Session H22: Focus Session: Fe-based Superconductors - Anisotropic Transport and Anisotropy

Temperature-dependent anisotropic resistivity in electron, hole and isoelectron - doped BaFe$_2$As$_2$ superconductors

Anisotropic electrical resistivity, $\rho(T)$, was studied in iron-arsenide superconductors, obtained by doping the parent BaFe$_2$As$_2$ compound on three different sites: (1) electron donor transition metal (Co,Ni,Rh,Pd) substitution of Fe [1,2]; (2) hole donor K substitution of Ba [3]; (3) isoelectron P substitution of As. For all three types of dopants a range of $T$-linear behavior is found at the optimal doping in both the in-plane and the inter-plane $\rho(T)$ above $T_c$. At some higher temperature this range of $T$-linear resistivity is capped by a slope-changing anomaly, which, by comparison with NMR, magnetic susceptibility and Hall effect measurements, can be identified with the onset of carrier activation over the pseudogap [1]. The doping-evolution of anisotropic temperature dependent $\rho(T)$ and of the pseudogap are quite different for three types of doping. A three-dimensional $T-H$ phase diagram summarizing our results will be presented. Furthermore, potential correlation of the anisotropic normal state transport and anisotropic superconducting state heat transport will be discussed. \\[4pt] In collaboration with N. Ni, A. Thaler, S.L.Bud'ko, P.C. Canfield, R. Prozorov, Bing Shen, Hai-Hu Wen, K. Hashimoto, S. Kasahara, T. Terashima, T. Shibauchi and Y. Matsuda. \\[4pt] [1] M.A.Tanatar et al. PRB \textbf{82}, 134528 (2010)\\[0pt] [2] M.A.Tanatar et al. PRB \textbf{84}, 014519 (2011)\\[0pt] [3] M.A.Tanatar et al. arXiv:1106.0533   

Enhanced conductance in the normal state of Fe pnictides {\&} chalcogenides measured by quasiparticle scattering spectroscopy (QPS): evidence of orbital fluctuations

QPS reveals a conductance enhancement at a temperature, T$_{onset}$,~for RFe$_{2}$As$_{2}$ (R=Ca, Sr, Ba) and Fe$_{1.13}$Te. For Ba/Sr Fe$_{2}$As$_{2}$ and Fe$_{1.13}$Te the enhancement survives well above the magnetic and structural transition temperatures (Ba: T$_{N}\sim $132 K, T$_{onset}\sim $175 K; Sr: T$_{N}\sim $192 K, T$_{onset}\sim $240 K; Fe$_{1.13}$Te: T$_{N}\sim $60 K, T$_{onset}\sim $75 K) while for most CaFe$_{2}$As$_{2 }$junctions it disappears below T$_{N}$ and T$_{S }$(11 junctions tested, only 2 showed weak enhancement above T$_{N})$. For Co underdoped BaFe$_{2}$As$_{2}$ the enhancement coexists with the superconducting Andreev peaks while it is not observed for Co overdoped Ba122. We construct a modified phase diagram for Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ to reflect the presence of this feature for the underdoped regime.\footnote{Arham et. al, arXiv:1108.2749.} We discuss this conductance enhancement in the context of non-Fermi liquid behavior of these compounds due to orbital fluctuations.\footnote{Lee et. al, arXiv:1110.5917.} This work is supported by the Center for Emergent Superconductivity, an Energy Frontier Research Center funded by the US DOE, Office of Science, Award No. DE-AC0298CH1088. Work at Cambridge supported by EPSRC. Work at BNL under DOE Award No.DE-AC0298CH10886. Ames Lab. operated by ISU for the U.S. DOE under Award No. DE-AC02-07CH11358.   

Nematic order in the vicinity of a vortex in superconducting FeSe

We present a phenomenological theory of the interplay between nematic order and superconductivity in the vicinity of a vortex induced by an applied magnetic field [1]. Nematic order can be strongly enhanced in the vortex core. As a result, the vortex cores become elliptical in shape. For the case where there is weak bulk nematic order at zero magnetic field, the field-induced eccentricity of the vortex core has a slow power-law decay away from the core. Conversely, if the nematic order is field induced, then the eccentricity is confined to the vortex core. We discuss the relevance of our results to recent scanning tunneling microscopy experiments on FeSe [2]. \\[4pt] [1] D. Chowdhury, E. Berg and S. Sachdev, to appear in Phys. Rev. B, arXiv: 1109.2600 (2011).\\[0pt] [2] Can- Li Song et al., Science 332, 1410 (2011).   

