Structural collapse and superconductivity in rare earth-doped CaFe2As2 (original) (raw)
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Structural anomaly in superconductivity of CaFe2As2 class of materials
2020
Quantum transitions in Fe-based systems are believed to involve spin, charge and nematic fluctuations. Complex structural phase diagram in these materials often emphasizes importance of covalency in their exotic properties, which is directly linked to the local structural network and barely understood. In order to address this outstanding issue, we investigate the evolution of the structural parameters and their implication in unconventional superconductivity of 122 class of materials employing extended x-ray absorption fine structure studies. The absorption spectral functions near the Fe K-edge and As K-edge of CaFe2As2 and its superconducting composition, CaFe1.9Co0.1As2 (Tc = 12 K) exhibit evidence of enhancement of Fe contributions near the Fermi level. As-Fe and Fe-Fe bondlengths derived from the experimental data show interesting changes with temperature across the magneto-structural transition; the evolution of these parameters in Co-doped composition is similar to its parent compound despite absence of magneto-structural transition in the studies of their bulk properties. These results reveal evidence of doping induced evolution to the proximity to critical behavior presumably leading to superconductivity in the system.
High pressure effects on the superconductivity in rare-earth-doped CaFe2As2
High Pressure Research, 2014
High-pressure superconductivity in a rare-earth doped Ca 0.86 Pr 0.14 Fe 2 As 2 single crystalline sample has been studied up to 12 GPa and temperatures down to 11 K using designer diamond anvil cell under a quasi-hydrostatic pressure medium. The electrical resistance measurements were complemented by high pressure and low temperature x-ray diffraction studies at a synchrotron source. The electrical resistance measurements show an intriguing observation of superconductivity under pressure, with T c as high as ~51 K at 1.9 GPa, presenting the highest T c reported in the intermetallic class of 1-2-2 ironbased superconductors. The resistive transition observed suggests a possible existence of two superconducting phases at low pressures of 0.5 GPa: one phase starting at T c1 ~48 K, and the other starting at T c2~1 6 K. The two superconducting transitions show distinct variations with increasing pressure. High pressure low temperature structural studies indicate that the superconducting phase is a collapsed tetragonal ThCr 2 Si 2-type (122) crystal structure. Our high pressure studies indicate that high T c state attributed to non-bulk superconductivity in rare-earth doped 1-2-2 iron-based superconductors is stable under compression over a broad pressure range.
Manipulating superconducting phases via current-driven magnetic states in rare-earth-doped CaFe2As2
NPG Asia Materials
Inhomogeneous superconductivity in rare-earth (RE)-doped CaFe 2 As 2 (Ca122) compounds leads to a novel state of matter in which the superconducting and magnetic states can be simultaneously controlled by using an electric current (I). Both La-and Ce-doped Ca122 single crystals show a very broad superconducting transition width (ΔT c) due to their non-bulk nature. Surprisingly, ΔT c becomes sharper or broader after an electric current larger than a threshold value (I t) is applied, with a concomitant change in the normal-state magnetism. The sharpened (broadened) ΔT c is accompanied by a decrease (an increase) in the amplitude of the ferromagnetic signals. The sensitive changes in the superconductivity and magnetism that occur when an external current is applied are related to the inhomogeneous electronic states that originate from the Fe magnetic state and/or self-organized superconducting/magnetic composites in Ca122 compounds. These discoveries shed new light on the role of Fe in Fe-based superconductors and will provide new ideas for the design of novel superconducting devices.
High Pressure Effects on the Interfacial Superconductivity in Rare Earth Doped CaFe2As2
2013
High-pressure superconductivity in a rare-earth doped Ca 0.86 Pr 0.14 Fe 2 As 2 single crystalline sample has been studied up to 12 GPa and temperatures down to 11 K using designer diamond anvil cell under a quasi-hydrostatic pressure medium. The electrical resistance measurements were complemented by high pressure and low temperature x-ray diffraction studies at a synchrotron source. The electrical resistance measurements show an intriguing observation of superconductivity under pressure, with T c as high as ~51 K at 1.9 GPa, presenting the highest T c reported in the intermetallic class of 1-2-2 ironbased superconductors. The resistive transition observed suggests a possible existence of two superconducting phases at low pressures of 0.5 GPa: one phase starting at T c1 ~48 K, and the other starting at T c2~1 6 K. The two superconducting transitions show distinct variations with increasing pressure. High pressure low temperature structural studies indicate that the superconducting phase is a collapsed tetragonal ThCr 2 Si 2-type (122) crystal structure. Our high pressure studies indicate that high T c state attributed to non-bulk superconductivity in rare-earth doped 1-2-2 iron-based superconductors is stable under compression over a broad pressure range.
