BiS2 Layers Research Articles

The Local Structure of the BiS2 Layer in RE(O,F)BiS2 Determined by In-Plane Polarized X-ray Absorption Measurements

We have investigated the local structure of BiS2-based layered materials by Bi L3-edge extended X-ray absorption fine structure (EXAFS) measurements performed on single crystal samples with polarization of the X-ray beam parallel to the BiS2 plane. The results confirm highly instable nature of BiS2 layer, characterized by ferroelectric like distortions. The distortion amplitude, determined by the separation between the two in-plane (Bi-S1) bonds, is found to be highest in LaO0.77F0.23BiS2 with ΔR∼0.26 Å and lowest in NdO0.71F0.29BiS2 with ΔR∼0.13 Å. Among the systems with intrinsic doping, CeOBiS2 shows smaller distortion (ΔR∼0.15 Å) than PrOBiS2 (ΔR∼0.18 Å) while the highest distortion appears for EuFBiS2 revealing ΔR∼0.22 Å. It appears that the distortion amplitude is controlled by the nature of the RE(O,F) spacer layer in the RE(O,F)BiS2 structure. The X-ray absorption near edge structure (XANES) spectra, probing the local geometry, shows a spectral weight transfer that evolves systematically with the distortion amplitude in the BiS2-layer. The results provide a quantitative measurements of the local distortions in the instable BiS2-layer with direct implication on the physical properties of these materials.

Enhancing thermoelectric performance of SrFBiS2−xSexvia band engineering and structural texturing

SrFBiS2 is a quaternary n-type semiconductor with rock-salt-type BiS2 and fluorite-type SrF layers alternately stacked along the c axis. The tunability of the crystal and electronic structures as well as the intrinsically low thermal conductivity make this compound a promising parent material for thermoelectric applications. In the current work, we show that alloying of Se and S in SrFBiS2 reduces the optical band gap with the second conduction band serving as an electron-transport medium, simultaneously increasing the electron concentration and effective mass. In addition, the raw material Bi2Se3 is shown to act as liquid adjuvant during the annealing process, favoring preferred-orientation grain growth and forming strengthen microstructural texturing in bulk samples after hot-pressed sintering. Highly ordered lamellar grains are stacked perpendicular to the pressure direction, leading to enhanced mobility along this direction. The synthetic effect results in a maximum power factor of 5.58 μW cm−1 K−2 at 523 K for SrFBiSSe and a peak zT = 0.34 at 773 K, enhancements of 180% compared with those of pristine SrFBiS2.

Electronic structure and optical anisotropy in Sr1−x BaxFBiS2 (x = 0, 0.25, 0.5, 0.75, 1) based solar cell materials

We conducted theoretical simulations for exploring the overall physical properties of the pristine SrFBiS2 as well as its derivative alloys (Sr1−x BaxFBiS2 (x = 0, 0.25, 0.5, 0.75, 1)) resulting from BiS2 layers via the compositional effect of substituting strontium with barium. In this respect, we computed the electronic and optical characteristics of Sr1−xBaxFBiS2 systems within the concentration limits of x varying from 0 to 1. Based on full-potential linear augmented plane wave scheme, we determined the structural properties of the Sr1−xBaxFBiS2 alloys, and the lattice parameter was found to increase with the augmentation of Ba concentration, which in turn agrees well with the available experimental works. The electronic band structures and density of states of Sr1−xBaxFBiS2 (0 ≤ x ≤ 1) alloys were also examined. According to our calculations, the parent compound SrFBiS2 has a semiconducting behavior and its energy gap is about 0.8 eV. The electronic band structures illustrated that the substitution of the divalent Ba+2 element instead of Sr+2 produces a semiconducting behavior for all Sr1−xBaxFBiS2 (0 ≤ x ≤ 1) alloys. When substituting Ba+2 with Sr+2, the acquired optical features, such as the dielectric function, refractive index, reflectivity, absorption coefficient, energy loss functions, and optical conductivity for Sr1−xBaxFBiS2 (0 ≤ x ≤ 1) alloys also exhibited a semiconducting nature. It is easily noticed that the maximum absorption coefficient changes with the variation of Ba concentration. Furthermore, a superior birefringence occurs in the concerned Sr1−xBaxFBiS2 alloys when the composition variation undergoes from x = 0 to 1. Intriguingly, the optical spectra for both parent compounds SrFBiS2 and BaFBiS2 and their ultimate Sr1−xBaxFBiS2 alloys displayed a high anisotropy. The viable significant role of the spin-orbit coupling in deciding the electronic structures and optical characteristics is due to the presence of heavy bismuth atoms. All our results determined for Sr1−xBaxFBiS2 systems are compared with the existing theoretical and experimental reports.

