By means of resonant inelastic x-ray scattering at the Cu $L_3$ edge, we measured the spin-wave dispersion along $\langle 100\rangle$ and $\langle 110\rangle$ in the undoped cuprate ${\mathrm{Ca}}_2{\mathrm{CuO}}_2{\mathrm{Cl}}_2$. The data yield a reliable estimate of the superexchange parameter $J=135\pm 4$ meV using a classical spin-1/2 two-dimensional Heisenberg model with nearest-neighbor interactions and including quantum fluctuations. Including further exchange interactions increases the estimate to $J=141$ meV. The 40 meV dispersion between the magnetic Brillouin zone boundary points (1/2, 0) and (1/4, 1/4) indicates that next-nearest-neighbor interactions in this compound are intermediate between the values found in ${\mathrm{La}}_2{\mathrm{CuO}}_4$ and ${\mathrm{Sr}}_2{\mathrm{CuO}}_2{\mathrm{Cl}}_2$. Due to the low-$Z$ elements composing ${\mathrm{Ca}}_2{\mathrm{CuO}}_2{\mathrm{Cl}}_2$, the present results may enable a reliable comparison with the predictions of quantum many-body calculations, which would improve our understanding of the role of magnetic excitations and of electronic correlations in cuprates.

In this work we outline a simple and numerically inexpensive approach to describe the spectral features of the single-impurity Anderson model. The method combines aspects of the density-matrix embedding theory (DMET) approach with a spectral broadening approach inspired by those used in numerical renormalization-group methods. At zero temperature for a wide range of $U$, the spectral function produced by this approach is found to be in good agreement with general expectations as well as more advanced and complex numerical methods such as DMRG-based schemes. The theory developed here is simply transferable to more complex impurity problems.

We investigated transport, magnetotransport, and broadband optical properties of the half-Heusler compound YbPtBi. Hall measurements evidence two types of charge carriers: highly mobile electrons with a temperature-dependent concentration and low-mobile holes; their concentration stays almost constant within the investigated temperature range from 2.5 to 300 K. The optical spectra (10 meV–2.7 eV) can be naturally decomposed into contributions from intra- and interband absorption processes, the former manifesting themselves as two Drude bands with very different scattering rates, corresponding to the charges with different mobilities. These results of the optical measurements allow us to separate the contributions from electrons and holes to the total conductivity and to implement a two-channel-conduction model for description of the magnetotransport data. In this approach, the electron and hole mobilities are found to be around 50 000 and $10{\mathrm{cm}}^2$/V s at the lowest temperatures (2.5 K), respectively.

We propose the concept of hyperbolic wires that overcome the actual limitation of metal-insulator-metal (MIM) patch antennas in terms of electromagnetic confinement and efficiency. The use of hyperbolic metamaterials allows one to realize miniaturized resonators sustaining bulk plasmon polaritons squeezed to only one-hundredth of the wavelength. Beyond this tenfold size reduction compared to MIM antennas, we propose a model allowing one to scale the resonant frequencies of hyperbolic wires over a broad spectral range by controlling the filling ratio in metals and dielectrics.

We show how to obtain the symmetry-imposed selection rules for plasmonic enhancement in surface- (SERS) and tip-enhanced Raman scattering (TERS). Plasmon-enhanced light scattering is described as a higher-order Raman process, which introduces a series of Hamiltonians representing the interaction between light, plasmons, electrons, and phonons. Using group theory, we derive the active representations for point group symmetries of exemplary plasmonic nanostructures. The phonon representations that are enhanced by SERS and TERS are then found as induced representations for the symmetry group of the molecule or another Raman probe. The selection rules are discussed for graphene that is coupled to a nanodisk dimer as an example for SERS and coupled to a tip as a TERS example. The phonon eigenmodes that are enhanced depend on the symmetry breaking when combining the plasmonic structures with graphene. We show that the most prominent optical phonon modes $(E_{2g}$ and $A_{1g})$ are allowed in all scattering configurations when using a nanodimer as a plasmonic hotspot. We predict the activation of the silent $B_{2g}$ as well as infrared-active $A_{2u}$ and $E_{1u}$ modes in SERS for crossed configurations of the incoming and scattered light. There is a systematic difference between spatially coherent and incoherent plasmon-enhanced Raman scattering, which is responsible for a dependence of TERS on the phonon coherence length.

