We investigate samples of compositionally disordered potassium-based perovskite single crystals with different composition and stoichiometry. The dielectric and Raman response is inspected over the nominal cubic-to-tetragonal long-range phase transition. The comparison between results shows that the occurrence of the phase transition is controlled by the temperature evolution of polar nanoregions (PNRs). We are able to correlate PNR order, formation, percolation, and freezing to the characteristic temperatures of each macroscopic crystal. The onset of the phase transition is found to occur when PNRs undergo a percolative ordering process, in correlation to the sudden arising of a specific high-frequency Raman mode.

Barium titanate (BT) based materials are at the forefront of materials being searched as possible candidates for the replacement of lead-based compositions in applications ranging from piezoelectrics to energy storage devices. Computational methods are very promising to increase the efficiency of materials discovery, provided that finite temperature properties can be realistically computed using, for example, molecular dynamics (MD). In this work, we present a systematic increase of the quality of MD simulations via an alternative way to calculate anharmonic contributions to the potential energy surface (PES) of barium titanate. A large number of first-principles calculations are performed, which are subsequently used to parametrize an effective Hamiltonian. To test the effects on various physical properties, MD simulations for the determination of transition temperatures, hysteresis, and permittivity of BT are shown. Furthermore, measurements were performed on BT single crystals to compare them directly with the MD simulations. It is observed that by incorporating a large number of anharmonic couplings, the description of the local minima in the PES becomes more accurate than in previous simulations. This leads to a better prediction of phase transition temperatures and shows the importance of anharmonic couplings in barium titanate. The presented approach can be directly adapted for other perovskite structures.

Socolar dodecagonal lattice is a quasicrystal closely related to the better-known Ammann-Beenker and Penrose lattices. The cut and project method generates this twelvefold rotationally symmetric lattice from the six-dimensional simple cubic lattice. We consider the vertex tight-binding model on this lattice and use the acceptance domains of the vertices in perpendicular space to count the frequency of strictly localized states. We numerically find that these states span $f_{\mathrm{Num}}\simeq 7.61\%$ of the Hilbert space. We give 18 independent localized state types and calculate their frequencies. These localized state types provide a lower bound of $f_{\mathrm{LS}}=\frac{10919-6304\sqrt{3}}{2}\simeq 0.075854$, accounting for more than $99\%$ of the zero-energy manifold. Numerical evidence points to larger localized state types with smaller frequencies, similar to the Ammann-Beenker lattice. On the other hand, we find sites forbidden by local connectivity to host localized states. Forbidden sites do not exist for the Ammann-Beenker lattice but are common in the Penrose lattice. We find a lower bound of $f_{\mathrm{Forbid}}\simeq 0.038955$ for the frequency of forbidden sites. Finally, all the localized state types we find can be chosen to have constant density and alternating signs over their support, another feature shared with the Ammann-Beenker lattice.

Recently, in the Luttinger liquid phase of the one-dimensional generalized antiferromagnetic Kitaev spin-1/2 model, it has been found that the Abelian bosonization formulas of the local spin operators only respect the exact discrete nonsymmorphic symmetry group of the model, not the emergent U(1) symmetry. In this work, we perform a renormalization group (RG) study to provide explanations for the origin of the U(1) breaking terms in the bosonization formulas. We find that the lack of U(1) symmetry originates from the wave-function renormalization effects in the spin operators along the RG flow induced by the U(1) breaking interactions in the microscopic Hamiltonian. In addition, the RG analysis can give predictions to the signs and order of magnitudes of the coefficients in the bosonization formulas. Our work is helping to understand the rich nonsymmorphic physics in one-dimensional Kitaev spin models.

