We examine recent magnetic torque measurements in two compounds, $\gamma -{\mathrm{Li}}_2{\mathrm{IrO}}_3$ and ${\mathrm{RuCl}}_3$, which have been discussed as possible realizations of the Kitaev model. The analysis of the reported discontinuity in torque, as an external magnetic field is rotated across the $c$ axis in both crystals, suggests that they have a translationally invariant chiral spin order of the form $\langle {\mathbf{S}}_i·\left({\mathbf{S}}_j\times {\mathbf{S}}_k\right)\rangle \ne 0$ in the ground state and persisting over a very wide range of magnetic field and temperature. An extraordinary $\left|B\right|B^2$ dependence of the torque for small fields, beside the usual $B^2$ part, is predicted by the chiral spin order. Data for small fields are available for $\gamma -{\mathrm{Li}}_2{\mathrm{IrO}}_3$ and are found to be consistent with the prediction upon further analysis. Other experiments such as inelastic scattering and thermal Hall effect and several questions raised by the discovery of chiral spin order, including its topological consequences, are discussed.

For environmental reasons there is a need for alternative Cd-free buffer materials in Cu(In,Ga)(S,${\mathrm{Se})}_2$ (CIGSSe) based thin film solar cells. In this context, $\beta -{\mathrm{In}}_2{\mathrm{S}}_3$ is one candidate material, whose optoelectronic properties can be affected by the presence of impurities. In this study, we investigate the impact of O and Cl impurities on the electronic and optical behavior of $\beta -{\mathrm{In}}_2{\mathrm{S}}_3$ by means of electronic structure calculations within density functional theory using hybrid functionals. We find that $\beta -{\mathrm{In}}_2{\mathrm{S}}_3$ is thermodynamically stable being in contact with both O and Cl reservoirs. Furthermore, we present evidence that O on interstitial sites (${\mathrm{O}}_{\mathrm{i}}$) and Cl on $8c$ In sites (${\mathrm{Cl}}_{\mathrm{In}}$) cause low-temperature persistent electron photoconductivity. At room temperature, defect levels associated with Cl on S sites (${\mathrm{Cl}}_{\mathrm{S}}, {\mathrm{Cl}}_{{\mathrm{S}}^{\prime }}$, and ${\mathrm{Cl}}_{{\mathrm{S}}^{\prime \prime }}$) get thermally ionized and release free electrons into the system. Thus, the $n$-type conductivity of the ${\mathrm{In}}_2{\mathrm{S}}_3$ buffer layer increases. O impurities on S sites, in contrast, are electrically inert. Hence, we conclude that intentional doping by Cl is a means to improve the properties of $\beta -{\mathrm{In}}_2{\mathrm{S}}_3$ serving as buffer material.

We present an ab initio study that identifies the main electron-phonon scattering channels in $n$-type PbTe. We develop an electronic transport model based on the Boltzmann transport equation within the transport relaxation time approximation, fully parametrized from first-principles calculations that accurately describe the dispersion of the electronic bands near the band gap. Our computed electronic mobility as a function of temperature and carrier concentration is in good agreement with experiments. We show that longitudinal optical phonon scattering dominates electronic transport in $n$-type PbTe, while acoustic phonon scattering is relatively weak. We find that scattering due to soft transverse optical phonons is by far the weakest scattering mechanism, due to the symmetry-forbidden scattering between the conduction band minima and the zone center soft modes. Soft phonons thus play the key role in the high thermoelectric figure of merit of $n$-type PbTe: they do not degrade its electronic transport properties although they strongly suppress the lattice thermal conductivity. Our results suggest that materials like PbTe with soft modes that are weakly coupled with the electronic states relevant for transport may be promising candidates for efficient thermoelectric materials.

We present a theory predicting how the linear magnetotransport of a two-dimensional (2D) electron gas is modified by a passive electromagnetic cavity resonator where no real photons are injected nor created. For a cavity photon mode with in-plane linear polarization, the dc bulk magnetoresistivity of the 2D electron gas is anisotropic. In the regime of high filling factors of the Landau levels, the envelope of the Shubnikov–de Haas oscillations is profoundly modified and the resistivity can be increased or reduced depending on the system parameters. In the limit of low magnetic fields, the resistivity along the cavity-mode polarization direction is enhanced in the ultrastrong light-matter coupling regime. Our work shows the crucial role of virtual polariton excitations in controlling the dc charge transport properties of cavity-embedded systems.

