The study of nuclear magnetic resonance and nuclear spin-lattice relaxation was conducted in an asymmetrically doped to $n\sim 1.8\times {10}^{11}{\mathrm{cm}}^{-2}16$ nm AlAs quantum well grown in the $\left[001\right]$ direction. The dynamic polarization of nuclear spins due to a hyperfine interaction resulted in the so-called Overhauser shift of two-dimensional conduction electron spin resonance. The maximum shifts achieved in the experiments are several orders of magnitude smaller than in GaAs-based heterostructures, indicating that the hyperfine interaction is weak. The nuclear spin-lattice relaxation time extracted from the decay of the Overhauser shift over time turned out to depend on the filling factor of the two-dimensional electron system. This observation indicates that nuclear spin-lattice relaxation is mostly due to the interaction between electron and nuclear spins. The Overhauser shift diminishes resonantly when the rf radiation of certain frequencies was applied to the sample. This effect served as an indirect, yet powerful, method for nuclear magnetic resonance detection: NMR quadrupole splitting of ${}^{75}\mathrm{As}$ nuclei was clearly resolved. Theoretical calculations performed describe well these experimental findings.

We argue that strain applied to a time-reversal and inversion breaking Weyl semimetal in a magnetic field can induce an electric current via the chiral magnetic effect. A tight-binding model is used to show that strain generically changes the locations in the Brillouin zone but also the energies of the band touching points (tips of the Weyl cones). Since axial charge in a Weyl semimetal can relax via intervalley scattering processes, the induced current will decay with a time scale given by the lifetime of a chiral quasiparticle. We estimate the strength and lifetime of the current for typical material parameters and find that it should be experimentally observable.

Using first-principles calculations, we analyze the structural properties of tungsten trioxide ${\mathrm{WO}}_3$. Our calculations rely on density functional theory and the use of the B1-WC hybrid functional, which provides very good agreement with experimental data. We show that the hypothetical high-symmetry cubic reference structure combines several ferroelectric and antiferrodistortive (antipolar cation motions, rotations, and tilts of oxygen octahedra) structural instabilities. Although the ferroelectric instability is the largest, the instability related to antipolar W motions combines with those associated to oxygen rotations and tilts to produce the biggest energy reduction, yielding a $P{2}_1/c$ ground state. This nonpolar $P{2}_1/c$ phase is only different from the experimentally reported $Pc$ ground state by the absence of a very tiny additional ferroelectric distortion. The calculations performed on a stoichiometric compound so suggest that the low-temperature phase of ${\mathrm{WO}}_3$ is not intrinsically ferroelectric and that the experimentally observed ferroelectric character might arise from extrinsic defects such as oxygen vacancies. Independently, we also identify never observed $R3m$ and $R3c$ ferroelectric metastable phases with large polarizations and low energies close to the $P{2}_1/c$ ground state, which makes ${\mathrm{WO}}_3$ a potential antiferroelectric material. The relative stability of various phases is discussed in terms of the anharmonic couplings between different structural distortions, highlighting a very complex interplay.

We study nonequilibrium thermoelectric transport properties of a correlated impurity connected to two leads for temperatures below the Kondo scale. At finite bias, for which a current flows across the leads, we investigate the differential response of the current to a temperature gradient. In particular, we compare the influence of a bias voltage and of a finite temperature on this thermoelectric response. This is of interest from a fundamental point of view to better understand the two different decoherence mechanisms produced by a bias voltage and by temperature. Our results show that in this respect the thermoelectric response behaves differently from the electric conductance. In particular, while the latter displays a similar qualitative behavior as a function of voltage and temperature, both in theoretical and experimental investigations, qualitative differences occur in the case of the thermoelectric response. In order to understand this effect, we analyze the different contributions in connection to the behavior of the impurity spectral function versus temperature. Especially in the regime of strong interactions and large enough bias voltages, we obtain a simple picture based on the asymmetric suppression or enhancement of the split Kondo peaks as a function of the temperature gradient. Besides the academic interest, these studies could additionally provide valuable information to assess the applicability of quantum dot devices as responsive nanoscale temperature sensors.

