Using density-functional methods, we study the electronic structure of ${\mathrm{Ba}}_3\mathrm{Cu}{\mathrm{Sb}}_2{\mathrm{O}}_9$, a candidate material for the quantum spin liquid behavior. We study both the triangular lattice as well as the recently proposed hexagonal lattice structures with flipped Cu-Sb dumbbells. The band structure near the Fermi energy is described very well by a tight-binding Hamiltonian involving the Cu ($e_g$) orbitals, confirming their central role in the physics of the problem. A minimal tight-binding Hamiltonian for the triangular structure is presented. The Cu ($d^9$) ions (a single $e_g$ hole in the band structure) present in the compound are expected to be Jahn-Teller centers, while the nature of the Jahn-Teller distortions in this material is still under debate. Solving a simple model by exact diagonalization, we show that electronic correlation effects in general enhance the tendency towards a Jahn-Teller distortion by reducing the kinetic energy due to correlation effects. Our density-functional calculations do indeed show a significant Jahn-Teller distortion of the $\mathrm{Cu}{\mathrm{O}}_6$ octahedra when we include the correlation effects within the Coulomb-corrected GGA+U method, so that the Jahn-Teller effect is correlation driven. We argue for the presence of a random static Jahn-Teller distortion in the hexagonal structure rather than a dynamical one because of the broken octahedral symmetry around the $\mathrm{Cu}{\mathrm{O}}_6$ octahedra and the potential fluctuations inherently present in the system caused by a significant disorder, which is believed to be present, in particular, due to the flipped Cu-Sb dumbbells.

In this work, we develop a time-dependent extension of the Landauer-Büttiker approach to study transient dynamics in time-dependent quantum transport through molecular junctions. A key feature of the approach is that it provides a closed integral expression for the time dependence of the density matrix of the molecular junction after switch-on of a bias in the leads or a perturbation in the junction, which in turn can be evaluated without the necessity of propagating individual single-particle orbitals or Green's functions. This allows for the study of time-dependent transport in large molecular systems coupled to wide-band leads. As an application of the formalism, we study the transient dynamics of zigzag and armchair graphene nanoribbons of different symmetries. We find that the transient times can exceed several hundreds of femtoseconds while displaying a long-time oscillatory motion related to multiple reflections of the density wave in the nanoribbons at the ribbon-lead interface. This temporal profile has a shape that scales with the length of the ribbons and is modulated by fast oscillations described by intraribbon and ribbon-lead transitions. Especially in the armchair nanoribbons there exists a sequence of quasistationary states related to reflections at the edge state located at the ribbon-lead interface. In the case of zigzag nanoribbons, there is a predominant oscillation frequency associated with virtual transitions between the edge states and the Fermi levels of the electrode. We further study the local bond currents in the nanoribbons and find that the parity of the edges strongly affects the path of the electrons in the nanoribbons. We finally study the behavior of the transients for various added potential profiles in the nanoribbons.

The photoemission spectrum of graphite is still debated. To help resolve this issue, we present photoemission measurements at high photon energy and analyze the results using a Green's function approach that takes into account the full complexity of the loss spectrum. Our measured data show multiple satellite replicas. We demonstrate that these satellites are of intrinsic origin, enhanced by extrinsic losses. The dominating satellite is due to the $\pi +\sigma$ plasmon of graphite, whereas the $\pi$ plasmon creates a tail on the high-binding energy side of the quasiparticle peak. The interplay between the two plasmons leads to energy shifts, broadening, and additional peaks in the satellite spectrum. We also predict the spectral changes in the transition from graphite towards graphene.

A graphene bilayer is known to perfectly reflect normally incident electrons due to their chirality. This is similar to Klein tunneling, which, in a monolayer, is instead responsible for perfect transmission at normal incidence. Stacking defaults turn each parabolic band crossing of a bilayer into pairs of Dirac cones. Here we show that, surprisingly, a stacking default (or shift) in a bilayer can result in perfect transmission at normal incidence as a result of Fabry-Pérot type resonances at zero energy. These constructive interferences only happen for a specific orientation of the Dirac cones with respect to the incident electron and for quantized values of their separation in reciprocal space. Our results provide a way to control transmission resonances in an undoped graphene bilayer structure by adjusting the layer stacking.

