Ferromagnetism and superconductivity in CeFeAs1−xPxO (

Anton Jesche

¹

,

Tobias Förster

²

,

J. Spehling

³

et al.

Abstract

We report on superconductivity in CeFeAs 1−x P x O and the possible coexistence with Ce ferromagnetism (FM) in a small homogeneity range around x = 30% with ordering temperatures of T SC ∼ = T C ∼ = 4 K. The antiferromagnetic (AFM) ordering temperature of Fe at this critical concentration is suppressed to T Fe N ≈ 40 K and does not shift to lower temperatures with a further increase of the P concentration. Therefore, a quantumcritical-point scenario with T Fe N → 0 K which is widely discussed for the iron based superconductors can be excluded for this alloy series. Surprisingly, thermal expansion and x-ray powder diffraction indicate the absence of an orthorhombic distortion despite clear evidence for short-range AFM Fe ordering from muonspin-rotation measurements. Furthermore, we discovered the formation of a sharp electron spin resonance signal unambiguously connected with the emergence of FM ordering. One interesting aspect of iron arsenides is the large variety of possible manipulations that result in superconductivity starting from antiferromagnetically (AFM) ordered parent compounds. Taking LaFeAsO as an example, superconductivity (SC) can be induced by pressure, 1 oxygen vacancies, 2 or substitution of any of the elements, e.g., La → Sr, 3 Fe → Co, 4 As → P, 5 or O → F. 6 The suppression of AFM ordering of Fe is leading to a superconducting ground state in all of these cases. This is a very general property of the Fe-based superconductors and in fact there are only a few examples known where SC does not show up despite vanishing Fe ordering [e.g., Mn-doped LaFeAsO 7 or BaFe 2 As 2 8 ]. In this sense CeFeAs 1−x P x O presents a unique situation because of the following: First, replacing As with P "usually" results in SC as shown for AFe 2 As 2 [A = Ca, 9 Ba, 10 Sr, 9 Eu 11 ] and RFeAsO [R = La, 5 Sm 12 ], and second, CeFeAsO becomes superconducting when substituting Fe with Co 13 or O with F 14 and also by inducing oxygen vacancies. 2 But combining P doping with R = Ce among the RFeAsO compounds results in exceptional behavior: The Fe ordering is suppressed, as shown by de la Cruz et al. 15 and Luo et al.,16 however, superconductivity had not yet been observed in CeFeAs 1−x P x O. Instead a crossover to ferromagnetic (FM) ordering of Ce was reported at a critical concentration of x ≈ 40%, where the AFM ordering of Fe vanishes. 16 In contrast or in addition to this results we have found the following: (1) long-range FM ordering coexisting with superconductivity in a small homogeneity range around x = 30%, (2) evidence against a quantum-critical scenario, (3) static AFM ordering of Fe moments in the absence of structural distortion, and (4) a clear electron spin resonance (ESR) signal connected to ferromagnetism.Polycrystalline and single crystalline materials were syn-

Hydrostatic and chemical pressure tuning of CeFeAs1−xPxO single crystals

“…At ambient pressure, R(T ) exhibits a maximum followed by a pronounced drop attributed to the onset of Fe-SDW ordering at T Fe N = 93 K and a further kink at T Ce N = 3.5 K due to the AFM ordering of Ce moments, which is in agreement with Ref. 17. Compared with CeFeAsO, 22% P substitution already suppresses T Fe N by about 50 K. The feature at T Fe N shifts to lower temperatures on application of hydrostatic pressure up to p = 1.95 GPa.…”

“…[12][13][14][15][16] Here, CeFeAs 1−x P x O is outstanding among the 1111-type iron-pnictide materials based on rare-earth elements: P substitution also suppresses the Fe-SDW ordering, but the rare-earth magnetism of the Ce moments changes from antiferromagnetic (AFM) at low (x < 0.3) to ferromagnetic (FM) at higher P concentrations (x 0.3). 17,18 Only recently, zero resistance was observed in CeFeAs 0.70 P 0.30 O single crystals close to the crossover from AFM to FM ordering of the Ce moments in an already ferromagnetically ordered sample. 17 The application of modest pressure leads to an increase of T c in doped LaFeAsO 1−x F x 8,9 and LaFePO 10 pnictide superconductors.…”

“…17,18 Only recently, zero resistance was observed in CeFeAs 0.70 P 0.30 O single crystals close to the crossover from AFM to FM ordering of the Ce moments in an already ferromagnetically ordered sample. 17 The application of modest pressure leads to an increase of T c in doped LaFeAsO 1−x F x 8,9 and LaFePO 10 pnictide superconductors. On the other hand, pressure induces a decrease of T c in F-doped CeFeAsO and an enhancement of T c in Co-doped CeFeAsO.…”

“…In the following we use the nominal P concentration x, which was found to be in good agreement with the actual composition. 17 Four-probe electrical-resistance measurements in the ab plane were carried out on single crystals with approximate dimensions 500 × 150 × 80 mm 3 using an LR700 resistance bridge. Temperatures down to 1.8 K and magnetic fields up to 14 T were achieved in a Quantum Design PPMS.…”

Probing the Mott physics inκ-(BEDT-TTF)2X salts via thermal expansion

“…As already mentioned in the introduction, thermal expansion coefficient measurements are a very useful and powerful tool to explore phase transitions in solid state physics research [3,4,47,[73][74][75][76][77][78][79][80][81][82][83][84][85]. For instance, combining specific heat and thermal expansion data one can employ the Ehrenfest relation to determine the pressure dependence of a secondorder phase transition temperature.…”