G.M. Ribeiro

Unusual sequence of commensurate-incommensurate phase transitions observed in KLiSO4

Abstract An unusual structural phase transition sequence is observed in KLiSO4 by EPR: high temperature prototype phase -incommensurately modulated phase - commensurately modulated phase - incommensurately modulated phase - low temperature prototype phase (reentrant phase). The low temperature phase has the same symmetry C66 of the high temperature prototype phase. The main feature of the modulation are rotations of the sulphate tetrahedra, which were precisely determined in the commensurately modulated phase.

Hopping conduction and persistent photoconductivity in Cu2ZnSnS4 thin films

The temperature dependence of electrical conductivity and the photoconductivity of polycrystalline Cu2ZnSnS4 were investigated. It was found that at high temperatures the electrical conductivity was dominated by band conduction and nearest-neighbour hopping. However, at lower temperatures, both Mott variable-range hopping (VRH) and Efros?Shklovskii VRH were observed. The analysis of electrical transport showed high doping levels and a large compensation ratio, demonstrating large degree of disorder in Cu2ZnSnS4. Photoconductivity studies showed the presence of a persistent photoconductivity effect with decay time increasing with temperature, due to the presence of random local potential fluctuations in the Cu2ZnSnS4 thin film. These random local potential fluctuations cannot be attributed to grain boundaries but to the large disorder in Cu2ZnSnS4.

High-resolution parameter space of an experimental chaotic circuit.

A high-resolution codimension-two parameter space showing the abundance of complex periodic structures of an experimental chaotic circuit is reported. Such resolution was propitiated by the use of a 0.5 mV step dc voltage source as one of the control parameters. Those complex periodic structures organize themselves in a period-adding bifurcation cascade that accumulates in a chaotic region. Numerical investigations on the dynamical model were also carried out to corroborate several new features observed in the experimental high-resolution parameter space.

EPR study of the ferroelastic phase transition in CsLiSO4

Abstract EPR spectra of SO-3 ion-radical in X-ray irradiated CsLiSO4 single crystals were used for the study of the ferroelastic phase transition at Tc = 203.0 K. The splitting (ΔH) of the SO3 line in the low-temperature ferroelastic phase has been interpreted as proportional to the square of the order parameter. The splitting shows the temperature dependence ΔH ∞ (Tc − T) 1.01± 0.01

Competition between negative and positive photoconductivity in silicon planar‐doped GaAs

Photo‐Hall free electron concentrations were measured on molecular beam epitaxy grown silicon planar‐doped GaAs samples, with silicon nominal concentration ranging from 1.4×1012 to 8.8×1013 cm−2, as function of temperature. We found conclusive results showing competition between positive photoconductivity and negative persistent photoconductivity effects. At temperatures below a critical Tc, the negative photoconductivity effect is dominant, while above Tc the positive effect dominates. We also found some evidence that the positive effect is related to spatial charge separation and that the negative effect is related to the DX center.

EPR study of central peak fluctuations in an ADA-ADP mixed crystal☆

Abstract The spontaneous dynamic symmetry breaking in the high temperature phase EPR spectra of AsO 4- 4 centers in antiferroelectric NH 4 H 2 AsO 4 - - NH 4 H 2 PO 4 mixed crystals was studied as a function of the concentration of paramagnetic defects. The results are not compatible with a Halperin-Varma type “relaxing” defect model. The width of the central peak seen by EPR strongly decreases at lower temperatures.

The Order Parameter for the Incommensurate Transition in K2SeO4: an EPR Lineshape Study

The EPR lineshape of the SeO 4 - center was investigated in the normal-incommensurate transition of K 2 SeO 4 . The value β=0.33 ±0.03 was obtained for the critical exponent of the order parameter. A summary of the measured values of the critical exponent β found in the literature for structurally incommensurate systems is presented. The average value of those measurements, β=0.345 ±0.004, is in excellent agreement with the calculated value of β for the 3d XY model. The lineshape analysis seems also to indicate that phase solitons appear in a large region of the incommensurate phase.

EPR study of the T2+ center in the paraelectric and antiferroelectric phases of NH4H2PO4

Abstract The EPR T 2+ center in NH4H2PO4 was studied in the range of temperature 85K – 175K which includes the antiferroelectric transition. The center has the symmetry of NH+4 site, where it goes substitutionally, in both the paraelectric and antiferroelectric phases. In the antiferr o electric phase there is a charge compensations mechanism involving the repulsion of one of the neighbor protons in the hydrogen bond. A measurement of the splitting of the resonance lines for the field along the cristallographic direction a is presented as a function of temperature. This splitting is proportional to the order parameter.

EPR studies of AsO44- centres in the ferroelectric CsH2AsO4 crystal

Two chemically different species (Q1 and Q2) of AsO44- centres in the ferroelectric CsH2AsO4 crystal were studied by X- and Q-band EPR techniques. Strong evidence is presented that the anomalous lineshape is due to super-hyperfine interaction with one neighbouring Cs+ ion. The spin Hamiltonian parameters support the hypothesis of a local condensation of the proton-heavy ions coupled ferroelectric mode, which exists even above Tc. The Q2 centre has either a proton deficiency or a meta-stabilised high-energy Slater configuration of the four protons around it.

Temperature evaluation of dental implant surface irradiated with high-power diode laser

The prevalence of peri-implantitis and the absence of a standard approach for decontamination of the dental implant surface have led to searches for effective therapies. Since the source of diode lasers is portable, has reduced cost, and does not cause damage to the titanium surface of the implant, high-power diode lasers have been used for this purpose. The effect of laser irradiation on the implants is the elevation of the temperature surface. If this elevation exceeds 47 °C, the bone tissue is irreversibly damaged, so for a safety therapy, the laser parameters should be controlled. In this study, a diode laser of GaAsAl was used to irradiate titanium dental implants, for powers 1.32 to 2.64 W (real) or 2.00 to 4.00 W (nominal), in continuous/pulsed mode DC/AC, with exposure time of 5/10 s, with/without air flow for cooling. The elevation of the temperature was monitored in real time in two positions: cervical and apical. The best results for decontamination using a 968-nm diode laser were obtained for a power of 1.65 and 1.98 W (real) for 10 s, in DC or AC mode, with an air flow of 2.5 l/min. In our perspective in this article, we determine a suggested approach for decontamination of the dental implant surface using a 968-nm diode laser.