Showing papers by "Joshua E. Goldberger published in 2021"

Coexisting ferromagnetic–antiferromagnetic state in twisted bilayer CrI3

Abstract: Moire engineering1–3 of van der Waals magnetic materials4–9 can yield new magnetic ground states via competing interactions in moire superlattices10–13. Theory predicts a suite of interesting phenomena, including multiflavour magnetic states10, non-collinear magnetic states10–13, moire magnon bands and magnon networks14 in twisted bilayer magnetic crystals, but so far such non-trivial magnetic ground states have not emerged experimentally. Here, by utilizing the stacking-dependent interlayer exchange interactions in two-dimensional magnetic materials15–18, we demonstrate a coexisting ferromagnetic (FM) and antiferromagnetic (AF) ground state in small-twist-angle CrI3 bilayers. The FM–AF state transitions to a collinear FM ground state above a critical twist angle of about 3°. The coexisting FM and AF domains result from a competition between the interlayer AF coupling, which emerges in the monoclinic stacking regions of the moire superlattice, and the energy cost for forming FM–AF domain walls. Our observations are consistent with the emergence of a non-collinear magnetic ground state with FM and AF domains on the moire length scale10–13. We further employ the doping dependence of the interlayer AF interaction to control the FM–AF state by electrically gating a bilayer sample. These experiments highlight the potential to create complex magnetic ground states in twisted bilayer magnetic crystals, and may find application in future gate-voltage-controllable high-density magnetic memory storage. In moire superlattice van der Waals magnetic materials, competing interactions emerge and can stabilize new magnetic states. Here, stacking-dependent interlayer exchange interactions in small-twist-angle CrI3 bilayers yield an ordered ground state with coexisting ferromagnetic and antiferromagnetic regions.

Highly efficient transverse thermoelectric devices with Re4Si7 crystals

Abstract: The principal challenges in current thermoelectric power generation modules are the availability of stable, diffusion-resistant, lossless electrical and thermal metal–semiconductor contacts that do not degrade at the hot end nor cause reductions in device efficiency. Transverse thermoelectric devices, in which a thermal gradient in a single material induces a perpendicular voltage, promise to overcome these problems. However, the measured material transverse thermoelectric efficiencies, zxyT, of nearly all materials to date has been far too low to confirm these advantages in an actual device. Here, we show that single crystals of Re4Si7, an air-stable, thermally robust, layered compound, have a transverse zxyT of 0.7 ± 0.15 at 980 K, a value that is on par with existing commercial longitudinal theremoelectrics today. Through constructing and characterizing a transverse power generation module, we prove that extrinsic losses through contact resistances are minimized in this geometry, and that no electrical contacts are needed at the hot side. This excellent transverse thermoelectric performance arises from the large, oppositely signed in-plane p-type and cross-plane n-type thermopowers. These large anisotropic thermopowers arise from thermal population of the highly anisotropic valence band and isotropic conduction band in this narrow gap semiconductor. Overall, this work establishes Re4Si7 as the “gold-standard” of transverse thermoelectrics, allowing future exploration of unique device architectures for waste heat recovery.

Computationally Guided Discovery of Axis-Dependent Conduction Polarity in NaSnAs Crystals

Abstract: Most electronic materials exhibit a single dominant charge carrier type, either holes or electrons, along all crystallographic directions. However, there are a small number of compounds, mostly met...

