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Materials irradiated with multiple femtosecond laser pulses in sub-ablation conditions are observed to develop various types of self-assembled morphologies that range from nano-ripples to periodic micro-grooves and quasi-periodic micro-spikes. Here, we present a physical scenario that couples electrodynamics, describing surface plasmon excitation, with hydrodynamics, describing Marangoni convection, to elucidate this important sub-ablation regime of light matter interaction in which matter is being modified, however, the underlying process is not yet fully understood. The proposed physical mechanism could be generally applicable to practically any conductive material structured by ultrashort laser pulses, therefore it can be useful for the interpretation of further critical aspects of light matter interaction., Comment: arXiv admin note: substantial text overlap with arXiv:1109.2568

Angle-resolved photoemission spectroscopy (ARPES) is one of most powerful techniques to unravel the electronic properties of layered materials and, in recent decades, it has lead to significant progress in the understanding of the band structures of cuprates, pnictides, and other materials of current interest. On the other hand, its application to Mott-Hubbard insulating materials where a Fermi surface is absent has been more limited. Here we show that in these latter materials, where electron spins are localized, ARPES may provide significant information on the spin correlations which can be complementary to the one derived from neutron-scattering experiments. Sr2Cu1-xZnxO2Cl2, a prototype of a diluted spin S=1/2 antiferromagnet (AF) on a square lattice, was chosen as a test case and a direct correspondence between the amplitude of the spectral weight beyond the AF zone boundary derived from ARPES and the spin-correlation length ξ estimated from Cl35 NMR was established. It was found that even for correlation lengths of a few lattice constants, a significant spectral weight in the backbended band is present which depends markedly on ξ. Moreover, the temperature dependence of that spectral weight is found to scale with the x-dependent spin stiffness. These findings prove that the ARPES technique is very sensitive to short-range correlations and its relevance in the understanding of the electronic correlations in cuprates is discussed.

The dynamical competition between the chemical mixing forced during energetic particle irradiation and thermally activated decomposition can lead to the stabilization of self-organized steady states in alloy systems comprised of immiscible elements. Continuum modeling and atomistic simulations predicted the stabilization of steady-state nanoscale compositional patterns for a well-defined range of ballistic mixing frequencies normalized by the irradiation-enhanced thermal atomic jump frequencies. Irradiation-induced compositional patterning has now indeed been observed experimentally, but a quantitative comparison has been lacking because models and simulations have relied on a simplified treatment with a fixed point-defect concentration. We overcome here this limitation by using a kinetic Monte Carlo code that includes the production, recombination, and elimination of point defects at sinks, as well as the chemical mixing forced by ballistic replacements. By varying the sink density and its efficiency, the temperature range of stabilization of steady-state compositional patterns is investigated in model binary alloys for point-defect regimes dominated by either recombination or elimination on sinks. We find that in the sink regime, compositional patterning can be extended to remarkably high temperatures. The results are discussed by analyzing the relative diffusivities of $A$ and $B$ species, and their dependencies on temperature.

Angle-resolved photoemission spectroscopy experiments reveal that the valence band splitting at $K$ point in transition metal dichalcogenides is primarily due to the spin-orbit coupling rather than inter-layer interactions. The authors have achieved convincing agreement between experimental results and theoretical calculations over a wide energy range.

We study the biexciton formation process in a single CdTe/ZnTe quantum dot. Consistently with previous studies, we find subquadratic dependence of the biexciton photoluminescence intensity on the excitation power. We quantitatively explain this dependence in terms of the interplay between two alternative biexciton formation mechanisms, including the formation from a long-lived dark exciton residing in the quantum dot. This mechanism is demonstrated to be a dominant one for a wide range of excitation intensities. Our study is complemented by determination of a characteristic biexciton formation time, which is shown to be governed by the spin relaxation dynamics of an excited electron pair.

Simulation evidence of an incipient triple point is reported for xenon submonolayers adsorbed on the (111) surface of platinum. This is in stark contrast to the "normal" triple point found in simulations and experiments for xenon on the basal plane surface of graphite. The motions of the atoms in the surface plane are treated with standard 2D "NVE" molecular dynamics simulations using modern interactions. The simulation evidence strongly suggests an incipient triple point in the 120-150 K range for adsorption on the Pt (111) surface while the adsorption on graphite shows a normal triple point at about 100 K., 5 pages, 4 figures

