Spin Density Wave Transition Research Articles

Theoretical Investigation of the Interplay of Superconductivity and Magnetism in Ba1−xKxFe2As2 Superconductor in a Two-Band Model by Using the Bogoliubov Transformation Formalism

The main focus of this article is to investigate the theoretical interplay of magnetism and superconductivity in a two-band model for the iron-based superconductor Ba1−xKxFe2As2. On the basis of experimental results, the two-band model Hamiltonian was considered. We obtained mathematical statements for the superconductor Ba1−xKxFe2As2 superconducting (SC) transition temperature, spin-density-wave (SDW), transition temperature, superconductivity order parameter, and SDW order parameter from the Bogoliubov transformation formalism and the model Hamiltonian. Furthermore, an expression for the dependence of the SDW transition temperature on the SDW order parameter and the dependence of the SC transition temperature on the SDW order parameter was obtained for Ba1−xKxFe2As2. By substituting the experimental and theoretical values of the parameters in the derived statements, phase diagrams of the SC transition temperature versus the SDW order parameter and the SDW transition temperature versus the SDW order parameters have been plotted to demonstrate the dependence of the SDW order parameter on transition temperatures. By combining the two-phase diagrams, we depicted the possible coexistence of superconductivity and magnetism for the Ba1−xKxFe2As2 superconductor. The phase diagrams of temperature versus SC order parameter and temperature versus SDW order parameter were also plotted to show the dependence of order parameters on temperature for the Ba1−xKxFe2As2 superconductor.

Spin density wave and antiferromagnetic transition in EuFe2As2: a high field transport and heat capacity study

Abstract Here we reported structural, electrical, magnetic, and thermal transport properties of ternary pnictide bulk iron based compound EuFe2As2 (Eu-122). This compound (Eu-122) crystallized in ThCr2Si2-type tetragonal phase structure with space group I4/mmm at ambient temperature. A promising divalent state (Eu2+) of Eu-ions was observed in the studied EuFe2As2. Magnetic ordering of Eu2+ ions takes place at very low temperature at around T N = 20 K in EuFe2As2. The ferromagnetic interactions between Fe–Fe ions were established at higher temperature which was revealed from magnetic susceptibility measurements with negative value of the paramagnetic Curie temperature. Both magnetic phase transitions (20 K and 190 K) were clearly established the intrinsic magnetic nature revealed by both electrical transport and specific heat measurement. However the phase transition at low temperature corresponds to the magnetic ordering of Eu2+ ions while the high transition temperature is due to the itinerant moment of Fe. EuFe2As2 is the only compound among various parent compounds of iron pnictide superconductor’s family, in which both spin density wave (SDW) of Fe and A-type antiferromagnetic (AFM) ordering of the localized Eu2+ magnetic moments take place simultaneously. We observed here that the localized character of Eu anti-ferromagnetism dominated via RKKY interactions, despite the largely itinerant nature of Fe magnetic interactions. The resistivity with applied magnetic field revealed that the AFM ordering temperature of Eu2+ ions suppress with applied magnetic field. Also resistivity under hydrostatic pressure measurements shows the T SDW (Fe) transition of the Fe moments shifts towards the lower temperatures while AFM ordering of Eu2+ decreases with pressure and the same is completely disappears at 2 GPa.

Transient gap generation in BaFe2As2 driven by coherent lattice vibrations.

Iron-based superconductors provide a rich platform to investigate the interplay between unconventional superconductivity, nematicity, and magnetism. The electronic structure and the magnetic properties of iron-based superconductors are highly sensitive to the pnictogen height. Coherent excitation of the phonon by femtosecond laser directly modulates the pnictogen height, which has been used to control the physical properties of iron-based superconductors. Previous studies show that the driven phonon resulted in a transient increase of the pnictogen height in BaFeAs, favoring an enhanced Fe magnetic moment. However, there are no direct observations on either the enhanced Fe magnetic moments or the enhanced spin-density wave (SDW) gap. Here, we use time-resolved broadband terahertz spectroscopy to investigate the dynamics of BaFeAs in the phonon-driven state. Below the SDW transition temperature, we observe a transient gap generation at early-time delays. A similar transient feature is observed in the normal state up to room temperature.

