arXiv subjects

Luisa Whittaker-Brooks

Publications and source records attributed to Luisa Whittaker-Brooks.

6 recordsLinked to original sources

arXiv2026

Non-relativistic spin splitting in a triangular metal-excess magnet Fe$_{1+δ}$Sb

Non-relativistic spin-splitting (NRSS) antiferromagnets have recently emerged as an important class of magnetic materials that combine compensated magnetism with momentum-dependent spin splitting, offering new opportunities for spintronic applications. Here, we investigate a NiAs-type Fe$_{1+δ}$Sb series ($δ= 0.17$-0.30) using neutron diffraction, pair distribution function, magnetometry and density functional theory calculations. Neutron diffraction establishes that Fe$_{1+δ}$Sb adopts a $120^\circ$ coplanar compensated magnetic order with a non-zero propagation vector $\mathbf{k}=(1/3,\,1/3,\,0)$. Increasing interstitial Fe suppresses the ordered magnetic moment while inducing local symmetry lowering, as revealed by pair distribution function refinements. Density functional theory predicts momentum-dependent spin splitting, dominated by an out-of-plane spin polarization with an odd-parity f-wave-like symmetry, establishing the material as a non-collinear NRSS antiferromagnet. Motivated by the structural similarities between Fe$_{1+δ}$Sb and a known altermagnet CrSb, we further investigate their solid solution and find that Cr substitution at intermediate concentrations gives rise to a ferromagnetic component and a cluster spin-glass behavior. These results establish Fe$_{1+δ}$Sb as a new platform for non-collinear NRSS antiferromagnetism and demonstrate metal interstitial and substitution as effective parameters for tuning the magnetic order and properties.

cond-mat.mtrl-sci↗

arXiv2026

Symmetry-Protected Weyl Nodal Loops in a Triangular Altermagnet

Weyl semimetals and altermagnets represent two distinct classes of quantum materials exhibiting nontrivial topological and magnetic order, respectively. Here we report the realization of a Weyl nodal-loop altermagnet in Cr$_7$Se$_8$, combining neutron diffraction and first-principles calculations. The hexagonal system hosts a coplanar $120^\circ$ compensated magnetic order on a triangular lattice, which breaks inversion-time-reversal and translation-time-reversal symmetries simultaneously while preserving a crystalline mirror plane. The resulting electronic structure features linearly dispersing nodal loops close to the Fermi level ($E_F$) confined to the mirror-invariant $k_z=0$ plane. Along high-symmetry directions, the crossings near $E_F$ form Dirac-like fourfold degeneracies in the absence of spin-orbit coupling; at generic momenta, these crossings split into twofold and form continuous Weyl-like nodal loops protected by mirror symmetry. The momentum-dependent spin polarization exhibits an $f$-wave-like pattern characteristic of odd-parity altermagnets.

cond-mat.mtrl-sci↗

arXiv2023

Exciton properties: learning from a decade of measurements on halide perovskites and transition metal dichalcogenides

The exciton binding energy ($E_b$) is a key parameter that governs the physics of many optoelectronic devices. At their best, trustworthy and precise measurements of $E_b$ challenge theoreticians to refine models, are a driving force in advancing the understanding a material system, and lead to efficient device design. At their worst, inaccurate $E_b$ measurements lead theoreticians astray, sew confusion within the research community, and hinder device improvements by leading to poor designs. This review article seeks to highlight the pros and cons of different measurement techniques used to determine $E_b$, namely, temperature-dependent photoluminescence, resolving Rydberg states, electroabsorption, magnetoabsorption, scanning tunneling spectroscopy, and fitting the optical absorption. Due to numerous conflicting $E_b$ values reported for halide perovskites (HP) and transition metal dichalcogenides (TMDC) monolayers, an emphasis is placed on highlighting these measurements in attempt to reconcile the variance between different measurement techniques. By considering the published data en masse, we argue the experiments with the clearest indicators are in agreement on the following values: ~350 - 450 meV for TMDC monolayers between SiO$_2$ and vacuum, ~150 - 200 meV for hBN-encapsulated TMDC monolayers, ~200 - 300 meV for common lead-iodide 2D HPs, and ~10 meV for methylammonium lead iodide.

