arXiv subjects

Mainak Das

Publications and source records attributed to Mainak Das.

5 recordsLinked to original sources

arXiv2026

Multicomponent Magnetic Domain Walls in Rhombohedral Graphene

Spatial textures of magnetic order, such as domain walls and skyrmions, are fundamental objects in magnetism. In rhombohedral multilayer graphene, magnetic order involves spin and valley degrees of freedom, opening the possibility of qualitatively new spatial textures. Here, we explore this possibility through a microscopic study of a one-dimensional domain wall in the valley-imbalanced quarter-metal phase of rhombohedral graphene. We uncover two different classes of domain walls. One resembles a conventional magnetic domain wall, locally rotating between the two bulk states, whereas the other is intrinsically multicomponent and explores states that are not occupied in either bulk domain. Which texture is realized is controlled by the competition between intervalley Hund's coupling and spin-orbit coupling, and we identify experimental signatures to distinguish them. We further show that, in a superconducting junction formed across the wall, the superconducting phase difference couples directly to the intervalley-coherent phase of the texture. Precession of this internal phase can therefore generate a voltage across the junction. Our theory shows that rhombohedral graphene indeed has magnetic textures beyond conventional magnet and that their dynamics can couple to superconducting transport.

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arXiv2025

Momentum-Space AC Josephson Effect and Intervalley Coherence in Multilayer Graphene

Electron transport driven by the phase coherence and interference of quantum many-body wavefunctions is a fascinating phenomenon with potential technological significance. Superconductivity, for example, enables dissipationless transport through macroscopic phase twisting. Similarly, in charge-density waves, once the phase degree of freedom-representing the collective position of electrons relative to the lattice-is depinned, it generates characteristic broadband noise and intriguing AC-DC interference patterns. In this work, we point out a phase-coherent dynamics in the intervalley coherent (IVC) state, also known as the bond-ordered or Kekul\'e distorted state, frequently reported in rhombohedral multilayer graphene. Under a static magnetic field, the IVC state responds with an oscillating intervalley current, which in turn causes oscillating orbital magnetization, thereby inducing a detectable AC Hall effect. This mechanism mirrors the AC Josephson effect observed in superconductors but now happening in momentum space. In this analogy, the static magnetic field acts as the DC voltage, while the oscillating intervalley current assumes the role of the AC Josephson current. We present detailed microscopic calculations for all the parameters of the phase-number free-energy in rhombohedral trilayer graphene, predicting an orbital magnetization oscillation frequency of approximately 12 GHz at 0.1 Tesla. We comment on this phase-coherent dynamics in other 2D materials like twisted homobilayer transition metal dichalcogenides.

cond-mat.mes-hall↗

arXiv2024

Superpolarized Electron-Hole Liquid and Multiferroicity in Multilayer Graphene

We introduce a many-body state termed superpolarized electron-hole liquid to explain the multiferroic properties observed in a recent experiment on rhombohedral pentalayer graphene by Han et al. [Nature 623, 41-47, 2023] . Superpolarization refers to a state where electrons and holes are fully polarized in opposite directions within the extended spin-valley space, resulting in a polarization per charge that exceeds the saturated value of one. This state exhibits multiferroic order, characterized by spontaneous spin, layer, and valley polarization. We demonstrate the independent control of valley polarization and orbital magnetization in this state through the application of electric-displacement field and weak magnetic fields. The large magnetoelectric effect observed in the experiment is attributed to the substantial Berry curvature concentrated near the band edge. The concept of superpolarization can be probed experimentally through magnetic oscillation and local current distribution measurements to determine the area of the Fermi surfaces and their magnetic moments.

cond-mat.mes-hall↗

arXiv2023

Quarter-Metal Phases in Multilayer Graphene: Ising-XY and Annular Lifshitz Transitions

Recent experiments have uncovered a distinctive magnetic metal in lightly-doped multilayer graphene, coined the \textit{quarter metal}. This quarter metal consolidates all the doped carriers, originally distributed evenly across the four (or twelve) Fermi surfaces of the paramagnetic state, into one expansive Fermi surface by breaking time-reversal and/or inversion symmetry. In this work, we map out a comprehensive mean-field phase diagram of the quarter-metal in rhombohedral trilayer graphene within the four dimensional parameter space spanned by the density $n_e$, interlayer electric potential $U$, external magnetic field parallel to the two-dimensional material plane $B_{\parallel}$ and Kane-Mele spin-orbit coupling $\lambda$. We found an annular Lifshitz phase transition and a Ising-XY phase transition and locate these phase boundaries on the experimental phase diagram. The movement of the Ising-XY phase boundary offers insights into $\lambda$. Our analysis reveals that it moves along the line $\partial n_e/\partial B_{\parallel} \sim -0.5\times 10^{11} \text{cm}^{-2}\text{T}^{-1}$ within the $n_e$-$B_{\parallel}$ parameter space when $\lambda=30\mu$eV. Additionally, we estimated the in-plane spin susceptibility of the valley-Ising quarter-metal $\chi_{_\parallel}\sim 8~\mu\text{eV} ~\text{T}^{-2}$. Beyond these quantitative findings, two general principles emerge from our study: 1) The valley-XY quarter metal's dominance in the $n_e-U$ parameter space grows with an increasing number of layers due to the reduce valley polarization variations within the Fermi sea. 2) Layer polarization near the band edge plays an important role in aiding the re-entrance of the paramagnetic state at low density. The insights derived from the quarter metal physics may shed light on the complex behaviors observed in other regions of the phase diagram.

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arXiv2023

Unconventional Metallic Magnetism: Non-analyticity and Sign-changing Behavior of Orbital Magnetization in ABC Trilayer Graphene

We study an unique form of metallic ferromagnetism in which orbital moments surpasses the role of spin moments in shaping the overall magnetization. This system emerges naturally upon doping a topologically non-trivial Chern band in the recently identified quarter metal phase of rhombohedral trilayer graphene. Our comprehensive scan of the density-interlayer potential parameter space reveals an unexpected landscape of orbital magnetization marked by two sign changes and a line of singularities. The sign change originates from an intense Berry curvature concentrated close to the band-edge, and the singularity arises from a topological Lifshitz transition that transform a simply connected Fermi sea into an annular Fermi sea. Importantly, these variations occur while the groundstate order-parameter (i.e. valley and spin polarization) remains unchanged. This unconventional relationship between the order parameter and magnetization markedly contrasts traditional spin ferromagnets, where spin magnetization is simply proportional to the groundstate spin polarization via the gyromagnetic ratio. We compute energy and magnetization curves as functions of collective valley rotation to shed light on magnetization dynamics and to expand the Stoner-Wohlfarth magnetization reversal model. We provide predictions on the magnetic coercive field that can be readily tested in experiments. Our results challenge established perceptions of magnetism, emphasising the important role of orbital moments in two-dimensional materials such as graphene and transition metal dichalcogenides, and in turn, expand our understanding and potential manipulation of magnetic behaviors in these systems.

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