We study a 2D measurement-only random circuit motivated by the Bacon-Shor error correcting code. We find a rich phase diagram as one varies the relative probabilities of measuring nearest-neighbor Pauli XX and ZZ check operators. In the Bacon-Shor code, these checks commute with a group of stabilizer and logical operators, which therefore represent conserved quantities. Described as a subsystem symmetry, these conservation laws lead to a continuous phase transition between an $X$-basis and $Z$-basis spin-glass order. The two phases are separated by a critical point where the entanglement entropy between two halves of an $L\times L$ system scales as $LlnL$, a logarithmic violation of the area law. We generalize to a model where the check operators break the subsystem symmetries (and the Bacon-Shor code structure). In tension with established heuristics, we find that the phase transition is replaced by a smooth crossover, and the $X$- and $Z$-basis spin-glass orders spatially coexist. Additionally, if we approach the line of subsystem symmetries away from the critical point in the phase diagram, some spin-glass order parameters jump discontinuously.

A spin-1/2 triangular-lattice antiferromagnet is a prototypical frustrated quantum magnet, which exhibits remarkable quantum many-body effects that arise from the synergy between geometric spin frustration and quantum fluctuations. It can host quantum frustrated magnetic topological phenomena such as quantum spin liquid (QSL) states, highlighted by the presence of fractionalized quasiparticles within a continuum of magnetic excitations. In this work, we use neutron scattering to study ${\mathrm{CoZnMo}}_3{\mathrm{O}}_8$, which has a triangular lattice of $J_{\mathrm{eff}}=1/2{\mathrm{Co}}^{2+}$ ions with octahedral coordination. We found a wave-vector-dependent excitation continuum at low energy that disappears with increasing temperature. Although these excitations are reminiscent of a spin excitation continuum in a QSL state, their presence in ${\mathrm{CoZnMo}}_3{\mathrm{O}}_8$ originates from magnetic intersite disorder-induced dynamic spin states with peculiar excitations. Our results, therefore, provide direct experimental evidence for the presence of a disorder-induced spin excitation continuum.

Bose-Fermi systems such as mixtures of electrons with excitons or exciton-polaritons are extensively discussed as candidates to host a variety of intriguing phenomena, including polaron formation, drag effects, supersolidity, and superconductivity. In this paper, assuming the strong-coupling regime between the semiconductor excitons and cavity photons, we develop the many-body theory approach addressing the interplay of different types of interaction among various species in such a mixture, wherein we take into account dynamical density responses of both the Bose-condensed exciton-polaritons and the two-dimensional electron gas inside an optical microcavity. As anticipated previously, at high enough polariton densities, the lower hybrid mode of the system's excitation spectrum acquires a roton minimum, making the system prone to superconducting pairing in the vicinity of the roton instability. We analyze the possibility of polariton-BEC-mediated superconductivity in the electron gas, taking into account full momentum and frequency dependence of the gap, as well as in the Eliashberg approach where the momentum dependence is neglected, and in the Bardeen-Cooper-Schrieffer approach that discards the frequency dependence and dynamical effects. Considering the interaction screening in Thomas-Fermi and random-phase approximations, we estimate the critical temperatures of superconductivity to be not larger than 0.1 K in the vicinity of instability. As possible realizations of the coupled polariton-electron system, semiconductor quantum wells and two-dimensional transition metal dichalcogenides are considered.

In conformal field theories, when the conformal symmetry is enhanced by a global Lie group symmetry, the original Virasoro algebra can be extended to Kac-Moody algebra. In this paper, we extend the lattice construction of the Kac-Moody generators introduced in Wang et al., [Phys. Rev. B 106, 115111 (2022)] to continuous systems and apply it to one-dimensional continuous boson systems. We justify this microscopic construction of Kac-Moody generators in two ways. First, through phenomenological bosonization, we express the microscopic construction in terms of the boson operators in the bosonization context, which can be related to the Kac-Moody generators in conformal field theories. Second, we study the behavior of the Kac-Moody generators in the integrable Lieb-Liniger model and reveal its underlying particle-hole excitation picture through Bethe ansatz solutions. Finally, we test the computation of the Kac-Moody generator in the continuous matrix product state simulations, paving the way for more challenging nonintegrable systems.