We study quantum transport through two-terminal nanoscale devices in contact with two particle reservoirs at different temperatures and chemical potentials. We discuss the general expressions controlling the electric charge current, heat currents, and the efficiency of energy transmutation in steady conditions in the linear regime. With focus in the parameter domain where the electron system acts as a power generator, we elaborate workable expressions for optimal efficiency and thermoelectric parameters of nanoscale devices. The general concepts are set at work in the paradigmatic cases of Lorentzian resonances and antiresonances, and the encompassing Fano transmission function: the treatments are fully analytic, in terms of the trigamma functions and Bernoulli numbers. From the general curves here reported describing transport through the above model transmission functions, useful guidelines for optimal efficiency and thermopower can be inferred for engineering nanoscale devices in energy regions where they show similar transmission functions.

It has been argued that stanene has lowest lattice thermal conductivity among two-dimensional (2D) group-IV materials because of its largest atomic mass, weakest interatomic bonding, and enhanced ZA phonon scattering due to the breaking of an out-of-plane symmetry selection rule. However, we show that, although the lattice thermal conductivity $\kappa$ for graphene, silicene, and germanene decreases monotonically with decreasing Debye temperature, unexpected higher $\kappa$ is observed in stanene. By enforcing all the invariance conditions in 2D materials and including Ge $3d$ and Sn $4d$ electrons as valence electrons for germanene and stanene, respectively, the lattice dynamics in these materials are accurately described. A large acoustic-optical gap and the bunching of the acoustic-phonon branches significantly reduce phonon scattering in stanene, leading to higher thermal conductivity than germanene. The vibrational origin of the acoustic-optical gap can be attributed to the buckled structure. Interestingly, a buckled system has two competing influences on phonon transport: the breaking of the symmetry selection rule leads to reduced thermal conductivity, and the enlarging of the acoustic-optical gap results in enhanced thermal conductivity. The size dependence of thermal conductivity is investigated as well. In nanoribbons, the $\kappa$ of silicene, germanene, and stanene is much less sensitive to size effect due to their short intrinsic phonon mean-free paths. This work sheds light on the nature of phonon transport in buckled 2D materials.

Epitaxial graphene growth on SiC surfaces is considered advantageous in terms of device application. However, the first graphitic layer on SiC transforms to a buffer layer because of strong coupling with the substrate. The properties of several subsequent layers are also significantly degraded. One method to decouple graphene from the substrate is Si intercalation. In the present work, we report observation and analysis of interface structures formed by Si intercalation in between the graphene layer and the SiC(0001) surface depending on Si coverage and influence of these interfaces on graphene electronic structure by means of low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), angle-resolved photoemission spectroscopy (ARPES), and theoretical first-principles calculations. The STM appearance of observed periodic interface structures strongly resembles previously known Si-rich phases on the SiC(0001) surface. Based on the observed range of interface structures we discuss the mechanism of graphene layer decoupling and differences in stability of the Si-rich phases on clean SiC(0001) and in the graphene/SiC(0001) interface region. We also discuss a possibility to tune graphene electronic properties by interface engineering.

We investigated the stability and mechanical and electronic properties of 15 metastable mixed $s{p}^2\text{−}s{p}^3$ carbon allotropes in the family of interpenetrating graphene networks (IGNs) using density functional theory (DFT). IGN allotropes exhibit nonmonotonic bulk and linear compressibilities before their structures irreversibly transform into new configurations under large hydrostatic compression. The maximum bulk compressibilities vary widely between structures and range from 3.6 to 306 ${\mathrm{TPa}}^{-1}$. We find all the IGN allotropes have negative linear compressibilities with maximum values varying from –0.74 to $–133{\mathrm{TPa}}^{-1}$. The maximal negative linear compressibility of Z33 ($–133{\mathrm{TPa}}^{-1}$ at 3.4 GPa) exceeds previously reported values at pressures higher than 1.0 GPa. IGN allotropes can be classified as either armchair or zigzag type, and these two types of IGNs exhibit different electronic properties. Zigzag-type IGNs are node-line semimetals, while armchair-type IGNs are either semiconductors or node-loop or node-line semimetals. Experimental synthesis of these IGN allotropes might be realized since their formation enthalpies relative to graphite are only 0.1–0.5 eV/atom (that of ${\mathrm{C}}_{60}$ fullerene is about 0.4 eV/atom), and energetically feasible binary compound pathways are possible.

