The effect of a single static impurity on the many-body states and on the spin and thermal transport in the one-dimensional anisotropic Heisenberg chain at finite temperatures is studied. Whereas the pure Heisenberg model reveals Poisson level statistics and dissipationless transport due to integrability, we show using the numerical approach that a single impurity induces Wigner-Dyson level statistics and at high enough temperature incoherent transport within the chain, whereby the relaxation time and d.c. conductivity scale linearly with length.
We consider the magnetic-field dependent spatial magnetization pattern around a general impurity embedded in a Heisenberg antiferromagnet using both an analytical and a numerical spin wave approach. The results are compared to quantum Monte Carlo simulations. The decay of the magnetization pattern away from the impurity follows a universal form which reflects the properties of the pure antiferromagnetic Heisenberg model. Only the overall magnitude of the induced magnetization depends also on the size of the impurity spin and the impurity coupling.
The finite temperature transport properties of a spin-1/2 anisotropic Heisenberg chain with an embedded spin-S impurity are studied. Using primarily numerical diagonalization techniques, we study the dependence of the dynamical spin and thermal conductivities on the lattice size, the magnitude of the impurity spin, the host-impurity coupling, the easy axis anisotropy, as well as the dependence on temperature. Particularly for the temperature dependence, we discuss the screening of the impurity by the chain eventually leading to the cutting or healing of the host chain. Numerical results are su…
We analyze the ground state properties of the two-dimensional quantum antiferromagnet with a S=1/2 Kondo impurity. Perturbation theory around the strong Kondo coupling limit is developed and the results compared with studies, based on exact diagonalization of small clusters. We find that at intermediate coupling the impurity is partially screened and the magnetization locally suppressed. A local singlet between the impurity and the host spin is formed asymptotically.
Using equivalencies between different models we reduce the model of two spin-1/2 Heisenberg chains crossed at one point to the model of free fermions. The spin-spin correlation function is calculated by summing the perturbation series in the interchain coupling. The result reveals a power law decay with a nonuniversal exponent.
We explore the effect of a (non) magnetic impurity on the thermal transport of the spin-1/2 Heisenberg chain model. This unique system allows to probe Kondo-type phenomena in a prototype strongly correlated system. Using numerical diagonalization techniques we study the scaling of the frequency dependent thermal conductivity with system size and host-impurity coupling strength as well as the dependence on temperature. We focus in particular on the analysis of cutting-healing of weak links or a magnetic impurity by the host chain via Kondo-like screening as the temperature is lowered.
We use a quantum Monte Carlo method (stochastic series expansion) to study the effects of a magnetic or nonmagnetic impurity on the magnetic susceptibility of the two-dimensional Heisenberg antiferromagnet. At low temperatures, we find a log-divergent contribution to the transverse susceptibility. We also introduce an effective few-spin model that can quantitatively capture the differences between magnetic and nonmagnetic impurities at high and intermediate temperatures.
In this letter, we construct a Hamiltonian of the impurity model within the framework of the open boundary Heisenberg XYZ spin chain. This impurity model is an exactly solved one and it degenerates to the integrable XXZ impurity model under the triangular limit. This approach is the first time to add the integrable impurities to the completely anisotropic Heisenberg spin model with the open boundary conditions.
We propose an integrable model of the spin-1/2 Heisenberg chain coupled to two impurity moments. With the open boundary conditions at the impurity sites, the model can be exactly solved for arbitrary impurity spin and arbitrary exchange constants between the bulk and the impurities. The absence of redundant terms in the hamiltonian makes the model very reasonable. The hamiltonian is diagonalized via algebraic Bethe ansatz. It is found that the impurity spins can only be screened (partially for $S>1/2$) for antiferromagnetic coupling between the impurity and the bulk. Otherwise the impurity spi…
We consider a magnetic impurity in the two-dimensional Heisenberg antifferomagnet with long range antiferromagnetic order. At low temperature the impurity magnetic susceptibility has a Curie term ($\propto 1/T$) and a logarithmic correction ($\propto \ln(T)$). We calculate the correction and derive related Ward identity for the impurity-spin-wave vertex.
