8 research outputs found

    Phase Field Modeling of Fracture and Stress Induced Phase Transitions

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    We present a continuum theory to describe elastically induced phase transitions between coherent solid phases. In the limit of vanishing elastic constants in one of the phases, the model can be used to describe fracture on the basis of the late stage of the Asaro-Tiller-Grinfeld instability. Starting from a sharp interface formulation we derive the elastic equations and the dissipative interface kinetics. We develop a phase field model to simulate these processes numerically; in the sharp interface limit, it reproduces the desired equations of motion and boundary conditions. We perform large scale simulations of fracture processes to eliminate finite-size effects and compare the results to a recently developed sharp interface method. Details of the numerical simulations are explained, and the generalization to multiphase simulations is presented

    Anisotropic thermal expansion and magnetostriction of YNi2_2B2_2C single crystals

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    We present results of anisotropic thermal expansion and low temperature magnetostriction measurements on YNi2_2B2_2C single crystals grown by high temperature flux and floating zone techniques. Quantum oscillations of magnetostriction were observed at low temperatures for HcH \| c starting at fields significantly below Hc2H_{c2} (H<0.7Hc2H < 0.7 H_{c2}). Large irreversible, longitudinal magnetostriction was seen in both, in-plane and along the c-axis, directions of the applied magnetic field in the intermediate superconducting state. Anisotropic uniaxial pressure dependencies of TcT_c were evaluated using results of zero field, thermal expansion measurements

    Evidence for structural and electronic instabilities at intermediate temperatures in κ\kappa-(BEDT-TTF)2_{2}X for X=Cu[N(CN)2_{2}]Cl, Cu[N(CN)2_{2}]Br and Cu(NCS)2_{2}: Implications for the phase diagram of these quasi-2D organic superconductors

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    We present high-resolution measurements of the coefficient of thermal expansion α(T)=lnl(T)/T\alpha (T)=\partial \ln l(T)/\partial T of the quasi-twodimensional (quasi-2D) salts κ\kappa-(BEDT-TTF)2_2X with X = Cu(NCS)2_2, Cu[N(CN)2_2]Br and Cu[N(CN)2_2]Cl. At intermediate temperatures (B), distinct anomalies reminiscent of second-order phase transitions have been found at T=38T^\ast = 38 K and 45 K for the superconducting X = Cu(NCS)2_2 and Cu[N(CN)2_2]Br salts, respectively. Most interestingly, we find that the signs of the uniaxial pressure coefficients of TT^\ast are strictly anticorrelated with those of TcT_c. We propose that TT^\ast marks the transition to a spin-density-wave (SDW) state forming on minor, quasi-1D parts of the Fermi surface. Our results are compatible with two competing order parameters that form on disjunct portions of the Fermi surface. At elevated temperatures (C), all compounds show α(T)\alpha (T) anomalies that can be identified with a kinetic, glass-like transition where, below a characteristic temperature TgT_g, disorder in the orientational degrees of freedom of the terminal ethylene groups becomes frozen in. We argue that the degree of disorder increases on going from the X = Cu(NCS)2_2 to Cu[N(CN)2_2]Br and the Cu[N(CN)2_2]Cl salt. Our results provide a natural explanation for the unusual time- and cooling-rate dependencies of the ground-state properties in the hydrogenated and deuterated Cu[N(CN)2_2]Br salts reported in the literature.Comment: 22 pages, 7 figure

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