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Interplay between Freezing and Superconductivity in the Optimally Doped LaEu0.20Sr0.15CuO4 under Hydrostatic Pressure

We study the electronic properties of a LaEu0.20Sr0.15CuO4 single crystal under hydrostatic pressure up to 2.9 GPa. Both the freezing of the Cu 3d moments and the structural transition from the orthorhombic (LTO) to the tetragonal (LTT) phase are observed via the relaxation of the nuclear magnetization of La nuclei. Resistivity and magnetic susceptibility measurements have been carried out under pressure on the same sample. The combination of all data reveals the connection between glassy dynamics, charge localization and the disappearance of superconductivity in the LTT phase.

We study the electronic properties of a La1.65Eu0.2Sr0.15CuO4 single crystal under hydrostatic pressure up to 2.9 GPa. Both the freezing of the Cu 3d moments and the structural transition from the orthorhombic (LTO) to the tetragonal (LTT) phase are observed via the relaxation of the nuclear magnetization of 139 La nuclei. Resistivity and magnetic susceptibility measurements have been carried out under pressure on the same sample. The combination of all data reveals the connection between glassy dynamics, charge localization and the disappearance of superconductivity in the LTT phase. The phenomenon of charge-stripe ordering in holedoped antiferromagnets has received much attention since its discovery in La 1.48 Nd 0.4 Sr 0.12 CuO 4 by elastic neutron scattering. [1] Much of this interest has been motivated by the proximity between stripe order and high temperature superconductivity and the potential relationship between these two states. [2,3,4] It is now accepted that competing electronic interactions can stabilize charge-stripe order, but the question of whether the order observed in La 1.48 Nd 0.4 Sr 0.12 CuO 4 is a genuine electronic phenomenon or specific to a particular distortion of the CuO 2 plane remains controversial.
Evidence for stripe ordering have been reported so far in the lanthanum cuprates La 2−x−y Nd y Sr x CuO 4 [1,5] and more recently in La 2−x Ba x CuO 4 . [6] These compounds undergo a first order transition from an orthorhombic (LTO) to a tetragonal (LTT) phase which affects superconductivity dramatically at optimal hole doping. [7] In both phases, the CuO 6 octahedras tilt collectively toward the c-axis of the perovksite structure. [7] This distortion produces a staggered buckling of the CuO 2 plane with a symmetry different for LTO and LTT phases. [7,8] Stripe order emerges in lieu of superconductivity when the tilt exceeds ≈ 3.6 • in the LTT phase. [1,9] Despite those observations, the reason for the disappearance of superconductivity is not understood. Neutron-scattering experiments in La 2−x−y Nd y Sr x CuO 4 have shown that the stripe arrangement mimics, to some extent, the symmetry of the tilt pattern of the LTT phase, and the detection of the stripe order in this compound is coincident with the first order structural transition. [1] This suggests strong pinning of the chargestripe order by the underlying lattice, which tends to localize charge carriers. Yet, this picture appears incomplete for La 1.65 Eu 0.2 Sr 0.15 CuO 4 where signs of ordering of the Cu 3d moments are observed only below 30K, while the LTT phase occurs at T LT =135K. [8,10,11] Neutron scattering has failed so far to find evidence of chargestripe order in La 1.65 Eu 0.2 Sr 0.15 CuO 4 but the slow dynamics of the Cu 3d moments, probed by NMR at low temperature, has been shown to be consistent with the picture of a glass-forming charge-stripe liquid state [11]. This is indicative of strong frustration, which could also be detrimental to superconductivity.
