UV/Ozone treatment to reduce metal-graphene contact resistance

We report reduced contact resistance of single-layer graphene devices by using ultraviolet ozone (UVO) treatment to modify the metal/graphene contact interface. The devices were fabricated from mechanically transferred, chemical vapor deposition (CVD) grown, single layer graphene. UVO treatment of graphene in the contact regions as defined by photolithography and prior to metal deposition was found to reduce interface contamination originating from incomplete removal of poly(methyl methacrylate) (PMMA) and photoresist. Our control experiment shows that exposure times up to 10 minutes did not introduce significant disorder in the graphene as characterized by Raman spectroscopy. By using the described approach, contact resistance of less than 200 {\Omega} {\mu}m was achieved, while not significantly altering the electrical properties of the graphene channel region of devices.Comment: 17 pages, 5 figure

The Impact of Carbon Nanotube Length and Diameter on their Global Alignment by Dead‐End Filtration

Dead-end filtration has proven to effectively prepare macroscopically (3.8 cm$^2$) aligned thin films from solutionbased single-wall carbon nanotubes (SWCNTs). However, to make this technique broadly applicable, the role of SWCNT length and diameter must be understood. To date, most groups report the alignment of unsorted, large diameter (≈1.4 nm) SWCNTs, but systematic studies on their small diameter are rare (≈0.78 nm). In this work, films with an area of A = 3.81 cm$^2$ and a thickness of ≈40 nm are prepared from length-sorted fractions comprising of small and large diameter SWCNTs, respectively. The alignment is characterized by cross-polarized microscopy, scanning electron microscopy, absorption and Raman spectroscopy. For the longest fractions (L$_{avg}$ = 952 nm ± 431 nm, Δ = 1.58 and L$_{avg}$ = 667 nm ± 246 nm, Δ = 1.55), the 2D order parameter, S2D, values of ≈0.6 and ≈0.76 are reported for the small and large diameter SWCNTs over an area of A = 625 µm$^2$, respectively. A comparison of Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory calculations with the aligned domain size is then used to propose a law identifying the required length of a carbon nanotube with a given diameter and zeta potential

Ring-Exchange Interaction Effects on Magnons in Dirac Magnet CoTiO3_3

In magnetically ordered materials with localized electrons, the fundamental magnetic interactions are due to exchange of electrons [1-3]. Typically, only the interaction between pairs of electrons' spins is considered to explain the nature of the ground state and its excitations, whereas three-, four-, and six-spin interactions are ignored. When these higher order processes occur in a loop they are called cyclic or ring exchange. The ring-exchange interaction is required to explain low temperature behavior in bulk and thin films of solid $^3$He [4-8]. It also plays a crucial role in the quantum magnet La$_2$CuO$_4$ [9,10]. Here, we use a combination of time domain THz (TDTS) and magneto-Raman spectroscopies to measure the low energy magnetic excitations in CoTiO$_3$, a proposed Dirac topological magnon material [11,12] where the origin of the energy gap in the magnon spectrum at the Brillouin zone center remains unclear. We measured the magnetic field dependence of the energies of the two lowest energy magnons and determine that the gap opens due to the ring-exchange interaction between the six spins in a hexagon. This interaction also explains the selection rules of the THz magnon absorption. Finally, we clarify that topological surface magnons are not expected in CoTiO$_3$. Our study demonstrates the power of combining TDTS and Raman spectroscopies with theory to identify the microscopic origins of the magnetic excitations in quantum magnets.Comment: 7 pages, 4 figures in main text, 26 pages and 11 figures in supplemen

Specifics of the Elemental Excitations in "True One-Dimensional" MoI3_3 van der Waals Nanowires

We report on the temperature evolution of the polarization-dependent Raman spectrum of exfoliated MoI$_3$, a van der Waals material with a "true one-dimensional" crystal structure that can be exfoliated to individual atomic chains. The temperature evolution of several Raman features reveals anomalous behavior suggesting a phase transition of a magnetic origin. Theoretical considerations indicate that MoI$_3$ is an easy-plane antiferromagnet with alternating spins along the dimerized chains and with inter-chain helical spin ordering. The calculated frequencies of the phonons and magnons are consistent with the interpretation of the experimental Raman data. The obtained results shed light on the specifics of the phononic and magnonic states in MoI$_3$ and provide a strong motivation for future study of this unique material with potential for spintronic device applications.Comment: 28 page

Evolution of Microscopic Localization in Graphene in a Magnetic Field from Scattering Resonances to Quantum Dots

Graphene is a unique two-dimensional material with rich new physics and great promise for applications in electronic devices. Physical phenomena such as the half-integer quantum Hall effect and high carrier mobility are critically dependent on interactions with impurities/substrates and localization of Dirac fermions in realistic devices. We microscopically study these interactions using scanning tunneling spectroscopy (STS) of exfoliated graphene on a SiO2 substrate in an applied magnetic field. The magnetic field strongly affects the electronic behavior of the graphene; the states condense into welldefined Landau levels with a dramatic change in the character of localization. In zero magnetic field, we detect weakly localized states created by the substrate induced disorder potential. In strong magnetic field, the two-dimensional electron gas breaks into a network of interacting quantum dots formed at the potential hills and valleys of the disorder potential. Our results demonstrate how graphene properties are perturbed by the disorder potential; a finding that is essential for both the physics and applications of graphene.Comment: to be published in Nature Physic