Correlating quasiparticle excitations with quantum femtosecond magnetism in photoexcited nonequilibrium states of insulating antiferromagnetic manganites

Abstract

We describe a mechanism for insulator-to-metal transition triggered by spin-canting following fs laserexcitation of insulating anti-ferromagnetic (AFM) states of colossal magneto-resistive (CMR) manganites. We show that photoexcitation of composite fermion quasi-particles dressed by spin fluctuations results in the population of a broad metallic conduction band due to canting of the AFM background spins via strong electron-spin local correlation. By inducing spin-canting, photoexcitation can increase the quasi-particle energy dispersion and quench the charge excitation energy gap. This increases the critical Jahn-Teller (JT) lattice displacement required to maintain an insulating state. We present fs-resolved pump-probe measurements showing bi-exponential relaxation of the differential reflectivity below the AFM transition temperature. We observe a nonlinear dependence of the ratio of the fs and ps relaxation component amplitudes at the same pump fluence threshold where we observe femtosecond magnetization photoexcitation. We attribute this correlation between nonlinear fs spin and charge dynamics to spin/charge/lattice coupling and population inversion between the polaronic majority carriers and metallic quasi-electron minority carriers as the lattice displacement becomes smaller than the critical value required to maintain an insulating state following laser-induced spin canting.

Photoinduced transitions in magnetoresistive manganites: A comprehensive view

“…The ferromagnetic correlations are found to emerge within the first 120 fs after excitation 23,24 and the antiferromagnetic magnetic order is suppressed on a time-scale faster than 250 fs 21 .…”

“…The spin ordering is also significantly altered after photo-excitation. The antiferromagnetic order is melted [20][21][22] and ferromagnetic correlations emerge 23,24 . Other excitation mechanism such as resonant excitation of the lattice 21,25,26 , pulsed magnetic fields 27 , intense…”

Room temperature hidden state in a manganite observed by time-resolved X-ray diffraction

“…In this arrangement, one Mn 3+ ion is surrounded by four Mn 4+ ions in the same a-b plane and two other Mn 3+ ions in the two neighboring a-b planes. 2,17,18 The e g orbitals on the Mn 3+ site are known to split into a lower 3x 2 −r 2 /3y 2 −r 2 level (e g 1 ) and a higher x 2 −z 2 /y 2 −z 2 level (e g 2 ) owing to Jahn-Teller (JT) distortion, with the e g electron occupying the lower one, while those e g orbitals on the Mn 4+ site are degenerate. After photon illumination in 1-2 eV range, the e g electron on Mn 3+ was suggested to be excited priorly.…”

“…However, because the spins within one zig-zag chain are parallel, femtosecond spin-flip/canting could not be induced by the intrachain charge transfer. 3,18,26 During the second route, a Mn 3+ to Mn 4+ charge transfer will instantly turn the sample into a JT inactive state within 100-200 fs presented as the first drop in reflectivity (Fig. 1c and S2).…”

Nonequilibrium Pair Breaking in Ba(Fe1−xCox)Yang1, Luo2, Mootz3 et al. 2018Phys. Rev. Lett.Self Cite350190View full textAdd to dashboardCite

“…Ultrafast optical tailoring of transient quantum states provides a new way to discover, design, and control exotic correlated materials phases. Recent examples include, among others, quantum femtosecond magnetism [1,2], light-induced superconductivity [3] and a metastable QP prethermalized phase hidden beneath SC [4]. This strategic approach is implemented by non-thermal separation, within a certain time window, of distinct coupled orders.…”