High-Pressure Electrical Resistivity Measurements of EuFe2As2 Single Crystals
N. Kurita, M. Kimata, K. Kodama, A. Harada, M. Tomita, H. S. Suzuki, T. Matsumoto, K. Murata, S. Uji, T. Terashima
aa r X i v : . [ c ond - m a t . s up r- c on ] A ug High-Pressure Electrical Resistivity Measurements ofEuFe As Single Crystals
N Kurita , , M Kimata , , K Kodama , , A Harada , M Tomita ,H S Suzuki , T Matsumoto , K Murata , S Uji , , and T Terashima , National Institute for Materials Science, Tsukuba, Ibaraki 305-0003, Japan JST, Transformative Research-Project on Iron Pnictides (TRIP), Chiyoda, Tokyo 102-0075,Japan Graduate School of Pure and Applied Sciences, University of Tsukuba, Ibaraki 305-0003, Japan Division of Molecular Materials Science, Graduate School of Science, Osaka City University,Osaka 558-8585, JapanE-mail:
Abstract.
High-pressure electrical resistivity measurements up to 3.0 GPa have been performed onEuFe As single crystals with residual resistivity ratios RRR = 7 and 15. At ambient pressure,a magnetic / structural transition related to FeAs-layers is observed at T = 190 K and 194 K forsamples with RRR = 7 and 15, respectively. Application of hydrostatic pressure suppresses T ,and then induces similar superconducting behavior in the samples with different RRR values.However, the critical pressure ∼ T →
0, for the samples with
RRR = 15 is slightlybut distinctly larger than ∼ RRR = 7.
1. Introduction
Since the discovery of superconductivity in LaFeAs(O,F) with T c = 26 K [1], a familyof Fe-pnictide superconductors has attracted much attention. In particular, A Fe As ( A = Ca, Sr, Ba, Eu, etc.) with a tetragonal ThCr Si -type structure has been intensively studiedbecause of the availability of stoichiometric single crystals with high quality. It turned outthat, in Fe-pnictide compounds, the superconducting (SC) ground state could appear inaccordance with the suppression of a magnetic/structural transition by doping [2]. In thephase diagrams, it is argued that the superconductivity could coexist and/or compete with theantiferromagnetism [3, 4]. However, a random potential introduced by doping could smear theintrinsic SC properties. For understanding the origin of the high- T c superconductivity with T c upto 55 K [5], it is of considerable importance to probe the systematic change of ground states usinghigh-quality single crystals. An alternative way to tune the ground state is to apply hydrostaticpressure ( P ). For instance, recent high- P ac-susceptibility and resistivity measurements haverevealed that A Fe As ( A = Sr, Eu) exhibits P -induced bulk superconductivity by suppressingthe magnetic/structural transition [6 − P is absent in CaFe As [11 − As [8, 15 − A Fe As series, EuFe As is quite unique because the localized Eu momentsorder antiferromagnetically at T N ∼
20 K, in addition to the magnetic/structural transitionrelated to FeAs-layers at T ∼
190 K [18 − momentsan be detected even in the SC state induced by doping or application of pressure, which couldbe a main reason for the novel reentrant-SC-like behavior [9, 10, 22 − P electrical resistivity measurements in EuFe As usingnewly grown single crystals with a residual resistivity ratio ( RRR ) as high as 15. At ambient P , the magnetic/structural transition occurs at a higher temperature of T = 194 K, comparedwith 190 K for single crystals with RRR = 7. Consequently, it is found that the higher qualitysingle crystal requires higher- P to suppress T , and to induce the SC ground state in EuFe As .
2. Experimental Details
Single crystals of EuFe As were grown by Bridgman method from a stoichiometric mixture ofthe constituent elements. In this study, we examined several crystals from two different batcheswith residual resistivity ratios RRR = 7 and 15, where
RRR is defined as ρ /ρ . Singlecrystals measured in Ref [10] were taken from a batch with RRR = 7. High-pressure resistivitymeasurements of samples with
RRR = 7 and 15 have been performed simultaneously up to3.0 GPa using a hybrid-type piston cylinder pressure device [26]. The resistivity was measuredby the four-probe method with an ac current I = 0.3 mA in the ab - plane. To generate hydrostaticpressure, Daphne 7474 (Idemitsu Kosan) oil, which remains in a liquid state up to 3.7 GPa atroom temperature [27], was used as a pressure-transmitting medium. Samples were cooled downin Oxford He system, slowly with an average rate of 0.5 K/min. Applied pressure was estimatedat 4.2 K from the resistance change of a calibrated Manganin wire [28].