DAE SOLID STATE PHYSICS SYMPOSIUM 2019, 2020
We investigate the role of ligand states in the electronic properties of CaFe2As2 using high-resolution hard x-ray photoemission spectroscopy (HAXPES) at different sample temperatures. Experimental results indicate that the binding energy of Ca is close to that for 2+ charge state of Ca atoms and the other constituent elements, Fe and As possess electronic configuration close to that in elemental systems. No difference is observed in the As 3p core level spectra with the change of emission angle and/or the change in sample temperature. This is surprising as the Ca atoms at the cleaved sample surface reorganizes itself to form linear structures which is expected to influence Ca-As hybridization leading to significant difference in surface and bulk electronic structures. Moreover, CaFe2As2 undergoes structural and magnetic phase transition at 170 K, and strong Fe-As hybridization provides pathways for electron dynamics. Clearly, further studies are required to resolve these puzzling observations.
Emergence of well-screened states in a superconducting material of the CaFe2As2 family
Physical Review B
Coupling among conduction electrons (e.g., Zhang-Rice singlet) are often manifested in the core level spectra of exotic materials such as cuprate superconductors, manganites, etc. These states are believed to play key roles in the ground state properties and appear as low binding energy features. To explore such possibilities in the Fe-based systems, we study the core level spectra of a superconductor CaFe 1.9 Co 0.1 As 2 (CaCo122) in the CaFe 2 As 2 (Ca122) family employing high-resolution hard x-ray photoemission spectroscopy. While As core levels show almost no change with doping and cooling, the Ca 2p peak of CaCo122 shows reduced surface contribution relative to Ca122 and a gradual shift of the peak position towards lower binding energies with cooling. In addition, we discover the emergence of a feature at the lower binding energy side of the well-screened Fe 2p signal in CaCo122. The intensity of this feature grows with cooling and indicates additional channels to screen the core holes. The evolution of this feature in the superconducting composition and its absence in the parent compound suggests relevance of the underlying interactions in the ground state properties of this class of materials. These results reveal another dimension in the studies of Fe-based superconductors and the importance of such states in the unconventional superconductivity in general.
Why is the superconducting Tc so high in rare-earth-doped CaFe2As2?
In rare-earth doped single crystalline CaFe2As2, the mysterious small volume fraction which superconducts up to 49 K, much higher than the bulk Tc ~ 30s K, has prompted a long search for a hidden variable that could enhance the Tc by more than 30% in iron-based superconductors of the same structure. Here we report a chemical, structural, and magnetic study of CaFe2As2 systematically doped with La, Ce, Pr, and Nd. Coincident with the high Tc phase, we find extreme magnetic anisotropy, accompanied by an unexpected doping-independent Tc and equally unexpected superparamagnetic clusters associated with As vacancies. These observations lead us to conjecture that the tantalizing Tc enhancement may be associated with naturally occurring chemical interfaces and may thus provide a new paradigm in the search for superconductors with higher Tc.
Pressure-induced volume-collapsed tetragonal phase of CaFe2As2 as seen via neutron scattering
Physical Review B, 2008
Recent investigations of the superconducting iron-arsenide families have highlighted the role of pressure, be it chemical or mechanical, in fostering superconductivity. Here we report that CaFe2As2 undergoes a pressure-induced transition to a non-magnetic, volume "collapsed" tetragonal phase, which becomes superconducting at lower temperature. Spin-polarized total-energy calculations on the collapsed structure reveal that the magnetic Fe moment itself collapses, consistent with the absence of magnetic order in neutron diffraction. PACS numbers: 61.50.Ks, 61.05.fm, 71.15.Nc, 74.62.Fj; Paper accepted for publication in Phys. Rev. B
Quantum oscillations in antiferromagnetic CaFe2As2on the brink of superconductivity
Journal of Physics: Condensed Matter, 2009
We report quantum oscillation measurements on CaFe2As2 under strong magnetic fields− recently reported to become superconducting under pressures of as little as a kilobar. The largest observed carrier pocket occupies less than 0.05 % of the paramagnetic Brillouin zone volume− consistent with Fermi surface reconstruction caused by antiferromagnetism. On comparing several alkali earth AFe2As2 antiferromagnets (with A = Ca, Sr and Ba), the dependence of both the Fermi surface cross-sectional area Fα and the effective mass m * α of the primary observed pocket on the antiferromagnetic/structural transition temperature Ts is found to be consistent with quasiparticles in a conventional spin-density wave model. These findings suggest that a conventional spin-density wave exists within close proximity to superconductivity in this series of compounds, which may have implications for the microscopic origin of unconventional pair formation.
Physical Review B, 2010
CaFe2As2 single crystals under uniaxial pressure applied along the c axis exhibit the coexistence of several structural phases at low temperatures. We show that the room temperature tetragonal phase is stabilized at low temperatures for pressures above 0.06 GPa, and its weight fraction attains a maximum in the region where superconductivity is observed under applied uniaxial pressure. Simultaneous resistivity measurements strongly suggest that this phase is responsible for the superconductivity in CaFe2As2 found below 10 K in samples subjected to non-hydrostatic pressure conditions.