arXiv subjects

Anjishnu Bose

Publications and source records attributed to Anjishnu Bose.

10 recordsLinked to original sources

arXiv2025

Symmetry constrained field theories for chiral spin liquid to spin crystal transitions

We consider the spin rotationally invariant Kalmeyer-Laughlin chiral spin liquid (CSL) in systems with broken time-reversal symmetry and explore symmetry constraints on possible conventional spin crystal states accessible via a direct transition. These constraints provide a framework to identify topological invariants of the magnetically ordered state. We show that the existence of a direct transition from a CSL requires a precise compatibility condition between the topological invariants of the ordered state and the anomaly of the CSL. The lattice symmetries also constrain the functional form of the low-energy theory to describe these transitions. This allows us to construct explicit Chern-Simons-matter field theories for the transition into a class of noncoplanar orders identified as candidates directly accessible from the CSL, including the octahedral spin crystal on the kagomé lattice, and the tetrahedral order on the triangular and honeycomb lattice. These transitions can either be described using coupled fractionalized $ \mathbb{CP}^1 $ theories or fractionalized matrix principal chiral models. We also discuss extensions to more general magnetic ordering transitions out of the CSL.

cond-mat.str-el↗

arXiv2025

Charge ordering and spontaneous topological Hall effect in bilayer skyrmion crystals

Magnetic skyrmion crystals with zero net skyrmion charge and zero topological Hall response are interesting candidate phases which can occur at a vanishing magnetic field in centrosymmetric systems. We study a minimal bilayer model of skyrmion crystals having opposite chirality and topological charge in the two layers, and show that it can host nearly flat electronic bands with quasi-uniform Berry curvature and quantum metric. Using Hartree-Fock theory, we show that weak to moderate short-range electron interactions induce two distinct types of symmetry breaking patterns depending on the band dispersion: an intra-unit-cell charge density modulation from Chern band mixing or a layer-imbalanced phase with a nonzero ferroelectric polarization. Both phases break inversion symmetry leading to a spontaneous and large net topological Hall effect, with the phase diagram tunable by external electric fields. Our results may be relevant to centrosymmetric skyrmion materials such as Gd$_2$PdSi$_3$ and Gd$_3$Ru$_4$Al$_{12}$ as well as artificially engineered heterostructures. We also discuss its relation to recent work on twisted transition metal dichalcogenide bilayers.

cond-mat.mes-hall↗

arXiv2025

Modified large-$N$ approach to gapless spin liquids, magnetic orders, and dynamics: Application to triangular lattice antiferromagnets

Recent work has shown that the triangular lattice spin-$1/2$ $J_1$-$J_2$ Heisenberg and XXZ antiferromagnets may exhibit coplanar or supersolid orders proximate to a gapless Dirac spin liquid phase. We explore a distinct $SU(2N)\!\!\times\!\!SU(M)$ fermionic parton approach, complemented by variational Monte Carlo calculations for the spin-$1/2$ model, to study the phase diagram of these models. We also calculate their dynamical spin response including parton interactions within a random phase approximation, and discuss implications for neutron scattering on triangular lattice cobaltates Ba$_3$CoSb$_2$O$_9$, Na$_2$BaCo(PO$_4$)$_2$, K$_2$Co(SeO$_3$)$_2$, Rb$_2$Co(SeO$_3$)$_2$, and Yb-based magnet KYbSe$_2$.

