Average time scale for Dome Fuji ice core, East Antarctica

Three different approaches to ice-core age dating are employed to develop a depth-age relationship at Dome F: (1) correlation of the ice-core isotope record to the geophysical metronome(Milankovich surface temperature cycle) inferred from the deep borehole temperature profile at Vostok,(2) importing a known chronology from another(Devils Hole) paleoclimatic signal, and(3) direct ice sheet flow modeling. Inverse Monte Carlo sampling is used to constrain the accumulation rate reconstruction and ice flow simulations in order to find the best-fit glaciological time scale matched with the two other chronologies. General uncertainty of the different age estimates varies from 2 to 6kyr on average and reaches 6-14kyr at maximum. Whatever the causes of this discrepancy might be, they are thought to be of different origins, and the age errors are assumed to be independent. Thus, the average time scale for the Dome F ice core down to a depth of 2500m(ice age of 335kyr) is deduced consistently with all three age-depth relationships within the standard deviation limits of ±3.3kyr, and its accuracy is estimated as 1.4kyr on average. The constrained ice-sheet flow model allows extrapolation of the ice age-depth curve further to the glacier bottom and predicts the ages at depths of 2800, 3000, and 3050m to be 615±70, 1560±531, and 2985±1568kyr, respectively

Magnetocaloric effect in the high-temperature antiferromagnet YbCoC2

The magnetic $H$-$T$ phase diagram and magnetocaloric effect in the recently discovered high-temperature heavy-fermion compound YbCoC$_2$ have been studied. With the increase in the external magnetic field YbCoC$_2$ experiences the metamagnetic transition and then transition to the ferromagnetic state. The dependencies of magnetic entropy change -$\Delta S_m (T)$ have segments with positive and negative magnetocaloric effects for $\Delta H \leq 6$~T. For $\Delta H = 9$~T magnetocaloric effect becomes positive with a maximum value of -$\Delta S_m (T)$ is 4.1 J / kg K and a refrigerant capacity is 56.6 J / kg

Towards an ab initio theory for the temperature dependence of electric field gradients in solids: Application to hexagonal lattices of Zn and Cd

Based on ab initio band-structure calculations we formulate a general theoretical method for description of the temperature dependence of an electric-field gradient in solids. The method employs a procedure of averaging multipole electron-density component (l≠0) inside a sphere vibrating with the nucleus at its center. As a result of averaging, each Fourier component (K≠0) on the sphere is effectively reduced by the square root of the Debye-Waller factor [exp(-W)]. The electric-field gradient related to a sum of K components most frequently decreases with temperature (T), but under certain conditions because of the interplay between terms of opposite signs, it can also increase with T. The method is applied to calculations of the temperature evolution of the electric-field gradients of pristine zinc and cadmium crystallized in the hexagonal lattice. For calculations within our model, of crucial importance is the temperature dependence of mean-square displacements which can be taken from experiment or obtained from the phonon modes in the harmonic approximation. For the case of Zn, we have used data obtained from single-crystal X-ray diffraction. In addition, for Zn and Cd, we have calculated mean-square displacements with the density-functional perturbation treatment of the quantum espresso package. With the experimental data for displacements in Zn, our calculations reproduce the temperature dependence of the electric-field gradient very accurately. Within the harmonic approximation of the quantum espresso package, the decrease in electric-field gradients in Zn and Cd with temperature is overestimated. Our calculations indicate that the anharmonic effects are of considerable importance in the temperature dependence of electric-field gradients. © 2020 American Physical Society.The work was supported by a Polish representative in the Joint Institute for Nuclear Research. The ab initio investigation of phonon frequencies was supported by the Russian Science Foundation (Grant No. 18-12-00438). Calculations were carried out using computing resources of the federal collective usage center “Complex for Simulation and Data Processing for Mega-science Facilities” at NRC “Kurchatov Institute” , the supercomputers at the Joint Supercomputer Center of the Russian Academy of Sciences , and the “Uran” supercomputer of IMM UB RAS

Ice core age dating and paleothermometer calibration based on isotope and temperature profiles from deep boreholes at Vostok Station (East Antarctica)

An interpretation of the deuterium profile measured along the Vostok (East ANtarctica) ice core down to 2755 m has been attempted on the basis of the borehole temperature analysis. An inverse problem is solved to infer a local 'geophysical metronome,' the orbital signal in the surface temperature oscillations expressed as a sum of harmonics of Milankovich periods. By correlating the smoothed isotopic temperature record to the metronome, a chronostratigraphy of the Vostok ice core is derived with an accuracy of ±3.0-4.5 kyr. The developed timescale predicts an age of 241 kyr at a depth of 2760 m. The ratio δD/δTi between deuterium content and cloud temperature fluctuations (at the top of the inversion layer) is examined by fitting simulated and measured borehole temperature profiles. The conventional estimate of the deuterium-temperature slope corresponding to the present-day spatial ratio (9 per mil/°C) is confirmed in general. However, the mismatch between modeled and easured borehole temperatures decreases noticeably if we allow surface temperature, responsible for the thermal state of the ice sheet, to undergo more intensive precession oscillations than those of the inversion temperature traced by isotope record. With this assumption, we obtain the long-term temporal deuterium-temperature slope to be 5.8-6.5 per il/°C which implies that the glacial-interglacial temperature increase over central Antarctica was about 15°C in the surface temperature and 10°C in the inversion temperature. Past variations of the accumulation rate and the corresponding changes in the ice-sheet surface elevation are simultaneously simulated