Silicene, the silicon-based counterpart of graphene with a two-dimensional honeycomb lattice, has attracted tremendous interest both theoretically and experimentally due to its significant potential industrial applications. From the aspect of theoretical study, the widely used classical molecular dynamics simulation is an appropriate way to investigate the transport phenomena and mechanisms in nanostructures such as silicene. Unfortunately, no available interatomic potential can precisely characterize the unique features of silicene. Here, we optimized the Stillinger-Weber potential parameters specifically for a single-layer Si sheet, which can accurately reproduce the low buckling structure of silicene and the full phonon dispersion curves obtained from ab initio calculations. By performing equilibrium and nonequilibrium molecular dynamics simulations and anharmonic lattice dynamics calculations with the new potential, we reveal that the three methods consistently yield an extremely low thermal conductivity of silicene and a short phonon mean-free path, suggesting silicene as a potential candidate for high-efficiency thermoelectric materials. Moreover, by qualifying the relative contributions of lattice vibrations in different directions, we found that the longitudinal phonon modes dominate the thermal transport in silicene, which is fundamentally different from graphene, despite the similarity of their two-dimensional honeycomb lattices.

We investigate the dynamics of two coupled macrospins connected to thermal baths at different temperatures. The system behaves like a diode which allows the propagation of energy and magnetization currents in one direction only. This effect is described by a simple model of two coupled nonlinear oscillators interacting with two independent reservoirs. It is shown that the rectification phenomenon can be interpreted as a a stochastic phase synchronization of the two spin oscillators. A brief comparison with realistic micromagnetic simulations is presented. This new effect yields promising opportunities in spin caloritronics and nanophononic devices.

We investigate the dynamic susceptibility of Fe${}_4$ single-molecule magnets with integer spin ($S=5$) in the form of pure crystals as well as diluted in crystals of isostructural, but nonmagnetic, Ga${}_4$ clusters. Below approximately 1 K, the spin-lattice relaxation becomes dominated by a temperature-independent process. The spin-lattice relaxation time $\tau$ measured in this “quantum regime” is 12 orders of magnitude shorter than the characteristic time scale of direct phonon-induced processes but agrees with the relaxation times of pure (i.e., not assisted by phonons) spin tunneling events. The present results show that the latter phenomenon, despite conserving the energy of the ensemble of electronic and nuclear spins, drives the thermalization of electronic spins at very low temperatures. The spin-lattice relaxation time scales with the concentration of Fe${}_4$, thus suggesting that the main effect of dipolar interactions is to block tunneling. The data show therefore no evidence for the contribution of collective phonon emission processes, such as phonon superradiance, to the spin-lattice relaxation.

Large local anomalies in the Earth's magnetic field have been observed in Norway, Sweden, and Canada. These anomalies have been attributed to the unusual magnetic properties of naturally occurring hemo-ilmenite, consisting of a paramagnetic ilmenite host ($\alpha$-Fe${}_2$O${}_3$-bearing FeTiO${}_3$) with exsolution lamellae ($\approx$$3\mu$m thick) of canted antiferromagnetic hematite (FeTiO${}_3$-bearing $\alpha$-Fe${}_2$O${}_3$) and the mutual exsolutions of the same phases on the micron to nanometer scale. The origin of stable natural remanent magnetization (NRM) in this system has been proposed to be uncompensated magnetic moments in the contact layers between the exsolution lamellae. This lamellar magnetism hypothesis is tested here by using polarized neutron diffraction to measure the orientation of hematite spins as a function of an applied magnetic field in a natural single crystal of hemo-ilmenite from South Rogaland, Norway. Polarized neutron diffraction clearly shows that the ilmenite spins do not contribute to the NRM and that hematite spins account for the full magnetization at ambient temperature. Hematite sublattice spins are shown to adopt an average angle of 56${}^{\circ }$ with respect to a saturating magnetic field, which is intermediate between the angle of 90${}^{\circ }$ predicted for a pure canted moment and the angle of 0${}^{\circ }$ predicted for a pure lamellar moment. The observed NRM is consistent with the vector sum of lamellar magnetism and canted antiferromagnetic contributions. The relative importance of the two contributions varies with the length scale of the microstructure, with the lamellar contribution increasing when exsolution occurs predominantly at the nanometer rather than the micrometer scale.

We report on a member of the spin-disordered honeycomb lattice antiferromagnet in a quasi-one-dimensional cobaltate ${\text{Ba}}_3$${\text{Co}}_2$${\text{O}}_6$(${\text{CO}}_3$)${}_{0.7}$. Resistivity exhibits as semimetallic along the face-sharing ${\text{CoO}}_6$ chains. Magnetic susceptibility shows strongly anisotropic Ising-spin character with the easy axis along the chain due to significant spin-orbit coupling and a trigonal crystal field. Nevertheless, ${}^{135}\mathrm{Ba}$ NMR detects no indication of the long-range magnetic order down to 0.48 K. Marginally itinerant electrons possess large entropy and low-lying excitations with a Wilson ratio $R_W=116$, which highlight interplays of charge, spin, and orbital in the disordered ground state.

