Recent News : Research Highlight

Authors: Soumya Mukherjee, Anjan Kumar NM, Subhadip Manna, Sambhu G Nath, Radha Krishna Gopal, Chiranjib Mitra, N. Kamaraju

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The interplay of magnetism and topological properties can provide a unique platform for realization of several novel quantum phenomena such as quantum anomalous Hall effect and axion insulator phases. Conventional methods of introducing magnetic order in topological materials like magnetic impurity doping, and magnetic proximity effects provide limited accessibility due to their very low operating temperature (sub kelvin regime) of investigation. With the recent introduction of the first stoichiometric anti-ferromagnetic (AFM) topological insulator (TI) MnBi 2 Te 4 , which hosts inherent magnetic ordering, the mentioned challenges have been repressed to a significant extent. Along with the non-trivial topological properties, MnBi 2 Te 4 provides an avenue to open an exchange gap in TSS due to magnetic ordering induced time reversal symmetry breaking below temperature of ~25 K. However, the nature and the occurrence of the exchange gap, are found to be highly sample specific and have invited contrasting views based on ARPES measurements and IR spectroscopic techniques. As the appearance of the exchange gap mostly influences the low energy (few to tens of meV) spectroscopic responses of MnBi 2 Te 4 , THz-TDS technique becomes instrumental for their in-depth characterizations. Moreover, the observation of IR active E u phonon mode (~1.5 THz, predicated in lattice dynamical calculations of MnBi 2 Te 4 ) and realization of its potential coupling with continuum of surface electron excitations are achievable through THz-TDS measurements.

Here, we have carried out time domain terahertz (THz) Spectroscopy measurements on PLD (pulsed laser deposition) grown epitaxial thin films of MnBi 2 Te 4 (MBT) and Sb doped MnBi 2 Te 4 (MBST) in a temperature range of 7K to 250K. We find that the extracted THz conductivity displays a strong Eu phonon absorption peak (at ∼1.5 THz) merged on top of the Drude like background contribution of bulk and surface electrons for both samples. The extracted parameters from the THz conductivity data fitted to the Drude-Fano-Lorentz model, show significant changes in their temperature dependence around the magnetic ordering Neel temperature of ∼ 25K, which is suggestive of the coupling between magnetic ordering and electronic band structure. The frequency of the Eu phonon displays an anomalous blue-shift with increasing temperatures by ∼ 0.1 THz (∼7%) for MnBi2Te4 and ∼0.2 THz (∼13%) for Sb doped MnBi2Te4. The line-shape of the Eu phonon mode in Sb doped MnBi2Te4 shows significant Fano asymmetry compared to that of MnBi2Te4, indicating that Sb doping plays an important role in the Fano interference between the bulk phonons and the surface electrons, here. A positive value of cubic anharmonicity induced phonon self-energy parameter is attributed to the anomalous blue shift of the Eu phonon mode in MnBi 2 Te 4 . Whereas the relatively higher anomalous shift in the phonon frequency of Sb doped MnBi2Te4 indicates the combined contribution of phonon anharmonicity and electron-phonon coupling, which is consistent with the emergence of Fano asymmetry in it. Our studies suggest THz-TDS is an efficient non-contact technique for probing the behaviour of charge carriers, phonons and their coupling in magnetic TIs. Furthermore, modulation of Fano asymmetry via doping and as a function of temperature has also been realized here. Thus, our investigations pave the way for fabricating a new class of quantum devices using MnBi 2 Te 4 , where both tunable Fano resonance and magnetic order influenced topological properties can be utilized.

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Posted on: November 2nd, 2024

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Title: Probing Shallow and Deep Oxygen Traps in Binary Transition Metal Oxide, CoV2O6, Using Ultrafast Pump Probe Spectroscopy

Authors: Anjan Kumar N M, Soumya Mukherjee, Souvick Chakraborty, Ayan Mondal, Debabrata Samanta, Goutam Dev Mukherjee, Venkataramanan Mahalingam, Satyabrata Raj, N. Kamaraju.

Journal: The Journal of Physical Chemistry C (https://pubs.acs.org/doi/epdf/10.1021/acs.jpcc.4c02975)

Transition metal oxides, while promising for optoelectronics, often suffer from performance limitations due to carrier trapping defects. Understanding and mitigating these traps is crucial for enhancing device efficiency.

