845 research outputs found
Unusual suppression of the superconducting energy gap and critical temperature in atomically thin NbSe2
It is well known that superconductivity in thin films is generally suppressed
with decreasing thickness. This suppression is normally governed by either
disorder-induced localization of Cooper pairs, weakening of Coulomb screening,
or generation and unbinding of vortex-antivortex pairs as described by the
Berezinskii-Kosterlitz-Thouless (BKT) theory. Defying general expectations,
few-layer NbSe2 - an archetypal example of ultrathin superconductors - has been
found to remain superconducting down to monolayer thickness. Here we report
measurements of both the superconducting energy gap and critical temperature in
high-quality monocrystals of few-layer NbSe2, using planar-junction tunneling
spectroscopy and lateral transport. We observe a fully developed gap that
rapidly reduces for devices with the number of layers N < 5, as does their
ctitical temperature. We show that the observed reduction cannot be explained
by disorder, and the BKT mechanism is also excluded by measuring its transition
temperature that for all N remains very close to Tc. We attribute the observed
behavior to changes in the electronic band structure predicted for mono- and
bi- layer NbSe2 combined with inevitable suppression of the Cooper pair density
at the superconductor-vacuum interface. Our experimental results for N > 2 are
in good agreement with the dependences of the gap and Tc expected in the latter
case while the effect of band-structure reconstruction is evidenced by a
stronger suppression of the gap and the disappearance of its anisotropy for N =
2. The spatial scale involved in the surface suppression of the density of
states is only a few angstroms but cannot be ignored for atomically thin
superconductors.Comment: 21 pages, including supporting informatio
Visualizing Poiseuille flow of hydrodynamic electrons
Hydrodynamics is a general description for the flow of a fluid, and is
expected to hold even for fundamental particles such as electrons when
inter-particle interactions dominate. While various aspects of electron
hydrodynamics were revealed in recent experiments, the fundamental spatial
structure of hydrodynamic electrons, the Poiseuille flow profile, has remained
elusive. In this work, we provide the first real-space imaging of Poiseuille
flow of an electronic fluid, as well as visualization of its evolution from
ballistic flow. Utilizing a scanning nanotube single electron transistor, we
image the Hall voltage of electronic flow through channels of high-mobility
graphene. We find that the profile of the Hall field across the channel is a
key physical quantity for distinguishing ballistic from hydrodynamic flow. We
image the transition from flat, ballistic field profiles at low temperature
into parabolic field profiles at elevated temperatures, which is the hallmark
of Poiseuille flow. The curvature of the imaged profiles is qualitatively
reproduced by Boltzmann calculations, which allow us to create a 'phase
diagram' that characterizes the electron flow regimes. Our results provide
long-sought, direct confirmation of Poiseuille flow in the solid state, and
enable a new approach for exploring the rich physics of interacting electrons
in real space
Propuesta de implementación de un modelo para la evaluación de la calidad del producto de software para una empresa consultora TI
El presente trabajo de investigación tiene como objetivo el estudio de la lÃnea de servicio de Certificación de Software de la empresa consultora TI y requiere resolver la problemática en la evaluación de los productos de desarrollo de software que impactan directamente en el cumplimiento de sus SLA, generando grandes pérdidas económicas.
En el análisis cuantitativo, se identificó que existen inadecuadas técnicas para la evaluación con un impacto en el cumplimiento de los umbrales definidos en cada SLA, lo cual ha generado grandes pérdidas económicas en los últimos años por penalización. Ante esta situación caótica, se propone la implementación de un modelo de evaluación de calidad del producto de Software, la cual propone lineamientos de acuerdo con estándares y prácticas internacionales para la evaluación de la calidad, ayudando a incrementar la calidad de sus productos y la satisfacción del cliente, debido que se evidencia pérdidas económicas que se están incrementando anualmente.
En este trabajo de investigación se hace una revisión general de los estándares de evaluación de calidad de producto de Software, se realiza una evaluación del cumplimiento de la norma ISO/IEC 25010 en la empresa y se propone un plan de mejora. Como conclusión, se recomienda la ejecución de la propuesta de implementación como apoyo estratégico al cumplimiento de los objetivos estratégicos de la empresa, reduciendo el riesgo de pérdidas económicas e incrementar la capacidad para ejecutar nuevas STD (Solicitudes Técnicas de Desarrollo), que permitirá a la empresa ser más rentable y brindar un servicio de mejor calidad.The following work of research has as main subject the study of the software certification service line from the consulting TI company and requires solving the problematic in the evaluation of software development products that has a direct impact in the fulfillment of their SLA, generating large economic losses.
