We develop a procedure to determine the portion of exact Hartree-Fock exchange interaction contained in a hybrid density functional to treat the range of electronic correlation governing the physics of a system as a function of a thermodynamical parameter. This includes systems that depend on physical parameters accessible to experiment (i.e., temperature, pressure, composition, etc.) or those composed of two or more materials such as heterostructures and interfaces. This approach is applied to ${\mathrm{LaMnO}}_3$, where we are able to simulate the high temperature insulator-to-metal transition (IMT) and observe a half-metallic orbital disorder ferromagnetic state using density functional theory. In particular, we show that the softening of the $Q_2$ Jahn-Teller mode plays a central role in driving the IMT. These findings are likely to motivate the investigation of heterostructures and bulk materials that contain a range of electronic correlation in similar material systems.

The magnetic skyrmion with the topological number of unity $\left(Q=1\right)$ is a well-known nanometric swirling spin structure in the nonlinear $\sigma$ model with the Dzyaloshinskii-Moriya interaction. Here we show that magnetic skyrmion with the topological number of two $\left(Q=2\right)$ can be created and stabilized by applying vertical spin-polarized current though it cannot exist as a static stable excitation. Magnetic skyrmion with $Q=2$ is a nonequilibrium dynamic object, subsisting on a balance between the energy injection from the current and the energy dissipation by the Gilbert damping. Once it is created, it becomes a topologically protected object against fluctuations of various variables including the injected current itself. Hence we may call it a topologically protected dissipative structure. We also elucidate the nucleation and destruction mechanisms of the magnetic skyrmion with $Q=2$ by studying the evolutions of the magnetization distribution, the topological charge density, as well as the energy density. Our results will be useful for the study of the nontrivial topology of magnetic skyrmions with higher topological numbers.

We discuss the possible existence of a spin singlet excitation with charge $\pm 2$ ($\eta$ mode) originating the $A_{1g}$ Raman resonance in cuprate superconductors. This $\eta$ mode relates the $d$-wave superconducting singlet pairing channel to a $d$-wave charge channel. We show that the $\eta$ boson forms a particle-particle bound state below the $2\mathrm{\Delta }$ threshold of the particle-hole continuum where $\mathrm{\Delta }$ is the maximum $d$-wave gap. Within a generalized random phase approximation and Bethe-Salpeter approximation study, we find that this mode has energies similar to the resonance observed with inelastic neutron scattering below the superconducting (SC) coherent peak at $2\mathrm{\Delta }$ in various SC cuprate compounds. We show that it is a very good candidate for the resonance observed in Raman scattering below the $2\mathrm{\Delta }$ peak in the $A_{1g}$ symmetry. Since the $\eta$ mode sits in the $S=0$ channel, it may be observable via Raman, x-ray, or electron energy loss spectroscopy probes.

By means of a multiscale first-principles approach, a description of the local electronic structure of two-dimensional and narrow phosphorene sheets with various types of modifications is presented. First, a rational argument based on the geometry of the pristine and modified P network, and supported by the Wannier functions formalism, is introduced to describe a hybridization model of the P atomic orbitals. Ab initio calculations show that nonisoelectronic foreign atoms form quasibound states at varying energy levels and create different polarization states depending on the number of valence electrons between P and the doping atom. The quantum transport properties of modified phosphorene ribbons are further described with great accuracy. The distortions on the electronic bands induced by the external species lead to strong backscattering effects on the propagating charge carriers. Depending on the energy of the charge carrier and the type of doping, the conduction may range from the diffusive to the localized regime. Interstitial defects at vacant sites lead to homogeneous transport fingerprints across different types of doping atoms. We suggest that the relatively low values of charge mobility reported in experimental measurements may have their origin in the presence of defects.

A fast and efficient way to calculate and generate an accurate surface energy database (of more than several million surface energy data points) for all bcc and fcc metals is proposed based on an effective pair-wise-potential model. The accuracy of this model is rigorously tested and verified by employing density functional theory calculations, which shows good agreement within a mean absolute error of 0.03 eV/atom. The surface energy database generated by this model is then visualized and mapped in various ways; namely, the surface energy as a function of relative orientation, a orientation-dependent stereographic projection (the so-called Wulff net), and Gibbs–Wulff construction of the equilibrium crystal shape, for comparison and analysis. The Wulff nets (drawn with several million surface energy data points) provide us with characteristic surface energy maps of these cubic metals. In an attempt to explain the surface energy anomaly in bcc Li, we demonstrate how our effective-pair-potential-derived Wulff net can clearly discriminate the strong influence of the second- and third-nearest-neighbor bonds on the high-Miller-index surface energetics of bcc Li.