Emergence of a noncollinear magnetic state in twisted bilayer CrI3

Abstract: The emergence of two-dimensional (2D) magnetic crystals and moir\'e engineering has opened the door for devising new magnetic ground states via competing interactions in moir\'e superlattices. Although a suite of interesting phenomena, including multi-flavor magnetic states, noncollinear magnetic states, moir\'e magnon bands and magnon networks, has been predicted, nontrivial magnetic ground states in twisted bilayer magnetic crystals have yet to be realized. Here, by utilizing the stacking-dependent interlayer exchange interactions in CrI3, we demonstrate in small-twist-angle bilayer CrI3 a noncollinear magnetic ground state. It consists of both antiferromagnetic (AF) and ferromagnetic (FM) domains and is a result of the competing interlayer AF coupling in the monoclinic stacking regions of the moir\'e superlattice and the energy cost for forming AF-FM domain walls. Above the critical twist angle of ~ 3{\deg}, the noncollinear state transitions abruptly to a collinear FM ground state. We further show that the noncollinear magnetic state can be controlled by gating through the doping-dependent interlayer AF interaction. Our results demonstrate the possibility of engineering moir\'e magnetism in twisted bilayer magnetic crystals, as well as gate-voltage-controllable high-density magnetic memory storage.

Synthesis and characterization of a new family of layered PbxSn4−xAs3 alloys

Abstract: Layered two-dimensional (2D) materials have attracted considerable interest for their exotic and anisotropic electronic behavior. One such material, Sn4As3, bears a resemblance in both structure and elemental composition to two other Sn- and As-containing layered materials that have recently demonstrated axis-dependent conduction polarity: NaSn2As2 and NaSnAs. Here, a new family of Pb-alloyed PbxSn4−xAs3 crystals was synthesized and the axis-dependent electronic and thermoelectric properties were evaluated. Up to one full equivalent of Pb could be alloyed into PbxSn4−xAs3 (0 < x < 1.06) before phase separation occurred. We establish the structural changes and the trends in the Raman spectra with increasing Pb substitution. These materials all exhibit metallic temperature-dependent resistivities and positive thermopowers along the in-plane and cross-plane directions. The absence of axis-dependent conduction polarity in these SnAs-layered materials is consistent with theoretical predictions, and illustrates that precise control over the atomic and electronic structure and doping is essential for realizing this phenomenon in new materials.

Anomalous electronic properties in layered, disordered ZnVSb

Abstract: New materials discovery is the driving force for progress in solid state physics and chemistry. Here we solve the crystal structure and comprehensively study physical properties of ZnVSb in the polycrystalline form. Synchrotron x-ray diffraction reveals that the compound attains a layered ZrSiS-type structure ($P4$/nmm, $a$ = 4.09021(2) \AA{}, $c$ = 6.42027(4) \AA{}). The unit cell is composed of a 2D vanadium network separated by Zn-Sb blocks that are slightly distorted from the ideal cubic arrangement. A considerable amount of vacancies were observed on the vanadium and antimony positions; the experimental composition is ${\mathrm{ZnV}}_{0.91}{\mathrm{Sb}}_{0.96}$. Low-temperature x-ray diffraction shows very subtle discontinuity in the lattice parameters around 175 K. Bonding V-V distance is below the critical separation of 2.97 \AA{} known from the literature, which allows for V-V orbital overlap and subsequent metallic conductivity. From ab initio calculations, we found that ZnVSb is a pseudogap material with an expected dominant vanadium contribution to the density of states at the Fermi level. The energy difference between the antiferromagnetic and nonordered magnetic configurations is rather small (0.34 eV/f.u.). X-ray photoelectron spectroscopy fully corroborates the results of the band structure calculations. Magnetic susceptibility uncovered that, in ZnVSb, itinerant charge carriers coexist with a small, localized magnetic moment of ca. 0.25 ${\ensuremath{\mu}}_{\mathrm{B}}$. The itinerant electrons arise from the ordered part of the vanadium lattice. Fractional localization, in turn, was attributed to V atoms neighboring vacancies, which hinder full V-V orbital overlap. Furthermore, the susceptibility and electrical resistivity showed a large hysteresis between 120 K and 160 K. The effect is not sensitive to magnetic fields up to 9 T. Curie-Weiss fitting revealed that the amount of itinerant charge carriers in ZnVSb drops with decreasing temperature below 160 K, which is accompanied by a concurrent rise in the localized magnetic moment. This observation together with the overall shape of the hysteresis in the resistivity allows for suggesting a plausible origin of the effect as a charge-transfer metal-insulator transition. Ab initio phonon calculations show the formation of a collective phonon mode at 2.8 THz (134 K). The experimental heat capacity reflected this feature by a boson peak with Einstein temperature of 116 K. Analysis of the heat capacity with both an ab initio perspective and Debye-Einstein model revealed a sizable anharmonic contribution to heat capacity, in line with disordered nature of the material. Further investigation of the electron and phonon properties for ZnVSb is likely to provide valuable insight into the relation between structural disorder and the physical properties of transition-metal-bearing compounds.

