Suchergebnisse

Investigating magnetic materials at realistic conditions with first-principles methods is a challenging task due to the interplay of vibrational and magnetic degrees of freedom. The most difficult contribution to include in simulations is represented by the longitudinal magnetic degrees of freedom [longitudinal spin fluctuation (LSF)] due to their inherent many-body nature; nonetheless, schemes that enable to take into account this effect on a semiclassical level have been proposed and employed in the investigation of magnetic systems. However, assessment of the effect of vibrations on LSF is lacking in the literature. For this reason, in this work we develop a supercell approach within the framework of constrained density functional theory to calculate self-consistently the size of local-environment-dependent magnetic moments in the paramagnetic, high-temperature state in the presence of lattice vibrations and for liquid Fe in different conditions. First, we consider the case of bcc Fe at the Curie temperature and ambient pressure. Then, we perform a similar analysis on bcc Fe at Earths inner-core conditions, and we find that LSFs stabilize nonzero moments which affect atomic forces and the electronic density of states of the system. Finally, we employ the present scheme on liquid Fe at the melting point at ambient pressure and at Earths outer-core conditions (p approximate to 200 GPa, T approximate to 6000 K). In both cases, we obtain local magnetic moments of sizes comparable to the solid-state counterparts. ; Funding Agencies|Swedish Research Council (VR) through International Career GrantSwedish Research Council [2014-6336, 2019-05403]; Marie Sklodowska Curie Actions [INCA 600398]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009 00971]; Knut and Alice Wallenberg Foundation (Wallenberg Scholar Grant) [KAW-2018.0194]; Swedish Foundation for Strategic Research through the Future Research Leaders 6 program [FFL 15-0290]

We study the origin of layer decoupling in ordered multilayer graphene grown by high temperature sublimation on C-face 4H-SiC. The mid-infrared optical Hall effect technique is used to determine the magnetic field dependence of the inter-Landau level transition energies and their optical polarization selection rules, which unambiguously show that the multilayer graphene consists of electronically decoupled layers. Transmission electron microscopy reveals no out-of-plane rotational disorder between layers in the stack, which is in contrast to what is typically observed for C-face graphene grown by low temperature sublimation. It is found that the multilayer graphene maintains AB-stacking order with increased interlayer spacing by 2.4%-8.4% as compared to highly oriented pyrolytic graphite. Electron energy loss spectroscopy mapping reveals Si atoms trapped in between layers, which are proposed to be the cause for the observed increased interlayer spacing leading to layer decoupling. Based on our results, we propose a defect-driven growth evolution mechanism for multilayer graphene on C-face SiC via high temperature sublimation. ; Funding Agencies|Swedish Research Council VRSwedish Research Council [2016-00889]; Swedish Foundation for Strategic ResearchSwedish Foundation for Strategic Research [FL12-0181, RIF 14-0074, RIF14-055]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [SFO Mat LiU 2009-00971]

We employ a first-principles cluster-expansion method in combination with canonical Monte Carlo simulations to study the effect of spin-orbit coupling on the alloying behavior of multilayer Bi1-xSbx. Our simulations reveal that spin-orbit coupling plays an essential role in determining the configurational thermodynamics of Bi and Sb atoms. Without the presence of spin-orbit coupling, Bi1-xSbx is predicted to exhibit at low-temperature chemical ordering of Bi and Sb atoms, leading to formation of an ordered structure at x approximate to 0.5. Interestingly, the spin-orbit-coupling effect intrinsically induced by the existence of Bi and Sb results in the disappearance of chemical ordering of the constituent elements within an immiscible region existing at T < 370 K, and consequently Bi1-xSbx displays merely a tendency toward local segregation of Bi and Sb atoms, resulting in coexistence of Bi-rich and Sb-rich Bi1-xSbx solid solutions without the formation of any ordered structure of Bi1-xSbx as predicted in the absence of spin-orbit coupling. These findings distinctly highlight an influence of spin-orbit coupling on the alloying behavior of Bi1-xSbx and probably other alloys composed of heavy elements, where the spin-orbit-coupling effect is supposed to be robust. ; Funding Agencies|Chulalongkorn UniversityChulalongkorn University [CUGR_62_66_23_26]; Thailand Toray Science Foundation (TTSF); Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009 00971]; Swedish Foundation for Strategic Research through the Future Research Leaders 6 program [FFL 15-0290]; Swedish Research Council (VR)Swedish Research Council [2019-05403]; Knut and Alice Wallenberg Foundation (Wallenberg Scholar Grant) [KAW-2018.0194]

