The subtle interplay of band topology and symmetry broken phase, induced by electron correlations, has immense contemporary relevance and potentially offers novel physical insights. Here, we provide evidence of possible charge density wave (CDW) in bulk ${\mathrm{Y}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ for $T<10$ K, and the Weyl semimetal (WSM) phase at higher temperatures. We observe the following characteristic properties of the CDW phase: (i) current induced nonlinear conductivity with negative differential resistance at low temperature, (ii) low-frequency Debye-like dielectric relaxation at low temperature with a large dielectric constant $\sim {10}^8$, and (iii) an anomaly in the temperature dependence of the thermal expansion coefficient. The WSM phase at higher temperature is analyzed using the dc and ac transport measurements, which show an inductive response at low frequencies. More interestingly, we show that by reducing the crystallite size, the low-temperature CDW phase can be eliminated leading to the restoration of the WSM phase.

A rank-2 toric code (R2TC) Hamiltonian in two dimensions can be constructed as a Higgsed descendant of rank-2 U(1) lattice gauge theory. As noted by the authors recently [Y.-T. Oh, J. Kim, E.-G. Moon, and J. H. Han, Phys. Rev. B 105, 045128 (2022)] the quasiparticles in that model show unusual braiding statistics that depends on the initial locations of the particles which participate in the braiding. We show that this kind of statistical phase captures the total dipole moment of quasiparticles encompassed in the braiding, in contrast to the conventional anyonic braiding seeing the total charge. An Aharonov-Bohm interpretation of such dipolar braiding statistics is made in terms of emergent, rank-1 vector potentials that are built out of the underlying rank-2 gauge fields. Pertinent field theories of the quasiparticle dynamics in the R2TC are developed, and the accompanying conservation laws derived. A dipolar BF theory of the rank-2 gauge fields is constructed and shown to correctly capture the dipolar braiding statistics, in contrast to the conventional BF theory capturing the monopolar braiding statistics of anyons in the rank-1 toric code.

Herein, we provide comprehensive understanding of the photoinduced thermal force between the metal coated tip and organic sample in the context of surface and volume responses where the photoinduced dipole force is typically small enough. Surface response is caused by near-field tip-enhanced thermal expansion, whereas volume response is generated via far-field bulk thermal expansion. In this paper, we quantitatively characterize the two responses, which are mediated via interatomic tip-sample forces in photoinduced force microscopy. The major contributors to the surface sensitivity are the field enhancement at the tip end, the small oscillation of the driving amplitude, and the noncontact/gentle tapping force. Other possible parameters such as the repetition period, and the pulse width of the light source are also discussed. This paper offers insights into the quantitative analysis of surface-sensitive and volume-sensitive spectroscopic nanoimaging, which is of great interest to the scientific community.

Silicon photoluminescence and lasing have been issues critical to breaking through bottlenecks in the understanding of luminescence mechanisms. Unfortunately, long-standing disputes about the exciton recombination mechanism and fluorescence lifetime remain unresolved, especially about whether silicon nanocrystals (Si NCs) can realize fast direct band gap–like optical transitions. Here, using ground-state and excited-state density functional theory (DFT), we obtained intrinsic phonon-free optical transitions at sizes from ${\mathrm{Si}}_{22}$ to ${\mathrm{Si}}_{705}$, showing that very small Si NCs can realize a strong direct optical transition. Orbital labeling results show that this rapid transition does not come from the $\mathbf{\Gamma }\text{−}\mathbf{\Gamma }$-like transition, contrary to the conclusions from the effective mass approximation, and that $\mathbf{\Gamma }\text{−}\mathit{X}$ mixing leads to a quasidirect band gap. This anomalous transition is particularly intense with decreasing size (or enhancement of quantum confinement). By investigating electron and hole distributions generated in the optical transition, localized state induced enhanced emission (LIEE) in Si NCs was proposed. Quantum confinement distorts the excited-state electron spatial distribution by localizing Bloch waves into the NC core, resulting in increased hole and electron overlap, thus inducing a fast optical process. This work resolves important debates and proposes LIEE to explain the anomalous luminescence—a phase transition from weak (or no) luminescent state to strong optical transition, which will aid attempts at realizing high radiative rate NC materials and application-level Si lasers.

