arXiv subjects

Subrata Mandal

Publications and source records attributed to Subrata Mandal.

8 recordsLinked to original sources

arXiv2026

Probing pairing symmetries through quasiparticle interference in chiral Bloch bands

Recent experiments in van der Waals multi-layer systems have demonstrated that superconductivity can emerge from symmetry-reduced, chiral normal states. We here provide a theory for quasiparticle interference (QPI) of superconductors with chiral Bloch bands. Our analysis reveals how the non-trivial quantum geometry of the Bloch states crucially affects the interference pattern even in the normal state, inducing significant sublattice dependence. In the superconducting state, the behavior becomes more complex due to the interplay of the quantum geometry of the Bogoliubov quasiparticles with the momentum-dependent phase of the order parameter. We reveal how the spatial dependence of the local spectral function around impurities can be used to distinguish between different candidate pairing states, both with zero and finite center-of-mass momentum. Our work thus provides guidance to interpreting QPI patterns in materials with chiral bands, which may be useful when probing the rich physics of pairing in such systems.

cond-mat.supr-con↗

arXiv2025

Exactly Solvable Models Hosting Altermagnetic Quantum Spin Liquids

We construct spin-$3/2$ and spin-$7/2$ models on the square-octagon and checkerboard lattices that are exactly solvable with Majorana representations. They give rise to spin-liquid phases with full spin-rotation and lattice-translational symmetries but broken time-reversal symmetry. Although non-zero on elementary plaquettes, the net orbital magnetic moment is guaranteed to vanish as a result of point symmetries; due to the analogy to long-range ordered altermagnets, these types of phases were dubbed altermagnetic spin liquids in [Phys. Rev. Research 7, 023152 (2025)]. For the spin-$3/2$ model, we find that a $g$-wave altermagnetic spin liquid emerges as the unique ground state. In contrast, the spin-7/2 model exhibits a significantly richer phase diagram, involving different types of chiral spin liquids competing with a $d$-wave altermagnetic spin liquid. Finally, we identify and characterize the topological and non-topological excitations, illustrating the rich physics of altermagnetic spin liquids resulting from the interplay of non-trivial topological and symmetry aspects of this novel phase of matter.

cond-mat.str-el↗

arXiv2025

Gyromagnetic ratio and Pauli form factor in anisotropic QED$_{2 + 1}$ at one-loop

The gyromagnetic ratio and Pauli form factor are important quantities that characterize the electric and magnetic moment distribution of a particle. The experimentally measured value for the gyromagnetic ratio or the spin g-factor of an electron in a vacuum agrees remarkably well with theoretical predictions, making it one of the biggest successes of quantum field theory. However, these factors may get modified compared to their vacuum counterparts due to the interaction present in the system. Motivating from that, in this article, we investigate the effects of interactions within the framework of $2+1$ dimensional Proca quantum electrodynamics. We demonstrate how the g-factor and Pauli form factors change with the electron's Fermi velocity and the mass of the vector fields within Proca quantum electrodynamics.

hep-th↗

arXiv2024

Fractionalized Altermagnets: from neighboring and altermagnetic spin-liquids to spin-symmetric band splitting

We study quantum-fluctuation-driven fractionalized phases in the vicinity of altermagnetic order. First, the long-range magnetic orders in the vicinity of collinear altermagnetism are identified; these feature a non-coplanar "orbital altermagnet" which has altermagnetic symmetries in spin-rotation invariant observables. We then describe neighboring fractionalized phases with topological order reached when quantum fluctuations destroy long-range spin order, within Schwinger-boson theory and an SU(2) gauge theory of fluctuating magnetism. Discrete symmetries remain broken in some of the fractionalized phases, with the orbital altermagnet becoming an "altermagnetic spin liquid". We compute the electronic spectral function in the doped system, which is characterized by split Fermi surfaces with preserved spin-rotation symmetry.

cond-mat.str-el↗

arXiv2023

$SO(8)$ unification and the large-N theory of superconductor-insulator transition of two-dimensional Dirac fermions

