Magnetoelectric effects in honeycomb antiferromagnet Co4Nb2O9 are investigated on the basis of symmetry analyses of Co ions in trigonal P-3c1 space group. For each Co ion, the possible spin dependence is classified by C3 point-group symmetry. This accounts for the observed main effect that an electric polarization rotates in the opposite direction at the twice speed relative to the rotation of the external magnetic field applied in the ab-plane. Inversion centers and twofold axes in the unit cell restrict the active spin-dependence of the electric polarization, which well explains the observed experimental results. Expected optical properties of quadrupolar excitation and various types of dichroism are also discussed.
The honeycomb antiferromagnet Co4Nb2O9 is known to exhibit an interesting magnetoelectric effect that the electric polarization rotates at the twice speed in the opposite direction relative to the rotation of the external magnetic field applied in the basal ab-plane. The spin-dependent electric dipole can be an origin of the magnetoelectric effect. It is described by the product of spin operators at different sites (type-I theory) or at the same site (type-II theory). We examine the electric polarization for the two cases on the basis of the symmetry analysis of the crystal structure of Co4Nb2…
By combining single crystal x-ray and neutron diffraction, and the magnetodielectric measurements on single crystal Fe4Nb2O9, we present the magnetic structure and the symmetry-allowed magnetoelectric coupling in Fe4Nb2O9. It undergoes an antiferromagnetic transition at TN=93 K, followed by a displacive transition at TS=70 K. The temperature-dependent dielectric constant of Fe4Nb2O9 is strongly anisotropic with the first anomaly at 93 K due to the exchange striction as a result of the long range spin order, and the second one at 70 K emanating from the structural phase transition primarily dri…
Through analysis of single crystal neutron diffraction data, we present the magnetic structures of magnetoelectric Co4Nb2O9 under various magnetic fields. In zero-field, neutron diffraction experiments below TN=27 K reveal that the Co2+ moments order primarily along the a* direction without any spin canting along the c axis, manifested by the magnetic symmetry C2/c'. The moments of nearest neighbor Co atoms order ferromagnetically with a small cant away from the next nearest neighbor Co moments along the c axis. In the applied magnetic field H//a, three magnetic domains were aligned with their…
Magnetoelectric phenomena are intimately linked to relativistic effects and also require the material to break spatial inversion symmetry and time reversal invariance. Magnetoelectric coupling can substantially affect light-matter interaction and lead to non-reciprocal light propagation. Here, we confirm on a fully experimental basis, without invoking either symmetry-based or material-specific assumptions, that the optical magnetoelectric effect in materials with non-parallel magnetization ($\boldsymbol{M}$) and electric polarization ($\boldsymbol{P}$) generates a trilinear term in the refract…
We study the effects of static magnetic and electric fields on Kitaev's honeycomb model. Using the electric polarization operator appropriate for Kitaev materials, we derive the effective Hamiltonian for the emergent Majorana fermions to second-order in both the electric and magnetic fields. We find that while individually each perturbation does not qualitatively alter Kitaev spin liquid, the cross-term induces a finite chemical potential at each Dirac node, giving rise to a Majorana-Fermi surface. We argue this gapless phase is stable and exhibits typical metallic phenomenology, such as linea…
Co4Ta2O9 exhibits a three-dimensional magnetic lattice based on the buckled honeycomb motif. It shows unusual magnetoelectric effects, including the sign change and non-linearity. These effects cannot be understood without the detailed knowledge of the magnetic structure. Herein, we report neutron diffraction and direction-dependent magnetic susceptibility measurements on Co4Ta2O9 single crystals. Below 20.3 K, we find a long-range antiferromagnetic order in the alternating buckled and flat honeycomb layers of Co2+ ions stacked along the c axis. Within experimental accuracy, the magnetic momen…
We report the discovery of linear magnetoelectric effect in the well-known green phase compound, Sm2BaCuO5, which crystallizes in the centrosymmetric orthorhombic (Pnma) structure. Magnetization and specific heat measurements reveal the long-range antiferromagnetic ordering of Cu2+ and Sm3+-ions moments at TN1 = 23 K and TN2 = 5 K, respectively. Applied magnetic field induces dielectric anomaly at TN1 whose magnitude increases with field, which results in significant (1.7%) magnetocapacitance effect. On the other hand, the dielectric anomaly observed in zero-applied magnetic field at TN2 shows…
