Open access peer-reviewed chapter

Emerging Superconductivity and Topological States in Bismuth Chalcogenides

By Jifeng Shao and Wenka Zhu

Submitted: September 29th 2017Reviewed: December 12th 2017Published: May 30th 2018

DOI: 10.5772/intechopen.73057

Downloaded: 373

Abstract

In this chapter, we review the recent experimental work in emerging superconductors, i.e., bismuth chalcogenides, including the newly discovered BiS(e)2-based layered superconductors and some topological superconductor candidates. Their crystal structure and various physical properties are reviewed in detail, with the correlation between structure and superconductivity as the main clue throughout this chapter. Bi2OS2 is the simplest structure in Bi─O─S compounds and probably the parent compound of this series. Superconductivity emerges when carriers are introduced by intercalation or chemical substitution. The superconducting layer is extended to BiSe2 layer in LaO1−xFxBiSe2, which has an improved superconductivity. Moreover, the topological insulator Bi2Se3 can be turned into superconductors by intercalating metal atoms into van der Waals space, e.g., SrxBi2Se3, a potential topological superconductor, whose quantum oscillations reveal a possible topological surface state. The intermediate external pressure can efficiently suppress superconductivity, which reemerges when pressure is further increased, while Tc is nearly invariant in high-pressure region, indicating an unconventional pairing state.

Keywords

  • bismuth chalcogenides
  • BiS(e)2-based superconductors
  • crystal structure
  • intercalation
  • topological superconductors
  • high pressure

1. Introduction

Superconductivity was first discovered in the resistivity measurement of mercury by Kamerlingh Onnes in 1911. Its resistance abruptly vanishes at 4.1 K. Zero resistance means no energy loss in electric transport, which could greatly solve the energy crisis in the future. Since then, superconductivity has been a long-lasting hot topic in condensed matter physics. Exploring room temperature superconductors is one of the ultimate dreams.

However, so far, only two kinds of unconventional superconducting systems have exceeded the Macmillan limit at ambient pressure, i.e., the cuprate and iron-based superconductors. In general, the correlation of structure and typical properties is always a useful guideline for effectively searching for special functional materials. In fact, the structure of both cuprate and iron-based superconductors can be characterized as a sandwiched “hamburger” model. It consists of superconducting layers (CuO2 plane, Fe2M2 (M = As, P, S, Se, and Te) layer) and spacer layers, which stack alternatively along the c-axis [1, 2]. Superconductivity occurs when the charged carriers are generated by the defects or substitution in superconducting layers or more commonly provided by the space layers; namely, a new superconducting layer probably means a new superconducting system. The spacer layer can be easily tuned by doping, substitution, intercalation, and pressure, which could affect superconductivity [3]. Therefore, materials with layered structure have been regarded as the most promising playground for exploring new high-Tc superconductors.

In 2010, superconductivity arising from the topological insulator Bi2Se3 by Cu intercalation was first reported [4]. It has drawn much attention since CuxBi2Se3 is proposed as a topological superconductor candidate, as evidenced by the zero-bias conductance peak and quantum oscillation experiment [5, 6]. Very recently, superconductivity with topological states was also reported in its isostructural compounds, SrxBi2Se3 and NbxBi2Se3 [7, 8]. In 2012, an exotic superconductivity was discovered in a new layered structure Bi4O4S3 with zero-resistance superconducting temperature at about 4.5 K [9]. Soon, another new BiS2-based superconductor LaO0.5F0.5BiS2 was reported, whose structure is more definite and the zero-resistance superconducting temperature is about 8 K for the samples annealed under high pressure [10]. As its structure is very similar to the iron-based superconductor LaOFeAs, this system has been intensively researched, and lots of isostructural superconductors have been synthesized, including ReO1−xFxBiS2 (Re: Ce, Pr, Nd, Yb), Sr1−xRexFBiS2 (Re: La, Ce), EuBiS2F, and Eu3Bi3S4F4 [11, 12, 13, 14, 15]. These researches are focused on tuning the spacer layers. The attempts to explore new superconducting layers only succeed in LaOxFxBiSe2 and Sr0.5La0.5FBiSe2 [16, 17, 18]. So far, the superconducting layer of this system has been extended to BiCh2 (Ch: S, Se). In this chapter, the crystal structure and superconducting properties of Bi─O─S superconductors, LaO1−xFxBiSe2 single crystals, and SrxBi2Se3 single crystals are briefly reviewed.

