arXiv subjects

Fabian Cadiz

Publications and source records attributed to Fabian Cadiz.

18 recordsLinked to original sources

arXiv2025

High-Temperature Activation of Single-Photon Emitters in monolayer WS2

Controlled activation of defect-bound excitonic states in two-dimensional semiconductors provides a route to isolated quantum emitters and a sensitive probe of defect physics. Here we demonstrate that \textit{in situ} high-temperature annealing of hBN-encapsulated monolayer WS$_2$ on a suspended microheater leads to the emergence of spectrally isolated single-photon emitters at cryogenic temperatures. Annealing at temperatures around 1100 K produces a sharp emission line, $X_L$, red-shifted by approximately 80 meV from the neutral exciton and exhibiting a linewidth below 200 $μ$eV. Photoluminescence excitation spectroscopy and power-dependent measurements show that $X_L$ originates from annealing-induced defects in the WS$_2$ monolayer, while second-order photon correlation measurements reveal clear antibunching with $g^{(2)}(0)<0.5$. These results establish high-temperature \textit{in situ} annealing as a controlled means to access defect-bound excitonic states and single-photon emission in van der Waals materials.

cond-mat.mtrl-sci↗

arXiv2025

Tunable electronic band structure in WSSe van der Waals Alloys

The electronic structure of semiconducting 2D materials such as transition metal dichalcogenides (TMDs) is known to be tunable by its environment, from simple external fields applied with electrical contacts up to complex van der Waals heterostructure assemblies. However, conventional alloying from reference binary TMD compounds to composition-controlled ternary alloys also offers unexplored opportunities. In this work, we use nano-angle resolved photoemission spectroscopy (nano-ARPES) and density functional theory (DFT) calculations to study the structural and electronic properties of different alloy compositions of bulk WS2(1-x)Se2x. Our results demonstrate the continuous variation of the band structure and the progressive evolution of the valence band splitting at the K points from 420 to 520 meV in bulk WS2(1-x)Se2x. We also carried out scanning tunneling microscopy (STM) measurements and DFT to understand the possible S or Se substitutions variants in WS2(1-x)Se2x alloys, with different local atomic configurations. Our work opens up perspectives for the fine control of the band dispersion in van der Waals materials and demonstrate how the band structure can be tuned in bulk TMDs. The collected information can serve as a reference for future applications.

cond-mat.mtrl-sci↗

arXiv2025

Crystal field tuned spin-flip luminescence in NiPS3

Layered magnetic materials potentially hold the key to future applications based on optical control and manipulation of magnetism. NiPS3, a prototype member of this family, is antiferromagnetic below 155 K and exhibits sharp photoluminescence associated to a transition between a triplet ground state and a singlet excited state. The nature of the luminescent transition is a matter of current debate and so is an eventual fundamental link of this excitation to magnetism. Here we provide answers through experiments and calculations. We fabricate samples with metal and ligand substitutions which alter the Neel transition temperature and measure the effects of these changes on the temperature dependent photoluminescence. We perform crystal field and charge transfer multiplet calculations to explain the origin of the excitation and identify the effects of the magnetic ground state on its properties. These measurements and calculations provide a comprehensive explanation for the observed properties and a template for finding similar materials exhibiting spin-flip luminescence.

cond-mat.mtrl-sci↗

arXiv2024

Using Light to Polarize and Detect Electron Spins in Silicon

Despite decades of research, demonstration of all-optical detection and control of free electron spins in silicon remains elusive. Here, we directly probe the electron spin properties in bulk silicon by measuring the polarization of luminescence following circularly polarized light excitation. The experiments performed for both direct and indirect gap excitation allow not only an experimental determination of the optical selection rules in silicon for the different phononassisted transitions but they also lead to the measurement of the spin relaxation of electrons in conditions which are not accessible using transport techniques. We also measure the spin properties of free excitons in bulk silicon, a very little explored field.

cond-mat.mtrl-sci↗

arXiv2024

High Strain Engineering of a Suspended WSSe Monolayer Membrane by Indentation and Measured by Tip-enhanced Photoluminescence

Straintronics involves the manipulation and regulation of the electronic characteristics of 2D materials through the use of macro- and nano-scale strain engineering. In this study, we utilized an atomic force microscope (AFM) coupled with an optical system to perform indentation measurements and tip-enhanced photoluminescence (TEPL), allowing us to extract the local optical response of a suspended monolayer membrane of ternary WSSe at various levels of deformation, up to strains of 10%. The photoluminescence signal is modelled considering the deformation, stress distribution and strain dependence of the WSSe band structure. We observe an additional TEPL signal that exhibits significant variation under strain, with 64 meV per percent of elongation. This peak is linked to the highly strained 2D material lying right underneath the tip. We discuss the amplification of the signal and its relation to the excitonic funnelling effect in a more comprehensive model. We will also compare the diffusion caused by Auger recombination against the radiative excitonic decay. We use TEPL to examine and comprehend the local physics of 2D semi-conducting materials subjected to extreme mechanical strain. Chemical vapour deposition-fabricated 2D ternaries possess high strain resistance, comparable to the benchmark MoS2, and a high Young's modulus of 273 GPa.

