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We report temperature-dependent transport and x-ray diffraction measurements of the influence of Ti hole doping on the charge density wave (CDW) in 1T-Ta1-xTixS2. Confirming past studies, we find that even trace impurities eliminate the low-temperature commensurate (C) phase in this system. Surprisingly, the magnitude of the in-plane component of the CDW wave vector in the nearly commensurate (NC) phase does not change significantly with Ti concentration, as might be expected from a changing Fermi surface volume. Instead, the angle of the CDW in the basal plane rotates, from 11.9° at x = 0 to 16.4° at x = 0.12. Ti substitution also leads to an extended region of coexistence between incommensurate (IC) and NC phases, indicating heterogeneous nucleation near the transition. Finally, we explain a resistive anomaly originally observed by Di Salvo [F. J. Di Salvo et al., Phys. Rev. B 12, 2220 (1975)] as arising from pinning of the CDW on the crystal lattice. Our study highlights the importance of commensuration effects in the NC phase, particularly at x ~ 0.08. [ABSTRACT FROM AUTHOR]

Rapid and cost-effective DNA sequencing at the single nucleotide level might be achieved by measuring a transverse electronic current as single-stranded DNA is pulled through a nanometer-sized pore. In order to enhance the electronic coupling between the nucleotides and the electrodes and hence the current signals, we employ a pair of single-walled close-ended (6,6) carbon nanotubes (CNTs) as electrodes. We then investigate the electron transport properties of nucleotides sandwiched between such electrodes by using first-principles quantum transport theory. In particular, we consider the extreme case where the separation between the electrodes is the smallest possible that still allows the DNA translocation. The benzene-like ring at the end cap of the CNT can strongly couple with the nucleobases and therefore it can both reduce conformational fluctuations and significantly improve the conductance. As such, when the electrodes are closely spaced, the nucleobases can pass through only with their base plane parallel to the plane of CNT end caps. The optimal molecular configurations, at which the nucleotides strongly couple to the CNTs, and which yield the largest transmission, are first identified. These correspond approximately to the lowest energy configurations. Then the electronic structures and the electron transport of these optimal configurations are analyzed. The typical tunneling currents are of the order of 50 nA for voltages up to 1 V. At higher bias, where resonant transport through the molecular states is possible, the current is of the order of several /xA. Below 1 V, the currents associated to the different nucleotides are consistently distinguishable, with adenine having the largest current, guanine the second largest, cytosine the third and, finally, thymine the smallest. We further calculate the transmission coefficient profiles as the nucleotides are dragged along the DNA translocation path and investigate the effects of configurational variations. Based on these results, we propose a DNA sequencing protocol combining three possible data analysis strategies. [ABSTRACT FROM AUTHOR]

We present temperature-, magnetic-field-, and pressure-dependent Raman scattering studies of single crystal Mn3O4, combined with temperature- and field-dependent x-ray diffraction studies, revealing the novel magnetostructural phases in Mn3O4. Our temperature-dependent studies show that the commensurate magnetic transition at T2 = 33 K in the binary spinel Mn3O4 is associated with a structural transition from tetragonal to orthorhombic structures. Field-dependent studies show that the onset and nature of this structural transition can be controlled with an applied magnetic field, and reveal evidence for a field-tuned quantum phase transition to a tetragonal spin-disordered phase for H||[11̄0]. Pressure-dependent Raman measurements indicate that the magnetic easy-axis direction in Mn3O4 can be controlled--and the ferrimagnetic transition temperature increased--with applied pressure. Finally, combined pressure- and magnetic-field-tuned Raman measurements reveal a rich magnetostructural phase diagram--including a pressure- and field-induced magnetically frustrated tetragonal phase in the P-H (pressure-magnetic field) phase diagram--that can be generated in Mn3O4 with applied pressure and magnetic field. [ABSTRACT FROM AUTHOR]

Changes in dynamic processes of relaxors due to a dc bias electric field have been studied in the reduced poly(vinylidene fluoride-trifluoroethylene), a copolymer system where relaxor-like behavior in the crystalline part can be separated from the glassy processes in the amorphous matrix. Strong electric field dependence of the Vogel-Fulcher temperature and the activation energy has been detected. Comparison of the results with those obtained in a classical inorganic relaxor, lanthanum-modified lead zirconate titanate ceramics, reveals that the nonlinear dielectric susceptibility dominantly influences the dielectric dynamics of relaxors in dc bias electric fields. We have additionally found that even low bias voltage effectively blocks the ac electrical conductivity of the studied copolymer. [ABSTRACT FROM AUTHOR]

Using angle-resolved photoemission spectroscopy, we studied the electronic structures of CsFe2As2 and RbFe2As2. Contrary to other iron-based superconductors where the band structures are usually depicted in the two-Fe Brillouin zone (BZ), we found that the distribution of electronic spectral weight in CsFe2As2 and RbFe2As2 follows the one-Fe BZ, and that the emerging band structure is qualitatively consistent with theoretical band calculations of the one-Fe BZ except for some shadow band effect. Our data suggest that the interlayer separation is an important tuning factor for the physics of FeAs layers, the increase of which can reduce the coupling between Fe and As and lead to the emergence of the electronic structure in accord with the one-Fe symmetry of the Fe square lattice. Our finding puts strong constraints on the theoretical models constructed on the basis of the one-Fe BZ. [ABSTRACT FROM AUTHOR]

