The absence of inversion symmetry leads to a strong spin-orbit splitting of the upper valence band of semiconducting single-layer transition-metal dichalchogenides such as ${\mathrm{MoS}}_2$ or ${\mathrm{WS}}_2$. This permits a direct comparison of the electron-phonon coupling strength in states that only differ by their spin. Here, the electron-phonon coupling in the valence band maximum of single-layer ${\mathrm{WS}}_2$ is studied by first-principles calculations and angle-resolved photoemission. The coupling strength is found to be drastically different for the two spin-split branches, with calculated values of ${\lambda }_K=0.0021$ and 0.40 for the upper and lower spin-split valence band of the freestanding layer, respectively. This difference is somewhat reduced when including scattering processes involving the Au(111) substrate present in the experiment but it remains significant, in good agreement with the experimental results.

We report an in-depth angle-resolved photoemission spectroscopy study on $2H\text{−}{\mathrm{TaS}}_2$, a canonical incommensurate charge density wave (CDW) system. This study demonstrates that just as in related incommensurate CDW systems, $2H\text{−}{\mathrm{TaSe}}_2$ and $2H\text{−}{\mathrm{NbSe}}_2$, the energy gap (${\mathrm{\Delta }}_{\text{CDW}}$) of $2H\text{−}{\mathrm{TaS}}_2$ is localized along the $K$-centered Fermi surface barrels and is particle-hole asymmetric. The persistence of ${\mathrm{\Delta }}_{\text{CDW}}$ even at temperatures higher than the CDW transition temperature ${\mathit{T}}_{\text{CDW}}$ in $2H\text{−}{\mathrm{TaS}}_2$, reflects the similar pseudogap behavior observed previously in $2H\text{−}{\mathrm{TaSe}}_2$ and $2H\text{−}{\mathrm{NbSe}}_2$. However, in sharp contrast to $2H\text{−}{\mathrm{NbSe}}_2$, where ${\mathrm{\Delta }}_{\text{CDW}}$ is nonzero only in the vicinity of a few “hot spots” on the inner $K$-centered Fermi surface barrels, ${\mathrm{\Delta }}_{\text{CDW}}$ in $2H\text{−}{\mathrm{TaS}}_2$ is nonzero along the entirety of both $K$-centered Fermi surface barrels. Based on a tight-binding model, we attribute this dichotomy in the momentum dependence and the Fermi surface specificity of ${\mathrm{\Delta }}_{\text{CDW}}$ between otherwise similar CDW compounds to the different orbital orientations of their electronic states that participate in the CDW pairing. Our results suggest that the orbital selectivity plays a critical role in the description of incommensurate CDW materials.

We study 't Hooft anomalies of discrete groups in the framework of (1+1)-dimensional multiscale entanglement renormalization ansatz states on the lattice. Using matrix product operators, general topological restrictions on conformal data are derived. An ansatz class allowing for optimization of MERA with an anomalous symmetry is introduced. We utilize this class to numerically study a family of Hamiltonians with a symmetric critical line. Conformal data is obtained for all irreducible projective representations of each anomalous symmetry twist, corresponding to definite topological sectors. It is numerically demonstrated that this line is a protected gapless phase. Finally, we implement a duality transformation between a pair of critical lines using our subclass of MERA.

We propose a diagnostic tool for detecting nontrivial symmetry-protected topological (SPT) phases protected by a symmetry group $G$ in $2+1$ dimensions. Our method is based on directly studying the $1+1$-dimensional anomalous edge conformal field theory (CFT) of SPT phases. We claim that if the CFT is the edge theory of an SPT phase, then there must be an obstruction to cutting it open. This obstruction manifests as the in-existence of boundary states in the CFT that preserves both the conformal symmetry and the global symmetry $G$. We discuss the relation between edgeability, i.e., the ability to find a consistent boundary state, and gappability, i.e., the ability to gap out a CFT, in the presence of $G$. We study several cases including time-reversal symmetric topological insulators, ${\mathbb{Z}}_N$ symmetric bosonic SPT phases, and ${\mathbb{Z}}_2\times {\mathbb{Z}}_2$ symmetric topological superconductors.

