Enhanced conductivity and weakened magnetism in Pb-doped Sr2IrO4

Zhi-Lai Yue(岳智来), Wei-Li Zhen(甄伟立), Rui Niu(牛瑞), Ke-Ke Jiao(焦珂珂),Wen-Ka Zhu(朱文卡),†, Li Pi(皮雳),‡, and Chang-Jin Zhang(张昌锦),3,§

1High Magnetic Field Laboratory,Hefei Institutes of Physical Science,Chinese Academy of Sciences,Hefei 230031,China

2Science Island Branch of Graduate School,University of Science and Technology of China,Hefei 230026,China

3Collaborative Innovation Center of Advanced Microstructures,Nanjing University,Nanjing 210093,China

Keywords: iridates,doping,conductivity,magnetism

1.Introduction

The search for possible room-temperature superconductors and the establishment of high-temperature superconducting mechanisms are two of the main scientific challenges facing the condensed matter physics community.[1]At ambient pressure,two classes of high-temperature superconducting materials, i.e., cuprate and iron-based superconductors,[2,3]have been discovered.The exploration of superconductors with transition temperature beyond the McMillan limit has attracted worldwide attention not only from the scientific community, but also from industry, as high-temperature superconductors have the potential to revolutionize electrical technology.[4]Besides the exploration of new superconducting materials, intensive experimental and theoretical investigations have been performed in order to unveil the mysterious pairing mechanisms of high-temperature superconductivity.[5–10]Despite the fact that significant progress has been achieved in the exploration of hightemperature superconducting mechanisms,the physical nature of the driving forces causing two electrons to form a superconducting pair is still under debate.Scientists are now eagerly expecting to discover a new family of high-temperature superconducting materials,which would greatly contribute to the final establishment of high-temperature superconducting mechanisms.

Sr2IrO4is a 5d transition metal oxide material that has a similar layered perovskite-type crystal structure to the parent compound of the cuprate high-temperature superconductor,La2CuO4.[11]Similar to theS=1/2 Mott insulating state in La2CuO4,the unique electronic and magnetic behaviors of Sr2IrO4could be accounted by a novelJeff= 1/2 Mott insulating state due to large relativistic spin–orbit coupling.[12]ThisJeff=1/2 Mott state leads to a canted antiferromagnetic ground state in Sr2IrO4, which is also reminiscent of the antiferromagnetic ground state in La2CuO4.Due to the similarities in the crystal structures and electronic and magnetic ground states of Sr2IrO4and La2CuO4, it is natural to anticipate that similar physical phenomena, especially the possible high-temperature superconductivity, could be realized upon modulating the physical behaviors of Sr2IrO4by chemical doping,pressure and other methods.Moreover,a series of theoretical work on the band structures and phase diagrams of doped Sr2IrO4have predicted that possible high-temperature superconductivity could be achieved upon proper chemical doping.[13–16]

Experimentally, the effects of chemical doping on the Sr2IrO4compound have been extensively investigated during the past several years.[17–28]Many intriguing physical phenomena, such as the formation of surface Fermi arcs and pseudogaps,[20]as well as the fragmented antiferromagnetic order,[22]have been observed.However, bulk superconductivity has not yet been achieved.Among the various possible dopants, La is the most often chosen element in investigations of the effects of doping on the physical properties of Sr2IrO4.[17,18,22,28]Generally, doping can result in structural modifications of the parent material,which in turn leads to alterations of its band filling.In Sr2IrO4,partial substitution of Sr by La induces a change in the bandwidth.[17,18]However,the low solubility of dopants in Sr2IrO4significantly limits the band filling control over a wide range.For instance, the transport data of a(Sr1−xLax)2IrO4single crystal withx=0.04 suggest that the insulating state could be driven into a metallic state.[29]However,no further data could be found with higher La doping level in Sr2IrO4single crystals.Besides La, other rare-earth elements and metal elements,such as K,Tb,Nd,Pr and Nb,have been chosen as dopants to investigate the physical behaviors of doped Sr2IrO4.[23–27]It was quite surprising to find that the insulating state is very robust against doping in Sr2IrO4.[26]Thus it is necessary to extend the investigation of effects of doping in the Sr2IrO4compound by choosing other possible elements as dopants.