We use quasiclassical methods of superconductivity to study the superconducting proximity effect from a topological $p$-wave superconductor into a disordered one-dimensional metallic wire. We demonstrate that the corresponding Eilenberger equations with disorder reduce to a closed non-linear equation for the superconducting component of the matrix Green's function. Remarkably, this equation is formally equivalent to a classical mechanical system (i.e., Newton's equations), with the Green function corresponding to a coordinate of a fictitious particle and the coordinate along the wire corresponding to time. This mapping allows to obtain exact solutions in the disordered nanowire in terms of elliptic functions. A surprising result that comes out of this solution is that the $p$-wave superconductivity proximity-induced into the disordered metal remains long-range, decaying as slowly as the conventional $s$-wave superconductivity. It is also shown that impurity scattering leads to the appearance of a zero-energy peak., 6 pages, 4 figures

The presence of a ${\mathrm{SiO}}_{2}$ layer on Si nanowires (SiNWs) has been found through molecular dynamics simulation to reduce their thermal conductivity $(\ensuremath{\kappa})$, with $\ensuremath{\kappa}$ approaching the amorphous limit of Si as the oxide layer thickness is increased. Through analysis of the phonon energy dispersion and vibrational density of states (VDOS) spectrum, this decrease in $\ensuremath{\kappa}$ was attributed to dispersionless vibrational states that appear in the low energy range below 4 THz as a result of the lattice vibration of Si atoms near the ${\mathrm{SiO}}_{2}/\mathrm{Si}$ interface. The ${\mathrm{SiO}}_{2}$ layer also induced a low-frequency tail in the VDOS spectrum, the length of which was more closely correlated to the reduction in $\ensuremath{\kappa}$ than the frequency-integrated value of the VDOS spectrum. These findings provide a more refined explanation for the decrease in $\ensuremath{\kappa}$ than has been previously observed, and contribute to providing a greater understanding of the anomalistic vibration near the interface that is critical to determining the heat conductivity in nanoscale materials.

The residual low-angle grain boundary (GB) network is still the most important current-limiting mechanism operating in biaxially textured rare-earth barium-copper-oxide (REBCO) coated conductors. While Ca doping is well established to improve supercurrent flow across low-angle GBs in weak fields at high temperatures, Ca doping also depresses ${T}_{c}$, making it so far impractical for high-temperature applications of REBCO coated conductors. On the other hand, high-field-magnet applications of REBCO require low temperatures. Here we systematically evaluate the effectiveness of Ca doping in improving the GB transparency, ${r}^{\mathrm{GB}}={J}_{c}^{\mathrm{GB}}/\phantom{\rule{0.0pt}{0ex}}{J}_{c}^{\mathrm{grain}}$, of low-angle $\mathrm{Y}{\mathrm{b}}_{1\ensuremath{-}x}\mathrm{C}{\mathrm{a}}_{x}\mathrm{BaCuO}$ [001] tilt bicrystal films down to 10 K and with magnetic fields perpendicular and parallel to the film surfaces, while varying the Ca and oxygen doping level. Using low-temperature scanning laser microscopy and magneto-optical imaging, we found ${r}^{\mathrm{GB}}$ to strongly depend on the angle between magnetic field and the GB plane and clearly identified regimes in which ${J}_{c}^{\mathrm{GB}}$ can exceed ${J}_{c}^{\mathrm{grain}}\phantom{\rule{0.28em}{0ex}}({r}^{\mathrm{GB}}g1)$ where the GB pinning is optimized by the field being parallel to the GB dislocations. However, even in this favorable situation, we found that ${r}^{\mathrm{GB}}$ became much smaller at lower temperatures. Calculations of the GB Ca segregation profile predict that the high-${J}_{c}$ channels between the GB dislocation cores are almost Ca free. It may be therefore that the positive effects of Ca doping seen by many authors near ${T}_{c}$ are partly a consequence of the higher ${T}_{c}$ of these Ca-free channels.

We report that the discovery of stable two-dimensional, earth-abundant, semiconducting materials is of great interest and may impact future electronic technologies. By combining global structural prediction and first-principles calculations, we have theoretically discovered several previously unknown semiconducting silicon phosphides (SixPy) monolayers, which could be formed stably at the stoichiometries of y/x ≥1. Unexpectedly, some of these compounds, i.e., P-6m2 Si1P1 and Pm Si1P2, have comparable or even lower formation enthalpies than their previously known bulk allotropes. The band gaps (Eg) of SixPy compounds can be dramatically tuned in an extremely wide range (0< Eg < 3 eV) by simply changing the number of layers or applying an in-plane strain. Furthermore, we find that carrier doping can drive the ground state of C2/m Si1P3 from a nonmagnetic state into a robust half-metallic spin-polarized state, originating from its unique valence band structure, which can extend the use of Si-related compounds for spintronics.

