arXiv subjects

M. J. Calderón

Publications and source records attributed to M. J. Calderón.

At least 19 recordsLinked to original sources

arXiv2026

Cascades in transport and optical conductivity of Twisted Bilayer Graphene

Using a combined Dynamical Mean Field Theory and Hartree (DMFT+H) calculation we study the transport and optical properties of the 8-band heavy fermion model for Twisted Bilayer Graphene (TBG) in the normal state. We find resistive states around integer fillings which resemble the ones observed in transport experiments. From a Drude fitting of the low frequency optical conductivity, we extract a very strongly doping-dependent Drude weight and scattering rate, resetting at the integers. For most dopings, particularly above the integers, the Drude scattering rate is high but notably smaller than that of the local electrons. This highlights the important role of itinerant electrons in the transport properties, despite their limited spectral weight on the flat bands. At far infrared frequencies, the optical conductivity exhibits cascades characterized by highly asymmetric resets of the intensity and oscillations in the interband peak frequencies.

cond-mat.str-el↗

arXiv2023

Heavy quasiparticles and cascades without symmetry breaking in twisted bilayer graphene

Among the variety of correlated states exhibited by twisted bilayer graphene, cascades in the spectroscopic properties and in the electronic compressibility occur over larger ranges of energy, twist angle and temperature compared to other effects. This suggests a hierarchy of phenomena. Using combined dynamical mean-field theory and Hartree calculations, we show that the spectral weight reorganisation associated with the formation of local moments and heavy quasiparticles can explain the cascade of electronic resets without invoking symmetry breaking orders. The phenomena reproduced here include the cascade flow of spectral weight, the oscillations of remote band energies, and the asymmetric jumps of the inverse compressibility. We also predict a strong momentum differentiation in the incoherent spectral weight associated with the fragile topology of twisted bilayer graphene.

cond-mat.str-el↗

arXiv2022

Mott correlations in ABC graphene trilayer aligned with hBN

The nature of the correlated phases found in some graphene heterostructures is under debate. We use dynamical mean-field theory (DMFT) to analyze the effect of local correlations close to half-filling on one of such systems, the ABC trilayer graphene aligned with hexagonal boron nitride (ABC/hBN), which presents a moiré superlattice. This system has shown insulating phases at integer fillings of the moiré lattice, precisely the fillings at which a sufficiently strong Coulomb interaction (U$_{\rm Mott}$) may produce a metal-insulator Mott transition. Our calculations show that the electronic states are strongly affected by a significant spectral weight transfer at interactions with magnitudes expected to be relevant in experiments. This effect, which emerges at interactions considerably smaller than U$_{\rm Mott}$ and does not require symmetry breaking, impacts the electronic properties at temperatures above the magnetic transitions producing anomalous temperature and doping dependences not present without alignment to hBN. Close to the Mott transition we find that onsite interactions promote an antiferromagnetic (AF) state, probably breaking the C$_3$ symmetry, that will compete with the ferromagnetism arising from intersite exchange interactions to determine the ground state.

cond-mat.str-el↗

arXiv2021

The role of orbital nesting in the superconductivity of Iron-based Superconductors

We analyze the magnetic excitations and the spin-mediated superconductivity in iron-based superconductors within a low-energy model that operates in the band basis but fully incorporates the orbital character of the spin excitations. We show how the orbital selectivity, encoded in our low-energy description, simplifies substantially the analysis and allows for analytical treatments, while retaining all the main features of both spin-excitations and gap functions computed using multiorbital models. Importantly, our analysis unveils the orbital matching between the hole and electron pockets as the key parameter to determine the momentum-dependence and the hierarchy of the superconducting gaps, instead of the Fermi surface matching as in the common nesting scenario.

cond-mat.supr-con↗

arXiv2021

Correlated states in magic angle twisted bilayer graphene under the optical conductivity scrutiny

