Spin caloritronics with superconductors: Enhanced thermoelectric effects, generalized Onsager response-matrix, and thermal spin currents

It has recently been proposed and experimentally demonstrated that it is possible to generate large thermoelectric effects in ferromagnet/superconductor structures due to a spin-dependent particle-hole asymmetry. Here, we theoretically show that quasiparticle tunneling between two spin-split superconductors enhances the thermoelectric response manyfold compared to when only one such superconductor is used, generating Seebeck coefficients ($\mathcal{S} > 1$ mV/K) and figures of merit ($ZT \simeq 40$) far exceeding the best bulk thermoelectric materials, and also becomes more resilient toward inelastic scattering processes. We present a generalized Onsager response-matrix which takes into account spin-dependent voltage and temperature gradients. Moreover, we show that thermally induced spin currents created in such junctions, even in the absence of a polarized tunneling barrier, also become largest in the case where a spin-dependent particle-hole asymmetry exists on both sides of the barrier. We determine how these thermal spin currents can be tuned both in magnitude and sign by several parameters, including the external field, temperature, and the superconducting phase-difference.Comment: 7 pages, 5 figures. v2: Added several new results, such as the response matrix for spin-dependent biases and the evaluation of thermal spin currents. Accepted for publication in Phys. Rev.

Conversion pathways of primary defects by annealing in proton-irradiated n-type 4H-SiC

The development of defect populations after proton irradiation of n-type 4H-SiC and subsequent annealing experiments is studied by means of deep level transient (DLTS) and photoluminescence (PL) spectroscopy. A comprehensive model is suggested describing the evolution and interconversion of irradiation-induced point defects during annealing below 1000{\deg}C. The model proposes the EH4 and EH5 traps frequently found by DLTS to originate from the (+/0) charge transition level belonging to different configurations of the carbon antisite-carbon vacancy (CAV) complex. Furthermore, we show that the transformation channel between the silicon vacancy (VSi) and CAV is effectively blocked under n-type conditions, but becomes available in samples where the Fermi level has moved towards the center of the band gap due to irradiation-induced donor compensation. The annealing of VSi and the carbon vacancy (VC) is shown to be dominated by recombination with residual self-interstitials at temperatures of up to 400{\deg}C. Going to higher temperatures, a decay of the CAV pair density is reported which is closely correlated to a renewed increase of VC concentration. A conceivable explanation for this process is the dissociation of the CAV pair into separate carbon anitisites and VC defects. Lastly, the presented data supports the claim that the removal of free carriers in irradiated SiC is due to introduced compensating defects and not passivation of shallow nitrogen donors

Cross-Sectional Carrier Lifetime Profiling and Deep Level Monitoring in Silicon Carbide Films Exhibiting Variable Carbon Vacancy Concentrations

The carrier lifetime control over 150 μm thick 4H-SiC epitaxial layers via thermal generation and annihilation of carbon vacancy (VC) related Z1/2 lifetime killer sites is reported. The defect developments upon typical SiC processing steps, such as high- and moderate-temperature anneals in the presence of a carbon cap, are monitored by combining electrical characterization techniques capable of VC depth-profiling, capacitance–voltage (CV) and deep-level transient spectroscopy (DLTS), with a novel all-optical approach of cross-sectional carrier lifetime profiling across 4H-SiC epilayer/substrate based on imaging time-resolved photoluminescence (TRPL) spectroscopy in orthogonal pump-probe geometry, which readily exposes in-depth efficacy of defect reduction and surface recombination effects. The lifetime control is realized by initial high-temperature treatment (1800 °C) to increase VC concentration to ≈1013 cm−3 level followed by a moderate-temperature (1500 °C) post-annealing of variable duration under C-rich thermodynamic equilibrium conditions. The post-annealing carried out for 5 h in effect eliminates VC throughout the entire ultra-thick epilayer. The reduction of VC-related Z1/2 sites is proven by a significant lifetime increase from 0.8 to 2.5 μs. The upper limit of lifetimes in terms of carrier surface leakage and the presence of other nonradiative recombination centers besides Z1/2, possibly related to residual impurities such as boron are discussed.publishedVersio

