In this Rapid Communication, we present the order of the phonon modes and the appearance of the reststrahlen bands for monoclinic symmetry materials with polar lattice vibrations. Phonon modes occur in associated pairs of transverse and longitudinal optical modes, and pairs either belong to inner or outer phonon modes. Inner modes are nested within outer modes. Outer modes cause polarization-dependent reststrahlen bands. Inner modes cause polarization-independent reststrahlen bands. The directional limiting frequencies within the Born-Huang approach are bound to within outer mode frequency regions not occupied by inner mode pairs. Hence, an unusual phonon mode order can occur where both lower-frequency as well as upper-frequency limits for the directional modes can be both transverse and/or longitudinal modes. We exemplify our findings using experimental data for the recently unraveled case of monoclinic symmetry $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ [Phys. Rev. B 93, 125209 (2016)] and demonstrate excellent agreement with results from density functional theory calculations.

We show that flat bands can be categorized into two distinct classes, that is, singular and nonsingular flat bands, by exploiting the singular behavior of their Bloch wave functions in momentum space. In the case of a singular flat band, its Bloch wave function possesses immovable discontinuities generated by the band-crossing with other bands, and thus the vector bundle associated with the flat band cannot be defined. This singularity precludes the compact localized states from forming a complete set spanning the flat band. Once the degeneracy at the band crossing point is lifted, the singular flat band becomes dispersive and can acquire a finite Chern number in general, suggesting a new route for obtaining a nearly flat Chern band. On the other hand, the Bloch wave function of a nonsingular flat band has no singularity, and thus forms a vector bundle. A nonsingular flat band can be completely isolated from other bands while preserving the perfect flatness. All one-dimensional flat bands belong to the nonsingular class. We show that a singular flat band displays a novel bulk-boundary correspondence such that the presence of the robust boundary mode is guaranteed by the singularity of the Bloch wave function. Moreover, we develop a general scheme to construct a flat band model Hamiltonian in which one can freely design its singular or nonsingular nature. Finally, we propose a general formula for the compact localized state spanning the flat band, which can be easily implemented in numerics and offer a basis set useful in analyzing correlation effects in flat bands.

A combination of the density functional theory and the single-site dynamical mean-field theory is employed to study the pressure dependence of the electronic structure for cubic phase californium. We predict that its $5f$ electrons could undergo an orbital-selective localized-itinerant transition under moderate pressure. The volume contraction causes remarkable charge redistribution and valence fluctuation behaviors, which are the driving forces of the divalent-trivalent transition. Additionally, we find that the angular momentum coupling mechanism is hardly affected by pressure. The $5f$ orbital occupancy is well described by the intermediate-coupling scheme.

Carbon nanotubes are one-dimensional nanoscale systems with strongly pronounced chirality-dependent optical properties with multiple excitonic transitions. We investigate the high-energy $G$ mode of semiconducting single-walled nanotubes of different chiralities at first excitonic transition by applying resonant Raman spectroscopy. The $G$ mode intensity dependence on excitation energy yielded asymmetric resonance Raman profiles similar to ones we reported for the second excitonic transition. We find the scattering efficiency to be strongest at the incoming Raman resonance. Still, the degree of asymmetry is different for the first and second transitions and the first transition profiles provide a narrower line shape due to longer exciton lifetimes. The overall scattering efficiency is up to a factor of 25 times more intense at first excitonic transition, compared to the second transition. The fifth-order perturbation theory, with implemented phonon scattering pathways between excitonic states, excellently reproduced experimental data.

Bismuth titanate, ${\mathrm{Bi}}_4{\mathrm{Ti}}_3{\mathrm{O}}_{12}$ (BiT), is a complex layered ferroelectric material that is composed of three perovskitelike units and one fluoritelike unit stacked alternatively along the transverse direction. The ground-state crystal structure is monoclinic with the spontaneous polarization ($\sim 50\mu \mathrm{C}/\mathrm{c}{\mathrm{m}}^2$) along the plane. BiT typically grows along the $c$ direction in thin-film form, and having the polarization vector aligned with the growth orientation can be beneficial for several potential device applications. It is well known that judicious doping of ferroelectrics is an effective method in adjusting the magnitude and the orientation of the spontaneous polarization. Here, we show using first-principles density-functional theory and a detailed phonon analysis that Bi atoms in the fluoritelike layers have significantly more impact on the magnitude and orientation of the spontaneous polarization vector as compared to the perovskitelike layer. The low-energy hard-phonon modes are characterized by fluoritelike layers experiencing transverse displacements and large changes in Born effective charges on Bi atoms. Thus, the breaking of symmetry caused by doping of Bi sites within the fluoritelike layer leads to the formation of uncancelled permanent dipole moments along the transverse direction. This provides an opportunity for doping the Bi site in the fluoritelike layer. Isovalent dopants P, As, and Sb were studied. P is found to be most effective in the reorientation of the spontaneous polarization. It leads to a threefold enhancement of the out-of-plane component of polarization and to a commensurate rotation of the spontaneous polarization vector by 36.2° towards the transverse direction.

