1,355 research outputs found
Filling constraints for spin-orbit coupled insulators in symmorphic and non-symmorphic crystals
We determine conditions on the filling of electrons in a crystalline lattice
to obtain the equivalent of a band insulator -- a gapped insulator with neither
symmetry breaking nor fractionalized excitations. We allow for strong
interactions, which precludes a free particle description. Previous approaches
that extend the Lieb-Schultz-Mattis argument invoked spin conservation in an
essential way, and cannot be applied to the physically interesting case of
spin-orbit coupled systems. Here we introduce two approaches, the first an
entanglement based scheme, while the second studies the system on an
appropriate flat `Bieberbach' manifold to obtain the filling conditions for all
230 space groups. These approaches only assume time reversal rather than spin
rotation invariance. The results depend crucially on whether the crystal
symmetry is symmorphic. Our results clarify when one may infer the existence of
an exotic ground state based on the absence of order, and we point out
applications to experimentally realized materials. Extensions to new situations
involving purely spin models are also mentioned.Comment: 9 pages + 5 page appendices, 4 figures, 2 tables; v4: a typo in
Figure 4 is correcte
Filling-Enforced Quantum Band Insulators in Spin-Orbit Coupled Crystals
While band insulators are usually described in wavevector space in terms of
fully filled bands, they are sometimes also described in terms of a
complementary Wannier picture in which electrons occupy localized, atom-like
orbitals. Under what conditions does the latter picture break down? The
presence of irremovable quantum entanglement between different sites can
obstruct a localized orbital description, which occurs in systems like Chern
and topological insulators. We collectively refer to such states as Quantum
Band Insulators (QBIs). Here we report the theoretical discovery of a
filling-enforced QBI - that is, a free electron insulator in which the band
filling is smaller than the minimum number dictated by the atomic picture.
Consequently such insulators have no representation in terms of filling
localized orbitals and must be QBIs. This is shown to occur in models of
certain cubic crystals with non-symmorphic space groups. Like topological
insulators, filling-enforced QBIs require spin-orbit coupling. However, in
contrast, they do not typically exhibit protected surface states. Instead their
nontrivial nature is revealed by studying the quantum entanglement of their
ground state wavefunction.Comment: 4.5 pages + 7.5 page Appendices, 2+4 figures; v3: Corrected Fig. 5 in
Appendix B; added discussion on surface states (Fig. 6
A proposal for a scalable universal bosonic simulator using individually trapped ions
We describe a possible architecture to implement a universal bosonic
simulator (UBS) using trapped ions. Single ions are confined in individual
traps, and their motional states represent the bosonic modes. Single-mode
linear operators, nonlinear phase-shifts, and linear beam splitters can be
realized by precisely controlling the trapping potentials. All the processes in
a bosonic simulation, except the initialization and the readout, can be
conducted beyond the Lamb-Dicke regime. Aspects of our proposal can also be
applied to split adiabatically a pair of ions in a single trap
Majorana fermions in ferromagnetic chains on the surface of bulk spin-orbit coupled -wave superconductors
Majorana fermion (MF) excitations in solid state system have non-Abelian
statistics which is essential for topological quantum computation. Previous
proposals to realize MF, however, generally requires fine-tuning of parameters.
Here we explore a platform which avoids the fine-tuning problem, namely a
ferromagnetic chain deposited on the surface of a spin-orbit coupled -wave
superconductor. We show that it generically supports zero-energy topological MF
excitations near the two ends of the chain with minimal fine-tuning. Depending
on the strength of the ferromagnetic moment in the chain, the number of MFs at
each end, , can be either one or two, and should be revealed by a robust
zero-bias peak (ZBP) of height in scanning tunneling microscopy (STM)
measurements which would show strong (weak) signals at the ends (middle) of the
chain. The role of an approximate chiral symmetry which gives an integer
topological invariant to the system is discussed.Comment: 9 pages, 4 figure
Scattering of coherent states on a single artificial atom
In this work we theoretically analyze a circuit QED design where propagating
quantum microwaves interact with a single artificial atom, a single Cooper pair
box. In particular, we derive a master equation in the so-called transmon
regime, including coherent drives. Inspired by recent experiments, we then
apply the master equation to describe the dynamics in both a two-level and a
three-level approximation of the atom. In the two-level case, we also discuss
how to measure photon antibunching in the reflected field and how it is
affected by finite temperature and finite detection bandwidth.Comment: 18 pages, 7 figure
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