We study the optical conductivity of a pristine two-dimensional electron system near an Ising-nematic quantum critical point. We discuss the relation between the frequency scaling of the conductivity and the shape of the Fermi surface, namely, whether it is isotropic, convex, or concave. We confirm the cancellation of the leading order terms in the optical conductivity for the cases of isotropic and convex Fermi surfaces and show that the remaining contribution scales as ${\left|\omega \right|}^{2/3}$ at $T=0$. On the contrary, the leading term, $\propto {\left|\omega \right|}^{-2/3}$, survives for a concave FS. We also address the frequency dependence of the optical conductivity near the convex-to-concave transition. Explicit calculations are carried out for the Fermi-liquid regime using the modified (but equivalent to the original) version of the Kubo formula, while the quantum-critical regime is accessed by employing the space-time scaling of the $Z=3$ critical theory.

The non-Hermitian skin effect (NHSE) has been intensely investigated over the past few years and has unveiled new topological phases, which have no counterparts in Hermitian systems. Here we consider the hybridization between the NHSE in an exciton-polariton waveguide and a localized defect mode. By tuning the non-Hermiticity, we find that the resulting ground state of the system is both spatially extended and energetically separated from other modes in the system. When polariton lasing occurs in the system, we find an enhanced spatial coherence (of typically 30 times longer) compared to regular waveguides, which is robust in the presence of disorder.

Two-dimensional (2D) dielectrics play a crucial role in the miniaturization of transistors. An optimal 2D dielectric should have a significant out-of-plane dielectric constant and appropriate band alignments to achieve superior performance. However, 2D materials generally exhibit poor out-of-plane dielectric constant. In this paper, we have identified 11 2D materials from the Computational 2D Materials Database with out-of-plane dielectric constants above 5.0, surpassing those of conventional dielectrics, i.e., ${\mathrm{SiO}}_2, {\mathrm{Al}}_2{\mathrm{O}}_3$, and BN. The band alignments show that ten monolayers ${\mathrm{SnNa}}_2{\mathrm{H}}_6{\mathrm{O}}_6, {\mathrm{GeLi}}_2{\mathrm{H}}_6{\mathrm{O}}_6, {\mathrm{HfNa}}_2{\mathrm{H}}_6{\mathrm{O}}_6, {\mathrm{ZrNa}}_2{\mathrm{H}}_6{\mathrm{O}}_6$, etc., are promising dielectrics when monolayer ${\mathrm{MoS}}_2/{\mathrm{WS}}_2$ serves as the channel. Moreover, four dielectrics have an out-of-plane dielectric constant that outperforms in-plane. This unusual feature indicates a stronger out-of-plane bond strength, resulting from the ionic bonds. We further provide a general guideline for quickly discovering high-performance dielectrics using structures and electronegativity tables.

The optical spectrum of a quantum dot is typically dominated by the fundamental transition between the lowest-energy configurations. However, the radiative Auger process can result in additional redshifted emission lines. The origin of these lines is a combination of Coulomb interaction and symmetry breaking in the quantum dot. In this paper, we present measurements of such radiative Auger lines for a range of InGaAs/GaAs self-assembled quantum dots. We account for the Auger lines with a tight-binding model with a configuration interaction including symmetry breaking via alloy disorder. We show that the model accounts for the intensities of the Auger lines and the changes from quantum dot to quantum dot. We relate our findings to group theory explaining how the reduction in symmetry caused by alloy disorder is essential for the appearance of the radiative Auger lines.

We study the effect of various configurations of vacancies on the magnetic properties of graphene nanoflake (GNF) with screened realistic long-range electron interaction [T. O. Wehling et al., Phys. Rev. Lett. 106, 236805 (2011)] within the functional renormalization group approach. In agreement with previous studies, the presence of vacancies in GNF yields a strong enhancement of spin-density-wave (SDW) correlations. We show, however, that only some part of the considered configurations of vacancies posses SDW ground state. The probability of a system with a random configuration of vacancies to be in the SDW ground state increases with increase of vacancy concentration. The disorder-averaged sublattice magnetization increases linearly with the concentration of vacancies. The ratio of the sublattice magnetizations at the center and edges of GNF, averaged over various realizations of disorder, depends only weakly on the number of vacancies. The effects of vacancies on the linear conductance and charge properties of GNF are discussed.

