The exciton coherent signal decay rate in GaAs quantum wells, as measured in four-wave mixing experiments, depends on the polarization of the excitation pulses. Using polarization-dependent two-dimensional coherent spectroscopy, we show that this behavior is due to the bosonic character of excitons. Interference between two different quantum mechanical pathways results in a smaller decay rate for cocircular and colinear polarization of the optical excitation pulses. This interference does not exist for cross-linearly polarized excitation pulses resulting in a larger decay rate. Our result shows that the bosonic nature of excitons must be considered when interpreting ultrafast spectroscopic studies of exciton dephasing in semiconductors. This behavior should be considered while interpreting results of ultrafast spectroscopy experiments involving bosonlike excitations.

The significance of the magnitude of the Prandtl number of a fluid in the propagation direction of induced convection rolls is elucidated. Specifically, we report on the physical mechanism to account for the formation and orientation of previously unexplored supra-wavelength periodic surface structures in dielectrics, following melting and subsequent capillary effects induced upon irradiation with ultrashort laser pulses. Counterintuitively, it is found that such structures exhibit periodicities, which are markedly, even multiple times, higher than the laser excitation wavelength. It turns out that the extent to which the hydrothermal waves relax depends upon the laser beam energy, produced electron densities upon excitation with femtosecond pulsed lasers, the magnitude of the induced initial local roll disturbances, and the magnitude of the Prandtl number with direct consequences on the orientation and size of the induced structures. It is envisaged that this elucidation may be useful for the interpretation of similar, albeit large-scale periodic or quasiperiodic structures formed in other natural systems due to thermal gradients, while it can also be of great importance for potential applications in biomimetics.

Measurements of electrical resistivity, $\rho \left(T\right)$, were performed under quasihydrostatic pressure up to $P\sim 2.2$ GPa to determine the pressure dependence of the so-called hidden order (HO) and large-moment antiferromagnetic (LMAFM) phases for the ${\mathrm{URu}}_{2-x}{\mathrm{Fe}}_x{\mathrm{Si}}_2$ system with $x$ = 0.025, 0.05, 0.10, 0.15, and 0.20. As the Fe concentration ($x$) is increased, we observed that a smaller amount of external pressure, $P_c$, is required to induce the HO $\to$ LMAFM phase transition. A critical pressure of $P_c\sim 1.2$ GPa at $x=0.025$ reduces to $P_c\sim 0$ at $x=0.15$, suggesting the ${\mathrm{URu}}_{2-x}{\mathrm{Fe}}_x{\mathrm{Si}}_2$ system is fully expressed in the LMAFM phase for $x\ge x_c^{*}=0.15$, where $x_c^{*}$ denotes the ambient pressure critical concentration of Fe. Using a bulk modulus calculation to convert $x$ to chemical pressure, $P_{\mathrm{ch}}\left(x\right)$, we consistently found that the induced HO $\to$ LMAFM phase transition occurred at various combinations of $x_c$ and $P_c$ such that $P_{\mathrm{ch}}\left(x_c\right)+P_c\approx 1.5$ GPa, where $x_c$ denotes those critical concentrations of Fe that induce the HO $\to$ LMAFM phase transition for the ${\mathrm{URu}}_{2-x}{\mathrm{Fe}}_x{\mathrm{Si}}_2$ compounds under pressure. We performed exponential fits of $\rho \left(T\right)$ in the HO and LMAFM phases in order to determine the pressure dependence of the energy gap, $\mathrm{\Delta }$, that opens over part of the Fermi surface in the transition from the paramagnetic (PM) phase to the HO/LMAFM phase at the transition temperature, $T_0$. The change in the pressure variation of $\mathrm{\Delta }\left(P\right)$ at the HO $\to$ LMAFM phase transition is consistent with the values of $P_c$ determined from the $T_0\left(P\right)$ phase lines at the PM $\to$ HO/LMAFM transition.

