Hydrostatic-pressure tuning of magnetic, nonmagnetic, and superconducting states in annealed Ca(Fe1−xCox)

Abstract

We report on measurements of the magnetic susceptibility and electrical resistance under He-gas pressure on single crystals of Ca(Fe1−xCox)2As2. We find that for properly heat-treated crystals with modest Co-concentration, x = 0.028, the salient ground states associated with iron-arsenide superconductors, i.e., orthorhombic/antiferromagnetic (o/afm), superconducting, and nonmagnetic collapsed-tetragonal (cT) states can be accessed all in one sample with reasonably small and truly hydrostatic pressure. This is possible owing to the extreme sensitivity of the o/afm (for T ≤ Ts,N ) and superconducting (T ≤ Tc) states against variation of pressure, disclosing pressure coefficients of dTs,N /dP = -(1100 ± 50) K/GPa and dTc/dP = -(60 ± 3) K/GPa, respectively. Systematic investigations of the various phase transitions and ground states via pressure tuning revealed no coexistence of bulk superconductivity (sc) with the o/afm state which we link to the strongly firstorder character of the corresponding structural/magnetic transition in this compound. Our results, together with literature results, indicate that preserving fluctuations associated with the o/afm transition to low enough temperatures is vital for sc to form.

Collapsed tetragonal phase transition in LaRu2P2

“…18 The electronic properties of the cT phase are vastly different than of the un-collapsed tetragonal (TET) phase due to the abrupt change of the electronic bandstructure, associated with the structural change 19 , e.g., the transition into the cT phase often leads to the loss of magnetism or SC. 8,9,[20][21][22][23] Therefore, studying the pressure-temperature phase diagrams and the different ground states is fundamental for understanding SC this family of materials. Some compounds with ThCr 2 Si 2 structure can support conventional superconductivity while still exhibiting intriguing ground states.…”

Thermal expansion of CaFe2As2: Effect of cobalt doping and postgrowth thermal treatment

“…This dT c /dP value is close to that directly measured under hydrostatic pressure. 20 The dT c /dp c ≈ −20 K/kbar value is phenomenally large and provides further evidence of the extreme pressure and stress sensitivity of these compounds.…”

“…18,19 The effects of annealing and quenching were shown to be similar to those of the pressure. [18][19][20] It was suggested that the annealing and quenching process, via the precipitates, controls the amount of strain built up in the samples and this way acts similarly to a moderate applied external pressure. It was shown 19 that by careful choice of Co concentration and annealing and quenching parameters, the Ca(Fe 1−x Co x ) 2 As 2 system can by systematically and reproducibly tuned to have one of four different ground states: orthorhombic and antiferromagnetic, tetragonal and superconducting, tetragonal and nonsuperconducting, and collapsed tetragonal without magnetic order or superconductivity.…”

“…The equivalence between annealing and quenching and pressure was explored further in Ref. 20, where three different low temperature phases were observed in one sample on application of a moderate hydrostatic pressure, below 3 kbar, and record-high in their absolute values, for inorganic compounds, pressure derivatives of the Néel temperature (T N ), superconducting transition temperature (T c ), and structural transition temperature to the collapsed tetragonal phase (T cT ) were measured.…”

Commentary: The Hash House Harriers and the winding path to materials discovery

“…13 In order to understand this compound, we had to commit to detailed analyses of process-property relations 14,15 as well as even pay attention to what type of grease we used to mount crystals for measurement. We found that CaFe 2 As 2 is a wonderful system, it is an inorganic superconductor with organic-like sensitivities, 16 it serves as a key example of FeAs-based superconductivity, but we had to be willing to pay the price and understand the trail we were jogging on.…”