For certain compositions Ni-Mn-Sn and related magnetic shape-memory alloys undergo a martensitic transition at temperatures in the range 300–400 K, with the emergence of novel magnetic properties below the transition. While $\mathrm{N}{\mathrm{i}}_{50}\mathrm{M}{\mathrm{n}}_{50}$ is an antiferromagnet, substitution of Sn on some fraction of the Mn sites in $\mathrm{N}{\mathrm{i}}_{50}\mathrm{M}{\mathrm{n}}_{50-x}\mathrm{S}{\mathrm{n}}_x$ leads to competing ferromagnetic (F) and antiferromagnetic (AF) phases at low temperatures. Details of this magnetic phase coexistence are, however, significantly lacking, particularly with respect to the AF phase. The present investigations use zero applied magnetic field ${}^{55}\mathrm{Mn}$ NMR as a local probe of the magnetic properties of the alloy $\mathrm{N}{\mathrm{i}}_{50}\mathrm{M}{\mathrm{n}}_{50-x}\mathrm{S}{\mathrm{n}}_x$ with $x=10$. Rich multipeak spectra are observed, and the various components are definitively assigned to nanoscale F or AF regions. Measurements of the static nuclear hyperfine field distributions as a function of temperature, and in small applied fields, together with nuclear relaxation rates provide detailed information on the size distributions, relative concentrations, and physical natures of these F and AF regions. The results show that the nanoscale magnetic features of the $x=10$ system are substantially more complex than previous studies have suggested. We argue that the general approach used in these experiments is applicable to other such complex metal alloys, and could yield many additional insights.

We study point-defect diffusion in crystalline silicon using the kinetic activation-relaxation technique (k-ART), an off-lattice kinetic Monte Carlo method with on-the-fly catalog building capabilities based on the activation-relaxation technique (ART nouveau), coupled to the standard Stillinger-Weber potential. We focus more particularly on the evolution of crystalline cells with one to four vacancies and one to four interstitials in order to provide a detailed picture of both the atomistic diffusion mechanisms and overall kinetics. We show formation energies, activation barriers for the ground state of all eight systems, and migration barriers for those systems that diffuse. Additionally, we characterize diffusion paths and special configurations such as dumbbell complex, di-interstitial (IV-pair+2I) superdiffuser, tetrahedral vacancy complex, and more. This study points to an unsuspected dynamical richness even for this apparently simple system that can only be uncovered by exhaustive and systematic approaches such as the kinetic activation-relaxation technique.

How does Fermi surface develop upon doping of cuprates, does it consist of large barrels or small pockets, which of them is responsible for superconductivity, and what is a role of the pseudogap? Those are actively debated questions, important for understanding of high temperature superconductivity. We study doping dependence of intrinsic tunneling in ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{Ca}}_{1-x}{\mathrm{Y}}_x{\mathrm{Cu}}_2{\mathrm{O}}_{8+\delta }$ cuprates, which allows independent analysis of Cooper pair and quasiparticle transport. We observe that with decreasing doping the supercurrent is rapidly decreasing at a rate much faster than the doping level, but the quasiparticle resistance at a sufficiently high bias remains almost doping independent. This remarkable discrepancy indicates that Cooper pairs and quasiparticles are originating from different parts of the Brillouin zone: Cooper pairs are residing on small pockets, which are progressively shrinking with decreasing doping, but the majority of the quasiparticle current is integrated over large barrels, which are only weakly doping dependent. The expanding pseudogap areas along the barrels do not contribute to pair current. This provides direct evidence for nonsuperconducting origin of the pseudogap. We present numerical calculations, taking into account Fermi surface topology, that support our conclusions.

$1T\text{−}{\mathrm{TiSe}}_2$ is a quasi-two-dimensional transition metal dichalcogenide, which exhibits a charge density wave transition at a critical temperature of $\sim 200$ K as well as low-temperature superconductivity induced by pressure or intercalation. The electronic energy dispersion measured by soft x-ray angle-resolved photoemission is not only momentum resolved parallel to the surface but also perpendicular to it. Experiments are compared to density functional theory based band structure calculations using different exchange-correlation functionals. The results reveal the importance of including spin-orbit coupling for a good description of the experimental bands. Compared to calculations within the local density approximation, the use of the modified Becke-Johnson (mBJ) exchange functional leads to a band structure that does not need an artificial downwards shift of the valence band to fit the experiment. The mBJ functional thus clearly appears as the most adapted functional for the theoretical description of the $1T\text{−}{\mathrm{TiSe}}_2$ band structure within the DFT framework.

