Microscopic theory of Raman spectra of heavy fermion systems in the normal state

In this communication, we develop the microscopic theory of Raman spectra of the heavy fermion (HF) system in its normal state at low temperatures. The system is described by the Periodic Anderson Model along with the coupling of the phonon to the bare f-electrons as well as to the hybridization between the conduction band and the f-electrons. The phonon Green's function and the phonon self-energy are evaluated by the equations of motion method of Zubarev. The phonon spectral density function (SDF) is evaluated at low temperatures in the long wavelength limit. The calculation depicts three Raman active modes one of which corresponds to the strongly renormalized phonon at the value of the reduced frequency ) around 0.57 denoted as (P₀), and the other two at low reduced frequencies of and in the SDF. The effect of electron–phonon (EP) coupling on Raman excitation peaks is investigated to probe the nature of the electronic states of the system.     

Phenomenology of antiferromagnetic metal-insulator transition in 3d transition metal dichalcogenides

A phenomenological theory is set up to describe the metal-insulator transition at T = 0 between phases which both exhibit cooperative antiferromagnetic ordering. A prediction which emerges relates the antiferromagnetic moment to Fermiology in the metal. Possible experiments on 3d transition metal dichalogenides are thereby suggested.     

Topological superconductivity in Janus monolayer transition metal dichalcogenides

The Janus monolayer transition metal dichalcogenides (TMDs) $MXY$ ($M={\rm Mo}$, W, $etc$. and $X, Y={\rm S}$, Se, $etc$.) have been successfully synthesized in recent years. The Rashba spin splitting in these compounds arises due to the breaking of out-of-plane mirror symmetry. Here we study the pairing symmetry of superconducting Janus monolayer TMDs within the weak-coupling framework near critical temperature $T_{\rm c}$, of which the Fermi surface (FS) sheets centered around both $ărGamma$ and $K (K')$ points. We find that the strong Rashba splitting produces two kinds of topological superconducting states which differ from that in its parent compounds. More specifically, at relatively high chemical potentials, we obtain a time-reversal invariant $s + f + p$-wave mixed superconducting state, which is fully gapped and topologically nontrivial, $i.e.$, a $\mathbb{Z}_2$ topological state. On the other hand, a time-reversal symmetry breaking $d + p + f$-wave superconducting state appears at lower chemical potentials. This state possess a large Chern number $|C|=6$ at appropriate pairing strength, demonstrating its nontrivial band topology. Our results suggest the Janus monolayer TMDs to be a promising candidate for the intrinsic helical and chiral topological superconductors.     

Analysis of IR and Raman spectra of transition metal telluromolybdates

The work presents the IR and Raman spectra in the range 400–4000 cm^?1 of simple and double salts of hexamolybdotelluric (VI) acid of the general formula M₃[TeMo₆O₂₄]·nH₂O and (NH₄)_2xM_3?x [TeMo₆O₂₄]·mH₂O respectively, and of substitutive telluromolybdates with the wolframite structure MTe_yMo_1?yO₄, where M = Co, Zn, Ni and Mn. In this range the modes have been assigned to stretching vibrations of appropriate Mo-O bonds. Approximate values of force constants for these bonds have been computed and compared with the literature values reported for transition metal molybdates and ammonium heptamolybdate.     

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.     

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.     

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.     

Microscopic mean-field theory of the jamming transition

Dense particle packings acquire rigidity through a nonequilibrium jamming transition commonly observed in materials from emulsions to sandpiles. We describe athermal packings and their observed geometric phase transitions by using equilibrium statistical mechanics and develop a fully microscopic, mean-field theory of the jamming transition for soft repulsive spherical particles. We derive analytically some of the scaling laws and exponents characterizing the transition and obtain new predictions for microscopic correlation functions of jammed states that are amenable to experimental verifications and whose accuracy we confirm by using computer simulations.     

Electron energy loss studies in solids; The transition metal dichalcogenides

The theory of characteristic electron energy losses is discussed in terms of the electronic band structure of a solid. The relationship between the observed plasmon energies, the average interband energy gap and the background dielectric constant of the solid is developed.

The transmission energy loss spectra of a number of the layer-type transition metal dichalcogenides, MX₂, where M=Zr, Hf, Nb, Ta, Mo and W and X=S and Se, have been measured in the range of 0–50 eV. In the experiments, a beam of 50 keV electrons is incident along the c-axis of the crystals and electrons inelastically scattered through an angle of 1 m radian are selected for energy analysis. This ensures that the momentum transfer and hence the electric vector for the excitations lies in the basal plane of the crystal (E⊥c).

