Catálogo de publicaciones - revistas

<jats:p>Symmetry breaking plays a pivotal role in modern physics. Although self-similarity is also a symmetry, and appears ubiquitously in nature, a fundamental question arises as to whether self-similarity breaking makes sense or not. Here, by identifying an important type of critical fluctuation, dubbed ‘phases fluctuations’, and comparing the numerical results for those with self-similarity and those lacking self-similarity with respect to phases fluctuations, we show that self-similarity can indeed be broken, with significant consequences, at least in nonequilibrium situations. We find that the breaking of self-similarity results in new critical exponents, giving rise to a violation of the well-known finite-size scaling, or the less well-known finite-time scaling, and different leading exponents in either the ordered or the disordered phases of the paradigmatic Ising model on two- or three-dimensional finite lattices, when subject to the simplest nonequilibrium driving of linear heating or cooling through its critical point. This is in stark contrast to identical exponents and different amplitudes in usual critical phenomena. Our results demonstrate how surprising driven nonequilibrium critical phenomena can be. The application of this theory to other classical and quantum phase transitions is also anticipated.</jats:p>

<jats:p>We experimentally analyze the heat capacity and thermal expansion of reference solids in a wide temperature range from several Kelvin to melting temperature, and establish a universal double-linear relation between the experimental heat capacity <jats:italic>C</jats:italic> <jats:sub>p</jats:sub> and thermal expansion <jats:italic>β</jats:italic>, which is different from the previous models. The universal behavior between heat capacity and thermal expansion is important to predict the thermodynamic parameters at constant pressure, and is helpful for understanding the nature of thermal properties in solids.</jats:p>

<jats:p>Topological defects in graphene induce structural and electronic modulations. Knowing exact nature of broken-symmetry states around the individual atomic defects of graphene is very important for understanding the electronic properties of this material. We investigate structural dependence on localized electronic states in the vicinity of topological defects on a highly oriented pyrolytic graphite (HOPG) surface, using scanning tunneling microscopy and spectroscopy. Several inherent topological defects on the HOPG surface and the local density of states surrounding them are explored, visualized as scattering wave-related (<jats:inline-formula> <jats:tex-math><?CDATA $\sqrt{3}\times \sqrt{3}$?></jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msqrt> <mml:mn>3</mml:mn> </mml:msqrt> <mml:mo>×</mml:mo> <mml:msqrt> <mml:mn>3</mml:mn> </mml:msqrt> </mml:mrow> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="cpl_38_2_027201_ieqn1.gif" xlink:type="simple" /> </jats:inline-formula>) R30° superstructures and honeycomb superstructures. In addition, the superstructures observed near the grain boundary have a much higher decay length at specific sites than that reported previously, indicating far greater electron scattering on the quasi-periodic grain boundary.</jats:p>

<jats:p>We investigate the electrical conductivity and thermal conductivity of polycrystalline gold nanofilms, with thicknesses ranging from 40.5 nm to 115.8 nm, and identify a thickness-dependent electrical conductivity, which can be explained via the Mayadas and Shatzkes (MS) theory. At the same time, a suppressed thermal conductivity is observed, as compared to that found in the bulk material, together with a weak thickness effect. We compare the thermal conductivity of suspended and supported gold films, finding that the supporting substrate can effectively suppress the in-plane thermal conductivity of the polycrystalline gold nanofilms. Our results indicate that grain boundary scattering and substrate scattering can affect electron and phonon transport in polycrystalline metallic systems.</jats:p>

<jats:p>The directional design of functional materials with multi-objective constraints is a big challenge, in which performance and stability are determined by a complicated interconnection of different physical factors. We apply multi-objective optimization, based on the Pareto Efficiency and Particle-Swarm Optimization methods, to design new functional materials directionally. As a demonstration, we achieve the thermoelectric design of 2D SnSe materials via the above methods. We identify several novel metastable 2D SnSe structures with simultaneously lower free energy and better thermoelectric performance in their experimentally reported monolayer structures. We hope that the results of our work on the multi-objective Pareto Optimization method will represent a step forward in the integrative design of future multi-objective and multi-functional materials.</jats:p>

<jats:p>The recent observation of high critical temperature <jats:italic>T</jats:italic> <jats:sub>c</jats:sub> in lanthanum and Yttrium hydrides confirms the key role of hydrogen cage (H-cage) in determining high superconductivity. Here, we present a new class of metastable H<jats:sub>12</jats:sub> clathrate structures based on the icosahedral <jats:italic>cI</jats:italic>24-Na that can be stabilized by incorporation of metal elements. Analysis shows that the charge transfer from metal atoms to H atoms contributes to forming the H<jats:sub>12</jats:sub> clathrate. Nine dynamically stable structures are identified to exhibit superconductivity, and a maximum <jats:italic>T</jats:italic> <jats:sub>c</jats:sub> of 28 K is found in voids-doped Mo<jats:sub>6</jats:sub>H<jats:sub>24</jats:sub>. Calculations reveal that the low <jats:italic>T</jats:italic> <jats:sub>c</jats:sub> is attributed to the weak interaction between H atoms in each cage due to the long H–H distance. The current results provide a possible route to design H-cage containing superconductors.</jats:p>

