We study a generalized honeycomb lattice spin-1/2 Heisenberg model with nearest-neighbor antiferromagnetic 2-spin exchange, and competing 4-spin interactions which serve to stabilize a staggered dimer state which breaks lattice rotational symmetry. Using a combination of quantum Monte Carlo numerics, spin wave theory, and bond operator theory, we show that this model undergoes a strong first-order transition between a Neel state and a staggered dimer state upon increasing the strength of the 4-spin interactions. We attribute the strong first order character of this transition to the spinless nature of the core of point-like Z(3) vortices obtained in the staggered dimer state. Unlike in the case of a columnar dimer state, disordering such vortices in the staggered dimer state does not naturally lead to magnetic order, suggesting that, in this model, the dimer and Neel order parameters should be thought of as independent fields as in conventional Landau theory.
Motivated by recent experiments on the spin-3/2 frustrated bilayer honeycomb antiferromagnet Bi$_3$Mn$_4$O$_{12}$(NO$_3$), we study the spin-S Heisenberg model on the honeycomb lattice with various additional exchange interactions which frustrate Néel order. Using spin wave theory, exact diagonalization, and bond operator theory, we consider the effects of (i) second-neighbor exchange, (ii) biquadratic exchange for $S = 3/2$ which leads to an AKLT valence bond solid, and (iii) bilayer coupling which leads to an interlayer dimer solid. We show that the resulting paramagnetic states undergo a tr…
We study a spin-1/2 SU(2) model on the honeycomb lattice with nearest-neighbor antiferromagnetic exchange $J$ that favors Néel order, and competing 6-spin interactions $Q$ which favor a valence bond solid (VBS) state in which the bond-energies order at the "columnar" wavevector ${\mathbf K} = (2π/3,-2π/3)$. We present quantum Monte-Carlo evidence for a direct continuous quantum phase transition between Néel and VBS states, with exponents and logarithmic violations of scaling consistent with those at analogous deconfined critical points on the square lattice. Although this strongly suggests a d…
We show the chiral d-wave RVB state on honeycomb lattice stands as a natural generalization of the staggered flux phase on square lattice. Although the state is generated from a time reversal symmetry broken mean field ansatz, it actually represents a fully symmetric spin liquid state with a positive definite wave function in the sense of Marshall sign rule for unfrustrated antiferromagnets. The evolution of the state with the parameter $Δ/χ$ follows exactly the same manner as that of the staggered flux phase on square lattice. The critical pairing strength corresponding to the $π$-flux phase …
Motivated by recent experiments on Bi$_3$Mn$_4$O$_{12}$(NO$_3$), and a broader interest arising from numerical work on the honeycomb lattice Hubbard model, we have studied the effect of a magnetic field on honeycomb lattice spin models with quantum paramagnetic ground states. For a model with frustrating second-neighbor exchange, $J_2$, we use a Lindemann-like criterion within spin wave theory to show that Néel order melts beyond a critical $J_2$. The critical $J_2$ increases with a magnetic field, implying the existence of a field-induced paramagnet-Néel transition over a range of $J_2$. We a…
Most of the searches for Kitaev materials deal with $4d/5d$ magnets with spin-orbit-coupled ${J=1/2}$ local moments such as iridates and $α$-RuCl$_3$. Here we propose the monoclinic YbCl$_3$ with a Yb$^{3+}$ honeycomb lattice for the exploration of Kiteav physics. We perform thermodynamic, $ac$ susceptibility, angle-dependent magnetic torque and neutron diffraction measurements on YbCl$_3$ single crystal. We find that the Yb$^{3+}$ ion exhibits a Kramers doublet ground state that gives rise to an effective spin ${J_{\text{eff}}=1/2}$ local moment. The compound exhibits short-range magnetic ord…
We use the coupled cluster method (CCM) to study a frustrated spin-$\frac{1}{2}$ $J_{1}$--$J_{2}$--$J_{1}^{\perp}$ Heisenberg antiferromagnet on a bilayer honeycomb lattice with $AA$ stacking. Both nearest-neighbor (NN) and frustrating next-nearest-neighbor antiferromagnetic (AFM) exchange interactions are present in each layer, with respective exchange coupling constants $J_{1}>0$ and $J_{2} \equiv κJ_{1} > 0$. The two layers are coupled with NN AFM exchanges with coupling strength $J_{1}^{\perp}\equiv δJ_{1}>0$. We calculate to high orders of approximation within the CCM the zero-field trans…
