The singular density of states and the two Fermi wave vectors resulting from a ring-shaped or “Mexican hat” valence band give rise to unique trends in the charged impurity scattering rates and charged impurity limited mobilities. Ring-shaped valence bands are common features of many monolayer and few-layer two-dimensional (2D) materials including the III-VI materials GaS, GaSe, InS, and InSe. The wave-vector dependence of the screening, calculated within the random phase approximation, is so strong that it is the dominant factor determining the overall trends of the scattering rates and mobilities with respect to temperature and hole density. Charged impurities placed on the substrate and in the 2D channel are considered. The different wave-vector dependencies of the bare Coulomb potentials alter both the magnitudes and the temperature dependencies of the mobilities. Moving the charged impurities 5 Å from the center of the channel to the substrate increases the mobility by an order of magnitude by suppressing the large wave-vector backscattering across the outer Fermi ring.

Two-dimensional layered materials have attracted tremendous attention due to their extraordinary physical and chemical properties. Using first-principles calculations and Boltzmann transport theory, we give an accurate prediction of the thermoelectric properties of hexagonal boron phosphide (BP) bilayer, where the carrier relaxation time is treated within the framework of electron-phonon coupling. It was found that the lattice thermal conductivity of BP bilayer is much lower than that of its monolayer structure, which can be attributed to the presence of van der Waals interactions. On the other hand, the graphene-like BP bilayer shows very high carrier mobility with a moderate band gap of 0.88 eV. As a consequence, a maximum $p$-type $\mathit{ZT}$ value of ∼1.8 can be realized along the $x$ direction at 1200 K, which is amazingly high for systems consisting of light elements only. Moreover, we obtain almost identical $p$- and $n$-type $\mathit{ZT}$ of ∼1.6 along the $y$ direction, which is very desirable for fabrication of thermoelectric modules with comparative efficiencies. Collectively, these findings demonstrate the great advantages of the layered structures containing earth-abundant elements for environmentally friendly thermoelectric applications.

We introduce a $p_z\text{−}d$ coupling model Hamiltonian for the $\pi$-graphene/Au bands that predicts a rather large intrinsic spin-orbit (SO) coupling as is being reported in recent experiments and DFT studies. Working within the analytical Slater-Koster tight-binding approach we were able to identify the overlapping orbitals of relevance in the enhancement of the SO coupling for both the sublattice symmetric (BC) and the ATOP (AC) stacking configurations. Our model effective Hamiltonian reproduces quite well the experimental spectrum for the two registries, and in addition, it shows that the hollow site configuration (BC), in which the A/B sites remain symmetric, yields the larger increase of the SO coupling. We also explore the Au-diluted case keeping the BC configuration and show that it renders the preservation of the SO gap with a similar SO interaction enhancement as the undiluted case but with a smaller graphene-gold distance.

Monolayer transition metal dichalcogenides (TMDS) are highly luminescent materials despite being subnanometer thick. This is due to the ultrashort ($<1\mathrm{ps}$) radiative lifetime of the strongly bound bright excitons hosted by these materials. The intrinsically short radiative lifetime results in a large broadening in the exciton band with a magnitude that is about two orders greater than the spread of the light cone itself. The situation calls for a need to revisit the conventional light cone picture. We present a modified light cone concept which places the light line $\left(\hslash cQ\right)$ as the generalized lower bound for allowed radiative recombination. A self-consistent methodology, which becomes crucial upon inclusion of large radiative broadening in the exciton band, is proposed to segregate the radiative and the nonradiative components of the homogeneous exciton linewidth. We estimate a fundamental radiative linewidth of $1.54\pm 0.17\mathrm{meV}$, owing purely to finite radiative lifetime in the absence of nonradiative dephasing processes. As a direct consequence of the large radiative limit, we find a surprisingly large ($\sim 0.27\mathrm{meV}$) linewidth broadening due to zero-point energy of acoustic phonons. This obscures the precise experimental determination of the intrinsic radiative linewidth and sets a fundamental limit on the nonradiative linewidth broadening at $T=0\mathrm{K}$.