We study the energy spectrum of tight-binding Hamiltonians for regular hyperbolic tilings. More specifically, we compute the density of states using the continued-fraction expansion of the Green's function on finite-size systems with more than ${10}^9$ sites and open boundary conditions. The coefficients of this expansion are found to quickly converge, so that the thermodynamic limit can be inferred quite accurately. This density of states is in stark contrast with the prediction stemming from the recently proposed hyperbolic band theory. Thus we conclude that the fraction of the energy spectrum described by the hyperbolic Bloch-like wave eigenfunctions vanishes in the thermodynamic limit.

On the basis of the self-consistent phonon theory and the special displacement method, we develop an approach for the treatment of anharmonicity in solids. We show that this approach enables the efficient calculation of temperature-dependent anharmonic phonon dispersions, requiring very few steps to achieve minimization of the system's free energy. We demonstrate this methodology in the regime of strongly anharmonic materials, which exhibit a multiwell potential energy surface, like cubic ${\mathrm{SrTiO}}_3$, ${\mathrm{CsPbBr}}_3$, ${\mathrm{CsPbI}}_3$, ${\mathrm{CsSnI}}_3$, and Zr. Our results are in good agreement with experiments and previous first-principles studies relying on stochastic nonperturbative and molecular dynamics simulations. We achieve a very robust workflow by using harmonic phonons of the polymorphous ground state as the starting point and an iterative mixing scheme of the dynamical matrix. We also suggest that the phonons of the polymorphous ground state might provide an excellent starting approximation to explore anharmonicity. Given the simplicity, efficiency, and stability of the present treatment to anharmonicity, it is especially suitable for use with any electronic structure code and for investigating electron-phonon couplings in strongly anharmonic systems.

We study the thermalization properties of one-dimensional open quantum systems coupled to baths at their boundary. The baths are driven to their thermal states via Lindblad operators, while the system undergoes Hamiltonian dynamics. We specifically consider multisite baths and investigate the extent to which the late-time steady state resembles a Gibbs state at some controllable temperature set by the baths. We study three models: a noninteracting fermion model accessible via free-fermion technology, and two interacting models, the XZ model and the chiral clock model, which are accessible via tensor network methods. We show that, by tuning towards the weak coupling and slow relaxation limits, one can engineer low temperatures in the bulk of the system provided the bath size is big enough. We use this capability to study energy transport in the XZ model at lower temperatures than previously reported. Our work paves the way for future studies of interacting open quantum systems at low temperatures.

Complex systems are characterized by strong coupling between different microscopic degrees of freedom. Photoexcitation of such materials can drive them into new transient and metastable hidden phases that may not have any counterparts in equilibrium. By exploiting femtosecond time- and angle-resolved photoemission spectroscopy, we probe the photoinduced transient phase and the recovery dynamics of the ground state in a complex material: the charge density wave (CDW)–Mott insulator $1T\text{−}{\mathrm{TaS}}_2$. We reveal striking similarities between the band structures of the transient phase and the (equilibrium) structurally undistorted metallic phase, with evidence for the coexistence of the low-temperature Mott insulating phase and high-temperature metallic phase. Following the transient phase, we find that the restorations of the Mott and CDW orders begin around the same time. This highlights that the Mott transition is tied to the CDW structural distortion, although earlier studies have shown that the collapses of Mott and CDW phases are decoupled from each other. Interestingly, as the suppressed order starts to recover, a metastable phase emerges before the material recovers to the ground state. Our results demonstrate that it is the CDW lattice order that drives the material into this metastable phase, which is indeed a commensurate CDW–Mott insulating phase but with a smaller CDW amplitude. Moreover, we find that the metastable phase emerges only under strong photoexcitation ($\sim 3.6$ mJ/${\mathrm{cm}}^2$) and has no evidence when the photoexcitation strength is weak ($\sim 1.2$ mJ/${\mathrm{cm}}^2$).

The nonreciprocal propagation of light typically requires use of materials like ferrites or magneto-optical media with a strong magnetic bias or methods based on material nonlinearities which require use of strong electromagnetic fields. A simpler possibility to produce nonreciprocity is to use spatiotemporal modulations to produce magnetic fields in synthetic dimensions. In this paper we show that dissipatively coupled systems can lead to considerable enhancement of nonreciprocity in synthetic fields. The enhancement comes about from the existence of a nearly nondecaying mode—a bound state in continuum (BIC)—in dissipatively coupled systems. The dissipative coupling occurs in a wide class of systems coupled via transmission lines, waveguides, or nano fibers. The systems could be optical resonators or microscopic qubits. Remarkably we find that for specific choice of the modulation amplitudes, the transmission say in forward direction is completely extinguished whereas in the backward direction it becomes maximum. The synthetic fields produce transmission resonances which show significant line narrowing which owe their origin to the existence of BIC's in dissipative systems.

