Twisted graphene bilayers (TGBs) with slight twist angle always have low-energy Van Hove singularities (VHSs) that are strongly localized around AA-stacked regions of the moiré pattern. The localized VHSs are strongly related to novel correlated quantum states, such as Mott-like insulating phase and unconventional superconductivity, observed in the slightly TGBs. Here we studied the electronic properties of a TGB with $\theta \sim 1.{64}^{\circ }$ and demonstrated that a VHS splits into two spin-polarized states flanking the Fermi energy when the VHS is close to the Fermi level. Such a result indicates that localized magnetic moments emerge in the AA-stacked regions of the TGB. Since the low-energy VHSs can be reached in slightly TGBs, our result therefore provides a facile direction to realize novel quantum phases in graphene system.

The composition dependence of the charge density wave (CDW) state is investigated in a solid solution of 1T-${\mathrm{TiSe}}_{2-x}{\mathrm{Te}}_2$ using neutron scattering and a pair density function analysis. It is observed that the CDW order is quickly suppressed with doping, between $0.2\le x\le 0.25$. The suppression of the CDW is coupled with a change in the local symmetry from hexagonal to monoclinic. In the monoclinic, high-temperature phase, a Jahn-Teller (JT) distortion is observed between Ti and Se bonds that is suppressed with Te doping. These observations favor a CDW that is driven by a cooperative JT effect, as Ti-Se bond shortening is crucial to CDW formation and excess chalcogen to Ti charge transfer inhibits the JT mechanism.

We show the occurrence of Dirac, triple-point, Weyl semimetal, and topological insulating phases in a single ternary compound using specific symmetry-preserving perturbations. Based on first-principles calculations, the $\mathit{k}·\mathit{p}$ model, and symmetry analysis, we show that alloying-induced precise symmetry breaking in SrAgAs (space group $\mathrm{P}{6}_3/mmc$) leads us to tune various low-energy excitonic phases transforming from Dirac to topological insulating via the intermediate triple-point and Weyl semimetal phases. We also consider the effect of external magnetic field, causing time reversal symmetry (TRS) breaking and analyze the effect of TRS on the realization of the Weyl state. Importantly, in this material, the Fermi level lies extremely close to the nodal point with no extra Fermi pockets, which further makes this compound an ideal platform for topological study. The multifold band degeneracies in these topological phases are analyzed based on point group representation theory. The topological insulating phase is further confirmed by calculating the ${\mathrm{Z}}_2$ index. Furthermore, the topologically protected surface states and Fermi arcs are investigated in some detail.

We demonstrate the possibility of drastically reducing the velocity of phonons in quasi-one-dimensional Bose-Einstein condensates. Our scheme consists of a dilute dark soliton “gas” that provide the trapping for the impurities that surround the condensate. We tune the interaction between the impurities and the condensate particles in such a way that the dark solitons result in an array of qutrits (three-level structures). We compute the phonon-soliton coupling and investigate the decay rates of these three-level qutrits inside the condensate. As such, we are able to reproduce the phenomenon of acoustic transparency based purely on matter-wave phononics, in analogy with the electromagnetically induced transparency effect in quantum optics. Thanks to the unique properties of transmission and dispersion of dark solitons, we show that the speed of an acoustic pulse can be brought down to ∼5 $\mu \mathrm{m}$/s, ∼${10}^3$ times lower than the condensate sound speed. This is a value that greatly underdoes most of the reported studies for phononic platforms. We believe the present work could pave the stage for a new generation of “stopped-sound”-based quantum information protocols.

The light-matter coupling between electromagnetic modes guided by a semiconductor nanowire and excitonic states of molecules localized in its surrounding media is studied from both classical and quantum perspectives, with the aim of describing the strong-coupling regime. Weakly guided modes (bare photonic modes) are found through a classical analysis, identifying those lowest-order modes presenting large electromagnetic fields spreading outside the nanowire while preserving their robust guided behavior. Experimental fits of the dielectric permittivity of an organic dye that exhibits excitonic states are used for realistic scenarios. A quantum model properly confirms through an avoided mode crossing that the strong-coupling regime can be achieved for this configuration, leading to Rabi splitting values above 100 meV. In addition, it is shown that the coupling strength depends on the fraction of energy spread outside the nanowire, rather than on the mode field localization. These results open up a new avenue towards strong-coupling phenomenology involving propagating modes in nonabsorbing media.

