281 research outputs found
Interlayer coherent composite Fermi liquid phase in quantum Hall bilayers
Composite fermions have played a seminal role in understanding the quantum
Hall effect, particularly the formation of a compressible `composite Fermi
liquid' (CFL) at filling factor nu = 1/2. Here we suggest that in multi-layer
systems interlayer Coulomb repulsion can similarly generate `metallic' behavior
of composite fermions between layers, even if the electrons remain insulating.
Specifically, we propose that a quantum Hall bilayer with nu = 1/2 per layer at
intermediate layer separation may host such an interlayer coherent CFL, driven
by exciton condensation of composite fermions. This phase has a number of
remarkable properties: the presence of `bonding' and `antibonding' composite
Fermi seas, compressible behavior with respect to symmetric currents, and
fractional quantum Hall behavior in the counterflow channel. Quantum
oscillations associated with the Fermi seas give rise to a new series of
incompressible states at fillings nu = p/[2(p \pm 1)] per layer (p an integer),
which is a bilayer analogue of the Jain sequence.Comment: 4 pages, 3 figure
Parafermionic edge zero modes in Z_n-invariant spin chains
A sign of topological order in a gapped one-dimensional quantum chain is the
existence of edge zero modes. These occur in the Z_2-invariant Ising/Majorana
chain, where they can be understood using free-fermion techniques. Here I
discuss their presence in spin chains with Z_n symmetry, and prove that for
appropriate coupling they are exact, even in this strongly interacting system.
These modes are naturally expressed in terms of parafermions, generalizations
of fermions to the Z_n case. I show that parafermionic edge zero modes do not
occur in the usual ferromagnetic and antiferromagnetic cases, but rather only
when the interactions are chiral, so that spatial-parity and time-reversal
symmetries are broken.Comment: 22 pages. v2: small changes, added reference
Phase diagram of bismuth in the extreme quantum limit
Elemental bismuth provides a rare opportunity to explore the fate of a
three-dimensional gas of highly mobile electrons confined to their lowest
Landau level. Coulomb interaction, neglected in the band picture, is expected
to become significant in this extreme quantum limit with poorly understood
consequences. Here, we present a study of the angular-dependent Nernst effect
in bismuth, which establishes the existence of ultraquantum field scales on top
of its complex single-particle spectrum. Each time a Landau level crosses the
Fermi level, the Nernst response sharply peaks. All such peaks are resolved by
the experiment and their complex angular-dependence is in very good agreement
with the theory. Beyond the quantum limit, we resolve additional Nernst peaks
signaling a cascade of additional Landau sub-levels caused by electron
interaction
Non-Abelian toplogical superconductors from topological semimetals and related systems under superconducting proximity effect
Non-Abelian toplogical superconductors are characterized by the existence of
{zero-energy} Majorana fermions bound in the quantized vortices. This is a
consequence of the nontrivial bulk topology characterized by an {\em odd} Chern
number. It is found that in topological semimetals with a single two-bands
crossing point all the gapped superconductors are non-Abelian ones. Such a
property is generalized to related but more generic systems which will be
useful in the search of non-Abelian superconductors and Majorana fermions
Topological orbital ladders
We unveil a topological phase of interacting fermions on a two-leg ladder of
unequal parity orbitals, derived from the experimentally realized double-well
lattices by dimension reduction. topological invariant originates simply
from the staggered phases of -orbital quantum tunneling, requiring none of
the previously known mechanisms such as spin-orbit coupling or artificial gauge
field. Another unique feature is that upon crossing over to two dimensions with
coupled ladders, the edge modes from each ladder form a parity-protected flat
band at zero energy, opening the route to strongly correlated states controlled
by interactions. Experimental signatures are found in density correlations and
phase transitions to trivial band and Mott insulators.Comment: 12 pages, 5 figures, Revised title, abstract, and the discussion on
Majorana numbe
Non-Abelian statistics and topological quantum information processing in 1D wire networks
Topological quantum computation provides an elegant way around decoherence,
as one encodes quantum information in a non-local fashion that the environment
finds difficult to corrupt. Here we establish that one of the key
operations---braiding of non-Abelian anyons---can be implemented in
one-dimensional semiconductor wire networks. Previous work [Lutchyn et al.,
arXiv:1002.4033 and Oreg et al., arXiv:1003.1145] provided a recipe for driving
semiconducting wires into a topological phase supporting long-sought particles
known as Majorana fermions that can store topologically protected quantum
information. Majorana fermions in this setting can be transported, created, and
fused by applying locally tunable gates to the wire. More importantly, we show
that networks of such wires allow braiding of Majorana fermions and that they
exhibit non-Abelian statistics like vortices in a p+ip superconductor. We
propose experimental setups that enable the Majorana fusion rules to be probed,
along with networks that allow for efficient exchange of arbitrary numbers of
Majorana fermions. This work paves a new path forward in topological quantum
computation that benefits from physical transparency and experimental realism.Comment: 6 pages + 17 pages of Supp. Mat.; 10 figures. Supp. Mat. has doubled
in size to establish results more rigorously; many other improvements as wel
Targeted Disruption of the Interaction between WD-40 Repeat Protein 5 (WDR5) and Mixed Lineage Leukemia (MLL)/SET1 Family Proteins Specifically Inhibits MLL1 and SETd1A Methyltransferase Complexes
