arXiv subjects

Mona Berciu

Publications and source records attributed to Mona Berciu.

At least 19 recordsLinked to original sources

arXiv2026

Magnon-induced hybridization brightens excitons in ferromagnetic semiconductors

Excitons in magnetic semiconductors have energies and spin structures that are sensitive to the underlying magnetic order. We study this coupling in a minimal one-dimensional lattice model of a conduction electron and a valence hole moving in a ferromagnetic background of localized quantum spins, treating the electron, the hole, and the magnon(s) as explicitly resolved degrees of freedom. At zero temperature, spin conservation closes the relevant Hilbert subspaces at the one- or two-magnon level, so the model can be solved essentially exactly with a real-space Green's function method. Comparison against a frozen-spin approximation in which the local moments are replaced by their ordered values identifies which effects are due to emission and absorption of quantum magnons. A frozen background already lifts the spin degeneracy of the exciton and, if the two carriers couple with different strengths to the local moments, it mixes the singlet with the spin-zero triplet exciton. The emission and absorption of quantum magnons goes qualitatively further: it splits and shifts the exciton energies nonlinearly in the exchange coupling even when all frozen-spin effects vanish; for carriers with unequal hopping integrals, it hybridizes the singlet and spin-zero triplet excitons, thereby brightening the dark triplet; and it increases the exciton radius, which in turn enhances the magnon dressing by removing an on-site cancellation. These quantum effects are therefore strongest for extended, Wannier-like excitons, which is precisely the regime that is commonly modeled assuming a static (frozen-spin) magnetic order.

cond-mat.str-el↗

arXiv2026

Phase-preserving steady-state operations beyond Lindblad

We consider a class of non-unitary operations that is naturally implemented via a combination of projective measurement and unitary operators. We implement these operations without using measurements by coupling the system to an infinite set of ancilla states and time-evolving with a single time-independent Hamiltonian. The infinite ancilla enables the main system to reach a local steady state that preserves initial phase information. We prove that these steady state operations are not describable as a mapping from initial to steady state of a time-independent Lindblad equation. As an additional degree of control, we show that the spectral resonance between the system and the ancilla acts as a switch that turns the operation on and off, and we derive a closed-form expression that quantifies the sharpness of this switch. We further find numerically that the phase preservation survives moderate disorder, over a time window set by the size of the ancilla. This Hamiltonian framework therefore provides a route to autonomous quantum control beyond what standard time-independent dissipative engineering can achieve.

quant-ph↗

arXiv2026

Universal thermal breakdown of polaron coherence in one, two, and three dimensions

How a polaron loses its quasiparticle coherence with increasing temperature is a long-standing open problem. Holstein addressed it in 1959 only in the extreme antiadiabatic, strong-coupling limit, while more recent numerically exact approaches are largely restricted to one dimension. Here we solve this problem on square and simple cubic lattices across the weak-, intermediate-, and strong-coupling regimes. We show that the polaron effective mass $m^*$ and inverse lifetime $1/τ$ increase monotonically with temperature until, at $T \sim 0.5\,Ω$, the quasiparticle peak dissolves into a broad incoherent thermal continuum. The phonon frequency $Ω$ therefore defines a universal coherence scale, independent of dimensionality and coupling strength, validating Holstein's prediction far beyond the regime in which it was derived. These results follow from a finite-temperature generalization of the Momentum Average (MA) approximation, yielding a closed-form, diagrammatically derived self-energy that is asymptotically exact in the strong-coupling limit at all temperatures. Benchmark comparisons demonstrate excellent quantitative agreement of the resulting 1D spectral functions with the numerically exact Variational Exact Diagonalization-Finite-Temperature Lanczos Method (VED-FTLM) and finite-$T$ Density Matrix Renormalization Group (DMRG).

cond-mat.str-el↗

arXiv2026

Efficient calculation of real-space lattice propagators in the presence of a Fermi sea

We present an efficient method for computing the real-space propagators (lattice Green's functions) of any tight-binding Hamiltonian in the presence of a Fermi sea with carrier concentration $x$. The method is valid for any lattice, dispersion, and dimension, provided the corresponding $x=0$ propagators are known. We show that suitable combinations of the finite-$x$ particle-addition propagators have a real or imaginary part trivially related to their $x=0$ counterpart, while the remaining part follows from a Kramers-Kronig relation that can be evaluated for all energies at once using the fast Fourier transforms. The computational cost is therefore independent of the dimensionality, unlike that of direct Brillouin-zone integration. The particle-removal propagators follow from general identities. We validate the method against direct integration for hypercubic lattices in one, two, and three dimensions, and use the 2D square lattice to illustrate how the shape of the Fermi surface is imprinted on the spatial structure of the propagators. In particular, at energies far outside the band, the propagator maps converge to the Fraunhofer diffraction pattern whose aperture is the unoccupied part of the Brillouin zone.

