1,698 research outputs found
Signatures of tunable Majorana-fermion edge states
Chiral Majorana-fermion modes are shown to emerge as edge excitations in a
superconductor--topological-insulator hybrid structure that is subject to a
magnetic field. The velocity of this mode is tunable by changing the
magnetic-field magnitude and/or the superconductor's chemical potential. We
discuss how quantum-transport measurements can yield experimental signatures of
these modes. A normal lead coupled to the Majorana-fermion edge state through
electron tunneling induces resonant Andreev reflections from the lead to the
grounded superconductor, resulting in a distinctive pattern of
differential-conductance peaks.Comment: (13 pages, Accepted for publication in New Journal of Physics, an
extension of and expansion on our previous work arXiv:1210.4057). arXiv admin
note: text overlap with arXiv:1210.405
Spin transport in a graphene normal-superconductor junction in the quantum Hall regime
The quantum Hall regime of graphene has many unusual properties. In
particular, the presence of a Zeeman field opens up a region of energy within
the zeroth Landau level, where the spin-up and spin-down states localized at a
single edge propagate in opposite directions. We show that when these edge
states are coupled to an s-wave superconductor, the transport of charge
carriers is spin-filtered. This spin-filtering effect can be traced back to the
interplay of specular Andreev reflections and Andreev retro-reflections in the
presence of a Zeeman field.Comment: 6 pages, 7 figure
Quantum transport signatures of chiral edge states in SrRuO
We investigate transport properties of a double quantum dot based Cooper pair
splitter, where the superconducting lead consists of SrRuO. The
proposed device can be used to explore the symmetry of the superconducting
order parameter in SrRuO by testing the presence of gapless chiral edge
states, which are predicted to exist if the bulk superconductor is described by
a chiral --wave state. The odd orbital symmetry of the bulk order parameter
ensures that we can realize a regime where the electrons tunneling into the
double dot system come from the chiral edge states and thereby leave their
signature in the conductance. The proposed Cooper pair splitter has the
potential to probe order parameters in unconventional superconductors.Comment: 5 page
Switchable valley filter based on a graphene - junction in a magnetic field
Low-energy excitations in graphene exhibit relativistic properties due to the
linear dispersion relation close to the Dirac points in the first Brillouin
zone. Two of the Dirac points located at opposite corners of the first
Brillouin zone can be chosen as inequivalent, representing a new valley degree
of freedom, in addition to the charge and spin of an electron. Using the valley
degree of freedom to encode information has attracted significant interest,
both theoretically and experimentally, and gave rise to the field of
valleytronics. We study a graphene - junction in a uniform out-of-plane
magnetic field as a platform to generate and controllably manipulate the valley
polarization of electrons. We show that by tuning the external potential giving
rise to the - junction we can switch the current from one valley
polarization to the other. We also consider the effect of different types of
edge terminations and present a setup, where we can partition an incoming
valley-unpolarized current into two branches of valley-polarized currents. The
branching ratio can be chosen by changing the location of the - junction
using a gate.Comment: 8 pages, 7 figure
Neutral edge modes in a superconductor -- topological-insulator hybrid structure in a perpendicular magnetic field
We study the low-energy edge states of a superconductor -- 3D
topological-insulator hybrid structure (NS junction) in the presence of a
perpendicular magnetic field. The hybridization of electron-like and hole-like
Landau levels due to Andreev reflection gives rise to chiral edge states within
each Landau level. We show that by changing the chemical potential of the
superconductor, this junction can be placed in a regime where the sign of the
effective charge of the edge state within the zeroth Landau level changes more
than once resulting in neutral edge modes with a finite value of the
guiding-center coordinate. We find that the appearance of these neutral edge
modes is related to the level repulsion between the zeroth and the first Landau
levels in the spectra. We also find that these neutral edge modes come in
pairs, one in the zeroth Landau level and its corresponding pair in the first.Comment: 5 page
Josephson Effect in a Coulomb-blockaded SINIS Junction
The problem of Josephson current through Coulomb-blocked nanoscale
superconductor-normal-superconductor structure with tunnel contacts is
reconsidered. Two different contributions to the phase-biased supercurrent are
identified, which are dominant in the limits of weak and strong Coulomb
