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We study the spin-half Heisenberg chain with alternating nearest neighborinteractions $J_1(1+\delta)$ and $J_1(1-\delta)$ and a uniform second neighborinteraction $J_2=y (1-\delta)$ by series expansions around the limit ofdecoupled dimers ($\delta=1$). By extrapolating to $\delta=0$ and tuning $y$,we study the critical point separating the power-law and spontaneouslydimerized phases of the spin-half antiferromagnet. We then focus on thedisorder line $y=0.5$, $0\le \delta\le 1$, where the ground states are knownexactly. We calculate the triplet excitation spectrum, their spectral weightsand wavevector dependent static susceptibility along this line. It is wellknown that as $\delta \to 0$, the spin-gap is still non-zero but the tripletsare replaced by spinons as the elementary excitations. We study this dynamicaltransition by analyzing the series for the spectral weight and the staticsusceptibility. In particular, we show that the spectral weight for thetriplets vanishes and the static spin-susceptibility changes from a simple poleat imaginary wavevectors to a branch cut at the transition.Comment: RevTeX, 3 figures, updated references and figure caption

physical review. b, condensed matter

Dynamical transition from triplets to spinon excitations: A series expansion study of the<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi>J</mml:mi></mml:mrow><mml:mrow><mml:mn>1</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mo>−</mml:mo><mml:mi>J</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mo>−</mml:mo><mml:mi>δ</mml:mi></mml:math>spin-<mml:math xmlns:mml="http…

Dynamical transition from triplets to spinon excitations: A series expansion study of theJ1−J2−δspin-