Electron beams in two-dimensional systems can provide a useful tool to study energy-momentum relaxation of electrons and to generate microwave radiation stemming from plasma-beam instabilities. Naturally, these two applications cannot coexist: if instability exists, it strongly distorts the distribution function of beam electrons; if scattering is strong, it typically suppresses plasma instabilities. Here, we study the role of inevitable electron-electron ($e-e$) collisions on possible plasma beam instabilities in graphene and show that scattering effects are far less trivial. We find that an unstable plasma mode associated with beam bunching is stabilized already by weak $e-e$ collisions. Quite surprisingly, further enhancement of $e-e$ collisions results in loss compensation and self-excitation of an ordinary graphene plasmon mode. Such instability is interpreted as viscous transfer of momentum from an electron beam to two-dimensional plasmons. Its growth rate reaches its maximum at hydrodynamic-to-ballistic crossover, when plasmon wavelength and electron mean free path are of the same order of magnitude.

We theoretically study a scheme to distinguish the two ground states of a one-dimensional topological superconductor, which could serve as a basis for the readout of Majorana qubits. The scheme is based on parity-to-charge conversion, i.e., the ground-state parity of the superconductor is converted to the charge occupation on a tunnel-coupled auxiliary quantum dot. We describe how certain error mechanisms degrade the quality of the parity-to-charge conversion process. We consider (i) leakage due to a strong readout tunnel pulse, (ii) incomplete charge Rabi oscillations due to slow charge noise, and (iii) charge relaxation due to phonon emission and absorption. To describe these effects, we use simple model Hamiltonians based on the ideal Kitaev chain, and draw conclusions to generic one-dimensional topological superconductors wherever possible. In general, the effects of the error mechanisms can be minimized by choosing a smooth shape and an optimal strength for the readout tunnel pulse. In a case study based on InAs heterostructure device parameters, we estimate that the parity-to-charge conversion error is mainly due to slow charge noise for weak tunnel pulses and leakage for strong tunnel pulses.

We report density functional theory based simulations of the final stages in the formation process of oxidized Ni nanocontacts along with their measurable conductance under mechanical strain. Not surprisingly, the presence of O atoms drastically changes the overall conductance as well as its spin polarization character. We have considered up to two O atoms in our simulations and found that their placement near the atomic constriction largely determines the conductance values. An overall picture emerges where the stability of the nanocontact improves with respect to that of pure Ni ones, supporting longer elongations as seen in the experiments. Also a remarkable effect takes place: the emergence of half metallicity (a fully spin polarized current), which seems to be a robust effect, but not necessarily translated into a very large magnetoresistance.

The two-dimensional topological insulator phase has been observed previously in single HgTe-based quantum wells with inverted subband ordering. In double quantum wells (DQWs), coupling between the layers introduces additional degrees of freedom leading to a rich phase picture. By studying local and nonlocal resistance in HgTe-based DQWs, we observe both the gapless semimetal phase and the topological insulator phase, depending on parameters of the samples and according to theoretical predictions. Our work establishes the DQWs as a promising platform for realization of multilayer topological insulators.

High-resolution thermal expansion and magnetostriction measurements were performed on single crystals of $\alpha -{\mathrm{RuCl}}_3$ in magnetic fields applied parallel to the Ru-Ru bonds. The length changes were measured in the direction perpendicular to the honeycomb planes. Our data show clear thermodynamic characteristics for the field-induced phase transition at the critical field ${\mu }_0{H}_{\mathrm{c}1}=7.8\left(2\right)\mathrm{T}$ where the antiferromagnetic zigzag order is suppressed. At higher fields, a kink in the magnetostriction coefficient signals an additional transition or crossover around ${\mu }_0{H}_{\mathrm{c}2}\approx 11\mathrm{T}$. The extracted Grüneisen ratio shows typical hallmarks for quantum criticality near $H_{\mathrm{c}1}$ but also displays anomalous behavior above $H_{\mathrm{c}1}$. We compare our experimental data with spin-wave calculations employing a minimal Kitaev-Heisenberg model in the semiclassical limit. Most of the salient features are in agreement with each other, however, the peculiar features in the region above $H_{\mathrm{c}1}$ cannot be accounted for in our semiclassical modeling and hence suggest a genuine quantum nature. We construct a phase diagram for $\alpha -{\mathrm{RuCl}}_3$ in a magnetic field along the Ru-Ru bonds, displaying a zigzag ordered state below $H_{\mathrm{c}1}$, a quantum paramagnetic regime between $H_{\mathrm{c}1}$ and $H_{\mathrm{c}2}$, and a semiclassical partially polarized state above $H_{\mathrm{c}2}$.