Investigation of the Interplay Between Spin Density Wave and Superconductivity in High‐Temperature Iron‐Based Superconductor Ba1−xRbxFe2As2

The current research work deals with the theoretical investigation of the interplay between spin density wave (SDW) and superconductivity in a two‐band model high‐temperature iron‐based superconductor by using the double‐time temperature‐dependent Green's function formalism. The superconductivity transition temperature () versus the spin density wave order parameter () for the electron and hole intrabands as well as the interband for the iron‐based superconductor has been plotted. With an increase in of the material, the superconductivity transition temperature decreases. In both intra‐ and interband pairings, the temperature at which the spin density wave transition () rises with increasing . The interplay between superconductivity and spin density wave of the electron and hole intraband interactions for . Furthermore, the phase diagram of the spin density wave parameter () versus temperature has been plotted, and the spin density wave parameter () is observed to decrease as the temperature rises and vanishes at the spin density wave transition temperature ().

Evolution of magnetic properties in iron-based superconductor Eu-doped CaFe2As2

This work presents the evolution of magnetic properties of EuxCa1−xFe2As2 (0 ≤ x ≤ 1, ECFA) samples. Unlike the resistivity data, that for magnetic susceptibility χ (T) does not show any clear evidence of the spin density wave (SDW) transition. When the Curie-Weiss contribution is subtracted, a weak anomaly appears at a temperature close to the SDW transition temperature (TSDW) determined from the resistivity data. To understand the magnetic orders arising from Fe-moments and Eu2+ spins order, we have studied the doping dependence of TSDW and Eu2+ antiferromagnetic order TN. It is found that TSDW increases almost linearly with increasing x and remains nearly unchanged above x ∼ 0.4, whereas TN first appears at x ∼ 0.4 and varies almost linearly with further increasing x. These observations suggest that magnetic orders due to two sublattices are coupled to each other. The results discussed here are helpful for understanding the magnetic properties of ECFA and other iron-based superconductors.

Effects of 4d transition metal Pd doping on the magnetic and superconducting properties of 112-type iron pnictide EuFeAs2

The recently discovered 112-type EuFeAs2 compound that shows complex Eu-spin magnetism provides a new platform to study the interplay between superconductivity (SC) and magnetism in iron pnictide superconductors. In this paper, by substituting Fe with the 4d transition metal (TM) Pd, we have synthesized a series of EuFe1−x Pd x As2 (0 ⩽ x ⩽ 0.08) samples and studied the doping effect on SC and magnetism by means of electrical transport and magnetization measurements. The systematic evolution of the lattice parameters indicates that the Pd atoms have been successfully substituted into the Fe sites. With Pd doping, the Fe-related spin density wave transition at in the parent phase is rapidly suppressed, and SC simultaneously emerges, exhibiting a domelike shape with a maximum onset transition temperature of around 18.5 K at x = 0.04. On the other hand, the Eu-related antiferromagnetic order at is suppressed very slowly and persists up to x = 0.08, covering the whole SC dome region. Also, the reentrant spin-glass and spin-reorientation transitions below remain unchanged with Pd doping. All these results indicate that the Eu spins have little effect on SC in EuFe1−x Pd x As2. Finally, based on the resistivity and magnetization data, the T − x phase diagram of EuFe1−x Pd x As2 is constructed and compared with those of 3d TMs Co/Ni and La doped ones.

Mn-induced Fermi-surface reconstruction in the SmFeAsO parent compound

The electronic ground state of iron-based materials is unusually sensitive to electronic correlations. Among others, its delicate balance is profoundly affected by the insertion of magnetic impurities in the FeAs layers. Here, we address the effects of Fe-to-Mn substitution in the non-superconducting Sm-1111 pnictide parent compound via a comparative study of SmFe_{1-x}Mn_{x}AsO samples with x(text{Mn})= 0.05 and 0.10. Magnetization, Hall effect, and muon-spin spectroscopy data provide a coherent picture, indicating a weakening of the commensurate Fe spin-density-wave (SDW) order, as shown by the lowering of the SDW transition temperature T_text{SDW} with increasing Mn content, and the unexpected appearance of another magnetic order, occurring at T^{*} approx 10 and 20 K for x=0.05 and 0.10, respectively. We attribute the new magnetic transition at T^{*}, occurring well inside the SDW phase, to a reorganization of the Fermi surface due to Fe-to-Mn substitutions. These give rise to enhanced magnetic fluctuations along the incommensurate wavevector varvec{Q}_2 =(pi pm delta ,pi pm delta ), further increased by the RKKY interactions among Mn impurities.

