The structure and dynamics of cleaved $\left(001\right)$ surfaces of potassium tantalates doped with niobium, $\mathrm{KT}{\mathrm{a}}_{1-x}\mathrm{N}{\mathrm{b}}_x{\mathrm{O}}_3$ (KTN), with $x$ ranging from $0\%$ to $30\%$, were measured by helium atom scattering (HAS). Through HAS time-of-flight (TOF) experiments, a dispersionless branch (Einstein phonon branch) with energy of $13-14\mathrm{meV}$ was observed across the surface Brillouin zone in all samples. When this observation is combined with the results from earlier experimental and theoretical studies on these materials, a consistent picture of the stable surface structure emerges: After cleaving the single-crystal sample, the surface should be composed of equal areas of KO and $\mathrm{Ta}{\mathrm{O}}_2/\mathrm{Nb}{\mathrm{O}}_2$ terraces. The data, however, suggest that ${\mathrm{K}}^{+}$ and ${\mathrm{O}}^{2-}$ ions migrate from the bulk to the surface, forming a charged KO lattice that is neutralized primarily by additional ${\mathrm{K}}^{+}$ ions bridging pairs of surface oxygens. This structural and dynamic modification at the $\left(001\right)$ surface of KTN appears due to its formally charged $\mathrm{KO}(-1)$ and $\mathrm{Ta}{\mathrm{O}}_2/\mathrm{Nb}{\mathrm{O}}_2(+1)$ layers and avoids a “polar catastrophe.” This behavior is contrasted with the $\left(001\right)$ surface behavior of the fluoride perovskite $\mathrm{KMn}{\mathrm{F}}_3$ with its electrically neutral KF and $\mathrm{Mn}{\mathrm{F}}_2$ layers.

An inelastic neutron scattering study of the spin waves corresponding to the stripe antiferromagnetic order in insulating ${\mathrm{Rb}}_{0.8}{\mathrm{Fe}}_{1.5}{\mathrm{S}}_2$ throughout the Brillouin zone is reported. The spin wave spectra are well described by a Heisenberg Hamiltonian with anisotropic in-plane exchange interactions. Integrating the ordered moment and the spin fluctuations results in a total moment squared of $27.6\pm 4.2{\mu }_B^2/\text{Fe}$, consistent with $S\approx 2$. Unlike $X{\mathrm{Fe}}_2{\mathrm{As}}_2$ ($X=\text{Ca}$, Sr, and Ba), where the itinerant electrons have a significant contribution, our data suggest that this stripe antiferromagnetically ordered phase in ${\mathrm{Rb}}_{0.8}{\mathrm{Fe}}_{1.5}{\mathrm{S}}_2$ is a Mott-like insulator with fully localized $3d$ electrons and a high-spin ground state configuration. Nevertheless, the anisotropic exchange couplings appear to be universal in the stripe phase of Fe pnictides and chalcogenides.

We have investigated polycrystalline samples of the zigzag chain system ${\mathrm{BaTb}}_2{\mathrm{O}}_4$ with magnetic susceptibility, heat capacity, neutron powder diffraction, and muon spin relaxation $\left(\mu \mathrm{SR}\right)$. No magnetic transitions are observed in the bulk measurements, while neutron diffraction reveals the presence of low-temperature, short-range, intrachain magnetic correlations between ${\text{Tb}}^{3+}$ ions. $\mu \mathrm{SR}$ indicates that these correlations are dynamic, as no signatures of static magnetism are detected by the technique down to 0.095 K. These combined findings provide strong evidence for a spin-liquid ground state in ${\mathrm{BaTb}}_2{\mathrm{O}}_4$.

It has been suggested that materials which break spatial inversion symmetry, but not time reversal symmetry, will be optically gyrotropic and display a nonlocal Hall effect. The associated optical rotary power and the suggested possibility of inducing a Kerr effect in such materials, in turn, are central to recent discussions about the nature of the pseudogap phases of various cuprate high-temperature superconductors. In this Rapid Communication, we show that optical gyrotropy and the nonlocal Hall effect provide a sensitive probe of broken inversion symmetry in $1T\text{−}{\mathrm{TiSe}}_2$. This material was recently found to possess a chiral charge-ordered phase at low temperatures, in which inversion symmetry is spontaneously broken, while time reversal symmetry remains unbroken throughout its phase diagram. We estimate the magnitude of the resulting gyrotropic constant and optical rotary power (the Faraday effect at zero applied field) and suggest that $1T\text{−}{\mathrm{TiSe}}_2$ may be employed as a model material in the interpretation of measurements on cuprate superconductors.