Some of the iron pnictides show coexisting superconductivity and spin-density-wave order. We study the superconducting pairing instability in the spin-density-wave phase. Assuming that the pairing interaction is due to spin fluctuations, we calculate the effective pairing interactions in the singlet and triplet channels by summing the bubble and ladder diagrams, taking the reconstructed band structure into account. The leading pairing instabilities and the corresponding superconducting gap structures are then obtained from the superconducting gap equation. We illustrate this approach for a minimal two-band model of the pnictides. Analytical and numerical results show that the presence of spin and charge fluctuations in the spin-density-wave phase strongly enhances the pairing. Over a limited parameter range, a $p_x$-wave state is the dominant instability. It competes with various states, which have mostly $s^{\pm }$-type structures. We analyze the effect of various symmetry-allowed interactions on the pairing in some detail.

In systems combining type-II superconductivity and magnetism the nonstationary magnetic field of moving Abrikosov vortices may excite spin waves, or magnons. This effect leads to the appearance of an additional damping force acting on the vortices. By solving the London and Landau-Lifshitz-Gilbert equations we calculate the magnetic-moment-induced force acting on vortices in ferromagnetic superconductors and superconductor-ferromagnet superlattices. If the vortices are driven by a dc force, magnon generation due to the Cherenkov resonance starts as the vortex velocity exceeds some threshold value. For an ideal vortex lattice this leads to an anisotropic contribution to the resistivity and to the appearance of resonance peaks on the current-voltage characteristics. For a disordered vortex array the current will exhibit a steplike increase at some critical voltage. If the vortices are driven by an ac force with a frequency $\omega$, the interaction with magnetic moments will lead to a frequency-dependent magnetic contribution ${\eta }_M$ to the vortex viscosity. If $\omega$ is below the ferromagnetic resonance frequency ${\omega }_F$, vortices acquire additional inertia. For $\omega >{\omega }_F$ dissipation is enhanced due to magnon generation. The viscosity ${\eta }_M$ can be extracted from the surface impedance of the ferromagnetic superconductor. Estimates of the magnetic force acting on vortices for the U-based ferromagnetic superconductors and cuprate-manganite superlattices are given.

We present a homogenization scheme for acoustic metamaterials that is based on reproducing the lowest orders of scattering amplitudes from a finite volume of metamaterials. This approach is noted to differ significantly from that of coherent potential approximation, which is based on adjusting the effective-medium parameters to minimize scatterings in the long-wavelength limit. With the aid of metamaterials’ eigenstates, the effective parameters, such as mass density and elastic modulus can be obtained by matching the surface responses of a metamaterial's structural unit cell with a piece of homogenized material. From the Green's theorem applied to the exterior domain problem, matching the surface responses is noted to be the same as reproducing the scattering amplitudes. We verify our scheme by applying it to three different examples: a layered lattice, a two-dimensional hexagonal lattice, and a decorated-membrane system. It is shown that the predicted characteristics and wave fields agree almost exactly with numerical simulations and experiments and the scheme's validity is constrained by the number of dominant surface multipoles instead of the usual long-wavelength assumption. In particular, the validity extends to the full band in one dimension and to regimes near the boundaries of the Brillouin zone in two dimensions.

The lightly hole-doped system CeOs${}_{1.94}$Re${}_{0.06}$Al${}_{10}$ has been studied by muon spin relaxation and neutron diffraction measurements. A long-range antiferromagnetic ordering of the Ce sublattice with a substantially reduced value of the magnetic moment $0.18\left(1\right){\mu }_B$ has been found below $T_N=21$ K. Similar to the undoped parent compound, the magnetic ground state of CeOs${}_{1.94}$Re${}_{0.06}$Al${}_{10}$ preserves the anomalous direction of the ordered moments along the $c$ axis. The obtained result reveals the crucial difference between electron- and hole-doping effects on the magnetic ordering in CeOs${}_2$Al${}_{10}$. The former suppresses the anisotropic $c\text{−}f$ hybridization and promotes localized Ce moments controlled by single ion anisotropy. On the contrary, the latter increases the hybridization, keeping the dominant role of the anisotropic exchange on the direction of the moments and shifts the system towards a delocalized nonmagnetic state.

The M-edge resonant magneto-optical Kerr effect of a transition metal film was measured using rotating analyzer ellipsometry and compared with theoretical calculations based on resonant scattering theory. A large Kerr rotation angle of over 10${}^{\circ }$ was observed at room temperature.

