We consider three-dimensional fermionic band theories that exhibit Weyl nodal surfaces defined as two-band degeneracies that form closed surfaces in the Brillouin zone. We demonstrate that topology ensures robustness of these objects under small perturbations of a Hamiltonian. This topological robustness is illustrated in several four-band models that exhibit nodal surfaces protected by unitary or antiunitary symmetries. Surface states and Nielsen-Ninomiya doubling of nodal surfaces are also investigated.

In this letter we report a thorough analysis of the exciton dispersion in bulk hexagonal boron nitride. We solve the ab initio GW Bethe-Salpeter equation at finite $\mathbf{q}\parallel \mathrm{\Gamma }K$, which is relevant for spectroscopic measurements. Simulations reproduce the dispersion and the intensity of recent high-accuracy electron energy loss data. We demonstrate that the excitonic peak comes from the interference of two groups of transitions involving the points $K$ and $K^{\prime }$ of the Brillouin zone. The number and the amplitude of these transitions determine variations in the peak intensity. Our results contribute to the understanding of electronic excitations in this system, unveiling a nontrivial relation between valley physics and excitonic properties. Furthermore, the methodology introduced in this study to regroup independent-particle transitions is completely general and can be applied successfully to the investigation of excitonic properties in any system.

We study theoretically the influence of Berry phase on the real-time dynamics of the single particle focusing on the diffusive dynamics, i.e., the time dependence of the distribution function. Our model can be applied to the real-time dynamics of intraband relaxation and diffusion of optically excited excitons, trions, or particle-hole pair. We found that the dynamics at the early stage is deeply influenced by the Berry curvature in real space ($B$), momentum space ($\mathrm{\Omega }$), and also the crossed space between these two ($C$). For example, it is found that $\mathrm{\Omega }$ induces the rotation of the wave packet and causes the time dependence of the mean square displacement of the particle to be linear in time $t$ at the initial stage; it is qualitatively different from the $t^3$ dependence in the absence of the Berry curvature. It is also found that $\mathrm{\Omega }$ and $C$ modify the characteristic time scale of the thermal equilibration of momentum distribution. Moreover, the dynamics under various combinations of $B,\mathrm{\Omega }$, and $C$ shows singular behaviors such as the critical slowing down or speeding up of the momentum equilibration and the reversals of the direction of rotations. The relevance of our model for time-resolved experiments in transition metal dichalcogenides is also discussed.

We present a theoretical study of electronic and optical properties of the layered $\mathrm{Re}{X}_2$ compounds ($X$ = S, Se) upon dimensional reduction. The effect on the band-gap character due to interlayer coupling is studied by means of the self-energy corrected $GW$ method for optimized and experimental sets of a structure's data. Induced changes on the optical properties as well as optical anisotropy are studied through optical spectra as obtained by solving the Bethe-Salpeter equation. At the $G_0{W}_0$ level of theory, when decreasing the thickness of the ${\mathrm{ReS}}_2$ sample from bulk to bilayer and to a freestanding monolayer, the band gap remains direct, despite a change of the band-gap nature, with values increasing from 1.6, 2.0, and 2.4 eV, respectively. For ${\mathrm{ReSe}}_2$, the fundamental band gap changes from direct for the bulk phase (1.38 eV) to indirect in the bilayer (1.73 eV) and becomes direct again for a single layer (2.05 eV). We discuss these results in terms of the renormalization of the band structure. We produce the polarization angular-dependent optical response to explore the optical anisotropy present in our results, as well as the fine structure of the lowest excitonic peaks present in the absorption spectra.

We present a treatment of charge dynamics in amorphous molecular semiconductors that accounts for the coupling of charges to all intramolecular phonon modes in a fully quantum mechanical way. Based on ab initio calculations, we derive charge transfer rates that improve on the widely used semiclassical Marcus rate and obtain benchmark results for the mobility and energetic relaxation of electrons and holes in three semiconductors commonly applied in organic light-emitting diodes. Surprisingly, we find very similar results when using the simple Miller-Abrahams rate. We conclude that extracting the disorder strength from temperature-dependent charge transport studies is very possible but extracting the reorganization energy is not.

