Terahertz spectroscopy evidence of possible 40 K superconductivity in rhenium-doped strontium ruthenates
Yurii Aleshchenko, Boris Gorshunov, Elena Zhukova, Andrey Muratov, Alexander Dudka, Rajendra Dulal, Serafim Teknowijoyo, Sara Chahid, Vahan Nikoghosyan, Armen Gulian
TTerahertz spectroscopy evidence of possible 40 K superconductivity in rhenium-dopedstrontium ruthenates
Yurii Aleshchenko, Boris Gorshunov, Elena Zhukova, Andrey Muratov, Alexander Dudka, Rajendra Dulal, Serafim Teknowijoyo, Sara Chahid, Vahan Nikoghosyan, and Armen Gulian ∗ V.L. Ginzburg Center for High-Temperature Superconductivity and Quantum Materials,P.N. Lebedev Physical Institute of the Russian Academy of Sciences, 53 Leninskiy Prospekt, 119991, Moscow, Russia Moscow Institute of Physics and Technology (National Research University), 141700 Dolgoprudny, Moscow Region, Russia Shubnikov Institute of Crystallography of Federal Scientific Research Centre “Crystallography andPhotonics” of Russian Academy of Sciences, Leninskiy Prospekt 59, 119333, Moscow, Russia Advanced Physics Laboratory, Institute for Quantum Studies,Chapman University, Burtonsville, MD 20866, USA (Dated: July 14, 2020)Strontium ruthenates have many similarities with copper oxide superconductors and are of partic-ular interest for the investigation of the mechanisms and conditions which lead to high-temperaturesuperconductivity. We report here on multiple experimental indications of superconductivity withonset at 40 K in strontium ruthenate doped by rhenium and selenium with chlorine used as the flux.The main experimental evidence arises from terahertz spectroscopy of this material followed by ACand DC magnetization, as well as measurements of its heat capacity and magnetoresistance. Struc-tural and morphological studies revealed the heterophase nature of this polycrystalline material aswell as the changes of lattice parameters relative to the original phases. Experimental data show ahigher critical temperature on the surface compared to that of the bulk of the sample.
I. INTRODUCTION
The fascinating properties of strontium ruthenatesSr n +1 Ru n O n +1 ( n = 1 , , ...,
8) have garnered enormousattention [1] since the discovery of high-temperaturesuperconductivity in cuprates. Superconductivity inruthenates was found only for n = 1 case [2] with T c as high as 1 .
5K [3]. Other representatives of thisRuddlesden-Popper family possess peculiar magneticproperties. The most pronounced magnetic order takesplace at n = ∞ : SrRuO is a ferromagnet with Curietemperature T C ∼
165 K; at n = 2, Sr Ru O isan anomalous paramagnet; at n = 3, Sr Ru O is ametamagnet [3]. The Cooper pairing in Sr RuO waswell known as a textbook example of the spin-tripletstate (odd parity S = 1, see reviews [3–6] and referencestherein). Recently, NMR spectroscopy reinvestigation [7]has given compelling evidence that the superconductivityin Sr RuO is likely to be even parity, which unequivo-cally demonstrates that research on the physical proper-ties of ruthenates is far from being complete.Another confirmation of this statement comes from therecent fascinating discovery of high-temperature super-conductivity in calcium ruthenate, Ca RuO [8]. Thestoichiometric composition of this material in a single-crystalline form is a Mott insulator, while single crys-tals with excess oxygen are metallic above 160 K [9].The excess amount of oxygen results in important crys-tallographic change: the lattice symmetry changes from Pbca to P /c with c − axis extension from 11 . . ∗ Corresponding author: [email protected] metal transition have been detected via infrared nano-imaging and optical-microscopy measurements on bulksingle crystal Ca RuO [10]. Much more drastic changesoccur when the thickness of Ca RuO crystal is re-duced to the nanometer range: novel quantum statesincluding high-temperature superconductivity via resis-tive and magnetic measurements at 64 K have been ob-served [8]. This remarkable