High-harmonic generation (HHG) from semiconductors and insulators has become a very active area of research due to its great potential for developing compact HHG devices. Here we show, that by growing monolayers (ML) of insulators on single-crystal metal surfaces, one can tune the harmonic spectrum by just varying the thickness of the ultrathin layer, rather than the laser properties. This is shown from numerical solutions of the time-dependent Schrödinger equation for Cu(111)/$n$-ML NaCl systems ($n=1-50$) based on realistic potentials. Remarkably, the harmonic cutoff increases linearly with $n$ and as much as an order of magnitude when going from $n=1$ to 30, while keeping the laser intensity low and the wavelength in the near-infrared range. The origin of this behavior is twofold: the initial localization of electrons in a Cu-surface state and the reduction of electronic “friction” when moving from the essentially discrete energy spectrum associated with a few-ML system to the continuous spectrum (bands) inherent in extended periodic systems. Our findings are valid for both few- and multicycle IR pulses and wavelengths $\sim 1–2 \mu \mathrm{m}$.

We present microscopically resolved, polarized spectroscopy of Raman scattering collected from tetragonal ${\mathrm{SrTiO}}_3$. The anisotropic response of first-order Raman peaks within a single tetragonal domain has been measured. From these data, we assign symmetries to the phonons seen in the first-order Raman spectrum which is normally complicated by uncontrolled domain structure. Using a translation stage, we map the local domain orientation of a $3\text{−}\mu {\mathrm{m}}^3$ crystal volume near the laser focus and compare it to wide-field polarized images. This technique can be performed with readily available instruments and is relevant to the study of a wide range of related materials, interfaces, and devices.

The composition-dependent crystal structure, volume, elastic constants, and electronic structure of $\delta \text{−}{\mathrm{Pu}}_{1-x}{M}_x$ $\left(M=\text{Ga and Al},0\le x\le 0.1\right)$ alloys are systematically studied by using first-principles EMTO-CPA calculations. It is shown that the fcc and $L{1}_2$ structures co-exist in the alloys with $x\le 0.04$ whereas for $x>0.04$, the $L{1}_2$ structure is more and more preferable and around $x=0.1$, it tends to be stabilized alone. The evaluated $V\sim x$ of the $L{1}_2$ structure, being negative deviation from Vegard's law, turns out to be in good agreement with the experimental result. For $x\le 0.04$, the estimated $E$, $G$, $\nu$, and $\mathrm{\Theta }$ of both the fcc and $L{1}_2$ structures are in line with the measured data, whereas when $x>0.04$, only those of the $L{1}_2$ structure are close to the experimental results. The electronic hybridization between Pu and $M$ atoms is dominated by Pu for the $s,d$, and $f$ states but $M$ for the $p$ state. The strong interactions between Pu and $M$ atoms in the same site of the $L{1}_2$ structure should be responsible for its relative stability in the alloys with $x>0.04$. The electron-phonon coupling further decreases the phase stability of $\delta \text{−}{\mathrm{Pu}}_{1-x}{M}_x$ with increasing $x$.