To clarify the relation between freezing, superconductivity and structural distortion, we use pressure to change the buckling of the CuO 2 plane without modifying doping or substitutional disorder. We have investigated a La 1.65 Eu 0.2 Sr 0.15 CuO 4 single crystal under pressure up to 2.9 GPa by means of the magnetization relaxation rate 139 T −1 1 of the 139 La nuclear spins, the magnetic ac susceptibility χ ac and the in-plane resistivity ρ ab . The pressure-temperature phase diagram of La 1.65 Eu 0.2 Sr 0.15 CuO 4 is obtained from the pressure dependence of four distinct quantities. First, the freezing temperature, T ω0 , is defined from the maximum of 139 T −1 1 where the frequency scale Ω of the magnetic hyperfine field fluctuations at the La site equals the Larmor frequency (ω 0 = 50MHz) of the 139 La nuclear spins. Second, the temperature T LT also is determined from 139 T −1 1 through its response to fluctuations of the surrounding electric field gradient near the first order structural transition [12]. Third, the superconducting transition temperature T c is determined from the temperature dependence of χ ac . Fourth, the amplitude of the upturn in ρ ab observed below 100K measures the tendency for charge carriers to localize on lowering temperature. This study shows as a whole that the disappearance of superconductivity in La 1.65 Eu 0.2 Sr 0.15 CuO 4 originates from long-time dynamical properties of the charge inhomogeneous phase. This anomalous long-time behavior appears to be controlled by the amplitude of the buckling of the CuO 2 layer which decreases under pressure. TLT and Tω 0 are marked with solid and dotted arrows respectively. We find that dTω 0 /dP = −9 K.GPa −1 up to P = 1.8GPa. The same fitting procedure as proposed in ref [11] has been used to extract the relaxation rates from the magnetization recovery This study has been carried out on the same single crystal used in ref [8,11]. Two similar clamp-type cells were used to generate hydrostatic pressure and, in both cases, fluorinert served as pressure medium. The pressure inside the cell was determined at room temperature by measuring the resistivity of a calibrated manganin gauge located next to the sample. Lowering the temperature from 300 to 4K induces pressure losses of ≈ 0.3GPa. [13] The 139 La(I = 7/2) NMR measurements were made on the central (m I = + 1 2 ↔ − 1 2 ) transition. 139 T −1 1 was measured up to 2.7 GPa by monitoring the recovery of the magnetization after an inversion pulse in a field of 83.5 kOe. After completing NMR experiments, the resistivity and magnetic susceptibility measurements were carried out simultaneously up to 2.9 GPa. Fig.1 shows the temperature dependence of the relaxation rate 139 T −1 1 for different values of pressure. The features observed at ambient pressure (P 0 ) have been examined already and discussed in detail [11]. They can be summarized as follows. The discontinuous drop of 139 T −1 1 around T LT ≈ 135 K indicates the occurrence of the first order structural transition from LTO to LTT. The broad peak in the relaxation rate 139 T −1 1 manifests a gradual freezing of the hyperfine field induced at the La site by the Cu 3d localized moments. We thus read from Fig.1 the pressure dependence of T LT and T ω0 and we observe that both decrease continuously with increasing pressure. It is striking, however, that T ω0 saturates near 3K for P 1.8 GPa while T LT continues to decrease at higher pres- sure. Fig.1 makes it obvious that 139 T −1 1 (T) below 30 K is unchanged between 2.3 and 2.7 GPa, whereas T LT is reduced by a factor of two. These differences establish the remarkable fact that the properties of the freezing of the Cu 3d moments no longer depend on the underlying lattice at high pressure.
When looking at Fig.1, a question arises as to whether the temperature of the glass transition, T g , remains finite or drops to zero under pressure. It should be emphasized that T g is conceptually different from T ω0 . Insofar as T ω0 indicates only the temperature at which Ω ≈ ω 0 , it can relate to dynamical properties at thermal equilibrium whereas T g marks the onset of a fully static nonequilibrium state, the system being frozen out into a glass. The relation between T ω0 and T g is a priori unknown but the two temperatures clearly differ at P 0 where T g 5K and T ω0 ≈ 25K [11]. From 139 T −1 1 alone, we can not determine the value of T g since 139 T −1 1 measures only the spectral weight of the fluctuations at frequencies close to ω 0 . Nevertheless, it can be seen in Fig.1 that the broad maximum of 139 T −1 1 occurs around 3K at high pressure. This means that Ω ≈ ω 0 in this temperature range while the system is frozen out at P 0 . Therefore, T g has dropped significantly and, hence disappearance of the glass phase under pressure is conceivable but remains to be proven. This said, one should keep in mind that despite the pressure dependence of 139 T −1 1 revealing a considerable enhancement of the Cu 3d moments fluctuations with increasing pressure, dynamics still remains extremely slow (≈ 10 −7 eV) compared to typical electron time scales. In particular, at low temperature, dynamics is certainly

Temperature (K)
FIG. 3: ρ ab versus temperature at P0(full circles) and at higher pressure (empty circles). ρ ab at P0 was measured after depressurization from 3.0 GPa. The continuous lines are linear fits described in the text and the solid arrow shows the temperature TLT when available from NMR data. The sharp drop at 30K shows filamentary superconductivity in our sample.
faster at pressures higher than P 0 but would nonetheless appear as static on the time scale ( 1 meV) of neutrondiffraction experiments. Thus, the variation seen in Fig.1 reflects properties of the inhomogeneous phase over long time scales i.e τ ω −1 0 . We show the amplitude of the diamagnetic shielding in Fig.2. Shielding of the applied magnetic field (equal to 10 Oe) manifests the presence of superconductivity in our La 1.65 Eu 0.2 Sr 0.15 CuO 4 single crystal and Fig.2 makes clear that T c increases under pressure. We find that dT c /dP = 12 ± 1 K.GPa −1 for P < P s = 1.5 ± 0.15 GPa and reduces to 0.6 ± 0.2 K.GPa −1 above. We emphasize, however, that the pressure dependence of the volume fraction of superconductivity cannot be inferred unambiguously from the data of Fig.2.
