arXiv subjects

Mirian Garcia-Fernandez

Publications and source records attributed to Mirian Garcia-Fernandez.

At least 19 recordsLinked to original sources

arXiv2026

Unveiling the nature of collective charge excitations in a cuprate superconductor

Emergent symmetry breakings in condensed matter systems are often intimately linked to collective excitations. For example, the intertwined spin-charge stripe order in cuprate superconductors is associated with spin and charge excitations. While the collective behavior of spin excitations is well established, the nature of charge excitations remains to be understood. Here we present a high-resolution resonant inelastic x-ray scattering (RIXS) study of charge excitations in the stripe-ordered cuprate La$_{1.675}$Eu$_{0.2}$Sr$_{0.125}$CuO$_4$. The RIXS spectra consist of both charge and phonon excitations around the charge ordering wave vector. By modeling the momentum-dependent phonon intensity, the charge-excitation spectral weight is extracted over a wide energy range. As such, we reveal the highly dispersive nature of the charge excitations, with an energy scale comparable to the spin excitations. Since charge order and superconductivity in cuprates are possibly driven by the same electronic correlations, determining the interaction strength underlying charge order is essential to establishing a comprehensive microscopic model of high-temperature superconductivity.

cond-mat.supr-con↗

arXiv2026

RIXS Identification of Optical Phonon-Spin Coupling Effects in CrSBr

Resonant inelastic x-ray scattering provides experimental signatures of spin-phonon coupling in CrSBr through temperature-dependent Cr $L$-edge spectra. Low-energy excitations are observed exclusively in the low-temperature antiferromagnetic phase as energy-loss features. A quasi-elastic peak at approximately 42 meV is observed under $π$-polarization. Density functional theory phonon-mode calculations identify these RIXS features as occurring within the same energy range as bond-bending optical phonon modes associated with distortions of the Cr--S--Cr network. The pronounced suppression of these low-energy excitations upon warming into the paramagnetic phase, together with their polarization dependence, the calculated phonon spectrum, and a spin-renormalized electron-phonon RIXS framework, indicates a strong interplay between magnetic correlations and lattice dynamics. While the loss features appear at energies characteristic of optical phonons, the significant overlap of the optical-phonon and magnon bands suggests that the temperature-dependent behavior should not be regarded as purely lattice-derived excitations. Instead, the room-temperature suppression of the low-energy RIXS peaks is explained in terms of a spin-phonon coupling effect on the $L$-edge electron-phonon RIXS mechanism. The interpretation is supported by the combined experimental observations, phonon calculations, and theoretical modeling, rather than by temperature contrast alone. These results support spin-phonon coupling as a plausible and consistent interpretation of the observed temperature-dependent RIXS response and demonstrate that magnetic order can strongly influence phonon-related spectral weight in the RIXS spectrum.

cond-mat.mtrl-sci↗

arXiv2026

$d$-wave altermagnetism revealed by resonant inelastic X-ray scattering

Altermagnetism defines a third fundamental class of collinear magnetic order, featuring compensated magnetic moments with antiparallel spin alignment, yet lifted Kramers degeneracy without the need for relativistic spin-orbit coupling. Its ability to host spin-polarized electronic bands and unconventional chiral magnons makes it a promising platform for functional materials. However, experimental verification has proven challenging; while circular dichroism in resonant inelastic X-ray scattering (RIXS) has been suggested as a signature of chiral magnons, it remains controversial whether this effect is an intrinsic property of altermagnetism or an artifact of experimental geometry. In this work, we resolve this debate and provide unambiguous experimental evidence of $d$-wave altermagnetism in the strongly correlated Lieb-lattice magnet La$_2$O$_3$Mn$_2$Se$_2$. The RIXS spectra exhibit a $d$-wave-symmetry circular dichroism in the magnetic excitations that vanishes in the paramagnetic phase. Through RIXS-operator symmetry analysis and exact-diagonalization calculations, we prove that the observed dichroism is a direct consequence of altermagnetic symmetry constraints, independent of magnon branch splitting. Our results provide definitive evidence for the experimental realization of $d$-wave altermagnetism in La$_2$O$_3$Mn$_2$Se$_2$ and establish circularly polarized RIXS as a highly symmetry-sensitive spectroscopic framework for detecting magnetic phases that evade conventional probes.