In the quantitative analysis, it was identified there are inadequate techniques for the evaluation with an impact of the fulfillment of the thresholds defined on each SLA, which has generated large economic losses in the last year due to penalty. Given this chaotic situation, the implementation of a software product quality evaluation model is proposed, which provides guidelines in accordance with international standards and practices for the evaluation of quality, helping to increase the quality of its products and customer satisfaction, because there is evidence of economic losses that are increasing annually.
In this research work a general review of the existing software product quality evaluation standards is made, an evaluation of compliance of the ISO/IEC 25010 norm in the company will be carried out and an improvement plan will be proposed. In conclusion, the implementation of the proposal is recommended as strategic support for the fulfillment of the strategic objectives of the company, reducing the risk of economic losses and increasing the capacity to execute new STD (Technical Development Requests), which will allow the company to be more profitable and provide a better quality service.Trabajo de investigació
Duration of adjuvant chemotherapy for stage III colon cancer
BACKGROUND
Since 2004, a regimen of 6 months of treatment with oxaliplatin plus a fluoropyrimidine has been standard adjuvant therapy in patients with stage III colon cancer. However, since oxaliplatin is associated with cumulative neurotoxicity, a shorter duration of therapy could spare toxic effects and health expenditures.
METHODS
We performed a prospective, preplanned, pooled analysis of six randomized, phase 3 trials that were conducted concurrently to evaluate the noninferiority of adjuvant therapy with either FOLFOX (fluorouracil, leucovorin, and oxaliplatin) or CAPOX (capecitabine and oxaliplatin) administered for 3 months, as compared with 6 months. The primary end point was the rate of disease-free survival at 3 years. Noninferiority of 3 months versus 6 months of therapy could be claimed if the upper limit of the two-sided 95% confidence interval of the hazard ratio did not exceed 1.12.
RESULTS
After 3263 events of disease recurrence or death had been reported in 12,834 patients, the noninferiority of 3 months of treatment versus 6 months was not confirmed in the overall study population (hazard ratio, 1.07; 95% confidence interval [CI], 1.00 to 1.15). Noninferiority of the shorter regimen was seen for CAPOX (hazard ratio, 0.95; 95% CI, 0.85 to 1.06) but not for FOLFOX (hazard ratio, 1.16; 95% CI, 1.06 to 1.26). In an exploratory analysis of the combined regimens, among the patients with T1, T2, or T3 and N1 cancers, 3 months of therapy was noninferior to 6 months, with a 3-year rate of disease-free survival of 83.1% and 83.3%, respectively (hazard ratio, 1.01; 95% CI, 0.90 to 1.12). Among patients with cancers that were classified as T4, N2, or both, the disease-free survival rate for a 6-month duration of therapy was superior to that for a 3-month duration (64.4% vs. 62.7%) for the combined treatments (hazard ratio, 1.12; 95% CI, 1.03 to 1.23; P=0.01 for superiority).
CONCLUSIONS
Among patients with stage III colon cancer receiving adjuvant therapy with FOLFOX or CAPOX, noninferiority of 3 months of therapy, as compared with 6 months, was not confirmed in the overall population. However, in patients treated with CAPOX, 3 months of therapy was as effective as 6 months, particularly in the lower-risk subgroup. (Funded by the National Cancer Institute and others.
