arXiv subjects

A. Revcolevschi

Publications and source records attributed to A. Revcolevschi.

At least 55 records · Page 3Linked to original sources

arXiv2006

Scaling of dynamic spin correlations in BaCu2SiGeO7

The magnetic dynamic structure factor of the one-dimensional S = 1/2 chain system BaCu2(Si0.5Ge0.5)2O7 is studied in a wide range of energy transfers and temperatures. Contrary to previous erroneous reports [T. Masuda et al., Phys. Rev. Lett. 93, 077206 (2004)], the scaling properties observed in the range 0.5-25 meV are found to be fully consistent with expectations for a Luttinger spin liquid. At higher energies, a breakdown of scaling laws is observed and attributed to lattice effects. The results are complementary to those found in literature for other S = 1/2-chain compounds, such as KCuF3 and Cu-benzoate.

cond-mat.str-el↗

arXiv2006

Observation of 300 K High Energy MagnetoDielectric Response in the Bilayer Manganite (La$_{0.4}$Pr$_{0.6}$)$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$

We observed a large HEMD effect in the bilayer manganite (La$_{0.4}$Pr$_{0.6}$)$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$, a direct consequence of field driven spin-glass insulator to ferromagnetic metal transition. The remnants of the transition can be used to achieve dielectric contrast at room temperature. This discovery suggests that electronic mechanisms such as the metal-insulator transition, charge ordering, and orbital ordering can be exploited to give substantial dielectric contrast in other materials.

cond-mat.mtrl-sci↗

arXiv2006

Magnetoelastic coupling across the metamagnetic transition in Ca$_{2-x}$Sr$_x$RuO$_4$ (0.2 < x < 0.5)

The magnetoelastic coupling in Ca$_{1.8}$Sr$_{0.2}$RuO$_4$ and in Ca$_{1.5}$Sr$_{0.5}$RuO$_4$ has been studied combining high-resolution dilatometer and diffraction techniques. Both compounds exhibit strong anomalies in the thermal-expansion coefficient at zero and at high magnetic field as well as an exceptionally large magnetostriction. All these structural effects, which are strongest in Ca$_{1.8}$Sr$_{0.2}$RuO$_4$, point to a redistribution of electrons between the different $t_{2g}$ orbitals tuned by temperature and magnetic field. The temperature and the field dependence of the thermal-expansion anomalies in Ca$_{1.8}$Sr$_{0.2}$RuO$_4$ yield evidence for a critical end-point lying close to the low-temperature metamagnetic transition; however, the expected scaling relations are not well fulfilled.

cond-mat.str-el↗

arXiv2006

Elastic and magnetic properties on field-induced Metal-Insulator transition of bilayer manganese oxide $(La_{1-z} Pr_z)_{1.2} Sr_{1.8} Mn_2 O_7$ ($z$=0.6)

The elastic and magnetic properties of a single crystal of 327 bilayer manganese oxide $(La_{1-z} Pr_z)_{1.2} Sr_{1.8} Mn_2 O_7$ with $z$=0.6 have been investigated by ultrasonic and high-field magnetization measurements. A distinct elastic anomaly was observed at low temperatures and in high magnetic fields when crossing the phase boundary. A pronounced elastic softening as a function of magnetic field ($H$) appears across the boundary of the low-temperature magnetic phase below around 40 K, accompanied by a distinct hysteresis. In the high field region, however, these elastic constants exhibit an increase with increasing a magnetic field in the high-field region. The high-field magnetization measurement characterizing the magnetic state clarified a strong connection between elastic strains and magnetic moments. The new metamagnetic transition was found in the high-field region above 40 K, accompanied with no hysteresis. The present results of elastic constants are discussed in terms of a coupling effect between elastic strains associated with sound waves and relevant magnetic moments, and relevant conduction bands. They are explained reasonably in terms of a strong coupling between elastic strains and magnetic susceptibility $chi$$_{m}$=$partial$$M$/$partial$$H$.

