Defect-induced photoconductivity in layered manganese oxides: a density functional theory study

Enhanced photoconductivity of layered Mn(IV)O2 containing protonated Mn(IV) vacancy defects has been recently demonstrated, suggesting new technological possibilities for photoelectric conversion based on visible light harvesting. Using spin-polarized density functional theory, we provide the first direct evidence that such defects can indeed facilitate photoconductivity by (i) reducing the band-gap energy and (ii) separating electron and hole states. Our results thus support the proposition that nanosheet MnO2 offers an attractive new material for a variety of photoconductivity applications.     

Substrate rigidity effects in mixed layered solids

The composition dependence of the d-spacing of Cs_1?xRb_x vermiculite has been determined for 0 ≤ × ≤ 1. It exhibits a step-like drop with increasing x at a composition that is unexpectedly lower than that at which Cs_1?xRb_xC₈ shows a more gradual drop. This results attributed to in-plane substrate distortions which preserve the rigidity of the vermiculite layer. Measurements of the torsional Raman mode of Cs_1?xRb_x vermiculite support this analysis.     

Diamagnetic enhancement in amorphous semiconductors

We analyse the diamagnetic susceptibility χ of a model two-dimensional semiconductor both in crystalline and amorphous phases using a linear combination of hybrids model. We show that the large diamagnetic enhancement in amorphous Si and Ge is due to the reduction of the Van Vleck type paramagnetic term.     

Giant photoplastic effect in vitreous semiconductors near the rigidity percolation transition

The negative giant photoplastic effect (giant photosoftening) has been experimentally observed in the As-Se system when films obtained by thermal evaporation of As₂₀Se₈₀ chalcogenide glass are irradiated by light from the region of the fundamental absorption edge. Correlation has been found between the photoplastic effect and rigidity percolation transition in the As-Se chalcogenide glass matrix. Such a correlation is not revealed when light irradiation changes the optical properties of these glass films. It has been shown that a nonlinear (non-Hookian) mechanism of the formation of the strain response is realized in the films subjected to the combined action of light and external mechanical loading.     

Large excitonic enhancement of optical refrigeration in semiconductors

We present a theoretical analysis for laser cooling of bulk GaAs based on a microscopic many-particle theory of absorption and luminescence of a partially ionized electron-hole plasma. Our cooling threshold analysis shows that, at low temperatures, the presence of the excitonic resonance in the luminescence is essential in competing against heating losses. The theory includes self-consistent energy renormalizations and line broadenings from both instantaneous mean-field and frequency-dependent carrier-carrier correlations, and it is applicable from the few-Kelvin regime to above room temperature.     

Thermal conductivity of GaS and GaSe layered semiconductors

This paper reports on the results of experimental investigations into the thermal conductivity of GaS and GaSe layered semiconductor crystals in directions parallel and perpendicular to the crystal layers in the temperature range 5–300 K. Specific features of the thermal conductivity of these crystals are analyzed.     

Magnetic susceptibility of layered 2H-WSe2 semiconductors

Magnetic susceptibility and anisotropy in magnetic susceptibility _L are studied in 2H-WSe₂ layered semiconductor crystals over the temperature interval 77–300 K and magnetic fields 1–10 kOe.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 20–23, November, 1989.The authors are indebted to L. P. Strakhov for his kind evaluation of the results of the study.     

Defect-induced ferromagnetism in fullerenes

Based on the ab initio electronic structure calculations the picture of ferromagnetism in polimerized C₆₀ is proposed which seems to explain the whole set of controversial experimental data. We have demonstrated that, in contrast with cubic fullerene, in rhombohedral C₆₀ the segregation of iron atoms is energetically unfavorable which is a strong argument in favor of intrinsic character of carbon ferromagnetism which can be caused by vacancies with unpaired magnetic electrons. It is shown that: (i) energy formation of the vacancies in the rhombohedral phase of C₆₀ is essentially smaller than in the cubic phase, (ii) there is a strong ferromagnetic exchange interactions between carbon cages containing the vacancies, (iii) presence of iron impurities can diminish essentially the formation energy of intrinsic defects, and (iv) the fusion of the magnetic single vacancies into nonmagnetic bivacancies is energetically favorable. The latter can explain a fragility of the ferromagnetism.     

