We investigate ultrafast coherent quantum dynamics of undoped $\mathrm{Ba}\mathrm{Bi}{\mathrm{O}}_3$ driven by a strong laser pulse. Our calculations demonstrate that in a wide range of radiation frequencies and intensities the system undergoes a transient change from the insulating to the metallic state, where the charge density wave and the corresponding energy spectrum gap vanish. The transition takes place on the ultrafast timescale of tens femtoseconds, comparable to the period of the corresponding lattice vibrations. The dynamics are determined by a complex interplay of the particle-hole excitation over the gap and of the tunneling through it, giving rise to the highly nontrivial time evolution which comprises high harmonics and reveals periodic reappearance of the gap. The time evolution is obtained by solving the dynamical mean-field theory equations with the realistic parameters for the system and radiation. Results are summarized in the phase diagram, helpful for a possible experimental setup to achieve a dynamical control over the conduction state of this and other materials with the similarly strong electron-phonon interaction.

Quantum computation promises an exponential speedup of certain classes of classical calculations through the preparation and manipulation of entangled quantum states. So far, most molecular simulations on quantum computers, however, have been limited to small numbers of particles. Here we prepare a highly entangled state on a 53-qubit IBM quantum computer, representing 53 particles, which reveals the formation of an exciton condensate of photon particles and holes. While the experimental realization of ground state exciton condensates remained elusive for more than 50 years, such condensates were recently achieved for electron-hole pairs in graphene bilayers and metal chalcogenides. Our creation of ground state photon condensates has the potential to further the exploration of exciton condensates, and this novel preparation may play a role in realizing efficient room-temperature energy transport.

The study of superconducting materials that also possess nontrivial correlations or interactions remains an active frontier of condensed matter physics. $\mathrm{Nb}{\mathrm{Se}}_2$ belongs to this class of superconductors and recent research has focused on the two-dimensional properties of this layered, superconducting material. Often, electronic transport measurements are used to investigate the superconducting properties of these materials, hence it is key to obtain a thorough understanding of transport in these systems. We investigate the transition between the superconducting and resistive state using radio-frequency AC excitation. Despite being a single piece of superconductor, the devices show novel dynamical Josephson effects behavior reminiscent of the AC Josephson effect observed in Josephson junctions. Detailed analysis uncovers the origin of this effect, identifying two types of vortex motion that categorize the transition to the normal state. Our results shed light on the nature of superconductivity in this material, unearthing exotic phenomena by exploiting nonequilibrium superconducting effects in atomically thin materials.

Quantum state transformations that are robust to experimental imperfections are important for applications in quantum information science and quantum sensing. Counterdiabatic (CD) approaches, which use knowledge of the underlying system Hamiltonian to actively correct for diabatic effects, are powerful tools for achieving simultaneously fast and stable state transformations. Protocols for CD driving have thus far been limited in their experimental implementation to discrete systems with just two or three levels, as well as bulk systems with scaling symmetries. Here, we extend the tool of CD control to a discrete synthetic lattice system composed of as many as nine sites. Although this system has a vanishing gap and thus no adiabatic support in the thermodynamic limit, we show that CD approaches can still give a substantial improvement in fidelity over naive, fast protocols.

Anisotropic dipole-dipole interaction often plays a key role in biological, soft, and complex matter. For it to induce nontrivial order in the system, there must be additional repulsive interactions or external potentials involved that partially or completely fix the positions of the dipoles. These positions can often be represented as an underlying lattice on which dipole interaction induces orientational ordering of the particles. On lattices in the Euclidean plane, dipoles have been found to assume different ground-state configurations depending on the lattice type, with a global ordering in the form of a macrovortex being observed in many cases. A similar macrovortex configuration of dipoles has recently been shown to be the sole ground state for dipoles positioned on spherical lattices based on solutions of the Thomson problem. At the same time, no symmetric configurations have been observed, even though the positional order of Thomson lattices exhibits a high degree of symmetry. Here, we show that a different choice of spherical lattices based on Caspar-Klug construction leads to ground states of dipoles with various degrees of symmetry, including the icosahedral symmetry of the underlying lattice. We analyze the stability of the highly symmetric metastable states, their symmetry breaking into subsymmetries of the icosahedral symmetry group, and present a phase diagram of symmetries with respect to lattice parameters. The observed relationship between positional order and dipole-induced symmetry breaking hints at ways of fine-tuning the structure of spherical assemblies and their design.

