We present a scheme for a single-photon transistor which can be implemented with only minor modifications of existing superconducting circuits. The proposal employs a three-level anharmonic ladder atom, e.g., a transmon qubit, placed in a cavity to mimic a $\Lambda$-type atom with two long-lived states. This configuration may enable a wide range of effects originally studied in quantum optical systems to be realized in superconducting systems, and in particular allow for single-photon transistors. We study analytically and numerically the efficiency and the gain of the proposed transistor as a function of the experimental parameters, in particular of the level anharmonicity and of the various decay and decoherence rates. State-of-the-art values for these parameters indicate that error probabilities of $\sim 1\%$ and gains of the order of hundreds can be obtained.

Real materials have structure at both the atomic or crystalline scale as well as at interfaces and defects at the larger scale of grains. There is a need for the study of materials at the “mesoscale,” the scale at which subgranular physical processes and intergranular organization couple to determine microstructure, crucially impacting constitutive response at the engineering macroscale. Diffractive imaging using photons that can penetrate multiple grains of material would be a transformative technique for the study of the performance of materials in dynamic extremes. Thicker samples imply higher energy photons of shorter wavelength, and imaging of multiple grains implies bigger spot sizes. Such imaging requires the use of future planned and proposed hard x-ray free electron lasers (such as the European XFEL) to provide both the spatial coherence transverse to the large spots and the peak brilliance to provide the short illumination times. The result is that the Fresnel number of the system becomes large and is no longer in the Fraunhofer far-field limit. The interrelated issues of diffractive imaging at large Fresnel number are analyzed, including proof that diffractive imaging is possible in this limit and estimates of the signal-to-noise possible. In addition, derivation of the heating rates for brilliant pulses of x rays are presented. The potential and limitations on multiple dynamic images are derived. This paper will present a study of x-ray interactions with materials in this new regime of spatially coherent but relatively large mesoscale spots at very hard energies. It should provide the theory and design background for the experiments and facilities required to control materials in extreme environments, in particular for the next generation of very-hard-x-ray free electron lasers.

The ab initio phase diagram of dense hydrogen is very sensitive to errors in the treatment of electronic correlation. Recently, it has been shown that the choice of the density functional has a large effect on the predicted location of both the liquid-liquid phase transition and the solid insulator-to-metal transition in dense hydrogen. To identify the most accurate functional for dense hydrogen applications, we systematically benchmark some of the most commonly used functionals using quantum Monte Carlo. By considering several measures of functional accuracy, we conclude that the van der Waals and hybrid functionals significantly outperform local density approximation and Perdew-Burke-Ernzerhof. We support these conclusions by analyzing the impact of functional choice on structural optimization in the molecular solid, and on the location of the liquid-liquid phase transition.

The elastic moduli of ${\mathrm{Fe}}_{1-c}{\mathrm{Co}}_c$ ($c\le 0.2$) alloys are found to decrease strongly with increasing temperature, but show very weak alloying effects for both low-temperature ferromagnetic and high-temperature paramagnetic states. For temperatures slightly below and around the Curie temperature of Fe, Co addition significantly increases the elastic moduli. The variation of the tetragonal shear elastic constant upon 20% Co addition increases from a small negative value to more than 135% as the temperature rises from 0 to 1200 K. The expected elastic softening in the case of Al doping is not confirmed. Both anomalous trends are ascribed to the interplay between intrinsic chemical effects, magnetism, and temperature.

The effect of an electric field on conduction in a disordered system is an old but largely unsolved problem. Experiments cover a wide variety of systems—amorphous/doped semiconductors, conducting polymers, organic crystals, manganites, composites, metallic alloys, double perovskites—ranging from strongly to weakly localized systems and from strongly to weakly correlated ones. Theories have singularly failed to predict any universal trend resulting in separate theories for separate systems. Here, we discuss a one-parameter scaling that has recently been found to give a systematic account of the field-dependent conductance in two diverse, strongly localized systems of conducting polymers and manganites. Except for a limited number of systems which are described by the hot electron models, the vast majority of different systems in various disorder regimes in two (2D)- and three (2D)-dimensions obey the scaling. The nonlinearity exponent $x$ associated with the scaling was found to be nonuniversal and exhibiting a structure. For 2D weakly localized systems, the nonlinearity exponent $x$ is $\ge$7 and is roughly inversely proportional to the sheet resistance. The existing theories of weak localization prove to be adequate and a complete scaling function is derived. In a 2D strongly localized system, a temperature-induced scaling-nonscaling transition (SNST) is revealed. For 3D strongly localized systems, the exponent lies between $-$1 and 1, and surprisingly is quantized ($x\approx 0.08n$). This poses a serious theoretical challenge. Various results are compared with predictions of the existing theories.

