Superconductivity, electronic phase diagram, and pressure effect in Sr1−xPrxFBiS2

Wei You, Lin Li, HaiYang Yang, JiaLu Wang, HongYing Mao, Li Zhang, ChuanYing Xi, Jie Cheng, YongKang Luo, JianHui Dai and YuKe Li

Citation: SCIENCE CHINA Physics, Mechanics & Astronomy 62, 957411 (2019); doi: 10.1007/s11433-018-9326-4

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•Article• May 2019 Vol. 62 No. 5: 957411https://doi.org/10.1007/s11433-018-9326-4

Superconductivity, electronic phase diagram, and pressure effect inSr1−xPrxFBiS2

Wei You1, Lin Li1, HaiYang Yang1, JiaLu Wang1, HongYing Mao1, Li Zhang5, ChuanYing Xi3,Jie Cheng4, YongKang Luo2, JianHui Dai1*, and YuKe Li1*

1 Department of Physics and Hangzhou Key Laboratory of Quantum Matter, Hangzhou Normal University, Hangzhou 311121, China;2 Wuhan National High Magnetic Field Center School of Physics, Huazhong University of Science and Technology, Wuhan 430074, China;

3 High Magnetic Field Laboratory, Chinese Academy of Sciences, Hefei 230031, China;4 College of Science, Center of Advanced Functional Ceramics, Nanjing University of Posts and Telecommunications, Nanjing 210023, China;

5 Department of Physics, China Jiliang University, Hangzhou 310018, China

Received September 12, 2018; accepted November 16, 2018; published online March 1, 2019

Based on a combination of X-ray diffraction, electrical transports, magnetic susceptibility, specific heat, and pressure-effectmeasurements, we report the results of experiments on a series of BiS2-based Sr1−xPrxFBiS2 superconductors with the maximumTc of 2.7 K for x=0.5 and at ambient pressure. Superconductivity appears only for 0.4≤x≤0.7 whereas the normal-state resistivityshows the semiconducting-like behaviors. The magnetic susceptibility χ(T) displays the low superconducting shielding volumefractions and C(T) shows no distinguishable anomaly near Tc, which suggests a filamentary superconductivity in the Pr-dopedpolycrystalline samples. By varying doping concentrations, an electronic phase diagram is established. Upon applying pressureon the optimally doped Sr0.5Pr0.5FBiS2 system, Tc is abruptly enhanced, reaches 8.5 K at the critical pressure of Pc=1.5 GPa, andincreases slightly to 9.7 K at 2.5 GPa. Accompanied by the enhancement of superconductivity from the low- to the high-Tcphases, the normal state undergoes a semiconductor-to-metal transition when under pressure. This scenario may be linked toenhanced overlap of the Bi-6p and S-p orbitals, which contributes to the enhanced superconductivity above Pc. The pressure-temperature phase diagram for Sr0.5Pr0.5FBiS2 is also presented.

BiS2-based superconductors, superconductivity, phase diagram, pressure effect

PACS number(s): 74.70.Xa, 74.25.F-, 74.62.Fj, 74.25.Dw

Citation: W. You, L. Li, H. Y. Yang, J. L. Wang, H. Y. Mao, L. Zhang, C. Y. Xi, J. Cheng, Y. K. Luo, J. H. Dai, and Y. K. Li, Superconductivity, electronic phasediagram, and pressure effect in Sr1−xPrxFBiS2, Sci. China-Phys. Mech. Astron. 62, 957411 (2019), https://doi.org/10.1007/s11433-018-9326-4

1 Introduction

The discovery of superconductivity with Tc=8.6 K [1] in thelayered crystal structure compound Bi4O4S3 has evokedconsiderable interest. Following this work, many BiS2-basedsuperconductors, including LnO1−xFxBiS2 (Ln=La, Ce, Pr,Nd) [2-6] with the highest Tc of 10 K, were reported and

studied. Similar to the copper-oxide and iron-based super-conductors with alternating stacks of superconducting andblocking layers, the crystal structure of BiS2-based com-pounds is composed of common superconducting BiS2 layersintercalated by various block layers; e.g., Bi4O4(SO4)1−x or[Ln2O2]

