Papers by David Perez de Lara
A Versatile Scanning Photocurrent Mapping System to Characterize Optoelectronic Devices based on 2D Materials
Small Methods
npj 2D Materials and Applications
Since their discovery, single-layer semiconducting transition metal dichalcogenides have attracte... more Since their discovery, single-layer semiconducting transition metal dichalcogenides have attracted much attention, thanks to their outstanding optical and mechanical properties. Strain engineering in these two-dimensional materials aims to tune their bandgap energy and to modify their optoelectronic properties by the application of external strain. In this paper, we demonstrate that biaxial strain, both tensile and compressive, can be applied and released in a timescale of a few seconds in a reproducible way on transition metal dichalcogenides monolayers deposited on polymeric substrates. We can control the amount of biaxial strain applied by letting the substrate expand or compress. To do this, we change the substrate temperature and choose materials with a large thermal expansion coefficient. After the investigation of the substrate-dependent strain transfer, we performed microdifferential spectroscopy of four transition metal dichalcogenides monolayers (MoS 2 , MoSe 2 , WS 2 , WSe 2 ) under the application of biaxial strain and measured their optical properties. For tensile strain, we observe a redshift of the bandgap that reaches a value as large as 95 meV/% in the case of single-layer WS 2 deposited on polypropylene. The observed bandgap shifts as a function of substrate extension/compression follow the order MoSe 2 < MoS 2 < WSe 2 < WS 2 . Theoretical calculations of these four materials under biaxial strain predict the same trend for the material-dependent rates of the shift and reproduce well the features observed in the measured reflectance spectra.
Characterization of highly crystalline lead iodide nanosheets prepared by room-temperature solution processing
Nanotechnology
Two-dimensional (2D) semiconducting materials are particularly appealing for many applications. A... more Two-dimensional (2D) semiconducting materials are particularly appealing for many applications. Although theory predicts a large number of 2D materials, experimentally only a few of these materials have been identified and characterized comprehensively in the ultrathin limit. Lead iodide, which belongs to the transition metal halides family and has a direct bandgap in the visible spectrum, has been known for a long time and has been well characterized in its bulk form. Nevertheless, studies of this material in the nanometer thickness regime are rather scarce. In this article we demonstrate an easy way to synthesize ultrathin, highly crystalline flakes of PbI2 by precipitation from a solution in water. We thoroughly characterize the produced thin flakes with different techniques ranging from optical and Raman spectroscopy to temperature-dependent photoluminescence and electron microscopy. We compare the results to ab initio calculations of the band structure of the material. Finally, we fabricate photodetectors based on PbI2 and study their optoelectronic properties.
High Throughput Characterization of Epitaxially Grown Single-Layer MoS2
Electronics, 2017
Micro-reflectance and transmittance spectroscopy: a versatile and powerful tool to characterize 2D materials
Journal of Physics D: Applied Physics, 2017
Highly responsive UV-photodetectors based on single electrospun TiO2 nanofibres
J. Mater. Chem. C, 2016
Electrospun TiO2 single-nanofibres are employed as photodetectors with state-of-the-art performan... more Electrospun TiO2 single-nanofibres are employed as photodetectors with state-of-the-art performance: high sensitivity in the UV (responsivity of 90 A W−1 for 375 nm wavelength), response time of ∼5 s and light polarisation sensitivity.
Josephson Junctions for ToF Mass Spectrometry
Vortex dynamics and vortex lattice reconfiguration in superconducting-magnetic hybrids
Amorphous superconducting films (Mo3Si) have been grown on top of array of nanometric magnets. Th... more Amorphous superconducting films (Mo3Si) have been grown on top of array of nanometric magnets. These periodic magnetic centers have been fabricated on Si substrates by Electron Beam Lithography and sputtering techniques. In the mixed state the competition between the intrinsic and random pinning potential of the superconducting film and the artificial induced periodic pinning potential governs the vortex lattice behavior. Close to critical temperature, the periodic potentials could overcome the random potentials, then the vortex lattice dynamics shows effects which are related with the array dimension and symmetry. We will show in these hybrid systems enhancements of matching effects between the vortex lattice and the array unit cell, and different vortex lattice configurations.
