Observation of Topologically Protected Surface States in -PdBi 2 Superconductor

Transcription

1 PC-20-INV Observation of Topologically Protected Surface States in -PdBi 2 Superconductor M. Sakano 1, K. Okawa 2, M. Kanou 2, H. Sanjo 1, T. Okuda 3, T. Sasagawa 2, K. Ishizaka 1 1 Quantum-Phase Electronics Center (QPEC) and Department of Applied Physics, The University of Tokyo, Tokyo , Japan 2 Materials and Structures Laboratory, Tokyo Institute of Technology, Kanagawa , Japan 3 Hiroshima Synchrotron Radiation Center, Hiroshima University, Higashi-Hiroshima , Japan The topological aspects of electrons in solids emerge in realistic matters, as represented by topological insulators. They are expected to show a variety of new magnetoelectric phenomena, and especially the ones hosting superconductivity are strongly desired as the candidate for topological superconductors. We report on the direct observation of topologically-protected surface states in a centrosymmetric layered superconductor, -PdBi2, investigated by spin- and angle-resolved photoemission spectroscopy. Besides the bulk bands, a clear Dirac-cone type surface band is observed at the binding energy of 2.4 ev, with symmetrically allowed in-plane spinpolarizations. In addition, a similarly spin-polarized surface state crossing the Fermi level is also recognized. These surface states are precisely evaluated to be topological, based on the first-principles band calculation and the Z2 invariant analysis in analogy to 3-dimensional strong topological insulators. -PdBi2 may offer a solid stage to investigate the topological aspect in the superconducting condensate.

2 PC-21 Superconducting Gap Symmetry of the BiCh-based Superconductor La(O 0.5 F 0.5 )BiCh 2 Naoki Kase *,1, Yusuke Terui 1, Kenta Saitoh 2, Tomohito Nakano 1, Naoya Takeda 2 1 Graduate school of science and technology, Niigata University 2 Deaprtment of materials science and technology, Niigata University Superconductivity of the BiCh-based layered compound La(O 0.5 F 0.5 )BiCh 2 (Ch = S, Se) has attracted much attention in the strong correlated system[1]. The crystal structure consists of the LaO layer and BiCh 2 layer, which is very similar to the FeAs-based high-t c superconductor. The BiCh-based superconductor has the relatively large upper critical field µ 0 H(0) = 24 T applying magnetic fields along ab-plane[2]. By analogy with the FeAs-based high-t c superconductor and the BiCh-based superconductor, it is highly possible that the origin of mechanism of the superconducting transition is unconventional. However, there is little report on the superconducting gap symmetry. Because synthesized single crystals are relatively small and lightweight, it is difficult to perform high precision measurements. In order to overcome the present situation, we constructed a hand-made sensitive calorimeter, which allows us to implement the high precision measurements, and is a powerful tool to reveal the superconducting symmetry. In this presentation, we discuss the superconducting gap symmetry of La(O 0.5 F 0.5 )BiS 1-x Se x by means of the sensitive calorimeter with one piece of single crystal (less than 0.5 mg). Single crystals of La(O 0.5 F 0.5 )BiS 2-x Se x were grown by a CsCl/KCl flux method. Electrical resistivity ρ(t) was observed by a standard dc-four-prove method. Specific heat C(T, H) was measured by a relaxation method with a 3 He cryostat. Sample was sandwiched between a thermometer and heater (RuO 2 ). The thermometer and heater are directly hanged by superconducting wire as electrode. The C(T)/T value of hand-made sensitive calorimeter is less than 2.0 nj/ K 2 at 0.4 K. The raw value of the sample and calorimeter is approximately 4.0 nj/k 2 at 0.4 K. The setup of the measurement of C(T) is presented in the inset of Fig. 1. Figure 1 shows T-dependence of specific heat C(T) divided by T of La(O 0.5 F 0.5 )BiSSe in a magnetic filed of 0 and 1 T. Clear jump is observed at T c = 3.9 K from C(T)/T in 0 T, suggesting that superconductivity is of bulk nature. The transition temperature is corresponding to that of electrical resistivity ρ(t). From the result, ΔC e /γt c is estimated to be approximately 1.7, suggesting a strong-coupling superconductor. Below T c, the electronic specific heat C e (T) behaves as an exponential dependence, suggesting that superconductivity is fully gapped. Detail analysis of T-dependence of C e (T) and H-dependence of C e (H) will be presented, and we discuss the superconducting symmetry of La(O 0.5 F 0.5 )BiS 2-x Se x. [1] Y. Mizuguchi et al., J. Phys. Soc. Jpn., (2012). [2] M. Tanaka et al., Appl. Phys. Lett., 106, (2015). Figure 1. T-dependence of specific heat C(T) divided by T of La(O 0.5 F 0.5 )BiSSe in a magnetic field of 0 and 1 T. Inset shows a picture of a hand-made sensitive calorimeter with the single crystal sample of La(O 0.5 F 0.5 )BiSSe.