The relative rotation and translation of graphene and graphite layers lead to remarkable physical and mechanical phenomena. One such phenomenon for graphite corresponds to the ultralow static and dynamic friction between incommensurate graphene layers, referred to as superlubricity. Even though many studies have been dedicated to this promising phenomenon in recent years, an experimental characterization and a quantitative determination of the effect of relative twist angles on microscale superlubricity are still lacking. The present paper investigates the superlubric properties of microscale graphite under different twist angels by shearing graphite with respect to a substrate. Experimentally, it is surprisingly found that the superlubricity of microscale graphite is almost invariant within a wide range of bicrystal twist angles $\left(6^{\circ }\le \theta \le {59}^{\circ }\right)$. This result is confirmed by carrying out molecular dynamics simulations. Further, the influences of twist angles and normal load on the incommensurate-to-commensurate transition are revealed. The estimated critical transition angle is less than 0.1 °. These results allow a better understanding of mesoscopic scale superlubricity and extend its application field.

We probe the existence of a many-body localized phase (MBL phase) in a spinless fermionic Hubbard chain with algebraically localized single-particle states, by investigating both static and dynamical properties of the system. This MBL phase can be characterized by an extensive number of integrals of motion which develop algebraically decaying tails, unlike the case of exponentially localized single-particle states. We focus on the implications for the quantum information propagation through the system. We provide evidence that the bipartite entanglement entropy after a quantum quench has an unbounded algebraic growth in time, while the quantum Fisher information grows logarithmically.

We fabricated spin-valve devices with a Co/iron phthalocyanine (FePc)/Co stacking structure. The spin-transport properties were systemically studied by varying the measurement temperature and magnetoresistance was confirmed. The estimated value of the spin-diffusion length for the FePc layer is around 30 nm. The existence of defect states was demonstrated based on the analysis of current-voltage curves. Carrier mobilities in the devices were calculated following a trap-filled space charge limited current model, showing the mobility could be up to $2.95\times {10}^{-5}\mathrm{c}{\mathrm{m}}^2/\mathrm{Vs}$ at 290 K. The molecular orientation growth of FePc on Co and the interfacial interaction for FePc/Co are also discussed. This work deepens the understanding of spin transport in organic spin devices.

We develop a theory for nonlocal spin transport through magnetic insulators that treats the coherent magnetoelastic interaction on equal footing with incoherent relaxation processes. In particular, our theory is able to describe the formation of magnon polarons, hybridized spin and elastic waves, near an interface where spin is injected into the magnetic insulator. Our theory is based on the stochastic Landau-Lifshitz-Gilbert equation coupled to stochastic equations of motion for the lattice displacement. By solving these equations, we obtain the charge voltage generated in a detector on one side of the magnetic insulator in response to spin biasing with an injector on the other side. We find that though magnon-polaron formation causes anomalous features in the spin transport, a length scale exists, however, below which magnetoelastic coupling does not affect the nonlocal spin current. This finding may motivate experiments to explore this aspect of magnon-phonon coupling in magnetic materials.

We state and compare four different definitions of magnetic permeability for periodic, artificial media, or metamaterials. The connections among them, and properties in general, are discussed in detail, including causality, passivity, symmetry, asymptotic behavior, and origin dependence. The analysis is limited to metamaterials made from linear and nonmagnetic constituents.

We investigate the recently introduced geometric quench protocol for fractional quantum Hall (FQH) states within the framework of exactly solvable quantum Hall matrix models. In the geometric quench protocol, a FQH state is subjected to a sudden change in the ambient geometry, which introduces anisotropy into the system. We formulate this quench in the matrix models and then we solve exactly for the postquench dynamics of the system and the quantum fidelity (Loschmidt echo) of the postquench state. Next, we explain how to define a spin-2 collective variable ${\widehat{g}}_{ab}\left(t\right)$ in the matrix models, and we show that for a weak quench (small anisotropy), the dynamics of ${\widehat{g}}_{ab}\left(t\right)$ agrees with the dynamics of the intrinsic metric governed by the recently discussed bimetric theory of FQH states. We also find a modification of the bimetric theory such that the predictions of the modified bimetric theory agree with those of the matrix model for arbitrarily strong quenches. Finally, we introduce a class of higher-spin collective variables for the matrix model, which are related to generators of the $W_{\infty }$ algebra, and we show that the geometric quench induces nontrivial dynamics for these variables.

