We study the excitation spectrum of a topological insulator in contact with an $s$-wave superconductor, starting from a microscopic model, and develop an effective low-energy model for the proximity effect. In the vicinity of the Dirac cone vertex, the effective model describing the states localized at the interface is well approximated by a model of Dirac electrons experiencing superconducting $s$-wave pairing. Away from the cone vertex, the induced pairing potential develops a $p$-wave component with a magnitude sensitive to the structure of the interface. Observing the induced $s$-wave superconductivity may require tuning the chemical potential close to the Dirac point. Furthermore, we find that the proximity of the superconductor leads to a significant renormalization of the original parameters of the effective model describing the surface states of a topological insulator.

Ab initio calculations of relativistic core-level electron-energy-loss spectra and x-ray absorption spectra in periodic systems are carried out using an impurity Green’s function formalism without the need for a supercell. The approach is based on a hybrid scheme incorporating a reciprocal space calculation of the multiple-scattering equations, together with a real-space calculation of the excitation spectrum with a statically screened core-hole potential. The approach accounts for core-hole effects in deep-core spectroscopies of periodic systems while circumventing the supercell convergence issues encountered with conventional band-structure codes. The approach is implemented in an extension of the real-space Green’s function code FEFF. Illustrative calculations are presented for the $K$ edges of C (diamond) and Cu, and the $\text{N}K$ edge of GaN.

We exploit the concept of strain-induced band-structure engineering in graphene through the calculation of its electronic properties under uniaxial, shear, and combined uniaxial-shear deformations. We show that by combining shear deformations to uniaxial strains it is possible modulate the graphene energy-gap value from zero up to 0.9 eV. Interestingly enough, the use of a shear component allows for a gap opening at moderate absolute deformation, safely smaller than the graphene failure strain.

Single-site and cluster dynamical mean-field theory methods are used to estimate the response of a doped Mott insulator to a charged impurity. The effect of correlations on the Thomas-Fermi screening properties is determined. The charge density, the on-site and near-neighbor spin-spin correlations in the vicinity of the impurity are compared to those far from it. The theory is used to address the question of the effect of the density perturbation induced by the muon charge on the local response functions of a high-temperature superconductor. For reasonable values of the background dielectric constant and basic correlation strength, a muon is shown to lead to an observable perturbation of the local spin dynamics, raising questions about the interpretation of muon-spin-rotation experiments in metallic high-temperature superconductors.

The longitudinal thermopower of ultrahigh mobility two-dimensional (2D) electrons has been measured at both zero magnetic field and at high fields in the compressible metallic state at filling factor $\nu =3/2$ and the incompressible fractional quantized Hall state at $\nu =5/2$. At zero field our results demonstrate that the thermopower is dominated by electron diffusion for temperatures below about $T=150\text{mK}$. A diffusion-dominated thermopower is also observed at $\nu =3/2$ and allows us to extract an estimate of the composite fermion effective mass. At $\nu =5/2$ both the temperature and magnetic field dependence of the observed thermopower clearly signal the presence of the energy gap of this fractional quantized Hall state. We find that the thermopower in the vicinity of $\nu =5/2$ exceeds that recently predicted under the assumption that the entropy of the 2D system is dominated by non-Abelian quasiparticle exchange statistics.

Doping of nanostructured materials using a clean, efficient, and site-selective route such as ion implantation would be hugely desirable for realization of large-scale production methods. Here, ion implantation is used to create uniform impurity-atom densities which are both dose and spatially controlled within multiwalled carbon nanotubes and graphene. The technique is demonstrated for a range of dopants, including silver, representing a likely candidate for optical enhancement, and boron, which is predicted to introduce a plasmon within the visible-frequency regime. Electron energy-loss spectroscopy performed within an aberration-corrected scanning transmission electron microscope, in combination with high-angle-annular-dark-field imaging, is used to pinpoint and identify the bonding configuration of single foreign species within the matrix.

Aging in martensite, which is accompanied by a gradual change in physical properties, has been observed in most shape-memory alloys for more than half a century. However, its microscopic mechanism has remained controversial due to a lack of experiments that can probe the atomic-level processes. By using a method which combines molecular-dynamics and Monte Carlo simulations, we clarify the atomic mechanism for one of the well-observed martensitic aging effects, martensitic stabilization. We successfully reproduce the observed effects using our method. Quantitative analysis of the atomic configurations during aging reveals that martensite stabilization is not associated with a change in the average martensite structure. It involves instead a gradual change in the short-range order of point defects so that the defect short-range order tends to adopt the same “symmetry” as the crystal symmetry of the host martensite lattice. Our simulation results are consistent with the symmetry-conforming short-range order model [X. Ren and K. Otsuka, Nature (London) 389, 579 (1997)].

