We report on the synthesis and characterization of BiFe${}_{0.5}$Mn${}_{0.5}$O${}_3$, a potential type-I multiferroic compound displaying temperature-induced magnetization reversal. Bulk samples were obtained by means of solid-state reaction carried out under the application of hydrostatic pressure of 6 GPa at 1100 °C. The crystal structure is a highly distorted perovskite with no cation order on the $B$ site, where, besides a complex scheme of tilt and rotations of the TM-O${}_6$ octahedra, large off-centering of the bismuth ions is detected. Below $T_1$ $=$ 420 K the compound undergoes a first weak ferromagnetic transition related to the ordering of iron-rich clusters. At lower temperatures (just below RT) a complex thermally activated mechanism induces at first an enhancement of the magnetization at $T_2$ $=$ 288 K, then a spontaneous reversal giving rise to a negative response. The complementary use of powder neutron diffraction, superconducting quantum interference device magnetometry, and Mössbauer spectroscopy allowed us to propose as a possible interpretation of the overall magnetic behavior the presence of an uncompensated competitive coupling between nonequivalent clusters of weakly ferromagnetic interactions characterized by different critical temperatures and resultant magnetizations.

Dynamical density and spin responses of a two-dimensional fermion system are studied with the self-consistent second-order perturbation theory using a model Hamiltonian. Special attention is paid to the density response at a large wave vector, where it was reported that a sharp mode develops below the particle-hole continuum in two-dimensional liquid ${}^3$He. We find that at a large wave vector, a peak in the dynamical density structure factor shifts to lower energy as interaction sets in, but it is not sharp enough to represent a well-defined mode because of the strong damping effect of the quasiparticles. We point out that it is necessary to treat the large damping effect properly in explaining the propagating mode found in two-dimensional liquid ${}^3$He.

Using density functional theory supplemented with on-site Coulomb $U$ interaction in combination with ab initio molecular dynamics simulations, we investigate the spin-crossover (SCO) properties of a Fe(II) based cyanide-bridged square molecular system, [Fe${}^{\mathrm{II}}$${}_4$($\mu$-CN)${}_4$(bpy)${}_4$(tpa)${}_2$](PF${}_6$)${}_4$ (where bpy $=$ $2,2^{\prime }$-bipyridine and tpa $=$ tris(2-pyridylmethyl)amine], exhibiting a two-step SCO transition. The ab initio calculated SCO temperatures are found to show remarkably good agreement with experimentally measured spin conversion temperatures [M. Nihei et al., Angew. Chem., Int. Ed. 44, 6484 (2005)]. Our theoretical study predicts further chemo switching of the spin state by introduction of guest molecules such as CO${}_2$, CS${}_2$, and H${}_2$O into the porous topology of the system, which would add another dimensionality to this interesting material.

A spin version of dynamical mean-field theory is extended for magnetically ordered states in the Heisenberg model. The self-consistency equations are solved with high numerical accuracy by means of the continuous-time quantum Monte Carlo with bosonic baths coupled to the spin. The resultant solution is critically tested by known physical properties. In contrast with the mean-field theory, soft paramagnons appear near the transition temperature. Moreover, the Nambu-Goldstone mode (magnon) in the ferromagnetic phase is reproduced reasonably well. However, antiferromagnetic magnons have an energy gap in contradiction to the Nambu-Goldstone theorem. The origin of this failure is discussed in connection with the artificial first-order nature of the transition.

We studied field and temperature-driven magnetic reversal of epitaxial spin-flop coupled Fe/MnPd bilayers. The onset temperatures for the spin-flop coupling, the loop shift, and the reversal asymmetry were found to be different for the same sample, indicating their sensitivity to different interface components. We show that the induced uniaxial anisotropy is a direct result of the spin-flop coupling, while the loop shift, observed along the bias, and the reversal asymmetry, measured perpendicular to the bias, are likely relevant to the local pinning environments and the remanence state of the ferromagnet. Further, the unidirectional bias can be manipulated by a controlled recooling process, while keeping the spin-flop property unaffected. Finally, we experimentally reveal the temperature-driven magnetization transitions in the thermal hysteresis of the bilayers and demonstrate the important role of competing energy terms on the observation of different types of thermal hysteresis behaviors.