Motivated by recent numerical results, we study the quantum phase transitions between Z_2 spin liquid, Neel ordered, and various valence bond solid (VBS) states on the honeycomb and square lattices, with emphasis on the staggered VBS. In contrast to the well-understood columnar VBS order, the staggered VBS is not described by an XY order parameter with Z_N anisotropy close to these quantum phase transitions. Instead, we demonstrate that on the honeycomb lattice, the staggered VBS is more appropriately described as an O(3) or CP(2) order parameter with cubic anisotropy, while on the square latt…
Tuning superconducting pairing symmetry via a staggered potential in the doped honeycomb Hubbard model
School of Physics and Astronomy, Beijing Normal University（北京师范大学物理学院）；Key Laboratory of Multiscale Spin Physics (Beijing Normal University), Ministry of Education（教育部多尺度自旋物理重点实验室（北京师范大学））；College of Physics, Hebei Normal University（河北师范大学物理学院）；Hebei Advanced Thin Films Laboratory（河北省先进薄膜实验室）
We explore the magnetic properties of isolated a-b planes of trinuclear organometallic crystals, Mo3S7(dmit)3, in which an interplay of strong electronic correlations and spin molecular-orbital coupling (SMOC) occurs. The magnetic properties can be described by a XXZ+120, S=1 Heisenberg model on a honeycomb lattice with single-spin anisotropy, D, which depends strongly on SMOC. Based on ab initio estimates of SMOC in Mo3S7(dmit)3 crystals, we predict that the honeycomb layers of Mo3S7(dmit)3 are Neel ordered. However, in materials with a greater degree of magnetic frustration, Neel order can g…
Li2MnO3 with a S=3/2 two-dimensional Mn honeycomb lattice has a Neel-type antiferromagnetic transition at TN=36 K with a broad maximum in the magnetic susceptibility at TM=48 K. We have investigated site percolation effects by replacing Mn with nonmagnetic Ti, and completed a full phase diagram of Li2Mn1-xTixO3 solid solution systems to find that the antiferromagnetic transition is suppressed continuously without a clear sign of changes in the Neel-type antiferromagnetic structure. The magnetic ordering eventually disappears at a critical concentration of xc=0.7. This experimental observation …
We study magnetically ordered phases and their phase boundaries in the $J_1-J_2-J_3$ Heisenberg models on the honeycomb lattice using series expansions around Néel and different colinear and non-colinear magnetic states. An Ising anisotropy ($λ=J_{\perp}/J_z\ne 1$) is introduced and ground state energy and magnetization order parameter are calculated as a power seies expansion in $λ$. Series extrapolation methods are used to study properties for the Heisenberg model ($λ=1$). We find that at large $J_3$ ($>0.6$) there is a first order transition between Néel and columnar states, in agreement wi…
We have carried out high magnetic field studies of single-crystalline Li$_2$MnO$_3$, a honeycomb lattice antiferromagnet. Its magnetic phase diagram was mapped out using magnetization measurements at applied fields up to 35 T. Our results show that it undergoes two successive meta-magnetic transitions around 9 T fields applied perpendicular to the ab-plane (along the c*-axis). These phase transitions are completely absent in the magnetization measured with field applied along the ab-plane. In order to understand this magnetic phase diagram, we developed a mean-field model starting from the cor…
We show that a magnetic field in the high-symmetry direction lifts the macroscopic classical ground-state degeneracy of the honeycomb $Γ$ model and induces a long-range magnetic order. While a simple spin-polarized state is stabilized for the ferromagnetic $Γ$-exchange, a periodic $\sqrt{3}\times \sqrt{3}$ magnetic order is selected by magnetic field for the antiferromagnetic interaction. We show that the complex spin structure of the tripled unit cell can be described by the magnetization vector and a Néel order parameter, similar to those for the spin-flop state of a bipartite antiferromagne…
We employ the spin cluster perturbation theory to investigate the dynamical properties of the antiferromagnetic $J_{1}$-$J_{2}$ Heisenberg model on the honeycomb lattice. We obtain the excitation spectra for all possible phases in the phase diagram, including the Néel phase, plaquette valence-bond-solid phase, dimer valence-bond-solid phase and stripe antiferromagnetic phase. In the Néel phase, besides the obvious renormalization of the magnon dispersion, we find that the spectrum exhibits a dome-shaped broad continuum around the second Brillouin zone (BZ) and the additional strong continuum c…
