Electron-Phonon Anomaly Related to Charge Stripes: Static Stripe Phase Versus Optimally Doped Superconducting La1.85Sr0.15CuO4

Inelastic neutron scattering was used to study the Cu-O bond-stretching vibrations in optimally doped La_1.85Sr_0.15CuO₄ (T_c = 35 K) and in two other cuprates showing static stripe order at low temperatures, i.e., La_1.48Nd_0.4Sr_0.12CuO₄ and La_1.875Ba_0.125CuO₄. All three compounds exhibit a very similar phonon anomaly, which is not predicted by conventional band theory. It is argued that the phonon anomaly reflects a coupling to charge inhomogeneities in the form of stripes, which remain dynamic in superconducting La_1.85Sr_0.15CuO₄ down to the lowest temperatures. These results show that the phonon effect indicating stripe formation is not restricted to a narrow region of the phase diagram around the so-called 1/8 anomaly but occurs in optimally doped samples as well.     

Neutron-Scattering Study of Impurity Effect on Stripe Correlations in La-Based 214 High-Tc Cuprate

We present results of neutron-scattering study of impurity effects on the static stripe correlations in La-based 214 high-T _c cuprate. We found that the superlattice peaks from the charge-density-wave (CDW) order was induced by doping magnetic Fe ions in the low-temperature-orthorhombic (LTO) phase of La_2−x Sr _x CuO₄ (LSCO). The spin-density-wave (SDW) order is also stabilized by Fe-doping, suggesting the appearance of stripe ordered state in the LTO phase, as is seen it the low-temperature-tetragonal (LTT) phase of La_1.875Ba_0.125Cu₄ (LBCO). Therefore, the stripe order can be potentially realized in the La-214 system irrespective of crystal structure. On the other hand, the onset temperature for the appearance of SDW and CDW orders is lowered by Zn-doping in La_1.875Ba_0.125CuO₄ system. This result suggests that the existing well-stabilized stripe order is degraded by substituting non-magnetic Zn ions.     

Charge Stripes and Superconductivity in La2−x Ba x CuO4 and La1.6−x Nd0.4Sr x CuO4

We report x-ray scattering study of charge density wave (stripe) order in La_2−x Ba _x CuO₄ (LBCO) and La_1.6−x Nd_0.4Sr _x CuO₄ (LNSCO). The doping dependence of the charge stripe ordering temperature and the low-temperature correlation length is compared with the bulk superconducting transition temperature. We find that the charge stripe ordering temperature and correlation length are quite different in these two families of cuprates, while there seems to be inverse correlation between the bulk superconducting temperature and the charge stripe correlation length.     

Lattice instability in the normal state of La2?xSrxCuO4

Novel softening has been found in the transverse elastic constant (C ₁₁–C ₁₂)/2 below 50 K in single-crystalline La_1.86Sr_0.14CuO₄ (LSCO) by high-precision ultrasonic measurements in magnetic fieldsH along thec axis. With decreasing temperature, this lattice softening persists down to the superconducting transition temperatureT _c(H), which is reduced to 14 K by applying fields up to 14 T. BelowT _c(H) the softening turns to rapid hardening. This behavior indicates the presence of lattice instability of the orthorhombic (Bmab) structure in the normal state of LSCO, which disappears in the superconducting state. This is evidence for the intimate interplay between high-T _c superconductivity and the lattice instability in LSCO.     

Spatial modulation of low-frequency spin fluctuations in hole-doped La2CuO4

Systematic neutron scattering measurements have been performed on the Sr-doped La_{2-x}Sr_xCuO₄ to study the doping dependence of spatially modulated dynamical spin correlations or so-called incommensurate spin fluctuations. The modulated spin correlations appear beyond x ≈ 0.05 which is close to the lower boundary of the superconducting phase. First evidence was observed for the linear relation between the degree of spatial modulation or the incommensurability Σ(x) and the maximumT _c atx. We present a universal curve for Σ(x) by adding data from other La₂CuO₂ systems such as oxygen-doped superconductors, oxygen-reduced or Zn-substituted nonsuperconductors, and La_1.6-xNd_0.4Sr_xCuO₄.     

