pinecone/core-2020-05-10-deduplication huggingface dataset:
Spin pumping through quantum dots

We propose schemes for generating spin currents into a semiconductor by
adiabatic or non-adiabatic pumping of electrons through interacting quantum
dots. The appeal of such schemes lies in the possibility to tune the pumping
characteristics via gate voltages that control the properties of the quantum
dot. The calculations are based on a systematic perturbation expansion in the
tunnel-coupling strength and the pumping frequency, expressed within a
diagrammatic real-time technique. Special focus is put on the possibility of
pure spin pumping, i.e., of pumping spin currents without charge currents.Comment: Feature Article for Physica Status Solidi
Towards combined transport and optical studies of the 0.7-anomaly in a
  quantum point contact

A Quantum Point Contact (QPC) causes a one-dimensional constriction on the
spatial potential landscape of a two-dimensional electron system. By tuning the
voltage applied on a QPC at low temperatures the resulting regular step-like
electron conductance quantization can show an additional kink near pinch-off
around 0.7(2$e^2$/h), called 0.7-anomaly. In a recent publication, we presented
a combination of theoretical calculations and transport measurements that lead
to a detailed understanding of the microscopic origin of the 0.7-anomaly.
Functional Renormalization Group-based calculations were performed exhibiting
the 0.7-anomaly even when no symmetry-breaking external magnetic fields are
involved. According to the calculations the electron spin susceptibility is
enhanced within a QPC that is tuned in the region of the 0.7-anomaly. Moderate
externally applied magnetic fields impose a corresponding enhancement in the
spin magnetization. In principle, it should be possible to map out this spin
distribution optically by means of the Faraday rotation technique. Here we
report the initial steps of an experimental project aimed at realizing such
measurements. Simulations were performed on a particularly pre-designed
semiconductor heterostructure. Based on the simulation results a sample was
built and its basic transport and optical properties were investigated.
Finally, we introduce a sample gate design, suitable for combined transport and
optical studies
Radiation defects in BaF$_2$-Cd

Radiation defects in barium fluoride single crystals doped with cadmium have
been investigated by luminescence and absorption spectroscopy, as well as by
electron spin resonance spectroscopy. Three types of Cd$^+$ centers differing
by the local environment with the point symmetries O$_h$, C$_3$$_v$ and
C$_2$$_v$ have been identified although in the crystals only the cubic centers
are usually formed. We believe that these features may arise from the
difference in the spacial distribution of the impurities in the crystals
Zinc Oxide - From Dilute Magnetic Doping to Spin Transport

During the past years there has been renewed interest in the wide-bandgap
II-VI semiconductor ZnO, triggered by promising prospects for spintronic
applications. First, ferromagnetism was predicted for dilute magnetic doping.
In comprehensive investigation of ZnO:Co thin films based on the combined
measurement of macroscopic and microscopic properties, we find no evidence for
carrier-mediated itinerant ferromagnetism. Phase-pure, crystallographically
excellent ZnO:Co is uniformly paramagnetic. Superparamagnetism arises when
phase separation or defect formation occurs, due to nanometer-sized metallic
precipitates. Other compounds like ZnO:(Li,Ni) and ZnO:Cu do not exhibit
indication of ferromagnetism.
  Second, its small spin-orbit coupling and correspondingly large spin
coherence length makes ZnO suitable for transporting or manipulating spins in
spintronic devices. From optical pump/optical probe experiments, we find a spin
dephasing time of the order of 15 ns at low temperatures which we attribute to
electrons bound to Al donors. In all-electrical magnetotransport measurements,
we successfully create and detect a spin-polarized ensemble of electrons and
transport this spin information across several nanometers. We derive a spin
lifetime of 2.6 ns for these itinerant spins at low temperatures, corresponding
well to results from an electrical pump/optical probe experiment.Comment: 11 pages, 6 figures, 99 reference
The rise of spin noise spectroscopy in semiconductors: From acoustic to GHz frequencies