We have theoretically and experimentally investigated the dispersion of the cavity-magnon-polariton (CMP) in a one-dimensional (1D) configuration, created by inserting a low damping magnetic insulator into a high-quality 1D microwave cavity. By simplifying the full-wave simulation based on the transfer matrix approach in the long wavelength limit, an analytic approximation of the CMP dispersion has been obtained. The resultant coupling strength of the CMP shows different dependence on the sample thickness as well as the permittivity of the sample, determined by the parity of the cavity modes. These scaling effects of the cavity and material parameters are confirmed by experimental data. Our work provides a detailed understanding of the 1D CMP, which could help to engineer a coupled magnon-photon system. [ABSTRACT FROM AUTHOR]

We investigated the thermodynamic properties of the Fe-based lightly disordered superconductor Ba0.05K0.95Fe2As2 in external magnetic field H applied along the FeAs layers (H||ab planes). The superconducting (SC) transition temperature for this doping level is Tc = 6.6 K. Our analysis of the specific heat C(T,H) measured for T < Tc implies a sign change of the superconducting order parameter across different Fermi pockets. We provide experimental evidence for the three components superconducting order parameter. We find that all three components have values which are comparable with the previously reported ones for the stoichiometric compound KFe2As2. Our data for C(T,H) and resistivity ρ(T,H) can be interpreted in favor of the dominant orbital contribution to the pair-breaking mechanism at low fields, while Pauli limiting effect dominates at high fields, giving rise to a gapless superconducting state with only the leading nonzero gap. [ABSTRACT FROM AUTHOR]

We report the results on the structure, transport, and magnetic properties of a layered oxysulfide CaOFeS with a stacked triangle lattice of Fe ions. The susceptibility data show a broad maximum near 120 K, indicating the existence of two-dimensional (2D) short-range ordering in this compound. Features associated with long-range antiferromagnetic (AFM) phase transition are seen below 40 K. Meanwhile, a very small heat-capacity anomaly is detected around 35 K, and most of the measured magnetic entropy is lost during the 2D ordering process. Both crystal and magnetic structures were studied by neutron powder diffraction at 300, 125,40, and 6 K. The structure was refined based on space group P63mc with a = 3.759 98(4)andc = 11.383 51(16) Å at ambient temperature. Low-temperature diffraction reveals 2D magnetic correlations between Fe moments without showing significant structural distortion. Warren peak shape analysis of the neutron-diffraction data at 2θ near 18° is employed to characterize the correlation length in the 2D magnetic state with lowering temperature. The geometrically frustrated compound is found to gradually condense into a partial long-range ordered state with AFM coupled Fe layers between 40.6 and 26 K. The resulting partially ordered magnetic structure is a G-type Ising AFM with a propagation vector of k = (1/2,1/2,0) and an ordered magnetic moment of 2.59(3)μB/Fe along c at 6 K. [ABSTRACT FROM AUTHOR]

We report a sudden reversal in the pressure dependence of Tc in the iron-based superconductor RbFe2As2, at a critical pressure Pc =11 kbar. Combined with our prior results on KFe2As2 and CsFe2As2, we find a universal V-shaped phase diagram for Tc versus P in these fully hole-doped 122 materials, when measured relative to the critical point (Pc,Tc). From measurements of the upper critical field Hc2(T) under pressure in KFe2As2 and RbFe2As2, we observe the same twofold jump in (1/Tc)(-∂Hc2/∂T)Tc across Pc, compelling evidence for a sudden change in the structure of the superconducting gap. We argue that this change is due to a transition from one pairing state to another, with different symmetries on either side of Pc. We discuss a possible link between scattering and pairing, and a scenario where a d-wave state favored by high-Q scattering at low pressure changes to a state with s± symmetry favored by low-Q scattering at high pressure. [ABSTRACT FROM AUTHOR]

A series of (Li0.8Fe0.2) OHFeSe1-xS x (0 ⩽ x ⩽ 1) samples were successfully synthesized via hydrothermal reaction method and the phase diagram is established. Magnetic susceptibility suggests that an antiferromagnetism arising from (Li0.8Fe0.2)OH layers coexists with superconductivity, and the antiferromagnetic transition temperature nearly remains constant for various S doping levels. In addition, the lattice parameters of the both a and c axes decrease and the superconducting transition temperature Tc is gradually suppressed with the substitution of S for Se, and eventually superconductivity vanishes at x = 0.90. The decrease of Tc could be attributed to the effect of chemical pressure induced by the smaller ionic size of S relative to that of Se, being consistent with the effect of hydrostatic pressure on (Li0.8Fe0.2) OHFeSe. But the detailed investigation on the relationships between Tc and the crystallographic facts suggests a very different dependence of Tc on anion height from the Fe2 layer or Ch-Eel-Ch angle from those in FeAs-based superconductors. [ABSTRACT FROM AUTHOR]

