Author

# José Luis Martínez

Other affiliations: Brookhaven National Laboratory, University of Córdoba (Spain), European Synchrotron Radiation Facility ...read more

Bio: José Luis Martínez is an academic researcher from Spanish National Research Council. The author has contributed to research in topics: Neutron diffraction & Magnetic susceptibility. The author has an hindex of 43, co-authored 448 publications receiving 7767 citations. Previous affiliations of José Luis Martínez include Brookhaven National Laboratory & University of Córdoba (Spain).

##### Papers published on a yearly basis

##### Papers

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TL;DR: Neutron-scattering experiments on YBa2- CU306+s and Laq —, (Ba, Sr), Cu04 show that for small doping, the antiferromagnetic long-range order is replaced by commensurate fluctuations with a correla- tion length of order of the interimpurity spacing.

Abstract: We use inelastic neutron scattering to establish the modulation vectors \ensuremath{\delta} and correlation lengths for the incommensurate magnetic fluctuations in metallic samples of ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$ with x=0.075 and 0.14. In notation appropriate for a square lattice where the magnetic instability in the undoped case occurs at (\ensuremath{\pi},\ensuremath{\pi}), the vectors \ensuremath{\delta} are along (\ensuremath{\pi},0) and (0,\ensuremath{\pi}). The correlation length \ensuremath{\xi} is larger than the distance between carriers, is weakly dependent on x, and changes significantly between 12 and 100 K for both compositions.

367 citations

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TL;DR: An overall survey of the structural and magnetic features of the La2NiO4+ delta system is presented as a result of neutron diffraction experiments in this article, where a tentative phase diagram is proposed.

Abstract: An overall survey of the structural and magnetic features of the La2NiO4+ delta system is presented as a result of neutron diffraction experiments. The stoichiometric compound ( delta =0) presents two structural phase transitions. At T0 approximately=770 K, La2NiO4 transforms from tetragonal (I4/mmm) to orthorhombic (Bmab); at T1 approximately=80 K, from orthorhombic to a new tetragonal (P42/ncm) phase. Associated with this second phase transition a strong microstrain produces anisotropic broadening of Bragg reflections. La2NiO4 is three-dimensional (3D) antiferromagnetically ordered at room temperature (TN=330 K). A weak ferromagnetic component appears below T1. Oxygen excess suppress the 3D magnetic ordering and the structural phase transformations, giving rise to a non-stoichiometric compound with interstitial oxygens. A tentative phase diagram is proposed.

365 citations

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TL;DR: In this article, a comparative study of the influence of the rare earth size on the structural parameters of R2BaCuO5 compounds with R = rare earth atom from Sm to Lu have been studied by neutron and X-ray powder diffraction.

264 citations

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TL;DR: In this article, the magnetic structures of the hexagonal oxides of the space group (P6}-3cm) have been studied by neutron-diffraction experiments on powder samples, and it has been complemented with magnetic susceptibility and specific heat measurements.

Abstract: The magnetic structures of the hexagonal ${\mathrm{YMnO}}_{3}$ and ${\mathrm{ScMnO}}_{3}$ oxides (space group ${P6}_{3}cm)$ have been studied by neutron-diffraction experiments on powder samples. This study has been complemented with magnetic susceptibility and specific-heat measurements. They confirm that ${\mathrm{YMnO}}_{3}$ and ${\mathrm{ScMnO}}_{3}$ become ordered below ${T}_{N}\ensuremath{\approx}70$ K and ${T}_{N}\ensuremath{\approx}129$ K, respectively, according to a triangular antiferromagnetic structure. The latter compound undergoes a second magnetic transition below 75 K, corresponding to a spin reorientation. The presence of weak ferromagnetism of Dzialoshinskii-Moriya type is observed in all the ordered region for the Sc material. In both compounds the moments are contained in the $(a,b)$ plane and oriented perpendicular $(R=\mathrm{Y})$ or parallel $(R=\mathrm{Sc})$ to the [100] directions. The low-temperature ordered moments are $2.90(2){\ensuremath{\mu}}_{B}$ $(R=\mathrm{Y})$ and $3.54(3){\ensuremath{\mu}}_{B}$ $(R=\mathrm{Sc})$ at $T=1.7$ K. The different behavior observed in the thermal evolution of the magnetic structure of both compounds is discussed in terms of geometrical parameters determined from high-resolution structural data. The origin of the weak ferromagnetism in ${\mathrm{ScMnO}}_{3}$ is also analyzed.

