Página 1 dos resultados de 5205 itens digitais encontrados em 0.051 segundos

## Aplicação de formulação baseada no método dos elementos finitos posicional na análise bidimensional elástica de compósitos particulados; Application of a positional finite element method based formulation on the elastic two-dimensional analysis of particulate composites

Fonte: Biblioteca Digitais de Teses e Dissertações da USP
Publicador: Biblioteca Digitais de Teses e Dissertações da USP

Tipo: Dissertação de Mestrado
Formato: application/pdf

Publicado em 05/05/2015
PT

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#Análise não-linear geométrica#Compósitos particulados#Concrete#Concreto#Geometric nonlinear analysis#Influence of particles#Influência das partículas#Método dos elementos finitos posicional#Particle composite materials#Positional finite element method#Sólidos bidimensionais

A utilização de materiais compósitos tornou-se uma alternativa importante em muitas aplicações dentro de diversas áreas da engenharia, pois seus constituintes podem agregar propriedades mecânicas, térmicas e acústicas ao compósito, garantindo eficiência e baixo custo. Com isso, faz-se necessário um maior conhecimento do comportamento mecânico desses materiais diante das solicitações, principalmente no que diz respeito aos campos de deslocamento, deformações e tensões. O presente trabalho tem por finalidade a análise, em nível macroscópico, de estruturas bidimensionais elásticas constituídas de materiais compósitos particulados, utilizando formulação desenvolvida no contexto do Grupo de Mecânica Computacional (GMEC), do Departamento de Engenharia de Estruturas (SET), da Escola de Engenharia de São Carlos (EESC), da Universidade de São Paulo (USP), no qual se insere a presente pesquisa. A formulação utilizada baseia-se no Método dos Elementos Finitos Posicional (MEFP) e foi desenvolvida em nível mesoscópico por tratar da interação entre matriz e partículas. Tal formulação possibilita a consideração da interação partícula-matriz sem a necessidade de coincidência entre as malhas da matriz e das partículas e sem o aumento do número de graus de liberdade dos problemas...

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## A nonzero gap two-dimensional carbon allotrope from porous graphene

Fonte: Universidade Estadual Paulista
Publicador: Universidade Estadual Paulista

Tipo: Conferência ou Objeto de Conferência
Formato: 79-84

ENG

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#Ab initio#Biphenylene#Carbon allotropes#Electronic mobility#Frontier orbitals#Gap values#Quantum molecular dynamics#Spontaneous formation#Synthetic routes#Calculations#Electronic properties

Graphene has been one of the hottest topics in materials science in the last years. Because of its special electronic properties graphene is considered one of the most promising materials for future electronics. However, in its pristine form graphene is a gapless semiconductor, which poses some limitations to its use in some transistor electronics. Many approaches have been tried to create, in a controlled way, a gap in graphene. These approaches have obtained limited successes. Recently, hydrogenated graphene-like structures, the so-called porous graphene, have been synthesized. In this work we show, based on ab initio quantum molecular dynamics calculations, that porous graphene dehydrogenation can lead to a spontaneous formation of a nonzero gap two-dimensional carbon allotrope, called biphenylene carbon (BC). Besides exhibiting an intrinsic nonzero gap value, BC also presents well delocalized frontier orbitals, suggestive of a structure with high electronic mobility. Possible synthetic routes to obtain BC from porous graphene are addressed. © 2012 Materials Research Society.

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## 2D and 3D periodic templates through holographic interference lithography : photonic and phononic crystals and biomimetic microlens arrays; Two-dimensional and three-dimensional periodic templates through holographic interference lithography

Fonte: Massachusetts Institute of Technology
Publicador: Massachusetts Institute of Technology

Tipo: Tese de Doutorado
Formato: 133 leaves; 6752048 bytes; 6757595 bytes; application/pdf; application/pdf

ENG

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In this thesis a simple technique for controlling structure via holographic interference lithography was established and implemented. Access to various space groups including such important structures as the level set approximations to the Diamond, the Schwartz P structure, the FCC, and the non centrosymmetric Gyroid structures were demonstrated. The ability to make 3D structures over a large area, with low defect densities and periodicities on the sub/i scale opens a whole range of opportunities including such diverse areas as photonic crystals, phononic crystals, drug delivery, microtrusses, tissue scaffolds, microfluidics and colloidal crystallization. A correlation between structure and photonic band gap properties was established by systematically exploring the 11 FCC space groups. This resulted in a technique to search for photonic band gap structures. It was found that a fundamental connectivity caused by simple Fourier elements tended to support gaps. 2-3, 5-6 and 8-9 gaps were opened in the f.c.c lattices. The F-RD and 216 structures were newly shown to have complete band gaps. Two of the three previously established champion photonic crystal structures, viz. the Diamond and the Gyroid presented practical fabrication challenges...

