arXiv subjects

Léo Gaspard

Publications and source records attributed to Léo Gaspard.

6 recordsLinked to original sources

arXiv2026

Paramagnon softening upon doping in oxychloride cuprate superconductors

Understanding how magnetic excitations and exchange interactions evolve with doping is crucial for deciphering the physics that shapes the phase diagram of high-temperature cuprate superconductors. Here, we clearly determine the behavior of the paramagnon dispersion in the oxychloride cuprate Na$_{x}$Ca$_{2-x}$CuO$_2$Cl$_2$ up to the highest doping level. We find that the paramagnon bandwidth, defined by the energy at the $X$ zone boundary $(0.5,0,0)$, remains unchanged within experimental uncertainty, even though it exhibits significant broadening. In contrast, along the nodal direction $Γ-M$ (\textit{i.e.}, from $(0,0,0)$ to $(0.5,0.5,0)$), the paramagnon energy shows a marked softening with doping, with a shift of up to $ΔE\sim150$ meV at the midpoint of the zone for the highest doping, which is about half of its initial value in the antiferromagnetic phase. Calculations of the dynamical spin structure factor for a one-band Hubbard model, including hopping terms up to the third-nearest neighbor ($t^{\prime\prime}$), explain this behavior and accurately reproduce the observed wave vector dependence and directional variation in the Brillouin zone.

cond-mat.supr-con↗

arXiv2026

Spin-polaron fingerprints in the optical conductivity of iridates

As a consequence of their spin-orbit entangled ground state, many $5d^{5}$ iridate materials display a peculiar double peak structure in optical transport quantities, such as absorption and conductivity. Their common interpretation is based on the presence of Hubbard subbands in the half-filled $j_{\mathrm{eff}}=1/2$ manifold. Herein, we challenge this picture, proposing a scenario based on the presence of spin-polaron (SP) quasiparticles, and assigning a dominant SP character to the first peak. We illustrate it by taking the materials Ba$_2$IrO$_4$ and Sr$_2$IrO$_4$ as paradigmatic examples, which we investigate within the dynamical mean-field theory and the self-consistent Born approximation. Both theories reproduce nontrivial features revealed by angle-resolved photoemission spectroscopy and optical transport measurements, supporting our interpretation. In the case of Sr$_2$IrO$_4$, we show how the SP scenario survives in the low-doped regime. Similar optical transport fingerprints are expected to be found in the wider class of $5d^5$ iridates and more generally in strongly correlated antiferromagnetic regimes, such as those found in cuprates.

cond-mat.str-el↗

arXiv2026

Affordable Five-Orbital Dynamical Mean-Field Theory for Layered Iridates and Rhodates

Full $d$-manifold DMFT with numerically exact solvers has remained computationally prohibitive for spin-orbit materials due their scaling and severe sign problem, forcing the community to rely on simplified one- and three-band models that omit the $e_g$ states despite their proximity with the $t_{2g}$ orbitals. We present the first full five-orbital Dynamical Mean-Field Theory (DMFT) calculations including spin-orbit coupling for the layered iridates and rhodates \bio~and \bro, revealing that the correlation effects shift significantly the $e_g$ states through static mean-field corrections rather than dynamical fluctuations. Motivated by this insight, we introduce hybrid-DMFT (hDMFT), which treats these orbitals and their coupling to the low-energy manifold at the mean-field level while maintaining near quantitative accuracy at a drastically reduced computational cost. These calculation establish hDMFT as a practical and accurate method for full $d$-manifold studies of layered iridates and rhodates, enabling systematic investigations of temperature, doping and pressure dependence that were previously computationally intractable.

cond-mat.str-el↗

arXiv2023

The rich phase diagram of the prototypical iridate Ba$_2$IrO$_4$: Effective low-energy models and metal-insulator transition

In the quest of new exotic phases of matter due to the interplay of various interactions, iridates hosting a spin-orbit entangled $j_{\mathrm{eff}}=1/2$ ground state have been in the spotlight in recent years. Also in view of parallels with the low-energy physics of high-temperature superconducting cuprates, the validity of a single- or few-band picture in terms of the $j_{\mathrm{eff}}$ states is key. However, in particular for its structurally simple member Ba$_2$IrO$_4$, such a systematic construction and subsequent analysis of minimal low-energy models are still missing. Here we show by means of a combination of different ab initio techniques with dynamical mean-field theory that a three-band model in terms of Ir-$j_{\mathrm{eff}}$ states fully retains the low-energy physics of the system as compared to a full Ir-$5d$ model. Providing a detailed study of the three-band model in terms of spin-orbit coupling, Hund's coupling and Coulomb interactions, we map out a rich phase diagram and identify a region of effective one-band metal-insulator transition relevant to Ba$_2$IrO$_4$. Compared to available angle-resolved photoemission spectra, we find good agreement of salient aspects of the calculated spectral function and identify features which require the inclusion of non-local fluctuations. In a broader context, we envisage the three- and five-band models developed in this study to be relevant for the study of doped Ba$_2$IrO$_4$ and to clarify further the similarities and differences with cuprates.

cond-mat.str-el↗

arXiv2021

Timescale of local moment screening across and above the Mott transition

A material's phase diagram typically indicates the types of realized long-range orders, corresponding to instabilities in static response functions. In correlated systems, however, key phenomena crucially depend on dynamical processes, too: In a Mott insulator, the electrons' spin moment fluctuates in time, while it is dynamically screened in Kondo systems. Here, we introduce a timescale $t_m$ characteristic for the screening of the local spin moment and demonstrate that it fully characterizes the dynamical mean-field phase diagram of the Hubbard model: The retarded magnetic response delineates the Mott transition and provides a new perspective on its signatures in the supercritical region above. We show that $t_m$ has knowledge of the Widom line and that it can be used to demarcate the Fermi liquid from the bad metal regime. Additionally, it reveals new structures inside the Fermi liquid phase: First, we identify a region with preformed local moments that we suggest to have a thermodynamic signature. Second, approaching the Mott transition from weak coupling, we discover a regime in which the spin dynamics becomes adiabatic, in the sense that it is much slower than valence fluctuations. Our findings provide resolution limits for magnetic measurements and may build a bridge to the relaxation dynamics of non-equilibrium states.

cond-mat.str-el↗

arXiv2021

Java Card Virtual Machine Memory Organization: a Design Proposal

The Java Card Virtual Machine (JCVM) platform is widely deployed on security-oriented components. JCVM implementations are mainly evaluated under security schemes. However, existing implementation are close-source without detail. We believe studying how to design JCVM will improve them and it can be reused by the community to improve Java Card security. In 2018, Bouffard et al. [6] introduced an Operating System (OS) which aims at running JCVM compatible implementation. This OS is compatible with several Commercially available Off-The-Shelf (COTS) components. This is a first step to design a secure JCVM platform. However, some important details are missing to design a secure-oriented Java Card platform. In this article, we focus on the JCVM memory. This memory contains everything required to run JCVM and applets. Currently, JCVM memory is out of the Java Card specification and each JCVM developer use his own approach. Based on the existing tools and documentation, we explain how to extract from the Java Card toolchain every data required by applets and JCVM. When data to store in memory are identified, this article introduces how to organize required data onto JCVM memory.

cs.CR↗