High Curie temperatures in (Ga,Mn)N from Mn clustering

The effect of microscopic Mn cluster distribution on the Curie temperature (Tc) is studied using density-functional calculations. We find that the calculated Tc depends crucially on the microscopic cluster distribution, which can explain the abnormally large variations in experimental Tc values from a few K to well above room temperature. The partially dimerized Mn_2-Mn_1 distribution is found to give the highest Tc > 500 K, and in general, the presence of the Mn_2 dimer has a tendency to enhance Tc. The lowest Tc values close to zero are obtained for the Mn_4-Mn_1 and Mn_4-Mn_3 distributions.Comment: To appear in Applied Phyiscs Letter

Sedimentation of binary mixtures of like- and oppositely charged colloids: the primitive model or effective pair potentials?

We study sedimentation equilibrium of low-salt suspensions of binary mixtures of charged colloids, both by Monte Carlo simulations of an effective colloids-only system and by Poisson-Boltzmann theory of a colloid-ion mixture. We show that the theoretically predicted lifting and layering effect, which involves the entropy of the screening ions and a spontaneous macroscopic electric field [J. Zwanikken and R. van Roij, Europhys. Lett. {\bf 71}, 480 (2005)], can also be understood on the basis of an effective colloid-only system with pairwise screened-Coulomb interactions. We consider, by theory and by simulation, both repelling like-charged colloids and attracting oppositely charged colloids, and we find a re-entrant lifting and layering phenomenon when the charge ratio of the colloids varies from large positive through zero to large negative values

Hydrophobicity within the three-dimensional Mercedes-Benz model: Potential of mean force

We use the three-dimensional Mercedes-Benz model for water and Monte Carlo simulations to study the structure and thermodynamics of the hydrophobic interaction. Radial distribution functions are used to classify different cases of the interaction, namely, contact configurations, solvent separated configurations, and desolvation configurations. The temperature dependence of these cases is shown to be in qualitative agreement with atomistic models of water. In particular, while the energy for the formation of contact configurations is favored by entropy, its strengthening with increasing temperature is accounted for by enthalpy. This is consistent with our simulated heat capacity. An important feature of the model is that it can be used to account for well-converged thermodynamics quantities, e.g., the heat capacity of transfer. Microscopic mechanisms for the temperature dependence of the hydrophobic interaction are discussed at the molecular level based on the conceptual simplicity of the model.Peer reviewe

First-principles calculations of Cu adsorption on an H-terminated Si surface

In this study, we use first-principles simulations to study the adsorption of copper onto H-terminated and partially OH-terminated silicon surfaces. We show that, in contrast to previous studies, copper adsorbs strongly to the H-terminated silicon surface and that the adsorption energy is significantly dependent on the local bonding environment. The addition of a hydroxide group increases the average adsorption energy while reducing the range of adsorption energies due to the strong interaction between copper and oxygen. Our results predict that copper will generally prefer to adsorb at dihydride sites on the surface, agreeing with experimental studies of copper nucleation. The adsorption energy hierarchy predicted by the calculations strongly supports the suggestion that copper acts as a micromask in wet chemical etching, blocking reactive sites.Peer reviewe

AFM tip characterization by Kelvin probe force microscopy

Reliable determination of the surface potential with spatial resolution is key for understanding complex interfaces that range from nanostructured surfaces to molecular systems to biological membranes. In this context, Kelvin probe force microscopy (KPFM) has become the atomic force microscope (AFM) method of choice for mapping the local electrostatic surface potential as it changes laterally due to variations in the surface work function or surface charge distribution. For reliable KPFM measurements, the influence of the tip on the measured electrostatic surface potential has to be understood. We show here that the mean Kelvin voltage can be used for a straightforward characterization of the electrostatic signature of neutral, charged and polar tips, the starting point for quantitative measurements and for tip-charge control for AFM manipulation experiments. This is proven on thin MgO(001) islands supported on Ag(001) and is supported by theoretical modeling, which shows that single ions or dipoles at the tip apex dominate the mean Kelvin voltage.Peer reviewe

Angle dependent molecular dynamics simulation of flux pinning in YBCO superconductors with artificial pinning sites