2. Crystal structure and superconducting properties

2.1. Bi─O─S superconductors

The element composition of Bi4O4S3 is the same as Bi4O4(SO4)xBi2S4 (x = 0.5), and its parent Bi6O8S5 is an oxide insulator composed of alternatively stacked BiS2 and Bi2O2 + SO4 + Bi2O2 layers along the c-axis. It has a tetragonal structure with I4/mmm space group and its schematic crystal structure is shown in Figure 1(c). Band calculations demonstrate that the half vacancy of SO4 layer generates electron carriers into BiS2 layer. The normal state of Bi4O4S3 is metallic and the superconductivity mainly originates from the Bi 6px and 6py orbitals in BiS2 layers. Therefore, the BiS2 layer is called the superconducting layer in this family.

Figure 1.

Crystal structures of (a) Bi2OS2, (b) Bi3O2S3, and (c) Bi4O4S3 [21].

However, the chemical composition studies show that it probably contains two new Bi─O─S phases, i.e., Bi2OS2 and Bi3O2S3. Their schematic structures can be seen in Figure 1(a) and (b). Bi2OS2 is an insulating phase and its content is less than 10%. Bi3O2S3 is the main phase and likely accounts for the 4.5 K superconductivity in Bi4O4S3. And the superconductivity can be suppressed by the amount of Bi2OS2-like stacking faults [19]. Once the quality of Bi3O2S3 sample is improved, the superconducting volume fraction will be enhanced with its zero-resistance superconducting temperature increased up to 4.9 K [20].

The crystal structure of Bi3O2S3 is similar to Bi4O4S3 with the same I4/mmm space group, a = 3.9674 Å and b = 41.2825 Å. The electron carriers are believed to be generated from S22− layers replacing the vacancy of SO42− layers in Bi4O4S3. The chemical composition of Bi2OS2 can also be expressed as BiOBiS2. Then we can see it is isostructural with LaOBiS2 with P4/nmm space group, a = b = 3.9744 Å and c = 13.7497 Å. BiOBiS2 has the simplest structure and composition, then it is probably the parent compound of this BiS2-based family. Besides, superconductivity is likely to be induced by introducing carriers into spacer layer. In fact, F-doped Bi2OS2 has been reported to exhibit bulk superconductivity below 5 K [21, 22].

Figure 2 shows the powder XRD patterns of Bi3O2S3, BiO1−xFxBiS2, and Bi2OS2 samples. We can see that samples of Bi─O─S compounds tend to contain impurities such as Bi2O3, Bi, and Bi2S3, because their synthesis temperature is relatively low (520°C for Bi4O4S3 and Bi3O2S3, and 400°C for Bi2(O,F)S2) [9, 19, 20, 21]. Besides, these samples can only be synthesized in a narrow temperature region. Another difficulty in detecting their actual composition and structure is that several strong diffraction peaks in the powder XRD patterns are very close to each other. Hence, bulk superconductivity is very important in this system. Up to now, high-quality samples, especially single crystals, are still needed to investigate the relationship of structure and properties, in view of the multiple competing low-energy crystal structures in this system.

Figure 2.

Powder XRD patterns of Bi3O2S3, Bi2OS2, and Bi2O1−xFxS2 polycrystalline samples. The special characters (*, #) represent the impurity phases.

The physical properties of Bi─O─S superconductors are introduced, taking Bi3O2S3 and F-doped Bi2OS2 for instance [20, 21]. Figure 3(a) shows the temperature dependence of resistivity and magnetoresistivity under different applied magnetic fields for Bi3O2S3. Its normal state is metallic-like and a sharp drop in resistivity appears at 5.8 K and quickly down to zero at 4.9 K. The upper critical field is estimated from resistivity versus temperature curves under different applied magnetic fields perpendicular to the sample surface, as seen in the insets of Figure 3(a). According to the Werthamer-Helfand-Hohenberg (WHH) formula, the upper critical field μ0Hc2(0) is evaluated to be about 4.84 T.

Figure 3.