We discuss effects of various impurities on the magnetic susceptibility and the specific heat of the quantum S=1/2 Heisenberg antiferromagnet on a two-dimensional square lattice. For impurities with spin S_i > 0 (here S_i=1/2 in the case of a vacancy or an added spin, and S_i=1 for a spin coupled ferromagnetically to its neighbors), our quantum Monte Carlo simulations confirm a classical-like Curie susceptibility contribution S_i^2/4T, which originates from an alignment of the impurity spin with the local Néel order. In addition, we find a logarithmically divergent contribution, which we attri…
Nuclear magnetic resonance and magnetization measurements were used to probe the magnetic features of single-crystalline Bi(Cu(1-x)Znx)2PO6 with 0<x<0.05 at temperatures between 2.6 K and 300 K. The simple lineshape of the 31P NMR signals of the pristine compound changes considerably for x>0 and we present clear evidence for a temperature dependent variation of the local magnetization close to the Zn sites. The generic nature of this observation is indicated by results of model calculations on appropriate spin systems of limited size employing QMC methods.
We theoretically investigate the Heisenberg antiferromagnet on a triangular lattice doped with nonmagnetic impurities. Two nontrivial effects resulting from collective impurity behavior are predicted. The first one is related to presence of uncompensated magnetic moments localized near vacancies as revealed by the low-temperature Curie tail in the magnetic susceptibility. These moments exhibit an anomalous growth with the impurity concentration, which we attribute to the clustering mechanism. In an external magnetic field, impurities lead to an even more peculiar phenomenon lifting the classic…
The local response to a uniform field around vacancies in the two-dimensional (2D) spin-1/2 Heisenberg antiferromagnet is determined by numerical quantum Monte Carlo simulations as a function of temperature. It is possible to separate the Knight shifts into uniform and staggered contributions on the lattice which are analyzed and understood in detail. The contributions show interesting long and short range behavior that may be of relevance in NMR and susceptibility measurements. For more than one impurity remarkable non-linear enhancement and cancellation effects take place. We predict that th…
The effect of non-uniform magnetic fields on the Ising-Heisenberg chain of a heterotrimetallic coordination compound $\mathrm{Fe-Mn-Cu}$, modeling a magnetic impurity on one dimer is studied. This impurity is configured by imposing non-uniform magnetic fields on each sites of $j-$th interstitial ionic dimer $\mathrm{Mn}^{2+}-\mathrm{Cu}^{2+}$ of the chain model. The quantum coherence and pairwise entanglement between spin-1/2 magnetic $\mathrm{Mn}^{2+}-\mathrm{Cu}^{2+}$ ion dimers clearly depend on the site which the impurity is located. It is demonstrated that when the magnetic impurity is co…
We analyze the logarithmic corrections due to ferromagnetic impurity ending bonds of open spin 1/2 antiferromagnetic chains, using the density matrix renormalization group technique. A universal finite size scaling $\sim {\frac 1 {L \log L}}$ for impurity contributions in the quasi-degenerate ground state energy is demonstrated for a zigzag spin 1/2 chain at the critical next nearest neighbor coupling and the standard Heisenberg spin 1/2 chain, in the long chain limit. Using an exact solution for the latter case it is argued that one can extract the impurity contributions to the entropy and sp…
This work is devoted to the study of how spin texture excitations are affected by the presence of a static nonmagnetic impurity whenever they lie on a conical support. We realize a number of novelties as compared to the flat plane case. Indeed, by virtue of the conical shape, the interaction potential between a soliton and an impurity appears to become stronger as long as the cone is tighten. As a consequence, a new kind of solitonic excitation shows up exhibiting lower energy than in the absence of such impurity. In addition, we conclude that such an energy is also dependent upon conical aper…
