We investigate electronic transport in graphene nanoelectromechanical systems, known also as graphene nanodrums or nanomembranes. We demonstrate that these devices, despite their small values of strain between $0.1\%$ and $1\%$, can be used as efficient and robust valley polarizers and filters. Their working principle is based on the pseudomagnetic field generated by the strain of the graphene membrane. They work for ballistic electron beams as well as for strongly dispersed ones and can be also used as electron beam collimators due to the focusing effect of the pseudomagnetic field. We show additionally that the current flow can be estimated by semiclassical trajectories which represent a computationally efficient tool for predicting the functionality of the devices.

Very recently, Janus two-dimensional (2D) materials have gathered enormous attention due to possessing unique properties caused by the lack of out-of-plane symmetry. Herein, based on the successful fabrication of 2D ${\mathrm{MoSi}}_2{\mathrm{N}}_4$ monolayer, we have reported 12 novel Janus ${M\mathrm{Si}}_2{\mathrm{P}}_x{\mathrm{As}}_y$ ($M$ = Mo, W; and $x+y=4$) structures with excellent dynamic stability and great potentials in spintronics and valleytronics applications. First of all, our first-principles calculations show that all structures demonstrate semiconductor characteristics with a direct band gap. The phonon dispersions confirm their dynamic stability. These Janus monolayers illustrate in-plane piezoelectric coefficients up to 4.53–6.89 pm/V and the broken mirror symmetry of these Janus structures induces out-of-plane piezoelectric coefficients up to 0.024 pm/V. Strong spin-orbit coupling (SOC) in these structures results in valley polarization at the edge of the valence band, which is stemmed from the breaking of the inversion symmetry. Constructing Janus structures induces an intrinsic electric field that gives rise to the Rashba-type spin splitting around the $\mathrm{\Gamma }$ point of the valence band and the $M$ valley of the conduction band. Comprehensive studies show that spin splitting is higher in compounds with heavier atoms like ${\mathrm{WSi}}_2{\mathrm{PAs}}_3$. According to the results, ${\mathrm{WSi}}_2{\mathrm{P}}_3\mathrm{As}$ and ${\mathrm{WSi}}_2\mathrm{PAsPAs}$ exhibit the largest values of the Rashba coefficient at the $\mathrm{\Gamma }$ point of the valence band. In summary, our predicted Janus ${M\mathrm{Si}}_2{\mathrm{P}}_x{\mathrm{As}}_y$ structures enhance the Janus 2D materials family and are promising candidates for future spintronics and valleytronics applications and also motivate related experimental works in the future.

Silicon nanochannel plates (NCPs) are employed in a variety of experiments as sources of cold positronium (Ps) via positron to Ps conversion. Despite the widespread use of NCPs, no comprehensive model of the dynamics of the cooling process of Ps within the nanochannels has been provided yet. In this work, we present a classical Monte Carlo model of Ps cooling produced in NCPs and validate it against experimental values present in the literature. Our model allows us to estimate crucial characteristics such as the cooling time in NCPs, the angular distribution of emitted Ps, and the fraction of Ps annihilating into three gamma quanta. The simulated energy spectra of emitted Ps closely match experimental results for NCPs held at room temperature. At cryogenic temperature the classical thermalization model does not accurately reproduce the measured spectra, which we attribute to a transition in the cooling process towards a regime where quantum effects play a significant role.

We investigate optical effects in a three-level $V$-type quantum system involving two closely situated upper levels (a doublet), interacting with a weak probe field while located near a two-dimensional array of metal-coated dielectric nanospheres. We demonstrate that the presence of the plasmonic nanostructure leads to a significant modification of the absorption and dispersion properties of the quantum $V$ system, yielding either very narrow resonances or induced transparency for the weak probe field. Introducing a weak incoherent pumping field can result in gain with and without population inversion in the quantum system, without the need for a strong coherent pump field. Such a gain can be controlled by varying the distance of the quantum system from the plasmonic nanostructure and by adjusting the amount of incoherent pumping, as well as the doublet splitting. The threshold limits are provided both analytically and numerically for achieving gain with and without inversion using the incoherent pumping. Our analysis paves the way toward further theoretical and experimental studies for mitigating dissipative losses in plasmonic modes when the gain is difficult to achieve due to impractical pumping requirements.

