We report on a study of the vortex creep in Fe${}_{1+\delta }($Te${}_x$Se${}_{1-x}$) single crystals ($x=0.5$ and $0.4$) down to 0.28 K ($\sim T_c/50$) and up to ${\mu }_0{H}_a=2$T. The relaxation of the current density [$J\left(t\right)$] has been measured during 20 hours and the decay of $J\left(t\right)$ can be well described by a $J\left(t\right)\propto {[ln(t/t_0)]}^{-1/\mu }$ law. We show that the relaxation exponent $\mu$ tends towards 0 for $T<2$K and ${\mu }_0{H}_a<0.1$T [i.e. $J\left(t\right)\to {(t_0/t)}^{\alpha }$] and increases for increasing $T$ and/or $H_a$. Our measurements strongly suggest that the logarithmic creep rate $R=-dln\left(J\right)/dln\left(t\right)$ remains finite at zero temperature ($R_{|T\to 0}\to 2$%) and hence that quantum creep plays a dominant role in the relaxation process at low temperature. A maximum is observed in both the temperature and field dependence of $R(t=100\text{s},T,H_a),$ which can be associated to a crossover from a single vortex (one-dimensional) to a bundle (three-dimensional) creep regime.

The conductivity of an electron gas can be alternatively calculated either from the current-current or from the density-density correlation function. Here, we compare these two frequently used formulations of the Kubo formula for the two-dimensional Dirac electron gas by direct evaluations for several special cases. Assuming the presence of weak disorder, we investigate perturbatively both formulas at and away from the Dirac point. While to zeroth order in the disorder amplitude both formulations give identical results, with some very strong assumptions though, they show significant discrepancies already in first order. At half-filling, we evaluate all second-order diagrams. Virtually none of the topologically identical diagrams yield the same corrections for both formulations. We conclude that a direct comparison of conductivities of disordered system calculated in both formulas is not possible. The density-density correlation function is preferable since it can be linked to diffusion via the Einstein relation.

We present a fully ab initio theory for coherent laser-induced ultrafast spin manipulation on the homodinuclear magnetic clusters FeOFe, FeOOFe, NiONi, and NiOONi. With the oxygen atom(s) bridging the magnetically active centers, the clusters tend to adopt asymmetric geometries due to the Jahn-Teller effect. Therefore they have well-defined spin localization which is required for local spin manipulation. We find that both spin-flip and spin-transfer scenarios can be achieved on the clusters FeOFe and FeOOFe while only spin flip is realized on NiONi and NiOONi, based on which we predict that the Fe-containing clusters are quite promising for logic operations. The different dynamical behavior on these systems is analyzed in detail from the viewpoint of their distinct electronic structures and related spectra. In striving for better magnetization dynamics control, the results obtained on these prototypic systems strongly indicate their great potential in spintronic device design and future practical applications.

The strain derivatives of $T_c$ along the $a$ and $c$ axes have been determined for HgBa${}_2$CuO${}_{4+\delta }$ (Hg1201), the simplest monolayer cuprate with the highest $T_c$ of all monolayer cuprates ($T_c$ = 97 K at optimal doping). The underdoped compound with the initial $T_c$ of 65 K has been studied as a function of pressure up to 20 GPa by magnetic susceptibility and x-ray diffraction. The observed linear increase in $T_c$ with pressure is the same as previously found for the optimally doped compound. The above results have enabled an investigation of the origins of the significantly different $T_c$ values of optimally doped Hg1201 and the well-studied compound La${}_{2-x}$Sr${}_x$CuO${}_4$ (LSCO), which has a maximal $T_c$ of 40 K, or only $40\%$ of that of Hg1201. Hg1201 can have almost identical CuO${}_6$ octahedra as LSCO if specifically strained. When the apical and in-plane CuO${}_2$ distances are the same for the two compounds, a large discrepancy in their $T_c$ remains. Differences in crystal structures and interactions involving the Hg-O charge reservoir layers of Hg1201 may be responsible for the different $T_c$ values exhibited by the two compounds.

Weyl semimetals possess massless chiral quasiparticles, and they are thus affected by the triangle anomalies. We discuss the features of the chiral magnetic and chiral vortical effects specific to Weyl semimetals, and then propose three phenomena caused by the triangle anomalies in this material: (i) anomaly cooling; (ii) charge transport by soliton waves as described by Burgers' equation, and (iii) the shift of the Berezinskii-Kosterlitz-Thouless phase transition of superfluid vortices coupled to Weyl fermions. In addition, we establish the conditions under which the chiral magnetic current exists in real materials.