We show that atomistic first-principles calculations based on real-time propagation within time-dependent density functional theory are capable of accurately describing electronic stopping of light projectile atoms in metal hosts over a wide range of projectile velocities. In particular, we employ a plane-wave pseudopotential scheme to solve time-dependent Kohn-Sham equations for representative systems of H and He projectiles in crystalline aluminum. This approach to simulate nonadiabatic electron-ion interaction provides an accurate framework that allows for quantitative comparison with experiment without introducing ad hoc parameters such as effective charges, or assumptions about the dielectric function. Our work clearly shows that this atomistic first-principles description of electronic stopping is able to disentangle contributions due to tightly bound semicore electrons and geometric aspects of the stopping geometry (channeling versus off-channeling) in a wide range of projectile velocities.

The combination of high-pressure ferromagnetic resonance (FMR) and an ab-initio scheme suitable for calculation of spin-orbit mediated anisotropic exchange interactions in molecular materials provides insights into the role of spin-orbit coupling (SOC) in a Se-based organic ferromagnet. FMR measurements reveal a continuous increase in the magnetic anisotropy with increasing pressure (up to 2.2 GPa), in excellent agreement with calculations based on the known pressure dependence of the structure. The large value of anisotropic exchange terms in this heavy atom organic ferromagnet emphasizes the important role of SOC in a wide range of organics where this effect is usually considered to be small.

Dispersionless (flat) electronic bands are investigated regarding their conductance properties. Due to "caging" of carriers these bands are usually insulating at partial filling, at least on the non-interacting level. Considering the specific example of a $\mathcal{T}_3$--lattice we study long-range Coulomb interactions. A non-trivial dependence of the conductivity on flat band filling is obtained, exhibiting an infinite number of zeros. Near these zeros, the conductivity rises linearly with carrier density. At densities half way in between adjacent conductivity-zeros, strongly enhanced conductivity is predicted, accompanying a solid-solid phase transition.

International audience; A bubble meta-screen, i.e. a single layer of gas inclusions in a soft solid, can be modeled as an acoustic open resonator, whose behavior is well captured by a simple analytical expression. We show that by tuning the parameters of the meta-screen, acoustic super-absorption can be achieved over a broad frequency range, which is confirmed by finite element simulations and experiments. Bubble meta-screens can thus be used as ultra thin coatings for turning acoustic reflectors into perfect absorbers.

We survey polaron formation in the BLF-SSH model using acoustic phonons in the adiabatic limit. Multiple different numerical optimization routines and strong coupling analytical calculations are used to find a robust ground state energy for a wide range of coupling strengths. The electronic configuration and accompanying ionic distortions of the polaron were determined, as well as a non-zero critical coupling strength for polaron formation in two and three dimensions., 9 pages, 6 figures

We develop a mean-field theory of the metallic phase near the many-body localization (MBL) transition, using the observation that a system near the MBL transition should become an increasingly slow heat bath for its constituent parts. As a first step, we consider the properties of a many-body localized system coupled to a generic ergodic bath whose characteristic dynamical timescales are much slower than those of the system. As we discuss, a wide range of experimentally relevant systems fall into this class; we argue that relaxation in these systems is dominated by collective many-particle rearrangements, and compute the associated timescales and spectral broadening. We then use the observation that the self-consistent environment of any region in a nearly localized metal can itself be modeled as a slowly fluctuating bath to outline a self-consistent mean-field description of the nearly localized metal and the localization transition. In the nearly localized regime, the spectra of local operators are highly inhomogeneous and the typical local spectral linewidth is narrow. The local spectral linewidth is proportional to the DC conductivity, which is small in the nearly localized regime. This typical linewidth and the DC conductivity go to zero as the localized phase is approached, with a scaling that we calculate, and which appears to be in good agreement with recent experimental results., 15 pages, 8 figures; major revisions, including a revised phase diagram

In this paper, we calculate the pairing interaction and the k dependence of the gap function associated with the nematic charge fluctuations of a CuO2 model.We find that the nematic pairing interaction is attractive for small momentum transfer and that it gives rise to d-wave pairing. Finally, as the doping p approaches a quantum critical point, the strength of this pairing increases and higher d-wave harmonics contribute to the k dependence of the superconducting gap function, reflecting the longer range nature of the nematic fluctuations.

