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A new phenomenon is detected in quasicrystals: it is shown that the so-called “exotic” properties of real quasicrystalline materials are due to the presence of metastable electron traps with the nonequilibrium population of levels in the ground state of the system.

Using transmission electron microscopy and X-ray diffraction, we established that the ordered η1-Al50Cu44Fe6 and φ-Al47.5Cu49.5Fe3 (Fmm2) alloys with nano-sized domain structure are formed by slowl...

Calorimetric and tunnel data for the icosahedral phases of the Al–Cu–Fe system have been jointly analyzed. It has been found that the field-dependent part of the tunnel conductance can be represented as the sum of elementary terms similar in nature to thermal Schottky anomalies. As a result, the features of the fine structure of tunnel spectra in the form of zero-bias anomalies, peaks, and humps can be due to the internal field emission and can indicate a wide distribution of two-level electron traps in the electronic structure of quasicrystals. It was previously assumed that these features constitute a direct image of the density of single-electron states of the conduction band.

A consistent reduction in the absolute value of the negative paramagnetic Curie temperature was found in a series of Al Cu Fe phases: β(CsCl)-solid state solution, noncanonical approximant (η-AlCu(Fe) phase), rational approximants (P1+P2-pentagonal phases) and icosahedral quasicrystal. For these Al Cu Fe phases, the decrease in the Curie temperature correlates with a reduction of the density of states at the Fermi level which was estimated from the low-temperature heat capacity measurements and rst-principles calculations. The observed correlation was related to the antiferromagnetic indirect exchange interaction (Ruderman Kittel Kasuya Yosida interaction) between the localized magnetic moments on Fe induced by the intrinsic structural defects in the Al Cu Fe phases. The weakening of Fe 3d Al s, p hybridization owing to the intrinsic structural defects such as vacancies, antisite defects and distortions of coordination polyhedrons is suggested to be the main mechanism of appearance of localized magnetic moments on Fe atoms.

Half-metallic ferromagnets and spin gapless semiconductors are promising materials for spintronic devices since a high degree of the spin polarization of charge carriers can be realized in such materials. Spin gapless semiconductors make it possible to combine the properties of half-metallic ferromagnets with semiconductor characteristics and to perform fine tuning of the energy gap value. The Mn2 MeAl (Me = Ti, V, Cr, Mn, Fe, Co, Ni) Heusler alloys can possess such features. We studied the electrical, magnetic and galvanomagnetic properties of the Mn2 MeAl (Me = Ti, V, Cr, Mn, Fe, Co, Ni) Heusler alloys from 4.2 K to 900 K and in magnetic fields up to 100 kOe. The features in the electronic and magnetic properties of Mn2MeAl Heusler alloys were observed, which can be a manifestation of the electronic energy spectrum peculiarities with occurrence of the half-metallic ferromagnet and/or spin gapless semiconductor states. © Published under licence by IOP Publishing Ltd. Russian Foundation for Basic Research, RFBR: 18-02-00739 Government Council on Grants, Russian Federation: 02, 211 Ministry of Science and Higher Education of the Russian Federation: AAAA-A18-1118020190095-4 Ural Branch, Russian Academy of Sciences, UB RAS: 18-10-2-37 The work was carried out within the state assignment of Ministry of Science and Higher Education of the Russian Federation (themes “Spin”, No. AAAA-A18-118020290104-2-2 “Magnet”, No. АААА-А18-118020290129-5 and “Quant”, No. AAAA-A18-1118020190095-4), supported in part by RFBR (projects No. 18-02-00739), the Complex Program of the UB RAS (Project No. 18-10-2-37) and the Government of the Russian Federation (Decree No. 211, Contract No. 02.A03.21.0006).

For icosahedral phases of the Al-Cu-Fe system, components of the electrical conductivity, magnetic susceptibility, Hall effect, and heat capacity associated with thermally induced charge carriers for the first time have been considered jointly over a wide temperature range. It has been shown that the full range of thermal effects can be understood in the framework of the unified concept, which is based on an inhomogeneous system of two-level electronic excitations. A model of the inhomogeneous electronic state and the mechanism of its formation with the dominant role of short-range order have been proposed.