Layered transition-metal trichalcogenides with the chemical formula $AB{X}_3$ have attracted recent interest as potential candidates for two-dimensional magnets. Using first-principles calculations within density functional theory, we investigate the magnetic ground states of monolayers of Mn- and Cr-based semiconducting trichalcogenides. We show that the second and third nearest-neighbor exchange interactions ($J_2$ and $J_3$) between magnetic ions, which have been largely overlooked in previous theoretical studies, are crucial in determining the magnetic ground state. Specifically, we find that monolayer ${\mathrm{CrSiTe}}_3$ is an antiferromagnet with a zigzag spin texture due to significant contribution from $J_3$, whereas ${\mathrm{CrGeTe}}_3$ is a ferromagnet with a Curie temperature of 106 K. Monolayers of Mn compounds (${\mathrm{MnPS}}_3$ and ${\mathrm{MnPSe}}_3$) always show antiferromagnetic Néel order. We identify the physical origin of various exchange interactions, and demonstrate that strain can be an effective knob for tuning the magnetic properties. Possible magnetic ordering in the bulk is also discussed. Our study suggests that $AB{X}_3$ can be a promising platform to explore two-dimensional magnetic phenomena.

In high magnetic fields, the exciton absorption spectrum of a semiconducting single-walled carbon nanotube splits as a result of Aharonov-Bohm magnetic flux. A magnetic field of 370 T, generated by the electromagnetic flux compression destructive pulsed magnet-coil technique, was applied to single-chirality semiconducting carbon nanotubes. Using streak spectroscopy, we demonstrated the separation of the independent band-edge exciton states at the $K$ and $K^{\prime }$ points of the Brillouin zone after the mixing of the dark and bright states above 150 T. These results enable a quantitative discussion of the whole picture of the Aharonov-Bohm effect in single-walled carbon nanotubes.

High-mobility and dilute two-dimensional electron systems exhibit metallic behavior down to the lowest experimental temperatures. In studies of ionic liquid gated insulating silicon, we have observed transitions to a metallic state in low-mobility samples at much higher areal carrier densities than found for samples of high mobility. We have also observed a mobility peak in metallic samples as the carrier density was increased beyond ${10}^{13}{\text{cm}}^{-2}$.

The geometrical Berry phase is widely recognized as having profound implications for the properties of electronic systems. Over the last decade or so, the Berry phase has been essential to our understanding of new materials such as graphene and topological insulators. In general, a nontrivial Berry phase is a result of band crossing as in the case of a massless Dirac point. The Berry phase can be accessed in quantum oscillation measurements as it contributes to the phase mismatch of electrons in their cyclotron orbits. With their enigmatic pseudogap and superconducting phases, the cuprates are materials where the Berry phase is thus far unknown. Based on quantum oscillation data in the high-field normal state of underdoped cuprates, we determined the Berry phase contribution to the phase mismatch unambiguously in this family of materials. In the hole-doped materials ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_y, {\mathrm{YBa}}_2{\mathrm{Cu}}_4{\mathrm{O}}_8$, and ${\mathrm{HgBa}}_2{\mathrm{CuO}}_{4+\delta }$, a trivial Berry phase of 0 $\mathrm{mod}(2\pi$) is systematically observed, while the electron-doped ${\mathrm{Nd}}_{2-x}{\mathrm{Ce}}_x{\mathrm{CuO}}_4$ exhibits a significant nonzero Berry phase of $1.4\pi$. Our results set significant constraints on the microscopic description of the high-field normal state and, in particular, do not support a nodal structure or broken time-reversal symmetry in the hole-doped compounds.