This study focuses on CoV2O6, a transition metal oxide with unique optical and magnetic properties. Using femtosecond pump-probe spectroscopy, we investigated the ultrafast optical dynamics of CoV2O6 microparticles. Our analysis revealed two distinct types of traps: shallow band-edge states and deeper mid-gap states. These traps were identified through steady-state absorption and photoluminescence measurements, supported by first-principles DFT calculations.

Kinetic modelling of the transient absorption data further confirmed the existence of these traps and their role in capturing photoexcited carriers. We found that shallow traps capture carriers within approximately 2 picoseconds, while deep traps capture them within 30 picoseconds. Our model predicts that approximately 44 % of photoexcited carriers are trapped in shallow traps, and 40 % in deep traps.

This research provides valuable insights into the carrier dynamics and trapping mechanisms in CoV2O6, paving the way for strategies to improve its optoelectronic performance.

The article is now online: https://pubs.acs.org/doi/epdf/10.1021/acs.jpcc.4c02975

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Posted on: September 2nd, 2024

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Patterning biomolecules (including proteins such as antigens and antibodies) in pre-designed, parallel architectures can be a game-changer in rapid, inexpensive diagnostics. However, typical patterning methodologies, including top-down approaches such as photolithography and e-beam lithography, or bottom-up approaches such as solgel nanofabrication, molecular self-assembly, and DNA-scaffolding - all have limitations in efficacy for various reasons ranging from damaging the biospecimens to being slow and inflexible.

In this context, Microbubble lithography (MBL) is a comparatively recent (about a decade-old) micro-patterning method that uses laser-generated and translated microbubbles to self-assemble and pattern materials of choice in real time. Thus, MBL has been used to pattern diverse mesoscopic organic and inorganic materials, allowing for various applications including the fabrication of plastic electronics, catalytic chips, and even biosensing. However, in spite of its success in patterning mesoscopic entities, MBL has not yet been employed in developing continuous patterns of living organisms. This is primarily due to the high temperatures associated with a bubble nucleated due to heat generated by tightly focusing a laser – which is at the heart of this technique.

In this paper, we report the successful creation of a strategy for immobilizing diverse biospecimens on transparent substrates using MBL. Thus, we develop a heterostructured platform made up of a scaffold to form microbubbles, a crosslinker to facilitate adherence, and finally the concerned immobilized biospecimen. Our deployment of fast laser translation speeds ensures no temperature-induced damage to the biospecimens since their contact time with the microbubbles is less than a hundred milliseconds. Our patterned biospecimens cover a wide range of biological macromolecules, such as reporter proteins, bacterial samples, and viruses at varying concentrations. Most importantly, we also performed experiments to determine the activity of the biospecimens, especially bacterial species and virus strains, post patterning – and obtained conclusive evidence of them retaining their vegetative activity.

Overall, this study sets the groundwork for manufacturing multilayer heterostructures of living matter using MBL, and may open a new paradigm in designing bioelectronic chips, and in fast, inexpensive, and even parallelized biosensing and diagnostics of pathogens, leveraging and highlighting the ubiquitous nature of MBL in terms of patterning ‘everything mesoscopic’.

Article details:

Title: Biologically Active Micropatterns of Biomolecules and Living Matter Using Microbubble Lithography

Authors: Anand Dev Ranjan, Sucharita Bhowmick, Arnab Gupta, Amirul Islam Mallick, Ayan Banerjee

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Posted on: July 12th, 2024

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Reference: Holographic entanglement renormalization for fermionic quantum matter. J. Phys. A: Math. Theor. 57, 275401 (2024), DOI: 10.1088/1751-8121/ad56e1

Authors: Abhirup Mukherjee, Siddhartha Patra and Siddhartha Lal

URL: https://t.ly/MaqVk

Juan Maldacena's version of the holographic principle posits a duality relation between a quantum field theory in d-spatial dimensions and a gravity theory in d+1-spatial dimensions. The additional dimension can be visualised as the geometrization of the scaling of various parameters, as well as the many-particle entanglement within, the (almost) critical field theory placed on a "boundary" and which extends into the bulk. This is also known as the AdS/CFT (or gauge-gravity) correspondence, and has been applied mostly as a conjecture reliant on a dictionary between two very different theories. However, employing the conjecture as a computational tool is not without its difficulties. As put forward eloquently by Andrew Green [1],

Indeed, the situation is even more delicate, since at present we cannot deduce the gravitational dual corresponding to any particular microscopic condensed matter Hamiltonian. ... Even accepting the power of this approach, there are good reasons to attempt to construct the gravitational dual to a given condensed matter system directly. Not least amongst these is the determined independence of condensed matter physicists. It rests rather uncomfortably to take anything upon the authority of mathematical tools with which one is unfamiliar. This is a tricky task. In constructing such a dual theory, we must, in a sense, solve precisely the problems that we in the condensed matter community have been struggling with.

