We employ an optical pump-probe technique to study coherent phonon oscillations in ${\mathrm{Ca}}_2{\mathrm{RuO}}_4$. We find that oscillation amplitude of an $A_g$ symmetric phonon mode is strongly suppressed at 260 K, a putative transition point of orbital ordering. The oscillation also shows a gradual but huge change in its phase, such that the oscillation even flips over with a ${180}^{\circ }$ change across the temperature. Density functional theory calculations indicate that the $A_g$ phonon has an eigenmode of octahedral distortion with conventional tilting along the $a$ axis and antipolar distortion of apical oxygen. Careful inspection of the lattice captures an unusually large antipolar distortion in low-temperature structures, which may play a crucial role for the phase transition at 260 K.

The unconventional temperature variation of the Knight shift (static spin susceptibility) that has been observed in Fe-based superconductors $A{\mathrm{Fe}}_2{\mathrm{As}}_2$ ($A=\text{K}$, Rb, Cs) is explained in terms of proximity to a van Hove singularity. Using the Hubbard model we show that when the Fermi energy is in the vicinity of a van Hove singularity, a downturn in spin susceptibility occurs as the temperature is lowered. This behavior is characterized by a temperature $T^{*}$ which is determined by the difference in energy between the Fermi level and the van Hove singularity. When vertex corrections are taken into account in a dynamical mean-field approximation, the effect of correlations amplifies the relative drop in the Knight shift and moves $T^{*}$ to lower temperatures.

The weakly screened electron-hole interactions in an atomically thin semiconductor not only downshift its excitation spectrum from a quasiparticle one, but also redistribute excitation energies and wave-function characters with profound effects on the diverse modes of the material response, including the exciton-phonon scattering processes accessible to resonant Raman measurements. Here, we develop a first-principles framework to calculate frequency-dependent resonant Raman intensities that includes excitonic effects and goes beyond the Placzek approximation. We show how excitonic effects in ${\mathrm{MoS}}_2$ strongly regulate Raman scattering amplitudes and thereby explain the puzzling near absence of a resonant Raman response around the $A$ and $B$ excitons (band-edge excitations which produce very strong signals in optical absorption), and also the pronounced strength of the resonant Raman response from the $C$ exciton (a higher-energy excitation arising from parallel valence and conduction bands). Furthermore, this efficient perturbative approach reduces the number of $GW$ plus Bethe-Salpeter-equation calculations from two per Raman mode (in finite displacement) to one for all modes and affords a natural extension to higher-order resonant Raman processes.

Crossings of energy bands in solids that are not pinned at symmetry points in the Brillouin zone and yet cannot be removed by perturbations are thought to be conditioned on the presence of a nonsymmorphic symmetry. In this article we show that such band crossings can actually appear also in a symmorphic crystal. A study of a class of tight-binding multiband one-dimensional lattice models of spinful electrons reveals that chiral, time-reversal, and site-mirror symmetries are sufficient to produce such movable but not removable band degeneracies.

Controllability of ultracold atomic gases has reached an unprecedented level, allowing for experimental realization of the long-sought-after Thouless pump, which can be interpreted as a dynamical quantum Hall effect. On the other hand, Weyl semimetals and Weyl nodal-line semimetals with touching points and rings in band structures have sparked tremendous interest in various fields in the past few years. Here, we show that dynamical Weyl points and dynamical four-dimensional (4D) Weyl nodal rings, which are protected by the first Chern number on a parameter surface formed by quasimomentum and time, emerge in a two-dimensional and three-dimensional system, respectively. We find that these dynamical topological gapless phenomena lead to a topological pump and the amount of pumped particles is not quantized and can be continuously tuned by controlling experimental parameters over a wide range, in stark contrast with the Thouless pump with quantized transport, which does not allow for such continuous tuning. We also propose an experimental scheme to realize the dynamical Weyl points and dynamical 4D Weyl nodal rings and to observe their corresponding topological pump in cold atomic gases.

