Article Cite This: Chem. Mater. 2018, 30, 5293−5304
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Superconductivity and Structural Conversion with Na and K Doping of the Narrow-Gap Semiconductor CsBi4Te6 Haijie Chen,†,‡ Helmut Claus,† Jin-Ke Bao,† Constantinos C. Stoumpos,‡ Duck Young Chung,† Wai-Kwong Kwok,† and Mercouri G. Kanatzidis*,†,‡ †
Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, United States Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States
‡
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S Supporting Information *
ABSTRACT: The monoclinic narrow-gap (∼0.08 eV) semiconductor CsBi4Te6 is a unique layered system which can be doped to achieve high thermoelectric performance as well as superconductivity. Here, we report superconductivity and structure change induced by alloying CsBi4Te6 single crystals with Na and K. Substitution of Na in CsBi4Te6 with doping levels ≥0.39 and of K with ≥0.63 transforms the original monoclinic structure (p-type) to the orthorhombic RbBi3.67Te6-type structure (n-type). When the K level is ≤0.18, the monoclinic structure type of CsBi4Te6 is retained. Transport and magnetic measurements on all as-synthesized doped single crystals demonstrate type-II, bulk superconductivity. A maximal superconducting transition at 5.07 K, which is the highest temperature in bismuth chalcogenidebased superconductors, was obtained in Cs0.82K0.18Bi4Te6 with a high upper critical field of ∼15 T. These findings suggest superconductivity may be induced by proper doping in narrow-gap semiconductors.
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INTRODUCTION The emergence of superconductivity in semiconductors with relatively low carrier density (1019−1020 cm−3), such as PbTe,1 Bi2Se3,2 Bi2Te3,3 IrTe2,4 TiSe2,5 SnTe,6 SrTiO3,7 and AxZrNCl (A = Li, Na, K),8 is an exceptional phenomenon because superconductors generally are metals with carrier densities above 1022 cm−3 and often have high density of states (DOS) at the Fermi level (EF). The observed superconductivity in semiconductors, which have very low DOS at EF, is counterintuitive and underscores the limited understanding we have on its mechanism and how to induce it. Therefore, it is important to discover and study new semiconductors that can be doped into superconductors to gain further insights into the evolution of this intriguing phenomenon. Thus, far, studies suggest that superconductivity can be induced in narrow-gap semiconductors by aliovalent doping with elements such as Tl in PbTe (superconducting transition temperature (Tc ∼ 1.5 K)),9 In in SnTe (Tc ∼ 4.5 K),10−13 Pt or Pd in IrTe2 (Tc ∼ 3.1 K),14,15 Tl in Bi2Te3 (Tc ∼ 2.3 K),16 F in LaOBiS2 (Tc ∼ 10.6 K),17 La in SrFBiS2 (Tc ∼ 10.6 K),18 and Ag in (PbSe)5(Bi2Se3)6 (Tc ∼ 1.7 K).19 Another approach is by intercalating Cu in TiSe2 (Tc ∼ 4.2 K)5,20 or Cu, Sr, and Nb in Bi2Se3 (Tc ∼ 2.5 K) in which potential topological superconductivity can emerge with fully gapped state in the bulk and gapless Majorana state at the surface.21−28 Moreover, high pressure is also a powerful technique to induce superconductivity in semiconductors, such as the topological parent material Bi2Te329 and the layered In2Se3.30 © 2018 American Chemical Society
