Landau-Zener quantum tunneling in disordered nanomagnets

We study Landau-Zener macroscopic quantum transitions in ferromagnetic metal nanoparticles containing on the order of 100 atoms. The model that we consider is described by an effective giant-spin Hamiltonian, with a coupling to a random transverse magnetic field mimicking the effect of quasiparticle excitations and structural disorder on the gap structure of the spin collective modes. We find different types of time evolutions depending on the interplay between the disorder in the transverse field and the initial conditions of the system. In the absence of disorder, if the system starts from a low-energy state, there is one main coherent quantum tunneling event where the initial-state amplitude is completely depleted in favor of a few discrete states, with nearby spin quantum numbers; when starting from the highest excited state, we observe complete inversion of the magnetization through a peculiar ``backward cascade evolution''. In the random case, the disorder-averaged transition probability for a low-energy initial state becomes a smooth distribution, which is nevertheless still sharply peaked around one of the transitions present in the disorder-free case. On the other hand, the coherent backward cascade phenomenon turns into a damped cascade with frustrated magnetic inversion.Comment: 21 pages, 7 figures, to be published in Phys.Rev.

Electron-magnon coupling and nonlinear tunneling transport in magnetic nanoparticles

We present a theory of single-electron tunneling transport through a ferromagnetic nanoparticle in which particle-hole excitations are coupled to spin collective modes. The model employed to describe the interaction between quasiparticles and collective excitations captures the salient features of a recent microscopic study. Our analysis of nonlinear quantum transport in the regime of weak coupling to the external electrodes is based on a rate-equation formalism for the nonequilibrium occupation probability of the nanoparticle many-body states. For strong electron-boson coupling, we find that the tunneling conductance as a function of bias voltage is characterized by a large and dense set of resonances. Their magnetic field dependence in the large-field regime is linear, with slopes of the same sign. Both features are in agreement with recent tunneling experiments.Comment: 4 pages, 2 figure

Theory of Tunneling Spectroscopy in a Mn12_{12} Single-Electron Transistor by Density-Functional Theory Methods

We consider tunneling transport through a Mn$_{12}$ molecular magnet using spin density functional theory. A tractable methodology for constructing many-body wavefunctions from Kohn-Sham orbitals allows for the determination of spin-dependent matrix elements for use in transport calculations. The tunneling conductance at finite bias is characterized by peaks representing transitions between spin multiplets, separated by an energy on the order of the magnetic anisotropy. The energy splitting of the spin multiplets and the spatial part of their many-body wave functions, describing the orbital degrees of freedom of the excess charge, strongly affect the electronic transport, and can lead to negative differential conductance.Comment: 4 pages, 3 figures, a revised version with minor change

Mean Field Theory of Sandpile Avalanches: from the Intermittent to the Continuous Flow Regime

We model the dynamics of avalanches in granular assemblies in partly filled rotating cylinders using a mean-field approach. We show that, upon varying the cylinder angular velocity $\omega$, the system undergoes a hysteresis cycle between an intermittent and a continuous flow regimes. In the intermittent flow regime, and approaching the transition, the avalanche duration exhibits critical slowing down with a temporal power-law divergence. Upon adding a white noise term, and close to the transition, the distribution of avalanche durations is also a power-law. The hysteresis, as well as the statistics of avalanche durations, are in good qualitative agreement with recent experiments in partly filled rotating cylinders.Comment: 4 pages, RevTeX 3.0, postscript figures 1, 3 and 4 appended

Physical descriptions of the bacterial nucleoid at large scales, and their biological implications.

Recent experimental and theoretical approaches have attempted to quantify the physical organization (compaction and geometry) of the bacterial chromosome with its complement of proteins (the nucleoid). The genomic DNA exists in a complex and dynamic protein-rich state, which is highly organized at various length scales. This has implications for modulating (when not directly enabling) the core biological processes of replication, transcription and segregation. We overview the progress in this area, driven in the last few years by new scientific ideas and new interdisciplinary experimental techniques, ranging from high space- and time-resolution microscopy to high-throughput genomics employing sequencing to map different aspects of the nucleoid-related interactome. The aim of this review is to present the wide spectrum of experimental and theoretical findings coherently, from a physics viewpoint. In particular, we highlight the role that statistical and soft condensed matter physics play in describing this system of fundamental biological importance, specifically reviewing classic and more modern tools from the theory of polymers. We also discuss some attempts toward unifying interpretations of the current results, pointing to possible directions for future investigation

Non-hermitean delocalization in an array of wells with variable-range widths

Nonhermitean hamiltonians of convection-diffusion type occur in the description of vortex motion in the presence of a tilted magnetic field as well as in models of driven population dynamics. We study such hamiltonians in the case of rectangular barriers of variable size. We determine Lyapunov exponent and wavenumber of the eigenfunctions within an adiabatic approach, allowing to reduce the original d=2 phase space to a d=1 attractor. PACS numbers:05.70.Ln,72.15Rn,74.60.GeComment: 20 pages,10 figure

Spinless particle in rapidly fluctuating random magnetic field

We study a two-dimensional spinless particle in a disordered gaussian magnetic field with short time fluctuations, by means of the evolution equation for the density matrix $$; in this description the two coordinates are associated with the retarded and advanced paths respectively. The static part of the vector potential correlator is assumed to grow with distance with a power $h$; the case $h = 0$ corresponds to a $\delta$-correlated magnetic field, and $h = 2$ to free massless field. The value $h = 2$ separates two different regimes, diffusion and logarithmic growth respectively. When $h < 2$ the baricentric coordinate $r = (1/2)(x^{(1)} + x^{(2)})$ diffuses with a coefficient $D_{r}$ proportional to $x^{-h}$, where $x$ is the relative coordinate: $x = x^{(1)} - x^{(2)}$. As $h > 2$ the correlator of the magnetic field is a power of distance with positive exponent; then the coefficient $D_{r}$ scales as $x^{-2}$. The density matrix is a function of $r$ and $x^2/t$,and its width in $r$ grows for large times proportionally to $log(t/x^2)$.Comment: latex2e; 2 figure