In a work that has appeared online just recently, Abhirup Mukherjee, Siddhartha Patra, and Siddhartha Lal have provided an explicit analytic demonstration of the holographic principle by studying a simple and tractable problem: a free fermion system with and without a mass gap (where the gap is an outcome of electronic correlations). The authors employ quantum information measures (such as entanglement entropy and multipartite information) to reveal hierarchical structure within the nature of entanglement possessed by the system. Further, they relate geometric parameters like distance and curvature of the holographic dimension to a renormalization group (RG) beta function of the mass gap, providing thereby an explicit analytic manifestation of the holographic principle.

Their work suggests that the conjectured principle is likely very useful in understanding the properties (e.g., quantum phase transitions) of systems of interacting fermions. Indeed, these transitions appear in the form of wormholes in the associated extra spatial dimension (see figure). More precisely, this wormhole is characterised by a change in the boundedness of the spatial dimension (i.e., its curvature) and corresponds to a change in a topological quantum number of the ground state of the many-electron system (i.e., the Luttinger volume). The additional (conformal) symmetry at the quantum phase transition links the emergent metric and the stress-energy tensor. Finally, in the presence of a strong transverse magnetic field, the Luttinger volume in the metallic state is found to transform into the Chern number of the insulating integer quantum Hall state.

References

[1]: An introduction to gauge-gravity duality and its application in condensed matter. A.G. Green. Contemporary Physics, 54:1, 33–48 (2013).

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Posted on: July 12th, 2024

Research Highlight: Light-Matter Lab

Title: Ultra stable three-dimensional Photophoretic trap in air facilitated by single Multimode fiber.

Authors: Souvik Sil, Anita Pahi, Aman Anil Punse, and Ayan Banerjee

We show that a multimode fiber with its speckle pattern (three figures in the top center) leads to ultrastable photophoretic traps – an intriguing result since such fibers are generally considered unusable for optical trapping. A single particle (observed as a fluorescent spot in the experimental sample chamber) can be trapped stably for hours as shown in the picture on the left. Particles are trapped in various locations of the speckles as shown in the bottom three pictures. Our data also suggest that other than the radial, an axial restoring force too acts on trapped particles – demanding new research on photophoretic forces.

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Posted on: January 8th, 2024

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This work discerns, for the first time, the effects of the electric and magnetic fields of light separately, by their work done on single highly birefringent particles that are optically trapped using vector beams of light.

More elaborately, the work describes an elegant scheme to discern the effects of the electric and magnetic fields of light separately, by their work done on single highly birefringent particles that are optically trapped using vector beams of light. Thus, the authors describe a scheme of generating pure transverse spin angular momentum (TSAM) in their system by tightly focusing radially and azimuthally polarized structured beams with no intrinsic angular momentum into a refractive index stratified medium. The TSAM is then transferred to single highly birefringent particles which spin in a direction parallel to the beam propagation direction. Such choice of beams ensures the breakdown of ‘electromagnetic democracy’ in the context of angular momentum in these systems, which helps demonstrate the action of the electric and magnetic fields on matter, separately.

This work is published in Lasers and Photonics Review with an Impact Factor of 11.

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Posted on: December 2nd, 2023

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In 1967, Sir Nevill F. Mott's pioneering work argued that the extended and the localized eigenstates in a 3-dimensional Anderson model are always separated by a critical energy called the mobility edge. The conventional wisdom has been that this energetic separation is true in any disordered low-dimensional system.

Our work shows that this is not always true in 1-dimensional systems. We identify the necessary criteria for realizing coexistent localized and extended states without forming any mobility edge: residual level repulsion, non-ergodicity of most bulk states, and spatial separation of localization centers. We demonstrate these ideas in a 1D disordered lattice governed by the Hamiltonians from the β-ensemble. We also explain the origin of the anomalous long-range correlations in the energy spectrum of the β-ensemble, which has been a long-standing puzzle in the random matrix theory community.

