arXiv subjects

Eva Y. Andrei

Publications and source records attributed to Eva Y. Andrei.

46 records · Page 3Linked to original sources

arXiv2011

Flame Synthesis of Graphene Films in Open Environments

Few-layer graphene (FLG) is grown on copper and nickel substrates at high rates using a novel flame synthesis method in open-atmosphere environments. Transmittance and resistance properties of the transferred films are similar to those grown by other methods, but the concentration of oxygen, as assessed by XPS, is actually less than that for CVD-grown graphene under near vacuum conditions. The method involves utilizing a multi-element inverse-diffusion-flame burner, where post-flame species and temperatures are radially-uniform upon deposition at a substrate. Advantages of the flame synthesis method are scalability for large-area surface coverage, increased growth rates, high purity and yield, continuous processing, and reduced costs due to efficient use of fuel as both heat source and reagent. Additionally, by adjusting local growth conditions, other carbon nanostructures (i.e. nanotubes) are readily synthesized.

cond-mat.mtrl-sci↗

arXiv2011

Self-navigation of STM tip toward a micron sized sample

We demonstrate a simple capacitive based method to quickly and efficiently locate micron size conductive samples on insulating substrates in a scanning tunneling microscope (STM). By using edge recognition the method is designed to locate and identify small features when the STM tip is far above the surface allowing for crash-free search and navigation. The method can be implemented in any STM environment even at low temperatures and in strong magnetic field, with minimal or no hardware modifications.

cond-mat.mes-hall↗

arXiv2011

Quantized Landau level spectrum and its density dependence

Scanning tunneling microscopy and spectroscopy in magnetic field was used to study Landau quantization in graphene and its dependence on charge carrier density. Measurements were carried out on exfoliated graphene samples deposited on a chlorinated SiO2 thermal oxide which allowed observing the Landau level sequences characteristic of single layer graphene while tuning the density through the Si backgate. Upon changing the carrier density we find abrupt jumps in the Fermi level after each Landau level is filled. Moreover, the Landau level spacing shows a marked increase at low doping levels, consistent with an interaction-induced renormalization of the Dirac cone.

cond-mat.mes-hall↗

arXiv2009

Fractional quantum Hall effect and insulating phase of Dirac electrons in graphene

In graphene, which is an atomic layer of crystalline carbon, two of the distinguishing properties of the material are the charge carriers two-dimensional and relativistic character. The first experimental evidence of the two-dimensional nature of graphene came from the observation of a sequence of plateaus in measurements of its transport properties in the presence of an applied magnetic field. These are signatures of the so-called integer quantum Hall effect. However, as a consequence of the relativistic character of the charge carriers, the integer quantum Hall effect observed in graphene is qualitatively different from its semiconductor analogue. As a third distinguishing feature of graphene, it has been conjectured that interactions and correlations should be important in this material, but surprisingly, evidence of collective behaviour in graphene is lacking. In particular, the quintessential collective quantum behaviour in two dimensions, the fractional quantum Hall effect (FQHE), has so far resisted observation in graphene despite intense efforts and theoretical predictions of its existence. Here we report the observation of the FQHE in graphene. Our observations are made possible by using suspended graphene devices probed by two-terminal charge transport measurements. This allows us to isolate the sample from substrate-induced perturbations that usually obscure the effects of interactions in this system and to avoid effects of finite geometry. At low carrier density, we find a field-induced transition to an insulator that competes with the FQHE, allowing its observation only in the highest quality samples. We believe that these results will open the door to the physics of FQHE and other collective behaviour in graphene.

cond-mat.mes-hall↗

arXiv2009

Scanning Tunneling Spectroscopy of Graphene on Graphite

We report low temperature high magnetic field scanning tunneling microscopy and spectroscopy of graphene flakes on graphite that exhibit the structural and electronic properties of graphene decoupled from the substrate. Pronounced peaks in the tunneling spectra develop with field revealing a Landau level sequence that provides a direct way to identify graphene and to determine the degree of its coupling to the substrate. The Fermi velocity and quasiparticle lifetime, obtained from the positions and width of the peaks, provide access to the electron-phonon and electron-electron interactions.

