We theoretically investigate the generation and dynamics of photocurrents induced by linearly polarized single-frequency light pulses in GaAs quantum wells. Our approach is based on the multiband semiconductor Bloch equations (SBE) formulated in the basis of eigenfunctions on the 14-band $\mathbf{k}·\mathbf{p}$ model and includes excitonic effects and carrier longitudinal-optical-phonon scattering processes. By solving the SBE, we obtain both shift and ballistic currents. For a particular excitation geometry, we obtain a ballistic current which is absent if the electron-hole attraction is neglected. Whereas in other cases excitonic effects quantitatively modify photocurrents that originate from single-particle properties, here we demonstrate the existence of a ballistic current which is absent in single-particle calculations. This photocurrent is of second order in the light-matter interaction and is purely caused by the asymmetric electron-hole Coulomb attraction that results from the inversion asymmetry of GaAs. Furthermore, we show that the coherent dynamics of excitonic wave packets gives rise to oscillations in the photocurrent transients.

We address the interaction between two quantum systems ($A$ and $B$) that is mediated by their common linear environment. If the environment is out of equilibrium, the resulting interaction violates Onsager relations and cannot be described by a Hamiltonian. In simple terms, the action of system $A$ on system $B$ does not necessarily produce a back action. We derive general quantum equations describing the situation, and we analyze in detail their classical correspondence. Changing the properties of the environment, one can easily change and engineer the resulting interaction. It is tempting to use this for quantum manipulation of the systems. However, the resulting quantum gate is not always unitary and may induce a loss of quantum coherence. For a relevant example, we consider systems $A$ and $B$ to be spins of arbitrary values and arrange the interaction to realize an analog of the two-qubit cnot gate. The direction of spin $A$ controls the rotation of spin $B$ while spin $A$ is not rotated experiencing no back action from spin $B$. We solve the quantum dynamics equations and analyze the purity of the resulting density matrix. The resulting purity essentially depends on the initial states of the systems. We attempt to find a universal characteristic of the purity, optimizing it for the worst choice of initial states. For both spins $s_A=s_B=1/2$, the optimized purity is bounded by 1/2 irrespective of the details of the gate. We also study in detail the semiclassical limit of large spins. In this case, the optimized purity is bounded by ${(1+\pi /2)}^{-1}\approx 0.39$. This is much better than the typical purity of a large spin state $\sim s^{-1}$. We conclude that although the quantum manipulation without back action inevitably causes decoherence of the quantum states, the actual purity of the resulting state can be optimized and made relatively high.

Motivated by recent progress on quantum engineered Kondo lattices, we numerically investigated the local magnetic properties of a hexagonal Anderson droplet consisting of multiple rings of magnetic atoms periodically arrayed on a triangular lattice. We demonstrated the tunability of the magnetic properties via their evolution with the droplet geometry for two types of systems with distinct local orbital occupancy profiles. We found that the local susceptibility of the droplet center of some types of droplets can be remarkably enhanced, in contrast to the conventionally rapid decrease due to spin correlations of surrounding droplet rings. The tunability of the magnetic properties is attributed to the charge redistribution with varying the droplet geometry enforced by the confined lattice with an open boundary. Our simulations complement the exploration of the novel artificial tunability of engineered lattice systems.

The magnetic, thermal, and transport properties as well as electronic band structure of MnPtSi are reported. MnPtSi is a metal that undergoes a ferromagnetic transition at $T_{\mathrm{C}}=340$(1) K and a spin-reorientation transition at $T_{\mathrm{N}}=326$(1) K to an antiferromagnetic phase. First-principles electronic structure calculations indicate a not-fully polarized spin state of Mn in a $d^5$ electron configuration with $J=S=3$/2, in agreement with the saturation magnetization of 3 ${\mu }_{\mathrm{B}}$ in the ordered state and the observed paramagnetic effective moment. A sizable anomalous Hall effect in the antiferromagnetic phase alongside the computational study suggests that the antiferromagnetic structure is noncollinear. Based on thermodynamic and resistivity data we construct a magnetic phase diagram. Magnetization curves $M(H$) at low temperatures reveal a metamagnetic transition of spin-flop type. The spin-flopped phase terminates at a critical point with $T_{\mathrm{cr}}\approx 300$ K and $H_{\mathrm{cr}}\approx 10$ kOe, near which a peak of the magnetocaloric entropy change is observed. Using Arrott plot analysis and magnetoresistivity data we argue that the metamagnetic transition is of a first-order type, whereas the strong field dependence of $T_{\mathrm{N}}$ and the linear relationship of the $T_{\mathrm{N}}$ with $M^2$ hint at its magnetoelastic nature.

