In cuprate high-temperature superconductors the small coherence lengths and high transition termperatures result in strong thermal fluctuations, which render the superconducting transition in applied magnetic fields into a wide continuous crossover. A state with zero resistance is found only below the vortex melting transition, which occurs well below the onset of superconducting correlations. Here we investigate the vortex phase diagram of the Fe-based superconductor in the form of a high-quality single crystal of Ba${}_{0.5}$K${}_{0.5}$Fe${}_2$As${}_2$, using three different experimental probes (specific heat, thermal expansion, and magnetization). We find clear thermodynamic signatures of a vortex melting transition, which shows that the thermal fluctuations in applied magnetic fields also have a considerable impact on the superconducting properties of iron-based superconductors.

The low energy excitations in ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ have been investigated using resonant inelastic x-ray scattering (RIXS). A magnetic excitation branch can be resolved, whose dispersion reaches a maximum energy of about 35 meV at the $\Gamma$ point. The momentum dependence of the excitation energy is much larger along the $\Gamma -X$ direction compared to that along the $\Gamma -Y$ direction. The observed dispersion relation is consistent with a recent theoretical prediction based on the Heisenberg-Kitaev model. At high temperatures, we find large contributions from lattice vibrational modes to our RIXS spectra, suggesting that a strong electron-lattice coupling is present in ${\mathrm{Na}}_2{\mathrm{IrO}}_3$.

The evolution of the magnetic structure for an inverse opal-like structure under an applied magnetic field is studied by small-angle neutron scattering. The samples were produced by filling the voids of an artificial opal film with Co. It is shown that the local configuration of magnetization is inhomogeneous over the basic element of the inverse opal-like lattice structure (IOLS) but follows its periodicity. Applying the “ice-rule” concept to the structure, we describe the local magnetization of this ferromagnetic three-dimensional lattice. We have developed a model of the remagnetization process predicting the occurrence of an unusual perpendicular component of the magnetization in the IOLS which is defined only by the direction and strength of the applied magnetic field.

We report a new magnetoresistance (MR) effect observed in a nominally nonmagnetic metal (Pt) thin film in contact with either a ferromagnetic insulator or a ferromagnetic metal. The resistivities with in-plane magnetic fields parallel (${\rho }_{\parallel }$) and transverse (${\rho }_{\mathrm{T}}$) to a current and a perpendicular field (${\rho }_{\perp }$) show the behavior of ${\rho }_{\perp }$ ≈ ${\rho }_{\parallel }$ > ${\rho }_{\mathrm{T}}$, distinctly different from all other known MR effects, including anisotropic MR with ${\rho }_{\parallel }$ > ${\rho }_{\mathrm{T}}$ ≈ ${\rho }_{\perp }$. We termed the new MR as hybrid MR, which appears in a metal in close proximity with a ferromagnet either insulating or metallic, and is associated with the induced magnetic moments at the interface.

The unoccupied states of complex materials are difficult to measure, yet they play a key role in determining their properties. We propose a technique that can measure the unoccupied states, called time-resolved Compton scattering, which measures the time-dependent momentum distribution (TDMD). Using a nonequilibrium Keldysh formalism, we study the TDMD for electrons coupled to a lattice in a pump-probe setup. We find a direct relation between temporal oscillations in the TDMD and the dispersion of the underlying unoccupied states, suggesting that both can be measured by time-resolved Compton scattering. We demonstrate the experimental feasibility by applying the method to a model of MgB${}_2$ with realistic material parameters.

Within the imaginary-time theory for nonequilibrium in quantum dot systems the calculation of dynamical quantities like Green's functions is possible via a suitable quantum Monte Carlo algorithm. The challenging task is to analytically continue the imaginary-time data for both complex voltage and complex frequency onto the real variables. To this end a function-theoretical description of dynamical observables is introduced and discussed within the framework of the mathematical theory of several complex variables. We construct a feasible maximum-entropy algorithm for the analytical continuation by imposing a continuity assumption on the analytic structure and provide results for spectral functions in stationary nonequilibrium and current-voltage characteristics for different values of the dot charging energy.

In this work we have investigated the structural and electronic properties of SnO, which is built up from layers kept together by van der Waals (vdW) forces. The combination of a vdW functional within density functional theory (DFT) and quasiparticle band structure calculations within the $GW$ approximation provides accurate values for the lattice parameters, atomic positions, and the electronic band structure including the fundamental (indirect) and the optical (direct) band gap without the need of experimental or empirical input. A systematic comparison is made between different levels of self-consistency within the $GW$ approach $\{$following the scheme of Shishkin et al. [Phys. Rev. B 75, 235102 (2007)]$\}$ and the results are compared with DFT and hybrid functional results. Furthermore, the effect of the vdW-corrected functional as a starting point for the $GW$ calculation of the band gap has been investigated. Finally, we studied the effect of the vdW functional on the electron charge density.

