Low temperature specific heat of intercalation compounds M x TiS2 (M = 3d transition metals)

Specific heats of intercalation compounds Fe_x TiS₂ (x = 0–1/3) and M_1/4 TiS₂ (M = Mn, Fe, Co and Ni) have been measured in the temperature range 0.35–5 K using an ac calorimetry method. The temperature dependence of the specific heats for these materials is written as the sum of electronic, lattice and anomalous terms. The anomalous specific heats show a Schottky type behavior, which is due to a very small amount of localized 3d atoms with crystal-field splittings, but the remaining majority of the intercalated guest 3d atoms are regarded as having itinerant electron character that contributes to the enhancement of the density of states at the Fermi energy or of electronic specific heat coefficient.     

The effects of bismuth intercalation on structure and thermal conductivity of TiS2

The structure and thermal conductivity of the bismuth (Bi) intercalated compounds Bi_xTiS₂ (0 ≤ x ≤ 0.25) were investigated by using X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and thermal conductivity measurements. The results indicated that besides lattice expansion and distortion, bismuth intercalation caused structural transition of Bi_xTiS₂ from stage-1 to stage-2 as x ≥ ∼0.1, which led to the appearance of D₄ and A₂ modes in Raman spectra. The enhancement of relative intensities of D₄ and A₂ peaks with increasing Bi content reflected increase of the concentration of stage-2 phase in the samples. The red shift of mode E_g as well as D₄ and A₂ would reflect weakening of intra-layer bonds, while the blue shift of A_1g after intercalation suggested the enhancement of chemical binding in the van der Waals gaps due to charge transfer. In addition, the weakening of A_1g intensity can be explained by the lattice distortion produced by bismuth intercalation. Remarkable reduction in (lattice) thermal conductivity of titanium disulfide (TiS₂) through Bi intercalation was realized, which could be attributed to the phonon scattering by “rattling” of the intercalated bismuth atoms in the van der Waals gaps of TiS₂.     

Development of novel thermoelectric materials by reduction of lattice thermal conductivity

Abstract

Thermal conductivity is one of the key parameters in the figure of merit of thermoelectric materials. Over the past decade, most progress in thermoelectric materials has been made by reducing their thermal conductivity while preserving their electrical properties. The phonon scattering mechanisms involved in these strategies are reviewed here and divided into three groups, including (i) disorder or distortion of unit cells, (ii) resonant scattering by localized rattling atoms and (iii) interface scattering. In addition, we propose construction of a ‘natural superlattice’ in thermoelectric materials by intercalating an MX layer into the van der Waals gap of a layered TX₂ structure which has a general formula of (MX)_1+x(TX₂)_n (M=Pb, Bi, Sn, Sb or a rare earth element; T=Ti, V, Cr, Nb or Ta; X=S or Se and n=1, 2, 3). We demonstrate that one of the intercalation compounds (SnS)_1.2(TiS₂)₂ has better thermoelectric properties compared with pure TiS₂ in the direction parallel to the layers, as the electron mobility is maintained while the phonon transport is significantly suppressed owing to the reduction in the transverse phonon velocities.     

Magnetic scattering of conduction carriers in 3d transition-metal intercalates of M x TiS2 (M=Mn,Fe, Co,and Ni)

Galvanomagnetic measurements on layer-structured 3d transition-metal intercalates of M_xTiS₂ (M = Mn, Fe, Co, and Ni;x0.33) have been made over the temperature range 0.34–20 K. Resistivity minima and negative magneto-resistances are observed for paramagnetic Mn, Fe, and Ni intercalates with low guest concentrations (x0.1), as found in well-known dilute alloys. However, in the case of the weak-ferromagnetic phase of Co_xTiS₂ (0.10x0.33) with the Curie temperatureT _c =120–140 K, such anomalous behaviors are also observed, which is not commonly found in the magnetically ordered phases of various magnetic materials. With these data we have given qualitative discussions on the magnetic scattering process of conduction carriers. Further, some of the experimental results are discussed in connection with the band calculations for M_1/3TiS₂.     

