We study spin transport in bilayer graphene (BLG), spin-orbit coupled to a tungsten di sulfide (${\mathrm{WS}}_{\text{2}}$) substrate, and measure a record spin lifetime anisotropy $\sim 40–70$, i.e., the ratio between the out-of-plane ${\tau }_{\perp }$ and in-plane spin relaxation time ${\tau }_{\left|\right|}$. We control the injection and detection of in-plane and out-of-plane spins via the shape anisotropy of the ferromagnetic electrodes. We estimate ${\tau }_{\perp }\sim 1–2\mathrm{ns}$ via Hanle measurements at high perpendicular magnetic fields and via a new tool we develop: oblique spin valve measurements. Using Hanle spin-precession experiments we find a low ${\tau }_{\left|\right|}\sim 30\mathrm{ps}$ in the electron-doped regime which only weakly depends on the carrier density in the BLG and conductivity of the underlying ${\mathrm{WS}}_{\text{2}}$, indicating proximity-induced spin-orbit coupling (SOC) in the BLG. Such high ${\tau }_{\perp }$ and spin lifetime anisotropy are clear signatures of strong spin-valley coupling for out-of-plane spins in $\mathrm{BLG}/{\mathrm{WS}}_{\text{2}}$ systems in the presence of SOC and unlock the potential of BLG/transition metal dichalcogenide heterostructures for developing future spintronic applications.

Twisted van der Waals heterostructures and the corresponding superlattices, moiré superlattices, are remarkable material platforms, in which electron interactions and excited-state properties can be engineered. Particularly, the band offsets between adjacent layers can separate excited electrons and holes, forming interlayer excitons that exhibit unique optical properties. In this work, we employ the first-principles GW-Bethe-Salpeter equation method to calculate quasiparticle band gaps, interlayer excitons, and their modulated excited-state properties in twisted $\mathrm{MoS}{\mathrm{e}}_2/\mathrm{WS}{\mathrm{e}}_2$ bilayers that are of broad interest currently. In addition to achieving good agreement with the measured interlayer exciton energies, we predict a more than 100-meV lateral quantum confinement on quasiparticle energies and interlayer exciton energies, guiding the effort on searching for localized quantum emitters and simulating the Hubbard model in two-dimensional twisted structures. Moreover, we find that the optical dipole oscillator strength and radiative lifetime of interlayer excitons are modulated by a few orders of magnitude across moiré supercells, highlighting the potential of using moiré crystals to engineer exciton properties for optoelectronic applications.

The full spatial 3D profile of Majorana bound states (MBS) in a nanowirelike setup featuring a semiconducting carbon nanotube (CNT) as the central element is discussed. By atomic tight-binding calculations, we show that the chiral nature of the CNT lattice is imprinted in the MBS wave function which has a helical structure, anisotropic in the transverse direction. The local spin-canting angle displays a similar spiral pattern, varying around the CNT circumference. We reconstruct the intricate 3D profile of the MBS wave function analytically, using an effective low-energy Hamiltonian accounting both for the electronic spin and valley degrees of freedom of the CNT. In our model, the four components of the Majorana spinor are related by the three symmetries of our Bogoliubov-de Gennes Hamiltonian, reducing the number of independent components to one. A Fourier transform analysis uncovers the presence of three contributions to the MBS, one from the $\mathrm{\Gamma }$-point and one from each of the Fermi points, with further complexity added by the presence of two valley states in each contribution.