We present a theoretical analysis of a Majorana-based qubit consisting of two topological superconducting islands connected via a Josephson junction. The qubit is operated by electrostatic gates which control the coupling of two of the four Majorana zero modes. At the end of the operation, readout is performed in the charge basis. Even though the operations are not topologically protected, the proposed experiment can potentially shed light on the coherence of the parity degree of freedom in Majorana devices and serve as a first step towards topological Majorana qubits. We discuss in detail the charge-stability diagram and its use for characterizing the parameters of the devices, including the overlap of the Majorana edge states. We describe the multilevel spectral properties of the system and present a detailed study of its controlled coherent oscillations, as well as decoherence resulting from coupling to a non-Markovian environment. In particular, we study a gate-controlled protocol where conversion between Coulomb-blockade and transmon regimes generates coherent oscillations of the qubit state due to the overlap of Majorana modes. We show that, in addition to fluctuations of the Majorana coupling, considerable measurement errors may be accumulated during the conversion intervals when electrostatic fluctuations in the superconducting islands are present. These results are also relevant for several proposed implementations of topological qubits which rely on readout based on charge detection.

We present a systematic phenomenological description of Gilbert damping in two-sublattice magnets. Our theory covers the full range of materials from ferro- via ferri- to antiferromagnets. Following a Rayleigh dissipation functional approach within a Lagrangian classical field formulation, the theory captures intra- as well as cross-sublattice terms in the Gilbert damping, parametrized by a $2\times 2$ matrix. When spin pumping into an adjacent conductor causes dissipation, we obtain the corresponding Gilbert damping matrix in terms of the interfacial spin-mixing conductances. Our model reproduces the experimentally observed enhancement of the ferromagnetic resonance linewidth in a ferrimagnet close to its compensation temperature without requiring an increased Gilbert parameter. It also predicts new contributions to damping in an antiferromagnet and suggests the resonance linewidths as a direct probe of the sublattice asymmetry, which may stem from boundary or bulk.

We compute the zero-temperature dynamical structure factor $S(\mathit{q},\omega )$ of the triangular lattice Heisenberg model (TLHM) using a Schwinger boson approach that includes the Gaussian fluctuations ($1/N$ corrections) of the saddle-point solution. While the ground state of this model exhibits a well-known ${120}^{\circ }$ magnetic ordering, experimental observations have revealed a strong quantum character of the excitation spectrum. We conjecture that this phenomenon arises from the proximity of the ground state of the TLHM to the quantum melting point separating the magnetically ordered and spin-liquid states. Within this scenario, magnons are described as collective modes (two-spinon bound states) of a spinon condensate (Higgs phase) that spontaneously break the SU(2) symmetry of the TLHM. Crucial to our results is the proper account of this spontaneous symmetry breaking. The main qualitative difference relative to semiclassical treatments ($1/S$ expansion) is the presence of a high-energy spinon continuum extending up to about three times the single-magnon bandwidth. In addition, the magnitude of the ordered moment ($m=0.224$) agrees very well with numerical results and the low-energy part of the single-magnon dispersion is in very good agreement with series expansions. Our results indicate that the Schwinger boson approach is an adequate starting point for describing the excitation spectrum of some magnetically ordered compounds that are near the quantum melting point separating this Higgs phase from the deconfined spin-liquid state.

We provide a full and unbiased solution to the Dyson-Schwinger equation illustrated for ${\varphi }^4$ theory in 2D. It is based on an exact treatment of the functional derivative $\partial \mathrm{\Gamma }/\partial G$ of the four-point vertex function $\mathrm{\Gamma }$ with respect to the two-point correlation function $G$ within the framework of the homotopy analysis method (HAM) and the Monte Carlo sampling of rooted tree diagrams. The resulting series solution in deformations can be considered as an asymptotic series around $G=0$ in a HAM control parameter $c_{0}G$, or even a convergent one up to the phase transition point if shifts in $G$ can be performed (such as by summing up all ladder diagrams). These considerations are equally applicable to fermionic quantum field theories and offer a fresh approach to solving functional integro-differential equations beyond any truncation scheme.

Graphene is known to be nonsuperconducting. However, surprising superconductivity is recently discovered in a flat band in a twisted bilayer graphene. Here, we show that superconductivity can be more easily realized in topological flat bands induced by strain in graphene through periodic ripples. Specifically, it is shown that by including correlation effects, the chiral $d$-wave superconductivity can be stabilized under strain even for slightly doped graphene. The chiral $d$-wave superconductivity generally coexists with charge density waves (CDW) and pair density waves (PDW) of the same period. Remarkably, a pure PDW state with doubled period that coexists with the CDW state is found to emerge at a finite-temperature region under reasonable strain strength. The emergent PDW state is shown to be superconducting with nonvanishing superfluid density, and it realizes the long-sought-after superconducting states with nonvanishing center-of-mass momentum for Cooper pairs.

