Abstract: The propensity of systems of charge and spin to form, under certain conditions, ‘stripe’ phases has recently attracted much attention, as it has been suggested that dynamically fluctuating stripe phases may be of central importance for an understanding of the physics of high-temperature superconductors1,2,3,4,5. A related phenomenon — static charge stripes — characterizes6 the insulating antiferromagnetic ground state of the manganese oxides, a class of materials which (like the copper oxide superconductors) have a perovskite structure, and are notable for their extraordinary electronic and magnetic properties, such as colossal magnetoresistance and charge ordering7,8. Here we report a different pattern of charge localization in the charge-ordered phase of the manganese oxide La1−xCaxMnO3 (x ⩾ 0.5). This pattern takes the form of extremely stable pairs of Mn3+O6 stripes, with associated large lattice contractions (due to the Jahn–Teller effect), separated periodically by stripes of non-distorted Mn4+O6 octahedra. These periodicities, which adopt integer values between 2 and 5 times the lattice parameter of the orthorhombic unit cell, correspond to the commensurate carrier concentrations (x = 1/2, 2/3, 3/4 and 4/5): for other values of x, the pattern of charge ordering is a mixture of the two adjacent commensurate configurations. These paired Jahn–Teller stripes appear therefore to be the fundamental building blocks of the charge-ordered state in the manganese oxides, and so may be expected to have profound implications for the magnetic and transport properties of these materials.

Abstract: We have investigated the magnetic-field-induced phase transitions of ${R}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}$ ($R=\mathrm{Pr}$ and Nd, $x=0.50,$ 0.45 and 0.50, 0.45, 0.40) by measurements of magnetization, magnetoresistance, and magnetostriction utilizing a nondestructive long-pulse magnet (generating up to 40 T). We observed processes where magnetic fields destroy the real-space ordering of the charge carriers and cause insulator-to-metal phase transitions over the whole temperature region below about 250 K. We found that the destruction of the charge ordering is accompanied with a structural phase transition as well as with the magnetic phase transition and the colossal magnetoresistance effect. The different profiles of the temperature vs transition field curve depending on the carrier concentration $x$ may be ascribed to the difference in the entropy between the commensurate and the discommensurate charge-ordered state. It turned out that the stability of the charge-ordered state is strongly correlated with the colinear antiferromagnetic ordering of the localized Mn moments.

Abstract: Manganites with the perovskite structure represent a very important family of oxides which are extensively studied for their colossal magnetoresistance (CMR) properties In the present review we discuss the different factors which govern the magnetic and transport properties of these materials: carrier concentration, average size of the interpolated cation, and mismatch effect on the A-site Three types of oxides are mainly examined: (i) the hole doped manganites Ln07A03MnO3 (A = Ca, Sr, Ba), (ii) the “charge ordered” Ln05A05MnO3 manganites, and (iii) the electron doped manganites Ca1-xLnxMnO3 and Ca1-xThxMnO3 The relationships between structural and magnetic transitions are discussed, and particular attention is paid to charge ordering phenomena The doping of the Mn sites by various elements (Al, Ga, In, Ti, Sn, Fe, Cr, Co, Ni) is systematically examined The beneficial effect of “Cr, Co, Ni” elements, which induce CMR properties in these perovskites, is emphasized

Abstract: The electrical resistivity, magnetization, dilatation and sound velocity have been measured for La 1- X Sr X MnO 3 (0.48≤ X ≤0.90) polycrystals. Observed anomalies in the sound velocity and the dilatation strongly suggest the occurrence of the charge ordering within the Sr concentration range 0.48≤ X ≤0.82. A phase diagram of the charge-ordered state in La 1- X Sr X MnO 3 as a function of temperature T and the Sr concentration X is proposed.

