High-quality single crystals of ${\mathrm{CoTiO}}_3$ are grown and used to elucidate in detail structural and magnetostructural effects by means of high-resolution capacitance dilatometry studies in fields up to 15 T which are complemented by specific heat and magnetization measurements. In addition, we refine the single-crystal structure of the ilmenite ($R\overline{3}$) phase. At the antiferromagnetic ordering temperature $T_{\mathrm{N}}$ pronounced $\lambda$-shaped anomaly in the thermal expansion coefficients signals shrinking of both the $c$ and $b$ axes, indicating strong magnetoelastic coupling with uniaxial pressure along $c$ yielding six times larger effect on $T_{\mathrm{N}}$ than pressure applied in-plane. The hydrostatic pressure dependency derived by means of Grüneisen analysis amounts to $\partial T_{\mathrm{N}}/\partial p\approx 2.7\left(4\right)\mathrm{K}/\mathrm{GPa}$. The high-field magnetization studies in static and pulsed magnetic fields up to 60 T along with high-field thermal expansion measurements facilitate in constructing the complete anisotropic magnetic phase diagram of ${\mathrm{CoTiO}}_3$. While the results confirm the presence of significant magnetodielectric coupling, our data show that magnetism drives the observed structural, dielectric, and magnetic changes both in the short-range ordered regime well above $T_{\mathrm{N}}$ as well as in the long-range magnetically ordered phase.

This paper reports a combined experimental and theoretical study of structural, electronic, magnetic, and mechanical properties of quaternary Heusler alloys ${\mathrm{Co}}_{2-x}{\mathrm{Cr}}_x\mathrm{FeGe}$ prepared by arc-melting with Cr concentrations $0\le x\le 1$. Single-phase microstructures are observed for Cr compositions from $x=0.25$ to $x=1$. Lower Cr concentrations are multiphased. X-ray diffraction patterns at room temperature reveal a face-centered cubic crystal structure in all single-phase samples. The low-temperature saturation magnetic moments, as determined from magnetization measurements, agree fairly well with our theoretical results and also obey the Slater-Pauling rule for half-metals, a prerequisite for half-metallicity. All alloys are observed to have high Curie temperatures that scale linearly with the saturation magnetic moments. Relatively high mechanical hardness values are also observed. First-principles calculations also predict a finite band gap in the minority spin channel of the alloys, increasing in size with increasing Cr concentration. Cr substitution brings the Fermi level toward the center of this gap while also increasing the majority spin density of states near the Fermi level. As a whole, ${\mathrm{Co}}_{2-x}{\mathrm{Cr}}_x\mathrm{FeGe}$ shows great promise as a half-metal with 100% spin polarization.

We investigate, within the framework of linear elasticity theory, edge Rayleigh waves of a two-dimensional elastic solid with broken time-reversal and parity symmetries due to a Berry term. As our prime example, we study the elastic edge wave traveling along the boundary of a two-dimensional skyrmion lattice hosted inside a thin-film chiral magnet. We find that the direction of propagation of the Rayleigh modes is determined not only by the chirality of the thin film, but also by the Poisson ratio of the crystal. We discover three qualitatively different regions distinguished by the chirality of the low-frequency edge waves, and study their properties. To illustrate the Rayleigh edge waves in real time, we have carried out finite-difference simulations of the model. Apart from skyrmion crystals, our results are also applicable to edge waves of gyroelastic media and screened Wigner crystals in magnetic fields. Our work opens a pathway towards controlled manipulation of elastic signals along boundaries of crystals with broken time-reversal symmetry.