Pressure Dependence of Tc in High Temperature Superconductors: Role of Interlayer Interactions

We have studied the role of interlayer interactions (W) in the pressure dependence of T _c of layered superconductors. The expressions for dT _c /dP are obtained by including the effects of layered structure within the framework of two different proposed models, namely the negative-U Hubbard model and the Hirsch model. We observe that the inclusion of interlayer interaction provides better explanation of pressure dependence of T _c . Our numerical results show that the systems having one CuO₂ layer per unit cell may be well described by small values of W while the larger values of W accounts for the systems having two or more superconducting layers in a unit cell. The calculated values of dT _c /dP vs. W are found to be in good agreement with those of experimental results obtained for various high T _c superconductors of cuprate family.     

New Results on the Anomalous Precursor Diamagnetism in the Underdoped La1.9Sr0.1CuO4 Superconductor

The new results summarized here, including a brief comparison with the paraconductivity, further suggest that the anomalous precursor (above T _c) diamagnetism recently observed in the underdoped La_1.9Sr_0.1CuO₄ superconductor could be attributed to the presence, in addition to the conventional superconducting pair fluctuations, of T _c-inhomogeneities with long characteristic lengths associated with chemical disorder.     

Transport and T c Anomalies around x=0.22 in La2−x SrxCu1−y M y O4 (M=Zn, Ga, Ni): Possibility of Charge and/or Spin Ordering

Transport and T _c anomalies around x=0.22 in La _2–x Sr _x CuO ₄ have been studied through the partial substitution of Zn, Ga and Ni for Cu. It has been found that the value of x where the anomalies occur shifts to lower x values through the Zn substitution, while it shifts to larger x values through the Ga substitution, and it disappears through the Ni substitution. There is a possibility that an order of holes and/or spins, such as the so-called stripe order, is formed or fluctuates around x=0.22 in La _2–x Sr _x CuO ₄.     

Evidence for onset of charge density wave in the La-based Perovskite superconductors

Recent studies have shown that structural instabilities play a vital role in high-T _c superconductivity. We have used a local structural probe, extended X-ray absorption fine structure (EXAFS), to investigate the instability of the CuO₂ plane in La-based high-T _c perovskite systems. Temperature-dependent CuK-edge EXAFS spectra measured on single crystals of La_1.85Sr_0.15CuO₄ (LSCO) and La₂CuO_4.1 (LCO) show a characteristic temperature (T _ls ) where the local structure of the system diverges from the average one. This temperature coincides with a crossover temperature where the system shows anomalous metal-insulator transition in transport measurements. We have discussed the EXAFS results as an evidence for a particular phase transition of the system with segregation of localized and itinerant charge carriers in a short coherence charge density wave (CDW). A phase diagram for this CDW has been proposed.     

Questioning the Assumptions of the Parabolic Model for Superconductive Cuprates

The assumptions of the parabolic model are questioned. These assumptions pertain to an expectation of universal T _c optima for cuprates at an experimental hole concentration of p = 0.16n, where n is the number of CuO₂ planes. This model was developed based on the T _c maximum for La_{2 – x} Sr _x CuO₄ at x = 0.16. However, a variety of cases are presented for higher optimal hole concentrations, including La₂CuO_4.16, where it is twice as high. Also, the success of a charge order model in universally predicting optimal T _c at formal stoichiometric holes, h = 0.5n, suggests a need for expansion of the parabolic model. By quantitatively taking into account the deleterious effect of the blocking layer, optimal T _c can be absolutely calibrated at a uniform optimal charge order with alternate holes.     