Synthesis and characterization of Ti-doped Bi4O4S3 superconductor

The substitution effects of 3d2 transition metal Ti on the Bi4O4S3 superconductor structure were studied. The presence of Ti in Bi2O2 layer spacer or superconducting BiS2 layers was investigated. For this study, the polycrystalline Bi4O4S3 superconductor was prepared following a two-step solid-state reaction. Grains consisted of lamellae, ~ 5–10 μm in the planar orientation and 100–500 nm thick, presenting a tetragonal crystalline structure, with space group I4/mmm. For the first time, Raman spectroscopy revealed three modes at room temperature, with no significant changes in the spectra with Ti doping. The superconductivity was observed by the Meissner effect from ZFC and FC magnetization measurements. For both samples, the critical temperature was around 3.5 K, showing that Ti doping did not change the TC value, probably due to fact that Ti is in the Bi2O2 layer spacer. Also, the doped sample presented a strong positive magnetic moment, probably due to the presence of ferromagnetic order.

Material Development and Physical Properties of BiS2-Based Layered Compounds

In 2012, we discovered superconductivity in Bi4O4S3 and LaO1-xFxBiS2, whose crystal structures are composed of alternate stacks of a BiS2-type conduction layer and a blocking layer. Since the discovery, many related superconductors and functional materials have been synthesized. Furthermore, the possibility of unconventional superconductivity has been proposed in recent theoretical and experimental studies. In addition, notable correlations between local structure and physical properties have been revealed in the BiS2-based systems. In this review article, we summarize the material development (probably all the compounds) and physical properties of BiS2-related materials developed in the last six years. The key parameters for the emergence of bulk superconductivity in the BiS2-based compounds are carrier doping to the parent phase (band insulator), pressure-induced structural transition, in-plane chemical pressure, and the suppression of in-plane disorder. We systematically review those parameters and the method of designing new superconductors with BiS2 layers. In addition, studies of BiS2-based compounds as thermoelectric materials are briefly reviewed.

Unconventional inner-TL electric polarization in TL-LaOBiS2 with ultrahigh carrier mobility.

Based on first principles calculations, we propose a new 2D ferroelectric material, triple-layer (TL) LaOBiS2, with an ultrahigh carrier mobility over 40 000 cm2 V-1 s-1 and large sunlight absorption. TL-LaOBiS2 is composed of a middle LaO layer and top-and-bottom BiS2 layers that can be possibly exfoliated from its bulk counterpart. We reveal that each BiS2 layer can hold spontaneous in-plane ferroelectric polarization that can be further enhanced by imposing extensive strain. Furthermore, we discover that TL-LaOBiS2 possesses unconventional inner-TL ferroelectric (FE), antiferroelectric (AFE) and orthogonal polarizations. The ground inner-TL AFE state can be flexibly driven into a nearly degenerate FE state. Moreover, the direct band gaps, optical absorption and the carrier mobilities of TL-LaOBiS2 can be effectively regulated by different ferroelectric polarization configurations. This finding of various ferroelectric states with ultrahigh mobility and excellent optical absorption in TL-LaOBiS2 provides a promising platform for future realization of two-dimensional ferroelectric photovoltaic devices.

Suppression of structural instability in LaOBiS2−xSex by Se substitution

Isovalent substitution of S by Se in LaOBiS2−xSex has a substantial effect on its electronic structure and thermoelectric properties. To investigate the possible role of BiS2 structural instability, we have studied the local structure of LaOBiS2−xSex () using temperature dependent Bi L3-edge extended x-ray absorption fine structure measurements. The results reveal that the local structure of the two compounds is significantly different. The BiS2 sub-lattice is largely distorted in LaOBiS2 (x = 0.0), with two in-plane Bi–S1 distances separated by ∼0.4 Å instead LaOBiSSe (x = 1.0) showing much smaller local disorder with two in-plane Bi–Se distances in the plane being separated by ∼0.2 Å. Temperature dependent study shows that the two Bi–S1 distances are characterized by different bond strength in LaOBiS2 (x = 0.0) while it is similar for the Bi–Se distances in LaOBiSSe (x = 1.0). The out of plane Bi–S2 bond is harder in LaOBiSSe indicating that the structural instability of BiS2 layer has large effect on the out-of-plane atomic correlations. The results suggest that the local structure of LaOBiS2−xSex is an important factor to describe differing electronic and thermal transport of the two compounds.

