Generating on-demand maximally entangled states is one of the cornerstones for quantum information processing. Parity measurements can serve to create Bell states and have been implemented via an electronic Mach-Zehnder interferometer among others. However, the entanglement generation is necessarily harmed by measurement-induced dephasing processes in one of the two parity subspaces. In this work, we propose two different schemes of continuous feedback for a parity measurement. They enable us to avoid both the measurement-induced dephasing process and the experimentally unavoidable dephasing, e.g., due to fluctuations of the gate voltages controlling the initialization of the qubits. We show that we can generate maximally entangled steady states in both parity subspaces. Importantly, the measurement scheme we propose is valid for implementation of parity measurements with feedback loops in various solid-state environments.

We consider the two-dimensional electron gas confined laterally to a narrow channel by a harmonic potential. As the Zeeman splitting matches the intersubband separation, the nonlocal spin polarization develops a minimum as reported by Frolov et al. [Nature (London) 458, 868 (2009)]. This phenomenon, termed ballistic spin resonance, is due to the degeneracy between the nearest oppositely polarized subbands that is lifted by spin-orbit coupling. We showed that the resonance survives the weak and short-range interaction. The latter detunes it, and as a result shifts the Zeeman splitting at which the minimum in spin polarization occurs. Here this shift is attributed to the absence of the Kohn theorem for the spin-sloshing collective mode. We characterized the shift due to weak interaction qualitatively by analyzing the spin-sloshing mode within the Fermi liquid phenomenology.

We perform first-principles calculations of elemental cerium and compute its pressure-temperature phase diagram, finding good quantitative agreement with the experiments. Our calculations indicate that, while a signature of the volume-collapse transition appears in the free energy already at low temperatures, at higher temperatures this signature is enhanced because of the entropic effects, and originates an actual thermodynamical instability. Furthermore, we find that the catalyst determining this feature is—in all temperature regimes—a pressure-induced effective reduction of the $f$-level degeneracy due to the spin-orbit coupling. Our analysis suggests also that the lattice vibrations might be crucial in order to capture the behavior of the pressure-temperature transition line at large temperatures.

The electronic structure across the metal-insulator (MI) transition of electron-doped ${\mathrm{V}}_{1-x}{\mathrm{W}}_x{\mathrm{O}}_2$ epitaxial films ($x=0-0.06$) grown on $\alpha \text{−}\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ substrates was studied by means of thermopower ($S$) measurements. Significant increase of $\left|S\right|$ values accompanied by MI transition was observed, and the transition temperatures of $S$ ($T_S$) decreased with $x$ in a good linear relation with MI transition temperatures. $\left|S\right|$ values of ${\mathrm{V}}_{1-x}{\mathrm{W}}_x{\mathrm{O}}_2$ films at $T>T_S$ were constant at low values of $23\mu \mathrm{V}{\mathrm{K}}^{-1}$ independently of $x$, which reflects a metallic electronic structure, whereas those at $T<T_S$ almost linearly decreased with logarithmic W concentrations. The gradient of $-213\mu \mathrm{V}{\mathrm{K}}^{-1}$ agrees well with $-k_B/e\ln 10(-198\mu \mathrm{V}{\mathrm{K}}^{-1})$, suggesting that ${\mathrm{V}}_{1-x}{\mathrm{W}}_x{\mathrm{O}}_2$ films have insulating electronic structures with a parabolic density of state around the conduction band bottom.

We analyze the influence of quantum critical fluctuations on single-particle excitations at the onset of incommensurate $2k_F$ charge- or spin-density wave order in two-dimensional metals. The case of a single pair of hot spots at high symmetry positions on the Fermi surface needs to be distinguished from the case of two hot spot pairs. We compute the fluctuation propagator and the electronic self-energy perturbatively in leading order. The energy dependence of the single-particle decay rate at the hot spots obeys non-Fermi-liquid power laws, with an exponent 2/3 in the case of a single hot spot pair, and exponent one for two hot spot pairs. The prefactors of the linear behavior obtained in the latter case are not particle-hole symmetric.