Polarized neutron reflectometry and x-ray reflectometry were used to determine the nanoscale magnetic and chemical depth profiles of the heavy rare-earth nitrides HoN, ErN, and DyN in the form of 15- to 40-nm-thick films. The net ferromagnetic components are much lower than the predictions of density-functional theory and Hund's rules for a simple ferromagnetic ground state in these $4f$ ionic materials, which points to the intrinsic contribution of crystal-field effects and noncollinear spin structures. The magnetic moment per rare-earth ion was determined as a function of temperature in the range 5–100 K at fields of 1–4 T. It is demonstrated that the films are stoichiometric within 1–3% and magnetically homogeneous on the nanometer scale.

By investigating the nonequilibrium transport across a metal/superconductor junction in both nonrelativistic and relativistic cases, we reveal that the asymmetric Andreev reflection with anisotropic superconductors is able to induce electric and thermal Hall-like effects in the absence of a magnetic field. That is, a longitudinal electric voltage or temperature bias can inducetransverse electric or thermal currents merely through the asymmetric Andreev reflection, respectively. In particular, a transverse thermoelectric effect, i.e., the Ettingshausen-like effect, is identified, although the conjugate Nernst effect is absent. The direction change of these electric and thermal Hall-like currents is also discussed. The Hall-like effects uncovered here do not require the conventional time-reversal symmetry breaking but, rather, originate from the mirror symmetry breaking with respect to the interface normal due to the anisotropic paring symmetry of the superconductor.

Even in the absence of Coulomb interactions, phase fluctuations induced by quantum size effects become increasingly important in superconducting nanostructures as the mean level spacing becomes comparable with the bulk superconducting gap. Here we study the role of these fluctuations, termed “quantum capacitance,” in the phase diagram of a one-dimensional ring of ultrasmall Josephson junctions at zero temperature by using path-integral techniques. Our analysis also includes dissipation due to quasiparticle tunneling and Coulomb interactions through a finite mutual and self-capacitance. The resulting phase diagram has several interesting features: A finite quantum capacitance can stabilize superconductivity even in the limit of only a finite mutual-capacitance energy, which classically leads to breaking of phase coherence. In the case of vanishing charging effects, relevant in cold-atom settings where Coulomb interactions are absent, we show analytically that superfluidity is robust to small quantum finite-size fluctuations and identify the minimum grain size for phase coherence to exist in the array. We have also found that the renormalization group results are in some cases very sensitive to relatively small changes of the instanton fugacity. For instance, a certain combination of capacitances could lead to a nonmonotonic dependence of the superconductor-insulator transition on the Josephson coupling.

We present a study of optical and dc properties of a highly frustrated organic conductor $\kappa$-(BEDT-TTF)${}_2$Hg(SCN)${}_2$Cl in the 300–10 K temperature range. At temperatures above 30 K, the material shows properties of a half-filled metal with strong electron-electron correlations. At 30 K, the compound undergoes a metal-insulator transition which we identify as a charge-ordering transition. We find that properties of $\kappa$-(BEDT-TTF)${}_2$Hg(SCN)${}_2$Cl are well explained by a model of a paired electron crystal.

We report significant changes of optical conductivity (${\sigma }_1$) in single-layer graphene induced by mild oxygen plasma exposure and explore the interplay between carrier doping, disorder, and many-body interactions from their signatures in the absorption spectrum. The first distinctive effect is the reduction of the excitonic binding energy that can be extracted from the renormalized saddle point resonance at 4.64 eV. Secondly, ${\sigma }_1$ is nearly completely suppressed (${\sigma }_1\ll {\sigma }_0$) below an exposure-dependent threshold in the near-infrared range. The clear steplike suppression follows the Pauli blocking behavior expected for doped monolayer graphene. The nearly zero residual conductivity below ω ∼ 2$E_F$ can be interpreted as arising from the weakening of the electronic self-energy. Our data shows that mild oxygen exposure can be used to controllably dope graphene without introducing the strong physical and chemical changes that are common in other approaches to oxidized graphene, allowing a controllable manipulation of the optical properties of graphene.

We report on the computation of the free-carrier absorption induced by static scatterers in asymmetric double quantum well structures when the electromagnetic wave propagates along the growth axis. We find that a Drude-like tail exists for this polarization. The absorption is found to be larger than when the wave propagates in the layer plane. Also, intrasubband scattering is found to be more efficient than intersubband scattering. The alloy scattering is found to be particularly efficient.