The control of ferroelastic, ferroelectric, and ferromagnetic domains through the corresponding fields has led to numerous applications taken for granted nowadays. Ferrotoroidal order on the other hand has been less explored due to its more complex character and due to the peculiar field needed to control it. LiFeSi${}_2$O${}_6$ exhibits a ferrotoroidal order that is particularly simple to analyze. By performing neutron-diffraction experiments with spherical polarization analysis we show that crossed electric and magnetic fields allow us to control ferrotoroidal moments.

We report the discovery of superconductivity in Nb${}_3$Pd${}_x$Se${}_7$ with an $x$-dependent superconducting transition temperature as high as $T_c\simeq 2.1$ K for $x\simeq 0.7$ (middle point of the resistive transition). Needlelike single crystals display anisotropic upper-critical fields with an anisotropy $\gamma =H_{c2}^b/H_{c2}^a$ as large as 6 between fields applied along their needle axis (or $b$ axis) or along the $a$ axis. As for the Fe based superconductors $\gamma$ is temperature-dependent, suggesting that Nb${}_3$Pd${}_{0.7}$Se${}_7$ is a multiband superconductor. This is supported by band structure calculations which reveal a Fermi surface composed of quasi-one-dimensional and quasi-two-dimensional sheets of hole character, as well as three-dimensional sheets of both hole and electron character. Remarkably, $H_{c2}^b$ is observed to saturate at $H_{c2}^b(T\to 0\text{K})\simeq 14.1$ T which is $4.26\times H_p$ where $H_p$ is the Pauli-limiting field in the weak-coupling regime. The synthesis procedure yields additional crystals belonging to the Nb${}_2$Pd${}_x$Se${}_5$ phase which also becomes superconducting when the fraction of Pd is varied. For both phases we find that superconductivity condenses out of an anomalous metallic state, i.e., displaying $\partial \rho /\partial T<0$ above $T_c$ similarly to what is observed in the pseudogap phase of the underdoped cuprates. An anomalous metallic state, low-dimensionality, multiband character, extremely high and anisotropic $H_{c2}$'s are all ingredients for unconventional superconductivity.

We reformulate the time-dependent Gutzwiller approximation by M. Schiró and M. Fabrizio, [Phys. Rev. Lett. 105, 076401 (2010)] in the framework of slave-boson mean-field theory, which is used to investigate the dynamical Mott transition of the generic two-band Hubbard model at half filling upon an interaction quench. Interorbital fluctuations lead to notable changes with respect to the single-band case. The singular dynamical transition is replaced by a broad regime of long-lived fluctuations between metallic and insulating states, accompanied by intriguing precursor behavior. A mapping to a spin model proves helpful to analyze the different regions in terms of the evolution of an Ising-like order parameter. Contrary to the static case, singlet occupations remain vital in the Mott-insulating regime with finite Hund's exchange.

With the help of first-principles-based computational techniques, we evaluate the structural, vibrational, and polar properties of a fictitious Ruddlesden-Popper-type Ba${}_2$TiO${}_4$ compound that is subjected to a range of epitaxial strains. We demonstrate that, depending on the value of the applied strain, this compound exhibits different types of incommensurate distortions, which does not happen in the similar, but better cation-size-balanced, Ca- and Sr-based structures. Furthermore, at moderate tensile strains this material develops spontaneous polarization (0.17 C/m${}^2$ at 1.5$\%$ strain) that is directed along the perovskite planes. By virtue of having the same symmetry, planar incommensurate and polar distortions can interact with each other, which results in a variety of complicated ground-state structural arrangements whose properties could be fine tuned by epitaxial strain engineering.

The dewetting properties of $\mathrm{Ge}/{\mathrm{SiO}}_2$ have been studied by low-energy electron microscopy and grazing incidence small-angle x-ray scattering in two temperature ranges characterized by the presence or the absence of {15 3 23} facets on the dewetting fronts. Thanks to a comparison with the $\mathrm{Si}/{\mathrm{SiO}}_2$ system, we show that the {15 3 23} facets: (i) play a role in the stabilization properties of Ge dewetting fronts, (ii) lead to a rotation of ${45}^{\circ }$ of the Ge fingers with respect to the Si fingers, and (iii) increase the Ge fingers’ stability delaying the formation of a three-dimensional Ge islands with respect to Si for the benefit of the formation of Ge nanowires. Studying the dewetting kinetics enables estimating the activation energy for Ge dewetting to $2.7\pm 0.2\mathrm{eV}$. The weak energetics differences between Si and Ge systems are sufficient to change the dewetting morphologies from a squared-void opening for $\mathrm{Si}/{\mathrm{SiO}}_2$ to multibranch dendrites for Ge with specific consequences on the relative dewetting velocities of the $\mathrm{Si}/{\mathrm{SiO}}_2$ and $\mathrm{Ge}/{\mathrm{SiO}}_2$ systems.

