We calculate nonequilibrium electron distributions created by the absorption of an intense, ultrashort laser pulse in a transparent model-dielectric using a Boltzmann-type collision approach. We compare electron energy and particle density, as well as electron-phonon energy transfer of a full nonequilibrium simulation with values of a simulation enforcing instant thermalization, testing the inherent assumption of temperature-based models. Finally, we calculate electron-phonon coupling parameters of nonequilibrium distributions and compare these with those of thermalized distributions of the same particle and energy density. We find that the strength as well as the relaxation of the electron-phonon coupling depends considerably on the excitation parameters.

We investigated the $ab$-plane optical properties of single crystals of ${\mathrm{WTe}}_2$ for light polarized parallel and perpendicular to the W-chain axis over a broad range of frequency and temperature. At far-infrared frequencies, we observed a striking dependence of the reflectance edge on light polarization, corresponding to anisotropy of the carrier effective masses. We quantitatively studied the temperature dependence of the plasma frequency, revealing a modest increase of the effective mass anisotropy in the $ab$ plane upon cooling. We also found strongly anisotropic interband transitions persisting to high photon energies. These results were analyzed by comparison with ab initio calculations. The calculated and measured plasma frequencies agree to within 10% for both polarizations, while the calculated interband conductivity shows excellent agreement with experiment.

Spin-orbit coupling effects occurring in noncentrosymmetric materials are known to be responsible for nontrivial spin configurations and a number of emergent physical phenomena. Ferroelectric materials may be especially interesting in this regard due to reversible spontaneous polarization making possible a nonvolatile electrical control of the spin degrees of freedom. Here, we explore a technologically relevant oxide material, $\mathrm{Hf}{\mathrm{O}}_2$, which has been shown to exhibit robust ferroelectricity in a noncentrosymmetric orthorhombic phase. Using theoretical modelling based on density-functional theory, we investigate the spin-dependent electronic structure of the ferroelectric $\mathrm{Hf}{\mathrm{O}}_2$ and demonstrate the appearance of chiral spin textures driven by spin-orbit coupling. We analyze these spin configurations in terms of the Rashba and Dresselhaus effects within the $\mathbf{k}·\mathbf{p}$ Hamiltonian model and find that the Rashba-type spin texture dominates around the valence-band maximum, while the Dresselhaus-type spin texture prevails around the conduction band minimum. The latter is characterized by a very large Dresselhaus constant ${\lambda }_D=0.578$ eV Å, which allows using this material as a tunnel barrier to produce tunneling anomalous and spin Hall effects that are reversible by ferroelectric polarization.

Strong spin-orbit coupling creates exotic electronic states such as Rashba and topological surface states, which hold promise for technologies involving the manipulation of spin. Only recently has the complexity of these surface states been appreciated: they are composed of several atomic orbitals with distinct spin textures in momentum space. A complete picture of the wave function must account for this orbital dependence of spin. We discover that symmetry constrains the way orbital and spin components of a state coevolve as a function of momentum, and from this, we determine the rules governing how the two degrees of freedom are interwoven. We directly observe this complexity in spin-resolved photoemission and ab initio calculations of the topological surface states of Sb(111), where the photoelectron spin direction near $\overline{\mathrm{\Gamma }}$ is found to have a strong and unusual dependence on photon polarization. This dependence unexpectedly breaks down at large $\left|k\right|$, where the surface states mix with other nearby surface states. However, along mirror planes, symmetry protects the distinct spin orientations of different orbitals. Our discovery broadens the understanding of surface states with strong spin-orbit coupling, demonstrates the conditions that allow for optical manipulation of photoelectron spin, and will be highly instructive for future spintronics applications.

