Heavily electron-doped and single-layer FeSe superconduct at much higher temperatures than bulk FeSe. There have been a number of proposals attempting to explain the origin of the enhanced transition temperature, including the proximity to magnetic, nematic, and antiferro-orbital critical points, as well as possible strong interfacial phonon coupling in the case of single-layer FeSe. In this paper, we examine the effect of the various mechanisms in an effective two-band model. Within our model, the fluctuations associated with these instabilities contribute to different parts of the effective multiband interactions. We propose to use the collective phase fluctuation between the bands–the Leggett mode–as a tool to identify the dominant effective pairing interaction in these systems. The Leggett mode can be resolved by means of optical probes such as electronic Raman scattering. We point out that the Leggett mode in these systems, if present, shall manifest in the Raman $B_{1g}$ channel.

We study the excitonic semimetal-insulator quantum phase transition in a three-dimensional Dirac semimetal in which the fermion dispersion is strongly anisotropic. After solving the Dyson-Schwinger equation for the excitonic gap, we obtain a global phase diagram in the plane spanned by the parameter for Coulomb interaction strength and the parameter for fermion velocity anisotropy. We find that excitonic gap generation is promoted as the interaction becomes stronger but is suppressed if the anisotropy increases. Applying our results to two realistic three-dimensional Dirac semimetals ${\mathrm{Na}}_3\mathrm{Bi}$ and ${\mathrm{Cd}}_3{\mathrm{As}}_2$, we establish that their exact zero-temperature ground state is gapless semimetal rather than excitonic insulator. Moreover, these two materials are far from the excitonic quantum critical point, and thus there should not be any observable evidence for excitonic insulating behavior. This conclusion is in general agreement with the existing experiments of ${\mathrm{Na}}_3\mathrm{Bi}$ and ${\mathrm{Cd}}_3{\mathrm{As}}_2$.

By performing first-principles calculations, we propose a stable direct band gap semiconductor Janus single-layer structure, ${\mathrm{In}}_2\mathrm{SSe}$. The binary analogs of the Janus structure, InS and InSe single layers are reviewed to evince the structural and electronic relation with ${\mathrm{In}}_2\mathrm{SSe}$. The structural optimization calculations reveal that a Janus ${\mathrm{In}}_2\mathrm{SSe}$ single layer has hexagonal geometry like the InS and InSe single layers, which are also its structural analogs. The Janus single layer is dynamically stable, as indicated by the phonon spectrum. The electronic band diagram of the Janus structure shows that an ${\mathrm{In}}_2\mathrm{SSe}$ single layer is a direct band gap semiconductor, in contrast to its analogs, InS and InSe single layers, which are indirect band gap semiconductors. Nevertheless, it is found that the strain effect on electronic properties of the InS and InSe single layers designates the electronic structure of the Janus single layer. A rough model for the construction of the electronic band diagram of the Janus structures is discussed, and it is indicated that the difference in work functions of chalcogenide sides in the Janus structure determines the construction of the electronic structure. It is found that the Janus structure is a robust direct gap semiconductor under tolerable strain; for that reason, the Janus ${\mathrm{In}}_2\mathrm{SSe}$ single layer is a candidate for optoelectronic nanodevice applications.

The state of electrons injected onto the surface of the Fermi sea depends on temperature. The state is pure at zero temperature and is mixed at finite temperature. In the case of a single-electron injection, such a transformation can be detected as a decrease in shot noise with increasing temperature. In the case of a multielectron injection, the situation is subtler. The mixedness helps the development of quantum-mechanical exchange correlations between injected electrons, even if such correlations are absent at zero temperature. These correlations enhance the shot noise, which in part counteracts the reduction of noise with temperature. Moreover, at sufficiently high temperatures, the correlation contribution to noise predominates over the contribution of individual particles. As a result, in the system of $N$ electrons, the apparent charge (which is revealed via the shot noise) is changed from $e$ at zero temperature to $Ne$ at high temperatures. It looks like the exchange correlations glue electrons into one particle of total charge and energy. This point of view is supported by both charge noise and heat noise. Interestingly, in the macroscopic limit, $N\to \infty$, the correlation contribution completely suppresses the effect of temperature on noise.