We show that strong electromagnon peaks can be found in absorption spectra of non-collinear magnets exhibiting a linear magnetoelectric effect. The frequencies of these peaks coincide with the frequencies of antiferromagnetic resonances and the ratio of the spectral weights of the electromagnon and antiferromagnetic resonance is related to the ratio of the static magnetoelectric constant and magnetic susceptibility. Using a Kagome lattice antiferromagnet as an example, we show that frustration of spin ordering gives rise to magnetoelastic instabilities at strong spin-lattice coupling, which tr…
Local multiferroic ordering at room temperature in collinear magnetoelectric antiferromagnets induced by flexo-Zeeman coupling
Helmholtz-Zentrum Dresden-Rossendorf e.V.（德累斯顿-罗森多夫亥姆霍兹中心）；Faculty of Physics of Dresden University of Technology（德累斯顿工业大学物理学院）；Helmholtz-Zentrum Berlin für Materialien und Energie（柏林亥姆霍兹材料与能源中心）；Department of Applied Physics, KTH Royal Institute of Technology（瑞典皇家理工学院应用物理系）
Imaging antiferromagnetic domains in LiCoPO$_4$ via the optical magnetoelectric effect
Budapest University of Technology and Economics（布达佩斯技术与经济大学）；RIKEN Center for Emergent Matter Science (CEMS)（理化学研究所新兴物质科学中心）；Leibniz IFW Dresden（莱布尼茨德累斯顿固体研究所）；University of Tokyo（东京大学）；Tokyo College, University of Tokyo（东京大学东京学院）；HUN-REN–BME Condensed Matter Physics Research Group, Budapest University of Technology and Economics（匈牙利研究与教育网络-布达佩斯技术与经济大学凝聚态物理研究组）；University of Augsburg（奥格斯堡大学）
All-optical control of antiferromagnetic domains via an inverse optical magnetoelectric effect
RIKEN Center for Emergent Matter Science (CEMS)（RIKEN 新兴物质科学中心）；Institut für Festkörperforschung（固体研究所）；Department of Advanced Materials Science, The University of Tokyo（东京大学先进材料科学系）；Center for Electronic Correlations and Magnetism, University of Augsburg（奥格斯堡大学电子关联与磁性中心）；Tokyo College and department of Applied Physics, The University of Tokyo（东京大学东京学院与应用物理系）
In the present study, we investigated magnetodielectric, magnetoelectric (ME), and angular-dependent polarization in single-crystal Fe4Nb2O9. The magnetodielectric effects in epsilon(x) (x//[100]), epsilon(y) (y//[120]), and epsilon(z) (z//[001]) are found to be significant only around T-N approximate to 95 K when magnetic fields are applied along three orthogonal x-, y- (y//[120]), and z directions. The finite polarization P-x, P-y, and P-z of 70, 100, and 30 mu C/m(2), respectively, can be induced in the antiferromagnetic phase when a finite magnetic field up to 9 T is applied along the thre…
We study the magnetocapacitance (MC) effect and magnetoelectric (ME) coupling in spin-flop driven antiferromagnet Co4Ta2O9. The magnetocapacitance data at high magnetic fields are analyzed by phenomenological Ginzburg-landau theory of ferroelectromagnets and it is found that change in dielectric constant is proportional to the square of magnetization. The saturation polarization and magnetoelectric coupling are estimated to be 52microC/m2 and $γ$ = 1.4 x10-3 (emu/g)-2 respectively at 6 Tesla. Electric polarization is achieved below Neel temperature only when the sample is cooled in the presenc…
The ground state of the frustrated A-site magnetic spinel CoAl2O4 has been a controversial issue whether it is a collinear antiferromagnetic ordering or a spiral spin - liquid state, as the ratio of the two competing interactions, J2/J1 lies close to the boundary between these two ground states. Here, we address the magnetic ground state in CoAl2O4 with different amount of Co2+/Al3+ site disorder from the study of magnetoelectric effect and Monte Carlo simulations. CoAl2O4 with low site disorder exhibits linear magnetoelectric effect below the magnetic ordering temperature. With increasing dis…
Motivated by the recent discovery of high temperature antiferromagnet SrRu$_2$O$_6$ and its potential to be the parent of a new superconductor, we construct a minimal $t_{2g}$-orbital model on a honeycomb lattice to simulate its low energy band structure. Local Coulomb interaction is taken into account through both random phase approximation and mean field theory. Experimentally observed Antiferromagnetic order is obtained in both approximations. In addition, our theory predicts that the magnetic moments on three $t_{2g}$-orbitals are non-collinear as a result of the strong spin-orbit coupling…