In iron-based superconductors, a spin-density-wave (SDW) magnetic order is suppressed with doping, and unconventional superconductivity appears in close proximity to the SDW instability. The optical response of the SDW order shows clear gap features: substantial suppression in the low-frequency optical conductivity, alongside a spectral weight transfer from low to high frequencies. Here, we study the detailed temperature dependence of the optical response in three different series of the Ba122 system [${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2, \mathrm{Ba}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_2{\mathrm{As}}_2$, and ${\mathrm{BaFe}}_2{\left({\mathrm{As}}_{1-x}{\mathrm{P}}_x\right)}_2$]. Intriguingly, we find that the suppression of the low-frequency optical conductivity and spectral weight transfer appear at a temperature $T^{*}$ much higher than the SDW transition temperature $T_{\mathrm{SDW}}$. Since this behavior has the same optical feature and energy scale as the SDW order, we attribute it to SDW fluctuations. Furthermore, $T^{*}$ is suppressed with doping, closely following the doping dependence of the nematic fluctuations detected by other techniques. These results suggest that the magnetic and nematic orders have an intimate relationship, in favor of the magnetic-fluctuation-driven nematicity scenario in iron-based superconductors.

A method for 3D interpolation between hard spheres is described. The function to be interpolated could be the charge density between atoms in condensed matter. Its electrostatic potential is found analytically, and so are various integrals. Periodicity is not required. The interpolation functions are localized structure-adapted linear combinations of spherical waves, the so-called unitary spherical waves (USWs), ${\psi }_{RL}\left(\varepsilon ,\mathbf{r}\right),$ centered at the spheres $R,$ where they have cubic-harmonic character $L.$ Input to the interpolation are the coefficients in the cubic-harmonic expansions of the target function at and slightly outside the spheres; specifically, the values and the three first radial derivatives labeled by $d=0$ (value) and 1–3 (derivatives). To fit this, we use USWs with four negative energies, $\varepsilon ={\epsilon }_1,{\epsilon }_2,{\epsilon }_3$, and ${\epsilon }_4.$ Each interpolation function, ${\varrho }_{dRL}\left(\mathbf{r}\right),$ is actually a linear combination of these four sets of USWs with the following properties. (1) It is centered at a specific sphere where it has a specific cubic-harmonic character and radial derivative. (2) Its value and the first three radial derivatives vanish at all other spheres and for all other cubic-harmonic characters, and is therefore highly localized, essentially inside its Voronoi cell. Value-and-derivative (v&d) functions were originally introduced and used by Methfessel [Phys. Rev. B 38, 1537 (1988)], but only for the first radial derivative. Explicit expressions are given for the v&d functions and their Coulomb potentials in terms of the USWs at the four energies, plus ${\epsilon }_0\equiv 0$ for the potentials. The coefficients, as well as integrals over the interstitial such as the electrostatic energy, are given entirely in terms of the structure matrix, $S_{RL,R^{\prime }{L}^{\prime }}\left({\epsilon }_n\right)$, describing the slopes of the USWs at the five energies and their expansions in Hankel functions. For open structures, additional constraints are installed to pinpoint the interpolated function deep in the interstitial. The strong localization of the v&d functions makes the method uniquely suited for complicated structures. Use of point- and space-group symmetries can significantly reduce matrix sizes and the number of v&d functions. As simple examples, we consider a constant density and the valence-electron densities in zinc-blende structured Si, ZnSe, and CuBr.

We propose two schemes to realize a highly efficient solid-state source of photon pairs using four-wave mixing and stimulated Raman adiabatic passage in a single quantum dot embedded in a microcavity. A resonant continuous-wave laser applied between biexciton and exciton states and a pulsed laser applied between a ground state and exciton state are utilized to facilitate coherent pumping. We show in the case of four-wave mixing that, although the probability of generating two photons in a cavity mode is small without cavity damping, two-photon-resonant emission is enhanced by cavity damping within the strong-coupling regime. For strong continuous-wave laser, a single photon from a pulsed laser and two-photon-resonant transition through a strongly coupled cavity mode lead to a (1+2)-type Raman transition through the generated Autler-Townes doublet. We also discuss the spectrum of the generated photon pair and the photon-photon correlations in the generated photon pair.

