We investigate the universal properties of quantum transport in graphene nanowires that engender subtle universal conductance fluctuations. We present results for three of the main microscopic models that describe the sublattice of graphene and generate, as we shall show, all the chiral universal symmetries. The results are robust and demonstrate the widely sought sign of chirality even in the regime of many open channels. The fingerprints pave the way to distinguish systems with sublattice symmetry such as topological insulators from ordinary ones by an order of magnitude. The experimental realization requires a single measurement of the chaotic fluctuations of the associated valleytronics conductant. Through the phase coherence length, our theoretical predictions are confirmed with the data from traditional measurements in the literature concerning quantum magnetotransport.

We introduce a family of $Z\left(N\right)$ multispin quantum chains with a free fermionic ($N=2$) or free parafermionic ($N>2$) eigenspectrum. The models have $(p+1)$ interacting spins ($p=1,2,\cdots$), which are Hermitian in the $Z\left(2\right)$ (Ising) case and non-Hermitian for $N>2$. We construct a set of mutually commuting charges that allows us to derive the eigenenergies in terms of the roots of polynomials generated by a recurrence relation of order $(p+1)$. In the critical limit we identify these polynomials with certain hypergeometric polynomials ${}_{p+1}{F}_p$. Also in the critical regime, we calculate the ground-state energy in the bulk limit and verify that they are given in terms of the Lauricella hypergeometric series. The models with special couplings are self-dual and at the self-dual point show a critical behavior with a dynamical critical exponent $z_c=\frac{p+1}{N}$.

In recent years, novel two-dimensional materials such as graphene, bismuthene, and transition-metal dichalcogenides have attracted considerable interest due to their unique physical properties. However, certain lattice geometries, such as the Lieb lattice, do not exist as atomic monolayers. Fortunately, a range of physical effects can be transferred to the realms of photonics by creating artificial photonic lattices emulating these two-dimensional materials. Here, exciton-polaritons in semiconductor microcavities offer an exciting opportunity to study a part-light, part-matter quantum fluid of light in a complex lattice potential. In this Rapid Communication, we study exciton-polaritons in a two-dimensional Lieb lattice of buried optical traps. The $S$ and $P_{xy}$ photonic orbitals of such a Lieb lattice give rise to the formation of two flatbands which are of greatest interest for the distortion-free storage of compact localized states. By using a well controlled etch-and-overgrowth technique, we manage to control the trapping as well as the site couplings with great precision. This allows us to spectroscopically monitor the flatness of the flatbands across the full Brillouin zone. Furthermore, we demonstrate experimentally that these flatbands can be directly populated by condensation under nonresonant laser excitation. Finally, using this advanced device approach we demonstrate resonant and deterministic excitation of flatband modes in transmission geometry. Our findings establish the exciton-polariton systems as a highly controllable, optical many-body system to study flatband effects and for distortion-free storage of compact localized states.

We present a numerical investigation of the performance of the micropillar cavity single-photon source in terms of collection efficiency and indistinguishability of the emitted photons in the presence of non-Markovian phonon-induced decoherence. We analyze the physics governing the efficiency using a single-mode model, and we optimize efficiency $\varepsilon$ and the indistinguishability $\eta$ on an equal footing by computing $\varepsilon \eta$ as function of the micropillar design parameters. We show that $\varepsilon \eta$ is limited to $\sim 0.96$ for the ideal geometry due to an inherent tradeoff between efficiency and indistinguishability. Finally, we subsequently consider the influence of realistic fabrication imperfections and Markovian pure dephasing noise on the performance.

We present a comprehensive first-principles electronic structure study of the magnetoelastic and magnetostrictive properties in Co-based ${\mathrm{Co}}_{2}X\mathrm{Al}$ ($X=\mathrm{V}$, Ti, Cr, Mn, Fe) full Heusler compounds. In addition to the commonly used total energy approach, we employ the torque method to calculate the magnetoelastic tensor elements. We show that the torque-based methods are, in general, computationally more efficient and allow us to unveil the atomic and orbital contributions to the magnetoelastic constants in an exact manner, as opposed to the conventional approaches based on second-order perturbation with respect to the spin-orbit coupling. The magnetostriction constants are in good agreement with available experimental data. The results reveal that the main contribution to the magnetostriction constants, ${\lambda }_{100}$ and ${\lambda }_{111}$, arises primarily from the strain-induced modulation of the $\langle d_{x^2-y^2}\left|{\widehat{L}}_z\right|d_{xy}\rangle$ and $\langle d_{z^2}\left|{\widehat{L}}_x\right|d_{yz}\rangle$ spin-orbit coupling matrix elements, respectively, of the Co atoms.

