( Sr / Ca ) 14 Cu 24 O 41 spin ladders studied by NMR under pressure

⁶³Cu-NMR measurements have been performed on two-leg hole-doped spin ladders Sr_14-xCa_xCu₂₄O₄₁ single crystals 0 ? x ? 12 at several pressures up to the pressure domain where the stabilization of a superconducting ground state can be achieved. The data reveal a marked decrease of the spin gap derived from Knight shift measurements upon Ca substitution and also under pressure and confirm the onset of low lying spin excitations around P _c as previously reported. The spin gap in Sr ₂ Ca ₁₂ Cu ₂₄ O ₄₁ is strongly reduced above 20 kbar. However, the data of an experiment performed at P = 36 kbar where superconductivity has been detected at 6.7 K by an inductive technique have shown that a significant amount of spin excitations remains gapped at 80 K when superconductivity sets in. The standard relaxation model with two and three-magnon modes explains fairly well the activated relaxation data in the intermediate temperature regime corresponding to gapped spin excitations using the spin gap data derived from Knight shift experiments. The data of Gaussian relaxation rates of heavily doped samples support the limitation of the coherence length at low temperature by the average distance between doped holes. We discuss the interplay between superconductivity and the spin gap and suggest that these new results support the exciting prospect of superconductivity induced by the interladder tunneling of preformed pairs as long as the pressure remains lower than the pressure corresponding to the maximum of the superconducting critical temperature. Received 8 March 2001 and Received in final form 27 July 2001     

New slow and short range magnetic correlations in superconducting La2-xSrxCuO4

Inelastic neutron scattering measurements have been performed on high quality single crystals of in order to study the spin dynamics of this compound. In addition to the well-established incommensurate magnetic response, we show the existence of a new set of low energy excitations present in the whole superconducting region of the phase diagram. This new feature of the dynamical cross section is characterized, below about 10 K, by very short range ( lattice spacing) antiferromagnetic correlations and by a low energy scale of meV. At higher temperatures these fluctuations become nearly Q-independent. Different possible origins of these new spin correlations are discussed. Received: 3 September 1997 / Revised: 21 November 1997 / Accepted: 26 January 1998     

Temperature dependence of the anisotropic electrodynamics in the ladder compounds

The optical reflectivity as a function of temperature of the title compound for x =0, 5 and 12 has been measured over a broad spectral range from 4 meV up to 12 eV. Our findings suggest a very anisotropic dynamics of the charge excitation spectrum when measuring along or perpendicular to the ladders. Moreover, a metal-insulator transition develops at low temperatures, leading to a suppression of Drude spectral weight in the far and mid-infrared spectral range. We identify this behaviour as a consequence of the localization effects for small Ca substitution, and of the possible formation of a charge density wave condensate for large Ca substitution. Received: 18 June 1998 / Accepted: 17 July 1998     

Optical study of RE 1 + x Ba 2 - x Cu 3 O 6 (RE = Nd, Sm) aaand YBa 2 Cu 3 O 6 in the mid infrared range

Infrared reflectance, and transmission measurements as well as Raman scattering have been used to study the RE_1+xBa_2-xCu₃O₆ (RE = Nd, Sm) and YBa₂Cu₃O₆ absorption bands in the 1100-1500 cm^-1 infrared range as a function of temperature and beam polarization. In addition to two-phonon absorption between 1100 and 1170 cm^-1, we observe excitations around 1400 cm^-1, occurring in oxygen rich enclosures within the samples, and assign them to an excitation involving two-phonons plus the 270 cm^-1 local mode related to Cu-O broken chains. Thus, the previously reported possible magnetic origin of the 1436 cm^-1 sharp absorption band in YBa₂Cu₃O₆ is contested. Received 14 February 2001 and Received in final form 12 April 2001     

Ageing, rejuvenation and memory phenomena in a Bi-2212 superconductor showing the paramagnetic Meissner effect

A melt-cast Bi ₂ Sr ₂ CaCu ₂ O ₈ sample showing the paramagnetic Meissner effect (PME) and an ageing phenomenon has been studied by magnetic relaxation and ac-susceptibility experiments. A memory behaviour is observed in the low frequency ac-suscpetibility and in the magnetisation vs. temperature curves measured on heating after certain cooling protocols. It is also found that large enough temperature shifts and positive temperature perturbations cause rejuvenation of the ageing system. All these observations show striking similarities with the ageing behaviour of spin glasses and indicate the existence of a low temperature glassy phase in this PME material. Received 27 February 2001     