cond-mat.str-el↗

arXiv2026

Strongly-coupled hybrid lattice-plasmons in layered cuprates

Metallic systems with delocalized valence electrons host collective charge density oscillations known as plasmons. On the other hand, conventional insulators do not have free electrons and the low energy charge degrees of freedom are pinned to the ions. The fate of the collective charge excitations in the intermediate regime is an outstanding question. This problem is especially important for strongly correlated systems such as the layered cuprates, where unconventional superconductivity and other emergent phenomena arise from valence electrons on the border between Mott localization and itinerancy. Using resonant inelastic X-ray scattering, we track this evolution in the prototypical electron-doped cuprate Nd2-xCexCuO4. We find a continuous transformation of the low-energy charge response: from an acoustic plasmon in the metallic regime, to a gapped hybrid mode at intermediate doping, and finally to a nearly dispersionless 139 meV excitation at half filling. Remarkably, the 139 meV excitation has approximately twice the energy of the oxygen breathing phonon responsible for the dispersion kink observed in angle-resolved photoemission spectroscopy, and is consistent with a putative 2-phonon excitation observed in Raman spectroscopy. These results establish a unified picture of collective charge excitations across the phase diagram of electron-doped cuprates, showing that such modes persist across the Mott transition via strong coupling to lattice degrees of freedom and revealing a missing link in the charge dynamics of carrier doped Mott insulators.

cond-mat.str-el↗

arXiv2026

Coherent spin waves in a maximal entropy phase

In solids, disorder is conventionally regarded as detrimental to coherence. It typically localizes and dampens collective excitations, as exemplified by Anderson localization or the broadening of magnetic modes in systems lacking long-range order. While high-entropy materials are specifically designed to harness disorder and stabilize homogeneous mixed-phase structures that can display unique properties, this same disorder is nonetheless expected to preclude the formation of coherent magnetic excitations. To test the limits of this picture, we selected the antiferromagnetic system YBaCuFeO5, as it features two distinct transition metal atoms with significantly different magnetic moments, rendering its spin dynamics exceptionally sensitive to local atomic ordering. Combining resonant inelastic x-ray scattering and linear spin wave theory, we reveal a surprising paradox: YBaCuFeO5 exhibits an unexpected, entropy-driven mixed phase, in which disorder, rather than reducing the lifetime of the collective excitations, favors coherence. In this mixed phase, the spin waves remain dispersive, markedly distinct from those expected for an ordered ground state, and exhibit well-defined acoustic and optical branches separated by a large optical gap. These results demonstrate that in entropy-stabilized magnets, disorder can favor coherent collective modes previously thought to be exclusive to low-entropy systems.

cond-mat.str-el↗

arXiv2026

$3d_{z^2}$ orbital delocalization and magnetic collapse in superconducting (La,Pr)$_3$Ni$_2$O$_{7-δ}$ films

The recent discovery of Ruddlesden--Popper (RP) nickelate thin-film superconductors has opened a new frontier in unconventional superconductivity. Its realization requires both compressive epitaxial strain and highly oxidative growth conditions, yet the microscopic pathway from the parent phase to the superconducting phase remains elusive. Here, X-ray absorption spectra and resonant inelastic X-ray scattering are employed to track this evolution by independently tuning strain and oxygen content in (La,Pr)$_3$Ni$_2$O$_{7-δ}$ thin films. We uncover a remarkable two-step narrative. First, signatures of delocalization emerge in the same way upon two independent tunings: Spectral weight transfers from a ''Upper Hubbard''-like peak to the hole-like peak associated with O $2p_z$ state, and in parallel, the initially localized Ni $3d_{z^2}$ orbital becomes more itinerant followed by the broadening and weakening of $dd$ orbital excitations. Second, as itinerancy increases, long-range spin-density-wave (SDW) order is suppressed in both intensity and correlation length, indicating direct competition with superconductivity. Yet, short-range magnons persist: they become damped but their bandwidth stays unchanged. Our results paint a coherent picture that both strain and oxygenation drive the RP bilayer nickelates towards the superconducting instability, where the O $2p_z$ and Ni $3d_{z^2}$ orbitals become delocalized. Concomitantly, the long-range magnetic order loses coherence and gets suppressed. These findings establish an orbital-selective route to RP nickelate superconductivity, in which the delocalization of the $2p_z$ and $3d_{z^2}$ orbitals and the robust short-range magnons upon the melting of SDW order are prerequisites, providing strong constraints for theory and the roadmap for designing nickelate superconductors.