Recently, a Dirac surface state (SS) was observed in Rashba semiconductor BiTeCl by angle-resolved photoemission spectroscopy (ARPES), which suggested strong inversion symmetry breaking therein, despite the absence of such symmetry breaking in existing first-principles calculations. To clarify the aforementioned conflict as well as understand the nature of such emergent phenomenon, we employ both high-field Shubnikov-de Haas (SdH) oscillation and Hall measurements to study BiTeCl single crystals. Both techniques yield consistent observation of a three-dimensional Fermi surface from a bulk state, while Dirac surface state contribution appears absent. Finally, we propose that various gating techniques could be used to explore the novel topological nature of this material. [ABSTRACT FROM AUTHOR]

We measured the in-plane resistivity anisotropy in the underdoped Ca1-xNa1-xFe2As2 single crystals. The anisotropy (indicated by Pb - Pa) appears below a temperature well above magnetic transition temperature TN, being positive (Pb - Pa > 0) as x = 0.14. With increasing the doping level to x = 0.19, an intersection between Pb and Pa is observed upon cooling, with Pb - Pa < 0 at low temperature deep inside a magnetically ordered state, while Pb - Pa > 0 at high temperature. Subsequently, further increase of hole concentration leads to a negative anisotropy Pb - Pa < 0 in the whole temperature range. These results manifest that the anisotropic behavior of resistivity in the magnetically ordered state depends strongly on the competition of the contributions from different mechanisms, and the competition between the two contributions results in a complicated evolution of the anisotropy of in-plane resistivity with doping level [ABSTRACT FROM AUTHOR]

The electronic structure of BaTi2As2O, a parent compound of the newly discovered titanium-based oxypnictide superconductors, is studied by angle-resolved photoemission spectroscopy and first-principles calculation. The experimental electronic structure shows a multiorbital nature and three-dimensional character, which is consistent with the calculated results. An anomalous temperature-dependent spectral weight redistribution and broad line shape 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. [ABSTRACT FROM AUTHOR]

We report a sudden reversal in the pressure dependence of Tc in the iron-based superconductor similar to that discovered recently in KFe2As2 [Tafti et ai, Nat. Phys. 9, 349 (2013)]. As in KFe2As2, we observe no change in the Hall coefficient at T → 0, again ruling out a Lifshitz transition across the critical pressure Pc. We interpret the Tc reversal in the two materials as a phase transition from one pairing state to another, tuned by pressure, and we investigate which parameters control this transition. Comparing samples of different residual resistivity p0, we find that a sixfold increase in impurity scattering does not shift Pc. From a study of x-ray diffraction on KFe2As2 under pressure, we report the pressure dependence of lattice constants and As-Fe-As bond angle. The pressure dependence of the various lattice parameters suggests that Pc should be significantly higher in CsFe2As2 than in KFe2As2, but we find on the contrary that Pc is lower in CsFe2As2, indicating that other factors control Tc. Resistivity measurements under pressure reveal a change of regime across Pc, suggesting a possible link between inelastic scattering and pairing symmetry. [ABSTRACT FROM AUTHOR]

High-quality K(Fe1-xCox)2As2 single crystals have been grown by using the KAs flux method. Instead of increasing the superconducting transition temperature Tc through electron doping, we find that Co impurities rapidly suppress Tc down to zero at only x ≈ 0.04. Such an effective suppression of Tc by impurities is quite different from that observed in Ba0.5K0.5Fe2As2 with multiple nodeless superconducting gaps. Thermal conductivity measurements in zero field show that the residual linear term K0/T only changes slightly with 3.4% Co doping, despite the sharp increase in the scattering rate. The implications of these anomalous impurity effects are discussed. [ABSTRACT FROM AUTHOR]

We report the discovery of superconductivity at ~9 K in Fe1.05Te single crystals that have been exposed to air for more than six months. The superconductivity is induced due to the oxygen incorporation and only exists in the surface layer of the samples. Our high-resolution transmission electron microscopy experiments and density functional theory calculations show the oxygen prefers to locate at the interstitial site in the Fe-Te layer. X-ray photoelectron spectra characterize an enhancement of the itinerant character of Fe 3d electrons, which may reduce the local moment and hence suppress the long-range antiferromagnetism associated with superexchange interactions. This work suggests the bicollinear antiferromagnetism in Fe1+yTe can be suppressed by oxygen incorporation to induce superconductivity and thus could be considered as the parent phase of the superconductivity. [ABSTRACT FROM AUTHOR]

We studied the doping and temperature (T) dependence of the infrared (IR) response of Ba(Fe1-xCox)2As2 single crystals. We show that a weak band around 1000 cm-1, that was previously interpreted in terms of interaction of the charge carriers with magnetic excitations or of a pseudogap, is rather related to low-energy interband transitions. Specifically, we show that this band exhibits a similar doping and T dependence as the hole pockets seen by angle resolved photoemission spectroscopy (ARPES). Notably, we find that it vanishes as a function of doping near the critical point where superconductivity is suppressed in the overdoped regime. Our IR data thus provide bulk specific information (complementary to the surface sensitive ARPES) for a Lifshitz transition. Our IR data also reveal a second low-energy band around 2300 cm-1 which further emphasizes the necessity to consider the multiband nature of these iron arsenides in the analysis of the optical response. [ABSTRACT FROM AUTHOR]

