Takashi Hotta

Colossal magnetoresistant materials: the key role of phase separation

The study of the manganese oxides, widely known as manganites, that exhibit the “colossal” magnetoresistance effect is among the main areas of research within the area of strongly correlated electrons. After considerable theoretical effort in recent years, mainly guided by computational and mean-field studies of realistic models, considerable progress has been achieved in understanding the curious properties of these compounds. These recent studies suggest that the ground states of manganite models tend to be intrinsically inhomogeneous due to the presence of strong tendencies toward phase separation, typically involving ferromagnetic metallic and antiferromagnetic charge and orbital ordered insulating domains. Calculations of the resistivity versus temperature using mixed states lead to a good agreement with experiments. The mixed-phase tendencies have two origins: (i) electronic phase separation between phases with different densities that lead to nanometer scale coexisting clusters, and (ii) disorder-induced phase separation with percolative characteristics between equal-density phases, driven by disorder near first-order metal–insulator transitions. The coexisting clusters in the latter can be as large as a micrometer in size. It is argued that a large variety of experiments reviewed in detail here contain results compatible with the theoretical predictions. The main phenomenology of mixed-phase states appears to be independent of the fine details of the model employed, since the microscopic origin of the competing phases does not influence the results at the phenomenological level. However, it is quite important to clarify the electronic properties of the various manganite phases based on microscopic Hamiltonians, including strong electron–phonon Jahn–Teller and/or Coulomb interactions. Thus, several issues are discussed here from the microscopic viewpoint as well, including the phase diagrams of manganite models, the stabilization of the charge/orbital/spin ordered half-doped correlated electronics (CE)-states, the importance of the naively small Heisenberg coupling among localized spins, the setup of accurate mean-field approximations, the existence of a new temperature scale T∗ where clusters start forming above the Curie temperature, the presence of stripes in the system, and many others. However, much work remains to be carried out, and a list of open questions is included here. It is also argued that the mixed-phase phenomenology of manganites may appear in a large variety of compounds as well, including ruthenates, diluted magnetic semiconductors, and others. It is concluded that manganites reveal such a wide variety of interesting physical phenomena that their detailed study is quite important for progress in the field of correlated electrons.

Ferromagnetic, A-type, and charge-ordered CE-type states in doped manganites using jahn-teller phonons

The two-orbital model for manganites with both noncooperative and cooperative Jahn-Teller phonons is studied at hole density x = 0.5 using Monte Carlo techniques. The phase diagram is obtained by varying the electron-phonon coupling and the t(2g)-spins exchange. The insulating CE-type charge- and orbital-ordered state with the z-axis charge stacking observed in narrow-bandwidth manganites is stabilized in the simulations. Its charge gap Delta(CO) is much larger than the critical temperature k(B)T(CO). Metalliclike A-type and ferromagnetic states are also obtained in the same framework, and the phase boundaries among them have first-order characteristics.

Self‐consistent calculation of electronic states in AlGaAs/GaAs/AlGaAs selectively doped double‐heterojunction systems under electric fields

The electronic states in AlGaAs/GaAs/AlGaAs selectively doped double‐heterojunction (SD‐DH) systems or single‐quantum‐well systems have been calculated self‐consistently for the case where an external gate voltage is applied perpendicularly to the surface. The transition from symmetrical to asymmetrical distributions of electrons by the external field is predicted. The calculated variations of electron concentration with the gate voltage are found to be in good agreement with the experiment. The shoulder structures observed in the gate voltage dependence of mobility have been successfully accounted for and ascribed to the onset of electron population in the upper subbands. This analysis is expected to provide powerful means to evaluate the performance of SD‐DH field‐effect transistors and to optimize their design.

A -type antiferromagnetic and C -type orbital-ordered states in LaMnO 3 using cooperative Jahn-Teller phonons

The effect of Jahn-Teller phonons on the magnetic and orbital structure of

A New AlGaAs/GaAs Heterojunction FET with Insulated Gate Structure (MISSFET)

{mathrm{LaMnO}}_{3}

Oscillatory magnetoresistance of GaAs/GaAlAs quantum well structures under parallel magnetic fields

is investigated using a combination of relaxation and Monte Carlo techniques on three-dimensional clusters of

PSEUDOGAP FORMATION IN AN ELECTRONIC SYSTEM WITH D-WAVE ATTRACTION AT LOW DENSITY

{mathrm{MnO}}_{6}

Bloch Electrons in a Jahn-Teller Crystal and an Orbital-Density-Wave State due to the Berry Phase

octahedra. In the physically relevant region of parameter space for

Superconductivity in the Alkali-Doped Fullerides: Competition of Phonon-Mediated Attractions with Coulomb Repulsions in Polaron Pairing

{mathrm{LaMnO}}_{3},

Role of the Berry Phase in the Formation of Stripes in Manganese Oxides

and after including small corrections due to tilting effects, the A-type antiferromagnetic and C-type orbital structures were stabilized, in agreement with experiments.