2,768 research outputs found
Interaction driven metal-insulator transition in strained graphene
The question of whether electron-electron interactions can drive a metal to
insulator transition in graphene under realistic experimental conditions is
addressed. Using three representative methods to calculate the effective
long-range Coulomb interaction between -electrons in graphene and solving
for the ground state using quantum Monte Carlo methods, we argue that without
strain, graphene remains metallic and changing the substrate from SiO to
suspended samples hardly makes any difference. In contrast, applying a rather
large -- but experimentally realistic -- uniform and isotropic strain of about
seems to be a promising route to making graphene an antiferromagnetic
Mott insulator.Comment: Updated version: 6 pages, 3 figure
The role of electron-electron interactions in two-dimensional Dirac fermions
The role of electron-electron interactions on two-dimensional Dirac fermions
remains enigmatic. Using a combination of nonperturbative numerical and
analytical techniques that incorporate both the contact and long-range parts of
the Coulomb interaction, we identify the two previously discussed regimes: a
Gross-Neveu transition to a strongly correlated Mott insulator, and a
semi-metallic state with a logarithmically diverging Fermi velocity accurately
described by the random phase approximation. Most interestingly, experimental
realizations of Dirac fermions span the crossover between these two regimes
providing the physical mechanism that masks this velocity divergence. We
explain several long-standing mysteries including why the observed Fermi
velocity in graphene is consistently about 20 percent larger than the best
values calculated using ab initio and why graphene on different substrates show
different behavior.Comment: 11 pages, 4 figure
An analysis of thermal comfort in primary schools in Vietnam
There is a trend of installing air conditioning systems in public primary schools that are currently naturally ventilated in Vietnam. A previous study conducted by the authors provided evidence that there is limited need for air conditioning in Vietnamese mid-season and the hottest season.
In this study, the authors investigated thermal comfort and users’ perceptions in three primary schools in Ho Chi Minh City during the hottest season (April 2016) and the coldest season (December 2016 – January 2017). In-situ spot and long-term measurements were recorded. Questionnaires were completed by 3,960 children (age range from 8 to 11 years) and the teachers to inform the study about their experiences and the extent of their interaction with the building in 97 free-running classrooms. The results were analysed by correlating the conditions measured and the comfort mean votes.
The neutral temperatures were respectively 31.7oC and 31.1oC for the hottest and coldest seasons. Children were observed to tolerate higher thermal comfort condition than the recommended values in the standards. Compared with the results of the hottest season, the thermal sensation mean vote reduced from (0.29) to (0.12) when the decrease of the mean temperature was from 33.3oC to 31.8oC in the coldest season. The temperature of 33oC was proposed for the overheating benchmark.
The results indicated that Vietnamese children adapted to hot climate and had higher thermal comfort tolerance than adults. Preliminary findings suggest that it is unnecessary to use air conditioning system all year round. These findings could help and encourage architects and engineers to deliver schools reaching acceptable comfort levels without the need of air conditioning system
The case for hybrid ventilated primary schools in Ho Chi Minh City in Vietnam
This study investigates indoor environmental quality and users’ perception in 14 classrooms of a mixed-mode ventilated primary school in Ho Chi Minh City, Vietnam, during the rainy season by qualitative and quantitative research methods. Spot and long-term measurements were recorded, covering a range of environmental parameters such as air temperature, relative humidity, CO2 concentration level, illuminance and sound level. A questionnaire was conducted and answered by 451 children (eight to eleven year olds) to investigate their perception of the thermal and visual comfort, indoor air quality, noise and overall comfort in the classrooms. In addition, 14 teachers were asked to inform the study about their experiences and behaviours in the classrooms. The results were analysed by cross relating the measured environmental conditions and the comfort vote on a seven-point scale
Children thermal comfort in primary schools in Ho Chi Minh City in Vietnam
Indoor environmental quality significantly impacts on students’ performance and productivity, particularly thermal comfort levels. Currently in Vietnam, very few studies have dealt with the issue and the current trend is to install energy-intensive air-conditioning in primary schools as this is perceived as more comfortable. In this study, the authors investigated the users’ perceptions of thermal comfort in three primary schools in Ho Chi Minh City during the mid-season (September 2015) and the hottest season (April 2016). In-situ spot and long-term measurements were recorded. Questionnaires were completed by 2,145 children (from 8 to 11 years-old) and 62 teachers to understand their experiences and the extent of their interaction with the building in 62 naturally ventilated classrooms. The results were analysed by correlating the conditions measured and the comfort mean votes. Throughout this study, children were observed to tolerate higher thermal comfort condition than the recommended values in the standards. Around 7% of the occupied time during academic year presented temperatures over 33oC, in which less than 80% of the children voted acceptable. The results indicated that Vietnamese children had higher thermal comfort tolerance than the comfort levels suggested in the standards. Using air conditioning system all year round was deemed unnecessary
Measuring valley polarization in two-dimensional materials with second-harmonic spectroscopy
A population imbalance at different valleys of an electronic system lowers
its effective rotational symmetry. We introduce a technique to measure such
imbalance - a valley polarization - that exploits the unique fingerprints of
this symmetry reduction in the polarization-dependent second-harmonic
generation (SHG). We present the principle and detection scheme in the context
of hexagonal two-dimensional crystals, which include graphene-based systems and
the family of transition metal dichalcogenides, and provide a direct
experimental demonstration using a 2H-MoSe monolayer at room temperature.
