We study theoretically the properties of the interacting Dirac liquid, a novel three-dimensional many-body system which was recently experimentally realized and in which the electrons have a chiral linear relativistic dispersion and a mutual Coulomb interaction. We find that the “intrinsic” Dirac liquid, where the Fermi energy lies exactly at the nodes of the band dispersion, displays unusual Fermi liquid properties similar to graphene, whereas the “extrinsic” system with finite detuning or doping behaves as a standard Landau Fermi liquid. We present analytical and numerical results for the self-energy and spectral function based on both Hartree-Fock and the random phase approximation (RPA) theories and compute the quasiparticle lifetime, residue, and renormalized Fermi velocity of the extrinsic Dirac liquid. A full numerical calculation of the extrinsic RPA spectral function indicates that the Fermi liquid description breaks down for large-energy excitations. Furthermore, we find an additional plasmaron quasiparticle sideband in the spectral function which is discontinuous around the Fermi energy. Our predictions should be observable in ARPES and STM measurements.

Field-theoretic methods are used to investigate the large-$U$ Hubbard model on the honeycomb lattice at half-filling and in the hole-doped regime. Within the framework of a functional-integral approach, we obtain the Lagrangian density associated with the charge and spin degrees of freedom. The Hamiltonian related to the charge degrees of freedom is exactly diagonalized. In the strong-coupling regime, we derive a perturbative low-energy theory suitable to describe the quantum antiferromagnetic phase (AF) as a function of hole doping. At half-filling, we deal with the underlying spin degrees of freedom of the quantum AF Heisenberg model by employing a second-order spin-wave analysis, in which case we have calculated the ground-state energy and the staggered magnetization; the results are in very good agreement with previous studies. Further, in the continuum, we derive a nonlinear $\sigma$ model with a topological Hopf term that describes the AF-VBS (valence bond solid) competition. Lastly, in the challenging doped regime, our approach allows the derivation of a $t\text{−}J$ Hamiltonian, and the analysis of the role played by charge and spin quantum fluctuations on the ground-state energy and, particularly, on the breakdown of the AF order at a critical hole doping; the results are benchmarked against recent Grassmann tensor product state simulations.

We propose an optimized set of quantum gates for a singlet-triplet qubit in a double quantum dot with two electrons utilizing the $S\text{−}{T}_{-}$ subspace. Qubit rotations are driven by the applied magnetic field and a field gradient provided by a micromagnet. We optimize the fidelity of this qubit as a function of the magnetic fields, taking advantage of “sweet spots” where the rotation frequencies are independent of the energy level detuning, providing protection against charge noise. We simulate gate operations and qubit rotations in the presence of quasistatic noise from charge and nuclear spins as well as leakage to nonqubit states. Our results show that, for silicon quantum dots, gate fidelities greater than $99\%$ should be realizable, for rotations about two nearly orthogonal axes.

Superstructured hyperbolic metamaterials (HMs) have been recently introduced to realize media with effective index $-1$ with the ultimate goal of designing flat lenses of super-resolution power for optical imaging applications. In this work, we analyze the impact on their effective optical properties of defect metallic layers periodically added in HMs. The effective index and losses are systematically calculated in both homogenization and diffractive regimes and with respect to the ratio of dielectric and metallic layers. Although the superstructuring can dramatically decrease the effective losses, we demonstrate that the extent of the hyperbolic dispersion curve in $k$ space plays an even more fundamental role for breaking the diffraction limit. Optimized superstructured HMs working in a regime between the homogenization and diffractive regimes are shown to present simultaneously low effective losses and a high optical resolution for visible light. These superstructured HMs present an effective index of $-5$ and extend the subwavelength focalization distance up to $2\lambda$, which is twice as large as for regular HMs.

Motivated by recent experiments on spin pumping from a ferromagnet into organic materials in which the charge transport is due to hopping, we study theoretically the generation and propagation of spin current in a hopping insulator. Unlike metals, the spin polarization at the boundary with ferromagnet is created as a result of magnon absorption within pairs of localized states and it spreads following the current-currying resistor network (although the charge current is absent). We consider a classic resonant mechanism of the ac absorption in insulators and adapt it to the absorption of magnons. A strong enhancement of pumping efficiency is predicted when the Zeeman splitting of the localized states in external magnetic field is equal to the frequency of ferromagnetic resonance. Under this condition the absorption of a magnon takes place within individual sites.

