We analyze the statistical characteristics of quasinonequilibrium two-dimensional electron-hole plasmas in graphene layers (GLs) and graphene bilayers (GBLs) and evaluate their heat capacity. The GL heat capacity of the weakly pumped intrinsic or weakly doped GLs normalized by the Boltzmann constant is equal to $c_{\text{GL}}\simeq 6.58$. With varying carrier temperature the intrinsic GBL carrier heat capacity $c_{\text{GBL}}$ changes from $c_{\text{GBL}}\simeq 2.37$ at $T\lesssim 300$ K to $c_{\text{GBL}}\simeq 6.58$ at elevated temperatures. These values are markedly different from the heat capacity of classical two-dimensional carriers with $c=1$. The obtained results can be useful for the optimization of different GL- and GBL-based high-speed devices.

Few-layer selenium and tellurium films have been recently prepared, and they provide a new platform to explore novel properties of two-dimensional (2D) elemental materials. In this work, we have performed a systematic first-principles study on the electronic, linear, and nonlinear optical (NLO) properties of atomically thin selenium and tellurium films within the density-functional theory with the generalized gradient approximation plus scissors correction using the band gaps from the relativistic hybrid Heyd-Scuseria-Ernzerhof functional calculations. The underlying atomic structures of these materials are determined theoretically using the strongly constrained and appropriately normed (SCAN) exchange-correlation functional. Interestingly, we find that few-layer Se and Te possess large second-harmonic generation (SHG), linear electro-optic effect, and bulk photovoltaic effect. In particular, trilayer (TL) Te possesses large SHG coefficient, being more than 65 times larger than that of GaN, a widely used NLO material. Bilayer (BL) Te has huge static SHG coefficient ${\chi }_{xyy}^{\left(2\right)}\left(0\right)$, being more than 100 times larger than that of GaN. Furthermore, monolayer (ML) Se possesses large SHG coefficient with ${\chi }_{xyy}^{\left(2\right)}$ being six times larger than that of GaN. Both ML Se and BL Te possess large linear electro-optic coefficients $r_{xyy}\left(0\right)$ and $r_{yzx}\left(0\right)$, which is about six times and five times larger than that of bulk GaN polytypes, respectively. Moreover, we predict that TL Te exhibits strong bulk photovoltaic effect (BPVE) with shift current conductivity of $\sim 440\mu \mathrm{A}$/${\mathrm{V}}^2$, being greater than that of GeS, a polar system with the largest BPVE found so far. Although the shift current conductivities of bulk and 2D Se are comparable, the shift current conductivities of TL Te are five times larger than those of bulk Te. Finally, an analysis of the calculated electronic band structures indicates that the strong NLO responses of 2D Se and Te materials are primarily derived from their low-dimensional structures with high anisotropy, directional covalent bonding, lone-pair electrons, and relatively small band gaps. These findings provide a practical strategy to search for excellent NLO and BPVE materials.

Magnetic field induced edge currents propagate in opposite directions at opposite edges of a Hall bar. We consider a system that possesses just one edge: a Möbius stripe. We show that if the carrier mean free path is less than the width of the stripe, magnetic field induced edge currents vanish. Instead, a stationary charge density wave is formed. Its profile is governed by the interplay between the magnetic field dependence of the chemical potential and the screened Coulomb repulsion of charge carriers. This effect offers an experimental tool for studies of the electron density of states in two-dimensional crystal films.

We investigate the spin-glass transition in the strongly frustrated well-known compound ${\mathrm{Fe}}_2{\mathrm{TiO}}_5$. A remarkable feature of this transition, widely discussed in the literature, is its anisotropic properties: The transition manifests itself in the magnetic susceptibly only along one axis, despite ${\mathrm{Fe}}^{3+} d^5$ spins having no orbital component. We demonstrate, using neutron scattering, that below the transition temperature $T_g=55$ K, ${\mathrm{Fe}}_2{\mathrm{TiO}}_5$ develops nanoscale surfboard-shaped antiferromagnetic regions in which the ${\mathrm{Fe}}^{3+}$ spins are aligned perpendicular to the axis which exhibits freezing. We show that the glass transition may result from the freezing of transverse fluctuations of the magnetization of these regions and we develop a mean-field replica theory of such a transition, revealing a type of magnetic van der Waals effect.

