Due to the nonlinear dielectric response within ${\mathrm{SrTiO}}_3$ (STO), an accumulation layer created by positive charges at the surface of the STO sample ($x=0$) has an electron density profile $n\left(x\right)$ that slowly decays as $1/x^{12/7}$. Here we show that the long tail of $n\left(x\right)$ causes the magnetization and the specific heat of the accumulation layer to diverge at large $x$. We explore the truncation of the tail by the finite sample width $W$, the transition from the nonlinear to linear dielectric response with dielectric constant $\kappa$, and the use of a back gate with a negative voltage $-\left|V\right|$. We find that both the magnetization and specific heat are anomalously large and obey nontrivial power law dependences on $W, \kappa$, or $\left|V\right|$. We conclude with a discussion of how the capacitance as a function of the back gate voltage may be used to study the shape of the $n\left(x\right)$ tail in thin samples.

We present results for the lifetime of the orbital transitions of Bi donors in Si, measured using both frequency-domain and time-domain techniques, allowing us to distinguish between homogeneous and inhomogeneous processes. The proximity of the energy of the optically allowed transitions to the optical phonon energy means that there is an unusually wide variation in the lifetimes and broadening mechanisms for this impurity, from fully homogeneous lifetime-broadened transitions to fully inhomogeneously broadened lines. The relaxation lifetime ($T_1$) of the states ranges from the low 10's to 100's of ps, and we find that there is little extra dephasing (so that $T_1$ is of the order of $T_2/2$) in each case.

Colloidal semiconductor nanocrystals (NCs) emitting simultaneously at two distinct wavelengths are of great interest for multiple applications ranging from ultrasensitive self-calibrating nanoscale sensing to light-emitting diodes and white-light emission. The physical mechanism governing dual emission in core/shell NCs composed of two materials is still under investigation; in particular, the roles of the volume of the shell and of the core/shell interface are not well understood. Here we compare three different PbS/CdS NCs with identical cores, but shells of different thicknesses and different core/shell interfaces. We study photoluminescence (PL) characteristics and ultrafast exciton dynamics with subpicosecond time resolution. We find that dual emission is possible for both thick and thin quantum-confined shells, and for different core/shell interfaces. We show that very thick shells allow the decoupling of shell excitons from core ones and we fully describe the dynamic exciton barrier, connected to efficient shell PL. We discuss the efficiency of shell PL in relation to the properties of the NCs and show that it can give rise to optical gain. Our results provide a comprehensive understanding of the physical phenomena governing dual-emission mechanisms in NCs.

It has been recently shown that long linear carbon chains (carbyne) can be formed inside multiwalled carbon nanotubes (CNTs). Encapsulation of carbyne inside the CNT affects the electronic structure of the chain by the long-range Coulomb interaction. This introduces an indirect band gap in the combined CNT-chain system and results in a change in the optical band gap. We study the excitonic optical response of the combined system using the Bethe-Salpeter and Wannier equations based on density functional theory and tight-binding band structures. The optical properties of isolated CNTs and chains are strongly affected by excitonic effects and the CNT-chain system follows a similar trend. The interaction between the CNT and chain results in new bright excitons as well as charge transfer excitons, where electrons are localized on the CNT and holes on the chain, yielding new dark excitons in the combined system.

Oxide thin films are of fundamental importance due to their applications in materials science, optics, corrosion protection, catalysis, and microelectronics. A multistep oxidation procedure is employed to precisely tune the alumina (${\mathrm{Al}}_2{\mathrm{O}}_3$) thickness on a NiAl(110) alloy from two atomic bilayers to $1.5\mathrm{nm}$. Structural changes are analyzed with x-ray photoelectron diffraction and low-energy electron diffraction. The long-range order does not relate to any bulk structure and is imposed by the crystallized interface. The large unit cell formed at the interface persists in thicker films. In contrast, the local order changes at a thickness above $0.5\mathrm{nm}$ from the complex structure of this prelayer under the formation of subnanometer-sized $\gamma -{\mathrm{Al}}_2{\mathrm{O}}_3$(111)-type nuclei. The band structure is monitored with angle-resolved photoelectron spectroscopy. Increasing film thickness results in a slight decrease of the work function, but does not lead to significant changes of the electronic band structure. The presented multistep procedure opens a route for the synthesis of thin film structures in general and in particular provides fundamental insight in the surface structure of spinel-based transition ${\mathrm{Al}}_2{\mathrm{O}}_3$.

