Perovskite SrTiO3 (STO) is an attractive photocatalyst for solar water splitting, but suffers from a limited photoresponse in the ultraviolet spectral range due to its wide band gap. By means of hybrid density functional theory calculations, we systematically study engineering its band gap via doping 4d and 5d transition metals M (M=Zr, Nb, Mo, Tc, Ru, Rh, Pd, Hf, Ta, W, Re, Os, Ir and Pt) and chalcogen elements Y (Y=S and Se). We find that transition metal dopant M either has no effect on STO band gap or introduces detrimental mid-gap states, except for Pd and Pt that are able to reduce the STO band gap. In contrast, doping S and Se significantly reduces STO's direct band gap, thus leading to appreciable optical absorption transitions in the visible spectral range. Our findings provide that Pd, S and Se doped STO are potential promising photocatalysts for water splitting under visible light irradiation, thereby providing insightful theoretical guides for experiments to improve the photocatalytic activity of STO.

Superconducting qubits are hampered by flux noise produced by surface spins from a variety of microscopic sources. Recent experiments indicated that hydrogen (H) atoms may be one of those sources. Using density functional theory calculations, we report that H atoms either embedded in, or adsorbed on, an α-Al2O3(0001) surface have sizable spin moments ranging from 0.81 to 0.87μB with energy barriers for spin reorientation as low as ∼10mK. Furthermore, H adatoms on the surface attract gas molecules such as O2, producing new spin sources. We propose coating the surface with graphene to eliminate H-induced surface spins and to protect the surface from other adsorbates.

Using density functional theory calculations, we investigated the magnetization and magnetic anisotropy of O2, H, and OH adsorbates on diamond surfaces. Our findings reveal that these adsorbates possess significant magnetic moments, with O2 having 2.0 μB and H and OH having 1.0 μB, respectively. Furthermore, they exhibit a non-negligible exchange coupling with the spin of the nitrogen-vacancy (NV) center positioned a few layers beneath the surface. Given the extremely small magnetic anisotropy energies of all of these systems (<1.0 μeV), the presence of magnetic adsorbates is expected to introduce noticeable noise in NV sensing, particularly when NV centers are positioned closer to the surface.

We report that anisotropic magnetoresistance (AMR) and anomalous Hall conductivity (AHC) in the Pd1-xPtx/Y3Fe5O12 (YIG) bilayers could be tuned by varying the Pt concentration (x) and also temperature (T). In particular, the AHC at low T changes its sign when x increases from 0 to 1, agreeing with the negative and positive AHC predicted by our ab initio calculations for the magnetic proximity (MP)-induced ferromagnetic Pd and Pt, respectively. The AMR ratio is enhanced by ten times when x increases from 0 to 1. Furthermore, the AMR of PdPt/YIG bilayers shows similar T-dependence as the magnetic susceptibility of the corresponding bulk Pd/Pt, also indicating the MP effect as the origin of the AMR. The present work demonstrates that the alloying of Pt and Pd not only offers tunable spin-orbit coupling but also is useful to reveal the nature of the AMR and AHC in Pt/YIG bilayers, which are useful for spintronics applications.

A major obstacle to using superconducting quantum interference devices (SQUIDs) as qubits is flux noise. We propose that the heretofore mysterious spins producing flux noise could be O_{2} molecules adsorbed on the surface. Using density functional theory calculations, we find that an O_{2} molecule adsorbed on an α-alumina surface has a magnetic moment of ~1.8 μ_{B}. The spin is oriented perpendicular to the axis of the O-O bond, the barrier to spin rotations is about 10 mK. Monte Carlo simulations of ferromagnetically coupled, anisotropic XY spins on a square lattice find 1/f magnetization noise, consistent with flux noise in Al SQUIDs.

This corrects the article DOI: 10.1103/PhysRevLett.115.077002.

