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Author / Affiliation / Email Article Menu Font Type Arial Georgia Verdana Open AccessArticle 1 Lebedev Physical Institute of the RAS, 119991 Moscow, Russia 2 National Research University Higher School of Economics, 101000 Moscow, Russia * Author to whom correspondence should be addressed. Received 21 October 2019 / Revised 11 November 2019 / Accepted 21 November 2019 / Published 26 November 2019 Abstract In this study, we grew Cu co-doped single crystals of a topological superconductor candidate Sr x Bi 2 Se 3 , and studied their structural and transport properties. We reveal that the addition of even as small an amount of Cu co-dopant as atomic %, completely suppresses superconductivity in Sr x Bi 2 Se 3 . Critical temperature ∼ K is rather robust with respect to co-doping. We show that Cu systematically increases the electron density and lattice parameters a and c. Our results demonstrate that superconductivity in Sr x Bi 2 Se 3 -based materials is induced by significantly lower Sr doping level x , superconductivity weakens and eventually disappears. Formation of a superconductive phase is shown to be related to some internal strain with a threshold-in-y character and triggers superconducting pairing mechanisms [33]. Two methods to obtain Cu-intercalated crystals—direct melt growth [8,29] and subsequent electrochemical intercalation [14,15] both produce rather nonuniform samples [11], with different superconductive volume x Bi 2 Se 3 is a different system, where already for small x . This small concentration is limited by the solubility of Sr atoms in the crystalline phase. All extra Sr, as seen from our EDX studies, drops into minority phases and plays no role in superconductivity. Fermi level is pinned by Sr in the bulk, and carrier density is rather small ∼ 2 × 10 19 cm − 3 .When Sr and Cu are superimposed in Bi 2 Se 3 matrix, and the amount of Cu is small y below these elements seems to act independently Cu—as an n-type dopant, as in Cu y Bi 2 Se 3 [29,34], and Sr—as a reason for superconductivity. Our paper shows that as the Cu content increases above that value, it suppresses superconductivity, probably due to its influence on Sr on our observations, along with the previous Hall measurements in Sr-doped Bi 2 Se 3 system, we suggest a "dome-like" phase diagram, shown in Figure 6b, where the horizontal axis is the Hall-effect electron density in Sr x Bi 2 Se 3 system, and the vertical axis is the temperature. In addition to our data, we plotted all available results on T c n H a l l in Sr x Bi 2 Se 3 [16,17] and Sr x Bi 2 Se 3 − y S y [19]. Despite intensive studies of this material, rather limited Hall data were published. [27] reported electron density significantly exceeding consolidated results published by all the other groups. Unfortunately, measurement details were not published in the paper. So, we disregard deviating results of [27] study demonstrates that we managed to add higher density points to this dome marked by arrows in Figure 6b. However, an extra copper definitely suppresses superconductivity. This suggests that the role of Cu co-dopants is two-fold they add up carriers and influence Sr atom arrangement. When the effect on Sr subsystem becomes too strong, superconductivity observation agrees well with recently confirmed low- k z -phonon mediated pairing mechanism [26]. Indeed, Sr, presumably hosting the Van der Waals gaps [16,17,28] provides some inter-layer bonds. This coupling, in turn, determines the phonon dispersion, and ultimately, electron-phonon interaction. As Cu intercalates the system, it perturbs the inter-layer data also suggest that that Cu promotes incorporation of Sr into the Bi 2 Se 3 lattice during the growth from the melt. Indeed, according to Crystal composition EDX data Table 1, Sr content enhances with addition of Cu by approximately 25% for Cu Sr Bi 2 Se 3 with respect to non-co-doped sample. Our X-ray data see Figure 3e show that Cu content y manages c-lattice parameter of Bi 2 Se 3 to grow even stronger than Sr composition x does alone. The available data on the direct effect of sole Cu on c-lattice parameter of Bi 2 Se3 are rather ambiguous in [8,15] c-parameter grows essentially with addition of Cu; in [35] it almost remains unchanged; in [36] it even decreases. Probably, this ambiguity is related to amphoteric character of Cu-dopant [29] and its small ionic radius. In our case of Sr-doped Bi 2 Se 3 , we believe, therefore, that the growth of c-lattice parameter, as well as all other y-dependent properties at least partially originate from surplus of Sr content in the lattice, provoked by also should discuss here the absence of T c n -dependence. According to Bardeen–Cooper–Schreifer BCS-theory, T c should grow dramatically with density of states, as it takes place, in superconductive