Nearest-neighbor Spin-spin Correlations Research Articles

Low-temperature enhancement of ferromagnetic Kitaev correlations in α−RuCl3

Kitaev-type interactions between neighbouring magnetic moments emerge in the honeycomb material ${\alpha}$-RuCl3. It is debated however whether these Kitaev interactions are ferromagnetic or antiferromagnetic. With electron energy loss spectroscopy (EELS) we study the lowest excitation across the Mott-Hubbard gap, which involves a d4 triplet in the final state and therefore is sensitive to nearest-neighbor spin-spin correlations. At low temperature the spectral weight of these triplets is strongly enhanced, in accordance with optical data. We show that the magnetic correlation function that determines this EELS spectral weight is directly related to a Kitaev-type spin-spin correlator and that the temperature dependence agrees very well with the results of a microscopic magnetic Hamiltonian for ${\alpha}$-RuCl3 with ferromagnetic Kitaev coupling.

Magnetism in spin crossover systems: Short-range order and effects beyond the Heisenberg model

To study non-Heisenberg effects in the vicinity of spin crossover in strongly correlated electron systems we derive an effective low-energy Hamiltonian for the two-band Kanamori model. It contains Heisenberg high-spin term proportional to exchange constant as well as low-spin term proportional to spin gap parameter $\varepsilon_s$. Using cluster mean field theory we obtain several non-Heisenberg effects. Near critical value of spin gap $\varepsilon^c_s$ there is a magnetic phase transition of first order. In the vicinity of $\varepsilon^c_s$ in the paramagnetic phase we observe non trivial behavior of the Curie constant in the paramagnetic susceptibility in the wide range of temperature. Reentrant temperature behavior of nearest-neighbor spin-spin correlations is observed at $\varepsilon_s > \varepsilon^c_s$. Finally, pressure-temperature magnetic phase diagram for ferroperriclase is obtained using the effective Hamiltonian.

Effective model with strong Kitaev interactions for α−RuCl3

We use an exact numerical diagonalization method to calculate the dynamical spin structure factors (DSFs) of three ab-initio models and one ab-initio-guided model for a honeycomb-lattice magnet $\alpha$-RuCl$_3$. We also use thermal pure quantum states to calculate the temperature dependence of the heat capacity, the nearest-neighbor (NN) spin-spin correlation function, and the static spin structure factor. From the results obtained from these four effective models, we find that, even when the magnetic order is stabilized at low temperature, the intensity at the $\Gamma$ point in the DSFs increases with increasing NN spin correlation. In addition, we find that the four models fail to explain heat-capacity measurements whereas two of the four models succeed in explaining inelastic-neutron-scattering (INS) experiments. In the four models, when temperature decreases, the heat capacity shows a prominent peak at a high temperature where the NN spin-spin correlation function increases. However, the peak temperature in heat capacity is too low in comparison with that observed experimentally. To address these discrepancies, we propose an effective model that includes strong ferromagnetic Kitaev coupling, and we show that this model quantitatively reproduces both INS experiments and heat-capacity measurements. To further examine the adequacy of the proposed model, we calculate the field dependence of the polarized terahertz spectra, which reproduces the experimental results: the spin-gapped excitation survives up to an onset field where the magnetic order disappears and the response in the high-field region is almost linear. Based on these numerical results, we argue that the low-energy magnetic excitation in $\alpha$-RuCl$_3$ is mainly characterized by interactions such as off-diagonal interactions and weak Heisenberg interactions between NN pairs, rather than by the strong Kitaev interactions.

Optical probe of Heisenberg-Kitaev magnetism inα−RuCl3

We report a temperature-dependent optical spectroscopic study of the Heisenberg-Kitaev magnet $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}_{3}$. Our measurements reveal anomalies in the optical response near the magnetic ordering temperature. At higher temperatures, we observe a redistribution of spectral weight over a broad energy range that is associated with nearest-neighbor spin-spin correlations. This finding is consistent with highly frustrated magnetic interactions and in agreement with theoretical expectations for this class of material. The optical data also reveal significant electron-hole interaction effects, including a bound excitonic state. These results demonstrate a clear coupling between charge and spin degrees of freedom and provide insight into the properties of thermally disordered Heisenberg-Kitaev magnets.

Probing inter- and intrachain Zhang-Rice excitons inLi2CuO2and determining their binding energy

Cuprate materials, such as those hosting high-temperature superconductivity, represent a famous class of materials where the correlations between the strongly entangled charges and spins produce complex phase diagrams. Several years ago, the Zhang-Rice singlet was proposed as a natural quasiparticle in hole-doped cuprates. The occurrence and binding energy of this quasiparticle, consisting of a pair of bound holes with antiparallel spins on the same ${\mathrm{CuO}}_{4}$ plaquette, depends on the local electronic interactions, which are fundamental quantities for understanding the physics of the cuprates. Here, we employ state-of-the-art resonant inelastic x-ray scattering (RIXS) to probe the correlated physics of the ${\mathrm{CuO}}_{4}$ plaquettes in the quasi-one-dimensional chain cuprate ${\mathrm{Li}}_{2}{\mathrm{CuO}}_{2}$. By tuning the incoming photon energy to the O $K$ edge, we populate bound states related to the Zhang-Rice quasiparticles in the RIXS process. Both intra- and interchain Zhang-Rice singlets are observed and their occurrence is shown to depend on the nearest-neighbor spin-spin correlations, which are readily probed in this experiment. We also extract the binding energy of the Zhang-Rice singlet and identify the Zhang-Rice triplet excitation in the RIXS spectra.

