Enhanced low-energy magnetic excitations via suppression of the itinerancy in Fe0.98-zCuzTe0.5Se0.5
Jinsheng Wen, Shichao Li, Zhijun Xu, Cheng Zhang, M. Matsuda, O. Sobolev, J. T. Park, A. D. Christianson, E. Bourret-Courchesne, Qiang Li, Genda Gu, Dung-Hai Lee, J. M. Tranquada, Guangyong Xu, R. J. Birgeneau
aa r X i v : . [ c ond - m a t . s up r- c on ] O c t Enhanced low-energy magnetic excitations via suppression of the itinerancy inFe . − z Cu z Te . Se . Jinsheng Wen,
1, 2, 3, ∗ Shichao Li, Zhijun Xu, Cheng Zhang, M. Matsuda, O. Sobolev, J. T. Park, A. D. Christianson, E. Bourret-Courchesne, Qiang Li, Genda Gu, Dung-Hai Lee,
2, 3
J. M. Tranquada, Guangyong Xu, and R. J. Birgeneau
2, 3, 8 Center for Superconducting Physics and Materials,National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093, China Department of Physics, University of California, Berkeley, California 94720, USA. Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA Condensed Matter Physics and Materials Science Department,Brookhaven National Laboratory, Upton, New York 11973, USA Quantum Condensed Matter Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA. Forschungsneutronenquelle Heinz Maier-Leibnitz (FRM-II), TU M¨unchen, D-85747 Garching, Germany Life Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA Department of Materials Science and Engineering,University of California, Berkeley, California 94720, USA. (Dated: November 5, 2018)We have performed resistivity and inelastic neutron scattering measurements on three samplesof Fe . − z Cu z Te . Se . with z = 0, 0.02, and 0.1. It is found that with increasing Cu doping thesample’s resistivity deviates progressively from that of a metal. However, in contrast to expectationsthat replacing Fe with Cu would suppress the magnetic correlations, the low-energy ( ≤
12 meV)magnetic scattering is enhanced in strength, with greater spectral weight and longer dynamical spin-spin correlation lengths. Such enhancements can be a consequence of either enlarged local momentsor a slowing down of the spin fluctuations. In either case, the localization of the conduction statesinduced by the Cu doping should play a critical role. Our results are not applicable to models thattreat 3 d transition metal dopants simply as effective electron donors. PACS numbers: 61.05.fg, 74.70.Xa, 75.25.–j, 75.30.Fv
I. INTRODUCTION
The effects of substitution of 3 d transition metals (suchas Co, Ni, Cu, etc) on the crystal and magnetic structure,Fermi-surface topology, superconductivity, and mag-netism in Fe-based superconductors have been widelydiscussed. Some initial studies on BaFe As (Ba122)have suggested that 3 d metals such as Co partially sub-stituted for Fe, act as effective electron donors. Suchapproaches typically describe the doping effects based ona rigid-band shift model.
However, such a model hasfaced serious challenges from both the experimental andtheoretical perspectives, each of which indicatesthe inadequacy of a rigid-band description. Furthermore,there is a dichotomy between Co/Ni and Cu substitutioneffects in terms of the emergence of the superconductiv-ity. In Ba122, with increasing Co or Ni doping, supercon-ductivity appears concomitant with the suppression ofthe lattice distortion and magnetic order.
