The notion of higher-order topological phases can have interesting generalizations to systems with subsystem symmetries that exhibit fractonic dynamics for charged excitations. In this work, we systematically study the higher-order topological phases protected by a combination of subsystem symmetries and ordinary global symmetries in two and three-dimensional interacting boson systems, with some interacting fermionic examples. In particular, we discover some important results, including (i) an Abelian subsystem symmetry group that does not allow for a nontrivial higher-order symmetry-protected topological phase in 2D without global symmetry. (ii) inhomogeneous subsystem symmetries do not allow for that either.

Positron annihilation in solid-state matter can be utilized to detect and identify open-volume defects. The momentum distribution of the annihilation radiation is an important observable in positron-based measurements and can reveal information on the chemical surroundings of the defect sites. In this paper, we present a variational quantum Monte Carlo method for simulation of the momentum densities of annihilating electron-positron pairs in semiconductors and insulators. We study finite-size effects, effects of lattice vibrations, and different levels of trial wave functions. Small simulation cells and simple wave function forms are found to be sufficient for accurate calculations in the simulation of pristine lattices, enabling cheap accumulation of results. We compare calculated predictions of the Doppler broadening of the 511-keV $2\gamma$ annihilation line in aluminium nitride and silicon against experimental data measured from reference samples. Our results achieve better agreement with experiments in these materials than conventional state-of-the-art methods and prove that direct modeling of the electron-positron correlations is important for a supporting theory of positron annihilation spectroscopies.

In general, optical and acoustic phonons have different energy scaling and are separated by an energy gap. However, the two phonon branches can also interact and provide an inherently poor thermal conductivity in complex minerals with a large number of atoms per unit cell. For instance, the copper-chalcogenide based minerals with high crystalline anharmonicity are inherently poor thermal conductors. We have studied large unit cell ${\mathrm{Cu}}_{26}{\mathrm{Nb}}_2{\mathrm{Sn}}_6{\mathrm{Se}}_{32}, {\mathrm{Cu}}_{26}{\mathrm{Nb}}_2{\mathrm{Sn}}_6{\mathrm{Se}}_{31.5}$, and ${\mathrm{Cu}}_{26}{\mathrm{Nb}}_2{\mathrm{Sn}}_6{\mathrm{Se}}_{30}{\mathrm{Te}}_2$ synthetic minerals with a strategically tailored anionic disorder. These compounds have a $p$-type degenerate behavior with carrier concentration $\left(\sim 2.7–\sim 15.3\right)\times {10}^{20}\mathrm{c}{\mathrm{m}}^{-3}$, at 300 K and a very low lattice thermal conductivity, $\left(\sim 0.76–\sim 1.49\right)\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ at $\sim 625\mathrm{K}$. Here, the softening of Cu-Se bonds and hence the crystal framework play an important role for very poor thermal conductivity. The existence of two low frequency Raman active optical modes (at $\sim 55$ and $72{\mathrm{cm}}^{-1}$) associated with soft Cu and Se atoms, three localized Einstein modes in specific heat, suggest a high scattering of acoustic and optical branches with very short phonon lifetime $(\sim 0.3–0.6\mathrm{ps})$. The excess vibrational density of states at low energies with compressed and flat optical branches strongly hinders the heat transport. The involvement of the Te atom at Se sites results in a lowering of the acoustic phonon cutoff frequency and the softening of optical phonons, significantly. Overall, ${\mathrm{Cu}}_{26}{\mathrm{Nb}}_2{\mathrm{Sn}}_6{\mathrm{Se}}_{30}{\mathrm{Te}}_2$ has the lowest thermal conductivity at $\sim 625\mathrm{K}$ and is a promising thermoelectric material because of robust acoustic-optical phonon scattering, very low sound velocity, and high crystalline anharmonicity.

A theoretical principle for explaining the peculiarity in “edge-free” wrinkled graphene has not been firmly established. Herein, we perform DFT calculations to verify the graphene nanowrinkle (GNW) feature on metal as a model system based on experimental observation. We unveil that the interfacial interaction between the graphene and the substrate plays a crucial role in leading to quantum confinement. The longitudinal direction and the effective confined length were investigated as key parameters to control the electronic structure of graphene by corrugation engineering. A series of standing waves corresponding to the “particle in a box” model was also confirmed by the charge densities of GNW.

We demonstrate the possibility of using time-space crystalline structures to simulate eight-dimensional systems based on only two physical dimensions. A suitable choice of system parameters allows us to obtain a gapped energy spectrum, making topological effects become relevant. The nontrivial topology of the system is evinced by considering the adiabatic state pumping along temporal and spatial crystalline directions. Analysis of the system is facilitated by rewriting the system Hamiltonian in a tight-binding form, thereby putting space, time, and the additional synthetic dimensions on an equal footing.

