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Preprints
8. Symmetry-driven Intrinsic Nonlinear Pure Spin Hall Effect, Sayan Sarkar, Sunit Das, Amit Agarwal, arXiv:2502.18226.
The generation of pure spin current, spin angular momentum transport without charge flow, is crucial for developing energy-efficient spintronic devices with minimal Joule heating. Here, we introduce the intrinsic nonlinear pure spin Hall effect (NPSHE), where both linear and second-order charge Hall currents vanish. We show intrinsic second-order spin angular momentum transport in metals and insulators through a detailed analysis of the quantum geometric origin of different spin current contributions. Our comprehensive symmetry analysis identifies 39 magnetic point groups that support NPSHE, providing a foundation for material design and experimental realization. We predict significant nonlinear pure spin Hall currents in Kramers-Weyl metals even at room temperature, positioning them as potential candidates for NPSHE-based spin-torque devices. Our work lays a practical pathway for realizing charge-free angular momentum transport for the development of next-generation, energy-efficient spintronic devices.
Fig.: Schematic of the pure spin Hall effect. In pure spin Hall effect, equal and opposite spin flows cancel the net charge response, ensuring the 'purity' of spin angular momentum flow is one and higher energy efficiency in spin-torque efficiency. The nonlinear pure spin Hall effect dominates where the linear response vanishes.
Fig. Band geometric origin of LNSM. (a) The anomalous spin magnetization component arises due to the electric field-induced spin dipole moment. (b) The change in the spin orientation during the real-space shift of wave packets gives rise to the spin-shift magnetization. (c) The velocity injection and spin injection contributions to spin magnetization arise from the change in the velocities of electrons and the change of spin expectation during transitions between a pair of bands. Additionally, multiple interband transitions of optically excited electrons generate the multiband spin magnetization.
7. Light-induced Nonlinear Resonant Spin Magnetization, Sayan Sarkar*, Sunit Das*, Debottam Mandal, Amit Agarwal, arXiv:2409.12142.
The optical generation of nonequilibrium spin magnetization plays a crucial role in advancing spintronics, providing ultrafast control of magnetization dynamics without the need for magnetic fields. Here, we demonstrate the feasibility of light-induced nonlinear spin magnetization (LNSM), which becomes a dominant effect in centrosymmetric materials. We reveal the quantum geometric origins of various LNSM contributions in both metallic and insulating systems. Through detailed symmetry analysis, we predict significant LNSM in the antiferromagnetic material CuMnAs. Notably, under circularly polarized light, the spin magnetization exhibits helicity-dependent behavior, reversing with opposite light helicity. These findings open up new possibilities for generating LNSM-driven nonlinear spin-orbit torques and developing innovative opto-spintronic devices.
6. Nonlinear Landau fan diagram and aperiodic magnetic oscillations in three-dimensional systems, Sunit Das, Suvankar Chakraverty, and Amit Agarwal, arXiv:2403.03765.
Quantum oscillations offer a powerful probe for the geometry and topology of the Fermi surface in metals. Onsager’s semiclassical quantization relation governs these periodic oscillations in 1/B, leading to a linear Landau fan diagram. However, higher-order magnetic susceptibility-induced corrections give rise to a generalized Onsager’s relation, manifesting in experiments as a nonlinear Landau fan diagram and aperiodic quantum oscillations. Here, we explore the generalized Onsager’s relation to three-dimensional (3D) systems to capture the B-induced corrections in the free energy and the Fermi surface. We unravel the manifestation of these corrections in the nonlinear Landau fan diagrams and aperiodic quantum oscillations by deriving the B-dependent oscillation frequency and the generalized Lifshitz-Kosevich equation, respectively. Our theory explains the necessary conditions to observe these fascinating effects and predicts the magnetic field dependence of the cyclotron mass. As a concrete example, we elucidate these effects in a 3D spin-orbit coupled system and extract zero-field magnetic response functions from analytically obtained Landau levels. Our comprehensive study deepens and advances our understanding of aperiodic quantum oscillations.
FIG. The schematic highlights the origin of the generalized Onsager’s quantization relation and its consequences. Physically, the generalization results from the magnetic field-induced modification of the Fermi surface and the magnetic susceptibility corrections in the Free energy. These corrections lead to three dominant effects in experiments: i) A nonlinear Landau fan diagram, where the relation between the Landau level index (n) and 1/B (inverse magnetic field strength) deviates from the conventional straight line. ii) Aperiodic magnetic oscillations in physical quantities (denoted as A) with 1/B, where the frequency of oscillations starts varying with the magnetic field. iii) The effective cyclotron mass (m_eff) becomes magnetic field-dependent.