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## Lattice deformations and spin-orbit effects in two dimensional materials

Fonte: Universidade Autônoma de Madrid
Publicador: Universidade Autônoma de Madrid

Tipo: Tese de Doutorado

SPA

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Tesis doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física de la Materia Condensada. Fecha de lectura: 18-09-2014; This thesis deals with the interplay between structural and electronic properties
of two-dimensional materials such as graphene, and the novel and very interesting
phenomena, both from the point of view of fundamental Physics and potential
applications, which emerge when lattice distortions such as strains or superlattice
modulations are combined with the dynamics of the electrons confined in two spatial
dimensions. The main microscopic ingredient which is behind all these phenomena
is the spin-orbit interaction. On the one hand, we analyze in detail how the spin-orbit
interaction modifies the electronic structure of these materials, and on the other, how
structural changes affect the spin-orbit interaction suffered by the electrons of the
solid, then modifying its electronic response in a very peculiar manner due to the
entanglement of the spin and orbital degrees of freedom.
The contents of the thesis are divided in three blocks. The first part is devoted to study
the effect of out-of-plane (flexural) vibration modes on the electronic properties of
graphene. We examine in detail the influence of the electron-phonon coupling on
the mobilities of suspended graphene samples...

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## A Bond-order Theory on the Phonon Scattering by Vacancies in Two-dimensional Materials

Fonte: Nature Publishing Group
Publicador: Nature Publishing Group

Tipo: Artigo de Revista Científica

Publicado em 28/05/2014
EN

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We theoretically investigate the phonon scattering by vacancies, including the impacts of missing mass and linkages () and the variation of the force constant of bonds associated with vacancies () by the bond-order-length-strength correlation mechanism. We find that in bulk crystals, the phonon scattering rate due to change of force constant is about three orders of magnitude lower than that due to missing mass and linkages . In contrast to the negligible in bulk materials, in two-dimensional materials can be 3–10 folds larger than . Incorporating this phonon scattering mechanism to the Boltzmann transport equation derives that the thermal conductivity of vacancy defective graphene is severely reduced even for very low vacancy density. High-frequency phonon contribution to thermal conductivity reduces substantially. Our findings are helpful not only to understand the severe suppression of thermal conductivity by vacancies, but also to manipulate thermal conductivity in two-dimensional materials by phononic engineering.

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## Deterministic transfer of two-dimensional materials by all-dry viscoelastic stamping

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

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Deterministic transfer of two-dimensional crystals constitutes a crucial step
towards the fabrication of heterostructures based on artificial stacking of
two-dimensional materials. Moreover, control on the positioning of
two-dimensional crystals facilitates their integration in complex devices,
which enables the exploration of novel applications and the discovery of new
phenomena in these materials. Up to date, deterministic transfer methods rely
on the use of sacrificial polymer layers and wet chemistry to some extent.
Here, we develop an all-dry transfer method that relies on viscoelastic stamps
and does not employ any wet chemistry step. This is found very advantageous to
freely suspend these materials as there are no capillary forces involved in the
process. Moreover, the whole fabrication process is quick, efficient, clean,
and it can be performed with high yield.; Comment: 34 pages, including detailed supporting information

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## Gas-like adhesion of two-dimensional materials onto solid surfaces

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

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The adhesion of two-dimensional (2D) materials to other surfaces is so far
believed to be a solid-solid mechanical contact. Here, we conduct both
atomistic simulations and theoretical modeling to show that there exists a
reversible conversion of energy between thermal and mechanical work in the
attachment/detachment of 2D materials on/off a surface, indicating that 2D
materials adhesion is fundamentally like gas adsorption rather than solid
adhesion. We reveal that the underlying mechanism of this intriguing gas-like
adhesion for 2D materials is the entropy difference between their freestanding
and adhered states. Both the theoretical model and atomistic simulations
predict that adhesion induced entropy difference increases with increasing
adhesion energy and decreasing equilibrium binding distance. The present
findings provide a fundamental guidance toward understanding the adhesion of 2D
materials, which is important for designing 2D materials based devices and may
have general implications for nanoscale efficient energy conversion.; Comment: 17 pages, 7 figures

