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Register a new userPressure effect on superconductivity and magnetism in FeSex
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L. Li
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In this paper, pressure effect on superconductivity and magnetism has been investigated in FeSex (x = 0.80, 0.88). The magnetization curves display anomaly at Ts1 106 K and Ts2 78 K except for the superconducting diamagnetic transition around Tc 8 K. The magnetic anomaly at Ts1 and Ts2 can be related to a ferromagnetic and an antiferromagnetic phase transition, respectively, as revealed by specific heat measurements. The application of pressure not only raises Tc, but also increases both Ts1 and Ts2. This system shows clear evidence that superconductivity arises in a phase with strong magnetic character and the superconductivity coexists with magnetism. In addition, the specific heat anomaly associated with the superconducting transition seems to be absent.
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Oxygen isotope (^{16}O/^{18}O) effects (OIEs) on the superconducting transition (T_c), the spin-glass ordering (T_g), and the antiferromagnetic ordering (T_N) temperatures were studied for Y_1-xPr_xBa_2Cu_3O_7-delta as a function of Pr content (0.0leq x leq 1.0). The OIE on T_c increases with increasing x up to xapprox0.55, where superconductivity disappears. For decreasing x the OIEs on T_N and T_g increase down to xapprox 0.7 where antiferromagnetic order and down to xapprox0.3 where spin-glass behavior vanish, respectively. The OIEs on T_g and T_N are found to have {it opposite signs} as compared to the OIE on T_c. All OIEs are suggested to arise from the isotope dependent mobility (kinetic energy) of the charge carriers.
Pressure effect on superconducting properties of two YB6 samples (Tc = 5.9 and 7.5 K) were investigated by measurements of electrical resistivity, magnetic susceptibility, and X-ray diffraction in the pressure range up to 320 kbar. Magnetoresistivity measurements down to 60 mK and up to 47 kbar have shown a negative pressure effect on Tc as well as on the third critical field Hc3 with the slopes dlnTc/dp = -0.59%/kbar and dlnHc3/dp = -1.1%/kbar, respectively. The magnetic susceptibility measurements evidenced that the slope of dlnTc/dp gradually decreases with pressure reaching 3 times smaller value at 112 kbar. The lattice parameter measurements revealed the volume reduction of 14% at 320 kbar. The pressure-volume dependence is described by the Rose-Vinet equation of state. The obtained relative volume dependence dlnTc/dlnV analyzed by the McMillan formula for Tc indicates that the reduction of the superconducting transition temperature is mainly due to hardening of the Einstein-like phonon mode responsible for the superconducting coupling. This is confirmed by the analysis of the resistivity measurements in the normal state up to T = 300 K performed at pressures up to 28 kbar.
An extended investigation of the electronic phase diagram of FeSe$_{1-x}$ up to pressures of $psimeq2.4$,GPa by means of ac and dc magnetization, zero field muon spin rotation (ZF $mu$SR), and neutron diffraction is presented. ZF $mu$SR indicates that at pressures $pgeq0.8$,GPa static magnetic order occurs in FeSe$_{1-x}$ and occupies the full sample volume for $pgtrsim 1.2$,GPa. ac magnetization measurements reveal that the superconducting volume fraction stays close to 100% up to the highest pressure investigated. In addition, above $pgeq1.2$,GPa both the superconducting transition temperature $T_{rm c}$ and the magnetic ordering temperature $T_{rm N}$ increase simultaneously, and both superconductivity and magnetism are stabilized with increasing pressure. Calculations indicate only one possible muon stopping site in FeSe$_{1-x}$, located on the line connecting the Se atoms along the $c$-direction. Different magnetic structures are proposed and checked by combining the muon stopping calculations with a symmetry analysis, leading to a similar structure as in the LaFeAsO family of Fe-based superconductors. Furthermore, it is shown that the magnetic moment is pressure dependent and with a rather small value of $muapprox 0.2,mu_B$ at $psimeq2.4$,GPa.
The recent discovery of pressure induced superconductivity in the binary helimagnet CrAs has attracted much attention. How superconductivity emerges from the magnetic state and what is the mechanism of the superconducting pairing are two important issues which need to be resolved. In the present work, the suppression of magnetism and the occurrence of superconductivity in CrAs as a function of pressure ($p$) were studied by means of muon spin rotation. The magnetism remains bulk up to $psimeq3.5$~kbar while its volume fraction gradually decreases with increasing pressure until it vanishes at $psimeq$7~kbar. At 3.5 kbar superconductivity abruptly appears with its maximum $T_c simeq 1.2$~K which decreases upon increasing the pressure. In the intermediate pressure region ($3.5lesssim plesssim 7$~kbar) the superconducting and the magnetic volume fractions are spatially phase separated and compete for phase volume. Our results indicate that the less conductive magnetic phase provides additional carriers (doping) to the superconducting parts of the CrAs sample thus leading to an increase of the transition temperature ($T_c$) and of the superfluid density ($rho_s$). A scaling of $rho_s$ with $T_c^{3.2}$ as well as the phase separation between magnetism and superconductivity point to a conventional mechanism of the Cooper-pairing in CrAs.
A hydrostatic pressure study was made on pure and Rh-doped specimens of the superconducting ferromagnetic compounds Ru1-xRhxSr2GdCu2O8 (x = 0-0.15) by means of measurement of electrical resistivity vs temperature, in pressures up to 2 GPa. Partial substitution of Rh for Ru decreases the magnetization of the material, lowers both the magnetic ordering temperature Tm, and the superconducting transition temperature Tc, and promotes granularity. The effect of pressure for all compositions is an increase in both the intra- and intergranular superconductivity transition temperatures, Tc and Tp respectively, as well as Tm. The rate of change of each transition temperature with pressure first drops for Rh concentrations near 5%, increasing latter for higher concentrations. While the rate of increase of Tc with pressure for all compositions is 2-3 times lower than in YBCO materials, the simultaneous increase of Tc and Tm with pressure could support the notion of competition between superconductivity and ferromagnetism in these materials. The effect of pressure on the weak-links was a significant improvement of inter-granular connectivity.
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