration of complex oxides on silicon presents opportunities to extend and enhance silicon technology with novel electronic, magnetic, and photonic properties. Among these materials, barium titanate (BaTiO3) is a particularly strong ferroelectric perovskite oxide with attractive dielectric and electro-optic properties. Here we demonstrate nanophotonic circuits incorporating ferroelectric BaTiO3 thin films on the ubiquitous silicon-on-insulator (SOI) platform. We grow epitaxial, single-crystalline BaTiO3 directly on SOI and engineer integrated waveguide structures that simultaneously confine light and an RF electric field in the BaTiO3 layer. Using on-chip photonic interferometers, we extract a large effective Pockels coefficient of 213 plus minus 49 pm/V, a value more than six times larger than found in commercial optical modulators based on lithium niobate. The monolithically integrated BaTiO3 optical modulators show modulation bandwidth in the gigahertz regime, which is promising for broadband applications.

cond-mat.mtrl-sci↗