Sr CuO Research Articles

Heat Transport in Spin Chains with Weak Spin-Phonon Coupling.

The heat transport in a system of S=1/2 large-J Heisenberg spin chains, describing closely Sr(2)CuO(3) and SrCuO(2) cuprates, is studied theoretically at T≪J by considering interactions of the bosonized spin excitations with optical phonons and defects. Treating rigorously the multiboson processes, we derive a microscopic spin-phonon scattering rate that adheres to an intuitive picture of phonons acting as thermally populated defects for the fast spin excitations. The mean-free path of the latter exhibits a distinctive T dependence reflecting a critical nature of spin chains and gives a close description of experiments. By the naturalness criterion of realistically small spin-phonon interaction, our approach stands out from previous considerations that require large coupling constants to explain the data and thus imply a spin-Peierls transition, absent in real materials.

Studies on nonlocal optical nonlinearity of Sr–CuO–polyvinyl alcohol nanocomposite thin films

Thermally induced nonlocal nonlinear optical properties of strontium (Sr) doped CuO-polyvinyl alcohol (PVA) nanocomposite thin films under continuous wave Helium–Neon laser illumination are investigated by single beam Z-scan method. Undoped and Sr doped CuO nanoparticles (NPs) using l-arginine as surface modifying agent have been synthesized by wet chemical method and their thin films with PVA as host matrix have been obtained by spin coating technique. Structure, morphology and purity of prepared CuO NPs and thin films have been studied by X-ray diffraction, high-resolution transmission electron microscopy, field emission scanning electron microscopy and energy dispersive X-ray absorption spectroscopy. Fourier transform infra-red spectrum attests the role of L-arginine as surface modifier and ultraviolet–visible absorption studies reveal that the excitonic absorption wavelengths are blue shifted for strontium doped CuO NPs. Sr doped CuO NPs with average particle size of 7nm and calculated optical band gap up to 2.54eV have been reported. All Sr doped CuO–PVA nanocomposite thin films show enhanced nonlinear refraction and absorption best suited for optical limiting applications. Observed effects have been attributed to thermal lensing effect.

Behaviour of the second magnetization peak in La2−Sr CuO4 single crystals upon entering the doping domain of static stripe order

Abstract The occurrence of a second magnetization peak (SMP) on the dc magnetization curves of superconducting single crystals with random quenched disorder in increasing external magnetic field H is quite common. Many models for this intriguing phenomenon have been proposed so far, but its origin is still under strong debate. Optimally doped and overdoped La2−xSrxCuO4 single crystals (x ≥ 0.15) exhibit an SMP over a large temperature interval, up to very close to the critical temperature. By decreasing doping, it was found that with H oriented along the c axis the SMP completely disappears in the doping domain of well developed static charge and spin stripes (x ∼ 1/8), reappearing for x ≤ 0.10. This behaviour follows the instability of the Bragg vortex glass generated by the static stripe order (as revealed by small-angle neutron scattering experiments), which is confirmed by our magnetization relaxation measurements. If H is along the (a, b) planes, the SMP is observed for all investigated specimens. The presented analysis supports the scenario in which the SMP is generated by the pinning-induced disordering of the low-field Bragg vortex glass in the dynamic conditions of dc magnetic measurements.

Magnetic characterisation of La2−Sr CuO4 at microwave frequencies

Abstract We have investigated the magnetisation response of a La 2− x Sr x CuO 4 epitaxial thin film at microwave frequencies as a function of temperature and external magnetic field using a copper loop as a resonator. At low field and low temperature a single negative peak and a sudden jump in the time and microwave frequency dependence of the magnetisation are observed for certain frequency values. Near T C , sharp negative peaks appear at different frequencies. With the increase of the magnetic field, the same structure of peaks appears at lower temperature. These phenomena are not observed when identical measurements are made either without radiation or when the sample is in the normal state. We attempt to identify them as the combined result of the vortex dynamics, a screening effect at the sample edge and a vortex shaking process caused by the microwave field.

Superconductivity in under-doped La2−Sr CuO4 under pressure

The out-of-plane resistivity ( ρ c ) has been measured in under-doped La 2− x Sr x CuO 4 under pressure to investigate the nature of the superconducting transition. The antiferromagnetic insulating sample with x =0.020 did not show superconductivity even under pressure. However, in the non-superconducting samples with x =0.035 and 0.045, an appreciable but incomplete drop of ρ c was observed with lowering temperature and increasing pressure. The drop of ρ c under pressure suggests the existence of the intrinsic phase separation or inhomogeneity around x ∼0.05.