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## Inorganic Graphenylene: A Porous Two-Dimensional Material With Tunable Band Gap

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 30/09/2014

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#Condensed Matter - Materials Science#Condensed Matter - Mesoscale and Nanoscale Physics#Physics - Chemical Physics#Physics - Computational Physics

By means of ab initio calculations we investigate the possibility of
existence of a boron nitride (BN) porous two-dimensional nanosheet which is
geometrically similar to the carbon allotrope known as biphenylene carbon. The
proposed structure, which we called Inorganic Graphenylene (IGP), is formed
spontaneously after selective dehydrogenation of the porous Boron Nitride (BN)
structure proposed by Ding et al. We study the structural and electronic
properties of both porous BN and IGP and it is shown that, by selective
substitution of B and N atoms with carbon atoms in these structures, the band
gap can be significantly reduced, changing their behavior from insulators to
semiconductors, thus opening the possibility of band gap engineering for this
class of two-dimensional materials.

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## Screening for Two dimensional MX$_2$ semiconductors with possible high room temperature mobility

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 21/05/2015

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We calculated the electron mobility of 14 two dimensional semiconductors with
composition of MX$_2$, where M (= Mo, W, Sn, Hf, Zr and Pt) is the transition
metal, and X is S, Se and Te. We treated the scattering matrix by deformation
potential approximation. Long wave longitudinal acoustical and optical phonon
scatterings are included. Piezoelectric scattering in the compounds without
inversion symmetry is also taken into account. We found that out of the 14
compounds, WSe$_2$, PtS$_2$ and PtSe$_2$, are promising regarding to the
possible high electron mobility and finite band gap. The phonon limited
mobility in PtSe$_2$ reaches about 3000 cm$^2$V$^{-1}$s$^{-1}$ at room
temperature which is the highest among the compounds. The bandgap under the
local density approximation is 1.25 eV. Our results can be a guide for
experiments to search for better two-dimensional materials for future
semiconductor devices.

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## Two-dimensional octagon-structure monolayer of nitrogen group elements and the related nano-structures

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

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In the purpose of expanding the family of two-dimensional materials, we
predict the existence of two-dimensional octa-structure of nitrogen group
elements that are composed of squares and octagons in first-principle method
based on density functional theory (DFT). From our calculations, electronic
structures of all monolayers show that they are semiconductors with indirect
(N, P, Bi) and direct (As, Sb) band gaps (0.57-2.61eV). Nano-ribbons of three
different unpassivated edges and their band structures are also investigated.
Because of the reconstruction on the edges and dangling bonds, there exist
ferromagnetic edge states in P, As, Sb nano-ribbons with different edges, and a
Dirac point near {\pi} is found in the band structure of one specific N
nano-ribbon. These structures may be useful in future applications, such as
semiconductor devices, spintronics, hydrogen storage and quantum computation.; Comment: 8 pages, 5 figures, 2 tables

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## Discrete differential geometry and the properties of conformal two-dimensional materials

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

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65.85%

Two-dimensional materials were first isolated no longer than ten years ago,
and a comprehensive understanding of their properties under non-planar shapes
is still being developed. Strictly speaking, the theoretical study of the
properties of graphene and other two-dimensional materials is the most complete
for planar structures and for structures with small deformations from
planarity. The opposite limit of large deformations is yet to be studied
comprehensively but that limit is extremely relevant because it determines
material properties near the point of failure. We are exploring uses for
discrete differential geometry within the context of graphene and other
two-dimensional materials, and these concepts appear promising in linking
materials properties to shape regardless of how large a given material
deformation is. A brief account of additional contributions arising from our
group to two-dimensional materials that include graphene, stanene and
phosphorene is provided towards the end of this manuscript.; Comment: Submitted on December 30, 2014 as an invited contribution to an
upcoming issue on Advances in Graphene Science and Engineering. Editors:
Jeanie Lau (UC-Riverside), Roland Kawakami (Ohio State) and Arthur Epstein
(Ohio State). Accepted version of the manuscript...

