We present the exact dimer ground state of a quantum antiferromagnet defined on a quasicrystal constructed from the Bronze-mean hexagonal quasicrystal. A coupling isotropy on the first and second-neighbor bonds is sufficient to stabilize a product state of singlets on the third-neighbor bonds. We also provide a systematic approach for constructing additional crystals, quasicrystals, and amorphous structures that can sustain an exact dimer ground state.
Quasicrystals are long-range ordered but not periodic, representing an interesting middle ground between order and disorder. We experimentally and numerically study the ground state of non- and weakly-interacting bosons in an eightfold symmetric quasicrystalline optical lattice. We find extended states for weak lattices but observe a localisation transition at a lattice depth of $V_0=1.78(2)\,E_{\mathrm{rec}}$ for the non-interacting system. We identify this transition by measuring the timescale required for adiabatic loading into the lattice, which diverges at the critical lattice depth for l…
Recent discovery of various magnetism in Tsai-type quasicrystalline approximants, in whose alloys rare-earth ions located on icosahedral apices are coupled with each other via the Ruderman--Kittel--Kasuya--Yosida interaction, opens an avenue to find novel magnetism originating from the icosahedral symmetry. Here we investigate classical and quantum magnetic states on an icosahedral cluster within the Heisenberg interactions of all bonds. Simulated annealing and numerical diagonalization are performed to obtain the classical and quantum ground states. We obtain qualitative correspondence of cla…
We consider the spin 1/2 Heisenberg antiferromagnet on a two dimensional quasiperiodic tiling. The T=0 state is long range ordered, with an inhomogeneous distribution of the staggered moment. An approximate real space renormalization scheme first developed for the square lattice is generalized and used to obtain information on the ground state energy as well as the distribution of local correlations in the quasicrystal.
We present the exact dimer ground state of a quantum antiferromagnet on the maple-leaf lattice. A coupling anisotropy for one of the three inequivalent nearest-neighbor bonds is sufficient to stabilize the dimer state. Together with the Shastry-Sutherland Hamiltonian, we show that this is the only other model with an exact dimer ground state for all two-dimensional lattices with uniform tilings.
We present a certain class of two-dimensional frustrated quantum Heisenberg spin systems with multiple ring exchange interactions which are rigorously demonstrated to have quantum disordered ground states without magnetic long-range order. The systems considered in this paper are s=1/2 antiferromagnets on a honeycomb and square lattices, and an s=1 antiferromagnet on a triangular lattice. We find that for a particular set of parameter values, the ground state is a short-range resonating valence bond state or a valence bond crystal state. It is shown that these systems are closely related to th…
We have investigated single crystals of two substitution series Ce(Ru$_{1-x}$Fe$_{x}$)PO and CeFe(As$_{1-y}$P$_{y}$)O in the vicinity to the quantum critical material CeFePO by means of magnetic-susceptibility and specific-heat measurements. We observe an antiferromagnetic ground state in the vicinity of the quantum critical point, with pronounced metamagnetic transitions for $H\parallel c$, which is the magnetically hard direction. Our results verify that a ferromagnetic quantum critical point is avoided in substituted CeFePO, because we clearly demonstrate that the ferromagnetic ground state…
We construct and study two frustrated quantum spin-1/2 models on square lattice, which are like the antiferromagnetic $J_1$-$J_2$ model with some additional four-spin exchange interactions. These models admit an exactly solvable case in which the ground state consists of four degenerate columnar-dimer singlet (CDS) configurations. Away from the exact case, we employ bond-operator mean-field theory to investigate the evolution of the ground state by varying the interaction parameters. The mean-field calculation reveals a quantum phase diagram in which the CDS phase undergoes a continuous phase …
By explicitly computing wavefunction overlap via exact diagonalization in finite systems, we provide evidence indicating that, in the limit of strong coupling, i.e., $Δ/t \to \infty$, the ground state of the Gutzwiller-projected BCS Hamiltonian (accompanied by proper particle-number projection) is identical to the exact ground state of the 2D antiferromagnetic Heisenberg model on the square lattice. This identity is adiabatically connected to a very high overlap between the ground states of the projected BCS Hamiltonian and the t-J model at moderate doping.
