Searcharxiv⌕ Search

arXiv subjects

A. Fontcuberta i Morral

Publications and source records attributed to A. Fontcuberta i Morral.

At least 19 recordsLinked to original sources
arXiv2022

Light emission properties of mechanical exfoliation induced extended defects in hexagonal boron nitride flakes

Recently hBN has become an interesting platform for quantum optics due to the peculiar defect-related luminescence properties. Concomitantly, hBN was established as the ideal insulating support for realizing 2D materials device, where, on the contrary, defects can affect the device performance. In this work, we study the light emission properties of hBN flakes obtained by mechanical exfoliation with particular focus on extended defects generated in the process. In particular, we tackle different issues as the light emission in hBN flakes of different thicknesses in the range of hundreds of nm, revealing a higher concentration of deep level emission in thinner area of the flake. We recognize the effect of crystal deformation in some areas of the flake with an important blue-shift (130 meV) of the room temperature near band edge emission of hBN and the concurrent presence of a novel emission at 2.36 eV related to the formation of array of dislocations. We studied the light emission properties by means of cathodoluminescence and sub-bandgap excitation photoluminescence of thickness steps with different crystallographic orientations, revealing the presence of different concentration of radiative centers. CL mapping allows to detect buried thickness steps, invisible to the SEM and AFM morphological analysis.

cond-mat.mtrl-sci↗
arXiv2020

Plasma-enhanced atomic layer deposition of nickel nanotubes with low resistivity and coherent magnetization dynamics for 3D spintronics

We report plasma-enhanced atomic layer deposition (ALD) to prepare conformal nickel thin films and nanotubes by using nickelocene as a precursor, water as the oxidant agent and an in-cycle plasma enhanced reduction step with hydrogen. The optimized ALD pulse sequence, combined with a post-processing annealing treatment, allowed us to prepare 30 nm thick metallic Ni layers with a resistivity of 8 $μΩ$cm at room temperature and good conformality both on the planar substrates and nanotemplates. Thereby we fabricated several micrometer-long nickel nanotubes with diameters ranging from 120 to 330 nm. We report on the correlation between ALD growth and functional properties of individual Ni nanotubes characterized in terms of magneto-transport and the confinement of spin wave modes. The findings offer novel perspectives for Ni-based spintronics and magnonic devices operated in the GHz frequency regime with a 3D device architecture.

physics.app-ph↗
arXiv2017

Observation of vortex-nucleated magnetization reversal in individual ferromagnetic nanotubes

The reversal of a uniform axial magnetization in a ferromagnetic nanotube (FNT) has been predicted to nucleate and propagate through vortex domains forming at the ends. In dynamic cantilever magnetometry measurements of individual FNTs, we identify the entry of these vortices as a function of applied magnetic field and show that they mark the nucleation of magnetization reversal. We find that the entry field depends sensitively on the angle between the end surface of the FNT and the applied field. Micromagnetic simulations substantiate the experimental results and highlight the importance of the ends in determining the reversal process. The control over end vortex formation enabled by our findings is promising for the production of FNTs with tailored reversal properties.

cond-mat.mes-hall↗
arXiv2017

Imaging stray magnetic field of individual ferromagnetic nanotubes

We use a scanning nanometer-scale superconducting quantum interference device to map the stray magnetic field produced by individual ferromagnetic nanotubes (FNTs) as a function of applied magnetic field. The images are taken as each FNT is led through magnetic reversal and are compared with micromagnetic simulations, which correspond to specific magnetization configurations. In magnetic fields applied perpendicular to the FNT long axis, their magnetization appears to reverse through vortex states, i.e.\ configurations with vortex end domains or -- in the case of a sufficiently short FNT -- with a single global vortex. Geometrical imperfections in the samples and the resulting distortion of idealized mangetization configurations influence the measured stray-field patterns.

cond-mat.mes-hall↗
arXiv2017

Revealing large-scale homogeneity and trace impurity sensitivity of GaAs nanoscale membranes

III-V nanostructures have the potential to revolutionize optoelectronics and energy harvesting. For this to become a reality, critical issues such as reproducibility and sensitivity to defects should be resolved. By discussing the optical properties of MBE grown GaAs nanomembranes we highlight several features that bring them closer to large scale applications. Uncapped membranes exhibit a very high optical quality, expressed by extremely narrow neutral exciton emission, allowing the resolution of the more complex excitonic structure for the first time. Capping of the membranes with an AlGaAs shell results in a strong increase of emission intensity but also to a shift and broadening of the exciton peak. This is attributed to the existence of impurities in the shell, beyond MBE-grade quality, showing the high sensitivity of these structures to the presence of impurities. Finally, emission properties are identical at the sub-micron and sub-millimeter scale, demonstrating the potential of these structures for large scale applications.

