The transport properties of the phase-change material ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ can be tuned by controlling its atomic structure and concentration of charge carriers. Moving away from the “225” stoichiometry or doping with atoms of different chemical species are major methods to reach this aim. The transport properties of these doped samples are challenging to study experimentally, since their crystalline phase generally possesses a complicated microstructure, consisting of grains with different compositions. They are also challenging to investigate by first-principles methods based on the calculation of Kohn-Sham wave functions, as larger supercells are needed to describe the unavoidable chemical disorder among Ge, Sb, dopant atoms, and vacancies. In this work, we perform first-principles calculations of the electronic structure and electrical conductivity of off-stoichiometric or Si-doped cubic ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ crystals, using the spin polarized relativistic Korringa-Kohn-Rostoker (KKR) method based on the multiple-scattering theory. The doped crystals have all been described with a rock-salt unit cell, in which the chemical disorder is taken into account through the coherent potential approximation (CPA). The accuracy of the results obtained using this method is verified by comparing, for several crystal compositions, the density of electronic states calculated with this method and with a method that uses Kohn-Sham wave functions and big supercells. We calculated the Bloch spectral function, which shows the dispersion of the electron states and its modification with the deviation from the 225 stoichiometry, silicon doping, and chemical disorder. We describe the composition dependence of the electrical conductivity, which we discuss in terms of the concentration of charge carriers and of the modification of their scattering by the intrinsic chemical disorder in the crystal. These results can be used to model real samples, the microstructure of which consists of grains with different concentrations of Ge, Sb, or Si atoms, each grain being described by a conductivity that depends on its composition.

The structural and electronic phase transitions of $\mathrm{C}{\mathrm{o}}_2\mathrm{T}{\mathrm{e}}_3{\mathrm{O}}_8$ spiroffite have been studied with a suite of in situ high-pressure characterization techniques including synchrotron x-ray diffraction, Raman, x-ray emission spectroscopy, UV-vis absorption, and electrical transport measurement. Two pressure-induced phase transitions were observed at about 6.9 and 14.4 GPa. The first transition is attributed to a small spin transition of Co along with discontinuity in unit-cell volume change, while the second one represents a first-order phase transition with a volume collapse of 4.5%. The latter transition is accompanied by the relaxation of distortion in $\mathrm{Co}{\mathrm{O}}_6$ octahedron, which enhances the crystal-field strength inhibiting the occurrence of spin transition. What is more, the competition between contributions of electrons and oxygen ion to the overall conductivity is observed and affected by the phase transition under high pressure. This demonstration provides insights into the relationship between the lattice-structural and spin degrees of freedom, and highlights the impact of pressure on the control of structural and electronic states of a given material for optimized functionalities.

The birefringent fermions possess a spectrum with two distinct Fermi velocities. Here based on the lattice model, we use the mean-field method to investigate the interaction-induced phase transitions of the birefringent fermions. We consider both the short-range nearest-neighbor (NN) and next-nearest-neighbor (NNN) repulsive interactions and calculate the phase diagrams under different conditions. We find that the NN interactions can induce the asymmetric charge density wave order, while the NNN interactions can drive the asymmetric quantum anomalous Hall order (AQAH) in the large limit of anisotropy in the hopping integrals, $\beta \to 1$. The AQAH order is characterized by the unequal loop currents connecting the NNN sites and by the appearance of a gap between the two conduction (valence) bands. Such asymmetric fermionic orders can be attributed to the specific lattice geometry of the birefringent fermions. The implications of our results for experiments are also discussed.

Rare-earth nickelates exhibit a remarkable metal-insulator transition accompanied by a symmetry-lowering structural distortion. Using model considerations and first-principles calculations, we present a theory of this phase transition which reveals the key role of the coupling between electronic and lattice instabilities. We show that the transition is driven by the proximity to an instability towards electronic disproportionation which couples to a specific structural distortion mode, cooperatively driving the system into the insulating state. This allows us to identify two key control parameters of the transition: the susceptibility to electronic disproportionation and the stiffness of the lattice mode. We show that our findings can be rationalized in terms of a Landau theory involving two coupled order parameters, with general implications for transition-metal oxides.

