Valley pseudospin relaxation of charged excitons in monolayer MoTe2.

Zeeman effect induced by the magnetic field introduces a splitting between the two valleys at K + and K - points of the Brillouin zone in monolayer semiconducting transition metal dichalcogenides. In consequence, the photoluminescence signal exhibits a field dependent degree of circular polarization. We present a comprehensive study of this effect in the case of a trion in monolayer MoTe2, showing that although time integrated data allows us to deduce a g-factor of the trion state, such an analysis cannot be substantiated by the timescales revealed in the time-resolved experiments.

Spontaneous Valley Polarization of Interacting Carriers in a Monolayer Semiconductor.

We report magnetoabsorption spectroscopy of gated WSe_{2} monolayers in high magnetic fields up to 60 T. When doped with a 2D Fermi sea of mobile holes, well-resolved sequences of optical transitions are observed in both σ^{±} circular polarizations, which unambiguously and separately indicate the number of filled Landau levels (LLs) in both K and K^{'} valleys. This reveals the interaction-enhanced valley Zeeman energy, which is found to be highly tunable with hole density p. We exploit this tunability to align the LLs in K and K^{'}, and find that the 2D hole gas becomes unstable against small changes in LL filling and can spontaneously valley polarize. These results cannot be understood within a single-particle picture, highlighting the importance of exchange interactions in determining the ground state of 2D carriers in monolayer semiconductors.

Temperature dependence of photoluminescence lifetime of atomically-thin WSe2 layer.

At cryogenic temperatures, the photoluminescence (PL) spectrum of monolayer WSe2 features a number of lines related to the recombination of so-called localized excitons (LEs). The intensity of these lines strongly decreases with increasing temperature. In order to understand the mechanism behind this phenomenon we carried out a time-resolved experiment, which revealed a similar trend in the PL decay time. Our results identify the opening of additional non-radiative relaxation channels as a primary cause of the observed temperature quenching of the LEs' PL.

Revealing exciton masses and dielectric properties of monolayer semiconductors with high magnetic fields.

In semiconductor physics, many essential optoelectronic material parameters can be experimentally revealed via optical spectroscopy in sufficiently large magnetic fields. For monolayer transition-metal dichalcogenide semiconductors, this field scale is substantial-tens of teslas or more-due to heavy carrier masses and huge exciton binding energies. Here we report absorption spectroscopy of monolayer [Formula: see text], and [Formula: see text] in very high magnetic fields to 91 T. We follow the diamagnetic shifts and valley Zeeman splittings of not only the exciton's [Formula: see text] ground state but also its excited [Formula: see text] Rydberg states. This provides a direct experimental measure of the effective (reduced) exciton masses and dielectric properties. Exciton binding energies, exciton radii, and free-particle bandgaps are also determined. The measured exciton masses are heavier than theoretically predicted, especially for Mo-based monolayers. These results provide essential and quantitative parameters for the rational design of opto-electronic van der Waals heterostructures incorporating 2D semiconductors.

Detection of thermodynamic "valley noise" in monolayer semiconductors: Access to intrinsic valley relaxation time scales.

Together with charge and spin, many novel two-dimensional materials also permit information to be encoded in an electron's valley degree of freedom-that is, in particular momentum states in the material's Brillouin zone. With a view toward valley-based (opto)electronic technologies, the intrinsic time scales of valley scattering are therefore of fundamental interest. Here, we demonstrate an entirely noise-based approach for exploring valley dynamics in monolayer transition-metal dichalcogenide semiconductors. Exploiting their valley-specific optical selection rules, we use optical Faraday rotation to passively detect the thermodynamic fluctuations of valley polarization in a Fermi sea of resident carriers. This spontaneous "valley noise" reveals narrow Lorentzian line shapes and, therefore, long exponentially-decaying intrinsic valley relaxation. Moreover, the noise signatures validate both the relaxation times and the spectral dependence of conventional (perturbative) pump-probe measurements. These results provide a viable route toward quantitative measurements of intrinsic valley dynamics, free from any external perturbation, pumping, or excitation.