We study a one-dimensional (1D) system with a power-law quasiparticle dispersion ${\propto \left|k\right|}^{\alpha }sgnk$ in the presence of a short-range-correlated random potential, and demonstrate that for $\alpha <1/2$ it exhibits a disorder-driven quantum phase transition with critical properties similar to those of the localization transition near the edge of the band of a semiconductor in high dimensions, as studied recently [Phys. Rev. Lett. 114, 166601 (2015); Phys. Rev. B 91, 035133 (2015)]. Despite the absence of localization in the considered 1D system, the disorder-driven transition manifests itself, for example, in a critical form of the disorder-averaged density of states. We confirm the existence of the transition by numerical simulations and find the critical exponents and the critical disorder strength as a function of $\alpha$. The proposed system thus presents a convenient platform for numerical studies of the recently predicted unconventional high-dimensional localization effects and has the potential for experimental realizations in chains of ultracold atoms in optical traps.

The layered compound ${\mathrm{CePd}}_{1-x}{\mathrm{Bi}}_2$ with the tetragonal ${\mathrm{ZrCuSi}}_2$-type structure was obtained from excess Bi flux. Magnetic susceptibility data of ${\mathrm{CePd}}_{1-x}{\mathrm{Bi}}_2$ show an antiferromagnetic ordering below 6 K and are anisotropic along the $c$ axis and the $ab$ plane. The anisotropy is attributed to crystal-electric-field (CEF) effects and a CEF model which is able to describe the susceptibility data is given. An enhanced Sommerfeld coefficient $\gamma$ of 0.191 J mol $\mathrm{Ce}{}^{-1} \mathrm{K}{}^{-2}$ obtained from specific-heat measurement suggests a moderate Kondo effect in ${\mathrm{CePd}}_{1-x}{\mathrm{Bi}}_2$. Other than the antiferromagnetic peak at 6 K, the resistivity curve shows a shoulderlike behavior around 75 K which could be attributed to the interplay between Kondo and CEF effects. Magnetoresistance and Hall-effect measurements suggest that the interplay reconstructs the Fermi-surface topology of ${\mathrm{CePd}}_{1-x}{\mathrm{Bi}}_2$ around 75 K. Electronic structure calculations reveal that the Pd vacancies are important to the magnetic structure and enhance the CEF effects which quench the orbital moment of Ce at low temperatures.

In the derivation of low-energy effective models for solids targeting the bands near the Fermi level, the constrained random-phase approximation (cRPA) has become an appreciated tool to compute the effective interactions. The Wick-ordered constrained functional renormalization group (cfRG) generalizes the cRPA approach by including all interaction channels in an unbiased way. Here we present applications of the cfRG to two simple multiband systems and compare the resulting effective interactions to the cRPA. First, we consider a multiband model for monolayer graphene, where we integrate out the $\sigma$ bands to get an effective theory for $\pi$ bands. It turns out that terms beyond cRPA are strongly suppressed by the different $xy$-plane reflection symmetry of the bands. In our model the cfRG corrections to cRPA become visible when one disturbs this symmetry difference slightly, however, without qualitative changes. This study shows that the embedding or layering of two-dimensional electronic systems can alter the effective interaction parameters beyond what is expected from screening considerations. The second example is a one-dimensional model for a diatomic system reminiscent of a CuO chain, where we consider an effective theory for Cu $3d$-like orbitals. Here the fRG data shows relevant and qualitative corrections compared to the cRPA results. We argue that the new interaction terms affect the magnetic properties of the low-energy model.

Weyl semimetals are three-dimensional topological states of matter, in a sense that they host paired monopoles and antimonopoles of Berry curvature in momentum space, leading to the chiral anomaly. The chiral anomaly has long been believed to give a positive magnetoconductivity or negative magnetoresistivity in strong and parallel fields. However, several recent experiments on both Weyl and Dirac topological semimetals show a negative magnetoconductivity in high fields. Here, we study the magnetoconductivity of Weyl and Dirac semimetals in the presence of short-range scattering potentials. In a strong magnetic field applied along the direction that connects two Weyl nodes, we find that the conductivity along the field direction is determined by the Fermi velocity, instead of by the Landau degeneracy. We identify three scenarios in which the high-field magnetoconductivity is negative. Our findings show that the high-field positive magnetoconductivity may not be a compelling signature of the chiral anomaly and will be helpful for interpreting the inconsistency in the recent experiments and earlier theories.

