The coherent spin dynamics of holes are investigated for ${\mathrm{CsPbBr}}_3$ and $\mathrm{CsPb}{(\mathrm{Cl},\mathrm{Br})}_3$ perovskite nanocrystals in a glass matrix using the time-resolved Faraday rotation/ellipticity techniques. In an external magnetic field, pronounced Larmor spin precession of the hole spins is detected across a wide temperature range from 5 to 300 K. The hole Landé $g$-factor varies in the range of $0.8–1.5$, in which it increases with increasing optical transition energy due to enhanced confinement in small nanocrystals. The hole spin dephasing time decreases from 1 ns to 50 ps in this temperature range. Nuclear spin fluctuations have a pronounced impact on the hole spin dynamics. The hyperfine interaction of the holes with nuclear spins modifies their spin polarization decay and induces their spin precession in zero external magnetic field. The results can be well described by the model developed in [I. A. Merkulov et al., Phys. Rev. B 65, 205309 (2002)], from which the hyperfine interaction energy of a hole spin with the nuclear spin fluctuation in the range of $2–5\mu \mathrm{eV}$ is evaluated.

The existence of exceptional points (EPs)—where both eigenvalues and eigenvectors converge—is a key characteristic of non-Hermitian physics. A newly discovered class of magnets—termed as altermagnets (AMs)—are characterized by a net zero magnetization as well as spin-split bands. In this study, we propose the emergence of non-Hermitian physics at AM-ferromagnet (FM) junctions. We discover that such a junction hosts tunable EPs. We demonstrate that the positions of these emergent EPs can be tuned using an external applied magnetic field and show that for a critical value of the applied magnetic field the EPs can annihilate. Notably, the number and position of the EPs crucially depends on the type of AM and its orientation with respect to the FM. Our work puts forth a promising platform of exploration of non-Hermitian physics in an emerging class of magnetic materials.

We theoretically study the electronic band structure and local optical conductivity of domain wall solitons in bilayer graphene (as well as twisted bilayer graphene) with arbitrary soliton angle, which characterizes the local strain direction. We demonstrate that the soliton angle provides an important yet underexplored degree of freedom that can strongly modify the local optical conductivity. The conductivity spectrum features resonance peaks associated with interband transitions involving the topological as well as high-energy soliton states. Two most prominent peaks exhibit continuous suppression and enhancement, respectively, with the soliton angle. The dependence of the peaks on Fermi energy provides important information about the soliton band structure. The local optical conductivity exhibits substantial spatial dependence, which can be used to study the spatial distribution of the soliton states. Furthermore, we show that the conductivity spectra for all soliton angles are broadly tunable by external pressure, which can double the energies of the resonance peaks in experimentally achievable pressure ranges.

Exploring advanced materials with extraordinary properties is necessary for developing high-performance devices. In this work we construct two-dimensional Janus $\mathrm{HfSi}{X}_3\mathrm{H} (X=\mathrm{N}/\mathrm{P}/\mathrm{As})$ monolayers and systematically examine their physical properties by utilizing first-principles calculations. Specifically, we focus on the stabilities, Raman spectra, electronic structures, piezoelectricity, and transport properties of the monolayers. As a result, the studied ${\mathrm{HfSiN}}_3\mathrm{H}, {\mathrm{HfSiP}}_3\mathrm{H}$, and ${\mathrm{HfSiAs}}_3\mathrm{H}$ monolayers are found as semiconductors with good structural, mechanical, dynamic, and thermodynamic stabilities. The piezoelectric coefficients ($e_{11}$ and $e_{31}$) are calculated to examine the piezoelectricity of the $\mathrm{HfSi}{X}_3\mathrm{H}$ monolayers. It is noted that besides the in-plane piezoelectricity, the three monolayers show additional out-of-plane piezoelectric effects because of their noncentrosymmetric structures. We also include fully anisotropic acoustic deformation potential, ionized impurity, piezoelectric, and polar electron–phonon scattering to calculate the total carrier mobilities of Janus $\mathrm{HfSi}{X}_3\mathrm{H}$. It is demonstrated that Janus $\mathrm{HfSi}{X}_3\mathrm{H}$ monolayers exhibit low carrier mobilities at room temperature $(<100{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1})$. Scattering from polar optical phonons is found to dominate the total mobilities of both electrons and holes.

