Raman phonon anomalies in Sr(Fe1−xCox)2As2

Yanxing Yang(杨彦兴) Hewei Zhang(张鹤巍) and Haizheng Zhuang(庄海正)

1School of Physics,Communication and Electronics,Jiangxi Normal University,Nanchang 330022,China2Department of Physics,New Jersey Institute of Technology,Newark 07102-1982,USA

Phonon anomalies have been reported in iron-pnictide superconductors indicating a diverse interplay between different orders in the materials. Here, we report Raman scattering measurements on Sr(Fe1−xCox)2As2 (x =0 and x =0.04)single crystals in the B2g symmetry with respect to a 1 Fe unit cell. Upon cooling, we observe a larger split (13 cm−1)of Eg Raman phonon modes pertaining to in-plane Fe and As displacements as the crystals undergo the tetragonal-toorthorhombic structural phase transition,although a considerable split(9 cm−1)has been reported in Ba(Fe1−xCox)2As2.Furthermore,the splitting of Eg phonon modes is strongly reduced upon doping. We perform an order-parameter analysis revealing a similar doping dependence of Eg phonon splitting as reported in other compounds of the 122 family,indicating these phonon anomalies widely exist in 122 iron-based superconductors and might share the same mechanisms.

Keywords: Raman scattering,superconductor,phonon,phase transition

1. Introduction

The discovery of superconductivity in the LaOFeP compound[1]in 2006 flipped the traditional understanding that Fe is harmful to superconductivity.In 2008,superconductivity was discovered in the compound LaO1−xFxFeAs with a maximalTcof 26 K[2]opening up a brand new family of high-Tcsuperconductors based on iron-arsenide superconducting layers. Since then, superconductivity in a large number of compounds based on iron-arsenide layers has been discovered withTcup to 56 K.[3–5]On the basis of their chemical formulae,these compounds are mainly referred to as the 1111 family(e.g., LaOFeAs[1]), 122 family (e.g., BaFe2As2[6]), 111 family(e.g.,LiFeAs[7])and 11 family(e.g.,FeSe[8]).The building block shared by all these compounds referred to as the FeAs plane,has been proposed to account for the magnetic and superconducting properties of these systems.[9]Unlike undoped cuprates behaving like Mott insulators,undoped iron-pnictides act like bad metals with a magnetic order of the spin density wave (SDW) at low temperatures. Superconductivity arises from suppressing the magnetic order and structural phase transition upon doping with electrons or holes.[10]More recently,superconductivity is induced in BaFe2As2compound without doping or even applying hydrostatic pressure,[11]providing a deeper insight into the interplay between magnetic/structural and superconducting transition.

It is clear that magnetic order and superconducting transition are mutually exclusive,however charge nematicity,SDW and structural transition emerge in almost the same region of the phase diagram, resulting in the ambiguity of these orders at transition points. The evolution of in-plane phonon modes,i.e.,EuandEg,with temperature have been extensively reported.[12–15]Specifically,upon cooling,a large splitting ofEgRaman phonon modes involving in-plane Fe and As displacements was observed in Ba(Fe1−xCox)2As2as the crystals undergo the tetragonal-to-orthorhombic transition.[12]Intuitively, doubly degenerate in-planeEgphonons split for the orthorhombic distortion of lattice during the structural transition. However, such splitting due to the lattice distortion has been suggested to be so tiny that it could not be observed under the current resolutions.[16]Hence, the observed large splitting must be driven by some other degrees of freedom.It has been proposed that charge nematicity is weakly coupled to the lattice,[17,18]thus spin-phonon coupling has been believed to be the driving force of the phonon anomalies.[12]Sr(Fe1−xCox)2As2compounds exhibit SDW order upon cooling as well as Ba(Fe1−xCox)2As2with higher transition temperatures, therefore we expect stronger phonon anomalies in Sr(Fe1−xCox)2As2.

Here, we report Raman scattering measurements on Sr(Fe1−xCox)2As2(x=0 andx=0.04) single crystals with Raman shifts from 9 cm−1up to 400 cm−1in theB2gsymmetry with respect to 1 Fe unit cell described elsewhere.[12]Particularly,the two single crystals do not show superconducting transition upon cooling down to 4.2 K, but exhibit magnetic order with Neel temperatures of 203 K and 137 K forx=0 andx=0.04 respectively. Meanwhile, they have tetragonal(I4/mmm)symmetry above and orthorhombic(Fmmm)symmetry below the Neel temperatures. Upon cooling, We observe a larger splitting of in-planeEgphonon modes than that of Ba(Fe1−xCox)2As2. Our analysis indicates that the anomalous splitting of in-plane phonon modes is a universal trend towards structural transition in 122 iron-based superconductorsand might share the same mechanisms.