Density-functional theory calculations utilizing the generalized Bloch theorem show that interfacial Dzyaloshinskii-Moriya interactions (DMI) at both interfaces of graphene/${\mathrm{Co}}_n$/Pt(111) multilayer heterostructures are decoupled for $n\ge 3$. Unlike the property of magnetocrystalline anisotropy, DMI in this system is not strongly affected by stacking defects in the Co layer. The effect of graphene (Gr) is to invert the chirality of the vaccum/Co interfacial DMI, overall reducing the DMI of the heterostructure, which is nevertheless dominated by the strong spin-orbit coupling (SOC) of Pt. A spectral analysis in reciprocal space shows that DMI at both the Gr/Co and Co/Pt interfaces have similar contributions from the strongly hybridized SOC-split $d$ bands. Our analysis proves that the DMI in this family of Gr-capped $3d/5d$ metal heterostructures displays richer physics than what is expected from the prediction of single-band models, such as the Rashba DMI model.

Unconventional superconductors have been long sought for their potential applications in quantum technologies and devices. A key challenge impeding this effort is the difficulty associated with probing and characterizing candidate materials and establishing their order parameter. Here we present a platform that allows one to spectroscopically probe unconventional superconductivity in thin-layer materials via the proximity effect. We show that inducing an $s$-wave gap in a sample with an intrinsic $d$-wave instability leads to the formation of bound states of quasiparticle pairs, which manifest as a collective mode in the $d$-wave channel. This finding provides a way to study the underlying pairing interactions vicariously through the collective mode spectrum of the system. Upon further cooling of the system we observe that this mode softens considerably and may even condense, signaling the onset of time-reversal symmetry-breaking superconductivity. Therefore, our proposal also allows for the creation and study of these elusive unconventional states.

We show that a plethora of topological insulating phases can appear in bilayer Dirac materials, including first-order topological insulators, topological mirror insulators, and second-order topological insulators. By considering doping and short-range attractive interactions, we show at a mean-field level that intrinsic odd-parity superconductivity can arise in such systems. Depending on the number and positions of Fermi surfaces in the normal state, we find that the resulting odd-parity superconductivity can lead to diverse time-reversal invariant topological superconducting phases, including topological superconductors and topological mirror superconductors. Our findings suggest that bilayer Dirac materials could be an excellent platform for investigating topological phases.

We numerically study the phase diagram of bosons tightly trapped in the lowest band of an optical lattice and dispersively coupled to a single-mode cavity field. The dynamics is encompassed by an extended Bose-Hubbard model. Here, the cavity-mediated interactions are described by a two-body potential term with a global range and by a correlated tunneling term where the hopping amplitude depends on a global observable. We determine the ground-state properties in one dimension by means of the density matrix renormalization group algorithm, focusing on the effects due to the correlated tunneling. The latter is responsible for the onset of bond orders, manifesting in one insulating and two gapless bond-ordered phases. We discuss the resulting phases for different geometries that correspond to different relative strengths of the correlated tunneling coefficient. We finally analyze the scaling of the entanglement entropy in the gapless bond-ordered phases that appear entirely due to global interactions and determine the corresponding central charges.

We study the Lindbladian dynamics of the Sachdev-Ye-Kitaev (SYK) model, where the SYK model is coupled to Markovian reservoirs with jump operators that are either linear or quadratic in the Majorana fermion operators. Here, the linear jump operators are nonrandom, while the quadratic jump operators are sampled from a Gaussian distribution. In the limit of large $N$, where $N$ is the number of Majorana fermion operators, and also in the limit of large $N$ and $M$, where $M$ is the number of jump operators, the SYK Lindbladians are analytically tractable, and we obtain their stationary Green's functions, from which we can read off the decay rate. For finite $N$, we also study the distribution of the eigenvalues of the SYK Lindbladians.