Strong impact of the electron-photon matrix element on angle-resolved photoelectron spectra of Sr 2 CuO 2 Cl 2

In recent years insight has been gained into the electronic structure of layered cuprates using angle-resolved photoelectron spectroscopy. In many of these studies it is assumed that the electron-photon matrix element follows the trends set by the atomic photoionization cross sections and does not influence lineshape, dispersion and the k-dependence of the spectral intensity. In this study using Sr₂CuO₂Cl₂ as an example it will be shown that the electron-photon matrix element can have a strong impact on both strength and shape of a feature in an angle-resolved photoelectron spectrum of a layered cuprate which can strongly affect information on character and the momentum-dependence of the energy and spectral weight of a state deduced from the spectra. The results of this study put an emphasis on the need to employ the whole parameter range of the ARPES method to get reliable information on the spectral function of cuprates for which purpose synchrotron radiation is an uniquely suited tool. Received 10 January 2000 and Received in final form 15 January 2001     

Doping and temperature dependence of the spin susceptibility in the p-d model

The spin magnetic susceptibility of the p-d model is calculated by means of a perturbation theory in the hybridization term V through a generalized cumulant expansion (GCE). The analysis is approached from the paramagnetic metallic phase. The results qualitatively reproduce some unusual magnetic properties in the normal state of the hole-doped cuprates, supporting the scenario of a Van Hove singularity near the Fermi level. Received 15 October 1998 and Received in final form 24 March 1999     

Evidence of critical fluctuations from the magnetoconductivity data in Bi2Sr2Ca2Cu3O10+x phase

The fluctuation-induced magnetoconductivity of the Bi₂Sr₂Ca₂Cu₃O_10+x phase is studied above zero-field critical temperature Tc(0) and for moderate magnetic fields. It is found that the Gaussian approximation for superconducting fluctuations underestimates the negative fluctuation magnetoconductance drastically in the Tc(0) < T < Tc(0) + 20 K temperature range. Taking into account the critical fluctuation contribution on the base of self-consistent Hartree approximation makes it possible to explain the data quantitatively in terms of the only Aslamazov-Larkin contribution for different magnetic fields and temperatures, consistently with the zero field data. Received 14 April 2000 and Received in final form 13 July 2000     

Magnetic order in LaEuSrCuO studied [2pt] by Fe Mössbauer spectroscopy

⁵⁷Fe M?ssbauer effect studies of La_1.65Eu_0.20Sr_0.15CuO₄ 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 La_2-xSr_xCuO₄. It indicates that the magnetic ordering temperature in the LTT phase of rare earth (RE) doped La_2-xSr_xCuO₄ 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 La_2-xSr_xCuO₄. Received: 23 June 1998 / Accepted: 14 July 1998     

Sum rule for the optical absorption of an interacting many-polaron gas

A sum rule for the first frequency moment of the optical absorption of a many-polaron system is derived, taking into account many-body effects in the system of constituent charge carriers of the many-polaron system. In our expression for the sum rule, the electron-phonon coupling and the many-body effects in the electron (or hole) system formally decouple, so that the many-body effects can be treated to the desired level of approximation by the choice of the dynamical structure factor of the electron (hole) gas. We calculate correction factors to take into account both low and high experimental cutoff frequencies. Received 26 April 2000 and Received in final form 5 December 2000     

Magnetization scaling in the ruthenate-cuprate RuSr2Eu1.4Ce 0.6Cu2O10-δ (Ru-1222)

The magnetic properties of the superconducting ruthenate-cuprate RuSr₂Eu _1.4Ce_0.6Cu₂O_10-δ (Ru-1222) have been studied by a scaling analysis of the dc magnetization and ac susceptibility measurements. Non-linear M(H) curves reveal the presence of nano-size clusters with a net magnetic moment of ∼10² μ_B at 180 K, near the deviation from a Curie-Weiss behavior. On cooling, no scaling was observed down to 90 K, discarding the possibility of magnetic phase separation of collinear ferromagnetic particles. We explain this result in terms of a variable number of contributing particles, with a temperature dependent net magnetic moment. For 70 K ≤T≤ 90 K the scaling plots evidence the emergence of a system of non-interacting particles, which couple on further cooling. The observed cluster-glass features are preserved down to the lowest measured temperature (10 K); no signature of long-range order was detected. The frequency shift of the peak in the real part of the ac susceptibility does not follow the Vogel-Fulcher type dependence, as previously reported. The puzzling temperature dependence of the coercive field, H_C(T), is correlated with the changes in the scaling factors.     