A molecular dynamics (MD) simulation to simulate the vortices in superconductors with artificial pinning sites is presented. The simulation reproduces the correct anisotropic behavior in angular dependence of critical current. We also show that the shape of the J(c)(B) curve depends on the size of the pinning sites and the change from p = 0.5 to p approximate to 1 is due to the breaking of the vortex lattice to individually acting vortices. The results beautifully correspond to experimental data. Furthermore, we found that the size and shape of the c-axis peak observed with columnar pinning sites in J(c)(theta) also depends on the size of the rods, larger pinning sites leading to wider peaks. The results obtained from the MD-simulation are similar to those of the much more computationally intensive Ginzburg-Landau simulations. Furthermore, the MD-simulations can provide insight to the vortex dynamics within the samples

GaAs:Mn nanowires grown by molecular beam epitaxy of (Ga,Mn)As at MnAs segregation conditions

GaAs:Mn nanowires were obtained on GaAs(001) and GaAs(111)B substrates by molecular beam epitaxial growth of (Ga,Mn)As at conditions leading to MnAs phase separation. Their density is proportional to the density of catalyzing MnAs nanoislands, which can be controlled by the Mn flux and/or the substrate temperature. Being rooted in the ferromagnetic semiconductor (Ga,Mn)As, the nanowires combine one-dimensional properties with the magnetic properties of (Ga,Mn)As and provide natural, self assembled structures for nanospintronics.Comment: 13 pages, 6 figure

An object oriented Python interface for atomistic simulations

Programmable simulation environments allow one to monitor and control calculations efficiently and automatically before, during, and after runtime. Environments directly accessible in a programming environment can be interfaced with powerful external analysis tools and extensions to enhance the functionality of the core program, and by incorporating a flexible object based structure, the environments make building and analysing computational setups intuitive. In this work, we present a classical atomistic force field with an interface written in Python language. The program is an extension for an existing object based atomistic simulation environment.&nbsp;</p

Clinical Value of 18F-fluorodeoxyglucose Positron Emission Tomography/Computed Tomography in Response Evaluation after Primary Treatment of Advanced Epithelial Ovarian Cancer

Aims: To prospectively evaluate the use of F-18-fluorodeoxyglucose positron emission tomography/computed tomography (F-18-FDG-PET/CT) in the definition of the treatment response after primary treatment of advanced epithelial ovarian cancer (EOC). Materials and methods: Forty-nine patients with advanced EOC had an F-18-FDG PET/CT scan before and after primary treatment. The treatment response was defined with the currently used radiological and serological Response Criteria in Solid Tumors (RECIST1.1/GCIC) criteria and the modified PET Response Criteria in Solid Tumors (PERCIST). The concordance of the two methods was analysed. If the patient had a complete response to primary treatment by conventional criteria, the end of treatment F-18-FDG PET/CT scan (etPET/CT) was not opened until retrospectively at the time of disease progression. The ability of etPET/CT to predict the time to disease recurrence was analysed. The recurrence patterns were observed with an F-18-FDG PET/CT at the first relapse. Results: The agreement of the RECIST1.1/GCIC and modified PERCIST criteria in defining the primary treatment response in the whole patient cohort was good (weighted kappa coefficient = 0.78 ). Of the complete responders (n = 28), 34% had metabolically active lesions present in the etPET/CT, most typically in the lymph nodes. The same anatomical sites tended to activate at disease relapse, but were seldom the only site of relapse. In patients with widespread intra-abdominal carsinosis at diagnosis, the definition of metabolic response was challenging due to problems in distinguishing the physiological FDG accumulation in the bowel loops from the residual tumour in the same area. The presence of metabolically active lesions in the etPET/CT did not predict earlier disease relapse in the complete responders. Conclusions: In the present study, etPET/CT revealed metabolically active lesions in complete responders after EOC primary therapy, but they were insignificant for the patient's prognosis. The current study does not favour routine use of F-18-FDG PET/CT after EOC primary treatment for complete responders. (C) 2018 The Royal College of Radiologists. Published by Elsevier Ltd. All rights reserved.Peer reviewe