${\mathrm{SmB}}_6$ has been predicted to be a strong topological Kondo insulator, and experimentally it has been confirmed that at low temperatures the electrical conductivity only takes place at the surfaces of the crystal. We study the temperature and magnetic field dependence of the NMR Knight shift and relaxation rate arising from the topological conduction states. For the clean surface the Landau quantization of the surface states gives rise to highly degenerate discrete levels for which the Knight shift is proportional to the magnetic field $B$ and inversely proportional to the temperature $T$. The relaxation rate $1/T_1$ is not Korringa-like. For the more realistic case of a surface with a low concentration of defects (dirty limit) the scattering of the electrons leads to a broadening of the Landau levels and hence to a finite density of states. The mildly dirty surface case leads to a $T$-independent Knight shift proportional to $B$ and a Korringa-like $1/T_1$ at low $T$. The wave functions of the surface states are expected to fall off exponentially with distance from the surface giving rise to a superposition of relaxation times, i.e., a stretched exponential. It is questionable that the experimental ${}^{11}\mathrm{B}$ Knight shift and relaxation rate arise from the surface states of the TKI. An alternative explanation is that the bulk susceptibility and the ${}^{11}\mathrm{B}$ NMR properties are the consequence of the in-gap bulk states originating from magnetic exciton bound states proposed by Riseborough [Phys. Rev. B 68, 235213 (2003)].

The maximal absorption rate of ultrathin films is 50% under the condition that the tangential electric (or magnetic) field is almost constant across the film in a symmetrical environment. However, with certain reflectors, the absorption rate can be greatly increased, even to perfect absorption (100%). In this paper, we explicitly derive the general conditions of the ultrathin absorptive film parameters to achieve perfect absorption with general types of reflectors under the condition that the tangential electric (or magnetic) field is almost constant across the film. We find that the parameters of the film can be classified into three groups, exhibiting (1) a large permittivity (permeability), (2) a near-zero permeability (permittivity), or (3) a suitable combination of permittivity and permeability, respectively. Interestingly, the latter two cases demonstrate extraordinary absorption in ultrathin films with almost vanishing losses. Our paper serves as a guide for designing ultrathin perfect absorbers with general types of reflectors.

We evaluate the impact that the amount of nonlocal Hartree-Fock (%HF) exchange included in a hybrid density functional has on the microscopic parameters (transfer integrals, band gaps, bandwidths, and effective masses) describing charge transport in high-mobility organic crystals. We consider both crystals based on a single molecule, such as pentacene, and crystals based on mixed-stack charge-transfer systems, such as dibenzo-TTF–TCNQ. In the pentacene crystal, the band gap decreases and the effective masses increase linearly with an increase in the amount of %HF exchange. In contrast, in the charge-transfer crystals, while the band gap increases linearly, the effective masses vary only slightly with an increase in %HF exchange. We show that the superexchange nature of the electronic couplings in charge-transfer systems is responsible for this peculiar evolution of the effective masses. We compare the density functional theory results with results obtained within the $G_0{W}_0$ approximation as a way of benchmarking the optimal amount of %HF exchange needed in a given functional.

We present a model of a qubit built of three coherently coupled quantum dots with three spins in a triangular geometry. The qubit states are encoded in the doublet subspace and they are controlled by a gate voltage, which breaks the triangular symmetry of the system. We show how to prepare the qubit and to perform one-qubit operations. A doublet blockade effect in the current will be discussed. The blockade is related to an asymmetry of transfer rates from the electrodes to different doublet states and is used to read out the dynamics of the qubit state. Our research also presents analysis of the Rabi oscillations, decoherence, and leakage processes in the doublets subspace.

Fundamental symmetries in thermoelectric quantum transport, beyond Onsagers relations, were predicted two decades ago but have to date not been observed in experiments. Recent works have predicted the symmetries to be sensitive to energy-dependent, inelastic scattering, raising the question whether they exist in practice. Here, we answer this question affirmatively by experimentally verifying the thermoelectric reciprocity relations in a four-terminal mesoscopic device where each terminal can be electrically and thermally biased individually. The linear-response thermoelectric coefficients are found to be symmetric under simultaneous reversal of magnetic field and exchange of injection and emission contacts. We also demonstrate a controllable breakdown of the reciprocity relations by increasing thermal bias, putting in prospect enhanced thermoelectric performance.

Application of a perpendicular magnetic field to charge neutral graphene is expected to result in a variety of broken symmetry phases, including antiferromagnetic, canted, and ferromagnetic. All these phases open a gap in bulk but have very different edge states and noncollinear spin order, recently confirmed experimentally. Here we provide an integrated description of both edge and bulk for the various magnetic phases of graphene Hall bars making use of a noncollinear mean field Hubbard model. Our calculations show that, at the edges, the three types of magnetic order are either enhanced (zigzag) or suppressed (armchair). Interestingly, we find that preformed local moments in zigzag edges interact with the quantum spin Hall like edge states of the ferromagnetic phase and can induce backscattering.

The formation of the roton-maxon excitation spectrum and the roton instability effect for a weakly correlated Bose gas of dipolar excitons in a semiconductor layer are predicted. The stability diagram is calculated. According to our numerical estimations, the threshold of the roton instability for Bose-Einstein condensed exciton gas with roton-maxon spectrum is achievable experimentally, e.g., in GaAs semiconductor layers.

