Table of contents

The low-temperature magnetic properties of the pyrochlore compound Dy₂Ti₂O₇ in magnetic fields applied along the [111] direction are reported. Below 1 K, a clear plateau has been observed in the magnetization process in the field range 2–9 kOe, followed by a sharp moment jump at around 10 kOe that corresponds to a breaking of the spin ice state. We found that the plateau state is disordered, with a residual entropy of nearly half the value for the zero-field state, whose macroscopic degeneracy comes from a frustration of the spins on the Kagomé layers perpendicular to the magnetic field.

We have investigated the quantum transport behaviour of Cu nanowires created by moving two macroscopic Cu wires into and out of contact. We have observed quantum conductance with steps of both e²/h and 2e²/h. We conclude that the spin degeneracy can be broken in non-magnetic Cu nanowires.

⟨001⟩-oriented 0.7Pb(Mg_1/3Nb_2/3)O₃–0.3PbTiO₃ single crystals were poled under two different electric fields, E_poling = 4 and 13 kV cm⁻¹. In addition to the temperature-dependent dielectric constant measurement, x-ray diffraction was also used to identify the poling-induced phase transitions. The results showed that the phase transition significantly depends on the poling intensity. The weaker field (E_poling = 4 kV cm⁻¹) was able to overcome the effect of the random internal field to produce the phase transition from the rhombohedral ferroelectric state with short-range ordering (microdomains), FE_SRO, to the rhombohedral ferroelectric state with long-range ordering (macrodomains), FE_LRO. But the rhombohedral ferroelectric-to-tetragonal ferroelectric phase transition originating from ⟨111⟩ to ⟨001⟩ polarization rotation could only be induced by the stronger field (E_poling = 13 kV cm⁻¹). The sample poled at E_poling = 4 kV cm⁻¹ showed a higher piezoelectric constant, d₃₃ ≥1500 pC N⁻¹, than the sample poled at E_poling = 13 kV cm⁻¹.

We have performed ab initio electronic structure and total-energy calculations for bcc, fcc, and hcp Al structures to study the equations of state, volume dependences of elastic constants, and relative stability diagram for these structures. A technique for elastic constant calculation in the case of initial isotropic pressure is presented. In this study we used the accurate full-potential linear muffin-tin orbital method to describe electrons of the crystal and the Debye treatment of the vibrating lattice. The volume dependence of the Debye temperature is derived from the volume dependence of the elastic constants. Our calculations show that at pressures of 1–2 Mbar and temperatures of about 1000 K and higher, the aluminium structure must have a lower symmetry than the structures considered.

We measured the transverse angular magnetoresistance and the Hall effect on high-quality NbSi₂ single crystals at low temperatures (4.2 K ≤ T ≤ 160 K) and high magnetic fields (B ≤ 20 T). The material behaves like a compensated metal, i.e. the magnetoresistance grows generally proportionally to B². For some current–field configurations, however, saturation of the magnetoresistance occurs, giving evidence for the presence of an open orbit on the Fermi surface of NbSi₂ parallel to the c axis. The Hall coefficient shows, as does the magnetoresistance, a low-field–high-field transition. It varies between −3.5 and −4.5 × 10¹⁰ m³ C⁻¹ in low magnetic field to −10.5 × 10⁻¹⁰ m³ C⁻¹ in the high-field region. We compare our results with those obtained for NbSi₂ thin films and discuss the validity of parameters deduced from transport data.

Band-structure calculations are performed for cubic Al_1−xGa_xN using the empirical pseudopotential method. The band gaps at Γ, X and L points and the electron effective masses of Γ and X valleys are calculated as a function of the gallium fraction x. It is found that there is no significant change in these electronic band parameters on taking into account the alloy disorder. On the basis of a model solid theory, we have calculated the band discontinuities for heterointerfaces between strained Al_{1 −x}Ga_xN and relaxed Al_1−yGa_yN. The latter calculations are extended to the whole range of compositions x and y. The information derived from this investigation will be useful in the design of lattice-mismatched heterostructures in blue-light optoelectronics applications.

Vanadium-doped semi-insulating Cd_1−xHg_xTe (x < 0.05) n-type crystals were grown by the Bridgman technique for the first time. Studies were carried out of the low-temperature optical and photoelectric properties of Cd_1−xHg_xTe (x = 0.014), which provided information on the deep anisotropic impurity centres and intrinsic defects. The nature and the position of their energy levels with respect to the crystal energy band were determined. It was shown that for the investigated crystals there are two photogeneration mechanisms of electrons from deep impurity levels: the auto-ionization of electrons from the discrete state, which is in resonance with the conduction band, and their direct photoionization. It was found that the photosensitivity region for Cd_0.986Hg_0.014Te:V crystals is about 1.3 μ m.

