arXiv subjects

Annica M. Black-Schaffer

Publications and source records attributed to Annica M. Black-Schaffer.

At least 19 recordsLinked to original sources

arXiv2026

Superconducting properties of transition metal dichalcogenides in proximity to a conventional superconductor

Transition metal dichalcogenides (TMDs) hold relevance for spin-triplet superconducting phases due to their inherent Ising spin-orbit coupling, but the majority of studies have so far focused on oversimplified models. In this work, we consider a TMD monolayer using a three-orbital model with anisotropic couplings and investigate the emergent superconducting properties when it is placed in proximity to a conventional spin-singlet $s$-wave superconductor. We find that the multiorbital nature of the TMDs lead to superconducting gaps not only at zero energy but also at higher energies, so-called hybridization gaps, which exhibit a complex structure due to the anisotropic couplings, challenging their spectral measurement. Moreover, we find that the inherent Ising spin-orbit coupling induces a spin splitting and a spin polarization along the $z$ direction, which correlates with the emergence of mixed spin-triplet superconducting pairs. These spin-triplet pair correlations appear in the monolayer as a proximity-induced effect due to the impact of the Ising spin-orbit field on conventional spin-singlet $s$-wave superconductivity. Taking realistic parameters for a $\text{MoS}_2$ monolayer, we show that the Ising field is strong enough to induce spin-triplet pair correlations of the same magnitude as their spin-singlet counterparts. We also include Rashba spin-orbit coupling, naturally emerging in a heterostructure and find that it induces equal spin-triplet superconducting pairs that compete with the mixed spin-triplet pairs induced by the Ising spin-orbit coupling. Our findings help understand the superconducting properties of TMDs in proximity to conventional superconductors.

cond-mat.supr-con↗

arXiv2026

Observation of end-to-end pumping in a quasiperiodic Fibonacci-type photonic chain

End-to-end pumps utilizing edge-localized states offer a promising route to operate as connecting buses, supplying efficient and robust connectivity between non-neighboring elements in a network. Here, we investigate a finite quasiperiodic Fibonacci-type photonic chain and demonstrate its ability for end-to-end pumping, with only small and simple changes to the system. First, we use a tight-binding formalism to numerically show that a localized pumping state can be transferred between opposite ends of the system, with only a small structural change to the chain. Then, we experimentally implement this end-to-end pump in an array of coupled optical waveguides, where light propagation is effectively described by the tight-binding model under the paraxial approximation, enabling direct correspondence between theory and experiment. We numerically simulate and experimentally demonstrate pumping by injecting light into a single waveguide at one end of the setup, which activates a localized pumping state. As the light propagates along the wave guide array, it is also pumped to the other end. We further show that pumping remains robust against structural deformation, such as controlled defects in the waveguide array. Our results establish that quasiperiodic Fibonacci-type photonic lattices are a robust and experimentally viable platform for disorder-resilient state transfer.

cond-mat.mes-hall↗

arXiv2026

Generalized conditions for odd-frequency pairing in superconducting systems

Odd-frequency superconducting pairing has been predicted to arise in many systems and is known to lead to phenomena such as paramagnetic Meissner response and long-range superconducting proximity effect. Here, we provide generalized necessary and sufficient conditions for odd-frequency pairing appearing in any superconducting system, from bulk superconductors to superconducting hybrid structures. This generalizes an earlier first-order expression in multiband bulk superconductors to all superconducting systems and to all orders, in both order parameter and frequency. We then apply the derived conditions to several systems, including superconducting-ferromagnet and superconducting Josephson junctions, as well as a transition metal dichalcogenide monolayer proximitized by a conventional superconductor, where the generalized conditions are used to understand the properties of the superconducting state.