The broad and Stokes-shifted photoluminescence line in ultra-wide-band gap semiconductors has numerous potential applications starting from white light generation, nanoscale optical thermometers to quantum information readout. Here, using density functional theory with an effective U approach and hybrid functional study, we establish the correlation between self-trapped hole polaron relaxation dynamics and broadness in photoluminescence spectra of ultra-wide-band gap oxide ${\mathrm{LiGaO}}_{\text{2}}$. After electron capture, self-trapped hole undergoes large lattice relaxation and gives rise to a broad and strongly Stokes-shifted emission band peaking at 4.17 eV. The donated holes from single acceptor defects form tightly bound states and serve as deep defects. These defects also provide additional stability to bound hole polarons and produce broader luminescence bands, with peaks in the infrared to the visible range of 2.4–4.1 eV. Our findings shed light on the fundamental properties of ultra-wide-band gap semiconductor oxides and provide insights into strategies for tuning luminescence bands for future optoelectronic applications.

The triple phase transitions or simultaneous transitions of three different phases, namely, topological, parity-time $\left(\mathcal{P}\mathcal{T}\right)$ symmetry breaking, and metal-insulator transitions, are observed in an extension of the $\mathcal{P}\mathcal{T}$ symmetric non-Hermitian Aubry-André-Harper model. In this model, besides the non-Hermitian complex quasiperiodic on-site potential, non-Hermiticity is also included in the nearest-neighbor hopping terms. Moreover, the nearest-neighbor hopping terms are also quasiperiodic. The presence of two non-Hermitian parameters, one from the on-site potential and another one from the hopping part, ensures $\mathcal{P}\mathcal{T}$ symmetry transition in the system. In addition, tuning these two non-Hermitian parameters, we identify a parameters regime, where we observe the triple phase transition. Following some recent studies, an electrical circuit based experimental realization of this model is also discussed.

We demonstrate that superconductors with broken inversion symmetry support a family of stable, spatially localized configurations of the self-knotted magnetic field. These solutions, which we term “toroflux,” are the superconducting counterparts of the Chandrasekhar-Kendall states (spheromaks) that appear in highly conducting, force-free astrophysical and nuclear-fusion plasmas. The superconducting torofluxes are solutions of superconducting models, in the presence of a parity-breaking Lifshitz invariant associated with the $O$ point-group symmetry. These solutions are characterized by a nonvanishing helicity of the magnetic field, and also by a toroidal dipole moment of the magnetic field. We demonstrate that a magnetic dipole or a ferromagnetic inclusion in the bulk of a noncentrosymmetric superconductor sources finite-energy toroflux solutions.

We study martensitic transformation behavior by considering the coupling of volumetric strain and the transformation strain in a group-subgroup transformation within a Ginzburg-Landau framework. Nonthermoelastic features, including large residual strain, large thermal hysteresis, and incomplete transformation gradually, appear with increasing coupling strength. The volume change associated with the transformation due to the coupling is demonstrated to be the essential physics for nonthermoelastic features that appear in an ideal thermoelastic martensitic transformation. A materials descriptor for features associated with a martensitic transformation is proposed based on insights from the model.

The compound CuTe (vulcanite) undergoes a quasi-one-dimensional charge density wave (CDW) at $T<T_{\mathrm{CDW}}=335$ K with a $5\times 1\times 2$ periodicity. The mechanism at its origin is debated. Several theoretical works claimed that semilocal functionals are unable to describe its occurrence and ascribed its formation only to strong electron-electron interaction. Moreover, the possible role of quantum anharmonicity has not been addressed. Here, by performing quantum-anharmonic calculations, we show that semilocal functionals correctly describe the occurrence of a CDW in CuTe if ultradense electron-momentum grids allowing for small electronic temperatures are used. The distortion is driven by the perfect nesting among one-dimensional Fermi surface sheets extending in the $k_y$ direction. Quantum anharmonic effects are important and tend to suppress both the distortion and $T_{\mathrm{CDW}}$. The quantum anharmonic structural minimization of the CDW phase in the generalized gradient approximation leads, however, to distorted Te-Te bond lengths in the low temperature phase that are $21\%$ of the experimental ones at $T=20$ K. This suggests that, even if the electron-electron interaction is not crucial for the mechanism of CDW formation, it is relevant to accurately describe the structural data for the low-$T$ phase. We assess the effect of electron-electron interaction on the CDW by using the $\text{DFT}+U+V$ approximation with parameters calculated from first principles. We find that electron-electron interaction enhances the Te-Te distortion, $T_{\mathrm{CDW}}$, and the total energy gain by the distortion.

