Copper Doping of BaNi_{2}As_{2}: Giant Phonon Softening and Superconductivity Enhancement
aa r X i v : . [ c ond - m a t . s up r- c on ] A p r Copper Doping of BaNi As : Giant Phonon Softening and SuperconductivityEnhancement Kazutaka Kudo, ∗ Masaya Takasuga, and Minoru Nohara † Research Institute for Interdisciplinary Science, Okayama University, Okayama 700-8530, Japan (Dated: November 6, 2018)The effects of copper doping on the structural and superconducting phase transitions ofBa(Ni − x Cu x ) As were studied by examining the resistivity, magnetic susceptibility, and specificheat. We found an abrupt increase in the superconducting transition temperature T c from 0.6 K inthe triclinic phase with less copper ( x ≤ x > (As − x P x ) [K. Kudo etal. , Phys. Rev. Lett. , 097002 (2012).], which markedly contrast the behavior of phosphorusand copper doping of the iron-based superconductor BaFe As . PACS numbers: 74.70.Xa, 74.25.Kc, 74.25.Dw, 74.25.-q, 74.25.Bt
Nickel-based 122-type compounds, such as BaNi As [1–8], SrNi As [9], BaNi P [10, 11], SrNi P [12–16],BaNi Ge [17, 18], and SrNi Ge [19] are nonmagneticanalogs of iron-based superconductors. Some of thesecompounds have provided opportunities for investigat-ing the interplay between structural instability and su-perconductivity upon chemical doping [7, 15, 16, 18].The prototype BaNi As compound crystallizes ina tetragonal ThCr Si -type structure (space group I /mmm , D h , No. 139) and exhibits a structural phasetransition from its tetragonal phase to a triclinic phase( P ¯1, C i , No. 2) at ≃
130 K, below which alternate Ni-Nibonds are formed in the Ni square lattice [3]. In this tri-clinic phase, weak-coupling Bardeen–Cooper–Schrieffer(BCS) superconductivity emerges at 0.7 K [1, 2]. Thesuperconducting transition temperature T c abruptly in-creases from 0.7 to 3.3 K upon phosphorus doping ofBaNi As [7]. The enhanced superconductivity, which isa strong-coupling type, is accompanied by the triclinic-to-tetragonal phase transition and pronounced phononsoftening that is induced by phosphorus doping at x =0.067 in BaNi (As − x P x ) [7]. Phosphorous and arsenichave the same number of valence electrons, but phos-phorus has a smaller ionic radius than arsenic. Thus,phosphorus doping was anticipated to produce chemicalpressure in BaNi (As − x P x ) [7], activating a mechanismsimilar to the pressure-enhanced superconductivity of el-emental tellurium at the rhombohedral β -Po to bcc phasetransition [20–22].For iron-based superconductors, the doping of variouschemical elements has been examined in order to demon-strate the suppression of antiferromagnetic and struc-tural phase transitions and the subsequent appearance ofsuperconductivity [23]. In the prototype BaFe As com-pound, potassium doping resulted in a maximum valueof T c = 38 K in Ba − x K x Fe As [24], while phospho-rus doping resulted in a maximum value of T c = 31 Kin BaFe (As − x P x ) [25, 26]. Interestingly, transition- metal doping resulted in superconductivity with max-imum transition temperatures of T c = 23 and 20.5 Kfor Ba(Fe − x Co x ) As [27, 28] and Ba(Fe − x Ni x ) As [29, 30], respectively. On the other hand, copper dopingresulted in a reduced superconducting transition tem-perature T c ≃ − x Cu x ) As [30–32].Then, a natural question arises whether transition-metal doping, specifically copper doping, of BaNi As can suppress the triclinic transition temperature and re-sult in enhanced superconductivity similar to that ob-served in the phosphorus doping of BaNi As . In this pa-per, we report the results of copper doping of BaNi As ,which revealed striking similarity with phosphorus dop-ing. In fact, the superconducting transition tempera-ture T c increased from 0.6 to 2.5–3.2 K as a result of thetriclinic-tetragonal phase transition at copper doping x =0.16 for Ba(Ni − x Cu x ) As . Strong-coupling type super-conductivity was observed in the tetragonal phase. Theenhanced superconductivity was accompanied by signifi-cant phonon softening, which was observed via the Debyeterm of specific heat.Single crystals of Ba(Ni − x Cu x ) As were grown