The charge-transfer compound ${(\mathrm{BEDT}-\mathrm{TTF})}_2\mathrm{Ag}{\left({\mathrm{CF}}_3\right)}_4\left(\mathrm{TCE}\right)$ crystallizes in three polymorphs with different alternating layers: While a phase with a $\kappa$ packing motif has a low superconducting transition temperature of $T_c=2.6\mathrm{K}$, two phases with higher $T_c$ of $9.5$ and 11 K are multilayered structures consisting of ${\alpha }^{\prime }$ and $\kappa$ layers. We investigate these three systems within density functional theory and find that the ${\alpha }^{\prime }$ layer shows different degrees of charge order for the two $\kappa -{\alpha }^{\prime }$ systems and directly influences the electronic behavior of the conducting $\kappa$ layer. We discuss the origin of the distinct behavior of the three polymorphs and propose a minimal tight-binding Hamiltonian for the description of these systems based on projective molecular Wannier functions.

The electronic structure and spin texture around the $\overline{\mathrm{M}}$ point of $\mathrm{Tl}/\mathrm{Si}\left(111\right)\text{−}(1\times 1)$ is investigated with spin- and angle-resolved inverse photoemission and density-functional theory. The $\mathrm{Tl}/\mathrm{Si}\left(111\right)\text{−}(1\times 1)$ surface exhibits a surface-derived unoccupied state, which extends over the complete surface Brillouin zone. It is known for its giant spin-orbit-induced splitting and unique symmetry- and spin-orbit-induced spin landscape: a rotating spin pattern in momentum space along $\overline{\mathrm{\Gamma }}\overline{\mathrm{K}}$ and pure in-plane spin polarization along $\overline{\mathrm{\Gamma }}\overline{\mathrm{M}}$. Our results demonstrate how the $\overline{\mathrm{M}}$ point is encircled by this state. Its spin texture shows a peculiar twist, which reflects the $C_{1h}$ symmetry of the $\overline{\mathrm{M}}$ point.

The dynamical competition between the chemical mixing forced during energetic particle irradiation and thermally activated decomposition can lead to the stabilization of self-organized steady states in alloy systems comprised of immiscible elements. Continuum modeling and atomistic simulations predicted the stabilization of steady-state nanoscale compositional patterns for a well-defined range of ballistic mixing frequencies normalized by the irradiation-enhanced thermal atomic jump frequencies. Irradiation-induced compositional patterning has now indeed been observed experimentally, but a quantitative comparison has been lacking because models and simulations have relied on a simplified treatment with a fixed point-defect concentration. We overcome here this limitation by using a kinetic Monte Carlo code that includes the production, recombination, and elimination of point defects at sinks, as well as the chemical mixing forced by ballistic replacements. By varying the sink density and its efficiency, the temperature range of stabilization of steady-state compositional patterns is investigated in model binary alloys for point-defect regimes dominated by either recombination or elimination on sinks. We find that in the sink regime, compositional patterning can be extended to remarkably high temperatures. The results are discussed by analyzing the relative diffusivities of $A$ and $B$ species, and their dependencies on temperature.

Machine learning methods are being increasingly used in condensed matter physics and materials science to classify crystals structures and predict material properties. However, the reliability of these methods for a given problem, especially when large data sets are unavailable, has not been well studied. By addressing the tasks of classifying crystal structure and predicting melting temperatures of the octet subset of AB solids, we performed such a study and found potential problems with using machine learning methods on relatively small data sets. At the same time, however, we can reaffirm the potential power of such methods for these tasks. In particular, we uncovered an important new material feature, the excess Born effective charge, that significantly increased the accuracy of the predictions for the classification problem we defined. This discovery leads us to propose a new scale for the degree of ionicity and covalency in these solids. More specifically, we partitioned the crystal structures of a set of 75 octet solids into those that are ionic and covalent bonded and thus performed a binary classification task. We found that using the standard indices $(r_{\sigma },r_{\pi })$, suggested by St. John and Bloch several decades ago, enabled an average success in classification of $92\%$. Using just $r_{\sigma }$ and the excess Born effective charge $\mathrm{\Delta }{Z}_A$ of the A atom enabled an average success of $97\%$, but we also found relatively large variations about these averages that were dependent on how certain machine learning methods were used and for which a standard deviation was not a proper measure of the degree of confidence we can place in either average. Instead, we calculated and report with $95\%$ confidence that the traditional classification pair predicts an accuracy in the interval $[89\%,95\%]$ and the accuracy of the new pair lies in the interval $[96\%,99\%]$. For melting temperature predictions, the size of our data set was 46. We estimate the root-mean-squared error of our resulting model to be $11\%$ of the mean melting temperature of the data, but we note that if the accuracy of this predicted error is itself measured, our estimated fitting error itself has a root-mean-square error of $50\%$. In short, what we illustrate is that classification and regression predictions can vary significantly, depending on the details of how machine learning methods are applied to small data sets. This variation makes it important, if not essential, to average the predictions and compute confidence intervals about these averages to report results meaningfully. However, when properly used, these statistical methods can advance our understanding and improve predictions of material properties even for small data sets.

