624 research outputs found
Back to square one for superfluidity
No full textSuperfluidity and superconductivity are striking examples of how quantum mechanics can affect the collective behaviour of large numbers of identical particles. When the temperature of a many-particle fluid is lowered below a critical threshold, thermal fluctuations are no longer able to prevent it from collapsing into a purely quantum state. If the particles are electrically neutral, like helium–3 and helium–4 atoms, such a 'coherent' state can flow without friction and is known as a superfluid. If, on the other hand, the particles are charged, the coherent state loses its resistance to electrical current and becomes a superconductor
Ab initio study of the LiH phase diagram at extreme pressures and temperatures
No full textThe effect of anharmonic vibrational contributions to the finite-temperature pressure-driven B1-B2 structural phase transition of LiH is studied by using the stochastic self-consistent harmonic approximation method in combination with ab initio density functional theory and the quasiharmonic approximation. Contrary to previous experimental results based on multiple-shock compression, we find that the B1-B2 transition pressure is not significantly reduced at high temperatures. Moreover, we find that the B2 phase is dynamically unstable at low temperatures within harmonic theory in a wide range of pressures where its enthalpy is lower than that of the B1 phase, and the inclusion of anharmonic effects stabilizes the B2 phase in this pressure range. Our results imply that a third, yet unknown phase must exist in the phase diagram of LiH, in addition to the B1 and B2 phases, in order to explain the shock compression result
Collective spin 1 singlet phase in high-pressure oxygen
No full textOxygen, one of the most common and important elements in nature, has an exceedingly well-explored phase diagram under pressure, up to and beyond 100 GPa. At low temperatures, the low-pressure antiferromagnetic phases below 8 GPa where O-2 molecules have spin S = 1 are followed by the broad apparently nonmagnetic epsilon phase from about 8 to 96 GPa. In this phase, which is our focus, molecules group structurally together to form quartets while switching, as believed by most, to spin S = 0. Here we present theoretical results strongly connecting with existing vibrational and optical evidence, showing that this is true only above 20 GPa, whereas the S = 1 molecular state survives up to about 20 GPa. The epsilon phase thus breaks up into two: a spinless epsilon(0) (20-96 GPa), and another epsilon(1) (8-20 GPa) where the molecules have S = 1 but possess only short-range antiferromagnetic correlations. A local spin liquid-like singlet ground state akin to some earlier proposals, and whose optical signature we identify in existing data, is proposed for this phase. Our proposed phase diagram thus has a first-order phase transition just above 20 GPa, extending at finite temperature and most likely terminating into a crossover with a critical point near 30 GPa and 200 K
Charge-density waves and surface Mott insulators for adlayer structures on semiconductors: Extended Hubbard modeling
No full textMotivated by the recent experimental evidence of commensurate surface charge-density waves (CDW) in Pb/Ge(lll) and Sn/Ge(lll) root 3-adlayer structures, as well as by the insulating states found on K/Si(lll):B and SiC(0001), we have investigated the role of electron-electron interactions, and also of electron-phonon coupling, on the narrow surface-state band originating from the outer dangling-bond orbitals of the surface. We model the root 3 dangling-bond lattice by an extended two-dimensional Hubbard model at half filling on a triangular lattice. The hopping integrals are calculated by fitting first-principle results for the surface band. We include an on-site Hubbard repulsion U and a nearest-neighbor Coulomb interaction V, plus a long-ranged Coulomb tail. The electron-phonon interaction is treated in the deformation potential approximation. We have explored the phase diagram of this model including the possibility of commensurate 3 X 3 phases, using mainly the Hartree-Fock approximation. For U larger than the bandwidth we find a noncollinear antiferromagnetic spin-density wave (SDW) insulator, possibly corresponding to the situation on the SiC and K/Si surfaces. For U comparable or smaller, a rich phase diagram arises, with several phases involving combinations of charge and spin-density-waves (SDW), with or without a net magnetization. We find that insulating, or partly metallic 3 X 3 CDW phases can be stabilized by two different physical mechanisms. One is the intersite repulsion V, which together with electron-phonon coupling can lower the energy of a charge modulation. The other is a magnetically-induced Fermi-surface nesting, stabilizing a net cell magnetization of 1/3, plus a collinear SDW, plus an associated weak CDW. Comparison with available experimental evidence, and also with first-principle calculations is made
