-
Emergent quasi-particles of spin-1 trimer chain
Authors:
Manodip Routh,
Anutosh Biswas,
Manoranjan Kumar
Abstract:
The recent experimental realization of emergent quasi-particles, such as spinons, doublons, and quartons, in a spin-$1/2$ trimer chain has spurred new interest in low dimensional magnetic systems. In this study, we investigate the dynamical properties of the isotropic spin-$1$ trimer chain with intra and inter-trimer antiferromagnetic exchange couplings, ($J >0$ and $J' >0$), respectively, unveili…
▽ More
The recent experimental realization of emergent quasi-particles, such as spinons, doublons, and quartons, in a spin-$1/2$ trimer chain has spurred new interest in low dimensional magnetic systems. In this study, we investigate the dynamical properties of the isotropic spin-$1$ trimer chain with intra and inter-trimer antiferromagnetic exchange couplings, ($J >0$ and $J' >0$), respectively, unveiling various quasi-particles: magnons, singletons, triplons, pentons, and heptons. For weak inter-trimer exchange coupling $J'/J \ll 1$, it behaves as an effective spin-$1$ chain with valence bond solid (VBS) ground state. Employing density matrix renormalization group (DMRG) techniques, we compute the dynamic structure factor (DSF) which reveals a gapped magnon band alongside weakly dispersive singleton, excited triplon, and penton excitations. The evolution of these excitations with inter-trimer coupling $J'$ is also examined, providing insight into the underlying excitation mechanisms. For spin-$1$ chain, these exotic quasi-particles eventually reduce to conventional magnon excitations as $J'/J \rightarrow 1$. Our results shed light on the rich and complex excitation spectrum of spin-$1$ trimer chains and offer unique perspectives on the dynamics in quantum spin systems.
△ Less
Submitted 22 September, 2025; v1 submitted 16 March, 2025;
originally announced March 2025.
-
Inter-chain Interactions, Multi-magnon condensation and Strain effect in chain compound NaVOPO$_4$
Authors:
Manoj Gupta,
Manodip Routh,
Manoranjan Kumar,
Tanusri Saha Dasgupta
Abstract:
Employing first-principles modelling and many-body methods, the magnetic properties of spin-1/2 chain compound NaVOPO$_4$ are explored. The extensive first-principles calculations establish an intricate three-dimensionally coupled model that consists of weakly alternating $J$-$J^{\prime}$ antiferromagnetic chains running along cris-cross directions between two consecutive $ab$ planes, connected vi…
▽ More
Employing first-principles modelling and many-body methods, the magnetic properties of spin-1/2 chain compound NaVOPO$_4$ are explored. The extensive first-principles calculations establish an intricate three-dimensionally coupled model that consists of weakly alternating $J$-$J^{\prime}$ antiferromagnetic chains running along cris-cross directions between two consecutive $ab$ planes, connected via two subleading couplings, a ferromagnetic exchange along the $c$ direction ($J_c$) and a weaker antiferromagnetic exchange ($J_a$) along the body diagonal direction. The exact diagonalization and density matrix renormalized group study has been carried out on a two-dimensional spin model with $J$-$J^{\prime}$-$J_c$ and effective $J_d$ couplings, constructed based on the full model, for numerical ease. The $J_c$-$J_d$ phase diagram is found to host a {\it disorder} phase with a finite spin gap for comparable values of $J_c$ and $J_d$, arising out of the competing nature of these two interactions, other than two ordered phases. The calculated thermodynamic properties of this model provide a fair description of experimentally measured data. The predominant manifestation of $J_c$ and $J_d$ in the disorder phase happens in the stabilisation of a multi-magnon condensed phase, upon gap closing by application of an external magnetic field. We further explore the effect of tensile uniaxial strain, which is found to drive the system from gapful to gapless ground state.
△ Less
Submitted 29 August, 2024; v1 submitted 15 March, 2024;
originally announced March 2024.
-
Emergent Quadrupolar Order in the Spin-$1/2$ Kitaev-Heisenberg Model
Authors:
Manodip Routh,
Sayan Ghosh,
Jeroen van den Brink,
Satoshi Nishimoto,
Manoranjan Kumar
Abstract:
Motivated by the largely unexplored domain of multi-polar ordered spin states in the Kitaev-Heisenberg (KH) systems we investigate the ground state dynamics of the spin-$\frac{1}{2}$ KH model, focusing on quadrupolar (QP) order in 2-leg ladder and two-dimensional honeycomb lattice geometries. Employing exact diagonalization and density-matrix renormalization group methods, we analyze the QP order…
▽ More
Motivated by the largely unexplored domain of multi-polar ordered spin states in the Kitaev-Heisenberg (KH) systems we investigate the ground state dynamics of the spin-$\frac{1}{2}$ KH model, focusing on quadrupolar (QP) order in 2-leg ladder and two-dimensional honeycomb lattice geometries. Employing exact diagonalization and density-matrix renormalization group methods, we analyze the QP order parameter and correlation functions. Our findings reveal a robust QP order across a wide range of the phase diagram, influenced by the interplay between Heisenberg and Kitaev interactions. Notably, we observe an enhancement of QP order near Kitaev quantum spin liquid (QSL) phases, despite the absence of long-range spin-spin correlations. This highlights a complex relationship between QP order and QSLs, offering new insights into quantum magnetism in low-dimensional systems. Our findings provide a rational explanation for the observed nonlinear magnetic susceptibility in $α$-RuCl$_3$.
△ Less
Submitted 21 January, 2024; v1 submitted 24 December, 2023;
originally announced December 2023.
