The dynamics of the charge-order domains has been investigated in La$_{1.48}$Nd$_{0.4}$Sr$_{0.12}$CuO$_{4}$, a prototypical stripe-ordered cuprate, using pulsed current injection. We first identify the regime in which nonthermal effects dominate over
simple Joule heating, and then demonstrate that, for small enough perturbation, pulsed current injection allows access to nonthermally-induced resistive metastable states. The results are consistent with pinning of the fluctuating charge order, with fluctuations being most pronounced at the charge-order onset temperature. The nonequilibrium effects are revealed only when the transition is approached from the charge-ordered phase. Our experiment establishes pulsed current injection as a viable and effective method for probing the charge-order dynamics in various other materials.
In-plane resistivity measurements as a function of temperature and magnetic field up to 35~T with precise orientation within the crystallographic $ac-$plane were used to study the upper critical field, $H_{c2}$, of the hole-doped iron-based supercond
uctor Ba$_{1-x}$K$_x$Fe$_2$As$_2$. Compositions of the samples studied were spanning from underdoped $x=$0.17 ($T_c$=12~K) and $x$=0.22 ($T_c$=20~K), both in the coexistence range of stripe magnetism and superconductivity, though optimal doping $x$=0.39 ($T_c$=38.4~K), $x$=0.47 ($T_c$=37.2~K), to overdoped $x$=0.65 ($T_c$=22~K), $x$=0.83 ($T_c$=10~K). We find notable doping asymmetry of the shapes of the anisotropic $H_{c2}(T)$ suggesting important role of paramagnetic limiting effects in $H parallel a$ configuration in overdoped compositions and multi-band effects in underdoped compositions.
A study of the conductance noise in a two-dimensional electron system (2DES) in Si at low temperatures (T) reveals the onset of large, non-Gaussian noise after cooling from an equilibrium state at a high T with a fixed carrier density n_s. This behav
ior, which signifies the falling out of equilibrium of the 2DES as T->0, is observed for n_s<n_g (n_g - glass transition density). A protocol where density is changed by a small value Delta n_s at low T produces the same results for the noise power spectra. However, a detailed analysis of the non-Gaussian probability density functions (PDFs) of the fluctuations reveals that Delta n_s has a qualitatively different and more dramatic effect than Delta T, suggesting that Delta n_s induces strong changes in the free energy landscape of the system as a result of Coulomb interactions. The results from a third, waiting-time (t_w) protocol, where n_s is changed temporarily during t_w by a large amount, demonstrate that non-Gaussian PDFs exhibit history dependence and an evolution towards a Gaussian distribution as the system ages and slowly approaches equilibrium. By calculating the power spectra and higher-order statistics for the noise measured over a wide range of the applied voltage bias, it is established that the non-Gaussian noise is observed in the regime of Ohmic or linear response, i.e. that it is not caused by the applied bias.
The relaxations of conductivity after a temporary change of carrier density n_s during the waiting time t_w have been studied in a strongly disordered two-dimensional electron system in Si. At low enough n_s < n_g (n_g - the glass transition density)
, the nonexponential relaxations exhibit aging and memory effects at low temperatures T. The aging properties change abruptly at the critical density for the metal-insulator transition n_c < n_g. The observed complex dynamics of the electronic transport is strikingly similar to that of other systems that are far from equilibrium.
We have investigated the evolution of the low temperature specific heat anomaly (TN=5.4K in zero field) in polycrystalline SmFeAsO samples with magnetic fields up to 35T. The anomaly remains very sharp up to 16T and becomes rounded with little shift
in temperature at higher fields. Doped (superconducting) SmFeAsO0.85F0.15 sample shows a similar behavior up to 16T. The initial slope of the critical field dBc/dT is 160T/K for undoped SmFeAsO and 70T/K for doped SmFeAsO0.85F0.15, with Bc(T) defined at the peak of the specific heat anomaly. The insensitivity to the application of an external magnetic field is unique to Sm and is not observed in CeFeAsO whose anomaly shifts with initial slope dBc/dT=5.7T/K. We argue that SmFeAsO(F) presents an unprecedented case of spin reorientation at the antiferromagnetic transition.