Serum Vitamin D Concentration and Markers of Bone Metabolism in Perimenopausal and Postmenopausal Women with Asthma and COPD.

Aging and menopause are closely related to hormonal and metabolic changes. Vitamin D is a crucial factor modulating several metabolic processes. The aim of this study was to evaluate biomarkers of bone metabolism in peri- and postmenopausal women with obstructive lung diseases. Sixty two female patients, 27 with asthma and 35 with COPD, aged over 45 years (median age 58 and 64 years, respectively) were enrolled into the study. The evaluation included lung function, bone mineral density, serum concentration of vitamin D, and bone metabolism markers. The study groups differed significantly in terms of forced expiratory volume in 1 s (FEV1); median values of 1.79 L vs. 1.16 L (p = 0.0001) and 71.2% vs. 53.0% predicted (p = 0.0072) and in vitamin D concentration (12.3 ng/ml vs. 17.6 ng/ml). Total bone mineral density (BMD) was lower in the COPD group (p = 0.0115). Serum vitamin D inversely correlated with the number of pack-years in asthma patients (r = -0.45, p = 0.0192). There was no correlation between serum vitamin D and disease duration or severity, and the Asthma Control Test (ACT) and the modified Medical Research Council (mMRC) dyspnea scores. The serum bone metabolism markers C-terminal cross-linked telopeptide of collagen type I (BCROSS), N-terminal propeptides of procollagen type-1 (tP1NP), and N-mid osteocalcin (OCN) inversely correlated with age in the COPD, but not asthma, patients (r = -0.38, p = 0.0264; r = -0.37, p = 0.0270; and r = -0.42, p = 0.0125, respectively). We conclude that peri- and postmenopausal women with obstructive lung diseases had a decreased serum concentration of vitamin D. Furthermore, vitamin D and body mineral density were appreciably lower in women with COPD than those with asthma.

Metabolic Syndrome as a Factor Affecting Systemic Inflammation in Patients with Chronic Obstructive Pulmonary Disease.

Chronic obstructive pulmonary disease (COPD) is a systemic disease which may be associated with other comorbidities. The aim of the study was to estimate the incidence of metabolic syndrome (MS) in COPD patients and to assess its impact on systemic inflammation and lung function. MS was diagnosed in accordance with the recommendations of the Polish Forum for the Prevention of Cardiovascular Diseases. The study group consisted of 267 patients with stable COPD in all stages of severity. All patients underwent spirometry with bronchial reversibility testing and 6 min walk test (6MWT). The following blood tests were evaluated: lipid profile, glucose and C-reactive protein as well as serum concentration of IL-6, leptin, adiponectin, and endothelin. MS was diagnosed in 93 patients (35.8%). No differences were observed in the incidence of MS in relation to airflow limitation severity (mild; moderate; severe and very severe: 38.9; 36.3; 35.2 and 25.0%, respectively). FEV1 (% predicted), FVC (% predicted), 6MWT distance (6MWD), age, and the number of pack-years were similar in patients with and without MS. MS was more frequent in males than females (38.7 vs. 28.4%, p > 0.05). Serum concentrations of IL-6, endothelin, leptin, and CRP were higher in the MS group, contrary to adiponectin concentration which was lower (p < 0.01). MS was more frequent in male COPD patients, but there were no differences in its frequency between patients with different severity of airflow limitation. We conclude that MS, as a comorbidity, occurs in all COPD stages and affects systemic inflammation. MS incidence does not depend on COPD severity.

Comparison of magneto-optical properties of various excitonic complexes in CdTe and CdSe self-assembled quantum dots.

We present a comparative study of two self-assembled quantum dot (QD) systems based on II-VI compounds: CdTe/ZnTe and CdSe/ZnSe. Using magneto-optical techniques we investigated a large population of individual QDs. The systematic photoluminescence studies of emission lines related to the recombination of neutral exciton X, biexciton XX, and singly charged excitons (X(+), X(-)) allowed us to determine average parameters describing CdTe QDs (CdSe QDs): X-XX transition energy difference 12 meV (24 meV); fine-structure splitting δ1=0.14 meV (δ1=0.47 meV); g-factor g = 2.12 (g = 1.71); diamagnetic shift γ=2.5 µeV T(-2) (γ =1.3 µeV T(-2)). We find also statistically significant correlations between various parameters describing internal structure of excitonic complexes.