cond-mat.str-el↗

arXiv2006

Dynamical spin chirality and spin anisotropy in gapped S=1/2 quantum systems

We have studied the spin anisotropy in spin-singlet ground state compounds and the magnetic chirality, as measured by inelastic polarized neutron scattering techniques, in the chain-sublattice of Sr14Cu24O41. In-plane and out of plane magnetic fluctuations are measured to be anisotropic and further discussed in the light of the current hypothesis of spin-orbit coupling. We show that under appropriate conditions of magnetic field and neutron polarization, the \textit{trivial} magnetic chirality selects only one of the Zeeman splitted triplet states for scattering and erases the other one that posses opposite helicity. Our analysis pertains to previous studies on dynamical magnetic chirality and chiral critical exponents, where the ground state is chiral itself, the so-called \textit{non-trivial} dynamical magnetic chirality. As it turns out, both \textit{trivial} and \textit{non-trivial} dynamical magnetic chirality have identical selection rules for inelastic polarized neutron scattering experiments and it is not at all evident that they can be distinguished in a paramagnetic compound.

cond-mat.mtrl-sci↗

arXiv2006

Magneto-optical investigation of the field-induced spin-glass insulator to ferromagnetic metallic transition of the bilayer manganite (La$_{0.4}$Pr$_{0.6}$)$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$

We measured the magneto-optical response of (La$_{0.4}$Pr$_{0.6}$)$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$ in order to investigate the microscopic aspects of the magnetic field driven spin-glass insulator to ferromagnetic metal transition. Application of a magnetic field recovers the ferromagnetic state with an overall redshift of the electronic structure, growth of the bound carrier localization associated with ferromagnetic domains, development of a pseudogap, and softening of the Mn-O stretching and bending modes that indicate a structural change. We discuss field- and temperature-induced trends within the framework of the Tomioka-Tokura global electronic phase diagram picture and suggest that controlled disorder near a phase boundary can be used to tune the magnetodielectric response. Remnants of the spin-glass insulator to ferromagnetic metallic transition can also drive 300 K color changes in (La$_{0.4}$Pr$_{0.6}$)$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$.

cond-mat.str-el↗

arXiv2006

Unidirectional Diagonal Order and 3D Stacking of Charge Stripes in Orthorhombic Pr1.67Sr0.33NiO4 and Nd1.67Sr0.33NiO4

The interplay between crystal symmetry and charge stripe order in Pr1.67Sr0.33NiO4 and Nd1.67Sr0.33NiO4 has been studied by means of single crystal x-ray diffraction. In contrast to tetragonal La1.67Sr0.33NiO4, these crystals are orthorhombic. The corresponding distortion of the NiO2 planes is found to dictate the direction of the charge stripes, similar to the case of diagonal spin stripes in the insulating phase of La2-xSrxCuO4. In particular, diagonal stripes seem to always run along the short a-axis, which is the direction of the octahedral tilt axis. In contrast, no influence of the crystal symmetry on the charge stripe ordering temperature itself was observed, with T_CO 240K for La, Pr, and Nd. The coupling between lattice and stripe degrees of freedom allows one to produce macroscopic samples with unidirectional stripe order. In samples with stoichiometric oxygen content and a hole concentration of exactly 1/3, charge stripes exhibit a staggered stacking order with a period of three NiO2 layers, previously only observed with electron microscopy in domains of mesoscopic dimensions. Remarkably, this stacking order starts to melt about 40K below T_CO. The melting process can be described by mixing the ground state, which has a 3-layer stacking period, with an increasing volume fraction with a 2-layer stacking period.

cond-mat.str-el↗

arXiv2006

Magnon Heat Conductivity and Mean Free Paths in Two-Leg Spin Ladders: A Model-Independent Determination

The magnon thermal conductivity $κ_{\mathrm{mag}}$ of the spin ladders in $\rm Sr_{14}Cu_{24-x}Zn_xO_{41}$ has been investigated at low doping levels $x=0$, 0.125, 0.25, 0.5 and 0.75. The Zn-impurities generate nonmagnetic defects which define an upper limit for $l_{\mathrm{mag}}$ and therefore allow a clear-cut relation between $l_{\mathrm{mag}}$ and $κ_{\mathrm{mag}}$ to be established independently of any model. Over a large temperature range we observe a progressive suppression of $κ_{\mathrm{mag}}$ with increasing Zn-content and find in particular that with respect to pure $\rm Sr_{14}Cu_{24}O_{41}$ $κ_{\mathrm{mag}}$ is strongly suppressed even in the case of tiny impurity densities where $l_{\mathrm{mag}}\lesssim 374$~Å. This shows unambiguously that large $l_{\mathrm{mag}}\approx 3000$~Å which have been reported for $\rm Sr_{14}Cu_{24}O_{41}$ and $\rm La_{5}Ca_9Cu_{24}O_{41}$ on basis of a kinetic model are in the correct order of magnitude.

