Session P22: Focus Session: Fe-based Superconductivity - Fe(Te,Se)

Electronic disorder and magnetic-field-induced superconductivity enhancement in Fe$_{1+y}$(Te$_{1-x}$Se$_{x})$

The iron chalcogenide Fe$_{1+y}$(Te$_{1-x}$Se$_{x})$ superconductor system exhibits a unique electronic and magnetic phase diagram distinct from those seen in iron pnictides: bulk superconductivity does not appear immediately following the suppression of long-range ($\pi $,0) AFM order. Instead, an intermediate phase with weak charge carrier localization appears between AFM order and bulk superconductivity (Liu \textit{et al.}, Nat. Mater. \textbf{9}, 719 (2010)). In this talk, we report our recent studies on the relationship between the normal state and superconducting properties in Fe$_{1+y}$(Te$_{1-x}$Se$_{x})$. We show that the superconducting volume fraction $V_{SC}$ and normal state metallicity significantly increase while the normal state Sommerfeld coefficient \textit{$\gamma $} and Hall coefficient $R_{H}$ drop drastically with increasing Se content in the underdoped superconducting region. Additionally, $V_{SC}$ is surprisingly enhanced by magnetic field in heavily underdoped superconducting samples. The implications of these results will be discussed. Our analyses suggest that the suppression of superconductivity in the underdoped region is associated with electronic disorder caused by incoherent magnetic scattering arising from ($\pi $,0) magnetic fluctuations.   

The structure of the oxygen-annealed Fe$_{1.08}$Te$_{0.55}$Se$_{0.45}$O$_{x}$ superconductor

Effect of oxygen annealing on the iron chalcogenide superconductor with excess Fe is studied and structure change is investigated by using electron microscopy. The as-grown single crystal Fe$_{1.08}$Te$_{0.55}$Se$_{0.45}$ with the tetragonal PbO-type structure is non-superconducting owing to the excess Fe. Superconductivity is induced after oxygen annealing with an onset and zero resistance transition temperature around 14.5 K and 11.5 K, respectively. The oxygen doping is evidenced by electron energy loss spectroscopy, and accompanied by improved homogeneity in the remaining PbO-type phase as well as an increase in the L$_{3}$/L$_{2}$ intensity ratio of the Fe-L$_{2,3}$ edge, indicating an increase in Fe valence. Local phase transformation from the tetragonal PbO-type phase to the hexagonal NiAs-type phase is also observed after oxygen annealing.   

Unusual Persistence of Superconductivity Against High Magnetic Fields in the Strongly-Correlated Iron-Chalcogenide Film FeTe:O$_{x}$

We report an unusual persistence of superconductivity against high magnetic fields in the iron chalcogenide film FeTe:O$_{x}$ below 2.52 K. Instead of saturating like a mean-field behavior with a single order parameter, the measured low-temperature upper criticial field increases progressively, suggesting a large supply of superconducting states accessible via magnetic field or low-energy thermal fluctuations. We demonstrate that superconducting states of finite momenta can be realized within the conventional theory, despite its questionable applicability. Our findings reveal a fundamental characteristic of superconductivity and electronic structure in the iron-based superconductors.   

Growth conditions for pulsed laser deposited FeTeO$_{y}$ superconducting thin films and observation of a low temperature structural transition

FeTe thin films were grown by pulsed laser deposition and oxygen was incorporated to make superconducting FeTeO$_{y}$. The Te concentration of the films was highly dependent on growth temperature, due to the evaporation of Te at higher temperatures. Films grown using a porous, unreacted Fe/Te target showed islanding, an open structure, and a strong c axis texture. Oxygen could be easily added or removed by low temperature anneals. Films grown using a dense, polycrystalline conglomerate for a target and a short target-substrate distance had better epitaxy, and were dense, continuous and smooth. Post processing of oxygen on these films was difficult, but we were able to control oxygen concentration during growth using a small oxygen partial pressure. Initial low temperature diffraction studies of these films using both x-rays and neutrons indicate a sudden change in c axis lattice parameter at the superconducting T$_{C}$, but continued exposure of the beam drives out oxygen. Dense epitaxial films should allow for studying this phase change.   

Growth and oxygen doping of thin film FeTe by Molecular Beam Epitaxy

FeTe is isomorphic to FeSe, a representative of the 11 family of iron based superconductors. While not a superconductor itself, FeTe, particularly in thin film form, undergoes a superconducting transition when doped with oxygen. In this presentation, we will discuss the growth of FeTe by MBE and various schemes we used to dope the samples. Evidence from our investigation suggests that FeTe films are doped via an oxygen diffusion process which is strongly activated by temperature.   