Although many metal hydrides were predicted by theory, very few of those were so far realized in experiment. Here, we systematically investigated the Ta-H system below 85 GPa, and found that the hexagonal close-packed $\mathrm{Ta}{\mathrm{H}}_{\sim 2}$ can be gradually transformed into $\mathrm{Ta}{\mathrm{H}}_{\sim 3}$ above 60 GPa at room temperature. With the help of density-functional theory calculations, the space group of $\mathrm{Ta}{\mathrm{H}}_{\sim 3}$ can be determined as a $I\overline{4}3d$ phase, which is isostructural to the Domeykite mineral. Such structure is rather rare and was not predicted in the earlier theoretical works due to the large unit cell. We also tried laser heating at high pressures and found that temperature plays a key role in tuning the $\mathrm{Ta}{\mathrm{H}}_{\sim 3}$ phase to $\mathrm{Ta}{\mathrm{H}}_{\sim 2}$ phase; however, no other new tantalum hydrides were synthesized. During decompression, the $\mathrm{Ta}{\mathrm{H}}_{\sim 3}$ was completely decomposed to the $\mathrm{Ta}{\mathrm{H}}_{\sim 2}$ below 30 GPa. Further experiments are still required for the synthesis of other tantalum polyhydrides at a higher pressure range and for the comparison with the theoretical calculations.

We report the discovery of bulk superconductivity in the ternary intermetallics ${\mathrm{YNiSi}}_3$ and ${\mathrm{LuNiSi}}_3$. High-quality single crystals were grown via the Sn-flux method and studied using magnetization, specific-heat, and resistivity measurements at low temperatures. The critical temperatures obtained from these different techniques are in very good agreement and yield $T_{\mathrm{c}}=1.36\left(3\right)\mathrm{K}$ and $T_{\mathrm{c}}=1.61\left(2\right)\mathrm{K}$ for ${\mathrm{YNiSi}}_3$ and ${\mathrm{LuNiSi}}_3$, respectively. Magnetization measurements indicate that both compounds are among the rare cases where type-I superconductivity occurs in a ternary intermetallic, however, the jump in the specific heat at the transition is lower than the value expected from BCS theory ($\mathrm{\Delta }{C}_{\mathrm{el}}/{\gamma }_{\mathrm{n}}{T}_{\mathrm{c}}=1.43$) in both materials and is equal to 1.14(9) and 0.71(5) for the Y and Lu compounds, respectively. Resistivity measurements exhibit sharp transitions but with critical fields ${\mu }_0{H}_{\mathrm{c}}\left(0\right)$ ($\approx 0.05\mathrm{T}$ for ${\mathrm{YNiSi}}_3$ and $\approx 0.08\mathrm{T}$ for ${\mathrm{LuNiSi}}_3$) considerably higher than those obtained from the magnetization and specific heat ($\approx 0.01\mathrm{T}$). First-principles density functional theory calculated electronic structure shows that these compounds have highly anisotropic and complex Fermi surfaces with one electronic and two holelike branches. One hole branch and the electron branch have a large cylindrical topology connecting the first Brillouin-zone boundaries, the former being built up by the hybridization of Y(Lu) $d$, Ni $d$, and Si $p$ states, and the latter being built up by Ni $d$ and Si $p$ states. The calculated phononic structures indicate that the coupling of the Y(Lu), Ni $d$, and Si $p$ electrons in the low-lying optical phonon branches is responsible for the formation of Cooper pairs and the observed superconducting state. Therefore, these compounds can be classified as anisotropic three-dimensional metals with multiband superconducting ground states in the weak-coupling regime.

Motivated by recent developments in the field of plasmonics, we develop the theory of radiation from one-dimensional electron liquids, showing that the spectrum of thermal radiation emitted from the system exhibits signatures of non-Fermi liquid behavior. We derive a multipole expansion for the radiation based on the Tomonaga-Luttinger liquid model. While the dipole radiation pattern is determined by the conductivity of the system, we demonstrate that the quadrupole radiation can reveal important features of the quantum liquid, such as the Luttinger parameter. Radiation offers a probe of the interactions of the system, including Mott physics as well as nonlinear Luttinger liquid behavior. We show that these effects can be probed in current experiments on effectively one-dimensional electron liquids, such as carbon nanotubes.

We study the homotopy classification of symmetry representations to describe the bulk topological invariants protected by the combined operation of a twofold rotation $C_{2z}$ and time-reversal $T$ symmetries. We define topological invariants as obstructions to having smooth Bloch wave functions compatible with a momentum-independent symmetry representation. When the Bloch wave functions are required to be smooth, the information on the band topology is contained in the symmetry representation. This implies that the $d$-dimensional homotopy class of the unitary matrix representation of the symmetry operator corresponds to the $d$-dimensional topological invariants. Here, we prove that the second Stiefel-Whitney number, a two-dimensional (2D) topological invariant protected by $C_{2z}T$, is the homotopy invariant that characterizes the second homotopy class of the matrix representation of $C_{2z}T$. As an application of our result, we show that the three-dimensional (3D) bulk topological invariant for the $C_{2z}T$-protected topological crystalline insulator proposed by C. Fang and L. Fu in Phys. Rev. B 91, 161105 (2015), which we call the 3D strong Stiefel-Whitney insulator, is identical to the quantized magnetoelectric polarizability. The bulk-boundary correspondence associated with the quantized magnetoelectric polarizability shows that the 3D strong Stiefel-Whitney insulator has chiral hinge states as well as 2D massless surface Dirac fermions. This shows that the 3D strong Stiefel-Whitney insulator has the characteristics of both the first- and the second-order topological insulators, simultaneously, which is in consistence with the recent classification of higher-order topological insulators protected by an order-two symmetry.

