Using density-functional theory (DFT), we study the growth of pristine and functionalized tin monolayers (Sn-MLs) on three different substrates, CdTe, InSb, and Si(111), and the impact these substrates have on the topological insulating properties of the electronic band structure. The presence of the substrate leads to strain and electronic charge transfer, which cause significant changes in the stability and electronic properties of the supported Sn-ML. Growth of pristine Sn-MLs on Si(111) leads to metallic behavior resembling that of the high-buckled Sn-ML phase; pristine Sn-MLs grown on InSb do not maintain a gap throughout the entire Brillouin zone; and pristine Sn-MLs grown on CdTe are unlikely to exhibit an experimentally observable gap. Provided the charge transfer from the substrate can be compensated, halogen-functionalized Sn-MLs grown on CdTe and InSb are topological insulators, albeit with a reduced band gap compared to their free-standing counterparts (from 0.34 eV for Sn-ML-I to 0.17 eV for InSb-Sn-ML-I). We employ ab initio thermodynamics calculations to study the thermodynamic stability of the halogenated InSb-Sn-MLs and CdTe-Sn-MLs surfaces for a temperature range of 100–1000 K under two extreme environments: ultrahigh vacuum (used in most of the laboratory characterization techniques) and rich-halogen conditions (10% vol. halogen environment). Our results indicate that it is possible to obtain stable topologically insulating Sn-MLs grown epitaxially on lattice-matched substrates.

A concrete strategy is presented for generating strong topological insulators in $d+d^{\prime }$ dimensions which have quantized physics in $d$ dimensions. Here, $d$ counts the physical and $d^{\prime }$ the virtual dimensions. It consists of seeking $d$-dimensional representations of operator algebras which are usually defined in $d+d^{\prime }$ dimensions where topological elements display strong topological invariants. The invariants are shown, however, to be fully determined by the physical dimensions, in the sense that their measurement can be done at fixed virtual coordinates. We solve the bulk-boundary correspondence and show that the boundary invariants are also fully determined by the physical coordinates. We analyze the virtual Chern insulator in $1+1$ dimensions realized in Y. E. Kraus et al., Phys. Rev. Lett. 109, 106402 (2012) and predict quantized forces at the edges. We generate a topological system in $(3+1)$ dimensions, which is predicted to have quantized magnetoelectric response.

At low temperatures, indirect excitons formed at the in-plane electron-hole interface in a coupled-quantum-well structure undergo a spontaneous transition into a spatially modulated state. We report on the control of the instability wavelength, measurement of the dynamics of the exciton emission pattern, and observation of the fluctuation and commensurability effect of the exciton density wave. We found that fluctuations are suppressed when the instability wavelength is commensurate with defect separation along the exciton density wave. The commensurability effect is also found in numerical simulations within the model describing the exciton density wave in terms of an instability due to stimulated processes.

We present a first-principles study of effects of small biaxial strain $\left(\left|\varepsilon \right|\le 5\%\right)$ and perpendicular electric field $(E$-field) on the electronic and phonon properties of low-buckled silicene and germanene. With an increase of the biaxial strain, the conduction bands at the high-symmetric $\mathrm{\Gamma }$ and $M$ points of the first Brillouin zone shift significantly towards the Fermi level in both silicene and germanene. In contrast, the $E$-field changes the band dispersions near the $\mathrm{\Gamma }$ and open a small band gap at the $K$ point in silicene. We found that the field-induced gap opening in silicene could be enhanced by a compressive strain while mitigated by a tensile strain. This result highlights the tunability of the electronic structure of silicene by combining mechanical strain and electric field.

The magnetoelectric antiferromagnet $\alpha \text{−}\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_3$ (chromia) is known to possess a roughness insensitive net equilibrium magnetization at the (0001) surface, called boundary magnetization (BM), which is coupled to the bulk antiferromagnetic order parameter. In order to verify whether this symmetry sensitive BM persists in alloys, we investigate the impact of diamagnetic dilution on chromia thin films alloyed with the isostructural $\alpha \text{−}\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ (alumina). Single-crystalline $\mathrm{C}{\mathrm{r}}_{2-x}\mathrm{A}{\mathrm{l}}_x{\mathrm{O}}_3$ thin films with (0001) surface orientation and varying stoichiometry have been grown by sputter codeposition in the concentration range between $x=0$ and $x=0.6$. For these samples, we find the corundum crystal structure, the antiferromagnetic ordering, and the boundary magnetization to be preserved. We also find that the critical temperature $T_{\mathrm{N}}$ can be tuned by alloying with $\alpha \text{−}\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$, using the BM as a probe to study the magnetic phase transition. Furthermore, we were able to evaluate the critical exponent and the absolute BM values for different samples. Both properties corroborate that the observed magnetic signals originate from the BM rather than the bulk of the samples.

