The magnetoresistance and magnetic torque of FeS are measured in magnetic fields $B$ of up to 18 T down to a temperature of 0.03 K. The superconducting transition temperature is found to be $T_c=4.1\mathrm{K}$, and the anisotropy ratio of the upper critical field $B_{c2}$ at $T_c$ is estimated from the initial slopes to be $\mathrm{\Gamma }\left(T_c\right)=6.9$. $B_{c2}\left(0\right)$ is estimated to be 2.2 and 0.36 T for $B\parallel ab$ and $c$, respectively. Quantum oscillations are observed in both the resistance and torque. Two frequencies $F=0.15$ and 0.20 kT are resolved and assigned to a quasi-two-dimensional Fermi surface cylinder. The carrier density and Sommerfeld coefficient associated with this cylinder are estimated to be $5.8\times {10}^{-3}$ carriers/Fe and $0.48\mathrm{mJ}/\left({\mathrm{K}}^2\mathrm{mol}\right)$, respectively. Other Fermi surface pockets still remain to be found. Band-structure calculations are performed and compared to the experimental results.

The high-pressure structural and elastic properties of ${\mathrm{MgCO}}_3, {\mathrm{CaCO}}_3$, and $\mathrm{MgCa}{\left({\mathrm{CO}}_3\right)}_2$ have been investigated with density functional theory. By computing the isotropic elastic constants, we found that the bulk and shear acoustic wave velocities are smaller in carbonates than in their respective silicates for a wide range of pressure. In terms of geophysics, these results provide strong evidence that the low-velocity zones, near the bottom of the Earth's lower mantle, could be associated with the presence of carbon element in the form of carbonate minerals. This is a different model, associated with chemical differentiation, to explain those low-velocity zones, in addition to several other recent ones proposed in the literature.

Electronic, structural, and magnetic properties of Pt dichalcogenide monolayers are investigated using first-principle calculations. We find that hydrogenation lifts the spin degeneracy in narrow antibonding Pt $5d$ subband electrons and transforms the nonmagnetic semiconductors $\mathrm{Pt}{X}_2(X=\mathrm{S},\mathrm{Se},\mathrm{Te})$ into ferromagnetic metals, $\mathrm{Pt}{X}_2$-1H; neither strain nor thin-film edges are necessary to support the transition. The trend towards ferromagnetism is most pronounced for $X=\mathrm{S}$, decreasing with increasing atomic weight of the chalcogens.

We present a comparative study of pair correlations and currents through superconducting-magnetic hybrid systems with a particular emphasis on the tunable Bloch domain wall of an exchange spring. This study of the Gor'kov functions contrasts magnetic systems with domain walls that change at discrete points in the magnetic region with those that change continuously throughout. We present results for misaligned homogeneous magnetic multilayers, including spin valves, for discrete domain walls, as well as exchange springs and helical domain walls—such as Holmium—for the continuous case. Introducing a rotating basis to disentangle the role of singlet and triplet correlations, we demonstrate that substantial amounts of (so-called short-range) singlet correlations are generated throughout the magnetic system in a continuous domain wall via the cascade effect. We propose a classification of $0-\pi$ transitions of the Josephson current into three types, according to the predominant pair correlations symmetries involved in the current. Properties of exchange springs for an experimental study of the proposed effects are discussed. The interplay between components of the Gor'kov function that are parallel and perpendicular to the local magnetization lead to a novel prediction about their role in a proximity system with a progressively twisting helix that is experimentally measurable.

Regular arrays of magnetic atoms on a superconductor provide a promising platform for topological superconductivity. In this work, we study the effects of disorder in these systems, focusing on vacancies realized by missing magnetic atoms. We develop approaches that allow treatment of ferromagnetic dense chains as well as long-range hopping ferromagnetic and helical Shiba chains at arbitrary subgap energies. Vacancies in magnetic chains play an analogous role to magnetic impurities in a clean $s$-wave superconductor. A single vacancy in a topological chain gives rise to a low-lying “anti-Shiba” state below the band edge of a regular magnetic chain. Proliferation of the anti-Shiba band formed by a finite density of hybridized vacancy states leads to deterioration of the topological phase, which exhibits unusual fragility in a particular parameter region in dilute chains. We also consider local fluctuation in the Shiba coupling and discuss how vacancy states could contribute to experimental verification of topological superconductivity.