In this work, we report the influence of Pb doping on the physical properties of Sr2IrO4.Pb is a group IV element which can be steadily coordinated at the A-site of ABO3-type perovskite compounds such as PbTiO3and Pb(Mg1/3Nb2/3)O3.[30,31]Since Sr2IrO4is crystallized in a K2NiF4-type perovskite structure where the coordination conditions of Sr-site ions are similar to those of A-site ions in ABO3-type perovskite materials, we anticipate that Pb ions could be incorporated into the Sr site of Sr2IrO4.It is found that the solid solubility of Pb in Sr2−xPbxIrO4could be as high asx=0.3.The magnetic data reveal that although the longrange canted antiferromagnetic ordering state is substantially suppressed by Pb doping,the net ferromagnetic moment could still be detected even in thex=0.3 sample.Quantitatively,the net ferromagnetic moment decreases from 0.042µB/Ir in the parent Sr2IrO4sample to 0.033µB/Ir in thex=0.3 sample.Compared to those reported in previous literatures, the suppression of magnetic ordering state in Pb-doped samples is the weakest among the doped Sr2IrO4compounds,suggesting that Pb ions do not severely alter the pristine ordered magnetic state in the Sr2IrO4system.The Sr2−xPbxIrO4polycrystalline samples exhibit an insulator-like behavior when the Pb doping content is less thanx=0.2.When the Pb doping level is higher than 0.2, a drastic increase of conductivity is seen, particularly in the low-temperature region.The greatly enhanced conductivity in Sr2−xPbxIrO4polycrystalline samples deserves further study through subsequent single-crystal growth and a comprehensive investigation of their physical properties.Taking into account the high solid solubility of Pb in the Sr2IrO4matrix,Sr2−xPbxIrO4compounds could serve as ideal material bases in future investigations of doping-induced physical phenomena in the Sr2IrO4system.

2.Experiment

The Sr2−xPbxIrO4polycrystalline samples were prepared via a conventional solid-state reaction technique.In detail,mixtures of high purity Sr2CO3,PbO and IrO2were weighed at a stoichiometric molar ratio of SrCO3:PbO:IrO2=(2−x):x:1.02 and thoroughly mixed in an agate mortar.In order to compensate for the possible loss of Ir at high temperature,we added 2%of additional IrO2in the starting materials.The mixtures were loaded into alumina crucibles and put into a high-temperature furnace,which was preheated to 1100°C for 24 hours.After another grinding,the mixtures were re-heated once again at 1150°C for 24 hours.Then the mixtures were ground and pressed into pellets.The pellets were finally sintered at 1150°C for 48 hours.

Powder x-ray diffraction(XRD)data were collected by a PANalytical X’Pert Pro MPD detector using monochromated CuKα1radiation as the x-ray source.The chemical compositions of the samples were determined with the aim of energy dispersive x-ray spectrometry analyses using an Oxford Swift 3000 spectrometer equipped with a Hitachi TM3000 scanning electron microscope.The temperature and field dependences of magnetization were measured on a Quantum Design superconducting quantum interference device magnetometer.The temperature dependence of resistivity was measured on a physical property measurement system using the standard four-probe method.

3.Results and discussion

Figure 1(a) shows the powder XRD patterns of the Sr2−xPbxIrO4samples.The parent material of Sr2IrO4crystallizes in a K2NiF4-type perovskite tetrahedral-phase structure,with space groupI41/acd.[11]It can be seen from Fig.1(a)that all the XRD diffraction peaks of the Sr2−xPbxIrO4samples are nearly identical to those of the Sr2IrO4parent sample,without any impurity peaks.This fact suggests that all Sr2−xPbxIrO4samples are crystallized into a single-phase crystal structure.In order to quantitatively determine the lattice parameters of the Sr2−xPbxIrO4samples,we performed detailed Rietveld refinements on the XRD data.Typical Rietveld refinement results of thex=0.3 sample are shown in Fig.1(b).It can be seen that the calculated curve can well reproduce the experimental data,suggesting that the refinements are reliable.Figure 1(c) gives the variations of thea- andc-axis lattice constants of the Sr2−xPbxIrO4samples.Thea-andc-axis lattice constants of the undoped Sr2IrO4sample are 5.4905 ˚A and 25.751 ˚A,respectively.These values are consistent with those reported in the literature.[11]With increasing Pb doping,botha-andc-axis lattice constants increase monotonously,reflecting the influence of Pb dopants on the lattice structures.

