arXiv subjects

Tomas A. Arias

Publications and source records attributed to Tomas A. Arias.

7 recordsLinked to original sources

arXiv2023

ZrNb(CO) RF superconducting thin film with high critical temperature in the theoretical limit

Superconducting radio-frequency (SRF) resonators are critical components for particle accelerator applications, such as free-electron lasers, and for emerging technologies in quantum computing. Developing advanced materials and their deposition processes to produce RF superconductors that yield nanoohms surface resistances is a key metric for the wider adoption of SRF technology. Here we report ZrNb(CO) RF superconducting films with high critical temperatures (Tc) achieved for the first time under ambient pressure. The attainment of a Tc near the theoretical limit for this material without applied pressure is promising for its use in practical applications. A range of Tc, likely arising from Zr doping variation, may allow a tunable superconducting coherence length that lowers the sensitivity to material defects when an ultra-low surface resistance is required. Our ZrNb(CO) films are synthesized using a low-temperature (100 - 200 C) electrochemical recipe combined with thermal annealing. The phase transformation as a function of annealing temperature and time is optimized by the evaporated Zr-Nb diffusion couples. Through phase control, we avoid hexagonal Zr phases that are equilibrium-stable but degrade Tc. X-ray and electron diffraction combined with photoelectron spectroscopy reveal a system containing cubic ZrNb mixed with rocksalt NbC and low-dielectric-loss ZrO2. We demonstrate proof-of-concept RF performance of ZrNb(CO) on an SRF sample test system. BCS resistance trends lower than reference Nb, while quench fields occur at approximately 35 mT. Our results demonstrate the potential of ZrNb(CO) thin films for particle accelerator and other SRF applications.

cond-mat.mtrl-sci↗

arXiv2023

Small-area Population Forecast in a Segregated City using Density-Functional Fluctuation Theory

Decisions regarding housing, transportation, and resource allocation would all benefit from accurate small-area population forecasts. While various tried-and-tested forecast methods exist at regional scales, developing an accurate neighborhood-scale forecast remains a challenge partly due to complex drivers of residential choice ranging from housing policies to social preferences and economic status that cumulatively cause drastic neighborhood-scale segregation. Here, we show how to forecast the dynamics of neighborhood-scale demographics by extending a novel statistical physics approach called Density-Functional Fluctuation Theory (DFFT) to multi-component time-dependent systems. In particular, this technique observes the fluctuations in neighborhood-scale demographics to extract effective drivers of segregation. As a demonstration, we simulate a segregated city using a Schelling-type segregation model, and found that DFFT accurately predicts how a city-scale demographic change trickles down to block scales. Should these results extend to actual human populations, DFFT could capitalize on the recent advances in demographic data collection and regional-scale forecasts to improve upon current small-area population forecasts.

physics.soc-ph↗

arXiv2021

Forecasting racial dynamics at the neighborhood scale using Density-functional Fluctuation Theory

Racial residential segregation is a defining and enduring feature of U.S. society, shaping inter-group relations, racial disparities in income and health, and access to high-quality public goods and services. The design of policies aimed at addressing these inequities would be better informed by descriptive models of segregation that are able to predict neighborhood scale racial sorting dynamics. While coarse regional population projections are widely accessible, small area population changes remain challenging to predict because granular data on migration is limited and mobility behaviors are driven by complex social and idiosyncratic dynamics. Consequently, to account for such drivers, it is necessary to develop methods that can extract effective descriptions of their impacts on population dynamics based solely on statistical analysis of available data. Here, we develop and validate a Density-Functional Fluctuation Theory (DFFT) that quantifies segregation using density-dependent functions extracted from population counts and uses these functions to accurately forecast how the racial/ethnic compositions of neighborhoods across the US are likely to change. Importantly, DFFT makes minimal assumptions about the nature of the underlying causes of segregation and is designed to quantify segregation for neighborhoods with different total populations in regions with different compositions. This quantification can be used to accurately forecast both average changes in neighborhood compositions and the likelihood of more drastic changes such as those associated with gentrification and neighborhood tipping. As such, DFFT provides a powerful framework for researchers and policy makers alike to better quantify and forecast neighborhood-scale segregation and its associated dynamics.