We look into the quantum phase diagram of a spin-$\frac{1}{2}$ antiferromagnet on the square lattice with degenerate Shastry-Sutherland ground states, for which only a schematic phase diagram is known so far. Many exotic phases were proposed in the schematic phase diagram by the use of exact diagonalization on very small system sizes. In our present work, an important extension of this antiferromagnet is introduced and investigated in the thermodynamic limit using triplon mean-field theory. Remarkably, this antiferromagnet shows a stable plaquette spin-gapped phase like the original Shastry-Su…
We study the ground state properties of a quantum antiferromagnet in the kagome lattice in the presence of a magnetic field, paying particular attention to the stability of the plateau at magnetization 1/3 of saturation. While the plateau is reinforced by certain deformations of the lattice, like the introduction of structural defect lines and against an Ising anisotropy, ground state correlations are seen to be quite different and the undistorted SU(2) case appears to be rather special.
We develop a generalization of the singlet sector valence bond basis projection algorithm of Sandvik, Beach, and Evertz (A. W. Sandvik, Phys. Rev. Lett. 95, 207203 (2005); K. S. D. Beach and A. W. Sandvik, Nucl. Phys. B750, 142 (2006); A. W. Sandvik and H. G. Evertz, arXiv:0807.0682, unpublished.) to cases in which the ground state of an antiferromagnetic Hamiltonian has total spin $S_{tot} = 1/2$ in a finite size system. We explain how various ground state expectation values may be calculated by generalizations of the estimators developed in the singlet case, and illustrate the power of the m…
The ground-state properties of the spin-1 pyrochlore antiferromagnet are studied by applying the VBS-like tetrahedron-unit decomposition to the original spin system. The symmetrization required on every vertex is taken into account by introducing a ferromagnetic coupling. The pairwise effective Hamiltonian between the adjacent tetrahedrons is obtained by considering the next nearest neighbor and the third neighbor exchange interactions. We find that the transverse component of the spin chirality exhibits a long-range order, breaking the parity symmetry of the tetrahedral group, while the chira…
We study the phase diagram of the spin-1 quantum bilinear-biquadratic antiferromagnet on the kagome lattice using exact diagonalization and the density matrix renormalization group algorithm. The SU(3)-symmetric point of this model Hamiltonian is a spontaneously trimerized state whose qualitative nature persists even at the Heisenberg point, a finding that contrasts previous proposals. We report the ground state energy per site of the Heisenberg model to be -1.410(2) and establish the presence of a spin gap.
We present an antiferromagnetic quantum spin-1/2 model on honeycomb lattice. It has two parts, one of which is the usual nearest-neighbor Heisenberg model. The other part is a certain multiple spin interaction term, introduced by us, which is exactly solvable for the ground state. Without the Heisenberg part, the model has an exact threefold degenerate dimer ground state. This exact ground state is also noted to exist for the general spin-S case. For the spin-1/2 case, we further carry out the triplon analysis in the ground state, to study the competition between the Heisenberg and the multipl…
The experimental observation of a spin-gap in the antiferromagnetic spin-1/2 compound $CaV_4O_9$ has been attributed to the formation of a plaquette resonating-valence-bond (PRVB) state in the underlying 1/5-depleted square lattice. We construct a spin-1/2 model on the 1/5-depleted lattice for which the PRVB state can be shown to be the exact ground state in a particular parameter regime. In a subspace of this parameter regime, the first excited state can be calculated exactly. In a different parameter regime, the dimer state is shown to be the exact ground state.
We have proposed the plaquette-singlet solid (PSS) ground state for the spin-1 antiferromagnetic Heisenberg two-leg ladder. Based on the PSS picture, we discuss the correspondence of the PSS state to the valence-bond solid (VBS) state of the ground state of spin-2 chain by introducing an appropriate composite spin picture. When the bond alternation is introduced, there occur quantum phase transitions and each phase can be identified with that in the dimerized spin-2 chain. Furthermore, we argue that the VBS picture of spin-2S chain can be applied to the ground state of spin-S two-leg ladder.