Zero average and net flows of vortices in hybrid nanostructures with asymmetric pinning potentials
We have fabricated hybrid nanostructures with superconducting film on top of an array of magnetic... more We have fabricated hybrid nanostructures with superconducting film on top of an array of magnetic nanotriangles. In these structures, non-zero DC and AC voltages (VDC, VAC) are generated by alternating currents injected in the hybrid device. The VDC and VAC behaviors give us an overall picture of the vortex dynamics and the rectification effects in these superconducting devices.
Aluminum Superconducting Tunnel Junction as X-ray detector: Technological aspects and phonon decoupling from the substrate
We have investigated Al/Al2O3/Al Superconducting Tunnel Junctions (STJs) for their use as X-ray d... more We have investigated Al/Al2O3/Al Superconducting Tunnel Junctions (STJs) for their use as X-ray detector. The junctions have been fabricated on sapphire substrates both with and without a SiO buffer layer. It was found that the presence of the SiO significantly changes the Al2O3 tunnel barrier properties. Experiments under X-ray irradiation from 55Fe X-ray source are reported. The spectra show that the SiO buffer layer succeeds in reducing the events coming from the phonons generated into the substrate. Besides, the charge yield is about 85% for STJs with the SiO buffer layer, quite high in comparison with the 5% of STJs without the buffer layer. .
EPJ Web of Conferences, 2010
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Employment of Submicron YBA2CU3O7-x Grain Boundary Junctions for the Fabrication of “ Quiet” Superconducting Flux-Qubits
NATO Science Series II: Mathematics, Physics and Chemistry, 2005
ABSTRACT
Transverse ratchet effect and superconducting vortices: simulation and experiment
New Journal of Physics, 2009
A transverse ratchet effect has been measured in magnetic/superconducting hybrid films fabricated... more A transverse ratchet effect has been measured in magnetic/superconducting hybrid films fabricated by electron beam lithography and magnetron sputtering techniques. The samples are Nb films grown on top of an array of Ni nanotriangles. Injecting an ac current parallel ...
Vortex lattice motion in the flux creep regime on asymmetric pinning potentials
Superconductor Science and Technology, 2013
ABSTRACT Vortex lattice dynamics is investigated in the adiabatic regime in superconducting/magne... more ABSTRACT Vortex lattice dynamics is investigated in the adiabatic regime in superconducting/magnetic nanostructures for strong and weak asymmetric pinning potentials, provided by a rectangular array of Ni triangles and a kagomé-like array of Ni dots, respectively. The AC voltage response of the AC-driven superconducting vortex lattice is measured in addition to the DC voltage response. The results allow us to link voltage rectification effects with the vortex lattice dynamics. In particular, for the weaker pinning kagomé-like array, a high-temperature zero critical current regime is identified, in which both AC and DC voltage responses can be described in terms of a simple model of thermal activation of single vortices over finite asymmetric barriers.
Vortex lattice matching effects in superconducting Mo $ _ {3} $ Si films with magnetic pinning centers
Bulletin of the American …, 2008
... Chair: Ilya Vekhter, Louisiana State University Abstract ID: BAPS.2008.MAR.W10.4. Abstract: W... more ... Chair: Ilya Vekhter, Louisiana State University Abstract ID: BAPS.2008.MAR.W10.4. Abstract: W10.00004 : Vortex lattice matching effects in superconducting Mo$_{3}$Si films with magnetic pinning centers. 3:30 PM–3:42 PM. Preview Abstract. Authors: David Perez de Lara (Dept ...