We report a comprehensive study of magnetic properties of Ni3TeO6. The system crystallizes in a noncentrosymmetric rhombohedral lattice, space group R3. There are three differently coordinated Ni atoms in the unit cell. Two of them form an almost planar honeycomb lattice, while the third one is placed between the layers. Magnetization and specific heat measurements revealed a single magnetic ordering at TN = 52 K. Below TN the susceptibility with the magnetic field parallel to the c-axis drops towards zero while the perpendicular susceptibility remains constant, a characteristic of antiferroma…
Recent experiments for linear magnetoelectric (ME) response in honeycomb antiferromagnet Co$_4$Nb$_2$O$_9$ revealed that the electric polarization can be manipulated by the in-plane rotating magnetic field in a systematic way. We propose the minimal model by extracting essential ingredients of Co$_4$Nb$_2$O$_9$ to exhibit such ME response. It is the three-orbital model with $xy$-type atomic spin-orbit coupling (SOC) on the single-layer honeycomb structure, and it is shown to reproduce qualitatively the observed field-angle dependence of the electric polarization. The obtained results can be un…
Properties of two ThCr2Si2-type materials are discussed within the context of their established structural and magnetic symmetries. Both materials develop collinear, G-type antiferromagnetic order above room temperature, and magnetic ions occupy acentric sites in centrosymmetric structures. We refute a previous conjecture that BaMn2As2 is an example of a magnetoelectric material with hexadecapole order by exposing flaws in supporting arguments, principally, an omission of discrete symmetries enforced by the symmetry of sites used by Mn ions and, also, improper classifications of the primary an…
We investigated the magnetic properties of the ilmenite-type iridate CdIrO$_3$ with a honeycomb lattice formed by Ir$^{4+}$ ions prepared via a solid-state metathesis. The magnetization measurements with using the powder sample reveal a large effective magnetic moment and a fairly strong antiferromagnetic interaction, indicating a deviation from the Kitaev model. Considering the relationship between magnetism and crystal structure in CdIrO$_3$ with comparing with the other ilmenite-type iridates ZnIrO$_3$ and MgIrO$_3$, we conclude that insulating CdIrO$_3$ cannot be describe as a $J_{\rm eff}…
Theoretical studies have predicted the existence of topological magnons in honeycomb compounds with zig-zag antiferromagnetic (AFM) order. Here we report the discovery of zig-zag AFM order in the layered and non-centrosymmetric honeycomb nickelate Ni$_2$Mo$_3$O$_8$ through a combination of magnetization, specific heat, x-ray and neutron diffraction and electron paramagnetic resonance measurements. It is the first example of such order in an integer-spin non-centrosymmetric structure ($P$$_6$3$mc$). Further, each of the two distinct sites of the bipartite honeycomb lattice has a unique crystal …
Strongly correlated materials with multiple order parameters provide unique insights into the fundamental interactions in condensed matter systems and present opportunities for innovative technological applications. A class of antiferromagnetic honeycomb lattices compounds, A4B2O9 (A = Co, Fe, Mn; B = Nb, Ta), have been explored owing to the occurrence of linear magnetoelectricity. We observe a highly nonlinear magnetoelectric effect on single crystals of Co4Ta2O9 (CTO), distinctive from the linear behavior in the isostructural Co4Nb2O9. Ferroelectricity emerges primarily along the [110] direc…
We have investigated the magnetic and dielectric properties of Ca$_2$CoSi$_2$O$_7$ crystal. The dielectricity and magnetism of Ca$_2$CoSi$_2$O$_7$ are strongly coupled below a canted antiferromagnetic transition temperature ($T_{\rm N}$): Magnetic fields induce electric polarization below $T_{\rm N}$. Interestingly, the magnetic-field-induced electric polarization is detected even without poling electric fields. Below $T_{\rm N}$, a canted antiferromagnetic-paramagnetic transition is induced by magnetic fields. The large magnetocapacitance is observed around $T_{\rm N}$. The origin of the larg…
We report the crystal structure and magnetic behavior of the $4d^3$ spin-$\frac32$ silicophosphate MoP$_3$SiO$_{11}$ studied by high-resolution synchrotron x-ray diffraction, neutron diffraction, thermodynamic measurements, and ab initio band-structure calculations. Our data revise the crystallographic symmetry of this compound and establish its rhombohedral space group ($R\bar 3c$) along with the geometrically perfect honeycomb lattice of the Mo$^{3+}$ ions residing in disconnected MoO$_6$ octahedra. Long-range antiferromagnetic order with the propagation vector $\mathbf k=0$ observed below $…