cond-mat.str-el↗

arXiv2025

Altermagnetism and superconductivity in a multiorbital t-J model

Motivated by exploring doped multi-orbital antiferromagnets (AFMs) and altermagnets (ALMs) we explore minimal $t$-$J$ models on the square-octagon lattice which favor such collinear magnetic orders in the regime where spin exchange dominates. While the AFM order breaks translational and time-reversal symmetries, the ALM state (equivalently, a `$d$-wave ferromagnet') features multipolar order which separately breaks time-reversal and crystal rotation symmetries but preserves their product leading to spin-split bands with zero net magnetization. We study the mean field phase diagram of these models as we vary doping and interactions, discovering regimes of weak and strong ALM order, superconductivity including uniform $s$-wave and $d$-wave pairing states, incipient $d$-wave pair density wave order, and phases with coexisting singlet-triplet pairing and AFM/ALM orders which appear unstable to phase separation and could host stripe order with longer-range interactions. We study the mean field phase diagram of these multiorbital models as we vary doping and interactions, discovering two types of ALM order: (i) itinerant weak-coupling ALM metals driven by quasi-1D van Hove singularities, as well as (ii) strong ALM order at half-filling. We also find regimes of superconductivity including uniform $s$-wave and $d$-wave pairing states, incipient $d_{xy}$-wave pair density wave order, and uniform phases with coexisting singlet-triplet pairing and ALM order. Our inhomogeneous mean field theory approach reveals that the coexistence phases are unstable to phase separation, but longer-range interactions could lead to stripe order. Our results may be relevant to doping or pressure studies of multiorbital ALM materials.

cond-mat.str-el↗

arXiv2024

Spin dynamics of an easy-plane Dirac spin liquid in a frustrated XY model: Application to honeycomb cobaltates

Recent work has shown that the honeycomb lattice spin-$1/2$ $J_1$-$J_3$ XY model, with nearest-neighbor ferromagnetic exchange $J_1$ and frustration induced by third-neighbor antiferromagnetic exchange $J_3$, may be relevant to a wide range of cobaltate materials. We explore a variational Monte Carlo study of Gutzwiller projected wavefunctions for this model and show that an easy-plane Dirac spin liquid (DSL) is a viable `parent' state for the competing magnetic orders observed in these materials, including ferromagnetic, zig-zag, spiral, and double zig-zag orders at intermediate frustration, and show that such broken symmetry states can be easily polarized by a weak in-plane magnetic field consistent with experiments. We formulate a modified parton theory for such frustrated spin models, and explore the potential instabilities of the DSL due to residual parton interactions within a random phase approximation (RPA), both at zero magnetic field and in a nonzero in-plane field. The broken symmetry states which emerge in the vicinity of this Dirac spin liquid include ferromagnetic, zig-zag, and incommensurate spiral orders, with a phase diagram which is consistent with VMC and density matrix renormalization group studies. We calculate the dynamical spin response of the easy-plane DSL, including RPA corrections, near the boundary of the ordered states, and present results for THz spectroscopy and inelastic neutron scattering, at zero field as well as in an in-plane magnetic field, and discuss experimental implications.

cond-mat.str-el↗

arXiv2023

Nematic order in topological SYK models

We study a class of multi-orbital models based on those proposed by Venderbos, Hu, and Kane which exhibit an interplay of topology, interactions, and fermion incoherence. In the non-interacting limit, these models exhibit trivial and Chern insulator phases with Chern number $C \geq 1$ bands as determined by the relative angular momentum of the participating orbitals. These quantum anomalous Hall insulator phases are separated by topological transitions protected by crystalline rotation symmetry, featuring Dirac or quadratic band-touching points. Here we study the impact of Sachdev-Ye-Kitaev (SYK) type interactions on these lattice models. Given the random interactions, these models display `average symmetries' upon disorder averaging, including a charge conjugation symmetry, so they behave as interacting models in topological class $\mathbf{D}$ enriched by crystalline rotation symmetry. The phase diagram of this model features a non-Fermi liquid at high temperature and an `exciton condensate' with nematic transport at low temperature. We present results from the free-energy, spectral functions, and the anomalous Hall resistivity as a function of temperature and tuning parameters. Our results are broadly relevant to correlated topological matter in multiorbital systems, and may also be viewed, with a suitable particle hole transformation, as an exploration of strong interaction effects on mean-field topological superconductors.

cond-mat.str-el↗

arXiv2023

Proximate Dirac spin liquid in honeycomb lattice $J_1$-$J_3$ XXZ model: Numerical study and application to cobaltates