(a) Temperature dependence of resistivity for Bi3O2S3. The lower inset shows the curves of resistivity versus temperature under different applied magnetic fields and the upper inset shows the field dependence of Tconset and Tczero. (b) Temperature dependence of magnetic susceptibility for Bi3O2S3 and the insets show the magnetic field dependence of magnetic susceptibility at 2 K. (c) Hall resistivity versus magnetic field at different temperatures. (d) Curves of C/T versus T2 in superconducting state (0 T) and normal state (9 T). The upper inset shows the data of normal state at low temperature region. The lower inset shows the temperature dependence of calculated electron specific heat in superconducting state [20].

The shielding volume fraction is about 100%, revealing bulk superconductivity, as seen in Figure 3(b). The divergence in temperature dependence of magnetic susceptibility and the M-H curves characterize Bi3O2S3 as a type-II superconductor. The Hall effect shows a remarkable nonlinear magnetic field dependence of transverse resistivity, which means it is likely a multiband superconductor [23]. However, the Hall resistivity at different temperatures is all negative, indicating that the dominant charge carriers are electron-type. The evaluated charge carrier density is about 1.5 × 1019 cm−3. It is much lower than those of cuprate and iron-based superconductors, implying a low superfluid density. Chemical substitution effects seem to increase the charge carrier density, but ultimately inhibit the superconductivity [24, 25, 26].

A clear specific heat anomaly appears around the superconducting transition temperature, as seen in Figure 3(d), confirming the bulk superconductivity in Bi3O2S3. The electronic specific heat coefficient γ and phonon specific heat coefficient β for the normal state under 9 T are obtained as 1.65 mJ/(mol K2) and 2.6 mJ/(mol K4), respectively, using linear fitting of C/T versus T2. As the phononic contribution to the heat capacity is generally independent of the external magnetic field, the electronic specific heat of superconducting state can be expressed by the equation

CeT=CT,H=0CT,H=9T+γT.E1

The estimated value of ΔCe/γTc is comparable to the BCS weak-coupling limit 1.43.

Undoped Bi2OS2 was predicted to be an insulating oxide by the band structure calculations. However, we can see it is almost metallic from 300 K to 30 K, and a weak semiconductor behavior emerges below 30 K, which may be originating from the impurities. The F-doping can significantly decrease the normal state resistivity and increase the shielding volume fraction, as shown in Figure 4. The best doping ratio is about 0.24. From the temperature dependence of magnetic susceptibility, the best doped sample has a bulk type-II-like superconductivity. When doping content exceeds 0.27, superconductivity disappears and the resistivity increases quickly. Besides, the quality of samples (x > 0.27) synthesized by conventional solid state reaction method begins to deteriorate with increasing doping content [21]. In fact, the Bi2(O,F)S2 samples synthesized by topotactic fluorination using XeF2 also contain bismuth impurity [22]. It is difficult to get pure samples because the optimal synthesis temperature is only around 400°C.

Figure 4.

(a) Temperature dependence of resistivity for BiO1−xFxBiS2. The inset shows the variation of Tc with different F-doping content. (b) Temperature dependence of magnetic susceptibility for BiO1−xFxBiS2 under ZFC process. The inset presents the FC and ZFC data for x = 0.24 sample [21].

2.2. Re(O,F)BiCh2 (Ch: S, Se) superconductors

Re(O,F)BiS2 (Re: La, Ce, Pr, Nd, Yb) superconductors have been intensively studied since the report of LaO0.5F0.5BiS2. Their structure is more definite and similar to “1111” phase of iron-based superconductors. Single crystals of this structure have been successfully synthesized [27]. Structure tuning is mainly concentrated on the spacer layers rather than the superconducting layer. And only the electron-doping into the insulating parent can induce superconductivity [28]. Here, we introduce the crystal structure and various physical properties of LaO1−xFxBiSe2 single crystals, which also firstly extend the superconducting layer to BiSe2 layer.

The powder XRD pattern and crystal structure of LaO0.59F0.41BiSe2 superconducting single crystal are presented in Figure 5. No impurity phase is found and each peak is indexed. It has a P4/nmm tetragonal lattice with the refined lattice constants a = b = 4.1377 Å and c = 14.1566 Å, which are larger than those of LaO0.5F0.5BiS2 for the larger ionic radius of Se2−. Figure 6 shows a comparison of the temperature dependence of resistivity for La(O,F)BiS2 and La(O,F)BiSe2 samples. LaOBiS2 can be described as an insulator while LaOBiSe2 is metallic. For LaO0.5F0.5BiS2, it exhibits a semiconducting behavior before the superconducting transition begins. The transport property of LaO0.5F0.5BiSe2 is similar to Bi3O2S3 but with a lower residual resistivity. Other isostructural compounds such as LaO0.5F0.5BiTe2 and LaO0.5F0.5SbS2 are also reported, but no superconductivity can be observed down to 1.7 K [16].

