We solved the Schrödinger problem for a two-dimensional electron gas (2DEG) with the Rashba spin-orbit interaction in the presence of a strong high-frequency electromagnetic field (dressing field). The found eigenfunctions and eigenenergies of the problem are used to describe the spin dynamics of the dressed 2DEG within the formalism of the density matrix response function. Solving the equations of spin dynamics, we show that the dressing field can switch the spin relaxation in the 2DEG between the cases corresponding to the known Elliott-Yafet and D'yakonov-Perel' regimes. As a result, the spin properties of the 2DEG can be tuned by a high-frequency electromagnetic field. The present effect opens an unexplored way for controlling the spin with light and, therefore, forms the physical prerequisites for creating light-tuned spintronics devices.

In this paper, optical and electronic properties of diamond-shaped graphene quantum dots (DQDs) have been studied by employing large-scale electron-correlated calculations. The computations have been performed using the $\pi$-electron Pariser-Parr-Pople model Hamiltonian, which incorporates long-range Coulomb interactions. The influence of electron-correlation effects on the ground and excited states has been included by means of the configuration-interaction approach used at various levels. Our calculations have revealed that the absorption spectra are redshifted with the increasing sizes of quantum dots. It has been observed that the first peak of the linear optical absorption, which represents the optical gap, is not the most intense peak. This result is in excellent agreement with the experimental data, but in stark contrast to the predictions of the tight-binding model, according to which the first peak is the most intense peak, pointing to the importance of electron-correlation effects.

Half-metallic ferromagnetism (FM) in single-crystal ${\mathrm{Ba}}_{0.39\left(1\right)}{\mathrm{Rb}}_{0.61\left(1\right)}{\mathrm{Mn}}_2{\mathrm{As}}_2$ below its Curie temperature $T_{\mathrm{C}}=103\left(2\right)$ K is reported. The magnetization $M$ versus applied magnetic field $H$ isotherm data at 1.8 K show complete polarization of the itinerant doped-hole magnetic moments that are introduced by substituting Rb for Ba. The material exhibits extremely soft FM, with unobservably small remanent magnetization and coercive field. Surprisingly, and contrary to typical itinerant FMs, the $M\left(H\right)$ data follow the Arrott-plot paradigm that is based on a mean-field theory of local-moment FMs. The in-plane electrical resistivity data are fitted well by an activated-$T^2$ expression for $T\le T_{\mathrm{C}}$, whereas the data sharply deviate from this model for $T>T_{\mathrm{C}}$. Hence the activated-$T^2$ resistivity model is an excellent diagnostic for determining the onset of half-metallic FM in this compound, which in turn demonstrates the presence of a strong correlation between the electronic transport and magnetic properties of the material. Together with previous data on 40% hole-doped ${\mathrm{Ba}}_{0.6}{\mathrm{K}}_{0.4}{\mathrm{Mn}}_2{\mathrm{As}}_2$, these measurements establish 61%-doped ${\mathrm{Ba}}_{0.39}{\mathrm{Rb}}_{0.61}{\mathrm{Mn}}_2{\mathrm{As}}_2$ as a prototype for a class of half-metallic ferromagnets in which all the itinerant carriers in the material are ferromagnetic.

I study the lattice dynamics and electron-phonon coupling in noncentrosymmetric quasi-one-dimensional ${\mathrm{K}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$ using density functional theory based first principles calculations. The phonon dispersions show stable phonons without any soft-mode behavior. They also exhibit features that point to a strong interaction of K atoms with the lattice. I find that the calculated Eliashberg spectral function shows a large enhancement around 50 ${\mathrm{cm}}^{-1}$. The phonon modes that show large coupling involve in-plane motions of all three species of atoms. The $\mathbf{q}$ dependent electron-phonon coupling decreases strongly away from the $q_z=0$ plane. The total electron-phonon coupling is large, with a value of ${\lambda }_{\mathrm{ep}}=3.0$, which readily explains the experimentally observed large mass enhancement.

We show that odd-parity superconductivity occurs in multilayer Rashba systems without requiring spin-triplet Cooper pairs. A pairing interaction in the spin-singlet channel stabilizes the odd-parity pair-density-wave (PDW) state in the magnetic field parallel to the two-dimensional conducting plane. It is shown that the layer-dependent Rashba spin-orbit coupling and the orbital effect play essential roles for the PDW state in binary and tricolor heterostructures. We demonstrate that the odd-parity PDW state is a symmetry-protected topological superconducting state characterized by the one-dimensional winding number in the symmetry class BDI. The superconductivity in the artificial heavy-fermion superlattice ${\mathrm{CeCoIn}}_5/{\mathrm{YbCoIn}}_5$ and bilayer interface ${\mathrm{SrTiO}}_3/{\mathrm{LaAlO}}_3$ is discussed.