Ab initio molecular dynamics simulations were performed to analyze changes in the structure and dynamics of molten Pb along the melting line for pressures ranging from ambient to 70 GPa. Common neighbor analysis reveals a local structural order of the underlying crystalline phase at the corresponding pressure, which increasingly competes with the existing icosahedral local order with pressure. Obtained dispersions of longitudinal and transverse collective excitations contain two branches of transverse modes for all pressures. Analysis of the pressure dependence of observed two-peak structure of the Fourier spectra of velocity autocorrelation functions allowed us to identify their peak locations with the frequencies of nonpropagating transverse branches in the second pseudo-Brillouin zone.

We study the bulk and edge properties of a driven Kitaev chain, where the driving is performed as instantaneous quenches of the on-site energies. We identify three periodic driving regimes: low period, which is equivalent to a static model, with renormalized parameters obtained from the Baker-Campbell-Hausdorff (BCH) expansion; intermediate period, where the first order BCH expansion breaks down; and high period when the quasienergy gap at $\omega /2$ closes. We investigate the dynamical localization properties for the case of quasiperiodic potential driving as a function of its amplitude and the pairing strength, obtaining regimes with extended, critical, and localized bulk states, if the driving is performed at high frequencies. In these we characterize wave-packet propagation, obtaining ballistic, subdiffusive, and absence of spreading, respectively. In the intermediate period regime, we find an additional region in the phase diagram with a mobility edge between critical and localized states. Furthermore, we investigate the stability of these phases under time aperiodicity on the drivings, observing that the system eventually thermalizes: It results in featureless random states which can be described by the symmetry of the Hamiltonian. In a system with open edges, we find that both Majorana and fermionic localized edge modes can be engineered with a spatially quasiperiodic potential, in similarity with the case of homogeneous on-site energies. Besides, we demonstrate the possibility of creating multiple Majorana 0 and $\pi$ modes in a driven setting, even if the underlying static Hamiltonian is in its trivial phase. Lastly, we study the robustness of the Majorana modes against the aperiodicity in the driving period, showing that the ones created via quasiperiodic potential are more robust to the decoherence. Moreover, we find an example where a Majorana mode displays high robustness, provided that it is chosen from a special point in the topological region.

We present an ab initio study of the spin-transfer torque in Fe/MgO/FePt/Fe magnetic tunnel junctions. We consider an FePt film with a thickness up to six unit cells, either in direct contact with the MgO spacer or with an intercalated ultrathin Fe seed layer. We find that in the FePt layer the torque is not attenuated as strongly as in the case of pure Fe. Moreover, in FePt the torque alternates sign at the Fe and Pt atomic planes throughout the stack for all FePt thicknesses considered. Finally, when Fe is intercalated between MgO and $L{1}_0$ FePt, the torque is sharply attenuated, and it is transferred to FePt only for an Fe seed layer that is less than two atomic planes thick. We attribute these features to the different spatial profiles of the exchange and correlation field and the induced nonequilibrium spin accumulation. The calculated tunneling magnetoresistance of the Fe/MgO/FePt/Fe junctions studied is enhanced with respect to the one of Fe/MgO/Fe, while it is reduced with Fe intercalation. Our work shows that $L{1}_0$ FePt junctions can be promising candidates for current-operated magnetic devices and that the magnetic texture at the atomic scale has an important effect on the spin-transfer torque.