We consider SU(3) fermions on the triangular lattice in the presence of a gauge potential which stabilizes a quantum Hall insulator (QHI) at the density of one particle per lattice site. We investigate the effect of the Hubbard interaction, favoring magnetic long-range order, and a three-sublattice potential (TSP), favoring a normal insulator (NI), on the system. For a weak TSP we find that the Hubbard interaction drives the QHI into a three-sublattice magnetic Mott insulator (MMI). For intermediate values of the TSP we identify two transition points upon increasing the Hubbard interaction. The first transition is from the NI to the QHI and the second transition is from the QHI to the MMI. For large values of the TSP a charge-ordered magnetic insulator (COMI) emerges between the NI and the QHI, leading to an interaction-driven COMI-to-QHI transition.

We derive a canonical form for 2-group gauge theory in $3+\text{1D}$ which shows that it is equivalent either to Dijkgraaf-Witten theory or to the so-called “EF1” topological order of Lan and Wen. According to that classification, recently argued from a different point of view by Johnson-Freyd, this amounts to a very large class of all $3+\text{1D}$ topological quantum field theories (TQFTs). We use this canonical form to compute all possible anomalies of 2-group gauge theory which may occur without spontaneous symmetry breaking, providing a converse of the recent symmetry-enforced-gaplessness constraints of Córdova and Ohmori and also uncovering some possible new examples. On the other hand, in cases where the anomaly is matched by a TQFT, we try to provide the simplest possible such TQFT. For example, with anomalies involving time reversal, ${\mathbb{Z}}_2$ gauge theory almost always works.

We introduce and develop a slave-spin mean-field technique for describing generic interacting two-level systems under time-dependent drivings, where an auxiliary $S=1$ spin is added to describe the localized character of the electrons. We show that the approach efficiently captures the main effects of the strong correlations as well as the dynamical nature of the driving, while remaining simple enough to allow for an analytical treatment. Our formalism provides a flexible solution method, which can be applied to different device configurations at an extremely small numerical cost. Furthermore, it leads to a very practical description of adiabatically driven systems in terms of frozen static solutions.

We apply the semiclassical Boltzmann formalism for the computation of transport properties to multilayer graphene. We compute the electrical conductivity as well as the thermal conductivity and thermopower for Bernal-stacked multilayers with an even number of layers. We show that the window for hydrodynamic transport in multilayer graphene is similar to the case of bilayer graphene. We introduce a simple hydrodynamic model which we dub the multifluid model and which can be used to reproduce the results for the electrical conductivity and thermopower from the semiclassical Boltzmann equation.

By pumping nonresonantly a ${\mathrm{MoS}}_2$ monolayer at 13 K under a circularly polarized continuous wave (CW) laser, we observe exciton energy redshifts that break the degeneracy between B excitons with opposite spin. The energy splitting increases monotonically with the laser power reaching as much as 18 meV, and it diminishes with the increase of temperature. The phenomenon can be explained theoretically by considering simultaneously the band-gap renormalization that gives rise to the redshift and exciton-exciton Coulomb exchange interaction that is responsible for the spin-dependent splitting. Our results offer a simple scheme to control the valley degree of freedom for ${\mathrm{MoS}}_2$ monolayers and provide an accessible method in investigating the many-body exciton-exciton interaction in such materials.

The $\mathrm{spin}-1$ Weyl point is formed by three bands touching at a single point in three-dimensional (3D) momentum space, with two of the bands showing conelike dispersion while the third band is flat. Such a triply degenerate point carries a higher topological charge $\pm 2$ and can be described by a three-band Hamiltonian. We first propose a tight-binding model of a 3D Lieb lattice with chiral interlayer coupling to form the spin-1 Weyl point. Then we design a chiral phononic crystal that carries these $\mathrm{spin}-1$ Weyl points and special straight-type acoustic Fermi arcs. We also demonstrate computationally the robust propagation of topologically protected surface states that can travel around a corner or defect without reflection. Our results pave the way to the manipulation of acoustic waves in 3D structures, and they provide a platform for exploring energy transport properties in 3D $\mathrm{spin}-1$ Weyl systems.

Using first-principles calculations, we reveal that the perpendicular magnetic anisotropy of ${\mathrm{NiI}}_2$ monolayer can be effectively enhanced via decreasing the interlayer distance of graphene/${\mathrm{NiI}}_2$ (Gr/${\mathrm{NiI}}_2$) van der Waals (vdW) heterostructures. Furthermore, by analyzing the atomic-resolved magnetocrystalline anisotropy energy (MAE), orbital hybridization-resolved MAE and the density of states we elucidate that this magnetic anisotropy enhancement mainly originated from the electronic states change of $5p$ orbitals of interfacial I atoms. At the same time, we find that the ${\mathrm{NiI}}_2$ substrate induces strong magnetic proximity effects on graphene and the quantum anomalous Hall effect (QAHE) can be acquired by decreasing the interlayer spacing. Our work demonstrates the control of magnetic anisotropy of two-dimensional ferromagnetic materials via tuning vdW interlayer distance, and provides a van der Waals system to realize the QAHE.

