Because disorders are inevitable in realistic nanodevices, the capability to quantitatively simulate the disorder effects on electron transport is indispensable for quantum transport theory. Here, we report a unified and effective first-principles quantum transport method for analyzing effects of chemical or substitutional disorder on transport properties of nanoelectronics, including averaged transmission coefficient, shot noise, and disorder-induced device-to-device variability. All our theoretical formulations and numerical implementations are worked out within the framework of the tight-binding linear muffin tin orbital method. In this method, we carry out the electronic structure calculation with the density functional theory, treat the nonequilibrium statistics by the nonequilbrium Green's function method, and include the effects of multiple impurity scattering with the generalized nonequilibrium vertex correction (NVC) method in coherent potential approximation (CPA). The generalized NVC equations are solved from first principles to obtain various disorder-averaged two-Green's-function correlators. This method provides a unified way to obtain different disorder-averaged transport properties of disordered nanoelectronics from first principles. To test our implementation, we apply the method to investigate the shot noise in the disordered copper conductor, and find all our results for different disorder concentrations approach a universal Fano factor $1/3$. As the second test, we calculate the device-to-device variability in the spin-dependent transport through the disordered Cu/Co interface and find the conductance fluctuation is very large in the minority spin channel and negligible in the majority spin channel. Our results agree well with experimental measurements and other theories. In both applications, we show the generalized nonequilibrium vertex corrections play a determinant role in electron transport simulation. Our results demonstrate the effectiveness of the first-principles generalized CPA-NVC for atomistic analysis of disordered nanoelectronics, extending the capability of quantum transport simulation.

We present the results of exact numerical calculations of the dielectric properties of tetrathiafulvalene-$p$-chloranil (TTF-CA) using the extended Hubbard model. The electronic polarization ${\overline{P}}_{\mathrm{el}}$ of the ionic ground state is obtained by directly calculating the adiabatic flow of current. The direction of ${\overline{P}}_{\mathrm{el}}$ is opposite to polarization ${\overline{P}}_{\mathrm{ion}}$ owing to ionic displacement, and $|{\overline{P}}_{\mathrm{el}}|$ is much larger than $|{\overline{P}}_{\mathrm{ion}}|$, showing that, in the ionic phase, TTF-CA is an electric ferroelectric. Furthermore, we numerically calculate the dynamics induced by THz pulse excitation. In the ionic phase, there exists an almost exact linear relationship between $\mathrm{\Delta }\rho \left(t\right)$ and $E\left(t\right)$, and between $\mathrm{\Delta }{P}_{\mathrm{el}}\left(t\right)$ and $E\left(t\right)$ in the realistic range of the excitation magnitude, where $\mathrm{\Delta }\rho \left(t\right)\left[\mathrm{\Delta }{P}_{\mathrm{el}}\left(t\right)\right]$ is the charge transfer (electric polarization) variation induced by the THz pulse and $E\left(t\right)$ is the electric field of the pulse at time $t$. The absolute value of $\mathrm{\Delta }\rho \left(t\right)$ in the neutral phase is much smaller than that in the ionic phase. These results are consistent with those of experiments and originate from the adiabatic nature of the THz pulse excited state.

Raman spectroscopy was used to measure the frequencies of phonons in ${\mathrm{Si}}_{24}$ with an open clathrate structure at temperatures from 80 to 400 K with simultaneous pressures of 0 to 8 GPa. The frequency shifts of the different phonons were substantially different under either temperature or pressure. The quasiharmonic behavior was isolated by varying pressure at low temperatures, and the anharmonic behavior was isolated by varying temperature at low pressures. Phonon modes dominated by bond bending were anomalous, showing stiffening with temperature and softening with pressure. Both the quasiharmonic behavior and the anharmonic behavior changed markedly with simultaneous changes in temperature $\mathrm{\Delta }T$ and pressure $\mathrm{\Delta }P$. With $\mathrm{\Delta }T=320\mathrm{K}$ and $\mathrm{\Delta }P=8\mathrm{GPa}$, some frequency shifts that scaled with the product $\mathrm{\Delta }T\mathrm{\Delta }P$ were as large as the shifts from $\mathrm{\Delta }T$ and $\mathrm{\Delta }P$ alone. The thermodynamic entropy of this material likely has a dependence on $\mathrm{\Delta }T$ and $\mathrm{\Delta }P$ that cannot be obtained by adding effects from quasiharmonicity and phonon-phonon anharmonicity.