We study the universal critical behavior of two-dimensional (2D) lattice bosonic gases at the Berezinskii-Kosterlitz-Thouless (BKT) transition, which separates the low-temperature superfluid phase from the high-temperature normal phase. For this purpose, we perform quantum Monte Carlo simulations of the hard-core Bose-Hubbard (BH) model at zero chemical potential. We determine the critical temperature by using a matching method that relates finite-size data for the BH model with corresponding data computed in the classical $XY$ model. In this approach, the neglected scaling corrections decay as inverse powers of the lattice size $L$, and not as powers of $1/\ln L$, as in more standard approaches, making the estimate of the critical temperature much more reliable. Then, we consider the BH model in the presence of a trapping harmonic potential, and we verify the universality of the trap-size dependence at the BKT critical point. This issue is relevant for experiments with quasi-2D trapped cold atoms.

The surface conductivity for conduction electrons with a fixed chirality in a topological insulator with elastic scattering is calculated. The surface excitations are described by the Weyl Hamiltonian. For a zero chemical potential, time reversal symmetry ensures that the integrated Berry phase curvature over the entire Brillouin zone vanishes. When the chemical potential is nonzero, $\mu >0$, the multiple scattering amplitude (for the conduction electrons) changes sign under the time reversal symmetry, $\mathcal{T}(\vec{k},-\vec{k})=-\mathcal{T}(-\vec{k},\vec{k})$; therefore antilocalization emerges. This result is consistent with the integrated Fermi surface Berry curvature of $\pi$. Using this model we compute conductivity as a function of frequency and phase coherence. We compare our results with the Raman line shift measurements for Bi${}_2$Se${}_3$. When the time reversal symmetry is broken the Berry phase curvature as well as the Fermi surface Berry phase are finite and modify the optical and Raman spectra.

A hallmark feature of topologically ordered states of matter is the dependence of ground-state degeneracy (GSD) on the topology of the manifold determined by the global shape of the system. Although the topology of a physical system is practically hard to manipulate, recently, it was shown that in certain topologically ordered phases, topological defects can introduce extra topological GSD. Here the topological defects can be viewed as effectively changing the topology of the physical system. Previous studies have been focusing on two spatial dimensions with pointlike topological defects. In three dimensions, linelike topological defects can appear. They are closed loops in the bulk that can be linked and knotted, effectively leading to complex three-dimensional manifolds in certain topologically ordered states. This paper studies the properties of such line defects in a particular context: the lattice dislocations. We give an analytical construction, together with support from exact numerical calculations, for the dependence of the GSD on dislocations of certain doubled versions of the exactly solvable Kitaev's toric code models in both two and three dimensions. We find that the GSD of the 3D model depends only on the total number of dislocation loops, no matter how they are linked or knotted. The results are extended to ${\mathbb{Z}}_n$ generalizations of the model. Additionally, we consider the phases in which the crystalline orders are destroyed through proliferation of double dislocations. The resulting phases are shown to host topological orders described by non-Abelian gauge theories.

The effect of strain in graphene is usually modeled by a pseudomagnetic vector potential which is, however, derived in the limit of small strain. In realistic cases deviations are expected in view of graphene's very high strain tolerance, which can be up to $25\%$. Here we investigate the pseudomagnetic field generated by a Gaussian bump and we show that it exhibits significant differences with numerical tight-binding results. Furthermore, we calculate the electronic states in the strained region for a hexagon shaped flake with armchair edges. We find that the sixfold symmetry of the wave functions inside the Gaussian bump is directly related to the different effects of strain along the fundamental directions of graphene: zigzag and armchair. Low energy electrons are strongly confined in the armchair directions and are localized on the carbon atoms of a single sublattice.

We develop a theory of spin fluctuations of exciton polaritons in a semiconductor microcavity under nonresonant unpolarized pumping. It is shown that the corresponding spin noise is sensitive to the scattering rates in the system, occupation of the ground state, statistics of polaritons, and interactions. The spin noise spectrum drastically narrows in the polariton lasing regime due to formation of a polariton condensate, while its shape can become non-Lorentzian owing to interaction-induced spin decoherence.