We have studied the evolution of atomic structure and electronic band dispersion of oxygen-induced superstructures on the Mo(110) surface by using low-energy electron diffraction, angle-resolved photoemission spectroscopy, and density functional theory. Our experiments show that adsorption of oxygen at an elevated temperature (473 K) gets rid of the mixed-phase coexisting region observed at room temperature and exhibits three distinct superstructures: $p(2\times 2$), $p(2\times 1$), and $p(2\times 6$) phases. Oxygenation of the Mo(110) surface leads to a confinement-induced gaplike opening at the zone center that can be tuned by the overlayer oxygen coverage. The “hole” pockets on the Fermi surface of the clean Mo(110) surface are found to persist upon oxygen adsorption with slightly changed volumes, while the “electron” pocket does not show any significant shift in momentum with no Fermi surface nesting behavior until saturation oxygen coverages. Apart from the significant modifications to the electronic states of the clean Mo(110) surface, we also observe the formation of new oxygen-induced bands. The evolution of the surface states near $E_F$ can be attributed to a change in the surface potential, while the evolution of oxygen-induced bands results from the hybridization between O-$2p(m_j=1/2$) and Mo-$4d$ ($m_j=1/2$) orbitals at the interface. The interlayer separation, bond length, and lateral repulsive interactions between the adatoms are found to play a crucial role in the selective chemisorption of the Mo(110) surface.

Thermoelectric materials enable us to harness dissipated energy and make electronic devices less energy-demanding. Heat-to-electricity conversion requires materials with a strongly suppressed thermal conductivity but still high electronic conduction. This goal is largely achieved with the help of nanostructured materials, even if the bulk counterpart is not highly efficient. In this work, we investigate how thermoelectric efficiency is enhanced by many-body effects in graphene nanoribbons at low temperature. To this end, starting from the Kane-Mele-Hubbard model within a mean-field approximation, we carry out an extensive numerical study of the impact of electron-electron interactions on the thermoelectric efficiency of graphene nanoribbons with armchair or zigzag edges. We consider two different regimes, namely trivial and topological insulators. We find that electron-electron interactions are crucial for the appearance of interference phenomena that give rise to an enhancement of the thermoelectric efficiency of the nanoribbons. Lastly, we also propose an experimental setup that would help to test the validity of our conclusions.

Following a proposed scheme for realizing the time-periodically modulated Rabi frequency in experiments, we suggest density-dependent hoppings of two-species hardcore bosons in a one-dimensional optical lattice. Distinct from the previous work [Phys. Rev. Res. 2, 013275 (2020)], we study effects in the first resonance region. In the effective Hamiltonian, the intraspecies hopping occurs only if the density discrepancy of the other species on these sites is zero, while the interspecies one is allowed once the relevant density discrepancy becomes nonzero. At integer-1 filling, the quantum phase diagram of the effective Hamiltonian is determined by the perturbation analysis together with numerical calculations. We find that in the limit of dominant $J_1$, the system becomes a double-degenerate dimerized state spontaneously breaking the translation symmetry. The interplay of $J_0, J_1$, and the fixed $\overline{U}=1$ leads to three BKT transition lines and a tricritical BKT point. Exact transition lines are obtained by the level spectroscopic technique. Besides, general physical properties, including the charge gap, neutral gap, superfluid density, and dimerization strength, are investigated as well.

The quantum anomalous Hall effect (QAHE) realizes dissipationless longitudinal resistivity and quantized Hall resistance without the need of an external magnetic field. However, when reducing the device dimensions or increasing the current density, an abrupt breakdown of the dissipationless state occurs with a relatively small critical current, limiting the applications of the QAHE. We investigate the mechanism of this breakdown by studying multiterminal devices and identified that the electric field created between opposing chiral edge states lies at the origin. We propose that electric-field-driven percolation of two-dimensional charge puddles in the gapped surface states of compensated topological-insulator films is the most likely cause of the breakdown.