Previously, we reported superconductivity in narrow-gap layered semiconductors RbBi3.67Te6 and CsBi4Te6 with low carrier densities of 1019 cm−3.31−34 The monoclinic CsBi4Te6 consists of unique Bi−Te slabs featuring Bi−Bi bonds, whereas RbBi3.67Te6 forms regular Bi−Te layers in the orthorhombic space group with Bi vacancies to maintain charge balance. In CsBi4Te6, superconductivity was induced via hole doping resulting in Tc of 4.4 K, while RbBi3.67Te6 is a superconductor below 3.2 K.35 Subsequently, superconductivity was also found in the homologous quaternary series AMmBi3Q5+m (m = 1, 2) (A = Rb, Cs; M = Pb, Sn; Q = Se, Te) semimetals with relatively lower Tc < 3 K.36,37 The above results, which demonstrate that the ternary semiconducting families of bismuth tellurides can harbor superconductivity despite their low carrier densities, have prompted the study of isovalent Na and K doping in the form of Cs1−xNaxBi4Te6 and Cs1−yKyBi4Te6 solid solutions. Our goal was to understand how isovalent doping on the Cs site, which does not alter the carrier concentration, transforms the crystal structures and charge-transport dynamics in this family of superconductors. Detailed single crystal X-ray diffraction and transport measurements demonstrate two completely different behaviors for Na and K. Specifically, Na does not act as an isovalent dopant but instead prefers the Bi site which has a Received: May 14, 2018 Revised: June 5, 2018 Published: June 20, 2018 5293
DOI: 10.1021/acs.chemmater.8b02030 Chem. Mater. 2018, 30, 5293−5304
Article
Chemistry of Materials
Table 1. Analyzed Composition, Approximate Yield, Carrier Density (n) at 300 and 10 K, Superconducting Transition Temperature (Tc), and Upper Critical Field (Hc2(0)) of Na- and K-Doped CsBi4Te6 Single Crystals doping level attempted
composition (EDS/ SEM analysis)
composition (refined crystal structure)
x=0 x = 0.1 x = 0.2 y = 0.1 y = 0.2 y = 0.3 y = 0.4 y = 0.5 y = 0.6 y = 0.7
CsBi4Te6 CsNa0.4Bi3.5Te6 CsNa0.5Bi3.5Te6 Cs0.9K0.1Bi4Te6 Cs0.8K0.2Bi4Te6 Cs0.8K0.55Bi3.5Te6 Cs0.7K0.6Bi3.5Te6 Cs0.65K0.65Bi3.5Te6 Cs0.6K0.7Bi3.5Te6 Cs0.56K0.8Bi3.5Te6
CsBi4Te6 CsBi3.55Na0.39Te6 CsBi3.52Na0.45Te6 Cs0.91K0.09Bi4Te6 Cs0.82K0.18Bi4Te6 Cs0.78K0.63Bi3.54Te6 Cs0.72K0.67Bi3.54Te6 Cs0.64K0.74Bi3.55Te6 Cs0.63K0.79Bi3.54Te6 Cs0.58K0.83Bi3.53Te6
structure type
approx. yield (%)
n at 300 K (1019 cm−3)
n at 10 K (1019 cm−3)
Tc (K)
Hc2(0) WHH fit (T)
Hc2(0) GL fit (T)
monoclinic orthorhombic orthorhombic monoclinic monoclinic orthorhombic orthorhombic orthorhombic orthorhombic orthorhombic
100 70 60 100 100 80 70 60 50 40
2.4 14.5 16.3 2.3 2.1 11.1 18.7 14.7 16.4 17.9
3.2 7.5 5.9 2.7 2.6 5.5 7.3 7.9 8.0 7.4
3.13 3.22 4.92 5.07 3.04 3.30 3.42 3.48 3.56
1.1 0.9 13.6 14.1 1.0 1.2 0.8 0.8 0.8
1.3 1.0 15.5 15.5 1.2 1.5 0.9 0.9 0.9
nearly identical ionic radius (rBi = 1.03 Å, rNa = 1.02 Å).38 Instead, introducing Na fractions of 0.39 and 0.45 induces a structural transformation from the monoclinic CsBi4Te6-type to an orthorhombic RbBi3.67Te6-type structure with a change in carrier type from p- to n-type. In contrast, K fraction < 0.18 substitutes only into the Cs site retaining the CsBi4Te6-type structure and induces superconductivity without changing the carrier type and density. With increasing K fraction in the range of 0.63 to 0.83, both the Cs sites and the Bi sites are affected, prompting a change from monoclinic CsBi4Te6-type to the orthorhombic RbBi3.67Te6-type structure. A noteworthy Tc of 5.07 K (highest in the bismuth chalcogenide class of superconductors) with an upper critical field (Hc2(0)) of ∼15 T was obtained in the monoclinic Cs0.82K0.18Bi4Te6.
