We study magnetism of a fcc lattice spin model with tetragonal distortions, with general exotic directional magnetic interactions allowed by symmetry. We consider two models, corresponding to a uniform tetragonal distortion, or a two-sublattice model with a tetragonal screw axis. We establish their low-temperature phase diagrams in the semiclassical limit using classical optimization and consideration of fluctuations both analytically and by Monte Carlo simulation. We found that both order-by-disorder and exchange-anisotropy mechanisms favor a $\langle 110\rangle$ easy axis for magnetization. We also show that spin-lattice coupling can give rise to an intermediate temperature paramagnetic nematic/orthorhombic phase, and discuss the transitions to/from this state. These results are relevant to a family of insulating magnetic double perovskites, and find immediate application to the ferromagnet ${\mathrm{Ba}}_2{\mathrm{NaOsO}}_6$.

We study the proximity-induced superconducting state in a general bilayer–conventional $s$-wave superconductor hybrid structure. For the bilayer we include a general parabolic dispersion, Rashba spin-orbit coupling, and finite-layer tunneling as well as the possibility to apply a bias potential and a magnetic Zeeman field, in order to address experimentally relevant bilayer systems, ranging from topological insulator thin films to generic double quantum well systems. By extracting the proximity-induced anomalous Green's function in the bilayer, we show a very rich structure for the superconducting pairing, including different spin states and odd-frequency pairing. Equal-spin spin-triplet $\left(p_x\pm i{p}_y\right)$-wave pairing is induced in both layers in the presence of a finite spin-orbit coupling and opposite-spin spin-triplet $s$-wave pairing with odd-frequency dependence appears for an applied magnetic Zeeman field. Finite interlayer pairing is also generally present in the bilayer. The interlayer pairing can be either even or odd in the layer index, with a complete reciprocity between parity in frequency and in layer index. We also find that a bilayer offers the possibility of sign reversal of the superconducting order parameters, both between the two layers and between multiple Fermi surfaces.

${\mathrm{YbCo}}_2{\mathrm{Si}}_2$ is considered to serve as a stable-valent, isoelectronic reference for the extensively studied heavy-fermion system ${\mathrm{YbRh}}_2{\mathrm{Si}}_2$ which is situated very close to an antiferromagnetic quantum critical point (QCP). The investigation of the Fermi surface (FS) topology of ${\mathrm{YbCo}}_2{\mathrm{Si}}_2$ and its comparison to ${\mathrm{YbRh}}_2{\mathrm{Si}}_2$ could help to unravel the strongly disputed nature of this quantum phase transition, whether it originates from a “local” or “itinerant” QCP. Here we study the electronic structure and FS of ${\mathrm{YbCo}}_2{\mathrm{Si}}_2$ by means of angle-resolved photoelectron spectroscopy (ARPES) and compare it to ab initio band structure calculations and FS modeling. Our approach allows the electronic structure at the surface and in the bulk to be disentangled. Identifying the bulk contribution, we demonstrate that ${\mathrm{YbCo}}_2{\mathrm{Si}}_2$ exhibits a “small” FS, confirming the formation of a “large” FS in ${\mathrm{YbRh}}_2{\mathrm{Si}}_2$. This favors an itinerant QCP, instead of the widely discussed local scenario. Our study demonstrates that ARPES is a reliable tool for the study of bulk electronic states in intermetallic Kondo lattice systems despite the complexity induced by their three-dimensional character and the presence of pronounced surface states.

We have in detail characterized the anisotropic charge response of the dimer Mott insulator $\kappa \text{−}\left(\text{BEDT-TTF}{)}_2{\mathrm{Cu}}_2\right(\text{CN}{)}_3$ by dc conductivity, Hall effect, and dielectric spectroscopy. At room temperature, the Hall coefficient is positive and close to the value expected from stoichiometry; the temperature behavior follows the dc resistivity $\rho \left(T\right)$. Within the planes the dc conductivity is well described by variable-range hopping in two dimensions; this model, however, fails for the out-of-plane direction. An unusually broad in-plane dielectric relaxation is detected below about 60 K; it slows down much faster than the dc conductivity following an Arrhenius law. At around 17 K, we can identify a pronounced dielectric anomaly concomitantly with anomalous features in the mean relaxation time and spectral broadening. The out-of-plane relaxation, on the other hand, shows a much weaker dielectric anomaly; it closely follows the temperature behavior of the respective dc resistivity. At lower temperatures, the dielectric constant becomes smaller both within and perpendicular to the planes; also, the relaxation levels off. The observed behavior bears features of relaxorlike ferroelectricity. Because heterogeneities impede its long-range development, only a weak-tunneling-like dynamics persists at low temperatures. We suggest that the random potential and domain structure gradually emerge due to the coupling to the anion network.

