Carlos J. Arguello

Visualizing Individual Nitrogen Dopants in Monolayer Graphene

Nitrogen atoms that replace carbon atoms in the graphene lattice strongly modify the local electronic structure. In monolayer graphene, substitutional doping during growth can be used to alter its electronic properties. We used scanning tunneling microscopy, Raman spectroscopy, x-ray spectroscopy, and first principles calculations to characterize individual nitrogen dopants in monolayer graphene grown on a copper substrate. Individual nitrogen atoms were incorporated as graphitic dopants, and a fraction of the extra electron on each nitrogen atom was delocalized into the graphene lattice. The electronic structure of nitrogen-doped graphene was strongly modified only within a few lattice spacings of the site of the nitrogen dopant. These findings show that chemical doping is a promising route to achieving high-quality graphene films with a large carrier concentration.

Visualization of electron nematicity and unidirectional antiferroic fluctuations at high temperatures in NaFeAs

Superconductivity in iron pnictides seems to be related to the formation of electronic nematic phases that break the rotational symmetry of the crystal lattice. But the nematic phase in NaFeAs is now shown to persist at high temperatures owing to the presence of antiferroic fluctuations.

Li(Zn,Mn)As as a new generation ferromagnet based on a I–II–V semiconductor

In a prototypical ferromagnet (Ga,Mn)As based on a III-V semiconductor, substitution of divalent Mn atoms into trivalent Ga sites leads to severely limited chemical solubility and metastable specimens available only as thin films. The doping of hole carriers via (Ga,Mn) substitution also prohibits electron doping. To overcome these difficulties, Masek et al. theoretically proposed systems based on a I-II-V semiconductor LiZnAs, where isovalent (Zn,Mn) substitution is decoupled from carrier doping with excess/deficient Li concentrations. Here we show successful synthesis of Li(1+y)(Zn(1-x)Mn(x))As in bulk materials. Ferromagnetism with a critical temperature of up to 50 K is observed in nominally Li-excess (y=0.05-0.2) compounds with Mn concentrations of x=0.02-0.15, which have p-type metallic carriers. This is presumably due to excess Li in substitutional Zn sites. Semiconducting LiZnAs, ferromagnetic Li(Zn,Mn)As, antiferromagnetic LiMnAs, and superconducting LiFeAs systems share square lattice As layers, which may enable development of novel junction devices in the future.

Spin ice: magnetic excitations without monopole signatures using muon spin rotation.

Theory predicts the low temperature magnetic excitations in spin ices consist of deconfined magnetic charges, or monopoles. A recent transverse-field (TF) muon spin rotation (μSR) experiment [S. T. Bramwell et al., Nature (London) 461, 956 (2009)] reports results claiming to be consistent with the temperature and magnetic field dependence anticipated for monopole nucleation-the so-called second Wien effect. We demonstrate via a new series of μSR experiments in Dy(2)Ti(2)O(7) that such an effect is not observable in a TF μSR experiment. Rather, as found in many highly frustrated magnetic materials, we observe spin fluctuations which become temperature independent at low temperatures, behavior which dominates over any possible signature of thermally nucleated monopole excitations.

Ferromagnetic ordering in superatomic solids.

In order to realize significant benefits from the assembly of solid-state materials from molecular cluster superatomic building blocks, several criteria must be met. Reproducible syntheses must reliably produce macroscopic amounts of pure material; the cluster-assembled solids must show properties that are more than simply averages of those of the constituent subunits; and rational changes to the chemical structures of the subunits must result in predictable changes in the collective properties of the solid. In this report we show that we can meet these requirements. Using a combination of magnetometry and muon spin relaxation measurements, we demonstrate that crystallographically defined superatomic solids assembled from molecular nickel telluride clusters and fullerenes undergo a ferromagnetic phase transition at low temperatures. Moreover, we show that when we modify the constituent superatoms, the cooperative magnetic properties change in predictable ways.

(La1-xBax)(Zn1-xMnx)AsO: A two-dimensional 1111-type diluted magnetic semiconductor in bulk form

We employ NMR techniques to investigate the nature of Mn spins in the I-II-V diluted magnetic semiconductor Li(Zn1−xMnx)P (x = 0.1, Curie temperature Tc = 25 K). We successfully identify the Li NMR signals arising from the Li sites adjacent to Mn, and probe the static and dynamic properties of Mn spins. From the NMR spin-lattice relaxation data, we show that the Mn spin-spin interactions extend over many unit cells.

Quasiparticle interference, quasiparticle interactions, and the origin of the charge density wave in 2H–NbSe2

We show that a small number of intentionally introduced defects can be used as a spectroscopic tool to amplify quasiparticle interference in 2H-NbSe2 that we measure by scanning tunneling spectroscopic imaging. We show, from the momentum and energy dependence of the quasiparticle interference, that Fermi surface nesting is inconsequential to charge density wave formation in 2H-NbSe2. We demonstrate that, by combining quasiparticle interference data with additional knowledge of the quasiparticle band structure from angle resolved photoemission measurements, one can extract the wave vector and energy dependence of the important electronic scattering processes thereby obtaining direct information both about the fermiology and the interactions. In 2H-NbSe2, we use this combination to confirm that the important near-Fermi-surface electronic physics is dominated by the coupling of the quasiparticles to soft mode phonons at a wave vector different from the charge density wave ordering wave vector.

A New Type Diluted Magnetic Semiconductor Li(Zn,Mn)As

Diluted magnetic semiconductor (DMS) exhibits unique magnetic and transport properties. The well known DMS is (Ga,Mn)As that is in thin film form so far only. However Mn dopant brings both localized spins and carriers in Ga1−xMnxAs, result in the lack of independent control of local moment and carrier densities. It is therefore difficult to study or control charge and spin variable quantitatively. Here we report that an individual control of carrier and spin can be realized in a new type of DMS for Mn-doped I-II-V type semiconductor Li1+yZn1-xMnxAs in bulk. The Li(Zn,Mn)As polycrystalline samples showed ferromagnetic transition temperature (TC) up to 50K and spontaneous magnetization up to 2.9 μB per Mn. The other advantage of new DMS over III-V DMSs is the availability of bulk specimens so far for these type of DMS Moreover the new DMS shows a soft magnetic behavior with the coercive force < 100Oe which would be promising for prospective application. Muon spin relaxtion (μSR) probed static magnetic order of full volume in the ferromagnetic region. Electrical transport properties showed Li1+y(Zn,Mn)As (y = 0.05–0.1) compounds have p-type carriers since excess Li substitutes Zn sites presumably[1].