Different proposals for adiabatic quantum motors (AQMs) driven by DC currents have recently attracted considerable interest. However, the systems studied are often based on simplified models with highly ideal conditions where the environment is neglected. Here, we investigate the performance (dynamics, efficiency, and output power) of a prototypical AQM, the Thouless motor. To include the effect of the surroundings on this type of AQMs, we extended our previous theory of decoherence in current-induced forces (CIFs) to account for spatially distributed decoherent processes. We provide analytical expressions that account for decoherence in CIFs, friction coefficients, and the self-correlation functions of the CIFs. We prove that the model is thermodynamically consistent, and we find that decoherence drastically reduces the efficiency of the motor mainly due to the increase in conductance, while its effect on the output power is not much relevant. The effect of decoherence on the current-induced friction depends on the length of the system, reducing the friction for small systems while increasing it for long ones. Finally, we find that reflections of the electrons at the boundary of the system induce additional conservative forces that affect the dynamics of the motor. In particular, this results in the hysteresis of the system and a voltage dependent switching.

We propose a series-expansion thermal tensor network (SETTN) approach for efficient simulations of quantum lattice models. This continuous-time SETTN method is based on the numerically exact Taylor series expansion of the equilibrium density operator $e^{-\beta H}$ (with $H$ the total Hamiltonian and $\beta$ the imaginary time), and is thus Trotter-error free. We discover, through simulating XXZ spin chain and square-lattice quantum Ising models, that not only the Hamiltonian $H$, but also its powers $H^n$, can be efficiently expressed as matrix product operators, which enables us to calculate with high precision the equilibrium and dynamical properties of quantum lattice models at finite temperatures. Our SETTN method provides an alternative to conventional Trotter-Suzuki renormalization-group (RG) approaches, and achieves a very high standard of thermal RG simulations in terms of accuracy and flexibility.

Motivated by a recent experiment on the rare-earth material ${\mathrm{YbMgGaO}}_4$ [Y. Li et al., Phys. Rev. Lett. 115, 167203 (2015)], which found that the ground state of ${\mathrm{YbMgGaO}}_4$ is a quantum spin liquid, we study the ground-state phase diagram of an anisotropic spin-$1/2$ model that was proposed to describe ${\mathrm{YbMgGaO}}_4$. Using the density matrix renormalization-group method in combination with the exact-diagonalization method, we calculate a variety of physical quantities, including the ground-state energy, the fidelity, the entanglement entropy and spin-spin correlation functions. Our studies show that in the quantum phase diagram, there is a ${120}^{\circ }$ phase and two distinct stripe phases. The transitions from the two stripe phases to the ${120}^{\circ }$ phase are of the first order. However, the transition between the two stripe phases is not of the first order, which is different from its classical counterpart. Additionally, we find no evidence for a quantum spin liquid in this model. Our results suggest that additional terms may also be important to model the material ${\mathrm{YbMgGaO}}_4$. These findings will stimulate further experimental and theoretical works in understanding the quantum spin-liquid ground state in ${\mathrm{YbMgGaO}}_4$.

In the framework of the weak-coupling renormalization-group technique we examine the possible instabilities of the extended quasi-one-dimensional electron-gas model with both intrachain and interchain electron-electron interactions, including the influence of umklapp scattering and the coupling of spins to a magnetic field. In the limit of purely repulsive intrachain interactions, we confirm the passage from singlet $d$-wave-like superconductivity to an inhomogeneous Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) state under magnetic field. The passage is accompanied by an anomalous increase of the upper critical field that scales with the antinesting distance from the quantum critical point joining superconductivity to antiferromagnetism in the phase diagram, as well as the strength of interactions. Adding weak repulsive interchain interactions promotes the passage from singlet to triplet $f$-wave superconductivity, which is expanded under field by the development of a triplet FFLO state with zero angular-momentum projection for the Cooper pairs. The connection between theory and experiment on the anomalous upper critical field in the Bechgaard salts is discussed.