The transition to a hidden metastable state in $1T-\mathrm{Ta}{\mathrm{S}}_2$ is investigated in real time using coherent time-resolved femtosecond spectroscopy. Relying on spectral differences between phonon modes in the equilibrium states and in the metastable state, and temperature-tuning the metastable state lifetime, we perform stroboscopic measurements of the electronic response and switching of coherent oscillation frequency through the transition. Very fast coherent switching of the collective-mode frequency is observed ($\sim 400\mathrm{fs}$), comparable to the electronic time scale $\sim 300\mathrm{fs}$. A slower, 4.7-ps process is attributed to lattice relaxation. The observations are described well by a fast electronic band structure transformation into the metastable state, consistent with a topological transition.

We present a theoretical scheme for multistability in planar microcavity exciton-polariton condensates under nonresonant driving. Using an excitation profile resulting in a spatially patterned condensate, we observe organized phase locking which can abruptly reorganize as a result of pump induced instability made possible by nonlinear interactions. For $\pi /2$ symmetric systems this reorganization can be regarded as a parity transition and is found to be a fingerprint of multistable regimes existing over a finite range of excitation strengths. The natural degeneracy of the planar equations of motion gives rise to parity bifurcation points where the condensate, as a function of excitation intensity, bifurcates into one of two anisotropic degenerate solutions. Deterministic transitions between multistable states are made possible using controlled nonresonant pulses, perturbing the solution from one attractor to another.

In this work, we present a mechanism that, based on quantum-mechanical principles, allows one to recover kinetic energy at the nanoscale. Our premise is that very small mechanical excitations, such as those arising from sound waves propagating through a nanoscale system or similar phenomena, can be quite generally converted into useful electrical work by applying the same principles behind conventional adiabatic quantum pumping. The proposal is potentially useful for nanoscale vibrational energy harvesting where it can have several advantages. The most important one is that it avoids the use of classical rectification mechanisms as it is based on what we call geometric rectification. We show that this geometric rectification results from applying appropriate but quite general initial conditions to damped harmonic systems coupled to electronic reservoirs. We analyze an analytically solvable example consisting of a wire suspended over permanent charges where we find the condition for maximizing the pumped charge. We also studied the effects of coupling the system to a capacitor including the effect of current-induced forces and analyzing the steady-state voltage of operation. Finally, we show how quantum effects can be used to boost the performance of the proposed device.

The selection rule on vibronic angular momentum of a $t_{1u}^n\otimes h_g$ Jahn-Teller problem ($n=1–5)$ is reinvestigated. It is shown that among three adiabatic orbitals only two have nonzero Berry phase. Thus, the Berry phase of adiabatic electronic configurations depends on the spin multiplicity as well as the number of electrons. On this basis, the general relation between the Berry phase and the angular momentum is described. It allows us, in particular, to clarify the nature of vibronic states arising from high spin configurations. In comparison with the previous solution for the low-lying vibronic states for bimodal systems, the present solutions correctly fulfill all the symmetry requirements.

We predict that the interplay between the spin-orbit coupling, stemming from the transverse electric–transverse magnetic energy splitting, and the Zeeman effect in semiconductor microcavities supporting exciton-polariton quasiparticles, results in the appearance of unidirectional linear topological edge states when the top microcavity mirror is patterned to form a truncated dislocated Lieb lattice of cylindrical pillars. Periodic nonlinear edge states are found to emerge from the linear ones. They are strongly localized across the interface and they are remarkably robust in comparison to their counterparts in honeycomb lattices. Such robustness makes possible the existence of nested unidirectional dark solitons that move steadily along the lattice edge.

We study the linear thermoelectric response of a quantum dot embedded in a constriction of a quantum Hall bar with fractional filling factors $\nu =1/m$ within Laughlin series. We calculate the figure of merit $ZT$ for the maximum efficiency at a fixed temperature difference. We find a significant enhancement of this quantity in the fractional filling in relation to the integer-filling case, which is a direct consequence of the fractionalization of the electron in the fractional quantum Hall state. We present simple theoretical expressions for the Onsager coefficients at low temperatures, which explicitly show that $ZT$ and the Seebeck coefficient increase with $m$.