interaction. Full expression for the free energy valid at arbitrary Coulomb
strength is found. The current derived from this free energy interpolates
between known results for weak and strong Coulomb interaction as phase bias
changes from 0 to pi. In the broad range of Coulomb strength the current-phase
relation is substantially non-sinusoidal and qualitatively different from the
case of semi-ballistic SNS junctions. Coulomb interaction leads to appearance
of a local minimum in the current at some intermediate value of phase
difference applied to the junction.Comment: 5 pages, 2 EPS figures, JETP Letters style file include
Variable electrostatic transformer: controllable coupling of two charge qubits
We propose and investigate a novel method for the controlled coupling of two
Josephson charge qubits by means of a variable electrostatic transformer. The
value of the coupling capacitance is given by the discretized curvature of the
lowest energy band of a Josephson junction, which can be positive, negative, or
zero. We calculate the charging diagram of the two-qubit system that reflects
the transition from positive to negative through vanishing coupling. We also
discuss how to construct a phase gate making use of the controllable coupling.Comment: final version, to appear in Phys. Rev. Let
On The Violation Of Marshall-Peierls Sign Rule In The Frustrated Heisenberg Antiferromagnet
We present a number of arguments in favor of the suggestion that the
Marshall-Peierls sign rule survives the frustration in the square-lattice
Heisenberg antiferromagnet with frustrating next-nearest-neighbor (diagonal)
bonds ( model) for relatively large values of the parameter
. Both the spin-wave analysis and the exact-diagonalization data
concerning the weight of Marshall states support the above suggestion.Comment: 8 pages, LaTex, 2 figurs on reques
The Effect of High Dose Total Body Irradiation on ACTH, Corticosterone, and Catecholamines in the Rat
Total body irradiation (TBI) or partial body irradiation is a distinct risk of accidental, wartime, or terrorist events. Total body irradiation is also used as conditioning therapy before hematopoietic stem cell transplantation. This therapy can result in injury to multiple tissues and might result in death as a result of multiorgan failure. The hypothalamic–pituitary–adrenal (HPA) axis could play a causative role in those injuries, in addition to being activated under conditions of stress. In a rat model of TBI, we have established that radiation nephropathy is a significant lethal complication, which is caused by hypertension and uremia. The current study assessed HPA axis function in rats undergoing TBI. Using a head-shielded model of TBI, we found an enhanced response to corticotropin-releasing hormone (CRH) in vitro in pituitaries from irradiated compared with nonirradiated rats at both 8 and 70 days after 10-Gy single fraction TBI. At 70, but not 8 days, plasma adrenocorticotrophic hormone (ACTH) and corticosterone levels were increased significantly in irradiated compared with nonirradiated rats. Plasma aldosterone was not affected by TBI at either time point, whereas plasma renin activity was decreased in irradiated rats at 8 days. Basal and stimulated adrenal steroid synthesis in vitro was not affected by TBI. In addition, plasma epinephrine was decreased at 70 days after TBI. The hypothalamic expression of CRH messenger RNA (mRNA) and hippocampal expression of glucocorticoid receptor mRNA were unchanged by irradiation. We conclude that the hypertension of radiation nephropathy is not aldosterone or catecholamine-dependent but that there is an abscopal activation of the HPA axis after 10 Gy TBI. This activation was attributable at least partially to enhanced pituitary ACTH production
Snake states and their symmetries in graphene
Snake states are open trajectories for charged particles propagating in two
dimensions under the influence of a spatially varying perpendicular magnetic
field. In the quantum limit they are protected edge modes that separate
topologically inequivalent ground states and can also occur when the particle
density rather than the field is made nonuniform. We examine the correspondence
of snake trajectories in single-layer graphene in the quantum limit for two
families of domain walls: (a) a uniform doped carrier density in an
antisymmetric field profile and (b) antisymmetric carrier distribution in a
uniform field. These families support different internal symmetries but the
same pattern of boundary and interface currents. We demonstrate that these
physically different situations are gauge equivalent when rewritten in a Nambu
doubled formulation of the two limiting problems. Using gauge transformations
in particle-hole space to connect these problems, we map the protected
interfacial modes to the Bogoliubov quasiparticles of an interfacial
one-dimensional p-wave paired state. A variational model is introduced to
interpret the interfacial solutions of both domain wall problems
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