David Rivas Gongora

• Venegas, Roberto Mario; Gongora, David Rivas & Treml, Eric (2025). Global climate-driven sea surface temperature and chlorophyll dynamics. Marine Environmental Research. ISSN 0141-1136. 204. doi: 10.1016/j.marenvres.2024.106856. Fulltekst i vitenarkiv Vis sammendrag

  Herein we study long-term changes in global sea surface temperature (SST) and chlorophyll-a concentration (CHL) in order to evaluate possible effects of climate change on the global marine ecosystems. Our approach is to analyze multi-model ensemble-means from global numerical-simulations available through the Coupled Model Intercomparison Project Phase 6 (CMIP6). A 250-year span consisting of the 1850–2014 historical period and the 2015–2099 climate-change projection was considered, where the Shared Socioeconomic Pathways (SSPs) 2.45 and 5.85 were selected as the projected climate-change scenarios. In the historical period, global linear trends show an SST increasing at 0.0024 °C year-1 and a CHL decreasing at −2.35x10-5 mg m-3 year-1, but by the last years (1985–2014) these changes become more abrupt: SST rising at 0.0146 °C year-1 and CHL declining at −1.49x10-4 mg m-3 year-1. During the intense climate-change scenario (SSP-5.85), SST increases at 0.0341 °C year-1 and CHL decreases at −0.0002 mg m-3 year-1, but in the last years (2070–2099) the warming is stronger (0.045 °C year-1) and the CHL decline is weaker (−0.0001 mg m-3 year-1). Additionally, Empirical Orthogonal Function (EOF) and dual Self-Organizing Maps (SOM) analyses on the model-data ensembles show: 1) significant correlations between SST and CHL patterns and climate teleconnection indices, 2) contracting polar and high latitude seascapes, 3) rising SST range (both high and low temperatures), 4) declining CHL in warming tropical seascapes, and 5) global expansion of low CHL levels. On the other hand, recent (2022–2023) global observed-SST anomalies mirror end-of-century projections, with extreme anomalies in tropical and subtropical regions and significant changes in near-polar regions. Thus, our findings emphasize the need to curb fossil fuel emissions in order to avoid irreparable consequences for the marine environment.

• Tafra, Emil; Novosel, Nikolina; Skoko, Željko; Ivek, Tomislav; Basletić, Mario & Mihaljević, Branimir [Vis alle 15 forfattere av denne artikkelen] (2025). Colossal magnetoresistance effect and spin-dependent variable-range hopping in the charge ordered phase of overdoped (La,Ca)MnO3 manganites. Physical review B (PRB). ISSN 2469-9950. 111(11). doi: 10.1103/PhysRevB.111.115107. Fulltekst i vitenarkiv Vis sammendrag

  Manganese oxides or manganites, widely known for their colossal magnetoresistance (CMR) effect, are systems in which spin, charge, lattice, and orbital interactions are simultaneously active, resulting in a complex phase diagram. Considerable research done mostly at low doping suggested that the CMR effect is the manifestation of the interplay of these interactions that occur in the phase-separated state composed of coexisting clusters of ferromagnetic and paramagnetic and antiferromagnetic phases. In order to get deeper insight in the physics at high doping, we have undertaken magnetoresistance and magnetization measurements of La1−𝑥⁢Ca𝑥⁢MnO3 thin-film samples with 𝑥=0.5, 0.52, 0.58, 0.62, 0.66, and 0.75, in the temperature range 1.5–300 K and in magnetic fields up to 16 T. We have observed similar characteristics of the phase diagram as for low doping: the CMR effect, ferromagnetism, and even metallic behavior for certain temperature, magnetic field, and doping values, demonstrating that the phase separation into ferromagnetic and antiferromagnetic clusters is here also in play. We find that the charge transport in the charge ordered phase takes place via spin-dependent variable-range hopping (VRH), which points towards Anderson-type localization of conducting electrons in a spin-disordered background of Mn magnetic moments. We also find a close relationship between the VRH activation energy, the CMR effect, the fraction of ferromagnetic clusters, and the charge ordering temperature, which implies that, aside from the spin disorder, the electron-electron and Jahn-Teller interactions also play an important role in the charge localization.

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• Skaar, Lars Kristian Gretland; Vines, Lasse & Gongora, David Rivas (2023). Formation of W-, C- and G-centers in silicon; towards single photon emitters. Universitetet i Oslo. Fulltekst i vitenarkiv Vis sammendrag

  Quantum technology utilize the fundamental properties of quantum mechanics in devices and applications. The most known fields of quantum technology include quantum computing, quantum sensing and quantum communication. Today, the communication between the individual devices are limited by the ability to transfer quantum information across larger distances. A solution for this problem is the development of single-photon emitters (SPEs) and to use individual photons to transfer the information. One quantum technology platform for such SPEs are quantum defects in semiconducting materials, where the emission can also be used to read out the state of the quantum defect, or qubit. The recent discoveries of SPEs in silicon have spiked an interest of the material as a potential host for SPEs. In this research, PL and DLTS have been used to examine the W-, C- and G-center to create the centers in low concentrations to prepare for future single-photon measurements. Systematic proton implantation and post-implant annealing resulted in successful formation of the centers in both FZ and CZ grown samples. The centers showed varying PL intensity depending on the implantation parameters and annealing method. DLTS measurements on the C-center in epitaxially grown samples resulted in a defect concentration of ∼ 2E13 cm−3 for samples implanted with a proton energy of 1.8 MeV and fluence of 1E13 cm−2. This low concentration of defects is promising for future single-photon measurements on individual centers.

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