We consider the two-dimensional (2D) quantum Heisenberg antiferromagnet at zero temperature with two types of locally frustrating perturbations - an isolated ferromagnetic bond (FMB) and a quantum impurity spin, coupled symmetrically to the two sublattices. An analytic technique is developed to study strong defect-environment interaction. In the case of a FMB we find that, contrary to the previous linear spin-wave result, the local magnetization stays finite even for strong frustration. For an antiferromagnetic coupling of a quantum impurity spin with its environment, we find a local ground st…
A quantum phase transition may occur in a system at zero temperature when a controlling parameter is tuned towards a critical point. An important question is whether such a critical point exists in a particular system and how stable it is. Here, we identify the critical point of a quantum phase transition as a singular point in the affine algebraic variety of the characteristic equation for the Hamiltonian describing the system, with an unstable critical point being associated with an isolated singular point which has a finite Tjurina number. The theory is illustrated by studying a model syste…
We consider a magnetic impurity in two different S=1/2 Heisenberg bilayer antiferromagnets at their respective critical inter-layer couplings separating N'eel and disordered ground states. We calculate the impurity susceptibility using a quantum Monte Carlo method. With intra-layer couplings in only one of the layers (Kondo lattice), we observe an anomalous Curie constant C*, as predicted on the basis of field-theoretical work [S. Sachdev et al., Science 286, 2479 (1999)]. The value C*=0.262 +/- 0.002 is larger than the normal Curie constant C=S(S+1)/3. Our low-temperature results for the symm…
We calculate the ground-state energy of one and two-dimensional spatially inhomogeneous antiferromagnetic Heisenberg models for spins 1/2, 1, 3/2 and 2. Our calculations become possible as a consequence of the recent formulation of density-functional theory for Heisenberg models. The method is similar to spin-density-functional theory, but employs a local-density-type approximation designed specifically for the Heisenberg model, allowing us to explore parameter regimes that are hard to access by traditional methods, and to consider complications that are important specifically for nanomagnetic…
The bulk properties and impurity effect in the one-dimensional S=1/2 Heisenberg model with dimerization and next-nearest-neighbor exchange are studied by the density matrix renormalization group (DMRG) and exact diagonalization methods. This model widely covers the S=1 Heisenberg model, two-leg ladders, bond-alternating chains and the S=1/2 Heisenberg model. Almost the whole parameter space belongs to the Haldane phase. In the non-dimerizing limit a transition to a kind of spin liquid takes place, characterized by the delocalized spin polarization. This transition causes the distinct impurity …
We study the effect of anisotropy of spin-$S$ impurities on an antiferromagnetic SU(2) Heisenberg chain. The magnetic impurities are assumed to have an XXZ-type anisotropic exchange with its neighboring sites. First, using density-matrix renormalization group technique. we examine spin-spin correlation function, instability of Néel order, and local spin susceptibility in the presence of a single spin-$S$ impurity. Based on the results, we find that the types of spin-$S$ impurities are classified into two groups: $\rm(\hspace{.18em}i\hspace{.18em})$ nonmagnetic and $S=1$ impurities enhance only…
Impurity effects on the triangular antiferromagnets Ni$_{1-x}M_x$Ga$_2$S$_4$ ($M=$ Mn, Fe, Co and Zn) are studied. The 2D frozen spin-disordered state of NiGa$_2$S$_4$ is stable against the substitution of Zn$^{2+}$ ($S=0$) and Heisenberg Fe$^{2+}$ ($S=2$) spins, and exhibits a $T^2$-dependent magnetic specific heat, scaled by the Weiss temperature. In contrast, the substitutions with Co$^{2+}$ ($S=3/2$) spin with Ising-like anisotropy and Heisenberg Mn$^{2+}$ ($S=5/2$) spin induce a conventional spin glass phase. From these comparisons, it is suggested that integer size of Heisenberg spins is…
We revisit the problem of a single hole moving in the background of the two dimensional Heisenberg antiferromagnet. The hole is loosely bound by an impurity potential. We show that the bound state is generically a parity doublet: there are parametrically close bound states of opposite parity. Due to the degeneracy the bound state readily breaks local symmetries of the square lattice and this leads to formation of the long range spiral distortion of the antiferromagnetic background. A direct analogy with van der Waals forces in atomic physics is discussed.