Suppression of superconductivity in Fe chalcogenides by annealing: A reverse effect to pressure

Superconductivity in FeTe$_{1-x}$Se$_{x}$ can be controlled by annealing, in the absence of extrinsic influences. Using neutron diffraction, we show that T$_{C}$ sensitively depends on the atomic configurations of the Te and Se ions. Low temperature annealing not only homogenizes the Te and Se ion distribution as previously observed, it suppresses T$_{C}$ because of changes in the chalcogen ions' z-parameter. In particular, the height of Te from the Fe basal plane is much reduced while that for Se shows a modest increase. These trends are reverse of the effects induced by pressure.   

Phase diagram and oxygen annealing effect of FeTe1-xSex iron-based superconductor

Phase diagrams of as-grown and O$_{2}$-annealed FeTe$_{1-x}$Se$_{x}$ decided by magnetic susceptibility measurement were obtained. For as-grown samples, the antiferromagnetic order was fully suppressed in the region of $x \quad \ge $ 0.15 and superconductivity appeared at $x \quad \ge $ 0.1. However, bulk superconductivity emerged at only $x$ = 0.5. Interestingly, for O$_{2}$-annealed samples, complete suppression of the magnetic order and bulk superconductivity was observed at $x \quad \ge $ 0.1. We found that O$_{2}$ annealing induces the bulk superconductivity for FeTe$_{1-x}$Se$_{x}$. The O$_{2}$ probably play a key role of a suppression of the magnetic order and appearance of bulk superconductivity.   

Magnetism and superconductivity in Pd$_{1-x}$Fe$_{x}$Te

PdTe is a long-known superconductor but its physical properties are almost unknown. We have recently studied its basic physical properties in both normal and superconducting states. While FeTe forms different crystallographic structure and is known to form spin density wave below T$_{N}$ = 70 K, we have successfully synthesized Pd$_{1-x}$Fe$_{x}$Te with x from 0 to 1. By measuring its electrical and magnetic properties, we establish the phase diagram of Pd$_{1-x}$Fe$_{x}$Te for the first time. With increasing x, we found that T$_{c }$is quickly suppressed. Ferromagnetism appears for the samples with x $\ge $ 0.02. For 0.25 $\le $ x $\le $ 1.0, the system exhibits antiferromagnetic ordering with T$_{N}$ increasing with x. This is a prototype system for studying the interplay between superconductivity and magnetism.   

FeSe$_{0.5}$Te$_{0.5}$ thin films with critical current density above 1MA/cm$^{2}$

High quality FeSe$_{0.5}$Te$_{0.5}$ thin films have been prepared on various substrates, such as SrTiO$_{3}$, LaAlO$_{3}$ and YSZ, some with buffer layers. $T_{c}$'s as high as 20K with superconducting transition widths of about 1K were obtained. These $T_{c}$'s are much higher than those of bulk FeSe$_{0.5}$Te$_{0.5}$ ($\sim $15K). Our films carry high critical current densities $J_{c}$'s (above 1MA/cm$^{2})$ at liquid helium temperature. These films hold $J_{c}$'s above 1$\times$10$^{5}$A/cm$^{2}$ and very low $J_{c}$ anisotropies ($<$ 3) under magnetic fields as high as 30T at 4.2K. We have also prepared textured FeSe$_{0.5}$Te$_{0.5}$ thin films on a buffered metal template with results similar to the ones mentioned above. This shows that iron chalcogenides have a very promising future for high-field applications at liquid helium temperatures. Pinning force analysis indicates the presence of a point defect flux-pinning mechanism, suggesting a straightforward approach to conductor optimization.   

London penetration depth measurements of Fe$_{1+y}$(Te$_{1-x}$Se $_{x})$ single-crystals at ultra-low temperatures

The evolution of the superconducting properties as derived from the in-plane penetration depth measurements as a function of temperature in single crystals of Fe$_{1.02}$(Te$_{1-x}$Se $_{x})$, with Se concentration spanning from 25{\%} to 45{\%}, was studied using a tunnel diode oscillator technique in a dilution refrigerator down to a temperature of 30mK. By using a set of two mutually coupled planar inductors parallel to the ab plane of the samples, the probing ac field is uniform across the sample along the c axis making the variation in susceptibility solely due to in-plane currents while significantly increasing the signal to noise ratio compared to usual inductors used in similar experiments The evolution of the topology of the superconducting gap from underdoped to optimally doped samples, as derived from exponential and power law behavior of $\lambda _{ab}$ at low temperatures, is presented.   