T-matrix Impurity Effects on Nematicity in Iron-Based Supeconductors

In iron-based superconductors, nematicity has been reported in transport measurements and a broad range of spectroscopies, including angle-resolved photoemission, neutron scattering, and scanning tunneling microscopy. These observed anisotropies of broken tetragonal symmetry have been attributed to pure spin physics or unequal occupation of the iron d-electron orbitals, referred to as orbital ordering. To address this issue, we use realistic multi-orbital tight-binding Hamiltonians and T-matrix formalism to explore the effects of non-magnetic impurities. In particular, we present a detailed examination of the local density of states around impurities, and highlight the interplay of magnetic and orbital degrees of freedom.   

Surface Structure of Stripe ordered 1x2 phase on (Ba, Ca)(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$

Low energy electron diffraction (LEED) and scanning tunneling microscopy/spectroscopy (STM/S) have been utilized to investigate the geometric structure of the stripe 1$\times $2 surface phase of (Ba, Ca)(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ iron pnictides. STM images show that the surface consists of competing ordered and disordered regions. The 1$\times $2 phase appears on the surface of all compounds but coexist with ($\surd $2 $\times \surd $2)R45$^{\circ}$ phase on the surface of Ba122. Quantitative structural analysis of LEED-I(V) using the fractional spots of the 1$\times $2 phase on both parent compounds as well as Ca(Fe$_{0.925}$Co$_{0.075})_{2}$As$_{2}$ gives a similar surface structure with a termination layer of 50{\%} Ca/Ba atoms. The surface Ca/Ba layer has a large inward relaxation $\sim $ 0.5 {\AA} and the underneath As-Fe-As layer displays a buckling distortion. The Pendry Rp factor ($\sim $ 0.24) obtained in the structural refinement is excellent for all three systems.   

Scanning tunneling spectroscopy in Co-doped BaFe$_{2}$As$_{2}$: what density functional theory can tell us

We use density functional theory to simulate the scanning tunneling spectra and topographic images of Co-doped BaFe$_{2}$As$_{2}$. The matrix element effects are evaluated and the specific contributions of the different surface atoms to the spectra are considered. The results give a better understanding of the measured spectra and assess the resolution of STS measurements in these systems.   

Scanning tunneling spectroscopic studies of the iron-arsenic superconducting Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ single crystals

Scanning tunneling spectra of Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ (x=0.06, 0.08, 0.12) single crystals are studied as a function of temperature ($T)$ and magnetic field ($H)$. For $H$ = 0, direct evidence for two-gap superconductivity at energies $\omega$ = $\Delta _{\beta}$ and $\Delta _{\alpha ,\gamma /\delta}$ ($\approx $ 2$\Delta _{\beta })$ and for magnetic resonance modes at $\Omega \quad \approx \quad \Delta _{\beta }+\Delta _{\alpha ,\gamma /\delta}$ are found for all samples at $T \quad < \quad T_{c}$. Fourier transformation of the tunneling spectra reveals $x$- and $\omega$-dependent quasiparticle interference (QPI) wave-vectors \textbf{q}$_{2}$ near ($\pm \pi $,0)/(0,$\pm \pi )$ and \textbf{q}$_{1}$ near ($\pm $2$\pi $,0)/(0,$\pm $2$\pi )$. The spectral intensity of \textbf{q}$_{2}$ exhibits strong $\omega$-dependence, peaking sharply at $\omega$ = $\Delta _{\beta }$, $\Delta _{\alpha ,\gamma /\delta}$ and $\Omega$. This is in stark contrast to the Bragg diffraction peaks that are independent of $\omega$, $T$ and $x$. For $H \quad >$ 0, additional QPI wave-vector \textbf{q}$_{3}$ appears near ($\pm \pi $,$\pm \pi )$. These findings are consistent with the sign-changing $s$-wave pairing symmetry. Additionally, for the optimally doped sample, a pseudogap at $\omega$ $\sim \quad \Delta _{\gamma /\delta }$ is found inside the vortex core, possibly due to coexisting superconductivity and spin density waves. This result is in contrast to the zero-bias conductance peaks observed inside the vortex core of (Ba$_{1-x}$K$_{x})$Fe$_{2}$As$_{2}$, implying asymmetry in the hole and electron-doping of the iron arsenides. This work was supported by NSF DMR-0907251.   

Examining multiple spectroscopic techniques with multiband Eliashberg theory in BaCo$_{x}$Fe$_{2-x}$As$_2$

A wealth of experimental data from multiple probes are available for the newly-discovered pnictide superconductors. Here, we exploit this fact and examine a multi-band Eliashberg model for optimal doped BaCo$_x$Fe$_{2-x}$As$_2$ assuming a broad spin-fluctuation boson spectrum and retaining the full 3D bandstructure. Our focus is on comparing the model directly with data collected by our group from ARPES, STM and optics experiments performed on samples from the same growth batch. We find that the model captures all of the important aspects of the three probes including the behavior of the gap structure in the dI/dV characteristics and contributions from low-energy interband transitions in the optical conductivity. Our results also indicate the role of matrix elements in establishing the strong particle-hole asymmetry observed in the dI/dV spectra.   

