6 Search Results

We combine synchrotron-based near-field infrared spectroscopy and first principles lattice dynamics calculations to explore the vibrational response of CrPS₄ in bulk, few-, and single-layer form. Analysis of the mode pattern reveals a C2 polar + chiral space group, no symmetry crossover as a function of layer number, and a series of non-monotonic frequency shifts in which modes with significant intralayer character harden on approach to the ultra-thin limit whereas those containing interlayer motion or more complicated displacement patterns soften and show inflection points or steps. This is different from MnPS3 where phonons shift as 1/size² and are sensitive to themore » three-fold rotation about the metal center that drives the symmetry crossover. We discuss these differences as well as implications for properties such as electric polarization in terms of presence or absence of the P–P dimer and other aspects of local structure, sheet density, and size of the van der Waals gap.« less

Compounds with two-dimensional (2D) layers of magnetic ions weakly connected by van der Waals bonding offer routes to enhance quantum behavior, stimulating both fundamental and applied interest. CrPS₄ is one such magnetic van der Waals material, however, it has undergone only limited investigation. Here we present a comprehensive series of neutron scattering measurements to determine the magnetic structure and exchange interactions. The observed magnetic excitations allow a high degree of constraint on the model parameters not normally associated with measurements on a powder sample. The results demonstrate the 2D nature of the magnetic interactions, while also revealing the importance ofmore » interactions along 1D chains within the layers. The subtle role of competing interactions is observed, which manifest in a nontrivial magnetic transition and a tunable magnetic structure in a small applied magnetic field through a spin-flop transition. Our results on the bulk compound provide insights that can be applied to an understanding of the behavior of reduced layer CrPS₄.« less

We employ synchrotron-based near-field infrared spectroscopy to reveal the vibrational properties of bulk, few-sheet, and single-sheet members of the MPS3 (M=Mn, Fe, Ni) family of materials and compare our findings with complementary lattice dynamics calculations. MnPS3 and the Fe analog are similar in terms of their symmetry crossovers, from C2/m to P3¯1m, as the monolayer is approached. These states differ as to the presence of a C3 rotation around the metal center. On the other hand, NiPS3 does not show a symmetry crossover, and the lack of a Bu symmetry mode near 450 cm-1 suggests that C3 rotational symmetry ismore » already present, even in the bulk material. We discuss these findings in terms of local symmetry and temperature effects as well as the curious relationship between these symmetry transformations and those that take place under pressure.« less

van der Waals materials are exceptionally responsive to external stimuli. Pressure-induced layer sliding, metallicity, and superconductivity are fascinating examples. Inspired by opportunities in this area, we combined high-pressure optical spectroscopies and first-principles calculations to reveal piezochromism in MnPS₃. Dramatic color changes (green → yellow → red → black) take place as the charge gap shifts across the visible regime and into the near infrared, moving systematically toward closure at a rate of approximately –50 meV/GPa. This effect is quenched by the appearance of the insulator–metal transition. In addition to uncovering an intriguing and tunable functionality that is likely to appearmore » in other complex chalcogenides, the discovery that piezochromism can be deterministically controlled at room temperature accelerates the development of technologies that take advantage of stress-activated modification of electronic structure.« less

Ionic liquid electrolytes are gaining widespread application as a gate dielectric used to control ion transport in functional materials. This letter systematically examines the important influence that device geometry in standard “side gate” 3-terminal geometries plays in device performance of a well-known oxygen ion conductor. We show that the most influential component of device design is the ratio between the area of the gate electrode and the active channel, while the spacing between these components and their individual shapes has a negligible contribution. Finally, these findings provide much needed guidance in device design intended for ionotronic gating with ionic liquids.

A FCC high-entropy alloy (HEA) that exhibits strong temperature dependence of strength at low homologous temperatures in sharp contrast to pure FCC metals like Ni that show weak temperature dependence is CrMnCoFeNi. In order to understand this behavior, elastic constants were determined as a function of temperature. From 300 K down to 55 K, the shear modulus (G) of the HEA changes by only 8%, increasing from 80 to 86 GPa. Moreover, this temperature dependence is weaker than that of FCC Ni, whose G increases by 12% (81–91 GPa). Therefore, the uncharacteristic temperature-dependence of the strength of the HEA ismore » not due to the temperature dependence of its shear modulus.« less