arXiv subjects

M. Koperski

Publications and source records attributed to M. Koperski.

15 recordsLinked to original sources

arXiv2024

Optical response of WSe$_2$-based vertical tunneling junction

Layered materials have attracted significant interest because of their unique properties. Van der Waals heterostructures based on transition-metal dichalcogenides have been extensively studied because of potential optoelectronic applications. We investigate the optical response of a light-emitting tunneling structure based on a WSe\textsubscript{2} monolayer as an active emission material using the photoluminescence (PL) and electroluminescence (EL) experiments performed at low temperature of 5~K. We found that the application of the bias voltage allows us to change both a sign and a value of free carriers concentrations. Consequently, we address the several excitonic complexes emerging in PL spectra under applied bias voltage. The EL signal was also detected and ascribed to the emission in a high-carrier-concentration regime. The results show that the excitation mechanisms in the PL and EL are different, resulting in various emissions in both types of experimental techniques.

cond-mat.mtrl-sci↗

arXiv2024

Atomic and electronic structure of defects in hBN: enhancing single-defect functionalities

Defect centers in insulators play a critical role in creating important functionalities in materials: prototype qubits, single-photon sources, magnetic field probes, and pressure sensors. These functionalities are highly dependent on their mid-gap electronic structure and orbital/spin wave-function contributions. However, in most cases, these fundamental properties remain unknown or speculative due to the defects being deeply embedded beneath the surface of highly resistive host crystals, thus impeding access through surface probes. Here, we directly inspected the atomic and electronic structures of defects in thin carbon-doped hexagonal boron nitride (hBN:C) using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). Such investigation adds direct information about the electronic mid-gap states to the well-established photoluminescence response (including single photon emission) of intentionally created carbon defects in the most commonly investigated van der Waals insulator. Our joint atomic-scale experimental and theoretical investigations reveal two main categories of defects: 1) single-site defects manifesting as donor-like states with atomically resolved structures observable via STM, and 2) multi-site defect complexes exhibiting a ladder of empty and occupied mid-gap states characterized by distinct spatial geometries. Combining direct probing of mid-gap states through tunneling spectroscopy with the inspection of the optical response of insulators hosting specific defect structures holds promise for creating and enhancing functionalities realized with individual defects in the quantum limit. These findings underscore not only the versatility of hBN:C as a platform for quantum defect engineering but also its potential to drive advancements in atomic-scale optoelectronics.

cond-mat.mtrl-sci↗

arXiv2023

Electrical excitation of carbon centers in hexagonal boron nitride with tuneable quantum efficiency

Defect centers in wide-band-gap crystals attracted considerable attention due to the realisations of qubits, sensors, or single photon emitters at room temperature. The family of these centers is constantly growing, including well-known examples such as nitrogen-vacancy centers in diamond, silicon-vacancy in silicon carbide, chromium substitutions in aluminium oxide, and many others. Unfortunately, such defect centers embedded in highly insulating crystals have been notoriously difficult to excite electrically. Herewith, we present a realisation of insulating light-emitting diodes based on carbon centers in hexagonal boron nitride. The rational design of the vertical tunnelling devices via van der Waals technology enabled us to control the charge dynamics related to non-radiative tunelling, defect-to-band electroluminescence, and intradefect electroluminescence. The fundamental understanding of the tunnelling events enabled us to achieve high efficiency of electrical excitation, which exceeded by a few orders of magnitude the efficiency of optical excitation in the sub-band-gap regime. A combination of a Stark effect and screening by band electrons provide a control knob for tuning the energy of emission. With this work, we solve an outstanding problem of creating electrically driven devices realised with defect centers in wide-band-gap crystals, which are relevant in the domain of optoelectronics, telecommunication, computation, or sensing.

cond-mat.mes-hall↗

arXiv2021

Carbon and vacancy centers in hexagonal boron nitride

Creation of defect with predetermined optical, chemical and other characteristics is a powerful tool to enhance the functionalities of materials. Herewith, we utilize density functional theory to understand the microscopic mechanisms of formation of defects in hexagonal boron nitride based on vacancies and substitutional atoms. Through in-depth analysis of the defect-induced band structure and formation energy in varying growth conditions, we uncovered a dominant role of interdefect electron paring in stabilization of defect complexes. The electron reorganization modifies the exchange component of the electronic interactions which dominates over direct Coulomb repulsion or structural relaxation effects making the combination of acceptor- and donor-type defect centers energetically favorable. Based on an analysis of a large number of defect complexes we develop a simple picture of the inheritance of electronic properties when individual defects are combined together to form more complex centers.

cond-mat.mtrl-sci↗

arXiv2021

ZnPSe$_3$ as ultrabright indirect bandgap system with microsecond excitonic lifetimes

We report an optical characterization of ZnPSe$_3$ crystals that demonstrates indirect band gap characteristics in combination with unusually strong photoluminescence. We found evidence of interband recombination from excitonic states with microsecond lifetimes. Through optical characterization, we reconstructed the electronic band scheme relevant for fundamental processes of light absorption, carrier relaxation and radiative recombination. The investigation of the radiative processes in the presence of magnetic field revealed spin polarization of fundamental electronic states. This observation opens a pathway towards controlling the spin of excitonic states in technologically relevant microsecond timescales.