We study the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between local magnetic moments in zigzag bilayer phosphorene nanoribbons (ZBLPNRs) under a perpendicular electric field. We evaluate the spatial and electric field dependency of the static spin susceptibility in real space in various configurations of the magnetic impurities at zero temperature. For large distances, the RKKY interaction falls off as $R^{-5}$ for moments located on the zigzag edge, whereas for those placed at interstitial sites it decays as $R^{-3}$. In the presence of a large bias potential, a beating pattern of the RKKY oscillations occurs when two magnetic impurities are located inside the ZBLPNR. The electrically tunable RKKY interaction of ZBLPNRs is expected to have important consequences on the spintronic application of a biased ZBLPNR. The electronic properties of ZBLPNRs in the presence of gate voltage are also obtained. In comparison to other two-dimensional materials such as graphene, silicene, etc., the ZBLPNR has two nearly degenerate quasiflat edge modes at the Fermi level, isolated from the bulk states. The band-gap modulation of ZBLPNRs by the ribbon width and perpendicular electric field is investigated. Due to the existence of these quasiflat bands at the Fermi level, in the absence of an electric field, a sharp peak in the RKKY interaction is seen. As is shown, the signatures of these unique quasiflat edge modes in ZBLPNRs could be explored by using the RKKY interaction.

A wide range of materials like graphene, topological insulators, and transition-metal dichalcogenides (TMDs) share an interesting property: the low-energy excitations behave as Dirac particles. This emergent behavior of Dirac quasiparticles defines a large class of media that are usually called Dirac materials. In this work we build the foundations of a way to study the linear optical properties of these two-dimensional media. Our approach is based on a Dirac-like formulation of the standard semiconductor Bloch equations used in semiconductor physics. We provide an explicit expression of the linear absorbance, which we call the relativistic Elliott formula, and use it to quantify the variation of the continuum absorbance spectrum with the strength of the Coulomb interaction (the Sommerfeld factor). Our calculations also show how the Coulomb enhancements scales with the band gap and vanishes for zero band gap, shedding light on the behavior of graphene for low-light intensities. The results presented are in good quantitative agreement with published experimental results. Our theory can help researchers to explore the nonlinear interactions of intense, ultrashort pulses with TMDs, and the framework is flexible enough to be adapted to different experimental situations, such as cavities, multilayers, heterostructures, and microresonators.

Hydrogen-rich materials have attracted great interest since the recent discovery of superconductivity at 203 K in highly compressed hydrogen sulfide. To probe the role of covalent bonding in determining the $T_{\mathrm{c}}$ of hydrogen-related superconductors, we systematically studied the crystal structure and superconductivity of ${\mathrm{H}}_6\mathrm{SSe}$, a hypothetical compound derived from ${\mathrm{H}}_3\mathrm{S}$ with half its S atoms replaced by group neighbor Se. First-principles structure searches identify three dynamically stable structures for ${\mathrm{H}}_6\mathrm{SSe}$ at 200 GPa. Interestingly, all three structures keep the main feature of the cubic $Im\overline{3}m$ structure of ${\mathrm{H}}_3\mathrm{S}$, but with different Se substitution positions. Electron-phonon coupling calculations reveal the superconductive potential of the three phases of ${\mathrm{H}}_6\mathrm{SSe}$, with $T_{\mathrm{c}}$ decreasing (from 195 to 115 K) upon the declining strength of the weakest covalent H-S or H-Se bonds in each structure, thereby highlighting the key role of covalent bonding in determining $T_{\mathrm{c}}$. For comparison, O-substituted ${\mathrm{H}}_6\mathrm{SO}$ was predicted to assume a semiconducting phase with entirely different structural features from ${\mathrm{H}}_6\mathrm{SSe}$. We attribute this difference to the much stronger electronegativity of O (3.44) compared with S (2.58) or Se (2.55).

