Welcome to Das group's home page

We are interested in understanding the following aspects of modern condensed matter physics:

1. Magnetism at nanometer scale - Artificial Spin Ice

   Nanoscale magnetic elements as well as arrays are of intense current interest, particularly due to their immense potential in several applications, e.g., memory arrays, high-density storage media, logic devices or miniaturized field sensors. In our group, we are interested in the understanding of switching processes and magnetization dynamics in nanoscale magnetic elements or arrays - which can be substantially controlled via the size, shape or density of elements in an array. One of our main interests is in understanding switching process in individual "small" magnetic elements so that the entire magnetic signal, without any loss due to averaging process, can be captured. Currently we are focusing on artificial spin ice systems.

   
   See our recent work on dipolar coupled highly shape anisotropic nanomagnets which are the building blocks of square artificial spin ice systems. Magnetically charged vertices in such systems can be considered as "sources" or "sinks" of magnetic flux which exhibit emergent magnetic monopole-like behavior. By performing magnetice force microscopy experiemnts, we have demonstrated controlled creation and annihniliation of such charged vertices in presence of an external magnetic field. Details in Scientific Reports, 2021. In another recent experiment, we fabricated high-sensitive Hall sensor devices based on 2-DEG in semiconductor heterostructures to measure the stray fields emanating from individual nanomagnets. Such measurements help us to understand the switching behavior in dipolar coupled nanomagnetic systems. Details in Appl. Phys. Lett., 2020. Currently, we are trying to understand how the spin waves propagate within such dipolar interaction mediated magnetic nanostrutucres (details to be updated soon!)

   

   
   
   
   
   
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   Experimental tool used:

   For our measurements of "tiny" magnetic signal, we use highly-sensitive micro-Hall magnetometry based on high-mobility 2-dimensional electron gas (2DEG) at the heterostructure of GaAs/AlGaAs. We fabricate our micron or sub-micron scale Hall devices from GaAs/AlGaAs heterostructures using electron beam lithography (EBL) and/or optical lithography. We use the maskless optial lithography as a well as EBL facilties of the centralized Nano Research Facility (NRF) of IIT Delhi. For details of the exisiting facilities at NRF please check NRF facilties of IIT Delhi

   ­­­In another work, we have recently shown how energetics of the building blocks of square artificial spin ice systems can be controlled by contolled alignment of an external magnetic field. See details in Phys. Rev. B, 2020.
   
   
   

   

   Currently, we are involved in understanding the spin-wave properties in such magnetic nanostructures.
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   

2. Magnetism at nanometer scale - Magnetic skyrmions

   Magnetic skyrmions are particle-like topological magnetic non colinear magnetic textures that are potential information carriers in future spintronic devices. In this project, we are intereted in the electronic transport behavior and their fluctuations in the skyrmionic phase of ultra thin film magnetic heterostructures. The project is funded by Indo-French Centre for Promotion of Advanced Scientific Research and Department of Science and Technology, India. Fig: Magnetic Force Microscopy image of magnetic skyrmion lattice in Ta|Pt|(Co|Al|Pt)X5|Pt multilayer ultra thin films measured at 95 mT (Courtsey: V. Cros).
   
   
   
   
   
   
   
   

3. Electronic phenomena of novel quantum materials down to atomic scale: Quantum materials are expected to drive the future technological development. The electron's behavior at the local (atomic) scale may provide wealth of information about the fundamental nature of the materils. In this project, we are interested in exploring the local electronic behavior by performing scanning tunneling microsopy and spectrocopy experiments at very low temperature and high magnetic field. Our reserch aims at developing an in-depth understanding of the global electronic behavior from the investigation at the very local scale.

   Experimental method:

   Scanning tunneling microscopy/spectroscopy (STM/S): STM/S is an important technique in the field of nanoscale (rather atomic scale!) research which works on the quantum mechanical principle of electron tunneling from one electrode to another through a barrier. By raster scanning one electrode (tip) while using the tunneling current as a feedback parameter, it is possible to obtain information on the surface topography at atomic scale resolution. Also, by measuring the tunneling current as a function of bias voltage applied between the two electrodes (tip and sample), it is possible to map the local density of states of a material. We have an access to a ultra-high vacuum (UHV)-low-temperature (LT) STM which can be operated at the base temperature down to 330 mK and in a magnetic field of up to 9 Tesla. The UHV-LT-STM is a part of the Central Research Facilities of IIT Delhi. Atomic resolution topography image, local density of states exhibiting superconducting energy gap and animage of a superconducting vortices of NbSe2 measured at T=330 mK is shown below.

   
   
   
   
   
   
   

4. Dynamical behavior of charge carriers in low-frequency regime

   In this project we aim to investigate the low-frequency (~ 0.001 Hz - 100 Hz) dynamics of charge carriers in material systems which exhibit electronic or magnetic phase transitions. By analyzing the fluctuations in the transport behavior in the time domain, we try to understand the intrinsic dynamics of the carriers as the system undergoes a phase transition.

   Experimental method:

   Fluctuation (Noise) spectroscopy

   We have developed a Noise measurement set-up in cryogenic and magnetic field environment. The measurements can be performed in the temperature range of ~ 6 K- 300 K as well as in a magnetic field of up to about 1 T. Soon, the facility will be upgrded to cover the temperature range of 300 mK-300 K and magnetic field of up to 8T.

   
   
   
   

   See Phys. Rev. B, 2012 for details of the studies on EuB₆.