arXiv subjects

Aline Ramires

Publications and source records attributed to Aline Ramires.

At least 19 recordsLinked to original sources

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

Structural reconstruction as the origin of the cuprate pseudogap

High-temperature superconductivity in the cuprates emerges from an enigmatic metallic state, known as the pseudogap, characterized by a reconstructed Fermi surface, reduced carrier density, and the appearance of Fermi arcs, whose origin remains unresolved. Here, we show that these defining signatures naturally arise from a structural reconstruction observed experimentally that introduces a symmetry-enforced sublattice degree of freedom. In the presence of spin-orbit coupling, the Fermi surface is reconstructed into small closed pockets, effectively reducing the carrier density. The same sublattice structure gives rise to matrix-element interference in angle-resolved photoemission spectroscopy, leading to the manifestation of Fermi arcs. Density functional theory calculations support this mechanism. These results demonstrate that lattice symmetry provides a unifying and experimentally verifiable framework for understanding the pseudogap regime in the cuprates.

cond-mat.supr-con↗

arXiv2025

Tuning macroscopic phase frustration in multiorbital superconductors

Time-reversal symmetry-breaking (TRSB) superconductivity has been reported in a growing number of materials. In some cases, TRSB arises naturally from chiral superconductivity, but in many low-symmetry systems this explanation is not viable. In these latter cases, TRSB is often attributed to phase frustration among multiple superconducting gaps on different Fermi surfaces. Yet, the microscopic conditions enabling such frustration remain poorly understood. Here, inspired by the TRSB reported in the superconducting state of iron-based materials, we demonstrate that a minimal two-orbital model can support a TRSB superconducting state via phase frustration. We identify the key microscopic parameters that stabilize TRSB in d-electron systems with orthorhombic symmetry and provide a framework to systematically enlarge the region of parameter space within which TRSB is expected in materials with other electronic content and crystalline symmetries. Our results offer a simple and experimentally relevant route to understand and control TRSB in multiorbital superconductors.

cond-mat.supr-con↗

arXiv2025

Symmetry Aspects of Chiral Superconductors

Recent developments in theory, synthesis, and experimental probes of quantum systems have revealed many suitable candidate materials to host chiral superconductivity. Chiral superconductors are a subset of unconventional superconductors which break time-reversal symmetry. Time-reversal symmetry breaking is possible given the order parameter's two-component nature, allowing for a complex relative phase. In this article, we focus on discussing the underlying symmetry aspects that allow for the development of chiral superconductivity. We provide an introductory account of key concepts in group theory and apply these to the classification of order parameters and the generalization of the Landau theory of phase transitions in the context of superconductivity.

cond-mat.supr-con↗

arXiv2025

Intertwining bulk and surface: the case of UTe$_2$

UTe$_2$ has been the focus of numerous experimental and theoretical studies in recent years, as it is recognized as an odd-parity bulk superconductor. Its surface has also been probed, revealing charge density wave (CDW), pair density wave (PDW), and time-reversal symmetry breaking (TRSB). In this work, we propose that the interplay between the order parameters observed on the surface and in the bulk of UTe$_2$ may be crucial in explaining some of the unusual features detected by surface probes in this material. Through a phenomenological analysis, we can account for three distinctive experimental signatures observed on the surface of UTe$_2$: i) the apparent suppression of CDW order at the upper critical field of the bulk superconducting state; ii) the magnetic field-induced imbalance of the Fourier peaks associated with the CDW; iii) the onset of TRSB at the bulk superconducting critical temperature and its field-trainability. Furthermore, we propose specific experimental checks to validate our conjecture, which we believe could be promptly achieved.

cond-mat.supr-con↗

arXiv2025

From pure to mixed: Altermagnets as intrinsic symmetry-breaking indicators

We investigate the impact of the presence of altermagnetism on physical observables in the presence of explicit or spontaneous symmetry-breaking fields. We focus on unconventional superconductivity as a potential source of spontaneous symmetry-breaking fields and derive the symmetry considerations for the transmutation of altermagnets from pure to mixed, which can give rise to unusual phenomena. Based on this analysis, we put forward scenarios that could explain the apparent onset of time-reversal symmetry breaking at the superconducting critical temperature in Sr$_2$RuO$_4$, and the hidden magnetic phase and magnetic memory in 4Hb-TaS$_2$.

cond-mat.supr-con↗

arXiv2024

Rotation of gap nodes in the topological superconductor Cu$_x$(PbSe)$_5$(Bi$_2$Se$_3$)$_6$

