Parent Compound Of Superconductor Research Articles

From NdNiO2 to a Mott Multiferroic BiNiO2

Motivated by the recently discovered superconductivity in Sr-doped nickelate oxides NdNiO2 reduced from NdNiO3 in experiments, we predict a Mott multiferroic BiNiO2 that may be obtained from BiNiO3. There is a ferroelectric structural transition for BiNiO2 from the nonpolar phase to the polar phase with the P4mm space group, which is driven by the lone pair on Bi. The lowest energy ground-state for polar BiNiO2 is a Hubbard Mott insulator with the G-type antiferromagnetic spin configurations. The replacement of Nd by Bi serves as a connecting link between a parent compound of high-temperature superconductor and a Mott multiferroic.

Tetragonal SnOFeSe: A possible parent compound of the FeSe-based superconductor

Recent experiments have reported that inserting metal atoms or small molecules in between the FeSe layers of $\beta$-FeSe can significantly enhance the superconducting transition temperature. Here, based on first-principles electronic structure calculations, we propose a stable compound SnOFeSe by alternatively stacking the SnO and $\beta$-FeSe layers. The predicted SnOFeSe has the same tetragonal structure as the well-known FeAs-based compound LaOFeAs, meanwhile their electronic structures in the nonmagnetic state are quite similar. The magnetic ground state of SnOFeSe is predicted to be the dimer antiferromagnetic (AFM) state, which is energetically only 2.77 (2.15) meV/Fe lower than the trimer (dimer-trimer-dimer-trimer) AFM state, indicating that strong magnetic fluctuations might be induced via slight modulation. Interestingly, SnOFeSe is at the verge of metal-insulator transition in these low-energy magnetic states, hence bridging the metallic parent compounds of iron-based superconductors and the insulating ones of cuprate superconductors. With the reduced dimensionality, monolayer SnOFeSe also shows great similarities in the electronic and magnetic properties to its bulk phase. Given that SnOFeSe is adjacent to magnetic frustration and resembles LaOFeAs in both crystal and electronic structures, we suggest that SnOFeSe is a possible superconductor parent compound, which may provide a promising platform to study the interplay between magnetism and unconventional superconductivity in FeSe-derived materials.

Structure and Physical Properties of the Layered Titanium-Based Pnictide Oxides (EuF)2Ti2Pn2O (Pn = Sb, Bi).

We report two novel titanium-based pnictide oxide compounds (EuF)2Ti2Pn2O (Pn = Sb, Bi), which are synthesized by replacing Sr2+ in (SrF)2Ti2Pn2O [Liu, R. H. Structure and Physical Properties of the Layered Pnictide-Oxides: (SrF)2Ti2Pn2O (Pn = As, Sb) and (SmO)2Ti2Sb2O. Chem. Mater. 2010, 22, 1503-1508] with Eu2+ using a solid-state reaction. (EuF)2Ti2Sb2O exhibits an obvious anomaly in resistivity and heat capacity at T ∼ 195 K, which may arise from the spin-density wave/charge-density wave instability. Similar features are also observed in BaTi2Pn2O, (SrF)2Ti2Pn2O, and Na2Ti2Pn2O (Pn = As and Sb) [Liu, R. H. Structure and Physical Properties of the Layered Pnictide-Oxides: (SrF)2Ti2Pn2O (Pn = As, Sb) and (SmO)2Ti2Sb2O. Chem. Mater. 2010, 22, 1503-1508, Ozawa, T. C. Chemistry of layered d-metal pnictide oxides and their potential as candidates for new superconductors. Sci. Technol. Adv. Mater. 2008, 9, 033003, Wang, X. F. Structure and physical properties for a new layered pnictide-oxide: BaTi2As2O. J. Phys.: Condens. Matter. 2010, 22, 075702, and Xu, H. C. Electronic structure of the BaTi2As2O parent compound of the titanium-based oxypnictide superconductor. Phys. Rev. B 2014, 89, 155108]. Magnetic susceptibility measurements indicate an antiferromagnetic transition at T ∼ 2.5 K for (EuF)2Ti2Sb2O. In particular, the electronic specific heat coefficients of both (EuF)2Ti2Sb2O and (EuF)2Ti2Bi2O are significantly enhanced compared to those of (SrF)2Ti2Pn2O, Na2Ti2Pn2O, and BaTi2Pn2O,1,5,6 which may be due to a strong electron correlation effect in this system. Thus, (EuF)2Ti2Pn2O (Pn = Sb, Bi) may provide new platforms for studying density wave, magnetic ordering, and electron correlation effects.