In this work an alternate pathway is demonstrated to form ultrathin cobalt ferrite (${\mathrm{Co}}_x{\mathrm{Fe}}_{3-x}{\mathrm{O}}_4$) films by interdiffusion of ${\mathrm{Fe}}_3{\mathrm{O}}_4$/CoO bilayers. Bilayer samples with different ${\mathrm{Fe}}_3{\mathrm{O}}_4$/CoO thickness ratios have been prepared by reactive molecular beam epitaxy on Nb-doped ${\mathrm{SrTiO}}_3$(001) substrates to obtain cobalt ferrite films of varied stoichiometry. Subsequently, oxygen-assisted postdeposition annealing experiments for consecutive temperature steps between $300{}^{\circ }\mathrm{C}$ and $600{}^{\circ }\mathrm{C}$ have been conducted monitoring the interdiffusion process by means of high-resolution x-ray reflectivity, soft and angle-resolved hard x-ray photoelectron, and x-ray absorption spectroscopy. Magnetic properties were characterized using superconducting quantum interference device magnetometry. The interdiffusion process starts from $300{}^{\circ }\mathrm{C}$ annealing temperature and is completed for temperatures above $500{}^{\circ }\mathrm{C}$. For completely interdiffused films with Co:Fe ratios larger than 0.84:2 a thin segregated CoO layer on top of the ferrite is formed. This CoO segregation is attributed to surface and interface effects. In addition, multiplet calculations of x-ray absorption spectra are performed to determine the occupancy of different sublattices. These results are correlated with the magnetic properties of the ferrite films. A stoichiometric ${\mathrm{CoFe}}_2{\mathrm{O}}_4$ film with partial inversion has been formed exhibiting homogeneously distributed ${\mathrm{Co}}^{2+}$ and mainly ${\mathrm{Fe}}^{3+}$ valence states if the initial Co:Fe content is 1.09:2. Thus, for the formation of stoichiometric cobalt ferrite by the proposed postdeposition annealing technique an initial Co excess has to be provided as the formation of a top CoO layer is inevitable.

Recent experiments with silicon qubits demonstrated strong coupling of a microwave resonator to the spin of a single electron in a double quantum dot, opening up the possibility of long-range spin-spin interactions. We present our theoretical calculation of effective interactions between distant quantum dot spins coupled by a resonator, and propose a protocol for fast, high-fidelity two-qubit gates consistent with experimentally demonstrated capabilities. Our simulations show that, in the presence of noise, spin-spin entangling gates significantly outperform cavity-mediated gates on charge qubits.

We investigate the light-cone-like spread of electronic correlations in a laser-driven quantum chain. Using the time-dependent density matrix renormalization group, we show that high-frequency driving leads to a Floquet-engineered spread velocity that determines the enhancement of density-density correlations when the ratio of potential and kinetic energies is effectively increased both by either a continuous or a pulsed drive. For large times we numerically show the existence of a Floquet steady state at not too long distances on the lattice with minimal heating. Intriguingly, we find a discontinuity of dynamically scaled correlations at the edge of the light cone, akin to the discontinuity known to exist for quantum quenches in Luttinger liquids. Our work demonstrates the potential of pump-probe experiments for investigating light-induced correlations in low-dimensional materials and puts quantitative speed limits on the manipulation of long-ranged correlations through Floquet engineering.

By using density functional theory calculations with an on-site Coulomb repulsion term combined with Boltzmann transport theory, we explore the effect of $t_{2g}$ orbital occupation on the electronic, magnetic, and thermoelectric properties of ${\left(\mathrm{Sr}X{\mathrm{O}}_3\right)}_1/{\left({\mathrm{SrTiO}}_3\right)}_n\left(001\right)$ superlattices with $n=1,3$ and $X=\mathrm{V}$, Cr, and Mn. In order to disentangle the effect of quantum confinement and octahedral rotations and to account for a wider temperature range, $P4/mmm$ (untilted) and $P{2}_1/c$ (tilted) phases are considered. We find that the ground-state superlattice geometries always display finite octahedral rotations, which drive an orbital reconstruction and a concomitant metal-to-insulator transition in confined ${\mathrm{SrVO}}_3$ and ${\mathrm{SrCrO}}_3$ single layers with ferro- and antiferromagnetic spin alignments, respectively. On the other hand, the confined ${\mathrm{SrMnO}}_3$ single layer exhibits electronic properties similar to bulk. We show that confinement enhances the thermoelectric properties, particularly for ${\mathrm{SrVO}}_3$ and ${\mathrm{SrCrO}}_3$ due to the emergent Mott phase. Large room-temperature Seebeck coefficients are obtained for the tilted superlattices, ranging from 500 to $600\mu \mathrm{V}/\mathrm{K}$ near the band edges. The estimated attainable power factors of $27.9\left(26.6\right)\mu \mathrm{W}{\mathrm{K}}^{-2}{\mathrm{cm}}^{-1}$ in plane for the ${\left({\mathrm{SrCrO}}_3\right)}_1/{\left({\mathrm{SrTiO}}_3\right)}_1\left(001\right)$ superlattice with $P4/mmm(P{2}_1/c)$ symmetry and $28.1\mu \mathrm{W}{\mathrm{K}}^{-2}{\mathrm{cm}}^{-1}$ cross plane for the ${\left({\mathrm{SrMnO}}_3\right)}_1/{\left({\mathrm{SrTiO}}_3\right)}_1\left(001\right)$ superlattice with $P{2}_1/c$ symmetry compare favorably with some of the best-performing oxide thermoelectrics. This demonstrates that the idea to use quantum confinement to enhance the thermoelectric response in correlated transition-metal oxide superlattices [Phys. Rev. Mater. 2, 055403 (2018)] can be applied to a broader class of materials combinations.

