A Real-Space Study of Flat Bands in Nanowires.

The flat electronic band has remarkable relevance in the strongly correlated phenomena mainly due to its reduced kinetic energy in comparison to the many-body potential energy. The formation of such bands in cubically structured nanowires is addressed in this article by means of a new independent channel method and a generalized convolution theorem developed for the Green's function including the first, second, and third neighbor interactions. A real-space renormalization method is further applied to address macroscopic-length aperiodic nanowires. We also determined the appearance condition of these flat bands, as well as their degeneracy and robustness in the face of perturbations, such as structural dislocations. Finally, the possible experimental detection of this flat band via the electronic specific heat is analyzed.

Independent Dual-Channel Approach to Mesoscopic Graphene Transistors.

Graphene field-effect transistors (GFETs) exhibit unique switch and sensing features. In this article, GFETs are investigated within the tight-binding formalism, including quantum capacitance correction, where the graphene ribbons with reconstructed armchair edges are mapped into a set of independent dual channels through a unitary transformation. A new transfer matrix method is further developed to analyze the electron transport in each dual channel under a back gate voltage, while the electronic density of states of graphene ribbons with transversal dislocations are calculated using the retarded Green's function and a novel real-space renormalization method. The Landauer electrical conductance obtained from these transfer matrices was confirmed by the Kubo-Greenwood formula, and the numerical results for the limiting cases were verified on the basis of analytical results. Finally, the size- and gate-voltage-dependent source-drain currents in GFETs are calculated, whose results are compared with the experimental data.

Estradiol to progesterone ratio is not a predictor of oocyte maturity at time of ovulation trigger.

PURPOSE: To evaluate the relationship between progesterone and oocyte maturity rate via estradiol to progesterone ratio (E/P) at the time of ovulatory trigger. METHODS: This is a retrospective cohort study of first autologous IVF cycles from January to December 2018 from a private practice fertility center. Serum estradiol and progesterone levels were measured on the day of ovulatory trigger. E/P was calculated to control for degree of response. Embryos were cultured to the blastocyst stage for trophectoderm biopsy. Preimplantation genetic testing for aneuploidy (PGT-A) was performed using next-generation sequencing (NGS). Oocyte retrieval rate (oocytes retrieved/follicles ≥ 13 mm), maturity rate (MII/oocytes retrieved), and euploid rate (euploid/total biopsied embryos) were calculated. Clinical pregnancy, ongoing pregnancy (> 10 weeks), and live births following frozen embryo transfer (FET) were examined in relation to E/P. Regression analyses were performed to analyze E/P as a categorical value (defined by quartile) on oocyte maturity. RESULTS: Two hundred eleven women underwent controlled ovarian hyperstimulation and had steroid levels at trigger available. Mean E at trigger was 3449 ± 2040 pg/mL while mean P was 1.13 ± 0.58 ng/mL, with mean E/P of 3.36 + 2.04. There were no differences between quartiles of E/P with respect to retrieval, maturity rate, or euploid rate. Two hundred eleven IVF cycles resulted in 138 euploid frozen embryo transfers. There were no differences between quartiles of E/P with respect to clinical pregnancy, ongoing pregnancy, or live birth rate. CONCLUSION: E/P ratio at the time of trigger does not impact oocyte retrieval rate, maturity rate, or euploid rate. Pregnancy and live birth outcomes were also not impacted.

Coarse-Grained Quantum Theory of Organic Photovoltaic Devices.

Understanding the exciton dissociation process in organic solar cells is a fundamental issue for the design of high-performance photovoltaic devices. In this article, a parameterized quantum theory based on a coarse-grained tight-binding model plus non-local electron-hole interactions is presented, while the diffusion and recombination of excitons are studied in a square lattice of excitonic states, where a real-space renormalization method on effective chains has been used. The Hamiltonian parameters are determined by fitting the measured quantum efficiency spectra and the theoretical short-circuit currents without adjustable parameters show a good agreement with the experimental ones obtained from several polymer:fullerene and polymer:polymer heterojunctions. Moreover, the present study reveals the degree of polymerization and the true driving force at donor-acceptor interface in each analyzed organic photovoltaic device.