We systematically studied in-plane optical conductivity of $\mathrm{Fe}{\mathrm{Se}}_{1-x}{\mathrm{Te}}_x$ thin films fabricated on $\mathrm{Ca}{\mathrm{F}}_2$ substrates for $x=0$, 0.1, 0.2, and 0.4. This system shows a large enhancement of superconducting transition temperature $T_{\mathrm{c}}$ at $x\approx 0.2$ and a gentle decrease in $T_{\mathrm{c}}$ with further increasing $x$. The low-energy optical conductivity spectrum is described by the sum of narrow and broad Drude components, associated with coherent and incoherent charge dynamics, respectively. With increasing Te content, the spectral weight of the narrow Drude component decreases, whereas the total weight of the two Drude components increases. As a consequence, the fraction of the narrow Drude weight significantly decreases, indicating that Te substitution leads to stronger electronic correlations. Below the nematic transition temperature, the narrow Drude weight decreases with decreasing temperature. This indicates the reduction of the coherent carrier density, resulting from the Fermi-surface modification induced by the development of the orbital order. The reduction of the narrow Drude weight with temperature stopped at $x\approx$ 0.2, corresponding to the disappearance of the nematic transition. Our result suggests that the increase in the coherent carrier density induced by the suppression of the nematic transition gives rise to the enhancement of $T_{\mathrm{c}}$. The decrease in $T_{\mathrm{c}}$ with further Te substitution likely arises from too strong electronic correlations, which are not favorable for superconductivity.

Topological crystalline metals and semimetals (TCMs) have stimulated great research interest, which broadens the classification of topological phases and provides a valuable platform to explore topological superconductivity. Here, we report the discovery of superconductivity and topological features in Pb-intercalated transition-metal dichalcogenide ${\mathrm{Pb}}_{1/3}\mathrm{Ta}{\mathrm{S}}_2$. Systematic measurements indicate that ${\mathrm{Pb}}_{1/3}\mathrm{Ta}{\mathrm{S}}_2$ is a quasi-two-dimensional (q-2D) type-II superconductor ($T{}_c\approx 2.8$ K) with a significantly enhanced anisotropy of upper critical field (${\gamma }_{H_{c2}}=H_{c2}^{ab}/H_{c2}^c\approx 17$). In addition, first-principles calculations reveal that ${\mathrm{Pb}}_{1/3}\mathrm{Ta}{\mathrm{S}}_2$ hosts multiple topological Dirac fermions in the electronic band structure. We discover four groups of Dirac nodal lines on the $k_z=\pi$ plane and two sets of Dirac points on the rotation or screw axes, which are protected by crystalline symmetries and robust against spin-orbit coupling (SOC). Dirac-cone-like surface states emerge on the (001) surface because of band inversion. Our work shows that the TCM candidate ${\mathrm{Pb}}_{1/3}\mathrm{Ta}{\mathrm{S}}_2$ is a promising arena to study the interplay between superconductivity and topological Dirac fermions.

The orthorhombic monochalcogenide SnSe has attracted much attention in recent years as a promising high-temperature thermoelectric material. We present a study of its thermal conductivity and specific heat of SnSe between 2 K and 300 K and quantify its anisotropic thermal diffusivity $D$. For both crystallographic orientations, thermal diffusivity remains above the recently identified Planckian limit ($D>v_s^2{\tau }_P$, where $v_s$ is the sound velocity and ${\tau }_P=\hslash /k_{B}T$) and its anisotropy in $D$ is set by the anisotropy of $v_s$. Comparison with cubic members of the IV-VI family leads to a consistent picture, where the diffusivity in all members of the family is set by the product of ${\mathrm{v}}_s$, ${\tau }_P$ and the ‘melting’ velocity derived from the melting temperature.