In-plane electronic anisotropy revealed by interlayer resistivity measurements on the iron-based superconductor parent compound CaFeAsF

Both cuprates and iron-based superconductors demonstrate nematicity, defined as the spontaneous breaking of rotational symmetry in electron systems. The nematic state can play a role in the high-transition-temperature superconductivity of these compounds. However, the microscopic mechanism responsible for the transport anisotropy in iron-based compounds remains debatable. Here, we investigate the electronic anisotropy of CaFeAsF by measuring its interlayer resistivity under magnetic fields with varying field directions. Counterintuitively, the interlayer resistivity was larger in the longitudinal configuration ($B \parallel I \parallel c$) than in the transverse one ($B \perp I \parallel c$). The interlayer resistivity exhibited a so-called coherence peak under in-plane fields and was highly anisotropic with respect to the in-plane field direction. At $T$ = 4 K and $B$ = 14 T, the magnetoresistance $\Delta\rho/\rho_0$ was seven times larger in the $B \parallel b_o$ than in the $B \parallel a_o$ configuration. Our theoretical calculations of the conductivity based on the first-principles electronic band structure qualitatively reproduced the above observations but underestimated the magnitudes of the observed features. The proposed methodology can be a powerful tool for probing the nematic electronic state in various materials.

Anomalous high-field magnetotransport in CaFeAsF due to the quantum Hall effect

CaFeAsF is an iron-based superconductor parent compound whose Fermi surface is quasi-two dimensional, composed of Dirac-electron and Schrödinger-hole cylinders elongated along the c axis. We measured the longitudinal and Hall resistivities in CaFeAsF with the electrical current in the ab plane in magnetic fields up to 45 T applied along the c axis and obtained the corresponding conductivities via tensor inversion. We found that both the longitudinal and Hall conductivities approached zero above ~40 T as the temperature was lowered to 0.4 K. Our analysis indicates that the Landau-level filling factor is ν = 2 for both electrons and holes at these high field strengths, resulting in a total filling factor ν = νhole − νelectron = 0. We therefore argue that the ν = 0 quantum Hall state emerges under these conditions.

Dynamics of collective modes in an unconventional charge density wave system BaNi2As2

BaNi2As2 is a non-magnetic analogue of BaFe2As2, the parent compound of a prototype pnictide high-temperature superconductor, displaying superconductivity already at ambient pressure. Recent diffraction studies demonstrated the existence of two types of periodic lattice distortions above and below the triclinic phase transition, suggesting the existence of an unconventional charge-density-wave (CDW) order. The suppression of CDW order upon doping results in a sixfold increase in the superconducting transition temperature and enhanced nematic fluctuations, suggesting CDW is competing with superconductivity. Here, we apply time-resolved optical spectroscopy to investigate collective dynamics in BaNi2As2. We demonstrate the existence of several CDW amplitude modes. Their smooth evolution through the structural phase transition implies the commensurate CDW order in the triclinic phase evolves from the high-temperature unidirectional incommensurate CDW, and may indeed trigger the structural phase transition. Excitation density dependence reveals exceptional resilience of CDW against perturbation, implying an unconventional origin of the underlying electronic instability.

A New Superconductor Parent Compound NaMn6Bi5 with Quasi-One-Dimensional Structure and Lower Antiferromagnetic-Like Transition Temperatures

Mn-based superconductors are very rare and their superconductivity has only been reported in three-dimensional MnP and quasi-one-dimensional KMn6Bi5 and RbMn6Bi5 with [Mn6Bi5]− columns under high pressures. Here we report the synthesis, magnetism, electrical resistivity, and specific heat capacity of the newly discovered quasi-one-dimensional NaMn6Bi5. Compared with other AMn6Bi5 (A = K, Rb, and Cs), NaMn6Bi5 has abnormal Bi-Bi bond lengths and two antiferromagnetic-like transitions at 47.3 K and 51.8 K. Anisotropic resistivity and low-temperature non-Fermi liquid behavior are observed. Heat capacity measurement reveals that the Sommerfeld coefficient for NaMn6Bi5 is unusually large. Using first-principles calculations, an unusual enhancement of density of states near the Fermi level is demonstrated for NaMn6Bi5. The features make NaMn6Bi5 a more suitable platform to explore the interplay of magnetism and superconductivity.