We report a study of the temperature-dependent electrical resistivity, Seebeck coefficient, thermal conductivity, specific heat, and ${}^{27}\mathrm{Al}$ nuclear magnetic resonance (NMR) in Heusler-type ${\mathrm{Ru}}_2\mathrm{TaAl}$, to shed light on its semimetallic behavior. While the temperature dependence of the electrical resistivity exhibits semiconductorlike behavior, the analysis of low-temperature specific heat reveals a residual Fermi-level density of states (DOS). Both observations can be realized by means of a semimetallic scenario with the Fermi energy located in the pseudogap of the electronic DOS. The NMR Knight shift and spin-lattice relaxation rate show activated behavior at higher temperatures, attributing to the thermally excited carriers across a pseudogap in ${\mathrm{Ru}}_2\mathrm{TaAl}$. From the first-principles band structure calculations, we further provide a clear picture that an indirect overlap between electron and hole pockets is responsible for the formation of a pseudogap in the vicinity of the Fermi level of ${\mathrm{Ru}}_2\mathrm{TaAl}$. In addition, an effort for improving the thermoelectric performance of ${\mathrm{Ru}}_2\mathrm{TaAl}$ has been made by investigating the thermoelectric properties of ${\mathrm{Ru}}_{1.95}{\mathrm{Ta}}_{1.05}\mathrm{Al}$. We found significant enhancements in the electrical conductivity and Seebeck coefficient and marked reduction in the thermal conductivity via the off-stoichiometric approach. This leads to an increase in the figure-of-merit $ZT$ value from $6.1\times {10}^{-4}$ in ${\mathrm{Ru}}_2\mathrm{TaAl}$ to $3.4\times {10}^{-3}$ in ${\mathrm{Ru}}_{1.95}{\mathrm{Ta}}_{1.05}\mathrm{Al}$ at room temperature. In this respect, a further improvement of thermoelectric performance based on ${\mathrm{Ru}}_2\mathrm{TaAl}$ through other off-stoichiometric attempts is highly probable.

The interplay of Ising spin-orbit coupling and nontrivial band topology in transition metal dichalcogenides (TMDs) produces anomalous transport and optical properties that are very different from a regular two-dimensional (2D) electron gas. The spin-momentum locking of optically excited carriers near a valley point can give rise to an anomalous spin-valley Hall current under the application of an in-plane electric field. TMDs also exhibit strong electron-nuclear hyperfine interactions, but their effect on spin-valley-locked currents remains unknown. Here, we show that hyperfine interactions can create a feedback mechanism in which spin-valley currents generate significant dynamical nuclear polarization which in turn Zeeman shifts excitonic transitions out of resonance with an optical driving field, saturating the production of spin-valley polarization. We propose an experimental signature of dynamic nuclear polarization which can be detected via measurements of the anomalous Hall current. Our results help to elucidate the interplay of valley polarization and nuclear-spin dynamics in TMDs.

We report on magnetotransport properties of a MgZnO/ZnO heterostructure subjected to weak direct currents. We find that in the regime of overlapping Landau levels, the differential resistivity acquires a quantum correction proportional to both the square of the current and the Dingle factor. The analysis shows that the correction to the differential resistivity is dominated by a current-induced modification of the electron distribution function and allows us to access both quantum and inelastic scattering rates.

Photoemission tomography (PT), the analysis of the photoemission intensity distribution within the plane wave final-state approximation, is being established as a useful tool for extracting the electronic and geometric structure of weakly interacting organic overlayers. Here we present a simple method for extending PT, which until now has been based on the calculations of isolated molecules. By including the substrate and a damped plane-wave final state, we are able to simulate the photoemission intensity distribution of two-dimensional molecular overlayers with both strong intermolecular and molecule-substrate interactions, here demonstrated for the model system 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA) on Cu(100). It is shown that the interaction and hybridization of the lowest unoccupied molecular orbital of PTCDA with substrate states leads to its occupation and the formation of a strongly dispersing intermolecular band, whose experimental magnitude of 1.1 eV and $k$-space periodicity is well reproduced theoretically.