We study the effect of the electromagnetic environment on the resonance frequency of plasmonic excitations in dimers of interacting metallic nanoparticles. The coupling between plasmons and vacuum electromagnetic fluctuations induces a shift in the resonance frequencies—analogous to the Lamb shift in atomic physics—which is usually not measurable in an isolated nanoparticle. In contrast, we show that this shift leads to sizable corrections to the level splitting induced by dipolar interactions in nanoparticle dimers. For the system parameters which we consider in this work, the ratio between the level splitting for the longitudinal and transverse hybridized modes takes a universal form dependent only on the interparticle distance and thus is highly insensitive to the precise fabrication details of the two nanoparticles. We discuss the possibility to successfully perform the proposed measurement using state-of-the-art nanoplasmonic architectures.

We study a graphene Hall probe located on top of a magnetic surface as a detector of skyrmions, using as a working principle the anomalous Hall effect produced by the exchange interaction of the graphene electrons with the noncoplanar magnetization of the skyrmion. We study the magnitude of the effect as a function of the exchange interaction, skyrmion size, and device dimensions. Our calculations for multiterminal graphene nanodevices, working in the ballistic regime, indicate that for realistic exchange interactions a single skyrmion would give Hall voltages well within reach of the experimental state of the art. The proposed device could act as an electrical transducer that marks the presence of a single skyrmion in a nanoscale region, paving the way towards the integration of skyrmion-based spintronics and graphene electronics.

The intricate interplay between optically dark and bright excitons governs the light-matter interaction in transition metal dichalcogenide monolayers. We have performed a detailed investigation of the “spin-forbidden” dark excitons in ${\mathrm{WSe}}_2$ monolayers by optical spectroscopy in an out-of-plane magnetic field ${\mathrm{B}}_{\mathrm{z}}$. In agreement with the theoretical predictions deduced from group theory analysis, magnetophotoluminescence experiments reveal a zero-field splitting $\delta =0.6\pm 0.1\mathrm{meV}$ between two dark exciton states. The low-energy state is strictly dipole forbidden (perfectly dark) at ${\mathrm{B}}_z=0$, while the upper state is partially coupled to light with $z$ polarization (“gray” exciton). The first determination of the dark neutral exciton lifetime ${\tau }^{\mathrm{D}}$ in a transition metal dichalcogenide monolayer is obtained by time-resolved photoluminescence. We measure ${\tau }^{\mathrm{D}}\sim 110\pm 10\mathrm{ps}$ for the gray exciton state, i.e., two orders of magnitude longer than the radiative lifetime of the bright neutral exciton at $T=12\mathrm{K}$.

Energy evaluation of charged defects is tremendously important in two-dimensional (2D) semiconductors for the industrialization of 2D electronic devices because of its close relation with the corresponding type of conductivity and its strength. Although the method to calculate the energy of charged defects in single-layer one-atom-thick systems of equilateral unit-cell geometry has recently been proposed, few-layer 2D semiconductors are more common in device applications. As it turns out, one may not apply the one-layer formalism to multilayer cases without jeopardizing accuracy. Here, we generalize the approach to 2D systems of arbitrary cell geometry and thickness and use few-layer black phosphorus to illustrate how defect properties, mainly group-VI substitutional impurities, are affected. Within the framework of density functional theory, we show that substitutional Te $\left(\mathrm{T}{\mathrm{e}}_{\mathrm{P}}\right)$ is the best candidate for $n$-type doping, and as the thickness increases, the ionization energy is found to decrease monotonically from 0.67 eV (monolayer) to 0.47 eV (bilayer) and further to 0.33 eV (trilayer). Although these results show the ineffectiveness of the dielectric screening at the monolayer limit, they also show how it evolves with increasing thickness whereby setting a new direction for the design of 2D electronics. The proposed method here is generally suitable to all the 2D materials regardless of their thickness and geometry.