However,superconductivity in the Cu-doped case has been ob-served only in one Cu concentration with T c ∼ In this work, we explore the effects of transition metalsubstitution by carrying out resistivity and inelastic neu-tron scattering measurements on a different Fe-basedsuperconductor system, namely Fe y Te − x Se x (labeledthe 11 system). We use Cu to substitute for Fe inFe . Te . Se . , and measure how both the transportproperties and low-energy magnetic excitations evolve as a function of Cu concentration. With increasing Cu sub-stitution, the system is driven towards an insulator. Thelow-energy ( ≤
12 meV) magnetic excitations respond tothe Cu doping by showing enhanced spectral weight andlonger dynamical spin-spin correlation lengths. This isin contrast to the expectation that using weakly (not)magnetic Cu to replace magnetic Fe suppresses themagnetic correlations. The behavior that we observe canbe naturally explained by assuming that the main effectof Cu doping is to localize conduction states, therebysuppressing the itinerancy. As a result, either the lo-cal moments are enhanced, or the spin fluctuation rateis reduced, and the spectral weight is transferred to lowenergies. Either case will give rise to an enhancement ofthe low-energy magnetic scattering. Our results demon-strate that Cu substitution in the 11 system cannot bedescribed by a rigid-band shift model. II. EXPERIMENTAL
The single-crystal samples of Fe − y − z Cu z Te . Se . were grown by the Bridgman method. We studied threesamples with nominal z = 0, 0.02, and 0.1, which we la-beled as Cu0, Cu02, and Cu10. To minimize the effectsof Fe interstitials, a nominal composition of y = 0 . The resistivity wasmeasured with a standard four-probe method. The lat-tice constants at room temperature are a = b ≈ . c = 6 . E f ) mode with E f = 14 . ′ –40 ′ –Sample–40 ′ –240 ′ , and 48 ′ –40 ′ –Sample–40 ′ –120 ′ respectively. Allthe measurements were performed in the ( HK
0) zone,with the scattering plane being defined by the [100] and[010] wave vectors, where we used reciprocal lattice units(rlu) of ( a ∗ , b ∗ , c ∗ ) = (2 π/a, π/b, π/c ). The measuredintensity I meas was normalized into absolute units of µ eV − /Fe by the integrated incoherent elastic scatter-ing intensity I inc from the sample, using the formula S ( Q , ω ) = I meas µ π I inc | f ( Q ) | p X j n j σ inc ,j , where µ B is the Bohr magneton, f ( Q ) is the wave-vector( Q ) dependent magnetic form factor of Fe , p is a con-stant of 0.27 × − cm, n j and σ inc ,j are the molar ratioand the incoherent cross section for the element in thecompound respectively. III. RESULTS AND DISCUSSIONS
We first present our results by showing the a - b planeresistivity vs. temperature ( ρ ab - T ) curves for Cu0, Cu02and Cu10 samples in Fig. 1. Without Cu substitution,the sample has the highest critical temperature T c of ∼
15 K among the Fe y Te − x Se x system. As shownin Fig. 1, for Se concentrations close to 0.5, the resistiv-ity decreases with decreasing temperature before it dropsto zero at T c , exhibiting a metallic behavior. The T c is suppressed rapidly by the Cu doping–with 0.02-Cusubstitution, it is reduced to 8 K. In the normal state,the temperature dependence of the resistivity differs fromthat in the Cu-free sample. Specifically, the resistivityincreases gradually with decreasing temperature, as in anarrow-band-gap semiconductor. Also, with 2% Cu dop-ing, the value of the resistivity has increased by almost anorder of magnitude. With 10%-Cu doping, the sample isno longer superconducting, and behaves like an insulator.The resistivity is about 4 orders of magnitude larger thanthat of the Cu-free sample. We fit the a - b plane resistivity( ρ ab ) for Cu02 and Cu10 samples with the Mott variablerange hopping formula ρ ab = ρ exp(T /T / (1+ d ) ), where ρ , T are constants, and d is the dimensionality. With d = 3, the data can be fitted reasonably well, as shown in Fig. 1 and its inset. This indicates that the Cu02 (inthe normal state) and Cu10 samples behave like three-dimensional Mott insulators, similar to the behavior inCu-doped FeSe for Cu doping larger than 4%. FIG. 1. (Color online) a - b plane resistivity ( ρ ab ) vs. tem-perature curve in the semi-log scale for Cu0, Cu02 and Cu10samples. The inset plots ln ρ ab against 1 /T / for Cu02 andCu10 samples. Lines through data are fits with the three-dimensional Mott variable range hopping formula, as de-scribed in the text. Given the dramatic change of the transport propertieswith Cu doping, it is important to explore the corre-sponding response of the magnetic excitations by carry-ing out inelastic neutron scattering measurements. Inone of our previous studies, we have done measure-ments in the low-temperature range and observed inter-esting connections between the occurrence of supercon-ductivity and the shape of the magnetic excitation spec-trum (see also Ref. 25): for the superconducting sam-ples, the spectra exhibit a two-vertical-line shape at hightemperatures, and transform to a “U” shape at temper-ature ∼ T c ; while for the nonsuperconducting ones, thespectra remain the two-vertical-line shape in the wholetemperature range. In this work, we will focus on thehigh temperature range with temperature T ≥
