Smooth electron waveguides in graphene

Copyright © 2010 American Physical SocietyWe present exact analytical solutions for the zero-energy modes of two-dimensional massless Dirac fermions fully confined within a smooth one-dimensional potential V(x)=−α/cosh(βx), which provides a good fit for potential profiles of existing top-gated graphene structures. We show that there is a threshold value of the characteristic potential strength α/β for which the first mode appears, in striking contrast to the nonrelativistic case. A simple relationship between the characteristic strength and the number of modes within the potential is found. An experimental setup is proposed for the observation of these modes. The proposed geometry could be utilized in future graphene-based devices with high on/off current ratios

The Kinematics of Molecular Cloud Cores in the Presence of Driven and Decaying Turbulence: Comparisons with Observations

In this study we investigate the formation and properties of prestellar and protostellar cores using hydrodynamic, self-gravitating Adaptive Mesh Refinement simulations, comparing the cases where turbulence is continually driven and where it is allowed to decay. We model observations of these cores in the C$^{18}$O$(2\to 1)$, NH$_3(1,1)$, and N$_2$H$^+(1\to 0)$ lines, and from the simulated observations we measure the linewidths of individual cores, the linewidths of the surrounding gas, and the motions of the cores relative to one another. Some of these distributions are significantly different in the driven and decaying runs, making them potential diagnostics for determining whether the turbulence in observed star-forming clouds is driven or decaying. Comparing our simulations with observed cores in the Perseus and $\rho$ Ophiuchus clouds shows reasonably good agreement between the observed and simulated core-to-core velocity dispersions for both the driven and decaying cases. However, we find that the linewidths through protostellar cores in both simulations are too large compared to the observations. The disagreement is noticably worse for the decaying simulation, in which cores show highly supersonic infall signatures in their centers that decrease toward their edges, a pattern not seen in the observed regions. This result gives some support to the use of driven turbulence for modeling regions of star formation, but reaching a firm conclusion on the relative merits of driven or decaying turbulence will require more complete data on a larger sample of clouds as well as simulations that include magnetic fields, outflows, and thermal feedback from the protostars.Comment: 18 pages, 12 figures, accepted to A

Optoelectronic properties of carbon-based nanostructures: Steering electrons in graphene by electromagnetic fields

Graphene has recently become the focus of enormous attention for experimentalists and theorists alike mainly due to its unique electronic properties. However, the limited way in which one can control these properties is a major obstacle for device applications. The unifying theme of this thesis is to propose and thoroughly justify ways to control the electronic properties of graphene and carbon nanotubes by light or static electric and magnetic fields and to harness these properties for optoelectronic applications. A linearly polarized excitation is shown to create a strongly anisotropic distribution of photoexcited carriers in graphene, where the momenta of photoexcited carriers are aligned preferentially normal to the polarization plane. This effect offers an experimental tool to generate highly directional photoexcited carriers which could assist in the investigation of "direction-dependent phenomena" in graphene-based nanostructures. The depolarization of hot photoluminescence is used to study relaxation processes in graphene, both free standing and grown on silicon carbide. This analysis is extended to include the effect of a magnetic field, thereby allowing one to obtain the momentum relaxation times of hot electrons. The analysis of momentum alignment in the high frequency regime shows that a linearly polarized excitation allows the spatial separation of carriers belonging to different valleys. Quasi-metallic carbon nanotubes are considered for terahertz applications. They are shown to emit terahertz radiation when a potential difference is applied across their ends and their spontaneous emission spectra have a universal frequency and bias voltage dependence. It is shown that the same intrinsic curvature which opens the gap in the quasi-metallic carbon nanotube energy spectrum also allows optical transitions in the terahertz range. The exciton binding energy in narrow-gap carbon nanotubes is calculated and found to scale with the band gap and vanishes as the gap decreases, even in the case of strong electron-hole attraction. Therefore, excitonic effects should not dominate in narrow-gap nanotubes. Contrary to widespread belief, it is shown that full confinement is possible for zero-energy states in pristine graphene. The exact analytical solutions for the zero-energy modes confined within a smooth one-dimensional potential V = α/ cosh (βx) are presented. This potential provides a good fit for the potential profiles of top-gated graphene structures. It is shown that there is a threshold value of the characteristic potential strength α/β for which the first mode appears, in striking contrast to the non-relativistic case. A relationship between the characteristic strength and the number of modes within the potential is found. An experimental setup is proposed for the observation of these modes. The proposed geometry could be utilized in future graphene-based devices with high on/off current ratios.EPSR

Excitons and interband terahertz transitions in narrow-gap carbon nanotubes

Copyright © 2013 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other users, including reprinting/ republishing this material for advertising or promotional purposes, creating new collective works for resale or redistribution to servers or lists, or reuse of any copyrighted components of this work in other works.2013 International Conference on Electromagnetics in Advanced Applications (ICEAA), Torino, Italy, 9 - 13 September 2013Post-print versionWe show, via a solution of the quasi-one-dimensional two-body problem applied to a Dirac system, that excitonic effects suppress the one-dimensional Van Hove singularity in single-walled carbon nanotubes with narrow band gaps.EPSRC (CAD) [Engineering and Physical Sciences Research Council]EU FP7 ITN SpinoptronicsFP7 IRSES project SPIN-METFP7 IRSES project QOCaNFP7 IRSES project CANTOREngineering and Physical Sciences Research Council: EPSRC (CAD)EU FP7 ITN SpinoptronicsFP7 IRSES project SPIN-METFP7 IRSES project QOCaNFP7 IRSES project CANTO