We report a theoretical study of time-dependent transport in a ballistic graphene field effect transistor. We develop a model based on Floquet theory describing Dirac electron transmission through a harmonically driven potential barrier. Photon-assisted tunneling results in excitation of quasibound states at the barrier. Under resonance conditions, the excitation of the quasibound states leads to promotion of higher-order sidebands and, in particular, an enhanced second harmonic of the source-drain conductance. The resonances in the main transmission channel are of the Fano form, while they are of the Breit-Wigner form for sidebands. For weak ac drive strength $Z_1$, the dynamic Stark shift scales as $Z_1^4$, while the resonance broadens as $Z_1^2$. We discuss the possibility of utilizing the resonances in prospective ballistic high-frequency devices, in particular frequency doublers operating at high frequencies and low temperatures.

In recent charge-pump experiments, single electrons are injected into quantum Hall edge channels at energies significantly above the Fermi level. We consider here the relaxation of these hot edge-channel electrons through longitudinal-optical-phonon emission. Our results show that the probability for an electron in the outermost edge channel to emit one or more phonons en route to a detector some microns distant along the edge channel suffers a double-exponential suppression with increasing magnetic field. This explains recent experimental observations. We also describe how the shape of the arrival-time distribution of electrons at the detector reflects the velocities of the electronic states post phonon emission. We show how this can give rise to pronounced oscillations in the arrival-time-distribution width as a function of magnetic field or electron energy.

We propose a model that is intended to qualitatively capture the electron-electron interaction physics of two-dimensional electron gases formed near transition-metal oxide heterojunctions containing $t_{2g}$ electrons with a density much smaller than one electron per metal atom. Two-dimensional electron systems of this type can be described perturbatively using a $GW$ approximation, which predicts that Coulomb interactions enhance quasiparticle effective masses more strongly than in simple two-dimensional electron gases, and that they reshape the Fermi surface, reducing its anisotropy.

Floquet topological insulators are systems in which the topology emerges only when a time-periodic perturbation is applied. In these systems one can define quasienergy states which replace the equilibrium stationary states. The system exhibits its nontrivial topology by developing edge-localized quasienergy states which lie in a gap of the quasienergy spectrum. These states represent a nonequilibrium analog of the topologically protected edge states in equilibrium topological insulators which exhibit an edge conductance of $2e^2/h$. Here we explore the transport properties of the edge states in a Floquet topological insulator. In stark contrast to the equilibrium result, we find that the two-terminal conductivity of these edge states is significantly different from $2e^2/h$. This fact notwithstanding, we find that for certain external potential strengths the conductivity is smaller than $2e^2/h$ and robust to the effects of disorder and smooth changes to the Hamiltonian's parameters. This robustness is reminiscent of the robustness found in equilibrium topological insulators. We provide an intuitive understanding of the reduction of the conductivity in terms of a picture where electrons in edge states are scattered by photons. We also consider the Floquet sum rule [A. Kundu and B. Seradjeh, Phys. Rev. Lett. 111, 136402 (2013)], which was proposed in a different context. The summed conductivity recovers the equilibrium value of $2e^2/h$ whenever edge states are present. We show that this sum rule holds in our system using both numerical and analytic techniques.

A well-known analytic model for Lorentzian broadening of metallic NMR lines by dilute localized magnetic centers embedded in a lattice has been applied to the case of the twofold-symmetry magnetism in ${\mathrm{URu}}_2{\mathrm{Si}}_2$ reported by R. Okazaki et al. [Science 331, 439 (2011)]. The observed Lorentzian spectra are accounted for with a simple formula giving the ${}^{29}\mathrm{Si}$ NMR linewidth in terms of the susceptibility of the magnetic-broadening centers and a Ruderman-Kittel-Kasuya-Yosida (RKKY) coupling parameter. The concentration of such centers is estimated as $c\sim 0.01$. A numerical simulation of these effects confirms Lorentzian broadening with no measurable NMR shift and a width in reasonable agreement with the analytical model. The simulation shows further that domain effects on these spectra are largely absent. A four-site extended model of the broadening centers gives an estimate of the twofold susceptibility within a factor of 2 of the torque value of the susceptibility. Hypothetical superlattice effects are shown to be easily smoothed over by convolution with background Lorentzian broadening.

