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See detailDephasing in the semiclassical limit is system-dependent
Petitjean, Cyril ULg; Jacquod, P.; Whitney, R. S.

in JETP LETTERS (2007), 86(10), 647-651

Dephasing in open quantum chaotic systems has been investigated in the limit of large system sizes to the Fermi wavelength ratio, L/lambda F >> 1. The weak localization correction g(wl) to the conductance ... [more ▼]

Dephasing in open quantum chaotic systems has been investigated in the limit of large system sizes to the Fermi wavelength ratio, L/lambda F >> 1. The weak localization correction g(wl) to the conductance for a quantum dot coupled to (i) an external closed dot and (ii) a dephasing voltage probe is calculated in the semiclassical approximation. In addition to the universal algebraic suppression g wl proportional to (1 + tau(D)/tau(phi))(-1) with the dwell time tau(D) through the cavity and the dephasing rate, we find an exponential suppression of weak localization by a factor of proportional to exp[-(tau) over tilde/tau(phi)], where is the system-dependent parameter. In the dephasing probe model, (tau) over tilde coincides with the Ehrenfest time, (tau) over tilde proportional to ln[L/lambda(F)], for both perfectly and partially transparent dot-lead couplings. In contrast, when dephasing occurs due to the coupling to an external dot, (tau) over tilde proportional to ln[L/xi] depends on the correlation length xi of the coupling potential instead of lambda(F). [less ▲]

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See detailNegative magnetoresistance in mixed-valence La0.6Y0.1Ca0.3MnO3: Evidence for charge localization governed by the Curie-Weiss law
Sergeenkov, S.; Bougrine, Hassan ULg; Ausloos, Marcel ULg et al

in JETP Letters (1999), 70(2), 141-146

The colossal negative magnetoresistance Delta rho observed in La0.6Y0.1Ca0.3MnO3 at B=1 T shows an unusual, nearly perfect symmetry (around the peak temperature T-0=160 K), suggestive of a Curie-Weiss ... [more ▼]

The colossal negative magnetoresistance Delta rho observed in La0.6Y0.1Ca0.3MnO3 at B=1 T shows an unusual, nearly perfect symmetry (around the peak temperature T-0=160 K), suggestive of a Curie-Weiss-dominated transport mechanism in this material both above and below the field-dependent Curie temperature T(C)equivalent to T-0. Attributing this symmetry to strong magnetic fluctuations below T-C (which are triggered by the Y substitution and cause a "bootstrap" destruction of the ferromagnetic phase), the data are interpreted in terms of the nonthermal spin hopping and magnetization-dependent charge carrier localization scenario leading to Delta rho=-rho(s)(1-e(-gamma M2)), with the magnetization M(T,B)=CB/\ T-T-C(B)\(nu). The separate fits through all the data points above and below T-C yield C(+)similar or equal to 2C(-) and nu(+)similar or equal to nu(-)similar or equal to 1, in agreement with the predictions of the Landau mean-field theory. (C) 1999 American Institute of Physics. [S0021-3640(99)01314-6]. [less ▲]

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See detailA sharp decrease of resistivity in La0.7Ca0.3Mn0.96Cu0.04O3: Evidence for Cu-assisted coherent tunneling of spin polarons
Sergeenkov, S. A.; Bougrine, Hassan ULg; Ausloos, Marcel ULg et al

in JETP Letters (1999), 69(11), 858-862

Nearly a 50% decrease of the resistivity rho(T,x) is observed upon just 4% Cu doping at the Mn site of La2/3Ca1/3Mn1-xCuxO3. When the observed phenomenon is attributed to a decrease of the spin-polaron ... [more ▼]

Nearly a 50% decrease of the resistivity rho(T,x) is observed upon just 4% Cu doping at the Mn site of La2/3Ca1/3Mn1-xCuxO3. When the observed phenomenon is attributed to a decrease of the spin-polaron energy E-sigma(x) below T-C(x), all of the data are found to be well fitted by the nonthermal coherent tunneling expression rho(T,x)=rho(0)e(-gamma M2(T,x)), assuming that the magnetization in the ferromagnetic state is given by the expression M(T,x)=M-R(x)+M-0(x)tanh{T-[root(C)(x)/T](2)-1}. The best fits through all the data points suggest M-0(x)similar or equal to root 1-xM(0)(0), M-R(x)similar or equal to x root M-0(0), and E-sigma(x)similar or equal to E-sigma(0)(1-x)(4) for the explicit x dependence of the Cu-induced modifications of the Mn-spin-dominated zero-temperature spontaneous magnetization, residual paramagnetic contribution, and spin-polaron tunneling energy, respectively, with E-sigma(0)=0.12 eV. (C) 1999 American Institute of Physics. [S0021-3640(99)00811-7]. [less ▲]

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See detailAnomalous temperature behavior of the resistivity in lightly doped manganites around a metal-insulator phase transition
Sergeenkov, S.; Ausloos, Marcel ULg; Bougrine, Hassan ULg et al

in JETP Letters (1999), 70(7), 481-487

An unusual temperature and concentration behavior of the resistivity ρ(T,x) in La0.7Ca0.3Mn1-xCuxO3 is observed at slight Cu doping (0 ≤x≤ 0.05). Namely, the introduction of copper results in a splitting ... [more ▼]

An unusual temperature and concentration behavior of the resistivity ρ(T,x) in La0.7Ca0.3Mn1-xCuxO3 is observed at slight Cu doping (0 ≤x≤ 0.05). Namely, the introduction of copper results in a splitting of the resistivity maximum around the metal-insulator transition temperature T0(x) into two differently evolving peaks. Unlike the original Cufree maximum, which steadily increases with doping, the second (satellite) peak remains virtually unchanged for x<xc, increases for x≥xc, and finally disappears at xm ≃ 2xc, with xc ≃ 0.03. The observed phenomenon is thought to arise from a competition between substitution-induced strengthening of the potential barriers (which hinder the charge hopping between neighboring Mn sites) and weakening of the kinetic energy of the carriers. The data are well fitted by assuming a nonthermal tunneling conductivity theory with randomly distributed hopping sites. © 1999 American Institute of Physics. [less ▲]

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