We report results of a microscopic calculation of a second-order phase transition into a state-breaking time-reversal and translational invariance along pair-breaking edges of $d$-wave superconductors. By solving a tight-binding model through exact diagonalization with the Bogoliubov–de Gennes method, we find that such a state with current loops having a diameter of a few coherence lengths is energetically favorable below $T^{*}$ between 10%–20% of $T_{\mathrm{c}}$ of bulk superconductivity, depending on model parameters. This extends our previous studies of such a phase crystal within the quasiclassical theory of superconductivity, and shows that the instability is not qualitatively different when including a more realistic band structure and the fast oscillations on the scale of the Fermi wavelength. Effects of size quantization and Friedel oscillations are not detrimental. We also report on a comparison with quasiclassical theory with the Fermi surfaces extracted from the tight-binding models used in the microscopic calculation. There are quantitative differences in for instance the value of $T^{*}$ between the different models, but we can explain the predicted transition temperature within each model as due to the different spectral weights of zero-energy Andreev bound states and the resulting gain in free energy by breaking time-reversal and translational invariance below $T^{*}$.

We analyze the stability of time-reversal ($\mathcal{T}$) and lattice fourfold ($C_4$) symmetry breaking three-dimensional higher-order topological (HOT) Dirac semimetals (DSMs) and the associated one-dimensional hinge modes in the presence of random pointlike charge impurities. Complementary real space numerical and momentum space renormalization group (RG) analyses suggest that a HOTDSM, while being a stable phase of matter for sufficiently weak disorder, undergoes a continuous quantum phase transition into a trivial metal at finite disorder. However, the corresponding critical exponents (numerically obtained from the scaling of the density of states) are extremely close to the ones found in a dirty, but first-order DSM that on the other hand preserves $\mathcal{T}$ and $C_4$ symmetries, and support two Fermi arc surface states. This observation suggests an emergent superuniversality (insensitive to symmetries) in the entire family of dirty DSMs, as also predicted by a leading-order RG analysis. As a direct consequence of the bulk-boundary correspondence, the hinge modes in a system with open boundaries gradually fade away with increasing randomness, and completely dissolve in the trivial metallic phase at strong disorder.

Odd-frequency $\left(\mathrm{odd}\text{−}\omega \right)$ electron pair correlations naturally appear at the interface between BCS superconductors and other materials. The detection of $\mathrm{odd}\text{−}\omega$ pairs, which are necessarily nonlocal in time, is still an open problem. The main reason is that they do not contribute to static measurements described by time-local correlation functions. Therefore, dynamical measurements, which depend on nonlocal time correlations, are suitable for detecting these pairs. In this work, we study the signatures of $\mathrm{odd}\text{−}\omega$ pairs in the supercurrent noise through a weak link between two superconductors at different superconducting phases. We show that the finite-frequency current noise can be decomposed into three different contributions coming from even-frequency $\left(\mathrm{even}\text{−}\omega \right),$ $\mathrm{odd}\text{−}\omega$ pair amplitudes, and electron-hole correlation functions. $\mathrm{Odd}\text{−}\omega$ pairing, which is interlead (between electrons at different sides of the junction), provides a positive contribution to the noise, becoming maximal at a superconducting phase difference of $\pi$. In contrast, intralead $\mathrm{even}\text{−}\omega$ pair amplitude tends to reduce the noise, except for a region close to $\pi$ controlled by the transmission of the junction.

