arXiv subjects

Mercè Roig

Publications and source records attributed to Mercè Roig.

18 recordsLinked to original sources

arXiv2026

Multi-orbital physics in inverse Lieb lattice altermagnets

The inverse Lieb lattice has recently emerged as a promising platform for altermagnetism, with several materials with this structure proposed as $d$-wave altermagnetic candidates. Here, we develop a symmetry-based microscopic Hamiltonian for these materials that includes both sublattice and orbital degrees of freedom, going beyond the sublattice-only minimal models that have been extensively used to study such altermagnets. We apply these models to examine multi-orbital electron correlation physics in the vanadium oxychalcogenide family altermagnets, which contain dominant $xy$ and $xz/yz$ orbitals character at the Fermi level in the altermagnetic state. We demonstrate that $xy$ orbitals are crucial to stabilize the altermagnetic state observed within a single V$_2$O layer, and altermagnetic order in the $xz/yz$ orbitals is induced through Hund's coupling. Additionally, we show that these multi-orbital models reveal topological regimes in which topological edge states are naturally orbital selective.

cond-mat.str-el↗

arXiv2026

Symmetry-required Orbital Selectivity in Monolayer FeSe

Orbital-selective correlations have been observed to play an important role in Fe-based superconductors. Here, in contrast to previous site-local Mott transition-based origins, we present a band-theory-based mechanism for orbital-selective physics in monolayer FeSe, for which only electron pockets appear. Underlying our mechanism is the observation in density functional theory (DFT) calculations that around the M point in the Brillouin zone, antiferromagnetic fluctuations are strongly coupled to electrons in $x^2-y^2$ orbitals but weakly coupled to those in $\{xz,yz\}$ orbitals. Symmetry-arguments reveal that this orbital selective coupling originates from the different intertwined orbital and Fe-site sublattice Bloch wavefunctions for these two sets of orbitals at the M point, specifically, the $x^2-y^2$ orbitals can be Fe-site localized. The strong coupling of electrons in $x^2-y^2$ orbitals to the magnetic fluctuations enables orbital-selective electronic renormalizations that can account for important features of our angle-resolved photoemission spectroscopy (ARPES) measurements. Our symmetry-required mechanism for orbital selective physics can be generalized to a range of crystal space groups with four-fold and six-fold screw axes.

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arXiv2026

Nonreciprocal Equilibrium Josephson Effect of Arbitrary Periodicity from Poor Man's Majorana Zero Modes

We show that the Josephson diode effect becomes possible when two coupled antiferromagnetic dimers of point-like magnetic adatoms are deposited on top of a Rashba superconductor. The degree of nonreciprocity is substantial when the arising Yu-Shiba-Rusinov (YSR) bound states approach zero energy. In this limit, these states behave as weakly coupled poor man's Majorana (PMM) excitations. This PMM regime is accompanied by highly dispersive and phase-bias asymmetric Andreev bound state dispersions. In turn, these result in a nonreciprocal Josephson current, whose diode efficiency can be controlled by varying the geometric details of the adatom's spatial configuration. In addition, thanks to spin-triplet pairing terms mixing different YSR states, the Josephson current can possess any periodicity in equilibrium, including $4π$. Our work opens the door to observing and harnessing Majorana behavior in currently experimentally accessible topologically trivial systems.

cond-mat.supr-con↗

arXiv2026

Inherent momentum-dependent gap structure of altermagnetic superconductors

Altermagnetic metals break time-reversal symmetry and feature spin-split Fermi surfaces generated by compensated Néel-ordered collinear magnetic moments. Being metallic, such altermagnets may undergo a further instability at low temperatures to a superconducting state, and it is an interesting open question what the salient features are of such altermagnetic superconductors. We address this question on the basis of realistic microscopic models that capture the altermagnetic sublattice degrees of freedom. We find that the sublattice structure can strongly affect the superconducting gap structure in altermagnetic superconductors. In particular, it imposes nodes in the gap on the Brillouin zone edges for superconductors stabilized by momentum-independent bare attraction channels. We contrast this to the case of superconductivity generated by extended range interactions where pairing is allowed on the Brillouin zone edges and both spin-singlet and equal-spin-pairing triplet states can be stabilized. Equal-spin-pairing triplet superconductivity is generically favored in the limit of large altermagnetic spin splitting of the bands compared to the superconducting gap scale, and features characteristic nonunitary properties arising from the altermagnetic order.