The interfacial atomic structure of a metallic ${\mathrm{LaNiO}}_3/{\mathrm{LaAlO}}_3$ superlattice grown on a ${\mathrm{LaSrAlO}}_4$ substrate was investigated using a combination of atomically resolved electron energy loss spectroscopy (EELS) at the Al $K$, Al $L_{2,3}$, Sr $L_{2,3}$, Ni $L_{2,3}$, La $M_{4,5}$, and O $K$ edges as well as hybridization mapping of selected features of the O $K$-edge fine structure. We observe an additional ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x{\mathrm{Al}}_{1-y}{\mathrm{Ni}}_y{\mathrm{O}}_3$ layer at the substrate-superlattice interface, possibly linked to diffusion of Al and Sr into the growing film or a surface reconstruction due to Sr segregation. The roughness of the ${\mathrm{LaNiO}}_3/{\mathrm{LaAlO}}_3$ interfaces is found to be on average around one pseudocubic unit cell. The O $K$-edge EELS spectra revealed reduced spectral weight of the prepeak derived from Ni-O hybridized states in the ${\mathrm{LaNiO}}_3$ layers. We rule out oxygen nonstoichiometry of the ${\mathrm{LaNiO}}_3$ layers and discuss changes in the Ni-O hybridization due to heterostructuring as possible origin.

Motivated by recent pump-probe spectroscopies, we study the effect of phonon dissipation and potential cooling on the nonequilibrium distribution function in a Floquet topological state. To this end, we apply a Floquet kinetic equation approach to study two-dimensional Dirac fermions irradiated by a circularly polarized laser, a system which is predicted to be in a laser-induced quantum Hall state. We find that the initial electron distribution shows an anisotropy with momentum-dependent spin textures whose properties are controlled by the switching-on protocol of the laser. The phonons then smoothen this out, leading to a nontrivial isotropic nonequilibrium distribution which has no memory of the initial state and initial switch-on protocol, and yet is distinct from a thermal state. An analytical expression for the distribution at the Dirac point is obtained that is relevant for observing quantized transport.

We study superconducting proximity effects in Weyl semimetals (WSM) with broken time-reversal symmetry by tunnel-coupling one of its surfaces to an $s$-wave superconductor using the Green's function approach. We find that the band structure develops coherence peaks, but despite the presence of metallic states in the bulk, the coherence peaks do not extend far into the bulk and remain confined to a few layers close to the interface, similar to the proximity effect in the topological insulators (TI) which are gapped in the bulk. The Weyl nodes remain unaffected, and in that sense, no true gap develops. We also study the induced $p$- and $s$-wave pairing amplitudes classified by their symmetries, as a function of the various parameters of the theory, and note the exponential decay of the induced pairings in the bulk both in the TI and the WSM, even at finite chemical potential.

The liquid-metal contact method (LMC method), which consists of immersing a carbon nanotube (CNT) placed on the apex of a metallic tip into a drop of liquid mercury (Hg), has been previously employed to show ballistic and quantum conductance properties of different kinds of CNTs. Using numerical simulations based on an analytical model of the mechanical interaction between the CNTs and the Hg surface, we show that the LMC method is unsuited for the analysis of ballistic conductance because most CNTs under realistic experimental conditions will not create a mobile contact this way. Furthermore, we show that the apparent electronic mean free path value deduced from these experiments will be exclusively due to elasticity induced CNT bending, geometry induced CNT sliding, and contact angle induced Hg meniscus related effects, and not to the electronic properties of the CNTs. These findings have strong consequences for the interpretation of previously reported works.

We show that the Landau levels in epitaxial graphene in the presence of localized defects are significantly modified compared to those of an ideal system. We report on magnetospectroscopy experiments performed on high-quality samples. Besides typical interband magneto-optical transitions, we clearly observe additional transitions that involve perturbed states associated with short-range impurities such as vacancies. Their intensity is found to decrease with an annealing process and a partial self-healing over time is observed. Calculations of the perturbed Landau levels by using a delta-like potential show electronic states both between and at the same energies as the Laudau levels of ideal graphene. The calculated absorption spectra involving all perturbed and unperturbed states are in very good agreement with the experiments.

By considering the Hubbard model on a honeycomb lattice, we develop a theory for intrinsic magnetic impurities (MIs) with quasilocalized spin moments induced by the vacancies in graphene. Because the intrinsic MIs are characterized by the zero modes that are orthogonal to the itinerant electrons, their properties are significantly different from those of Anderson MIs with well-localized spin moments.