finding in micronanocrystalsdemonstrated how rich the superconducting phenomenain ruthenates can be. It also invigorates the value of poly-crystalline materials (ceramics), in which the samples in[8] were originally prepared before subsequent sonifica-tion to obtain nanomicrocrystals.In this rapid communication, we present spectroscopicdata in the THz − FIR range obtained for polycrys-talline samples of initial stoichiometric composition ofSr Ru − x Re x O − y Se y . The choice of this compositionwas made in result of series of experiments in which theoxygen was partially replaced by S or Se in presence ofCl as a flux at the synthesis of Sr RuO − x S(Se) x andsubsequent cationic substitutions for Ru (Fig. 1).As follows from Fig. 1, typical ρ ( T ) dependence ofSr RuO drastically changes into typical strange metalbehavior [11] at application of Cl − flux and vacuum dur-ing synthesis (details can be found in [12]). Moreover,inclination towards zero resistivity at T → − ions [12]. At the timeof publication [12], the critical temperatures in the rangeof 20 −
30 K appeared very unusual for ruthenates; how-ever they were later supported by the findings of Ref. [8].Our current spectroscopic data is in support of supercon- a r X i v : . [ c ond - m a t . s up r- c on ] J u l FIG. 1. Drastic change in resistivity of Sr RuO ce-ramic samples. While pure samples typically demonstratesemiconductor-type temperature dependence, the chlorineflux and vacuum treated samples demonstrate the so-calledstrange metal behavior, and chalcogen addition introduces adownturn. Both features are intriguing (more details can befound in [11]). The curve corresponding to Re, as well asthe modification of its resistivity in the magnetic field, arediscussed in the text. ducting phase in polycrystalline Sr Ru − x Re x O − y Se y . II. EXPERIMENTAL DETAILS
Details on the preparation of Sr Ru − x Re x O − y Se y samples can be found in [12]. Here, we will briefly sum-marize them. The precursors, RuO , SrSe, ReO , SrCO ,and SrCl · O were powdered and mixed in stoichio-metric proportions. A combination of hand and mechan-ical grinding and mixing was applied. The powder wascalcinated at 695 ◦ C for 10 hours which incurred 6% ofweight loss. The calcinated powder was again powder-ized and heat treated in air, linearly increasing tempera-ture up to 1350 ◦ C and down during 8 hours with 25% ofweight loss. This powder was pelletized and heat treatedagain at 1350 ◦ C in air for 5 hours with linear temperatureincrease and decrease at a similar rate (4 hours each, witha weight loss ∼ ∼ − mbar, 650 ◦ C, 500 min). The weight of thepellet did not change noticeably, but the resistivity be-came smaller. No changes in the sample’s characteristicswere obtained at further vacuum heat treatments.The crystalline structure of the sample is shown inFig. 2. Using EDX well-matching data taken by mul-tiple analysis of various crystalline areas of this sam-ple, the composition of the sample was determinedas Sr . (Ru . , Re . )O . In view of >
25% of
FIG. 2. Polycrystalline surface morphology of the measuredsample (JEOL JCM 6000Plus SEM). The rectangle indicatesone of the regions from which the compositional data weretaken. weight loss at the thermal treatment quoted above, thechange of the initial stoicheometry is not surprising(for the convenience, we will keep calling our sampleSr Ru − x Re x O − y Se y ). For Se content, EDX microanal-ysis is not sufficiently sensitive. WDX analysis revelas Seof the amount less than 0 . α line in the angle interval 2 θ =3 − ◦ with the step 0 . ◦ and scanning rate 0 . − . ◦ /s;phase content and refinement of atomic structural mod-els by the Panalytical HighScore Plus 3.0e software),0 . of sample was ground in agath mortar. Anal-ysis revealed the presence of three phases: Sr Ru O (50 − RuO (20 − (10 − (Ru , Re)(O , Se) are: a = b = 3 . c = 12 . / mmm. For comparison, the pure n = 1 phase has pa-rameters a = b = 3 . c = 12 . .