Kramers-Kronig analysis has been applied to the energy loss data to deduce the complex dielectric function of each material. From this function, all other ‘optical’ constants, such as the reflectivity, and the oscillator integral function and joint density of states function have been calculated.

The results give substantial support to the existing band model for the family of materials and, in addition, provide the basis for a quantitative understanding of the band structure of individual compounds.     

Improving the device performances of two-dimensional semiconducting transition metal dichalcogenides: Three strategies

Two-dimensional (2D) semiconductors are emerging as promising candidates for the next-generation nanoelectronics. As a type of unique channel materials, 2D semiconducting transition metal dichalcogenides (TMDCs), such as MoS₂ and WS₂, exhibit great potential for the state-of-the-art field-effect transistors owing to their atomically thin thicknesses, dangling-band free surfaces, and abundant band structures. Even so, the device performances of 2D semiconducting TMDCs are still failing to reach the theoretical values so far, which is attributed to the intrinsic defects, excessive doping, and daunting contacts between electrodes and channels. In this article, we review the up-to-date three strategies for improving the device performances of 2D semiconducting TMDCs: (i) the controllable synthesis of wafer-scale 2D semiconducting TMDCs single crystals to reduce the evolution of grain boundaries, (ii) the ingenious doping of 2D semiconducting TMDCs to modulate the band structures and suppress the impurity scatterings, and (iii) the optimization design of interfacial contacts between electrodes and channels to reduce the Schottky barrier heights and contact resistances. In the end, the challenges regarding the improvement of device performances of 2D semiconducting TMDCs are highlighted, and the further research directions are also proposed. We believe that this review is comprehensive and insightful for downscaling the electronic devices and extending the Moore’s law.     

Raman spectra of IVB and VIB transition metal disulfides using laser energies near the absorption edges

In this paper, we present Raman spectra of ZrS₂, HfS₂, MoS₂ and WS₂ using laser energies near the energies of the absorption edges. The Raman spectra probe the properties of the first-excited electronic state and the nature of the electron-phonon coupling. The spectra of the IVB disulfides are independent of the laser excitation energy, suggesting weak electron-phonon interaction. In contrast, additional Raman bands appear in the spectra of the VIB disulfides as the laser energy approaches the band gap energy. The new modes in the spectra of MoS₂ and WS₂ cannot be assigned as first-order processes nor as combination bands of the phonons with zero momentum. The resonance Raman scattering of MoS₂ is analyzed in terms of second-order scattering due to the coupling of phonon modes of nonzero momentum with an electronic transition associated with excitonic states.     

Joint density of states and charge density wave in 2H-structured transition metal dichalcogenides

The joint density of states of two different 2H-structured transition metal dichalcogenides (TMDs) with and without charge density wave (CDW), Na_0.05TaS₂ and Cu_0.09NbS₂, respectively, are compared. While there is a clear maximum at the 3×3 charge density wavevector for Na_0.05TaS₂, the joint density of states for Cu_0.09NbS₂ does not show such behavior, consistent with the absence of CDW in the system. Our results illustrate that the joint density of states well represents the charge instability in 2D systems.     

Microscopic theory of covalent-ionic transition at metal-semiconductor interfaces

The microscopic mechanism responsible for the reduction of Schottky barrier heights at interfaces between metals and covalent semiconductors id not effective at similar interfaces between metals and ionic semiconductors. The critical polarizability ?_c at which the transition between one regime and the other takes place is calculated theoretically to be 7, in good agreement with experimental values of 5–6.     

Microscopic theory of covalent-ionic transition at metal-semiconductor interfaces

The microscopic mechanism responsible for the reduction of Schottky barrier heights at interfaces between metals and covalent semiconductors is not effective at similar interfaces between metals and ionic semiconductors. The critical polarizability ?_cat which the transition between one regime and the other takes place is calculated theoretically to be 7, in good agreement with experimental values of 5 to 6.     

Fundamental absorption spectra of transition metal chlorides

The optical absorption spectra of MnCl₂, FeCl₂, CoCl₂, and NiCl₂ have been measured over the energy range from 2 to 30 eV. The gross features of the spectra, especially broad bands above 10eV, are alike in all of the four materials. The charge transfer bands due to the electronic transitions from the 3p level of chlorine to the 3d and 4s levels of metal ions and the band due to the 3d → 4s transition are assigned in the spectra.     

Photoelectron spectra of some transition metal pentachlorides

He(I) and He(II) excited photoelectron spectra of the transition metal pentachlorides MCl₅, where M  Nb, Ta, Mo, W and Re, are reported. De