The structural and electronic response of $\mathrm{LaAl}{\mathrm{O}}_3$ to biaxial strain in the (111) plane is studied by density functional theory (DFT) and compared with strain in the (001) plane and isostatic strain. For (111) strain, in-plane rotations are stabilized by compressive strain and out-of-plane rotations by tensile strain. This is an opposite splitting of the modes compared with (001) strain. Furthermore, for compressive (111) strain, in-plane rotations are degenerate with respect to the rotation axis, giving rise to Goldstone-like modes. We rationalize these changes in octahedral rotations by analyzing the $V_A/V_B$ polyhedral volume ratios. Finally, we investigate how strain affects the calculated band gap, and find a 28% difference between the strain planes under 4% tension. This effect is attributed to different A-site dodecahedral crystal field splitting for (001) and (111) strains.

The density matrix renormalization group (DMRG) is one of the most powerful numerical methods available for many-body systems. It has been applied to solve many physical problems, including the calculation of ground states and dynamical properties. In this work, we develop a perturbation theory of the DMRG (PT-DMRG) to greatly increase its accuracy in an extremely simple and efficient way. Using the canonical matrix product state (MPS) representation for the ground state of the considered system, a set of orthogonal basis functions $\left\{|{\psi }_i\rangle \right\}$ is introduced to describe the perturbations to the ground state obtained by the conventional DMRG. The Schmidt numbers of the MPS that are beyond the bond dimension cutoff are used to define these perturbation terms. The perturbed Hamiltonian is then defined as ${\tilde{H}}_{ij}=\langle {\psi }_i|\widehat{H}|{\psi }_j\rangle$; its ground state permits us to calculate physical observables with a considerably improved accuracy compared to the original DMRG results. We benchmark the second-order perturbation theory with the help of a one-dimensional Ising chain in a transverse field and the Heisenberg chain, where the precision of the DMRG is shown to be improved $O\left(10\right)$ times. Furthermore, for moderate $L$ the errors of the DMRG and PT-DMRG both scale linearly with $L^{-1}$ (with $L$ being the length of the chain). The linear relation between the dimension cutoff of the DMRG and that of the PT-DMRG at the same precision shows a considerable improvement in efficiency, especially for large dimension cutoffs. In the thermodynamic limit we show that the errors of the PT-DMRG scale with $\sqrt{L^{-1}}$. Our work suggests an effective way to define the tangent space of the ground-state MPS, which may shed light on the properties beyond the ground state. This second-order PT-DMRG can be readily generalized to higher orders, as well as applied to models in higher dimensions.

We present a theoretical approach to study nonequilibrium quantum heat transport in molecular junctions described by a spin-boson type model. Based on the Feynman-Vernon path integral influence functional formalism, expressions for the average value and high-order moments of the heat current operators are derived, which are further obtained directly from the auxiliary density operators (ADOs) in the hierarchical equations of motion (HEOM) method. Distribution of the heat current is then derived from the high-order moments. As the HEOM method is nonperturbative and capable of treating non-Markovian system-environment interactions, the method can be applied to various problems of nonequilibrium quantum heat transport beyond the weak coupling regime.

Magnetic domain wall (DW) motion induced by a localized Gaussian temperature profile is studied in a Permalloy nanostrip within the framework of the stochastic Landau-Lifshitz-Bloch equation. The different contributions to thermally induced DW motion, entropic torque and magnonic spin transfer torque, are isolated and compared. The analysis of magnonic spin transfer torque includes a description of thermally excited magnons in the sample. A third driving force due to a thermally induced dipolar field is found and described. Finally, thermally induced DW motion is studied under realistic conditions by taking into account the edge roughness. The results give quantitative insights into the different mechanisms responsible for domain wall motion in temperature gradients and allow for comparison with experimental results.

We present a theory of the anisotropy of tunneling magnetoresistance (ATMR) phenomenon in magnetic tunnel junctions (MTJs) attributed to Rashba spin-orbit interaction in the insulating barrier. ATMR represents the difference of tunnel magnetoresistance (TMR) amplitude measured with in-plane and out-of-plane magnetic configurations. It is demonstrated that within the spin-polarized free-electron model the change of conductance associated with the ATMR is exactly twice the change of conductance measured at full saturation (i.e., in parallel configuration of magnetizations) between in-plane and out-of-plane configuration, i.e., the tunneling anisotropic magnetoresistance (TAMR). Both ATMR and TAMR are closely related to the TMR amplitude and spin-orbit constant. The predicted ATMR phenomenon is confirmed experimentally, showing a few percent value in the case of the widely studied CoFeB/MgO/CoFeB based MTJ.