Electron microscopy has revealed that, upon the formation of a stable icosahedral (i) AlCuFe phase, there arise a large number of planar defects. The revealed defects are nanometer-sized coherent intergrowths of the P1-pentagonal approximant. It has been found that the formation of these defects is the result of a nonequilibrium intermediate transformation. The intergrowths are located in the planes with the fivefold symmetry axis and give rise to an elastic-strain state of the i-phase. Analysis of the diffraction contrast has revealed the presence of phason and phonon atomic displacements due to the mismatch between the quasiperiodic (i-phase) and periodic (P1-phase) lattices. The formed structure exhibits a higher electrical resistance as compared to the i-phase and has been considered as a model state of the imperfect icosahedral phase with preferred phonon displacements.

This study was supported by the Russian Foundation for Basic Research (project no. 10-02-00602-a). The heat capacity of icosahedral quasicrystals Al63Cu25Fe12 and Al62Cu25.5Fe12.5 has been studied at high temperatures up to 1700 K, which is by almost 400 K higher than the melting point of the material. It has been shown that the melt exhibits an excess heat capacity with respect to that determined by the Dulong-Petit law and that is a direct extension of the excess heat capacity of the solid state. It has been concluded that the excess heat capacity is related, as a whole, to the short-range order in the quasicrystal structure. This circumstance allows the identification of the orbital hybridization as the most probable mechanism of formation of the pseudogap in the electronic structure of the quasicrystals.

Experimental evidence is presented which indicates that the total heat capacity in the Al63Cu25Fe12 icosahedral phase contains a contribution associated with inconstancy of the charge carrier concentration. The contribution has an oscillating temperature dependence and is supposedly traceable to two Schottky-like heat anomalies.

The precise measurements of the specific heat and the linear expansion coefficient of polygrain samples of the ordered icosahedral phase Al63Cu25Fe12 have been performed in the temperature range 1.8–400.0 K. The deviations from the Gruneisen law, according to which the temperature dependences of the lattice specific heat at constant volume and the linear expansion coefficient are identical to each other, have been analyzed. The proofs that the specific heat of the quasicrystals contains latent electronic and lattice contributions of the Schottky type have been obtained. The revealed contributions can be thermodynamic consequences of the fractal energy spectra.

This study was supported by the Russian Foundation for Basic Research (grant no. 07-02-01049). Results are given of an investigation of magnetic susceptibility, electrical resistance, and density of Al62Cu25.5 5Fe(12.5) alloy in the solid and liquid states. It is found that the behavior of the investigated properties at temperatures above the melting point is abnormal. The results are discussed assuming the existence of long-lived (metastable) microregions in the melts.

Transmission electron microscopy was used to investigate structural and phase transformations and defects of the icosahedral (ι) phase that is formed upon isothermal annealings (Tann = 550–700°C) of quenched quasicrystal-forming alloys Al61Cu26Fe13 and Al63Cu25Fe12 (β solid solution + ι phase). It has been established that in the Al63Cu25Fe12 alloy there occurs a reversible ι-R-approximant transformation, whereas in the Al61Cu26Fe13 alloy there is formed a single-phase ι structure with regions with a high density of randomly distributed planar defects (Tann = 550°C), which are partially annealed at 650°C. The observed defects are, mainly, ultrathin interlayers (“intergrowths” to 3–5 nm in thickness) on quasicrystal planes with A5ι axes with an imperfect decagonal structure. As the basic mechanism, the growth mechanism of the formation of defects during the β → i transformation is proposed. The role of the alloy composition and low-temperature β → 3C-phase transformation in the realization of this mechanism is discussed.

In the icosahedral phase of Al-Cu-Fe alloys in the temperature range above 300 K there has been found an interrelation between the heat capacity, magnetic susceptibility, and conductivity not characteristic of typical metals. It is supposed that these phases are systems whose properties at finite temperatures are predominantly determined by excited electrons rather than by electrons with a Fermi energy EF.

On the basis of the results of the experimental investigation of the correlations between the electric, magnetic, and galvanomagnetic properties, the nature of the electronic phenomena leading to the difference of icosahedral quasicrystals from typical metals and insulators (low conductivity and negative temperature coefficient of resistance, diamagnetism of the ground state and its decrease with increasing temperature, and existence of residual and thermally induced charge carriers) is discussed.