The record high photoinduced current and power conversion efficiencies of organic solar cells (OSCs) should be attributed to the significant contribution of non-fullerene electron acceptors via hole transfer to electron donors and/or a pronounced decrease in energy losses for exciton dissociation by aligned highest occupied molecular orbitals (HOMOs) or lowest unoccupied molecular orbitals (LUMOs). However, the hole transfer mechanism in those highly efficient non-fullerene OSCs with small HOMO offsets has not been extensively studied and fully understood, yet. Herein, we comparatively study the hole transfer kinetics in two OSCs with a positive (0.05 eV) and a negative (-0.07 eV) HOMO offset (Delta HOMO) based on polymer donor PTQ10 paired with non-fullerene acceptors ZITI-C or ZITI-N. Short-circuit current densities (J(sc)) of 20.42 and 12.81 mA cm(-2) are achieved in the OSCs based on PTQ10:ZITI-C (Delta HOMO = 0.05 eV) and PTQ10:ZITI-N (Delta HOMO = -0.07 eV) with an optimized donor (D):acceptor (A) ratio of 1:1, respectively, despite the small and even negative Delta HOMO. Results from time-resolved transient absorption spectroscopy show slower hole transfer (14.3 ps) in PTQ10:ZITI-N than that (3.7 ps) in PTQ10:ZITI-C. To understand the decent J(sc) value in the OSCs of PTQ10:ZITI-N, the temperature and electric field dependences of hole transfer are investigated in low-donor-content OSCs (D:A ratio of 1:9) in which photocurrent is dominated by the contribution via hole transfer from ZITI-N to PTQ10. Devices based on PTQ10:ZITI-C and PTQ10:ZITI-N show similar free charge generation behavior as a function of temperature, whereas the external quantum efficiencies of the PTQ10:ZITI-N device exhibit a much stronger bias dependence than that of PTQ10:ZITI-C, which suggests that the electric field facilitates exciton dissociation in PTQ10:ZITI-N where the energetic driving force alone cannot efficiently dissociate excitons. ; Funding Agencies|Swedish Government Strategic Research Area in Material Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU ) [200900971]; Swedish Research CouncilSwedish Research Council [2017-04123]; Knut and Alice Wallenberg FoundationKnut & Alice Wallenberg Foundation [2016.0059]; China Scholarship Council (CSC)China Scholarship Council; National Key R&D Program of China [2019YFA0705900, 2017YFA0204701]; National Natural Science Foundation of ChinaNational Natural Science Foundation of China [21572234, 21661132006, 91833304]

Ytterligare forskningsfinansiärer: Knut and Alice WallenbergFoundation for a Fellowship Grant and Project funding (KAW 2015.0043), Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University (Faculty Grant SFO MatLiUNo. 2009 00971), Swedish Research Council VR-RFI (#2017-00646_9), Swedish Foundation for Strategic Research(contract RIF14-0053)

We report on optimization of growth conditions of GaAs/GaNAs/GaAs core/shell/shell nanowire (NW) structures emitting at ~1 μm, aiming to increase their light emitting efficiency. A slight change in growth temperature is found to critically affect optical quality of the active GaNAs shell and is shown to result from suppressed formation of non-radiative recombination (NRR) centers under the optimum growth temperature. By employing the optically detected magnetic resonance spectroscopy, we identify gallium vacancies and gallium interstitials as being among the dominant NRR defects. The radiative efficiency of the NWs can be further improved by post-growth annealing at 680 °C, which removes the gallium interstitials. ; Funding agencies: Swedish Energy AgencySwedish Energy Agency [P40119-1]; Swedish Research CouncilSwedish Research Council [2015-05532]; Swedish Foundation for International Cooperation in Research and Higher Education (STINT) [JA2014-5698]; Swedish Government Strategic Res