${\mathrm{CrGeTe}}_3$ recently emerges as a new two-dimensional (2D) ferromagnetic semiconductor that is promising for spintronic device applications. Unlike ${\mathrm{CrSiTe}}_3$ whose magnetism can be understood using the 2D-Ising model, ${\mathrm{CrGeTe}}_3$ exhibits a smaller van der Waals gap and larger cleavage energy, which could lead to a transition of magnetic mechanism from 2D to 3D. To confirm this speculation, we investigate the critical behavior of ${\mathrm{CrGeTe}}_3$ around the second-order paramagnetic-ferromagnetic phase transition. We obtain the critical exponents estimated by several common experimental techniques including the modified Arrott plot, Kouvel-Fisher method, and critical isotherm analysis, which show that the magnetism of ${\mathrm{CrGeTe}}_3$ follows the tricritical mean-field model with the critical exponents $\beta$, $\gamma$, and $\delta$ of $0.240\pm 0.006$, $1.000\pm 0.005$, and $5.070\pm 0.006$, respectively, at the Curie temperature of 67.9 K. We therefore suggest that the magnetic phase transition from 2D to 3D for ${\mathrm{CrGeTe}}_3$ should locate near a tricritical point. Our experiment provides a direct demonstration of the applicability of the tricritical mean-field model to a 2D ferromagnetic semiconductor.

Spin drag measurements were performed in a two-dimensional electron system set close to the crossed spin helix regime and coupled by strong intersubband scattering. In a sample with an uncommon combination of long spin lifetime and high charge mobility, the drift transport allows us to determine the spin-orbit field and the spin mobility anisotropies. We used a random walk model to describe the system dynamics and found excellent agreement for the Rashba and Dresselhaus couplings. The proposed two-subband system displays a large tuning lever arm for the Rashba constant with gate voltage, which provides a new path towards a spin transistor. Furthermore, the data show large spin mobility controlled by the spin-orbit constants setting the field along the direction perpendicular to the drift velocity. This work directly reveals the resistance experienced in the transport of a spin-polarized packet as a function of the strength of anisotropic spin-orbit fields.

The quantum hydrodynamic theory is a promising method for describing microscopic details of macroscopic systems. The hydrodynamic equation can be partially obtained from a single-particle Kohn-Sham equation and improved by adding a viscoelastic kinetic-exchange-correlation tensor term, so that broadening of collective excitation can be taken into account, as well as a correction to the plasmon dispersion. The result is an accurate self-consistent and computationally efficient hydrodynamic description of the free-electron gas. A very accurate agreement with full quantum calculations is shown.

A detailed first-principles study has been performed to evaluate the electronic and optical properties of single-layer (SL) transition metal dichalcogenides (TMDCs) ($MX{}_2$; $M$ = transition metal such as Mo, W, and $X$ = S, Se, Te), in the presence of vacancy defects (VDs). Defects usually play an important role in tailoring electronic, optical, and magnetic properties of semiconductors. We consider three types of VDs in SL TMDCs: (i) $X$ vacancy, (ii) $X_2$ vacancy, and (iii) $M$ vacancy. We show that VDs lead to localized defect states (LDS) in the band structure, which in turn gives rise to sharp transitions in in-plane and out-of-plane optical susceptibilities, ${\chi }_{\parallel }$ and ${\chi }_{\perp }$. The effects of spin-orbit coupling (SOC) are also considered. We find that SOC splitting in LDS is directly related to the atomic number of the transition metal atoms. Apart from electronic and optical properties we also find magnetic signatures (local magnetic moment of $\sim {\mu }_B$) in ${\mathrm{MoSe}}_2$ in the presence of the Mo vacancy, which breaks the time-reversal symmetry and therefore lifts the Kramers degeneracy. We show that a simple qualitative tight-binding model (TBM), involving only the hopping between atoms surrounding the vacancy with an on-site SOC term, is sufficient to capture the essential features of LDS. In addition, the existence of the LDS can be understood from the solution of the two-dimensional Dirac Hamiltonian by employing infinite mass boundary conditions. In order to provide a clear description of the optical absorption spectra, we use group theory to derive the optical selection rules between LDS for both ${\chi }_{\parallel }$ and ${\chi }_{\perp }$.

