We study collective modes in anisotropic double-layer systems. To this end, we derive a zero-$T$ dynamic polarization function for a conductor with a paraboloidal dispersion. Following this, we demonstrate the dependence of the plasmonic modes on the relative orientation, doping, and system anisotropy.

We provide a microscopic description of the magnetic properties of ${\text{UGe}}_2$ and, in particular, of its both classical and quantum critical behavior. Namely, we account for all the critical points: the critical ending point (CEP) at the metamagnetic phase transition, the tricritical point, and the quantum critical end point at the ferromagnetic to paramagnetic phase transition. Their position agrees quantitatively with experiment. Additionally, we predict that the metamagnetic CEP can be traced down to zero temperature and becomes quantum critical point by a small decrease of both the total electron concentration and the external pressure. The system properties are then determined by the quantum critical fluctuations appearing near the instability point of the Fermi surface topology.

Cluster dynamical mean field methods are used to calculate the normal and anomalous components of the electron self-energy of the two-dimensional Hubbard model. Issues associated with the analytical continuation of the normal and anomalous parts of the gap function are discussed. Methods of minimizing the uncertainties associated with the pseudogap-related pole in the self-energy are discussed. From these the evolution of the superconducting gap and the momentum-dependent electron spectral function across the phase diagram are determined. In the pseudogap regime, decreasing the temperature into the superconducting state leads to a decrease in the energy gap and the formation of a “peak-dip-hump” structure in the electronic density of states. The peak feature disperses very weakly. The calculated spectral functions are in good qualitative agreement with published data. The mathematical origin of the behavior is found to be the effect of the superconductivity on the pole structure giving rise to the normal state pseudogap. In particular the “hump” feature is found to arise from a zero crossing of the real part of the electron self-energy rather than from an onset of scattering. The effect of superconductivity on the zone diagonal spectra is presented.

We successfully synthesized the zinc-verdazyl complex $\left[{\text{Zn(hfac)}}_2\right]·(o-\mathrm{Py}-V)$ [hfac = 1,1,1,5,5,5-hexafluoroacetylacetonate; $o$-Py-V = 3-(2-pyridyl)-1,5-diphenylverdazyl], which is an ideal model compound with an $S=\frac{1}{2}$ ferromagnetic-antiferromagnetic alternating Heisenberg chain (F-AF AHC). Ab initio molecular-orbital (MO) calculations indicate that two dominant interactions $J_{\mathrm{F}}$ and $J_{\mathrm{AF}}$ form the $S=\frac{1}{2}$ F-AF AHC in this compound. The magnetic susceptibility and magnetic specific heat of the compound exhibit thermally activated behavior below approximately 1 K. Furthermore, its magnetization curve is observed up to the saturation field and directly indicates a zero-field excitation gap of 0.5 T. These experimental results provide evidence for the existence of a Haldane gap. We successfully explain the results in terms of the $S=\frac{1}{2}$ F-AF AHC through quantum Monte Carlo calculations with $|J_{\mathrm{AF}}/J_{\mathrm{F}}|=0.22$. The ab initio MO calculations also indicate a weak AF interchain interaction $J^{\prime }$ and that the coupled F-AF AHCs form a honeycomb lattice. The $J^{\prime }$ dependence of the Haldane gap is calculated, and the actual value of $J^{\prime }$ is determined to be less than $0.01|J_{\mathrm{F}}|$.

In addition to the absorption due to known optical phonons, we found a temperature-dependent absorption band at 2 THz in garnet-type $\mathrm{H}{\mathrm{o}}_3\mathrm{G}{\mathrm{a}}_5{\mathrm{O}}_{12}$, at temperatures in the range of 450–540 K. The optical and electrical properties reveal that the absorption band at 2 THz is not produced by mechanisms related to soft phonons, rattling phonons, impurities, or charge density waves, but rather to polaron conduction. Our analysis of the scattering rates and optical mobility shows that electron transport via the intermediate polaron is significant in this absorption band. Electrical measurements also support the existence of polarons in $\mathrm{H}{\mathrm{o}}_3\mathrm{G}{\mathrm{a}}_5{\mathrm{O}}_{12}$. This paper facilitates a method to investigate charge carrier transport from an optical point of view using terahertz time-domain spectroscopy, which we demonstrate using $\mathrm{H}{\mathrm{o}}_3\mathrm{G}{\mathrm{a}}_5{\mathrm{O}}_{12}$.

Quantum entanglement plays a ubiquitous role in theoretical physics, from the characterization of novel phases of matter to understanding the efficacy of numerical algorithms. As such, there have been extensive studies on the entanglement spectrum (ES) of free-fermion systems, particularly in the relation between its spectral flow and topological charge pumping. However, far less has been studied about the spacing between adjacent entanglement eigenenergies, which affects the truncation error in numerical computations involving matrix product states or projected entangled pair states. In this paper, we shall hence derive asymptotic bounds for the ES spacings through an interpolation argument that utilizes known results on Wannier function decay. For translationally invariant systems, the entanglement energies are shown to decay at a rate monotonically related to the complex gap between the filled and occupied bands. This interpolation also demonstrates the one-to-one correspondence between the ES and the edge states. Our results also provide asymptotic bounds for the eigenvalue distribution of certain types of block Toeplitz matrices common in physics, even for those not arising from entanglement calculations.