-
Proximate Dirac spin liquid in honeycomb lattice $J_1$-$J_3$ XXZ model: Numerical study and application to cobaltates
Authors:
Anjishnu Bose,
Manodip Routh,
Sreekar Voleti,
Sudeep Kumar Saha,
Manoranjan Kumar,
Tanusri Saha-Dasgupta,
Arun Paramekanti
Abstract:
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 den…
▽ More
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.
△ Less
Submitted 3 November, 2023; v1 submitted 26 December, 2022;
originally announced December 2022.
-
Spin Peierls transition of $J_{1}-J_{2}$ and extended models with ferromagnetic $J_{1}$.Sublattice dimerization and thermodynamics of zigzag chains in $β$-TeVO$_{4}$
Authors:
Manodip Routh,
Sudip Kumar Saha,
Manoranjan Kumar,
Zoltán G. Soos
Abstract:
The spin$-1/2$ chain with ferromagnetic exchange $J_1 < 0$ between first neighbors and antiferromagnetic $J_2 > 0$ between second neighbors supports two spin-Peierls (SP) instabilities depending on the frustration $α= J_2/\vert J_1\vert$. Instead of chain dimerization with two spins per unit cell, $J_1-J_2$ models with $α> 0.65$ and linear spin-phonon coupling are unconditionally unstable to subla…
▽ More
The spin$-1/2$ chain with ferromagnetic exchange $J_1 < 0$ between first neighbors and antiferromagnetic $J_2 > 0$ between second neighbors supports two spin-Peierls (SP) instabilities depending on the frustration $α= J_2/\vert J_1\vert$. Instead of chain dimerization with two spins per unit cell, $J_1-J_2$ models with $α> 0.65$ and linear spin-phonon coupling are unconditionally unstable to sublattice dimerization with four spins per unit cell. Unequal $J_1$ to neighbors to the right and left extends the model to gapped ($γ> 0$) chains with conditional SP transitions at $T_{SP}$ to dimerized sublattices and a weaker specific heat $C(T)$ anomaly. The spin susceptibility $χ(T)$ and $C(T)$ are obtained in the thermodynamic limit by a combination of exact diagonalization of small systems with $α> 0.65$ and density matrix renormalization group (DMRG) calculations of systems up to $N \sim 100$ spins. Both $J_1-J_2$ and $γ> 0$ models account quantitatively for $χ(T)$ and $C(T)$ in the paramagnetic phase of $β$-TeVO$_{4}$ for $T > 8$ K, but lower $T$ indicates a gapped chain instead of a $J_1-J_2$ model as previously thought. The same parameters and $T_{SP} = 4.6$ K generate a $C(T)/T$ anomaly that reproduces the anomaly at the $4.6$ K transition of $β$-TeVO$_{4}$, but not the weak $χ(T)$ signature.
△ Less
Submitted 22 June, 2022;
originally announced June 2022.
-
Low temperature thermodynamics of the antiferromagnetic $J_1-J_2$ model: Entropy, critical points and spin gap
Authors:
Sudip Kumar Saha,
Manodip Routh,
Manoranjan Kumar,
Zoltán G. Soos
Abstract:
The antiferromagnetic $J_1-J_2$ model is a spin-1/2 chain with isotropic exchange $J_1 > 0$ between first neighbors and $J_2 = αJ_1$ between second neighbors. The model supports both gapless quantum phases with nondegenerate ground states and gapped phases with $Δ(α) > 0$ and doubly degenerate ground states. Exact thermodynamics is limited to $α= 0$, the linear Heisenberg antiferromagnet (HAF). Ex…
▽ More
The antiferromagnetic $J_1-J_2$ model is a spin-1/2 chain with isotropic exchange $J_1 > 0$ between first neighbors and $J_2 = αJ_1$ between second neighbors. The model supports both gapless quantum phases with nondegenerate ground states and gapped phases with $Δ(α) > 0$ and doubly degenerate ground states. Exact thermodynamics is limited to $α= 0$, the linear Heisenberg antiferromagnet (HAF). Exact diagonalization of small systems at frustration $α$ followed by density matrix renormalization group (DMRG) calculations returns the entropy density $S(T,α,N)$ and magnetic susceptibility $χ(T,α,N)$ of progressively larger systems up to $N = 96$ or 152 spins. Convergence to the thermodynamics limit, $S(T,α)$ or $χ(T,α)$, is demonstrated down to $T/J \sim 0.01$ in the sectors $α< 1$ and $α> 1$. $S(T,α)$ yields the critical points between gapless phases with $S^\prime(0,α) > 0$ and gapped phases with $S^\prime(0,α) = 0$. The $S^\prime(T,α)$ maximum at $T^*(α)$ is obtained directly in chains with large $Δ(α)$ and by extrapolation for small gaps. A phenomenological approximation for $S(T,α)$ down to $T = 0$ indicates power-law deviations $T^{-γ(α)}$ from $\exp(-Δ(α)/T)$ with exponent $γ(α)$ that increases with $α$. The $χ(T,α)$ analysis also yields power-law deviations, but with exponent $η(α)$ that decreases with $α$. $S(T,α)$ and the spin density $ρ(T,α) = 4Tχ(T,α)$ probe the thermal and magnetic fluctuations, respectively, of strongly correlated spin states. Gapless chains have constant $S(T,α)/ρ(T,α)$ for $T < 0.10$. Remarkably, the ratio decreases (increases) with $T$ in chains with large (small) $Δ(α)$.
△ Less
Submitted 28 June, 2021;
originally announced June 2021.