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.

We study the entanglement spectrum of a translationally invariant lattice system under a random partition, implemented by choosing each site to be in one subsystem with probability $p\in [0,1]$. We apply this random partitioning to a translationally invariant (i.e., clean) topological state, and argue on general grounds that the corresponding entanglement spectrum captures the universal behavior about its disorder-driven transition to a trivial localized phase. Specifically, as a function of the partitioning probability $p$, the entanglement Hamiltonian $H_A$ must go through a topological phase transition driven by the percolation of a random network of edge states. As an example, we analytically derive the entanglement Hamiltonian for a one-dimensional topological superconductor under a random partition, and demonstrate that its phase diagram includes transitions between Griffiths phases. We discuss potential advantages of studying disorder-driven topological phase transitions via the entanglement spectra of random partitions.

In nonsuperconducting, metallic paramagnetic ${\mathrm{SrCo}}_2{\mathrm{As}}_2$, inelastic neutron scattering measurements have revealed strong stripe-type $\mathbf{q}=(\pi ,0)$ antiferromagnetic (AFM) spin correlations. Here, using nuclear magnetic resonance (NMR) measurements on ${}^{59}\mathrm{Co}$ and ${}^{75}\mathrm{As}$ nuclei, we demonstrate that stronger ferromagnetic (FM) spin correlations coexist in ${\mathrm{SrCo}}_2{\mathrm{As}}_2$. Our NMR data are consistent with density functional theory (DFT) calculations which show enhancements at both $\mathbf{q}=(\pi ,0)$ and the in-plane FM $\mathbf{q}=0$ wave vectors in static magnetic susceptibility $\chi \left(\mathbf{q}\right)$. We suggest that the strong FM fluctuations prevent superconductivity in ${\mathrm{SrCo}}_2{\mathrm{As}}_2$, despite the presence of stripe-type AFM fluctuations. Furthermore, since DFT calculations have consistently revealed similar enhancements of the $\chi \left(\mathbf{q}\right)$ at both $\mathbf{q}=(\pi ,0)$ and $\mathbf{q}=0$ in the iron-based superconductors and parent compounds, our observation of FM correlations in ${\mathrm{SrCo}}_2{\mathrm{As}}_2$ calls for detailed studies of FM correlations in the iron-based superconductors.

The recent discovery of superconductivity in the quasi-one-dimensional compound ${\mathrm{K}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$, which consists of double-walled tubes of ${\left[{\left({\mathrm{Cr}}_3{\mathrm{As}}_3\right)}^{2-}\right]}^{\infty }$ that run along the $c$ axis, has attracted immediate attention as a potential system for studying superconductors with reduced dimensionality. Here we report clear experimental evidence for the unconventional nature of the superconducting order parameter in ${\mathrm{K}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$, by precisely measuring the temperature dependence of the change in the penetration depth $\mathrm{\Delta }\lambda \left(T\right)$ using a tunnel-diode oscillator. Linear behavior of $\mathrm{\Delta }\lambda \left(T\right)$ is observed for $T\ll T_c$, instead of the exponential behavior of conventional superconductors, indicating that there are line nodes in the superconducting gap. This is strong evidence for unconventional behavior and may provide key information for identifying the pairing state of this super-conductor.

Aluminum has been used prolifically as an impedance matching standard in the multimegabar regime (1 Mbar = 100 GPa), particularly in nuclear driven, early laser driven, and early magnetically driven flyer plate experiments. The accuracy of these impedance matching measurements depends upon the knowledge of both the Hugoniot and release or reshock response of aluminum. Here, we present the results of several adiabatic release measurements of aluminum from $\sim 400\text{–}1200$ GPa states along the principal Hugoniot using full density polymethylpentene (commonly known as TPX), and both $\sim 190$ and $\sim 110$ mg/cc silica aerogel standards. These data were analyzed within the framework of a simple, analytical model that was motivated by a first-principles molecular dynamics investigation into the release response of aluminum, as well as by a survey of the release response determined from several tabular equations of state for aluminum. Combined, this theoretical and experimental study provides a method to perform impedance matching calculations without the need to appeal to any tabular equation of state for aluminum. As an analytical model, this method allows for propagation of all uncertainty, including the random measurement uncertainties and the systematic uncertainties of the Hugoniot and release response of aluminum. This work establishes aluminum for use as a high-precision standard for impedance matching in the multimegabar regime.

