arXiv subjects

E. Granado

Publications and source records attributed to E. Granado.

32 records · Page 2Linked to original sources

arXiv2018

Damping of Landau levels in neutral graphene at low magnetic fields: A phonon Raman scattering study

Landau level broadening mechanisms in electrically neutral and quasineutral graphene were investigated through micro-magneto-Raman experiments in three different samples, namely, a natural single-layer graphene flake and a back-gated single-layer device, both deposited over Si/SiO2 substrates, and a multilayer epitaxial graphene employed as a reference sample. Interband Landau level transition widths were estimated through a quantitative analysis of the magnetophonon resonances associated with optically active Landau level transitions crossing the energy of the E_2g Raman-active phonon. Contrary to multilayer graphene, the single-layer graphene samples show a strong damping of the low-field resonances, consistent with an additional broadening contribution of the Landau level energies arising from a random strain field. This extra contribution is properly quantified in terms of a pseudomagnetic field distribution Delta_B = 1.0-1.7 T in our single-layer samples.

cond-mat.mtrl-sci↗

arXiv2017

Nematic Fluctuations and Phase Transitions in LaFeAsO: a Raman Scattering Study

Raman scattering experiments on LaFeAsO with splitted antiferromagnetic (T_AFM = 140 K) and tetragonal-orthorhombic (T_S = 155 K) transitions show a quasi-elastic peak (QEP) in B2g symmetry (2 Fe tetragonal cell) that fades away below ~T_AFM and is ascribed to electronic nematic fluctuations. A scaling of the reported shear modulus with the T-dependence of the QEP height rather than the QEP area indicates that magnetic degrees of freedom drive the structural transition. The large separation between T_S and T_AFM in LaFeAsO compared with their coincidence in BaFe2As2 manifests itself in slower dynamics of nematic fluctuations in the former.

cond-mat.str-el↗

arXiv2015

Magnetic dimers and trimers in the disordered $S=3/2$ spin system BaTi$_{1/2}$Mn$_{1/2}$O$_{3}$

We report a structural/magnetic investigation by X-ray absorption spectroscopy (XAS), neutron diffraction, dc-susceptibility ($χ_{\mbox{dc}}$) and electron spin resonance (ESR) of the 12R-type perovskite BaTi$_{1/2}$Mn$_{1/2}$O$_{3}$. Our structural analysis by neutron diffraction supports the existence of structural trimers with chemically disordered occupancy of Mn$^{4+}$ and Ti$^{4+}$ ions, with the valence of the Mn ions confirmed by the XAS measurements. The magnetic properties are explored by combining dc-susceptibility and $X$-band ($9.4$ GHz) electron spin resonance, both in the temperature interval of $2\leq T\leq1000$ K. A scenario is presented under which the magnetism is explained by considering magnetic dimers and trimers, with exchange constants $J_{a}/k_{B}=200(2)$ K and $J_{b}/k_{B}=130(10)$ K, and orphan spins. Thus, BaTi$_{1/2}$Mn$_{1/2}$O$_{3}$ is proposed as a rare case of an intrinsically disordered $S=3/2$ spin gap system with a frustrated ground state.

cond-mat.str-el↗

arXiv2012

Two-Dimensional Magnetic Correlations and Partial Long-Range Order in Geometrically Frustrated Sr2YRuO6

Neutron diffraction on the double perovskite Sr2YRuO6 with a quasi-face-centered cubic (FCC) lattice of Ru moments reveals planar magnetic correlations that condense into a partial long-range ordered state with coupled alternate antiferromagnetic (AFM) YRuO4 square layers coexisting with short-range correlations below T{N1}=32 K. A second transition to a fully ordered AFM state below T{N2}=24 K is observed. These observations are ascribed to the cancellation of the magnetic coupling between consecutive AFM square layers in FCC antiferromagnets, notably the simplest three-dimensional frustrated magnet model system. Inhomogeneous superconductivity in Sr2YRu_{1-x}Cu_{x}O6, reported in earlier works, may be nucleated in magnetically disordered YRu_{1-x}Cu_{x}O4 layers with strong spin fluctuations.

cond-mat.str-el↗

arXiv2012

Pressure and chemical substitution effects in the local atomic structure of BaFe2As2

The effects of K and Co substitutions and quasi-hydrostatic applied pressure (P<9 GPa) in the local atomic structure of BaFe2As2, Ba(Fe{0.937}Co{0.063})2As2 and Ba{0.85}K{0.15}Fe2As2 superconductors were investigated by extended x-ray absorption fine structure (EXAFS) measurements in the As K absorption edge. The As-Fe bond length is found to be slightly reduced (<~ 0.01 Angstroms) by both Co and K substitutions, without any observable increment in the corresponding Debye Waller factor. Also, this bond is shown to be compressible (k = 3.3(3)x10^{-3} GPa^{-1}). The observed contractions of As-Fe bond under pressure and chemical substitutions are likely related with a reduction of the local Fe magnetic moments, and should be an important tuning parameter in the phase diagrams of the Fe-based superconductors.