Cs2CuCl4 is known to possess a quantum spin liquid phase with antiferromagnetic interaction below 2.8 K. We report the observation of a new metastable magnetic phase of the triangular frustrated quantum spin system Cs2CuCl4 induced by the application of hydrostatic pressure. We measured the magnetic properties of Cs2CuCl4 following the application and release of pressure after 3 days. We observed a previously unknown ordered magnetic phase with a transition temperature of 9 K. Furthermore, the recovered sample with new magnetic ground state possesses an equivalent crystal structure to the unco…
Artificial quasicrystals are nowadays routinely manufactured, yet only two naturally occurring examples are known. We present a class of systems with the potential to be realized both artificially and in nature, in which the lowest energy state is a one-dimensional quasicrystal. These systems are based on incommensurately charge-ordered materials, in which the quasicrystalline phase competes with the formation of a regular array of discommensurations as a way of interpolating between incommensurate charge order at high temperatures and commensurate order at low temperatures. The nonlocal corre…
We present local probe results on the honeycomb lattice antiferromagnet Ba3CuSb2O9. Muon spin relaxation measurements in zero field down to 20 mK show unequivocally that there is a total absence of spin freezing in the ground state. Sb NMR measurements allow us to track the intrinsic susceptibility of the lattice, which shows a maximum at around 55 K and drops to zero in the low-temperature limit. The spin-lattice relaxation rate shows two characteristic energy scales, including a field-dependent crossover to exponential low-temperature behavior, implying gapped magnetic excitations.
We describe the zero temperature phases and phase boundaries of an effective lattice model for frustrated, easy-plane, spin S=1/2 quantum antiferromagnets. The model contains states with Neel and bond-centered charge order and fractionalization in different limiting regimes, and we describe how these orders compete at intermediate coupling. The complex Berry phase terms render this model unsuitable for direct simulation; however, a duality mapping leads to a model of interacting current loops with positive weights, which is then studied by Monte Carlo simulations.
The ground state of the stacked triangular antiferromagnet CsFeBr$_3$ is a spin singlet due to the large single ion anisotropy $D(S^z)^2$. The field-induced magnetic ordering in this compound was investigated by the magnetic susceptibility, the magnetization process and specific heat measurements for an external field parallel to the $c$-axis. Unexpectedly, two phase transitions were observed in the magnetic field $H$ higher than 3 T. The phase diagram for temperature versus magnetic field was obtained. The mechanism leading to the successive phase transitions is discussed.
In an ideal classical pyrochlore antiferromagnet without perturbations, an infinite degeneracy at a ground state leads to absence of a magnetic order and spin-glass transition. Here we present Na$_3$Mn(CO$_3$)$_2$Cl as a new candidate compound where classical spins are coupled antiferromagnetically on the pyrochlore lattice, and report its structural and magnetic properties.The temperature dependences of the magnetic susceptibility and heat capacity, and the magnetization curve are consistent with those of an $S$ = 5/2 pyrochlore lattice antiferromagnet with nearest-neighbor interactions of 2 …
Muon spin relaxation and rotation ($μ$SR) experiments on single crystals of the structurally perfect triangular antiferromagnet YbMgGaO$_4$ indicate the absence of both static long-range magnetic order and spin freezing down to 0.048 K in zero field. Below 0.4 K, the $μ^{+}$ spin relaxation rates, which are proportional to the dynamic correlation function of the Yb$^{3+}$ spins,exhibit temperature-independent plateaus. Same behavior is revealed by temperature-independent local susceptibilities extracted from the Knight shifts of the $μ^{+}$ spin rotation frequencies under a transverse field of…
Entanglement renormalization techniques are applied to numerically investigate the ground state of the spin-1/2 Heisenberg model on a kagome lattice. Lattices of N={36,144,inf} sites with periodic boundary conditions are considered. For the infinite lattice, the best approximation to the ground state is found to be a valence bond crystal (VBC) with a 36-site unit cell, compatible with a previous proposal. Its energy per site, E=-0.43221, is an exact upper bound and is lower than the energy of any previous (gapped or algebraic) spin liquid candidate for the ground state.