Using pulsed magnetic fields up to $30\mathrm{T}$ we have measured the bulk magnetization and electrical polarization of ${\mathrm{LiNiPO}}_4$ and have studied its magnetic structure by time-of-flight neutron Laue diffraction. Our data establish the existence of a reentrant magnetoelectric phase between $19\mathrm{T}$ and $21\mathrm{T}$. We show that a magnetized version of the zero field commensurate structure explains the magnetoelectric response quantitatively. The stability of this structure suggests a field-dependent spin anisotropy. Above $21\mathrm{T}$, a magnetoelectrically inactive, short-wavelength incommensurate structure is identified. Our results demonstrate the combination of pulsed fields with epithermal neutron Laue diffraction as a powerful method to probe even complex phase diagrams in strong magnetic fields.

The foldover effect of nonlinear eigenmodes in feedback ring systems was observed. Experiments made use of a system that consisted of a magnetic thin film strip, which supported the propagation of spin waves, and a microwave amplifier, which amplified the signal from the output of the magnetic strip and then fed it back to the input of the magnetic strip. The signal amplitude vs frequency response in this ring system showed resonant peaks which resulted from ring eigenmodes. With an increase in the resonance amplitude, those resonant peaks evolved from symmetric peaks to asymmetric ones and then folded over to higher frequencies. The experimental observations were reproduced by theoretical calculations that took into account the nonlinearity-produced frequency shift of the traveling spin waves.

A model of a clean two-band $s$-wave superconductor with cylindrical Fermi surfaces, different Fermi velocities $v_{1,2}$, and a general $2\times 2$ coupling matrix $V_{\alpha \beta }$ is used to study the order parameter distribution in vortex lattices. The Eilenberger weak coupling formalism is used to calculate numerically the spatial distributions of the pairing amplitudes ${\mathrm{\Delta }}_1$ and ${\mathrm{\Delta }}_2$ of the two bands for vortices parallel to the Fermi cylinders. For generic values of the interband coupling $V_{12}$, it is shown that, independently of the couplings $V_{\alpha \beta }$, of the ratio $v_1/v_2$, of the temperature, and the applied field, the length scales of spatial variation of ${\mathrm{\Delta }}_1$ and of ${\mathrm{\Delta }}_2$ are the same within the accuracy of our calculations. The only exception from this single length-scale behavior is found for $V_{12}\ll V_{11}$, i.e., for nearly decoupled bands.

Topological Weyl semimetals (TWS) can be classified as type I TWS, in which the density of states vanishes at the Weyl nodes, and type II TWS, in which an electron pocket and a hole pocket meet at a singular point of momentum space, allowing for distinct topological properties. We consider various minimal lattice models for type II TWS. The simplest time-reversal-breaking band structure, with a pair of Weyl nodes sharing a single electron pocket and a single hole pocket (hydrogen model), exhibits relics of surface Fermi arc states only away from the Fermi energy, with no topological protection. Topologically protected Fermi arcs can be restored by an additional term (hydrogen model) that produces a bulk structure where the electron and hole pockets of each Weyl point are disjoint. In time-reversal-symmetric but inversion-breaking models, we identify nontopological surface track states that arise out of the topological Fermi arc states at the transition from type I to type II and persist in the type II TWS. The distinctions among these minimal models can aid in distinguishing between generic and model-dependent behavior in studies of superconductivity, magnetism, and quantum oscillations of type II Weyl semimetals.

While ${\mathrm{SnS}}_2$ is an earth-abundant large-band-gap semiconductor material, the indirect nature of the band gap limits its applications in light harvesting or detection devices. Here, using density functional theory in combination with the many-body perturbation theory, we report indirect-to-direct band-gap transition in bulk ${\mathrm{SnS}}_2$ under moderate, $2.98\%$ uniform biaxial tensile (BT) strain. Further enhancement of the BT strain up to $9.75\%$ leads to a semiconductor-to-metal transition. The strain-induced weakening of the interaction of the in-plane orbitals modifies the dispersion as well as the character of the valence- and the conduction-band edges, leading to the transition. A quasiparticle direct band gap of 2.17 eV at the $\mathrm{\Gamma }$ point is obtained at $2.98\%$ BT strain. By solving the Bethe-Salpeter equation to include excitonic effects on top of the partially self-consistent ${\mathrm{GW}}_0$ calculation, we study the dielectric functions, optical oscillator strength, and exciton binding energy as a function of the applied strain. At $2.98\%$ BT strain, our calculations show the relatively high exciton binding energy of 170 meV, implying strongly coupled excitons in ${\mathrm{SnS}}_2$. The effect of strain on vibrational properties, including Raman spectra, is also investigated. The Raman shift of both in-plane ($E_{2g}^1$) and out-of plane ($A_{1g}$) modes decreases with the applied BT strain, which can be probed experimentally. Furthermore, ${\mathrm{SnS}}_2$ remains dynamically stable up to $9.75\%$ BT strain, at which it becomes metallic. A strong coupling between the applied strain and the electronic and optical properties of ${\mathrm{SnS}}_2$ can significantly broaden the applications of this material in strain-detection and optoelectronic devices.

