We present numerical results for the \(J_1\)-\(J_2\) Heisenberg model on a triangular lattice at finite temperatures \(T>0\). In contrast to unfrustrated lattices we reach much lower \(T \sim 0.15 ...J_1\). In static quantities the novel feature is a quite sharp low-\(T\) maximum in the specific heat. Dynamical spin structure factor \(S({\bf q},\omega)\) allows for the extraction of the effective spin-wave energies \(\omega_{\bf q}(T)\) and their damping \(\gamma_{\bf q}(T)\). While for \(J_2=0\) our results are consistent with \(T=0\) spin ordering, \(J_2/J_1 \sim 0.1 \) induces additional frustration with a signature of spin liquid ground state. In the latter case, results for spin-lattice relaxation rate indicate in the low-\(T\) accesible regime on \(1/T_1 \propto T^{\alpha}\) with \(\alpha \geq 1\), as observed in recent spin-liquid materials on a triangular lattice.
The theory of homogeneous nucleation in incommensurate structures is presented. The critical radius of the nucleus, the nucleation energy and the nucleation rate are evaluated. Quantitative analysis ...for Rb2ZnCI4 is performed.
V prispevku so predstavljena konstruktivna razmišljanja o nekaterih pomanjkljivostih nedavno sprejetih zakonov s področij urejanja prostora in graditve objektov. Kritike zakonov so v zvezi z ...nedoslednim uveljavljanjem koncepta deregulacije, vsebino prostorskih aktov, mehanizmi motivacije, aspiracij in dobičkov ter zanemarjanjem pobud, ki so motorji urejanja prostora. Posebno sta izpostavljena nekomunikativnost zakonov in pomanjkanje mehanizma za doseganje kakovostne arhitekture in urejenosti okolja.
The anomalous spin diffusion of the integrable easy-axis Heisenberg chain originates in the ballistic transport of symmetry sectors with nonzero magnetization. Ballistic transport is replaced by ...normal dissipative transport in all magnetization sectors upon introducing the integrability-breaking perturbations, including external driving. Such behavior implies that the diffusion constant obtained for the integrable model is relevant for the spread of spin excitations but not for the spin conductivity. We present numerical results for closed systems and driven open systems, indicating that the diffusion constant shows a discontinuous variation as the function of perturbation strength.
We study the electronic thermal conductivity \(\kappa_\textrm{el}\) and the thermal diffusion constant \(D_\textrm{Q,el}\) in the square lattice Hubbard model using the finite-temperature Lanczos ...method. We exploit the Nernst-Einstein relation for thermal transport and interpret the strong non-monotonous temperature dependence of \(\kappa_\textrm{el}\) in terms of that of \(D_\textrm{Q,el}\) and the electronic specific heat \(c_\textrm{el}\). We present also the results for the Heisenberg model on a square lattice and ladder geometries. We study the effects of doping and consider the doped case also with the dynamical mean-field theory. We show that \(\kappa_\textrm{el}\) is below the corresponding Mott-Ioffe-Regel value in almost all calculated regimes, while the mean free path is typically above or close to lattice spacing. We discuss the opposite effect of quasi-particle renormalization on charge and heat diffusion constants. We calculate the Lorenz ratio and show that it differs from the Sommerfeld value. We discuss our results in relation to experiments on cuprates. Additionally, we calculate the thermal conductivity of overdoped cuprates within the anisotropic marginal Fermi liquid phenomenological approach.
We study real-time dynamics of a charge carrier introduced into undoped Mott insulator propagating under a constant electric field F on the t-J ladder and square lattice. We calculate quasistationary ...current. In both systems adiabatic regime is observed followed by the positive differential resistivity (PDR) at moderate fields where carrier mobility is determined. Quantitative differences between ladder and 2-dimensional (2D) system emerge when at large fields both systems enter negative differential resistivity (NDR) regime. In the ladder system Bloch-like oscillations prevail, while in 2D the current remains finite, proportional to 1/F. The crossover between PDR and NDR regime in 2D is accompanied by a change of the spatial structure of the propagating spin polaron.
Phys. Rev. Lett. 93, 207202 (2004) A theory of the local magnetic response of a nonmagnetic impurity in a doped
antiferromagnet, as relevant to the normal state in cuprates, is presented. It
is based ...on the assumption of the overdamped collective mode in the bulk system
and on the evidence, that equal-time spin correlations are only weakly
renormalized in the vicinity of the impurity. The theory relates the Kondo-like
behavior of the local susceptibility to the anomalous temperature dependence of
the bulk magnetic susceptibility, where the observed increase of the Kondo
temperature with doping reflects the crossover to the Fermi liquid regime and
the spatial distribution of the magnetization is given by bulk
antiferromagnetic correlations.
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