5,671 research outputs found
Universal properties of many-body delocalization transitions
We study the dynamical melting of "hot" one-dimensional many-body localized
systems. As disorder is weakened below a critical value these non-thermal
quantum glasses melt via a continuous dynamical phase transition into classical
thermal liquids. By accounting for collective resonant tunneling processes, we
derive and numerically solve an effective model for such quantum-to-classical
transitions and compute their universal critical properties. Notably, the
classical thermal liquid exhibits a broad regime of anomalously slow
sub-diffusive equilibration dynamics and energy transport. The subdiffusive
regime is characterized by a continuously evolving dynamical critical exponent
that diverges with a universal power at the transition. Our approach elucidates
the universal long-distance, low-energy scaling structure of many-body
delocalization transitions in one dimension, in a way that is transparently
connected to the underlying microscopic physics.Comment: 12 pages, 6 figures; major changes from v1, including a modified
approach and new emphasis on conventional MBL systems rather than their
critical variant
Fluidization of collisionless plasma turbulence
In a collisionless, magnetized plasma, particles may stream freely along
magnetic-field lines, leading to phase "mixing" of their distribution function
and consequently to smoothing out of any "compressive" fluctuations (of
density, pressure, etc.,). This rapid mixing underlies Landau damping of these
fluctuations in a quiescent plasma-one of the most fundamental physical
phenomena that make plasma different from a conventional fluid. Nevertheless,
broad power-law spectra of compressive fluctuations are observed in turbulent
astrophysical plasmas (most vividly, in the solar wind) under conditions
conducive to strong Landau damping. Elsewhere in nature, such spectra are
normally associated with fluid turbulence, where energy cannot be dissipated in
the inertial scale range and is therefore cascaded from large scales to small.
By direct numerical simulations and theoretical arguments, it is shown here
that turbulence of compressive fluctuations in collisionless plasmas strongly
resembles one in a collisional fluid and does have broad power-law spectra.
This "fluidization" of collisionless plasmas occurs because phase mixing is
strongly suppressed on average by "stochastic echoes", arising due to nonlinear
advection of the particle distribution by turbulent motions. Besides resolving
the long-standing puzzle of observed compressive fluctuations in the solar
wind, our results suggest a conceptual shift for understanding kinetic plasma
turbulence generally: rather than being a system where Landau damping plays the
role of dissipation, a collisionless plasma is effectively dissipationless
except at very small scales. The universality of "fluid" turbulence physics is
thus reaffirmed even for a kinetic, collisionless system
Cavity method for force transmission in jammed disordered packings of hard particles
The force distribution of jammed disordered packings has always been
considered a central object in the physics of granular materials. However, many
of its features are poorly understood. In particular, analytic relations to
other key macroscopic properties of jammed matter, such as the contact network
and its coordination number, are still lacking. Here we develop a mean-field
theory for this problem, based on the consideration of the contact network as a
random graph where the force transmission becomes a constraint optimization
problem. We can thus use the cavity method developed in the last decades within
the statistical physics of spin glasses and hard computer science problems.
This method allows us to compute the force distribution for random
packings of hard particles of any shape, with or without friction. We find a
new signature of jamming in the small force behavior , whose exponent has attracted recent active interest: we find a
finite value for , along with . Furthermore, we relate
the force distribution to a lower bound of the average coordination number of jammed packings of frictional spheres with
coefficient . This bridges the gap between the two known isostatic limits
(in dimension ) and by extending the naive Maxwell's counting argument to
frictional spheres. The theoretical framework describes different types of
systems, such as non-spherical objects in arbitrary dimensions, providing a
common mean-field scenario to investigate force transmission, contact networks
and coordination numbers of jammed disordered packings
Hydrodynamics of operator spreading and quasiparticle diffusion in interacting integrable systems
We address the hydrodynamics of operator spreading in interacting integrable
lattice models. In these models, operators spread through the ballistic
propagation of quasiparticles, with an operator front whose velocity is locally
set by the fastest quasiparticle velocity. In interacting integrable systems,
this velocity depends on the density of the other quasiparticles, so
equilibrium density fluctuations cause the front to follow a biased random
walk, and therefore to broaden diffusively. Ballistic front propagation and
diffusive front broadening are also generically present in non-integrable
systems in one dimension; thus, although the mechanisms for operator spreading
are distinct in the two cases, these coarse grained measures of the operator
front do not distinguish between the two cases. We present an expression for
the front-broadening rate; we explicitly derive this for a particular
integrable model (the "Floquet-Fredrickson-Andersen" model), and argue on
kinetic grounds that it should apply generally. Our results elucidate the
microscopic mechanism for diffusive corrections to ballistic transport in
interacting integrable models.Comment: Published versio
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