"Absence of mobility edge in short-range uncorrelated disordered model: Coexistence of localized and extended states"

Adway Kumar Das, Anandamohan Ghosh, Ivan M. Khaymovich Published online 18 October 2023, appeared in 20 October 2023 issue: Physical Review Letters (Vol. 131, No. 16):

URL: https://link.aps.org/doi/10.1103/PhysRevLett.131.166401 DOI: 10.1103/PhysRevLett.131.166401

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Posted on: November 21st, 2023

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Simultaneous demonstration of the forward (left) and inverse (right) spin Hall effect of light in a waveguiding plasmonic crystal

Joint research of bioNap & Light Matter Lab., DPS

“Spin-Direction-Spin Coupling of Quasiguided Modes in Plasmonic Crystals” has been published in Phys. Rev. Lett. 131, 193803 (2023) and featured in the cover page. Jeeban Kumar Nayak, a PhD student of DPS and a former BS-MS student is the primary author of this paper. The other students who were involved in this research are, Shyamal Guchhait (PhD), Harley Suchiang (former BS-MS student), Subir Kumar Ray (former PhD student).

Brief description: The coupling of spin (SAM) and orbital angular momentum (OAM) degrees of freedom of light, the so-called spin orbit interactions (SOI) have led to a number of fundamental photonic effects in various light-matter interactions. These have not only provided new insights on the universal SOI phenomena in classical optical settings but also have opened up novel routes for the development of spin-orbit photonic devices. Among the various SOI effects, photonic spin-momentum locking has attracted particular attention due to its fundamental nature and potential device applications. We have observed an unusual spin-direction-spin coupling phenomena using the leaky quasiguided modes of a waveguided plasmonic crystal system. We have observed simultaneous spin controlled directional guiding of waves (spin-direction coupling) and its reciprocal effect of wavevector-dependent spin acquisition of the scattered light (direction-spin coupling) from the same nanophotonic system, which we have defined as `spin-direction-spin coupling' phenomena. While the former effect manifests as the conventional photonic spin Hall effect (SHE), the latter is known as the inverse SHE of light, which is rather new and has not been explored much. The inverse SHE of light reported in our system can be seen as an optical analogue of the spin-injection in the solid-state spintronic devices; where with unpolarized excitation (no input SAM), the scattered light acquires a particular spin polarization according to the scattering direction. These SOI effects are shown to be mediated by the evolution of space-varying polarization due to highly non-paraxial focused light and its subsequent interaction with the polarization-anisotropic quasiguided modes of the plasmonic crystal system.

The fundamental origin and the unconventional manifestation of the spin-direction-spin coupling phenomena from a relatively simple system, ability to probe and interpret the resulting phenomena in the far field through momentum-domain polarization analysis, and their regulated control in plasmonic-photonic crystals open exciting avenues in spin-orbit photonic research.

Link to the paper: https://journals.aps.org/prl/issues/131/19.

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Posted on: November 13th, 2023

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Research Highlights from Ultrafast and Terahertz spectroscopy (UFTS) group led by Dr. Kamaraju

Transition metal dichalcogenides (TMDC) in monolayer or few-layers form have attracted significant interest due to their intriguing physical properties, such as layer-dependent band structure, high exciton binding energy, valley selective optical excitation and applications like electrocatalysis, photocatalysis, optoelectronics. The preparation of TMDCs via colloidal synthesis method has been drawing considerable attention recently due to their extremely low-cost fabrication, efficient control over size and shape, and easy implementation in device fabrication via methods such as drop-casting, and spray-coating. Despite several advantages, unintentionally formed defect states in these colloidally synthesized TMDCs are crucial in device- performance and need proper characterization for efficient device formation.

In this work we have employed ultrafast time-resolved non degenerate pump–probe spectroscopy to explore the role of defects in carrier dynamics of colloidally synthesized few-layer single-phase 2H-MoSe2 and mixed-phase 1T/2H-MoSe2 nanosheets. Metastable metallic 1T-phase has been introduced via Cr doping to coexist with stable 2H-phase to form stable mixed-phase 1T/2H-MoSe2 nanosheets which has been selected for pump-probe studies to reveal the physical processes governing its enhanced photocatalytic activities compared to 2H-MoSe2. After above bandgap excitation and probing at A-exciton peak, the differential probe transmission data for both the samples show photo-induced bleaching at earlier pump-probe delay followed by photo-induced absorption unveiling signatures of exciton-state filling, exciton trapping, defect-mediated photo-induced probe absorption and recombination of defect bound excitons. Pump-fluence dependent reduction of decay time constants of bound-exciton-recombination and further modelling of exciton population using higher order kinetic rate equation reveals that the two-body exciton-exciton annihilation governs the exciton recombination process initially with a decay rate of ∼10−8 cm3 s −1. Our analysis also suggests that the fraction of total excitons that decay via long decay channel decreases with increasing exciton density for 2H–MoSe2, in contrast to 1T/2H–MoSe2 where the fraction of excitons decaying via long decay channel remains constant, consistent with its higher photocatalytic efficiency. This work estimating crucial parameters such as trapping time, exciton-exciton annihilation rate, and shares of many-body and single-body processes in the exciton-decay should be instrumental in optimizing properties of colloidally synthesized mixed-phase MoSe2 nanosheets for applications like optoelectronics and photocatalysis.