High-frequency ESR results on the S=1/2 Heisenberg hexagonal antiferromagnet InCu2/3V1/3O3 are reported. This compound appears to be a rare model substance for the honeycomb lattice antiferromagnet with very weak interlayer couplings. The high-temperature magnetic susceptibility can be interpreted by the S=1/2 honeycomb lattice antiferromagnet, and it shows a magnetic-order-like anomaly at TN=38 K. Although, the resonance field of our high-frequency ESR shows the typical behavior of the antiferromagnetic resonance, the linewidth of our high-frequency ESR continues to increase below TN, while i…
The magnetic and magnetocaloric (MCE) properties were studied in a stuffed honeycomb antiferromagnet GdInO3 polycrystalline. No long-range magnetic ordering was observed with only a sharp upturn in the temperature dependent magnetization curves at TN ~ 2.1 K. The large value of frustration index value ~ 5.0 suggests short-range antiferromagnetic interactions existing between the Gd3+ moments in this frustrated magnetic system. Negligible thermal and magnetic hysteresis suggest a second-order feature of phase transition and a reversible magnetocaloric effect (MCE) in GdInO3 compound. In the mag…
Thermomagnetic and inelastic neutron scattering measurements on Na2PrO3 are reported. This material is an antiferromagnetic honeycomb magnet based on the tetravalent lanthanide Pr4+ and has been proposed to host dominant antiferromagnetic Kitaev interactions. These measurements reveal magnetic fluctuations in Na2PrO3 below an energy of 2 meV as well as crystal-field excitations around 230 meV. The latter energy is comparable to the scale of the spin-orbit interaction and explains both the very small effective moment of around 1.0 μB per Pr4+ and the difficulty to uncover any static magnetic sc…
The magnetic excitations in CoPS$_3$, a two-dimensional van der Waals (vdW) antiferromagnet with spin $S=3/2$ on a honeycomb lattice, has been measured using powder inelastic neutron scattering. Clear dispersive spin waves are observed with a large spin gap of ~13 meV. The magnon spectra were fitted using an $XXZ$-type $J_1-J_2-J_3$ Heisenberg Hamiltonian with a single-ion anisotropy assuming no magnetic exchange between the honeycomb layers. The best-fit parameters show ferromagnetic exchange $J_1=-2.08$ meV and $J_2=-0.26$ meV for the nearest and second-nearest neighbors and a sizeable antif…
Hexagonal perovskite 15R-BaMnO2.99 with a ratio of cubic to hexagonal layers of 1/5 in the unit cell is an antiferromagnetic insulator that orders at a Néel temperature TN = 220 K. Here we report structural, magnetic, dielectric and thermal properties of single crystal BaMnO2.99 and its derivatives BaMn0.97Li0.03O3 and Ba0.97K0.03MnO3. The central findings of this work are: (1) these materials possess a usually large, high-temperature magnetoelectric effect that amplifies the dielectric constant by more than an order of magnitude near their respective Néel temperature; (2) Li and K doping can …
Elastic and inelastic neutron scattering studies reveal details of the antiferromagnetic tansition and intriguing spin-dynamics in the magneto-electric effect single crystal LiMnPO$_4$. The elastic scattering studies confirm the system is antiferromagnetic (AFM) below $T_N$=33.75 K with local magnetic moments (Mn$^{2+}$; $S = 5/2$) that are aligned along the crystallographic a-axis. The spin-wave dispersion curves propagating along the three principal axes, determined by inelastic scattering, are adequately modeled in the linear spin-wave framework assuming a spin-Hamiltonian that is parameter…
Hexagonal YMnO$_{3}$ is a prototype antiferromagnet which exhibits multiferroic behavior with the ferroelectric and magnetic transitions occurring at different temperatures. We observe an out-of-plane canting of the Mn$^{3+}$ magnetic moments using resonant X-ray diffraction (RXD) in a single crystal of this material. These canted moments result in the symmetry-forbidden (0,0,1) magnetic Bragg reflection, which is observed at the Mn $L_{2,3}$ absorption edges. We also observe an unexpected difference in the RXD spectral shapes at different temperatures. Using ab initio calculations, we explore…
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…
