First-principles density functional calculations show that the ${\text{SrRuO}}_3/{\text{PbTiO}}_3/{\text{SrRuO}}_3$ multiferroic junction with asymmetric (RuO${}_2$/PbO and TiO${}_2$/SrO) interfaces has a large ferroelectric depolarizing field, whose switching changes the interface transmission probabilities for tunneling electrons, leading to electroresistance modulation by over two orders of magnitude. The switching further affects the interface spin density, naturally driving magnetoresistance as well as modulated spin-dependent in-plane resistivity, which may be exploited in field-effect devices.

Highly accurate quantum electron dynamics calculations demonstrate that energy can be efficiently transferred between quantum dots. Specifically, in a double quantum dot an incoming electron is captured by one dot and the excess energy is transferred to the neighboring dot and used to remove an electron from this dot. This process is due to long-range electron correlation and shown to be operative at rather large distances between the dots. The efficiency of the process is greatly enhanced by preparing the double quantum dot such that the incoming electron is initially captured by a two-electron resonance state of the system. In contrast to atoms and molecules in nature, double quantum dots can be manipulated to achieve this enhancement. This mechanism leads to a surprisingly narrow distribution of the energy of the electron removed in the process which is explained by resonance theory. We argue that the process could be exploited in practice.

We apply diffusion quantum Monte Carlo to a broad set of solids, benchmarking the method by comparing bulk structural properties (equilibrium volume and bulk modulus) to experiment and density functional theory (DFT) based theories. The test set includes materials with many different types of binding including ionic, metallic, covalent, and van der Waals. We show that, on average, the accuracy is comparable to or better than that of DFT when using the new generation of functionals, including one hybrid functional and two dispersion corrected functionals. The excellent performance of quantum Monte Carlo on solids is promising for its application to heterogeneous systems and high-pressure/high-density conditions. Important to the results here is the application of a consistent procedure with regards to the several approximations that are made, such as finite-size corrections and pseudopotential approximations. This test set allows for any improvements in these methods to be judged in a systematic way.

We study a simple, effective, and robust method for entangling two separate stationary quantum dot spin qubits with high fidelity using multiphoton Gaussian state. The fluorescence signals from the two dots interfere at a beam splitter. The bosonic nature of photons leads, in analogy with the Hong-Ou-Mandel effect, to selective pairing of photon holes (photon absences in the fluorescent signals). As a result, two odd photon number detections at the outgoing beams herald trion entanglement creation, and subsequent reduction of the trions to the spin ground states leads to spin-spin entanglement. The robustness of the Gaussian states is evidenced by the ability to compensate for photon absorption and noise by a moderate increase in the number of photons at the input. We calculate the entanglement generation rate in the ideal, nonideal, and near-ideal detector regimes and find substantial improvement over single-photon schemes in all three regimes. Fast and efficient spin-spin entanglement creation can form the basis for a scalable quantum dot quantum computing network. Our predictions can be tested using current experimental capabilities.

We report the stabilization of a nonmagnetic spin-zero state of Ru${}^{4+}$ in SrRuO${}_3$ thin films under tensile strain, resulting in suppression of the saturation magnetization relative to bulk. The magnitude of suppression as a function of strain magnitude agrees well with density functional theory predictions of the magnetic ground state of (100) oriented films. In addition to the (100) orientation, we report on the effects of tensile strain on the magnetic properties of (110) and (111) oriented SrRuO${}_3$ thin films. All films show reduced saturation magnetization as well as enhancement of the Curie temperature, a universal in-plane magnetic easy axis, and anisotropy of the saturation magnetization. Films of all orientations exhibit characteristics of a magnetic easy axis as well as higher saturation magnetizations when magnetized in the film plane rather than along the film normal. We show that all of these effects can be attributed to the tensile strain induced by growth on KTaO${}_3$ substrates and explain them in terms of an anisotropic modification of the crystal field.