A method with which to calculate the Gilbert damping parameter from a real-space electronic structure method is reported here. The anisotropy of the Gilbert damping with respect to the magnetic moment direction and local chemical environment is calculated for bulk and surfaces of ${\mathrm{Fe}}_{50}{\mathrm{Co}}_{50}$ alloys from first-principles electronic structure in a real-space formulation. The size of the damping anisotropy for ${\mathrm{Fe}}_{50}{\mathrm{Co}}_{50}$ alloys is demonstrated to be significant. Depending on details of the simulations, it reaches a maximum-minimum damping ratio as high as 200%. Several microscopic origins of the strongly enhanced Gilbert damping anisotropy have been examined, where in particular interface/surface effects stand out, as do local distortions of the crystal structure. Although theory does not reproduce the experimentally reported high ratio of 400% [Phys. Rev. Lett. 122, 117203 (2019)], it nevertheless identifies microscopic mechanisms that can lead to huge damping anisotropies.

We present an extended microcanonical Lanczos method for a direct evaluation of the diffusion constant and its frequency dependence within the disordered Anderson model of noninteracting particles. The method allows to study systems beyond ${10}^6$ sites of hypercubic lattices in $d=3–7$ dimensions. Below the transition to localization where we confirm dynamical scaling behavior of interest is a wide region of incoherent diffusion, similar to percolating phenomena and to interacting many-body localized systems.

We reveal a transition within the strong coupling regime and below the crossover to ultrastrong coupling, which alters the coupling mechanism in multimode cavities. We show that this transition drastically modifies the Hamiltonian describing the polaritons, such that different cavity modes are either entangled via the material or completely decoupled. This decoupling transition occurs due to the competition between the dissipation in the material and the finite group velocity, which governs the propagation of information across the cavity and among the molecules. The results indicate that the velocity of light, which is often not taken into account in cavity quantum electrodynamics, plays a crucial role in the formation of cavity polaritons and their dynamics.

We theoretically calculated ultrafast propagations of spin waves channeled in antiferromagnetic Bloch-type domain walls based on a phenomenological theory developed by Hals et al. [Phys. Rev. Lett. 106, 107206 (2011)]. Our analytical derivation demonstrates that the dispersion relation of such channeled spin waves exhibits an extremely high group velocity, up to ∼18.7 km/s, without any forbidden gap. Further, such electromagnetic-wave-like dispersion relation allows for dispersionless and translational transmission of trains of low-frequency, long-wavelength digital signals in the channel. Our results offer guidelines for the development of ultrafast information signal processing in nanoscale magnonic circuits composed of antiferromagnetic domain walls.

We study the effects of the flux configurations on the emergent Majorana fermions in the $S=\frac{1}{2}$ Kitaev model on a honeycomb lattice, where quantum spins are fractionalized into itinerant Majorana fermions and localized fluxes. A quantum spin liquid appears as the ground state of the Kitaev model in the flux-free sector, which has intensively been investigated so far. In this flux sector, the Majorana fermion system has linear dispersions and shows power-law behavior in the Majorana correlations. On the other hand, periodically arranged flux configurations yield low-energy excitations in the Majorana fermion system, which are distinctly different from those in the flux-free state. We find that one of the periodically arranged flux states results in the gapped Majorana dispersion and the exponential decay in the Majorana correlations. The Kitaev system with another flux configuration exhibits a semi-Dirac-type dispersion, leading to the power-law decay with a smaller power than that in the flux-free sector along symmetry axes. We also examine the effect of the randomness in the flux configurations and clarify that the Majorana density of states is filled by increasing the flux density, and power-law decay in the Majorana correlations remains. The present results could be important to control the motion of Majorana fermions, which carries the spin excitations, in the Kitaev candidate materials.