241 citations

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TL;DR: The presence of a moderate level of antisite disorder is at the very root of low field magnetoresistance although effects such as disorder distribution, connectivity, or morphology add their contribution.

Abstract: We search for general patterns that explain the low field magnetoresistance at low temperatures in the system ${A}_{(2\ensuremath{-}x)}{A}_{x}^{\ensuremath{'}}{\mathrm{FeMoO}}_{6}$. The observed linear dependence of the low field magnetoresistance with the saturation magnetization for the series is related to the antisite disorder at the $\mathrm{Fe}$ and $\mathrm{Mo}$ sites. This is explained in terms of a spin dependent crossing of intragranular barriers originated from the presence of antiferromagnetic ${\mathrm{SrFeO}}_{3}$ patches that naturally develop when antisite disorder occurs in the double perovskite. The presence of a moderate level of antisite disorder is at the very root of low field magnetoresistance although effects such as disorder distribution, connectivity, or morphology add their contribution.

223 citations

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TL;DR: In this article, the main formulas governing the analysis of the Bragg magnetic scattering are summarized and shortly discussed and the method of profile fitting without a structural model to get precise integrated intensities and refine the propagation vector(s) of the magnetic structure is discussed.

Abstract: In spite of intrinsic limitations, neutron powder diffraction is, and will still be in the future, the primary and most straightforward technique for magnetic structure determination. In this paper some recent improvements in the analysis of magnetic neutron powder diffraction data are discussed. After an introduction to the subject, the main formulas governing the analysis of the Bragg magnetic scattering are summarized and shortly discussed. Next, we discuss the method of profile fitting without a structural model to get precise integrated intensities and refine the propagation vector(s) of the magnetic structure. The simulated annealing approach for magnetic structure determination is briefly discussed and, finally, some features of the program FullProf concerning the magnetic structure refinement are presented and discussed. The different themes are illustrated with simple examples.

11,923 citations

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Abstract: Recent research activities on the linear magnetoelectric (ME) effect?induction of magnetization by an electric field or of polarization by a magnetic field?are reviewed. Beginning with a brief summary of the history of the ME effect since its prediction in 1894, the paper focuses on the present revival of the effect. Two major sources for 'large' ME effects are identified. (i) In composite materials the ME effect is generated as a product property of a magnetostrictive and a piezoelectric compound. A linear ME polarization is induced by a weak ac magnetic field oscillating in the presence of a strong dc bias field. The ME effect is large if the ME coefficient coupling the magnetic and electric fields is large. Experiments on sintered granular composites and on laminated layers of the constituents as well as theories on the interaction between the constituents are described. In the vicinity of electromechanical resonances a ME voltage coefficient of up to 90?V?cm?1?Oe?1 is achieved, which exceeds the ME response of single-phase compounds by 3?5 orders of magnitude. Microwave devices, sensors, transducers and heterogeneous read/write devices are among the suggested technical implementations of the composite ME effect. (ii) In multiferroics the internal magnetic and/or electric fields are enhanced by the presence of multiple long-range ordering. The ME effect is strong enough to trigger magnetic or electrical phase transitions. ME effects in multiferroics are thus 'large' if the corresponding contribution to the free energy is large. Clamped ME switching of electrical and magnetic domains, ferroelectric reorientation induced by applied magnetic fields and induction of ferromagnetic ordering in applied electric fields were observed. Mechanisms favouring multiferroicity are summarized, and multiferroics in reduced dimensions are discussed. In addition to composites and multiferroics, novel and exotic manifestations of ME behaviour are investigated. This includes (i) optical second harmonic generation as a tool to study magnetic, electrical and ME properties in one setup and with access to domain structures; (ii) ME effects in colossal magnetoresistive manganites, superconductors and phosphates of the LiMPO4 type; (iii) the concept of the toroidal moment as manifestation of a ME dipole moment; (iv) pronounced ME effects in photonic crystals with a possibility of electromagnetic unidirectionality. The review concludes with a summary and an outlook to the future development of magnetoelectrics research.