cond-mat.mes-hall↗

arXiv2019

Magnetic 2D materials and heterostructures

The family of 2D materials grows day by day, drastically expanding the scope of possible phenomena to be explored in two dimensions, as well as the possible van der Waals heterostructures that one can create. Such 2D materials currently cover a vast range of properties. Until recently, this family has been missing one crucial member - 2D magnets. The situation has changed over the last two years with the introduction of a variety of atomically-thin magnetic crystals. Here we will discuss the difference between magnetic states in 2D materials and in bulk crystals and present an overview of the 2D magnets that have been explored recently. We will focus, in particular, on the case of the two most studied systems - semiconducting CrI$_3$ and metallic Fe$_3$GeTe$_2$ - and illustrate the physical phenomena that have been observed. Special attention will be given to the range of novel van der Waals heterostructures that became possible with the appearance of 2D magnets, offering new perspectives in this rapidly expanding field.

cond-mat.mtrl-sci↗

arXiv2018

Fine structure of $\mathrm{K}$-excitons in multilayers of transition metal dichalcogenides

Reflectance and magneto-reflectance experiments together with theoretical modelling based on the $\mathbf{k\cdot p}$ approach have been employed to study the evolution of direct bandgap excitons in MoS$_2$ layers with a thickness ranging from mono- to trilayer. The extra excitonic resonances observed in MoS$_2$ multilayers emerge as a result of the hybridization of Bloch states of each sub-layer due to the interlayer coupling. The properties of such excitons in bi- and trilayers are classified by the symmetry of corresponding crystals. The inter- and intralayer character of the reported excitonic resonances is fingerprinted with the magneto-optical measurements: the excitonic $g$-factors of opposite sign and of different amplitude are revealed for these two types of resonances. The parameters describing the strength of the spin-orbit interaction are estimated for bi- and trilayer MoS$_2$.

cond-mat.mes-hall↗

arXiv2018

Orbital, spin and valley contributions to Zeeman splitting of excitonic resonances in MoSe$_2$, WSe$_2$ and WS$_2$ monolayers

We present a comprehensive optical study of the excitonic Zeeman effects in transition metal dichalcogenide monolayers, which are discussed comparatively for selected materials: MoSe$_2$, WSe$_2$ and WS$_2$. We introduce a simple semi-phenomenological description of the magnetic field evolution of individual electronic states in fundamental sub-bands by considering three additive components: valley, spin and orbital terms. We corroborate the validity of the proposed description by inspecting the Zeeman-like splitting of neutral and charged excitonic resonances in absorption-type spectra. The values of all three terms are estimated based on the experimental data, demonstrating the significance of the valley term for a consistent description of magnetic field evolution of optical resonances, particularly those corresponding to charged states. The established model is further exploited for discussion of magneto-luminescence data. We propose an interpretation of the observed large g-factor values of low energy emission lines, due to so-called bound/localized excitons in tungsten based compounds, based on the brightening mechanisms of dark excitonic states.

cond-mat.mes-hall↗

arXiv2016

Optical properties of atomically thin transition metal dichalcogenides: Observations and puzzles

Recent results on the optical properties of mono- and few-layers of semiconducting transition metal dichalcogenides are reviewed. Experimental observations are presented and discussed in the frame of existing models, highlighting the limits of our understanding in this emerging field of research. We first introduce the representative band structure of these systems and their interband optical transitions. The effect of an external magnetic field is then considered to discuss Zeeman spectroscopy and optical pumping experiments, both revealing phenomena related to the valley degree of freedom. Finally, we discuss the observation of single photon emitters in different types of layered materials, including wide band gap hexagonal boron nitride. While going through these topics, we try to focus on open questions and on experimental observations, which do not yet have a clear explanation.

cond-mat.mes-hall↗

arXiv2016

Direct determination of zero-field splitting for single Co$^{2+}$ ion embedded in a CdTe/ZnTe quantum dot

When Co$^{2+}$ impurity is embedded in semiconductor structure, crystal strain strongly influences zero-filed splitting between Co$^{2+}$ states with spin projection $S_z = \pm 3/2$ and $S_z = \pm 1/2$. Experimental evidences of this effect have been given in previous studies, however direct measurement of the strain induced zero-field splitting has been inaccessible so far. Here this splitting is determined thanks to magneto-optical studies of individual Co$^{2+}$ ion in epitaxial CdTe quantum dot in ZnTe barrier. Using partially allowed optical transitions we measure strain induced zero-field splitting of Co$^{2+}$ ion directly on excitonic photoluminescence spectrum. Moreover, by observation of anticrossing of $S_z = + 3/2$ and $S_z = - 1/2$ Co$^{2+}$ spin states in magnetic field, we determine axial and in-plane components of crystal field acting on Co$^{2+}$. Proposed technique can be applied for optical determination of zero-field splitting of other transition metal ions in quantum dots.