Precise muon-spin-relaxation measurements on the Cu-spin dynamics in La2−Sr CuO4

Abstract Precise zero-field muon-spin-relaxation measurements on La 2− x Sr x CuO 4 have revealed that the dynamical muon-spin depolarization rate starts to increase monotonically with decreasing temperature at a high temperature around 100 K . Compared with the resistivity results in the normal state, it has been concluded that the localization of holes leads to the suppression of the Cu-spin fluctuations.

Interpretation of anomalous normal state optical conductivity of La–Sr–CuO cuprates

The observed frequency dependent optical response of hole doped cuprate La 1.85Sr 0.15CuO 4 (T c ≈40 K) superconductors, has been theoretically analysed. Starting from an effective two-dimensional (2D) interaction potential for superlattice of hole doped cuprates treated as a layered electron gas, the spectral function is formulated. Calculations of the optical conductivity σ( ω) have been made within the two-component scheme: one is the coherent Drude carriers responsible for superconductivity and the other is incoherent motion of carriers from one site to other leads to a pairing between Drude carriers. The approach accounts for the anomalies reported (frequency dependence of optical conductivity) in the optical measurements for the normal state. Estimating the effective mass from specific heat measurement and ε ∞ from band structure calculations for the low-energy charge density waves, the model has only one free parameter, the relaxation rate. The frequency dependent relaxation rates are expressed in terms of memory functions, and the coherent Drude carriers from the effective interaction potential lead to a sharp peak at zero frequency and a long tail at higher frequencies, i.e. in the infrared (IR) region. However, the hopping of carriers from one site to other (incoherent motion of doped carriers) yields a peak value in the optical conductivity centred at mid-IR (MIR) region. We find that both the Drude and hopping carriers in the superlattice of cuprates will contribute to the optical process of conduction in the CuO 2 planes and shows similar results on optical conductivity in the MIR as well as IR frequency regions as those revealed from experiments.

Scanning tunneling microscopy and spectroscopy on La2−Sr CuO4

Scanning tunneling microscopy and spectroscopy (STM/STS) measurements have been performed on the nearly optimally doped La1.84Sr0.16CuO4 (LSCO) single crystal with Tc=39 K. The surface topography has been observed for the first time and revealed the step structures on the c-face whose edge was parallel to the aHTT-axis (Cu–O bonding direction). The value of the superconducting gap Δ has been distributed around 8 meV, which corresponds to 2Δ/kBTc≈4.8.

An intralayer pairing mechanism for the coexistence of charge- and spin-density waves induced superconductivity in LaSrCuO

Starting from an effective two-dimensional dynamic interaction that includes screening of holes as carriers by charge density fluctuations and by optical phonons, we investigate the nature of a d-wave pairing mechanism leading to superconductivity in layered La-based cuprates. We consider the La–Sr–CuO system as an ionic solid containing layers of holes as carriers with a single CuO2 layer in a unit cell, where the localized spins form an antiferromagnetic (AF) order. The electron–phonon interaction matrix element in the case of an ordinary unit cell without the local AF order yields s-wave superconductivity. While for the unit cell with AF order, the wave-vector dependence of the intralayer effective interaction potential shows the sign reversal to create d-wave pairing due to localized antiferromagnetic spin order for the screened phonons. Following the strong coupling theory, the superconducting transition temperature, the isotope exponent, coherence length and magnetic penetration depth are also estimated. The implications of the intralayer pairing model and its analysis are discussed.

High field transport properties of La Sr CuO epitaxial thin films

We have studied the temperature dependent resistivity ρ( T ) of La2-xSrxCuO4 epitaxial thin films in the doping range 0.045 ⩽ x ⩽ 0.25 in pulsed magnetic fields up to 50 T. The zero-field resistivity ρ( T ) of these samples in the pseudogap regime, can be scaled onto one single universal curve in a broad temperature range by using a linear transformation of both temperature and resistivity. The high field data ρ( T ) reveal a metal to insulator transition (MIT) at low temperatures, well into the overdoped regime. For samples having k F l < 1, with kF the Fermi wave vector and l the mean free path, this low temperature insulating behavior of the resistivity is described by the variable range hopping conductivity (VRH). For samples with k F l > 1, the divergence follows ρ( T ) ∼ ln (1/ T ) or a power law, depending upon the Sr-content. We further found that the residual conductivity at the minimum in ρ( T ), appearing due to the MIT, follows a linear behavior with respect to the Sr-content. It is argued that the unusual MIT in compounds with k F l > 1, is most probably associated with the pseudogap and the behavior of charge stripes at low temperatures.