Co-doping effects on magnetism and superconductivity in the 112-type EuFeAs2 system

The discovery of EuFeAs2, currently the only charge-neutral parent phase of the 112-type iron-pnictide system, provides a new platform for the study of elemental doping effects on magnetism and superconductivity (SC). In this study, a series of polycrystalline EuFe1-yCoyAs2 and Eu0.9Pr0.1Fe1-yCoyAs2 samples are synthesized through solid-state reaction, and the evolutions of SC and magnetism with Co doping in EuFeAs2 and Eu0.9Pr0.1FeAs2 are investigated by electrical transport and magnetic susceptibility measurements. For EuFe1-yCoyAs2, the Eu-related antiferromagnetic (AFM) transition around 40 K is barely affected by Co doping, while the Fe-related spin density wave (SDW) transition temperature drops rapidly. Meanwhile, SC is induced by a trace amount of Co doping, with a highest transition temperature Tc ~ 28 K found in EuFe0.9Co0.1As2. For the Eu0.9Pr0.1Fe1-yCoyAs2 series, the magnetism and superconductivity show similar evolutions upon Co doping, and the highest Tc is enhanced to 30.6 K with an optimum doping level y ~ 0.07. Our results shed light on the competition between SC and SDW with Co doping in the 112-type EuFeAs2 system.

Old Donors for New Molecular Conductors: Combining TMTSF and BEDT-TTF with Anionic (TaF6)1−x/(PF6)x Alloys

Tetramethyl-tetraselenafulvalene (TMTSF) and bis(ethylenedithio)-tetrathiafulvalene (BEDT-TTF) are flagship precursors in the field of molecular (super)conductors. The electrocrystallization of these donors in the presence of (n-Bu4N)TaF6 or mixtures of (n-Bu4N)TaF6 and (n-Bu4N)PF6 provided Bechgaard salts formulated as (TMTSF)2(TaF6)0.84(PF6)0.16, (TMTSF)2(TaF6)0.56(PF6)0.44, (TMTSF)2(TaF6)0.44(PF6)0.56 and (TMTSF)2(TaF6)0.12(PF6)0.88, together with the monoclinic and orthorhombic phases δm-(BEDT-TTF)2(TaF6)0.94(PF6)0.06 and δo-(BEDT-TTF)2(TaF6)0.43(PF6)0.57, respectively. The use of BEDT-TTF and a mixture of (n-Bu4N)TaF6/TaF5 afforded the 1:1 phase (BEDT-TTF)2(TaF6)2·CH2Cl2. The precise Ta/P ratio in the alloys has been determined by an accurate single crystal X-ray data analysis and was corroborated with solution 19F NMR measurements. In the previously unknown crystalline phase (BEDT-TTF)2(TaF6)2·CH2Cl2 the donors organize in dimers interacting laterally yet no organic-inorganic segregation is observed. Single crystal resistivity measurements on the TMTSF based materials show typical behavior of the Bechgaard phases with room temperature conductivity σ ≈ 100 S/cm and localization below 12 K indicative of a spin density wave transition. The orthorhombic phase δo-(BEDT-TTF)2(TaF6)0.43(PF6)0.57 is semiconducting with the room temperature conductivity estimated to be σ ≈ 0.16–0.5 S/cm while the compound (BEDT-TTF)2(TaF6)2·CH2Cl2 is also a semiconductor, yet with a much lower room temperature conductivity value of 0.001 to 0.0025 S/cm, in agreement with the +1 oxidation state and strong dimerization of the donors.