Interband absorption in layered semiconductors with a smooth random field

The effect of a smooth random field on interband absorption is studied in highly anisotropic semiconductors. Such fields can arise because of semimacroscopic defects induced during production and randomly distributed throughout the sample. The calculations are performed using a semiclassical approximation.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 45–47, September, 1981.We wish to thank V. L. Bonch-Bruevich for discussions of these results and his constant attention to the work reported here.     

Motional enhancement of exciton magnetic moments in zinc-blende semiconductors

We report a remarkable enhancement of the magnetic moments of excitons as a result of their motion. This surprising result, which we have observed in magneto-optical studies of three distinct zinc-blende semiconductors, GaAs, CdTe, and ZnSe, becomes significant as the kinetic energy of the exciton becomes comparable with its Rydberg energy and is attributed to motionally induced changes in the internal structure of the exciton. The enhancement of the magnetic moment as a function of the exciton translational wave vector can be represented by a universal equation.     

Defect-induced instabilities in condensed media

Some universal responses of condensed media to intense loads are studied. This behavior is attributed to the evolution of ensembles of mesoscopic defects (microcracks, microshears). It is shown that there exist several types of attractors which control the evolution of ensembles of defects under conditions of nonequilibrium transitions. The role of these transitions in the development of anomalies of the deformational behavior of solids and instabilities during fluid flow is discussed. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 9, 714–721 (10 May 1998)     

Defect-induced electric fields in ZnS

The optical absorption edge in cubic ZnS shows both phonon- and impurity-induced effects which determine its precise shape. Applying external electric fields and measuring the shift of the absorption edge makes it also possible to compute the impurity excitonic mass which was found to be ? 0.02m₀. Comparing the edge shifts due to the impurity-induced electric fields with those due to the external fields, one finds the approximate value for the average impurity-induced field as being about 3 × 10⁴ V/cm.     

Search for giant Franz-Keldysh-like effects in GaSe and other layered semiconductors

Recently, an anomalously large redshift of the absorption edge with electric field was claimed for -GaSe_1–x S _x layered crystals. We have studied Bridgman grown -GaSe crystals as well as vapor transport grown -GaS, and 2H-WSe₂. While we have observed a shift in the absorption edge for -GaSe similar to that reported in previous work, our results demonstrate that the redshift arises from Joule heating, and is thus temperature induced, rather than intrinsic. For -GaS, much larger resistivities virtually eliminate Joule heating, and our measurements of the electric field induced absorption edge shift yield an estimated upper limit of approximately 0.04 meV · cm/kV for a field of 2.4×10³ kV/cm, in good agreement with the theoretical value expected for the Franz-Keldysh effect.Also at University of Konstanz     

Anomalous thermal expansion of InSe layered semiconductors in the low-temperature region

The temperature dependence of the linear thermal expansion coefficient (TEC) of an InSe single crystal in the (001) plane is measured in the temperature range 7–50 K. A peak in the thermal expansion is detected near T = 10 K, after which the sample shrinks upon heating. The effect of an external magnetic field of up to 6 T, which is parallel to the (001) plane, on the TEC is investigated. The observed partial suppression of the peak and crystal compression by the field indicates the relation of these anomalies to possible electron ordering in InSe layers.     

Photoacoustic saturation spectra of GaSe,GaTe and InSe layered semiconductors

The photoacoustic spectra in GaSe, GaTe and InSe semiconductors have been measured in the region of energy greater than the fundamental absorption edge. In this range the photoacoustic signal magnitude can be regarded as independent of the absorption coefficient, i.e. photoacoustic spectroscopy in saturation region, and the dips in the spectra are ascribed to the optical reflection effect inherent in semiconductors interband transitions. The photoacoustic signal phase spectra have been also measured; they are shown to be a useful check of the structures observed in the magnitude saturation spectra. The experimental results are in good agreement with the allowed interband transition energies as observed in thermo-reflectance, electroreflectance and normal reflectivity experiments.     

Na adsorption on the layered semiconductors SnS2 and WSe2

The interaction of Na metal deposits with UHV cleaved van der Waals surfaces (0001) of SnS₂ and WSe₂ was investigated with SXPS and LEED. For SnS₂ an insertion reaction (intercalation) of Na leads to a reduced Sn-species. For WSe₂“non-reactive” intercalation occurs at room temperature which follows the rigid band model. At 120 K a metallic Na overlayer is formed. The observed reactivity is discussed in terms of bulk thermodynamics and the obtained difference is tentatively related to differences in the electronic structure of the compounds.