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NaBiTe2 crystals along with an air-sensitive mixture of possibly Na(Cs)2Tez/Bi. Synthesis of K-Doped CsBi4Te6. Nominal compositions of Cs1−yKyBi4Te6 (y = 0.1−0.7) were attempted as described above. As a representative example, Cs0.9K0.1Bi4Te6 (y = 0.1) was synthesized by a stoichiometric mixture of Bi2Te3 (0.160 g, 0.2 mmol), Cs (0.120 g, 0.9 mmol), K (0.008 g, 0.1 mmol), Te (0.689 g, 5.4 mmol), and Bi (0.752 g, 3.6 mmol) which were all sealed in the same sized silica tube as above. The mixture was heated to 700 °C over 10 h, dwelled there for 2 h, and then slowly cooled down to 400 °C over 50 h. The resulting crystal morphology of the K-doped CsBi4Te6 is needle-like. Plate-like crystals of the binary Bi2Te3 were observed to form as a side product when y ≥ 0.3, and its fraction increased as y increased. Compositions with higher doping level (y > 0.7) were attempted but produced only Bi2Te3 crystals and a black air-sensitive residue which probably is a mixture of K(Cs)2Tez/Bi. The approximate yield of the needle phase for each composition is listed in Table 1. Single Crystal X-ray Diffraction. Needle-shaped single crystals were carefully separated from the ingot and cut to an appropriate size for X-ray diffraction data collection. The quality of the crystals was assessed using a small number of diffraction data frames on a STOE IPDS 2 single-crystal diffractometer equipped with graphitemonochromatized Mo Kα radiation (λ = 0.71073 Å). Full sphere data were collected at room temperature (293 K) on the best quality crystal. The data was reduced, integrated, and corrected for absorption using the STOE X-Area suite.39 The crystal structure was solved and refined by full-matrix least-squares on F2 using the Jana2006 package40,41 and the SHELXTL program.42 Powder X-ray Diffraction (PXRD). The obtained ingots were ground for powder X-ray diffraction (PXRD) for phase identification and yield analysis (Figure S1). The PXRD patterns were collected on a Rigaku Miniflex600 powder X-ray diffractometer (Cu Kα radiation, λ = 1.5406 Å) operating at 40 kV/15 mA with a Kβ foil filter. Energy Dispersive X-ray Spectroscopy and Scanning Electron Microscopy (EDS/SEM). Elemental analysis of the crystals was performed by EDS/SEM using a Hitachi S3400N-II scanning electron microscope equipped with an Oxford Instruments INCAx-act SDD EDS detector. Several needle crystals with fresh, clean surfaces were mounted on a carbon tape and examined with an accelerating voltage of 20 kV. Data was recorded using 120 s acquisition times. The average compositions obtained from EDS/SEM measurements are compared with the nominal compositions in Table 1 for all needle phases. Transport Properties. Single crystals of needle phase Na- and Kdoped CsBi4Te6 were selected for temperature-dependent resistivity and heat capacity measurements in the Physical Property Measurement System (PPMS Dynacool, Quantum Design). Since the surfaces of the crystals are very air-sensitive, electrical contacts were mounted under inert conditions in a glovebox filled with Ar gas and were made with gold wires attached to the sample surface using Dupont 4929N silver. The current was applied along the needle direction and the magnetic field transverse to the needle direction.
