arXiv subjects

Allana G. Iwanicki

Publications and source records attributed to Allana G. Iwanicki.

6 recordsLinked to original sources

arXiv2026

Enabling the Ambient Pressure Growth of ScB2 Crystals for AlGaN Power Electronics

Here we report the growth of single crystalline ScB2, an ultrahigh-temperature ceramic, at ambient pressure in a laser-heated Optical Floating Zone via the travelling solvent method. Crystals have been grown from both Sc-rich (55-65 at% Sc) and B-rich self-flux (80-83 at% B) at growth rates in the range of 0.2-2 mm/hr. The structure of grown crystals is in good agreement with an AlB2-type layered hexagonal phase, space group P6/mmm, with lattice constants a = 3.1423(2) Å (resp. 3.1502(3) Å) and c = 3.5084(3) Å (resp. 3.5041(3) Å) for crystals grown under Sc-rich (resp. B-rich) conditions. Crystals natively grow along the in-plane [100] direction. Electron backscattered diffraction shows that Sc-flux growth results in boules with multiple domains containing Sc inclusions, with the domains highly aligned. In contrast, B-flux boules are single domain after the initial nucleation region. Rocking-curve measurements of B-flux crystals for the (h000) and (000l) reflections show single peaks, establishing that the crystals are free from grain boundaries; the asymmetry in the scattered-intensity tails suggests the presence of point defects. Surface X-ray photoemission spectroscopy shows that the electronic environment in B-flux crystals is superior to that of Sc-flux crystals and produces highly resolved binding-energy peaks for B 1s and Sc 2p. Work-function measurements for the (11-20) plane give a value of approximately 5 eV, consistent with the highly electrically conductive nature of ScB2. These results demonstrate the viable ambient-pressure growth of ScB2, establish it as a lattice-matched substrate candidate for Al-rich AlGaN power microelectronics, and show that this growth route enables scalable manufacturing of ScB2 substrates.

cond-mat.mtrl-sci↗

arXiv2026

Scandium diboride: a semi-metallic, lattice, thermally matched substrate for vertical AlGaN power electronics

We report the properties of hexagonal (space group P6/mmm) scandium diboride ($\mathrm{ScB}_2$) single crystals grown by a laser diode floating zone method at growth rates of ~1mm/hr under B-rich conditions with (002) rocking curve widths $Δω$=38'' approaching the quality of commercial SiC/GaN substrates. Lattice expansion measurements reveal matching to $\mathrm{Al_{0.55}Ga_{0.45}N}$ with a coefficient of thermal expansion ~5ppm/K at typical AlGaN growth temperatures, enabling thick AlGaN layers for ultra-wide bandgap (UWBG) power electronics >1kV. We measure semi-metallic room temperature resistivity ~15$μΩ$ cm, climbing to ~93$μΩ$ cm at 773K with a $T^2$ dependence effectively eliminating substrate parasitic resistance, the limiting factor in exploiting the full potential of UWBG. The Debye temperature $θ_{D,ScB_2}$ from heat capacity and lattice expansion is ~850K well matched to $θ_{D,ScB_2}$, but lower than the 1100K measured for Sc-rich growth conditions. We discuss Debye matching as a key substrate codesign criterion providing significant overlap in phonon modes for heat removal and thermal matching during AlGaN growth. The competitive thermal conductivity at room temperature 53W/mK is half that from full first principles calculations, a discrepancy we attribute to the presence of Sc-vacancies generated by B-rich growth. while the resistivity is ~2x the theoretical value, indicating that both electrons and phonons play equal role in thermal transport. The smooth ~2.5nm rms roughness surface enables advanced heat removal modalities through engineered phonon bridges and phonon polaritons in $\mathrm{ScB}_2$/AlGaN interfacial heterostructures, potentially allowing ~10-100x increase in power handling over state-of-the-art GaN/SiC.

cond-mat.mtrl-sci↗

arXiv2026

Stripping Symmetry: Electrochemical Oxidation to a Superconducting Polar Metal in Au2Pb0.914P2

Polar metals and noncentrosymmetric superconductors are exceptionally rare, yet their broken inversion symmetry can give rise to emergent electronic phenomena including mixed singlet-triplet superconducting pairing. As only a few such materials have been found among known compounds, accessing new examples requires synthetic strategies that go beyond conventional crystal growth. Here, we use electrochemical topotactic deintercalation to remove Pb from the centrosymmetric parent compound Au$_2$PbP$_2$, producing the polar metal Au$_2$Pb$_{0.914}$P$_2$. Unlike conventional chemical doping, this transformation actively drives structural symmetry-breaking: the partial removal of Pb triggers a cooperative electronic and geometric rearrangement, mediated by a second-order Jahn-Teller effect and stereochemically active lone pairs, that locks the product into a polar, noncentrosymmetric superspace group Ama2(01g)ss0. We solve the complete (3+1)D modulated structure by synchrotron single-crystal X-ray diffraction and confirm the polar assignment through nonlinear electronic transport. Below T$_c$ = 1.52 K, Au$_2$Pb$_{0.914}$P$_2$ becomes a type-II superconductor whose heat capacity and AC susceptibility both exhibit power-law behavior, suggestive of a gap structure governed by the broken inversion symmetry of the host lattice. This work establishes electrochemical oxidation as a rational route to metastable noncentrosymmetric superconductors through chemically directed symmetry-breaking.