cond-mat.mes-hall↗

arXiv2015

Tuning valley polarization in a WSe2 monolayer with a tiny magnetic field

In monolayers of semiconducting transition metal dichalcogenides, the light helicity ($σ^+$ or $σ^-$) is locked to the valley degree of freedom, leading to the possibility of optical initialization of distinct valley populations. However, an extremely rapid valley pseudospin relaxation (at the time scale of picoseconds) occurring for optically bright (electric-dipole active) excitons imposes some limitations on the development of opto-valleytronics. Here we show that inter-valley scattering of excitons can be significantly suppressed in a $\mathrm{WSe}_2$ monolayer, a direct-gap two-dimensional semiconductor with the exciton ground state being optically dark. We demonstrate that the already inefficient relaxation of the exciton pseudospin in such system can be suppressed even further by the application of a tiny magnetic field of $\sim$100 mT. Time-resolved spectroscopy reveals the pseudospin dynamics to be a two-step relaxation process. An initial decay of the pseudospin occurs at the level of dark excitons on a time scale of 100 ps, which is tunable with a magnetic field. This decay is followed by even longer decay ($>1$ ns), once the dark excitons form more complex objects allowing for their radiative recombination. Our finding of slow valley pseudospin relaxation easily manipulated by the magnetic field open new prospects for engineering the dynamics of the valley pseudospin in transition metal dichalcogenides.

cond-mat.mes-hall↗

arXiv2014

Single photon emitters in exfoliated WSe2 structures

Crystal structure imperfections in solids often act as efficient carrier trapping centers which, when suitably isolated, act as sources of single photon emission. The best known examples of such attractive imperfections are wellwidth or composition fluctuations in semiconductor heterostructures (resulting in a formation of quantum dots) and coloured centers in wide bandgap (e. g., diamond) materials. In the case of recently investigated thin films of layered compounds, the crystal imperfections may logically be expected to appear at the edges of commonly investigated few-layer flakes of these materials, exfoliated on alien substrates. Here, we report on comprehensive optical microspectroscopy studies of thin layers of tungsten diselenide, WSe2, a representative semiconducting dichalcogenide with a bandgap in the visible spectral range. At the edges of WSe2 flakes, transferred onto Si/SiO2 substrates, we discover centers which, at low temperatures, give rise to sharp emission lines (0.1 meV linewidth). These narrow emission lines reveal the effect of photon antibunching, the unambiguous attribute of single photon emitters. The optical response of these emitters is inherently linked to two-dimensional properties of the WSe2 monolayer, as they both give rise to luminescence in the same energy range, have nearly identical excitation spectra and very similar, characteristically large Zeeman effects. With advances in the structural control of edge imperfections, thin films of WSe2 may provide added functionalities, relevant for the domain of quantum optoelectronics.

cond-mat.mes-hall↗

arXiv2014

Coherent precession of an individual 5/2 spin

We present a direct observation of a coherent spin precession of an individual Mn$^{2+}$ ion, having both electronic and nuclear spins equal to 5/2, embedded in a CdTe quantum dot and placed in magnetic field. The spin state evolution is probed in a time-resolved pump-probe measurement of absorption of the single dot. The experiment reveals subtle details of the large-spin coherent dynamics, such as non-sinusoidal evolution of states occupation, and beatings caused by the strain-induced differences in energy levels separation. Sensitivity of the large-spin impurity on the crystal strain opens the possibility of using it as a local strain probe.

cond-mat.mes-hall↗

arXiv2013

Introducing single Mn2+ ions into spontaneously coupled quantum dot pairs

We present the photoluminescence excitation study of the self-assembled CdTe/ZnTe quantum dots doped with manganese ions. We demonstrate the identification method of spontaneously coupled quantum dots pairs containing single Mn2+ ions. As the result of the coupling, the resonant absorption of the photon in one quantum dot is followed by the exciton transfer into a neighboring dot. It is shown that the Mn2+ ion might be present in the absorbing, emitting or both quantum dots. The magnetic properties of the Mn2+ spin are revealed by a characteristic sixfold splitting of the excitonic line. The statistics of the value of this splitting is analyzed for the large number of the dots and gives the information on the maximum density of the neutral exciton wave function.

cond-mat.mes-hall↗

arXiv2013

Designing quantum dots for solotronics

Solotronics, optoelectronics based on solitary dopants, is an emerging field of research and technology reaching the ultimate limit of miniaturization. It aims at exploiting quantum properties of individual ions or defects embedded in a semiconductor matrix. As already shown, optical control of a spin of a magnetic ion is feasible employing photo-generated carriers confined in a quantum dot. A non-radiative recombination, regarded as a severe problem, limited development of quantum dots with magnetic ions. Our photoluminescence studies on, so far unexplored, individual CdTe dots with single cobalt ions and individual CdSe dots with single manganese ions show, however, that even if energetically allowed, the single ion related non-radiative recombination is negligible in such zero-dimensional structures. This opens solotronics for a wide range of even not yet considered systems. Basing on the results of our single spin relaxation experiments and on the material trends, we identify optimal magnetic ion-quantum dot systems for implementation of a single-ion based spin memory.

cond-mat.mes-hall↗