The behavior of long space-time excitations in many-electron systems with ground state degeneracy is explored via multiscale analysis. The analysis starts with an ansatz for the wave function's dual dependence on the N-electron configuration (i.e., both by direct means and by indirect means via a set of order parameters). It is shown that a Dirac-like equation form of the wave equation emerges in the limit where the ratio epsilon (of the average nearest-neighbor distance to the characteristic length of the long-scale phenomenon of interest) is small. Examples of the long scale are the size of a quantum dot, nanotube, or wavelength of a density disturbance. The velocities in the Dirac-like equation are the transition moments of the single-particle momentum operator connecting degenerate ground states. While detailed band structure and the independent quasi-particle picture could underlie the behavior of some systems (as commonly suggested for graphene), the present scaling law results show it is not necessarily the only explanation. Rather, it can follow from the scaling properties of low-lying, long spatial scale excitations and ground state degeneracy, even in strongly interacting systems. The generality of our findings suggests graphene may be just one of many examples of Dirac-like equation behavior. A preliminary validation of our quantum scaling law for molecular arrays is presented. As our scaling law constitutes a coarse-grained wave equation, path integral or other methods derived from it hold great promise for calibration-free, long-time simulation of many-particle quantum systems.
The single-particle hopping between two chains is investigated by exact-diagonalization techniques supplemented by finite-size scaling analysis. In the case of two coupled strongly correlated chains of spinless fermions, the Taylor expansion of the expectation value of the single-particle interchain hopping operator of an electron at momentum
kF in powers of the interchain hopping ${t}_{\ensuremath{\perp}}$ is shown to become unstable in the thermodynamic limit. The single-chain anomalous exponent $\ensuremath{\alpha}$ [characterizing the low-energy density of state $N(\ensuremath{\omega})\ensuremath{\sim}{\ensuremath{\omega}}^{\ensuremath{\alpha}}]$ is shown to be the key parameter that governs the finite-size scaling behavior. In the regime $\ensuremath{\alpha}<{\ensuremath{\alpha}}_{2p}$ $({\ensuremath{\alpha}}_{2p}\ensuremath{\simeq}0.41)$ where transverse two-particle hopping is less relevant than single-particle hopping, the finite-size effects can be described in terms of a universal scaling function. From this analysis it is found that the single-particle transverse hopping behaves as ${t}_{\ensuremath{\perp}}^{\ensuremath{\alpha}/(1\ensuremath{-}\ensuremath{\alpha})},$ in agreement with a random-phase-approximation--like treatment of the interchain coupling. For $\ensuremath{\alpha}>{\ensuremath{\alpha}}_{2p},$ the scaling law is proved to change its functional form, thus signaling the onset of coherent transverse two-particle hopping.
We calculate the single-particle momentum distribution of a quantum many-particle system in the presence of the Coulomb interaction and a confining potential. The region of intermediate momenta, where the confining potential dominates, marks a crossover from a Gaussian distribution valid at low momenta to a power-law behavior valid at high momenta. We show that for all momenta the momentum distribution can be parametrized by a
q-Gaussian distribution whose parameters are specified by the confining potential. The real-space pair-correlation function calculated in this way can, in principle, be used to construct improved exchange-correlation functionals to solve electronic structure problems. Furthermore, we find that the functional form of the probability of transitions between the confined ground state and the
nth excited state is invariant under scaling of the ratio ${Q}^{2}/{\ensuremath{\nu}}_{n}$, where
Q is the transferred momentum and ${\ensuremath{\nu}}_{n}$ is the corresponding excitation energy. Using the scaling variable ${Q}^{2}/{\ensuremath{\nu}}_{n}$ the maxima of the transition probabilities can also be expressed in terms of a
q-Gaussian.