Magnetism is widely considered to be a key ingredient of unconventional superconductivity.In contrast to cuprate high-temperature superconductors, antiferromagnetism in Fe-based superconductors (FeSCs) is characterized by a pair of magnetic propagation vectors 1, 2 . Consequently, three different types of magnetic order are possible. Of theses, only stripe-type spin-density wave (SSDW) and spin-charge-density wave (SCDW) orders have been observed 2-4 . A realization of the proposed spin-vortex crystal (SVC) order is noticeably absent. We report a magnetic phase consistent with the hedgehog variation of SVC order in Ni-and Co-doped CaKFe 4 As 4 based on thermodynamic, transport, structural and local magnetic probes combined with symmetry analysis. The exotic SVC phase is stabilized by the reduced symmetry of the CaKFe 4 As 4 structure.Our results suggest that the possible magnetic ground states in FeSCs have very similar energies, providing an enlarged configuration space for magnetic fluctuations to promote high-temperature superconductivity.

We report the temperature-pressure phase diagram of CaKFe4As4 established using high pressure electrical resistivity, magnetization and high energy x-ray diffraction measurements up to 6 GPa. With increasing pressure, both resistivity and magnetization data show that the bulk superconducting transition of CaKFe4As4 is suppressed and then disappears at p 4 GPa. High pressure x-ray data clearly indicate a phase transition to a collapsed tetragonal phase in CaKFe4As4 under pressure that coincides with the abrupt loss of bulk superconductivity near 4 GPa. The x-ray data, combined with resistivity data, indicate that the collapsed tetragonal transition line is essentially vertical, occuring at 4.0(5) GPa for temperatures below 150 K. Band structure calculations also find a sudden transition to a collapsed tetragonal state near 4 GPa, as As-As bonding takes place across the Ca-layer. Bonding across the K-layer only occurs for p ≥ 12 GPa. These findings demonstrate a new type of collapsed tetragonal phase in CaKFe4As4: a half-collapsed-tetragonal phase.

By performing pressure simulations within density functional theory for the family of iron-based superconductors AeAFe4As4 with Ae = Ca, Sr, Ba and A = K, Rb, Cs we predict in these systems the appearance of two consecutive half-collapsed tetragonal transitions at pressures Pc 1 and Pc 2 , which have a different character in terms of their effect on the electronic structure. We find that, similarly to previous studies for CaKFe4As4, spin-vortex magnetic fluctuations on the Fe sublattice play a key role for an accurate structure prediction in these materials at zero pressure. We identify clear trends of critical pressures and discuss the relevance of the collapsed phases in connection to magnetism and superconductivity. Finally, the intriguing cases of EuRbFe4As4 and EuCsFe4As4, where Eu magnetism coexists with superconductivity, are discussed as well in the context of halfcollapsed phases. PACS numbers:Introduction.-The so-called 122 Fe-based pnictides ( Fig. 1b) (AFe 2 As 2 , AeFe 2 As 2 , EuFe 2 As 2 ) with A alkali and Ae alkaline-earth cations crystallize at room temperature in a body-centered tetragonal ThCr 2 Si 2 structure (I4/mmm) 1,2 where As sites from the neighboring Fe-As blocks face each other across the A or Ae plane. The As-As interlayer distance in these systems can be then tuned by either mechanical or chemical pressure down to sufficiently small values allowing the formation of As-As p z bonds. This is accompanied by a structural phase transition to a collapsed tetragonal (cT) phase where the c/a ratio is significantly reduced due to a dramatic contraction of the c-lattice parameter and a slight expansion of the a-lattice parameter. This process is known to suppress superconductivity or/and long-range "stripe" magnetic order ( Fig. 1d) due to the crossover to a more three-dimensional structure and the loss of spin fluctuations and local Fe moments caused by a compression of Fe-As bonds. 3 In the 122 materials, the transition to a cT phase affects the whole structure leading to As-As p z bond formation across each cation spacer layer.In contrast, a half-collapsed tetragonal (hcT) phase transition was recently reported for the 1144 material CaKFe 4 As 4 4 ( Fig. 1c) where the periodic arrangement of Ca and K spacer layers produces two different kinds of As sites 5-7 and the tetragonal structure (P 4/mmm) shows a layer-selective collapse upon application of pressure. First, at 4 GPa the As-As p z bonding across the Ca layer induces a collapsed tetragonal transition with disappearance of superconductivity while a second collapsed transition across the K layer was predicted around ∼12 GPa. Furthermore, Ref. 4 showed that "hedgehog" (spin-vortex, Fig. 1e) magnetism had to be invoked in the pressure-dependent density functional theory (DFT) simulations in order to predict the observed structural transitions. This magnetic order has been recently measured upon electron-doping CaKFe 4 As 4 . 8 FIG. 1: (a) Cations of alkali (1+) and alkaline-earth (2+) elements, as well as divalent Eu, together with their...

