arXiv subjects

Cristina Bena

Publications and source records attributed to Cristina Bena.

At least 19 recordsLinked to original sources

arXiv2026

An iterative method bridging DFT, disorder averaging, and experiment in intercalated materials: application to Au-intercalated graphene

Intercalation can strongly modify the electronic dispersion of a host material, as directly revealed by angle-resolved photoemission spectroscopy (ARPES). We develop a general iterative method combining density functional theory (DFT), tight-binding (TB), disorder averaging within the self-consistent T-matrix approximation (SCTMA), and experiment, to construct an effective model of the intercalated system. DFT identifies the relevant microscopic degrees of freedom and constrains selected model parameters, while comparison of SCTMA calculations with experiment guides their further refinement. We apply this method to graphene intercalated with Au clusters and show that it reproduces the main ARPES signatures of the Au-cluster phase, including the broadening of the V12an Hove singularity and the emergence of kink-like features in the dispersion. The essential microscopic ingredients identified by the analysis are the hybridization between selected intercalant orbitals and the graphene states, together with an intercalation-induced local scattering potential.

cond-mat.mes-hall↗

arXiv2026

Effects of gold cluster intercalation in graphene: stationary waves and modified QPI features

Gold intercalation beneath epitaxial graphene on SiC produces a cluster phase with unusual standing waves and quasiparticle-interference (QPI) features concentrated near the graphene M points. We show that this can be explained by Au intercalation below graphene hollow sites, which induces a local scattering potential on the six surrounding carbon atoms. Within a T-matrix treatment, this ring-like scatterer produces elliptical QPI structures centered near M, in agreement with the experimental FT-STS measurements. We further show that these QPI features naturally generate the nearly stationary standing-wave patterns observed in real space. Finally, we compute the local-density-of-states contrast on and off a small cluster and show that its sign and magnitude are strongly energy dependent, consistent with the experimental observations.

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arXiv2025

Quasiparticle interference and spectral function of the UTe$_2$ superconductive surface band

We compute the (0-11) surface spectral function, the surface density of states (DOS), and the quasiparticle interference (QPI) patterns, both in the normal state and superconducting (SC) state of UTe$_2$. We consider all possible non-chiral and chiral order parameters (OPs) that could in principle describe the superconductivity in this compound. We describe the formation of surface states whose maximum intensity energy depends on the nature of the pairing. We study also the QPI patterns resulting from the scattering of these surface states. We show that the main feature distinguishing between various OPs is a QPI peak that is only observed experimentally in the superconducting state. The energy dispersion and the stability of this peak is consistent among the non-chiral OPs only with a $B_{3u}$ pairing. Moreover, $B_{3u}$ is the only non-chiral pairing that shows a peak at zero energy in the DOS, consistent with the experimental observations.

cond-mat.supr-con↗

arXiv2025

Odd-Parity Quasiparticle Interference in the Superconductive Surface State of UTe2

Although no known material exhibits intrinsic topological superconductivity, wherein spin-triplet odd-parity electron pairing occurs, UTe2 is now the leading representative of this class. Conventionally, the parity of the superconducting order parameter may be established by using Bogoliubov quasiparticle interference (QPI) imaging. However, odd-parity superconductors should support a topological quasiparticle surface band (QSB) at energies within the maximum superconducting energy gap. QPI would then be dominated by the electronic structure of the QSB and only reveal the characteristics of the bulk order parameter excursively. Here, we visualize quasiparticle interference patterns of UTe2 and find that, at the (0-11) cleave surface, a new band of Bogoliubov quasiparticles appears only in the superconducting state. QPI visualization then allows study of dispersion of states within this QSB, which we demonstrate exists only within the range of Fermi momenta projected onto the (0-11) surface. Finally, we develop a theoretical framework to predict the QPI signatures of such a QSB at the (0-11) surface of UTe2. Its predictions are most consistent with the experimental results if the bulk superconducting gap function exhibits time-reversal conserving, odd-parity, a-axis nodal, B3u symmetry.

