A M Jayannavar
Articles written in Resonance – Journal of Science Education
Volume 22 Issue 7 July 2017 pp 659-676 General Article
Second law of thermodynamics can be apparently violated insystems whose dynamics depend on information acquired bymeasurement. However, when one considers measurementand erasure together with the system, it saves the second law.We consider the simple example of an information machine,where information is used as a resource to increase the machine’s performance. The system is connected to two baths, awork source, and a moving tape which is used as an informationreservoir. The performance of the device is autonomous.The system acts as an engine, erasure or refrigerator. Evencombination of any two is possible. All these possibilities areallowed by the generalized second law.
Volume 22 Issue 8 August 2017 pp 787-800 General Article
Topological insulators are new class of materials which arecharacterized by a bulk band gap like ordinary band insulatorsbut have protected conducting states on their edgesor surfaces. These states emerge due to the combination ofspin-orbit coupling and time reversal symmetry. Also, thesestates are insensitive to scattering by non-magnetic impurities.A two-dimensional topological insulator has one dimensionaledge states in which the spin-momentum locking ofthe electrons give rise to quantum spin Hall effect. A threedimensionaltopological insulator supports novel spin-polarized2D Dirac fermions on its surface. These topological insulatormaterials have been theoretically predicted and experimentallyobserved in a variety of 2D and 3D systems, includingHgTe quantum wells, BiSb alloys, and Bi2Te3, Bi2Se3 crystals.Moreover, proximity induced superconductivity in these systemscan lead to a state that supports zero energy Majoranafermions, and the phase is known as topological superconductors.In this article, the basic idea of topological insulatorsand topological superconductors are presented alongwith their experimental development.
Volume 23 Issue 5 May 2018 pp 573-589 General Article
Fluctuation theorems are a group of exact relations that remain valid irrespective of how far the system has been driven away from equilibrium. Other than having practical applications, like determination of equilibrium free energy change from nonequilibrium processes, they help in our understandiing of the second law and the emergence of irreversibility from time-reversible equations of motion at microscopic level. A vast number of such theorems have been proposed in literature, ranging from Hamiltonian to stochastic systems, from systems in steady state to those in transient regime, and for both open and closed quantum systems. In this article, we discuss about a few such relations, when the system evolves under Hamiltonian dynamics.
Volume 23 Issue 7 July 2018 pp 759-786 General Article
The delay time associated with a scattering process is one ofthe most important dynamical aspects in quantum mechanics.A commonmeasure of this is theWigner delay time basedon the group velocity description of a wave packet, which mayeasily indicate superluminal or even negative times of interactionthat are unacceptable. Many other measures such asdwell times have been proposed, but also suffer from seriousdeficiencies, particularly for evanescent waves. One importantway of realising timescales that are causally connected tothe spatial region of interest relies on utilising the dynamicalevolution of extra degrees of freedom, called quantum clocks,such as the precession of the spin of an electron in an appliedmagnetic field or the coherent decay or growth of light in anabsorptive or amplifying medium placed within the region ofinterest. Here we provide a review the several approaches developedto answer the basic question “how much time doesa quantum particle (or wave) spend in a specified region ofspace?” While a unique answer still evades us, importantprogress has been made in understanding the timescales andobtaining positive definite times of interaction by noting thatall such clocks are affected by spurious scattering concomitantwith the very clock potentials, however, weak they be,and by eliminating the spurious scattering.