We analyze the polar Kerr effect in an itinerant electron system on a square lattice in the presence of a composite charge order proposed for the pseudogap state in underdoped cuprates. This composite charge order preserves discrete translational symmetries, and is “chiral nematic” in the sense that it breaks time-reversal symmetry, mirror symmetries in $x$ and $y$ directions, and $C_4$ lattice rotation symmetry. The Kerr angle ${\theta }_K$ in $C_4$-symmetric system is proportional to the antisymmetric component of the anomalous Hall conductivity ${\sigma }_{xy}-{\sigma }_{yx}$. We show that this result holds when $C_4$ symmetry is broken. We show that in order for ${\sigma }_{xy}$ and ${\sigma }_{yx}$ to be nonzero the mirror symmetries in $x$ and $y$ directions have to be broken, and that for ${\sigma }_{xy}-{\sigma }_{yx}$ to be nonzero time-reversal symmetry has to be broken. The chiral-nematic charge order satisfies all these conditions, such that a nonzero signal in a polar Kerr effect experiment is symmetry allowed. We further show that to get a nonzero ${\theta }_K$ in a one-band spin-fluctuation scenario, in the absence of disorder, one has to extend the spin-mediated interaction to momenta away from $(\pi ,\pi )$ and has to include particle-hole asymmetry. Alternatively, in the presence of disorder, one can get a nonzero ${\theta }_K$ from impurity scattering: either due to skew scattering (with non-Gaussian disorder) or due to particle-hole asymmetry in case of Gaussian disorder. The impurity analysis in our case is similar to that in earlier works on Kerr effect in $p_x+i{p}_y$ superconductors, however, in our case, the magnitude of ${\theta }_K$ is enhanced by the flattening of the Fermi surface in the “hot” regions, which mostly contribute to charge order.

We investigate the electronic structure of a perovskite-type Pauli paramagnet ${\mathrm{SrMoO}}_3$ (${t_{2g}}^2$) thin film using hard x-ray photoemission spectroscopy and compare the results to realistic calculations that combine density functional theory within the local-density approximation (LDA) with dynamical mean-field theory (DMFT). Despite the clear signature of electron correlations in the electronic specific heat, the narrowing of the quasiparticle bands is not observed in the photoemission spectrum. This is explained in terms of the characteristic effect of Hund's rule coupling for partially filled $t_{2g}$ bands, which induces strong quasiparticle renormalization already for values of Hubbard interaction which are smaller than the bandwidth. This interpretation is supported by DMFT model calculations including Hund's rule coupling, which show a renormalization of low-energy quasiparticles without affecting the overall bandwidth. The photoemission spectra show additional spectral weight around $-2.5$ eV that is not present in the LDA+DMFT results, pointing to a source of correlations that is not present in our calculations that include only on-site interactions. We interpret this weight as a plasmon satellite, which is supported by the measured core-level spectra that all show satellites at this energy.

We have established a pressure-temperature phase diagram of $\mathrm{TTF}{\left[\mathrm{Ni}{\left(\mathrm{dmit}\right)}_2\right]}_2$ based on spin susceptibility at ambient pressure as well as longitudinal and transverse resistivity measurements under pressure up to 30 kbar. The data were analyzed on the basis of first-principles density functional theory calculations. We were able to find several phase transitions and identify three different charge density wave (CDW) states which all coexist with a metallic state in a wide temperature range and superconductivity at the lowest temperatures. This metallicity arises from the development, upon cooling, of a two-dimensional band associated to the $\mathrm{Ni}{\left(\mathrm{dmit}\right)}_2$ chains. At low pressure, two successive CDW transitions have been clearly identified and are associated to the successive nesting of two strongly one-dimensional bands: The LUMO and ${\mathrm{HOMO}}_{\mathrm{I}}$ of the $\mathrm{Ni}{\left(\mathrm{dmit}\right)}_2$ chains. These two transitions merge into a single one at 12 kbar which probably corresponds to the partial nesting of a bunch of LUMOs into the ${\mathrm{HOMO}}_{\mathrm{I}}$'s. A maximum of this unique CDW transition temperature is observed at 19 kbar. The CDW instability associated to the LUMO band is announced by an important regime of Peierls-like fluctuations in the metallic state which give rise to the progressive development of a pseudogap in the spin susceptibility which has been quantitatively analyzed using the Lee-Rice-Anderson theory.