The assignment of epitaxy types by Wang et al. [Phys. Rev. B 103, 245430 (2021)] is critically examined. In particular, a ${\mathrm{C}}_{60}$ phase grown on Cd(0001) is denoted by them as “incommensurate”; however, it is most likely higher order commensurate. The crucial distinction between these epitaxy types is discussed here. Further, the strain values given by Wang et al. with subpercent accuracy for ${\mathrm{C}}_{60}$ monolayers are revisited. Systematic errors are identified due to the unjustified negligence of the temperature dependencies of the involved lattice constants, the largest contribution stemming from the bulk lattice constant of ${\mathrm{C}}_{60}$ used by Wang et al. as a reference value for strain calculations.

Cobaltates with $3d$ based layered honeycomb structure were recently proposed as Kitaev magnets and putative candidates to host the long-sought Kitaev spin liquid. Here we present inelastic neutron scattering results down to 50 mK for powder samples of ${\mathrm{Na}}_3{\mathrm{Co}}_2{\mathrm{SbO}}_6$ and ${\mathrm{Na}}_2{\mathrm{Co}}_2{\mathrm{TeO}}_6$, with high resolution in regions of low momentum and energy transfers. We compare the experimental data below the antiferromagnetic zigzag ordering temperature with dynamical structure factors obtained within spin wave theory. We search the wide parameter range of a $K\text{−}{J}_1\text{−}\mathrm{\Gamma }\text{−}{\mathrm{\Gamma }}^{\prime }\text{−}{J}_3$ spin 1/2 model and identify the best fits to constant momentum cuts of the inelastic neutron data. The powder average limits our ability to uniquely select a best-fit model, but we find that the experimental data is matched equally well by two classes of parameters: one with a dominant $K<0, |K/J_1|\sim 5...25$, and another with $K>0, |K/J_1|\sim 1$. We show that these classes are equivalent under the exact self-duality transformation identified by Chaloupka and Khalliulin [Phys. Rev. B 92, 024413 (2015)]. This model symmetry unifies a number of previous parameter estimates. Though the two cases are indistinguishable by our experiment, there is evidence in favor of the $K<0$ case. A purely isotropic Heisenberg model is incompatible with our results.

Parametrically driven oscillators can display period tripling in response to a drive at thrice the resonance frequency. In contrast to the parametric instability for period doubling, the symmetric fixed point corresponding to the state of rest remains stable at arbitrary strong driving for the tripling transition. Previously, it has been shown that fluctuations can circumvent this and induce a period-tripling instability. In this article, we explore an alternative way of inducing a period-tripling transition by investigating properties of period tripling due to parametric down-conversion beyond the rotating-wave approximation. We show that despite the absence of an instability threshold, off-resonant frequency contributions can induce a period-tripling transition by activating the parametric down-conversion. Moreover, we study the subsequent period-tripled states of the Josephson potential and discuss the asymmetry between the clockwise and counterclockwise rotating fixed points that only arises beyond the rotating-wave approximation.

Achieving superconductivity in hydrides at lower pressures is a long-standing scientific challenge. Here, we propose that reducing their effective dimensionality might be a possible route to achieve this goal. First-principles structural search calculations identify several two-dimensional transition metal hydrides (TMHs) with phonon-mediated superconductivity. Among them, the two ${\mathrm{CuH}}_2$ phases with $P\text{−}3m1$ and $P\text{−}6m2$ symmetries are predicted to have the highest $T_{\mathrm{c}}$ values up to 44.4 and 47.8 K, comparable to the well-known ${\mathrm{MgB}}_2$ superconductor. Their superconductivity mainly originates from the coupling of Cu $3d_{xz}/d_{yz}$ and $\mathrm{H}\text{−}1s$ electrons with in-plane H-vibrational phonons. Interestingly, both ${\mathrm{CuH}}_2$ structures show a unique superconducting energy gap by solving the anisotropic Eliashberg equations. Furthermore, the superconductivity of TMHs is closely related to the polarity of the TM-H bond. Our paper paves the way for finding hydride superconductors in low-dimensional materials.