We consider antiferromagnets breaking both time-reversal $\left(\Theta \right)$ and a primitive-lattice translational symmetry $\left(T_{1/2}\right)$ of a crystal but preserving the combination $S=\Theta T_{1/2}$. The $S$ symmetry leads to a ${\mathbb{Z}}_2$ topological classification of insulators, separating the ordinary insulator phase from the “antiferromagnetic topological insulator” phase. This state is similar to the “strong” topological insulator with time-reversal symmetry and shares with it such properties as a quantized magnetoelectric effect. However, for certain surfaces the surface states are intrinsically gapped with a half-quantum Hall effect $\left[{\sigma }_{xy}=e^2/\left(2h\right)\right]$, which may aid experimental confirmation of $\theta =\pi$ quantized magnetoelectric coupling. Step edges on such a surface support gapless, chiral quantum wires. In closing we discuss GdBiPt as a possible example of this topological class.

We have investigated the spin preparation efficiency by optical pumping of individual Mn atoms embedded in CdTe/ZnTe quantum dots. Monitoring the time dependence of the intensity of the fluorescence during the resonant optical pumping process in individual quantum dots allows to directly probe the dynamics of the initialization of the Mn spin. This technique presents the convenience of including preparation and readout of the Mn spin in the same step. Our measurements demonstrate that Mn spin initialization, at zero magnetic field, can reach an efficiency of 75% and occurs in the tens of nanoseconds range when a laser resonantly drives at saturation one of the quantum-dot transition. We observe that the efficiency of optical pumping changes from dot-to-dot and is affected by a magnetic field of a few tens of millitesla applied in Voigt or Faraday configuration. This is attributed to the local strain distribution at the Mn location which predominantly determines the dynamics of the Mn spin under weak magnetic field. The spectral distribution of the spin-flip-scattered photons from quantum dots presenting a weak optical pumping efficiency reveals a significant spin relaxation for the exciton split in the exchange field of the Mn spin.

The influence of magnetic layer thickness on the interlayer coupling through a tunnel barrier is investigated in Co/MgO/Co structures with perpendicular anisotropy. Despite the rather large MgO thickness, a clear antiferromagnetic coupling is observed. It oscillates with increasing magnetic layer thickness, in agreement with theories on indirect coupling through insulating spacers. Although the average interlayer coupling strength decreases with increasing annealing temperature, constant period and amplitude of these oscillations are observed for all annealing temperatures.

In situ real-time electron microscope observations of metals irradiated with ultrahigh-energy electrons or energetic ions show that the dynamics of microstructural evolution in these materials is strongly influenced by long-range elastic interactions between mobile nanoscale radiation defects. Treating long-range interactions is also necessary for modeling microstructures formed in ex situ high-dose-rate ion-beam irradiation experiments, and for interpolating the ion-beam irradiation data to the low-dose-rate limit characterizing the neutron irradiation environments of fission or fusion power plants. We show that simulations, performed using an algorithm where nanoscale radiation defects are treated as interacting Langevin particles, are able to match and explain the real-time dynamics of nanodefects observed in in situ electron microscope experiments.