We observe metastable localized spin configurations with topological charges ranging from $Q=-3$ to $Q=2$ in a (${\mathrm{Pt}}_{0.95}{\mathrm{Ir}}_{0.05}$)/Fe bilayer on a $\mathrm{Pd}\left(111\right)$ surface by performing spin dynamics simulations, using a classical Hamiltonian parametrized by ab initio calculations. We demonstrate that the frustration of the isotropic exchange interactions is responsible for the creation of these various types of skyrmionic structures. The Dzyaloshinsky-Moriya interaction present due to the breaking of inversion symmetry at the surface energetically favors skyrmions with $Q=-1$, distorts the shape of the other objects, and defines a preferred orientation for them with respect to the underlying lattice.

The crystal structure, magnetic ground state, and the temperature-dependent microscopic spin-spin correlations of the frustrated honeycomb lattice antiferromagnet $\mathrm{N}{\mathrm{a}}_2\mathrm{C}{\mathrm{o}}_2\mathrm{Te}{\mathrm{O}}_6$ have been investigated by powder neutron diffraction. A long-range antiferromagnetic (AFM) ordering has been found below $T_{\mathrm{N}}\sim 24.8\mathrm{K}$. The magnetic ground state, determined to be zigzag antiferromagnetic and characterized by a propagation vector $\mathbf{k}=(1/200)$, occurs due to the competing exchange interactions up to third-nearest neighbors within the honeycomb lattice. The exceptional existence of a limited magnetic correlation length along the $c$ axis (perpendicular to the honeycomb layers in the $ab$ planes) has been found even at 1.8 K, well below the $T_{\mathrm{N}}\sim 24.8\mathrm{K}$. The observed limited correlation along the $c$ axis is explained by the disorder distribution of the Na ions within the intermediate layers between honeycomb planes. The reduced ordered moments $m_{\mathrm{Co}\left(1\right)}=2.77\left(3\right){\mu }_{\mathrm{B}}/\mathrm{C}{\mathrm{o}}^{2+}$ and $m_{\mathrm{Co}\left(2\right)}=2.45\left(2\right){\mu }_{\mathrm{B}}/\mathrm{C}{\mathrm{o}}^{2+}$ at 1.8 K reflect the persistence of spin fluctuations in the ordered state. Above $T_{\mathrm{N}}\sim 24.8\mathrm{K}$, the presence of short-range magnetic correlations, manifested by broad diffuse magnetic peaks in the diffraction patterns, has been found. Reverse Monte Carlo analysis of the experimental diffuse magnetic scattering data reveals that the spin correlations are mainly confined within the two-dimensional honeycomb layers ($ab$ plane) with a correlation length of $\sim 12\AA$ at 25 K. The nature of the spin arrangements is found to be similar in both the short-range and long-range ordered magnetic states. This implies that the short-range correlation grows with decreasing temperature and leads to the zigzag AFM ordering at $T\le T_{\mathrm{N}}$. The present study provides a comprehensive picture of the magnetic correlations over the temperature range above and below the $T_{\mathrm{N}}$ and their relation to the crystal structure. The role of intermediate soft Na layers on the magnetic coupling between honeycomb planes is discussed.

