Research
Current Research Areas
A device for angle dependent transport measurement in exfoliated vdW flakes.
2D magnetic van der Waals systems
How do magnetic moments order in a two-dimensional solid? This question lies at the heart of fundamental physics with implications beyond its realm such as in the study of topology, device applications, etc. The well-known Mermin-Wagner theorem rules out the possibility of long range ordering in 2D Heisenberg ferromagnets with continuous rotation symmetry and short range interaction. However introduction of anisotropy could gap out the long wavelength excitations and stabilize magnetic ordering. Restricting the spin dimensionality to two, as in Heisenberg XY model systems, opens up possibility of topological phase transition without any symmetry breaking. Enhanced fluctuations in such systems can potentially lead to novel ordered phases. Recent discovery of 2D magnetism in van-der Waals materials has created opportunity of addressing these questions using controlled experiments.
Spin based nanoelectronics
The discovery of Giant Magnetoresistance (GMR) in the late 1980's, for which Albert Fert and Peter Grunberg were awarded the Nobel prize for physics in 2007, and the subsequent research activities that ensued gave birth to the area of spin-based nanoelectronics. This whole-new branch of condensed matter physics, which is proving to be a magnificent laboratory of testing out and applying our acquired knowledge on magnetism in particular, which combines our newly-found ability to `see' and `design' matter at the atomic level, possessing extra-ordinarily rich physical properties, promises to stretch the horizon of our understanding and at the same time utilize them for commercial purposes.
Spintronics, also known as spin electronics, is an emerging solid-state device technology that exploits the intrinsic spin properties of an electron and its associated magnetic moment, in addition to the electron charge. Conventional electronic and semiconductor devices rely on the transport of electron charge carriers. Whereas, spintronics deal with spin-charge coupling in metallic systems with implications in the efficiency of data storage and transfer.
We are interested in generation and detection of pure spin current or to understand the spin-charge interconversion process in magnetic-nonmagnetic ultra-thin film heterostructures. Pure spin current is diffusion of spins without any associated charge current. There are various ways in which spin current can be generated such as using the `non-local' geometry, spin pumping etc. Spin Hall effect and spin Seebeck effect are the other alternatives to generate pure spin current without using any magnetic field. While the generation of spin current involves energy dissipation, it can be limited to a remote location away from the device which in turn can be manipulated by the relatively less dissipative pure spin currents. Discovery of inverse spin Hall effect (ISHE) has been extremely useful in that now an all-electrical generation and detection of spin current is possible. In our laboratory, we study the spin charge interconversion in nano-meter scale thin film heterostructures and vdW heterostructures using ferromagnetic resonance (FMR) induced spin pumping and ISHE.
Strongly Correlated Topological systems
More recently, topological phases of matter with symmetry protected surface states have attracted significant interest from the fundamental physics viewpoint albeit within the framework of weakly-interacting systems with strong spin orbit coupling. However, the recent discovery of 3D topological semimetals with gapless surface as well as bulk states makes them susceptible to electron correlation.
This has facilitated the possibility of integrating two hitherto mutually exclusive domains of condensed matter physics. The convergence of coulomb correlation and topology has opened up a new frontier in quantum materials research: correlated topological materials. For example, in a strongly correlated Weyl semimetal, the low-energy electronic excitations are expected to involve spin degrees of freedom which could be used in memory devices. Indeed, such systems have been discovered recently. A concrete example is Weyl Kondo semimetals, in which the quasiparticles near the Weyl nodes in the bulk and on the surface Fermi arc emerge due to Kondo effect.
Topological Kondo Systems
Kondo systems are realized when we put trace amount of magnetic impurities in metal. We observe non-interacting local moment formation at high temperature, and many body singlet formation leading to local moment screening below a characteristic temperature scale T_K , called the Kondo temperature. Such systems are said to be in the dilute Kondo regime. If the impurity concentration is increased slightly in the metal, there is a possibility that the local moments begin to talk to each other via the conduction electrons leading to competing RKKY interaction.