Black phosphorene (BP) is an emerging two-dimensional material that has potentially wide applications in nano- and optoelectronics. Despite the strong excitonic effects observed in few-layer BP, there is still a lack of understanding regarding exciton-phonon (ex-ph) interaction and exciton mobility. In this work, using $\mathit{GW}$ plus real-time Bethe-Salpeter equation approach, it is revealed that the out-of-plane acoustic phonon mode $\mathit{ZA}$, and the optical phonon modes $A_g^1$ and $A_g^2$ couple strongly with the exciton. Among them, the $A_g^2$ mode has a uniquely significant influence on the oscillator strength. Driven by these ex-ph interactions, the exciton becomes more localized accompanied with strengthened binding and the absorption spectrum undergoes a notable renormalization at 300 K. In the framework of deformation potential theory, the exciton mobility of monolayer BP (MLBP) is found to be largely suppressed compared to the free-charge carriers, showing quasi-one-dimensional character. This work provides rich knowledge of ex-ph interaction and exciton mobility in MLBP at the ab initio level, offering insights for the understanding and controlling the exciton behavior in BP-based devices.

This work presents electronic properties of the kagome metal ${\mathrm{ScV}}_6{\mathrm{Sn}}_6$ using de Haas–van Alphen (dHvA) oscillations and density-functional theory (DFT) calculations. The torque signal with the applied fields up to 43 T shows clear dHvA oscillations with six major frequencies, five of them are below 400 T (low frequencies) and one is nearly 2800 T (high frequency). The Berry phase calculated using the Landau level fan diagram near the quantum limit is approximately $\pi$, which suggests the nontrivial band topology in ${\mathrm{ScV}}_6{\mathrm{Sn}}_6$. To explain the experimental data, we computed the electronic band structure and Fermi surface using DFT in both the pristine- and charge-density wave (CDW) phases. Our results confirm that the CDW phase is energetically favorable, and the Fermi surface undergoes a severe reconstruction in the CDW state. Furthermore, the angular dependence of the dHvA frequencies are consistent with the DFT calculations. The detailed electronic properties presented here are invaluable for understanding the electronic structure and CDW order in ${\mathrm{ScV}}_6{\mathrm{Sn}}_6$, as well as in other vanadium-based kagome systems.

Semihydrogenated graphene (${\mathrm{C}}_2\mathrm{H}$) and semifluorinated graphene (${\mathrm{C}}_2\mathrm{F}$), both in the chair conformation, were predicted by first-principles calculations to be ferromagnetic and antiferromagnetic. It is unclear what is the underlying mechanism leading to such distinct magnetic orders of the two materials. We show that a single-orbital tight-binding model of the graphene lattice up to the next-nearest-neighbor hopping term, with one carbon sublattice coupled to a single active orbital of the adatoms (hydrogen for ${\mathrm{C}}_2\mathrm{H}$, and fluorine for ${\mathrm{C}}_2\mathrm{F}$), supplemented by the Hubbard interactions on the carbon sites, correctly reproduces the distinct magnetic orders of ${\mathrm{C}}_2\mathrm{H}$ and ${\mathrm{C}}_2\mathrm{F}$. In terms of a fairly good approximation to the low-energy band of the nonmagnetic state, we analytically elucidate how a finite next-nearest-neighbor hopping term makes the bandwidth of the low-energy band of ${\mathrm{C}}_2\mathrm{H}$ much smaller than that of ${\mathrm{C}}_2\mathrm{F}$. Competition between the reduction of the interaction energy and the increase of the band energy then favors the ferromagnetic state for ${\mathrm{C}}_2\mathrm{H}$ and the antiferromagnetic state for ${\mathrm{C}}_2\mathrm{F}$. Implications of the mechanism, including electric field tuning of the bandwidth and application to elemental analogs of graphene such as silicene, are analyzed.

