565 research outputs found
Scaling Approach to the Phase Diagram of Quantum Hall Systems
We present a simple classification of the different liquid and solid phases
of quantum Hall systems in the limit where the Coulomb interaction between the
electrons is significant, i.e. away from integral filling factors. This
classification, and a criterion for the validity of the mean-field
approximation in the charge-density-wave phase, is based on scaling arguments
concerning the effective interaction potential of electrons restricted to an
arbitrary Landau level. Finite-temperature effects are investigated within the
same formalism, and a good agreement with recent experiments is obtained.Comment: 4 pages, 3 figures; to be published in Europhys. Lett.; new version
contains more detailed description of finite-temperature effect
Bias dependence of perpendicular spin torque and of free and fixed layer eigenmodes in MgO-based nanopillars
We have measured the bias voltage and field dependence of eigenmode
frequencies in a magnetic tunnel junction with MgO barrier. We show that both
free layer (FL) and reference layer (RL) modes are excited, and that a
cross-over between these modes is observed by varying external field and bias
voltage. The bias voltage dependence of the FL and RL modes are shown to be
dramatically different. The bias dependence of the FL modes is linear in bias
voltage, whereas that of the RL mode is strongly quadratic. Using modeling and
micromagnetic simulations, we show that the linear bias dependence of FL
frequencies is primarily due to a linear dependence of the perpendicular spin
torque on bias voltage, whereas the quadratic dependence of the RL on bias
voltage is dominated by the reduction of exchange bias due to Joule heating,
and is not attributable to a quadratic dependence of the perpendicular spin
torque on bias voltage
Temperature dependence of linewidth in nano-contact based spin torque oscillators: effect of multiple oscillatory modes
We discuss the effect of mode transitions on the current (I) and temperature
(T) dependent linewidth (\Delta f) in nanocontact based spin torque oscillators
(STOs). At constant I, \Delta f exhibits an anomalous temperature dependence
near the mode transitions; \Delta f may either increase or decrease with T
depending on the position w.r.t. the mode transition. We show that the behavior
of \Delta f as a function of I can be fitted by the single mode analytical
theory of STOs, even though there are two modes present near the mode
transition, if the nonlinear amplification is determined directly from the
experiment. Using a recently developed theory of two coupled modes, we show
that the linewidth near mode transition can be described by an "effective"
single-oscillator theory with an enhanced nonlinear amplification that carries
additional temperature dependence, which thus qualitatively explain the
experimental results.Comment: 8 page
Lorentz shear modulus of fractional quantum Hall states
We show that the Lorentz shear modulus of macroscopically homogeneous
electronic states in the lowest Landau level is proportional to the bulk
modulus of an equivalent system of interacting classical particles in the
thermodynamic limit. Making use of this correspondence we calculate the Lorentz
shear modulus of Laughlin's fractional quantum Hall states at filling factor
( an odd integer) and find that it is equal to ,
where is the density of particles and the sign depends on the direction of
magnetic field. This is in agreement with the recent result obtained by Read in
arXiv:0805.2507 and corrects our previous result published in Phys. Rev. B {\bf
76}, 161305 (R) (2007).Comment: 8 pages, 3 figure
Universal Equilibrium Currents in the Quantum Hall Fluid
The equilibrium current distribution in a quantum Hall fluid that is
subjected to a slowly varying confining potential is shown to generally consist
of strips or channels of current, which alternate in direction, and which have
universal integrated strengths. A measurement of these currents would yield
direct independent measurements of the proper quasiparticle and quasihole
energies in the fractional quantum Hall states.Comment: 4 pages, Revte
Atomic and Electronic Structure of CoFeB/MgO Interface from First Principles
First-principles calculations of the atomic and electronic structure of crystalline CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) are performed to understand the effect of B on spin-dependent transport in these junctions. The authors find that it is energetically favorable for B atoms to reside at the crystalline CoFeB/MgO interface rather than remain in the bulk of the crystalline CoFeB electrode. The presence of B at the interfaces is detrimental to tunneling magnetoresistance (TMR) because it significantly suppresses the majority-channel conductance through states of symmetry. Preventing B segregation to the interfaces during annealing should result in an enhanced TMR in CoFeB/MgO/CoFeB MTJs
Density functional theory of the phase diagram of maximum density droplets in two-dimensional quantum dots in a magnetic field
We present a density-functional theory (DFT) approach to the study of the
phase diagram of the maximum density droplet (MDD) in two-dimensional quantum
dots in a magnetic field. Within the lowest Landau level (LLL) approximation,
analytical expressions are derived for the values of the parameters (number
of electrons) and (magnetic field) at which the transition from the MDD to
a ``reconstructed'' phase takes place. The results are then compared with those
of full Kohn-Sham calculations, giving thus information about both correlation
and Landau level mixing effects. Our results are also contrasted with those of
Hartree-Fock (HF) calculations, showing that DFT predicts a more compact
reconstructed edge, which is closer to the result of exact diagonalizations in
the LLL.Comment: ReVTeX 3.
Highest weight Macdonald and Jack Polynomials
Fractional quantum Hall states of particles in the lowest Landau levels are
described by multivariate polynomials. The incompressible liquid states when
described on a sphere are fully invariant under the rotation group. Excited
quasiparticle/quasihole states are member of multiplets under the rotation
group and generically there is a nontrivial highest weight member of the
multiplet from which all states can be constructed. Some of the trial states
proposed in the literature belong to classical families of symmetric
polynomials. In this paper we study Macdonald and Jack polynomials that are
highest weight states. For Macdonald polynomials it is a (q,t)-deformation of
the raising angular momentum operator that defines the highest weight
condition. By specialization of the parameters we obtain a classification of
the highest weight Jack polynomials. Our results are valid in the case of
staircase and rectangular partition indexing the polynomials.Comment: 17 pages, published versio
The Atomic Slide Puzzle: Self-Diffusion of an Impure Atom
In a series of recent papers van Gastel et al have presented first
experimental evidence that impure, Indium atoms, embedded into the first layer
of a Cu(001) surface, are not localized within the close-packed surface layers
but make concerted, long excursions visualized in a series of STM images. Such
excursions occur due to continuous reshuffling of the surface following the
position exchanges of both impure and host atoms with the naturally occuring
surface vacancies. Van Gastel et al have also formulated an original
lattice-gas type model with asymmetric exchange probabilities, whose numerical
solution is in a good agreement with the experimental data. In this paper we
propose an exact lattice solution of several versions of this model.Comment: Latex, 4 pages, 2 figures, to appear in Phys. Rev. E (RC
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