We theoretically study exotic superconducting phases in graphene-like single-sheet material ${\mathrm{BC}}_3$ doped to its type-II van Hove singularity whose saddle-point momenta are not time-reversal invariant. From combined renormalization group analysis and random-phase approximation calculations, we show that the dominant superconducting instability induced by weak repulsive interactions is in the time-reversal-invariant $p+ip$ pairing channel because of the interplay among dominant ferromagnetic fluctuations, subleading spin fluctuations at finite momentum, and spin-orbit coupling. Such time-reversal-invariant $p+ip$ superconductivity has nontrivial ${\mathbb{Z}}_2$ topological invariant. Our results show that doped ${\mathrm{BC}}_3$ provides a promising route to realizing a genuine two-dimensional helical $p+ip$ superconductor.

We demonstrate that the zero-temperature superconducting phase diagram of underdoped cuprates can be quantitatively understood in the strong binding limit, using only the experimental spectral function of the “normal” pseudogap phase without any free parameter. In the prototypical $\left({\mathrm{La}}_{1-x}{\mathrm{Sr}}_x\right){}_2{\mathrm{CuO}}_4$, a kinetics-driven $d$-wave superconductivity is obtained above the critical doping ${\delta }_c\sim 5.2\%$, below which complete loss of superfluidity results from local quantum fluctuation involving local $p$-wave pairs. Near the critical doping, an enormous mass enhancement of the local pairs is found responsible for the observed rapid decrease of phase stiffness. Finally, a striking mass divergence is predicted at ${\delta }_c$ that dictates the occurrence of the observed quantum critical point and the abrupt suppression of the Nernst effects in the nearby region.

We demonstrate that use of the field effect enables tuning of the effective optical parameters of a layered hyperbolic metamaterial at optical frequencies. Field-effect gating electrically modulates the permittivity in transparent conductive oxides via changes in the carrier density. These permittivity changes lead to active modulation of the effective electromagnetic parameters along with active control of the anisotropic dispersion surface of hyperbolic metamaterials and enable the opening and closing of photonic band gaps. Tunability of the effective electric permittivity and magnetic permeability also leads to topological transitions in the optical dispersion characteristics.

Recently, a relativistic phase shift model (RPSM) was introduced [D. V. Fedorov et al., Phys. Rev. B 88, 085116 (2013)] to describe the skew-scattering mechanism of the spin Hall effect caused by impurities in bulk crystals. Here, we present its analog derived for two-dimensional (2D) systems. The proposed 2D-RPSM is applied to one-monolayer noble-metal films with various substitutional impurities and the obtained results are compared with those of corresponding first-principles calculations. We demonstrate that, in contrast to the three-dimensional RPSM, the considered model does not provide a sufficient qualitative description of the transport properties. Therefore, an ab initio treatment is necessary for the description of the spin Hall effect in two-dimensional crystals.

Improving the magnetic stability of single-molecule magnets is a key challenge facing molecular spintronics. We use x-ray magnetic circular dichroism to explore the possibility of magnetically stabilizing ${\mathrm{TbPc}}_2$ molecules by attaching them to ultrathin Fe and FeMn films. We show that ${\mathrm{TbPc}}_2$ deposited on antiferromagnetic FeMn films exhibits magnetic hysteresis and exchange bias as a consequence of coupling to the uncompensated interfacial Fe spins. The FeMn-thickness dependence of the coercive field and exchange bias of ${\mathrm{TbPc}}_2$ is similar to that of inorganic ferromagnetic/antiferromagnetic systems. The magnetic remanence is comparable with the fraction of molecules attached to pinned interfacial Fe spins. The Tb magnetic moments are antiferromagnetically coupled to the Fe thin films as well as to the uncompensated Fe spins at the FeMn interface. The sign of the coupling changes from antiferromagnetic to ferromagnetic after doping the interface with electron-donor Li atoms.

We predict an existence of a dissipation channel leading to attenuation of ultrasound in disordered conductors and superconductors with perfect electroneutrality. It is due to slow diffusion of thermal energy. We show that in doped silicon ultrasound attenuation may be enhanced by a factor about 100. A similar effect is also studied for $s$-wave and $d$-wave superconductors. The latter case is applied to BSCCO family where a strong enhancement of the ultrasound attenuation is predicted. For usual $s$-wave superconductors, this dissipation channel might be important for very low-electron-density materials near the BCS-BEC crossover.