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arXiv2025

Probing quasiparticle excitations in a doped Mott insulator via Friedel oscillations

In this work, we investigate impurity-induced Friedel oscillations in the doped two-dimensional Hubbard model, focusing on the role of holon and doublon excitations. We show that weak impurities, due to the non-fermionic nature of the underlying quasiparticles, induce Friedel oscillations whose behavior is consistent with an effective non-interacting theory for these quasiparticles, and whose wavevector reflects the violation of Luttinger's theorem. At larger impurity strength, the system transitions to a phase-separated state composed of coexisting Mott-insulating (half-filled) and hole-rich regions. Within the composite operator framework, this phase separation arises from a competition between the kinetic energy of holons and the tendency to form tightly bound holon-doublon pairs. Our results offer new insights into the nature of charge carriers and the emergent electronic phases in the doped Mott regime.

cond-mat.str-el↗

arXiv2025

Impurity-induced Mott ring states and Mott zeros ring states in the Hubbard operator formalism

We study the formation of subgap impurity states in strongly correlated Mott insulators. We use a composite operator method that gives us access to both the bulk Green's function, as well as to the real-space Green's function in the presence of an impurity. Similar to the non-interacting systems, we show that the formation of impurity subgap states at large impurity potential ("Mott ring states") depends rather on the band-mixing, than on the topological character of the system. Thus even a trivial Mott insulator can under certain conditions exhibit ring states. For the system studied here the band mixing is that between the holon and doublon elementary excitations rather than an orbital mixing. Moreover we study the formation of bands of zeros in the correlated Green's function, believed to exhibit a free quasiparticle-like behavior. We show that in the presence of an impurity the same conclusion can be applied, i.e. ``Mott zeros ring states" form in the presence of topological bands of zeros, but also for trivial quasi-flat bands of zeros with band mixing.

cond-mat.str-el↗

arXiv2023

Superconductivity in twisted bilayer graphene: possible pairing symmetries, impurity-induced states and Chern number

We consider the most energetically favorable symmetry-allowed spin-singlet and spin-triplet superconducting pairing symmetries in twisted bilayer graphene at the magic angle, whose normal state physics is described by a six-band effective tight-binding model. We compute the Chern number as a function of the superconducting order parameter strength and the chemical potential and we find a topological phase transition only for the chiral $p+ip'$ superconducting state. Different from the regular graphene systems for which this happens at the van Hove singularity, for TBG the topological phase transition arises at the point where the Fermi surface becomes tangent to the boundary of the first Brillouin zone. For each pairing symmetry we study the formation of subgap impurity states for both scalar and magnetic impurities. We analyze the number of subgap states as well as their spin polarized density of states that we find to exhibit peculiar properties that allows one to distinguish between spin-singlet and triplet pairing. Thus only triplet-paired states may exhibit opposite-energy impurity states with the same spin, same as for regular graphene systems\, moreover we find that this spin may flip at the twist-induced van Hove singularity.

cond-mat.supr-con↗

arXiv2022

Superconductivity in monolayer and few-layer graphene: I. Review of possible pairing symmetries and basic electronic properties

We review all symmetry-allowed spin-singlet and spin-triplet superconducting (SC) order parameters in graphene ($s$-wave, $d$-wave, $p$-wave, and $f$-wave) generated by generic onsite, nearest-neighbor (NN), and next-to-nearest-neighbor (NNN) pairing interactions in a tight-binding model. For each pairing channel, we calculate both the band structure and the dependence of the density of states on energy, chemical potential, and on the pairing strength. In particular, we distinguish between nodal superconducting states and fully gapped states and study the dependence of gap closing points on the chemical potential and the superconducting pairing strength. We further investigate the difference between mono-, bi-, and tri-layer ABC and ABA graphene, including accounting for the effects of trigonal warping.