The interplay of quantum and classical fluctuations in the vicinity of a quantum critical point (QCP) gives rise to various regimes or phases with distinct quantum character. In this work, we show that the Rényi entropy is a precious tool to characterize the phase diagram of critical systems not only around the QCP but also away from it, thanks to its capability to detect the emergence of local moments at finite temperature. For an efficient evaluation of the Rényi entropy, we introduce an algorithm based on a path-integral Langevin dynamics combined with a previously proposed thermodynamic integration method built on regularized paths. We apply this framework to study the critical behavior of a linear chain of anharmonic oscillators, a particular realization of the ${\varphi }^4$ model. We fully resolved its phase diagram, as a function of both temperature and interaction strength. At finite temperature, we find a sequence of three regimes—para, disordered, and antiferro—met as the interaction is increased. The Rényi entropy divergence coincides with the crossover between the para and disordered regimes, which shows no temperature dependence. The occurrence of the antiferro regime, on the other hand, is temperature dependent. The two crossover lines merge in proximity of the QCP, at zero temperature, where the Rényi entropy is sharply peaked. Via its subsystem-size scaling, we confirm that the transition belongs to the two-dimensional Ising universality class. This phenomenology is expected to happen in all ${\varphi }^4$-like systems, as well as in the elusive water-ice transition across phases VII, VIII, and X.

Searching for single-atom systems with large magnetic anisotropy energies and tunable magnetic states is of vital importance for both fundamental research of magnetism at the atomic scale and realization of future spin-based quantum computation or information storage schemes. Single $5f$ electron based actinide atoms are potential candidates for inducing large magnetic anisotropy energies (MAEs), yet they have been much less studied as compared with $3d$ or $4f$ single-atom systems. Here we present the adsorptive, electronic, and magnetic properties of a single $5f$ electron based uranium atom on two-monolayer MgO/Ag(001) by combining scanning tunneling microscopy/spectroscopy (STM/STS) and density functional theory. Our results reveal that single U atoms spontaneously adsorb at the hollow sites of the MgO/Ag(001) surface and they can be controllably switched between the hollow and the O-top sites of MgO/Ag(001) via STM atom manipulation. Most importantly, single U atoms at the O-top sites reveal complex tunneling spectral features, including a symmetric dip at the Fermi energy, which is the manifestation of the existence of a relatively large $5f$-driven magnetic anisotropy energy, whereas single U atoms at the hollow sites exhibit a two-lobe subatomic structure stemming from the valence electron orbitals of U itself and show no signs related with magnetic anisotropy. This work proves that single $5f$ electron based U atoms can possess a considerable uniaxial magnetic anisotropy via adsorbing at the appropriate sites on the carefully chosen supporting surface, and their magnetic states can be tuned by atom manipulation techniques.

Internal and crystalline symmetries play critical roles in the classification of topological materials. Recently, the crystalline symmetries are found to allow projective representations by gauge fields and bring about new topological phases, such as Möbius insulators in spinless systems. Here, we report an observation of a Klein bottle insulator (KBI) phase in phononic crystals under ${\mathbb{Z}}_2$ gauge fields. This intriguing insulator phase possesses, in momentum space rather than real space, a nonsymmorphic glide symmetry, under which the fundamental domain of the Brillouin zone is topologically equivalent to a Klein bottle. We exploit a bilayer structure that can be decomposed into a time-reversal-broken KBI and its time-reversal counterpart. In the acoustic KBI, a pair of edge states with a nonlocal twist are observed in experiment. All the theoretical and experimental results are in agreement and consistently validate the existence of the KBI for acoustic waves. Our work will promote the discovery of unexplored phases induced by gauge fields and offer possible applications in topological materials.

We show that high harmonic generation (HHG) in carbon nanotubes (CNTs) can be efficiently controlled using the Aharonov-Bohm (AB) effect. When a static magnetic field ($B$) is applied along the tube, electronic wave functions acquire complex phases along the circumferential direction (the AB effect), which modifies the band structure. When the magnetic field is applied to metallic CNTs, which can be regarded as one-dimensional massless Dirac systems, realistic values of $B$ lead to a nonzero gap in the THz regime. We demonstrate that such a change from gapless to gapped Dirac systems drastically increases the HHG intensity in the THz regime. In the gapless Dirac system, the velocity of each electron never changes under the electric field, and thus there is no HHG. On the other hand, the gap opening activates both the interband and intraband currents, which strongly contribute to HHG. Our work demonstrates a unique way to manipulate HHG in nanotubes by tuning electronic wave functions using the magnetic field and the tube structure.

