A sharp decrease of resistivity in La0.7Ca0.3Mn0.96Cu0.04O3: Evidence for Cu-assisted coherent tunneling of spin Polarons

Anomalous temperature behavior of the resistivity in lightly doped manganites around a metal-insulator phase transition

“…As we shall see, the data are reasonably well fitted (for all T and x) by a unique (nonthermal) tunneling expression for the resistivity assuming a random (Gaussian) distribution of hopping sites and an explicit form for the temperature and doping dependent effective potential U ef f (T, x) = U (x)−E(T, x). Besides, the Cu doping induced competition between the barrier's height profile U (x) and the previously found [17] behavior of the carrier's kinetic energy E 0 (x) ≡ E(0, x) results in emergence of a satellite peak in the temperature behavior of the observed resistivity on the insulating side.…”

“…The observed phenomenon is thought to arise from competition between substitution induced strengthening of potential barriers (which hamper the charge hopping between neighboring M n sites) and weakening of carrier's kinetic energy. The data are well fitted assuming a nonthermal tunneling conductivity theory with randomly distributed hopping sites.PACS numbers: 71.30.+h, 75.50.Cc, 71.27.a To clarify the underlying microscopic transport mechanisms in exhibiting colossal magnetoresistance manganites, numerous studies (both experimental and theoretical) have been undertaken during the past few years [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] which revealed a rather intricate correlation of structural, magnetic and charging properties in these materials based on a crucial role of the M n 3+ −O−M n 4+ network. In addition to the so-called double-exchange (DE) mechanism (allowing conducting electrons to hop from the singly occupied e 2g orbitals of M n 3+ ions to empty e 2g orbitals of neighboring M n 4+ ions), these studies emphasized the important role of the Jahn-Teller (JT) mechanism associated with the distortions of the network's bond angle and length and leading to polaron formation and electron localization in the paramagnetic insulating region.…”

“…Given the growing experimental evidence [14,15] that polaronic distortions (evident in the paramagnetic state) persist in the ferromagnetic phase as well, we consider the observed resistivity to arise from tunneling of small spin polarons through the doping created potential barriers. According to a conventional picture [5][6][7]14,17], the conductivity due to tunneling of a carrier through an effective barrier of height U ef f and width R reads…”

Nature of temperature‐ and magnetic‐field‐dependent conduction mechanism in electron‐doped Ca0.85R0.15MnO3 (R = Pr, La) manganites

“…To study the temperature‐ and magnetic‐field‐dependent resistivity behavior of the semiconducting/insulator polycrystalline sample, different types of hopping model such as small‐polaron hopping, Mott‐type variable‐range hopping (VRH), E‐S type VRH, etc. have been adopted . Here, we have also reported the magnetoresistance data with an applied field as high as 15 T and the field‐dependent resistance has also been analyzed with a phenomenological model .…”

“…However, much less is known about the conduction mechanism of such electrondoped manganites (i.e., x < 0.2) up to 5 K and also with the application of high magnetic fields up to 15 T. Our plan in the present study is to concentrate on the conduction properties of electron-doped CO-antiferromagnetic insulating system Ca 0.85 R 0.15 MnO 3 (R ¼ Pr, La) with and without an applied magnetic field. To study the temperature-and magnetic-field-dependent resistivity behavior of the semiconducting/insulator polycrystalline sample, different types of hopping model such as small-polaron hopping, Mott been adopted [12][13][14][15]. Here, we have also reported the magnetoresistance data with an applied field as high as 15 T and the field-dependent resistance has also been analyzed with a phenomenological model [16].…”