The temperature in a ferromagnetic nanostripe with a notch subject to Joule heating has been studied in detail. We first performed an experimental real-time calibration of the temperature versus time as a 100 ns current pulse was injected into a Permalloy nanostripe. This calibration was repeated for different pulse amplitudes and stripe dimensions and the set of experimental curves were fitted with a computer simulation using the Fourier thermal conduction equation. The best fit of these experimental curves was obtained by including the temperature-dependent behavior of the electrical resistivity of the Permalloy and of the thermal conductivity of the substrate $\left(\mathrm{Si}{\mathrm{O}}_2\right)$. Notably, a nonzero interface thermal resistance between the metallic nanostripe and the substrate was also necessary to fit the experimental curves. We found this parameter pivotal to understand our results and the results from previous works. The higher current density in the notch, together with the interface thermal resistance, allows a considerable increase of the temperature in the notch, creating a large horizontal thermal gradient. This gradient, together with the high temperature in the notch and the larger current density close to the edges of the notch, can be very influential in experiments studying the current assisted domain wall motion.

We present a theory of the dc Josephson effect in contacts between Fe-based and spin-singlet $s$-wave superconductors. The method is based on the calculation of temperature Green's function in the junction within the tight-binding model. We calculate the phase dependencies of the Josephson current for different orientations of the junction relative to the crystallographic axes of Fe-based superconductor. Further, we consider the dependence of the Josephson current on the thickness of an insulating layer and on temperature. Experimental data for ${\mathrm{PbIn}/\mathrm{Ba}}_{1-x}{\mathrm{K}}_x({\mathrm{FeAs})}_2$ point-contact Josephson junctions are consistent with theoretical predictions for $s_{\pm }$ symmetry of an order parameter in this material. The proposed method can be further applied to calculations of the dc Josephson current in contacts with other new unconventional multiorbital superconductors, such as ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ and the superconducting topological insulator ${\mathrm{Cu}}_x{\mathrm{Bi}}_2{\mathrm{Se}}_3$.

The observation of an anomalous quadrupolar signal in x-ray magnetic circular dichroism (XMCD) at the Fe $K$-edge of iron phthalocyanine (FePc) films is reported. All ground states previously suggested for FePc are incompatible with the experimental data. Based on ab initio molecular orbital multiplet calculations of the isolated FePc molecule, we propose a model for the magnetic ground state of the FePc film that explains the XMCD data and reproduces the observed values of the orbital moments in the perpendicular and planar directions.

We address the question of whether individual nonmagnetic impurities can induce zero-energy states in time-reversal-invariant topological superconductors, and define a class of symmetries which guarantee the existence of such states for a specific value of the impurity strength. These symmetries allow the definition of a position-space topological ${\mathbb{Z}}_2$ invariant, which is related to the standard bulk topological ${\mathbb{Z}}_2$ invariant. Our general results are applied to the time-reversal-invariant $p$-wave phase of the doped Kitaev-Heisenberg model, where we demonstrate how a lattice of impurities can drive a topologically trivial system into the nontrivial phase.

