RESUMEN
We present a general method to construct translation-invariant and SU(2) symmetric antiferromagnetic parent Hamiltonians of valence bond crystals (VBCs). The method is based on a canonical mapping transforming S=1/2 spin operators into a bilinear form of a new set of dimer fermion operators. We construct parent Hamiltonians of the columnar and the staggered VBCs on the square lattice, for which the VBC is an eigenstate in all regimes and the exact ground state in some region of the phase diagram. We study the departure from the exact VBC regime upon tuning the anisotropy by means of the hierarchical mean field theory and exact diagonalization on finite clusters. In both Hamiltonians, the VBC phase extends over the exact regime and transits to a columnar antiferromagnet (CAFM) through a window of intermediate phases, revealing an intriguing competition of correlation lengths at the VBC-CAFM transition. The method can be readily applied to construct other VBC parent Hamiltonians in different lattices and dimensions.
RESUMEN
We present a canonical mapping transforming physical boson operators into quadratic products of cluster composite bosons that preserves matrix elements of operators when a physical constraint is enforced. We map the 2D lattice Bose-Hubbard Hamiltonian into 2×2 composite bosons and solve it within a generalized Hartree-Bogoliubov approximation. The resulting Mott insulator-superfluid phase diagram reproduces well quantum Monte Carlo results. The Higgs boson behavior in the superfluid phase along the unit density line is unraveled and in remarkable agreement with experiments. Results for the properties of the ground and excited states are competitive with other state-of-the-art approaches, but at a fraction of their computational cost. The composite boson mapping here introduced can be readily applied to frustrated many-body systems where most methodologies face significant hurdles.
RESUMEN
CONTEXT: Prehabilitation is emerging as a method of preparing patients physically and mentally for the often disabling effects of cancer treatment. AIMS: This study aims to assess the feasibility and to explore the potential effects of a prehabilitation program consisting of educational physical exercise sessions in patients with rectal cancer undergoing neoadjuvant chemoradiotherapy treatment (NCRT). SETTINGS AND DESIGN: This was a pilot study with 12 patients (3 males and 9 females, age 61 ± 7 years). SUBJECTS AND METHODS: The program included six educational sessions of exercise during NCRT. Adherence to the intervention; quality of life (QoL); anxiety and depression; body mass index; physical fitness (peak oxygen uptake (VO2peak), handgrip and dynamic leg strength); and physical activity (PA) levels were measured. STATISTICAL ANALYSIS USED: Data are reported as the mean ± standard deviation or medians and interquartile ranges for questionnaire-derived data. Secondary outcome measures were compared using the nonparametric Wilcoxon test. The threshold P value for significance was calculated after correction for multiple comparisons using the Bonferroni method. RESULTS: Adherence to the program was 64 of 72 possible exercise education sessions completed, i.e., 89%. We detected a trend toward a significant improvement in VO2peak after the intervention (P = 0.015), together with reduced scores for both depression (P = 0.017) and the QoL domain "emotional function" (P = 0.027). Mean levels of moderate to vigorous PA tended to increase after the exercise program (P = 0.091). CONCLUSIONS: Exercise might be an effective prehabilitation strategy for surgery during the period of NCRT.