Excitations of impurity complexes in semiconductors cannot only provide a route to fill the terahertz gap in optical technologies but can also play a role in connecting local quantum bits efficiently to scale up solid-state quantum-computing devices. However, taking into account both the interactions among electrons/holes bound at the impurities and the host band structures is challenging. Here we combine first-principles band-structure calculations with quantum-chemistry methodology to evaluate the ground and excited states of a pair of phosphorous (shallow donors) impurities in silicon within a single framework. We account for the electron-electron interaction within a broken-symmetry Hartree-Fock approach, followed by a time-dependent Hartree-Fock method to compute the excited states. We adopt a Hamiltonian for each conduction-band valley including an anisotropic kinetic energy term, which splits the $2p_0$ and $2p_{\pm }$ transitions of isolated donors by $\sim 4$ meV, in good agreement with experiments. Our single-valley calculations show the optical response is a strong function of the optical polarization and suggest the use of valley polarization to control optics and reduce oscillations in exchange interactions. When taking into account all valleys, we have included valley-orbital interactions that split the energy levels further. We find a gap opens between the $1s\to 2p$ transition and the low-energy charge-transfer states within $1s$ manifolds (which become optically allowed because of interdonor interactions). In contrast to the single-valley case, we also find charge-transfer excited states in the triplet sector, thanks to the extra valley degrees of freedom. Our computed charge-transfer excited states have a qualitatively correct energy as compared with previous experimental findings; additionally, we predict a set of excitations below 20 meV. Calculations based on a statistical average of nearest-neighbor pairs at different separations suggest that THz radiation could be used to excite the donor pairs spin-selectively. Our approach can readily be extended to other types of donors such as arsenic, and more widely to other semiconducting host materials such as germanium, zinc oxides, and gallium nitride, etc.

We consider an interferometer based on artificially induced topological superconductivity and chiral one-dimensional (1D) Majorana fermions. The (nontopological) superconducting island inducing the superconducting correlations in the topological substrate is assumed to be floating. This allows probing the physics of interfering Majorana modes via microwave response, i.e., the frequency-dependent impedance between the island and the earth. Namely, charging and discharging of the island is controlled by the time-delayed interference of chiral Majorana excitations in both normal and Andreev channels. We argue that microwave measurements provide a direct way to observe the physics of 1D chiral Majorana modes.

The three-dimensional Dirac semimetal is a parent phase for a variety of topological phases that can be generated by tuning parameters in material growth or device operation. Notably, it has recently been found that cadmium arsenide, which is ordinarily a three-dimensional Dirac semimetal, can nevertheless realize a three-dimensional topological insulator in (001)-oriented films about 50-nm thick. In this work, we study the quantum Hall effect in thin (001)-oriented cadmium arsenide films, their thickness ranging from 12 to 24 nm. When the carrier density is kept approximately constant across the different films, quantum transport reveals an identical underlying picture. The result is shown to be consistent with the transport's origin in the surface states of a three-dimensional topological insulator, but problematic for a perspective in which the quantum Hall effect originates from the confined subbands of the bulk band structure. These thin-film results complement previous studies of the quantum Hall effect in 50-nm-thick films.

We study phenomena driven by electron-electron interactions in the minimally twisted bilayer graphene (mTBLG) with a perpendicular electric field. The low-energy degrees of freedom in mTBLG are governed by a network of one-dimensional domain-wall states, described by two channels of one-dimensional linearly dispersing spin-$1/2$ fermions. We show that the interaction can realize a spin-gapped interchannel charge density wave (CDW) state at low temperatures, forming a “Coulomb drag” between the channels and leaving only one charge conducting mode. For sufficiently high temperatures, power-law-in-temperature resistivity emerges from the charge Umklapp scatterings within a domain wall. Remarkably, the presence of the CDW states can strengthen the charge Umklapp scattering and induce a resistivity minimum at an intermediate temperature corresponding to the CDW correlation energy. We further discuss the conditions that resistivity of the network is dominated by the domain walls. In particular, the power-law-in-temperature resistivity results can apply to other systems that manifest topological domain-wall structures.

Topologically nontrivial phases have recently been reported on self-similar structures. Here we investigate the effect of local structure, specifically the role of the coordination number, on the topological phases on self-similar structures embedded in two dimensions. We study a geometry dependent model on two self-similar structures having different coordination numbers, constructed from the Sierpinski gasket. For different nonspatial symmetries present in the system, we numerically study and compare the phases on both structures. We characterize these phases by the localization properties of the single-particle states, their robustness to disorder, and by using a real-space topological index. We find that both structures host topologically nontrivial phases and the phase diagrams are different on the two structures. This suggests that, in order to extend the present classification scheme of topological phases to nonperiodic structures, one should use a framework which explicitly takes the coordination of sites into account.

