Studies of Iron-Based Superconductor Thin Films

Transcription

1 MBE Growth and STM Studies of Iron-Based Superconductor Thin Films Wei Li 1, Canli Song 1,2, Xucun Ma 2, Xi Chen 1*, Qi-Kun Xu 1 State Key Laboratory of Low-Dimensional Quantum Physics, Department of Physics, Tsinghua University, Beijing , China 2 Institute of Physics, Chinese Academy of Sciences, Beijing , China *xc@mail.tsinghua.edu.cn ue 1,2 Qi-Kun Xue received his PhD degree in condensed matter physics from Institute of Physics, The Chinese Academy of Sciences (CAS) in From 1994 to 2000, he worked as a research associate in Tohoku University, Japan and North Carolina State University, USA. He became a professor at Institute of Physics, CAS in He was elected into The Chinese Academy of Sciences in Since 2005, he has been a professor in Department of Physics, Tsinghua University. He is now the Chair of Department of Physics and the Dean of School of Sciences, Tsinghua University. His research interests include scanning tunneling microcopy/spectroscopy, molecular beam epitaxy growth of semiconductor and superconductor thin films, lowdimensional superconductivity, topological insulators. Studies of iron-based superconductors usually suffer from various imperfections in materials. We have successfully solved this problem by molecular beam epitaxy (MBE) growth under well-controlled conditions. The MBE grown FeSe and K x Fe 2-y Se crystalline films of extremely high quality together with in situ scanning tunneling microscopy (STM) as a local probe in an MBE-STM combined system avoid the complexity caused by uncertainties in stoichiometry and defects of these materials. We show that our approach is very powerful and unique for a study of novel phenomena in quantum materials. 1. Introduction The discovery of superconductivity in iron pnictides [1] has opened a new avenue for revealing the secrets of high-tc superconductors. However, the uncertainty in the stoichiometry of samples has made it challenging to obtain the intrinsic properties of the superconducting and normal states of the materials. To avoid this complexity, we combine

2 molecular beam epitaxy (MBE) with a low-temperature scanning tunneling microscope (STM) in the same ultrahigh-vacuum (UHV) environment. MBE, one of the most powerful techniques for thin films growth, is employed to obtain ultra high-quality samples and in situ low-temperature STM is used as the most sensitive local probes of the samples. By giving two examples, FeSe [2,3] and K x Fe 2-y Se 2 [4], we show that an STM?MBE combined system is powerful and unique for studies of low-dimensional and new functional materials. 2. FeSe PbO-type -FeSe with a common structural motif of FeX4 (X: Se, As, P) tetrahedral is the simplest iron-based superconductor [5, 6]. Under an ambient pressure, its transition temperature of Tc is ~8 K. Suffering from significant nonstichiometry, disorder and clustering pathologies, the pairing symmetry of this material remains elusive. 2.1 MBE Growth and Observation of Nodal Gap Function The FeSe film was prepared on the graphitized SiC(0001) substrate. High-purity Fe (99.995%) and Se ( %) were evaporated from two standard Knudsen cells. The growth was carried out under Se-rich conditions with a nominal Se/Fe flux ratio of ~20. The MBE growth of the FeSe films is characterized by a typical layer-by-layer mode. The STM topographic images (Fig. 1, a and b) showed atomically flat and defect-free Se-terminated (001) surfaces with large terraces. The selenium atom spacing of the (1 1)?Se lattice (Fig. 1b) in the topmost layer was 3.8?, consistent with the previous reports. Fig. 1. STM characterization of the as-grown FeSe film. (a) Topographic image of FeSe a film (~30 unit cells thick). (b) Atomic-resolution STM image of FeSe film (5nm 5nm). (c) Temperature dependence of differential conductance spectra. Scanning tunneling spectroscopy (STS) probes the quasiparticle density of states. STS (Fig. 1c) on FeSe films at 0.4 K revealed an unexpected V-shape gap structure, which is quite similar to cuprates, and is in contrast to the U-shape gap of FeSe 0.5 Te 0.5 [7]. This feature explicitly reveals the existence of line nodes in the superconducting gap function. We expect that the small chalcogen-height in FeSe [8] enhances the exchange interaction between the