cond-mat.other↗

arXiv2026

Phonon spectral functions of low-density polaron metals

We use the density matrix renormalization group (DMRG) to compute the phonon spectral function of a one-dimensional spinless Holstein model doped with a low but finite carrier concentration, $x \leq 0.15$, as a function of the electron-phonon coupling $λ$. To the best of our knowledge, these are the first such results in this regime, complementing extensive prior work at the single-polaron level ($x\to 0$). We find that significant phonon spectral weight is transferred both below, all the way down to $ω=0$, and above the bare phonon energy $Ω$, in stark contrast with the Kohn-anomaly phenomenology expected in the Migdal limit, where weight remains centered near $Ω$ with a kink at $q=2k_F$. No signature of this $2k_F$ kink appears in our results. This behavior is captured qualitatively by the Random Phase Approximation (RPA), and semi-quantitatively, at negligible extra computational cost, by a ``dressed RPA'' scheme in which the electron addition propagator is renormalized using the Momentum Average (MA) approximation for the low-density electron-polaron. By contrast, adding the lowest-order vertex correction to this dressed scheme produces unphysical negative spectral weight, signaling that vertex and propagator dressings must be treated consistently once the propagators are dressed nonperturbatively. Our results provide an efficient approximation for the phonon spectral functions of low-density polaron metals, a regime relevant to weakly doped insulators.

cond-mat.str-el↗

arXiv2026

Magnetic susceptibility of diluted magnetic semiconductors at low carrier densities

We calculate the static longitudinal and the transverse dynamic magnetic susceptibilities of (III,Mn)V diluted magnetic semiconductors, using the random phase approximation, for a simple impurity band model appropriate for the low charge carrier concentration regime. The magnetic susceptibilities are shown to depend sensitively on the amount of positional disorder of the Mn impurities. The results we obtain are consistent with previous studies of the spin wave spectrum and of the spatially inhomogeneous ferromagnetic state of these materials.

cond-mat.mtrl-sci↗

arXiv2026

Hallmark Signatures of Electronic Pairing in Two-Photon Two-Electron Coincidence Angle-Resolved Photoemission Spectroscopy

Understanding strongly correlated quantum materials remains a central challenge in condensed matter physics and materials science. While angle-resolved photoemission spectroscopy (ARPES) has become an indispensable probe of single-quasiparticle excitations, it accesses electronic correlations only indirectly. Here we show that unlike one-photon in, two-electrons out coincidence ARPES ($γ\!\rightarrow\!2e$ 2eARPES), the two-photon in, two-electron out $2γ\!\rightarrow\!2e$ 2eARPES provides a direct and unambiguous probe of electronic pairing. We establish this on general theoretical grounds and substantiate it through large-scale numerical simulations of strongly correlated models with both paired and unpaired ground states. The key result is a model-independent separation in the $(ω_1,ω_2)$ plane of the two photoelectrons' energies, between signal from electrons emitted from the \emph{same} pair and signal from electrons emitted from \emph{different} pairs; this follows from energy conservation alone and is independent of any material-specific assumptions. Our findings demonstrate that $2γ\!\rightarrow\!2e$ 2eARPES can identify pairing and extract the pair binding energy as well as the energy of the 'glue' boson without any sophisticated data analysis or complementary measurements.

cond-mat.str-el↗

arXiv2026

Fingerprints of preformed pairs in two-electron angle-resolved photoemission spectroscopy

We use variational exact diagonalization (VED) to calculate the two-electron removal spectral weight for the Hubbard-Holstein model, starting from the ground-state with two electrons on a one-dimensional chain. We argue that this spectral weight provides a valuable proxy for the intensity of 2eARPES processes. Our results show that when contrasted to the presumably larger signal due to two electrons ejected from two different pairs, the presumably weaker signal due to two electrons ejected from the same pair (i) is segregated in energy, appearing at a lower binding energy, and (ii) has a very characteristic momentum dependence, with a different symmetry than that of the signal corresponding to two electrons emitted from two different pairs. We verify that these fingerprints appear for pairs with different symmetries, and prove that they arise as a direct consequence of momentum and energy conservation, therefore they are generic for any model with electron-boson coupling that can lead to formation of electron pairs. Experimental observation of these fingerprints will confirm the existence of pairs. Moreover, the momentum dependence map allows one to distinguish whether the pairs are coherent (superconducting) or not. Finally, we argue that these considerations generalize to finite but low electron concentrations, finite temperatures and higher dimensions.