The phase diagram of magnesium has been investigated to 211 GPa at 300 K, and to 105 GPa at 4500 K, by using a combination of x-ray diffraction and resistive and laser heating. The ambient pressure hcp structure is found to start transforming to the bcc structure at $\sim 45$ GPa, with a large region of phase-coexistence that becomes smaller at higher temperatures. The bcc phase is stable to the highest pressures reached. The hcp-bcc phase boundary has been studied on both compression and decompression, and its slope is found to be negative and steeper than calculations have previously predicted. The laser-heating studies extend the melting curve of magnesium to 105 GPa and suggest that, at the highest pressures, the melting temperature increases more rapidly with pressure than previously reported. Finally, we observe some evidence of a new phase in the region of 10 GPa and 1200 K, where previous studies have reported a double-hexagonal-close-packed (dhcp) phase. However, the additional diffraction peaks we observe cannot be accounted for by the dhcp phase alone.

We investigated the surface morphology and magnetic properties of Co films grown on MgO (100) substrates before and after treatment with a NaOH solution. The surface morphology evolves from a continuous film into isolated hexagonal nanodisks resulting from the formation of cobalt hydroxide. In the as-grown Co films the magnetic anisotropy behaves as a superposition of fourfold magnetocrystalline anisotropy and uniaxial magnetic anisotropy (UMA), and the magnetization reversal proceeds by two-step domain-wall motion. After the NaOH treatment the UMA is reduced and the magnetization reversal appears to be dominated by magnetization rotation. In addition, the exchange bias effect, which in the as-deposited films results from the presence of a cobalt oxide layer, disappears after the NaOH treatment, and can be reinduced by annealing the sample. The observed significant changes in the magnetic properties can be related to the disappearance and reappearance of a cobalt oxide top layer.

The local distribution of electrons and holes in the ${\mathrm{CuO}}_2$ plane of the high-temperature superconducting cuprates is determined with nuclear magnetic resonance through the quadrupole splittings of ${}^{17}\mathrm{O}$ and ${}^{63}\mathrm{Cu}$. Based on new data for single crystals of electron-doped ${\mathrm{Pr}}_{2-x}{\mathrm{Ce}}_x{\mathrm{CuO}}_4$ $(x=0$, 0.05, 0.10, 0.15) as well as ${\mathrm{Nd}}_{2-x}{\mathrm{Ce}}_x{\mathrm{CuO}}_4$ $(x=0$, 0.13) the changes in hole contents $n_d$ of Cu ${3d}_{x^2-y^2}$ and $n_p$ of O $2p_{\sigma }$ orbitals are determined and these account for the stoichiometrically doped charges, similar to hole-doped ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$. It emerges that while $n_d+2n_p=1$ in all parent materials as expected, $n_d$ and $n_p$ vary substantially between different groups of materials. Doping holes increases predominantly $n_p$, but also $n_d$. To the contrary, doping electrons predominantly decreases $n_d$ and only slightly $n_p$. However, $n_p$ for the electron-doped systems is higher than that in hole-doped ${\mathrm{La}}_{1.85}{\mathrm{Sr}}_{0.15}{\mathrm{CuO}}_4$. Cuprates with the highest maximum $T_{\mathrm{c}}$'s appear to have a comparatively low $n_d$ and high $n_p$. The rather high oxygen hole content of the ${\mathrm{Pr}}_2{\mathrm{CuO}}_4$ and ${\mathrm{Nd}}_2{\mathrm{CuO}}_4$ with the low $n_d$ seems to make them ideal candidates for hole doping to obtain a potentially high $T_{\mathrm{c}}$.

Strain-induced and chemical interactions of interstitial carbon atoms in bcc or $\alpha$-Fe are obtained in first-principles calculations. Subsequent Monte Carlo simulations show that at low temperatures, carbon atoms prefer to occupy at least two different octahedral sublattices, which is due to quite strong attractive interactions of carbon atoms at the corresponding coordination shells. The direct total-energy calculations of one of the obtained ordered structures with composition ${\mathrm{Fe}}_{16}{\mathrm{C}}_2$, show that it is more stable than the predicted earlier structure with the same composition but carbon atoms occupying only one octahedral sublattice. This indicates that the long-existing thermodynamic mean-field theory of ordering of carbon in $\alpha$-Fe assuming strong preference of carbon atoms to occupy only one octahedral sublattice is deficient. It is shown that the presence of carbon atoms only at one octahedral sublattice in the experimentally observed martensitic phase, ${\alpha }^{\prime }$-Fe, is a self-trapping effect. It occurs during a displacive martensitic transformation from $\gamma$- to $\alpha$-Fe, which kinematically transfers the carbon atoms from a single fcc octahedral sublattice to one of three octahedral sublattices, where they appear to be locked by a consequent tetragonal distortion minimizing elastic energy of the phase. The latter creates a strong preference for carbon atoms to be only at one already occupied octahedral sublattice preventing them from further distribution over the other sublattices.