Fig.1.(a)Powder XRD patterns of the Sr2−xPbxIrO4 samples detected at room temperature.(b)Typical Rietveld refinements to the x=0.3 sample.The black hollow circles are experimental data.The red curve is the fitting to experimental data.The green curve is the difference between the experimental data and the fitting results.(c)The variations of lattice constants with increasing Pb doping.(d)The lattice constant ratio of the c-to a-axis(c/a ratio)and the in-plane Ir–O–Ir bond angle at different Pb doping levels.

It is well known that the IrO6octahedra in Sr2IrO4are rotated around the crystallographiccaxis by about 11°.[11]As a consequence of this rotation, the in-plane Ir–O–Ir bond is significantly bent away from a perfect IrO2plane.The resultant in-plane Ir–O–Ir bond angle is about 157.9°in the pristine Sr2IrO4sample.It is not yet clear whether or not the bent character of the Ir–O–Ir bond plays a crucial role in determining the transport and magnetic properties of the Sr2IrO4compound.In order to learn the influence of Pb doping on the Ir–O–Ir bond of the Sr2−xPbxIrO4samples,we calculated the in-plane Ir–O–Ir bond angle of the Pb-doped samples.The results are plotted in Fig.1(d).It can be seen that the introduction of Pb results in a monotonous increase of the in-plane Ir–O–Ir bond angle.The monotonous strengthening of the inplane Ir–O–Ir bond indicates that the rotation of IrO6octahedra is substantially released in Pb-doped samples.In Fig.1(d)we give the lattice constant ratio of thec- to thea-axis (c/aratio) at different Pb doping levels.It is found that thec/aratio monotonously increases with increasing Pb doping.In a layered perovskite oxide, the enhancedc/aratio suggests that the material becomes more two-dimensional.This twodimensional feature is consistent with the increased Ir–O–Ir bond angle.

In order to know whether or not the Pb ions are uniformly distributed and to what extent the Pb ions are melted into the samples, we performed energy dispersive x-ray spectrometry(EDX)analyses on the Sr2−xPbxIrO4samples.Figure 2 shows the typical EDX spectrum of thex=0.3 sample.It can be seen from the elemental mapping data of Figs.2(a)–2(c) that the Sr, Pb and Ir ions are all nearly uniformly distributed in the Sr2−xPbxIrO4polycrystalline samples, with no evidence of phase separations.The distribution of Pb is regular, confirming the successful incorporation of Pb in the Sr2IrO4matrix.Figure 2(d)gives a typical EDS spectrum of thex=0.3 sample.The quantitative analyses of the EDS spectra of the Sr2−xPbxIrO4samples confirm that the real compositions of the obtained samples are comparable to the nominal compositions.Combining the XRD data and the EDS results, it is suggested that the Pb ions are uniformly incorporated into the Sr2IrO4matrix,with solid solubility as high asx=0.3.

Fig.2.(a)–(c) The energy-dispersive x-ray spectroscopy (EDS) mapping data of the Sr(a),Pb(b)and Ir(c)elements of the Sr1.7Pb0.3IrO4 sample.(d)A typical EDS spectrum confirming the chemical composition of Sr1.7Pb0.3IrO4.