First-order topological phase transition of the Haldane-Hubbard model

We study the interplay of topological band structure and conventional magnetic long-range order in spinful Haldane model with onsite repulsive interaction. Using the dynamical cluster approximation with clusters of up to 24 sites we find evidence of a first order phase transition from a Chern insulator at weak coupling to a topologically trivial antiferromagnetic insulator at strong coupling. These results call into question a previously found intermediate state with coexisting topological character and antiferromagnetic long-range order. Experimentally measurable signatures of the first order transition include hysteretic behavior of the double occupancy, single-particle excitation gap and nearest neighbor spin-spin correlations. This first order transition is contrasted with a continuous phase transition from the conventional band insulator to the antiferromagnetic insulator in the ionic Hubbard model on the honeycomb lattice.

Probing orbital fluctuations inRVO3(R=Y, Gd, or Ce) by ellipsometry

We study optical excitations across the Mott gap in the multiorbital Mott-Hubbard insulators $R$VO${}_{3}$. The multipeak structure observed in the optical conductivity can be described consistently in terms of the different $3{d}^{3}$ multiplets or upper Hubbard bands. The spectral weight is very sensitive to nearest-neighbor spin-spin and orbital-orbital correlations and thus shows a pronounced dependence on both temperature and polarization. Comparison with theoretical predictions based on either rigid orbital order or strong orbital fluctuations clearly rules out that orbital fluctuations are strong in $R$VO${}_{3}$. Both the line shape and the temperature dependence give clear evidence for the importance of excitonic effects.

Spin correlations and doublon production rate for fermionic atoms in modulated optical lattices

We compute the integrated doublon production rate in response to a lattice modulation for two component fermions in an optical lattice. We derive a general formula for the integrated intensity, valid in the presence of inhomogeneous potentials such as the trap, which gives the integrated intensity in terms of equal time correlation functions only. Such a formula is thus well suited for direct numerical calculations. We show that, in the limit of large repulsion for commensurate fillings, or for temperature ranges for which the hopping is incoherent, the integrated doublon spectrum is directly related to the nearest neighbor spin-spin correlation function. We compute its temperature dependence in this regime using finite temperature quantum Monte Carlo calculation.

Spin-12J1-J2Heisenberg antiferromagnet on a square lattice: A plaquette renormalized tensor network study

We apply the plaquette renormalization scheme of tensor network states [Phys. Rev. E 83, 056703 (2011)] to study the spin-1/2 frustrated Heisenberg ${J}_{1}$-${J}_{2}$ model on an $L\ifmmode\times\else\texttimes\fi{}L$ square lattice with $L$ $=$ 8, 16, and 32. By treating tensor elements as variational parameters, we obtain the ground states for different ${J}_{2}/{J}_{1}$ values, and investigate staggered magnetizations, nearest-neighbor spin-spin correlations, and plaquette order parameters. In addition to the well-known N\'eel order and collinear order at low and high ${J}_{2}/{J}_{1}$, we observe a plaquettelike order at ${J}_{2}/{J}_{1}\ensuremath{\approx}0.5$. A continuous transition between the N\'eel order and the plaquettelike order near ${J}_{2}^{{c}_{1}}\ensuremath{\approx}0.40{J}_{1}$ is observed. The collinear order emerges at ${J}_{2}^{{c}_{2}}\ensuremath{\approx}0.62{J}_{1}$ through a first-order phase transition.

Dzyaloshinsky-Moriya Anisotropy in the Spin-1/2 Kagome CompoundZnCu3(OH)6Cl2

We report the determination of the Dzyaloshinsky-Moriya interaction, the dominant magnetic anisotropy term in the kagome spin-1/2 compound ZnCu3(OH)6Cl2. Based on the analysis of the high-temperature electron spin resonance (ESR) spectra, we find its main component |Dz|=15(1) K to be perpendicular to the kagome planes. Through the temperature dependent ESR linewidth, we observe a building up of nearest-neighbor spin-spin correlations below approximately 150 K.