First-principles calculations have been performed to study the evolution of the inverted bands and the topological phase diagrams of monoclinic transition-metal dichalcogenide monolayers ($1T^{\prime }\text{−}M{X}_2$ with $M=\text{Mo}$, W and $X=\text{S}$, Se, Te) under strain. We find that the band topology undergoes a nontrivial to trivial transition in compressed systems due to the strain-sensitive inverted $p$-orbital and $d$-orbital bands, which exhibit an anisotropic evolution behavior with respect to the strain orientation. In $M{\mathrm{Te}}_2$, the normally ordered $p_y$ and $d$ bands at the $X$ point are inverted mainly by compressive strain along the $y$ direction (${\epsilon }_y$), which, together with the unchanged inverted bands at the $\mathrm{\Gamma }$ point, turns the topology trivial. In $M{\mathrm{S}}_2$ and $M{\mathrm{Se}}_2$, the inverted $p_x$ and $d$ bands at $\mathrm{\Gamma }$ become normally ordered under a large compressive strain along the $x$ direction (${\epsilon }_x$). $M{\mathrm{Te}}_2$ acquires a much smaller critical strain for the topological phase transition (TPT) than S- and Se-based systems due to strain-sensitive head-to-head bonding between the $p_y$ orbitals. Particularly, the critical compressive ${\epsilon }_y$ can be further reduced by applying tensile ${\epsilon }_x$ for $M{\mathrm{Te}}_2$. Our results provide a concrete mechanism behind the nontrivial band topology in $1T^{\prime }\text{−}M{X}_2$ and a guide for applying strain to control the TPT.

We study the radiative heat transfer between multilayer structures made by a periodic repetition of a graphene sheet and a hexagonal boron nitride (hBN) slab. Surface plasmons in a monolayer graphene can couple with hyperbolic phonon polaritons in a single hBN film to form hybrid polaritons that can assist photon tunneling. For periodic multilayer graphene/hBN structures, the stacked metallic/dielectric array can give rise to a further effective hyperbolic behavior, in addition to the intrinsic natural hyperbolic behavior of hBN. The effective hyperbolicity can enable more hyperbolic polaritons that enhance the photon tunneling and hence the near-field heat transfer. However, the hybrid polaritons on the surface, i.e., surface plasmon-phonon polaritons, dominate the near-field heat transfer between multilayer structures when the topmost layer is graphene. The effective hyperbolic regions can be well predicted by the effective medium theory (EMT), thought EMT fails to capture the hybrid surface polaritons and results in a heat transfer rate much lower compared to the exact calculation. The chemical potential of the graphene sheets can be tuned through electrical gating and results in an additional modulation of the heat transfer. We found that the near-field heat transfer between multilayer structures does not increase monotonously with the number of layers in the stack, which provides a way to control the heat transfer rate by the number of graphene layers in the multilayer structure. The results may benefit the applications of near-field energy harvesting and radiative cooling based on hybrid polaritons in two-dimensional materials.

When the coverage of the second atomic layer of Fe in an Fe/W(110) ultrathin film reaches a critical value, the system moves suddenly from a frustrated magnetic state without long-range order to an in-plane ferromagnetic state with long-range order, and displays many features of a percolation transition. Measurements of the magnetic susceptibility as the films are grown at 255 K show power-law scaling that is limited by noise at low deposition and by the dynamics of the paramagnetic, frustrated state at high deposition. Because the measurements represent a system driven by a finite field oscillating at a finite frequency, it is demonstrated that the threshold deposition for percolation is bounded by the depositions where the real and imaginary components of the susceptibility have maxima. Fitting for the critical exponent of the static susceptibility at these bounds gives a bounded value for ${\gamma }_p=2.39\pm 0.04$, in agreement with theory.