We measured the resistivity and the specific heat of CsFe2As2 single crystals, which were grown by using a self-flux method. The CsFe2As2 crystal shows a sharp superconducting transition at 1.8 K with a transition width of 0.1 K. The sharp superconducting transition and pronounced jump in specific heat indicate high quality of the crystals. A large ΔCp/Tc accompanying a very low Tc, nonexponential dependence of the lowest temperature ΔCp/ T and an H1/2 dependence of Δγ(H) are observed in the superconducting-state specific heat; Combined with the fitting results by the α model for the superconducting-state specific heat, a pairing symmetry with nodal gaps was suggested in CsFe2As2. [ABSTRACT FROM AUTHOR]

The thermal conductivity of iron-based superconductor CsFe2As2 single crystal (Tc=1.81 K) was measured down to 50 mK. A significant residual linear term κ0/T=1.27 mW K-2 cm-1 is observed in zero magnetic field, which is about 1/10 of the normal-state value in upper critical field Hc2. In low magnetic field, κ0/T increases rapidly with field. The normalized κ0/T(H) curve for our CsFe2As2 (with residual resistivity ρ0=1.80 μΩ cm) lies between the dirty KFe2As2 (with ρ0=3.32 μΩ cm) and the clean KFe2As2 (with ρ0=0.21 μΩ cm), which is consistent with its impurity level. These results strongly suggest a nodal superconducting gap in CsFe2As2, similar to its sister compound KFe2As2. [ABSTRACT FROM AUTHOR]

The H-T phase diagrams of single crystalline electron-doped K0.83Fe1.83Se2 (KFS1), K0.8Fe2Se2 (KFS2) and hole-doped Eu0.5K0.5Fe2As2 (EKFA) have been deduced from tunnel diode oscillator-based contactless measurements in pulsed magnetic fields up to 57 T for the interplane (H∥c) and in-plane (H∥ab) directions. The temperature dependence of the upper critical magnetic field Hc2(T) relevant to EFKA is accounted for by Pauli model including an anisotropic Pauli paramagnetic contribution (μBHp=114 T for H∥ab and 86 T for H∥c). This is also the case of KFS1 and KFS2 for H∥ab whereas a significant upward curvature, accounted for by a two-gap model, is observed for H∥c. Despite the presence of antiferromagnetic lattice order within the superconducting state of the studied compounds, no influence of magnetic ordering on the temperature dependence of Hc2(T) is observed. [ABSTRACT FROM AUTHOR]

Using angle-resolved photoemission spectroscopy, we studied the evolution of electronic structure of NaFe1-xCoxAs from an optimally doped superconducting compound (x = 0.028) to a heavily overdoped nonsuperconducting one (x = 0.109). As in "122"-type iron pnictides, our data suggest that the Co dopant in NaFe1-xCoxAs supplies extra charge carriers and shifts the Fermi level accordingly. The overall band renormalization remains basically the same throughout the doping range we studied, suggesting that the local magnetic and electronic correlations are not affected by carrier doping. In the x = 0.109 compound, the holelike bands around the zone center Γ move to deeper binding energies and an electron pocket appears instead, resulting in a Fermi surface topology similar to that of AxFe2-ySe2 (A = K, Cs, Rb, Tl). Our data suggest that a balance between itinerant properties of mobile carriers and local interactions plays an important role for the superconductivity in these materials. [ABSTRACT FROM AUTHOR]

Sizable single-crystalline samples of Ca10(Pt3As8)((Fe1-x:Ptx)2As2)5 (the 10-3-8 phase) with 0 ≤ x < 0.1 have been grown and have been systematically characterized via x-ray diffraction, magnetic, and transport measurements. The unsubstituted sample is a heavily doped semiconductor with no sign of magnetic order down to 2 K. With increasing Pt content, the metallic behavior appears, and superconductivity is realized for x ≥ 0.023. Tc rises to its maximum of approximately 13.6 K at the doping level of x ∼ 0.06f and then decreases for higher x values. The electronic phase diagram of the 10-3-8 phase was mapped out based on the magnetic and transport measurements. The mass anisotropy parameter &Ggr; ∼ 10, obtained from resistive measurements in magnetic fields, indicates a relatively large anisotropy in the iron-based superconductor family. This strong two-dimensional character may lead to the absence of magnetic order. A linear T dependence of susceptibility at high temperatures is observed, indicating that spin fluctuations exist in the underdoped region as in most of the Fe-pnictide superconductors. [ABSTRACT FROM AUTHOR]