EXPERIMENTAL SECTION
Reagents. The following chemicals were used as purchased: cesium metal (99.9%, Strem Chemicals, Inc.), potassium metal (98%, Sigma-Aldrich), sodium metal (98%, Sigma-Aldrich), bismuth metal (99.9%, Strem Chemicals, Inc.), and tellurium shots (99.999%, American Elements). Bi2Te3. A stoichiometric mixture (∼10 g) of elemental Bi and Te corresponding to Bi2Te3 was sealed in a silica tube (15 mm O.D. × 13 mm I.D.) at a residual pressure of 100%), generating unstable refinements. Herein, all Bi and Te atoms were set with full occupancies. Hall Effect. We performed Hall effect measurements to obtain the carrier concentration in carefully selected single crystals of these compounds. For the orthorhombic parent compound, the ideal charge balanced formula is ABi3.67Te6 (A = Cs, Rb, K, Na) achieved through Bi vacancies.34 The fact that Na occupies only Bi octahedral sites while K substitutes both Cs and Bi sites48−51 raises the following consideration: if K substitutes for Cs, in principle the carrier density should remain unaffected because Cs and K are isovalent. On the other hand, when Na or K substitutes into the Bi sites, they are not isovalent and can act either as an acceptor or donor depending on whether they substitute Bi atoms or fill the vacancies in the Bi sites. To probe this behavior, we conducted Hall measurements to determine the carrier type and concentration, on all assynthesized single crystals (see Figures S2−S4). For
CsBi3.55Na0.39Te6 and CsBi3.52Na0.45Te6, the Hall resistivity (ρxy) is negative at positive magnetic field, demonstrating ntype behavior. The carrier concentration n increases from ∼1019 cm−3 in CsBi4Te6 to ∼1020 cm−3 at 300 K for both compounds (see Figure 2a,b). As shown in Figure S3c,d, for Cs0.91K0.09Bi4Te6 and Cs0.82K0.18Bi4Te6, ρxy is positive at positive magnetic fields, indicating holes as the dominant carriers. The carrier density is 2.3 × 1019 and 2.1 × 1019 cm−3 for Cs0.91K0.09Bi4Te6 and Cs0.82K0.18Bi4Te6 respectively at 300 K which is nearly unchanged from 2.4 × 1019 cm−3 for the undoped CsBi4Te6. Since there is no significant effect on the dominant carrier type and carrier density, this result is consistent with isovalent K doping in Cs sites as obtained from the single crystal X-ray refinement. For Cs0.78K0.63Bi3.54Te6, Cs0.72K0.67Bi3.54Te6, Cs0.64K0.74Bi3.55Te6, Cs0.63K0.79Bi3.54Te6 and Cs0.58K0.83Bi3.53Te6, the Hall coefficients (ρH = dρxy/dH) are negative (n-type) and n is also ∼1020 cm−3 at 300 K, which is similar as CsBi3.55Na0.39Te6 and CsBi3.52Na0.45Te6. Na/K occupation in Bi sites in the orthorhombic structure can create holes when Na/K substitutes for Bi atoms or electrons when Na/K fills only the Bi vacancies. Judging from the results of the Hall measurements, vacancy filling appears to be more dominant than substitution of Bi with Na/K. Superconductivity. Superconductivity was observed in all compounds from resistivity measurements on single crystals. 5298
DOI: 10.1021/acs.chemmater.8b02030 Chem. Mater. 2018, 30, 5293−5304
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Chemistry of Materials
Figure 4. (a) Temperature dependence of magnetic susceptibility (4πχ) of Cs0.82K0.18Bi4Te6 under zero-field-cooled (ZFC) and field-cooled (FC) procedures with applied magnetic field of 0.5 mT below 6 K. (b) Small field (0−50 mT) dependent 4πχ at different temperatures (2.0−4.4 K). (c) Calculation of the lower critical field at zero temperature (Hc1(0)), which is determined to be ∼4 mT. (d) Field dependent resistivity from 1.8 to 6 K. (e) Calculation of upper critical field (Hc2(T)) using the Ginzburg−Landau (GL) theory and the Werthamer−Helfand−Hohenberg (WHH) formula. The two models yield an upper critical field of ∼15.6 and 14.1 T, respectively. (f) Comparison of Hc2(0) for Cs0.91K0.09Bi4Te6 and Cs0.82K0.18Bi4Te6.