Magnetic ground state of an individual Fe(2+) ion in strained semiconductor nanostructure.

Single impurities with nonzero spin and multiple ground states offer a degree of freedom that can be utilized to store the quantum information. However, Fe(2+) dopant is known for having a single nondegenerate ground state in the bulk host semiconductors and thus is of little use for spintronic applications. Here we show that the well-established picture of Fe(2+) spin configuration can be modified by subjecting the Fe(2+) ion to high strain, for example, produced by lattice mismatched epitaxial nanostructures. Our analysis reveals that high strain induces qualitative change in the ion energy spectrum and results in nearly doubly degenerate ground state with spin projection Sz= ± 2. We provide an experimental proof of this concept using a new system: a strained epitaxial quantum dot containing individual Fe(2+) ion. Magnetic character of the Fe(2+) ground state in a CdSe/ZnSe dot is revealed in photoluminescence experiments by exploiting a coupling between a confined exciton and the single-iron impurity. We also demonstrate that the Fe(2+) spin can be oriented by spin-polarized excitons, which opens a possibility of using it as an optically controllable two-level system free of nuclear spin fluctuations.

Coherent Precession of an Individual 5/2 Spin.

We present 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 a 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 nonsinusoidal 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.

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. It has already been shown that optical control of a magnetic ion spin is feasible using the carriers confined in a quantum dot. However, a serious obstacle was the quenching of the exciton luminescence by magnetic impurities. Here we show, by photoluminescence studies on thus-far-unexplored individual CdTe dots with a single cobalt ion and CdSe dots with a single manganese ion, that even if energetically allowed, nonradiative exciton recombination through single-magnetic-ion intra-ionic transitions is negligible in such zero-dimensional structures. This opens solotronics for a wide range of as yet unconsidered systems. On the basis of 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.

Quantum interference in exciton-Mn spin interactions in a CdTe semiconductor quantum dot.

We show theoretically and experimentally the existence of a new quantum-interference effect between the electron-hole interactions and the scattering by a single Mn impurity. The theoretical model, including electron-valence-hole correlations, the short- and long-range exchange interaction of a Mn ion with the heavy hole and with electron and anisotropy of the quantum dot, is compared with photoluminescence spectroscopy of CdTe dots with single magnetic ions. We show how the design of the electronic levels of a quantum dot enables the design of an exciton, control of the quantum interference, and hence engineering of light-Mn interaction.

Optical manipulation of a single Mn spin in a CdTe-based quantum dot.

Two coupled CdTe quantum dots, selected from a self-assembled system, one of them containing a single Mn ion, were studied by continuous wave and modulated photoluminescence, photoluminescence excitation, and photon correlation experiments. Optical writing of information on the spin state of the Mn ion has been demonstrated, using the orientation of the Mn spin by spin-polarized carriers transferred from the neighboring quantum dot. Mn spin orientation time values from 20 to 100 ns were measured, depending on the excitation power. Storage time of the information on the Mn spin was found to be enhanced by application of a static magnetic field of 1 T, reaching hundreds of microseconds in the dark. Simple rate equation models were found to describe correctly the static and dynamical properties of the system.

Magnetization dynamics down to a zero field in dilute (Cd,Mn)Te quantum wells.

The evolution of the magnetization in (Cd,Mn)Te quantum wells after a short pulse of magnetic field was determined from the giant Zeeman shift of spectroscopic lines. The dynamics in the absence of a static magnetic field was found to be up to 3 orders of magnitude faster than that at 1 T. Hyperfine interaction and strain are mainly responsible for the fast decay. The influence of a hole gas is clearly visible: at zero field anisotropic holes stabilize the system of Mn ions, while in a magnetic field of 1 T they are known to speed up the decay by opening an additional relaxation channel.