We performed high-field magnetotransport and magnetization measurements on a single crystal of the 122-phase iron pnictide Ba(Fe1-xCox)2As2. Unlike the HTS cuprates and 1111-phase oxypnictides, Ba(Fe1-xCox)2As2 showed practically no broadening of the
resistive transitions under magnetic fields up to 45 T. The mass anisotropy gamma = Hc2ab/Hc2c deduced from the slopes of the upper critical field dHc2ab/dT = 4.9T/K and dHc2c/dT = 2.5T/K decreases from ~2 near Tc, to ~1.5 at lower temperatures. We observed the irreversibility field close to Hc2, and a rather unusual symmetric volume pinning force curve Fp(H) suggestive of strong pinning nano-structure.
Here, we report an overview of the phase diagram of single layered and double layered Fe arsenide superconductors at high magnetic fields. Our systematic magnetotransport measurements of polycrystalline SmFeAsO$_{1-x}$F$_x$ at different doping levels
confirm the upward curvature of the upper critical magnetic field $H_{c2}(T)$ as a function of temperature $T$ defining the phase boundary between the superconducting and metallic states for crystallites with the ab planes oriented nearly perpendicular to the magnetic field. We further show from measurements on single crystals that this feature, which was interpreted in terms of the existence of two superconducting gaps, is ubiquitous among both series of single and double layered compounds. In all compounds explored by us the zero temperature upper critical field $H_{c2}(0)$, estimated either through the Ginzburg-Landau or the Werthamer-Helfand-Hohenberg single gap theories, strongly surpasses the weak coupling Pauli paramagnetic limiting field. This clearly indicates the strong coupling nature of the superconducting state and the importance of magnetic correlations for these materials. Our measurements indicate that the superconducting anisotropy, as estimated through the ratio of the effective masses $gamma = (m_c/m_{ab})^{1/2}$ for carriers moving along the c-axis and the ab planes, respectively, is relatively modest as compared to the high-$T_c$ cuprates, but it is temperature, field and even doping dependent. Finally, our preliminary estimations of the irreversibility field $H_m(T)$, separating the vortex-solid from the vortex-liquid phase in the single layered compounds, indicates that it is well described by the melting of a vortex lattice in a moderately anisotropic uniaxial superconductor.
We present measurements of the resistivity and the upper critical field H_c2 of Nd(O_0.7F_0.3)FeAs single crystals in strong DC and pulsed magnetic fields up to 45 T and 60 T, respectively. We found that the field scale of H_c2 is comparable to ~100
T of high T_c cuprates. H_c2(T) parallel to the c-axis exhibits a pronounced upward curvature similar to what was extracted from earlier measurements on polycrystalline samples. Thus this behavior is indeed an intrinsic feature of oxypnictides, rather than manifestation of vortex lattice melting or granularity. The orientational dependence of H_c2 shows deviations from the one-band Ginzburg-Landau scaling. The mass anisotropy decreases as T decreases, from 9.2 at 44K to 5 at 34K. Spin dependent magnetoresistance and nonlinearities in the Hall coefficient suggest contribution to the conductivity from electron-electron interactions modified by disorder reminiscent that of diluted magnetic semiconductors. The Ohmic resistivity measured below T_c but above the irreversibility field exhibits a clear Arrhenius thermally activated behavior over 4-5 decades. The activation energy has very different field dependencies for H||ab and Hperp ab. We discuss to what extent different pairing scenarios can manifest themselves in the observed behavior of H_{c2}, using the two-band model of superconductivity. The results indicate the importance of paramagnetic effects on H_c2(T),which may significantly reduce H_c2(0) as compared toH_c2(0)~200-300 T based on extrapolations of H_c2(T) near T_c down to low temperatures.
A c-axis magnetotransport and resistance noise study in La_{1.97}Sr_{0.03}CuO_{4} reveals clear signatures of glassiness, such as hysteresis, memory, and slow, correlated dynamics, but only at temperatures (T) well below the spin glass transition tem
perature T_{sg}. The results strongly suggest the emergence of charge glassiness, or dynamic charge ordering, as a result of Coulomb interactions.
The recent discovery of a new class of superconducting oxypnictides with high transition temperatures may have profound implications for understanding unconventional high-temperature superconductivity. Like the cuprates, the oxypnictides seem to mani
fest an interleaving of charge donor and superconducting layers emerging upon doping of an antiferromagnetic parent semi-metal. Here we report magneto-transport measurements of three rare earth (Re = La, Nd, Sm) oxypnicide compounds with the transition temperatures near the maximum reported to date, in very high DC and pulsed magnetic fields up to 45 and 54 T, respectively. Our resistivity, Hall coefficient and critical magnetic fields data suggest that these oxypnictide superconductors bridge the gap between MgB$_2$ and YBaCu$_3$O$_{7-x}$ as far as electromagnetic and vortex properties are concerned.