Hybrid quantum systems, which combine quantum-mechanical systems with macroscopic mechanical oscillators, have attracted increasing interest as they are well suited as high-performance sensors or transducers in quantum computers. A promising candidate is based on diamond cantilevers, whose motion is coupled to embedded nitrogen-vacancy (NV) centers through crystal deformation. Even though this type of coupling has been investigated intensively in the past, several inconsistencies exist in available literature, and no complete and consistent theoretical description has been given thus far. To clarify and resolve these issues, we here develop a complete and consistent formalism to describe the coupling between the NV spin degree of freedom and crystal deformation in terms of stress, defined in the crystal coordinate system $XYZ$, and strain, defined in the four individual NV reference frames. We find that describing crystal deformation in $XYZ$ significantly simplifies the formalism and therefore constitutes the preferred approach for future advances in the field. Furthermore, we illustrate how the developed formalism can be employed to extract values for the spin-stress and spin-strain coupling constants from data published by Teissier et al. [Phys. Rev. Lett. 113, 020503 (2014)].

A useful experimental signature of the ordinary spin Hall effect is the spin accumulation it produces at the sample edges. The superspin Hall current [Phys. Rev. B 96, 094512 (2017)] is a transverse equilibrium spin current which is induced by a charge supercurrent. We study the superspin Hall current numerically, and find that it does not give rise to a similar edge magnetization. We also predict and numerically confirm the existence of the inverse superspin Hall effect, which produces a transverse charge supercurrent in response to an equilibrium spin current. We verify the existence of the inverse superspin Hall effect both for a spin-polarized charge supercurrent and an exchange spin current, and propose that a ${\varphi }_0$ junction produced by the inverse superspin Hall effect can be used to directly and electrically measure the spin polarization of a charge supercurrent. This provides a possible way to solve the long-standing problem of how to directly detect the spin polarization of supercurrents carried by triplet Cooper pairs.

Silicon carbide (SiC)-based defects are promising for quantum communications, quantum information processing, and for the next generation of quantum sensors, as they feature long coherence times, frequencies near the telecom, and optical and microwave transitions. For such applications, the efficient initialization of the spin state is necessary. We develop a theoretical description of the spin-polarization process by using the intersystem crossing of the silicon vacancy defect, which is enabled by a combination of optical driving, spin-orbit coupling, and interaction with vibrational modes. By using distinct optical drives, we analyze two spin-polarization channels. Interestingly, we find that different spin projections of the ground state manifold can be polarized. This paper helps in understanding initialization and readout of the silicon vacancy and explains some existing experiments with the silicon vacancy center in SiC.

We study $d$-wave superconductivity in twisted bilayer graphene and reveal phenomena that arise due to the moiré superlattice. In the $d$-wave pairing, the relative motion (RM) of two electrons in a Cooper pair can have either $d+id$ or $d-id$ symmetry with opposite angular momenta. Due to the enlarged moiré superlattice, the center-of-mass motion (COMM) can also carry a finite angular momentum while preserving the moiré periodicity. By matching the total angular momentum, which has contributions from both the RM and the COMM, Cooper pairs with $d+id$ and $d-id$ RMs are intrinsically coupled in a way such that the COMM associated with one of the RMs has a spontaneous vortex-antivortex lattice configuration. Another phenomenon is that the chiral $d$-wave state carries spontaneous bulk circulating supercurrent. The chiral $d$-wave superconductors are gapped and also topological as characterized by an integer Chern number. Nematic $d$-wave superconductors, which could be stabilized, for example, by uniaxial strain, are gapless with point nodes.