We present an ab initio investigation of the $\Lambda$-process-based ultrafast spin manipulation on positively charged two-magnetic-center molecular ions with a nonmagnetic bridging atom O in-between. Multiple spin-switching and spin-transfer scenarios are derived on the quasilinear structure [Fe-O-Co]${}^{+}$, which are used to build two closed, irreversible spin-dynamics cycles with respect to the spin localization and orientation. A mechanism addressing the “ERASE” functionality is proposed by properly exploiting the irreversibility of some laser-induced spin-manipulation scenarios, and the resulting Shannon entropy change is analyzed. Such controllable spin-dynamics cycles and logic functionality demonstrate promising applications in the design of spintronic devices.

The influence of the antiferromagnetic thickness and dilution on the training effect is investigated with the use of an atomistic model for the magnetic interaction for constant temperature. We analyze the phenomenology in both ferromagnet and antiferromagnet in terms of hysteresis loop quantities and stable spin populations during training. While for small antiferromagnetic layer thicknesses we observe thermal training, an increase in the AFM thickness leads to athermal training. In contrast an increase in the AFM dilution leads to athermal training, while in low dilution we observe thermal training. At a value of dilution in the range of $30–40\%$, we observe the largest exchange-bias field with the smallest training effect. The domain structure of the antiferromagnet changes rapidly with dilution, which is shown to give large changes in the training effect.

We present a generic theoretical model for torque differential magnetometry (TDM)—an experimental method for determining the magnetic properties of a magnetic specimen by recording the resonance frequency of a mechanical oscillator, on which the magnetic specimen has been mounted, as a function of the applied magnetic field. The effective stiffness change, and hence the resonance frequency shift, of the oscillator due to the magnetic torque on the specimen is calculated, treating the magnetic specimen as a single magnetic domain. Our model can deal with an arbitrary magnetic free-energy density characterizing the specimen, as well as any relative orientation of the applied magnetic field, the specimen, and the oscillator. Our calculations agree well with published experimental data. The theoretical model presented here allows one to take full advantage of TDM as an efficient magnetometry method.

We report ${}^{75}$As and ${}^{51}$V nuclear magnetic resonance (NMR) measurements on the iron-based superconductor ${\text{Sr}}_2$${\text{VFeAsO}}_3$ with alternating stacks structure. We find that the ${}^{75}$As nuclear spin-spin relaxation rate ($1/T_2$) shows a pronounced peak at $T_N$ = 165 K, below which the resonance peak shifts to a higher frequency due to the onset of an internal magnetic field. The ${}^{51}$V spectrum does not shift, but is broadened below $T_N$. We conclude that the Fe electrons order antiferromagnetically below $T_N$ with a magnetic moment $m_{Fe}$ $\sim$ 0.4 ${\mu }_B$. Application of external pressure up to 2.4 GPa reduces $T_N$ at a rate of $-$40 K/GPa, and enhances the superconducting transition temperature $T_c$ at a rate of 2 K/GPa. The pressure-temperature phase diagram for ${\text{Sr}}_2$${\text{VFeAsO}}_3$ shows that superconductivity coexists with antiferromagnetism over a wide pressure range with an unprecedented high $T_c$ up to 36.5 K.

We study the timely issue of charge order checkerboard patterns observed in a variety of cuprate superconductors. We suggest a minimal model in which strong quantum fluctuations in the vicinity of a single antiferromagnetic quantum critical point generate the complexity seen in the phase diagram of cuprates superconductors and, in particular, the evidenced charge order. The Fermi surface is found to fractionalize into hot spots and antinodal regions, where physically different gaps are formed. In the phase diagram, this is reflected by three transition temperatures for the formation of pseudogap, charge density wave, and superconductivity (or quadrupole density wave if a sufficiently strong magnetic field is applied). The charge density wave is characterized by modulations along the bonds of the CuO lattice with wave vectors connecting points of the Fermi surface in the antinodal regions. These features, previously observed experimentally, are so far unique to the quantum critical point in two spatial dimensions and shed a different light on the interplay between strongly fluctuating critical modes and conduction electrons in high-temperature superconductors.

We generalize the recently introduced dual-fermion (DF) formalism for disordered fermion systems by including the effect of interactions. For an interacting disordered system the contributions to the full vertex function have to be separated into crossing-asymmetric and crossing-symmetric scattering processes, and addressed differently when constructing the DF diagrams. By applying our approach to the Anderson-Falicov-Kimball model and systematically restoring the nonlocal correlations in the DF lattice calculation, we show a significant improvement over the dynamical mean-field theory and the coherent potential approximation for both one-particle and two-particle quantities.

In the giant Rashba semiconductor BiTeI, electronic surface scattering with Lorentzian linewidth is observed that shows a strong dependence on surface-termination and surface-potential shifts. A comparison with the topological insulator ${\mathrm{Bi}}_2$${\mathrm{Se}}_3$ evidences that surface-confined quantum well states are the origin of these processes. We notice an enhanced quasiparticle dynamics of these states with scattering rates that are comparable to polaronic systems in the collision-dominated regime. The $E_g$ symmetry of the Lorentzian scattering contribution is different from the chiral (RL) symmetry of the corresponding signal in topological insulators, although both systems have spin-split surface states.

