Quantum Condensed Matter

 

The bizarre behavior of Quantum mechanics on macroscopic scales fascinating physicists for almost a century after the discovery of superconductivity by Kammerlingh Onnes in 1911.The enigmatic quantum world reveals itself here on a macroscopic scale in an peculiar condensate phase of Cooper pairs and this condensation on the other hand effect a host of response functions and transport properties in a non trivial way. The story did not stop there. The discovery of Quantum Hall Effect(1980, 1982) and the observation of Bose-Einstein condensation of cold atoms(1995), discovery of the wonder material graphene, discovery of topological insulato, in the last few decades provided us one after another interesting opportunities, where  we can  enrich ourselves with the knowledge of quantum phases that atomic and subatomic constituents of matter can form over an extended range of external paramaters. Experiments and theory have been going hand in hand in both these branches and possible technological applications in future. Over the past few years I have been learning about these quantum phases of ultra-cold material with my collaborators. Here are some of my personal favourites from present and past.

Current Research Interest :

1.     Ultra Cold atoms: Ultra cold atomic systems are  considered as one of the purest quantum system where macroscopic quantum effect can be observed at temperature close to absolute zero. A number of quantum phenomena that had been predicted for conventional electronic solid state systems or even in the field of particle and nuclear physics were “Quantum Simulated( experimentally realized under a highly controllable situation) in such ultra cold atomic systems.

             Effect of artificial gauge field on ultra cold supersolid: I am particularly interested in understanding the effect of artificial gauge field (usual magnetic field and its fancier generalizations) on the quantum phases of ultra cold atoms. Since these ultra cold atoms are charge neutral, their orbital degrees of freedom cannot sense the real magnetic field directly. In recent times artificial magnetic field is created for such system through the engineering of the atom-laser interactions in several state of the art experiments. In a set of papers with my Ph.D. student Rashi Sachdeva we particularly studied  the effect of such artificial gauge field on one of the most enigmatic quantum phases known as supersolid phase where crystalline order and superfluidity is supposed to co-exist. Almost forty years of effort is yet to clearly confirm the existence of supersolidity and ultra cold atomic system is being projected as the most likely places where they can be hunted down. In this regard we provided some interesting results showing how the study of the effect of artificial gauge field on ultra cold atomic system can help to identify supersolidity in such systems. Here are the link for further reading: 1.http://pra.aps.org/abstract/PRA/v82/i6/e063617  2.http://pra.aps.org/abstract/PRA/v85/i1/e013642 3.http://arxiv.org/abs/1308.1592.

            Ultra Cold atoms in a cavity: Another system of interest in Ultra Cold atoms which we are currently working on is the cavity quantum electrodynamics/ cavity opto-mechanics with such ultra cold atoms. As the name suggests, essentially this means putting the ultra cold atoms in side a Fabry Perot cavity and allow it a interact with  selected mode(s) inside the cavity. The cavity transmission spectrums brings rich information about the ultra cold atomic system inside without doing a measurement directly on the atoms. Our research goal is to propose ways that extend the scope existing techniques of cavity QED/cavity opto-mechanics to learn more exotic phases of ultra cold atoms. Collaboration here includes undergraduate student (Bikash Padhi), former M. Sc. students ( Adhip Agarwala and Madhurima Nath), Ph.D. student ( Jasleen Lugani) and colleague ( Prof. K. Thyagarajan). For example in a recent paper with Bikash Padhi we for the first time predicted how Shubnikov de Haas oscillation can be observed for neutral ultra cold atoms using the tools of cavity opto-mechanics. Here are the links for further readings on our work:1.http://prl.aps.org/abstract/PRL/v111/i4/e043603 2.  http://pra.aps.org/abstract/PRA/v85/i6/e063606

2. Dirac Physics in Condensed Matter

      ( Graphene): With my colleague Dr. Manish Sharma, Ph.D. student Neetu Agarwal(Garg) and research associate Sameer Grover    (currently pursuing Ph.D. in TIFR), we have been developing a theory of ballistic transport of electrons in mono and bilayer graphene through inhomogenous magnetic field dubbed as magnetic barriers. Our particular achievement is that, we systematically develop an analogy of such transport with light propagation through a medium with varying dielectric constant. We also predicted a number of interesting physical phenomena and there are possibilities of novel device fabrications using our theory. A number of interesting results is reported in a series of papers and some of these papers are well cited. Some part of this work is supported by the SERC division of DST Govt. of India and we recently completed a project funded by them successfully. Here we list our papers for your further reading on this subject. 1. http://iopscience.iop.org/0953-8984/21/29/292204/  (cited 70 times) 2.http://iopscience.iop.org/0953-8984/23/5/055501/  (cited 30 times) 3. http://iopscience.iop.org/0953-8984/24/17/175003/  4. http://iopscience.iop.org/0965-0393/20/4/045010/ 5. http://link.springer.com/article/10.1140%2Fepjb%2Fe2013-40278-9 6. http://www.worldscientific.com/doi/abs/10.1142/S0217979213410038 (invited review article). Puja Mondal and Manisha Arora recently joined the group as Ph.D. students and will be working primarily on Topological Insulators and Graphene.

 

To summarize my current interests are:

1.     Cold atomic gases (A useful site for BEC watchers)

2.    Graphene (To know more about graphene click here)

3.    Ultra cold atoms in optical cavity 

4.    Topological Insulators.

 

Previous Research: ( Before joining IIT Delhi)

       posbul1a  BOSE-EINSTEIN CONDENSATION: The above figures reveal some of the interesting aspects we have learnt in this process of studying cold atoms (~nK). The first figure from the left, for example, depicts the interplay between the dipole moments associated with certain type of cold atoms and the consequent modulation of their density profile (PRL, 98,260403(2007)) with (M. Takahashi, T. Mizushima and K. Machida). The second figure tells about the temporal evolution of the momentum distribution of such cold atoms restricted in a narrow one dimensional channel as it passes through a disorder potential created by superimposing two or more laser beams( cond-mat/0610579). The third picture depicts, what will happen the time evolution of such cold atoms in a disorder potential if they form a lump like object called soliton (With Eric Akkermans and Ziad Musslimani). The last two pictures (with Assa Auerbach and Dan Arovas) are the schematic diagram of a vortex-antivortex pair created out of such cold atoms a sphere ( a geometry which does not have any edge) and their excitation spectrum (Phys. Rev. B, 74, 064511). We have used this data to evaluate how such vortex ( such quantum matter can rotate only by creating vortex like whirlpool) tunnels from one point to another. To summarize with my collaborators I have investigated how various types of interactions and external potential modifies the quantum phases of these cold atoms and how those various  phases manifest themselves in certain properties, for example transport.

 

posbul1a   QUANTUM HALL EFFECT: Here I have mostly studied the effect of spin and spin like degrees (with R. Rajaraman) of freedom (on the quantum mechanical ground states of two dimensional electron system in a transverse magnetic field at very low temperature ( ~mK) . Currently I am looking at similar properties of graphene, where a relativistic two-dimensional electron system can be formed . The above picture depicts an interesting topological object, dubbed as CP3 soliton (see the above figure), where spin and layer degrees of freedom of two dimensional get intertwined to provide a exotically textured object. Some experimental signatures of this object was also found at later stage.