Published
FIG. The oscillating MR for EuO/KTO and LVO/KTO for different configurations of magnetic field. The oscillating resistance is an aperiodic function of 1/B, and can not be fitted with the conventional Lifshitz-Kosevich equation. As a consequence of the aperiodic oscillation, the Landau fan diagram (Landau level vs. 1/B) becomes nonlinear, instead of the conventional linear behavior.
5. Observation of the Magnetic Field Induced Fermi Surface Expansion in Aperiodic Quantum Oscillations, Anamika Kumari, Harsha Silotia, Sunit Das, Shama Monga, Vivek Kumar Malik, Amit Agarwal, and Suvankar Chakraverty, Adv. Funct. Mater. 2422986 (2025).
Magnetic field-induced quantum oscillations in resistivity have been extensively used to explore Fermi surfaces in quantum materials. This is enabled by the robustness of the Fermi surface to a magnetic field and the validity of the semiclassical Onsager’s quantization relation for Landau levels(LLs). Challenging this conventional understanding, evidence of magneticfield-induced expansion of the Fermi surface is presented. This is captured by your observations of magnetic field induced aperiodic quantum oscillations in resistivity at the conducting interfaces of LaVO3-KTaO3 (LVO-KTO) and EuO-KTaO3 (EO-KTO). It is showed that these aperiodic oscillations occur insystems with a small Fermi surface, where the magneticsusceptibility-induced corrections to the Fermi surface become substantial. Physically, these arise from the magnetic susceptibility-induced modificationsin the Free energy, resulting in the generalization of the semiclassical quantization rule for LLs. The magnetic field-induced Fermi surface expansion is corroborated via the measurements of nonlinear Landau fan diagrams, deviations in the Lifshitz–Kosevich (LK) fit of longitudinal magnetoresistivity,and the magnetic field dependence of the effective cyclotron mass extracted from transport measurements. These findings provide a new perspective and fresh insights into the intriguing physics of magnetic field-based probes of the Fermi surface.
4. Giant gate-controlled room temperature odd-parity magnetoresistance in magnetized bilayer graphene, Divya Sahani, Sunit Das, Kenji Watanabe, Takashi Taniguchi, Amit Agarwal, Aveek Bid, Phys. Rev. Lett. 134, 106301 (2025).
Magnetotransport measurements are crucial for understanding the Fermi surface properties, magnetism, and topology in quantum materials. Here, we report the discovery of giant room temperature odd-parity magnetoresistance (OMR) in a bilayer graphene (BLG) heterostructure interfaced with Cr2Te2Ge6(CGT). Using magnetotransport measurements, we demonstrate that the BLG/CGT heterostructure exhibits a significant antisymmetric longitudinal magnetoresistance, indicative of intrinsic time-reversal symmetry (TRS) breaking in the system. We show that the OMR is tunable via electrostatic gating. Additionally, the OMR is pronounced near the band edges and diminishes with increasing charge carrier density in graphene. Our theoretical analysis reveals that this phenomenon arises from the coupling of the out-of-plane components of Berry curvature and orbital magnetic moment to the applied magnetic field in a TRS-broken system. Our findings establish OMR as a significant probe for TRS breaking in quantum materials in which the crystal symmetries preclude the appearance of anomalous Hall effect.
FIG. Device schematic and antisymmetric magnetoresistance: (a) Schematic of the layers used to create the heterostructure. (b) Sketch of the device structure, including the contact numbers and the measurement configurations. (c) Comparison of the longitudinal magnetoresistance R14,65 = (V65/I14) for a hBN/BLG/hBN heterostructure (red line; right-axis) and a hBN/BLG/CGT heterostructure (green line, left-axis). The data were measured at 2 K. Inset: Optical image of the device.
FIG. Schematic for 2D planar Hall effect (2DPHE). Layered 2D materials host hidden planar Berry curvature and planar orbital magnetic moment arising from inter-layer tunneling. The planar Berry cuvrature and orbital magnetic moment combine with the in-plane electric and magnetic field to induce a longitudinal and transverse current in the 2D plane.
3. Planar Hall Effect in Quasi-Two-Dimensional Materials, Koushik Ghorai*, Sunit Das*, Harsh Varshney, and Amit Agarwal, Phys. Rev. Lett. 134, 026301 (2025). [Editors' Suggestion]
The planar Hall effect in 3D systems is an effective probe for their Berry curvature, topology, and electronic properties. However, the Berry curvature-induced conventional planar Hall effect is forbidden in 2D systems as the out-of-plane Berry curvature cannot couple to the band velocity of the electrons moving in the 2D plane. Here, we demonstrate a unique 2D planar Hall effect (2DPHE) originating from the hidden planar components of the Berry curvature and orbital magnetic moment in quasi-2D materials. We identify all planar band geometric contributions to 2DPHE and classify their crystalline symmetry restrictions. Using gated bilayer graphene as an example, we show that in addition to capturing the hidden band geometric effects, 2DPHE is also sensitive to the Lifshitz transitions. Our work motivates further exploration of hidden planar band geometry-induced 2DPHE and related transport phenomena for innovative applications.