3 nearest-neighbor Fe atoms and results in a dominant pairing symmetry with nodes on the electron pockets. The maximum of the superconducting gap 0 = 2.2 mev is half of the energy between the two conductance peaks. The gap disappears above Tc, as shown in the various temperature STS, and the ratio of 2 0 /k B T c is ~ 5.7 (k B is the Boltzmann constant), exceeding the s-wave weak coupling BCS ratio ~ 3.5. Furthermore, well-controlled layer-by-layer growth allows us to investigate the thicknessdependent superconductivity of FeSe [3]. We show that the superconductivity transition temperature Tc of FeSe films scales inversely with the film s thickness. 2.2 Two-Fold Pairing Symmetry We could also apply a magnetic field and introduce single impurities to gain further insight into the pairing symmetry. When a magnetic field is applied perpendicularly to the FeSe sample surface, the field can enter the superconductor in the form of vortices (Fig. 2a). The di/dv curve at the center of a vortex in FeSe showed a pronounced zero-bias peak, as shown in Fig. 2b. Figure 2c is a zero-bias di/dv mapping near a single vortex, which reflects the spatial distribution of the peak. Surprisingly, this resonance state elongates along the a-axis (presumably the direction with nodes). Anisotropic distribution of the core state can be understood by the difference between coherence lengths along the a- and b-directions, which mainly stems from the twofold symmetry of the gap function.

4 Fig.2. Two folded vortex and impurity-induced bound states. (a) Vortex lattices under a magnetic field of 4T (0.4K). (b) STS on the center of a vortex core. (c) Zero-bias mapping for a single vortex. (d to f) STM topography, di/dv spectrum, and density of states map of a single Fe adatom. The twofold symmetry of the FeSe gap function is further supported by the impurity-induced resonance states inside the superconducting gap. Fe atoms were deposited on FeSe surface (Fig. 2d) at low temperature (about 50 K). On a single Fe adatom, STS reveals two resonance states at?1.4 mev and?0.4 mev (Fig. 2e). The density of states map in Fig. 2f again shows the twofold symmetry. 3. K x Fe 2-y Se 2 The newly discovered alkali-doped iron selenide superconductors [9] not only reach a superconducting transition temperature as high as 32 K, but also exhibit unique characteristics that are absent from other iron-based superconductors, such as antiferromagnetically ordered insulating phases [10,11], extremely high N?el transition

5 temperatures [12] and the presence of Fe vacancies and ordering [13-15]. The key debate regarding this system is whether the superconductivity can coexist with magnetism. 3.1 MBE Growth and Observation of Phase Separation We extended the unique capability of the combined STM/MBE system to study ternary iron selenide. K x Fe 2-y Se 2 was prepared on a graphitized 6H-SiC(0001) substrate, which follows the island growth mode. The size of an island is typically 100 nm 100 nm. Two distinct regions (marked by I and II in Fig. 3b), coexisting side by side, are clearly revealed on each island, indicating that phase separation occurs. Fig. 3. K x Fe 2-y Se 2 MBE film and phase separation of K x Fe 2-y Se 2. (a) The crystal structure of KFe 2 Se 2. (b) STM topographic images of a K x Fe 2-y Se 2 film. Two distinct regions are labeled by I and II, respectively. (c) Atomic-resolution STM topography of region I (5 nm 5 nm). (d) Atomic structure of (110) plane. K and Se atoms are in the topmost layer. Fe atoms are in