We study superconducting three-band systems within strong-coupling Eliashberg theory. In particular, we search for phase-frustrated superconducting systems with spontaneous time-reversal symmetry breaking (TRSB) states. The emergence of TRSB states in multiband systems has so far been studied using microscopic weak-coupling BCS theory or more phenomenological effective field theories such as multicomponent Ginzburg-Landau theories. For systems with three disjoint Fermi surfaces whose electrons experience interactions mediated by phonons, we present a microscopic analysis showing that TRSB states also exist within a strong-coupling microscopic theory. The systems we consider have sizable electron-phonon couplings, putting them into the strong-coupling regime. They are thus a fitting description for strong-coupling materials such as some of the iron pnictides. Moreover, as the TRSB states are challenging to find numerically, we calculate the free energy of multiband systems within strong-coupling theory and make explicit use of it to pin down the TRSB states' elusive nature. Since Eliashberg theory is well incorporated with first-principles calculations, our strong-coupling approach might help facilitate a more efficient search for candidate materials that can exhibit TRSB.

Investigations of the superconducting properties of $\mathrm{Fe}{\mathrm{Se}}_{0.4}{\mathrm{Te}}_{0.6}$ single crystals have demonstrated that these iron-based superconductors hold the potential to host Dirac-cone-type surface states and two-dimensional (2D) topological superconducting states. Experimental observations detected Berezinsky-Kosterlitz-Thouless transition and transport vortex dynamics, suggesting a dominant role of topological surface states in magnetotransport at low temperatures. In the normal state, $\mathrm{Fe}{\mathrm{Se}}_{0.4}{\mathrm{Te}}_{0.6}$ demonstrated a weak antilocalization transport feature due to the topological nature at temperatures $T$ $<16.5$ K and negative magnetoresistance characteristic due to the suppression of spin-disorder scattering induced by the Kondo effect as the temperature reached 40 K. Additionally, the annealing process of $\mathrm{Fe}{\mathrm{Se}}_{0.4}{\mathrm{Te}}_{0.6}$ single crystals caused a shift from the initial three-dimensional transport pattern to one that more closely resembles a 2D transport system. These findings support the notion that $\mathrm{Fe}{\mathrm{Se}}_{0.4}{\mathrm{Te}}_{0.6}$ holds promise as a platform for investigating topological superconductivity.

We study the spectral properties of disordered superconducting transition metal dichalcogenide monolayers, known as Ising superconductors, subjected to in-plane magnetic fields. In addition to the conventional singlet pairing, we also consider the recently proposed equal-spin triplet pairing, which couples to the singlet at finite in-plane magnetic fields. While both singlet and triplet order parameters are immune to intravalley scattering, they are significantly affected by intervalley scattering. In the realistic regime of strong intrinsic Ising spin-orbit coupling, we find that the properties of the superconductor are well described by a simple formula reminiscent of the well-known Abrikosov-Gor'kov theory, but with a modified self-consistency condition. Our results enable straightforward self-consistent calculation of singlet and triplet order parameters and the density of states of disordered Ising superconductors, which can be particularly useful for interpreting recent tunneling spectroscopy experiments in these systems. We also investigate the high-energy features in the density of states, the so-called mirage gaps, and discuss how they are modified by triplet pairing.