Point-Contact Andreev Reflection Spectroscopy in Pt-Substituted BaFe2As2

We have investigated the superconducting order parameter of BaFe2-xPtxAs2 using point-contact Andreev reflection spectroscopy (PCAR). The samples used were large single crystals with measured Pt concentrations consistent with optimal doping (x = 0.15). Junctions were made between gold or lead tips and the c-axis of the superconducting samples. Conductivity spectra were measured over a range of temperatures and fit to curves generated using the Blonder-Tinkham-Klapwijk (BTK) model for two gaps, one isotropic and one angle-dependent gap with nodes.   

Scanning tunneling microscopy study of K-doped iron selenide superconductor film by MBE

The alkali-doped iron selenide superconductors have generated considerable excitements as well as confusions, regarding the delicate interplay between Fe vacancies, magnetism and superconductivity. We have grown high-quality K$_{x}$Fe$_{2-y}$Se$_{2}$ thin film with (001) surface orientation on STO substrate by molecular beam epitaxy. The scanning tunneling microscopy (STM) measurement demonstrates that there are two superconducting phases: striped KFe$_{2}$Se$_{2}$ in adjacent to the phase with $\surd 5 \times \surd $5 Fe vacancy order and doped KFe$_{2}$Se$_{2}$ with Fe and K vacancies. Both phases have a superconducting gap of 9 meV. These findings elucidate the existing controversies on the role of $\surd 5 \times \surd $5 Fe vacancy order in superconducting K$_{x}$Fe$_{2-y}$Se$_{2}$. Based on the atomic level information by STM, we will discuss the mechanism of the two different superconducting phases.   

The Surface Structure of FeTe$_{1-x}$Se$_{x}$: A LEED/STM Study

We have utilized low energy electron diffraction (LEED $I-V)$ and scanning tunneling microscopy (STM) to investigate the structural properties of FeTe$_{1-x}$Se$_{x}$ ($x$ = 0 and 0.45) surface. The LEED pattern indicates there is no surface reconstruction on both parent and doped compounds. However, the detailed surface structure calculations from LEED $I-V$ show the cleaved surface of FeTe is Te termination with a 0.06 {\AA} compression on the top layer. The STM topography shows the extreme flatness of the FeTe surface with a corrugation of less than 8 pm. The high resolution STM topography indicates that nanoscale chemical phase separation between Te and Se atoms is unambiguously revealed on the surface of FeTe$_{0.55}$Se$_{0.45}$. Chemical phase separation on the nanoscale makes the LEED $I-V$ surface structural analysis of FeTe$_{1-x}$Se$_{x}$ (001) very challenging. We will discuss the solution of the LEED analysis and the STM observations as a function of $x.$   

Cryomagnetic Point-Contact Andreev Reflection Spectroscopy on Single Crystal Iron-Chalcogenide Superconductors

We report on cryomagnetic point-contact Andreev reflection spectroscopy performed on single crystals of superconducting FeTe$_{1-x}$S$_{x}$ and FeTe$_{1-x}$Se$_{x}$. The samples are cleaved in-situ and the measurements are carried out at temperatures down to 4.2K and in a field up to 9T. At base temperature and zero field, we observe a cone-shaped hump at lower voltages in the conductance spectra with no dips at zero bias and a linear background at higher voltages. The spectral evolution of gap size, zero-bias conductance, and excess spectral area are analyzed as a function of temperature and field. Further spectral analysis is carried out using theoretical models of conductance spectra in multiband superconductors [1,2] and of gap symmetry in Fe-based superconductors [3]. The role of interstitial iron is also considered, by comparison with atomically-resolved scanning tunneling spectroscopy data.\\[4pt] [1] V. Lukic and E.J. Nicol, PRB 76, 144508 (2007) [2] A. Golubov \emph{et al.}, PRL 103, 077003 (2009) [3] P.J. Hirschfeld \emph{et al.}, RPP 74, 124508 (2011)   

Transport properties of disordered iron-pnictides in case of coexistence between superconducting and spin-density wave order

We present a theoretical description of the transport properties of a dirty multi-band superconductor in the case when both superconducting and spin-density wave orders coexist. We focus on differential conductance spectra of normal metal-superconductor junctions. In pure SC phase, we demonstrate that the interband impurity scattering broadens the coherent peak near the superconducting gap and significantly reduces its height even at relatively low scattering rates. This broadening is consistent with a number of recent experiments performed for both tunnel junctions and larger diffusive contacts. We further analyze the effect of the SDW order parameter on the differential conductance and other transport properties in the coexistence phase.