We study the semiclassical limit of the $Sp(N)$ generalization of the pyrochlore lattice Heisenberg antiferromagnet by expanding about the $N \to \infty$ saddlepoint in powers of a generalized inverse spin. To leading order, we write down an effective Hamiltonian as a series in loops on the lattice. Using this as a formula for calculating the energy of any classical ground state, we perform Monte-Carlo simulations and find a unique collinear ground state. This state is not a ground state of linear spin-wave theory, and can therefore not be a physical (N=1) semiclassical ground state.
We study the nearest-neighbor Heisenberg antiferromagnet on a face-centered cubic lattice with arbitrary spin S. The model exhibits degenerate classical ground states including two collinear structures AF1 and AF3 described by different propagation vectors that are prime candidates for the quantum ground state. We compute the energy for each of the two states as a function of S using the spin-wave theory that includes magnon-magnon interaction in a self-consistent way and the numerical coupled cluster method. Our results unambiguously demonstrate that quantum fluctuations stabilize the AF1 sta…
We present a Density Matrix Renormalization Group (DMRG) study of the ground-state properties of spin-1/2 frustrated $J_1-J_3$ Heisenberg $n_l$-leg ladders (with $n_l$ up to 8). For strong frustration ($J_3/J_1\simeq 0.5$), both even- and odd-leg ladders display a finite gap to spin excitations, which we argue remains finite in the two-dimensional limit. In this regime, on odd-leg ladders the ground state is spontaneously dimerized, in agreement with the Lieb-Schultz-Mattis prediction, while on even-leg ladders the dimer correlations decay exponentially. The magnitude of the dimer order parame…
A rare example of a two dimensional Heisenberg model with an exact dimerized ground state is presented. This model, which can be regarded as a variation on the kagome lattice, has several features of interest: it has a highly (but not macroscopically) degenerate ground state; it is closely related to spin chains studied by earlier authors; in particular, it is probably the first genuinely two-dimensional quantum system to exhibit domain-wall-like ``kink'' excitations normally found only in one-dimensional systems. In some limits it decouples into non-interacting chains, purely dynamically and …
A connection between quantum antiferromagnetism and high $T_C$ superconductivity is theoretically investigated by analyzing the t-J model and its relationships to the Gutzwiller-projected BCS Hamiltonian. After numerical corroboration via exact diagonalization, it is analytically shown that the ground state of the t-J model at half filling (i.e., the 2D antiferromagnetic Heisenberg model) is entirely equivalent to the ground state of the Gutzwiller-projected BCS Hamiltonian with strong pairing. Combined with the high wavefunction overlap between the ground states of the t-J model and the proje…
The (three-dimensional) pyrochlore lattice antiferromagnet with Heisenberg spins of large spin length $S$ is a highly frustrated model with an macroscopic degeneracy of classical ground states. The zero-point energy of (harmonic order) spin wave fluctuations distinguishes a subset of these states. I derive an approximate but illuminating {\it effective Hamiltonian}, acting within the subspace of Ising spin configurations representing the {\it collinear} ground states. It consists of products of Ising spins around loops, i.e has the form of a $Z_2$ lattice gauge theory. The remaining ground sta…
A linear spin-wave approach, a variational method and exact diagonlization are used to investigate the magnetic long-range order (LRO) of the spin-1/2 Heisenberg antiferromagnet on a two-dimensional 1/7-depleted triangular (maple leaf) lattice consisting of triangles and hexagons only. This lattice has z=5 nearest neighbors and its coordination number z is therefore between those of the triangular (z=6) and the kagome (z=4) lattices. Calculating spin-spin correlations, sublattice magnetization, spin stiffness, spin-wave velocity and spin gap we find that the classical 6-sublattice LRO is stron…