physics.soc-ph↗

arXiv2020

$\textit{Ab Initio}$ Mismatched Interface Theory of Graphene on $α$-RuCl$_3$: Doping and Magnetism

Recent developments in twisted and lattice-mismatched bilayers have revealed a rich phase space of van der Waals systems and generated excitement. Among these systems are heterobilayers which can offer new opportunities to control van der Waals systems with strong in plane correlations such as spin-orbit-assisted Mott insulator $α$-RuCl$_3$. Nevertheless, a theoretical $\textit{ab initio}$ framework for mismatched heterobilayers without even approximate periodicity is sorely lacking. We propose a general strategy for calculating electronic properties of such systems, mismatched interface theory (MINT), and apply it to the graphene/$α$-RuCl$_{3}$ (GR/$α$-RuCl$_{3}$) heterostructure. Using MINT, we predict uniform doping of 4.77$\%$ from graphene to $α$-RuCl$_3$ and magnetic interactions in $α$-RuCl$_3$ to shift the system toward the Kitaev point. Hence we demonstrate that MINT can guide targeted materialization of desired model systems and discuss recent experiments on GR/$α$-RuCl$_{3}$ heterostructures.

cond-mat.str-el↗

arXiv2017

Grand canonical electronic density-functional theory: algorithms and applications to electrochemistry

First-principles calculations combining density-functional theory and continuum solvation models enable realistic theoretical modeling and design of electrochemical systems. When a reaction proceeds in such systems, the number of electrons in the portion of the system treated quantum mechanically changes continuously, with a balancing charge appearing in the continuum electrolyte. A grand-canonical ensemble of electrons at a chemical potential set by the electrode potential is therefore the ideal description of such systems that directly mimics the experimental condition. We present two distinct algorithms, a self-consistent field method (GC-SCF) and a direct variational free energy minimization method using auxiliary Hamiltonians (GC-AuxH), to solve the Kohn-Sham equations of electronic density-functional theory directly in the grand canonical ensemble at fixed potential. Both methods substantially improve performance compared to a sequence of conventional fixed-number calculations targeting the desired potential, with the GC-AuxH method additionally exhibiting reliable and smooth exponential convergence of the grand free energy. Finally, we apply grand-canonical DFT to the under-potential deposition of copper on platinum from chloride-containing electrolytes and show that chloride desorption, not partial copper monolayer formation, is responsible for the second voltammetric peak.

physics.chem-ph↗

arXiv2010

Classical density-functional theory for water

We introduce a new computationally efficient and accurate classical density-functional theory for water and apply it to hydration of hard spheres and inert gas atoms. We find good agreement with molecular dynamics simulations for the hydration of hard spheres and promising agreement for the solvation of inert gas atoms in water. Finally, we explore the importance of the orientational ambiguity in state-of-the-art continuum theories of water, which are based on the molecular density only.

cond-mat.stat-mech↗

arXiv2004

A tunable carbon nanotube electromechanical oscillator

Nanoelectromechanical systems (NEMs) hold promise for a number of scientific and technological applications. In particular, NEMs oscillators have been proposed for use in ultrasensitive mass detection, radio-frequency signal processing, and as a model system for exploring quantum phenomena in macroscopic systems. Perhaps the ultimate material for these applications is a carbon nanotube. They are the stiffest material known, have low density, ultrasmall cross-sections and can be defect-free. Equally important, a nanotube can act as a transistor and thus may be able to sense its own motion. In spite of this great promise, a room-temperature, self-detecting nanotube oscillator has not been realized, although some progress has been made. Here we report the electrical actuation and detection of the guitar-string-like oscillation modes of doubly clamped nanotube oscillators. We show that the resonance frequency can be widely tuned and that the devices can be used to transduce very small forces.

cond-mat.mes-hall↗