cuprates. Correspondingly, there are two options. Either the pairing mechanism is unusual that was suggested in [26], or density of states weakly depends on the Fermi level. The latter case corresponds to quasi-two-dimensional, cylinder-like Fermi surface. Such Fermi surface topology has already been observed in superconductive Cu x Bi 2 Se 3 [30,34] with larger carrier density. We expect that in Sr x Bi 2 Se 3 quasi-two-dimensional Fermi-surface may also have to discuss here another relative superconducting material Nb z Bi 2 Se 3 [37,38]. SC in this material is air-stable, similarly to Sr x Bi 2 Se 3 . Electron density is about 2 × 10 20 cm − 3 , similarly to Cu y Bi 2 Se 3 . The experimental manifestations of this system are somewhat controversial. [38] reports crystals with a very pronounced "nematic" phenomenology, similarly to the best Sr x Bi 2 Se 3 [20,25]. However, this research does not contain any structural XRD data; that makes direct comparison with Sr x Bi 2 Se 3 system problematic. [37], on the contrary, reports multi-phase crystals. These crystals demonstrate a growth of c lattice parameter with z d c / d z = 2 pm/% for nominal z < and unchanged a-parameter, similar to our observations for Sr x Bi 2 Se 3 . The similarities point to the common predominant metal impurity location in the Van der Waals gap as an important ingredient required for intercalating dopant natures of Sr, Cu and Nb impurities in the Bi 2 Se 3 matrix seems to be the key factor for superconductivity. Indeed, many other cationic impurities in Bi 2 Se 3 , such as Ag [39] or In [40], readily substitute for bismuth and do not bring superconductivity at ambient pressure. 5. ConclusionsIn our study, we tried Cu co-doping as a novel strategy to tune the properties of topological superconductor candidate Sr x Bi 2 Se 3 . Cu co-dopants were shown to serve as additional electron donors. However, increase of carrier concentration did not affect the critical temperature, indicating an unusual character of SC in the system, in-line with numerous experiments by the other groups. We have shown that the crystals are non-uniform and typically contain micrometer-sized, Sr-enriched inclusions. Real Sr concentration within the crystalline body is rather small ∼ atomic %, and as the atomic concentration of Cu co-dopant exceeds this value, superconductivity is suppressed. We relate this effect to structural modification caused by Cu in the Sr–Bi–Se subsystem. Author supervision and writing—original draft preparation; and conceptualization; and resources crystal growth; XRD investigations; and transport investigations; SEM and EDX investigations; all authors, writing—review and work was funded by the Russian Science Foundation grant number 17-12-01544.AcknowledgmentsMagnetotransport, XRD and SEM/EDX measurements were performed using the equipment of the LPI shared facility center. The authors are thankful to G. Dimitrakopulos Aristotle University of Thessaloniki, Greece for the preliminary EDX analysis of our samples, which motivated us to do thorough EDX of InterestThe authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the following abbreviations are used in this manuscriptSCSuperconductivityXRDX-ray diffractionEDXEnergy-dispersive X-ray spectroscopySEMScanning Electron MicroscopeRRRResidual Resistivity Ratio, ratio of the room-T resistivity to the lowest value at low-T yet above SC.BCSBardeen–Cooper–SchreiferReferencesQi, Y.; Naumov, Ali, Rajamathi, Schnelle, W.; Barkalov, O.; Hanfland, M.; Wu, Shekhar, C.; Sun, Y.; et al. Superconductivity in Weyl semimetal candidate MoTe2. Nat. Commun. 2016, 7, 11038. [Google Scholar] [CrossRef] [PubMed]Novak, M.; Sasaki, S.; Kriener, M.; Segawa, K.; Ando, Y. Unusual nature of fully gapped superconductivity in In-doped SnTe. Phys. Rev. B 2013, 88, 140502R. 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SEM images with 100× magnification of the 001 cleaved surface of the pristine Bi 2 Se 3 —sample 272 a, Sr Bi 2 Se 3 —sample 306 b, and Cu Sr Bi 2 Se 3 —sample 328 c. Figure 2. SEM image at 250× magnification a for sample 328 with nominal composition Cu Sr Bi 2 Se 3 . Green crosses show the examples of the points where crystal phase composition was analysed. The insert shows an example of a zoom-in of a relevant region. Panels b–e show the corresponding EDX element distribution maps b Bi map; c Se map; d Sr map; e Cu map. Blue circles highlight the representative domains where anti-correlatiosn between Sr and Bi distribution are seen. The maps were collected for 48 h. Figure 2. SEM image at 250× magnification a for sample 328 with nominal composition Cu Sr Bi 2 Se 3 . Green crosses show the examples of the points where crystal phase composition was analysed. The insert shows an example of a zoom-in of a relevant region. Panels b–e show the corresponding EDX element distribution maps b Bi map; c