It is well established that when energetic ions are moving in crystals, they may penetrate much deeper if they happen to be directed in some specific crystal directions. This ‘channeling’ effect is utilized for instance in certain ion beam analysis methods and has been described by analytical theories and atomistic computer simulations. However, there have been very few systematic studies of channeling in directions other than the principal low-index ones. We present here a molecular dynamics-based approach to calculate ion channeling systematically over all crystal directions, providing ion ‘channeling maps’ that easily show in which directions channeling is expected. The results show that channeling effects can be quite significant even at energies below 1 keV, and that in many cases, significant planar channeling occurs also in a wide range of crystal directions between the low-index principal ones. In all of the cases studied, a large fraction ($\sim 20–60\%$) of all crystal directions show channeling. A practical implication of this is that modern experiments on randomly oriented nanostructures will have a large probability of channeling. It also means that when ion irradiations are carried out on polycrystalline samples, channeling effects on the results cannot a priori be assumed to be negligible. The maps allow for easy selection of good ‘nonchanneling’ directions in experiments or alternatively finding wide channels for beneficial uses of channeling. We implement channeling theory to also give the fraction of channeling directions in a manner directly comparable to the simulations. The comparison shows good qualitative agreement. In particular, channeling theory is very good at predicting which channels are active at a given energy. This is true down to sub-keV energies, provided the penetration depth is not too small.

We consider the nonequilibrium dynamics arising after a quench of the transverse magnetic field of a quantum Ising chain, together with the sudden switch-on of a long-range interaction term. The dynamics after the quantum quench is mapped onto a fully connected model of hard-core bosons, after a suitable combination of a Holstein-Primakoff transformation and of a low-density expansion in the quasiparticles injected by the quench. This mapping holds for a broad class of initial states and for quenches which do not cross the critical point of the transverse field Ising model. We then study the algebraic relaxation in time of a number of observables towards a metastable, prethermal state, which becomes the asymptotic steady state in the thermodynamic limit.

In approximate density-functional theory (DFT), the self-interaction error is an electron delocalization anomaly associated with underestimated insulating gaps. It exhibits a predominantly quadratic energy-density curve that is amenable to correction using efficient, constraint-resembling methods such as DFT + Hubbard $U$ (DFT+$U$). Constrained DFT (cDFT) enforces conditions on DFT exactly, by means of self-consistently optimized Lagrange multipliers, and while its use to automate error corrections is a compelling possibility, we show that it is limited by a fundamental incompatibility with constraints beyond linear order. We circumvent this problem by utilizing separate linear and quadratic correction terms, which may be interpreted either as distinct constraints, each with its own Hubbard $U$ type Lagrange multiplier, or as the components of a generalized DFT+$U$ functional. The latter approach prevails in our tests on a model one-electron system, ${\text{H}}_2{}^{+}$, in that it readily recovers the exact total energy while symmetry-preserving pure constraints fail to do so. The generalized DFT+$U$ functional moreover enables the simultaneous correction of the total energy and ionization potential, or the correction of either together with the enforcement of Koopmans' condition. For the latter case, we outline a practical, approximate scheme by which the required pair of Hubbard parameters, denoted as $U_1$ and $U_2$, may be calculated from first principles.

The time needed for metals to respond structurally to electronic excitation is usually considered to be set by picosecond-long electron-phonon coupling, hence limiting the rapid achievement of structural phase changes. Via time-resolved ellipsometry, we show that fs laser excitation of tungsten determines unexpectedly fast optical and structural transformations, almost on the time scale of the laser pulse, with sub-ps destructuring of matter. If at low energies, below the damage threshold, Fermi redistribution within the $d$-band pseudogap populates localized states that screen ions, in ablative ranges, at electronic energies above 4 eV, a charge deficit appears on bonding orbitals generating bond-softening and ionic repulsion in spite of the metallic environment. Along pressure gradients, this rapidly destabilizes the structure. Sub-ps ablation occurs, challenging thermally driven scenarios of phonon activation. First-principles and hydrodynamic models show that ablation interrelates nonclassical charge distortions, electronic stress, and classical electron-phonon dynamics, with the coexistence of thermal and nonthermal effects on scales believed to be prohibitive for the former.