Elasto-Raman scattering: Arsenic optical phonon as a probe of nematicity in BaFe2As2

We report a Raman scattering study of nematic degrees of freedom in the iron-based superconductor parent compound BaFe$_2$As$_2$ under tunable uniaxial strain. We demonstrate that the polarization resolved arsenic (As) phonon intensity can be used to monitor the nematic order parameter as a function of both temperature and strain. At low temperature in the nematic ordered phase we use it to track the continuous and reversible orientation of nematic domains under variable strain. At higher temperature, the evolution of the As phonon intensity under strain reflects an enhanced nematic susceptibility close to the nematic transition $T_S$. Its temperature dependence under strong strain follows qualitatively the expected behavior of an Ising order parameter under a symmetry breaking field. Our elasto-Raman study illustrates the interest of combining selective anisotropic strain with a symmetry resolved probe like Raman scattering. Elasto-Raman scattering can be applied to a wide variety of quantum materials where uniaxial strain tunes electronic orders.

Reinvestigation of crystal symmetry and fluctuations in La2CuO4

New surprises continue to be revealed about La$_2$CuO$_4$, the parent compound of the original cuprate superconductor. Here we present neutron scattering evidence that the structural symmetry is lower than commonly assumed. The static distortion results in anisotropic Cu-O bonds within the CuO$_2$ planes; such anisotropy is relevant to pinning charge stripes in hole-doped samples. Associated with the extra structural modulation is a soft phonon mode. If this phonon were to soften completely, the resulting change in CuO$_6$ octahedral tilts would lead to weak ferromagnetism. Hence, we suggest that this mode may be the "chiral" phonon inferred from recent studies of the thermal Hall effect. We also note the absence of interaction between the antiferromagnetic spin waves and low-energy optical phonons, in contrast to what is observed in hole-doped samples.

Prediction of antiferromagnetism in barium chromium phosphide confirmed after synthesis

We have carried out density-functional theory (DFT) calculations for the chromium pnictide BaCr2P2, which is structurally analogous to BaFe2As2, a parent compound for iron-pnictide superconductors. Evolutionary methods combined with DFT predict that the chromium analog has the same crystal structure as the latter. DFT also predicts Néel antiferromagnetic order on the chromium sites. Comparison with a simple electron-hopping model over a square lattice of chromium atoms suggests that it is due to residual nesting of the Fermi surfaces. We have confirmed the DFT predictions directly after the successful synthesis of polycrystalline samples of BaCr2P2. X-ray diffraction recovers the predicted crystal structure to high accuracy, while magnetic susceptibility and specific-heat measurements are consistent with a transition to an antiferromagnetically ordered state below K.

125Te-Mössbauer study of Fe1.1Te and FeTe0.5Se0.5 superconductor

A parent compound of iron-based superconductor, Fe1.1Te, and a Se-doped superconductor, FeTe0.5Se0.5, were investigated using 125Te-Mossbauer spectroscopy. By measuring precise temperature dependence of the spectra, discontinuous line broadening in the Fe1.1Te near the antiferromagnetic transition temperature (TN) of Fe was observed. While the broadening of the line width were observed in both samples as the temperature decreases, the temperature dependences of the line width were well reproduced as that of the recoilless fraction, assuming the Debye model with a Debye temperature of about 200 K. The discontinuous additional line broadening in the Fe1.1Te can be considered as the internal magnetic fields of Te below TN, since the temperature dependence of internal magnetic fields of Te is similar to that of Fe in 57Fe-Mossbauer spectroscopy. Therefore, hybridization of the electronic orbital of Te and Fe was suggested and the transferred magnetic moment of Te was induced through the antiferromagnetic order of Fe at TN.

