Session W23: Focus Session: Fe-based Superconductors - AFeAs and RFeAsO Families

Quantum oscillations in LiFeAs

Quantum oscillations are a powerful technique to establish with accuracy the three-dimensional topology of the Fermi surface and it has been successfully used in the study of iron-based superconductors. Here we report quantum oscillations in the 111 pnictide superconductor, LiFeAs, with T$\sim $18K, by using highly sensitive torque magnetometry in high magnetic fields (up to 58T) and at very low temperatures (0.3-4.2K). We observe clearly three different orbits around 1.5kT, 2.4kT and 2.9kT. By comparing the angular dependence of the measured frequencies with the predictions given by the first principle band calculations we conclude that the observed orbits belong to the electronic bands. The values of the quasiparticle masses for these orbits are significantly enhanced as compared with the band masses (a factor 4-5) suggesting that either that electron-electron and/or electron-phonon correlations are significant. We will compare our data with available APRES data on the same material and discuss the effect of the spin-orbit coupling. The details of the Fermi surface of LiFeAs will be compared with other iron-based superconductors. This work was supported by EPSRC (UK), EuroMagNET II, KAKENHI from JSPS and National Science Foundation, State of Florida and the U.S. Department of Energy.   

Effects of correlations in LiFeAs and LiFeP

We will discuss the role of electronic correlations in the iron-based superconductors LiFeAs and LiFeP by considering the effects on band structure, mass enhancements, and Fermi surface in the framework of density functional theory combined with dynamical mean field theory calculations. We show that LiFeAs shows characteristics of a moderately correlated metal and that the strength of correlations is mainly controlled by the value of the Hund's rule coupling $J$. The hole pockets of the Fermi surface show a distinctive change in form and size with implications for the nesting properties. We discuss our results in view of recent angle-resolved photoemission spectroscopy and de Haas-van Alphen experiments.   

Anisotropic Energy-Gaps of Iron-based Superconductivity from Intra-band Quasiparticle Interference in LiFeAs, Part I

Cooper pairing in the iron-based high-T$_c$ superconductors is thought to occur due to the projection of the antiferromagnetic interactions between neighboring iron atoms onto the complex momentum-space electronic structure. It is thus pivotal to have an exact measurement of the electronic structure in these materials. In this talk, I will introduce intra-band Bogoliubov quasiparticle scattering interference (QPI) to iron-based superconductor studies. We report a precise determination of the low energy band structure of LiFeAs using QPI. We observe three hole-like bands, in qualitative agreement with dHvA and ARPES studies (``$\gamma$, $\alpha_2$ \& $\alpha_1$''). The quantitative determination of the bandstructure is the foundation that we later use to measure the superconducting gap structure with QPI.   

Non-magnetic impurity effects in LiFeAs studied by STM/STS

Detecting the possible sign reversal of the superconducting gap in iron-based superconductors is highly non-trivial. Here we use non-magnetic impurity as a sign indicator. If the sign of the superconducting gap is positive everywhere in momentum space, in-gap bound state should not be observed near the impurity site unless it is magnetic. On the other hand, if there is a sign-reversal in the gap, even non-magnetic impurity may create in-gap bound state~[1]. We performed STM/STS experiments on self-flux and Sn-flux grown LiFeAs crystals and examined the effects of Sn impurity. In STM images of Sn-flux grown samples, we found a ring-like object which may represent Sn. Tunneling spectrum taken at this defect site exhibits in-gap bound state. Together with flat-bottom superconducting gap observed far from the defects, sign-reversing $s$-wave gap is the most plausible gap structure in LiFeAs. [1] T. Kariyado and M. Ogata, JPSJ {\bf 79}, 083704 (2010).   

Bogoliubov Electronic Structure at Individual Impurity Atoms in LiFeAs

Individual impurity atoms in a superconductor can strongly perturb the surrounding electronic environment and can therefore be used to probe high-temperature superconductivity at the atomic scale. Spectroscopic imaging scanning tunneling microscopy (SI-STM) is an ideal technique for the study of such effects. This has provided the motivation for several theoretical studies predicting the spatial and energetic structure of Bogoliubov electronic states at single impurity atoms in Fe-based superconductors. Here we report on a type of impurity atom that forms a strong in-gap quasi-particles interference (QPI) pattern nearby in the Fe-based superconductor LiFeAs. Our result puts a number of restrictions on theoretical studies of Fe-based superconducting mechanism.   

Theory of quasiparticle vortex bound states in Fe-based superconductors: application to LiFeAs

Spectroscopy of vortex bound states can provide valuable information on the structure of the superconducting order parameter. Quasiparticle wavefunctions are expected to leak out in the directions of gap minima or nodes, if they exist, and scanning tunneling spectroscopy (STS) on these low-energy states should probe the momentum dependence of the gap. Anisotropy can also arise from band structure effects, however. We perform a quasiclassical calculation of the density of states of a single vortex in an anisotropic superconductor, and show that if the gap itself is not highly anisotropic, the Fermi surface anisotropy dominates, preventing direct observation of superconducting gap features. This serves as a cautionary message for the analysis of STS data on the vortex state on Fe-based superconductors, in particular LiFeAs, which we treat explicitly. YW and PJH were supported by the DOE under DE-FG02-05ER46236, and I. V. under DE-FG02-08ER46492.   