Sub-bandgap voltage electroluminescence and magneto-oscillations in a WSe2 light-emitting van der Waals heterostructure
We report on experimental investigations of an electrically driven WSe2 based
light-emitting van der Waals heterostructure. We observe a threshold voltage
for electroluminescence significantly lower than the corresponding single
particle band gap of monolayer WSe2. This observation can be interpreted by
considering the Coulomb interaction and a tunneling process involving excitons,
well beyond the picture of independent charge carriers. An applied magnetic
field reveals pronounced magneto-oscillations in the electroluminescence of the
free exciton emission intensity with a 1/B-periodicity. This effect is ascribed
to a modulation of the tunneling probability resulting from the Landau
quantization in the graphene electrodes. A sharp feature in the differential
conductance indicates that the Fermi level is pinned and allows for an
estimation of the acceptor binding energy.Comment: Accepted for publication in Nano Letter
Tunnel field-effect transistors for sensitive terahertz detection
The rectification of electromagnetic waves to direct currents is a crucial
process for energy harvesting, beyond-5G wireless communications, ultra-fast
science, and observational astronomy. As the radiation frequency is raised to
the sub-terahertz (THz) domain, ac-to-dc conversion by conventional electronics
becomes challenging and requires alternative rectification protocols. Here we
address this challenge by tunnel field-effect transistors made of bilayer
graphene (BLG). Taking advantage of BLG's electrically tunable band structure,
we create a lateral tunnel junction and couple it to an antenna exposed to THz
radiation. The incoming radiation is then down-converted by the tunnel junction
nonlinearity, resulting in high-responsivity (> 4 kV/W) and low-noise (0.2
pW/}) detection. We demonstrate how switching from
intraband Ohmic to interband tunneling regime can raise detectors' responsivity
by few orders of magnitude, in agreement with the developed theory. Our work
demonstrates a potential application of tunnel transistors for THz detection
and reveals BLG as a promising platform therefor
Highly confined In-plane Exciton-Polaritons in monolayer semiconductors
2D materials support unique excitations of quasi-particles that consist of a material excitation and photons called polaritons. Especially interesting are in-plane propagating polaritons, which can be confined to a single monolayer and carry large momentum. In this work, we theoretically predict the existence of a new type of in-plane propagating polariton, supported on monolayer transition-metal-dicalcogonides (TMDs) in the visible spectrum. This 2D in-plane exciton-polariton (2DEP) is described by the coupling of an electromagnetic light field with the collective oscillations of the excitons supported by monolayer TMDs. We experimentally demonstrate the specific conditions required for the excitation of the 2DEP and show that these can be achieved if the TMD is encapsulated with hexagonal-boron-nitride (hBN) and cooled to cryogenic temperatures. In addition, we compare the properties of the 2DEP with those of the surface-plasmon-polariton at the same spectral range, and find that the 2DEP exhibit over two orders-of-magnitude larger wavelength confinement. Finally, we propose and numerically demonstrate two configurations for the possible experimental observation of 2DEPs.- I E thanks Dr Fabien Vialla. J H and D R acknowledge the funding support by the NSF MRSEC program through Columbia in the Center for Precision Assembly of Superstratic and Superatomic Solids (DMR-1420634). H G and B F acknowledge support from ERC advanced grant COMPLEXPLAS. F H L K acknowledges financial support from the Spanish Ministry of Economy and Competitiveness, through the "Severo Ochoa" Programme for Centres of Excellence in R&D (SEV-2015-0522), support by Fundacio Cellex Barcelona, Generalitat de Catalunya through the CERCA program, and the Mineco grants Plan Nacional (FIS2016-81044-P) and the Agency for Management of University and Research Grants (AGAUR) 2017 SGR 1656. Furthermore, the research leading to these results has received funding from the European Union Seventh Framework Programme under grant agreement no.785219 (Core2) and no. 881603 (Core3) Graphene Flagship. This work was supported by the ERC TOPONANOP under grant agreement no. 726001
Evidence of Flat Bands and Correlated States in Buckled Graphene Superlattices
Two-dimensional atomic crystals can radically change their properties in
response to external influences such as substrate orientation or strain,
resulting in essentially new materials in terms of the electronic structure. A
striking example is the creation of flat-bands in bilayer-graphene for certain
'magic' twist-angles between the orientations of the two layers. The quenched
kinetic-energy in these flat-bands promotes electron-electron interactions and
facilitates the emergence of strongly-correlated phases such as
superconductivity and correlated-insulators. However, the exquisite fine-tuning
required for finding the magic-angle where flat-bands appear in twisted-bilayer
graphene, poses challenges to fabrication and scalability. Here we present an
alternative route to creating flat-bands that does not involve fine tuning.
Using scanning tunneling microscopy and spectroscopy, together with numerical
simulations, we demonstrate that graphene monolayers placed on an
atomically-flat substrate can be forced to undergo a buckling-transition,
resulting in a periodically modulated pseudo-magnetic field, which in turn
creates a post-graphene material with flat electronic bands. Bringing the
Fermi-level into these flat-bands by electrostatic doping, we observe a
pseudogap-like depletion in the density-of-states, which signals the emergence
of a correlated-state. The described approach of 2D crystal buckling offers a
strategy for creating other superlattice systems and, in particular, for
exploring interaction phenomena characteristic of flat-bands.Comment: 22 pages, 15 figures. arXiv admin note: substantial text overlap with
arXiv:1904.1014
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