We investigate the effect of introducing nearest-neighbor $p$-wave superconducting pairing to both the static and kicked extended Harper model with two periodic phase parameters acting as artificial dimensions to simulate three-dimensional systems. It is found that in both the static model and the kicked model, by varying the $p$-wave pairing order parameter, the system can switch between a fully gapped phase and a gapless phase with point nodes or line nodes. The topological property of both the static and kicked model is revealed by calculating corresponding topological invariants defined in the one-dimensional lattice dimension. Under open boundary conditions along the physical dimension, Majorana flat bands at energy zero (quasienergy zero and $\pi$) emerge in the static (kicked) model at the two-dimensional surface Brillouin zone. For certain values of pairing order parameter, (Floquet) Su-Schrieffer-Heeger-like edge modes appear in the form of arcs connecting different (Floquet) Majorana flat bands. Finally, we find that in the kicked model, it is possible to generate two controllable Floquet Majorana modes, one at quasienergy zero and the other at quasienergy $\pi$, at the same parameter values.

The Chern-Simons axion coupling of a bulk insulator is only defined modulo a quantum of $e^2/h$. The quantized part of the coupling is uniquely defined for a bounded insulating sample, but it depends on the specific surface termination. Working in a slab geometry and representing the valence bands in terms of hybrid Wannier functions, we show how to determine that quantized part from the excess Chern number of the hybrid Wannier sheets located near the surface of the slab. The procedure is illustrated for a tight-binding model consisting of coupled quantum anomalous Hall layers. By slowly modulating the model parameters it is possible to transfer one unit of Chern number from the bottom to the top surface over the course of a cyclic evolution of the bulk Hamiltonian, changing the surface anomalous Hall conductivity by a quantum of conductance $e^2/h$. When the evolution of the surface Hamiltonian is also cyclic, the Chern pumping is obstructed by chiral touchings between valence and conduction surface bands.

Topological semimetals are gapless states of matter which have robust surface states and characteristic electromagnetic responses. In this paper, we consider the electromagnetic response of gapless phases in $3+1$ dimensions with line nodes. We show through a layering approach that an intrinsic antisymmetric tensor ${\mathcal{B}}_{\mu \nu }$ (2-form), which is determined by the geometry and energy-embedding of the nodal lines, emerges in the effective response field theory. ${\mathcal{B}}_{\mu \nu }$ is shown to be simply related to the charge polarization and orbital magnetization of the sample; hence the geometry of the line nodes determines these electromagnetic observables. We conclude by discussing the relevance for recently proposed materials and heterostructures with line-node fermi-surfaces.

The magnetic correlations within the cuprates have undergone intense scrutiny as part of efforts to understand high-temperature superconductivity. We explore the evolution of the magnetic correlations along the nodal direction of the Brillouin zone in ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$, spanning the doping phase diagram from the antiferromagnetic Mott insulator at $x=0$ to the metallic phase at $x=0.26$. Magnetic excitations along this direction are found to be systematically softened and broadened with doping, at a higher rate than the excitations along the antinodal direction. This phenomenology is discussed in terms of the nature of the magnetism in the doped cuprates. Survival of the high-energy magnetic excitations, even in the overdoped regime, indicates that these excitations are marginal to pairing, while the influence of the low-energy excitations remains ambiguous.

Doping silicon nanocrystals (Si NCs) embedded in silicon dioxide ($\mathrm{Si}{\mathrm{O}}_2$) with boron (B) and phosphorus (P) is a promising way of tuning the properties of Si NCs. Here we take advantage of density functional theory to investigate the dependence of the structural and electronic properties of Si NCs embedded in $\mathrm{Si}{\mathrm{O}}_2$ on the doping of B and P. The locations and energy-level schemes are examined for singularly B-doped or B/P-codoped Si NCs embedded in $\mathrm{Si}{\mathrm{O}}_2$ with a perfect or defective Si/$\mathrm{Si}{\mathrm{O}}_2$ interface at which a Si dangling bond exists. A dangling bond plays an important role in the doping of Si NCs with B or B/P. The doping behavior of B in Si NCs embedded in $\mathrm{Si}{\mathrm{O}}_2$ vastly differs from that of P. The electronic structure of a B/P-codoped Si NC largely depends on the distribution of the dopants in the NC.