74% along the c − axis, andextended by 0 . ab − plane. This can be as-sociated with an influence of the uniaxial pressure whichsignificantly affects the T c of Sr RuO [14]. The latticeparameters for the phase Sr (Ru , Re) (O , Se) are: a = b = 3 . c = 20 . / mmm.Reference data for n = 2 [15] are: a = b = 3 . c = 20 . .
17% for a , 0 .
33% for c ).Physical characterization of properties of this sample’smagnetoresistance, heat capacity, DC and AC magneticsusceptibility were reported in [12] (sample μ m) disk-type slice was dry cut from cylindrical sam-ple μ m grade to ob-tain a shiny, highly planar (within 1 − ◦ accuracy) sur-face. The infrared reflectivity spectra were measurednear normal incidence ( ≈ ◦ ) in the spectral range of40 −
670 cm − (5 −
83 meV) at various temperatures be-tween 5 K and 300 K using a conventional Fourier trans-form IR spectrometer (IFS 125HR, Bruker) equippedwith a liquid He − cooled Si − bolometer and a multi-layermylar beam splitter. For IR measurements, the polishedslice was mounted with the STYCAST 2850ft epoxy glueto the tip of the cone to avoid parasitic back-reflection.A similar cone supports the gold reference mirror. Bothcones were attached to the two-position sample holder onthe cold finger of the vertical Konti Spectro A continuous-flow cryostat with TPX windows. The design of thecryostat provides a sliding heat exchanger with the pre-cision positioning system controlled by stepping motors.The possible uncertainties related to misalignments dur-ing the taking of reference measurements, especially atlow frequencies, can be greatly reduced in relative mea-surements by cycling the temperature without movingthe sample [16]. The advantage of this technique is thatall temperature-driven distortions of the optical set-upare already frozen around 20 K, thus making it unneces-sary to take a reference measurement at every tempera-ture in the range 5-45 K. At energies below 5 meV , thesmall size of the sample, combined with strong oscilla-tions that stem from standing waves between the opticalelements of the spectrometer and cryostat windows, pre-vent accurate measurements and set a lower limit in ourexperiment. III. RESULTS
Final outcome of our FTIR spectroscopic study isshown in Fig. 3 ( top panel). The spectral curves forSr Ru − x Re x O − y Se y were taken at 5, 10, 15, 20, 25,35 and 45 K temperatures, then normalized by the 45 Kcurve. For wavenumbers above 220 cm − the set of curvesfor different temperatures becomes horizontal up to smallvertical translations due to noise. At the lower range ofwave numbers (40 −
225 cm − ), one can observe a morecomplex structure. The most noticeable is the first dipwhich occurs within the range 75 −
175 cm − and afterthe peak at 50 −
60 cm − . The curve corresponding to T = 35 K has the lowest deviation ( i.e ., it does not haveas deep of a dip compared to other curves). The othercurves are more packed together and reach roughly thesame peak elevation for the range 50 −
60 cm − . More-over, one can notice that the lower the temperature, thehigher the peak. This behavior closely resembles thatof a typical superconductor ( e.g. boron-doped diamond[17, 18]) as shown in the bottom panel of Fig 3. It is FIG. 3. Reflectance of Sr Ru − x Re x O − y Se y ( top panel)and of superconducting boron-doped diamond ( bottom panel,from Ref. [17]). In both cases, reflectance is normalized by itsnormal-state value. important to note that for the graph in the bottom panelthe curves correspond to temperatures both above andbelow the critical temperature ( T c = 6 K). In our case, T c was not known; however, one can suggest, based onthe relative flatness of the T = 35 K curve, that T c shouldbe slightly below 45 K.A second dip, though less strong in amplitude, occurswithin the range 200 −
225 cm − . This dip is not accom-panied by as high of a peak as the one that was discussedabove. One can theorize that this is due to another,larger gap that occurred within the range 75 −
175 cm − .Based on the fact that the opening of a superconductinggap below T c results in the behavior shown in the bottom panel, one can suggest that in the top panel at T c ∼
35 K,two gaps of different magnitude opened up.The dip in the case of the boron-doped diamond is
FIG. 4. Broad BCS-type singularity in heat capacity whichdisappears with the application of high magnetic field. located at ∼
18 cm − . The major dip in the case ofSr Ru − x Re x O − y Se y corresponds to ∼
125 cm − , i.e. ,the gap is by a factor of seven larger than that of thedoped diamond. This means that T c should be about bya factor of seven higher as well: T c ∼
42 K. Physically,the dip in reflectance corresponds to the maximum of ab-sorption, which takes place at the photon energy ω = 2∆in the “dirty” limit. Let us estimate the gap value fromour data. The wave number k ∼