Abstract: We introduce and study the nodal liquid, a novel zero-temperature quantum phase obtained by quantum-disordering a d-wave superconductor. It has numerous remarkable properties which lead us to suggest it as an explanation of the pseudo-gap state in underdoped high-temperature superconductors. In the absence of impurities, these include power-law magnetic order, a T-linear spin susceptibility, nontrivial thermal conductivity, and two- and one-particle charge gaps, the latter evidenced, e.g. in transport and electron photoemission (which exhibits pronounced fourfold anisotropy inherited from the d-wave quasiparticles). We use a (2+1)-dimensional duality transformation to derive an effective field theory for this phase. The theory is comprised of gapless neutral Dirac particles living at the former d-wave nodes, weakly coupled to the fluctuating gauge field of a dual Ginzburg–Landau theory. The nodal liquid interpolates naturally between the d-wave superconductor and the insulating antiferromagnet, and our effective field theory is powerful enough to permit a detailed analysis of a panoply of interesting phenomena, including charge ordering, antiferromagnetism, and d-wave superconductivity. We also discuss the zero-temperature quantum phase transitions which separate the nodal liquid from various ordered phases.

Abstract: The commensurability of the charge ordering process of the Ln0.50Ca0.50MnO3 manganites (Ln=Pr, Nd, Sm, Eu, and Gd) has been studied by electron diffraction and lattice imaging versus temperature. For the as-synthesized phases, it is shown that an incommensurate state at low temperature (92 K) is favored as the size of the lanthanide increases. A large transition width (ΔT ranging from 40 to 110 K) is observed which is closely related to the size of Ln3+, and also to the size mismatch between Ln3+ and Ca2+ cations. The most important result concerns the effects of oxygen stoichiometry, varied via different annealings carried out at low temperature (T⩽600 °C) under Ar/H2 flow, air or O2 at PO2=100b. It is shown that very tiny deviations in the oxygen stoichiometry, which are not measurable using conventional techniques, affect significantly the transition width and commensurability, keeping the magnetic properties nearly unchanged. It is demonstrated that oxidized samples are systematically incommensurate w...

Abstract: We demonstrate that the half-doped bilayer manganite La_{1}Sr_{2}Mn_{2}O_{7} exhibits CE-type charge-ordered and spin-ordered states below $T_{N, CO}^A = 210$ K and below $T_{N}^{CE} \sim 145$ K, respectively. However, the volume fraction of the CE-type ordering is relatively small, and the system is dominated by the A-type spin ordering. The coexistence of the two types of ordering is essential to understand its transport properties, and we argue that it can be viewed as an effective phase separation between the metallic $d(x^{2}-y^{2})$ orbital ordering and the charge-localized $d(3x^{2}-r^{2})/d(3y^{2}-r^{2})$ orbital ordering.

Abstract: A variable temperature neutron diffraction study has been carried out on the layered perovskite YBaMn2O5 between 100 and 300 K. A broad peak indexed as (1/2, 1/2, 1) on the nuclear cell is consistent with short range MnII /MnIII valence ordering. The magnetic structure below the ferrimagnetic ordering temperature of 167 K has been determined and is in agreement with a previously proposed model. Changes in lattice parameters, bond lengths and angles show evidence of an exchange striction at TC .

Abstract: We suggest that the phase transition observed in NaV_2O_5 at T_c = 34K is not a spin-Peierls transition, but a charge ordering transition, related to the formal presence in this system of equal number of V^4+ and V^4- ions. Below T_c, V^4+ ions form a zigzag structure, which is consistent with the experimentally observed doubling of the lattice period in a and b directions. We show that this charge ordering also results in the alternation of spin exchange constants along the b-direction, which opens a gap in the spin excitation spectrum. We emphasize the role of lattice distortions around V ions both in the formation of the charged ordered state and in the spin-gap opening.

