Recent photoemission (ARPES) experiments on cuprate superconductors provide important guidelines for a theory of electronic excitations in the stripe phase. Using a cluster perturbation theory, where short-distance effects are accounted for by exact cluster diagonalization and long-distance effects by perturbation (in the hopping), we calculate the single-particle Green's function for a striped t-J model. The data obtained quantitatively reproduce salient (ARPES-) features and may serve to rule out "bond-centered" in favor of "site-centered" stripes. [4]. At present, there is an intensive discussion and controversy, whether stripes are directly connected with and beneficial for the microscopic mechanism in HTSC [5][6][7][8][9][10][11]. What is clear, however, is that the apparent presence of static or dynamic stripes crucially influences low-energy excitations and thus the foundations for such a microscopic theory. Evidence for this has recently been accumulated by angleresolved photoemission spectroscopy (ARPES) both on the static stripes in the Nd-LSCO system [3] and on dynamic domain walls in the LSCO compound [12,13]. The electronic structure revealed by ARPES contains characteristic features consistent with other cuprates, such as the flat band at low energy near the Brillouin zone face (k = (π, 0)). In Nd-LSCO, the frequency-integrated spectral weight is confined inside 1D-segments in k-space, deviating strongly from the more rounded Fermi surface expected from band calculations.In this Letter, we present a numerical study of the singleelectron excitations in a striped phase via cluster perturbation theory (CPT) [14] and a detailed comparison with recent ARPES results. The basic idea of our application of the CPT is indicated in Fig.(1): it is based on dividing the 2D plane into alternating clusters of metallic stripes and AF domains. The individial clusters are modeled by the microscopic t-J hamiltonian and solved exactly via exact diagonalization (ED). Then the intercluster hopping linking the alternating metallic and AF domains is incorporated perturbatively via CPT on the basis of the exact cluster Green's functions thus yielding the spectral function of the infinite 2D plane in a striped phase. Using this CPT approach, the important short-distance interaction effects within the stripes are taken into account exactly while longer-ranged hopping effects are treated perturbatively. A study of the properties of experimentally observed stripe phases solely by ED [15] is precluded by the prohibitively large unit cells.The manageable clusters for ED are simply too small to accommodate even a single such unit cell.Our main results are: (i) close to k = (π, 0) we see, like in experiments, a two-component electronic feature (see Fig.(2)): a sharp low-energy feature close to E F and a more broad feature at higher binding energies. Both features can be explained by the mixing of metallic and antiferromagnetic bands at this k-point. (ii) the excitation near (π/2, π/2) is at higher binding energies than the lowene...
The problem of spin-charge separation is analyzed numerically in the metallic phase of the one-band Hubbard model in one dimension by studying the behavior of the single-particle Green's function and of the spin and charge susceptibilities. We first analyze the Quantum-Monte Carlo data for the imaginary-time Green's function within the Maximum Entropy method in order to obtain the spectral function at real frequencies. For some values of the momentum sufficiently away from the Fermi surface two separate peaks are found, which can be identified as charge and spin excitations. In order to improve our accuracy and to be able to extend our study to a larger portion of the Brillouin zone, we also fit our data with the imaginary-time Green's function obtained from the Luttinger-model solution with two different velocities as fitting parameters. The excitation energies associated with these velocities turn out to agree, in a broad range of momenta, with the ones calculated from the charge and spin susceptibilities. This allows us to identify these single-particle excitations as due to a separation of spin and charge. Remarkably, the range of momenta where spin-charge separation is seen extends well beyond the region of linear dispersion about the Fermi surface. We finally discuss a possible extension of our method to detect spin-charge separation numerically in two dimensions.
We have developed a low temperature magnetic force microscope capable of operation down to 6 K in vacuum by using piezoresistive cantilevers. We use the non-contact frequency modulation technique to detect the magnetic force gradient between an iron-coated tip and the sample. We demonstrate the operation of this new instrument by obtaining images of magnetic domains in VHS tape at room temperature, 77 and 6 K. This microscope is ideally suited for the characterization of thin films of high temperature superconductors.
By comparing single-particle spectral functions of t-J and Hubbard models with recent angle-resolved photoemission results for La 2Ϫx Sr x CuO 4 ͑LSCO͒ and Nd-LSCO, we can decide where holes go as a function of doping, and more specifically, which type of stripe ͑bond-, site-centered͒ is present in these materials at a given doping. For dopings greater than about 12% our calculation shows that the holes prefer to proliferate out of the metallic stripes into the neighboring antiferromagnetic domains. The spectra were calculated by a cluster-perturbation technique, for which we present an alternative formulation. Implications for the theory for high-T c superconductivity are discussed.
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