arXiv subjects

C. Gomez

Publications and source records attributed to C. Gomez.

At least 19 recordsLinked to original sources

arXiv2025

Thermal tuning of dynamic response in Ag-based nanowire networks

Self-assembled networks of metallic nanowires (NWs) are being intensively explored as test benches for neuromorphic proposals. In this work, we study the electric transport properties of dense self-assembled networks of Ag-based NWs (AgNWNs) coated with a thin insulating layer, using DC and AC stimuli. The building blocks of this network are the metallic NWs and the NW-NW junctions, either metallic or memristive. In the pristine state, frequency independence of the impedance reveals an over-percolated purely resistive network. A combination of low-temperature annealing and AC stimulus is shown to drastically affect the resistivity of the sample (interpreted as a depopulation of purely metallic junctions), unveiling a rich dynamic response. This procedure triggers the achievement of a capacitive response, which is successfully rationalized using a previously introduced 'two junction model'. Thermal treatment appears to be an indirect strategy to effectively modify the humidity content at the NW-NW intersections and, consequently, enable multiple switching schemes suitable for brain-like processing alternatives.

cond-mat.mtrl-sci↗

arXiv2023

Marijuana on Main Streets? The Story Continues in Colombia: An Endogenous Three-part Model

Cannabis is the most common illicit drug, and understanding its demand is relevant to analyze the potential implications of its legalization. This paper proposes an endogenous three-part model taking into account incidental truncation and access restrictions to study demand for marijuana in Colombia, and analyze the potential effects of its legalization. Our application suggests that modeling simultaneously access, intensive and extensive margin is relevant, and that selection into access is important for the intensive margin. We find that younger men that have consumed alcohol and cigarettes, living in a neighborhood with drug suppliers, and friends that consume marijuana face higher probability of having access and using this drug. In addition, we find that marijuana is an inelastic good (-0.45 elasticity). Our results are robust to different specifications and definitions. If marijuana were legalized, younger individuals with a medium or low risk perception about marijuana use would increase the probability of use in 3.8 percentage points, from 13.6% to 17.4%. Overall, legalization would increase the probability of consumption in 0.7 p.p. (2.3% to 3.0%). Different price settings suggest that annual tax revenues fluctuate between USD 11.0 million and USD 54.2 million, a potential benchmark is USD 32 million.

econ.GN↗

arXiv2023

Centralised Design and Production of the Ultra-High Vacuum and Laser-Stabilisation Systems for the AION Ultra-Cold Strontium Laboratories

This paper outlines the centralised design and production of the Ultra-High-Vacuum sidearm and Laser-Stabilisation systems for the AION Ultra-Cold Strontium Laboratories. Commissioning data on the residual gas and steady-state pressures in the sidearm chambers, on magnetic field quality, on laser stabilisation, and on the loading rate for the 3D Magneto-Optical Trap are presented. Streamlining the design and production of the sidearm and laser stabilisation systems enabled the AION Collaboration to build and equip in parallel five state-of-the-art Ultra-Cold Strontium Laboratories within 24 months by leveraging key expertise in the collaboration. This approach could serve as a model for the development and construction of other cold atom experiments, such as atomic clock experiments and neutral atom quantum computing systems, by establishing dedicated design and production units at national laboratories.

physics.atom-ph↗

arXiv2023

Entanglement, Observers and Cosmology: a view from von Neumann Algebras

Infinite entanglement fluctuations appear when a quantum field theory on a causally complete domain of space-time is a type $III$ factor. In the weak gravity limit $G_N=0$ this factor can be transformed into a crossed product type $II$ factor with finite entanglement fluctuations by adding a physical reference frame system (observer). The use of a physical reference frame to define a regularization of divergent entanglement is formally identical to the quantum information approach to superselection charges. In that case the added reference frame allows quantum superpositions between different superselection sectors. For the case of cosmological horizons we map the primordial inflationary slow rolling phase into a type $II$ modification of the pure de Sitter type $III$ factor and we use the so defined type $II$ finite entanglement fluctuations to predict the primordial power spectrum of scalar curvature fluctuations. For the Hagedorn high temperature phase of large $N$ Yang Mills, the type $II$ description of the Hagedorn phase accounts, in the large $N$ limit, for the quantum fluctuations of the interval of the corresponding matrix model eigenvalue distribution.