cond-mat.supr-con↗

arXiv2026

Hydroflux Crystal Growth of Alkali Tellurate Oxide-Hydroxides

This study investigates the synthesis of novel magnetic materials via hydroflux synthesis, a method that combines flux-based and hydrothermal techniques. Single crystals of three novel alkali tellurate oxide hydroxides were synthesized. One, CsTeO3(OH), is nonmagnetic and a new member of the series ATeO3(OH) (A = alkali). The other two phases contain magnetic Cu-Te substructures, one of which, KCu2Te3O8(OH), is structurally three-dimensional and undergoes several magnetic ordering transitions. The other, Cs2Cu3Te2O10, is structurally two-dimensional and remains paramagnetic above T = 2 K. These exploratory investigations of novel phase spaces reveal key factors, including hydroxide concentration, precursor solubility, and oxidizing power of the solution, in the formation and composition of alkali tellurate oxide hydroxides.

cond-mat.mtrl-sci↗

arXiv2025

Atomically Modulating Competing Exchange Interactions in Centrosymmetric Skyrmion Hosts GdRu2X2 (X = Si, Ge)

Magnetic skyrmions are topologically protected spin states enabling high-density, low-power spin electronics. Despite growing efforts to find new skyrmion host systems, the microscopic mechanisms leading to skyrmion phase transitions at specific temperatures and magnetic fields remain elusive. Here, we systematically study the isostructural centrosymmetric magnets- GdRu2X2 (X = Si and Ge), and the role of X-p orbitals in modifying magnetic exchange interactions. GdRu2Ge2 single crystals, synthesized by arc melting, exhibit two high-entropy pockets associated with skyrmion phases at 0.9 T < H < 1.2 T and 1.3 T < H < 1.7 T, 2 K < T < 30 K-more accessible condition at lower fields and higher temperatures than that in the Si counterpart. Entropy estimations from heat capacity measurements align with magnetization data, and transport studies confirm a topological Hall effect, highlighting the system's nontrivial spin textures and Berry curvature. Compared to GdRu2Si2, electronic structure and exchange interaction evaluations reveal the more extended Ge-4p orbitals enhance competing exchange interactions in GdRu2Ge2, thereby manifesting the rich skyrmion behavior. This work demonstrates how modifying exchange interactions at the atomic level enables the tunability of topologically nontrivial electronic states while advancing our understanding of skyrmion formation mechanisms for future spintronics.

cond-mat.mtrl-sci↗

arXiv2024

Hydroflux-Controlled Growth of Magnetic K-Cu-Te-O(H) Phases

Innovative synthetic approaches can yield new phases containing novel structural and magnetic motifs. In this work, we show the synthesis and magnetic characterization of three new and one previously reported layered phase in the K-Cu-Te-O(H) phase space using a tunable hydroflux technique. The hydroflux, with a roughly equal molar ratio of water and alkali hydroxide, is a highly oxidizing, low melting solvent which can be used to isolate metastable phases unattainable through traditional solid state or flux techniques. The newly synthesized phases, K$_{2}$Cu$_{2}$TeO$_{6}$, K$_{2}$Cu$_{2}$TeO$_{6}$ $\cdot$ H$_{2}$O, and K$_{6}$Cu$_{9}$Te$_{4}$O$_{24}$ $\cdot$ 2 H$_{2}$O, contain Cu$^{2+}$ within CuO$_{4}$ square planar plaquettes and TeO$_{6}$ octahedra ordering to form structural honeycomb layers isolated by interlayer K$^{+}$ ions and H$_{2}$O molecules. We find the synthesized structures display varying tilt sequences of the CuO$_{4}$ plaquettes, leading to distinct Cu$^{2+}$ magnetic motifs on the structural honeycomb lattice and a range of effective magnetic dimensionalities. We find that K$_{2}$Cu$_{2}$TeO$_{6}$ $\cdot$ H$_{2}$O does not order and displays alternating chain Heisenberg antiferromagnetic (AFM) behavior, while K$_{2}$Cu$_{2}$TeO$_{6}$ and K$_{6}$Cu$_{9}$Te$_{4}$O$_{24}$ $\cdot$ 2 H$_{2}$O order antiferromagnetically (T$_{N}$ = 100 K and T$_{N}$ = 6.5 K respectively). The previously known phase, K$_{2}$CuTeO$_{4}$(OH)$_{2}$ $\cdot$ H$_{2}$O, we find contains structurally and magnetically one-dimensional CuO$_{4}$ plaquettes leading to uniform chain Heisenberg AFM behavior and shows no magnetic order down to T = 0.4 K. We discuss and highlight the usefulness of the hydroflux technique in novel syntheses and the interesting magnetic motifs that arise in these particular phases.

cond-mat.str-el↗