2−. Subsequently, another family of BiS2-based su-perconductors Sr1−xLnxFBiS2 (Ln=La, Ce) with the tetra-gonal crystal structure was synthesized and investigated[7,8]. The parent compound SrFBiS2 is a band insulator

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SCIENCE CHINAPhysics, Mechanics & Astronomy

*Corresponding authors (YuKe Li, email: yklee@hznu.edu.cn; JianHui Dai, email:daijh@hznu.edu.cn)

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without detectable antiferromagnetic transition or structuralphase transition [9,10]. Superconductivity with Tc=2.8 Kwas induced by rare-earth elements La or Ce doping into thelattice, but its normal state is semiconducting like, even inthe optimal superconducting sample [5,11]. The suppressionof the semiconducting behavior by increasing the carrierdensity was claimed to contribute to the enhanced super-conductivity [12]. Therefore, most studies of the BiS2-basedsystem mainly focused on the effect of chemical substitutionand the superconducting transition temperature [13].Pressure is considered as a clean way to tune the lattice

parameter and the electronic band structure, which can en-hance superconductivity and tune the normal-state char-acteristics. Among the BiS2-based superconductors,LaO1−xFxBiS2 is superconducting below Tc=2.6 K and am-bient pressure, but Tc reaches a maximum of 10 K at P=2 GPa [14]. This finding seems to be a universal feature andis also observed in most BiS2-based superconductors with adramatic enhancement of Tc under pressure [14-16]. Thisresult is possibly explained by X-ray diffraction, which in-dicates that the system undergoes a structural phase transi-tion from a tetragonal phase to a monoclinic phase at thecritical pressure [17]. Hall Effect measurements in Eu3F4-Bi2S4 also reveal a change in electronic structure across thesuperconducting phase under pressure [16]. At present,Sr1−xLnxFBiS2 (Ln=rare-earth elements) has exhibited un-ique properties but is less investigated [7]. An interestingobservation is that La doping can induce a semiconductor-to-metal transition in the Sr0.5La0.5FBiS2 system, accompaniedwith a change in sign of the Hall coefficient, although itsband structure is similar to that of LaO1−xFxBiS2 [10]. InSr0.5Ce0.5FBiS2, the diluted-Ce ions form ferromagnetic or-dering at about 7.5 K, and coexist with superconductivitybelow 2.8 K [8]. Therefore, studying the substitution effectfor other magnetic rare-earth elements such as Pr in SrFBiS2system is highly desirable, in particular for super-conductivity, normal-state properties, and the pressure effect.In this paper, we report the successful synthesis of Pr-

doped Sr1−xPrxFBiS2 (0≤x≤0.7) and investigate its super-conductivity, the detailed pressure effect, and its normal-state properties. The results indicate that Pr doping stronglydecreases the resistivity and induces superconductivity witha maximum Tc of 2.7 K for x≥0.4, but the normal state stillshows the semiconducting-like behaviors. Data on χ(T) andC(T) suggest that these superconducting samples shouldexhibit filamentary superconductivity because of the lowshielding-volume fractions and the absence of the super-conducting transition jumps in the specific heat. The presentexperimental results give the electronic state diagram. Anapplied pressure causes a rapid increase in Tc to 8.5 K at acritical pressure Pc=1.5 GPa, and then slightly increases to9.7 K at 2.5 GPa. Accompanied with superconductivitycrossing from low- to high-Tc phase, the normal state re-

sistivity undergoes a semiconductor-to-metal transition un-der pressure. Meanwhile, the thermal activation energy Egdecreases gradually to zero near Pc. The upper critical fieldBc2(0) at high pressure is over ten times greater than at am-bient pressure. The ratio Bc2(0)/Tc dramatically increasesfrom 0.6 T/K at P=0 to 2.6 T/K at P=2.5 GPa, which is incontrast with that observed in the isostructural EuFBiS2.Those results imply that the nature of superconductivity inboth the low- and high-Tc phases may be distinct in thepresent system. The low-Tc phase maintains filamentarysuperconductivity whereas the higher pressures significantlyenhance superconductivity in the high-Tc phase because ofthe structural phase transition.