Motivated by the superconductivity of $M_x{\mathrm{Bi}}_2{\mathrm{Se}}_3$, we study topological excitations in a nematic superconductor using Ginzburg-Landau theory. An isolated excitation at low field is shown to be either a distorted phase vortex or a tightly-bounded pair of half quantum vortices. Close to upper critical field $H_{c2}$, the vortex lattice is shown to be always hexagonal in the extreme type-II limit. Due to the different symmetries of the vortex lattice states, at least two phase transitions must take place when the external field is lowered from $H_{c2}$.

The advent of topological phases of matter revealed a variety of observed boundary phenomena, such as chiral and helical modes found at the edges of two-dimensional (2D) topological insulators. Antichiral states in 2D semimetals, i.e., copropagating edge modes on opposite edges compensated by a counterpropagating bulk current, are also predicted, but, to date, no realization of such states in a solid-state system has been found. Here, we put forward a procedure to realize antichiral states in twisted van der Waals multilayers, by combining the electronic Dirac-cone spectra of each layer through the combination of the orbital moiré superstructure, an in-plane magnetic field, and interlayer bias voltage. In particular, we demonstrate that a twisted van der Waals heterostructure consisting of graphene/two layers of hexagonal boron nitride $\left[{\left(\mathrm{hBN}\right)}_2\right]/\mathrm{graphene}$ will show antichiral states at in-plane magnetic fields of 8 T, for a rotation angle of $0.2^{\circ }$ between the graphene layers. Our findings engender a controllable procedure to engineer antichiral states in solid-state systems, as well as in quantum engineered metamaterials.

We investigate the interactions between exciton-polaritons in $N$ two-dimensional semiconductor layers embedded in a planar microcavity. In the limit of low-energy scattering, where we can ignore the composite nature of the excitons, we obtain exact analytical expressions for the spin-triplet and spin-singlet interaction strengths, which go beyond the Born approximation employed in previous calculations. Crucially, we find that the strong light-matter coupling enhances the strength of polariton-polariton interactions compared to that of the exciton-exciton interactions, due to the Rabi coupling and the small photon-exciton mass ratio. We furthermore obtain the dependence of the polariton interactions on the number of layers $N$, and we highlight the important role played by the optically dark states that exist in multiple layers. In particular, we predict that the singlet interaction strength is stronger than the triplet one for a wide range of parameters in most of the currently used transition metal dichalcogenides. This has consequences for the pursuit of polariton condensation and other interaction-driven phenomena in these materials.

Density functional theory (DFT) calculations are used to study the magnetic and ferroelectric properties of ${\mathrm{Sr}}_{1-x}{\mathrm{Ba}}_x\mathrm{Mn}{\mathrm{O}}_3$, with focus on $x=0.5$, under isotropic volume expansion or compression and biaxial strain. Our results indicate that, unexpectedly, Ba substitution alters the electronic structure in a way that, at fixed lattice parameter, notably enhances the interatomic magnetic exchange interactions. However, increasing Ba content also causes a volume expansion which tends to weaken these interactions, leading to a net effect of weakly suppressed magnetism, as observed in experiments. The ferroelectric properties, on the other hand, are found to be less affected by changes in the electronic structure and can largely be understood in terms of the volume expansion caused by Ba substitution. The calculated electric polarization as a function of biaxial strain in ${\mathrm{Sr}}_{1-x}{\mathrm{Ba}}_x\mathrm{Mn}{\mathrm{O}}_3$ for $x=0$ and $x=0.5$ shows that the difference between the two is mainly due to differences in the magnetic order at certain strain values, accompanied by enormous magnetoelectric coupling.

