We propose a path-integral variant of the DMRG method to calculate real-time correlation functions at arbitrary finite temperatures. To illustrate the method we study the longitudinal autocorrelation function of the XXZ-chain. By comparison with exact results at the free fermion point we show that our method yields accurate results up to a limiting time which is determined by the spectrum of the reduced density matrix. The density-matrix renormalization group (DMRG) 1 is today a well established numerical method to study ground state properties of one-dimensional quantum systems. Within the last few years the DMRG method has been generalized to allow also for the calculation of spectral functions 2 and, quite recently, to incorporate directly real-time evolution.3 The most powerful variant of DMRG to calculate thermodynamic properties is the density-matrix renormalization group applied to transfer matrices (TMRG). This method has been proposed by Bursill et al. 4 and has later been improved considerably.
5The main idea of TMRG is to express the partition function Z of a one-dimensional quantum model by that of an equivalent two-dimensional classical model obtained by a Trotter-Suzuki decomposition. 6 Thermodynamic quantities can then be calculated by considering a suitable transfer matrix T for the classical model. The main advantage of this method is that the thermodynamic limit (chain length L → ∞) can be performed exactly and that the free energy in the thermodynamic limit is determined solely by the largest eigenvalue of T . The TMRG has been applied to calculate static thermodynamic properties for a variety of one-dimensional systems including spin chains, the Kondo lattice model, the t − J chain and ladder, and also spin-orbital models.
7,8,9The Trotter-Suzuki decomposition of a onedimensional quantum system yields a two-dimensional classical model with one axis corresponding to imaginary time (inverse temperature). It is therefore straightforward to calculate imaginary-time correlation functions (CFs) using the TMRG algorithm. Although the results for the imaginary-time CFs obtained by TMRG are very accurate, the results for real-times (real-frequencies) involve large errors because the analytical continuation poses an ill-conditioned problem. In practice it has turned out that the maximum entropy method is the most efficient and reliable way to obtain spectral functions from TMRG data. The combination of TMRG and maximum entropy has been used to calculate spectral functions for the XXZ-chain 10 and the Kondo-lattice model. 8 However, this method involves intrinsic errors due to the analytical continuation which cannot be resolved.Here we propose a method to calculate directly realtime CFs at finite temperature by a modified TMRG algorithm thus avoiding an analytical continuation. We start by considering the two-point CF for an operator O r (t) at site r and time tTr (e −βH ) where ǫ = β/M so that the partition function Z = exp(−βH) becomes(3) With it → τ in Eq.(1) and inserting the identity operator at e...