Electron-phonon coupling in layered FeSe compounds

Iron-chalcogenide superconductors, showing many characteristic physical properties, can serve as a model materials to study the electron-pairing mechanism for all iron-based superconductivity. Layered iron-chalcogenide systems including single layer FeSe, bulk FeSe, K-intercalated FeSe, were studied using first principle pseudopotential density functional based approach. Electronic structure, vibrational properties and electron-phonon coupling strength were studied for the cases with and without iron magnetic moment ordering. The latter is incorporated using local spin density approximation. Our results show significant changes to electronic structure resulting in much higher electron-phonon coupling for spin-resolved configurations. Electron-phonon matrix elements for particular phonon mode of A1g symmetry are showing dramatic increase. Superconducting transition temperature estimates based on McMillan's equation are showing values significantly higher then previously reported, but still not high enough to account for the experimental results.   

Magnetism and superconductivity in modulated FeTe systems.

We examined the interplay between magnetism and superconductivity by monitoring the non-superconducting chalcogenide FeTe. We studied its transitions under insertion of oxygen, iron and vacancies of iron using spin-polarized band structure methods (LSDA with GGA) starting from the collinear and bicollinear magnetic arrangements. A supercell with 8-Fe and 8-Te atoms was used so that it can capture local changes in magnetic moments. The calculated values of magnetic moments agree well with available experimental data while some of the modulations lead to significant changes in the bicollinear or collinear magnetic moments/arrangements. The total energies of these systems indicate that the collinear-derived structure is more favorable prior to a possible superconducting transition.   

Revealing the dual nature of magnetism in iron pnictides and iron chalcogenides using x-ray emission spectroscopy

We present a Fe K$\beta$ x-ray emission spectroscopy study of local magnetic moments in various iron-based superconductors in their paramagnetic phases. Our findings show that a local magnetic moments exists in all samples studied: PrFeAsO, $\rm Ba(Fe,Co)_2As_2$, LiFeAs, Fe$_{1+x}$(Te,Se), and $\rm A_2Fe_4Se_5$ (where A = K, Rb, and Cs). The moment size is independent of temperature or carrier concentration, but varies significantly across different families. Specifically, all iron pnictides samples have local moments of about 1$\mu_B$/Fe, while FeTe and $\rm K_2Fe_4Se_5$ families have much larger local moments of $\sim\!\!2 \mu_B$/Fe and $\sim\!\! 3.3 \mu_B$/Fe, respectively. Our results illustrate the importance of multi-orbital physics in describing magnetism of these compounds.   

Electronic Structure of A$_y$Fe$_{2-x}$Se2 from First Principles

The newly discovered A$_y$Fe$_{2-x}$Se2 iron-selenide material family was studied in a series of first-principles simulations. The electronic structure and magnetic properties of possible vacancy-superstructure phases A$_2$Fe$_3$Se$_4$ and A$_2$Fe$_4$Se$_5$, as well as the possible parental phase of superconductivity AFe2Se2 were examined. It was discovered that AFe2Se2 ground state is a SDW-AFM metal without hole Fermi-surface, thus FS-nesting is absent and its magnetism is very likely to be local moment; A$_2$Fe$_3$Se$_4$ is a Mott insulator with SDW-AFM magnetism; A$_2$Fe$_4$Se$_5$ shows block-spin AFM ground state with 400$\sim$600 meV band gap. Under high pressure, the A$_2$Fe$_4$Se$_5$ phase exhibits rich and exotic physical properties. \\[4pt] [1] Chao Cao and Jianhui Dai, Block Spin Ground State and Three-Dimensionality of (K,Tl)$_y$Fe$_{1.6}$Se$_2$, \textit{Phys. Rev. Lett.} \textbf{107}, 056401 (2011)\\[0pt] [2] Chao Cao and Jianhui Dai, Electronic structure and Mott localization of iron-deficient TlFe$_{1.5}$Se$_2$ with superstructures, \textit{Phys. Rev. B} \textbf{83}, 193104 (2011)\\[0pt] [3] Chao Cao and Jianhui Dai, Electronic Structure of KFe$_2$Se$_2$ from First-Principles Calculations, \textit{Chin. Phys. Lett.} \textbf{28}, 057402 (2011)