Among the family of odd-parity topological superconductors derived from $\mathrm{Bi}_{2}\mathrm{Se}_{3}$, $\mathrm{Cu}_{x}(\mathrm{PbSe})_{5}(\mathrm{Bi}_{2}\mathrm{Se}_{3})_{6}$ (CPSBS) has been elucidated to have gap nodes. Although the nodal gap structure has been established by specific-heat and thermal-conductivity measurements, there has been no direct observation of the superconducting gap of CPSBS using scanning tunnelling spectroscopy (STS). Here we report the first STS experiments on CPSBS down to 0.35 K, which found that the vortices generated by out-of-plane magnetic fields have an elliptical shape, reflecting the anisotropic gap structure. The orientation of the gap minima is found to be aligned with the bulk direction when the surface lattice image shows twofold symmetry, but, surprisingly, it is rotated by 30$^{\circ}$ when twofold symmetry is absent. In addition, the superconducting gap spectra in zero magnetic field suggest that the gap nodes are most likely lifted. We argue that only an emergent symmetry at the surface, allowing for a linear superposition of gap functions with different symmetries in the bulk, can lead to the rotation of the gap nodes. The absence of inversion symmetry at the surface additionally lifts the nodes. This result establishes the subtle but crucial role of crystalline symmetry in topological superconductivity.

cond-mat.supr-con↗

arXiv2024

Still mystery after all these years -- Unconventional superconductivity of Sr2RuO4 --

This review describes recent significant research developments made on the layered perovskite Sr2RuO4 and discusses current issues from both experimental and theoretical perspectives. Since the discovery of superconductivity in Sr2RuO4 in 1994, studies using high-quality single crystals quickly revealed it to be an archetypal unconventional superconductor among strongly correlated electron systems. In particular, it was thought that the spin-triplet chiral p-wave superconducting state, which breaks time-reversal symmetry, was a prominent possibility. In 2019, however, a new development overturned the past experimental results, and spin-singlet-like behavior became conclusive. Furthermore, innovation in uniaxial strain devices has stimulated researchers to explore changes in the superconducting state by controlling the symmetry and dimensionality of the Fermi surfaces and enhancing the superconducting transition temperature Tc from 1.5 K to 3.5 K. A spin-singlet chiral d-wave superconducting state is consistent with most of these recent experimental results. Nevertheless, there are still unnatural aspects that remain to be explained. The focus of this review is on unraveling this mystery. Unlike many other unconventional superconductors, the normal state of Sr2RuO4 exhibits typical Fermi liquid behavior. Nevertheless, to elucidate its superconducting state, it may be essential to go beyond the traditional framework of unconventional superconductivity and recast the theory explicitly considering the multi-orbital aspects of its electronic states. In this review, we describe the frontiers of superconductivity research in Sr2RuO4 and discuss how the remaining issues may be resolved.

cond-mat.supr-con↗

arXiv2023

Ferromagnetic quantum critical point in a locally noncentrosymmetric and nonsymmorphic Kondo metal

Quantum critical points (QCPs), zero-temperature phase transitions, are windows to fundamental quantum-mechanical phenomena associated with universal behaviour and can provide parallels to the physics of black holes. Magnetic QCPs have been extensively investigated in the vicinity of antiferromagnetic order. However, QCPs are rare in metallic ferromagnets due to the coupling of the order parameter to electronic soft modes [1,2]. Recently, antisymmetric spin-orbit coupling in noncentrosymmetric systems was suggested to protect ferromagnetic QCPs [3]. Nonetheless, multiple centrosymmetric materials host FM QCPs, suggesting a more general mechanism behind their protection. In this context, CeSi$_{2-\delta}$, a dense Kondo lattice crystallising in a centrosymmetric structure, exhibits ferromagnetic order when Si is replaced with Ag. We report that the Ag-substitution controls the strength of the Kondo coupling, leading to a transition between paramagnetic and ferromagnetic Kondo phases. Remarkably, a ferromagnetic QCP accompanied by concurrent strange-metal behaviour emerges. Herein, we suggest that, despite the centrosymmetric structure, spin-orbit coupling arising from the local noncentrosymmetric structure, in combination with nonsymmorphic symmetry, can protect ferromagnetic QCPs. Our findings present a unique example of Kondo coupling-driven ferromagnetic QCP through chemical doping and offer a general guideline for discovering new ferromagnetic QCPs.