cond-mat.supr-con↗

arXiv2026

Dissecting superconductivity in the Ruddlesden-Popper nickelates: The role of electron correlation and interlayer magnetic exchange

The discovery of superconductivity in the Ruddlesden-Popper (RP) nickelates has opened a new chapter in the search for high superconducting transition temperatures ($T_\mathrm{c}$) materials. A central and puzzling feature of this family is the wide variation in $T_\mathrm{c}$ despite their common NiO$_2$ building blocks, as highlighted by the recent observation of superconductivity at $\sim$ 30 K in trilayer $\mathrm{La_4Ni_3O_{10}}$, significantly lower than 80 K reported in bilayer $\mathrm{La_3Ni_2O_7}$. Understanding the factors that control $T_\mathrm{c}$ in this family is therefore of paramount importance. Here, we use resonant inelastic x-ray scattering (RIXS) to investigate the electronic and magnetic excitations of $\mathrm{La_4Ni_3O_{10}}$ in direct comparison with its bilayer counterpart. Our results reveal a markedly different landscape. $\mathrm{La_4Ni_3O_{10}}$ exhibits a more itinerant character, evidenced by broader Ni $dd$ orbital excitations and a strong Ni 3$d$ fluorescence continuum, suggesting weaker electronic correlations than in the bilayer. Despite this, well-defined collective spin excitations persist, including dispersive acoustic and optical magnon branches alongside an incommensurate spin density wave. Using linear spin wave theory, we extract the interlayer superexchange interaction ($J_z$) to be $\sim$ 22 meV, much smaller than that in $\mathrm{La_3Ni_2O_7}$. The weaker correlation and reduced interlayer exchange together provide a consistent explanation for the substantially lower $T_\mathrm{c}$ in the trilayer compound. Our findings establish interlayer magnetic coupling and electronic correlation as key parameters governing superconductivity in layered nickelates and offer critical constraints for understanding the pairing mechanism in this emerging family.

cond-mat.supr-con↗

arXiv2026

X-ray magnetic circular dichroism evidence of intrinsic $d$-wave altermagnetism in rutile-structure NiF$_2$

We present the x-ray magnetic circular dichroism (XMCD) at the Ni $L_{2,3}$-edge as an evidence of the $d$-wave altermagnetism in rutile-structure NiF$_2$. Sizable XMCD signal is observed in excellent agreement with theoretical simulations. Owing to a considerable net magnetization due to spin canting, the XMCD spectrum consists of an altermagnetic signal as well as a non-negligible ferromagnetic contribution. We verify experimentally that the XMCD spectrum can be written as a sum of contributions from altermagnetism and weak ferromagnetism. Two experimental methods to isolate the ferromagnetic contribution are shown to yield essentially the same result. These are dependence of XMCD on applied magnetic fields below the Néel temperature and the XMCD measured in applied field above the Néel temperature. Our results demonstrate the utility of XMCD as a probe for altermagnetic materials with the coexisting weak ferromagnetism induced by the relativistic spin-orbit coupling.

cond-mat.str-el↗

arXiv2026

Disorder-induced damping of spin excitations in Cr-doped BaFe$_2$As$_2$

In doped Hund's metals, such as the iron-based superconductors, effects like charge doping and chemical pressure are often considered the dominant factors. Partial chemical substitution, however, inevitably introduces disorder. Here, we investigate spin excitations in Ba(Fe$_{1-x}$Cr$_x$)$_2$As$_{2}$ (CrBFA) by high-resolution resonant inelastic x-ray scattering (RIXS) for samples with $x = 0, 0.035,$ and $ 0.085$. In CrBFA, Cr acts as a hole dopant, but also introduces localized spins that compete with Fe-derived magnetic excitations. We found that the Fe-derived magnetic excitations are softened and damped, becoming overdamped for $x = 0.085$. At this doping level, complementary angle-resolved photoemission spectroscopy measurements (ARPES) show increased electronic localization and a suppression of the nematic $d_{xz}/d_{yz}$ band splitting present in the parent compound. We thus propose a localized spin model that explicitly incorporates substitutional disorder and Cr local moments, successfully reproducing our key observations. Our findings reveal a case where disorder dominates over charge doping in the case of a Hund's metal.

