arXiv subjects

Alexander Valladares

Publications and source records attributed to Alexander Valladares.

16 recordsLinked to original sources

arXiv2026

The structure of a melt: The case of liquid bismuth

Molecular Dynamics (MD) is performed on supercells of 216 atoms of bismuth, going from 300 K to 573 K in 100 steps and maintaining it in the liquid state, at 573 K, during 500 steps using the Materials Studio (MS) suite of codes. The Pair Distribution Functions (PDFs) and the Plane Angle Distributions (PADs) of the last 1, 10, 25 and 100 steps of the MD have been obtained. Averaging the last 100 steps, as representative of the liquid, Reverse Monte Carlo (RMC) was applied to obtain 4 atomic structures, one for each set of initial random velocities. Then, a detailed structural study of liquid bismuth at 573 K was undertaken; PDFs and PADs are calculated and reported. Two noticeable peaks appear for the PDFs, at 3.25 and 6.55 Å, along with a pseudo peak (shoulder) at 4.6 Å. This shoulder (after the first peak) of the PDFs is found to be related to the third and fourth neighbor peaks of the crystalline Wyckoff structure and also to the diagonal distances in deformed squares in the liquid structure. For completeness J(r)s are also reported. Two prominent peaks in the MS PADs are observed: 53° and 85°; and two for the RMC PADs: 58° and 90°, suggesting the existence of deformed triangles and squares. Less abundant are higher-order geometrical structures.

cond-mat.mtrl-sci↗

arXiv2025

Edge-Dependent Superconductivity in Twisted Bismuth Bilayers

Twisted bilayers offer a compelling and, at times, confounding platform for the engineering of new twistronic materials. Whereas standard studies almost exclusively focus on the explicit enigma that is presented by twist-angles, perhaps better epitomized by the related phenomena that have been observed in twisted bilayer graphene, functional devices necessarily face a fundamental concern: boundary heterogeneity in their structures. In this study, we address this concern by strictly investigating the electronic properties of twisted bismuth bilayers at the flake's edges and the vibrational properties of the flake. Twisted flakes exhibit continuous variations of these properties, away from the bulk, as we herein report using ab initio density functional theory, by systematically mapping the drastic evolution of band topology, electronic density of states, and possible superconductivity. Our work reveals a dramatic, non-fortuitous consequence of the structural disorder at the edges of the flakes: an enhanced electronic density of states at the Fermi level. This enhancement reaches a maximum of 10 times that of perfect-crystalline bismuth. Given that the superconducting critical temperature, Tc, is exponentially dependent on the electronic density of states at the Fermi level, this substantial structural variation immediately suggests a powerful mechanism for vastly increasing Tc. We also identify the twist-angle as a new critical parameter in designing novel engineering devices with topologically enhanced properties. Our results provide a necessary theoretical framework for interpreting new data for the upcoming generation of twistronic heterogeneous materials, and pave the way to search for atomic disordered metastable structures that could lead to enhanced superconducting transition temperatures.

cond-mat.supr-con↗

arXiv2024

The Low-Temperature Electronic Specific Heats of Disordered Ag-Au Alloys, Revisited

Disordered alloys of silver and gold have been in the interest of the condensed matter community for decades since they are the prototype of the ideal solid solution due to the chemical similarity of their constituents and due to their potential industrial applications. Although they are considered well-known materials, surprises have appeared that have not been well understood despite several studies performed. One example are the experimental results of the electronic specific heat at low temperatures of disordered Ag-Au alloys. In 1966, Green and Valladares [Phys. Rev. 142, 379 (1966)] conducted experimental studies of $γ$, the coefficient of the temperature in the expression for the electronic specific heat at low temperatures, finding a parabolic behavior as a function of the concentration, when a linear interpolation between the pure-element values was expected. This detonated several ulterior experiments that corroborated this parabolic behavior, and theoretical attempts followed that did not satisfactorily succeed at the explanation. It is our hope that this paper will contribute to the understanding of the experimental results; old problems can be reanalyzed with the help of new tools.