We develop a field-theoretic approach to massless Dirac fermions in a supercritical Coulomb potential. By introducing an Aharonov–Bohm solenoid at the potential center, the critical Coulomb charge can be made arbitrarily small for one partial-wave sector, where a perturbative renormalization group analysis becomes possible. We show that a scattering amplitude for reflection of particle at the potential center exhibits the renormalization group limit cycle, i.e., log-periodic revolutions as a function of the scattering energy, revealing the emergence of discrete scale invariance. This outcome is further incorporated in computing the induced charge and current densities, which turn out to have power-law tails with coefficients log-periodic with respect to the distance from the potential center. Our findings are consistent with the previous prediction obtained by directly solving the Dirac equation and can in principle be realized by graphene experiments with charged impurities.

The quantum anomalous Hall effect (QAHE) originates from a combination of the spin-orbital coupling and the breaking of time-reversal symmetry due to intrinsic ferromagnetic ordering and was recently observed in Cr and V doped magnetic topological insulators (TIs). However, it was only observed at extremely low temperatures due to the low ferromagnetic Curie temperature and the tiny magnetically induced gap. To fully understand the mechanism of the ferromagnetic ordering, thereby improving the ferromagnetism, we investigated $4f$ transition metal doped $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$, using density functional theory approaches. We predict that Eu and Sm can introduce stable long-range ferromagnetic states in $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$, with large magnetic moments and low impurity disorders. Additionally, codoping is proposed to tune the Fermi level into the gap, which simultaneously improves the magnetic moment and the incorporation of magnetic ions. Our findings, thus, offer a step in facilitating the realization of QAHE in TI systems.

A recent quasielastic neutron scattering (QENS) study of hydrogen in heavily deformed fcc palladium provided the first direct measurement of hydrogen pipe diffusion, which has a significantly higher diffusivity and lower activation barrier than in bulk. While ab initio estimates of hydrogen diffusion near a dislocation corroborated the experimental values, open questions remain from the Chudley-Elliott analysis of the QENS spectra, including significant nonmonotonic changes in jump distance with temperature. We calculate the spherically averaged incoherent scattering function at different temperatures using our ab initio data for the network of site energies, jump rates, and jump vectors to directly compare to experiment. Diffusivities and jump distances are sensitive to how a single Lorentzian is fit to the scattering function. Using a logarithmic least squares fit over the range of experimentally measured energies, our diffusivities and jump distances agree well with those measured by experiment. However, these calculated quantities do not reflect barriers or distances in our dislocation geometry. This computational approach allows for validation against experiment, along with a more detailed understanding of the QENS results.

We investigate the nonequilibrium charge dynamics of a triple quantum dot and demonstrate how electron transport through these systems can give rise to nontrivial tunneling paths. Using a real-time charge sensing method, we establish tunneling pathways taken by particular electrons under well-defined electrostatic configurations. We show how these measurements map to the chemical potentials for different charge states across the system. We use a modified Hubbard Hamiltonian to describes the system dynamics and show is reproduces all experimental observations.

Physical properties induced by exchange interactions (Curie temperature and spin stiffness) and spin-orbit coupling (anomalous Hall effect, anisotropic magnetoresistance, and Gilbert damping) in the diluted (Ga,Mn)As ferromagnetic semiconductor are studied from first principles. Recently developed Kubo-Bastin transport theory and nonlocal torque operator formulation of the Gilbert damping as formulated in the tight-binding linear muffin-tin orbital method are used. The first-principles Liechtenstein mapping is employed to construct an effective Heisenberg Hamiltonian and to estimate Curie temperature and spin stiffness in the real-space random-phase approximation. Good agreement of calculated physical quantities with experiments on well-annealed samples containing only a small amount of compensating defects is obtained.

