Papers by Cristian Martinez
THz to Inspect Graphene and Thin Film Materials
2019 44th International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz), 2019
In this paper, we present a system that provides meso-scale characterization of thin film materia... more In this paper, we present a system that provides meso-scale characterization of thin film materials, covering the gap between nano-scale and macro-scale methods. Nano-scale methods are slow and cannot characterize large surfaces. Macroscale methods generate characterization that averages the magnitudes and, thus, cannot provide localized information. Our system works in reflection as opposed to state-of-the-art methods and provides mobility, carrier density, and conductance maps in the THz range. Moreover, it can be integrated with reactors and enables monitoring of the fabrication of materials in real-time, supporting, for instance, the production of graphene at industrial scale.
Physical Review D, 2021
We discuss various novel features of n(≥ 4)-dimensional spacetimes sourced by a massless (non-)ph... more We discuss various novel features of n(≥ 4)-dimensional spacetimes sourced by a massless (non-)phantom scalar field in general relativity. Assuming that the metric is a warped product of static two-dimensional Lorentzian spacetime and an (n − 2)-dimensional Einstein space K n−2 with curvature k = 0, ±1, and that the scalar field depends only on the radial variable, we present a complete classification of static solutions for both signs of kinetic term. Contrary to the case with a non-phantom scalar field, the Fisher solution is not unique, and there exist two additional metrics corresponding to the generalizations of the Ellis-Gibbons solution and the Ellis-Bronnikov solution. We explore the maximal extension of these solutions in detail by the analysis of null/spacelike geodesics and singularity. For the phantom Fisher and Ellis-Gibbons solutions, we find that there inevitably appear parallelly propagated (p.p) curvature singularities in the parameter region where there are no scalar curvature singularities. Interestingly, the areal radius blows up at these p.p curvature singularities, which are nevertheless accessible within a finite affine time along the radial null geodesics. It follows that only the Ellis-Bronnikov solution describes a regular wormhole in the two-sided asymptotically flat spacetime. Using the general transformation relating the Einstein and Jordan frames, we also present a complete classification of solutions with the same symmetry coupled to a conformal scalar field. Additionally, by solving the field equations in the Jordan frame, we prove that this classification is genuinely complete.
Anales de la Facultad de Medicina, Nov 30, 2017
Resumen Introducción. En un estudio previo se realizó una reducción a 16 horas en la evaluación d... more Resumen Introducción. En un estudio previo se realizó una reducción a 16 horas en la evaluación de la ley exponencial de la dinámica cardiaca caótica, mostrando su efectividad en la caracterización de enfermedad y normalidad. Objetivo. Confirmar la aplicabilidad clínica de la ley matemática exponencial para evaluar la dinámica cardiaca caótica a partir de los registros Holter en 16 horas, observando su utilidad diagnóstica al disminuir su tiempo de evaluación. Diseño. Estudio observacional de corte trasversal donde se avaluó los parámetros electrocardiográficos mediante metodologías físico matemáticas inductivas con una confirmación estadística. Metodología. Se tomaron 100 registros Holter con diferentes tipos de patología, y 40 Holter que fueron diagnosticados como normales. Para cada Holter se construyó un atractor caótico, y midiendo sus espacios de ocupación y dimensión fractal se aplicó la evaluación matemática para diferenciar normalidad de enfermedad. Finalmente se realizaron medidas de concordancia diagnostica respecto al estándar de oro. Resultados. La ocupación espacial de todos los atractores estuvieron dentro de los valores esperados; los registros normales presentaron en la rejilla Kp valores entre 205 y 423. Para los registros con enfermedad aguda, estos valores oscilaron entre 21 y 65; y para los registros de enfermedad crónica estos valores estuvieron entre 104 y 195. Los valores de sensibilidad y especificidad fueron de 100% y el coeficiente Kappa fue de 1. Conclusión. El presente estudio muestra la aplicabilidad clínica de esta metodología para la evaluación en 16 horas de registros electrocardiográficos continuos o Holter.