Polycrystalline samples of ${\text{Sr}}_2{\text{Mn}}_2{\text{CuAs}}_2{\text{O}}_2$, ${\text{Sr}}_2{\text{Mn}}_3{\text{As}}_2{\text{O}}_2$, and ${\text{Sr}}_2{\text{Zn}}_2{\text{MnAs}}_2{\text{O}}_2$ were synthesized. Their temperature- and applied magnetic field-dependent structural, transport, thermal, and magnetic properties were characterized by means of x-ray and neutron diffraction, electrical resistivity $\rho$, heat capacity, magnetization, and magnetic susceptibility measurements. These compounds have a body-centered-tetragonal crystal structure (space group $I4/mmm$) that consists of $M{\text{O}}_2$ ($M=\text{Zn}$ and/or Mn) oxide layers similar to the ${\text{CuO}}_2$ layers in high superconducting transition temperature $T_c$ cuprate superconductors, and intermetallic $M\text{As}$ ($M=\text{Cu}$ and/or Mn) layers similar to the FeAs layers in high-$T_c$ pnictides. These two types of layers alternate along the crystallographic $c$ axis and are separated by Sr atoms. The site occupancies of Mn, Cu, and Zn were studied using Rietveld refinements of x-ray and neutron powder diffraction data. The temperature dependences of $\rho$ suggest metallic character for ${\text{Sr}}_2{\text{Mn}}_2{\text{CuAs}}_2{\text{O}}_2$ and semiconducting character for ${\text{Sr}}_2{\text{Mn}}_3{\text{As}}_2{\text{O}}_2$ and ${\text{Sr}}_2{\text{Zn}}_2{\text{MnAs}}_2{\text{O}}_2$. ${\text{Sr}}_2{\text{Mn}}_2{\text{CuAs}}_2{\text{O}}_2$ is inferred to be a ferrimagnet with a Curie temperature $T_{\text{C}}=95\left(1\right)\text{K}$. Remarkably, we find that the magnetic ground-state structure changes from a $\text{G}$-type antiferromagnetic structure in ${\text{Sr}}_2{\text{Mn}}_3{\text{As}}_2{\text{O}}_2$ to an $\text{A}$-type ferrimagnetic structure in ${\text{Sr}}_2{\text{Mn}}_2{\text{CuAs}}_2{\text{O}}_2$ in which the Mn ions in each layer are ferromagnetically aligned but are antiferromagnetically aligned between layers.

Since the lattices of $ABA$-stacked graphene multilayers with an even number of layers, as well as that of monolayer graphene, satisfy spatial-inversion symmetry, their electronic bands must be spin degenerate in the presence of time-inversion symmetry. In intrinsic monolayer and bilayer graphene, when symmetry is not broken by external fields, the only spin-orbit coupling present at low energy near the corner of the Brillouin zone is the Kane-Mele term, that opens a bulk energy gap but does not break the spin degeneracy of the energy bands [C. L. Kane and E. J. Mele, Phys. Rev. Lett. 95, 226801 (2005)]. However, spin splitting is allowed in multilayers with an odd number of layers $\left(\ge 3\right)$ because their lattices do not satisfy spatial-inversion symmetry. We show that, in trilayer graphene, in addition to the Kane-Mele term, there is a second type of intrinsic spin-orbit coupling present at low energy near the corner of the Brillouin zone. It introduces a Zeeman-type spin splitting of the energy bands at each valley with an opposite sign of the effective magnetic field in the two valleys. We estimate the magnitude of the effective field to be $\sim 2\text{T}$.

The layered ternary $sp$ conductor NaAlSi, possessing the iron-pnictide “111” crystal structure, superconducts at 7 K. Using density-functional methods, we show that this compound is an intrinsic (self-doped) low-carrier-density semimetal with a number of unusual features. Covalent Al-Si valence bands provide the holes, and free-electronlike $\text{Al}3s$ bands, which propagate in the channel between the neighboring Si layers, dip just below the Fermi level to create the electron carriers. The Fermi level (and therefore the superconducting carriers) lies in a narrow and sharp peak within a pseudogap in the density of states. The small peak arises from valence bands which are nearly of pure Si, quasi-two-dimensional, flat, and coupled to Al conduction bands. Isostructural NaAlGe, which is not superconducting above 1.6 K, has almost exactly the same band structure except for one missing piece of small Fermi surface. Certain deformation potentials induced by Si and Na displacements along the $c$ axis are calculated and discussed. It seems likely that the mechanism of pairing is related to that of several other lightly doped two-dimensional nonmagnetic semiconductors (TiNCl, ZrNCl, HfNCl), which is not well understood but apparently not of phonon origin.

One-dimensional nanobeam photonic crystal cavities are fabricated in silicon dioxide with silicon nanocrystals. Quality factors of over $9\times {10}^3$ are found in experiment, matching theoretical predictions, with mode volumes of $1.5{\left(\lambda /n\right)}^3$. Photoluminescence from the cavity modes is observed in the visible wavelength range 600–820 nm. Studies of the lossy characteristics of the cavities are conducted at varying temperatures and pump powers. Free carrier absorption effects are found to be significant at pump powers as low as a few hundred nanowatts.