In phonon mediated conventional $s$-wave superconductors, higher-frequency phonon (or smaller atomic mass) leads to a higher superconducting transition temperature, known as the isotope effect. However, in correlated systems, various competing electronic order (such as spin-density-wave, charge-density-wave, and unconventional superconductivity) arises and the effect of electron-phonon coupling on these orders is a long-standing problem. Using the functional renormalization group, here we investigated the interplay between the electron correlation and electron-phonon coupling in the Hubbard-Holstein model on a square lattice. At half-filling, we found spin-density-wave and charge-density-wave phases and the transition between them, while no superconducting phase arises. Upon finite doping, $d$-wave/$s$-wave superconductivity emerges in proximity to the spin-density-wave/charge-density-wave phase. Surprisingly, lower-frequency Holstein phonons are either less destructive or even beneficial to the various phases, resulting in a negative isotope effect. For the superconducting phases, such an effect is apparently beyond the Bardeen-Cooper-Schrieffer theory.

We have performed density functional theory calculations to investigate the core-level shifts on the Au/Si(111)-($5\times 2$) surface. The employed structural model, which was recently proposed as containing seven Au atoms per ($5\times 2$) unit cell [Phys. Rev. Lett. 113, 086101 (2014)], is found to well reproduce the Si $2p$ and Au $4f$ surface core-level shifts reported by previous x-ray photoelectron spectroscopy studies, thereby clarifying their atomic origins.

We propose a system of real-space envelope function equations without fitting parameters for modeling the electronic spectrum and wave functions of a phosphorus donor atom embedded in silicon. The approach relies on the Burt-Foreman envelope function representation and leads to coupled effective-mass Schrödinger equations containing smooth effective potentials. These potentials result from the spatial filtering imposed on the exact potential energy matrix elements in the envelope function representation. The corresponding filter function is determined from the definition of the envelope function. The resulting effective potentials and the system of envelope functions jointly reproduce the valley-orbit coupling effect in doped silicon. It is found that including the valley-orbit coupling not only of the $1s$ but also for $2s$ atomic orbitals, as well as static dielectric screening, is crucial to accurately reproduce experimental data. The measured binding energies are recovered with a maximum relative error of 1.53%. The computed wave functions are in good agreement with experimental measurements of the electron density provided by scanning tunneling microscopy.

In two-dimensional electron systems confined to wide GaAs quantum wells we observe a remarkable sequence of transitions of the fractional quantum Hall states in the filling factor range $1<\nu <3$ near the crossing of two $N=0$ Landau levels from different subbands and with opposite spins. The transitions attest to the interplay between the spin and subband degrees of freedom and can be explained as pseudospin polarization transitions where the pseudospins are the two crossing levels. The magnetic field positions of the transitions yield a quantitative measure of the transition energies or, equivalently, the composite fermions' discrete energy level separations. Surprisingly, these energies are much larger than those reported for other systems with SU(2) symmetry; moreover, they are larger when the electron system is less spin polarized.

A Raman spectroscopy study on the half-doped single-layer manganite ${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{1.5}{\mathrm{MnO}}_4$ has been performed in combination with x-ray diffraction, resistivity, magnetization, and specific heat measurements. The results provide insight into the underlying mechanisms of phenomena that arise from correlations between lattice, charge, orbital, and spin degrees of freedom. The appearance of a new Raman mode at $366{\mathrm{cm}}^{-1}$, a visible jump in the resistivity, and a doubling of the unit cell signify the onset of charge/orbital ordering at 320 K. This transition is also marked by a sharp peak in the magnetic susceptibility and specific heat, suggesting strong spin-charge coupling. Our structural analysis suggests that the charge disproportionation below 320 K is small. Orbital fluctuations below 320 K are evidenced by the evolution with temperature of the Jahn-Teller Raman mode (situated at $533{\mathrm{cm}}^{-1}$ at 320 K). A coincidence between the onset of two-dimensional short-range antiferromagnetic order at 215 K and anomalies in both the temperature dependence of the Jahn-Teller mode and the Mn-O bonding pattern in the $ab$ plane indicate that the short-range magnetic order plays a role in stabilizing the orbital fluctuations. Below the Néel temperature of 127 K, the softening of both the $366{\mathrm{cm}}^{-1}$ mode and an octahedral tilting mode at $214{\mathrm{cm}}^{-1}$ mark the onset of three-dimensional antiferromagnetic ordering. The estimated spin-phonon coupling constants for these two modes are 2.6 and $6.8{\mathrm{cm}}^{-1}$, respectively. This study highlights the remarkable coupling of charge, orbital, and spin degrees of freedom to the lattice in single-layer ${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{1.5}{\mathrm{MnO}}_4$.