The electronic spectra of the valence band and core levels of the surface of polygrain alloys with the icosahedral structure and the β-(CsCl)-type solid solution of Al50Cu33Fe17 were investigated by X-ray photoelectron spectroscopy (XPS). The obtained XPS spectra of the Al62.5Cu25Fe12.5 alloy, in comparison with those of the crystalline Al50Cu33Fe17 alloy demonstrate narrowing and a decrease in asymmetry of the Fe2p core level and a decrease in the electron state density N(EF) near the Fermi level, features expected for the poorly conducting icosahedral phase. The XPS data are compared with the estimates of N(EF) based on the low-temperature specific heat measurements.

Electron microscopy has been used to study orientation relationships (ORs) and stable misorientations between corresponding directions of the icosahedral ι phase and the β phase (CsCl type structure) of the solid solution in quenched two-phase Al61Cu26Fe13 alloys with a dendritic structure. Calculated values of the azimuthal misorientations have been obtained for the directions of the reciprocal lattices of the ι and β phases in diffraction patterns A5l,[113]β; A2l, [111]A5β for the basic \(OR[110]_\beta \left\| {A5_l , [\bar 11\bar 1]_\beta } \right\|A2_l \) and close orientation conditions such as \([110]_\beta \left\| {A5_l , [\bar 110]_\beta } \right\|A2_l ; [11\bar 1]_\beta \left\| {A3_l , [\bar 110]_\beta } \right\|A2_l ; [111]_\beta \left\| {A2_l , [\bar 110]_\beta } \right\|A2_l \). Two possible mechanisms of stabilization of the azimuthal misorientations revealed are discussed; first, the realization of the \(OR[111]_\beta \left\| {A2_l , [\bar 110]_\beta } \right\|A2_l \) in quenched Al-Cu-Fe alloys and, second, the stability of the arising misorientations as a result of the formation of a layer with a modulated icosahedral structure at the boundaries between the β and gi phases.

By dilatometric experiments on rapidly quenched quasicrystalline Al–Cu–Fe, in the course of their annealing it is proposed that icosahedral (I-) phases with free carriers of different signs (either electrons or holes) have different senses of the volume changes upon ordering. The effect discovered serves as a direct experimental support that the structural defects in I-quasicrystals are electrically active centers. This circumstance allows the identification of intrinsic and extrinsic properties of I-phases and opens novel opportunities for understanding the physical properties of real quasicrystals.

The results of the experimental investigation the purpose of which was to determine the thermodynamic coefficients of quasicrystalline alloy in the limit of zero residual conductivity are presented. The experimental picture of properties is obtained which is in concordance with critical character of localized electron states.

The electric conductivity and magnetic susceptibility of the quasicrystalline alloy Al62Cu25.5Fe12.5 are studied in the temperature range from 3.9 to 1200 K. The results are analysed under the assumption that the thermal activation of carriers is the main temperature effect.

Electron microscopic investigations of the structural state have been carried out for the β(CsCl-type)-phase in the quenched quasicrystal-forming two-phase (ι + β) alloy Al 61 Cu 26 Fe 13 . We conclude that in the vicinity of compositions at which the icosahedral phase (i) is formed, at deviations from the composition Al 3 (Cu,Fe) 2 towards Al(Cu,Fe), the β(CsCl-type)-solid solution has a heterogeneous structure (a state similar to the diffuse incommensurable ω-phase in b.c.c.-solid solutions). This structure includes the β-matrix with fine regions of correlated ω-like displacements which in a certain way are interrelated with the τ 3 -short-range order of substitution. It is concluded that the β(CsCl-type)-phase of Al 61-x Cu 26+y Fe 13-z reveals structural signs typical for electron-stabilized solid solutions, and because of this, the β-phase demonstrates a dielectric behavior of its electronic properties. An assumption has been made that the regions with ω-displacements are involved in the structural transformation β-phase → ι-phase.

Gruneisen’s law according to which the temperature dependence of the lattice specific heat at constant volume is similar to that of the thermal expansion coefficient is checked experimentally for the icosahedral phase of Al63Cu25Fe12. The specific heat CP(T) and linear thermal expansion coefficient αL(T) are measured in the temperature interval 1.8–400K. It is found that Gruneisen’s law holds quite satisfactorily in the temperature range 200–400K. Significant deviations from this law are found at lower temperatures. These deviations are due to the presence of “excess” specific heat of the Schottky anomaly type. It is shown that this contribution is of electronic origin.