We report magnetotransport measurements for the fractional quantum Hall state at filling factor $\nu =$ 5/2 as a function of applied parallel magnetic field ($B_{\left|\right|}$). As $B_{\left|\right|}$ is increased, the 5/2 state becomes increasingly anisotropic, with the in-plane resistance along the direction of $B_{\left|\right|}$ becoming more than 30 times larger than in the perpendicular direction. Despite the very large resistance anisotropy, the anisotropy ratio remains constant over a relatively large temperature range, yielding an energy gap which is the same for both directions. Our data are qualitatively consistent with a fractional quantum Hall nematic phase.

We study theoretically the transport of linearly polarized exciton-polaritons in a quasi-one-dimensional microcavity channel separating two polariton condensates generated by optical pumping. The direction and value of mass and spin currents are controlled by the relative phase and polarization of two condensates, as in the stationary Josephson effect. However, due to dissipation and particle-particle interactions, the current density is inhomogeneous: it strongly depends on the coordinate along the axis of the channel. A stationary spin domain can be created in the channel, its position would be sensitive to the phase difference between two bordering condensates.

We theoretically study the dynamic polarization of lattice nuclear spins in GaAs double quantum dots containing two electrons. In our prior work [Phys. Rev. Lett. 104, 226807 (2010)], we identified three regimes of long-term dynamics, including the buildup of a large difference in the Overhauser fields across the dots, the saturation of the nuclear polarization process associated with the formation of so-called dark states, and the elimination of the difference field. In particular, when the dots are different sizes, we found that the Overhauser field becomes larger in the smaller dot. Here we present a detailed theoretical analysis of these problems, including a model of the polarization dynamics and the development of a numerical method to efficiently simulate semiclassical-central-spin problems. When nuclear spin noise is included, the results agree with our prior work indicating that large difference fields and dark states are stable configurations, while the elimination of the difference field is unstable; however, in the absence of noise, we find all three steady states are achieved depending on parameters. These results are in good agreement with dynamic nuclear polarization experiments in double quantum dots.

Armchair single-wall carbon nanotubes (SWCNTs) are theoretically predicted to be metallic in the simple zone-folding scheme and protected from the curvature effects. They are, however, found to be small-gap semiconductors, experimentally. Lacking the genuine band-gap opening mechanism for armchair SWCNTs, the Mott insulator concept was proposed previously for the nominally metallic carbon nanotubes. Using the ab initio local density approximation method, we show, in this paper, that the carbon nanotubes without corrugation are, in fact, not metastable. The corrugated structures are always lower in energy than those without corrugation. The size of corrugation increases rapidly with the nanotubes’ curvature and vanishes for flat graphene. The corrugation breaks the local symmetry between two types of carbon sites, and a true band gap starts to develop once graphene sheets are rolled up into nanotubes.

Coherent phonon (CP) dynamics and electronic relaxation in single-walled carbon nanotubes (SWNTs) are investigated in femtosecond pump-probe experiments. Using a sensitive multichannel lock-in amplifier, chirality-specific electronic relaxation and vibrational dynamics are resolved in SWNT ensembles composed of several chiral systems without the need for selective isolation of the different species by purification. The dynamics of vibrational wave packets are studied based on oscillatory changes in the absorbance of the systems. Modulations corresponding to the radial breathing mode (RBM), observed in the time traces of the absorbance change for the four chiral systems (6,4), (6,5), (7,5), and (8,3), have been analyzed in detail. The vibrational modes of the CP spectra are identified from the two-dimensional distribution of probe photon energy versus Fourier frequency. Resonance conditions and mode frequencies lead to definite chirality assignments. Coherent RBM phonon generation is analyzed using the probe photon energy-dependent amplitude profiles as a result of the spectral shift induced by wave-packet motion on the potential surface. The present study clarifies that the observed probe photon energy dependence is due to both the imaginary and real parts of the third-order susceptibility, corresponding to Raman (and Raman-like) gain and loss processes and to molecular phase modulation, respectively. The imaginary part is the dominant contribution to the modulation in the difference absorbance. It shows probe photon energy dependence in the form of a difference in absorbed photon energy between the spectra that are shifted and unshifted with vibrational frequency. The size of the Huang-Rhys factors from the difference fitting to the (6,4), (6,5), and (8,3) systems are 0.26, 0.32, and 0.75, respectively. The trend of the factors originates in the stiffness differences of the SWNT structures. The real part depends on the derivative of the absorbed photon energy spectrum due to cross-phase modulation, resulting from the change in refractive index during the molecular vibrations. This process induces a probe spectral change, as evidenced by first-derivative fitting using a small number of data points of probe photon energies. The effective nonlinear refractive index for each chiral system is determined to range from 0.2 to $3.1\times {10}^{-17}$ cm${}^2$/W.