$\mathrm{SrV}{\mathrm{O}}_3$ and $\mathrm{SrNb}{\mathrm{O}}_3$ are perovskite-type transition-metal oxides with the same $d^1$ electronic configuration. Although $\mathrm{SrNb}{\mathrm{O}}_3$ $\left(4d^1\right)$ has a larger $d$ orbital than $\mathrm{SrV}{\mathrm{O}}_3$ $\left(3d^1\right)$, the reported electrical resistivity of $\mathrm{SrNb}{\mathrm{O}}_3$ is much higher than that of $\mathrm{SrV}{\mathrm{O}}_3$, probably owing to nonstoichiometry. In this paper, we grew epitaxial, high-conductivity stoichiometric $\mathrm{SrNb}{\mathrm{O}}_3$ using pulsed laser deposition. The growth temperature strongly affected the Sr/Nb ratio and the oxygen content of the films, and we obtained stoichiometric $\mathrm{SrNb}{\mathrm{O}}_3$ at a very narrow temperature window around 630 °C. The stoichiometric $\mathrm{SrNb}{\mathrm{O}}_3$ epitaxial thin films grew coherently on $\mathrm{KTa}{\mathrm{O}}_3$ (001) substrates with high crystallinity. The room-temperature resistivity of the stoichiometric film was $2.82\times {10}^{-5}\mathrm{\Omega }\mathrm{cm}$, one order of magnitude lower than the lowest reported value of $\mathrm{SrNb}{\mathrm{O}}_3$ and comparable with that of $\mathrm{SrV}{\mathrm{O}}_3$. We observed a $T$-square dependence of resistivity below $T^{*}=180\mathrm{K}$ and non-Drude behavior in near-infrared absorption spectroscopy, attributable to the Fermi-liquid nature caused by electron correlation. Analysis of the $T$-square coefficient $A$ of resistivity experimentally revealed that the $4d$ orbital of Nb that is larger than the $3d$ ones certainly contributes to the high electrical conduction of $\mathrm{SrNb}{\mathrm{O}}_3$.

We study the charge conductivity of the one-dimensional repulsive Hubbard model at finite temperature using the method of dynamical quantum typicality, focusing at half filling. This numerical approach allows us to obtain current autocorrelation functions from systems with as many as 18 sites, way beyond the range of standard exact diagonalization. Our data clearly suggest that the charge Drude weight vanishes with a power law as a function of system size. The low-frequency dependence of the conductivity is consistent with a finite dc value and thus with diffusion, despite large finite-size effects. Furthermore, we consider the mass-imbalanced Hubbard model for which the charge Drude weight decays exponentially with system size, as expected for a nonintegrable model. We analyze the conductivity and diffusion constant as a function of the mass imbalance and we observe that the conductivity of the lighter component decreases exponentially fast with the mass-imbalance ratio. While in the extreme limit of immobile heavy particles, the Falicov-Kimball model, there is an effective Anderson-localization mechanism leading to a vanishing conductivity of the lighter species, we resolve finite conductivities for an inverse mass ratio of $\eta \gtrsim 0.25$.

We combine the concept of phase-gradient metasurfaces with fluorescence directionality control of an ensemble of incoherent emitters. We design a periodic metasurface to control the scattering amplitude of the lattice in momentum space. The lattice is embedded in a waveguiding layer doped with organic fluorophores. In contrast to the usual symmetric directionality that plasmonic lattices impart on emission, we find that the phase gradient translates into asymmetric directional emission into the far field, determined by scattering on a subset of the reciprocal lattice vectors. The measured asymmetry is well explained by analytical modeling.

The structural properties of amorphous ${\mathrm{GeTe}}_4$ are studied within the framework of first-principles molecular dynamics combined with density functional theory. Four different theoretical schemes are considered, each one intended to correspond to a distinct structural model. These are obtained by selecting either the PBE (Perdew, Burke, and Ernzerhof) or the BLYP (Becke, Lee, Yang, and Parr) exchange-correlation functionals and, for each one of the two, by disregarding or including van der Waals dispersion forces. Based on the comparison with experimental total structure factor $S\left(k\right)$ and total pair correlation function $g\left(r\right)$, one can infer the quantitative character of such models, thereby extracting information on the underlying structures. We found that the inclusion of the vdW forces improves the predictive power of our approach for the PBE and (to a smaller extent) the BLYP exchange-correlation functionals. Overall, BLYP performs better than PBE in reproducing the available experimental quantities, providing a tetrahedral atomic-scale network profoundly different from the one predicted by PBE, in which tetrahedral and octahedral motifs do coexist. Our work demonstrates that a careful choice of the exchange-correlation functional, combined with the account of van der Waals forces, is crucial to achieve realistic structural predictions for glassy ${\mathrm{GeTe}}_4$.