While the semiclassical Boltzmann approach has been widely adopted to study chiral-anomaly-related transport in Weyl semimetals (WSMs), it predicts an unphysical diverging electrical conductivity when $E_F\to 0$, where $E_F$ is the Fermi energy. Here, we develop a modified semiclassical equation of motion which includes both the diagonal and off-diagonal contributions of the Berry curvature. On this basis, we derive an undivergent classical formula for the positive longitudinal magnetoconductivity in a WSM which resolves the conflict between the classical and ultraquantum approaches. We demonstrate that the chiral anomaly is closely related to the topological pumping effect and it can be realized even in the absence of a magnetic field. With our findings we propose a different perspective to understand the topological properties of WSMs and suggest a way to measure the chiral anomaly using transport.

Recent studies suggest that the nonlinear transport properties in Weyl semimetal may be a measurable consequence of its chiral anomaly. Nonlinear responses in transport are estimated to be substantial, because in real materials such as TaAs or ${\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x$, the Fermi level resides near the Weyl nodes where the chiral charge pumping is said to diverge. However, this work presents semiclassical Boltzmann analysis that indicates that the chiral charge pumping is nondivergent even at the zero-temperature limit. We demonstrate that the divergence in a common semiclassical calculation scheme is not a problem of the scheme itself, but occurs because a commonly used approximation of the change in particle number breaks down near the Weyl nodes. Our result suggests the possibility that the nonlinear properties in Weyl semimetals can be overestimated, and it provides the validity condition for the conventional approximation. We also show the distinct Fermi level dependencies of the chiral magnetic effect and the negative longitudinal magnetoresistance, as a consequence of nondiverging chiral charge pumping.

Singlet-triplet qubits typically require large magnetic field gradients on the order of militeslas to achieve high-fidelity electrically-controlled qubit operations. However, such large magnetic field gradients in quantum dot systems also increase charge noise and provide a relaxation pathway from the triplet to singlet qubit state, making high-fidelity readout challenging. Recently, shelving and latched readout have been employed in gate-defined quantum dots and donor-dot systems to achieve readout fidelities of 80% and 99.86%, respectively. In this paper, we theoretically examine shelving and latched singlet-triplet readout techniques for multidonor-based qubits in silicon where the large phosphorus hyperfine interaction of the order of 100 MHz gives rise to large effective magnetic field gradients (equivalent to tens of mT) but where it can change in time due to the presence of nuclear spin flips. Using numerical simulations, we show that shelving readout does not work giving a zero average visibility for mutidonor quantum dots, due to the time-varying nuclear spin polarization. To remedy this we propose adding a calibration step, in which we derive the nuclear spin polarization from a single shelving readout of a singlet state, before every qubit operation. The derived information can then be used via a feed-forward protocol to apply correct readout mapping, greatly improving the overall readout fidelity to $>99\%$. We also simulate the latched readout mechanism, which is resistant to nuclear polarization changes and is thus promising for achieving high visibility readout. Here we observe a nonzero readout visibility irrespective of the nuclear spin flipping. Finally, we discuss how to optimize the readout visibility in the presence of strong hyperfine interactions and show that for both readout methods we can obtain readout fidelity $>99\%$. These results demonstrate that singlet-triplet qubits based on multidonor quantum dots are a promising route for future electrically controlled qubits in silicon.

In this paper we explore the effects of nonadiabatic external driving on the dynamics of an electronic system coupled to two electronic leads and to a phonon mode, with and without damping. In the limit of slow driving, we establish nonadiabatic corrections to thermodynamic and transport quantities. In particular, we study the first-order correction to the work done by the driving, the charge current, and the vibrational excitation using a perturbative expansion. We then compare the results to the numerically exact hierarchical equations of motion (HEOM) approach. Furthermore, the HEOM analysis spans both the weak and strong system-bath coupling regime and the slow- and fast-driving limits. We show that the electronic friction and the nonadiabatic corrections to the charge current provide a clear indicator for the Franck-Condon effect and for nonresonant tunneling processes. We also discuss the validity of the approximate quantum master equation approach and the benefits of using HEOM to study nonadiabatically driven open quantum systems out of equilibrium.