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Posted on: November 3rd, 2023

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Harvesting and use of solar energy as an alternate source of green energy has generated renewed interest in fundamental research on optoelectronic conversion. Before using the material for making an optoelectronic device, it is essential to check the stability of the material under the external environment (pressure, temperature, etc.). In this work, temperature-dependent Raman scattering measurement demonstrates softening of certain Raman modes, which however remains incomplete as the modes do not reach zero. We propose that the onset of a new structure scatters the soft phonon modes and hence leads to incomplete soft mode behaviour. The presence of strong electron-phonon coupling, which hampers device performance, is also discussed in detail. Reference: Debabrata Samanta, Aritra Mazumder, Sonu Pratap Chaudhary, Bishnupada Ghosh, Pinku Saha, Sayan Bhattacharyya, and Goutam Dev Mukherjee, Phys. Rev. B 108, 054104 – Published 16 August 2023

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Posted on: August 21st, 2023

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Exploration of planets in our own solar system and the discovery of more than 5000 exoplanets has the scientific community excited about the possibility of life beyond Earth. The possibility of life and the ultimate fate of planets such as the Earth are influenced by diverse factors including the nature of the stars they orbit. The strength of the invisible magnetic field - known as the magnetosphere - that surrounds a planet, as well as the strength of the magnetic field of its host star, which for us, is the Sun, have a surprisingly important role on this context, a new study from Department of Physical Sciences and the Center of Excellence in Space Sciences India finds.

In the study published in the Astrophysical Journal, PhD student Sakshi Gupta, Research Scientist Arnab Basak and Prof. Dibyendu Nandi present computer simulations exploring the interplay of cosmic magnetic fields that can result in the loss of a planet's atmospheres and their ability to host life. They go on to establish a mathematical equation which relates atmospheric mass loss rate to the relative strengths of stellar and planetary magnetic fields. Their findings are expected to aid in the identification of habitable star-exoplanet systems and catalyse observational efforts for measuring exoplanetary magnetic fields.

Reference: Gupta, S., Basak, A. and Nandy, D. 2023, “Impact of Changing Stellar and Planetary Magnetic Fields on (Exo)planetary Environments and Atmospheric Mass Loss”, Astrophysical Journal, Volume 953, Page 70

Journal: https://iopscience.iop.org/article/10.3847/1538-4357/acd93b

Arxiv: https://arxiv.org/abs/2303.04770

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Posted on: August 5th, 2023

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On June 29, an international group of scientists recorded the hum of the gravitational waves of the universe using six of the world’s most sensitive radio telescopes, including India’s Giant Metrewave Radio Telescope (GMRT), located near Narayangaon, Pune.

Gravitational waves were proposed by Albert Einstein in 1916 but were first detected by Laser Interferometer Gravitational-Wave Observatory (LIGO) in 2015.

Radio telescopes detected the gravitational hum by studying pulsars, a type of rapidly rotating neutron stars, which form when a massive star runs out of fuel and collapses. A pulsar acts as a cosmic lighthouse, which emits radio beams that hit Earth.

Gravitational waves are thought to change the arrival times of these radio flashes, allowing researchers to document the hum.

Fazal Kareem a Final year BS-MS Student at IISER Kolkata was involved in the discovery of the stochastic background gravitational waves, the humming of the universe, along with 40+ other researchers of the Indian Pulsar Timing Array Collaboration. The team reported significant evidence for the presence of GWB in their pulsar timing dataset. Fazal was involved with the observation of pulsars (the most important part of the experiment), data analysis, noise analysis and combination of the European Pulsar Timing Array data with InPTA data.

Figure: Artist's rendering of black hole binaries emitting gravitational waves. As the waves overlap, they produce a background of gravitational waves that creates a distinctive correlation pattern in the timing of pulses coming from pairs of pulsars.