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arXiv2026

Superconductivity induced by altermagnetic spin fluctuations in high-pressure MnB$_4$

Recent experiments found superconductivity in nonmagnetic MnB$_4$ with a high critical temperature ($T_{c}$) reaching 14 K at 158 GPa. However, ab initio calculations of the electron-phonon coupling predict a $T_{c}$ below 1 K, suggesting that a conventional mechanism cannot explain this phenomenon. In this Letter, we find that MnB$_4$ is close to an altermagnetic instability in density-functional theory calculations. We propose that the superconductivity is driven by altermagnetic spin fluctuations. To verify the pairing symmetry, we have constructed a two-orbital tight-binding model, where boron states at the Fermi level are integrated out. Using this model, we identify an extended-$s$ symmetry as the leading pairing instability. If confirmed, this will be the first reported case of superconductivity driven by altermagnetic spin fluctuations.

cond-mat.supr-con↗

arXiv2025

Origin of sublattice particle-hole asymmetry in monolayer FeSe superconductors

In iron-based superconductors, the two Fe atoms in the unit cell are typically related by crystal symmetries; therefore, we expect no intra-unit cell variations in the superconducting gap. However, recent experiments have challenged this expectation, reporting intra-unit cell variations in the gap with an unusual particle-hole asymmetry. Here, we examine the origin of this asymmetry between the two Fe sublattices in monolayer FeSe grown on SrTiO$_3$. We reveal that, in addition to the substrate-induced broken inversion symmetry, substrate nematic symmetry breaking is key to observing this asymmetry. We further identify two possible mechanisms through which this can occur. The first is through an odd-parity gap function that coexists with an extended $s$-wave function. The second is via a nodeless $d$-wave gap function that develops in the presence of a symmetry-breaking substrate. We argue that the latter mechanism is more physical. To test our theory, we performed scanning tunneling spectroscopy measurements across the nematic domain walls, which exhibit a clear enhancement of the asymmetry between the two Fe sublattices. In addition, we reveal that the observed sublattice particle-hole asymmetry is associated with odd-frequency pairing correlations, providing an experimental realization of this unusual pairing correlation.

cond-mat.supr-con↗

arXiv2025

Odd-parity Magnetism Driven by Antiferromagnetic Exchange

Realizing odd-parity, time-reversal-preserving, non-relativistic spin splitting is a central goal for spintronics applications. We propose a group-theory-based microscopic framework to induce odd-parity spin splitting from coplanar antiferromagnetic (AFM) states without spin-orbit coupling (SOC). We develop phenomenological models for 421 conventional period-doubling AFM systems in non-symmorphic space groups and construct minimal microscopic models for 119 of these. We find that these AFM states can attain three possible competing ground states. These ground states all break symmetries in addition to those broken by the usual AFM order. Specifically, they give rise to either odd-parity spin-splitting, nematic order, or scalar odd-parity order related to multiferroicity. Our microscopic theories reveal that the odd-parity spin-splitting energy scale is generically large and further reveal that the scalar odd-parity order gives a non-zero Berry curvature dipole without SOC. We identify 67 materials in the Magndata database for which our theory applies. We provide DFT calculations on FeSe that reveal an $h$-wave spin splitting consistent with our symmetry arguments and apply our microscopic model to determine the non-relativistic Edelstein response for CeNiAsO.