cond-mat.supr-con↗

arXiv2025

The doping evolution of the charge density wave and charge density fluctuations in La$_{2-x}$Sr$_x$CuO$_4$

Cuprate superconductors show various collective charge correlations that are intimately connected with their electronic properties. In particular, charge order in the form of an incommensurate charge density wave (CDW) order with an in-plane wavevector $δ_{\text{CDW}} \approx $ 0.23--0.35~r.l.u. appears to be universally present. In addition to CDW, dynamic charge density fluctuations (CDF) are also present with wavevectors comparable to $δ_{\text{CDW}}$. CDFs are present up to $\sim300\;$K and have relatively short correlation lengths of $ξ\sim 20$\;Å. Here we use Cu-$L_3$ and O-$K$ resonant inelastic X-ray scattering (RIXS) to study the doping dependence of CDW and CDFs in La$_{2-x}$Sr$_x$CuO$_4$. We fit our data with (quasi)elastic peaks resulting from the CDW and up to four inelastic modes associated with oxygen phonons that can be strongly coupled to the CDFs. Our analysis allows us to separate the charge correlations into three components: the CDW with wavevector $δ_{4a-\text{CDW}} \approx 0.24$ and two CDF components with $δ_{4a-\text{CDF}} \approx 0.24$ and $δ_{3a-\text{CDF}} \approx 0.30$. We find that for $T \approx T_c$ the CDW coexists with the CDFs for dopings near $x=p \sim 1/8$. The $4a$-CDW disappears beyond $x=0.16$ and the $4a$-CDF beyond $x=0.19$, leaving only a weak $3a$-CDF at the highest doping studied, $x=0.22$. Our data suggest that low-energy charge fluctuations exist up to doping $x=0.19=p^{\star}$, where the pseudogap disappears, however, we find no evidence that they are associated with a quantum critical point.

cond-mat.supr-con↗

arXiv2025

Persistent paramagnons in high-temperature infinite-layer nickelate superconductors

The recent discovery of high-temperature superconductivity in hole-doped SmNiO$_2$, exhibiting the record-high transition temperature $T_c$ among infinite-layer (IL) nickelates, has opened a new avenue for exploring design principles of superconductivity. Experimentally determining the electronic structure and magnetic interactions in this new system is crucial to elucidating the mechanism behind the enhanced superconductivity. Here, we report a Ni $L$-edge resonant inelastic x-ray scattering (RIXS) study of superconducting Sm-based IL nickelate thin films Sm$_{1-x-y-z}$Eu$_x$Ca$_y$Sr$_z$NiO$_2$ (SECS). Dispersive paramagnonic excitations are observed in both optimally and overdoped SECS samples, supporting a spin-fluctuation-mediated pairing scenario. However, despite the two-fold enhancement of $T_c$ in the Sm-based nickelates compared to their Pr-based counterparts, the effective exchange coupling strength is reduced by approximately $20\%$. This behavior contrasts with hole-doped cuprates, where magnetic interactions correlate positively with $T_c$, highlighting essential differences in their superconducting mechanisms.

cond-mat.supr-con↗

arXiv2025

Probing d-wave superconducting gap of high-$T_\mathrm{c}$ cuprate $\mathrm{Bi}_2\mathrm{Sr}_2\mathrm{Ca}_2\mathrm{Cu}_3\mathrm{O}_{10+δ}$ by resonant inelastic X-ray scattering