Work, moments of work, and heat flux are studied for the generic case of a strongly driven two-level system immersed in a bosonic heat bath in domains of parameter space where perturbative treatments fail. This includes in particular the interplay between non-Markovian dynamics and moderate to strong external driving. Exact data are compared with predictions from weak-coupling approaches. Further, the role of system-bath correlations in the initial thermal state and their impact on the heat flux are addressed. The relevance of these results for current experimental activities on solid-state devices is discussed.

A high-pressure study of the lattice dynamics in the filled skutterudite ${\mathrm{Eu}}_{0.84}{\mathrm{Fe}}_4{\mathrm{Sb}}_{12}$ was carried out by means of x-ray powder diffraction and nuclear inelastic scattering. The anharmonicity of particular phonon modes was characterized by mode and element specific Grüneisen parameters. The large anharmonicity of the rattling optical mode that is hybridized with the acoustical phonons at ambient pressure is reduced at high pressure as the phonon modes decouple. This result suggests that anharmonic coupling between acoustic and optical phonon modes plays a central role in the reduced thermal conductivity.

Magnonic vortex crystals are studied via scanning transmission x-ray microscopy and ferromagnetic-resonance spectroscopy. We investigate a two-dimensional vortex crystal by imprinting waves with tunable wave vectors. The dispersion relation $\omega \left(k\right)$ is determined via ferromagnetic-resonance spectroscopy with a tunable frequency and wave vector for two vortex core polarization patterns that are adjusted by self-organized state formation prior to the measurement. We demonstrate that the band structure of the crystal is reprogrammed by tuning the vortex polarizations.

We report elastic and inelastic neutron-scattering measurements of the high-$T_{\mathrm{C}}$ ferromagnet ${\mathrm{Mn}}_{1+\delta }\mathrm{Sb}$. Measurements were performed on a large, $T_{\mathrm{C}}=434$ K, single crystal with an interstitial Mn content of $\delta \approx 0.13$. The neutron-diffraction results reveal that the interstitial Mn has a magnetic moment, and that it is aligned antiparallel to the main Mn moment. We perform density functional theory calculations including the interstitial Mn and find the interstitial to be magnetic, in agreement with the diffraction data. The inelastic neutron-scattering measurements reveal two features in the magnetic dynamics: (i) a spin-wave-like dispersion emanating from ferromagnetic Bragg positions $\left(\mathrm{H}\mathrm{K}2n\right)$, and (ii) a broad, nondispersive signal centered at forbidden Bragg positions $\left(\mathrm{H}\mathrm{K}2n+1\right)$. The inelastic spectrum cannot be modeled by simple linear spin-wave-theory calculations and appears to be significantly altered by the presence of the interstitial Mn ions. The results show that the influence of the interstitial Mn on the magnetic state in this system is more important than previously understood.

The $\mathrm{Ce}T{X}_3$ compounds belong to the intensively studied group of materials revealing such properties as valence fluctuations, heavy-fermion behavior, or pressure-induced superconductivity. So far, magnetic structure was investigated in only a few of these. We performed a series of neutron-diffraction experiments on a structurally well-defined ${\mathrm{CeCuAl}}_3$ single crystal. The ordered non-centro-symmetric tetragonal ${\mathrm{BaNiSn}}_3$-type (I4mm) crystal structure as well as the antiferromagnetic order below $T_{\mathrm{N}}\approx 2.7$ K were corroborated by our diffraction experiments. The amplitude modulated magnetic structure with in-plane magnetic moments described by the propagation vector $\mathbf{k}=(0.4,0.6,0)$ was revealed. The relation between structural parameters and magnetic structures in the family of isostructural $\mathrm{Ce}T{X}_3$ compounds is discussed.