cond-mat.dis-nn↗

arXiv2023

Superconductivity in twisted bismuth bilayers

First-principles calculations for two twisted bismuth bilayers, each with 120 atoms, were studied by means of the electronic density of states and vibrational density of states. Metallic character at the Fermi level was found for the non-rotated sample as well as for each sample rotated 0.5°, 1.0°, 1.5°, 2.0°, 2.5°, 3.0°, 4.0°, 5.0°, 6.0°, 7.0°, 8.0° and 10° with respect to the static bilayer. Assuming that the superconductivity is BCS-type and the invariance of the Cooper pairing potential, we predict a maximum superconducting temperature T_c ~ 1.8 K for a magic angle of 0.5° degrees between the two bilayers, increasing the superconducting transition temperature from the experimentally measured value of 0.53 mK for the Wyckoff structure of crystalline bismuth.

cond-mat.supr-con↗

arXiv2022

Short-range Atomic Topology of Ab initio Generated Amorphous PdSi Alloys

Since the pioneering efforts of Duwez and coworkers in 1965, when a solid amorphous phase of Pd-Si ($a$-PdSi) was obtained in the vicinity of the eutectic concentration, much work has been done. However, some points related to the atomic structures remain to be systematized. In this work, 8 amorphous Pd$_{100-c}$Si$_{c}$ alloys (c = 2.5, 5, 10, 13.34, 15, 17.5, 20, and 22 at %) were generated by molecular dynamics $ab$ $initio$ simulations; the short-range structure is analyzed using several correlation functions, like Pair Distribution Functions, reduced Pair Distribution Functions, Plane Angle Distribution Functions. Other related properties, like nearest-neighbors and some Frank-Kasper polyhedra are reported. The generated samples correctly reproduce the scarce experimental pair correlation functions that are reported in the literature. An unexpected outcome is the appearance of structural changes in the neighborhood of Pd$_{86.66}$Si$_{13.34}$, which may be related to the magnetic changes reported in the liquid and amorphous Pd-Si alloys previously reported. The explicit amorphous topologies are reported.

cond-mat.mtrl-sci↗

arXiv2022

Amorphous Boron Nitride: Ab initio Study of its vibrational properties

Boron nitride (BN) is a structurally versatile insulator since it can be found in several crystalline structures with interesting mechanical and electrical properties, making this material attractive for technological applications. Seeking to improve its features, experimental and simulational studies for the amorphous phase (a-BN) have been carried out by some groups, focusing on the electrical and structural properties, pressure-induced phase transformations, and hydrogenated a-BN. In this work two amorphous structures are computationally generated and studied using ab initio Molecular Dynamics on a 216-atom supercell with two different densities, 2.04 and 2.80 g cm$^{-3}$. Our undermelt-quench approach is followed, since it has proven to give good structures for disordered materials and their properties. The topology, the vibrational density of states and some thermodynamic properties of the two samples are reported and compared with existing experiment. Some computational results are also revisited.

cond-mat.mtrl-sci↗

arXiv2022

Nanomagnetism in porous amorphous palladium, a sequel. A possible light-weight magnet

Magnetism is a very relevant subject that permeates our everyday lives. However, magnetism keeps taking us from surprise to surprise which seems to indicate that it is a phenomenon not well understood. For example, we found that bulk amorphous palladium becomes magnetic; so, naturally one should ask, could defective palladium develop magnetism? In particular, would amorphous porous palladium become magnetic? Here we show that the answer to that question is affirmative, this defective topology of Pd is magnetic, with a magnetism that depends on the amount of sample porosity and on the topology of their structures. Clearly, if magnetism exists in porous amorphous palladium, this indicates the possibility of developing light-weight magnets, useful when a maximized magnetism/weight ratio is demanded, well suited for space and aeronautical applications.

cond-mat.mtrl-sci↗

arXiv2022

Could Negative Pressures Turn Bismuth into a Metal? The Case of the Expanded

Materials may behave in non-expected ways when subject to unexpected conditions. For example, when Bi was turned into an amorphous phase (\textit{a}-Bi) unexpectedly it became a superconductor at temperatures below $10$ K. We provided an explanation as to why \textit{a}-Bi superconducts and the crystalline (\textit{c}-Bi) had not been found to do so: we computer calculated their electronic properties and found that \textit{a}-Bi has a larger electron density of states, eDoS, at the Fermi surface than \textit{c}-Bi and this explained the phenomenon. We even predicted an upper limit for the superconducting $T_c$ of the crystalline phase, which was experimentally corroborated within the following year. We now decided to investigate what happens to crystalline (Wyckoff structure) and amorphous Bi when pressures below the atmospheric are applied (expansion). Here we show that when expanded, \textit{c}-Bi becomes more metallic, since the eDoS increases when the volume increases for the Wyckoff structure, while the amorphous eDoS decreases. If the crystalline structure is maintained its $T_c$ would rise under expansion, whereas it would diminish for the \textit{a}-Bi. Expansion can be obtained in the laboratory by chemically etching Bi-based alloys, a process also known as dealloying, for example.