22 K superconductivity in BaFe2As2 exposed to F2

Several previous reports on undoped $AE\mathrm{}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ ($AE=$ alkaline earth metal) point to drops in resistivity indicative of possible superconductivity and some degree of Meissner effect in the magnetic susceptibility. Also, based on both resistivity and magnetic susceptibility measurements, controlled exposure to water vapor has been shown to induce superconductivity in $AE{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$. In our study, $\mathrm{Ba}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ single crystals grown using the self-flux method showed a full resistive drop around 22 K and magnetic shielding, when exposed to fluorine gas postgrowth. Our measurements indicate electron (donor) doping via atomic substitution of F for As is concurrent with the observed superconductivity, which sheds light on the likely effect of exposure to water vapor in previous work. Like doping experiments (such as substitution of Co for Fe, P for As, or K for Ba) in $AE{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ to date, the present work is consistent with suppression of the spin density wave transition coincident with the appearance of ${T}_{c}$. In addition to answering the puzzle of superconductivity in undoped or water vapor exposed $AE{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$, our results also represent a fast (20 min exposure to 5% F in He) and reliable method suitable for inducing superconductivity in thin films of $AE{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$. Some supportive work on $\mathrm{Ba}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ exposed to ${\mathrm{Cl}}_{2}$ is also presented.

Doping dependence and high-pressure studies on EuxCa1 − xFe2As2 (0 ≤x ≤ 1)

We have investigated the electrical transport and magnetic properties of EuxCa1 − xFe2As2 (ECFA) (0 ≤x ≤ 1) at ambient and under high-pressure. Temperature dependence of resistance shows a distinct anomaly at TSDW∼ 165 − 190 K due to the spin density wave (SDW) transition. Initially, TSDW increases rapidly with x up to 0.45, and then more slowly at higher x until it saturates at TSDW∼ 190 K for x = 1. Also, there exists another transition at low temperature, TN∼ 10 − 20 K, due to the antiferromagnetic order of Eu2 + spins. Magnetic susceptibility follows the Curie-Weiss, 1/T behavior. However, it does not show clear evidence of TSDW while exhibiting a sharp anomaly at TN. The TN values in both the resistance and magnetization measurements are comparable to one another and vary linearly with x. At ambient pressure, none of the ECFA samples show properties of superconductivity down to 1.8 K. With the application of external pressure, the SDW order is gradually suppressed and superconductivity with a critical temperature as high as, Tc∼ 19 K is induced in the ECFA samples (x = 0.14, 0.31, and 0.45). We have studied the pressure dependence of Tc and TSDW, and we present the pressure-temperature phase diagram.

The Superconductivity Mechanism in Nd-1111 Iron-Based Superconductor Doped by Calcium

We describe the effect of nonmagnetic impurity on the superconductivity behavior of the NdFeAsO0.8F0.2 iron-based superconductor. The resistivity measurements show that the superconductivity is suppressed upon increasing the low amounts of calcium impurity (x ≤ 0.05). Also, the TC decreases with the increase in the residual resistivity. Such behavior is qualitatively described by the Abrikosov–Gorkov theory and confirms that these impurities act as scattering centers. Moreover, we present the phase diagram of our synthesized samples for the various calcium dopings. We find that according to the increase in the calcium impurities and the decrease in the spin-density wave transition tempeθrature (TSDW), Fe ions are arranged stripe-antiferromagnetic at lower temperatures and also the superconducting transition temperature (TC) declines. Based on our results and in agreement with the available theories as is explained in the text, since the S++ state has no effect on the impurity-doped samples, and for low amounts of calcium, the S± state that is attributed to the spin-fluctuations causes the superconductivity suppression. So, it confirms the role of the spin-fluctuations as a dominant pairing mechanism in our synthesized samples.

Theoretical investigation of the coexistence of superconductivity and spin density wave (SDW) in two-band model for the iron-based superconductor BaFe2(As1−xPx)2

This study focuses on the theoretical investigation of the coexistence of superconductivity and spin density wave (SDW) in a two-band model for the iron-based superconductor BaFe2(As1−xPx)2. By employing a system Hamiltonian in a two-band model for a given system and by using the double time temperature dependent Green’s function formalism at the Bardeen-Cooper-Schrieffer (BCS) mean field approximation (MFA) level, we obtained mathematical expressions for the dependence of superconducting transition temperature (TC) on SDW order parameter (ΔSDW) in the electron and hole intra-band interactions, and in the inter-band interactions for superconductor BaFe2(As1−xPx)2. Similarly, we have obtained an expression for the dependence of SDW transition temperature (TSDW) on the SDW order parameter (ΔSDW) for BaFe2(As1−xPx)2. By using experimental values and considering some plausible approximations of the parameters in the obtained expressions, phase diagrams of TC versus ΔSDW in the electron and hole intra-band interactions, and in the inter-band interactions are plotted for the BaFe2(As1−xPx)2 superconductor. Similarly, a phase diagram of TSDW versus ΔSDW is plotted for the same superconductor. By merging the two phase diagrams, we have demonstrated the possible coexistence of superconductivity and SDW in the electron and hole intra-band interactions, and in the inter-band interactions for BaFe2(As1−xPx)2. Furthermore, we have investigated the dependence of ΔSDW on temperature in pure SDW region and the phase diagram is plotted for the variation of ΔSDW with temperature for superconductor BaFe2(As1−xPx)2. Our findings are in agreement with previous experimental results.