We evidence magnetic field triggered polariton lasing in a microcavity containing semimagnetic quantum wells. This effect is associated with a decrease of the polariton lasing threshold power in magnetic field. The observed magnetic field dependence of the threshold power systematically exhibits a minimum which only weakly depends on the zero-field photon-exciton detuning. These results are interpreted as a consequence of the polariton giant Zeeman splitting which in magnetic field leads to a decrease of the number of accessible states in the lowest polariton branch by a factor of 2, and substantially changes the photon-exciton detuning.

The universality of the shear viscosity of alkali metals is studied up to high pressure. Equilibrium molecular dynamics simulations are used to calculate the stress autocorrelation function, which allows us to obtain the value of shear viscosity using the Green-Kubo formula. Atomic interactions are computed from Fiolhais pseudopotential and are validated by comparison between pair distribution functions and mean-squared displacements obtained from classical and ab initio molecular dynamics simulations. The description of the interactions is accurate at least up to 12 GPa, 9.4 GPa, 6.6 GPa, and 3 GPa for Na, K, Rb, and Cs, respectively, and to a lesser extent up to 4.8 GPa for Li. A good agreement between simulation and experimental viscosity results along the liquid-gas coexistence curve is found. The viscosity appears to be a universal property over a wide range of the liquid phase of the phase diagram, between 0.85 and 1.5 times the ambient melting density and up to seven times the ambient melting temperature. Scaling laws are proposed following relations formulated in [Meyer, Xu, and Wax, Phys. Rev. B 93, 214203 (2016)] so that it is possible to predict the viscosity value of any alkali metal with an accuracy better than 10% over the corresponding density and temperature range.

We present an ab initio study of the electronic stopping power of protons and helium ions in an insulating material, ${\mathrm{HfO}}_2$. The calculations are carried out in channeling conditions with different impact parameters by employing Ehrenfest dynamics and real-time, time-dependent density functional theory. The satisfactory comparison with available experiments demonstrates that this approach provides an accurate description of electronic stopping power. The velocity-proportional stopping power is predicted for protons and helium ions in the low-energy region, which conforms to the linear response theory. Due to the existence of a wide band gap, a threshold effect in the extremely low velocity regime below excitation is expected. For protons, the threshold velocity is observable, while it does not appear in the case of helium ions. This indicates the existence of extra energy-loss channels beyond the electron-hole pair excitation when helium ions are moving through the crystal. To analyze it, we checked the charge state of the moving projectiles and an explicit charge exchange behavior between the ions and host atoms was found. The missing threshold effect for helium ions is attributed to the charge transfer, which also contributes to energy loss of the ion.

Long-wavelength spin spiral structures are ubiquitous in a large variety of magnetic materials. The detailed magnetic structure can take many variations owing to their different physical origins. Therefore, the unambiguous structural determination is crucial for understanding these spin systems, though such a task is experimentally challenging. Here, we show that ordered spin spiral structures can be fully determined in a single measurement by dichroic resonant elastic x-ray scattering using circularly polarized light. It is found that at certain geometrical conditions, the circular dichroism of the diffraction vanishes completely, revealing a one-to-one correspondence with the spin structure. We demonstrate both theoretically and experimentally this experimental principle, which allows for unambiguous structure determination immediately from the measured signal, whereby no modeling-based data refinement is needed. This largely expands the capabilities of conventional magnetic characterization techniques.