${\mathrm{Bi}}_2{\mathrm{Se}}_3$ is a three-dimensional topological insulator which has been extensively studied because it has a single Dirac cone on the surface, inside a relatively large bulk band gap. However, the effect of two-dimensional topological insulator Bi bilayers on the properties of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ and vice versa, has not been explored much. Bi bilayers are often present between the quintuple layers of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, since ${\left({\mathrm{Bi}}_2\right)}_n{\left({\mathrm{Bi}}_2{\mathrm{Se}}_3\right)}_m$ form stable ground-state structures. Moreover, ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ is a good substrate for growing ultrathin Bi bilayers. By first-principles techniques, we first show that there is no preferable surface termination by either Bi or Se. Next, we investigate the electronic structure of Bi bilayers on top of, or inside a ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ slab. If the Bi bilayers are on top, we observe a charge transfer to the quintuple layers that increases the binding energy of the surface Dirac cones. The extra states, originating from the Bi bilayers, were declared to form a topological Dirac cone, but here we show that these are ordinary Rashba-split states. This result, together with the appearance of a new Dirac cone that is localized slightly deeper, might necessitate the reinterpretation of several experimental results. When the Bi bilayers are located inside the ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ slab, they tend to split the slab into two topological insulators with clear surface states. Interface states can also be observed, but an energy gap persists because of strong coupling between the neighboring quintuple layers and the Bi bilayers.

The topological crystalline insulator SnTe belongs to the recently discovered class of materials in which a crystalline symmetry ensures the existence of topologically protected surface states. The bulk band structure of SnTe is characterized by a band inversion at the four equivalent $L$ points, giving rise to a mirror Chern number of $n_{\mathcal{M}}=-2$. The (001) surface exhibits four Dirac cones which lie at non-time-reversal-invariant points close to $\overline{X}$ and $\overline{Y}$ and are protected by the $\left(\overline{1}10\right)$ and (110) mirror symmetries. In contrast to topological insulators, this symmetry can be broken via deformations of the crystal. This opens up new possibilities for manipulating the Dirac states and inducing a controllable gap. Here, we have employed density-functional theory to investigate the response of the Dirac states to applied distortions from first principles. Our calculations show that a local gap of up to $\approx 30\mathrm{meV}$ can be introduced via lattice deformations that break at least one of the underlying mirror symmetries. It is formed at either all four or just two cones, depending on the direction of the displacement vector, making it possible to create either a global gap or a state where opened and intact Dirac cones coexist. Notably, applying these deformations at the surface only can already induce the gap. If the complete slab is distorted, bulk bands are pushed into the gap making the whole system metallic.

We calculate the terahertz radiation that would be emitted from rotating current densities coherently injected by two-color optical pulses in GaAs and graphene samples in a magnetic field. This is done for realistic experimental geometries and parameters, with scattering and relaxation processes taken into account phenomenologically. Results are presented in the time domain for the expected terahertz signal observed at a detector. We include predictions for bilayer graphene as well as monolayer graphene, and compare with results expected in the absence of a magnetic field.

The Si(111)$4\times 1$-In surface is composed of metallic atomic wires, which undergo a transition into a charge density wave phase at a transition temperature $\left(T_c\right)$ of 125 K. This $T_c$ was reported recently to substantially increase upon the oxygen adsorption, for which the underlying mechanism is not understood. We investigate the structures of oxygen adsorbates on the Si(111)$4\times 1$-In surface by scanning tunneling microscopy (STM) and density-functional theory calculations. We identify three distinct atomic-scale structures induced by the oxygen adsorption with high-resolution STM topography. The calculations find two energetically favorable adsorption sites on and between In zigzag chains, respectively. In conjunction with an additional adsorption configuration, where O is buried underneath the In chain, three stable structures are thus identified that reproduce very well the characteristic bias-dependent STM images. Experimentally, a switching between two specific adsorption structures is observed and is consistent with the structure models proposed. The structural distortions and the charge transfer of In atomic wires around the adsorbates are also characterized. This work provides a solid basis for the microscopic understanding of the intriguing oxygen impurity effect on the phase transition.