The superconducting gap is a characteristic feature of high-T$_c$ superconductors and provides crucial information on the pairing mechanism underlying high-temperature superconductivity. Here, we employ high-resolution resonant inelastic X-ray scattering (RIXS) at the Cu $L_3$-edge to investigate the superconducting gap in the overdoped cuprate $\mathrm{Bi}_2\mathrm{Sr}_2\mathrm{Ca}_2\mathrm{Cu}_3\mathrm{O}_{10+δ}$ ($T_\mathrm{c}$ = 107 K). By analyzing antisymmetrized, temperature-dependent RIXS spectra over a range of in-plane momentum transfers, we observe a clear suppression of low-energy spectral weight below T$_c$, indicative of superconducting gap formation. This suppression is most pronounced at small momentum transfers ($|\boldsymbol{q}_\parallel| \leq 0.18$ r.l.u.) and corresponds to a gap size of approximately 2$Δ_0 \sim$ 130 meV. Comparison with theoretical calculations of the momentum-dependent charge susceptibility supports a d-wave symmetry of the superconducting gap, while an isotropic s-wave gap fails to reproduce key experimental features. These findings establish RIXS as a powerful, bulk-sensitive probe of superconducting gap symmetry and highlight its utility for studying materials beyond the reach of surface-sensitive techniques such as ARPES and STM.

cond-mat.supr-con↗

arXiv2025

Unconventional polaronic ground state in superconducting LiTi$_2$O$_4$

Geometrically frustrated lattices can display a range of correlated phenomena, ranging from spin frustration and charge order to dispersionless flat bands due to quantum interference. One particularly compelling family of such materials is the half-valence spinel Li$B_2$O$_4$ materials. On the $B$-site frustrated pyrochlore sublattice, the interplay of correlated metallic behavior and charge frustration leads to a superconducting state in LiTi$_2$O$_4$ and heavy fermion behavior in LiV$_2$O$_4$. To date, however, LiTi$_2$O$_4$ has primarily been understood as a conventional BCS superconductor despite a lattice structure that could host more exotic groundstates. Here, we present a multimodal investigation of LiTi$_2$O$_4$, combining ARPES, RIXS, proximate magnetic probes, and ab-initio many-body theoretical calculations. Our data reveals a novel mobile polaronic ground state with spectroscopic signatures that underlie co-dominant electron-phonon coupling and electron-electron correlations also found in the lightly doped cuprates. The cooperation between the two interaction scales distinguishes LiTi$_2$O$_4$ from other superconducting titanates, suggesting an unconventional origin to superconductivity in LiTi$_2$O$_4$. Our work deepens our understanding of the rare interplay of electron-electron correlations and electron-phonon coupling in unconventional superconducting systems. In particular, our work identifies the geometrically frustrated, mixed-valence spinel family as an under-explored platform for discovering unconventional, correlated ground states.

cond-mat.supr-con↗

arXiv2025

Doping dependence of 2-spinon excitations in the doped 1D cuprate Ba$_2$CuO$_{3+δ}$

Recent photoemission experiments on the quasi-one-dimensional Ba-based cuprates suggest that doped holes experience an attractive potential not captured using the simple Hubbard model. This observation has garnered significant attention due to its potential relevance to Cooper pair formation in high-$T_c$ cuprate superconductors. To scrutinize this assertion, we examined signatures of such an attractive potential in doped 1D cuprates Ba$_2$CuO$_{3+δ}$ by measuring the dispersion of the 2-spinon excitations using Cu $L_3$-edge resonant inelastic X-ray scattering (RIXS). Upon doping, the 2-spinon excitations appear to weaken, with a shift of the minimal position corresponding to the nesting vector of the Fermi points, $q_F$. Notably, we find that the energy scale of the 2-spinons near the Brillouin zone boundary is substantially softened compared to that predicted by the Hubbard model in one-dimension. Such a discrepancy implies missing ingredients, which lends support for the presence of an additional attractive potential between holes.

cond-mat.str-el↗

arXiv2024

Emergence of interfacial magnetism in strongly-correlated nickelate-titanate superlattices

