We describe the dynamics of a single fermion in a dispersionless band coupled to the 2+1 dimensional conformal field theory (CFT) describing the quantum phase transition of a bosonic order parameter with $N$ components. The fermionic spectral functions are expected to apply to the vicinity of quantum critical points in two-dimensional metals over an intermediate temperature regime where the Landau damping of the order parameter can be neglected. Some of our results are obtained by a mapping to an auxiliary problem of a CFT containing a defect line with an external field which locally breaks the global O$\left(N\right)$ symmetry.

Detecting an itinerant microwave photon with high efficiency is an outstanding problem in microwave photonics and its applications. We present a scheme to detect an itinerant microwave photon in a transmission line via the nonlinearity provided by a transmon in a driven microwave resonator. With a single transmon we achieve 84% distinguishability between zero and one microwave photons and 90% distinguishability with two cascaded transmons by performing continuous measurements on the output field of the resonator. We also show how the measurement diminishes coherence in the photon number basis thereby illustrating a fundamental principle of quantum measurement: The decoherence rate increases as the detector is made more effective.

We report an observation of a highly anisotropic Dirac-cone structure, and its unconventional electronic behaviors, in high quality ${\mathrm{SrMnBi}}_2$ crystal by angle-resolved photoemission spectroscopy measurement. We observed a well-defined sharp quasiparticle peak in a wide energy range, linearly dispersive, forming a highly anisotropic holelike Dirac cone $(v_{F_1}/v_{F_2}\sim 5–6)$. The density of state remains linear, and the scattering rate of the quasiparticle increases linearly as a function of binding energy up to higher binding energy. The results contrast with the expectation for a two-dimensional (2D) or three-dimensional (3D) Fermi-liquid theory and rather suggest the existence of a two-dimensional electron gas system in this 3D material, making ${\mathrm{SrMnBi}}_2$ a potentially useful 2D-electron-gas-encapsulated material.

Recently, a method based on stochastic integration on the surfaces of steepest descent of the action was introduced to tackle the sign problem in quantum-field theories. We show how this method can be used in many-body theories to perform fully nonperturbative calculations of quantum corrections about mean-field solutions. We discuss an explicit algorithm for implementing our method, and present results for the repulsive Hubbard model away from half-filling at intermediate temperatures. Our results are consistent with those from other state-of-the-art calculations.

We demonstrate adiabatic rapid passage on a subpicosecond time scale in a single semiconductor quantum dot, enabling the exploration of a regime of strong (and rapidly varying) Rabi energies for optical control of excitons. An observed dependence of the exciton inversion efficiency on the sign of the pulse chirp demonstrates the dominance of phonon-mediated dephasing, which is suppressed for positive chirp at low temperature. Our findings will support the realization of dynamical decoupling strategies and suggest that multiphonon emission and/or non-Markovian effects should be taken into account.

The self-consistent rate theory for surface growth in the submonolayer regime is generalized from mono- to multicomponent systems, which are formed by codeposition of different types of atoms or molecules. The theory requires the introduction of pair density distributions to enable a symmetric treatment of reactions among different species. The approach is explicitly developed for binary systems and tested against kinetic Monte Carlo simulations. Using a reduced set of rate equations, only a few differential equations need to be solved to obtain good quantitative predictions for island and adatom densities, as well as densities of unstable clusters.

The surface potential of the herringbone reconstruction on Au(111) is known to guide surface-state electrons along the potential channels. Surprisingly, we find by scanning tunneling spectroscopy that hot electrons with kinetic energies twenty times larger than the potential amplitude (38 meV) are still guided. The efficiency even increases with kinetic energy, which is reproduced by a tight-binding calculation taking the known reconstruction potential and strain into account. The guiding is explained by diffraction at the inhomogeneous electrostatic potential and strain distribution provided by the reconstruction.

We propose a third-order time-dependent perturbation theory approach to describe the chemical surface-enhanced Raman spectroscopy of molecules interacting with two-dimensional (2D) surfaces such as an ideal 2D metal and graphene, which are both 2D metallic monolayers. A detailed analysis is performed for all the possible scattering processes involving both electrons and holes and considering the different time orderings for the electron-photon and electron-phonon interactions. We show that for ideal 2D metals a surface enhancement of the Raman scattering is possible if the Fermi energy of the surface is near the energy of either the HOMO or the LUMO states of the molecule and that a maximum enhancement is obtained when the Fermi energy matches the energy of either the HOMO or the LUMO energies plus or minus the phonon energy. The graphene-enhanced Raman spectroscopy effect is then explained as a particular case of a 2D surface, on which the density of electronic states is not constant, but increases linearly with the energy measured from the charge neutrality point. In the case of graphene, the Raman enhancement can occur for any value of the Fermi energy between the HOMO and LUMO states of the molecule. The proposed model allows for a formal approach for calculating the Raman intensity of molecules interacting with different 2D materials.