Dielectric properties of perovskite crystals

The soft mode dynamical model has been used to study the dielectric properties of Perovskite-type crystals. The model Hamiltonian proposed by Pytte has been modified and designed in terms of creation and annihilation operators. The correlations appearing in the dynamical equation have been evaluated using double time thermal retarded Green’s function and Dyson’s equation. Without any decoupling the higher order correlations have been evaluated using the renormalized Hamiltonian and thus, all possible interactions among phonons have been taken into account. The expressions for phonon frequencies and widths have beenMcalculated. Using appropriate parameters the softening of different modes at different transition temperatures give rise to a series of transitions from cubic to tetragonal, orthorhombic or trigonal phases. The significantly temperature-dependent modes are considered responsible for damping constant, dielectric constant, tangent loss and attenuation constant for these crystals. The dielectric properties are directly related to the optical phonon frequencies and widths and acoustic attenuation to the acoustic and optical phonon widths. Using suitable approximations, the model explains the experimental results on dielectric properties and acoustic attenuation reported for LiNbO₃, SrTiO₃, BaTiO₃ and LaAlO₃.     

The verification of the existence of TiS2

The intrinsic electronic properties of TiS₂ are currently under some debate. It has not been conclusively demonstrated if TiS₂ is a semimetal or a degenerate semiconductor. We have thoroughly studied this problem by first characterizing the material composition and then determining the electronic properties. Precision x-ray diffraction, mass density, and chemical analysis have been used to characterize TiS₂. Trends in the Ti_xS₂ series of nonstoichiometric compounds have been monitored by intercalation, scanning electron microscope, magnetic susceptibility and electron transport measurements. We conclude from the total body of data collected that TiS₂ exists as a stoichiometric compound and that its semimetallic properties are intrinsic.     

Development of novel thermoelectric materials by reduction of lattice thermal conductivity

Thermal conductivity is one of the key parameters in the figure of merit of thermoelectric materials. Over the past decade, most progress in thermoelectric materials has been made by reducing their thermal conductivity while preserving their electrical properties. The phonon scattering mechanisms involved in these strategies are reviewed here and divided into three groups, including (i) disorder or distortion of unit cells, (ii) resonant scattering by localized rattling atoms and (iii) interface scattering. In addition, we propose construction of a ‘natural superlattice’ in thermoelectric materials by intercalating an MX layer into the van der Waals gap of a layered TX₂ structure which has a general formula of (MX)_1+x(TX₂)_n (M=Pb, Bi, Sn, Sb or a rare earth element; T=Ti, V, Cr, Nb or Ta; X=S or Se and n=1, 2, 3). We demonstrate that one of the intercalation compounds (SnS)_1.2(TiS₂)₂ has better thermoelectric properties compared with pure TiS₂ in the direction parallel to the layers, as the electron mobility is maintained while the phonon transport is significantly suppressed owing to the reduction in the transverse phonon velocities.     

Lattice dynamics and phonon dispersion in Hg2Br2 model ferroelastic crystals

Interatomic force constants have been determined, acoustic and optical phonon frequencies have been calculated, phonon dispersion curves have been constructed, and the density of states in the phonon spectrum has been obtained for Hg₂Br₂ model ferroelastic crystals. The effect of hydrostatic pressure on the acoustic and optical phonon frequencies and their dispersion has been theoretically analyzed. It is established that an increase in the pressure leads to significant softening of the slowest acoustic TA branch (soft mode) at the X point of the Brillouin zone, which agrees with the phenomenological Landau theory of phase transitions and is consistent with the available experimental data.     

Lattice dynamics, force constants, and phonon dispersion in model ferroelastics Hg2I2

The dispersion dependences of the frequencies of acoustic and optical phonons have been calculated and plotted, and the density of states of the phonon spectrum of Hg₂I₂ crystals has been obtained. The influence of hydrostatic pressure on the frequency of acoustic and optical phonons and their dispersion has been theoretically analyzed. The slowest acoustic TA branch (soft mode) at the X point of the Brillouin-zone boundary exhibits strong softening with an increase in pressure.     