cond-mat.str-el↗

arXiv2023

Superconductivity-induced improper orders

The study of improper phases in the context of multiferroic materials has a long history, but superconductivity has yet to be connected to the network of ferroic orders. In this work, we highlight an overlooked mechanism that couples superconducting order parameters to odd-parity orders in the charge or spin sectors such that the latter emerge as improper orders. For that, we explore a novel perspective of nonsymmorphic symmetries based on extended symmetry groups in real space. We highlight how nonsymmorphic symmetries can generate rather nonintuitive couplings between order parameters. In particular, we find that a bilinear in the superconducting order parameter can couple linearly to odd-parity orders in centrosymmetric systems. Our findings can account for the unusual phenomenology of CeRh$_2$As$_2$, a recently discovered heavy fermion superconductor, and open the door for exploring nonsymmorphic symmetries in the broader context of improper orders with potential applications to functional materials.

cond-mat.supr-con↗

arXiv2023

Decoding the drive-bath interplay: A guideline to enhance superconductivity

Driven-dissipative physics lie at the core of quantum optics. However, the full interplay between a driven quantum many-body system and its environment remains relatively unexplored in the solid state realm. In this work, we inspect this interplay beyond the commonly employed stroboscopic Hamiltonian picture based on the specific example of a driven superconductor. Using the Shirley-Floquet and Keldysh formalisms as well as a generalization of the notion of superconducting fitness to the driven case, we show how a drive which anti-commutes with the superconducting gap operator generically induces an unusual particle-hole structure in the spectral functions from the perspective of the thermal bath. Concomitant with a driving frequency which is near resonant with the intrinsic cutoff frequency of the underlying interaction, this spectral structure can be harnessed to enhance the superconducting transition temperature. Our work paves the way for further studies for driven-dissipative engineering of exotic phases of matter in solid-state systems.

cond-mat.supr-con↗

arXiv2023

In-plane magnetic penetration depth in Sr$_2$RuO$_4$: muon-spin rotation/relaxation study

We report on measurements of the in-plane magnetic penetration depth ($λ_{\rm ab}$) in single crystals of Sr$_2$RuO$_4$ down to $\simeq 0.015$ K by means of muon-spin rotation/relaxation. The linear temperature dependence of $λ^{-2}_{\rm ab}$ for $T\lesssim 0.7$ K suggests the presence of nodes in the superconducting gap. This statement is further substantiated by observation of the Volovik effect, $i.e.$ the reduction of $λ_{ab}^{-2}$ as a function of the applied magnetic field. The experimental zero-field and zero-temperature value of $λ_{\rm ab}=124(3)$ nm agrees with $λ_{\rm ab}\simeq 130$ nm, calculated based on results of electronic structure measurements reported in [Phys. Rev X 9, 021048 (2019)]. Our analysis reveals that a simple nodal superconducting energy gap, described by the lowest possible harmonic of a gap function, does not capture the dependence of $λ_{\rm ab}^{-2}$ on $T$, so the higher angular harmonics of the energy gap function need to be introduced.

cond-mat.supr-con↗

arXiv2023

$μ$SR measurements on Sr$_2$RuO$_4$ under $\langle 110 \rangle$ uniaxial stress

Muon spin rotation/relaxation ($μ$SR) and polar Kerr effect measurements provide evidence for a time-reversal symmetry breaking (TRSB) superconducting state in Sr$_2$RuO$_4$. However, the absence of a cusp in the superconducting transition temperature ($T_{\rm c}$) vs. stress and the absence of a resolvable specific heat anomaly at TRSB transition temperature ($T_{\rm TRSB}$) under uniaxial stress challenge a hypothesis of TRSB superconductivity. Recent $μ$SR studies under pressure and with disorder indicate that the splitting between $T_{\rm c}$ and $T_{\rm TRSB}$ occurs only when the structural tetragonal symmetry is broken. To further test such behavior, we measured $T_\text{c}$ through susceptibility measurements, and $T_\text{TRSB}$ through $μ$SR, under uniaxial stress applied along a $\langle 110 \rangle$ lattice direction. We have obtained preliminary evidence for suppression of $T_\text{TRSB}$ below $T_\text{c}$, at a rate much higher than the suppression rate of $T_\text{c}$.

