Low-temperature quantum magnetotransport of graphene on SiC (0 0 0 1) in pulsed magnetic fields up to 30 T

Resistivity, ρ(T), and magnetoresistance (MR) are investigated in graphene grown on SiC (0 0 0 1), at temperatures between T ~ 4-85 K in pulsed magnetic fields of B up to 30 T. According to the Raman spectroscopy and Kelvin-probe microscopy data, the material is a single-layer graphene containing ~20% double-layer islands with a submicron scale and relatively high amount of intrinsic defects. The dependence of ρ(T) exhibits a minimum at temperature T _m ~ 30 K. The low-field Hall data have yielded a high electron concentration, n _R  ≈  1.4  ×  10¹³ cm^-2 connected to intrinsic defects, and a mobility value of µ _H ~ 300 cm² (Vs)^-1 weakly depending on T. Analysis of the Shubnikov-de Haas oscillations of MR, observed between B ~ 10-30 T, permitted to establish existence of the Berry phase β  ≈  0.55 and the cyclotron mass, m _c  ≈  0.07 (in units of the free electron mass) close to expected values for the single-layer graphene, respectively. MR at 4.2 K is negative up to B ~ 9 T, exhibiting a minimum near 3 T. Analysis of MR within the whole range of B  =  0-10 T below the onset of the SdH effect has revealed three contributions, connected to (i) the classical MR effect, (ii) the weak localization, and (iii) the electron-electron interaction. Analysis of the ρ(T) dependence has confirmed the presence of the contributions (ii) and (iii), revealing a high importance of the electron-electron scattering. As a result, characteristic relaxation times were obtained; an important role of the spin-orbit interaction in the material has been demonstrated, too.

Hopping magnetotransport of the band-gap tuning Cu2Zn(Sn x Ge1-x )Se4 crystals

Resistivity, ρ(T, x), of Cu2Zn(Sn x Ge1-x )Se4 (CZTGeSe) single crystals with x  =  0-1, investigated at temperatures between T ~ 10-320 K, exhibits an activated character within the whole temperature range, attaining a minimum at x  =  0.47. Magnetoresistance (MR) of CZTGeSe with x  =  0.26, 0.47 and 0.64 is positive (pMR) in all measured fields of B up to 20 T at any T between ~40-320 K, whereas MR of samples with x  =  0 and 1 contains a negative contribution (nMR). The dependence of ρ(T) at B  =  0 gives evidence for a nearest-neighbor hopping (NNH) conductivity in high-temperature intervals within T ~ 200-320 K depending on x, followed by the Mott variable-range hopping (VRH) charge transfer with lowering temperature. The pMR law of lnρ(B) [Formula: see text] B (2) is observed in both hopping conductivity regimes above, provided that the nMR contribution is absent or saturated. Analysis of the ρ(T) and MR data has yielded the values of the NNH activation energy and the VRH characteristic temperature, as well as those of the acceptor band width, the acceptor concentration, the localization radii of holes and the density of the localized states (DOS) at the Fermi level. All the parameters above exhibit a systematic non-monotonous dependence on x. Their extremums, lying close to x  =  0.64, correspond to the minimum of a lattice disorder along with the maximum of DOS and of the acceptor concentration, as well as a highest proximity to the metal-insulator transition.

Irreversible magnetic properties of nanocarbon

Magnetization M(T, B) of powder and glassy samples containing carbon nanoparticles, not intentionally doped and doped with Ag, Au and Co, is investigated at temperatures T between - 3-300 K in magnetic fields B up to 5T. According to atomic force microscopy data, a system of carbon particles has a broad size distribution, given by the average and the maximum radii of -60 nm and - 110 nm, respectively. In low fields of B << B(K), where B(K) - 1T is the mean anisotropy field, M(T) exhibits large irreversibility or deviation of zero-field cooled and field-cooled magnetizations, which is suppressed completely at B > B(K). The dependence of M(B) saturates above B - 2T at T - 150-300 K and contains a large paramagnetic-like response below - 50-150 K. Hysteresis is observed already at 300 K and is characterized by a power-law temperature decay of the coercive field, B(c)(T). This is described by the exponent n approximately 0.8 and by the low-temperature values of B(c) (0) increasing from -36-53 mT in the undoped sample and those doped with Ag and Au, up to 80 mT in the Co-doped material, yielding the blocking temperatures T(b) approximately 400-580 K. Analysis of the experimental magnetization data above suggests distribution of the magnetization close to the surface of the particles, yielding a thickness of the near-surface layer, h, filled with localized magnetic moments, micro1 - microB, to be close to the average distance, a, between the moments, h approximately a - 1 nm. This is consistent with the origin of magnetism in nanocarbon being presumably due to intrinsic near-surface defects.

