Hour-glass magnetic spectrum in a stripeless insulating transition metal oxide

Abstract

An hour-glass-shaped magnetic excitation spectrum appears to be a universal characteristic of the high-temperature superconducting cuprates. Fluctuating charge stripes or alternative band structure approaches are able to explain the origin of these spectra. Recently, an hourglass spectrum has been observed in an insulating cobaltate, thus favouring the charge stripe scenario. Here we show that neither charge stripes nor band structure effects are responsible for the hour-glass dispersion in a cobaltate within the checkerboard charge-ordered regime of La 2 À x Sr x CoO 4 . The search for charge stripe ordering reflections yields no evidence for charge stripes in La 1.6 Sr 0.4 CoO 4 , which is supported by our phonon studies. With the observation of an hour-glass-shaped excitation spectrum in this stripeless insulating cobaltate, we provide experimental evidence that the hour-glass spectrum is neither necessarily connected to charge stripes nor to band structure effects, but instead, probably intimately coupled to frustration and arising chiral or non-collinear magnetic correlations.

Electronic and Magnetic Nano Phase Separation in Cobaltates La2−x Sr x CoO4

“…1b [13,14]. Also, in La 1.6 Sr 0.4 CoO 4 , the charge ordering peaks were not observed at the incommensurate positions that correspond to the magnetic peak positions, but, instead at half-integer peak positions typical for CBCO [8,14]. The CBCO character of the charge correlations in La 5/3 Sr 1/3 CoO 4 and La 1.6 Sr 0.4 CoO 4 is also reflected in the topmost 1 Co-O bond-stretching phonon dispersions that resemble the one in the half-doped CBCO ordered cobaltate La 1.5 Sr 0.5 CoO 4 where an anomalous phonon softening can be observed at half-integer propagation vectors, see magenta arrow in Fig.…”

“…We proposed that frustration effects in disordered CBCO phases produce the incommensurabilities in La 2−x Sr x CoO 4 [8]. Even more intriguing is the microscopic origin behind this mechanism.…”

“…The absence of charge stripes and Fermi surfaces in La 5/3 Sr 1/3 CoO 4 and La 1.6 Sr 0.4 CoO 4 forced us to find an alternative mechanism for the emergence of the hour-glass spectrum [8,13]. We proposed that frustration effects in disordered CBCO phases produce the incommensurabilities in La 2−x Sr x CoO 4 [8].…”

“…In contrast to the hightemperature superconducting cuprates La 2−x Sr x CuO 4 , the cobaltates remain insulating up to high hole-doping levels of x ∼ 1 [7]. Also, the antiferromagnetic correlations with La 2 CoO 4 -like character which are centered at the planar antiferromagnetic wavevector persist to very high holedoping ranges [8], higher than in the cuprates where incommensurate magnetic satellites have been observed already above ∼2 % Sr-doping [9]. A clearly incommensurate antiferromagnetic regime with well-separated incommensurate antiferromagnetic peaks can be observed only above ∼33 % of hole-doping in the cobaltates [8].…”

“…Also, the antiferromagnetic correlations with La 2 CoO 4 -like character which are centered at the planar antiferromagnetic wavevector persist to very high holedoping ranges [8], higher than in the cuprates where incommensurate magnetic satellites have been observed already above ∼2 % Sr-doping [9]. A clearly incommensurate antiferromagnetic regime with well-separated incommensurate antiferromagnetic peaks can be observed only above ∼33 % of hole-doping in the cobaltates [8]. At half-doping, a robust checkerboard charge order (CBCO) has been reported in these cobaltates (La 1.5 Sr 0.5 CoO 4 ) with a charge ordering temperature of the order of ∼800 K [10,11].…”