<jats:p>Two-dimensional (2D) magnetic materials have been experimentally recognized recently, however, the Curie temperatures (<jats:italic>T</jats:italic> <jats:sub>C</jats:sub>) of known 2D systems are quite low. Generally, magnetic systems can be seen as constituent magnetic elements providing spins and the non-magnetic elements providing frameworks to host the magnetic elements. Short bond lengths between the magnetic and non-magnetic elements would be beneficial for strong magnetic interactions and thus high <jats:italic>T</jats:italic> <jats:sub>C</jats:sub>. Based on this, we propose to combine the magnetic element Cr and the non-magnetic element boron to design novel 2D magnetic systems. Using our self-developed software package IM<jats:sup>2</jats:sup>ODE, we design a series of chromium-boride based 2D magnetic materials. Nine stable magnetic systems are identified. Among them, we find that CrB<jats:sub>4</jats:sub>-I, CrB<jats:sub>4</jats:sub>-II and CrB<jats:sub>5</jats:sub>-I with common structural units [CrB<jats:sub>8</jats:sub>] are ferromagnetic metals with estimated <jats:italic>T</jats:italic> <jats:sub>C</jats:sub> of 270 K, 120 K and 110 K, respectively. On the other hand, five CrB<jats:sub>3</jats:sub> phases with structural units [Cr<jats:sub>2</jats:sub>B<jats:sub>12</jats:sub>] are antiferromagnetic metals. Additionally, we also find one antiferromagnetic semiconductor CrB<jats:sub>2</jats:sub>-I. Our work may open new directions for identifying 2D magnetic systems with high <jats:italic>T</jats:italic> <jats:sub>C</jats:sub>.</jats:p>

<jats:p>An ultra-wideband metamaterial absorber is developed, which is polarized-insensitive and angular-stable. Three layers of square resistive films comprise the proposed metamaterial. The optimal values of geometric parameters are obtained, such that the designed absorber can achieve an ultra-broadband absorption response from 4.73 to 39.04 GHz (relative bandwidth of 156.7%) for both transverse electricity and transverse magnetic waves. Moreover, impedance matching theory and an equivalent circuit model are utilized for the absorption mechanism analysis. The compatibility of equivalent circuit calculation results, together with both full-wave simulation and experimental results, demonstrates the excellent performance and applicability of the proposed metamaterial absorber.</jats:p>

<jats:p>Fabrication of atomic dopant wires at large scale is challenging. We explored the feasibility to fabricate atomic dopant wires by nano-patterning self-assembled dopant carrying molecular monolayers via a resist-free lithographic approach. The resist-free lithography is to use electron beam exposure to decompose hydrocarbon contaminants in vacuum chamber into amorphous carbon that serves as an etching mask for nanopatterning the phosphorus-bearing monolayers. Dopant wires were fabricated in silicon by patterning diethyl vinylphosphonate monolayers into lines with a width ranging from 1 μm down to 8 nm. The dopants were subsequently driven into silicon to form dopant wires by rapid thermal annealing. Electrical measurements show a linear correlation between wire width and conductance, indicating the success of the monolayer patterning process at nanoscale. The dopant wires can be potentially scaled down to atomic scale if the dopant thermal diffusion can be mitigated.</jats:p>

<jats:p>The <jats:italic>sp</jats:italic> <jats:sup>2</jats:sup>–<jats:italic>sp</jats:italic> <jats:sup>3</jats:sup>-hybridized carbon allotropes with the advantage of two hybrid structures possess rich and fascinating electronic and mechanical properties and they have received long-standing attention. We design a class of versatile <jats:italic>sp</jats:italic> <jats:sup>2</jats:sup>–<jats:italic>sp</jats:italic> <jats:sup>3</jats:sup> carbons composed of graphite and diamond structural units with variable sizes. This class of <jats:italic>sp</jats:italic> <jats:sup>2</jats:sup>–<jats:italic>sp</jats:italic> <jats:sup>3</jats:sup> carbons is energetically more favorable than graphite under high pressure, and their mechanical and dynamical stabilities are further confirmed at ambient pressure. The calculations of band structure and mechanical properties indicate that this class of <jats:italic>sp</jats:italic> <jats:sup>2</jats:sup>–<jats:italic>sp</jats:italic> <jats:sup>3</jats:sup> carbons not only exhibits peculiar electronic characteristics adjusted from semiconducting to metallic nature but also presents excellent mechanical characteristics, such as superhigh hardness and high ductility. These <jats:italic>sp</jats:italic> <jats:sup>2</jats:sup>–<jats:italic>sp</jats:italic> <jats:sup>3</jats:sup> carbons have desirable properties across a broad range of potential applications.</jats:p>