The ultrathin Pb layer on the Si(110) surface might be converted into hut-shaped nanowires extending in a unique $\left[\overline{1}10\right]$ direction. Despite the fact that about 2.6 ML of lead is used to form an ordered array of nanowires, this system shows not only anisotropy of the electronic structure but also a small band gap in the [001] direction. Among several quasi-one-dimensional bands, two of them show pure one-dimensional character. Electron dispersion maps measured with photoelectron spectroscopy are supported by scanning tunneling spectroscopy data which confirm the absence of electronic states at the Fermi level in areas between lead nanowires. Based on first-principles density functional theory calculations four different structural models of lead nanowires are proposed reflecting a different number of Pb atoms in the unit cell.

We investigate the crossover of the entanglement entropy toward its thermal value in nearly integrable systems. We employ equations-of-motion techniques to study the entanglement dynamics in a lattice model of weakly interacting spinless fermions after a quantum quench. For weak enough interactions we observe a two-step relaxation of the entanglement entropies of finite subsystems. Initially, the entropies follow a nearly integrable evolution, approaching the value predicted by the generalized Gibbs ensemble (GGE) of the unperturbed model. Then, they start a slow drift toward the thermal stationary value described by a standard Gibbs ensemble (GE). While the initial relaxation to the GGE is independent of the interaction, the slow drift from GGE to GE values happens on timescales proportional to the inverse interaction squared. For asymptotically large times and subsystem sizes the dynamics of the entropies can be predicted using a modified quasiparticle picture that keeps track of the evolution of the fermionic occupations caused by the integrability breaking. This picture gives a quantitative description of the results as long as the integrability-breaking timescale is much larger than the one associated with the (quasi)saturation to the GGE. In the opposite limit, the quasiparticle picture still provides the correct late-time behavior, but it underestimates the initial slope of the entanglement entropy.

We have investigated the pressure effect on magnetic ordering of the ferromagnetic double perovskite ${\mathrm{Lu}}_2{\mathrm{NiMnO}}_6$ by magnetization, ac magnetic susceptibility, and neutron diffraction experiments up to 8.0 GPa in order to understand the ferromagnetic-to-antiferromagnetic phase transition by substitution of the $A$ sites in $A_2{\mathrm{NiMnO}}_6$ from rare-earth to indium or scandium ions. Strong ferromagnetic spin correlation seen in the susceptibility at low pressure is significantly suppressed by increasing pressure. In a neutron diffraction experiment, the magnetic Bragg reflections associated with ferromagnetic ordering disappear above 4.5 GPa. For the high-pressure region above 4.5 GPa, an antiferromagnetic ordering with long-period incommensurate modulation appears, which is coexistent with ferromagnetic short-range order. From mean-field calculations, we infer that pressure modification of the delicate balance of the competing next-nearest-neighbor exchange interactions between Ni and Ni, or Mn and Mn, plays an important role in the phase transition from ferromagnetic to antiferromagnetic ordering in $A_2{\mathrm{NiMnO}}_6$.

We report studies of the magnetic properties of a staggered stacked triangular lattice ${\mathrm{Ba}}_2\mathrm{Mn}\mathrm{Te}{\mathrm{O}}_6$ using magnetic susceptibility, specific heat, neutron powder diffraction, inelastic neutron scattering measurements, and first-principles density functional theory calculations. Neutron diffraction measurements reveal ${\mathrm{Ba}}_2\mathrm{Mn}\mathrm{Te}{\mathrm{O}}_6$ to be antiferromagnetically ordered with a propagation vector $\mathit{k}=(0.5,0.5,0)$ and Néel transition temperature of $T_{\text{N}}\approx 20$ K. The dominant interaction derived from the Curie-Weiss fitting to the inverse DC susceptibility is antiferromagnetic. Modeling of the inelastic neutron scattering data with linear spin wave theory yielded magnetic exchange interactions for the nearest intralayer, nearest interlayer, and next-nearest interlayer of $J_1=0.27\left(3\right)$, meV $J_2=0.27\left(3\right)$ meV, and $J_3=-0.05\left(1\right)$ meV, respectively, and a small value of easy-axis anisotropy of $D_{zz}=-0.01$ meV. We derive a magnetic phase diagram that reveals a collinear stripe-type antiferromagnetic order that is stabilized by the competition between $J_1,J_2$, and $J_3$.

