Resolving the Spatial Structures of Bound Hole States in Black Phosphorus.

Understanding the local electronic properties of individual defects and dopants in black phosphorus (BP) is of great importance for both fundamental research and technological applications. Here, we employ low-temperature scanning tunnelling microscope (LT-STM) to probe the local electronic structures of single acceptors in BP. We demonstrate that the charge state of individual acceptors can be reversibly switched by controlling the tip-induced band bending. In addition, acceptor-related resonance features in the tunnelling spectra can be attributed to the formation of Rydberg-like bound hole states. The spatial mapping of the quantum bound states shows two distinct shapes evolving from an extended ellipse shape for the 1s ground state to a dumbbell shape for the 2px excited state. The wave functions of bound hole states can be well-described using the hydrogen-like model with anisotropic effective mass, corroborated by our theoretical calculations. Our findings not only provide new insight into the many-body interactions around single dopants in this anisotropic two-dimensional material but also pave the way to the design of novel quantum devices.

Two-dimensional exciton properties in monolayer semiconducting phosphorus allotropes.

Excitons play a key role in technological applications since they have a strong influence on determining the efficiency of photovoltaic devices. Recently, it has been shown that the allotropes of phosphorus possess an optical band gap that can be tuned over a wide range of values including the near-infrared and visible spectra, which would make them promising candidates for optoelectronic applications. In this work we carry out ab initio many-body perturbation theory calculations to study the excitonic effects on the optical properties of two-dimensional phosphorus allotropes: the case of blue and black monolayers. We elucidate the most relevant optical transitions, exciton binding energy spectrum as well as real-space exciton distribution, particularly focusing on the absorption spectrum dependence on the incident light polarization. In addition, based on our results, we use a set of effective hydrogenic models, in which the electron-hole Coulomb interaction is included to estimate exciton binding energies and radii. Our results show an excellent agreement between the many-body methodology and the effective models.

Multiferroic Two-Dimensional Materials.

The relation between unusual Mexican-hat band dispersion, ferromagnetism, and ferroelasticity is investigated using a combination of analytical, first-principles, and phenomenological methods. The class of material with Mexican-hat band edge is studied using the α-SnO monolayer as a prototype. Such a band edge causes a van Hove singularity diverging with 1/sqrt[E], and a charge doping in these bands can lead to time-reversal symmetry breaking. Herein, we show that a material with Mexican-hat band dispersion, α-SnO, can be ferroelastic or paraelastic depending on the number of layers. Also, an unexpected multiferroic phase is obtained in a range of hole density for which the material presents ferromagnetism and ferroelasticity simultaneously.

[Macular drusen variability: multimodal imaging potential]. / Variabel'nost' makulyarnykh druz: vozmozhnosti mul'timodal'noi vizualizatsii.

Insufficient and controversial knowledge about the macular drusen (MD), a lack of scientifically proven management methods for drusen and their strong correlation with AMD active progression makes MD an important area of research. AIM: The purpose of the study ­ to assess clinical feature of MD using modern digital imaging technologies. MATERIAL AND METHODS: Patients with both hard and soft drusen were studied using fluorescein angiography, swept-source optical coherence tomography (OCT), OCT-angiography, autofluorescence (both short-wavelength and near infra-red), scanning laser ophthalmoscopy in Multicolor mode. The retina, choroid and vitreoretinal interface were assessed on 50 patients with AMD and drusen using different imaging modalities. An additional group of the study was presented by 5 patients with geographic atrophy (GA) formed as a result of soft drusen fading, where retrospective assessment of the OCT scans was performed with special attention to the signs of soft drusen regression associated with atrophy of the overlying RPE. RESULTS: Two types of hard drusen were defined as the reticular pseudodrusen and the cuticular drusen. The qualitative and comparative analysis of data for each type of MD was performed. Vitreoretinal interface evaluation demonstrated the correlation between vitreomacular adhesion and mixed reticular and cuticular drusen. The choroidal thickness assessment in 9 different macular sectors in drusenoid eyes does not reveal a significant difference with control group. All of the analysed drusen-faded-eyes initially had been presented with OCT patterns of "nascent" GA. CONCLUSION: The modern retinal imaging techniques enable new approach to the diagnostic differentiation and description of various macular drusen types. The value of these methods for AMD prognosis is yet to be further investigated.

Tunneling Plasmonics in Bilayer Graphene.