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arXiv2022

Superconductivity in monolayer and few-layer graphene: III Impurity-induced subgap states and quasi-particle interference patterns

We consider the most energetically favorable symmetry-allowed spin-singlet and spin-triplet superconducting pairing symmetries in monolayer and few-layer graphene, and for each calculate the energy spectrum in the presence of a scalar or magnetic impurity. We find that two doubly degenerate subgap states exist for scalar impurities for all types of pairing, except for the spin-singlet $s$-wave state. For magnetic impurities, two or four subgap states may form depending on the order parameter symmetry. We find that the spin polarization of these states allows one to distinguish between spin-singlet and triplet pairing, for example, only the spin-triplet states show opposite-energy subgap states with the same spin. We also calculate the quasi-particle interference patterns associated with the subgap states and find that they exhibit features that could distinguish between different types of pairing symmetries, especially a breaking of rotational symmetry for nodal states, stronger for the spin-singlet $d_{xy}$ and $d_{x^2-y^2}$ than for the spin-triplet $p_x$ and $p_y$ states.

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arXiv2022

Superconductivity in monolayer and few-layer graphene: II. Topological edge states and Chern numbers

We study the emergence of electronic edge states in superconducting (SC) monolayer, bilayer, and trilayer graphene for both spin-singlet and spin-triplet SC order parameters. We focus mostly on the gapped chiral $p+ip'$- and $d+id'$-wave SC states that show a non-zero Chern number and a corresponding number of edge states. For the $p+ip'$-wave state, we observe a rich Chern phase diagram when tuning the chemical potential and the SC order parameter amplitudes, which depends strongly on the number of layers and their stacking, and is also modified by trigonal warping. At small parameter values we observe a region whose Chern number is unique to rhombohedrally stacked graphene, and is independent of the number of layers. Our results can be understood in relation not only to the SC order parameter winding as expected, but also to the normal state band structure. This observation establishes the importance of the normal state characteristics for understanding the topology in SC graphene systems.

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arXiv2021

Instability of Majorana states in Shiba chains due to leakage into a topological substrate

We revisit the problem of Majorana states in chains of scalar impurities deposited on a superconductor with a mixed s-wave and p-wave pairing. We also study the formation of Majorana states for magnetic impurity chains. We find that the magnetic impurity chains exhibit well-localized Majorana states when the substrate is trivial, but these states hybridize and get dissolved in the bulk when the substrate is topological. Most surprisingly, and contrary to previous predictions, the scalar impurity chain does not support fully localized Majorana states except for very small and finely tuned parameter regimes, mostly for a non-topological substrate close to the topological transition. Our results indicate that a purely p-wave or a dominant p-wave substrate are not good candidates to support either magnetic or scalar impurity topological Shiba chains.

cond-mat.supr-con↗

arXiv2021

Surface states and quasiparticle interference in Bernal and rhombohedral graphite with and without trigonal warping

We use an exact analytical technique [Phys. Rev. B \textbf{101}, 115405 (2020), Phys. Rev. B \textbf{102}, 165117 (2020)] to recover the surface Green's functions for Bernal (ABA) and rhombohedral (ABC) graphite. For rhombohedral graphite we recover the predicted surface flat bands. For Bernal graphite we find that the surface state spectral function is similar to the bilayer one, but the trigonal warping effects are enhanced, and the surface quasiparticles have a much shorter lifetime. We subsequently use the T-matrix formalism to study the quasiparticle interference patterns generated on the surface of semi-infinite ABA and ABC graphite in the presence of impurity scattering. We compare our predictions to experimental STM data of impurity-localized states on the surface of Bernal graphite which appear to be in a good agreement with our calculations.

cond-mat.mes-hall↗

arXiv2021

Sublattice dependence and gate-tunability of midgap and resonant states induced by native dopants in Bernal-stacked bilayer graphene

The properties of semiconductors can be crucially impacted by midgap states induced by dopants, which can be native or intentionally incorporated in the crystal lattice. For Bernal-stacked bilayer graphene (BLG), which has a tunable bandgap, the existence of midgap states induced by dopants has been conjectured, but never confirmed experimentally. Here, we report scanning tunneling microscopy and spectroscopy results, supported by tight-binding calculations, that demonstrate the existence of midgap states in BLG. We show that the midgap state in BLG -- for which we demonstrate gate-tunability -- appears when the dopant is hosted on the non-dimer sublattice sites. We further evidence the presence of narrow resonances at the onset of the high energy bands (valence or conduction, depending on the dopant type) when the dopants lie on the dimer sublattice sites. These results suggest that dopants/defects can play an important role in the transport and optical properties of multilayer graphene samples, especially at energies close to the band extrema.