We significantly improve the physical models underlying atomistic Monte Carlo (MC) simulations, through the use of ab initio fitted high-dimensional neural network potentials (NNPs). In this way, we can incorporate energetics derived from density functional theory (DFT) in MC, and avoid using empirical potentials that are very challenging to design for complex alloys. We take significant steps forward from a recent work where artificial neural networks (ANNs), exclusively trained on DFT vacancy migration energies, were used to perform kinetic MC simulations of Cu precipitation in Fe. Here, a more extensive transfer of knowledge from DFT to our cohesive model is achieved via the fitting of NNPs, aimed at accurately mimicking the most important aspects of the ab initio predictions. Rigid-lattice potentials are designed to monitor the evolution during the simulation of the system energy, thus taking care of the thermodynamic aspects of the model. In addition, other ANNs are designed to evaluate the activation energies associated with the MC events (migration towards first-nearest-neighbor positions of single point defects), thereby providing an accurate kinetic modeling. Because our methodology inherently requires the calculation of a substantial amount of reference data, we design as well lattice-free potentials, aimed at replacing the very costly DFT method with an approximate, yet accurate and considerably more computationally efficient, potential. The binary FeCu and FeCr alloys are taken as sample applications considering the extensive literature covering these systems.

The presence of a molecule at the $A$ site of an organic perovskite leads to unusual behavior compared to its inorganic counterpart. Considering the case of ${\mathrm{CH}}_3{\mathrm{NH}}_3$, we find that it is both the size of the molecule as well as its orientation in the cage formed by the Pb and Br atoms which determine the favored structure. At the microscopic level, the basic energetics which come into play are steric effects as well as hydrogen bonding. While the molecule is asymmetrically placed in the cuboctahedral cavity, a mapping of the ab initio band structure to a tight-binding model reveals that the movement of the amine end of the molecule towards the Br atoms is driven primarily by electrostatic considerations. While the hydrogen bonding is responsible for driving the octahedral tilts, the energy lowering considerations do not follow a simple prescription of minimizing H-Br bond lengths. The presence of several competing energetics results in a complex low-energy landscape with deep valleys and high barriers between them which could explain the glassy dynamics seen even at low temperatures in the orthorhombic structure where the dipoles are believed to be frozen.

I critically examine Goodenough's explanation for the experimentally observed phase transition in ${\mathrm{LiVO}}_2$ using microscopic calculations based on density functional and dynamical mean field theories. The high-temperature rhombohedral phase exhibits both magnetic and dynamical instabilities. Allowing a magnetic solution for the rhombohedral structure does not open an insulating gap, and an explicit treatment of the on-site Coulomb $U$ interaction is needed to stabilize an insulating rhombohedral phase. The non-spin-polarized phonon dispersions of the rhombohedral phase show two unstable phonon modes at the wave vector $(\frac{1}{3},-\frac{1}{3},0)$ that corresponds to the experimentally observed trimer forming instability. A full relaxation of the supercell corresponding to this instability yields a nonmagnetic state containing ${\mathrm{V}}_3$ trimers. These results are consistent with Goodenough's suggestion that the high-temperature phase is in the localized-electron regime and the transition to the low-temperature phase in the itinerant-electron regime is driven by V-V covalency.

We report ultrafast spectroscopic measurements of intersite charge transfer in a single crystal of the hydrogen-bonded material quinhydrone showing anticorrelated dynamics of vibrational coherences at 172 and $216{\mathrm{cm}}^{-1}$. To explain these coherent dynamics we derive a density matrix model in the presence of higher order electron-vibration coupling. Given the symmetry of vibrations calculated using density functional theory, the Huang-Rhys parameter of the Raman-active vibration found from spontaneous resonance light scattering measurements, and previously reported nonresonant impulsive stimulated Raman scattering measurements on quinhydrone, we restrict the density matrix model to three levels in the excited state of this material to simulate the observed dynamics with a density matrix approach. The close agreement between the experiment and our theoretical treatment leads us to conclude that the measured behavior corresponds to intermolecular Rabi-like oscillatory coherence transfer. These results provide foundational knowledge into the capability of functional organic materials to support quantum coherent transport of charge and energy as well as shed light on recent experimental and theoretical investigations of room temperature organic ferroelectrics.