We employ group theory to study the properties of the lattice vibrations and magnetic order in the antiferromagnetic topological insulator $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$, both in bulk and few-layer form. In the paramagnetic phase, we obtain the degeneracies, the selection rules, and real-space displacements for the lattice vibrational modes for different stacking configurations. We discuss how magnetism influences these results. As a representative example, we consider a double septuple layer system and obtain the form of the magnetic order allowed by the symmetries of the crystal. We derive the allowed coupling terms that describe the paramagnetic to antiferromagnetic transition. Finally, we discuss the implications of our results for Raman scattering and other optical measurements. Our work sets the stage for a deeper understanding of the interplay of lattice modes, band topology, and magnetic order in $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$ and other symmetry-related materials.

Given the development of hybrid spintronic-photonic devices and optical manipulation of chiral magnetic textures, a combined interest in single-pulse all-optical switching (AOS) of magnetization and current-induced domain wall motion in synthetic ferrimagnetic structures with strong Dzyaloshinskii-Moriya Interaction (DMI) is emerging. In this work, we explore the role of the DMI on the AOS process as well as the stability of optically written micromagnetic domains using specially engineered Co/Gd-based multilayer structures. Quantitative insight is obtained by measuring the interesting dynamics in moon-shaped structures written by two successive laser pulses. The stability of domains resulting from an interplay between the dipolar interaction and domain-wall energy are compared to simple analytical models and micromagnetic simulations. A shortening process occurring at ns time scale is confirmed by our computational results, and a stabilizing role of DMI on microscopic AOS-written domains is experimentally demonstrated.

The Berezinskii-Kosterlitz-Thouless (BKT) mechanism describes universal vortex unbinding in many two-dimensional systems, including the paradigmatic XY model. However, most of these systems present a complex interplay between excitations at different length scales that complicates theoretical calculations of nonuniversal thermodynamic quantities. These difficulties may be overcome by suitably modifying the initial conditions of the BKT flow equations to account for noncritical fluctuations at small length scales. In this work, we perform a systematic study of the validity and limits of this two-step approach by constructing optimised initial conditions for the BKT flow. We find that the two-step approach can accurately reproduce the results of Monte Carlo simulations of the traditional XY model. To systematically study the interplay between vortices and spin-wave excitations, we introduce a modified XY model with increased vortex fugacity. We present large-scale Monte Carlo simulations of the spin stiffness and vortex density for this modified XY model and show that even at large vortex fugacity, vortex unbinding is accurately described by the nonperturbative functional renormalization group.

The helicity modulus of the quasi-one-dimensional classical $\mathrm{XY}$ model is examined in connection with liquid He confined in nanopores. The helicity modulus ${\mathrm{{\rm Y}}}_{\mathrm{p}}$ is suppressed with phase slippage, and its suppression is found to be well described with the scaling form obtained from the Tomonaga-Luttinger liquid theory in the one-dimensional limit. For phase slippage to occur, the system has to overcome an energy barrier, and the helicity modulus ${\mathrm{{\rm Y}}}_{\mathrm{p}}$ suppressed by phase slippage is not necessarily observed in experiments. The energy barrier for phase slippage is explicitly calculated. Using the result, the helicity modulus ${\mathrm{{\rm Y}}}_{\mathrm{s}}$ protected by the energy barrier is found to survive up to a higher temperature than ${\mathrm{{\rm Y}}}_{\mathrm{p}}$. The relation of the present results to experiments on liquid He in nanopores is also discussed.

A theory of photoelectron escape time $\tau$ from crystals is presented that combines the one-step photoemission theory with the Wigner delay. Equations are formally derived and illustrated with an exactly solvable model. An ab initio implementation with the augmented-plane-wave-based scattering method is applied to emission from Mg(0001). The theory is in reasonable agreement with streaking measurements. Lattice scattering is found to cause rapid variations of $\tau$ with photon energy both for the valence band and for the $2p$ core band emission, and a strong dependence on the initial state is revealed.

