Universal Fermi velocity in highly compressed hydride superconductors

The Fermi velocity vF is one of the primary characteristics of any conductor,including any superconductor.For conductors at ambient pressure,several experimental techniques have been developed to measure vF,and,for instance,Zhou et al.[Nature 423,398(2003)]reported that high-Tc cuprates exhibited a universal nodal Fermi velocity vF,univ=(2.7±0.5)×10^(5) m/s.However,there have been no measurements of vF in highly compressed near-room-temperature superconductors(NRTS),owing to experimental challenges.Here,to answer the question of the existence of a universal Fermi velocity in NRTS materials,we analyze the full inventory of data on the ground-state upper critical field Bc2(0)for these materials and find that this class of superconductors exhibits a universal Fermi velocity vF,univ=(1/1.3)×[2Δ(0)/kBTc]×10^(5) m/s,whereΔ(0)is the ground-state amplitude of the energy gap.The ratio 2Δ(0)/kBTc varies within a narrow range 3.2≤2Δ(0)/kBTc≤5,and so vF,univ in NRTS materials lies in the range 2.5×10^(5) m/s≤vF,univ≤3.8×10^(5) m/s,which is similar to the range of values found for the high-Tc cuprate counterparts of these materials.

Elastic Moduli in Cadmium Selenide Doped with Chromium

Temperature dependence of elastic moduli , , and the latter for the piezo-active and non-piezo-active versions, have been measured in the interval of 4 - 180 K at 28 - 262 MHz in a CdSe: Cr2+ crystal. Anomalies below 40 K have been found for all the moduli, except . The interpretation of the results has been carried out involving the Jahn-Teller effect and relaxation between the equivalent distortions of the tetrahedral CrSe4 centers.

Bending sensor based on flexible spin valve

Flexible spin valves were prepared by magnetron sputtering on polyimide substrates. The buffer layer that reduces significantly the effect of the polymer substrate on the spin valve microstructure and magnetoresistive properties was revealed. Bending deformation was applied to the microobjects based on the flexible spin valves in parallel to anisotropy axes. It was revealed that during the bend the magnetoresistance changes due to the joint impact of both the change of the magnetic field projection on the film plane and the change of the magnetic properties of the ferromagnetic layers. The obtained dependences have been used in construction of bending sensor, in which the flexible spin valve microstripes were united into the Wheatstone bridge.

Correlation of microstructure and magnetic properties in Sm(Co_(bal)Fe_(0.1)Cu_(0.1)Zr_(0.033))_(6.93) magnets solution-treated at different temperatures

The correlation of microstructure and magnetic properties in Sm(Co_(bal)Fe_(0.1)Cu_(0.1)Zr_(0.033))_(6.93) magnets solution-treated at different temperatures was systematically investigated. It is found that the magnets solution-treated at 1219℃ possess a single 1:7 H phase, exhibiting the homogeneous cellular structure during further aging treatment, leading to the optimum magnetic properties. However, for the magnets solution-treated at 1211 and 1223℃,2:17 H or 1:5 H secondary phase will also form besides 1:7 H main phase, which cannot transform into cellular structure,thus deteriorating the magnetic properties greatly. The irreversible magnetization investigations with recoil loops also propose a non-uniform pinning in the magnets induced by the secondary precipitates. At proper solution temperature, Zr is supposed to occupy the Fe-Fe dumbbell sites in the form of Zr-vacancy pairs, leading to the minimum c/a ratio and thus stabilizing the 1:7 H phase. Finally,Sm(Co_(bal)Fe_(0.1)Cu_(0.1)Zr_(0.033))_(6.93) magnets with the maximum energy product and intrinsic coercivity at 550℃ up to 60.73 kJ·m^(-3) and 553.88 kA·m^(-1) were prepared by powder metallurgy method.

