Two-dimensional Heisenberg Antiferromagnet Research Articles

Special transition and extraordinary phase on the surface of a two-dimensional quantum Heisenberg antiferromagnet

Continuous phase transitions exhibit richer critical phenomena on the surface than in the bulk, because distinct surface universality classes can be realized at the same bulk critical point by tuning the surface interactions. The exploration of surface critical behavior provides a window looking into higher-dimensional boundary conformal field theories. In this work, we study the surface critical behavior of a two-dimensional (2D) quantum critical Heisenberg model by tuning the surface coupling strength, and discover a direct special transition on the surface from the ordinary phase into an extraordinary phase. The extraordinary phase has a long-range antiferromagnetic order on the surface, in sharp contrast to the logarithmic decaying spin correlations in the 3D classical O(3) model. The special transition point has a new set of critical exponents, y_{s}=0.86(4)ys=0.86(4) and \eta_{\parallel}=-0.33(1)η∥=−0.33(1), which are distinct from the special transition of the classical O(3) model and indicate a new surface universality class of the 3D O(3) Wilson-Fisher theory.

Impact of organic amine cations on photoluminescence and magnetic properties in Dion-Jacobson hybrid manganese halide perovskites

A strenuous effort has been made to design multifunctional lead-free organic–inorganic hybrid (OIH) halide compounds, which are envisioned as next generation solar cell materials. However, it is challenging to design OIH halides that can exhibit both long-range magnetic ordering and high photoluminescence quantum yield (PLQY) since the dimensionality of the compounds has a contrasting effect on them. In this article, we have shown an approach to enhance PLQY in two-dimensional (2D) Heisenberg antiferromagnets by increasing the alkylene chain length of [H3N–(CH2)m–NH3]MnCl4 (m = 2, 3, and 4) compounds. All these compounds exhibit 2D layers of corner-sharing MnCl6 octahedra where the organic cations are intercalated between them. These compounds exhibit long-range antiferromagnetic ordering confirmed by the DC magnetic susceptibility and heat capacity measurements. The Néel temperature (TN) decreases with increasing the length of spacer cations due to a decrease in interlayer exchange interactions; however, interestingly, the lifetime of photoexcited electrons and PLQY enhances from 24 to 56 µs and 8% to 23%, respectively. Furthermore, the temperature-dependent photoluminescence measurements provide insight into thermal quenching and exciton binding energy. We believe this study can help to design new OIH halides with long-range magnetic ordering and high PLQY.

Nonequilibrium phase transition in a driven-dissipative quantum antiferromagnet

A deeper theoretical understanding of driven-dissipative interacting systems and their nonequilibrium phase transitions is essential both to advance our fundamental physics understanding and to harness technological opportunities arising from optically controlled quantum many-body states. This paper provides a numerical study of dynamical phases and the transitions between them in the nonequilibrium steady state of the prototypical two-dimensional Heisenberg antiferromagnet with drive and dissipation. We demonstrate a nonthermal transition that is characterized by a qualitative change in the magnon distribution, from subthermal at low drive to a generalized Bose-Einstein form including a nonvanishing condensate fraction at high drive. A finite-size analysis reveals static and dynamical critical scaling at the transition, with a discontinuous slope of the magnon number versus driving field strength and critical slowing down at the transition point. Implications for experiments on quantum materials and polariton condensates are discussed.

Supermagnonic Propagation in Two-Dimensional Antiferromagnets.

We investigate the propagation of magnons after ultrashort perturbations of the exchange interaction in the prototype two-dimensional Heisenberg antiferromagnet. Using the recently proposed neural quantum states, we predict highly anisotropic spreading in space constrained by the symmetry of the perturbation. Interestingly, the propagation speed at the shortest length scale and timescale is up to 40% higher than the highest magnon velocity. We argue that the enhancement stems from extraordinary strong magnon-magnon interactions, suggesting new avenues for manipulating information transfer on ultrashort length scales and timescales.

Spectral damping without quasiparticle decay: The dynamic structure factor of two-dimensional quantum Heisenberg antiferromagnets

Two-dimensional Heisenberg antiferromagnets play a central role in quantum magnetism, yet the nature of dynamic correlations in these systems at finite temperature has remained poorly understood for decades. We solve this long-standing problem by using a novel quantum-classical duality to calculate the dynamic structure factor analytically and, paradoxically, find a broad frequency spectrum despite the very long quasiparticle lifetime. The solution reveals new multi-scale physics whereby an external probe creates a classical radiation field containing infinitely-many quanta. Crucially, it is the multi-scale nature of this phenomenon which prevents a conventional renormalization group approach. We also challenge the common wisdom on static correlations and perform Monte Carlo simulations which demonstrate excellent agreement with our theory.

