Antiferromagnetic and nematic phase transitions inBaFe2(As1−xPx

Abstract

We study the antiferromagnetic (AFM) and structural phase transitions in single crystal BaFe 2 (As 1-x P x ) 2 ( 0, 0.3 x ) at temperatures T N and T S , respectively, by high resolution ac microcalorimetry and SQUID magnetometry. The specific heat measurements of both as grown and annealed BaFe 2 As 2 displays a sharp peak at the AFM/Structural transitions. A kink in the entropy of annealed BaFe 2 As 2 gives evidence for splitting of the two transitions by approximately 0.5K. No additional features could be identified in the specific heat of both BaFe 2 As 2 and BaFe 2 (As 0.7 P 0.3 ) 2 in the temperature regions around T* > T S where torque measurements [S. Kasahara et al., Nature 486, 382 (2012)] had revealed the "true" nematic phase transition, indicating that the behavior at T* does not represent a 2 nd order phase transition, and that the phase transition of BaFe 2 (As 1-x P x ) 2 into the orthorhombic phase does occur at T S .

Electronic nematic correlations in the stress-free tetragonal state ofBaFe2−xNixAs2

“…5(h)]. Since our neutron Larmor diffraction measurements showed no additional anomaly in lattice parameters and lattice distortion above T s , we conclude that there is no thermodynamic phase transition at T * and T * * in agreement with recent heat capacity measurements [21]. The resistivity anisotropy seen in the stress free detwinned samples below T * * on warming across T N is then due to local spin nematic correlations and associated lattice distortions arising from the magnetoelastic coupling through the collinear AF state below T N .…”

“…These results suggest that the resistivity anisotropy observed in the external uniaxial pressure free tetragonal state of BaFe 2−x Ni x As 2 arises from a strong magnetoelastic coupling induced by AF order, and such anisotropy would vanish in a strain free paramagnetic tetragonal phase. Furthermore, our data indicate no additional thermodynamic phase transitions above T s , consistent with earlier work [21,31]. Therefore, the Ising-nematic correlations, a state with no magnetic long-range order (stag-gered magnetization M = 0) but with local broken C 4 symmetry lattice distortion [32][33][34], is the driving force for the observed resistivity anisotropy (Fig.…”

“…However, since the uniaxial strain necessary to detwin the sample also enhances T N [19] and introduces an explicit symmetry breaking field, it is unclear if there will be resistivity anisotropy in the stress free tetragonal state below T * upon releasing the applied external uniaxial strain as expected in an ideal electronic nematic phase. From transport [16][17][18], inelastic neutron scattering [20], and thermodynamic measurements [21], T * is believed to mark a temperature range of nematic fluctuations with structure and magnetic phase transitions occurring at T s and T N , respectively. On the other hand, magnetic torque and X-ray diffraction experiments on supposedly strain free samples of BaFe 2 As 2 and BaFe 2 (As 1−x P x ) 2 suggest that T * is a signature of a "true" second-order nematic phase transition from the high-temperature tetragonal phase to a low-energy orthorhombic phase, and there are no additional C 4 symmetry breaking structural transitions below T * [22].…”

Surface nematic order in iron pnictides

“…These observations are also consistent with finite orbital splitting persisting above the bulk transition found for unstressed Ba(Fe 1−x Co x ) 2 As 2 crystals by ARPES. 29 On the other hand, the recent high-resolution specific heat measurements 35 clearly excluded possibility of the bulk phase transition in this temperature range.…”

Nematic Fluctuations in Optimally Doped BaFe1.87Co0.13As2 Observed in Photoinduced Reflectivity Change

“…In contrast, Kasahara et al, reported that the magneto‐torque and X‐ray diffraction measurements on a micro‐sized single‐domain crystal of BaFe 2 (As,P) 2 show the nematic properties above the macroscopic crystallographic phase‐transition temperature T S even without any external pressure and suggested that a weak but static nematic phase transition should be detected in the specific heat measurement. However, no clear phase transition has been detected in the AC magneto‐calorimetry measurements on BaFe 2 (As,P) 2 above T S …”