Characterizing the many-body localization (MBL) transition in strongly disordered and interacting quantum systems is an important issue in the field of condensed matter physics. We study the single particle Greens functions for a disordered interacti
ng system in one dimension using exact diagnonalization in the infinite temperature limit. We provide strong evidence that the typical values of the local density of states and the scattering rate, evaluated using the computed eigenstate Greens functions and self energies, can be used to track the delocalization to MBL transition. In the delocalized phase, the typical values of the local density of states and the scattering rate are of the order of the corresponding average values while in the MBL phase, the typical values for both the quantities become vanishingly small. The probability distribution functions of the local density of states and the scattering rate are broad log-normal distributions in the delocalized phase while the distributions become very narrow and sharply peaked close to zero in the MBL phase. We also study the eigenstate Greens function for all the many-body eigenstates and demonstrate that both, the energy resolved typical scattering rate and the typical local density of states, carry signatures of the many-body mobility edges.
We report magnetization, electron spin resonance (ESR), and muon spin relaxation ($mu $SR) measurements on single crystals of the $S=1/2$ (Cu$% ^{+2}$) kagom{e} compound Cu(1,3-benzendicarboxylate). The $mu $SR is carried to temperatures as low as 45
mK. The spin Hamiltonian parameters are determined from the analysis of the magnetization and ESR data. We find that this compound has anisotropic ferromagnetic interactions. Nevertheless, no spin freezing is observed even at temperatures two orders of magnitude lower than the coupling constants. In light of this finding, the relation between persistent spin dynamics and spin liquid are reexamined.
The Kagome Heisenberg antiferromagnet is mapped onto an effective Hamiltonian on the star superlattice by Contractor Renormalization. Comparison of ground state energies on large lattices to Density Matrix Renormalization Group justifies truncation o
f effective interactions at range 3. Within our accuracy, magnetic and translational symmetries are not broken (i.e. a spin liquid ground state). However, we discover doublet spectral degeneracies which signal the onset of p6 - chirality symmetry breaking. This is understood by simple mean field analysis. Experimentally, the p6 chiral order parameter should split the optical phonons degeneracy near the zone center. Addition of weak next to nearest neighbor coupling is discussed.