Recent significant progress in inelastic neutron scattering (INS) has rendered this technique even more useful for the characterization of magnetic systems, including molecular spin clusters. By so-called four-dimensional INS on single-crystal probes, excitation spectra can be recorded in large portions of momentum-transfer (Q) and energy-transfer (E) space. Spin-selection rules permit $\mathrm{\Delta }S=0,\pm 1$ transitions between different spin multiplets. Additional selection rules can be imposed by point-group symmetry but were not discussed yet. As most synthetic spin clusters with interesting magnetic properties have high molecular symmetry, a clear understanding of this issue will be helpful for interpreting INS spectra. Here we discuss point-group INS selection rules for magnetically isotropic or anisotropic spin clusters. Rings and a number of spin polyhedra with cubic or icosahedral symmetry are chosen as illustrative and relevant examples. These systems exhibit a significant number of point-group selection rules in isotropic spin models, and most of them maintain a smaller number of selection rules in anisotropic spin models. We also explain how the Q dependence of certain excitations depends exclusively on the point-group symmetry of the states involved in the transition, an aspect that had thus far only been detailed for spin rings. We provide the universal Q-dependent intensity functions (and their powder-averaged forms) for a set of polyhedra (cube, icosahedron, truncated tetrahedron, cuboctahedron, dodecahedron, icosidodecahedron, and truncated icosahedron). Overall, these results help to disentangle the relevant dynamical information contained in INS spectra from those features that are entirely determined by molecular symmetry.

We study hybrid Josephson junctions between a multiband ${\mathrm{Ba}}_{1-x}{\mathrm{Na}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ iron-pnictide and Nb. We observe that the insertion of a Cu interlayer in such junctions leads to a dramatic enhancement of the $I_c{R}_n$ product, despite the weaker proximity-induced superconductivity of Cu. This counterintuitive phenomenon is attributed to the differences in Fermi surface geometries of Nb and Cu, which affect the selectivity of tunneling in sign-reversal $s_{\pm }$ bands of pnictide. Our results indicate that the sensitivity to Fermi surface geometries provides a new tool for phase-sensitive studies and paves the way to conscious Fermi surface engineering of pnictide junctions.

We present fresh evidence for the presence of discrete quantum time crystals in two spatial dimensions. Discrete time crystals are intricate quantum systems that break discrete time translation symmetry in driven quantum many-body systems undergoing nonequilibrium dynamics. They are stabilized by many-body localization arising from disorder. We directly target the thermodynamic limit using instances of infinite tensor network states, and we implement disorder in a translationally invariant setting by introducing auxiliary systems at each site. We discuss how such disorder can be realized in programmable quantum simulators: This gives rise to the interesting situation in which a classical tensor network simulation can contribute to devising a blueprint of a quantum simulator featuring prethermal time crystalline dynamics, one that will ultimately have to be built in order to explore the stability of this phase of matter for long times.

We consider the mapping of tight-binding electronic structure theory to a local spin Hamiltonian, based on the adiabatic approximation for spin degrees of freedom in itinerant-electron systems. Local spin Hamiltonians are introduced in order to describe the energy landscape of small magnetic fluctuations, locally around a given spin configuration. They are designed for the linear response near a given magnetic state and, in general, are insufficient to capture arbitrarily strong deviations of spin configurations from the equilibrium. In order to achieve this mapping, we include a linear term in the local spin Hamiltonian that together with the usual bilinear exchange tensor, produces an improved accuracy of effective magnetic Weiss fields for noncollinear states. We also provide examples from tight-binding electronic structure theory, where our implementation of the calculation of exchange constants is based on constraining fields that stabilize an out-of-equilibrium spin configuration. We check our formalism by means of numerical calculations for iron dimers and chains.

The microscopic mechanism for electron pairing in heavy-fermion superconductors remains a major challenge in quantum materials. Some form of magnetic mediation is widely accepted with spin fluctuations as a prime candidate. A novel mechanism, “composite pairing” based on the cooperative two-channel Kondo effect directly involving the $f$-electron moments, has also been proposed for some heavy-fermion compounds including ${\mathrm{CeCoIn}}_5$. The origin of the spin-resonance peak observed in neutron-scattering measurements on ${\mathrm{CeCoIn}}_5$ is still controversial and the corresponding hump-dip structure in the tunneling conductance is missing. This is in contrast to the cuprate and Fe-based high-temperature superconductors, where both characteristic signatures are observed, indicating spin fluctuations are likely involved in the pairing process. Here, we report results from planar tunneling spectroscopy along three major crystallographic orientations of ${\mathrm{CeCoIn}}_5$ over wide ranges of temperature and magnetic field. The pairing gap opens at $T_{\mathrm{p}}\sim 5\mathrm{K}$, well above the bulk $T_{\mathrm{c}}=2.3\mathrm{K}$, and its directional dependence is consistent with $d_{x2-y2}$ symmetry. With increasing magnetic field, this pairing gap is suppressed as expected but, intriguingly, a gaplike structure emerges smoothly, increasing linearly up to the highest field applied. This field-induced gaplike feature is only observed below $T_{\mathrm{p}}$. The concomitant appearance of the pairing gap and the field-induced gaplike feature, along with its linear increase with field, indicates that the $f$-electron local moments are directly involved in the pairing process in ${\mathrm{CeCoIn}}_5$.

