Despite being usually considered two competing phenomena, charge-density wave and superconductivity coexist in few systems, the most emblematic one being the transition-metal dichalcogenide $2H-{\mathrm{NbSe}}_2$. This unusual condition is responsible for specific Raman signatures across the two phase transitions in this compound. While the appearance of a soft phonon mode is a well-established fingerprint of the charge-density-wave order, the nature of the sharp subgap mode emerging below the superconducting temperature is still under debate. In this work we use external pressure as a knob to unveil the delicate interplay between the two orders, and consequently the nature of the superconducting mode. Thanks to an advanced extreme-conditions Raman technique, we are able to follow the pressure evolution and the simultaneous collapse of the two intertwined charge-density-wave and superconducting modes. The comparison with microscopic calculations in a model system supports the Higgs-type nature of the superconducting mode and suggests that charge-density wave and superconductivity in $2H-{\mathrm{NbSe}}_2$ involve mutual electronic degrees of freedom. These findings fill the knowledge gap on the electronic mechanisms at play in transition-metal dichalcogenides, a crucial step to fully exploit their properties in few-layer systems optimized for device applications.

We investigated several FeSe single crystals grown by two different methods by utilizing experimental techniques, namely, resistivity, magnetoresistance, specific heat, scanning tunneling microscopy, and spectroscopy. The residual resistivity ratio (RRR) shows systematic differences between samples grown by chemical vapor transport and flux vapor transport, indicating variance in the amount of scattering centers. Although the superconducting transition temperature $T_c$ is not directly related to RRR, our study evidences subtle differences in the features of an incipient ordering mode related to a depletion of density of states at the Fermi level. For instance, the onset temperature of anisotropic spin fluctuations at $T^{*}\approx 75$ K, and the temperature of the opening up of a partial gap in the density of states at $T^{**}\approx 30$ K, are not discernible in the samples with lower RRR. Further, we show that the functional dependence of the electronic specific heat below 2 K, which allows us to determine the nodal features as well as the small superconducting gap, differs significantly in crystals grown by these two different methods. Our investigation suggests that some of the controversies about the driving mechanism for the superconducting gap or its structure and symmetry are related to minute differences in the crystals arising due to the growth techniques used and the total amount of scattering centers present in the sample.

The spin-$\frac{1}{2}$ Ising-Heisenberg pentagonal chain is investigated with use of the star-triangle transformation, which establishes a rigorous mapping equivalence with the effective spin-$\frac{1}{2}$ Ising zigzag ladder. The investigated model has a rich ground-state phase diagram including two spectacular quantum antiferromagnetic ground states with a fourfold broken symmetry. It is demonstrated that these long-period quantum ground states arise due to a competition between the effective next-nearest-neighbor and nearest-neighbor interactions of the corresponding spin-$\frac{1}{2}$ Ising zigzag ladder. The concurrence is used to quantify the bipartite entanglement between the nearest-neighbor Heisenberg spin pairs, which are quantum-mechanically entangled in two quantum ground states with or without spontaneously broken symmetry. The pair correlation functions between the nearest-neighbor Heisenberg spins as well as the next-nearest-neighbor and nearest-neighbor Ising spins were investigated with the aim to bring insight into how a relevant short-range order manifests itself at low enough temperatures. It is shown that the specific heat displays temperature dependencies with either one or two separate round maxima.

Recently synthesized metastable tetragonal CoSe, isostructural to the FeSe superconductor, offers a new avenue for investigating systems in close proximity to the iron-based superconductors. We present magnetic and transport property measurements on powders and single crystals of CoSe. High field magnetic susceptibility measurements indicate a suppression of the previously reported 10 K ferromagnetic transition with the magnetic susceptibility, exhibiting time dependence below the proposed transition. Dynamic scaling analysis of the time dependence yields a critical relaxation time of ${\tau }^{*}=0.064\pm 0.008$ s which in turn yields activation energy $E_a^{*}=14.84\pm 0.59$ K and an ideal glass temperature $T_0^{*}=8.91\pm 0.09$ K from Vogel-Fulcher analysis. No transition is observed in resistivity and specific heat measurements, but both measurements indicate that CoSe is metallic. These results are interpreted on the basis of CoSe exhibiting frustrated magnetic ordering arising from competing magnetic interactions. Arrott analysis of single crystal magnetic susceptibility has indicated the transition temperature occurs in close proximity to previous reports and that the magnetic moment lies solely in the $ab$ plane. The results have implications for understanding the relationship between magnetism and transport properties in the iron chalcogenide superconductors.

We study a simple model of photovoltaic energy harvesting across a Mott-insulating gap consisting of a correlated layer connected to two metallic leads held at different chemical potentials. We address, in particular, the issue of impact ionization, whereby a particle photoexcited to the high-energy part of the upper Hubbard band uses its extra energy to produce a second particle-hole excitation. We find a drastic increase of the photocurrent upon entering the frequency regime where impact ionization is possible. At large values of the Mott gap, where impact ionization is energetically not allowed, we observe a suppression of the current and a piling up of charge in the high-energy part of the upper Hubbard band. Our study is based on a Floquet dynamical mean-field theory treatment of the steady state with the so-called auxiliary master equation approach as impurity solver. We verify that an additional approximation, taking the self-energy diagonal in the Floquet indices, is appropriate for the parameter range we are considering.

