33 research outputs found
Tuning the polarized quantum phonon transmission in graphene nanoribbons
We propose systems that allow a tuning of the phonon transmission function
T() in graphene nanoribbons by using C isotope barriers, antidot
structures, and distinct boundary conditions. Phonon modes are obtained by an
interatomic fifth-nearest neighbor force-constant model (5NNFCM) and
T() is calculated using the non-equilibrium Green's function formalism.
We show that by imposing partial fixed boundary conditions it is possible to
restrict contributions of the in-plane phonon modes to T() at low
energy. On the contrary, the transmission functions of out-of-plane phonon
modes can be diminished by proper antidot or isotope arrangements. In
particular, we show that a periodic array of them leads to sharp dips in the
transmission function at certain frequencies which can be
pre-defined as desired by controlling their relative distance and size. With
this, we demonstrated that by adequate engineering it is possible to govern the
magnitude of the ballistic transmission functions T in graphene
nanoribbons. We discuss the implications of these results in the design of
controlled thermal transport at the nanoscale as well as in the enhancement of
thermo-electric features of graphene-based materials
The role of atomic vacancies and boundary conditions on ballistic thermal transport in graphene nanoribbons
Quantum thermal transport in armchair and zig-zag graphene nanoribbons are
investigated in the presence of single atomic vacancies and subject to
different boundary conditions. We start with a full comparison of the phonon
polarizations and energy dispersions as given by a fifth-nearest-neighbor
force-constant model (5NNFCM) and by elasticity theory of continuum membranes
(ETCM). For free-edges ribbons we discuss the behavior of an additional
acoustic edge-localized flexural mode, known as fourth acoustic branch (4ZA),
which has a small gap when it is obtained by the 5NNFCM. Then, we show that
ribbons with supported-edges have a sample-size dependent energy gap in the
phonon spectrum which is particularly large for in-plane modes. Irrespective to
the calculation method and the boundary condition, the dependence of the energy
gap for the low-energy optical phonon modes against the ribbon width W is found
to be proportional to 1/W for in-plane, and 1/W for out-of-plane phonon
modes. Using the 5NNFCM, the ballistic thermal conductance and its
contributions from every single phonon mode are then obtained by the non
equilibrium Green's function technique. We found that, while edge and central
localized single atomic vacancies do not affect the low-energy transmission
function of in-plane phonon modes, they reduce considerably the contributions
of the flexural modes. On the other hand, in-plane modes contributions are
strongly dependent on the boundary conditions and at low temperatures can be
highly reduced in supported-edges samples. These findings could open a route to
engineer graphene based devices where it is possible to discriminate the
relative contribution of polarized phonons and to tune the thermal transport on
the nanoscale
Dirichlet boundary conditions in a noncommutative theory
We study the problem of imposing Dirichlet-like boundary conditions along a
static spatial curve, in a planar Noncommutative Quantum Field Theory model.
After constructing interaction terms that impose the boundary conditions, we
discuss their implementation at the level of an interacting theory, with a
focus on their physical consequences, and the symmetries they preserve. We also
derive the effect they have on certain observables, like the Casimir energies.Comment: 19 pages, 1 figure, pdflate
Screening Acute HIV Infections among Chinese Men Who Have Sex with Men from Voluntary Counseling & Testing Centers
Recent studies have shown the public health importance of identifying acute HIV infection (AHI) in the men who have sex with men (MSM) of China, which has a much higher risk of HIV transmission. However, cost-utility analyses to guide policy around AHI screening are lacking.An open prospective cohort was recruited among MSM living in Liaoning Province, Northeast China. Blood samples and epidemiological information were collected every 10 weeks. Third-generation ELISA and rapid test were used for HIV antibody screening, western blot assay (WB) served for assay validation. Antibody negative specimens were tested with 24 mini-pool nucleic acid amplification testing (NAAT). Specimens with positive ELISA but negative or indeterminate WB results were tested with NAAT individually without mixing. A cost-utility analysis of NAAT screening was assessed. Among the 5,344 follow-up visits of 1,765 MSM in 22 months, HIV antibody tests detected 114 HIV chronic infections, 24 seroconverters and 21 antibody indeterminate cases. 29 acute HIV infections were detected with NAAT from 21 antibody indeterminate and 1,606 antibody negative cases. The HIV-1 prevalence and incidence density were 6.6% (95% CI: 5.5–7.9) and 7.1 (95% CI: 5.4–9.2)/100 person-years, respectively. With pooled NAAT and individual NAAT strategy, the cost of an HIV transmission averted was 3,366 per gained quality-adjusted life year (QALY). The input-output ratio of NAAT was about 1∶16.9.The HIV infections among MSM continue to rise at alarming rates. Despite the rising cost, adding pooled NAAT to the HIV antibody screening significantly increases the identification of acute HIV infections in MSM. Early treatment and target-oriented publicity and education programs can be strengthened to decrease the risk of HIV transmission and to save medical resources in the long run