M = Pb, Sn; Q = Se, Te) semimetals with n ∼ 1021 cm−3 where electrons are the dominant carriers.33,34 This implies that introduction of more carriers can lead to a deterioration of superconductivity in the orthorhombic phases, which is contrary to the normal behavior of superconductivity in narrow-gap semiconductors,13,17,19,52 suggestive of a potentially unusual superconductivity mechanism in the ternary semiconductors A−Bi−Te (A = Rb, Cs). To see the resistivity behavior in the normal state, the temperature dependence of the resistivity for monoclinic Cs0.82K0.18Bi4Te6 and orthorhombic Cs0.78K0.63Bi3.54Te6 above Tc was measured, Figure 3c,d. The monoclinic phase displays a concave curve in the resistivity data as a function of temperature, whereas the orthorhombic phase shows a convex
Figure 3a shows temperature dependence of the normalized resistance for CsBi3.55Na0.39Te6 and CsBi3.52Na0.45Te6. Tc increases with Na doping, going from Tc ∼ 3.13 to 3.22 K. Figure 3b shows the temperature dependence of the normalized resistance (ρ/ρ 6K ) for Cs 0.91 K 0.09 Bi 4 Te 6 , Cs0.82K0.18Bi4Te6, Cs0.78K0.63Bi3.54Te6, Cs0.72K0.67Bi3.54Te6, Cs0.64K0.74Bi3.55Te6, Cs0.63K0.79Bi3.54Te6, and Cs0.58K0.83Bi3.53Te6, corresponding to nominal y = 0.1−0.7, respectively. Generally, Tc of the monoclinic phases (∼5 K) are higher than the orthorhombic phases (∼3 K) with the highest Tc of 5.07 K obtained for Cs0.82K0.18Bi4Te6 (y = 0.2). In each group of monoclinic and orthorhombic phases, Tc increases with K doping. For comparison, superconductivity appears below 3 K in a series of AMmBi3Q5+m (m = 1, 2) (A = Rb, Cs; 5299
DOI: 10.1021/acs.chemmater.8b02030 Chem. Mater. 2018, 30, 5293−5304
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Chemistry of Materials
Figure 5. (a) Temperature dependence of magnetic susceptibility (4πχ) of Cs0.78K0.63Bi3.54Te6 under zero-field-cooled (ZFC) and field-cooled (FC) procedures with applied magnetic field of 0.5 mT below 5 K. (b) Small field (0−50 mT) dependent 4πχ at different temperatures (2.0−3.0 K). (c) Calculation of the lower critical field at zero temperature (Hc1(0)), which is determined to be ∼2.5 mT. (d) Field dependent resistivity from 1.8 to 4 K. (e) Calculation of upper critical field (Hc2(T)) using the Ginzburg−Landau (GL) theory and the Werthamer−Helfand− Hohenberg (WHH) formula. The two models yield an upper critical field of ∼1.2 and 1 T, respectively. (f) Comparison of Hc2(0) for CsBi3.55Na0.39Te6 and CsBi3.52Na0.45Te6 (olive and violet colors) and Cs0.78K0.63Bi3.54Te6, Cs0.72K0.67Bi3.54Te6, Cs0.64K0.74Bi3.55Te6, Cs0.63K0.79Bi3.54Te6, and Cs0.58K0.83Bi3.53Te6 (black and red colors).
filament arising from Bi thin film or CsBi2, we conducted magnetic susceptibility measurements on Cs0.82K0.18Bi4Te6 and Cs0.78K0.63Bi3.54Te6 as representative single crystals. For Cs0.82K0.18Bi4Te6, the magnetization observed at 2 K in the zero-field cooled (ZFC) measurement is estimated to be about 10% of that expected for full diamagnetism, Figure 4a. A higher superconducting volume fraction is expected below 2 K because the diamagnetism is still increasing steeply with decreasing temperature. From the low-field dependence magnetic susceptibility data (Figure 4b), a zero-temperature lower critical field Hc1(0) ∼ 4 mT was obtained (Figure 4c).