Hybrid qubit systems combining electronic spins with nearby (“proximate”) nuclear spin registers offer a promising avenue towards quantum information processing, with even multispin error-correction protocols recently demonstrated in diamond. However, for the important platform offered by spins of donor atoms in cryogenically cooled silicon, decoherence mechanisms of ${}^{29}\mathrm{Si}$ proximate nuclear spins are not yet well understood. The reason is partly because proximate spins lie within a so-called “frozen core” region where the donor electronic hyperfine interaction strongly suppresses nuclear dynamics. We investigate the decoherence of a central proximate nuclear qubit arising from quantum spin baths outside, as well as inside, the frozen core around the donor electron. We consider the effect of a very large nuclear spin bath comprising many $\left(\gtrsim {10}^8\right)$ weakly contributing pairs outside the frozen core (the “far bath”). We also propose that there may be an important contribution from a few (of order 100) symmetrically sited nuclear spin pairs (“equivalent pairs”), which were not previously considered because their effect is negligible outside the frozen core. If equivalent pairs represent a measurable source of decoherence, nuclear coherence decays could provide sensitive probes of the symmetries of electronic wave functions. For the phosphorus donor system, we obtain $T_{2n}$ values of order 1 second for both the far-bath and equivalent-pair models, confirming the suitability of proximate nuclei in silicon as very-long-lived spin qubits.

A general theory of forward volume magnetostatic spin-wave multiple scattering by a finite two-dimensional ensemble of cylindrical magnetic inclusions in a ferromagnetic film (matrix) metallized from both sides is developed. It is predicted that bound spin-wave modes are excited around these inclusions. The set of self-consistent equations for spin-wave multiple scattering inside the matrix with inclusions is obtained by using an invariant addition theorem for the cylindrical wave functions. This set of equations is solved by an iterative process. We derive simple expressions for eigenmodes (bound modes) in a form of Bloch-like spin waves propagating along the inclusion circular array. The theory is illustrated on practically significant examples: (i) bound-mode excitation by propagating spin waves; (ii) helix components of the spin-wave-scattered field around magnetic inclusions; and (iii) a high-quality spin-wave resonator based on a small number of magnetic inclusions.

We study the electronic structure of vortex core states of FeSe superconductors based on a ${\mathrm{t}}_{2g}$ three-orbital model by solving the Bogoliubov-de Gennes (BdG) equation self-consistently. The orbital-resolved vortex core states of different pairing symmetries manifest themselves as distinguishable structures due to different quasiparticle wave functions. The obtained vortices are classified in terms of the invariant subgroups of the symmetry group of the mean-field Hamiltonian in the presence of a magnetic field. Isotropic $s$ and anisotropic $s$-wave vortices have $G_5$ symmetry for each orbital, whereas $d_{x^2-y^2}$-wave vortices show $G_6^{*}$ symmetry for $d_{xz/yz}$ orbitals and $G_5^{*}$ symmetry for $d_{xy}$ orbital. In the case of $d_{x^2-y^2}$-wave vortices, hybridized-pairing between $d_{xz}$ and $d_{yz}$ orbitals gives rise to a relative phase difference in terms of gauge transformed pairing order parameters between $d_{xz/yz}$ and $d_{xy}$ orbitals, which is essentially caused by a transformation of co-representation of $G_5^{*}$ and $G_6^{*}$ subgroups. The calculated local density of states (LDOS) of $d_{x^2-y^2}$-wave vortices shows a qualitatively similar pattern with the experimental results. The phase difference of $\frac{\pi }{4}$ between $d_{xz/yz}$ and $d_{xy}$ orbital-resolved $d_{x^2-y^2}$-wave vortices can be verified by further experimental observation.