Temperature dependence of the anomaly in the excitation spectrum of the 2D quantum Heisenberg antiferromagnet

It is well established that in the low-temperature limit, the two-dimensional quantum Heisenberg antiferromagnet on a square lattice (2DQHAFSL) exhibits an anomaly in its spectrum at short-wavelengths on the zone-boundary. In the vicinity of the point the pole in the one-magnon response exhibits a downward dispersion, is heavily damped and attenuated, giving way to an isotropic continuum of excitations extending to high energies. The origin of the anomaly and the presence of the continuum are of current theoretical interest, with suggestions focused around the idea that the latter evidences the existence of spinons in a two-dimensional system. Here we present the results of neutron inelastic scattering experiments and Quantum Monte Carlo calculations on the metallo-organic compound Cu(DCOO)D2O (CFTD), an excellent physical realisation of the 2DQHAFSL, designed to investigate how the anomaly at evolves up to finite temperatures . Our data reveal that on warming the anomaly survives the loss of long-range, three-dimensional order, and that it is thus a robust feature of the two-dimensional system. With further increase of temperature the zone-boundary response gradually softens and broadens, washing out the anomaly. This is confirmed by a comparison of our data with the results of finite-temperature Quantum Monte Carlo simulations where the two are found to be in good accord. In the vicinity of the antiferromagnetic zone centre, there was no significant softening of the magnetic excitations over the range of temperatures investigated.

Spin-wave study of entanglement and Rényi entropy for coplanar and collinear magnetic orders in two-dimensional quantum Heisenberg antiferromagnets

We use modified linear spin-wave theory (MLSWT) to study ground-state entanglement for a length-$L$ line subsystem in $L\times L$ square- and triangular-lattice quantum Heisenberg antiferromagnets with coplanar spiral magnetic order with ordering vector $\mathbf{Q}=(q,q)$ and $N_G=3$ Goldstone modes, except if $q=\pi$ (collinear order, $N_G=2$). Generalizing earlier MLSWT results for $q=\pi$ to commensurate spiral order with $s\geq 3$ sublattices ($q=2\pi r/s$ with $r$ and $s$ coprime), we find analytically for large $L$ a universal and $n$-independent subleading term $(N_G/2)\ln L$ in the R\'{e}nyi entropy $S_n$, associated with $L^{1/2}$ scaling of $\lambda_0$ and $\lambda_{\pm q}$, with $\lambda_0\neq \lambda_{\pm q}$ for spiral order; here $\{\lambda_{k_y}\}$ are the $L$ mode occupation numbers of the entanglement Hamiltonian. The term $(3/2)\ln L$ in $S_n$ agrees with a nonlinear sigma model (NLSM) study of $s=3$ spiral order ($q=2\pi/3$). These and other properties of $S_n$ and $\lambda_{k_y}$ are explored numerically for an anisotropic nearest-neighbor triangular-lattice model for which $q$ varies in the spiral phase.

Nonordinary criticality at the edges of planar spin-1 Heisenberg antiferromagnets

Dangling edge spins of dimerized two-dimensional spin-1 Heisenberg antiferromagnets are shown to exhibit nonordinary quantum critical correlations, akin to the scaling behavior observed in recently explored spin-1/2 systems. Based on large-scale quantum Monte Carlo simulations, we observe remarkable similarities between these two cases, and also examine the crossover to the fundamentally distinct behavior in the one-dimensional limit of strongly coupled edge spins. We complement our numerical analysis by a cluster mean-field theory that encompasses the qualitatively similar behavior for the spin-1 and the spin-1/2 case, and its dependence on the spatial edge spin configuration in a generic way.

Extinction of the long-range order on site diluted quantum two-dimensional antiferromagnet in Bose–Einstein condensation

We study the two-dimensional Heisenberg antiferromagnet with ion single anisotropy in the square lattice in the presence of nonmagnetic impurities at T=0 using the SU(3) Schwinger boson theory. In particular we discuss the influence of site disorder on the quantum phase transition of this model at Dc that separates the Néel phase, D<Dc, which is gapless, from the disordered phase, gapped phase, D>Dc. We find that the long-range order in D<Dc for the model without impurities is destroyed for a concentration of nonmagnetic impurities xc≈0.15.

Effect of Dzyaloshinskii–Moriya interaction on quantum entanglement in superconductors models of high Tc

The modified spin-wave (MSW) theory and SU(N) Schwinger boson theory (SBW) are employed to study the quantum entanglement in one- (1D) and two-dimensional (2D) Heisenberg antiferromagnets with Dzyaloshinskii–Moriya (DM) interaction which are models to superconducting materials of high critical temperature T c such as La2 CuO4 . For the 1D case, we consider integer spin and for 2D case, since the behavior is independent on the spin value, we consider the one-half-spin and square lattice. We get the entanglement entropy in function of the temperature T where we have not gotten large variation of the quantum entanglement with the changing of the anisotropy Δ and DM interaction constant D .

