We report a study of the topological Hall effect (THE) in Fe-doped MnSi and compare it with results from pure MnSi under pressure. We find that Fe doping increases the THE, indicating an enhancement of the magnitude of the emergent gauge field. This is consistent with the concurrent reduction in the length scale of the skyrmion lattice. For both pressurized and doped samples, we calculate the emergent magnetic field based on the size of the measured THE and compare it with a theoretical upper bound. We find that the ratio of these two remains more or less constant with pressure or Fe doping but differs greatly from that of pure MnSi at ambient pressure. We discuss the implications of this ratio with respect to trends in the saturated magnetic moment and helical pitch length as $T_C\to 0$ via doping and pressure, respectively.

The effect of hydrogen adsorption on the magnetic properties of an Fe${}_3$ cluster immersed in a Cu(111) surface has been calculated using density functional theory and the results used to parametrize an Alexander-Anderson model which takes into account the interaction of $d$ electrons with itinerant electrons. A number of adatom configurations containing one to seven H atoms were analyzed. The sequential addition of hydrogen atoms is found to monotonically reduce the total magnetic moment of the cluster with the effect being strongest when the H atoms sit at low-coordinated sites. Decomposition of the charge density indicates a transfer of 0.3 electrons to each of the H atoms from both the Fe atoms and from the copper substrate, irrespective of adsorption site and coverage. The magnetic moment of only the nearest neighbor Fe atoms is reduced, mainly due to increased population of minority spin $d$ states. This can be modeled by increased indirect coupling of $d$ states via the conduction $s$ band in the Alexander-Anderson model.

Thin-film ferromagnetic disks present a vortex spin structure whose dynamics, added to the small size ($\approx$10 nm) of their core, has earned them intensive study. Here we use a scanning nitrogen-vacancy (NV) center microscope to quantitatively map the stray magnetic field above a $1-\mu$m-diameter disk of permalloy, unambiguously revealing the vortex core. Analysis of both probe-to-sample distance and tip motion effects through stroboscopic measurements allows us to compare directly our quantitative images to micromagnetic simulations of an ideal structure. Slight perturbations with respect to the perfect vortex structure are clearly detected either due to an applied in-plane magnetic field or imperfections of the magnetic structures. This work demonstrates the potential of scanning NV microscopy to map tiny stray field variations from nanostructures, providing a nanoscale, nonperturbative detection of their magnetic texture.

We classify ground states and normal modes for $n$-component superconductors with frustrated intercomponent Josephson couplings, focusing on $n=4$. The results should be relevant not only to multiband superconductors, but also to Josephson-coupled multilayers and Josephson-junction arrays. It was recently discussed that three-component superconductors can break time-reversal symmetry as a consequence of phase frustration. We discuss how to classify frustrated superconductors with an arbitrary number of components. Although already for the four-component case there are a large number of different combinations of phase-locking and phase-antilocking Josephson couplings, we establish that there are a much smaller number of equivalence classes where properties of frustrated multicomponent superconductors can be mapped to each other. This classification is related to the graph-theoretical concept of Seidel switching. Numerically, we calculate ground states, normal modes, and characteristic length scales for the four-component case. We report conditions of appearance of new accidental continuous ground-state degeneracies.

Since their discovery, it has been suggested that pairing in pnictides can be mediated by spin fluctuations between hole and electron bands. In this view, multiband superconductivity would substantially differ from other systems like MgB${}_2$, where pairing is predominantly intraband. Indeed, interband-dominated pairing leads to the coexistence of bonding and antibonding superconducting channels. Here, we show that this has profound consequences on the nature of the low-energy superconducting collective modes. In particular, the so-called Leggett mode for phase fluctuations is absent in the usual two-band description of pnictides. On the other hand, when also the repulsion between the hole bands is taken into account, a more general three-band description should be used, and a Leggett mode is then allowed. Such a model, which has been proposed for strongly hole-doped 122 compounds, can also admit a low-temperature $s+is$ phase that breaks the time-reversal symmetry. We show that the (quantum and thermal) transition from the ordinary superconductor to the $s+is$ state is accompanied by the vanishing of the mass of Leggett-like phase fluctuations, regardless the specific values of the interaction parameters. This general result can be obtained by means of a generalized construction of the effective action for the collective degrees of freedom that allows us also to deal with the nontrivial case of dominant interband pairing.