2 Experimental

Polycrystalline samples of Sr1−xPrxFBiS2 were synthesizedby using a two-step solid-state-reaction method. The detailedsynthesis method is available [7]. The crystal structure wascharacterization by powder X-ray diffraction (XRD) at roomtemperature using a D/Max-rA diffractometer with Cu Kαradiation and a graphite monochromator. Lattice parameterswere obtained by Rietveld refinements. The (magneto) re-sistivity under several magnetic fields was measured byusing a standard four-terminal method over the temperaturerange 0.5-300 K in a commercial Quantum Design PPMS-9system with a 3He refrigeration insert. The temperature de-pendence of the dc magnetization was measured by using aQuantum Design SQUID-VSM-7T. The resistivity wasmeasured under pressure up to 2.5 GPa on a PPMS-9T byusing a HPC-33 piston-type pressure cell with the QuantumDesign DC resistivity and AC transport options. The samplewas immersed in a pressure-transmitting medium (DaphneOil) and was covered with a Teflon cap. Annealed Au wireswere fixed with silver epoxy to the contact surfaces on eachsample in a standard four-wire configuration.

3 Results and discussion

3.1 Electronic phase diagram and superconductivity inSr1−xPrxFBiS2 (0≤x≤0.7)

Figure 1 shows powder XRD patterns for the Sr1−xPrxFBiS2(0≤x≤0.7) samples. The main diffraction peaks in thesesamples can be well indexed based on a tetragonal crystalstructure with the P4/nmm space group. For x≥0.5, extra tinypeaks arise from the impurity phase of Bi2S3 [18], and thecontent gradually increases with Pr doping. About 70% Prdoping produces much more of the impurities phase, reach-ing the limit of the solid solubility of Sr/Pr. An enlargedimage for the (004) peak is shown in Figure 1(b). This peakclearly shifts toward high angles but retains its position when

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x>0.6, implying that Pr succeeds in doping into the latticeand that its maximum doping concentration is close to 0.7.Figure 1(c) shows the lattice constants refined by Rietveldstructural analysis. Compared with the mother compoundSrFBiS2 [7], the a axis decreases very slowly and almostbecomes constant in the high-doping regime, whereas the caxis rapidly decreases with Pr doping concentration. Thus,the cell volume decreases significantly. The remarkable de-crease in the lattice parameters indicates the successfulsubstitution of Sr by Pr, similar to the case of Ce doped inSr0.5Ce0.5FBiS2 [8] and LaO1−xFxBiS2 [2].The temperature dependence of resistivity ρ(T) in

Sr1−xPrxFBiS2 is mapped in Figure 2(a). For the parentcompound, the resistivity exhibits a semiconducting-likebehavior down to low temperatures. Upon Pr doping, theresistivity in the whole temperature regime decreases sharplyto 0.3 because of the introduction of extra electrons. Asx≥0.4, a sharp superconducting transition occurs at lowtemperature. The maximum Tc of 2.7 K occurs at x=0.5 inFigure 2(b); the normal-state resistivity exhibits the mini-mum values but retains its semiconducting-like character.Such a feature seems to be universal in BiS2-based super-conductors [2,5]. Increasing the Pr doping content to 0.7, thenormal-state resistivity reverses and begins to increase, andTc decreases significantly without zero resistivity down to2 K. By fitting with the thermal activation formulaρ(T)=ρ0exp(Eα/kBT) for temperatures ranging from 120 to

300 K, the thermal activation energy Eg, decreases graduallywith Pr doping. When x≥0.5, the increase in Eg and re-sistivity upon increasing the doping concentration can beascribed to increased disorder, impurity phases, and thegrain-boundary effect in the polycrystalline samples.Note that, in a previous report [19], superconductivity was