The cubic-to-rhombohedral (C–R) phase transition of the ferroelectric relaxor Pb(Zn_1/3Nb_2/3O₃) is investigated by means of a high-resolution x-ray diffraction study on single crystals. The phase transition is diffuse and spreads over the temperature range T_CR = 385 ± 5 K to T_R = 325 ± 5 K. Below T_CR, the cubic phase transforms progressively into rhombohedral domains whose average size, about 60–70 nm in the [111] direction, remains unchanged as temperature is lowered, so no growth of the R-domains is observed. At T_R, the nanometric R-domains fully pave the crystal but structural mismatches between adjacent domains generate stresses, which increase as temperature is lowered. The anomalous broadening of diffraction peaks of the R-phase, which originates from size and strain effects, can be suppressed by applying a dc electric field, along the [111] direction, which transforms the polydomain state into a rhombohedral quasi-monodomain state.

Our measurements of the magnetic susceptibility, resistivity and specific heat of CePd₂B₂C, a quaternary borocarbide, show clearly that Ce is in a stable trivalent state in this compound and orders antiferromagnetically at T_N ∼ 4.5 K. In contrast, our preliminary investigations of CePt₂B₂C suggest that replacement of Pd by Pt led to the formation of a heavy-fermion state without magnetic ordering down to 2 K.

The structural change between substitution-type ordered structures basically includes the competition of pairwise interactions, which can describe the stability of such structures. The purpose of this study is to investigate the frustration effect resulting from competition related to the L1₂ → D0₂₂ structural change in the Ni₃Al_0.52V_0.48 alloy and to discuss the pattern bifurcation in the pseudobinary Ni₃(Al, V) alloy. The present data obtained by transmission electron microscopy show that the change in microstructures during the structural change is characterized by the appearance and the subsequent annihilation of D0₂₂ regions in the L1₂ matrix. The final product of the structural change is the L1₂ single phase including topological defects, which is different from the equilibrium state known as the L1₂ + D0₂₂ checkerboard pattern. This indicates that the suppression of the L1₂ → D0₂₂ structural change occurs in this alloy. Such suppression can be explained as being due to an inability to accommodate the strain field in the vicinity of the L1₂/D0₂₂ boundaries when D0₂₂ precipitation occurs. The trigger of this inability is thought to be strong frustration originating from a small concentration deviation in the initial L1₂ matrix from the stoichiometry of the L1₂ structure. It is concluded that the formation of the L1₂ single phase as the final state, i.e. spontaneous L1₂ single crystallization, results from an effort by the alloy to accommodate the strain field, the occurrence of which is related to the frustration effect appearing during the L1₂ → D0₂₂ structural change.

The phototransferred thermoluminescence (PTTL) of PWO and PWO:Y crystals was studied in the range of 30–300^oC. From the experiments, one can conclude that in PWO crystals there are abundant traps and that some traps must be very deep. Visible light (blue, green and red) illumination can change the distribution of the trapped charges—namely, some charges trapped by deep traps can be transferred to shallow traps. These traps above room temperature are in practice detrimental to the light yield and its stability. Doping with Y³⁺ is a good way to reduce the number of these traps, as can be concluded from the γ-irradiation-induced thermoluminescence and PTTL curves. This paper provides us with some clues as regards how to enhance the stability of the light yield, which is an important requirement for PWO in high-energy physics detection applications.

A novel ternary phase, Sn_yNi₄Sb_12−xSn_x, has been characterized and found to exhibit a wide range of homogeneity (at 250 °C, 2.4 ≤ x ≤ 5.6, 0 ≤ y ≤ 0.31; at 350 °C, 2.7 ≤ x ≤ 5.0, 0 ≤ y ≤ 0.27). Sn_yNi₄Sb_12−xSn_x crystallizes in a skutterudite-based structure in which Sn atoms are found to occupy two crystallographically inequivalent sites: (a) Sn and Sb atoms randomly share the 24g site; and (b) a small fraction of Sn atoms occupy the 2a (0, 0, 0) position, with an anomalously large isotropic atomic displacement parameter. Eu_0.8Ni₄Sb_5.8Sn_6.2, Yb_0.6Ni₄Sb_6.7Sn_5.3 and Ni₄As_9.1Ge_2.9 are isotypic skutterudites. Depending on the particular composition, metallic as well as semiconducting states appear. The crossover from semiconducting to metallic behaviour is discussed in terms of a temperature-dependent carrier concentration employing a simple model density of states with the Fermi energy slightly below a narrow energy gap. This model accounts for the peculiar temperature-dependent electrical resistivity. These skutterudites are characterized by a number of lattice vibrations, which were elucidated by Raman measurements and compared to the specific heat data. The Eu-containing compound exhibits long-range magnetic order at T_mag ≈ 6 K, arising from the Eu²⁺ ground state.

We describe the effects of disorder in a two-dimensional electron gas in the presence of a parallel magnetic field. We argue that localized states lead to the formation of local moments. The parallel magnetic field for complete spin polarization depends on the electron density and the density of local moments and we can explain the observed dependence of the saturation field of the magnetoresistance on the sample quality. The magnetic properties of the electron spins are described in terms of a Curie-like paramagnetism due to localized moments and a Pauli paramagnetism due to itinerant electrons.