The Goldstone mode in a helical magnetic phase, also known as the helimagnon, is a propagating mode with a highly anisotropic dispersion relation. Here we study theoretically the helimagnon excitations in a complex chiral ground state of pyrochlore antiferromagnets such as spinel CdCr2O4 and itinerant magnet YMn2. We show that the effective theory of the soft modes in the helical state possesses a symmetry similar to that of smectic liquid crystals. We compute the low-energy spin-wave spectrum based on a microscopic spin Hamiltonian and find a dispersion relation characteristic of the helimagn…
The Dirac spin liquid ground state of the spin 1/2 Heisenberg kagome antiferromagnet has potential instabilities[1-4]. This has been suggested as the reason why it is not strongly supported in large-scale numerical calculations[5]. However, previous attempts to observe these instabilities have failed. We report on the discovery of a projected BCS state with lower energy than the projected Dirac spin liquid state which provides new insight into the stability of the ground state of the kagome antiferromagnet. The new state has three remarkable features. First, it breaks both spatial symmetry in …
In the pyrochlore lattice Heisenberg antiferromagnet, for large spin length $S$, the massive classical ground state degeneracy is partly lifted by the zero-point energy of quantum fluctuations at harmonic order in spin-waves. However, there remains an infinite manifold of degenerate collinear ground states, related by a gaugelike symmetry. We have extended the spin-wave calculation to quartic order, assuming a Gaussian variational wavefunction (equivalent to Hartree-Fock approximation). Quartic calculations \emph{do} break the harmonic-order degeneracy of periodic ground states. The form of th…
A Kosterlitz-Thouless phase transition in the ground state of an antiferromagnetic spin-$\frac{1}{2}$ Heisenberg chain with nearest and next-nearest-neighbor interactions is re-investigated from a different perspective: An unequivocal correspondence is found between components of the scalar product $\langle Ψ_0| Ψ_0 \rangle$ and geometrical objects. One can classify these objects according to whether any two of them can be transformed into each other in a continuous way (belong to the same homotopy class). A finite size scaling of the "connection term`` $\langle Ψ_0|\partial Ψ_0 \rangle$ with …
We report muSR experiments on Mg{x}Cu{4-x}(OH)6Cl2 with x \sim 1, a new material isostructural to Herbertsmithite exhibiting regular kagome planes of spin 1/2 (Cu^{2+}), and therefore a candidate for a spin liquid ground state. We evidence the absence of any magnetic ordering down to 20 mK (\sim J/10^4). We investigate in detail the spin dynamics on well characterized samples in zero and applied longitudinal fields and propose a low T defect based interpretation to explain the unconventional dynamics observed in the quantum spin liquid phase.
We use series expansion methods to study ground- and excited-state properties in the helically ordered phase of spin-1/2 frustrated antiferromagnets on an anisotropic triangular lattice. We calculate the ground state energy, ordering wavevector, sublattice magnetization and one-magnon excitation spectrum for parameters relevant to Cs_2CuCl_4 and Cs_2CuBr_4. Both materials are modeled in terms of a Heisenberg model with spatially anisotropic exchange constants; for Cs_2CuCl_4 we also take into account the additional Dzyaloshinskii-Moriya (DM) interaction. We compare our results for Cs_2CuCl_4 w…
$S$ = 1/2 Ising kagome antiferromagnets (KAFs) are rare in reality. In this work, we report the magnetic ground state and field-induced transitions of an $S$ = 1/2 Ising antiferromagnet with a stacked kagome geometry, Na$_5$Co$_{15.5}$Te$_6$O$_{36}$. Upon cooling, magnetic susceptibility measurement reveals three successive magnetic transitions at $T_1$ = 45.2 K, $T_2$ = 35.4 K, and $T_3 \sim$ 20 K. When the magnetic field is applied along the Ising $c$ axis, a strikingly anomalous initial magnetization lying outside the hysteresis loop is observed below $T_3$. Through the detailed characteriz…
Spin liquids are exotic phases of quantum matter challenging Landau's paradigm of symmetry-breaking phase transitions. Despite strong exchange interactions, spins do not order or freeze down to zero temperature. While well-established for 1D quantum antiferromagnets, in higher dimension where quantum fluctuations are less acute, realizing and understanding such states represent major issues, both theoretical and experimental. In this respect the simplest nearest-neighbor Heisenberg antiferromagnet Hamiltonian on the highly frustrated kagome lattice has proven to be a fascinating and inspiring …