Electrical and thermal spin currents excited by an electric current have been systematically investigated in lateral spin valves consisting of CoFeAl and ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ (Py) wires bridged by a Cu strip. In the electrical spin signal, the reciprocity between the current and voltage probes was clearly confirmed. However, a significant nonreciprocity was observed in the thermal spin signal. This provides clear evidence that a large spin-dependent Seebeck coefficient is more important than the spin polarization for efficient thermal spin injection and detection. We demonstrate that the spin-dependent Seebeck coefficient can be simply evaluated from the thermal spin signals for two configurations. Our experimental description paves a way for evaluating a small spin-dependent Seebeck coefficient for conventional ferromagnets without using complicated parameters.

The friction behavior of atomically stepped metal surfaces under contact loading is studied using molecular dynamics simulations. While real rough metal surfaces involve roughness at multiple length scales, the focus of this paper is on understanding friction of the smallest scale of roughness: atomic steps. To this end, periodic stepped Al surfaces with different step geometry are brought into contact and sheared at room temperature. Contact stress that continuously tries to build up during loading, is released with fluctuating stress drops during sliding, according to the typical stick-slip behavior. Stress release occurs not only through local slip, but also by means of step motion. The steps move along the contact, concurrently resulting in normal migration of the contact. The direction of migration depends on the sign of the step, i.e., its orientation with respect to the shearing direction. If the steps are of equal sign, there is a net migration of the entire contact accompanied by significant vacancy generation at room temperature. The stick-slip behavior of the stepped contacts is found to have all the characteristic of a self-organized critical state, with statistics dictated by step density. For the studied step geometries, frictional sliding is found to involve significant atomic rearrangement through which the contact roughness is drastically changed. This leads for certain step configurations to a marked transition from jerky sliding motion to smooth sliding, making the final friction stress approximately similar to that of a flat contact.

We calculate the probability distribution of entanglement entropy $S$ across a cut of a finite one-dimensional spin chain of length $L$ at an infinite-randomness fixed point using Fisher's strong randomness renormalization group (RG). Using the random transverse-field Ising model as an example, the distribution is shown to take the form $p\left(S\right|L)\sim L^{-\psi \left(k\right)}$, where $k\equiv S/ln\left[L/L_0\right]$, the large deviation function $\psi \left(k\right)$ is found explicitly, and $L_0$ is a nonuniversal microscopic length. We discuss the implications of such a distribution on numerical techniques that rely on entanglement, such as matrix-product-state-based techniques. Our results are verified with numerical RG simulations, as well as the actual entanglement entropy distribution for the random transverse-field Ising model which we calculate for large $L$ via a mapping to Majorana fermions.

Spin waves are promising candidates for information carriers in advanced technology. The interactions between spin waves and acoustic waves in magnetic nanostructures are of much interest because of their potential application for spin-wave generation, amplification, and transduction. We investigate numerically the dynamics of magnetoelastic excitations in a one-dimensional magnonic-phononic crystal consisting of alternating layers of permalloy and cobalt. We use the plane-wave method and the finite-element method for frequency- and time-domain simulations, respectively. The studied structure is optimized for hybridization of specific spin-wave and acoustic dispersion branches in the entire Brillouin zone in a broad frequency range. We show that this type of periodic structure can be used for efficient generation of high-frequency spin waves.

The synchronized magnetization dynamics in ferromagnets on a nonmagnetic heavy metal caused by the spin Hall effect is investigated theoretically. The direct and inverse spin Hall effects near the ferromagnetic/nonmagnetic interface generate longitudinal and transverse electric currents. The phenomenon is known as the spin Hall magnetoresistance effect, whose magnitude depends on the magnetization direction in the ferromagnet due to the spin transfer effect. When another ferromagnet is placed onto the same nonmagnet, these currents are again converted to the spin current by the spin Hall effect and excite the spin torque to this additional ferromagnet, resulting in the excitation of the coupled motions of the magnetizations. The in-phase or antiphase synchronization of the magnetization oscillations, depending on the value of the Gilbert damping constant and the field-like torque strength, is found in the transverse geometry by solving the Landau-Lifshitz-Gilbert equation numerically. On the other hand, in addition to these synchronizations, the synchronization having a phase difference of a quarter of a period is also found in the longitudinal geometry. The analytical theory clarifying the relation among the current, frequency, and phase difference is also developed, where it is shown that the phase differences observed in the numerical simulations correspond to that giving the fixed points of the energy supplied by the coupling torque.