3. Results and Discussions
Figure 1 shows the temperature ( T ) dependence of electrical resistivity scaled at 300 K ( ρ/ρ )in EuFe As single crystals with RRR = 7 and 15, where
RRR is determined as ρ /ρ . Themeasurement was performed in zero field at ambient pressure outside a pressure device withcurrent direction I k ab . To our knowledge, RRR = 15 is the largest value in EuFe As singlecrystals [9, 21, 29]. Overall T -variations of the resistivity in the samples with RRR = 7 and 15 ρ / ρ Κ EuFe As T T N RRR = 7
RRR = 15 I || ab , 0 GPa ρ / ρ Κ T ρ / ρ Κ T N Figure 1. (Color online) The scaled electrical resistivity ρ/ρ versus temperature inEuFe As single crystals with RRR = 7 and 15. The measurement was carried out in zero-field at ambient pressure with the current direction I k ab . Upper left and lower right insetsrepresent the expanded views around T = T and T N , respectively. The data for the sample with RRR = 7 in the lower right inset is arbitrarily shifted in vertical direction for clarity.re qualitatively similar to each other, and are consistent with previous results [9, 20, 21, 29]. Itis worthwhile to mention that, as shown in the upper left inset, a magnetic/structural transitiontemperature T = 194 K for the sample with RRR = 15 is higher than T = 190 K for the samplewith RRR = 7. This would be the reason why samples with
RRR = 15 needs higher pressure( P ) to suppress T , as will be discussed below. The N´eel temperature T N of the localized Eu moments for the sample with RRR = 15 is slightly higher than the value for the sample with
RRR = 7, as can be seen in the lower right inset.Next, we turn to the pressure effect on the electrical resistivity for the samples with
RRR = 7(Fig. 2) and 15 (Fig. 3), which are simultaneously measured in the same pressure device. Withincreasing P , the resistivity peak related to the magnetic/structural transition is suppressedto a lower temperature in both samples as shown in Figs. 2(a) and 3(a). For the sample with RRR = 7, a reminiscence of the peak is clearly recognized at 2.38 GPa around 100 K, and faintlyvisible at 2.46 GPa around 70 K as shown in Fig. 2(a). At 2.55 GPa, there is no detectableanomaly, which implies that the critical pressure P c , where T →
0, may be about 2.5 GPa.For the sample with
RRR = 15, P c would be ∼ T -linear behavior above T c at 2.55 and 2.77 GPa ( P ∼ P c ) forsamples with RRR = 7 and 15, respectively, as guided by a dashed line. A similar T -variation ofresistivity was also reported in several optimally-doped Fe-pnictide superconductors [4, 30 − RRR = 7, a resistivity upturn and a small maximum, as indicated by anarrow in Figs. 2(b), in the broad SC transition below 31 K are observed at P = 2.38 GPa ( < P c ).It suggests that the superconductivity is suppressed by the magnetic order of the Eu moments;consequently, reentrant-SC-like behavior appears. A similar behavior is also slightly seen for thesample with RRR = 15 (Figs. 3(b)), but more smeared out. At
P > P c , resistivity exhibits sharpSC transitions to zero-resistivity with T c ∼
30 K for both samples. With increasing P , the SC ρ ( µ Ω c m ) EuFe As P (GPa) 2.38 2.46 2.55 (a) RRR = 7 ∝ Τ ρ ( µ Ω c m ) EuFe As P (GPa) 2.46 2.55 2.77 3.00 (b) RRR = 7
Figure 2. ρ vs T of a EuFe As singlecrystal ( RRR = 7) up to 3.0 GPa in thetemperature ranges (a) 30 −
120 K and(b) 15 −
40 K. An arrow in (b) indicatesan anomaly attributed to T N ρ ( µ Ω c m ) EuFe As (a) RRR = 15 P (GPa) 2.46 2.69 2.77 ∝ Τ ρ ( µ Ω c m ) EuFe As P (GPa) 2.69 2.77 3.00 (b) RRR = 15
Figure 3. ρ vs T of a EuFe As singlecrystal ( RRR = 15) up to 3.0 GPa in thetemperature ranges (a) 30 −
120 K and(b) 15 −
40 K. Arrows in (b) indicateanomalies attributed to T N .ransitions persist up to 3.00 GPa although the T c continuously decreases. Thus, the P -variationof the resistive behavior between the samples with different quality is qualitatively similar toeach other, and is consistent with the previous result [10]. However, P c ∼ RRR = 15 is slightly but distinctly larger than ∼ RRR = 7,which may be as a consequence of the larger value of T for the higher-quality sample at ambient- P . We have repeated similar high- P resistivity measurements using several single crystals, andconfirmed that the observed difference in the magnitude of T and P c between the sampleswith RRR = 7 and 15 is beyond the error of the pressure estimation ( ± × − GPa) [28].Another meaningful issue, which probably relates to the sample quality, is the width of a SCtransition ∆ T c . The minimum values of ∆ T c are 1 K and 0.8 K for samples with RRR = 7 and15, respectively. These facts suggest that the higher-quality single crystals have larger values of T and P c as well as a sharper SC transition in EuFe As .Until now, there has been no report concerning the quantum oscillation in EuFe As , despitethe importance for understanding the Fermi surface topology and mass renormalization. Infact, we have already tried de Haas-van Alphen (dHvA) measurements of EuFe As using thesamples with RRR = 7 at 0.6 K with fields up to 35 T, but could not detect any dHvA oscillation.Given that quantum oscillations were successfully detected in SrFe As ( RRR ∼
8) [33] andBaFe As ( RRR = 10) [34], it is worthwhile to perform the dHvA measurement of EuFe As using the newly grown single crystals with RRR = 15.
4. Conclusions
We have performed high-pressure electrical resistivity measurements up to 3.0 GPa in EuFe As single crystals with RRR = 7 and 15. At ambient pressure, a magnetic/structural transitionoccurred at T = 190 K and 194 K for the samples with RRR = 7 and 15, respectively. Although P -induced superconductivity was confirmed in the samples with different RRR values, thecritical pressure P c ∼ RRR = 15 was slightly but distinctly largerthan ∼ RRR = 7.
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