Zsolt Jenei

Observation of fluorine-vacancy complexes in silicon

We show direct evidence, obtained by positron annihilation spectroscopy, for the complexing of fluorine with vacancies in silicon. Both float zone and Czochralski silicon wafers were implanted with ...

Role of intericosahedral chains on the hardness of sputtered boron carbide films

The relationship between the structure and mechanical properties of sputter-deposited boron carbide films was investigated. Changes in the structure induced by annealing were characterized in terms of chemical composition, chemical bonding, and concentrations of defects and trapped impurities. The creation of intericosahedral chains for higher annealing temperatures was revealed by infrared and Raman measurements, and the intensity of the infrared band at 1500 cm−1 was found to be related to the hardness. The presence of residual trapped Ar atoms and of open-volume defects is insensitive to relatively high annealing temperatures and does not influence the recovery of the hardness. Our results suggest postdeposition annealing as a pathway to enhance the mechanical properties of boron carbide films.

Melting and phase transitions of nitrogen under high pressures and temperatures

Dense nitrogen exhibits fascinating molecular and extended polymorphs as well as an anomalous melt maximum at high temperatures. However, the exact solid-liquid phase boundary is still the subject of debate, as both creating and probing hot dense nitrogen, solid and fluid alike, poses unique experimental challenges. Raman studies of nitrogen were performed to investigate the melting curve and solid-solid phase transitions in the pressure-temperature range of 25 to 103 GPa and 300 to 2000 K. The solid-liquid phase boundary has been probed with time-resolved Raman spectroscopy on ramp heated nitrogen in diamond anvil cell (DAC), showing a melting maximum at 73 GPa and 1690 K. The solid-solid phase boundaries have been measured with spatially resolved micro-confocal Raman spectroscopy on resistively heated DAC, probing the δ-ɛ phase line to 47 GPa and 914 K. At higher pressures the θ-phase was produced upon a repeated thermal heating of the ζ-phase, yet no evidence was found for the ι-phase. Hence, the present results signify the path dependence of dense nitrogen phases and provide new constraints for the phase diagram.

High-temperature experiments using a resistively heated high-pressure membrane diamond anvil cell.

We describe a reliable high performance resistive heating method developed for the membrane diamond anvil cell. This method generates homogenous high temperatures at high pressure in the whole sample for extended operation period. It relies on two mini coil heaters made of Pt-Rh alloy wire mounted around the diamond anvils and gasket, while temperature is monitored by two K-type thermocouples mounted near the sample. The sample, diamonds, and tungsten-carbide seats are thermally insulated from the piston and cylinder keeping the cell temperature below 750 K while the sample temperature is 1200 K. The cell with the heaters is placed in a vacuum oven to prevent oxidation and unnecessary heat loss. This assembly allows complete remote operation, ideally suited for experiments at synchrotron facilities. Capabilities of the setup are demonstrated for in situ Raman and synchrotron x-ray diffraction measurements. We show experimental measurements from isothermal compression at 900 K and 580 K to 100 GPa and 185 GPa, respectively, and quasi-isobaric compression at 95 GPa over 1000 K.

Pressure dependence of Ce valence in CeRhIn5

We have studied the Ce valence as a function of pressure in CeRhIn5 at 300 K and at 22 K using x-ray absorption spectroscopy in partial fluorescent yield mode. At room temperature, we found no detectable change in Ce valence greater than 0.01 up to a pressure of 5.5 GPa. At 22 K, the valence remains robust against pressure below 6GPa, in contrast to the predicted valence crossover at P=2.35 GPa. This work yields an upper limit for the change in Ce-valence and suggests that the critical valence fluctuation scenario, in its current form, is unlikely.We have studied the Ce valence as a function of pressure in CeRhIn5 at 300 K and at 22 K using x-ray absorption spectroscopy in partial fluorescent yield mode. At room temperature, we found no detectable change in Ce valence greater than 0.01 up to a pressure of 5.5 GPa. At 22 K, the valence remains robust against pressure below 6 GPa, in contrast to the predicted valence crossover at P  =  2.35 GPa. This work yields an upper limit for the change in Ce-valence and suggests that the critical valence fluctuation scenario, in its current form, is unlikely.

Superconducting Bi2Te: Pressure-induced universality in the (Bi2)m(Bi2Te3)n series

Using high-pressure magnetotransport techniques we have discovered superconductivity in Bi2Te, a member of the infinitely adaptive (Bi2)m(Bi2Te3)n series, whose end members, Bi and Bi2Te3, can be tuned to display topological surface states or superconductivity. Bi2Te has a maximum Tc = 8.6 K at P = 14.5 GPa and goes through multiple high pressure phase transitions, ultimately collapsing into a bcc structure that suggests a universal behavior across the series. High-pressure magnetoresistance and Hall measurements suggest a semi-metal to metal transition near 5.4 GPa, which accompanies the hexagonal to intermediate phase transition seen via x-ray diffraction measurements. In addition, the linearity of Hc2 (T) exceeds the Werthamer-Helfand-Hohenberg limit, even in the extreme spin-orbit scattering limit, yet is consistent with other strong spin-orbit materials. Furthermore, considering these results in combination with similar reports on strong spin-orbit scattering materials seen in the literature, we suggest the need for a new theory that can address the unconventional nature of their superconducting states.