ABCD. Arquivos Brasileiros de Cirurgia Digestiva (São Paulo), 2019
Background: Laparoscopic gastrectomy has numerous perioperative advantages, but the long-term sur... more Background: Laparoscopic gastrectomy has numerous perioperative advantages, but the long-term survival of patients after this procedure has been less studied. Aim: To compare survival, oncologic and perioperative outcomes between completely laparoscopic vs. open gastrectomy for early gastric cancer. Methods: This study was retrospective, and our main outcomes were the overall and disease-specific 5-year survival, lymph node count and R0 resection rate. Our secondary outcome was postoperative morbidity. Results: Were included 116 patients (59% men, age 68 years, comorbidities 73%, BMI 25) who underwent 50 laparoscopic gastrectomies and 66 open gastrectomies. The demographic characteristics, tumour location, type of surgery, extent of lymph node dissection and stage did not significantly differ between groups. The overall complication rate was similar in both groups (40% vs. 28%, p=ns), and complications graded at least Clavien 2 (36% vs. 18%, p=0.03), respiratory (9% vs. 0%, p=0.03) and wound-abdominal wall complications (12% vs. 0%, p=0.009) were significantly lower after laparoscopic gastrectomy. The lymph node count (21 vs. 23 nodes; p=ns) and R0 resection rate (100% vs. 96%; p=ns) did not significantly differ between groups. The 5-year overall survival (84% vs. 87%, p=0.31) and diseasespecific survival (93% vs. 98%, p=0.20) did not significantly differ between the laparoscopic and open gastrectomy groups. Conclusion: The results of this study support similar oncologic outcome and long-term survival for patients with early gastric cancer after laparoscopic gastrectomy and open gastrectomy. In addition, the laparoscopic approach is associated with less severe morbidity and a lower occurrence of respiratory and wound-abdominal wall complications. RESUMO-Racional: A gastrectomia laparoscópica tem numerosas vantagens perioperatórias, mas a sobrevivência em longo prazo após este procedimento tem sido menos estudada. Objetivo: Comparar resultados de sobrevivência, oncológica e perioperatória entre a gastrectomia completamente laparoscópica vs. aberta para câncer gástrico precoce. Método: Este estudo foi retrospectivo e os principais resultados foram a sobrevivência global e específica de cinco anos, contagem de linfonodos e taxa de ressecção R0. Resultado secundário foi a morbidade pós-operatória. Resultados: Foram incluídos 116 pacientes (59% homens, idade 68 anos, comorbidades 73%, IMC 25) que foram submetidos a 50 gastrectomias laparoscópicas e 66 gastrectomias abertas. As características demográficas, a localização do tumor, o tipo de operação, a extensão da dissecção dos linfonodos e do estágio não diferiram significativamente entre os grupos. A taxa geral de complicações foi semelhante em ambos os grupos (40% vs. 28%, p=ns) e complicações classificadas Clavien 2 (36% vs. 18%, p=0,03), respiratórias (9% vs. 0%, p=0,03) e as da parede abdominal (12% vs. 0%, p=0,009) foram significativamente menores após a gastrectomia laparoscópica. A contagem de linfonodos (21 contra 23, p=ns) e a taxa de ressecção R0 (100% vs. 96%; p=ns) não diferiram significativamente entre os grupos. A sobrevida global de cinco anos (84% vs. 87%, p=0,31) e a sobrevida específica (93% vs. 98%, p=0,20) não diferiram significativamente entre os grupos de gastrectomia laparoscópica e aberta. Conclusão: Estes resultados suportam resultados oncológicos similares e sobrevida em longo prazo para pacientes com câncer gástrico precoce após gastrectomia laparoscópica e gastrectomia aberta. Além disso, a abordagem laparoscópica está associada com morbidade menos grave e menor ocorrência de complicações respiratórias e da parede abdominal.
Classical and Quantum Gravity, 2019
We present higher-dimensional generalizations of the Buchdahl and Janis-Robinson-Winicour transfo... more We present higher-dimensional generalizations of the Buchdahl and Janis-Robinson-Winicour transformations which generate static solutions in the Einstein-Maxwell system with a massless scalar field. While the former adds a nontrivial scalar field to a vacuum solution, the latter generates a charged solution from a neutral one with the same scalar field. Applying these transformations to (i) a static solution with an Einstein base manifold, (ii) a multi-center solution, and (iii) a four-dimensional cylindrically symmetric solution, we construct several new exact solutions.
