26 research outputs found
The effect of the Abrikosov vortex phase on spin and charge states in magnetic semiconductor-superconductor hybrids
We explore the possibility of using the inhomogeneous magnetic field carried
by an Abrikosov vortex in a type-II superconductor to localize spin-polarized
textures in a nearby magnetic semiconductor quantum well. We show how
Zeeman-induced localization induced by a single vortex is indeed possible, and
use these results to investigate the effect of a periodic vortex array on the
transport properties of the magnetic semiconductor. In particular, we find an
unconventional Integer Quantum Hall regime, and predict directly testable
experimental consequences due to the presence of the periodic spin polarized
structure induced by the superconducting vortex lattice in the magnetic
semiconductor.Comment: 12 pages, 15 figure
Nanoscale Zeeman localization of charge carriers in diluted magnetic semiconductor-permalloy hybrids
We investigate the possibility of charge carrier localization in magnetic
semiconductors due to the presence of a highly inhomogeneous external magnetic
field. As an example, we study in detail the properties of a magnetic
semiconductor-permalloy disk hybrid system. We find that the giant Zeeman
respose of the magnetic semiconductor in conjuction with the highly non-uniform
magnetic field created by the vortex state of a permalloy disk can lead to
Zeeman localized states at the interface of the two materials. These trapped
state are chiral, with chirality controlled by the orientation of the core
magnetization of the permalloy disk. We calculate the energy spectrum and the
eigenstates of these Zeeman localized states, and discuss their experimental
signatures in spectroscopic probes.Comment: 4 pages, 1 figur
Binding energy of shallow donors in a quantum well in the presence of a tilted magnetic field
We present results of variational calculations of the binding energy of a
neutral donor in a quantum well in the presence of a magnetic field tilted
relative to the QW plane. Assuming that the donor is located in the center of
the QW, we perform calculations for parameters typical of a II-VI wide-gap
semiconductor heterostructure, using as an example the case of a rectangular
CdTe quantum well with CdMgTe barriers. We present the dependence of the
binding energy of a neutral donor on the tilt angle and on the magnitude of the
applied magnetic filed. As a key result, we show that measurement of the
binding energy of a donor at two angles of the magnetic field with respect to
the quantum well plane can be used to unambiguously determined the conduction
band offset of the materials building up heterostructure.Comment: 6 pages, 5 figure
Enhanced robustness and dimensional crossover of superradiance in cuboidal nanocrystal superlattices
Cooperative emission of coherent radiation from multiple emitters (known as
superradiance) has been predicted and observed in various physical systems,
most recently in CsPbBr nanocrystal superlattices. Superradiant emission is
coherent and occurs on timescales faster than the emission from isolated
nanocrystals. Theory predicts cooperative emission being faster by a factor of
up to the number of nanocrystals (). However, superradiance is strongly
suppressed due to the presence of energetic disorder, stemming from nanocrystal
size variations and thermal decoherence. Here, we analyze superradiance from
superlattices of different dimensionalities (1D, 2D and 3D) with variable
nanocrystal aspect ratios. We predict as much as a thirty-fold enhancement in
robustness against realistic values of energetic disorder in three-dimensional
(3D) superlattices composed of cuboid-shaped, as opposed to cube-shaped,
nanocrystals. Superradiance from small two-dimensional (2D)
superlattices is up to 10 times more robust to static disorder and up to twice
as robust to thermal decoherence than three-dimensional (3D) superlattices with
the same . As the number of increases, a crossover in the robustness of
superradiance occurs from 2D to 3D superlattices. For large , the
robustness in 3D superlattices increases with , showing cooperative
robustness to disorder. This opens the possibility of observing superradiance
even at room temperature in large 3D superlattices, if nanocrystal size
fluctuations can be kept small
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Spin-resolved spectra of Shiba multiplets from Mn impurities in MgB2
We study the effect of magnetic Mn ions on the two-band superconductor MgB2, and compute both the total and spin-resolved scanning tunneling spectrum in the vicinity of the magnetic impurity. We show that when the internal structure of the Mn ion’s d-shell is taken into account, multiple Shiba states appear in the spectrum. The presence of these multiplets could alter significantly the overall interpretation of local tunneling spectra for a wide range of superconducting hosts and magnetic impurities.Physic
Heterogeneous Fluorescence Intermittency in Single Layer Reduced Graphene Oxide
We provide, for the first time, direct experimental evidence for heterogeneous blinking in reduced graphene oxide (rGO) during photolysis. The spatially resolved intermittency originates from regions within individual rGO sheets and shows 1/f-like power spectral density. We describe the evolution of rGO blinking using the multiple recombination center (MRC) model that captures common features of nanoscale blinking. Our results illustrate the universal nature of blinking and suggest a common microscopic origin for the effect
Origin of the pseudogap phase: Precursor superconductivity versus a competing energy gap scenario
In the last few years evidence has been accumulating that there are a
multiplicity of energy scales which characterize superconductivity in the
underdoped cuprates. In contrast to the situation in BCS superconductors, the
phase coherence temperature Tc is different from the energy gap onset
temperature T*. In addition, thermodynamic and tunneling spectroscopies have
led to the inference that the order parameter is to be
distinguished from the excitation gap ; in this way, pseudogap effects
persist below Tc. It has been argued by many in the community that the presence
of these distinct energy scales demonstrates that the pseudogap is unrelated to
superconductivity. In this paper we show that this inference is incorrect. We
demonstrate that the difference between the order parameter and excitation gap
and the contrasting dependences of T* and Tc on hole concentration and
magnetic field follow from a natural generalization of BCS theory. This
simple generalized form is based on a BCS-like ground state, but with self
consistently determined chemical potential in the presence of arbitrary
attractive coupling . We have applied this mean field theory with some
success to tunneling, transport, thermodynamics and magnetic field effects. We
contrast the present approach with the phase fluctuation scenario and discuss
key features which might distinguish our precursor superconductivity picture
from that involving a competing order parameter.Comment: 4 pages, 2 EPS figures, use LaTeX package espcrc2.sty from Elsevier,
submitted to SNS'01 conference proceeding
The electronic specific heat in the pairing pseudogap regime
When pairing correlations in a quasi two dimensional electron system induce a
pseudogap in the single particle density of states, the specific heat must also
contain a sizeable pair contribution. The theoretically calculated specific
heat for such a system is compared to the experimental results of Loram and his
collaborators for underdoped YBa_2Cu_3O_{6+x} and La_{2-x}Sr_{x}CuO_4 samples.
The size and doping dependence of the extracted pseudogap energy scale for both
materials is comparable to the values obtained from a variety of other
experiments.Comment: 4 pages, 5 eps figure
Resonant multiple-phonon absorption causes efficient anti-Stokes photoluminescence in CsPbBr nanocrystals
Lead-halide perovskite nanocrystals such as CsPbBr, exhibit efficient
photoluminescence (PL) up-conversion, also referred to as anti-Stokes
photoluminescence (ASPL). This is a phenomenon where irradiating nanocrystals
up to 100 meV below gap results in higher energy band edge emission. Most
surprising is that ASPL efficiencies approach unity and involve single photon
interactions with multiple phonons. This is unexpected given the statistically
disfavored nature of multiple-phonon absorption. Here, we report and
rationalize near-unity anti-Stokes photoluminescence efficiencies in CsPbBr
nanocrystals and attribute it to resonant multiple-phonon absorption by
polarons. The theory explains paradoxically large efficiencies for
intrinsically disfavored, multiple-phonon-assisted ASPL in nanocrystals.
Moreover, the developed microscopic mechanism has immediate and important
implications for applications of ASPL towards condensed phase optical
refrigeration