The electronic structure and intrinsic magnetic properties of ${\mathrm{Fe}}_2{\mathrm{AlB}}_2$-related compounds and their alloys have been investigated using density functional theory. For ${\mathrm{Fe}}_2{\mathrm{AlB}}_2$, the crystallographic $a$ axis is the easiest axis, which agrees with experiments. The magnetic ground state of ${\mathrm{Mn}}_2{\mathrm{AlB}}_2$ is found to be ferromagnetic in the basal $ab$ plane, but antiferromagnetic along the $c$ axis. All $3d$ dopings considered decrease the magnetization and Curie temperature in ${\mathrm{Fe}}_2{\mathrm{AlB}}_2$. Electron doping with Co or Ni has a stronger effect on the decreasing of Curie temperature in ${\mathrm{Fe}}_2{\mathrm{AlB}}_2$ than hole doping with Mn or Cr. However, a larger amount of Mn doping on ${\mathrm{Fe}}_2{\mathrm{AlB}}_2$ promotes the ferromagnetic to antiferromagnetic transition. A very anisotropic magnetoelastic effect is found in ${\mathrm{Fe}}_2{\mathrm{AlB}}_2$: the magnetization has a much stronger dependence on the lattice parameter $c$ than on $a$ or $b$, which is explained by electronic-structure features near the Fermi level. Dopings of other elements on B and Al sites are also discussed.

We show theoretically that $\pi$ Josephson junctions may be constructed by use of antiferromagnetic (AF) metals between superconducting electrodes. We argue that the AF magnetic ordering introduces the energy difference of electrons in a Cooper pair due to the effect of the exchange field varying in space. Such an energy difference is quadratic in the amplitude of exchange field and this is sufficient to change the behavior of a Josephson junction from 0 to $\pi$ junction if the width of the AF metal is big enough. The advantage of using an AF barrier instead of a ferromagnetic one is that it does not suppress Cooper pairing in superconducting electrodes as much as the ferromagnet barrier does. However, to reach $\pi$-junction regime the AF metal should be a clean one with the electron mean free path bigger than the junction width.

Flat bands (FB) are strictly dispersionless bands in the Bloch spectrum of a periodic lattice Hamiltonian, recently observed in a variety of photonic and dissipative condensate networks. FB Hamiltonians are fine-tuned networks, still lacking a comprehensive generating principle. We introduce a FB generator based on local network properties. We classify FB networks through the properties of compact localized states (CLS) which are exact FB eigenstates and occupy $U$ unit cells. We obtain the complete two-parameter FB family of two-band $d=1$ networks with nearest unit cell interaction and $U=2$. We discover a novel high symmetry sawtooth chain with identical hoppings in a transverse dc field, easily accessible in experiments. Our results pave the way towards a complete description of FBs in networks with more bands and in higher dimensions.

The $\nu =12/5$ fractional quantum Hall plateau observed in $\mathrm{GaAs}$ semiconductor wells is a suspect in the search for non-Abelian Fibonacci anyons. Using the infinite density matrix renormalization group, we find clear evidence that fillings $\nu =12/5$ and $\nu =13/5$ are in the $k=3$ Read-Rezayi phase in the absence of particle-hole symmetry breaking effects. The lowest energy charged excitation is identified as a non-Abelian Fibonacci anyon, distinguished from its Abelian counterpart by its local quadrupole moment. However, several experiments at $\nu =13/5$ observe a reentrant integer quantum Hall effect, implying particle-hole symmetry is broken. We rule out spin polarization as the origin of the asymmetry. Further, we point out extremely close energetic competition between the Read-Rezayi phase and a reentrant integer quantum Hall phase. This competition suggests that even small particle-hole symmetry breaking perturbations can explain the experimentally observed asymmetry between $\nu =12/5$ and $13/5$. We find that at $\nu =12/5$ Landau level mixing favors the Read-Rezayi phase over the reentrant phase.