Relationship Between Physical Performance and Anthropometry in College Athletes Depending on Sex
PONTE International Scientific Researchs Journal, 2017
El objetivo de este trabajo es presentar un modelo computacional para "asignar en forma óptima" a... more El objetivo de este trabajo es presentar un modelo computacional para "asignar en forma óptima" aulas del Campus Universitario a las asignaturas que las requieran. Se plantea un modelo matemático de asignación equilibrado y un algoritmo heurístico basado en Simulated Annealing codificado en PERL y usando MySQL como motor de base de datos. Para cada día de la semana (d) el horario de clase disponible, se divide en 14 franjas de 1 hora cada una, se determina la franja horaria (k) más requerida, y es ella la que se optimiza, asignando también la misma aula si la comisión (teoría o práctica) tiene el mismo horario otro día de la semana. A partir de esa franja (k) óptima, se continúa optimizando las franjas vecinas (k-1) y (k+1), hasta completar todo el horario requerido para ese día. Se realiza el mismo procedimiento para los otros días de la semana. Los resultados obtenidos muestran la conveniencia del modelo y del uso de esta técnica heurística para resolver problemas de asignación y en particular el problema en estudio, ya que el porcentaje de sillas vacías o cantidad de alumnos sin bancos fue notoriamente inferior respecto a la asignación manual.
Physical Review D, 2015
The general stationary cylindrically symmetric solution of Einstein-massless scalar field system ... more The general stationary cylindrically symmetric solution of Einstein-massless scalar field system with a non-positive cosmological constant is presented. It is shown that the general solution is characterized by four integration constants. Two of these essential parameters have a local meaning and characterize the gravitational field strength. The other two have a topological origin, as they define an improper coordinate transformation that provides the stationary solution from the static one. The Petrov scheme is considered to explore the effects of the scalar field on the algebraic classification of the solutions. In general, these spacetimes are of type I. However, the presence of the scalar field allows us to find a non-vacuum type O solution and a wider family of type D spacetimes, in comparison with the vacuum case. The mass and angular momentum of the solution are computed using the Regge-Teitelboim method in the case of a negative cosmological constant. In absence of a cosmological constant, the curvature singularities in the vacuum solutions can be removed by including a phantom scalar field, yielding non-trivial locally homogeneous spacetimes. These spacetimes are of particular interest, as they have all their curvature invariants constant.
Medical Science Monitor, 2014
This study was supported by Sahmyook University Background: Treadmill gait analysis was more adva... more This study was supported by Sahmyook University Background: Treadmill gait analysis was more advantageous than over-ground walking because it allowed continuous measurements of the gait parameters. The purpose of this study was to investigate the concurrent validity and the test-retest reliability of the OPTOGait photoelectric cell system against the treadmill-based gait analysis system by assessing spatio-temporal gait parameters. Material/Methods: Twenty-six stroke patients and 18 healthy adults were asked to walk on the treadmill at their preferred speed. The concurrent validity was assessed by comparing data obtained from the 2 systems, and the test-retest reliability was determined by comparing data obtained from the 1 st and the 2 nd session of the OPTOGait system. Results: The concurrent validity, identified by the intra-class correlation coefficients (ICC [2, 1]), coefficients of variation (CV ME), and 95% limits of agreement (LOA) for the spatial-temporal gait parameters, were excellent but the temporal parameters expressed as a percentage of the gait cycle were poor. The test-retest reliability of the OPTOGait System, identified by ICC (3, 1), CV ME , 95% LOA, standard error of measurement (SEM), and minimum detectable change (MDC 95 %) for the spatio-temporal gait parameters, was high. Conclusions: These findings indicated that the treadmill-based OPTOGait System had strong concurrent validity and test-retest reliability. This portable system could be useful for clinical assessments.