We present a comprehensive study on ${\mathrm{PrCaFeO}}_4$ using macroscopic methods, neutron and x-ray diffraction, as well as inelastic neutron scattering. One polycrystalline and two single-crystal samples were investigated exhibiting structural phase transitions from a high-temperature tetragonal phase to an intermediate orthorhombic phase (space group $Bmeb$) at $510{}^{\circ }\mathrm{C}$ (783 K). At approximately 240 K a second structural phase transition takes place into the space group $Pccn$ where the tilt axis of the ${\mathrm{FeO}}_6$ octahedra changes from the [100] to the $\langle 110\rangle$ directions. Due to strong diffuse scattering at high temperatures neutron powder diffraction can only safely state that $T_N$ is above 330 K. ${\mathrm{PrCaFeO}}_4$ exhibits a magnetic spin-flop phase transition where the magnetic moments turn from the $b$ axis to the $c$ axis upon cooling. However, the transition temperatures and the width of this magnetic transition are strikingly different between the investigated samples, suggesting a strong influence from the real structure. Indeed, a significant difference in the oxygen content was deduced by single-crystal x-ray diffraction. The magnon dispersion was studied by inelastic neutron scattering revealing a nearest-neighbor interaction comparable to that in ${\mathrm{LaSrFeO}}_4$ but with smaller anisotropy gaps. A clear interlayer dispersion was observed resulting from the structural distortions and the relief of geometrical frustration due to the orthorhombic splitting.

Using Monte Carlo simulations, we explore the magnetic phase diagram of triangular spin tubes coupled with a ferromagnetic intertube interaction for ${\mathrm{CsCrF}}_4$. The planar structure of the coupled tubes is topologically equivalent to the kagome-triangular lattice, which induces nontrivial frustration effects in the system. We particularly find that, depending on the intertube coupling, various ordered phases are actually realized, such as incommensurate order, ferromagnetic order, and cuboc order, which is characterized by the noncoplanar spin structure of the 12 sublattices accompanying the spin chirality breaking. We also discuss the relevance of the results to recent experiments on ${\mathrm{CsCrF}}_4$.

We investigate the Kondo effect in a quadruple-quantum-dot device of coupled double quantum dots (DQDs), which simultaneously contain intra-DQD and inter-DQD coupling. A variety of novel behaviors are observed. The differential conductance dI/dV is measured in the upper DQDs as a function of source drain bias. It is found to exhibit multiple peaks, including a zero-bias peak, where the number of peaks exceeds five. Alternatively, tuning the lower DQDs yielded regions of four peaks. In addition, a Kondo effect switcher is demonstrated, using the lower DQDs as the controller.

In metallic ferromagnets, the spin-transfer torque and spin-motive force are known to exhibit a reciprocal relationship. Recent experiments on ferromagnets with strong spin-orbit coupling have revealed a rich complexity in the interaction between itinerant charge carriers and magnetization, but a full understanding of this coupled dynamics is lacking. Here, we develop a general phenomenology of the two reciprocal processes of charge pumping by spin-motive forces and current-driven magnetization dynamics. The formalism is valid for spin-orbit coupling of any strength and presents a systematic scheme for deriving all possible torque and charge-pumping terms that obey the symmetry requirements imposed by the point group of the system. We demonstrate how the different charge pumping and torque contributions are connected via the Onsager reciprocal relations. The formalism is applied to two important classes of systems: isotropic ferromagnets with nonuniform magnetization and homogeneous ferromagnets described by the point group $C_{2v}$.

The longitudinal and transverse ultrasound speeds and attenuation were measured in a MnSi single crystal in the temperature range of 2–40 K and magnetic fields up to 7 Tesla. The magnetic phase diagram of MnSi in applied magnetic field appears to depend on the experimental setups, which is related to a difference in demagnetization factors arising due to the disk shape of the sample. The magnetic phase transition in MnSi in zero magnetic field is signified by a quasidiscontinuity in the $c_{11}$ elastic constant, which varies significantly with magnetic field. It is notable that the region where the $c_{11}$ discontinuity almost vanishes closely corresponds to the extent of skyrmion phase along the magnetic to paramagnetic transition. This implies that the $c_{11}$ elastic constant is almost continuous through the transition from the skyrmion to paramagnetic phases. A recovery of the discontinuity of $c_{11}$ and enhanced sound absorption occur at the crossing of the phase transition line and the line of minima in $c_{11}$. The powerful fluctuations at the minima of $c_{11}$ make the mentioned crossing point similar to a critical end point, where a second order phase transition meets a first order one.

