71 research outputs found
Vortices on demand in multicomponent Bose-Einstein condensates
We present a simple mechanism to produce vortices at any desired spatial
locations in harmonically trapped Bose-Einstein condensates (BEC) with
multicomponent spin states coupled to external transverse and axial magnetic
fields. The vortices appear at the spatial points where the spin-transverse
field interaction vanishes and, depending on the multipolar magnetic field
order, the vortices can acquire different predictable topological charges. We
explicitly demonstrate our findings, both numerically and analytically, by
analyzing a 2D BEC via the Gross-Pitaevskii equation for atomic systems with
either two or three internal states. We further show that, by an spontaneous
symmetry breaking mechanism, vortices can appear in any spin component, unless
symmetry is externally broken at the outset by an axial field. We suggest that
this scenario may be tested using an ultracold gas of Rb occupying all
three states in an optical trap.Comment: 11 pages, 9 figures, (Accepted in PRA
Wien effect in interfacial water dissociation through proton-permeable graphene electrodes
Strong electric fields can accelerate molecular dissociation reactions. The
phenomenon known as the Wien effect was previously observed using high-voltage
electrolysis cells that produced fields of about 10^7 V m-1, sufficient to
accelerate the dissociation of weakly bound molecules (e.g., organics and weak
electrolytes). The observation of the Wien effect for the common case of water
dissociation (H2O = H+ + OH-) has remained elusive. Here we study the
dissociation of interfacial water adjacent to proton-permeable graphene
electrodes and observe strong acceleration of the reaction in fields reaching
above 10^8 V m-1. The use of graphene electrodes allow measuring the proton
currents arising exclusively from the dissociation of interfacial water, while
the electric field driving the reaction is monitored through the carrier
density induced in graphene by the same field. The observed exponential
increase in proton currents is in quantitative agreement with Onsager's theory.
Our results also demonstrate that graphene electrodes can be valuable for the
investigation of various interfacial phenomena involving proton transport
Sieving hydrogen isotopes through two dimensional crystals
One-atom-thick crystals are impermeable to atoms and molecules, but hydrogen
ions (thermal protons) penetrate through them. We show that monolayers of
graphene and boron nitride can be used to separate hydrogen ion isotopes.
Employing electrical measurements and mass spectrometry, we find that deuterons
permeate through these crystals much slower than protons, resulting in a
separation factor of ~10 at room temperature. The isotope effect is attributed
to a difference of about 60 meV between zero-point energies of incident protons
and deuterons, which translates into the equivalent difference in the
activation barriers posed by two dimensional crystals. In addition to providing
insight into the proton transport mechanism, the demonstrated approach offers a
competitive and scalable way for hydrogen isotope enrichment.Comment: early version of an accepted repor
Exponentially selective molecular sieving through angstrom pores
Two-dimensional crystals with angstrom-scale pores are widely considered as
candidates for a next generation of molecular separation technologies aiming to
provide extreme, exponentially large selectivity combined with high flow rates.
No such pores have been demonstrated experimentally. Here we study gas
transport through individual graphene pores created by low intensity exposure
to low kV electrons. Helium and hydrogen permeate easily through these pores
whereas larger species such as xenon and methane are practically blocked.