The transient single-particle spectral function of ${\mathrm{BaFe}}_2{\mathrm{As}}_2$, a parent compound of iron-based superconductors, has been studied by time- and angle-resolved photoemission spectroscopy with an extreme-ultraviolet laser generated by higher harmonics from Ar gas, which enables us to investigate the dynamics in the entire Brillouin zone. We observed electronic modifications from the spin density wave (SDW) ordered state within $\sim 1$ ps after the arrival of a 1.5 eV pump pulse. We observed optically excited electrons at the zone center above $E_F$ at 0.12 ps, and their rapid decay. After the fast decay of the optically excited electrons, a thermalized state appears and survives for a relatively long time. From a comparison with the density-functional theory band structure for the paramagnetic and SDW states, we interpret the experimental observations as the melting of the SDW. Exponential decay constants for the thermalized state to recover back to the SDW ground state are $\sim 0.60\mathrm{ps}$ both around the zone center and the zone corner.

A fully predictive kinetic-collective model using first principles phonon spectra and relaxation times is presented. Thermal conductivity values obtained for Si, Ge, C (diamond), and GaAs in a wide range of sizes and temperatures are in good agreement with experimental data without the use of any fitting parameter. These results open the door to discuss how the precise combination of kinetic and collective contributions to heat transport could provide a useful framework to interpret recent complex experiments displaying non-Fourier behavior.

Electron-phonon coupling (EPC) in graphite was investigated via high-resolution electron energy-loss spectroscopy and first-principles band calculations. We found strong resonance enhancements in the magnitude of specific phonon scatterings at specific primary energies of the incident electron. This result is attributed to a new scattering mechanism, where the incident electron is trapped at an unoccupied band, is scattered into another band via EPC with a specific phonon, and is finally detected by the analyzer. The scattering intensity is related to the electron-phonon matrix element wherein the energy and momenta of both electron and phonon are resolved.

A theory of relativistic space-charge-limited current (SCLC) is formulated to determine the SCLC scaling, $J\propto V^{\alpha }/L^{\beta }$, for a finite band-gap Dirac material of length $L$ biased under a voltage $V$. In one-dimensional (1D) bulk geometry, our model allows ($\alpha ,\beta$) to vary from (2,3) for the nonrelativistic model in traditional solids to (3/2,2) for the ultrarelativistic model of massless Dirac fermions. For 2D thin-film geometry we obtain $\alpha =\beta$, which varies between 2 and 3/2, respectively, at the nonrelativistic and ultrarelativistic limits. We further provide rigorous proof based on a Green's-function approach that for a uniform SCLC model described by carrier-density-dependent mobility, the scaling relations of the 1D bulk model can be directly mapped into the case of 2D thin film for any contact geometries. Our simplified approach provides a convenient tool to obtain the 2D thin-film SCLC scaling relations without the need of explicitly solving the complicated 2D problems. Finally, this work clarifies the inconsistency in using the traditional SCLC models to explain the experimental measurement of a 2D Dirac semiconductor. We conclude that the voltage scaling $3/2<\alpha <2$ is a distinct signature of massive Dirac fermions in a Dirac semiconductor and is in agreement with experimental SCLC measurements in ${\mathrm{MoS}}_2$.