It is known that ultrafast laser pulses can be used to deterministically switch magnetization and create skyrmions; however, the deterministic creation of a single Néel skyrmion after ultrafast demagnetization remains an open question. Here we show domain wall skyrmions also emerge in systems with broken inversion symmetry after exposure to an ultrafast laser pulse, carrying an integer topological charge. While domain wall skyrmions do not appear in the relaxed state due to quick thermal decay following an Arrhenius law, they play a key role in controlling the final skyrmion population through annihilations with skyrmions of opposite topological charge, with the resultant skyrmion states following a Poisson distribution. Using single-shot linearly polarized laser pulses, as well as a train of circularly polarized laser pulses, we show that when a high degree of disorder is created, the possibility of nucleating a single Néel skyrmion is accompanied by the possibility of nucleating a skyrmion with domain wall skyrmion pair, which results in a self-annihilation collapse.

In the present Rapid Communication, we explore superconductivity in $\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_2$ and $\mathrm{La}\mathrm{Ni}{\mathrm{O}}_2$ employing a first-principles derived low-energy model Hamiltonian, consisting of two orbitals: Ni $x^2-y^2$, and an axial orbital. The axial orbital is constructed out of Nd/La $d$, Ni $3z^2-r^2$, and Ni $s$ characters. Calculation of the superconducting pairing symmetry and pairing eigenvalue of the spin-fluctuation mediated pairing interaction underlines the crucial role of the interorbital Hubbard interaction in superconductivity, which turns out to be orbital selective. The axial orbital brings in material dependence to the problem, making $\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_2$ different from $\mathrm{La}\mathrm{Ni}{\mathrm{O}}_2$, thereby controlling the interorbital Hubbard interaction-assisted superconductivity.

Broken inversion symmetry in combination with the spin-orbit interaction generates a finite Dzyaloshinskii-Moriya interaction (DMI), which can induce noncollinear spin textures of chiral nature. The DMI is characterized by an interaction vector whose magnitude, direction, and symmetries are crucial to determine the stability of various spin textures, such as skyrmions and spin spirals. The DMI can be measured from the nonreciprocity of spin waves in ferromagnets, which can be probed via inelastic scattering experiments. In a ferromagnet, the DMI can modify the spin-wave dispersion, moving its minimum away from the $\mathrm{\Gamma }$ point. Spin waves propagating with opposite wave vectors are then characterized by different group velocities, energies, and lifetimes, defining their nonreciprocity. Here, we address the case of complex spin textures, where the manifestation of DMI-induced chiral asymmetries remains to be explored. We discuss such nonreciprocal effects and propose ways of accessing the magnitude and direction of the DMI vectors in the context of spin-polarized or spin-resolved inelastic scattering experiments. We show that only when a periodic magnetic system has finite net magnetization, that is, when the vector sum of all magnetic moments is nonzero, can it present a total nonreciprocal spin-wave spectrum. However, even zero-net-magnetization systems, such as collinear antiferromagnets and cycloidal spin spirals, can have spin-wave modes that are individually nonreciprocal, while the total spectrum remains reciprocal.

We report the electronic properties of single crystals of the candidate nodal-line semimetal CaAgP. The transport properties of CaAgP are understood within the framework of a hole-doped nodal-line semimetal. In contrast, Pd-doped CaAgP shows a drastic increase of magnetoresistance at low magnetic fields and a strong decrease of electrical resistivity at low temperatures probably due to weak antilocalization. Hall conductivity data indicated that the Pd-doped CaAgP has not only hole carriers induced by the Pd doping, but also high-mobility electron carriers in the proximity of the Dirac point. Electrical resistivity of Pd-doped CaAgP also showed a superconducting transition with onset temperature of $1.7-1.8\mathrm{K}$.

The silicon-vacancy (SiV) and nitrogen-vacancy (NV) centers in diamond are commonly regarded as prototypical defects for solid-state quantum information processing. Here, we show that when silicon and nitrogen are simultaneously introduced into the diamond lattice, these defects can strongly interact and form larger complexes. Nitrogen atoms strongly bind to Si and SiV centers, and complex formation can occur. Using a combination of hybrid density functional theory and group theory, we analyze the electronic structure and provide various useful physical properties, such as hyperfine structure, quasilocal vibrational modes, and a zero-phonon line, to enable experimental identification of these complexes. We demonstrate that the presence of substitutional silicon adjacent to nitrogen significantly shifts the donor level toward the conduction band, resulting in an activation energy for the SiN center that is comparable to phosphorus. We also find that the neutral SiNV center is of particular interest due to its photon emission at $\sim 1530\mathrm{nm}$, which falls within the $C$ band of telecom wavelengths and its paramagnetic nature. In addition, the optical transition associated with the ${\mathrm{SiNV}}^0$ color center exhibits very small electron-phonon coupling (Huang-Rhys factor = 0.78) resulting in high quantum efficiency (Debye-Waller factor = 46%) for single-photon emission. These features render this new center very attractive for a potential application in scalable quantum telecommunication networks.