We investigate the two-magnon Raman scattering from an anisotropic S=1/2 triangular Heisenberg antiferromagnet Cs2CuCl4. We find that the Raman response is very sensitive to magnon-magnon interactions and to scattering geometries, a feature that is in remarkable contrast with the polarization-independent Raman signal from the isotropic triangular Heisenberg antiferromagnet. Since a spin-liquid ground state gives rise to a similar rotationally invariant Raman response, our results on the polarization dependence of the scattering spectrum suggest that Raman spectroscopy provides a useful probe, …
We investigated the crystal structure of Rb$_2$Cu$_3$SnF$_{12}$ and its magnetic properties using single crystals. This compound is composed of Kagome layers of corner-sharing CuF$_{6}$ octahedra with a 2a x 2a enlarged cell as compared with the proper Kagome layer. Rb$_2$Cu$_3$SnF$_{12}$ is magnetically described as an $S$=1/2 modified Kagome antiferromagnet with four kinds of neighboring exchange interaction. From magnetic susceptibility and high-field magnetization measurements, it was found that the ground state is a disordered singlet with the spin gap, as predicted from a recent theory. …
Using the coupled cluster method for high orders of approximation and Lanczos exact diagonalization we study the ground-state phase diagram of a quantum spin-1/2 J1-J2 model on the square lattice with plaquette structure. We consider antiferromagnetic (J1>0) as well as ferromagnetic (J1<0) nearest-neighbor interactions together with frustrating antiferromagnetic next-nearest-neighbor interaction J2>0. The strength of inter-plaquette interaction lambda varies between lambda=1 (that corresponds to the uniform J1-J2 model) and lambda=0 (that corresponds to isolated frustrated 4-spin plaquettes). …
We study a frustrated spin-$S$ staggered-dimer Heisenberg model on square lattice by using the bond-operator representation for quantum spins, and investigate the emergence of classical magnetic order from the quantum mechanical (staggered-dimer singlet) ground state for increasing $S$. Using triplon analysis, we find the critical couplings for this quantum phase transition to scale as $1/S(S+1)$. We extend the triplon analysis to include the effect of quintet dimer-states, which proves to be essential for establishing the classical order (Néel or collinear in the present study) for large $S$,…
Using the coupled-cluster method (CCM) and the rotation-invariant Green's function method (RGM), we study the influence of the interlayer coupling $J_\perp$ on the magnetic ordering in the ground state of the spin-1/2 $J_1$-$J_2$ frustrated Heisenberg antiferromagnet ($J_1$-$J_2$ model) on the stacked square lattice. In agreement with known results for the $J_1$-$J_2$ model on the strictly two-dimensional square lattice ($J_\perp=0$) we find that the phases with magnetic long-range order at small $J_2< J_{c_1}$ and large $J_2> J_{c_2}$ are separated by a magnetically disordered (quantum parama…
Elastic neutron scattering, ac susceptibility, and specific heat experiments on the pyrochlores Er$_{2}$Ge$_{2}$O$_{7}$ and Yb$_{2}$Ge$_{2}$O$_{7}$ show that both systems are antiferromagnetically ordered in the $Γ_5$ manifold. The ground state is a $ψ_{3}$ phase for the Er sample and a $ψ_{2}$ or $ψ_{3}$ phase for the Yb sample, which suggests "Order by Disorder"(ObD) physics. Furthermore, we unify the various magnetic ground states of all known R$_{2}$B$_{2}$O$_{7}$ (R = Er, Yb, B = Sn, Ti, Ge) compounds through the enlarged XY type exchange interaction $J_{\pm}$ under chemical pressure. The…
The S=1/2 and S=1 two-dimensional quantum Heisenberg antiferromagnets on the anisotropic dimerized square lattice are investigated by the quantum Monte Carlo method. By finite-size-scaling analyses on the correlation lengths, the ground-state phase diagram parametrized by strengths of the dimerization and of the spatial anisotropy is determined much more accurately than the previous works. It is confirmed that the quantum critical phenomena on the phase boundaries belong to the same universality class as that of the classical three-dimensional Heisenberg model. Furthermore, for S=1, we show th…
Using the infinite Projected Entangled Pair States (iPEPS) algorithm, we study the ground-state properties of the spin-$1/2$ quantum Heisenberg antiferromagnet on the star lattice in the thermodynamic limit. By analyzing the ground-state energy of the two inequivalent bonds of the lattice in different unit-cell structures, we identify two competing Valence-Bond-Solid (VBS) phases for different antiferromagnetic Heisenberg exchange couplings. More precisely, we observe (i) a VBS state which respects the full symmetries of the Hamiltonian, and (ii) a resonating VBS state which, in contrast to pr…