A theory of interference-induced quantum corrections to conductivity is developed for two-dimensional systems with chiral spin textures including skyrmions. The effect of exchange interaction between electrons and spin textures on weak localization of electronic waves is studied. The spin dephasing rates are calculated as functions of the spin texture size. The anomalous magnetoresistance is shown to be governed by the size and magnetization spatial distribution of the spin textures. The effect of average magnetization-induced spin splitting on weak localization is analyzed. The sign-alternating weak-antilocalization magnetoresistance is demonstrated for skyrmion crystals. We argue that analysis of the low-field magnetoresistance might assist the experimental detection of chiral spin textures and, in particular, skyrmions.

In this paper, we study the topological properties of a 3D lattice dimer model. We demonstrate that the dimer model on a bipartite lattice possesses topological defects, which are exactly characterized by the Hopf invariant. We derive its explicit algebraic expression in terms of the effective magnetic field of a dimer configuration. Thus we solve the problem of topological classification of possible states in a 3D lattice dimer model. Furthermore, since the lattice dimer model is known to be dual to spin ice, our work can be viewed as a proposal to search for hopfions in classical, as well as artificial spin ice and related materials.

Recently, it has been proposed that a mechanism for the appearance of nonequilibrium superconductivity in a resonantly driven semiconductor with repulsive interband interactions exists [25]. The underlying microscopic model relies on the appearance of a specific fermionic dissipation mechanism and the careful simultaneous tailoring of the electronic dispersion relation and electron-electron interactions. We, instead, show that the phenomenon is rather general and does not need a special fine tuning of parameters. By considering a pair of bands with locally the same sign of concavity, we demonstrate that interband pairing arises under the natural assumption of the presence of phononic baths and radiative recombination. In light of these findings, we demonstrate how the emergence of superconductivity can be understood in terms of standard equilibrium interband BCS theory.

The existence of band gaps in Mott insulators such as perovskite oxides with partially filled 3d shells has been traditionally explained in terms of strong, dynamic interelectronic repulsion codified by the on-site repulsion energy U in the Hubbard Hamiltonian. The success of the “DFT+U approach” where an empirical on-site potential term U is added to the exchange- and correlation density functional theory (DFT) raised questions on whether U in DFT+U represents interelectronic correlation in the same way as it does in the Hubbard Hamiltonian, and if empiricism in selecting U can be avoided. Here we illustrate that ab initio DFT without any U is able to predict gapping trends and structural symmetry breaking (octahedra rotations, Jahn-Teller modes, bond disproportionation) for all $3dAB{\mathrm{O}}_3$ perovskites from titanates to nickelates in both spin-ordered and spin-disordered paramagnetic phases. Thus, the mechanism of gap formation due to the Hubbard Hamiltonian dynamic interelectronic correlation is not a requirement in these $3d$ electron compounds. We describe the paramagnetic phases as a supercell where individual sites can have different local environments thereby allowing DFT to develop finite moments on different sites as long as the total cell has zero moment. We use the recently developed strongly constrained appropriately normed exchange and correlation functional (SCAN) that is sanctioned by the usual single-determinant, mean-field DFT paradigm with static correlations, but has a more precise rendering of self-interaction cancelation. Our results suggest that strong dynamic electronic correlations are not playing a universal role in gapping of $3dAB{\mathrm{O}}_3$ Mott insulators, and opens the way for future applications of DFT for studying a plethora of complexity effects that depend on the existence of gaps, such as doping, defects, and band alignment in $AB{\mathrm{O}}_3$ oxides.

We present a Raman spectroscopic study of the layered antiferromagnetic Mott insulator ${\mathrm{V}}_{0.9}{\mathrm{PS}}_3$ and demonstrate the evolution of the spectra with applied quasihydrostatic pressure. Clear features in the spectra are seen at the pressures identified as corresponding to a structural transition between 20 and 80 kbar and the insulator-metal transition at 120 kbar. The feature at 120 kbar can be understood as a stiffening of interplanar vibrations, linking the metallization to a crossover from two- to three-dimensionality. Theoretical ab initio calculations, using the previously determined high-pressure structures, were able to reproduce the measured spectra and map each peak to specific vibration modes. We additionally show calculations of the high-pressure band structure in these materials, where the opening of a band gap with an included Hubbard $U$ term and its subsequent closing with pressure are clearly demonstrated. This little-studied material shows great promise as a model system for the fundamental study of low-dimensional magnetism and Mott physics.