Crystal of ${\mathrm{GaV}}_4{\mathrm{S}}_8$, a multiferroic system hosting a Néel-type skyrmion lattice phase, has been investigated by polarized Raman and IR spectroscopy above and below the ferroelectric phase transition. Counts of the observed IR and Raman-active modes belonging to distinct irreducible representations agree quite well with group-theory predictions. Phonon spectra are assigned and interpreted with the aid of ab initio calculations of the phonon spectra in the ferroelectric phase. Results allow appreciation of phonon frequencies of the modes involved in Jahn-Teller distortion and their contribution to the spontaneous polarization.

First-principles simulations of the porous layered calcogenide materials ${\mathrm{Li}}_2{\mathrm{SnO}}_3$ and ${\mathrm{Li}}_2{\mathrm{SnS}}_3$ are used to study their structures, Li ion mobilities, and their interactions with excess Li. The pristine materials are characterized by a regular pattern of voids within the calcogenide layers which are occupied by intralayer Li ions. The energetically most favorable Li ion migration processes for both materials result in a net motion perpendicular to the layers and involve intralayer Li ions and nearby interstitial sites. The ideal lattice has eight symmetry related stable interstitial sites within the conventional unit cell which, in addition to participating in the Li ion migration processes, are also important for accommodating excess Li during lithiation processes. Consistent with experimental findings, the simulations find that the addition of Li atoms to ${\mathrm{Li}}_2{\mathrm{SnO}}_3$ results in a disruption of the calcogen lattice with the breaking of Sn-O bonds. The estimated voltage versus bcc Li for this system is in qualitative agreement with experiment provided that Sn/Li disorder is taken into account. By contrast, the simulations predict that the addition of Li atoms to ${\mathrm{Li}}_2{\mathrm{SnS}}_3$ results in a stable metallic material up to a stoichiometry of ${\mathrm{Li}}_3{\mathrm{SnS}}_3$. This prediction has not yet been studied experimentally. Simulations of surfaces of these materials find that it is energetically favorable to add a small amount of excess surface Li. However, interfaces of these materials with Li metal are found to be reactive. Some of the findings may be relevant to other materials having the same crystal structure such as ${\mathrm{Li}}_2{\mathrm{MnO}}_3$ and ${\mathrm{Li}}_2{\mathrm{TiO}}_3$.

The density of states $\varrho \left(E\right)$ of graphene is investigated within the tight-binding (Hückel) approximation in the presence of vacancies. They introduce a nonvanishing density of zero modes $n_{\text{zm}}$ that act as midgap states, $\varrho \left(E\right)=n_{\text{zm}}\delta \left(E\right)+\text{smooth}$. As is well known, the actual number of zero modes per sample can, in principle, exceed the sublattice imbalance, $N_{\text{zm}}\ge |N_{\text{A}}-N_{\text{B}}|$, where $N_{\text{A}},N_{\text{B}}$ denote the number of carbon atoms in each sublattice. In this paper, we establish a stronger relation that is valid in the thermodynamic limit and that involves the concentration of zero modes, $n_{\text{zm}}>|c_{\text{A}}-c_{\text{B}}|$, where $c_{\text{A}}$ and $c_{\text{B}}$ denote the concentration of vacancies per sublattice; in particular, $n_{\text{zm}}$ is nonvanishing even in the case of balanced disorder, $N_{\text{A}}/N_{\text{B}}=1$. Adopting terminology from benzoid graph theory, the excess modes associated with the current carrying backbone (percolation cluster) are called supernumerary. In the simplest cases, such modes can be associated with structural elements such as carbon atoms connected with a single bond, only. Our result suggests that the continuum limit of bipartite hopping models supports nontrivial “supernumerary” terms that escape the present continuum descriptions.

