K. I. Kugel

Metal-insulator transition induced by oxygen isotope exchange in the magnetoresistive perovskite manganites

Perovskite manganites derived from LaMnO3 have recently become the subject of intensive study following the discovery of ‘colossal’ magnetoresistance (a magnetically induced change in electrical resistance of up to several orders of magnitude) in several members of this family of compounds. The manganites exhibit a broad range of electronic and magnetic phases, ranging from low-resistance ferromagnetic metals to high-resistance insulators, which are extremely sensitive to variation of composition, temperature and pressure. A recent study showed that such sensitivity also extends to oxygen isotope exchange: replacing 16O with 18O in La0.8Ca0.2MnO3 produces an unusually large change in the magnetic properties (a 21-kelvin decrease in the Curie temperature). The magnitude of this isotope shift is evidence for the essential role played by electron–phonon coupling in determining the transport properties of these materials. Here we show that this sensitivity to oxygen isotope exchange can be even more extreme. In its normal state, the compound La0.175Pr0.525Ca0.3MnO3 undergoes an insulator-to-metal transition as it is cooled below ∼95 K. But we find that, after substituting 18O for 16O, the compound remains an insulator down to 4.2 K, so providing a vivid demonstration of the importance of lattice vibrations in these materials.

Resistivity and 1/f noise in nonmetallic phase-separated manganites

A simple model is proposed to calculate resistivity, magnetoresistance, and noise spectrum in non-metallic phase-separated manganites containing small metallic droplets (magnetic polarons). The system is taken to be far from the percolation transition into a metallic state. It is assumed that the charge transfer occurs due to electron tunneling from one droplet to another through the insulating medium. As a result of this tunneling, the droplets acquire or lose extra electrons forming metastable two-electron and empty states. In the framework of this model, explicit expressions for dc conductivity and noise power of the system are derived. It is shown that the noise spectrum has 1/f form in the low-frequency range.

Elastic interactions and superstructures in manganites and other Jahn-Teller systems

The role of elastic interactions between Jahn-Teller ions in formation of various orbital- and charge-ordered structures in manganites and related compounds is analyzed. It is shown that such interactions alone are often sufficient to reproduce the structures observed in different regions of the phase diagram. Special attention is focused on stripe structures at high doping levels.

METAL-INSULATOR TRANSITION INDUCED BY 16O-18O OXYGEN ISOTOPE EXCHANGE IN COLOSSAL NEGATIVE MAGNETORESISTANCE MANGANITES

The effect of 16O→18O isotope exchange on the electric resistivity was studied for (La1−yPry)0.7Ca0.3MnO3 ceramic samples. It was found that at y=0.75, the substitution of 16O by 18O results in the reversible transition from a ferromagnetic metal (FM) to charge ordered (CO) insulator at zero magnetic field. The applied magnetic field (H⩾2 T) transformed the sample with 18O again to the metallic state and caused the increase in the FM transition temperature TC of the 16O sample. As a result, the isotope shift of TC at H=2 T was as high as 63 K. Such unique sensitivity of the system to oxygen isotope exchange, giving rise even to the metal–insulator transition, is discussed in terms of the isotope dependence of the effective electron bandwidth which shifts the balance between the CO and FM phases.

Phase separation in systems with charge ordering

A simple model of charge ordering is considered. It is explicitly shown that at any deviation from half-filling (n≠1/2), the system is unstable with respect to the phase separation into the charge ordered regions with n=1/2 and the metallic regions with a smaller electron or hole density. A possible structure of this phase-separated state (metallic droplets in a charge ordered matrix) is discussed. The model is extended to account for the strong Hund-rule onsite coupling and the weaker intersite antiferromagnetic exchange. The analysis of this extended model allows us to determine the magnetic structure of the phase-separated state and to reveal the characteristic features of the manganites and other substances with charge ordering.

Modification of the ground state in Sm-Sr manganites by oxygen isotope substitution

The effect of O-16-->O-18 isotope substitution on electrical resistivity and magnetic susceptibility of Sm1-xSrxMnO3 manganites is analyzed. It is shown that the oxygen isotope substitution drastically affects the phase diagram at the crossover region between the ferromagnetic metal state and that of antiferromagnetic insulator (0.4

Charge inhomogeneities and transport in semiconductor heterostructures with a Mn δ -layer

We study experimentally and theoretically the effects of disorder, nonlinear screening, and magnetism in semiconductor heterostructures containing a

The effect of oxygen isotope substitution on magnetic properties of (La1-yPry)0.7Ca0.3MnO3 manganites

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The effect of oxygen isotope substitution on the phase diagram of nearly half-doped R1-xSrxMnO3 manganites (R = Sm, NdTb, NdEu)

-layer of Mn, where the charge carriers are confined within a quantum well and hence both ferromagnetism and transport are two-dimensional (2D) and differ qualitatively from their bulk counterparts. Anomalies in the electrical resistance observed in both metallic and insulating structures can be interpreted as a signature of significant ferromagnetic correlations. The insulating samples turn out to be the most interesting as they can give us valuable insights into the mechanisms of ferromagnetism in these heterostructures. At low charge carrier densities, we show how the interplay of disorder and nonlinear screening can result in the organization of the carriers in the 2D transport channel into charge droplets separated by insulating barriers. Based on such a droplet picture and including the effect of magnetic correlations, we analyze the transport properties of this set of droplets, compare it with experimental data, and find a good agreement between the model calculations and experiment. Our analysis shows that the peak or shoulder-like features observed in temperature dependence of resistance of 2D heterostructures

Tunneling magnetoresistance of phase-separated manganites

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