Role of local structure distortion in the suppression of superconductivity for Eu3-xSrxBi2S4F4 system

As new family of superconductors with a layered structure, BiS2-based materials have attracted much interest. Eu3Bi2S4F4 is a newly discovered member which shows superconductivity at 1.5 K without extrinsic doping, however, superconductivity is strongly suppressed by Sr-doping. In this contribution, we present, in terms of local atomic displacement, an insight into the relationship between Sr-doping and the suppression of superconductivity in Eu3-xSrxBi2S4F4 by Bi and Eu L3-edge X-ray absorption spectroscopy. Upon Sr-doping, the distance between Eu3F4 blocking layers and BiS2 layers is significantly elongated, accompanied by the breaking of Eu-S-Bi channel, which makes it more difficult to transfer electrons into BiS2 layers. Moreover, distortions in the Bi-S1 plane increase with Sr content, reducing the overlap among Bi(6px/6py)-S(3p) orbitals within the superconducting plane. The decoupling between blocking and BiS2 layers, as well as the distortions in the superconducting Bi-S1 plane, may contribute to the suppression of superconductivity in Sr-doped Eu3Bi2S4F4 compounds.

Direct evidence of hidden local spin polarization in a centrosymmetric superconductor LaO0.55 F0.45BiS2

Conventional Rashba spin polarization is caused by the combination of strong spin–orbit interaction and spatial inversion asymmetry. However, Rashba–Dresselhaus-type spin-split states are predicted in the centrosymmetric LaOBiS2 system by recent theory, which stem from the local inversion asymmetry of active BiS2 layer. By performing high-resolution spin- and angle-resolved photoemission spectroscopy, we have investigated the electronic band structure and spin texture of superconductor LaO0.55F0.45BiS2. Here we present direct spectroscopic evidence for the local spin polarization of both the valence band and the conduction band. In particular, the coexistence of Rashba-like and Dresselhaus-like spin textures has been observed in the conduction band. The finding is of key importance for fabrication of proposed dual-gated spin-field effect transistor. Moreover, the spin-split band leads to a spin–momentum locking Fermi surface from which superconductivity emerges. Our demonstration not only expands the scope of spintronic materials but also enhances the understanding of spin–orbit interaction-related superconductivity.

Evaluation of Bi defect concentration in LnO1-xFxBiCh2 by scanning tunneling microscopy

We examined a concentration of Bi defects in layered BiCh2 based superconductors LnO1-xFxBiCh2 (Ln = La, Ce, Nd Ch = S, Se). These materials show superconductivity by electron carrier doping into BiCh2 layer. Since Bi defects affect the carrier concentration directly, an examination of the concentration is important to evaluate an actual carrier concentration. In this paper, the concentration of Bi defects on the BiCh2 layers in BiCh2 based superconductors was evaluated by scanning tunneling microscopy measurements in real space. We found the samples with BiSe2 layers have less Bi defects than those with BiS2 layers. Furthermore, the concentration of Bi defects was found to be almost constant regardless of F concentration and Ln ion.

Synchrotron powder X-ray diffraction and structural analysis of Eu0.5La0.5FBiS2-xSex

Eu0.5La0.5FBiS2-xSex is a new BiS2-based superconductor system. In Eu0.5La0.5FBiS2-xSex, electron carriers are doped to the BiS2 layer by the substitution of Eu by La. Bulk superconductivity in this system is induced by increasing the in-plane chemical pressure, which is controlled by the Se concentration (x). In this study, we have analysed the crystal structure of Eu0.5La0.5FBiS2-xSex using synchrotron powder diffraction and the Rietveld refinement. The precise determination of the structural parameters and thermal factors suggest that the emergence of bulk superconductivity in Eu0.5La0.5FBiS2-xSex is achieved by the enhanced in-plane chemical pressure and the decrease in in-plane disorder.

Electron Transfer and Local Atomic Displacement in Sr1–xCexFBiS2 Revealed by X-ray Absorption Spectroscopy

Superconductivity in quasi-two-dimensional crystal structures has attracted sustained interest among condensed matter community, primarily due to their exotic superconducting properties. Recently, superconductivity of BiS2-based compounds with the desired layered crystal structure has been reported. In this contribution, Ce-doping effect on the electronic structure and local atomic displacement of Sr1–xCexFBiS2 was investigated by Ce and Bi L3-edge X-ray absorption spectroscopy. Upon Ce-doping both the Ce–S2 and Bi–S2 bonds shrink, enhancing the coupling between Sr/CeF and superconducting BiS2 layers. Therefore, Ce substitution provides electron carriers to the superconducting BiS2 layers through the enhanced Ce–S2–Bi channel. On the other hand, Ce-doping effectively modulates the in-plane structure of Bi–S1 plane. In Ce-doped compounds both the shortening of in-plane Bi–S1 bond and the flatter Bi–S1 plane would enhance overlap of Bi(6px/6py)–S(3p) orbitals within the superconducting plane. Therefore, Ce-...

