190 research outputs found
Activity-composition relations in the system CaCO3-MgCO3 predicted from static structure energy calculations and Monte Carlo simulations
Thermodynamic mixing properties and subsolidus phase relations of the rhombohedral carbonate system, (1 - x) CaCO3 - x MgCO3, were modelled in the temperature range of 623-2023 K with static structure energy calculations based on well-parameterised empirical interatomic potentials. Relaxed static structure energies of a large set of randomly varied structures in a 4 x 4 x 1 supercell of R3c calcite (a = 19.952A , c = 17.061A ) were calculated with the General Utility Lattice Program (GULP). These energies were cluster expanded in a basis set of 12 pair-wise effective interactions. Temperature-dependent enthalpies of mixing were calculated by the Monte Carlo method. Free energies of mixing were obtained by thermodynamic integration of the Monte Carlo results. The calculated phase diagram is in good agreement with experimental phase boundaries
Spin susceptibility of charge ordered YBa2Cu3Oy across the upper critical field
The value of the upper critical field Hc2, a fundamental characteristic of
the superconducting state, has been subject to strong controversy in high-Tc
copper-oxides. Since the issue has been tackled almost exclusively by
macroscopic techniques so far, there is a clear need for local-probe
measurements. Here, we use 17O NMR to measure the spin susceptibility
of the CuO2 planes at low temperature in charge ordered
YBa2Cu3Oy. We find that increases (most likely linearly) with
magnetic field H and saturates above field values ranging from 20 to 40 T. This
result is consistent with Hc2 values claimed by G. Grissonnanche et al. [Nat.
Commun. 5, 3280 (2014)] and with the interpretation that the
charge-density-wave (CDW) reduces Hc2 in underdoped YBa2Cu3Oy. Furthermore, the
absence of marked deviation in at the onset of long-range CDW
order indicates that this Hc2 reduction and the Fermi-surface reconstruction
are primarily rooted in the short-range CDW order already present in zero
field, not in the field-induced long-range CDWorder. Above Hc2, the relatively
low values of at T=2 K show that the pseudogap is a ground-state
property, independent of the superconducting gap.Comment: To appea
ОБСЛУГОВУВАННЯ ЧИТАЧІВ У БІБЛІОТЕКАХ НАВЧАЛЬНИХ ЗАКЛАДІВ ДОРЕВОЛЮЦІЙНОГО КАТЕРИНОСЛАВА
Висвітлено історичний розвиток бібліотек навчальних закладів краю, приділена увага
процесу обслуговування читацького контингенту освітніх закладів, напрямкам, формам, та
характерним рисам. Визначено місце і роль бібліотечних працівників-викладачів.Historical development of educational institutions libraries of Yekaterinoslav region is shown,
special attentionis paidto readers’ service in national education, its trends,forms andpeculiar features.
The place and role of librarian teachers is defined
Thermodynamics of pyrope-majorite, Mg3Al2Si3O12-Mg4Si4O12, solid solution from atomistic model calculations
Static lattice energy calculations, based on empirical pair potentials have been performed for a large set of different structures with compositions between pyrope and majorite, and with different states of order of octahedral cations. The energies have been cluster expanded using pair and quaternary terms. The derived ordering constants have been used to constrain Monte Carlo simulations of temperature-dependent properties in the ranges of 1073 3673K and 0 20 GPa. The free energies of mixing have been calculated using the method of thermodynamic integration. At zero pressure the cubic/tetragonal transition is predicted for pure majorite at 3300 K. The transition temperature decreases with the increase of the pyrope mole fraction. A miscibility gap associated with the transition starts to develop at about 2000K and xmaj 0.8, and widens with the decrease in temperature and the increase in pressure. Activity composition relations in the range of 0 20 GPa and 1073 2673K are described with the help of a high-order Redlich Kister polynomial
Spin-echo and quantum versus classical critical fluctuations in TmVO
Using spin-echo Nuclear Magnetic Resonance in the model Transverse-Field
Ising system TmVO, we show that low frequency quantum fluctuations at the
quantum critical point have a very different effect on V nuclear-spins
than classical low-frequency noise or fluctuations that arise at a finite
temperature critical point. Spin-echos filter out the low frequency classical
noise but not the quantum fluctuations. This allows us to directly visualize
the quantum critical fan and demonstrate the persistence of quantum
fluctuations at the critical coupling strength in TmVO to high temperatures
in an experiment that remains transparent to finite temperature classical phase
transitions. These results show that while dynamical decoupling schemes can be
quite effective in eliminating classical noise in a qubit, a quantum critical
environment may lead to rapid entanglement and decoherence.Comment: 10 pages, 5 figure
Justification for effective water planning and management in the north of the Sinai Peninsula, Egypt
