We have studied the appearance of vortex arrays in a rotating ${}^4\mathrm{He}$ nanodroplet at zero temperature within density functional theory. Our results are compared with those for classical rotating fluid drops used to analyze the shape and vorticity in recent experiments [L. F. Gomez et al., Science 345, 906 (2014)], where vortices have been directly seen in superfluid droplets for the first time. In agreement with the experiments, we have found that the shape of the droplet changes from pseudospheroid, oblatelike for a small number of vortices to a peculiar “wheel-like” shape, delimited by nearly flat upper and lower surfaces, when the number of vortices is large. Also in agreement with the experiments, we have found that the droplet remains stable well above the stability limit predicted by classical theories for axially symmetric shapes.

Muon spin relaxation and rotation $\left(\mu \mathrm{SR}\right)$ measurements have been performed to study the superconducting and magnetic properties of ${\mathrm{Nb}}_2{\mathrm{PdS}}_5$. Zero-field $\mu \mathrm{SR}$ data show that no sizable spontaneous magnetization arises with the onset of superconductivity in ${\mathrm{Nb}}_2{\mathrm{PdS}}_5$, which indicates that the time-reversal symmetry is probably preserved in the superconducting state of this system. A strong diamagnetic shift is observed in the transverse-field $\mu \mathrm{SR}$ data practically ruling out a dominant triplet-pairing superconducting state in ${\mathrm{Nb}}_2{\mathrm{PdS}}_5$. The temperature dependence of magnetic penetration depth evidences the existence of a single $s$-wave energy gap $\Delta \left(0\right)$ with a gap value of 1.07(4) meV at zero temperature. The ratio $\Delta \left(0\right)/k_{\mathrm{B}}{T}_{\mathrm{c}}=2.02\left(9\right)$ indicates that ${\mathrm{Nb}}_2{\mathrm{PdS}}_5$ should be considered as a moderately strong-coupling superconductor. The magnetic penetration depth at zero temperature is 785(20) nm, indicating a very low superfluid density consistent with the quasi-one-dimensional nature of this system.

We develop an analytical wave approach to describe the physics properties of multiresonant metamaterials for Lamb waves propagating in plates. The metamaterial that we characterize consists of a 10 by 10 uniform, periodic array of long rods attached to the surface of the plate that forms the substrate in which antisymmetric $A_0$ Lamb waves are excited. We show that the $A_0$ Lamb wave propagation through the metamaterial can be accurately modeled using a simplified theory that replaces the two-dimensional array with a one-dimensional beam with a linear array of 10 rods. The wave propagation problem is solved rigorously for this one-dimensional system using the scattering matrix for a single rod. The exact eigenvalues of the system are approximated in a long wavelength expansion to determine a simple expression for the effective wave number and dispersion of the metamaterial. The modeled dispersion is compared with an experimental measurement of the dispersion inside the metamaterial with excellent agreement. The multiresonant rods, restricted to longitudinal vibration consistent with $A_0$ Lamb waves excited in the plate, produce two wide stop bands in the frequency domain from 0 to 10 kHz where the stop or passband boundaries align with the minima and maxima of the rod's impedance. We show that a negative effective density is obtained in the stop band. With the simple yet highly accurate relations given in this paper we have a tool to develop more complex metamaterials with rods and plates of different properties.

We present an experimental and theoretical study of magnonic band gaps in planar one-dimensional magnonic crystals of two types, with a periodic array of metallic stripes or a periodic array of grooves on an yttrium iron garnet film. Propagating magnetostatic surface spin waves are excited in the considered periodic magnetic structures and measured by microstripe transducers with a vector network analyzer and with a Brillouin light scattering spectroscopy. The properties of magnonic band gaps are explained and illustrated by finite element method calculations. We demonstrate a major influence of the nonreciprocal spin-wave dispersion properties induced by metallic stripes on the width of the magnonic band gap and its dependence on the external magnetic field. The indirect character of the band gap and the higher group velocity of spin waves in the metallic magnonic crystal are identified as the main causes of the wider band gap and steeper decrease in its width with increasing magnetic field in this structure as compared to the grooved magnonic crystal. Potential applications of both types of magnonic crystals and prospects for their miniaturization are discussed as well.

In turbulent superfluid He II, the quantized vortex lines interact via the classical Biot-Savart law to form a complicated vortex tangle. We show that vortex tangles with the same vortex line density will have different energy spectra, depending on the normal fluid which feeds energy into the superfluid component, and identify the spectral signature of two forms of superfluid turbulence: Kolmogorov tangles and Vinen tangles. By decomposing the superfluid velocity field into local and nonlocal contributions, we find that in Vinen tangles the motion of vortex lines depends mainly on the local curvature, whereas in Kolmogorov tangles the long-range vortex interaction is dominant and leads to the formation of clustering of lines, in analogy to the “worms” of ordinary turbulence.