Mean-field glassy systems have a complicated energy landscape and an enormous number of different Gibbs states. In this paper, we introduce a generalization of the cavity method in order to describe the adiabatic evolution of these glassy Gibbs states as an external parameter, such as the temperature, is tuned. We give a general derivation of the method and describe in details the solution of the resulting equations for the fully connected $p$-spin model, the XOR-satisfiability (SAT) problem and the antiferromagnetic Potts glass (coloring problem). As direct results of the states following method we present a study of very slow Monte Carlo annealings, the demonstration of the presence of temperature chaos in these systems and the identification of an easy/hard transition for simulated annealing in constraint optimization problems. We also discuss the relation between our approach and the Franz-Parisi potential, as well as with the reconstruction problem on trees in computer science. A mapping between the states following method and the physics on the Nishimori line is also presented.

Using x-ray magnetic circular dichroism (XMCD), we determine the element-specific character and polarization of unoccupied states near the Fermi level in (Ga,Mn)As and (In,Ga,Mn)As thin films. The XMCD at the $\text{As}K$ absorption edge consists of a single peak located on the low-energy side of the edge, which increases with the concentration of ferromagnetic Mn moments. The XMCD at the $\text{Mn}K$ edge is more detailed and is strongly concentration dependent, which is interpreted as a signature of hole localization for low Mn doping. The results indicate a markedly different character of the polarized holes in low-doped insulating and high-doped metallic films, with a transfer of the hole orbital magnetic moment from Mn to As sites on crossing the metal-insulator transition.

We study the disorder dependence of the phase coherence time of quasi-one-dimensional wires and two-dimensional (2D) Hall bars fabricated from a high mobility GaAs/AlGaAs heterostructure. Using an original ion implantation technique, we can tune the intrinsic disorder felt by the 2D electron gas and continuously vary the system from the semiballistic regime to the localized one. In the diffusive regime, the phase coherence time follows a power law as a function of diffusion coefficient as expected in the Fermi-liquid theory, without any sign of low-temperature saturation. Surprisingly, in the semiballistic regime, it becomes independent of the diffusion coefficient. In the strongly localized regime we find a diverging phase coherence time with decreasing temperature, however, with a smaller exponent compared to the weakly localized regime.

Commensurate-incommensurate transitions are ubiquitous in physics and are often accompanied by intriguing phenomena. In few-layer graphene (FLG) systems, commensurability between honeycomb lattices on adjacent layers is regulated by their relative orientation angle $\theta$, which is in turn dependent on sample preparation procedures. Because incommensurability suppresses interlayer hybridization, it is often claimed that graphene layers can be electrically isolated by a relative twist, even though they are vertically separated by a fraction of a nanometer. We present a theory of interlayer transport in FLG systems which reveals a richer picture in which the specific conductance depends sensitively on $\theta$, single-layer Bloch-state lifetime, in-plane magnetic field, and bias voltage. We find that linear and differential conductances are generally large and negative near commensurate values of $\theta$, and small and positive otherwise. We show that accounting for interlayer coupling may be essential for describing transport in FLG despite its physically insignificant effect on the band structure of the system.

We consider an isotropic Fermi liquid in two dimensions near the $n=2$ Pomeranchuk instability in the charge channel. The order parameter is a quadrupolar stress tensor with two bosonic shear modes with polarizations longitudinal and transverse to the quadrupolar momentum tensor. Longitudinal and transverse bosonic modes are characterized by dynamical exponents $z_{\parallel }=3$ and $z_{\perp }=2$, respectively. Previous studies have found that such a system exhibits multiscale quantum criticality with two different energy scales $\omega \sim {\xi }^{-z_{\parallel ,\perp }}$, where $\xi$ is the correlation length. We study the impact of the multiple energy scales on the electron Green’s function. The interaction with the critical $z_{\parallel }=3$ mode is known to give rise to a local self-energy that develops a non-Fermi-liquid form, $\Sigma \left(\omega \right)\sim {\omega }^{2/3}$ for frequencies larger than the energy scale $\omega \sim {\xi }^{-3}$. We find that the exchange of transverse $z_{\perp }=2$ fluctuations leads to logarithmically singular renormalizations of the quasiparticle residue $Z$ and the vertex $\Gamma$. We derive and solve renormalization-group equations for the flow of $Z$ and $\Gamma$, and show that the system develops an anomalous dimension at the nematic quantum critical point (QCP). As a result, the spectral function at a fixed $\omega$ and varying $k$ has a non-Lorentzian form. Away from the QCP, we find that the flow of $Z$ is cut at the energy scale ${\omega }_{\text{FL}}\propto {\xi }^{-1}$, associated with the $z=1$ dynamics of electrons. The $z_{\perp }=2$ energy scale, $\omega \sim {\xi }^{-2}$, affects the flow of $Z$ only if one includes into the theory self-interaction of transverse fluctuations.