125 cm − correspondsto photon energies ∼ . T c ∼
42 K, i.e ., about3 . /T c ∼ .
3, which is not far from theBCS value 3 .
53; typically, higher T c materials have 2∆ /T c ratio higher than 3 . IV. DISCUSSION
Let us consider how this spectroscopic result relateswith other facts reported previously on possible super-conductivity in this material [12]. We will first compareit with the heat capacity measurement which we will re-plot in a more elucidating way (Fig. 4). The curve inthis figure is compatible with the BCS behavior of theheat capacity of a superconductor with a broad transi-tion, which most likely characterizes superconductivity inthe heterophase Sr Ru − x Re x O − y Se y . To characterizeits behavior, as shown in Fig. 4, we applied a 5 T mag-netic field to the sample, which reduced the supercon-ducting phase volume mimicking its normal state value FIG. 5. AC magnetic moment M = M (cid:48) +iM (cid:48)(cid:48) ofSr Ru − x Re x O − y Se y . Measurement with AC field ofamplitude 5 Oe and frequency 337 Hz. Inset: Magneticmoment measured by a DC SQUID magnetometer in a 5 Tfield. for the heat capacity. One can conclude that at about23 K , the heat capacity has an upturn compared to itsnormal value, and far below transition, it has values lowerthan in its normal state (as should be expected from thequalitative BCS pattern of superconductivity). An im-portant question here is why the critical temperature atthis measurement is smaller by a factor of two than in thespectroscopic case. A possible answer can be found in therecent results on superconductivity in calcium ruthen-ates [8]. Unlike 60 K superconductivity in Ca RuO mi-crocrystals, superconductivity in bulk polycrystalline (aswell as in macroscopically large crystalline samples) isfully absent [9]. If the mechanism of superconductivityin Sr Ru − x Re x O − y Se y is similar to that of Ca RuO (it is hard to expect that the mechanisms are much dif-ferent!) then T c in the bulk of Sr Ru − x Re x O − y Se y pellet may easily be lower than at the surface layer, evenby a factor greater than 2: the heat capacity reflects thebulk property while the IR reflectance is related with thesurface layer. The micro-crystallites in the surface layershould be relatively free from the effects of the surround-ing material.The magnetization measurements support this conclu-sion, Fig. 5. Typically, for AC magnetic susceptibilitymeasurements, the superconducting transition reveals it-self as a small jump at T = T c of the imaginary part ofthe magnetic susceptibility [19]. Such a jump is indeedobservable on the M (cid:48)(cid:48) − curve of our sample at about 40 K,which comes close to the FIR data (Fig. 3). After fur-ther cooling, the polycrystalline samples with intergran-ular connections may have a broad hump [20–23] similarto the one seen in Fig. 5. Interestingly, the major down-turn of the real part of M (cid:48) , as well as the downturn ofthe magnetic moment measured by the DC magnetome-ter, starts at about 20 K. It appears that the relativecontribution of the surface effects is small in the caseof these quantities, similar to the heat capacity (Fig. 4).These downturns may be indicative of the Meissner effect.These curves have no hysteresis: ZFC and FC curves co-incide, which means that the applied magnetic field isabove H c .Resistive transitions in our material are incomplete,which is most likely the result of intergranular