Non-magnetic B-site Impurities Induce Ferromagnetic Tendencies in CE Manganites

Using a two-orbital model and Monte Carlo simulations, we investigate the effect of nonmagnetic B-site substitution on half-doped CE-type manganites. The lattice defects induced by this substitution destabilize the CE phase, which transforms into (1) the ferromagnetic (FM) metallic competing state, or (2) a regime with short-range FM clusters, or (3) a spin-glass state, depending on couplings and on the valence of the B-site substitution. While a C-type antiferromagnetic state is usually associated with an average $e_{\rm g}$ charge density less than 0.5, the nonmagnetic B-site substitution that lowers the $e_{\rm g}$ charge density is still found to enhance the FM tendency in our simulations. The present calculations are in qualitative agreement with experiments and provide a rationalization for the complex role of nonmagnetic B-site substitution in modulating the phase transitions in manganites.Comment: 8 pages, 5 figure

Colossal Magnetoresistant Materials: The Key Role of Phase Separation

The study of the manganese oxides, widely known as manganites, that exhibit the ``Colossal'' Magnetoresistance (CMR) 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. 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.Comment: 76 pages, 21 PNG files with figures. To appear in Physics Report

Influence of B - site Disorder in La0.5Ca0.5Mn1−xBxO3La_{0.5}Ca_{0.5}Mn_{1-x}B_{x}O_{3} (B = Fe, Ru, Al and Ga) Manganites

We have investigated the influence of B - site doping on the crystal and magnetic structure in $La_{0.5}Ca_{0.5}Mn_{1-x}B_{x}O_{3}$ (B= Fe, Ru, Al and Ga) compounds using neutron diffraction, SANS, magnetization and resistivity techniques. The B - site doped samples are isostructural and possess an orthorhombic structure in \textit{Pnma} space group at 300K. A structural transition from orthorhombic to monoclinic is found to precede the magnetic transition to CE - type antiferromagnetic state in few of these samples. On doping with Fe, charge and orbitally ordered CE - type antiferromagnetic state is suppressed, followed by the growth in ferromagnetic insulating phase in $0.02\leq x\leq0.06$ compounds. At higher Fe doping in $x>0.06$, the ferromagnetic state is also suppressed and no evidence of long range magnetic ordering is observed. In Ru doped samples $(0.01\leq x\leq0.05)$, the ferromagnetic metallic state is favored at $T{}_{C}\approx200K$ and $T_{MI}\approx125K$ and no significant change in $T_{C}$ and $T_{MI}$ as a function of Ru doping is found. In contrast, with non magnetic Al substitution for $0.01\leq x\leq0.03$, the charge ordered CE - type antiferromagnetic state coexists with the ferromagnetic metallic phase. With further increase in Al doping $(0.05\leq x\leq0.07)$, both CE - type antiferromagnetic and ferromagnetic phases are gradually suppressed. This behavior is accompanied by the evolution of A - type antiferromagnetic insulating state. Eventually, at higher Al doping $(0.10\leq x\leq0.13)$, this phase is also suppressed and signature of spin glass like transition are evident in M(T). Likewise, substitution with Ga is observed to induce similar effects as described for Al doped samples. The presence of short ranged ferromagnetic ordering has been further explored using SANS measurements in few of the selected samples.Comment: To appear in Journal of Physics: Condensed Matte

Disorder-induced orbital ordering in doped manganites

We study the effect of quenched disorder on the ordering of orbital and magnetic degrees of freedom in a two-dimensional, two-band double-exchange model for $e_g$ electrons coupled to Jahn-Teller distortions. Using a real-space Monte Carlo method, we find that disorder can induce a short-range ordering of the orbital degrees of freedom near 30% hole doping. The most striking consequence of this short range ordering is a strong increase in the low temperature resistivity. The real-space approach allows to analyze the spatial patterns of the charge, orbital, and magnetic degrees of freedom, and the correlations among them. The magnetism is inhomogeneous on the nanoscale in the short-range orbitally ordered state.Comment: 7 pages, 7 figure

Fermi-liquid instabilities at magnetic quantum phase transitions

This review discusses instabilities of the Fermi-liquid state of conduction electrons in metals with particular emphasis on magnetic quantum critical points. Both the existing theoretical concepts and experimental data on selected materials are presented; with the aim of assessing the validity of presently available theory. After briefly recalling the fundamentals of Fermi-liquid theory, the local Fermi-liquid state in quantum impurity models and their lattice versions is described. Next, the scaling concepts applicable to quantum phase transitions are presented. The Hertz-Millis-Moriya theory of quantum phase transitions is described in detail. The breakdown of the latter is analyzed in several examples. In the final part experimental data on heavy-fermion materials and transition-metal alloys are reviewed and confronted with existing theory.Comment: 62 pages, 29 figs, review article for Rev. Mod. Phys; (v2) discussion extended, refs added; (v3) shortened; final version as publishe