Silicon-vacancy qubits in silicon carbide (SiC) are emerging tools in quantum-technology applications due to their excellent optical and spin properties. In this paper, we explore the effect of temperature and strain on these properties by focusing on the two silicon-vacancy qubits, V1 and V2, in 4H-SiC. We apply density-functional theory beyond the Born-Oppenheimer approximation to describe the temperature-dependent mixing of electronic excited states assisted by phonons. We obtain a polaronic gap of around 5 and 22 meV for the V1 and V2 centers, respectively, which results in a significant difference in the temperature-dependent dephasing and zero-field splitting of the excited states, which explains recent experimental findings. We also compute how crystal deformations affect the zero-phonon line of these emitters. Our predictions are important ingredients in any quantum applications of these qubits sensitive to these effects. ; Funding Agencies|National Excellence Program of Quantum-Coherent Materials Project (Hungarian NKFIH Grant) [KKP129866]; European Union QuantERA Q-Magine Project [127889]; QuantERA Nanospin Project [127902]; European Union Horizon 2020 Quantum Technology Flagship project ASTERIQS [820394]; National Quantum Technology Program [2017-1.2.1NKP-2017-00001]; Swedish Research CouncilSwedish Research Council [VR 2016-04068]; Swedish Energy AgencySwedish Energy Agency [43611-1]; Knut and Alice Wallenberg FoundationKnut & Alice Wallenberg Foundation [KAW 2018.0071]; European Union Horizon 2020 project QuanTELCO [862721]; Japan Society for the Promotion of ScienceMinistry of Education, Culture, Sports, Science and Technology, Japan (MEXT)Japan Society for the Promotion of Science [JSPS KAKENHI 17H01056, JSPS KAKENHI 18H03770]; U.S. Department of Energy, Office of ScienceUnited States Department of Energy (DOE) [DE-SC0019174]; Swedish National Infrastructure for Computing at the National Supercomputer Centre [SNIC 2019/3667, LiU-2015-00017-60]

Quantum dots (QDs) formed in semiconductor nanowires (NWs) form a basis for studying interesting quantum phenomena and provide an exciting platform for various device applications, where in-depth understanding of the formation and electronic properties of such QDs is a requirement. In this work we present detailed investigations of structural, electronic, and optical properties of nitrogen-induced self-assembled QDs in novel Ga(N,As,P) NWs. We show that the three-dimensional quantum confinement of the excitons caused by short-range fluctuations in the N content leads to single-photon emission. We demonstrate that valence band character varies between the QD emitters and attribute it to a varying degree of hole localization within the QDs induced by fluctuations in the As/P ratio. These fluctuations in the chemical compositions are also believed to be at least partly responsible for the observed large variation in exciton lifetimes of the QDs, ranging between 0.5 and 10 ns. The rather long lifetimes demonstrate the resilience of the QDs to nonradiative recombination-a merit for efficient single-photon emitters. Furthermore, we show that the principal axes of the formed QDs are oriented radially, normal to the NW side facets, in contrast to the N-induced QDs in GaNAs NWs which are primarily aligned along the NW axis. Our results, therefore, underline the impacts of the host alloy on the QD formation, which could provide an additional degree of freedom for band structure engineering in nanowires. ; Funding Agencies|Linkoping University; Swedish Research CouncilSwedish Research Council [2016-05091]; Swedish Energy AgencySwedish Energy Agency [P40119-1]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009 00971]; Swiss National Foundation through the NCCR QSIT

We perform a comprehensive theoretical study of the pressure-induced evolution of the electronic structure, magnetic state, and phase stability of the late transition metal monoxides MnO, FeO, CoO, and NiO using a fully charge self-consistent DFT-Pdynamical mean-field theory method. Our results reveal that the pressure-induced Mott insulator-to-metal phase transition in MnO-NiO is accompanied by a simultaneous collapse of local magnetic moments and lattice volume, implying a complex interplay between chemical bonding and electronic correlations. We compute the pressure-induced evolution of relative weights of the different valence states and spin-state configurations. Employing the concept of fluctuating valence in a correlated solid, we demonstrate that in MnO, FeO, and CoO a Mott insulator-metal transition and collapse of the local moments is accompanied by a sharp crossover of the spin-state and valence configurations. Our microscopic explanation of the magnetic collapse differs from the accepted picture and points out a remarkable dynamical coexistence (frustration) of the high-, intermediate-, and low-spin states. In particular, in MnO, the magnetic collapse is found to be driven by the appearance of the intermediate-spin state (IS), competing with the low-spin (LS) state; in FeO, we observe a conventional high-spin to low-spin (HS-LS) crossover. Most interestingly, in CoO, we obtain a remarkable (dynamical) coexistence of the HS and LS states, i.e., a HS-LS frustration, up to high pressure. Our results demonstrate the importance of quantum fluctuations of the valence and spin states for the understanding of quantum criticality of the Mott transitions. ; Funding Agencies|Russian Science FoundationRussian Science Foundation (RSF) [18-12-00492]; Minobrnauki of Russia [AAAA-A18-118020190098-5]; Knut and Alice Wallenberg FoundationKnut & Alice Wallenberg Foundation [KAW-2018.0194]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFOMatLiU) [2009 00971]; Swedish e-Science Research Centre (SeRC)