Unusual behavior of reentrant spin-glass (RSG) compound $\mathrm{L}{\mathrm{u}}_2\mathrm{MnNi}{\mathrm{O}}_6$ has been investigated by magnetometry and neutron diffraction. The system possesses a ferromagnetic (FM) ordering below 40 K and undergoes a RSG transition at 20 K. Additionally, $\mathrm{L}{\mathrm{u}}_2\mathrm{MnNi}{\mathrm{O}}_6$ retains memory effect above the glassy transition till spins sustain ordering. A novel critical behavior with unusual critical exponents ($\beta =\sim 0.241$ and $\gamma \sim 1.142$) is observed that indicates a canting in the spin structure below the ferromagnetic transition ($T_C$). A comprehensive analysis of temperature-dependent neutron diffraction data and first-principles calculations divulge that a structural distortion induced by an octahedral tilting results in a canted spin structure below $T_C$.

We study the effect of a single impurity on the transport properties of a one-dimensional quantum liquid highly excited away from its ground state by a sudden quench of the bulk interaction. In particular we compute the time-dependent dc current to leading order in the impurity potential, using bosonization and starting both from the limit of a uniform system and from the limit of two decoupled semi-infinite systems. Our results reveal that the nonequilibrium excitation of bulk modes induced by the global quench has important effects on the conductor-insulator quantum phase transition, turning it into a crossover for any small quench amplitude and destroying the exact duality between the conducting and insulating fixed points. In addition, the current displays a faster decay towards the steady state than the equilibrium case, a signature of quench-induced decoherence.

We study systems of multiple interacting quantum impurities deposited on a metallic surface in a three-dimensional host. For the real-space two-impurity problem, using numerical renormalization group calculations, a rich range of behavior is shown to arise due to the interplay between Kondo physics and effective Ruderman-Kittel-Kasuya-Yosida interactions—provided the impurity separation is small. Such calculations allow identification of the minimum impurity separation required for a description in terms of independent impurities, and thereby the onset of the “dilute-impurity limit” in many-impurity systems. A “dilute-cluster” limit is also identified in systems with higher impurity density, where interimpurity interactions are important only within independent clusters. We calculate the quasiparticle interference due to two and many impurities, and explore the consequences of the independent impurity and cluster paradigms. Our results provide a framework to investigate the effects of disorder due to interacting impurities at experimentally relevant surface coverages.

We studied the electronic structures of antiferromagnetic (AFM) GdBiPt with propagating vectors $\overrightarrow{Q_1}=(0,0,\pi$) (A-type) and $\overrightarrow{Q_2}=(\pi ,\pi ,\pi$) (G-type) by performing first-principles calculation based on density-functional theory with modified Becke and Johnson local-density approximation potentials plus Hubbard U (MBJLDA+U). With the total energy calculation, the G-type AFM spin-ordered state is relatively more stable than the A-type AFM spin-ordered state, although the difference in total energy is minute. Our band-structure calculation predicts that the A-type AFM state is topologically nontrivial due to a single $s$-character band inversion at the $\mathrm{\Gamma }$ point, which is similar to the band inversions in half-Heusler topological insulator candidates and bulk HgTe semiconductors, while the G-type AFM state is topologically trivial due to the absence of $s/p$ band inversion. With a realistic tight-binding model calculation with 20 bands coupled to an AFM Zeeman field, GdBiPt with A-type AFM spin order presents a metallic surface state on the terminations with AFM aligned Gd ions, and this surface state is independent of the strength of the AFM Zeeman field, i.e., this surface state will be preserved in a nonmagnetic case. Upon terminating the ferromagnetic spin-aligned Gd ions, the surface state is dependent on the strength of the Zeeman field, and the metallic surface can recover when the Zeeman field approaches zero.