Inelastic Light Scattering From Correlated Electrons

Inelastic light scattering is an intensively used tool in the study of electronic properties of solids. Triggered by the discovery of high temperature superconductivity in the cuprates and by new developments in instrumentation, light scattering both in the visible (Raman effect) and the X-ray part of the electromagnetic spectrum has become a method complementary to optical (infrared) spectroscopy while providing additional and relevant information. The main purpose of the review is to position Raman scattering with regard to single-particle methods like angle-resolved photoemission spectroscopy (ARPES), and other transport and thermodynamic measurements in correlated materials. Particular focus will be placed on photon polarizations and the role of symmetry to elucidate the dynamics of electrons in different regions of the Brillouin zone. This advantage over conventional transport (usually measuring averaged properties) indeed provides new insights into anisotropic and complex many-body behavior of electrons in various systems. We review recent developments in the theory of electronic Raman scattering in correlated systems and experimental results in paradigmatic materials such as the A15 superconductors, magnetic and paramagnetic insulators, compounds with competing orders, as well as the cuprates with high superconducting transition temperatures. We present an overview of the manifestations of complexity in the Raman response due to the impact of correlations and developing competing orders. In a variety of materials we discuss which observations may be understood and summarize important open questions that pave the way to a detailed understanding of correlated electron systems.Comment: 62 pages, 48 figures, to appear in Rev. Mod. Phys. High-resolution pdf file available at http://onceler.uwaterloo.ca/~tpd/RMP.pd

Time-resolved optical studies of colossal magnetoresistance and charge -density wave materials

This thesis presents measurements of collective modes and ultrafast carrier relaxation dynamics in charge-density-wave (CDW) conductors and colossal magnetoresistance (CMR) manganites. A femtosecond laser pump pulse excites a broad frequency spectrum of low-energy collective modes and electron-hole pairs thereby changing its optical properties. The low-energy collective excitations and quasiparticle relaxation and recombination processes are monitored by measuring the resulting photoinduced absorption as a function of probe pulse wavelength and time delay.;A general model was developed for the photogeneration and detection mechanism of collective modes based on light absorption in two-color pump-probe experiments. A broad spectrum of collective modes (phasons and amplitudons) with frequencies down to a few GHz is excited and propagates normal to the surface into the material. The dispersion of the long-wavelength phason and amplitudon can be measured by changing the probe wavelength.;The first pump-probe spectroscopy was performed from the ultraviolet to mid-infrared wavelength range to study low-frequency collective excitations, including temperature evolution, dispersion, damping, and anisotropy of amplitude mode and transverse phason in quasi-one dimensional CDW conductors, K 0.3MoO3 and K0.33MoO3 on ultrafast time scale. The transverse phason exhibits an acoustic-like dispersion relation in the frequency range from 5--40 GHz. The phason velocity is strongly anisotropic with a very weak temperature dependence. In contrast, the amplitude mode exhibits a weak (optic-like) dispersion relation with a frequency of 1.66 THz at 30 K.;The studies were extended to doped perovskite manganite thin films and single crystals. A low-energy collective mode is observed and discussed in terms of the opening of a pseudogap resulting from charge/orbital ordering phases. The softening of the collective mode is necessary to explain by combining a cooperative Jahn-Teller type distortion of the MnO6 octahedra with the collective mode. The quasiparticle dynamics in the vicinity of the metal-insulator transition is strongly affected by the presence of a pseudogap, phase separation and percolation, which are strongly dependent on temperature. A very long-lived relaxation process is observed due to a slow spin relaxation process. The dynamics of the spin system is further investigated in strained and unstrained thin films, which show a strong strain effect

Estudos de distorções da rede e correlações electrónicas em manganites utilizando sondas lociais