In addition to the well-known chiral anomaly, Dirac semimetals have been argued to exhibit a mirror anomaly, a close analog to the parity anomaly of ($2+1$)-dimensional massive Dirac fermions. The observable response of such anomaly is manifested in a singular steplike anomalous Hall response across the mirror-symmetric plane in the presence of a magnetic field. Although this result seems to be valid in type-II Dirac semimetals (strictly speaking, in the linearized Hamiltonian), we find that type-I Dirac semimetals do not possess such an anomaly in anomalous Hall response even at the level of the linearized Hamiltonian. In particular, we show that the anomalous Hall response continuously approaches zero as one approaches the mirror symmetric angle in a type-I Dirac semimetal as opposed to the singular Hall response in a type-II Dirac semimetal. Moreover, we show that, under certain conditions, the anomalous Hall response may vanish in a linearized type-I Dirac semimetal, even in the presence of time-reversal symmetry breaking.

We employ the multiorbital dynamical mean-field theory to examine the ground state of a three-orbital Hubbard model with a relativistic spin-orbit coupling (SOC) at four electrons per site. We demonstrate that the interplay between the strong electron correlations and the SOC induces a Van Vleck-type nonmagnetic insulator and its magnetic exciton condensation. In the moderate electron correlation regime, the SOC induces another type of a nonmagnetic excitonic insulator, in addition to a relativistic band insulator. Most interestingly, we find that this nonmagnetic excitonic insulator exhibits a high-order multipole ordering. The characteristic features among these insulators are manifested in the momentum-resolved single-particle excitations, thus accessible by angle-resolved photoemission spectroscopy experiments.

Optical pump-probe techniques are used to generate and measure electron spin polarization in a gallium arsenide epilayer in which the electron spin coherence time exceeds the mode-locked laser repetition period. Resonant spin amplification occurs at magnetic fields at which the electron spin polarization excited by successive laser pulses adds constructively. Measurements of Kerr rotation as a function of applied magnetic field and pump-probe time delay reveal nuclear spin polarization that aligns either with or against the external magnetic field depending on whether the applied magnetic field is being decreased or increased. Furthermore, the nuclear spin polarization magnitude varies in proportion to the perpendicular net electron spin polarization as the latter changes due to resonant spin amplification and other causes. We also report an experimental technique that reveals a minutes-long memory of precise field history in the electron-nuclear spin system.

A great deal of theoretical and experimental efforts have been devoted in the last decades to the study of long-wavelength photodetection mechanisms in field-effect transistors hosting two-dimensional (2D) electron systems. A particularly interesting subclass of these mechanisms is intrinsic and based on the conversion of the incoming electromagnetic radiation into plasmons, which resonantly enhance the photoresponse, and subsequent rectification via hydrodynamic nonlinearities. In this paper, we show that such a conversion and subsequent rectification occur well beyond the frequency regime in which hydrodynamic theory applies. We consider the nonlinear optical response of generic 2D electron systems and derive pseudo-Euler equations of motion for suitable collective variables. These are solved in one- and two-dimensional geometries for the case of graphene and the results are compared with those of hydrodynamic theory. Significant qualitative differences are found, which are amenable to experimental studies. Our theory expands the knowledge of the fundamental physics behind long-wavelength photodetection.

Models of interacting Majorana modes may be realized in vortex lattices in superconducting films in contact with topological insulators and may be tuned to the strong-interaction regime by adjusting the chemical potential. Extending the results on one- and two-dimensional Majorana-Hubbard models, here we determine the phase diagrams of two- and four-leg ladders using both field theory and the density-matrix renormalization group (DMRG) methods.

A significant increase in conductivity under an applied magnetic field or large negative magnetoresistance up to several percent is observed at low temperatures in epitaxial perovskite $\mathrm{NdNi}{\mathrm{O}}_3$ films. The (20–100 nm) strained films are grown on compressive $\left(001\right)\mathrm{LaAl}{\mathrm{O}}_3$ and tensile $\left(001\right)\left(\mathrm{L}{\mathrm{a}}_{0.3}\mathrm{S}{\mathrm{r}}_{0.7}\right)\left(\mathrm{A}{\mathrm{l}}_{0.65}\mathrm{T}{\mathrm{a}}_{0.35}\right){\mathrm{O}}_3$ substrates. The negative magnetoresistance is found in the insulating phase, where the hopping mechanism of conductivity is revealed. It is shown that the presence and Zeeman splitting of localized states are responsible for conductivity and magnetoresistance. The localized states are suggested to emerge due to thermally induced local disorder in the Ni-O bond disproportionation. Such disorder can also lead to a phase coexistence in a broad temperature range.