We describe the uniform and staggered magnetization distributions around a vacancy in a quantum critical two-dimensional S=1/2 antiferromagnet. The distributions are delocalized across the entire sample with a universal functional form arising from an uncompensated Berry phase. The numerical results, obtained using quantum Monte Carlo simulations of the Heisenberg model on bilayer lattices with up to approximately 10^5 spins, are in good agreement with the proposed scaling structure. We determine the exponent eta'=0.40 +/- 0.02, which governs both the staggered and uniform magnetic structure a…
We study the Heisenberg ferromagnetic spin chain coupled with a boundary impurity. Via Bethe ansatz solution, it is found that (i) for J>0, the impurity spin behaves as a diamagnetic center and is completely screened by 2S bulk spins in the ground state, no matter how large the impurity spin is; (ii) The specific heat of the local composite (impurity plus 2S bulk spins which form bound state with it) shows a simple power law $C_{loc}\sim T^{3/2}$. (iii)For J<0, the impurity is locked into the critical behavior of the bulk. Possible phenomena in higher dimensions are discussed.
The magnetic susceptibility and specific heat of the one-dimensional S=1 bilinear-biquadratic Heisenberg model are calculated using the transfer matrix renormalization group. By comparing the results with the experimental data of ${\rm LiVGe_2O_6}$ measured by Millet et al. (Phys. Rev. Lett. {\bf 83}, 4176 (1999)), we find that the susceptibility data of this material, after subtracting the impurity contribution, can be quantitatively explained with this model. The biquadratic exchange interaction in this material is found to be ferromagnetic, i.e. with a positive coupling constant.
We study an open Heisenberg XXZ spin chain with long-range Ising-type interaction, which is incoherently driven at its boundaries and therefore a far-from-equilibrium steady state current is induced. Quantum Monte Carlo techniques make a regime accessible, where the system size is large enough to examine the transition from a diffusive to a ballistic transport regime. We find that the chain lengths for this transition are increasing with decreasing range of the Ising-type interactions between distant spins, while the strength of the incoherent driving does not have a relevant effect on the tra…
The effects of spin-1 impurities introduced in dimerized antiferromagnetic spin chains are studied using Lanczos diagonalization and quantum Monte Carlo simulations. Firstly, the magnitude of the interaction between a Ni-ion replacing a Cu-ion is derived for the case study system Cu-O-Cu. Then, it is shown that the introduction of S=1 impurities leads to the suppression of the original spin gap of the dimerized system and to the enhancement of the antiferromagnetic correlations close to the impurity. These features are interpreted in terms of a valence bond picture of the ground state of both …
We study the boundary impurity-induced critical behavior in an integrable SU(2)-invariant model consisting of an open Heisenberg chain of arbitrary spin-$S$ (Takhatajian-Babujian model) interacting with an impurity of spin $\vec{S'}$ located at one of the boundaries. For $S=1/2$ or $S'=1/2$, the impurity interaction has a very simple form $J\vec{S}_1\cdot\vec{S'}$ which describes the deformed boundary bond between the impurity $\vec{S'}$ and the first bulk spin $\vec{S}_1$ with an arbitrary strength $J$. With a weak coupling $0<J<J_0/[(S+S')^2-1/4]$, the impurity is completely compensated, und…
We study the energy and spin transport of the classical spin liquid hosted by the pyrochlore Heisenberg antiferromagnet in the large $S$ limit. Molecular dynamics calculation suggests that both the energy and spin diffusion constants approach finite limits as the temperature tends to zero. We explain our results in terms of an effective disorder model, where the energy/spin-carrying normal modes propagate in a quasi-static disordered spin background. The finite zero temperature limits of the diffusion constants are then naturally understood as a result of the finite mean free path of the norma…
Using the density matrix renormalization-group technique, we calculate the impurity energy levels for two different effective models of off-chain doping for quasi-one-dimensional Heisenberg chain compound Y$_2$ Ba Ni O$_5$: ferromagnetic bond doping and antiferromagnetic site spin-1/2 doping. Thresholds of the impurity strength for the appearance of localized states are found for both models. However, the ground-state and low-energy excitations for weak impurity strength are different for these two models and the difference can be detected by experiments.