This article gives an overview on the advance of spin noise spectroscopy (SNS) in semiconductors in the past 8 years from the first measurements in bulk n-GaAs [Oestreich et al., Phys. Rev. Lett. 95, 216603 (2005)] up to the recent achievement of optical detection of the intrinsic spin fluctuations of a single hole confined in an individual self-assembled quantum dot [Dahbashi et al., arXiv:1306.3183 (2013)]. We discuss the general technical implementation of optical SNS and the invaluable profit of the introduction of real-time fast Fourier transform analysis into the data acquisition. By now, the full spin dynamic from the milli- to picosecond timescales can be addressed by SNS and the technique quickly strides ahead to enable real quantum non-demolition measurements in semiconductors. Spin noise spectra recorded in 2005 in bulk n-GaAs with approximately 109 electron spins (Oestreich et al.) and 2013 (Dahbashi et al.) for a single hole spin. The integration time for the latter is more than a factor of 40 shorter due to the significant advances in the measurement technique
Magnetic polaron formation in graphene-based single-electron transistor

We theoretically study the magnetic polaron (MP) formation in a ferromagnetic graphene single-electron transistor (SET), which consists of a graphene quantum dot (QD) between two insulating layers, i.e., a ferromagnetic insulator EuO and a nonmagnetic substrate such as SiC. In the lateral direction, the QD is electrically coupled to source, drain and gate electrodes, so that the number of electrons can be controlled by the gate voltage. Due to a proximity effect at the EuO/graphene interface, i.e., the exchange interaction between the charge carriers in graphene and the localized magnetic electrons in EuO, there is a magnetic coupling between the graphene QD and EuO. Using Green's function technique an expression for the total free energy of the SET is derived, which is then minimized at the given temperature and gate voltage. This leads to the MP formation and a consequent local enhancement of the ferromagnetic properties of EuO at the EuO/graphene interface. The MP formation is enforced by the large Coulomb interaction between the carriers in graphene. The spin polarization at the graphene/EuO interface in the QD area can be controlled by the gate voltage of the SET.Peer reviewe
Quantum interference in HgTe structures

We study quantum transport in HgTe/HgCdTe quantum wells under the condition
that the chemical potential is located outside of the bandgap. We first analyze
symmetry properties of the effective Bernevig-Hughes-Zhang Hamiltonian and the
relevant symmetry-breaking perturbations. Based on this analysis, we overview
possible patterns of symmetry breaking that govern the quantum interference
(weak localization or weak antilocalization) correction to the conductivity in
two dimensional HgTe/HgCdTe samples. Further, we perform a microscopic
calculation of the quantum correction beyond the diffusion approximation.
Finally, the interference correction and the low-field magnetoresistance in a
quasi-one-dimensional geometry are analyzed.Comment: 15 pages, 6 figures; feature article (topical review) for a special
  issue "Semiconductor Spintronics" (DFG-SPP 1285) of Physica Status Solidi B:
  Basic Solid State Physics; partly overviews PRB 86, 125323 (2012) and PRB 90,
  085401 (2014) (arXiv:1207.4102 and arXiv:1402.7097, respectively
Exact results for a generalized spin-1/2 Ising-Heisenberg diamond chain
  with the second-neighbor interaction between nodal spins

The ground state and thermodynamics of a generalized spin-1/2
Ising-Heisenberg diamond chain with the second-neighbor interaction between
nodal spins are calculated exactly using the mapping method based on the
decoration-iteration transformation. Rigorous results for the magnetization,
susceptibility, and heat capacity are investigated in dependence on temperature
and magnetic field for the frustrated diamond spin chain with the
antiferromagnetic Ising and Heisenberg interactions. It is demonstrated that
the second-neighbor interaction between nodal spins gives rise to a greater
diversity of low-temperature magnetization curves, which may include an
intermediate plateau at two-third of the saturation magnetization related to
the classical ferrimagnetic (up-up-up-down-up-up-...) ground state with
translationally broken symmetry besides an intermediate one-third magnetization
plateau reflecting the translationally invariant quantum ferrimagnetic
(monomer-dimer) spin arrangement.Comment: 14 pages, 8 figure
Influence of the electron density on the thickness-dependent energy gap
  oscillations in superconducting metallic nanofilms