cond-mat.mes-hall↗
arXiv2017

Imaging magnetic vortex configurations in ferromagnetic nanotubes

We image the remnant magnetization configurations of CoFeB and permalloy nanotubes (NTs) using x-ray magnetic circular dichroism photo-emission electron microscopy. The images provide direct evidence for flux-closure configurations, including a global vortex state, in which magnetization points circumferentially around the NT axis. Furthermore, micromagnetic simulations predict and measurements confirm that vortex states can be programmed as the equilibrium remnant magnetization configurations by reducing the NT aspect ratio.

cond-mat.mes-hall↗
arXiv2016

Vectorial scanning force microscopy using a nanowire sensor

Self-assembled nanowire (NW) crystals can be grown into nearly defect-free nanomechanical resonators with exceptional properties, including small motional mass, high resonant frequency, and low dissipation. Furthermore, by virtue of slight asymmetries in geometry, a NW's flexural modes are split into doublets oscillating along orthogonal axes. These characteristics make bottom-up grown NWs extremely sensitive vectorial force sensors. Here, taking advantage of its adaptability as a scanning probe, we use a single NW to image a sample surface. By monitoring the frequency shift and direction of oscillation of both modes as we scan above the surface, we construct a map of all spatial tip-sample force derivatives in the plane. Finally, we use the NW to image electric force fields distinguishing between forces arising from the NW charge and polarizability. This universally applicable technique enables a form of atomic force microscopy particularly suited to mapping the size and direction of weak tip-sample forces.

cond-mat.mes-hall↗
arXiv2015

Dynamic cantilever magnetometry of individual CoFeB nanotubes

We investigate single CoFeB nanotubes with hexagonal cross-section using dynamic cantilever magnetometry (DCM). We develop both an analytical model based on the Stoner-Wohlfarth approximation and a broadly applicable numerical framework for analyzing DCM measurements of magnetic nanostructures. Magnetometry data show the presence of a uniformly magnetized configuration at high external fields with $μ_0 M_s =1.3 \pm 0.1$ T and non-uniform configurations at low fields. In this low-field regime, comparison between numerical simulations and DCM measurements supports the existence of flux-closure configurations. Crucially, evidence of such configurations is only apparent because of the sensitivity of DCM to single nanotubes, whereas conventional measurements of ensembles are often obscured by sample-to-sample inhomogeneities in size, shape, and orientation

cond-mat.mes-hall↗
arXiv2015

Magnetization reversal of an individual exchange biased permalloy nanotube

We investigate the magnetization reversal mechanism in an individual permalloy (Py) nanotube (NT) using a hybrid magnetometer consisting of a nanometer-scale SQUID (nanoSQUID) and a cantilever torque sensor. The Py NT is affixed to the tip of a Si cantilever and positioned in order to optimally couple its stray flux into a Nb nanoSQUID. We are thus able to measure both the NT's volume magnetization by dynamic cantilever magnetometry and its stray flux using the nanoSQUID. We observe a training effect and temperature dependence in the magnetic hysteresis, suggesting an exchange bias. We find a low blocking temperature $T_B = 18 \pm 2$ K, indicating the presence of a thin antiferromagnetic native oxide, as confirmed by X-ray absorption spectroscopy on similar samples. Furthermore, we measure changes in the shape of the magnetic hysteresis as a function of temperature and increased training. These observations show that the presence of a thin exchange-coupled native oxide modifies the magnetization reversal process at low temperatures. Complementary information obtained via cantilever and nanoSQUID magnetometry allows us to conclude that, in the absence of exchange coupling, this reversal process is nucleated at the NT's ends and propagates along its length as predicted by theory.

cond-mat.mes-hall↗
arXiv2014

Nonlinear motion and mechanical mixing in as-grown GaAs nanowires

We report nonlinear behavior in the motion of driven nanowire cantilevers. The nonlinearity can be described by the Duffing equation and is used to demonstrate mechanical mixing of two distinct excitation frequencies. Furthermore, we demonstrate that the nonlinearity can be used to amplify a signal at a frequency close to the mechanical resonance of the nanowire oscillator. Up to 26 dB of amplitude gain are demonstrated in this way.

cond-mat.mes-hall↗
arXiv2014

Quantum dot opto-mechanics in a fully self-assembled nanowire

We show that fully self-assembled optically-active quantum dots (QDs) embedded in MBE-grown GaAs/AlGaAs core-shell nanowires (NWs) are coupled to the NW mechanical motion. Oscillations of the NW modulate the QD emission energy in a broad range exceeding 14 meV. Furthermore, this opto-mechanical interaction enables the dynamical tuning of two neighboring QDs into resonance, possibly allowing for emitter-emitter coupling. Both the QDs and the coupling mechanism -- material strain -- are intrinsic to the NW structure and do not depend on any functionalization or external field. Such systems open up the prospect of using QDs to probe and control the mechanical state of a NW, or conversely of making a quantum non-demolition readout of a QD state through a position measurement.

cond-mat.mes-hall↗
arXiv2013

Untangling the role of oxide in Ga-assisted growth of GaAs nanowires on Si substrates