We present an alternative scheme to determine the local density of states (LDOS) of a sample using data obtained via scanning tunneling spectroscopy (STS). Using either the tunneling current as a function of applied bias voltage or the tip-sample separation as a function of applied bias voltage, the LDOS can be determined via a numerical fitting algorithm. This fitting algorithm makes use of the one-dimensional Simmons tunnel barrier model without introducing any further mathematical approximations. By ways of a simulated LDOS, the proposed method is compared to existing LDOS extraction methods for both positive and negative biases, and the differences between the methods are discussed.

We present a detailed Raman study of defective graphene samples containing specific types of defects. In particular, we compared ${\mathit{sp}}^3$ sites, vacancies, and substitutional Boron atoms. We find that the ratio between the $D$ and $G$ peak intensities, $I$($D$)/$I$($G$), does not depend on the geometry of the defect (within the Raman spectrometer resolution). In contrast, in the limit of low defect concentration, the ratio between the $D^{\prime }$ and $G$ peak intensities is higher for vacancies than ${\mathit{sp}}^3$ sites. By using the local activation model, we attribute this difference to the term $C$${}_{S,x}$, representing the Raman cross section of $I$($x$)/$I$($G$) associated with the distortion of the crystal lattice after defect introduction per unit of damaged area, where $x$ $=$ $D$ or $D^{\prime }$. We observed that $C_{S,D}=0$ for all the defects analyzed, while $C_{S,D^{\prime }}$ of vacancies is 2.5 times larger than $C_{S,D^{\prime }}$ of ${\mathit{sp}}^3$ sites. This makes $I$($D$)/$I$($D^{\prime }$) strongly sensitive to the nature of the defect. We also show that the exact dependence of $I$($D$)/$I$($D^{\prime }$) on the excitation energy may be affected by the nature of the defect. These results can be used to obtain further insights into the Raman scattering process (in particular for the $D^{\prime }$ peak) in order to improve our understanding and modeling of defects in graphene.

We consider the effect of quantum friction at zero absolute temperature resulting from polaritonic interactions in closely positioned two-dimensional arrays of polarizable atoms (e.g., graphene sheets) or thin dielectric sheets modeled as such arrays. The arrays move one with respect to another with a nonrelativistic velocity $v\ll c$. We confirm that quantum friction is inevitably related to material dispersion, and that such friction vanishes in nondispersive media. In addition, we consider a classical analog of the quantum friction which allows us to establish a link between the phenomena of quantum friction and classical parametric generation. In particular, we demonstrate how the quasiparticle generation rate typically obtained from the quantum Fermi golden rule can be calculated classically.

We theoretically investigate phonon dispersion in AA-stacked, AB-stacked, and twisted bilayer graphene with various rotation angles. The calculations are performed using the Born–von Karman model for the intralayer atomic interactions and the Lennard-Jones potential for the interlayer interactions. It is found that the stacking order affects the out-of-plane acoustic phonon modes the most. The difference in the phonon densities of states in the twisted bilayer graphene and in AA- or AB-stacked bilayer graphene appears in the phonon frequency range 90–110 cm${}^{-1}$. Twisting bilayer graphene leads to the emergence of different phonon branches—termed hybrid folded phonons—which originate from the mixing of phonon modes from different high-symmetry directions in the Brillouin zone. The frequencies of the hybrid folded phonons depend strongly on the rotation angle and can be used for noncontact identification of the twist angles in graphene samples. The obtained results and the tabulated frequencies of phonons in twisted bilayer graphene are important for the interpretation of experimental Raman data and in determining the thermal conductivity of these material systems.

The spin-dependent dynamical blockade was investigated in a lateral quantum dot in a magnetic field. Spin-polarized edge channels in the two-dimensional leads and the spatial distribution of Landau orbitals in the dot modulate the tunnel coupling of the quantum dot level spectrum. In a measurement of the electron shot noise we observe a pattern of super-Poissonian noise which is correlated to the spin-dependent competition between different transport channels.