cond-mat.supr-con↗

arXiv2012

Investigation of element-specific and bulk magnetism, electronic and crystal structures of La{0.70}Ca{0.30}Mn{1-x}Cr{x}O{3}

The magnetic interactions in La{0.70}Ca{0.30}Mn{1-x}Cr{x}O{3} (x = 0.15, 0.50 and 0.70) are investigated by x-ray absorption spectroscopy (XAS), x-ray magnetic circular dichroism (XMCD), high-resolution x-ray powder diffraction, and bulk magnetization measurements. XAS in the Mn and Cr L{2,3} edges support stable single valent Cr{3+} ions and a varying Mn valence state with x, while the O K edge XAS spectrum reveals local maxima in the O 2p density of states close to the Fermi level due to mixing with Mn and Cr 3d states. A robust antiferromagnetic state is found for x=0.70 below TN = 258 K. For x=0.15, combined XMCD and bulk magnetization measurements indicate a fully polarized ferrimagnetic state for the Mn and Cr spins below Tc=224 K. For x=0.50, a reduced ferrimagnetic component dominated by Mn spins is present below Tc=154 K. No evidence of lattice anomalies due to cooperative charge and orbital orderings is found by x-ray diffraction for all samples. The magnetic properties of this system are rationalized in terms of a competition of ferromagnetic Mn-Mn double exchange and antiferromagnetic Cr-Cr and Cr-Mn superexchange interactions.

cond-mat.mtrl-sci↗

arXiv2012

Spin-Electron-Phonon Excitation in Re-based Half-Metallic Double Perovskites

A remarkable hardening (~ 30 cm-1) of the normal mode of vibration associated with the symmetric stretching of the oxygen octahedra for the Ba2FeReO6 and Sr2CrReO6 double perovskites is observed below the corresponding magnetic ordering temperatures. The very large magnitude of this effect and its absence for the anti-symmetric stretching mode provide evidence against a conventional spin-phonon coupling mechanism. Our observations are consistent with a collective excitation formed by the combination of the vibrational mode with oscillations of local Fe or Cr 3d and Re 5d occupations and spin magnitudes.

cond-mat.str-el↗

arXiv2011

Co-substitution effects on the Fe-valence in the BaFe2As2 superconducting compound: A study of hard x-ray absorption spectroscopy

The Fe K X-ray absorption near edge structure (XANES) of BaFe2-xCoxAs2 superconductors was investigated. No appreciable alteration in shape or energy position of this edge was observed with Co substitution. This result provides experimental support to previous ab initio calculations in which the extra Co electron is concentrated at the substitute site and do not change the electronic occupation of the Fe ions. Superconductivity may emerge due to bonding modifications induced by the substitute atom that weakens the spin-density-wave ground state by reducing the Fe local moments and/or increasing the elastic energy penalty of the accompanying orthorhombic distortion.

cond-mat.supr-con↗

arXiv2008

Unconventional Landau levels in bulk graphite revealed by Raman spectroscopy

The electronic Raman scattering of bulk graphite at zero magnetic field reveals a structureless signal characteristic of a metal. For T<~100 K and B > 2 T, several peaks at energies scaling linearly with magnetic field were observed and ascribed to transitions from the lowest energy Landau level(s) (LL) to excited states belonging to the same ladder. The LLs are equally (unequally) spaced for high (low) quantum numbers, being surprisingly consistent with the LL sequence from massive Dirac Fermions (m* = 0.033(2) m_e) with Berry's phase 2πfound in graphene bilayers. These results provide spectroscopic evidence that much of the unconventional physics recently revealed by graphene multilayers is also shared by bulk graphite.

cond-mat.mes-hall↗

arXiv2007

Magnetoelastic and thermal effects in the BiMn2O5 lattice: a high-resolution x-ray diffraction study

High-resolution synchrotron x-ray diffraction measurements were performed on single crystalline and powder samples of BiMn2O5. A linear temperature dependence of the unit cell volume was found between T_{N}=38$ K and 100 K, suggesting that a low-energy lattice excitation may be responsible for the lattice expansion in this temperature range. Between T* ~ 65 K and T_{N}, all lattice parameters showed incipient magnetoelastic effects, due to short-range spin correlations. An anisotropic strain along the a-direction was also observed below T*. Below T_{N}, a relatively large contraction of the a-parameter following the square of the average sublattice magnetization of Mn was found, indicating that a second-order spin hamiltonian accounts for the magnetic interactions along this direction. On the other hand, the more complex behaviors found for $b$ and $c$ suggest additional magnetic transitions below T_{N} and perhaps higher-order terms in the spin hamiltonian. Polycrystalline samples grown by distinct routes and with nearly homogeneous crystal structure above T_{N} presented structural phase coexistence below T_{N}, indicating a close competition amongst distinct magnetostructural states in this compound.

