The resonance fluorescence spectrum (RFS) of a hybrid system consisting of a $p$-doped semiconductor quantum dot (QD) coupled to a metallic nanoparticle (MNP) is analyzed. The quantum dot is described as a four-level atomlike system using the density matrix formalism. The lower levels are Zeeman-split hole spin states and the upper levels correspond to positively charged excitons containing a spin-up, spin-down hole pair and a spin electron. A linearly polarized laser field drives two of the optical transitions of the QD and produces localized surface plasmons in the nanoparticle, which act back upon the QD. The frequencies of these localized plasmons are very different along the two principal axes of the nanoparticle, thus producing an anisotropic modification of the spontaneous emission rates of the allowed optical transitions, which is accompanied by very minor local field corrections. This manifests into dramatic modifications in the RFS of the hybrid system in contrast to the one obtained for the isolated QD. The RFS is analyzed as a function of the nanoparticle's aspect ratio, the external magnetic field applied in the Voigt geometry, and the Rabi frequency of the driving field. It is shown that the spin of the QD is imprinted onto certain sidebands of the RFS, and that the signal at these sidebands can be optimized by engineering the shape of the MNP.

We develop a quantitative model of the self-catalyzed vapor-liquid-solid growth of GaAs nanowires, that depends on only a few a priori unknown physical parameters. The model is based on the sole consideration of the As species and incorporates all relevant mechanisms of exchange of As between the vapor, liquid, and solid phases, namely direct impingement of molecules on the droplet, their re-emission by the neighboring surfaces, evaporation from the droplet, and nucleation at the solid-liquid interface. It reproduces quantitatively all salient features of our experimental study, namely the variations of nanowire growth rate with As flux, temperature, and nanowire radius. From these optimized fits, we extract a complete set of model parameters, in particular the nucleus edge energy. We also determine quantities so far inaccessible to experiment, such as the As concentration in the droplet (about 1%), supersaturation of the liquid, and nucleation barrier, for individual nanowires. The model can then be used to predict the growth rate (and all quantities of interest) for an arbitrary GaAs nanowire of given geometry in arbitrary growth conditions, including conditions not yet explored experimentally, provided the fraction of the As flux re-emitted by its environment is known. Although largely ignoring group III elements, our model captures most of the physics of self-catalyzed nanowire growth.

We perform a general group-theoretical study of the Kondo problem in monolayer and bilayer graphene around the charge neutrality point. Utilizing the group representation theory, we derive from symmetry considerations a family of the Kondo models for all symmetric placements with either a 3- or a 6-fold rotational axis of an impurity atom in an arbitrary orbital state. We find six possible classes of the partially anisotropic four-channel Kondo model. As the key result, we argue several possibilities to realize the regime of the dominant channel-symmetric two-channel Kondo effect, protected by the local symmetry and specifics of the graphene band structure. Our findings open prospects for the observation of the rich multichannel Kondo physics in graphene and the associated non-Fermi-liquid behavior.

Here we present a Brownian ratchet based on plasmonic interactions. By periodically turning on and off a laser beam that illuminates a periodic array of plasmonic nanostructures with broken spatial symmetry, the random thermal motion of a subwavelength dielectric bead is rectified into one direction. By means of the molecular dynamics technique we show a statistical directed drift in particle flow.

The successful isolation of graphene ten years ago has evoked a rapidly growing scientific interest in the nature of two-dimensional (2D) crystals. A number of different 2D crystals has been produced since then, with properties ranging from superconductivity to insulating behavior. Here, we predict the possibility for realizing ferromagnetic 2D crystals by exfoliating atomically thin films of K${}_2$CuF${}_4$. From a first-principles theoretical analysis, we find that single layers of K${}_2$CuF${}_4$ form exactly 2D Kosterlitz-Thouless systems. The 2D crystal can form a free-standing membrane, and exhibits an experimentally accessible transition temperature and robust magnetic moments of 1${\mu }_B$ per formula unit. 2D K${}_2$CuF${}_4$ unites ferromagnetic and insulating properties and is a demonstration of principles for nanoelectronics such as novel 2D-based heterostructures.