Motivated by recent experiments exploring the spin-orbit-coupled magnetism in $4d$- and $5d$-band transition metal oxides, we study magnetic interactions in Ir- and Rh-based compounds. In these systems, the comparable strength of spin-orbit coupling, crystal-field splitting, and Coulomb and Hund's coupling leads to a rich variety of magnetic exchange interactions, leading to new types of ground states. Using a strong coupling approach, we derive effective low-energy superexchange Hamiltonians from the multiorbital Hubbard model by taking full account of the Coulomb and Hund's interactions in the intermediate states. We find that in the presence of strong SOC and lattice distortions the superexchange Hamiltonian contains various kinds of magnetic anisotropies. Here we are primarily interested in the magnetic properties of Sr${}_2$IrO${}_4$ and Sr${}_2$Ir${}_{1-x}$Rh${}_x$O${}_4$ compounds. We perform a systematic study of how magnetic interactions in these systems depend on the microscopic parameters and provide a thorough analysis of the resulting magnetic phase diagram. Comparison of our results with experimental data shows good agreement. Finally, we discuss the parameter space in which the spin-flop transition in Sr${}_2$IrO${}_4$, experimentally observed under pressure, can be realized.

ZeTe${}_3$ belongs to the ample class of chainlike chalcogenide charge-density-wave (CDW) materials. Its peculiarity consists in the formation of the CDW condensate at $T_{\mathrm{CDW}}$ along the crystallographic directions perpendicular to the $b$-axis, Zr-Zr chains. Ni and Cu intercalation of ZrTe${}_3$ leads to the onset of bulk superconductivity at $T_c<T_{\mathrm{CDW}}$, raising questions about the possible coexistence as well as competition between both broken-symmetry ground states. Here, we report on the optical investigation of Ni${}_{0.05}$ZrTe${}_3$ and Cu${}_{0.05}$ZrTe${}_3$ single crystals performed with electromagnetic radiation polarized along the in-plane crystallographic $a$ and $b$ axes, over a broad spectral range and as a function of temperature. The charge dynamics of the CDW state displays a polarization dependence within the $ab$ plane and gives evidence for a partial gapping of the Fermi surface, which affects almost exclusively the crystallographic direction parallel to the $a$ axis. We provide a complementary analysis of the spectral weight distribution, disentangling its reshuffling in the excitation spectrum across the symmetry-breaking CDW transition. While the superconducting energy gap lies outside our measurable spectral range, we observe nonetheless that the CDW-gap feature persists at $T<T_c$. We propose that CDW and superconductivity coexist along the $a$ axis.

We report measurements of electrical resistivity, magnetic susceptibility, specific heat, and thermoelectric power on the system Pr${}_{1-x}$Ce${}_x$Pt${}_4$Ge${}_{12}$. Superconductivity is suppressed with increasing Ce concentration up to $x=0.5$, above which there is no evidence for superconductivity down to 1.1 K. The Sommerfeld coefficient $\gamma$ increases with increasing $x$ from $\sim$48 mJ/mol K${}^2$ up to $\sim$120 mJ/mol K${}^2$ at $x=$ 0.5, indicating an increase in strength of electronic correlations. The temperature dependence of the specific heat at low temperatures evolves from roughly $T^3$ for $x=$ 0 to e${}^{-\Delta /T}$ behavior for $x=$ 0.05 and above, suggesting a crossover from a nodal to a nodeless superconducting energy gap or a transition from multiband to single-band superconductivity. Fermi-liquid behavior is observed throughout the series in low-temperature magnetization, specific heat, and electrical resistivity measurements.

The low temperature ($T$ = 5–40 K) capture of free electrons into hydrogenlike antimony centers in germanium has been studied by a time-resolving experiment using the free electron laser FELBE. The analysis of the pump-probe signal reveals a typical capture time of about 1.7 ns that decreases with pump energy to less than 1 ns while the number of ionized donors increases. The dependence on the pump-pulse energy is well described by an acoustic phonon-assisted capture process. In the cases when (i) a significant number of the electrons is in the conduction band (flux densities larger than 5 × 10${}^{25}$ photons/(cm${}^2$ s), (ii) the lattice temperature is above ∼20 K, or (iii) a static electric field above ∼2 V/cm is applied to the crystal, the pump-probe technique reveals an additional intraband relaxation process with a characteristic time of ∼100 ps, which is much shorter than that of the capture of free electrons into the antimony ground state.