Classification of possible quantum spin liquid (QSL) states of interacting spin-1/2's in two dimensions has been a fascinating topic of condensed matter for decades, resulting in enormous progress in our understanding of low-dimensional quantum matter. By contrast, relatively little work exists on the identification, let alone classification, of QSL phases for spin-1 systems in dimensions higher than one. Employing the powerful ideas of tensor network theory and its classification, we develop general methods for writing QSL wave functions of spin-1 respecting all the lattice symmetries, spin rotation, and time reversal with trivial gauge structure on the square lattice. We find $2^5$ distinct classes characterized by five binary quantum numbers. Several explicit constructions of such wave functions are given for bond dimensions $D$ ranging from two to four, along with thorough numerical analyses to identify their physical characters. Both gapless and gapped states are found. The topological entanglement entropy of the gapped states is close to zero, indicative of topologically trivial states. In $D=4$, several different tensors can be linearly combined to produce a family of states within the same symmetry class. A rich “phase diagram” can be worked out among the phases of these tensors, as well as the phase transitions among them. Among the states we identified in this putative phase diagram is the plaquette-ordered phase, gapped resonating valence bond phase, and a critical phase. A continuous transition separates the plaquette-ordered phase from the resonating valence bond phase.

In the prototypical $4d$ system $(\mathrm{Sr},\mathrm{Ca}){\mathrm{RuO}}_3$, the degree and origin of electron correlations, and how they correlate with physical properties, still remain elusive, though extensive studies have been performed. In this work we present a comparative electronic structure study of high-quality epitaxial ${\mathrm{CaRuO}}_3$ and ${\mathrm{SrRuO}}_3$ thin films, by means of reactive molecular beam epitaxy and in situ angle-resolved photoemission spectroscopy. We found that while ${\mathrm{SrRuO}}_3$ possesses sharp features signaling the Fermi liquid state, the isostructural ${\mathrm{CaRuO}}_3$ exhibits broad features and its spectral weight is markedly transferred from the Fermi level to $-1.2$ eV forming a “hump” structure which resembles the Mott-Hubbard system $(\mathrm{Sr},\mathrm{Ca}){\mathrm{VO}}_3$. We suggest that this hump is the precursor of the lower Hubbard band, and the $U/W$ ($U$ and $W$ represent the on-site Coulomb interactions and bandwidth, respectively) of our ${\mathrm{CaRuO}}_3$ thin film is much larger than that of ${\mathrm{SrRuO}}_3$. In addition, we discuss the origin of electron correlations as well as the ferromagnetism in ${\mathrm{SrRuO}}_3$ which is absent in ${\mathrm{CaRuO}}_3$. Our findings put constraints on future studies, and also show that perovskite ruthenates are indeed an experimentally tunable system for the study of electron correlations.

Molecular adsorbates on metal surfaces exchange energy with substrate phonons and low-lying electron-hole pair excitations. In the limit of weak coupling, electron-hole pair excitations can be seen as exerting frictional forces on adsorbates that enhance energy transfer and facilitate vibrational relaxation or hot-electron-mediated chemistry. We have recently reported on the relevance of tensorial properties of electronic friction [M. Askerka et al., Phys. Rev. Lett. 116, 217601 (2016)] in dynamics at surfaces. Here we present the underlying implementation of tensorial electronic friction based on Kohn-Sham density functional theory for condensed phase and cluster systems. Using local atomic-orbital basis sets, we calculate nonadiabatic coupling matrix elements and evaluate the full electronic friction tensor in the Markov limit. Our approach is numerically stable and robust, as shown by a detailed convergence analysis. We furthermore benchmark the accuracy of our approach by calculation of vibrational relaxation rates and lifetimes for a number of diatomic molecules at metal surfaces. We find friction-induced mode-coupling between neighboring CO adsorbates on Cu(100) in a $c(2\times 2)$ overlayer to be important for understanding experimental findings.