Motivated by the recent numerical simulations for doped $t$-$J$ model on the honeycomb lattice, we study superconductivity of singlet and triplet pairing on the honeycomb lattice Hubbard model. We show that a superconducting state with coexisting spin-singlet and spin-triplet pairings is induced by the antiferromagnetic order near half filling. The superconducting state we obtain has a topological phase transition that separates a topologically trivial state and a nontrivial state with Chern number two. Possible experimental realization of such a topological superconductivity is also discussed…
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 topological properties of massive and massless fermionic quasiparticles have been intensively investigated over the past decade in topological materials without magnetism. Recently, the bosonic analogs of such quasiparticles arising from spin waves have been reported in the two-dimensional (2D) honeycomb lattice ferromagnet/antiferromagnet and the 3D antiferromagnet. Here we use time-of-flight inelastic neutron scattering to study spin waves of the S = 1 honeycomb lattice antiferromagnet BaNi2(AsO4)2, which has a zig-zag antiferromagnetic (AF) ground state identical to that of the Kitaev q…
We theoretically propose possible magnetism-induced negative thermal expansion in honeycomb-lattice antiferromagnets with edge-sharing networks of $MX_6$ octahedra where $M$ and $X$ are transition-metal and ligand ions, respectively. In this crystal structure, the nearest-neighbor exchange interaction is composed of two competing contributions, i.e., the antiferromagnetic contribution from a direct 180$^\circ$ $M$-$M$ bond and the ferromagnetic contribution from 90$^\circ$ $M$-$X$-$M$ bonds, amplitudes of which have different bond-length dependence. Numerical analysis of the spin-lattice model…
The linear magnetoelectric (ME) effect allows for the selection or switching between two antiferromagnetic (AFM) states via the application of large electric ($E$) and magnetic ($H$) fields. Once an AFM state is selected, it is preserved by an energy barrier, even when the fields are removed. Using a simple phenomenological model, we find that this energy barrier, needed to switch the AFM state, is proportional to the product of the $E$ and $H$ coercive fields $(EH)_{\rm C}$. We measured the field and temperature dependence of $(EH)_{\rm C}$ in LiCoPO$_4$ for two different field configurations…
Magnetoelectric and magnetoelastic phenomena correlated with a phase transition into noncollinear magnetic phase have been investigated for single crystals of CuFeO2 with a frustrated triangular lattice. CuFeO2 exhibits several long-wavelength magnetic structures related to the spin frustration, and it is found that finite electric polarization, namely inversion symmetry breaking, occurs with noncollinear but not at collinear magnetic phases. This result demonstrates that the noncollinear spin structure is a key role to induce electric polarization, and suggests that frustrated magnets which o…
We have studied spin excitations in a single-domain crystal of antiferromagnetic LiCoPO4 by THz absorption spectroscopy. By analyzing the selection rules and comparing the strengths of the absorption peaks in the different antiferromagnetic domains, we found electromagnons and magnetoelectric spin resonances besides conventional magnetic-dipole active spin-wave excitations. Using the sum rule for the magnetoelectric susceptibility we determined the contribution of the spin excitations to all the different off-diagonal elements of the static magnetoelectric susceptibility tensor in zero as well…
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 …
Electric-field controlled exchange bias in a heterostructure composed of the ferromagnetic manganite La0.7Sr0.3MO3 and the ferroelectric antiferromagnetic BiFeO3 has recently been demonstrated experimentally. By means of a microscopic model Hamiltonian we provide a possible explanation of the origin of this magnetoelectric coupling. We find, in agreement with experimental results, a net ferromagnetic moment at the BiFeO3 interface. The induced ferromagnetic moment is the result of the competition between the e_g-electrons double exchange and the t_2g-spins antiferromagnetic superexchange that …
Frustrated bilayer quantum magnets have attracted attention as flat-band spin systems with unconventional thermodynamic properties. We study the low-temperature properties of a frustrated honeycomb-lattice bilayer spin-$\frac{1}{2}$ isotropic ($XXX$) Heisenberg antiferromagnet in a magnetic field by means of an effective low-energy theory using exact diagonalizations and quantum Monte Carlo simulations. Our main focus is on the magnetization curve and the temperature dependence of the specific heat indicating a finite-temperature phase transition in high magnetic fields.