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arXiv2025

Gapless superconductivity from extremely dilute magnetic disorder in 2H-NbSe2-xSx

Most superconducting materials exhibit a vanishing density of states at the Fermi level and Anderson's theorem posits that the superconducting gap is robust against nonmagnetic disorder. Although dilute magnetic impurities lead to localized in-gap states, these states typically have no bearing on the material's bulk superconducting properties. However, numerous experiments reveal a finite density of states at the Fermi level in systems with an apparently negligible number of magnetic impurities. Here, using scanning tunneling microscopy and self-consistent Bogoliubov-de Gennes calculations, we find that gapless superconductivity emerges in 2H-NbSe2-xSx at remarkably low magnetic impurity concentrations. Furthermore, our density functional theory calculations and in-gap quasiparticle interference measurements demonstrate that the Se-S substitution significantly modifies the band structure. This modification favours nesting and dictates the in-gap scattering for x>0, in stark contrast to the dominant charge density wave interactions in pure 2H-NbSe2. Our findings reveal an unusual superconducting response to disorder and highlight the importance of incorporating material-specific band structures in the understanding of a superconductor's response to even very low concentrations of magnetic impurities.

cond-mat.supr-con↗

arXiv2025

Local signatures of altermagnetism

Altermagnets constitute a class of collinear compensated Néel ordered magnets that break time-reversal symmetry and feature spin-split band structures. Based on versatile microscopic models able to capture the altermagnetic sublattice degrees of freedom, we study characteristic local signatures of altermagnetism near disorder sites. We give a complete list of two-dimensional models that exhibit altermagnetism classified by their corresponding layer groups. Specifically, we calculate the local density of states in the vicinity of pointlike nonmagnetic impurities and expose its spatial dependence for two minimal models showcasing $d$-wave and $g$-wave altermagnetism. The momentum structure of the nodes ($d$-wave, $g$-wave, etc.) is directly imprinted on the total local density of states, thus measurable by scanning tunneling conductance experiments. This signature is present both in the spin-resolved as well as the spin-summed local density of states. We find a weaker response in the nonmagnetic state from the anisotropic crystal environment and uncover the importance of the sublattice degree of freedom to model altermagnets. We also study coexistence phases of altermagnetism and superconductivity and provide predictions for the local impurity response of in-gap bound states. The response of impurity bound states strongly enhances the distinct altermagnetic signature.

cond-mat.str-el↗

arXiv2025

Quasi-symmetry Constrained Spin Ferromagnetism in Altermagnets

Altermagnets break time-reversal symmetry and their spin-orbit coupling (SOC) allow for an anomalous Hall effect (AHE) that depends on the direction of the Néel ordering vector. The AHE and the ferromagnetic spin moment share the same symmetry and hence are usually proportional. However, density functional theory (DFT) calculations find that the AHE exists with negligible ferromagnetic spin moment for some compounds, whereas it reaches sizable values for other altermagnets. By examining realistic minimal models for altermagnetism in which the DFT phenomenology is captured, we uncover a general SOC-enabled quasi-symmetry, the uniaxial spin space-group, that provides a natural explanation for the amplitude of the ferromagnetic spin moment across the vast range of different altermagnetic materials. Additionally, we derive analytic expressions for the magnetic anisotropy energy, providing a simple means to identify the preferred Néel vector orientation for altermagnets.

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arXiv2025

Correlation Enhanced Electron-Phonon Coupling in FeSe/SrTiO$_3$ at a Magic Angle

While a predictive theory for unconventional superconductivity in Fe-based superconductors remains elusive, an extensively debated aspect is the interaction between phonons and strongly correlated electrons, and its potential role in the pairing mechanism. Here, through the combination of first principles dynamical mean field theory calculations and epitaxial growth of the single-layer FeX (X=Se, S, Te) on SrTiO$_3$ (STO)(001) substrate, which facilitates the controlled distortion of the FeX$_4$ tetrahedron, we demonstrate an unique superconducting dome where the superconducting gap peaks at a `magic' angle of the FeX$_4$ tetrahedron and the electron-phonon coupling (EPC) for the A$_{1g}$ mode is maximized for the FeSe film. Our findings uncover a significant role of electronic correlations in strengthening Cooper pairing in unconventional superconductors by enhancing EPC.