Groundwater is virtually the only source of water supply to the north of the Sinai Peninsula. Propagation conditions for Quaternary aquifers were characterised. Information was updated for all existing hydrogeological wells in the North Sinai area. Well logs for the development of the main Quaternary aquifers were compiled; statistical analysis of permeability parameter values for water-bearing rocks was carried out, based on which the zoning was identified according to the permeability coefficient value
Second order Zeeman interaction and ferroquadrupolar order in TmVO
TmVO exhibits ferroquadrupolar order of the Tm 4f electronic orbitals
at low temperatures, and is a model system for Ising nematicity that can be
tuned continuously to a quantum phase transition via magnetic fields along the
-axis. Here we present V nuclear magnetic resonance data in magnetic
fields perpendicular to the -axis in a single crystal that has been
carefully cut by a plasma focused ion beam to an ellipsoidal shape to minimize
the inhomogeneity of the internal demagnetization field. The resulting dramatic
increase in spectral resolution enabled us to resolve the anisotropy of the
electric field gradient and to measure the magnetic and quadrupolar relaxation
channels separately. Perpendicular magnetic fields nominally do not couple to
the low energy degrees of freedom, but we find a significant nonlinear
contribution for sufficiently large fields that give rise to a rich phase
diagram. The in-plane magnetic field can act either as an effective transverse
or longitudinal field to the Ising nematic order, depending on the orientation
relative to the principle axes of the quadrupole order, and leads to a marked
in-plane anisotropy in both relaxation channels. We find that the small
in-plane transverse fields initially enhance the ferroquadrupolar ordering
temperature but eventually suppress the long-range order. We tentatively
ascribe this behavior to the competing effects of field-induced mixing of
higher energy crystal field states and the destabilizing effects of
field-induced quantum fluctuations.Comment: 15 pages, 13 figure
Locally commensurate charge-density wave with three-unit-cell periodicity in YBCO
In order to identify the mechanism responsible for the formation of
charge-density waves (CDW) in cuprate superconductors, it is important to
understand which aspects of the CDW's microscopic structure are generic and
which are material-dependent. Here, we show that, at the local scale probed by
NMR, long-range CDW order in YBa2Cu3Oy is unidirectional with a commensurate
period of three unit cells (lambda = 3b), implying that the incommensurability
found in X-ray scattering is ensured by phase slips (discommensurations).
Furthermore, NMR spectra reveal a predominant oxygen character of the CDW with
an out-of-phase relationship between certain lattice sites but no specific
signature of a secondary CDW with lambda = 6b associated with a putative
pair-density wave. These results shed light on universal aspects of the cuprate
CDW. In particular, its spatial profile appears to generically result from the
interplay between an incommensurate tendency at long length scales, possibly
related to properties of the Fermi surface, and local commensuration effects,
due to electron-electron interactions or lock-in to the lattice.Comment: Original submission (revised version available upon request
Competition between spin ordering and superconductivity near the pseudogap boundary in La2−xSrxCuO4: Insights from NMR
When superconductivity is suppressed by high magnetic fields in La2−xSrxCuO4, striped antiferromagnetic (AFM) order becomes the magnetic ground state of the entire pseudogap regime, up to its end at the doping p∗ [Frachet, Vinograd et al., Nat. Phys. 16, 1064 (2020)]. Glass-like freezing of this state is detected in 139La NMR measurements of the spin-lattice relaxation rate T−11. Here, we present a quantitative analysis of T−11 data in the hole-doping range p=x=0.12−0.171, based on the Bloembergen-Purcell-Pound (BPP) theory, modified to include statistical distribution of parameters arising from strong spatial inhomogeneity. We observe spin fluctuations to slow down at temperatures T near the onset of static charge order and, overall, the effect of the field B may be seen as equivalent to strengthening stripe order by approaching p=0.12 doping. In details, however, our analysis reveals significant departure from usual field-induced magnetic transitions. The continuous growth of the amplitude of the fluctuating moment with increasing B suggests a nearly-critical state in the B→0 limit, with very weak quasistatic moments possibly confined in small areas like vortex cores. Further, the nucleation of spin order in the vortex cores is shown to account quantitatively for both the value and the p dependence of a field scale characterizing bulk spin freezing. The correlation time of the fluctuating moment appears to depend exponentially on B/T (over the investigated range). This explains the timescale dependence of various experimental manifestations, including why, for transport measurements, the AFM moments may be considered static over a considerable range of B and T. These results make the high-field magnetic ground state up to p∗ an integral part of the discussion on putative quantum criticality
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