Local electronic structure near a vortex in LiFeAs within self-consistent BdG

A major question in Fe-based superconductors is whether the pairing is of an unconventional nature with a sign change. The electronic structure in the presence of vortices can serve as a platform for phase sensitive measurements to answer this question. However as Fe-based superconductors are in the intermediate regime of correlation strength, a delicate balance between band structure effects and interaction effects may challenge a simple guess. We perform a microscopic self-consistent BdG calculation for LiFeAs in the presence of a perpendicular magnetic field and calculate the energy-dependent local electronic structure near a vortex. We use a band structure in agreement with recent experiments and compare different gap-symmetry possibilities. We find the low-energy local density of states to be dominated by the geometry of the Fermi surface, with tails along the directions perpendicular to the flat portions of the Fermi surface. These are the directions of the gap {\it maxima} on the square-like hole pocket around the $\Gamma$ point according to recent observations. We discuss how the gap symmetry affects high-energy local density of states.   

Elastic neutron scattering on Co-doped NaFeAs superconductors

NaFeAs and LiFeAs are the only two members in the Pnictide superconductors 111 family. Without doping, NaFeAs shows filamentary superconductivity coexisting with static antiferromagnetic (AF) order .In addition to the superconducting transition at 9 K, structural and AF transitions appear at $\sim $50 K and $\sim $ 40 K, respectively. By gradually doping Co on Fe sites, the AF order is suppressed and bulk superconductivity occurs. Although the electronic phase diagram of Co-doped NaFeAs is similar to the phase diagram of Co-doped 122 family, the NaFeAs system is much simpler because it does not have strong c-axis magnetic coupling and therefore a detailed investigation of this system will shed new light to our understanding on the common features amongst different classes of Fe-based superconductors. We will report our elastic neutron scattering results about this system.   

Temperature-dependent resistivity in single crystals Na$_{1-\delta}$Fe$_{1-x}$Co$_x$

Stoichiometric NaFeAs superconductor is representative of the slightly underdoped part of the doping phase diagram, with a sequence of tetragonal-to-orthorhombic, $T_{s} \approx$ 60 K, magnetic, $T_{m}$=45 K, and superconducting, $T_c$=12~K transitions. Doping level in the compound can be tuned with Co substitution of Fe, acting as electron donor. This doping suppresses structural and magnetic instabilities and induces superconductivity with $T_c$ up to 25~K. Doping with Co allows for studying complete doping phase diagram. We performed systematic measurements of the temperature-dependent in-plane, $\rho _a(T)$, and inter-plane, $\rho _c(T)$, electrical resistivities in the compounds. At optimal doping, both $\rho _a(T)$ and $\rho _c(T)$ show close to $T$-linear temperature dependence above the superconducting $T_c$. With doping this dependence gradually evolves towards $T^2$. At much higher temperatures a slope-change is observed in $\rho _a(T)$, which we relate with onset of carrier activation over a pseudogap.   

Oxidative deintercalation of single crystal Na$_{1-\delta}$FeAs upon Interaction with the environment

Due to high mobility of Na ions, NaFeAs superconductor exhibits pronounced reaction with the environment, leading to a bulk change change in stoichiometry. We study the doping evolution of the same single crystals as a function of time of the environmental exposure. In NaFeAs, a controlled reaction with air increases the superconducting transition temperature, $T_{c}$, from the initial value of 12 K to 27 K as probed by transport and magnetic measurements. Temperature dependent resistivity, $\rho_{a}$(T), shows a dramatic change with the exposure time. In freshly prepared samples, $\rho_{a}$(T) reveals clear features at the structural, $T_{s} \approx$ 60 K, and magnetic, $T_{m}$=45 K, transitions and superconductivity with onset $T_{c;ons} \approx$16 K and offset $T_{c;off} \approx$12 K. The exposed samples show $T-$linear variation of $\rho_{a}$(T) above $T_{c;ons} \approx$30 K ($T_{c;off} \approx$26 K). This suggests bulk doping and implies the existence of a quantum critical point at the optimal doping. The resistivity for different doping levels is affected below $\sim$ 200 K suggesting the existence of a characteristic energy scale that caps the $T-$linear regime, which could be identified with a pseudogap.   

Some magnetic and electronic properties of Vanadium (hole) doped NaFeAs

We examine the magnetic properties of the hole doped system V$_{x}$Na$_{1-x}$FeAs. It's known that the Na111 system exhibits structural, magnetic, and superconducting phase transitions. As opposed to the better studied electron doped systems, we successfully grew homogeneous single crystals with holes doped into the iron sites. Preliminary results show a total suppression of the superconducting phase and partial suppression of the AF order with doping as low as 1{\%}. I will present a range of susceptibility measurements to examine the effect of Vanadium doping on T$_{c}$ and the Neel Temperature.   

