S. N. Bose National Centre for Basic Sciences

Under Department of Science and Technology, Govt. of India

Department of Physics of Complex Systems

Glimpse of departmental research activities

The Department has three major areas of research:

  1. Biological and soft matter systems
  2. Equilibrium and nonequilibrium statistical physics
  3. Quantum information, mesoscopic and many-body systems

Keywords: Bio-molecular and active matter, bacterial chemotaxis, mass aggregation processes, frustrated magnets, topological phases, entanglement, superposition principle, nonlocality, and transport in quantum mesoscopic systems, etc.

We work in the broad interdisciplinary area of "complex systems" - those with many interacting components whose collective behaviour cannot be inferred simply from the behaviour of their components. On a smaller scale, the fundamental laws of nature (Newtonian laws or Schrodinger's equation) govern the constituent particles in a system. Interestingly, to a statistical physicist, "motility of microorganisms" could be as fundamental as Newton's laws of motion or Schrodinger equation. On larger scales, however, complexity emerges. From experiments and our day-to-day observations, we immediately find that complexity arises at much larger scales. Indeed, the microscopic dynamics of a large number of interacting (complicated and otherwise disorderly) components, quite astonishingly, can produce coherent macroscopic relaxation and an intricate long-ranged order or pattern; in the simple words of Nobel laureate P. W. Anderson, "more is different"!

As a physicist, we would like to discover, and understand, the fundamental as well as the phenological laws of nature. So what are the examples of such "complex systems"? Well, that is simply complex! More specifically, they could be biological systems (tissues, organs, and proteins, etc.), weather systems (e.g., clouds and storms), fractal structures in nature (e.g., coastlines), percolating porous media, economic systems, disordered systems, entanglement, magnets, and topological states, etc. We address the following broad questions:

  • How does complexity arise in the first place?
  • Can one use the "reductionist hypothesis" to construct matter and materials from, say, atoms and molecules?

However, bridging the gap between the microscopic and macroscopic scales remains a major challenge in physics! This is mainly because of the following two reasons:

  1. One has to handle the microscopic dynamics to explain the macroscopic properties.
  2. And that too for a "huge number" of interacting components!

So how does one overcome these issues? We discuss here briefly about the activities of various subgroups, which would give some flavour of the answers we have come up with.

We are engaged in various research activities, such as developing statistical mechanical theory of soft and active matter, understanding collective behaviours in driven systems by formulating general principles and classification schemes through studies of minimal model systems, understanding emergent properties of strongly correlated electron systems and non-classical aspects of elementary quantum systems, studies of phase fluctuations in mesoscopic quantum systems, etc. More specifically, our goal is to comprehend a wide range of physical phenomena such as:

  • self-organization, clustering / pattern formation in passive as well as active matter,
  • the emergence of spontaneous order (e.g., exotic topological phases) in condensed matter,
  • the ubiquity of long-ranged spatio-temporal correlations in nature and 1/f noise,
  • feedback and adaptation in biological systems (e.g., micro-organisms like E. coli),
  • entanglement, non-locality and phase fluctuations in elementary / mesoscopic quantum systems,
  • the occurrence of simple phenomenological laws (e.g., "Pareto's law") in social and financial systems, etc.,
  • the secondary structure of molten protein globules and their binding properties to ligands,
  • temperature-dependent interaction mediated long-range order developed in a particular system,
  • design of a "quantum Switch" to build "Quantum Internet",
  • collective behaviors of self-propelled particles (e.g. bacterial or Janus swimmers),

We ask how complexity or an intricate structure arises from the fundamental laws of nature and address the issue by attempting to bridge the gap between "microscopic" scales (atomic, molecular, micron, …) and "macroscopic" scales (millimetre, micron, metre, …). More specifically, we have studied, and developed theory of, the bio-molecular and active matter, bacterial chemotaxis, collective behaviours of driven systems, emergent properties of strongly-correlated electronic systems, "non-classical" aspects of elementary quantum systems and response characteristics of mesoscopic systems to applied electric and magnetic field. To this end, we resort to exact theoretical analysis of microscopic models, exact diagonalization, coarse-grained hydrodynamics and mean-field theories, and the Monte Carlo and molecular dynamics simulations, etc. Our research can have applications and relevance in our daily lives, such as in drug design and delivery, developing mechanical and bio-molecular sensors, understanding epidemic spreading, quantum entanglement, developing quantum protocols in internet and cryptography, phase spectroscopy, and building room-temperature superconductors, etc.