Narrow Conduction Band Research Articles

Suppression of electron-phonon interaction in narrow-band crystals

Taking the inelastic nature of the interaction of electrons with acoustic phonons into account is crucial in crystals having a conduction band width Δɛ comparable to the maximum acoustic phonon energy\(\bar \varepsilon \). In view of this, the laws of conservation of energy and wave vector impose stringent constraints on possible electron scattering processes, and single-phonon scattering becomes impossible for\(\Delta \varepsilon< \bar \varepsilon \). The phonon contribution to the resistance may be negligible, therefore, in narrow-band conductors.

Nonmagnetic molecular Jahn-Teller Mott insulators

Narrow-band conductors may turn insulating and magnetic as a consequence of strong electron-electron correlation. In molecular conductors, the concomitance of a strong Jahn-Teller coupling may give rise to the alternative possibility of a non-magnetic insulator, with or without a static cooperative Jahn-Teller distortion. In the latter case the insulator has Mott-like properties, with an interesting interplay between electron-electron repulsion and the Jahn-Teller effect, which is dynamical. We study this kind of non-magnetic insulator in a very simple $E\otimes e$ Jahn-Teller model and we discuss its general properties in a more general context, also in connection with the insulating state of K_{4}C_{60} and Rb_{4}C_{60}.

Destruction of superconductivity in the narrow-band metalK3C60s

Transport data on ${\mathrm{K}}_{3}$ ${\mathrm{C}}_{60}$ prepared with different amounts of disorder are presented, indicating that as the disorder is increased the material undergoes a superconductor-insulator transition, a result which does not occur in conventional wide-band metals. We speculate that this behavior is a consequence of the narrow conduction band and low electron density in ${\mathrm{K}}_{3}$ ${\mathrm{C}}_{60}$ , which gives rise to an extreme sensitivity to disorder and correlation effects.

Alkali spin-orbit interactions and the width of the conduction-electron spin resonance in superconducting alkali-metal fullerides.

It is shown that conduction-electron spin relaxation in the superconducting alkali-metal fullerides in due to spin-orbit interactions within the alkali ion $p$ orbitals that are admixed into the ${\mathrm{C}}_{60}$-based conduction band states when these band states are orthogonalized to the alkali orbitals, as required to satisfy the Pauli exclusion principle. The calculated electron-spin-resonance (ESR) linewidths support experimental findings that the broader of the two ESR lines often observed in these compounds is due to conduction electrons. The anomalous increase in the width of this line with decreasing temperature is due to strong scattering in a narrow conduction band.

Swift, Heavy Ions in Insulating and Conducting Oxides: Tracks and Physical Properties

Transition metal oxides belong to the class of iono-covalent materials in which electrons are, to first order, localized in tight metal-oxygen bonds. They appear to be quite different from classical metals in which electrons are assumed to be delocalized in a free electron gas. Nevertheless, due to exchange interactions and hybridization of atomic orbitals, oxides exhibit many interesting physical properties like magnetic ordering and various electron transport properties that extend from wide gap insulators to narrow band conductors and high-temperature superconducting (HTS) superconductors. This class of materials appears well-suited to the study of the damage induced by heavy ion irradiation and its consequence on physical properties like electrical resistivity.In this article, we will describe the results of heavy ion irradiations in magnetic oxides, exhibiting a wide palette of electron transport properties from insulators to metals through hopping semiconductors and in HTS copper oxides that are narrow-band metals aboveTc. We will show that heavy ion irradiation, in addition to disordering matter, can induce changes in some physical properties like magnetic ordering and orientation of magnetic fields, and can produce large increases of the critical current. To account for these results, the thermal spike model will be considered as a possible way to understand the damage creation in iono-covalent compounds.

51V NMR Study of A15 Superconductor V3Si

The Knight shift K and the spin-lattice relaxation rate 1/ T 1 of 51 V nuclei, and the macroscopic magnetic susceptibility have been measured in an A15 superconductor V 3 Si. From the K -χ plot, the hyperfine coupling constant and the temperature independent Van Vleck term have been estimated. In the normal state, 1/ T 1 T decreases drastically with increasing temperature. In the superconducting state, 1/ T 1 shows no coherence peak just below T c (=17.1 K), and obeys the T 3 -law down to T c /3, but deviates from it with decreasing temperature. It is shown that the temperature dependences of χ and 1/ T 1 T are well reproduced by the numerical calculation based on the narrow-band model at Fermi level, and that the band width should be much smaller than that given by the band calculation of Mattheiss and Weber. T 1 T K 2 d varies with temperature, which should also be explained with the narrow conduction band.