Slumping Dynamics of Tilled Sandy Soils in North-East Thailand

International audienceRecompaction of tilled layers, under the effect of rainfall or irrigation only (i.e. without any external loading), was called slumping by Mullins et al. (1990). It has been observed in various soil types with negative effects on plant production. Our objective was to characterise the dynamics of slumping at the ploughed layer scale in a sandy soil of North East Thailand. An experimental field was tilled to two depths (20 and 40 cm) with or without ridges and furrows and was submitted i) to natural rainfall during two months (214mm in June and July 2007) or ii) to experimental flood irrigation (100 or 200 mm over some hours). Changes in bulk density with time were observed, particularly under flooding and after heavy daily rainfall. Final bulk density of 1.60 Mg m 3 has been measured over 20 cm depth while initial bulk density after tillage was 1.25 Mg m 3. Bulk density profiles were often characterised with two maximum values, either in the top layer (0-5 cm) or at the bottom of the ploughed layer (15-20 or 35-40 cm). We demonstrated that several processes occurred simultaneously: i) a redistribution of sand particles from the top of ridges to the bottom of furrows that decreased soil roughness, ii) a 2 to 5 cm topsoil collapse when water infiltrated, iii) a soil collapse at greater depths due to overburden pressure. These phenomena agree with the theory of granular material and the decrease in capillary forces between sand grains during wetting. The specific changes in bulk density profiles induced by rainfall should allow the occurrence of slumping to be predicted or identified as a function of soil, climate and tillage conditions

Self-monitoring blood pressure in hypertension, patient and provider perspectives: A systematic review and thematic synthesis.

OBJECTIVE: To systematically review the qualitative evidence for patient and clinician perspectives on self-measurement of blood pressure (SMBP) in the management of hypertension focussing on: how SMBP was discussed in consultations; the motivation for patients to start self-monitoring; how both patients and clinicians used SMBP to promote behaviour change; perceived barriers and facilitators to SMBP use by patients and clinicians. METHODS: Medline, Embase, PsycINFO, Cinahl, Web of Science, SocAbs were searched for empirical qualitative studies that met the review objectives. Reporting of included studies was assessed using the COREQ framework. All relevant data from results/findings sections of included reports were extracted, coded inductively using thematic analysis, and overarching themes across studies were abstracted. RESULTS: Twelve studies were included in the synthesis involving 358 patients and 91 clinicians. Three major themes are presented: interpretation, attribution and action; convenience and reassurance v anxiety and uncertainty; and patient autonomy and empowerment improve patient-clinician alliance. CONCLUSIONS: SMBP was successful facilitating the interaction in consultations about hypertension, bridging a potential gap in the traditional patient-clinician relationship. PRACTICE IMPLICATIONS: Uncertainty could be reduced by providing information specifically about how to interpret SMBP, what variation is acceptable, adjustment for home-clinic difference, and for patients what they should be concerned about and how to act

Slumping dynamics in tilled sandy soils under natural rainfall and experimental flooding

International audienceCompaction of tilled layers under the single effect of rainfall or irrigation was called slumping. Slumping affects strongly root development and plant biomass production. It has been observed in different soil types, but sandy soils appear particularly prone to this physical degradation. Our objectives in this study were (i) to measure in the field the changes in soil structure and water status simultaneously, (ii) to study the effects of rainfall and management practices on slumping, and (iii) to propose a conceptual model for sandy soil slumping. An experimental site was selected in Northeast Thailand and we studied the effect of tillage depth and initial water content on slumping dynamic. Plots (9 m × 15 m each) were tilled at (i) two depths (20 and 40 cm, called S and previous termDnext term respectively) in dry conditions, (ii) at 20 cm depth in dry or wet conditions (called Y and W respectively). These plots were submitted to natural rainfall for 20 or 61 days to get different total rainfall amounts (114 and 212 mm respectively). In addition, smaller plots (0.24 m2 each) were used for experimental flooding irrigation (similar to measured rainfalls, i.e. 100 and 200 mm). Soil bulk density, soil surface elevation, soil water content and matric potential were measured. A decrease in soil elevation was observed in all treatments. In the absence of erosion it was interpreted as a loss of porosity which resulted from slumping. Bulk density increased in all layers of the tilled profile (from 1.38 to 1.57 g cm−3). In the surface layer (0-5 cm) this increase was systematically higher compared to deeper layer. No significant difference in final bulk density was found between the S and W treatments, and between the Y and W treatments. Bulk density increased more rapidly in the Y and W treatments than in the S and previous termDnext term treatments, even though the cumulative rainfall was lower. After the flooding experime

Multi‐Element Surface Coating of Layered Ni‐Rich Oxide Cathode Materials and Their Long‐Term Cycling Performance in Lithium‐Ion Batteries

The energy density of layered oxide cathode materials increases with their Ni content, while the stability decreases and degradation becomes more severe. A common strategy to mitigate or prevent degradation is the application of protective coatings on the particle surfaces. In this article, a room-tem-perature, liquid-phase reaction of trimethylaluminum (TMA) and tetraethyl orthosilicate (TEOS) with adsorbed moisture on either LiNi0.85Co0.10Mn0.05O2or LiNiO2, yielding a hybrid coating that shows synergetic benefits compared to coatings from TMA and TEOS individually, is reported. The surface layer is investigated in long-term pouch full-cell studies as well as by electron micros-copy, X-ray photoelectron spectroscopy, and differential electrochemical mass spectrometry, demonstrating that it prevents degradation primarily by a fluorine-scavenging effect, and by reducing the extent of rock salt-type phase formation