We present a density-functional-theory analysis of crystalline and amorphous Zn- and Sn-based oxide systems which focuses on the electronic defect states within the band gap. A comparison of these electronic levels reveals that the hybrid functionals PBE0, HSE06, or B3LYP agree with a self-interaction corrected (SIC) local-density-approximation functional on occupied defect levels when similar treatments of the self-interaction are considered. However, for unoccupied levels, the hybrid functionals and the SIC approach lead to very different predictions. We show that a prerequisite for the determination of the energetic position of subgap states in these oxides is that a functional needs to predict correctly the electronic band structure over a wide energy range and not just close to the band gap. We conclude that for accurate defect levels, an adequate treatment of the self-interaction problem is required especially in the presence of nearby metal-metal interactions.

We utilize ultrafast optical spectroscopy to study the quasiparticle relaxation in stoichiometric LiFeAs crystals. According to our temperature-dependent studies, we have observed three electronic phases in LiFeAs. Below the superconducting (SC) temperature ${T}_{c}$ (=15 K), the relaxation time of quasiparticles due to the SC gaps is far longer than 50 ps in our experimental conditions. In addition to SC gaps, we have also found a gaplike feature in the SC state. This gaplike feature is evident up to 40 K, which is above the SC temperature. The quasiparticle relaxation time due to this gaplike feature is in the range of 1--2 ps. We suggest the gaplike feature in LiFeAs is induced by magnetic fluctuations.

We performed positron annihilation spectroscopy measurements on uranium dioxide irradiated with 45 MeV $\ensuremath{\alpha}$ particles. The positron lifetime was measured as a function of the temperature in the 15--300 K range. The experimental results were combined with electronic structure calculations of positron lifetimes of vacancies and vacancy clusters in ${\mathrm{UO}}_{2}$. Neutral and charged defects consisting of from one to six vacancies were studied computationally using the $\mathrm{DFT}\phantom{\rule{0.16em}{0ex}}+\phantom{\rule{0.16em}{0ex}}U$ method to take into account strong correlations between the $5f$ electrons of uranium. The two-component density functional theory with two different fully self-consistent schemes was used to calculate the positron lifetimes. All defects were relaxed taking into account the forces due to the creation of defects and the positron localized in the vacancy. The interpretation of the experimental observations in the light of the DFT + $U$ results and the positron trapping model indicates that neutral ${\mathrm{V}}_{\text{U}}\phantom{\rule{0.16em}{0ex}}+\phantom{\rule{0.16em}{0ex}}2{\mathrm{V}}_{\text{O}}$ trivacancies (bound Schottky defects) are the predominant defects detected in the 45 MeV $\ensuremath{\alpha}$ irradiated ${\mathrm{UO}}_{2}$ samples. Our results show that the coupling of a precise experimental work and calculations using carefully chosen assumptions is an effective method to bring further insight into the subject of irradiation induced defects in ${\mathrm{UO}}_{2}$.

We report on nonlinear transport measurements in a GaAs/AlGaAs quantum well exhibiting a colossal negative magnetoresistance effect. Under applied dc bias, the magnetoresistance becomes nonmonotonic, exhibiting distinct extrema that move to higher magnetic fields with increasing current. In the range of magnetic fields corresponding to the resistivity minimum at zero bias, the resistivity increases linearly with current and the rate of this increase scales with the inverse magnetic field. The latter observation is consistent with the theory, proposed more than 35 years ago, considering classical memory effects in the presence of strong, dilute scatterers.

We consider theoretically the dynamics of electric currents optically injected in carbon nanotubes. Although the plasma oscillations are not seen in these systems, the main effect on the carrier's motion is due to strongly nonuniform space-charge Coulomb forces produced by time-dependent separation of injected electron and hole densities. We calculate evolution of the dipole moment characterizing the time- and coordinate-dependent charge density distributions and analyze different regimes of the dynamics. The developed time-dependent dipole moment leads to a dipole radiation in the THz frequency range for typical parameters of injected currents., 7 pages, 6 figures

We calculate the multiphonon transition rate from the excited state of a single-occupied two-level quantum dot. The electron interacts with certain optical phonon modes which in turn transfer the transition energy to the bath of other phonon modes decoupled from the electron states. Our theory covers the previously unexplored range of transition energies several times larger than the optical phonon energy and systematically studies the role of quantum interference of the processes involving different virtual polaron states.

Ge-based spin-photodiodes have been employed to investigate the spectral dependence of optical spin orientation in germanium, in the range 400--1550 nm. We found the expected maximum in the spin polarization of photocarriers for excitation at the direct gap in $\ensuremath{\Gamma}$ (1550 nm) and a second sizable peak due to photogeneration in the $L$ valleys (530 nm). Data suggest distinct spin depolarization mechanisms for excitation at $\ensuremath{\Gamma}$ and $L$, with shorter spin relaxation times whether the $X$ point is involved. These devices can be used as integrated photon-helicity detectors over a wide spectral range.