Point-contact spectroscopy for itinerant-magnetic material of Fe x TiS2

The nonlinear current-voltage characteristics of a homojunction point contact of 1T-CdI₂-type layered crystal TiS₂ and its intercalation compound of itinerantmagnetic Fe _x TiS₂ have been measured at 1.4 K. For the magnetic samples, both a maximum and a minimum in thedI/dV-V curves are observed within a restricted range of the initial differential resistancesR _d(0). Electron-magnon scatterings are responsible for the appearance of the minimum at a bias voltageV _B(min), which depends onR _d(0). These results are discussed using the conventional spherical spreading out model, together with a heating model. The agreement between experiment and theory is apparently good if a large background is taken into account, but with unusually large andR _d(0)-dependent Lorenz number.     

Supporting electrolytes containing tetrabutylammonium ions in Li/TiS cells intercalation of tetrabutylammonium ions in TiS

In context with the development of $\text{Li}\text{TiS}{}_2$ cells, the role of tetrabutylammonium bromide (TBAB) was investigated as Supporting Salt in LiSCN-1:3 dioxolane organic electrolytes. TBAB had little effect on the overall conductivity. A deterioration of cell performance was noted upon addition of TBAB to the electrolyte in spite of an improvement in the lithium half-cell. This behavior is attributed to the intercalation of TBA⁺ ions which have been shown in independent experiments to insert into TiS₂ to form the new compound TBA_x⁺ TiS₂.     

Coherent Optical Phonon Oscillations in GaMnAs

Coherent phonon oscillations are investigated in a series of Ga_1–xMn_xAs layers with different Mn and hole concentrations. The longitudinal optical (LO)-phonon-hole-plasmon coupled mode oscillations are observed in layers with large Mn concentrations. The energies of these coupled modes depend on the hole concentrations, which may prove useful for determining this parameter in ferromagnetic alloys. Finally, electron-plasmon coupling with LO phonon, observed under optical excitation, is compared in GaMnAs and in LT-GaAs.     

Effect of doping concentration on normal state resistivity of La2 −x (Ba,Sr) x CuO4 superconductors

Based on free electron layered electron gas model of quasi two dimensional CuO₂ layers in La(Ba/Sr)CuO superconductors a model potentialV(q) is developed earlier with the electron-electron and electron-phonon interactions. The model approach facilitates the dielectric functions and the dispersion relations of 2D acoustic phonon and plasmon modes. We have then worked out the coupling strength (γ) linking electrons to the 2D acoustic phonon mode (ħω_) from the residue at the pole ofV(q). Furthermore, the scattering time (τ _e−ph) during electron-phonon interaction (EPI) for this simplified system is also estimated. The contribution to the normal state in plane resistivity due to EPI is then evaluated. Finally, the variations ofτ andρ is studied with the doping concentration (x) and temperature (T) and the results obtained by us show reasonably good agreement with the available experimental data.     

Lithium intercalation in TiS2

We report the intercalation of lithium in Li_XTiS₂ corresponding to 0 ≤ x ≤ 3. Intercalation to x = 3 results in a new phase whose charge-discharge behavior displays hysteresis. This is in contrast to the behavior of Li_xTiS₂ for 0 ≤ x ≤ 2.     

SrTi1−x Fe x O3 nanoparticle: a study of structural,optical, impedance and magnetic properties

We investigate the effects of Fe-dopant concentration on the structure, as well as optical, electrical transport and magnetic properties of SrTi_{1? x} Fe _x O₃ (x?=?0.00, 0.10, 0.20, 0.30, 0.40 and 0.50) nanoparticles prepared by sol–gel method. The dopant-induced changes are studied by X-ray diffraction (XRD), Raman, optical absorption, impedance and magnetic measurements. The results show an average particle size of about 15–30?nm, depending on the Fe-doped concentration. The decrease in lattice parameters and the change of phonon modes related to structural changes, decreasing gap with increasing dopant concentration in conjunction with increasing grain boundary contribution to the impedance. The Fe-doped content has affected the structure, absorption and Raman spectroscopy of the samples. These indicate that the Fe ion has replaced the site of Ti in unit cell. By this method, we have decreased the annealing temperature considerably than that in the conventional solid-state reaction.     

