We present a theoretical investigation of the voltage-driven metal-insulator transition based on solving coupled Boltzmann and Hartree-Fock equations to determine the insulating gap and the electron distribution in a model system: a one-dimensional charge density wave. Electric fields that are parametrically small relative to energy gaps can shift the electron distribution away from the momentum-space region where interband relaxation is efficient, leading to a highly nonequilibrium quasiparticle distribution even in the absence of Zener tunneling. The gap equation is found to have regions of multistability; a nonequilibrium analog of the free energy is constructed and used to determine which phase is preferred.

The linewidths of the $\pi$ and $\sigma$ bands originating from the electron-phonon coupling in graphene are analyzed based on model calculations and experimental angle-resolved photoemission spectroscopy (ARPES) data. We find evidence for crucial contributions to the lifetime broadening from interband scattering $\pi \to \sigma$ and $\sigma \to \pi$, respectively, driven by the out-of-plane ZA acoustic phonons. The essential features of the calculated $\sigma$ band linewidths are in agreement with recent published ARPES data [Mazzola et al., Phys. Rev. B 95, 075430 (2017)] and of the $\pi$ band linewidth with ARPES data presented here.

A modified scheme of scanning tunneling spectroscopy data analysis has been used to reconstruct the band structure of a strongly dispersive two-dimensional unoccupied electronic state located at the commensurate interface between a monolayer of 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA) and Ag(111) surface up to the fourth band in the reduced Brillouin zone of the molecular monolayer. The two-dimensional scattering potential, evaluated with the help of the measured band structure data, is highly corrugated. This shows that the accepted picture, describing the interface state of PTCDA/Ag(111) as an upshifted Shockley state of Ag(111), is incomplete. Our findings indicate that the hybridization of unoccupied molecular orbitals of PTCDA on one of the two sublattices with the metal states of Ag(111) substantially influences the interface state properties.

Diamond-based quantum magnetometers are more sensitive to oscillating (ac) magnetic fields than static (dc) fields because the crystal impurity-induced ensemble dephasing time $T_2^{*}$, the relevant sensing time for a dc field, is much shorter than the spin coherence time $T_2$, which determines the sensitivity to ac fields. Here we demonstrate measurement of dc magnetic fields using a physically rotating ensemble of nitrogen-vacancy centers at a precision ultimately limited by $T_2$ rather than $T_2^{*}$. The rotation period of the diamond is comparable to $T_2$ and the angle between the nitrogen-vacancy (NV) axis and the target magnetic field changes as a function of time, thus upconverting the static magnetic field to an oscillating field in the physically rotating frame. Using spin-echo interferometry of the rotating NV centers, we are able to perform measurements for over 100 times longer compared to a conventional Ramsey experiment. With modifications our scheme could realize dc sensitivities equivalent to demonstrated NV center ac magnetic field sensitivities of order $0.1\mathrm{nT}{\mathrm{Hz}}^{-1/2}$.

The effect of differently oriented magnetic field on chiral incommensurate helimagnet UPtGe is studied both experimentally and theoretically. The magnetization measurements up to the field above the saturation have revealed an isotropic magnetic response below 20 T and a remarkable nonmonotonic anisotropy in high fields. Moreover, the two principally different phase transitions from the noncollinear incommensurate to the field-induced ferromagnetic state have been observed. These properties are successfully explained by density-functional theory calculations taking into account the noncollinearity of the magnetic structures, arbitrary directed magnetic field, and relativistic effects. We also estimate the strength of different competing magnetic interactions and discuss possible scenarios of the field-induced phase transformations.

Hopfions are three-dimensional (3D) topological textures characterized by the integer Hopf invariant $Q_H$. Here we present the realization of a zero-field, stable hopfion spin texture in a magnetic system consisting of a chiral magnet nanodisk sandwiched by two films with perpendicular magnetic anisotropy. The preimages of the spin texture and numerical calculations of $Q_H$ show that the hopfion has $Q_H=1$. Furthermore, another nontrivial state that includes a monopole-antimonopole pair (MAP) is also stabilized in this system. By applying an external magnetic field, hopfion and MAP states with the same polarization can be switched between each other. The topological transition between the hopfion and the MAP state involves a creation (annihilation) of the MAP and twist of the preimages. Our work paves the way to study nontrivial 3D topological spin textures and stimulates more investigations in the field of 3D spintronics.

We address the dynamics of a localized molecular spin under the influence of external voltage pulses using a generalized spin equation of motion which incorporates anisotropic fields, nonequilibrium conditions, and nonadiabatic dynamics. We predict a recurring $4\pi$-periodic switching of the localized spin by application of a voltage pulse of temporal length $\tau$. The switching phenomena can be explained by dynamical exchange interactions, internal transient fields, and self-interactions acting on the localized spin moment.

