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Published on November 26, 2007

Author: Wanderer

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Charge Ordering in Transition Metal Oxides:  Charge Ordering in Transition Metal Oxides Mohammad E. Ghazi Shahrood University Of Technology, Iran In the name of God Slide2:  Oxides display a range of different structural, electronic and magnetic phases due to electron correlations • superconductivity in copper oxides: La2-xSrxCuO4 • stripe phases in nickel oxides : La2-xSrxNiO4; La2NiO4+ •colossal magnetoresistance in manganese oxides: La1-xCaxMnO3, Nd1-xSrxMnO3 A common theme of charge, spin and orbital ordering La2-xSrxCuO4 and La2-xSrxNiO4:  La2-xSrxCuO4 and La2-xSrxNiO4 La2-xSrxNiO4 does not exhibit superconductivity for any value of x. But displays charge ordering (periodic ordering of doped holes at low temperature). Attraction : To understand the possible relationship between charge ordering and superconductivity The parent compounds of La2CuO4 and La2NiO4 are an AFM insulator Doping with Sr and creating hole: In cuprites >5% causes the formation of a superconducting phase. The properties of La2-xSrxNiO4:  The properties of La2-xSrxNiO4 Resistivity and susceptibility measurements show that x = 1/3 is special. However for x = 1/3 there are no structural phase transitions at low temperatures. But strong effect of charge ordering on physical properties Temperature derivative of logarithmic resistivity (upper panel) and susceptibility multiplied by temperature (lower panel) versus temperature for Sr concentrations of 0.2 x 0.4 [Phys. Rev. B 49,7088,1994]. Properties of La5/3Sr1/3NiO4:  Properties of La5/3Sr1/3NiO4 Charge ordering at TCO  240 K and ordering of the Nickel spins at TSO  190 K. The stripes run along diagonal directions in NiO2 planes The transition into the charge striped phase is entirely second order. Anomalies associated with charge and spin ordering in these materials have also been observed in sound velocity, specific heat, resistivity, and magnetic susceptibility. Stripe correlations are maximised at x = 1/3 and x = 1/2. Mechanisms of Stripe Ordering:  Mechanisms of Stripe Ordering Leads to : Competition between long-range Coulombic repulsion of doped holes and short-range attractive interactions such as electron-lattice coupling and magnetic confinement effect, etc. Polaron ordering due to the large magnetic coupling between Ni and hole spin with large electron phonon coupling induces self-localisation of holes (a small polaron) Frustrated phase separation: The holes minimise their kinetic energy by attempting to separate into hole-rich regions and regions with local antiferromagnetic correlations Experimental Probes of Charge Stripes:  Experimental Probes of Charge Stripes Neutron Diffraction Used for study of phonons , spin ordering etc. but low intensity, low resolution and needs large samples. • Electron Diffraction Sensitive to small charge/lattice distortions, but low resolution and intensities difficult to interpret X-rays as a probe of charge stripes:  X-rays as a probe of charge stripes • Higher wavevector resolution, Higher intensities, and can use smaller crystal (better quality) • Mainly using synchrotron radiation at BM28, ESRF, and SRS, in the UK at Daresbury . Experiments involve cryostat, triple crystal geometry with high-resolution analyser crystals. Results and Discussion 1: Intensity:  Results and Discussion 1: Intensity Intensity of charge stripe reflections close to the melting point fit the power law TCO = 239.2 ±0.2 2 = 0.23 ±0.02 • Exponent indicates 2D nature of charge stripes and critical scattering above TCO. New, weak satellites found at low temperatures surrounding Bragg reflections at non-integer positions such as (4.66, 0,5), (5.33, 0, 7). These disappear above 240 K. All indexed as q = (h±2, 0, l) with h = even and l = odd.  = 0..33 (i.e. Sr stoichiometry). Results and Discussion 2: Width:  Results and Discussion 2: Width The width of the charge stripe satellites gives us a measure of the inverse correlation length. Above TCO the width increases dramatically because of critical scattering. Fit to the power law TCO = 239.2 H,K = 1.08±0.2 ; L = 1.005 ±0.03 Results and Discussion 3: Order, Disorder & Liquid:  Results and Discussion 3: Order, Disorder & Liquid At low temperatures the stripes are correlated, but this does not improve as the temperature is decreased, suggesting quenched and partially ordered stripes. Above the melting point (240 K) scattering observed due to fluctuations into the charge stripe phase. Below the melting temperature displays an anisotropic broadening suggestive of an order-disorder transition. What happens away from x = 1/3?:  What happens away from x = 1/3? The width (wavevector) of the stripes is both stoichiometry and temperature dependant. The deviation away from nh =  starts below x = 1/3 and increases as x decreases. The density of the holes within an average stripe is nh/ .. So the variation in  is inversely related to the hole density (nh is a constant). The variation of  occurs principally at higher temperatures and ceases at low temperatures. Manganese Oxides, Nd0.5 Sr0.5 MnO3:  Manganese Oxides, Nd0.5 Sr0.5 MnO3 Attractions: CMR, charge-, spin-and orbital ordering due to the presence of the multi valence Mn (Mn+3, Mn+4) ion. Mn +3 (d4)has an electronic configuration with 3 electrons in the lower t2g band and the outer electron in the degenerate eg orbital. Different phases include ferromagnetic metal, antiferromagnetic insulators, canted antiferromagnetic etc. Spin and orbital ordering is common over a wide range of stoichiometry but charge ordering exists only over a narrow range around Nd½Sr½MnO3. Slide14:  Nd0.5Sr0:5MnO3 is a ferromagnetic metal with a TC of ~ 250 K and transforms to an insulating CO state around 160 K. The CO transition is first order. The FMM – CO transition is accompanied by spin and orbital ordering, and the CO insulator is antiferromagnetic (CE type). Manganese Oxides, Nd0.5 Sr0.5 MnO3 Manganese Oxides, Nd0.5 Sr0.5 MnO3:  Below TCO~160 K,, (Periodic ordering of Mn3+ and Mn4+ ions at low temperature) a structural modulation arises from the Jahn-Teller (J-T) distortion associated with charge and orbital ordering. This causes the neighbouring undistorted Mn4+O6 octahedra to displace in opposite directions and a doubling of the unit cell along the a-axis . Manganese Oxides, Nd0.5 Sr0.5 MnO3 The projection of Nd0.5Sr0.5MnO3 superstructure in the ac-plane at low temperature. Arrows show the displacements of the Mn4+O6 octahedra [P. G. Radaelli et al.]. We report X-ray scattering measurements of J-T distortion ordering satellite in the low temperature phase . These peaks had a wavevector (1/2, 0, 0) and an intensity of approximately 10-3 that of the Bragg reflections. First-order structural phase transition in Nd½Sr½MnO3:  First-order structural phase transition in Nd½Sr½MnO3 Measurement of Bragg reflections show transition from ferromagnetic metallic phase into low temperature charge ordered phase. The structural transition is first order and displays considerable hysteresis. Melting of charge order satellites with temperature:  Melting of charge order satellites with temperature Inverse width (correlation length) of charge order satellites also displays a dramatic collapse at the structural transition. • Intensity of charge order satellites as a function of temperature displays large hysteresis, typical of first-order structural phase transition Charge, Spin and Orbital Ordering in Transition Metal Oxides:  Charge, Spin and Orbital Ordering in Transition Metal Oxides Different types of charge, spin or orbital ordering have been found in a large number of different transition metal oxides. Iron oxides, Fe3O4, La1-xSrxFeO3, LuFe2O4 Cobalt Oxides, YBa2Co2O5, La2-xSrxCoO4, Vanadium Oxides, V2O3, NaV2O5, • In nearly all cases, such ordering causes dramatic changes in physical properties, metal-insulator transitions, magnetic and electrical properties etc. • Charge and spin ordering is found to exist in some high-TC oxide superconductors. • The understanding of charge, spin and orbital ordering is a new, and very lively, area of research. Acknowledgements and collaborators:  Acknowledgements and collaborators Prof. Peter Hatton (Durham, UK) Dr. S. Wilkins, and Dr. P. Spencer Prof. S-W. Cheong (Rutgers, USA) Dr. A. Boothroyd (Oxford) Dr. S. Brown ( XMaS, ESRF, Grenoble) Dr. S. Collins (16.3 SRS, UK) And: Thank you for your attention

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