cond-mat.supr-con↗

arXiv2022

Nonunitary Superconductivity in Complex Quantum Materials

We present a comprehensive discussion on nonunitary superconductivity in complex quantum materials. Starting with a brief review of the notion of nonunitary superconductivity, we discuss its spectral signatures in simple models with only the spin as an internal degree of freedom. In complex materials with multiple internal degrees of freedom, there are many more possibilities for the development of nonunitary order parameters. We provide examples focusing on d-electron systems with two orbitals, applicable to a variety of materials. We discuss the consequences for the superconducting spectra, highlighting that gap openings of band crossings at finite energies can be attributed to a nonunitary order parameter if this is associated with a finite superconducting fitness matrix $\hat{F}_C(\mathbf{k})$. We speculate that nonunitary superconductivity in complex quantum materials is in fact very common and can be associated with multiple cases of time-reversal symmetry breaking superconductors.

cond-mat.supr-con↗

arXiv2022

Sensitivity of superconducting states to the impurity location in layered materials

The family of multi-layered superconductors derived from the doped topological insulator Bi$_2$Se$_3$ has been found to be unusually robust against non-magnetic disorder. Recent experimental studies have highlighted the fact that the location of impurities could play a critical role for this puzzling robustness. Here we investigate the effects of four different types of impurities, on-site, interstitial, intercalated and polar, on the superconducting critical temperature. We find that different components of the scattering potential are active depending on the impurity configuration and choice of orbitals for the effective low-energy description of the normal state. For the specific case of Bi$_2$Se$_3$-based superconductors, we find that only the symmetric share of impurity configurations contribute to scattering, such that polar impurities are completely inactive. We also find that a more dominant mass-imbalance term in the normal-state Hamiltonian can make the superconducting state more robust to intercalated impurities, in contrast to the case of on-site or interstitial impurities.

cond-mat.supr-con↗

arXiv2021

The effects of strain in multi-orbital superconductors: the case of Sr$_2$RuO$_4$

Uniaxial strain experiments have become a powerful tool to unveil the character of unconventional phases of electronic matter. Here we propose a combination of the superconducting fitness analysis and density functional theory (DFT) calculations in order to dissect the effects of strain in complex multi-orbital quantum materials from a microscopic perspective. We apply this framework to the superconducting state of Sr$_2$RuO$_4$, and argue that the recently proposed orbitally anti-symmetric spin-triplet (OAST) order parameter candidate has unique signatures under strain which are in agreement with recent observations. In particular, we can account for the asymmetric splitting of the critical temperatures for compressive strain along the $\langle 100\rangle$ direction, and the reduction of the critical temperatures for compressive strain along the $\langle 001\rangle$ and $\langle 110\rangle$ directions with a single free parameter.

cond-mat.supr-con↗

arXiv2021

Nodal gaps from local interactions in Sr$_2$RuO$_4$

Sr$_2$RuO$_4$ has been under intensive scrutiny over the past years after new NMR measurements unveiled that the superconducting state might be a spin singlet. One of the best order parameter candidates in light of these new experiments is a chiral d-wave state with $d_{xz}+id_{yz}$ symmetry. This order parameter has been overlooked given the strong two-dimensional character of the normal state electronic structure. Recently, a phenomenological proposal based on local interactions with a three-dimensional electronic structure showed that a chiral d-wave state can be stable in Sr$_2$RuO$_4$ once momentum-dependent spin-orbit coupling is properly taken into account. Here we discuss the origin of the nodes and dips in this order parameter as inherited from the normal state Hamiltonian, showing that a nodal gap can emerge out of purely local interactions and connect the presence of nodes with the superconducting fitness measure.

cond-mat.supr-con↗

arXiv2021

Designing spin-textured flat bands in twisted graphene multilayers via helimagnet encapsulation

Twisted graphene multilayers provide tunable platforms to engineer flat bands and exploit the associated strongly correlated physics. The two-dimensional nature of these systems makes them suitable for encapsulation by materials that break specific symmetries. In this context, recently discovered two-dimensional helimagnets, such as the multiferroic monolayer NiI$_2$, are specially appealing for breaking time-reversal and inversion symmetries due to their nontrivial spin textures. Here we show that this spin texture can be imprinted on the electronic structure of twisted bilayer graphene by proximity effect. We discuss the dependence of the imprinted spin texture on the wave-vector of the helical structure, and on the strength of the effective local exchange field. Based on these results we discuss the nature of the superconducting instabilities that can take place in helimagnet encapsulated twisted bilayer graphene. Our results put forward helimagnetic encapsulation as a powerful way of designing spin-textured flat band systems, providing a starting point to engineer a new family of correlated moire states.

cond-mat.mes-hall↗