Stress-optical studies of VI B transition metal dichalcogenides

The effect of a uniaxial stress on the excitonic optical spectra are studied for MoS₂, WS₂, MoSe₂ and WSe₂. Stress dichroism appears in the A′, B′ excitons in diselenides, while it is absent in the A, B excitons in the four compounds. The A′, B′ excitons shift oppositely to the A, B excitons, indicating that A, B and A′, B′ are not pair excitons split by interlayer interaction.     

Optical transitions in group V transition metal dichalcogenides

A comparison of the correspondent structures of thermoreflectance spectra in NbSe₂, TaSe₂, NbTe₂ gives indications in favour d-d transitions as responsible of the main features of the optical spectra at relatively low energies. Besides, a temperature broadened saddle point in the joint density of states in NbSe₂ is identified.     

Chemical bonding and structure in layered transition metal dichalcogenides

Recent infrared reflectivity measurements on the transition metal dichalcogenides show striking differences among these compounds. The consequences of these differences on their band structures is discussed.     

Phonon dispersion curves of VI B group metals

The phonon dispersion curves of chromium, molybdenum and tungsten have been computed along the principal symmetry directions using an angular force model with volume forces. The results obtained are in very good agreement with the experimental observations.The author is grateful to Dr. V. Ramamurthy of I.I.T. Delhi for many valuable and stimulating discussions.     

Cointercalation of titanium dichalcogenides with transition metals and copper

Physics of the Solid State - Сointerсalated materials are studied, obtained by introducing copper into a TiSe2 lattice preintercalated with transition metals M = Mn, Fe, Co, or Ni. The...     

Structural phase transition in mixed IV–VI compounds

On the basis of a linear chain model and CPA, structural phase transition in mixed IV–VI compound A_χB_1?χTe (where A and B stand for Pb, Sn or Ge) is discussed. It is suggested that observed non-linear nature of T_c?χ relation may be attributed to the change in the electronic state with χ.     

Charge-density waves and superlattices in the metallic layered transition metal dichalcogenides

The d1 layer metals TaS 2 , TaSe 2 , NbSe 2 , in all their various polytypic modifications, acquire, below some appropriate temperature, phase conditions that their electromagnetic properties have previously revealed as 'anomalous'. Our present electron-microscopic studies indicate that this anomalous behaviour usually included the adoption, at some stage, of a superlattice. The size of superlattice adopted often is forecast in the pattern of satellite spotting and strong diffuse scattering found above the transition. Our conclusions are that charge-density waves and their concomitant periodic structural distortions occur in all these 4d 1 /5d 1 dichalcogenides. We have related the observed periodicities of these CDW states to the theoretical form of the parent Fermi surfaces. Particularly for the 1T octahedrally coordinated polytypes the Fermi surface is very simple and markedly two-dimensional in character, with large near-parallel walls. Such a situation is known theoretically to favour the formation of charge and spin-density waves. When they first appear, the CDWs in the 1T (and 4Hb) polytypes are incommensurate with the lattice. This condition produes a fair amount of gapping in the density of states at the Fermi level. For the simplest case of 1T-TaSe 2 , the room temperature superlattice is realized when this existing CDW rotates into an orientation for which it then become commensurate. At this first-order transition the Fermi surface energy gapping increases beyond that generated by the incommensurate CDW, as is clearly evident in the electromagnetic properties. For the trigonal prismatically coordinated polytypes, CDW formation is withheld to low temperatures, probably because of the more complex band structures. This CDW state (in the cases measured) would seem at once commensurate, even though the transition is, from a wide variety of experiments, apparently second order. A wide range of doped and intercalated materials have been used to substantiate the presence of CDWs in these compounds, and to clarify the effect that their occurrence has on the physical properties. The observations further demonstrate the distinctiveness of the transition metal dichalcogenide layer compounds, and of the group VA metals in particular.     

Electron and positron characteristics of dioxides of transition metals of group IV

The linear method of MT-orbitals is used to perform a self-consistent calculation of the band structure of dioxides of titanium, zirconium, and hafnium. Calculations are performed to determine total and partial densities of state, partial charges, total energy, and optical conductivity. Quasi-free positron states, positron lifetime, and contributions to the process of the annihilation of different electronic states are computed, and a study is made of the effect of oxygen vacancies and substitutional metallic impurities (Y, Mg) on the positron characteristics of cubic zirconium dioxide.Institute of the Physics of Strength and Materials Science, Siberian Branch, Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 97–104, September, 1993.     

Phonon dispersion curves of V B group of metals

The phonon frequencies of vanadium, niobium and tantalum along the principal symmetry directions are computed using an angular force model. The calculated dispersion curves are in very good agreement with the experimental results.The author is grateful to Dr. V. Ramamurthy and Dr. S. B. Rajendraprasad of IIT Delhi for many valuable and stimulating discussions.     