4,315 citations

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TL;DR: In this paper, a review of the physics of high-temperature superconductors from the point of view of the doping of a Mott insulator is presented, with the goal of putting the resonating valence bond idea on a more formal footing.

Abstract: This article reviews the physics of high-temperature superconductors from the point of view of the doping of a Mott insulator. The basic electronic structure of cuprates is reviewed, emphasizing the physics of strong correlation and establishing the model of a doped Mott insulator as a starting point. A variety of experiments are discussed, focusing on the region of the phase diagram close to the Mott insulator (the underdoped region) where the behavior is most anomalous. The normal state in this region exhibits pseudogap phenomenon. In contrast, the quasiparticles in the superconducting state are well defined and behave according to theory. This review introduces Anderson's idea of the resonating valence bond and argues that it gives a qualitative account of the data. The importance of phase fluctuations is discussed, leading to a theory of the transition temperature, which is driven by phase fluctuations and the thermal excitation of quasiparticles. However, an argument is made that phase fluctuations can only explain pseudogap phenomenology over a limited temperature range, and some additional physics is needed to explain the onset of singlet formation at very high temperatures. A description of the numerical method of the projected wave function is presented, which turns out to be a very useful technique for implementing the strong correlation constraint and leads to a number of predictions which are in agreement with experiments. The remainder of the paper deals with an analytic treatment of the $t\text{\ensuremath{-}}J$ model, with the goal of putting the resonating valence bond idea on a more formal footing. The slave boson is introduced to enforce the constraint againt double occupation and it is shown that the implementation of this local constraint leads naturally to gauge theories. This review follows the historical order by first examining the U(1) formulation of the gauge theory. Some inadequacies of this formulation for underdoping are discussed, leading to the SU(2) formulation. Here follows a rather thorough discussion of the role of gauge theory in describing the spin-liquid phase of the undoped Mott insulator. The difference between the high-energy gauge group in the formulation of the problem versus the low-energy gauge group, which is an emergent phenomenon, is emphasized. Several possible routes to deconfinement based on different emergent gauge groups are discussed, which leads to the physics of fractionalization and spin-charge separation. Next the extension of the SU(2) formulation to nonzero doping is described with a focus on a part of the mean-field phase diagram called the staggered flux liquid phase. It will be shown that inclusion of the gauge fluctuation provides a reasonable description of the pseudogap phase. It is emphasized that $d$-wave superconductivity can be considered as evolving from a stable U(1) spin liquid. These ideas are applied to the high-${T}_{c}$ cuprates, and their implications for the vortex structure and the phase diagram are discussed. A possible test of the topological structure of the pseudogap phase is described.

3,246 citations

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TL;DR: In this article, the authors examined the possibility that this effect is related to dynamical two-dimensional spin correlations, incommensurate with the crystal lattice, that have been observed in La2-SrxCuO4 by neutron scattering.

Abstract: ONE of the long-standing mysteries associated with the high-temperature copper oxide superconductors concerns the anomalous suppression1 of superconductivity in La2-xBaxCuO4 (and certain related compounds) when the hole concentration x is near . Here we examine the possibility that this effect is related to dynamical two-dimensional spin correlations, incommensurate with the crystal lattice, that have been observed in La2-xSrxCuO4 by neutron scattering2–4. A possible explanation for the incommensurability involves a coupled, dynamical modulation of spin and charge in which antiferromagnetic 'stripes' of copper spins are separated by periodically spaced domain walls to which the holes segregate5–9. An ordered stripe phase of this type has recently been observed in hole-doped La2NiO4 (refs 10–12). We present evidence from neutron diffraction that in the copper oxide material La1.6-xNd0.4SrxCuO4, with x = 0.12, a static analogue of the dynamical stripe phase is present, and is associated with an anomalous suppression of superconductivity13,14. Our results thus provide an explanation of the '
' conundrum, and also support the suggestion15 that spatial modulations of spin and charge density are related to superconductivity in the copper oxides.

2,449 citations