hep-th↗

arXiv2021

Penetration depth of Cooper pairs in the IrMn antiferromagnet

Suppression of superconductivity due to the proximity effect between a superconductor and a ferromagnet can be partially alleviated when a Cooper pair simultaneously samples different directions of the short-range exchange field. The superconductor's critical temperature, TC, is therefore expected to partially recover when the ferromagnet is in a multi-domain state, as opposed to a single-domain state. Here, we discuss series of experiments performed with ferromagnet(Pt/Co)/spacer(IrMn and Pt)/superconductor(NbN) heterostructures. By tuning the various parameters in play, e.g., superconducting coherence length-to-thicknesses ratio, and domain sizes, we obtained up to 10 percent recovery of the superconducting critical temperature {\Delta}TC/TC. This large-scale recovery made novel investigations possible. In particular, from the spacer thickness-dependence of {\Delta}TC/TC, it was possible to deduce the characteristic length for Cooper pair penetration in an IrMn antiferromagnet. This information is crucial for electronic transport, and up to now has been difficult to access experimentally for antiferromagnets.

cond-mat.mtrl-sci↗

arXiv2019

Spin pumping as a generic probe for linear spin fluctuations: demonstration with ferromagnetic and antiferromagnetic orders, metallic and insulating electrical states

We investigated spin injection by spin pumping from a spin-injector(NiFe) into a spin-sink to detect spin fluctuations in the spin-sink. By scanning the ordering-temperature of several magnetic transitions, we found that enhanced spin pumping due to spin fluctuations applies with several ordering states: ferromagnetic(Tb) and antiferromagnetic(NiO, NiFeOx, BiFeO3, exchange-biased and unbiased IrMn). Results also represent systematic experimental investigation supporting that the effect is independent of the metallic and insulating nature of the spin-sink, and is observed whether the spin current probe involves electronic or magnonic transport, facilitating advances in material characterization and engineering for spintronic applications.

cond-mat.mtrl-sci↗

arXiv2018

Unstable and stable regimes of polariton condensation

Modulational instabilities play a key role in a wide range of nonlinear optical phenomena, leading e.g. to the formation of spatial and temporal solitons, rogue waves and chaotic dynamics. Here we experimentally demonstrate the existence of a modulational instability in condensates of cavity polaritons, arising from the strong coupling of cavity photons with quantum well excitons. For this purpose we investigate the spatiotemporal coherence properties of polariton condensates in GaAs-based microcavities under continuous-wave pumping. The chaotic behavior of the instability results in a strongly reduced spatial and temporal coherence and a significantly inhomogeneous density. Additionally we show how the instability can be tamed by introducing a periodic potential so that condensation occurs into negative mass states, leading to largely improved coherence and homogeneity. These results pave the way to the exploration of long-range order in dissipative quantum fluids of light within a controlled platform.

cond-mat.mes-hall↗

arXiv2017

Measuring topological invariants from generalized edge states in polaritonic quasicrystals

We investigate the topological properties of Fibonacci quasicrystals using cavity polaritons. Composite structures made of the concatenation of two Fibonacci sequences allow investigating generalized edge states forming in the gaps of the fractal energy spectrum. We employ these generalized edge states to determine the topological invariants of the quasicrystal. When varying a structural degree of freedom (phason) of the Fibonacci sequence, the edge states spectrally traverse the gaps, while their spatial symmetry switches: the periodicity of this spectral and spatial evolution yields direct measurements of the gap topological numbers. The topological invariants that we determine coincide with those assigned by the gap-labeling theorem, illustrating the direct connection between the fractal and topological properties of Fibonacci quasicrystals.

cond-mat.mes-hall↗

arXiv2016

Monolithic AlGaAs second-harmonic nanoantennas

We demonstrate monolithic aluminum gallium arsenide (AlGaAs) optical anoantennas. Using a selective oxidation technique, we fabricate such epitaxial semiconductor nanoparticles on an aluminum oxide substrate. Second harmonic generation from an AlGaAs nanocylinder of height h=400 nm and varying radius pumped with femtosecond pulses delivered at 1554-nm wavelength has been measured, revealing a peak conversion efficiency exceeding 10-5 for nanocylinders with an otpimized geometry.

physics.optics↗

arXiv2015

Coherent control of a solid-state quantum bit with few-photon pulses

Single photons are the natural link between the nodes of a quantum network: they coherently propagate and interact with many types of quantum bits including natural and artificial atoms. Ideally, one atom should deterministically control the state of a photon and vice-versa. The interaction between free space photons and an atom is however intrinsically weak and many efforts have been dedicated to develop an efficient interface. Recently, it was shown that the propagation of light can be controlled by an atomic resonance coupled to a cavity or a single mode waveguide. Here we demonstrate that the state of a single artificial atom in a cavity can be efficiently controlled by a few-photon pulse. We study a quantum dot optimally coupled to an electrically-controlled cavity device, acting as a near optimal one-dimensional atom. By monitoring the exciton population through resonant fluorescence, we demonstrate Rabi oscillations with a $π$-pulse of only 3.8 photons on average. The probability to flip the exciton quantum bit with a single photon Fock state is calculated to reach 55% in the same device.