We demonstrate that the parity-time ($\mathcal{PT}$) symmetric interfaces formed between non-Hermitian amplifying (“gainy”) and lossy topological crystals exhibit $\mathcal{PT}$ phase transitions separating phases of lossless and decaying/amplifying topological edge transport. The spectrum of these interface states exhibits exceptional points (EPs) separating (i) a $\mathcal{PT}$ symmetric real-valued regime with an evenly distributed wave function in both gainy and lossy domains and (ii) a $\mathcal{PT}$ broken complex-valued regime, in which edge states asymmetrically localize in one of the domains. Despite its complex-valued character, the edge spectrum remains gapless and connects complex-valued bulk bands through the EPs. We find that the regimes exist when the real edge spectrum is embedded into the bulk continuum without mixing, indicating that the edge states are protected against leakage into the bulk by the $\mathcal{PT}$ symmetry. Two exemplary $\mathcal{PT}$ symmetric systems, exhibiting valley and Chern topological phases, respectively, are investigated and the connection with the corresponding Hermitian systems is established. Interestingly, despite the complex bulk spectrum of the Chern insulator, the bulk-interface correspondence principle still holds, as long as the topological gap remains open. The proposed systems are experimentally feasible in photonics, which is evidenced by our rigorous full-wave simulations of $\mathcal{PT}$ symmetric silicon-based photonic graphene.

Due to mirror symmetry breaking, two-dimensional Janus transition metal dichalcogenides $MXY$ ($M=\text{Mo,W}$; $X,Y=\text{S,Se,Te}$) present charming electronic properties. However, there have not been many related studies as of yet, and the intrinsic physical pictures are unclear. Here, we use first-principles calculations to explore the universality of electronic characteristics and photocatalyst applications of Janus $MXY$, finding that the induced dipole moment, vibrational frequency, Rashba parameters, and direct-indirect band transition of monolayer $MXY$ are deeply associated with the atomic radius and electronegativity differences of chalcogen $X$ and $Y$ elements. The internal electric field renders Janus $MXY$ the ideal photocatalysts. Moreover, the stacking-dependent on/off switch of the dipole moment further confirms that asymmetric Janus $MXY$ serves as a promising candidate for highly efficient photocatalysts within a broad range from infrared, visible, to ultraviolet light.

The properties of graphene can be significantly modified by interaction with a commensurate substrate. Via proximity effects, both band gaps and substantial spin-orbit coupling can be induced, opening a route to novel applications. In this work, we ask whether the linear and nonlinear optical response can be applied to probe the underlying symmetry of gapped and proximitized graphene. Answering in the affirmative, we show that, in particular, optical second-harmonic generation is a highly sensitive tool for characterization. Importantly, the spin-orbit sublattice symmetry is clearly revealed by the optical spectra. In addition, the topological phase of the materials is revealed via linear and nonlinear spin-optical effects.

We discuss a network of Kitaev wires coupled to several individually-tunable quantum dots as an extension of the recent experiments on a quantum dot coupled to a nanowire hosting Majorana zero modes [M. T. Deng et al., Science 354, 1557 (2016); Phys. Rev. B 98, 085125 (2018)]. The setup features localized Majorana modes with exact zero energy and we show that they can be manipulated by solely acting on the quantum dots. A braiding process can be obtained by arranging three wires as a trijunction and a charge readout of the quantum dots can be used to reveal the non-Abelian statistics of Majorana zero modes. The setup can be scaled up to serve the more advanced purposes of topological quantum computation.