The $S=1/2$ antiferromagnetic Heisenberg spin chain compound Sr$_2$CuO$_3$ doped with $1 \%$ and $2 \%$ of Ni impurities has been studied by means of $^{63}$Cu nuclear magnetic resonance. A strong decrease of the spin-lattice relaxation rate $T_1^{-1}$ at low temperatures points toward a spin gap, while a stretching exponent $λ< 1$ and a frequency dependence of $T_1^{-1}$ indicate that this spin gap varies spatially and should rather be characterized as a spin pseudogap. The magnitude of the spin pseudogap scales with doping level. Our results therefore evidence the finite-size character of th…
The interplay of different scattering mechanisms can lead to novel effects in transport. We show theoretically that the interplay of weak impurity and Umklapp scattering in spin-1/2 chains leads to a pronounced dip in the magnetic field dependence of the thermal conductivity $κ$ at a magnetic field $B \sim T$. In sufficiently clean samples, the reduction of the magnetic contribution to heat transport can easily become larger than 50% and the effect is predicted to exist even in samples with a large exchange coupling, J >> B, where the field-induced magnetization is small. Qualitatively, our th…
We study the impurity-induced phase transitions in a quasi-one-dimensional Heisenberg antiferromagnet doped with magnetic spin-1/2 impurities and non-magnetic ones. The impurity-induced transition temperature determined by the quantum Monte Carlo method with the continuous-time loop algorithm is monotonically increasing as a function of the magnitude of the impurity spin. To these results, we give discussions based on the valence-bond solid-like picture for the pure system and the inspection of the local magnetic structure around the impurities.
Density functional calculations are performed to study the magnetic order of the severely distorted square planar cupric oxide (CuO) and local spin disorder in it in the presence of the transition metal impurities M (= Cr, Mn, Fe, Co and Ni). The distortion in the crystal structure, arisen to reduce the band energy by minimizing the covalent interaction, creates two crisscrossing zigzag spin-1/2 chains. From the spin dimer analysis we find that while the spin chain along [10$\bar 1$] has strong Heisenberg type antiferromagnetic coupling (J ~ 127 meV), along [101] it exhibits weak, but robust, …
We report low-temperature specific heat, $C(T)$, measurements on (Yb$_{1-x}$Lu$_x$)$_4$As$_3$ with $x=0.01$ and $x=0.03$, where nonmagnetic Lu atoms are randomly distributed on antiferromagnetic $S=1/2$ Heisenberg chains with $J/k_{\mathrm B}=28$ K. The observed reduction of $C$ below 15 K with increasing $x$ is accurately described by quantum transfer matrix simulations without any adjustable parameter, implying that the system is an excellent experimental realization of segmented quantum spin chains. Finite-size effects consistent with conformal-field theory predictions are leading to the fo…
We study the non-equilibrium transport properties of a highly anisotropic two-dimensional lattice of spin-1/2 particles governed by a Heisenberg XXZ Hamiltonian. The anisotropy of the lattice allows us to approximate the system at finite temperature as an array of incoherently coupled one-dimensional chains. We show that in the regime of strong intrachain interactions, the weak interchain coupling considerably boosts spin transport in the driven system. Interestingly, we show that this enhancement increases with the length of the chains, which is related to superdiffusive spin transport. We de…
We study the impact of a weak bond disorder on the spinon heat transport in the S=1/2 antiferromagnetic (AFM) Heisenberg chain material Sr_{1-x}Ca_xCuO_2. We observe a drastic suppression in the magnetic heat conductivity kappa_mag even at tiny disorder levels (i.e., Ca-doping levels), in stark contrast to previous findings for kappa_mag of S=1/2 two-dimensional square lattice and two-leg spin-ladder systems, where a similar bond disorder has no effect on kappa_mag. Hence, our results underpin the exceptional role of integrability of the S=1/2 AFM Heisenberg chain model and suggest that the bo…