cond-mat.supr-con↗

arXiv2021

\textit{Ab initio} Studies of Magnetism and Topology in solid Pd-rich a-PdSi Alloys

In 1965 Duwez \textit{et al.} reported having generated an amorphous, stable phase of palladium-silicon in the region 15 to 23 atomic percent (at. \%) silicon. These pioneering efforts have led to the development of solid materials that are now known as Bulk Metallic Glasses (BMG). In 2019 we discovered, computationally, that bulk amorphous Pd becomes magnetic, and so does porous/amorphous Pd. Puzzled by our results we undertook the study of several solid binary systems in the Pd-rich zone; in particular, the study of the glassy metallic alloy $a$-Pd$_{100-c}$Si$_{c}$, for $0 \leq c \leq 22$, ($c$ in at. \%) to see what their topology is, what their electronic properties are and to inquire about their magnetism. Here we show that this metallic glass is in fact magnetic in the region $0 \leq c < 15$. Collaterally we present $α$ and $β$ magnetization curves that manifest the net magnetic moment observed. We also discuss the topology and the position of the first few peaks of the pair distribution functions, which agrees well with experiment. The BMGs produced experimentally so far are limited in size, but despite this limitation, recent industrial efforts have developed some useful devices that may revolutionize technology.

cond-mat.mtrl-sci↗

arXiv2021

Superconductivity versus magnetism in the palladium 'ides': Pd$_{1-c}$(H/D/T)$_{c}$

In general, conventional superconductivity and magnetism are competing phenomena. In some alloys this competition is a function of the concentration of the elements. Here we show that in the palladium alloys Pd$_{1-c}$(H/D/T)$_{c}$ (Pd-ides) the increase in the concentration $c$ of the ides: hydrogen, deuterium, tritium (H/D/T), lowers the predicted magnetism of amorphous palladium ($a$-Pd) gradually, allowing superconductivity to appear for $ c \approx 40\%$. This magnetism explains why superconductivity does not manifest for smaller values of $c$ ($c \leq 40\%$) in these Pd alloys. Also, these results validate indirectly our predicted magnetism in the amorphous/porous palladium ($a/p$-Pd). The understanding of the interplay between magnetism and superconductivity may contribute to the comprehension of the magnetic behavior in materials, especially in high $T_{c}$ superconductors, with the corresponding implications.

cond-mat.supr-con↗

arXiv2020

Correlation: An Analyzing Tool for Liquids and for Amorphous Solids

For almost a century, since Bernalś attempts at a molecular theory of liquid structure(Bernal [1]), correlation functions have been the bridge to compare theoretical calculations with experimental measurements in the study of disordered materials. Pair Distribution Functions (g(r)), Radial Distribution Functions (J(r)), Plane Angle Distributions (g(θ)) and Coordination Numbers (nc) have been widely used to characterize amorphous and liquid materials (Waseda [2]; Elliott [3]; Valladares et al. [4]) and, in particular Bulk Metallic Glasses (Miller and Liaw [5]; Galván-Colín et al. [6]). Correlation is an Open-Source software designed to analyze liquid structures and amorphous solids; the software is user-friendly, the modular design makes it easy to integrate in High-Throughput Computing (HTC) to process structures with a large number of constituents in a standardized fashion. Correlation is ready to be used in Windows,Linux and Mac. Currently, we support DMol3 (CAR), CASTEP (CELL), ONETEP (DAT)and VASP (POSCAR) structure files. The code can handle up to 25,000 atoms, so it can be used to analyze both classical and first-principles simulations. At the end, the output of every single correlation function is exported to the corresponding comma-separated value file (CSV), to further analyze the results.

physics.comp-ph↗

arXiv2019

Emergence of magnetism in bulk amorphous palladium

Magnetism in palladium has been the subject of much work and speculation. Bulk crystalline palladium is paramagnetic with a high magnetic susceptibility. Palladium under pressure and palladium nanoclusters have generated interest to scrutinize its magnetic properties. Here we report another possibility: Palladium may become an itinerant ferromagnet in the amorphous bulk phase at atmospheric pressure. Atomic palladium is a d$^{10}$ element, whereas bulk crystalline Pd is a d$^{10-x}$(sp)$^{x}$ material; this, together with the possible presence of 'unsaturated bonds' in amorphous materials, may explain the remnant magnetism reported herein. This work presents and discusses magnetic effects in bulk amorphous palladium.