We use the inchworm quantum Monte Carlo method to investigate the full counting statistics of particle and energy currents in a strongly correlated quantum dot. Our method is used to extract the heat fluctuations and entropy production of a quantum thermoelectric device, as well as cumulants of the particle and energy currents. The energy-particle current cross correlations reveal information on the preparation of the system and the interplay of thermal and electric currents. We furthermore demonstrate the signature of a crossover from Coulomb blockade to Kondo physics in the energy current fluctuations, and show how the conventional master equation approach to full counting statistics systematically fails to capture this crossover.

The dual boson approach to strongly correlated systems generally involves a dynamic (frequency-dependent) interaction in the auxiliary impurity model. In this work, we explore the consequences of forcing this interaction to be instantaneous (frequency independent) via the use of a self-consistency condition on the instantaneous susceptibility. The result is a substantial simplification of the impurity model, especially with an eye on realistic multiband implementations, while keeping desirable properties of the dual boson approach, such as the charge conservation law, intact. We show and illustrate numerically that this condition enforces the absence of phase transitions in finite systems, as should be expected from general physical considerations, and respects the Mermin-Wagner theorem. In particular, the theory does not allow the metal-to-insulator phase transition associated with the formation of the magnetic order in a two-dimensional system. At the same time, the metal-to-charge-ordered phase transition is allowed, as it is not associated with the spontaneous breaking of a continuous symmetry, and is accurately captured by the introduced approach.

The properties of the Hofstadter butterfly, a fractal, self-similar spectrum of a two-dimensional electron gas, are studied in the case where the system is additionally illuminated with monochromatic light. This is accomplished by applying Floquet theory to a tight-binding model on the honeycomb lattice subjected to a perpendicular magnetic field and either linearly or circularly polarized light. It is shown how the deformation of the fractal structure of the spectrum depends on intensity and polarization. Thereby, the topological properties of the Hofstadter butterfly in the presence of the oscillating electric field are investigated. A thorough numerical analysis of not only the Chern numbers but also the $W_3$ invariants gives the appropriate insight into the topology of this driven system. This includes a comparison of a direct $W_3$ calculation to the method based on summing up Chern numbers of the truncated Floquet Hamiltonian.

Atomically thin two-dimensional heterostructures are a promising, novel class of materials with ground-breaking properties. The possibility of choosing many constituent components and their proportions allows optimization of these materials to specific requirements. The wide adaptability comes with a cost of large parameter space making it hard to experimentally test all the possibilities. Instead, efficient computational modeling is needed. However, large range of relevant time and length scales related to physics of polycrystalline materials poses a challenge for computational studies. To this end, we present an efficient and flexible phase-field crystal model to describe the atomic configurations of multiple atomic species and phases coexisting in the same physical domain. We extensively benchmark the model for two-dimensional binary systems in terms of their elastic properties and phase boundary configurations and their energetics. As a concrete example, we demonstrate modeling lateral heterostructures of graphene and hexagonal boron nitride. We consider both idealized bicrystals and large-scale systems with random phase distributions. We find consistent relative elastic moduli and lattice constants, as well as realistic continuous interfaces and faceted crystal shapes. Zigzag-oriented interfaces are observed to display the lowest formation energy.