We report experimental signatures of plasmonic effects due to electron tunneling between adjacent graphene layers. At subnanometer separation, such layers can form either a strongly coupled bilayer graphene with a Bernal stacking or a weakly coupled double-layer graphene with a random stacking order. Effects due to interlayer tunneling dominate in the former case but are negligible in the latter. We found through infrared nanoimaging that bilayer graphene supports plasmons with a higher degree of confinement compared to single- and double-layer graphene, a direct consequence of interlayer tunneling. Moreover, we were able to shut off plasmons in bilayer graphene through gating within a wide voltage range. Theoretical modeling indicates that such a plasmon-off region is directly linked to a gapped insulating state of bilayer graphene, yet another implication of interlayer tunneling. Our work uncovers essential plasmonic properties in bilayer graphene and suggests a possibility to achieve novel plasmonic functionalities in graphene few-layers.

Spin-orbit proximity effect in graphene.

The development of spintronics devices relies on efficient generation of spin-polarized currents and their electric-field-controlled manipulation. While observation of exceptionally long spin relaxation lengths makes graphene an intriguing material for spintronics studies, electric field modulation of spin currents is almost impossible due to negligible intrinsic spin-orbit coupling of graphene. In this work, we create an artificial interface between monolayer graphene and few-layer semiconducting tungsten disulphide. In these devices, we observe that graphene acquires spin-orbit coupling up to 17 meV, three orders of magnitude higher than its intrinsic value, without modifying the structure of the graphene. The proximity spin-orbit coupling leads to the spin Hall effect even at room temperature, and opens the door to spin field effect transistors. We show that intrinsic defects in tungsten disulphide play an important role in this proximity effect and that graphene can act as a probe to detect defects in semiconducting surfaces.

Strain-induced gap modification in black phosphorus.

The band structure of single-layer black phosphorus and the effect of strain are predicted using density functional theory and tight-binding models. Having determined the localized orbital composition of the individual bands from first principles, we use the system symmetry to write down the effective low-energy Hamiltonian at the Γ point. From numerical calculations and arguments based on the crystal structure of the material, we show that the deformation in the direction normal to the plane can be used to change the gap size and induce a semiconductor-metal transition.

Tunable phonon polaritons in atomically thin van der Waals crystals of boron nitride.

van der Waals heterostructures assembled from atomically thin crystalline layers of diverse two-dimensional solids are emerging as a new paradigm in the physics of materials. We used infrared nanoimaging to study the properties of surface phonon polaritons in a representative van der Waals crystal, hexagonal boron nitride. We launched, detected, and imaged the polaritonic waves in real space and altered their wavelength by varying the number of crystal layers in our specimens. The measured dispersion of polaritonic waves was shown to be governed by the crystal thickness according to a scaling law that persists down to a few atomic layers. Our results are likely to hold true in other polar van der Waals crystals and may lead to new functionalities.

Electronic and plasmonic phenomena at graphene grain boundaries.

Graphene, a two-dimensional honeycomb lattice of carbon atoms of great interest in (opto)electronics and plasmonics, can be obtained by means of diverse fabrication techniques, among which chemical vapour deposition (CVD) is one of the most promising for technological applications. The electronic and mechanical properties of CVD-grown graphene depend in large part on the characteristics of the grain boundaries. However, the physical properties of these grain boundaries remain challenging to characterize directly and conveniently. Here we show that it is possible to visualize and investigate the grain boundaries in CVD-grown graphene using an infrared nano-imaging technique. We harness surface plasmons that are reflected and scattered by the graphene grain boundaries, thus causing plasmon interference. By recording and analysing the interference patterns, we can map grain boundaries for a large-area CVD graphene film and probe the electronic properties of individual grain boundaries. Quantitative analysis reveals that grain boundaries form electronic barriers that obstruct both electrical transport and plasmon propagation. The effective width of these barriers (∼10-20 nm) depends on the electronic screening and is on the order of the Fermi wavelength of graphene. These results uncover a microscopic mechanism that is responsible for the low electron mobility observed in CVD-grown graphene, and suggest the possibility of using electronic barriers to realize tunable plasmon reflectors and phase retarders in future graphene-based plasmonic circuits.

Gate-tuning of graphene plasmons revealed by infrared nano-imaging.

Surface plasmons are collective oscillations of electrons in metals or semiconductors that enable confinement and control of electromagnetic energy at subwavelength scales. Rapid progress in plasmonics has largely relied on advances in device nano-fabrication, whereas less attention has been paid to the tunable properties of plasmonic media. One such medium--graphene--is amenable to convenient tuning of its electronic and optical properties by varying the applied voltage. Here, using infrared nano-imaging, we show that common graphene/SiO(2)/Si back-gated structures support propagating surface plasmons. The wavelength of graphene plasmons is of the order of 200 nanometres at technologically relevant infrared frequencies, and they can propagate several times this distance. We have succeeded in altering both the amplitude and the wavelength of these plasmons by varying the gate voltage. Using plasmon interferometry, we investigated losses in graphene by exploring real-space profiles of plasmon standing waves formed between the tip of our nano-probe and the edges of the samples. Plasmon dissipation quantified through this analysis is linked to the exotic electrodynamics of graphene. Standard plasmonic figures of merit of our tunable graphene devices surpass those of common metal-based structures.