Figure 5.

(a) Powder XRD pattern (black circles) with the Rietveld refinement (red curve) and Miller indices for LaO0.59F0.41BiSe2. The inset table summarizes the structural parameters. (b) Crystal structure of LaO0.59F0.41BiSe2. The rectangle indicates the unit cell [17].

Figure 6.

A comparison of the temperature dependence of resistivity between (a) La(O,F)BiS2 and (b) La(O,F)BiSe2.

Fluorine doping effect on the superconductivity of LaO1−xFxBiSe2 single crystals is shown in Figure 7(a) and (b). F-doping can significantly decrease the resistivity of normal state and increase the superconducting transition temperature and shielding volume fraction. Unfortunately, the flux method can only grow single crystals with the largest F content of about 0.5. For example, the sample with F-doping amount of 0.52 was grown by a nominal component of 0.9. The magnetic susceptibility measurement shows LaO1−xFxBiSe2 has a bulk superconductivity and belongs to the type-II superconductors. Upper critical magnetic field can be evaluated from the resistivity versus temperature under various magnetic fields. As seen in Figure 7(c) and (d), the upper critical fields at zero temperature are estimated to be 29 T and 1 T for H∥ab and H⊥ab, respectively, which indicate large anisotropy.

Figure 7.

Superconducting properties of LaO1−xFxBiSe2 single crystals with different F-doping contents. (a) Temperature dependence of resistivity and an enlarged view near the superconducting transition temperature for all samples. (b) ZFC and FC magnetic susceptibility versus temperature with magnetic field applied parallel to ab-plane for all samples. (c) and (d) Resistivity versus temperature with magnetic field applied perpendicular to and parallel to ab-plane, respectively, for the x = 0.52 sample [17].

The anisotropy parameter γs of the LaO1−xFxBiSe2 superconducting single crystal is investigated by measuring the angular dependence of resistivity under various magnetic fields at 3 K (see Figure 8). Note that the angle θ describes the deviation of magnetic field with respect to the ab-plane of single crystal. Only the data with magnetic field below 1 T are selected for the reduced magnetic field, because the HC2(0) for H⊥ab is about 1 T. The reduced magnetic field is calculated by the equation

Hred=Hsin2θ+γs2cos2θ.E2

Figure 8.

Anisotropy of LaO1−xFxBiSe2 superconducting single crystal. (a) Angular dependence of resistivity taken under magnetic fields from 0.1 T to 6 T at 3 K for LaO0.48F0.52BiSe1.93 single crystal. (b) Scaling of the resistivity vs. the reduced magnetic field Hred [17].

According to the Ginzburg-Landau theory [29], the curves of resistivity versus reduced magnetic field under different magnetic fields should merge into one. The resultant anisotropy parameter at 3 K is about 30 (see Figure 8(b)), which is close to the result of upper critical field within the ab-plane.

Considering that the Tc of LaO0.5F0.5BiS2 is increased from 2.7 K to 10.6 K under a hydrostatic pressure of 1.68 GPa [30], the highest Tc among the BiS2-based superconductors, higher Tc, above 10.6 K is expected for LaO0.5F0.5BiSe2 under external pressure since its zero-resistance temperature is about 3.5 K. However, we find that its superconductivity and shielding volume fraction decrease unexpectedly with increasing pressure below 1 GPa hydrostatic pressure, as seen in Figure 9(a). Another experiment with higher pressure shows that a new superconducting phase emerges at about 1.2 GPa and Tc reaches about 6.5 K at 2.17 GPa [31]. Accompanied by this crossover, the normal state is switched from that with a low temperature resistivity upturning to a metallic one. Accordingly, the normal state resistivity also shows a nonmonotonic change with the external pressure. These facts suggest that the BiSe2-based system is very different from the BiS2-based system.

Figure 9.