Heusler-type ${\mathrm{Co}}_x{\mathrm{Cr}}_{78-x}{\mathrm{Ga}}_{11}{\mathrm{Si}}_{11}$ alloys exhibit the reentrant martensitic transition, i.e., the forward martensitic transition from the paramagnetic austenite (Para-A) phase into the paramagnetic martensite (Para-M) phase and the successive reverse martensitic transition from the Para-M phase into the ferromagnetic austenite (Ferro-A) phase with decreasing temperature $T$. In the present paper, this complicated transition is analyzed by use of phenomenological free energy, which is expanded in powers of two order parameters, i.e., the tetragonal distortion $e_3$ and the magnetization $M$. It is assumed that an interference between $e_3$ and $M$ through the repulsive magnetostructural interaction, $G{e}_3^2{M}^2(>0)$, in the free energy is strong enough to suppress out the ferromagnetic martensite (Ferro-M) phase with both nonvanishing $e_3$ and $M$. With the presence of this repulsive interaction, the observed phase diagram in the $T-x$ plane is found to be successfully explained. The $T$ dependencies of $e_3$ and the entropy are also calculated to see the equilibrium state. The inverse magnetic susceptibility in the martensite phase ${\chi }^{-1}$ is derived by taking into account that the term $G{e}_3^2{M}^2(>0)$ opposes the magnetization by an external magnetic field in the martensite phase. The calculated ${\chi }^{-1}$ exhibits a negative paramagnetic Curie temperature as was observed, which verifies directly the existence of $G{e}_3^2{M}^2(>0)$. An essential role of $G{e}_3^2{M}^2(>0)$ in the phase stabilities is also found in the calculated $T$ dependence of the elastic constant $(c_{11}-c_{12})$. On the basis of the present analyses, the conditions for the appearance of the reentrant martensitic transition are described.

Near-field radiative heat transfer exhibits various effects, such as amplification due to the geometry of the system. In this work, we construct a periodic layered structure which consists of multiple layers of alternating materials. Radiative heat transfer (RHT) between nanoparticles placed on each side of an intermediate structure is studied. Thermal energy exchange between nanoparticles is assisted by transmitted evanescent fields, which is theoretically included in the system's Green's function. We show that the resulting heat transfer with the assistance of a multilayered structure is more than five orders of magnitude higher than that in the absence of the multilayered structure at the same interparticle distance. This increase is observed over a broad range of distances ranging from near to far field. This is due to the fact that the intermediate multilayered structure supports hyperbolic phonon polaritons, where the edge frequencies of the type-I and type-II reststrahlen bands coincide at a value that is approximately equal to the nanoparticle resonance. This allows high-$k$ evanescent modes to resonate with the nanoparticles. The effects of the number of layers and fill factor in the multilayered structure on RHT are examined. Finally, we show that when there is a lateral distance between the two particles assisted by the interference of surface waves, RHT conductance exhibits an oscillating and nonmonotonic behavior with respect to the lateral distance between nanoparticles. These results illustrate a powerful method for regulating energy transport in particle systems and can be relevant for effective energy management at nano-micro scales.

We investigate dynamical evolution of a topological memory that consists of two $p$-wave superconducting wires separated by a nontopological junction, focusing on the primary errors (i.e., qubit loss) and secondary errors (bit and phase flip) that arise due to nonadiabaticity. On the question of qubit loss we examine the system's response to both periodic boundary driving and deliberate shuttling of the Majorana bound states. In the former scenario, we show how the frequency-dependent rate of qubit loss is strongly correlated with the local density of states at the edge of wire, a fact that can make systems with a larger gap more susceptible to high-frequency noise. In the second scenario we confirm previous predictions concerning superadiabaticity and critical velocity, but see no evidence that the coordinated movement of edge boundaries reduces qubit loss. Our analysis on secondary bit-flip errors shows that it is necessary that nonadiabaticity occurs in both wires and that interwire tunneling be present for this error channel to be open. We also demonstrate how such processes can be minimized by disordering central regions of both wires. Finally, we identify an error channel for phase-flip errors, which can occur due to mismatches in the energies of states with bulk excitations. In the noninteracting system considered here, this error systematically opposes the expected phase rotation due to finite-size splitting in the qubit subspace.

Motivated by the recent development of terahertz pump-probe experiments, we investigate the short-time dynamics in superconductors with multiple attractive pairing channels. Studying a single-band square lattice model with a spin-spin interaction as an example, we find the signatures of collective excitations of the pairing symmetries (known as Bardasis-Schrieffer modes) as well as the order parameter amplitude (Higgs mode) in the short-time dynamics of the spectral gap and quasiparticle distribution after an excitation by a pump pulse. We show that the polarization and intensity of the pulse can be used to control the symmetry of the nonequilibrium state as well as frequencies and relative intensities of the contributions of different collective modes. We find particularly strong signatures of the Bardasis-Schrieffer mode in the dynamics of the quasiparticle distribution function. Our work shows the potential of modern ultrafast experiments to address the collective excitations in unconventional superconductors and highlights the importance of subdominant interactions for the nonequilibrium dynamics in these systems.