Spin crossover plays a central role in the structural instability, net magnetic moment modification, metallization, and even in superconductivity in corresponding materials. Most reports on the pressure-induced spin crossover with a large volume collapse have so far focused on compounds with a single transition metal. Here we report a comprehensive high-pressure investigation of a mixed Fe-Mn perovskite ${\mathrm{La}}_2{\mathrm{FeMnO}}_6$. Under pressure, the strong coupling between Fe and Mn leads to a combined valence/spin transition: $\mathrm{F}{\mathrm{e}}^{3+}\left(S=5/2\right)\to \mathrm{F}{\mathrm{e}}^{2+}\left(S=0\right)$ and $\mathrm{M}{\mathrm{n}}^{3+}\left(S=2\right)\to \mathrm{M}{\mathrm{n}}^{4+}\left(S=3/2\right)$, with an isostructural phase transition. The spin transitions of both Fe and Mn are offset by $\sim 20$ GPa of the onset pressure, and the lattice collapse occurs in between. Interestingly, ${\mathrm{Fe}}^{3+}$ ion shows an abnormal behavior when it reaches a lower valence state (${\mathrm{Fe}}^{2+}$) accompanied by a +0.5 eV energy shift in the Fe K-absorption edge at 15 GPa. This process is associated with the charge-spin-orbital state transition from high spin ${\mathrm{Fe}}^{3+}$ to low spin ${\mathrm{Fe}}^{2+}$, caused by the significantly enhanced $t_{2g}\text{−}{e}_g$ crystal field splitting in the compressed lattice under high pressure. Density functional theory calculations confirm the energy preference of the high-pressure state with charge redistribution accompanied by spin state transition of Fe ions. Moreover, ${\mathrm{La}}_2{\mathrm{FeMnO}}_6$ maintains semiconductor behaviors even when the pressure reached 144.5 GPa as evidenced by the electrical transport measurements, despite the huge resistivity decreasing seven orders of magnitude compared with that at ambient pressure. The investigation carried out here demonstrates high flexibility of double perovskites and their good potentials for optimizing the functionality of these materials.

The spin structure and spin dynamics of excitons in an ensemble of (In,Al)As/AlAs quantum dots (QDs) with type-I band alignment, containing both direct and indirect band gap dots, are studied. Time-resolved and spectral selective techniques are used to distinguish between the direct and indirect QDs. The exciton fine structure is studied by means of optical alignment and optical orientation techniques in magnetic fields applied in the Faraday or Voigt geometries. A drastic difference in emission polarization is found for the excitons in the direct QDs involving a $\mathrm{\Gamma }$-valley electron and the excitons in the indirect QDs contributed by an $X$-valley electron. We show that in the direct QDs the exciton spin dynamics is controlled by the anisotropic exchange splitting, while in the indirect QDs it is determined by the hyperfine interaction with nuclear field fluctuations. The anisotropic exchange splitting is determined for the direct QD excitons and compared with model calculations.

The entanglement spectrum (ES) provides a barometer of quantum entanglement and encodes physical information beyond that contained in the entanglement entropy. In this paper, we explore the ES of stabilizer codes, which furnish exactly solvable models for a plethora of gapped quantum phases of matter. Studying the ES for stabilizer Hamiltonians in the presence of arbitrary weak local perturbations thus allows us to develop a general framework within which the entanglement features of gapped topological phases can be computed and contrasted. In particular, we study models harboring fracton order, both type-I and type-II, and compare the resulting ES with that of both conventional topological order and of (strong) subsystem symmetry protected topological (SSPT) states. We find that nonlocal surface stabilizers (NLSS), a set of symmetries of the Hamiltonian which form on the boundary of the entanglement cut, act as purveyors of universal nonlocal features appearing in the entanglement spectrum. While in conventional topological orders and fracton orders, the NLSS retain a form of topological invariance with respect to the entanglement cut, subsystem symmetric systems—fracton and SSPT phases—additionally show a nontrivial geometric dependence on the entanglement cut, corresponding to the subsystem symmetry. This sheds further light on the interplay between geometric and topological effects in fracton phases of matter and demonstrates that strong SSPT phases harbour a measure of quasilocal entanglement beyond that encountered in conventional SPT phases. We further show that a version of the edge-entanglement correspondence, established earlier for gapped two-dimensional topological phases, also holds for gapped three-dimensional fracton models.

We present a detailed model for the nonadiabatic coupling between ions and electrons in energetic ion-solid interactions over a wide range of energies in concentrated solid-solution fcc alloys of the $3d$ transition metals Ni, Co, Fe, and Cr. The model is based on general statistical mechanical principles and results in a stochastic modification of the classical nuclei motion which is parameterized by the first-principles calculation of a dissipation function produced by explicit time-dependent electronic evolution. This model provides a full picture of an entire collision process, from the ballistic to the thermal phases of a cascade, giving a detailed description of the energy exchange between ions and electrons till their final thermalization, removing in this way some ad hoc assumptions used in the state-of-the-art atomistic two-temperature models. This work is separated in two papers; in the present Part I, we report on the ab initio methodology used to translate stopping power and electron-phonon interaction into a parameterized dissipation function; Part II, to be published, addresses the nonadiabatic ion dynamics using our modified Langevin dynamics [Tamm et al. Phys. Rev. Lett. 120, 185501 (2018)] applying the dissipation functions developed here to specific collision cascade events.