$\mathrm{Ti}{\mathrm{O}}_2$ is an oxygen-deficient, intrinsically $n$-type material, but it is often debated whether the electrons are donated by oxygen vacancies $\left({\mathrm{V}}_{\mathrm{O}}\right)$ or titanium interstitials $\left(\mathrm{T}{\mathrm{i}}_{\mathrm{i}}\right)$. Investigating this issue is complicated by the fact that rutile can self-trap electrons in intrinsic small polaron states, while bulk anatase cannot. The screened hybrid functional HSE06 was proven to account for this phenomenon and has provided quantitatively correct results for ${\mathrm{V}}_{\mathrm{O}}$ in our earlier study. Here, we use it for $\mathrm{T}{\mathrm{i}}_{\mathrm{i}}$ in both rutile and anatase, allowing full spin and symmetry freedom, to shed light on the similarities and differences to ${\mathrm{V}}_{\mathrm{O}}$. We find that these two defects give rise to very similar fingerprints in electron paramagnetic resonance, infrared absorption, or photoelectron spectra. In weakly reduced rutile, the ground state of both defects is $(2+)$, with two electrons in polaronic traps, bound loosely to the defect. Most of the time, only these latter states (crudely resembling a hydrogenic series, with increasing distance from the defect) are likely to be detected. In anatase, both ${\mathrm{V}}_{\mathrm{O}}$ and $\mathrm{T}{\mathrm{i}}_{\mathrm{i}}$ can be expected to be ionized at room temperature (singly and doubly, respectively), and the next vertical ionization energy is similar in the two defects—and very close to the ionization energy of the bound polarons in rutile. Most signals in paramagnetic resonance experiments on rutile must also be related to the polaron states, and, in general, very special conditions have to be fulfilled to detect electrons localized to ${\mathrm{V}}_{\mathrm{O}}$ or $\mathrm{T}{\mathrm{i}}_{\mathrm{i}}$ itself. We show that, in thermal equilibrium, the dominant defect in intrinsic samples is ${\mathrm{V}}_{\mathrm{O}}$, and $\mathrm{T}{\mathrm{i}}_{\mathrm{i}}$ can be the majority defect only in strongly reduced anatase, or in case of $p$-type doping.

We present the study of the spin-lattice relaxation of an isolated ${\mathrm{Mn}}^{2+}$ ion in a single CdTe/ZnTe quantum dot. The measurements are done in a wide range of magnetic field. The spin-lattice relaxation rate is determined in a time resolved experiment. The ion spin state is driven out of equilibrium using optical orientation of the ${\mathrm{Mn}}^{2+}$ spin in a system of two coupled dots. Then the light is switched off and the ${\mathrm{Mn}}^{2+}$ ion spin relaxes. The ${\mathrm{Mn}}^{2+}$ spin state is measured after switching the light on again. We discuss the magnetic field dependence of the spin-relaxation rate in light of two theoretical models: one based on scattering of transverse acoustic phonons in the presence of a finite uniaxial ${\mathrm{Mn}}^{2+}$ spin anisotropy, and the second relying on presence of quantum dot charge state fluctuations.

We used density functional theory (DFT) calculations to model the interaction of hydrogen atoms and molecules with strained bonds and neutral oxygen vacancies in amorphous silica $\left({\text{a-SiO}}_2\right)$. The results demonstrate that the interaction of atomic hydrogen with strained Si–O bonds in defect-free ${\text{a-SiO}}_2$ networks results in the formation of two distinct defect structures, which are referred to as the ${[{\text{SiO}}_4/\text{H}]}^0$ and the hydroxyl ${\text{E}}^{\prime }$ center. To study the distribution of each defect's properties, up to 116 configurations of each center were calculated. We show that the hydroxyl $E^{\prime }$ center can be thermodynamically stable in the neutral charge state. In order to understand the origins and reactions of this defect, different mechanisms of formation, passivation, and depassivation have been investigated. The interaction of H with a single-oxygen vacancy in ${\text{a-SiO}}_2$ was studied in 144 configurations, all resulting in the hydrogen bridge defect. The reaction of the hydrogen bridge defect with the second H atom is barrierless and fully passivates the O vacancy. The latter defect reacts with atomic H with a small barrier, restoring the hydrogen bridge defect. These results provide a better understanding of how atomic and molecular hydrogen can both passivate existing defects and create new electrically active defects in amorphous-silica matrices.