In this paper we apply an optimal control technique to derive control fields that transfer an electron between ends of a chain of donors or quantum dots. We formulate the transfer as an optimal steering problem, and then derive the dynamics of the optimal control. A numerical algorithm is developed to effectively generate control pulses. We apply this technique to transfer an electron between sites of a triple quantum dot and an ionized chain of phosphorus dopants in silicon. Using the optimal pulses for the spatial shuttling of phosphorus dopants, we then add hyperfine interactions to the Hamiltonian and show that a 500 G magnetic field will transfer the electron spatially as well as transferring the spin components of two of the four hyperfine states of the electron-nuclear spin pair.

The universal scaling of plasmon coupling in metallic nanostructures is now a well-established feature. However, if the interaction between dipolar plasmon modes has been intensively studied, this is not the case of the coupling between higher order ones. Using Mie theory extended to second harmonic generation, we investigate the coupling between quadrupolar plasmon modes in metallic nanoshells. Like in the case of dipolar plasmon modes, a universal scaling behavior is observed in agreement with the plasmon hybridization model.

We performed circular dichroism angle-resolved two-photon photoemission experiments on the quantum-well system one monolayer Pb on Cu(111) to investigate the orbital angular momentum (OAM) structure of the unoccupied quantum-well state. The Pb/Cu(111) system exhibits a small Rashba-type spin-orbit splitting of the unoccupied band around the $\overline{\Gamma }$ point and can be referred to as a material system with an intermediate spin-orbit coupling (SOC) parameter. As predicted by Kim et al. [Phys. Rev. B 85, 195402 (2012)], systems with such intermediate SOC strength should show a reversal of the OAM direction for the inner band of a spin-split system. Our results show dichroism values up to $\pm 20\%$ and also the expected change of the dichroism signal at a finite $k_{\parallel }$ point. The circular dichroism angle-resolved photoemission results therefore support the sensitivity of circular dichroism to OAM texture additionally to spin texture, and also that local OAM must be included to describe the experimentally observed large spin splittings in Rashba systems.

We report on the observation of the giant photocurrents in HgTe/HgCdTe quantum well (QW) of critical thickness at which a Dirac spectrum emerges. At an exciting QW of 6.6 nm width by terahertz (THz) radiation and sweeping magnetic field we detected a resonant photocurrent. Remarkably, the position of the resonance can be tuned from negative ($-$0.4 T) to positive (up to 1.2 T) magnetic fields by means of optical doping. The photocurrent data, accompanied by measurements of radiation transmission as well as Shubnikov–de Haas and quantum Hall effects, prove that the photocurrent is caused by cyclotron resonance in a Dirac fermion system, which allows us to obtain the effective electron velocity $v\approx 7.2\times {10}^5$ m/s. We develop a microscopic theory of the effect and show that the inherent spin-dependent asymmetry of light-matter coupling in the system of Dirac fermions causes the electric current to flow.

Using ab initio density functional theory we study the structure and electronic properties of hexagonal BN (h-BN) on Ni(111) and Co(0001) surfaces. Our calculations show that while dispersion interactions play an important role in stabilizing h-BN on the Ni(111) surface, on the Co(0001) surface it is primarily the covalent interactions. For h-BN on Ni(111) we show that semiempirical van der Waals correction proposed by Grimme to total energies obtained from density functional theory can correctly capture both the strong chemisorption minima closer to the surface and the weak physisorption minima further away from the surface. On both surfaces, the h-BN sheet becomes weakly ferrimagnetic. Additionally, on Ni(111) the h-BN sheet becomes half metallic and on Co(0001) it becomes metallic.

Density functional theory simulations with conventional (PBE) and hybrid (HSE06) functionals were performed to investigate the structural and electronic properties of MXene monolayers, ${\mathrm{Ti}}_{n+1}{\mathrm{C}}_n$ and ${\mathrm{Ti}}_{n+1}{\mathrm{N}}_n$ ($n=1$–9) with surfaces terminated by O, F, H, and OH groups. We find that PBE and HSE06 give similar results. Without functional groups, MXenes have magnetically ordered ground states. All the studied materials are metallic except for ${\mathrm{Ti}}_2{\mathrm{CO}}_2$, which we predict to be semiconducting. The calculated density of states at the Fermi level of the thicker MXenes ($n⩾5$) is much higher than for thin MXenes, indicating that properties such as electronic conductivity and surface chemistry will be different. In general, the carbides and nitrides behave differently with the same functional groups.

We show that the key formula in the works of Ding et al. [K.-H. Ding, Z.-G. Zhu, and J. Berakdar, Phys. Rev. B 79, 045405 (2009); K.-H. Ding, Z.-G. Zhu, and J. Berakdar, Phys. Rev. B 84, 115433 (2011); K.-H. Ding, Z.-G. Zhu, Z.-H. Zhang, and J. Berakdar, Phys. Rev. B 82, 155143 (2010)] is invalid in the extended graphene system they investigated. The correct formalism in the extended infinite system is also presented in this Comment.

