Charge Melting and Polaron Collapse inLa1.2Sr1.8Mn2

Abstract

X-ray and neutron scattering measurements directly demonstrate the existence of polarons in the paramagnetic phase of optimally-doped colossal magnetoresistive oxides. The polarons exhibit short-range correlations that grow with decreasing temperature, but disappear abruptly at the ferromagnetic transition because of the sudden charge delocalization. The "melting" of the charge ordering as we cool through TC occurs with the collapse of the quasi-static polaron scattering, and provides important new insights into the relation of polarons to colossal magnetoresistance.PACS numbers: 75.30. Vn, 75.30.Et, 71.30.+h, 71.38.+i Manganese oxides have attracted tremendous interest because they exhibit colossal magnetoresistance (CMR) -a dramatic increase in the electrical conductivity when they order ferromagnetically. The basic relationship between ferromagnetism and conductivity in doped manganese oxides has been understood in terms of the doubleexchange mechanism [1,2], where an itinerant e g electron hops between Mn 4+ ions, providing both the ferromagnetic exchange and electrical conduction. In addition, an important aspect of the physics of manganese oxides is the unusually strong coupling among spin, charge, and lattice degrees of freedom [2,3]. These couplings can be tuned by varying the electronic doping, electronic bandwidth, and disorder, giving rise to a complex phase diagram in which structural, magnetic, and transport properties are intimately intertwined. The charge-ordered phases represent one of the most intriguing results of balancing these couplings, and have been observed at low temperature in insulating, antiferromagnetically ordered manganites, but are incompatible with double exchangemediated ferromagnetism seen in optimally-doped CMR systems.In comparison to the cubic manganites such as La 1−x A x MnO 3 (A=Sr, Ca, Ba), the two-layer Ruddlesden-Popper compounds La 2−2x Sr 1+2x Mn 2 O 7 [4], where x is the nominal hole concentration, are advantageous to study because the reduced dimensionality strongly enhances the spin and charge fluctuations. The crystal structure is body-centered tetragonal (space group I4/mmm) [5] with a ≃ 3.87Å and c ≃ 20.15 A, and consists of MnO 2 bilayers separated by (La,Sr)O sheets. In the intermediate doping regime (0.32 ≤ x < 0.42), the ground state is a ferromagnetic metal, and the magnetoresistance is found to be strongly enhanced near the combined metal-insulator and Curie transition at T C (112 K for the x=0.4 system of present interest [6]). The present results reveal diffuse scattering associated with lattice distortions around localized charges, i.e. polarons, in the paramagnetic phase. The formation of lattice polarons above the ferromagnetic transition temperature T C has been inferred from a variety of measurements [7], but detailed observation via diffuse x-ray or neutron scattering in single crystals has been lacking until now [8]. Through such measurements, we have observed the collapse of quasi-static polaron scattering when the metallic, ferromagnetic sta...

Theory for charge and orbital density-wave states in manganite La0.5Sr1.5MnO4Yao1, Chen2, Gao3 et al. 2013Phys. Rev. B4010View full textAdd to dashboardCite

“…There are other ARPES experiments that also indicate nesting-induced charge/orbital ordering. In an early ARPES measurement of the bilayer manganite La 1.2 Sr 1.8 Mn 2 O 7 14 , the nesting wavevector (0.6π, 0) is found to be consistent with the modulation vector observed by x-ray and neutron experiments 15 . Another very recent ARPES measurement on bilayer manganite (La 1−z Pr z ) 1.2 Sr 1.8 Mn 2 O 7 shows addition evidence of the FS nesting induced ordering, where the observed FSs are almost straight lines, and the nesting wave vectors (π/2, 0) is confirmed as a modulation vector above the ferromagnetic transition temperature by elastic high energy x-ray diffraction measurement 16 .…”

Transient differential reflectivity of ferromagnetic and paramagnetic phases in the bilayered manganite La1.24Sr1.76Mn2O7

“…In the present system, there has been no report on phase separation in the ferromagnetic metallic region, thus it would be unjustified to attribute our fast component to the same origin [13]. A possible origin for this fast component is photoexcitation and a trapping process of carriers to quasistatic Jahn-Teller (JT) distortion [8,9,10], which interacts with localizing carriers. The photon energy of 1.5eV, which is near our photon energy, is assigned to a d-d transition in optical spectral weight [21,22,23].…”

“…Although there is some debate on whether this d-d transition is an inter-or intra-atomic transition, both interpretations can explain the optical transition at 1.5eV as arising from JT split-levels. Above T C , quasistatic JT distortion [8,9,10] has 6 been reported on a timescale of about 1ps, which is a comparable or longer than the timescale of our laser pulsewidth (0.2ps). Our laser pulse will be able to excite and observe the transition between JT split-levels.…”

“…24 June 2018 tic neutron scattering have revealed that an additional mechanism for CMR is strong electron-lattice coupling due to Jahn-Teller interaction [7,8,9,10]. Ultrafast optical measurement using a femtosecond laser pulse is also a promising technique for investigation of such electron-lattice coupling.…”

The role of CDW gap on the magnetic phase transition in CMR materials

“…The pseudogap may be a kind of dynamic charge density wave (CDW) gap originating from the Fermi surface instability. It has been observed as super lattice reflection in X-ray and neutron scattering experiments [10][11][12]. Recently some theoretical model calculations have been used to explain the CMR effects in manganites taking band J-T effect as one of the pseudo-gaps [13][14][15].…”