Pressure Dependence Of Superconductivity Research Articles

Pressure Dependence of Superconductivity in Alkaline Earth Metal-Doped FeSe: toward Completion of the Phase Diagram of Superconducting Transition Temperature Versus FeSe Layer Distance

The pressure dependence of superconductivity in alkaline earth metal-doped FeSe, (NH3)yAExFeSe (AE: Ca, Sr, and Ba), prepared by the liquid ammonia (NH3) technique was investigated over a wide pressure range, in order to clarify the correlation between superconducting transition temperature Tc and FeSe layer distance. Doping with an alkaline earth metal atom implies the preparation of FeSe doped with a divalent metal atom. The correlation between the Tc and layer distance has been investigated thus far for FeSe doped with a monovalent metal atom, i.e., alkaline metal-doped FeSe ((NH3)yAxFeSe (A: Na, K, Rb, and Cs)). The Tc value of (NH3)yAxFeSe increased with an increase in the lattice constant c, which corresponds to twice of the FeSe layer distance. In (NH3)yAExFeSe (AE: Ca, Sr, and Ba), Tc values decreased with pressures up to 10 GPa, and superconductivity was not clearly observed above 10 GPa. Pressure-dependent X-ray diffraction of (NH3)yAExFeSe indicated no structural phase transitions up to ∼25 GPa, with a rapid decrease in c with pressure and saturation above 10 GPa. Tc–c plots of (NH3)yAExFeSe were drawn from Tc–p and c–p plots obtained in this study. Moreover, we analyze Tc–c plots of (NH3)yAExFeSe based on those depicted for both monovalent and divalent atom-doped FeSe, demonstrating a clear linear relationship in the Tc–c plot with similar slopes. Simply put, the different valence between monovalent and divalent atoms does not remarkably affect the c dependence of Tc, and a universal rule that Tc increases with an increase in c is unambiguously established for both compounds.

Pressure dependence of superconductivity in alkali-Bi compounds KBi2 and RbBi2.

The structural and superconducting properties of alkali-Bi-based compounds, KBi2 and RbBi2, were investigated over a wide pressure range for the first time. The samples of KBi2 and RbBi2 were prepared using a liquid ammonia (NH3) technique, and demonstrated superconductivity with superconducting transition temperatures, Tc, of 3.50 and 4.21 K at ambient pressure, respectively. The onset superconducting transition temperature, Tconset, of KBi2 decreased slightly; however, it suddenly jumped at 2 GPa and increased gradually with pressure, indicating the presence of two superconducting phases in the low-pressure range. The pressure-dependent X-ray diffraction patterns indicate that the KBi2 sample decomposed into KBi and Bi at pressures higher than 2.5 GPa. Moreover, a discontinuous change in Tconset was observed for KBi2 at 9 GPa, which reflects the decomposition of KBi2 into KBi and Bi. By contrast, the value of Tconset of RbBi2 was almost constant over a pressure range of 0-8 GPa. Thus, the superconducting properties and stability of alkali-Bi-based compounds against pressure were comprehensively explored in this study. In addition, the superconducting Cooper pair symmetry was investigated from the magnetic field dependence of Tc of KBi2 at 0.790 and 2.32 GPa, and of RbBi2 at 1.17 GPa, indicating the exact deviation from the simple s-wave paring model, which may be due to the complex electronic structure of Bi. The results elucidated the exotic superconducting properties of KBi2 and RbBi2 based on the pressure and magnetic field dependence of Tc and verified the chemical stability of KBi2 under pressure.

Preparation and characterization of superconducting Ba1−xCsxTi2Sb2O, and its pressure dependence of superconductivity

A new superconducting sample, Ba1−xCsxTi2Sb2O, was prepared and characterized in a wide pressure range. The maximum value of superconducting transition temperature, Tc, of Ba1−xCsxTi2Sb2O was 4.4 K for x = 0.25 at ambient pressure. The crystal structure was determined to be tetragonal [space group of P4/mmm (No. 123)]. The charge density wave/spin density wave transition was observed at 44 K for Ba1−xCsxTi2Sb2O (nominal x = 0.25) at ambient pressure, but the transition was suppressed by applying pressure. The pressure dependent X-ray diffraction showed no structural phase transition up to 23.4 GPa, but very interesting Tc–pressure (p) behavior was observed, i.e. the Tc decreases with an increase in pressure up to 4.0 GPa, but it increased above 4.0 GPa, suggestive of non-BCS type behavior. Thus, the systematic study on new pnictide superconductor, Ba1−xCsxTi2Sb2O, was achieved, and the fascinating behavior of superconductivity against pressure was discovered.