The effect of disorder on magnonic transport in low-dimensional magnetic materials is studied in the framework of a classical spin model. Numerical investigations give insight into scattering properties of the systems and show the existence of Anderson localization in one dimension and weak localization in two dimensions, potentially affecting the functionality of magnonic devices.

Inelastic neutron scattering is used to study the finite-temperature scaling behavior of the local dynamic structure factor in the quasi-one-dimensional quantum antiferromagnet $\text{Ni}[N,N^{\prime }-\text{bis}(\text{3-aminopropyl}\left)\text{propane-1,3-diamine}\right](\mu \text{−}{\mathrm{NO}}_2){\mathrm{ClO}}_4$ (NTENP), at its field-induced Ising quantum critical point. The validity and the limitations of the theoretically predicted scaling relations are tested.

We report the results of a study on the crystal and magnetic structure of ${\mathrm{Li}}_2\mathrm{Ni}({\mathrm{WO}}_4$)${}_2$ with a neutron diffraction technique. The ${\mathrm{Ni}}^{2+}$ spins of S = 1 for ${\mathrm{NiO}}_6$ octahedra are coupled via corner-sharing, nonmagnetic double tungstate groups in a super-superexchange route. Two magnetic anomalies at $T_{N1}\sim$ 18 K and $T_{N2}\sim$ 13 K are revealed from the measured magnetic susceptibility $\chi$(T), and $T_{N2}$ is confirmed to be the onset of a commensurate long-range antiferromagnetic (AF) ordering through neutron diffraction. A negative thermal expansion phenomenon is observed below $T_{N2}$, which has been interpreted as a result of competing normal thermal contraction and long-range AF spin ordering through counterbalanced ${\mathrm{WO}}_4$ and ${\mathrm{NiO}}_6$ polyhedral local distortion. The AF spin structure has been modeled and used to show that Ni spins with a saturated magnetic moment of $\sim 1.90\left(27\right){\mu }_B$ that lies in the $a\text{−}c$ plane approximately ${46}^{\circ }(\pm {10}^{\circ })$ off the $a$ axis. The experimental results are compared and found to be consistent with theoretical calculations using density-functional theory with a generalized gradient approximation plus on-site Coulomb interaction.

We performed quantum manipulations of the multilevel spin system $S=5/2$ of a ${\mathrm{Mn}}^{2+}$ ion, by means of a two-tone pulse drive. The detuning between the excitation and readout radio frequency pulses allows one to select the number of photons involved in a Rabi oscillation as well as increase the frequency of this nutation. Thus detuning can lead to a resonant multiphoton process. Our analytical model for a two-photon process as well as a numerical generalization fit well the experimental findings, with implications for the use of multilevel spin systems as tunable solid state qubits.

We propose a way to control the Josephson energy of a single Josephson junction embedded in a one-dimensional superconducting metamaterial: an inhomogeneous superconducting loop, made out of a superconducting nanowire or a chain of Josephson junctions. The Josephson energy is renormalized by the electromagnetic modes propagating along the loop. We study the behavior of the modes as well as of their frequency spectrum when the capacitance and the inductance along the loop are spatially modulated. We show that, depending on the amplitude of the modulation, the renormalized Josephson energy is either larger or smaller than the one found for a homogeneous loop. Using typical experimental parameters for Josephson junction chains and superconducting nanowires, we conclude that this mode engineering can be achieved with currently available metamaterials.