The resistivity ρ ab measured in the CuO 2 plane, is shown in Fig.3. We analyze the data as follows. For each value of pressure, we fit ρ ab in the temperature interval 150-300K to the linear form ρ 0 + γT and extract the quantity ρ Up (T)= ρ ab(T) − ρ 0 − γT . We find that the residual term ρ 0 varies smoothly with pressure. At P 0 , ρ 0 = 2.5 ± 0.001 mΩ.cm and decreases at the rate of −0.13 mΩ.cm.GPa −1 . The slope of the linear term, γ = 5.08 ± 0.02 µΩ.cm at P 0 and drops to 4.03 ± 0.02 at 0.67 GPa, which is the first pressure point in our experiment. γ remains unchanged at higher pressure, the variation being less than 0.4%. This linear term in the resistivity is typical of the metallic phase of optimally doped cuprates. It is considered to be an evidence for a non-Fermi liquid behavior, which might be related to a quantum-phase transition at T = 0. [14]In La 1.65 Eu 0.2 Sr 0.15 CuO 4 it can be seen in Fig.3 that this feature is unaffected by pressure.
Most of the effect of increasing pressure on ρ ab is reflected in the amplitude of the upturn. This upturn is characteristic of the LTT phase around optimal Sr doping and its amplitude increases with rare-earth substitution. [9,15]. Because superconductivity is suppressed also with increasing rare-earth content, the upturn might be related to a competing order, but the mechanism by which localization occurs is still unknown. Nevertheless, we can estimate the amplitude of the upturn from Fig.3 as the maximum of the term ρ Up . We find that it decays as e −P/Ps with P s = 1.5 ± 0.1 GPa, as shown in the lower panel of Fig.4.
The relevance of P s becomes clear when comparing T LT , T c , T ω0 and the maximum of ρ Up as a function of pressure in Fig.4. We discern two distinct regimes on either side of P s , which differ in the way variations of T LT alter the electronic properties of the CuO 2 layer. To better appreciate the role of the lattice, we compare T c and T LT as a function of Eu content in La 2−x−y Eu y Sr x CuO 4 [15] with that obtained in La 1.65 Eu 0.2 Sr 0.15 CuO 4 under pressure, in the inset of Fig.4. The correspondence between the two sets of data is clear: increasing pressure is equivalent to reducing Eu content and, by the same token, to the buckling of the CuO 2 layer [9]. It is, therefore, the amplitude of the structural distortion that controls the interplay between freezing and superconductivity in La 1.65 Eu 0.2 Sr 0.15 CuO 4 . However, the two compete with each other only when P ≤ P s , where T LT varies slowly with pressure. Above P s , the two phenomena no longer interfere and, both freezing and superconductivity depend only weakly on pressure. Since the temperature T LT is connected to the amplitude of the staggered buckling of the CuO 2 layer [9], the sharp contrast above P s between the rapid drop of T LT and the saturation of T c and T ω0 reveals the important conclusion that the coexistence between slow dynamics of the Cu 3d moments and superconductivity at high pressure is electronic in origin. Also, the slow dynamics and the presence of buckling of the CuO 2 layer do not limit T c which reaches its optimal value above P s . It is then clear that the key insight provided by our work is that the mechanism of the dramatic suppression of T c in La 1.65 Eu 0.2 Sr 0.15 CuO 4 is to be found in the long time (t ≥ ω −1 0 ) dynamical properties of the inhomogeneous phase at low pressure or equivalently, at high buckling amplitude.
At P 0 , we have shown that the onset of slow dynamics of the Cu 3d moments below 30K can be understood in terms of a glass-forming stripe-liquid phase. [11] Following this idea, a possible interpretation of the decrease of T ω0 under pressure is that frustration causing charge stripes to freeze out into a glass is gradually released under pressure. The reduction of the structural distortion is more likely to increase the mobility of charge carriers in the CuO 2 layer, thus enhancing the fluidity of the electronic matrix. [16] This idea is also well supported by the fact that most of the reduction of the resistivity upturn is found where T ω0 decreases, which implies a connection between freezing and localization of the charge carriers. Insofar as the slow dynamics is driven by slowly moving charge stripes in the CuO 2 plane, we can infer from the present data that glassiness, rather than stripes, is what dominantly competes with superconductivity in La 1.65 Eu 0.2 Sr 0.15 CuO 4 . This leaves open the question of what makes charge stripes glassy. The pressure dependence of the glass transition temperature T g particularly in relation to the qualitative changes occurring around P s , remains to be investigated to test these ideas.
To conclude, we report a detailed investigation of the structural and electronic properties of a La 1.65 Eu 0.2 Sr 0.15 CuO 4 single crystal under pressure by means of 139 La-NMR, resistivity and magnetic ac susceptibility measurements. Our primary result is clear evidence that the sharp suppression of superconductivity in La 1.65 Eu 0.2 Sr 0.15 CuO 4 originates from anomalous long-time dynamics of the charge inhomogeneous phase. This reveals the key role of frustration in the interplay between static inhomogeneities and superconductivity in cuprates.
The work at Los Alamos National Laboratory and Brookhaven was performed under the auspices of the US Department of Energy (Contract No. DE-AC02-98CH10886).