Preparation of new superconductors by metal doping of two-dimensional layered materials using ethylenediamine

We have studied new superconductors prepared by metal doping of two-dimensional (2D) layered materials, FeSe and FeSe0.5Te0.5, using ethylenediamine (EDA). The superconducting transition temperatures (T(c)s) of metal-doped FeSe and metal-doped FeSe0.5Te0.5, i.e., (EDA)(y)MxFeSe and (EDA)(y)MxFeSe0.5Te0.5 (M: Li, Na, and K), were 31-45 K and 19-25 K, respectively. The stoichiometry of each sample was clarified by energy dispersive x-ray (EDX) spectroscopy, and the x-ray powder diffraction pattern indicated a large expansion of lattice constant c, indicating the cointercalation of metal atoms and EDA. The pressure dependence of superconductivity in (EDA)(y)NaxFeSe0.5Te0.5 has been investigated at a pressure of 0-0.8GPa, showing negative pressure dependence in the same manner as (NH3)(y)NaxFeSe0.5Te0.5. The T-c-c phase diagrams of MxFeSe and MxFeSe0.5Te0.5 were drawn afresh from the T-c and c of (EDA)(y)MxFeSe and (EDA)(y)MxFeSe0.5Te0.5, showing that the T-c increases with increasing c but that extreme expansion of c reverses the T-c trend.

High Pressure Studies on the Transport Properties and Upper Critical Field (H c2) of Hole-Doped Pr0.8Sr0.2FeAsO Iron Pnictides

We report the pressure dependence of superconductivity for hole-doped polycrystalline Pr 0.8Sr 0.2FeAsO by magnetization and the electrical resistivity using cubic anvil. The superconducting transition temperature is initially increased from 16 K (P = 0) to 32 K (P = 6 GPa) with positive pressure coefficient and then it decreases on increasing the pressure further which is similar to an electrondoped systems. The upper critical field of the sample with T c = 16 K was estimated to be ∼56(1) T. Our results suggest that the hole-doped samples may have higher upper critical fields comparing to the electrondoped ones, due to the higher quasi particle density of states at the Fermi level.

Predicted superconductivity of Ni2VAl and pressure dependence of superconductivity in Ni2NbX (X = Al, Ga and Sn) and Ni2VAl

A first-principles study of the electronic and superconducting properties of the Ni2VAl Heusler compound is presented. The electron–phonon coupling constant of is obtained, which leads to a superconducting transition temperature of K (assuming a Coulomb pseudopotential ), which is a relatively high transition temperature for Ni based Heusler alloys. The electronic density of states reveals a significant hybridization between Ni-eg and V-t2g states around the Fermi level. The Fermi surface, consisting of two electron pockets around the X-points of the Brillouin zone, exhibits nesting and leads to a Kohn anomaly of the phonon dispersion relation for the transverse acoustic mode TA2 along the (1, 1, 0) direction, which is furthermore found to soften with pressure. As a consequence, and vary non-monotonically under pressure. The calculations are compared to similar calculations performed for the Ni2NbX (X = Al, Ga and Sn) Heusler alloys, which experimentally have been identified as superconductors. The experimental trend in is well reproduced, and reasonable quantitative agreement is obtained. The calculated of Ni2VAl is larger than either calculated or observed s of any of the Nb compounds. The Fermi surfaces of Ni2NbAl and Ni2NbGa consist of only a single electron pocket around the X point, however under compression a second electron pocket similar to that of Ni2VAl emerges Ni2NbAl and the increases non-monotonically in all the compounds. Fermi surface nesting and associated Kohn anomalies are a common feature of all four compounds, albeit weakest in Ni2VAl.

High Pressure Effect on Superconductivity of Hole-Doped Pr0.75Sr0.25FeAsO Iron Pnictides

We report the pressure dependence of superconductivity for hole-doped polycrystalline Pr0.75Sr0.25FeAsO by the resistivity study using high pressure cubic and diamond anvil pressure cells. The superconducting transition temperature is initially increased from 17 K (P=0) to 34 K (P=6 GPa) with positive pressure coefficient, which is remarkable among other pnictide superconductors. At high pressure, there is no incredible enhancement of T c; in other words, it has a negative pressure effect on superconductivity, like in an electron-doped compounds. A systematic investigation of superconductivity for this 1111 type hole-doped iron pnictides is further carefully analyzed and discussed in detail using superconducting phase diagram under high pressure chart.

Pressure Dependence of Superconductivity in FeSe studied by DC Magnetic Measurements

Pressure dependence of superconductivity in FeSe has been investigated by DC magnetic measurements under high pressure up to 5 GPa using miniature diamond anvil cell combined with commercial SQUID magnetometer. The specimens with nominal composition FeSex (x = 0.80–1.00) were prepared from iron pieces and selenium shot firing at 1075 °C. The specimens containing no impurity phase of hexagonal FeSe were obtained for 0.80≤x≤0.96. For all of the specimens, it has been found that Tc increases in two steps by the application of pressure P, showing a local maximum of Tc∼12 K at P∼1 GPa, and Tc is nearly pressure independent above ∼3 GPa. The maximum value Tcmax above 3 GPa is 15–20 K for 0.80≤x≤0.98 but ∼25 K for x≥0.99.