3 PC-22 Transport properties of FeSe 1-x Te x thin films under finite magnetic fields Yuichi Sawada *, Fuyuki Nabeshima, Daisuke Asami, Ryo Ogawa, Yoshinori Imai, Atsutaka Maeda (Dept. of Basic Science, The Univ. of Tokyo) Since the discovery of iron-based superconductors, iron-chalcogenide superconductors FeSe 1-x Te x have been studied energetically. Because FeSe 1-x Te x consist of only two-dimensional conducting planes, it is suitable for studying the mechanism of iron-based superconductors. However, it was impossible to obtain single-phase samples with 0.1 < x < 0.4 because of phase separation[1], which hinders systematic studies. Recently we demonstrated the fabrication of FeSe 1-x Te x epitaxial thin films with the whole range of it using pulsed laser deposition and proposed new phase diagram of this system. Surprisingly, we observed a giant enhancement of T c in the region of x = , and drastic change of T c in 0.1 < x < 0.2[2]. To investigate the origins of them, we measured the temperature dependence of the electrical resistivity under finite magnetic fields and Hall effect for our FeSe 1-x Te x thin films on CaF 2. From these measurements, we estimated upper critical field and Hall coefficient. In 0.2 < x < 0.4, which is optimal region for FeSe 1-x Te x thin films on CaF 2, resistive broadening, which is familiar in cuprate superconductors, was observed, while the width of superconducting transition was almost the same with increasing magnetic field in x = In addition, temperature dependence of Hall coefficient drastically changed, on the boundary between 0.1 < x < 0.2 at low temperature, indicating change of electrical structure. [1] M. H. Fang et al., Phys. Rev. B 78, (2008). [2] Y. Imai et al., Proc. Natl. Acad. Sci. U.S.A 112, 1937 (2015).

4 PC-23 Doping Evolution of the Optical Spectrum in Ba(Fe 1 x TM x ) 2 As 2 (TM=Cr, Mn, and Co) Tatsuya Kobayashi *,1, Masamichi Nakajima 1, Shigeki Miyasaka 1, Setsuko Tajima 1 ( 1 Department of Physics, Graduate School of Science, Osaka University, Toyonaka, Osaka , Japan) In iron-based superconductors, it is still unclear how the chemical substitution and carrier doping affect the charge transport. We have investigated the doping dependence of the optical spectrum in Ba(Fe 1-xTM x) 2As 2 (TM=Cr, Mn, and Co, TM-Ba122). In Mn- and Co-Ba122, the low-energy optical conductivity of the paramagnetic-tetragonal (PT) phase can be decomposed into a narrow Drude part and a broad incoherent term. The Drude component increases with Co-substitution as previously reported [1], while it broadens with doping level in Mn-Ba122, as shown in Fig. 1. On the other hand, the narrow Drude component disappears and the incoherent component dominates the low-energy conductivity in Cr-Ba122. These results suggest that the narrow Drude component is attributed to electron carries, and is enhanced by electron doping in Co-Ba122. On the other hand, this component is broadened by a strong impurity scattering in Mn-Ba122, and it is reduced by hole doping in Cr-Ba122. The different doping dependence of the low-energy conductivity is consistent with the recent Hall resistivity measurement and may lead to the different transport property [2]. [1] M. Nakajima, S. Ishida, T. Tanaka, K. Kihou, Y. Tomioka, T. Saito, C. H. Lee, H. Fukazawa, Y. Kohori, T. Kakeshita, A. Iyo, T. Ito, H. Eisaki, and S. Uchida, Sci. Rep. 4, 5873 (2014). [2] T. Kobayashi, K. Tanaka, S. Miyasaka, and S. Tajima, to be published in J. Phys. Soc. Jpn. arxiv: Conductivity ( -1 cm -1 ) Ba(Fe 0.91 Cr 0.09 ) 2 As 2 T AFO =90K T=150K exp total Drude Lorentz (cm -1 ) Ba(Fe 0.96 Mn 0.04 ) 2 As 2 T AFO =112K T=150K exp total Drude1 Drude2 Lorentz (cm -1 ) Ba(Fe 0.96 Co 0.04 ) 2 As 2 T AFO =80K T=150K exp total Drude1 Drude2 Lorentz (cm -1 ) Figure 1. Doping evolution of optical conductivity in Ba(Fe 1 xtm x) 2As 2 (TM=Cr, Mn, and Co).