2. Chiral anomalies in 3D spin-orbit coupled metals: electrical, thermal, and gravitational anomaly [Sunit Das, Kamal Das, and Amit Agarwal, Phys. Rev. B 108, 045405 (2023)]
Abstract: The discovery of a chiral anomaly in Weyl semimetals, the non-conservation of chiral charge and energy across two opposite chirality Weyl nodes, has sparked immense interest in understanding its impact on various physical phenomena. Here, we demonstrate the existence of electrical, thermal, and gravitational quantum chiral anomalies in 3D spin-orbit coupled systems. Notably, these anomalies involve chiral charge transfer across two Fermi surfaces linked to a single Weyl-like point, rather than across opposite chirality Weyl nodes as in Weyl semimetals. Our findings reveal that the Berry curvature flux piercing the Fermi surface plays a critical role in distinguishing the `chirality' of the carriers and the corresponding chiral charge and energy transfer. Importantly, we demonstrate that these quantum chiral anomalies lead to interesting thermal spin transport such as the spin Nernst effect. Our results suggest that 3D spin-orbit coupled metals offer a promising platform for investigating the interplay between quantum chiral anomalies and charge and spin transport in non-relativistic systems.
FIG. Depiction of the quantum chiral anomalies in (a) Weyl semi-metals and (b) 3D spin-orbit coupled metals or Kramers-Weyl metals. Both systems experience chiral charge and energy pumping, manifesting as electrical, thermal, and gravitational anomalies, when subjected to a magnetic field and collinear electric field or a temperature gradient. In contrast to Weyl semimetals, the chiral charge pumping in 3D spin-orbit coupled metals occurs between two different Fermi surfaces associated with a single ‘Kramers-Weyl’ node, but with opposite Berry curvature flux passing through them.
FIG. (a) A schematic of the second harmonic generation [σ (2ω)] in the presence of a quantizing magnetic field. Panel (b) presents a summary of the various tilt orientations in WSMs, and the corresponding nonlinear responses. We show that the nonlinear longitudinal response is finite only when both the space inversion symmetry and time-reversal symmetry in WSMs are broken, with the tilt direction in the Weyl nodes of opposite chirality being aligned with each other.
1. Nonlinear magnetoconductivity in Weyl and multi-Weyl semimetals in quantizing magnetic field, [Sunit Das*, Kamal Das*, and Amit Agarwal, Phys. Rev. B 105, 235408 (2022).]
Abstract: Magnetotransport and magneto-optics experiments offer a very powerful probe for studying the physical properties of materials. Here, we investigate the second-order nonlinear magnetoconductivity of the tilted type-I Weyl and multi-Weyl semimetals. In contrast to the existence of chiral charge pumping in the linear response regime, we reproduce the absence of chiral charge pumping in the nonlinear transport regime, using the Boltzmann transport framework with the Landau levels. We predict that an inversion symmetry broken and tilted Weyl semimetal can support finite longitudinal nonlinear magnetoconductivity, which is otherwise absent in untilted Weyl semimetals. The nonlinear magnetoconductivity vanishes in the ultraquantum limit, oscillates in the intermediate magnetic field regime, and saturates in the semiclassical limit. The nonlinear magnetoconductivity depends intricately on the tilt orientation, and it can be used to determine the tilt orientation in Weyl and multi-Weyl semimetals, via nonlinear magnetoresistivity or second harmonic generation experiments.
Outreach Activities
Outreach Activities
6) Presented a poster titled ``Planar Hall Effect in Quasi-Two-Dimensional Materials'' at the international conference Graphene2025, held at San Sebastian, Spain.
5) Presented a poster titled ``Planar Hall Effect in Quasi-Two-Dimensional Materials'' at the PMRF symposium 2025, held at IIT Hyderabad.
4) Presented a poster titled ``Planar Hall Effect in Quasi-Two-Dimensional Materials'' at the national conference QMAT 2025, held at IIT Guwahati.
3) Delivered a talk in the in-house symposium, condensed matter theory division, IIT Kanpur.
2) Presented a poster titled ``Nonlinear magnetoconductivity in Weyl semimetals in quantizing magnetic field'' at the international conference Emergent phenomena in van der Waals heterostructures, held at TIFR, Mumbai.
1) Presented a poster and worked as a volunteer in the local organizing committee in QMAT 2022, held at IIT Kanpur.
*Equal contribution.
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