We put forward a theory of the weak localization in two dimensional graphene layers which explains experimentally observable transition between positive and negative magnetoresistance. Calculations are performed for the whole range of classically weak magnetic field with account on intervalley transitions. Contribution to the quantum correction which stems from closed trajectories with few scatterers is carefully taken into account. We show that intervalley transitions lead not only to the transition from weak antilocalization to the weak localization, but also to the non-monotonous dependence of the conductivity on the magnetic field.

Intermediate-band (IB) photovoltaic materials are designed to absorb a wider range of the solar spectrum by dividing the gap with a set of bands that plays a mediating role in two-step absorption processes. Thus far, the conventional model of IB absorbers has been focused on a single half-filled IB. We show that a multiple-IB picture with filled and empty bands provides a more convincing explanation of the experimental findings in some cases, and it should be considered as a possible scenario in understanding and engineering IB solar cells. Toward that end, we report the formation of two sets of IBs in a Ti-substituted ${\mathrm{CuGaS}}_{2}$ compound. Using hybrid functional calculations within the density functional theory framework, we show that the lower IBs are occupied and mainly derive from the Ti $3{d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ states, while the higher ones originate from the Ti $3{d}_{{z}^{2}}$ states and are unoccupied. The positions of the IBs within the gap are found to depend weakly on the dopant concentration and the relative distances of the dopant ions. In addition, for a more pertinent comparison to real materials, we present a practical way to combine and analyze the densities of states resulting from the energetically most probable microscopic structures. This multi-intermediate-band picture provides a platform to comprehend unexplained features of the recent experimental study on the material [X. Lv et al., Solar Energy 103, 480 (2014)].

To explore the doping dependence of the recently discovered charge-density-wave (CDW) order in YBa2Cu3Oy, we present a bulk-sensitive high-energy x-ray study for several oxygen concentrations, including strongly underdoped YBa2Cu3O6.44. Combined with previous data around the so-called 1/8 doping, we show that bulk CDW order exists at least for hole concentrations (p) in the CuO2 planes of 0.078 less than or similar to p less than or similar to 0.132. This implies that CDW order exists in close vicinity to the quantum critical point for spin-density-wave (SDW) order. In contrast to the pseudogap temperature T*, the onset temperature of CDW order decreases with underdoping to T-CDW similar to 90 K in YBa2Cu3O6.44. Together with a weakened order parameter this suggests a competition between CDW and SDW orders. In addition, the CDW order in YBa2Cu3O6.44 shows the same type of competition with superconductivity as a function of temperature and magnetic field as samples closer to p = 1/8. At low p the CDW incommensurability continues the previously reported linear increasing trend with underdoping. In the entire doping range the in-plane correlation length of the CDW order in b axis direction depends only very weakly on the hole concentration, and appears independent of the type and correlation length of the oxygen-chain order. The onset temperature of the CDW order is remarkably close to a temperature T-dagger that marks the maximum of 1/(T1T) in planar Cu-63 NQR/NMR experiments, potentially indicating a response of the spin dynamics to the formation of the CDW. Our discussion of these findings includes a detailed comparison to the charge stripe order in La2-xBaxCuO4.

Using resonant x-ray scattering, this group maps the evolution of charge density wave correlations in a series of YBa${}_{2}$Cu${}_{3}$O${}_{6+x}$ single crystals at a wide range of doping levels. Their data add important pieces to the cuprate phase diagram puzzle.

We present a flexible model for a generic straight transport barrier in graphene in the specular limit. The specular limit applies for scattering of carriers sufficiently close to the Fermi level in three quarters of the transport barriers. Using the Lippmann-Schwinger equation, we obtain the wave function, from which we derive the reflection and transmission probabilities as a function of the angle of incidence. The results, which are compared to those from exact multichannel tight-binding quantum transport calculations, show that some barriers exhibit a broader absence of reflection across a wide range of incident angles, whereas other barriers are highly reflective. The power of our model is that it could be used even when the precise atomistic detail of the transport barriers is unknown.