Using the standard Bardeen-Cooper-Schrieffer (BCS) theory, we revise microscopic derivation of the superconductor-insulator boundary conditions for the Ginzburg-Landau (GL) model. We obtain a negative contribution to free energy in the form of surface integral. Boundary conditions for the conventional superconductor have the form $\mathbf{n}·\nabla \psi =\text{const}\psi$. These are shown to follow from considering the order parameter reflected in the boundary. The boundary conditions are also derived for more general GL models with higher-order derivatives and pair-density-wave states. It shows that the boundary states with higher critical temperature and the boundary gap enhancement, found recently in BCS theory, are also present in microscopically derived GL theory. In the case of an applied external field, we show that the third critical magnetic-field value $H_{c3}$ is higher than what follows from the de Gennes boundary conditions and is also significant in type-I regime.

When the quantum critical transverse-field Ising chain is perturbed by a longitudinal field, a quantum integrable model emerges in the scaling limit with massive excitations described by the exceptional $E_8$ Lie algebra. Using the corresponding analytical form factors of the quantum $E_8$ integrable model, we systematically study the spin dynamic structure factor of the perturbed quantum critical Ising chain, where particle channels with total energy up to $5m_1$ ($m_1$ being the mass of the lightest $E_8$ particle) are exhausted. In addition to the significant single-particle contributions to the dynamic spectrum, each two-particle channel with different masses is found to exhibit an edge singularity at the threshold of the total mass and decays with an inverse square root of energy, which is attributed to the singularity of the two-particle density of states at the threshold. The singularity is absent for particles with equal masses due to a cancellation mechanism involving the structure of the form factors. As a consequence, the dynamic structure factor displays a cascade of bumping peaks in the continuum region with clear singular features which can serve as a solid guidance for the material realization of the quantum $E_8$ model.

The Fibonacci topological order is the simplest platform for a universal topological quantum computer. While the $\nu =12/5$ fractional quantum Hall (QH) state has been proposed to support a Fibonacci sector, a dynamical picture of how a pure Fibonacci state may emerge in a QH system has been lacking. We use non-Abelian dualities to construct a Fibonacci state of bosons at filling $\nu =2$ starting from a trilayer of integer QH states. Our parent theory consists of bosonic composite vortices coupled to fluctuating $U\left(2\right)$ gauge fields, which is dual to the theory of Laughlin quasiparticles. The Fibonacci state is obtained by interlayer clustering of the composite vortices, along with flux attachment. We use this framework to motivate a wave function for the Fibonacci state.

We perform a comprehensive analysis of both the chemical and correlated electronic structure reconstruction of rhombohedral $\mathrm{Cr}{X}_3$ $(X=\text{Br}, \mathrm{Cl}, \mathrm{I})$ van der Waals bulk crystals. Using the generalized gradient approximation (GGA) plus dynamical mean-field theory we explicitly demonstrate the importance of local dynamical correlations for a consistent understanding of emergent Kondo quasiparticles and Mott localized electronic states, showing the interplay between material-dependent one-electron GGA line-shape and multiorbital electronic interactions. To probe the correlated paramagnetic electronic state we performed x-ray absorption spectroscopy measurements for ${\mathrm{CrCl}}_3$ and ${\mathrm{CrBr}}_3$ bulk crystals. Our correlated many-body study is relevant to understanding the electronic structure reconstruction of paramagnetic Cr-trihalide crystals and should be widely applicable to other van der Waals magnetic materials.