arXiv subjects

S. Larochelle

Publications and source records attributed to S. Larochelle.

9 recordsLinked to original sources

arXiv2007

High resolution x-ray study of nematic-smectic-A and Smectic-A-reentrant nematic transitions in liquid crystal+aerosil gels

We have studied the effects of quenched random disorder created by dispersed aerosil nanoparticle gels on the nematic to smectic-A (N-SmA) and smectic-A to reentrant nematic (SmA- RN) phase transitions of thermotropic liquid crystal mixtures of 6OCB (hexyloxycyanobiphenyl) and 8OCB (octyloxycyanobiphenyl). These effects are probed using high-resolution synchrotron x-ray diffraction techniques. We find that the reentrant characteristics of the system are largely unchanged by the presence of the aerosil gel network. By comparing measurements of the smectic static structure amplitude for this 8OCB-6OCB+aerosil system with those for butyloxybenzilidene-octylaniline (4O.8)+aerosil gels, we find that the short-range smectic order in the smectic-A phase is significantly weaker in the reentrant system. This result is consistent with the behavior seen in pure 8OCB-6OCB mixtures. The strength of the smectic ordering decreases progressively as the 6OCB concentration is increased. Detailed line shape analysis shows that the high- and low-temperature nematic phases (N and RN) are similar to each other.

cond-mat.soft↗

arXiv2006

Quasi-stationary criticality of the Order-Parameter of the d=3 Random-Field Ising Antiferromagnet Fe(0.85)Zn(0.15)F2: A Synchrotron X-ray Scattering Study

The critical exponent beta =0.17(1) for the three-dimensional random-field Ising model (RFIM) order parameter upon zero-field cooling (ZFC) has been determined using extinction-free magnetic x-ray scattering techniques for Fe(0.85)Zn(0.15)F2. This result is consistent with other exponents determined for the RFIM in that Rushbrooke scaling is satisfied. Nevertheless, there is poor agreement with equilibrium computer simulations, and the ZFC results do not agree with field-cooling (FC) results. We present details of hysteresis in Bragg scattering amplitudes and line shapes that help elucidate the effects of thermal cycling in the RFIM, as realized in dilute antiferromagnets in an applied field. We show that the ZFC critical-like behavior is consistent with a second-order phase transitions, albeit quasi-stationary rather than truly equilibrium in nature, as evident from the large thermal hysteresis observed near the transition.

cond-mat.dis-nn↗

arXiv2004

Structural and magnetic properties of the single-layer manganese oxide La{1-x}Sr{1+x}MnO4

Using x-ray and neutron scattering, we have studied the structural and magnetic properties of the single-layer manganite La{1-x}Sr{1+x}MnO4 (0 < x < 0.7). Single crystals were grown by the traveling-solvent floating-zone method at 18 La/Sr concentrations. The low-temperature phase diagram can be understood by considering the strong coupling of the magnetic and orbital degrees of freedom, and it can be divided into three distinct regions: low (x < 0.12), intermediate (0.12< x < 0.45), and high (x > 0.45) doping. LaSrMnO_4 (x=0) is an antiferromagnetic Mott insulator,and its spin-wave spectrum is well-described by linear spin-wave theory for the spin-2 square-lattice Heisenberg Hamiltonian with Ising anisotropy. Upon doping, as the e_g electron concentration (1-x) decreases, both the two-dimensional antiferromagnetic spin correlations in the paramagnetic phase and the low-temperature ordered moment decrease due to an increase of frustrating interactions, and Neel order disappears above xc = 0.115(10). In the intermediate region, there exists neither long-range magnetic nor superstructural order. Short-range-correlated structural "nanopatches" begin to form above x ~ 0.25. At high doping (x > 0.45), the ground state of La{1-x}Sr{1+x}MnO4 exhibits long-range superstructural order and a complex (CE-type) antiferromagnetic order which differs from that at low doping. The superstructural order is thought to arise from charge and orbital ordering on the Mn sites. For x > 0.50, the superstructural order becomes incommensurate with the lattice, with a modulation wavevector that depends linearly on the e_g electron concentration. On the other hand, the magnetic order remains commensurate, but loses its long-range coherence upon doping beyond x = 0.50.