cond-mat.supr-con↗

arXiv2026

A Superconducting Peierls Instability

The Peierls instability is a foundational mechanism in condensed matter physics, showing how the electron--phonon interaction can transform a simple metal into an insulating state with a new lattice periodicity. In a one-dimensional (1D) metal the Peierls instability follows from the enhanced electronic response at wavevector $Q=2k_F$ (connecting the Fermi points) producing a Kohn anomaly: a softening of the phonon mode at the same wavevector. Condensation of this mode then generates the tell-tale Peierls periodic lattice distortion and charge-density wave (CDW) that gaps the electronic spectrum. Here we show an analogous instability at the edge of a two-dimensional (2D) superconductor, where a dispersing Andreev bound state (ABS) hosts Bogoliubov Fermi points at $\pm k_c$. The enhanced quasiparticle response at the connecting wavevector $Q=2k_c$ couples directly to pairing-fluctuations and produces a superconducting Kohn anomaly: a softening of a pairing mode at the same wavevector. Condensation of this mode then generates an edge pair-density wave (PDW) that gaps the ABS. We refer to this as a superconducting Peierls instability and identify the boundary quasiparticle structure that enables it. As a concrete realization, we consider a square-lattice extended Hubbard model whose mixed-symmetry $s+d+ip$ state hosts a dispersing ABS with zero-energy crossings at finite edge momenta. Using self-consistent Bogoliubov--de Gennes (BdG) calculations we show that the order parameter develops an edge PDW, with the wavevector set by the superconducting Kohn anomaly, that gaps the ABS crossings. Our results identify a novel mechanism for spontaneous translation-symmetry breaking in a superconductor. As this superconducting Peierls mechanism does not require an extensive zero-energy flat band or topologically protected edge states, it may apply broadly to unconventional superconductors.

cond-mat.supr-con↗

arXiv2026

Composite quantum geometry of superconductors

The interplay of superconductivity and the quantum geometry of the normal state has recently been the subject of an array of studies, especially regarding the superfluid weight. In this work, we turn our attention to the quantum geometry of the superconducting state itself, set by the Bogoliubov-de Gennes (BdG) Hamiltonian, which dictates the geometric and topological properties of superconductivity. We show that under three general conditions, namely superconducting fitness, orbital uniformity of the superconducting pairing, and absence of normal-state spin-flip terms, the BdG quantum geometry exactly separates into a sum of the normal-state quantum geometry and an additional pairing quantum geometry, thereby displaying a simple composite structure. We show that this separation holds for all spin-singlet and -triplet pairings, including nonunitary spin-triplet pairing. We further provide explicit analytical formulas for the pairing quantum geometry for all these cases. These results establish how superconducting pairing alone easily drives both topology and a finite quantum metric, thus being present even in topological trivial or flat band superconductors, with no normal state quantum geometry. To complement these results, we also derive the BdG quantum geometry of a general two-orbital spin-singlet superconductor with non-uniform pairing and finite superconducting fitness. Here, our explicit analytical results establish a non-separable composite BdG quantum geometry, with the normal state and pairing contributions generally intertwining, thereby producing even more possibilities for finite quantum geometry. Our results provide design rules for creating superconductors and superconducting hybrid structures with nontrivial topology and finite quantum metric and will additionally help in the experimental diagnosis of unconventional superconductivity.

cond-mat.supr-con↗

arXiv2026

Disorder-induced superconductivity in graphene

Correlated phases of matter are typically investigated in crystalline systems, where disorder is considered to be detrimental. However, intriguing exceptions exist, such as superconductivity being enhanced in amorphous realizations of Al and Bi. Here, we demonstrate that superconductivity can even emerge entirely from disorder, using monolayer graphene as an example. In the clean limit, the semi-metallic nature of graphene requires prohibitively strong electronic interactions to achieve superconductivity. Despite the inherently random nature of disorder, we show that introducing low concentrations of vacancies or hydrogenation in graphene provides a large density of low-energy states that easily induce superconductivity. For conventional $s$-wave pairing, disorder induces a finite superconducting order parameter for arbitrarily weak attractive interactions. Rather than forming isolated superconducting puddles, global phase coherence is established through a finite superfluid weight of purely geometrical origin. Away from the chiral limit of vacancies, hydrogenation similarly yields a finite transition temperature and nonzero superfluid weight for weak interactions. For unconventional nearest-neighbor pairing, typically more disrupted by disorder, superconductivity exhibits quantum-critical-like behavior, yet phase coherence persists at low interaction strengths, with mixed $d$-wave symmetries. Our work demonstrates the robust emergence of macroscopic superconducting phase coherence engineered entirely from microscopic disorder.

cond-mat.supr-con↗

arXiv2026

Kagome edge states under lattice termination, spin-orbit coupling, and magnetic order