not seen in Sr0.5Pr0.5FBiS2 at ambient pressure. To determinethe bulk superconductivity in our sample, Figure 3(a) showsthe dc magnetic susceptibility with zero-field cooling (ZFC)modes under 5 Oe. Below Tc, the diamagnetic signal appearsclearly for x≥0.4. From ZFC data, the estimated super-conducting shielding volume fractions for all super-conducting samples are less than 25%, implying that thosesamples may exhibit filamentary or weak superconductivitythrough Pr doping, similar to the case for LaO0.5F0.5FBiS2[2]. A relatively small volume fraction is commonly found inBiS2-based polycrystalline samples, which we ascribe to alow-Tc phase [2,20] or to in-plane disorder [21].Figure 3(b) shows the magnetic susceptibility as a function

of temperature for 0.4≤x≤0.7. Overall, χ(T) for these sam-ples rapidly increases and follows the Curie-Weiss law uponcooling. Down to 2 K, no anomaly associated with themagnetic ordering of Pr moments appears. In contrast,Sr0.5Ce0.5FBiS2 displays ferromagnetic ordering at 7.5 K [8]and EuFBiS2 undergoes an antiferromagnetic transition at2.3 K [22]. Such divergence implies that Pr3+ ions with in-teger angular momentum (J=4) may result in a Kramers

Figure 1 (Color online) (a) Powder X-ray diffraction patterns for Sr1−xPrxFBiS2 (0≤x≤0.7) at room temperature. The * peak positions designate theimpurity phase of Bi2S3. (b) Enlarged plots of (004) peak. (c) Lattice constants vs Pr doping content.

Figure 2 (Color online) (a) Temperature dependence of resistivity for the Sr1−xPrxFBiS2 (0≤x≤0.7) samples. (b) Enlarged plot of resistivity around Tc.

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nonmagnetic ground state in those samples, similar to thecase of superconducting PrNiAsO [23]. According to theCurie-Weiss law: χ(T)=χ(0)+C/T−θ, where χ(0) is the tem-perature-independent term, C is the Curie-Weiss constant,and θ is the paramagnetic Curie temperature. We therefore fitχ(T) above 50 K. By subtracting the temperature-in-dependent term χ(0), the term (χ−χ0)

−1 is linear in tempera-ture (see Figure 3(b)), which suggests that the fit is reliable.As shown in Table 1, the fitted effective moments μeff(Pr) forx=0.4-0.7 approach the theoretical value of 3.57 for a freePr3+.Figure 4(a) shows specific-heat data for x=0.4-0.7 super-

conducting samples below 10 K. No peak originating fromthe superconducting transition or from magnetic ordering ofPr moments appears in the temperature regime studied,which confirms that Pr doping may induce filamentary su-perconductivity but does not lead to long-range magneticordering. The total specific heat can be written as C(T)=γ0T+βT 3+Cmag, where γ0 and β are coefficients of the electron andphonon contributions, while Cmag is the Pr-4f magnetic term.Considering the negligible magnetic contributions of Pr-4f,we fit the data and plot C(T)/T vs T 2 in Figure 4(b). Above7 K, a good linear fit for all samples is obtained, which yieldsthe Sommerfeld coefficient and the Debye temperature (θD)listed in Table 1. θD in the Pr-doped samples is much lessthan in Sr0.5(La,Ce)0.5FBiS2 (265 K, 220 K), which is con-sistent with the substitution of lighter Sr ions by heavier Prions. The derived γ0~154 mJ (mol K2) for x=0.5 is surpris-ingly large: 108 times that of Sr0.5La0.5FBiS2(1.4 mJ mol−1 K−2), and over twice that of Sr0.5Ce0.5FBiS2(58.6 mJ mol−1 K−2) and EuFBiS2 (73.3 mJ mol−1 K−2). Notethat a similar large γ0 was reported in the isostructurePrO0.5F0.5BiS2 superconductor (286.36 mJ mol−1 K−2) [6].The strongly enhanced Sommerfeld coefficient suggests thatthe main contributions originate from the hybridization be-tween conduction electrons and the Pr-4f electrons. Themagnetic entropy Smag can be obtained by integrating Cmag/Tover T up to 10 K (see result in the inset of Figure 4(b)). Thereleased entropy is about 2.2%-4.2% of the expected valuefor J=4 in the present samples. Unlike the ferromagnetic

