arXiv subjects

F. Cordero

Publications and source records attributed to F. Cordero.

33 records · Page 2Linked to original sources

arXiv2004

Polar and non-polar atomic motions in the relaxor ferroelectric PLZT from dielectric, anelastic and NMR relaxation

Dielectric, anelastic and ^139La NMR relaxation measurements have been made on the relaxor ferroelectric Pb(1-3x/2)La(x)Zr(0.2)Ti(0.8)O(3) (PLZT) with x = 0.22. The dielectric susceptibility exihibits the frequency dispersive maximum due to the freezing of the polar degrees of freedom around T ~250 K. The anelastic and especially NMR relaxation, besides this maximum, indicate an intense and broad component at lower temperatures, attributed to rotational modes of the O octahedra, weakly coupled to the polar modes. It is discussed why such short range rotational instabilities, known to occur in the Zr-rich rhombohedral region of the PLZT phase diagram, might appear also in the Ti-rich region.

cond-mat.mtrl-sci↗

arXiv2004

Memory of multiple aging stages above the freezing temperature in the relaxor ferroelectric PLZT

The dynamic dielectric susceptibility and elastic compliance of the relaxor ferroelectric PLZT 9/65/35 have been measured under different cooling and heating protocols, in order to study aging and memory. The memory of multiple aging stages has been found (several dips in the susceptibility curves on heating), as in spin glass systems below the glass transition. Remarkably, in PLZT the memory is retained also above the freezing temperature deduced from the dynamic susceptibilities. The results are discussed in the light of the existing models of aging and memory in spin and dipolar glasses.

cond-mat.dis-nn↗

arXiv2003

Formation and mobility of oxygen vacancies in RuSr2GdCu2O8

Oxygen vacancies are introduced in the RuO2 and possibly CuO2 planes of RuSr2GdCu2O8 by annealing in vacuum above 600 K. The diffusive jumps of the O vacancies are accompanied by a reorientation of the local distortion, and are probed by measuring the elastic energy loss and modulus versus temperature at 1-10 kHz. An intense acoustic absorption peak develops near 670 K at 1 kHz and finally stabilizes after heating up to 920 K in vacuum. The analysis of the peak shows a barrier for the O diffusion of ~1.4 eV, and a slowing down of Curie-Weiss type, with Tc = 400-470 K, due to the interaction among the O vacancies. A secondary peak is attributed to O vacancies trapped at defects in the RuO2 planes, or to vacancies in the CuO2 planes. No sign of structural phase transformation is found up to 920 K.

cond-mat.supr-con↗

arXiv2002

Anelastic relaxation process of polaronic origin in La{2-x}Sr{x}CuO{4}: interaction between the charge stripes and pinning centers

The evolution of an anelastic relaxation process occurring around 80 K in La{2-x}Sr{x}CuO{4} at a measuring frequency of ~1 kHz has been followed from x = 0.0075 to the overdoped region, x = 0.2, where it disappears. The dependence of the peak intensity on doping is consistent with a polaronic mechanism, identified with the disordered charge stripes overcoming pinning centers. A marked decrease of the peak amplitude occurs at x > 0.045, the same doping range where a change of the stripe order from parallel to diagonal with respect to the Cu-O bonds has been observed by neutron diffraction. Both the energy barrier and peak amplitude also exhibit a rise near x = 1/8.

cond-mat.supr-con↗

arXiv2002

Anelastic spectroscopy study of the spin-glass and cluster spin-glass phases of La$_{2-x}$Sr$_{x}$CuO$_{4}$ $(0.015<x<0.03)$

The anelastic spectra of La$_{2-x}$Sr$_{x}$CuO$_{4}$ have been measured at liquid He temperatures slightly below and above the concentration $% x_{c}\simeq 0.02$ which is considered to separate the spin-glass phase from the cluster spin-glass (CSG) phase. For $x\le x_{c}$ all the elastic energy loss functions show a step below the temperature $T_{g}(x=0.02) $ of freezing into the CSG state, similarly to what found in samples well within the CSG phase, but with a smaller amplitude. The excess dissipation in the CSG state is attributed to the motion of the domain walls between the clusters of antiferromagnetically correlated spin. These results are in agreement with the recent proposal, based on inelastic neutron scattering, of an electronic phase separation between regions with $x\sim 0$ and $x\sim 0.02$, at least for $x>0.015$

