Suppression Of Charge Ordering Research Articles

Atomically-resolved interlayer charge ordering and its interplay with superconductivity in YBa2Cu3O6.81

High-temperature superconductive (SC) cuprates exhibit not only a SC phase, but also competing orders, suppressing superconductivity. Charge order (CO) has been recognized as an important competing order, but its microscopic spatial interplay with SC phase as well as the interlayer coupling in CO and SC phases remain elusive, despite being essential for understanding the physical mechanisms of competing orders and hence superconductivity. Here we report the achievement of direct real-space imaging with atomic-scale resolution of cryogenically cleaved YBa2Cu3O6.81 using cross-sectional scanning tunneling microscopy/spectroscopy. CO nanodomains are found embedded in the SC phase with a proximity-like boundary region characterized by mutual suppression of CO and superconductivity. Furthermore, SC coherence as well as CO occur on both CuO chain and plane layers, revealing carrier transport and density of states mixing between layers. The CO antiphase correlation along the c direction suggests a dominance of Coulomb repulsion over Josephson tunneling between adjacent layers.

Irreversible Meta‐Magnetic Transition in Charge Ordered Nd0.5Ca0.5MnO3 Manganite

The investigation of charge ordering (CO) suppression and consequence effects in manganites are of interest to the scientific community as they help them to understand the interactions among the spin, charge and orbital degrees of freedom of conduction electrons. In this work, the suppression of CO in small bandwidth half‐doped manganite, Nd0.5Ca0.5MnO3 (NCMO) has been demonstrated by reducing the particle size. Bulk and nano forms of NCMO materials are prepared by sol‐gel technique and characterized by various physicochemical techniques. The magnetic and magnetotransport properties of bulk samples show the exciting features, viz., CO, training and irreversible metamagnetic effects, whereas these features are significantly suppressed in the nanoscale sample. The observed behavior in the present investigation is explained in terms of magnetic phase separation and spin memory effects. Our investigation establishes a combined effect of surface spin disorder and lattice strains for the suppression of CO behavior in NCMO manganite system.

Effect of grain size on charge and spin correlations in Bi0.5Ca0.5MnO3 manganite nanoparticles

In this work we report the electron spin resonance (ESR) and magnetization (M) studies to understand the effect of grain size (GS) on the charge ordering and spin correlations in Bi0.5Ca0.5MnO3 manganite synthesized by sol–gel method. The suppression of charge ordering (CO), long-range antiferromagnetic (AFM) state, shifting of ferromagnetic (FM)-cluster glass (CG) transition towards higher temperatures and evolution of different magnetic correlations with decrease in GS are discussed in view of the changes in surface to volume ratio of nano-grains.

Charge order suppression, emergence of ferromagnetism and absence of exchange bias effect in Bi0.25Ca0.75MnO3 nanoparticles: Electron paramagnetic resonance and magnetization studies

We report the results of magnetization and electron paramagnetic resonance (EPR) studies on nanoparticles (average diameter ∼ 30 nm) of Bi0.25Ca0.75MnO3 (BCMO) and compare them with the results on bulk BCMO. The nanoparticles were prepared using the nonaqueous sol-gel technique and characterized by XRD and TEM analysis. Magnetization measurements were carried out with a commercial physical property measurement system (PPMS). While the bulk BCMO exhibits a charge ordering transition at ∼230 K and an antiferromagnetic (AFM) transition at ∼130 K, in the nanoparticles, the CO phase is seen to have disappeared and a transition to a ferromagnetic (FM) state is observed at Tc ∼ 120 K. However, interestingly, the exchange bias effect observed in other nanomanganite ferromagnets is absent in BCMO nanoparticles. EPR measurements were carried out in the X-band between 8 and 300 K. Lineshape fitting to a Lorentzian with two terms (accounting for both the clockwise and anticlockwise rotations of the microwave field) was employed to obtain the relevant EPR parameters as functions of temperature. The results confirm the occurrence of ferromagnetism in the nanoparticles of BCMO.

Electron spin resonance and magnetization studies on Bi0.5Ca0.5Mn0.95TM0.05O3 (TM = Cr, Fe, Co and Ni)

In this work, we present the temperature dependent ESR and magnetization measurements on 3d TM doped Bi-manganite samples of composition Bi0.5Ca0.5Mn0.95TM0.05O3 (TM = Cr, Fe, Co and Ni) synthesized by solid state reaction method. The temperature dependent double integrated intensity (DI) of the ESR signal and the magnetic moment (M) have been used to estimate the charge ordering temperature (TCO), antiferromagnetic ordering (AFM) onset temperature TO and Neel temperature, TN. The 1/DI and 1/M versus T plots show FM correlations in the temperature range T > TCO and coexistence of ferromagnetic (FM) and AFM correlations in the temperature range TCO > T > TO. The magnetization data give TCO values close to the values obtained from ESR data. The data are analyzed in view of electronic phase separation model. The suppression of charge ordering is explained in view of the changes in short range magnetic interactions between Mn ions and the possible change in the band structure as a result of TM ion doping.

Magnetic dynamics of charge ordered Nd 0.80Na 0.20MnO 3 compound

The magnetic dynamics of charge ordered Nd 0.8Na 0.2MnO 3 compound was studied by measuring the temperature variation of magnetization for different magnetic fields up to 7 T and, the field variation of magnetization at different temperatures down to 5 K. This sample exhibits a charge-ordering transition at 180 K, followed by a weak ferromagnetic (FM) transition at around 100 K and a spin glass like transition below 40 K. Suppression of charge-ordering and spin glass like transition and increase in FM T C were observed with an increase in magnetic field. A reversible metamagnetic transition above a threshold field ( H f ) of 4.5 T was observed at 130 K, followed by a saturation magnetization of 3.2 μ B /f.u. However at 5 K, an irreversible field induced first order phase transition from charge ordered state to FM state was observed at H f =5 T. For comparison, the temperature and field variations of magnetization were studied on a FM compound from the same series with the composition Nd 0.90Na 0.10MnO 3. A clear FM transition with a T C of 113 K and a saturation magnetization of 4.3 μ B /f.u was observed.