We use the coupled cluster method to study the zero-temperature quantum phase diagram of the spin-$\frac{1}{2}$ $J_{1}$--$J_{2}$--$J_{1}^{\perp}$ model on the honeycomb bilayer lattice. In each layer we include both nearest-neighbor and frustrating next-nearest-neighbor antiferromagnetic exchange couplings, of strength $J_{1}>0$ and $J_{2} \equiv κJ_{1} > 0$, respectively. The two layers are coupled by an interlayer nearest-neighbor exchange, with coupling constant $J_{1}^{\perp} \equiv δJ_{1}>0$. We calculate directly in the infinite-lattice limit both the ground-state energy per spin and the…
Inspired by the recently theoretical development relevant to the experimental data of TlCuCl$_3$, particularly those associated with the universal scaling between the Néel temperature $T_N$ and the staggered magnetization density $M_s$, we carry a detailed investigation of 3-dimensional (3D) dimerized quantum antiferromagnets using the first principles quantum Monte Carlo calculations. The motivation behind our study is to better understand the microscopic effects on these scaling relations of $T_N$ and $M_s$, hence to shed some light on some of the observed inconsistency between the theoretic…
Using the coupled cluster method we study the phase diagram of the spin-1/2 Heisenberg antiferromagnet on a honeycomb lattice with nearest-neighbor exchange coupling $J_{1}>0$ and frustrating next-nearest-neighbor coupling $J_{2} \equiv xJ_{1}>0$. In the range $0<x<1$ we find four phases exhibiting respectively Néel, 6-spin plaquette, staggered dimer, and Néel-II orderings, with quantum critical points at $x_{c_{1}} \approx 0.207(3), x_{c_{2}} \approx 0.385(10)$, and $x_{c_{3}} \approx 0.65(5)$. The transitions at $x_{c_{1}}$ and $x_{c_{3}}$ appear to be continuous (and hence deconfined) ones,…
Puzzled by the indication of a new critical theory for the spin-1/2 Heisenberg model with a spatially staggered anisotropy on the square lattice as suggested in \cite{Wenzel08}, we study a similar anisotropic spin-1/2 Heisenberg model on the honeycomb lattice. The critical point where the phase transition occurs due to the dimerization as well as the critical exponent $ν$ are analyzed in great detail. Remarkly, using most of the available data points in conjunction with the expected finite-size scaling ansatz with a sub-leading correction indeed leads to a consistent $ν= 0.691(2)$ with that ca…
Recent work shows that a quantum spin liquid can arise in realistic fermionic models on a honeycomb lattice. We study the quantum spin-1/2 Heisenberg honeycomb model, considering couplings J1, J2, and J3 up to third nearest neighbors. We use an unbiased pseudofermion functional renormalization group method to compute the magnetic susceptibility and determine the ordered and disordered states of the model. Aside from antiferromagnetic, collinear, and spiral order domains, we find a large paramagnetic region at intermediate J2 coupling. For larger J2 within this domain, we find a strong tendency…
Single crystals of SrMn2P2 and CaMn2P2 were grown using Sn flux and characterized by single-crystal x-ray diffraction, electrical resistivity rho, heat capacity Cp, and magnetic susceptibility chi = M/H measurements versus temperature T and magnetization M versus applied magnetic field H isotherm measurements. The x-ray diffraction results show that both compounds adopt the trigonal CaAl2Si2-type structure. The rho(T) measurements demonstrate insulating ground states for both compounds. The chi(T) and Cp(T) data reveal a weak first-order antiferromagnetic (AFM) transition at the Neel temperatu…
By using a variational Monte Carlo technique based upon Gutzwiller-projected fermionic states, we investigate the dynamical structure factor of the antiferromagnetic $S=1/2$ Heisenberg model on the honeycomb lattice, in presence of first-neighbor ($J_1$) and second-neighbor ($J_2$) couplings, for ${J_2 < 0.5 J_1}$. The ground state of the system shows long-range antiferromagnetic order for ${J_2/J_1 \lesssim 0.23}$, plaquette valence-bond order for ${0.23 \lesssim J_2/J_1 \lesssim 0.36}$, and columnar dimer order for ${J_2/J_1 \gtrsim 0.36}$. Within the antiferromagnetic state, a well-defined …
The zero-temperature quantum phase diagram of the spin-$\frac{1}{2}$ $J_{1}$--$J_{2}$--$J_{1}^{\perp}$ model on an $AA$-stacked bilayer honeycomb lattice is investigated using the coupled cluster method (CCM). The model comprises two monolayers in each of which the spins, residing on honeycomb-lattice sites, interact via both nearest-neighbor (NN) and frustrating next-nearest-neighbor isotropic antiferromagnetic (AFM) Heisenberg exchange iteractions, with respective strengths $J_{1} > 0$ and $J_{2} \equiv κJ_{1}>0$. The two layers are coupled via a comparable Heisenberg exchange interaction be…
