Phase separation and charge ordering in doped manganite perovskites: Projection perturbation and mean-field approaches

A theory is developed to explain various types of electronic collective behaviors in doped manganites R 1-xX xMnO 3 (R = La, Pr, Nd, etc. and X=Ca, Sr, Ba, etc.). Starting from a realistic electronic model, we derive an effective Hamiltonian by utilizing the projection perturbation techniques and develop a spin-charge-orbital coherent-state theory, in which the Jahn-Teller effect and the orbital degeneracy of e g electrons in Mn ions are taken into account. Physically, the experimentally observed charge-ordering state and electronic phase separation are two macroscopic quantum phenomena with opposite physical mechanisms, and their physical origins are elucidated in this theory. The interplay of the Jahn-Teller effect, the lattice distortion, as well as the double-exchange mechanism leads to different magnetic structures and to different charge-ordering patterns and phase separation. ©2000 The American Physical Society.published_or_final_versio

Interference in transport through double barriers in interacting quantum wires

We investigate interference effects of the backscattering current through a double-barrier structure in an interacting quantum wire attached to noninteracting leads. Depending on the interaction strength and the location of the barriers, the backscattering current exhibits different oscillation and scaling characteristics with the applied voltage in the strong and weak interaction cases. However, in both cases, the oscillation behaviors of the backscattering current are mainly determined by the quantum mechanical interference due to the existence of the double barriers.Comment: 6 pages, 3 fig

Berry phase and its induced charge and spin currents in a ring of a double-exchange system

A ring of double-exchange system is investigated to explore the Berry phase acquired by the interplay of localized and conduction electrons. The competition between the double-exchange ferromagnetism and the superexchange antiferromagnetic coupling from the localized electrons leads to a phase transition from a ferromagnetic state to a spin spiral state. The spin spiral state acquires a nonzero Berry phase along the ring, and induces both charge and spin currents simultaneously. It is predicted that both the Aharonov-Bohm effect and Aharonov-Cashier effect will be exhibited spontaneously in the system. © 1999 The American Physical Society.published_or_final_versio

Does the Tail Wag the Dog?: The Effect of Credit Default Swaps on Credit Risk

We use credit default swaps (CDS) trading data to demonstrate that the credit risk of reference firms, reflected in rating downgrades and bankruptcies, increases significantly upon the inception of CDS trading, a finding that is robust after controlling for the endogeneity of CDS trading. Additionally, distressed firms are more likely to file for bankruptcy if they are linked to CDS trading. Furthermore, firms with more “no restructuring” contracts than other types of CDS contracts (i.e., contracts that include restructuring) are more adversely affected by CDS trading, and the number of creditors increases after CDS trading begins, exacerbating creditor coordination failure in the resolution of financial distress.postprin

Ferrimagnetism in the organic polymeric Hubbard model: Quantum Monte Carlo simulation

The ground-state properties of organic polymers are studied by means of the quantum Monte Carlo simulation. The polymer doped by transition-metal impurities at every other radical site of the chain is described by the quasi-one-dimensional polymeric Hubbard chain. The topological structure of the chain possesses a flat-band structure of the energy band. The spin-spin correlation function and the static magnetic susceptibility are investigated in the case of half filling. Our analysis shows that the on-site Coulomb repulsions in the chain and/or in the radical lead to the coexistence of ferromagnetic and antiferromagnetic order, i.e., the ferrimagnetic order. The on-site Coulomb repulsion (U d) of electrons at the radicals plays a more significant role in stabilizing the ferromagnetic order than that (U) on the chain does, while U has a stronger impact on the antiferromagnetic order. © 1999 The American Physical Society.published_or_final_versio

Hole dispersions in the G- and C-type orbital ordering backgrounds: Doped manganese oxides

In the framework of the linear spin-wave theory and orbital-charge separation, we calculate quasiparticle (QP) dispersions for two different antiferromagnetic orbital structures in the fully saturated spin phase of manganese oxides. Although with the same orbital wave excitations, the QP bands of C- and G-type orbital structures exhibit completely different shapes. The pseudogap observed in the density of states and spectral functions around ω=0 is related with the large antiferromagnetic orbital fluctuation. The minimal band energy for G-type is lower than that for C-type orbital order, while these band curves almost coincide in some momentum points. Larger energy splitting occurs between the two branches of k z=0 and k z=π when increasing the superexchange coupling J, suggesting that the orbital scattering plays an essential role in the QP dispersions. ©2000 The American Physical Society.published_or_final_versio

Orbital ordering and two ferromagnetic phases in low-doped La 1-xSr xMnO 3

We present a theory for the transition between two ferromagnetic phases observed experimentally in lightly doped La 1-xSr xMnO 3. Starting from an electronic model, the instabilities to various types of orbital orderings are studied within the random-phase approximation. In most cases, the instabilities occur in the region of strong correlations. A phase diagram is calculated in the case of strong correlation by means of the projected perturbation technique and the Schwinger boson technique. A phase transition between two types of orbital ordering occurs at a low doping, which may be closely relevant to recent experimental observations.published_or_final_versio

Phase diagram of an extended Kondo lattice model for manganites: The Schwinger-boson mean-field approach

We investigate the phase diagram of an extended Kondo lattice model for doped manganese oxides in the presence of strong but finite Hund's coupling and on-site Coulomb interaction. By means of the Schwinger-boson mean-field approach, it is found that, besides magnetic ordering, there will be nonuniform charge distributions, such as charge ordering and phase separation, if the interaction between electrons prevails over the hybridization. Which of the charge ordering and phase separation appears is determined by a competition between effective repulsive and attractive interactions due to virtual processes of electron hopping. Calculated results show that strong electron correlations caused by the on-site Coulomb interaction as well as the finite Hund's coupling play an important role in the magnetic ordering and charge distribution at low temperatures. ©2000 The American Physical Society.published_or_final_versio

Behavior of a movable electrode in piezo-response mode of an atomic force microscope

Author name used in this publication: C. H. XuAuthor name used in this publication: C. H. WooAuthor name used in this publication: S. Q. ShiAuthor name used in this publication: Y. Wang2003-2004 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe

Spin and orbital excitations in undoped manganites

We develop a theory for spin and orbital excitations in undoped manganites to account for the spin and orbital orderings observed experimentally. It is found that the anisotropy of the magnetic structure is closely related to the orbital ordering, and the Jahn-Teller effect stabilizes the orbital ordering. The phase diagram and the low-energy excitation spectra for both spin and orbital orderings are obtained. The calculated critical temperatures can be quantitatively comparable to the experimental data. © 2000 American Institute of Physics.published_or_final_versio