Multifold degenerate fermions in electronic structures of topological materials give rise to many interesting physics properties. Among them, three-, six-, and eightfold degenerate fermions in condensed matter systems are considered important because these fermions are not allowed in high-energy physics due to restriction posed by Poincáre symmetry. Phonons are the basic emergent boson of the crystalline lattice. Moreover, topological phonons also exist in crystalline solids, like fermionic electrons, due to the crystal symmetry constraints. Two-, three-, and fourfold degenerate phonons were predicted previously. This study proposes degenerate phonons with the maximum fold, i.e., sixfold; we find them in five space groups (with numbers 218, 220, 222, 223, and 230) through a detailed symmetry analysis. We also propose a series of realistic materials with above-mentioned space group numbers that host sixfold degenerate nodal-point phonons at the high-symmetry points $H$ (or $R$) point using first-principles calculations. Finally, as examples, we investigate the surface phonon spectrum of ${\mathrm{C}}_3{\mathrm{N}}_4$, ${\mathrm{Sc}}_4{\mathrm{C}}_3$, ${\mathrm{Y}}_4{\mathrm{Sb}}_3$, and ${\mathrm{K}}_8{\mathrm{Si}}_{46}$ materials and find two obvious projected phonon surface states on (110) or (100) surfaces.

When an energy gap is induced in monolayer graphene the valley degeneracy is broken and the energy spectrum of a confined system such as a quantum dot, becomes rather complex exhibiting many irregular level crossings and small energy spacings, which are very sensitive to the applied magnetic field. Here we study the energy spectrum of a graphene quantum dot that is formed between Landau levels, and show that for the appropriate potential well the dot energy spectrum in the first Landau gap can have a simple pattern with energies coming from one of the two valleys only. This part of the spectrum has no crossings, has specific angular momentum numbers, and the energy spacing can be large enough, consequently, it can be probed with standard spectroscopic techniques. The magnetic field dependence of the dot levels as well as the effect of the mass-induced energy gap are examined, and some regimes leading to a controllable quantum dot are specified. At high magnetic fields and negative angular momentum a simple approximate method to the Dirac equation is developed, which gives further insight into the physics. The approximate energies exhibit the correct trends and agree well with the exact energies.

Plasmonic dimer cavities can induce extreme electric-field hot spots that allow one to access ultrastrong coupling regimes using Raman-type spectroscopy on single vibrating molecules. Using a generalized master equation, we study resonant Raman scattering in the strong coupling regime of cavity quantum electrodynamics, when also in the vibrational ultrastrong coupling regime, leading to “phonon-dressed polaritons.” The master equation rigorously includes spectral baths for the cavity and vibrational degrees of freedom, as well as a pure dephasing bath for the resonant two-level system, which play a significant role. Employing realistic parameters for gold dimer cavity modes, we investigate the emission spectra in several characteristic strong coupling regimes, leading to extremely rich spectral resonances due to an interplay of phonon-modified polariton states and bath-induced resonances. We also show explicitly the failure of the standard master equation in these quantum nonlinear regimes.

Transparent conductors—nearly an oxymoron—are in pressing demand, as ultrathin-film technologies become ubiquitous commodities. With current solutions relying on nonabundant elements, it has recently been suggested that perovskites—such as $\mathrm{Sr}\mathrm{V}{\mathrm{O}}_3$ and $\mathrm{Sr}\mathrm{Nb}{\mathrm{O}}_3$—could serve as next generation transparent conductors. In these, strong electronic correlations push the plasma frequency below the visible spectrum to curtail absorption. Our ab initio calculations and analytical insights establish, however, that the required effective mass enhancement unavoidably comes at a price: An increased scattering that causes substantial optical absorption above the plasma edge. We circumvent this dilemma and identify high-grade perovskite transparent conductors, relying on an interplay of hole doping, larger bandwidths, separation of bands, but only mild mass enhancements.

