Abhay Shukla

Persistent photoconductivity is often observed in single-layer MoS2 devices and has a bearing on device applications because it alters device conductivity and characteristics. It originates in the trapping and detrapping of photo-generated charges and may be useful in devices serving neuromorphic computing. The dependence of persistent photoconductivity on the nature of the substrate, traps, and working parameters including doping, gate voltage, and temperature is the subject of debate. Here, we measure transport characteristics in single-layer MoS2 on SiO2/Si substrates as a function of temperature and gating, following illumination. Analysis of the multiexponential decay of the photocurrent shows that extrinsic impurities at the device–substrate interface act as traps when ionized. For shallow traps probably formed from Na impurities, we deduce an activation energy of about 80 meV, while deep traps are estimated to have activation energies of a few hundred meV. We give a coherent and self-consistent explanation of the microscopic mechanisms governing the phenomenon.

Layered magnetic materials provide the opportunity for fundamental investigations of magnetism in the two-dimensional limit. NiPS_{3}, a prototype member of this family, is antiferromagnetic below 155 K and exhibits sharp photoluminescence associated with 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. We synthesize samples with metal and ligand substitutions altering the Néel transition temperature and measure the effects of these changes on temperature dependent photoluminescence. We perform crystal field and charge transfer multiplet calculations to explain the origin of the excitation and identify it as spin flip luminescence originating in Ni crystal field levels. Through observations of phonon side bands we confirm that it is localized at the Ni site as opposed to a possible coherent delocalized excitation between hybridized Zhang-Rice states. Alloying with Zn and Se shows a lack of correlation between the temperature dependence of the luminescence and the Néel temperature, belying its supposedly magnetic nature.

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.

Ammonium fluoride (NH₄F) exhibits a variety of crystalline phases depending on temperature and pressure. By employing Raman spectroscopy and synchrotron X-ray diffraction beyond megabar pressures (up to 140 GPa), we have here observed a novel dense solid phase of NH₄F, characterised by the tetragonal P4/nmm structure also observed in other ammonium halides under less extreme pressure conditions, typically a few GPa. Using detailed ab-initio calculations and reevaluating earlier theoretical models pertaining to other ammonium halides, we examine the microscopic mechanisms underlying the transition from the low-pressure cubic phase (P-43m) to the newly identified high-pressure tetragonal phase (P4/nmm). Notably, NH₄F exhibits distinctive properties compared to its counterparts, resulting in a significantly broader pressure range over which this transition unfolds, facilitating the identification of its various stages. Our analysis points to a synergistic interplay driving the transition to the P4/nmm phase, which we name phase VIII. At intermediate pressures (around 40 GPa), a displacive transition of fluorine ions initiates a tetragonal distortion of the cubic phase. Subsequently, at higher pressures (around 115 GPa), every second ammonium ion undergoes a rotational shift, adopting an anti-tetrahedral arrangement. This coupled effect orchestrates the transition process, leading to the formation of the tetragonal phase.

MoTe₂ has a stable hexagonal semiconducting phase (2H) as well as two semimetallic phases with monoclinic (1T') and orthorhombic (T_d) structures. A structural change can thus be accompanied by a significant change in electronic transport properties. The two semimetallic phases are connected by a temperature driven transition and could exhibit topological properties. Here we make extensive Raman measurements as a function of layer thickness, temperature, and electrostatic doping on few layer 2H-MoTe₂ and also on 1T'-MoTe₂ and T_d-WTe₂. Recent work in MoTe₂ has raised the possibility of a 2H-1T' transition through technology compatible pathways. It has been claimed that such a transition, of promise for device applications, is activated by electrostatic gating. We investigate this claim and find that few-layer tellurides are characterized by high mobility of Te ions, even in ambient conditions and especially through the variation of external parameters like electric field or temperature. These can generate Te clusters, vacancies at crystalline sites, and facilitate structural transitions. We however find that the purported 2H-1T' transition in MoTe₂ cannot be obtained by a pure electrostatic field.

The superconductor-insulator transition in two dimensions is a prototype continuous quantum phase transition at absolute zero, driven by a parameter other than temperature. Here we reveal this transition in one unit-cell Bi_2.1Sr_1.9CaCu₂O_8+x by space charge doping, a field effect electrostatic doping technique. We determine the related critical parameters and develop a reliable way to estimate doping in the nonsuperconducting region, a crucial and central problem in these materials. Finite-size scaling analysis yields a critical doping of 0.057 holes/Cu, a critical resistance of ~6.85 kΩ and a scaling exponent product νz ~ 1.57. These results, together with earlier work in other materials, provide a coherent picture of the superconductor-insulator transition and its bosonic nature in the underdoped regime of emerging superconductivity in high critical temperature superconductors.

Pressure- and temperature-dependent Raman scattering in GeSe, SnSe, and GeTe for pressures beyond 50 GPa and for temperatures ranging from 78 to 800 K allow us to identify structural and electronic phase transitions, similarities between GeSe and SnSe, and differences with GeTe. Calculations help to deduce the propensity of GeTe for defect formation and the doping that results from it, which gives rise to strong Raman damping beyond anomalous anharmonicity. These properties are related to the underlying chemical bonding and consistent with a recent classification of bonding in several chalcogenide materials that puts GeTe in a separate class of “incipient” metals.

Significance: Two-dimensional electron gases are an essential building block of today’s technology and attract broad interest in the context of material science or nanoengineering. Their widespread applications in many strategical sectors call for a direct visualization of hot electrons relaxation in an accumulation layer. In this work, we make use of time-resolved photoelectron spectroscopy to acquire snapshots of the electronic distribution after a strong and impulsive drive. We identified and quantified the remote coupling of the confined electrons to phonon modes of the adjacent polar material. The far-reaching outcomes provide insights on the screening of electron–phonon coupling in constrained dimensions and will be of high relevance for the development of aggressively downscaled circuits.