Unveiling new magnetic phases of undoped and doped manganites

Canted magnetic ground state of quarter-doped manganites R 0.75Ca0.25MnO3 (R = Y, Tb, Dy, Ho, and Er)

Polycrystalline samples of the quarter-doped manganites R _0.75Ca_0.25MnO₃ (R  =  Y, Tb, Dy, Ho, and Er) were studied by x-ray diffraction and AC/DC susceptibility measurements. All five samples are orthorhombic and exhibit similar magnetic properties: enhanced ferromagnetism below T ₁ (∼80 K) and a spin glass (SG) state below T _SG (∼30 K). With increasing R ³⁺ ionic size, both T ₁ and T _SG generally increase. The single crystal neutron diffraction results on Tb_0.75Ca_0.25MnO₃ revealed that the SG state is mainly composed of a short-range ordered version of a novel canted (i.e. noncollinear) antiferromagnetic spin state. Furthermore, calculations based on the double exchange model for quarter-doped manganites reveal that this new magnetic phase provides a transition state between the ferromagnetic state and the theoretically predicted spin-orthogonal stripe phase.

Orbital-selective Mott phases of a one-dimensional three-orbital Hubbard model studied using computational techniques

A recently introduced one-dimensional three-orbital Hubbard model displays orbital-selective Mott phases with exotic spin arrangements such as spin block states [J. Rincón et al., Phys. Rev. Lett. 112, 106405 (2014)PRLTAO0031-900710.1103/PhysRevLett.112.106405]. In this publication we show that the constrained-path quantum Monte Carlo (CPQMC) technique can accurately reproduce the phase diagram of this multiorbital one-dimensional model, paving the way to future CPQMC studies in systems with more challenging geometries, such as ladders and planes. The success of this approach relies on using the Hartree-Fock technique to prepare the trial states needed in CPQMC. We also study a simplified version of the model where the pair-hopping term is neglected and the Hund coupling is restricted to its Ising component. The corresponding phase diagrams are shown to be only mildly affected by the absence of these technically difficult-to-implement terms. This is confirmed by additional density matrix renormalization group and determinant quantum Monte Carlo calculations carried out for the same simplified model, with the latter displaying only mild fermion sign problems. We conclude that these methods are able to capture quantitatively the rich physics of the several orbital-selective Mott phases (OSMP) displayed by this model, thus enabling computational studies of the OSMP regime in higher dimensions, beyond static or dynamic mean-field approximations.

Narrowing of topological bands due to electronic orbital degrees of freedom

The fractional quantum Hall effect has been predicted to occur in the absence of magnetic fields and at high temperature in lattice systems that have flat bands with a nonzero Chern number. We demonstrate that orbital degrees of freedom in frustrated lattice systems lead to a narrowing of topologically nontrivial bands. This robust effect does not rely on fine-tuned long-range hopping parameters and is directly relevant to a wide class of transition-metal compounds.

Unified picture for magnetic correlations in iron-based superconductors

The varying metallic antiferromagnetic correlations observed in iron-based superconductors are unified in a model consisting of both itinerant electrons and localized spins. The decisive factor is found to be the sensitive competition between the superexchange antiferromagnetism and the orbital-degenerate double-exchange ferromagnetism. Our results reveal the crucial role of Hund's rule coupling for the strongly correlated nature of the system and suggest that the iron-based superconductors are closer kin to manganites than cuprates in terms of their diverse magnetism and incoherent normal-state electron transport. This unified picture would be instrumental for exploring other exotic properties and the mechanism of superconductivity in this new class of superconductors.

First order colossal magnetoresistance transitions in the two-orbital model for manganites

Large-scale Monte Carlo simulation results for the two-orbital model for manganites, including Jahn-Teller lattice distortions, are presented here. At hole density x=1/4 and in the vicinity of the region of competition between the ferromagnetic metallic and spin-charge-orbital ordered insulating phases, the colossal magnetoresistance (CMR) phenomenon is observed with a magnetoresistance ratio ∼10,000%. Our main result is that this CMR transition is found to be of first order in some portions of the phase diagram, in agreement with early results from neutron scattering, specific heat, and magnetization, thus solving a notorious discrepancy between experiments and previous theoretical studies. The first order characteristics of the transition survive, and are actually enhanced, when weak quenched disorder is introduced.

Striped multiferroic phase in double-exchange model for quarter-doped manganites

The phase diagram of quarter-hole-doped perovskite manganites is investigated using the double-exchange model. An exotic striped type-II multiferroic phase, where 25% of the nearest-neighbor spin couplings are orthogonal to each other, is found in the narrow-bandwidth region. Comparing with the spiral-spin ordering phase of undoped manganites, the multiferroic Curie temperature of the new phase is estimated to be approximately 4 times higher, while the ferroelectric polarization is similar in magnitude. Our study provides a path for noncollinear spin multiferroics based on electronic self-organization, different from the traditional approach based on superexchange frustration.

Dual nature of improper ferroelectricity in a magnetoelectric multiferroic

Using first-principles calculations, we study the microscopic origin of ferroelectricity (FE) induced by magnetic order in the orthorhombic HoMnO3. We obtain the largest ferroelectric polarization observed in the whole class of improper magnetic ferroelectrics to date. We find that the two proposed mechanisms for FE in multiferroics, lattice and electronic based, are simultaneously active in this compound: a large portion of the ferroelectric polarization arises due to quantum-mechanical effects of electron orbital polarization, in addition to the conventional polar atomic displacements. An interesting mechanism for switching the magnetoelectric domains by an electric field via a 180 degrees coherent rotation of Mn spins is also proposed.

Ferroelectricity in the magnetic E-phase of orthorhombic perovskites

We show that the symmetry of the spin zigzag chain E phase of the orthorhombic perovskite manganites and nickelates allows for the existence of a finite ferroelectric polarization. The proposed microscopic mechanism is independent of spin-orbit coupling. We predict that the polarization induced by the E-type magnetic order can potentially be enhanced by up to 2 orders of magnitude with respect to that in the spiral magnetic phases of TbMnO3 and similar multiferroic compounds.

Continuous charge modulated diagonal phase in manganites

We present a novel ground state that explains the continuous charge modulated diagonal order recently observed in manganese oxides, at hole concentrations x larger than one-half. In this diagonal phase the charge is modulated with a predominant Fourier component inversely proportional to 1-x. Magnetically this state consists of antiferromagnetically coupled zigzag chains. For a wide range of physical parameters such as electron-phonon coupling, antiferromagnetic interaction between Mn ions, and on-site Coulomb repulsion, the diagonal phase is the ground state of the system. Also we find that the diagonal modulation of the electron density is only a small fraction of the average charge, a much smaller modulation than the one obtained by distributing Mn+3 and Mn+4 ions. We discuss also the spin and orbital structure properties of this new diagonal phase.

Relevance of cooperative lattice effects and stress fields in phase-separation theories for CMR manganites

Previous theoretical investigations of colossal magnetoresistance (CMR) materials explain this effect using a "clustered" state with preformed ferromagnetic islands that rapidly align their moments with increasing external magnetic fields. While qualitatively successful, explicit calculations indicate drastically different typical resistivity values in two- and three-dimensional lattices, contrary to experimental observations. This conceptual bottleneck in the phase-separated CMR scenario is resolved here considering the cooperative nature of the Mn-oxide lattice distortions. This effectively induces power-law correlations in the quenched disorder used in toy models with phase competition. When these effects are incorporated, resistor-network calculations reveal very similar results in two and three dimensions, qualitatively modifying previous scenarios and solving the puzzle.