connec-tions in which proximitized superconductivity is sup-pressed by the internal magnetic field that builds up be-low 165 K [12] due to the presence of SrRuO − phase inour heterophase sample. At the same time, the resistivitydownturn becomes suppressed at the lower temperatures,as Fig. 1 indicates (see also the enlarged pattern in itsinset).To complete our discussion, we should mention thatfeatures similar to the ones which we mentioned here havebeen reported in the past for the composition Sr Ru O [15]. They were attributed to magnetic fluctuations, sim-ilar to other cases [24–28]. Leaving aside the applicabilityof the magnetic fluctuations to all other facts pointing to-wards superconductivity in our samples, it is hardly pos-sible that magnetic fluctuations would be able to quanti-tatively explain the spectroscopic data presented in Sec-tion 3 (with the ratio of the gap to the temperature ofits opening being close to the BCS-value). V. SUMMARY
Terahertz spectroscopy, taken together with other ob-servational data on Sr Ru − x Re x O − y Se y , delivered in-dications of high temperature superconductivity with T onset c ∼ K (Fig. 3). This spectroscopic value for T c comes close to the estimate of T c from the measurementsof an imaginary part of the AC susceptibility (Fig. 5).Both properties are likely to be determined by the sur-face layer of our polycrystalline sample. Bulk character-istics, such as the heat capacity (Fig. 4) or the real partof the magnetic susceptibility (Fig. 5) also point towardssuperconductivity and reveal themselves at lower temper-atures. That means that the bulk properties are differentfrom the properties of the surface layer, which sets up abridge between our findings and the recently discoveredsuperconductivity at 60 K in solitary micronanocrystalsof Ca RuO [8]. It is very likely that the mechanism ofsuperconductivity is the same in both cases. ACKNOWLEDGMENTS
A.D. acknowledges support by the Ministry of Sci-ence and Higher Education of the Russian Federation(project RFMEFI62119X0035 and the State assignmentof the FSRC “Crystallography and Photonics” RAS) andthe Shared Research Center FSRC “Crystallography andPhotonics” RAS in part of X-rays diffraction study.The work of Yu.A. and A.M. is carried out within thestate assignment of the Ministry of Science and HigherEducation of the Russian Federation (theme ”Physics ofhigh-temperature superconductors and novel quantummaterials”, No. 0023-2019-0005). FIR measurementswere done using research equipment of the Shared Fa-cilities Center at LPI.The work of Chapman U. research team is supportedby the ONR grants N00014-16-1-2269, N00014-17-1-2972,N00014-18-1-2636 and N00014-19-1-2265. [1] N. P. Armitage, Superconductivity mystery turns 25, Na-ture , 386387 (2019).[2] Y. Maeno, H. Hashimoto, K. Yoshida, S. Nishizaki,T. Fujita, J. G. Bednorz, and F. Lichtenberg, Supercon-ductivity in a layered perovskite without copper, Nature , 532 (1994).[3] A. P. Mackenzie and Y. Maeno, The superconductivityof Sr RuO and the physics of spin-triplet pairing, Rev.Mod. Phys. , 657 (2003).[4] Y. Maeno, S. Kittaka, T. Nomura, S. Yonezawa, andK. Ishida, Evaluation of Spin-Triplet Superconductivityin Sr RuO , Journal of the Physical Society of Japan ,011009 (2012).[5] C. Kallin, Chiral p-wave order in Sr RuO , Reports onProgress in Physics , 042501 (2012).[6] Y. Liu and Z.