We present an ab initio study of boron nitride as well as carbon allotropes. Their relative thermodynamic stabilities and structural phase transitions from low- to high-density phases are investigated. Pressure-temperature phase diagrams are calculated and compared to experimental findings. The calculations are performed using quantum chemical wave-function-based as well as density functional theories. Our findings reveal that predicted energy differences often depend significantly on the choice of the employed method. Comparison between calculated and experimental results allows for benchmarking the accuracy of various levels of theory. The produced results show that quantum chemical wave-function-based theories allow for achieving systematically improvable estimates. We find that on the level of coupled cluster theories the low- and high-density phases of boron nitride become thermodynamically degenerate at 0 K. This is in agreement with recent experimental findings, indicating that cubic boron nitride is not the thermodynamically stable allotrope at ambient conditions. Furthermore, we employ the calculated results to assess transition probabilities from graphitic low-density to diamondlike high-density phases in an approximate manner. We conclude that the stacking order of the parent graphitic material is crucial for the possible formation of metastable wurtzite boron nitride and hexagonal carbon diamond also known as lonsdaleite.

Heat in phononic crystals (PnCs) is carried by phonons, which can behave coherently (wave-like) or incoherently (particle-like) depending on the modes, temperature, and length scales. By comparing the measured thermal conductivity of PnCs with theories, recent works suggest that thermal conductivity of PnCs can be explained by considering only surface and boundary scatterings, which not only backscatter phonons but also break their coherence. The logic here is that since average phonon wavelength at room temperature is only a few nanometers, the roughness at the surfaces and boundaries make the scattering diffusive (break the phase coherence of phonons), and thus only very long wavelength (low frequency) phonons with negligible contribution to total thermal conductivity remain coherent. Here, we theoretically show that in a thin film PnCs, the low frequency coherent phonons could significantly contribute to thermal conductivity when assuming the three-phonon scattering model for intrinsic scattering because of their extremely large density of states that result from the low dimensional nature. Yet, further analysis shows the contribution of the low frequency coherent phonons is still negligible within temperature range from 130 to 300 K due to the Akhiezer mechanism, which properly answers the question why the thermal conductivity of PnCs can be explained by considering only scattering of incoherent phonons at these temperatures.

Paramagnons are supposed to provide the pairing glue for unconventional superconductors. For the heavy-fermion superconductor ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$, there is indeed good evidence from inelastic neutron scattering (INS) that spin fluctuations drive the superconductivity. Here, we present the INS measurement of the inelastic response of the antiferromagnetic parent compound, ‘A-type’ ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$, to probe the relation to the excitations of the superconducting (‘S-type') sample. We find that the dispersion is very similar in the antiferromagnetic state and in the normal state of the superconducting sample. Pronounced differences to the response in the superconducting state exist at low energies around the zone center. These findings are in line with observations of other unconventional superconductors.

We investigate the ground-state and the finite-temperature properties of the bond-random $s=1/2$ Heisenberg model on a square lattice with frustrating nearest- and next-nearest-neighbor antiferromagnetic interactions, $J_1$ and $J_2$, by the exact diagonalization and the Hams–de Raedt methods. The ground-state phase diagram of the model is constructed in the randomness versus the frustration ($J_2/J_1$) plane, with the aim of clarifying the effects of randomness and frustration in stabilizing a variety of phases. We find that the randomness induces the gapless quantum spin liquid (QSL)-like state, the random-singlet state, together with the spin-glass state in a certain range of parameter space. The spin-glass state might be stabilized by employing the lattice directional degrees of freedom associated with the stripe-type magnetic order of the regular model. Possible implications to recent experiments on the square-lattice mixed-crystal antiferromagnet ${\mathrm{Sr}}_2\mathrm{Cu}\left({\mathrm{Te}}_{1-x}{\mathrm{W}}_x\right){\mathrm{O}}_6$ exhibiting the gapless QSL-like behaviors are discussed.