Skyrmions and merons in two-dimensional antiferromagnetic systems

We have investigated (numerically) skyrmions constituted by two merons in two-dimensional Heisenberg antiferromagnets. The behavior and stability of these topological pseudo-particles are analyzed in discrete and finite lattices. The influence of lattice defects and external magnetic fields is also studied. The topological stability dictates that the two merons must be kept apart but processes of annihilation and fusion of merons are observed for finite lattices.

Effect of the Higgs mode in the spin transport of a Heisenberg antiferromagnet on a square lattice

We study spin transport at zero temperature in the two-dimensional spin-one Heisenberg antiferromagnet on a square lattice with easy-plane single-ion anisotropy in the Néel phase. The regular part of the spin conductivity is calculated at zero temperature using the SU(3) Schwinger boson formalism and the Kubo theory. Three magnon processes provide the dominant contribution to the spin conductivity. We find evidence of the Higgs mode contribution to the spin conductivity.

Renormalization of spin excitations in hexagonal HoMnO3 by magnon-phonon coupling

Hexagonal HoMnO3, a two-dimensional Heisenberg antiferromagnet, has been studied via inelastic neutron scattering. A simple Heisenberg model with a single-ion anisotropy describes most features of the spin-wave dispersion curves. However, there is shown to be a renormalization of the magnon energies located at around 11 meV. Since both the magnon-magnon interaction and magnon-phonon coupling can affect the renormalization in a noncollinear magnet, we have accounted for both of these couplings by using a Heisenberg XXZ model with 1=S expansions [1] and the Einstein site phonon model [13], respectively. This quantitative analysis leads to the conclusion that the renormalization effect primarily originates from the magnon-phonon coupling, while the spontaneous magnon decay due to the magnon-magnon interaction is suppressed by strong two-ion anisotropy.

Observation of ferroelastic domains in layered magnetic compounds using birefringence imaging

The two-dimensional Heisenberg antiferromagnet (C2H5NH3)2CuCl4 is a candidate compound for the coexistence of ferroelectricity and ferroelasticity; however, the microscopic observations of multiferroic domains may still be unclear. In-plane birefringence imaging measurements were performed to observe the manner in which the ferroelectric and the ferroelastic domains change during phase transitions between 15 K and 300 K. It was found that 90° ferroelastic domains appeared in the ab-plane at 300 K. As the temperature decreased toward 15 K, each domain inverted at a certain temperature (Ta) without structural or magnetic phase transitions. The value of Ta was found to be significantly influenced by external stresses; therefore, birefringence imaging techniques are useful for investigating variations in ferroelastic domains with temperature. Furthermore, a structural phase transition from orthorhombic to monoclinic or triclinic occurred at 230 ~ 240 K; however, no spontaneous polarization appeared in the ab-plane over the entire investigated range.

Entanglement in site diluted quantum two-dimensional antiferromagnet

The von Neumann entanglement entropy is studied for the two-dimensional Heisenberg antiferromagnet with ion single anisotropy in the square lattice and in the presence of nonmagnetic impurities, at T=0 using the SU(3) Schwinger boson theory. In particular, we discuss the influence of site disorder on the quantum entanglement of this model that is characterized by a quantum phase transition at Dc, that separates a Néel phase, D<Dc, which is gapless, from a disordered phase, gapped phase, D>Dc. We find that the quantum entanglement in D<Dc for the model is destroyed for a concentration of nonmagnetic impurities xc≈0.5.

Dzyaloshinsky–Moriya Interaction and the Ground State in S = 3/2 Perfect Kagome Lattice Antiferromagnet KCr3(OH)6(SO4)2 (Cr-Jarosite) Studied by X-Band and High-Frequency ESR

A single-crystal S = 3/2 perfect kagome lattice antiferromagnet, KCr3(OH)6(SO4)2 (Cr-jarosite), has been studied by X-band and high-frequency electron spin resonance (ESR). The g-values perpendicular to the kagome plane (c-axis) and in the plane were determined to be gc = 1.9704 ± 0.0002 and gξ = 1.9720 ± 0.0003, respectively, by high-frequency ESR observed at 265 K. Antiferromagnetic resonances (AFMRs) with an antiferromagnetic gap of 120 GHz were observed at 1.9 K, which is below TN = 4.5 K. The analysis of AFMR modes using the conventional molecular field theory gave dp = 0.27 K and dz = 0.07 K, where dp and dz are in-plane and out-of-plane components of d vector of the Dzyaloshinsky–Moriya (DM) interaction, respectively. On the basis of these results and the exchange interaction of J = 6.15 K estimated by Okuta et al., the ground state of Cr-jarosite was discussed in connection with the Monte Carlo simulation results with classical Heisenberg spins on the kagome lattice by Elhajal et al. Finally, the angular dependence of the linewidth and lineshape observed at 296 K by X-band ESR showed the typical behavior of a two-dimensional Heisenberg antiferromagnet, suggesting the good two-dimensionality of Cr-jarosite.

