Person: Díaz García, Elena
Universidad Complutense de Madrid
Faculty / Institute
Física de Materiales
Ciencia de los Materiales e Ingeniería Metalúrgica
Now showing 1 - 10 of 56
PublicationSuper Bloch oscillations in the Peyrard-Bishop-Holstein model(Elsevier, 2012-01-09) Herrero-Gomez, C.; Díaz García, Elena; Domínguez-Adame Acosta, FranciscoRecently, polarons in the Peyrard-Bishop-Holstein model under DC electric fields were established to perform Bloch oscillations, provided the charge-lattice coupling is not large. In this work, we study this model when the charge is subjected to an applied field with both DC and AC components. Similarly to what happens in the rigid lattice, we find that the carrier undergoes a directed motion or coherent oscillations when the AC field is resonant or detuned with respect to the Bloch frequency, respectively. The electric density current and its Fourier spectrum are also studied to reveal the frequencies involved in the polaron dynamics. PublicationEffective nonlinear model for electron transport in deformable helical molecules(American Physical Society, 2018-11-21) Díaz García, Elena; Contreras, A.; Hernández, J.; Domínguez-Adame Acosta, FranciscoThe helical conformation of electric dipoles in some chiral molecules, such as DNA and bacteriorhodopsin, induces a spin-orbit coupling that results in a sizable spin selectivity of electrons. The local deformation of the molecule about the moving electron may affect the spin dynamics due to the appearance of bright solitons with well-defined spin projection onto the molecule axis. In this work, we introduce an effective model for electron transport in a deformable helical molecular lattice that resembles the nonlinear Kronig-Penney model in the adiabatic approximation. In addition, the continuum limit of our model is achieved when the dipole-dipole distance is smaller than the spatial extent of the bright soliton, as discussed by E. Diaz et al. [N. J. Phys. 20, 043055 (2018)]. In this limit, our model reduces to an extended Davydov model. Finally, we also focus on perturbations to the bright soliton that arise naturally in the context of real helical molecules. We conclude that the continuum approximation provides excellent results in more complex scenarios. PublicationMany-impurity scattering on the surface of a topological insulator(Nature publishing group, 2021-03-11) Hernando Grande, Antonio; Baba, Yuriko Caterina; Díaz García, Elena; Domínguez-Adame Acosta, FranciscoWe theoretically address the impact of a random distribution of non-magnetic impurities on the electron states formed at the surface of a topological insulator. The interaction of electrons with the impurities is accounted for by a separable pseudo-potential method that allows us to obtain closed expressions for the density of states. Spectral properties of surface states are assessed by means of the Green's function averaged over disorder realisations. For comparison purposes, the configurationally averaged Green's function is calculated by means of two different self-consistent methods, namely the self-consistent Born approximation (SCBA) and the coherent potential approximation (CPA). The latter is often regarded as the best single-site theory for the study of the spectral properties of disordered systems. However, although a large number of works employ the SCBA for the analysis of many-impurity scattering on the surface of a topological insulator, CPA studies of the same problem are scarce in the literature. In this work, we find that the SCBA overestimates the impact of the random distribution of impurities on the spectral properties of surface states compared to the CPA predictions. The difference is more pronounced when increasing the magnitude of the disorder. PublicationSpin dynamics in helical molecules with nonlinear interactions(IOP Publishing, 2018-04-27) Díaz García, Elena; Estévez, P. G.; Cerveró, J. M.; Gaul, C.; Díez, E.; Domínguez-Adame Acosta, FranciscoIt is widely admitted that the helical conformation of certain chiral molecules may induce a sizable spin selectivity observed in experiments. Spin selectivity arises as a result of the interplay between a helicity-induced spin-orbit coupling (SOC) and electric dipole fields in the molecule. From the theoretical point of view, different phenomena might affect the spin dynamics in helical molecules, such as quantum dephasing, dissipation and the role of metallic contacts. With a few exceptions, previous studies usually neglect the local deformation of the molecule about the carrier, but this assumption seems unrealistic to describe charge transport in molecular systems. We introduce an effective model describing the electron spin dynamics in a deformable helical molecule with weak SOC. We find that the electron-lattice interaction allows the formation of stable solitons such as bright solitons with well defined spin projection onto the molecule axis. We present a thorough study of these bright solitons and analyze their possible impact on the spin dynamics in deformable helical molecules. PublicationExcitons, trions and Rydberg states in monolayer MoS_2 revealed by low-temperature photocurrent spectroscopy(Nature, 2020-10-30) Vaquero, Daniel; Clericò, Vito; Salvador Sánchez, Juan; Martín Ramos, Adrián; Díaz García, Elena; Domínguez-Adame Acosta, Francisco; Meziani, Yahya M; Díez, Enrique; Quereda, JorgeExciton physics in two-dimensional semiconductors are typically studied by photoluminescence spectroscopy. However, this technique does not allow for direct observation of non-radiating excitonic transitions. Here, we use low-temperature photocurrent spectroscopy as an alternative technique to investigate excitonic transitions in a high-quality monolayer MoS_2 phototransistor. The resulting spectra presents excitonic peaks with linewidths as low as 8 meV. We identify spectral features corresponding to the ground states of neutral excitons (X^A_(1s) and X^B_(1s) and charged trions (T^A and T^B) as well as up to eight additional spectral lines at energies above the X^B_(1s) transition, which we attribute to the Rydberg series of excited states of X^A and X^B. The intensities of the spectral features can be tuned by the gate and drain-source voltages. Using an effective-mass theory for excitons in two-dimensional systems we are able to accurately fit the measured spectral lines and unambiguously associate them with their corresponding Rydberg states. PublicationTunable slow- and fast-light devices based on molecular-aggregate nanofilms(Amer Physical Soc, 2018-10-09) Díaz García, Elena; Villas, A.; Cabrera Granado, E.; Calderon, O. G.We study the tunability of nanofilms composed of linear molecular aggregates for slow- and fast-light performance. In order to describe a wide range of intensity field regimes, we consider a two-exciton model for the molecular aggregate where exciton creation or annihilation may occur. Our simulations show fractional delays and advancements of the order of those found in longer propagating media. This nanometric device presents two functionalities: (i) it allows the existence of slow or fast light in a different bandwidth of pulsed signals and (ii) such dual performance can be tuned by a small perturbation of the input signal amplitude for a fixed pulse temporal width. Both effects are tested under the usual presence of disorder in these molecular systems. This study concludes that a molecular-aggregate nanofilm presents a wide control of group velocity in the GHz and THz bandwidths. PublicationControl of upconversion luminescence by gold nanoparticle size: from quenching to enhancement(RSC, 2019-08-07) Méndez González, Diego; Melle Hernández, Sonia; Gómez Calderón, Óscar; Laurenti, Marco; Cabrera Granado, Eduardo; Egatz-Gómez, Ana; López Cabarcos, Enrique; Rubio Retama, Jorge; Díaz García, ElenaMetallic nanostructures have the potential to modify the anti-Stokes emission of upconverting nanoparticles (UCNPs) by coupling their plasmon resonance with either the excitation or the emission wavelength of the UCNPs. In this regard gold nanoparticles (AuNPs) have often been used in sensors for UCNP luminescence quenching or enhancement, although systematic studies are still needed in order to design optimal UCNP–AuNP based biosensors. Amidst mixed experimental evidence of quenching or enhancement, two key factors arise: the nanoparticle distance and nanoparticle size. In this work, we synthesize AuNPs of different sizes to assess their influence on the luminescence of UCNPs. We find that strong luminescence quenching due to resonance energy transfer is preferentially achieved for small AuNPs, peaking at an optimal size. A further increase in the AuNP size is accompanied by a reduction of luminescence quenching due to an incipient plasmonic enhancement effect. This enhancement counterbalances the luminescence quenching effect at the biggest tested AuNP size. The experimental findings are theoretically validated by studying the decay rate of the UCNP emitters near a gold nanoparticle using both a classical phenomenological model and the finite-difference time-domain method. Results from this study establish general guidelines to consider when designing sensors based on UCNPs–AuNPs as donor–quencher pairs, and suggest the potential of plasmon-induced luminescence enhancement as a sensing strategy. PublicationStable Bloch oscillations of cold atoms with time-dependent interaction(American Physical Society, 2009-06-26) Gaul, C.; Lima, R. P. A.; Díaz García, Elena; Mueller, C. A.; Domínguez-Adame Acosta, FranciscoWe investigate Bloch oscillations of interacting cold atoms in a mean-field framework. In general, atom-atom interaction causes dephasing and destroys Bloch oscillations. Here we show that Bloch oscillations are persistent if the interaction is modulated harmonically with suitable frequency and phase. For other modulations, Bloch oscillations are rapidly damped. We explain this behavior in terms of collective coordinates whose Hamiltonian dynamics permits one to predict a whole family of stable solutions. In order to describe also the unstable cases, we carry out a stability analysis for Bogoliubov excitations. Using Floquet theory, we are able to predict the unstable modes as well as their growth rate, found to be in excellent agreement with numerical simulations. PublicationPublisher's Note: Spin-selective transport through helical molecular systems (vol 85, 081404, 2012)(American Physical Society, 2012-05-11) Gutierrez, R.; Díaz García, Elena; Naaman, R.; Cuniberti, G. PublicationContact effects in spin transport along double-helical molecules(American Physical Society, 2014-04-28) Ai-Min, Guo; Díaz García, Elena; Gaul, C.; Gutierrez, R.; Domínguez-Adame Acosta, Francisco; Cuniberti, G.; Sun, Qing-fengWe report on spin transport along double-helical molecular systems by considering various contact configurations and asymmetries between the two helical strands in the regime of completely coherent charge transport. Our results reveal that no spin polarization appears in two-terminal molecular devices when coupled to one-dimensional electrodes. The same holds in the case of finite-width electrodes if there is a bottleneck of one single site in the system electrode-molecule-electrode. Then, additional dephasing is necessary to induce spin-filtering effects. In contrast, nonzero spin polarization is found in molecular devices with multiple terminals or with two finite-width electrodes, each of them connected to more than one site of the molecule. The magnitude of spin polarization can be enhanced by increasing the asymmetry between the two strands. We point out that the spin-filtering effects could emerge in double-helical molecular devices at low temperature without dephasing by a proper choice of the electrode number and the connection between the molecule and the electrodes.