The detailed analysis of resistivity $\left(\rho \right)$ for single-crystalline ${\mathrm{EuTi}}_{1-x}{\mathrm{Nb}}_x{\mathrm{O}}_3(x=0.10$–0.20), an itinerant ferromagnet with very low Curie temperature $\left(T_C\right)$, reveals that the charge conduction in this system is extremely sensitive to Nb concentration and dominated by several scattering mechanisms. Well below $T_C$, where the spontaneous magnetization follows Bloch's $T^{3/2}$ law, $\rho$ exhibits $T^2$ dependence with a large coefficient $(\sim {10}^{-1}\mu \mathrm{\Omega }$ cm ${\mathrm{K}}^{-2})$ due to the electron-magnon scattering. Remarkably, all the studied samples exhibit a unique resistivity minimum at $T=T_{\min }$, below which $\rho$ shows a logarithmic increment with $T$ (for $T_{\mathrm{C}}<T<T_{\min })$ due to the Kondo scattering of $4d^1$ itinerant electrons of Nb by the localized $4f$ moments of ${\mathrm{Eu}}^{2+}$ which suppresses strongly with applied magnetic field. In the paramagnetic state, $T^2$ and $T^{3/2}$ dependence of the resistivity have been observed, suggesting an unusual crossover from a Fermi-liquid to a non-Fermi-liquid behavior with increasing $T$.

We present a general method to accelerate ab initio atomic structural relaxations. In this method, the conjugate gradient ab initio relaxation is preconditioned by a force field relaxation. This force field is constructed on-the-flight with the information of the ab initio forces and the current atomic configuration. At each ab initio relaxation step, the so-constructed force field is used to prerelax the system, with the relaxation direction as the preconditioned direction for the ab initio conjugated gradient method. The force field model and its parameters are rather general making this method applicable for a wide range of systems commonly used in material simulations. More than 80 different systems have been tested representing different cases in material simulations. Across the board, we find accelerations mostly over a factor of 2 and for some large systems with a factor of $6\sim 9$. There is no case where the relaxation becomes worse. The code and lib for this method are provided, which can be used as a plugin in a standard ab initio atomic relaxation code.

A quarter-wave impedance-matching element inserted between two dissimilar media allows full-power transmission, but this phenomenon occurs only when the mode of the incident and transmitted waves remains unaltered, either longitudinal (L) or transverse (T). Here, we report our asymptotic bimodal quarter-wave impedance-matching theory for full-power mode-converting transmission. It requires that two coupled quarter waves should be simultaneously formed inside a matching element possessing specific anisotropy for phase matching and L-to-T (T-to-L) mode conversion. Simulations using designed metamaterial matching elements show that nearly full mode-converting transmissions can be realized within a single medium or between two dissimilar media. We expect that our theory can be critically useful for medical and industrial ultrasonic applications wherever highly efficient mode conversion and high-powered transverse waves are required.

In this paper, the lattice thermal conductivities of bulk ZnO, GaN, and AlN in the normal-pressure wurtzite phase and high-pressure rocksalt phase at room temperature are investigated by solving the phonon Boltzmann transport equation based on first-principles calculations. The results show that the thermal conductivities of the rocksalt ZnO, GaN, and AlN close to the phase-transition pressure are reduced by 73%, 91%, and 34%, respectively, compared to that of corresponding normal-pressure wurtzite structures. Additionally, the thermal conductivity of wurtzite ZnO and GaN exhibits unexpectedly nonmonotonic dependence on pressure; however, for wurtzite and rocksalt AlN the thermal conductivity increases monotonically with the increase of pressure. Detailed phonon mode analysis reveals that the enhanced lattice anharmonicity is responsible for the depressed thermal conductivity of the rocksalt phase. By evaluating the relative contributions of dominant factors affecting lattice thermal conductivity, we find that the nonmonotonic response of thermal conductivity to pressure in wurtzite ZnO and GaN is attributed to the interplay of group velocity and phonon relaxation time. This study provides quantitative understanding of lattice thermal conductivity of ZnO, GaN, and AlN considering the pressure-induced phase transition and highlights the importance of pressure in manipulating lattice thermal conductivity.