cond-mat.str-el↗

arXiv2004

Phase Decomposition and Chemical Inhomogeneity in Nd2-xCexCuO4

Extensive X-ray and neutron scattering experiments and additional transmission electron microscopy results reveal the partial decomposition of Nd2-xCexCuO4 (NCCO) in a low-oxygen-fugacity environment such as that typically realized during the annealing process required to create a superconducting state. Unlike a typical situation in which a disordered secondary phase results in diffuse powder scattering, a serendipitous match between the in-plane lattice constant of NCCO and the lattice constant of one of the decomposition products, (Nd,Ce)2O3, causes the secondary phase to form an oriented, quasi-two-dimensional epitaxial structure. Consequently, diffraction peaks from the secondary phase appear at rational positions (H,K,0) in the reciprocal space of NCCO. Additionally, because of neodymium paramagnetism, the application of a magnetic field increases the low-temperature intensity observed at these positions via neutron scattering. Such effects may mimic the formation of a structural superlattice or the strengthening of antiferromagnetic order of NCCO, but the intrinsic mechanism may be identified through careful and systematic experimentation. For typical reduction conditions, the (Nd,Ce)2O3 volume fraction is ~1%, and the secondary-phase layers exhibit long-range order parallel to the NCCO CuO2 sheets and are 50-100 angstromsthick. The presence of the secondary phase should also be taken into account in the analysis of other experiments on NCCO, such as transport measurements.

cond-mat.supr-con↗

arXiv2004

Susceptibilities and Spin Gaps of Weakly-coupled Spin Ladders

We calculate the uniform and staggered susceptibilities of two-chain spin-1/2 Heisenberg ladders using Monte-Carlo simulations. We show that the gap extracted from the uniform susceptibility and the saturation value of the staggered susceptibility are independent of the sign of the inter-chain coupling J_perp in the asymptotic limit |J_perp|/J -> 0. Furthermore, we examine the existence of logarithmic corrections to the linear scaling of the gap with |J_perp|.

cond-mat.str-el↗

arXiv2003

Spurious magnetism in high-Tc superconductor

One challenge in condensed-matter physics is to unravel the interplay between magnetism and superconductivity in copper oxides with a high critical temperature (Tc). Kang et al. claim to have revealed a quantum phase transition from the superconducting to an antiferromagnetic state in the electron-doped material Nd2-xCexCuO4 (NCCO) based on the observation of magnetic-field-induced neutron scattering intensity at (1/2,1/2,0), (1/2,0,0), and related reflections. Here we argue that the observed magnetic intensity is due to a secondary phase of (Nd,Ce)2O3. We therefore contend that the effect is spurious and not intrinsic to superconducting NCCO.

cond-mat↗

arXiv2002

Order Parameter Criticality of the d=3 Random-Field Ising Antiferromagnet Fe(0.85)Zn(0.15)F2

The critical exponent beta =0.16 +- 0.02 for the random-field Ising model order parameter is determined using extinction-free magnetic x-ray scattering for Fe(0.85)Zn(0.15)F2 in magnetic fields of 10 and 11 T. The observed value is consistent with other experimental random-field critical exponents, but disagrees sharply with Monte Carlo and exact ground state calculations on finite-sized systems.

cond-mat.dis-nn↗

arXiv2002

Nature of $e_g$ Electron Order in La$_{1-x}$Sr$_{1+x}$MnO$_4$

Synchrotron x-ray scattering measurements of the low-temperature structure of the single-layer manganese oxide La$_{1-x}$Sr$_{1+x}$MnO$_4$, over the doping range $0.33 \le x \le 0.67$, indicate the existence of three distinct regions: a disordered phase ($x < 0.4$), a charge-ordered phase ($x \ge 0.5$), and a mixed phase ($0.4 \le x < 0.5$). For $x>0.5$, the modulation vector associated with the charge order is incommensurate with the lattice and depends linearly on the concentration of $e_g$ electrons. The primary superlattice reflections are strongly suppressed along the modulation direction and the higher harmonics are weak, implying the existence of a largely transverse and nearly sinusoidal structural distortion, consistent with a charge density wave of the $e_g$ electrons.

cond-mat.str-el↗

arXiv2001

Preparation and Characterization of Homogeneous YBCO Single Crystals with Doping Level near the SC-AFM Boundary

High-purity and homogeneous YBa2Cu3Oy single crystals with carrier doping level near the AFM-SC boundary have been obtained in the oxygen content range between y = 6.340 and 6.370. The crystals are ortho-II phase at room temperature and undergo the orthorhombic to tetragonal transition at about 140_Degree_C. They show sharp superconducting transitions, with Tc between 4 and 20 K. Tc changes by 0.8 K when the oxygen content y is changed by 0.001, and is also sensitive to annealing conditions near room temperature, due to the dependence of doping on oxygen ordering correlation lengths. Crystals with oxygen content y lower than 6.345 are non-superconducting.

cond-mat.supr-con↗