Superconductivity in Dense Rashba Semiconductor BiTeCl Jian-Jun Ying,1,2 Viktor V. Struzhkin,2 Alexander F. Goncharov,2 Ho-Kwang Mao,2,1 Fei Chen,3,1 Xian-Hui Chen,3,4 Alexander G. Gavriliuk,5,6 and Xiao-Jia Chen1,2,∗ 1Center for High Pressure Science and Technology Advanced Research, Shanghai 201203, China 2Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC 20015, U.S.A. 3Hefei National Laboratory for Physical Sciences at Microscale and Key Laboratory of Strongly-Coupled Quantum Matter Physics, 5 Chinese Academy of Sciences, School of Physical Sciences, 1 University of Science and Technology of China, Hefei 230026, China 0 4Collaborative Innovation Center of Advanced Microstructures, Nanjing 210093, China 2 5Institute of Crystallography, Russian Academy of Sciences, Leninsky pr. 59, Moscow 119333, Russia n 6Institute for Nuclear Research, Russian Academy of Sciences, Troitsk, Moscow 142190, Russia a (Dated: January 27, 2015) J Layered non-centrosymmetric bismuth tellurohalides are being examined as candidates for topo- 5 logical insulators. Pressure is believed to be essential for inducing and tuning topological order in 2 these systems. Through electrical transport and Raman scattering measurements, we find super- conductivityin twohigh-pressure phasesof BiTeCl with thedifferent normal state features, carrier ] n characteristics, and upper critical field behaviors. Superconductivity emerges when the resistivity o maximum or charge density wave is suppressed by the applied pressure and then persists till the c highestpressureof51GPameasured. Thehugeenhancementoftheresistivitywiththreemagnitude - of orders indicates the possible achievement of the topological order in the dense insulating phase. r p Thesefindingsnotonlyenrichthesuperconductingfamily fromtopological insulatorsbutalsopave u theroad on thesearch of topological superconductivityin bismuth tellurohalides. s . PACSnumbers: 74.62.Fj,74.25.Dw,74.70.-b t a m - Topological insulators represent the newly discovered resolved photoemission spectroscopy (ARPES) experi- d phase of matter with insulating bulk state but topolog- ment [24]. This was soon supported by the transport n ically protected metallic surface state due to the time- measurement[25]. Unlikethepreviouslydiscoveredthree o reversal symmetry and strong spin-orbital interaction dimensionaltopologicalinsulatorswithinversionsymme- c [ [1, 2]. Searching for topological superconductivity is one try, the inversion symmetry is naturally broken by the ofthehottesttopicsduetotheexplorationoffundamen- crystal structure in IATI. It is highly possible to realize 1 tal physics and the potential applications in topological the topological magneto-electric effects and the topolog- v 3 quantumcomputation[3,4]. Superconductivityhasbeen ical superconductivity [24, 26, 27]. However, quantum 0 found in some topological insulators such as compressed oscillationmeasurementsexcludedthe existenceofDirac 2 A B compoundsincludingBi Te [5–9],Bi Se [10,11], surface state in BiTeCl single crystals [28, 29]. Such 2 3 2 3 2 3 6 Sb Te [12], and Sb Se [13] and substituted Cu Bi Se contradictionmay come fromthe strongsurface polarity 0 2 3 2 3 x 2 3 [14] together with related materials such as YPtBi [15]. whichwouldgeneratelargeeffective pressurealongthe c . 1 However,theidentificationoftheirtopologicalsupercon- axis. This pressure could drive the severalsurface layers 0 ductivity is still a hard task and under debate [7, 9]. In into a topological insulator as the case in BiTeI [21, 24]. 5 1 most cases, pressure is needed to drive topological in- Therefore, pressure must be used in experiments to ex- : sulators to superconductors. Superconductivity is usu- aminewhethertopologicalphasetransitioncouldhappen v ally accompaniedby the electronic topologicaltransition in this kind of materials and whether superconductivity i X and/or structural transition [6, 11]. It remains unclear would be induced after that. Meanwhile, the discoveries r whether sucha transitionis essentialfor inducing super- of pressure-induced superconductivity from topological a conductivity in topological insulators. insulators were limited in A B -type compounds. Find- 2 3 ing new superconducting family from topological insula- The class of non-centrosymmetric bismuth telluro- tors wouldadd a new opportunity for exploring topolog- halides (BiTeX with X=Cl, Br, I) exhibit large Rashba- ical superconductivity. type splittings in the bulk bands [16–20], and they are potential candidates for building the spintronic de- In this Letter, we address the above mentioned issues vices. Pressure-inducedtopologicalquantumphasetran- by investigating the high-pressure behaviors