Moiré systems displaying flat bands have emerged as novel platforms to study correlated electron phenomena. Insulating and superconducting states appear upon doping magic angle twisted bilayer graphene (TBG), and there is evidence of correlation induced effects at the charge neutrality point (CNP) which could originate from spontaneous symmetry breaking. Our theoretical calculations show how optical conductivity measurements can distinguish different symmetry breaking states, and reveal the nature of the correlated states. In the specific case of nematic order, which breaks the discrete rotational symmetry of the lattice, we find that the Dirac cones are displaced, not only in momentum space but also in energy, inducing finite Drude weight at the CNP. We also show that the sign of the dc conductivity anisotropy induced by a nematic order depends on the degree of lattice relaxation, the doping and the nature of the symmetry breaking.

cond-mat.str-el↗

arXiv2020

Interactions in the 8-orbital model for twisted bilayer graphene

We calculate the interactions between the Wannier functions of the 8-orbital model for twisted bilayer graphene (TBG). In this model, two orbitals per valley centered at the AA regions, the AA-p orbitals, account for the most part of the spectral weight of the flats bands. Exchange and assisted-hopping terms between these orbitals are found to be small. Therefore, the low energy properties of TBG will be determined by the density-density interactions. These interactions decay with the distance much faster than in the two orbital model, following a 1/r law in the absence of gates. The magnitude of the largest interaction in the model, the onsite term between the flat band orbitals, is controlled by the size of the AA regions and is estimated to be ~ 40 meV. To screen this interaction, the metallic gates have to be placed at a distance smaller than 5 nm. For larger distances only the long-range part of the interaction is substantially screened. The model reproduces the band deformation induced by doping found in other approaches within the Hartree approximation. Such deformation reveals the presence of other orbitals in the flat bands and is sensitive to the inclusion of the interactions involving them.

cond-mat.str-el↗

arXiv2020

Lifting of Spin Blockade by Charged Impurities in Si-MOS Double Quantum Dot Devices

One obstacle that has slowed the development of electrically gated metal-oxide-semiconductor (MOS) singlet-triplet qubits is the frequent lack of observed spin blockade, even in samples with large singlet-triplet energy splittings. We present theoretical and experimental evidence that the cause of this problem in MOS double quantum dots is the stray positive charges in the oxide inducing accidental levels near the device's active region that allow spin blockade lifting. We also present evidence that these effects can be mitigated by device design modifications, such as overlapping gates.

cond-mat.mes-hall↗

arXiv2020

The nature of correlations in the insulating states of twisted bilayer graphene

The recently observed superconductivity in twisted bilayer graphene emerges from insulating states believed to arise from electronic correlations. While there have been many proposals to explain the insulating behaviour, the commensurability at which these states appear suggests that they are Mott insulators. Here we focus on the insulating states with $\pm 2$ electrons or holes with respect to the charge neutrality point. We show that the theoretical expectations for the Mott insulating states are not compatible with the experimentally observed dependence on temperature and magnetic field if, as frequently assumed, only the correlations between electrons on the same site are included. We argue that the inclusion of non-local (inter-site) correlations in the treatment of the Hubbard model can bring the predictions for the magnetic and temperature dependencies of the Mott transition to an agreement with experiments and have consequences for the critical interactions, the size of the gap, and possible pseudogap physics. The importance of the inter-site correlations to explain the experimental observations indicates that the observed insulating gap is not the one between the Hubbard bands and that antiferromagnetic-like correlations play a key role in the Mott transition.

cond-mat.str-el↗

arXiv2018

Spin-orbit effects surfacing on manganites

Spin-orbit coupling in magnetic systems lacking inversion symmetry can give rise to non trivial spin textures. Magnetic thin films and heterostructures are potential candidates for the formation of skyrmions and other non-collinear spin configurations as inversion symmetry is inherently lost at their surfaces and interfaces. However, manganites, in spite of their extraordinarily rich magnetic phase diagram, have not yet been considered of interest within this context as their spin-orbit coupling is assumed to be negligible. We demonstrate here, by means of angular dependent X-ray linear dichroism experiments and theoretical calculations, the existence of a noncollinear antiferromagnetic ordering at the surface of ferromagnetic La$_{2/3}$Sr$_{1/3}$MnO$_3$ thin films whose properties can only be explained by an unexpectedly large enhancement of the spin-orbit interaction. Our results reveal that spin-orbit coupling, usually assumed to be very small on manganites, can be significantly enhanced at surfaces and interfaces adding a new twist to the possible magnetic orders that can arise in electronically reconstructed systems.