We study the coupling between the ferroelectric polarization and the magnetization of a granular ferromagnetic film using a phenomenological model of a combined multiferroic system consisting of a granular ferromagnetic film placed above a ferroelectric (FE) layer. The coupling is due to the screening of the Coulomb interaction in the granular film by the FE layer. Below the FE Curie temperature, the magnetization has a hysteresis as a function of the electric field. Below the magnetic ordering temperature, the polarization has a hysteresis as a function of the magnetic field. We study the magnetoelectric coupling for weak and strong spatial dispersion of the FE layer. The effect of mutual influence decreases with increasing spatial dispersion of the FE layer. For weak dispersion, the strongest coupling occurs in the vicinity of the ferroelectric-paraelectric phase transition. For strong dispersion, the situation is the opposite. We study the magnetoelectric coupling as a function of the distance between the FE layer and the granular film. For large distances, the coupling decays exponentially due to the exponential decrease of the electric field produced by the oscillating charges in the granular ferromagnetic film.

We analyze the spin-resolved transport properties of a triangular quantum dot molecule weakly coupled to external ferromagnetic leads. The calculations are performed by using the real-time diagrammatic technique up to the second-order of perturbation theory, which enables a description of both the sequential and cotunneling processes. We study the behavior of the current and differential conductance in the parallel and antiparallel magnetic configurations, as well as the tunnel magnetoresistance (TMR) and the Fano factor in both the linear and nonlinear response regimes. It is shown that the transport characteristics depend greatly on how the system is connected to external leads. Two specific geometrical configurations of the device are considered—the mirror one, which possesses the reflection symmetry with respect to the current flow direction and the fork one, in which this symmetry is broken. In the case of first configuration we show that, depending on the bias and gate voltages, the system exhibits both enhanced TMR and super-Poissonian shot noise. On the other hand, when the system is in the second configuration, we predict a negative TMR and a negative differential conductance in certain transport regimes. The mechanisms leading to those effects are thoroughly discussed.

Density functional theory and group-theoretical methods are used to explore the origin for ferroelectricity in cation ordered ${\mathrm{LaSrMnO}}_4$ with the Ruddlesden-Popper structure. The equilibrium phase exhibits the polar $Pca{2}_1$ space group where small polar displacements of $d^4{\mathrm{Mn}}^{3+}$ coexist with antiferrodistortive octahedral rotations and Jahn-Teller distortions. We find that the octahedral rotations and Jahn-Teller distortion stabilize the polar structure and induce polar displacements through high-order anharmonic interactions among the three modes, making ${\mathrm{LaSrMnO}}_4$ a hybrid-improper ferroelectric material. The rotations result from the ionic size mismatch between $A$ cations and Mn whereas the Jahn-Teller distortions are energetically favored owing to the coupling between the local $e_g$ orbital polarization of the two nearest-neighboring Mn cations in the two-dimensional ${\mathrm{MnO}}_2$ sheets. Our results indicate that anharmonic interactions among multiple centric modes can be activated by cation ordering to induce polar displacements in layered oxides, making it a reliable approach for realizing acentric properties in artificially constructed materials.

The elastic moduli, elastic anisotropy coefficients, sound velocities and Poisson's ratio of hcp solid helium have been calculated using density functional theory in generalized gradient approximation (up to 30 TPa), and pair + triple semiempirical potentials (up to 100 GPa). Zero-point vibrations have been treated in the Debye approximation assuming ${}^4\mathrm{He}$ isotope (we exclude the quantum-crystal region at very low pressures from consideration). Both methods give a reasonable agreement with the available experimental data. Our calculations predict significant elastic anisotropy of helium ($▵P\approx 1.14,▵S_1\approx 1.7,▵S_2\approx 0.93$ at low pressures). Under terapascal (TPa) pressures helium becomes more elastically isotropic. At the metallization point, there is a sharp feature in the elastic modulus $C_S$, which is the stiffness with respect to the isochoric change of the $c/a$ ratio. This is connected with the previously obtained sharp minimum of the $c/a$ ratio at the metallization point. Our calculations confirm the previously measured decrease of the Poisson's ratio with increasing pressure. This is not a quantum effect, as the same sign of the pressure effect was obtained when we disregarded zero-point vibrations. At TPa pressures, Poisson's ratio reaches the value of $0.31$ at the theoretical metallization point ($V_{mol}=0.228{\mathrm{cm}}^3$/mol, $p=17.48$ TPa) and $0.29$ at 30 TPa. For $p=0$, we predict a Poisson's ratio of $0.38$, which is in excellent agreement with the low-$p$-low-$T$ experimental data.