Path-integral Monte Carlo calculations were performed to study the adsorption of ${}^4$He atoms on $\alpha$-graphyne. We find that one ${}^4$He atom can be embedded onto the in-plane center of each hexagon of the graphyne. In the first ${}^4$He layer above the ${}^4$He-embedded graphyne surface, a Mott-insulating state was observed at an areal density of 0.0706 Å${}^{-2}$ with three ${}^4$He atoms occupying each hexagonal cell while the helium atoms form a commensurate triangular solid at a density of 0.0941 Å${}^{-2}$. Here we show that the Ising pseudospin symmetry introduced for two degenerate configurations of three ${}^4$He atoms in a hexagonal cell can be broken by additional ${}^4$He atoms placed at the hexagon vertices, and the Mott-insulator to commensurate-solid transition is a transition from a nonmagnetic spin liquid of frustrated antiferromagnets to a spin-aligned ferromagnet under a particle-induced pseudomagnetic field.

We consider ferromagnetic instabilities of two-dimensional helical Dirac fermions hosted on the surface of three-dimensional topological insulators. We investigate ways to increase the role of interactions by means of modifying the bulk properties which in turn changes both the surface Dirac theory and the screening of interactions. We discuss both the long-ranged part of the Coulomb interactions controlled by the dimensionless coupling constant $\alpha =e^2/\left(\hslash \epsilon v_F^{\mathrm{surf}}\right)$ as well as the effects of local interactions parametrized by the ratio $U_{\mathrm{surf}}/D_{\mathrm{surf}}$ of a local interaction on the surface $U_{\mathrm{surf}}$ and the surface bandwidth $D_{\mathrm{surf}}$. If large compared to 1, both mechanisms can induce spontaneously surface ferromagnetism, thereby gapping the surface Dirac metal and inducing an anomalous quantum Hall effect. We investigate two mechanisms which can naturally lead to small Fermi velocities $v_F^{\mathrm{surf}}$ and a corresponding small bandwidth $D_{\mathrm{surf}}$ at the surface when the bulk band gap is reduced. The same mechanisms can, however, also lead to an enhanced screening of surface interactions. While in all considered cases the long-ranged part of the Coulomb interaction is screened efficiently, $\alpha \lesssim 1$, we discuss situations where $U_{\mathrm{surf}}/D_{\mathrm{surf}}$ becomes parametrically large compared to 1, thus inducing surface magnetism.

Acoustic frequency combs are optically excited and detected in silicon membranes covered with thin aluminum layers by femtosecond pump-probe spectroscopy. The various frequency combs consist of 11 up to 45 modes ranging in frequency from 10 up to 500 GHz. Evaluating the different modes of the combs allows us to quantify the dynamic properties of this two-layer system with great precision. Deviations of the frequencies of higher modes from a linear relation can be quantitatively understood. The time domain traces show clearly defined pulses which are detected in regular time intervals after each roundtrip in the acoustic cavity formed by the membrane and the metal film. By analyzing the individual reflected pulses and their evolution in time, damping times for the whole frequency range are determined. We analytically derive a deviation of the individual comb modes from integer values of the fundamental frequency which is corroborated by the experiments.

We investigate the conductivity of doped single-layer graphene in the semiclassical Boltzmann limit, as well as the conductivity minimum in neutral graphene within the self-consistent transport theory, pointing up the effects due to both the structure of charged impurities near graphene and the structure of the surrounding dielectrics. Using the hard-disk model for a two-dimensional (2D) distribution of impurities allows us to investigate structures with large packing fractions, which are shown to give rise to both strong increase in the slope of conductivity at low charge carrier densities in graphene and a strongly sublinear behavior of the conductivity at high charge carrier densities when the correlation distance between the impurities is large. On the other hand, we find that a superlinear dependence of the conductivity on charge carrier density in heavily doped graphene may arise from increasing the distance of impurities from graphene or allowing their clustering into disklike islands, whereas the existence of an electric dipole polarizability of impurities may give rise to an electron-hole asymmetry in the conductivity. Using the electrostatic Green's function for a three-layer structure of dielectrics, we show that finite thickness of a dielectric layer in the top-gating configuration, as well as the existence of nonzero air gap(s) between graphene and the nearby dielectric(s), exerts strong influences on the conductivity and its minimum. While a decrease in the dielectric thickness is shown to increase the conductivity in doped graphene and even gives rise to finite conductivity in neutral graphene for a 2D distribution of impurities, we find that an increase in the dielectric thickness gives rise to a superlinear behavior of the conductivity when impurities are homogeneously distributed throughout the dielectric. Moreover, the dependence of graphene's mobility on its charge carrier density is surprisingly strongly affected, quantitatively and qualitatively, by the graphene-dielectric gap(s) when combined with the precise position of a 2D distribution of charged impurities. Finally, we show that the conductivity minimum in neutral graphene is increased by increasing the correlation distance between the impurities, reduced by increasing the graphene-dielectric gap, and increased by decreasing the dielectric thickness in a top-gated configuration, even though the corresponding residual charge carrier density is reduced by decreasing the dielectric thickness.