Se map; d Sr map; e Cu map. Blue circles highlight the representative domains where anti-correlatiosn between Sr and Bi distribution are seen. The maps were collected for 48 h. Figure 3. a–c 2 θ / -scans for three representative samples a 272 Bi 2 Se 3 , b 317 nominal composition Sr Bi 2 Se 3 and c 320 nominal composition Cu Sr Bi 2 Se 3 . d Rocking curve for the multi-block 325 sample nominal composition Cu Sr Bi 2 Se 3 taken at two reflections—0 0 6 red line and 0 0 15 wine line. The arrows indicate the angular positions of different blocks. e 2 θ / scans at 0 0 15 reflection for different crystals nominal compositions are indicated in the panel. Figure 3. a–c 2 θ / -scans for three representative samples a 272 Bi 2 Se 3 , b 317 nominal composition Sr Bi 2 Se 3 and c 320 nominal composition Cu Sr Bi 2 Se 3 . d Rocking curve for the multi-block 325 sample nominal composition Cu Sr Bi 2 Se 3 taken at two reflections—0 0 6 red line and 0 0 15 wine line. The arrows indicate the angular positions of different blocks. e 2 θ / scans at 0 0 15 reflection for different crystals nominal compositions are indicated in the panel. Figure 4. Lattice parameters c panel a and a panel b as a function of Cu nominal content y. Figure 4. Lattice parameters c panel a and a panel b as a function of Cu nominal content y. Figure 5. a Resistivity as a function of temperature for representative samples from our study nominal compositions are indicated in the panel; b zoom-in of low-temperature region, where SC transition occurs; c photo of the sample, mounted for resistivity and Hall-effect measurements. Figure 5. a Resistivity as a function of temperature for representative samples from our study nominal compositions are indicated in the panel; b zoom-in of low-temperature region, where SC transition occurs; c photo of the sample, mounted for resistivity and Hall-effect measurements. Figure 6. a Electron density as a function of the nominal co-dopant content y in our Cu y Sr x Bi 2 Se 3 samples empty boxes compared with Cu y Bi 2 Se 3 reflectivity data of [29] stars. The straight and dashed lines are just guides to the eye to demonstrate a tendency for the density to grow with y. b Phase diagram of superconductivity T c versus electron density according to our results and [16,17,19]. Figure 6. a Electron density as a function of the nominal co-dopant content y in our Cu y Sr x Bi 2 Se 3 samples empty boxes compared with Cu y Bi 2 Se 3 reflectivity data of [29] stars. The straight and dashed lines are just guides to the eye to demonstrate a tendency for the density to grow with y. b Phase diagram of superconductivity T c versus electron density according to our results and [16,17,19]. Table 1. Summary of crystal structure and transport parameters. NSC means that the crystal is not superconductive. Electron density and mobility are collected at T = 4 K. Table 1. Summary of crystal structure and transport parameters. NSC means that the crystal is not superconductive. Electron density and mobility are collected at T = 4 K. Sample Nom. Compositionn,10 19 cm − 2 μ , cm 2 /Vs T c , KRRR0015 c, Å205 a, ÅAvg. CompositionCrystal Composition272Bi 2 Se 3 Se 3 Bi Se 3 306Sr Bi 2 Se 3 Bi Se 3 Sr Bi Se 3 317Sr Bi 2 Se 3 Bi Se 3 Sr Bi Se 3 329Cu Sr Bi 2 Se 3 Sr Bi Se 3 Cu Sr Bi Se 3 325Cu Sr Bi 2 Se 3 Sr Bi Se 3 Cu Sr Bi Se 3 324Cu Sr Bi 2 Se 3 Sr Bi Se 3 Cu Sr Bi Se 3 328Cu Sr Bi 2 Se 3 Sr Bi Se 3 Cu Sr Bi Se 3 320Cu Sr Bi 2 Se 3 Sr Bi Se 3 Cu Sr Bi Se 3 Table 2. Summary on EDX compositional data in atomic % taken at six representative points P1–P6 within the regions with perfect morphology of the 001 surface for the most disordered sample—number 320 nominal composition Cu Sr Bi 2 Se 3 . Table 2. Summary on EDX compositional data in atomic % taken at six representative points P1–P6 within the regions with perfect morphology of the 001 surface for the most disordered sample—number 320 nominal composition Cu Sr Bi 2 Se 3 . Element, TermP1P2P3P4P5P6AverageErrCu © 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution CC BY license Share and Cite MDPI and ACS Style Kuntsevich, Martovitskii, Rybalchenko, Selivanov, Bannikov, Sobolevskiy, Chigevskii, Superconductivity in Cu Co-Doped SrxBi2Se3 Single Crystals. Materials 2019, 12, 3899. AMA Style Kuntsevich AY, Martovitskii VP, Rybalchenko GV, Selivanov YG, Bannikov MI, Sobolevskiy OA, Chigevskii EG. Superconductivity in Cu Co-Doped SrxBi2Se3 Single Crystals. Materials. 2019; 12233899. Chicago/Turabian Style Kuntsevich, Aleksandr Yu., Victor P. Martovitskii, George V. Rybalchenko, Yuri G. Selivanov, Mikhail I. Bannikov, Oleg A. Sobolevskiy, and Evgenii G. Chigevskii. 2019. "Superconductivity in Cu Co-Doped SrxBi2Se3 Single Crystals" Materials 12, no. 23 3899. Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here. Article Metrics

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