We theoretically discuss electronic transport via Majorana states in magnetic topological insulator-superconductor junctions with an asymmetric split of the applied bias voltage. We study normal-superconductor-normal (NSN) junctions made of narrow (wirelike) or wide (filmlike) magnetic topological insulator slabs with a central proximitized superconducting sector. The occurrence of charge-nonconserving Andreev processes entails a nonzero conductance related to an electric current flowing to ground from the proximitized sector of the NSN junction. We show that topologically protected Majorana modes require an antisymmetry of this conductance with respect to the point of equally split bias voltage across the junction.

Using the tight-binding approach, we calculate the thermoelectric properties of phosphorene nanorings in the absence and presence of a perpendicular magnetic field, and we investigate the effect of symmetrically and asymmetrically positioned leads. Our findings indicate that the symmetry/asymmetry of our designed nanostructures, the geometrical characteristics of the ring, and the magnetic flux are three important factors in controlling their thermoelectric properties. Our results show that when zigzag phosphorene nanoribbons (ZPNRs) are coupled symmetrically/asymmetrically to rectangular rings, a large band gap is induced in the electronic conductance due to the suppression of the contribution of edge states. This gives rise to a remarkable increase in the thermopower response compared to the case of pristine ZPNRs. More intriguingly, we realized that the significantly smaller electronic thermal conductance and the reduced phonon thermal conductance of these phosphorene-based nanostructures result in a remarkable improvement in the figure of merit. Moreover, for asymmetric connection configurations with armchair-edged leads, we found that although the thermopower is almost intact, a remarkable reduction of the electronic and phononic thermal conductance leads to a notable improvement in the figure of merit. Also, our numerical calculations showed that by changing the magnetic flux through the nanoring, a drastic increase in the thermopower is observed near an antiresonance point. We demonstrate the tunability of the thermopower and the possibility to switch on and off the thermoelectric response with magnetic flux. Our results suggest phosphorene nanorings as promising candidate nanostructures for thermoelectric applications.

Heterostructures composed of different elements are a suitable platform for many novel physical phenomena. Here, quantum oscillations related to the lattice symmetry in hexagonal close-packed (hcp) ${\mathrm{Co}}_2/{\mathrm{Si}}_n$ superlattices, such as lattice parameters, electronic structure, and magnetism, are revealed and studied by density-functional theory. Densely packed atomic layers stack in an $ABAB$ sequence in the superlattice and reach a bilayer-like structure by interlayer coupling in the presence of the Co/Si interface. The lattice symmetry depends on the parity of $n$, and an odd number of $n$ benefits interlayer coupling and the interface effect, causing odd-even quantum oscillations. These observations indicate that changing lattice symmetry by designing component type, layer number, and stacking sequence is a feasible method of effectively regulating the performance of ultrathin heterojunctions.

The high-quality structures containing semiconducting transition-metal dichalcogenide (S-TMD) monolayers (MLs) required for optical and electrical studies are achieved by their encapsulation in hexagonal BN (hBN) flakes. To examine the effect of hBN thickness in these systems, we consider a model with an S-TMD ML placed between a semi-infinite in the out-of-plane direction substrate and complex top cover layers: a layer of finite thickness, adjacent to the ML, and a semi-infinite in the out-of-plane direction top part. We obtain the expression for the Coulomb potential for such a structure. Using this result, we demonstrate that the energies of excitonic $s$ states in the structure with a ${\mathrm{WSe}}_2$ ML change significantly for the top hBN with thickness less than 30 layers for different substrate cases, such as hBN and ${\mathrm{SiO}}_2$. For the larger thickness of the top hBN flake, the binding energies of the excitons are saturated to their values of the bulk hBN limit.

We present a prediction of chiral topological metals with several classes of unconventional quasiparticle fermions in a family of SrGePt-type materials in terms of first-principles calculations. In these materials, fourfold spin-3/2 Rarita-Schwinger-Weyl (RSW) fermion, sixfold excitation, and Weyl fermions coexist around the Fermi level as spin-orbit coupling is considered, and the Chern number for the first two kinds of fermions is the maximal value four. We found that large Fermi arcs from spin-3/2 RSW fermion emerge on the (010) surface, spanning the whole surface Brillouin zone. Moreover, there exist Fermi arcs originating from Weyl points, which further overlap with trivial bulk bands. In addition, we revealed that the large spin Hall conductivity can be obtained, which attributed to the remarkable spin Berry curvature around the degenerate nodes and band splitting induced by spin-orbit coupling. Our findings indicate that the SrGePt family of compounds provide an excellent platform for studying on topological electronic states and the intrinsic spin Hall effect.

