Research Highlights

We investigated magnetoelastic coupling through the field-driven transition to the fully polarized magnetic state in quasi-two-dimensional [Cu(HF)₂)(pyz)₂]BF₄ by magnetoinfrared spectroscopy. This transition modifies out-of-plane ring distortion and bending vibrational modes of the pyrazine ligand. The extent of these distortions increases with the field, systematically tracking the low-temperature magnetization. These distortions weaken the antiferromagnetic spin exchange, a finding that provides important insight into magnetic transitions in other copper halides.

We measured the far infrared vibrational properties of bulk and nanoscale MoS₂ in order to investigate finite length scale effects and chemical bonding in these materials. From an analysis of frequencies, oscillator strengths, and high frequency dielectric constants, we extract Born and local effective charges for both materials. In the intralayer direction, we find that the Born effective charge of the nanoparticles is decreased significantly compared to the layered bulk, a difference that we attribute to structural strain (and resulting change in polarizability) in the nanoparticles. This paper was reprinted in the May 18, 2009 issue of Virtual Journal of Nanoscale Science and Technology.

We investigated the series of temperature and field-driven transitions in LuFe₂O₄ by optical and Mossbauer spectroscopies, magnetization, and x-ray scattering in order to understand the interplay between charge, structure, and magnetism in this multiferroic material. We demonstrate that charge fluctuation has an onset well below the charge ordering transition, supporting the "order by fluctuation" mechanism for the development of charge order superstructure. Bragg splitting and large magneto optical contrast suggest a low temperature monoclinic distortion that can be driven by both temperature and magnetic field.

Application of a magnetic field offers an incisive opportunity to tune competing interactions in complex materials. Here we probe field-induced changes in the local structure of DyMn₂O₅ by using magnetoinfrared spectroscopy. The high tunability of the dielectric constant and ferroelectric polarization with field is well documented in the literature, but the lattice response on the microscopic level remains unknown. In this work, we reveal the dynamic nature of the local structural response to field and analyze it in terms of calculated mode displacements and local lattice distortions.

We measured the optical properties of mixed valent vanadium oxide nanoscrolls and their metal-exchanged derivatives in order to investigate the charge dynamics in these compounds. In contrast to the prediction of a metallic state for the metal-exchanged derivatives within a rigid band model, we find that the injected charges in Mn²⁺-exchanged vanadium oxide nanoscrolls are pinned. A low-energy electronic excitation associated with the pinned carriers appears in the far infrared and persists at low temperature, suggesting that the nanoscrolls are weak metals in their bulk form, dominated by inhomogeneous charge disproportionation and Madelung energy effects.

To investigate phonon confinement in nanoscale metal dichalcogenides, we measured the low-temperature specific heat of layered and nanoparticle WS₂. Below 9K, the specific heat of the nanoparticles deviates from that of the bulk counterpart. Further, it deviates from the usual T³ dependence below 4 K due to finite size effects that eliminate long wavelength acoustic phonons and interparticle-motion entropy. This separation of nanoscale effects from T³ dependence can be modeled by assuming that the phonon density of states is flexible, changing with size and shape. We invoke relationships between the low-temperature T³ phonon term, Young's modulus, and friction coefficient to assess the difference in the tribological properties. On the basis of this analysis, we conclude that the improved lubrication properties of the nanoparticles are extrinsic.