25 research outputs found
Prediction of a novel monoclinic carbon allotrope
A novel allotrope of carbon with symmetry was identified during an
\emph{ab-initio} minima-hopping structural search which we call -carbon.
This structure is predicted to be more stable than graphite at pressures above
14.4 GPa and consists purely of bonds. It has a high bulk modulus and is
almost as hard as diamond. A comparison of the simulated X-ray diffraction
pattern shows a good agreement with experimental results from cold compressed
graphite.Comment: 3 pages, 3 figure
Emergence of hidden phases of methylammonium lead-iodide (CHNHPbI) upon compression
We perform a thorough structural search with the minima hopping method (MHM)
to explore low-energy structures of methylammonium lead iodide. By combining
the MHM with a forcefield, we efficiently screen vast portions of the
configurational space with large simulation cells containing up to 96 atoms.
Our search reveals two structures of methylammonium iodide perovskite (MAPI)
that are substantially lower in energy than the well-studied experimentally
observed low-temperature orthorhombic phase according to density
functional calculations. Both structures have not yet been reported in the
literature for MAPI, but our results show that they could emerge as
thermodynamically stable phases via compression at low temperatures. In terms
of the electronic properties, the two phases exhibit larger band gaps than the
standard perovskite-type structures. Hence, pressure induced phase selection at
technologically achievable pressures (i.e., via thin-film strain) is a route
towards the synthesis of several MAPI polymorph with variable band gaps
Primeras etapas en el crecimiento de pelÃculas delgadas de teloruro de cadmio (CdTe) por sublimación en espacio cerrado
Elemental Phosphorus: structural and superconducting phase diagram under pressure
Pressure-induced superconductivity and structural phase transitions in
phosphorous (P) are studied by resistivity measurements under pressures up to
170 GPa and fully crystal structure and superconductivity
calculations up to 350 GPa. Two distinct superconducting transition temperature
(T) vs. pressure () trends at low pressure have been reported more
than 30 years ago, and for the first time we are able to reproduce them and
devise a consistent explanation founded on thermodynamically metastable phases
of black-phosphorous. Our experimental and theoretical results form a single,
consistent picture which not only provides a clear understanding of elemental P
under pressure but also sheds light on the long-standing and unsolved
superconductivity trend. Moreover, at higher pressures we predict a
similar scenario of multiple metastable structures which coexist beyond their
thermodynamical stability range. Metastable phases of P experimentally
accessible at pressures above 240 GPa should exhibit T's as high as 15 K,
i.e. three times larger than the predicted value for the ground-state crystal
structure. We observe that all the metastable structures systematically exhibit
larger transition temperatures than the ground-state ones, indicating that the
exploration of metastable phases represents a promising route to design
materials with improved superconducting properties.Comment: 14 pages, 4 figure
Enhancing the superconducting transition temperature of BaSi2 by structural tuning
We present a joint experimental and theoretical study of the superconducting
phase of the layered binary silicide BaSi2. Compared with the layered AlB2
structure of graphite or diboride-like superconductors, in the hexagonal
structure of binary silicides the sp3 arrangement of silicon atoms leads to
corrugated sheets. Through a high-pressure synthesis procedure we are able to
modify the buckling of these sheets, obtaining the enhancement of the
superconducting transition temperature from 4 K to 8.7 K when the silicon
planes flatten out. By performing ab-initio calculations based on density
functional theory we explain how the electronic and phononic properties of the
system are strongly affected by changes in the buckling. This mechanism is
likely present in other intercalated layered superconductors, opening the way
to the tuning of superconductivity through the control of internal structural
parameters.Comment: Submitte
Rare-earth magnetic nitride perovskites
We propose perovskite nitrides with magnetic rare-earth metals as novel materials with a range of technological applications. These materials appear to be thermodynamically stable and, in spite of possessing different crystal structures and different atomic environments, they retain the magnetic moment of the corresponding elemental rare-earth metal. We find both magnetic metals and semiconductors, with a wide range of magnetic moments and some systems posses record high magnetic anisotropy energies. Further tuning of the electronic and magnetic properties can also be expected by doping with other rare-earths or by creating solid solutions. The synthesis of these exotic materials with unusual compositions would not only extend the accepted stability domain of perovskites, but also open the way for a series of applications enabled by their rich physics
A Perspective on Conventional High-Temperature Superconductors at High Pressure: Methods and Materials
Two hydrogen-rich materials, HS and LaH, synthesized at megabar
pressures, have revolutionized the field of condensed matter physics providing
the first glimpse to the solution of the hundred-year-old problem of room
temperature superconductivity. The mechanism underlying superconductivity in
these exceptional compounds is the conventional electron-phonon coupling. Here
we describe recent advances in experimental techniques, superconductivity
theory and first-principles computational methods which have made possible
these discoveries. This work aims to provide an up-to-date compendium of the
available results on superconducting hydrides and explain how the synergy of
different methodologies led to extraordinary discoveries in the field. Besides,
in an attempt to evidence empirical rules governing superconductivity in binary
hydrides under pressure, we discuss general trends in the electronic structure
and chemical bonding. The last part of the Review introduces possible
strategies to optimize pressure and transition temperatures in conventional
superconducting materials as well as future directions in theoretical,
computational and experimental research.Comment: 68 pages, 30 figures, Preprint submitted to Physics Report