We show that an interlayer exciton condensate doped into a strongly correlated Mott insulator exhibits a remarkable enhancement of the bandwidth of the magnetic excitations (triplons). This triplon is visible in the dynamical magnetic susceptibility and can be measured using resonant inelastic x-ray scattering. The bandwidth of the triplon scales with the exciton superfluid density, but only in the limit of strong correlations. As such the triplon bandwidth acts as a probe of exciton-spin interactions in the condensate.

Following the work in graphene, we report a first-principles study of electron-phonon coupling (EPC) in low-buckled monolayer silicene and germanene. Despite the similar honeycomb atomic arrangement and linear-band dispersion, the EPC matrix-element squares of the $\Gamma$-$E_g$ and $K$-$A_1$ modes in silicene are only about 50% of those in graphene. However, the smaller Fermi velocity in silicene compensates for this reduction by providing a larger joint electronic density of states near the Dirac point, giving rise to comparable phonon linewidths. We predict that Kohn anomalies associated with these two optical modes are significant in silicene. In addition, the EPC-induced frequency shift and linewidth of the Raman-active $\Gamma$-$E_g$ mode in silicene are calculated as a function of doping. The results are comparable to those in graphene, indicating a similar nonadiabatic dynamical origin. In contrast, the EPC in germanene is found to be much reduced.

Since the recent discovery of plutonium-based superconductors such as PuCoIn${}_5$, systematic studies of the electronic properties for plutonium compounds are providing insight into the itinerancy-localization crossover of Pu 5$f$ electrons. We are particularly interested in understanding the Fermi surface properties of the PuIn${}_3$ compound, which serves as the building block for the PuCoIn${}_5$ superconductor. Motivated by the first observation of quantum oscillation and renewed interest in the de Haas–van Alphen (dHvA) measurements on PuIn${}_3$, we study the Fermi surface (FS) topology and the band dispersion in both the paramagnetic and antiferromagnetic states of PuIn${}_3$, based on density-functional theory with a generalized gradient approximation. We compare the results with its isostructural paramagnetic compound PuSn${}_3$. We present the detailed Fermi surfaces of the compounds PuIn${}_3$ and PuSn${}_3$, and conclude that the FS topology of an antiferromagnetic PuIn${}_3$ agrees better with dHvA measurements. In addition, we show the magnetization of the antiferromagnetic order can alter the field-angle dependence and values of the effective mass for the dHvA orbits. Our results suggest that the accurate determination of the magnetic order orientation with respect to the crystal orientation is crucial to advance the understanding of the electronic structure of the PuIn${}_3$ compound.

A general discussion of the ratio of thermal and electrical conductivities in non-Fermi liquid metals is given. In metals with sharp Drude peaks, the relevant physics is correctly organized around the slow relaxation of almost-conserved momenta. While in Fermi liquids both currents and momenta relax slowly, due to the weakness of interactions among low-energy excitations, in strongly interacting non-Fermi liquids typically only momenta relax slowly. It follows that the conductivities of such non-Fermi liquids are obtained within a fundamentally different kinematics to Fermi liquids. Among these strongly interacting non-Fermi liquids we distinguish cases with only one almost-conserved momentum, which we term quasi-hydrodynamic metals, and with many patchwise almost-conserved momenta. For all these cases, we obtain universal expressions for the ratio of conductivities that violate the Wiedemann-Franz law. We further discuss the case in which long-lived “cold” quasiparticles, in general with unconventional scattering rates, coexist with strongly interacting hot spots, lines, or bands. For these cases, we characterize circumstances under which non-Fermi liquid transport, in particular a linear in temperature resistivity, is and is not compatible with the Wiedemann-Franz law. We suggest the likely outcome of future transport experiments on CeCoIn${}_5$, YbRh${}_2$Si${}_2$, and Sr${}_3$Ru${}_2$O${}_7$ at their critical magnetic fields.