From the quasisymmetry-group perspective [J. Ren et al., Phys. Rev. Lett. 126, 120604 (2021)], we show the universal existence of collective, coherent modes of excitations in many-body scar models in the degenerate limit, where the energy spacing in the scar tower vanishes. The number of these modes, as well as the quantum numbers carried by them, are given, not by the symmetry of the Hamiltonian, but by the quasisymmetry of the scar tower: hence the name quasi-Nambu-Goldstone modes. Based on this, we draw a concrete analogy between the paradigm of spontaneous symmetry breaking and the many-body scar physics in the degenerate limit.

In quantum many-body spin systems, the interplay between the entangling effect of multiqubit Pauli measurements and the disentangling effect of single-qubit Pauli measurements may give rise to two competing effects. By introducing a randomized measurement pattern with such bases, a phase transition can be induced by altering the ratio between them. In this work, we numerically investigate a measurement-based model associated with the $(2+1)\mathrm{D}{\mathbb{Z}}_2$ Fradkin-Shenker Hamiltonian that encompasses the deconfining, confining, and Higgs phases. We determine the phase diagram in our measurement-only model by employing entanglement measures. For the bulk topological order, we use the topological entanglement entropy. We also use the mutual information between separated boundary regions to diagnose the boundary phase transition associated with the Higgs or the bulk symmetry-protected topological (SPT) phase. We observe the structural similarity between our phase diagram and the one in the standard quantum Hamiltonian formulation of the Fradkin-Shenker model with the open rough boundary. First, a deconfining phase is detected by nonzero and constant topological entanglement entropy. Second, we find a (boundary) phase transition curve separating the Higgs=SPT phase from the rest. In certain limits, the topological phase transitions reside at the critical point of the formation of giant homological cycles in the bulk three-dimensional (3D) space-time lattice, as well as the bond percolation threshold of the boundary 2D space-time lattice when it is effectively decoupled from the bulk. Additionally, there are analogous mixed-phase properties at a certain region of the phase diagram, emerging from how we terminate the measurement-based procedure. Our findings pave an alternative pathway to study the physics of Higgs=SPT phases on quantum devices in the near future.

Berry phases have long been known to significantly alter the properties of periodic systems, resulting in anomalous terms in the semiclassical equations of motion describing wave-packet dynamics. In non-Hermitian systems, generalizations of the Berry connection have been proposed and shown to have novel effects on dynamics and transport. In this paper, we consider perturbing fields that are themselves non-Hermitian, in the form of complex external electric fields, which are realizable as gain/loss gradients. We derive the full set of semiclassical equations of motion and show that the anomalous velocity depends not only on the Berry curvature, but on the entirety of the quantum geometric tensor, including the quantum metric. This quantum metric dependent velocity appears regardless of whether the unperturbed Hamiltonian is Hermitian or not. These analytical results are compared with numerical lattice simulations, which reveal these anomalous terms even in one dimension. Our paper expands the range of phenomena expected to be detectable in experimental setups, which should be realizable in currently available metamaterials and classical wave systems, including mechanical, acoustic, and optical.

Kagome metals show rich competing quantum phases due to geometry frustration, flat bands, many-body effects, and nontrivial topology. Recently, a novel charge density wave (CDW) was discovered deep inside the antiferromagnetic phase of FeGe, attracting intense attention due to the close relation with magnetism. Here, via a scanning tunneling microscope (STM), we find the 2 × 2 CDW in FeGe is very fragile and can be readily disrupted into the initial 1 × 1 phase; small √3 × √3 CDW puddles are found to coexist with the 2 × 2 CDW in as-grown samples, and can also be induced in the intermediate process of CDW disruption, which will eventually transform into the initial 1 × 1 phase. Moreover, an exotic intermediate CDW state and standalone CDW nuclei appear unexpectedly during the disruption process. Our first-principles calculations find equal softening of a flat optical phonon mode in a large momentum region around the CDW wave vector, corresponding to numerous competing CDWs with close energies. This might lead to strong instability of the CDW ground state, responsible for STM observations. Our findings provide more interesting experimental aspects to understand the CDW in FeGe and suggest that FeGe-like kagome metals are ideal platforms for studying the physics of competing CDW instabilities.

Frustrated itinerant magnets often exhibit complex noncollinear or noncoplanar magnetic orders which support topological electronic structures. A canonical example is the anomalous quantum Hall state with a chiral spin order stabilized by electron-spin interactions on a triangular lattice. While a long-range magnetic order cannot survive thermal fluctuations in two dimensions, the chiral order which results from the breaking of a discrete Ising symmetry persists even at finite temperatures. We present a scalable machine learning (ML) framework to model the complex electron-mediated spin-spin interactions that stabilize the chiral magnetic domains in a triangular lattice. Large-scale dynamical simulations, enabled by the ML force-field models, are performed to investigate the coarsening of chiral domains after a thermal quench. While the chiral phase is described by a broken $Z_2$ Ising-type symmetry, we find that the characteristic size of chiral domains increases linearly with time, in stark contrast to the expected Allen-Cahn domain growth law for a nonconserved Ising order-parameter field. The linear growth of the chiral domains is attributed to the orientational anisotropy of domain boundaries. Our work also demonstrates the promising potential of ML models for large-scale spin dynamics of itinerant magnets.