The influence of the oxide in Ga-assisted growth of GaAs nanowires on Si substrates is investigated. Three different types of oxides with different structure and chemistry are considered. We observe that the critical oxide thicknesses needed for achieving nanowire growth depends on the nature of oxide and how it is processed. Additionally, we find that different growth conditions such as temperature and Ga rate are needed for successful nanowire growth on different oxides. We generalize the results in terms of the characteristics of the oxides such as surface roughness, stoichiometry and thickness. These results constitute a step further towards the integration of GaAs technology on the Si platform.

cond-mat.mtrl-sci↗
arXiv2013

Reversal mechanism of an individual Ni nanotube simultaneously studied by torque and SQUID magnetometry

Using an optimally coupled nanometer-scale superconducting quantum interference device, we measure the magnetic flux originating from an individual ferromagnetic Ni nanotube attached to a Si cantilever. At the same time, we detect the nanotube's volume magnetization using torque magnetometry. We observe both the predicted reversible and irreversible reversal processes. A detailed comparison with micromagnetic simulations suggests that vortex-like states are formed in different segments of the individual nanotube. Such stray-field free states are interesting for memory applications and non-invasive sensing.

cond-mat.mes-hall↗
arXiv2013

Nanoscale multifunctional sensor formed by a Ni nanotube and a scanning Nb nanoSQUID

Nanoscale magnets might form the building blocks of next generation memories. To explore their functionality, magnetic sensing at the nanoscale is key. We present a multifunctional combination of a scanning nanometer-sized superconducting quantum interference device (nanoSQUID) and a Ni nanotube attached to an ultrasoft cantilever as a magnetic tip. We map out and analyze the magnetic coupling between the Ni tube and the Nb nanoSQUID, demonstrate imaging of an Abrikosov vortex trapped in the SQUID structure - which is important in ruling out spurious magnetic signals - and reveal the high potential of the nanoSQUID as an ultrasensitive displacement detector. Our results open a new avenue for fundamental studies of nanoscale magnetism and superconductivity.

cond-mat.supr-con↗
arXiv2009

Raman spectroscopy of wurtzite and zinc-blende GaAs nanowires: polarization dependence, selection rules and strain effects

Polarization dependent Raman scattering experiments realized on single GaAs nanowires with different percentages of zinc-blende and wurtzite structure are presented. The selection rules for the special case of nanowires are found and discussed. In the case of zinc-blende, the transversal optical mode E1(TO) at 267 cm-1 exhibits the highest intensity when the incident and analyzed polarization are parallel to the nanowire axis. This is a consequence of the nanowire geometry and dielectric mismatch with the environment, and in quite good agreement with the Raman selection rules. We also find a consistent splitting of 1 cm-1 of the E1(TO). The transversal optical mode related to the wurtzite structure, E2H, is measured between 254 and 256 cm-1, depending on the wurtzite content. The azymutal dependence of E2H indicates that the mode is excited with the highest efficiency when the incident and analyzed polarization are perpendicular to the nanowire axis, in agreement with the selection rules. The presence of strain between wurtzite and zinc-blende is analyzed by the relative shift of the E1(TO) and E2H modes. Finally, the influence of the surface roughness in the intensity of the longitudinal optical mode on {110} facets is presented.

cond-mat.mes-hall↗
arXiv2009

Structural and optical properties of high quality zinc-blende/wurtzite GaAs hetero-nanowires

The structural and optical properties of 3 different kinds of GaAs nanowires with 100% zinc-blende structure and with an average of 30% and 70% wurtzite are presented. A variety of shorter and longer segments of zinc-blende or wurtzite crystal phases are observed by transmission electron microscopy in the nanowires. Sharp photoluminescence lines are observed with emission energies tuned from 1.515 eV down to 1.43 eV when the percentage of wurtzite is increased. The downward shift of the emission peaks can be understood by carrier confinement at the interfaces, in quantum wells and in random short period superlattices existent in these nanowires, assuming a staggered band-offset between wurtzite and zinc-blende GaAs. The latter is confirmed also by time resolved measurements. The extremely local nature of these optical transitions is evidenced also by cathodoluminescence measurements. Raman spectroscopy on single wires shows different strain conditions, depending on the wurtzite content which affects also the band alignments. Finally, the occurrence of the two crystallographic phases is discussed in thermodynamic terms.

cond-mat.mes-hall↗
arXiv2009

Photocurrent and Photoconductance Properties of a GaAs Nanowire

We report on photocurrent and photoconductance processes in a freely suspended p-doped single GaAs nanowire. The nanowires are grown by molecular beam epitaxy (MBE), and they are electrically contacted by a focused ion beam (FIB) deposition technique. The observed photocurrent is generated at the Schottky contacts between the nanowire and metal source-drain electrodes, while the observed photoconductance signal can be explained by a photogating effect induced by optically generated charge carriers located at the surface of the nanowire. Both optoelectronic effects are sensitive to the polarization of the exciting laser field, enabling polarization dependent photodetectors.

cond-mat.mes-hall↗