A quantum critical (QC) fluid exhibits universal subleading corrections to the area law of its entanglement entropies. In two dimensions, when the partition involves a corner of angle $\theta$, the subleading term is logarithmic with coefficient $a_{\alpha }\left(\theta \right)$ for the $\alpha$-Rényi entropy. In the smooth limit $\theta \to \pi ,a_1\left(\theta \right)$ yields the central charge of the stress tensor when the QC point is described by a conformal field theory (CFT). For general Rényi indices and angles, $a_{\alpha }\left(\theta \right)$ is richer and few general results exist. We study $a_{\alpha }\left(\theta \right)$ focusing on two benchmark CFTs, the free Dirac fermion and boson. We perform numerical lattice calculations to obtain high precision results in $\theta ,\alpha$ regimes hitherto unexplored. We derive field theory estimates for $a_{\alpha }\left(\theta \right)$, including exact results, and demonstrate an excellent quantitative match with our numerical calculations. We also develop and test strong lower bounds, which apply to both free and interacting QC systems. Finally, we comment on the near collapse of $a_{\alpha }\left(\theta \right)$ for various theories, including interacting $O\left(N\right)$ models.

Spin-polarized current through helimagnets and the conductance modulation due to the chirality mismatch are studied numerically. The one-dimensional spiral magnet structure is obtained by taking into account the Dzyaloshinskii-Moriya interaction and the ferromagnetic interaction. Although the spiral magnetic structure consists of the $y\text{−}z$ components of the magnetization, the conduction electron through the spiral magnet is polarized in the $x$ direction, and its sign depends on the chirality of the spiral structure. We also investigate charge transport through the junction system consisting of two helimagnets. Similar to the giant magnetoresistance in the uniform ferromagnet, the conductance is reduced significantly by attaching the helimagnets with different chiralities. It is possible that our proposed mechanism can make use of the chirality measuring method by using electron transport and an alternative type of magnetoresistance using a topological property.

The $\alpha -T_3$ model interpolates between the pseudospin $S=1/2$ honeycomb lattice of graphene and the pseudospin $S=1$ dice lattice via parameter $\alpha$. We present calculations of the magnetic properties of this hybrid pseudospin model, namely the absorptive magneto-optical conductivity and the Hofstadter butterfly spectra. In the magneto-optics curves, signatures of the hybrid system include a doublet structure present in the peaks, resulting from differing Landau level energies in the $K$ and $K^{\prime }$ valleys. In the Hofstadter spectra, we detail the evolution of the Hofstadter butterfly as it changes its periodicity by a factor of three as we vary between the two limiting cases of the $\alpha -T_3$ model.

Neutron diffraction, Mössbauer spectroscopy, magnetometry, and in-field x-ray diffraction are employed to investigate the magnetoelastic phase transition in hexagonal ${\left(\mathrm{Mn},\mathrm{Fe}\right)}_2\left(\mathrm{P},\mathrm{Si}\right)$ compounds. ${\left(\mathrm{Mn},\mathrm{Fe}\right)}_2\left(\mathrm{P},\mathrm{Si}\right)$ compounds undergo for certain compositions a second-order paramagnetic (PM) to a spin-density-wave (SDW) phase transition before further transforming into a ferromagnetic (FM) phase via a first-order phase transition. The SDW-FM transition can be kinetically arrested, causing the coexistence of FM and untransformed SDW phases at low temperatures. Our in-field x-ray diffraction and magnetic relaxation measurements clearly reveal the metastability of the untransformed SDW phase. This unusual magnetic configuration originates from the strong magnetoelastic coupling and the mixed magnetism in hexagonal ${\left(\mathrm{Mn},\mathrm{Fe}\right)}_2\left(\mathrm{P},\mathrm{Si}\right)$ compounds.