Using series expansions around the dimer limit, we show that the ground state of the Heisenberg Antiferromagnet on the Kagome Lattice appears to be a Valence Bond Crystal (VBC) with a 36-site unit cell, and an energy per site of $E/J=-0.433\pm0.001$. It is a honeycomb lattice of `perfect hexagons' as discussed by Nikolic and Senthil. The energy difference between the ground state and other ordered states with the maximum number of `perfect hexagons', such as a stripe-ordered state, is of order $0.001 J$. The energy of the 36-site system with periodic boundary conditions is further lowered by a…
The vanadium oxyfluoride [NH4]2[C7H14N][V7O6F18] (DQVOF) is a geometrically frustrated magnetic bilayer material. The structure consists of S=1/2 kagome planes of V4+ d1 ions with S=1 V3+ d2 ions located between the kagome layers. Muon spin relaxation measurements demonstrate the absence of spin freezing down to 40 mK despite an energy scale of 60 K for antiferromagnetic exchange interactions. From magnetization and heat capacity measurements we conclude that the S=1 spins of the interplane V3+ ions are weakly coupled to the kagome layers, such that DQVOF can be viewed as an experimental model…
Examples of materials where an "order by disorder" mechanism is at play to select a particular ground state are scarce. It has recently been proposed, however, that the antiferromagnetic XY pyrochlore Er2Ti2O7, reveals a most convincing case of this mechanism. Observation of a spin gap at zone centers has recently been interpreted as a corroboration of this physics. In this paper, we argue, however, that the anisotropy generated by the interaction-induced admixing between the crystal-field ground and excited levels provides for an alternative mechanism. It especially predicts the opening of a …
We use the matrix product approach to construct all optimum ground states of general anisotropic spin-2 chains with nearest neighbour interactions and common symmetries. These states are exact ground states of the model and their properties depend on up to three parameters. We find three different antiferromagnetic Haldane phases, one weak antiferromagnetic and one weak ferromagnetic phase. The antiferromagnetic phases can be described as spin liquids with exponentially decaying correlation functions. The variety of phases found with the matrix product ansatz also gives insight into the behavi…
We apply the coupled cluster method to high orders of approximation and exact diagonalizations to study the ground-state properties of the triangular-lattice spin-$s$ Heisenberg antiferromagnet. We calculate the fundamental ground-state quantities, namely, the energy $e_0$, the sublattice magnetization $M_{\rm sub}$, the in-plane spin stiffness $ρ_s$ and the in-plane magnetic susceptibility $χ$ for spin quantum numbers $s=1/2, 1, \ldots, s_{\rm max}$, where $s_{\rm max}=9/2$ for $e_0$ and $M_{\rm sub}$, $s_{\rm max}=4$ for $ρ_s$ and $s_{\rm max}=3$ for $χ$. We use the data for $s \ge 3/2$ to e…
We use the coupled cluster method to high orders of approximation in order to calculate the ground-state phase diagram of the XXZ spin-$s$ kagome antiferromagnet with easy-plane anisotropy, i.e. the anisotropy parameter $Δ$ varies between $Δ=1$ (isotropic Heisenberg model) and $Δ=0$ ($XY$ model). We find that for the extreme quantum case $s=1/2$ the ground state is magnetically disordered in the entire region $0 \le Δ\le 1$. For $s=1$ the ground state is disordered for $0.818 < Δ\le 1$, it exhibits $\sqrt{3}\times\sqrt{3}$ magnetic long-range order for $0.281 < Δ<0.818$, and $q=0$ magnetic lon…
We study low energy excitations in the quantum breathing pyrochlore antiferromagnet Ba$_3$Yb$_2$Zn$_5$O$_{11}$ by combination of inelastic neutron scattering (INS) and thermodynamical properties measurements. The INS spectra are quantitatively explained by spin-1/2 single-tetrahedron model having $XXZ$ anisotropy and Dzyaloshinskii-Moriya interaction. This model has a two-fold degeneracy of the lowest-energy state per tetrahedron and well reproduces the magnetization curve at 0.5 K and heat capacity above 1.5 K. At lower temperatures, however, we observe a broad maximum in the heat capacity ar…