Coupling of Longitudinal and Transverse Stripe Fluctuations

The interplay of transverse and longitudinal fluctuations of a stripe is analyzed using bosonization techniques. We show the existence of a charge-density-wave instability for quarter-filled stripes if the underlying lattice potential has a zigzag symmetry. Our results are in accordance with the observation of static stripes formation in underdoped La_2–x–y Nd _y Sr _X CuO₄ exactly at the onset of the low-temperature-tetragonal transition.     

Lattice instability in the normal state of La2−x Sr x CuO4

Novel softening has been found in the transverse elastic constant (C ₁₁–C ₁₂)/2 below 50 K in single-crystalline La_1.86Sr_0.14CuO₄ (LSCO) by high-precision ultrasonic measurements in magnetic fieldsH along thec axis. With decreasing temperature, this lattice softening persists down to the superconducting transition temperatureT _c(H), which is reduced to 14 K by applying fields up to 14 T. BelowT _c(H) the softening turns to rapid hardening. This behavior indicates the presence of lattice instability of the orthorhombic (Bmab) structure in the normal state of LSCO, which disappears in the superconducting state. This is evidence for the intimate interplay between high-T _c superconductivity and the lattice instability in LSCO.     

Polarons and strong electron-phonon coupling in the optical response of La{2-x}SrxCuO4SrxCuO4

A multiliquid approach, based on free carriers with strong electron-phonon interaction, localized polaronic states near 0.15 eV, and a mid-infrared band at 0.5 eV, is applied to model the optical response of La_{2-x}Sr_xCuO₄. The normal state reflectance and absorbance of La_1.83Sr_0.17CuO₄ are investigated and their temperature dependence is explained. Both the ac and dc response are recovered, and the quasilinear behavior of the optical scattering rate up to 3000–4000 cm^-1 is found to be consistent with strong electron-phonon interaction, which also accounts for the value ofT _c . In the superconducting state a comparison with optical data indicates the presence of additional carriers in the conduction band. Model calculations suggest the 0.5 eV mid-infrared band as the source of these carriers and indicate a bipolaronic origin of this band.     

Dynamic Inhomogeneities in the La2CuO4-Based Superconductors

Polarization-dependent X-ray-absorption fine-structure (XAFS) measurements on the local structure of the La₂CuO₄-based high-T _c superconductors La_2–x Sr _x CuO₄, La_2–x Ba _x CuO₄, and La_1.6–x Sr _x Nd_0.4CuO₄ find, among others, orientation disorder induced in the Cu–O₂ planes by doping Sr, Ba, and alloying Nd atoms, all such atoms residing in La-sites. The orientation disorder is of two types: mostly static-buckling disorder, and dynamic disordering of the tilt angles of the Cu–O₆ octahedra correlated in nanoscale regions, with respect to neighboring nanoscale regions. Buckling disorder in the Cu–O₂ planes has the greatest detrimental effect on T _c and conductivity for such foreign atoms.     

Superconductivity and scanning tunneling microscopy (STM) / spectroscopy (STS) of transition metal oxide artificial lattices

High T_c superconducting artificial lattices and superlattices have been constructed by a laser molecular beam epitaxy. Layer-by-layer growth method controlled by RHEED and STM is a promising method for the construction of these artificial lattices. Formation and properties of superconducting artificial lattices and super-conducting / ferromagnetic superlattices are presented as the typical examples. In order to investigate the electronic state of carrier doped strongly correlated electronic system, STS measurements have been performed on La_2–xSr_xCuO₄, La_2–xSr_xNiO₄, and La_1-xSr_xMnO₃ thin films. Based on the similarities and the differences between the STS spectra of the three types of the films, electronic states of the strongly correlated system are discussed.     

Effects of Higher Hopping Terms on the Stripe States in the Two-Dimensional Hubbard Model

Using the variational Monte Carlo method for the two-dimensional t-t′-t″-U Hubbard model, we discuss the ground state of the underdoped region on the high-T _c cuprates. We found that, while stripe states with the periodicity consistent with experiments for La_{2− x} Sr _x CuO₄ are stabilized in the case of t′/t < 0, the positive t′/t makes the stripe state unstable with the lowest energy state being the commensurate AF state, which is consistent with experiments on the electron doping system such as Nd_{2− x} Ce _x CuO₄. We also show that the stripe state is sensitive to the value of t″/t > 0. These results indicate that the nesting condition is a critical factor to the stripe instability.     