cond-mat.mes-hall↗

arXiv2021

Enhancement of valley polarization at high photoexcited densities in MoS2 monolayers

We have investigated the steady-sate valley polarization and valley coherence of encapsulated MoS2 monolayer as a function of the temperature and the power density with a continuous wave laser excitation. Both valley polarization and coherence exhibit a non-monotonic dependence on sample temperature, attaining a local maximum at T=40 K. This has been recently attributed to a motional narrowing effect: an enhancement of the valley relaxation time occurs when the scattering rate increases. At a fixed temperature of T=6 K, a two-fold increase of the steady-state valley polarization is achieved by increasing the laser excitation power, which we attribute to a local heating induced by the energy relaxation of photoexcited excitons outside the light cone and to an increase in the exciton-exciton scattering rate. In contrast, in the same power range only a moderate enhancement of valley coherence is observed. Further increasing the excitation power leads to a small reduction of valley polarization but a dramatic loss of valley coherence. Supported by spatial imaging of the excitonic luminescence and polarization, we attribute this behaviour to the detrimental role of exciton-exciton interactions on the pure dephasing rate.

cond-mat.mtrl-sci↗

arXiv2021

Efficient valley polarization of charged excitons and resident carriers in MoS2 monolayers by optical pumping

We investigate with polarized microphotoluminescence the optical pumping of the valley degree of freedom in charge-tunable MoS2 monolayers encapsulated with hexagonal boron nitride at cryogenic temperatures. We report a large steady state valley polarization of the different excitonic complexes following circularly-polarized laser excitation 25 meV above the neutral exciton transition. For the first time in this material we reveal efficient valley pumping of positively-charged trions, which were so far elusive in non-encapsulated monolayers due to defect and laser-induced large electron doping. We find that negatively-charged trions present a polarization of 70 % which is unusually large for non-resonant excitation. We attribute this large valley polarization to the particular band structure of MoS2, where an optically dark exciton ground state coexists with a bright conduction band ordering in the single-particle picture, leading to a supression of the valley relaxation for negatively-charged trions. In addition, we demonstrate that circular excitation induces a dynamical polarization of resident electrons and holes, as recently shown in tungsten-based monolayers. This manifest itself as a variation in the intensity of different excitonic complexes under circular and linear excitation.

cond-mat.mes-hall↗

arXiv2021

Imaging Seebeck drift of excitons and trions in MoSe2 monolayers

Hyperspectral imaging at cryogenic temperatures is used to investigate exciton and trion propagation in MoSe$_2$ monolayers encapsulated with hexagonal boron nitride (hBN). Under a tightly focused, continuous-wave laser excitation, the spatial distribution of neutral excitons and charged trions strongly differ at high excitation densities. Remarkably, in this regime the trion distribution develops a halo shape, similar to that previously observed in WS2 monolayers at room temperature and under pulsed excitation. In contrast, the exciton distribution only presents a moderate broadening without the appereance of a halo. Spatially and spectrally resolved luminescence spectra reveal the buildup of a significant temperature gradient at high excitation power, that is attributed to the energy relaxation of photoinduced hot carriers. We show, via a numerical resolution of the transport equations for excitons and trions, that the halo can be interpreted as thermal drift of trions due to a Seebeck term in the particle current. The model shows that the difference between trion and exciton profiles is simply understood in terms of the very different lifetimes of these two quasiparticles.

cond-mat.mtrl-sci↗

arXiv2021

Spin/Valley pumping of resident electrons in WSe2 and WS2 monolayers

Monolayers of transition metal dichalcogenides are ideal materials to control both spin and valley degrees of freedom either electrically or optically. Nevertheless, optical excitation mostly generates excitons species with inherently short lifetime and spin/valley relaxation time. Here we demonstrate a very efficient spin/valley optical pumping of resident electrons in n-doped WSe2 and WS2 monolayers. We observe that, using a continuous wave laser and appropriate doping and excitation densities, negative trion doublet lines exhibit circular polarization of opposite sign and the photoluminescence intensity of the triplet trion is more than four times larger with circular excitation than with linear excitation. We interpret our results as a consequence of a large dynamic polarization of resident electrons using circular light.

