We propose a mechanism to explain the phenomenon of positive exchange bias on magnetic bilayered systems. The mechanism is based on the formation of a domain wall at a disordered interface during field cooling, which induces a symmetry breaking of the antiferromagnet, without relying on any ad hoc assumption about the coupling between the ferromagnetic (FM) and antiferromagnetic (AFM) layers. The domain wall is a result of the disorder at the interface between FM and AFM, which reduces the effective anisotropy in the region. We show that the proposed mechanism explains several known experimental facts within a single theoretical framework. This result is supported by Monte Carlo simulations on a microscopic Heisenberg model, by micromagnetic calculations at zero temperature, and by mean-field analysis of an effective Ising-like phenomenological model.

We compute by means of quantum Monte Carlo simulations the equation of state of bulk solid para-hydrogen extrapolated to zero temperature, up to a pressure of $\sim$2 Mbar. We compare the equation of state yielded by three different pair potentials, namely, the Silvera-Goldman (SG), Buck, and one recently proposed by Moraldi, modified at short distances to include a repulsive core, missing in the originally proposed potential. The Moraldi pair potential yields an equation of state in very good agreement with experiment at megabar pressures, owing to its softer core, and is at least as accurate as the SG or the Buck at saturated vapor pressure. Estimates for the experimentally measurable kinetic energy per molecule are provided for all pair potentials.

We report the lattice dynamic properties of CaF${}_2$ and CeO${}_2$ obtained using a direct method in combination of a mixed-space approach accounting for the vibration-induced dipole-dipole interactions. We overcame the overestimation of $T_{1u}$ TO mode shown in prior calculations using the linear response theory. For CaF${}_2$, we employed the Perdew-Burke-Ernzerhof functional and achieved a significant improvement over previous linear response calculations by comparing the results with experimentally measured phonon dispersion curves. For CeO${}_2$, we adopted the Heyd-Scuseria-Ernzerhof hybrid functional and an elongated supercell and obtained results in excellent agreement with experimental measurements. We attributed the improved calculation results to the convenience of the direct method in implementing new exchange-correlation functionals and use of the mixed-space approach for treating the long-range dipole-dipole interactions.

We use combined high resolution neutron diffraction (HRPD) with density functional theory (DFT) to investigate the exchange striction at the Curie temperature ($T_C$) of Fe${}_2$P and to examine the effect of boron and carbon doping on the P site. We find a significant contraction of the basal plane on heating through $T_C$ with a simultaneous increase of the $c$ axis that results in a small overall volume change of $\sim 0.01\%$. At the magnetic transition the Fe${}_I$-Fe${}_I$ distance drops significantly and becomes shorter than Fe${}_I$-Fe${}_{II}$. The shortest metal-metalloid (Fe${}_I$-P${}_I$) distance also decreases sharply. Our DFT model reveals the importance of the latter as this structural change causes a redistribution of the Fe${}_I$ moment along the $c$ axis (Fe-P chain). We are able to understand the site preference of the dopants, the effect of which can be linked to the increased moment on the Fe${}_I$ site, brought about by strong magnetoelasticity and changes in the electronic band structure.

Within the framework of exact diagonalization (ED), we compute the ground state of Anderson impurity problem using the variational approach based on the configuration interaction (CI) expansion. We demonstrate that an accurate ground state can be obtained by iteratively diagonalizing a matrix with the dimension that is less than 10$\%$ of the full Hamiltonian. The efficiency of the CI expansion for different problems is analyzed. By way of example, we apply this method to the single-site dynamical mean-field theory using ED as the impurity solver. Specifically, to demonstrate the usefulness of this approach, we solve the attractive Hubbard model in the grand-canonical ensemble, where the $s$-wave superconducting solution is explicitly obtained.

We investigate the low temperature ($T<2$ K) electronic structure of the heavy fermion superconductor CeCoIn${}_5$ ($T_c=2.3$ K) by angle-resolved photoemission spectroscopy (ARPES). The hybridization between conduction electrons and $f$ electrons, which ultimately leads to the emergence of heavy quasiparticles responsible for the various unusual properties of such materials, is directly monitored and shown to be strongly band dependent. In particular the most two-dimensional band is found to be the least hybridized one. A simplified multiband version of the periodic Anderson model (PAM) is used to describe the data, resulting in semiquantitative agreement with previous bulk sensitive results from de Haas–van Alphen measurements.