In this study, we investigate the magnetoelectric (ME) effect of the newly discovered antiferromagnetic (AFM) compound Ce$_{3}$TiBi$_{5}$ with a hexagonal structure (space group $P6_{3}$/$mcm$). In this system, Ce ions form zig-zag chains that extend along the $c$-axis. We focus on the lack of the local inversion symmetry at the Ce site, although the crystal structure has an inversion center. A theoretical study has predicted that magnetization can be induced by an electric current in the AFM ordering on the zig-zag chain. We conducted magnetization measurements on Ce$_{3}$TiBi$_{5}$ under an …
We discover a new type of multiple-$q$ state, "ripple state", in a frustrated honeycomb-lattice Heisenberg antiferromagnet under magnetic fields. The ground state has an infinite ring-like degeneracy in the wavevector space, exhibiting a cooperative paramagnetic state, "ring-liquid" state. We elucidate that the system exhibits the ripple state as a new low-temperature thermodynamic phase via a second-order phase transition from the ring-liquid state, keeping the ring-like spin structure factor. The spin texture in real space looks like a "water ripple" and can induce a giant electric polarizat…
A systematic low-energy effective field theory for hole-doped antiferromagnets on the honeycomb lattice is constructed. The formalism is then used to investigate spiral phases in the staggered magnetization as well as the formation of two-hole bound states.
It is shown that the semimetallic state of the two-dimensional honeycomb lattice with a point-like Fermi surface is unstable towards a canted antiferromagnetic insulator upon application of an in-plane magnetic field. This instability is already present at the mean-field level; the magnetic field shifts the up- and the down-spin cones in opposite directions thereby generating a finite density of states at the Fermi surface and a perfect nesting between the up- and the down-spin Fermi sheets. This perfect nesting triggers a canted antiferromagnetic insulating state. Our conclusions, based on me…
We present a spin-rotation-invariant Green-function theory for the dynamic spin susceptibility in the spin-1/2 antiferromagnetic Heisenberg model on a stacked honeycomb lattice. Employing a generalized mean-field approximation for arbitrary temperatures, the thermodynamic quantities (two-spin correlation functions, internal energy, magnetic susceptibility, staggered magnetization, N'eel temperature, correlation length) and the spin-excitation spectrum are calculated by solving a coupled system of self-consistency equations for the correlation functions. The temperature dependence of the magnet…
We study the spin-1/2 Heisenberg antiferromagnet on a bilayer honeycomb lattice including interlayer frustration. Using a set of complementary approaches, namely Schwinger bosons, dimer series expansion, bond operators, and exact diagonalization, we map out the quantum phase diagram. Analyzing ground state energies and elementary excitation spectra, we find four distinct phases, corresponding to three collinear magnetic long range ordered states, and one quantum disordered interlayer dimer phase. We detail, that the latter phase is adiabatically connected to an "exact" singlet product ground s…
The spin$-1/2$ Heisenberg antiferromagnet on the honeycomb bilayer lattice is shown to display a rich variety of semiclassical and genuinely quantum phases, controlled by the interplay between intralayer frustration and interlayer exchange. Employing a complementary set of techniques, comprising spin rotationally invariant Schwinger boson mean field theory, bond operators, and series expansions we unveil the quantum phase diagram, analyzing low-energy excitations and order parameters. By virtue of Schwinger bosons we scan the complete range of exchange parameters, covering both long range orde…
The spin-3/2 Heisenberg antiferromagnet on the bilayer honeycomb lattice is a minimal model to describe the magnetic behavior of Bi$_3$Mn$_4$O$_{12}$(NO$_3$). We study this model with frustrating inter-layer second-neighbor couplings, taking into account quantum and thermal fluctuations. We use a path integral formulation in terms of coherent states to describe the low energy physics of the model. We show that for a particular point in the parameter space, close to the experimental estimated couplings, a continuum classical degeneracy is lifted by both quantum and thermal fluctuations, and a c…