The thickness-dependent energy gap oscillations in the metallic nanofilms are
investigated by the use of the self-consistent numerical solutions of the
Bogoliubov-de Gennes equations. It is shown, that the oscillations are induced
by the quasi-particle energy quantization triggered by the confinement of
electrons in the direction perpendicular to the sample. We have analyzed, how
the changes in the electron density of states ($n_e$) and the electron-phonon
coupling constant ($g$) influence the amplitude of the considered oscillations.
It has been found, that the increase in $n_e$ and the decrease in $g$, can lead
to a significant reduction of the oscillations amplitude. As a result, for the
values of the mentioned parameters corresponding to some of the realistic
situations the thickness-dependent superconducting gap oscillations can be
almost completely suppressed
Fidelity of optically induced single-spin rotations in semiconductor
  quantum dots in the presence of nuclear spins

We examine the influence of nuclear spins on the performance of optically
induced rotations of single electron spins in semiconductor quantum dots. We
consider Raman type optical transitions between electron spin states and take
into account the additional effect of the Overhauser field. We calculate
average fidelities of rotations around characteristic axes in the presence of
nuclear spins analytically with perturbation theory up to second order in the
Overhauser field. Moreover, we calculate the fidelity using numerical averaging
over the nuclear field distribution, including arbitrary orders of the
hyperfine interaction.Comment: 7 pages, 5 figure
A realistic generic model for anti-tetrachiral systems

This work has been funded through\ud
the European Union 6th Framework Programme STREP project\ud
NMP3-CT-2005-013641 (CHISMACOMB). This research has\ud
been carried out using computational facilities procured through\ud
the European Regional Development Fund, Project ERDF-080\ud
“A Supercomputing Laboratory for the University of Malta”. The\ud
authors would also like to acknowledge the financial help received\ud
by the Malta Council of Research and Technology (MCST) and the\ud
University of Malta.Chiral systems are a class of structures, which may exhibit the anomalous property of a negative Poisson's ratio. Proposed by Wojciechowski and implemented later by Lakes, these structures have aroused interest due to their remarkable mechanical properties and numerous potential applications. In view of this, this paper investigates the on-axis mechanical properties of the general forms of the flexing anti-tetrachiral system through analytical and finite element models. The results suggest that these are highly dependent on the geometry (the ratio of ligament lengths, thicknesses, and radius of nodes) and material properties of the constituent materials. We also show that the rigidity of an anti-tetrachiral system can be changed without altering the Poisson's ratio.peer-reviewe
Absolutely continuous energy bands and extended electronic states in an
  aperiodic comb-shaped nanostructure

The nature of electronic eigenstates and quantum transport in a comb-shaped
Fibonacci nanostructure model is investigated within a tight-binding framework.
Periodic linear chains are side-attached to a Fibonacci chain, giving it the
shape of an aperiodic comb. The effect of the side-attachments on the usual
Cantor set energy spectrum of a Fibonacci chain is analyzed using the Greens
function technique. A special correlation between the coupling of the
side-attached chain with the Fibonacci chain and the inter-atomic coupling of
the Fibonacci chain results in a dramatic triggering of the fragmented Cantor
set energy spectrum into multiple sets of continuous sub-bands of extended
eigenstates. The result is valid even for a disordered comb and turns out to be
a rare exception of the conventional Anderson localization problem. The
electronic transport thus can be made selectively ballistic within desired
energy regimes. The number and the width of such continuous sub-bands can be
easily controlled by tuning the number of atomic sites in the side-coupled
periodic linear chains. This gives us a scope of proposing such aperiodic
nanostructures as potential candidates for prospective energy selective
nanoscale filtering devices.Comment: 7 pages, 7 figures, Revtex versio
Spin noise in a quantum dot ensemble: from a quantum mechanical to a
  semi-classical description

Spin noise spectroscopy is a promising technique for revealing the
microscopic nature of spin dephasing processes in quantum dots. We compare the
spin-noise in an ensemble of singly charged quantum dots calculated by two
complementary approaches. The Chebyshev polynomial expansion technique (CET)
accounts for the full quantum mechanical fluctuation of the nuclear spin bath
and a semi-classical approach (SCA) is based on the averaging the electron spin
dynamics over all different static Overhauser field configurations.
  We observe a remarkable agreement between both methods in the high-frequency
part of the spectra, while the low-frequency part is determined by the long
time fluctuations of the Overhauser field. We find small differences in the
spectra depending on the distribution of hyperfine couplings. The spin-noise
spectra in strong enough magnetic fields where the nuclear dynamics is quenched
calculated by two complimentary approaches are in perfect agreement.Comment: 6 pages, 3 figure
Characterization of gas phase iron phthalocyanine with X-ray photoelectron and absorption spectroscopies