Quantum dots in silicon are promising candidates for the implementation of solid-state quantum information processing. It is important to understand the effects of the multiple conduction band valleys of silicon on the properties of these devices. Here we present a systematic effective mass theory of valley-orbit coupling in disordered silicon systems. This theory reveals valley-orbit hybridization effects that are detrimental for storing quantum information in the valley degree of freedom, including nonvanishing dipole matrix elements between valley states and altered intervalley tunneling.

The electronic structure of the recently synthesized ($3\times 3$) reconstructed silicene on ($4\times 4$) Ag(111) is investigated by first-principles calculations. New states emerge due to the strong hybridization between silicene and Ag. Analyzing the nature and composition of these hybridized states, we show that (i) it is possible to clearly distinguish them from states coming from the Dirac cone of free-standing silicene or from the $sp$ bands of bulk Ag and (ii) assign their contribution to the description of the linearly dispersing band observed in photoemission. Furthermore, we show that silicene atoms contribute to the Fermi level, which leads to similar scanning tunneling microscopy patterns as observed below or above the Fermi level. Our findings are crucial for the proper interpretation of experimental observations.

I analyze the correlation function of currents generated by a periodically driven quantum capacitor emitting single electrons and holes into a chiral waveguide. I compare adiabatic and nonadiabatic transient working regimes of a single-electron emitter and find a striking difference between the correlation functions in the two regimes. Quite generally, for a system driven with frequency $\Omega$ the correlation function depends on two frequencies $\omega$ and $\ell \Omega -\omega$, where $\ell$ is an integer. For an emitter driven nonadiabatically the correlation functions for different $\ell$ are similar and almost symmetric in $\omega$, while in the case of adiabatic driving the correlation functions for $\ell \ne 0$ are highly asymmetric in $\omega$ and exceed significantly the one corresponding to $\ell =0$. Under optimal operating conditions the correlation function for odd $\ell$ is zero.

An ideal emitter of entangled photon pairs combines the perfect symmetry of an atom with the convenient electrical trigger of light sources based on semiconductor quantum dots. Our source consists of strain-free GaAs dots self-assembled on a triangular symmetric (111)$A$ surface. The emitted photons reveal a fidelity to the Bell state as high as 86($\pm 2$)% without postselection. We show a violation of Bell's inequality by more than five times the standard deviation, a prerequisite to test a quantum cryptography channel for eavesdropping. Due to the strict nonlocal nature the source can be used for real quantum processing without any postprocessing. The remaining decoherence channel of the photon source is ascribed to random charge and nuclear spin fluctuations in and near the dot.

The existence of deconfined spinons is the defining property of quantum spin liquids. These exotic excitations have a (fractionalized) spin quantum number and no electric charge, and have been proposed to form Fermi surfaces in the recently discovered organic spin liquid candidates. However, direct probes for them are still lacking. Here we propose to experimentally identify the spinons by measuring the spin current flowing through the spin liquid candidate materials, which would be a direct test for the existence of spin-carrying mobile excitations. By the nonequilibrium Green's function technique we evaluate the spin current through the interface between a Mott insulator and a metal under a spin bias, and find that different kinds of Mott insulators, including quantum spin liquids, can be distinguished by different relations between the spin bias and spin current. We also discuss relations to experiments and estimate experimentally relevant parameters.