2. Experimental details

Sr(Fe1−xCox)2As2(x= 0 andx= 0.04) single crystals were grown by a self-flux method. Single crystals of two different compositions (x= 0 andx= 0.04) were studied.The parent compound SrFe2As2was annealed at 700°C for three weeks and then cooled down to 300°C. The magnetic transition temperatures (TN) were determined to be 203 K and 137 K forx= 0 andx= 0.04 respectively by transport and magnetization measurements, which are consistent with previous studies.[19]Additionally, we notice that there is no detectable discrepancy between the structural and magnetic transition temperatures in Sr(Fe1−xCox)2As2in contrast to Ba(Fe1−xCox)2As2.[20,21]

Fig.1. Raman spectrum of SrFe2As2 at 315 K in B2g symmetry. Inset: optical image of the freshly cleaved undoped single crystal SrFe2As2 (left)and geometry of polarization configuration of the B2g symmetry(right).

Single crystals were cleaved under optical microscopy to gain high-quality surfaces (see the inset of Fig. 1) and then sealed in a vacuum of～10−6mbar cooled by a closecycle refrigerator. Raman scattering experiments were performed using diode pumped solid state laser ofλ=532 nm and argon-krypton laser ofλ=488 nm. An incident laser beam of 10 mW was focused on an elliptical spot of dimension 40 µm×120 µm accounting for a power density of～220 W/cm2. Reported temperatures were calibrated by taking the laser heating into account. The laser heating was estimated to be 1 K±0.2 per mW by monitoring the temperature dependencies of phonon frequencies accounting for 4.5×10−2K±10−2per W/cm2in terms of power density.The scattered beam was resolved by a triple stage spectrometer (JY-T64000) equipped with 1800 lines/mm gratings and detected by a liquid nitrogen cooled CCD detector. All the spectra reported here were corrected by eliminating the instrumental response. Polarizations of incident (ei) and scattered(es) lights aligned perpendicular to each other (ei⊥es) and along the Fe–Fe bonds (see the inset of Fig. 1) referred to asB2gsymmetry in this study.

3. Results and discussion

The imaginary part of the Raman response is derived byχ′′～[1+n(ω,T)]×I,[22]whereIis the Raman intensity measured from experiment,n(ω,T)the Bose–Einstein distribution andχ′′the imaginary part of Raman response. Figure 1 shows a typical Raman spectrum of SrFe2As2inB2gsymmetry at room temperature. The crystal has a tetragonal symmetry pertaining toI4/mmmspace group above its Neel temperatureTN,wherein the irreducible presentation of the phonon modes isA1g+B1g+2Eg+2A2u+2Eu.[23]Specifically,A1gandB1gmodes correlate with the displacements of As and Fe along thecaxis respectively, and twoEgmodes are doubly degenerate involving the displacements of Fe and As in theabplane for the equivalence ofaandcaxis in the tetragonal phase.A2uandEumodes are not Raman-active and will not be discussed here. Since the intensity of theB1gphonon mode is proportional to sin22αforei⊥esconfiguration,whereαis the angle between the polarization of incident lighteiandaaxis. Noticing thataaxis is defined as the diagonal of the 1 Fe unit cell schematized in the Fig.1,αis 45°for the polarization configuration used here resulting in a maximal Raman intensity of theB1gphonon. In Fig.1,we notice a sharp peak appearing at 210 cm−1, and the intensity of this peak is proved to be sensitive to the angleαthrough rotating the sample by a motor equipped in the refrigerator. Therefore,the peak at 210 cm−1is identified asB1gphonon mode(see Fig.1). SinceA1gmode could be only detected byei‖espolarization configuration,we expect there will be no peak corresponding toA1gphonon in this study. The peak at 118 cm−1is indexed to be the LowfrequencyEgphonon (see Fig. 1) since the incident light is polarized along Fe–Fe bond with a finite projection along thecaxis as described eleswhere.[12]High-frequencyEgphonon modes seems too weak to be detected here.