Despite the numerous studies dedicated to phthalocyanine molecules adsorbed on surfaces, in monolayer or thin film, very few works have been focused on the characterization of vapors of these molecules. In this article we present the C 1s, N 1s and Fe 2p photoemission results as well as N K-edge X-ray absorption data of iron phthalocyanine (FePc) in gas phase. Presented comparison of X-ray photoelectron spectroscopy and X-ray absorption spectroscopy spectra of FePc films show a great similarity with the gas phase results, con- firming the molecular character of thick films. The Fe 2p photoemission spectrum of the gas phase FePc, shown for the first time, can be considered as a fingerprint of the Fe(II) ionic state of the central metal of the iron phthalocyanine. The performed multiplet calculations for describing the Fe 2p XP spectrum indicate 3 Eg (a1g 2 eg 3 b2g 1 ) state as the most probable ground state for thick film of iron phthalocyanine
Theoretical electron energy loss spectroscopy of isolated graphene

A thorough understanding of the electronic structure is a necessary first
step for the design of nanoelectronics, chemical/bio-sensors, electrocatalysts,
and nanoplasmonics using graphene. As such, theoretical spectroscopic
techniques to describe collective excitations of graphene are of fundamental
importance. Starting from density functional theory (DFT), linear response time
dependent DFT in frequency-reciprocal space within the random phase
approximation (TDDFT-RPA) is used to describe the loss function
-Im{1/{\epsilon}(q,{\omega})} for isolated graphene. To ensure any spurious
interactions between layers are removed, both a radial cutoff of the Coulomb
kernel, and extra vacuum directly at the TDDFT-RPA level are employed. A
combination of both methods is found to provide a correct description of the
electron energy loss spectra of isolated graphene, at a significant reduction
in computational cost compared to standard methods.Comment: 6 pages, 5 figures, International Winter School on Electronic
  Properties of Novel Materials (IWEPNM) 201
Resistivity in Co-doped Ba-122: comparison of thin films and single
  crystals

The temperature dependence of the resistivity of epitaxial
Ba(Fe_(1-x)Co_x)2As2 thin films (with nominal doping x = 0.08, 0.10 and 0.15)
has been analyzed and compared with analogous measurements on single crystals
taken from literature. The rho(T) of thin films looks different from that of
single crystals, even when the cobalt content is the same. All rho(T) curves
can be fitted by considering an effective two-band model (with holes and
electrons bands) in which the electrons are more strongly coupled with the
bosons (spin fluctuations) than holes, while the effect of impurities is mainly
concentrated in the hole band. Within this model the mediating boson has the
same characteristic energy in single crystals and thin films, but the shape of
the transport spectral function at low energy has to be very different, leading
to a "hardening" of the electron-boson spectral function in thin films,
associated with the strain induced by the substrate.Comment: 13 pages, 4 figure
Scanning probe microscopy and spectroscopy of graphene on metals

Graphene, a two-dimensional (2D) material with unique electronic properties,
appears to be an ideal object for the application of surface-science methods.
Among them, a family of scanning probe microscopy methods (STM, AFM, KPFM) and
the corresponding spectroscopy add-ons provide information about the structure
and electronic properties of graphene on the local scale (from inline imagem to
atoms). This review focuses on the recent applications of these
microscopic/spectroscopic methods for the investigation of graphene on metals
(interfaces, intercalation-like systems, graphene nanoribbons, and quantum
dots, etc.). It is shown that very important information about interaction
strength at the graphene/metal interfaces as well as about modification of the
electronic spectrum of graphene at the Fermi level can be obtained on the local
scale. The combination of these results with those obtained by other methods
and comparison with recent theoretical data demonstrate the power of this
approach for the investigation of the graphene-based systems.Comment: Feature Article in pss (b
A Study of the Inclusion of Prelayers in InGaN/GaN Single and Multiple Quantum Well Structures