We measured the resistivity and magnetic susceptibility to map out the phase diagram of single crystalline NaFe1-xCoxAs. Replacement of Fe by Co suppresses both the structural and magnetic transitions; however, it also enhances the superconducting transition temperature (Tc) and superconducting component fraction. Magnetic susceptibility exhibits temperature-linear dependence in high temperatures up to 500 K for all the superconducting samples, but such behavior suddenly breaks down for the nonsuperconducting overdoped crystal, suggesting that the superconductivity is closely related to the T-linear dependence of susceptibility. Analysis on the superconducting-state specific heat of the optimally doped crystal provides strong evidence for a two-band s-wave order parameter with gap amplitudes of &Dgr;1(0)/kBTC = 1.78 and &Dgr;2(0)/kBTC = 3.11, being consistent with the nodeless gap symmetry revealed by angle-resolved photoemission spectroscopy experiment. [ABSTRACT FROM AUTHOR]

Although a nodeless superconducting gap has been observed on the large Fermi pockets around the zone corner in KxFe2-ySe2, whether its pairing symmetry is s wave or nodeless d wave is still under intense debate. Here we report an isotropic superconducting gap distribution on the small electron Fermi pocket around the Z point in KxFe2-ySe2, which favors the s-wave pairing symmetry. [ABSTRACT FROM AUTHOR]

Using muon spin rotation and infrared spectroscopy, we investigated the recently discovered superconductor K0.73Fe1.67Se2 with Tc &ap; 32 K. We show that the combined data can be consistently described in terms of a macroscopically phase-segregated state with a matrix of ∼88% volume fraction that is insulating and strongly magnetic and inclusions with an ∼12% volume fraction, which are metallic, superconducting, and nonmagnetic. The electronic properties of the latter, in terms of the normal state plasma frequency and the superconducting condensate density, appear to be similar as in other iron selenide or arsenide superconductors. [ABSTRACT FROM AUTHOR]

We report the magnetic and transport measurements of CaCo2As2 and Ca0.9Sr0.1Co2As2 single crystals. An antiferromagnetic transition was observed at about 70 K and 90 K for CaCo2As2 and Ca0.9Sr0.1Co2As2, respectively. Magnetic and magnetoresistance measurements reveal a metamagnetic transition from an antiferromagnetic state to a ferromagnetic state with a critical field of 3.5 and 1.5 Ô for these two samples, respectively along the c axis at low temperature. For the field applied along the ab plane, spins can be fuUy polarized above a field of 4.5 T for Ca0.9Sr0.1Co2As2. While for CaCo2As2, spins cannot be fully polarized up to 7 T. We propose the cobalt moments of these two materials should be ordered ferromagnetically within the ab plane but antiferromagnetically along the c axis (A-type antiferromagnetic). [ABSTRACT FROM AUTHOR]

We measured the transport properties and susceptibility of single-crystal Ca1-xLaxFe2As2 (x = 0, 0.05, 0.1, 0.15, 0.19, and 0.25). Large in-plane resistivity anisotropy similar to that in Co-doped 122 iron pnictides is observed although no transition metals were introduced in the FeAs plane. The in-plane resistivity anisotropy gradually increases with La doping below TSDW, being different from the hole-doped 122 superconductors. The susceptibilities of the samples show that La doping leads to suppression of the spin-density wave (SDW) and induces a Curie-Weiss-like behavior at low temperature, which is much stronger than the other 122 iron-based superconductors. [ABSTRACT FROM AUTHOR]

We establish the phase diagram of the EUEu1-xLaxFe2As2 system as a function of doping level x and pressure by measuring the resistivity and magnetic susceptibility. Increased pressure can suppress the spin-density-wave (SDW) and structural transitions very efficiently, while enhancing the antiferromagnetic (AFM) transition temperature TN of Eu2+. Superconductivity coexists with SDW order at low pressure and always coexists with EU2+ AFM order. The results suggest that the Eu2+ spin dynamics is unconnected with the superconducting pairing taking place in the two-dimensional Fe-As plane, but it can strongly affect superconducting coherence along the c axis. [ABSTRACT FROM AUTHOR]

We report magnetic properties in the Co- and Zn-doped spin-1/2 honeycomb-lattice compound In3Cu2VO9. Magnetic susceptibility and specific heat experiments show no long-range ordering down to 2 K in In3Cu2VO9. Analyses of electronic structures and states suggest that the ground state of undoped In3Cu2VO9 is probably a Néel antiferromagnet (AFM). When Cu2+ ions are partially substituted by Co2+ ions, both the impurity potential scattering and magnetic scattering induced by the magnetic Co2+ ions enhance the three-dimensional character of the magnetic coupling and lead to an AFM long-range order. Replacement of Cu2+ with nonmagnetic Zn2+ions weakens the AFM correlation between Cu2+ ions, leading to suppression of the AFM state. [ABSTRACT FROM AUTHOR]

Magnetic impurities are crucial for probing spin-polarized topological surface states. Scanning tunneling microscopy (STM) studies of Fe on the Bi2Te3(l 11) surface are preformed and unveil distinct impurity structures of unknown origin which exhibit temperature-dependent characteristics. Using density functional theory with spin-orbit coupling, we show that Fe prefers highly coordinated subsurface configurations. By comparing simulated STM images we can explain the experimental results of both low-temperature deposition, in which Fe exists in metastable states of which only the transition-metal Bi split interstitial is STM visible, and after room-temperature annealing, in which iron forms substitutional FeBi. [ABSTRACT FROM AUTHOR]