We consider the $S=1/2$ Heisenberg model with nearest-neighbor interaction $J$ and an additional multi-spin interaction $Q_3$ on the square lattice. The $Q_3$ term consists of three bond-singlet projectors and is chosen to favor the formation of a valence-bond solid (VBS) where the valence bonds (singlet pairs) form a staggered pattern. The model exhibits a quantum phase transition from the Néel state to the VBS as a function of $Q_3/J$. We study the model using quantum Monte Carlo (stochastic series expansion) simulations. The Néel-VBS transition in this case is strongly first-order in nature…
Frustrated spin-$\frac{1}{2}$ Heisenberg magnet on an $AA$-stacked honeycomb bilayer: High-order study of the collinear magnetic phases of the $J_{1}$--$J_{2}$--$J_{1}^{\perp}$ model
We study the ground-state (gs) phase diagram of the frustrated spin-1/2 $J_{1}$-$J_{2}$-$J_{3}$ antiferromagnet with $J_{2} = J_{3} =κJ_1$ on the honeycomb lattice, using coupled-cluster theory and exact diagonalization methods. We present results for the gs energy, magnetic order parameter, spin-spin correlation function, and plaquette valence-bond crystal (PVBC) susceptibility. We find a Néel antiferromagnetic (AFM) phase for $κ< κ_{c_{1}} \approx 0.47$, a collinear striped AFM phase for $κ> κ_{c_{2}} \approx 0.60$, and a paramagnetic PVBC phase for $κ_{c_{1}} \lesssim κ\lesssim κ_{c_{2}}$. …
The spin-1/2 Heisenberg antiferromagnet on the distorted honeycomb (DHC) lattice is studied by means of the tensor renormalization group method. It is unveiled that the system has a quantum phase transition of second-order between the gapped quantum dimer phase and a collinear Neel phase at the critical point of coupling ratio α_{c} = 0.54, where the quantum critical exponents ν= 0.69(2) and γ= 1.363(8) are obtained. The quantum criticality is found to fall into the O(3) universality class. A ground-state phase diagram in the field-coupling ratio plane is proposed, where the phases such as the…
The ground state phase diagram of the dimerized spin-1/2 XX honeycomb model in presence of a transverse magnetic field (TF) is known. With the absence of the magnetic field, two quantum phases, namely, the Néel and the dimerized phases have been identified. Moreover, canted Néel and the paramagnetic (PM) phases also emerge by applying the magnetic field. In this paper, using two complementary numerical exact techniques, Lanczos exact diagonalization, and Density matrix renormalization group (DMRG) methods, we study this model by focusing on the quantum correlations, the concurrence, and the qu…
We use Quantum Monte-Carlo methods to study the ground state phase diagram of a S=1/2 honeycomb lattice magnet in which a nearest-neighbor antiferromagnetic exchange J (favoring Néel order) competes with two different multi-spin interaction terms: a six-spin interaction Q_3 that favors columnar valence-bond solid (VBS) order, and a four-spin interaction Q_2 that favors staggered VBS order. For Q_3 ~ Q_2 >> J, we establish that the competition between the two different VBS orders stabilizes Néel order in a large swathe of the phase diagram even when J is the smallest energy-scale in the Hamilto…
On the precise boundary of the Néel antiferromagnetic phase in the Shastry-Sutherland model
Key Laboratory of Artificial Structures and Quantum Control (Ministry of Education), School of Physics and Astronomy, Shanghai Jiao Tong University（上海交通大学物理与天文学院人工结构与量子控制重点实验室（教育部））；Tsung-Dao Lee Institute, Shanghai Jiao Tong University（上海交通大学汤川研究所）；Hefei National Laboratory（合肥国家实验室）
High field electron spin resonance, nuclear magnetic resonance and magnetization studies addressing the ground state of the quasi two-dimensional spin-1/2 honeycomb lattice compound InCu{2/3}V{1/3}O{3} are reported. Uncorrelated finite size structural domains occurring in the honeycomb planes are expected to inhibit long range magnetic order. Surprisingly, ESR data reveal the development of two collinear antiferromagnetic (AFM) sublattices below ~ 20 K whereas NMR results show the presence of the staggered internal field. Magnetization data evidence a spin reorientation transition at ~ 5.7 T. …
Heavy fermion systems represent a prototypical setting to study magnetic quantum phase transitions. A particular focus has been on the physics of Kondo destruction, which captures quantum criticality beyond the Landau framework of order-parameter fluctuations. In this context, we study the spin one-half Kondo-Heisenberg model on a honeycomb lattice at half filling. The problem is approached from the Kondo destroyed, antiferromagnetically ordered insulating phase. We describe the local moments in terms of a coarse grained quantum non-linear sigma model, and show that the skyrmion defects of the…