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Posted on: July 10th, 2023

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Research Highlights from Ultrafast and Terahertz spectroscopy (UFTS) group led by Dr. Kamaraju Natarajan

Covellite (CuS) is a direct band gap p-type semiconductor with a high intrinsic hole population causing localized surface plasmon resonance (LSPR) in NIR regime. Though these highly dense (~10 21 /cm 3 ) hole populations have immense potential to enhance photocatalytic efficiency, their ultrafast decay after photoexcitation causes a significant drawback. To enhance the photocatalytic efficiency of CuS, the relaxation of hot carriers needs to get delayed, and this can be achieved via transferring these hot carriers to another material with which CuS can stay in a composite formation. CuS/ is one such photocatalytic efficiency-enhanced nanocomposite, as observed in an earlier report. In this work, we have conducted non-degenerate ultrafast pump-probe transmission studies of CuS nanoparticles and CuS/Ag 2 S nanocomposites with varying pump fluence to explore their hot hole relaxation dynamics and determine the underlying physics of photocatalytic efficiency enhancement. The measured transient transmission data for CuS is found to originate from both photoinduced bleaching of intrinsic holes and photoinduced absorption of excited electrons. But, the localization of hot electrons in the conduction band of Ag 2 S is found to cause the disappearance of photoinduced absorption in the transient transmission of CuS/Ag 2 S at all the pump fluences except for the highest one. The first-order relaxation process based multi-exponential (ME) modeling of transient transmission data for CuS reveals three hole decay channels (∼240-410 fs, ∼24-144 ps, ≳10 µs) along with the electron decay channel of ∼1-2.6 ps. Whereas for CuS/Ag 2 S, the electron decay channel disappears, and a new hole decay channel appears making a total of four decay channels for holes (∼230-470 fs, ∼10-49 ps, ≳10 µs, ∼132-326 ps). The decay time constants are found to vary considerably with increasing pump fluence (wide range of pump fluence ranging from 0.04 mJ/cm 2 to 1.22 mJ/cm 2 ), indicating the involvement of higher-order decay processes and suggesting the need of modeling using kinetic rate equation that involves appropriate higher order terms. Extraction of transient hole densities from pump-probe measurement and modeling them using a suitable kinetic rate equation (KRE) model capable of detecting higher-order processes have been performed to reveal the exact involvement. KRE modeling extracts first, second and third order decay rate constants. The first order decay agrees with the long decay time obtained from ME fitting. And the second and third order decay rates for CuS/ get reduced by almost one order compared to CuS, indicating a reduction in overall decay rates for CuS/ consistent with its enhanced photocatalytic activity. These results are found to indicate the presence of fast trapping of hot holes and a three-hole Auger kind of process involving two free holes and one trapped hole. These observations are very much relevant for fabricating efficient light harvesting devices using CuS and its composites.

Cite This: https://doi.org/10.1021/acsaom.3c00153

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Posted on: July 10th, 2023

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Frustration. We think we understand it, and how to deal with it. But what does frustration refer to in a quantum system? Consider classical spins (i.e., spins that have only two configurations, say, pointing up and pointing down) placed on an equilateral triangle and interacting with one another through a nearest neighbour antiferromagnetic Ising exchange interaction. It is easy to see that Neel (anti-parallel) ordering is no longer possible: for any two spins that are anti-aligned with one another, the third is left confused on which direction to choose (see Fig.1). This confusion is labelled as the frustration of the classical Neel order. Frustration can also been seen in quantum mechanical systems: while two quantum spin 1/2s with an antiferromagnetic Heisenberg interaction will form a maximally-entangled singlet state, introducing a third spin-1/2 creates a dilemma (see Fig.2) - the two-spin singlet cannot accommodate another spin (often curiously referred to as entanglement monogamy). Since the spin-flip quantum fluctuations of the system will want to lower the energy of the system by entangling all three spins, no spin can be left free and the two-spin singlet cannot be the true ground-state.

The multichannel Kondo problem involves a local antiferromagnetic Heisenberg interaction between a single spin-1/2 impurity and the electrons of several conduction bath channels (see Fig.3). Had there been only one conduction bath, the impurity moment would form a singlet together with a “cloud” of electrons from the bath. We refer to this as the screening of the impurity moment (as the singlet has no magnetisation). However, in the multichannel Kondo problem described above, the formation of a singlet between the impurity and electron from one of the conduction channels is frustrated. As a result, the so-called Kondo screening of the impurity spin’s magnetic moment is hampered. Indeed, if the total spin of the conduction bath is greater than the spin of the impurity, the multichannel Kondo problem is said to be over-screened. The screening process, as well as its breakdown, are truly many-body in nature: a macroscopic number of conduction electrons interact with a single quantum impurity, and are therefore “aware” of one another. A proper description of the physics thus requires a field-theoretic treatment of the impurity-bath interactions, and the problem has been studied using a wide variety of powerful analytic and numerical methods.