We successfully synthesized two new compounds, RbxFe2-yS2 and CsxFe2-yS2, which are isostructural with a KxFe2-ySe2 superconductor. We systematically investigated the resistivity, magnetism, and thermoelectric power of AxFe2-yS2 (A=K, Rb, and Cs) single crystals. High-temperature resistivity and magnetic measurements show anomalies above 500 K depending on A which are similar to AxFe2-ySe2. A discrepancy between zero-field-cooling (ZFC) and field-cooling (FC) curves was observed in KxFe2-yS2 and RbxFe2-yS2, whereas it disappears in CsxFe2−yS2. Our results indicate similar magnetism between AxFe2−yS2 and AxFe2−ySe2 at high temperature. [ABSTRACT FROM AUTHOR]

We discover superconductivity in alkali-earth metal-doped phenanthrene. The superconducting critical temperatures Tc are 5.6 and 5.4 K for Sr1.5phenanthrene and Ba1.5phenanthrene, respectively. The shielding fraction of Ba1.5phenanthrene exceeds 65%. The Raman spectra show 8 and 7 cm-1/electron downshifts for the mode at 1441 cm-1 due to the charge transfer to organic molecules from the dopants of Ba and Sr. Similar behavior has been observed in A3phenanthrene and A3C60 (A = K and Rb). The positive-pressure effect in Sr1.5phenanthrene and Ba1.5phenanthrene together with the lower Tc with a larger lattice indicates unconventional superconductivity in this organic system. [ABSTRACT FROM AUTHOR]

We study through the time evolution of magnetization the low-temperature (7) dynamics of the metastable coexisting phases created by traversing different paths in magnetic field H and T space in a shape memory alloy system, Ni45Co5Mn38Sn12. It is shown that these coexisting phases consisting of a fraction of kinetically arrested austenite phase and a remaining fraction of low-T equilibrium martensitic phase undergo a slow relaxation to low magnetization (martensitic) state but with very different thermomagnetic history-dependent rates at the same T and H. We discovered that, when the nucleation of the martensitic phase is initiated at much lower T through the de-arrest of the glasslike arrested state contrasted with the respective first-order transformation (through super cooling at much higher T), the long-time relaxation rate scales with the nonequilibrium phase fraction but has a very weak dependence on T. This is explained on the basis of the H-T path dependent size of the critical radii of the nuclei and the subsequent growth of the equilibrium phase through the motion of the interface. [ABSTRACT FROM AUTHOR]

The mixed-metal-network layered compound TaFe1+xTe3 has the TaFeTe3 layers formed by Ta-Fe bonded network (Fe1) sandwiched with tellurium layers and the excess iron atoms (Fe2) partially occupying the interstitial site of the tetrahedral (Ta,Fe)Te at random, which are similar to the interstitial iron atoms in the iron-based high-Tc, superconductor Fe1+xTe. The antiferromagnetic (AFM) transition of Fel zigzag chains and spin flop of these interstitial Fe2 atoms under a high magnetic field are studied through susceptibility, magnetoresistance (MR), the Hall effect, and specific heat measurements in high-quality single-crystal TaFe1+yTe3. These properties suggest that the high-temperature AFM transition of the TaFeTe3 layers should be a spin-density-wave-type AFM order. Below TN, the spin flop of these interstitial Fe2 from antiferromagnetism to ferromagnetism induces a sharp drop on resistivity and an anomalous Hall effect. It can be inferred from the spin flop of Fe2 that the local moment of Fe2 atom is about 4 μB/Fe. The possible magnetic structure of TaFe1+yTe3 is proposed from the susceptibility, MR, and Hall effect. The properties related to the spin flop of Fe2 supply a good opportunity to study the coupling between Fe1 and Fe2 in these TaFe1+yTe3 or Fe1+yTe compounds with interstitial Fe2. [ABSTRACT FROM AUTHOR]

The magnetic structures adopted by the Fe and Sm sublattices in SmFeAsO have been investigated using element-specific x-ray resonant and nonresonant magnetic scattering techniques. Between 110 and 5 K, the Sm and Fe moments are aligned along the c and a directions, respectively, according to the same magnetic representation Γ5 and the same propagation vector (1 0 1/2). Below 5 K, the magnetic order of both sublattices changes to a different magnetic structure, and the Sm moments reorder in a magnetic unit cell equal to the chemical unit cell. Modeling of the temperature dependence for the Sm sublattice, as well as a change in the magnetic structure below 5 K, provides clear evidence of a surprisingly strong coupling between the two sublattices, and indicates the need to include anisotropic exchange interactions in models of SmFeAsO and related compounds. [ABSTRACT FROM AUTHOR]

We report the results of an experimental study of dc and low-frequency magnetic properties of a K0.8Fe2-ySe2 single crystal when the dc magnetic field is applied parallel to the ab plane. From the data obtained, we deduce the full H-T phase diagram, which consists of all three Hc1(T), Hc2(T), and Hc3(T) critical magnetic-field plots. The two Hc1(T) and Hc2(T) curves were obtained from dc magnetic measurements, whereas the surface critical-field Hc3(T) line was extracted by ac susceptibility studies. It appears that near Tc, the Hc3(T)/Hc2(T) ratio is ≈4.4, which is much larger than expected. [ABSTRACT FROM AUTHOR]