Apparent power-law behavior of conductance in disordered quasi-one-dimensional systems.

The dependence of hopping conductance on temperature and voltage for an ensemble of modestly long one-dimensional wires is studied numerically using the shortest-path algorithm. In a wide range of parameters this dependence can be approximated by a power law rather than the usual stretched-exponential form. The relation to recent experiments and prior analytical theory is discussed.

On the origin of the genetic code: signatures of its primordial complementarity in tRNAs and aminoacyl-tRNA synthetases.

If the table of the genetic code is rearranged to put complementary codons face-to-face, it becomes apparent that the code displays latent mirror symmetry with respect to two sterically different modes of tRNA recognition. These modes involve distinct classes of aminoacyl-tRNA synthetases (aaRSs I and II) with recognition from the minor or major groove sides of the acceptor stem, respectively. We analyze the anticodon pairs complementary to the face-to-face codon couplets. Taking into account the invariant nucleotides on either side (5' and 3'), we consider the risk of anticodon confusion and subsequent erroneous aminoacylation in the ancestral coding system. This logic leads to the conclusion that ribozymic precursors of tRNA synthetases had the same two complementary modes of tRNA aminoacylation. This surprising case of molecular mimicry (1) shows a key potential selective advantage arising from the partitioning of aaRSs into two classes, (2) is consistent with the hypothesis that the two aaRS classes were originally encoded by the complementary strands of the same primordial gene and (3) provides a 'missing link' between the classic genetic code, embodied in the anticodon, and the second, or RNA operational, code that is embodied mostly in the acceptor stem and is directly responsible for proper tRNA aminoacylation.

[Prevention of complications in combined (laser and extrascleral) treatment of retinal detachment]. / Profilaktika oslozhnenií pri kombinirovannom (lazernom i ékstraskleral'nom) lechenii otsloek setchatki.

The main factors of risk were analyzed and indications for preventive laser coagulation were specified in the paper. An approach was defined to ensure a maximally sparing surgical and stage-based laser treatment in retinal detachment, which is targeted at preventing its relapses and at improving the functional results.

[Results of photodynamic therapy of subretinal neovascular membranes according to the data of fluorescence angiography and optical coherence tomography]. / Rezul'taty fotodinamicheskoi terapii pri subretinal'nykh neovaskuliarnykh membranakh po dannym fliuorestsentnoi angiografii i opticheskoi kogerentnoi tomografii.

We studied the possibilities of the method of photodynamic therapy (PDT) in the treatment of subretinal new-vascular membranes. The results of PDT in 28 patients (28 eyes) with subretinal new-vascular membranes of different etiologies were evaluated by using the techniques of fluorescent angiography and optic coherent tomography. A reduced membrane area was noted in 17 patients (17 eye), and an improved visual acuity was registered in 7 patients (7 eyes). The use of PDT makes it possible, in a number of cases, to prevent the reduction of visual functions and even to improve them in patients with subretinal new-vascular membranes. The use of diagnostic opportunities of fluorescent angiography and optic coherent tomography is an indispensable part belonging to the monitoring of the conducted treatment.

Cancerous hyper-mutagenesis in p53 genes is possibly associated with transcriptional bypass of DNA lesions.

The database of tumor-associated p53 base substitutions includes about 5% of tumors with two or more base substitutions. These multiplet base substitutions in one tumor are evidence for hyper-mutagenesis. Our retrospective analysis of this database indicates that most multiplets arise from a single transient hyper-mutagenic event in one cell that subsequently proliferated into a clonal tumor. The hyper-mutagenesis, 1.8 x 10(-4) substitutions per base pair, is detected as multiple mutations in p53 genes of tumors. It requires one strongly tumorigenic p53 substitution, usually missense, called the driver mutation. The occurrence frequencies of ancillary base substitutions, those that hitch-hike along with the driver mutation, are independent of their amino acid coding properties. In this respect, they act like neutral mutations. In support of this neutrality, we find that the frequency distribution of hitch-hiking CpG transitions along the p53 exons, their mutational spectrum, approximates the spontaneous pre-selection mutational spectrum of most human tissues and is correlated with the mutational spectrum of p53 pseudogenes in mammalian germ cells. The driver substitutions of multiplets predominantly originate along the transcribed strand while the ancillary substitutions tend to originate along the non-transcribed strand. This data is consistent with a model of time-dependent mutagenesis in non-dividing stem cells for generating multiple strand-asymmetric p53 mutations in tumors. By transcriptional bypass of DNA lesions with concomitant misincorporation, transcriptional mutagenesis generates a transient mutant p53 mRNA. The associated mutant p53 protein could allow the host cell a growth advantage, release from G1-arrest. Then, during subsequent DNA replication and misreading of the same lesion, the damaged base along the transcribed DNA strand would serve as the origin of the p53 base substitution that drives the hyper-mutagenic event leading to tumors with multiple p53 mutations.