curve. This is consistent with a normal single band degenerately doped metal-like behavior for the former and possibly a multiband based behavior for the latter, similar to the behavior of the resistivity in Ba1−xKxFe2As2.53 Below 50K the resistivities exhibit a Fermi liquid behavior,54 characterized by a T2 temperature dependence for both compounds, and can be well fitted with ρ = 0.84 + 1.5 × 10−4T2 for x = 0.2 and ρ = 0.2 + 4.4 × 10−5T2 for x = 0.3, respectively. It is known that superconductivity exists in Bi thin films (Tc ∼ 6 K)55 and the very air and moisture sensitive CsBi2 (Tc ∼ 4.8 K).56 In order to determine if the samples exhibit bulk superconductivity and exclude the possible superconducting 5300
DOI: 10.1021/acs.chemmater.8b02030 Chem. Mater. 2018, 30, 5293−5304
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Chemistry of Materials
Figure 6. (a) Specific heat for Cs0.63K0.79Bi3.54Te6 divided by temperature (C/T) as a function of T2. (b) Temperature dependent plot of electron heat capacity divided by γT (Cel/γT; Cel = C(T) − β1T3 − β2T5) where γT and β1T3 + β2T5 are the electron and phonon contributions to the specific heat, respectively. A Cel/γTc value of 1.64 was obtained.
Figure 7. (a) Phase diagram with superconducting transition temperature (Tc) as a function of Na and K content alloyed in CsBi4Te6. (b) Comparison of the maximal Tc and upper critical field Hc2(0) from this work with the other Bi−Q (Q = Se, Te)-based superconductors (SrxBi2Se3,25 CuxBi2Se3,2 NbxBi2Se3,28,57 Tl0.6Bi2Te3,16 CsBi4Te6,33 and RbBi6.67Te634).
33 nm, λ(0) ∼ 450 nm, and κ ∼ 14, suggesting also a type-II superconductor. For the other orthorhombic compounds, field dependent resistivities are shown in Figures S7−S10. All Hc2(0) are plotted in Figure 5f. Generally, GL theory gives minor larger Hc2(0) than the values calculated by the WHH formula. Hc2(0) of CsBi3.52Na0.45Te6 is determined to be 1.02 T from GL theory and 0.89 T from WHH formula which is smaller compared to that in CsBi3.55Na0.39Te6 (1.3 T from GL theory and 1.08 T from WHH formula). This demonstrates that increasing the Na fraction into CsBi4Te6 has negative effect on Hc2(0). As for the orthorhombic K doped compounds, an increased Hc2(0) of 1.54 T (GL theory) and 1.25 T (WHH formula) was obtained in Cs0.72K0.67Bi3.54Te6. When more K is introduced, Hc2(0) is much decreased, with 0.9 and 0.86 T for Cs0.64K0.74Bi3.55Te6, 0.86 and 0.8 T for Cs0.63K0.79Bi3.54Te6, and 0.86 and 0.71 T Cs0.58K0.83Bi3.53Te6, calculated by GL theory and the WHH formula, respectively. The effect on Hc2(0) of K doping shows a dome-like behavior in the orthorhombic crystals, which is different from that in the monoclinic crystals as discussed above. We note that WHH/GL extrapolations may give an overevaluated Hc2(0) because it is valid when only the orbital
Detailed temperature dependent resistivity curves under various applied magnetic fields were measured to evaluate the upper critical field Hc2(0). As shown in Figure 4d, Tc decreases monotonically with increasing field from 0−7 T. Ginzburg− Landau (GL) theory43 and Werthamer−Helfand−Hohenberg (WHH) formula45 fittings give a high Hc2(0) ∼ 15.6 and 14.1 T, respectively. The calculated GL coherence ξ(0) ∼ 4.7 nm and the penetration depth λ(0) ∼ 460 nm. The corresponding GL parameter κ (λ(0)/ξ(0)) of 98 identifies this material as an extreme type-II superconductor, similar to Nb doped Bi2Se3.28,57 Cs0.9K0.1Bi4Te6 has a similar value of Hc2(0) ∼ 15.5 (GL) and 13.6 T (WHH) based on similar analysis, Figure 