of BiTeCl sitionwaspredictedforRashbasemiconductorBiTeI[21]. throughthecombinationoftheelectricaltransportprop- However, controversial conclusions were drawn from the erties and Raman scattering measurements. A huge en- followingexperimentsonthismaterial[22,23]. Recently, hancement of the resistivity with three magnitude of BiTeCl was discovered to be the first example of inver- orders is observed with the application of pressure fol- sionasymmetrictopologicalinsulator(IATI)fromangle- lowed by a phase transition at relatively low pressure of 2 6.5 GPa 8.4 GPa 1000 6.5 GPa 1000 11.0 GPa ()mcm100 4.7 GPa ()mcm100 11119753....1031 GGGGPPPPaaaa 2T 2.7 GPa 3 T 24.1 GPa 10 0.6 GPa 10 )m 5 T 50.1 GPa (a) (b) 0 100 200 300 0 100 200 300 c T) 100 19.1 GPa Tempera ture (K) 2 Tempera ture (K) 32.9 GPa m H (C2 ( 20.8 GPa 35.8 GPa 1 T 0.5 T 0 T 22.4 GPa 38.8 GPa ()mcm 10 22229864....5041 GGGGPPPPaaaa ()mcm1 444320...611 GGGPPPaaa TC (K) 31.0 GPa 4486..61 GGPPaa 50.8 GPa (c) (d) 10 100 200 300 00 100 200 300 Temperature (K) Temperature (K) Temperature (K) FIG. 1: (Color online) Temperature dependence of the resis- FIG. 2: (Color online) Temperature dependence of the re- tivity of BiTeCl at various pressures. (a) Thesudden change sistivity of BiTeCl at various magnetic fields and pressure of of the resistivity behavior indicates a phase transition from 50.8 GPa. Inset: Upper critical field Hc2 for the pressure of Rashba semiconductor to insulator around 5 GPa. (b) The 24.1GPaand50.8GPa,respectively. Tcwasdeterminedfrom resistivity was gradually suppressed with increasing pressure the 90% resistivity transition. The color area represents the and superconductivity emerges above 13 GPa. The normal calculated Hc2 from WHH theory. statebehaveslikeaninsulator(c)andametal(d)uponheavy compression. resistivity maximumaround150K [Fig. 1(a)]. It clearly indicates a phase transition from a Rashba semiconduc- 5 GPa, pointing to the possible realization of topologi- tor to an insulator around 5 GPa in BiTeCl. Further cal order in the insulating phase suggested from recent increasing pressure, the resistivity is gradually reduced experiment. Two following superconducting phases are and the resistivity maximum shifts downwards to lower discovered with the highest critical temperature Tc of 7 temperatures. Interestingly, superconductivity emerges K at 15 GPa. These results indicate that high-pressure when the resistivity maximum is completely suppressed study offers a new opportunity for uncovering the novel around13GPa,thoughthenormalstatestillexhibitsan physical properties in bismuth tellurohalides. insulating behavior. This behavior is different from the High quality single crystals of BiTeCl were grown by other superconducting phases obtained from topological a self-flux method [20]. Pressure was applied at room insulatorsunderpressure[6,11]. Furtherincreasingpres- temperature using the miniature diamondanvilcell [30]. sure, the normal state behaves gradually in a metallic Diamond anvil with 300 µm culet and c-BN gasket with wayabove28GPa. Superconductivity persists up to the sample chambers of diameter 130µm were used. BiTeCl highestpressure of50.8 GPa studied. Superconductivity single crystal was cut with the dimensions of 65×65×15 in compressed BiTeCl has never been reported before. µm3. Four Pt wires were adhered to the sample using Such a discovery with novel normal-state properties in the silver epoxy. Daphne oil 7373was usedas a pressure BiTeCl enriches the physics besides large Rashba-type transmitting medium. Pressure was calibrated by using splittings and topological properties. therubyfluoresceshiftatroomtemperature. Resistivity The obtainedsuperconductivityinBiTeClwasfurther and Hall coefficient were measured using the Quantum supportedbytheevolutionoftheresistivity-temperature Design PPMS-9. Diamond anvil with 300 µm culet was curve with the applied magnetic fields (Fig. 2). The used for high-pressure Raman measurements with inci- curve gradually shifts towards the lower temperatures dent laser wavelength of 488 nm. Neon was loaded as with increasing magnetic field. The magnetic field was the pressure transmitting medium. applied along the crystallographic c axis. It seems likely Figure 1 shows the temperature dependence of the re- that 5 T is sufficient to suppress superconductivity at sistivity for BiTeCl at various pressures up to 50.8 GPa. 50.8 GPa. However, much higher field is needed to sup- At low pressures below 4 GPa, the resistivity shows a press superconductivity at 24.1 GPa. These two pres- metallic behavior similar to that at ambient pressure as sures yield different normal-state behaviors − metallic