Small-angle diffraction of polarized neutrons and resonant contribution to diffraction of synchrotron radiation have been applied to prove chirality of the crystal and magnetic structures of the magnetoelectric insulator ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$. Similarly to other chiral magnets with $P{2}_{1}3$ crystal structure the corresponding chiralities are linked to each other via the phenomenological Dzyaloshinskii-Moriya interaction. The crystal and magnetic structures for ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$ have the same chirality as is observed for MnSi, Mn${}_{1-x}$Fe${}_x$Si, and MnGe. However, the combination of chiralities is opposite to that proposed from a recent theoretical consideration. The chiral link between structure and magnetism previously found for metallic compounds is also proved to exist for an insulator (${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$), allowing us to conclude that the conducting electrons play no role in a possible common microscopic mechanism of this specific magnetolattice interaction.

We propose a method of measuring the fermion parity lifetime of Majorana fermion modes due to quasiparticle poisoning. We model quasiparticle poisoning by coupling the Majorana modes to electron reservoirs, explicitly breaking parity conservation in the system. This poisoning broadens and shortens the resonance peak associated with Majorana modes. In a two-lead geometry, the poisoning decreases the correlation in current noise between the two leads from the maximal value characteristic of crossed Andreev reflection. The latter measurement allows for calculation of the poisoning rate even if the temperature is much higher than the resonance width.

The polar nature of twin boundaries (TBs) of inherently nonpolar ferroelastic ${\mathrm{CaTiO}}_3$ is examined by the confocal second harmonic generation (SHG) microscope. Three-dimensional SHG images of samples with the pseudocubic (110) and (1-11) planes confirm that both crystallographically prominent TB ($W$ plane) and nonprominent TB ($W^{\prime }$ plane) are SHG active and consequently polar. The directions of the electric polarization $P_S$ are determined from the anisotropy of SHG intensities. In the $W^{\prime }$ plane, the SHG polar diagrams are well fitted using the point-group symmetry 2, which reveals that the $P_S$ directions parallel to the twofold axis do not lie in the $W^{\prime }$ plane but are tilted from it. In the $W$ plane, the $P_S$ direction lies in the mirror plane of the point-group symmetry $m$.

We have studied the microwave-assisted domain-wall (DW) motion driven by the alternating spin-polarized current. It is found that the adiabatic spin-transfer torque alone can sustain a steady-state wall motion without Walker breakdown if a transverse microwave field is applied. The direction and velocity of the domain-wall motion can be effectively manipulated by tuning the phase difference between the ac and the microwave field. The wall motion is very sensitive to the microwave frequency and can be resonantly enhanced by exciting the DW thickness oscillation mode. The characteristics of this kind of DW motion are well accounted for by an analytical model.

Muon spin rotation and relaxation ($\mu$SR) experiments have been carried out to characterize magnetic and superconducting ground states in the ${\text{Pr}}_{1-x}$${\text{Nd}}_x$${\text{Os}}_4$${\text{Sb}}_{12}$ alloy series. In the ferromagnetic end compound ${\text{NdOs}}_4$${\text{Sb}}_{12}$ the spontaneous local field at positive-muon (${\mu }^{+}$) sites below the ordering temperature $T_C$ is greater than expected from dipolar coupling to ferromagnetically aligned Nd${}^{3+}$ moments, indicating an additional indirect RKKY-like transferred hyperfine mechanism. For $0.45\le x\le 0.75$, ${\mu }^{+}$ spin relaxation rates in zero and weak longitudinal applied fields indicate that static fields at ${\mu }^{+}$ sites below $T_C$ are reduced and strongly disordered. We argue this is unlikely to be due to reduction of Nd${}^{3+}$ moments, and speculate that the Nd${}^{3+}$-${\mu }^{+}$ interaction is suppressed and disordered by Pr doping. In an $x=0.25$ sample, which is superconducting below $T_c=1.3$ K, there is no sign of “spin freezing” (static Nd${}^{3+}$ magnetism), ordered or disordered, down to 25 mK. Dynamic ${\mu }^{+}$ spin relaxation is strong, indicating significant Nd-moment fluctuations. The ${\mu }^{+}$ diamagnetic frequency shift and spin relaxation in the superconducting vortex-lattice phase decrease slowly below $T_c$, suggesting pair breaking and/or possible modification of Fermi-liquid renormalization by Nd spin fluctuations. For $0.25\le x\le 0.75$, the $\mu$SR data provide evidence against phase separation; superconductivity and Nd${}^{3+}$ magnetism coexist on the atomic scale.