cond-mat.mes-hall↗

arXiv2021

Direct Visualization of Native Defects in Graphite and Their Effect on the Electronic Properties of Bernal-Stacked Bilayer Graphene

Graphite crystals used to prepare graphene-based heterostructures are generally assumed to be defect free. We report here scanning tunneling microscopy results that show graphite commonly used to prepare graphene devices can contain a significant amount of native defects. Extensive scanning of the surface allows us to determine the concentration of native defects to be 6.6$\times$10$^8$ cm$^{-2}$. We further study the effects of these native defects on the electronic properties of Bernal-stacked bilayer graphene. We observe gate-dependent intravalley scattering and successfully compare our experimental results to T-matrix-based calculations, revealing a clear carrier density dependence in the distribution of the scattering vectors. We also present a technique for evaluating the spatial distribution of short-scale scattering. A theoretical analysis based on the Boltzmann transport equation predicts that the dilute native defects identified here are an important extrinsic source of scattering, ultimately setting the mobility at low temperatures.

cond-mat.mes-hall↗

arXiv2021

Analytical and semianalytical tools to determine the topological character of Shiba chains

We introduce three new analytical and semi-analytical tools that allow one to determine the topological character of impurity Shiba chains. The analytical methods are based on calculating the effective Green's function of an infinite embedded chain using the T-matrix formalism and describing the chain as a {\it line impurity}. We thus provide a solution to the longstanding size-effects problem affecting the only general alternative method, the numerical tight-binding analysis. As an example we consider a chain of magnetic impurities deposited on an s-wave superconducting substrate with Rashba spin-orbit and we calculate its topological phase diagram as a function of the magnetic impurity strength and the chemical potential. We find a perfect agreement between all our new techniques and a numerical analysis.

cond-mat.supr-con↗

arXiv2021

Interface states at the boundary between ABC and ABA multilayer graphene structures

We study the interfaces between ABC and ABA regions in multilayer graphene, in particular we consider regions in which the transition between the ABA and ABC structure arises due to the local compression in one of the graphene layers for a zigzag interface, and to a local shear of the atoms for the armchair interface. When we consider an infinite ribbon configuration we find that interface bands form in these regions, more pronouncedly for the armchair-type interfaces. We note that these states are not topological. For fully-finite-size structures the continuous interface bands transform into sets of quantized levels.

cond-mat.mes-hall↗

arXiv2021

Quasiparticle interference patterns in bilayer graphene with trigonal warping

We calculate the form of quasiparticle interference patterns in bilayer graphene within a low-energy description, taking into account perturbatively the trigonal warping terms. We introduce four different types of impurities localized on the A and B sublattices of the first and the second layer, and we obtain closed-form analytical expressions both in real and Fourier spaces for the oscillatory corrections to the local density of states generated by the impurities. Finally, we compare our findings with recent experimental and semi-analytical T-matrix results from arXiv:2104.10620 and we show that there is a very good agreement between our findings and the previous results, as well as with the experimental data.

cond-mat.mes-hall↗

arXiv2020

Exact lattice-model calculation of boundary modes for Weyl semimetals and graphene

We provide an exact analytical technique to obtain within a lattice model the wave functions of the edge states in zigzag- and bearded-edge graphene, as well as of the Fermi-arc surface states in Weyl semimetals described by a minimal bulk model. We model the corresponding boundaries as an infinite scalar potential localized on a line, and respectively within a plane. We use the T-matrix formalism to obtain the dispersion and the spatial distribution of the corresponding boundary modes. Furthermore, to demonstrate the power of our approach, we write down the surface Green's function of the considered Weyl semimetal model, and we calculate the quasiparticle interference patterns originating from an impurity localized at the respective surface.

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