The time evolution of the ferromagnetic resonance output signal in the arrays of permalloy circular dots of submicron sizes was measured near the critical fields of the vortex nucleation and annihilation. Surprisingly short times of the transition from the quasiuniform to the vortex magnetization state (several milliseconds) were detected. The observed effects are explained by overcoming the field dependent energy barriers in the process of vortex core nucleation. The energy barrier values found from the time dependences of the ferromagnetic resonance peak intensities were compared with the ones calculated within the rigid vortex model. The rigid vortex model overestimates the nucleation barriers and a more adequate magnetization reversal model is needed. There is a strong dependence of the stable, metastable energy minima and energy barriers on the magnetic field and dot geometrical parameters.

We show that under certain conditions, multiband superconductors with broken time reversal symmetry have a vortex viscosity-generating mechanism which is different from that in conventional superconductors. It appears due to the existence of a mixed superfluid phase-density mode inside the vortex core. This contribution is dominant near the time reversal symmetry-breaking phase transition. The results could be relevant for the three-band superconductor ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$.

By performing atomistic calculations with a tight-binding potential, we study the displacement field induced by a screw dislocation lying along a free $\langle 011\rangle$ Cu cylindrical nanowire. For this anisotropic orientation that is often encountered experimentally, we show that the displacement field $u_z$ along the nanowire can be seen as the superposition of three different fields: the screw dislocation field in an infinite medium, the warping displacement field caused by the so-called Eshelby twist, and an additional image field induced by the free surfaces. A Fourier series analysis of this latter image displacement and stress fields is given. For a circular cross section of the wire, this image field corresponds mainly to an additional warping displacement $u_z\propto xy$. The dissociation mechanism of the dislocation into partials and the surface stress effects being also captured in our simulations, the present study enables one to quantify the various contributions to the formation of the x-ray diffractograms.

A source of uncertainty in the state of the art calculations of defect levels is the inaccurate prediction of band-gap energies. Several approaches were developed to surpass this problem. However, another source of uncertainty remains: the small number of clustered atoms imposed by the computational restrictions. In this work, the LDA-1/2 method is explored in an attempt to overcome both problems with a small computational cost. We considered the self-interstitial defects in silicon as a benchmark for calculating defect states and charge-transition levels of point defects in semiconductors. We found neutral formation energies, including reaction barriers, of 4.65, 4.49, and 4.87 eV, for hexagonal, split $\langle 110\rangle$ and $C_{3v}$ configurations, respectively, in good agreement with most experimental results.

Na${}_{0.5}$Bi${}_{0.5}$TiO${}_3$ (NBT) and its derivatives have prompted a great surge in interest owing to their potential as lead-free piezoelectrics. In spite of five decades since its discovery, there is still a lack of clarity on crucial issues such as the origin of significant dielectric relaxation at room temperature, structural factors influencing its depoling, and the status of the recently proposed monoclinic ($Cc$) structure vis-à-vis the nanosized structural heterogeneities. In this work, these issues are resolved by comparative analysis of local and global structures on poled and unpoled NBT specimens using electron, x-ray, and neutron diffraction in conjunction with first-principles calculation, dielectric, ferroelectric, and piezoelectric measurements. The reported global monoclinic ($Cc$) distortion is shown not to correspond to the thermodynamic equilibrium state at room temperature. The global monocliniclike appearance rather owes its origin to the presence of local structural and strain heterogeneities. Poling removes the structural inhomogeneities and establishes a long-range rhombohedral distortion. In the process the system gets irreversibly transformed from a nonergodic relaxor to a normal ferroelectric state. The thermal depoling is shown to be associated with the onset of incompatible in-phase tilted octahedral regions in the field-stabilized long range rhombohedral distortion.

There is proposed a phenomenological description of quasielastic neutron scattering in the ferromagnetic metals UGe${}_2$ and UCoGe based on their property that magnetization supported by the moments located at uranium atoms is not a conserved quantity relaxing to equilibrium by the interaction with an itinerant electron subsystem. As a result the linewidth of quasielastic neutron scattering at $q\to 0$ acquires nonvanishing value at all temperatures but the Curie temperature.