Implanted muons have been used as a local probe to detect the magnetic ordering in the molecular magnetic nanodisk system Fe${}_{19}$. Two distinct groups of muon sites are identified from the relaxation data, reflecting sites near the magnetic core and sites distributed over the rest of the molecule. Dipole field calculations and Monte Carlo simulations confirm that the observed transition in Fe${}_{19}$ is consistent with magnetic ordering driven by interactions between molecules that are predominantly dipolar in nature. The triclinic crystal structure of this system gives the dipolar field a significant component transverse to the easy spin axis and the parallel component provides a dipolar bias closely tuned to the first level crossing of the system. These factors enhance the quantum tunneling between levels, thus enabling the system to avoid spin freezing at low temperatures and efficiently reach the dipolar ordered state.

We propose an analog superconducting quantum simulator for a one-dimensional model featuring momentum-dependent (nonlocal) electron-phonon couplings of Su-Schrieffer-Heeger and “breathing-mode” types. Because its corresponding coupling vertex function depends on both the electron and phonon quasimomenta, this model does not belong to the realm of validity of the Gerlach-Löwen theorem that rules out any nonanalyticities in single-particle properties. The superconducting circuit behind the proposed simulator entails an array of transmon qubits and microwave resonators. By applying microwave driving fields to the qubits, a small-polaron Bloch state with an arbitrary quasimomentum can be prepared in this system within times several orders of magnitude shorter than the typical qubit decoherence times. We demonstrate that—by varying the externally tunable parameters—one can readily reach the critical coupling strength required for observing the sharp transition from a nondegenerate (single-particle) ground state corresponding to zero quasimomentum ($K_{\mathrm{gs}}=0$) to a twofold-degenerate small-polaron ground state at nonzero quasimomenta $K_{\mathrm{gs}}$ and $-K_{\mathrm{gs}}$. Through exact numerical diagonalization of our effective Hamiltonian, we show how this nonanalyticity is reflected in the relevant single-particle properties (ground-state energy, quasiparticle residue, average number of phonons). We also show that the proposed setup provides an ideal testbed for studying the nonequilibrium dynamics of small-polaron formation in the presence of strongly momentum-dependent electron-phonon interactions.

Superconducting phases of $\left({\mathrm{NH}}_3\right){}_{y}M{}_x{\mathrm{FeSe}}_{1-z}{\mathrm{Te}}_z$ have been synthesized by the intercalation of metal atoms $(M:\mathrm{Li},\mathrm{Na},\mathrm{Ca})$ into FeSe and ${\mathrm{FeSe}}_{0.5}{\mathrm{Te}}_{0.5}$ using a low-temperature liquid ${\mathrm{NH}}_3$ technique. The superconducting transition temperature $\left(T_{\mathrm{c}}\right)$ is 31.5 K for Na-doped FeSe, and for Li-, Na-, and Ca-doped ${\mathrm{FeSe}}_{0.5}{\mathrm{Te}}_{0.5}$ it is 26, 22, and 17 K, respectively. The 31.5 K superconducting is the superconducting phase in ammoniated Na-doped FeSe. The $T_{\mathrm{c}}$ is lower than that (onset $T_{\mathrm{c}}=46\mathrm{K})$ of the superconducting phase reported previously. The reason why the $T_{\mathrm{c}}$ of this phase is lower is discussed based on the structure. The pressure dependences of $T_{\mathrm{c}}$ in the $\left({\mathrm{NH}}_3\right){}_y{\mathrm{Na}}_{0.5}\mathrm{FeSe}$ and $\left({\mathrm{NH}}_3\right){}_y{\mathrm{Na}}_{0.4}{\mathrm{FeSe}}_{0.5}{\mathrm{Te}}_{0.5}$ samples have been measured and a negative pressure dependence is observed; i.e., a decrease in lattice constant $c$ leads to a decrease in $T_{\mathrm{c}}$, consistent with the behavior of $\left({\mathrm{NH}}_3\right){}_y{\mathrm{Cs}}_{0.4}\mathrm{FeSe}$ reported previously by our group. Furthermore, the magnetic behavior of $\left({\mathrm{NH}}_3\right){}_y{\mathrm{Na}}_{0.4}{\mathrm{FeSe}}_{0.5}{\mathrm{Te}}_{0.5}$ has been fully investigated at different applied magnetic fields $(H$) to determine the critical magnetic field. This is a successful metal intercalation into ${\mathrm{FeSe}}_{1-z}{\mathrm{Te}}_z$ $(z\ne 0)$ and an observation of superconductivity.

An unconventional type of metal-insulator transition is realized in the FeSi${}_{1-x}$Ge${}_x$ alloys which is driven by the magnetic instability of the paramagnetic insulator. Recently, a minimal model for this type of transition has been introduced and studied in the limit of vanishing temperature. Here we explore the finite-temperature properties of the minimal model in the mean-field approximation. We show that the predictions of the model are in qualitative agreement with experimental data on FeSi${}_{1-x}$Ge${}_x$ and on FeGe under pressure.

