We present a functional renormalization group calculation for the two-dimensional (t,t )-Hubbard model at Van Hove filling. Using a channel decomposition we describe the momentum and frequency dependence of the vertex function in the normal phase. Compared to previous studies that neglect frequency dependencies we find higher pseudocritical scales and a smaller region of d-wave superconductivity. A large contribution to the effective interaction is given by a forward-scattering process with finite frequency exchange. We test different frequency parametrizations and in a second calculation include the frequency dependence of the imaginary self-energy. We also generalize the channel decomposition to frequency-dependent fermion-boson vertex functions.
We study the two-dimensional repulsive Hubbard model by functional RG
methods, using our recently proposed channel decomposition of the interaction
vertex. The main technical advance of this work is that we calculate the full
Matsubara frequency dependence of the self-energy and the interaction vertex in
the whole frequency range without simplifying assumptions on its functional
form, and that the effects of the self-energy are fully taken into account in
the equations for the flow of the two-body vertex function. At Van Hove
filling, we find that the Fermi surface deformations remain small at fixed
particle density and have a minor impact on the structure of the interaction
vertex. The frequency dependence of the self-energy, however, turns out to be
important, especially at a transition from ferromagnetism to d-wave
superconductivity. We determine non-Fermi-liquid exponents at this transition
point.Comment: 48 pages, 18 figure
The functional renormalization group (RG) in combination with Fermi surface patching is a well-established method for studying Fermi liquid instabilities of correlated electron systems. In this article, we further develop this method and combine it with mean-field theory to approach multiband systems with spin-orbit coupling, and we apply this to a tight-binding Rashba model with an attractive, local interaction. The spin dependence of the interaction vertex is fully implemented in a RG flow without SU(2) symmetry, and its momentum dependence is approximated in a refined projection scheme. In particular, we discuss the necessity of including in the RG flow contributions from both bands of the model, even if they are not intersected by the Fermi level. As the leading instability of the Rashba model, we find a superconducting phase with a singlet-type interaction between electrons with opposite momenta. While the gap function has a singlet spin structure, the order parameter indicates an unconventional superconducting phase, with the ratio between singlet and triplet amplitudes being plus or minus one on the Fermi lines of the upper or lower band, respectively. We expect our combined functional RG and mean-field approach to be useful for an unbiased theoretical description of the low-temperature properties of spin-based materials.
We propose a circuit-level modeling approach for the threshold voltage shift in PMOS devices due to the negative-bias temperature instability (NBTI). The model is suitable forapplication in analog circuit design and reproduces the results of existing digital-stress NBTI models in the limit of two-level stress signals. It accounts for recovery effects during intervals of low stress, and it predicts a stress-pattern dependent saturation of the degradation at large operation times. Since the model can be solved numerically in an efficient way, we have direct access to the threshold voltage shift at arbitrary times, in particular to the exact solution at large operation times, without any approximation. We implement the model via the Cadence Spectre URI. Finally, we make use of the model to compare the aging properties of several analog stress patterns. We furthermorepresent the results of an analog circuit-level NBTI simulation of a ring oscillator
We experimentally and theoretically investigate the NBTI degradation of pMOS devices due to analog stress voltages and thus go beyond existing NBTI studies for digital stress. As a result, we propose a physics-based compact model for analogstress NBTI which builds upon the extensive TCAD analysis of our ultra-short-delay experimental data. The numerical efficiency of the compact model allows its direct coupling to electric circuit simulators and permits to accurately account for NBTI degradation already during circuit design. Our model enables the calculation of the time-dependent NBTI variability of single device and of circuit performance parameters. We demonstrate our NBTI model on an operational amplifier and calculate the mean drift and variability of its offset voltage
In this paper we discuss the criticality of a quantum Ising spin chain with competing random ferromagnetic and antiferromagnetic couplings. Quantum fluctuations are introduced via random local transverse fields. First we consider the chain with couplings between first and second neighbors only and then generalize the study to a quantum analog of the Viana-Bray model, defined on a small world random lattice. We use the Dasgupta-Ma decimation technique, both analytically and numerically, and focus on the scaling of the lattice topology, whose determination is necessary to define any infinite disorder transition beyond the chain. In the first case, at the transition the model renormalizes towards the chain, with the infinite disorder fixed point described by Fisher. This corresponds to the irrelevance of the competition induced by the second neighbors couplings. As opposed to this case, this infinite disorder transition is found to be unstable towards the introduction of an arbitrary small density of long range couplings in the small world models.
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