We report on the resonance fluorescence and phase-dependent spectra in singly charged $n$-doped quantum dots in the presence of a strong external magnetic field in the Voigt geometry. The use of a nonresonant laser field driving two of the four active transitions results in the obtention of fluorescent photons along the four channels. The fluorescent photons coming from the two undriven transitions exhibit sidebands whose spectral location can be tuned through the Rabi frequency of the driving field and, most interestingly, their linewidths remain subnatural even for large values of the Rabi frequency of the laser field. It is shown that those sidebands present quantum fluctuations below the fundamental limit set by vacuum fluctuations. The numerical findings are fully explained by considering how the dressed states evolve in terms of the external physical magnitudes: the detuning, the Rabi frequency, and the dephasing rates.

It has been proposed that interactions lead to ferromagnetism on a zigzag edge of a graphene sheet. While not yet directly studied experimentally, dramatically improving techniques for making and studying clean zigzag edges may soon make this possible. So far, most theoretical investigations of this claim have been based on mean-field theories or more exact calculations using the Hubbard model. But long-range Coulomb interactions are unscreened in graphene, so it is important to consider their effects. We study rather general nonlocal interactions, including of the Coulomb $1/r$ form, using the technique of projection to a strongly interacting edge Hamiltonian, valid at first order in the interactions. The ground states as well as electron/hole and exciton excitations are studied in this model. Our results indicate that ferromagnetism survives with unscreened Coulomb interactions.

The recent experimental discovery of ${\phi }_0$ Josephson junctions by Szombati et al. [Nat. Phys. 12, 568 (2016)], characterized by a finite phase offset in the supercurrent, requires the same ingredients as topological superconductors, which suggests a profound connection between these two distinct phenomena. Here, we show that a quantum dot ${\phi }_0$ Josephson junction can serve as a qualitative indicator for topological superconductivity: microscopically, we find that the phase shift in a junction of $s$-wave superconductors is due to the spin-orbit induced mixing of singly occupied states on the quantum dot, while for a topological superconductor junction it is due to singlet-triplet mixing. Because of this important difference, when the spin-orbit vector of the quantum dot and the external Zeeman field are orthogonal, the $s$-wave superconductors form a $\pi$ Josephson junction, while the topological superconductors have a finite offset ${\phi }_0$ by which topological superconductivity can be distinguished from conventional superconductivity. Our prediction can be immediately tested in nanowire systems currently used for Majorana fermion experiments and thus offers a realistic approach for detecting topological bound states.

We describe solvable models that capture how impurity scattering in certain fractional quantum Hall edges can give rise to a neutral mode—i.e., an edge mode that does not carry electric charge. These models consist of two counterpropagating chiral Luttinger liquids together with a collection of discrete impurity scatterers. Our main result is an exact solution of these models in the limit of infinitely strong impurity scattering. From this solution, we explicitly derive the existence of a neutral mode and we determine all of its microscopic properties including its velocity. We also study the stability of the neutral mode and show that it survives at finite but sufficiently strong scattering. Our results are applicable to a family of Abelian fractional quantum Hall states of which the $\nu =2/3$ state is the most prominent example.

Using the recently introduced time-local formulation of the nonequilibrium cluster perturbation theory (CPT), we construct a generalization of the approach such that macroscopic conservation laws are respected. This is achieved by exploiting the freedom for the choice of the starting point of the all-order perturbation theory in the intercluster hopping. The proposed conserving CPT is a self-consistent propagation scheme which respects the conservation of energy, particle number, and spin, which treats short-range correlations exactly up to the linear scale of the cluster, and which represents a mean-field-like approach on length scales beyond the cluster size. Using Green's functions, conservation laws are formulated as local constraints on the local spin-dependent particle and the doublon density. We consider them as conditional equations to self-consistently fix the time-dependent intracluster one-particle parameters. Thanks to the intrinsic causality of the CPT, this can be set up as a step-by-step time propagation scheme with a computational effort scaling linearly with the maximum propagation time and exponentially in the cluster size. As a proof of concept, we consider the dynamics of the two-dimensional, particle-hole-symmetric Hubbard model following a weak interaction quench by simply employing two-site clusters only. Conservation laws are satisfied by construction. We demonstrate that enforcing them has strong impact on the dynamics. While the doublon density is strongly oscillating within plain CPT, a monotonic relaxation is observed within the conserving CPT.