cond-mat.str-el↗

arXiv2026

Limits of the non-Hermitian description of decay models

We present a general proof that non-Hermitian dynamics and Lindblad dynamics with only decay terms are equivalent in the highest particle subspace. We then propose an unbiased method to determine if a system's dynamics in the highest-particle subspace is non-Hermitian. We exemplify this for a simple two-site decay system connected to two baths, and find that the exact solution is well approximated by non-Hermitian dynamics only in the weak-coupling and in the singular-coupling limits, where a Lindbladian description was already known to be accurate. The fact that an accurate non-Hermitian description is so limited, even for such a simple system, raises doubts about how valid such descriptions are for more complicated systems away from these asymptotic limits. Finally, we prove that for models with a nondegenerate system Hamiltonian, exceptional points cannot occur in the weak-coupling limit. This result is relevant for the design of experiments that aim to identify such exceptional points.

quant-ph↗

arXiv2026

Sharp transitions in the spectra of small Frenkel-like excitons for multi-orbital lattice systems

We propose a method for calculating exciton spectra and wavefunctions for model lattice Hamiltonians, based on real-space electron-hole propagators. We verify that our results agree with those of the continuum approximation in the limit of large Wannier excitons, and propose a simple criterion to estimate the exciton size above which the continuum approximation is quantitatively accurate. We then investigate simple one- and two-dimensional multi-orbital lattice models and show that small, Frenkel-like excitons, whose size approaches the lattice constant, can display physics that disagrees with the simplest continuum descriptions (a single-valley quadratic expansion around the minimum gap) not just quantitatively, but qualitatively. Specifically, we identify sharp transitions in the character and momentum of the lowest-energy exciton, enabled by the multi-orbital nature of the lattice models.

cond-mat.str-el↗

arXiv2025

Advantage of Warm Starts for Electron-Phonon Systems on Quantum Computers

Simulating electron-phonon interactions on quantum computers remains challenging, with most algorithmic effort focused on Hamiltonian simulation and circuit optimization. In this work, we study the single-electron Holstein model and propose an initial-state ansatz that substantially enhances ground state overlap in the strong coupling regime, thereby reducing the number of iterations required in standard quantum phase estimation. We further show that this ansatz can be implemented efficiently and yields an exponential reduction in overall circuit costs relative to conventional initial guesses. Our results highlight the practical value of incorporating physical intuition into initial state preparation for electron-phonon coupled systems.

quant-ph↗

arXiv2025

Extended Hubbard Model realized in 2D clusters of molecular anions

The Hubbard model, despite its simplicity, is remarkably successful at describing numerous many-body phenomena. However, due to the small class of problems which can be solved exactly, there has been substantial interest in quantum simulations of extended Hubbard models to in turn, simulate materials and the interaction-driven phases they host. Here, we study small clusters of molecular anions of 3,4,9,10-perylene tetracarboxylic dianhydride on NaCl bilayers on Ag(111) using non-contact Atomic Force Microscopy, Electrostatic Force Spectroscopy, and Scanning Tunnelling Microscopy and Spectroscopy, and show that the occupation and transition energies are well described by an extended Hubbard model. In particular, asymmetric clusters of four molecules require the addition of differing inter-site electrostatic interaction terms and on-site potentials, as well as asymmetric hoping terms. With $t<<U$, occupation asymmetry is driven by these terms, independent of U. The good agreement between the model and the data indicate such molecular anion clusters could be used to probe larger systems and a more varied phase space of realistic fermionic Hubbard models.

cond-mat.mes-hall↗

arXiv2025

Angle-resolved photoemission intensity for multi-orbital bands: Complex interplay between the self-energy matrix and the optical matrix elements

We use a simple one-dimensional two-band model with electron-phonon coupling to illustrate some of the complications that arise in multi-band systems when trying to extract a self-energy using the typical approach used for single-band systems when analyzing angle-resolved photoemission spectroscopy (ARPES) data. The underlying reason is that in multi-band models the self-energy is a matrix, not a scalar, and the result obtained from the ARPES analysis is a complicated function of all these self-energy matrix elements, weighted by different dipole matrix elements of the relevant Wannier orbitals. We contrast the results for Holstein and Peierls electron-phonon couplings to further illustrate differences between models with a local versus non-local self-energy matrix.

cond-mat.str-el↗

arXiv2025

Nature of Spinons in 1D Spin Chains

We provide an intuitive understanding of the collective low-energy spin excitation of the one-dimensional spin-1/2 antiferromagnetic Heisenberg chain, known as the spinon. To this end, we demonstrate how a single spinon can be excited by adding one extra spin to the ground state. This procedure accurately reproduces all key features of the spinon's dispersion. These follow from the vanishing norm of the excited state which is triggered by the ground state entanglement. Next, we show that the spinon dispersion can be approximately reproduced if we replace the true ground state with the simplest valence-bond solid. This proves that the spinon of the one-dimensional Heisenberg model can be understood as a single spin flowing through a valence-bond solid.