We discuss a two-dimensional system under the perturbation of a moiré potential, which takes the same geometry and lattice constant as the underlying lattices but mismatches up to relative rotation. Such a self-dual model belongs to the orthogonal class of a quasiperiodic system whose features have been evasive in previous studies. We find that such systems enjoy the same scaling exponent as the one-dimensional Aubry-André model $\nu \approx 1$, which saturates the Harris bound $\nu >2/d=1$ in two dimensions. Meanwhile, there exists a continuous and rapid change for the inverse participation ratio in the eigenstate-disorder plane, different from the typical one-dimensional situation where only a few or no steplike contours show up. An experimental scheme based on optical lattices is discussed. It allows for using lasers of arbitrary wavelengths and therefore is more applicable than the one-dimensional situations requiring laser wavelengths close to certain incommensurate ratios.

During the past few years high-order harmonic generation (HHG) has opened up the field of ultrafast spectroscopy to an ever larger community by providing a table-top and affordable femtosecond extreme ultraviolet (EUV) and soft-x-ray source. In particular, the field of femtomagnetism has largely benefited from the development of these sources. However, the use of x-ray magnetic circular dichroism (XMCD) as a probe of magnetization, the most versatile and reliable one, has been constrained by the lack of polarization control at HHG sources, so studies have relied on more specific magneto-optical effects. Even the recent developments on the generation of elliptically polarized harmonics have only resulted in a few time-resolved experiments relying on this powerful technique since they add complexity to already-difficult measurements. In this article we show how to easily probe magnetization dynamics with linearly polarized EUV or soft-x-ray light with a versatility similar to XMCD by exploiting the Faraday effect. Static and time-resolved measurements of the Faraday effect are presented around the Co $M$ edges. Using simple theoretical considerations, we show how to retrieve the samples magnetization dynamics from the Faraday rotation and ellipticity transients. Ultrafast demagnetization dynamics of a few nanometers in Co-based samples are measured with this method in out-of-plane as well as in-plane magnetization configurations, showing its great potential for the study of femtomagnetism.

Iridate oxides on a honeycomb lattice are considered promising candidates for realization of quantum spin liquid states. We investigate the magnetic couplings in a structural model for a honeycomb iridate ${\mathrm{K}}_2{\mathrm{IrO}}_3$, with $C_3$ point-group symmetry at the Ir sites, which is an end member of the recently synthesized iridate family ${\mathrm{K}}_x{\mathrm{Ir}}_y{\mathrm{O}}_2$. Using ab initio quantum chemical methods, we elucidate the subtle relationship between the real space symmetry and magnetic anisotropy and show that the higher point-group symmetry leads to high frustration with strong magnetic anisotropy driven by the unusually large off-diagonal exchange couplings ($\mathrm{\Gamma }$'s) as opposed to other spin-liquid candidates considered so far. Consequently, large quantum fluctuations imply lack of magnetic ordering consistent with the experiments. Exact diagonalization calculations for the fully anisotropic $K-J-\mathrm{\Gamma }$ Hamiltonian reveal the importance of the off-diagonal anisotropic exchange couplings in stabilizing a spin liquid state and highlight an alternative route to stabilize spin liquid states for ferromagnetic $K$.

We study spin transport in the one-dimensional anisotropic $S=1$ Heisenberg model. Particular emphasis is given to dynamics at infinite temperature, where current autocorrelations and spatiotemporal correlation functions are obtained by means of an efficient pure-state approach based on the concept of typicality. Our comprehensive numerical analysis unveils that high-temperature spin transport is diffusive in the easy-axis regime for strong exchange anisotropies. This finding is based on the combination of numerous signatures, such as (i) Gaussian spreading of correlations, (ii) a time-independent diffusion coefficient, (iii) power-law decay of equal-site correlations, (iv) exponentially decaying long-wavelength modes, and (v) Lorentzian line shapes of the dynamical structure factor. Moreover, we provide evidence that some of these signatures are not exclusively restricted to the infinite-temperature limit but can persist at lower temperatures as well, where we complement our results by additional quantum Monte Carlo simulations of large systems. In contrast to the easy-axis regime, we show that in the case of an isotropic chain, the signatures (i)–(v) are much less pronounced or even entirely absent, suggesting the existence of anomalous spin transport despite the nonintegrability of the model. Eventually, upon introducing a random on-site magnetic field, we observe a breakdown of diffusion and distinctly slower dynamics. In particular, our results exhibit qualitative similarities to disordered spin-$1/2$ chains and might be consistent with the onset of many-body localization in the $S=1$ model for sufficiently strong disorder.