The order parameter and pairing mechanism for superconductivity in heavy-fermion compounds are still poorly understood. Scanning tunneling microscopy and spectroscopy at ultralow temperatures can yield important information about the superconducting order parameter and the gap structure. Here, we study the first heavy-fermion superconductor, ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$. Our data show the superconducting gap which is not fully formed and exhibits features that point to a multigap order parameter. Spatial mapping of the zero-bias conductance in magnetic field reveals the vortex lattice, which allows us to unequivocally link the observed conductance gap to superconductivity in ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$. The vortex lattice is found to be predominantly triangular with distortions at fields close to $\sim 0.7H_{c2}$.

A method and nanodevice are introduced that allows us to rotate the single electron spin confined in a gated electrostatic InSb nanowire quantum dot. The proposed method does not require the application of any (oscillating or static) external magnetic fields. Our proposal instead employs spatial and time modulation of confining potential induced by electric gates, which, in turn leads to oscillating Rashba-type spin-orbit coupling. Moving electron back and forth in such a variable Rashba field allows for the realization of spin rotations around two different axes separately without using an external magnetic field. The results are supported by realistic three-dimensional time-dependent Poisson-Schrödinger calculations for systems and material parameters corresponding to experimentally accessible structures.

Interatomic Coulombic decay (ICD) is an ionization phenomena that takes place due to the Coulombic electronic repulsion force that correlates the ionized electrons with the other electrons. Here we present an analytical analysis of the ICD in two coupled quantum wells (QWs) which shows that the ICD ionization of an electron is in the perpendicular direction to the semiconductor layers. This happens when two conditions are satisfied. The first condition requires the excitation of the two electrons from the valence band. The first electron is excited to the bottom of the excited subband in one quantum well, and the second to the bottom of the first subband of the adjacent QW. The second condition ensures that the distance between the wells is large enough. In our case the two QWs are far apart when the distance between the two QWs is larger than 80 Å. Only vertical currents are expected in such a case. These conditions can be verified by future experiments.

The motion of [100](010) screw dislocations via a kink-pair mechanism is investigated in high-pressure ${\mathrm{MgSiO}}_3$ perovskite by means of atomistic calculations and an elastic interaction model for kink nucleation. Atomistic calculations based on the nudged elastic band method provide the Peierls potential, which is shown to be dynamically asymmetric and stress dependent. The elastic interaction model adjusted to match kink width computed atomistically, is used to evaluate the critical nucleation enthalpy. We demonstrate that the kink-pair mechanism in ${\mathrm{MgSiO}}_3$ perovskite is controlled by the nucleation of kinks along the [100] screw dislocation.

High-energy vibrational scattering in the binary systems UC and US is measured using time-of-flight inelastic neutron scattering. A clear set of well-defined peaks equally separated in energy is observed in UC, corresponding to harmonic oscillations of the light C atoms in a cage of heavy U atoms. The scattering is much weaker in US and only a few oscillator peaks are visible. We show how the difference between the materials can be understood by considering the neutron scattering lengths and masses of the lighter atoms. Monte Carlo ray tracing is used to simulate the scattering, with near quantitative agreement with the data in UC, and some differences with US. The possibility of observing anharmonicity and anisotropy in the potentials of the light atoms is investigated in UC. Overall, the observed data is well accounted for by considering each light atom as a single atom isotropic quantum harmonic oscillator.

We study the competition between $\sqrt{3}\times \sqrt{3}$ (RT3) and $q=0$ (Q0) magnetic orders in spin-1 and $\text{spin}-\frac{3}{2}$ kagome-lattice XXZ antiferromagnets with varying XY anisotropy parameter $\mathrm{\Delta }$, using series expansion methods. The Hamiltonian is split into two parts: an $H_0$ which favors the classical order in the desired pattern and an $H_1$, which is treated in perturbation theory by a series expansion. We find that the ground state energy series for the RT3 and Q0 phases are identical up to sixth order in the expansion, but ultimately a selection occurs, which depends on spin and the anisotropy $\mathrm{\Delta }$. Results for ground state energy and the magnetization are presented. These results are compared with recent spin-wave theory and coupled-cluster calculations. The series results for the phase diagram are close to the predictions of spin-wave theory. For the spin-1 model at the Heisenberg point ($\mathrm{\Delta }=1$), our results are consistent with a vanishing order parameter, that is, an absence of a magnetically ordered phase. We also develop series expansions for the ground state energy of the spin-1 Heisenberg model in the trimerized phase. We find that the ground state energy in this phase is lower than those of magnetically ordered ones, supporting the existence of a spontaneously trimerized phase in this model.