Biomolecular systems that involve thousands of atoms are difficult to address with standard density functional theory (DFT) calculations. With the development of sparse-matter methods such as the van der Waals density functional (vdW-DF) method [M. Dion et al., Phys. Rev. Lett. 92, 246401 (2004)], it is now possible to include the dispersive forces in DFT which are necessary to describe the cohesion and behavior of these systems. vdW-DF implementations can be as efficient as those for traditional DFT. Yet, the computational costs of self-consistently determining the electron wave functions and hence the kinetic-energy repulsion still limit the scope of sparse-matter DFT. We propose to speed up sparse-matter calculations by using the Harris scheme [J. Harris, Phys. Rev. B 31, 1770 (1985)]; that is, we propose to perform electronic relaxations only for separated fragments (molecules) and use a superposition of fragment densities as a starting point to obtain the total energy non-self-consistently. We evaluate the feasibility of this approach for an adaption of the Harris scheme for non-self-consistent vdW-DF (sfd-vdW-DF). We study four molecular dimers with varying degrees of polarity and find that the sfd scheme accurately reproduces standard non-self-consistent vdW-DF for van der Waals dominated systems but is less accurate for those dominated by polar interactions. Results for the S22 set of typical organic molecular dimers are promising.

In this article, we present a theoretical framework that provides predictable results on band gap modifications due to the addition of dopants into CdSe quantum dots (QDs). A theoretical model is developed that predicts a lowering of the conduction band minimum (CBM) due to hybridization. We then use x-ray absorption spectroscopy (XAS) at the Cd $M_3$-edge to determine the effects of chemical doping on the conduction band (CB) of the QD. Analysis of the XAS onset energy provides evidence for a lowering of the CBM, with our calculations yielding results comparable to experiment within 0.02 eV for tested materials. Also present in the XAS data is a distinct shift of the Cd $M_3$-edge peak maximum as a function of particle size, suggesting this peak can be used as a tracer to probe the angular momentum resolved shifts in the CB states due to quantum confinement. Our theoretical model can model a variety of dopants and theoretically predict the shift in the energy levels, and should be generalizable towards predicting similar behavior in other materials.

It is demonstrated both analytically and numerically that the properties of spin wave modes excited by a current-driven nanocontact of length $L$ in a quasi-one-dimensional magnetic waveguide magnetized by a perpendicular bias magnetic field $H_e$ are qualitatively different from the properties of spin waves excited by a similar nanocontact in a two-dimensional unrestricted magnetic film (“free layer”). In particular, there is an optimum nanocontact length $L_{\mathrm{opt}}$ corresponding to the minimum critical current of the spin wave excitation. This optimum length is determined by the magnitude of $H_e$, the exchange length, and the Gilbert dissipation constant of the waveguide material. Also, for $L<L_{\mathrm{opt}}$ the wavelength λ (and the wave number $k$) of the excited spin wave can be controlled by the variation of $H_e$ (λ decreases with the increase of $H_e$), while for $L>L_{\mathrm{opt}}$ the wave number $k$ is fully determined by the contact length $L$ ($k\sim 1/L$), similar to the case of an unrestricted two-dimensional free layer.

Motivated by a proposed experimental search for the electric dipole moment of the neutron (nEDM) utilizing neutron-${}^3$He capture in a dilute solution of ${}^3$He in superfluid ${}^4$He, we derive the transport properties of dilute solutions in the regime where the ${}^3$He are classically distributed and rapid ${}^3$He-${}^3$He scatterings keep the ${}^3$He in equilibrium. Our microscopic framework takes into account phonon-phonon, phonon-${}^3$He, and ${}^3$He-${}^3$He scatterings. We then apply these calculations to measurements by Rosenbaum et al. [J. Low Temp. Phys. 16, 131 (1974)] and by Lamoreaux et al. [Europhys. Lett. 58, 718 (2002)] of dilute solutions in the presence of a heat flow. We find satisfactory agreement of theory with the data, serving to confirm our understanding of the microscopics of the helium in the future nEDM experiment.

The melting curve of Ni up to 100 GPa has been calculated using first-principles methods based on density functional theory (DFT). We used two complementary approaches: (i) coexistence simulations with a reference system and then free-energy corrections between DFT and the reference system, and (ii) direct DFT coexistence using simulation cells including 1000 atoms. The calculated zero pressure melting temperature is slightly underestimated at $1637\pm 10$ K (experimental value is 1728 K), and at high pressure is significantly higher than recent measurements in diamond-anvil cell experiments [Phys. Rev. B 87, 054108 (2013)]. The zero pressure DFT melting slope is calculated to be $30\pm 2$ K, in good agreement with the experimental value of 28 K.

