Symmetry Lowering in LaOBiS<sub>2</sub>: A Mother Material for BiS<sub>2</sub>-Based Layered Superconductors

Sagayama, Ryoko; Sagayama, Hajime; Kumai, Reiji; Murakami, Youichi; Asano, Tsunaki; Kajitani, Joe; Higashinaka, Ryuji; Matsuda, Tatsuma D.; Aoki, Yuji

doi:10.7566/jpsj.84.123703

Cited by 58 publications

(49 citation statements)

References 31 publications

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“…This suggests that there are some factors to stabilize the supermodulation period of 5 a 0 in BiCh 2 materials. Recently, a supermodulation with the same period of 5 a 0 has been reported in the synchrotron X-ray diffraction measurements in LaO 1-x F x BiS 2 [51]. This result also supports the stability of the supermodulation with the period of 5 a 0 in these materials.…”

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confidence: 77%

Observation of Supermodulation in LaO_0.5F_0.5BiSe₂ by Scanning Tunneling Microscopy and Spectroscopy

et al. 2017

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We observed surface and electronic structure of LaO 0.5 F 0.5 BiSe 2 single crystal by scanning tunneling microscopy/spectroscopy (STM/STS) at 4.2 K. Square lattice composed of Bi atoms was observed at a positive sample bias voltage on the surface prepared by cleavage. At a negative sample bias voltage, a stripe structure running along Bi-Bi directions was observed as in the previous report on NdO 0.7 F 0.3 BiS 2 . Furthermore, we observed a supermodulation running along the diagonal directions with the period of about 5 times of the lattice constant. This seems to be indicative of structural instability of this system rather than electronic instability attributed to a nesting picture.Key word: Superconductivity, BiS 2 -superconductor, Scanning tunneling microscopy, Scanning tunneling spectroscopy 2 Recently, various kinds of layered BiS 2 -based superconductors, LnOBiS 2 (Ln = La, Pr, Ce, Nd, Yb, and Bi), have been discovered . These materials have been intensively investigated because their layered structure, which is composed of superconducting layers and blocking layers, is similar to those of cuprates and iron-based superconductors. Although, the superconducting transition temperature (T c ) of these BiS 2 -based materials is 3-5 K, T c strongly depends on external pressure or high-pressure annealing [2,[28][29][30][31][32][33]. For instance, LaO 1-x F x BiS 2 showing T c of around 3 K shows superconductivity at around 10 K when an external pressure is applied. This higher T c remains at ambient pressure after as-grown polycrystalline samples are annealed under high pressure. These high-pressure effects indicate that the crystal structure of these materials is rather unstable against the external perturbations.Theoretical calculations also predicted electronic and structural instability against the formation of charge density wave (CDW) [34][35][36][37][38]. A calculation of band structures predicted due to nesting at the F concentration of x=0.5, where the Fermi surface connects at around (/2, /2) in the k space [34,35]. The CDW is expected to be formed along the (, ) direction in the k space, whose direction corresponds to the diagonal directions of Bi square lattice in the real space with the period of about √2 times of the lattice constant. A calculated phonon dispersion curve found the existence of plateau at the negative energy between Γ and M point in k space, indicating a structural instability at F concentration of x=0.5 [35][36][37][38]. Although the direction of the instability agrees with that of the electronic instability, because of the plateau at the negative energy, the period of the CDW cannot be defined from the dispersion curve.These theoretical calculations suggest the superconductivity in BiS 2 based materials realizes near CDW ground state.Although the existence of the CDW was suggested by transport experiments in EuFBiS 2 , this has not been confirmed by direct observations [10]. Thus, the relation between the superconductivity and the CDW is still unclear. 3To investigate...

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confidence: 77%

Observation of Supermodulation in LaO_0.5F_0.5BiSe₂ by Scanning Tunneling Microscopy and Spectroscopy

et al. 2017

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“…The decrease in n with increasing concentration of Se may result from changes in the electronic structure. Recently, synchrotron X-ray diffraction experiments revealed that a LaOBiS2 single crystal has a monoclinic, rather than tetragonal, crystal structure [6]. In fact, we observed (via neutron diffraction) the short-range distortions of the Bi-S plane of the LaOBiS2 polycrystalline sample [7].…”

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confidence: 63%

Electronic Origins of Large Thermoelectric Power Factor of LaOBiS₂₋_xSe_x

et al. 2016

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We examined the electrical transport properties of densified LaOBiS2-xSex, which constitutes a new family of thermoelectric materials. The power factor increased with increasing concentration of Se, i.e., Se substitution led to an enhanced electrical conductivity, without suppression of the Seebeck coefficient. Hall measurements indicated that the low electrical resistivity resulted from increases in the carrier mobility, and the decrease in carrier concentration led to large absolute values of the Seebeck coefficient of the system. (at ~650 K) for LaOBiSSe [5]. In contrast to the typically observed trend, the electrical resistivity of LaOBiS2-xSex was suppressed and the absolute value of the Seebeck coefficient was increased by Se substitution [2]. We therefore sought to clarify the electronic origins of the Se-substitution-induced enhancement of the power factor (PF = S 2 /) of LaOBiS2-xSex; this was done by determining the electrical resistivity (), Seebeck coefficient (S), carrier concentration (n), and mobility () of the system. Polycrystalline samples of LaOBiS2-xSex (x = 0, 0.4, 0.6, 0.8, and 1) were prepared using the solid-state-reaction method described in Ref. 2. The samples were densified by hot-pressing (HP) under a uniaxial pressure of 50 MPa. The resulting LaOBiS2-xSex powders were sealed in a 15-mm-diameter graphite capsule during HP annealing. The powders were annealed for 1 h at 700 ºC and 800 ºC for x = 0.4-1 and x = 0, respectively. This annealing