High-pressure effect on the superconductivity of LaO0.5F0.5BiSe2 single crystal. (a) High-pressure effect on the temperature dependence of magnetic susceptibility. (b) and (c) High-pressure effect on the transport properties of two single crystal samples of LaO0.5F0.5BiSe2 [31].

2.3. MxBi2Ch2 (Ch: Se, Te) superconductors

Topological insulator has linearly dispersive band structures and its topological surface state exhibits metallic properties while the bulk state is insulating. If its spin-momentum locking effect combines with superconductivity, Majorana fermion may exist, which is useful for quantum computing. At first, the topological superconductors were mostly focused on the proximity-induced superconductivity. The discovery of CuxBi2Se3 superconductor opens a new gate to topological superconductors, i.e., superconductors induced by doping into topological insulators, which are expected to be the candidate of three-dimensional topological superconductors. Recently, a series of superconductors based on the topological insulators have been reported, such as Cux(PbSe)5(Bi2Se3)6 [32], SrxBi2Se3 [7], NbxBi2Se3 [8], and TlxBi2Te3 [33]. Here, we put emphasis on the crystal structure and physical properties of SrxBi2Se3 single crystals.

The structure of SrxBi2Se3 is similar to that of CuxBi2Se3 and isomorphic to the parent Bi2Se3. Sr atoms may act as a bipolar dopant that can be embedded in the van der Waals space or randomly substitute for Bi. The actual Sr doping content of SrxBi2Se3 is very little so that it is hard to define its precise position. Nevertheless, the lattice constants of SrxBi2Se3 are a little larger than those of Bi2Se3, while the lattice constants of Bi2−xSrxSe3 are smaller. The c-axis lattice constant of Bi2−xSrxSe3 decreases slightly with increasing doping content (see Figure 10(b)). In addition, all samples grown in Bi2−xSrxSe3 ratio show no signs of superconductivity at 1.8 K, as seen in Figure 11(a). Therefore, we could use Figure 10(a) as the schematic structure diagram.

Figure 10.

Crystal structure of SrxBi2Se3 superconductors. (a) Schematic diagram of SrxBi2Se3 crystal structure. (b) Powder XRD patterns of SrxBi2Se3, Bi2Se3, and Bi2−xSrxSe3 [7].

Figure 11.

Superconducting properties of SrxBi2Se3. (a) Temperature dependence of resistivity for SrxBi2Se3 and Bi2−xSrxSe3. (b) Hall resistivity versus magnetic field curves measured at different temperatures. (c) Temperature dependence of estimated Hall coefficient and charge carrier density. (d) Temperature dependence of susceptibility for samples with different Sr contents. (e) Plot of Tconset, Tczero, and shielding volume fraction as a function of Sr content [7].

The linear curves of Hall resistivity versus magnetic field indicate that SrxBi2Se3 has only one electron-like bulk carrier. The carrier density increases slightly with decreasing temperature. Its average is around 2.3 × 1019 cm−3, about 1–2 orders of magnitude lower than CuxBi2Se3. Figure 11(d) and (e) shows that the Tc of superconducting samples changes little with different Sr contents, but the shielding volume fraction is very different. Only those samples with Sr content above 0.06 have a large shielding volume fraction. Moreover, the superconductivity is very stable in air, as evidenced by the almost unchanged shielding volume fraction for the sample placed in air even for a month. This provides great convenience for experimental research.

The topological surface state of SrxBi2Se3 single crystal has been investigated through Shubnikov-de Haas oscillation measurements. Clear oscillations in resistivity and Hall resistivity can be observed under high magnetic field at different temperatures, as shown in Figure 12(a) and (c). The oscillation amplitudes become more pronounced for higher magnetic field and lower temperature. However, the oscillatory periods measured at different temperatures remain constant, so only the data at 0.35 K with the most noticeable oscillations are selected to deduce the Landau level indices. In fact, the measured resistivity and Hall resistivity actually contain contributions from both the surface and bulk conductance when a large parallel bulk conduction channel is present. Therefore, the least confusing method is to convert resistivity into conductance to determine the Landau index because its components are additive [34]. The following equations are used to calculate conductance

Gxx=RxxRxx2+Rxy2,Gxy=RxyRxx2+Rxy2.E3

Figure 12.