We report an infrared spectroscopy study on K0.83Fe1.53Se2, a semiconducting parent compound of the 245 iron selenide superconductors. The major spectral features are found to be distinctly different from all other Fe-based superconducting systems. Our measurement revealed two peculiar spectral structures: a double-peak structure between 4000 and 6000 cm-1 and abundant phonon modes-much more than expected for a 122 structure. We elaborate that those features could be naturally explained from the blocked antiferromagnetism due to the presence of Fe vacancy ordering as determined by recent neutron-diffraction experiments. The double peaks reflect the coexistence of ferromagnetic and antiferromagnetic couplings between the neighboring Fe sites. [ABSTRACT FROM AUTHOR]

We successfully grew the high-quality single crystals of AxFe2Se2 (A = K and Cs) by self-flux method. Sharp superconducting transition was observed for both types of crystals. The crystals show the onset superconducting transition temperatures (Tc) of 31 and 30 K for K and Cs compounds, respectively, with nearly 100% superconductive shielding fraction. The crystals show quite high resistivity in the normal state of more than 160 and 1300 mΩ cm maximum resistivity for KxFe2Se2 and CsxFe2Se2 single crystals, respectively. Much larger upper critical field Hc2 is inferred from low-temperature isomagnetic-field magnetoresistance in these crystals than in FeSe. The anisotropy Hc2ab(0)/Hc2c(0) is estimated to be around 3 for both of the two materials. Anisotropic peculiar magnetic behavior in normal state has been found for CsxFe2Se2. [ABSTRACT FROM AUTHOR]

The new layered manganese oxychalcogenides (LaO)2Mn2Se2O and (BaF)2Mn2Se2O, isostructural to (LaO)2Fe2Se2O, were synthesized by solid state reaction method. The single crystals of the former compound were also successfully grown using fusion method. The polycrystalline samples show the semiconducting behavior with the activation energy gaps of about 278 and 416 meV for (LaO)2Mn2Se2O and (BaF)2Mn2Se2O, respectively. The magnetic susceptibility and specific heat indicate an antiferromagnetic (AFM) transition at around 160 ± 1 K for the former compound and 100 ± 1 K for the latter compound. The strong anisotropic magnetic properties below TN of the former compound suggests a long-range canted AFM ordering. A broad maximum of the susceptibility can be observed for the two compounds at high temperatures of 360 and 210 K, respectively, suggesting that strong frustrated magnetic correlation gives rise to low-dimensional AFM or short-range ordering at high temperatures in these rare transition metal oxychalcogenides with an AFM checkerboard spin lattice. [ABSTRACT FROM AUTHOR]

The specific heat down to 0.08 K of the iron pnictide superconductor KFe2As2 was measured on a single-crystal sample with a residual resistivity ratio of ~650, with a Tconset determined by a specific heat of 3.7 K. The zero-field normal-state specific heat divided by temperature, C/T, was extrapolated from above Tc to T=0 by insisting on agreement between the extrapolated normal-state entropy at Tc, Snextrap(Tc), and the measured superconducting-state entropy at Tc, Ssmeas(Tc), since for a second-order phase transition the two entropies must be equal. This extrapolation would indicate that this rather clean sample of KFe2As2 exhibits non-Fermi-liquid behavior; i.e., C/T increases at low temperatures, in agreement with the reported non-Fermi-liquid behavior in the resistivity. However, specific heat as a function of magnetic field shows that the shoulder feature around 0.7 K, which is commonly seen in KFe2As2 samples, is not evidence for a second superconducting gap as has been previously proposed but instead is due to an unknown magnetic impurity phase, which can affect the entropy balance and the extrapolation of the normal-state specific heat. This peak (somewhat larger in magnitude) with similar field dependence is also found in a less pure sample of KFe2As2, with a residual resistivity ratio of only 90 and Tconset=3.1 K. These data, combined with the measured normal-state specific heat in field to suppress superconductivity, allow the conclusion that an increase in the normal-state specific heat as T→0 is in fact not seen in KFe2As2; i.e., Fermi-liquid behavior is observed. [ABSTRACT FROM AUTHOR]

We carried out high-resolution angle-resolved photoemission measurements on the electronic structure and superconducting gap of K0.68Fe1.79Se2 (Tc=32 K) and (Tl0.45K0.34)Fe1.84Se2 (Tc=28 K) superconductors. In addition to the electron-like Fermi surface near M(π,π), two electron-like Fermi pockets are revealed around the zone center Γ(0,0) in K0.68Fe1.79Se2. This observation makes the Fermi surface topology of K0.68Fe1.79Se2 consistent with that of (Tl, Rb)xFe2-ySe2 and (Tl, K)xFe2-ySe2 compounds. A nearly isotropic superconducting gap Δ is observed along the electron-like Fermi pocket near the M point in K0.68Fe1.79Se2 (Δ∼9 meV) and (Tl0.45K0.34)Fe1.84Se2 (Δ ∼ 8 meV). The establishment of a universal picture on the Fermi surface topology and superconducting gap in the AxFe2-ySe2 (A=K, Tl, Cs, Rb, etc.) superconductors will provide important information for understanding the superconductivity mechanism of iron-based superconductors. [ABSTRACT FROM AUTHOR]

