arXiv subjects

M. M. Scherer

Publications and source records attributed to M. M. Scherer.

4 recordsLinked to original sources

arXiv2018

Excitonic instability and unconventional pairing in the nodal-line materials ZrSiS and ZrSiSe

We use a functional renormalization group (fRG) approach to investigate potential interaction-induced instabilities in a two-dimensional model for the Dirac nodal-line materials ZrSiS and ZrSiSe employing model parameters derived from {\it ab initio} calculations. Our results confirm that the excitonic instability recently found in random-phase approximation for ZrSiS is indeed the leading instability. In the simplest modeling, spin- and charge-excitonic states are degenerate. Beyond this, we show that the fRG analysis produces an energy scale for the onset of the instability in good agreement with the experimentally observed mass enhancement. Additionally, by exploring the parameter space of the model we find that reducing the band splitting increases the instability scale and gives the chance to drive the system into an unconventional superconducting pairing state. The model parameters for the case of the structurally similar material ZrSiSe suggest the $d$-wave superconducting state as the leading instability with a very small critical scale.

cond-mat.str-el↗

arXiv2016

Functional renormalization and mean-field approach to multiband systems with spin-orbit coupling: Application to the Rashba model with attractive interaction

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.

cond-mat.str-el↗

arXiv2016

Tight-binding Rashba model and statistical field theory

This document contains information supplementary to the article [1], but it is self-contained and can be read independently as a pedagogical review. In the first part, we explain our conventions for the tight-binding description of electronic states on the hexagonal Bravais lattice in two dimensions. We derive the Rashba spin splitting from elementary symmetry conditions, and subsequently construct a minimal tight-binding model which displays Rashba spin splitting near the center of the Brillouin zone. Furthermore, we derive the corresponding symmetry conditions for a two-particle interaction in a second-quantized framework. In the second part, we describe our conventions for the temperature Green functions, in terms of which the renormalization group equations in [1] have been formulated. We also provide a brief summary of the most important definitions and properties of temperature Green functions. [1] G. A. H. Schober, K.-U. Giering, M. M. Scherer, C. Honerkamp, and M. Salmhofer, arXiv:1409.7087.

cond-mat.str-el↗

arXiv2010

Modified Fermi-sphere, pairing gap and critical temperature for the BCS-BEC crossover

We investigate the phase diagram of two-component fermions in the BCS-BEC crossover. Using functional renormalization group equations we calculate the effect of quantum fluctuations on the fermionic self-energy parametrized by a wavefunction renormalization, an effective Fermi radius and the gap. This allows us to follow the modifications of the Fermi surface and the dispersion relation for fermionic excitations throughout the whole crossover region. We also determine the critical temperature of the second order phase transition to superfluidity. Our results are in agreement with BCS theory including Gorkov's correction for small negative scattering length a and with an interacting Bose gas for small positive a. At the unitarity point the result for the gap at zero temperature agrees well with Quantum-Monte-Carlo simulations while the critical temperature differs.

cond-mat.quant-gas↗