Stavros Christopoulos

Change ΔS of the entropy in natural time under time reversal: Complexity measures upon change of scale

The entropy S in natural time as well as the entropy in natural time under time reversal have already found useful applications in the physics of complex systems, e.g., in the analysis of electrocardiograms (ECGs). Here, we focus on the complexity measures which result upon considering how the statistics of the time series changes upon varying the scale l. These scale-specific measures are ratios of the standard deviations and hence independent of the mean value and the standard deviation of the data. They focus on the different dynamics that appear on different scales. For this reason, they can be considered complementary to other standard measures of heart rate variability in ECG, like SDNN, as well as other complexity measures already defined in natural time. An application to the analysis of ECG —when solely using NN intervals— is presented: We show how can be used to separate ECG of healthy individuals from those suffering from congestive heart failure and sudden cardiac death.

Minima of the fluctuations of the order parameter of global seismicity

It has been recently shown [N. V. Sarlis, Phys. Rev. E 84, 022101 (2011) and N. V. Sarlis and S.-R. G. Christopoulos, Chaos 22, 023123 (2012)] that earthquakes of magnitude M greater or equal to 7 are globally correlated. Such correlations were identified by studying the variance κ1 of natural time which has been proposed as an order parameter for seismicity. Here, we study the fluctuations of this order parameter using the Global Centroid Moment Tensor catalog for a magnitude threshold Mthres = 5.0 and focus on its behavior before major earthquakes. Natural time analysis reveals that distinct minima of the fluctuations of the order parameter of seismicity appear within almost five and a half months on average before all major earthquakes of magnitude larger than 8.4. This phenomenon corroborates the recent finding [N. V. Sarlis et al., Proc. Natl. Acad. Sci. U.S.A. 110, 13734 (2013)] that similar minima of the seismicity order parameter fluctuations had preceded all major shallow earthquakes in Japan. Moreover, on the basis of these minima a statistically significant binary prediction method for earthquakes of magnitude larger than 8.4 with hit rate 100% and false alarm rate 6.67% is suggested.

VV and VO2 defects in silicon studied with hybrid density functional theory

The formation of VO (A-center), VV and VO2 defects in irradiated Czochralski-grown silicon (Si) is of technological importance. Recent theoretical studies have examined the formation and charge states of the A-center in detail. Here we use density functional theory employing hybrid functionals to analyze the formation of VV and VO2 defects. The formation energy as a function of the Fermi energy is calculated for all possible charge states. For the VV and VO2 defects double negatively charged and neutral states dominate, respectively.

Defect processes in Li2ZrO3: insights from atomistic modelling

Lithium zirconate (Li2ZrO3) is an important material that is considered as an anode in lithium-ion batteries and as a nuclear reactor breeder material. The intrinsic defect processes and doping can impact its material properties. In the present study we employ density functional theory calculations to calculate the defect processes and doping in Li2ZrO3. Here we show that the lithium Frenkel is the dominant intrinsic defect process and that dopants substituting in the zirconium site strongly associate with oxygen vacancies. In particular, it is calculated that divalent dopants more strongly bind with oxygen vacancies, with trivalent dopants following in binding energies and even tetravalent dopands having significant binding energies. The results are discussed in view of the application of Li2ZrO3 in energy applications.

Controlling A-center concentration in silicon through isovalent doping: mass action analysis

Abstract It has been determined experimentally that doping silicon with large isovalent dopants such as tin can limit the concentration of vacancy-oxygen defects, this in turn, can be deleterious for the materials properties and its application. These results have been supported by recent calculations based on density functional theory employing hybrid functional. In the present study, we employ mass action analysis to calculate the impact of germanium, tin and lead doping on the relative concentrations of vacancy-oxygen defects and defect clusters in silicon under equilibrium conditions. In particular, we calculate how much isovalent doping is required to constrain vacancy-oxygen concentration in silicon and conclude that Sn and Pb doping are the most effective isovalent dopants. The results are discussed in view of recent experimental and computational results.

A roadmap of strain in doped anatase TiO 2

Anatase titanium oxide is important for its high chemical stability and photocatalytic properties, however, the latter are plagued by its large band gap that limits its activity to only a small percentage of the solar spectrum. In that respect, straining the material can reduce its band gap increasing the photocatalytic activity of titanium oxide. We apply density functional theory with the introduction of the Hubbard + U model, to investigate the impact of stress on the electronic structure of anatase in conjunction with defect engineering by intrinsic defects (oxygen/titanium vacancies and interstitials), metallic dopants (iron, chromium) and non-metallic dopants (carbon, nitrogen). Here we show that both biaxial and uniaxial strain can reduce the band gap of undoped anatase with the use of biaxial strain being marginally more beneficial reducing the band gap up to 2.96 eV at a tensile stress of 8 GPa. Biaxial tensile stress in parallel with doping results in reduction of the band gap but also in the introduction of states deep inside the band gap mainly for interstitially doped anatase. Dopants in substitutional positions show reduced deep level traps. Chromium-doped anatase at a tensile stress of 8 GPa shows the most significant reduction of the band gap as the band gap reaches 2.4 eV.

Isovalent doping and the CiOi defect in germanium

Oxygen–carbon defects have been studied for decades in silicon but are less well established in germanium. In the present study we employ density functional theory calculations to study the structure of the CiOi defect in germanium. Additionally, we investigate the interaction the CiOi defect with isovalent dopants such as silicon and tin. It is calculated that the CiOi defects will preferentially form near isovalent dopants in germanium. Interestingly the structure of the dopant-CiOi defects is different with the Sn residing next to the Oi whereas the Si atom bonds with the Ci. The differences in the structure of CiOi defects in the vicinity of isovalent dopants are discussed.

Intrinsic defect processes and elastic properties of Ti3AC2 (A = Al, Si, Ga, Ge, In, Sn) MAX phases

Mn+1AXn phases (M = early transition metal; A = group 13–16 element and X = C or N) have a combination of advantageous metallic and ceramic properties, and are being considered for structural applications particularly where high thermal conductivity and operating temperature are the primary drivers: for example in nuclear fuel cladding. Here, we employ density functional theory calculations to investigate the intrinsic defect processes and mechanical behaviour of a range of Ti3AC2 phases (A = Al, Si, Ga, Ge, In, Sn). Based on the intrinsic defect reaction, it is calculated that Ti3SnC2 is the more radiation-tolerant 312 MAX phase considered herein. In this material, the C Frenkel reaction is the lowest energy intrinsic defect mechanism with 5.50 eV. When considering the elastic properties of the aforementioned MAX phases, Ti3SiC2 is the hardest and Ti3SnC2 is the softest. All the MAX phases considered here are non-central force solids and brittle in nature. Ti3SiC2 is elastically more anisotropic and Ti3A...

Relative concentrations of carbon related defects in silicon

Irradiation induced defects in silicon are technologically important as they impact the electronic properties. Calculations based on density functional theory employing hybrid functionals have been previously used to investigate the structures and relative energies of defect clusters formed between vacancies, self-interstitials, carbon and oxygen atoms in silicon. In this study we employ a model to calculate the relative concentrations of carbon related defects in silicon. It is calculated that the carbon content has a significant impact upon the concentration of carbon-related defects. The CiCs defect is the most populous for all the conditions considered followed by the CiOiSiI and the CiOi defects. CiOiSiI and the CiOi become increasingly important for silicon with high carbon concentrations.