We report direct observation of the development of long-range spatial intensity correlations and the growth of intensity fluctuations inside the random media. We fabricated quasi-two-dimensional disordered photonic structures and probed the light transport from a third dimension. Good agreements between experiment and theory are obtained. We were able to manipulate the long-range intensity correlations and intensity fluctuations inside the disordered waveguides by simply varying the waveguide geometry., Comment: 5 figures

In Chap. 4, we noted that the emergent incommensurate (IC) short-range antiferromagnetic (AF) order near optimal superconductivity of Ba(Fe\(_{1-x}\)Co\(_{x}\))\(_2\)As\(_{2}\) heavily modifies the electronic properties of this doping region and preempts the purported quantum critical point (QCP). Therefore, it is of particular interest to investigate the nature of the IC AF order.

Superconducting Pr$_2$CuO$_x$, $x\simeq 4$ (PCO) films with $T^\prime$ structure and a $T_c$ of 27 K have been investigated by various optical methods in a wide frequency (7 - 55000 cm$^{-1}$) and temperature (2 to 300 K) range. The optical spectra do not reveal any indication of a normal-state gap formation. A Drude-like peak centered at zero frequency dominates the optical conductivity below 150 K. At higher temperatures, it shifts to finite frequencies. The detailed analysis of the low-frequency conductivity reveals that the Drude peak and a far-infrared (FIR) peak centered at about 300 cm$^{-1}$ persist at all temperatures. The FIR-peak spectral weight is found to grow at the expense of the Drude spectral weight with increasing temperature. The temperature dependence of the penetration depth follows a behavior typical for $d$-wave superconductors. The absolute value of the penetration depth for zero temperature is 1.6 $\mu$m, indicating a rather low density of the superconducting condensate.

Motivated by the novel electronic behaviors seen in transition metal oxides, we look for physical insight into disordered, strongly-correlated systems by exploring the atomic limit. In recent work, the atomic limit has provided a useful reference point in systems with strong {\em local} interactions. For comparison with experiments, the exploration of {\em non}local interactions is of interest. In the atomic limit, both the case of on-site interactions alone and the case of infinite-range ($1/r$) interactions are well understood; however, not so the intervening possibilities. Here we study the atomic limit of the extended Anderson-Hubbard model using classical Monte Carlo to calculate the single-particle density of states. We show that the combination of nearest-neighbor interactions and site disorder produce a zero-bias anomaly caused by residual charge ordering, and the addition of on-site interactions has a non-monotonic effect on the depth of this zero-bias anomaly. A key conclusion is that the form of the density of states in this classical system strongly resembles density of states results obtained for the full extended Anderson-Hubbard model when $U, 8 pages, 6 figures

We report in-plane thermoelectric power measurements on single crystals of (Ba1−xKx)Fe2As2 (0.44 x 1). We observe a minimum in the S|T =const versus x at x ∼ 0.55 that can be associated with the change in the topology of the Fermi surface, a Lifshitz transition, related to the electron pockets at the center of M point crossing the Fermi level. This feature is clearly observable below ∼75 K. Thermoelectric power also shows a change in the x ∼ 0.8–0.9 range, where the maximum in the thermoelectric power collapses into a plateau. This Lifshitz transition is most likely related to the reconstruction of the Fermi surface associated with the transformation of the hole pockets at the M point into four blades as observed by ARPES measurements.

After an initial transient period, the conduction regime in a 1D superconducting wire that carries a fixed current is either normal or periodic or stationary. The phase diagram for these possibilities was studied in Phys. Rev. Lett. {\bf 99}, 167003 (2007) for particular values of the length and the material parameters. We have extended this study to arbitrary length and to a range of material parameters that includes realistic values. Variation of the length leads to scaling laws for the phase diagram. Variation of the material parameters leads to new qualitative features and new phases, including a parameter region in which all three regimes are possible.

Noncontact atomic force microscopy (NC-AFM) features the measurement of forces with highest spatial resolution and sensitivity, resolving forces of the order of pico-Newtons with submolecular resolution. However, the measured total force is a mixture composed of various interactions. While some interactions such as electrostatic or magnetic forces can be excluded by a careful design of the experiment, the subtraction of van der Waals forces, which mainly originate from London dispersion interactions between the macroscopic tip shank and the bulk sample, remains a challenge. We present the determination of the inherently present van der Waals forces in total interaction force data from fitting a suitable model, allowing for extraction of the short-range force component. We compare the applicability of several van der Waals models based on experimental interaction data from the calcite(1014) surface. The feasibility to fit these models to experimental data is critically discussed. We furthermore introduce criteria to assess the transition point from pure long-range interaction to mixed shortand long-range forces based on the variance of lateral and vertical force data. This determination allows us to extract the short-range interaction forces, which remained a challenge so far in NC-AFM experiments.