A one-parameter family of long-range resonating-valence-bond (RVB) state on the square lattice was previously proposed to describe a critical spin liquid (SL) phase of the spin-1/2 frustrated Heisenberg model. We provide evidence that this RVB state in fact also realizes a topological (long-range entangled) ${\mathbb{Z}}_2$ SL, limited by two transitions to critical SL phases. The topological phase is naturally connected to the ${\mathbb{Z}}_2$ gauge symmetry of the local tensor. This Rapid Communication shows that, on one hand, spin-1/2 topological SL with $C_{4v}$ point-group symmetry and $SU\left(2\right)$ spin rotation symmetry exists on the square lattice and, on the other hand, criticality and nonbipartiteness are compatible. We also point out that strong similarities between our phase diagram and the ones of classical interacting dimer models suggest both can be described by similar Kosterlitz-Thouless transitions. This scenario is further supported by the analysis of the one-dimensional boundary state. Forms of parent Hamiltonians hosting the ${\mathbb{Z}}_2$ SL are suggested.

Local spin fluctuations provide the glue for orbital-singlet spin-triplet pairing in the doped Mott insulating regime of multiorbital Hubbard models. At large Hubbard repulsion $U$, the pairing susceptibility is nevertheless tiny because the pairing interaction cannot overcome the suppression of charge fluctuations. Using nonequilibrium dynamical mean field simulations of the two-orbital Hubbard model, we show that out of equilibrium the pairing susceptibility in this large-$U$ regime can be strongly enhanced by creating a photoinduced population of the relevant charge states. This enhancement is supported by the long lifetime of photodoped charge carriers and a built-in cooling mechanism in multiorbital Hubbard systems.

The conventional Kibble-Zurek scaling describes the scaling behavior in the driven dynamics across a single critical region. In this paper, we study the driven dynamics across an overlapping critical region, in which a critical region (Region A) is overlaid by another critical region (Region B). We develop a hybridized Kibble-Zurek scaling (HKZS) to characterize the scaling behavior in the driven process. According to the HKZS, the driven dynamics in the overlapping region can be described by the critical theories for both Region A and Region B simultaneously. This results in a constraint on the scaling function in the overlapping critical region. We take the quantum Ising chain in an imaginary longitudinal field as an example. In this model, the critical region of the Yang-Lee edge singularity and the critical region of the ferromagnetic-paramagnetic phase transition overlap with each other. We numerically confirm the HKZS by simulating the driven dynamics in this overlapping critical region. The HKZSs in other models are also discussed.

Local NMR relaxation rates in the vortex state of chiral and helical $p$-wave superconductors are investigated by the quasiclassical Eilenberger theory. We calculate the spatial and resonance frequency dependences of the local NMR spin-lattice relaxation rate $T_1^{-1}$ and spin-spin relaxation rate $T_2^{-1}$. Depending on the relation between the NMR relaxation direction and the $d$-vector symmetry, the local $T_1^{-1}$ and $T_2^{-1}$ in the vortex core region show different behaviors. When the NMR relaxation direction is parallel to the $d$-vector component, the local NMR relaxation rate is anomalously suppressed by the negative coherence effect due to the spin dependence of the odd-frequency $s$-wave spin-triplet Cooper pairs. The difference between the local $T_1^{-1}$ and $T_2^{-1}$ in the site-selective NMR measurement is expected to be a method to examine the $d$-vector symmetry of candidate materials for spin-triplet superconductors.

We present an experimental study for polycrystalline samples of the diluted magnetic semiconductor $\mathrm{M}{\mathrm{n}}_x\mathrm{G}{\mathrm{a}}_{1-x}\mathrm{N}\left(x\le 0.04\right)$ in order to address some of the existing controversial issues. X-ray and neutron diffraction, x-ray absorption near-edge structure, and electron paramagnetic resonance experiments were used to characterize the structural, electronic, and magnetic properties of the samples, and inelastic neutron scattering was employed to determine the magnetic excitations associated with Mn monomers and dimers. Our main conclusions are as follows: (i) The valence of the Mn ions is $2+$. (ii) The $\mathrm{M}{\mathrm{n}}^{2+}$ ions experience a substantial single-ion axial anisotropy with parameter $D=0.027\left(3\right)\mathrm{meV}$. (iii) Nearest-neighbor $\mathrm{M}{\mathrm{n}}^{2+}$ ions are coupled antiferromagnetically. The exchange parameter $J=-0.140\left(7\right)\mathrm{meV}$ is independent of the Mn content $x$; i.e., there is no evidence for hole-induced modifications of $J$ towards a potentially high Curie temperature postulated in the literature.