We investigate charge transport and photocarrier dynamics in graphene–mesoporous silicon nanocomposites using optical-pump terahertz-probe measurements. The nanocomposite material consists of a free-standing mesoporous silicon membrane whose specific surface is coated with a few-layer graphene shell. Temporal decays of the differential transmission measurements are reproduced using a biexponential function with an initial decay time of 5 ps and a longer decay time of about 25 ps. These decay times are significantly reduced compared to the values of ${\tau }_1\sim 74$ ps and ${\tau }_2\sim 730$ ps obtained for the uncoated mesoporous silicon membrane and this is attributed to the introduction of additional surface defects formed during the graphene deposition process. Based on the influence of the laser fluence on the time-resolved differential transmission curves, a capture/recombination model is proposed to describe the photocarrier dynamics in these nanocomposite materials. Frequency-dependent complex photoconductivity data curves are extracted from the terahertz waveforms taken at different optical-pump THz-probe delays. These data curves are well reproduced using a modified Drude-Smith model taking into account diffusive-restoring currents. The $c$ parameter of this model, which describes the degree of carrier localization, is about $-0.73$ for the uncoated porous Si membrane and is approaching $-1$ for graphene–mesoporous Si nanocomposites formed at temperatures above $800{}^{\circ }\mathrm{C}$. For all the nanocomposites, the characteristics of the photoconductive material, in terms of photocarrier capture/recombination time and effective mobility, are of interest for the fabrication of pulsed terahertz devices.

Suspended carbon nanotubes display at cryogenic temperatures a distinct interaction between the quantized longitudinal vibration of the macromolecule and its embedded quantum dot, visible via Franck-Condon conductance sidebands in transport spectroscopy. We present data on such sidebands at known absolute number $N_{\text{el}}=1$ and $N_{\text{el}}=2$ of conduction band electrons and, consequently, well-defined electronic ground and excited states in a clean nanotube device. The interaction evolves only at a finite axial magnetic field and displays a distinct magnetic-field dependence of the Franck-Condon coupling parameter, different for different electronic base states and indicating a valley dependence. Reshaping of the electronic wave function by the magnetic field is discussed as a possible cause of our observations; its impact is demonstrated in a model calculation reproducing the field-dependent coupling.

Studies of the electronic spectral function in cuprates by angle-resolved photoemission spectroscopy (ARPES) reveal unusual features in the pseudogap phase that persist in the superconducting phase. Here we address these observations based on the recently proposed idea that the pseudogap is due to the fractionalization of modulated particle-particle pairs (a pair density wave) into uniform particle-particle and modulated particle-hole pairs. The constraint that appears between these two types of pairs can be seen has an amplitude for the pseudogap energy scale. This constraint directly modifies the electronic spectral function in the pseudogap phase. We derive a self-consistent equation for the pseudogap amplitude and show that it leads to the formation of Fermi arcs. The band dispersion obtained in the antinodal region is in good agreement with experimental ARPES observations in ${\mathrm{Pb}}_{0.55}{\mathrm{Bi}}_{1.5}{\mathrm{Sr}}_{1.6}{\mathrm{La}}_{0.4}{\mathrm{CuO}}_{6+\delta }\left(\mathrm{Bi}2201\right)$ and present a back-bending that goes to the Fermi level as we go away from the antinodal region. We also discuss the temperature dependence of the ARPES spectrum in the pseudogap and in the superconducting state.

We study theoretically the ultrafast electron dynamics of three-dimensional Weyl semimetals in the field of a laser pulse. For a circularly polarized pulse, such dynamics is governed by topological resonance, which manifests itself as a specific conduction band population distribution in the vicinity of the Weyl points. The topological resonance is determined by the competition between the topological phase and the dynamic phase and depends on the handedness of a circularly polarized pulse. Also, we show that the conduction band population induced by a circularly polarized pulse that consists of two oscillations with opposite handedness is highly chiral, which represents the intrinsic chirality of the Weyl points.