We apply the unbiased weak-coupling continuous time quantum Monte Carlo (CTQMC) method to review the physics of a single magnetic impurity coupled to s-wave superconducting leads described by the BCS reduced Hamiltonian. As a function of the superconducting gap $Δ$, we study the signature of the first order transition between the singlet and doublet (local moment) states on various quantities. In particular we concentrate on the Josephson current with 0 to $π$ phase shift, the crossing of the Andreev bound states in the single particle spectral function, as well as the local dynamical spin str…
We analyse a model of a one-dimensional electron gas interacting with an antiferromagnetic Heisenberg spin-1/2 chain via the spin exchange interactions. Using a solution at a special limit, we characterize the gapless modes of the spin gap fixed point at weak coupling $ J_{K}\ll J_{H},E_{F} $. we show that the only gapless pairing mode with divergent susceptibility is a composite odd-parity odd-frequency singlet pairing order parameter, while the ordinary BCS even-parity singlet pairing mode is incoherent. For 2-leg ladder systems, we note that it is possible to have a range of doping where th…
The spectral and thermodynamic response of segmented quantum spin chains is analyzed using a combination of numerical techniques and finite-size scaling arguments. Various distributions of segment lengths are considered, including the two extreme cases of quenched and annealed averages. As the impurity concentration is increased, it is found that (i) the integrated spectral weight is rapidly reduced, (ii) a pseudo-gap feature opens up at small frequencies, and (iii) at larger frequencies a discrete peak structure emerges, dominated by the contributions of the smallest cluster segments. The cor…
We compute the magnetic susceptibility and specific heat of the spin-1/2 Heisenberg model on the kagome lattice with high-temperature expansions and exact diagonalizations. We compare the results with the experimental data on ZnCu3(OH)6Cl2 obtained by Helton et al. [Phys. Rev. Lett. 98, 107204 (2007)]. Down to k_BT/J~0.2, our calculations reproduce accurately the experimental susceptibility, with an exchange interaction J~190K and a contribution of 3.7% of weakly interacting impurity spins. The comparison between our calculations of the specific heat and the experiments indicate that the low-t…
By means of first principles calculations we investigate the nature of exchange coupling in ferromagnetic bcc Fe on a microscopic level. Analyzing the basic electronic structure reveals a drastic difference between the $3d$ orbitals of $E_g$ and $T_{2g}$ symmetries. The latter ones define the shape of the Fermi surface, while the former ones form weakly-interacting impurity levels. We demonstrate that, as a result of this, in Fe the $T_{2g}$ orbitals participate in exchange interactions, which are only weakly dependent on the configuration of the spin moments and thus can be classified as Heis…
We study the possibility of d0 ferromagnetism in the compound MgO doped with nitrogen (N). The Haldane-Anderson impurity model is formulated within the tight-binding approximation for determining the host band-structure and the impurity-host hybridization. Using the quantum Monte Carlo technique, we observe a finite local moment for an N impurity, and long-range ferromagnetic correlations between two N impurities. The ferromagnetic correlations are strongly influenced by the impurity bound state. When the ferromagnetic correlation between a pair of impurities is mapped onto the isotropic Heise…
Strange metals possess highly unconventional transport characteristics, such as a linear-in-temperature ($T$) resistivity, an inverse Hall angle that varies as $T^2$ and a linear-in-field ($H$) magnetoresistance. Identifying the origin of these collective anomalies has proved profoundly challenging, even in materials such as the hole-doped cuprates that possess a simple band structure. The prevailing dogma is that strange metallicity in the cuprates is tied to a quantum critical point at a doping $p*$ inside the superconducting dome. Here, we study the high-field in-plane magnetoresistance of …
