Two-Mode Variational Monte Carlo Study of Quasiparticle Excitations in Cuprate Superconductors

Abstract

Recent measurements of quasiparticles in hole-doped cuprates reveal highly unusual features: 1) the doping-independent Fermi velocity, 2) two energy scales in the quasiparticle spectral function, and 3) a suppression of the low energy spectral weight near the zone center. The underlying mechanism is under hot debate. We addressed these important issues by a novel two-mode variational Monte Carlo (VMC) study of the t-J model. We obtained results in agreement with the experiments but without invoking extrinsic effects. Besides, we resolved a long standing issue of the sum rule for quasiparticle spectral weights in VMC studies. The electron doped case was also discussed. PACS numbers: 74.25.Jb,71.10.Li,71.10.Fd,71.27.+a Understanding the quasiparticle excitations is a preclude to unravel the mechanism of high-Tc superconductivity. Recent angle-resolved photo-emission spectra (ARPES) in cuprates reveal highly unusual features and lead to hot debates. For example, on the low energy quasiparticle band the nodal Fermi velocity does not seem to increase with increasing doping, and is called a "universal nodal Fermi velocity".[1] This seems to challenge the concept of doped Mott insulators which would naively predict a vanishing Fermi velocity with decreasing doping. On the other hand, ARPES measurements up to higher energies [2][3][4][5][6][7][8][9] revealed that the quasi-particle dispersion along the nodal direction breaks up near the momentum (π/4, π/4) at an energy around 0.3 ∼ 0.4 eV (or the low energy spectral weight near the Γ seems to disappear), and reappeared around 1eV. The two bands are connected in a waterfall fashion in the momentum-energy space. This high energy anomaly has intrigued many theoretical studies and disputes.[2, 10-17] A central concern is whether the anomaly is caused by extrinsic effects due to phonons, or is an intrinsic property of the strongly correlated electrons themselves. We summarize the main important issues that will concern us in this Letter: 1) the doping-independent Fermi velocity, 2) two energy scales in the quasiparticle spectral function, and 3) a suppression of the low energy spectral weight near the zone center.We address the above important issues within the commonly accepted one-band t-J model without invoking other extrinsic effects. The theoretical machinery we use is the Gutzwiller projection variational Monte Carlo (VMC). It proves to give good energy and supports dwave pairing symmetry. [18][19][20][21][22] As for quasiparticle excitations under concern, a projected mean-field excited state is commonly used in the literature.[23-25] For a given momentum k and spin σ, only one such quasiparticle can be constructed. We call such an approach as a single-mode approach (SMA). Unfortunately, the SMA gives only a single low energy band and predicts that the spectral weight is maximal at the center of the Brillouine zone (the Γ point), in contrast to the two energy scales and suppression of low energy spectral weight revealed by ARPES. The spectral weight below the Fermi...

Projective symmetry group classification of chiral spin liquids

“…Apart from energetics, various other physically interesting properties may be calculated from the projected wave function, such as static or dynamic spin structure factors, excitation gaps, modular matrices, etc. 15,66,[113][114][115][116][117][118][119][120] The invariant gauge group (IGG) is an important concept in the phenomenology of quantum spin liquid phases when we view H 0 as a low-energy effective theory. It is defined as the subgroup of gauge transformations G that leave the spinon Hamiltonian H 0 invariant, i.e., g(u) = u for all g ∈ IGG u .…”

Single-Particle Spectral Function Formulated and Calculated by Variational Monte Carlo Method with Application to d -Wave Superconducting State

“…Meanwhile, methods of calculating the spin and charge dynamical structure factors utilizing the variational wave functions for ground and excited states have been proposed recently [24][25][26][27][28]. Some attempts have been made to calculate the excitation spectrum on larger clusters of the t-J model [29,30].…”

Differences in the high-energy kink between hole- and electron-doped high-Tcsuperconductors

“…In order to elucidate the mechanism of the HEK, it is necessary to look into electron-doped HTSCs, too, and to identify the differences and similarities between the hole-and electron-doped HTSCs. Theoretical studies using the t-J model have demonstrated that there is no HEK in the electron-doped HTSCs due to the lack of the incoherent part in the photoemission spectra [18,19], while other theoretical studies have demonstrated that a HEK of the electron-doped HTSCs exists in a high-energy region compared with that of the hole-doped ones due to a charge modulation mechanism [20] or due to the different magnetic susceptibilities of the hole-and electrondoped HTSCs according to a paramagnon-induced HEK mechanism [15]. Since most of the studies of the HEK have been performed in a limited region of momentum space, more systematic investigations of the momentum dependence of the HEK are necessary to understand the origin of the HEK.…”