We employ the determinant projector quantum Monte Carlo method to investigate the ground-state magnetic properties in the Mott insulating states of the half-filled $SU\left(4\right)$ and $SU\left(6\right)$ Fermi-Hubbard model in the two-dimensional square lattice, which is free of the sign problem. The long-range antiferromagnetic Neel order is found for the $SU\left(4\right)$ case with a small residual Neel moment. Quantum fluctuations are even stronger in the $SU\left(6\right)$ case. Numeric results are consistent with either a vanishing or even weaker Neel ordering than that of $SU\left(4\right)$.

PdCrO${}_2$ is material which has attracted interest due to the coexistence of metallic conductivity associated with itinerant Pd $4d$ electrons and antiferromagnetic order arising from localized Cr spins. A central issue is determining to what extent the magnetic order couples to the conduction electrons. Here we perform angle-resolved photoemission spectroscopy (ARPES) to experimentally characterize the electronic structure. We find that the Fermi surface has contributions from both bulk and surface states, which can be experimentally distinguished and theoretically verified by slab band structure calculations. The bulk Fermi surface shows no signature of electronic reconstruction in the antiferromagnetic state. This observation suggests that there is negligible interaction between the localized Cr spin structure and the itinerant Pd electrons measured by ARPES.

Spin selectivity in angle-resolved Auger photoelectron coincidence spectroscopy (AR-APECS) is used to probe electronic structure in antiferromagnetic thin films. In particular, exploiting the AR-APECS capability to discriminate Auger electron emission events characterized by a different spin of the ion in its final state, a sharp multiplet structure in the Ni $\mathit{MVV}$ Auger line shape of NiO/Ag(001) thin films is measured below the critical Néel temperature. The assignment of multiplet terms follows from a close comparison of the experimental AR-APECS line shapes with the predictions based on semiempirical calculations on a cluster model and an open-band extension of the Cini-Sawatzky approach. In analogy to CoO, also in NiO, above the Néel temperature a more featureless Auger spectrum appears and AR-APECS does not disentangle anymore high-spin and low-spin contributions to the total Auger intensity. Such a behavior, which seems to be a general result for metal oxide antiferromagnetic systems, is discussed.

We investigate the ground-state properties of the highly degenerate noncoplanar phase of the classical bilinear-biquadratic Heisenberg model on the triangular lattice with Monte Carlo simulations. For that purpose, we introduce an Ising pseudospin representation of the ground states, and we use a simple Metropolis algorithm with local updates, as well as a powerful cluster algorithm. At sizes that can be sampled with local updates, the presence of long-range order is surprisingly combined with an algebraic decay of correlations and the complete disordering of the chirality. It is only thanks to the investigation of unusually large systems (containing $\sim$${10}^8$ spins) with cluster updates that the true asymptotic regime can be reached and that the system can be proven to consist of equivalent (i.e., equally ordered) sublattices. These large-scale simulations also demonstrate that the scalar chirality exhibits long-range order at zero temperature, implying that the system has to undergo a finite-temperature phase transition. Finally, we show that the average distance in the order parameter space, which has the structure of an infinite Cayley tree, remains remarkably small between any pair of points, even in the limit when the real space distance between them tends to infinity.

We report the synthesis of hexagonal LaV${}_x$Cr${}_{1-x}$Ge${}_3$ ($x=0$–0.21, 1.00) single crystals and present a systematic study of this series by measurements of temperature- and field-dependent magnetic susceptibility, magnetization, resistivity, and specific heat. Ferromagnetism has been observed for $x=0$–0.21. The Curie temperature declines monotonically as the V concentration increases. Single-crystalline samples could only be grown for $x$ values up to 0.21 for which the transition temperature was suppressed down to 36 K. Although we could not fully suppress $T_{\mathrm{C}}$ via V substitution, for $x=0.16$, we performed magnetization measurements under pressure. The ferromagnetic state is suppressed under pressure at an initial rate of $d{T}_{\mathrm{C}}/dp\simeq -11.7$ K/GPa and vanishes by 3.3 GPa. The increase of the Rhodes-Wohlfarth ratio suggests that the ferromagnetism in this system evolves toward itinerant as the V concentration increases.