Human lung cancer and p53: the interplay between mutagenesis and selection.

It is an almost consensus opinion that the major carcinogenic risk of tobacco smoke is in its direct mutagenic action on DNA of cancer-related genes. The key data supposedly linking smoke-induced mutations to lung cancer were obtained from the adduct spectrum of the p53 tumor suppressor gene. Results of our analysis of p53 mutations compiled from the International Agency for Research on Cancer p53 database (April 1999 update) and from the literature point to a different causative link. Our new analytical tests focused on complementary base substitutions and showed that it is strand-specific repair of primary lesions and site-specific selection of the resultant mutations that determine the lung cancer-specific hot spots of G:C to T:A transversions along the p53 gene and also their increased abundance in lung tissues as compared with smoke-inaccessible tissues. However, on each of the two strands of p53 DNA, our tests revealed no significant difference between smokers and nonsmokers, either in the frequency of different types of mutations or in the frequency of their occurrence along the p53 gene. Moreover, in both smokers and nonsmokers, there was the same frequency of lung tumors with silent p53 mutations. Accordingly, we offer here a selection-based explanation of why lung cancers with nonsilent p53 mutations are more common in smokers than in nonsmokers. We conclude that physiological stresses (not necessarily genotoxic) aggravated by smoking are the leading risk factor in the p53-associated etiology of lung cancer.

CpG transition strand asymmetry and hitch-hiking mutations as measures of tumorigenic selection in shaping the p53 mutation spectrum.

By the genetic code, the average protein perturbation expected from a CpG-->TpG transition is strand-specific and smallest when it originates with the C on the transcribed (noncoding) strand. To distinguish the effects of selection from mutagenesis, we measured strand asymmetry for CpG-->TpG transitions fixed in active p53 genes and pseudogenes during vertebrate evolution, and for p53 genes from human tumors with one (singlet) and two (doublet) p53 point mutations. Mutagenesis appears to generate the transitions symmetrically while selection usually acts asymmetrically being most sensitive to the larger protein perturbations. Tumorigenic selection acting on the central domain of the p53 gene appears exceptional in that it often senses gain of function amino acid substitutions whose altered function is unrelated to degree of protein perturbation. In doublets, the selection on some gain of function substitutions is relaxed as evidenced by a return to the transition strand symmetry.

Strand asymmetry of CpG transitions as indicator of G1 phase-dependent origin of multiple tumorigenic p53 mutations in stem cells.

In dividing cells, expression of mutations is DNA strand symmetric. Of all mutations originating de novo in nondividing cells, only those in the transcribed (noncoding) strand are immediately expressed in mRNA and protein. In contrast, any new mutation in the nontranscribed (coding) strand remains unexpressed until the cells enter S phase and begin proliferation. This previously unrecognized difference enables us to examine the cell cycle-dependent origin of multiple tumorigenic mutations in stem cells. The human p53 gene, which acts as a gatekeeper in the control of G1 to S phase transition, was chosen for the analysis. Of all multiple mutations contained in p53 databases, we have tested in detail CpG transitions. Three features of CpG sites dictate this choice: C --> T transitions at methylated mCpG are the direct product of mC deamination and are replication-independent; it is easy to identify the strand bearing a primary mC --> T event because C --> T on the transcribed strand appears as G --> A on the nontranscribed strand; and CpG transitions are the most frequent (as both singular and multiple occurrences) tumor-related p53 mutations. The origin of double nonsilent CpG transitions in nondividing cells predicts a significant excess of the heterostrand (C --> T, G --> A) doublets over the homostrand (C --> T, C --> T and G --> A, G --> A) doublets. For p53, we found such an excess. Based on this result, along with the results of three other tests reported here, we conclude that the majority of multiple p53 mutations from human tumors occurred in quiescent stem cells.