intwo ways. A mixture with a ratio of Ba:NiAs:Cu:As =1:4(1 − x ):4 x :4 x [7] or 1:2(1 − x ):2 x :2 x was placed in analumina crucible and sealed in an evacuated quartz tube.The former mixture was heated at 700 ◦ C for 3 h, slowlyheated to 1150 ◦ C, and cooled from 1150 to 1000 ◦ C at arate of 2 ◦ C/h, followed by furnace cooling [7]. The lattermixture was heated at 700 ◦ C for 3 h and then at 1000 ◦ Cfor 72 h, followed by cooling to room temperature over24 h. In both cases, single crystals with a typical di-mension of 0.5–1.0 × × were obtained.The results of powder x-ray diffraction, performed usinga Rigaku RINT-TTR III x-ray diffractometer with Cu K α radiation, showed that all specimens were in a singlephase. Energy dispersive x-ray spectrometry (EDS) was r ab / r ab ( K ) T (K) T s Ba(Ni x Cu x ) As x = 0.3630.2920.1560.0820.0650.000 FIG. 1. (Color online) Temperature dependence of the elec-trical resistivity parallel to the ab plane, ρ ab , normalized bythe value at 300 K for Ba(Ni − x Cu x ) As . The data mea-sured upon heating and cooling are plotted. For clarity, ρ ab /ρ ab (300K) is shifted by 0.15 with respect to all data. T s is the phase transition temperature at which the tetragonal-to-triclinic phase transition takes place. used to determine the copper content x . Samples with0.485 < x < − x Cu x ) As in this doping range re-sulted in phase separation between x = 0.485 and 0.939,indicating a miscibility gap of BaNi As –BaCu As solidsolution. The samples were treated in a glove box filledwith dried argon gas. The magnetization M was mea-sured using a Quantum Design MPMS. The electricalresistivity ρ ab (parallel to the ab plane) and specific heat C were measured using a Quantum Design PPMS.Figure 1 shows the temperature dependence of the elec-trical resistivity ρ ab for Ba(Ni − x Cu x ) As . As previ-ously reported, pure BaNi As exhibits a transition at130 K with a thermal hysteresis accompanying a suddenincrease in resistivity upon cooling [1, 3, 6, 7]. For x = 0.156 copper doping, the transition was significantlysuppressed to 40 K, and the resistivity jump was smalland broad. The transition appeared to be absent for x =0.292. These results suggested suppression of the triclinicphase at x ≃ T c = 2.5–3.2K emerged as soon as the triclinic phase was suppressedwith copper doping, while T c < x < x = 0.292, the bulk superconductivity was evidentfrom the full-shielding diamagnetic signal and sharp re-sistivity transition at 3.2 K. On the other hand, the resis-tivity transition was broad for x = 0.156, which occurred C e / T ( m J / K m o l ) T (K) x = 0.000 0.292 0.363 (c)1.51.00.50.0 r ab / r ab ( K ) x = 0.156 0.292 0.363 (b)-1.0-0.50.0 π M / H ( e m u / c m ) (a) Ba(Ni x Cu x ) As
30 Oe x = 0.156 0.165 0.292 0.363 FIG. 2. (Color online) (a) Temperature dependence of dcmagnetization M measured in a magnetic field H of 30 Oefor Ba(Ni − x Cu x ) As under zero-field cooling and field cool-ing. (b) Temperature dependence of the electrical resistivityparallel to the ab plane, ρ ab , normalized by the value at 5 Kfor Ba(Ni − x Cu x ) As . (c) Temperature dependence of theelectronic specific heat divided by the temperature, C e /T , forBa(Ni − x Cu x ) As . C e was determined by subtracting thephonon contribution βT from the total specific heat C , asshown in Fig. 3. The specific-heat data for BaNi As ( x =0.0) are taken from Ref. [7]. at the critical phase boundary of the triclinic and tetrago-nal phases. Superconductivity persisted up to x = 0.363,while T c decreased slightly to 2.75 K. Superconductivitywas not observed for x = 0.485, which was the solubilitylimit of Cu for Ni. The low-temperature specific-heatdata, shown in Fig. 2(c) and 3, provided further evi-dence of the enhanced superconductivity in the tetrag-onal phase. Pure BaNi As exhibits a specific-heat jumpat 0.6 K, as reported previously [1–3, 7]. In the tetrago-nal phase at x = 0.292, the specific-heat jump appearedat an elevated temperature of 3.2 K, in a consistent man-ner with the magnetic susceptibility and resistivity data.The specific-heat jump was significantly broadened for x = 0.363, suggesting a reduced superconducting volumefraction and the disappearance of superconductivity be-fore reaching the solubility limit at x = 0.485.Figure 3 shows the specific heat divided by the tem-perature C/T as a function of the squared temperature T . The normal-state data above T c could be well fitted C / T ( m J / K m o l ) T (K ) x = 0.0000.1320.2920.363Ba(Ni x Cu x ) As FIG. 3. (Color online) The specific heat divided by the tem-perature,