We report results of joint experimental and theoretical studies on magnetoelectric (ME) compounds ${\mathrm{Co}}_4{\mathrm{Nb}}_2{\mathrm{O}}_9$ and ${\mathrm{Co}}_4{\mathrm{Ta}}_2{\mathrm{O}}_9$. On the experimental side, we present results of the magnetization and dielectric permittivity measurements in the magnetic field. On the theoretical side, we construct the low-energy Hubbard-type model for the magnetically active $\mathrm{Co}3d$ bands in the Wannier basis, using the input of the first-principles electronic structure calculations, solve this model in the mean-field Hartree-Fock approximation, and evaluate the electric polarization in terms of the Berry phase theory. Both experimental and theoretical results suggest that ${\mathrm{Co}}_4{\mathrm{Ta}}_2{\mathrm{O}}_9$ is magnetically softer than ${\mathrm{Co}}_4{\mathrm{Nb}}_2{\mathrm{O}}_9$. Therefore, it is reasonable to expect that the antiferromagnetic structure of ${\mathrm{Co}}_4{\mathrm{Ta}}_2{\mathrm{O}}_9$ can be easier deformed by the external magnetic field, yielding larger polarization. This trend is indeed reproduced by our theoretical calculations, but does not seem to be consistent with the experimental behavior of the polarization and dielectric permittivity. Thus, we suggest that there should be a hidden mechanism controlling the ME coupling in these compounds, probably related to the magnetic striction or a spontaneous change of the magnetic structure, which breaks the inversion symmetry. Furthermore, we argue that unlike in other ME systems (e.g., ${\mathrm{Cr}}_2{\mathrm{O}}_3$), in ${\mathrm{Co}}_4{\mathrm{Nb}}_2{\mathrm{O}}_9$ and ${\mathrm{Co}}_4{\mathrm{Ta}}_2{\mathrm{O}}_9$ there are two crystallographic sublattices, which contribute to the ME effect. These contributions are found to be of the opposite sign and tend to compensate each other. The latter mechanism can be also used to control and reverse the electric polarization in these compounds.

We show that Berezinskii's classification of the symmetries of Cooper pair amplitudes holds for driven systems even in the absence of translation invariance. We then consider a model Hamiltonian for a superconductor coupled to an external driving potential and, treating the drive as a perturbation, we investigate the corrections to the anomalous Green's function, density of states, and spectral function. We find that in the presence of an external drive the anomalous Green's function develops terms that are odd in frequency and that the same mechanism responsible for these odd-frequency terms generates additional features in the density of states and spectral function.

We demonstrate amplification (and attenuation) of a probe signal by a driven two-level quantum system in the Landau-Zener-Stückelberg-Majorana regime by means of an experiment, in which a superconducting qubit was strongly coupled to a microwave cavity, in a conventional arrangement of circuit quantum electrodynamics. Two different types of flux qubit, specifically a conventional Josephson junctions qubit and a phase-slip qubit, show similar results, namely, lasing at the working points where amplification takes place. The experimental data are explained by the interaction of the probe signal with Rabi-like oscillations. The latter are created by constructive interference of Landau-Zener-Stückelberg-Majorana (LZSM) transitions during the driving period of the qubit. A detailed description of the occurrence of these oscillations and a comparison of obtained data with both analytic and numerical calculations are given.

A longstanding controversy for relaxor ferroelectrics has been the origin of the “waterfall” effect in the phonon dispersion curves, in which low-energy transverse phonons cascade into vertical columns. Originally interpreted as phonons interacting with polar nanoregions (PNRs), it was later explained as an interference effect of coupling damped optic and acoustic phonons. In light of a recently discovered PNR vibrational mode near the “waterfall” wave vector [M. E. Manley, J. W. Lynn, D. L. Abernathy, E. D. Specht, O. Delaire, A. R. Bishop, R. Sahul, and J. D. Budai, Nat. Commun. 5, 3683 (2014)], we have reexamined this feature using neutron scattering on [100]-poled PMN-30%PT $[0.6\mathrm{Pb}\left(\mathrm{M}{\mathrm{g}}_{1/3}\mathrm{N}{\mathrm{b}}_{2/3}\right){\mathrm{O}}_3-0.3\mathrm{PbTi}{\mathrm{O}}_3]$. We find that the PNR mode couples to both optic and acoustic phonons and that this results in complex patterns in the dynamic structure factor, including intensity pockets and peaks localized in momentum-energy space. These features are fully explained by extending the mode-coupling model to include three coupled damped harmonic oscillators representing the transverse optic, acoustic, and PNR modes.