Core/shell nanowire (NW) heterostructures based on III-V semiconductors and related alloys are attractive for optoelectronic and photonic applications owing to the ability to modify their electronic structure via bandgap and strain engineering. Post-growth thermal annealing of such NWs is often involved during device fabrication and can also be used to improve their optical and transport properties. However, effects of such annealing on alloy disorder and strain in core/shell NWs are not fully understood. In this work we investigate these effects in novel core/shell/shell GaAs/GaNAs/GaAs NWs grown by molecular beam epitaxy on (111) Si substrates. By employing polarization-resolved photoluminescence measurements, we show that annealing (i) improves overall alloy uniformity due to suppressed long-range fluctuations in the N composition; (ii) reduces local strain within N clusters acting as quantum dot emitters; and (iii) leads to partial relaxation of the global strain caused by the lattice mismatch between GaNAs and GaAs. Our results, therefore, underline applicability of such treatment for improving optical quality of NWs from highly-mismatched alloys. They also call for caution when using ex-situ annealing in strain-engineered NW heterostructures. ; Funding Agencies|Swedish Energy AgencySwedish Energy Agency [P40119-1]; Swedish Research CouncilSwedish Research Council [2015-05532]; Swedish Foundation for International Cooperation in Research and Higher Education (STINT) [JA2014-5698]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009 00971]; KAKENHI from the Japan Society for the Promotion of ScienceMinistry of Education, Culture, Sports, Science and Technology, Japan (MEXT)Japan Society for the Promotion of ScienceGrants-in-Aid for Scientific Research (KAKENHI) [16H05970, 23686004]; Kato Foundation for Promotion of Science; Kurata Memorial Hitachi Science and Technology Foundation; Murata Science Foundation; Linkoping University

Indium nitride (InN) is characterized by its high electron mobility, making it a ground-breaking material for high frequency electronics. The difficulty of depositing high-quality crystalline InN currently impedes its broad implementation in electronic devices. Herein, we report a new highly volatile In(III) triazenide precursor and demonstrate its ability to deposit high-quality epitaxial hexagonal InN by atomic layer deposition (ALD). The new In(III) precursor, the first example of a homoleptic triazenide used in a vapor deposition process, was easily synthesized and purified by sublimation. Thermogravimetric analysis showed single step volatilization with an onset temperature of 145 degrees C and negligible residual mass. Strikingly, two temperature intervals with self-limiting growth were observed when depositing InN films. In the high-temperature interval, the precursor underwent a gas-phase thermal decomposition inside the ALD reaction chamber to produce a more reactive In(III) compound while retaining self-limiting growth behavior. Density functional theory calculations revealed a unique two-step decomposition process, which liberates three molecules of each propene and N-2 to give a smaller tricoordinated In(III) species. Stoichiometric InN films with very low levels of impurities were grown epitaxially on 4H-SiC. The InN films deposited at 325 degrees C had a sheet resistivity of 920 Omega/sq. This new triazenide precursor enables ALD of InN for semiconductor applications and provides a new family of M-N bonded precursors for future deposition processes. ; Funding Agencies|Swedish foundation for Strategic Research through the project "Time-resolved low temperature CVD for III-nitrides" [SSF-RMA 15-0018]; Knut and Alice Wallenberg foundation through the project "Bridging the THz gap" [KAW 2013.0049]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO Mat LiU) [2009 00971]

Although perovskite light-emitting diodes (PeLEDs) have recently experienced significant progress, there are only scattered reports of PeLEDs with both high efficiency and long operational stability, calling for additional strategies to address this challenge. Here, we develop perovskite-molecule composite thin films for efficient and stable PeLEDs. The perovskite-molecule composite thin films consist of in-situ formed high-quality perovskite nanocrystals embedded in the electron-transport molecular matrix, which controls nucleation process of perovskites, leading to PeLEDs with a peak external quantum efficiency of 17.3% and half-lifetime of approximately 100 h. In addition, we find that the device degradation mechanism at high driving voltages is different from that at low driving voltages. This work provides an effective strategy and deep understanding for achieving efficient and stable PeLEDs from both material and device perspectives. ; Funding Agencies|ERC Starting GrantEuropean Research Council (ERC) [717026]; European Commission Marie Skodowska-Curie Actions [691210]; Swedish Foundation for International Cooperation in Research and Higher Education [CH2018-7736]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009-00971]; China Scholarship CouncilChina Scholarship Council; Jardine Foundation; Cambridge Trust; Linkoping University