Transitions between the two states of an AC Stark-split doublet are forbidden in centro-symmetric systems, and thus almost impossible to observe in experiments performed with atomic clouds. However, electrons trapped in nanoscopic heterostructures can behave as artificial atoms, with the advantage that the wavefunction symmetry can be broken by using asymmetric confining potentials. Here we develop the many-body theory describing the intra-doublet emission of a resonantly pumped intersubband transition in a doped asymmetric quantum well, showing that in such a system the intra-doublet emission can be orders of magnitude higher than in previously studied systems. This emission channel, which lies in the terahertz range, and whose frequency depends upon the pump power, opens the way to the realization of a new class of monolithic and tunable terahertz emitters., Comment: 10 pages, 7 figures

We study localized modes at a surface of a multilayer structure made of graphene layers separated by dielectric layers. We demonstrate the existence of deeply subwavelength surface modes that can be associated with the electromagnetic Tamm states, with the frequencies in the THz frequency range the negative group velocities. We suggest that the dispersion properties of these Tamm surface modes can be tuned by varying the thickness of a dielectric cap layer.

The reststrahlen bands of rare-earth garnets were investigated in the terahertz regime. Through this, it was found that the crystal orientation and light polarization directly influence the reststrahlen-related dielectric absorptions of ${\text{Gd}}_{3}$${\text{Ga}}_{5}$${\text{O}}_{12}$ (GGG), which is attributed to restriction in the vibrational motion of Gd ions. A theoretical fit to the dielectric absorption revealed that the phonon-polariton frequency in GGG crystals at around 85.5 cm${}^{\ensuremath{-}1}$ exhibits a narrow spectral width of 100 GHz at room temperature, as estimated using a Lorentz oscillator model. In addition, it was found that the reststrahlen bands can be readily manipulated within a range of 0.7--3.7 THz by changing the chemical composition of the garnet.

The long-range surface structure of the dichalcogenide $\mathrm{Mo}{\mathrm{S}}_{2}$ is probed with nanometer-length spatial resolution using low-energy electron microscopy (LEEM) and microprobe low-energy electron diffraction (\ensuremath{\mu}-LEED). The quality of two differently prepared types of $\mathrm{Mo}{\mathrm{S}}_{2}$, single-layer and multilayer exfoliated crystals, as well as single-layer chemical-vapor-deposition (CVD)--grown crystals, is examined. The effects induced by a supporting interface are examined by utilizing two different substrates, $\mathrm{Si}{\mathrm{O}}_{2}$ and native-oxide-covered Si. In addition, the role of impurities is also studied by way of in situ deposition of the alkali-metal potassium. Microprobe measurements reveal that, unlike exfoliated $\mathrm{Mo}{\mathrm{S}}_{2}$, CVD-grown $\mathrm{Mo}{\mathrm{S}}_{2}$ may, in some instances, exhibit large-scale grain-boundary alterations due to the presence of surface strain during growth. However, real-space probing by LEEM in conjunction with $k$-space probing by \ensuremath{\mu}-LEED shows that the quality of CVD-grown $\mathrm{Mo}{\mathrm{S}}_{2}$ can be comparable to that of exfoliated $\mathrm{Mo}{\mathrm{S}}_{2}$.

We study theoretically the relative importance of short-range disorder in determining the low-temperature 2D mobility in GaAs-based structures with respect to Coulomb disorder which is known to be the dominant disorder in semiconductor systems. We give results for unscreened and screened short-range disorder effects on 2D mobility in quantum wells and heterostructures, comparing with the results for Coulomb disorder and finding that the asymptotic high-density mobility is always limited by short-range disorder which, in general, becomes effectively stronger with increasing `carrier density' in contrast to Coulomb disorder. We also predict an intriguing re-entrant metal-insulator transition at very high carrier densities in Si-MOSFETs driven by the short-range disorder associated with surface roughness scattering.

An antiferromagnetic ordering in Pb3TeCo3V2O14 takes place through formation of short range correlation regime with T* ~ 10.5 K and succession of second order phase transition at TN1 = 8.9 K and first order phase transition at TN2 = 6.3 K. An external magnetic field rapidly destroys magnetic structure at T < TN2 and influences the magnetic order at TN2 < T < TN1 resulting in complex magnetic phase diagram of Pb3TeCo3V2O14 as derived from magnetization and specific heat measurements. The first principles calculations indicate that in variance with layered crystal structure the magnetic subsystem of Pb3TeCo3V2O14 is quasi-one-dimensional and highly unusual consisting of weakly coupled triangular tubes.