We have reported on the structural, elastic, electronic, lattice dynamical and electron-phonon interaction properties of $\text{La}{T}_4{X}_{12}$ ($T=\text{Ru}$, Pt and $X=\text{P}$, As, Ge) by using the generalized gradient approximation of the density functional theory and the plane-wave pseudopotential method. These lanthanum-filled skutterudites are found to be characterized with a flat band, resulting in a sharp peak in the electronic density of states, near the Fermi level. The lattice dynamical properties of these materials change considerably when the P atom is replaced by larger As or Ge atoms. The Migdal-Eliashberg approach is used to determine the Eliashberg spectral function for all the considered compounds. Using the calculated Eliashberg spectral function, the value of average electron-phonon coupling parameter is found to be 0.74 for ${\mathrm{LaRu}}_4{\mathrm{P}}_{12}$, 1.03 for ${\mathrm{LaRu}}_4{\mathrm{As}}_{12}$, and 1.08 for ${\mathrm{LaPt}}_4{\mathrm{Ge}}_{12}$. The superconducting critical temperature ($T_c$) values for ${\mathrm{LaRu}}_4{\mathrm{P}}_{12}, {\mathrm{LaRu}}_4{\mathrm{As}}_{12}$, and ${\mathrm{LaPt}}_4{\mathrm{Ge}}_{12}$ are estimated to be 6.95, 11.56, and 8.32 K, respectively, which compare well enough with their experimentally measured values of 7.2, 10.45, and 8.23 K.

The phase diagram of the ${\mathrm{LaFeAs}}_{1-x}{\mathrm{P}}_x\mathrm{O}$ system has been extensively studied through hole and electron doping as well as As/P substitution. It has been revealed that there are three different superconducting phases with different Fermi surface (FS) topologies and thus with possibly different pairing glues. One of them is well understood as spin fluctuation-mediated superconductivity within a FS nesting scenario. Another one with the FSs in a bad nesting condition must be explained in a different context such as orbital or spin fluctuation in a strongly correlated electronic system. In both phases, $T$-linear resistivity was commonly observed when the superconducting transition temperature $T_{\mathrm{c}}$ becomes the highest value, indicating that the strength of bosonic fluctuation determines $T_{\mathrm{c}}$. In the last superconducting phase, the nesting condition of FSs and the related bosonic fluctuation are moderate. Variety of phase diagram characterizes the multiple orbital nature of the iron-based superconductors which are just near the boundary between weak and strong correlation regimes.

In this paper, we use an elastic model in order to study the elastically driven cooperative switching of spin-crossover materials after femtosecond laser excitation. In this model, the molecules occupy a triangular lattice in open-boundary systems, and they are connected by springs. The volume change of a molecule between its two possible spin states, low-spin and high-spin, determines a variation of the spring length and therefore modifies elastic interactions between molecules, which propagates throughout the whole sample as elastic distortions. This model is able to reproduce the multistep out-of-equilibrium response to ultrashort laser excitation and especially the elastically driven cooperative response. It is developed in order to predict the behavior of the system as a function of its different physical parameters, such as the magnitude of the elastic constant or the homogeneity of the photoexcitation. The contribution of the reorganization of the molecular states during elastic steps, leading to clusters of high-spin molecules toward the edge or the corners, is also revealed.

We found anomalous relaxation behaviors in a two-dimensional magnet ${\mathrm{La}}_5{\mathrm{Mo}}_4{\mathrm{O}}_{16}$, in a pure crystalline form in both the resistivity and magnetization, whose characteristics are similar to those of disordered materials. We also found that such anomalous relaxation behaviors disappear in the impurity-doped sample. We propose a possible scenario for the relaxation behavior that the charge rearrangement of ${\mathrm{Mo}}^{4+}$ and ${\mathrm{Mo}}^{5+}$ causes a change in the interlayer magnetic interaction in this compound.

In this experiment, we couple a superconducting transmon qubit to a high-impedance $645\mathrm{\Omega }$ microwave resonator. Doing so leads to a large qubit-resonator coupling rate $g$, measured through a large vacuum Rabi splitting of $2g\simeq 910$ MHz. The coupling is a significant fraction of the qubit and resonator oscillation frequencies $\omega$, placing our system close to the ultrastrong coupling regime ($\overline{g}=g/\omega =0.071$ on resonance). Combining this setup with a vacuum-gap transmon architecture shows the potential of reaching deep into the ultrastrong coupling $\overline{g}\sim 0.45$ with transmon qubits.