We construct the complete set of boundary states of two-dimensional fermionic CFTs using that of the bosonic counterpart. We see that there are two groups of boundary conditions, which contributes to the open-string partition function by characters with integer coefficients, or with $\sqrt{2}$ times integer coefficients. We argue that, using the argument of P. B. Smith and D. Tong [J. High Energy Phys. 09 (2020) 018], this $\sqrt{2}$ indicates a single unpaired Majorana zero mode and that these two groups of boundary conditions are mutually incompatible. We end the paper by mentioning a possible interpretation of the result in terms of the entanglement entropy.

The self-energy method for quantum impurity models expresses the correlation part of the self-energy in terms of the ratio of two Green's functions and allows for a more accurate calculation of equilibrium spectral functions than is possible directly from the one-particle Green's function [Bulla et al., J. Phys.: Condens. Matter 10, 8365 (1998)], for example, within the numerical renormalization group method. In addition, the self-energy itself is a central quantity required in the dynamical mean field theory of strongly correlated lattice models. Here, we show how to generalize the self-energy method to the time-dependent situation for the prototype model of strong correlations, the Anderson impurity model. We use the equation-of-motion method to obtain closed expressions for the local Green's function in terms of a time-dependent correlation self-energy, with the latter being given as a ratio of a one-particle time-dependent Green's function and a higher-order correlation function. We benchmark this self-energy approach to time-dependent spectral functions against the direct approach within the time-dependent numerical renormalization group method. The self-energy approach improves the accuracy of time-dependent spectral function calculations, and the closed-form expressions for the Green's function allow for a clear picture of the time evolution of spectral features at the different characteristic time scales. The self-energy approach is of potential interest also for other quantum impurity solvers for real-time evolution, including time-dependent density matrix renormalization group and continuous-time quantum Monte Carlo techniques.

Using ion-implanted ${}^8\mathrm{Li} \beta$-detected NMR, we study the evolution of the correlated metallic state of ${\mathrm{LaNiO}}_3$ in a series of ${\mathrm{LaNiO}}_3/{\mathrm{LaAlO}}_3$ superlattices as a function of bilayer thickness. Spin-lattice relaxation measurements in an applied field of $6.55\mathrm{T}$ reveal two equal amplitude components: one with metallic ($T$ linear) $1/T_1$ and a second with a more complex $T$ dependence. The metallic character of the slow relaxing component is only weakly affected by the ${\mathrm{LaNiO}}_3$ thickness, while the fast component is much more sensitive, exhibiting the opposite temperature dependence (increasing toward low $T$) in the thinnest, most magnetic samples. The origin of this bipartite relaxation is discussed in terms of electronic phase separation.

A highly superlinear in radiation intensity photoconductance induced by continuous wave terahertz laser radiation with low intensities has been observed in quantum point contacts made of GaAs quantum wells operating in the deep tunneling regime. For very low values of the normalized dark conductance $G_{\mathrm{dark}}/G_0\approx {10}^{-6}$, with the conductance quantum $G_0=2e^2/h$, the photoconductance scales exponentially with the radiation intensity, so that already at $100\mathrm{mW}/{\mathrm{cm}}^2$, it increases by almost four orders of magnitude. This effect is observed for a radiation electric field oriented along the source drain direction. We provide model considerations of the effect and attribute it to the variation of the tunneling barrier height by the radiation field made possible by local diffraction effects. We also demonstrate that cyclotron resonance due to an external magnetic field manifests itself in the photoconductance, completely suppressing the photoresponse.

The field of thermotronics aims to develop thermal circuits that operate with temperature biases and heat currents just as how electronic circuits are based on voltages and electric currents. Here, we investigate a thermal half-wave rectifier based on a quantum two-level system (a qubit) that is driven by a periodically modulated temperature difference across it. To this end we present a nonequilibrium Green's function technique, which we extend to the time domain to account for the time-dependent temperature in one of two thermal reservoirs connected to the qubit. We find that the qubit acts a thermal diode in parallel with a thermal capacitor, whose capacitance is controlled by the coupling to the reservoirs. These findings are important for the efforts to design nonlinear thermal components such as heat rectifiers and multipliers that operate with more than one diode.

The false vacuum decay has been a central theme in physics for half a century with applications to cosmology and to the theory of fundamental interactions. This fascinating phenomenon is even more intriguing when combined with the confinement of elementary particles. Due to the astronomical timescales involved, the research has so far focused on theoretical aspects of this decay. The purpose of this Letter is to show that the false vacuum decay is accessible to current optical experiments as quantum analog simulators of spin chains with confinement of the elementary excitations, which mimic the high energy phenomenology but in one spatial dimension. We study the nonequilibrium dynamics of the false vacuum in a quantum Ising chain and in an XXZ ladder. The false vacuum is the metastable state that arises in the ferromagnetic phase of the model when the symmetry is explicitly broken by a longitudinal field. This state decays through the formation of “bubbles” of true vacuum. Using infinite volume time evolving block decimation (iTEBD) simulations, we are able to study the real-time evolution in the thermodynamic limit and measure the decay rate of local observables. We find that the numerical results agree with the theoretical prediction that the decay rate is exponentially small in the inverse of the longitudinal field.

