The suppression of magnetism and the development of superconductivity within the collapsed tetragonal phase of Ca0.67Sr0.33Fe2As2 at high pressure

Structural and electronic characterization of (Ca0.67Sr0.33)Fe2As2 has been performed as a func- tion of pressure up to 12 GPa using conventional and designer diamond anvil cells. The compound (Ca0.67Sr0.33)Fe2As2 behaves intermediate between its end members-CaFe2As2 and SrFe2As2- displaying a suppression of magnetism and the onset of superconductivity. Like other members of the AEFe2As2 family, (Ca0.67Sr0.33)Fe2As2 undergoes a pressure-induced isostructural volume collapse, which we associate with the development of As-As bonding across the mirror plane of the structure. This collapsed tetragonal phase abruptly cuts off the magnetic state, giving rise to superconductivity with a maximum Tc=22.2 K. The maximum Tc of the superconducting phase is not strongly correlated with any structural parameter, but its proximity to the abrupt suppression of magnetism as well as the volume collapse transition suggests that magnetic interactions and structural inhomogeneity may play a role in its development. The pressure-dependent evolution of the ordered states and crystal structures in (Ca,Sr)Fe2As2 provides an avenue to understand the generic behavior of the other members of the AEFe2As2 family.Comment: 9 pages, 9 figure

Diurnal and seasonal variations of <i>hm</i>F2 deduced from digitalionosonde over New Delhi and its comparison with IRI 2001

International audienceUsing digital ionosonde observations at a low mid-latitude station, New Delhi (28.6°N, 77.2°E, dip 42.4°N), we have derived hourly monthly values of hmF2 (the real height corresponding to the peak electron density in the F2-region), employing both the Dudeney (1983) and Bilitza (1990) empirical formulations for the period from January 2001 to August 2002. The diurnal and seasonal variations of hmF2 are analyzed. Further, to assess the predictability of the latest available model, International Reference Ionosphere, (IRI-2001), we have obtained the median values of hmF2 derived from M(3000)F2 for each hour during different seasons and compare these with the model. Our results show that both the Dudeney (1983) and Bilitza (1990) formulations reveal more or less a similar diurnal trend of hmF2, with higher values around midnight and lower during sunrise, in all the seasons. It is also noted that the hmF2 shows a larger variability around midnight than by daytime, in all the seasons. Further, the study shows that median values of observed hmF2, using both formulations, are somewhat larger than those predicted by the IRI, in all seasons and at all local times. During summer, the IRI values agree comparatively well with the observations, especially during daytime. Major discrepancies occur when the IRI underestimates observed hmF2 for local times from about 14:00 LT to 18:00 LT and 04:00 LT to 05:00 LT during winter and equinox, where the percentage deviation of the observed hmF2 values with respect to the IRI model varies from 15 to 25%. The difference between the model and observations, outside this time period, remains less than 20% during all the seasons. Key words. Ionosphere (modelling and forecasting; equatorial ionosphere

Pressure-induced superconductivity in the giant Rashba system BiTeI

At ambient pressure, BiTeI is the first material found to exhibit a giant Rashba splitting of the bulk electronic bands. At low pressures, BiTeI undergoes a transition from trivial insulator to topological insulator. At still higher pressures, two structural transitions are known to occur. We have carried out a series of electrical resistivity and AC magnetic susceptibility measurements on BiTeI at pressure up to ~40 GPa in an effort to characterize the properties of the high-pressure phases. A previous calculation found that the high-pressure orthorhombic P4/nmm structure BiTeI is a metal. We find that this structure is superconducting with Tc values as high as 6 K. AC magnetic susceptibility measurements support the bulk nature of the superconductivity. Using electronic structure and phonon calculations, we compute Tc and find that our data is consistent with phonon-mediated superconductivity.Comment: 7 pages, 7 figure

The lifetime cost to English students of borrowing to invest in a medical degree:a gender comparison using data from the Office for National Statistics

OBJECTIVE: To evaluate this impact on male and female English medical graduates by estimating the total time and amount repaid on loans taken out with the UK's Student Loans Company (SLC). SETTING: UK. PARTICIPANTS: 4286 respondents with a medical degree in the Labour Force Surveys administered by the Office for National Statistics (ONS) between 1997 and 2014. OUTCOMES: Age-salary profiles were generated to estimate the repayment profiles for different levels of initial graduate debt. RESULTS: 2195 female and 2149 male medical graduates were interviewed by the ONS. Those working full-time (73.1% females and 96.1% males) were analysed in greater depth. Following standardisation to 2014 prices, average full-time male graduates earned up to 35% more than females by the age of 55. The initial graduate debt from tuition fees alone amounts to £39 945.69. Owing to interest charges on this debt the average full-time male graduate repays £57 303 over 20 years, while the average female earns less and so repays £61 809 over 26 years. When additional SLC loans are required for maintenance, the initial graduate debt can be as high as £81 916 and, as SLC debt is written off 30 years after graduation, the average female repays £75 786 while the average male repays £110 644. CONCLUSIONS: Medical graduates on an average salary are unlikely to repay their SLC debt in full. This is a consequence of higher university fees and as SLC debt is written off 30 years after graduation. This results in the average female graduate repaying more when debt is low, but a lower amount when debt is high compared to male graduates

High-Pressure Amorphous Nitrogen

The phase diagram and stability limits of diatomic solid nitrogen have been explored in a wide pressure--temperature range by several optical spectroscopic techniques. A newly characterized narrow-gap semiconducting phase $\eta$ has been found to exist in a range of 80--270 GPa and 10--510 K. The vibrational and optical properties of the $\eta$ phase produced under these conditions indicate that it is largely amorphous and back transforms to a new molecular phase. The band gap of the $\eta$ phase is found to decrease with pressure indicating possible metallization by band overlap above 280 GPa.Comment: 5 pages, 4 figure

Thermal Stabilization of the HCP Phase in Titanium

We have used a tight-binding model that is fit to first-principles electronic-structure calculations for titanium to calculate quasi-harmonic phonons and the Gibbs free energy of the hexagonal close-packed (hcp) and omega crystal structures. We show that the true zero-temperature ground-state is the omega structure, although this has never been observed experimentally at normal pressure, and that it is the entropy from the thermal population of phonon states which stabilizes the hcp structure at room temperature. We present the first completely theoretical prediction of the temperature- and pressure-dependence of the hcp-omega phase transformation and show that it is in good agreement with experiment. The quasi-harmonic approximation fails to adequately treat the bcc phase because the zero-temperature phonons of this structure are not all stable