Permeating gases experience activation barriers that increase quadratically
with molecules' kinetic diameter, and the effective diameter of the created
pores is estimated as ~2 angstroms, about one missing carbon ring. Our work
reveals stringent conditions for achieving the long sought-after exponential
selectivity using porous two-dimensional membranes and suggests limits on their
possible performance
Atomically-thin micas as proton conducting membranes
Monolayers of graphene and hexagonal boron nitride (hBN) are highly permeable
to thermal protons. For thicker two-dimensional (2D) materials, proton
conductivity diminishes exponentially so that, for example, monolayer MoS2 that
is just three atoms thick is completely impermeable to protons. This seemed to
suggest that only one-atom-thick crystals could be used as proton conducting
membranes. Here we show that few-layer micas that are rather thick on the
atomic scale become excellent proton conductors if native cations are
ion-exchanged for protons. Their areal conductivity exceeds that of graphene
and hBN by one-two orders of magnitude. Importantly, ion-exchanged 2D micas
exhibit this high conductivity inside the infamous gap for proton-conducting
materials, which extends from 100 C to 500 C. Areal conductivity of
proton-exchanged monolayer micas can reach above 100 S cm-2 at 500 C, well
above the current requirements for the industry roadmap. We attribute the fast
proton permeation to 5 A-wide tubular channels that perforate micas' crystal
structure which, after ion exchange, contain only hydroxyl groups inside. Our
work indicates that there could be other 2D crystals with similar nm-scale
channels, which could help close the materials gap in proton-conducting
applications
Exponentially selective molecular sieving through angstrom pores
From Springer Nature via Jisc Publications RouterHistory: received 2021-09-18, accepted 2021-11-12, registration 2021-11-17, collection 2021-12, pub-electronic 2021-12-09, online 2021-12-09Publication status: PublishedFunder: National Natural Science Foundation of China (National Science Foundation of China); doi: https://doi.org/10.13039/501100001809; Grant(s): 51920105002Funder: RCUK | Engineering and Physical Sciences Research Council (EPSRC); doi: https://doi.org/10.13039/501100000266; Grant(s): EP/S030719Funder: EC | EU Framework Programme for Research and Innovation H2020 | H2020 Priority Excellent Science | H2020 European Research Council (H2020 Excellent Science - European Research Council); doi: https://doi.org/10.13039/100010663; Grant(s): 786532 VANDERFunder: Lloyd’s Register Foundation (LRF); doi: https://doi.org/10.13039/100008885Abstract: Two-dimensional crystals with angstrom-scale pores are widely considered as candidates for a next generation of molecular separation technologies aiming to provide extreme, exponentially large selectivity combined with high flow rates. No such pores have been demonstrated experimentally. Here we study gas transport through individual graphene pores created by low intensity exposure to low kV electrons. Helium and hydrogen permeate easily through these pores whereas larger species such as xenon and methane are practically blocked. Permeating gases experience activation barriers that increase quadratically with molecules’ kinetic diameter, and the effective diameter of the created pores is estimated as ∼2 angstroms, about one missing carbon ring. Our work reveals stringent conditions for achieving the long sought-after exponential selectivity using porous two-dimensional membranes and suggests limits on their possible performance
Proton transport through nanoscale corrugations in two-dimensional crystals
Defect-free graphene is impermeable to all atoms1,2,3,4,5 and ions6,7 under ambient conditions. Experiments that can resolve gas flows of a few atoms per hour through micrometre-sized membranes found that monocrystalline graphene is completely impermeable to helium, the smallest atom2,5. Such membranes were also shown to be impermeable to all ions, including the smallest one, lithium6,7. By contrast, graphene was reported to be highly permeable to protons, nuclei of hydrogen atoms8,9. There is no consensus, however, either on the mechanism behind the unexpectedly high proton permeability10,11,12,13,14 or even on whether it requires defects in graphene’s crystal lattice6,8,15,16,17. Here, using high-resolution scanning electrochemical cell microscopy, we show that, although proton permeation through mechanically exfoliated monolayers of graphene and hexagonal boron nitride cannot be attributed to any structural defects, nanoscale non-flatness of two-dimensional membranes greatly facilitates proton transport. The spatial distribution of proton currents visualized by scanning electrochemical cell microscopy reveals marked inhomogeneities that are strongly correlated with nanoscale wrinkles and other features where strain is accumulated. Our results highlight nanoscale morphology as an important parameter enabling proton transport through two-dimensional crystals, mostly considered and modelled as flat, and indicate that strain and curvature can be used as additional degrees of freedom to control the proton permeability of two-dimensional materials
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