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arXiv2025

Altermagnetism from coincident Van Hove singularities: application to $κ$-Cl

Realizing two-dimensional (2D) altermagnets is important for spintronics applications. Here we propose a microscopic template for stabilizing 2D altermagnetism through Van Hove singularities that are coincident in both energy and momentum. These coincident Van Hove singularities are a generic consequence of non-symmorphic symmetries in nine 2D space groups. Due to nontrivial symmetry properties of the Hamiltonian, these coincident Van Hove singularities allow new hopping interactions between the Van Hove singularities that do not appear in analogous Van Hove singularity based patch models for cuprates and graphene. We show these new interactions can give rise to various weak coupling, and BCS-based instabilities, including altermagnetism, nematicity, inter-band d-wave superconductivity, and orbital altermagnetic order. We apply our results to quasi-2D organic $κ$-Cl in which altermagnetism is known to appear.

cond-mat.str-el↗

arXiv2024

Minimal Models for Altermagnetism

Altermagnets feature vanishing net magnetization, like antiferromagnets, but exhibit time-reversal symmetry breaking and momentum-dependent spin-split band structures. Motivated by the prevalence of altermagnetic materials with non-symmorphic symmetry-dictated band degeneracies, we provide realistic minimal models for altermagnetism by constructing tight-binding models for nonsymmorphic space groups with a sublattice defined by two magnetic atoms. These models can be applied to monoclinic, orthorhombic, tetragonal, rhombohedral, hexagonal, and cubic materials and can describe d-wave, g-wave, and i-wave altermagnetism. By examining the altermagnetic susceptibility and mean field instabilities within a Hubbard model we reveal that these models have altermagnetic ground states and yield a Berry curvature that is linear in the spin-orbit coupling. We apply our models to RuO$_2$, MnF$_2$, FeSb$_2$, $κ$-Cl, CrSb, and MnTe.

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arXiv2023

Superconducting diodes from magnetization gradients

The superconducting diode effect may exist in bulk systems as well as in junctions when time-reversal and inversion symmetries are simultaneously broken. Magnetization gradients and textures satisfy both requirements and therefore also allow for superconducting diodes. We concretely demonstrate such possibilities in two-dimensional superconductors. We first consider superconducting Rashba metals in the presence of an inhomogeneous out-of-plane exchange field. Using analytical arguments, we reveal that such magnetization gradients stabilize a helical superconducting ground state, similar to homogeneous in-plane magnetic fields. Our predictions are confirmed by employing self-consistent real-space numerical lattice simulations exemplified through the cases of a uniform magnetization gradient or a ferromagnetic domain wall. Furthermore, by considering a phase difference, we determine the nonreciprocal current-phase relations and explore their parameter dependence. Our calculations show that planar devices with out-of-plane magnetization gradients may be as efficient supercurrent rectifiers as their analogs induced by uniform in-plane fields. In addition, they feature the advantage that by means of tailoring the spatial profile of the out-of-plane magnetization, one may optimize and spatially control the diode effect. Finally, we show that superconducting diodes may become also accessible even in the absence of spin-orbit coupling by means of suitable spatially-varying magnetization fields.

cond-mat.supr-con↗

arXiv2023

Time-reversal symmetry breaking from lattice dislocations in superconductors

Spontaneous generation of time-reversal symmetry breaking in unconventional superconductors is currently a topic of considerable interest. While chiral superconducting order is often assumed to be the source of such signatures, they can sometimes also arise from nonmagnetic disorder. Here we perform a theoretical study of the impact of dislocations on the superconducting order parameter within a microscopic one-band model which, in the homogeneous case, features either extended $s$-wave, $d$-wave, or $s+id$-wave superconductivity depending on the electron concentration. We find that the dislocations minimize their impact on the superconducting condensate by inducing localized supercurrents pinned by the dislocations, even well outside the $s+id$ regime. We map out the parameter and density dependence of the induced currents. From these results we conclude that quite generically unconventional superconductors hosting dislocations tend to break time-reversal symmetry locally.

