Carrier Density Collapse and Colossal Magnetoresistance in Doped Manganites

Abstract

A novel ferromagnetic transition, accompanied by carrier density collapse, is found in doped chargetransfer insulators with strong electron-phonon coupling. The transition is driven by an exchange interaction of polaronic carriers with localized spins; the strength of the interaction determines whether the transition is first or second order. A giant drop in the number of current carriers during the transition, which is a consequence of bound pairs formation in the paramagnetic phase close to the transition, is extremely sensitive to an external magnetic field. This carrier density collapse describes the resistivity peak and the colossal magnetoresistance of doped manganites. 71.30.+h, 71.38.+i, 72.20.Jv, 75.50.Pp, 75.70.Pa The interplay of the electron-phonon and exchange interactions [1-3] is thought to be responsible for many exotic properties of oxides ranging from high-T c superconductivity in cuprates [4] to colossal magnetoresistance (CMR) and ferromagnetism in doped manganites [5][6][7][8][9]. A huge negative magnetoresistance was observed in doped perovskite manganites La 1−x D x MnO 3 (D=Ca, Sr, Ba) close to the ferromagnetic transition in a certain range of doping x ≈ 0.15 − 0.4 [6][7][8][9], and this raised a question of possible applications.The metal-insulator transition in lanthanum manganites has long been thought to be the consequence of a double exchange (DEX), which results in a varying band width of holes doped into the Mn 3+ d-shell [10], as function of the doping concentration and temperature. Recently it has been realized [11], however, that the effective carrier-spin interaction in DEX model is too weak to lead to a significant reduction of the electron band width and, therefore, cannot account for the observed scattering rate [12] (see also Ref. [13]) or for localization induced by slowly fluctuating spin configurations [14]. In view of this problem, it has been suggested [11] that the essential physics of perovskite manganites lies in the strong coupling of carriers to Jahn-Teller lattice distortions. The argument [11] was that in the high-temperature state the electron-phonon coupling constant λ is large (so that the carriers are polarons) while the growing ferromagnetic order increases the bandwidth and thus decreases λ sufficiently for metallic behavior to set in below the Curie temperature T c . A giant isotope effect [15], the sign anomaly of the Hall effect, and the Arrhenius behavior of the drift and Hall mobilities [16] over a temperature range from 2T c to 4T c unambiguously confirmed the polaronic nature of carriers in manganites.However, an early established unusual relation between magnetization and transport below T c have led to a conclusion that the polaronic hopping is the prevalent conduction mechanism also below T c [17]. Lowtemperature optical [18][19][20], electron-energy-loss (EELS) [21] and photoemission spectroscopies [22] showed that the idea [11,14] of a 'metalization' of manganites below T c is not tenable. A broad incoherent spectral feature [18][19][20]22]...

Phase Coexistence and Resistivity near the Ferromagnetic Transition of Manganites

“…The earlier of the above observations [3][4][5][6]9] led to a novel theory of ferromagnetic-paramagnetic phase transition and CMR, based on the so-called current-carrier density collapse (CCDC) [12], confirmed by later observations. In the CCDC model, the p holes are bound into heavy bipolarons above the Curie temperature T C due to the Fröhlich electron-phonon interaction, which is written in the realspace representation as…”

“…In particular, the oxygen isotope effect has been observed in the low-temperature resistivity of La 0:75 Ca 0:25 MnO 3 and Nd 0:7 Sr 0:3 MnO 3 and explained by CCDC with polaronic carriers in the ferromagnetic phase [16]. The current-carrier density collapse has been directly observed using the Hall data in La 0:67 Ca 0:33 MnO 3 and La 0:67 Sr 0:33 MnO 3 [17], and the first-order phase transition at T C , predicted by the theory [12], has been firmly established in the specific heat measurements [18]. Importantly, the character of the magnetic phase transition in Tl 2 Mn 2 O 7 pyrochlores has also been determined to be of the first order [19] and attributed to the tendency of small polarons to phase separation at finite carrier density.…”

Alexandrov and Bratkovsky Reply:

“…Irrespective of any theoretical arguments, the existence of polarons in CMR materials is unambiguously confirmed experimentally, including very low mobility, activated dc and ac transport in the paramagnetic phase, and the giant isotope effect (for references, see [1]). Contrary to Nagaev's claim, the theory [1] well describes the optical properties of manganites, in particular the giant spectral weight transfer with temperature and magnetic field [6].…”

“…• This was published in the journal, Physical Review Let- Alexandrov and Bratkovsky Reply: In our Letter [1] we proposed a theory of ferromagnetism and colossal magnetoresistance (CMR) based on the idea of a current carrier density collapse due to pairing of polaronic carriers in doped ferromagnetic semiconductors. Nagaev [2] has neglected the fact that the theory is largely independent of the type of electron-phonon interaction and of a particular orbital state of holes.…”

Phase separation and charge ordering in doped manganite perovskites: Projection perturbation and mean-field approaches

“…Most theoretical efforts focus on understanding metallic ferromagnetism and its connection to unusual transport properties [12][13][14][15][16][17] . The scenario of double exchange mechanism is extensively accepted to explain the metallic ferromagnetism.…”