Orbital-dependent electron dynamics in Fe-pnictide superconductors

We report on orbital-dependent quasiparticle dynamics in EuFe$_2$As$_2$, a parent compound of Fe-based superconductors and a novel way to experimentally identify this behavior, using time- and angle-resolved photoelectron spectroscopy across the spin density wave transition. We observe two different relaxation time scales for photo-excited d$_x$$_z$/d$_y$$_z$ and d$_x$$_y$ electrons. While d$_x$$_z$/d$_y$$_z$ electrons relax faster through the electron-electron scattering channel, showing an itinerant character, d$_x$$_y$ electrons form a quasi-equilibrium state with the lattice due to their localized character, and the state decays slowly. Our findings suggest that electron correlation in Fe-pnictides is an important property, which should be taken into careful account when describing the electronic properties of both parent and electron-doped compounds, and therefore establish a strong connection with cuprates.

Magnetism and superconductivity in Eu(Fe1−xNix)As2 (x = 0, 0.04)

We report Eu-local-spin magnetism and Ni-doping-induced superconductivity (SC) in a 112-type ferroarsenide system Eu(Fe$_{1-x}$Ni$_{x}$)As$_2$. The non-doped EuFeAs$_2$ exhibits two primary magnetic transitions at $\sim$100 and $\sim$ 40 K, probably associated with a spin-density-wave (SDW) transition and an antiferromagnetic ordering in the Fe and Eu sublattices, respectively. Two additional successive transitions possibly related to Eu-spin modulations appear at 15.5 and 6.5 K. For the Ni-doped sample with $x$ = 0.04, the SDW transition disappears, and SC emerges at $T_\mathrm{c}$ = 17.5 K. The Eu-spin ordering remains at around 40 K, followed by the possible reentrant magnetic modulations with enhanced spin canting. Consequently, SC coexists with a weak spontaneous magnetization below 6.2 K in Eu(Fe$_{0.96}$Ni$_{0.04}$)As$_2$, which provides a complementary playground for the study of the interplay between SC and magnetism.

Thermoelectric anisotropy in the iron-based superconductor Ba(Fe1−xCox)2As2

We report the in-plane anisotropy of the Seebeck and Nernst coefficients as well as of the electrical resistivity determined for the series of the strain-detwinned single crystals of Ba(Fe1-xCox)2As2. Two underdoped samples (x = 0.024, 0.045) exhibiting the transition from the tetragonal paramagnetic phase to the orthorhombic spin density wave (SDW) phase (at Ttr = 100 and 60 K, respectively) show an onset of the Nernst anisotropy at temperatures above 200 K, which is significantly higher than Ttr. In the optimally doped sample (x = 0.06) the transport properties also appear to be in-plane anisotropic below T = 120 K, despite the fact that this particular composition does not show any evidence of long-range magnetic order. However, the anisotropy observed in the optimally doped crystal is rather small and for the Seebeck and Nernst coefficients the difference between values measured along and across the uniaxial strain has opposite sign to those observed for underdoped crystals with x = 0.024 and 0.045. For these two samples, insensitivity of the Nernst anisotropy to the SDW transition suggests that the nematicity might be of other than magnetic origin.