We deliberately use the simplest possible setup, where a single pulsed laser
beam simultaneously controls the valley imbalance and tracks the SHG process.
We further developed a model of the transient population dynamics which
analytically describes the valley-induced SHG rotation in very good agreement
with the experiment. In addition to providing the first experimental
demonstration of the effect, this work establishes a conceptually simple,
com-pact and transferable way of measuring instantaneous valley polarization,
with direct applicability in the nascent field of valleytronics
Foreword by guest editors for the Special Issue on the 2013 ICUFN Conferencs
Jeong, S.; Rodrigues, JJPC.; Cano Escribá, JC. (2014). Foreword by guest editors for the Special Issue on the 2013 ICUFN Conferencs. Wireless Personal Communications. 78(4):1827-1831. doi:10.1007/s11277-014-2046-yS1827183178
Topological Vortices in Chiral Gauge Theory of Graphene
Generation mechanism of energy gaps between conductance and valence bands is
at the centre of the study of graphene material. Recently Chamon, Jackiw, et
al. proposed a mechanism of using a Kekul\'{e} distortion background field and its induced gauge potential to generate energy gaps. In
this paper various vortex structures inhering in this model are studied.
Regarding as a generic background field rather than a fixed
Nielson-Oleson type distribution, we have found two new types of vortices on
the graphene surface --- the velocity field vortices and the monopole-motion
induced vortices --- from the inner structure of the potential . These
vortex structures naturally arise from the motion of the Dirac fermions instead
of from the background distortion field.Comment: 12 page
Ancilla-based quantum simulation
We consider simulating the BCS Hamiltonian, a model of low temperature
superconductivity, on a quantum computer. In particular we consider conducting
the simulation on the qubus quantum computer, which uses a continuous variable
ancilla to generate interactions between qubits. We demonstrate an O(N^3)
improvement over previous work conducted on an NMR computer [PRL 89 057904
(2002) & PRL 97 050504 (2006)] for the nearest neighbour and completely general
cases. We then go on to show methods to minimise the number of operations
needed per time step using the qubus in three cases; a completely general case,
a case of exponentially decaying interactions and the case of fixed range
interactions. We make these results controlled on an ancilla qubit so that we
can apply the phase estimation algorithm, and hence show that when N \geq 5,
our qubus simulation requires significantly less operations that a similar
simulation conducted on an NMR computer.Comment: 20 pages, 10 figures: V2 added section on phase estimation and
performing controlled unitaries, V3 corrected minor typo
Structure-function relationships at the human spinal disc-vertebra interface.
Damage at the intervertebral disc-vertebra interface associates with back pain and disc herniation. However, the structural and biomechanical properties of the disc-vertebra interface remain underexplored. We sought to measure mechanical properties and failure mechanisms, quantify architectural features, and assess structure-function relationships at this vulnerable location. Vertebra-disc-vertebra specimens from human cadaver thoracic spines were scanned with micro-computed tomography (μCT), surface speckle-coated, and loaded to failure in uniaxial tension. Digital image correlation (DIC) was used to calculate local surface strains. Failure surfaces were scanned using scanning electron microscopy (SEM), and adjacent sagittal slices were analyzed with histology and SEM. Seventy-one percent of specimens failed initially at the cartilage endplate-bone interface of the inner annulus region. Histology and SEM both indicated a lack of structural integration between the cartilage endplate (CEP) and bone. The interface failure strength was increased in samples with higher trabecular bone volume fraction in the vertebral endplates. Furthermore, failure strength decreased with degeneration, and in discs with thicker CEPs. Our findings indicate that poor structural connectivity between the CEP and vertebra may explain the structural weakness at this region, and provide insight into structural features that may contribute to risk for disc-vertebra interface injury. The disc-vertebra interface is the site of failure in the majority of herniation injuries. Here we show new structure-function relationships at this interface that may motivate the development of diagnostics, prevention strategies, and treatments to improve the prognosis for many low back pain patients with disc-vertebra interface injuries. © 2017 The Authors. Journal of Orthopaedic Research® Published by Wiley Periodicals, Inc. on behalf of Orthopaedic Research Society. J Orthop Res 36:192-201, 2018
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