Physics Letters B, 2008
A Kallen-Lehman approach to 3D Ising model is analyzed numerically both at low and high temperatu... more A Kallen-Lehman approach to 3D Ising model is analyzed numerically both at low and high temperature. It is shown that, even assuming a minimal duality breaking, one can fix three parameters of the model to get a very good agreement with the MonteCarlo results at high temperatures. With the same parameters the agreement is satisfactory both at low and near critical temperatures. How to improve the agreement with Monte-Carlo results by introducing a more general duality breaking is shortly discussed.
The Effects of Interday Rest on Adaptation to 6 Weeks of Plyometric Training in Young Soccer Players
Journal of Strength and Conditioning Research, 2015
International Transactions in Operational Research, 2004
The Chilean State delivers essential meal services at schools for low‐income students. Junta Naci... more The Chilean State delivers essential meal services at schools for low‐income students. Junta Nacional de Auxilio Escolar y Becas, the institution in charge of covering 1,300,000 children, leases the meal service to private enterprises. We developed an integer linear programming model to assign the meal contracts, in a process known as combinatorial auctions. The resulting model, which is NP‐hard, led to significant improvements in efficiency and also contributed to making the process more transparent. The results are apparent in substantial improvements in quality and coverage of the service, and important savings to the country, which are equivalent to feeding 300,000 children in addition. We developed techniques to solve the combinatorial models and also to analyze and compare multiple scenarios to find robust solutions. For the objective function of this problem, we analyzed several options to consider different kinds of social benefits. In this paper, we describe the problem, th...
Conductancia diferencial y estados cuasiligados de un contacto sobre una interfase metal normal-superconductor
Revista Colombiana de …, 2009
Page 1. Revista Colombiana de Física, vol. 41, No. 2, Abril 2009 1 Conductancia diferencial y est... more Page 1. Revista Colombiana de Física, vol. 41, No. 2, Abril 2009 1 Conductancia diferencial y estados cuasiligados de un contacto sobre una interfase metal normal-superconductor Differential conductance and quasibound ...
Annals of Physics, 2007
We examine anti-de Sitter gravity minimally coupled to a selfinteracting scalar field in D ≥ 4 di... more We examine anti-de Sitter gravity minimally coupled to a selfinteracting scalar field in D ≥ 4 dimensions when the mass of the scalar field is in the range m 2 * ≤ m 2 < m 2 * + l −2. Here, l is the AdS radius, and m 2 * is the Breitenlohner-Freedman mass. We show that even though the scalar field generically has a slow fall-off at infinity which back reacts on the metric so as to modify its standard asymptotic behavior, one can still formulate asymptotic conditions (i) that are anti-de Sitter invariant; and (ii) that allows the construction of well-defined and finite Hamiltonian generators for all elements of the anti-de Sitter algebra. This requires imposing a functional relationship on the coefficients a, b that control the two independent terms in the asymptotic expansion of the scalar field. The anti-de Sitter
Physical Review D, 2010
Recently, the asymptotic behaviour of three-dimensional anti-de Sitter gravity with a topological... more Recently, the asymptotic behaviour of three-dimensional anti-de Sitter gravity with a topological mass term was investigated. Boundary conditions were given that were asymptotically invariant under the two-dimensional conformal group and that included a fall-off of the metric sufficiently slow to consistently allow pp-wave type of solutions. Now, pp-waves can have two different chiralities. Above the chiral point and at the chiral point, however, only one chirality can be considered, namely the chirality that has the milder behaviour at infinity. The other chirality blows up faster than AdS and does not define an asymptotically AdS spacetime. By contrast, both chiralities are subdominant with respect to the asymptotic behaviour of AdS spacetime below the chiral point. Nevertheless, the boundary conditions given in the earlier treatment only included one of the two chiralities (which could be either one) at a time. We investigate in this paper whether one can generalize these boundary conditions in order to consider simultaneously both chiralities below the chiral point. We show that this is not possible if one wants to keep the two-dimensional conformal group as asymptotic symmetry group. Hence, the boundary conditions given in the earlier treatment appear to be the best possible ones compatible with conformal symmetry. In the course of our investigations, we provide general formulas controlling the asymptotic charges for all values of the topological mass (not just below the chiral point).
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Papers by Cristian Martinez