Epitaxial strain, layer confinement, and inversion symmetry breaking have emerged as powerful new approaches to control the electronic and atomic-scale structural properties of complex metal oxides. Trivalent rare-earth (RE) nickelate $RE{\mathrm{NiO}}_3$ heterostructures have been shown to be exemplars since the orbital occupancy, degeneracy, and, consequently, electronic/magnetic properties can be altered as a function of epitaxial strain, layer thickness, and superlattice structure. One recent example is the tricomponent ${\mathrm{LaTiO}}_3–{\mathrm{LaNiO}}_3–{\mathrm{LaAlO}}_3$ superlattice which exhibits charge transfer and orbital polarization as the result of its interfacial dipole electric field. A crucial step towards control of these parameters for future electronic and magnetic device applications is to develop an understanding of both the magnitude and range of the octahedral network's response towards interfacial strain and electric fields. An approach that provides atomic-scale resolution and sensitivity towards the local octahedral distortions and orbital occupancy is therefore required. Here, we employ atomic-resolution imaging coupled with electron spectroscopies and first-principles theory to examine the role of interfacial charge transfer and symmetry breaking in a tricomponent nickelate superlattice system. We find that nearly complete charge transfer occurs between the ${\mathrm{LaTiO}}_3$ and ${\mathrm{LaNiO}}_3$ layers, resulting in a mixed ${\mathrm{Ni}}^{2+}/{\mathrm{Ni}}^{3+}$ valence state. We further demonstrate that this charge transfer is highly localized with a range of about 1 unit cell within the ${\mathrm{LaNiO}}_3$ layers. We also show how Wannier-function-based electron counting provides a simple physical picture of the electron distribution that connects directly with formal valence charges. The results presented here provide important feedback to synthesis efforts aimed at stabilizing new electronic phases that are not accessible by conventional bulk or epitaxial film approaches.

The exotic normal state of iron chalcogenide superconductor FeSe, which exhibits vanishing magnetic order and possesses an electronic nematic order, triggered extensive explorations of its magnetic ground state. To understand its novel properties, we study the ground state of a highly frustrated spin-1 system with bilinear-biquadratic interactions using an unbiased large-scale density matrix renormalization group. Remarkably, with increasing biquadratic interactions, we find a paramagnetic phase between Néel and stripe magnetic ordered phases. We identify this phase as a candidate of nematic quantum spin liquid by the compelling evidences, including vanished spin and quadrupolar orders, absence of lattice translational symmetry breaking, and a persistent nonzero lattice nematic order in the thermodynamic limit. The established quantum phase diagram naturally explains the observations of enhanced spin fluctuations of FeSe in neutron scattering measurement and the phase transition with increasing pressure. This identified paramagnetic phase provides a possibility to understand the novel properties of FeSe.

The main factor that determines which of the two domains forms upon reconstruction of the Si(110) “$16\times 2$” surface has been investigated. Low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM) images showed that the domain orientation was independent of the heating current direction used to induce the Si(110) “$16\times 2$” reconstruction. Reciprocal-space lattice models of the reconstruction allowed for the correct identification of domain orientations in the LEED images, and they confirmed that the reconstruction is two dimensionally chiral. It is proposed that the domain orientation upon surface reconstruction is determined by the direction of monatomic steps present on the Si(110) plane. This is determined in turn by the direction at which the surface is polished off-axis from the (110) plane.

The adsorption of azobenzene (AB), ortho fluoro-azobenzene (FAB) and ortho chlor-azobenzol (ClAB), in both the cis and trans isomers, at the Au(111) and Ag(111) surfaces is investigated using plane-wave density functional calculations with the revPBE-vdW-DF functional. The resulting adsorption energies and internal structures of AB adsorbed to both metal surfaces are in broad agreement with available experimental data. In the gas phase, FAB and ClAB feature a significant reduction in the energy difference between the two isomeric states, compared with AB. This relative reduction in the energy difference is still significant for the adsorbed form of FAB but is only weakly apparent for ClAB. The absorption spectra of the molecules have also been calculated, with the halogen substituents generating significant changes in the gas phase, but only a modest difference for the adsorbed molecules.