Through fluctuations in the local composition, InAsP quantum dots embedded within site-selected InP nanowires are observed to display biexciton binding energies spanning a range between $-0.3\mathrm{meV}$ and $+2.9\mathrm{meV}$. From this range we select dots having energy-degenerate exciton and biexciton emission and observe an excitation rate-mediated transition from sub- to super-Poissonian second-order correlation statistics. Under pulsed excitation, $g^{\left(2\right)}(\tau =0)$ is found to increase from 0.5 at high excitation levels, rising to 28 as the excitation is reduced by two orders of magnitude. The observed second-order correlation statistics are interpreted using both a stochastic model and a rate equation model of the competition between the various excitonic emission processes. Our results demonstrate that nanowire quantum dots represent a promising approach to the efficient generation of twin-photon states.

Electron-electron interactions (EEIs) in 2D van der Waals (vdW) nanostructures is a topic of high current interest, with implications in both fundamental physics and nanoelectronics. In this Rapid Communication, we report the observation of a negative parabolic magnetoresistance (MR) in the multilayer 2D semiconductor InSe beyond the low-field weak localization/antilocalization regime, and provide evidence for the EEI origin of this MR behavior. Further, we analyze this negative parabolic MR and other observed quantum transport signatures of EEIs (temperature-dependent conductance and Hall coefficient) within the framework of Fermi-liquid theory and extract the gate voltage tunable Fermi-liquid parameter $F_0^{\sigma }$ which quantifies the electron spin-exchange interaction strength. This work opens up different directions for investigations of EEI effects in the electron transport of 2D vdW nanostructures.

Using time-resolved two-photon photoemission, we systematically study temperature effects on the Shockley-derived unoccupied interface states of perylene- and naphthalene-tetracarboxylic dianhydride (PTCDA and NTCDA) adsorbed on Ag(111). With increasing sample temperatures from 100 to 300 K, we observe a continuous decrease of the interface-state energy by 0.06 and 0.1 eV, respectively, for the two different molecule/metal systems. We relate this energy shift to a molecular lifting off the metal surface. The lifting is accompanied with an increase of the interface-state lifetime from 28 to 53 fs for PTCDA and from 60 to 110 fs for NTCDA. This is in qualitative agreement with the reduction of phase space for electron scattering associated with the thermal energy shift. The magnitude of the experimentally observed lifetime increase, however, is much stronger than would be expected from the energy shift alone.

We theoretically demonstrate that dc electron flow across the junction of two-dimensional electron systems leads to excitation of edge magnetoplasmons. The threshold current for such plasmon excitation does not depend on contact effects and approaches zero for ballistic electron systems, which makes a strong distinction from the well-known Dyakonov-Shur and Cerenkov-type instabilities. We estimate the competing plasmon energy gain from dc current and loss due to electron scattering. We show that plasmon self-excitation is feasible in GaAs-based heterostructures at $T\lesssim 200\mathrm{K}$ and magnetic fields $B\lesssim 10\mathrm{T}$.

We study an open Heisenberg XXZ spin chain with long-range Ising-type interaction, which is incoherently driven at its boundaries and, therefore, a far-from-equilibrium steady-state current is induced. Quantum Monte Carlo techniques make a regime accessible where the system size is large enough to examine the transition from a diffusive to a ballistic transport regime. We find that the chain lengths for this transition are increasing with decreasing range of the Ising-type interactions between distant spins, whereas the strength of the incoherent driving does not have a relevant effect on the transport transition. The transition can be explained by the suppression of ferromagnetic domains at the edges of the chain.

We present an exact solution for the heat conductance along a harmonic chain connecting two reservoirs at different temperatures. In this model, the end points correspond to Brownian particles with different damping coefficients. Such analytical expression for the heat conductance covers its behavior from mesoscopic to very long one-dimensional quantum chains and validates the ballistic nature of the heat transport in the latter example. This implies the absence of the Fourier law for classical and quantum harmonic chains. We also provide a thorough analysis of the normal modes of system which helps us to satisfactorily interpret these results.

We study Cooper-pair transport through a quantum point contact between a superconductor and a quantum Hall edge state at integer and fractional filling factors. We calculate the tunneling current and its finite-frequency noise to the leading order in the tunneling amplitude for dc and ac bias voltage in the limit of low temperatures. At zero temperature and in the case of tunneling into a single edge channel both the conductance and differential shot noise vanish as a result of the Pauli exclusion principle. In contrast, in the presence of two edge channels, this Pauli blockade is softened and a nonzero conductance and shot noise are revealed.

We unveil the connection between magnetic anisotropy and band structures in $\mathrm{\Gamma }$-specialized ${\mathrm{In}}_2{\mathrm{Se}}_3/{\mathrm{Fe}}_3{\mathrm{GeTe}}_2$ heterostructure, through first-principles calculations. A significant band distortion occurs near the $\mathrm{\Gamma }$ point with a formed valley. As the band-distortion-caused valley moves relative to the quasivalence band maximum, the magnetic anisotropy switches between in-plane and out-of-plane magnetic anisotropy. Such a rule applies to either the strained or polarization-switched ${\mathrm{In}}_2{\mathrm{Se}}_3/{\mathrm{Fe}}_3{\mathrm{GeTe}}_2$ heterostructure. These findings demonstrate the feasibility of predicting the magnetic anisotropy by band structure modification in two-dimensional magnetic systems.