We calculate the energy spectrum of a Dirac double layer, where each layer has the Dirac electronic dispersion, in the presence of a tilted magnetic field and small interlayer tunneling. We show that the energy splitting between the Landau levels has an oscillatory dependence on the in-plane magnetic field and vanishes at a series of special tilt angles of the magnetic field. Using a semiclassical analysis, we show that these special tilt angles are determined by the Berry phase of the Dirac Hamiltonian. The interlayer tunneling conductance also exhibits an oscillatory dependence on the magnetic field tilt angle, known as the angular magnetoresistance oscillations (AMRO). Our results are applicable to graphene double layers and thin films of topological insulators.

We describe and analyze leakage errors of singlet-triplet qubits. Even though leakage errors are a natural problem for spin qubits encoded using quantum dot arrays, they have obtained little attention in previous studies. We describe the realization of leakage correction protocols that can be implemented together with the quantum error correction protocol of the surface code. Furthermore we construct explicit leakage reduction units that need, in the ideal setup, as few as three manipulation steps. Our study shows that leakage errors can be corrected without the need of measurements and at the cost of only a few additional ancilla qubits and gate operations compared to standard quantum error correction codes.

Silicon nitride is a bulk and a coating material exhibiting excellent mechanical properties. We present a transferable reduced tight-binding (TB) model for the silicon nitride system, developed within the framework of coarse graining the electronic structure from density-functional theory (DFT) to tight binding (TB) to bond-order potentials (BOPs). The TB bond integrals are obtained directly from mixed-basis DFT projections of wave functions onto a minimal basis of atom-centered orbitals. This approach reduces the number of overall parameters to be fitted. Furthermore, applying the reduced TB approximation automatically leads to a single $\sigma$ bond order that contributes to the bond energy. DFT binding energies of ground state and metastable crystal structures are used as the benchmark to which the TB repulsive parameters are fitted. The quality of the TB models is demonstrated by comparing their predictions for the binding energies, heats of formation, elastic constants, and defect energies with DFT and experimental values.

The variational stochastic self-consistent harmonic approximation is combined with the calculation of third-order anharmonic coefficients within density functional perturbation theory and the $2n+1$ theorem to calculate anharmonic properties of crystals. It is demonstrated that in the perturbative limit, the combination of these two methods yields the perturbative phonon linewidth and frequency shift in a very efficient way, avoiding the explicit calculation of fourth-order anharmonic coefficients. Moreover, it also allows calculating phonon lifetimes and inelastic neutron-scattering spectra in solids where the harmonic approximation breaks down and a nonperturbative approach is required to deal with anharmonicity. To validate our approach, we calculate the anharmonic phonon linewidth in the strongly anharmonic palladium hydrides. We show that due to the large anharmonicity of hydrogen optical modes, the inelastic neutron-scattering spectra are not characterized by a Lorentzian line shape, but by a complex structure including satellite peaks.

We show that the interplay of strong Hubbard interaction $U$ and spin-orbit coupling $\lambda$ in systems with $d^4$ electronic configuration leads to several unusual magnetic phases. Most notably, we find that competition between superexchange and spin-orbit coupling leads to a phase transition from a nonmagnetic state predicted by atomic physics to a novel magnetic state in the large-$U$ limit. We show that the local moment changes dramatically across this phase transition, challenging the conventional wisdom that local moments are robust against small perturbations in a Mott insulator. The Hund's coupling plays an important role in determining the nature of the magnetism. We identify candidate materials and present predictions for resonant x-ray scattering signatures of the unusual magnetism in $d^4$ Mott insulators.

We consider bipartite $\mathrm{SU}\left(N\right)$ spin Hamiltonians with a fundamental representation on one sublattice and a conjugate to fundamental on the other sublattice. By mapping these antiferromagnets to certain classical loop models in one higher dimension, we provide a practical strategy to write down a large family of $\mathrm{SU}\left(N\right)$ symmetric spin Hamiltonians that satisfy Marshall's sign condition. This family includes all previously known sign-free $\mathrm{SU}\left(N\right)$ spin models in this representation and in addition provides a large set of new models that are Marshall positive and can hence be studied efficiently with quantum Monte Carlo methods. As an application of our idea to the square lattice, we show that in addition to Sandvik's $Q$ term, there is an independent nontrivial four-spin $R$ term that is sign free. Using numerical simulations, we show how the $R$ term provides a new route to the study of quantum criticality of Néel order.

