Conductance of a quantum point contact based on spin-density-functional theory

We present full quantum mechanical conductance calculations of a quantum point contact (QPC) performed in the framework of the density functional theory (DFT) in the local spin-density approximation (LDA). We show that a spin-degeneracy of the conductance channels is lifted and the total conductance exhibits a broad plateau-like feature at 0.5*2e^{2}/h. The lifting of the spin-degeneracy is a generic feature of all studied QPC structures (both very short and very long ones; with the lengths in the range 40<l<500 nm). The calculated conductance also shows a hysteresis for forward- and backward sweeps of the gate voltage. These features in the conductance can be traced to the formation of weakly coupled quasi-bound states (magnetic impurities) inside the QPC (also predicted in previous DFT-based studies). A comparison of obtained results with the experimental data shows however, that while the spin-DFT based "first-principle" calculations exhibits the spin polarization in the QPC, the calculated conductance clearly does not reproduce the 0.7 anomaly observed in almost all QPCs of various geometries. We critically examine major features of the standard DFT-based approach to the conductance calculations and argue that its inability to reproduce the 0.7 anomaly might be related to the infamous derivative discontinuity problem of the DFT leading to spurious self-interaction errors not corrected in the standard LDA. Our results indicate that the formation of the magnetic impurities in the QPC might be an artefact of the LDA when localization of charge is expected to occur. We thus argue that an accurate description of the QPC structure would require approaches that go beyond the standard DFT+LDA schemes.Comment: 9 pages, 5 figure

Quantum Interaction ϕ44\phi^4_4: the Construction of Quantum Field defined as a Bilinear Form

We construct the solution $\phi(t,{\bf x})$ of the quantum wave equation $\Box\phi + m^2\phi + \lambda:\!\!\phi^3\!\!: = 0$ as a bilinear form which can be expanded over Wick polynomials of the free $in$-field, and where $:\!\phi^3(t,{\bf x})\!:$ is defined as the normal ordered product with respect to the free $in$-field. The constructed solution is correctly defined as a bilinear form on $D_{\theta}\times D_{\theta}$, where $D_{\theta}$ is a dense linear subspace in the Fock space of the free $in$-field. On $D_{\theta}\times D_{\theta}$ the diagonal Wick symbol of this bilinear form satisfies the nonlinear classical wave equation.Comment: 32 pages, LaTe

Evidence for a single hydrogen molecule connected by an atomic chain

Stable, single-molecule conducting-bridge configurations are typically identified from peak structures in a conductance histogram. In previous work on Pt with H$_2$ at cryogenic temperatures it has been shown that a peak near 1 $G{_0}$ identifies a single molecule Pt-H$_{2}$-Pt bridge. The histogram shows an additional structure with lower conductance that has not been identified. Here, we show that it is likely due to a hydrogen decorated Pt chain in contact with the H$_2$ molecular bridge.Comment: 4 pages, 4 figure

Inelastic Scattering in Metal-H2-Metal Junctions

We present first-principles calculations of the dI/dV characteristics of an H2 molecule sandwiched between Au and Pt electrodes in the presence of electron-phonon interactions. The conductance is found to decrease by a few percentage at threshold voltages corresponding to the excitation energy of longitudinal vibrations of the H2 molecule. In the case of Pt electrodes, the transverse vibrations can mediate transport through otherwise non-transmitting Pt $d$-channels leading to an increase in the differential conductance even though the hydrogen junction is characterized predominately by a single almost fully open transport channel. In the case of Au, the transverse modes do not affect the dI/dV because the Au d-states are too far below the Fermi level. A simple explanation of the first-principles results is given using scattering theory. Finally, we compare and discuss our results in relation to experimental data.Comment: Accepted in Phys. Rev.

First report of HIV-related oral manifestations in Mali

Introduction: In 2004, the sudden availability of free antiretroviral therapy (ART in Mali, within the context of an already overburdened health care&nbsp; system created gaps in individual patient quality of care. The objective of this study was to determine the prevalence of HIV-related oral manifestations (OM) during the first month of ART therapy in a Malian health facility. Methods: Medical records of adult patients who initiated ART regimens at the Gabriel Tour&eacute; Hospital, Mali (2001 to 2008) were randomly identified. Multiple logistic regression models were used to evaluate the relationship between the presence of OM during the first month of ART and selected variables, including CD4 counts and WHO clinical staging at ART initiation. Results: Out of 205 patients on ART (mean age 39 &plusmn; 10 years), 71.0% were females and 36.1% had no formal education. 40.6% were in WHO clinical stage III. OM prevalence during the first month of HIV care was 31.4%, being oral candidiasis the commonest lesion. 73.2% and 82.5% of the patients with OM had CD4 count &lt; 200 cells/mm3 and were classified as WHO clinical stage III or IV. WHO clinical stage III and VI patients had 5.4-fold increased odds of having any OM (both p&lt; 0.01) when controlling for age, ethnicity, gender, marital status, and CD4 counts. Conclusion: OM detected in people with low CD4 count and WHO clinical stage III and IV at ART initiation suggested that they were very immune- compromised when initiating HIV care. Early identification of OM could improve the quality of care and guarantee the benefits of ART