The c-axis complex permittivity and electrical impedance in BaFe2As2: Experimental examination on transformation validity**Project supported by the National Natural Science Foundation of China (Grant No. 11004106), the National Basic Research Program of China (Grant No. 2015CB946502), the Chinese Academy of Sciences (Grant No. SPRP-B: XDB25000000), and the Youth Innovation Promotion Association of Chinese Academy of Sciences.

Complex permittivity and electrical impedance have been measured along the c-axis in single crystals BaFe2As2, which are the conductors known as the parent compound of 122-type iron superconductor. The resultant relative errors defined in the study indicate the existence of the transformation between complex permittivity and electrical impedance in the conductors, and these two physics quantities possibly reveal different aspects of the consistent superconductivity-relevant physics picture.

Study of Annealing Effect on Momentum Distribution Functions in Parent Compound of Electron-Doped High-Tc Superconductor by High Resolution Compton Scattering

Study of Annealing Effect on Momentum Distribution Functions in Parent Compound of Electron-Doped High-Tc Superconductor by High Resolution Compton Scattering

CsMn4As3: A Layered Tetragonal Transition-Metal Pnictide Compound with an Antiferromagnetic Ground State.

We report the synthesis and properties of a new layered tetragonal ternary compound CsMn4As3 (structure type, KCu4S3; space group, P4/ mmm, no. 123; and Z = 2). The material is a small band gap semiconductor and exhibits an antiferromagnetic ground state associated with Mn spins. The compound exhibits a signature of a distinct magnetic moment canting event at 150(5) K with a canting angle ≈ 0.3°. Although some features of the magnetic characteristics of this new compound are qualitatively similar to those of the related BaMn2As2, the underlying Mn sublattices of the two materials are quite different. While the Mn square-lattice layers in BaMn2As2 are equally spaced along the c-direction with the interlayer distance dLBa = 6.7341(4) Å, the Mn sublattice forms bilayers in CsMn4As3 with the interlayer distance within a bilayer being dLCs = 3.1661(6) Å; the distance between the two adjacent bilayers is dB = 7.290(6) Å. This difference in the Mn sublattice is bound to significantly alter the energy balance among the J1, J2, and J c exchange interactions within the J1- J2- J c model compared to those in BaMn2As2 and the other related 122 compounds, including the well-known iron-arsenide superconductor parent compound BaFe2As2. Owing to the novelty of its transition-metal sublattice, this new addition to the family of tetragonal materials related to the iron-based superconductors brings prospects for doping and pressure studies in the search of new superconducting phases as well as other exciting correlated electron properties.

Magnetism of PrFeAsO parent compound for iron-based superconductors: Mössbauer spectroscopy study

Magnetism of PrFeAsO parent compound for iron-based superconductors: Mössbauer spectroscopy study

Local and collective magnetism ofEuFe2As2

We present an experimental study of the local and collective magnetism of $\mathrm{EuFe_2As_2}$, that is isostructural with the high temperature superconductor parent compound $\mathrm{BaFe_2As_2}$. In contrast to $\mathrm{BaFe_2As_2}$, where only Fe spins order, $\mathrm{EuFe_2As_2}$ has an additional magnetic transition below 20 K due to the ordering of the Eu$^{2+}$ spins ($J =7/2$, with $L=0$ and $S=7/2$) in an A-type antiferromagnetic texture (ferromagnetic layers stacked antiferromagnetically). This may potentially affect the FeAs layer and its local and correlated magnetism. Fe-K$_\beta$ x-ray emission experiments on $\mathrm{EuFe_2As_2}$ single crystals reveal a local magnetic moment of 1.3$\pm0.15~\mu_B$ at 15 K that slightly increases to 1.45$\pm0.15~\mu_B$ at 300 K. Resonant inelastic x-ray scattering (RIXS) experiments performed on the same crystals show dispersive broad (in energy) magnetic excitations along $\mathrm{(0, 0)\rightarrow(1, 0)}$ and $\mathrm{(0, 0)\rightarrow(1, 1)}$ with a bandwidth on the order of 170-180 meV. These results on local and collective magnetism are in line with other parent compounds of the $\mathrm{AFe_2As_2}$ series ($A=$ Ba, Ca, and Sr), especially the well characterized $\mathrm{BaFe_2As_2}$. Thus, our experiments lead us to the conclusion that the effect of the high magnetic moment of Eu on the magnitude of both Fe local magnetic moment and spin excitations is small and confined to low energy excitations.