Hard x-ray photoelectron spectroscopy (HAXPES) has been used to study metal-oxide-semiconductor (MOS) structures fabricated with both high (Ni) and low (Al) work-function metals on 8-nm thick Al${}_2$O${}_3$ dielectric layers, deposited on sulfur passivated $n$- and $p$-doped GaAs substrates. A binding energy difference of 0.6 eV was measured between the GaAs core levels of the $n$- and $p$-doped substrates in the absence of gate metals, indicating different Fermi level positions in the band gap. Subsequent photoemission measurements made on the MOS structures with the different work-function metals displayed very limited change in the GaAs core level binding energies, indicating that the movement of the Fermi level at the Al${}_2$O${}_3$/GaAs interface is restricted. Using a combination of HAXPES measurements and theoretical calculations, the Fermi level positions in the band gap have been determined to be in the range of 0.4–0.75 eV and 0.8–1.11 eV above the valence band maximum for $p$- and $n$-type GaAs, respectively. Analysis of capacitance voltage (C-V) measurements on identically prepared samples yield very similar Fermi level positions at zero applied gate bias. The C-V analysis also indicates a higher interface defect density ($D$${}_{it}$) in the upper half of the GaAs bandgap.

The results of experimental study of interference induced magnetoconductivity in narrow quantum well HgTe with a normal energy spectrum are presented. Analysis is performed by taking into account the conductivity anisotropy. It is shown that the fitting parameter ${\tau }_{\varphi }$ corresponding to the phase relaxation time increases in magnitude with the increasing conductivity ($\sigma$) and decreasing temperature following the $1/T$ law. Such a behavior is analogous to that observed in the usual two-dimensional systems with a simple energy spectrum and corresponds to the inelasticity of electron-electron interaction as the main mechanism of the phase relaxation. However, it drastically differs from that observed in the wide HgTe quantum wells with the inverted spectrum, in which ${\tau }_{\varphi }$, being obtained by the same way, is practically independent of $\sigma$. It is presumed that a different structure of the electron multicomponent wave function for the inverted and normal quantum wells could be the reason for such a discrepancy.

In this work we study the effect of the hexagonal warping term on the optical conductivities of topological insulators (TIs). The possible optical signatures in the spectra are also discussed in detail. We begin with systematical analyses of the optical conductivities of the TIs by using Kubo's formula. Then, based on the features of the optical conductivities, we study the possible optical signatures due to the warping term. We explore the reflection spectra from a TI thin film growing on a metamaterial. We find that by properly tuning the refraction index of the metamaterial, all features of the warping effect on the conductivities can be detected and identified from the reflection spectra at an experimentally accessible level. Finally, we discuss the possibility of using the Goos-Hänchen effect and the Brewster angle measurement to determine the possible optical signatures of the warping term on the imperfectly quantized optical Hall conductivity.

The nature of triclinic to orthorhombic phase transition, at which colossal negative thermal expansion is observed, and the magnetic ordering of Bi${}_{1-x}$La${}_x$NiO${}_3$ have been investigated with neutron powder diffraction (NPD) and x-ray absorption spectroscopy techniques. The presence of a charge-transfer transition from (Bi/La)${}^{3+}$${}_{0.5}$Bi${}^{5+}$${}_{0.5}$Ni${}^{2+}$O${}_3$ to (Bi/La)${}^{3+}$Ni${}^{3+}$O${}_3$, accompanied by the simultaneous structural distortion, was confirmed. The NPD data also revealed that magnetic ordering is present only in the insulating triclinic phase. The metallic orthorhombic phase was found to be nonmagnetic down to 10 K.

The question raised recently about the mechanisms of the pressure-induced $B$4-$B$1 transition of GaN is investigated using a density functional-based metadynamics method. A homogeneous deformation path through a tetragonal intermediate structure is found to be energetically favorable under hydrostatic pressure conditions. This transition path is initialized by the soft phonon modes that appear along the transverse acoustic branches near the transition point. However, under certain nonhydrostatic pressure conditions an elastic instability replaces the phonon instability to become a dominant mechanism for the structural transformation of GaN. The phase transition would avoid the tetragonal path when a uniaxial stress is applied on the $c$ axis but follows a different path through a hexagonal intermediate structure. The selection of the transformation mechanisms, determined by specific stress conditions applied, highlights the important role the competition between phonon and elastic instabilities plays in the reconstructive phase transitions.