We experimentally and theoretically investigated the second-harmonic (SH) radiation from spherical gold nanoparticles as function of the particle radius in the range from 40 to 100 nm. By fitting the measured SH intensity to an analytical model, we demonstrated size-dependent second-order susceptibility tensor components, which significantly increase by reducing the size of the particles. Finally, we found new SH polarization diagrams for large particle radii arising from the interference among the different multipolar terms.

We report pressure-dependent reflection and transmission measurements on ZnCr$_2$Se$_4$, HgCr$_2$S$_4$, and CdCr$_2$O$_4$ single crystals at room temperature over a broad spectral range 200-24000 cm$^{-1}$. The pressure dependence of the phonon modes and the high-frequency electronic excitations indicates that all three compounds undergo a pressure-induced structural phase transition with the critical pressure 15 GPa, 12 GPa, and 10 GPa for CdCr$_2$O$_4$, HgCr$_2$S$_4$, and ZnCr$_2$Se$_4$, respectively. The eigenfrequencies of the electronic transitions are very close to the expected values for chromium crystal-field transitions. In the case of the chalcogenides pressure induces a red shift of the electronic excitation which indicates a strong hybridization of the Cr d-bands with the chalcogenide bands.

We present a theory of electron-phonon interaction and phonon decay in Si nanocrystals based on $s{p}^{3}{d}^{5}{s}^{*}$ empirical tight-binding model and anharmonic Keating model. We demonstrate that the time of optical phonon emission by hot carriers in Si nanocrystal lies in the subpicosecond time range. However, due to the fast phonon recycling, the energy relaxation rate is determined not by the phonon emission but by the phonon decay rate. The decay rate of the optical phonon into two phonons of smaller energy is found to be in the 1--10 ps range. It is this anharmonic phonon decay process that may control the energy relaxation rate of excited carriers in Si nanocrystals.

A cubic ${\text{Er}}_{2}$${\text{Ge}}_{2}$${\text{O}}_{7}$ pyrochlore was prepared under high-pressure and high-temperature conditions and its magnetic ground state was investigated by measurements of specific heat, dc and ac magnetic susceptibility as functions of temperature, pressure, and magnetic field. We found that ${\text{Er}}_{2}$${\text{Ge}}_{2}$${\text{O}}_{7}$ undergoes a long-range antiferromagnetic transition at ${T}_{N}$ \ensuremath{\approx} 1.4 K, which can be further enhanced by applying external physical pressure. On the other hand, application of external magnetic fields suppresses the antiferromagnetic order to zero temperature around ${H}_{c}$ \ensuremath{\approx} 2.3 T, where a magnetic-field-induced spin-flop transition was observed. ${H}_{c}$ increases accordingly with increasing ${T}_{N}$ under external pressure. A comparison of the magnetic ground states and structural variations along the isostructural series ${\text{Er}}_{2}$${B}_{2}$${\text{O}}_{7}$ ($B$ = Sn, Ti, Ge) together with the high-pressure study on ${\text{Er}}_{2}$${\text{Ge}}_{2}$${\text{O}}_{7}$ indicated that the magnetic properties of these highly frustrated XY pyrochlore antiferromagnets are very sensitive to the minute structural changes that determine the anisotropic exchange interactions and the local crystal-electric-field environments of Er${}^{3+}$ ions.

We present a detailed ac susceptibility investigation of the fluctuation regime in the insulating cubic helimagnet Cu2OSeO3. For magnetic fields mu H-0 >= 200 mT, and over a wide temperature (T) range, the system behaves according to the scaling relations characteristic of the classical three-dimensional Heisenberg model. For lower magnetic fields, the scaling is preserved only at higher T and becomes renormalized in a narrow-T range above the transition temperature. Contrary to the well-studied case of MnSi, where the renormalization has been interpreted within the Brazovskii theory, our analysis of the renormalization at H = 0 shows the fluctuation regime in Cu2OSeO3 to lie closer to that expected within the Wilson-Fischer scenario.

Image charge effects in nanoscale junctions with strong electron-phonon coupling open the way to unexplored physical scenarios. We propose a simple and still accurate many-body approach to deal with the simultaneous occurrence of the Franck-Condon blockade and the screening-induced enhancement of the polaron mobility. A transparent analytic expression for the polaron decay rate is derived and the dependence on the strength and range of the screening is highlighted. This allows us to interpret and explain several transient and steady-state features of the electrical current. Remarkably, we find that the competition between the charge blocking due to the electron-phonon interaction and the charge deblocking due to the image charges gives rise to a novel mechanism of negative differential conductance. An experimental setup to observe this phenomenon is discussed.