Strongly-correlated transition-metal oxides are widely known for their various exotic phenomena. This is exemplified by rare-earth nickelates such as LaNiO$_{3}$, which possess intimate interconnections between their electronic, spin, and lattice degrees of freedom. Their properties can be further enhanced by pairing them in hybrid heterostructures, which can lead to hidden phases and emergent phenomena. An important example is the LaNiO$_{3}$/LaTiO$_{3}$ superlattice, where an interlayer electron transfer has been observed from LaTiO$_{3}$ into LaNiO$_{3}$ leading to a high-spin state. However, macroscopic emergence of magnetic order associated with this high-spin state has so far not been observed. Here, by using muon spin rotation, x-ray absorption, and resonant inelastic x-ray scattering, we present direct evidence of an emergent antiferromagnetic order with high magnon energy and exchange interactions at the LaNiO$_{3}$/LaTiO$_{3}$ interface. As the magnetism is purely interfacial, a single LaNiO$_{3}$/LaTiO$_{3}$ interface can essentially behave as an atomically thin strongly-correlated quasi-two-dimensional antiferromagnet, potentially allowing its technological utilisation in advanced spintronic devices. Furthermore, its strong quasi-two-dimensional magnetic correlations, orbitally-polarized planar ligand holes, and layered superlattice design make its electronic, magnetic, and lattice configurations resemble the precursor states of superconducting cuprates and nickelates, but with an $S \rightarrow 1$ spin state instead.

cond-mat.str-el↗

arXiv2024

Unified Description of Charge Density Waves in Electron- and Hole-doped Cuprate Superconductors

High-temperature cuprates superconductors are characterised by the complex interplay between superconductivity (SC) and charge density wave (CDW) in the context of intertwined competing orders. In contrast to abundant studies for hole-doped cuprates, the exact nature of CDW and its relationship to SC was much less explored in electron-doped counterparts. Here, we performed resonant inelastic x-ray scattering (RIXS) experiments to investigate the relationship between CDW and SC in electron-doped La$_{2-x}$Ce$_x$CuO$_4$. The short-range CDW order with a correlation length $\sim35$~Å~was found in a wide range of temperature and doping concentration. Near the optimal doping, the CDW order is weakened inside the SC phase, implying an intimate relationship between the two orders. This interplay has been commonly reported in hole-doped La-based cuprates near the optimal doping. We reconciled the diverging behaviour of CDW across the superconducting phase in various cuprate materials by introducing the CDW correlation length as a key parameter. Our study paves the way for establishing a unified picture to describe the phenomenology of CDW and its relationship with SC in the cuprate family.

cond-mat.supr-con↗

arXiv2024

Electronic and magnetic excitations in La$_3$Ni$_2$O$_7$

The striking discovery of high-temperature superconductivity (HTSC) of 80 K in a bilayer nickelate La$_3$Ni$_2$O$_7$ under a moderately high pressure of about 14 GPa ignited a new wave of studying HTSC in nickelates. The properties of the parental phase at ambient pressure may contain key information on basic interactions therein and bosons that may mediate pairing giving birth to superconductivity. Moreover, the bilayer structure of La$_3$Ni$_2$O$_7$ may suggest a distinct minimal model in comparison to cuprate superconductors. Here using X-ray absorption spectroscopy and resonant inelastic X-ray scattering, we studied La$_3$Ni$_2$O$_7$ at ambient pressure, and found that Ni 3$d_{x^2-y^2}$, Ni 3$d_{z^2}$, and ligand oxygen 2$p$ orbitals dominate the low-energy physics with a small charge-transfer energy. Remarkably, well-defined optical-like magnetic excitations were found to soften into a quasi-static spin-density-wave ordering, evidencing the strong electronic correlations and rich magnetic properties. Based on a Heisenberg spin model, we found that the inter-layer effective magnetic superexchange interaction is much larger than the intra-layer ones, and proposed two viable magnetic structures. Our results set the foundation for further exploration of La$_3$Ni$_2$O$_7$ superconductor.

cond-mat.supr-con↗

arXiv2023

Weak-signal extraction enabled by deep-neural-network denoising of diffraction data

Removal or cancellation of noise has wide-spread applications for imaging and acoustics. In every-day-life applications, denoising may even include generative aspects, which are unfaithful to the ground truth. For scientific use, however, denoising must reproduce the ground truth accurately. Here, we show how data can be denoised via a deep convolutional neural network such that weak signals appear with quantitative accuracy. In particular, we study X-ray diffraction on crystalline materials. We demonstrate that weak signals stemming from charge ordering, insignificant in the noisy data, become visible and accurate in the denoised data. This success is enabled by supervised training of a deep neural network with pairs of measured low- and high-noise data. We demonstrate that using artificial noise does not yield such quantitatively accurate results. Our approach thus illustrates a practical strategy for noise filtering that can be applied to challenging acquisition problems.

eess.IV↗