MLL1 belongs to the SET1 family of histone H3 lysine 4 (H3K4) methyltransferases, composed of MLL1–4 and SETd1A/B. MLL1 translocations are present in acute leukemias, and mutations in several family members are associated with cancer and developmental disorders. MLL1 associates with a subcomplex containing WDR5, RbBP5, ASH2L, and DPY-30 (WRAD), forming the MLL1 core complex required for H3K4 mono- and dimethylation and transcriptional activation. Core complex assembly requires interaction of WDR5 with the MLL1 Win (WDR5 interaction) motif, which is conserved across the SET1 family. Agents that mimic the SET1 family Win motif inhibit the MLL1 core complex and have become an attractive approach for targeting MLL1 in cancers. Like MLL1, other SET1 family members interact with WRAD, but the roles of the Win motif in complex assembly and enzymatic activity remain unexplored. Here, we show that the Win motif is necessary for interaction of WDR5 with all members of the human SET1 family. Mutation of the Win motif-WDR5 interface severely disrupts assembly and activity of MLL1 and SETd1A complexes but only modestly disrupts MLL2/4 and SETd1B complexes without significantly altering enzymatic activity in vitro. Notably, in the absence of WDR5, MLL3 interacts with RAD and shows enhanced activity. To further probe the role of the Win motif-WDR5 interaction, we designed a peptidomimetic that binds WDR5 (Kd ∼3 nm) and selectively inhibits activity of MLL1 and SETd1A core complexes within the SET1 family. Our results reveal that SET1 family complexes with the weakest Win motif-WDR5 interaction are more susceptible to Win motif-based inhibitors
Coulomb-assisted braiding of Majorana fermions in a Josephson junction array
We show how to exchange (braid) Majorana fermions in a network of
superconducting nanowires by control over Coulomb interactions rather than
tunneling. Even though Majorana fermions are charge-neutral quasiparticles
(equal to their own antiparticle), they have an effective long-range
interaction through the even-odd electron number dependence of the
superconducting ground state. The flux through a split Josephson junction
controls this interaction via the ratio of Josephson and charging energies,
with exponential sensitivity. By switching the interaction on and off in
neighboring segments of a Josephson junction array, the non-Abelian braiding
statistics can be realized without the need to control tunnel couplings by gate
electrodes. This is a solution to the problem how to operate on topological
qubits when gate voltages are screened by the superconductor
Topologically non-trivial superconductivity in spin-orbit coupled systems: Bulk phases and quantum phase transitions
Topologically non-trivial superconductivity has been predicted to occur in
superconductors with a sizable spin-orbit coupling in the presence of an
external Zeeman splitting. Two such systems have been proposed: (a) s-wave
superconductor pair potential is proximity induced on a semiconductor, and (b)
pair potential naturally arises from an intrinsic s-wave pairing interaction.
As is now well known, such systems in the form of a 2D film or 1D nano-wires in
a wire-network can be used for topological quantum computation. When the
external Zeeman splitting crosses a critical value , the
system passes from a regular superconducting phase to a non-Abelian topological
superconducting phase. In both cases (a) and (b) we consider in this paper the
pair potential is strictly s-wave in both the ordinary and the
topological superconducting phases, which are separated by a topological
quantum critical point at , where is the chemical potential. On the other hand, since , the Zeeman splitting required for the topological phase () far exceeds the value () above which an s-wave
pair potential is expected to vanish (and the system to become
non-superconducting) in the absence of spin-orbit coupling. We are thus led to
a situation that the topological superconducting phase appears to set in a
parameter regime at which the system actually is non-superconducting in the
absence of spin-orbit coupling. In this paper we address the question of how a
pure s-wave pair potential can survive a strong Zeeman field to give rise to a
topological superconducting phase. We show that the spin-orbit coupling is the
crucial parameter for the quantum transition into and the robustness of the
topologically non-trivial superconducting phase realized for .Comment: as published in the focus issue on Topological Quantum Computation,
New J. Phys. 13 (2011
Identification of Thioaptamer Ligand against E-Selectin: Potential Application for Inflamed Vasculature Targeting
Active targeting of a drug carrier to a specific target site is crucial to provide a safe and efficient delivery of therapeutics and imaging contrast agents. E-selectin expression is induced on the endothelial cell surface of vessels in response to inflammatory stimuli but is absent in the normal vessels. Thus, E-selectin is an attractive molecular target, and high affinity ligands for E-selectin could be powerful tools for the delivery of therapeutics and/or imaging agents to inflamed vessels. In this study, we identified a thiophosphate modified aptamer (thioaptamer, TA) against E-selectin (ESTA-1) by employing a two-step selection strategy: a recombinant protein-based TA binding selection from a combinatorial library followed by a cell-based TA binding selection using E-selectin expressing human microvascular endothelial cells. ESTA-1 selectively bound to E-selectin with nanomolar binding affinity (KD = 47 nM) while exhibiting minimal cross reactivity to P- and L-selectin. Furthermore, ESTA-1 binding to E-selectin on the endothelial cells markedly antagonized the adhesion (over 75% inhibition) of sLex positive HL-60 cells at nanomolar concentration. ESTA-1 also bound specifically to the inflamed tumor-associated vasculature of human carcinomas derived from breast, ovarian, and skin but not to normal organs, and this binding was highly associated with the E-selectin expression level. Similarly, intravenously injected ESTA-1 demonstrated distinct binding to the tumor vasculature in a breast cancer xenograft model. Together, our data substantiates the discovery of a thioaptamer (ESTA-1) that binds to E-selectin with high affinity and specificity, thereby highlighting the potential application of ESTA-1 for E-selectin targeted delivery
- …