We present a model for the role of electrons in collision cascades in solids in which the coupling between ions and electrons is calculated using first-principles models and introduced into the classical ion equations of motion using our modified Langevin dynamics [A. Tamm et al., Phys. Rev. Lett. 120, 185501 (2018)]. This model gives a full picture of the entire collision process, from the ballistic to the thermal phases of a cascade, giving a detailed representation of the energy exchange between ions and electrons until their final thermalization, removing in this way some ad hoc assumptions used in the state-of-the-art two-temperature model. This work is separated into two papers: Part I [M. Caro et al., Phys. Rev. B 99, 174301 (2019)] reports on the ab initio methodology used to translate stopping power into the parametrized dissipation function. Part II applies the nonadiabatic ion dynamics using the dissipation functions developed in Part I to specific collision cascade events.

The crystalline electric field (CEF) influence is usually described by considering an ideally symmetrical rare-earth site. In the case of cage compounds, ample excursions of the rare earth inside the cage require an adapted CEF description. A corrective, position-dependent, CEF term accounts for the deviation from the perfect symmetry. In the paramagnetic range, a CEF level with orbital degeneracy thus acquires a width reflecting the rare-earth spatial distribution. In the case, frequent in cubic systems, of an orbitally degenerate CEF ground state at the center, this width introduces an additional energy scale, influential at low temperature. A spherical simplification allows to identify the major consequences of a cage-split ground multiplet: a Schottky-type anomaly appears in the specific heat with associated reduction of the magnetic entropy and alteration of the magnetic properties. Concomitantly, a centrifugal Jahn-Teller effect develops that expands the distribution of the magnetic ion and softens the rattling phonons. These effects are confronted with anomalous paramagnetic properties of rare-earth cage compounds, notably rare-earth-filled skutterudites and hexaborides.

We study anomalous mesoscopic transport effects at the onset of the superconducting transition focusing on the observed large Nernst-Ettinghausen signal in disordered thin films. In the vicinity of the transition, as the Ginzburg-Landau coherence length of preformed Cooper pairs diverges, short-range mesoscopic fluctuations are equivalent to local fluctuations of the critical temperature. As a result, the dynamical susceptibility function of pair propagation acquires a singular mesoscopic component, and consequently, superconducting correlations give rise to enhanced mesoscopic fluctuations of thermodynamic and transport characteristics. In contrast to disordered normal metals, the rms value of mesoscopic conductivity fluctuations ceases to be universal and displays strong dependence on dimensionality and temperature and under certain conditions can exceed its quantum normal-state value by a large factor. Interestingly, we find different universality as magnetic susceptibility, conductivity, and transverse magnetic thermopower coefficients all display the same temperature dependence. Finally, we discuss an enhancement of mesoscopic effects in the Seebeck thermoelectricity and Hall conductivity fluctuations as mediated by emergent superconductivity.

Description of elasticity of iron at the ultrahigh pressures is a challenging task for physics, with a potential strong impact on other branches of science. In the present work, we calculate the elastic properties of hcp iron in the pressure range of 50–340 GPa beyond the linear elasticity approximation, conventionally assumed in theoretical studies. We define the higher order elastic constants and present expressions for the long-wave acoustic modes Grüneisen parameters of a compressed hcp crystal. We obtain the second and third order elastic constants of the hcp Fe in the considered pressure interval, as well as its Grüneisen parameters for the high-symmetry directions. The latter are directly compared with the Grüneisen parameters derived from the volume dependences of the vibrational frequencies calculated in the quasiharmonic approximation. The obtained results are used for the stability analysis of the hcp phase of iron at high pressures.

We study the spin-$\frac{1}{2}$ Kitaev-Heisenberg (KJ) model in a two-leg ladder. Without a Heisenberg interaction, the Kitaev phase in the ladder model has Majorana fermions with local $Z_2$ gauge fields, and is usually described as a disordered phase without any order parameter. Here we identify a long-range nonlocal string order parameter (SOP) in the Kitaev phase which survives with a finite Heisenberg interaction. The SOP is obtained by relating the Kitaev ladder, through a nonlocal unitary transformation, to a one-dimensional $XY$ chain with an Ising coupling to a dangling spin at every site. This differentiates the Kitaev phases from other nearby phases including a rung singlet. Two phases with nonzero SOP corresponding to ferromagnetic and antiferromagnetic Kitaev interactions are identified. The full phase diagram of the KJ ladder is determined using exact diagonalization and density matrix renormalization group methods, which shows a striking similarity to the KJ model on a two-dimensional honeycomb lattice.