The structure of silicene fabricated on a Ag(111) surface was determined using reflection high-energy positron diffraction with a linac-based brightness-enhanced intense positron beam. From the rocking curve analysis, the silicene was verified to have a buckled structure with a spacing of $0.83\AA$ between the top and the bottom Si layers. The distance between the bottom Si layer in the silicene and the first Ag layer was determined to be $2.14\AA$. These results agree with the theoretically predicted values from a previous study [Phys. Rev. Lett. 108, 155501 (2012)] within an error of $\pm 0.05\AA$.

Localized magnetic moments with crystal-field doublet or pseudospin-$\frac{1}{2}$ may arise in correlated insulators with even number of electrons and strong spin-orbit coupling. Such a non-Kramers pseudospin-$\frac{1}{2}$ is the consequence of crystalline symmetries as opposed to the Kramers doublet arising from time-reversal invariance, and is necessarily a composite of spin and orbital degrees of freedom. We investigate possible spin-orbital liquids with fermionic spinons for such non-Kramers pseudospin-$\frac{1}{2}$ systems on the kagome lattice. Using the projective symmetry group analysis, we find 10 phases that are not allowed in the corresponding Kramers systems. These phases are allowed due to unusual action of the time-reversal operation on non-Kramers pseudospins. We compute the spin-spin dynamic structure factor that shows characteristic features of these non-Kramers spin-orbital liquids arising from their unusual coupling to neutrons, which is therefore relevant for neutron scattering experiments. We also point out possible anomalous broadening of Raman scattering intensity that may serve as a signature experimental feature for gapless non-Kramers spin-orbital liquids.

A magnetization study of a Tm${}_2$Fe${}_{17}$ single crystal and aligned powder of the deuteride Tm${}_2$Fe${}_{17}$D${}_{3.2}$ has been carried out in steady (14 T) and pulsed (60 T and, in one case, up to 74 T) magnetic fields at temperatures between 1.5 and 300 K. Tm${}_2$Fe${}_{17}$ is a ferrimagnet with $T_C$ $=$ 295 K and a spontaneous moment of 22 ${\mu }_B$/f.u. at $T$ $=$ 4.2 K. Of particular interest are low-temperature magnetization curves along the sixfold crystal axis $c$, which is an easy direction in Tm${}_2$Fe${}_{17}$ and a hard direction in the deuteride. In either case the magnetization increases with magnetic field undulatorily in broad steps whose height is a multiple of the atomic moment of Tm, ${\mu }_{\mathrm{Tm}}$ $=$ 7 ${\mu }_B$. In Tm${}_2$Fe${}_{17}$, the positions of the steps yield information on the Fe-Tm molecular field, 48 T on the Tm 2$d$ site and 60 T on the Tm 2$b$ site, whereas the crystal field parameter $A_{66}$ is found from the widths of the steps: $A_{66}$($b$) $=$ $-$35 K$a_0^{-6}$ and $A_{66}$($d$) $=$ $-$26 K$a_0^{-6}$ (here $a_0$ is the Bohr radius). It also proves possible to estimate the other sixth-order crystal field parameter: $A_{60}$ ∼ −4 K$a_0^{-6}$ (on average for both sites). Less information can be extracted from the powder data for the deuteride. Thus, the mean molecular field on Tm in Tm${}_2$Fe${}_{17}$D${}_{3.2}$ is found to be 49 T or 9$\%$ less than in the parent binary compound.

The flux quantization in the intermediate state of a type-I superconducting Pb film is studied by using scanning Hall probe microscopy. The vorticity of flux tubes can be tuned by changing the cooling field through the flux expulsion process, and single flux quantum vortices coexisting with multiple quantized flux tubes are observed at low enough fields. However, the minimum fluxoid observed through flux penetration is found to contain more than one flux quantum, and its vorticity increases with decreasing temperature. By combining these two processes it is possible to stabilize flux tubes of opposite polarity, and single flux quantum vortices are created through the annihilation process under the drive of the Lorentz force. Our results give strong evidence that single quantum vortices can be thermodynamically stabilized in the intermediate state of type-I superconductors.