We report the ${}^{139}\mathrm{La}$ nuclear magnetic resonance (NMR), electron spin resonance (ESR), and complementary magnetometry results on the Ir-based double perovskite compounds ${\mathrm{La}}_{2}B{\mathrm{IrO}}_6$ with strongly magnetic ${\mathrm{Co}}^{2+}$ ($3d^7$, $S=3/2$) and nonmagnetic ${\mathrm{Zn}}^{2+}$ ($3d^{10}$, $S=0$) ions at the $B$ site. ESR signals from ${\mathrm{Ir}}^{4+}$ ions in ${\mathrm{La}}_2{\mathrm{ZnIrO}}_6$ and a collective ESR mode of coupled ${\mathrm{Co}}^{2+}$ spins $S=3/2$ and ${\mathrm{Ir}}^{4+}$ pseudospins $j_{\mathrm{eff}}=1/2$ were observed and studied as a function of temperature and excitation frequency. These signals gradually evolve into a resonance mode of the uncompensated ferromagnetic moments in the antiferromagnetically (AFM) ordered state below $T_{\mathrm{N}}\approx 11\mathrm{K}$ and 98 K for ${\mathrm{La}}_2{\mathrm{ZnIrO}}_6$ and ${\mathrm{La}}_2{\mathrm{CoIrO}}_6$, respectively. The temperature evolution of the ${}^{139}\mathrm{La}$ NMR spectra and the nuclear spin relaxation $T_1^{-1}\left(T\right)$ reveal three distinct regimes for ${\mathrm{La}}_{2}B{\mathrm{IrO}}_6$ ($B=\text{Co}$, Zn): (i) a paramagnetic state at high temperatures; (ii) a short-range AFM order in an extended temperature region above $T_{\mathrm{N}}$ where the ESR modes first appear; (iii) magnetic long-range order at lower $T$. The extended short-range-ordered regime at temperatures much higher than $T_{\mathrm{N}}$ constitutes evidence of substantial magnetic frustration which appears to be a salient feature of both studied materials.

The half-metallic $\mathrm{Cr}{\mathrm{O}}_2$ with nearly 100% spin polarization is an ideal system to study magnetic domain-wall resistance, which differs from the resistance (or resistivity) inside a single domain. To experimentally measure the domain-wall resistance, we design and prepare a special $\mathrm{Cr}{\mathrm{O}}_2$ epitaxial nanostructure with an asymmetrical weak link to localize a domain wall, by using the techniques of chemical vapor deposition and selective-area growth. This structure provides a capability to generate and annihilate a domain wall near the weak link. By contrasting the resistance between a single-domain state and a domain-wall state, we observe a repeatable and reversible resistance jump, namely domain-wall resistance, in half-metallic $\mathrm{Cr}{\mathrm{O}}_2$. Using the Levy-Zhang model, we further obtain the spin asymmetry ratio ${\rho }_0^{\uparrow }/{\rho }_0^{\downarrow }$ between resistivities in the two spin channels. The ratio, 4256 ± 388 at 5.0 K, is much larger than that of conventional ferromagnetic metals, attesting to the half metallicity of $\mathrm{Cr}{\mathrm{O}}_2$.

Geometric magnetic frustration has attracted substantial interest due to the exotic physics and rich phase diagrams revealed by the cancellation of normally dominant magnetic interactions, giving impetus for the search for novel frustrated systems, most often based on antiferromagnetic correlations between magnetic ions decorating triangular or tetrahedral lattices. We report here low-temperature magnetic susceptibility and muon spin relaxation results on ${\mathrm{Li}}_4{\mathrm{MgOsO}}_6$ and ${\mathrm{Li}}_3{\mathrm{Mg}}_2{\mathrm{OsO}}_6$, members of the $A_5{B\text{O}}_6$ rock-salt ordered family of frustrated materials. In ${\mathrm{Li}}_3{\mathrm{Mg}}_2{\mathrm{OsO}}_6$ we find spin freezing below 12 K. In ${\mathrm{Li}}_4{\mathrm{MgOsO}}_6$, which can crystallize into either orthorhombic $Fddd$ or monoclinic $C2/m$ crystal symmetries depending on synthesis conditions, we find magnetism consistent with glassylike behavior dominating below 2 K, with partial ordering and evidence for dynamics at somewhat higher temperatures.

Recent photoemission spectroscopy measurements (T. J. Reber et al., arXiv:1509.01611) of cuprate superconductors have inferred that the self-energy exhibits critical scaling over an extended doping regime, thereby calling into question the conventional wisdom that critical scaling exists only at isolated points. In particular, this new state of matter, dubbed a power-law liquid, has a self-energy whose imaginary part scales as ${\mathrm{\Sigma }}^{\prime \prime }\sim {({\omega }^2+{\pi }^2{T}^2)}^{\alpha }$, with $\alpha =1$ in the overdoped Fermi-liquid state and $\alpha \le 0.5$ in the optimal to underdoped regime. Previously, we showed that this self-energy can arise from interactions between electrons and unparticles, a scale-invariant sector that naturally emerges from strong correlations. Here, taking the self-energy as a given, we first reconstruct the real part of the self-energy. We find that the resultant quasiparticle weight vanishes for any doping level less than optimal, implying an absence of particlelike excitations in the underdoped regime. Consequently, the Fermi velocity vanishes and the effective mass diverges for $\alpha \le \frac{1}{2}$, in agreement with earlier experimental observations. We then use the self-energy to reconstruct the spectral function and compute the superconducting $T_c$ within the BCS formalism. We find that the $T_c$ has a domelike structure, implying that broad scale invariance manifested in the form of a power-law liquid is the likely cause of the superconducting dome in the cuprates.