The generation of a DC voltage in single layers of permalloy (${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$) when the magnetization is undergoing ferromagnetic resonance is investigated in a series of samples with thickness varying from 4.0 to 150 nm. By sweeping the external field at a fixed microwave frequency, we measure a DC voltage at the ends of the layer as a function of the in-plane angle for each sample. The asymmetric voltage signal generated at the resonance field is a superposition of symmetric Lorentzian and antisymmetric Lorentzian derivative line shapes. The in-plane dependence of both symmetric and antisymmetric signals cannot be explained as due to spin rectification (SRE) only. The results are well explained by a model that takes into account in addition to the SRE the contribution of the recent discovered effect of magnonic charge pumping that converts magnetization dynamics into charge current by means of the spin orbit coupling.

Nanocontact spin-torque oscillators are devices in which the generation of propagating spin waves can be sustained by spin transfer torque. In the present paper, we perform combined electrical and optical measurements in a single experimental setup to systematically investigate the excitation of spin waves by a nanocontact spin-torque oscillator and their propagation in a $\mathrm{N}{\mathrm{i}}_{80}\mathrm{F}{\mathrm{e}}_{20}$ extended layer. By using microfocused Brillouin light scattering we observe an anisotropic emission of spin waves, due to the broken symmetry imposed by the inhomogeneous Oersted field generated by the injected current. In particular, spin waves propagate on the side of the nanocontact where the Oersted field and the in-plane component of the applied magnetic field are antiparallel, while propagation is inhibited on the opposite side. Moreover, propagating spin waves are efficiently excited only in a limited frequency range corresponding to wavevectors inversely proportional to the size of the nanocontact. This frequency range obeys the dispersion relation for exchange-dominated spin waves in the far field, as confirmed by micromagnetic simulations of similar devices. The present results have direct consequences for spin wave based applications, such as synchronization, computation, and magnonics.

We theoretically study hydrogen-bonded molecular conductors synthesized recently, $\kappa -{\mathrm{H}}_3{(\text{Cat}-\text{EDT}-\mathrm{TTF})}_2$ and its diselena analog, $\kappa -{\mathrm{H}}_3{(\text{Cat}-\text{EDT}-\mathrm{ST})}_2$, by first-principles density functional theory calculations. In these crystals, two H(Cat-EDT-TTF/ST) units share a hydrogen atom with a short O–H–O hydrogen bond. The calculated band structure near the Fermi level shows a quasi-two-dimensional character with a rather large interlayer dispersion due to the absence of insulating layers, in contrast with conventional molecular conductors. We discuss effective low-energy models based on H(Cat-EDT-TTF/ST) units and its dimers, respectively, where the microscopic character of the orbitals composing them are analyzed. Furthermore, we find a stable structure which is different from the experimentally determined structure, where the shared hydrogen atom becomes localized to one of the oxygen atoms, in which charge disproportionation between the two types of H(Cat-EDT-TTF) units is associated. The calculated potential energy surface for the H atom is very shallow near the minimum points; therefore the probability of the H atom can be delocalized between the two O atoms.

There are several possible theoretically allowed non-Abelian fractional quantum Hall (FQH) states that could potentially be realized in one- and two-component FQH systems at total filling fraction $\nu =n+2/3$, for integer $n$. Some of these states even possess quasiparticles with non-Abelian statistics that are powerful enough for universal topological quantum computation, and are thus of particular interest. Here we initiate a systematic numerical study, using both exact diagonalization and variational Monte Carlo, to investigate the phase diagram of FQH systems at total filling fraction $\nu =n+2/3$, including in particular the possibility of the non-Abelian $Z_4$ parafermion state. In $\nu =2/3$ bilayers we determine the phase diagram as a function of interlayer tunneling and repulsion, finding only three competing Abelian states, without the $Z_4$ state. On the other hand, in single-component systems at $\nu =8/3$, we find that the $Z_4$ parafermion state has significantly higher overlap with the exact ground state than the Laughlin state, together with a larger gap, suggesting that the experimentally observed $\nu =8/3$ state may be non-Abelian. Our results from the two complementary numerical techniques agree well with each other qualitatively.

The iron chalcogenide $\mathrm{F}{\mathrm{e}}_{1+y}\mathrm{T}{\mathrm{e}}_{1-x}\mathrm{S}{\mathrm{e}}_x$ on the Te-rich side is known to exhibit the strongest electron correlations among the Fe-based superconductors and is nonsuperconducting for $x<0.1$. In order to understand the origin of such behaviors, we have performed angle-resolved photoemission spectroscopy studies of $\mathrm{F}{\mathrm{e}}_{1+y}\mathrm{T}{\mathrm{e}}_{1-x}\mathrm{S}{\mathrm{e}}_x \left(x=0,0.1,0.2,\mathrm{and}0.4\right)$. The obtained mass renormalization factors for different energy bands are qualitatively consistent with density functional theory and dynamical mean-field theory calculations. Our results provide evidence for strong orbital dependence of mass renormalization and systematic data which help us to resolve inconsistencies with other experimental data. The unusually strong orbital dependence of mass renormalization in Te-rich $\mathrm{F}{\mathrm{e}}_{1+y}\mathrm{T}{\mathrm{e}}_{1-x}\mathrm{S}{\mathrm{e}}_x$ arises from the dominant contribution to the Fermi surface of the $d_{xy}$ band, which is the most strongly correlated and may contribute to the suppression of superconductivity.