A systematic investigation of the effects of disorder on the BCS-BEC crossover at the lowest order in the impurity potential is presented for the normal phase above the critical temperature $T_c$. Starting with the $t$-matrix approach for the clean system, by which pairing correlations between opposite-spin fermions evolve from the weak-coupling (BCS) to the strong-coupling (BEC) limits by increasing the strength of the attractive interparticle interaction, all possible diagrammatic processes are considered where the effects of a disordered potential are retained in the self-energy at the lowest order. An accurate numerical investigation is carried out for all these diagrammatic terms, to determine which of them are mostly important throughout the BCS-BEC crossover. Explicit calculations for the values of $T_c$, the chemical potential, and the Tan's contact are carried out. In addition, the effect of disorder on the single-particle spectral function is analyzed, and a correlation is found between an increase of $T_c$ and a widening of the pseudogap energy at $T_c$ on the BCS side of unitarity in the presence of disorder, while on the BEC side of unitarity the presence of disorder favors the collapse of the underlying Fermi surface. The present investigation is meant to orient future studies when the effects of disorder will be considered at higher orders, with the purpose of limiting the proliferation of diagrammatic terms in which interaction and disorder are considered simultaneously.

We measure the electrical resistance of $o$-TaS${}_3$ between 1 and 300 K under pressures up to 20 GPa. We observe a gradual decrease of the charge density wave transition temperature $T_{\mathrm{CDW}}$ with increasing pressure $P$ following a mean field quantum fluctuation power law $T_{\mathrm{CDW}}=215\mathrm{K}{\left[\left(P_C-P\right)/P_C\right]}^{\gamma }$ with a quantum critical point at a pressure $P_C=11.5\mathrm{GPa}$ and $\gamma \approx 0.5$. Around the quantum critical point (QCP) we observe a superconducting dome with a maximum superconducting transition temperature $T_c=3.1\mathrm{K}$. Such dome is similar to superconducting domes around other types of order suggesting that the QCP is directly responsible for the enhancement of superconductivity through a universal mechanism still not well understood.

We have studied the electrodynamic response of strongly disordered superconducting TiN films using microwave resonators, where the disordered superconductor is the resonating element in a high-quality superconducting environment of NbTiN. We describe the response assuming an effective pair-breaking mechanism modifying the density of states and compare this to local tunneling spectra obtained using scanning tunneling spectroscopy. For the least disordered film ($k_{\mathrm{F}}l=8.7$, $R_{\mathrm{s}}=13\Omega$), we find good agreement, whereas for the most disordered film ($k_{\mathrm{F}}l=0.82$, $R_{\mathrm{s}}=4.3\mathrm{k}\Omega$), there is a strong discrepancy, which signals the breakdown of a model based on uniform properties.

High-pressure x-ray diffraction measurements were performed at room temperature on single crystals of the highly magnetostrictive alloy Fe${}_{0.81}$Ga${}_{0.19}$ (galfenol). This alloy has a bcc crystal structure at ambient pressure but undergoes a bcc-to-hcp phase transition at 24 GPa on compression. A large hysteresis loop is observed in which the reversed transition occurs at 13 GPa on decompression. The midpoint of this transition is 18.5 GPa. The measured bulk modulus of this material is 182 (±17) GPa, which is comparable to that of pure iron. As with iron, the hcp structure of the alloy can be derived from a compression of the bcc lattice along [001] that is accompanied by shearing along $\left[1\overline{1}0\right]$. Our results indicate that the addition of Ga shifts the bcc-to-hcp transition from 13 GPa in pure iron to 18.5 GPa, and we speculate that this is due to the larger atomic radius of Ga. A uniaxial loading of 3 GPa completely suppresses the diffuse scattering in Fe${}_{0.81}$Ga${}_{0.19.}$ We ascertain that the magnetostrictive properties of the alloy are reduced under pressure.

There are two commonly discussed points of view in theoretical description of cuprate superconductors: (i) Cuprates can be described by the modified $t-J$ model; (ii) overdoped cuprates are close to the regime of normal Fermi liquid (NFL). We argue that recent resonant inelastic x-ray scattering data challenge both points. While the modified $t-J$ model describes well the strongly underdoped regime, it fails to describe high energy magnetic excitations when approaching optimal doping. This probably indicates failure of the Zhang-Rice singlet picture. In the overdoped regime the momentum-integrated spin structure factor $S\left(\omega \right)$ has the same intensity and energy distribution as that in an undoped parent compound. This implies that the entire spin spectral sum rule is saturated at $\omega \approx 2J$, while in an NFL the spectral weight should saturate only at the total bandwidth which is much larger than $2J$.