The coupling of charge, lattice, orbital, and spin degrees of freedom was studied in a $p$-electron mixed-valence Cs${}_4$O${}_6$ compound by the electron paramagnetic resonance (EPR) and ${}^{133}$Cs nuclear magnetic resonance (NMR). The dramatic differences in the evolution of the EPR and ${}^{133}$Cs NMR spectra measured under different cooling protocols reveal two competing low-temperature phases: quenched high-temperature cubic and the low-temperature low-symmetry phases, respectively. They differ in the orientation of O${}_2^{-}$ anion axes, the ordering of ${\pi }^{*}$ molecular orbitals, and the superexchange interactions through the Cs${}^{+}$ bridges. The transformation between the two phases involves large amplitude reorientations of O${}_2$ groups and is extremely sluggish, thus explaining the coexistence of both phases and the deviations of the magnetic susceptibility from the simple Curie-Weiss dependence at low temperatures.

The compound KTi(SO${}_4$)${}_2·$H${}_2$O was recently reported as a quasi-one-dimensional spin-1/2 compound with competing antiferromagnetic nearest-neighbor exchange $J_1$ and next-nearest-neighbor exchange $J_2$ along the chain with a frustration ratio $\alpha =J_2/J_1\approx$ 0.29 [G. J. Nilsen, H. M. Rønnow, A. M. Läuchli, F. P. A. Fabbiani, J. Sanchez-Benitez, K. V. Kamenev, and A. Harrison, Chem. Mater. 20, 8 (2008)]. Here, we report a microscopically based magnetic model for this compound derived from density functional theory (DFT) based electronic structure calculations along with respective tight-binding models. Our (LSDA+$U_d$) calculations confirm the quasi-one-dimensional nature of the system with antiferromagnetic $J_1$ and $J_2$, but suggest a significantly larger frustration ratio ${\alpha }_{\mathrm{DFT}}\approx 0.94$—1.4, depending on the choice of $U_d$ and structural parameters. Based on transfer matrix renormalization group (TMRG) calculations we find ${\alpha }_{\mathrm{TMRG}}=1.5$. Due to an intrinsic symmetry of the $J_1$-$J_2$ model, our larger frustration ratio $\alpha$ is also consistent with the previous thermodynamic data. To identify the frustration ratio $\alpha$ unambiguously, we propose performing high-field magnetization and low-temperature susceptibility measurements.

We study the hydrodynamics of a nonresonantly pumped polariton condensate in a quasi-one-dimensional quantum wire taking into account the spin degree of freedom. We clarify the relevance of the Landau criterion for superfluidity in this dissipative two-component system. Two Cherenkov-like critical velocities are identified corresponding to the opening of different channels of radiation: one of (damped) density fluctuations and another of (weakly damped) polarization fluctuations. We determine the drag force exerted onto an external obstacle and propose experimentally measurable consequences of the specific features of the fluctuations of polarization.

A large class of symmetry-protected topological phases (SPT) in boson/spin systems have been recently predicted by the group cohomology theory. In this work, we consider bosonic SPT states at least with charge symmetry [U(1) or Z${}_N$] or spin-$S^z$ rotation symmetry [U(1) or Z${}_N$] in two (2D) and three dimensions (3D) and the surface of 3D. If both are U(1), we apply external electromagnetic field/“spin gauge field” to study the charge/spin response. For the SPT examples we consider {i.e., U${}_c$(1)$⋊$Z${}_2^T$, U${}_s$(1)$\times$Z${}_2^T$, U${}_c$(1)$\times$[U${}_s$(1)$⋊$Z${}_2$]; subscripts $c$ and $s$ are short for charge and spin; Z${}_2^T$ and Z${}_2$ are time-reversal symmetry and $\pi$ rotation about $S^y$, respectively}, many variants of Witten effect in the 3D SPT bulk and various versions of anomalous surface quantum Hall effect are defined and systematically investigated. If charge or spin symmetry reduces to Z${}_N$ by considering charge-$N$ or spin-$N$ condensate, instead of the linear response approach, we gauge the charge/spin symmetry, leading to a dynamical gauge theory with some remaining global symmetry. The 3D dynamical gauge theory describes a symmetry-enriched topological phase (SET), i.e., a topologically ordered state with global symmetry which admits nontrivial ground-state degeneracy depending on spatial manifold topology. For the SPT examples we consider, the corresponding SET states are described by dynamical topological gauge theory with topological $\mathit{BF}$ term and axionic $\Theta$ term in 3D bulk. In addition, the surface of SET is described by the chiral boson theory with quantum anomaly.