We studied the finite-size effects on the magnetic behavior of the quasi-one-dimensional spin $S=\frac{1}{2}$ Heisenberg antiferromagnets ${\text{Sr}}_2{\text{CuO}}_3$, ${\mathrm{Sr}}_2{\mathrm{Cu}}_{0.99}{M}_{0.01}{\mathrm{O}}_3$ ($M=\text{Zn}$ and Ni), and ${\text{SrCuO}}_2$. Magnetic susceptibility data were analyzed to estimate the concentration of chain breaks due to extrinsically doped defects and/or due to slight oxygen off-stoichiometry. We show that the susceptibility of ${\text{Sr}}_2{\text{Cu}}_{0.99}{\text{Ni}}_{0.01}{\text{O}}_3$ can be described by considering ${\text{Ni}}^{2+}$ as a scalar defect ($S_{\mathrm{eff}}=0$) indicating that the Ni spin is screened. In ${\text{Sr}}_2{\text{Cu}}_{0.99}{\text{Zn}}_{0.01}{\text{O}}_3$ susceptibility analysis yields a defect concentration smaller than the nominal value which is in good qualitative agreement with crystal growth experiments. Influence of doping on the low-temperature long-range spin ordered state is studied. In the compound ${\text{SrCuO}}_2$, consisting of zigzag $S=\frac{1}{2}$ chains, the influence of spin frustration on the magnetic ordering and the defect concentration determined from the susceptibility data is discussed.

We study the “Higgs” amplitude mode in the relativistic quantum $\mathrm{O}\left(N\right)$ model in two space dimensions. Using the nonperturbative renormalization group and the Blaizot–Méndez-Galain–Wschebor approximation (which we generalize to compute four-point correlation functions), we compute the $\mathrm{O}\left(N\right)$-invariant scalar susceptibility at zero temperature in the vicinity of the quantum critical point. In the ordered phase, we find a well-defined Higgs resonance for $N=2$ and 3 and determine its universal properties. No resonance is found for $N\ge 4$. In the disordered phase, the spectral function exhibits a threshold behavior with no Higgs-like peak. We also show that for $N=2$, the Higgs mode manifests itself as a very broad peak in the longitudinal susceptibility in spite of the infrared divergence of the latter. We compare our findings with results from quantum Monte Carlo simulations and $\epsilon =4-(d+1)$ expansion near $d=3$.

Quantum Hall systems have recently been shown to possess a quantity sensitive to the spatial geometry and topology of the system, dubbed the Hall viscosity ${\eta }_H$. Despite the extensive theoretical discussions on its properties, the question of how to measure ${\eta }_H$ still poses a challenge. In this paper, we present a general relation between Hall viscosity and susceptibility for systems with Galilean invariance. Thus, it allows for determination of ${\eta }_H$ through density response signatures. The relations are verified in the integer quantum Hall example and are further illustrated in an effective hydrodynamic analysis. Since the derivation is based on Kubo formulas and assumes no more than conservation laws and translational symmetry, the results are applicable to a wide range of systems.

Using first-principles density functional theory calculations, we investigated the effect of charge doping in a ${\mathrm{LaNiO}}_3/{\mathrm{SrTiO}}_3$ superlattice. The detailed analysis based on two different doping simulation methods clearly shows that the electronic and structural properties change in a systematic way, and that the orbital polarization (i.e., relative occupation of two $\mathrm{Ni}-e_g$ orbitals) is reduced and the Ni to apical oxygen distance is enlarged as the number of doped electrons increases. Also, the rotation angles of the ${\mathrm{NiO}}_6/{\mathrm{TiO}}_6$ octahedra strongly and systematically depend on the doping. Our results not only suggest a possible way to control the orbital and structural properties by means of charge doping, but also provide useful information for understanding experiments under various doping situations such as oxygen vacancy.

We investigate the ground-state properties of monolayer ${\mathrm{MoS}}_2$ incorporating the Coulomb interaction together with a short-range intervalley interaction between charged particles between two valleys within the Hartree-Fock approximation. We consider four variables as independent parameters, namely, homogeneous charge (electron or hole) density, averaged dielectric constant, spin degree of freedom, and, finally, the Hubbard repulsion coefficient, which originates mostly from $4d$ orbits of Mo atoms. We find the electronic charge compressibility within the mean-field approximation and show that nonmonotonic behavior of the compressibility as a function of carrier density which is rather different from those of the two-dimensional electron gas. We also explore a paramagnetic-to-ferromagnetic quantum phase transition for the wide range of the electron density in the parameter space.