We investigate grain boundaries (GBs) in the cubic inverse Heusler phase ${\mathrm{Fe}}_2\mathrm{CoGa}$ by means of first-principles calculations based on density functional theory. Besides the energetic stability, the analysis focuses on the magnetic properties of a set of 16 GB structures in this intermetallic phase. We determine the integrated excess magnetization across the GB and analyze it in terms of the projected local magnetic moments of the atoms and their local Voronoi volumes. The results are systematically compared to those of corresponding GBs in body-centered-cubic (bcc) Fe. The studied GBs in ${\mathrm{Fe}}_2\mathrm{CoGa}$ may have a considerably increased magnetization at the GB, up to more than twice as much as in bcc Fe, depending on the GB type, while geometrical quantities such as GB widening or local GB excess volume distributions are similar for both phases. We explain this difference by the higher flexibility of the ternary ${\mathrm{Fe}}_2\mathrm{CoGa}$ phase in compensating the disturbance of a crystal defect by structural relaxation. The GB structures therefore have a lower energy accompanied by increased local magnetic moments of the Co and half of the Fe atoms within a distance of a few $\text{Å}$ around the GB plane.

This paper discusses Floquet engineering with arbitrary polarization in $\alpha \text{−}{\mathrm{RuCl}}_3$. We describe the influence of arbitrary polarization and the limiting cases of linear and circular polarization. The corresponding model is derived via perturbation theory up to fourth order. Starting from linear and circular polarization, we bridge the gap between those two limiting cases. We then study more complex Lissajous figures and general trends arising for them. In theory these Lissajous figures should inherit effects from both linear and circular polarized light and therefore yield tuning possibilities not yet explored in the context of candidate Kitaev materials.

In this work, we report low-temperature Raman spectroscopic measurement to investigate the temperature dependence of phonons and magnetic as well as electronic excitations in ${\left({\mathrm{Sm}}_{1-x}{\mathrm{Bi}}_x\right)}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ for $x=0$, 0.02, 0.035, 0.05, and 0.10. A strong spin-phonon coupling induced phonon renormalization is observed below magnetic transition temperature $T_N$ for samples with $x\le 0.035$. An underlying broad continuum attributed to pronounced spin-spin correlations is observed below $T_N$ for pure ${\mathrm{Sm}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$. Significant phonon anomalies at lower temperatures, much below $T_N$, for $x=0$ and 0.02, are observed and are attributed to a possible crossover into the Weyl semimetal state. Most interestingly, we observe clear signatures of quadratic band touching in $x=0.05$ and 0.10 systems in terms of a broad electronic continuum at low temperatures analyzed using the Raman susceptibility for a non-Fermi liquid, corroborating the inference from recent transport measurements.

We characterize the magnetic ground state of the topological kagome metal ${\mathrm{GdV}}_6{\mathrm{Sn}}_6$ via resonant x-ray diffraction. Previous magnetoentropic studies of ${\mathrm{GdV}}_6{\mathrm{Sn}}_6$ suggested the presence of a modulated magnetic order distinct from the ferromagnetism that is easily polarized by the application of a magnetic field. Diffraction data near the Gd-$L_2$ edge directly resolve a $c$-axis modulated spin structure order on the Gd sublattice with an incommensurate wave vector that evolves upon cooling toward a partial lock-in transition. While equal moment (spiral) and amplitude (sine) modulated spin states cannot be unambiguously discerned from the scattering data, the overall phenomenology suggests an amplitude modulated state with moments predominantly oriented in the $ab$ plane. Comparisons to the “double-flat” spiral state observed in Mn-based $R{\mathrm{Mn}}_6{\mathrm{Sn}}_6$ kagome compounds of the same structure type are discussed.

Pair-density waves (PDWs) are superconducting states that spontaneously break translation symmetry in systems with time-reversal symmetry (TRS). Evidence for PDWs has been seen in several recent experiments, as well as in the pseudogap regime in cuprates. Theoretical understanding of PDWs has been largely restricted to phenomenological and numerical studies, while microscopic theories typically require strong coupling or fine tuning. In this work, we provide a symmetry-based mechanism under which PDWs emerge as a weak-coupling instability of a two-dimensional TRS metal. Combining mean-field and renormalization-group analyses, we identify a weak-coupling instability towards a triplet PDW realized in the $\pi$-flux square lattice model with on-site repulsion and moderate nearest-neighbor attraction when the Fermi level crosses Van Hove singularities at 1/4 and 3/4 fillings. This PDW is protected by the magnetic translation symmetries characteristic of Hofstadter systems, of which the $\pi$-flux lattice is a special time-reversal-symmetric case.