Surface and bulk ripplocations in layered nanomaterials have recently attracted the attention of researchers because they possess the properties of topological solitons, which are capable of efficient transportof mass and energy and of mediating plastic deformation. In a ripplocation, one or a few layers at the surface or in the bulk of a material are bent or folded. So far, only the static properties of ripplocations have been analyzed. In the present study, the dynamics of graphene bubbles and folds on a graphite substrate are analyzed by full-atomic molecular dynamics and with the help of the two-dimensional chain model. It is demonstrated that such objects, classified as surface ripplocations, are robust solitary waves that propagate while practically radiating no energy. Energy and geometrical parameters of the ripplocations are calculated as the functions of their propagation velocity. In the presence of thermal fluctuations the ripplocations can be accelerated or decelerated, showing a random-walk-like dynamics. Collisions of ripplocations result in their merger. Overall, our results reveal that layered materials can support surface ripplocations that are highly mobile topological solitary waves efficiently transporting mass and energy.

The formation of periodic structures in ultrafast laser-irradiated surfaces implies dynamic coupling between the incoming light and the light-driven material. To capture the mutual influence and feedback between light and evolving surface topographies, we investigate numerically the evolution of metal surfaces irradiated by multiple femtosecond laser pulses of sub-, near-, and slightly above-threshold ablation fluence. The multiphysical model combines Maxwell equations and thermohydrodynamic approach based on electron-ion heat transfer and compressible Navier-Stokes equations and allows us to account for interpulse feedback on the resulting surface topographies. First pulses of the subthreshold energy lead to material swelling, nanocavitation few tens of nanometers below the surface, and, as a result, nanoroughness formation on the initially flat surface. Further pulses contribute to the development of periodic surface structures, enhanced absorption, and increased removal rate. Cavitation in the tips of ripples is found to play a crucial role in modification and regulation of surface topography for sub- and near-threshold ablation fluences. At higher laser pulse energy, thermal ablation is mostly involved in surface modification, and the ablation rate per power reaches its maximum at three to five times the ablation threshold fluence, resulting from the optimal heat penetration depth for laser ablation. The numerical results offer a better understanding of the surface topography modifications upon multipulse femtosecond laser irradiation.

I theoretically study the influence of impurity scattering on the electrical and thermal transport of borophane layer, a two-dimensional anisotropic Dirac semimetal with two tilted and anisotropic Dirac cones. In a systematic framework, I have calculated the electrical conductivity and thermoelectric coefficients of borophane in the presence of the short-range, long-range charged impurity and the short-range electromagnetic (SREM) scatterers, by using the exact solution of the Boltzmann transport equation within the linear-response theory. Contrary to the large electron-hole asymmetry in borophane, its electron-hole conductivity is nearly symmetric. Interestingly, in the case of short-range scattering, just like graphene, the conductivities of borophane were found to have constant values, independent of the chemical potential, while the conductivities of the SREM scatterers are linearly dependent on the chemical potential. Regardless of the impurity type, the electrical conductivity of borophane is highly anisotropic, while the Seebeck coefficient and figure of merit ($\mathit{ZT}$) are isotropic. Along with the ambipolar nature of the borophane thermopower, a very high value of $\mathit{ZT}$ around unity is obtained at room temperature, due to the large asymmetry between electrons and holes in borophane. More importantly, borophane attains its maximum value of $\mathit{ZT}$ at very low chemical potentials, in the vicinity of the charge neutrality point. In comparison to phosphorene, a highly unique anisotropic 2D material, borophane with a higher anisotropy ratio (${\sigma }_{xx}/{\sigma }_{yy}\sim 10$) is an unprecedented anisotropic material. This high anisotropy ratio together with the large figure of merit suggest that borophane is promising for the thermoelectric applications and transport switching in the Dirac transport channels.

Machine learning algorithms based on artificial neural networks have proven very useful for a variety of classification problems. Here we apply them to a well-known problem in crystallography, namely the classification of x-ray diffraction (XRD) patterns of inorganic powder specimens by the respective crystal system and space group. Over ${10}^5$ theoretically computed powder XRD patterns were obtained from inorganic crystal structure databases and used to train a deep dense neural network. For space group classification, we obtain an accuracy of around 54% on experimental data. Finally, we introduce a scheme where the network has the option to refuse the classification of XRD patterns that would be classified with a large uncertainty. This enhances the accuracy on experimental data to 82% at the expense of having half of the experimental data unclassified. With further improvements of neural network architecture and experimental data availability, machine learning constitutes a promising complement to classical structure determination methodology.