Low-dimensional ferroelectricity has attracted tremendous attention due to its huge potential in device applications. Here, based on first-principles calculations, we predict the existence of spontaneous in-plane electrical polarization and ferroelectricity in monolayer $\beta$-GeSe, a polymorph of GeSe with a boat conformation newly synthesized in experiment. The magnitude of the polarization is about $0.16nC/m$, which is comparable to that of monolayer SnTe studied in recent experiment, and the intrinsic Curie temperature is estimated to be above 200 K. Interestingly, owing to its puckered structure, the physical properties of $\beta$-GeSe can be easily controlled by strain. The Curie temperature can be raised above room temperature by applying a 1% tensile strain, and the magnitude of polarization can be largely increased by strains in either the armchair or zigzag direction. Furthermore, we find that for the case with electron doping, applying strain can readily tune the anisotropic electric transport with the preferred conducting direction rotated by ${90}^{\circ }$, which is connected to a strain-induced Lifshitz transition. The ratio between the effective masses along the two in-plane directions can undergo a dramatic change of two orders of magnitude even by a 2% strain. Our result reveals monolayer $\beta$-GeSe is a promising platform for exploring ferroelectricity in two dimensions and for nanoscale mechanoelectronic device applications.

Theoretical results for the magnetization dynamics of a magnonic crystal formed by grooves on the surface of a ferromagnetic film, called a surface-modulated magnonic crystal, are presented. For such a system, the role of the periodic dipolar field induced by the geometrical modulation is addressed by using the plane-wave method. The results reveal that, under the increasing of the depth of the grooves, zones with magnetizing and demagnetizing fields act on the system in such a way that magnonic band gaps are observed in both Damon-Eshbach and backward volume geometries. Particularly, in the backward volume configuration, high-frequency band gaps and low-frequency flat modes are obtained. By taking into account the properties of the internal field induced by the grooves, the flattening of the modes and their shift towards low frequencies are discussed and explained. To test the validity of the model, the theoretical results of this work are confirmed by micromagnetic simulations, and good agreement between both methods is achieved. The theoretical model allows for a detailed understanding of the physics underlying these kinds of systems, thereby providing an outlook for potential applications on magnonic devices.

How to make a model of a non-Fermi-liquid metal with efficient current dissipation is a long-standing problem. Results from holographic duality suggest a framework where local critical fermionic degrees of freedom provide both a source of decoherence for the Landau quasiparticle, and a sink for its momentum. This leads us to study a Kondo lattice type model with SYK models in place of the spin impurities. We find evidence for a stable phase at intermediate couplings.

Motivated by recent experimental observations, we explore electron transport properties of a ferroelectric tunnel junction (FTJ) with an embedded head-to-head ferroelectric domain wall, using first-principles density-functional theory calculations. We consider a FTJ with $\mathrm{L}{\mathrm{a}}_{0.5}\mathrm{S}{\mathrm{r}}_{0.5}\mathrm{Mn}{\mathrm{O}}_3$ electrodes separated by a $\mathrm{BaTi}{\mathrm{O}}_3$ barrier layer and show that an in-plane charged domain wall in the ferroelectric $\mathrm{BaTi}{\mathrm{O}}_3$ can be induced by polar interfaces. The resulting $\mathsf{V}$-shaped electrostatic potential profile across the $\mathrm{BaTi}{\mathrm{O}}_3$ layer creates a quantum well and leads to the formation of a two-dimensional electron gas, which stabilizes the domain wall. The confined electronic states in the barrier are responsible for resonant tunneling as is evident from our quantum-transport calculations. We find that the resonant tunneling is an orbital selective process, which leads to sharp spikes in the momentum- and energy-resolved transmission spectra. Our results indicate that domain walls embedded in FTJs can be used to control the electron transport.