We derive the effective $\mathbf{k}·\mathbf{p}$ Hamiltonian for an electron in monolayer $T^{\prime }\text{−}{\mathrm{MoS}}_2$ near the Fermi level in the presence of spin-orbit coupling and a perpendicular electric field. The $4\times 4\mathbf{k}·\mathbf{p}$ Hamiltonian is capable of describing the nonparabolic energy band dispersion, perpendicular electric field effect, and spin texture, in good agreement with first-principles calculations. This $\mathbf{k}·\mathbf{p}$ Hamiltonian provides a simple and convenient way to understand and manipulate the spin transport properties of monolayer $T^{\prime }\text{−}{\mathrm{MoS}}_2$ under the perpendicular electric field. Surprisingly, our model predicts a significantly large out-of-plane spin polarization $\left(S_z\right)$ induced by the charge current along the $x$ direction. We also prove that the mirror symmetry $\left({\sigma }_h\left(yz\right)\right)$ leads to the forbidden of spin polarization $\left(S_z\right)$ induced by the charge current along the $y$ direction.

In this paper, we analyze the band structure of two-dimensional (2D) halide perovskites by considering structures related to the simpler case of the series, (${\mathrm{BA})}_2{\mathrm{PbI}}_4$, in which ${\mathrm{PbI}}_4$ layers are intercalated with butylammonium $[\mathrm{BA}={\mathrm{CH}}_3$(${\mathrm{CH}}_2$)${}_3{\mathrm{NH}}_3]$ organic ligands. We use density-functional-theory (DFT) based calculations and tight-binding (TB) models aiming to discover a simple description of the bands within 1 eV below the valence-band maximum and 2 eV above the conduction-band minimum, which, including the energy gap, is about a $\mathrm{\Delta }E=5$ eV energy range. The bands in this $\mathrm{\Delta }E$ range are those expected to contribute to the transport phenomena, photoconductivity, and light emission in the visible spectrum, at room and low temperature. We find that the atomic orbitals of the butylammonium chains have negligible contribution to the Bloch states which form the conduction and valence bands in the above defined $\mathrm{\Delta }E$ range. Our calculations reveal a rather universal, i.e., independent of the intercalating BA, rigid-band picture inside the above $\mathrm{\Delta }E$ range characteristic of the layered perovskite “matrix” (i.e., ${\mathrm{PbI}}_4$ in our example). Besides demonstrating the above conclusion, the main goal of this paper is to find accurate TB models which capture the essential features of the DFT bands in this $\mathrm{\Delta }E$ range. First, we ignore electron hopping along the $c$ axis and the octahedral distortions and this increased symmetry (from ${\mathrm{C}}_2$ to ${\mathrm{C}}_4$) halves the Bravais lattice unit cell size and the Brillouin zone unfolds to a ${45}^{\circ }$ rotated square and this allows some analytical handling of the 2D TB Hamiltonian. The Pb $6s$ and I $5s$ orbitals are far away from the above $\mathrm{\Delta }E$ range and, thus, we integrate them out to obtain an effective model which only includes hybridized Pb $6p$ and I $5p$ states. Our TB-based treatment (a) provides a good quantitative description of the DFT band structure, (b) helps us conceptualize the complex electronic structure in the family of these materials in a simple way, and (c) yields the one-body part to be combined with appropriately screened electron interaction to describe many-body effects, such as excitonic bound states.

We study transport and relaxation of spinless fermions with long-range Coulomb interactions at high temperatures through numerical simulations of out-of-equilibrium dynamics. We find that the transport and relaxation are continuously slowing down for increasing coupling $V$, and that there is a transition in the type of transport. For intermediate couplings, the system exhibits normal diffusive transport, but the timescale for the onset of that is long. For large couplings, it exhibits subdiffusive transport, while at the same time, the relaxation time diverges exponentially with system lengths, featuring a many-body-localization (MBL)-like phase. We attribute the slow transport to formation of slow bound states and stable clusters of particles. For few-particle systems, we prove the existence, visualize the slowness, and analyze the collision properties of the bound states. For many particles at high densities, there should be a hierarchy of clusters of particles on many different length scales. We argue that at large couplings, the average maximal size of the stable clusters should scale linearly with the length of the lattice, which is in accordance with the MBL-like behavior.

The spin of the conduction-band electrons in a quantum well constitutes a promising two-level system for realizing a quantum bit in the solid state. One of the important parameters in the spin dynamics is their relaxation time. It is desired to develop models and semiconductor structures in order to control it. There are several reported examples of spin lifetime control in quantum wells. Double quantum wells in a semiconductor heterostructure allow the modulation of the relaxation time. By changing the relative thicknesses of the wells in the double quantum well system, the wave function for the lowest-energy level of the conduction band can be distributed symmetrically or asymmetrically with respect to the coupling barrier between the quantum wells to control the Dresselhaus and Rashba components of the spin-orbit coupling and, consequently, the relaxation time. To detect and characterize this effect, circularly polarized photoluminescence was used at low temperatures (19 K). The results reported in the present paper suggest that this coupled double quantum well system enables the design of structures that allows for the control of the relaxation time and the performance of the electron spin as a quantum bit.