Motivated by recent experiments with ultra-cold quantum gases in optical lattices we study the decay of the staggered moment in the one-dimensional Fermi-Hubbard model starting from a perfect Neel state using exact diagonalization and the iTEBD method. This extends previous work in which the same problem has been addressed for pure spin Hamiltonians. As a main result, we show that the relaxation dynamics of the double occupancy and of the staggered moment are different. The former is controlled by the nearest-neighbor tunneling rate while the latter is much slower and strongly dependent on the…
We investigate effects of doped holes' hopping on Neel order in the two-dimensional Hubbard model. Semiclassical staggered moments are computed by solving saddle point equations derived from a path-integral formalism. Effects of quantum fluctuations are taken into account by the Schwinger boson mean field theory. We argue that hopping of doped holes is ineffective in suppressing Neel order compared to rapid supprestion of Neel order in high-temperature superconductors. After destruction of Neel order, the quantum disordered phase sets in. Taking the strong coupling limit in the quantum disorde…
We present inelastic neutron scattering measurements of the antiferromagnetic insulator CaMn$_2$Sb$_2$, which consists of corrugated honeycomb layers of Mn. The dispersion of magnetic excitations has been measured along the H and L directions in reciprocal space, with a maximum excitation energy of $\approx$ 24 meV. These excitations are well described by spin waves in a Heisenberg model, including first and second neighbor exchange interactions, J$_{1}$ and J$_{2}$, in the Mn plane and also an exchange interaction between planes. The determined ratio J$_{2}$/J$_{1}$ $\approx$ 1/6 suggests tha…
Using the density-matrix renormalization group, we determine the phase diagram of the spin 1/2 Heisenberg antiferromagnet on a honeycomb lattice with a nearest neighbor interaction J1 and a frustrating, next-neighbor exchange J2. As frustration increases, the ground state exhibits Neel, plaquette and dimer orders, with critical points at J2/J1 = 0.22 and 0.35. We observe that both the spin gap and the corresponding order parameters vanish continuously at both the critical points, indicating the presence of deconfined quantum criticality.
The continuous imaginary-time quantum Monte Carlo method with the worm update algorithm is applied to explore the ground state properties of the spin-1/2 Heisenberg model with antiferromagnetic (AF) coupling $J>0$ and ferromagnetic (F) coupling $J^{\prime}<0$ along zigzag and armchair directions, respectively, on honeycomb lattice. It is found that by enhancing the F coupling $J^{\prime}$ between zigzag AF chains, the system is smoothly crossover from one-dimensional zigzag spin chains to a two-dimensional magnetic ordered state. In absence of an external field, the system is in a stripe order…
We study the ground-state phase diagram of the frustrated quantum $J_1-J_2$ Heisenberg antiferromagnet on the honeycomb lattice using a mean field approach in terms of the Schwinger boson representation of the spin operators. We present results for the ground-state energy, local magnetization, energy gap and spin-spin correlations. The system shows magnetic long range order for $0\leq J_{2}/J_{1}\lesssim 0.2075$ (Néel) and $0.398\lesssim J_{2}/J_{1}\leq 0.5$ (spiral). In the intermediate region, we find two magnetically disordered phases: a gapped spin liquid phase which shows short-range Néel…
We present an extensive quantum Monte Carlo study of the Néel-valence bond solid (VBS) phase transition on rectangular and honeycomb lattice SU($N$) antiferromagnets in sign problem free models. We find that in contrast to the honeycomb lattice and previously studied square lattice systems, on the rectangular lattice for small $N$ a first order Néel-VBS transition is realized. On increasing $N\geq 4$, we observe that the transition becomes continuous and with the {\em same} universal exponents as found on the honeycomb and square lattices (studied here for $N=5,7,10$), providing strong support…
Within the linear flavor-wave theory, we show that, caused by quantum order-by-disorder mechanism, the spin-1 bilinear-biquadratic Heisenberg model defined on a honeycomb lattice can spontaneously develop a columnar dimer order with a non-bipartite structure. The low-lying excitations above this novel ground state form several flat bands separated by nonzero energy gaps. Our results suggest that the quantum phase transition separating this dimerized phase with the nearby Néel-order one may be of first order.