Magnetization, vortex state and specific heat in the superconducting state of RuSr2GdCu2O8

Based on field-cooled dc-magnetization measurements in a SQUID magnetometer with carefully controlled magnetic-field profiles, we present evidence that diamagnetism is missing in the superconducting state (T < 50 K) of the (weakly) ferromagnetic (T_M ≈130 K) superconductor RuSr₂GdCu₂O₈ (Ru1212). Nevertheless, taking into account the granular nature of the samples investigated so far, this cannot be taken as evidence for the lack of a Meissner state or bulk superconductivity. It is shown that for low applied magnetic fields a vortex state most probably involves the intergrain area (area between the grains) rather than the individual grains (bulk Ru1212). Furthermore, the wide superconducting transition of Ru1212 (T_c,onset = T_c ≈48 K, T_s(R = 0) = T_s = 32 K) realized in resistance measurements in zero applied magnetic field can be readily understood as the effect of resistive grain contacts and is not necessarily related to the movement of spontaneously induced vortices in bulk Ru1212, as it has been suggested previously. A comparison of the low-temperature specific heat of Sr₂GdRuO₆ (Sr2116), the precursor for the preparation of Ru1212 and thus a possible impurity phase, with previously reported data for Ru1212 shows that it is unlikely that Sr2116 is responsible for the specific-heat features attributed to the superconductivity of Ru1212 and supports the existence of a bulk superconducting state in the latter compound.     

自旋梯状化合物Sr14(Cu1-y Fey)24O41的拉曼散射谱研究

利用常规的固相合成法制备了三种Sr14(Cu1-y-Fey)24O41(y=.,0.03,0.05)多晶样品,其中y=0.03是单纯Fe3 掺杂,y=0.05是Fe3 和Fe2 混合掺杂.在100-1500 cm-1频移范围内测量了这三种样品的偏振拉曼光谱.首先,通过与同类结构化合物的谱线比较,确定了在由两个子品格组成的简单Fmmm结构样品中,拉曼光谱的测量存在(4Ag 4B1g 3B2gB3G)模式.其次,实验观察到这三种样品都在1170 cm-1附近出现一弱峰,解释为非拉曼散射的红外活性的纵光学声子模,其中峰强的变化反映了空穴在自旋链与自旋梯之间的重新分布.导电性较强的y=0.03样品在1170 Cm-1附近的散射峰强度十分微弱,原因是CuO2自旋链上的空穴转移到了Cu2O3自旋梯上.     

Coexistence of ferromagnetism and superconductivity in Cu-rich lanthanum Cu-oxides

We have measured the zero field and field cooled magnetization of the lightly oxygen doped Cu-rich La₂CuO _{4 + δ} in a wide temperature range (5 K to 350 K). The data together with the evolution of the magnetic hysteresis loop suggest that the ferromagnetism with Curie temperature of 280 K coexists with superconductivity below the transition temperature ∼ 34 K. The coexistence occurs in the hole-rich clusters of size ? 150 nm, which are electronic phase separated from the hole-poor antiferromagnetic background. Received 17 October 2001     

Infrared transmission study of Pr<Subscript><Emphasis Type="Bold">2</Emphasis></Subscript>CuO<Subscript><Emphasis Type="Bold">4</Emphasis></Subscript> crystal-field excitations

We present an infrared crystal-field study of Pr₂CuO₄ single crystals and thin films. Excitations from the ground state multiplet ³H₄ to the ³H₅, ³H₆, ³F₂ and ³F₃ excited multiplets are observed in both single crystals and thin films. A precise set of crystal-field parameters, that reproduces the energy and the symmetry of the levels, is determined. Received 25 April 2001     

Effects of Ca substitution on the phonon anomalies in the Raman scattering of YBa<Subscript>2</Subscript>Cu<Subscript>4</Subscript>O<Subscript>8</Subscript>