Band properties of metallic corundum-phase V2O3

Band structure results for metallic corundum-phase V2O3 feature narrow ( approximately 2.7 eV) t2g conduction bands and a density-of-states peak at EF such that the heat capacity to band ratio is approximately 4.4. The predominant V(3d) character of conduction electrons suggests that transport properties will be especially sensitive to cation disorder. It is proposed that the structural and resistivity anomalies that occur in Cr-doped samples may arise from dopants that are interstitial (i.e., at NiAs-type cation sites which are vacant in the corundum structure) rather than substitutional.

Possible influence of the Fermi level position inside the conduction band of high-T c oxides on the temperature dependence of their resistivity

The systematic variation of the temperature dependence of the second temperature derivative of the electrical resistivity d 2ϱ dT 2 is reproduced by model calculations, assuming the existence of a narrow conduction band in the vicinity of the Fermi level E F. The analysis of the characteristic features on the d 2ϱ/d T 2 vs. T curve makes it possible to determine both the half-band width T 2=( ϵ 2- ϵ 1)/2 K B , where ϵ 1, ϵ 2 are the band edges, and the Fermi level position with respect to the middle of the band T 1=[ 1 2 (ϵ 1+ϵ 2)-E F]/K B . It is shown that d 2 ϱ/ dT 2 is negative in this model at certain temperatures below T 2 if the condition 0.563 T 2<| T 1|< T 2 is fulfilled, and positive at all temperatures if | T 1| is outside this interval, while dϱ/ dT>0 in both cases. This fact may be responsible for the complex behaviour of the ϱ( T) dependence caused by the change of composition during the metal-insulator transition in high- T c oxides.

Electron-Phonon Vertex Corrections and Superconductivity in Alkali-Metal-Doped C60 Solids

The narrow conduction band of C60 leads to the breakdown of the Migdal theorem and the McMillan formula is no longer applicable. In this letter, the second-order electron-phonon vertex correction is considered to explain the superconductivity in doped C60 solids and the transition temperature Tc thus obtained is in accordance with the experimental results.

Hole filling in a narrow conduction band in YBa2Cu3-xCoxO7- delta (x

Measurements of the temperature dependence of the absolute thermopower (S) on samples of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Co}}_{\mathit{x}}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ with 0\ensuremath{\le}x\ensuremath{\le}0.3 and \ensuremath{\delta}0.05 show that, between 100--200 K, S has a maximum that depends on x, according to the relation ${\mathit{S}}_{\mathit{m}}$(x)\ensuremath{\approxeq}(${\mathit{k}}_{\mathit{B}}$/\ensuremath{\Vert}e\ensuremath{\Vert}) ln[(1+x)/(1-x)]. This is the expected saturation value of S, for noninteracting carriers in a narrow conduction band with x doped electrons above half-filling. The temperature dependence of the resistivity (\ensuremath{\rho}) exhibits a crossover from wide-band to narrow-band behavior, in a conductor with a mean free path limited by impurities. The drop of S at higher temperatures, accompanied by a drop in d\ensuremath{\rho}/dT, can be attributed to excitation of electrons from the conduction band to higher-energy levels associated with the dopant, or to the contribution of activated hopping to the transport coefficients.

51V nuclear spin-lattice relaxation in an A-15 superconductor V 3Si

The temperature dependence of 51V nuclear spin-lattice relaxation rate 1/ T 1 was measured in a strong electron-phonon coupling superconductor V 3Si. In the normal state, 1/ T 1 T decreases drastically with increasing temperature, reflecting a narrow conduction band. 1/ T 1 presents T 3 dependence below T c .

Thin films of lithium cobalt oxide

Thin films of lithium cobalt oxide have been prepared by rf sputtering from crystalline LiCoO 2. Chemical analysis shows that the films as deposited are deficient in lithium. X-ray diffraction suggests that the films retain the basic rhombohedral structure of LiCoO 2. A plausible structure defect model for these lithium deficient films is that of a disordered α-NaFeO 2 structure. Little absorption and moderate reflectivity are found in the transmission and reflectance spectra in the infrared range. The visible absorption corresponds to a direct allowed optical transition with an energy gap of 2.2 eV. Electrical conductivity measurements of the films show that at 300 K the films behave as semiconductors, with an activation energy of 0.12 eV. Ionic conductivity does not contribute significantly to the measured conductivity. Thermopower measurements indicate that the films are p-type conductors and suggest a narrow conduction band.

Tunneling of Charged Particles through a Crystal with Narrow Conduction Band in the Presence of an Electric Field

The energy spectrum and tunneling probability are obtained for charged particle tunneling through a crystal with narrow conduction band in an electric field. It is also shown that the current–voltage characteristic of such a system has a region with negative differential resistance which does not arise in the usual way (due to a resonant level) but due to the small width of the conduction band. Also the role of surface interaction is investigated. [Russian text ignoured]

A narrow conduction band in YBa2Cu3O7−δ

LDA band-structure calculations of YBa2Cu3O7−δ show wide bands (of several eV) spanning EF. However, experiments, and especially transport measurements, contradict this and indicate the presence of a narrow conduction band (about a tenth of an eV wide).

