52 research outputs found
Measure of Diracness in two-dimensional semiconductors
We analyze the low-energy properties of two-dimensional direct-gap
semiconductors, such as for example the transition-metal dichalcogenides
MoS, WS, and their diselenide analogues MoSe, WSe, etc., which
are currently intensively investigated. In general, their electrons have a
mixed character -- they can be massive Dirac fermions as well as simple
Schr\"odinger particles. We propose a measure (Diracness) for the degree of
mixing between the two characters and discuss how this quantity can in
principle be extracted experimentally, within magneto-transport measurements,
and numerically via ab initio calculations.Comment: 6 pages, 2 figures ; new version (with minor modifications) accepted
for publication in EP
Landau levels, response functions and magnetic oscillations from a generalized Onsager relation
A generalized semiclassical quantization condition for cyclotron orbits was
recently proposed by Gao and Niu \cite{Gao}, that goes beyond the Onsager
relation \cite{Onsager}. In addition to the integrated density of states, it
formally involves magnetic response functions of all orders in the magnetic
field. In particular, up to second order, it requires the knowledge of the
spontaneous magnetization and the magnetic susceptibility, as was early
anticipated by Roth \cite{Roth}. We study three applications of this relation
focusing on two-dimensional electrons. First, we obtain magnetic response
functions from Landau levels. Second we obtain Landau levels from response
functions. Third we study magnetic oscillations in metals and propose a proper
way to analyze Landau plots (i.e. the oscillation index as a function of
the inverse magnetic field ) in order to extract quantities such as a
zero-field phase-shift. Whereas the frequency of -oscillations depends on
the zero-field energy spectrum, the zero-field phase-shift depends on the
geometry of the cell-periodic Bloch states via two contributions: the Berry
phase and the average orbital magnetic moment on the Fermi surface. We also
quantify deviations from linearity in Landau plots (i.e. aperiodic magnetic
oscillations), as recently measured in surface states of three-dimensional
topological insulators and emphasized by Wright and McKenzie \cite{Wright}.Comment: 31 pages, 8 figures; v2: SciPost style; v3: several references added,
small corrections, typos fixed; v4: abstract changed, generalized
quantization condition called Roth-Gao-Niu; v5: minor modifications, 2
references adde
Tunable orbital susceptibility in - tight-binding models
We study the importance of interband effects on the orbital susceptibility of
three bands - tight-binding models. The particularity of
these models is that the coupling between the three energy bands (which is
encoded in the wavefunctions properties) can be tuned (by a parameter )
without any modification of the energy spectrum. Using the gauge-invariant
perturbative formalism that we have recently developped, we obtain a generic
formula of the orbital susceptibility of - tight-binding
models. Considering then three characteristic examples that exhibit either
Dirac, semi-Dirac or quadratic band touching, we show that by varying the
parameter and thus the wavefunctions interband couplings, it is
possible to drive a transition from a diamagnetic to a paramagnetic peak of the
orbital susceptibility at the band touching. In the presence of a gap
separating the dispersive bands, we show that the susceptibility inside the gap
exhibits a similar dia to paramagnetic transition.Comment: 15 pages,5 figs. Proceedings of the International Workshop on Dirac
Electrons in Solids 2015Proceedings of the International Workshop on Dirac
Electrons in Solids 201
Cumulative identical spin rotation effects in collisionless trapped atomic gases
We discuss the strong spin segregation in a dilute trapped Fermi gas recently
observed by Du et al. with "anomalous" large time scale and amplitude. In a
collisionless regime, the atoms oscillate rapidly in the trap and average the
inhomogeneous external field in an energy dependent way, which controls their
transverse spin precession frequency. During interactions between atoms with
different spin directions, the identical spin rotation effect (ISRE) transfers
atoms to the up or down spin state, depending on their motional energy. Since
low energy atoms are closer to the center of the trap than high energy atoms,
the final outcome is a strong correlation between spins and positions.Comment: 4 pages, 2 figures; v2: comparison to experimental data adde
Extended coherence time on the clock transition of optically trapped Rubidium
Optically trapped ensembles are of crucial importance for frequency
measurements and quantum memories, but generally suffer from strong dephasing
due to inhomogeneous density and light shifts. We demonstrate a drastic
increase of the coherence time to 21 s on the magnetic field insensitive clock
transition of Rb-87 by applying the recently discovered spin self-rephasing.
This result confirms the general nature of this new mechanism and thus shows
its applicability in atom clocks and quantum memories. A systematic
investigation of all relevant frequency shifts and noise contributions yields a
stability of 2.4E-11 x tau^(-1/2), where tau is the integration time in
seconds. Based on a set of technical improvements, the presented frequency
standard is predicted to rival the stability of microwave fountain clocks in a
potentially much more compact setup.Comment: 5 pages, 4 figure
Spin self-rephasing and very long coherence times in a trapped atomic ensemble
We perform Ramsey spectroscopy on the ground state of ultra-cold 87Rb atoms
magnetically trapped on a chip in the Knudsen regime. Field inhomogeneities
over the sample should limit the 1/e contrast decay time to about 3 s, while
decay times of 58 s are actually observed. We explain this surprising result by
a spin self-rephasing mechanism induced by the identical spin rotation effect
originating from particle indistinguishability. We propose a theory of this
synchronization mechanism and obtain good agreement with the experimental
observations. The effect is general and susceptible to appear in other physical
systems.Comment: Revised version; improved description of the theoretical treatmen
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