4f (and Figures S5 and S6). This demonstrates that the introduction of more K in the structure has little effect on Hc2(0) in the monoclinic structure. As shown in Figure 5a, the superconducting volume fraction at 2 K for Cs0.78K0.63Bi3.54Te6 is almost 100%, confirming full bulk superconductivity at this temperature. The temperature and field dependent magnetic susceptibility (Figure 5b) suggests a value of ∼2.5 mT for Hc1(0) (Figure 5c). Tc also decreases monotonically with increasing field from 0 to 0.4 T, Figure 5d. GL and WHH models give Hc2(0) 1.2 and 1 T, respectively. From similar analysis as above, we obtain ξ(0) ∼ 5301
DOI: 10.1021/acs.chemmater.8b02030 Chem. Mater. 2018, 30, 5293−5304
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Chemistry of Materials
carriers from holes (p-type) to electrons (n-type). Superconductivity is present in all compositions, and Tc increases with increasing level of alkali metal fractions. Moreover, the effect of Na and K alloying on the upper critical field Hc2(0) results in different behaviors in the two different structures. Magnetization and heat capacity results demonstrate bulk, type-II superconductivity. The Na and K doped ternary CsBi4Te6 system provides an interesting framework for investigation of complex multinary phases with respect to superconductivity arising, amazingly, from narrow-gap semiconductors.
effects are considered. Most superconducting materials with a spin-singlet pairing have a Pauli paramagnetic limiting field (Hc2(0) ∼ 1.85Tc) which breaks the singlet Cooper pair. However, the Pauli limiting field value only considers the simple one-band BCS superconductor case. There are other factors such as multiband and strong spin−orbit coupling which can also affect the upper critical field of a superconductor. To more precisely determine the upper critical fields, these samples need to be further investigated at even lower temperatures and higher magnetic fields. Heat Capacity. To evaluate the electronic contribution to the superconductivity, we conducted specific heat measurements on Cs0.63K0.79Bi3.54Te6 single crystals. The data confirms the bulk superconducting transition around 3.4 K, which is consistent with the resistivity results presented above, Figure 6. The normal-state data below 4 K (in blue dash line) is well fitted by C(T) = γT + β1T3 + β2T5, where γT and β1T3 + β2T5 are the electron and phonon contributions to the specific heat, respectively. The calculated coefficients are γ = 8.49 mJ mol−1 K−2, β1 = 8.37 mJ mol−1 K−4, and β2 = 0.19 mJ mol−1 K−6. The Debye temperature is low, ΘD = (12π4NR/5β)1/3 ∼ 135.9 K, and consistent with the heavy element nature of the compounds. The electron m* is estimated to be 11.4 m0 from the expression γ = π2/3κB2N(EF) = 1.36 × 10−4 × Vmol2/3nγ1/3m*/m0,51 where Vmol is the molar volume, nγ is the carrier concentration per atom, and m*/m0 is the effective mass. As shown in Figure 6b, the electronic contribution Cel = C(T) − β1T3 − β2T5 is normalized by the normal state contribution γC(T). The dimensionless specific-heat jump (Cel/(γTc)) is evaluated to be ∼1.64 which is close to the theoretical value (1.43) from the well-known Bardeen− Cooper−Schrieffer (BCS) theory. Phase Diagram. All of the characteristic parameters (Tc, structure, carrier types, and Na/K ratios) are summarized in a phase diagram plotted in Figure 7a as a function of Na and K content. Cs0.91K0.09Bi4Te6 and Cs0.82K0.18Bi4Te6 have the p-type monoclinic CsBi4Te6-type structure with Tc ∼ 5 K. For CsBi 