One-dimensional systems often possess multiple channels or bands arising from the excitation of transverse degrees of freedom. In this work, we study the specific processes that dominate the equilibration of multichannel Fermi gases at low temperatures. Focusing on the case of two channels, we perform an analysis of the relaxation properties of these systems by studying the spectrum and eigenmodes of the linearized collision integral. As an application of this analysis, a detailed calculation of the bulk viscosity is presented. The dominant scattering processes obey an unexpected conservation law which is likely to affect the hydrodynamic behavior of these systems.

The orthogonality catastrophe (OC) problem is considered solved for 50 years. It has important consequences for numerous dynamic phenomena in fermionic systems, including Kondo effect, x-ray spectroscopy, and quantum diffusion of impurities, and is often used in the context of metals. However, the key assumptions on which the known solution is based—impurity potentials with finite cross section and noninteracting fermions—are both highly inaccurate for problems involving charged particles in metals. As far as we know, the OC problem for the “all Coulomb” case has never been addressed systematically, leaving it unsolved for the most relevant practical applications. In this paper we include effects of dynamic screening in a consistent way and demonstrate that for short-range impurity potentials the noninteracting Fermi-sea approximation radically overestimates the power-law decay exponent of the overlap integral. We also find that the dynamically screened Coulomb potential leads to a larger exponent than the often used static Yukawa potential. Finally, by employing the diagrammatic Monte Carlo technique, we quantify effects of a finite impurity mass and reveal how OC physics leads to small, but finite, impurity residues.

The Kondo lattice model is a paradigmatic model for the description of local moment systems, a class of materials exhibiting a range of strongly correlated phenomena including heavy fermion formation, magnetism, quantum criticality, and unconventional superconductivity. Conventional theoretical approaches invoke fractionalization of the local moment spin through large-$N$ and slave particle methods. In this work we develop a formalism based on noncanonical degrees of freedom, building upon a recently developed approach for strongly correlated electrons [E. Quinn, Phys. Rev. B 97, 115134 (2018)]. Specifically, we demonstrate that higher dimensional representations of $su\left(2\right|2)$ correspond to a splitting of the electronic degree of freedom on the Kondo lattice, in a manner which entwines the conduction electrons with the local moment spins. This provides a powerful means of organizing correlations, and offers a perspective on heavy fermion formation. Unlike slave-particle methods, noncanonical degrees of freedom generically allow for a violation of the Luttinger sum rule, and we interpret recent angle resolved photoemission experiments on Ce-115 systems in view of this.

The effects of several adatom dopants (H, Be, C, N, Si, and Zn) on the electronic structure of low-energy surface reconstructions of InAs—(001, 110, 111, and 112) low-index planes—were investigated using ab initio density functional theory calculations. It was observed that the local electronic structure can be controlled in such materials, either reducing or enhancing surface states depending on the dopant and the unique surface reconstruction. The local bonding environment, in particular whether the bonding has an aromatic character, was found to influence both surface reconstruction and a dopant's affinity for a surface. Beryllium and Zinc were often found to reduce the surface states around the band gap, which could improve the charge-carrier mobility in various electronic components. However, these dopants can also cause further surface reconstruction/reconfiguration inducing different surface states.

Rare-earth deposition on Si(001) substrates and thermal treatment lead to the formation of metastable silicide nanowires with extremely high aspect ratios and quasi-one-dimensional metallicity. In this work, the prototypical erbium silicide nanowires are investigated theoretically by ab initio thermodynamics, considering a multitude of possible nanowire structures. The calculations indicate that the nanowires consist of hexagonal ${\mathrm{ErSi}}_2$ with two possible orientations with respect to the substrate, that they are partially buried into the substrate, and that they are electronically decoupled from the substrate. However, wires of finite width are predicted to be less favorable than planar silicide structures, demonstrating that the wires are metastable and suggesting that their formation is more governed by kinetic effects than by thermodynamics.

$X$-waves are an example of a localized wave packet solution of the homogeneous wave equation and can potentially arise in any area of physics exhibiting wave phenomena, such as acoustics, electromagnetism, and quantum mechanics. They have been predicted in condensed matter systems such as atomic Bose-Einstein condensates in optical lattices and were recently observed in exciton-polariton condensates. Here we show that polariton $X$-waves result from an interference between two separating wave packets that arise from the combination of a locally hyperbolic dispersion relation and nonlinear interactions. We show that similar $X$-wave structures could also be observed in expanding spin-orbit coupled Bose-Einstein condensates.