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تسجيل مستخدم جديدFormation of Collapsed Tetragonal Phase in EuCo2As2 under High Pressure
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The structural properties of EuCo2As2 have been studied up to 35 GPa, through the use of x-ray diffraction in a diamond anvil cell at a synchrotron source. At ambient conditions, EuCo2As2 (I4/mmm) has a tetragonal lattice structure with a bulk modulus of 48 +/-4 GPa. With the application of pressure, the a-axis exhibits negative compressibility with a concurrent sharp decrease in c-axis length. The anomalous compressibility of the a-axis continues until 4.7 GPa, at which point the structure undergoes a second-order phase transition to a collapsed tetragonal (CT) state with a bulk modulus of 111 +/- 2 GPa. We found a strong correlation between the ambient pressure volume of 122 parents of superconductors and the corresponding tetragonal to collapsed tetragonal phase transition pressures
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We present high-energy x-ray diffraction data under applied pressures up to p = 29 GPa, neutron diffraction measurements up to p = 1.1 GPa, and electrical resistance measurements up to p = 5.9 GPa, on SrCo2As2. Our x-ray diffraction data demonstrate
that there is a first-order transition between the tetragonal (T) and collapsed-tetragonal (cT) phases, with an onset above approximately 6 GPa at T = 7 K. The pressure for the onset of the cT phase and the range of coexistence between the T and cT phases appears to be nearly temperature independent. The compressibility along the a-axis is the same for the T and cT phases whereas, along the c-axis, the cT phase is significantly stiffer, which may be due to the formation of an As-As bond in the cT phase. Our resistivity measurements found no evidence of superconductivity in SrCo2As2 for p <= 5.9 GPa and T >= 1.8 K. The resistivity data also show signatures consistent with a pressure-induced phase transition for p >= 5.5 GPa. Single-crystal neutron diffraction measurements performed up to 1.1 GPa in the T phase found no evidence of stripe-type or A-type antiferromagnetic ordering down to 10 K. Spin-polarized total-energy calculations demonstrate that the cT phase is the stable phase at high pressure with a c/a ratio of 2.54. Furthermore, these calculations indicate that the cT phase of SrCo2As2 should manifest either A-type antiferromagnetic or ferromagnetic order.
By performing pressure simulations within density functional theory for the family of iron-based superconductors $Ae{}A$Fe$_4$As$_4$ with $Ae$ = Ca, Sr, Ba and $A$ = K, Rb, Cs we predict in these systems the appearance of two consecutive half-collaps
ed tetragonal transitions at pressures $P_{c_1}$ and $P_{c_2}$, which have a different character in terms of their effect on the electronic structure. We find that, similarly to previous studies for CaKFe$_4$As$_4$, spin-vortex magnetic fluctuations on the Fe sublattice play a key role for an accurate structure prediction in these materials at zero pressure. We identify clear trends of critical pressures and discuss the relevance of the collapsed phases in connection to magnetism and superconductivity. Finally, the intriguing cases of EuRbFe$_4$As$_4$ and EuCsFe$_4$As$_4$, where Eu magnetism coexists with superconductivity, are discussed as well in the context of half-collapsed phases.
Single crystals of Ca(Fe1-xRux)2As2 (0<x<0.065) and Ca1-yLay(Fe0.973Ru0.027)2As2 (0<y<0.2) have been synthesized and studied with respect to their structural, electronic and magnetic properties. The partial substitution of Fe by Ru induces a decrease
of the c-axis constant leading for x<0.023 to a suppression of the coupled magnetic and structural (tetragonal to orthorhombic) transitions. At x_cr=0.023 a first order transition to a collapsed tetragonal (CT) phase is found, which behaves like a Fermi liquid and which is stabilized by further increase of x. The absence of superconductivity near x_cr is consistent with truly hydrostatic pressure experiments on undoped CaFe2As2. Starting in the CT regime at x=0.027 we investigate the additional effect of electron doping by partial replacement of Ca by La. Most remarkably, with increasing y the CT phase transition is destabilized and the system is tuned back into a tetragonal ground state at y>0.08. This effect is ascribed to a weakening of interlayer As-As bonds by electron doping. Upon further electron doping filamentary superconductivity with Tc of 41 K at y=0.2 is observed.
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