Magnetoresistance of rolled-up Fe3Si nanomembranes

Magnetotransport of individual rolled-up Fe(3)Si nanomembranes is investigated in a broad temperature range from 4.2 K up to 300 K in pulsed magnetic fields up to 55 T. The observed magnetoresistance (MR) has the following pronounced features: (i) MR is negative in the investigated intervals of temperature and magnetic field; (ii) its magnitude increases linearly with the magnetic field in a low-field region and reveals a gradual trend to saturation when the magnetic field increases; (iii) the MR effect becomes more pronounced with increasing temperature. These dependences of MR on the magnetic field and temperature are in line with predictions of the spin-disorder model of the spin-flip s-d interaction assisted with creation or annihilation of magnons, which is expected above a certain critical temperature. Comparison of the MR features in rolled-up and planar samples reveals a substantial increase of the critical temperature in the rolled-up tube, which is attributed to a new geometry and internal strain arising in the rolled-up nanomembranes, influencing the electronic and magnetic properties of the material.

Variable-range hopping conductivity of La(1 - x)Sr(x)Mn(1 - y)Fe(y)O₃

The temperature dependence of the resistivity, ρ, of ceramic La(1 - x)Sr(x)Mn(1 - y)Fe(y)O(3) (LSMFO) samples with x = 0.3 and y = 0.03, 0.15, 0.20 and 0.25 (or simply #03, #15, #20 and #25, respectively) is investigated between temperatures T ∼ 5 and 310 K in magnetic fields B up to 8 T. Metallic conductivity in #03 is changed eventually to activated in #25. In #15 and #20 the behavior of ρ(T) is more complicated, comprising of two extremes, divided by an interval of metallic behavior in #15, and two inflections of ρ(T) in #20 within similar intervals ΔT below approximately 100 K. Mott variable-range hopping (VRH) conductivity is observed in #15 above the ferromagnetic Curie temperature, T(C). In #20 the Mott VRH conductivity takes place in three different temperature intervals at T > T(C), T close to T(C) and T < T(C). In #25, the Mott VRH conductivity is observed in two different intervals, above and below T(C), divided by an intermediate interval of the Shklovskii-Efros VRH conduction regime. Analysis of the VRH conductivity yielded the values of the localization radius, α, and the dependence of α and of the density of the localized states, g, near the Fermi level, on B. Above T(C) the localization radius in all samples at B = 0 has similar values, α approximately 1.0-1.2 Å, which is enhanced to α approximately 3.3 Å (#20) and 2.0 Å (#25) below T(C). The sensitivity of α and g to B depend on y and T. The complicated behavior of the mechanisms of the hopping charge transfer, as well as of the microscopic parameters α and g, is attributable to different electronic and magnetic phases of LSMFO varying with temperature and Fe doping.

Hopping conductivity in CaCu(2)O(3) single crystals

The resistivity, ρ, of the spin-ladder compound CaCu(2)O(3) is investigated between T∼130-450 K. The ρ(T) data measured for [Formula: see text] (along the Cu-O-Cu leg) and [Formula: see text] (along the Cu-O-Cu rungs), ρ(a)(T)>ρ(b)(T), exhibit an activated dependence, similar in both directions and characterized by a nearest-neighbour hopping followed by a variable-range hopping (VRH) regime when T is decreased. A detailed analysis of ρ(T) demonstrates that conventional d-dimensional models of the hopping conductivity, based on the electron localization in disordered systems, cannot interpret the experimental data at any d = 1, 2 or 3, leading to the mismatch of the characteristic energies and/or unphysical values of the characteristic length scales. The observed VRH conductivity law on the low-temperature interval, lnρ∼T(-3/4), contradicts the models above, too. Instead, it is found that this law can be substantiated and the correct matching of the energy and length scales can be found within a model of Fogler et al (2004 Phys. Rev. B 69 035413) by treating CaCu(2)O(3) as a three-dimensional array of quasi-one-dimensional electron crystals.

Lattice distortions, magnetoresistance and hopping conductivity in LaMnO(3+δ)

Structural and transport properties of ceramic LaMnO(3+δ) are investigated for δ = 0-0.154. According to x-ray diffraction measurements at room temperature the crystal structure of this compound varies from orthorhombic (Pbnm) for δ = 0 to rhombohedrally distorted cubic (Pm3m) for δ = 0.065-0.112 and to rhombohedral ([Formula: see text]) crystal symmetry for δ = 0.125-0.154. These structural modifications are confirmed by the Raman micro-spectroscopy measurements. The resistivity displays in the range δ = 0-0.154 an activated behaviour both above and below the paramagnetic (PM) to ferromagnetic transition temperature, T(C). In the field of 8 T the relative magnetoresistance, Δρ(B)/ρ(0), reaches at δ = 0.154 the values of -88% near T(C) and -98% at [Formula: see text] K. The resistivity of the PM phase of LaMnO(3+δ) with δ = 0.100-0.154 satisfies the Shklovskii-Efros-like variable-range hopping (VRH) conductivity law between [Formula: see text] K and the VRH onset temperature [Formula: see text] K. The resistivity is governed by a complex energy dependence of the density of the localized states near the Fermi level, comprising a soft Coulomb gap [Formula: see text] eV and a rigid gap [Formula: see text] eV, the latter being connected to formation of small polarons.