The anomalous Hall effect (AHE), and magnetic and electronic transport properties were investigated in a series of amorphous transition metal thin films - FexSi1-x, FexGe1-x, CoxGe1-x, CoxSi1-x, and Fe1-yCoySi. The experimental results are compared with density functional theory calculations of the density of Berry curvature and intrinsic anomalous Hall conductivity. In all samples, the longitudinal conductivity (σxx), magnetization (M), and Hall resistivity (ρxy) increase with increasing transition metal concentration; due to the structural disorder σxx is lower in all samples than a typical crystalline metal. In the systems with Fe as the transition metal (including Fe1-yCoySi), the magnetization and AHE are large and in some cases greater than the crystalline analog. In all samples, the AHE is dominated by the intrinsic mechanism, arising from a nonzero, locally derived Berry curvature. The anomalous Hall angle (AHA) (=σxy/σxx) is as large as 5% at low temperature. These results are compared with the AHAs reported in a broad range of crystalline and amorphous materials. Previous work has shown that in a typical crystalline ferromagnet the Hall conductivity (σxy) and σxx are correlated and are usually either both large or both small, resulting in an AHA that decreases with increasing σxy. By contrast, the AHA increases linearly with increasing σxy in the amorphous systems. This trend is attributed to a generally low σxx, while σxy varies and can be large. In the amorphous systems, σxx and σxy are not coupled, and there may thus exist the potential to further increase the AHA by increasing σxy.

The amorphous iron-germanium system (a-FexGe1-x) lacks long-range structural order and hence lacks a meaningful Brillouin zone. The magnetization of a-FexGe1-x is well explained by the Stoner model for Fe concentrations x above the onset of magnetic order around x=0.4, indicating that the local order of the amorphous structure preserves the spin-split density of states of the Fe-3d states sufficiently to polarize the electronic structure despite k being a bad quantum number. Measurements reveal an enhanced anomalous Hall resistivity ρxyAH relative to crystalline FeGe; this ρxyAH is compared to density-functional theory calculations of the anomalous Hall conductivity to resolve its underlying mechanisms. The intrinsic mechanism, typically understood as the Berry curvature integrated over occupied k states but shown here to be equivalent to the density of curvature integrated over occupied energies in aperiodic materials, dominates the anomalous Hall conductivity of a-FexGe1-x (0.38≤x≤0.61). The density of curvature is the sum of spin-orbit correlations of local orbital states and can hence be calculated with no reference to k space. This result and the accompanying Stoner-like model for the intrinsic anomalous Hall conductivity establish a unified understanding of the underlying physics of the anomalous Hall effect in both crystalline and disordered systems.

Off-stoichiometry, epitaxial FexSi1-x thin films (0.5 < x < 1.0) exhibit D03 or B2 chemical order, even far from stoichiometry. Theoretical calculations show the magnetic moment is strongly enhanced in the fully chemically disordered A2 phase, while both theoretical and experimental results show that the magnetization is nearly the same in the B2 and D03 phases, meaning partial chemical disorder does not influence the magnetism. The dependencies of the magnetic moments are directly and nonlinearly linked to the number of Si atoms, primarily nearest neighbor but also to a lesser extent (up to 10%) next nearest neighbor, surrounding Fe, explaining the similarities between B2 and D03 and the strong enhancement for the A2 structure. The calculated electronic density of states shows many similarities in both structure and spin polarization between the D03 and B2 structures, while the A2 structure exhibits disorder broadening and a reduced spin polarization.

Magnetic flux noise is a dominant source of dephasing and energy relaxation in superconducting qubits. The noise power spectral density varies with frequency as 1/fα, with α1, and spans 13 orders of magnitude. Recent work indicates that the noise is from unpaired magnetic defects on the surfaces of the superconducting devices. Here, we demonstrate that adsorbed molecular O2 is the dominant contributor to magnetism in superconducting thin films. We show that this magnetism can be reduced by appropriate surface treatment or improvement in the sample vacuum environment. We observe a suppression of static spin susceptibility by more than an order of magnitude and a suppression of 1/f magnetic flux noise power spectral density of up to a factor of 5. These advances open the door to the realization of superconducting qubits with improved quantum coherence.