Recent theoretical and experimental work suggest that the honeycomb cobaltates, initially proposed as candidate Kitaev quantum magnets, are in fact described by a pseudospin-$1/2$ easy-plane spin Hamiltonian with nearest neighbor ferromagnetic (FM) exchange $J_1$ being frustrated by antiferromagnetic third-neighbor exchange $J_3$ and weaker compass anisotropies. Using exact diagonalization and density-matrix renormalization group (DMRG) calculations, we show that this model exhibits FM order at small $J_3/J_1$ and zig-zag (ZZ) order at large $J_3/J_1$, separated by an intermediate phase, which we label as $\widetilde{\mathrm{SL}}$. This $\widetilde{\mathrm{SL}}$ phase is shown to exhibit spin-liquid-like correlations in DMRG, although we cannot preclude weak broken symmetries, e.g. weak Ising type Néel order, given the limits on our explored system sizes. Using a modified parton mean field theory and variational Monte Carlo on Gutzwiller projected wavefunctions, we show that the optimal FM and ZZ orders as well as the intermediate $\widetilde{\mathrm{SL}}$ state are proximate to a `parent' Dirac spin liquid (SL). This Dirac SL is shown to capture the broad continuum in the temperature and magnetic field dependent terahertz spectroscopy of BaCo$_2$(AsO$_4$)$_2$, and the reported low temperature metallic thermal conductivity in Na$_2$Co$_2$TeO$_6$ and BaCo$_2$(AsO$_4$)$_2$ upon incorporating disorder induced broadening.

cond-mat.str-el↗

arXiv2022

Chiral Broken Symmetry Descendants of the Kagomé Lattice Chiral Spin Liquid

The breaking of chiral and time-reversal symmetries provides a pathway to exotic quantum phenomena and topological phases. In particular, the breaking of chiral (mirror) symmetry in quantum materials has been shown to have important technological applications. Recent work has extensively explored the resulting emergence of chiral charge orders and chiral spin liquids on the kagomé lattice. Such chiral spin liquids are closely tied to bosonic fractional quantum Hall states and host anyonic quasiparticles; however, their connection to nearby magnetically ordered states has remained a mystery. Here, we show that two distinct non-coplanar magnetic orders with uniform spin chirality, the XYZ umbrella state and the Octahedral spin crystal, emerge as competing orders in close proximity to the kagomé chiral spin liquid. Our results highlight the intimate link between a many-body topologically ordered liquid and broken symmetry states with nontrivial real-space topology.

cond-mat.str-el↗

arXiv2021

Selection rules for the S-Matrix bootstrap

We examine the space of allowed S-matrices on the Adler zeros' plane using the recently resurrected (numerical) S-matrix bootstrap program for pion scattering. Two physical quantities, an averaged total scattering cross-section, and an averaged entanglement power for the boundary S-matrices, are studied. Emerging linearity in the leading Regge trajectory is correlated with a reduction in both these quantities. We identify two potentially viable regions where the S-matrices give decent agreement with low energy S- and P-wave scattering lengths and have leading Regge trajectory compatible with experiments. We also study the line of minimum averaged total cross section in the Adler zeros' plane. The Lovelace-Shapiro model, which was a precursor to modern string theory, is given by a straight line in the Adler zeros' plane and, quite remarkably, we find that this line intersects the space of allowed S-matrices near both these regions.

hep-th↗

arXiv2020

Relative entropy in scattering and the S-matrix bootstrap

We consider entanglement measures in 2-2 scattering in quantum field theories, focusing on relative entropy which distinguishes two different density matrices. Relative entropy is investigated in several cases which include $ϕ^4$ theory, chiral perturbation theory ($χPT$) describing pion scattering and dilaton scattering in type II superstring theory. We derive a high energy bound on the relative entropy using known bounds on the elastic differential cross-sections in massive QFTs. In $χPT$, relative entropy close to threshold has simple expressions in terms of ratios of scattering lengths. Definite sign properties are found for the relative entropy which are over and above the usual positivity of relative entropy in certain cases. We then turn to the recent numerical investigations of the S-matrix bootstrap in the context of pion scattering. By imposing these sign constraints and the $ρ$ resonance, we find restrictions on the allowed S-matrices. By performing hypothesis testing using relative entropy, we isolate two sets of S-matrices living on the boundary which give scattering lengths comparable to experiments but one of which is far from the 1-loop $χPT$ Adler zeros. We perform a preliminary analysis to constrain the allowed space further, using ideas involving positivity inside the extended Mandelstam region, and elastic unitarity.

hep-th↗