We study the edge state properties of a two-dimensional kagome lattice using a tight-binding approach, focusing on the role of lattice termination, spin-orbit coupling, and magnetic order. In the pristine limit, we show that the existence of localized edge states is highly sensitive to boundary geometry, with certain terminations completely suppressing edge modes. Kane-Mele spin-orbit coupling opens a bulk gap and stabilizes topologically protected helical edge states, yielding a robust $\mathbb{Z}_2$ insulating phase that is insensitive to termination details. In contrast, the combined effect of a Zeeman field and Rashba spin-orbit coupling drives the system into Chern insulating phases, with Chern numbers consistent with the number of chiral edge modes. We further demonstrate that non-coplanar magnetic textures generate multiple Chern phases through finite scalar spin chirality, with Kane-Mele coupling strongly tuning the topological gaps. Our results provide important insights into the tunability of edge states in the kagome lattice, which can be key to designing materials with novel electronic properties and topological phases.

cond-mat.mtrl-sci↗

arXiv2026

Quantum metric and localization in a quasicrystal

We use the quantum metric to understand the properties of quasicrystals, represented by the one-dimensional (1D) Fibonacci chain. We show that the quantum metric can relate the localization properties of the eigenstates to the self-similarity of both the chain and its energy spectrum. In particular, the quantum metric incorporates information about distances between the local symmetry centers of each eigenstate, making it much more sensitive to the localization properties of quasicrystals than other measures of localization, such as the inverse participation ratio. Importantly, we further find that a complete description of localization requires us to, in addition, introduce a new phasonic component to the quantum metric, along with a similarly mixed phason-position Chern number. Using this, we show that the sum of both position and phasonic components of the quantum metric is lower-bounded by the gap labels associated with each energy gap of the Fibonacci chain, which stem from the Chern number. This establishes a direct link through the quantum geometry between spatial localization and fractal energy spectrum of quasicrystals. Taken together, quantum geometry provides a unifying, yet accessible, understanding of quasicrystals, rooted in their self-similarity and with intriguing consequences also for many-body physics.

cond-mat.mes-hall↗

arXiv2026

Quantum geometric anomalous Hall response in orbitally nonunitary superconductors

We investigate the anomalous Hall response (AHR) in a multiband superconductor at optical frequencies, a phenomenon intimately related to the polar Kerr effect, a key probe of time-reversal symmetry breaking in superconductors. In translationally invariant multiband systems with purely intraband pairing, Galilean invariance decouples center-of-mass and relative motion of Cooper pairs, leading to the widespread expectation that a finite AHR requires either disorder or finite interband pairing amplitudes. However, this restriction can be lifted by the quantum geometric effects inherent to multiband Bloch states. Using a honeycomb lattice tight-binding model with Kane-Mele spin-orbit coupling, we analyze the AHR for the time-reversal symmetry broken chiral $d$-wave spin-singlet and chiral $p$-wave equal-spin-triplet pairing states with intraband pairing only. We demonstrate, through both analytical and numerical calculations, that the spin-singlet state yields a vanishing AHR, even with its broken time-reversal symmetry, whereas the equal-spin triplet state exhibits a finite AHR, even when it is spin-unitary. We attribute the latter to orbital nonunitarity, which, in the presence of spin-orbit coupling, generates the spin-polarized Bogoliubov quasiparticle states required for a finite AHR. The response is mediated by interband velocity matrix elements governed by the quantum geometry. This finding establishes that spin-unitary, but orbitally nonunitary pairing, can generate a finite AHR even without interband pairing and thereby revises the criteria for Kerr signals in superconductors.

cond-mat.supr-con↗

arXiv2026

Nonuniform superconducting states from Majorana flat bands

Zero-energy flat bands within the superconducting gap can give rise to competing ordered phases. We investigate such phases in topological superconductors based on the magnetic adatom platform hosting a flat band of Majorana edge states. Our self-consistent calculations of the superconducting order parameter show the emergence of both a pair density wave with edge-localized amplitude modulations and a phase crystal characterized by edge-localized phase modulations. These two phases lower the free energy of the system by gapping out the Majorana flat band, as dictated by winding numbers, which are primarily tuned by the chemical potential. In fact, at zero temperature the uniform superconducting solution with Majorana flat band never survives and the phase diagram features a pair density wave, while the order parameter transitions into a phase crystal when amplitude modulations are insufficient to hybridize all the Majorana states. A broad intermediate region connects these two phases with comparable modulations in both amplitude and phase. At finite temperatures, the pair density wave survives up to around 80% of the bulk superconducting transition temperature, while the phase crystal only appears at lower temperatures and the intermediate region is strongly suppressed. Our findings establish the ubiquity of emergent nonuniform superconducting phases and their temperature-dependent behavior in topological superconductors.