We consider the influence of a spin accumulation in a normal metal on the magnetic statics and dynamics in an adjacent magnetic insulator. In particular, we focus on arbitrary angles between the spin accumulation and the easy axis of the magnetic insulator. Based on Landau-Lifshitz-Gilbert phenomenology supplemented with magnetoelectronic circuit theory, we find that the magnetic texture twists into a stable configuration that turns out to be described by a virtual, or image, domain wall configuration, i.e., a domain wall outside the ferromagnet. We show that even when the spin accumulation is perpendicular to the anisotropy axis, the magnetic texture develops a component parallel to the spin accumulation for sufficiently large spin bias. The emergence of this parallel component gives rise to threshold behavior in the spin Hall magnetoresistance and nonlocal magnon transport. This threshold can be used to design novel spintronic and magnonic devices that can be operated without external magnetic fields.

We apply the concepts of stochastic thermodynamics combined with transition-state theory to develop a framework for evaluating local heat distributions across the assemblies of interacting magnetic nanoparticles (MPs) subject to time-varying external magnetic fields. We show that additivity of entropy production in the particle state-space allows separating the entropy contributions and evaluating the heat produced by the individual MPs despite interactions. Using MP chains as a model system for convenience, without losing generality, we show that the presence of dipolar interactions leads to significant heat distributions across the chains. Our study also suggests that the typically used hysteresis loops cannot be used as a measure of energy dissipation at the local particle level within MP clusters, aggregates, or assemblies, and explicit evaluation of entropy production based on appropriate theory, such as developed here, becomes necessary.

The quadruple perovskite ${\mathrm{CaMn}}_7{\mathrm{O}}_{12}$ is a topical multiferroic in which the hierarchy of electronic correlations driving structural distortions, modulated magnetism, and orbital order is not well known and may vary with temperature. X-ray resonant elastic scattering (XRES) provides a momentum-resolved tool to study these phenomena, even in very small single crystals, with valuable information encoded in its polarization and energy dependence. We present an application of this technique to ${\mathrm{CaMn}}_7{\mathrm{O}}_{12}$. By polarization analysis, it is possible to distinguish superstructure reflections associated with magnetic order and orbital order. Given the high momentum resolution, we resolve a previously unknown splitting of an orbital-order superstructure peak, associated with a distinct locked-in phase at low temperatures. A second set of orbital-order superstructure peaks can then be interpreted as a second-harmonic orbital signal. Surprisingly, the intensities of the first- and second-harmonic orbital signal show disparate temperature and polarization dependence. This orbital reordering may be driven by an exchange mechanism that becomes dominant over the Jahn-Teller instability at low temperature.

We present a systematic experimental and theoretical investigation of the magnonic band structure in dense arrays of both asymmetric and symmetric cross-section trilayered Fe(10 nm)/Cu( $t$)/Py(10 nm) nanowires (NWs). The Cu spacer thickness ($t$) is varied in the range between 0 and 10 nm. The frequency dispersion of collective spin-wave excitations in the above trilayered NW arrays has been studied by the Brillouin light-scattering technique while sweeping the wave vector perpendicularly to the nanowire length over four Brillouin zones of the reciprocal space. The experimental results have been quantitatively reproduced by an original numerical model that includes a two-dimensional Green's function description of the dipole field of the dynamic magnetization and exchange coupling between the layers. We found that, depending on $t$, the Py and Fe magnetic layers within the same nanowire are coupled by either the interlayer exchange or dipolar interactions. This has an impact on both the magnetization reversal and the collective dynamical properties of the artificial crystal. In particular, it is possible to stabilize a magnetization configuration where the layer magnetization vectors point in the same or in the opposite direction over a field range that varies with the Cu thickness. In addition, several modes are detected whose propagation properties (i.e., stationary or dispersive) through the array depend on static magnetization configuration as well as on the relative phase (in-phase or out-of-phase) of dynamic magnetizations between the two layers within the same nanowire.

