244 research outputs found
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Structural and electronic properties of SrZrO3 and Sr(Ti,Zr) O3 alloys
Using hybrid density functional calculations, we study the electronic and structural properties of SrZrO3 and ordered Sr(Ti,Zr)O3 alloys. Calculations were performed for the ground-state orthorhombic (Pnma) and high-temperature cubic (Pm3m) phases of SrZrO3. The variation of the lattice parameters and band gaps with Ti addition was studied using ordered SrTixZr1-xO3 structures with x=0, 0.25, 0.5, 0.75, and 1. As Ti is added to SrZrO3, the lattice parameter is reduced and closely follows Vegard's law. On the other hand, the band gap shows a large bowing and is highly sensitive to the Ti distribution. For x=0.5, we find that arranging the Ti and Zr atoms into a 1×1SrZrO3/SrTiO3 superlattice along the [001] direction leads to interesting properties, including a highly dispersive single band at the conduction-band minimum (CBM), which is absent in both parent compounds, and a band gap close to that of pure SrTiO3. These features are explained by the splitting of the lowest three conduction-band states due to the reduced symmetry of the superlattice, lowering the band originating from the in-plane Ti 3dxy orbitals. The lifting of the t2g orbital degeneracy around the CBM suppresses scattering due to electron-phonon interactions. Our results demonstrate how short-period SrZrO3/SrTiO3 superlattices could be exploited to engineer the band structure and improve carrier mobility compared to bulk SrTiO3
First-principles calculations of hyperfine interaction, binding energy, and quadrupole coupling for shallow donors in silicon
Spin qubits based on shallow donors in silicon are a promising quantum information technology with enormous potential
scalability due to the existence of robust silicon-processing infrastructure. However, the most accurate theories of donor electronic
structure lack predictive power because of their reliance on empirical fitting parameters, while predictive ab initio methods have so
far been lacking in accuracy due to size of the donor wavefunction compared to typical simulation cells. We show that density
functional theory with hybrid and traditional functionals working in tandem can bridge this gap. Our first-principles approach
allows remarkable accuracy in binding energies (67 meV for bismuth and 54 meV for arsenic) without the use of empirical fitting.
We also obtain reasonable hyperfine parameters (1263 MHz for Bi and 133 MHz for As) and superhyperfine parameters. We
demonstrate the importance of a predictive model by showing that hydrostatic strain has much larger effect on the hyperfine
structure than predicted by effective mass theory, and by elucidating the underlying mechanisms through symmetry analysis of the
shallow donor charge density
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Unusual Formation of Point-Defect Complexes in the Ultrawide-Band-Gap Semiconductor β-Ga2 O3
Understanding the unique properties of ultra-wide band gap semiconductors requires detailed information about the exact nature of point defects and their role in determining the properties. Here, we report the first direct microscopic observation of an unusual formation of point defect complexes within the atomic-scale structure of β-Ga2O3 using high resolution scanning transmission electron microscopy (STEM). Each complex involves one cation interstitial atom paired with two cation vacancies. These divacancy-interstitial complexes correlate directly with structures obtained by density functional theory, which predicts them to be compensating acceptors in β-Ga2O3. This prediction is confirmed by a comparison between STEM data and deep level optical spectroscopy results, which reveals that these complexes correspond to a deep trap within the band gap, and that the development of the complexes is facilitated by Sn doping through increased vacancy concentration. These findings provide new insight on this emerging material's unique response to the incorporation of impurities that can critically influence their properties
Tailoring ferromagnetic chalcopyrites
If magnetic semiconductors are ever to find wide application in real
spintronic devices, their magnetic and electronic properties will require
tailoring in much the same way that band gaps are engineered in conventional
semiconductors. Unfortunately, no systematic understanding yet exists of how,
or even whether, properties such as Curie temperatures and band gaps are
related in magnetic semiconductors. Here we explore theoretically these and
other relationships within 64 members of a single materials class, the Mn-doped
II-IV-V2 chalcopyrites, three of which are already known experimentally to be
ferromagnetic semiconductors. Our first-principles results reveal a variation
of magnetic properties across different materials that cannot be explained by
either of the two dominant models of ferromagnetism in semiconductors. Based on
our results for structural, electronic, and magnetic properties, we identify a
small number of new stable chalcopyrites with excellent prospects for
ferromagnetism.Comment: 6 pages with 4 figures, plus 3 supplementary figures; to appear in
