We report low-temperature, high-field magnetotransport measurements of SrTiO 3 gated by an ionic gel electrolyte. A saturating resistance upturn and negative magnetoresistance that signal the emergence of the Kondo effect appear for higher applied gate voltages. This observation, enabled by the wide tunability of the ionic gel-applied electric field, promotes the interpretation of the electric field-effect-induced 2D electron system in SrTiO 3 as an admixture of magnetic Ti 3þ ions, i.e., localized and unpaired electrons, and delocalized electrons that partially fill the Ti 3d conduction band.The Coulomb interaction amongst electrons and ions in a solid can spontaneously generate internal magnetic fields and effective magnetic interactions. Unexpected magnetic phenomena may emerge whenever we consider a new system where interactions are important. In recent years, predictions for and observations of magnetism originating in the two-dimensional (2D) system of electrons at the interface between SrTiO 3 (STO) and LaAlO 3 (LAO) have attracted much attention [1][2][3][4][5][6][7], particularly the prediction of charge disproportionation and the emergence of þ3-valent Ti sites with unpaired spin [1] and direct measurements of in-plane magnetization [6,7]. The conducting electrons at the LAO=STO interface are believed to be induced by polar LAO's strong internal electric fields, and to reside on the Ti sites on the STO side of the interface, partially filling the lowest-lying Ti 3d bands [8-10]. Questions remain, however, over the role of the growth process, in particular, whether oxygen vacancy formation or cation intermixing are in fact responsible for the observed n-type conduction [11][12][13][14].Other than growing a polar overlayer, a 2D system of electrons in STO can be made by chemical doping with Nb, La, or oxygen vacancies [15][16][17][18], or purely electrostatic charging in an electric double layer transistor (EDLT) [19,20]. If electronic reconstruction in response to overlayer polarity is an accurate description for LAO=STO, then that system can be closely modeled by field-effectinduced electrons in undoped STO, where confounding questions over growth conditions do not arise, and the applied electric field can be widely tuned.In this Letter, we expand on the body of evidence for Ti 3þ magnetism in STO that conducts in two dimensions. We demonstrate a gate-controlled Kondo effect in the 2D electron system in undoped STO formed beneath the bare surface by the electric field from an ionic gel electrolyte, and interpret this system as an admixture of magnetic Ti 3þ ions (unpaired and localized electrons) and delocalized electrons partially filling the Ti 3d conduction band, as predicted theoretically [2,21]. The Kondo effect is an archetype for the emergent magnetic interactions amongst localized and delocalized electrons in conducting alloys [22,23], and the ability to produce and tune the effect by purely electrostatic means in any conducting system is of interest in its own right [24,25]. The observed appear...

Rational design of long-period artificial lattices yields effects unavailable in simple solids. The moiré pattern in highly aligned graphene/hexagonal boron nitride (h-BN) heterostructures is a lateral superlattice with high electron mobility and an unusual electronic dispersion whose miniband edges and saddle points can be reached by electrostatic gating. We investigated the dynamics of electrons in moiré minibands by measuring ballistic transport between adjacent local contacts in a magnetic field, known as the transverse electron focusing effect. At low temperatures, we observed caustics of skipping orbits extending over hundreds of superlattice periods, reversals of the cyclotron revolution for successive minibands, and breakdown of cyclotron motion near van Hove singularities. At high temperatures, electron-electron collisions suppress focusing. Probing such miniband conduction properties is a necessity for engineering novel transport behaviors in superlattice devices.

Graphene monolayers are known to display domains of anisotropic friction with twofold symmetry and anisotropy exceeding 200%. This anisotropy has been thought to originate from periodic nanoscale ripples in the graphene sheet, which enhance puckering around a sliding asperity to a degree determined by the sliding direction. Here we demonstrate that these frictional domains derive not from structural features in the graphene but from self-assembly of environmental adsorbates into a highly regular superlattice of stripes with period 4–6 nm. The stripes and resulting frictional domains appear on monolayer and multilayer graphene on a variety of substrates, as well as on exfoliated flakes of hexagonal boron nitride. We show that the stripe-superlattices can be reproducibly and reversibly manipulated with submicrometre precision using a scanning probe microscope, allowing us to create arbitrary arrangements of frictional domains within a single flake. Our results suggest a revised understanding of the anisotropic friction observed on graphene and bulk graphite in terms of adsorbates.