MODIFICATION OF TRANSPORT AND MAGNETIC BEHAVIOR IN HALF-DOPED CHARGE-ORDERED NANOCRYSTALLINE MANGANITE Sm0.5Ca0.5MnO3

Half-doped Sm0.5Ca0.5MnO3 nanoparticles with varying particle size down to ~14 nm are synthesized by chemical "pyrophoric reaction" route and their electronic transport and magnetotransport behaviors are investigated in detail in order to understand the nanodimensional size effect on the charge ordering in manganites. The experimental result shows that reduction of particle size (having same chemical composition) down to nanometer scale destabilizes the charge ordering. Weak ferromagnetism and metal–insulator transition are observed in the sample having lowest particle size (14 nm) as a consequence of such suppression of charge ordering. The observed phenomena have been attributed to enhanced surface pressure in the nanoparticles.

D−dexcitations and charge ordering inLa5/3Sr1/3NiO4

We report the temperature dependence of the crystal-field-excitation spectrum in ${\text{La}}_{5/3}{\text{Sr}}_{1/3}{\text{NiO}}_{4}$ by resonant inelastic x-ray scattering at the $\text{Ni}\text{ }K$ edge. Two distinct excitations are observed in the 1--3 eV energy range, in agreement with optical conductivity studies. The temperature dependence of the low-energy spectral weight is found to be closely related to the appearance and suppression of charge ordering. The relation between these crystal-field excitations and the stripe ordering is discussed.

Suppression of charge order and exchange bias effect in Nd0.5Ca0.5MnO3 nanocrystalline

An Nd0.5Ca0.5MnO3 (NCMO) sample (average diameter ∼45 nm) is synthesized by the sol–gel method. The temperature dependence of magnetization indicates that the charge order state is suppressed and a ferromagnetic (FM) transition occurs at ∼100 K. In addition, the magnetic hysteresis loop at 10 K under a cooling field of 10 kOe shifts to both the horizontal and the vertical directions when the measure field is 10 kOe. With an increase in the measure field, both the horizontal and the vertical shifts decrease. When the measure field is 50 kOe, the vertical shift vanishes but the horizontal shift still exists. The observed exchange bias effect is attributed to the exchange coupling between the antiferromagnetic core and the FM shell which embodies spin glass-like surface layers.

Ionic relaxation contribution to the electronic reconstruction at then-typeLaAlO3/SrTiO3interface

Density-functional theory calculations reveal that the compensationmechanism at the isolated n-type interface in LaAlO(3)/SrTiO(3)superlattices involves both ionic and electronic degrees of freedom.Strong polar distortions screen the local electric field and reduce theband discontinuity across the interface. We find that the electronicreconstruction depends sensitively on whether structural optimization isperformed within GGA (conventional exchange and correlation effects) orGGA+U (which includes strong intra-atomic interactions). For astructural optimization within GGA+U the excess charge is confined tothe interface TiO(2) layer with a charge-ordered, orbitally polarizedarrangement of Ti(3+) and Ti(4+). While the charge-ordered phaserepresents the ground state, optimization within GGA leads to morepronounced lattice polarization, suppression of charge order (withremaining d(xy)-orbital occupation in the interface layer), and adelocalization of the excess charge extending over a few SrTiO(3)layers.

Suppression of charge-ordering and appearance of magnetoresistance in a spin-cluster glass manganite La 0.3Ca 0.7Mn 0.8Cr 0.2O 3

The magnetic properties of electron-doped manganite La 0.3Ca 0.7MnO 3 and La 0.3Ca 0.7Mn 0.8Cr 0.2O 3 polycrystalline samples prepared by sol–gel technique have been investigated between 5 and 300 K in magnetic fields ranging from 0 to 5 T. The transition at 260 K, attributed to charge ordering in La 0.3Ca 0.7MnO 3, is completely suppressed in the Cr-substituted sample while the onset of a magnetic remanence followed by the appearance of a magnetic irreversibility at lower temperatures is observed in both samples. These features indicate that ferromagnetic clusters coexist with either an antiferromagnetic phase for La 0.3Ca 0.7MnO 3 or a spin-cluster glass phase for La 0.3Ca 0.7Mn 0.8Cr 0.2O 3 at the lowest temperatures. The exponential temperature dependence of the resistivity for the Cr-substituted sample is consistent with the small polaron hopping model for 120 K< T<300 K, while the data are better described by Mott's hopping mechanism for T<120 K. Whereas the parent compound La 0.3Ca 0.7MnO 3 is known to show no magnetoresistance, a large negative magnetoresistance is observed in the La 0.3Ca 0.7Mn 0.8Cr 0.2O 3 sample below 120 K. The appearance of the CMR is attributed to spin dependent hopping between spin clusters and/or between ferromagnetic domains.

The nature of magnetoelastic anomalies in Nd0.5Sr0.5MnO3 upon suppression of charge ordering by a magnetic field

This paper reports on the results of investigations into the magnetic and magnetoelastic properties of Nd0.5Sr0.5MnO3 single crystals in pulsed magnetic fields up to 250 kOe, the magnetic and kinetic properties of these crystals in weak static magnetic fields, and their thermal expansion. It is demonstrated that the studied properties exhibit a number of anomalies due to a transition from the antiferromagnetic semiconducting state to the ferromagnetic metallic state upon suppression of charge ordering.