Lucky Number 13: A 13-Layer Polytype of the Alkyne Hydrogenation Catalyst CaGaGe.

Abstract: Polytypism, the ability of materials to form crystal structures with different stacking sequences, occasionally causes materials with the same stoichiometry and similar local structures to have profoundly different properties. Herein, we discover a metastable 13-layer trigonal (13T) polytype of CaGaGe, a layered intermetallic phase comprised of [GaGe]2- honeycombs separated by Ca2+. 13T-CaGaGe is synthesized from arc-melting the elements, and its structure is elucidated via neutron powder diffraction. Air-stable 13T-CaGaGe has one misaligned [GaGe]2- layer for every 13 and transforms into the more stable 4-layer hexagonal (4H) CaGaGe polytype after annealing at 500 °C. Transition-metal-free 13T-CaGaGe shows remarkable activity in the catalytic hydrogenation of phenylacetylene to styrene and ethylbenzene, much higher than the 4H polytype. This work identifies the first 13-layer polytype for any crystal structure and further establishes the influence of polytypism on catalysis.

Alkyne Hydrogenation Catalysis across a Family of Ga/In Layered Zintl Phases.

Abstract: Transition-metal-free Zintl-Klemm phases have received little attention as heterogeneous catalysis. Here, we show that a large family of structurally and electronically similar layered Zintl-Klemm phases built from honeycomb layers of group 13 triel (Tr) or group 14 tetrel (Tt) networks separated by electropositive cations (A) and having a stoichiometry of ATr2 or ATrTt (A = Ca, Ba, Y, La, Eu; Tr = Ga, In; Tt = Si, Ge) exhibit varying degrees of activity for the hydrogenation of phenylacetylene to styrene and ethylbenzene at 51 bar H2 and 40-100 °C across a variety of solvents. The most active catalysts contain Ga with, formally, a half-filled pz orbital, and minimal bonding between neighboring Tr2 or TrTt layers. A 13-layer trigonal polytype of CaGaGe (13T-CaGaGe) was the most active, cyclable, and robust catalyst and under modest conditions (1 atm H2, 40 °C) had a surface specific activity (590 h-1) comparable to a commercial Lindlar's catalyst. Additionally, 13T-CaGaGe maintained 100% conversion of phenylacetylene to styrene at 51 bar H2, even after 5 months of air exposure. This work reveals the structural design elements that lead to particularly high catalytic activity in Zintl-Klemm phases, further establishing them as a promising materials platform for hydrogen-based heterogeneous catalysis.

Dynamics of Two Distinct Exciton Populations in Methyl-functionalized Germanane.

Abstract: Methyl-substituted Germanane is an emerging material that has been proposed for novel applications in optoelectronics, photoelectrocatalysis, and biosensors. It is a two-dimensional semiconductor with a strong above-gap fluorescence associated with water intercalation. Here, we use time-resolved photoluminescence spectroscopy to understand the mechanism causing this fluorescence. We show that it originates from two distinct exciton populations. Both populations recombine exponentially, accompanied by the thermally activated transfer of exciton population from the shorter- to the longer-lived type. The two exciton populations involve different electronic levels and couple to different phonons. The longer-lived type of exciton migrates within the disordered energy landscape of localized recombination centers. These outcomes shed light on the fundamental optical and electronic properties of functionalized germanane, enabling the groundwork for future applications in optoelectronics, light-harvesting, and sensing.