The rare-earth ditellurides are known to form a two-dimensional square lattice where the strong Fermi surface nesting leads to structural modulation. In contrast to charge density waves, the supercell modulation is accompanied by the formation of the periodic Te vacancy network, where the Te deficiency affects the nesting vector, i.e., the supercell size, via tuning the chemical potential. In this work, first-principles electronic-structure calculations for the $\sqrt{5}\times \sqrt{5}$ supercell, which commonly appears in this family of tellurides, reveal interesting electronic and transport properties of the Te defect lattice in ${\mathrm{DyTe}}_{1.8}$. The reconstruction of the Te-deficient square lattice, consisting of a single Te dimer and a pair of Te trimers per unit cell, gives rise to an out-of-plane polarization, whose direction depends on the position of the dimer. This results in various close-in-energy parallel and antiparallel polarization configurations of successive Te layers depending on the dimer positions. We predict that the orientation of the Te dimers, and hence the corresponding structural motifs, can be reversibly switched between two in-plane perpendicular directions under tensile epitaxial strain via a piezoelectric substrate, resulting in a colossal conductivity switching. Furthermore, the Te-dimer orientations result in an asymmetric Fermi surface, which can be confirmed by quantum oscillations measurements.

Transition metal dichalcogenide/graphene (TMDC/Gr) heterojunction devices exhibit significantly higher photoresponsivity compared to TMDC devices alone, making them promising for optoelectronic applications. However, experiments demonstrated that graphene cannot prolong the photogenerated carrier lifetime of TMDC/Gr heterojunctions and, further, that the high density of sulfur vacancies in TMDCs complicates photogenerated carrier dynamics, leaving the underlying physical mechanism behind the high photoresponsivity unclear. Herein, we investigate photogenerated carrier transfer and recombination of $\mathrm{Mo}{\mathrm{S}}_2$/Gr and $\mathrm{W}{\mathrm{S}}_2$/Gr heterojunctions through nonadiabatic molecular dynamics simulations. Instead of conventional speculation that sulfur vacancies of TMDCs store photogenerated carriers to enhance optoelectronic performance, we find that the hybridization between defect states of TMDC and Dirac points of graphene induces fast photoelectron transfer from TMDC to graphene, promoting high carrier gain in TMDC/Gr heterojunctions. Fast carrier transfer of heterojunctions derives from the excitation of low-frequency in-plane phonon modes. Meanwhile, graphene does not drastically reduce the photogenerated carrier lifetime of TMDC/Gr heterojunctions. Therefore, the faster photogenerated electrons transfer and long carrier lifetime lead to photogenerated carrier gain, resulting in superior optoelectronic performance of TMDC/Gr heterojunctions. This study provides a comprehensive understanding of photogenerated carrier dynamics in TMDC/Gr heterojunctions, laying the foundation for design of high-performance TMDC optoelectronic devices.

Lattice gauge theories (LGTs) were introduced in 1974 by Wilson to study quark confinement. These models have been shown to exhibit (de)confined phases, yet it remains challenging to define experimentally accessible order parameters. Here we propose percolation-inspired order parameters (POPs) to probe confinement of dynamical matter in ${\mathbb{Z}}_2$ LGTs using electric field basis snapshots accessible to quantum simulators. We apply the POPs to study a classical ${\mathbb{Z}}_2$ LGT and find a confining phase up to temperature $T=\infty$ in two dimensions (critical $T_c$, i.e., finite-$T$ phase transition, in three dimensions) for any nonzero density of ${\mathbb{Z}}_2$ charges. Further, using quantum Monte Carlo we demonstrate that the POPs reproduce the square lattice Fradkin-Shenker phase diagram at $T=0$ and explore the phase diagram at $T>0$. The correlation length exponent coincides with the one of the three-dimensional Ising universality class and we determine the POP critical exponent characterizing percolation. Our proposed POPs provide a geometric perspective of confinement and are directly accessible to snapshots obtained in quantum simulators, making them suitable as a probe for quantum spin liquids.