We investigate the low-energy spectral properties and robustness of the topological phase of color code, which is a quantum spin model for the aim of fault-tolerant quantum computation, in the presence of a uniform magnetic field or Ising interactions, using high-order series expansion and exact diagonalization. In a uniform magnetic field, we find first-order phase transitions in all field directions. In contrast, our results for the Ising interactions unveil that for strong enough Ising couplings, the ${\mathbb{Z}}_2\times {\mathbb{Z}}_2$ topological phase of color code breaks down to symmetry broken phases by first- or second-order phase transitions.

We present measurements of the microwave surface impedance of the single-layer cuprate Tl${}_2$Ba${}_2$CuO${}_{6+\delta }$, deep in the overdoped regime, with $T_c\approx 25$ K. Measurements have been made using cavity perturbation of a dielectric resonator at 17 discrete frequencies ranging from 2.50 to 19.16 GHz, and at temperatures from 0.12 to 27.5 K. From the surface impedance we obtain the microwave conductivity, penetration depth, and superfluid density. The superfluid density displays a strong linear temperature dependence from 2 to 14 K, indicative of line nodes in the energy gap. The microwave data are compared with theoretical predictions for a $d$-wave superconductor with pointlike impurities, with the conclusion that disorder in Tl${}_2$Ba${}_2$CuO${}_{6+\delta }$ acts predominantly in the weak-to-intermediate-strength scattering regime, and that small-angle scattering is important.

It has been suggested that the nearest-neighbor antiferromagnetic Heisenberg model on the Kagome lattice may be a good starting point for understanding the spin-liquid behavior discovered in herbertsmithite. In this work, we investigate possible quantum spin liquid phases in the presence of spin-rotation symmetry-breaking perturbations such as Dzyaloshinskii-Moriya and Ising interactions, as well as second-neighbor antiferromagnetic Heisenberg interactions. Experiments suggest that such perturbations are likely to be present in herbertsmithite. We use the projective symmetry group analysis within the framework of the slave-fermion construction of quantum spin liquid phases and systematically classify possible spin liquid phases in the presence of perturbations mentioned above. The dynamical spin-structure factor for relevant spin liquid phases is computed and the effect of those perturbations are studied. Our calculations reveal dispersive features in the spin structure factor embedded in a generally diffuse background due to the existence of fractionalized spin-$1/2$ excitations called spinons. For two of the previously proposed $Z_2$ states, the dispersive features are almost absent, and diffuse scattering dominates over a large energy window throughout the Brillouin zone. This resembles the structure factor observed in recent inelastic neutron-scattering experiments on singlet crystals of herbertsmithite. Furthermore, one of the $Z_2$ states with the spin structure factor with mostly diffuse scattering is gapped, and it may be adiabatically connected to the gapped spin liquid state observed in recent density-matrix renormalization group calculations for the nearest-neighbor antiferromagnetic Heisenberg model. The perturbations mentioned above are found to enhance the diffuse nature of the spin structure factor and reduce the momentum dependencies of the spin gap. We also calculate the electron spin resonance (ESR) absorption spectra that further characterize the role of spin-rotation symmetry-breaking perturbations in these states. We suggest that the measurement of ESR spectra can shed more light into the nature of the ground state in herbertsmithite.

Unconventional superconductors are most frequently identified by the observation of power-law behavior on low-temperature thermodynamic or transport properties, such as specific heat. Here, we show that, in addition to the usual point and line nodes, a much wider class of different nodal types can occur. These new types of nodes typically occur when there are transitions between different types of gap node topology, for example, when point or line nodes first appear as a function of some physical parameter. We identify anomalous, noninteger thermodynamic power laws associated with these new nodal types, and give physical examples of superconductors in which they might be observed experimentally, including the noncentrosymmetric superconductor Li${}_2$Pd${}_{3-x}$Pt${}_x$B.