Influence of dilution in the spin transport in the quantum anisotropic two-dimensional Heisenberg antiferromagnet

We study the influence of the site disorder in the long range order and in the spin transport in the two-dimensional Heisenberg antiferromagnet with ion-single anisotropy, in the square lattice in T=0 using the SU(3) Schwinger boson theory. We analyze these properties in the regime of Bose–Einstein condensation, where the bosons tz are condensed: 〈tz〉=〈tz†〉=t. In particular, we discuss the influence of the site disorder in the spin transport of this model and in the critical properties, where Dc separates Néel's phase, D<Dc, gapless phase, of the disordered phase, gapped phase, D>Dc, in the spin conductivity. We find that the behavior of the long-range order for D<Dc in the model without impurities with the concentration of nonmagnetic impurities does not affect the spin conductivity.

Stabilization of Long-Range Order by Additional Anisotropic Spins in Two-Dimensional Isotropic Heisenberg Antiferromagnets —A Possible Model of an Organic Compound with Magnetic Anions—

We examine a two-dimensional (2D) coupled antiferromagnetic (AF) Heisenberg model that consists of two subsystems: an isotropic S = 1/2 spin subsystem with strong AF exchange interactions (main system), and a uniaxial S = 5/2 spin subsystem with weak exchange interactions. This model is an example in which additional semiclassical degrees of freedom affect a quantum system; it also describes a possible stabilization mechanism of AF long-range order (LRO) in the 2D organic compound λ-(BETS)2FeCl4, where BETS stands for bis(ethylenedithio)tetraselenafulvalene. Previous experimental studies have revealed that 3d spins on FeCl4 anions passively follow the AF LRO of the π-electron system in the BETS layers, although the AF LRO is stabilized by the 3d spins themselves. To explain this paradoxical behavior, we examine a scenario in which the uniaxial anisotropy of the 3d spins stabilizes the AF LRO on an isotropic 2D π-spin system. We extend Green’s function theory, called the Tyablikov approximation, to the present system, which describes spin-wave excitations and is consistent with the Mermin–Wagner theorem. It is shown that even extremely weak interactions with the uniaxial subsystem efficiently stabilize the AF LRO in the main system, even in the absence of AF exchange interactions in the uniaxial subsystem. The AF LRO is triggered by the uniaxial subsystem, but the sublattice magnetization remains smaller than that of the main system in the high-temperature region. These results are consistent with experimental data for λ-(BETS)2FeCl4 and λ-(BETS)2GaCl4; the latter does not have the 3d spins and does not exhibit the AF LRO.

Spin-half Heisenberg antiferromagnet on two archimedian lattices: From the bounce lattice to the maple-leaf lattice and beyond

We investigate the ground state of the two-dimensional Heisenberg antiferromagnet on two Archimedean lattices, namely, the maple-leaf and bounce lattices as well as a generalized $J$-$J'$ model interpolating between both systems by varying $J'/J$ from $J'/J=0$ (bounce limit) to $J'/J=1$ (maple-leaf limit) and beyond. We use the coupled cluster method to high orders of approximation and also exact diagonalization of finite-sized lattices to discuss the ground-state magnetic long-range order based on data for the ground-state energy, the magnetic order parameter, the spin-spin correlation functions as well as the pitch angle between neighboring spins. Our results indicate that the "pure" bounce ($J'/J=0$) and maple-leaf ($J'/J=1$) Heisenberg antiferromagnets are magnetically ordered, however, with a sublattice magnetization drastically reduced by frustration and quantum fluctuations. We found that magnetic long-range order is present in a wide parameter range $0 \le J'/J \lesssim J'_c/J $ and that the magnetic order parameter varies only weakly with $J'/J$. At $J'_c \approx 1.45 J$ a direct first-order transition to a quantum orthogonal-dimer singlet ground state without magnetic long-range order takes place. The orthogonal-dimer state is the exact ground state in this large-$J'$ regime, and so our model has similarities to the Shastry-Sutherland model. Finally, we use the exact diagonalization to investigate the magnetization curve. We a find a 1/3 magnetization plateau for $J'/J \gtrsim 1.07$ and another one at 2/3 of saturation emerging only at large $J'/J \gtrsim 3$.

Dynamics of the two-dimensional Heisenberg antiferromagnet in an external magnetic field

In this paper we study the dynamics of the two-dimensional Heisenberg antiferromagnet in an external magnetic field at zero temperature, using the memory function formalism. The imaginary part of the memory function, which is related to the damping, shows a one peak structure. The dynamical structure factor shows a smooth peak well defined separated from another very small peak.