The phase-field crystal model is one of the most successful models with which to describe crystallization at a near-atomic scale with a larger timescale compared with other atomistic methods when direct correlation functions (DCFs) for the desired lattice are specified. However, the DCFs are, in general, not known and are hard to obtain from essential material information, such as primitive lattice vectors and atom positions. In this paper, we propose a method of obtaining two-point DCFs for desired two-dimensional lattices. The proposed optimization scheme is simple in that it minimizes the temporal change in free energy with respect to the target lattice using a gradient descent. In numerical experiments, we successfully obtained DCFs not only for well-known two-dimensional lattices (i.e., triangular, square, rectangular, honeycomb, and kagome lattices) but also for five nontrivial lattices (i.e., maple leaf, ladybug, trellis, Lieb, and CaVO lattices). We also show that these five lattices can be simulated using a phase-field crystal with at least five modes.

Longitudinal sound velocities (and elastic moduli) were determined in soda lime glass (SLG) at peak shock stresses ranging between 40 and 90 GPa. Laser interferometry was used to obtain particle velocity histories and sound velocities by impacting SLG samples on lithium fluoride (LiF) optical windows. In all experiments, the SLG response consists of a sharp jump to a constant state followed by a release wave. The measured longitudinal sound velocities and moduli showed a marked decrease between 52 and 58 GPa, providing experimental evidence for the transformation from an amorphous solid to a liquid in shock-compressed SLG. The stress threshold (∼55 GPa) for melting in SLG is considerably lower than the threshold reported in shock-compressed fused silica (∼72 GPa), showing the effect of network-modifying cations on the onset of melting. The relative values of sound velocities, shock velocities, and the Hugoniot slopes—between 58 and 90 GPa—are fully consistent with the thermodynamic response of a shock-compressed liquid. Using the experimental results, the Grüneisen parameter (Γ) values were determined for liquid SLG to 90 GPa and then used to provide the Mie-Grüneisen equation of state for liquid SLG.

This work investigates the effects of further neighbor exchange and cubic anisotropy on the magnetic excitations of the fcc kagome antiferromagnet $\mathrm{Ir}{\mathrm{Mn}}_3$. Spin wave frequencies are calculated using the torque equation and the dynamical structure factor $S(\mathbf{Q},\omega )$ is determined by a Green's function method as an extension of our previous work [LeBlanc et al. Phys. Rev. B 90, 144403 (2014)]. These spin wave calculations are compared with inelastic neutron scattering data of polycrystalline $\mathrm{Ir}{\mathrm{Mn}}_3$, where magnetic Mn ions occupy lattice sites of ABC-stacked kagome planes. The data are best explained by a spin wave model including exchange interactions up to fourth nearest neighbor and strong cubic anisotropy, as previously predicted by density functional theory calculations.

Detailed analysis reveals that an incorporation of stabilized defect complexes within metallic thin films, though a highly disordering and nonequilibrium process, gives rise to superconductivity, Fermi-liquid (FL) transport, and a universal correlation among them. This remarkable manifestation of correlated macroscopic quantum effects is attributed to a phonon-mediated electron-electron, e-e, scattering channel which encompasses both Koshino-Taylor and Bergmann's pseudo-Umklapp processes. This channel—denoted below as pseudo-Umklapp e-e scattering channel—is distinctly different from traditional ones in that disorder leads to a breakdown of lattice momentum conservation (significantly enlarging available phase space), to a spectral weight transfer towards lower frequencies (modifying electron-phonon coupling constant $\lambda$), and to a relaxation of kinematic constraints (all phonic polarization modes become available for mediation). On modeling the distorted structure in terms of Hosemann's paracrystal and using standard quantum many-body techniques, we demonstrate the role of distortion and softening in establishing this pseudo-Umklapp channel and, consequently, the surge of superconductivity, the FL transport, and the correlation of their parameters. This unifying approach allows us to derive analytical expressions for $T_c\left({\rho }_{\circ }\right)$ (hallmark of superconductivity), the coefficient $A\left({\rho }_{\circ }\right)$ (hallmark of FL transport), and the universal kinematic scaling relation $\ln \left(\frac{T_c}{\theta }\right)\propto A^{\frac{-1}{2}}$: All are in satisfactory agreement with experiments ($\theta$ is an energy scale; residual resistivity ${\rho }_{\circ }$ measures the extent of disorder).