reportedbefore[31,32],thoughthismaterialwasthought for the former but insulating for the latter (Fig. 1). The to be a Rashba semiconductor. When pressure is in- upper critical field Hc2 can be determined from these creased to 5 GPa, the resistivity suddenly increases al- measurements. most 3 orders, and shows an insulating behavior with a Withintheweak-couplingBCStheory,theuppercriti- 3 I II III 39.4 GPa 37.4 GPa K) 32.0 GPa (C T s) 26.3 GPa nit u sity (Arb. 1181..39 GGPPaa SC en 1 Int 8.5 GPa 1000 5.7 GPa 0 C) 4.8 GPa cm)100 -1 3 mm/ 2.6 GPa m 2 10 ( 10 (H 50 100 150 200 250 300 -2 R -1 Raman shift (cm ) 1 -3 0 10 20 30 40 50 FIG. 3: (Color online) Raman spectra of BiTeCl measured under pressure. The vibration modes in phase I are marked Pressure (GPa) byarrows. Thedifferentspectraabove5GPaandtheabsence of Raman peaks above39 GPa indicate two different phases. FIG. 4: (Color online) Phase diagram of BiTeCl under pres- sure (upper panel) and pressure dependence of the resistiv- ity and Hall coefficient measured at 10 K (lower panel). The calfieldatT=0KcanbedeterminedbytheWerthamer- dashedregionsrepresentthebordersoftwophasetransitions. Helfand-Hohenberg (WHH) equation [33]: H (0) = Two high-pressurephases havelarge overlapping region. c2 0.693[−(dH /dT)] T . The grey and red areas shown c2 Tc c inthe insetofFig. 2aretemperaturedependence ofH c2 calculated based on the WHH theory for the supercon- modes in phase I below 5 GPa. The obtained modes ductivityat24.1and50.8GPa,respectively. TheH (0) marked by the arrows are also similar to those reported c2 at 50.8 GPa is about 3.5 T. This value is comparable previously at ambient pressure [32, 35]. withthatofsuperconductingBi2Se3 phase[11]. Thecal- Above5 GPa,the spectrasuddenly change,indicating culated Hc2(0) of almost 8 T at 24.1 GPa is twice larger the appearance of a new high-pressure phase. The sim- than that at 50.8 GPa. The large difference of Hc2(0) at ilar phase transition to an orthorhombic Pnma struc- 24.1and50.8GPaindicatesthedifferentoriginsofsuper- ture has been reported in the sister system BiTeI [36]. conductivity of two high-pressure phases. Differing from ThePnmastructurehasmoreRamanvibrationalmodes A2B3-type topologicalmaterials, the normal state of su- (6Ag+3B1g+6B2g+3B3g). This phase was predicted to perconductingBiTeClshowsbothinsulatingandmetallic be a semiconductor in BiTeI [36]. The temperature de- behaviors with different values of Hc2(0). pendence of the resistivity of BiTeCl shown in Fig. 1(b) Raman spectroscopy is a very powerful tool to probe exhibits a more complicated feature. There is a maxi- the changes in lattice and thus can provide valuable in- mumaround150Kforthepressureof6.5GPa. Applying formationonstructure. Figure3showsthe Ramanspec- pressure shifts it down to lower temperatures. However, tra of BiTeCl at various pressures up to 39.4 GPa. At the maximum feature is suppressed when superconduc- ambient pressure, BiTeCl crystalizes in a trigonal layer tivity appears by applied pressure. The emergence of structurewithspacegroupofP6 mc(phaseI)[34]. This superconductivity and the destruction of the resistivity 3 phase has seven Raman active modes (2A +2E +3E ). maximum at the same pressure indicate a close connec- 1 1 2 Both E-type modes belong to the in-plane vibration of tionbetweenthem. Theresistivitymaximumisthecom- the Bi, Te, and Cl layers, while the lower E mode has a monfeatureforchargedensitywave. Thissuperconduct- largecontributionoftheClatomvibration. Thetwoout- ingphaseis possibletocome afterthe suppressionofthe of-plane A modes include the vibrationwith higher fre- charge density wave. The evolution of charge density 1 quencies. Our Raman measurements produced all these wave with pressure has been observed in 1T-TiSe [37]. 2 4 Above 39 GPa, no Raman modes were detected, sug- and holes and the formation of Bose-condensate of the gesting a phase transition to a high symmetric structure electron-hole pairs. For all the features observed,partic- with a possible cubic unit cell. This is consistent with ularly for the evolution of the resistivity maximum with the obtained metallic normal state of this high-pressure pressureandtheemergenceofsuperconductivity,BiTeCl phase(phaseIII)fromtheresistivitymeasurements[Fig. is considered as a new excitonic insulator and excitonic 1(d)]. Previously, an orthorhombic P4/nmm structure superconductor. Meanwhile,the Rashbasemiconducting has been suggested for phase III of BiTeI from com- phase at ambient pressure can be tuned into an insu- bined XRD measurements and theoretical calculations lating phase above 5 GPa. The huge enhancement of [36]. This P4/nmm structure corresponds to more rich the resistivity with three magnitude of orders in phase I Raman active modes with nine E modes besides A , indicates the possible realization of the topological insu- g 1g B ,andB . OurRamandatadoesnotsupporttheex- latoruponcompressionsuggestedfromearlyexperiment. 