3.55 Na 0.39 Te6 , CsBi 3.52Na 0.45 Te 6, Cs0.78K 0.63 Bi3.54 Te 6 , Cs0.72K0.67Bi3.54Te6, Cs0.64K0.74Bi3.55Te6, Cs0.63K0.79Bi3.54Te6, and Cs0.58K0.83Bi3.53Te6, the structure changes to n-type RbBi3.67Te6 structure with Tc ∼ 3 K. Within the same structure type, Tc increases with increasing Na and K fractions. For comparison, the maximal Tc and Hc2(0) with the previous well-known bismuth chalcogenide-based superconductors of Sr x Bi 2 Se 3 , 2 5 Cu x Bi 2 Se 3 , 2 Nb x Bi 2 Se 3 , 2 8 ,5 7 Tl0.6Bi2Te3,16 CsBi4Te6,33 and RbBi6.67Te634 are reported to be 2.9 K, 2.1 T; 3.8 K, 3 T; 3.1 K, 1.8 T; 2.3 K, 1.1 T; 4.4 K, 9.7 T; and 3.2 K, 0.5 T; respectively. As shown in Figure 7b, it is obvious that both values obtained from Cs0.82K0.18Bi4Te6 are much higher. For Cu x Bi 2 Se 3 , 2 Nb x Bi 2 Se 3 , 2 8 ,5 7 and Tl0.6Bi2Te3,16 the carrier density is determined to be around 1020 cm−3 which is a magnitude higher than that in Cs0.82K0.18Bi4Te6. This further points to an unusual nature of the superconducting mechanism in CsBi4Te6, which suggests additional experimentation needed in the future.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b02030. Crystal data, structure refinement, and atomic coordinates (×104) and equivalent isotropic displacement parameters (Å 2 × 103) with estimated standard deviations for CsBi3.52Na0.45Te6, Cs0.82K0.18Bi4Te6, Cs0.72K0.67Bi3.54Te6, Cs0.64K0.74Bi3.55Te6, Cs0.63K0.79Bi3.54Te6, and Cs0.58K0.83Bi3.53Te6 at 293 K; powder X-ray diffraction patterns (PXRD) for K-doped CsBi4Te6; temperature dependence of Hall resistivity for CsBi3.55Na0.39Te6, CsBi3.52Na0.45Te6, Cs0.78K0.63Bi3.54Te6, Cs0.72K0.67Bi3.54Te6, Cs0.64K0.74Bi3.55Te6, Cs0.63K0.79Bi3.54Te6, and Cs0.58K0.83Bi3.53Te6; and calculation of upper critical field (H c 2 (T)) for CsBi3.55Na0.39Te6, CsBi3.52Na0.45Te6, Cs0.78K0.63Bi3.54Te6, Cs0.72K0.67Bi3.54Te6, Cs0.64K0.74Bi3.55Te6, Cs0.63K0.79Bi3.54Te6, and Cs0.58K0.83Bi3.53Te6 using the Werthamer−Helfand−Hohenberg (WHH) theory and the Ginzburg−Landau (GL) theory (PDF) Crystallographic data in CIF format (ZIP)
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AUTHOR INFORMATION
Corresponding Author
*(M.G.K.) E-mail:
[email protected]. ORCID
Haijie Chen: 0000-0003-3567-1763 Jin-Ke Bao: 0000-0001-5522-3605 Constantinos C. Stoumpos: 0000-0001-8396-9578 Duck Young Chung: 0000-0002-1315-2631 Mercouri G. Kanatzidis: 0000-0003-2037-4168 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was primarily performed in the Materials Science Division of Argonne National Laboratory and supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division. SEM/ EDS was conducted by the use of the EPIC, Keck-II, and/or SPID facility(ies) of Northwestern University’s NUANCE Center, which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-1542205); the MRSEC program (NSF DMR1121262) at the Materials Research Center; the International Institute for Nanotechnology (IIN); and the Keck Foundation; and the State of Illinois, through the IIN.
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CONCLUSIONS CsBi4Te6 is a narrow-gap, low crystal symmetry semiconductor that exhibits superconductivity through an extensive range of alloying. With the exception for low K (y ≤ 0.18), the monoclinic CsBi4Te6 structure is destabilized and transforms to the very different orthorhombic RbBi3.67Te6-type structure with Na or K doping with a concomitant change in dominant 5302
DOI: 10.1021/acs.chemmater.8b02030 Chem. Mater. 2018, 30, 5293−5304
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