cond-mat.mes-hall↗

arXiv2009

Towards ballistic transport in graphene

Graphene is a fascinating material for exploring fundamental science questions as well as a potential building block for novel electronic applications. In order to realize the full potential of this material the fabrication techniques of graphene devices, still in their infancy, need to be refined to better isolate the graphene layer from the environment. We present results from a study on the influence of extrinsic factors on the quality of graphene devices including material defects, lithography, doping by metallic leads and the substrate. The main finding is that trapped Coulomb scatterers associated with the substrate are the primary factor reducing the quality of graphene devices. A fabrication scheme is proposed to produce high quality graphene devices dependably and reproducibly. In these devices, the transport properties approach theoretical predictions of ballistic transport.

cond-mat.mes-hall↗

arXiv2008

Suspended Graphene: a bridge to the Dirac point

The recent discovery of methods to isolate graphene, a one-atom-thick layer of crystalline carbon, has raised the possibility of a new class of nano-electronics devices based on the extraordinary electrical transport and unusual physical properties of this material. However, the experimental realization of devices displaying these properties was, until now, hampered by the influence of the ambient environment, primarily the substrate. Here we report on the fabrication of Suspended Graphene devices and on studies of their electrical transport properties. In these devices, environmental disturbances were minimized allowing unprecedented access to the intrinsic properties of graphene close to the Dirac Point (DP) where the energy dispersion of the carriers and their density-of-states vanish linearly giving rise to a range of exotic physical properties. We show that charge inhomogeneity is reduced by almost one order of magnitude compared to that in Non-Suspended Graphene devices. Moreover, near the DP, the mobility exceeds 100,000 cm2/Vs, approaching theoretical predictions for evanescent transport in the ballistic model.

cond-mat.mes-hall↗

arXiv2007

Josephson Current and Multiple Andreev Reflections in Graphene SNS Junctions

The Josephson Effect and Superconducting Proximity Effect were observed in Superconductor -Graphene-Superconductor (SGS) Josephson junctions with coherence lengths comparable to the distance between the superconducting leads. By comparing the measured temperature and doping dependence of the supercurrent and the proximity induced sub-gap features (multiple Andreev reflections) to theoretical predictions we find that, contrary to expectations, the ballistic transport model fails to describe the SGS junctions. In contrast, the diffusive junction model yields close quantitative agreement with the results. This conclusion is consistent with transport measurements in the normal state, which yield mean free paths in the graphene link that are much shorter than the junction length. We show that all devices fabricated on SiO2 substrates so far (our own as well as those reported by other groups) fall in the diffusive junction category.

cond-mat.mes-hall↗

arXiv2007

Observation of Landau levels of Dirac fermions in graphite

The low energy electronic excitations in single layer and bilayer graphite (graphene) resemble quantum-relativistic particles also known as Dirac Fermions (DF). They possess an internal degree of freedom, chirality, that leads to unusual Landau Level (LL) energy sequences in a magnetic field and profoundly alters the magneto-transport properties. One of the consequences is an anomalous Quantum-Hall effect, recently detected in both single layer and bi-layer graphene. However the underlying cause, the unusual LL sequence, was never observed. Here we report the direct observation of LL of DF by means of low temperature Scanning-Tunnelling-Spectroscopy (STS) on the surface of graphite in fields up to 12 Tesla. We find evidence of coexistence of massless and massive DF, and identify the zero-energy LL which is a unique consequence of their quantum-relativistic nature. Surprisingly these strictly two-dimensional properties emerge even on bulk graphite in samples where the interlayer coupling is weak.

cond-mat.mes-hall↗

arXiv2007

Ageing memory and glassiness of a driven vortex system

Many systems in nature, glasses, interfaces and fractures being some examples, cannot equilibrate with their environment, which gives rise to novel and surprising behaviour such as memory effects, ageing and nonlinear dynamics. Unlike their equilibrated counterparts, the dynamics of out-of- equilibrium systems is generally too complex to be captured by simple macroscopic laws. Here we investigate a system that straddles the boundary between glass and crystal: a Bragg glass formed by vortices in a superconductor. We find that the response to an applied force evolves according to a stretched exponential, with the exponent reflecting the deviation from equilibrium. After the force is removed, the system ages with time and its subsequent response time scales linearly with its age (simple ageing), meaning that older systems are slower than younger ones. We show that simple ageing can occur naturally in the presence of sufficient quenched disorder. Moreover, the hierarchical distribution of timescales, arising when chunks of loose vortices cannot move before trapped ones become dislodged, leads to a stretched-exponential response.

cond-mat.supr-con↗