A Landau-theoretical approach is utilized to model the magnetic field induced reversal of the antiferromagnetic order parameter in thin films of magnetoelectric antiferromagnets. A key ingredient of this peculiar switching phenomenon is the presence of a robust spin polarized state at the surface of the antiferromagnetic films. Surface or boundary magnetization is symmetry allowed in magnetoelectric antiferromagnets and experimentally established for chromia thin films. It couples rigidly to the antiferromagnetic order parameter and its Zeeman energy creates a pathway to switch the antiferromagnet via magnetic field application. In the framework of a minimalist Landau free energy expansion, the temperature dependence of the switching field and the field dependence of the transition width are derived. Least-squares fits to magnetometry data of $\left(0001\right)$ textured chromia thin films strongly support this model of the magnetic reversal mechanism.

We have studied the magnetic ordering in ${\mathrm{Ba}}_{1-x}{\mathrm{Na}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ with $0.25\le x\le 0.4$ by unpolarized and polarized neutron diffraction using single crystals. Unlike most FeAs-based compounds that magnetically order, Na-doped ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ exhibits two successive magnetic transitions: For $x=0.35$, upon cooling, magnetic order occurs at $\sim 70$ K with in-plane magnetic moments being arranged as in pure or Ni-, Co-, or K-doped ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ samples. At a temperature of $\sim 46$ K a second phase transition occurs, which the single-crystal neutron-diffraction experiments can unambiguously identify as a spin reorientation. At low temperatures, the ordered magnetic moments in ${\mathrm{Ba}}_{0.65}{\mathrm{Na}}_{0.35}{\mathrm{Fe}}_2{\mathrm{As}}_2$ point along the $c$ direction. The two nearly degenerate magnetic states document orbital degeneracy to persist in the superconducting phase.

Inelastic neutron scattering (INS) and specific heat measurements have been performed on the intermediate valence compound ${\mathrm{CeRuSi}}_3$, which is isostructural to the noncentrosymmetric pressure-induced superconductors ${\mathrm{CeRhSi}}_3,{\mathrm{CeIrSi}}_3$, and ${\mathrm{CeCoGe}}_3$. INS measurements at 7 K reveal a broad peak at $(58.5 \pm 1.4)$ meV, while at 300 K, broad quasielastic scattering is observed. This indicates a large Kondo temperature of $T_{\mathrm{K}}\sim 680$ K. The magnetic contribution to the specific heat $\left(C_{\mathrm{mag}}\right)$ has a value of $\gamma =62.5\left(1\right)\mathrm{mJ}/\mathrm{mol}{\mathrm{K}}^2$ at low temperatures and above about 100 K can be well accounted for by the Coqblin-Schrieffer model with a characteristic temperature of $T_0=680$ K, which is further evidence that ${\mathrm{CeRuSi}}_3$ is in the intermediate valence regime.

We present a femtosecond optical pump-probe study of the multiferroic manganite ${\text{Eu}}_{0.75}{\text{Y}}_{0.25}{\text{MnO}}_3$. The optical response of the material at pump energies of 1.55 and 3.1 eV is dominated by the $d\text{−}d$ and $p\text{−}d$ transitions of the ${\mathrm{Mn}}^{3+}$ ions. The relaxation of photoexcited electrons includes the relaxation of the Jahn-Teller distortion and polaron trapping at ${\mathrm{Mn}}^{2+}$ and ${\mathrm{Mn}}^{4+}$ sites. Ultrafast switching of superexchange interactions due to modulated $e_g$ orbital occupancy creates a localized spin excitation, which then decays on a time scale of tens of picoseconds at low temperatures. The localized spin state decay appears as a tremendous increase in the amplitude of the photoinduced reflectance, due to the strong coupling of optical transitions to the spin-spin correlations in the crystalline $a\text{−}b$ plane.

We present a theory of Galilean-invariant conventional and chiral $p_x\pm i{p}_y$ fermionic superfluids at zero temperature in two spatial dimensions in terms of a dual gauge theory. Our formulation is general coordinate invariant. The parity-violating effects are encoded in the Wen-Zee term that gives rise to the Hall viscosity and edge current. We show that the relativistic superfluid with the Euler current reduces to the chiral superfluid in the limit $c\to \infty$. Using Newton-Cartan geometry, we construct the covariant formulation of the effective theory and calculate the energy current.

We studied the quantum critical behavior of the second antiferromagnetic (AF) phase in the heavily electron-doped high-$T_c$ pnictide, ${\mathrm{LaFeAsO}}_{1-x}{\mathrm{H}}_x$, by using a ${}^{75}\mathrm{As}$ and ${}^1\mathrm{H}$ nuclear magnetic resonance (NMR) technique. In the second AF phase, we observed a spatially modulated spin-density-wave-like state up to $x=0.6$ from the NMR spectral line shape and detected a low-energy excitation gap from the nuclear relaxation time $T_1$ of ${}^{75}\mathrm{As}$. The excitation gap closes at the AF quantum critical point (QCP) at $x\approx 0.49$. The superconducting phase in a lower-doping regime contacts the second AF phase only at the AF QCP, and both phases are segregated from each other. The absence of AF critical fluctuations and the enhancement of the in-plane electric anisotropy are key factors for the development of superconductivity.