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“…S1 in Supplemental Materials [33].). For x = 0 and 0.1, the XRD patterns were refined using a monoclinic space group of (P21/m), which is the same as the high-pressure phase of LaO0.5F0.5BiS2 [10] and single-crystal LaOBiS2 [34]. The refined lattice constant of  is 90.189 (2) According to the phase transition from monoclinic to tetragonal, a and b of x = 0.1 approaches each other.…”

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confidence: 99%

Intrinsic Phase Diagram of Superconductivity in the BiCh₂-Based System Without In-Plane Disorder

et al. 2017

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We have investigated the crystal structure and physical properties of LaO1-xFxBiSSe to reveal the intrinsic superconductivity phase diagram of the BiCh2-based layered compound family. From synchrotron X-ray diffraction and Rietveld refinements with anisotropic displacement parameters, we clearly found that the in-plane disorder in the BiSSe layer was fully suppressed for all x. In LaO1-xFxBiSSe, metallic conductivity and superconductivity are suddenly induced by electron doping even at x = 0.05 and 0.1 with a monoclinic structure. In addition, x (F concentration) dependence of the transition temperature (Tc) for x = 0.2-0.5 with a tetragonal structure shows an anomalously flat phase diagram. With these experimental facts, we have proposed the intrinsic phase diagram of the ideal BiCh2-based superconductors with less in-plane disorder.Since the discovery of Bi4O4S3 and REO1-xFxBiS2 (RE: rare earth) superconductors, BiCh2-based (Ch: chalcogen) superconductors have been drawing much attention as a new class of layered superconductors [1][2][3]. Since the crystal structure composed of alternate stacks of the electrically conducting BiCh2 layers and the insulating (blocking) layers resembles those of the Cuprate and the FeAs-based high-transition-temperature (high-Tc) superconductors [4,5], many experimental and theoretical studies have been performed to clarify the superconductivity mechanisms of this system and to increase Tc. However, the mechanisms have not been understood completely. Recently, Morice et al. proposed that a 2 weak-coupling electron-phonon mechanism cannot explain the emerging Tc, as high as 11 K, in the BiCh2-based systems, from first principle calculations [6]. Therefore, full understandings of the basic characteristics of the superconductivity in the BiCh2-based system are crucial.The parent phase of the BiCh2-based superconductor is a band insulator [1,7,8]. On the basis of the calculated band structure, electrons carriers are doped into the bands mainly composed of Bi-6px and Bi-6py components. Electron-doped BiCh2-based compounds are expected to become metallic. Indeed, superconductivity is experimentally observed in electron-doped compounds [1][2][3]. However, the real situations are not simple as expected from the band structure. Although the superconductivity in BiCh2-based compounds is emerged by carrier doping, metallic transport is sometimes absent, and weakly localized behavior (semiconducting-like behavior) is observed in electrical resistivity measurements; a good example would be optimally doped LaO0.5F0.5BiS2. In semiconducting-like samples of LaO0.5F0.5BiS2, bulk superconductivity is not observed, while weak (filamentary) superconductivity is observed. To induce bulk superconductivity in the LaO0.5F0.5BiS2 system, external pressure effects [9][10][11][12][13][14][15][16] and/or element substitution at the La site [17][18][19][20], which optimize the crystal structure, are available. Namely, both electron carrier doping and crystal structure optimization are required for the e...

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Symmetry Lowering in LaOBiS₂: A Mother Material for BiS₂-Based Layered Superconductors

Cited by 58 publications

References 31 publications

Observation of Supermodulation in LaO_0.5F_0.5BiSe₂ by Scanning Tunneling Microscopy and Spectroscopy

Observation of Supermodulation in LaO_0.5F_0.5BiSe₂ by Scanning Tunneling Microscopy and Spectroscopy

Electronic Origins of Large Thermoelectric Power Factor of LaOBiS₂₋_xSe_x

Intrinsic Phase Diagram of Superconductivity in the BiCh₂-Based System Without In-Plane Disorder

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Symmetry Lowering in LaOBiS2: A Mother Material for BiS2-Based Layered Superconductors

Cited by 58 publications

References 31 publications

Observation of Supermodulation in LaO0.5F0.5BiSe2 by Scanning Tunneling Microscopy and Spectroscopy

Observation of Supermodulation in LaO0.5F0.5BiSe2 by Scanning Tunneling Microscopy and Spectroscopy

Electronic Origins of Large Thermoelectric Power Factor of LaOBiS2−xSex

Intrinsic Phase Diagram of Superconductivity in the BiCh2-Based System Without In-Plane Disorder

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Symmetry Lowering in LaOBiS₂: A Mother Material for BiS₂-Based Layered Superconductors

Observation of Supermodulation in LaO_0.5F_0.5BiSe₂ by Scanning Tunneling Microscopy and Spectroscopy

Observation of Supermodulation in LaO_0.5F_0.5BiSe₂ by Scanning Tunneling Microscopy and Spectroscopy

Electronic Origins of Large Thermoelectric Power Factor of LaOBiS₂₋_xSe_x

Intrinsic Phase Diagram of Superconductivity in the BiCh₂-Based System Without In-Plane Disorder