The effects of uniaxial strain and hydrostatic pressure on Hg 0.83 Re 0.18 Ba 2 Ca 2.4 Cu 3.6 O 14 [Hg 0.83 (Re 0.18)-1223] were investigated by AC magnetic measurements under stress corresponding to a pressure of 20 GPa at maximum. According to a previous thermal study based on the Ehrenfest relation, in-plane contraction should increase the superconducting transition temperature (T c), whereas out-of-plane contraction should decrease T c. This suggests that the increase in T c under hydrostatic-pressure contraction must be smaller than that under in-plane contraction. However, the present uniaxial-strain experiments revealed enhancement of T c under both in-plane and out-of-plane contraction, and the largest enhancement was observed under hydrostatic-pressure contraction. According to a band calculation, all contraction styles induce hole doping from the HgO blocks to the CuO 2 blocks, and hydrostatic-pressure contraction yields the largest hole doping among three contractions. This behavior explains well a series of changes in T c in the stress region of below 8 GPa. More specifically, under hydrostatic-pressure contraction, T c exhibited an increase, a decrease, and another increase with increasing pressure, and this multistep change is similar to that observed in Bi-2223-type cuprate superconductors, suggesting that it is necessary to distinguish the effect of strain on the middle CuO 2 plane in the three-CuO 2-plane package from that on the outer planes.
In the solid state the iodo-substituted bisdiselenazolyl radical 1c orders as a ferromagnet with T = 10.5 K. With the application of pressure T rises rapidly, reaching a value of 27.5 K at 2.4 GPa. The accompanying structural and magnetic changes have been examined by high resolution powder X-ray diffraction and by DFT calculations of magnetic exchange interactions.
We study the effects of hydrostatic pressure (HP) compression on the superconducting transition of severely strained Nb samples, whose grain sizes are reduced to the submicrometer level. Engineered granularity by high-pressure torsion (HPT) treatment changes the strength of coupling between submicrometer-scale grains and introduces lattice strain. We attempt to utilize the initially accumulated shear strain in the starting material for increasing the superconducting transition temperature Tc under HP compression. The HP effects on non-strained Nb have already been investigated in the pressure regime over 100 GPa by Struzhkin et al. [Phys. Rev. Lett. 79, 4262 (1997)], and Tc reportedly exhibited an increase from 9.2 to 9.9 K at approximately 10 GPa. (1) Slightly strained Nb in the HPT treatment exhibits the increase in Tc under HP due to the strengthening of the intergrain coupling, so the pressure scale of the pressure response observed by Struzhkin et al. is reduced to approximately one-seventh at the maximum. (2) Prominently strained Nb in the HPT treatment exhibits the increase in Tc under HP due to a reduction in structural symmetry at the unit-cell level: In a Nb sample subjected to HPT (6 GPa, 10 revolutions), Tc exceeds 9.9 K at approximately 2 GPa. According to our first-principle calculations, the reduction in the structural symmetry affords an increase in the density of states at the Fermi energy, thereby yielding a prominent increase in Tc at low pressures.
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