We study spin resonance in the superconducting state of recently discovered alkali-intercalated iron selenide materials $A$${}_x$Fe${}_{2-y}$Se${}_2$ ($A=\text{K}$, Rb, Cs) in which the Fermi surface has only electron pockets. Recent angle-resolved photoemission spectroscopy (ARPES) studies [M. Xu et al., Phys. Rev. B 85, 220504(R) (2012)] were interpreted as strong evidence for $s$-wave gap in these materials, while the observation of the resonance peak in neutron scattering measurements [G. Friemel et al., Phys. Rev. B 85, 140511 (2012)] suggests that the gap must have different signs at Fermi surface points connected by the momentum at which the resonance has been observed. We consider recently proposed unconventional $s^{+-}$ superconducting state of $A_x$Fe${}_{2-y}$Se${}_2$ with superconducting gap changing sign between the hybridized electron pockets. We argue that such a state supports a spin resonance. We compute the dynamical structure factor and show that it is consistent with the results of inelastic neutron scattering.

Electron states in Ge/Si heterostructures with double quantum dots were studied by use of the electron spin resonance (ESR) method. It was demonstrated that the spatial localization of electrons as well as the localization in the momentum space can be controlled by the change of spacer thickness between quantum dots layers. New ESR signals, indicating the electron localization on the base edges of quantum dots, were obtained for the structures with the double layers of vertically aligned Ge quantum dots separated by a 2-nm-thick Si layer. Anisotropy of the $g$ factor is typical for electron states in ${\Delta }^{100}$ and ${\Delta }^{\overline{1}00}$ valleys. Strain distribution in the structures under study makes the localization of electrons in these valleys at the base edges of quantum dots energetically favorable. The broadening of the ESR line due to electron-hole exchange interaction was detected. Theoretical estimation of the exchange interaction magnitude based on the experimental data gives $J\approx 0.1$ $\mu$eV.

The recent experimental observations of designer Dirac fermions and topological phases in molecular graphene are addressed theoretically. Using scattering theory, we calculate the electronic structure of finite lattices of scattering centers dual to the honeycomb lattice. In good agreement with experimental observations, we obtain a V-shaped electron density of states around the Fermi energy. By varying the lattice parameter we simulate electron and hole doping of the structure, and by adding and removing scattering centers we simulate, respectively, vacancy and impurity defects. Specifically, for the vacancy defect we verify the emergence of a sharp resonance near the Fermi energy for increasing strength of the scattering potential.

We present experimental observations of picosecond Joule heating inside the gold metallization of ErAs:GaAs photoconductive switches. Femtosecond laser time-domain thermoreflectance is employed to resolve the fast thermal dynamics in the central switch electrode during generation and transmission of rf electrical pulses. Sharp features in the thermoreflectance signal scaling quadratically with the bias/pulse voltage reveal Joule heating with durations $\le$5 ps inside the gold metal. The temporal shape and rise time of the signals is in excellent agreement with the theoretical pulse wave form and subpicosecond carrier lifetime of the active medium. Probing different locations on the electrode shows the propagation, attenuation, and dispersion of the electrical pulses along the coplanar waveguide structure. Overall, the presented photothermal experiments demonstrate great potential to reveal the internal dynamics of photoconductive switches, characterize transmission lines, and study ultrafast heating in metals.