Two kinds of in-plane resistivity anisotropy inFe1+δTe(δ=0.09)as seen via synchrotron radiation x-ray diffraction andin situresistivity measurements

We have investigated correlation between structural and electronic anisotropies in a parent compound of Fe-chalcogenide superconductor ${\mathrm{Fe}}_{1+\ensuremath{\delta}}\mathrm{Te}$ with $\ensuremath{\delta}=0.09$ by means of synchrotron x-ray diffraction and in situ in-plane resistivity anisotropy measurements with uniaxial stress applied along a tetragonal $a$ axis. This system is known to exhibit a tetragonal-to-monoclinic structural transition at ${T}_{\mathrm{S}}\ensuremath{\sim}60$ K. We have confirmed that the in-plane resistivity anisotropy in the low-temperature monoclinic phase is attributed to the asymmetry in volume fractions of the monoclinic domains, as was suggested in a previous study [Jiang et al., Phys. Rev. B 88 115130 (2013)]. On the other hand, we found another in-plane resistivity anisotropy above ${T}_{\mathrm{S}}$. The present x-ray diffraction and resistivity anisotropy measurements have revealed that this anisotropy is not due to an onset of the low-temperature monoclinic phase but to the lattice softening enhanced toward ${T}_{\mathrm{S}}$. As one of the possibilities, we suggest that the orbital fluctuation contributes to the lattice softening and the resistivity anisotropy above ${T}_{\mathrm{S}}$.

Anisotropic magnetic form factor in a detwinned single crystal ofBaFe2As2

We have performed neutron diffraction measurement on a single crystal of parent compound of iron-based superconductor, BaFe$_2$As$_2$ at 12~K. In order to investigate in-plane anisotropy of magnetic form factor in the antiferromagnetic phase, the detwinned single crystal is used in the measurement. The magnetic structure factor and magnetic form factor are well explained by the spin densities consisting of $3d_{yz}$ electrons with a fraction of about 40~\% and the electrons in the other four $3d$ orbitals with each fraction of about 15~\%. Such anisotropic magnetic form factor is qualitatively consistent with the anisotropic magnetic behaviors observed in the antiferromagnetic phase of the parent compound of iron-based superconductor.

Electronic structure of theBaTi2As2Oparent compound of the titanium-based oxypnictide superconductor

The electronic structure of BaTi2As2O, a parent compound of the newly discovered titanium-based oxypnictide superconductors, is studied by angle-resolved photoemission spectroscopy. The electronic structure shows multi-orbital nature and possible three-dimensional character. An anomalous temperature-dependent spectral weight redistribution and broad lineshape indicate the incoherent nature of the spectral function. At the density-wave-like transition temperature around 200 K, a partial gap opens at the Fermi patches. These findings suggest that BaTi2As2O is likely a charge density wave material in the strong interaction regime.

General model for explicitly hole-doped superconductor parent compounds: Electronic structure of Ca2−xNaxCuO2Cl2as a case study

Periodic density functional theory (DFT)+U calculations using sufficiently large supercells to explicitly account for dopant disorder are reported for undoped Ca${}_{2}$CuO${}_{2}$Cl${}_{2}$ and for the doped Ca${}_{2\ensuremath{-}x}$Na${}_{x}$CuO${}_{2}$Cl${}_{2}$ ($x$ $=$ 0.125 and $x$ $=$ 0.250). For the undoped material, the charge-transfer antiferromagnetic character is properly reproduced. For the doped systems, a metallic character is predicted with the conduction band dominated by the O(2p) states, with doping having a small effect on the antiferromagnetic order of the Cu${}^{2+}$ cations within the CuO${}_{2}$ planes while maintaining the diamagnetic character in the rest of centers. The analysis of the spin density plots for the doped material evidences the appearance of a new feature reminiscent of the so-called Zhang-Rice singlet, thus providing unbiased independent support to the corresponding model Hamiltonian. However, it is also worth pointing out that the present DFT picture provides only an average static representation of what is expected to be a dynamic electronic structure.