We report a domelike superconductivity and quantum criticality in the 12442-type hybrid-structure superconductor $\mathrm{KC}{\mathrm{a}}_2\mathrm{F}{\mathrm{e}}_4\mathrm{A}{\mathrm{s}}_4{\mathrm{F}}_2$ under hydrostatic pressures up to 15 GPa. It is found that the superconducting transition temperature ($T_c$) increases from $\sim 33.5\mathrm{K}$ at ambient pressure to a maximum of $\sim 36.5\mathrm{K}$ at 2 GPa in the background of enhanced resistivity of the $\left(\mathrm{K},\mathrm{Rb},\mathrm{Cs}\right)\mathrm{C}{\mathrm{a}}_2\mathrm{F}{\mathrm{e}}_4\mathrm{A}{\mathrm{s}}_4{\mathrm{F}}_2$ at ambient pressure (AP), and then shows a slope change near 8 GPa with anomalous characteristics of resistivity. $T_c$ is found to be inversely proportional to the characteristic temperature of the coherence-incoherence crossover. The plots of resistivity at various temperatures and pressures and high-pressure x-ray diffraction confirm no pressure-induced half collapse of the tetragonal structure below 15 GPa. The pressure-induced crossover from non-Fermi liquid (the temperature exponent $n\sim 1.05\pm 0.1$ at AP) to Fermi liquid ($n\sim 1.92\pm 0.1$ at 15 GPa) is accompanied by the reduction of the linear temperature coefficient of normal-state resistivity, nearly two orders of magnitude compared to AP. These characteristics are ascribed to the weakening electronic correlation and/or critical fluctuations by pressure.

In the layered iron-based superconductor ${\mathrm{YFe}}_2{\mathrm{Ge}}_2$, a high Sommerfeld ratio of $\sim 100\mathrm{mJ}/\mathrm{mol}{\mathrm{K}}^2$ and a $T^{3/2}$ temperature dependence of the electrical resistivity at low temperature $T$ indicate strong electronic correlations and point toward an unconventional pairing state. We have investigated the role of composition and annealing conditions in optimizing the growth of high-quality ${\mathrm{YFe}}_2{\mathrm{Ge}}_2$. Our findings confirm that bulk superconductivity is observed in samples with disorder scattering rates less than $2k_B{T}_c/\hslash$. Fe deficiency on the Fe site is identified as the dominant source of disorder, which can be minimized by precipitating from a slightly iron-rich melt, followed by annealing.

We have investigated the influence of electron beam generated defects on the structure of periodic lattice distortions (PLDs) which accompany charge density wave modulations in $1T\text{−}\mathrm{Ta}{\mathrm{S}}_2$ and $1T\text{−}\mathrm{TaS}{\mathrm{e}}_2$. Lattice defects were generated through irradiation with high-energy electrons in a transmission electron microscope (TEM). Using atomically resolved high-resolution TEM imaging, we investigate the PLD structure and the changes in this structure with prolonged exposure to the electron beam. We observe the formation of dislocationlike topological defects in the PLD structure. Prolonged exposure to the electron beam also leads to an increase in density of these defects. This is also accompanied by an increase in structural disorder of the PLD. Density functional theory calculations were also performed in order to understand sulfur (S) and selenium (Se) vacancy defect formation in $1T\text{−}\mathrm{TaS}{\mathrm{e}}_2$ and $1T\text{−}\mathrm{Ta}{\mathrm{S}}_2$ and their effects on the PLD structure. The formation energy of Se/S vacancies was calculated to be lowest for the highly displaced S/Se atoms in the vicinity of PLD maxima. Vacancies formed at the less displaced sites near the PLD minima were found to have lower formation energy. The calculations also showed that an increase in the S/Se vacancies leads to the formation of dislocations and an increase in disorder in the PLD structures. This supports the experimental observations.

A spring-mass model arranged in a diamond structure—mechanical diamond—is analyzed in terms of topology in detail. We find that additional springs connecting the next-nearest-neighbor (NNN) pairs of mass points and the modulation of the mass parameters to the pristine mechanical diamond generate multiple pairs of Weyl points in the frequency dispersion. Evolution of the Weyl point positions in the Brillouin zone against the uniform outward tension is tracked and explained by the point group symmetry, especially tetrahedral symmetry of the NNN springs. Interestingly, a rapid transmutation of the monopole charges of the Weyl points occurs as the tension varies. We also show surface Fermi arcs in the case with anisotropy in the NNN springs.