We present a detailed calculation of the electronic structure of SrVO${}_3$ based on the $GW$+DMFT method. We show that a proper inclusion of the frequency-dependent Hubbard $U$ and the nonlocal self-energy via the $GW$ approximation, as well as a careful treatment of the Fermi level, are crucial for obtaining an accurate and coherent picture of the quasiparticle band structure and satellite features of SrVO${}_3$. The $GW$+DMFT results for SrVO${}_3$ are not attainable within the $GW$ approximation or the LDA+DMFT scheme. We also compare the results of $GW$+DMFT to DMFT calculations based on the $GW$ quasiparticle bands.

We systematically explore the ground-state phase diagram of the Kane-Mele-Kondo lattice model on the honeycomb lattice; particularly, we focus on its magnetic properties, which are ubiquitous and important in many realistic heavy-fermion materials but have not been clearly studied in the previous publication [X. Y. Feng, J. Dai, C. H. Chung, and Q. Si, Phys. Rev. Lett. 111, 016402 (2013)]. Besides the Kondo insulator found in that paper, two kinds of antiferromagnetic spin-density-wave phases are discovered. One is the normal antiferromagnetic spin-density-wave state and the other is a nontrivial topological antiferromagnetic spin-density-wave state, which shows the unexpected $Z_2$ topological order with a quantized spin Hall conductance and a helical edge state similar to the hotly studied topological insulators. The quantum spin Hall insulator in the work of Feng et al. is found to be absent since it is always unstable to antiferromagnetic spin-density-wave states at least at the mean-field level in our model. Interestingly, the transition between the two spin-density-wave phases is an exotic topological quantum phase transition, whose critical behavior is described by an emergent three-dimensional quantum electrodynamics, in which conduction electrons contribute to the low-energy Dirac fermions while the spin-wave fluctuation of local spins gives rise to an effective dynamic $U\left(1\right)$ gauge field. Such nontrivial transition shows radical critical thermodynamic, transport, and single-particle behaviors, which provide a fingerprint for this transition. Additionally, the transition of antiferromagnetic spin-density-wave states to the Kondo insulator is found to be first order. The introduction of two novel magnetic phases and their topological quantum phase transition show rich and intrinsic physics involved in the Kane-Mele-Kondo lattice model.

We perform an instability analysis of the Emery three-band model at hole doping and weak coupling within a channel-decomposed functional renormalization group flow proposed in Phys. Rev. B 79, 195125 (2009). In our approach, momentum dependences are taken into account with improved precision compared to previous studies of related models. Around a generic parameter set, we find a strong competition of antiferromagnetic and $d$-wave Cooper instabilities with a smooth behavior under a variation of doping and additional hopping parameters. For increasingly incommensurate ordering tendencies in the magnetic channel, the $d$-wave pairing gap is deformed at its maxima. Comparing our results for the Emery model to those obtained for the two-dimensional one-band Hubbard model with effective parameters, we find that, despite considerable qualitative agreement, multiorbital effects have a significant impact on a quantitative level.

We present a theoretical study on iron pyrite using density-functional theory (DFT) and the $GW$ approximation to many-body perturbation theory. The fundamental band gap of iron pyrite is determined by iron 3$d$ states at the valence band edge and a sulfur 3$p$-dominated conduction band at $\Gamma$. The gap is quite sensitive to structural changes as well as to the applied electronic structure method. We found that this $p$-dominated band does not play a significant role for the optical absorption, leading to a large difference between the optical and fundamental band gaps of iron pyrite. As a consequence the $GW$-corrected energies result in no considerable change of the optical band gap as compared to standard DFT, both being in reasonable agreement with experiment. However, we show that the fundamental band gap is reduced to about $0.3$ eV in $GW$, which may contribute to the low open-circuit voltage of about $0.2$ V observed in iron pyrite solar cells, representing a serious bottleneck for photovoltaic applications. To demonstrate that this unconventional reduction of the $p$-$d$ gap is not unique for iron pyrite, similarities for FeS${}_2$ in the marcasite structure are presented.