cond-mat.str-el↗

arXiv2025

Spectral signatures of residual electron pairing in the extended-Hubbard-Su-Schrieffer-Heeger model

We study the electron addition spectrum of the one-dimensional extended Hubbard-Su-Schrieffer-Heeger (HSSH) model in the dilute limit using the density matrix renormalization group method. In addition to the expected renormalization to the band structure, we find that the electron-phonon (e-ph) interaction produces an anomalous spectral feature when electrons are added in the singlet channel but which is absent in the triplet channel. By comparing these results with those obtained from perturbation theory in the antiadiabatic limit, we demonstrate that this anomalous feature is a remnant of the strong electron-electron interaction mediated by the SSH coupling previously derived in the two-particle limit. By studying the evolution of this feature as a function of doping, we track the fate of this attraction to higher carrier concentrations and provide predictions for the spectral features to help guide future searches for strong e-ph mediated pairing.

cond-mat.str-el↗

arXiv2025

Intertwined charge and spin instability of La$_3$Ni$_2$O$_7$

Research on nickel-based superconductors has progressed from infinite-layer LaNiO$_2$ to finite-layer La$_{6}$Ni$_{5}$O$_{12}$, and most recently to the Ruddlesden-Popper phase La$_3$Ni$_2$O$_7$, which was found to exhibits onset of superconductivity at $\sim$80\,K under a pressure of $\sim$16\,GPa. Unlike the superconductivity mainly driven by the $d_{x^2-y^2}$ orbital in infinite-layer nickelates, the Ni-$d_{z^2}$ and O-2$p$ orbitals contribute significantly to the low energy states and potentially to the superconducting electron pairing mechanism of La$_3$Ni$_2$O$_7$. Employing density functional calculations and multi-orbital multi-atom cluster exact diagonalization including local exchange and Coulomb interactions, here we analyze the pressure dependent low-energy electronic states of the Ni$_2$O$_9$ cluster, relevant for the bilayer phase of La$_3$Ni$_2$O$_7$. The various possible spin states and the exchange and superexchange mechanisms of the Ni$_2$O$_9$ cluster are quantified via the involvement of the Ni-$3d_{3z^2-r^2}$ orbitals and the atomic Hund's rule exchange, the apical bridging O-$2p_z$ orbitals, and the orbitals involved in the formation of local Zhang-Rice singlet like states. We find that the leading configurations contributiong to the cluster ground-states both for nominal valence and also with local charge fluctuations, do not involve occupation of the apical oxygen, instead they favor formation of in-plane Zhang-Rice singlet like states between an O ligand hole and the Ni $3d_{z^2-y^2}$ orbital. We also highlight two possible charge and spin ordered states suggested by our cluster results, that are nearly degenerate at all relevant pressures within our modelling.

cond-mat.supr-con↗

arXiv2025

Signature of preformed pairs in angle-resolved photoemission spectroscopy

We use density matrix renormalization group (DMRG) and variational exact diagonalization (VED) to calculate the single-electron removal spectral weight for the Hubbard-Holstein model at low electron densities. Tuning the strength of the electron-phonon coupling and of the Hubbard repulsion allows us to contrast the results for a liquid of polarons versus a liquid of bipolarons. The former shows spectral weight up to the Fermi energy, as expected for a metal. The latter has a gap in its spectral weight, set by the bipolaron binding energy, although this is also a (strongly correlated) metal. This difference suggests that angle-resolved photoemission spectroscopy could be used to identify liquids of pre-formed pairs. Furthermore, we show that the one-dimensional liquid of incoherent bipolarons is well approximated by a "Bose sea" of bosons that are hard-core in momentum space, occupying the momenta inside the Fermi sea but otherwise non-interacting. This new proposal for a strongly-correlated many-body wavefunction opens the way for studying various other properties of incoherent (non-superconducting) liquids of pre-formed pairs in any dimension.

cond-mat.str-el↗

arXiv2024

Investigating the role of anion polarizability in Fe-based superconductors via light-matter interaction

The polarizability of nearby ions may have a significant impact on electron interactions in solids, but only limited experimental data are available to support this picture. In this work, using a highly simplified description of the prototypical FeAs superconducting layer, we show how external optical excitation of the As 4p-5s splitting can lead to a significant modulation of the polarization-mediated effective interactions between carriers. Our results suggest that even perturbative external fields, approximately two orders of magnitude smaller than the internal field generated by charge carriers, might enable the exploration of the role of the anion's polarizability in determining the correlated physics, although more detailed modeling is needed to decide optimal ways to achieve this.

cond-mat.supr-con↗