quant-ph↗

arXiv2015

Near optimal single photon sources in the solid state

Single-photons are key elements of many future quantum technologies, be it for the realisation of large-scale quantum communication networks for quantum simulation of chemical and physical processes or for connecting quantum memories in a quantum computer. Scaling quantum technologies will thus require efficient, on-demand, sources of highly indistinguishable single-photons. Semiconductor quantum dots inserted in photonic structures are ultrabright single photon sources, but the photon indistinguishability is limited by charge noise induced by nearby surfaces. The current state of the art for indistinguishability are parametric down conversion single-photon sources, but they intrinsically generate multiphoton events and hence must be operated at very low brightness to maintain high single photon purity. To date, no technology has proven to be capable of providing a source that simultaneously generates near-unity indistinguishability and pure single photons with high brightness. Here, we report on such devices made of quantum dots in electrically controlled cavity structures. We demonstrate on-demand, bright and ultra-pure single photon generation. Application of an electrical bias on deterministically fabricated devices is shown to fully cancel charge noise effects. Under resonant excitation, an indistinguishability of $0.9956\pm0.0045$ is evidenced with a $g^{2}(0)=0.0028\pm0.0012$. The photon extraction of $65%$ and measured brightness of $0.154\pm0.015$ make this source $20$ times brighter than any source of equal quality. This new generation of sources open the way to a new level of complexity and scalability in optical quantum manipulation.

quant-ph↗

arXiv2015

Detection of precursor charge-density-wave order in 2H-NbSe2 by muSR

We demonstrate the sensitivity of transverse-field muon spin rotation (TF-muSR) to static charge-density-wave (CDW) order in the bulk of 2H-NbSe2. In the presence of CDW order the quadrupolar interaction of the 93Nb nuclei with the local electric-field gradient is modified, and this in turn affects the magnetic dipolar coupling of the positive muon to these nuclei. For a weak magnetic field applied parallel to the c-axis, we observe a small enhancement of the muon depolarization rate at temperatures below the established CDW phase transition. Aligning the applied field perpendicular to the c-axis, we observe a sensivity to static CDW order in regions of the sample extending up to nearly 3 times the CDW transition temperature. The results suggest that the muon is mobile over the temperature range explored above the superconducting transition temperature (Tc), and becomes trapped in the vicinity of defects.

cond-mat.supr-con↗

arXiv2015

Magnetic properties of the helimagnet Cr1/3NbS2 observed by muSR

We have performed muon spin rotation/relaxation (muSR) measurements on single crystals of the chiral helimagnet Cr1/3NbS2 at zero to low magnetic field. The transition from the paramagnetic to helical magnetically ordered phase at zero field is marked by the onset of a coherent oscillation of the zero-field muon spin polarization below a critical temperature Tc. An enhancement of the muon spin precession frequency is observed below T ~ 50K, where anomalous behavior has been observed in bulk transport measurements. The enhanced precession frequency indicates a low-temperature modification of the helical magnetic structure. A Landau free energy analysis suggests that the low-temperature change in the magnetic structure is caused by a structural change, whereas the magnetic order above Tc is the result of an attractive interaction between the ferromagnetic moment induced by the applied field and the magnetic moments of the helical structure. We also suggest a longer periodicity of helicity below T ~ 50K, which can be verified by neutron scattering.

cond-mat.mtrl-sci↗

arXiv2015

Preliminary Report of the AMS analysis of tsunami deposits in Tohoku -- Japan -- 18th to the 21st Century