Atomically precise epitaxial structures are unique systems for tunneling spectroscopy that minimize extrinsic effects of disorder. We present a systematic tunneling spectroscopy study, over a broad doping, temperature, and bias range, in epitaxial $c$-axis $\mathrm{L}{\mathrm{a}}_{2-x}\mathrm{S}{\mathrm{r}}_x\mathrm{Cu}{\mathrm{O}}_4/\mathrm{L}{\mathrm{a}}_2\mathrm{Cu}{\mathrm{O}}_4/\mathrm{L}{\mathrm{a}}_{2-x}\mathrm{S}{\mathrm{r}}_x\mathrm{Cu}{\mathrm{O}}_4$ heterostructures. The behavior of these superconductor/insulator/superconductor (SIS) devices is unusual. Down to 20 mK there is complete suppression of $c$-axis Josephson critical current with a barrier of only 2 nm of $\mathrm{L}{\mathrm{a}}_2\mathrm{Cu}{\mathrm{O}}_4$, and the zero-bias conductance remains at 20–30% of the normal-state conductance, implying a substantial population of in-gap states. Tunneling spectra show greatly suppressed coherence peaks. As the temperature is raised, the superconducting gap fills in rather than closing at $T_c$. For all doping levels, the spectra show an inelastic tunneling feature at ∼80 meV, suppressed as T exceeds $T_c$. These nominally simple epitaxial cuprate junctions deviate markedly from expectations based on the standard Bardeen-Cooper-Schrieffer theory.

Fully self-consistent GW ($sc$-GW) methods are now available to evaluate quasiparticle and spectral properties of various molecular and bulk systems. However, such techniques based on the full matrix of G and W are computationally demanding. The routinely used single-shot GW approximation ($G_0{W}_0$) has an undesirable dependency on the choice of initial exchange-correlation functional. In the literature, many so-called self-consistent GW methods are based on diagonal approximation of G and low-ranking approximation of W. It is thus worth checking how good such approximations are in comparison with the full matrix method. In this work, we consider AlAs, AlP, GaP, and ZnS as the prototype systems to perform $sc$-GW calculations by expressing the full G matrix using a plane-wave basis set. We compared our sc-GW results with the diagonal G and subspace W approximated $sc$-GW results ($sc$-GW-diagG and $sc$-GW-subW methods). In the $sc$-GW-diagG method, interacting G is expanded in the eigenvectors of noninteracting G such that only diagonal elements are retained, whereas the number of eigenmodes is truncated in $sc$-GW-subW calculations. A systematic analysis of the results obtained from the above techniques is presented. The differences in the quasiparticle band gap between the approximated and the full matrix sc-GW approaches are mostly less than 1.7%, which validates such widely adopted approximations, and also shows how such low-ranking approximation can be used to include higher-order terms such as the vertex correction.

Coupled-wire constructions offer particularly simple and powerful models to capture the essence of strongly correlated topological phases of matter. They often rely on effective theories valid in the low-energy and strong-coupling limits, which impose severe constraints on the physical systems where they could be realized. We investigate the microscopic relevance of a class of coupled-wire models and their possible experimental realization in cold-atom experiments. We connect with earlier results and prove the emergence of fractional quantum Hall states in the limit of strong interwire tunneling. Contrary to previous studies relying on renormalization group arguments, our microscopic approach exposes the connection between coupled-wire constructions and model wave functions in continuum Landau levels. Then, we use exact-diagonalization methods to investigate the appearance of these fractional quantum Hall states in more realistic settings. We examine the parameter regimes where these strongly correlated phases arise, and provide a way to detect their appearance in cold-atom experiments through standard time-of-flight measurements. Motivated by this experimental probe, we finally propose a realization of our model with cold atoms in spin-dependent optical lattices. Our estimates show that the previous fractional quantum Hall phases lie within experimentally accessible parameter regimes, giving a viable route toward their experimental study.

Electronic properties of ${\mathrm{V}}_2{\mathrm{OPO}}_4$ have been investigated by means of hard x-ray photoemission spectroscopy (HAXPES) and subsequent theoretical calculations. The V $1s$ and $2p$ HAXPES spectra are consistent with the charge ordering of ${\mathrm{V}}^{2+}$ and ${\mathrm{V}}^{3+}$. The binding energy difference between the ${\mathrm{V}}^{2+}$ and ${\mathrm{V}}^{3+}$ components is unexpectedly large, indicating large bonding-antibonding splitting between them in the final states of core level photoemission. The V $1s$ HAXPES spectrum exhibits a charge transfer satellite which can be analyzed by configuration interaction calculations on a ${\mathrm{V}}_2{\mathrm{O}}_9$ cluster. The V $3d$ spectral weight near the Fermi level is assigned to the $3d{t}_{2g}$ orbitals of the ${\mathrm{V}}^{2+}$ site. The broad V $3d$ spectral distribution is consistent with the strong hybridization between ${\mathrm{V}}^{2+}$ and ${\mathrm{V}}^{3+}$ in the ground state. The core level and valence band HAXPES results indicate substantial charge transfer from the ${\mathrm{V}}^{2+}$ site to the ${\mathrm{V}}^{3+}$ site.