1,497 research outputs found
Electron Localization in the Insulating State
The insulating state of matter is characterized by the excitation spectrum,
but also by qualitative features of the electronic ground state. The insulating
ground wavefunction in fact: (i) sustains macroscopic polarization, and (ii) is
localized. We give a sharp definition of the latter concept, and we show how
the two basic features stem from essentially the same formalism. Our approach
to localization is exemplified by means of a two--band Hubbard model in one
dimension. In the noninteracting limit the wavefunction localization is
measured by the spread of the Wannier orbitals.Comment: 5 pages including 3 figures, submitted to PR
The Quantum-Mechanical Position Operator in Extended Systems
The position operator (defined within the Schroedinger representation in the
standard way) becomes meaningless when periodic boundary conditions are adopted
for the wavefunction, as usual in condensed matter physics. We show how to
define the position expectation value by means of a simple many-body operator
acting on the wavefunction of the extended system. The relationships of the
present findings to the Berry-phase theory of polarization are discussed.Comment: Four pages in RevTe
Theory of Orbital Magnetization in Solids
In this review article, we survey the relatively new theory of orbital
magnetization in solids-often referred to as the "modern theory of orbital
magnetization"-and its applications. Surprisingly, while the calculation of the
orbital magnetization in finite systems such as atoms and molecules is straight
forward, in extended systems or solids it has long eluded calculations owing to
the fact that the position operator is ill-defined in such a context.
Approaches that overcome this problem were first developed in 2005 and in the
first part of this review we present the main ideas reaching from a Wannier
function approach to semi-classical and finite-temperature formalisms. In the
second part, we describe practical aspects of calculating the orbital
magnetization, such as taking k-space derivatives, a formalism for
pseudopotentials, a single k-point derivation, a Wannier interpolation scheme,
and DFT specific aspects. We then show results of recent calculations on Fe,
Co, and Ni. In the last part of this review, we focus on direct applications of
the orbital magnetization. In particular, we will review how properties such as
the nuclear magnetic resonance shielding tensor and the electron paramagnetic
resonance g-tensor can elegantly be calculated in terms of a derivative of the
orbital magnetization
‘We are all in the same boat’: How societal discontent affects intention to help during the COVID-19 pandemic
‘We are all in the same boat’: How societal discontent affects intention to help during the COVID-19 pandemic
Non-Abelian Braiding of Lattice Bosons
We report on a numerical experiment in which we use time-dependent potentials
to braid non-abelian quasiparticles. We consider lattice bosons in a uniform
magnetic field within the fractional quantum Hall regime, where , the
ratio of particles to flux quanta, is near 1/2, 1 or 3/2. We introduce
time-dependent potentials which move quasiparticle excitations around one
another, explicitly simulating a braiding operation which could implement part
of a gate in a quantum computation. We find that different braids do not
commute for near and , with Berry matrices respectively
consistent with Ising and Fibonacci anyons. Near , the braids commute.Comment: 5 pages, 1 figur
Nested Kirkpatrick–Baez (Montel) optics for hard X-rays
A comprehensive description and ray-tracing simulations are presented for symmetric nested Kirkpatrick-Baez (KB) mirrors, commonly used at synchrotrons and in commercial X-ray sources. This paper introduces an analytical procedure for determining the proper orientation between the two surfaces composing the nested KB optics. This procedure has been used to design and simulate collimating optics for a hard-X-ray inverse Compton scattering source. The resulting optical device is composed of two 12 cm-long parabolic surfaces coated with a laterally graded multilayer and is capable of collimating a 12 keV beam with a divergence of 5 mrad (FWHM) by a factor of ~250. A description of the ray-tracing software that was developed to simulate the graded multilayer mirrors is included
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