Both structural glasses and disordered crystals are known to exhibit anomalous thermal, vibrational, and acoustic properties at low temperatures or low energies, what is still a matter of lively debate. To shed light on this issue, we studied the halomethane family ${\mathrm{CBr}}_n{\mathrm{Cl}}_{4-n}$ ($n=0,1,2$) at low temperature where, despite being perfectly translationally ordered stable monoclinic crystals, glassy dynamical features had been reported from experiments and molecular dynamics simulations. For $n=1,2$ dynamic disorder originates by the random occupancy of the same lattice sites by either Cl or Br atoms, but not for the ideal reference case of ${\mathrm{CCl}}_4$. Measurements of the low-temperature specific heat ($C_p$) for all these materials are here reported, which provide evidence of the presence of a broad peak in Debye-reduced $C_p\left(T\right)/T^3$ and in the reduced density of states ($g\left(\omega \right)/{\omega }^2$) determined by means of neutron spectroscopy, as well as a linear term in $C_p$ usually ascribed in glasses to two-level systems in addition to the cubic term expected for a fully ordered crystal. Being ${\mathrm{CCl}}_4$ a fully ordered crystal, we also performed density functional theory (DFT) calculations, which provide unprecedented detailed information about the microscopic nature of vibrations responsible for that broad peak, much alike the “'boson peak” of glasses, finding it to essentially arise from a piling up (at around $3–4$ meV) of low-energy optical modes together with acoustic modes near the Brillouin-zone limits.

The intermediate-valence compound CeNi experiences a pressure-induced first-order structural phase transition with an abrupt change in the unit cell volume, and as such constitutes an attractive system to study pressure-driven $f$-electron delocalization behavior in a system with an unstable $f$-electron shell. The inelastic neutron scattering study of the dynamic magnetic response of CeNi reveals that the structural transition increases the characteristic energy of magnetic fluctuations due to enhanced Ce $4f$–Ni $3d$ hybridization. At the same time, the inelastic $4f$ magnetic form factor remains unchanged.

Multiband superconductors have a sufficient number of degrees of freedom to allow topological excitations characterized by skyrmionic topological invariants. In the most common, clean $s$-wave multiband systems, the interband Josephson and magnetic couplings favor composite vortex solutions, without a skyrmionic topological charge. It was discussed recently that certain kinds of anisotropies lead to hybridization of the interband phase difference (Leggett) mode with magnetic modes, dramatically changing the hydromagnetostatics of the system. Here we report this effect for a range of parameters that substantially alter the nature of the topological excitations, leading to solutions characterized by a nontrivial skyrmionic topological charge. The solutions have a form of a coreless texture formed of spatially separated but bound excitations in each band, namely fractional vortices, each carrying a fraction of the flux quantum. We demonstrate that in this regime there is a rich spectrum of skyrmion solutions, with various topological charges, that are robust with respect to changes of parameters of the system and present for a wide range of anisotropies.

We show that a Fermi gas, in three dimensions, at temperatures above the superconducting phase transition $\left(T_c\right)$ but below the Fermi temperature $\left(T_F\right)$, cannot be described by Fermi liquid theory (FLT) in the unitary limit where the scattering length diverges. The result follows by showing that there are no effective field theory descriptions that both behave like a Fermi liquid and properly nonlinearly realize the spontaneously broken boost and conformal invariance of the system. We have also derived an exact result for the beta function for the coupling function in the unitary limit.

${\mathrm{CeMnNi}}_4$ exhibits an unusually large spin polarization, but its origin has baffled researchers for more than a decade. We use bulk sensitive hard x-ray photoelectron spectroscopy (HAXPES) and density functional theory based on the Green's function technique to demonstrate the importance of electron-electron correlations of both the Ni $3d$ ($U_{\text{Ni}}$) and Mn $3d$ ($U_{\text{Mn}}$) electrons in explaining the valence band of this multiply correlated material. We show that Mn-Ni antisite disorder as well as $U_{\text{Ni}}$ play a crucial role in enhancing its spin polarization: Antisite disorder broadens a Ni $3d$ minority-spin peak close to the Fermi level ($E_F$), while an increase in $U_{\text{Ni}}$ shifts it toward $E_F$, both leading to a significant increase of minority-spin states at $E_F$. Furthermore, the rare occurrence of a valence state transition between the bulk and the surface is demonstrated highlighting the importance of HAXPES in resolving the electronic structure of materials unhindered by surface effects.

We extend the formalism of matrix product states (MPS) to describe one-dimensional gapped systems of fermions with both unitary and antiunitary symmetries. Additionally, systems with orientation-reversing spatial symmetries are considered. The short-ranged entangled phases of such systems are classified by three invariants, which characterize the projective action of the symmetry on edge states. We give interpretations of these invariants as properties of states on the closed chain. The relationship between fermionic MPS systems at a renormalization group fixed point and equivariant algebras is exploited to derive a group law for the stacking of fermionic phases. The result generalizes known classifications to symmetry groups that are nontrivial extensions of fermion parity and time-reversal.