Temperature, detriment, or advantage for memory emergence: The case of ZnO.

Despite the widespread emergence of memory effects in solid systems, understanding the basic microscopic mechanisms that trigger them is still puzzling. We report how ingredients of solid state transport in polycrystalline systems, such as semiconductor oxides, become sufficient conditions for a memristive response that points to the natural emergence of memory, discernible under an adequate set of driving inputs. The experimental confirmation of these trends will be presented along with a compact analytical theoretical picture that allows discerning the relative contribution of the main building blocks of memory and the effect of temperature, in particular. These findings can be extended to a vast universe of materials and devices, providing a unified physical explanation for a wide class of resistive memories and pinpointing the optimal driving configurations for their operation.

Resonant Tunneling Diodes: Mid-Infrared Sensing at Room Temperature.

Resonant tunneling diode photodetectors appear to be promising architectures with a simple design for mid-infrared sensing operations at room temperature. We fabricated resonant tunneling devices with GaInAsSb absorbers that allow operation in the 2-4 µm range with significant electrical responsivity of 0.97 A/W at 2004 nm to optical readout. This paper characterizes the photosensor response contrasting different operational regimes and offering a comprehensive theoretical analysis of the main physical ingredients that rule the sensor functionalities and affect its performance. We demonstrate how the drift, accumulation, and escape efficiencies of photogenerated carriers influence the electrostatic modulation of the sensor's electrical response and how they allow controlling the device's sensing abilities.

Robust room temperature emissions of trion in darkish WSe2 monolayers: effects of dark neutral and charged excitonic states.

Owing to nonzero charge and spin degrees of freedom, trions offer unprecedented tunability and open new paths for applications in devices based on 2D semiconductors. However, in monolayer WSe2, the trion photoluminescence is commonly detected only at low temperatures and vanishes at room temperature, which undermines practical applications. To unveil how to overcome this obstacle, we have developed a comprehensive theory to probe the impact of different excitonic channels on the trion emission in WSe2 monolayers, which combines ab initio tight-binding formalism, Bethe-Salpeter equation and a set of coupled rate equations to describe valley dynamics of excitonic particles. Through a systematic study in which new scattering channels are progressively included, we found that, besides the low electron density, strong many-body correlations between bright and dark excitonic states quenches the trion emission in WSe2. Therefore, the reduction of scatterings from bright to dark states is required to achieve trion emission at room temperature for experimentally accessible carrier concentrations.

Dark-exciton valley dynamics in transition metal dichalcogenide alloy monolayers.

We report a comprehensive theory to describe exciton and biexciton valley dynamics in monolayer Mo1-xWxSe2 alloys. To probe the impact of different excitonic channels, including bright and dark excitons, intravalley biexcitons, intervalley scattering between bright excitons, as well as bright biexcitons, we have performed a systematic study from the simplest system to the most complex one. In contrast to the binary WSe2 monolayer with weak photoluminescence (PL) and high valley polarization at low temperatures and the MoSe2, that presents high PL intensity, but low valley polarization, our results demonstrate that it is possible to set up a ternary alloy with intermediate W-concentration that holds simultaneously a considerably robust light emission and an efficient optical orientation of the valley pseudospin. We find the critical value of W-concentration, xc, that turns alloys from bright to darkish. The dependence of the PL intensity on temperature shows three regimes: while bright monolayer alloys display a usual temperature dependence in which the intensity decreases with rising temperature, the darkish alloys exhibit the opposite behavior, and the alloys with x around xc show a non-monotonic temperature response. Remarkably, we observe that the biexciton enhances significantly the stability of the exciton emission against fluctuations of W-concentration for bright alloys. Our findings pave the way for developing high-performance valleytronic and photo-emitting devices.

Nanothermometer Based on Resonant Tunneling Diodes: From Cryogenic to Room Temperatures.

Sensor miniaturization together with broadening temperature sensing range are fundamental challenges in nanothermometry. By exploiting a large temperature-dependent screening effect observed in a resonant tunneling diode in sequence with a GaInNAs/GaAs quantum well, we present a low dimensional, wide range, and high sensitive nanothermometer. This sensor shows a large threshold voltage shift of the bistable switching of more than 4.5 V for a temperature raise from 4.5 to 295 K, with a linear voltage-temperature response of 19.2 mV K(-1), and a temperature uncertainty in the millikelvin (mK) range. Also, when we monitor the electroluminescence emission spectrum, an optical read-out control of the thermometer is provided. The combination of electrical and optical read-outs together with the sensor architecture excel the device as a thermometer with the capability of noninvasive temperature sensing, high local resolution, and sensitivity.

Voltage-driven ring confinement in a graphene sheet: assessing conditions for bound state solutions.

We have systematically studied the single-particle states in quantum rings produced by a set of concentric circular gates over a graphene sheet placed on a substrate. The resulting potential profiles and the interaction between the graphene layer and the substrate are considered within the Dirac Hamiltonian in the framework of the envelope function approximation. Our simulations allow microscopic mapping of the character of the electron and hole quasi-particle solutions according to the applied voltage. General conditions to control and operate the bound state solutions are described as functions of external and controllable parameters that will determine the optical properties ranging from metallic to semiconductor phases. Contrasting behaviors are obtained when comparing the results for repulsive and attractive voltages as well as for variation of the relative strength of the graphene-substrate coupling parameter.

Magneto-optical properties in IV-VI lead-salt semimagnetic nanocrystals.

: We present a systematic study of lead-salt nanocrystals (NCs) doped with Mn. We have developed a theoretical simulation of electronic and magneto-optical properties by using a multi-band calculation including intrinsic anisotropies and magnetic field effects in the diluted magnetic semiconductor regime. Theoretical findings regarding both broken symmetry and critical phenomena were studied by contrasting two different host materials (PbSe and PbTe) and changing the confinement geometry, dot size, and magnetic doping concentration. We also pointed out the relevance of optical absorption spectra modulated by the magnetic field that characterizes these NCs.

Tailoring electronic transparency of twin-plane 1D superlattices.

The structural properties of twin-plane superlattices in InP nanowires are systematically analyzed. First, we employ molecular dynamics simulations to determine the strain fields in nanowires grown in the [111] direction. These fields are produced by the formation of twin-planes and by surface effects. By using the stress tensor obtained from molecular dynamics simulations, we are able to describe changes on the electronic structure of these nanowires. On the basis of the resulting electronic structure, we confirm that a one-dimensional superlattice is indeed formed. Furthermore, we describe the transport properties of both electrons and holes in the twin-plane superlattices. In contrast to the predicted transparency of Γ-electrons in heterolayered III-V semiconductor superlattices, we verify that surface effects in 1D systems open up possibilities of electronic structure engineering and the modulation of their transport and optical responses.

Circular polarization in a non-magnetic resonant tunneling device.

We have investigated the polarization-resolved photoluminescence (PL) in an asymmetric n-type GaAs/AlAs/GaAlAs resonant tunneling diode under magnetic field parallel to the tunnel current. The quantum well (QW) PL presents strong circular polarization (values up to -70% at 19 T). The optical emission from GaAs contact layers shows evidence of highly spin-polarized two-dimensional electron and hole gases which affects the spin polarization of carriers in the QW. However, the circular polarization degree in the QW also depends on various other parameters, including the g-factors of the different layers, the density of carriers along the structure, and the Zeeman and Rashba effects.