Recently, using a first principles approach, we predicted that zinc blende boron arsenide (BAs) will have an ultrahigh lattice thermal conductivity, κ, of over 2000 Wm${}^{-1}$K${}^{-1}$ at room temperature (RT), comparable to that of diamond. Here, we provide a detailed ab initio examination of phonon thermal transport in boron arsenide, contrasting its unconventional behavior with that of other related materials, including the zinc blende crystals boron nitride (BN), boron phosphide, boron antimonide, and gallium nitride (GaN). The unusual vibrational properties of BAs contribute to its weak phonon-phonon scattering and phonon-isotope scattering, which are responsible for its exceptionally high κ. The thermal conductivity of BAs has contributions from phonons with anomalously large mean free paths (∼2 μm), two to three times those of diamond and BN. This makes κ in BAs sensitive to phonon scattering from crystal boundaries. An order of magnitude smaller RT thermal conductivity in a similar material, zinc blende GaN, is connected to more separated acoustic phonon branches, larger anharmonic force constants, and a large isotope mixture on the heavy rather than the light constituent atom. The striking difference in κ for BAs and GaN demonstrates the importance of using a microscopic first principles thermal transport approach for calculating κ. BAs also has an advantageous RT coefficient of thermal expansion, which, combined with the high κ value, suggests that it is a promising material for use in thermal management applications.

The semiclassical motion of electrons in phase space $\mathbf{x}=(\mathbf{R},\mathbf{k})$ is influenced by Berry phases described by a six-component vector potential $\mathbf{A}=({\mathbf{A}}^{\mathrm{R}},{\mathbf{A}}^{\mathrm{k}})$. In chiral magnets, Dzyaloshinskii-Moriya (DM) interactions induce slowly varying magnetic textures (helices and skyrmion lattices) for which all components of $\mathbf{A}$ are important, inducing effectively a curvature in mixed position and momentum space. We show that for smooth textures and weak spin-orbit coupling, phase-space Berry curvatures determine the DM interactions and give important contributions to the charge. Using ab initio methods, we calculate the strength of DM interactions in MnSi in good agreement with experiment and estimate the charge of skyrmions.

We present the magnetic phase diagram of the multiferroic geometrically frustrated antiferromagnet CuCrO${}_2$ determined using dielectric constant and ultrasonic velocity measurements with the magnetic field parallel to the $\left[1\overline{1}0\right]$ direction. According to these measurements, at zero field the magnetoelectric phase induced by a proper screw magnetic ordering is observed below $T_{N1}=23.4$ K, while the velocity measurements reveal another transition at $T_{N2}=24.3$ K. As the dielectric and velocity measurements were performed simultaneously, our results confirm the presence of an intermediate nonferroelectric magnetic state between the magnetoelectric and paramagnetic phases. Moreover, our observations indicate that this intermediate phase persists up to 7 T for a field applied along the $\left[1\overline{1}0\right]$ direction. Based on similar observations obtained on the multiferric compounds CuO and MnWO${}_4$, this intermediate state is most likely a collinear antiferromagnetic phase. Apart from two phases observed at zero field, the ultrasonic velocity measured in the magnetoelectric phase as a function of the magnetic field reveals a spin-flop transition at $H_{\mathrm{flop}}=5$ T at 23 K, which is attributed to the flop of the spin spiral from the (110) plane to the $\left(1\overline{1}0\right)$ plane. In this paper, we also present a detailed analysis of the elastic properties of CuCrO${}_2$. The analysis of the data using a Landau-type free energy indicates that CuCrO${}_2$ undergoes a first-order pseudoproper ferroelastic transition leading to a $\overline{3}m\rightharpoonup 2/m$ structural transition at $T_{N2}$. According to the model, the order parameter of the ferroelastic-antiferromagnetic transition at $T_{N2}$ belongs to the $E_g$ irreducible representation of the trigonal $\overline{3}m$ point group, and the magnetic moments must act as a secondary order parameter.

We report on an approach to improving the performance of spin-torque nano-oscillators (STNOs) that utilizes power-dependent negative feedback to achieve significantly enhanced dynamic damping. In combination with a sufficiently slow variation of frequency with power, this can result in a quasilinear STNO, with weak nonlinear coupling of power and phase fluctuations over a range of bias currents and fields. An implementation of this approach that utilizes nonuniform spin torque applied to the oscillating magnetic free layer of a nanoscale spin-valve device demonstrates that highly coherent room-temperature STNOs can be achieved while retaining sufficient tunability.