Thermal expansion, magnetostriction, and magnetization measurements under magnetic field and hydrostatic pressure were performed on a ${\mathrm{UAu}}_2{\mathrm{Si}}_2$ single crystal. They revealed a large anisotropy of magnetoelastic properties manifested by prominent length changes, leading to a collapse of the unit-cell volume accompanied by breaking the fourfold symmetry (similar to that in ${\mathrm{URu}}_2{\mathrm{Si}}_2$ in the hidden-order state) in the antiferromagnetic state as consequences of strong magnetoelastic coupling. The magnetostriction curves measured at higher temperatures confirm a bulk character of the $50\mathrm{K}$ weak ferromagnetic phase. The large positive pressure change of the ordering temperature predicted from Ehrenfest relation contradicts the more than an order of magnitude smaller pressure dependence observed by the magnetization and specific heat measured under hydrostatic pressure. A comprehensive magnetic phase diagram of ${\mathrm{UAu}}_2{\mathrm{Si}}_2$ in magnetic field applied along the $c$ axis is presented. The ground-state antiferromagnetic phase is suppressed by a field-induced metamagnetic transition that changes its character from second to first order at the tricritical point.

While spin textures in materials exhibiting zero net magnetization, such as antiferromagnetic domain walls, have attracted much interest lately due to their robustness against external magnetic noise, their generic detection via conventional magnetometry remains a challenging task. Here, we propose quantum relaxometry as a route to image spin textures by probing the collective spin modes harbored by them. We investigate the Goldstone modes hosted by an antiferromagnetic domain wall and assess the relaxation rate of a quantum-spin sensor interacting with them. We show that such modes can be detected via relaxometry in some common antiferromagnets. Moreover, based on symmetry considerations, we propose a strategy for probing the individual dynamics of each mode.

The ultrafast dynamics of periodic ripples are studied during their formation on the surface of a gold film with a prefabricated nanogroove. These transient ripples are induced by a single 800-nm, 50-fs laser pulse and are observed by a collinear pump-probe imaging method. When the laser polarization is parallel to the nanogroove, transient ripples begin appearing after an elapsed time of 25–80 ps, and become clear and regular at 400–600 ps. The ripple period increases from 685 to 770 nm as the laser fluence $F$ increases from 0.73 to $3.42\mathrm{J}/\mathrm{c}{\mathrm{m}}^2$. The evolution of temperature and lattice temperature are theoretically studied using the two-temperature model. When the laser fluence $F$ is above $0.73\mathrm{J}/\mathrm{c}{\mathrm{m}}^2$, the electron temperature rises to several ${10}^4\mathrm{K}$, and the collision frequency rises above ${10}^{16}/\mathrm{s}$, which further causes the localization of hot electrons. Moreover, the $d$-band electrons can be excited through two-photon absorption and become free electrons. Using the dielectric constant of the excited states, which includes the effects of hot-electron localization and $d$-band transitions, the period predicted by the surface-plasmon-polarization (SPP) model accords well with experimental results. Both theory and experiment give support to SPP excitations playing a prominent role in the formation of periodic ripples induced by femtosecond laser pulses.

By using density functional theory calculations we determined the influence of anharmonic effects on the infrared reflectance, transmittance, and emittance of MgO. The goal is to determine the limit of validity of a perturbative (multiphonon) approach. MgO is chosen as a test material because of the availability of different kinds of radiative properties measured experimentally. Nonanalytic terms of the three-phonon scattering coefficients are explicitly calculated and do not provide measurable effects. The agreement is overall very good to such an extent that, already at room temperature, one can clearly identify regions in which four-phonon scattering processes are dominant with respect to the three-phonon ones. The influence of isotopic disorder at cryogenic temperatures is also settled.

We present a comprehensive and systematic magnetization and ac susceptibility study of ${\mathrm{Mn}}_{1-x}{\mathrm{Fe}}_x\mathrm{Si}$ over an extensive range of ten Fe concentrations between $x=0$ and $x=0.32$. With increasing Fe substitution, the critical temperature decreases but the magnetic phase diagrams remain qualitatively unaltered for $x\le x^{*}\approx 0.11$ with clear boundaries between the helical, conical, and skyrmion lattice phase as well as an enhanced precursor phase. A notably different behavior sets in for $x=0.11,0.13$, and 0.14, where certain characteristics of helimagnetic correlations persist, but without clear phase boundaries. Although a qualitative change already sets in at $x^{*}$, the transition temperature and spontaneous magnetization vanish only at $x_c=0.17$ where also the average magnetic interactions change sign. Although the Curie-Weiss temperature reaches $-12$ K for $x=0.32$, no signature of long-range magnetic order is found down to the lowest temperature, indicating a possible significant role for quantum fluctuations in these systems.