cond-mat.str-el↗

arXiv2018

Signatures of atomic-scale structure in the energy dispersion and coherence of a Si quantum-dot qubit

We report anomalous behavior in the energy dispersion of a three-electron double-quantum-dot hybrid qubit and argue that it is caused by atomic-scale disorder at the quantum-well interface. By employing tight-binding simulations, we identify potential disorder profiles that induce behavior consistent with the experiments. The results indicate that disorder can give rise to "sweet spots" where the decoherence caused by charge noise is suppressed, even in a parameter regime where true sweet spots are unexpected. Conversely, "hot spots" where the decoherence is enhanced can also occur. Our results suggest that, under appropriate conditions, interfacial atomic structure can be used as a tool to enhance the fidelity of Si double-dot qubits.

cond-mat.mes-hall↗

arXiv2018

2-dimensional semiconductors pave the way towards dopant based quantum computing

Since the 1998 proposal to build a quantum computer using dopants in semiconductors as qubits, much progress has been achieved on semiconductors nano fabrication and control of charge and spins in single dopants. However, an important problem remains, which is the control at the atomic scale of the dopants positioning. We propose to circumvent this problem by using 2 dimensional materials as hosts. Since the first isolation of graphene in 2004, the number of new 2D materials with favorable properties for electronics has been growing. Dopants in 2 dimensional systems are more tightly bound and potentially easier to position and manipulate. Considering the properties of currently available 2D materials, we access the feasibility of such proposal in terms of the manipulability of isolated dopants (for single qubit operations) and dopant pairs (for two qubit operations). Our results indicate that a wide variety of 2D materials may perform at least as well as the currently studied bulk host for donor qubits.

cond-mat.mes-hall↗

arXiv2017

Entanglement control and magic angles for acceptor qubits in Si

Full electrical control of quantum bits could enable fast, low-power, scalable quantum computation. Although electric dipoles are highly attractive to couple spin qubits electrically over long distances, mechanisms identified to control two-qubit couplings do not permit single-qubit operations while two-qubit couplings are off. Here we identify a mechanism to modulate electrical coupling of spin qubits that overcomes this drawback for hole spin qubits in acceptors,that is based on the electrical tuning of the direction of the spin-dependent electric dipole by a gate. In this way, inter-qubit coupling can be turned off electrically by tuning to a "magic angle" of vanishing electric dipole-dipole interactions, while retaining the ability to manipulate the individual qubits. This effect stems from the interplay of the $\rm T_d$ symmetry of the acceptor state in the Si lattice with the magnetic field orientation, and the spin-3/2 characteristic of hole systems. Magnetic field direction also allows to greatly suppress spin relaxation by phonons that limit single qubit performance, while retaining sweet spots where the qubits are insensitive to charge noise. Our findings can be directly applied to state-of-the-art acceptor based architectures, for which we propose suitable protocols to practically achieve full electrical tunability of entanglement and the realization of a decoherence-free subspace.

cond-mat.mes-hall↗

arXiv2017

Spin qubit manipulation of acceptor bound states in group IV quantum wells

The large spin-orbit coupling in the valence band of group IV semiconductors provides an electric field knob for spin-qubit manipulation. This fact can be exploited with acceptor based qubits. Spin manipulation of holes bound to acceptors in engineered SiGe quantum wells depends very strongly on the electric field applied and on the heterostructure parameters. The g-factor is enhanced by the Ge content and can be tuned by shifting the hole wave-function between the heterostructure constituent layers. The lack of inversion symmetry induced both by the quantum well and the electric fields together with the g-factor tunability allows the possibility of different qubit manipulation methods such as electron spin resonance, electric dipole spin resonance and g-tensor modulation resonance. Rabi frequencies up to hundreds of MHz can be achieved with heavy-hole qubits, and of the order of GHz with light-hole qubits.

cond-mat.mes-hall↗

arXiv2016

Strong correlations and the search for high-Tc superconductivity in chromium pnictides and chalcogenides

