AC field driven population inversion

Non-equilibrium physics can offer much richer phenomena than equilibrium physics. One such example is the population inversion: high energy states are occupied while the low energy ones become emptied, effectively a negative temperature phenomena. It is expected that, many exotic things might be explored with this. For instance, bandwidth vanishing. A new paper reports how to achieve this using non-adiabatic switching of ac field [PRL 106, 236401 (2011)].

We show theoretically that the sudden application of an appropriate ac field to correlated lattice fermions flips the band structure and effectively switches the interaction from repulsive to attractive. The nonadiabatically driven system is characterized by a negative temperature with a population inversion. We
numerically demonstrate the converted interaction in an ac-driven Hubbard model with the nonequilibrium dynamical mean-field theory solved by the continuous-time quantum Monte Carlo method. Based on this, we propose an efficient ramp-up protocol for ac fields that can suppress heating, which leads to an effectively attractive Hubbard model with a temperature below the superconducting transition temperature of the equilibrium system.

Ballastic and Diffusive motions

The impacts of heat bath on a small system embedded in it are clear on macroscopic and stationary scale, but they remain a challenging subject from the microscopic and dynamic point of view. A simple example is the Brownian motion of a single particle placed in a air or other medium. The equilibrium statistical theory was forwarded by Einstein a century ago, yet what actually take place over very short periods are still under intensive study (see previous entries). Here comes a new report [Science, 332:802(2011)]:

For many years after Einstein's contributions, it was expected that the transition from ballistic to diffusive motion would be quite sharp, corresponding to an exponential decay of the particle's memory of its earlier velocity. However, about 50 years ago, hints from computer simulations and theory started to suggest a more complex scenario. In particular, hydrodynamic vortices in the liquid created by the particle's motion lead to memory effects, and the particle's velocity decays much more slowly than exponentially, exhibiting a t^−3/2 “long-time-tail” (12). Detailed analysis by Huang et al. of data like that shown in the second figure, panel B, where the ballistic-to-diffusive transition spans more than three decades in time, has now provided a thorough verification of the full, complicated hydrodynamic theory (13, 14). Although several previous experiments had observed the breakdown of the simple diffusion picture [e.g., (15)], the present studies extend into the ballistic regime.

What next? Li et al. mention the fascinating prospect of laser cooling a trapped particle to a temperature at which quantization of the energy of this mesoscopic object could be observed (16). Huang et al. suggest extending their measurements to Brownian motion in confined regions and heterogeneous media. Here, understanding the details of prediffusive motion over subnanometer distances could well be relevant to some biological processes, such as the lock-and-key mechanism of enzyme action.

Cellular networks

Cellular networks are common in nature, such as seen in the cells, bubbles, and polycrystals. In all these problems, a central question is, how do the cell boundaries distribute in time in their geometric structures and textures ? A key phenomena is that, at large times, the evolution of the distribution ubiquitously leads to a steady state that follows Boltzman's law. Here is an attempt to address this problem. The approach is traditional and phenomenological.

Mesoscale experiment and simulation permit harvesting information about both geometric features and texture in polycrystals. The grain boundary character distribution (GBCD) is an empirical distribution of the relative length [in two dimensions (2D)] or area (in 3D) of an interface with a given lattice misorientation and normal. During the growth process, an initially random distribution of boundary types reaches a steady state that is strongly correlated to the interfacial energy density. In simulation, it is found that if the given energy density depends only
on lattice misorientation, then the steady-state GBCD and the energy are related by a Boltzmann distribution. This is among the simplest nonrandom distributions, corresponding to independent trials with respect to the energy. In this paper, we derive an entropy-based theory that suggests that the evolution of the GBCD satisfies a Fokker-Planck equation, an equation whose stationary state is a Boltzmann distribution. Cellular structures coarsen according to a local evolution law, curvature-driven growth, and are limited by space-filling constraints. The interaction between the evolution law and the constraints is governed primarily by the force balance at triple junctions, the natural boundary condition associated with curvature-driven growth, and determines a dissipation relation. A simplified coarsening model is introduced that is driven by the boundary conditions and reflects the network level dissipation relation of the grain growth system. It resembles an ensemble of inertia-free spring-mass dash pots. Application is made of the recent characterization of Fokker-Planck kinetics as a gradient flow for a free energy in deriving the theory. The theory predicts the results of large-scale two-dimensional simulations and
is consistent with experiment. [PHYSICAL REVIEW B 83, 134117 (2011)]

1D is special

1D means enough space but little mobility. One always blocks another even if a bit of interaction gets in the way. And this makes the boundary between bosons and fermions fuzzy. Fermions naturally set hurdles to their compatriots for exclusion principle. While bosons, although without that famous principle, will also demobilize their partners in the presence of strong repulsions. If so, fermions and bosons will resemble each other. This is indeed what happens in this setup proposed in this work [Phys. Rev. Lett. 106, 153601 (2011) ]!