The ground-state properties of the square-lattice spin-1/2 Heisenberg antiferromagnet with spatially anisotropic couplings are investigated by Green's-function projection approaches. The staggered magnetization and the two-spin correlators are calculated; the competition between magnetic long- and short-range order is discussed in comparison with experiments on $\rm Sr_2[Ca_2]CuO_3$.
We study the Neel temperature of quasi one-dimensional S=1/2 antiferromagnets containing non-magnetic impurities. We first consider the temperature dependence of the staggered susceptibility of finite chains with open boundary conditions, which shows an interesting difference for even and odd length chains. We then use a mean field theory treatment to incorporate the three dimensional inter-chain couplings. The resulting Neel temperature shows a pronounced drop as a function of doping by up to a factor of 5.
We use a combination of analytical and numerical techniques to study the phase diagram of the frustrated Heisenberg model on the bilayer honeycomb lattice. Using the Schwinger boson description of the spin operators followed by a mean field decoupling, the magnetic phase diagram is studied as a function of the frustration coupling $J_{2}$ and the interlayer coupling $J_{\bot}$. The presence of both magnetically ordered and disordered phases is investigated by means of the evaluation of ground-state energy, spin gap, local magnetization and spin-spin correlations. We observe a phase with a spin…
Using variational wave functions and Monte Carlo techniques, we study the antiferromagnetic Heisenberg model with first-neighbor $J_1$ and second-neighbor $J_2$ antiferromagnetic couplings on the honeycomb lattice. We perform a systematic comparison of magnetically ordered and nonmagnetic states (spin liquids and valence-bond solids) to obtain the ground-state phase diagram. Néel order is stabilized for small values of the frustrating second-neighbor coupling. Increasing the ratio $J_2/J_1$, we find strong evidence for a continuous transition to a nonmagnetic phase at $J_2/J_1 \approx 0.23$. C…
The magnetic behaviors of a spin-1/2 alternating Heisenberg antiferromagnetic chain in a staggered transverse magnetic field is studied by means of the density-matrix renormalization group method and Jordan-Wigner transformation. Quantum phase transitions of different types are observed in the S=1 Neel and XY-like gapless phases, which result from the competitions between the staggered transverse field and magnetic orders induced by anisotropy and alternating interactions. The results are compared with the mean-field and some exactly resolved results.
We investigate the behavior of antiferromagnetic integer-spin chains in a staggered magnetic field, by means of the density-matrix renormalization group, carefully addressing the role of finite-size effects within the Haldane phase at small fields. In the case of spin S=2, we determine the dependence of the groundstate energy and magnetization on the external field, in the thermodynamic limit, and show how the peculiar finite-size behavior can be connected with the crossover in the groundstate from a spin liquid to a polarized Néel state.
We consider the quasi-two-dimensional pseudo-spin-1/2 Kitaev - Heisenberg model proposed for A2IrO3 (A=Li, Na) compounds. The spin-wave excitation spectrum, the sublattice magnetization, and the transition temperatures are calculated in the random phase approximation (RPA) for four different ordered phases, observed in the parameter space of the model: antiferomagnetic, stripe, ferromagnetic, and zigzag phases. The Néel temperature and temperature dependence of the sublattice magnetization are compared with the experimental data on Na2IrO3.