Spin dynamics in hole-doped two-dimensional S = 1/2 Heisenberg antiferromagnets: Cu NQR relaxation in La Sr CuO for

The effects on the correlated Cu^{2+} S = 1/2 spin dynamics in the paramagnetic phase of La_{2-x}Sr_xCuO_4 (for $x \lesssim 0.04$) due to the injection of holes are studied by means of ^{63}Cu NQR spin-lattice relaxation time T_1 measurements. The results are discussed in the framework of the connection between T_1 and the in-plane magnetic correlation length $\xi_{2D}(x,T)$. It is found that at high temperatures the system remains in the renormalized classical regime, with a spin stiffness constant $\rho_s(x)$ reduced by small doping to an extent larger than the one due to Zn doping. For $x\gtrsim 0.02$ the effect of doping on $\rho_s(x)$ appears to level off. The values for $\rho_s(x)$ derived from T_1 for $T\gtrsim 500$ K are much larger than the ones estimated from the temperature behavior of sublattice magnetization in the ordered phase ($T\leq T_N$). It is argued that these features are consistent with the hypothesis of formation of stripes of microsegregated holes.

Magnetic order in La Eu Sr CuO studied [2pt] by Fe Mössbauer spectroscopy

57Fe Mossbauer effect studies of La1.65Eu0.20Sr0.15CuO4 doped with 0.5 at% 57Fe performed in the temperature region 300 K > T > 4.2 K give an onset temperature for magnetic ordering of K. This temperature practically is the same as that found in Nd doped La2-xSrxCuO4. It indicates that the magnetic ordering temperature in the LTT phase of rare earth (RE) doped La2-xSrxCuO4 is independent of the RE moment. The direction of the 57Fe magnetic moment in the magnetically ordered state is within the CuO2 plane, while it has been found to be parallel to the c-axis in Nd doped La2-xSrxCuO4.

Probing of the density of states near the Fermi level in (Ca,Sr) CuO 2 infinite layer thin films by STM/STS

The carrier distribution in the topmost CuO 2 sheets of infinite layer (Ca 0.3, Sr 0.7)CuO 2 thin films has been investigated by ultra-high-vacuum scanning tunneling microscopy/spectroscopy (STM/STS). STM images at different values of the tip-sample bias provide a map which suggests an inhomogeneous distribution of the energy states near the Fermi level. The appearance of energy states at the Fermi level is observed by STS for a one unit-cell thick layer of SrCuO y on a charge reservoir layer of Bi 2Sr 2CuO 6.

GROWTH AND PROPERTIES OF Sr1−xNdxCuO2 INFINITE-LAYER PHASE THIN FILMS

Thins films of Sr 1−x Nd x CuO 2 (infinite CuO 2 layer) compounds with x=0.10–0.15 have been prepared by the rf-magnetron sputtering method. X-ray diffraction analysis showed that for the parent compound of SrCuO 2, there is a solubility limit x~0.11 for Nd. Corresponding to this limit, a single infinite- CuO 2-layer phase (Nd, Sr)CuO 2 thin film with smallest c-axis length was obtained. Exceeding this limit, infinite-layer phases with longer c-axis and long c-axis tetragonal phase (the second phase) coexisted. Further, the superconducting indication presented by both resistive transition (at ~28 K) and Meissner effect was also found for a thin-film sample with x~0.11.

Evidence of magnetic order of Cu moments in the superconducting and normal region of La2−Sr CuO4: Observation of Cu-NQR for 0.12≦x≦0.40

The observation of Cu-NQR in La2−xSrxCuO4 revealed that the spectra were splitted into two peaks at low temperatures and that its separation decreased with increasing x. This fact indicates that the well-defined hyperfine field due to magnetic ordering of Cu moments exists in the superconducting and normal metallic regions. The nuclear spin-lattice relaxation rate, 1/T1 of the Cu spectra is suppressed apparently by the superconducting energy gap for x≦0.20, while its temperature dependence obeys the Korringa relation for the normal region of x≧0.25. The nuclear spin-spin relaxation rate, 1/T2 increases proportionally with increasing hyperfine field, indicating the dominant indirect unclear spin coupling via virtual excitation of antiferromagnetic spin waves. The coexistence of superconductivity and the magnetic order of Cu d-electrons with a tiny moment is demonstrated by the NQR study in the La-Sr-Cu-O system

Phonon Density-of-States for the High-Tc Superconductor La1.85Sr0.15CuO4, its Non-Superconducting Reference La2CuO4 and for YBa2Cu3O7

We present results of inelastic neutron-scattering experiments on high-Tc La–Sr–Cu-O and Ba–Y–Cu–O superconductors and on reference samples La–Cu–O for T=300 K and T=6 K. From these results generallized phonon densities-of-states (PDOS) are deduced, which for the La–Sr–Cu–O-compound bear clear indications of increased coupling between electrons and high-frequency oxygen vibrations but do not show pronounced soft-mode behaviour. For the higher Tc Y-compound the important oxygen breathing vibrations lie at still lower frequencies, which at least partially could be due to a still stronger electron-phonon-coupling.