Gapless spin excitations in nanographene-based antiferromagnetic spin-1/2 Heisenberg chains
Chenxiao Zhao, Lin Yang, João C. G. Henriques, Mar Ferri-Cortés, Gonçalo Catarina, Carlo A. Pignedoli, Ji Ma, Xinliang Feng, Pascal Ruffieux, Joaquín Fernández-Rossier, Roman Fasel
备注：
5 figures. arXiv admin note: text overlap with arXiv:2402.13590
Haldane's seminal work established two fundamentally different types of excitation spectra for antiferromagnetic Heisenberg quantum spin chains: gapped excitations in integer-spin chains and gapless excitations in half-integer-spin chains. In finite-length half-integer spin chains, quantization, however, induces a gap in the excitation spectrum, with the upper bound given by the Lieb-Schulz-Mattis (LSM) theorem. Here, we investigate the length-dependent excitations in spin-1/2 Heisenberg chains obtained by covalently linking olympicenes--Olympic rings shaped nanographenes carrying spin-1/2--into one-dimensional chains. The large exchange interaction (J~38 mV) between olympicenes and the negligible magnetic anisotropy in these nanographenes make them an ideal platform for studying quantum spin excitations, which we directly measure using inelastic electron tunneling spectroscopy. We observe a power-law decay of the lowest excitation energy with increasing chain length L, remaining below the LSM boundary. In a long chain with L = 50, a nearly V-shaped excitation continuum is observed, reinforcing the system's gapless nature in the thermodynamic limit. Finally, we visualize the standing wave of a single spinon confined in odd-numbered chains using low-bias current maps. Our results provide compelling evidence for the realization of a one-dimensional analog of a gapless spin liquid.
Quantum Critical Dynamics of a Heisenberg-Ising Chain in a Longitudinal Field: Many-Body Strings versus Fractional Excitations
利用高分辨率太赫兹光谱研究了反铁磁Heisenberg-Ising自旋链化合物BaCo $_2$ V $_2$ O $_8$的量子自旋动力学随温度和纵向磁场的变化.在低于$T_N\simeq 5.5$ K的反铁磁相中观察到了受限的自旋激发。在一个以上的$B_c=3.8$ T的场诱导的无带隙阶段，我们确定多体弦激发以及低能量分数psinon/antipsinon激发通过比较Bethe-Anglesia计算。在$B_c$附近，高能量弦激发被发现是动态占主导地位的分数激发。