The dynamical conductivity of interacting multiband electronic systems derived by Kupčić et al. [J. Phys.: Condens. Matter 90, 145602 (2013)] is shown to be consistent with the general form of the Ward identity. Using the semiphenomenological form of this conductivity formula, we have demonstrated that the relaxation-time approximation can be used to describe the damping effects in weakly interacting multiband systems only if local charge conservation in the system and gauge invariance of the response theory are properly treated. Such a gauge-invariant response theory is illustrated on the common tight-binding model for conduction electrons in doped graphene. The model predicts two distinctly resolved maxima in the energy-loss-function spectra. The first one corresponds to the intraband plasmons (usually called the Dirac plasmons). On the other hand, the second maximum $(\pi$ plasmon structure) is simply a consequence of the Van Hove singularity in the single-electron density of states. The dc resistivity and the real part of the dynamical conductivity are found to be well described by the relaxation-time approximation, but only in the parametric space in which the damping is dominated by the direct scattering processes. The ballistic transport and the damping of Dirac plasmons are thus the problems that require abandoning the relaxation-time approximation.

The technological potential of functionalized graphene has recently stimulated considerable interest in the study of the adsorption of metal atoms on graphene. However, a complete understanding of the optimal adsorption pattern of metal atoms on a graphene substrate has not been easy because of atomic relaxations at the interface and the interaction between metal atoms. We present a partial particle swarm optimization technique that allows us to efficiently search for the equilibrium geometries of metal atoms adsorbed on a substrate as a function of adatom concentration. Using Li deposition on graphene as an example we show that, contrary to previous works, Li atoms prefer to cluster, forming four-atom islands, irrespective of their concentration. We further show that an external electric field applied vertically to the graphene surface or doping with boron can prevent this clustering, leading to the homogeneous growth of Li.

The tunneling anisotropic magnetothermopower (TAMT) and the tunneling anisotropic spin-Seebeck (TASS) effects are studied for a magnetic tunnel junction (MTJ) composed of a ferromagnetic electrode, a zinc-blende semiconductor, and a normal metal. We develop a theoretical model for describing the dependence of a thermally induced tunneling current across the MTJ on the in-plane orientation of the magnetization in the ferromagnetic layer. The model accounts for the specific Bychkov-Rashba and Dresselhaus spin-orbit interactions present in these systems, which are responsible for the $C_{2v}$ symmetry we find in the TAMT and the TASS.

Polarization-analyzed small-angle neutron scattering methods are used to determine the spin morphology in high crystalline anisotropy, 11 nm diameter ${\mathrm{CoFe}}_2{\mathrm{O}}_4$ nanoparticle assemblies with randomly oriented easy axes. In moderate to high magnetic fields, the nanoparticles adopt a uniformly canted structure, rather than forming domains, shells, or other arrangements. The observed canting angles agree quantitatively with those predicted from an energy model dominated by Zeeman and anisotropy competition, with implications for the technological use of such nanoparticles.

Interstitial oxygen critical to the emergence of superconductivity in ${\mathrm{Fe}}_{1+y}\mathrm{Te}{\mathrm{O}}_x$ thin films has been detected. Its location and concentration are measured by atomic-resolution electron-energy-loss spectroscopy with $x=0.09$. The density functional theory calculations show that oxygen incorporation leads to local disorder in the magnetic moments of Fe, hole doping by oxygen forming ionic bonds with Fe, and a large magnetic- and position-dependent increase or reduction in the Te-Fe-Te bond angles. An examination of bonding based on charge density further reveals covalent charge between Fe and Te, and its reduction with O doping.

We study the dynamics of a tilted one-dimensional Bose-Hubbard model for two distinct protocols using numerical diagonalization for a finite sized system ($N\le 18$). The first protocol involves periodic variation of the effective electric field $E$ seen by the bosons which takes the system twice (per drive cycle) through the intermediate quantum critical point. We show that such a drive leads to nonmonotonic variation of the excitation density $D$ and the wave function overlap $F$ at the end of a drive cycle as a function of the drive frequency ${\omega }_1$, relate this effect to a generalized version of Stückelberg interference phenomenon, and identify special frequencies for which $D$ and $1-F$ approach zero leading to near-perfect dynamic freezing phenomenon. The second protocol involves a simultaneous linear ramp of both the electric field $E$ (with a rate ${\omega }_1$) and the boson hopping parameter $J$ (with a rate ${\omega }_2$) starting from the ground state for a low effective electric field up to the quantum critical point. We find that both $D$ and the residual energy $Q$ decrease with increasing ${\omega }_2$; our results thus demonstrate a method of achieving near-adiabatic protocol in an experimentally realizable quantum critical system. We suggest experiments to test our theory.