3,288 research outputs found
Magnitude of Magnetic Field Dependence of a Possible Selective Spin Filter in ZnSe/Zn_{1-x}Mn_{x}Se Multilayer Heterostructure
Spin-polarized transport through a band-gap-matched ZnSe/Zn_{1-x}Mn_{x}
Se/ZnSe/Zn_{1-x}Mn_{x}Se/ZnSe multilayer structure is investigated. The
resonant transport is shown to occur at different energies for different spins
owing to the split of spin subbands in the paramagnetic layers. It is found
that the polarization of current density can be reversed in a certain range of
magnetic field, with the peak of polarization moving towards a stronger
magnetic field for increasing the width of central ZnSe layer while shifting
towards an opposite direction for increasing the width of paramagnetic layer.
The reversal is limited in a small-size system. A strong suppression of the
spin up component of the current density is present at high magnetic field. It
is expected that such a reversal of the polarization could act as a possible
mechanism for a selective spin filter device
Topological Nematic States and Non-Abelian Lattice Dislocations
An exciting new prospect in condensed matter physics is the possibility of
realizing fractional quantum Hall (FQH) states in simple lattice models without
a large external magnetic field. A fundamental question is whether
qualitatively new states can be realized on the lattice as compared with
ordinary fractional quantum Hall states. Here we propose new symmetry-enriched
topological states, topological nematic states, which are a dramatic
consequence of the interplay between the lattice translation symmetry and
topological properties of these fractional Chern insulators. When a partially
filled flat band has a Chern number N, it can be mapped to an N-layer quantum
Hall system. We find that lattice dislocations can act as wormholes connecting
the different layers and effectively change the topology of the space. Lattice
dislocations become defects with non-trivial quantum dimension, even when the
FQH state being realized is by itself Abelian. Our proposal leads to the
possibility of realizing the physics of topologically ordered states on high
genus surfaces in the lab even though the sample has only the disk geometry.Comment: 10 pages, 6 figures. Several new sections added in v2, including
sections on even/odd effect for numerical diagnostics, analysis of domain
walls, and effective topological field theor
Universality and scaling in multi-field -attractor preheating
We explore preheating in multi-field models of inflation in which the
field-space metric is a highly curved hyperbolic manifold. One broad family of
such models is called -attractors, whose single-field regimes have been
extensively studied in the context of inflation and supergravity. We focus on a
simple two-field generalization of the -model, which has received renewed
attention in the literature. Krajewski et al. concluded, using lattice
simulations, that multi-field effects can dramatically speed-up preheating. We
recover their results and further demonstrate that significant analytical
progress can be made for preheating in these models using the WKB approximation
and Floquet analysis. We find a simple scaling behavior of the Floquet
exponents for large values of the field-space curvature, that enables a quick
estimation of the -model reheating efficiency for any large value of the
field-space curvature. In this regime we further observe and explain universal
preheating features that arise for different values of the potential steepness.
In general preheating is faster for larger negative values of the field-space
curvature and steeper potentials. For very highly curved field-space manifolds
preheating is essentially instantaneous.Comment: 43 pages, 21 figures; v2: published version with analysis extende
Superconformal defects in the tricritical Ising model
We study superconformal defect lines in the tricritical Ising model in 2
dimensions. By the folding trick, a superconformal defect is mapped to a
superconformal boundary of the N=1 superconformal unitary minimal model of
c=7/5 with D_6-E_6 modular invariant. It turns out that the complete set of the
boundary states of c=7/5 D_6-E_6 model cannot be interpreted as the consistent
set of superconformal defects in the tricritical Ising model since it does not
contain the "no defect" boundary state. Instead, we find a set of 18 consistent
superconformal defects including "no defect" and satisfying the Cardy
condition. This set also includes some defects which are not purely
transmissive or purely reflective.Comment: 25 pages, 3 figures. v2: typos corrected. v3: clarification about
spin structure aligned theory added, references adde
Kondo effect in carbon nanotube quantum dots with spin-orbit coupling
Motivated by recent experimental observation of spin-orbit coupling in carbon
nanotube quantum dots [F. Kuemmeth \textsl{et al.}, Nature (London) {\bf 452},
448 (2008)], we investigate in detail its influence on the Kondo effect. The
spin-orbit coupling intrinsically lifts out the fourfold degeneracy of a single
electron in the dot, thereby breaking the SU(4) symmetry and splitting the
Kondo resonance even at zero magnetic field. When the field is applied, the
Kondo resonance further splits and exhibits fine multipeak structures resulting
from the interplay of spin-orbit coupling and Zeeman effect. A microscopic
cotunneling process for each peak can be uniquely identified. Finally, a purely
orbital Kondo effect in the two-electron regime is also obtained.Comment: published version, 5 pages, 4 figure
- …