cond-mat.str-el↗

arXiv2006

Octahedral tilts and electronic correlations in La_7/8Sr_1/8MnO_3

We present a resonant x-ray scattering study of the octahedral tilt order between 50K and 310K in La_7/8Sr_1/8MnO_3. At the La L_II-edge the resonant (300) reflection probes cooperative tilts of the MnO_6-octahedra in this material, as verified by a model caclulation as well as a LDA+U study. The investigation of the octahedral tilts as a function of temperature and the comparison to the lattice parameters, the magnetization and the superlattice reflections related to charge and/or orbital order reveal an intimate coupling between electronic and tilt degrees of freedom in La_7/8Sr_1/8MnO_3.

cond-mat.str-el↗

arXiv2006

Pressure-induced melting of the orbital polaron lattice in La1-xSrxMnO3

We report on the pressure effects on the orbital polaron lattice in the lightly doped manganites $\mathrm{La_{1-x}Sr_xMnO_{3}}$, with $x\sim 1/8$. The dependence of the orbital polaron lattice on $negative$ chemical pressure is studied by substituting Pr for La in $\mathrm{(La_{1-y}Pr_y)_{7/8}Sr_{1/8}MnO_{3}}$. In addition, we have studied its hydrostatic pressure dependence in $\mathrm{(La_{0.9}Pr_{0.1})_{7/8}Sr_{1/8}MnO_{3}}$. Our results strongly indicate that the hopping $t$ significantly contributes to the stabilization of the orbital polaron lattice and that the orbital polarons are ferromagnetic objects which get stabilized by local double exchange processes. The analysis of short range orbital correlations and the verification of the Grueneisen scaling by hard x-ray, specific heat and thermal expansion data reinforces our conclusions.

cond-mat.str-el↗

arXiv2006

Revisiting the chain magnetism in Sr14Cu24O41: Experimental and numerical results

We study the magnetism of the hole doped CuO2 spin chains in Sr14Cu24O41 by measuring the Electron Spin Resonance (ESR) and the static magnetization M in applied magnetic fields up to 14 T. In this compound, the dimerized ground state and the charge order in the chains are well established. Our experimental data suggest that at low temperatures the Curie-like increase of M as well as the occurrence of the related ESR signal are due to a small amount of paramagnetic centers which are not extrinsic defects but rather unpaired Cu spins in the chain. These observations qualitatively confirm recent ab initio calculations of the ground state properties of the CuO2 chains in Sr14Cu24O41. Our complementary quantum statistical simulations yield that the temperature and field dependence of the magnetization can be well described by an effective Heisenberg model in which the ground state configuration is composed of spin dimers, trimers, and monomers.

cond-mat.str-el↗

arXiv2006

The metal-insulator transition of the ferromagnetic La7/8Sr1/8MnO3 probed by spin waves: a 2D stripe superstructure

We report a study of spin waves in hole-doped ferromagnetic La7/8Sr1/8MnO3, in the metallic state (165K) below TC (181K) and through the puzzling metal-insulator transition which occurs at TO'O''=159K. They reveal very unusual excitations. Propagating spin waves are observed in the small q-range up to q=0.25 (lambda=4a), and, beyond, four dispersionless levels. Both types of excitations have a quasi two-dimensional (2D) character. The transition is revealed by a folding of the dispersed magnon branch at q=1/8. In the metallic state, the dispersionless levels reveal ferromagnetic domains with 4 lattice spacings for their size along a and b. They lead to a picture of charge segregation with hole-poor domains surrounded with hole-rich paths. Within this description, the transition appears as the ordering of domains, which can be interpreted in terms of a 2D superstructure of orthogonal stripes.

cond-mat.str-el↗

arXiv2006

Charge density waves in Sr_14-xCa_xCu_24O_41: electron correlations vs. structural effects

We report inelastic light scattering experiments in the spin-chain and ladder system Sr_14-xCa_xCu_24O_41. A depletion of an electronic background is observed for 0 < x < 5 at low temperatures and with light polarization both parallel to the leg and rung directions. This points to the formation of 2D charge density wave in the ladders. Furthermore, the Ca-substitution dependence of the 305- and 249-cm-1 modes, sensitive to the lattice modulations of the respective chain and ladder system, suggests that the charge dynamics in the ladders is strongly influenced by the structural incommensurability.

