### 2D QUANTUM MATERIALS

Over the last decades, artificial nanostructures grown with atomic-scale precision have become the cutting edge of materials physics. One of their salient features is the posibility of restricting the motion of the electrons to two, one or zero dimensions.

### Feature papers

### 2021

Fast response photogating in monolayer MoS

_{2}phototransistorsD. Vaquero, V. Clericò, J. Salvador-Sánchez, E. Díaz, F. Domínguez-Adame, L. Chico, Y. M. Meziani, E. Diez and J. Quereda

Nanoscale

**13**, 16156 (2021)We investigate the photoresponse of a fully h-BN encapsulated monolayer MoS

_{2}phototransistor. In contrast with previous understanding, we identify a rapidly-responding photogating effect mechanism that becomes the dominant contribution to photoresponse under high-frequency light modulation. Using a Hornbeck–Haynes model for the photocarrier dynamics, we fit the illumination power dependence of this photogating effect and estimate the energy level of the involved traps. The resulting energies are compatible with shallow traps in MoS_{2}caused by the presence of sulfur vacancies. Rashba coupling and spin switching through surface states of Dirac semimetals

### 2020

Excitons, trions and Rydberg states in monolayer MoS

_{2}revealed by ...D. Vaquero, V. Clericò, J. Salvador-Sánchez, A. Martín-Ramos, F. Domínguez-Adame, Y. M. Meziani, E. Diez and J. Quereda

Communications Physics 3

**3**, 194 (2020)Exciton physics in two-dimensional semiconductors are typically studied by photoluminescence spectroscopy. However, this technique does not allow for direct observation of non-radiating excitonic transitions. Here, we use low-temperature photocurrent spectroscopy as an alternative technique to investigate excitonic transitions in a high-quality monolayer MoS

_{2}phototransistor. The resulting spectra presents excitonic peaks with linewidths as low as 8 meV. We identify spectral features corresponding to the ground states of neutral excitons (X_{A1s}and X_{B1s}) and charged trions (T_{A}and T_{B}) as well as up to eight additional spectral lines at energies above the X_{B1s}transition, which we attribute to the Rydberg series of excited states of X_{A}and X_{B}. The intensities of the spectral features can be tuned by the gate and drain-source voltages. Using an effective-mass theory for excitons in two-dimensional systems we are able to accurately fit the measured spectral lines and unambiguously associate them with their corresponding Rydberg states. Tuning the thermoelectric reponse of silicene nanoribbons with vacancies

### 2019

Electric field manipulation of surface states in topological semimetals

Y. Baba, A. Fernández-Díaz, E. Díaz, F. Domínguez-Adame and R. A. Molina

Physical Review B

**100**, 165105 (2019)We investigate the consequences of applying electric fields perpendicularly to thin films of topological semimetals. In particular, we consider Weyl and Dirac semimetals in a configuration such that their surface Fermi arcs lie on opposite edges of the films. We develop an analytical approach based on perturbation theory and a single-surface approximation and we compare our analytical results with numerical calculations. The effect of the electric field on the dispersion is twofold: It shifts the dispersion relation and renormalizes the Fermi velocity, which would, in turn, have direct effects on quantum transport measurements. Additionally, it modifies the spatial decay properties of surface states which will impact the connection of the Fermi arcs in opposite sides of a narrow thin.

Quantum nanoconstrictions fabricated by cryo-etching in encapsulated graphene

### 2018

Topologically protected states in δ-doped junctions with band inversion

A. Díaz-Fernández, N. del Valle, E. Díaz and F. Domínguez-Adame

Physical Review B

**98**, 085424 (2021)A topological boundary can be formed at the interface between a trivial and a topological insulator. The difference in the topological index across the junction leads to robust gapless surface states. Optical studies of these states are scarce in the literature, the reason being the difficulty in isolating their response from that of the bulk. In this work, we propose to deposit a δ layer of donor impurities in close proximity to a topological boundary to help in detecting gapless surface states. As we will show, gapless surface states are robust against this perturbation and they enhance intraband optical transitions as measured by the oscillator strength. These results help us to understand the interplay of surface and bulk states in topological insulators.

Robust midgap states in band-inverted junctions under electric and magnetic fields