6 the second layer. (e) Differential conductance spectrum in region I. (f) Differential conductance spectrum in region II. (g) Atomic-resolution STM topography of region II (10 nm 10 nm). (h) The structure of 5 5 Fe vacancy pattern as seen from (110) plane. The positions of Fe vacancies are marked by crosses. The STM topography with atomic resolution (Fig. 3c) of region I exhibits a centered rectangular lattice structure. The periods along the two orthogonal directions are 5.5 and 14.1?, respectively. Compared to the X-ray diffraction data, we realized that the orientation of the film is (110) (see Fig. 3d) instead of the natural cleavage plane (001). The K atoms are visible at positive bias and form atomic rows 7.05? apart (Fig. 3c). Furthermore, our STM observation shows that very few defects in region I could be found. We therefore identify region I as the stoichiometric KFe 2 Se 2. STS (Fig. 3e) at 0.4 K on region I exhibits a superconducting gap centered at the Fermi energy and two characteristic coherence peaks, indicating the stoichiometric KFe 2 Se 2 is a superconductor. Region II shows a periodic stripe pattern. STS in Fig. 3f exhibits an energy gap up to 0.43 ev across the Fermi level, suggesting that this region is insulating. As well as the K atomic rows in the topmost layer, there is a superposed striped structure with a period of 14.0? in the STM image (Fig. 3g). The stripes are along the c axis and perpendicular to the K atomic rows. We attribute this superstructure to the 5 5 pattern of Fe vacancies in the second atomic layer and this vacancy order leads to a composition of K x Fe 1.6 Se 2, where x is either 1 or Magnetic Order in Superconducting Region According to the above STM study, we demonstrate that a K x Fe 2-y Se 2 sample contains two distinct phases: an insulating phase with well-defined 5 5 order of Fe vacancies, and a superconducting KFe 2 Se 2 phase containing no Fe vacancies. By annealing the sample at 450 C for several hours, a few iron vacancies could be injected into the superconducting region from the insulating K x Fe 1.6 Se 2 region (Fig. 4a). By examining the registration of iron sites with respect to the Se lattice in the topmost layer, the iron atoms in the (110) plane can be divided into two interpenetrating sublattices. As shown in Fig. 4b, the iron vacancies on two different sublattices are labeled by A and B, respectively.

7 Fig. 4. Fe vacancy-induced bound states. (a, b) STM topography (12 nm 12 nm) and atomic structure of Fe vacancies. Two types of vacancies are labeled by A and B respectively. (c) di/dv spectrum of a single Fe vacancy. (d to f) Magnetic field dependence of the bound states. An iron vacancy carries spin and breaks superconducting pairing in the singlet channel through spin-flip scattering. STS on a single iron vacancy (Fig. 4c) shows the strongly suppressed coherence peaks and a pair of resonances inside the superconducting gap, i.e., an electron-like bound state at 1.9 mev and a hole-like bound state at -1.9 mev. Expecting to reveal the difference in spin orientation between the two types of vacancies, we apply a magnetic field perpendicular to the sample surface to break the rotational symmetry. To date, the magnetic field effect on defect-induced sub-gap resonance has not been observed. The energy of a subgap state has a linear dependence on magnetic field B (Figs. 4d to 4f): E=E 0 +gµ B B S, where µ B is the Bohr magneton and g the Land? factor. Fitting the data to a line gives g= , close to of a free electron. The most striking behavior of the magnetic field effect is that the peaks on type-a and B vacancies shift to completely opposite directions with field. The opposite shifting can be attributed to the different spin orientations of the two types of vacancies, implying a magnetically-related bipartite order in the tetragonal iron lattice. 4. Summary

8 Using MBE, we could not only obtain high-quality and defect-free samples, but we could also control the stoichiometry, doping level, and crystallographic orientation of the sample. Combined with low-temperature STM, many problems can be resolved in low-dimensional and novel quantum materials [16]. References and Notes 1. Y. Kamihara, T. Watanabe, M. Hirano, H. Hosono, J. Am. Chem. Soc. 130, 3296 (2008). 2. C.-L. Song et al., Science 332, 1410 (2011). 3. C.-L. Cong et al., Phys. Rev. B 84, (2011). 4. W. Li et al., Nature Phys 8, (2012). 5. F. C. Hsu et al., Proc. Natl. Acad. Sci. U. S. A. 105, (2008). 6. S. Medvedev et al., Nat. Mater. 8, 630 (2009). 7. T. Hanaguri, S. Niitaka, K. Kuroki, H. Takagi, Science 328, 474 (2010). 8. K. Kuroki et al., Phys. Rev. B 79, (2009). 9. Guo, J. G. et al. Phys. Rev. B 79, (R) (2010). 10. M.-H. Fang et al., Europhys. Lett. 94, (2011). 11. Z. G. Chen et al., Phys. Rev. B 83, (R) (2011). 12. W. Bao et al., Chin. Phys. Lett. 28, (2011). 13. Z. Wang et al., Phys. Rev. B 83, (R) (2011). 14. P. Zavalij et al., Phys. Rev. B 83, (2011). 15. X.-W. Yan, M. Gao, Z.-Y. Lu, T. Xiang, Phys. Rev. B 83, (2011). 16. Jin-Feng Jia, Xucun Ma, Xi Chen, Qi-Kun Xue, J. Phys. D: Appl. Phys (2011).