Figure 2 shows the evolution of the Raman spectrum ranged from 9 cm−1to 400 cm−1of undoped SrFe2As2.The spectra are vertically shifted for clearness. The most remarkable feature of the spectra presented in the Fig. 2 is the large splitting of the low-energyEgphonon across the phase transition. A similar set of spectra is extracted for the Sr(Fe1−xCox)2As2(x=0.04),wherein theEgphonon splits at a lower temperature consistent with its lower transition temperature (not shown). In order to gain some insights into the splitting of theEgphonon mode with temperature, we fit the imaginary part of Raman response around the peaks at 118 cm−1by the superposition of two Lorentzian peaks (see the inset of Fig.2)

whereω1andω2are the resonant frequencies of the Lorentz oscillators,γ1andγ2the line widths. TheEgphonon modes exhibit tiny mean square errors in the fitting,indicating a typical Lorentzian peak ofEgphonons without any detectable asymmetry.

Fig.2. Evolution with temperature of Raman spectrum of SrFe2As2 at 315 K in B2g symmetry. Inset: zoom on the splitting Eg phonon modes with fitting curves by two Lorentzian peaks.

TheEgphonon frequencies shown in Fig. 3 are the resonant frequencies fitted by the above equation. The amplitudes of the phonon splitting reach up to 13 cm−1and 8.76 cm−1forx= 0 andx= 0.04 respectively. As discussed earlier, the amplitude of the in-plane phonon splitting caused by orthorhombic lattice distortion during the structural transition is negligible. Specifically, a simple estimation from the relation between phonon frequencyωand bond lengthl(ω2～(1/l3))[24]would result in a splitting less than 1 cm−1based on crystallographic data.[25]Remarkably, the largest amplitude of the splitting (13 cm−1) observed in the Sr(Fe1−xCox)2As2compounds here is much larger than that reported in the Ba(Fe1−xCox)2As2compounds (9 cm−1).[12]Moreover, the amplitude of the phonon splitting depends on both temperature and doping. Specifically,the amplitudes increase upon cooling and suppressed by doping, which is in good agreement with the Ba(Fe1−xCox)2As2.Interestingly,the maximal amplitude of theEgphonon splitting in the samplex=0.04 is very close to the undoped BaFe2As2.[12]Considering they have similar structural transition temperaturesTs,which characterize the energy scale(kBTs)of the driving force of the transition, anomalous in-plane phonon splitting in the 122 family might share the same mechanism regardless of the type of the earth-Alcaline, supporting the argument that the FeAs plane control the properties of these systems.[9]

The amplitude of the phonon splitting support a good order parameter to gain some insights into the tetragonalorthorhombic transition. An order parameter analysis is performed by fitting the temperature dependence of the amplitude of theEgphonon splitting with Δω(T)=Δω(T=0 K)×(1−(T/Ts))β[26]as shown in the Fig. 4. Several tentative values ofβranging from 0.1 to 0.2 produce a set of similarTsin a good agreement with the structural transition temperature measured in previous study.[19]As a result, the splitting ofEgphonon in Sr(Fe1−xCox)2As2displays the same temperature and doping dependences as in Ba(Fe1−xCox)2As2,implying universally existed in-plane phonon mode anomalies in the 122 iron-based superconductors.

Fig.3. Evolution of the Eg phonon frequency with temperature for x=0(a)and x=0.04(b).

Fig.4. Evolution of the amplitude of the Eg phonon splitting with temperature for x=0(black square)and x=0.04(red circle). The dashed lines are fits using the order-parameter power law described in the text using β =0.12.

4. Conclusion

We performed Raman measurements on the Sr(Fe1−xCox)2As2(x=0 andx=0.04) single crystals. We discovered a large splitting of theEgphonon modes as the crystals undergo the tetragonal-orthorhombic structural transition, which is similar to what was previously reported for Ba(Fe1−xCox)2As2.[12]As proposed previously, the large splitting cannot simply be driven by the orthorhombic distortion, and strong spin-phonon coupling has been proposed to be a candidate to explain the phonon anomalies. However, we found a much larger maximal amplitude of the splitting in the Sr(Fe1−xCox)2As2(13 cm−1) than the one in the Ba(Fe1−xCox)2As2(9 cm−1). Moreover, the sample of Sr(Fe1−xCox)2As2(x= 0.04) displays a similar splitting amplitude and transition temperature as undoped BaFe2As2,implying the amplitude of the in-planeEgphonon splitting depends only on the transition temperature, which characterizes the energy scale (kBTs) of the driven force, regardless of the type of earth-Alcaline. The order parameter analysis exhibits the same doping dependence as reported in the Ba(Fe1−xCox)2As2, implying that the anomalies of theEgphonon mode might be universal in the 122 family and share the same mechanism.