Figure 3(a)gives the temperature dependence of dc magnetic susceptibility (χ–T) of the Sr2−xPbxIrO4samples measured under zero-field-cooling (ZFC) and field-cooling (FC)processes.For the parent Sr2IrO4sample,theχ–Tcurves exhibit a paramagnetic-like behavior at high temperature, with theχ–Tcurves measured under the ZFC and FC conditions nearly identical.BelowTN～240 K, theχ–Tcurves exhibit a drastic upturn with decreasing temperature.In addition,theχ–Tcurves measured under ZFC condition and FC condition diverge, consistent with the formation of a canted antiferromagnetic state.[12]Previous studies have revealed that the Ir 5d orbitals are strongly hybridized with the neighboring O 2p orbitals and thus significant parts of the moments are canceled in the canted antiferromagnetic ordered state,leading to a net ferromagnetism in Sr2IrO4.[11,12]

A remarkable observation is that the N´eel temperatureTNstays at about 240 K in all Pb-doped samples.The nearly unchanged N´eel temperature in the Pb-doped samples is sharply different from that in many other elements (such as La, K,Tb, Sm, Pr, and Nb)doped samples, where the incorporation of dopants readily depresses the antiferromagnetic ordering state.[17,21–26]Interestingly,we find that the nearly unchangedTNhints at a certain similarity to the cases in Fe and Sc doping at the Ir-site of Sr2IrO4.[32,33]In Fe-doped samples, it is suggested that the Fe dopants have only aS=1 spin, thus carrying very small magnetic moments that are too weak to effectively affect the superexchange interactions between the Ir ions.[32]In the Pb-doped samples,the charge-transfer effect between the Sr(Pb)O layer and the IrO2layer is weak; thus the superexchange interactions within the InO2layer is hardly influenced.

It should be noted that with the incorporation of Pb, the absolute value of magnetic susceptibility exhibits a slight decrease.The decrease of the absolute value of magnetic susceptibility means that the net ferromagnetic moment is slightly decreased in the Pb-doped samples.Figure 3(b) gives the irreversibility curve of the Sr2−xPbxIrO4samples obtained by subtracting the ZFC data from the FC data.It can be seen that the amplitude of the irreversibility data decreases with increasing Pb doping content, consistent with the decrease of net ferromagnetic moment.The decrease of net ferromagnetic moment is due to the strong coupling of the magnetic moment and lattice.[34–36]Due to the coupling of the magnetic moment and lattice, a relaxed Ir–O–Ir bond angle (Fig.1(c)) inevitably weakens the Dzyaloshinskii–Moriya interaction that drives the canted antiferromagnetic ordering state in Sr2IrO4,leading to the decrease of net ferromagnetic moment.It can be also seen from Fig.3(b)that the decrease of net ferromagnetic moment is small in all Pb-doped samples.Quantitatively,the net ferromagnetic moment decreases from 0.042µB/Ir in the undoped Sr2IrO4sample to 0.033µB/Ir in thex=0.3 sample.This small decrease of net ferromagnetic moment suggests that the introduction of Pb dopants only moderately suppresses its magnetic ordered state.Figure 3(c)gives the isothermal magnetization (M–H) curve of thex=0.3 sample.The apparent magnetic hysteresis loop displayed in theM–Hcurve of thex=0.3 sample confirms that a substantial amount of net ferromagnetic moment still survives in heavily Pb-doped samples.

Fig.3.(a)Temperature dependence of DC magnetic susceptibility of the Sr2−xPbxIrO4 samples measured under zero-field-cooling(ZFC)and field-cooling (FC) processes.The applied magnetic field is 1500 Oe.(b) The irreversibility curve of the DC magnetic susceptibility of the Sr2−xPbxIrO4 samples obtained by subtracting the ZFC data from the FC data.(c) Magnetic field dependence of DC magnetization of the x=0.3 sample measured at 10 K.

Figure 4(a) shows the temperature dependence of resistivity (ρ–T) of the Sr2−xPbxIrO4samples.The undoped Sr2IrO4sample exhibits a typical insulator-like behavior over the whole temperature range, consistent with its spin–orbit coupling drivenJeff=1/2 Mott insulating state.[12]With the incorporation of Pb, the resistivity decreases in the 100 K–300 K temperature region,suggesting an enhanced conductivity.For thex ≤0.15 samples, they exhibit typical insulatorlike behavior over the whole temperature range,similar to the undoped Sr2IrO4sample.However, for thex ≥0.2 samples,theρ–Tcurves exhibit different features compared to those of thex ≤0.15 samples.For example, the resistivity only displays a small increase with decreasing temperature, showing typical semiconductor-like behavior.In addition, there is an inflection point at around 100 K.These different features suggest that the incorporation of a sufficient amount of Pb could alter the electronic conductivity of the Sr2−xPbxIrO4compounds.

