Florentin Popescu

Critical temperatures of the two-band model for diluted magnetic semiconductors

Using dynamical mean field theory and Monte Carlo simulations, we study the ferromagnetic transition temperature (T{sub c}) of a two-band model for diluted magnetic semiconductors (DMS), varying coupling constants, hopping parameters, and carrier densities. We found that T{sub c} is optimized at all fillings p when both impurity bands fully overlap in the same energy range, namely when the exchange couplings J and bandwidths are identical. The optimal T{sub c} is found to be about twice larger than the maximum value obtained in the one-band model, showing the importance of multiband descriptions of DMS at intermediate Js.

Dynamical mean-field study of the ferromagnetic transition temperature of a two-band model for colossal magnetoresistance materials

The ferromagnetic transition temperature (T{sub c}) of a two-band double-exchange (DE) model for colossal magnetoresistance materials is studied using dynamical mean-field theory in wide ranges of coupling constants, hopping parameters, and carrier densities. The results are shown to be in good agreement with Monte Carlo simulations. When the bands overlap, the value of T{sub c} is found to be much larger than in the one-band case, for all values of the chemical potential within the energy overlap interval. A nonzero off-diagonal hopping produces an additional boost of T{sub c}, showing the importance of these terms, as well as the concomitant use of multiband models, to increase the critical temperatures in DE-based theories.

Study of strongly correlated electrons in nanoscopic and bulk forms: some recent results

Recent progress in the numerical study of various strongly correlated electronic systems is reviewed. The study of transport in single molecule conductors and quantum dots is addressed with a recently proposed adaptive time-dependent density-matrix-renormalization group (DMRG). Experiments involving non-local spin control and their numerical simulation are also discussed. A section is devoted to recent efforts in the study of spin-fermion models for colossal magnetoresistive manganites, where we present insights on the effect of disorder and electron–phonon coupling. Finally, using a dynamical mean field approach, we review calculations in the area of diluted magnetic semiconductors that provides guidelines on how the Curie temperature could be increased in these itinerant ferromagnetic systems.

Short-Range Ordered Phase of the Double-Exchange Model in Infinite Dimensions

Using dynamical mean-field theory, we have evaluated the magnetic instabilities and T=0 phase diagram of the double-exchange model on a Bethe lattice in infinite dimensions. In addition to ferromagnetic (FM) and antiferromagnetic (AF) phases, we also study a class of disordered phases with magnetic short-range order (SRO). In the weak-coupling limit, a SRO phase has a higher transition temperature than the AF phase for all fillings p below 1 and can even have a higher transition temperature than the FM phase. At T=0 and for small Hunds coupling J_H, a SRO state has lower energy than either the FM or AF phases for 0.26\le p 0 limit but appears for any non-zero value of J_H.

p-WAVE PAIRING IN FERMI SYSTEMS WITH UNEQUAL POPULATION NEAR FESHBACH RESONANCE

We explore p-wave pairing in a single-channel two-component Fermi system with unequal population near Feshbach resonance. Our analytical and numerical study reveal a rich superfluid (SF) ground state structure as a function of imbalance. In addition to the state Δ±1 ∝ Y1±1, a multitude of “mixed” SF states formed of linear combinations of Y1ms give global energy minimum under a phase stability condition; these states exhibit variation in energy with the relative phase between the constituent gap amplitudes. States with local energy minimum are also obtained. We provide a geometric representation of the states. A T = 0 polarization vs. p-wave coupling phase diagram is constructed across the BEC-BCS regimes. With increased polarization, the global minimum SF state may undergo a quantum phase transition to the local minimum SF state.

Magnetic instabilities and phase diagram of the double-exchange model in infinite dimensions

Dynamical mean-field theory is used to study the magnetic instabilities and phase diagram of the double-exchange (DE) model with Hunds coupling JH > 0 in infinite dimensions. In addition to ferromagnetic (FM) and antiferromagnetic (AF) phases, the DE model also supports a broad class of short-range ordered (SRO) states with extensive entropy and short-range magnetic order. For any site on the Bethe lattice, the correlation parameter q of an SRO state is given by the average q = sin2 (θi/2), where θi is the angle between any spin and its neighbours. Unlike the FM (q = 0) and AF (q = 1) transitions, the transition temperature of an SRO state with 0 < q < 1 cannot be obtained from the magnetic susceptibility. But a solution of the coupled Greens functions in the weak-coupling limit indicates that an SRO state always has a higher transition temperature than the AF for all fillings p below 1 and even has a higher transition temperature than the FM for 0.26 ≤ p ≤ 0.39. For 0.39 < p < 0.73, where both the FM and AF phases are unstable for small JH, an SRO phase has a nonzero transition temperature except close to p = 0.5. As JH increases, the SRO transition temperature eventually vanishes and the FM phase dominates the phase diagram. For small JH, the T = 0 phase diagram of the DE model is greatly simplified by the presence of the SRO phase. An SRO phase is found to have lower energy than either the FM or AF phases for 0.26 ≤ p < 1. Phase separation (PS) disappears as JH → 0 but appears for any nonzero coupling. For fillings near p = 1, PS occurs between an AF with p = 1 and either an SRO or a FM phase. The stability of an SRO state at T = 0 can be understood by examining the interacting density-of-states, which is gapped for any nonzero JH in an AF but only when JH exceeds a critical value in an SRO state.