We discuss the nonlinear current of an interacting quantum dot coupled to normal and superconducting reservoirs with applied voltage and temperature differences. Due to the particle-hole symmetry introduced by the superconducting lead, the pure (subgap) thermoelectric response vanishes. However, we show that the Andreev bound states shift as the thermal gradient increases. As a consequence, the $I\text{−}V$ characteristic can be tuned with a temperature bias if the system is simultaneously voltage biased. This is a cross effect that occurs beyond linear response only. Furthermore, we emphasize the role of quasiparticle tunneling processes in the generation of high thermopower sensitivities.

Temperature-dependent magnetization, specific-heat, and electrical-resistivity measurements were performed on single crystals of ${\mathrm{PrNi}}_2{\mathrm{Cd}}_{20}$ and ${\mathrm{PrPd}}_2{\mathrm{Cd}}_{20}$. Neither compound shows any evidence for magnetic order above 2 K. Magnetization measurements suggest that Pr ions assume a nonmagnetic ${\Gamma }_1$ singlet or non-Kramers ${\Gamma }_3$ doublet ground state. A broad peak, which is identified as a Schottky anomaly, is observed in the specific heat at low temperature. Low-lying excitations involving the $4f$ electrons persist down to 2 K for both ${\mathrm{PrNi}}_2{\mathrm{Cd}}_{20}$ and ${\mathrm{PrPd}}_2{\mathrm{Cd}}_{20}$ and related features are also observed in the magnetization and electrical resistivity.

We study thermoelectric transport in ultrathin topological insulators under the application of circularly polarized off-resonant light of frequency $\Omega$ and amplitude $A$. We derive analytical expressions for the band structure, orbital magnetization $M_{\text{orb}}$, and the thermal (${\kappa }_{xy}$) and Nernst (${\alpha }_{xy}$) conductivities. Reversing the light polarization from right to left leads to an exchange of the conduction and valence bands of the symmetric and antisymmetric surface states and to a sign change in $M_{\text{orb}},{\alpha }_{xy}$, and ${\kappa }_{xy}$. Varying the sample thickness or $A/\Omega$ leads to a strong enhancement of $M_{\text{orb}}$ and ${\alpha }_{xy}$. These effects, accessible to experiments, open the possibility for selective, state-exchanged excitations under light and the conversion of heat to electric energy.

We investigated the scattering dynamics of hot electrons at the atomically well-defined In-rich $(2\times 4)$-reconstructed InP(100) surface in the presence of different surface states. Using energy- and time-resolved femtosecond two-photon photoemission spectroscopy, we determined the electron transfer between three-dimensional bulk states and the two-dimensional surface state C2, located high above the conduction band minimum (CBM) avoiding energetic overlap with relaxed bulk electrons. At excitation energies, where C2 was populated through hot bulk electrons, relaxing towards the CBM, we found evidence that the energy loss rate of the photoexcited electron distribution is mostly determined by inelastic electron-phonon scattering between bulk states. This was confirmed by measurements repeated after quenching the surface states with oxygen. For resonant photoexcitation, on the other hand, we measured a time constant of ${\tau }_{\mathrm{fast}}=20\mathrm{fs}$ for the depopulation of C2, showing that electron-phonon scattering between bulk and surface states might have direct consequence on ultrafast relaxation dynamics for materials with a high surface-to-bulk ratio.

We theoretically investigate the trion-polariton and the effects of a two-dimensional electron gas on its single-particle properties. Focusing on the trion and exciton transitions, we set up an effective model and calculate the optical absorption of the quantum well containing the two-dimensional electron gas (2DEG). Including the light-matter coupling, we compute the Rabi splitting and polariton line shapes as a function of 2DEG density. The role of finite temperature is investigated. The spatial extent of the trion-polariton is also calculated. We find a substantial charge buildup at short distances as long as the Rabi frequency does not exceed the trion binding energy. All our calculations take into account the Fermi edge singularity and the Anderson orthogonality catastrophe.

We depict a family of one-dimensional systems where one can create and detect fractional charges. These charges are produced via interacting one-dimensional conductors connected to noninteracting leads. At certain electron densities, some distinct modes develop an energy gap due to electron-electron interactions. This gap allows for tunneling events inside the conductor, which generate electric current noise. The resulting fractional noise's Fano factor depends only on the identification of the gapped mode, and is insensitive to additional interactions in the conductor. These effects can be realized in either quantum wires or edges of quantum Hall systems.

The development of numerical methods capable of simulating realistic materials with strongly correlated electrons, with controllable errors, is a central challenge in quantum many-body physics. Here we describe a framework for a general multiscale method based on embedding a self-energy of a strongly correlated subsystem into a self-energy generated by a method able to treat large weakly correlated systems approximately. As an example, we present the embedding of an exact diagonalization self-energy into a self-energy generated from self-consistent second-order perturbation theory. Using a quantum impurity model, generated from a cluster dynamical mean field approximation to the two-dimensional Hubbard model, as a benchmark, we illustrate that our method allows us to obtain accurate results at a fraction of the cost of typical Monte Carlo calculations. We test the method in multiple regimes of interaction strengths and dopings of the model. The general embedding framework we present avoids difficulties such as double counting corrections, frequency-dependent interactions, or vertex functions. As it is solely formulated at the level of the single-particle Green's function, it provides a promising route for the simulation of realistic materials that are currently difficult to study with other methods.