${\mathrm{Co}}_2\mathrm{FeAl}$ (CFA) thin films, of various thicknesses ($3\text{nm}\le t\le 50\mathrm{nm}$), have been grown by sputtering on (001) MgO single-crystal substrates and annealed at different temperatures ($\text{RT}\le T_a\le 600{}^{\circ }\mathrm{C}$, where RT is the room temperature). The influence of the CFA thickness ($t$), as well as ex situ annealing temperature ($T_a$), on the magnetic and structural properties has been investigated by x-ray diffraction (XRD), vibrating sample magnetometry, and broadband microstrip ferromagnetic resonance (MS-FMR). The XRD revealed an epitaxial growth of the films with the cubic [001] CFA axis normal to the substrate plane and that the chemical order varies from the $B2$ phase to the $A2$ phase when decreasing $t$ or $T_a$. The deduced lattice parameters showed an in-plane tetragonal distortion and in-plane and out-plane strains that increase with $T_a$ and $1/t$. For all $T_a$ values, the variation of the effective magnetization, deduced from the fit of MS-FMR measurements, shows two different regimes separated by a critical thickness, which is $T_a$ dependent. It decreases (increases) linearly with the inverse thickness ($1/t$) in the first (second) regime due to the contribution of the magnetoelastic anisotropy to surface (to volume) anisotropy. The observed behavior has been analyzed through a model allowing for the separation of the magnetocrystalline, magnetoelastic, and Néel-type interface anisotropy constants to the surface and the volume anisotropies. Similar behavior has been observed for the effective fourfold anisotropy field which governs the in-plane anisotropy present in all the samples. Finally, the MS-FMR data also allow one to conclude that the gyromagnetic factor remains constant and that the exchange stiffness constant increases with $T_a$.

Exchange bias and coercivity are both studied numerically in antiferromagnetic/ferromagnetic (AFM/FM) bilayers in the presence of a rough interface. The roughness is modeled by an AFM atomic mesa of variable width, in a periodic bidimensional system. Unlike the flat interface case, roughness can favor the presence of magnetic interfacial frustration or the formation of sharp magnetic domain walls pinned within the first AFM planes, inside the AFM mesa, in a Peierls potential well. We demonstrate by using athermal steepest-descent calculations that irreversible processes can occur during the hysteresis loops, when the AFM mesa width is less than half of the system period. In this case, the depinning of the domain wall from the Peierls potential well during the descending branch is not followed by its rewinding in a certain range of the AFM anisotropy. This leads to a large increase of both exchange bias and coercivity at low temperature and to an athermal training effect. When the thermal activation is taken into account by using Monte Carlo simulations, we show that a random walk of the domain wall occurs within the AFM layer. These processes induce changes in the AFM spin configuration when the system is cycled several times and produce a thermally activated training effect. Our simulations, interpreted in the context of periodic Peierls potential, provide an explanation for two important features of the exchange bias phenomenon, i.e., the thermal variation of its characteristic fields and the different contributions giving rise to the training effect (AFM bulk vs interface). More generally, the presence of interfacial atomic roughness reduces both exchange bias and coercivity with respect to the perfect interface case.

We utilize a combination of an ab initio calculation of effective Coulomb interactions and a DFT+DMFT calculation of total energy to study the structural properties of pure actinides. We first show that the effective direct Coulomb interactions in plutonium and americium are much smaller than usually expected. Secondly, we emphasize the key role of Hund's exchange in combination with the spin-orbit coupling in determining the structural parameters of $\delta$-plutonium and americium. Thirdly, using this ab initio description, we reproduce the experimental transition from low volume early actinides (uranium, neptunium, $\alpha$-plutonium) to high-volume late actinides ($\delta$-plutonium, americium, and curium) without the need of an artificial magnetism. Finally, we compare the energies and structural properties of $\alpha , \gamma , \epsilon$, and $\delta$ phases of plutonium to experimental data.

We present a simple technique to calculate spin-orbit coupling, $\langle \mathbf{L}·\mathbf{S}\rangle$, and branching ratio measured in x-ray absorption spectroscopy. Our method is for first-principles electronic structure calculation, and its implementation is straightforward for any of the standard formulations and codes. We applied this technique to several different large spin-orbit coupling iridates. The calculated $\langle \mathbf{L}·\mathbf{S}\rangle$ and branching ratio of a prototype $j_{\mathrm{eff}}=1/2$ Mott insulator, ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$, are in good agreement with recent experimental data over the wide range of Rh doping. Three different double-perovskite iridates (namely, ${\mathrm{Sr}}_2{\mathrm{MgIrO}}_6, {\mathrm{Sr}}_2{\mathrm{ScIrO}}_6$, and ${\mathrm{Sr}}_2{\mathrm{TiIrO}}_6$) are also well described. This technique can serve as a promising tool for studying large spin-orbit coupling materials from first principles and for understanding experiments.