The local electronic state at every crystallographically independent molecular site was separately investigated in the organic superconductor $\alpha -\left(\text{BEDT-TTF}{)}_2{\mathrm{NH}}_4\mathrm{Hg}\right(\mathrm{SCN}{)}_4$ using site-selective ${}^{13}\mathrm{C}$-NMR spectroscopy. The density of states (DOS) at three molecular sites shows independent temperature variation, which is caused by the long-range Coulomb interaction. Comparing the present results with those for a nonsuperconducting counterpart $\alpha -\left(\text{BEDT-TTF}{)}_2\mathrm{Rb}\mathrm{Hg}\right(\mathrm{SCN}{)}_4$, we found that the uniform DOS at two of three independent molecular sites is crucial to induce superconductivity. Taking the site-dependent DOS as a parameter, we constructed a phase diagram for a superconductor with long-range Coulomb interaction, an $\alpha$-type BEDT-TTF family, and summarized the competition between charge instability and superconductivity.

We carried out a high-resolution x-ray diffraction experiment on ${\mathrm{Ce}}_{0.7}{\mathrm{La}}_{0.3}{\mathrm{B}}_6$ that exhibits staggered order of $4f$ octupole moments below $T_O=1.4$ K. Theoretically, ferroquadrupole moments that accompany the antiferro-octupole order and a resultant rhombohedral deformation of the cubic lattice were predicted. In contrast, experimentally, no direct evidence of the rhombohedral lattice has been obtained. We observe the splitting of Bragg peaks below $T_O$ and find that the unit cell is a rhombohedron being elongated along the [111] axis. The response of rhombohedral domains to magnetic fields also well agrees with theoretical calculations. A particular outcome of this experiment is that the magnitude of the induced quadrupole moments is precisely evaluated from the obtained shear strain. Using this result, the magnitude of the hidden octupole moments is also discussed.

The dc Josephson current across a multichannel superconductor–normal-superconductor (SNS) junction is computed by summing contributions from subgap Andreev bound states, as well as from continuum states propagating within the superconducting leads. We show that, in a long multichannel SNS junction, at low temperatures, all these contributions add up, so that the current can be entirely expressed in terms of single-particle normal and Andreev reflection amplitudes at the Fermi level at both SN interfaces. Our derivation applies to a generic number of channels in the normal region and/or in the superconducting leads, without assumptions about scattering processes at the SN interfaces: if the channels within the central region have the same dispersion relation, it leads to simple analytical formulas for the current at low temperatures; if the channels within the central region have different dispersion relations, it allows for expressing the current in terms of a simple integral involving only scattering amplitudes at the Fermi level. Our result motivates using a low-energy effective boundary Hamiltonian formalism for computing the current, which is crucial for treating Luttinger liquid interaction effects.

Using density functional theory with added on-site interactions, we study the electronic structure of bulk, monolayer, and bilayer of the layered transition-metal dichalcogenide $1T\text{−}\mathrm{TaS}{}_2$. We show that a two-dimensional spin-$\frac{1}{2}$ Mott phase exists for the monolayer in the charge density wave (CDW) state and that such a phase is systematically destroyed by packing of the distorted layers leading to a one-dimensional metal for bulk, CDW-distorted $\mathrm{TaS}{}_2$. The latter finding is in contrast with previous dynamical mean-field theory predictions—we explain the disagreement by the weak effective interaction felt by the electrons in the CDW state. Experimental observations of insulating behavior may arise from disorder due to stacking faults.

We have measured both the ordinary and extraordinary refractive index of $m$-plane cuts of wurtzite ZnO, GaN, and AlN single crystals at room temperature and as a function of hydrostatic pressure up to 8 GPa. For that purpose we have developed an alternative optical interference method, called bisected-beam method, which leads, in general, to high contrast interference fringes. Its main feature, however, is to be particularly suitable for high pressure experiments with the diamond anvil cell, when the refractive index of the sample is low and similar to that of diamond and/or the pressure transmitting medium, as is the case here. For all three wide-gap materials we observe a monotonous decrease of the ordinary and extraordinary refractive indices with increasing pressure, being most pronounced for GaN, less marked for ZnO, and the smallest for AlN. The frequency dependence of the refractive indices was extrapolated to zero energy using a critical-point-plus-Lorentz-oscillator model of the ordinary and extraordinary dielectric function. In this way, we determined the variation with pressure of the electronic part (no-phonon contribution) of the static dielectric constant ${\varepsilon }_{\infty }$. Its volume derivative, $r=dln{\varepsilon }_{\infty }/dlnV$, serves as single scaling coefficient for comparison with experimental and/or theoretical results for other semiconductors, regarding the pressure effects on the dielectric properties. We have obtained an ordinary/extraordinary average value $\overline{r}$ of 0.49(15) for ZnO, 1.22(9) for GaN, and 0.32(4) for AlN. With the values for the ordinary and extraordinary case being within experimental uncertainty, there is thus no apparent change in dielectric anisotropy under pressure for these wurtzite semiconductors. Results are discussed in terms of the pressure-dependent electronic band structure of the materials.