5 PC-24 Anisotropy of the Thermopower in Iron Arsenide T. Fujii *, T. Shirachi, and A. Asamitsu (University of Tokyo) Recently, an anomalous in-plane anisotropy of the resistivity have been reported in an iron arsenide Ba(Fe 1-xCo x) 2As 2[1] below the temperature T*, which is far above the magnetostructural transition temperature T AFO. The origin of the anisotropy have been ascribed to the reconstructed Fermi surface in the antiferromagnetic spin order. On the other hand, it is reported that the magnitude of anisotropy is proportional to the residual resistivity, which indicate the resistivity anisotropy originates from the anisotropic impurity scattering[2]. In this study, in order to investigate the origin of the anisotropy, the anisotropy of the thermopower was measured for the detwined single crystals of Ba(Fe 1-xCo x) 2As 2,. The thermopower does not depend on the scattering time, when there are no energy dependences of scattering time. Single crystals of Ba(Fe 1-xCo x) 2As 2 were grown by self-flux method. By gluing the sample on the side wall of a piezo stack, strains can be applied by the deformation of the piezo stack. As seen in Fig. (a) and (b), the anisotropy of the resistivity was clearly observed below T*, which is lower along antiferromagnetic and longer a-axis than along ferromagnetic and shorter b-axis. In the case of thermopower (Fig. (d) and (e)), no anisotropy was observed between T* and T AFO. Since the thermopower does not depend on the scattering time, the anisotropy of the resistivity between T* and T AFO is considered to due to the anisotropic impurity scattering. On the other hand, clear anisotropy can be observed below T AFO, which indicate the anisotropy in the thermopower below T AFO is ascribed to the reconstructed Fermi surface. [1] Science; 329, 824 (2010). [2] Phys. Rev. Lett.; 110, (2013). (T)/ (300K) a-axis b-axis T* (T)/ (200K) T * (T)/ (200K) x=0 (a) x=0.05 (b) x=0.08 (c) S(T) / S(300K) T AFO S(T) / S(200K) (d) Temperature (K) T AFO S(T) / S(200K) -1.2 (e) T (K) (f) T (K) Figure: Anisotropy of the resistivity and thermopower in Ba(Fe1-xCox)2As2.

6 PC-25 Growth and characterization of millimetersized single crystals of CaFeAsF Gang Mu *,1, Yonghui Ma 1,2, Hui Zhang 3, Xiaoming Xie 1 ( 1 State Key Laboratory of Functional Materials for Informatics and Shanghai Center for Superconductivity, Shanghai Institute of Microsystemand Information Technology, Chinese Academy of Sciences, Shanghai, China, 2 School of Physical Science and Technology, ShanghaiTech University, Shanghai, China, 2 CAS Key Laboratory of Materials for Energy Conversion, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, China) High-quality and sizable single crystals are crucial for studying the intrinsic properties of unconventional superconductors, which are lacking in the 1111 phase of the Fe-based superconductors. Here we report the successful growth of CaFeAsF single crystals with the sizes of 1-2 mm using the self-flux method. Owning to the availability of the high-quality single crystals, the structure and transport properties were investigated with a high reliability. The structure was refined by using the single-crystal x-ray diffraction data, which confirms the reports earlier on the basis of powder data. A clear anomaly associated with the structural transition was observed at 121 K from the resistivity, magnetoresistance, and magnetic susceptibility measurements. Another kink-feature at 110 K, most likely an indication of the antiferromagnetic transition, was also detected in the resistivity data. The superconducting crystals were also obtained by the similar method in the Co-doped samples. Our results supply a basis to propel the physical investigations on the 1111 phase of the Fe-based superconductors. Figure 1. X-ray diffraction pattern measured on the CaFeAsF single crystal with the x-ray incident on the ab-plane. The inset is the schematic of the crystal structure of CaFeAsF.