100 K. InFig. 2, we plot contour maps for a series of scans around(0.5, 0.5) and (0.5, 0) at constant energy of 6 meV at 100and 300 K for each of the Cu0, Cu02, and Cu10 samples.Similar as previous studies, for Se content close to50%, there is not much spectral weight around (0.5, 0),and there is neither static magnetic order near (0.5, 0.5)nor (0.5, 0). At this temperature range, the scattering isincommensurate, with the strongest scattering occurringat wave vectors displaced from (0.5, 0.5) along the [1¯10]direction. From the 100-K data [Fig. 2(a), (c), and (e)], itis clear that the spectral weight is greatly enhanced in theCu10 sample compared to that of the Cu0 and Cu02 sam-ples. For each sample, as the temperature increases from100 to 300 K, the magnetic excitations become broader,especially along the [110] direction. However the temper-ature dependence of the Cu10 sample differs from that ofthe other two in an important way. As shown in Fig. 3,while the scattering intensity for all three samples is com-parable at 300 K, only in the case of Cu10 does it growsubstantially on cooling to 100 K. We have studied theenergy dependence of the enhancement on the integratedintensities for the [1¯10] scan for the Cu10 sample at 100 Kcompared to that at 300 K, and the results are shown inthe inset of Fig. 3(c). At low energies, for example, from2 to 10 meV, the integrated intensities are almost dou-bled. In Fig. 4, we plot linear scans through (0.5, 0.5, 0)along the [1¯10] direction at a constant energy of 6 meVat temperatures ranging from 100 to 300 K, where wefind that the spectral weight is enhanced gradually uponcooling. For temperatures below 100 K, the scatteringintensity appears to be saturated.
FIG. 2. (Color online) Contour plots of the magnetic scatter-ing at 6 meV at 100 K (upper panels) and 300 K (bottom) forCu0 (left column), Cu02 (middle), and Cu10 sample (right).
In Fig. 5(a) we plot on linear scans along [1¯10] direc-tion through (0.5, 0.5, 0) at 100 K for the three sam-ples. Comparing the scattering intensities between Cu02and Cu0, it is apparent that there is some enhancement,but the margin is small. However, in the Cu10 sam-ple, the intensities are almost doubled. In Fig. 5(b) weplot scans along the [110] direction through one of thetwo incommensurate peaks (0.7, 0.3, 0), where it is alsoquite clear that the peak of the Cu10 sample is muchstronger. We extract the dynamical spin-spin correla-tion lengths at this temperature from the Gaussian fitsto the [1¯10] scan through the peak (0.3, 0.7, 0), and (0.7,0.3, 0), and average the correlation lengths as ξ T . Thecorrelation length extracted from the [110] scan through(0.7, 0.3, 0) is denoted as ξ L . The so-obtained valuesfor the three samples are given in Table I. For the Cu0and Cu02 samples, the dynamical correlation lengths arebasically identical. However, the Cu10 sample does ap-pear to exhibit a longer dynamical spin-spin correlationlength, especially when one looks at the scan along the[110] direction through (0.7, 0.3, 0) ( ξ L ). We have per- FIG. 3. (Color online) (a), (b), and (c), constant-energy scansof 6 meV through (0.5, 0.5) along [1¯10] direction at 100 and300 K for Cu0, Cu02, and Cu10 respectively. The scan tra-jectories are the same as depicted in Fig. 5(a). Lines throughdata are fits with Gaussian functions. In the inset of (c) weplot the ratio ( R ) of enhancement on the 100-K integratedintensities (I ) to that of 300-K (I ) for these scans atdifferent energies, with R = (I − I ) / I . The linethrough data is a guide to the eyes.TABLE I. Dynamical spin-spin correlation lengths at 100 Kextracted from Gaussian fits to the [1¯10] scan through (0.3,0.7, 0) and (0.7, 0.3, 0) and averaged as ( ξ T ), and [110] scanthrough (0.7, 0.3, 0) ( ξ L ) for Cu0, Cu02, and Cu10 samples.The dynamical correlation length ξ is estimated using 1 /κ ,where κ is the instrument resolution corrected full width athalf maximum. The uncertainties of the correlation lengthsare obtained from the resolution-convoluted Gaussian fits.Cu0 Cu02 Cu10 ξ T (˚A) 2.9 ± .