A generalized tight-binding model is utilized to study Landau-level spectra of monolayer molybdenum disulfide. The intrinsic spin-orbit coupling effectively gives rise to multiple splitting of Landau levels. With a close inspection of wave-function characteristics, these levels can be classified into specific groups in terms of their orbital, spin, and valley signatures. In the calculation of magnetoabsorption spectra, the physical origin of the optical selection rules is clearly resolved. In particular, crossings of absorption lines are available and serve as a unique feature of the spin-orbit coupling. Our numerical results clearly demonstrate the magnetic control of spin and valley charge carriers and provide a basis for future experiments.

Electron distributions produced by grazing impact of fast protons on Mg(0001), Cu(111), Ag(111), and Au(111) surfaces are investigated, focusing on the effects of the electronic band structure. The process is described within the band-structure-based approximation, which is a perturbative method that includes an accurate representation of the electron-surface interaction, incorporating information of the electronic band structure of the solid. For all the studied surfaces, the presence of partially occupied surface electronic states produces noticeable structures in double-differential—energy- and angle-resolved—electron emission probabilities from the valence band. For Mg, Cu, and Ag these structures remain visible in electron emission spectra after adding contributions coming from core electrons, which might make possible their experimental detection, but for Au they are hidden by inner-shell emission.

Angle-resolved two-photon photoemission is used to probe the image potential states on monolayers of metal-free phthalocyanine (H${}_2$Pc) and cobalt phthalocyanine (CoPc) on the Ag(100) and Ag(111) surfaces. We find that in the plane of the surface the normally flat potential landscape becomes significantly corrugated in the presence of the phthalocyanine lattice, causing the opening of a band gap in the first image potential state, in agreement with previous findings. Interestingly, the data show that the absence of the electron dense metal center does not qualitatively alter this picture. Experiments conducted on Ag(100) and Ag(111) crystals demonstrate that the surface band structure and symmetry of the metal surface plays a minor role in the band folding phenomenon. A two-dimensional model that takes into account both the band structure of the substrate and the corrugation of the potential landscape in the surface plane due to the molecular lattice is presented and compared to experiment. This model enables the observation that, counter to intuition, the Co metal center at the CoPc/Ag interface actually smooths the interfacial potential relative to that of the H${}_2$Pc/Ag interface. We suggest that the strong corrugation of the potential at the organic/metal interface measured herein may account for the recent observation of surface umklapp scattering in recent ultraviolet photoemission experiments on organic/metal interfaces.

The surface electrostatic potential of Au(111) has been evaluated by using site-specific barrier-height measurements in a scanning tunneling microscope. From the spatial variation of the barrier height, sharp enlargement and reduction of the surface potential are obtained at the upper and lower step edges, respectively, which are characterized as the Smoluchowski effect. We also observe the potential with oscillatory decay away from the step, known as the Friedel potential oscillation. In addition, the periodic potential modulation induced by the surface reconstruction is evaluated quantitatively, which is confirmed by the energy shift of the bulk electronic states.

We show that in transition-metal compounds containing structural metal dimers there may exist in the presence of different orbitals a special state with partial formation of singlets by electrons on one orbital, while others are effectively decoupled and may give, e.g., long-range magnetic order or stay paramagnetic. A similar situation can be realized in dimers spontaneously formed at structural phase transitions, which can be called the orbital-selective Peierls transition. This can occur in the case of strongly nonuniform hopping integrals for different orbitals and small intra-atomic Hund's rule coupling $J_H$. Yet another consequence of this picture is that for an odd number of electrons per dimer there exists competition between the double-exchange mechanism of ferromagnetism and the formation of a singlet dimer by the electron on one orbital, with the remaining electrons giving a net spin of a dimer. The first case is realized for strong Hund's rule coupling, typical for $3d$ compounds, whereas the second is more plausible for 4$d$-5$d$ compounds. We discuss some implications of these phenomena, and consider examples of real systems, in which the orbital-selective phase seems to be realized.

A combination of density functional and dynamical mean-field theory is applied to the perovskites ${\mathrm{SrVO}}_3$, ${\mathrm{LaTiO}}_3$, and ${\mathrm{LaVO}}_3$. We show that DFT + DMFT in conjunction with the standard fully localized-limit (FLL) double-counting predicts that ${\mathrm{LaTiO}}_3$ and ${\mathrm{LaVO}}_3$ are metals even though experimentally they are correlation-driven (“Mott”) insulators. In addition, the FLL double counting implies a splitting between oxygen $p$ and transition metal $d$ levels, which differs from experiment. Introducing into the theory an ad hoc double counting correction, which reproduces the experimentally measured insulating gap leads also to a $p$-$d$ splitting consistent with experiment if the on-site interaction $U$ is chosen in a relatively narrow range ($\sim$$6\pm 1$ eV). The results indicate that these early transition metal oxides will serve as critical test for the formulation of a general ab initio theory of correlated electron metals.