The impact of local cubic anisotropy on the magnetic states of the Heisenberg model on the fcc kagome lattice are examined through classical Monte Carlo simulations. Previous simulations revealed that the macroscopic degeneracy of the two-dimensional (2D) kagome exchange-coupled co-planar spin system partially persists in the 3D case of ABC stacked layers giving rise to a discontinuous phase transition. Local cubic anisotropy is shown to remove this degeneracy by re-orienting the spins out of the co-planar configuration. In addition, the re-oriented states are shown to carry a uniform magnetic moment. Simulation results indicate that the effect of anisotropy is to transform the first-order phase transition to a continuous one. These results are relevant to Ir-Mn alloys which have been widely used by the magnetic storage industry in thin-film form as the antiferromagnetic pinning layer in spin valves.

We present experimental results for the density of states (DOS) of a superconducting microwave Dirac billiard which serves as an idealized model for the electronic properties of graphene. The DOS exhibits two sharp peaks which evolve into van Hove singularities with increasing system size. They divide the band structure into regions governed by the relativistic Dirac equation and by the nonrelativistic Schrödinger equation, respectively. We demonstrate that in the thermodynamic limit, a topological transition appears as a neck-disrupting Lifshitz transition in the number susceptibility and as an excited-state transition in the electronic excitations. Furthermore, we recover the finite-size scaling typical for excited-state quantum phase transitions involving logarithmic divergences and identify a quasiorder parameter.

Electromagnons are known from multiferroics as spin waves excited by the electric component of electromagnetic radiation. We report the discovery of an excitation in the far-infrared spectra of $\varepsilon -{\mathrm{Fe}}_2{\mathrm{O}}_3$, which we attribute to an electromagnon appearing below 110 K where the ferrimagnetic structure becomes incommensurately modulated. Inelastic neutron scattering shows that the electromagnon energy corresponds to that of a magnon from the Brillouin-zone boundary. Dielectric measurements did not reveal any sign of ferroelectricity in $\varepsilon -{\mathrm{Fe}}_2{\mathrm{O}}_3$ down to 10 K, despite its acentric crystal structure. This shows that the activation of an electromagnon requires, in addition to the polar ferrimagnetic structure, a modulation of the magnetic structure. We demonstrate that a combination of inelastic neutron scattering with infrared and/or terahertz (THz) spectroscopies allows detecting electromagnons in ceramics where no crystal-orientation analysis of THz and infrared spectra is possible.

We investigate the ground-state phase diagram and critical properties of the $S=1/2$ two-leg Heisenberg spin ladder system with a negative four-spin interaction using a numerical exact diagonalization method. Using the perturbation theory in the strong negative rung-coupling limit, we derive an $S=1$ bilinear-biquadratic chain as an effective model. We discuss the ground-state phase diagram in this limit. Next we numerically determine a phase boundary between the rung singlet phase and the columnar dimer (CD) phase by the phenomenological renormalization group method, and one between the CD phase and the Haldane phase by the twisted boundary condition method. We confirm that the phase transition between the CD phase and the Haldane phase is of second order and this universality class is described by the $k=2$ $SU\left(2\right)$ Wess-Zumino-Novikov-Witten nonlinear $\sigma$ model, calculating the central charge and scaling dimensions.

The evolution of low-energy spin dynamics in the iron-based superconductor LaFeAsO${}_{1-x}$F${}_x$ was studied over a broad doping, temperature, and magnetic field range ($x=$ 0–0.15, $T\le$ 480 K, ${\mu }_{0}H\le$ 30 T) by means of ${}^{75}$As nuclear magnetic resonance. An enhanced spin-lattice relaxation rate divided by temperature ${\left(T_{1}T\right)}^{-1}$ in underdoped superconducting samples ($x=$ 0.045, 0.05, and 0.075) suggests the presence of antiferromagnetic spin fluctuations, which are strongly reduced in optimally doped ($x=0.10$) and completely absent in overdoped ($x=0.15$) samples. In contrast to previous analysis, Curie-Weiss fits are shown to be insufficient to describe the data over the whole temperature range. Instead, a Bloembergen-Purcell-Pound (BPP) model is used to describe the occurrence of a peak in ${\left(T_{1}T\right)}^{-1}$ clearly above the superconducting transition, reflecting a progressive slowing down of the spin fluctuations down to the superconducting phase transition.