Sedimentary records of tsunamis are a precious tool to assess the occurrence of past events, as attested by an abundant literature, which has seen a particular 'boom' in the aftermath of the 2004 Indian Ocean tsunami and the 2011 Tohoku tsunami. Despite an extensive literature, there is very little to no understanding of the role that the changing coastal environment is playing on the record of a tsunami, and for a given location, it is still unclear whether the largest tsunamis leave the largest amount of deposits. To research this question, the present study took place in Japan, in the Tohoku Region at Agawa-pond, because the pond act as a sediment trap. Using a sediment-slicer, a 1 m thick deposit was retrieved, from which 4 tsunami sequences were identified, including the latest 2011 tsunami. Using a series of sedimentary proxies: the AMS (Anisotropy of Magnetic Susceptibility), grain size analysis, quartz morphoscopy (morphology and surface characteristics) and the analysis of microfossils, disparities between the tsunami deposits were identified and most importantly a clear thinning of the tsunami deposit towards the top. Provided the present evidences, the authors discuss that the upward fining is due to at least two components that are seldom assessed in tsunami research (1) a modification of the depositional environment, with the progressive anthropization of the coast, providing less sediments to remobilize; and (2) a progressive filling of the Agawa pond, which progressively loses its ability to trap tsunami materials.

physics.geo-ph↗

arXiv2015

Black Hole Formation and Classicalization in Ultra-Planckian 2 -> N Scattering

We establish a connection between the ultra-Planckian scattering amplitudes in field and string theory and unitarization by black hole formation in these scattering processes. Using as a guideline an explicit microscopic theory in which the black hole represents a bound-state of many soft gravitons at the quantum critical point, we were able to identify and compute a set of perturbative amplitudes relevant for black hole formation. These are the tree-level N-graviton scattering S-matrix elements in a kinematical regime (called classicalization limit) where the two incoming ultra-Planckian gravitons produce a large number N of soft gravitons. We compute these amplitudes by using the Kawai-Lewellen-Tye relations, as well as scattering equations and string theory techniques. We discover that this limit reveals the key features of the microscopic corpuscular black hole N-portrait. In particular, the perturbative suppression factor of a N-graviton final state, derived from the amplitude, matches the non-perturbative black hole entropy when N reaches the quantum criticality value, whereas final states with different value of N are either suppressed or excluded by non-perturbative corpuscular physics. Thus we identify the microscopic reason behind the black hole dominance over other final states including non-black hole classical object. In the parameterization of the classicalization limit the scattering equations can be solved exactly allowing us to obtain closed expressions for the high-energy limit of the open and closed superstring tree-level scattering amplitudes for a generic number N of external legs. We demonstrate matching and complementarity between the string theory and field theory in different large-s and large-N regimes.

hep-th↗

arXiv2014

L'-band AGPM vector vortex coronagraph's first light on LBTI/LMIRCam

We present the first observations obtained with the L'-band AGPM vortex coronagraph recently installed on LBTI/LMIRCam. The AGPM (Annular Groove Phase Mask) is a vector vortex coronagraph made from diamond subwavelength gratings. It is designed to improve the sensitivity and dynamic range of high-resolution imaging at very small inner working angles, down to 0.09 arcseconds in the case of LBTI/LMIRCam in the L' band. During the first hours on sky, we observed the young A5V star HR\,8799 with the goal to demonstrate the AGPM performance and assess its relevance for the ongoing LBTI planet survey (LEECH). Preliminary analyses of the data reveal the four known planets clearly at high SNR and provide unprecedented sensitivity limits in the inner planetary system (down to the diffraction limit of 0.09 arcseconds).

astro-ph.EP↗

arXiv2014

Zero-Field muSR Search for a Time-Reversal-Symmetry-Breaking Mixed Pairing State in Superconducting Ba1-xKxFe2As2

We report the results of a zero-field muon spin relaxation (ZF-muSR) study of superconducting Ba1-xKxFe2As2 (0.5 < x < 0.9) in search of weak spontaneous internal magnetic fields associated with proposed time-reversal-symmetry breaking mixed pairing states. The measurements were performed on polycrystalline samples, which do not exhibit the mesoscopic phase separation previously observed in single crystals of Ba1-xKxFe2As2. No evidence of spontaneous internal magnetic fields is found in any of the samples at temperatures down to T ~ 0.02 K.

cond-mat.supr-con↗

arXiv2009

QPACE -- a QCD parallel computer based on Cell processors

QPACE is a novel parallel computer which has been developed to be primarily used for lattice QCD simulations. The compute power is provided by the IBM PowerXCell 8i processor, an enhanced version of the Cell processor that is used in the Playstation 3. The QPACE nodes are interconnected by a custom, application optimized 3-dimensional torus network implemented on an FPGA. To achieve the very high packaging density of 26 TFlops per rack a new water cooling concept has been developed and successfully realized. In this paper we give an overview of the architecture and highlight some important technical details of the system. Furthermore, we provide initial performance results and report on the installation of 8 QPACE racks providing an aggregate peak performance of 200 TFlops.

hep-lat↗