The properties of semiconductors, insulators, and photonic crystals are defined by their electronic or photonic bands and the gaps between them. When the material is disordered, Lifshitz tails appear: these are localized states that bifurcate from the band edge and act to effectively close the band gap. While Lifshitz tails are well understood when the disorder is spatially uncorrelated, there has been recent interest in the case of hyperuniform disorder, i.e., when the disorder fluctuations are highly correlated and approach zero at long length scales. In this paper, we analytically solve the Lifshitz tail problem for hyperuniform systems using a path-integral and instanton approach. We find the functional form of the density of states as a function of the energy difference from the band edge. We also examine the effect of hyperuniform disorder on the density of states of Weyl semimetals, which do not have a band gap.

Surface electric dynamics influences the quantum coherence of near-surface spin centers (i.e., $T_1$ and $T_2$ times) through spatial and temporal fluctuations of the surface charge density and the electrostatic potential at the crystal surface that are described by the spectral noise density $S\left(\omega \right)$. We find that the electric noise's $S\left(\omega \right)$ dependence on the depth $d$ of a spin center below the surface follows a power law $d^{-\gamma }$ not solely determined by whether the surface charges move as localized pointlike charges (monopoles) or dipoles. The exponent $\gamma$ instead reveals the structure of the surface charge and surface potential correlation functions. For surface dynamics originating from diffusion, the spin center's relaxation and decoherence times exhibit a temporal crossover near the correlation time of the fluctuators and thus provide a quantitative fingerprint of the diffusive behavior of charged particles at surfaces.

The low-energy quasiparticle states in the mixed state of most superconductors remain Bloch waves due to the presence of supercurrent around vortex cores. In contrast, the Weyl superconductor (WSC) may display Dirac-Landau levels in the presence of a vortex lattice. Here, we investigate the Landau-level (LL) structure and optical conductivity in the mixed state of a generic WSC using a heterostructure model, where the tilt of the Bogoliubov-Weyl (BW) cones can be tuned, yielding either type-I (undertilted) or type-II (overtilted) cones. We find that, in a magnetic field, the tilted type-I BW cone in the mixed state may exhibit squeezed LLs with reduced spacings. On the other hand, the spectrum of type-II cones shows a dependence on the angle between the magnetic field and the tilt direction; LL quantization is only possible if the projection of the tilt in the plane perpendicular to the magnetic field does not exceed the Fermi velocity. For zero tilt the optical conductivity in the mixed state of the WSC shows peaks only at photon frequencies ${\omega }_n\propto \sqrt{n}+\sqrt{n+1}$, with a linear background. However, the tilt of BW cones results in the emergence of optical transitions beyond the usual dipolar ones. For type-II BW cones, unique intraband transition conductivity peaks emerge at low frequency, which can serve as an indicator to distinguish between type-I and type-II BW cones in WSCs.

Identifying the superconducting gap symmetry and topological signatures in the putative spin-triplet superconductor ${\mathrm{UTe}}_2$ is an important issue. Especially, a smoking-gun detection scheme for Majorana surface states hallmarking topological superconductivity in ${\mathrm{UTe}}_2$ is still lacking. In this study, we examine the surface spin susceptibility of ${\mathrm{UTe}}_2$ with a particular focus on the contribution of the surface states. We find that Majorana surface states contribute significantly to the surface spin susceptibility, and give rise to an Ising-like anisotropy and anomalous enhancement in the surface spin susceptibility. We calculate the surface spin susceptibility as well as the local density of states using the recursive Green's function method and examine the anisotropy of the surface spin susceptibility in terms of the topological surface states and symmetry for all irreducible representations of odd-parity pairing states. Our results indicate that the Ising anisotropy and the anomalous enhancement are attributed to the Majorana surface state protected by the crystalline symmetry. These findings suggest the possibility of detecting the Majorana surface state via magnetic measurements.

We propose an optomechanical scheme for reaching quantum entanglement in vibration polaritons. The system involves $N$ molecules, whose vibrations can be fairly entangled with plasmonic cavities. We find that the vibration-photon entanglement can exist at room temperature and is robust against thermal noise. We further demonstrate the quantum entanglement between the vibrational modes through the plasmonic cavities, which shows a delocalized nature and an incredible enhancement with the number of molecules. Moreover, we find that the vibration-vibration entanglement reaches its maximum when the number of molecules in the two collective modes is equal, and it can be proved analytically. The underlying mechanism for the entanglement generation is attributed to the strong vibration-cavity coupling which possesses collectivity. Our results provide a molecular optomechanical scheme which offers a promising platform for the study of noise-free quantum resources and macroscopic quantum phenomena.