Plasmons in nanostructured metals are widely utilized to trigger strong light-matter interactions with quantum light sources. While the nonclassical behavior of such quantum emitters (QEs) is well understood in this context, the role of quantum and surface effects in the plasmonic resonator is usually neglected. Here, we combine the Green's tensor approach with the Feibelman $d$-parameter formalism to theoretically explore the influence of quantum surface effects in metal-dielectric layered nanostructures on the relaxation dynamics of a proximal two-level QE. Having identified electron spill-out as the dominant source of quantum effects in jellium-like metals, we focus our study on sodium. Our results reveal a clear splitting in the emission spectrum, indicative of having reached the strong coupling regime, and, more importantly, non-Markovian relaxation dynamics of the emitter. Our findings establish that strong light-matter coupling is not suppressed by the emergence of nonclassical surface effects in the optical response of the metal.

Pursuing room-temperature superconductors has been a major theme in physics and material sciences since the discovery of superconductivity in 1911. In light of the tremendous progress that has been made in hydrogen-based superconductors with record $T_c\sim 260\mathrm{K}$ in binary fcc $\mathrm{La}{\mathrm{H}}_{10}$, it is natural to explore more similar phases in hydrogen-rich materials. Y and La are conventionally grouped together with the lanthanide elements due to the similarities in their outer electron configurations and properties. The yttrium-hydrogen system is the most attractive candidate with high $T_c$ among all binary metal-hydrogen systems theoretically studied so far. Here we report the synthesis of the $I4/mmm\text{−}\mathrm{Y}{\mathrm{H}}_4$ phase and determine its superconducting transition with maximum $T_c\sim 88\mathrm{K}$ at 155 GPa. The decrease in critical temperature under an external magnetic field suggests the upper critical magnetic field of about 14–22 T at megabar pressures consistent with theoretical data. By making the overlapping analysis, we elucidate that the stabilization of clathrate structures through Y atoms and the high-frequency hydrogen sublattice vibrations contribute effectively to strong electron-phonon coupling and therefore high-temperature superconductivity in the $I4/mmm\text{−}\mathrm{Y}{\mathrm{H}}_4$ and $Im\overline{3}m\text{−}\mathrm{Y}{\mathrm{H}}_6$ phases.

We report the experimental observation of Ising superconductivity in three-dimensional ${\mathrm{NbSe}}_2$ stacked with single-layer ${\mathrm{MoS}}_2$. The angular dependence of the upper critical magnetic field and the temperature dependence of the upper parallel critical field confirm the appearance of two-dimensional Ising superconductivity in three-dimensional ${\mathrm{NbSe}}_2$ with a single-layer ${\mathrm{MoS}}_2$ overlay. We show that the superconducting phase has strong Ising spin-orbit correlations which make the holes spin nondegenerate. Our observation of Ising superconductivity in heterostructures of few-layer ${\mathrm{NbSe}}_2$ of thickness $\sim 15$ nm with single-layer ${\mathrm{MoS}}_2$ raises the interesting prospect of observing topological chiral superconductors with nontrivial Chern numbers in a momentum-space spin-split fermionic system.

Integrable Floquet spin chains are known to host strong zero and $\pi$ modes which are boundary operators that respectively commute and anticommute with the Floquet unitary generating stroboscopic time evolution, in addition to anticommuting with a discrete symmetry of the Floquet unitary. Thus the existence of strong modes implies a characteristic pairing structure of the full spectrum. Weak interactions modify the strong modes to almost strong modes that almost commute or anticommute with the Floquet unitary. Manifestations of strong and almost strong modes are presented in two different Krylov subspaces. One is a Krylov subspace obtained from a Lanczos iteration that maps the time evolution generated by the Floquet Hamiltonian onto dynamics of a single particle on a fictitious chain with nearest-neighbor hopping. The second is a Krylov subspace obtained from the Arnoldi iteration that maps the time evolution generated directly by the Floquet unitary onto dynamics of a single particle on a fictitious chain with longer-range hopping. While the former Krylov subspace is sensitive to the branch of the logarithm of the Floquet unitary, the latter obtained from the Arnoldi scheme is not. The effective single-particle models in the Krylov subspace are discussed, and the topological properties of the Krylov chain that ensure stable zero and $\pi$ modes at the boundaries are highlighted. The role of interactions is discussed. Expressions for the lifetime of the almost strong modes are derived in terms of the parameters of the Krylov subspace, and are compared with exact diagonalization.