The transport properties described previously [Troć et al., Phys. Rev. B 85, 224434 (2012)] as well as the magnetic and thermal properties presented in this paper, observed for single-crystalline UCu${}_2$Si${}_2$, are discussed by assuming a dual (localized-itinerant) scenario. The electronic states of the localized 5$f$ electrons in UCu${}_2$Si${}_2$ are constructed using the effective Hamiltonian known for ionic systems, allowing us to treat the Coulomb, spin-orbital, and crystal-field interactions on equal footing. The space of parameters has been restricted in the initial steps with the aid of the angular overlap model approximation. The final crystal-field parameters, obtained from the refined steps of calculations, are relatively large (in absolute values), which we attribute to the hybridization characteristic of the metallic systems on the verge of localization. The proposed crystal-field model reproduces correctly with satisfactory accuracy the magnetic and thermal properties of UCu${}_2$Si${}_2$ in agreement also with the transport properties reported previously. Considerable crystal-field splitting of the ground multiplet of 2760 K is responsible for a large anisotropy in the magnetic behavior, observed in the whole temperature range explored.

Photoinduced insulator-metal transition in VO${}_2$ and the related transient and multi-time-scale structural dynamics upon photoexcitation are explained within a unified framework. Holes created by photoexcitation weaken the V-V bonds and eventually break V-V dimers in the $M_1$ phase of VO${}_2$ when the laser fluence reaches a critical value. The breaking of the V-V bonds in turn leads to an immediate electronic phase transition from an insulating to a metallic state while the crystal lattice remains monoclinic in shape. The coupling between excited electrons and the 6.0-THz phonon mode is found to be responsible for the observed zigzag motion of V atoms upon photoexcitation and is consistent with coherent phonon experiments.

We present a renormalized second-order perturbation theory (rPT2), based on a Kohn-Sham (KS) reference state, for the electron correlation energy that includes the random-phase approximation (RPA), second-order screened exchange (SOSEX), and renormalized single excitations (rSE). These three terms all involve a summation of certain types of diagrams to infinite order, and can be viewed as ``renormalization'' of the second-order direct, exchange, and single-excitation (SE) terms of Rayleigh-Schrödinger perturbation theory based on a KS reference. In this work, we establish the concept of rPT2 and present the numerical details of our SOSEX and rSE implementations. A preliminary version of rPT2, in which the renormalized SE (rSE) contribution was treated approximately, has already been benchmarked for molecular atomization energies and chemical reaction barrier heights and shows a well-balanced performance [J. Paier et al., New J. Phys. 14, 043002 (2012)]. In this work, we present a refined version of rPT2, in which we evaluate the rSE series of diagrams rigorously. We then extend the benchmark studies to noncovalent interactions, including the rare-gas dimers, and the S22 and S66 test sets, as well as the cohesive energy of small copper clusters, and the equilibrium geometry of 10 diatomic molecules. Despite some remaining shortcomings, we conclude that rPT2 gives an overall satisfactory performance across different electronic situations, and is a promising step towards a generally applicable electronic-structure approach.

Majorana fermions hold promise for quantum computation, because their non-Abelian braiding statistics allows for topologically protected operations on quantum information. Topological qubits can be constructed from pairs of well-separated Majoranas in networks of nanowires. The coupling to a superconducting charge qubit in a transmission line resonator (transmon) permits braiding of Majoranas by external variation of magnetic fluxes. We show that readout operations can also be fully flux controlled, without requiring microscopic control over tunnel couplings. We identify the minimal circuit that can perform the initialization-braiding-measurement steps required to demonstrate non-Abelian statistics. We introduce the Random Access Majorana Memory (RAMM), a scalable circuit that can perform a joint parity measurement on Majoranas belonging to a selection of topological qubits. Such multiqubit measurements allow for the efficient creation of highly entangled states and simplify quantum error correction protocols by avoiding the need for ancilla qubits.