order of Ce3+ in Sr0.5Ce0.5FBiS2 and the antiferromagneticorder of Eu2+ in EuFBiS2 with its large magnetic entropy, thePr-doped samples exhibit low magnetic entropy Smag (seeTable 1), which is reasonably consistent with the non-magnetic ground state in the present samples.Figure 5 shows the electronic phase diagram for the Pr-

doped system. At low Pr doping concentrations (x<<0.3), thesample is semiconductor like over the entire temperatureregime. Superconductivity starts to emerge only when x=0.4.For x>>0.4, the normal state becomes semiconducting like,and Tc attains its maximum value at x=0.5 and then decreasesslightly as x increases to 0.7. Conversely, the thermal acti-vation energy Eg estimated from the resistivity data decreasesrapidly in the low-doping regime (<<0.3), and increasesabnormally at x=0.4. The minimum Eg occurs at x=0.5,where Tc is the highest. Further increasing x to 0.7 causes Egto reverse and increase again. The combined change of Tcand Eg with Pr doping concentration suggests that super-conductivity competes with the semiconducting normal statein the present system and that 40% Pr doping may induce achange in the Fermi surface or a possible Lifshitz transition,leading to the appearance of superconductivity. Such achange also occurs in the Sr1−xLaxFBiS2 system [11]. Cor-respondingly, the ratio c/a of lattice parameters drops slightly

Figure 3 (Color online) (a) Magnetic susceptibility as a function of temperature under 5 Oe and (b) 1000 Oe magnetic fields with zero-field-cooling modefor Sr1−xPrxFBiS2 samples.

Table 1 Superconducting transition temperature Tc derived from (T),electronic coefficient γ0, θD, and magnetic entropy Smag obtained from zero-field specific-heat data for Sr1−xPrxFBiS2. As a comparison, the corre-sponding parameters for Sr0.5(La,Ce)0.5FBiS2 [7,8] and EuFBiS2 [22]samples are also summarized

x Tc(K)

γ0(mJ mol−1 K−2)

θD(K)

μeff(μB)

Smag(J mol−1 K−1)

0.4 2.23 104 206 3.45 0.76

0.5 2.72 154 215 3.47 0.45

0.6 2.65 128 196 3.49 0.42

0.7 2.44 162 207 3.52 0.4

Sr0.5La0.5 2.8 1.42 265 – –

Sr0.5Ce0.5 2.7 117.2 220 2.53 2.7

EuFBiS2 0.3 73.3 201 7.2 12.4

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and the resistivity ρ10K exhibits an anomaly at x=0.4 (seeFigure 5(a)), implying that a slight change in the lattice maypartly contribute to the emergence of superconductivity inthis system. As a comparison, in the Sr1−xLaxFBiS2 system[11], La doping can result in a rapid increase in the metallicconductivity and induce bulk superconductivity with Ladoping, but in Sr1−xPrxFBiS2 the semiconducting-like normalstate is rather robust, even at x=0.7, and evidence of the bulksuperconductivity is absent. Such a feature occurs in someBiS2-based superconductors [2,3] and is ascribed to the weakoverlap of Bi-6p and S-p orbitals associated with the robustsemiconducting-like normal state [21,24].

3.2 Effect of external pressure on superconductivity ofSr0.5Pr0.5FBiS2

A strategy to enhance orbital overlap is to apply high pres-sure. Several cases in the BiS2 systems [14-16] have beenachieved to enhance superconductivity under pressure. Wethus made pressure measurements at optimal doping (x=0.5).

Figure 6 shows the resistivity as a function of temperature atvarious pressures. Figure 6(a) shows ρ(T) upon increasingthe gauge pressure from zero to 2.5 GPa. Semiconductingbehavior clearly occurs at lower pressures and is graduallysuppressed with increasing pressure, until the system finallyturns metallic at 2.5 GPa. Accompanied with a decrease inresistivity in the normal state, Tc is enhanced under pressure.Figure 6(b) shows an expanded view of resistivity around Tc.As illustrated in Figure 6(b), for P≤0.8 GPa, Tc slowly in-creases with increasing pressure. For pressure in the range0.8-1.3 GPa, the superconducting transition broadens sig-nificantly, and two successive superconducting transitionsoccur for 1.15 GPa: one at 5.5 K and one at 8.5 K. Furtherincreasing pressure to 1.5 GPa makes the superconductingtransition sharp again and the onset of Tc increases sig-nificantly to about 9 K. For pressure greater than 1.5 GPa,Tc