We present a time- and angle-resolved photoemission study of the transition-metal dichalcogenide ${\mathrm{WTe}}_2$, a candidate type-II Weyl semimetal exhibiting extremely large magnetoresistence. Using femtosecond light pulses, we characterize the unoccupied states of the electron pockets above the Fermi level. Following the ultrafast carrier relaxation in distinct parts of the Brillouin zone, we report remarkably similar decay dynamics for electrons and holes. Our results confirm that charge compensation between electron and hole pockets—a key effect to explain the nonsaturating magnetoresistance of this material—is a distinctive feature of ${\mathrm{WTe}}_2$ even in an out-of-equilibrium regime.

We investigate the crystal structure and the electronic properties of the trilayer nickelate ${\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$ by means of quantum-mechanical calculations in the framework of the density-functional theory. We find that, at low temperature, ${\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$ undergoes a hitherto unreported structural phase transition and transforms to a new monoclinic $P{2}_1/a$ phase. This phase exhibits electronic properties in agreement with recent angle-resolved photoemission spectroscopy data reported in H. Li et al., [Nat. Commun. 8, 704 (2017)] and should be considered in models focused on explaining the observed $\sim 140$ K metal-to-metal phase transition.

Electronic structure and electron dynamics in the ternary topological insulator $\mathrm{B}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_2\mathrm{Se}$ are studied with time- and angle-resolved photoemission spectroscopy using optical pumping. An unoccupied surface resonance split off from the bulk conduction band previously indirectly observed in scanning tunneling measurements is spectroscopically identified. Furthermore, an unoccupied topological surface state (TSS) is found, which is serendipitously located at about 1.5 eV above the occupied TSS, thereby facilitating direct optical transitions between the two surface states at $\hslash \omega =1.5\mathrm{eV}$ in an $n$-type topological insulator. An appreciable nonequilibrium population of the bottom of the bulk conduction band is observed for longer than 15 ps after the pump pulse. This leads to a long recovery time of the lower TSS, which is constantly populated by the electrons coming from the bulk conduction band. Our results demonstrate $\mathrm{B}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_2\mathrm{Se}$ to be an ideal platform for designing future optoelectronic devices based on topological insulators.

Raman spectra of graphene and ${\mathrm{MoS}}_2$ are calculated for incident and scattered circularly polarized light. In the case of graphene, the well known $G$-band Raman spectra have a not well known property that the helicity of the incident circularly polarized light changes to another helicity in the scattered light. Using the electron-photon and electron-phonon matrix elements by first-principles calculation, we calculate resonant Raman spectra of graphene and ${\mathrm{MoS}}_2$ for circularly polarized light which are compared with recent experiments. The Raman intensity for circularly polarized light is relevant to optical valley polarization in the case of ${\mathrm{MoS}}_2$. We also discuss how the helicity-selection rule can be modified by applying stress to graphene.

The properties of the low-lying eigenvalues of the entanglement Hamiltonian and their relation to the localization length of a disordered interacting one-dimensional many-particle system are studied. The average of the first entanglement Hamiltonian level spacing is proportional to the ground-state localization length and shows the same dependence on the disorder and interaction strength as the localization length. This is the result of the fact that entanglement is limited to distances of order of the localization length. The distribution of the first entanglement level spacing shows a Gaussian-type behavior as expected for level spacings much larger than the disorder broadening. For weakly disordered systems (localization length larger than sample length), the distribution shows an additional peak at low-level spacings. This stems from rare regions in some samples which exhibit metalliclike behavior of large entanglement and large particle-number fluctuations. These intermediate microemulsion metallic regions embedded in the insulating phase are discussed.

${\mathrm{ThTaN}}_3$, a rare cubic perovskite nitride semiconductor, has been studied using ab initio methods. Spin-orbit coupling (SOC) results in band inversion and a band gap of 150 meV at the zone center. Despite trivial $Z_2$ indices, two pairs of spin-polarized surface bands cross the gap near the zone center, indicating that this system is a topological crystalline insulator with the mirror Chern number of $|{\mathcal{C}}_m|=2$ protected by the mirror and $C_4$ rotational symmetries. Additionally, SOC doubles the Seebeck coefficient, leading to a maximum of $\sim 400\mu \mathrm{V}/\mathrm{K}$ at 150 K for carrier-doping levels of several ${10}^{17}/{\mathrm{cm}}^3.{\mathrm{ThTaN}}_3$ combines excellent bulk thermopower with parallel conduction through topological surface states that may point toward new possibilities for platforms for engineering devices with larger figures of merit.