Application of one-band concepts to the diborides of group IV transition metals

Results are presented on the electrical conductivity, thermal emf, thermal conductivity, Hall coefficient, and Ettingshausen-Nernst coefficient for the diborides of Ti, Zr, and Hf. The one-band model is used to calculate the concentration, mobility, effective mass, Fermi energy, etc for the conduction electrons. It is concluded that this model is applicable to these compounds.     

Structural dependence of the superconducting transition temperature in liquid-quenched alloys based on the group IV B elements

The structural modifications and superconducting transition temperatures in TiPd, ZrPd, ZrMo, and HfPd thin-foil alloys (containing up to ～95 at.% of the group IV B elements) obtained from the liquid state have been investigated. Contrary to some previously reported results, a ‘single phase’ of the body-centered cubic β structure cannot be retained in alloys containing more than 90 at.% of the IV B elements at room temperature. The present findings provide a more reliable assessment of the transition temperatures for the cubic β phase.     

Photoacoustic saturation spectra of some semiconducting group IV-A transition metal dichalcogenides

Photoacoustic saturation spectra in some semiconductors as HfSe₂, ZrS₂ and HfS₂ have been measured at energies above the fundamental absorption edge, where the photoacoustic signal is independent of the optical coefficient and the dips detected in the photoacoustic spectra may be ascribed to optical reflection effects inherent to the band structure. The comparison between our experimental data and those obtained by means of conventional and modulatory spectroscopic techniques proves that such method is a useful tool in order to determine the electronic structure of photoacoustically active semiconductors.     

Temperature dependence of specific electrical resistivity of germanides of transition metals groups IV–VI

The temperature dependence of specific electrical resistivity of germanium phases of Ti, Zr, Hf, V, Nb, Ta, Cr, and Mo was determined. The study attempts to explain the temperature course of the curves of electrical resistivity of germanides by the low location of Fermi level, i. e., by the state of degeneracy of conduction electrons.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 59–64, May, 1972.     

Lithium intercalation complexes of layered transition metal dichalcogenides: An NMR survey of physical properties

NMR studies of lithium intercalation complexes of layered disulfides and diselenides of Group IVb and Vb transition metals establish that the inclusion of a guest lithium atom between the host dichalcogenide layers is accompanied by the donation of approximately one electron to the host. Measurements of the electrostatic and magnetic interactions between the lithium atom and the host and the rate of lithium atom motion are discussed.     

Anomalous resistivity in structural phase transition of IV–VI

The contribution of a carrier-soft optical phonon interaction to the electrical resistivity of IV–VI compounds is discussed. The calculated resistivity based on a soft mode theory shows a large increase near the transition temperature T_c. It is foound that the observed increase of resistivity of p-SnTe is reproduced by the carrier-soft TO phonon interaction.     

Calculation of oscillator strengths for P III,S IV,Cl V,and Ar VI

We use an analytic potential model adjusted to experimental energy levels to generate wave functions for the valence and excited states of the isoelectronic ions P III, S IV, Cl V, and Ar VI of the aluminum sequence. Using these wave functions in conjuction with the Born approximation and the LS-coupling scheme, we calculate optical oscillator strengths for various excitations of the valence electron. Good agreement is obtained with experimental and theoretical data.     

Experimental transition probabilities in N III,N IV and N V spectra

We have obtained transition probabilities (Einstein's A values) of thirteen transitions in doubly (N III), six in triply (N IV) and two in four times (N V) ionized nitrogen spectra belonging to the 3s-3p and 3p-3d transitions using a relative line intensity ratio (RLIR) technique. The linear low-pressure pulsed arc was used as an optically thin plasma source operated at 51 400 K electron temperature and 2.2×10²³ m^-3 electron density in nitrogen plasma. Our A values are compared to recent theoretical and experimental data. Received 18 December 2001 / Received in final form 29 January 2002 Published online 28 June 2002     

Structural instabilities and transverse effective charges in IV–VI compounds

Transverse effective charges for some IV–VI, III–V and II–VI compounds are investigated on the basis of the interband electron-phonon coupling with the use of the Penn model for electron energy bands. The results show that the large transverse effective charges of IV–VI compounds are related with strong electron-phonon interaction as well as with the softening of TO phonon in IV–VI compounds.     

The influence,on lithium electrochemical intercalation,of bond ionicity in layered chalcogenophosphates of transition metals

In the course of lithium electrochemical intercalation in the host structure of layered M∥PX₃ phases (M = V, Mn, Fe, Co, Ni, X = S, Se), it was shown that the best energy yield was obtained from low ionicity bond materials. The absorption edge energy, along with the free energy of the intercalation reactions have been correlated in a satisfactory way to the ionicity fi of the M-X bonds. These diagrams indicate which phases have to be looked at to obtain the maximum electrochemical yields.