Azimuthal anisotropy of charged jet production in root s(NN)=2.76 TeV Pb-Pb collisions

We present measurements of the azimuthal dependence of charged jet production in central and semi-central root s(NN) = 2.76 TeV Pb-Pb collisions with respect to the second harmonic event plane, quantified as nu(ch)(2) (jet). Jet finding is performed employing the anti-k(T) algorithm with a resolution parameter R = 0.2 using charged tracks from the ALICE tracking system. The contribution of the azimuthal anisotropy of the underlying event is taken into account event-by-event. The remaining (statistical) region-to-region fluctuations are removed on an ensemble basis by unfolding the jet spectra for different event plane orientations independently. Significant non-zero nu(ch)(2) (jet) is observed in semi-central collisions (30-50% centrality) for 20 <p(T)(ch) (jet) <90 GeV/c. The azimuthal dependence of the charged jet production is similar to the dependence observed for jets comprising both charged and neutral fragments, and compatible with measurements of the nu(2) of single charged particles at high p(T). Good agreement between the data and predictions from JEWEL, an event generator simulating parton shower evolution in the presence of a dense QCD medium, is found in semi-central collisions. (C) 2015 CERN for the benefit of the ALICE Collaboration. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).Peer reviewe

Forward-central two-particle correlations in p-Pb collisions at root s(NN)=5.02 TeV

Two-particle angular correlations between trigger particles in the forward pseudorapidity range (2.5 2GeV/c. (C) 2015 CERN for the benefit of the ALICE Collaboration. Published by Elsevier B. V.Peer reviewe

Event-shape engineering for inclusive spectra and elliptic flow in Pb-Pb collisions at root(NN)-N-S=2.76 TeV

Peer reviewe

Elliptic flow of muons from heavy-flavour hadron decays at forward rapidity in Pb-Pb collisions at root s(NN)=2.76TeV

The elliptic flow, v(2), of muons from heavy-flavour hadron decays at forward rapidity (2.5 <y <4) is measured in Pb-Pb collisions at root s(NN)= 2.76TeVwith the ALICE detector at the LHC. The scalar product, two- and four-particle Q cumulants and Lee-Yang zeros methods are used. The dependence of the v(2) of muons from heavy-flavour hadron decays on the collision centrality, in the range 0-40%, and on transverse momentum, p(T), is studied in the interval 3 <p(T)<10 GeV/c. A positive v(2) is observed with the scalar product and two-particle Q cumulants in semi-central collisions (10-20% and 20-40% centrality classes) for the p(T) interval from 3 to about 5GeV/c with a significance larger than 3 sigma, based on the combination of statistical and systematic uncertainties. The v(2) magnitude tends to decrease towards more central collisions and with increasing pT. It becomes compatible with zero in the interval 6 <p(T)<10 GeV/c. The results are compared to models describing the interaction of heavy quarks and open heavy-flavour hadrons with the high-density medium formed in high-energy heavy-ion collisions. (C) 2015 CERN for the benefit of the ALICE Collaboration. Published by Elsevier B.V.Peer reviewe

Pseudorapidity and transverse-momentum distributions of charged particles in proton-proton collisions at root s=13 TeV

The pseudorapidity (eta) and transverse-momentum (p(T)) distributions of charged particles produced in proton-proton collisions are measured at the centre-of-mass energy root s = 13 TeV. The pseudorapidity distribution in vertical bar eta vertical bar <1.8 is reported for inelastic events and for events with at least one charged particle in vertical bar eta vertical bar <1. The pseudorapidity density of charged particles produced in the pseudorapidity region vertical bar eta vertical bar <0.5 is 5.31 +/- 0.18 and 6.46 +/- 0.19 for the two event classes, respectively. The transverse-momentum distribution of charged particles is measured in the range 0.15 <p(T) <20 GeV/c and vertical bar eta vertical bar <0.8 for events with at least one charged particle in vertical bar eta vertical bar <1. The evolution of the transverse momentum spectra of charged particles is also investigated as a function of event multiplicity. The results are compared with calculations from PYTHIA and EPOS Monte Carlo generators. (C) 2015 CERN for the benefit of the ALICE Collaboration. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).Peer reviewe

