Sliding charge-density wave in manganites

Abstract

The so-called stripe phase of the manganites is an important example of the complex behaviour of metal oxides, and has long been interpreted as the localisation of charge at atomic sites 1,2,3,4 . Here, we demonstrate via resistance measurements on La 0.50 Ca 0.50 MnO 3 that this state is in fact a prototypical charge density wave (CDW) which undergoes collective transport. Dramatic resistance hysteresis effects and broadband noise properties are observed, both of which are typical of sliding CDW systems. Moreover, the high levels of disorder typical of manganites result in behaviour similar to that of well-known disordered CDW materials. Our discovery that the manganite superstructure is a CDW shows that unusual transport and structural properties do not require exotic physics, but can emerge when a well-understood phase (the CDW) coexists with disorder.The stripe phase in manganites of the form La 1−x Ca x MnO 3 appears as the temperature is lowered through T ≃ 240 K, and the superstructure wavevector settles on a final value of q≃ (1 − x)a * 1 (where a * is the reciprocal lattice vector) for 0.5 ≤ x < 0.85, at T ≃ 120 K 2 . Based on the insulating nature of the manganites up to room temperature, and the observation of stripes of charge order in transmission electron microscopy (TEM) images, early studies concluded that the superstructure arose from localisation of charge at atomic sites 3,4 . However, neutron and xray studies found the degree of charge localisation at Mn sites to be small, and subsequent theoretical work suggested that a CDW model may be more applicable 5 . This suggestion is supported by the observation that q/a * is strongly temperature dependent 2,6 , indicating that a model in which the superstructure periodicity is derived from the sample stoichiometry cannot be valid. In addition, heat capacity peaks at the transition to the stripe phase can be modelled as "dirty Peierls transitions", expected in a disordered CDW system 7 . However, the possibility of the stripe phase exhibiting sliding behaviour, as seen in many other CDW systems 8 , could not be probed without the ability to make orientation-dependent resistivity measurements. Here we describe the first such measurements on the manganite stripe phase, which reveal dramatic

Lattice-form-dependent orbital shape and charge disproportionation in charge- and orbital-ordered manganites

“…This result is in contrast with the belief at the early stage of the research of the CO-OO phenomena, but in accord with the recent results 16,17,18,19,20,21,22,23,24,25 . This indicates that various interactions, such as electron-lattice, Coulomb repulsion, and magnetic interactions, should be taken into account to correctly understand the nature of chargeordered state.…”

“…Many diffraction and theoretical studies have reported the checkerboard type charge order of Mn 3+ and Mn 4+ ions with charge disproportionation and stripe-type orbital order, explaining a complex antiferromagnetic spin order at low temperatures in half-doped manganites 7,8,9,10,11,12,13,14,15,16 . On the other hand, another model with less distinct charge disproportionation has also been proposed 17,18,19,20,21,22,23,24,25 . For example, taking into account the on-site Coulomb interaction, Brink et al have suggested 17 that maximum value of charge disproportionation is as little as 20%.…”

X-ray-induced persistent photoconductivity in vanadium dioxide

“…With discrete sites, the relaxation becomes stretched [42]. However, our observed value of β = 0.69 is slightly larger than the expected value of 3 5 for 3-dimensional systems [43]. This increase is often observed near and above the MIT temperature where electron correlations are reduced and simple (Debye) exponential relaxation is recovered [44].…”

“…Metal oxides exhibit diverse ground states and phenomena including high-temperature superconductivity [1], colossal magnetoresistance [2], charge density waves [3], Verwey transitions [4], and metal-insulator transitions (MIT) [5]. Significant attention has been given to the deceptively simple MIT, which has to date been driven with temperature, electric field [6], pressure [7], chemical doping [8], ultrafast laser excitation [9], and ionizing radiation [10].…”

Crossover of angular dependent magnetoresistance with the metal-insulator transition in colossal magnetoresistive manganite films

“…1 The huge change in conductivity could be attributed to melting of long-range and/or short-range charge/orbital ordering ͑COO͒, a percolation transition, or a sliding charge-density wave. 2 However, no consensus in the microscopic mechanism of the MIT has been reached yet.…”