With single-crystal x-ray diffraction studies, we compare the structures of three samples showing optimal superconductivity: K0.775(4)Fe1.613(1)Se2, K0.737(6)Fe1.631(3)Se2, and Cs0.748(2)Fe1.626(1)Se2. All have an almost identical ordered vacancy structure with a (√5 × √5 × 1) supercell. The tetragonal unit cell, space group I4/m, possesses lattice parameters at 250 K of a = b = 8.729(2) Å and c = 14.120(3) Å, a = b = 8.7186(12) Å and c = 14.0853(19) Å, and at 295 K, a = b = 8.8617(16) Å and c = 15.304(3) Å for the three crystals, respectively. The structure contains two iron sites; one is almost completely empty while the other is fully occupied. There are similarly two alkali metal sites that are occupied in the range of 72.2(2)%-85.3(3)%. The inclusion of alkali metals and the presence of vacancies within the structure allows for considerable relaxation of the FeSe4 tetrahedron, compared with members of the Fe(Te, Se, S) series, and the resulting shift of the Se-Fe-Se bond angles to less distorted geometry could be important in understanding the associated increase in the superconducting transition temperature. The structure of these superconductors are distinguished from the structure of the nonsuperconducting phases by an almost complete absence of Fe on the (0 0.5 0.25) site, as well as lower alkali metal occupancy that ensures an exact Fe2+ oxidation state, which are clearly critical parameters in the promotion of superconductivity. [ABSTRACT FROM AUTHOR]

We successfully grew the high-quality single crystal of Rb0.88Fe1.81 Se2, which shows a clear superconducting transition in magnetic susceptibility and electrical resistivity. Resistivity shows the onset superconducting transition Tc at 32.1 K and zero resistivity at 30 K. From the low-temperature iso-magnetic-field magnetoresistance, the large upper critical field Hc2(0) has been estimated to be as high as 180 T for the field applied within the ab plane and 59 T for the field applied along the c axis. The anisotropy Hc2(0)/Hc2(0) is around 3.0, lying right between those observed in KxFe2Se2 and CsxFe2Se2. [ABSTRACT FROM AUTHOR]

We present a comprehensive experimental and theoretical study of the electronic and magnetic properties of two quasi-two-dimensional (2D) honeycomb lattice monoclinic compounds A3Ni2SbO6(A = Li, Na). Magnetic susceptibility and specific heat data are consistent with the onset of antiferromagnetic long-range order with Néel temperatures of ~14 and 16 K for Li3Ni2SbO6 and NA3Ni2SbO6, respectively. The effective magnetic moments of 4.3µB/f.u.(Li3Ni2SbO6) and 4.4µB/f.u.(NA3Ni2SbO6), where f.u. is formula units, indicate that Ni2+ is in a high-spin configuration (S = 1). The temperature dependence of the inverse magnetic susceptibility follows the Curie-Weiss law in the high-temperature region and shows positive values of the Weiss temperature, ~8K (Li3Ni2SbO6) and ~12 K (NA3Ni2SbO6), pointing to the presence of nonnegligible ferromagnetic interactions, although the system orders antiferromagnetically at low temperatures. In addition, the magnetization curves reveal a field-induced (spin-flop type) transition below TN that can be related to the magnetocrystalline anisotropy in these systems. These observations are in agreement with density functional theory calculations, which show that both antiferromagnetic and ferromagnetic intralayer spin exchange couplings between Ni2+ ions are present in the honeycomb planes, supporting a zigzag antiferromagnetic ground state. Based on our experimental measurements and theoretical calculations, we propose magnetic phase diagrams for the two compounds. [ABSTRACT FROM AUTHOR]

We report on 7Li and Se77 nuclear magnetic resonance (NMR) measurements in the newly discovered iron-based superconductor (Li0.8Fe0.2)OHFeSe. Both remarkable change in spectrum and divergence of spin-spin relaxation (T2) are observed with decreasing temperature, suggesting a static magnetic ordering in this material. Moreover, the broadened 7Li spectrum also shows a characteristic three-peak structure. Comparison of linewidth between 7Li and Se77 spectra indicates that there is a ferromagnetic component along the external field direction and it happens on Fe sites in the (Li0.8Fe0.2)OH layer rather than the FeSe layer, which is also responsible for the three-peak structure of the 7Li spectrum with (Li0.8Fe0.2) random occupation. The field-dependent spectrum and T2 suggest that the above ferromagnetic component is induced by external magnetic field. Our present results indicate that the superconductivity in (Li0.8Fe0.2)OHFeSe is very robust against the observed field-induced ferromagnetism and both of them could coexist under external magnetic field. [ABSTRACT FROM AUTHOR]