We study flat bands in bipartite tight-binding networks with discrete translational invariance. Chiral flat bands with chiral symmetry eigenenergy $E=0$ and host compact localized eigenstates for finite range hopping. For a bipartite network with a majority sublattice chiral flat bands emerge. We present a simple generating principle of chiral flat-band networks and as a showcase add to the previously observed cases a number of new potentially realizable chiral flat bands in various lattice dimensions. Chiral symmetry respecting network perturbations—including disorder and synthetic magnetic fields—preserve both the flat band and the modified compact localized states. Chiral flat bands are spectrally protected by gaps and pseudogaps in the presence of disorder due to Griffiths effects.

The electron spin resonance (ESR) of two-dimensional electrons confined in a high-quality, 16-nm AlAs quantum well was investigated near the filling factor $\nu =2/3$ of the fractional quantum Hall effect (FQHE). The spin resonance was robust in the vicinity of the fractional filling $\nu =2/3$, indicating that the $\nu =2/3$ state is at least partially spin polarized. The formation of the $2/3$ FQHE state did not result in any modifications of the ESR linewidth and, hence, of the electron spin relaxation rate. Yet the nuclear spin-lattice relaxation rate extracted from the time decay of the ESR Overhauser shift demonstrated a strong nonmonotonic dependence on the electron filling factor with a minimum near $\nu =2/3$. This observation suggests the enhancement of the energy gap in the spin excitation spectrum of two-dimensional electrons at the $\nu =2/3$ state.

A Weyl semimetal discovered recently, NbP, exhibits two groups of Weyl points with one group lying inside the $k_z=0$ plane and the other group staying away from this plane. All Weyl points have been assumed to be type I, in which the Fermi surface (Fs) shrinks into a point as the Fermi energy crosses the Weyl point. In this paper, we have revealed that the second group of Weyl points are actually type II, which are found to be touching points between the electron and hole pockets in the FS. Corresponding Weyl cones are strongly tilted along a line approximately ${17}^{\circ }$ off the $k_z$ axis in the $k_x-k_z$ (or $k_y-k_z$) plane, violating the Lorentz symmetry but still giving rise to Fermi arcs on the surface. Therefore, NbP exhibits both type-I ($k_z=0$ plane) and type-II ($k_z\ne 0$ plane) Weyl points.

We examine the magnetic correlations in quantum spin models that were derived recently as effective low-energy theories for electronic correlation effects on the edge states of graphene nanoribbons. For this purpose, we employ quantum Monte Carlo simulations to access the large-distance properties, accounting for quantum fluctuations beyond mean-field-theory approaches to edge magnetism. For certain chiral nanoribbons, antiferromagnetic interedge couplings were previously found to induce a gapped quantum disordered ground state of the effective spin model. We find that the extended nature of the intraedge couplings in the effective spin model for zigzag nanoribbons leads to a quantum phase transition at a large, finite value of the interedge coupling. This quantum critical point separates the quantum disordered region from a gapless phase of stable edge magnetism at weak intraedge coupling, which includes the ground states of spin-ladder models for wide zigzag nanoribbons. To study the quantum critical behavior, the effective spin model can be related to a model of two antiferromagnetically coupled Haldane-Shastry spin-half chains with long-ranged ferromagnetic intrachain couplings. The results for the critical exponents are compared also to several recent renormalization-group calculations for related long-ranged interacting quantum systems.

A hydrodynamic theory of screening in a generic electron gas of arbitrary dimensionality is given that encompasses all previously studied cases and clarifies the predictions of the many-body approach. We find that long-wavelength plasma oscillations are classical phenomena with quantum-mechanical effects playing no explicit role. The character of the oscillations is solely dictated by the dimensionality of the electron system and its equation of state in the neutral limit. Materials whose excitations are described by the Dirac dispersion law—such as doped graphene or a Weyl semimetal—are no exception to this rule.