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## Emerging Two-dimensional Materials: graphene and its other structural analogues

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 03/06/2013

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The study of graphene, since its discovery around 2004, is possibly the
largest and fastest growing field of research in material science, because of
its exotic mechanical, thermal, electronic, optical and chemical properties.
The studies of graphene have also led to further research in exploring the
field of two dimensional (2D) systems in general. For instance, a number of
other 2D crystals (not based on carbon, e.g., boronitrene, silicone, graphane,
etc.) have been synthesized or predicted theoretically in recent years.
Further, theoretical studies have predicted the possibility of other 2D
hexagonal crystals of Ge, SiC, GeC, AlN, GaN, etc. The properties of these 2D
materials are very different from their bulk. We shall present the general
exotic properties of graphene like 2D systems followed by our computational
results on the structural and electronic properties of some of them.; Comment: Invited talk given in "Workshop on New and Nano Materials
(WNNM)-2012, 20th-21st January 2012", Institute of Materials Science,
Planetarium Building, Bhubaneswar-751013, Odisha, India. 12 pages including
the cover page; 8 figures

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## Harnessing quadratic optical response of two-dimensional materials through active microcavities

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 26/06/2014

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We propose a method for efficiently harnessing the quadratic optical response
of two-dimensional graphene-like materials by theoretically investigating
second harmonic generation from a current biased sheet placed within a planar
active microcavity. We show that, by tuning the cavity to resonate at the
second harmonic frequency, a highly efficient frequency doubling process is
achieved (several order of magnitude more efficient than the free-standing
sheet). The efficiency of the process is not due to phase-matching, which is
forbidden by the localization of the nonlinear quadratic response on the
two-dimensional atomic layered material, but it stems from the interplay
between the two-dimensional planar geometry of the nonlinear medium and the
field oscillation within the active cavity near its threshold. The suggested
method can easily be extended to different waves interactions and
nonlinearities and therefore it can represent a basic tool for efficiently
exploiting nonlinear optical properties of two-dimensional materials.; Comment: 5 pages, 4 figures

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## Electron counting and a large family of two-dimensional semiconductors

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 28/02/2015

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Comparing with the conventional semiconductors, the choice of the two
dimensional semiconductor (2DSC) materials is very limited. Based on proper
electron counting, we propose a large family of 2DSCs, all adopting the same
structure and consisting of only main group elements. Using advanced density
functional calculations, we demonstrate the attainability of these materials,
and show that they cover a large range of lattice constants, band gaps and band
edge states, therefore are good candidate materials for heterojunctions. This
family of two dimensional materials may pave a way toward fabrication of 2DSC
devices at the same thriving level as 3D semiconductors.

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## Substrate-free layer-number identification of two-dimensional materials: A case of Mo$_{0.5}$W$_{0.5}$S$_2$ alloy

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 15/05/2015

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Any of two or more two-dimensional (2D) materials with similar properties can
be alloyed into a new layered material, namely, 2D alloy. Individual monolayer
in 2D alloys are kept together by Van der Waals interactions. The property of
multilayer alloys is a function of their layer number. Here, we studied the
shear (C) and layer-breathing (LB) modes of Mo$_{0.5}$W$_{0.5}$S$_2$ alloy
flakes and their link to the layer number of alloy flakes. The study reveals
that the disorder effect is absent in the C and LB modes of 2D alloys, and the
monatomic chain model can be used to estimate the frequencies of the C and LB
modes. We demonstrated how to use the C and LB mode frequency to identify the
layer number of alloy flakes deposited on different substrates. This technique
is independent of the substrate, stoichiometry, monolayer thickness and complex
refractive index of 2D materials, offering a robust and substrate-free approach
for layer-number identification of 2D materials.; Comment: 5 pages, 4 figures

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## Piezoelectricity in Two-Dimensional Group III Monochalcogenides

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 25/03/2015

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We find that several layer-phase group-III monochalcogenides, including GaS,
GaSe and InSe, are piezoelectric in the monolayer form. First-principles
calculations reveal that the piezoelectric coefficients of monolayer GaS, GaSe
and InSe are on the same order of magnitude as the earlier discovered
two-dimensional piezoelectric materials, such as BN and MoS2 monolayers. Our
study expands the family of two dimensional piezoelectric materials, suggesting
that strong piezoelectric response can occur in a wide range of two dimensional
materials with broken inversion symmetry. The co-existence of piezoelectricity
and superior photo-sensitivity in these two-dimensional semiconductors enables
the integration of electromechanical and optical sensors on the same material
platform.