Cerium-Iron Magnetic Coupling in Single Crystal CeFeAsO at Low Temperatures

Neutron and synchrotron resonant X-ray scattering techniques have been used to determine the intricate magnetic structure of single crystal CeFeAsO at low temperatures, way below the spin-density wave (SDW) transition around 130 K associated with Fe moments ordering. Our synchrotron X-ray scattering results at the Ce $L_{II-}$edge clearly show a magnetic transition that is specific to the Ce ordering at $T_{Ce}$ = 4 K, whereas neutron diffraction data indicate a transition at $T*$ = 12 K with unusual order parameter. Detailed order parameter measurements on the (100) {\&} (101) magnetic reflections by neutrons show an anomaly at $T$ = 4 K which we associate with the Ce ordering. The successive transitions at $T_{Ce}$ and $T$* can also be clearly identified by two anomalies in heat capacity measurements. We argue that the higher transition temperature observed in neutron measurements reflects Fe-Ce combined rearrangement prior to the complete ordering of the Ce. The effect of the weak Ce-Fe coupling on the rearrangement of Fe ordering is yet another example of the vulnerability of the Fe-SDW as influenced by minute doping or by relatively low applied pressures.   

FT-IR evaluation of SmFeAsO$_{1-x}$F$_{x}$ (x = 0, 0.069)

Optical properties of superconducting SmFeAsO$_{1-x}$F$_{x}$ (x=0, 0.069) were demonstrated by reflection measurement with FT- IR method. Polycrystalline SmFeAsO$_{1-x}$F$_{x}$ samples were synthesized using two-step solid state reaction described elsewhere [New J. Phys.\textbf{12}, 033005 (2010)]. Purity of samples was checked by X-ray diffraction patterns using Cu K- alpha radiation. The reflection measurement was performed at the range from 9000 cm$^{-1}$ to 18000 cm$^{-1}$ that was corresponded to an energy region from 1.12 eV to 2.25 eV. A photoconductivity of SmFeAsO$_{1-x}$F$_{x}$ was determined by Kramer-Kroning (KK) relation. Reflectivity and photoconductivity measurements, as well as by FT-IR, at various areas were performed to define an energy level of materials [EPL, \textbf{84} 67013 (2008), and J. Phys. Soc. Jpn. \textbf{80} 013707 (2011)]. Obtained photoconductivity and reflection spectra were similar to those of LaFeAsO$_{1-x}$F$_{x}$ that was a basic compound of LnFeAsO$_{1-x}$F$_{x}$ (Ln=La, Ce, Sm), reported by Z. G. Chen et al [Phys. Rev. B \textbf{81}, 100502 (2010)]. Our result suggests that the energy band structure of SmFeAsO was affected by F-doping even in visible area. Details and temperature dependence of the reflection and photoconductivity spectra will be presented at the conference.   

Superconducting and normal state properties of single crystalline RFeAsO-based materials

We report electrical resistivity, Hall effect, magnetoresistance, magnetization, and specific heat measurements on single crystals of several RFeAsO-based materials (R = rare-earth). We also discuss the effects of lattice compression on the magnetic and superconducting transition temperatures. These single crystal studies benefit from significantly sharper signatures of the transitions, compared to earlier studies on polycrystalline samples.   

Growth conditions of oxypnictide compounds LaFePnO Pn=\{P,As,Sb\}

The discovery of superconductivity in layered ferro-oxynictides LaFePO $(T_c\sim4\ K)$ and LaFeAsO$_{1-x}$F$_x$ $(T_c\sim26\ K)$ lead to an intense search for other iron based superconducting materials. For the hypothetical compound LaFeSbO, ab initio density functional theory (DFT) calculations predict an enhanced density of states at the Fermi level together with increased nesting between the electron and hole sheets in the tetragonal structure (isostructural to LaFeAsO) and an enhanced spin-density wave ground state in a closely related orthorhombic structure; indicating the potential for superconductivity with a higher transition temperature [1]. We report ab initio DFT calculations of the phase stability of the oxypnictides LaFe$Pn$O $Pn$=\{P,As,Sb\} and find growth conditions where LaFePO and LaFeAsO are thermodynamically stable, but LaFeSbO is unstable with respect to the formation of La$_2$SbO$_2$. Indeed, our attempt to synthesize LaFeSbO led to the synthesis and characterization of La$_2$SbO$_2$. The phonon spectrum of hypothetical LaFeSbO shows no soft modes, indicating that LaFeSbO is potentially metastable and leaving open the possibilty of a nonequilibrium synthesis route. \\[4pt] [1] C-Y. Moon, S.~Y. Park, and H. J. Choi, Phys. Rev. B, {\bf 78}, 212507 (2008)