We investigate the influence of Pearl vortices in the vicinity of an edge-type Josephson junction for a superconducting thin-film loop in the form of an annulus, under uniform magnetic field. Specifically, we obtain the exact analytic formulation that allows one to describe the circulating current density and the gauge invariant phase increment $\mathrm{\Delta }\varphi$ across the junction. The main properties of $\mathrm{\Delta }\varphi$ and their influence on the critical current pattern $I_c\left(B\right)$ are described quantitatively in terms of the loop's width-to-radius ratio $W/R$ and of the vortex position within the loop ${\mathbf{r}}_v$. It is shown that narrow loops ($W/R<0.3$) may be well described by the straight geometry limit. However, such approximation fails to predict a number of distinctive features captured by our formulation, such as the node lifting effect of the $I_c\left(B\right)$ pattern in wide loops or the actual influence of a vortex pinned at different positions.

We develop the plasmon-pole approximation (PPA) theory for calculating the carrier self-energy of extrinsic graphene as a function of doping density within analytical approximations to the $GW$ random phase approximation ($GW$-RPA). Our calculated self-energy shows excellent quantitative agreement with the corresponding full $GW$-RPA calculation results in spite of the simplicity of the PPA, establishing the general validity of the plasmon-pole approximation scheme. We also provide a comparison between the PPA and the hydrodynamic approximation in graphene, and comment on the experimental implications of our findings.

The lower-symmetry trilayer AAB-stacked graphene exhibits rich electronic properties and diverse Coulomb excitations. Three pairs of unusual valence and conduction bands create nine available interband excitations for the undoped case, in which the imaginary (real) part of the polarizability shows one-dimensional square root asymmetric peaks and two-dimensional shoulder structures (pairs of antisymmetric peaks and logarithm-type symmetric peaks). The low-frequency acoustic plasmon, being revealed as a prominent peak in the energy-loss spectrum, can survive in a narrow-gap system with the large density of states from the valence band. This type of plasmon mode is similar to that in a narrow-gap carbon nanotube. However, the decisive mechanism governing this plasmon is the intraband conduction state excitations. Its frequency, intensity and critical momentum exhibit a nonmonotonic dependence on the Fermi energy. The well-defined electron-hole excitation boundaries and the higher frequency optical plasmons are transformed by varying the Fermi energy. Many substantial differences between the electronic properties of AAB and other trilayer graphene stackings are discussed.

Epitaxial hexagonal boron nitride on Ir(111) is significantly modified by adsorption and intercalation of alkali-metal atoms. Regarding geometry, intercalation lifts the two-dimensional layer from its substrate and reduces the characteristic corrugation imprinted by direct contact with the metal substrate. Moreover, the presence of charged species in close proximity to the hexagonal boron nitride (hBN) layer strongly shifts the electronic structure (valence bands and core levels). We used scanning tunneling microscopy, low-energy electron diffraction, x-ray photoelectron spectroscopy (XPS), and the x-ray standing wave technique to study changes in the atomic structure induced by Cs adsorption and intercalation. Depending on the preparation, the alkali-metal atoms can be found on top and underneath the hexagonal boron nitride in ordered and disordered arrangements. Adsorbed Cs does not change the morphology of hBN/Ir(111) significantly, whereas an intercalated layer of Cs decouples the two-dimensional sheet and irons out its corrugation. XPS and angle-resolved photoelectron spectroscopy reveal a shift of the electronic states to higher binding energies, which increases with increasing density of the adsorbed and intercalated Cs. In the densest phase, Cs both intercalates and adsorbs on hBN and shifts the electronic states of hexagonal boron nitride by 3.56 eV. As this shift is not sufficient to move the conduction band below the Fermi energy, the electronic band gap must be larger than 5.85 eV.

Manipulating the orbital occupation of valence electrons via epitaxial strain in an effort to induce new functional properties requires considerations of how changes in the local bonding environment affect the band structure at the Fermi level. Using synchrotron radiation to measure the x-ray linear dichroism of epitaxially strained films of the correlated oxide ${\mathrm{CaFeO}}_3$, we demonstrate that the orbital polarization of the Fe valence electrons is opposite from conventional understanding. Although the energetic ordering of the Fe $3d$ orbitals is confirmed by multiplet ligand field theory analysis to be consistent with previously reported strain-induced behavior, we find that the nominally higher energy orbital is more populated than the lower. We ascribe this inverted orbital polarization to an anisotropic bandwidth response to strain in a compound with nearly filled bands. These findings provide an important counterexample to the traditional understanding of strain-induced orbital polarization and reveal a method to engineer otherwise unachievable orbital occupations in correlated oxides.