One of the unique properties of graphene is its extremely high mechanical strength. Several studies have shown that the mechanical failure of a graphene sheet under a tensile strain is due to the enhancement of the Kohn anomaly of the zone boundary transverse optical phonon modes. In this work, we derive an analytical expression of the Kohn anomaly parameter ${\alpha }_{\vec{K}}$ of these phonons in graphene deformed by a uniaxial strain along the armchair direction. We show that the tilt of Dirac cones, induced by the strain, contributes to the enhancement of the Kohn anomaly under a tensile deformation and gives rise to a dominant contribution of the so-called outer intervalley-mediated phonon processes. Moreover, the Kohn anomaly is found to be anisotropic with respect to the phonon wave vectors around the $K$ point. This anisotropy may be at the origin of the light polarization dependence of the Raman 2D band of the strained graphene. Our results not only uncover the role of the Kohn anomaly in the anisotropic mechanical failure of the graphene sheet under strains applied along the armchair and zigzag directions, but they also shed light on the doping-induced strengthening of strained graphene.

We investigate the generation and temperature-induced evolution of optically driven spin photocurrents in ${\mathrm{WTe}}_2$ and ${\mathrm{MoTe}}_2$. By correlating the scattering-plane dependence of the spin photocurrents with the symmetry analysis, we find that a sizable spin photocurrent can be controllably driven along the chain direction by optically exciting the system in the high-symmetry $y-z$ plane. Temperature dependence measurements show that pronounced variations in the spin photocurrent emerge at temperatures that coincide with the onset of anomalies in their transport and optical properties. The decreasing trend in the temperature dependence starting below $\sim 150\mathrm{K}$ is attributed to the temperature-induced Lifshitz transition. The sign inversion of the spin photocurrent, observed around 50 K in ${\mathrm{WTe}}_2$ and around 120 K in ${\mathrm{MoTe}}_2$, may have its origin in an interaction that involves multiple kinds of carriers.

We report on a combined measurement of high-resolution x-ray diffraction on powder and Raman scattering on single crystalline ${\mathrm{NiS}}_{2-x}{\mathrm{Se}}_x$ samples that exhibit the insulator-metal transition with Se doping. Via x rays, an abrupt change in the bond length between Ni and S (Se) ions was observed at the transition temperature, in sharp contrast to the almost constant bond length between chalcogen ions. Raman scattering, a complementary technique with the unique sensitivity to the vibrations of chalcogen bonds, revealed no anomalies in the phonon spectrum, consistent with the x-ray diffraction results. This indicates the important role of the interaction between Ni and S (Se) in the insulator-metal transition. The potential implication of this interpretation is discussed in terms of current theoretical models.

The Chern insulator phase is shown to emerge in two-dimensional arrays of polariton rings where time-reversal symmetry is broken due to the application of an out-of-plane magnetic field. The interplay of Zeeman splitting with the photonic analog of spin-orbit coupling [the transverse-electric–transverse-magnetic (TE-TM) splitting] inherently present in this system leads to the appearance of synthetic U(1) gauge field and the opening of topologically nontrivial spectral gaps. This results in the onset of topologically protected chiral edge states similar to those forming in quantum Hall effect. In one-dimensional zigzag arrays of polariton rings edge states similar to those appearing in the Su-SchriefferHeeger (SSH) model are formed.

Recently, replacing or mixing organic molecules in hybrid halide perovskites with inorganic Cs or Rb cations was reported to increase the material stability with comparable solar cell performance. In this work, we systematically investigate the electronic and optical properties of all-inorganic alkali iodide perovskites ${\mathrm{Rb}}_{1-x}{\mathrm{Cs}}_x{\mathrm{PbI}}_3$ using first-principles virtual-crystal approximation calculations. Our calculations show that with increasing the Cs content $x$, band gaps, exciton binding energies, and effective masses of charge carriers decrease following the quadratic (linear for effective masses) functions, while lattice constants and static dielectric constants increase following the quadratic function, indicating an enhancement of solar cell performance upon the Rb addition to ${\mathrm{CsPbI}}_3$. By including the many-body interaction within the GW approximation and incorporating the spin-orbit coupling (SOC), we obtain a more reliable band gap compared with experiment for ${\mathrm{CsPbI}}_3$, highlighting the importance of using the GW+SOC approach for the all-inorganic and organic-inorganic hybrid halide perovskite materials.

The honeycomb iridate ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ has received much attention as a candidate to realize a quantum spin liquid state, but the nature of its insulating state remains controversial. We found that the material exhibits structural transitions at 3 and 10 GPa. The former is accompanied by 166-meV suppression of the activation gap, but the energies for the low-lying interband transitions change by less than 10 meV. This can be reconciled in a picture in which the application of high pressure barely shifts the electronic bands, but rather merely broadens them. First-principles calculations uncover a strong correlation between the band gap and the $\beta$ angle of the monoclinic structure, indicating non-negligible interlayer coupling. These results offer clear evidence for a spin-orbit Mott insulating state in ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ and are inconsistent with the quasimolecular orbital model.