The optical absorption of ${\mathrm{CdWO}}_4$ is reported at high pressures up to 23 GPa. The onset of a phase transition was detected at 19.5 GPa, in good agreement with a previous Raman spectroscopy study. The crystal structure of the high-pressure phase of ${\mathrm{CdWO}}_4$ was solved at 22 GPa, employing single-crystal synchrotron x-ray diffraction. The symmetry changes from space group $P2/c$ in the low-pressure wolframite phase to $P{2}_1/c$ in the high-pressure postwolframite phase accompanied by a doubling of the unit-cell volume. The octahedral oxygen coordination of the tungsten and cadmium ions is increased to [7]-fold and [6+1]-fold, respectively, at the phase transition. The compressibility of the low-pressure phase of ${\mathrm{CdWO}}_4$ has been reevaluated with powder x-ray diffraction up to 15 GPa, finding a bulk modulus of $B_0=123$ GPa. The direct band gap of the low-pressure phase increases with compression up to 16.9 GPa at 12 meV/GPa. At this point an indirect band gap crosses the direct band gap and decreases at $-2$ meV/GPa up to 19.5 GPa where the phase transition starts. At the phase transition the band gap collapses by 0.7 eV and another direct band gap decreases at –50 meV/GPa up to the maximum measured pressure. The structural stability of the postwolframite structure is confirmed by ab initio calculations, finding the postwolframite-type phase to be more stable than the wolframite at 18 GPa. Lattice dynamic calculations based on space group $P{2}_1/c$ explain well the Raman-active modes previously measured in the high-pressure postwolframite phase. The pressure-induced band gap crossing in the wolframite phase as well as the pressure dependence of the direct band gap in the high-pressure phase are further discussed with respect to the calculations.

We evaluate entropy production in a photovoltaic cell that is modeled by four electronic levels resonantly coupled to thermally populated field modes at different temperatures. We use a formalism recently proposed, the so-called multiple parallel worlds, to consistently address the nonlinearity of entropy in terms of density matrix. Our result shows that entropy production is the difference between two flows: a semiclassical flow that linearly depends on occupational probabilities, and another flow that depends nonlinearly on quantum coherence and has no semiclassical analog. We show that entropy production in the cells depends on environmentally induced decoherence time and energy detuning. We characterize regimes where reversal flow of information takes place from a cold to hot bath. Interestingly, we identify a lower bound on entropy production, which sets limitations on the statistics of dissipated heat in the cells.

Recently, nonvolume conserving or non-Joulian magnetostriction has been reported in Fe-Ga alloys [H. D. Chopra and M. Wuttig, Nature (London) 521, 340 (2015); H. D. Chopra and M. Wuttig, Nature (London) 538, 416 (2016)] that increase their volume by expanding in magnetic fields. Here, we report a complementary set of contracting non-Joulian crystals (Fe-Ge, Fe-Al) that decrease their volume. Results provide the critical compositional degree of freedom necessary for designing new alloys (beyond the initially reported Fe-Ga alloys) and establish this family of functional materials based on non-Joulian magnetostriction.

We investigate spin pumping phenomena in polycrystalline CoFeB/Pt and CoFeB/Ta bilayers and the correlation between the effective spin-mixing conductance $g_{\mathrm{eff}}^{\uparrow \downarrow }$ and the obtained voltages generated by the spin-to-charge current conversion via the inverse spin Hall effect in the Pt and Ta layers. For this purpose, we measure the in-plane angular dependence of the generated voltages on the external static magnetic field and we apply a model to separate the spin pumping signal from the one generated by the spin rectification effect in the magnetic layer. Our results reveal a dominating role of anomalous Hall effect for the spin rectification effect with CoFeB and a lack of correlation between $g_{\mathrm{eff}}^{\uparrow \downarrow }$ and inverse spin Hall voltages pointing to a strong role of the magnetic proximity effect in Pt in understanding the observed increased damping. This is additionally reflected on the presence of a linear dependency of the Gilbert damping parameter on the Pt thickness.

Magnetic structures of nanosized (20 to 70 nm) powders of $\mathrm{MnW}{\mathrm{O}}_4$ and $\mathrm{MnW}{\mathrm{O}}_4$:Mo were studied using neutron powder diffraction (NPD). Sizes and shapes of the crystallites calculated from anisotropic peak broadening of diffraction peaks were found to be orthogonal parallelepipedlike with the longest edge along the $c$ axis and the shortest along the $b$ axis. SQUID measurements indicate the presence of two magnetic transitions around 8 and 12 K. Rietveld refinement of the NPD data below the magnetic transition was consistent with the presence of an incommensurate spiral-like ($ac\text{−}\mathrm{AF}2$) phase. A commensurate phase AF1 was not observed down to 2.5 K for all of the samples.