Interchain electron states in polyethylene
No full textWe present a theoretical study of the nature of the lowest empty conduction-band states in crystalline polyethylene (PE), conducted through density-functional electronic structure calculations. Results reveal that the wave function of the conduction-band edge is of interchain character, as opposed to the intrachain char- acter of all the filled valence-band states. Thus, while a hole added to neutral PE will mainly belong to the PE chain backbone bonds, an added electron in PE will mostly reside between the chains, and far from the existing bonds. Moreover, the added electron state charge centroid is predicted to move further out from the chain backbone towards the low-density interstitial region, if and when the chains are pried apart. This suggests that injected electrons will naturally flow to low-density regions inside real PE, and that the experimentally estab- lished propensity of PE to expel electrons out of the bulk, should be directly related to the interchain nature of the conduction states
Surface charge density waves and the Mott insulators for sqrt(3)xsqrt(3) adlayers on (111) semiconductor surfaces
No full textWe discuss our recent studies on the low-temperature instabilities of the root3 x root3? phase of tetravalent adatoms on (111) semiconductor surfaces. The cases of interest include the surface charge density wave (CDW) systems Pb/Ge(1 1 1) and Sn/Ge(1 1 1), as well as the Mott insulators Si/SiC(0 0 0 1) and K/Si(1 1 1):B. We have approached the problem in two ways: first, by employing a one-band model Hamiltonian of the Hubbard-Holstein type, in order to understand general features of the phase diagram as a function of the strength of electron-electron (e-e) and electron-phonon (e-ph) interactions; second, by performing realistic ab initio calculations within the local spin density approximation (LSDA) for the case of Sn/Si(1 1 1), and of a hypothetical root3 x root3 Si/Si(1 1 1) mimicking K/Si(1 1 1):B. The collinear LSDA calculation for both Sn/Ge(1 1 1) and Si/Si(1 1 1) predicts a spin density wave (SDW) state with a uniform magnetization m(z) = 1/3 and a small secondary CDW. We discuss and stress the likely important role played by e-e interactions in explaining the phenomenology of all these systems, as opposed to the secondary role played by the e-ph coupling, which would at most drive the lattice, for Pb-Sn/Ge(1 1 1), after the electrons have caused the transition
International Editorial Advisory Board
No full textProfessor Sandro Scandolo, ItalyProfessor Alessandro Laio, ItalyProfessor Joel Koech, KenyaProfessor Jerome Nriagu, USADr. Arch. Barnabas Nawangwe, UgandaDr. Tomlins Keith, UKDr. Olaniran Fasina, USADr. (Mrs.) Prajeab Jammne, IndiaMr. V. Prakash, IndiaSubscription
International Editorial Advisory Board
No full textª¤? Professor Sandro Scandolo, Italyª¤? ª¤?Professor Alessandro Laio, Italyª¤? ª¤?Professor Joel Koech, Kenyaª¤? ª¤?Professor Jerome Nriagu, USAª¤? ª¤?Dr. Arch. Barnabas Nawangwe, Ugandaª¤? ª¤?Dr. Tomlins Keith, UKª¤? ª¤? Dr. Olaniran Fasina, USAª¤? Dr. (Mrs.) Prajeab Jammne, Indiaª¤? ª¤?Mr. V. Prakash, Indiaª¤?ª¤
Pressure Dependence of Hydrogen-Bond Dynamics in Liquid Water Probed by Ultrafast Infrared Spectroscopy
No full textClarifying the structure/dynamics relation of water hydrogen-bond network has been the aim of extensive research over many decades. By joining anvil cell high-pressure technology, femtosecond 2D infrared spectroscopy, and molecular dynamics simulations, we studied, for the first time, the spectral diffusion of the stretching frequency of an HOD impurity in liquid water as a function of pressure. Our experimental and simulation results concordantly demonstrate that the rate of spectral diffusion is almost insensitive to the applied pressure. This behavior is in contrast with the previously reported pressure-induced speed up of the orientational dynamics, which can be rationalized in terms of large angular jumps involving sudden switching between two hydrogen-bonded configurations. The different trend of the spectral diffusion can be, instead, inferred considering that the first solvation shell preserves the tetrahedral structure with pressure and the OD stretching frequency is only slight perturbed
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