Pressure dependence of superconductivity in doped two-leg ladder cuprates

Within the kinetic energy driven superconducting mechanism, the effect of the pressure on superconductivity in the doped two-leg ladder cuprates is studied. It is shown that the superconducting transition temperature in the doped two-leg ladder cuprate superconductors increases with increasing pressure in the underpressure regime, and reaches a maximum in the optimal pressure, then decreases in the overpressure regime. This domed shape of the pressure dependence of the superconducting transition temperature is similar to that of the pressure dependence of the longitudinal part of the superconducting gap parameter, indicating that the pressure dependence of superconductivity in the doped two-leg ladder cuprates is mainly produced by the development of the pairing correlation along legs.

Pressure dependence of superconductivity in Ba4C60

In BaxC60 superconductivity (sc) at Tc=(6.8±0.2) K is associated with the x=4 phase. Here we present pressure studies of Tc of a BaxC60 sample (dTc/dp= −1.9 K/GPa) which exhibits a large volume fraction of the x=4 phase. In the alkali-fullerides K60C60 and Rb3C60 Tc is governed by the electronic density of states N(EF) (Tc ∝exp{−1/N(EF)V}). The dependence of N(EF) on volume can be approximated by N(EF)=N0 d eff 3, where deff is the surface to surface distance of neighboured C60-molecules. This relationship is corroborated by (TC,d)-data of various compounds of the form A3−xBxC60 (A: K, Rb; B: K, Rb, Cs; x≤2). Strong deviation from this behavior is found for the alkaline-fulleride superconductor Ba4C60. This deviation is attributed to the hybridization of alkaline-metal s-, d-states and C60 π-states.

Pressure dependence of superconductivity in the pseudo-one-dimensional compound Tl 2Mo 6Se 6

Measurements of the temperature and pressure dependences of the resistivity of the pseudo-one-dimensional ternary compound Tl 2Mo 6Se 6 are presented. We find that the conductivity parallel to the highly conducting c-axis is enhanced by pressure and the superconducting transition temperature T c is suppressed by pressure at a rate ϑT c ϑP =−7.6×10 −5 kbar −1. These results are discussed in relation to the current models of transport in one-dimensional conductors.

Pressure dependence of superconductivity in amorphous La100?xAlx alloys

The pressure dependence of the superconducting transition temperatureTc of amorphous La100−xAlx alloys (x=16.5, 20, 25, 30, 34) has been studied under hydrostatic pressure to 3.5 GPa and, additionally, forx=16.5 under quasihydrostatic pressure to 12 GPa. For amorphous samples having Al-concentrationx>20%, ∂Tc/∂P is positive in sign and decreases linearly with increasingx, extrapolating atx=0 to the same value (+1.1 K/Gpa) determined for pure crystalline La. For thex=16.5 sample, however, ∂Tc/∂P is observed to be anomalously large, more than twice the value expected. The present results are compared with previous studies on amorphous and crystalline La-systems.

Pressure dependence of superconductivity in non‐transition metals

AbstractAn expression for the volume dependence of electron‐phonon coupling strength is derived by considering the corresponding variation of Fermi momentum explicitly. Numerical data for the critical volumes deduced from the formulation compare fairly well with the existing theories. The calculated values of logarithmic volume derivative of effective interaction strength are in good accord with experiments. The volume dependence of superconducting transition temperature evaluated for three representative cases of weak, medium, and strong coupling superconductors leads to the conclusion that the theory reproduces the experimental data satisfactorily.

Strong-Coupling Effects in the Pressure Dependence of Superconductivity

We discuss the effect of hydrostatic pressure on the superconducting properties of the strong-coupling metals Pb and Hg. We solve the \'Eliashberg gap equations at zero and at finite pressures to obtain the gap ${\ensuremath{\Delta}}_{0}$ in the excitation spectrum at $T=0$ and the critical temperature ${T}_{c}$. The calculations are effected for kernels at zero pressure and repeated with kernels appropriate to a 5% volume decrease. At nonzero pressure the phonon parts of the kernels are rescaled for the upward shift in the phonon frequencies and adjusted for changes in the electron-ion pseudopotential form factor. It is found that ${\ensuremath{\Delta}}_{0}$ and ${T}_{c}$ do not scale in the same way under pressure. The gap is relatively more affected, so that the ratio $\frac{2{\ensuremath{\Delta}}_{0}}{{k}_{B}{T}_{c}}$ tends towards the BCS weak-coupling limit of 3.52 with decreasing volume and, hence, decreasing electron-phonon interaction. In Pb, our results are in good agreement with experiment for both the change in ${T}_{c}$ and in the ratio $\frac{2{\ensuremath{\Delta}}_{0}}{{k}_{B}{T}_{c}}$. In Hg, no experimental results exist at the moment, but we predict similar strong-coupling effects.