cond-mat.str-el↗

arXiv2019

A facile approach to calculating superconducting transition temperatures in the bismuth solid phases

All solid phases of bismuth under pressure, but one, have been experimentally found to superconduct. From Bi-I to Bi-V, avoiding Bi-IV, they become superconductors and perhaps Bi-IV may also become superconductive. To investigate the influence of the electronic properties N(E) and the vibrational properties F(ω) on their superconductivity we have ab initio calculated them for the corresponding experimental crystalline structures, and using a BCS approach have been able to determine their critical temperatures Tc obtaining results close to experiment: For Bi-I (The Wyckoff Phase) we predicted a transition temperature of less than 1.3 mK and a year later a Tc of 0.5 mK was measured; for Bi-II Tc is 3.9 K measured and 3.6 K calculated; Bi-III has a measured Tc of 7 K and 6.5 K calculated for the structure reported by Chen et al., and for Bi-V Tc ~ 8 K measured and 6.8 K calculated. Bi-IV has not been found to be a superconductor but we have recently predicted a Tc of 4.25 K.

cond-mat.supr-con↗

arXiv2017

Possible superconductivity in Bismuth (111) bilayers. Its electronic and vibrational properties from first principles

Using a 72-atom supercell we report ab initio calculations of the electronic and vibrational densities of states for the bismuth (111) bilayers (bismuthene) with periodic boundary conditions and a vacuum of 5 Å, 10 Å and 20 Å. We find that the electronic density of states shows a metallic character at the Fermi level and that the vibrational density of states manifests the expected gap due to the layers. Our results indicate that a vacuum down to 5 Å does not affect the electronic and vibrational structures noticeably. A comparison of present results with those obtained for the Wyckoff structure is displayed. Assuming that the Cooper pairing potential is similar for all phases and structures of bismuth, an estimate of the superconducting transition temperature gives 2.61 K for the bismuth bilayers.

cond-mat.supr-con↗

arXiv2017

Superconductivity in the solid phases of Bi. Is Bi-IV a superconductor?

The first successful theory of superconductivity was the one proposed by Bardeen, Cooper and Schrieffer in 1957. This breakthrough fostered a remarkable growth of the field that propitiated progress and questionings, generating alternative theories to explain specific phenomena. For example, it has been argued that Bismuth, being a semimetal with a low number of carriers, does not comply with the basic hypotheses underlying BCS and therefore a different approach should be considered. Nevertheless, in 2016 based on BCS we put forth a prediction that Bi at ambient pressure becomes a superconductor at 1.3 mK [1]. A year later an experimental group corroborated that in fact Bi is a superconductor with a transition temperature of 0.53 mK [2], a result that eluded previous work. So, since Bi is superconductive in almost all the different structures and phases, the question is why Bi-IV has been elusive and has not been found yet to superconduct? Here we present a study of the electronic and vibrational properties of Bi-IV and infer its possible superconductivity using a BCS approach. We predict that if the Bi-IV phase structure were cooled down to liquid helium temperatures it would also superconduct at a Tc of 4.25 K.

cond-mat.supr-con↗

arXiv2017

Compressed Crystalline Bismuth and Superconductivity-An ab initio computational Simulation

Bismuth displays puzzling superconducting properties. In its crystalline equilibrium phase, it does not seem to superconduct at accessible low temperatures. However, in the amorphous phase it displays superconductivity at ~ 6 K. Under pressure bismuth has been found to superconduct at Tcs that go from 3.9 K to 8.5 K depending on the phase obtained. So the question is: what electronic or vibrational changes occur that explains this radical transformation in the conducting behavior of this material? In a recent publication we argue that changes in the density of electronic and vibrational states may account for the behavior observed in the amorphous phase with respect to the crystal. We have now undertaken an ab initio computational study of the effects of pressure alone maintaining the original crystalline structure and compressing our supercell computationally. From the results obtained we infer that if the crystal structure remains the same (except for the contraction), no superconductivity will appear.

cond-mat.mtrl-sci↗