101 research outputs found
Transparent dense sodium
Under pressure, metals exhibit increasingly shorter interatomic distances.
Intuitively, this response is expected to be accompanied by an increase in the
widths of the valence and conduction bands and hence a more pronounced
free-electron-like behaviour. But at the densities that can now be achieved
experimentally, compression can be so substantial that core electrons overlap.
This effect dramatically alters electronic properties from those typically
associated with simple free-electron metals such as lithium and sodium, leading
in turn to structurally complex phases and superconductivity with a high
critical temperature. But the most intriguing prediction - that the seemingly
simple metals Li and Na will transform under pressure into insulating states,
owing to pairing of alkali atoms - has yet to be experimentally confirmed. Here
we report experimental observations of a pressure-induced transformation of Na
into an optically transparent phase at 200 GPa (corresponding to 5.0-fold
compression). Experimental and computational data identify the new phase as a
wide bandgap dielectric with a six-coordinated, highly distorted
double-hexagonal close-packed structure. We attribute the emergence of this
dense insulating state not to atom pairing, but to p-d hybridizations of
valence electrons and their repulsion by core electrons into the lattice
interstices. We expect that such insulating states may also form in other
elements and compounds when compression is sufficiently strong that atomic
cores start to overlap strongly.Comment: Published in Nature 458, 182-185 (2009
Ionic high-pressure form of elemental boron
Boron is an element of fascinating chemical complexity. Controversies have
shrouded this element since its discovery was announced in 1808: the new
'element' turned out to be a compound containing less than 60-70 percent of
boron, and it was not until 1909 that 99-percent pure boron was obtained. And
although we now know of at least 16 polymorphs, the stable phase of boron is
not yet experimentally established even at ambient conditions. Boron's
complexities arise from frustration: situated between metals and insulators in
the periodic table, boron has only three valence electrons, which would favour
metallicity, but they are sufficiently localized that insulating states emerge.
However, this subtle balance between metallic and insulating states is easily
shifted by pressure, temperature and impurities. Here we report the results of
high-pressure experiments and ab initio evolutionary crystal structure
predictions that explore the structural stability of boron under pressure and,
strikingly, reveal a partially ionic high-pressure boron phase. This new phase
is stable between 19 and 89 GPa, can be quenched to ambient conditions, and has
a hitherto unknown structure (space group Pnnm, 28 atoms in the unit cell)
consisting of icosahedral B12 clusters and B2 pairs in a NaCl-type arrangement.
We find that the ionicity of the phase affects its electronic bandgap, infrared
adsorption and dielectric constants, and that it arises from the different
electronic properties of the B2 pairs and B12 clusters and the resultant charge
transfer between them.Comment: Published in Nature 453, 863-867 (2009
Superconductivity at 36 K in beta-Fe1.01Se with the compression of the interlayer separation under pressure
In this letter, we report that the superconductivity transition temperature
in beta-Fe1.01Se increases from 8.5 to 36.7 K under applied pressure of 8.9
GPa. It then decreases at higher pressure. A dramatic change in volume is
observed at the same time Tc rises, due to a collapse of the separation between
the Fe2Se2 layers. A clear transition to a linear resistivity normal state is
seen on cooling at all pressures. No static magnetic ordering is observed for
the whole p-T phase diagram. We also report that at higher pressure (starting
around 7 GPa and completed at 38 GPa), Fe1.01Se transforms to a hexagonal
NiAs-type structure and displays non-magnetic, insulating behavior. The
inclusion of electron correlation in band structure caculations is necessary to
describe this behavior, signifying that such correlations are important in this
chemical system. Our results strongly support unconventional superconductivity
in beta-Fe1.01Se.Comment: 17 pages, 4 figure
Unexpectedly high pressure for molecular dissociation in liquid hydrogen by electronic simulation
The study of the high pressure phase diagram of hydrogen has continued with renewed effort for about one century as it remains a fundamental challenge for experimental and theoretical techniques. Here we employ an efficient molecular dynamics based on the quantum Monte Carlo method, which can describe accurately the electronic correlation and treat a large number of hydrogen atoms, allowing a realistic and reliable prediction of thermodynamic properties. We find that the molecular liquid phase is unexpectedly stable, and the transition towards a fully atomic liquid phase occurs at much higher pressure than previously believed. The old standing problem of low-temperature atomization is, therefore, still far from experimental reach
Superconductivity in diamond
We report the discovery of superconductivity in boron-doped diamond
synthesized at high pressure (8-9 GPa) and temperature (2,500-2,800 K).