Nature Material
Field-induced water electrolysis switches an oxide semiconductor from an insulator to a metal
Here we demonstrate that water-infiltrated nanoporous glass electrically
switches an oxide semiconductor from an insulator to metal. We fabricated the
field effect transistor structure on an oxide semiconductor, SrTiO3, using
100%-water-infiltrated nanoporous glass - amorphous 12CaO*7Al2O3 - as the gate
insulator. For positive gate voltage, electron accumulation, water electrolysis
and electrochemical reduction occur successively on the SrTiO3 surface at room
temperature, leading to the formation of a thin (~3 nm) metal layer with an
extremely high electron concentration of 10^15-10^16 cm^-2, which exhibits
exotic thermoelectric behaviour.Comment: 21 pages, 12 figure
Measurement of the spin temperature of optically cooled nuclei and GaAs hyperfine constants in GaAs/AlGaAs quantum dots
Deep cooling of electron and nuclear spins is equivalent to achieving polarization degrees close to 100% and is a key requirement in solid state quantum information technologies. While polarization of individual nuclear spins in diamond and SiC reaches 99% and beyond, it has been limited to 60-65% for the nuclei in quantum dots. Theoretical models have attributed this limit to formation of coherent "dark" nuclear spin states but experimental verification is lacking, especially due to the poor accuracy of polarization degree measurements. Here we measure the nuclear polarization in GaAs/AlGaAs quantum dots with high accuracy using a new approach enabled by manipulation of the nuclear spin states with radiofrequency pulses. Polarizations up to 80% are observed - the highest reported so far for optical cooling in quantum dots. This value is still not limited by nuclear coherence effects. Instead we find that optically cooled nuclei are well described within a classical spin temperature framework. Our findings unlock a route for further progress towards quantum dot electron spin qubits where deep cooling of the mesoscopic nuclear spin ensemble is used to achieve long qubit coherence. Moreover, GaAs hyperfine material constants are measured here experimentally for the first time
Hydrogen-doped Brookite TiO2 Nanobullets Array as a Novel Photoanode for Efficient Solar Water Splitting
As a representative photocatalyst for photoelectrochemical solar water splitting, TiO2 has been intensively studied but most researches have focused on the rutile and anatsase phases because brookite, another important crystalline polymorph of TiO2, rarely exists in nature and is difficult to synthesize. In this work, hydrogen doped brookite (H:brookite) nanobullet arrays were synthesized via a well-designed solution reaction for the first time. H:brookite shows highly improved PEC properties with excellent stability, enhanced photocurrent, and significantly high Faradaic efficiency for overall solar water splitting. To support the experimental data, ab initio density functional theory calculations were also conducted. At the interstitial doping site that has minimum formation energy, the hydrogen atoms act as shallow donors and exist as H+. which has the minimum formation energy among three states of hydrogen (H+. H0, and H-). The calculated density of states of H:brookite shows a narrowed bandgap and an increased electron density compared to the pristine brookite. The combined experimental and theoretical results provide frameworks for the exploration of the PEC properties of doped brookite and extend our knowledge regarding the undiscovered properties of brookite of TiO2.ope
Isotope sensitive measurement of the hole-nuclear spin interaction in quantum dots
Decoherence caused by nuclear field fluctuations is a fundamental obstacle to
the realization of quantum information processing using single electron spins.
Alternative proposals have been made to use spin qubits based on valence band
holes having weaker hyperfine coupling. However, it was demonstrated recently
both theoretically and experimentally that the hole hyperfine interaction is
not negligible, although a consistent picture of the mechanism controlling the
magnitude of the hole-nuclear coupling is still lacking. Here we address this
problem by performing isotope selective measurement of the valence band
hyperfine coupling in InGaAs/GaAs, InP/GaInP and GaAs/AlGaAs quantum dots.
Contrary to existing models we find that the hole hyperfine constant along the
growth direction of the structure (normalized by the electron hyperfine
constant) has opposite signs for different isotopes and ranges from -15% to
+15%. We attribute such changes in hole hyperfine constants to the competing
positive contributions of p-symmetry atomic orbitals and the negative
contributions of d-orbitals. Furthermore, we find that the d-symmetry
contribution leads to a new mechanism for hole-nuclear spin flips which may
play an important role in hole spin decoherence. In addition the measured
hyperfine constants enable a fundamentally new approach for verification of the
computed Bloch wavefunctions in the vicinity of nuclei in semiconductor
nanostructures
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