Transition metal dichalcogenide heterostructures can host interlayer excitons (IXs), which consist of electrons and holes spatially separated in different layers. IXs possess permanent dipoles and have proven to offer a wealth of novel physics. We develop a discrete, random-walk model which includes annihilation and repulsion interactions among IXs. Using this model, we simulate the trapping of IXs in traps of different depths, densities, and shapes. Our results show that dipole-dipole interactions play an important role in regulating IX trapping. The effects of dipole interactions can be mitigated with small, deep traps which are realizable with atomic defects and moiré potentials.

Thermal transport in nanocrystalline Si is of great importance for many advanced applications. A better understanding of the modal thermal conductivity of nanocrystalline Si will be expected. In this work, the efficient variance reduced Monte Carlo simulation with full band phonon dispersion is applied to study the modal thermal conductivity of nanocrystalline Si. Importantly, the phonon modal transmissions across the grain boundaries which are modeled by the amorphous Si interface are calculated by the mode-resolved atomistic Green's function method. The predicted ratios of thermal conductivity of nanocrystalline Si to that of bulk Si agree well with that of the experimental measurements in a wide range of grain size. The ratio of thermal conductivity of nanocrystalline Si is decreased from 54% to 3% and the contribution of phonons with mean free path larger than the grain size increases from 30% to 96% as the grain size decreases from 550 to 10 nm. This work demonstrates that the full band Monte Carlo simulation using phonon modal transmission by the mode-resolved atomistic Green's function method can effectively capture the phonon transport picture in complex nanostructures, and therefore can provide guidance for designing Si based devices with better performance.

Phase transitions are characterized by a sharp change in the type of dynamics of microparticles, and their description usually requires quantum mechanics. Recently, a peculiar type of conductor was discovered in which two-dimensional (2D) electrons form a viscous fluid. In this paper we reveal that such an electron fluid in high-quality samples can be formed from ballistic electrons via a phase transition. For this purpose, we theoretically study the evolution of a ballistic flow of 2D weakly interacting electrons with an increase of magnetic field and trace an emergence of a fluid fraction at a certain critical field. Such a restructuring of the flow manifests itself in a kink in the magnetic field dependencies of the longitudinal and the Hall resistances. It is remarkable that the studied phase transition has a classical-mechanical origin and is determined by both the ballistic size effects and electron-electron scattering. Our analysis shows that this effect was apparently observed in recent transport experiments on 2D electrons in graphene and high-mobility GaAs quantum wells.

We study the fidelity and the entanglement entropy for the ground states of quantum systems that have infinite-order quantum phase transitions. In particular, we consider the quantum O(2) model with a spin-$S$ truncation, where there is an infinite-order Gaussian (IOG) transition for $S=1$ and there are Berezinskii-Kosterlitz-Thouless (BKT) transitions for $S\ge 2$. We show that the height of the peak in the fidelity susceptibility (${\chi }_F$) converges to a finite thermodynamic value as a power law of $1/L$ for the IOG transition and as $1/ln\left(L\right)$ for BKT transitions. The peak position of ${\chi }_F$ resides inside the gapped phase for both the IOG transition and BKT transitions. On the other hand, the derivative of the block entanglement entropy with respect to the coupling constant ($S_{vN}^{\prime }$) has a peak height that diverges as ${ln}^2\left(L\right)$ for $S=1$ and ${ln}^3\left(L\right)$ for $S\ge 2$ and can be used to locate both kinds of transitions accurately. We include higher-order corrections for finite-size scalings and obtain the value of the central charge consistent with $c=1$ predicted by conformal field theory. The crossing point of ${\chi }_F$ between different system sizes is at the IOG point for $S=1$ but is inside the gapped phase for $S\ge 2$, while those of $S_{vN}^{\prime }$ are at the phase-transition points for all $S$ truncations. Our work elaborates on how to use the finite-size scaling of ${\chi }_F$ or $S_{vN}^{\prime }$ to detect infinite-order quantum phase transitions and discusses the efficiency and accuracy of the two methods.