Electrical resistivity, magnetic susceptibility, specific heat, and
field-dependent resistance measurements show that boron-doped diamond is a
bulk, type-II superconductor below the superconducting transition temperature
Tc=4 K; superconductivity survives in a magnetic field up to Hc2(0)=3.5 T. The
discovery of superconductivity in diamond-structured carbon suggests that Si
and Ge, which also form in the diamond structure, may similarly exhibit
superconductivity under the appropriate conditions.Comment: 13 pages, 4 figure
Experimental pressure-temperature phase diagram of boron: resolving the long-standing enigma
Boron, discovered as an element in 1808 and produced in pure form in 1909, has still remained the last elemental material, having stable natural isotopes, with the ground state crystal phase to be unknown. It has been a subject of long-standing controversy, if α-B or ÎČ-B is the thermodynamically stable phase at ambient pressure and temperature. In the present work this enigma has been resolved based on the α-B-to- ÎČ-B phase boundary line which we experimentally established in the pressure interval of âŒ4 GPa to 8 GPa and linearly extrapolated down to ambient pressure. In a series of high pressure high temperature experiments we synthesised single crystals of the three boron phases (α-B, ÎČ-B, and Îł-B) and provided evidence of higher thermodynamic stability of α-B. Our work opens a way for reproducible synthesis of α-boron, an optically transparent direct band gap semiconductor with very high hardness, thermal and chemical stability
Quantum simulation of low-temperature metallic liquid hydrogen
The melting temperature of solid hydrogen drops with pressure above ~65âGPa, suggesting that a liquid state might exist at low temperatures. It has also been suggested that this low-temperature liquid state might be non-molecular and metallic, although evidence for such behaviour is lacking. Here we report results for hydrogen at high pressures using ab initio methods, which include a description of the quantum motion of the protons. We determine the melting temperature as a function of pressure and find an atomic solid phase from 500 to 800âGPa, which melts at <200âK. Beyond this and up to 1,200âGPa, a metallic atomic liquid is stable at temperatures as low as 50âK. The quantum motion of the protons is critical to the low melting temperature reported, as simulations with classical nuclei lead to considerably higher melting temperatures of ~300âK across the entire pressure range considered
Unusually complex phase of dense nitrogen at extreme conditions
Nitrogen exhibits an exceptional polymorphism under extreme conditions, making it unique amongst the elemental diatomics and a valuable testing system for experiment-theory comparison. Despite attracting considerable attention, the structures of many high-pressure nitrogen phases still require unambiguous determination. Here, we report the structure of the elusive high-pressure high-temperature polymorph at 56âGPa and ambient temperature, determined by single crystal X-ray diffraction, and investigate its properties using ab initio simulations. We find that is characterised by an extraordinarily large unit cell containing 48 molecules. Geometry optimisation favours the experimentally determined structure and density functional theory calculations find to have the lowest enthalpy of the molecular nitrogen polymorphs that exist between 30 and 60âGPa. The results demonstrate that very complex structures, similar to those previously only observed in metallic elements, can become energetically favourable in molecular systems at extreme pressures and temperatures
Understanding Novel Superconductors with Ab Initio Calculations
This chapter gives an overview of the progress in the field of computational
superconductivity.
Following the MgB2 discovery (2001), there has been an impressive
acceleration in the development of methods based on Density Functional Theory
to compute the critical temperature and other physical properties of actual
superconductors from first-principles. State-of-the-art ab-initio methods have
reached predictive accuracy for conventional (phonon-mediated) superconductors,
and substantial progress is being made also for unconventional superconductors.
The aim of this chapter is to give an overview of the existing computational
methods for superconductivity, and present selected examples of material
discoveries that exemplify the main advancements.Comment: 38 pages, 10 figures, Contribution to Springer Handbook of Materials
Modellin
Band Gap Closure, Incommensurability and Molecular Dissociation of Dense Chlorine
Molecular systems are predicted to transform into atomic solids and be metallic at high pressure; this was observed for the diatomic elements iodine and bromine. Here the authors access the higher pressures needed to observe the dissociation in chlorine, through an incommensurate phase, and provide evidence for metallization
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