Discovery of BiS2-Based Superconductor and Material Design Concept

In 2012, we discovered new layered superconductors whose superconducting states emerge in the BiS2 layers. Since their crystal structure, composed of alternate stacks of BiS2 conduction layers and electrically insulating (blocking) layers, is similar to those of cuprate and Fe-based superconductors, many researchers have explored new BiS2-based superconductors and have studied the physical and chemical properties of the BiS2-based superconductors. In this paper, we present the histories of the discovery of the first BiS2-based superconductor, Bi4O4S3, and the second one, LaO1−xFxBiS2. The structural variation of the BiS2-based superconductor family is briefly introduced. Then, we show the material design concept for the emergence of bulk superconductivity in BiS2-based compounds. At the end, a possible strategy for the enhancement of the transition temperature in the BiS2-based superconductors is proposed.

Electrostatic electron-doping yields superconductivity in LaOBiS2

Electrostatic carrier-doping is attracting serious attention as a meaningful technique for producing interesting electronic states in two-dimensional (2D) layered materials. Ionic-liquid gating can provide the critical carrier density required to induce the metal-insulator transition and superconductivity. However, the physical properties of only a few materials have been controlled by the electrostatic carrier-doping during the past decade. Here, we report an observation of superconductivity in a 2D layered material, LaOBiS2, achieved by the electrostatic electron-doping. The electron doping of LaOBiS2 induced metallic conductivity in the normally insulating LaOBiS2, ultimately led to superconductivity. The superconducting transition temperature, Tc, was 3.6 K, higher than the 2.7 K seen in LaO1-xFxBiS2 with an electron-doped BiS2 layer. A rapid drop in resistance (R) was observed at low temperature, which disappeared with the application of high magnetic fields, implying a superconducting state. This study reveals that electron-doping is an important technique for inducing superconductivity in 2D layered BiS2 materials.

RVB states in doped band insulators from Coulomb forces: theory and a case study of superconductivity in BiS2 layers

Doped band insulators, HfNCl, WO3, diamond, Bi2Se3, BiS2 families, STO/LAO interface, gate doped SrTiO3, MoS2 and so on are unusual superconductors. With an aim to build a general theory for superconductivity in doped band insulators, we focus on the BiS2 family which was discovered by Mizuguchi et al in 2012. While maximum Tc is only ∼11 K in , a number of experimental results are puzzling and anomalous in the sense that they resemble high Tc and unconventional superconductors. Using a two orbital model of Usui, Suzuki and Kuroki, we show that the uniform low density free Fermi sea in is unstable towards formation of the next nearest neighbor Bi-S-Bi diagonal valence bond (charged −2e Cooper pair) and their Wigner crystallization. Instability to this novel state of matter is caused by unscreened nearest neighbor coulomb repulsions (V ∼ 1 eV) and a hopping pattern with sulfur mediated diagonal next nearest neighbor Bi-S-Bi hopping t’ ∼ 0.88 eV, as well as larger than nearest neighbor Bi-Bi hopping, t ∼ 0.16 eV. Wigner crystals of Cooper pairs quantum melt for doping around x = 0.5 and stabilize certain resonating valence bond states and superconductivity. We study a few variational RVB states and suggest that BiS2 family members are latent high Tc superconductors, but challenged by competing orders and the fragile nature of many body states sustained by unscreened Coulomb forces. One of our superconducting states has dXY symmetry and a gap. We also predict a 2d Bose metal or vortex liquid normal state, as charged −2e valence bonds survive in the normal state.

