Aliovalent rare earth substitution into the alkaline earth site of CaFe2As2 single-crystals is used to fine-tune structural, magnetic and electronic properties of this iron-based superconducting system. Neutron and single crystal x-ray scattering experiments indicate that an isostructural collapse of the tetragonal unit cell can be controllably induced at ambient pressures by choice of substituent ion size. This instability is driven by the interlayer As-As anion separation, resulting in an unprecedented thermal expansion coefficient of 180 × 10 −6 K −1 . Electrical transport and magnetic susceptibility measurements reveal abrupt changes in the physical properties through the collapse as a function of temperature, including a reconstruction of the electronic structure. Superconductivity with onset transition temperatures as high as 47 K is stabilized by the suppression of antiferromagnetic order via chemical pressure, electron doping or a combination of both. Extensive investigations are performed to understand the observations of partial volume-fraction diamagnetic screening, ruling out extrinsic sources such as strain mechanisms, surface states or foreign phases as the cause of this superconducting phase that appears to be stable in both collapsed and uncollapsed structures.
We report a Fe Kβ x-ray emission spectroscopy study of local magnetic moments in the rare-earth doped iron pnictide Ca1−xRExFe2As2 (RE=La, Pr, and Nd). In all samples studied the size of the Fe local moment is found to decrease significantly with temperature and goes from ∼0.9µB at T = 300 K to ∼0.45µB at T = 70 K. In the collapsed tetragonal (cT) phase of Nd-and Pr-doped samples (T<70K) the local moment is quenched, while the moment remains unchanged for the La-doped sample, which does not show lattice collapse. Our results show that Ca1−xRExFe2As2 (RE= Pr and Nd) exhibits a spin-state transition and provide direct evidence for a non-magnetic Fe 2+ ion in the cT-phase, as predicted by Yildirim. We argue that the gradual change of the the spin-state over a wide temperature range reveals the importance of multiorbital physics, in particular the competition between the crystal field split Fe 3d orbitals and the Hund's rule coupling.PACS numbers: 74.70. Xa, 75.20.Hr, 78.70.En, 75.30.Wx The interesting orbital physics found in many 3d and 4d transition metal compounds, such as manganites [1,2] and ruthenates [2], seems to play an important role in the iron based superconductors as well [3][4][5][6][7][8][9]. In the iron pnictides, many low-energy probes such as transport [10], scanning tunnelling microscopy [11], inelastic neutron scattering [12], angle-resolved photoemission spectroscopy [13,14], and most recently magnetic torque measurements [15] have reported a strong in-plane anisotropy of electronic properties. These results have spurred a great deal of interest in the orbital physics of the iron pnictides, in particular the possibility of orbital order [3][4][5][6][7][8][9].An important aspect of the orbital physics is the competition between the Hund's rule coupling constant J H and the crystal field splitting, ∆ CF . In the case of LaCoO 3 , the energy scales of ∆ CF and J H are similar, resulting in spin-state transition; Co 3+ ions take on a low-spin state (S=0) at low temperature, but go into thermally excited high/intermediate-spin (S=2 or S=1) states at elevated temperature [16,17] Among the iron based superconductors, CaFe 2 As 2 offers perhaps the best system to investigate the competition between ∆ CF [20] and J H , and its effect on the spin-state. Like many iron pnictides, CaFe 2 As 2 goes from a high temperature tetragonal phase (T-phase) to an orthorhombic and antiferromagnetically (AFM) ordered phase, below T N ≈ 170 K [21]. More importantly, CaFe 2 As 2 takes on yet another structural phase at low temperatures through application of a modest pressure of 0.35 GPa [22] or chemical doping, with rare-earths [23] or phosphorus [24]. Upon entering this phase, known as the collapsed tetragonal phase (cT-phase), the lattice undergoes a ∼ 10% reduction along the c-axis and an ∼2% increase along the a-axis. This is accompanied by a disappearance of the AFM order [22], supression of spin fluctuations [25], and recovery of Fermi liquid behavior [24]. It is thus clear that an unusually dramatic lattice i...
We report superconductivity in single crystals of the new iron-pnictide system BaFe(1.90)Pt(0.10)As(2) grown by a self-flux solution method and characterized via x-ray, transport, magnetic and thermodynamic measurements. The magnetic ordering associated with a structural transition at 139 K present in BaFe(2)As(2) is completely suppressed by substitution of 5% Fe with Pt and superconductivity is induced at a critical temperature T(c) = 23 K. Full diamagnetic screening in the magnetic susceptibility and a jump in the specific heat at T(c) confirm the bulk nature of the superconducting phase. All properties of the superconducting state-including the transition temperature T(c), the lower critical field H(c1) = 200 mT, the upper critical field H(c2)≈ 65 T, and the slope ∂H(c2)/∂T-are comparable in value to those found in other transition metal-substituted BaFe(2)As(2) series, indicating the robust nature of superconductivity induced by substitution of Group VIII elements.
The electronic nematic phase, wherein electronic degrees of freedom lower the crystal rotational symmetry, is a common motif across a number of high-temperature superconductors. However, understanding the role and influence of nematicity and nematic fluctuations in Cooper pairing is often complicated by the coexistence of other orders, particularly long-range magnetic order. Here we report the enhancement of superconductivity in a model electronic nematic system absent of magnetism, and show that the enhancement is directly born out of strong nematic fluctuations emanating from a tuned quantum phase transition. We use elastoresistance measurements of the Ba1−xSrxNi2As2 substitution series to show that strontium substitution promotes an electronically driven B1g nematic order in this system, and that the complete suppression of that order to absolute zero temperature evokes a dramatic enhancement of the pairing strength, as evidenced by a sixfold increase in the superconducting transition temperature. The direct relation between enhanced pairing and nematic fluctuations in this model system, as well as the interplay with a unidirectional charge density wave order comparable to that found in the cuprates, offers a means to elucidate the role of nematicity in boosting superconductivity. I.High-temperature superconductivity in both cuprate [1, 2] and iron-based materials [3-5] emerges from a notably complex normal state. Though magnetic spin fluctuations are commonly believed to drive Cooper pairing in both of these families, the common occurrence of a rotational symmetry-breaking nematic phase has captured increasing attention in recent years [6,7]. In contrast to a conventional structural transition, overwhelming evidence suggests that the nematic phase in these compounds is promoted by an electronic instability rather than lattice softening [8,9].Theoretical analyses have shown that fluctuations associated with such an electronic nematic phase, particularly near a putative quantum critical point, can significantly enhance superconductivity [10][11][12][13][14]. Being peaked at zero wave-vector, nematic fluctuations favor pairing instabilities in several symmetry channels, in contrast to the case of magnetic fluctuations. Experiments have indeed shown a striking enhancement of nematic fluctuations centered at optimal tuning of superconductivity in a number of iron-based superconductors [8,9], and a strong tendency towards nematicity in high T c cuprate materials [15][16][17]. However, the overarching presence of magnetic fluctuations emanating from proximate antiferromagnetic instabilities complicates drawing any isolated relation between enhanced pairing and nematicity in most nematic materials. The FeSe 1−x S x substitution series is one exception, where the system exhibits both superconductivity and nematicity in the absence of magnetic order [18]. However, in this series, the superconducting transition temperature T c is virtually unaffected by tuning through the nematic quantum critical point [18,19], leaving ...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.