Physical Review B, 2010
Magnetic vortex dynamics in lithographically prepared nanodots is currently a subject of intensiv... more Magnetic vortex dynamics in lithographically prepared nanodots is currently a subject of intensive research, particularly after recent demonstration that the vortex polarity can be controlled by in-plane magnetic field. This has stimulated the proposals of non-volatile vortex magnetic random access memories. In this work, we demonstrate that triangular nanodots offer a real alternative where vortex chirality, in addition to polarity, can be controlled. In the static regime, we show that vortex chirality can be tailored by applying in-plane magnetic field, which is experimentally imaged by means of Variable-Field Magnetic Force Microscopy. In addition, the polarity can be also controlled by applying a suitable out-of-plane magnetic field component. The experiment and simulations show that to control the vortex polarity, the out-of-plane field component, in this particular case, should be higher than the in-plane nucleation field. Micromagnetic simulations in the dynamical regime show that the magnetic vortex polarity can be changed with short-duration magnetic field pulses, while longer pulses change the vortex chirality.
Physical Review B, 2011
Triangular arrays of Ni nanotriangles embedded in superconducting Nb films exhibit unexpected dyn... more Triangular arrays of Ni nanotriangles embedded in superconducting Nb films exhibit unexpected dynamical vortex effects. Collective pinning with a vortex lattice configuration different from the expected fundamental triangular "Abrikosov state" is found. The vortex motion which prevails against the triangular periodic potential is produced by channelling effects between triangles. Interstitial vortices coexisting with pinned vortices in this asymmetric potential, lead to ratchet reversal, i.e. a DC output voltage which changes sign with the amplitude of an applied alternating drive current. In this landscape, ratchet reversal is always observed at all magnetic fields (all numbers of vortices) and at different temperatures. The ratchet reversal is unambiguously connected to the presence of two locations for the vortices: interstitial and above the artificial pinning sites.
Erratum: Vortex ratchet reversal: Role of interstitial vortices [Phys. Rev. B <span xmlns:xlink="http://www.w3.org/1999/xlink" style="font-weight: bold;">83</span>, 174507 (2011)]
Physical Review B, 2013
Physical Review B, 2006
We have investigated theoretically and experimentally a small annular Josephson junction with thr... more We have investigated theoretically and experimentally a small annular Josephson junction with three leads, where the third lead is added to one of the ring-shaped electrodes to apply a control ͑injection͒ current and thereby create a local magnetic field. We study the static case, namely, we derive the general expression for the Josephson critical current in the presence of both an injection current and an external parallel magnetic field. Concerning the theoretical investigation of the dynamic case, we obtain an analytical expression for the Fiske steps amplitude as a function of the injection current intensity and the angle separating the two injection leads ͑ 1 ͒. The theoretical results show that a perfect analogy with the behavior of a rectangular junction in an external uniform magnetic field can be established for any orientation of the injector leads in the static case, while in the dynamic case the analogy holds only when the injector leads have a separation angle of 1 = . We present experimental dependences of the Josephson critical current and Fiske steps amplitudes on the injected current for two separation angles 1 = / 2 and 1 = . The analysis and the comparison with the experiments confirm the theoretical predictions. The Fiske step measurements, presented for the case 1 = / 2, have no straightforward analogous for the rectangular junction; however, we show a very good agreement between theory and experiments also in this case.
Physical Review B, 2009
A ratchet effect (the rectification of an ac injected current) which is purely magnetic in origin... more A ratchet effect (the rectification of an ac injected current) which is purely magnetic in origin has been observed in a superconducting-magnetic nanostructure hybrid. The hybrid consists of a superconducting Nb film in contact with an array of nanoscale magnetic triangles, circular rings or elliptical rings. The arrays were placed into welldefined remanent magnetic states by application of different magnetic field cycles. The stray fields from these remanent states provide a magnetic landscape which influences the motion of superconducting vortices. We examined both randomly varying landscapes from demagnetized samples, and ordered landscapes from samples at remanence after saturation in which the magnetic rings form parallel onion states containing two domain walls. The ratchet effect is absent if the rings are in the demagnetized state or if the vortices propagate parallel to the magnetic reflection symmetry axis (perpendicular to the magnetic domain walls) in the ordered onion state. On the other hand, when the vortices move perpendicular to the magnetic reflection symmetry axis in the ordered onion state (parallel to the domain walls) a clear ratchet effect is observed. This behavior differs qualitatively from that observed in samples containing arrays of triangular Ni nanostructures, which show a ratchet of structural origin.
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Papers by David Perez de Lara