The ferromagnetic resonance (FMR) linewidth and its anisotropy in ${\mathrm{F}}_1/\mathrm{f}/{\mathrm{F}}_2$/AF multilayers, where spacer f has a low Curie point compared to the strongly ferromagnetic ${\mathrm{F}}_1$ and ${\mathrm{F}}_2$, is investigated. The role of the interlayer exchange coupling in magnetization relaxation is determined experimentally by varying the thickness of the spacer. It is shown that stronger interlayer coupling via thinner spacers enhances the microwave energy exchange between the outer ferromagnetic layers, with the magnetization of ${\mathrm{F}}_2$ exchange dragged by the resonance precession in ${\mathrm{F}}_1$. A weaker mirror effect is also observed: the magnetization of ${\mathrm{F}}_1$ can be exchange dragged by the precession in ${\mathrm{F}}_2$, which leads to antidamping and narrower FMR linewidths. A theory is developed to model the measured data, which allows separating various contributions to the magnetic relaxation in the system. Key physical parameters, such as the interlayer coupling constant, in-plane anisotropy of the FMR linewidth, and dispersion of the magnetic anisotropy fields, are quantified. These results should be useful for designing high-speed magnetic nanodevices based on thermally assisted switching.

We study the low-energy sector of the Heisenberg necklace model. Using the field-theory methods, we estimate how the coupling of the electronic spins with the paramagnetic Kondo spins affects the overall spin dynamics and evaluate its dependence on a magnetic field. We are motivated by the experimental realizations of the spin-1/2 Heisenberg chains in ${\mathrm{SrCuO}}_2$ and ${\mathrm{Sr}}_2{\mathrm{CuO}}_3$ cuprates, which remain one-dimensional Luttinger liquids down to temperatures much lower than the in-chain exchange coupling $J$. We consider the perturbation of the energy spectrum caused by the interaction $\gamma$ with nuclear spins ($I=3/2$) present on the same sites. We find that the resulting necklace model has a characteristic energy scale, $\mathrm{\Lambda }\sim J^{1/3}{\left(\gamma I\right)}^{2/3}$, at which the coupling between (nuclear) spins of the necklace and the spins of the Heisenberg chain becomes strong. This energy scale is insensitive to a magnetic field $B$. For ${\mu }_{B}B>\mathrm{\Lambda }$ we find two gapless bosonic modes that have different velocities, whose ratio at strong fields approaches a universal number, $\sqrt{2}+1$.

In this paper, we investigate the quasiparticle scattering interference (QPI) in the nematic phase of iron pnictides, based on the magnetic and orbital scenarios of nematicity, respectively. In the spin density wave (SDW) state, the QPI pattern exhibits a dimer structure in the energy region of the SDW gap, with its orientation along the ferromagnetic direction of the SDW order. When the energy is increased to be near the Fermi level, it exhibits two sets of dimers along the same direction. The dimer structure of the QPI patterns persists in the magnetically driven nematic phase, although the two dimers tend to merge together with energies closing to the Fermi level. While in the orbital scenario, the QPI patterns exhibit a dimer structure in a wide energy region. It undergoes a $\pi /2$ rotation with the increasing of energy, which is associated with the inequivalent energies of the two Dirac nodes induced by the orbital order. These distinct features may be used to probe or distinguish two kinds of scenarios of the nematicity.

The environment of a quantum dot that is connected to two leads is modeled by telegraph noise, i.e., random Markovian jumps of the (spinless) electron energy on the dot between two levels. The temporal evolutions of the charge on the dot and of the currents in the leads are studied using a recently developed single-particle basis approach, which is particularly convenient for the averaging over the histories of the noise. In the steady-state limit, we recover the Landauer formula. At a very fast jump rate between the two levels, the noise does not affect the transport. As the jump rate decreases, the effective average transmission crosses over from the transmission through a single (average) level to an incoherent sum of the transmissions through the two levels. The transient temporal evolution towards the steady state is dominated by the displacement current at short times, and by the Landauer current at long times. It contains oscillating terms that decay to zero faster than for the case without noise. When the average chemical potential on the leads equals the dot's “original” energy, without the noise, the oscillations disappear completely and the transient evolution becomes independent of the noise.