cond-mat.supr-con↗

arXiv2026

Unifying description of competing chiral and nematic superconducting states in twisted bilayer graphene

We reveal a striking correspondence between electron- and phonon-driven pairing in twisted bilayer graphene (TBG) by mapping an atomistic electronically driven pairing model onto an effective inter-valley, intra-Chern description, originally proposed for phonon-mediated superconductivity. Within the unified framework of intra-Chern pairing, we analyze the competition between nematic and chiral superconducting states. The latter corresponds to the extreme Chern-polarized limit and thus hosts unpaired flat bands within the superconducting gap, which generally disfavors it relative to the nematic states. Crucially, nematic order is locally preferred at each momenta, but the optimal nematic directions are incompatible across the Brillouin zone due to the broken rotation symmetry. This momentum-space frustration enables a chiral ground state at large fillings or weak interactions. Our results thereby both provide a unified understanding of superconductivity in TBG, with a natural cooperation of electron- and phonon-mediated pairing, and clarify the microscopic origin of the competition between the chiral and nematic superconducting states.

cond-mat.supr-con↗

arXiv2026

Laser-induced topological phases in monolayer amorphous carbon

Driving non-topological materials out of equilibrium using time-periodic perturbations, such as circularly-polarized laser light, is a compelling way to engineer topological phases. At the same time, topology has traditionally only been considered for crystalline materials. Here we propose an experimentally feasible way of driving monolayer amorphous carbon topological.We show that circularly polarized laser light induces both regular and anomalous edge modes at quasienergies $0$ and $\pm π$, respectively. We also obtain a complete topological characterization using an energy- and space-resolved topological marker based on the spectral localizer. Additionally, by introducing atomic coordination defects in the amorphous carbon, we establish the importance of the local atomic coordination in topological amorphous materials. Our work establishes amorphous systems, including carbon, as a versatile and abundant playground to engineer topological phases.

cond-mat.mes-hall↗

arXiv2026

Josephson tunneling through a Yu-Shiba-Rusinov state: Interplay of $π$-shifts in Josephson current and local superconducting order parameter

An impurity hosting a magnetic moment coupled to a conventional $s$-wave superconductor gives rise to so-called Yu-Shiba-Rusinov (YSR) states with energies inside the superconducting gap. Depending on the coupling between the impurity and the superconductor, the system can have two distinct quantum ground states separated by a quantum phase transition (QPT). We investigate the interplay of two effects observed at the QPT. First, the tunneling supercurrent through the impurity reverses its sign at the QPT, denoted as a $π$-shift in the current-phase relation. Secondly, the local superconducting order parameter at the impurity site is suppressed and becomes negative at the QPT, generally termed a $π$-shift in the local superconducting order parameter. We find that both these effects are governed by the presence of the YSR state, however, they do not significantly depend or influence each other. In particular, we establish that the $π$-shift in the superconducting order parameter does not induce a $π$-shift in the tunneling Josephson current, nor can the Josephson current and its spatial behavior be used to directly probe the impurity-induced changes in the local superconducting order parameter, which occur on a length scale substantially shorter than the superconducting coherence length.

cond-mat.supr-con↗

arXiv2026

Topology and energy dependence of Majorana bound states in a photonic cavity

Light-matter interaction plays a crucial role in modifying the properties of quantum materials. In this work, we investigate the effect of cavity induced photon fields on a topological superconductor hosting Majorana bound states (MBS). We model the system using a Peierls substitution of the photonic operator in the kinetic and spin-orbit terms, and utilize an exact diagonalization of Hamiltonian for a finite number of photons to investigate the coupled system. We find that the MBS persist even in the presence of a cavity field and notably appear at finite and tunable energy, in contrast to a usual 1D topological superconductor. The MBS energy is shifted by two processes: the cavity photon energy adds a constant energy shift, while the light-matter interaction induces additional parameter dependencies, such that the MBS experience a pseudo-dispersion as a function of both light-matter interaction and magnetic field. Additionally, we find that the MBS energy oscillations are suppressed with increasing light-matter interaction and that disorder stability is not impacted by the light-matter interaction. Combined, these offer additional tunability and stability of the MBS. As a second result, we establish a modified spectral localizer formalism as an essential tool for topological characterization of quantum matter in a cavity. The spectral localizer allows characterization at arbitrary energies, which is needed for probing different photon sectors. However, hybridization between different photon sectors in the low-frequency regime limits a straightforward application of a standard spectral localizer. We fully resolve this issue by judiciously applying an energy shift to the spectral localizer. Our work thus introduces a new avenue for controlling MBS via light-matter coupling and provides a framework for exploring cavity-modified topologies.