In this work we show that light-matter excitations (polaritons) generated inside a hollow-core onedimensional fiber filled with two types of atoms, can exhibit Luttinger liquid behavior. We first explain how to prepare and drive this quantum-optical system to a strongly interacting regime, described by a bosonic two-component Lieb-Liniger model. Utilizing the connection between strongly interacting bosonic and fermionic systems, we then show how spin-charge separation could be observed by probing the correlations in the polaritons. This is performed by first mapping the polaritons to propagating photon pulses and then measuring the effective photonic spin and charge densities and velocities by analyzing the correlations in the emitted photon spectrum. The necessary regime of interactions is achievable with
current quantum-optical technology.

How gels sediment

This experiment studies how gel particles move during the course of sedimentation.

"Depending on the kind of colloidal particles it contains, a gel will sediment in a matter of minutes or days. Understanding how shifts in the positions of the typically submicron sized particles affect the more macroscopic sedimentation process (and vice versa) could be helpful in designing industry-use gels. So far, however, no experiments have provided simultaneous access to these vastly different length scales.

Now, a group of scientists in France and Italy report in Physical Review Letters the use of light scattering to capture both the microscopic and macroscopic pictures of a gel collapsing under its own weight.

Giovanni Brambilla of the Université Montpellier, France, and colleagues filled a tall glass column with about 10 mm of a water-based gel. The sticky, colloidal particles in the gel slowly rearranged as the gel started to sediment, altering the specklelike pattern of laser light that the team scattered through a vertical slice of the gel. Over the course of ten days, Brambilla et al. captured this speckle pattern at various heights along the column and used an algorithm to extract such parameters as the particle relaxation rate, sedimentation velocity and density.

The team finds, at least in the slowly settling gels they studied, that both the microscopic and macroscopic dynamics mimic what is found in glassy polymers. Brambilla et al.’s data should thus provide a solid basis on which to test the theory of gels. – Jessica Thomas " [http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.106.118302]

Trends: Climate Modelling

Brad Marston [Physics, 4:20(2011)] suggests physicists think about tackling climate problems, e.g., how to model climate. He wrote, "In this article, I discuss specific advances in nonequilibrium statistical physics that have direct applications on efforts to understand and predict the climate. (For an excellent introduction to the general science of climate change, see David Archer’s Global Warming: Understanding the Forecast [3].) But first, let’s look at the sort of question a statistical description of the climate system would be expected to answer."

Nonlinear dynamics are not easy !!

This review [http://www.nature.com/nature/journal/v470/n7335/full/470475a.html?WT.ec_id=NATURE-20110224#/references] explains why I claim that !

The formal problem of the stability of rotating flow was first addressed by Lord Rayleigh in the late nineteenth century⁷. Rayleigh found that if the rotational velocity of a fluid decreases more rapidly with radius than the reciprocal of the distance from the axis of rotation, such a system is unstable to infinitesimal perturbations. Astrophysical disks, by this criterion, should be stable. But Rayleigh's analysis was restricted to vanishingly small disturbances, and the geometrical shape of the perturbations was in the form of rings with cylindrical symmetry. It is still not known what types of flow that are formally stable by this Rayleigh criterion might still be unstable to more general forms of disturbance; it is known, however, that some types of Rayleigh-stable flow certainly can be destabilized^{4, 8}. The issue of interest is whether the rotation of an astrophysical gas disk about a central mass falls into this unstable category.

This problem can be investigated in the laboratory by studying what is known as Couette flow. In a Couette apparatus, water is confined to flow in the space between two coaxial cylinders. There should be no motion along the central axis, only rotational flow about the axis. The cylinders rotate independently of one another, so that small frictional viscous forces near the cylindrical walls will set up a hydrodynamical flow in which the rotational velocity depends on the distance from the rotation axis. By choosing the rotational velocities of the rotating cylinders appropriately, a small section of an astrophysical disk can be mimicked in the laboratory. In such a disk, the flow velocity is inversely proportional to the square root of the distance from the centre, a pattern known as Keplerian flow. The question to be answered is whether Keplerian flow, formally stable by the Rayleigh criterion, actually breaks down into turbulence.
......

It is this question that Paoletti and Lathrop¹ have sought to address. When a Couette flow becomes turbulent, one of the consequences is a greatly enhanced outward flux of angular momentum, which is imparted to the outer cylinder in the form of a torque. In their experiment, the authors measure this torque directly. An earlier investigation⁹ had claimed to detect this torque, but the new experiment¹ was conducted under conditions in which (undesirable) viscous effects were more effectively minimized.

Close on the heels of Paoletti and Lathrop's claim, however, comes a report by Schartman et al.¹⁰ on a related experiment. These investigators found no transition to turbulence for Keplerian flow with the same controlled level of viscosity. This null result was first reported³ in 2006, and the most recent paper maintains its original conclusion that there is no evidence of a turbulent breakdown of Keplerian-like laminar flow for very small values of the viscosity.