cond-mat.str-el↗

arXiv2005

Thermodynamic Properties of (Ca,Sr)$_2$RuO$_4$ in Magnetic Fields

We have studied the influence of a magnetic field on the thermodynamic properties of Ca$_{2-x}$Sr$_{x}$RuO$_4$ in the intermediate metallic region with tilt and rotational distortions ($0.2\leq x \leq 0.5$). We find strong and anisotropic thermal expansion anomalies at low temperatures, which are suppressed and even reversed by a magnetic field. The metamagnetic transition of Ca$_{1.8}$Sr$_{0.2}$RuO$_4$ is accompanied by a large magnetostriction. Furthermore, we observe a strong magnetic-field dependence of $c_p/T$, that can be explained by magnetic fluctuations.

cond-mat.str-el↗

arXiv2005

Quasiparticle-like peaks, kinks, and electron-phonon coupling at the ($π$,0) regions in the CMR oxide La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$

Using Angle-Resolved Photoemission (ARPES), we present the first observation of sharp quasiparticle-like peaks in a CMR manganite. We focus on the ($π$,0) regions of k-space and study their electronic scattering rates and dispersion kinks, uncovering the critical energy scales, momentum scales, and strengths of the interactions that renormalize the electrons. To identify these bosons we measured phonon dispersions in the energy range of the kink by inelastic neutron scattering (INS), finding a good match in both energy and momentum to the oxygen bond-stretching phonons.

cond-mat.str-el↗

arXiv2005

Magnetic resonance study of the spin-reorientation transitions in the quasi-one-dimensional antiferromagnet BaCu2Si2O7

A quasi-one dimensional antiferromagnet with a strong reduction of the ordered spin component, BaCu2Si2O7, is studied by the magnetic resonance technique in a wide field and frequency range. Besides of conventional spin-flop transition at the magnetic field parallel to the easy axis of spin ordering, magnetic resonance spectra indicate additional spin-reorientation transitions in all three principal orientations of magnetic field. At these additional transitions the spins rotate in the plane perpendicular to the magnetic field keeping the mutual arrangement of ordered spin components. The observed magnetic resonance spectra and spin-reorientation phase transitions are quantitatively described by a model including the anisotropy of transverse susceptibility with respect to the order parameter orientation. The anisotropy of the transverse susceptibility and the strong reduction of the anisotropy energy due to the quantum spin fluctuations are proposed to be the reason of the spin reorientations which are observed.

cond-mat.str-el↗

arXiv2005

Orbital polaron lattice formation in lightly doped La1-xSrxMnO3

By resonant x-ray scattering at the Mn K-edge on La7/8Sr1/8MnO3, we show that an orbital polaron lattice (OPL) develops at the metal-insulator transition of this compound. This orbital reordering explains consistently the unexpected coexistence of ferromagnetic and insulating properties at low temperatures, the quadrupling of the lattice structure parallel to the MnO2-planes, and the observed polarization and azimuthal dependencies. The OPL is a clear manifestation of strong orbital-hole interactions, which play a crucial role for the colossal magnetoresistance effect and the doped manganites in general.

cond-mat.str-el↗

arXiv2005

Optical study of orbital excitations in transition-metal oxides

The orbital excitations of a series of transition-metal compounds are studied by means of optical spectroscopy. Our aim was to identify signatures of collective orbital excitations by comparison with experimental and theoretical results for predominantly local crystal-field excitations. To this end, we have studied TiOCl, RTiO3 (R=La, Sm, Y), LaMnO3, Y2BaNiO5, CaCu2O3, and K4Cu4OCl10, ranging from early to late transition-metal ions, from t_2g to e_g systems, and including systems in which the exchange coupling is predominantly three-dimensional, one-dimensional or zero-dimensional. With the exception of LaMnO3, we find orbital excitations in all compounds. We discuss the competition between orbital fluctuations (for dominant exchange coupling) and crystal-field splitting (for dominant coupling to the lattice). Comparison of our experimental results with configuration-interaction cluster calculations in general yield good agreement, demonstrating that the coupling to the lattice is important for a quantitative description of the orbital excitations in these compounds. However, detailed theoretical predictions for the contribution of collective orbital modes to the optical conductivity (e.g., the line shape or the polarization dependence) are required to decide on a possible contribution of orbital fluctuations at low energies, in particular in case of the orbital excitations at about 0.25 eV in RTiO3. Further calculations are called for which take into account the exchange interactions between the orbitals and the coupling to the lattice on an equal footing.

cond-mat.str-el↗