Abstract: The oxides Sm0.5Ca0.5Mn1–xCrxO3 , with 0≤x≤0.07, have been studied by electron diffraction and electron microscopy versustemperature from 90 K to 300 K, in connection with their magnetic and transport properties. An incommensurate charge ordering appears atTCO=275 K for the undoped compound, then a progressive transition from an incommensurate to a commensurate modulated structure is observed as Tdecreases from 275 K to 170 K. But the most important point is that chromium doping enhances the incommensurability of the structure: theq vector of the low temperature form decreases and TCO decreases abruptly as the chromium content x increases, so that the charge ordering process is weakened and finally disappears for x=0.05. The structure of the low temperature form of these manganites is characterized by a double supercell, 2ap √2×2ap ×ap √2, similar to that observed for other Ln0.5Ca0.5MnO3 manganites with Ln=La, Pr, Nd, Tb. However, in these series, one observes similar space groups (P2mmorPmmm) for Ln=Nd, Sm, different from those observed for Ln=La, Tb.

Abstract: Recent X-ray analysis and NMR- experiments have shown that alpha'- NaV2O5 is not a conventional spin-Peierls (SP) compound because above the transition at 33K it is in a homogeneous mixed valent (MV) state and atomic spin chains do not exist. Furthermore, thermal expansion experiments have identified the existence of a double phase transition. We present a theoretical model which qualitatively explains these observations. We derive an effective Hamiltonian for the insulating state which describes the transitions as primary charge order leading to linear spin chains in b- direction and a secondary SP- transition. We show that this scenario explains in a natural way the anomalous BCS-ratio observed in alpha'- NaV2O5.

Abstract: Doping of Mn-site by chromium, cobalt and nickel has been investigated in the calcium rich manganites Pr0.4Ca0.6MnO3 and Pr0.3Ca0.7MnO3. Whatever the nature of the doping element, a rapid disappearance of the charge ordered (CO) state is observed, T CO decreasing as the doping rate increases. But the most important result concerns the Cr-doped compounds Pr0.4Ca0.6Mn1‒x Cr x O3 for which a re-entrant insulator to metal transition at 90-120 K is observed for x = 0.10‒0.12, in a zero magnetic field. The possibility to induce colossal magnetoresistance (CMR) properties for high Mn(IV) contents (Mn(IV)/Mn(III) ≥ 3/2) for x = 0.07‒0.12 is shown for the first time, the resistance ratios |R0/R7T reaching 2 × 106 at 30 K. This study also shows differently that the small size of the A-site cation (Pr, Ca) is not a redhibitory obstacle to the appearance of CMR properties in manganites, in contrast with previously established phase diagrams.

Abstract: We report on a comparative study of three manganites: Pr0.5Sr0.5MnO3, Pr0.5Ca0.5MnO3, and Pr0.5Sr0.3Ca0.2MnO3, which all exhibit the same Mn3+/Mn4+=1:1 concentration but show a very distinct magnetic behavior. The Pr0.5Sr0.5MnO3 sample is a ferromagnet (FM) below TC=265 K and becomes the A-type antiferromagnet at TN=140 K. In Pr0.5Ca0.5MnO3 the charge order develops at TCO=245 K and the CE-type antiferromagnetism occurs below TN=175 K. The Pr0.5Sr0.3Ca0.2MnO3 sample exhibits the FM transition at TC=225 K and the CE type antiferromagnetism and charge ordering occur simultaneously at TN=TCO=160 K (185 K) upon cooling (heating), respectively. The study is complemented by the data on two systems of 30% Mn4+, the ferromagnetic metal Pr0.7Sr0.1Ca0.2MnO3, and insulator Pr0.85K0.15MnO3, both with similar Curie temperature TC∼140 K. The electrical resistivity, thermoelectric power, thermal conductivity, and volume thermal expansion data are analyzed with respect to observed magnetic transitions and reveal the sign...

Abstract: Giant magnetoresistance and related properties such as charge-ordering in manganates have been investigated with great vigor in the last two years. The manganates have turned out to be truly exciting systems, exhibiting a wide variety of electronic and magnetic properties.

Abstract: Some unusual electronic properties of doped manganites are described. The low temperature resistivity of the metallic ferromagnet has a large $T^2$ term. The residual resistivity can be much larger than the Mott maximum, and depends exponentially on spontaneous magnetization. There is much evidence for strong spatial and temporal fluctuation effects. The available theories are discussed in the light of these phenomena, and it is suggested that a novel localizing mechanism is at work, leading to extreme sensitivity of physical properties to small atomic changes.