The band structure of a monolayer ${\mathrm{MoS}}_2$ comprises spin-split subbands, owing to the mutual presence of broken inversion symmetry and strong spin-orbit coupling. In the conduction band, spin-valley coupled subbands cross each other at finite momenta, and they are valley degenerate. When exposed to surface acoustic waves, the emerging strain-induced effective magnetic fields can give rise to spin-flip transitions between the spin-split subbands in the vicinity of the subband crossing point, resulting in the emergence of a spin-acoustic resonance and the acoustoelectric current. Resonance peaks occur at acoustic frequencies of gigahertz range in the vicinity of the subband crossing point. An external magnetic field breaks the valley degeneracy, resulting in the valley-selective splitting of spin-acoustic resonances, both in surface acoustic wave absorption and acoustoelectric current.

Many topological band crossings (BCs) have been predicted efficiently utilizing the symmetry properties of wave-functions at high-symmetry points. Among various BCs, the so-called hourglass BCs (with the low-energy excitations dubbed as hourglass fermions) are fascinating since they can be guaranteed to exist under specific symmetry conditions even without realistic calculations. Such novel property renders the theoretical prediction on magnetic topological metals with hourglass BC (which can be Weyl point, Dirac point, lying in nodal loop, and so on) independent on the calculation methods and only determined by the symmetry of crystal and magnetic structure, namely, the magnetic space group (MSG). To date, there have been no magnetic material verified with hourglass fermions. Here we first list all symmetry conditions that allow hourglass BCs in the 1651 MSGs and 528 magnetic layer groups (MLGs) with spin-orbital coupling (SOC): Only 331 MSGs and 53 MLGs can host hourglass BCs. Among these results, the essential hourglass BCs are highlighted, whose MSGs are then applied to predict hundreds of magnetic materials from the MAGNDATA magnetic materials database and first-principles calculations in the frame of LDA + SOC + U verify the hourglass BCs for different values of $U$. We take ${\mathrm{CsMn}}_2{\mathrm{F}}_6$, synthesized recently with a distorted pyrochlore structure to illustrate the hourglass band structure in detail which is very clear around the Fermi level and the topologically protected surface drumhead states of (100) surface are found to spread over more than one half surface Brillouin zone and only appear in a narrow energy window ($\sim 30$ meV), which could induce intriguing stability by prominent electronic correlation. The symmetry-guaranteed existence of hourglass fermion in the predicted magnetic materials is expected to be applied in manipulating band topology by applying appropriate external fields moving the hourglass BC.

Within many-body perturbation theory, Hedin's formalism offers a systematic way to iteratively compute the self-energy $\mathrm{\Sigma }$ of any dynamically correlated interacting system, provided one can evaluate the interaction vertex $\mathrm{\Gamma }$ exactly. This is, however, impossible, in general, for it involves the functional derivative of $\mathrm{\Sigma }$ with respect to the Green's function. Here, we analyze the structure of this derivative, splitting it into four contributions and outlining the type of quasiparticle interactions that each of them generate. Moreover, we show how, in the implementation of self-consistency, the action of these contributions can be classified into two: A quantitative renormalization of previously included interaction terms and the inclusion of qualitatively distinct interaction terms through successive functional derivatives of $\mathrm{\Gamma }$ itself. Implementing this latter type of self-consistency can extend the validity of perturbative approximations based on Hedin's equations toward the high interaction limit, as we show in the example of the Hubbard dimer. Our analysis also provides a unifying perspective on the perturbation theory landscape, showing how the T-matrix approach is completely contained in Hedin's formalism.

Introducing internal degrees of freedom in the description of topological insulators has led to a myriad of theoretical and experimental advances. Of particular interest are the effects of periodic perturbations, either in time or space, as they considerably enrich the variety of electronic responses, with examples such as Thouless's charge pump and its higher-dimensional cousins, or higher-order topological insulators. Here, we develop a semiclassical approach to transport and accumulation of general spinor degrees of freedom, such as physical spin, valley, or atomic orbits, in adiabatically driven, weakly inhomogeneous insulators of dimensions 1, 2, and 3, under external electromagnetic fields. Specifically, we derive the spinor current and density up to third order in the spatiotemporal modulations of the system and relate the induced responses to geometrical and topological objects—the spinor-Chern fluxes and numbers—defined over the higher-dimensional phase space of the system, i.e., its combined position-momentum-time coordinates. Furthermore, we provide a connection between our semiclassical analysis and the modern theory of multipole moments by introducing spinor analogs of the electric dipole, quadrupole, and octupole moments. The results are showcased in concrete tight-binding models where spinor transport and accumulation are calculated analytically.