The spin Seebeck effect (SSE) consists in the generation of a spin current by a temperature gradient applied in a magnetic film. The SSE is usually detected by an electric voltage generated in a metallic layer in contact with the magnetic film resulting from the conversion of the spin current into charge current by means of the inverse spin Hall effect. The SSE has been widely studied in bilayers made of the insulating ferrimagnet yttrium iron garnet (YIG) and metals with large spin-orbit coupling such as platinum. Recently the SSE has been observed in bilayers made of the antiferromagnet $\mathrm{Mn}{\mathrm{F}}_2$ and Pt, revealing dependences of the SSE voltage on temperature and field very different from the ones observed in YIG/Pt. Here we present a theory for the SSE in structures with an antiferromagnetic insulator (AFI) in contact with a normal metal (NM) that relies on the bulk magnon spin current created by the temperature gradient across the thickness of the AFI/NM bilayer. The theory explains quite well the measured dependences of the SSE voltage on the sample temperature and on the applied magnetic field in $\mathrm{Mn}{\mathrm{F}}_2/\mathrm{Pt}$.

We report on interfacial contributions to the anisotropic magnetoresistance (AMR) in Co layers sandwiched between Pt. Utilizing the Fuchs-Sondheimer formalism interface contributions can be separated from bulklike AMR. We demonstrate that for all-metal systems interfacial AMR is also present when varying the magnetization within the film plane. This interfacial in-plane AMR is two times smaller than the contribution that arises when the magnetization is varied within the plane perpendicular to the current direction. This finding is in contrast to the spin Hall MR found for ferromagnetic insulator/Pt bilayers revealing the existence of different MR effects at the interfaces of Pt with conducting and insulating ferromagnets.

A spin-1 Heisenberg model on trimerized kagome lattice is studied by doing a low-energy bosonic theory in terms of plaquette triplons defined on its triangular unit cells. The model considered has an intratriangle antiferromagnetic exchange interaction $J$ (set to 1) and two intertriangle couplings $J^{\prime }>0$ (nearest neighbor) and $J^{″}$ (next nearest neighbor; of both signs). The triplon analysis performed on this model investigates the stability of the trimerized singlet ground state (which is exact in the absence of intertriangle couplings) in the $J^{\prime }-J^{″}$ plane. It gives a quantum phase diagram that has two gapless antiferromagnetically ordered phases separated by the spin-gapped trimerized singlet phase. The trimerized singlet ground state is found to be stable on $J^{″}=0$ line (the nearest-neighbor case), and on both sides of it for $J^{″}\ne 0$, in an extended region bounded by the critical lines of transition to the gapless antiferromagnetic phases. The gapless phase in the negative $J^{″}$ region has a coplanar ${120}^{\circ }$ antiferromagnetic order with $\sqrt{3}\times \sqrt{3}$ structure. In this phase, all the magnetic moments are of equal length, and the angle between any two of them on a triangle is exactly ${120}^{\circ }$. The magnetic lattice in this case has a unit cell consisting of three triangles. The other gapless phase, in the positive $J^{″}$ region, is found to exhibit a different coplanar antiferromagnetic order with ordering wave vector $\mathbf{q}=(0,0)$. Here, two magnetic moments in a triangle are of the same magnitude, but shorter than the third. While the angle between two short moments is ${120}^{\circ }-2\delta$, it is ${120}^{\circ }+\delta$ between a short and the long one. Only when $J^{″}=J^{\prime }$, their magnitudes become equal and the relative angles ${120}^{\circ }$. The magnetic lattice in this $\mathbf{q}=(0,0)$ phase has the translational symmetry of the kagome lattice with triangular unit cells of reduced (isosceles) symmetry. This reduction in the point-group symmetry is found to show up as a difference in the intensities of certain Bragg peaks, whose ratio $I_{(1,0)}/I_{(0,1)}=4{sin}^2(\frac{\pi }{6}+\delta )$ presents an experimental measure of the deviation $\delta$ from the ${120}^{\circ }$ order.

The strongly correlated spin-electron system on a diamond chain containing localized Ising spins on its nodal lattice sites and mobile electrons on its interstitial sites is exactly solved in a magnetic field using the transfer-matrix method. We have investigated in detail all available ground states, the magnetization processes, the spin-spin correlation functions around an elementary plaquette, fermionic quantum concurrence, and spin frustration. It is shown that the fermionic entanglement between mobile electrons hopping on interstitial sites and the kinetically induced spin frustration are closely related yet independent phenomena. In the ground state, quantum entanglement only appears within a frustrated unsaturated paramagnetic phase, while thermal fluctuations can promote some degree of quantum entanglement above the nonfrustrated ground states with saturated paramagnetic or classical ferrimagnetic spin arrangements.