Starting from the three-band Hubbard model for the cuprates, we calculate analytically the four-spin cyclic exchange in the limit of infinite on-site Coulomb repulsion and zero O-O hopping $t_{pp}$ using two methods: (i) perturbation theory in $t_{pd}/\mathrm{\Delta }$, where $t_{pd}$ is the Cu-O hopping and $\mathrm{\Delta }$ the Cu-O charge transfer energy and (ii) exact solution of a ${\mathrm{Cu}}_4{\mathrm{O}}_4$ plaquette. The latter method coincides with the first to order eight in $t_{pd}$ and permits us to extend the results to $t_{pd}/\mathrm{\Delta }$ of order one. The results are relevant to recent experimental and theoretical research that relate the splitting of certain spin excitations with $\mathrm{\Delta }$ and the superconducting critical temperature.

We consider the propagation of a single particle in a random chain, assisted by the coupling to dispersive bosons. Time evolution treated with rate equations for hopping between localized states reveals a qualitative difference between dynamics due to noninteracting bosons and hard-core bosons. In the first case the transient dynamics is subdiffusive, but multiboson processes allow for long-time normal diffusion, while hard-core effects suppress multiboson processes leading to persistent subdiffusive transport, consistent with numerical results for a full many-body evolution. In contrast, analogous study for a quasiperiodic potential reveals a stable long-time diffusion.

We present a combined experimental and theoretical study of InAs/InGaAs/GaAs quantum dots capable of single photon emission in the technologically important telecom C-band for temperatures of up to $77\mathrm{K}$. The structure and morphology of the dots is investigated by means of atomic force microscopy and transmission electron microscopy. Based on these results, the single-particle transition energies are modeled utilizing an atomistic tight-binding method. These energies are compared to experimental data, thereby explaining the mechanism leading to the emission in the C-band as well as the inhomogeneous broadening resulting from fluctuations of size and concentration of individual quantum dots. Additionally, for a single quantum dot, the corresponding multiexciton transition energies are identified. An analysis of the multiexciton states explains the intensity dependence and activation energies of the ensemble spectra at elevated temperatures. As a result, the alternating behavior of specific single dot transitions at different temperatures can be interpreted. Finally, the preservation of single-photon emission up to liquid-nitrogen temperatures of $77\mathrm{K}$ is demonstrated.

We investigated a possible correlation between spin Hall angles and exchange bias in $\mathrm{N}{\mathrm{i}}_{81}\mathrm{F}{\mathrm{e}}_{19}/\mathrm{IrMn}$ samples by performing spin-torque ferromagnetic resonance measurements. This correlation is probed by patterning of $\mathrm{N}{\mathrm{i}}_{81}\mathrm{F}{\mathrm{e}}_{19}/\mathrm{IrMn}$ bilayers in different relative orientations with respect to the exchange bias direction. The measured voltage spectra allow a quantitative determination of spin Hall angles, which are independent of the exchange bias orientation around $2.8\pm 0.3\%$.

We show that multiband superconductors with broken time-reversal symmetry can produce spontaneous currents and magnetic fields in response to the local variations of pairing constants. Considering the iron pnictide superconductor ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ as an example we demonstrate that both the point-group symmetric $s+is$ state and the $C_4$-symmetry-breaking $s+id$ states produce, in general, the same magnitudes of spontaneous magnetic fields. In the $s+is$ state these fields are polarized mainly on an $ab$ crystal plane, whereas in the $s+id$ state their $ab$-plane and $c$-axis components are of the same order. The same is true for the random magnetic fields which are produced by the order parameter fluctuations near the critical point of the time-reversal symmetry-breaking phase transition. Our findings can be used as a direct test of the $s+is/s+id$ dichotomy and the additional discrete symmetry-breaking phase transitions with the help of muon spin-relaxation experiments.

This work presents and analyzes electrical resistivity data on the organic superconductor ${\left(\mathrm{TMTSF}\right)}_2{\mathrm{ClO}}_4$ and their anion-substituted alloys ${\left(\mathrm{TMTSF}\right)}_2{\left({\mathrm{ClO}}_4\right)}_{1-x}{\left({\mathrm{ReO}}_4\right)}_x$ along the least conducting $c^{★}$ axis. Nonmagnetic disorder introduced by finite size domains of anion ordering on non-Fermi-liquid character of resistivity is investigated near the conditions of quantum criticality taking place at $x=0$. The evolution of the $T$-linear resistivity term with anion disorder shows a limited decrease in contrast with the complete suppression of the critical temperature $T_c$ as expected for unconventional superconductivity beyond a threshold value of $x$. The resulting breakdown of scaling between both quantities is compared to the theoretical predictions of a linearized Boltzmann equation combined to the scaling theory of umklapp scattering in the presence of disorder-induced pair breaking for the carriers. The theory shows that quantum antiferromagnetic fluctuations, which are at the core of unconventional Cooper pairing and inelastic scattering of $T$-linear resistivity in these systems, are weakly affected by disorder while the phase coherence responsible for a finite $T_c$ can be completely suppressed.