To observe a Weyl point (WP) possessing the maximum charge number and to search for single-pair WPs in realistic materials are two challenges in topology physics. Here, we have successfully designed an acoustic crystal to realize single-pair WPs with the maximum charge number. Protected by time-reversal symmetry and point group symmetries, a WP with charge $-4$ and another WP with charge $+4$ are located at two different high-symmetry points of an acoustic crystal, without any other kinds of WPs. Moreover, the single-pair WPs obtained here are designed by the simplest two-band mode, and the related quadruple-helicoid surface arcs can be observed clearly in experiments, since they are not hybridized by other kinds of WPs. Our theoretical work puts forward an ideal acoustic crystal to study the topological properties of single-pair WPs with the maximum charge number.

Mixed iodine-bromine perovskites used in solar cells undergo below a critical temperature an intrinsic demixing into phases with different iodine-bromine compositions. In addition, under illumination they show nucleation of an iodine-rich phase. We predict from thermodynamic considerations that in mixed iodine-bromine perovskites like $\mathrm{MAPb}{\left({\mathrm{I}}_{1-x}{\mathrm{Br}}_x\right)}_3$ the interplay of these effects can lead to coexistence of a bromine-rich, iodine-rich, and nearly iodine-pure nucleated phase. This three-phase coexistence occurs in a region in the composition-temperature phase diagram near the critical point for intrinsic demixing. We investigate the hysteresis in the evolution of this coexistence when temperature or illumination intensity are cycled. Depending on the particular way the coexistence is established, nearly iodine-pure nuclei should form either in the iodine-rich phase only or both in the bromine-rich and iodine-rich phases. Experimental verification of this fundamentally novel type of light-tunable three-phase coexistence should be possible by a combination of absorption and photoluminescence experiments.

The modeling of the intraband dynamics of conduction electrons in gold induced by a femtosecond laser pulse is addressed. Current approaches, based on the numerical resolution of the Boltzmann equation, are only able to describe electron excitation $\mathit{or}$ relaxation processes. In the present paper, within a single formalism, our kinetic model accounts quantitatively for both excitation and relaxation processes, i.e., photon absorption, thermal conductivity, and electron-phonon coupling coefficient. We suggest that such an approach can only be built by including Umklapp processes in the description of electron collisions. In addition to normal electronic collisional processes with phonons and other electrons, the present theoretical description further includes these mechanisms: (i) a conduction electron can be scattered in the second Brillouin zone after a collision, contributing significantly to the photon absorption, and (ii) the influence of the discrete and periodic nature of the lattice on electron collisions is considered, enabling in particular the coupling between electron and transverse phonons. A good agreement of predictions of this unified modeling with experimental observations for absorption and energy transfer from electrons to the lattice is obtained. We have investigated the influence of the Umklapp processes on the imaginary part of the dielectric function, the electron-phonon coupling parameter, and the transient shape of the electron energy distribution.

Topological phase in magnetic materials yields a quantized contribution to the Hall effect known as the topological Hall effect, which is often caused by skyrmions, with each skyrmion creating a magnetic flux quantum $\pm h/e$. The control and understanding of topological properties in nanostructured materials is the subject of immense interest for both fundamental science and technological applications, especially in spintronics. In this work, the electron-transport properties and spin structure of exchange-coupled cobalt nanoparticles with an average particle size of 13.7 nm are studied experimentally and theoretically. Magnetic and Hall-effect measurements identify topological phase transitions in the exchange-coupled cobalt nanoparticles and were used to discover a qualitatively new type of hysteresis in the topological Hall effect—namely, Berry-phase hysteresis. Micromagnetic simulations reveal the origin of the topological Hall effect—namely, the chiral domains, with domain-wall chirality quantified by an integer skyrmion number. These spin structures are different from the skyrmions formed due to Dzyaloshinskii–Moriya interactions in B20 crystals and multilayered thin films, and caused by cooperative magnetization reversal in the exchange-coupled cobalt nanoparticles. An analytical model is developed to explain the underlying physics of Berry-phase hysteresis, which is strikingly different from the iconic magnetic hysteresis and constitutes one aspect of 21st-century reshaping of our view on nature at the borderline of physics, chemistry, mathematics, and materials science.