Excitation and recombination dynamics of the photoexcited charge carriers in porous silicon membranes were studied using a femtosecond pump-probe technique. Near-infrared pulses (800 nm, 60 fs) were used for the pump while, for the probe, we employed different wavelengths in the range between 3.4 and $5\mu \text{m}$ covering the medium wavelength infrared range. The data acquired in these experiments consist of simultaneous measurements of the transmittance and reflectance as a function of the delay time between the pump and probe for different pump fluences and probe wavelengths. To evaluate the results, we developed an optical model based on the two-dimensional Maxwell-Garnett formula, incorporating the free-carrier Drude contribution and nonuniformity of the excitation by the Wentzel-Kramers-Brillouin model. This model allowed the retrieval of information about the carrier density as a function of the pump fluence, time, and wavelength. The carrier density data were analyzed to reveal that the recombination dynamics is governed by Shockley-Read-Hall and Auger processes, whereas the diffusion has an insignificant contribution. We show that, in porous silicon samples, the Auger recombination process is greatly enhanced at the wavelength corresponding to the infrared-active vibrational modes of the molecular impurities on the surface of the pores. This observation of surface-vibration-assisted Auger recombination is not only for porous silicon in particular, but for low-dimension and bulk semiconductors in general. We estimate the time constants of Shockley-Read-Hall and Auger processes, and demonstrate their wavelength dependence for the excited carrier density in the range of ${10}^{18}–{10}^{19}1/{\text{cm}}^3$. We demonstrate that both processes are enhanced by up to three orders of magnitude with respect to the bulk counterpart. In addition, we provide a plethora of the physical parameters evaluated from the experimental data, such as the dielectric function and its dependence on the injection level of the free carriers, charge-carrier scattering time related high-frequency conductivity, and the free-carrier absorption at the midwave infrared range.

The vacancy-induced chiral zero energy modes (CZEMs) of chiral-unitary-class (AIII) and chiral-symplectic-class (CII) two-dimensional $\left(2\mathrm{D}\right)$ disordered Dirac semimetals realized on a square bipartite lattice are investigated numerically by using the Kubo-Greenwood formula with the kernel polynomial method. The results show that, for both systems, the CZEMs exhibit the critical delocalization. The CZEM conductivity remains a robust constant (i.e., ${\sigma }_{{}_{\mathrm{CZEM}}}\approx 1.05e^2/h)$, which is insensitive to the sample sizes, the vacancy concentrations, and the numbers of moments of Chebyshev polynomials, i.e., the dephasing strength. For both kinds of chiral systems, the CZEM conductivities are almost identical. However, they are not equal to that of graphene (i.e., $4e^2/\pi h)$, which belongs to the chiral orthogonal class (BDI) semimetal on a $2\mathrm{D}$ hexagonal bipartite lattice. In addition, for the case that the vacancy concentrations are different in the two sublattices, the CZEM conductivity vanishes, and thus both systems exhibit localization at the Dirac point. Moreover, a band gap and a mobility gap open around zero energy. The widths of the energy gaps and mobility gaps are increasing with larger vacancy concentration difference. The width of the mobility gap is greater than that of the band gap, and a $\delta$-function-like peak of density of states emerges at the Dirac point within the band gap, implying the existence of numerous localized states.

It is known that sputter yields of elemental materials depend on the target curvature. Here we explore to what extent this effect is modified for alloy targets using both Monte Carlo and molecular dynamics simulations. For the exemplary case of amorphous ${\mathrm{Si}}_c{\mathrm{Ge}}_{1-c}$ spheres, we find that, in the limit of small curvatures, the curvature dependence of the sputter preferentiality is negligible. This finding can be explained by a natural extension of the existing analytical theory of curvature-dependent sputtering for elemental materials. For large curvatures, however, the sputter preferentiality strongly depends on curvature. In this case, molecular-dynamics simulations also predict strong spike effects which increase the sputter yields. Sputter yield amplification—an increase in partial sputter yield with decreasing concentration of the element—occurs for Si-rich alloys.

We study both analytically and numerically the role of orbital effects caused by a magnetic field applied along the axis of a semiconducting Rashba nanowire in the topological regime hosting Majorana fermions. We demonstrate that the orbital effects can be effectively taken into account in a one-dimensional model by shifting the chemical potential and thus modifying the topological criterion. We focus on the energy splitting between two Majorana fermions in a finite nanowire and find a striking interplay between orbital and Zeeman effects on this splitting. In the limit of strong spin-orbit interaction, we find regimes where the amplitude of the oscillating splitting stays constant or even decays with increasing the magnetic field, in stark contrast to the commonly studied case where orbital effects of the magnetic field are neglected. The period of these oscillations is found to be almost constant in many parameter regimes.