cond-mat.mes-hall↗

arXiv2026

Boosting superconductivity in ultrathin YBa$_2$Cu$_3$O$_{7-δ}$ films via nanofaceted substrates

In cuprate high-temperature superconductors the doping level is fixed during synthesis, hence the charge carrier density per CuO$_2$ plane cannot be easily tuned by conventional gating, unlike in 2D materials. Strain engineering has recently emerged as a powerful tuning knob for manipulating the properties of cuprates, in particular charge and spin orders, and their delicate interplay with superconductivity. In thin films, additional tunability can be introduced by the substrate surface morphology, particularly nanofacets formed by substrate surface reconstruction. Here we show a remarkable enhancement of the superconducting onset temperature $T_{\mathrm{c}}^{\mathrm{on}}$ and the upper critical magnetic field $H_{c,2}$ in nanometer-thin YBa$_2$Cu$_3$O$_{7-δ}$ films grown on a substrate with a nanofaceted surface. We theoretically show that the enhancement is driven by electronic nematicity and unidirectional charge density waves, where both elements are captured by an additional effective potential at the interface between the film and the uniquely textured substrate. Our findings show a new paradigm in which substrate engineering can effectively enhance the superconducting properties of cuprates. This approach opens an exciting frontier in the design and optimization of high-performance superconducting materials.

cond-mat.supr-con↗

arXiv2025

Quantum state transfer and maximal entanglement between distant qubits using a minimal quasicrystal pump

Coherent quantum state transfer over macroscopic distances between non-neighboring elements in quantum circuits is a crucial component to increase connectivity and simplify quantum information processing. To facilitate such transfers, an efficient and easily controllable quantum pump would be highly beneficial. In this work, we demonstrate such a quantum pump based on a one-dimensional quasicrystal Fibonacci chain~(FC). In particular, we utilize the unique properties of quasicrystals to pump the edge-localized winding states between the two distant ends of the chain by only minimal manipulation of the FC at its end points. We establish the necessary conditions for successful state transfer within a fully time-dependent picture and also demonstrate robustness of the transfer protocol against disorder. We then couple external qubits to each end of the FC and establish highly adaptable functionality as a quantum bus with both on-demand switching of the qubit states and generation of maximally entangled Bell states between the qubits. Thanks to the minimal control parameters, the setup is well-suited for implementation across diverse experimental platforms, thus establishing quasicrystals as an efficient platform for versatile quantum information processing.

cond-mat.mes-hall↗

arXiv2025

Probing Topological Stability with Nonlocal Quantum Geometric Markers

Spatially resolved local quantum geometric markers play a crucial role in the diagnosis of topological phases without long-range translational symmetry, including amorphous systems. Here, we focus on the nonlocality of such markers. We demonstrate that they behave as correlation functions independently of the material's structure, showing sharp variations in the vicinity of topological transitions and exhibiting a unique pattern in real space for each transition. Notably, we find that, even within the same Altland-Zirnbauer class, distinct topological transitions generate qualitatively different spatial signatures, enabling a refined, class-internal probe of topological stability. As such, nonlocal quantum geometric indicators provide a more efficient and versatile tool to understand and predict the stability of topological phase transitions.

cond-mat.mes-hall↗

arXiv2025

Temperature dependent energy gap for Yu-Shiba-Rusinov states at the quantum phase transition

Motivated by recent experiments, which allow for fine tuning of the effective magnetic interaction between the impurity and the superconductor, we investigate the regime around the quantum phase transition where the system's ground state changes from a weakly coupled free spin to a screened spin regime. At this transition we find that the Yu-Shiba-Rusinov (YSR) states remain at finite energies at low temperatures, thereby generating a gap in the spectrum, which is inconsistent with predictions of the original YSR theory. We investigate various gap-generating scenarios and determine that the local suppression of the order parameter, only captured by self-consistent calculations, generates the gap.

cond-mat.supr-con↗