08 3.0 ± .
07 3.9 ± . ξ L (˚A) 3.9 ± .
22 4.2 ± .
13 5.8 ± . formed similar scans at other energies from 2 to 12 meV,and the results are similar. Combining the results fromFigs. 2 and 5, and Table I, we conclude that in the Cu10sample, the magnetic scattering is significantly strength-ened.Such an enhancement of the low-energy magnetic scat-terings by Cu doping is intriguing. Normally, one wouldexpect that substituting Cu for Fe would suppress themagnetic correlations. One plausible interpretation ofthe results is that in the Fe-based superconductors, themagnetic excitations have contributions from both local- FIG. 4. (Color online) Linear scans through (0.5, 0.5, 0) alongthe [1¯10] direction at a constant energy of 6 meV at varioustemperatures for the Cu10 sample.FIG. 5. (Color online) Linear scans at 6 meV for Cu0, Cu02and Cu10 samples at 100 K. Lines through data are fits withGaussian functions. The scan trajectories are shown in theinset. ized spins and itinerant electrons, and more impor-tantly, these two components are entangled with eachother. Thus, by tuning one component, one can affectthe other. In our case, we infer that when the itinerancyof the system is suppressed by the addition of Cu, thelocal moments can be enhanced, which in turn strength-ens the low-energy magnetic excitations. We note that inFe y Te, the enhancement of instantaneous moments oc-curred together with the transition from a more coherentto a less coherent electronic state. In the case of Cu-doped FeSe, it has also been sug-gested that Cu substitution introduces local moments,and when Cu doping equals 0.12, the sample exhibitsa spin-glass transition. The results are interpreted byChadov et al. who show that Cu is in a d state,and with increasing Cu doping, the system evolvesfrom a weak-moment itinerant state to a local-momentmagnet. Due to the Anderson localization, a metal-insulator transition can be induced. These results are consistent with our observations.There is another possibility, though, which is that theoverall moments of the system are not enhanced, butdue to the localization effects of Cu, the system tendsto order magnetically. In this case, the spin fluctuationslows down, and the spectral weight is shifted to lowerenergies. Upon heating, the spectral weight redistributes.Carrying out time-of-flight measurements on the Cu10sample with energy extending up to the top of the band( >
200 meV) will be helpful to distinguish whether themoments of the system are enhanced or not. This workis under way.In any case, our results show clearly that the maineffect of Cu doping is to introduce localization into thesystem, and suppress the itinerancy. This indicates lim-itation of applying a rigid-band shift model, where thechemical substitution is treated as charge carrier dop-ing. There have been some X-ray emission/absorptionspectroscopy works on the Cu-doped Ba122 system, andit has been shown that the Cu 3 d states are located atthe bottom of the valence band in a localized shell. This is consistent with what we see here for the Cu-dopedFe y Te − x Se x case; this suggests that Cu may have auniversal localizing effect across different Fe-based super-conductor systems. IV. SUMMARY
In summary, we have found that the substitution ofFe by Cu in Fe . Te . Se . drives the system from ametallic to an insulating state. Concomitant with thesuppression of the system’s itinerancy, there is an en-hancement of the low-energy magnetic excitations. Ourresults are consistent with the idea that Cu doping doesnot introduce extra carriers into the system, but ratherenhances a more localized state. Such observations donot favor a simple band shift model. Given the reportsthat Co/Ni and Cu may act differently when substitutedinto Ba122, it will be interesting to study the ef-fects of Co- and Ni-doping effects on Fe y Te − x Se x , andspecifically to compare those results with the Cu-dopingeffects presented here. V. ACKNOWLEDGMENTS
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