Knowledge of the plasmonic band structure is crucial for the fundamental understanding and better control of electromagnetic waves in periodic optical metamaterials. Here, we probe the plasmonic band structure of the important fishnet metamaterial by angle- and polarization-resolved optical spectroscopy. We show that multiple “magnetic” plasmon bands exist in the fishnet metamaterial, and they display interesting dispersion behavior. We can understand the plasmon bands qualitatively using a band-structure zone-folding picture. Interestingly, some plasmon resonances have wave-vector-dependent coupling, and the corresponding plasmon bands exhibit anticrossing behavior due to hybridization of the plasmonic waves.

We explore the electron-phonon coupling and possible superconductivity in Li-intercalated borocarbide materials using precise Wannier interpolation-based first-principles technique. We find a $T_c$ of 36.8 K for the previously suggested superconductor Li${}_2$B${}_3$C, however, we also propose another material Li${}_4$B${}_5$C${}_3$ with an estimated $T_c$ of 16.8 K. Replacing BC layers with BC${}_3$ in Li${}_2$B${}_3$C allows the $\pi$ electronic states to be dominant at the Fermi level for Li${}_4$B${}_5$C${}_3$. We analyze wave-vector-resolved electron-phonon coupling parameters and suggest that Li${}_4$B${}_5$C${}_3$ may be more suitable for experimental fabrication than Li${}_2$B${}_3$C.

We propose a Hanbury Brown and Twiss (HBT) experiment of Cooper pairs on the edge channels of quantum spin Hall insulators. The helical edge channels provide a well-defined beam of Cooper pairs and perfect Andreev reflections from superconductors. This allows our setup to be identical in spirit to the original HBT experiment. Interestingly, the cross correlation is always negative and provides no hint of the bosonic nature of Cooper pairs. This counterintuitive result is attributed to the perfect Andreev reflection and the true beam splitter in the setup.

An ultrasonic measurement has been performed on the spin-liquid organic candidate $\kappa$-(BEDT-TTF)${}_2$Cu${}_2$(CN)${}_3$ at low temperatures. A softening anomaly was observed on the longitudinal velocity along a direction perpendicular to the organic planes with a consistent one on the attenuation. The velocity anomaly appears as a frequency dependent softening peak below 20 K with a maximum value found near 5–6 K. A magnetic field affects predominantly the peak only below a temperature $T_p=5.9$ K that is associated with the 6 K transition found on other physical properties. We suggest to relate the frequency dependent velocity softening at low temperatures to a spinon-phonon coupling, softening that is reduced below 5.9 K due to a pairing instability transition of the spinons.

A kinetic model is derived and used to analyze recently published works and new data on the temperature dependence of the spectrally integrated photoluminescence (PL) from thick-film formulations of PbSe colloidal quantum dots (QDs), with particular attention to the effects of air exposure. The model assumes that the excitons thermalize within a ground-state manifold of states and treats the distribution of radiative and nonradiative decay rates within the distribution as generally as possible, while using a minimal number of free parameters. By adjusting the parameters of the model, good fits are obtained for the wide range of integrated PL behaviors reported in [J. Phys. Chem. Lett. 2, 889 (2011); ACS Nano 6, 5498 (2012); Phys. Rev. B 82, 165435 (2010)] and the new data presented in this manuscript. By comparing the extracted parameters we deduce the following: (i) All of the samples in the first two references emit from two distinct clusters of states separated by an energy of 55 to 80 meV regardless of air exposure, while there is only one cluster of emissive states that contributes to the emission reported in the third reference. (ii) In the absence of intentional air exposure, the nonradiative decay from all samples can be described by a single Arrhenius-like process. (iii) Although air-exposure effects are reversible in some samples and irreversible in others, the changes in integrated PL behavior brought about by air-exposure forces the introduction of a common, low-activation-energy nonradiative pathway in all cases. (iv) The low-lying emissive cluster of the two-emissive-cluster samples exhibits behavior similar to the single emissive cluster of the other samples. (v) Many hours of air exposure do not trend either the radiative or nonradiative behavior of the dual-emissive-cluster samples towards the behavior of the single-emissive-cluster samples.