We report on the effects on the optical properties of blue-light emitting InGaN/GaN single- and multiple-quantum-well structures including a variety of prelayers. For each single-quantum-well structure containing a Si-doped prelayer, we measured a large blue shift of the photoluminescence peak energy and a significant increase in radiative recombination rate at 10 K. Calculations of the conduction and valence band energies show a strong reduction in the built-in electric field across the quantum well (QW) occurs when including Si-doped prelayers, due to enhancement of the surface polarization field which opposes the built-in field. The reduction in built-in field across the QW results in an increase in the electron–hole wavefunction overlap, increasing the radiative recombination rate, and a reduction in the strength of the quantum confined Stark effect, leading to the observed blue shift of the emission peak. The largest reduction of the built-in field occurred for an InGaN:Si prelayer, in which the additional InGaN/GaN interface of the prelayer, in close proximity to the QW, was shown to further reduce the built-in field. Study of multiple QW structures with and without an InGaN:Si prelayer showed the same mechanisms identified in the equivalent single-quantum-well structure.This work was carried out with the financial support of the United Kingdom Engineering and Physical Sciences Research Council under Grant Numbers EP/I012591/1 and EP/H011676/1.This is the accepted manuscript. The final version's available from Wiley at http://dx.doi.org/10.1002/pssb.20145153
Investigation of unintentional indium incorporation into GaN barriers of InGaN/GaN quantum well structures

High resolution transmission electron microscopy has been employed to investigate the impact of the GaN bar-rier growth technique on the composition profile of InGaN quantum wells (QWs). We show that the profiles deviate from their nominal configuration due to the pres-ence of an indium tail at the upper interface of the QW. This indium tail, thought to be associated with a segrega-tion effect from the indium surfactant layer, has been shown to strongly depend on the growth method. The ef-fect of this tail has been investigated using a self-consistent Schrödinger-Poisson simulation. For the simu-lated conditions, a graded upper interface has been found to result in a decreased electron-hole wavefunction over-lap of up to 31 % compared to a QW with a rectangular profile, possibly leading to a decrease in radiative-recombination rate. Therefore in order to maximize the efficiency of a QW structure, it is important to grow the active region using a growth method which leads to QW interfaces which are as abrupt as possible. The results of this experiment find applications in every study where the emission properties of a device are correlated to a particular active region design.The authors acknowledge support from the EPSRC under EP/H0495331.This is the final version. It was first published by Wiley at http://onlinelibrary.wiley.com/doi/10.1002/pssb.201451543/abstrac
Low defect large area semi-polar (11-22) GaN grown on patterned (113) silicon

We report on the growth of semi-polar GaN (11¯22)
templates on patterned Si (113) substrates. Trenches
were etched in Si (113) using KOH to expose Si
{111} sidewalls. Subsequently an AlN layer to pre-
vent meltback etching, an AlGaN layer for stress
management, and finally two GaN layers were de-
posited. Total thicknesses up to 5 μm were realised
without cracks in the layer. Transmission electron
microscopy showed that most dislocations propagate
along [0001] direction and hence can be covered by
overgrowth from the next trench.
The defect densities were below 108 cm−2 and stack-
ing fault densities less than 100 cm−1. These num-
bers are similar to reports on patterned r-plane sap-
phire. Typical X-ray full width at half maximum
(FHWM) were 500” for the asymmetric (00.6) and
450” for the (11.2) reflection. These FHWMs were
50% broader than reported for patterned r-plane
sapphire which is attributed to different defect struc-
ture and total thicknesses. The surface roughness
shows strong variation on templates. For the final
surface roughness the roughness of the sidewalls of
the GaN ridges at the time of coalescence are criti-
cal.This work was supported by EU-
FP7 ALIGHT No. NMP-2011-280587 and the UK En-
gineering and Physical Sciences Research Council No.
EP/I012591/1.This is the final version. It was first published by http://onlinelibrary.wiley.com/doi/10.1002/pssb.201451591/abstrac
Entanglement generation in microcavity polariton devices

Entanglement generation in microcavity exciton-polaritons is an interesting
application of the peculiar properties of these half-light/half-matter
quasiparticles. In this paper we theoretically investigate their luminescence
dynamics and entanglement formation in single, double, and triple cavities. We
derive general expressions and selection rules for polariton-polariton
scattering. We evaluate a number of possible parametric scattering schemes in
terms of entanglement, and identify the ones that are experimentally most
promising.Comment: 25 pages, 8 figure