Classical molecular-dynamics simulations were used to examine the effect of an electric field on the mobility of oxygen ions in the model crystalline oxide ${\mathrm{CeO}}_2$. Simulation cells containing oxygen vacancies were subjected at temperatures $1000\le T/\mathrm{K}\le 1600$ to electric field strengths $0.1\le E/\mathrm{MV}{\mathrm{cm}}^{-1}\le 40$ to obtain the oxygen-ion mobility $u_{\mathrm{i}}(E,T)$. In addition, static nudged-elastic-band calculations were performed to obtain directly the forward/reverse barriers for oxygen-ion migration, $\mathrm{\Delta }{H}_{\mathrm{mig}}^{\mathrm{f}/\mathrm{r}}$. Qualitatively, $u_{\mathrm{i}}$ behaves as expected: independent of $E$ at low values of $E$ and exponentially dependent on $E$ at high values. The quantitative (standard) Mott-Gurney treatment, however, underestimates $\mathrm{\Delta }{H}_{\mathrm{mig}}^{\mathrm{f}}$ at high $E$ and thus overestimates $u_{\mathrm{i}}$. A new, superior analytical expression for $u_{\mathrm{i}}(E,T)$ is consequently derived.

The $5f$ electrons of uranium in the uranium mononitride (UN) compound are described in the literature as either localized or fully itinerant. Motivated by these contradictory statements, we studied low-temperature specific heat and high-field magnetization of single-crystalline UN in magnetic fields up to 9 and 65 T, respectively. Our detailed analysis of the magnetic contribution to the specific heat of UN revealed that its real ground state is complex and the $5f$ electrons seem to have a dual nature; i.e., they possess simultaneously local and itinerant characters in two substates. High-field experiments allowed us to construct a tentative magnetic phase diagram of UN with a metamagnetic transition from antiferromagnetism to ferrimagnetism at a magnetic field as high as 58 T at 2 K. Such a field only enables a reversal of 1 of the 12 antiferromagnetically coupled ferromagnetic layers in the direction of the magnetic field. Any further steplike transitions require application of ever higher magnetic fields, which is beyond the experimental possibilities. We show that the magnetic phase diagram can be successfully reproduced considering a layer model of the Ising spins. That model allows rough estimation of a phase transition into fully induced ferromagnetism at a field as high as about 258 T. It gives rise to a giant coupling between ferromagnetically ordered layers in UN. The obtained characteristics are presented, together with the results of recent x-ray photoemission spectroscopy and transport property measurements. They are analyzed and compared with a number of earlier experiments and band structure calculations that were performed for this compound and are widely described in the literature. We show that different experiments probe different substates of the uranium $5f$ electrons in UN (itinerant or localized), which supports our hypothesis on their dual nature.

We studied the novel multiferroic material ${\mathrm{Sr}}_2{\mathrm{FeSi}}_2{\mathrm{O}}_7$ and found three absorption modes above the magnetic ordering transition temperature using time-domain terahertz spectroscopy. These absorption modes can be explained as the optical transitions between the spin-orbit coupling and crystal-field split $3d^6{\mathrm{Fe}}^{2+}$ ground-state term in this material. Consideration of the compressed tetrahedral environment of the ${\mathrm{Fe}}^{2+}$ site is crucial to understand the excitations. We point out, however, discrepancies between the single-site atomic picture and the experimental results.

Motivated by experimental realizations of integer quantized charge pumping in one-dimensional superlattices [Nat. Phys. 12, 350 (2016); Nat. Phys. 12, 296 (2016)], we generalize and propose the adiabatic pumping of a fractionalized charge in interacting bosonic systems. This is achieved by dynamically sweeping the modulated potential in a class of one-dimensional interacting systems. As concrete examples, we show the charge pumping of interacting bosons at certain fractionally occupied fillings. We find that, for a given ground state, the charge pumping in a complete potential cycle is quantized to the fractional value related to the corresponding Chern number, characterized by the motion of the charge polarization per site. Moreover, the difference between charge polarizations of two ground states is quantized to an intrinsic constant revealing the fractional elementary charge of quasiparticle.