The Spectroscopy of Quantum Materials Group at IACS Kolkata, led by Prof. Mintu Mondal, explores emergent quantum phenomena in low-dimensional materials, with a particular emphasis on how electronic topology, symmetry breaking, and collective order intertwine in van der Waals (vdW) quantum materials and quasi-one-dimensional systems [A. Bera, et al, PRB 112, L180502 (2025)]. A defining strength of our research group is the development of in-house fluctuation-sensitive experimental techniques, including “1/f conductance fluctuation spectroscopy”, alongside electron and thermal transport measurements. Together, these approaches provide a unique route to access critical dynamics [Sk Kalimuddin, et al, PRL 132, 266504 (2024)], uncover hidden electronic instabilities [Sk Kalimuddin, et al, PRB 113 (4), 045114 (2026)], and reveal nonequilibrium dynamics, often invisible in conventional measurements. These allowed us to explore a broad range of quantum phases, including charge density waves, topological electronic states, and low-dimensional magnetism [S. Bera, et al, PRB 108, 115122 (2023)]. 

Quasi–1D compounds constitute a remarkable class of quantum materials where reduced dimensionality amplifies the effects of electronic correlations and lattice interactions. In many such systems, conduction electrons self-organize into a spatially modulated condensate—the charge density wave (CDW)—accompanied by a periodic lattice distortion driven by strong electron–phonon coupling. The collective dynamics of this ordered state exhibit rich low-energy behavior when subjected to external electric, optical, or magnetic perturbations. The nonlinear response of the CDW phase manifests in diverse emergent phenomena, including nonlinear conductivity, giant low-frequency dielectric constants (often exceeding 110^9), and broadband electrical noise under nonequilibrium conditions.

Beyond their electronic order, quasi-1D systems hosting localized spins display an equally rich landscape of magnetic phenomena. For example, spin-½ chains exhibit gapless spin excitations characteristic of a Luttinger liquid, whereas spin-1 chains possess a finite excitation gap, consistent with the Haldane conjecture. The absence of inversion symmetry in certain quasi-1D lattices allows for higher-order exchange interactions, notably the Dzyaloshinskii–Moriya (DM) interaction, which couples spin textures to underlying electronic polarity. Such magnetoelectric couplings can induce spin canting and stabilize noncollinear or spiral magnetic configurations, even in systems that are nominally centrosymmetric. Together, these intertwined charge, spin, and lattice degrees of freedom make quasi-1D materials an exceptional platform for exploring correlated quantum dynamics and emergent collective order. 

 To explore the novel physics in quasi-1D systems, we are investigating the family of quasi-1D linear-chain compounds (MX4)nI [M=Nb, Ta ; X= S, Se, Te and n=2, 3, and 10/3] and M2PdXy [M= Nb and Ta, X=S, Se, Te; y = 5, 6] in details for their wide varieties of phase transitions and phase co-existences. Typically, in the (MX4)nI group, M atoms from long 1D chains, which are surrounded by rectangular X antiprisms and stabilized by I ions between the chains. The filling of the metal dz2 band governs the electronic properties determined by n as (n − 1)/2n.  These quasi-1D materials show some unexpected phenomena, like unusual dynamic properties in optical spectroscopy and large thermopower that changes signs at finite temperature, and incomplete phonon softening near the phase transition.

The discovery of topological materials—including topological insulators and Dirac/Weyl semimetals—has unveiled a new class of quantum systems characterized by nontrivial band topology. These materials host robust, dissipationless edge or surface states, giving rise to a range of exotic quantum phenomena. Layered topological semimetals such as WTe2 and MoTe2 serve as model platforms for exploring low-dimensional topological transport and nonequilibrium dynamics. Subtle lattice distortions, strain, or electrostatic gating can rive transitions across topological phase boundaries, enabling direct experimental access to the interplay between topology, symmetry breaking, and collective order.  

Emergent phenomena:

• Chiral magnetic effects -- generation of an electrical current parallel to an applied magnetic field due to the imbalance between Weyl nodes of opposite chirality, reflecting the chiral anomaly in condensed matter systems. 

• Chiral photogalvanic response -- helicity-dependent photocurrent arising from circularly polarized light in non-centrosymmetric topological systems, providing a direct probe of Berry curvature and topological band geometry. 

• Quantum anomalous Hall effect -- quantized Hall conductance in the absence of an external magnetic field, originating from intrinsic magnetization and Berry phase effects in spin-polarized topological bands. 

• Unconventional magneto-oscillations -- oscillatory transport and thermodynamic responses that deviate from the Lifshitz–Kosevich formalism, often linked to Berry phase accumulation, Fermi-arc states, or nonlinear band topology. 

Our ongoing efforts aim to elucidate how spin–orbit coupling, electronic correlations, and structural instabilities together govern topological protection, nonequilibrium dynamics, and dissipationless transport in quasi-2D systems.

Two-dimensional (2D) magnetic materials offer a rich platform to explore quantum phenomena such as the quantum anomalous Hall effect, long-range spin entanglement, and topological magnetic phase transitions. Their spin-polarized electronic conduction makes them highly promising for spintronic applications, with potential impact on nanoscale information storage, low-power computing, and quantum devices. However, the critical temperatures of most 2D magnets remain low, constrained by the Mermin–Wagner theorem, which forbids long-range magnetic order in isotropic 2D systems at finite temperatures.

Our group investigates promising 2D magnetic candidates, particularly FenGeTe2, combining magnetization measurements with magneto-optical spectroscopy (including Raman and optical Kerr rotation techniques) to probe their Curie temperatures, magnetic ground states, and domain dynamics as a function of layer thickness. We aim to understand the microscopic origins of magnetic anisotropy and interlayer coupling that stabilize long-range order, and to explore pathways for enhancing the Curie temperature toward room-temperature 2D magnetism, thereby overcoming the constraints imposed by the Mermin–Wagner theorem.

Quantum materials with engineered atomic precision enable control over their electronic, magnetic, and optical properties, leading to phenomena such as superconductivity, quantum confinement, and topological insulating behavior. These properties make them ideal for energy-efficient electronics, quantum computing, and next-generation memory devices.

A key focus is on resistive memory (memristors), where the intrinsic resistive switching in quantum materials enables non-volatile data storage. The nanoscale structure and defect landscape critically determine switching speed, stability, and endurance. By tuning interfaces and quantum transport pathways, our work aims to develop scalable, low-power quantum device platforms that bridge fundamental physics with functional technology.