cond-mat.supr-con↗

arXiv2022

Heat capacity double transitions in time-reversal symmetry broken superconductors

Standard superconductors display a ubiquitous discontinuous jump in the electronic specific heat at the critical superconducting transition temperature. In a growing class of unconventional superconductors, however, a second order parameter component may get stabilized and produce a second heat capacity jump at a lower temperature, typically associated with the spontaneous breaking of time-reversal symmetry. The splitting of the two specific heat discontinuities can be controlled by external perturbations such as chemical substitution, hydrostatic pressure, or uniaxial strain. We develop a theoretical quantitative multi-band framework to determine the ratio of the heat capacity jumps, given the band structure and the order parameter momentum structure. We discuss the conditions of the gap profile which determine the amplitude of the second jump. We apply our formalism to the case of Sr$_2$RuO$_4$, and using the gap functions from a microscopic random phase approximation calculation, we show that recently-proposed accidentally degenerate order parameters may exhibit a strongly suppressed second heat capacity jump. We discuss the origin of this result and consider also the role of spatial inhomogeneity on the specific heat. Our results provide a possible explanation of why a second heat capacity jump has so far evaded experimental detection in Sr$_2$RuO$_4$.

cond-mat.supr-con↗

arXiv2022

Revisiting superconductivity in the extended one-band Hubbard model: pairing via spin and charge fluctuations

The leading superconducting instabilities of the two-dimensional extended repulsive one-band Hubbard model within spin-fluctuation pairing theory depend sensitively on electron density, band and interaction parameters. We map out the phase diagrams within a random phase approximation (RPA) spin- and charge-fluctuation approach, and find that while $B_{1g}$ ($d_{x^2-y^2}$) and $B_{2g}$ ($d_{xy}$) pairing dominates in the absence of repulsive longer-range Coulomb interactions $V_{\rm NN}$, the latter induces pairing in other symmetry channels, including e.g $A_{2g}$ ($g$-wave), nodal $A_{1g}$ (extended $s$-wave), or nodal $E_u$ ($p$-wave) spin-triplet superconductivity. At the lowest temperatures, transition boundaries in the phase diagrams between symmetry-distinct spin-singlet orders generate complex time-reversal symmetry broken superpositions. By contrast, we find that boundaries between singlet and triplet regions are characterized by first-order transitions. Finally, motivated by recent photoemission experiments, we have determined the influence of an additional explicitly attractive nearest-neighbor interaction, $V_{\rm NN}<0$, on the superconducting gap structure. Depending on the electronic filling, such an attraction boosts $E_u$ ($p$-wave) spin-triplet or $B_{1g}$ ($d_{x^2-y^2}$) spin-singlet ordering.

cond-mat.supr-con↗

arXiv2022

Superconductivity in multiorbital systems with repulsive interactions: Hund's pairing vs. spin-fluctuation pairing

Hund's pairing refers to Cooper pairing generated by onsite interactions that become attractive due to large Hund's exchange $J$. This is possible in multiorbital systems even when all local bare interactions are repulsive, since attraction in specific channels are given by certain linear combinations of interaction parameters. On the other hand, pairing processes such as the exchange of spin fluctuations, are also present. We compare these two mechanisms on an equal footing using electronic bands appropriate for different classes of multiorbital systems over a wide range of interaction parameters. We find that for systems without clear nesting features, the superconducting state generated by the Hund's mechanism agrees well with that from the full fluctuation exchange vertex when Hund's exchange and spin-orbit coupling are sufficiently large. On the other hand, for systems characterized by a peaked finite-momentum particle-hole susceptibility, spin-fluctuation pairing generally dominates over Hund's pairing. We conclude that Hund's pairing states are unlikely to be realized in systems like Sr$_2$RuO$_4$ and generic iron-based superconductors.

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