7,921 research outputs found
Pinned Bilayer Wigner Crystals with Pseudospin Magnetism
We study a model of \textit{pinned} bilayer Wigner crystals (WC) and focus on
the effects of interlayer coherence (IC) on pinning. We consider both a
pseudospin ferromagnetic WC (FMWC) with IC and a pseudospin antiferromagnetic
WC (AFMWC) without IC. Our central finding is that a FMWC can be pinned more
strongly due to the presence of IC. One specific mechanism is through the
disorder induced interlayer tunneling, which effectively manifests as an extra
pinning in a FMWC. We also construct a general "effective disorder" model and
effective pinning Hamiltonian for the case of FMWC and AFMWC respectively.
Under this framework, pinning in the presence of IC involves
\textit{interlayer} spatial correlation of disorder in addition to intralayer
correlation, leading to \textit{enhanced} pinning in the FMWC. The pinning mode
frequency (\wpk) of a FMWC is found to decease with the effective layer
separation, whereas for an AFMWC the opposite behavior is expected. An abrupt
drop of \wpk is predicted at a transition from a FMWC to AFMWC. Possible
effects of in-plane magnetic fields and finite temperatures are addressed.
Finally we discuss some other possible ramifications of the FMWC as an
electronic supersolid-like phase.Comment: Slightly revised. The final version is published on PR
Polycrystalline graphene and other two-dimensional materials
Graphene, a single atomic layer of graphitic carbon, has attracted intense
attention due to its extraordinary properties that make it a suitable material
for a wide range of technological applications. Large-area graphene films,
which are necessary for industrial applications, are typically polycrystalline,
that is, composed of single-crystalline grains of varying orientation joined by
grain boundaries. Here, we present a review of the large body of research
reported in the past few years on polycrystalline graphene. We discuss its
growth and formation, the microscopic structure of grain boundaries and their
relations to other types of topological defects such as dislocations. The
review further covers electronic transport, optical and mechanical properties
pertaining to the characterizations of grain boundaries, and applications of
polycrystalline graphene. We also discuss research, still in its infancy,
performed on other 2D materials such as transition metal dichalcogenides, and
offer perspectives for future directions of research.Comment: review article; part of focus issue "Graphene applications
Existence of negative differential thermal conductance in one-dimensional diffusive thermal transport
We show that in a finite one-dimensional (1D) system with diffusive thermal
transport described by the Fourier's law, negative differential thermal
conductance (NDTC) cannot occur when the temperature at one end is fixed. We
demonstrate that NDTC in this case requires the presence of junction(s) with
temperature dependent thermal contact resistance (TCR). We derive a necessary
and sufficient condition for the existence of NDTC in terms of the properties
of the TCR for systems with a single junction. We show that under certain
circumstances we even could have infinite (negative or positive) differential
thermal conductance in the presence of the TCR. Our predictions provide
theoretical basis for constructing NDTC-based devices, such as thermal
amplifiers, oscillators and logic devices
Formation of ultracold LiRb molecules by photoassociation near the Li (2s 2S1/2) + Rb (5p 2P1/2) asymptote
We report the production of ultracold 7Li85Rb molecules by photoassociation
(PA) below the Li (2s 2S1/2) + Rb (5p 2P1/2) asymptote. We perform PA
spectroscopy in a dual-species 7Li-85Rb magneto-optical trap (MOT) and detect
the PA resonances using trap loss spectroscopy. We observe several strong PA
resonances corresponding to the last few bound states, assign the lines and
derive the long range C6 dispersion coefficients for the Li (2s 2S1/2) + Rb (5p
2P1/2) asymptote. We also report an excited-state molecule formation rate
(P_LiRb) of ~10^7 s^-1 and a PA rate coefficient (K_PA) of ~4x10^-11 cm^3/s,
which are both among the highest observed for heteronuclear bi-alkali
molecules. These suggest that PA is a promising route for the creation of
ultracold ground state LiRb molecules.Comment: 6 page
Nonlinear thermal transport and negative differential thermal conductance in graphene nanoribbons
We employ classical molecular dynamics to study the nonlinear thermal
transport in graphene nanoribbons (GNRs). For GNRs under large temperature
biases beyond linear response regime, we have observed the onset of negative
differential thermal conductance (NDTC). NDTC is tunable by varying the manner
of applying the temperature biases. NDTC is reduced and eventually disappears
when the length of the GNR increases. We have also observed NDTC in triangular
GNRs, where NDTC exists only when the heat current is from the narrower to the
wider end. These effects may be useful in nanoscale thermal managements and
thermal signal processing utilizing GNRs.Comment: 5 pages, 4 figure
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