We have measured temperature ($T$)- and power-dependent electron spin resonance in bulk single-wall carbon nanotubes to determine both the spin-lattice and the spin-spin relaxation times, $T_1$ and $T_2$. We observe that $T_1^{-1}$ increases linearly with $T$ from 4 K to 100 K, whereas $T_2^{-1}$ decreases by over a factor of two when $T$ is increased from 3 K to 300 K. We interpret the $T_1^{-1}\propto T$ trend as spin-lattice relaxation via interaction with conduction electrons (Korringa law) and the decreasing $T$ dependence of $T_2^{-1}$ as motional narrowing. By analyzing the latter, we find the spin hopping frequency to be 285 GHz. Last, we show that the Dysonian line shape asymmetry follows a three-dimensional variable-range hopping behavior from 3 K to 20 K; from this scaling relation, we extract a localization length of the hopping spins to be $\sim$100 nm.

We perform first-principles calculations on CaFe${}_2$As${}_2$ under hydrostatic pressure. Our total-energy calculations show that though the striped antiferromagnetic (AFM) orthorhombic (OR) phase is favored at $P=0$, a nonmagnetic collapsed tetragonal (cT) phase with diminished $c$ parameter is favored for $P>0.36$ GPa, in agreement with experiments. Rather than a mechanical instability, this is an enthalpically driven transition from the higher volume OR phase to the lower volume cT phase. A simple thermodynamic model provides an interpretation of the finite-temperature phase boundaries of the cT phase. Calculations of electronic density of states reveal pseudogaps in both OR and cT phases. Band-structure analysis provides insight into the origin of the pseudogaps while revealing the location and nature of hybridized Fe-$d$ and As-$p$ bonding orbitals.

While the bulk, stoichiometric Bi${}_2$Te${}_3$ single crystals often exhibit $p$-type metallic electrical conduction due to the Bi${}_{\mathrm{Te}}$-type antisite defects, doping by thallium (Bi${}_{2-x}$Tl${}_x$Te${}_3$, $x=0-0.30$) progressively changes the electrical conduction of single crystals from $p$ type (0 ≤ $x$ ≤ 0.08) to $n$ type (0.12 ≤ $x$ ≤ 0.30). This is observed via measurements of both the Seebeck coefficient and the Hall effect performed in the crystallographic (0001) plane in the temperature range of 2–300 K. Since any kind of substitution of Tl on the Bi or Te sublattices would result in an enhancement of the density of holes rather than its decrease, and because simple incorporation of Tl at interstitial sites or in the van der Waals gap is unlikely as it would increase the lattice parameters which is not observed in experiments, incorporation of Tl likely proceeds via the formation of TlBiTe${}_2$ fragments coexisting with the quintuple layer structure of Bi${}_2$Te${}_3$. At low levels of Tl, 0 ≤ $x$ ≤ 0.05, the temperature-dependent in-plane ($I\perp c$) electrical resistivity maintains its metallic character as the density of holes decreases. Heavier Tl content with 0.08 ≤ $x$ ≤ 0.12 drives the electrical resistivity into a prominent nonmetallic regime displaying characteristic metal-insulator transitions upon cooling to below ∼100 K. At the highest concentrations of Tl, 0.20 ≤ $x$ ≤ 0.30, the samples revert back into the metallic state with low resistivity. Thermal conductivity measurements of Bi${}_2$Te${}_3$ single crystals containing Tl, as examined by the Debye-Callaway phonon conductivity model, reveal a generally stronger point-defect scattering of phonons with the increasing Tl content. The systematic evolution of transport properties suggests that the Fermi level of Bi${}_2$Te${}_{3,}$ which initially lies in the valence band (for $x$ = 0), gradually shifts toward the top of the valence band (for 0.01 ≤ $x$ ≤ 0.05), then moves into the band gap (for 0.08 ≤ $x$ ≤ 0.12), and eventually intersects the conduction band (for 0.20 ≤ $x$ ≤ 0.30).

Silicene, which is the silicon equivalent of carbon-based graphene and shares some unique properties with graphene, has been attracting more and more attention since its successful synthesis. Using Green's function perturbation theory, many-body effects in silicene, hydrogenated silicene (silicane), fluorinated silicene (fluorosilicene), as well as armchair silicene nanoribbons (ASiNRs) are studied. Optical resonances in silicene have been aroused by excitonic effects: The $\pi \to {\pi }^{*}$ excitonic resonance at 1.23 eV is contributed by the characteristic dispersion of Dirac fermions, while the one at 3.75 eV is due to the $\sigma \to {\pi }^{*}$ transition. Hydrogenation or fluorination of silicene removes the conductivity at the Dirac point and causes band-gap opening. In addition to the remarkable self-energy effects, optical absorption properties of silicane, fluorosilicene, and ASiNRs are dominated by strong excitonic effects with formation of bound excitons with considerable binding energies.