Graphene samples with thicknesses ranging from monolayer to few layer graphene grown on the C-face of SiC by Si flux-assisted molecular beam epitaxy were studied to understand their stacking structure. Particular attention was put on determining the size, thickness, spatial distribution, and orientation relative to the SiC of the graphene domains. A complete electronic characterization of the graphene films down to submicrometer grains was obtained by using synchrotron-based conventional and nanoresolved photoelectron spectroscopies. These measurements were completed with scanning probe techniques like atomic force and scanning tunneling microscopies. By probing exactly the same region of the samples using angular-resolved and core-level photoelectron spectroscopy imaging and point modes, we were able to identify two types of grains constituting the graphene films with radically different thickness, stacking and orientation. The size, distribution, and registry with the substrate for each type of grain were determined. Most interestingly, we have evidenced that multilayer graphene grains with Bernal stacking coexist with areas composed of twisted bilayer graphene grains.

We present the temperature–pressure–magnetic-field phase diagram of the itinerant-electron metamagnet UCoAl determined from the ac susceptibility measurement. The quantum critical point (QCP) was found to be located at $2.9\pm 0.2$ GPa ($\equiv P_{\mathrm{QCP}}$) and $130\pm 5$ kOe. In addition to the determination of the phase diagram, we report magnetic and transport properties in the paramagnetic state and around the phase boundary. The critical divergence of the magnetic susceptibility is suppressed toward the QCP. From the longitudinal magnetoresistance and the resistivity under magnetic fields, enhancement of the resistivity is observed in the so-called supercritical region. In the paramagnetic region, the magnetoresistance and temperature dependence of the resistivity exhibit complex behaviors, especially in the vicinity of 1.5 GPa and $P_{\mathrm{QCP}}$.

We investigate numerically the charge, spin, and entanglement dynamics of two electrons confined in a gated semiconductor nanowire. The electrostatic coupling between electrons in the nanowire and the induced charge on the metal electrodes leads to a self-trapping of the electrons, which results in solitonlike properties. We show that the interplay of an all-electrically controlled coherent transport of the electron solitons and of the exchange interaction can be used to realize ultrafast SWAP and entangling $\sqrt{\text{SWAP}}$ gates for distant spin qubits. We demonstrate that the latter gate can be used to generate a maximally entangled spin state of spatially separated electrons. The results are obtained by quantum-mechanical time-dependent calculations with exact inclusion of electron-electron correlations.

For certain nonmagnetic solids with low symmetry the occurrence of spin-polarized longitudinal currents is predicted. These arise due to an interplay of spin-orbit interaction and the particular crystal symmetry. This result is derived using a group-theoretical scheme that allows investigating the symmetry properties of any linear response tensor relevant to the field of spintronics. For the spin conductivity tensor it is shown that only the magnetic Laue group has to be considered in this context. Within the introduced general scheme also the spin Hall and additional related transverse effects emerge without making reference to the two-current model. Numerical studies confirm these findings and demonstrate for ${\left({\text{Au}}_{1-x}{\text{Pt}}_x\right)}_4\text{Sc}$ that the longitudinal spin conductivity may be on the same order of magnitude as the conventional transverse one. The presented formalism only relies on the magnetic space group and therefore is universally applicable to any type of magnetic order.

Quantifying the spin-spin interactions which influence electronic transitions in organic semiconductors is crucial for understanding their magneto-optoelectronic properties. By combining a theoretical model for three spin interactions in the coherent regime with pulsed electrically detected magnetic resonance experiments on $\pi$-conjugated polymer diodes (poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]), we quantify the spin coupling within complexes comprising three spin-1/2 particles. We determine that these particles form triplet-exciton/polaron pairs, where the polaron-exciton exchange is over five orders of magnitude weaker ($<170$ MHz) than that within the exciton. This approach provides a direct spectroscopic approach for distinguishing between coupling regimes, and to test hypotheses relating microscopic properties to bulk characteristics of organic electronic devices.