Centrality evolution of the charged-particle pseudorapidity density over a broad pseudorapidity range in Pb-Pb collisions at root s(NN)=2.76TeV

Peer reviewe

Point defects in silicon carbide for quantum technologies: Identification, tuning and control

Point defects strongly affect the electrical and optical properties of semiconductors, and are therefore of vast importance for device performance. Over recent years, however, point defects have been shown to possess properties that are highly suitable for applications related to quantum computing, sensing and communication. Single-photon emission and coherent spin manipulation at room temperature have been established for several systems, with the nitrogen-vacancy center in diamond counting among the first solid-state and semiconductor-based quantum platforms. Despite the long coherence times and established entanglement protocols of diamond-based qubit systems, diamond is only marginally compatible with advanced device fabrication methodology, and methods for integration of quantum emitters with electrically and optically controlled devices remain immature. For this reason, silicon carbide (SiC) has gained the attention of the quantum community, having a wide band gap, low spin-orbit coupling, mature device fabrication, and playing host to several promising quantum emitters of both extrinsic and intrinsic type. In this work, electrically and optically active point defects in silicon carbide have been studied using a combination of theoretical and experimental methods, with the aim of elucidating the role of different defects in power electronics and quantum technology devices. Hybrid density functional theory (DFT) calculations were employed to establish defect formation energies and chargestate transition levels, explore defect migration, and develop a new framework for studying the effect of electric fields on defect quantum emission. The calculations are correlated to experimental findings, where deep level transient spectroscopy (DLTS) and photo/cathodoluminescence (PL/CL) measurements reveal electrical and optical defect properties, respectively. The thesis places a particular emphasis on the silicon vacancy (VSi) in SiC, a room temperature single-photon source and qubit candidate exhibiting long spin coherence times. By monitoring VSi emission and comparing to DLTS spectra of proton-irradiated 4H-SiC samples, the VSi(-/2-) and VSi(2-/3-) charge-state transitions are assigned to the S-center, enabling electrical control over the VSi charge state. Depositing Schottky barrier diodes (SBDs) on the 4H-SiC sample surface enhances VSi emission by almost an order of magnitude, and sequential biasing of the SBD results in VSi charge-state switching, as detected by monitoring the V1 and V10 zero-phonon lines attributed to the negatively charged VSi at a hexagonal lattice site. The framework of bulk 4H-SiC epitaxial layers is compared to that of a microparticle matrix of predominantly the 6H polytype, with the former ensuring a homogeneous environment for the qubit defect and the latter enabling self-assembly, flexibility and ease of addressability. Importantly, both external and internal perturbations to the solid-state matrix wherein the VSi is embedded are shown to influence the emitted photon energies, as evidenced by an electric field-induced Stark effect and strain tuning in SiC microparticles. Furthermore, a set of emitters observed in the vicinity of the V1/V10 lines and having consistent subset spacings of 1.45 meV and 1.59 meV are tentatively attributed to vibronic replicas of the VSi emission. The VSi is unstable at elevated temperatures, and this thesis addresses the topics of VSi conversion and defect migration in p-type, intrinsic and n-type 4H-SiC material at 400 °C and above. Indeed, we find that hydrogen and VSi are likely to form complexes in the case that both species are present and in close proximity. In the absence of H, the VSi may convert to the carbon antisitevacancy (CAV) pair in p-type material, however, temperatures above 1000 °C are needed in n-type 4H-SiC, where both recombination with interstitials and divacancy formations prove to be more favorable annealing pathways for the VSi. The carbon vacancy (VC) is far more stable than VSi, and by comparing the two defect species using muon spin rotation (μSR) spectroscopy, we establish the μSR technique as a powerful tool for distinguishing different defect relaxation mechanisms and probing near-surface semiconductor defects in a non-destructive and depth-resolved manner. Annealing temperatures above 1200 °C are shown to be needed to induce VC migration, which is further demonstrated to be anisotropic in 4H-SiC, with the VC favoring in-plane atomic hops over the axial migration path. Finally, above temperatures of 2300 °C the lattice atoms themselves become mobile, and secondary ion mass spectrometry (SIMS) is employed to investigate the influence of a carbon cap covering the surface during annealing on self-diffusion of Si and C in 4H-SiC