The cuprate superconductors: Narrow correlated-electron bands and interlayer pairing via plane-chain charge transfer

The cuprate superconductors are modelled by two metallic CuO2planes, separated by insulating layers, in an extended Hubbard Hamiltonian. Hybridization of O(2p) and Cu(d) orbitals splits the wide bands of LDA theory, yielding a narrow conduction band of antibonding holes. Holes on the two CuO2 planes are correlated via interplane hopping, giving a non-magnetic normal Fermi liquid. Charge exchange between the planes and the intervening layers generates attraction and a BCS condensation.

Core-level lineshapes in photoemission from transition-metal intercalates of TaS2

The photoemission lineshapes of the Ta 4f levels in 2H-TaS2 and its intercalates Mn14/TaS2 and Co13/TaS2 have been studied using synchrotron radiation. The peaks show marked asymmetries because of the many-electron screening response to the core-hole potential. The different lineshapes are discussed in terms of the rigid band model of the electronic structure of the intercalates, and of the effects of charge transfer into the narrow conduction band on the many-electron response. Calculations of the lineshapes using a simple model for the conduction band density of states allow the experimental lineshapes to be understood in terms of intrinsic losses through multiple excitations within the narrow conduction band, together with some extrinsic plasma losses.

Negative magnetoresistance in magnetic semiconductors with narrow degenerate conduction bands

We propose a model to explain electrical conductivity anomalies in magnetic chalcogenide spinels near the Curie point. The model is based on strong electronelectron correlations between d-electrons in the narrow, orbitally degenerate conduction band. We demonstrate that in this system spin inhomogeneities produce scattering and lead to strong negative magnetoresistance. Experimental data on the electrical conductivity of the CdCr2Se4 compound in a magnetic field is compared to numerical computation. We explain the absence of magnetoresistance anomalies in p-type samples.

Ground-state properties of dilute magnetic alloys.

We have investigated the problem of impurity atoms in metals with a narrow conduction band of constant density of states. The chains of equations for the double-time finite-temperature Green's functions are solved by the Fredholm method of kernel for inhomogeneous integral equations. We thus obtain the correlations in motion of the electrons in the conduction band and the impurity state. Expressions for the ground-state energy and the average number of electrons in the impurity state have been obtained. The condition for localization in the impurity state at low temperature has been obtained. Magnetic susceptibility of the impurity shows a residual value at zero degree Kelvin, thus giving an estimate of the Kondo temperature. The specific heat at low temperature has a Schottky-type behavior.

Theory of Gunn effect in the narrow band systems

The exact theory of Gunn Effect in narrow band conductors (organic conductors, molecular crystals, polymers, superlattices, superionic conductors etc.) is developed using the Fokker-Plank kinetic equation. The exact analytical solutions for a moving domain structure are obtained and their stability is investigated. The analytical expression for a current-voltage characteristic of a moving domain structure is found.

Valence states and magnetic properties of LaNi1-xMnxO3 (for 0⩽x⩽0.2 and x=0.5)

Single-phase LaNi1-xMnxO3 samples in the compositional range 0&lt;or=x&lt;or=0.2 were prepared by the decomposition of precursor hydroxide solid solutions, and La2NiMnO6 was prepared from a mixture of carbonates. All the samples contained a disordered array of nickel and manganese ions. X-ray absorption and XPS measurements indicate the presence of Mn(III) and Ni(III) valence states at room temperature for x=0.1, 0.2 and 0.5. Magnetic susceptibility data suggest Mn4+ solute ions and Stoner-enhanced Pauli paramagnetism of the metallic solvent for x=0.01 with a smooth transition to Mn3+ solute ions and a spontaneously magnetised solvent conduction band at x=0.05. Below 200K, the x=0.05 sample forms superparamagnetic clusters, and below 40K there is evidence for an antiferromagnet spin-density wave. Comparisons with LaCo0.95Mn0.05O3 and La0.98Sr0.02CoO3 confirm that the long-range magnetic coupling occurs via solvent electrons in a narrow conduction band. The conductivity changes from that of a narrow-band metal for x&lt;0.01 to that more characteristic of diffusive motion for x&gt;0.05, but any motional enthalpy appears to remain small ( Delta Hm approximately=0). The x=0.1 sample exhibits ferrimagnetic spin glass behaviour below 40K, and the ferromagnetic interactions increase with manganese concentration. The oxide with x=0.50 is ferromagnetic with a well defined Curie temperature.