Abstract: An exact-diagonalization technique on small clusters is used to calculate the dynamical density correlation functions of the dimerized t - J chain and coupled anisotropic t - J ladders (trellis lattice) at quarter-filling, i.e., the systems regarded as a network of pairs (dimers or rungs) of sites coupled weakly via the hopping and exchange interactions. We thereby demonstrate that the intersite Coulomb repulsions between the pairs induce a low-energy collective mode in the charge excitations of the systems where the internal charge degrees of freedom of the pairs play an essential role. Implications to the electronic states of NaV 2 O 5 , i.e., fluctuations of the valence state of V ions and phase transition as a charge ordering, are discussed.

Abstract: We investigate the ground state properties of electronic models for doped transition-metal oxides. An effective $t\ensuremath{-}J$ like Hamiltonian is derived from the case of strong Hund coupling between the conduction electrons and localized spins by means of the projection technique. An attractive interaction for conduction electrons and an antiferromagnetic coupling of the localized spin are obtained. A large ratio of the attraction to effective electron hopping, which is modulated by the spin background, will lead to the phase separation. The antiferromagnetic phase and the phase separation appear in the case of either high or low density of electrons. The possible relevance of the phase separation to the charge stripe phase in doped transition-metal oxides is discussed.

Abstract: The colossal-magnetoresistance compound ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}$ shows a transition to commensurate charge ordering $({\mathrm{Mn}}^{3+}{\char21{}\mathrm{Mn}\mathrm{}}^{4+})$ for $x=50%.$ In the range $x=42\char21{}50%,$ the high-temperature paramagnetic insulating phase transforms first into a conducting ferromagnetic phase at the Curie temperature ${T}_{C},$ and at somewhat lower temperature ${T}_{\mathrm{CO}}$ into the charge-ordered phase. The oxygen isotope effect on ${T}_{C}$ is small in this range, ${\ensuremath{\alpha}}_{O}=\ensuremath{-}d\mathrm{ln}{T}_{C}/d{\mathrm{lnm}}_{O}\ensuremath{\approx}0.14.$ In contrast to this, the charge-ordering transition shows a large and negative isotope effect, i.e., the heavier mass ${(}^{18}\mathrm{O})$ favors charge ordering. At the border of the existence of the charge-ordered phase, ${}^{18}\mathrm{O}$ substitution can induce this transition in an ${}^{16}\mathrm{O}$ sample that does not show this transition. The effect can be reversed by back exchange. These results show the extreme mass dependence of the charge-ordering transition in this system.

Abstract: Clear changes of the Yb 4f photoemission spectra with temperature are observed in Yb 4 As 3 by high resolution photoemission spectroscopy. The photoemission intensity is very low at the Fermi level and decreases as the temperature is lowered in accord with the narrowing of the 4f spectra. Bulk and surface contributions are deconvoluted in the 4f 13 final state spectra. In comparison with the corresponding results in Yb 4 (As 1- x Sb x ) 3 , the spectral changes of Yb 4 As 3 with temperature are confirmed to be beyond simple thermal broadening effects and related to the charge ordering effects. Although the change from the charge ordering temperature ( T t ) down to about T t /2 is consistent with a theoretical prediction, the change below it is rather unusual and may be ascribed to the incompleteness of the charge ordering effect. Spectral changes with Yb 4 Sb 3 -substitution are qualitatively discussed.

Abstract: It has been established as a result of successful direct experimental studies of the symmetry of the superconducting order parameter that the pairing symmetry in the compounds YBCO, GdBCO, Tl2201, and Bi2212 is of the d-wave type. In this paper, experimental and theoretical results concerning the d-wave superconducting state and the peculiarities of magnetic properties of the d-type superconductor such as pseudogap, charge ordering, and unusual structure of vortex excitations differing from that for s-states are analyzed. Several mechanisms of d-pairing and related microscopic theories are discussed and possible experiments for their verification are proposed.