Undoped iron superconductors accommodate $n=6$ electrons in five d-orbitals. Experimental and theoretical evidence shows that the strength of correlations increases with hole-doping, as the electronic filling approaches half-filling with $n=5$ electrons. This evidence delineates a scenario in which the parent compound of iron superconductors is the half-filled system, in analogy to cuprate superconductors. In cuprates the superconductivity can be induced upon electron or hole doping. In this work we propose to search for high-Tc superconductivity and strong correlations in chromium pnictides and chalcogenides with $n<5$ electrons. By means of ab-initio, slave spin and multi-orbital RPA calculations we analyse the strength of the correlations and the superconducting and magnetic instabilities in these systems with main focus on LaCrAsO. We find that electron-doped LaCrAsO is a strongly correlated system with competing magnetic interactions, being $(π,π)$ antiferromagnetism and nodal d-wave pairing the most plausible magnetic and superconducting instabilities, respectively.

cond-mat.str-el↗

arXiv2015

Interface effects on acceptor qubits in silicon and germanium

Dopant-based quantum computing implementations often require the dopants to be situated close to an interface to facilitate qubit manipulation with local gates. Interfaces not only modify the energies of the bound states but also affect their symmetry. Making use of the successful effective mass theory we study the energy spectra of acceptors in Si or Ge taking into account the quantum confinement, the dielectric mismatch and the central cell effects. The presence of an interface puts constraints to the allowed symmetries and lead to the splitting of the ground state in two Kramers doublets [J. Mol et al, App. Phys. Lett. 106, 203110 (2015)]. Inversion symmetry breaking also implies parity mixing which affects the allowed optical transitions. Consequences for acceptor qubits are discussed.

cond-mat.mes-hall↗

arXiv2015

Donor Wavefunctions in Si Gauged by STM Images

The triumph of effective mass theory in describing the energy spectrum of dopants does not guarantee that the model wavefunctions will withstand an experimental test. Such wavefunctions have recently been probed by scanning tunneling spectroscopy, revealing localized patterns of resonantly enhanced tunneling currents. We show that the shape of the conducting splotches resemble a cut through Kohn-Luttinger (KL) hydrogenic envelopes, which modulate the interfering Bloch states of conduction electrons. All the non-monotonic features of the current profile are consistent with the charge density fluctuations observed between successive {001} atomic planes, including a counterintuitive reduction of the symmetry - a heritage of the lowered point group symmetry at these planes. A model-independent analysis of the diffraction figure constrains the value of the electron wavevector to $k_0 = (0.82 \pm 0.03)(2π/a_{Si})$. Unlike prior measurements, averaged over a sizeable density of electrons, this estimate is obtained directly from isolated electrons. We further investigate the model-specific anisotropy of the wave function envelope, related to the effective mass anisotropy. This anisotropy appears in the KL variational wave function envelope as the ratio between Bohr radii b=a. We demonstrate that the central cell corrected estimates for this ratio are encouragingly accurate, leading to the conclusion that the KL theory is a valid model not only for energies but for wavefunctions as well.

cond-mat.mes-hall↗

arXiv2014

Correlation, doping and interband effects on the optical conductivity of iron superconductors

Electronic interactions in multiorbital systems lead to non-trivial features in the optical spectrum. In iron superconductors the Drude weight is strongly suppressed with hole-doping. We discuss why the common association of the renormalization of the Drude weight with that of the kinetic energy, used in single band systems, does not hold in multi-orbital systems. This applies even in a Fermi liquid description when each orbital is renormalized differently, as it happens in iron superconductors. We estimate the contribution of interband transitions at low energies. We show that this contribution is strongly enhanced by interactions and dominates the coherent part of the spectral weight in hole-doped samples at frequencies currently used to determine the Drude weight.

cond-mat.str-el↗

arXiv2014

Theory of one and two donors in Silicon

We provide here a roadmap for modeling silicon nano-devices with one or two group V donors (D). We discuss systems containing one or two electrons, that is, D^0, D^-, D_2^+ and D_2^0 centers. The impact of different levels of approximation is discussed. The most accurate instances -- for which we provide quantitative results -- are within multivalley effective mass including the central cell correction and a configuration interaction account of the electron-electron correlations. We also derive insightful, yet less accurate, analytical approximations and discuss their validity and limitations -- in particular, for a donor pair, we discuss the single orbital LCAO method, the Huckel approximation and the Hubbard model. Finally we discuss the connection between these results and recent experiments on few dopant devices.

cond-mat.mes-hall↗