Showing papers by "Joshua E. Goldberger published in 2019"

Pressure-controlled interlayer magnetism in atomically thin CrI3.

Abstract: Stacking order can influence the physical properties of two-dimensional van der Waals materials1,2. Here we applied hydrostatic pressure up to 2 GPa to modify the stacking order in the van der Waals magnetic insulator CrI3. We observed an irreversible interlayer antiferromagnetic-to-ferromagnetic transition in atomically thin CrI3 by magnetic circular dichroism and electron tunnelling measurements. The effect was accompanied by a monoclinic-to-rhombohedral stacking-order change characterized by polarized Raman spectroscopy. Before the structural change, the interlayer antiferromagnetic coupling energy can be tuned up by nearly 100% with pressure. Our experiment reveals the interlayer ferromagnetic ground state, which is established in bulk CrI3 but not observed in native exfoliated thin films. The observed correlation between the magnetic ground state and the stacking order is in good agreement with first principles calculations3–8 and suggests a route towards nanoscale magnetic textures by moire engineering3,9. Pressure-induced changes in the magnetic order of atomically thin van der Waals crystals are revealed and attributed to changes in the stacking arrangement.

Pressure-controlled interlayer magnetism in atomically thin CrI3

Abstract: Stacking order can significantly influence the physical properties of two-dimensional (2D) van der Waals materials. The recent isolation of atomically thin magnetic materials opens the door for control and design of magnetism via stacking order. Here we apply hydrostatic pressure up to 2 GPa to modify the stacking order in a prototype van der Waals magnetic insulator CrI3. We observe an irreversible interlayer antiferromagnetic (AF) to ferromagnetic (FM) transition in atomically thin CrI3 by magnetic circular dichroism and electron tunneling measurements. The effect is accompanied by a monoclinic to a rhombohedral stacking order change characterized by polarized Raman spectroscopy. Before the structural change, the interlayer AF coupling energy can be tuned up by nearly 100% by pressure. Our experiment reveals interlayer FM coupling, which is the established ground state in bulk CrI3, but never observed in native exfoliated thin films. The observed correlation between the magnetic ground state and the stacking order is in good agreement with first principles calculations and suggests a route towards nanoscale magnetic textures by moire engineering.

Decomposition-Induced Room-Temperature Magnetism of the Na-Intercalated Layered Ferromagnet Fe3-xGeTe2.

Abstract: The creation of 2D van der Waals materials with ferromagnetism above room temperature is an essential goal toward their practical utilization in spin-based applications. Recent studies suggest that intercalating lithium in exfoliated flakes of the ferromagnet Fe3-xGeTe2 induces a nonzero magnetization at T ∼ 300 K. However, the nanoscale nature of such experiments precludes precise observations of structural and chemical changes upon intercalation. Here, we report the preparation of sodium-intercalated NaFe2.78GeTe2 as well as the investigation into its structure and magnetic properties. Sodium readily intercalates into the van der Waals gap, as revealed by synchrotron X-ray diffraction. Concurrently, the Fe2.78GeTe2 layer becomes heavily charge doped and strained via chemical pressure, yet retains its structure and ferromagnetic transition temperature of ∼140 K. However, we observe the presence of a ferromagnetic amorphous iron germanide impurity over a wide range of synthetic conditions, leading to room-temperature magnetization. This work highlights the importance of strain and electronic control for manipulating the Curie temperature in 2D ferromagnets, while emphasizing the need for careful chemical analysis when exploring phenomena in exfoliated layers.

The Fermi surface geometrical origin of axis-dependent conduction polarity in layered materials.

Abstract: Electronic materials generally exhibit a single isotropic majority carrier type, electrons or holes. Some superlattice1,2 and hexagonal3-5 materials exhibit opposite conduction polarities along in-plane and cross-plane directions due to multiple electron and hole bands. Here, we uncover a material genus with this behaviour that originates from the Fermi surface geometry of a single band. NaSn2As2, a layered metal, has such a Fermi surface. It displays in-plane electron and cross-plane hole conduction in thermopower and exactly the opposite polarity in the Hall effect. The small Nernst coefficient and magnetoresistance preclude multi-band transport. We label this direction-dependent carrier polarity in single-band systems 'goniopolarity'. We expect to find goniopolarity and the Fermi surface geometry that produces it in many metals and semiconductors whose electronic structure is at the boundary between two and three dimensions. Goniopolarity may enable future explorations of complex transport phenomena that lead to unprecedented device concepts.

Transition Metal-Free Alkyne Hydrogenation Catalysis with BaGa2, a Hydrogen Absorbing Layered Zintl Phase

Abstract: Inexpensive, transition metal-free intermetallic compounds have received almost no attention as heterogeneous catalysts. Here, we show that BaGa2, a Zintl–Klemm compound composed of honeycomb sheet...

Distinct magneto-Raman signatures of spin-flip phase transitions in CrI3

Abstract: The discovery of 2-dimensional (2D) materials, such as CrI3, that retain magnetic ordering at monolayer thickness has resulted in a surge of research in 2D magnetism from both pure and applied perspectives. Here, we report a magneto-Raman spectroscopy study on multilayered CrI3, focusing on two new features in the spectra which appear at temperatures below the magnetic ordering temperature and were previously assigned to high frequency magnons. We observe a striking evolution of the Raman spectra with increasing magnetic field in which clear, sudden changes in intensities of the modes are attributed to the interlayer ordering changing from antiferromagnetic to ferromagnetic at a critical magnetic field. Our work highlights the sensitivity of the Raman modes to weak interlayer spin ordering in CrI3. In addition, we theoretically examine potential origins for the new modes, which we deduce are unlikely single magnons.

Identification of turbostratic twisting in germanane

Abstract: Germanane, a van der Waals layered material predicted to have a direct band gap and high carrier mobility, is a promising two-dimensional material with applications in optoelectronics. The electronic properties of germanane have been well studied; however, experimentally measured properties are orders of magnitude lower than predicted values, potentially limiting future device applications. The structure of germanane contains an inherent disorder along the c-axis, resulting in a diffuse halo with hexagonal symmetry in the electron diffraction pattern. The origin of this disorder is not well understood, further limiting the device application of germanane. Here, we have used experimental and simulated electron diffraction patterns to show that this diffuse scattering arises from turbostratic disorder present in the germanane structure with rotational disorder as the main contribution. The maximum rotation angle in the examined germanane crystal is limited to three degrees. For larger angles, germanane would become unstable as DFT calculations show. DFT calculations also indicate that small angle rotations cause a change in charge distribution in Ge and H atoms, and thus should affect the electronic properties of germanane considerably. This study explains for the first time the origin of the c-axis disorder in this van der Waals structure and establishes computationally analyzed diffraction patterns as a tool to quantify turbostratic disorder.

Germanane analogs and optoelectronic devices using the same

Abstract: The present invention provides novel two-dimensional van der Waals materials and stacks of those materials. Also provided are methods of making and using such materials.