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## Phonon transport and thermal conductivity in two-dimensional materials

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

75.92%

Two-dimensional materials, such as graphene, boron nitride and transition
metal dichalcogenides, have attracted increased interest due to their potential
applications in electronics and optoelectronics. Thermal transport in
two-dimensional materials could be quite different from three-dimensional bulk
materials. This article reviews the progress on experimental measurements and
theoretical modeling of phonon transport and thermal conductivity in
two-dimensional materials. We focus our review on a few typical two-dimensional
materials, including graphene, boron nitride, silicene, transition metal
dichalcogenides, and black phosphorus. The effects of different physical
factors, such as sample size, strain and defects, on thermal transport in
Two-dimensional materials are summarized. We also discuss the environmental
effect on the thermal transport of two-dimensional materials, such as substrate
and when two-dimensional materials are presented in heterostructures and
intercalated with inorganic components or organic molecules.; Comment: 67 pages, 18 figures. Submitted to Annual Review of Heat Transfer

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## Two-Dimensional Van der Waals Epitaxy Kinetics in a Three-Dimensional Perovskite Halide

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 27/09/2015

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The exploration of emerging materials physics and prospective applications of
two-dimensional materials greatly relies on the growth control of their
thickness, phases, morphologies and film-substrate interactions. Though
substantial progresses have been made for the development of two-dimensional
films from conventional layered bulky materials, particular challenges remain
on obtaining ultrathin, single crystalline, dislocation-free films from
intrinsically non-Van der Waals-type three-dimensional materials. In this
report, with the successful demonstration of single crystalline ultrathin large
scale perovskite halide material, we reveal and identify the favorable role of
weak Van der Waals film-substrate interaction on the nucleation and growth of
the two-dimensional morphology out of non-layered materials compared to
conventional epitaxy. We also show how the bonding nature of the
three-dimensional material itself affects the kinetic energy landscape of
ultrathin films growth. By studying the formation of fractal perovskites
assisted with Monte Carlo simulations, we demonstrate that the competition
between the Van der Waals diffusion and surface free energy of the perovskite
leads to film thickening, suggesting extra strategies such as surface
passivation may be needed for the growth of monolayer and a few layers films.; Comment: Crystal Growth & Design 2015

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## Two-Dimensional Honeycomb Monolayer of Nitrogen Group Elements and the Related Nano-Structure: A First-Principle Study

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

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Because of its novel physical properties, two-dimensional materials have
attracted great attention. From first-principle calculations and vibration
frequenceis analysis, we predict a new family of two-dimensional materials
based on the idea of octet stability: honeycomb lattices of pnictogens (N, P,
As, Sb, Bi). The buckled structures of materials come from the sp3
hybridization. These materials have indirect band gap ranging from 0.43eV to
3.7eV. From the analysis of projected density of states, we argue that the s
and p orbitals together are sufficient to describe the electronic structure
under tight-binding model, and the tight-binding parameters are obtained by
fitting the band structures to first-principle results. Surprisingly large
on-site spin-orbit coupling is found for all the pnictogen lattices except
nitrogen. Investigation on the electronic structures of both zigzag and
armchair nanoribbons reveals the possible existence of spin-polarized
ferromagnetic edge states in some cases, which are rare in one-dimensional
systems. These edge states and magnetism may exist under the condition of high
vaccum and low temperature. This new family of materials would have promising
applications in electronics, optics, sensors, and solar cells.; Comment: 22 pages...

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## Nature of the quantum metal in a two-dimensional crystalline superconductor

Fonte: Nature Publishing Group
Publicador: Nature Publishing Group

Tipo: Artigo de Revista Científica

EN_US

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Two-dimensional (2D) materials are not expected to be metals at low temperature owing to electron localization. Consistent with this, pioneering studies on thin films reported only superconducting and insulating ground states, with a direct transition between the two as a function of disorder or magnetic field. However, more recent works have revealed the presence of an intermediate quantum metallic state occupying a substantial region of the phase diagram, whose nature is intensely debated. Here, we observe such a state in the disorder-free limit of a crystalline 2D superconductor, produced by mechanical co-lamination of NbSe2 in an inert atmosphere. Under a small perpendicular magnetic field, we induce a transition from superconductor to the quantum metal. We find a unique power-law scaling with field in this phase, which is consistent with the Bose-metal model where metallic behaviour arises from strong phase fluctuations caused by the magnetic field.

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