Hybrid systems of superconductors and magnets display several intriguing properties, both from a fundamental physics point of view and with practical applications. Promising applications in superconducting spintronics motivate a search for systems where superconductivity can survive larger in-plane critical magnetic fields than the conventional limit. The Chandrasekhar-Clogston (CC) limit applies to thin-film conventional superconductors with in-plane magnetic fields such that orbital effects may be ignored. For a magnetic field strength comparable to the superconducting gap at zero temperature and zero field, a spin-split normal state attains lower free energy than the superconducting state. A multiband superconductor with a flat band placed just below the Fermi surface has been shown to surpass the CC limit using weak-coupling theory. Since the dimensionless coupling determining the critical temperature scales with the density of states, it is natural to anticipate corrections from strong-coupling theory in flat-band systems, owing to the large density of states of the flat bands. We derive Eliashberg equations and the free energy for a multiband superconductor in a magnetic field. First, we show that the CC limit can be exceeded by a small amount in one-band strong-coupling superconductors due to self-energy renormalization of the magnetic field. Next, we consider a two-band system with one flat band and find that the CC limit can be exceeded by a large amount also in strong-coupling theory, even when including hybridization between bands that intersect.

The noncentrosymmetric Weyl semimetal ${\mathrm{PtBi}}_{2-x}$ $\left(t\text{−}{\mathrm{PtBi}}_{2-x}\right)$ exhibits various interesting technologically important physical properties. We report the experimental investigation of ${\mathrm{PtBi}}_{1.6}$ via second harmonic generation (SHG), single-crystal x-ray diffraction, magnetic susceptibility, and electrical resistivity measurements. While bulk structural, magnetic, and electrical properties show no phase transitions below room temperature, the temperature dependence of the SHG intensity reveals two anomalies: one at $T^{*}\sim 60$ K and another at $T_x\sim 200$ K. Quantitative analysis indicates that the SHG signal results from both the buckled Bi1 surface termination with the $3m$ symmetry and flat Bi2 surface termination with the $m$ symmetry. However, the anomalies are mainly driven by Bi1 on the surface: (1) $T^{*}$ marks the onset of surface states which is also manifested in the $c$-axis resistivity drop and (2) $T_x$ corresponds to the lowest thermal contraction of the structure and enhanced magnetic susceptibility. This study demonstrates that SHG is a powerful technique for probing surface properties even for noncentrosymmetric materials.

We study measurement-induced symmetry-protected topological (SPT) order in a wide class of quantum random circuit models by combining calculations within the stabilizer formalism with tensor network simulations. We construct a family of quantum random circuits, generating the out-of-equilibrium version of all generalized cluster models, and derive a set of nonlocal string order parameters to distinguish different SPT phases. We apply this framework to investigate a random circuit realization of the XZX cluster model, and use the string order parameter to demonstrate that the phase diagram is stable against extending the class of unitary gates in the circuit, from Clifford gates to Haar unitaries. We then turn to the XZZX generalized cluster model, and demonstrate the coexistence of SPT order and spontaneous symmetry breaking, by relying on string order parameters and a connected correlation function.

The advancement of spin-orbit torque (SOT)-based spintronic devices hinges on the ability to optimize interfacial spin-orbit coupling phenomena. This work elucidates the modulation of key spintronic parameters by inserting an Ir layer at Pt/Co and PtCr/Pt(0.5)/Co interfaces. A comprehensive investigation into the thickness-dependent perpendicular magnetic anisotropy, Dzyaloshinskii-Moriya interaction, spin-orbit torque efficiency, and switching current density is undertaken. The results reveal that an ultrathin 0.1 nm Ir insertion layer can enhance the dampinglike SOT efficiency by over 26% in Pt/Co and 10% in PtCr/Co systems, attributed to increased interfacial impurity scattering. Strikingly, the magnetic anisotropy and Dzyaloshinskii-Moriya interaction exhibit a nonmonotonic relationship with Ir thickness, minimized at 0.4–0.5 nm, explained by interfacial roughness effects evidenced through first-principles calculations. The critical switching current density becomes widely tunable from $1.6–3.5\times {10}^7$ $\left({\text{A/cm}}^2\right)$ in Pt/Ir/Co and $1.0–2.0\times {10}^7$ $\left({\text{A/cm}}^2\right)$ in PtCr/Ir/Co, respectively, reduced by nearly 50% with a 0.4 nm Ir insertion. These findings provide significant physical insights into interfacial spin-orbit coupling mechanisms and interface engineering of spintronic devices using insertion layers. The capability to substantially increase SOT efficiency while reducing the switching current highlights the promise of an Ir insertion layer to unlock the full potential of SOT-MRAM technology.