The family of multilayered superconductors derived from doped topological insulators in the family of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ has been found to be unusually robust against nonmagnetic disorder. Recent experimental studies have highlighted the fact that the location of impurities could play a critical role in this puzzling robustness. Here we investigate the effects of four different types of impurities, on-site, interstitial, intercalated, and polar, on the superconducting critical temperature. We find that different components of the scattering potential are active depending on the impurity configuration and choice of orbitals for the effective low-energy description of the normal state. For the specific case of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$-based superconductors, we find that only the symmetric share of impurity configurations contribute to scattering, such that polar impurities are completely inactive. We also find that a more dominant mass-imbalance term in the normal-state Hamiltonian can make the superconducting state more robust to intercalated impurities, in contrast to the case of on-site or interstitial impurities.

The direct measurement of flexoelectric coefficients in epitaxial thin films is an unresolved problem, due to the clamping effect of substrates which induces a net strain (and hence parasitic piezoelectricity) in addition to strain gradients and flexoelectricity. Herein, we propose and demonstrate the use of van der Waals epitaxy as a successful strategy for measuring the intrinsic (clamping-free = flexoelectric coefficients of epitaxial thin films. We have made, measured, and compared ${\mathrm{BaTiO}}_3$ and ${\mathrm{SrTiO}}_3$ thin film capacitor heterostructures grown both by conventional oxide-on-oxide epitaxy and by van der Waals oxide-on-mica epitaxy, and found that, whereas the former is dominated by parasitic piezoelectricity, the response of the latter is truly flexoelectric. The results are backed by theoretical calculations of the film-substrate mechanical interaction, as well as by direct measurements that confirm the strain-free state of the films. van der Waals epitaxy thus emerges as powerful new tool in the study of flexoelectricity and, in particular, they finally allow exploring flexoelectric phenomena at the nanoscale (where strain gradients are highest) with direct experimental knowledge of the actual flexoelectric coefficients of thin films.

Bipartite quantum entanglement of electron spins of a Cooper pair in a dirty $s$-wave superconductor is investigated at absolute zero temperature in terms of the exchange interaction and potential scattering as well as the relative distance between the two electrons. In the case of transition-metal impurities, we utilize the T-matrix approach to obtain the relevant Green's functions. We employ the two-electron spin-space density matrix, which is associated with the perturbed Green's functions. We show that a two-spin state can be described by the Werner state for dirty superconductors as well as for clean ones. We find that both the first and second quantum phase transitions (QPTs) are generated by the competition between potential scattering and other interactions. Meanwhile, it is not possible to determine the allowable value for each interaction individually. In the case of rare-earth impurities, concurrence, as a measure of bipartite entanglement, is discussed in terms of the relative distance and collision times for all finite and infinite Debye frequencies. The results confirm that quantum correlation and QPTs can be tuned by impurities.

We study the subgap Yu-Shiba-Rusinov (YSR) states in the Richardson's model of a superconductor with a magnetic impurity for different electron pairing strengths from the weak-coupling Bardeen-Cooper-Schrieffer (BCS) regime to the strong-coupling Bose-Einstein-condensate (BEC) regime. We observe that the effect of the increasing pairing strength on the YSR excitation spectrum as a function of hybridization strength and impurity onsite potential is only quantitative and, in fact, rather weak when the results are appropriately rescaled. We furthermore show that the problem is analytically solvable in the deep BEC limit which is equivalent to flat-band superconductivity. This exact solution can be related to a zero-bandwidth (ZBW) effective BCS mean-field Hamiltonian where the superconductor is described by a single electron level with onsite pairing. The small difference between the BCS and BEC regimes of the Richardson's model explains the success of the simple ZBW calculations for BCS mean-field Hamiltonians. A ZBW model requires only a suitable parameter rescaling to become useful as a quantitative predictive tool for the full problem.