We use a recently developed fRG method (extendend Coupled-Ladder Approximation) to study the 0.7 analog in quantum point contacts, arising at the crossing of the first and second band at sufficiently high magnetic fields. We reproduce the main features of the experimentally observed magnetic field dependence of the conductance at the 0.7 analog, using a QPC model with two bands and short-range interactions. In particular, we reproduce the fact that this dependence is qualitatively different, depending on whether the analog is approached from higher or lower magnetic fields. We show that this effect can be explained qualitatively within a simple Hartree picture for the influence of the lowest electrons.

We derive the symmetrized current-noise spectrum of a quantum dot, which is weakly tunnel-coupled to an electron reservoir and driven by a slow time-dependent gate voltage. This setup can be operated as an on-demand emitter of single electrons into a mesoscopic conductor. By extending a real-time diagrammatic technique which is perturbative in the tunnel coupling, we obtain the time-resolved finite-frequency noise as well as its decomposition into noise harmonics in the presence of both strong Coulomb interaction and slow time-dependent driving. We investigate the noise over a large range of frequencies and point out where the interplay of Coulomb interaction and driving leads to unique signatures in finite-frequency noise spectra, in particular in the first harmonic. Besides that, we employ the first noise harmonic as a spectroscopic tool to access individual fluctuation processes. We discuss how the inverse noise frequency sets a time scale for fluctuations, which competes with time scales of the quantum-dot relaxation dynamics as well as the driving.

Propagating surface plasmons in the laboratory can have basic properties quite different from those of the ideal surface plasmon polariton, which is a bound mode at the interface between two infinite media. Features such as field confinement and propagation length are affected by the configuration used for coupling to the far field. We experimentally investigate how the Otto and Kretschmann configurations influence the linear coupling properties of smooth metal layers and the nonlinear generation of the second harmonic. The results are compared to calculations from electrodynamic theory based on the hydrodynamic model, which confirm that the nonlinear yield is an order of magnitude greater in the Kretschmann configuration than in the Otto configuration.

Recent ab initio theoretical calculations of the electrical performance of several two-dimensional materials predict a low-field carrier mobility that spans several orders of magnitude (from $26000$ to 35 ${\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$, for example, for the hole mobility in monolayer phosphorene) depending on the physical approximations used. Given this state of uncertainty, we review critically the physical models employed, considering phosphorene, a group-V material, as a specific example. We argue that the use of the most accurate models results in a calculated performance that is at the disappointing lower end of the predicted range. We also employ first-principles methods to study high-field transport characteristics in monolayer and bilayer phosphorene. For thin multilayer phosphorene we confirm the most disappointing results, with a strongly anisotropic carrier mobility that does not exceed $\sim 30$ ${\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ at 300 K for electrons along the armchair direction. We also discuss the dependence of low-field carrier mobility on the thickness of multilayer phosphorene.

We propose a spinless symmetry-based three-band tight-binding model with the coexistence of nodal-line and Weyl points, after considering spin-orbital coupling and different magnetization directions. It is confirmed that the number of Weyl points and nodal lines can be tuned by the magnetization direction. The combination of time-reversal symmetry and mirror reflection symmetry plays a crucial role in protecting the degeneracy of nodal lines. Moreover, we put forward a different class of materials ${\mathrm{C}}_4\mathrm{Cr}{X}_3$ ($X=\text{Ge}$ and Si) which host such a nontrivial semimetallic phase.

The tensor network representation of many-body quantum states, given by local tensors, provides a promising numerical tool for the study of strongly correlated topological phases in two dimensions. However, the representation may be vulnerable to instabilities caused by small variations in the local tensors. For example, the topological order in the tensor network representations of the toric code ground state has been shown in Chen, Zeng, Gu, Chuang, and Wen, Phys. Rev. B 82, 165119 (2010) to be unstable if the variations break certain $Z_2$ symmetry of the tensor. In this work, we ask whether other types of topological orders suffer from similar kinds of instability and if so, what is the underlying physical mechanism and whether we can protect the order by enforcing certain symmetries on the tensor. We answer these questions by showing that the tensor network representations of all string-net models are indeed unstable, but the matrix product operator (MPO) symmetries of the tensors identified in Şahinoğlu, Williamson, Bultinck, Mariën, Haegeman, Schuch, and Verstraete, arXiv:1409.2150 can help to protect the order. In particular, we show that a subset of variations that break the MPO symmetries lead to instability by inducing the condensation of bosonic quasiparticles, which destroys the topological order in the wave function. Therefore such variations must be forbidden for the encoded topological order to be reliably extracted from the local tensors. On the other hand, if a tensor network based variational algorithm is used to simulate the phase transition due to boson condensation, such variation directions may prove important to access the continuous transition correctly.