Infrared-active lattice mode properties of melt-grown high-quality single bulk crystals of ZnGa2O4 are investigated by combined spectroscopic ellipsometry and density functional theory computation analysis. The normal spinel structure crystals are measured by spectroscopic ellipsometry at room temperature in the range of 100cm(-1)-1200cm(-1). The complex-valued dielectric function is determined from a wavenumber-by-wavenumber approach, which is then analyzed by the four-parameter semi-quantum model dielectric function approach augmented by impurity mode contributions. We determine four infrared-active transverse and longitudinal optical mode pairs, five localized impurity mode pairs, and the high frequency dielectric constant. All four infrared-active transverse and longitudinal optical mode pairs are in excellent agreement with results from our density functional theory computations. With the Lyddane-Sachs-Teller relationship, we determine the static dielectric constant, which agrees well with electrical capacitance measurements performed on similarly grown samples. We also provide calculated parameters for all Raman-active and for all silent modes and, thereby, provide a complete set of all symmetry predicted Brillouin zone center modes. ; Funding Agencies|National Science FoundationNational Science Foundation (NSF) [DMR 1808715]; Air Force Office of Scientific ResearchUnited States Department of DefenseAir Force Office of Scientific Research (AFOSR) [FA9550-18-1-0360]; Nebraska Materials Research Science and Engineering Center [DMR 1420645]; Swedish Research Council VRSwedish Research Council [2016-00889]; Swedish Foundation for Strategic ResearchSwedish Foundation for Strategic Research [RIF14-055, EM16-0024]; Swedish Governmental Agency for Innovation Systems VINNOVA under Competence Center Program [2016-05190]; Knut and Alice Wallenbergs FoundationKnut & Alice Wallenberg Foundation; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University, Faculty Grant SFO Mat LiU [2009-00971]; University of Nebraska Foundation; J. A. Woollam Foundation

Organic electrochemical transistors (OECTs) have shown great promise in a variety of applications ranging from digital logic circuits to biosensors and artificial synapses for neuromorphic computing. The working mechanism of OECTs relies on the mixed transport of ionic and electronic charge carriers, extending throughout the bulk of the organic channel. This attribute renders OECTs fundamentally different from conventional field effect transistors and endows them with unique features, including large gate-to-channel capacitance, low operating voltage, and high transconductance. Owing to the complexity of the mixed ion-electron coupling and transport processes, the OECT device physics is sophisticated and yet to be fully unraveled. Here, we give an account of the one- and two-dimensional drift-diffusion models that have been developed to describe the mixed transport of ions and electrons by finite-element methods and identify key device parameters to be tuned for the next developments in the field. ; Funding Agencies|Swedish Foundation for Strategic ResearchSwedish Foundation for Strategic Research [SE13-0045]; Swedish Research CouncilSwedish Research Council [2016-03979]; AForsk [18-313, 19-310]; Olle Engkvists Stiftelse [204-0256]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009-00971]

The electronic structure, chemical bonding, and interface component in ZrN-AlN nanocomposites formed byphase separation during thin film deposition of metastable Zr1−xAlxN (x = 0.0, 0.12, 0.26, 0.40) are investigatedby resonant inelastic x-ray scattering, x-ray emission, and x-ray absorption spectroscopy and compared to firstprinciples calculations including transitions between orbital angular momentum final states. The experimentalspectra are compared with different interface-slab model systems using first principles all-electron full-potentialcalculations where the core states are treated fully relativistically. As shown in this work, the bulk sensitivity andelement selectivity of x-ray spectroscopy enables one to probe the symmetry and orbital directions at interfacesbetween cubic and hexagonal crystals. We show how the electronic structure develops from local octahedralbond symmetry of cubic ZrN that distorts for increasing Al content into more complex bonding. This results inthree different kinds of bonding originating from semicoherent interfaces with segregated ZrN and lamellar AlNnanocrystalline precipitates. An increasing chemical shift and charge transfer between the elements takes placewith increasing Al content and affects the bond strength and increases resistivity. ; Fulltext published under the terms of the Creative Commons Attribution 4.0 International license. https://creativecommons.org/licenses/by/4.0/ No changes have been made to the fulltext. Funded by Bibsam. Funding agencies: Swedish Research Council (VR) LiLi-NFM Linnaeus EnvironmentSwedish Research Council [621-2009-5258]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009 00971, 2