SdH oscillations under high magnetic field for SrxBi2Se3 single crystal. (a) and (c) Magnetic field dependence of resistivity and Hall resistivity at different temperatures. (b) and (d) Magnetic field dependence of the fitted longitudinal and Hall conductivity at 0.35 K [7].

After removing the nonoscillatory background, the oscillatory components are obtained and plotted as a function of 1/B. The frequencies are 146 T for longitudinal conductance and 144.8 T for Hall conductance, which are comparable to those of Bi2Se3 but smaller than CuxBi2Se3. The integer Landau index n corresponds to the valleys in ΔGxx, while the valleys in ΔGxy are assigned to n + 1/4 [see Figure 13(a) and (c)]. The 1/4 shift arises to match the valleys in dΔGxy/dB with the valleys in ΔGxx [34]. The obtained intercepts of the linear fittings for n versus 1/B are both close to the value for an ideal Dirac system, i.e., −0.5 rather than 0 or 1 (see Figure 13(b) and (d)). Thus, it provides transport evidence for the existence of Dirac fermions in SrxBi2Se3 superconductor.

Figure 13.

(a) and (c) Oscillatory component of the longitudinal and Hall conductivity at 0.35 K plotted against 1/B. (b) The Landau index n versus 1/B, where n and n + 1/2 correspond to the valleys and peaks of ΔGxx. (d) n versus 1/B derived from (c), where n + 1/4 corresponds to the valleys of ΔGxy [7].

The superconductivity of SrxBi2Se3 is very sensitive to external pressure below 1 GPa, as seen in Figure 14(a) and (b). With the increasing applied pressure, the Tc and shielding volume fraction decrease but the normal state resistivity increases. This depression of superconductivity can be attributed to the reduction of charge carrier density, which is apparent from the normal state resistivity. However, if the pressure continues to increase, the normal state resistivity begins to decrease and a sign of superconducting transition occurs at 6 GPa. Then, the Tconset and the charge carrier density estimated from the normal state resistivity gradually increase with the increasing pressure, and Tconset reaches around 8 K when P > 14 GPa. But unfortunately, the Tconset remains almost constant for the pressure up to 40 GPa, although the normal state resistivity keeps decreasing. The reemerging superconductivity is very robust and the Tconset still changes little under 80 GPa [35]. In fact, the whole process contains three structural phases, i.e., R-3 m, C2/m, and I4/mmm, as seen in Figure 14(d). The structural transitions and pressure-invariant Tc are very similar to the parent compound Bi2Se3, which needs further investigations.

Figure 14.

(a) Temperature dependence of magnetic susceptibility under different pressures. (b) and (c) Temperature dependence of resistance under high pressure. (d) The structural phase diagram on pressure for SrxBi2Se3 [35].

3. Conclusions

The discovery of superconductivity in layered compound Bi4O4S3 brings in a new BiS2-based superconducting family, including the Bi─O─S compounds, Re(O,F)BiS2, and MFBiS2 superconductors. The superconducting layer is extended to BiSe2 layer in LaO1−xFxBiSe2 and Sr1−xLaxFBiSe2. The crystal structure and various superconducting properties are reviewed for selective systems. Hall effect and specific heat suggest that they are probably multiband superconductors and can be described by BCS weak-coupling theory. Moreover, bismuth chalcogenide topological insulators can be turned into superconductors by doping, which are potential candidates for 3D topological superconductors. For example, the topological surface state of SrxBi2Se3 is well supported by SdH oscillations under high magnetic field. The intermediate external pressure can efficiently suppress the superconductivity, which reemerges when pressure is further increased, while Tc is nearly invariant in high-pressure region, indicating an unconventional pairing state.

Acknowledgments

We acknowledge the support from the National Natural Science Foundation of China (Grant nos. 51603207, U1532267, and 11574288).

J.S. would like to dedicate his best love to his dear wife, Lv Youyou, who always encourages and supports his research and life. Best wishes for their love and coming baby. W.Z. would like to express his special thanks to his dearest daughters, Lily and Amy.

Conflict of interest

The authors declare no competing financial interests.

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Jifeng Shao and Wenka Zhu (May 30th 2018). Emerging Superconductivity and Topological States in Bismuth Chalcogenides, Superfluids and Superconductors, Roberto Zivieri, IntechOpen, DOI: 10.5772/intechopen.73057. Available from:

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