The core of the vortex in the Neel order parameter for an easy-plane antiferromagnet on honeycomb lattice is demonstrated to bind two zero-energy states. Remarkably, a single electron occupying this mid-gap band has its spin fragmented between the two sublattices: Whereas it yields a vanishing total magnetization it shows a finite Neel order, orthogonal to the one of the assumed background. The requisite easy-plane anisotropy may be introduced by a magnetic field parallel to the graphene layer, for example. The results are relevant for spin-1/2 fermions on graphene's or optical honeycomb latti…
We investigate a microscopic origin of nonreciprocal magnon that is distinct from the Dzyaloshinskii-Moriya interaction in a honeycomb antiferromagnet. The key ingredient is a symmetric anisotropic exchange interaction depending on the bond direction, which results in a valley-type nonreciprocal magnon excitations under the staggered antiferromagnetic ordering. Furthermore, we find this type of nonreciprocal magnon exhibits a peculiar magnetic-field response; the nonreciprocal direction can be manipulated by the in-plane rotating magnetic field. The obtained results can be accounted for the em…
In the present paper we present some new data supporting the existence of a spin-disordered phase in the Heisenberg model on the honeycomb lattice with antiferromagnetic interactions up to third neighbors along the line J2=J3, predicted in [Phys. Rev. B 83, 094506 (2011)]. We use the Schwinger boson technique followed by a mean field decoupling and exact diagonalization for small systems to show the existence of an intermediate phase with a spin gap and short range Néel correlations in the strong quantum limit (S=1/2).
We calculate the two-magnon Raman scattering spectra in antiferromagnetic phases of several frustrated spin models defined on the honeycomb lattice. These include the Néel antiferromagnetic phase of a $J_1$-$J_2$-$J_3$ model and the stripe phase of the Heisenberg-Kitaev model. We show that both the magnetic frustration and the anisotropy of interactions may significantly affect the Raman spectra. We further discuss the implications of our results to the magnetic excitations of the iron-based compound BaFe$_2$Se$_2$O and show how the magnetic interactions can be extracted from fit to the Raman …
We explore the effect of local charge fluctuations on the spin response of a Mott insulator by deriving an effective spin model, and studying it using Schwinger boson mean field theory. Applying this to La$_2$CuO$_4$, we show that an accurate fit to the magnon dispersion relation, measured by Coldea {\em et al.} [Phys. Rev. Lett. {\bf 86}, 5377 (2001)] is obtained with Hubbard model parameters $U \approx 2.34 eV$, and $t \approx 360 meV$. These parameters lead to estimates of the staggered magnetization ($m_s \approx 0.25$), spin wave velocity ($c\approx 800 meV$-Å), and spin stiffness ($ρ_s \…
We study the ground-state phase diagram of the quantum $J_1-J_2$ model on the honeycomb lattice by means of an entangled-plaquette variational ansatz. Values of energy and relevant order parameters are computed in the range $0\le {J_2}/{J_1} \le 1$. The system displays classical order for ${J_2}/{J_1}\lesssim 0.2$ (Néel), and for $J_2/J_1 \gtrsim 0.4$ (collinear). In the intermediate region, the ground-state is disordered. Our results show that the reduction of the half-filled Hubbard model to the model studied here does not yield accurate predictions.
We investigate the spin-1/2 Heisenberg model on a rectangular lattice, using the Gutzwiller projected variational wave function known as the staggered flux state. Using Monte Carlo techniques, the variational parameters and static spin-structure factor for different coupling anisotropies $γ=J_y/J_x$ are calculated. We observe a gradual evolution of the ground state energy towards a value which is very close to the 1D estimate provided by the Bethe ansatz and a good agreement between the finite size scaling of the energies. The spin-spin correlation functions exhibit a power-law decay with vary…
We discuss the effect of site dilution on both the magnetization and the density of states of quantum spins in the honeycomb lattice, described by the antiferromagnetic Heisenberg spin-S model. For this purpose a real-space Bogoliubov-Valatin transformation is used. In this work we show that for the S>1/2 the system can be analyzed in terms of linear spin wave theory. For spin S=1/2, however, the linear spin wave approximation breaks down. In this case, we have studied the effect of dilution on the staggered magnetization using the Stochastic Series Expansion Monte Carlo method. Two main resul…
We use density matrix renormalization group (DMRG) algorithm to study the phase diagram of the spin-1/2 Heisenberg model on honeycomb lattice with first (J1) and second (J2) neighbor antiferromagnetic interactions, where a Z2 spin liquid region has been proposed. By implementing SU(2) symmetry in the DMRG code, we are able to obtain accurate results for long cylinders with width slightly over 15 lattice spacings and torus up to the size N=2*6*6. With increasing J2, we find a Neel phase with vanishing spin gap and a plaquette valence-bond (PVB) phase with non-zero spin gap. By extrapolating the…