Despite extensive studies on coupled spin and valley physics, occurrence of these properties is currently limited to transition-metal dichalcogenides (TMDCs) and graphene. Therefore, exploration of materials beyond TMDCs and graphene is necessary for a further advancement in valleytronics. In this work, the $\mathrm{Hf}{\mathrm{N}}_2$ monolayer, a theoretically reported semiconductor having a direct band gap has been investigated in-depth for its applications in valleytronics and spintronics. It exhibits a large valley spin splitting (VSS) ∼314 meV at the conduction band (CB) due to strong spin-orbit coupling (SOC). Such a large VSS at the CB is unique and comparable to that of tungsten-based dichalcogenide monolayers at its valence band (VB). The spin splitting is nearly insensitive to small in-plane strain, whereas the optical transition frequencies between the spin states $({\omega }_{\mathrm{up}},{\omega }_{\mathrm{down}})$ in the CB and VB is effectively strain tunable. Inspired by the experimentally observed higher exciton lifetime 1.8 ns in $\mathrm{MoS}{\mathrm{e}}_2/\mathrm{WS}{\mathrm{e}}_2$ type-II van der Waals heterostructure (vdWH) than in the monolayers ∼2.1 (2.5) $ps$ in monolayer $\mathrm{Mo}{\mathrm{S}}_2$ $\left(\mathrm{W}{\mathrm{S}}_2\right)$, a type-II (vdWH) prototype has been demonstrated by stacking $\mathrm{Hf}{\mathrm{N}}_2$ monolayer over $1\mathrm{H}-\mathrm{WS}{\mathrm{e}}_2$ monolayer for an extended lifetime of valley polarized excitons and large magnitude of spin splitting at both CB and VB. The elastic properties highlight the robustness of $\mathrm{Hf}{\mathrm{N}}_2$ monolayer and vdWH, as the bending modulus and critical buckling strain are found to be superior to that in graphene. Further, carrier mobility calculated using the deformation potential theory in $\mathrm{Hf}{\mathrm{N}}_2/\mathrm{WS}{\mathrm{e}}_2$ is as high as ${\mu }_h=16\times {10}^3\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^1{\mathrm{s}}^{-1}$. Moreover, this work introduces the $\mathrm{Hf}{\mathrm{N}}_2$ monolayer as an exceptionally promising valleytronic material having valley properties complementary to that of Group VI TMDC. This is a report on valley contrasting physics and VSS at the CB, which would strongly motivate experimentalists to synthesize and explore this predicted two-dimensional material.

This paper reports an original approach allowing us to simulate the compression at finite temperature of nanostructures, based on the combination of external forces with Car-Parrinello molecular dynamics. An example of a successful application is described: the compression of buckminsterfullerene ${\mathrm{C}}_{60}$ at room temperature. It is shown that the ${\mathrm{C}}_{60}$ shell breaks at much lower strains than previously predicted, with a maximum contact force of 30 nN. This simple example demonstrates the potential of this approach, which can be especially useful to determine the mechanical properties of nanoparticles and clusters with an outstanding accuracy.

We analytically and numerically study the Loschmidt echo and the dynamical order parameters in a spin chain with a deconfined phase transition between a dimerized state and a ferromagnetic phase. For quenches from a dimerized state to a ferromagnetic phase, we find that the model can exhibit a dynamical quantum phase transition characterized by an associating dimerized order parameter. In particular, when quenching the system from the Majumdar-Ghosh state to the ferromagnetic Ising state, we find an exact mapping into the classical Ising chain for a quench from the paramagnetic phase to the classical Ising phase by analytically calculating the Loschmidt echo and the dynamical order parameters. By contrast, for quenches from a ferromagnetic state to a dimerized state, the system relaxes very fast so that the dynamical quantum transition may exist on only a short timescale. We reveal that the dynamical quantum phase transition can occur in systems with two broken symmetry phases and the quench dynamics may be independent of equilibrium phase transitions.