cond-mat.supr-con↗

arXiv2002

Acoustic measurement of the low-energy excitations in Nd2-xCexCuO4

The complex dynamic Young's modulus of ceramic Nd2-xCexCuO4 with x = 0, 0.05 and 0.20 has been measured from 1.5 to 100 K at frequencies of 1-10 kHz. In the undoped sample the modulus starts decreasing below ~20 K, instead of approaching a constant value as in a normal solid. The modulus minimum has been interpreted in terms of paraelastic contribution from the relaxation of the Nd^3+ 4f electrons between the levels of the ground state doublet, which is split by the interaction with the antiferromagnetically ordered Cu sublattice. The value of the splitting is found to be 0.34 meV, in excellent agreement with inelastic neutron scattering, infrared and specific heat experiments. With doping, the anomaly shifts to lower temperature and decreases in amplitude, consistently with a reduction of the local field from the Cu sublattice.

cond-mat.supr-con↗

arXiv2001

A search for incipient lattice instabilities in MgB$_{2}$ by anelastic spectroscopy

We measured the complex dynamic elastic modulus of MgB2 between 1.3 and 650 K at frequencies included between 5 and 70 kHz. The main purpose of this work is to search for possible lattice instabilities, which could contribute to enhance the electron-phonon coupling. The Young's modulus indeed presents an anomalous softening on cooling below 400 K, and hysteresis between cooling and heating. Such anomalies, however, can be accounted for by intense relaxation processes with maxima between 50 and 150 K, whose nature is not yet clarified, and by geometric defects generated by the anisotropic thermal expansion between the grains during thermal cycling. Additional intense relaxation processes are observed above room temperature, and their possible origin is discussed. The influence of the highly anharmonic in-plane vibration modes of the B atoms on the elastic and anelastic properties of MgB2 is discussed in detail.

cond-mat.supr-con↗

arXiv2001

Glassy dynamics of the inhomogeneous metallic phase in La1-xCaxMnO3

The anelastic spectra (elastic energy loss and dynamic modulus versus temperature) of La1-xCaxMnO3 have been measured for 0.33< x < 0.5. A peak in the imaginary part of the elastic susceptibility is found slightly below the Curie temperature, whose temperature and frequency dependences are typical of the dielectric and magnetic susceptibilities of relaxor ferroelectrics, spin glasses and other inhomogeneous frustrated systems. The fluctuations can be associated with the magnetic clusters which are observed by several techniques or with charge-ordered insulating domains in the low temperature inhomogeneous phase. The temperature dependent distribution function of the fluctuation energy barriers has been extracted by the analysis of the experimental curves.

cond-mat.str-el↗

arXiv2000

Cluster Spin Glass Distribution Functions in La$_{2-x}$Sr$_x$CuO$_4$

Signatures of the cluster spin glass have been found in a variety of experiments, with an effective onset temperature $T_{on}$ that is frequency dependent. We reanalyze the experimental results and find that they are characterized by a distribution of activation energies, with a nonzero glass transition temperature $T_g(x)<T_{on}$. While the distribution of activation energies is the same, the distribution of weights depends on the process. Remarkably, the weights are essentially doping independent.

cond-mat.supr-con↗

arXiv2000

Pinning of the domain walls of the cluster spin-glass phase in LTT La_{2-x}Ba_xCuO_4

We compare the low frequency (~1 kHz) anelastic spectra of La_{2-x}Sr_xCuO_4 and La_{2-x}Ba_xCuO_4 at x = 0.03 and 0.06 in the temperature region where the freezing into the cluster spin-glass (CSG) phase occurs and is accompanied by an increase of the acoustic absorption. The dependence of the amplitude of the anelastic relaxation on doping is explained in terms of movement of the domain walls (DW) in the CSG phase between the Sr (Ba) pinning points. The LBCO sample at x = 0.06 transforms into the LTT structure below 40 K and the amplitude of the anelastic anomaly is 7 times smaller than expected, indicating pinning of the DW which run parallel to the LTT modulation. Such DW can be identified with the stripes of high hole density, and the present measurements show that they are mobile between the Sr (Ba) pinning points down to few kelvin, but become static in the presence of LTT modulation also far from the condition x = 1/8 for commensurability between stripe and lattice periodicities.

cond-mat.supr-con↗

arXiv2000

Anelastic relaxation and $^{139}$La NQR in La$_{2-x}$Sr$_x$CuO$_4$ around the critical Sr content x=0.02