${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ is an excitonic insulator (EI) exhibiting bound electron-hole pairs condensed at room temperature, which transforms to a small-gap semiconducting state above $T=325$ K simultaneously undergoing a phonon-related structural transition. Despite the clear experimental evidence for strong exciton-phonon coupling, its role and the origin of EI state in terms of BCS versus Bose-Einstein condensation mechanisms are unclear. Motivated by the tunability of these mechanisms with pressure, we report Raman experiments under pressure of ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ and first-principles theoretical analysis of two pressure-induced transitions at 1 and 3 GPa. We present a simple method to derive the exciton-phonon coupling within density functional theory and show using a model Hamiltonian that reducing strength of this coupling relative to electronic gap and phonon frequency destabilizes the EI state with pressure. In addition to connecting with the Raman anomalies observed under pressure, our simple picture explains the recently observed phonon-coupled state of exciton condensate.

The predicted interplay between Kondo physics and nontrivial topology in ${\mathrm{SmB}}_6$ has stimulated many experimental reports, some of which are in apparent contradiction. The origin of the dispute may lie in the fragility of the Kondo insulating phase in the presence of Sm vacancies (Kondo holes) and/or natural impurities, such as ${\mathrm{Gd}}^{3+}$. In this work, we locally investigate this fragility for Al flux grown ${\mathrm{Sm}}_{1-x}{\mathrm{Gd}}_x{\mathrm{B}}_6$ single crystals ($0\le x\le 0.02$) by combining electron spin resonance (ESR) and complementary bulk measurements. The ${\mathrm{Gd}}^{3+}$ ESR spectra in a highly dilute regime ($x\sim 0.0004$) display the features of an insulating cubic environment. Remarkably, a metallic ESR line shape is observed for more concentrated samples ($x\ge 0.004$), even though these systems are still in a reasonably dilute regime and show insulating dc electrical resistivity. Our data indicate that the Kondo insulating state is destroyed locally around impurities before a global percolation occurs. This result not only explains the discrepancy between dc and ac conductivity but also provides a scenario to explain the presence of quantum oscillations in magnetization in the absence of quantum oscillations in electrical resistivity.

In silicon spin qubits, the valley splitting must be tuned far away from the qubit Zeeman splitting to prevent fast qubit relaxation. In this work, we study in detail how the valley splitting depends on the electric and magnetic fields as well as the quantum dot geometry for both ideal and disordered Si/SiGe interfaces. We theoretically model a realistic electrostatically defined quantum dot and find the exact ground and excited states for the out-of-plane electron motion. This enables us to find the electron envelope function and its dependence on the electric and magnetic fields. For a quantum dot with an ideal interface, the slight cyclotron motion of electrons driven by an in-plane magnetic field slightly increases the valley splitting. Importantly, our modeling makes it possible to analyze the effect of arbitrary configurations of interface disorders. In agreement with previous studies, we show that interface steps can significantly reduce the valley splitting. Interestingly, depending on where the interface steps are located, the magnetic field can increase or further suppress the valley splitting. Moreover, the valley splitting can scale linearly or, in the presence of interface steps, nonlinearly with the electric field.

In ${\mathrm{Eu}}_2\mathrm{ZnIr}{\mathrm{O}}_6$, effectively two atoms are active; i.e., Ir is magnetically active, which results in complex magnetic ordering within the Ir sublattice at low temperature. On the other hand, although Eu is a Van Vleck paramagnet, it is active in the electronic channels involving $4f^6$ crystal-field split levels. Phonons, quanta of lattice vibration involving vibration of atoms in the unit cell, are intimately coupled with both magnetic and electronic degrees of freedom (DOF). Here, we report a comprehensive study focusing on the phonons as well as intraconfigurational excitations in double-perovskite ${\mathrm{Eu}}_2\mathrm{ZnIr}{\mathrm{O}}_6$. Our studies reveal strong coupling of phonons with the underlying magnetic DOF reflected in the renormalization of the phonon self-energy parameters well above the spin-solid phase ($T_N\sim 12\mathrm{K}$) until temperature as high as $\sim 3T_N$ evidences broken spin rotational symmetry deep into the paramagnetic phase. In particular, all the observed first-order phonon modes show softening of varying degree below $\sim 3T_N$, and low-frequency phonons become sharper, while the high-frequency phonons show broadening attributed to the additional available magnetic damping channels. We also observed a large number of high-energy modes, 39 in total, attributed to the electronic transitions between $4f$ levels of the rare-earth ${\mathrm{Eu}}^{3+}$ ion and these modes shows anomalous temperature evolution as well as mixing of the crystal-field split levels attributed to the strong coupling of electronic and lattice DOF.

