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We report spin wave and DMRG studies of the ground and low-lying excitedstates of uniform and dimerized alternating spin chains. The DMRG procedure isalso employed to obtain low-temperature thermodynamic properties of the system.The ground state of a 2N spin system with spin-1 and spin-1/2 alternating fromsite to site and interacting via an antiferromagnetic exchange is found to beferrimagnetic with total spin $s_G=N/2$ from both DMRG and spin wave analysis.Both the studies also show that there is a gapless excitation to a state withspin $s_G-1$ and a gapped excitation to a state with spin $s_G+1$.Surprisingly, the correlation length in the ground state is found to be verysmall from both the studies for this gapless system. For this very reason, weshow that the ground state can be described by a variational ``ansatz'' of theproduct type. DMRG analysis shows that the chain is susceptible to aconditional spin-Peierls' instability. The DMRG studies of magnetization,magnetic susceptibility ($\chi$) and specific heat show strong magnetic-fielddependence. The product $\chi T$ shows a minimum as a function oftemperature($T$) at low-magnetic fields and the minimum vanishes athigh-magnetic fields. This low-field behaviour is in agreement with earlierexperimental observations. The specific heat shows a maximum as a function oftemperature and the height of the maximum increases sharply at high magneticfields. It is hoped that these studies will motivate experimental studies athigh-magnetic fields.Comment: 22 pages in latex; 16 eps figures available upon reques

Low-lying excited states and low-temperature properties of an alternating spin-1–spin-1/2 chain: A density-matrix renormalization-group study