Excitation spectrum gap and spin-wave velocity ofXXZHeisenberg chains: Global renormalization-group calculation

The anisotropic $XXZ$ spin-$\frac{1}{2}$ Heisenberg chain is studied using renormalization-group theory. The specific heats and nearest-neighbor spin-spin correlations are calculated throughout the entire temperature and anisotropy ranges in both ferromagnetic and antiferromagnetic regions, obtaining a global description and quantitative results. We obtain, for all anisotropies, the antiferromagnetic spin-liquid spin-wave velocity and the Ising-like ferromagnetic excitation spectrum gap, exhibiting the spin-wave to spinon crossover. A number of characteristics of purely quantum nature are found: The in-plane interaction ${s}_{i}^{x}{s}_{j}^{x}+{s}_{i}^{y}{s}_{j}^{y}$ induces an antiferromagnetic correlation in the out-of-plane ${s}_{i}^{z}$ component, at higher temperatures in the antiferromagnetic $XXZ$ chain, dominantly at low temperatures in the ferromagnetic $XXZ$ chain, and, in-between, at all temperatures in the $XY$ chain. We find that the converse effect also occurs in the antiferromagnetic $XXZ$ chain: an antiferromagnetic ${s}_{i}^{z}{s}_{j}^{z}$ interaction induces a correlation in the ${s}_{i}^{xy}$ component. As another purely quantum effect, (i) in the antiferromagnet, the value of the specific heat peak is insensitive to anisotropy and the temperature of the specific heat peak decreases from the isotropic (Heisenberg) with introduction of either type (Ising or $XY$) of anisotropy; and (ii) in complete contrast, in the ferromagnet, the value and temperature of the specific heat peak increase with either type of anisotropy.

Mott-Hubbard versus charge-transfer behavior inLaSrMnO4studied via optical conductivity

figuration 10Dq= 1.2 eV, eg= 1.4 eV, and t2g = 0.2 eV. The spectral weight of the lowest absorption feature at 1 – 2 eV changes by a factor of 2 as a function of temperature, which can be attributed to the change of the nearest-neighbor spin-spin correlation function across the Neel temperature TN = 133 K. Interpreting LaSrMnO4 effectively as a Mott-Hubbard insulator naturally explains this strong temperature dependence, the relative weight of the different absorption peaks, and the pronounced anisotropy. By means of transmittance measurements, we determine the onset of the optical gap opt = 0.4– 0.45 eV at 15 K and 0.1– 0.2 eV at 300 K. Our data show that the crystal-field splitting is too large to explain the anomalous temperature dependence of the c-axis lattice parameter by thermal occupation of excited crystal-field levels. Alternatively, we propose that a thermal population of the upper Hubbard band gives rise to the shrinkage of the c-axis lattice parameter.

Spin diffusion and the anisotropic spin-12Heisenberg chain

Measurements of the spin-lattice relaxation rate $1∕{T}_{1}$ by nuclear magnetic resonance for the one-dimensional Heisenberg antiferromagnet ${\mathrm{Sr}}_{2}\mathrm{Cu}{\mathrm{O}}_{3}$ have provided evidence for a diffusionlike contribution at finite temperature and small wave vector. By analyzing real-time data for the auto- and nearest-neighbor spin-spin correlation functions obtained by the density-matrix renormalization group I show that such a contribution indeed exists for temperatures $T>J$, where $J$ is the coupling constant, but that it becomes exponentially suppressed for $T⪡J$. I present evidence that the frequency dependence of $1∕{T}_{1}$ in the Heisenberg case is smoothly connected to that in the free fermion case where the exponential suppression of the diffusionlike contribution is easily understood.

Temperature and thickness dependence of magnetic moments in NiO epitaxial films

We show that linear polarized x-ray-absorption spectroscopy can be used to measure the temperature and thickness dependence of magnetic moments in NiO thin films. We demonstrate that both the long-range order and the nearest-neighbor spin-spin correlations can be revealed. NiO (100) films with thicknesses of 5, 10, and 20 monolayers epitaxially grown on MgO (100) are studied. The N\'eel temperature is found to be strongly reduced from the bulk value even for the 20 monolayer film.

High-temperature expansion along the self-dual line of three-dimensionalZ(2)spin-gauge theory

We exploit the self-duality of the three-dimensional Ising spin-gauge theory to develop an eighth-order high-temperature expansion for the partition function along the self-dual line. This generates a high-temperature series for the gauge-invariant, nearest-neighbor spin-spin correlation function. A Pad\'e analysis of this series reveals a pole along the self-dual line. Recent Monte Carlo simulations indicate that this theory has a first-order self-dual line emerging from a triple point. We interpret the Pad\'e pole as a theoretical estimate of the end point of this self-dual line.

Effects of Mechanical Stretching and Quadratic Coupling on Critical Behavior

Assuming the existence of certain limits we give an exact solution of a two-dimensional elastic Ising model with quadratic coupling. The main results are these: (I) If the lattice is slightly stretched, thermodynamic instability occurs for temperatures in a neighborhood of the critical point. (II) Outside this neighborhood, ${\ensuremath{\kappa}}_{T}$, $\ensuremath{\alpha}$, and ${C}_{p}$ diverge. (III) The nearest-neighbor spin-spin correlation function is nonzero for any pressure when $T$ becomes infinite. (IV) Antiferromagnetic as well as ferromagnetic behavior can occur for weak coupling at any pressure. (V) For strong coupling, at 0 pressure the model can have a Curie and N\'eel point or no critical point.