Hybrid organic-inorganic perovskites, such as methylammonium lead tri-iodide (${\mathrm{MAPbI}}_3$), are interesting candidates for efficient absorber materials in next-generation solar cells, partly due to an unusual combination of low exciton-binding energy and strong optical absorption. Excitonic effects in this material have been subject to debate both for experiment and theory, indicating a need for better understanding of the screening mechanisms that act upon the electron-hole interaction. Here, we use cutting-edge first-principles theoretical spectroscopy, based on density-functional and many-body perturbation theory, to study atomic geometries, electronic structure, and optical properties of three ${\mathrm{MAPbI}}_3$ polymorphs and find good agreement with earlier results and experiment. We then study the influence of free electrons on the electron-hole interaction and show that this explains consistently smaller exciton-binding energies, compared to those in the material without free electrons. Interestingly, we also find that the absorption line shape strongly resembles that of the spectrum without free electrons up to high free-electron concentrations. We explain this unexpected behavior by formation of Mahan excitons that dominate the absorption edge, making ${\mathrm{MAPbI}}_3$ robust against free-electron-induced changes observed in other semiconductors.

A magnetophotoluminescence study of the carrier transfer with hybrid InAs/GaAs-quantum-dot–InGaAs-quantum-well structures is carried out where we observe an unusual dependence of the photoluminescence (PL) on the GaAs barrier thickness at strong magnetic field and excitation density. For the case of a thin barrier the quantum-well (QW) PL intensity is observed to increase at the expense of a decrease in the quantum-dot (QD) PL intensity. This is attributed to changes in the interplane carrier dynamics in the QW and the wetting layer (WL) resulting from increasing the magnetic field along with changes in the coupling between QD excited states and exciton states in the QW and the WL.

We study theoretically the phonon-induced relaxation and decoherence processes in the hybrid qubit in silicon. A hybrid qubit behaves as a charge qubit when the detuning is close to zero and as a spin qubit for large detuning values. It is realized starting from an electrostatically defined double quantum dot where three electrons are confined and manipulated through only electrical tuning. By employing a three-level effective model for the qubit and describing the environment bath as a series of harmonic oscillators in the thermal equilibrium states, we extract the relaxation and decoherence times as a function of the bath spectral density and of the bath temperature using the Bloch-Redfield theory. For Si quantum dots the energy dispersion is strongly affected by the physics of the valley, i.e., the conduction band minima, so we also included the contribution of the valley excitations in our analysis. Our results offer fundamental information on the system decoherence properties when the unavoidable interaction with the environment is included and temperature effects are considered.

Topological properties of quantum systems could provide protection of information against environmental noise, and thereby drastically advance their potential in quantum information processing. Most proposals for topologically protected quantum gates are based on many-body systems, e.g., fractional quantum Hall states, exotic superconductors, or ensembles of interacting spins, bearing an inherent conceptual complexity. Here, we propose and study a topologically protected quantum gate, based on a one-dimensional single-particle tight-binding model, known as the Su-Schrieffer-Heeger chain. The proposed $Y$ gate acts in the two-dimensional zero-energy subspace of a Y junction assembled from three chains, and is based on the spatial exchange of the defects supporting the zero-energy modes. With numerical simulations, we demonstrate that the gate is robust against hopping disorder but is corrupted by disorder in the on-site energy. Then we show that this robustness is topological protection, and that it arises as a joint consequence of chiral symmetry, time-reversal symmetry, and the spatial separation of the zero-energy modes bound to the defects. This setup will most likely not lead to a practical quantum computer; nevertheless it does provide valuable insight to aspects of topological quantum computing as an elementary minimal model. Since this model is noninteracting and nonsuperconducting, its dynamics can be studied experimentally, e.g., using coupled optical waveguides.