Lattice dynamics play a crucial role in the physics of moiré systems. In twisted bilayer graphene (TBG), it was shown that, in addition to the graphene phonons, there is another set of gapless excitations termed moiré phonons [Koshino and Son, Phys. Rev. B 100, 075416 (2019)] reflecting the lattice dynamics at the moiré superlattice level. These modes were later suggested to be phasons due to the incommensurate stacking of the two graphene layers [Ochoa, Phys. Rev. B 100, 155426 (2019)]. In this work, we elucidate the equivalence of these two seemingly distinct perspectives by identifying an underlying symmetry, which we dub mismatch symmetry, that exists for any twist angle. For commensurate angles, this is a discrete symmetry whereas, for incommensurate angles, it is equivalent to a continuous phase symmetry giving rise to phason modes. In the small-angle limit, such symmetry becomes a continuous local symmetry whose spontaneous breaking gives rise to moiré phonons as its Goldstone mode. We derive an effective field theory for these collective modes in TBG in precise agreement with the full model and discuss their different properties. Our analysis is then generalized to twisted multilayer graphene (TMG) where we identify higher-order mismatch and deduce the count of gapless modes including graphene phonons, moiré phonons, and phasons. Especially, we study twisted mirror-symmetric trilayer graphene with an alternating twist angle $\theta$ and find that it can be mapped to a TBG with the rescaled twist angle $\sqrt{2/3}\theta$, hosting the same moiré phonon modes in the even mirror sector with an additional set of gapped modes in the odd sector. Our work presents a systematic study of lattice symmetries in TMG providing insights into its unique lattice dynamics.

It is known that Josephson junction (JJ) hosting scattering centers with energy-dependent scattering amplitudes, which breaks the $\omega \to -\omega$ symmetry (where $\omega$ is the excitation energy of electron about the Fermi level) exhibits finite thermoelectric response. In contrast, here we show that even in a ballistic JJ this symmetry is broken and it leads to a nonzero thermal-bias induced charge current above the gap, when the junction length is of the order of coherence length of the superconductor and the corresponding response confirms to the universal sinusoidal dependence on ${\varphi }_{12}$, where ${\varphi }_{12}$ is the superconducting phase bias. In presence of multiple scatterers in the junction region, we have numerically shown that the sign of the even-in-${\varphi }_{12}$ part of the this response fluctuates violently from one disorder configuration to another hence averaging to vanishingly small values while the odd part tends towards the universal sinusoidal dependence on ${\varphi }_{12}$ as we approach the large disorder limit under disorder averaging.

We consider the influence of topological phases, or their vicinity, on the spin density and spin polarization through a chiral chain. We show the quantization of the Berry phase in a one-dimensional polarization helix structure, under the presence of an external magnetic field, and show its influence on the spin density. The polar angle of the momentum space spin density becomes quantized in the regime that the Berry phase is quantized, as a result of the combined effect of the induced spin-orbit coupling and the external transverse magnetic field, while the edge states do not show the polar angle quantization, in contrast with the bulk states. Under appropriate conditions, the model can be generalized to have similarities with a chain with nonhomogeneous Rashba spin-orbit couplings, with zero- or low-energy edge states. Due to the breaking of time-reversal symmetry, we recover the effect of chiral-induced spin polarization and spin transport across the chiral chain, when coupling to external leads. Some consequences of the quantized spin polarization and low-energy states on the spin transport are discussed.