We propose an accurate variational Monte Carlo method applicable in the presence of the strong spin-orbit interactions. The algorithm is applicable even in a wider class of Hamiltonians that do not have the spin-rotational symmetry. Our variational wave functions consist of generalized Pfaffian-Slater wave functions that involve mixtures of singlet and triplet Cooper pairs, Jastrow-Gutzwiller-type projections, and quantum number projections. The generalized wave functions allow describing states including a wide class of symmetry-broken states, ranging from magnetic and/or charge ordered states to superconducting states and their fluctuations, on equal footing without any ad hoc ansatz for variational wave functions. We detail our optimization scheme for the generalized Pfaffian-Slater wave functions with complex-number variational parameters. Generalized quantum number projections are also introduced, which imposes the conservation of not only the momentum quantum number but also Wilson loops. As a demonstration of the capability of the present variational Monte Carlo method, the accuracy and efficiency is tested for the Kitaev and Kitaev-Heisenberg models, which lack the SU(2) spin-rotational symmetry except at the Heisenberg limit. The Kitaev model serves as a critical benchmark of the present method: The exact ground state of the model is a typical gapless quantum spin liquid far beyond the reach of simple mean-field wave functions. The newly introduced quantum number projections precisely reproduce the ground state degeneracy of the Kitaev spin liquids, in addition to their ground state energy. An application to a closely related itinerant model described by a multiorbital Hubbard model with the spin-orbit interaction also shows promising benchmark results. The strong-coupling limit of the multiorbital Hubbard model is indeed described by the Kitaev model. Our framework offers accurate solutions for the systems where strong electron correlation and spin-orbit interaction coexist.

It is known that a nucleus with charge $Ze$, where $Z>170$, creates electron-positron pairs from the vacuum. Electrons collapse onto the nucleus resulting in a net charge $Z_n<Z$ while the positrons are emitted. This effect is due to the relativistic dispersion law. The same reason leads to the collapse of electrons to the donor cluster with a large charge number $Z$ in narrow-band-gap semiconductors, Weyl semimetals, and graphene. In this paper, a similar effect of electron collapse and charge renormalization is found for a donor cluster in ${\text{SrTiO}}_3$ (STO), but with a different origin. At low temperatures, STO has an enormously large dielectric constant and the nonlinear dielectric response becomes dominant when the electric field is still small. This leads to the collapse of electrons into a charged spherical donor cluster with radius $R$ when its total charge number $Z$ exceeds a critical value $Z_c\simeq R/a$ where $a$ is the lattice constant. The net charge $Z_{n}e$ grows with $Z$ until $Z$ exceeds $Z^{*}\simeq {(R/a)}^{9/7}$. After this point, the charge of the compact core $Z_n$ remains $\simeq Z^{*}$, while the rest $Z^{*}$ electrons form a sparse Thomas-Fermi electron atmosphere around it. We show that the thermal ionization of such two-scale atoms easily strips the outer atmosphere while the inner core remains preserved. We extend our results to the case of long cylindrical clusters. We discuss how our predictions can be tested by measuring the conductivity of chains of disks of charge on the STO surface.

The carbon nanoscroll is a unique topologically open configuration of graphene nanoribbons possessing outstanding properties and application perspectives due to its morphology. However, the molecular dynamics study of nanoscrolls with more than a few coils is limited by computational power. Here, we propose a simple model of the molecular chain moving in the plane, allowing us to describe the folded and rolled packaging of long graphene nanoribbons. The model is used to describe a set of possible stationary states and the low-frequency oscillation modes of isolated single-layer nanoribbon scrolls as a function of the nanoribbon length. Possible conformational changes of scrolls due to thermal fluctuations are analyzed and their thermal stability is examined. Using the full-atomic model, the frequency spectrum of thermal vibrations is calculated for the scroll and compared to that of the flat nanoribbon. It is shown that the density of phonon states of the scroll differs from the one of the flat nanoribbon only in the low $(\omega <100$ ${\mathrm{cm}}^{-1})$ and high $(\omega >1450$ ${\mathrm{cm}}^{-1})$ frequency ranges. Finally, the linear thermal expansion coefficient for the scroll outer radius is calculated from the long-term dynamics with the help of the developed planar chain model. The scrolls demonstrate an anomalously high coefficient of thermal expansion and this property can find new applications.

Using the tight-binding formalism, we study spin and charge transport through a zigzag silicene ribbon subject to an external electric field $E_z$. The effect of an exchange field $M_z$ is also taken into account and its consequences on the band structure as well as spin transport are evaluated. We show that the band structure lacks spin inversion symmetry in the presence of intrinsic spin-orbit interaction in combination of $E_z$ and $M_z$ fields. Our quantum transport calculations indicate that for certain energy ranges of the incoming electrons the silicene ribbon can act as a controllable high-efficiency spin polarizer. The polarization maxima occur simultaneously with the van Hove singularities of the local density of states. In this case, the combination of electric and exchange fields is the key to achieving nearly perfect spin polarization, which also leads to the appearance of additional narrow plateaus in the quantum conductance. Moreover, we demonstrate that the output current still remains completely spin-polarized for low-energy carriers even when a few edge vacancies are present.