30 research outputs found
Phase separation in a lattice model of a superconductor with pair hopping
We have studied the extended Hubbard model with pair hopping in the atomic
limit for arbitrary electron density and chemical potential. The Hamiltonian
considered consists of (i) the effective on-site interaction U and (ii) the
intersite charge exchange interactions I, determining the hopping of electron
pairs between nearest-neighbour sites. The model can be treated as a simple
effective model of a superconductor with very short coherence length in which
electrons are localized and only electron pairs have possibility of
transferring. The phase diagrams and thermodynamic properties of this model
have been determined within the variational approach, which treats the on-site
interaction term exactly and the intersite interactions within the mean-field
approximation. We have also obtained rigorous results for a linear chain (d=1)
in the ground state. Moreover, at T=0 some results derived within the random
phase approximation (and the spin-wave approximation) for d=2 and d=3 lattices
and within the low density expansions for d=3 lattices are presented. Our
investigation of the general case (as a function of the electron concentration
and as a function of the chemical potential) shows that, depending on the
values of interaction parameters, the system can exhibit not only the
homogeneous phases: superconducting (SS) and nonordered (NO), but also the
phase separated states (PS: SS-NO). The system considered exhibits interesting
multicritical behaviour including tricritical points.Comment: 15 pages, 9 figures; pdf-ReVTeX, final version, corrected typos;
submitted to Journal of Physics: Condensed Matte
The effects of the next-nearest-neighbour density-density interaction in the atomic limit of the extended Hubbard model
We have studied the extended Hubbard model in the atomic limit. The
Hamiltonian analyzed consists of the effective on-site interaction U and the
intersite density-density interactions Wij (both: nearest-neighbour and
next-nearest-neighbour). The model can be considered as a simple effective
model of charge ordered insulators. The phase diagrams and thermodynamic
properties of this system have been determined within the variational approach,
which treats the on-site interaction term exactly and the intersite
interactions within the mean-field approximation. Our investigation of the
general case taking into account for the first time the effects of
longer-ranged density-density interaction (repulsive and attractive) as well as
possible phase separations shows that, depending on the values of the
interaction parameters and the electron concentration, the system can exhibit
not only several homogeneous charge ordered (CO) phases, but also various phase
separated states (CO-CO and CO-nonordered). One finds that the model considered
exhibits very interesting multicritical behaviours and features, including
among others bicritical, tricritical, critical-end and isolated critical
points.Comment: 12 pages, 7 figures; final version, pdf-ReVTeX; corrected typos in
reference; submitted to Journal of Physics: Condensed Matte
Low-Temperature Phase of the CdReO Superconductor: Ab initio Phonon Calculations and Raman Scattering
Using an {\it ab initio} approach, we report a phonon soft mode in the
tetragonal structure described by the space group of the K
superconductor CdReO. It induces an orthorhombic distortion to a
crystal structure described by the space group which hosts the
superconducting state. This new phase has a lower total energy than the other
known crystal structures of CdReO. Comprehensive temperature
dependent Raman scattering experiments on isotope enriched samples,
CdReO, not only confirm the already known structural
phase transitions but also allow us to identify a new characteristic
temperature regime around K, below which the Raman spectra undergo
remarkable changes with the development of several sharp modes and mode
splitting. Together with the results of the \textit{ab initio} phonon
calculations we take these observations as strong evidence for another phase
transition to a novel low-temperature crystal structure of CdReO.Comment: 7 pages, 8 figure
New AMS 14C dates track the arrival and spread of broomcorn millet cultivation and agricultural change in prehistoric Europe
Broomcorn millet (Panicum miliaceum L.) is not one of the founder crops domesticated in Southwest Asia in the early Holocene, but was domesticated in northeast China by 6000 bc. In Europe, millet was reported in Early Neolithic contexts formed by 6000 bc, but recent radiocarbon dating of a dozen 'early' grains cast doubt on these claims. Archaeobotanical evidence reveals that millet was common in Europe from the 2nd millennium bc, when major societal and economic transformations took place in the Bronze Age. We conducted an extensive programme of AMS-dating of charred broomcorn millet grains from 75 prehistoric sites in Europe. Our Bayesian model reveals that millet cultivation began in Europe at the earliest during the sixteenth century bc, and spread rapidly during the fifteenth/fourteenth centuries bc. Broomcorn millet succeeds in exceptionally wide range of growing conditions and completes its lifecycle in less than three summer months. Offering an additional harvest and thus surplus food/fodder, it likely was a transformative innovation in European prehistoric agriculture previously based mainly on (winter) cropping of wheat and barley. We provide a new, high-resolution chronological framework for this key agricultural development that likely contributed to far-reaching changes in lifestyle in late 2nd millennium bc Europe
Phase Separations in the Narrow-Bandwidth Limit of the Penson-Kolb-Hubbard Model at Zero Temperature
In this work we study the ground state of the Penson-Kolb-Hubard model in the limit of narrow-bandwidth. We present phase diagrams of the model for fixed chemical potential and concentration (involving various phase separations). The results are derived within the Hartree-Fock approximation (HFA) in the narrow-bandwidth regime and compared with the exact ones in the atomic limit and the high-dimension regime. Our investigation reveals that the HFA can reconstruct the exact diagram at the ground state when the bandwidth approaches to zero
Interplay and competition between superconductivity and charge orderings in the zero-bandwidth limit of the extended Hubbard model with pair hopping and on-site attraction
This is an author-created, un-copyedited version of an article accepted for publication in Journal of Superconductivity and Novel Magnetism.We present studies of an effective model which is a simple generalization of the standard model of a local pair superconductor with on-site pairing (i.e., the model of hard core bosons on a lattice) to the case of finite pair binding energy. The tight binding Hamiltonian consists of (i) the effective on-site interaction U, (ii) the intersite density-density interactions W between nearest-neighbours, and (iii) the intersite charge exchange term I, determining the hopping of electron pairs between nearest-neighbour sites. In the analysis of the phase diagrams and thermodynamic properties of this model we treat the intersite interactions within the mean-field approximation. Our investigations of the U0 case show that, depending on the values of interaction parameters, the system can exhibit three homogeneous phases: superconducting (SS), charge-ordered (CO) and nonordered (NO) as well as the phase separated SS-CO state.National Science Center (NCN) as a research project in years 2011-2013, under grant No. DEC-2011/01/N/ST3/00413;
European Commission and Ministry of Science and Higher Education (Poland) - partial financial support from European Social Fund – Operational
Programme "Human Capital" – POKL.04.01.01-00-133/09-00 – "Proinnowacyjne kształcenie, kompetentna kadra, absolwenci przyszłości";
The Fundation of Adam Mickiewicz University in Pozna
Ground State Phase Diagram of the Extended Hubbard Model with Pair-Hopping Interaction in the Limit of Very Narrow Bandwidth
The extended Hubbard model with the pair-hopping interaction, i.e. the Penson-Kolb-Hubbard model, is one of the conceptually simplest phenomenological models for studying correlations and for description of superconductivity in very narrow-band systems with short-range, almost unretarded pairing. We present ground state phase diagrams of the model derived within the broken-symmetry Hartree-Fock approximation in the narrow-bandwidth regime and compare these results with the exact ones in the atomic limit for the limit of high dimensions. The investigation of the diagrams of the model show that results obtained within both approaches are consistent, although for the case of the finite single-electron hopping phases with magnetic long-range order also occur