Optical phonons and symmetry of Hg<Subscript>2</Subscript>F<Subscript>2</Subscript>

We have experimentally observed even optical phonon modes in the Raman spectra of mercury(I) fluoride (Hg₂F₂) polycrystals. Based on the investigation of X-ray diffraction patterns and group-theory analysis, D _4h ¹⁷ symmetry is assigned to these crystals and the selection rules for their vibrational spectra are established. The results are interpreted and discussed in comparison to the spectra of isomorphous Hg₂Hal₂ (Hal = Cl, Br, I) analogs.     

Superconductivity and electrical resistivity in alkali metal doped fullerides: Phonon mechanism

We consider a two- peak model for the phonon density of states to investigate the nature of electron pairing mechanism for superconducting state in fullerides. We first study the intercage interactions between the adjacent C₆₀ cages and expansion of lattice due to the intercalation of alkali atoms based on the spring model to estimate phonon frequencies from the dynamical matrix for the intermolecular alkali- C₆₀ phonons. Electronic parameter as repulsive parameter and the attractive coupling strength are obtained within the random phase approximation. Transition temperature,T _c, is obtained in a situation when the free electrons in lowest molecular orbital are coupled with alkali-C₆₀ phonons as 5 K, which is much lower as compared to reportedT _c (≈ 20 K). The superconducting pairing is mainly driven by the high frequency intramolecular phonons and their effects enhance it to 22 K. To illustrate the usefulness of the above approach, the carbon isotope exponent and the pressure effect are also estimated. Temperature dependence of electrical resistivity is then analysed within the same model phonon spectrum. It is inferred from the two- peak model for phonon density of states that high frequency intramolecular phonon modes play a major role in pairing mechanism with possibly some contribution from alkali-C₆₀ phonon to describe most of the superconducting and normal state properties of doped fullerides.     

Symmetry, mechanism of the pairing, and the isotope effect in the cuprates

This paper is concerned mainly with the problem of the origin of highT _c .s-Wave vs.d-wave pairing, the isotope effect, and the strong coupling of the carriers to low-frequency optical phonon modes are discussed. This latter strong coupling is sufficient to provide highT _c .The work of VZK was sponsored by the U.S. Office of Naval Research Contract No. N00014-92-F0006.     

The Interplay of Phonon and Magnetic Mechanism of Pairing in Strongly Correlated Electron System of High-<Emphasis Type="Italic">T</Emphasis><Subscript><Emphasis Type="Italic">c</Emphasis></Subscript> Cuprates

We consider magnetic mechanism of superconducting pairing in the effective low energy t−t′−t″−J ^* model with all parameters calculated ab initio. Interaction of strongly correlated electrons with different phonon modes is also incorporated. In a BCS type theory, the d_x²-y²d_{x^{2}-y^{2}} gap is given by a sum of magnetic and phonon contributions. The main contribution to the only fitting parameter G is determined by a competition of the breathing and buckling modes. Fitting the parameter G from the isotope effect, we obtain that magnetic and phonon contributions to the critical temperature T _c work together and are of the same order of magnitude.     

A nonlinear dynamics of stimulated phonon emission in a nonautonomous acoustic quantum generator under superlow-frequency-modulated pumping conditions

A nonlinear resonance was experimentally observed in a ruby acoustic quantum generator operating in the region of 10¹⁰ Hz with electromagnetic pumping modulated at a superlow frequency. The resonance is manifested by slow regular self-detunings in the microwave spectra of stimulated phonon emission. The self-detuning period T _SD strongly depends on Δ_L≡ω_m-ω_L, where ω_m is the modulation frequency and ω_L is the resonance frequency varying from 9.8 to ∼5 Hz when the magnetic field detuning grows from 0 to 60 Oe. The direction of motion of a mode cluster along the frequency axis is uniquely determined by the sign of Δ_L. As the |Δ_L| value decreases to 0.05 Hz, the self-detuning period increases to very large values T _SD > 100 s. These large-scale collective motions take place against the background of small-scale low-frequency chaotic oscillations in intensity of the generated phonon modes, while the mode widths remain almost as narrow (<1 kHz) as those in the autonomous regime.