We report 75 As nuclear magnetic resonance (NMR) studies on a new iron-based superconductor CaKFe4As4 with Tc = 35 K. 75 As NMR spectra show two distinct lines corresponding to the As(1) and As(2) sites close to the K and Ca layers, respectively, revealing that K and Ca layers are well ordered without site inversions. We found that nuclear quadrupole frequencies νQ of the As(1) and As(2) sites show an opposite temperature (T ) dependence. Nearly T independent behavior of the Knight shifts K are observed in the normal state, and a sudden decrease in K in the superconducting (SC) state suggests spin-singlet Cooper pairs. 75 As spin-lattice relaxation rates 1/T1 show a power law T dependence with different exponents for the two As sites. The isotropic antiferromagnetic spin fluctuations characterized by the wavevector q = (π, 0) or (0, π) in the single-iron Brillouin zone notation are revealed by 1/T1T and K measurements. Such magnetic fluctuations are necessary to explain the observed temperature dependence of the 75 As quadrupole frequencies, as evidenced by our first-principles calculations. In the SC state, 1/T1 shows a rapid decrease below Tc without a Hebel-Slichter peak and decreases exponentially at low T , consistent with an s ± nodeless two-gap superconductor.PACS numbers:

The spinel/perovskite heterointerface γ-Al2O3/SrTiO3 hosts a two-dimensional electron system (2DES) with electron mobilities exceeding those in its all-perovskite counterpart LaAlO3/SrTiO3 by more than an order of magnitude despite the abundance of oxygen vacancies which act as electron donors as well as scattering sites. By means of resonant soft x-ray photoemission spectroscopy and ab initio calculations we reveal the presence of a sharply localized type of oxygen vacancies at the very interface due to the local breaking of the perovskite symmetry. We explain the extraordinarily high mobilities by reduced scattering resulting from the preferential formation of interfacial oxygen vacancies and spatial separation of the resulting 2DES in deeper SrTiO3 layers. Our findings comply with transport studies and pave the way towards defect engineering at interfaces of oxides with different crystal structures.The search for high-mobility two-dimensional electron systems (2DES) at atomically engineered transition metal oxide heterointerfaces is an ongoing endeavor, since the strong electronic correlations in partially occupied d-orbitals promise an even richer physical behavior than found in conventional semiconductor heterostructures [1]. However, the charge carrier mobilities in the most prominent complex oxide 2DES, found at the perovskite-perovskite heterointerface between the band insulators LaAlO 3 and SrTiO 3 , still fall short of those in semiconductors by several orders of magnitude [2]. The hitherto-highest mobility in SrTiO 3 -based structures (140,000 cm 2 /Vs at 2 K) is found at the spinel/perovskite heterointerface between γ-Al 2 O 3 thin films and SrTiO 3 [3, 4], thus making it a promising candidate for applications in oxide electronics or fundamental studies of quantum transport. The mechanism of 2DES formation in SrTiO 3 -based heterostructures has been under debate for many years.