The nonadditive dispersion contribution to the binding energy of three one-dimensional (1D) wires is investigated using wires modeled by (i) chains of hydrogen atoms and (ii) homogeneous electron gases. We demonstrate that the nonadditive dispersion contribution to the binding energy is significantly enhanced compared with that expected from Axilrod-Teller-Muto–type triple-dipole summations and follows a different power-law decay with separation. The triwire nonadditive dispersion for 1D electron gases scales according to the power law $d^{-\beta }$, where $d$ is the wire separation, with exponents $\beta \left(r_s\right)$ smaller than 3 and slightly increasing with $r_s$ from 2.4 at $r_s=1$ to 2.9 at $r_s=10$, where $r_s$ is the density parameter of the 1D electron gas. This is in good agreement with the exponent $\beta =3$ suggested by the leading-order charge-flow contribution to the triwire nonadditivity, and is a significantly slower decay than the $\sim d^{-7}$ behavior that would be expected from triple-dipole summations.

Resonant in situ photoemission from Mn $3d$ states in Ga${}_{1-x}$Mn${}_x$As is reported for Mn concentrations down to the very dilute level of 0.1%. Concentration-dependent spectral features are analyzed on the basis of first-principles calculations for systems with selected impurity positions as well as for random alloys. Effects of direct Mn-Mn interaction are found for concentrations as low as 2.5%, and are ascribed to statistical (nonuniform) distribution of Mn atoms.

We have studied the surface phonon modes of the reconstructed Si(111)-($7\times 7$) surface by polarized Raman spectroscopy. Six surface vibration modes are observed in the frequency range between 62.5 and 420.0 cm${}^{-1}$. The mode frequencies agree very well with reported calculation results. This enables their attribution to calculated eigenmodes, whose elongation patterns are dominated by specific atomic sites: the two most characteristic novel fingerprints of the ($7\times 7$) reconstruction are sharp Raman peaks from localized adatom vibrations, located at 250.9 cm${}^{-1}$, and collective vibrations of the adatoms and first- and second-layer atoms, located at 420.0 cm${}^{-1}$. While the sharp localized adatom vibration peak is a substantial refinement of an earlier broad spectral structure from electron energy-loss spectroscopy, no spectroscopic features were reported before in the collective-vibration frequency region. Furthermore, we observe in-plane wagging vibrations in the range from 110 to 140 cm${}^{-1}$, and finally the backfolded acoustic Rayleigh wave at 62.5 cm${}^{-1}$, which coincides with helium atom scattering data. Moreover, the Raman peak intensities of the surface phonons show a mode-specific dependence on the polarization directions of incident and scattered light. From this polarization dependence the relevant symmetry components in the Raman scattering process ($A_1$ and/or $E$ symmetry) are deduced for each mode.

The evolution of extended states of two-dimensional electron gas with white-noise randomness and field is numerically investigated by using the Anderson model on square lattices. Focusing on the lowest Landau band we establish an anti-levitation scenario of the extended states: As either the disorder strength $W$ increases or the magnetic field strength $B$ decreases, the energies of the extended states move below the Landau energies pertaining to a clean system. Moreover, for strong enough disorder, there is a disorder-dependent critical magnetic field $B_c\left(W\right)$ below which there are no extended states at all. A general phase diagram in the $W\text{−}1/B$ plane is suggested with a line separating domains of localized and delocalized states.

Density functional theory calculations are employed to investigate the energetics of point defects—monovacancy, self-interstitials (tetrahedral, octahedral, and dumbbell), and Frenkel pairs—in aluminum under tensile hydrostatic stresses. Our study suggests that the defect core energy of a vacancy, which is governed solely by the electronic structure at the core, significantly depends on the macroscopic hydrostatic stress, and that this constitutes an important contribution to the formation enthalpy, especially in the regime of extreme tensile hydrostatic stresses. This finding is in contrast to widely used elastic formulations of point defects based on formation volume that ignore the defect core-energy contribution. The formation enthalpies of all point defects considered in the present study monotonically decrease with increasing tensile hydrostatic stress. Furthermore, we find that the formation enthalpies of vacancies and Frenkel pairs are negative beyond critical tensile hydrostatic stresses (9 GPa for vacancies and 12 GPa for Frenkel pairs), which suggests a spontaneous nucleation of these point defects and this has important implications to nucleation mechanisms leading to spall failure. In particular, the present findings suggest two possible defect nucleation mechanisms leading to spall failure: (i) a heterogeneous nucleation of vacancies from defect sources and (ii) a homogeneous nucleation of Frenkel pairs at higher hydrostatic stresses.