92 research outputs found
Nanoscopy of pairs of atoms by fluorescence in a magnetic field
Spontaneous emission spectra of two initially excited closely spaced
identical atoms are very sensitive to the strength and the direction of the
applied magnetic field. The relevant schemes are considered that ensure the
determination of the mutual spatial orientation of the atoms and the distance
between them by entirely optical means. A corresponding theoretical description
is given accounting for the dipole-dipole interaction between the two atoms in
the presence of a magnetic field and for polarizations of the quantum field
interacting with magnetic sublevels of the two-atom system
Superfluidity of identical fermions in an optical lattice: atoms and polar molecules
In this work, we discuss the emergence of -wave superfluids of identical
fermions in 2D lattices. The optical lattice potential manifests itself in an
interplay between an increase in the density of states on the Fermi surface and
the modification of the fermion-fermion interaction (scattering) amplitude. The
density of states is enhanced due to an increase of the effective mass of
atoms. In deep lattices, for short-range interacting atoms, the scattering
amplitude is strongly reduced compared to free space due to a small overlap of
wavefunctions of fermions sitting in the neighboring lattice sites, which
suppresses the -wave superfluidity. However, we show that for a moderate
lattice depth there is still a possibility to create atomic -wave
superfluids with sizable transition temperatures. The situation is drastically
different for fermionic polar molecules. Being dressed with a microwave field,
they acquire a dipole-dipole attractive tail in the interaction potential.
Then, due to a long-range character of the dipole-dipole interaction, the
effect of the suppression of the scattering amplitude in 2D lattices is absent.
This leads to the emergence of a stable topological superfluid of
identical microwave-dressed polar molecules.Comment: 14 pages, 4 figures; prepared for proceedings of the IV International
Conference on Quantum Technologies (Moscow, July 12-16, 2017); the present
paper summarizes the results of our studies arXiv:1601.03026 and
arXiv:1701.0852
Resistivity of non-Galilean-invariant Fermi- and non-Fermi liquids
While it is well-known that the electron-electron (\emph{ee}) interaction
cannot affect the resistivity of a Galilean-invariant Fermi liquid (FL), the
reverse statement is not necessarily true: the resistivity of a
non-Galilean-invariant FL does not necessarily follow a T^2 behavior. The T^2
behavior is guaranteed only if Umklapp processes are allowed; however, if the
Fermi surface (FS) is small or the electron-electron interaction is of a very
long range, Umklapps are suppressed. In this case, a T^2 term can result only
from a combined--but distinct from quantum-interference corrections-- effect of
the electron-impurity and \emph{ee} interactions. Whether the T^2 term is
present depends on 1) dimensionality (two dimensions (2D) vs three dimensions
(3D)), 2) topology (simply- vs multiply-connected), and 3) shape (convex vs
concave) of the FS. In particular, the T^2 term is absent for any quadratic
(but not necessarily isotropic) spectrum both in 2D and 3D. The T^2 term is
also absent for a convex and simply-connected but otherwise arbitrarily
anisotropic FS in 2D. The origin of this nullification is approximate
integrability of the electron motion on a 2D FS, where the energy and momentum
conservation laws do not allow for current relaxation to leading
--second--order in T/E_F (E_F is the Fermi energy). If the T^2 term is
nullified by the conservation law, the first non-zero term behaves as T^4. The
same applies to a quantum-critical metal in the vicinity of a Pomeranchuk
instability, with a proviso that the leading (first non-zero) term in the
resistivity scales as T^{\frac{D+2}{3}} (T^{\frac{D+8}{3}}). We discuss a
number of situations when integrability is weakly broken, e.g., by inter-plane
hopping in a quasi-2D metal or by warping of the FS as in the surface states of
Bi_2Te_3 family of topological insulators.Comment: Submitted to a special issue of the Lithuanian Journal of Physics
dedicated to the memory of Y. B. Levinso
Protection of helical transport in Quantum Spin Hall samples: the role of symmetries on edges
Time-reversal invariant two-dimensional topological insulators, often dubbed
Quantum Spin Hall systems, possess helical edge modes whose ballistic transport
is protected by physical symmetries. We argue that, though the time-reversal
symmetry (TRS) of the bulk is needed for the existence of helicity, protection
of the helical transport is actually provided by the spin conservation on the
edges. This general statement is illustrated by specific examples. One example
demonstrates the ballistic conductance in the setup where the TRS on the edge
is broken. It shows that attributing the symmetry protection exclusively to the
TRS is not entirely correct. Another example reveals weakness of the protection
in the case where helical transport is governed by a space-fluctuating
spin-orbit interaction. Our results demonstrate the fundamental importance of
the spin conservation analysis for the identification of mechanisms which may
(or may not) lead to suppression of the ballistic helical transport.Comment: Substantially re-written version: improved explanation of the setup,
Abstract, and Conclusions; more detailed proof of the main result;
re-arranged and substantially extended examples and Supplemental Materials;
extended Bibliography; 11 pages, 2 figure
Protection of edge transport in quantum spin Hall samples: spin-symmetry based general approach and examples
- …