We apply time-dependent density functional theory to calculate the transverse magnetic susceptibility of bcc Cr and ${\mathrm{Cr}}_2{\mathrm{O}}_3$, which constitute prototypical examples of antiferromagnets with itinerant and localized magnetic moments, respectively. The exchange-correlation kernel is rescaled in order to enforce the Goldstone condition and the magnon dispersion relations are extracted based on a symmetry analysis relying on the generalized Onsager relation. Doing so, our calculations yield the characteristic linear magnon dispersion of antiferromagnets in the long-wavelength limit. In the case of ${\mathrm{Cr}}_2{\mathrm{O}}_3$, we find that the adiabatic local density approximation yields a good qualitative agreement with the measured dispersion, but overestimates the magnon velocity and bandwidth by a factor of 2. Including a Hubbard correction improves the magnon velocity, but at the expense of the overall qualitative agreement with the experimental magnon dispersion. For bcc Cr we find a sharp acoustic magnon mode at low energies with a velocity in agreement with previously reported values. At higher energies, the acoustic magnon mode becomes subject to strong Landau damping and rapidly vanishes once it enters the Stoner continuum. In addition to the acoustic magnon mode, we also observe an additional collective mode along the $\mathrm{\Gamma }\to \mathrm{R}$ direction with an energy of $\sim 1$ eV, which is located inside the Stoner continuum, but appears to elude the effect of Landau damping.

High-order Raman scattering is a typical photophysical process in understanding the electron–phonon coupling (EPC) in materials. In a “soft” polar lattice, due to the strong EPC, the excited electron-hole pairs can be captured by the lattice deformation potential, forming the self-trapped exciton (STE). Although high-order Raman scattering mediated by STE has been predicted by theory, there are rare experimental reports, especially in the double perovskite ${\mathrm{Cs}}_2{\mathrm{Ag}}_{0.4}{\mathrm{Na}}_{0.6}{\mathrm{InCl}}_6$, with highly efficient white light emission. We observed high-order Raman mode up to 12 orders at 4 K in ${\mathrm{Cs}}_2{\mathrm{Ag}}_{0.4}{\mathrm{Na}}_{0.6}{\mathrm{InCl}}_6$ by resonance excitation. We propose a physical picture of high-order Raman scattering mediated by STE to explain well the linear dependence of frequency and linewidth with order number. A reduction in the EPC with a temperature increase is attributed to the breakdown of momentum conservation during high-order scattering and the delocalization of the STE. Our work deepens the understanding of the EPC in STE and inspires the research of the excited-state decay process.

Floquet-Weyl semimetals (FWSMs) generated by irradiation of a continuous-wave laser with left-hand circular polarization (rotating in counterclockwise sense with time) on the group II–V narrow-gap semiconductor ${\mathrm{Zn}}_3{\mathrm{As}}_2$ are theoretically investigated, where the frequency of the laser is set nearly resonant with the band gap of the crystal. It is found that the excitation of the crystal by such a laser induces two types of FWSM phases that differ absolutely in character. To be specific, the associated two pairs of Weyl points are stably formed by band touching between Floquet sidebands ascribable to a valence band labeled $J_z=\pm 3/2$ and a conduction band labeled $J_z=\pm 1/2$, where $J_z$ represents the $z$ component of total angular-momentum quantum number of the $\mathrm{\Gamma }$ point and a double sign corresponds. Here, one FWSM state composed of the up-spin Floquet sidebands relevant to $J_z=3/2$ and $1/2$ shows almost quadratic band-touching in the vicinity of the associated pair of Weyl points, while the other FWSM state composed of the down-spin Floquet sidebands relevant to $J_z=-3/2$ and $-1/2$ shows linear band-touching. Furthermore, it is revealed that both up-spin and down-spin sidebands host nontrivial two-dimensional surface states that are pinned to the respective pairs of the Weyl points. Both surface states also show different energy dispersions and physical properties. A more detailed discussion is presented in the text on the origin of the above findings, the chirality of the FWSM phases, the alteration of topological order, laser-induced magnetic properties, and so on.

The variety of excitonic states in tungsten-based dichalcogenide monolayers stems from unique interplay between the spin and valley degrees of freedom. One of the exciton species is the indirect exciton (momentum or valley dark), which is responsible for a series of resonances when the monolayer is charge neutral. We investigate the short-range electron-hole exchange interaction of the indirect exciton, as well as its recombination mechanism mediated by impurities. The analysis provides thorough understanding of the energy and polarization of the zero-phonon indirect exciton resonance in the emission spectrum.