The energetic proximity of crystal electric field (CEF) to phonon excitations in rare-earth materials can lead to strong electron-phonon coupling, exotic behavior, and technologically useful properties. We present a temperature-, pressure-, and magnetic field-dependent Raman scattering study of ${\mathrm{Pr}}_2{\mathrm{O}}_3$ that reveals soft phonon mode behavior and anomalies associated with CEF excitations, which are indicative of strong phonon-CEF coupling. We also show that both an applied magnetic field and pressure induce a similar splitting of the doubly degenerate ${\mathrm{\Gamma }}_3$ crystal-field level, providing evidence for pressure-induced magnetostructural effects in ${\mathrm{Pr}}_2{\mathrm{O}}_3$. These results demonstrate the importance of low-lying CEF excitations and strong CEF-phonon coupling in governing the phases and properties of rare-earth materials.

Non-Hermitian skin effect and critical skin effect are unique features of non-Hermitian systems. In this work, we explore novel phenomena associated with the ${\mathbb{Z}}_2$ skin effect and critical skin effect in open quantum systems. For the open system with the corresponding damping matrix exhibiting ${\mathbb{Z}}_2$ skin effect, we identify the existence of helical damping, characterized by the evolution of relative particle number with exponentially and algebraically decreasing intervals distinguished by sharp wave fronts with opposite propagation directions. When the time-reversal symmetry is broken by adding perturbations, we find the occurrence of dynamical critical skin effect, which causes the disappearance of the helical damping in the thermodynamic limit although it can survive in small size systems. We also demonstrate the existence of anomalous critical skin effect when we couple two identical systems with ${\mathbb{Z}}_2$ skin effect. With the help of non-Bloch band theory, we unveil that the change of generalized Brillouin zone equation is the origin of critical skin effect.

Fracton phases exhibit striking behavior which appears to render them beyond the standard topological quantum field theory (TQFT) paradigm for classifying gapped quantum matter. Here, we explore fracton phases from the perspective of defect TQFTs and show that topological defect networks—networks of topological defects embedded in stratified $3+1$-dimensional $(3+1\mathrm{D})$ TQFTs—provide a unified framework for describing various types of gapped fracton phases. In this picture, the subdimensional excitations characteristic of fractonic matter are a consequence of mobility restrictions imposed by the defect network. We conjecture that all gapped phases, including fracton phases, admit a topological defect network description and support this claim by explicitly providing such a construction for many well-known fracton models, including the X-cube and Haah's B code. To highlight the generality of our framework, we also provide a defect network construction of a fracton phase hosting non-Abelian fractons. As a byproduct of this construction, we obtain a generalized membrane-net description for fractonic ground states as well as an argument that our conjecture implies no topological fracton phases exist in $2+1$-dimensional gapped systems. Our paper also sheds light on techniques for constructing higher-order gapped boundaries of $3+1\mathrm{D}$ TQFTs.

Based on a topological transition of the symmetry-protected topological phase (SPT), an interaction-induced topological charge pump (iTCP) is proposed with the symmetry-breaking parameter as a synthetic dimension. It implies that the phase boundary of the SPT is a topological obstruction, although iTCP and the gap closing singularity are stable for symmetry-breaking perturbations. As for the iTCP, an interaction is essential since the pumped charge is trivial for a noninteracting system. We have confirmed the bulk-edge correspondence for this iTCP using density matrix renormalization group for the Rice-Mele model with nearest-neighbor interactions. As for a realization in optical lattices, an interaction sweeping pump protocol is proposed as well.