J. Valenta

Absence of quantum confinement effects in the photoluminescence of Si3N4–embedded Si nanocrystals

Superlattices of Si-rich silicon nitride and Si3N4 are prepared by plasma-enhanced chemical vapor deposition and, subsequently, annealed at 1150 °C to form size-controlled Si nanocrystals (Si NCs) embedded in amorphous Si3N4. Despite well defined structural properties, photoluminescence spectroscopy (PL) reveals inconsistencies with the typically applied model of quantum confined excitons in nitride-embedded Si NCs. Time-resolved PL measurements demonstrate 105 times faster time-constants than typical for the indirect band structure of Si NCs. Furthermore, a pure Si3N4 reference sample exhibits a similar PL peak as the Si NC samples. The origin of this luminescence is discussed in detail on the basis of radiative defects and Si3N4 band tail states in combination with optical absorption measurements. The apparent absence of PL from the Si NCs is explained conclusively using electron spin resonance data from the Si/Si3N4 interface defect literature. In addition, the role of Si3N4 valence band tail states as...

Quantum ferromagnet in the proximity of the tricritical point

Echoes of quantum phase transitions at finite temperatures are theoretically and experimentally challenging and unexplored topics. Particularly in metallic quantum ferromagnets the experimental investigations are hampered by an intricate preparation of sufficiently pure samples and the access to the proper coordinates in parameter space. The present study shows that it is possible to tune a specific system at easily accessible conditions to the vicinity of its quantum phase transition. The physics is demonstrated on Ru-doped UCoAl, driven by pressure or substitution to and across the tricritical point and follows the first-order transition line to the theoretically presumed quantum phase transition. These findings open the possibilities for further in-depth studies of classical and quantum critical phenomena at easily reachable conditions.Quantum phase transitions: Tuned in metallic ferromagnetsClean ferromagnetic systems are predicted to exhibit quantum phase transitions (QPTs) rather than critical points. QPTs happen at zero temperature due to quantum fluctuations between the phases, and can be triggered by non-thermal perturbations such as hydrostatic pressure, chemical composition or magnetic fields. Jan Prokleška at Czesh Charles University and colleagues from Czech Republic and Germany demonstrate that it is possible to tune the QPT of the metallic ferromagnet UCo1-xRuxAl by pressure or weak Ru doping. The experimental study of QPTs in metallic ferromagnets is typically hindered by the extreme conditions required to drive the system into the transition, or by the presence of additional phases such as superconductivity. Instead, UCo1-xRuxAl allows to get access to the QPT at easily accessible experimental conditions, opening the possibility of studying in detail quantum critical phenomena.

Antiferromagnetism and phase transitions in noncentrosymmetric UIrSi3

Magnetization and specific heat measurements on a UIrSi3 single crystal reveal Ising-like antiferromagnetism below T

Electrical resistivity across the tricriticality in itinerant ferromagnet

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Presence of parimagnetism in HoCo2 under hydrostatic pressure

= 41.7 K with easy magnetization direction along the c-axis of tetragonal structure. The antiferromagentic ordering is suppressed by magnetic fields > H

Slow luminescence and hole burning of porous silicon

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Time-resolved photoluminescence study of AgCl:Cd2+ solid state nuclear track detectors

({\mu}

Picosecond and millisecond dynamics of photoexcited carriers in porous silicon.

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Experimental study of magnetocaloric effect in the two-level quantum system KTm(MoO 4 ) 2

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Properties of the divalent-Yb compound YbAu2Si2 under extreme conditions

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