Magnonics has been receiving significant attention in magnetism and spintronics because of its premise for devices using spin current carried by magnons, the quanta of spin-wave excitations of macroscopically ordered magnetic media. Although magnonics has a clear energywise advantage over conventional electronics due to the absence of Joule heating, inherent magnon-magnon interactions give rise to a finite lifetime of magnons, which has been hampering the efficient realization of magnonic devices. To promote magnonics, it is imperative to identify the delocalized magnon modes that are minimally affected by magnon-magnon interactions and thus possess a long lifetime and use them to achieve efficient magnon transport. Here, we suggest that quasicrystals may offer the solution to this problem via critical magnon modes that are neither extended nor localized. We find that a critical magnon exhibits fractal characteristics that are absent in conventional magnon modes in regular solids such as a unique power-law scaling and a self-similar distribution of distances showing perfect magnon transmission. Moreover, critical magnons have longer lifetimes compared to the extended ones in a periodic system, by suppressing the magnon-magnon interaction decay rate. Such an enhancement of the magnon stability originates from the presence of the quasiperiodicity and intermediate localization behavior of critical magnons. Thus, we offer the utility of quasicrystals and their critical spin-wave functions in magnonics as unique fractal transport characteristics and enhanced stability.

The superconducting symmetry of ${\mathrm{Sr}}_2\mathrm{Ru}{\mathrm{O}}_4$ remains a puzzle. Time-reversal symmetry breaking $d_{x^2-y^2}+i{g}_{xy(x^2-y^2)}$ pairing has been proposed for reconciling multiple key experiments. However, its stability remains unclear. In this work, we theoretically study the superconducting instabilities in ${\mathrm{Sr}}_2\mathrm{Ru}{\mathrm{O}}_4$, including the effects of spin-orbit coupling (SOC), in the presence of both local and longer-range interactions within a random-phase approximation. We show that the inclusion of second-nearest-neighbor repulsions, together with nonlocal SOC in the $B_{2g}$ channel or orbital-anisotropy of the nonlocal interactions, can have a significant impact on the stability of both $d_{x^2-y^2}$- and $g$-wave pairing channels. We analyze the properties, such as Knight shift and spontaneous edge current, of the realized $d_{x^2-y^2}+ig, s^{\prime }+i{d}_{xy}$, and mixed helical pairings in different parameter spaces, and we find that the $d_{x^2-y^2}+ig$ solution is in better agreement with the experimental data.

Preferred orientations of the macroscopic high-spin (HS) low-spin (LS) interfaces appearing in spin transition molecular crystals during their phase transitions are explained through the study of a generalized 3D version of the electroelastic model accounting for an anisotropic change of the lattice parameters, $a, b$, and $c$ at the transition. The investigations are performed at 0 K by analyzing the energy landscape of a lattice made of two HS and LS phases, separated by a tilted interface with variable orientation, $\theta$ as a function of the anisotropy ratio, $\lambda =\frac{\mathrm{\Delta }b}{\mathrm{\Delta }a}=\frac{\mathrm{\Delta }b}{\mathrm{\Delta }c}$, where $\mathrm{\Delta }x=x_{\mathrm{HS}}-x_{\mathrm{LS}}$ is the lattice misfit along $x(=a,b,c)$ direction between HS and LS states. For large $\lambda <0$, the $\theta$ dependence of the relaxed total elastic energy depicts a symmetric double-well structure with two stable positions, ${\theta }_{\min }$, and an unstable orientation ${\theta }_{\max }={90}^{\circ }$. Beyond a critical value, ${\lambda }_C^{-}<0$, only one minimum subsists at $\theta ={90}^{\circ }$, thus recalling the behavior of an order parameter of a 2nd order phase transition. On increasing $\lambda$, this minimum survives until a second threshold value ${\lambda }_C^{+}>0$ above which, the elastic energy recovers a double well configuration with two new preferential interface orientations, highlighting the existence of a re-entrant phenomenon. We demonstrate that the behavior of ${\theta }_{\min }$ versus $\lambda$ follows the same universality class as that of a second-order phase transition, for which we calculate the critical exponents $\beta$ and $\nu$ through a finite size scaling analysis. Overall, these investigations reveal that in switchable molecular solids with anisotropic unit cell deformation between the LS and HS states, there exists a stress-free interface orientation ensuring their integrity upon a large number of thermal cycles or loads during their practical utilization