Polytypism inLaOBiS2-type compounds based on different three-dimensional stacking sequences of two-dimensionalBiS2layers

LaOBiS2-type materials have drawn much attention recently because of various interesting physical properties, such as low-temperature superconductivity, hidden spin polarization, and electrically tunable Dirac cones. However, it was generally assumed that each LaOBiS2-type compound has a unique and specific crystallographic structure (with a space group P4/nmm) separated from other phases. Using first-principles total energy and stability calculations we confirm that the previous assignment of the centrosymetric P4/nmm structure to LaOBiS2 is incorrect as a phonon instability renders this structure impossible. Furthermore, we find that the unstable structure is replaced by a family of energetically closely spaced modifications (polytypes) differing by the layer sequences and orientations. We find that the local Bi-S distortion leads to three polytypes of LaOBiS2 with different stacking patterns of the distorted BiS2 layers. The energy difference between the polytypes of LaOBiS2 is merely ~1 meV/u.c., indicating the possible coexistence of all polytypes in the real sample and that the particular distribution of polytypes may be growth-induced. The in-plane distortion can be suppressed by pressure, leading to a phase transition from polytypes to the high-symmetry P4/nmm structure with a pressure larger than 2.5 GPa. In addition, different choices of the intermediate atoms (replacing La) or active atoms (BiS2) could also manifest different ground state structures. One can thus tune the distortion and the ground state by pressure or substituting covalence atoms in LaOBiS2-family.

BiS2-Layer Gives Giant Birefringence: First-Principles Calculations

Birefringence attracts increasing attention due to its extensive applications in the imaging spectrometer, laser devices and optical components. Stimulated by the discovery of the giant birefringence (GBF) in LaOBiS2 and AFBiS2, we explore its origin by checking three compounds with and without the BiS2 layer: Bi2OS2 (i.e., BiOBiS2), Bi2O2S and LaOAlS2. It is demonstrated that GBF appears only in the compounds with the BiS2 layer and is furthermore confirmed in MOBiS2 (M=Al, Ga, In and Sc) compounds. The result is useful for discovering/developing new-type birefringent crystal or even multifunctional materials.

Hydrostatic pressure effect on new BiS2 based Bi4O4S3 and ReO/FBiS2 (Re = La, Pr, Nd, Sm) Superconductors

ABSTRACTDiscovery of superconductivity in BiS2 layers based systems has attracted tremendous interest of both experimentalists and theoreticians from condensed matter physics community. In this article, a review of our high pressure studies on BiS2 based superconductors is given. The pressure effects on magnetic, transport properties and superconducting transitions are discussed for different types of doped and undoped BiS2-based compounds such as Bi4O4S3 and ReO/FBiS2 (Re = rare-earth). Pressure tends to decrease the magnetic transition temperature in the undoped or only slightly doped compounds. The superconducting Tc increases with low pressure for under doped BiS2 based compounds, remains approximately constant for optimal doping, and decreases linearly in the overdoped range. Under pressure, the semiconducting behavior in the normal state is suppressed markedly and monotonically, whereas the evolution of Tc is nonlinear, the superconductivity in the BiS2 layer favors the Fermi surface at the boundary between the semiconducting and metallic behaviors. However, strong suppression of the semiconducting and induced metallic behavior without doping in ReO/FBiS2 suggests that the Fermi surface is located in the vicinity of some instability. Furthermore, notable properties under pressure in the BiS2 family are reported. The prospects for raising Tc in this family are proposed on the basis of experimental and theoretical studies.

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

We report X-ray diffraction studies of single-crystal samples of LaOBiS2 and CeOBiS2, which are mother materials for BiS2-based layered superconductors. The structural symmetry in LaOBiS2 is lowered from tetragonal to monoclinic, whereas no such symmetry lowering occurs in CeOBiS2. Crystal structure analysis demonstrates that the space group of LaOBiS2 is monoclinic P21m. The in-plane S atoms uniformly shift along the a-axis in each BiS2 layer. Because the directions of the shift are opposite in the two adjacent BiS2 layers, the spatial inversion symmetry remains. The distortion is attributed to the in-plane lattice mismatch between the BiS2 layers and the block layers.

Coexistence of superconductivity and complex 4 f magnetism in Eu0.5Ce0.5BiS2F

EuBiS2F is a self-doped superconductor due to the mixed valence of Eu. Here we report that, with the Ce substitution for Eu by 50 at.%, the material exhibits ferromagnetic ordering at 8 K for the Ce-4 f moment, superconductivity at 2.2 K in the BiS2 layers and possibly antiferromagnetic ordering at 2.1 K for the Eu-4 f spins. The Eu valence is essentially divalent with the Ce incorporation. We tentatively interpret the coexistence of ferromagnetism and superconductivity by considering different Bi-6p orbitals that are responsible for the superconductivity itself and for mediating the ferromagnetic interaction, respectively. We argue that the antiferromagnetic ordering of the Eu-4 f spins is most likely due to a magnetic dipole-dipole interaction.