Our contribution in this work was to show that the fascinating properties of the N-channel Kondo problem could be linked to those of the associated skeletal problem: a central quantum spin-1/2 coupled to N quantum spin-1/2s (corresponding to the N conduction channels) through identical antiferromagnetic Heisenberg exchange couplings. Such a model is often referred to as a star graph, and it can be identified as a limit of the multichannel problem in which the kinetic energy of the itinerant electrons has been switched off. We show in our work that certain properties of the star graph, such as the ground-state degeneracy and the magnetisation, are linked to bulk thermodynamic properties. The star graph also sets the scattering phase shift of the conduction electrons, and the scattering phase shift then dictates how the quantum fluctuations resolve themselves in order to lead to novel features. In this way, the quantum frustration inherent in the underlying simple quantum mechanical problem is seen to offer great insights into a many-body problem which looks quite daunting otherwise. Please read our work to find out more.

Authors: Siddhartha Patra, Abhirup Mukherjee, Anirban Mukherjee, N. S. Vidhyadhiraja, A. Taraphder, and Siddhartha Lal Journal Reference: Journal of Physics: Condensed Matter 35, 315601 (2023) Link: https://iopscience.iop.org/article/10.1088/1361-648X/acd09c

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Posted on: May 23rd, 2023

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In this work, the researchers from ultrafast and THz lab at IISER Kolkata have prepared five free-standing films of PVA-GO-Ag composites by varying AgNP weight fractions (0.0 wt. % to 0.64 wt. %) and investigated them using our home-built THz-TDS setup in 0.45 to 2.45 THz range. Complex THz conductivities of these materials have been fitted using universal dielectric relaxation (UDR) and Drude-Smith (DS) models, both of which have widespread appreciation in modelling AC conductivity of disordered solids. Interpretations of the fitting parameters for these two models along with percolation theory helped them to construct a microscopic picture of AC conduction mechanism of these composites. Strong localization of charge carriers, which mostly reduces with increment AgNP weight fraction, are found in these systems. A comparison between the applicability and utility of the UDR and DS model in the context of analysing our THz conductivity data has also been made. Their studies indicate that the DS model is a better fit for explaining THz conductivity of the disordered solids due to its unambiguous interpretations and the ability to provide a more detailed picture of the charge transport mechanism. The samples being lightweight and flexible and having sub-100 µm thickness, show electromagnetic interference shielding efficiency (SE) of ∼ 2-3 dB below 1 THz that increases linearly between 4-8 dB above 1 THz. These SE values are found to arise mainly from the absorption and they are estimated to increase to ∼ 10-50 dB of magnitude when the thickness is ∼ 500 µm indicating their high potential application as shielding materials in rapidly growing THz communication technology. Congratulations to the whole team As a first author of this work, Mr. Soumya Mukherjee presented this work as a poster at the recently conducted Indo-France Conference 2023 at Mahindra University, Hyderabad ((ifcfpm.com/user/home)) and won the best poster award. Congratulations to Soumya Mukherjee. "Investigation of conductivity and shielding efficiency of the free-standing PVA-GO-Ag composite thin films in terahertz regime using time domain terahertz spectroscopy " has been published in Applied Physics A (APA). PhD students of DPS, Soumya Mukherjee and Anjankumar NM, and Prof. B. Karthikeyan from NIT, Trichy were involved in this research from UFTS group @ IISER kolkata.

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Posted on: April 23rd, 2023

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"Position-position classical entanglement of light" has been published in Lasers & Photonics Reviews (LPR). LPR covers breakthrough research in the domain of optical physics and photonics. PhD students of DPS, Niladri Modak and Shyamal Guchhait and former BS-MS students, S. Ashutosh and Sayantan Das were involved in this research. In this work, the group has demonstrated inherent non-separability of the longitudinal Goos–Hänchen and the transverse Imbert–Fedorov beam shifts of light in the simple case of a partially reflecting Gaussian laser beam from a dielectric interface. This manifests as a position–position classically entangled state of light field. This discovery is expected to throw new light into our understanding of classical entanglement of light, which may also have practical interests in applications involving metrology, information processing, and communication.