By performing density functional theory based first-principles calculations, the electronic, vibrational, elastic, and piezoelectric properties of two dynamically stable crystal phases of monolayer Janus MoSTe, namely $1H$-MoSTe and $1T^{\prime }$-MoSTe, are investigated. Vibrational frequency analysis reveals that the other possible crystal structure, $1T$-MoSTe, of this Janus monolayer does not exhibit dynamical stability. The $1H$-MoSTe phase is found to be an indirect band-gap semiconductor while $1T^{\prime }$-MoSTe is predicted as small-gap semiconductor. Notably, in contrast to the direct band-gap nature of monolayers $1H-{\mathrm{MoS}}_2$ and $1H-{\mathrm{MoTe}}_2, 1H$-MoSTe is found to be an indirect gap semiconductor driven by the induced surface strains on each side of the structure. The calculated Raman spectrum of each structure shows unique character enabling us to clearly distinguish the stable crystal phases via Raman measurements. The systematic piezoelectric stress and strain coefficient analysis reveals that out-of-plane piezoelectricity appears in $1H$-MoSTe and the noncentral symmetric $1T^{\prime }$-MoSTe has large piezoelectric coefficients. Static total-energy calculations show clearly that the formation of $1T^{\prime }$-MoSTe is feasible by using $1T^{\prime }-{\mathrm{MoTe}}_2$ as a basis monolayer. Therefore, we propose that the Janus MoSTe structure can be fabricated in two dynamically stable phases which possess unique electronic, dynamical, and piezoelectric properties.

A design paradigm of topology has recently emerged to manipulate the flow of phonons. At its heart lies a topological transition to a nontrivial state with exotic properties. This framework has been limited to linear lattice dynamics so far. Here we show a topological transition in a nonlinear regime and its implication in emerging nonlinear solutions. We employ nonlinearity management such that the system consists of masses connected with two types of nonlinear springs, “stiffening” and “softening” types, alternating along the length. We show, analytically and numerically, that the lattice makes a topological transition simply by changing the excitation amplitude and invoking nonlinear dynamics. Consequently, we witness the emergence of a family of finite-frequency edge modes, not observed in linear phononic systems. We also report the existence of kink solitons at the topological transition point. These correspond to heteroclinic orbits that form a closed curve in the phase portrait separating the two topologically distinct regimes. These findings suggest that nonlinearity can be used as a strategic tuning knob to alter topological characteristics of phononic crystals. These also provide fresh perspectives towards understanding a different family of nonlinear solutions in light of topology.

Energy transport and conversion at nanoscale have become an important topic of fundamental and applied research, in particular for conceiving ground-breaking solutions in energy-aware digital electronics and energy production. In this work, we propose a formal framework to address time-dependent energy transport inside quantum networks. The approach permits us to investigate how energy transferred to electrons by a femtosecond laser pulse is stored and released in a molecular circuit consisting of two donor-acceptor branches connected to an acceptor chain. Additionally, the two donors may be coupled, creating a loop inside the circuit. Time-resolved analysis reveals that when a difference exists between the two donor-acceptor branches, a loop current occurs and persists during relaxation, while only a small amount of current flows through the acceptor chain. A long-lasting energy flow thus emerges from the asymmetry of the molecular structure.

A universal quantum computing scheme, with a universal set of logical gates, is proposed based on networks of one-dimensional (1D) quantum systems. The encoding of information is in terms of universal features of gapped phases, for which effective field theories such as sine-Gordon field theory can be employed to describe a qubit. Primary logical gates are from twist, pump, glue, and shuffle operations that can be realized in principle by tuning parameters of the systems. Our scheme demonstrates the power of 1D quantum systems for robust quantum computing.

Structural and magnetic properties of iron-free and iron-substituted $\mathrm{SmC}{\mathrm{o}}_5$ have been investigated theoretically and experimentally. The nanocrystalline ribbons of ${\mathrm{SmCo}}_{5-x}{\mathrm{Fe}}_x\left(0\le x\le 2\right)$, which were produced by rapid solidification, crystallize in the hexagonal $\mathrm{CaC}{\mathrm{u}}_5$ structure for $x\le 0.75$. Small Fe additions $\left(x=0.25\right)$ substantially improve the coercivity, from 0.45 to 2.70 T, which we interpret as combined intrinsic and extrinsic effect. Most of our findings are consistent with past samarium-cobalt research, but some are at odds with findings that have seemingly been well established through decades of rare-earth transition-metal research. In particular, our local spin-density approximation with Hubbard parameter calculations indicate that the electronic structure of the Sm atoms violates Hund's rules and that the orbital moment is strongly quenched. Possible reasons for the apparent disagreement between theory and experiment are discussed. We explicitly determine the dependence of the Sm $4f$ charge distribution, arguing that an accurate density-functional description of $\mathrm{SmC}{\mathrm{o}}_5$ is a challenge to future research.