We show how to realize topologically protected crossings of three energy bands, integer-spin analogs of Weyl fermions, in three-dimensional optical lattices. Our proposal only involves ultracold atom techniques that have already been experimentally demonstrated and leads to isolated triple-point crossings (TPCs) which are required to exist by a novel combination of lattice symmetries. The symmetries also allow for a new type of topological object, the type-II, or tilted, TPC. Our Rapid Communication shows that spin-1 Weyl points, which have not yet been observed in the band structure of crystals, are within reach of ultracold atom experiments.

A theoretical framework is developed for better understanding the time-dependent soft-x-ray response of dissipative quantum many-body systems. It is shown how x-ray absorption and resonant inelastic x-ray scattering (RIXS) at transition-metal $L$ edges can provide insight into ultrafast intersystem crossings of importance for energy conversion, ultrafast magnetism, and catalysis. The photoinduced doublet-to-quartet spin crossover on cobalt in Fe-Co Prussian blue analogs is used as a model system to demonstrate how the x-ray response is affected by the nonequilibrium dynamics on a femtosecond time scale. Changes in local spin and symmetry and the underlying mechanism are reflected in strong broadenings, a collapse of clear selection rules during the intersystem crossing, fluctuations in the isotropic branching ratio in x-ray absorption, crystal-field collapse and/or oscillations, and time-dependent anti-Stokes processes in RIXS.

The tetragonal phase of CuMnAs progressively appears as one of the key materials for antiferromagnetic spintronics due to efficient current-induced spin-torques whose existence can be directly inferred from crystal symmetry. Theoretical understanding of spintronic phenomena in this material, however, relies on the detailed knowledge of electronic structure (band structure and corresponding wave functions) which has so far been tested only to a limited extent. We show that AC permittivity (obtained from ellipsometry) and UV photoelectron spectra agree with density functional calculations. Together with the x-ray diffraction and precession electron diffraction tomography, our analysis confirms recent theoretical claim [Phys. Rev. B 96, 094406 (2017)] that copper atoms occupy lattice positions in the basal plane of the tetragonal unit cell.

Superradiance explains the collective enhancement of emission, observed when nanophotonic emitters are arranged within subwavelength proximity and perfect symmetry. Thermal superradiant emitter assemblies with variable photon far-field coupling rates are known to be capable of outperforming their conventional, nonsuperradiant counterparts. However, due to the inability to account for assemblies comprising emitters with various materials and dimensional configurations, existing thermal superradiant models are inadequate and incongruent. In this paper, a generalized thermal superradiant assembly for nanophotonic emitters is developed from first principles. Spectral analysis shows that not only does the proposed model outperform existing models in power delivery, but also portrays unforeseen and startling characteristics during emission. These electromagnetically induced transparency like (EIT-like) and superscattering-like characteristics are reported here for a superradiant assembly, and the effects escalate as the emitters become increasingly disparate. The fact that the EIT-like characteristics are in close agreement with a recent experimental observation involving the superradiant decay of qubits strongly bolsters the validity of the proposed model.

The recent realization of single-electron sources lets us envision performing electron quantum optics experiments, where electrons can be viewed as flying qubits propagating in a ballistic conductor. To date, all electron sources operate in a periodic electron injection mode, leading to energy spectrum singularities in various physical observables which sometimes hide the bare nature of physical effects. To go beyond this, we propose a spread-spectrum approach where electron flying qubits are injected in a nonperiodic manner following a pseudorandom binary bit pattern. Extending the Floquet scattering theory approach from periodic to spread-spectrum drive, the shot noise of pseudorandom binary sequences of single-electron injection can be calculated for leviton and nonleviton sources. Our new approach allows us to disentangle the physics of the manipulated excitations from that of the injection protocol. In particular, the spread-spectrum approach is shown to provide better knowledge of electronic Hong-Ou-Mandel correlations and to clarify the nature of the pulse train coherence and the role of the dynamical orthogonality catastrophe for noninteger charge injection.

Ab initio density-functional calculations are used to investigate the response of four face-centered-cubic (fcc) high-entropy alloys (HEAs) to tensile and shear loading. The ideal tensile and shear strengths (ITS and ISS) of the HEAs are studied by employing first-principles alloy theory formulated within the exact muffin-tin orbital method in combination with the coherent-potential approximation. We benchmark the computational accuracy against literature data by studying the ITS under uniaxial [110] tensile loading and the ISS for the $\left[11\overline{2}\right]\left(111\right)$ shear deformation of pure fcc Ni and Al. For the HEAs, we uncover the alloying effect on the ITS and ISS. Under shear loading, relaxation reduces the ISS by $\sim 50\%$ for all considered HEAs. We demonstrate that the dimensionless tensile and shear strengths are significantly overestimated by adopting two widely used empirical models in comparison with our ab initio calculations. In addition, our predicted relationship between the dimensionless shear strength and shear instability are in line with the modified Frenkel model. Using the computed ISS, we derive the half-width of the dislocation core for the present HEAs. Employing the ratio of ITS to ISS, we discuss the intrinsic ductility of HEAs and compare it with a common empirical criterion. We observe a strong linear correlation between the shear instability and the ratio of ITS to ISS, whereas a weak positive correlation is found in the case of the empirical criterion.