Silicene, germanene, and stanene are two-dimensional topological insulators exhibiting helical edge states. We investigate global and local manipulations at the edges by exposing them to (i) a charge-density-wave order, (ii) a superconductor, (iii) an out-of-plane antiferromagnetic, and (iv) an in-plane antiferromagnetic field. We show that these perturbations affect the helical edge states in a different fashion. As a consequence one can realize quantum spin-Hall effect without edge states. In addition, these edge manipulations lead to very promising applications: a giant magnetoresistance and a perfect spin filter. We also investigate the effect of manipulations on a very few edge sites of a topological insulator nanodisk.

We have investigated electronic and magnetic properties of ultrathin ${\mathrm{SrRuO}}_3$ (SRO) films grown on (111) ${\mathrm{SrTiO}}_3$ substrates using the ab initio electronic structure calculations. Ru-terminated SRO (111) films suffer from strong surface atomic relaxations, while ${\mathrm{SrO}}_3$-terminated ones preserve the surface structure of ideal perovskites. Both Ru- and ${\mathrm{SrO}}_3$-terminated SRO (111) films show unexpected interlayer antiferromagnetic (AFM) structures at the surface, but with different characters and mechanisms. The AFM structure for the former results from the large surface atomic relaxation, whereas that for the latter results from the truncated film effect. Interestingly, for the ${\mathrm{SrO}}_3$-termination case, the half-metallic nature emerges despite the interlayer AFM structure. Upon reducing the thickness, the collapsing behavior of magnetic anisotropy from out-of-plane to in-plane easy axis is found to occur for the Ru-termination case, which, however, does not pertain to the ${\mathrm{SrO}}_3$-termination case.

We identify hyperfine-induced electron and nuclear spin cross-relaxation as the dominant physical mechanism for the longitudinal electron spin relaxation time $\left(T_1\right)$ of the spin-filtering $G{a}_i^{2+}$ defects in GaNAs alloys. This conclusion is based on our experimental findings that $T_1$ is insensitive to temperature over 4–300 K, and its exact value is directly correlated with the hyperfine coupling strength of the defects that varies between different configurations of the $G{a}_i^{2+}$ defects present in the alloys. These results thus provide a guideline for further improvements of the spin-filtering efficiency by optimizing growth and processing conditions to preferably incorporate the $G{a}_i^{2+}$ defects with a weak hyperfine interaction and by searching for new spin-filtering defects with zero nuclear spin.

The low-energy theory of the surface of the topological crystalline insulator (TCI) is characterized by four Dirac cones anisotropic into the $x$ and $y$ directions. Recent experiments have shown that the band gap can be introduced in these Dirac cones by crystal distortion by applying strain to the crystal structure. The TCI surface provides us with a way to valleytronics when gaps are given to Dirac cones. Indeed, the system has the Chern number and three valley-Chern numbers. We investigate the optical absorption on the TCI surface. It shows a strong elliptic dichroism though the four Dirac cones have the same chiralities. Namely, it is found that the absorptions of the right- and left-polarized light are different, depending on the sign of mass and the location of the Dirac cones, owing to the anisotropy of the Dirac cone. By measuring this elliptic dichroism it is possible to determine the anisotropy of a Dirac cone experimentally.

Bloch oscillation and Zener tunneling are investigated in helicoidal molecules, with DNA as the representative example, in the presence of spin-orbit coupling induced by electrical charges accumulated along the structure of the molecule. We show that the presence of the spin-orbit coupling does not destroy the Bloch oscillations and, further, it induces the appearance of nontrivial Bloch oscillation frequencies associated with resonances among Wannier-Stark states. The Zener tunneling between the spin states is also studied here by looking at the time evolution of the polarization of the wave packet. The results show that the polarization also oscillates with nontrivial well-determined frequencies.

We present an efficient approach to the electron correlation problem that is well suited for strongly interacting many-body systems, but requires only mean-field-like computational cost. The performance of our approach is illustrated for one-dimensional Hubbard rings with different numbers of sites, and for the nonrelativistic quantum-chemical Hamiltonian exploring the symmetric dissociation of the H${}_{50}$ hydrogen chain.

The octahedral tilting and ferroelectric-like structural transition of ${\mathrm{LiOsO}}_3$ metallic perovskite [Y. Shi et al., Nat. Mater. 12, 1024 (2013)] is studied using first-principles density-functional theory. In ${\mathrm{LiOsO}}_3$, the $a^{-}$$a^{-}$$a^{-}$ octahedral tilting mode is responsible for cubic ($Fm\overline{3}m$) to rhombohedral structure ($R\overline{3}c$), which is stable phase at room temperature, and at low temperature, noncentrosymmetric transition to $R3c$ rhombohedral transition is realized as a result of zone center phonon softening. By using density-functional calculation and phonon calculation, the phase transition behavior of ${\mathrm{LiOsO}}_3$ can be fully understood. We also present electronic structure change and Fermi surface change due to electron lattice coupling effect. The change of carrier density of state across phase transition is associated with experimental temperature dependence of resistivity, heat capacity, and susceptibility.