In a tight-binding lattice model with $n$ orbitals (single-particle states) per site, Wannier functions are $n$-component vector functions of position that fall off rapidly away from some location, and such that a set of them in some sense span all states in a given energy band or set of bands; compactly supported Wannier functions are such functions that vanish outside a bounded region. They arise not only in band theory, but also in connection with tensor-network states for noninteracting fermion systems, and for flat-band Hamiltonians with strictly short-range hopping matrix elements. In earlier work, it was proved that for general complex band structures (vector bundles) or general complex Hamiltonians—that is, class A in the tenfold classification of Hamiltonians and band structures—a set of compactly supported Wannier functions can span the vector bundle only if the bundle is topologically trivial, in any dimension $d$ of space, even when use of an overcomplete set of such functions is permitted. This implied that, for a free-fermion tensor network state with a nontrivial bundle in class A, any strictly short-range parent Hamiltonian must be gapless. Here, this result is extended to all ten symmetry classes of band structures without additional crystallographic symmetries, with the result that in general the nontrivial bundles that can arise from compactly supported Wannier-type functions are those that may possess, in each of $d$ directions, the nontrivial winding that can occur in the same symmetry class in one dimension, but nothing else. The results are obtained from a very natural usage of algebraic $K$-theory, based on a ring of polynomials in $e^{\pm i{k}_x},e^{\pm i{k}_y},...,$ which occur as entries in the Fourier-transformed Wannier functions.

The existence of pseudomagnetic helicons is predicted for strained Dirac and Weyl materials. The corresponding collective modes are reminiscent of the usual helicons in metals in strong magnetic fields but can exist even without a magnetic field due to a strain-induced background pseudomagnetic field. The properties of both pseudomagnetic and magnetic helicons are investigated in Weyl matter using the formalism of the consistent chiral kinetic theory. It is argued that the helicon dispersion relations are affected by the electric and chiral chemical potentials, the chiral shift, and the energy separation between the Weyl nodes. The effects of multiple pairs of Weyl nodes are also discussed. A simple setup for experimental detection of pseudomagnetic helicons is proposed.

Pseudocapacitors are energy-storage devices characterized by fast and reversible redox reactions that enable them to store large amounts of electrical energy at high rates. We simulate the response of pseudocapacitive electrodes under realistic conditions to identify the microscopic factors that determine their performance, focusing on ruthenia $\left({\mathrm{RuO}}_2\right)$ as a prototypical electrode material. Electronic-structure methods are used together with a self-consistent continuum solvation (SCCS) model to build a complete data set of free energies as the surface of the charged electrode is gradually covered with protons under applied voltage. The resulting data set is exploited to compute hydrogen-adsorption isotherms and charge-voltage responses by means of grand-canonical sampling, finding close agreement with experimental voltammetry. These simulations reveal that small changes on the order of $5\mu \mathrm{F}/{\mathrm{cm}}^2$ in the intrinsic double-layer capacitance of the electrode-electrolyte interface can induce variations of up to $40\mu \mathrm{F}/{\mathrm{cm}}^2$ in the overall pseudocapacitance.

An ordered state in the spin sector that breaks parity without breaking time-reversal symmetry, i.e., that can be considered dynamically generated spin-orbit coupling, was proposed to explain puzzling observations in a range of different systems. Here, we derive severe restrictions for such a state that follow from a Ward identity related to spin conservation. It is shown that $l=1$ spin-Pomeranchuk instabilities are not possible in nonrelativistic systems since the response of spin-current fluctuations is entirely incoherent and nonsingular. This rules out relativistic spin-orbit coupling as an emergent low-energy phenomenon. We illustrate the exotic physical properties of the remaining higher-angular-momentum analogs of spin-orbit coupling and derive a geometric constraint for spin-orbit vectors in lattice systems.