Nonequilibrium Many-Particle Systems

Far-from-equilibrium growth in nature: the snowflake (credit: Kenneth Libbrecht, SnowCrystals.com).
I-V characteristics of a molecular device under finite bias voltage (grey-carbon, white-hydrogen, yellow-sulfur; Nikolic group).
Shot-noise-to-current ratio modulated by spin-dependent quantum interference effects in semiconductor nanorings (Nikolic group).
Different levels of approximation for the many-body self-energy of Coulomb interaction employed in quantum transport studies.

The description of quantum and classical systems far from equilibrium is one the most fundamental challenges for  theoretical and experimental physics in the XXI century. When energy is continually supplied to systems with many interacting constituents, the outcome generally differs strikingly from the unchanging state that characterizes equilibrium. From the molecular processes on the nanoscale that form the basis of life, to the dynamically changing climate on this planet, to the clustering of matter within the universe as a whole, a myriad of phenomena owe their existence to being not just slightly away from equilibrium, but far from it.

Phenomena such as turbulence, earthquakes, fracture, and life itself only occur far from equilibrium. Subjecting materials to conditions far from equilibrium leads to otherwise-unattainable properties. For example, rapid cooling is a key process in the manufacture of the strongest metallic alloys and toughest plastics. Processes that occur far from equilibrium also create some of the most intricate structures known, from snowflakes to the highly organized structures of life.

While much is understood about systems at or near equilibrium, scientists are just beginning to uncover the basic principles governing systems far from equilibrium. Breakthroughs in this area of condensed matter  physics will affect virtually every discipline in the physical sciences, life sciences, and engineering.

Despite some success in  formulating Gibbsian description of the steady-state nonequilibrium classical or quantum statistics, only limited progress has been ahieved  without widely applicable algorithms. Even a simple nonequilibrium situation---the current transport through an interacting junction at finite bias voltage is not fully understood. The Coulomb blockade  and advent of the experimental realizations of the Kondo effect in such devices requires a many-body description at low temperatures. Landauer-Buttiker-type approaches to quantum transport problems  treat the charging effect only on a mean-field level by mapping the strongly interacting quantum problem onto a model of noninteracting fictitious particles, unsuitable to describe phenomena such as the Coulomb-blockade physics and other intricate many-particle effects.

Another example of a genuine nonequilibrium phenomenon is quantum shot noise. Over the past two decades, the exploration of the shot noise accompanying charge currents in nanostructures has become one of the major tools for gathering information about microscopic mechanisms of transport and correlations  between charge carriers which cannot be extracted from traditional conductance measurements. These nonequilibrium time-dependent fluctuations arise due to discreetness of charge, persist down to zero temperature (in contrast to thermal fluctuations which vanish at T=0 K), and  require stochasticity induced by quantum-mechanical backscattering of charge carriers. The quantum shot noise is observed only in meso- and nano-scale conductors whose electrons can be driven out of equilibrium, while in macroscopic conductors it is averaged to zero by electron scattering off lattice vibrations which efficiently drain external  electric-field-supplied energy from the electron subsystem thereby bringing it into local equilibrium.

Research Projects:

  • vibrating granular media (Nowak)
  • fluctuations and noise in electronic transport through nanostructures far from equilibrium (Nowak, Nikolic)
  • nonequilibrium quantum statistical mechanics techniques for nonlinear and time-dependent transport problems (Nikolic)
  • nonequilibrium phase transitions in quantum many-particle systems (Nikolic).
Experiment: 
Theory & Computation: 
Selected Publications: 

D. A. Areshkin and B. K. Nikolic, I-V curve signatures of nonequilibrium-driven band gap collapse in magnetically ordered zigzag graphene nanoribbon two-terminal devices, Phys. Rev. B 79, 205430 (2009). [PDF]

S.-H. Chen, C.-R. Chang, J. Q. Xiao, and B. K. Nikolic, Spin and charge pumping in magnetic tunnel junctions with precessing magnetization: A nonequilibrium Green function approach, Phys. Rev. B 79, 054424 (2009). [PDF]

B. K. Nikolic and R. L. Dragomirova, What can we learn about the dynamics of transported spins by measuring shot noise in spin-orbit-coupled nanostructures?, Semicond. Sci. Tech. 24, 064006 (2009), review article for the special issue on "The effects of spin-orbit interaction on charge transport." [URL]

E. R. Nowak, A. Grushin, A. C. B. Barnum, and M. B. Weissman, Density-noise power fluctuations in vibrated granular media, Physical Review E 63, 020301(R) (2001). [PDF]