Topological insulator films are promising materials for optoelectronics due to a strong optical absorption and a thickness-dependent band gap of the topological surface states. They are superior candidates for photodetector applications in the THz-infrared spectrum, with a potential performance higher than graphene. Using a first-principles $k·p$ Hamiltonian, incorporating all symmetry-allowed terms to second order in the wave vector $k$, first order in the strain $\epsilon$, and of order $\epsilon k$, we demonstrate a significantly improved optoelectronic performance due to strain. For ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ films of variable thickness, the surface-state band gap, and thereby the optical absorption, can be effectively tuned by the application of uniaxial strain ${\epsilon }_{zz}$, leading to a divergent band-edge absorbance for ${\epsilon }_{zz}\gtrsim 6\%$. Shear strain breaks the crystal symmetry and leads to an absorbance varying significantly with polarization direction. Remarkably, the directional average of the absorbance always increases with strain, independent of material parameters.

The van der Waals (vdW) density functional (vdW-DF) method [Rep. Prog. Phys. 78, 066501 (2015)] describes dispersion or vdW binding by tracking the effects of an electrodynamic coupling among pairs of electrons and their associated exchange-correlation holes. This is done in a nonlocal-correlation energy term $E_{\mathrm{c}}^{\mathrm{nl}}$, which permits density functional theory calculation in the Kohn-Sham scheme. However, to map the nature of vdW forces in a fully interacting materials system, it is necessary to also account for associated kinetic-correlation energy effects. Here, we present a coupling-constant scaling analysis, which permits us to compute the kinetic-correlation energy $T_{\mathrm{c}}^{\mathrm{nl}}$ that is specific to the vdW-DF account of nonlocal correlations. We thus provide a more complete spatially resolved analysis of the electrodynamical-coupling nature of nonlocal-correlation binding, including vdW attraction, in both covalently and noncovalently bonded systems. We find that kinetic-correlation energy effects play a significant role in the account of vdW or dispersion interactions among molecules. Furthermore, our mapping shows that the total nonlocal-correlation binding is concentrated to pockets in the sparse electron distribution located between the material fragments.

Fractons are emergent particles which are immobile in isolation, but which can move together in dipolar pairs or other small clusters. These exotic excitations naturally occur in certain quantum phases of matter described by tensor gauge theories. Previous research has focused on the properties of small numbers of fractons and their interactions, effectively mapping out the “standard model” of fractons. In the present work, however, we consider systems with a finite density of either fractons or their dipolar bound states, with a focus on the $U\left(1\right)$ fracton models. We study some of the phases in which emergent fractonic matter can exist, thereby initiating the study of the “condensed matter” of fractons. We begin by considering a system with a finite density of fractons, which we show can exhibit microemulsion physics, in which fractons form small-scale clusters emulsed in a phase dominated by long-range repulsion. We then move on to study systems with a finite density of mobile dipoles, which have phases analogous to many conventional condensed matter phases. We focus on two major examples: Fermi liquids and quantum Hall phases. A finite density of fermionic dipoles will form a Fermi surface and enter a Fermi liquid phase. Interestingly, this dipolar Fermi liquid exhibits a finite-temperature phase transition, corresponding to an unbinding transition of fractons. Finally, we study chiral two-dimensional phases corresponding to dipoles in “quantum Hall” states of their emergent magnetic field. We study numerous aspects of these generalized quantum Hall systems, such as their edge theories and ground state degeneracies.

We introduce and compare three different Monte Carlo determinantal algorithms that allow one to compute dynamical quantities, such as the self-energy, of fermionic systems in their thermodynamic limit. We show that the most efficient approach expresses the sum of a factorial number of one-particle-irreducible diagrams as a recursive sum of determinants with exponential complexity. By comparing results for the two-dimensional Hubbard model with those obtained from state-of-the-art diagrammatic Monte Carlo, we show that we can reach higher perturbation orders and greater accuracy for the same computational effort.