Diffusion of ${\mathrm{Li}}^{+}$ in (111) oriented thin films of the spinels ${\mathrm{Li}}_4{\mathrm{Ti}}_5{\mathrm{O}}_{12}$ and ${\mathrm{LiTi}}_2{\mathrm{O}}_4$ has been studied with ${}^8\mathrm{Li}\beta$-detected NMR in the temperature range between 5 and 310 K. In ${\mathrm{Li}}_4{\mathrm{Ti}}_5{\mathrm{O}}_{12}$, the spin-lattice relaxation rate ($1/T_1$) versus temperature shows a clear maximum around 100 K ($=T_{\max }$) which we attribute to magnetic freezing of dilute ${\mathrm{Ti}}^{3+}$ local magnetic moments, consistent with the results of magnetization and muon spin relaxation (${\mu }^{+}\mathrm{SR}$) measurements. The decrease in $1/T_1$ with temperature above $T_{\max }$ indicates that ${\mathrm{Li}}^{+}$ starts to diffuse with a thermal activation energy ($E_a$) of 0.11(1) eV. In ${\mathrm{LiTi}}_2{\mathrm{O}}_4$, on the contrary, as temperature increases from 200 K, $1/T_1$ increases monotonically up to 310 K. This suggests that Li also starts to diffuse above 200 K with $E_a=0.16\left(2\right)\mathrm{eV}$ in ${\mathrm{LiTi}}_2{\mathrm{O}}_4$. Comparison with conventional Li-NMR on ${\mathrm{Li}}_4{\mathrm{Ti}}_5{\mathrm{O}}_{12}$ implies that both $\beta$-NMR and ${\mu }^{+}\mathrm{SR}$ sense short-range Li motion, i.e., a jump diffusion of ${\mathrm{Li}}^{+}$ to the nearest neighboring sites.

Cu(pyz)${\left({\mathrm{NO}}_3\right)}_2$ is a quasi-one-dimensional molecular antiferromagnet that exhibits three-dimensional long-range magnetic order below $T_{\mathrm{N}}=110\mathrm{mK}$ due to the presence of weak interchain exchange couplings. Here, we compare calculations of the three largest exchange coupling constants in this system using two techniques based on plane-wave basis-set density functional theory: (i) a dimer fragment approach and (ii) an approach using periodic boundary conditions. The calculated values of the large intrachain coupling constant are found to be consistent with experiment, showing the expected level of variation between different techniques and implementations. However, the interchain coupling constants are found to be smaller than the current limits on the resolution of the calculations. This is due to the computational limitations on convergence of absolute energy differences with respect to basis set, which are larger than the interchain couplings themselves. Our results imply that errors resulting from such limitations are inherent in the evaluation of small exchange constants in systems of this sort, and that many previously reported results should therefore be treated with caution.

The exceptional ability of carbon to form ${\mathit{sp}}^2$ and ${\mathit{sp}}^3$ bonding states leads to a great structural and chemical diversity of carbon-bearing phases at nonambient conditions. Here we use laser-heated diamond-anvil cells combined with synchrotron x-ray diffraction, Raman spectroscopy, and first-principles calculations to explore phase transitions in $\mathrm{CaC}{\mathrm{O}}_3$ at $P>40\mathrm{GPa}$. We find that postaragonite $\mathrm{CaC}{\mathrm{O}}_3$ transforms to the previously predicted $P{2}_1/c\mathrm{CaC}{\mathrm{O}}_3$ with ${\mathit{sp}}^3$-hybridized carbon at 105 GPa ($\sim 30\mathrm{GPa}$ higher than the theoretically predicted crossover pressure). The lowest-enthalpy transition path to $P{2}_1/c\mathrm{CaC}{\mathrm{O}}_3$ includes reoccurring ${\mathit{sp}}^2$ and ${\mathit{sp}}^3\mathrm{CaC}{\mathrm{O}}_3$ intermediate phases and transition states, as revealed by our variable-cell nudged-elastic-band simulation. Raman spectra of $P{2}_1/c\mathrm{CaC}{\mathrm{O}}_3$ show an intense band at $1025\mathrm{c}{\mathrm{m}}^{-1}$, which we assign to the symmetric C-O stretching vibration based on empirical and first-principles calculations. This Raman band has a frequency that is $\sim 20\%$ lower than the symmetric C-O stretching in ${\mathit{sp}}^2\mathrm{CaC}{\mathrm{O}}_3$ due to the C-O bond length increase across the ${\mathit{sp}}^2-{\mathit{sp}}^3$ transition and can be used as a fingerprint of tetrahedrally coordinated carbon in other carbonates.