In their Comment, Y. Zhou and M. W. Wu claim that the fundamental transport equation relating the current to the transmission function, used by us and in fact by numerous other researchers, is invalid for extended systems and should be corrected. They provide a “correct” new formula for transport in extended systems. This would be indeed a surprising new aspect of quantum transport theory. Here we show mathematically, however, that the “new formula” is a misconception resulting from adding an energy- and momentum-dependent function that has to vanish, due to fundamental reasons. Results and conclusions stemming from adding this function are irrelevant. The known established formulas for quantum transport are consistent with each other under the well-documented conditions.

We explore the optical properties of periodic layered media containing left-handed metamaterials. This study is based on several analogies between the propagation of light in metamaterials and charge transport in graphene. We derive the conditions when these two problems become equivalent, i.e., the equations and the boundary conditions for the corresponding wave functions coincide. We show that the photonic band-gap structure of a periodic system built of alternating left- and right-handed dielectric slabs contains conical singularities similar to the Dirac points in the energy spectrum of charged quasiparticles in graphene. Such singularities in the zone structure of the infinite systems give rise to rather unusual properties of light transport in finite samples. In an insightful numerical experiment (the propagation of a Gaussian beam through a mixed stack of normal and metadielectrics), we simultaneously demonstrate four Dirac point-induced anomalies: (i) diffusionlike decay of the intensity at forbidden frequencies, (ii) focusing and defocussing of the beam, (iii) absence of the transverse shift of the beam, and (iv) a spatial analogue of the Zitterbewegung effect. All of these phenomena take place in media with nonzero average refractive index and can be tuned by changing either the geometrical and electromagnetic parameters of the sample or the frequency and the polarization of light.

We introduce an alternative approach for obtaining smooth finite-temperature spectral functions of quantum impurity models using the numerical renormalization group (NRG) technique. It is based on calculating first the Green's function on the imaginary-frequency axis, followed by an analytic continuation to the real-frequency axis using Padé approximants. The arbitrariness in choosing a suitable kernel in the conventional broadening approach is thereby removed and, furthermore, we find that the Padé method is able to resolve fine details in spectral functions with less artifacts on the scale of $\omega \sim T$. We discuss the convergence properties with respect to the NRG calculation parameters (discretization $\Lambda$, $z$-averaging, truncation cutoff) and the number of Matsubara points taken into account in the analytic continuation. We test the technique on the single-impurity Anderson model and the Hubbard model (within the dynamical mean-field theory). For the Anderson impurity model, we discuss the shape of the Kondo resonance and its temperature dependence. For the Hubbard model, we discuss the inner structure of the Hubbard bands in metallic and insulating solutions at half-filling, as well as in the doped Mott insulator. Based on these test cases, we conclude that the Padé approximant approach provides improved results for spectral functions at low-frequency scales of $\omega \lesssim T$ and that it is capable of resolving sharp spectral features also at high frequencies.

Experimental X-ray absorption spectra at the fluorine K and transition metal $L$${}_{2,3}$ absorption edges of the $M$F${}_2$ ($M=\text{Cr}$-Ni) family are presented. Ligand field calculations in D${}_{4h}$ symmetry show very good agreement with the transition metal $L$${}_{2,3}$ XAS spectra. To successfully explain nominal Cr${}^{2+}$ $L$${}_{2,3}$ XAS spectrum in CrF${}_2$, the inclusion of Cr${}^{+}$ and Cr${}^{3+}$ was needed implying the presence of a disproportionation reaction. The multiplet calculations were then modified to remove the structure of the 2$p$ hole in the calculated $M$ $2p\to 3d$ absorption spectra. These results for the $3d^{n+1}$ states are in one to one correspondence with the leading edge structures found at the fluorine K edge. A direct comparison with the metal $L$${}_{2,3}$ edges also indicates that there is evidence of the metal multiplet at the fluorine K pre-edge structures.

We present a comprehensive experimental and theoretical study of photoluminescence of single Cd${}_{1-x}$Mn${}_x$Te quantum dots with Mn content $x$ ranging from 0.01 to 0.2. We distinguish three stages of the equilibration of the exciton–Mn ion spin system and show that the intermediate stage, in which the exciton spin is relaxed while the total equilibrium is not attained, gives rise to a specific asymmetric shape of the photoluminescence spectrum. From an excellent agreement between the measured and calculated spectra we are able to evaluate the exciton localization volume, number of paramagnetic Mn ions, and their temperature for each particular dot. We discuss the values of these parameters and compare them with results of other experiments. Furthermore, we analyze the dependence of average Zeeman shifts and transition linewidths on the Mn content and point out specific processes, which control these values at particular Mn concentrations.