Fractionalized Fermi liquids (${\mathrm{FL}}^{*}$) have been introduced as non-Fermi-liquid metallic phases, characterized by coexisting electron-like charge carriers and local moments which form a fractionalized spin liquid. Here we investigate a Kondo lattice model on the honeycomb lattice with Kitaev interactions among the local moments, a concrete model hosting ${\mathrm{FL}}^{*}$ phases based on Kitaev's ${\mathbb{Z}}_2$ spin liquid. We characterize the ${\mathrm{FL}}^{*}$ phases via perturbation theory, and we employ a Majorana-fermion mean-field theory to map out the full phase diagram. Most remarkably we find nematic triplet superconducting phases which mask the quantum phase transition between fractionalized and conventional Fermi liquid phases. Their pairing structure is inherited from the Kitaev spin liquid; i.e., superconductivity is driven by Majorana glue.

Rational tuning the anisotropic electronic properties of monolayer phosphorene is essential to their applications in electronic and optoelectronic devices. By combining the density functional theory and the nonequilibrium Green's function method, we developed a strategy to tune the anisotropic transport properties of phosphorene by designing stable arsenic-phosphorene ($\mathrm{A}{\mathrm{s}}_x{\mathrm{P}}_{1-x}$) superlattice (SL). It was found that, with a careful design of As:P ratio and atomic arrangement, the anisotropic transport properties could be tuned in a wide range. The transport current along the zigzag direction, which is very low in pristine phosphorene, was gradually enhanced by increasing the As:P ratio, and even became larger than that along armchair direction when the As:P ratio achieved 1:1 under a given arrangement of As atoms in $\mathrm{A}{\mathrm{s}}_x{\mathrm{P}}_{1-x}$ SL. The tunable anisotropic transport properties of $\mathrm{A}{\mathrm{s}}_x{\mathrm{P}}_{1-x}$ SL are attributed to the interplay between the different scattering rates related to the number and orientation of As-P interfaces. This finding demonstrates that the $\mathrm{A}{\mathrm{s}}_x{\mathrm{P}}_{1-x}$ SL design could be an effective approach to tune the anisotropic electronic properties of monolayer phosphorene, which is important for the development of high-performance electronic and optoelectronic devices based on phosphorene.

We report tip-enhanced Raman spectroscopy and tip-enhanced photoluminescence studies of monolayer and bilayer $\mathrm{Mo}{\mathrm{S}}_2$ in which we characterize photoluminescence and first- and second-order Raman spectra in monolayer, bilayer, and inhomogeneously strained $\mathrm{Mo}{\mathrm{S}}_2$. From the transition of unstrained $\mathrm{Mo}{\mathrm{S}}_2$ from monolayer to bilayer, we determine a spatial resolution of approximately 100 nm through the peak positions of the first-order Raman modes. The strain dependence of the second-order Raman modes reveals changes in the electronic band structure in strained $\mathrm{Mo}{\mathrm{S}}_2$ that are directly observed through changes in the Raman peak positions and peak area ratios, which are corroborated through density functional theory calculations.

We consider an antiferromagnetic insulator that is in contact with a metal. Spin accumulation in the metal can induce spin-transfer torques on the staggered field and on the magnetization in the antiferromagnet. These torques relate to spin pumping: the emission of spin currents into the metal by a precessing antiferromagnet. We investigate how the various components of the spin-transfer torque are affected by spin-independent disorder and spin-flip scattering in the metal. Spin-conserving disorder reduces the coupling between the spins in the antiferromagnet and the itinerant spins in the metal in a manner similar to Ohm's law. Spin-flip scattering leads to spin-memory loss with a reduced spin-transfer torque. We discuss the concept of a staggered spin current and argue that it is not a conserved quantity. Away from the interface, the staggered spin current varies around a 0 mean in an irregular manner. A network model explains the rapid decay of the staggered spin current.