Doutoramento em FísicaNesta tese apresenta-se um estudo experimental das distorções locais e correlações electrónicas em óxidos magnéticos com magnetoresistência colossal. A técnica de sonda local – Correlação Angular Perturbada – é utilizada em amostras caracterizadas quanto às suas propriedades macroscópicas nomeadamente propriedades estruturais, magnéticas e eléctricas, tendo em vista a obtenção de informação microscópica relevante via gradiente de campo eléctrico e campo magnético hiperfino, focando em particular os seguintes aspectos: -Distorções de rede e agregados de polarões no sistema LaMnO3+∆. É apresentado um estudo dos gradientes de campo eléctrica e campo magnético hiperfino em amostras representativas do sistema LaMnO3+∆, correlacionando estas propriedades locais com a caracterização das propriedades macroscópicas, efectuada nas mesmas amostras. Desta forma, foi possível estudar a natureza microscópica das distorções polarónicas. Foi dada especial atenção ao composto com composição LaMnO3.12 uma vez que este é um sistema padrão de uma manganite ferromagnética-isoladora que apresenta uma transição estrutural romboédrica (R)-ortorrômbica (O) perto da temperatura ambiente. O estudo revelou que agregados de distorções locais sobrevivem até 776 K, na fase de estrutura média mais simétrica (romboédrica), onde, por simetria, os octaedros MnO6 deveriam ser regulares. Estas distorções são semelhantes às observadas no sistema LaMnO3 onde os octaedros MnO6 apresentam uma distorção Jahn-Teller colectiva. Com a diminuição da temperatura observa-se um aumento contínuo destes agregados. Abaixo de uma temperatura crítica estas distorções relaxam acomodando-se numa estrutura com reduzidas distorções Jahn-teller. Verificou-se também que a transição estrutural (macroscópica) pode ser entendida como uma transição de percolação dos ambientes microscópicos. -Coexistência das ordens eléctrica e magnética no sistema Pr1-xCaxMnO3. É apresentado o primeiro estudo de gradiente de campo eléctrico no sistema Pr1-xCaxMnO3. Este estudo foi efectuado numa larga gama de temperaturas permitindo estudar localmente as diversas transições que este sistema apresenta. Em particular, na região do diagrama de fases onde existe ordenamento de carga e orbital (0.32<x<0.90), a componente principal do gradiente de campo eléctrico apresenta uma descontinuidade perto desta transição. Esta anomalia no gradiente de campo eléctrico é relacionada com possiveis distorções da simetria local sugerindo a presença de uma polarização eléctrica espontânea a temperaturas inferiores às de ordenamento de carga.This thesis presents an experimental study on lattice distortions and electronic correlations in colossal magnetoresistive magnetic oxides. The Perturbed Angular Correlation local probe technique is used to study selected manganite systems in order to obtain relevant insight into microscopic phenomena responsible for their macroscopic properties. Complementary structural, magnetic and electric characterization was performed. The work is focused on the following aspects: - Lattice distortions and polaron clusters in LaMnO3+∆ system. A study of the electric field gradient and magnetic hyperfine field was performed in representative samples of the LaMnO3+∆ system, and correlated with macroscopic information obtained in the same samples. Particular attention was given to the LaMnO3.12 sample since this compound is a prototype of a ferromagnetic-insulator manganite, presenting a rhombohedricorthorhombic structural phase transition near room temperature. We found that random distributed polaron clusters survive in the undistorted Rhombohedric phase, up to temperatures as high as 776 K. These distortions are as strong as those observed in the orbital ordered LaMnO3. Lowering temperature, the clusters continuously expand until a microscopic transition takes place. Below the transition, the distortions are accommodated into a weaker JT distorted phase. Additionally, the macroscopic structural phase transition can be viewed as a percolation transition of the microscopic environments. - Coexistence of electric and magnetic order in the Pr1-xCaxMnO3 system. The electrical field gradient (EFG) was studied for several compositions of the Pr1-xCaxMnO3 system. This local probe analysis was complemented with the study of the magnetic and structural properties. This allowed the determination of the electrical field gradient phase diagram for this system. The problematic of the charge/orbital order was also studied. Perturbed Angular Correlation studies were used to infer about atomic-scale distortions in a wide temperature range encompassing the charge/orbital and magnetic ordering transitions. The electrical-field gradient generated by the charge distribution around the probe shows strong anomalies when the system undergoes the charge-order (CO) transition. In particular, the principal component of the EFG presents a sharp discontinuity below the CO transition. The anomaly of EFG below TCO was related with the displacements of the ions causing a distortion of the local symmetry and thus connected with the existence of a local electric polarization

Orbital ordering phenomena in dd- and ff-electron systems

In recent decades, novel magnetism of $d$- and $f$-electron compounds has been discussed very intensively both in experimental and theoretical research fields of condensed matter physics. It has been recognized that those material groups are in the same category of strongly correlated electron systems, while the low-energy physics of $d$- and $f$-electron compounds has been separately investigated rather in different manners. One of common features of both $d$- and $f$-electron systems is certainly the existence of active orbital degree of freedom, but in $f$-electron materials, due to the strong spin-orbit interaction in rare-earth and actinide ions, the physics seems to be quite different from that of $d$-electron systems. In general, when the number of internal degrees of freedom and relevant interactions is increased, it is possible to obtain rich phase diagram including large varieties of magnetic phases by using several kinds of theoretical techniques. However, we should not be simply satisfied with the reproduction of rich phase diagram. It is believed that more essential point is to seek for a simple principle penetrating complicated phenomena in common with $d$- and $f$-electron materials, which opens the door to a new stage in orbital physics. In this sense, it is considered to be an important task of this article to explain common features of magnetism in $d$- and $f$-electron systems from a microscopic viewpoint, using a key concept of orbital ordering, in addition to the review of the complex phase diagram of each material group.Comment: 112 pages, 38 figure