Experiments are performed to determine the magnetic susceptibility of the icosahedral phase itself in the alloy Al62Cu25.5Fe12.5 in the temperature range 3.9–1100 K. A new regularity is observed — the curves of the temperature dependences of the susceptibility and electrical conductivity are congruent.

The mechanisms of energy absorption by metallic alloys with long-range aperiodic lattice order and electronic properties of marginal metals are studied. The heat capacity and linear expansion coefficient of the Al63Cu25Fe12 icosahedral phase in the temperature range 300–1000 K are measured for the first time. Disagreement between the measured heat capacity and predictions made from the Debye model is found and analyzed. It is shown that the excess heat capacity observed at the temperatures of the experiment is fitted by Einstein’s function in the approximation T ΘE.

Experimental proofs are obtained for the fact that the intrinsic conductivity of quasicrystals at low temperatures satisfies the T3/2 law.

New results of dilatometric experiments with rapidly quenched Al-Cu-Fe quasicrystalline alloys in the course of their thermal annealing are presented. It is established that the icosahedral (I) phases with different types of carriers exhibit different signs of volume changes with ordering. The observed effect is a direct experimental proof of the fact that structural defects in icosahedral quasicrystals are electrically active centers.

This work was supported by the Russian Foundation for Basic Research, project no. 10-02-00602-a. The electric resistance, magnetic susceptibility, and specific heat of the icosahedral phases of the Al-Cu-Fe system have been examined in the melt region. It has been shown that the features of the properties of a homogeneous solid state, as well as correlations between these features, hold in melts up to temperatures above the melting point by several hundreds of degrees. The results indicate that the short-range order and orbital hybridization determine the mechanism responsible for the electronic spectrum and ultrahigh-resistance state of quasicrystals.

We present the first Raman investigation of icosahedral quasicrystals. Broad structured bands in the energy range up to ~ 500 $cm^{-1} have been observed in a series of AlFeCu, AlPdMn and AlPdRe systems. Original information on the vibrational density of states g(omega) was obtained for AlPdRe; for AlFeCu and AlPdMn estimated g(omega) shows a good agreement with the previous neutron results, but demonstrates finer structure. Strong increase in the parameter of electron-vibrational coupling for the low-energy vibrations and its correlation with changes in electronic conductivity has been observed in the series from AlFeCu to AlPdRe. This suggests the increase of the degree of localization for these vibrational excitations and involved electronic states., 5 pages, 5 figures

The temperature dependences of resistivity of a number of transition metal Ti-based alloys have been studied in detail. It is shown that the conductivity of these alloys consists of three additive contributions, viz. (1) a T-independent metallic part, (2) a thermally activated part which obeys the Mott T 1/4 law, and (3) a thermally activated part with T-independent activation energy. This coexistence of metallic and thermally activated conductivities is quite unusual and requires a theoretical explanation.

The superconducting transition temperature Tc′ the magnetic susceptibility χ at temperatures between 77 and 1300 K, the resistivity ϱ, and the Hall emf at temperatures between 77 and 300 K are measured for solid solutions Nb1—xTaxCy close to stoichiometry (1—y < 0.05). The concentration dependences of ϱ(x) and Tc(x) show that the empirical rules established earlier for binary equilibrium solid solutions of metals are also true for solid solutions of niobium and tantalum monocarbides and can be explained by the negative contribution of the impurity correction λimp ∼ x(1—x) to the electron—phonon interaction parameter λ. Die Supraleitungsubergangstemperatur Tc′ die magnetische Suszeptibilitat χ bei Temperaturen zwischen 77 und 1300 K, der Widerstand ϱ und die Hall-EMK bei Temperaturen zwischen 77 und 300 K werden fur Nb1—xTaxCy-Mischkristalle in der Nahe der Stochiometrie (1—y < 0,05) gemessen. Die Konzentrationsabhangigkeit von ϱ(x) und Tc(x) zeigt, das die fruher fur binare feste Gleichgewichtslosungen von Metallen aufgestellten empirischen Regeln auch fur feste Losungen von Niobium- und Tantalmonokarbide gultig sind und durch den negativen Beitrag der Storstellenkorrektur λimp ∼ x(1—x) zum Elektron—Phonon-Wechselwirkungsparameter λ erklart werden konnen.