1g 2g istenceoftheP4/nmmstructureforBiTeCl. Themetal- Although the estimated pressure of the surface polarity lic phaseofBiTeClis possiblyasubstitutionalalloysim- along the c axis for BiTeCl is only about 1 GPa in the ilar to Bi Te system [38]. The Raman spectra provide ARPES measurement [24], our measurements were per- 2 3 evidence for the existence of two phase transitions and formedatthequasi-hydrostaticpressureconditionwhich the features of different phases in BiTeCl. isquitedifferentwiththe nonhydrostaticenvironmentin Combining the resistivity and Raman measurements, the ARPES experiment. The pressure-induced topolog- we can map out the phase diagram of BiTeCl (Fig. 4). ical phase transition at 4-5 GPa has also been observed Below5GPa,BiTeClkeepsitsinitialphaseIasaRashba in the sister BiTeI [36, 39]. These results together con- semiconductor but behaves in reality like a metal or tribute BiTeCl the possible topological feature in its in- semimetal [Fig. 1(a)]. Above that, it evolves to a su- sulating state and even superconducting state. perconductor with non-metallic normal state in phase II In conclusion, we have reported an experimental find- up to the phase boundary starting at 28 GPa. Above 39 ing of superconductivity in dense Rashba semiconductor GPa, superconductivity in phase III remains unchanged BiTeCl. Superconductivityemergesafterthesuppression with pressure but the material possesses a metallic nor- oftheresistivitymaximumorchargedensitywavearound malstate. T exhibitsadome-likeshapewithpressurein c 13GPaandpersistsabove51GPa. The interestingelec- phase II, while it becomes almost constant upon heavy tricaltransportpropertiesandthecrossoverofthecarrier compression in phase III. Accompanying the T change, c characteristicsupon compressionmake BiTeCl to be the the sign of the measured Hall coefficient at 10 K grad- candidate for a new excitonic insulator and excitonic su- ually changes from negative to positive above 28 GPa perconductor. The significant enhancement of the resis- (lowerpanelofFig. 4). Thisprovidesthedominantelec- tivity in the insulating phase indicates the possible real- tron and hole carrier character for the superconducting ization of the topological order. These results demon- phase II and III. The large hole carrier density in phase strate that BiTeCl possesses novel physical properties III is consistent with the character of the substitutional under pressure besides the extraordinary large Rashba- alloyindicatedfromRamanmeasurement. Therelatively typesplittingsandtopologicalproperties. Ourworkthus weakpressuredependence ofthecarrierdensityprovides paves the road on searching topological superconductiv- a natural explanation for the almost constant T in this c ity in BiTeX (X=Cl, Br, I) compounds. phase. When examining the electrical transportproperties at ThisworkwassupportedbyEFree,anEnergyFrontier differenthigh-pressurephasesofBiTeCl,onecanseethat Research Center funded by the U.S. Department of En- the so-called Rashba semiconducting phase at ambient ergy(DOE),OfficeofScience,OfficeofBasicEnergySci- pressure behaves like a metal [31, 32] or a semimetal ences (BES) under Award Number DE-SG0001057. The due to the apperrance of the resistivity maximum [Fig. resistance measurements were supported by the DOE 1(a)]. Superconductivity emerges when the resistivity under Grant No. DE-FG02-02ER45955. Raman mea- maximum or charge density wave is suppressed by ap- surements were supported by the U.S. National Science plied pressure. The superconducting phase II possesses Foundation Earth Sciences Instrumentation and Facil- an insulating or a semiconducting normal state [Figs. ities (EAR/IF) and DARPA. The sample design and 1(b) and 1(c)]. This dramatic change of the electrical growth were supported by the National Natural Sci- transport with pressure is also reflected by the resistiv- ence Foundation of China (Grant No. 11190021), the ity value. The lower panel of Fig. 4 also summarizes “Strategic Priority Research Program (B)” of the Chi- the resistivity measured at 10 K. An anomaly is clearly nese Academy of Sciences (Grant No. XDB04040100). observed across the phase boundary of phase I and II. A.G.G. acknowledges the support from Russian Foun- The semiconductor-semimetal transition serves a possi- dation for Basic Research (Grant No. 14-02-00483-a), bility for an excitonic insulator. 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