Ageing memory and glassiness of a driven vortex system

Many systems in nature, glasses, interfaces and fractures being some examples, cannot equilibrate with their environment, which gives rise to novel and surprising behaviour such as memory effects, ageing and nonlinear dynamics. Unlike their equilibrated counterparts, the dynamics of out-of- equilibrium systems is generally too complex to be captured by simple macroscopic laws. Here we investigate a system that straddles the boundary between glass and crystal: a Bragg glass formed by vortices in a superconductor. We find that the response to an applied force evolves according to a stretched exponential, with the exponent reflecting the deviation from equilibrium. After the force is removed, the system ages with time and its subsequent response time scales linearly with its age (simple ageing), meaning that older systems are slower than younger ones. We show that simple ageing can occur naturally in the presence of sufficient quenched disorder. Moreover, the hierarchical distribution of timescales, arising when chunks of loose vortices cannot move before trapped ones become dislodged, leads to a stretched-exponential response.Comment: 16 pages, 5 figure

Where does the transport current flow in Bi2Sr2CaCu2O8 crystals?

A new measurement technique for investigation of vortex dynamics is introduced. The distribution of the transport current across a crystal is derived by a sensitive measurement of the self-induced magnetic field of the transport current. We are able to clearly mark where the flow of the transport current is characterized by bulk pinning, surface barrier, or a uniform current distribution. One of the novel results is that in BSCCO crystals most of the vortex liquid phase is affected by surface barriers resulting in a thermally activated apparent resistivity. As a result the standard transport measurements in BSCCO do not probe the dynamics of vortices in the bulk, but rather measure surface barrier properties.Comment: 11 pages, 4 figures, accepted for publication in Natur

Anisotropy dependence of the fluctuation spectroscopy in the critical and gaussian regimes in superconducting NaFe1-xCoxAs single crystals

We investigate thermal fluctuations in terms of diamagnetism and magnetotransport in superconducting NaFe1-xCoxAs single crystals with different doping levels. Results show that in the case of optimal doped and lightly overdoped (x= 0.03, 0.05) crystals the analysis in the critical as well as in the Gaussian fluctuation regions is consistent with the Ginzburg-Landau 3D fluctuation theory. However, in the case of strongly overdoped samples (x >= 0.07) the Ullah-Dorsey scaling of the fluctuation induced magnetoconductivity in the critical region confirms that thermal fluctuations exhibit a 3D anisotropic nature only in a narrow temperature region around T-c(H). This is consistent with the fact that in these samples the fluctuation effects in the Gaussian region above T-c may be described by the Lawrence-Doniach approach. Our results indicate that the anisotropy of these materials increases significantly with the doping level

Scanning tunneling microscopy observation of a square Abrikosov lattice in LuNi2B2

We present scanning tunneling microscopy measurements of the (001) surface of a LuNi(2)B2C borocarbide single crystal at 4.2 K. In zero field, the conductance versus voltage characteristics recorded at various locations on the sample reproducibly provide a gap value of 2.2 meV. In a magnetic field of 1.5 and 0.375 T, the recordings of the conductance as a function of position reveal a regular square vortex lattice tilted by 45 degrees with respect to the crystalline a axis. This unusual result is correlated with an in-plane anisotropy of the upper critical field H(c2)parallel to(45 degrees)/H(p)arallel to(c2)(0) = 0.92 at 4.2 K and is analyzed within the framework of Ginzburg-Landau theory.This article is published as De Wilde, Yannick, Maria Iavarone, U. Welp, Vitali Metlushko, Alexei E. Koshelev, I. Aranson, George W. Crabtree, and Paul C. Canfield. "Scanning tunneling microscopy observation of a square Abrikosov lattice in LuNi 2 B 2 C." Physical Review Letters 78, no. 22 (1997): 4273. DOI: 10.1103/PhysRevLett.78.4273. Copyright 1997 American Physical Society. Posted with permission

Magnetoresistance Scaling and the Origin of H -Linear Resistivity in BaFe2 (As1-x Px)2

We explore field and temperature scaling of magnetoresistance in underdoped (x=0, x=0.19) and optimally doped (x=0.31) samples of the high-temperature superconductor BaFe2(As1-xPx)2. In all cases, the magnetoresistance is H linear at high fields. We demonstrate that the data can be explained by an orbital model in the presence of strongly anisotropic quasiparticle spectra and scattering time due to antiferromagnetism. In optimally doped samples, the magnetoresistance is controlled by the properties of small regions of the Fermi surface called "hot spots,"where antiferromagnetic excitations induce a large quasiparticle scattering rate. The anisotropic scattering rate results in hyperbolic H/T magnetoresistance scaling, which competes with the more conventional Kohler scaling. We argue that these results constitute a coherent picture of magnetotransport in BaFe2(As1-xPx)2, which links the origin of H-linear resistivity to antiferromagnetic hot spots. Implications for the T-linear resistivity at zero field are discussed