cond-mat.mes-hall↗

arXiv2020

Charge and spin transport over record distances in GaAs metallic n-type nanowires : I photocarrier transport in a dense Fermi sea

We have investigated charge and spin transport in n-type metallic GaAs nanowires (~ 10^17 cm^-3 doping level), grown by hydride vapor phase epitaxy (HVPE) on Si substrates. This was done by exciting the nanowire by tightly-focussed circularly-polarized light and by monitoring the intensity and circular polarization spectrum as a function of distance from the excitation spot. The spin-polarized photoelectrons give rise to a well-defined feature in the nearbandgap spectrum, distinct from the main line due to recombination of the spin-unpolarized electrons of the Fermi sea with the same minority photoholes. At a distance of 2 (micro)m, only the main line remains, implying that photoelectrons have reached a charge thermodynamic equilibrium with the Fermi sea. However, although no line is present in the intensity spectrum at the corresponding energy, the circular polarization is still observed at the same energy in the spectrum, implying that photoelectrons have preserved their spin orientation and that the two spin reservoirs remain distinct. Investigations as a function of distance to the excitation spot show that, depending on excitation power, a photoelectron spin polarization of 20% can be transported over a record distance of more than 20 (micro)m. This finding has potential applications for long distance spin transport in n-type doped nanowires.

cond-mat.mes-hall↗

arXiv2020

Electrical Detection of Light Helicity using a Quantum Dots based Hybrid Device at Zero Magnetic Field

Photon helicity-dependent photocurrent is measured at zero magnetic field on a device based on an ensemble of InGaAs/GaAs quantum dots that are embedded into a GaAs-based p-i-n diode. Our main goal is to take advantage of the long electron spin relaxation time expected in these nano-objects. In these experiments, no external magnetic field is required thanks to the use of an ultrathin magnetic CoFeB/MgO electrode, presenting perpendicular magnetic anisotropy (PMA). We observe a clear asymmetry of the photocurrent measured under respective right and left polarized light that follows the hysteresis of the magnetic layer. The amplitude of this asymmetry at zero magnetic field decreases with increasing temperatures and can be controlled with the bias. Polarization-resolved photoluminescence is detected in parallel while the device is operated as a photodetector. This demonstrates the multifunctional capabilities of the device and gives valuable insights into the spin relaxation of the electrons in the quantum dots.

physics.app-ph↗

arXiv2018

Fine Structure and Lifetime of Dark Excitons in Transition Metal Dichalcogenide Monolayers

The intricate interplay between optically dark and bright excitons governs the light-matter interaction in transition metal dichalcogenide monolayers. We have performed a detailed investigation of the "spin-forbidden" dark excitons in WSe2 monolayers by optical spectroscopy in an out-of-plane magnetic field Bz. In agreement with the theoretical predictions deduced from group theory analysis, magneto-photoluminescence experiments reveal a zero field splitting $δ=0.6 \pm 0.1$ meV between two dark exciton states. The low energy state being strictly dipole forbidden (perfectly dark) at Bz=0 while the upper state is partially coupled to light with z polarization ("grey" exciton). The first determination of the dark neutral exciton lifetime $τ_D$ in a transition metal dichalcogenide monolayer is obtained by time-resolved photoluminescence. We measure $τ_D \sim 110 \pm 10$ ps for the grey exciton state, i.e. two orders of magnitude longer than the radiative lifetime of the bright neutral exciton at T=12 K.

cond-mat.mtrl-sci↗

arXiv2016

Ultra-low power threshold for laser induced changes in optical properties of 2D Molybdenum dichalcogenides

The optical response of traditional semiconductors depends on the laser excitation power used in experiments. For two-dimensional (2D) semiconductors, laser excitation effects are anticipated to be vastly different due to complexity added by their ultimate thinness, high surface to volume ratio, and laser-membrane interaction effects. We show in this article that under laser excitation the optical properties of 2D materials undergo irreversible changes. Most surprisingly these effects take place even at low steady state excitation, which is commonly thought to be non-intrusive. In low temperature photoluminescence (PL) we show for monolayer (ML) MoSe2 samples grown by different techniques that laser treatment increases significantly the trion (i.e. charged exciton) contribution to the emission compared to the neutral exciton emission. Comparison between samples exfoliated onto different substrates shows that laser induced doping is more efficient for ML MoSe2 on SiO2/Si compared to h-BN and gold. For ML MoS2 we show that exposure to laser radiation with an average power in the $μ$W/$μ$m$^2$ range does not just increase the trion-to-exciton PL emission ratio, but may result in the irreversible disappearance of the neutral exciton PL emission and a shift of the main PL peak to lower energy.