The effects of impurities in spin-1/2 Heisenberg chains are recently experiencing a renewed interest due to experimental realizations in solid state systems and ultra-cold gases. The impurities effectively cut the chains into finite segments with a discrete spectrum and characteristic correlations, which have a distinct effect on the dynamic structure factor. Using bosonization and the numerical Density Matrix Renormalization Group we provide detailed quantitative predictions for the momentum and energy resolved structure factor in doped systems. Due to the impurities, spectral weight is shift…
We investigate a quantum Heisenberg model with both antiferromagnetic and disordered nearest-neighbor couplings. We use an extended dynamical mean-field approach, which reduces the lattice problem to a self-consistent local impurity problem that we solve by using a quantum Monte Carlo algorithm. We consider both two- and three-dimensional antiferromagnetic spin fluctuations and systematically analyze the effect of disorder. We find that in three dimensions for any small amount of disorder a spin-glass phase is realized. In two dimensions, while clean systems display the properties of a highly …
We study high-temperature spin transport through an anisotropic spin-1/2 Heisenberg chain in which integrability is broken by a single impurity close to the center of the chain. For a finite impurity strength, the level spacing statistics of this model is known to be Wigner-Dyson. Our aim is to understand if this integrability breaking is manifested in the high-temperature spin transport. We focus first on the nonequilibrium steady state (NESS), where the chain is connected to spin baths that act as sources and sinks for spin excitations at the boundaries. Using a combination of open quantum s…
We present a Quantum Monte Carlo study for Heisenberg spin-1/2 two leg ladder systems doped with non magnetic impurities. The simulations are applied to the doped spin ladder compound SrCu2O3 doped with Zn, where a large broadening of the 65Cu NMR lines has been observed in experiment at low temperatures but far above the Neel temperature. We find that interladder-couplings with a sizeable coupling in stacking direction are required to describe the line broadening, which can not be explained by considering a single ladder only. Around a single impurity, spin correlations cause an exponentially…
Using Schwinger-boson mean-field theory, we calculate the dynamic spin structure factor at low temperatures $0<T\ll J$ for the spin-$1/2$ antiferromagnetic Heisenberg kagome model, within the gapped $\mathbb{Z}_2$ spin liquid phase Ansatz. We find that the spectral gap rapidly fills with temperature, with robust low-energy spectral weight developing by a temperature of $Δ/3$, where the spin gap is $2Δ$ (i.e., $Δ$ is the spinon gap), before any appreciable rise in spinon density or change in zero-temperature mean-field parameters. This is due to deconfinement of spinons which leads to terms sup…
Using the DMRG method we calculate the surface spin correlation function, $C_L(l)=< S_l^z S_{L+1-l}^z>$, in the spin $S=3/2$ antiferromagnetic Heisenberg chain. For comparison we also investigate the $S=1/2$ chain with S=1 impurity end spins and the S=1 chain. In the half-integer spin models the end-to-end correlations are found to decay to zero logarithmically, $C_L(1)\sim (\log L)^{-2d}$, with $d=0.13(2)$. We find no surface order, in clear contrast with the behavior of the S=1 chain, where exponentially localized end spins induce finite surface correlations. The lack of surface order implie…
We analyze the experimental data for the magnetic susceptibility of the material ZnCu3(OH)6Cl2 in terms of the Kagome Lattice Heisenberg model (KLHM), discussing possible role of impurity spins, dilution, exchange anisotropy, and both out-of-plane and in-plane Dzyaloshinsky-Moriya (DM) anisotropies, with explicit theoretical calculations using the Numerical Linked Cluster (NLC) method and exact diagonalization (ED). The high-temperature experimental data are well described by the pure Heisenberg model with J=170 K. We show that the sudden upturn in the susceptibility around T=75 K is due to DM…