cond-mat.mtrl-sci↗

arXiv2021

Franz-Keldysh and Stark Effects in Two-Dimensional Metal Halide Perovskites

As the field of metal halide perovskites (MHP) matures, state-of-the-art techniques to measure basic properties such as the band gap and exciton binding energy continue to produce inconsistent values. This issue is persistent even for 2D MHPs wherein the large separation between exciton and continuum states should make such measurements more straightforward. In this study, we revert to the established theory of a 2D Wannier exciton in a uniform electric field to analyze the electroabsorption response of an archetypal 2D MHP system, phenethylammonium lead iodide (PEA2PbI4). The high level of agreement between the electroabsorption simulation and measurement allows for a deepened understanding of the exciton's redshift according to the quadratic Stark effect and the continuum wavefunction leaking according to the Franz-Keldysh effect. We find the field-dependency of each of these effects to be rich with information, yielding measurements of the exciton's Bohr radius, transition dipole moment, polarizability, and reduced effective mass. Most importantly, the exciton binding energy is unambiguously determined with 2% uncertainty. The high precision of these new measurement methods opens the opportunity for future studies to accurately determine the influence of chemical and environmental factors on the optoelectronic properties of MHPs and thereby increase the tunability of this important class of materials.

cond-mat.mes-hall↗

arXiv2021

N-type doping of LPCVD-grown \b{eta}-Ga2O3 thin films using solid-source germanium

We report on the growth and characterization of Ge-doped \b{eta}-Ga2O3 thin films using a solid germanium source. \b{eta}-Ga2O3 thin films were grown using a low-pressure chemical vapor deposition (LPCVD) reactor with either an oxygen or gallium delivery tube. Films were grown on 6 degree offcut sapphire and (010) \b{eta}-Ga2O3 substrates with growth rates between 0.5 - 22 μm/hr. By controlling the germanium vapor pressure, a wide range of Hall carrier concentrations between 10^17 - 10^19 cm-3 were achieved. Low-temperature Hall data revealed a difference in donor incorporation depending on the reactor configuration. At low growth rates, germanium occupied a single donor energy level between 8 - 10 meV. At higher growth rates, germanium doping predominantly results in a deeper donor energy level at 85 meV. This work shows the effect of reactor design and growth regime on the kinetics of impurity incorporation. Studying donor incorporation in \b{eta}-Ga2O3 is important for the design of high-power electronic devices.

cond-mat.mtrl-sci↗

arXiv2020

Low Temperature Homoepitaxy Of (010) $β$-Ga$_2$O$_3$ By Metalorganic Vapor Phase Epitaxy : Expanding The Growth Window

In this work, we report on the growth of high-mobility $β$-Ga$_2$O$_3$ homoepitaxial thin films grown at a temperature much lower than the conventional growth temperature window for metalorganic vapor phase epitaxy. Low-temperature $β$-Ga$_2$O$_3$ thin films grown at 600$^{\circ}$C on Fe-doped (010) bulk substrates exhibits remarkable crystalline quality which is evident from the measured room temperature Hall mobility of 186 cm$^2$/Vs for the unintentionally doped films. N-type doping is achieved by using Si as a dopant and controllable doping in the range of 2$\times$10$^{16}$ - 2$\times$10$^{19}$ cm$^{-3}$ is studied. Si incorporation and activation is studied by comparing silicon concentration from secondary ion mass spectroscopy (SIMS) and electron concentration from temperature-dependent Hall measurements. The films exhibit high purity (low C and H concentrations) with a very low concentration of compensating acceptors (2$\times$10$^{15}$ cm$^{-3}$) even at this growth temperature. Additionally, abrupt doping profile with forward decay of $\sim$ 5nm/dec (10 times improvement compared to what is observed for thin films grown at 810$^{\circ}$C) is demonstrated by growing at a lower temperature.

physics.app-ph↗