-Q. Mao, Unconventional superconductivityin Sr RuO , Physica C: Superconductivity and its Appli-cations , 339 (2015).[7] A. Pustogow, Y. Luo, A. Chronister, Y.-S. Su, D. A.Sokolov, F. Jerzembeck, A. P. Mackenzie, C. W. Hicks,N. Kikugawa, S. Raghu, E. D. Bauer, and S. E. Brown,Constraints on the superconducting order parameter in Sr RuO from oxygen-17 nuclear magnetic resonance,Nature , 72 (2019).[8] H. Nobukane, K. Yanagihara, Y. Kunisada, Y. Oga-sawara, K. Isono, K. Nomura, K. Tanahashi, T. Nomura,T. Akiyama, and S. Tanda, Co-appearance of supercon-ductivity and ferromagnetism in a Ca RuO nanofilmcrystal, Scientific Reports , 3462 (2020).[9] M. Braden, G. Andr´e, S. Nakatsuji, and Y. Maeno, Crys-tal and magnetic structure of Ca RuO : Magnetoelasticcoupling and the metal-insulator transition, Phys. Rev.B , 847 (1998).[10] J. Zhang, A. S. McLeod, Q. Han, X. Chen, H. A. Bech-tel, Z. Yao, S. N. Gilbert Corder, T. Ciavatti, T. H.Tao, M. Aronson, G. L. Carr, M. C. Martin, C. Sow,S. Yonezawa, F. Nakamura, I. Terasaki, D. N. Basov,A. J. Millis, Y. Maeno, and M. Liu, Nano-ResolvedCurrent-Induced Insulator-Metal Transition in the MottInsulator Ca RuO , Phys. Rev. X , 011032 (2019).[11] J. A. N. Bruin, H. Sakai, R. S. Perry, and A. P. Macken-zie, Similarity of Scattering Rates in Metals Showing T-Linear Resistivity, Science , 804 (2013). [12] A. M. Gulian and V. R. Nikoghosyan, Serendipitous vs.systematic search for room-temperature superconductiv-ity, Quantum Studies: Mathematics and Foundations ,161 (2018).[13] J. J. Neumeier, M. F. Hundley, M. G. Smith, J. D.Thompson, C. Allgeier, H. Xie, W. Yelon, and J. S. Kim,Magnetic, thermal, transport, and structural propertiesof Sr RuO δ : Enhanced charge-carrier mass in a nearlymetallic oxide, Phys. Rev. B , 17910 (1994).[14] A. Steppke, L. Zhao, M. E. Barber, T. Scaffidi,F. Jerzembeck, H. Rosner, A. S. Gibbs, Y. Maeno, S. H.Simon, A. P. Mackenzie, and C. W. Hicks, Strong peakin T c of Sr RuO under uniaxial pressure, Science (2017).[15] S.-I. Ikeda, Y. Maeno, S. Nakatsuji, M. Kosaka, andY. Uwatoko, Ground state in Sr Ru O : Fermi liquidclose to a ferromagnetic instability, Phys. Rev. B ,R6089 (2000).[16] A. Perucchi, L. Baldassarre, B. Joseph, S. Lupi, S. Lee,C. B. Eom, J. Jiang, J. D. Weiss, E. E. Hellstrom,and P. Dore, Transmittance and reflectance measure-ments at terahertz frequencies on a superconductingBaFe . Co . As ultrathin film: an analysis of the op-tical gaps in the Co-doped BaFe As pnictide, The Eu-ropean Physical Journal B , 274 (2013).[17] S. Lupi, Terahertz Spectroscopy of Novel Supercon-ductors, Advances in Condensed Matter Physics ,816906 (2011).[18] M. Ortolani, S. Lupi, L. Baldassarre, U. Schade,P. Calvani, Y. Takano, M. Nagao, T. Takenouchi, andH. Kawarada, Low-Energy Electrodynamics of Supercon-ducting Diamond, Phys. Rev. Lett. , 097002 (2006).[19] M. Couach, A. Khoder, and F. Monnier, Study of su-perconductors by a.c. susceptibility, Cryogenics , 695(1985).[20] R. B. Goldfarb, M. Lelental, and C. A. Thompson,Alternating-Field Susceptometry and Magnetic Suscep-tibility of Superconductors, in Magnetic Susceptibility ofSuperconductors and Other Spin Systems , edited by R. A. Hein, T. L. Francavilla, and D. H. Liebenberg (SpringerUS, Boston, MA, 1991) pp. 49–80.[21] L. Civale, T. K. Worthington, L. Krusin-Elbaum, andF. Holtzberg, Nonlinear A.C. Susceptibility ResponseNear the Irreversibility Line, in
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