Biomedical subjects

Rajeev Ahuja

Publications and source records attributed to Rajeev Ahuja.

6 recordsLinked to original sources

PubMed2006

Structure of phase change materials for data storage.

Phase change materials based on chalcogenide alloys play an important role in optical and electrical memory devices. Both applications rely on the reversible phase transition of these alloys between amorphous and metastable cubic states. However, their atomic arrangements are not yet clear, which results in the unknown phase change mechanism of the utilization. Here using ab initio calculations we have determined the atomic arrangements. The results show that the metastable structure consists of special repeated units possessing rocksalt symmetry, whereas the so-called vacancy positions are highly ordered and layered and just result from the cubic symmetry. Finally, the fast and reversible phase change comes from the intrinsic similarity in the structures of the amorphous and metastable states.

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PubMed2004

New probe of the electronic structure of amorphous materials.

Here we show that electrochemical equilibrium voltage curves of amorphous WO3 and TiO2 coatings exhibit fine structure in striking agreement with the density of states in the conduction bands, as obtained by ab initio calculations for the crystalline counterparts. We suggest that localization of the band states is essential for observing the electronic structure. Our highly sensitive electrochemical method opens new vistas for studying the electronic structure of nonmetallic disordered materials that can be intercalated with an ionic species.

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PubMed2004

Structural phase transitions in heavy alkali metals under pressure.

We performed a theoretical study of the crystal structures of cesium and rubidium under high compressions. Our results confirm the recent high-pressure experimental observations of new complex crystal structures for the Cs III and Rb III phases. The calculated transition pressures agree extremely well with the measured data. Thus, it is now certain that the famous isostructural phase transition in cesium is actually a new crystallographic phase transition. A d-orbital occupation number of about 0.52 is crucial for the occurrence of these complex structures.

Cesium↗

PubMed2003

Ferromagnetism above room temperature in bulk and transparent thin films of Mn-doped ZnO.

The search for ferromagnetism above room temperature in dilute magnetic semiconductors has been intense in recent years. We report the first observations of ferromagnetism above room temperature for dilute (<4 at.%) Mn-doped ZnO. The Mn is found to carry an average magnetic moment of 0.16 mu(B) per ion. Our ab initio calculations find a valance state of Mn(2+) and that the magnetic moments are ordered ferromagnetically, consistent with the experimental findings. We have obtained room-temperature ferromagnetic ordering in bulk pellets, in transparent films 2-3 microm thick, and in the powder form of the same material. The unique feature of our sample preparation was the low-temperature processing. When standard high-temperature (T > 700 degrees C) methods were used, samples were found to exhibit clustering and were not ferromagnetic at room temperature. This capability to fabricate ferromagnetic Mn-doped ZnO semiconductors promises new spintronic devices as well as magneto-optic components.

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PubMed2003

Stability of the body-centred-cubic phase of iron in the Earth's inner core.

Iron is thought to be the main constituent of the Earth's core, and considerable efforts have therefore been made to understand its properties at high pressure and temperature. While these efforts have expanded our knowledge of the iron phase diagram, there remain some significant inconsistencies, the most notable being the difference between the 'low' and 'high' melting curves. Here we report the results of molecular dynamics simulations of iron based on embedded atom models fitted to the results of two implementations of density functional theory. We tested two model approximations and found that both point to the stability of the body-centred-cubic (b.c.c.) iron phase at high temperature and pressure. Our calculated melting curve is in agreement with the 'high' melting curve, but our calculated phase boundary between the hexagonal close packed (h.c.p.) and b.c.c. iron phases is in good agreement with the 'low' melting curve. We suggest that the h.c.p.-b.c.c. transition was previously misinterpreted as a melting transition, similar to the case of xenon, and that the b.c.c. phase of iron is the stable phase in the Earth's inner core.

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PubMed2002

Experimental evidence for sub-3-fs charge transfer from an aromatic adsorbate to a semiconductor.

The ultrafast timescale of electron transfer processes is crucial to their role in many biological systems and technological devices. In dye-sensitized solar cells, the electron transfer from photo-excited dye molecules to nanostructured semiconductor substrates needs to be sufficiently fast to compete effectively against loss processes and thus achieve high solar energy conversion efficiencies. Time-resolved laser techniques indicate an upper limit of 20 to 100 femtoseconds for the time needed to inject an electron from a dye into a semiconductor, which corresponds to the timescale on which competing processes such as charge redistribution and intramolecular thermalization of excited states occur. Here we use resonant photoemission spectroscopy, which has previously been used to monitor electron transfer in simple systems with an order-of-magnitude improvement in time resolution, to show that electron transfer from an aromatic adsorbate to a TiO(2) semiconductor surface can occur in less than 3 fs. These results directly confirm that electronic coupling of the aromatic molecule to its substrate is sufficiently strong to suppress competing processes.

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