C/T , as a function of T for Ba(Ni − x Cu x ) As .The dashed lines denote fits by C/T = γ + βT , where γ isthe electronic specific-heat coefficient and β is a constant cor-responding to the Debye phonon contributions. The specific-heat data for BaNi As ( x = 0.0) are taken from Ref. [7]. by C/T = γ + βT , where γ was the electronic specific-heat coefficient and β was the coefficient of phonon con-tributions from which the Debye temperature Θ D wasestimated. As shown in Fig. 3, the slope of the C/T vs T lines was almost unchanged in the triclinic phase for x < x = 0.292copper doping, suggesting significant phonon softening inthe tetragonal phase. The slope decreased upon furthercopper doping in the tetragonal phase. Figure 4(c) showsthe estimated Debye temperature Θ D as a function of thecopper content x . In particular, Θ D showed significantreduction from 250 K for x = 0.0 to 170 K for x = 0.292.In contrast to the strong dependence of both Θ D and T c on doping, the electronic specific-heat coefficient γ (= 14 mJ/mol K ) was almost independent of the cop-per content x , as shown from the almost unchanged in-tercept of the C/T vs T lines along the C/T axis inFig. 3. These observations suggested that the structuralphase transition, as well as the enhanced superconductiv-ity in the tetragonal phase, originated in the enhancedelectron-phonon coupling due to soft phonons [7]. In-deed, the normalized specific-heat jump ∆
C/γT c ≃ As in the triclinic phase, determined fromthe data shown in Fig. 2(c), was comparable to the valueof the weak-coupling limit (= 1.43), while ∆ C/γT c wasenhanced in the tetragonal phase with a value of 1.9 at x = 0.292, indicating strong-coupling superconductivity.Our observations in Ba(Ni − x Cu x ) As are summa- SC1 SC2 T ( K ) Ba(Ni x Cu x ) As (b) tetragonaltriclinic T s T c x 10 a p a r a m e t e r ( Å ) c p a r a m e t e r ( Å ) a c (a)3002001000 Θ D ( K ) x (EDS) (c) FIG. 4. (Color online) (a) Lattice parameters a and c as afunction of copper content x at 300 K for Ba(Ni − x Cu x ) As .(b) Electronic phase diagram of Ba(Ni − x Cu x ) As . The(blue) closed circles represent the superconducting transitiontemperatures T c . For clarity, the values of T c are scaled by afactor of 10. SC1 and SC2 denote the superconducting phases.The (red) open and closed diamonds represent the tetragonal-to-triclinic structural transition temperatures T s upon coolingand heating, respectively. (c) The Debye temperature Θ D asa function of phosphorous content x for Ba(Ni − x Cu x ) As .Θ D is determined from the slope of the C/T vs T curves inFig. 3. rized in the electronic phase diagram shown in Fig. 4(b).The triclinic phase transition temperature in pureBaNi As was gradually suppressed with copper doping.As a result, the superconducting transition temperaturewas enhanced from T c = 0.6 K in pure BaNi As to 2.5–3.2 K in the tetragonal phase at x > − x Cu x ) As werestrikingly similar to those observed in phosphorus-dopedBaNi (As − x P x ) [7]. For BaNi (As − x P x ) , the tri-clinic phase transition temperature was suppressed withphosphorus doping, and the superconducting transitiontemperature was enhanced to 3.2–3.3 K in the tetragonalphase at x > γ wasalmost unchanged upon phosphorous doping, and the De-bye temperature Θ D exhibited significant reduction from250 to 150 K at the structural phase boundary of x =0.067.Dissimilarity between copper and phosphorus dopingcould be observed in the opposite doping dependence oflattice parameters. For Ba(Ni − x Cu x ) As , as shown inFig. 4(a), the a parameter increased with copper dop-ing x , while c decreased with ratios of (1 /a )( da/dx ) =+0 .