In a cubic environment, the ground state of spin-orbit coupled $5d^2$ ions is a non-Kramers $E_g$ doublet, which hosts quadrupole and octupole moments. A series of $5d^2$ osmium double perovskites ${\mathrm{Ba}}_{2}M{\mathrm{OsO}}_6$ $(M=\text{Mg}, \mathrm{Ca}, \mathrm{Zn}, \mathrm{Cd})$ have recently been proposed to exhibit multipolar orders. We investigate the structural properties of these materials using $ab$ initio calculations and find that the cubic structure is unstable for the Cd compound while the Mg, Ca, and Zn materials retain $Fm\overline{3}m$ symmetry. We show that ${\mathrm{Ba}}_2{\mathrm{CdOsO}}_6$ favors a rhombohedral $R\overline{3}$ structure characterized by $a^{-}{a}^{-}{a}^{-}$ octahedral tiltings as indicated by unstable ${\mathcal{T}}_{1g}$ phonon modes. Trigonal distortions split the excited $T_{2g}$ triplet into an $E_g^{\prime }$ doublet and an $A_g$ singlet, which may cross energy levels with the $E_g$ doublet and suppress the multipolar physics. We find a window where $E_g$ remains the lowest energy state under trigonal distortion, enabling the emergence of multipole phases in noncubic crystal environments.

It is now widely recognized that the toric code is a pure gauge-theory model governed by a projective Hamiltonian with topological orders. In this paper, we extend the interplay between quantum information system and gauge-theory model from the viewpoint of subsystem code, which is suitable for gauge systems including matter fields. As an example, we show that $Z_2$ lattice gauge-Higgs model in ($2+1$)-dimensions with specific open boundary conditions is nothing but a kind of subsystem code. In the system, Gauss-law constraints are stabilizers, and order parameters identifying Higgs and confinement phases exist and they are nothing but logical operators in subsystem codes residing on the boundaries. Mixed anomaly of them dictates the existence of boundary-zero modes, which is a direct consequence of symmetry-protected topological order in Higgs and confinement phases. After identifying phase diagram, subsystem codes are embedded in the Higgs and confinement phases. As our main findings, we give an explicit description of the code (encoded qubit) in the Higgs and confinement phases, which clarifies duality between Higgs and confinement phases. The degenerate structure of subsystem code in the Higgs and confinement phases remains even in very high-energy levels, which is analogous to notion of strong-zero modes observed in some interesting condensed-matter systems. Numerical methods are used to corroborate analytically-obtained results and the obtained spectrum structure supports the analytical description of various subsystem codes in the gauge theory phases.

Although the ground-state energy of the Kitaev spin liquid can be calculated exactly, the associated vison gap energy has to date only been calculated numerically from finite size diagonalization. Here we show that the phase shift for scattering Majorana fermions off a single bond flip can be calculated analytically, leading to a closed-form expression for the vison gap energy $\mathrm{\Delta }=0.2633J$. Generalizations of our approach can be applied to Kitaev spin liquids on more complex lattices such as the three-dimensional hyperoctagonal lattice.

Electron states above the vacuum level are known to play an important role in secondary electron processes, such as photoelectron emission and secondary electron emission where they act as “final” (or better “intermediate”) states from which an electron is emitted to the vacuum. However, despite their relevance, these states are typically not well known, nor independently investigated, mostly due to a lack of proper spectroscopic techniques. Here, we present a spectroscopy study on crystalline pentacene, used as a model system to investigate the influence of these states on secondary electron processes. Using low-energy electron (LEE) spectroscopy, we first gauge the spectrum of such states in few-monolayer pentacene films. We, subsequently, relate these states to photoelectron and secondary electron emission. Specifically, photoemission experiments (Hg lamp) show a decrease in intensity with each additional pentacene layer grown. Given an absence of increase in the ionization energy or change in the crystal structure with increasing layer count, we relate the decrease in photoemission intensity to the emergence of a band gap just above the vacuum level as observed in LEE reflectivity spectra. Second, we study the energy distribution of secondary electrons. We use electron-beam damage to cause controlled changes in the band structure, and find a clear correlation between the evolution of the LEE spectra and the distribution of secondary electrons.