Using variational wave functions and Monte Carlo techniques, we study the antiferromagnetic Heisenberg model with first-neighbor $J_1$ and second-neighbor $J_2$ antiferromagnetic couplings on the honeycomb lattice. We perform a systematic comparison of magnetically ordered and nonmagnetic states (spin liquids and valence-bond solids) to obtain the ground-state phase diagram. Néel order is stabilized for small values of the frustrating second-neighbor coupling. Increasing the ratio $J_2/J_1$, we find strong evidence for a continuous transition to a nonmagnetic phase at $J_2/J_1\approx 0.23$. Close to the transition point, the Gutzwiller-projected uniform resonating valence-bond state gives an excellent approximation to the exact ground-state energy. For $0.23\lesssim J_2/J_1\lesssim 0.36$, a gapless $Z_2$ spin liquid with Dirac nodes competes with a plaquette valence-bond solid. In contrast, the gapped spin liquid considered in previous works has significantly higher variational energy. Although the plaquette valence-bond order is expected to be present as soon as the Néel order melts, this ordered state becomes clearly favored only for $J_2/J_1\gtrsim 0.3$. Finally, for $0.36\lesssim J_2/J_1\le 0.5$, a valence-bond solid with columnar order takes over as the ground state, being also lower in energy than the magnetic state with collinear order. We perform a detailed finite-size scaling and standard data collapse analysis, and we discuss the possibility of a deconfined quantum critical point separating the Néel antiferromagnet from the plaquette valence-bond solid.

We investigated quantum critical behaviors in the nonequilibrium steady state of a XXZ spin chain with boundary Markovian noise using Fisher information. The latter represents the distance between two infinitesimally close states, and its superextensive size scaling witnesses a critical behavior due to a phase transition since all the interaction terms are extensive. Perturbatively, in the noise strength, we found superextensive Fisher information at anisotropy $\left|\mathrm{\Delta }\right|⩽1$ and irrational $\frac{arccos\mathrm{\Delta }}{\pi }$ irrespective of the order of two noncommuting limits, i.e., the thermodynamic limit and the limit of sending $\frac{arccos\mathrm{\Delta }}{\pi }$ to an irrational number via a sequence of rational approximants. From this result we argue the existence of a nonequilibrium quantum phase transition with a critical phase $\left|\mathrm{\Delta }\right|⩽1$. From the nonsuperextensivity of the Fisher information of reduced states, we infer that this nonequilibrium quantum phase transition does not have local order parameters but has nonlocal ones, at least at $\left|\mathrm{\Delta }\right|=1$. In the nonperturbative regime for the noise strength, we numerically computed the reduced Fisher information which lower bounds the full-state Fisher information and is superextensive only at $\left|\mathrm{\Delta }\right|=1$. From the latter result, we derived local order parameters at $\left|\mathrm{\Delta }\right|=1$ in the nonperturbative case. The existence of critical behavior witnessed by the Fisher information in the phase $\left|\mathrm{\Delta }\right|<1$ is still an open problem. The Fisher information also represents the best sensitivity for any estimation of the control parameter, in our case the anisotropy $\mathrm{\Delta }$, and its superextensivity implies enhanced estimation precision which is also highly robust in the presence of a critical phase.

We present a formalism to calculate the orbital magnetization of interacting Dirac fermions under a magnetic field. In this approach, the divergence difficulty is overcome with a special limit of the derivative of the thermodynamic potential with respect to the magnetic field. The formalism satisfies the particle-hole symmetry of the Dirac fermions system. We apply the formalism to the interacting Dirac fermions in graphene. The charge and spin orderings and the exchange interactions between all the Landau levels are taken into account by the mean-field theory. The results for the orbital magnetization of interacting Dirac fermions are compared with that of noninteracting cases.