The situation becomes more interesting when there is a dense concentration of local moments in metal. When the local magnetic moments are arranged in a regular pattern, an antiferromagnetic RKKY interaction tends to order the local moments. On the other hand, due to the translation symmetry, coherent scattering off the Kondo singlets leads to dramatic reduction of resistivity below certain coherence temperature T^*. According to Doniach’s conjecture, competition between Kondo coupling and RKKY interaction leads to a continuous phase transition or a QCP (quantum critical point) between a heavy Fermi liquid state and AFM ordered state. Such systems are broadly categorized as Kondo lattice systems. Throwing magnetic frustration into the mix spices up the Physics even more, leading to complicated global phase diagrams and novel non-Fermi liquid phases. For example, the discoveries of strongly correlated pyrochlore materials with coexisting d and f electron element have opened up realistic opportunity to study emergent quantum criticality with a variety of novel quantum phases including topological phases in geometrically frustrated Kondo lattice system.
One of the primary objectives of our group is to explore this emerging frontier of correlated topological materials for novel and exciting physics. Another aim is to integrate such 3D correlated topological materials into functional devices at nanoscale with dramatically enhanced correlation effects, quantum fluctuations and topological effects.
Multiferroicity and Magneto-Electricity (ME)
Electric polarization (P) and magnetization (M) in a solid are of distinctly different origins. The mutual coupling between the two order parameters, whereby one can tune the magnetization using electric field and electric polarization by magnetic field, is called magnetoelectricity. The coupling between electric polarization and magnetization is governed by the symmetries of these two order parameters, which are very different. The electric polarization and electric field change sign on inversion of position coordinates but remain invariant under time reversal. On the other hand, the magnetization and magnetic field are invariant under space inversion while they change sign on time reversal. Because of this difference, the coexistence of and the linear coupling between the two order parameters are only possible in a system where both space and time inversion symmetries are broken.
There are multiple ways to measure magneto-electric coupling: first, the indirect method wherein one can study the changes in either magnetization near a ferroelectric transition temperature or the dielectric constant near a magnetic transition temperature; second, more difficult and direct method wherein the magnetic response to an applied electric field or an electrical response to applied magnetic field is measured. For example, in the latter case, the time-integrated current per unit area represents the magnetically induced change of polarization.
In recent years, control of ferromagnetic (FM) devices by electric fields has received a lot of attention due to very promising prospects for low-power memory and logic devices. The magnetoelectric effect gives rise to one such possibility of controlling magnetic properties by electric field.
Spin Liquids
A spin configuration is frustrated when one cannot find a configuration of spins to fully satisfy the interaction (bond) between every pair of spins. In other words, the minimum of the total energy does not correspond to the minimum of each bond. This situation arises when there is a competition between different kinds of interactions acting on a spin by its neighbors or when the lattice geometry does not allow to satisfy all the bonds simultaneously. Ground state degeneracies are the defining characteristic of frustration. The most interesting among frustrated spin systems is the so called Quantum spin liquids where quantum fluctuations prevent long range magnetic order even at absolute zero! Quantum spin liquid can be defined as having long range entangled phase with topological order and emergent quasiparticles with fractional quantum statistics.
How to identify a QSL candidate beyond? Initial experimental tests for the QSL state in a material consist of looking for the absence of a symmetry breaking phase transition or even a spin glass transition. Thermodynamic measurements showing that the spins interact strongly and yet failing to order or freeze, even at temperatures well below the interaction energy scale, would be consistent with a QSL state. One can also identify a QSL by probing the low energy excitation. For example, insulating materials where spin excitations are behaving as nearly free fermions rather than as magnons can manifest in a T-linear contribution to thermal conductivity. The most direct and unambiguous way to identify as QSL material is to design experiments to demonstrate the quantum non-locality (long-range entanglement) in the material.