Valley-spin coupling in transition-metal dichalcogenides (TMDs) can result in unusual spin transport behaviors under an external magnetic field. Nonlocal resistance measured from two-dimensional materials such as TMDs via electrical Hanle experiments is predicted to exhibit nontrivial features compared with results from conventional materials due to the presence of intervalley scattering as well as a strong internal spin-orbit field. Here we report the all-electrical injection and nonlocal detection of spin-polarized carriers in trilayer ${\mathrm{MoS}}_2$ films. We calculate the Hanle curves theoretically when the separation between spin injector and detector is much larger than spin diffusion length ${\lambda }_s$. The experimentally observed curve matches the theoretically predicted Hanle shape under the regime of slow intervalley scattering. The estimated spin lifetime was found to be around 110 ps at 30 K.

Raman scattering has been employed to investigate lattice and magnetic excitations of the honeycomb Kitaev material $\alpha -{\mathrm{RuCl}}_3$ and its Heisenberg counterpart ${\mathrm{CrCl}}_3$. Our phonon Raman spectra give evidence for a first-order structural transition from a monoclinic to a rhombohedral structure for both compounds. Significantly, only $\alpha -{\mathrm{RuCl}}_3$ features a large thermal hysteresis, consistent with the formation of a wide phase of coexistence. In the related temperature interval of $70–170$ K, we observe a hysteretic behavior of magnetic excitations as well. The stronger magnetic response in the rhombohedral compared to the monoclinic phase evidences a coupling between the crystallographic structure and low-energy magnetic response. Our results demonstrate that the Kitaev magnetism concomitant with fractionalized excitations is susceptible to small variations of bonding geometry.

We present a slave-particle mean-field study of the mixed boson+fermion quantum dimer model introduced by Punk et al. [Proc. Natl. Acad. Sci. USA 112, 9552 (2015)] to describe the physics of the pseudogap phase in cuprate superconductors. Our analysis naturally leads to four charge $e$ fermion pockets whose total area is equal to the hole doping $p$ for a range of parameters consistent with the $t-J$ model for high-temperature superconductivity. Here we find that the dimers are unstable to $d$-wave superconductivity at low temperatures. The region of the phase diagram with $d$-wave rather than $s$-wave superconductivity matches well with the appearance of the four fermion pockets. In the superconducting regime, the dispersion contains eight Dirac cones along the diagonals of the Brillouin zone.

Momentum-resolved photoemission spectroscopy indicates the instability of the Dirac surface state upon deposition of gold on the (0001) surface of the topological insulator ${\mathrm{Bi}}_2{\mathrm{Se}}_3$. Based on the structure model derived from extended x-ray absorption fine structure experiments showing that gold atoms substitute bismuth atoms, first-principles calculations provide evidence that a gap appears due to hybridization of the surface state with gold $d$ states near the Fermi level. Our findings provide insights into the mechanisms affecting the stability of the surface state.

Critical properties of the dynamical phase transition in the quenched generalized Bose-Anderson impurity model are studied in the mean-field limit of an infinite number of channels. The transition separates the evolution toward ground state and toward the branch of stable excited states. We perform numerically exact simulations of a close vicinity of the critical quench amplitude. The relaxation constant describing the asymptotic evolution toward ground state, as well as asymptotic frequency of persistent phase rotation and number of cloud particles at stable excited state are power functions of the detuning from the critical quench amplitude. The critical evolution (separatrix between the two regimes) shows a non-Lyapunov power-law instability arising after a certain critical time. The observed critical behavior is attributed to the irreversibility of the dynamics of particles leaving the cloud and to memory effects related to the low-energy behavior of the lattice density of states.

The magnetic order induced by the pressure was studied in FeSe by means of muon spin rotation ($\mu \mathrm{SR}$) technique. By following the evolution of the oscillatory part of the $\mu \mathrm{SR}$ signal as a function of angle between the initial muon spin polarization and 101 axis of the studied FeSe sample, it was found that the pressure-induced magnetic order in FeSe corresponds either to the collinear (single-stripe) antiferromagnetic order as observed in parent compounds of various FeAs-based superconductors or to the bi-collinear order as obtained in the FeTe system, but with the Fe spins turned by ${45}^{\mathrm{o}}$ within the $ab$ plane. The value of the magnetic moment per Fe atom was estimated to be $\simeq 0.13–0.14{\mu }_{\mathrm{B}}$ at $p\simeq 1.9$ GPa.