We report on the temperature dependence of the frequencies and linewidths in the phonon Raman scattering for Y _1-y Ca y Ba ₂ Cu ₄ O ₈ system ( y = 0 - 0.15). The phonon anomalies above T _c of the frequencies are observed for the out-of-phase O(2)-O(3) A _g and O(4) A _g modes, and these onset temperatures decrease with increasing Ca content. These features are consistent with the spin-gap behaviors associated with Ca doping reported previously. Furthermore, we find that the smaller gap exists near or just above the Ba phonon frequency at in the undoped samples and its energy increases with Ca doping. Received 9 June 1999     

First-principles calculation of electric field gradients and hyperfine couplings in YBa2Cu3O7

The local electronic structure of YBa₂Cu₃O₇ has been calculated using first-principles cluster methods. Several clusters embedded in an appropriate background potential have been investigated. The electric field gradients at the copper and oxygen sites are determined and compared to previous theoretical calculations and experiments. Spin polarized calculations with different spin multiplicities have enabled a detailed study of the spin density distribution to be made and a simultaneous determination of magnetic hyperfine coupling parameters. The contributions from on-site and transferred hyperfine fields have been disentangled with the conclusion that the transferred spin densities essentially are due to nearest neighbour copper ions only with marginal influence of ions further away. This implies that the variant temperature dependencies of the planar copper and oxygen NMR spin-lattice relaxation rates are only compatible with commensurate antiferromagnetic correlations. The theoretical hyperfine parameters are compared with those derived from experimental data. Received 10 April 2001 and Received in final form 19 June 2001     

Magnetic transition and Meissner shielding in ferromagnetic high-Tc superconductor RuSr2Gd1.4Ce0.6Cu2O10-δ

The magnetic superconductor RuSr₂Gd_1.4Ce_0.6Cu₂O_{10- δ} was studied by means of detailed magnetization measurements in order to clarify its complex magnetic and superconducting properties. Our experiments agree with a model indicating that the in-plane component of the Ru moment might transform from a spin-glass ground state to a ferromagnetically coupled excited state by application of a small magnetic field below the magnetic transition temperature. The experiments also show that a full Meissner state is present at the low temperature, the low magnetic field region of the phase diagram. This is in contrast to the previous prediction that no Meissner state exists in this material for an internal magnetic field of several hundred Oersteds in the CuO₂ planes exerted by the Ru sublattice and exceeding the lower critical field.     

Electronic structure of the Sr0.4Ca13.6Cu24O41 incommensurate compound

We extracted, from strongly-correlated ab-initio calculations, a complete model for the chain subsystem of the Sr0.4Ca13.6Cu24O41 incommensurate compound. A second neighbor t-J+V model has been determined as a function of the fourth crystallographic parameter , for both low and room temperature crystallographic structures. The analysis of the obtained model shows the crucial importance of the structural modulations on the electronic structure through the on-site energies and the magnetic interactions. The structural distortions are characterized by their long range effect on the cited parameters that hinder the reliability of analyses such as BVS. One of the most striking results is the existence of antiferromagnetic nearest-neighbor interactions for metal-ligand-metal angles of 90. A detailed analysis of the electron localization and spin arrangement is presented as a function of the chain to ladder hole transfer and of the temperature. The obtained spin arrangement is in agreement with antiferromagnetic correlations in the chain direction at low temperature.     

Synthesis and microwave properties of TI2Ba2CaCu2O8 superconducting films grown by MOCVD

We report on the synthesis, structural and electrical characterization of high quality Tl₂Ba₂Ca₁Cu₂O₈ (Tl-2212) superconducting films. The samples have been grown ex-situ on mm² LaAlO₃ (100) substrates by a combined approach of metal-organic chemical vapor deposition (MOCVD) and thallium vapor diffusion. The morphological and compositional nature of the c-axis oriented films has been investigated by SEM and X-ray analyses. Typical values of K and MA/cm² at 77 K have been measured. Microwave measurements have been performed at f = 87 GHz inserting the film in a copper cavity and at f =1.5 GHz on patterned samples using a microstrip resonator technique. A penetration depth nm is evaluated by fitting the microwave data with phenomenological equations. The minimum value of the surface resistance measured at 4.2 K is 60 and 6 m at 1.5 GHz and 87 GHz respectively. The microwave data are described in the context of a modified two fluid model. An evaluation of the temperature dependence of the scattering rate has been performed through the simultaneous measurement of the surface resistance and the penetration depth. Received 16 December 1999 and Received in final form 17 March 2000