arXiv subjects

João C. Inácio

Publications and source records attributed to João C. Inácio.

4 recordsLinked to original sources

arXiv2026

Probing metal-insulator criticality with cavity photons

The Hubbard model on the honeycomb lattice is a pristine realisation of a semimetal-to-insulator Mott transition belonging to the O(3) Gross-Neveu universality class. We couple this system to a single linearly polarised, uniform cavity photon mode. For this interacting light-matter model, we formulate a negative-sign-free fermion quantum Monte Carlo algorithm that allows for unbiased results on finite system sizes. Our numerical results show that the coupling to the cavity is irrelevant at criticality, even at strong electron-photon coupling. On the other hand, we observe, and show analytically, that the photon spectrum acquires finite corrections in the form of the optical conductivity. In the semimetal phase, gapless particle-hole excitations produce a low-energy polaritonic mode, which is suppressed in the Mott insulator phase. Furthermore, we show that the photon spectra can be used to detect universal information about the quantum critical point. Thereby, our results establish that a single, uniform cavity photon mode acts as a contact-free non-invasive probe for Mott criticality. Our results can be generalised to any cavity where an intensive number of uniform linearly polarised photon modes couple to the electronic system.

cond-mat.str-el↗

arXiv2026

Dynamical magnetotropic susceptibility as a new probe of Kitaev materials and beyond

The magnetotropic susceptibility, $k(ω)$, probes ultra-low-frequency uniform ($\boldsymbol{q}=0$) spin and charge fluctuations in a crystal mounted on an oscillating cantilever: its real part shifts the oscillation frequency, while its imaginary part characterises the induced damping. We derive $k(ω)$ within linear response theory for a generic correlated-electron Hamiltonian, showing that its real part is sensitive to magnetic anisotropy and its imaginary part encodes the uniform dynamical spin susceptibility, even for spin-symmetric insulators, while in metals it reveals eddy-current damping conditions. Using auxiliary-field quantum Monte Carlo, we compute $k(ω)$ for microscopic models of $α$-RuCl$_3$, finding that the low-temperature $k(ω=0)/T$ scaling with $B/T$ is a signature of dominant Kitaev coupling, robust to optical phonons, while the dynamical response shows local-moment-like features. We highlight applications to Kondo destruction quantum criticality, relevant to strange metallicity and unconventional superconductivity.

cond-mat.str-el↗

arXiv2026

Unconstrained compact lattice QED$_{2+1}$ coupled to phonons: Gauss sectors, orthogonal semimetal, and deconfined criticality

Gauge theoretic descriptions of 2D quantum magnets are defined on a restricted physical Hilbert space by imposing Gauss's law. In this work, we study an unconstrained compact lattice QED$_{2+1}$ coupled to bond phonons, in which local Gauss operators are conserved, but their eigenvalues are not a priori fixed. Using exact auxiliary-field quantum Monte Carlo simulations at $N_f = 2$, we investigate how the physical Gauss sector becomes energetically favourable, and how this process is connected to confinement and symmetry breaking. We identify an orthogonal semimetallic (OSM) phase, in which fermions striped off their gauge charge and form a Dirac liquid. This phase mixes various Gauss sectors, and becomes unstable when Gauss's law is explicitly enforced. Upon the imposition of Gauss's law, compactness allows for monopole excitations carrying antiferromagnetic (AFM) and valence-bond-solid (VBS) quantum numbers. Gauge field fluctuations and spinon-phonon coupling tune between the competition of such monopoles and generate a phase diagram with OSM, AFM and VBS orders. The transitions out of the OSM phase coincide with the dynamical generation of Gauss's law, while the competition between AFM and VBS charged monopoles produces a transition consistent with deconfined quantum criticality. Thus our results establish that upon the projection to the physical Hilbert space, deconfinement in compact lattice QED$_{2+1}$ is always unstable towards either AFM or VBS order.

cond-mat.str-el↗

arXiv2025

Finite temperature fermion Monte Carlo simulations of frustrated spin-Peierls systems

The Abrikosov fermion representation of the spin-1/2 degree of freedom allows for auxiliary-field quantum Monte Carlo simulations of frustrated spin systems. This approach provides a manifold of equivalent actions over which the negative sign problem can be optimised. As a result, we can reach temperature scales well below the magnetic scale. Here, we show how to generalise this algorithm to spin-Peierls systems. In contrast to exact diagonalisation approaches, Monte Carlo methods are not Hilbert space bound such that the computational effort per sweep remains invariant when adding phonons. However, the computational effort required to generate independent configurations increases in the presence of phonons. We also show that, for the specific case of the Kitaev-Heisenberg model, the inclusion of phonons does not render the negative sign problem more severe. This new algorithm hence allows us to investigate the interplay between phonon degrees of freedom and magnetic frustration. We present results for frustrated and non-frustrated spin systems.

cond-mat.str-el↗