Similar glassy features in the NMR response of pure and disordered La1.88Sr0.12CuO4

High Tc superconductivity in La2-xSrxCuO4 coexists with (striped and glassy) magnetic order. Here, we report NMR measurements of the 139La spin-lattice relaxation, which displays a stretched-exponential time dependence, in both pure and disordered x=0.12 single crystals. An analysis in terms of a distribution of relaxation rates T1^-1 indicates that i) the spin-freezing temperature is spatially inhomogeneous with an onset at Tg(onset)=20 K for the pristine samples, and ii) the width of the T1^-1 distribution in the vicinity of Tg(onset) is insensitive to an ~1% level of atomic disorder in CuO2 planes. This suggests that the stretched-exponential 139La relaxation, considered as a manifestation of the systems glassiness, may not arise from quenched disorder.Comment: 7 pages, to be published in Phys. Rev.

Stability of the spiral phase in the 2D extended t-J model

We analyze the t-t'-t''-J model at low doping by chiral perturbation theory and show that the (1,0) spiral state is stabilized by the presence of t',t'' above critical values around 0.2J, assuming t/J=3.1. We find that the (magnon mediated) hole-hole interactions have an important effect on the region of charge stability in the space of parameters t',t'', generally increasing stability, while the stability in the magnetic sector is guaranteed by the presence of spin quantum fluctuations (order from disorder effect). These conclusions are based on perturbative analysis performed up to two loops, with very good convergence.Comment: 7 pages, 6 figure

Absence of large nanoscale electronic inhomogeneities in the Ba(Fe1-xCox)2As2 pnictide

75As NMR and susceptiblity were measured in a Ba(Fe1-xCox)2As2 single crystal for x=6% for various field H values and orientations. The sharpness of the superconducting and magnetic transitions demonstrates a homogeneity of the Co doping x better than +-0.25%. On the nanometer scale, the paramagnetic part of the NMR spectra is found very anisotropic and very narrow for H//ab which allows to rule out the interpretation of Ref.[6] in terms of strong Co induced electronic inhomogeneities. We propose that a distribution of hyperfine couplings and chemical shifts due to the Co effect on its nearest As explains the observed linewidths and relaxations. All these measurements show that Co substitution induces a very homogeneous electronic doping in BaFe2As2, from nano to micrometer lengthscales, on the contrary to the K doping.Comment: 6 pages, 4 figure

Van der Waals forces in density functional theory: perturbational long-range electron interaction corrections

Long-range exchange and correlation effects, responsible for the failure of currently used approximate density functionals in describing van der Waals forces, are taken into account explicitly after a separation of the electron-electron interaction in the Hamiltonian into short- and long-range components. We propose a "range-separated hybrid" functional based on a local density approximation for the short-range exchange-correlation energy, combined with a long-range exact exchange energy. Long-range correlation effects are added by a second-order perturbational treatment. The resulting scheme is general and is particularly well-adapted to describe van der Waals complexes, like rare gas dimers.Comment: 8 pages, 1 figure, submitted to Phys. Rev.

A smoothing monotonic convergent optimal control algorithm for NMR pulse sequence design

The past decade has demonstrated increasing interests in using optimal control based methods within coherent quantum controllable systems. The versatility of such methods has been demonstrated with particular elegance within nuclear magnetic resonance (NMR) where natural separation between coherent and dissipative spin dynamics processes has enabled coherent quantum control over long periods of time to shape the experiment to almost ideal adoption to the spin system and external manipulations. This has led to new design principles as well as powerful new experimental methods within magnetic resonance imaging, liquid-state and solid-state NMR spectroscopy. For this development to continue and expand, it is crucially important to constantly improve the underlying numerical algorithms to provide numerical solutions which are optimally compatible with implementation on current instrumentation and at same time are numerically stable and offer fast monotonic convergence towards the target. Addressing such aims, we here present a smoothing monotonically convergent algorithm for pulse sequence design in magnetic resonance which with improved optimization stability lead to smooth pulse sequence easier to implement experimentally and potentially understand within the analytical framework of modern NMR spectroscopy

Coherent oscillations of electrons in tunnel-coupled wells under ultrafast intersubband excitation

Ultrafast intersubband excitation of electrons in tunnell-coupled wells is studied depending on the structure parameters, the duration of the infrared pump and the detuning frequency. The temporal dependencies of the photoinduced concentration and dipole moment are obtained for two cases of transitions: from the single ground state to the tunnel-coupled excited states and from the tunnel-coupled states to the single excited state. The peculiarities of dephasing and population relaxation processes are also taken into account. The nonlinear regime of the response is also considered when the splitting energy between the tunnel-coupled levels is renormalized by the photoexcited electron concentration. The dependencies of the period and the amplitude of oscillations on the excitation pulse are presented with a description of the nonlinear oscillations damping.Comment: 8 pages, 12 figure

A numerical approach to large deviations in continuous-time

We present an algorithm to evaluate the large deviation functions associated to history-dependent observables. Instead of relying on a time discretisation procedure to approximate the dynamics, we provide a direct continuous-time algorithm, valuable for systems with multiple time scales, thus extending the work of Giardin\`a, Kurchan and Peliti (PRL 96, 120603 (2006)). The procedure is supplemented with a thermodynamic-integration scheme, which improves its efficiency. We also show how the method can be used to probe large deviation functions in systems with a dynamical phase transition -- revealed in our context through the appearance of a non-analyticity in the large deviation functions.Comment: Submitted to J. Stat. Mec