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Posted on: April 22nd, 2023

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V2O5 is a very promising material with a wide range of applications, including light harvesting, energy storage, photocatalysis, photodetectors, solar cells, light-emitting diodes, and wave guides. However, for optimal device performance as well as from a fundamental physics point of view, a proper understanding of carrier dynamics in V2O5 based systems is essential. Despite such huge potential, not much time resolved experiments were performed on these materials, leaving the ultrafast dynamics obscure. This article is focused to unravel the ultrafast dynamics of photo-excited carriers in V2O5 microparticles. We have used time-resolved non-degenerate pump-probe transmission spectroscopy to investigate the photo-excited carrier dynamics in V2O5 microparticles. Tri exponential function has been used to fit the time-resolved absorption data derived from transient transmission data. A closer look utilizing pump fluence-dependent analysis reveals that the decay is considerably impacted by the pump fluence, indicating that fitting with first order decay process is insufficient to explain the decay dynamics. To understand further, a numerically solved first order coupled differential equations based polaronic kinetic model is used on the time-resolved absorption data to study the formation of polarons and their various decay channels. Through this analysis, it is found that the free carriers are trapped in polaronic traps within an average time scale of ~4.5ps, and these polarons subsequently interact with the free carriers to undergo polaron-assisted bi-molecular decay. Using the pump fluence-dependent experiments, the variation of the various underlying processes could be explained through the carrier screening concepts. It is estimated that the average density of polarons here, is ∼2.58×1017cm−3 in our study.

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Posted on: April 21st, 2023

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The behavior of primordial quantum fluctuations generated during inflation dictates the distribution of dark matter haloes in the universe at later times. These haloes attract gas clouds which gravitate to form galaxies and clusters of galaxies that we observe today. In this work, it is shown that in a specific class of inflationary models, called ultra slow-roll inflation, the primordial scalar fluctuations can be nullified at a specific scale so that the corresponding distribution of dark matter haloes will be suppressed over that scale. This effect gets imprinted as a dip in the spectrum of 21-cm lines emitted by hydrogen gas clouds that are captured by these haloes during the Dark Ages. Interestingly, such a dip can be detected by future 21-cm observatories such as an array of radio detectors on the far side of the moon. If detected, such a dip in the signal will confirm an ultra-slow roll model of inflation. This in turn shall have interesting implications for other phenomena predicted by the model such as the amplification of primordial tensor fluctuations that propagate as stochastic gravitational waves. Journal Reference: Shyam Balaji, H. V. Ragavendra, Shiv K. Sethi, Joseph Silk, and L. Sriramkumar, Phys. Rev. Lett. 129, 261301 – Published 22 December 2022

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Posted on: February 17th, 2023

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A team of researchers led by Prof. Rajesh Kumble Nayak and Prof. Bhavtosh Bansal from the Department of Physical Sciences (DPS) of Indian Institute of Science Education and Research (IISER) Kolkata, has been able to demostrate experimentally the slowing down and enhancement of fluctuations at an abrupt phase transition (APT) using thermally induced Mott transition. The research which has been published in Physical Review Letters shows that the strong abrupt transition is controlled by a critical-like singularity in the hysteretic metastable phase.

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Posted on: March 16th, 2020

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A new research finding by Dr. Rumi De from the Department of Physical Sciences (DPS) of Indian Institute of Science Education and Research (IISER) Kolkata explains the puzzling observations about the orientations of focal adhesions under different applied stretches. The research which is published in Nature Communications Biology, could lead to further understaning of many cellular processes, wound healing, tissue engineering, and regenerative medicine. Read more here (Journal DOI: 10.1038/s42003-018-0084-9) .

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Posted on: June 29th, 2018

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A team of researchers from the Department of Physical Sciences (DPS) of Indian Institute of Science Education and Research (IISER) Kolkata and their collaborators developed a hidden Markov model based integrated framework for optimum precancer detection. Read more in this news article from The Hindu. Journal DOI: 10.1117/1.JBO.22.10.105005

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Posted on: December 24th, 2017

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A team of researchers led by Dr. Dibyendu Nandi from the Department of Physical Sciences (DPS) of Indian Institute of Science Education and Research (IISER) Kolkata and CESSI predicts large-scale coronal structure for Great American Solar Eclipse of 2017, and gets it almost right. Read more in this news article from The Telegraph. Technical details are available from CESSI webpage and in this arXiv article. https://arxiv.org/abs/1708.05996

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Posted on: August 25th, 2017

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A team of researchers led by Dr. Supratim Sengupta from the Department of Physical Sciences (DPS) of Indian Institute of Science Education and Research (IISER) Kolkata finds that that incidents of bribery can be considerably reduced in a network-structured populations compared to mixed populations. Read more in this news article from the Business Standard. Journal DOI: 10.1038/srep42735

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Posted on: March 7th, 2017