Electrons on honeycomb or pi-flux lattices obey effective massless Dirac equation at low energies and at the neutrality point, and should suffer quantum phase transitions into various Mott insulators and superconductors at strong two-body interactions. We show that 35 out of 36 such order parameters that provide Lorentz-invariant mass-gaps to Dirac fermions can be organized into a single irreducible tensor representation of the $SO(8)$ symmetry of the two-dimensional Dirac Hamiltonian for the spin-1/2 lattice fermions. The minimal interacting Lagrangian away from the neutrality point has the $SO(8)$ symmetry reduced to $U(1) \times SU(4)$ by finite chemical potential, and it allows only two independent interaction terms. When the Lagrangian is nearly $SO(8)$-symmetric and the ground state insulating at the neutrality point, we argue it turns superconducting at the critical value of the chemical potential through a ``flop" between the tensor components. The theory is exactly solvable when the $SU(4)$ is generalized to $SU(N)$ and $N$ taken large. A lattice Hamiltonian that may exhibit this transition, parallels with the Gross-Neveu model, and applicability to related electronic systems are briefly discussed.

cond-mat.str-el↗

arXiv2021

Time reversal symmetry breaking and $d$-wave superconductivity of triple-point fermions

We study the possibility of complex tensor ($d$-wave) superconducting order in three-dimensional semimetals with chiral spin-1/2 triple-point fermions, which have an effective orbital angular momentum of $L=1$ arising from a crossing of three bands. Retaining the first three lowest order terms in momentum and assuming rotational symmetry we show that the resulting mean-field $d$-wave ground state breaks time reversal symmetry, and depends crucially on the coefficients of the two quadratic terms in the Hamiltonian. The phase diagram at a finite chemical potential displays both the "cyclic" and the "ferromagnetic" states, distinguished by the average value of the magnetization; in the former state it is minimal (zero), whereas in the latter it is maximal (two). In both states we find mini Bogoliubov-Fermi surfaces in the quasiparticle spectrum, conforming to recent general arguments.

cond-mat.supr-con↗

arXiv2021

p-wave superconductivity and the axi-planar phase of triple-point fermions

We consider weak-coupling superconductivity in the inversion- and rotation-symmetric system of two pseudospin $s=1$ low-energy fermions of opposite chirality. General contact interactions can lead to Cooper instabilities towards d-wave or towards a novel $2\times 3$ p-wave matrix order parameter. We compute the Ginzburg-Landau free energy for the latter. Remarkably, in this case the Ginzburg-Landau free energy can be minimized exactly, with the resulting ordered state being analogous to the ``axi-planar" p-wave, which exhibits extra degeneracy, and generally breaks time reversal symmetry. Whereas a generic Ginzburg-Landau free energy for our order parameter has only $U(1) \times SO(2) \times SO(3)$ symmetry, at weak coupling we find it displaying the enlarged $U(1)\times SO(3) \times U(1) \times SO(3)$ symmetry, broken down to $SO(2)\times SO(2)$ in the axi-planar ordered phase, and leading therefore to six Goldstone bosons. We show how the lattice, once restored, fixes the allowed values of the magnetization in the superconducting state by locking the spatial directions implicit in the order parameter to its high-symmetry axes.

cond-mat.supr-con↗

arXiv2019

Ground state of the three-dimensional BCS d-wave superconductor

We determine the mean-field ground state of the three-dimensional rotationally symmetric d-wave ($\ell=2$) superconductor at weak coupling. It is a non-inert state, invariant under the symmetry $C_{2}$ only, which breaks time reversal symmetry almost maximally, and features a high, but again less than maximal average magnetization. The state obtained by minimization of the expanded sixth-order Ginzburg--Landau free energy is found to be an excellent approximation to the true ground state. The coupling to a parasitic s-wave component has only a minuscule quantitative and no qualitative effect on the ground state.

cond-mat.supr-con↗