onset slowly increases with pressure and reaches 9.7 K at2.5 GPa. A similar pressure effect was reported for the su-perconducting isostructures EuFBiS2 and LaO1−xFxBiSe2[25,26], whose metallic conductivity is enhanced underpressure.In contrast, a semiconductor-to-metal transition occurs

upon increasing the pressure in the present Sr0.5Pr0.5FBiS2sample. As shown in Figure 6(c) and (d), the resistivity ex-hibits a metallic behavior at high temperature, and then un-dergoes a metal-to-semiconductor transition near thecharacteristic temperature Tmin at 1.0 GPa. Upon further in-creasing the pressure, Tmin gradually shifts to lower tem-perature and finally vanishes above 1.5 GPa. A similarfeature was reported for the Sr1−xLaxFBiS2 system, where thesemiconductor-metal transition is induced by La doping[11,27]. A first-principles calculation [28] suggests thatLnO1−xFxBiS2 undergoes a pressure-induced semiconductor-metal transition. Conversely, accompanied with the en-hancement of metallicity under pressure, the resistivityshows a clear linear temperature dependence above Tmin,which differs from the case of EuFBiS2 under pressure [25].

Figure 4 (Color online) (a) Low-temperature specific heat as a function of temperature for Sr1−xPrxFBiS2 (0.4≤x≤0.7) samples. (b) C/T vs T 2 in the mainplane; the inset shows the magnetic entropy for these samples.

Figure 5 (Color online) Phase diagram of Sr1−xPrxFBiS2 system with Pr-doing concentrations.

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Note that as pressure ranges from 1.0 to 1.3 GPa, a clear kinkappears in the resistivity at about 250 K, as illustrated inFigure 6(c). A similar feature occurred in the EuFBiS2compound and was tentatively ascribed to a possible charge-density-wave transition [22].Figure 7 shows the resistivity as a function of temperature

under various magnetic fields at ambient pressure and at2.5 GPa. At ambient pressure, the sharp superconductingtransition slowly broadens and Tc gradually shifts to below2 K as the external magnetic fields increase from zero to 3 T(see Figure 7(a)). For B≥5 T, superconductivity completelydisappears and resistivity recovers a weak semiconducting-

like feature, similar to the case of the Bi4O4S3 system [29].For P=2.5 GPa, the high-Tc of 9.7 K occurs at zero field. Asthe magnetic field is applied, Tc shifts slightly to lowertemperatures and the superconducting transition graduallybroadens. As magnetic fields exceed 4 T, a shoulder in theρ(T) curve appears, as marked by arrows in Figure 7(b). Asimilar characteristic has been reported for the high-pressurephases of EuFBiS2 and Eu3F4Bi2S4 [16,25], which was in-terpreted as the anisotropy in the upper critical field. How-ever, superconductivity seems to be rather robust bymaintaining a strong signal above 2 K although the magneticfield is as high as 9 T. In comparison, for both EuFBiS2 and

Figure 6 (Color online) (a) Resistivity as a function of temperature for several representative pressures for Sr0.5Pr0.5FBiS2 from 2 to 300 K. (b) Enlargedview of superconducting transition in resistivity below 13 K. (c) and (d) Enlarged plots of ρ in high-temperature regime.

Figure 7 (Color online) (a) Superconducting transition vs temperature T under various magnetic fields and at ambient pressure and (b) 2.5 GPa. (c) Uppercritical field Hc2(T)-T phase diagram at ambient pressure and 2.5 GPa, respectively. Tc

onset and Tc90% are determined by the onset of the superconducting

transition and when ρ(T) reaches 90% of its normal-state value.

