arXiv subjects

D. S. Almeida

Publications and source records attributed to D. S. Almeida.

3 recordsLinked to original sources

arXiv2026

Competing spin-1 and spin-2 regimes in a frustrated four-leg spin-1/2 ladder

We investigate a frustrated four-leg spin-$1/2$ ladder using density matrix renormalization group calculations. The uniform system displays three regimes: short-range antiferromagnetic legs, short-range ferromagnetic legs, and an effective spin-2 Heisenberg chain, separated by a crossover and a first-order transition. The spin-2 regime is confirmed through its finite string order parameter, edge-localized excitations, and excellent agreement with a projected $S_r=2$ effective Hamiltonian. Recasting the model as two frustrated two-leg ladders coupled by rung and diagonal interactions, we track how the trivial and Haldane phases of an isolated ladder evolve as interladder couplings are introduced. The resulting phase diagrams reveal crossover and first-order lines whose locations are captured by the spin-2 projection and show how singlet- and triplet-dominated regimes reorganize when two ladders merge into a four-leg structure, clarifying the emergence of effective spin-1 versus spin-2 behavior.

cond-mat.str-el↗

arXiv2026

Mixed-spin Heisenberg ladders in a magnetic field

In this work, we study alternating mixed-spin $(s,S)$ Heisenberg ladders in the magnetic field $h$ using density matrix renormalization group and linear spin-wave calculations. The $h$ \textit{versus} interchain coupling $J_\perp$ phase diagram for the $(1/2,1)$ case is investigated in detail. { In particular, we demonstrate the compatibility between the critical line estimates and magnetic ordering by analyzing chains with variable values of $J_\perp$ and of $h$ along the chain, $J_\perp$ and $h$ scans, and considering the usual case of chains with uniform couplings}. The magnetization plateau at 1/3 of saturation magnetization, 1/3 - plateau, is observed for $J_\perp>0$ and in a limited range for $J_\perp<0$. The critical Kosterlitz-Thouless transition point, where the 1/3 - plateau closes, is identified through a finite-size analysis of the transverse spin correlation functions.

cond-mat.str-el↗

arXiv2023

Quantum bicritical point and phase separation in a frustrated Heisenberg ladder

We use the density matrix renormalization group (DMRG) and a hard-core boson map to investigate the quantum phase transitions present in the phase diagram of the frustrated Heisenberg ladder in a magnetic field. The quantum bicritical point is observed at the end of a first-order transition line, which is at the meeting of the two second-order transition lines that bound the fully polarized plateau. The characterization of the bicritical point was made using a hard-core boson mapping of the low-energy excitations from the fully polarized phase and through DMRG by studying the probability density of finding the rung spins in a singlet or a triplet state with zero spin component along the magnetic field. In particular, we give conditions for the exchange couplings for the presence of the first-order transition line and the bicritical point in the phase diagram. Moreover, we unveil the phase-separated states for magnetization values inside the magnetization jump and, in particular, the dependence on the system size of the energy curve as a function of magnetization. Finite-size scaling analysis of the transverse spin correlation functions has been used to estimate the critical points of the Kosterlitz-Thouless transitions from the fractional magnetization plateau $m=1/2$ to the respective gapless Luttinger liquid phases for some sets of parameters.

cond-mat.str-el↗