Anelastic relaxation and $^{139}$La NQR relaxation measurements in La$_{2-x}$Sr$_x$CuO$_4$ for Sr content x around 2 and 3 percent, are presented and discussed in terms of spin and lattice excitations and ordering processes. It is discussed how the phase diagram of La$_{2-x}$Sr$_x$CuO$_4$ at the boundary between the antiferromagnetic (AF) and the spin-glass phase (x = 0.02) could be more complicate than previous thought, with a transition to a quasi-long range ordered state at T = 150 K, as indicated by recent neutron scattering data. On the other hand, the $^{139}$La NQR spectra are compatible with a transition to a conventional AF phase around T = 50 K, in agreement with the phase diagram commonly accepted in the literature. In this case the relaxation data, with a peak of magnetic origin in the relaxation rate around 150 K at 12 MHz and the anelastic counterparts around 80 K in the kHz range, yield the first evidence in La$_{1.98}$Sr$_{0.02}$CuO$_4$ of freezing involving simultaneously lattice and spin excitations. This excitation could correspond to the motion of charged stripes.

cond-mat.supr-con↗

arXiv2000

Observation of the cluster spin-glass phase in La_{2-x}Sr_{x}CuO_{4} by anelastic spectroscopy

An increase of the acoustic absorption is found in La_{2-x}Sr_{x}CuO_{4} (x = 0.019, 0.03 and 0.06) close to the temperatures at which freezing of the spin fluctuations in antiferromagnetic-correlated clusters is expected to occur. The acoustic absorption is attributed to changes of the sizes of the quasi-frozen clusters induced by the vibration stress through magnetoelastic coupling.

cond-mat.str-el↗

arXiv1999

A study of the mechanisms of the semi-insulating conversion of InP by anelastic spectroscopy

Elastic energy absorption measurements versus temperature on semiconducting, semi-insulating (SI) and Fe-doped InP are reported. A thermally activated relaxation process is found only in the SI state, which is identified with the hopping of H atoms trapped at In vacancies. It is proposed that the presence of In vacancies in InP prepared by the liquid encapsulated Czochralski method is due to the lowering of their energy by the saturation of the P dangling bonds with H atoms dissolved from the capping liquid containing H2O. The conversion of iron-free InP to the SI state following high temperature treatments would be due to H loss with the transformation of the H-saturated In vacancies, V_In-H_4 donors, into neutral and acceptor V_In-H_n complexes with n < 4. Such complexes would produce the observed anelastic relaxation process and may also act as deep acceptors which neutralize unwanted donor impurities.

cond-mat.mtrl-sci↗

arXiv1999

Tunneling driven tilt modes of the O octahedra in La{2-x}Sr{x}CuO{4}: strong dependence on doping

The anelastic spectrum of La{2-x}Sr{x}CuO{4} (x = 0, 0.008, 0.019, 0.032) has been measured down to 1.5 K, in order to see the effect of doping on the intrinsic lattice fluctuations already found in stoichiometric La{2}CuO{4}, and identified with tunneling driven tilt modes of the O octahedra. Slight doping with Sr causes a drastic increase of the transition rates and relaxation strength of the tunneling systems. The influence of doping on the relaxation rate is interpreted in terms of direct coupling between between the tilts of the octahedra and the hole excitations. However, the observed fast dependence of the rate on temperature cannot be explained in terms of the ususal models of coupling between a tunneling system and the conduction electrons.

cond-mat.supr-con↗

arXiv1999

Tilt waves dynamics of the oxygen octahedra in La2CuO4 from anelastic and 139La NQR relaxation

The anharmonic vibrational dynamics in nearly stoichiometric La2CuO4+delta is studied by means of anelastic and ^{139}La NQR relaxation. In the absorption component of the elastic susceptibility as well as in the nuclear relaxation rate a peak is detected as a function of temperature, and a relaxation time tau = 1.7 10^{-12} exp [(2800 K)/T] s is derived. The relaxation processes are attributed to tilt motion of the CuO6 octahedra in doublewell potentials, whose cooperative character increases the effective energy barrier to the observed value. The analysis of the relaxation mechanisms has been carried out by reducing the dynamics of the interacting octahedra to a one-dimensional equation of motion. The soliton-like solutions correspond to parallel walls separating domains of different tilt patterns and give rise to pseudo-diffusive modes which appear as a central component in the spectral density of the motion of the octahedra. The tilt waves may be considered to correspond to the dynamical lattice stripes observed in La-based and Bi-based high-Tc superconductors.

cond-mat.supr-con↗