032 and (1 /c )( dc/dx ) = − . /V )( dV /dx ) = +0 . /a )( da/dx ) = − . /c )( dc/dx ) = +0 . /V )( dV /dx ) = − .
072 forBaNi (As − x P x ) [7], as expected from the small ionicradius of phosphorus and resultant chemical pressure.Thus, the reduction of the triclinic structural transitiontemperature upon copper/phosphorus doping could notbe ascribed to the volume effect. This behavior was con-sistent with the weak effect of hydrostatic pressure onboth the triclinic and superconducting transition tem-peratures [33].Finally, we discuss the results of further doping. A mis-cibility gap of the copper content was observed between x = 0.485 and 0.939 for Ba(Ni − x Cu x ) As . Superconduc-tivity was absent when nickel was completely replaced bycopper in BaCu As [34]. It has been reported that Cu 3 d orbitals are fully occupied for BaCu As [35]. Similarly,a miscibility gap of the phosphorus content existed above x = 0.13 for BaNi (As − x P x ) [7]. Superconductivity at T c = 2.5 K appeared when arsenic was completely re-placed by phosphorus in BaNi P [10, 11, 16, 18], butit was weak-coupling type superconductivity as demon-strated in BaNi (Ge − x P x ) [18].In conclusion, our studies demonstrated that enhancedsuperconductivity associated with phonon softening andthe subsequent structural phase transition occurred uponcopper doping of BaNi As . Specifically, the increase in T c from 0.6 to 2.5–3.2 K was related to the giant phononsoftening of ∼
50% at the triclinic-to-tetragonal phasetransition induced by copper doping. These features werestrikingly similar to those observed in phosphorus dopingof BaNi As [7]. The similarity of copper and phospho-rus doping of the nickel-based superconductor BaNi As and dissimilarity of copper and phosphorus doping of theiron-based superconductor BaFe As could provide fur-ther understanding of the role of chemical substitution inrealizing high-temperature superconductivity.Part of this work was performed at the Advanced Sci-ence Research Center, Okayama University. This workwas partially supported by Grants-in-Aid for ScientificResearch (No. 26287082, 15H01047, 15H05886, and16K05451) provided by the Japan Society for the Pro-motion of Science (JSPS) and the Program for Advanc-ing Strategic International Networks to Accelerate theCirculation of Talented Researchers from JSPS. ∗ [email protected] † [email protected][1] F. Ronning, N. Kurita, E. D. Bauer, B. L. Scott, T. Park,T. Klimczuk, R. Movshovich, and J. D. Thompson, J.Phys.: Condens. Matter , 342203 (2008).[2] N. Kurita, F. Ronning, Y. Tokiwa, E. D. Bauer,A. Subedi, D. J. Singh, J. D. Thompson, and R.Movshovich, Phys. Rev. Lett. , 147004 (2009).[3] A. S. Sefat, M. A. McGuire, R. Jin, B. C. Sales, D. Man-drus, F. Ronning, E. D. Bauer, and Y. Mozharivskyj,Phys. Rev. B , 094508 (2009).[4] A. Subedi and D. J. Singh, Phys. Rev. B , 132511(2008).[5] I. R. Shein and A. L. Ivanovskii, Phys. Rev. B , 054510(2009).[6] Z. G. Chen, G. Xu, W. Z. Hu, X. D. Zhang, P. Zheng,G. F. Chen, J. L. Luo, Z. Fang, and N. L. Wang, Phys.Rev. B , 094506 (2009).[7] K. Kudo, M. Takasuga, Y. Okamoto, Z. Hiroi, and M.Nohara, Phys. Rev. Lett. , 097002 (2012).[8] Y. Yamakawa, S. Onari, and H. Kontani, J. Phys. Soc.Jpn. , 094704 (2013).