${AX}_2$-type compounds exhibit a rich and complex series of phase transitions under compression. The high-pressure polymorphism of rutile-type ${AX}_2$ difluorides can be characterized by a typical sequence from $\mathrm{rutile}\mathrm{type}\to \mathrm{Ca}{\mathrm{Cl}}_2\mathrm{type}\to \mathrm{HP}\text{−}\mathrm{Pd}{\mathrm{F}}_2\mathrm{type}\to \mathrm{cotunnite}\mathrm{type}$. Relative to dioxides, the reduced valence and ionic radius of the ${\mathrm{F}}^{-}$ anion in rutile-type ${\mathrm{MgF}}_2$ (sellaite) results in lowered transition pressures, making it a useful analog for ${\mathrm{SiO}}_2$ and other dioxides. In this work, ${\mathrm{MgF}}_2$ single crystals were shock-compressed along the [100] direction to 24 to 120 GPa using plate impact techniques, and wave profiles were measured using laser interferometry. At low stresses (24 to 44 GPa), we observe features consistent with an elastic–inelastic response, followed by a phase transformation. Peak stress-density states in this stress range are consistent with those expected for the modified fluorite-type (HP-${\mathrm{PdF}}_2$) phase or a related structure. At higher stresses (69 to 91 GPa), we observe a two-wave structure with peak stress-density states consistent with transformation to a denser and likely more highly coordinated phase such as the cotunnite-type structure. At 120 GPa, only a single wave structure is observed, indicating that the wave profile features observed at lower stresses are overdriven.

We prepare two-dimensional states generated by shallow circuits composed of (1) one layer of the two-qubit controlled-Z (CZ) gate or (2) a few layers of the two-qubit random Clifford gate. After measuring all of the bulk qubits, we study the entanglement structure of the remaining qubits on the one-dimensional boundary. In the first model, we observe that the competition between the bulk $X$ and $Z$ measurements can lead to an entanglement phase transition between an entangled volume law phase and a disentangled area law phase. We numerically evaluate the critical exponents and generalize this idea to other qudit systems with a local Hilbert space dimension larger than 2. In the second model, we observe the entanglement transition by varying the density of the two-qubit gate in each layer. We give an interpretation of this transition in terms of the random bond Ising model in a similar shallow circuit composed of random Haar gates.

Magnetic topological materials, in which strong correlations between magnetic and electronic properties of matter, give rise to various exotic phenomena such as the anomalous Hall effect (AHE), topological Hall effect (THE), and skyrmion lattice. Here, we report on the electronic, magnetic, and topological properties of ${\mathrm{Mn}}_{3-\mathit{x}}{\mathrm{Fe}}_{\mathit{x}}\mathrm{Sn}$ single crystals ($\mathit{x}=0$, 0.25, and 0.35). Low-temperature magnetic properties have been significantly changed with Fe doping. Most importantly, we observe that large uniaxial magnetocrystalline anisotropy that is induced by the Fe doping in combination with competing magnetic interactions at low temperature produce nontrivial spin texture, leading to a large topological Hall effect in the doped systems at low temperatures. Our studies further show that the topological properties of ${\mathrm{Mn}}_{3-\mathit{x}}{\mathrm{Fe}}_{\mathit{x}}\mathrm{Sn}$ are very sensitive to the Fe doping.

Bi-isotropic media constitute a proper scenario for scrutinizing optical effects stemming from magnetoelectric parameters. Chiral magnetic current is a macroscopic effect arising from the chiral magnetic effect that enriches the phenomenology of a classical dielectric medium. This paper examines the optical aspects of bi-isotropic media in the presence of Ohmic and magnetic conductivity. The full isotropic scenario manifests circular birefringence described by a dispersive rotatory power that changes sign at a given frequency. For a bi-isotropic medium with antisymmetric magnetic conductivity, an intricate dispersive rotatory power is attained, supplied with sign reversal as well. This scenario occurs in the absence or presence of Ohmic conductivity. It also indicates a handedness reversion of the medium, an unusual property in dielectrics, which may work as a signature of bi-isotropic media supporting chiral magnetic current.

We study theoretically electron dynamics of a graphene nanoring placed in the field of an ultrashort optical pulse. We describe the graphene nanoring within an effective model with infinite mass boundary conditions. For an optical pulse with a duration of just a few femtoseconds, the electron dynamics is coherent and is described by a time-dependent Schrödinger equation. If the optical pulse is circularly polarized, then two valleys of graphene are populated differently, resulting in a finite valley polarization of the system after the pulse. Such a valley polarization is a unique property of graphene nanoscale systems, while for a graphene monolayer, a circularly polarized pulse does not produce any valley polarization. The valley polarization of the graphene nanoring depends on parameters of the system, such as inner and outer radii. With the system's size increasing, the valley polarization monotonically decreases, converging to its zero value for the infinite graphene monolayer.