Fingerprints of a position-dependent Fermi velocity on scanning tunnelling spectra of strained graphene.

Nonuniform strain in graphene induces a position dependence of the Fermi velocity, as recently demonstrated by scanning tunnelling spectroscopy experiments. In this work, we study the effects of a position-dependent Fermi velocity on the local density of states (LDOS) of strained graphene, with and without the presence of a uniform magnetic field. The variation of LDOS obtained from tight-binding calculations is successfully explained by analytical expressions derived within the Dirac approach. These expressions also rectify a rough Fermi velocity substitution used in the literature that neglects the strain-induced anisotropy. The reported analytical results could be useful for understanding the nonuniform strain effects on scanning tunnelling spectra of graphene, as well as when it is exposed to an external magnetic field.

Low-energy theory for strained graphene: an approach up to second-order in the strain tensor.

An analytical study of low-energy electronic excited states in uniformly strained graphene is carried out up to second-order in the strain tensor. We report a new effective Dirac Hamiltonian with an anisotropic Fermi velocity tensor, which reveals the graphene trigonal symmetry being absent in first-order low-energy theories. In particular, we demonstrate the dependence of the Dirac-cone elliptical deformation on the stretching direction with respect to graphene lattice orientation. We further analytically calculate the optical conductivity tensor of strained graphene and its transmittance for a linearly polarized light with normal incidence. Finally, the obtained analytical expression of the Dirac point shift allows a better determination and understanding of pseudomagnetic fields induced by nonuniform strains.

Raman scattering by confined optical phonons in Si and Ge nanostructures.

A microscopic theory of the Raman scattering based on the local bond-polarizability model is presented and applied to the analysis of phonon confinement in porous silicon and porous germanium, as well as nanowire structures. Within the linear response approximation, the Raman shift intensity is calculated by means of the displacement-displacement Green's function and the Born model, including central and non-central interatomic forces. For the porous case, the supercell method is used and ordered pores are produced by removing columns of Si or Ge atoms from their crystalline structures. This microscopic theory predicts a remarkable shift of the highest-frequency of first-order Raman peaks towards lower energies, in comparison with the crystalline case. This shift is discussed within the quantum confinement framework and quantitatively compared with the experimental results obtained from porous silicon samples, which were produced by anodizing p--type (001)-oriented crystalline Si wafers in a hydrofluoric acid bath.

Oxygen absorption in free-standing porous silicon: a structural, optical and kinetic analysis.

Porous silicon (PSi) is a nanostructured material possessing a huge surface area per unit volume. In consequence, the adsorption and diffusion of oxygen in PSi are particularly important phenomena and frequently cause significant changes in its properties. In this paper, we study the thermal oxidation of p+-type free-standing PSi fabricated by anodic electrochemical etching. These free-standing samples were characterized by nitrogen adsorption, thermogravimetry, atomic force microscopy and powder X-ray diffraction. The results show a structural phase transition from crystalline silicon to a combination of cristobalite and quartz, passing through amorphous silicon and amorphous silicon-oxide structures, when the thermal oxidation temperature increases from 400 to 900 °C. Moreover, we observe some evidence of a sinterization at 400 °C and an optimal oxygen-absorption temperature about 700 °C. Finally, the UV/Visible spectrophotometry reveals a red and a blue shift of the optical transmittance spectra for samples with oxidation temperatures lower and higher than 700 °C, respectively.

Renormalization plus convolution method for atomic-scale modeling of electrical and thermal transport in nanowires.

Based on the Kubo-Greenwood formula, the transport of electrons and phonons in nanowires is studied by means of a real-space renormalization plus convolution method. This method has the advantage of being efficient, without introducing additional approximations and capable to analyze nanowires of a wide range of lengths even with defects. The Born and tight-binding models are used to investigate the lattice thermal and electrical conductivities, respectively. The results show a quantized electrical dc conductance, which is attenuated when an oscillating electric field is applied. Effects of single and multiple planar defects, such as a quasi-periodic modulation, on the conductance of nanowires are also investigated. For the low temperature region, the lattice thermal conductance reveals a power-law temperature dependence, in agreement with experimental data.