We report the first principles investigations on the phonons of three polytypes of BaTiO${}_3$ (BTO): paraelectric (PE) cubic $Pm\overline{3}m$ and two ferroelectric (FE) phases, tetragonal $P$4$mm$ and rhombohedral $R$3$m$. The phonon frequencies were calculated using various exchange-correlation functionals, including density functional theory, Hartree-Fock approximation, and their hybrids were reviewed. The pressure-induced interplays between the modes from individual phases were explored by calculating the phonon modes as a function of pressure, $P$ from $-$15 to 230 GPa. The pressure-sensitive modes of the FE phases showed softening and converged to the modes of the PE phase at pressures below $\sim$10 GPa. These results on the FE phases can be interpreted as phonon precursors for a change in symmetry from low- to high-symmetry and partly as a theoretical explanation for the pressure-induced mode-coupling behaviors reported by Sood et al. [Phys. Rev. B 51, 8892 (1995)]. As pressure is applied further beyond $\sim$50 GPa to the cubic PE phase, the lowest $F_{1u}$ mode softens again and diverges into two separate modes of tetragonal FE $P$4$mm$ at above $\sim$150 GPa. These phonon-branching behaviors at high pressures provide a clear reconfirmation of the re-entrant ferroelectricity predicted in earlier papers [Phys. Rev. Lett. 95, 196804 (2005); Phys. Rev. B 74, 180101 (2006); Phys. Rev. B 85, 054108 (2012)]. The high-pressure-re-entrant FE polarization was not found in the rhombohedral structure. Instead, the centrosymmetric $R\overline{3}m$ phase was favored at above $\sim$30 GPa. The phonon modes calculated for the phonon-propagation vectors in the high-symmetry directions show that the $Pm\overline{3}m$ phase exhibits polar instability at the $\Gamma$ point and nonpolar instability at the $X$, $M$, and $R$ points under high pressure.

We investigate field-driven domain-wall dynamics in thin ferromagnetic layers in the precessional regime using an analytical model that goes beyond the rigid wall approximation and takes into account the flexing modes of domain walls. This model allows us to turn on and off the stray field from domains adjacent to the domain wall in order to elucidate its role. We determine the eigenfrequencies of the flexing modes. The domain-wall flexing instabilities are shown to exist even without the stray field. The amplitude of the flexing modes shows a maximum when the magnetization precession frequency is equal to their eigenfrequency. At maximum amplitude of the flexing modes, the domain-wall velocity exhibits bumps even without the stray field. The stray field further enhances the velocity and broadens the bumps. This study is completed by micromagnetic simulations that show excellent agreement with the analytical model. The role of the various spatial modes in the velocity enhancement is clarified. Additional fractional frequencies appear in the time-dependent amplitude of the flexing modes, in analogy with the behavior of parametric oscillators.

Fits were performed to diffuse neutron scattering intensity patterns to determine anisotropic exchange coupling constants for Tb${}_2$Ti${}_2$O${}_7$. A symmetry-preserving map between non-Kramers Tb${}^{3+}$ states and spin-1/2 states is described. Using second-order perturbation theory, the Tb${}^{3+}$ nearest-neighbor exchange model is mapped to a spin-1/2 model, and the anisotropic exchange coupling constants for this model are calculated. Tb${}_2$Ti${}_2$O${}_7$ is found to be a spin-1/2 spin ice with corrections arising from transverse exchange terms.

We study a series of spectra measured in the superconducting state of optimally doped Bi${}_2$Sr${}_2$Ca${}_2$Cu${}_3$O${}_{10+\delta }$ (Bi-2223) by scanning tunneling spectroscopy. Each spectrum, as well as the average of spectra presenting the same gap, is fitted using a strong-coupling model taking into account the band structure, the BCS gap, and the interaction of electrons with the spin resonance. After describing our measurements and the main characteristics of the strong-coupling model, we report the whole set of parameters determined from the fits, and we discuss trends as a function of the gap magnitude. We also simulate angle-resolved photoemission spectra, and compare with recent experimental results.