7 PC-26-INV First-Principles Study of the High-T c Superconductivity in Compressed Sulfur Hydrides Ryosuke Akashi *,1, Mitsuaki Kawamura 1, Shinji Tsuneyuki 1,2, Yusuke Nomura 3,4, and Ryotaro Arita 3,5 ( 1 Dept. Phys., The Univ. of Tokyo, Japan, 2 ISSP, The Univ. of Tokyo, Japan 3 RIKEN-CEMS, Japan, 4 Dept. Appl. Phys., The Univ. of Tokyo, Japan, 5 JST-ERATO, Tohoku Univ., Japan) Recently, it has been discovered that sulfur hydride exhibit superconductivity under high pressures at ~200K, which is the new record of the superconducting transition temperature (T c ) [1]. Since the observed T c is subject to the hydrogen isotope effect, this superconducting phase has been supposed to be induced by the phonon-induced conventional mechanism [1]. In fact, there have been ab initio calculations which have predicted strong electron-phonon coupling [2,3]. However, the most intriguing structure inducing the observed high T c is yet to be determined experimentally. Although theoretical calculations have reported various candidate compositions and crystal structure capable of supporting strong electron-phonon coupling, quantitative comparison of their possible T c, especially that including the pair-breaking electron-electron Coulomb interaction, is still scarce. Such lack of information of the high-t c phase has hampered further exploration of higher T c. We then study various possible structural phases of H 2 S and H 3 S [4] using the density functional theory for superconductors [5,6]. This theory enables us to calculate T c without any empirical parameter and therefore evaluate the effect of the screened Coulomb interaction on T c. In the pressure range P<~150GPa, the experimentally observed high T c (>130 K) is accurately reproduced by assuming that H 2 S decomposes into R3m-H 3 S and S, which indicates the relevance of R3m-H 3 S phase (and irrelevance of H 2 S phases) in this pressure regime. For the higher pressures, the calculated T c s for Im-3m-H 3 S are systematically higher than those for R3m-H 3 S and the experimentally observed maximum value (203 K), which suggests the possibility of higher T c. [1] A. P. Drozdov, M. I. Eremets, I. A. Troyan, arxiv: ; A. P. Drozdov, M. I. Eremets, I. A. Troyan, V. Ksenofontov, and S. I. Shylin, arxiv: [2] Y. Li et al, J. Chem. Phys. 140, (2014). [3] D. Duan et al., Sci. Reports 4, 6968 (2014). [4] R. Akashi, M. Kawamura, S. Tsuneyuki, Y. Nomura, and R. Arita, Phys. Rev. B 91, (2015). [5] M. Lüders et al., Phys. Rev. B 72, (2005). [6] R. Akashi and R. Arita, Phys. Rev. Lett. 111, (2013) ; J. Phys. Soc. Jpn. 83, (2014). Figure 1. Superconducting transition temperature calculated from the first principles (filled circle and square), compared with the experimental data (open circle and square).

8 PC-27 Ab initio self-consistent Eliashberg study on high-t c superconductivity in sulfur hydrides W. Sano *,1,2, T. Koretsune 2, R. Arita 2 ( 1 Department of Applied Physics, The University of Tokyo, 2 CEMS, RIKEN) Motivated by the recent report of superconductivity (SC) with extremely high transition temperature (T c ~200K) in sulfur hydrides [1], we perform first-principles calculation based on the Migdal-Eliashberg theory. So far, T c of sulfur hydrides has been calculated by two types of approaches based on density functional theory (DFT). One is calculation using the McMillan formula or the Eliashberg equations, and the other is based on DFT for superconductors [2]. While both approaches succeed in reproducing or even predicting high T c s in sulfur hydrides [3], there are several remaining issues for fully non-empirical calculation of T c. In the former, one introduces an adjustable parameter μ* empirically representing the retardation effect of the Coulomb repulsion. On the other hand, although the latter is free from such empirical parameters, self-consistent treatment of the normal self-energy has not been established yet. Since a sharp peak in DOS accidentally locating at Fermi level plays a crucial role in the high T c SC, self-consistent calculation of the mass enhancement and the level shift due to the electron-phonon coupling is indispensable for accurate estimate of T c. In the presentation, we solve the Eliashberg equations from first-principles, where we take account of both the retardation and the self-consistent normal self-energy. Without introducing μ*, we get T c ~170K for H 3 S under 250GPa in the non-self-consistent calculation. In the self-consistent calculation, T c is enhanced by 25K. We further considered the large zero point motion (ZPM) of hydrogen atoms which causes shifts of ~280meV in one-electron energy. We find that ZPM causes another enhancement of T c by 10K.. [1] A.P. Drozdov, M. I. Eremets, and I. A. Troyan. arxiv: [2] M. Lüders et al., Phys. Rev. B 72, (2005); M. A. L. Marques et al., Phys. Rev. B 72, (2005).. [3] D. Duan et al., Sci. Rep. 4, 6968 (2014); I. Errea et al., Phys. Rev. Lett. 114, (2015); J. Flores-Lives, A. Sanna, and E. Gross, arxiv: ; R. Akashi et al., Phys. Rev. B 91, (2015).