In order to learn more about the conductive mechanisms of the Sr2−xPbxIrO4samples,we perform a fitting to theρ–Tcurves of the samples.The fitting is performed according to the segmentation fitting method proposed by Kiniet al.[37]In the high temperature region (210 K–300 K), theρ–Tcurves of Sr2IrO4could be fitted using the logarithmic relation ofρ(T) =ρ0e−αT, whereρ0andαare constants dependent on the samples.The lnρ–Tcurves of the Sr2−xPbxIrO4samples are plotted in Fig.4(b).It can be seen that the lnρ–Tcurves of all Sr2−xPbxIrO4samples yield nearly linear behavior in the 210 K–300 K regions, suggesting that the logarithmic relation could well explain the transport mechanisms of the Sr2−xPbxIrO4samples.In the intermediate temperature region of 110 K–190 K,Kiniet al.suggest that the conduction mechanism of Sr2IrO4could be described using a thermally activated model according to an Arrhenius type relation ofρ(T)=ρ0eΔ/(2kBT),whereΔis a constant relating to the energy gap of the sample andkBis the Boltzmann constant.The nearly linear lnρ–T−1curves displayed in Fig.4(c) suggest that the conduction mechanism of the Sr2−xPbxIrO4samples could be ascribed using the thermally activated model in the 110 K–190 K temperature region.According to the slope of the lnρ–T−1curves,the energy gap of the Sr2−xPbxIrO4samples can be derived.The energy gap of the Sr2IrO4sample is 115 meV,which is consistent with the small theoretically calculated gap value.[38]The energy gap decreases monotonously with increasing Pb doping.For example, the energy gaps of thex=0.1 and 0.2 samples are 104 meV and 38 meV,respectively.For thex=0.3 sample,the energy gap is only 4.1 meV.The continuous decrease of energy gap could be connected to the increase of Ir–O–Ir bond angle with Pb doping.In Sr2IrO4,the significantly bent Ir–O–Ir bond hinders the free motion of charge carriers within the IrO2conductive plane.With the incorporation of Pb,the in-plane Ir–O–Ir bond becomes straightened.The straightened Ir–O–Ir bond favors orbital hybridization between the Ir 4f electrons and the O 2p electrons,leading to a monotonous decrease of the energy gap and an enhancement of the electrical conductivity in Sr2−xPbxIrO4samples.

Fig.4.(a)Temperature dependence of resistivity of the Sr2−xPbxIrO4 polycrystalline samples.(b)The lnρ versus T curves in the 210–300 K temperature region.(c)The lnρ versus T−1 curves in the 110–190 K temperature region.

4.Conclusion

We report the synthesis and physical properties of a series of Sr2−xPbxIrO4samples.The detailed Rietveld refinements to the x-ray diffraction data and the energy-dispersive x-ray spectrometry analyses provide dual evidence that the Pb ions are successfully incorporated into the Sr2IrO4matrix with solid solubility up tox=0.3.The incorporation of Pb results in a moderate destruction of the canted antiferromagnetic state of the Sr2IrO4compound.The incorporation of Pb substantially enhances the conductivity of Sr2−xPbxIrO4samples at high temperature.The energy gap decreases monotonously with increased Pb doping.However,the energy gap could not reach zero even in the heaviest Pb-doped samples.Despite the greatly enhanced conductivity,complete metallization remains unrealized in Sr2−xPbxIrO4samples.In future, the growth of single crystals and the investigation of the physical properties of Sr2−xPbxIrO4single-crystal samples are needed.In addition, preparing the Pb-doped samples withx ＞0.3 using unconventional synthetic techniques such as the high-pressure high-temperature method is highly anticipated in order to realize complete metallization and,more importantly,superconductivity.

Acknowledgements

Project supported by the National Key R&D Program of China(Grant Nos.2022YFA1403203 and 2021YFA1600201),the National Natural Science Foundation of China (Grant Nos.11974356 and 12274414), the Joint Funds of the National Natural Science Foundation of China and the Chinese Academy of Sciences Large-Scale Scientific Facility (Grant No.U1932216).