[9] E. D. Bauer, F. Ronning, B. L. Scott, and J. D. Thomp-son, Phys. Rev. B , 172504 (2008).[10] T. Mine, H. Yanagi, T. Kamiya, Y. Kamihara, M. Hi-rano, and H. Hosono, Solid State Commun. , 111(2008).[11] Y. Tomioka, S. Ishida, M. Nakajima, T. Ito, H. Kito, A.Iyo, H. Eisaki, and S. Uchida, Phys. Rev. B , 132506(2009).[12] V. Keimes, D. Johrendt, A. Mewis, C. Huhnt, and W.Schlabitz, Z. Anorg. Allg. Chem. , 1699 (1997).[13] F. Ronning, E. D. Bauer, T. Park, S.-H. Baek, H. Sakai,and J. D. Thompson, Phys. Rev. B , 134507 (2009).[14] N. Kurita, F. Ronning, C. F. Miclea, E. D. Bauer, K.Gofryk, J. D. Thompson, and R. Movshovich, Phys. Rev.B , 094527 (2011).[15] V. Hlukhyy, A. V. Hoffmann, V. Grinenko, J. Scheiter,F. Hummel, D. Johrendt, and T. F. F¨assler, Phys. StatusSolidi B , 1600351 (2017).[16] K. Kudo, Y. Kitahama, K. Iba, M. Takasuga, and M.Nohara, J. Phys. Soc. Jpn. , 035001 (2017).[17] V. Hlukhyy, D. Trots, and T. F. F¨assler, Inorg. Chem. , 1173 (2017).[18] D. Hirai, F. von Rohr, and R. J. Cava, Phys. Rev. B ,100505(R) (2012).[19] T. L. Hung, I. A. Chen, C. H. Huang, C. Y. Lin, C. W.Chen, Y. B. You, S. T. Jian, M. C. Yang, Y. Y. Hsu, J.C. Ho, Y. Y. Chen, and H. C. Ku, J. Low Temp. Phys. , 148 (2013).[20] F. Mauri, O. Zakharov, S. de Gironcoli, S. G. Louie, andM. L. Cohen, Phys. Rev. Lett. , 1151 (1996).[21] S. P. Rudin, A. Y. Liu, J. K. Freericks, and A. Quandt,Phys. Rev. B , 224107 (2001).[22] N. Suzuki and M. Otani, J. Phys.: Condens. Matter ,125206 (2007).[23] H. Hosono, K. Tanabe, E. Takayama-Muromachi, H.Kageyama, S. Yamanaka, H. Kumakura, M. Nohara, H.Hiramatsu, and S. Fujitsu, Sci. Technol. Adv. Mater. ,033503 (2015).[24] M. Rotter, M. Tegel, and D. Johrendt, Phys. Rev. Lett. , 107006 (2008).[25] S. Kasahara, T. Shibauchi, K. Hashimoto, K. Ikada, S.Tonegawa, R. Okazaki, H. Shishido, H. Ikeda, H. Takeya,K. Hirata, T. Terashima, and Y. Matsuda, Phys. Rev. B , 184519 (2010).[26] S. Jiang, H. Xing, G. F. Xuan, C. Wang, Z. Ren, C.M. Feng, J. H. Dai, Z. A. Xu, and G. H. Cao, J. Phys.:Condens. Matter , 382203 (2009).[27] A. S. Sefat, R. Jin, M. A. McGuire, B. C. Sales, D. J.Singh, and D. Mandrus, Phys. Rev. Lett. , 117004(2008).[28] N. Ni, M. E. Tillman, J.-Q. Yan, A. Kracher, S. T. Han-nahs, S. L. Bud’ko, and P. C. Canfield, Phys. Rev. B ,214515 (2008).[29] L. J. Li, Y. K. Luo, Q. B. Wang, H. Chen, Z. Ren, Q.Tao, Y. K. Li, X. Lin, M. He, Z. W. Zhu, G. H. Cao, and Z. A. Xu, New J. Phys. , 025008 (2009).[30] N. Ni, A. Thaler, J. Q. Yan, A. Kracher, E. Colombier,S. L. Bud’ko, P. C. Canfield, and S. T. Hannahs, Phys.Rev. B , 024519 (2010).[31] T. M. Garitezi, P. F. S. Rosa, C. Adriano, and P. G.Pagliuso, J. Appl. Phys. , 17D704 (2014).[32] M. M. Piva, M. Besser, K. Mydeen, T. M. Garitezi, P. F.S. Rosa, C. Adriano, T. Grant, Z. Fisk, R. R. Urbano, M.Nicklas, and P. G. Pagliuso, J. Phys.: Condens. Matter , 145701 (2015).[33] T. Park, H. Lee, E. D. Bauer, J. D. Thompson, and F.Ronning, J. Phys.: Conf. Ser. , 012155 (2010).[34] B. Saparov and A. S. Sefat, J. Solid State Chem. ,213 (2012).[35] S. F. Wu, P. Richard, A. van Roekeghem, S. M. Nie, H.Miao, N. Xu, T. Qian, B. Saparov, Z. Fang, S. Biermann,A. S. Sefat, and H. Ding, Phys. Rev. B91