First-principles calculations within the framework of the density functional theory are used to construct realistic models for the surface of glassy $\mathrm{GeS}{}_2\left(g\text{−}\mathrm{GeS}{}_2\right)$. Both calculations at $T=0$ K and at finite temperature ($T=300$ K) are considered. This allows for a comparison between the structural and electronic properties of surface and bulk $g\text{−}\mathrm{GeS}{}_2$. Although the $g\text{−}\mathrm{GeS}{}_2$ surface recovers the main tetrahedral structural motif of bulk $g\text{−}\mathrm{GeS}{}_2$, the number of fourfold coordinated Ge atoms and twofold coordinated S atoms is smaller than in the bulk. On the contrary, the surface system features a larger content of overcoordinated S atoms and threefold coordinated Ge atoms. This effect is more important for the $g\text{−}\mathrm{GeS}{}_2$ surface relaxed at 0 K. Maximally localized Wannier functions (WF) are used to inspect the nature of the chemical bonds of the structural units present at the $g\text{−}\mathrm{GeS}{}_2$ surface. We compare the ability of several charge derivation methods to capture the atomic charge variations induced by a coordination change. Our estimate for the charges allows exploiting the first-principles results as a data base to construct a reliable interatomic force field.

We report the results of an experimental study of the dc and ac magnetic properties of superconducting Nb thin-walled cylinders in parallel to the axis magnetic fields. The magnetization curves at various temperatures are measured. Surprisingly, at 4.5 K, for magnetic fields much lower than $H_{c1}$, avalanchelike jumps of the magnetization are observed. The position of the jumps is not reproducible and changes from one experiment to another, resembling vortex lattice instabilities usually observed for magnetic fields larger than $H_{c1}$. At temperatures larger than 6.5 K, the measured magnetization curves become smooth. ac response is measured in constant and swept dc magnetic fields. A phenomenological model that describes the ac response of the surface superconducting states is proposed. This model assumes that the observed ac response in dc fields larger than $H_{c2}$ is due to the relaxation of surface superconducting states with nonzero current in the walls to the state with zero current, and the existence of a critical current below which this relaxation is absent.

Two-dimensional arrays of bicomponent structures made of cobalt and permalloy elliptical dots with thickness of 25 nm, length 1 μm, and width of 225 nm, have been prepared by a self-aligned shadow deposition technique. Brillouin light scattering has been exploited to study the frequency dependence of thermally excited magnetic eigenmodes on the intensity of the external magnetic field, applied along the easy axis of the elements. Several modes have been observed while sweeping the field along the major and minor hysteresis loops, encompassing both the parallel and the antiparallel alignment of the magnetization in adjacent permalloy and cobalt dots. Micromagnetic simulations based on the dynamic matrix method enabled us to successfully reproduce the measured evolution of the frequencies with the field, as well as to identify the spatial profiles of the modes. A marked difference in the field dependence of the frequency of some modes has been observed for parallel and antiparallel magnetization configurations, suggesting the possibility of tuning the dynamic response in a reprogrammable way. The role of both static and dynamic magnetic coupling in determining the mode frequency is discussed in detail by studying the frequency evolution as a function of the gap size between the dots.

We have observed the eigenmodes of coupled transverse domain walls in a pair of ferromagnetic nanowires. Although the pair is coupled magnetostatically, its spectrum is determined by a combination of pinning by edge roughness and dipolar coupling of the two walls. Because the corresponding energy scales are comparable, the coupling can be observed only at the smallest wire separations. A model of the coupled wall dynamics reproduces the experiment quantitatively, allowing for comparisons with the estimated pinning and domain wall coupling energies. The results have significant implications for the dynamics of devices based on coupled domain walls.

We demonstrate an unexpected tenfold reduction in the lasing threshold of an organic vertical microcavity under subpicosecond optical excitation. In contrast to conventional theory of lasing, we find that the lasing threshold depends on the rate at which excitons are created rather than the total energy delivered within the exciton lifetime. The threshold reduction is discussed in the context of microcavity-enhanced super-radiant coupling between the excitons. The interpretation of super-radiance is supported by the temporal relaxation dynamics of the microcavity emission, which follows the super-radiance time rather than the cavity lifetime. This demonstration suggests that room-temperature super-radiant effects could generally lower the threshold in four-level lasing systems of similar relaxation dynamics.

The magnetoresistance and Hall coefficient of a doped graphene layer are investigated in the presence of a tilted magnetic field. We consider a graphene layer assembled by either another graphene layer or by a two-dimensional electron gas (2DEG) layer and with the interlayer electron-electron interaction modeled within the random phase approximation. Our calculated magnetoresistances show different interlayer screening effects between decoupled graphene-graphene and graphene-2DEG systems. We also analyze the dependence of dielectric materials as well as the distance between the layers on magnetoresistances. The angle dependence of the Hall coefficient is studied and we show that a quite large Hall resistivity occurs in the graphene layer.