Magnetite represents a prototype compound with a mixed valence of iron cations. Its structure and electron ordering below the Verwey transition have been studied for decades. A recently published precise crystallographic structure [Senn et al., Nature (London) 481, 173 (2012)] accompanied by a suggestion of a “trimeron” model has given a new impulse to magnetite research. Here we investigate hyperfine field anisotropy in the $Cc$ phase of magnetite by quantitative reanalysis of published measurements of the dependences of the ${}^{57}\mathrm{Fe}$ nuclear magnetic resonance frequencies on the external magnetic field direction. Further, ab initio density-functional-theory-based calculations of hyperfine field depending on the magnetization direction using the recently reported crystal structure are carried out, and analogous hyperfine anisotropy data linked to particular crystallographic sites are determined. These two sets of data are compared, and mutually matching groups of the iron B sites in the 8:5:3 ratio are found. Moreover, information on electronic structure is obtained from the ab initio calculations. Our results are compared with the trimeron model and with an alternative analysis [Patterson, Phys. Rev. B 90, 075134 (2014)] as well.

Self-consistent solutions of the Dyson equation are obtained using a plane-wave basis set for seven small molecules. Such self-consistent solutions can help to unify the different $GW$ self-consistent schemes, reduce the scatter of results in current $GW$ calculations, and shed light on the true effects of $GW$ self-consistency. Unlike other works of self-consistent $GW$ calculations, in the present work the Green's function is expressed as a matrix under the plane-wave basis set. The algorithmic details which enable such calculations are presented. The ability to solve the full Green's function using a plane-wave basis set may open the door for future beyond-$GW$ many-body perturbation theory calculations.

One of the key factors that determine the fates of quantum many-body systems in the zero temperature limit is the competition between kinetic energy that delocalizes particles in space and interaction that promotes localization. While one dominates over the other in conventional metals and insulators, exotic states can arise at quantum critical points where none of them clearly wins. Here we present a class of metallic states that emerge at an antiferromagnetic (AF) quantum critical point in the presence of one-dimensional Fermi surfaces embedded in space dimensions three and below. At the critical point, interactions between particles are screened to zero in the low-energy limit and at the same time the kinetic energy is suppressed in certain spatial directions to the leading order in a perturbative expansion that becomes asymptotically exact in three dimensions. The resulting dispersionless and interactionless state exhibits distinct quasilocal strange metallic behaviors due to a subtle dynamical balance between screening and infrared singularity caused by spontaneous reduction of effective dimensionality. The strange metal, which is stable near three dimensions, shows enhanced fluctuations of bond density waves, $d$-wave pairing, and pair density waves.

Graphene plasmonics is a rapidly growing field with multiple potential applications. One of the standard ways to study plasmons in graphene is by fabricating an array of graphene nanoribbons where nanoribbon edges provide the efficient photon-plasmon coupling. We systematically analyze the problem of optical plasmonic response in such systems and demonstrate the purely geometric nature of the size quantization condition for graphene plasmons. Accurate numerical calculations allowed us to tabulate the universal geometric parameters of plasmon size quantization, which is expected to become useful in analysis of experimental data on plasmonic response of graphene nanoribbons. A simple analytical theory has also been developed, which accurately reproduces all the qualitative features of optical plasmonic response of graphene nanoribbons.

We report a theoretical study of the structural phase transition and associated metal-to-insulator transition in $\mathrm{Nb}{\mathrm{O}}_2$ within density functional theory using the transition state theory as formulated in the generalized solid-state nudged elastic band method. Consistent with prior experimental work, we demonstrate that niobium dimerization is primarily driven by soft modes at the P point of the rutile Brillouin zone. In light of our results, it appears that the metal-to-insulator transition in $\mathrm{Nb}{\mathrm{O}}_2$ is driven by a second-order structural transition of the Peierls type.

We performed point-contact spectroscopy tunneling measurements on ${\mathrm{Cu}}_x\mathrm{TiSe}{}_2$ bulk with $x=0.02$ and $0.06$ at temperatures ranging from $T=4-40$ K and observe a suppression in the density of states around zero bias that we attribute to enhanced Coulomb interactions due to disorder. We find that the correlation gap associated with this suppression is related to the zero-temperature resistivity. We use our results to estimate the disorder-free transition temperature and find that the clean limit $T_{c0}$ is close to the experimentally observed $T_c$ at optimal doping.

We use a first-principles based self-consistent momentum-resolved density fluctuation (MRDF) model to compute the combined effects of electron-electron and electron-phonon interactions to describe the superconducting dome in the correlated ${\mathrm{MoS}}_2$ thin flake and ${\mathrm{TiSe}}_2$. We find that without including the electron-electron interaction, the electron-phonon coupling and the superconducting transition temperature ($T_c$) are overestimated in both these materials. However, once the full angular and dynamical fluctuations of the spin and charge density induced quasiparticle self-energy effects are included, the electron-phonon coupling and $T_c$ are reduced to the experimental value. With doping, both electronic correlation and electron-phonon coupling grows, and above some doping value, the former becomes so large that it starts to reduce the quasiparticle-phonon coupling constant and $T_c$, creating a superconducting dome, in agreement with experiments.