Exciton-polaron quenching (EPQ) is a major efficiency loss process in organic optoelectronic devices, in particular at high excitation densities. Within commonly used models, the rate is assumed to be given by the product of the exciton density, the polaron density, and a constant EPQ rate coefficient, which is proportional to the polaron diffusion coefficient and an EPQ capture radius. In this work, we study the effects of polaron diffusion on the EPQ rate in energetically disordered materials with a Gaussian density of states using kinetic Monte Carlo simulations, and show that the effective rate coefficient can depend strongly on the polaron concentration and on the electric field. We furthermore find that under realistic conditions, the effective value of the capture radius can exceed the expected value of $\sim 1$ nm by up to two orders of magnitude. To a first approximation, the simulation results can be understood from macroscopic diffusion theory, adapted at finite electric fields to include the observed “polaron wind” effect. However, for strongly disordered systems we find distinct deviations from that theory, related to the very small time and spatial scales involved in the capture process.

We consider the problem of spin-triplet $p$-wave superfluid pairing with total spin projection $m_s=0$ in atomic Fermi gas across the Feshbach resonance. We allow for imbalanced populations and take into account the effects due to the presence of a harmonic trapping potential. Within the mean-field approximation for the one- and two-channel pairing models we show that the $p$-wave superfluid state has the lowest energy up to some distance from the trap center, the distance depending on population imbalance and the side of the Feshbach resonance (BCS or BEC) the system is in. Beyond this, the system is in the normal state. Since the system, even in the presence of the harmonic trap, is invariant under O(3) space rotation, the superfluid cores which emerge in the trap are comprised of states that are degenerate with respect to the “orbital ferromagnet” state, $p_x\pm i{p}_y$, as well as the ones in which all angular momentum components of the order parameter are nonzero, provided the modulus of the order parameter maintains a fixed magnitude.

The Ginzburg-Landau free energy of a conventional superconductor coupled to a helimagnet is microscopically derived using functional field integral techniques. We show that the spin texture leads to a Lifshitz invariant in the free energy, which couples the momentum density of the superconducting condensate to the magnetization of the helimagnet. For helimagnets with a conical texture, the Lifshitz invariant yields a spatial modulation of the superconducting phase along the helical wave vector of the magnetic texture. Based on self-consistent numerical calculations, we verify the theoretical formalism by investigating a superconductor that contains a helical Yu-Shiba-Rusinov (YSR) chain. We demonstrate that the texture-induced magnetoelectric coupling produces a strong supercurrent along the YSR chain, which induces a detectable magnetic field.

Water and ammonia have long been seen as the main species of extraterrestrial space, especially on solar giants, moons, comets, and numerous extrasolar planets. The phases formed by their admixtures under temperature and pressure conditions of the giant planets are important for understanding many observable properties (gravitational moments, atmospheric composition, and magnetic field). Here we employ a Monte Carlo packing algorithm combined with first-principles calculations to search the low-energy crystal structures of ammonia dihydrate (ADH). At high pressure above 11.81 GPa, we predict an unusual ionic phase (tetragonal, $I{4}_{1}cd)$ consisting of three alternating layers of ${\mathrm{H}}_2\mathrm{O},{\mathrm{NH}}_4{}^{+}$, and $\mathrm{O}{\mathrm{H}}^{-}$. The occurrence of ionic phase is attributed to the ${\mathrm{NH}}_4{}^{+}$ and $\mathrm{O}{\mathrm{H}}^{-}$ electrostatic interaction induced volume reduction, which lowers the energy barrier of molecular to ionic phase transition. Analysis of proton transfer under pressure further supports the transformation mechanism between molecular and ionic phase. According to the mobility of hydrogen atoms from ab initio molecular dynamics, this ionic crystal will transform into a superionic phase under high temperature and high pressure. The existence of ionic or superionic ADH may have important implications for understanding the interiors of Neptune, Uranus, and many extrasolar planets.

We present molecular dynamics simulations based on finite-temperature density functional theory that determine self-consistently the dynamic ion structure factor and the electronic form factor in lithium. Our comprehensive data set allows for the calculation of the dispersion relation for collective excitations, the calculation of the sound velocity, and the determination of the ion feature from the total electronic form factor and the ion structure factor. The results are compared with available experimental x-ray and neutron scattering data. Good agreement is found for both the liquid metal and warm dense matter domain. Finally, we study the impact of possible target inhomogeneities on x-ray scattering spectra.