Coercivity kinetics upon step annealing of sintered Sm(Co0.88-xFexCu0.09Zr0.03)7 magnets

Behavior of the coercivity of the high-temperature Sm(Co0.88-xFexCu0.09Zr0.03)7 magnets depending on the temperature and time of annealing with the temperature decreasing stepwise from 700 to 400℃ was investigated.It is shown that the growth rate of coercivity abruptly increases at the initial stage of annealing in the vicinity of the Curie temperature of the SmCo5 phase.The origin of the effect is the counter diffusion of Cu and Co atoms through dislocation tubes,which form because of enhanced stresses and a partial breakage of coherent coupling at the interface of the Sm2 Co17 and SmCo5 phases.Diffusive enrichment of the SmCo5 phase in Cu close to the interface with Sm2 Co17 leads to relaxation of stresses and increases in the gradient of the magnetic domain-wall energy and coercivity.

Magnetic properties of melt-spun ribbons(Sm1-xZrx)(Fe0.92Ti0.08)10 with ThMn12 structure and their hydrides

The structure and magnetic hysteresis properties of the cast Sm1-xZrx(Fe0.92Ti0.08)10(x = 0-0.3)alloys and melt-spun ribbons prepared from them were studied.In the cast alloy with x>0.2, a considerable amount of the eutectic phase is found in the SEM micrographs.Analysis of the temperature dependences of the magnetic susceptibility and XRD patterns allows amorphous state in the as-spun ribbons with x>0.2 to be determined.The specific magnetization measured in a field of 17 kOe and remanence decrease with increasing annealing temperature from 800 to 900 ℃ and weakly depend on Zr concentration.The maximal value of coercivity Hc = 4.7 kOe is obtained on the ribbons with x = 0.2 after annealing at 850℃ for 10 min.After additional hydrogenation of the ribbons,both the coercivity and remanence increase by 54% and 7%,respectively.

Development of high-coercivity state in melt-spun Fe41Pd41B8Si6P4 ribbons

The phase transformation and magnetic hysteresis properties of melt-spun Fe41 Pd41 B8 Si6 P4 ribbons subjected to the annealing at temperatures of 500-550℃were studied after holding for 0.1-60.0 h by transmission electron microscopy(TEM),X-ray diffraction(XRD)and thermomagnetic analysis.The additions of P,B and Si to the FePd alloy allowed us to achieve the coercivity of124 kA·m-1,which is 2.6 times higher than that of the melt-spun ribbons of the binary equiatomic FePd alloy.The high-coercivity Fe41Pd41B8 Si6 P4 alloy is nanocrystalline and is composed of the ordered L10-phase grains approximately 40 nm in size and inclusions of the Fe2(P,B)and Pd2(Si,B)phases.The coercivity kinetics is controlled by the phase transformation which can be divided into three stages:transformation from the bcc structure to nanosized regions of the fee and Fe2 P phases;transformation from the fee to L10 nanosized regions with somewhat different degrees of tetragonality and their ordering;and extensive growth of the weight fraction of L10 phase from the fee nanosized regions.P and B atoms occupy interstitial sites in the iron plane of L10 lattice,thus decreasing its Curie temperature(TC).

Charge disproportionation and site-selective local magnetic moments in the post-perovskite-type Fe_(2)O_(3) under ultra-high pressures

The archetypal 3d Mott insulator hematite,Fe_(2)O_(3),is one of the basic oxide components playing an important role in mineralogy of Earth’s lower mantle.Its high pressure-temperature behavior,such as the electronic properties,equation of state,and phase stability is of fundamental importance for understanding the properties and evolution of the Earth’s interior.Here,we study the electronic structure,magnetic state,and lattice stability of Fe_(2)O_(3)at ultra-high pressures using the density functional plus dynamical mean-field theory(DFT+DMFT)approach.In the vicinity of a Mott transition,Fe_(2)O_(3) is found to exhibit a series of complex electronic,magnetic,and structural transformations.In particular,it makes a phase transition to a metal with a post-perovskite crystal structure and site-selective local moments upon compression above 75 GPa.We show that the site-selective phase transition is accompanied by a charge disproportionation of Fe ions,with Fe^(3±δ)and δ~0.05–0.09,implying a complex interplay between electronic correlations and the lattice.Our results suggest that site-selective local moments in Fe_(2)O_(3) persist up to ultra-high pressures of~200–250 GPa,i.e.,sufficiently above the core-mantle boundary.The latter can have important consequences for understanding of the velocity and density anomalies in the Earth’s lower mantle.