An Internet-Based Tool for Use in Assessing the Likely Effect of Intensification on Losses of Nitrogen to the Environment

The EU Nitrates, Habitat and National Emissions Ceilings directives and the Kyoto Agreement mean that agricultural losses of NO3, NH3 and N2O are under scrutiny by national and international environmental authorities. When farmers wish to intensify their operations, the authorities must then assess the likely environmental impact of the change in operation. The FARM-N internet tool was developed to help farmers and authorities agree how the farm will be structured and managed in the future, and to provide an objective assessment of the environmental losses that will result

Protostellar accretion traced with chemistry. High resolution C18O and continuum observations towards deeply embedded protostars in Perseus

Context: Understanding how accretion proceeds is a key question of star formation, with important implications for both the physical and chemical evolution of young stellar objects. In particular, very little is known about the accretion variability in the earliest stages of star formation. Aims: To characterise protostellar accretion histories towards individual sources by utilising sublimation and freeze-out chemistry of CO. Methods: A sample of 24 embedded protostars are observed with the Submillimeter Array (SMA) in context of the large program "Mass Assembly of Stellar Systems and their Evolution with the SMA" (MASSES). The size of the C$^{18}$O emitting region, where CO has sublimated into the gas-phase, is measured towards each source and compared to the expected size of the region given the current luminosity. The SMA observations also include 1.3 mm continuum data, which are used to investigate whether a link can be established between accretion bursts and massive circumstellar disks. Results: Depending on the adopted sublimation temperature of the CO ice, between 20% and 50% of the sources in the sample show extended C$^{18}$O emission indicating that the gas was warm enough in the past that CO sublimated and is currently in the process of refreezing; something which we attribute to a recent accretion burst. Given the fraction of sources with extended C$^{18}$O emission, we estimate an average interval between bursts of 20000-50000 yr, which is consistent with previous estimates. No clear link can be established between the presence of circumstellar disks and accretion bursts, however the three closest known binaries in the sample (projected separations <20 AU) all show evidence of a past accretion burst, indicating that close binary interactions may also play a role in inducing accretion variability.Comment: Accepted for publication in A&A, 21 pages, 13 figure

Effect of bonding of a CO molecule on the conductance of atomic metal wires

We have measured the effect of bonding of a CO molecule on the conductance of Au, Cu, Pt, and Ni atomic contacts at 4.2 K. When CO gas is admitted to the metal nano contacts, a conductance feature appears in the conductance histogram near 0.5 of the quantum unit of conductance, for all metals. For Au, the intensity of this fractional conductance feature can be tuned with the bias voltage, and it disappears at high bias voltage (above $\sim$ 200 mV). The bonding of CO to Au appears to be weakest, and associated with monotomic Au wire formation.Comment: 6 figure

Transport through quasi-ballistic quantum wires: the role of contacts

We model one-dimensional transport through each open channel of a quantum wire by a Luttinger liquid with three different interaction parameters for the leads, the contact regions and the wire, and with two barriers at the contacts. We show that this model explains several features of recent experiments, such as the flat conductance plateaux observed even at finite temperatures and for different lengths, and universal conductance corrections in different channels. We discuss the possibility of seeing resonance-like structures of a fully open channel at very low temperatures.Comment: revtex, 5 pages, 1 eps figure; clarifications added in light of new experiment

Benchmark density functional theory calculations for nanoscale conductance

We present a set of benchmark calculations for the Kohn-Sham elastic transmission function of five representative single-molecule junctions. The transmission functions are calculated using two different density functional theory methods, namely an ultrasoft pseudopotential plane-wave code in combination with maximally localized Wannier functions and the norm-conserving pseudopotential code SIESTA which applies an atomic orbital basis set. All calculations have been converged with respect to the supercell size and the number of k ʈ points in the surface plane. For all systems we find that the SIESTA transmission functions converge toward the plane-wave result as the SIESTA basis is enlarged. Overall, we find that an atomic basis with double zeta and polarization is sufficient ͑and in some cases, even necessary͒ to ensure quantitative agreement with the plane-wave calculation. We observe a systematic downshift of the SIESTA transmission functions relative to the plane-wave results. The effect diminishes as the atomic orbital basis is enlarged; however, the convergence can be rather slow