Charge Density Wave and Spin Density Wave in Two-Orbital Model for Iron-Based Superconductors

We present a mean-field theoretical model study for the coexistence of the two strongly interacting charge density wave (CDW) and spin density wave (SDW) for iron-based superconductors in the under-doped region before the onset of the superconductivity in the system. The analytic expressions for the temperature dependence of the CDW and SDW order parameters are derived by using Zubarev’s technique of double-time single particle Green’s functions and solved self-consistently. Their interplay is studied by varying both the CDW and SDW coupling constants. Further, the electronic density of states (DOS) for the conduction electrons are studied in the pure CDW and SDW states and coexistence state for the cases, where the CDW transition temperature is greater than the SDW transition temperature and vice versa, which show two gap parameters.

Collinear spin density wave order and anisotropic spin fluctuations in the frustrated J1−J2 chain magnet NaCuMoO4(OH)

The phase diagram of the quasi-one-dimensional magnet NaCuMoO$_4$(OH) is established through single-crystal NMR and heat-capacity measurements. The $^{23}$Na and $^1$H NMR experiments indicate a spiral and a collinear spin-density-wave (SDW) order below and above $B_c$ = 1.5-1.8 T, respectively. Moreover, in the paramagnetic state above the SDW transition temperature, the nuclear spin-lattice relaxation rate $1/T_1$ indicates anisotropic spin fluctuations that have gapped excitations in the transverse spectrum but gapless ones in the longitudinal spectrum. These static and dynamic properties are well described by a theoretical model assuming quasi-one-dimensional chains with competing ferromagnetic nearest-neighbor interactions $J_1$ and antiferromagnetic next-nearest-neighbor interactions $J_2$ ($J_1$-$J_2$ chains). Because of the excellent crystal quality and good one dimensionality, NaCuMoO$_4$(OH) is a promising compound to elucidate the unique physics of the frustrated $J_1$-$J_2$ chain.

Direct response of the spin-density wave transition and superconductivity to anion height inSrFe2As2

We explored the effects of isovalent doping on the evolution of structure, spin-density wave (SDW), and superconductivity (SC) in three series of Ba-doped $\mathrm{SrF}{\mathrm{e}}_{2}\mathrm{A}{\mathrm{s}}_{2\ensuremath{-}y}{\mathrm{P}}_{y}$ ($y=0.5,\phantom{\rule{0.16em}{0ex}}0.7,\phantom{\rule{0.16em}{0ex}}1.0$). For the $y=0.5$ series, the SDW and SC can coexist in the all-Ba-doped $\mathrm{SrF}{\mathrm{e}}_{2}\mathrm{A}{\mathrm{s}}_{1.5}{\mathrm{P}}_{0.5}$, while the SC first decreases and then saturates in the case of $y=0.7$. Besides, it is strikingly found that the SC of Ba-doped ${\mathrm{SrFe}}_{2}\mathrm{AsP}$ exhibits a dome-shaped ${T}_{\mathrm{c}}$, in which the anion height, ${h}_{\mathrm{pn}}$, approaching the optimal value of 1.308(2) \AA{} and the ${T}_{\mathrm{c}}$ simultaneously increase to the maxima. The phase diagrams of SDW and SC as a function of ${h}_{\mathrm{pn}}$ and Pn-Fe-Pn angle are first constructed here. It is argued that the isovalent substitutions in FeAs-based superconductors could significantly alter SC and ground states equally as well as carrier doping. The theoretical calculations show that electronic band structure is strongly dependent on the parameter of ${h}_{\mathrm{pn}}$ without introducing carriers by doping. The transition from three-dimensional-like to two-dimensional electronic states and the nesting of the Fermi surface can be realized by changing the ${h}_{\mathrm{pn}}$.

Nonlinear transport associated with spin density wave dynamics in Ca3Co4O9

We have carried out the transient nonlinear transport measurements on the layered cobalt oxide Ca$_3$Co$_{4}$O$_9$, in which a spin density wave (SDW) transition is proposed at $T_{\rm SDW} \simeq 30$ K. We find that, below $T_{\rm SDW}$, the electrical conductivity systematically varies with both the applied current and the time, indicating a close relationship between the observed nonlinear conduction and the SDW order in this material. The time dependence of the conductivity is well analyzed by considering the dynamics of SDW which involves a low-field deformation and a sliding motion above a threshold field. We also measure the transport properties of the isovalent Sr-substituted systems to examine an impurity effect on the nonlinear response, and discuss the obtained threshold fields in terms of thermal fluctuations of the SDW order parameter.