Magnetic resonance imaging of hyperpolarized nuclei provides high image contrast with little or no background signal. To date, in-vivo applications of pre-hyperpolarized materials have been limited by relatively short nuclear spin relaxation times. Here, we investigate silicon nanoparticles as a new type of hyperpolarized magnetic resonance imaging agent. Nuclear spin relaxation times for a variety of Si nanoparticles are found to be remarkably long, ranging from many minutes to hours at room temperature, allowing hyperpolarized nanoparticles to be transported, administered, and imaged on practical time scales. Additionally, we demonstrate that Si nanoparticles can be surface functionalized using techniques common to other biologically targeted nanoparticle systems. These results suggest that Si nanoparticles can be used as a targetable, hyperpolarized magnetic resonance imaging agent with a large range of potential applications.

Electrolyte gating is a powerful technique for accumulating large carrier densities at a surface. Yet this approach suffers from significant sources of disorder: electrochemical reactions can damage or alter the sample, and the ions of the electrolyte and various dissolved contaminants sit Angstroms from the electron system. Accordingly, electrolyte gating is well suited to studies of superconductivity and other phenomena robust to disorder, but of limited use when reactions or disorder must be avoided. Here we demonstrate that these limitations can be overcome by protecting the sample with a chemically inert, atomically smooth sheet of hexagonal boron nitride. We illustrate our technique with electrolyte-gated strontium titanate, whose mobility when protected with boron nitride improves more than 10-fold while achieving carrier densities nearing 1014 cm−2. Our technique is portable to other materials, and should enable future studies where high carrier density modulation is required but electrochemical reactions and surface disorder must be minimized.

We investigate the low-field relaxation of nuclear hyperpolarization in undoped and highly doped silicon microparticles at room temperature following removal from high field. For nominally undoped particles, two relaxation time scales are identified for ambient fields above 0.2 mT. The slower, T_1s, is roughly independent of ambient field; the faster, T_1f, decreases with increasing ambient field. A model in which nuclear spin relaxation occurs at the particle surface via a two-electron mechanism is shown to be in good agreement with the experimental data, particularly the field-independence of T_1s. For boron-doped particles, a single relaxation time scale is observed. This suggests that for doped particles, mobile carriers and bulk ionized acceptor sites, rather than paramagnetic surface states, are the dominant relaxation mechanisms. Relaxation times for the undoped particles are not affected by tumbling in a liquid solution.Comment: related papers at http://marcuslab.harvard.ed

We describe the synthesis, materials characterization and dynamic nuclear polarization (DNP) of amorphous and crystalline silicon nanoparticles for use as hyperpolarized magnetic resonance imaging (MRI) agents. The particles were synthesized by means of a metathesis reaction between sodium silicide (Na4Si4) and silicon tetrachloride (SiCl4) and were surface functionalized with a variety of passivating ligands. The synthesis scheme results in particles of diameter ~10 nm with long size-adjusted 29Si spin lattice relaxation (T1) times (> 600 s), which are retained after hyperpolarization by low temperature DNP.

Electrolyte gating using ionic liquid electrolytes has recently generated considerable interest as a method to achieve large carrier density modulations in a variety of materials. In noble metal thin films, electrolyte gating results in large changes in sheet resistance. The widely accepted mechanism for these changes is the formation of an electric double layer with a charged layer of ions in the liquid and accumulation or depletion of carriers in the thin film. We report here a different mechanism.In particular, we show using x-ray absorption near edge structure (XANES) that the previously reported large conductance modulation in gold films is due to reversible oxidation and reduction of the surface rather than the charging of an electric double layer. We show that the double layer capacitance accounts for less than 10% of the observed change in transport properties. These results represent a significant step towards understanding the mechanisms involved in electrolyte gating.