We present a detailed investigation of ultrafast (subpicosecond) exciton dynamics in the lattice model of a donor/acceptor heterojunction. Exciton generation by means of a photoexcitation, exciton dissociation, and further charge separation are treated on equal footing. The experimentally observed presence of space-separated charges at $\lesssim 100\mathrm{fs}$ after the photoexcitation is usually attributed to ultrafast transitions from excitons in the donor to charge-transfer and charge-separated states. Here, we show, however, that the space-separated charges appearing on $\lesssim 100$-fs time scales are predominantly directly optically generated. Our theoretical insights into the ultrafast pump-probe spectroscopy challenge usual interpretations of pump-probe spectra in terms of ultrafast population transfer from donor excitons to space-separated charges.

We investigate the occurrence of magnetic and charge density wave instabilities in rhombohedral-stacked multilayer (three to eight layers) graphene by first principles calculations including exact exchange. Neglecting spin polarization, an extremely flat surface band centered at the special point $\mathbf{K}$ of the Brillouin zone occurs at the Fermi level. Spin polarization opens a gap in the surface state by stabilizing an antiferromagnetic state. The top and the bottom surface layers are weakly ferrimagnetic in-plane (net magnetization smaller than ${10}^{-3}{\mu }_B$) and are antiferromagnetic coupled to each other. This coupling is propagated by the out-of-plane antiferromagnetic coupling between the nearest neighbors. The gap is very small in a spin-polarized generalized gradient approximation, while it is proportional to the amount of exact exchange in hybrid functionals. For trilayer rhombohedral graphene it is 38.6 meV in PBE0, in agreement with the 42 meV gap found in experiments. We study the temperature and doping dependence of the magnetic gap. At electron doping of $n\sim 7\times {10}^{11} {\mathrm{cm}}^{-2}$ the gap closes. Charge density wave instabilities with $\sqrt{3}\times \sqrt{3}$ periodicity do not occur.

Motivated by recent studies that reported the successful synthesis of monolayer $\mathrm{Mg}{\left(\mathrm{OH}\right)}_2$ [Suslu et al., Sci. Rep. 6, 20525 (2016)] and hexagonal $\left(h\text{−}\right)\text{AlN}$ [Tsipas et al., Appl. Phys. Lett. 103, 251605 (2013)], we investigate structural, electronic, and optical properties of vertically stacked $h$-AlN and $\mathrm{Mg}{\left(\mathrm{OH}\right)}_2$, through ab initio density-functional theory (DFT), many-body quasiparticle calculations within the GW approximation and the Bethe-Salpeter equation (BSE). It is obtained that the bilayer heterostructure prefers the $A{B}^{\prime }$ stacking having direct band gap at the $\mathrm{\Gamma }$ with Type-II band alignment in which the valance band maximum and conduction band minimum originate from different layer. Regarding the optical properties, the imaginary part of the dielectric function of the individual layers and heterobilayer are investigated. The heterobilayer possesses excitonic peaks, which appear only after the construction of the heterobilayer. The lowest three exciton peaks are analyzed in detail by means of band decomposed charge density and the oscillator strength. Furthermore, the wave function calculation shows that the first peak of the heterobilayer originates from spatially indirect exciton where the electron and hole localized at $h$-AlN and $\mathrm{Mg}{\left(\mathrm{OH}\right)}_2$, respectively, which is important for the light harvesting applications.

A single metallic slit is the simplest plasmonic structure for basic physical understanding of electromagnetic field confinement. By reducing the gap size, the field enhancement is expected to first go up and then go down when the gap width becomes subnanometer because of the quantum tunneling effects. A fundamental question is whether we reach the classical limit of field enhancement before entering the quantum regime, i.e., whether the quantum effects undercut the highest field enhancement classically possible. Here, by performing terahertz time domain spectroscopy on single slits of widths varying from 1.5 nm to 50 µm, we show that ultimate field enhancement determined by the wavelength of light and film thickness can be reached before we hit the quantum regime. Our paper paves way toward designing a quantum plasmonic system with maximum control yet without sacrificing the classical field enhancements.