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Eu3F4Bi2S4 at high pressure [16,25], a magnetic field ofabout 3.5 T completely quenches the superconductivity. Theupper critical fields of P=0 GPa and 2.5 GPa are shown inFigure 7(c), where the criteria of Tc

onset and Tc90% are used to

determine μ0Hc2. The zero-temperature limit estimated by theWerthamer-Helfand-Hohenberg formula H T T( ) = 0.69 ×c2 c

HT

T

c2

c

is about 1.6 and 25 T for ambient pressure and

2.5 GPa, respectively. Thus, Bc2(0)/Tc dramatically increasesfrom 0.6 T/K at P=0 to 2.6 T/K at P=2.5 GPa, suggestingdifferent origins of superconductivity in the correspondingsuperconducting phase. In contrast, the isostructural Eu-FBiS2 has a comparable Tc of 9 K under 2.4 GPa, but itsupper critical field is rather small, and Bc2(0)/Tc decreasesabnormally under applied pressure [25].Based on these experimental results, the phase diagram of

Sr0.5Pr0.5FBiS2 under pressure is summarized in Figure 8. Atlower pressure, Tc increases slightly with increasing pres-sure. As pressure crosses the critical value of Pc≈1.5 GPa, Tcincreases dramatically from 3 K to around 9 K, and a high-Tcsuperconducting phase emerges at higher pressure. Twodistinct superconducting regions, including the low-Tc phase(SC1) below Pc and the high-Tc phase (SC2) above Pc areclearly distinguished, sharing a common feature in the mostBiS2-based superconductors [14,16,25]. Such a feature islikely related to a structural transition from a tetragonalphase to a monoclinic phase at a critical pressure Pc [17]. Bycoincidence, both the Eg and Tmin decrease suddenly and tendto approach zero at the critical pressure Pc. Our resultssuggest that, in the low-pressure regime (<Pc), the low-Tcphase is associated with filamentary superconductivity, butin the high-pressure regime (>Pc), enhances the super-conductivity in the high-Tc phase because of the structuralphase transition.

4 Conclusions

In summary, we report herein the results of resistivity,magnetic susceptibility, specific heat, and pressure mea-surements of the BiS2-based Sr1−xPrxFBiS2 system. The ex-perimental results show that Pr doping significantlydecreases the sample’s resistivity and induces super-conductivity with a maximum Tc of 2.7 K at x=0.5, but itsnormal-state resistivity retains semiconducting-like char-acteristics. However, both the low superconducting shieldingvolume fractions in χ(T) and the absence of the super-conducting transition jump in C(T) point clearly to fila-mentary superconductivity in the Pr doping samples.Applying pressure can abruptly increase Tc from 2.7 K atambient pressure to about 8.5 K at the critical pressure Pc=1.5 GPa, reaching about 9.7 K at 2.5 GPa. Meanwhile, the

normal-state resistivity undergoes a semiconductor-to-metaltransition and both Tmin and Eg decrease and approach zero atPc. Two superconducting states, including the low- and thehigh-Tc phases, are separated by a coexisting phase at thecritical pressure Pc. We conclude that the low-Tc phase isfilamentary superconductivity because of weak orbitaloverlap, whereas enhanced superconductivity may occur inthe high-Tc phase because of the structural transition fromtetragonal phase to trigonal phase under pressure. Moreover,the upper critical field (μ0Hc2(0)) estimated in the high-Tcphase at 2.5 GPa is over ten times greater than that at ambientpressure. Therefore, the ratio of Bc2(0)/Tc dramatically in-creases from 0.6 T/K at P=0 to 2.6 T/K at P=2.5 GPa. Theseresults suggest that the two different superconducting phasesmay possess different origins, which mean that they shouldbe investigated by further experimentation.

This work was supported by the National Natural Science Foundation ofChina (Grant Nos. 11474082, 61401136, and 61376094), the Natural Sci-ence Foundation of Zhejiang Province (Grant No. LY18F010019), and theQianJiang Talents Program of Zhejiang Province. YuKe Li was supported byan open program from Wuhan National High Magnetic Field Center (GrantNo. 2016KF03). Jie Cheng was supported by the General Program ofNatural Science Foundation of Jiangsu Province of China (Grant No.BK20171440). YongKang Luo acknowledges the support from the 1000Youth Talents Plan of China.

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