We use the variational exact diagonalization to investigate the single polaron properties for four different dual models, combining a short-range off-diagonal (Peierls) plus a longer-range diagonal (Holstein or breathing-mode) coupling. This allows us to investigate the sensitivity of various polaron properties both to the range of the diagonal coupling and to the specific diagonal coupling chosen. We find strong sensitivity to the range for all dual models as the adiabatic limit is approached; however, considerable sensitivity is observed for some quantities even in the antiadiabatic limit. Also, strong dependence of the results on the specific form of the diagonal coupling is observed everywhere in the parameter space. Taken together, these results suggest that a careful consideration must be given to the specific coupling and its proper range, when quantitative comparisons with experiments are sought.

We study the planar magnetic textures coupled to a two-dimensional Dirac surface state, where both the magnetic texture and surface state form in the magnetic topological insulator surface layer. It is shown that the radial vortex with winding number $w=\pm 1$ leads to the confinement of Dirac states, where a mapping to the Schrödinger equation of a two-dimensional hydrogen atom is found. The fully spin-polarized zero-energy bound state forms a flat band that resembles the zeroth Landau level of Dirac electrons in a uniform out-of-plane magnetic field. Remarkably, the number of the zero-energy state is topologically robust. Interestingly, when such a system is proximity coupled to an $s$-wave superconductor, the existence of Majorana zero modes at the Abrikosov vortex depends only on the relative value of the magnetic exchange coupling and the pairing strength. We conclude with a brief discussion on the physical realization with such magnetic textures.

Innovations in imaging technology involve finding strategies and materials suitable for detection applications over the entire infrared range. Herein, we propose a new design concept based on the unique feature of an excitonic insulator, namely, negative exciton transition energy ($E_t$). We demonstrate this concept using first-principles $GW$-Bethe-Salpeter equation calculations on one-dimensional organometallic wire ${\left(\mathrm{CrBz}\right)}_{\infty }$. The pristine ${\left(\mathrm{CrBz}\right)}_{\infty }$ exhibits an excitonic instability due to a negative $E_t$ for the lowest exciton. Substitutional doping can continuously tune the $E_t$ from $\sim 0$ to $\sim 0.6$ eV, which shows the ability of photon detection from terahertz to near infrared. This type of detector has advantages of outstanding wavelength selectivity, reduced thermal disturbance, and elevated working temperature. Our paper not only adds another member in the family of rare one-dimensional excitonic insulators, but also opens a new avenue for the development of high-performance infrared photodetectors in the future.

Exploring the mechanism of unconventional superconductivity has been one of the most important topics in condensed matter physics, while studying the magnetic and electronic properties of the parent compounds of unconventional superconductors can provide helpful clues. Recently, superconductivity in the orthorhombic-phase MnSe was successfully induced by applying high pressure, which makes MnSe the second Mn-based superconductor after MnP. Based on the spin-polarized density functional theory calculations, we have studied the magnetic and electronic properties of the orthorhombic-phase MnSe under high pressure. We show that there may exist strong antiferromagnetic (AFM) fluctuations in a narrow energy window (less than 3 meV/Mn) among an AFM state dubbed AFM3 and a series of staggered $n$-mer AFM states. Here the $n$-mer means that a set of $n$ adjacent spins on a line are parallelly aligned. In the AFM3 state and the $n$-mer AFM states, the Mn spins show AFM coupling along the $x$ axis and ferromagnetic (FM) coupling along the $y$ axis, but respectively host FM and staggered $n$-mer AFM correlations along the $z$ axis. Our calculations indicate that the orthorhombic-phase MnSe exhibits a metallic behavior in the low-energy magnetic states, in good accordance with the previous experimental observations. We also map the calculated energies onto an effective Heisenberg model and obtain the exchange couplings $J$, whose values can serve as a reference for analyzing the data from magnetic measurements. Two usual mechanisms like Fermi surface nesting and electron-phonon coupling can be ruled out as the origin of superconductivity. The magnetic properties in the orthorhombic-phase MnSe under high pressure need future in-depth experimental examination.