The electronic properties of $\pi$-conjugated two-dimensional (2D) polymers near the Fermi level are determined by structural topology and chemical composition. Thus tight-binding (TB) calculations of the corresponding fundamental network can be used to explore the parameter space to find configurations with intriguing properties before designing the atomistic 2D polymer network. The vertex-transitive fes lattice, which is also called a square-octagon, 4-8, or 4.8${}^2$ lattice, is rich in interesting topological features including Dirac points and flat bands. Herein, we study its electronic and topological properties within the TB framework using representative parameters for chemical systems. Secondly, we demonstrate that the rational implementation of band structure features obtained from TB calculations in 2D polymers is feasible with a family of 2D polymers possessing fes structure. A one-to-one band structure correspondence between the fundamental network and 2D polymers is found. Moreover, changing the relative length of linkers connecting the triangulene units in the 2D polymers reflects tuning of hopping parameters in the TB model. These perturbations allow sizable local band gaps to open at various positions in the Brillouin zone. From analysis of the Berry curvature flux, none of the polymers exhibits a large topologically nontrivial band gap. However, we find a particular configuration of semimetallic characteristics with separate electron and hole pockets, which possess very low effective masses both for electrons (as small as $m_{\mathrm{e}}^{*}=0.05$) and for holes (as small as $m_{\mathrm{h}}^{*}=0.01$).

We predict the occurrence of unexpected magnetic ground states in ultrathin Mn films due to the frustration of higher-order interactions. Based on density functional theory we show that significant chiral-chiral interactions occur in hexagonal Mn monolayers due to large topological orbital moments which interact with the emergent magnetic field. Due to the competition with biquadratic and four-spin interactions superposition states of spin spirals such as the $2Q$ state or a distorted $3Q$ state arise. Simulations of spin-polarized scanning tunneling microscopy images suggest that the distorted $3Q$ state could be the magnetic ground state of a Mn monolayer on Re(0001).

Coulomb interactions in atomically thin transition metal dichalcogenides can be dynamically engineered by exploiting the dielectric environment to control the optical and electronic properties. Here we demonstrate an optically tunable giant band-gap renormalization (BGR) $\sim 1200$ and 850 meV from the edge of the conduction band and complete suppression of the exciton absorption in large-area single-layer (1L) and three-layer (3L) ${\mathrm{MoS}}_2$, respectively. The observed giant BGR is two orders of magnitude larger than that in the conventional semiconductors, and it persists for tens of ps. Strikingly, our results demonstrate photoinduced transparency at the electronic band gap using an intense optical field at room temperature. Exciton bleach recovery in 1L and 3L show a contrasting fluence-dependent response, demonstrating the layer-dependent optical tuning of exciton lifetime in a way that would be both reversible and real time. We find that the optical band gap (exciton resonance peak) shows a transient redshift followed by an anomalous blueshift from the lowest energy point as a function of the photo-generated carrier density. The observed exciton energy shift is analogous to atom-atom interactions, and it varies as a Lennard-Jones like potential as a function of the interexciton separation.

Modern problems in magnetization dynamics require more and more the numerical determination of the spin-wave spectra and dispersion in magnetic systems where analytic theories are not yet available. Micromagnetic simulations can be used to compute the spatial mode profiles and oscillation frequencies of spin waves in magnetic system with almost arbitrary geometry and different magnetic interactions. Although numerical approaches are very versatile, they often do not give the same insight and physical understanding as analytical theories. For example, it is not always possible to decide whether a certain feature (such as dispersion asymmetry, for example) is governed by one magnetic interaction or the other. Moreover, since numerical approaches typically yield the normal modes of the system, it is not always feasible to disentangle hybridized modes. In this paper, we build a bridge between numerics and analytics by presenting a methodology to calculate the individual contributions to general spin-wave dispersions in a fully numerical manner. We discuss the general form of any spin-wave dispersion in terms of the effective (stiffness) fields produced by the modes. Based on a special type of micromagnetic simulation, the numerical dynamic-matrix approach, we show how to calculate each stiffness field in the respective dispersion law, separately for each magnetic interaction. In particular, it becomes possible to disentangle contributions of different magnetic interactions to the dispersion asymmetry in systems where nonreciprocity is present. At the same time, dipolar-hybridized modes can be easily disentangled. Since this methodology is independent of the geometry or the involved magnetic interactions at hand, we believe it is attractive for experimental and theoretical studies of magnetic systems where there are no analytics available yet, but also to aid the development of new analytical theories.

Third-harmonic generation (THG) experiments on superconductors can be used to investigate collective excitations like the amplitude mode of the order parameter known as Higgs mode. These modes are visible due to resonances in the THG signal if the driving frequency matches the energy of the mode. In real materials, multiple modes can exist giving rise to additional THG contributions, such that it is difficult to unambiguously interpret the results. In this paper, we additionally analyze the phase of the THG signal for the first time, which contains microscopic details beyond classical resonances as well as signatures of couplings between modes that are difficult to observe in the amplitude alone. We investigate how the Higgs mode, impurities, or Coulomb interaction affects the phase response and consider exemplary two systems with additional modes. We argue that extracting this phase information could be valuable in future experiments.