arXiv subjects

Pit Bermes

Publications and source records attributed to Pit Bermes.

4 recordsLinked to original sources

arXiv2026

Two-channel physics in a lightly doped antiferromagnetic Mott insulator revealed by two-hole spectroscopy

Understanding pairing in the strong-coupling regime of doped Mott insulators remains an open problem in the context of cuprate superconductors. We perform ultra-high resolution numerical simulations of spectral functions in the highly underdoped $t-J$ model and discover two coupled branches of hole pairs emerging at low energies in the largely unexplored two-particle spectrum. As spin anisotropy is tuned from the Ising limit to the $SU(2)$-symmetric Heisenberg regime, the lowest $d$-wave pair evolves from a single bipolaronic branch into two hybridized branches separated by an avoided crossing. We explain this behaviour using an effective two-channel model involving a tightly bound bipolaronic state and a second channel associated with two magnetic polarons. The model reproduces the qualitative low-energy spectra and implies near-resonant $d$-wave interactions in the $SU(2)$-symmetric $t-J$ model, consistent with proximity to an emergent Feshbach-type resonance. To probe these predictions experimentally, we propose a Raman spectroscopy scheme for the attractive Hubbard model that can be directly implemented using ultracold atoms in optical lattices. Our work establishes two-particle spectroscopy, beyond single-particle Green's functions, as a powerful tool for revealing the microscopic origins of unconventional superconductivity.

cond-mat.str-el↗

arXiv2024

Operator Valued Flow Equation Approach to the Bosonic Lattice Polaron: Dispersion Renormalization Beyond the Fr\"ohlich Paradigm

We consider the ground state properties of a lattice Bose polaron, a quasiparticle arising from the interaction between an impurity confined to an optical lattice and a surrounding homogeneous Bose-Einstein condensate hosting phononic modes. We present an extension of Wegner's and Wilson's flow equation approach, the operator valued flow equation approach, which allows us to calculate the renormalized dispersion of the polaron and assess the role of two-phonon scattering processes on the dispersion. The results obtained in this way are compared to a variational mean-field approach. We find that in certain impurity phonon interaction regimes the shape of the dispersion is significantly altered by the inclusion of two-phonon scattering events as opposed to only single-phonon scattering events. Moreover, our results predict that a polaronic bound state may emerge, which is not present in Fr\"ohlich-type models that only consider single-phonon scattering events.

cond-mat.quant-gas↗

arXiv2024

Magnetic polarons beyond linear spin-wave theory: Mesons dressed by magnons

When a mobile hole is doped into an antiferromagnet, its movement will distort the surrounding magnetic order and yield a magnetic polaron. The resulting complex interplay of spin and charge degrees of freedom gives rise to very rich physics and is widely believed to be at the heart of high-temperature superconductivity in cuprates. In this paper, we develop a quantitative theoretical formalism, based on the phenomenological parton description, to describe magnetic polarons in the strong coupling regime. We construct an effective Hamiltonian with weak coupling to the spin-wave excitations in the background, making the use of standard polaronic methods possible. Our starting point is a single hole doped into an AFM described by a 'geometric string' capturing the strongly correlated hopping processes of charge and spin degrees of freedom, beyond linear spin-wave approximation. Subsequently, we introduce magnon excitations through a generalized 1/S expansion and derive an effective coupling of these spin-waves to the hole plus the string (the meson) to arrive at an effective polaron Hamiltonian with density-density type interactions. After making a Born-Oppenheimer-type approximation, this system is solved using the self-consistent Born approximation to extract the renormalized polaron properties. We apply our formalism (i) to calculate beyond linear spin-wave ARPES spectra, (ii) to reveal the interplay of ro-vibrational meson excitations, and (ii) to analyze the pseudogap expected at low doping. Moreover, our work paves the way for exploring magnetic polarons out-of equilibrium or in frustrated systems, where weak-coupling approaches are desirable and going beyond linear spin-wave theory becomes necessary.

cond-mat.str-el↗

arXiv2023

Scattering theory of mesons in doped antiferromagnetic Mott insulators: Multichannel perspective and Feshbach resonance

Modeling the underlying pairing mechanism of charge carriers in strongly correlated electrons, starting from a microscopic theory, is among the central challenges of condensed-matter physics. Hereby, the key task is to understand what causes the appearance of superconductivity at comparatively high temperatures upon hole doping an antiferromagnetic (AFM) Mott insulator. Recently, it has been proposed that at strong coupling and low doping, the fundamental one- and two-hole meson-type constituents -- magnetic polarons and bipolaronic pairs -- likely realize an emergent Feshbach resonance producing near-resonant $d_{x^2-y^2}$ interactions between charge carriers. Here, we provide detailed calculations of the proposed scenario by describing the open and closed meson scattering channels in the $t$-$t'$-$J$ model using a truncated basis method. After integrating out the closed channel constituted by bipolaronic pairs, we find $d_{x^2-y^2}$ attractive interactions between open channel magnetic polarons. The closed form of the derived interactions allows us analyze the resonant pairing interactions and we find enhanced (suppressed) attraction for hole (electron) doping in our model. The formalism we introduce provides a framework to analyze the implications of a possible Feshbach scenario, e.g. in the context of BEC-BCS crossover, and establishes a foundation to test quantitative aspects of the proposed Feshbach pairing mechanisms in doped antiferromagnets.

cond-mat.str-el↗