In this work, we study the conductance and the thermoelectric properties of a quantum dot embedded between two metallic leads with a side-coupled triple quantum dot molecule under a magnetic field. We focus on the spin polarization and thermoelectric quantities. Our results show the possibility of designing an efficient spin-filter device in addition to a noticeable enhancement of the Seebeck coefficient driven by the asymmetry in the quantum dot energy levels, and a tunable pure spin-Seebeck effect is obtained. This behavior also holds in the interacting case, where a pure spin-Seebeck effect can be obtained for fixed values of the embedded quantum dot energy level. Our findings could lead to the implementation of a new pure spin energy conversion and capable spin-filter devices working with weak magnetic fields.

We show that the sensitivity and depth selectivity of neutron reflectometry can be greatly enhanced through a waveguide design that takes advantage of the spin-dependent magnetic neutron scattering potential to steer spin-up and spin-down neutrons into waveguide modes with different depth profiles. Using a bilayer of manganate and ruthenate ferromagnets, we demonstrate that a magnetic waveguide structure with sharp spin-up and spin-down modes centered in the two different layers can be generated by adding a magnetically inactive capping layer. The resulting reflectometric data allow accurate and reliable determination of a small in-plane magnetization in the ruthenate layer, despite its immediate proximity to the manganate layer with much larger magnetization. Magnetic neutron waveguides thus enable depth-sensitive measurements of small electronic spin polarizations in a large variety of magnetic multilayers and devices.

In order to point out experimentally accessible signatures of spin-orbit interaction, we investigate numerically the Andreev spectrum of a multichannel mesoscopic quantum wire (N) with high spin-orbit interaction coupled to superconducting electrodes (S), contrasting topological and nontopological behaviors. In the nontopological case (square lattice with Rashba interactions), we find that the Kramers degeneracy of Andreev levels is lifted by a phase difference between the S reservoirs except at multiples of $\pi$, when the normal quantum wires can host several conduction channels. The level crossings at these points invariant by time-reversal symmetry are not lifted by disorder. Whereas the dc Josephson current is insensitive to these level crossings, the high-frequency admittance (susceptibility) at finite temperature reveals these level crossings and the lifting of their degeneracy at $\pi$ by a small Zeeman field. We have also investigated the hexagonal lattice with intrinsic spin-orbit interaction in the range of parameters where it is a two-dimensional topological insulator with one-dimensional helical edges protected against disorder. Nontopological superconducting contacts can induce topological superconductivity in this system characterized by zero-energy level crossing of Andreev levels. Both Josephson current and finite-frequency admittance carry then very specific signatures at low temperature of this disorder-protected Andreev level crossing at $\pi$ and zero energy.

We have conducted degenerate pump-pump-probe experiments on semimetal bismuth with femtosecond time resolution, varying both pump-probe and interpump time delays. The observed phonon dynamics, including the amplitude, damping, frequency, and phase, can be modeled by the interference of two chirped damped oscillators. No lattice anharmonicity along the trigonal axis is observed. We also find evidence of phonon-mediated relaxation of the energy density at the sample surface.

We study the spin-$S$ Kitaev model in the classical ($S\to \infty$) limit using Monte Carlo simulations combined with semiclassical spin dynamics. We discuss differences and similarities in the dynamical structure factors of the spin-$1/2$ and the classical Kitaev liquids. The quantum behavior is restricted to low temperatures where a gap protects visons from decohering the system. Once this quantum gap is breached, at low temperatures compared to the coupling constant, significant entropic disorder decoheres the Majorana fermions and the system is described quantitatively by classical dynamics. The low-temperature and low-energy spectrum of the classical model exhibits a finite-energy peak, which is the precursor of the one produced by the Majorana modes of the $S=1/2$ model. The classical peak is spectrally narrowed compared to the quantum result and can be explained by magnon excitations within fluctuating one-dimensional manifolds (loops). Hence the difference from the classical limit to the quantum limit can be understood by the fractionalization of magnons propagating in one-dimensional manifolds. Moreover, we show that the momentum-space distribution of the low-energy spectral weight of the $S=1/2$ model follows the momentum-space distribution of zero modes of the classical model.