cond-mat.mtrl-sci↗

arXiv2016

Surface-enhanced gallium arsenide photonic resonator with a quality factor of six million

Gallium Arsenide and related compound semiconductors lie at the heart of optoelectronics and integrated laser technologies. Shaped at the micro and nano-scale, they allow strong interaction with quantum dots and quantum wells, and promise to result in stunning devices. However gallium arsenide optical structures presently exhibit lower performances than their silicon-based counterparts, notably in nanophotonics where the surface plays a chief role. Here we report on advanced surface control of miniature gallium arsenide optical resonators, using two distinct techniques that produce permanent results. One leads to extend the lifetime of free-carriers and enhance luminescence, while the other strongly reduces surface absorption originating from mid-gap states and enables ultra-low optical dissipation devices. With such surface control, the quality factor of wavelength-sized optical disk resonators is observed to rise up to six million at telecom wavelength, greatly surpassing previous realizations and opening new prospects for Gallium Arsenide nanophotonics.

physics.optics↗

arXiv2016

Well separated trion and neutral excitons on superacid treated MoS2 monolayers

Developments in optoelectronics and spin-optronics based on transition metal dichalcogenide monolayers (MLs) need materials with efficient optical emission and well-defined transition energies. In as-exfoliated MoS2 MLs the photoluminescence (PL) spectra even at low temperature consists typically of broad, overlapping contributions from neutral, charged excitons (trions) and localized states. Here we show that in superacid treated MoS2 MLs the PL intensity increases by up to 60 times at room temperature. The neutral and charged exciton transitions are spectrally well separated in PL and reflectivity at T=4 K, with linewidth for the neutral exciton of 15 meV, but with similar intensities compared to the ones in as-exfoliated MLs at the same temperature. Time resolved experiments uncover picoseconds recombination dynamics analyzed separately for charged and neutral exciton emission. Using the chiral interband selection rules, we demonstrate optically induced valley polarization for both complexes and valley coherence for only the neutral exciton.

cond-mat.mtrl-sci↗

arXiv2014

VLS-HVPE growth of ultra-long and defect-free GaAs nanowires investigated by ab initio simulation coupled to near-field microscopy

High aspect ratio, rod-like and single crystal phase GaAs nanowires (NWs) were grown by gold catalyst-assisted hydride vapor phase epitaxy (HVPE). High resolution transmission electron microscopy (HRTEM) and micro-Raman spectroscopy revealed polytypism-free zinc blende NWs over lengths of several tens of micrometers for diameters ranging between 50 and 150 nm. Micro-photoluminescence studies of individual NWs showed linewidths smaller than those reported elsewhere which is consistent with the crystalline quality of the NWs. HVPE makes use of chloride growth precursors of which high decomposition frequency, after adsorption onto the catalyst particle, favors a direct and rapid introduction of the Ga atoms from the vapor phase into the catalyst liquid droplet. This yields high axial growth rate (more than 100 micron/h) of NWs. The fast diffusion of the Ga atoms in the droplet towards the interface between the liquid and the solid nanowire was investigated by using density functional theory calculations. The diffusion coefficient of Ga atoms was estimated to be 3x10-9 m2/s, which matches the experimental observations.

cond-mat.mes-hall↗

arXiv2013

RGE Effects on neutrino masses in partial split supersymmetry

We show that the running of the Higgs-gaugino-higgsino couplings present in Partial Split Supersymmetry can severely affect the neutrino masses generated through Bilinear R-parity Violation. We find a working scenario where the predicted neutrino observables satisfy the experimental constraints when the running is neglected. After including the running, we show that already with a split supersymmetric scale of 10000 GeV the atmospheric mass leaves the allowed experimental window, and that the solar mass leaves it even earlier, with a split supersymmetric scale of 1000 GeV. This shows that the correct prediction of neutrino observables in these models necessitates the inclusion of the running of these couplings.

hep-ph↗

arXiv2012

Imaging ambipolar diffusion of photocarriers in GaAs thin films

Images of the steady-state luminescence of passivated GaAs self-standing films under excitation by a tightly-focussed laser are analyzed as a function of light excitation power. While unipolar diffusion of photoelectrons is dominant at very low light excitation power, an increased power results in a decrease of the diffusion constant near the center of the image due to the onset of ambipolar diffusion. The results are in agreement with a numerical solution of the diffusion equations and with a physical analysis of the luminescence intensity at the centre of the image, which permits the determination of the ambipolar diffusion constant as a function of electron concentration.

cond-mat.mes-hall↗