The equilibrium topology of superconducting and normal domains in flat type-I superconductors is investigated. Important improvements with respect to previous work are that (1) the energy of the external magnetic field, as deformed by the presence of superconducting domains, is calculated in the same way for three different topologies and (2) calculations are made for arbitrary orientation of the applied field. A phase diagram is presented for the minimum-energy topology as a function of applied field magnitude and angle. For small (large) applied fields, normal (superconducting) tubes are found, while for intermediate fields, parallel domains have a lower energy. The range of field magnitudes for which the superconducting-tubes structure is favored shrinks when the field is more in-plane oriented.

We report on muon spin rotation ($\mu$SR) studies of the superconducting and magnetic properties of the ternary intermetallic stannide Ca${}_3$Ir${}_4$Sn${}_{13}$. This material has recently been the focus of intense research activity due to a proposed interplay of ferromagnetic spin fluctuations and superconductivity. In the temperature range $T=1.6$–200 K, we find that the zero-field muon relaxation rate is very low and does not provide evidence for spin fluctuations on the $\mu$SR time scale. The field-induced magnetization cannot be attributed to localized magnetic moments. In particular, our $\mu$SR data reveal that the anomaly observed in thermal and transport properties at $T^{*}\approx 38$ K is not of magnetic origin. Results for the transverse-field muon relaxation rate at $T=0.02$–12 K suggest that superconductivity emerges out of a normal state that is not of a Fermi-liquid type. This is unusual for an electronic system lacking partially filled $f$-electron shells. The superconducting state is dominated by a nodeless order parameter with a London penetration depth of ${\lambda }_{\mathrm{L}}=385\left(1\right)$ nm and the electron-phonon pairing interaction is in the strong-coupling limit.

Several examples of metallic electron-doped polycyclic aromatic hydrocarbons (PAHs) molecular crystals have recently been experimentally proposed. Some of them have superconducting components, but most other details are still unknown beginning with structure and the nature of metallicity. We carried out ab initio density functional calculations for La-Phenanthrene (La-PA), here meant to represent a generic case of three-electron doping, to investigate structure and properties of a conceptually simple case. To our surprise we found first of all that the lowest energy state is not metallic but band insulating, with a disproportionation of two inequivalent PA molecular ions and a low $P1$ symmetry, questioning the common assumption that three electrons will automatically metallize a PAH crystal. Our best metallic structure is metastable and slightly higher in energy, and retains equivalent PA ions and a higher $P{2}_1$ symmetry—the same generally claimed for metallic PAHs. We show that a “dimerizing” periodic distortion opens very effectively a gap in place of a symmetry related degeneracy of all $P{2}_1$ structures near the Fermi level, foreshadowing a possible role of that special intermolecular phonon in superconductivity of metallic PAHs. A Hubbard-Fröhlich model describing that situation is formulated for future studies.

We consider quasiballistic electron transmission in a one-dimensional quantum wire subject to both time-independent and periodic potentials of a finger gate that results in a local time-dependent Rashba-type spin-orbit coupling. A spin-dependent conductance is calculated as a function of external constant magnetic field, the electric field frequency, and potential strength. The results demonstrate the effect of the gate-driven electric dipole spin resonance in a transport phenomenon such as spin-flip electron transmission.

The population dynamics of single exciton and biexciton states in a simple model of a spherical semiconductor nanostructure is modeled numerically in the presence of Coulomb coupling between single and two exciton states and a dissipation channel in order to study the transient biexciton population that occurs in an optically excited semiconductor nanocrystal. The results show that the system evolution strongly changes if the dissipation is included. In a certain range of parameters, the growth of the exciton number (multiple exciton generation process) is fast (on picosecond time scale) and the following decay (Auger process) is much slower (hundreds of picoseconds). In some cases, the maximum occupation of the biexciton state increases when dissipation is included. The dynamics of an ensemble of nanostructures with a certain size dispersion is studied by averaging over the energy of the biexciton state which can be different for each single nanostructure. The validity of Markov and secular approximation is also verified.