A Comparative Study of Angle Dependent Magnetoresistance in [001] and [110] La<Subscript>2/3</Subscript>Sr<Subscript>1/3</Subscript>MnO<Subscript>3</Subscript>

The angle dependent magnetoresistance study on [001] and [110] La_2/3Sr_1/3MnO₃ thin films shows that the anisotropy energy of [110] films is higher than that of a [001] oriented La_2/3Sr_1/3MnO₃ film of similar thickness. The data have been analyzed in the light of a multidomain model and it is seen that this model correctly explains the observed behavior.     

Lattice Instability in High-Temperature Superconducting Cuprates: Polarons Probed by EXAFS

Lattice instability in the planar Cu–O bond in high-temperature superconducting cuprates is probed by the Cu K-edge extended X-ray absorption fine structure (EXAFS). Refined temperature-dependent polarized EXAFS data for high-quality La_1.85Sr_0.15CuO₄ (LSCO) single crystals grown by MBE and TSFZ methods are analyzed and compared with the transport properties. Temperature-dependent oxygen displacement shows a signature of bond splitting at T ^* and a sharp drop due to superconductivity coherence at T _c. Detailed analysis of the anomalous oxygen displacement demonstrates the Cu–O bond splitting, i.e., long and short bonds (ΔR≈0.12 Å), which is in good agreement with the recent cluster calculation. The results indicate that the origin of pseudogap is related to the formation of charged dynamical lattice distortions (polarons) that are coexistent with metallic (superconducting) domains.     

Roles of oxygen defects in copper oxide superconductors

Oxygen vacancy and interstitial defects can have a profound effect on the superconducting properties of copper oxide compounds. Recent work on compounds such as La₂CuO_4+x and HgBa₂CuO_4+x has provided new insight into the role of interstitial oxygen defects as a doping mechanism. The number of carriers created by each interstitial defect depends on the local defect structure. Studies of (La, Sr, Ca)₃Cu₂O_6+x with various metal compositions and metalsite ordering show that interstitial oxygen defects that form between the CuO₂ layers in this structure systematically lowerT _c and eventually destroy superconductivity. Conversely, oxygen vacancies in the CuO₂ planes have surprisingly little effect at concentrations below 3%. The infinite-layer compounds, ACuO₂, where A=La, Sr, Ca, Nd, etc., in solid-solution combinations, could offer a similar environment for the formation of interstitial oxygen defects between the CuO₂ planes, allowing interstitial oxygen defects to contribute to the doping of these compounds. However, neutron diffraction experiments on Sr_0.9La_0.1CuO₂ (T _c = 42 K) have not found any interstitial oxygen.     

On the Presence of 1D-Like Stripes in Superconducting Epitaxial La2-xSrxCuO4 thin Films

A large positive magnetoresistance (up to tens of percents) is observed in both underdoped (x < 0.15) and overdoped superconducting La_{2− x}Sr_xCuO₄ (LSCO) epitaxial thin films, at temperatures far above the superconducting critical temperature T _c . In the underdoped regime, this magnetoresistance cannot be described by the Kohler rule and is due to the influence of superconducting fluctuations. On the other hand, in the overdoped regime, the Kohler rule does not seem to be violated. The strong magnetoresistance above T _c can be related to the preformed superconducting pairs existing well above T _c but forming a phase coherent superconducting state below T _c . The observations support the idea of a close relation between the pseudogap and the superconducting gap and provide evidence for the presence of pre-pairs above T _c . Both the observed fluctuations and the observed magnetoresistance are in accordance with the existence of 1D-like stripes. These results are further supported by recent high magnetic field measurements (up to 55 T) of the transverse magnetoconductivity σ _xy, which goes to zero for T→0 K.