cond-mat.mtrl-sci↗

arXiv2006

Magnetic structure and critical behavior of GdRhIn$_{5}$: resonant x-ray diffraction and renormalization group analysis

The magnetic structure and fluctuations of tetragonal GdRhIn5 were studied by resonant x-ray diffraction at the Gd LII and LIII edges, followed by a renormalization group analysis for this and other related Gd-based compounds, namely Gd2IrIn8 and GdIn3. These compounds are spin-only analogs of the isostructural Ce-based heavy-fermion superconductors. The ground state of GdRhIn5 shows a commensurate antiferromagnetic spin structure with propagation vector tau = (0,1/2, 1/2), corresponding to a parallel spin alignment along the a-direction and antiparallel alignment along b and c. A comparison between this magnetic structure and those of other members of the Rm(Co,Rh,Ir)n In3m+2n family (R =rare earth, n = 0, 1; m = 1, 2) indicates that, in general, tau is determined by a competition between first-(J1) and second-neighbor(J2) antiferromagnetic (AFM) interactions. While a large J1 /J2 ratio favors an antiparallel alignment along the three directions (the so-called G-AFM structure), a smaller ratio favors the magnetic structure of GdRhIn5 (C-AFM). In particular, it is inferred that the heavy-fermion superconductor CeRhIn5 is in a frontier between these two ground states, which may explain its non-collinear spiral magnetic structure. The critical behavior of GdRhIn5 close to the paramagnetic transition at TN = 39 K was also studied in detail. A typical second-order transition with the ordered magnetization critical parameter beta = 0.35 was experimentally found, and theoretically investigated by means of a renormalization group analysis.

cond-mat.str-el↗

arXiv2006

Physical properties and magnetic structure of TbRhIn5 intermetallic compound

In this work we report the physical properties of the new intermetallic compound TbRhIn5 investigated by means of temperature dependent magnetic susceptibility, electrical resistivity, heat-capacity and resonant x-ray magnetic diffraction experiments. TbRhIn5 is an intermetallic compound that orders antiferromagnetically at TN = 45.5 K, the highest ordering temperature among the existing RRhIn5 (1-1-5, R = rare earth) materials. This result is in contrast to what is expected from a de Gennes scaling along the RRhIn5 series. The X-ray resonant diffraction data below TN reveal a commensurate antiferromagnetic (AFM) structure with a propagation vector (1/2 0 1/2) and the Tb moments oriented along the c-axis. Strong (over two order of magnitude) dipolar enhancements of the magnetic Bragg peaks were observed at both Tb absorption edges LII and LIII, indicating a fairly high polarization of the Tb 5d levels. Using a mean field model including an isotropic first-neighbors exchange interaction J(R-R) and the tetragonal crystalline electrical field (CEF), we were able to fit our experimental data and to explain the direction of the ordered Tb-moments and the enhanced TN of this compound. The evolution of the magnetic properties along the RRhIn5 series and its relation to CEF effects for a given rare-earth is discussed.

cond-mat.str-el↗

arXiv2003

Inhomogeneous Magnetism in La-doped CaMnO3. (II) Mesoscopic Phase Separation due to Lattice-coupled FM Interactions

A detailed investigation of mesoscopic magnetic and crystallographic phase separation in Ca(1-x)La(x)MnO3, 0.00<=x<=0.20, is reported. Neutron powder diffraction and DC-magnetization techniques have been used to isolate the different roles played by electrons doped into the eg level as a function of their concentration x. The presence of multiple low-temperature magnetic and crystallographic phases within individual polycrystalline samples is argued to be an intrinsic feature of the system that follows from the shifting balance between competing FM and AFM interactions as a function of temperature. FM double-exchange interactions associated with doped eg electrons are favored over competing AFM interactions at higher temperatures, and couple more strongly with the lattice via orbital polarization. These FM interactions thereby play a privileged role, even at low eg electron concentrations, by virtue of structural modifications induced above the AFM transition temperatures.

cond-mat.mtrl-sci↗

arXiv2002

Unconventional Metallic Magnetism in LaCrSb{3}

Neutron-diffraction measurements in LaCrSb{3} show a coexistence of ferromagnetic and antiferromagnetic sublattices below Tc=126 K, with ordered moments of 1.65(4) and 0.49(4) Bohr magnetons per formula unit, respectively (T=10 K), and a spin reorientation transition at ~95 K. No clear peak or step was observed in the specific heat at Tc. Coexisting localized and itinerant spins are suggested.

cond-mat↗