62,344 research outputs found
Transport in gapped bilayer graphene: the role of potential fluctuations
We employ a dual-gated geometry to control the band gap \Delta in bilayer
graphene and study the temperature dependence of the resistance at the charge
neutrality point, RNP(T), from 220 to 1.5 K. Above 5 K, RNP(T) is dominated by
two thermally activated processes in different temperature regimes and exhibits
exp(T3/T)^{1/3} below 5 K. We develop a simple model to account for the
experimental observations, which highlights the crucial role of localized
states produced by potential fluctuations. The high temperature conduction is
attributed to thermal activation to the mobility edge. The activation energy
approaches \Delta /2 at large band gap. At intermediate and low temperatures,
the dominant conduction mechanisms are nearest neighbor hopping and
variable-range hopping through localized states. Our systematic study provides
a coherent understanding of transport in gapped bilayer graphene.Comment: to appear in Physical Review B: Rapid Com
Colossal negative magnetoresistance in dilute fluorinated graphene
Adatoms offer an effective route to modify and engineer the properties of
graphene. In this work, we create dilute fluorinated graphene using a clean,
controlled and reversible approach. At low carrier densities, the system is
strongly localized and exhibits an unexpected, colossal negative
magnetoresistance. The zero-field resistance is reduced by a factor of 40 at
the highest field of 9 T and shows no sign of saturation. Unusual "staircase"
field dependence is observed below 5 K. The magnetoresistance is highly
anisotropic. We discuss possible origins, considering quantum interference
effects and adatom-induced magnetism in graphene.Comment: 21 pages, 4 figures, including supplementary informatio
Quantum phase transition in easy-axis antiferromagnetic Heisenberg spin-1 chain
The fidelity and entropy in an easy-axis antiferromagnetic Heisenberg spin-1
chain are studied numerically. By using the method of density-matrix
renormalization group, the effects of anisotropy on fidelity and entanglement
entropy are investigated. Their relations with quantum phase transition are
analyzed. It is found that the quantum phase transition from the Haldane spin
liquid to N\'eel spin solid can be well characterized by the fidelity. The
phase transition can be hardly detected by the entropy but it can be
successfully detected by the first deviation of the entropy.Comment: 3 figure
Spin and Current Variations in Josephson Junctions
We study the dynamics of a single spin embedded in the tunneling barrier
between two superconductors. As a consequence of pair correlations in the
superconducting state, the spin displays rich and unusual dynamics. To properly
describe the time evolution of the spin we derive the effective Keldysh action
for the spin. The superconducting correlations lead to an effective spin
action, which is non-local in time, leading to unconventional precession. We
further illustrate how the current is modulated by this novel spin dynamics
Magnitude of Magnetic Field Dependence of a Possible Selective Spin Filter in ZnSe/Zn_{1-x}Mn_{x}Se Multilayer Heterostructure
Spin-polarized transport through a band-gap-matched ZnSe/Zn_{1-x}Mn_{x}
Se/ZnSe/Zn_{1-x}Mn_{x}Se/ZnSe multilayer structure is investigated. The
resonant transport is shown to occur at different energies for different spins
owing to the split of spin subbands in the paramagnetic layers. It is found
that the polarization of current density can be reversed in a certain range of
magnetic field, with the peak of polarization moving towards a stronger
magnetic field for increasing the width of central ZnSe layer while shifting
towards an opposite direction for increasing the width of paramagnetic layer.
The reversal is limited in a small-size system. A strong suppression of the
spin up component of the current density is present at high magnetic field. It
is expected that such a reversal of the polarization could act as a possible
mechanism for a selective spin filter device
Proximity effect in atomic-scaled hybrid superconductor/ferromagnet structures: crucial role of electron spectra
We study the influence of the configuration of the majority and minority spin
subbands of electron spectra on the properties of atomic-scaled
superconductor-ferromagnet S-F-S and F-S-F hybrid structures. At low
temperatures, the S/F/S junction is either a 0 or junction depending on the
energy shift between S and F materials and the anisotropy of the Fermi
surfaces. We found that the spin switch effect in F/S/F system can be reversed
if the minority spin electron spectra in F metal is of the hole-like type
Graded Orbital Occupation near Interfaces in a La2NiO4 - La2CuO4 Superlattice
X-ray absorption spectroscopy and resonant soft x-ray reflectivity show a
non-uniform distribution of oxygen holes in a La2NiO4 - La2CuO4 (LNO-LCO)
superlattice, with excess holes concentrated in the LNO layers. Weak
ferromagnetism with Tc = 160 K suggests a coordinated tilting of NiO6
octahedra, similar to that of bulk LNO. Ni d3z2-r2 orbitals within the LNO
layers have a spatially variable occupation. This variation of the Ni valence
near LNO-LCO interfaces is observed with resonant soft x-ray reflectivity at
the Ni L edge, at a reflection suppressed by the symmetry of the structure, and
is possible through graded doping with holes, due to oxygen interstitials taken
up preferentially by inner LNO layers. Since the density of oxygen atoms in the
structure can be smoothly varied with standard procedures, this orbital
occupation, robust up to at least 280 K, is tunable.Comment: 11 pages, 8 figure
- …